L 3312A :5; 1’”
Michifixfw‘; 9:3 '23
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V.

This is to certify that the J“
thesis entitled "’1; ~
SYNTHESIS AND ABSORPTION 0F PORPHYRINS AND
PORPHYRIN INTERMEDIATES ON THE INTRACRYSTAL

SURFACES 0F MONTMORILLONITE
presented by

Shannaine S . Cady

has been accepted towards fulfillment
of the requirements for

Ph 0 Do degree in Chemistry

1/] (ZWW
y

Major professor

Dani/414141174

0-7839

 

 

 

 

 

 

 

m/Ui ABSTRACT

SYNTHESIS AND ABSORPTION OF PORPHYRINS AND
PORPHYRIN INTERMEDIATES ON THE INTRACRYSTAL
SURFACES OF MONTMORILLONITE
BY

Sharmaine S. Cady

The adsorption of pyrrole and benzaldehyde on the
interlayer surfaces of various cation exchanged
montmorillonites under aerobic and anaerobic aqueous
conditions at room temperature results in the formation of
a porphyrin intermediate which is assigned a porphodimethene
structure. This condensation to a macrocyclic structure is
achieved at pyrrole and benzaldehyde concentrations of
0.015 M or less and Mn+ to pyrrole molar ratios of 1:2,
where Mn+ = Fe3+, Cu2+, Zn2+, 002+, VO2+, Mg2+, and Na+.

The H+ and TPA+ exchange forms of montmorillonite are also
investigated.

During the course of the condensation reaction, the
aqueous clay slurries gradually change from pale pink
to a deep purple over a 24-hour period. The elapsed
time before the initial appearance of the pink color is
dependent on the acidity of the cation in the interlamellar
region. Fe(III), Cu(II), VO(II), and H+ require only minutes
to initiate the condensation while the other metal ions

require a half hour or more before any color change is

 

Sharmaine S. Cady
observed. TPA+ montmorillonitedoesnot catalyze the
reaction.

Freeze-drying of the clays with the adsorbed reaction
product yields purple clays whose Nujol mull, visible
absorption spectra exhibit an intense absorption near
500 nm characteristic of porphyrin intermediates with a
dipyrromethene chromophore. X-ray diffraction data indicate
a basal spacing of 5.1 X, which is the approximate thickness
of a porphyrin ring and indicates the intermediate is
oriented parallel to the silicate layers. This "sandwiching"
of the intermediate between the layers may result in the
flattening of the pyrrole and benzene rings which increases
the resonance interaction of the phenyl rings with the
porphyrin nucleus. This increased resonance interaction
accounts for the bathochromic shift of the visible absorption
band observed for the intermediate adsorbed on montmorillonite
from that obtained in acetic acid solution.

Desorption of the intermediate from the montmorillonite
surface in the presence of a large excess of exchangeable
cation results in its oxidation to the free base porphyrin
and/or the diprotonated cation of the porphyrin. The
highest yields of porphyrin(10 to 12 per cent based on
pyrrole) are obtained when the condensation reaction is
allowed to proceed in the presence of an exchangeable cation.

The porphyrin intermediate may be adsorbed onto the
external surfaces of Cu(II) and TPA+ montmorillonites from

a chloroform solution that is 1.1 x 10'"2 M in pyrrole and

 

Sharmaine S. Cady
benzaldehyde and contains 0.1 ml concentrated HC1.
Maximum absorption occurs at 485 nm for TPA+ montmoril—
lonite and at 505 nm for Cu(II) montmorillonite. The
bathochromic shift of the absorption in the case of Cu(II)
montmorillonite is believed to arise from chelation of the
porphyrin intermediate with the Cu(II) ions.

In studies on the adsorption of free base mggg—tetra—
phenylporphyrin by metal ion exchange forms of montmoril-
lonite, solutions of porphyrin in acetone are allowed to
react with montmorillonite at M2+ to porphyrin molar ratios
of 14:1, 2:1, 1:1, and 1:2. The adsorbed free base
porphyrin undergoes two distinct reactions with the metal
exchange ions: formation of the dication, which remains
mineral-bound, and formation of metalloporphyrins, which
are desorbed into solution.

Dication formation is found to be dependent on the
acidity of the exchangeable cation, as measured by its
ability to hydrolyze in the interlamellar region. X-ray
diffraction data, infrared pleochroic measurements, and
visible absorption spectra for the dication exchange form
of montmorillonite indicate that the dication orients
itself parallel to the silicate layers upon intercalation.

Cu(II), Zn(II), and Co(II) are the only metal exchange
ions incorporated into the porphyrin ring with the order
of the rate of incorporation being given by Cu(II)> Zn(II)>>
Co(II). Dication formation also occurs during metallo—

porphyrin formation; however, the metallation reaction

 

 

Sharmaine S. Cady
predominates. The neutral porphyrin complexes CuTPP
and CoTPP are physically adsorbed onto the external surfaces

of montmorillonite.

SYNTHESIS AND ABSORPTION OF PORPHYRINS AND
PORPHYRIN INTERMEDIATES ON THE INTRACRYSTAL

SURFACES OF MONTMORILLONITE

By

Sharmaine S. Cady

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Chemistry

1976

 

 

To
my parents
and

my husband

ii

 

ACKNOWLEDGMENTS

I wish to express my appreciation to Professor
Thomas J. Pinnavaia for the patience, guidance, and
support which he has shown me during the past five years.

I wish to thank Professor C. H. Brubaker, Jr. for being
my second reader and overseeing my grammar. I also wish
to thank Professor M. M. Mortland of the Department of
Crops and Soil Science for the interest he has expressed
in my research and for the helpful suggestions he has
given me.

A special note of thanks is due Mr. Vaughn E. Berkheiser
for his invaluable assistance in obtaining and interpreting
the eray diffraction data.

Finally, I am deeply grateful to my husband, Skip,
for the love and encouragement he has given me throughout
my graduate studies. I am especially grateful to my
parents, Mr. and Mrs. Leon J. Shive, for their continued
love and encouragement and for the many sacrifices made on

my behalf which have enabled me to pursue my education.

iii

 

 

Chapter

1.

2.

3.

TABLE OF CONTENTS

STATEMENT OF PURPOSE . . . . . . . . . . . . .
INTRODUCTION . . . . . . . . . . . . . . . . .
2.1 Clay Minerals . . . . . . . . . . . . . .
Definition . . . .
Structure . . . . .

Exchange cations .
Swelling properties

0 O O O
O O O O
O O O O
O O O O
O O O O
O O O O
O O O O
O O O O
O O O O
O O O O
O O O O

2.2 Porphyrins . . . . . . .

O
O
O
O
O
O
O
0

Structure . . . .

Ionization O O O O O O O O O O O O O 0 0
Metalloporphyrins . . . . . . . . . . . .
Spectra . . . . . . . . . . . . . . . . .
Adsorption on clay minerals . . . . . . .

2.3 meso-Tetraphenylporphyrin .

SYDtheSis 0 O O O O O O O O O 0 O O 0 0 O
Mechanisms of formation . . . . . . . . .

2.4 Chemical Evolution of Porphyrins . . . .
2.5 Prebiotic Syntheses on Clay Minerals . .
EXPERIMENTAL SECTION . . . . . . . . . . . . .
3.1 Preparation of Hbmoionic Montmorillonites

Mp++montmorillonites . . . . . . . . . .
TPA +and H montmorillonites . . . . . .

TPPHQ montmorillonite . . . . . . . . .
3.2 Synthesis of TPPH2 . . . . . . . . . . .

3.3 Adsorption of TPPH onto Montmorillonite

2

iv

of?

\O QONub-P J? uL‘ H

 

 

Chapter

3.4

3.5

3.6

3.7

3.9

Reaction between Pyrrole and Benzaldehyde
in the Presence of Montmorillonite . . .

Aerobic synthesis . . . . . . . . . . . .
Anaerobic synthesis . . . . . . . . . . .

Reaction between Pyrrole and Benzaldehyde
in Water . . . . . . . . . . . . . . . .

Desorption of the Condensation Product .

Method A
Method B
Method C
Method D

O O O O
O O O O
O O O O
O I O O
O O O O
O O O O
0 O O O
O O O O
O O O O
O O O O

Porphyrin Intermediates . . . . . . . . .

Synthes ls O O O O O O O O O O O 0 O O 0
Adsorption on montmorillonite . . . . . .

Spectroscopy . . . . . . . . . . . . . .

Visible absorption . .
Infrared absorption . .
Electron spin resonance
X-ray diffraction . . .

O O O O
O O O O
O O 0 O
O O O O
O O O O
O O O O
O O O O
O O O O
O O O 0

Chemical Analyses . . . . . . . . . . . .

RESULTS AND DISCUSSION . . . . . . . . . . . .

4.1

Interaction of meso-Tetraphenylporphyrin
with Montmorillonite . . . . . . . . . .

V0 II
Cu II
Co II
Na I) and Mg(II) montmorillonite .

and FeéIII) montmorillonite .
Mechanism of meso-tetraphenylporphyrin

and Zn II) montmorillonite .
montmorillonite . . . . .

 

adsorption . . . . . . . . . . .
Conclusion . . . . . . . . . . . .

O O O O

Page

40
40
41
42
42
42
43
44
44
45

45
46

51
51
51
51
51
51

52

 

 

 

Chapter Page

4.2 Synthesis of Porphyrin Intermediates on the
Intracrystal Surfaces of Montmorillonite 75

Visible absorption spectra . . . . . . . 75
Desorption studies . . . 81
Adsorption of the porphyrin intermediate

from salution I O O O O O O O 0 O O O O 81‘
Mechanism of meso-tetraphenylporpho-

dimethene formation . . . . . . . . . . . 86
0011011181011 0 O O O O O O O O O O O O O O O 92

BIBLIOGRAPHY O O O O O O O O O O O O O O O O O O O O O 9 3

vi

 

Table
1.

3.

LIST OF TABLES

Page

Ultraviolet and Visible Absorption Spectra

for the Reaction of TPPH2 with Montmorillonite . 54

Hydrolysis Constants for Selected Metal Ions . . 7O
Visible Absorption Spectra of the Pyrrole-

Benzaldehyde Condensation Product Adsorbed on
Various Montmorillonites . . . . . . . . . . . . 76

vii

 

LIST OF FIGURES

Figure

1.

2.

Structure of montmorillonite. . . . . . . . . . .

Molecular structures of (a) porphine, (b) chlorin,
20; tetrahydroporphyrin, (d) por h inogen,

e porphomethene, (f) phlorin, fgyrpor hodi-
methene, (h) phlorin monocation, and (i porpho-
dimethene dication. . . . . . . . . . . . . . . .

Molecular structures of (a) meso-tetraphenyl-
porphyrin, (b) meso-tetraphenylporphyrin dication,
and (c) metal chelate of meso-tetraphenyl-
porphyrin . . . . . . . . . . . . . . . . . . . .

 

 

Mechanism proposed by Badger gt a; for the
Rothemund reaction. . . . . . . . . . . . . . . .

Summary of Dolphin's proposed mechanism for
meso-tetraarylporphyrin formation . . . . . . . .

Electronic absorption spectra of TPPH++ in lacial
acetic acid, TPPH montmorillonite, d Fe III)
montmorillonite a ter adsorption of TPPH2 . . . .

Infrared absorption spectra of 8A) Cu(II) mont-
morillonite dried 1 week at 110 C and (E Cu(II)
montmorillonite after exchange with TPPH and
drying 4 weeks at 130° C to remove adsorfied
acetone . . . . . . . . . . . . . . . . . . . . .

Electronic absorption spectra of an HCl-CHCI
solution of pyrrole and benzaldehyde after 2
hours of reaction time and after exposure to
air overnight 0 O O O O O O O O O O O O O O O O 0

3

Electronic absorption spectra of TPA+ montmoril-
lonite after the adsorption of the porphyrin
intermediate from HCl-CHCl solution and Cu(II)
montmorillonite after adsoPption of the interme-
diate from HCl-CHCl3 solution . . . . . . . . . .

viii

Page

11

21

23

26

37

39

48

SO

 

Figure Page

10. Schematic drawing showing the orientation of the
porphyrin skeleton between montmorillonite layers 56

11. (A) Spectrophotometric changes after various time
intervals in the absorption spectrum of the TPPH2-
acetone solution after the addition of Cu(II)
montmorillonite. (B) and (C) Visible and Soret
absorptions of the TPPH -acetone solution before
and 24 hours after the Eddition of Cu(II)
montmorillonite . . . . . . . . . . . . . . . . . 59

12. (A) Spectrophotometric changes after various time
intervals in the absorption spectrum of the TPPH -
acetone solution after the addition of Zn(II)
montmorillonite. (B) and (C) Visible and Soret
absorptions of the TPPH -acetone solution before
and 24 hours after the addition of Zn(II)
montmorillonite . . . . . . . . . . . . . . . . . 61

13. Electronic absorption spectrum of CoTPP on Co(II)
montmorillonite C O O O O O O O O O O O O O O O O S

14. (A) Spectrophotometric changes after various time
intervals in the absorption spectrum of the TPPH2-
acetone solution after the addition of Co(II)
montmorillonite at reflux temperature. (B) and
(C) Visible and Soret absorptions of the TPPH —
acetone solution before and 24 hours after th
addition of Co(II) montmorillonite. . . . . . . . 67

15. Electronic absorption spectra of the pyrrole-
benzaldehyde condensation pr duct adsorbed on
Cu(II) montmorillonite and H montmorillonite . . 78

16. Electronic absorption spectra of the chloroform
extraction solution 2 hours and 24 hours after the
desorption of the pyrrole-benzaldehyde condensa-
tion product frgm Cu(II) montmorillonite in the
presence of TPA . . . . . . . . . . . . . . . . . 83

17. Mechanism of meso-tetraphenylporphodimethene
formation on the intracrystal surfaces of
montmorillonite . . . . . . . . . . . . . . . . . 89

 

ix

 

CHAPTER 1

STATEMENT OF PURPOSE

The interaction of porphyrins with clay minerals
has been studied by petroleum geochemists to determine
if clay surfaces played a role in the degradation of
chlorophyll to a homologous series of porphyrins similar
to that found in petroleum, oil shale, and coal. Although
numerous researchers have concentrated on the relation
between porphyrins and clay minerals in the origin of
petroleum, no attention has been given to the formation
of porphyrins in the presence of clay minerals from
simple precursor molecules available on the prebiotic,
primitive Earth 4.5 to 3.5 billion years ago. The
presence of porphyrins on the primitive Earth was necessary
for the emergence of the first living organism and the
subsequent plant and animal life that developed. The
research presented here provides in part a plausible
mechanism for the prebiotic synthesis of porphyrins by
reaction of pyrrole and an aldehyde on a clay mineral
surface.

The kinetics of mggg—tetraarylporphyrin formation

indicate that at least one step in the reaction mechanism

2

is acid-catalyzed.1-4 Since transition metal ions
exchanged onto the intracrystal surfaces of montmorillonite
generate an acidic medium in the interlamellar region by
transferring protons to interlayer water molecules, it

is feasible that montmorillonite may act as an effective
heterogeneous catalyst for the prebiotic synthesis of
porphyrins or their intermediate precursors from simple
molecules present during chemical evolution.

The Mn+ montmori1lonite-pyrrole-benzaldehyde-water
system was chosen for investigation, where Mn+ = Fe3+,
Cu2+, Zn2+, 002+, VO2+, Mg2+, and Na+. An expanding
layer lattice silicate, such as montmorillonite, was
required for the experiments in order to allow the reactant
molecules to penetrate the interlamellar region and come in
contact with the catalytic surface. water was selected as
the solvent to simulate the environment provided by the
prebiotic ocean. In order for synthesis to occur, the
montmorillonite clay must concentrate the reactant molecules
from the aqueous solution, catalyze their condensation to
macrocycles, and then provide for the storage of the
reaction products until their incorporation into the first
living cell.

Pyrrole and benzaldehyde were ideal for the reactant
molecules because the reactions between pyrroles and
benzaldehydes offer several diversified pathways and
intermediates leading to the formation of mggg—tetraaryl-

Porphyrins. It was of interest to determine whether the

3
structure of the silicate layer lattices of clay minerals
and the increased acidic nature of the interlayer water
molecules would influence the mechanism by which porphyrin
synthesis may occur.

Since free base meso-tetraphenylporphyrin, the diacid,

 

and several of the reaction intermediates have distinct and
characteristic absorption spectra, the condensation
products formed on the clay surface could be easily
identified from their electronic absorption spectra. In
addition, X-ray diffraction data could be employed to
determine whether the products were intercalated or only
externally adsorbed.

It was also necessary in the present studies to
prepare free base mggg—tetraphenylporphyrin and study its
adsorption on montmorillonite saturated with various metal
ions in order to compare its interaction with the clay
surface with that exhibited by the condensation reaction

products.

 

CHAPTER 2

INTRODUCTION

2.1 Clay Minerals

Definition. Naturally occurring clay minerals
invariably contain varying percentages of coarse, nonclay-
mineral material. A clay particle size of 2u.is frequently
the optimum size fraction for the best separation of the
clay-mineral components from the nonclay-mineral components
in the natural source materials. The term "clay" in the
context of the research presented here indicates that the
minerals employed does not exceed 2u.as the upper limit.

Structure. Clay minerals are essentially composed of
silica, alumina, and water, often with appreciable quantities
of iron, alkali cations, and alkaline earth cations.
Based on structural characteristics, clay minerals can be
classified into two groups: amorphous and crystalline.
The crystalline group can be further subdivided into
layer- and chain-type structures. Montmorillonite is a
crystalline-type, expanding layer lattice clay mineral
'with the approximate unit cell formula Na0.66[318(A13.34'

Mgo.ee>°20(ofl)e]-
Two basic structural units compose the lattices of

5
most of the clay minerals. One unit consists of a central
aluminum, magnesium or iron atom octahedrally coordinated
to six oxygen atoms or hydroxyl groups. When aluminum is
the central atom, only two thirds of the octahedral positions
are filled to maintain electrical neutrality in the
octahedral sheet. If magnesium is substituted for aluminum
in the octahedral layer, all the positions must be filled
to balance the electrical charge.

The second structural unit is the silica tetrahderon.
Each tetrahedron contains a central silicon atom bound to
the four oxygen atoms occupying the apices of the tetrahedron.
Occasionally a hydroxyl group is located at one of the apices
in order to balance the charge. These silica tetrahedra
are arranged in an open hexagonal network which is repeated
indefinitely to form a sheet. In addition, the tips of all
the tetrahedra point in the same direction, and the
triangular bases all lie in the same plane.

The montmorillonite lattice structure is composed of
three-layer units consisting of two silica tetrahedral
sheets with a central alumina octahedral sheet. The
tetrahedral and octahedral layers are arranged so that the
tips of the silica tetrahedra and one of the hydroxyl
layers of the octahedral sheet form a common layer, and
each tetrahedron shares its apex oxygen with an octahedron.

The layers composing the montmorillonite lattice are
continuous in the a and y directions and are stacked one

above the other in the c direction. The g-axis dimension

 

 

6
o
of the three-layer structural unit is 9.6 A. Figure 1

shows a diagrammatic sketch of this structure of
montmorillonite.

Exchange cations. The layer lattice units always
have a net negative charge per unit cell of -O.66
associated with them as a result of substitutions within

4* in the tetrahedral sheet

2+

the lattice of Al3+ for Si

2+ in the octahedral

and/or Mg for Al3+ or Li+ for Mg
sheet. This charge deficiency is balanced by exchangeable
cations adsorbed between the unit silicate layers and

around their edges.

These exchangeable cations are of fundamental
importance in clay chemistry for they have a pronounced
effect on many of the physical properties of clay minerals.
The graxis spacing of completely dehydrated montmorillonite
is dependent to some extent on the size of the intercalated
cation, the graxis spacing being larger for larger cations.
It is also of significant importance to the research
presented here that the intercalated cations adsorbed in
the natural state can be exchanged for other cations by
treatment with such ions in a water solution or other
suitable solvent. The exchange reaction is also
stoichiometric with montmorillonite having an approximate
exchange capacity of 90 meg/100 g clay.

Swelling properties. Co-adsorbed with the exchangeable

cations into the interlayer region are the solvent molecules,

usually water. The thickness of the water layer between

Figure 1. Structure of montmorillonite.

 

 

 

 

 

 

 

Oxygens

Hydroxyls

Si4+, occasionally A13+

 

9

the silicate units is dependent on the nature of the
exchangeable cation at a given relative humidity; however,
the thickness of the water layer is normally restricted
to an integral number of molecular water layers, usually
one or two.

The existence of a very weak bond between the
silica-alumina-silica structural units in the 2 direction
allows the penetration of inorganic and organic molecules
between the silicate layers. This ability of the
montmorillonite lattice to expand in the 3 direction is
of primary importance in studies of heterogeneous catalysis

on clay surfaces.

2.2 Porphyrins

Structure. Porphyrins are a class of highly conjugated
tetrapyrrole macrocycles which are formally derived from
the parent free base compound, porphine(PH2)(Fig. 2a), by
substitution of some or all of the hydrogen atoms in
positions 1-8 and/or the mgggr positions a—o. The
porphyrins of animal origin, including the metalloporphyrins
such as the iron heme complexes, are completely substituted
at all eight pyrrole carbon atoms. Substitution at the
mgggf positions has not been found in the naturally
occurring porphyrins.

5’6 show it

Crystal studies on the porphine molecule
to be essentially planar; however, the porphyrin ring

system is flexible, having a low energy barrier with

 

10

Figure 2. Molecular structures of (a) porphine, (b) chlorin,

c tetrahydroporphyrin, (d) porph inogen,

e porphomethene, (f) phlorin (g porphodimethene,
h phlorin monocation, and (i) porphodimethene
dication.

 

 

11

7 2 H II
II II
5 B
3 ll
5 4 “II
II
a b c

 

12
respect to deviations from planarity. Intramolecular

crystal interactions and bulky substituents can greatly
influence the geometry of the molecule. Falk7 estimates
the porphyrin skeleton to be 8.5 X in diameter and 4.7 3
thick based on the crystal structure obtained for the
Ni(II) aetioporphyrin I complex.8

Reduction of the porphyrin nucleus in which some of
the pyrrole carbon atoms are hydrogenated yields dihydro-
porphyrins or tetrahydroporphyrins(Fig. 2c). Chlorin
(Fig. 2b) is a dihydroporphyrin from which chlorophylls
g and b are derived while bacteriochlorophyll is related
to the tetrahydroporphyrin structure.

Hydrogenation of some or all of the pyrrolenine
nitrogen atoms and bridge carbon atoms yields porphyrinogens
and related compounds as the reduction products. The
porphyrinogens are hexahydroporphyrins having the four
methene bridge carbon atoms and the two pyrrolenine
nitrogen atoms hydrogenated(Fig. 2d). The conjugation of
the porphyrin ring is absent in porphyrinogens, and they
therefore lack the planarity and color of the porphyrin
molecule. The porphyrinogens may be chemically or
photolytically oxidized to porphyrins, with the oxidation
pathway involving tetra- and dihydroporphyrin intermediates.

The tetrahydroporphyrin structure known as a
porphomethene(Fig. 2e) has three methene bridge carbons
and one pyrrolenine nitrogen hydrogenated. There is some

9-12

evidence to suggest that tetrahydroporphyrins are

 

13
intermediates in the biological oxidation of porphyrinogens

to porphyrins.

The dihydroporphyrins phlorin(Fig. 2f) and porpho-
dimethene(Fig. 2g) are isomers. In the porphodimethene
only two methene bridge carbons are hydrogenated while
the phlorin structure requires that one pyrrolenine
nitrogen be hydrogenated in addition to the methene bridge
carbon. The protonation of a monocationic phlorin salt
(Fig. 2b) to form a dicationic porphodimethene(Fig. 21)
is known to occur under strongly acidic conditions.13
Both the phlorin and the porphodimethene, including their
cations, can be oxidized to porphyrins by chemical and
photolytic methods.

Ionization. Porphyrins are amphoteric in nature with
ionization occurring at either the two pyrrolenine nitrogen
atoms(-N=) or at the two pyrrole-type nitrogens(-NH-).

Both the pyrrolenine and pyrrole-type nitrogens are capable
of accepting a proton and therefore acting as basic centers.
The pyrrole-type nitrogens are also capable of acting as
acidic centers through the loss of their protons. Thus,
2-

there are six charged species possible: PH", P

mi", mg“, and P32”. Only three of these ionic porphyrins

.have been observed spectroscopically, namely, P2-, PH;, and

IEHfi+; and only the dianion and dication have been chemically

+
3 PH},

isolated as salts.
Porphyrins are extremely weak acids even in concentrated

sodium.hydroxide solutions, the dianion being formed only in

14
the more strongly basic alkoxide solutions. Porphyrins
are, however, readily soluble in dilute mineral acids in
which the formation of the dication is usually observed.

Metalloporphyrins. The two protons attached to the
pyrrole-type nitrogens in porphyrins are readily displaced
by metal ions to form metalloporphyrins. It is in this
form that the macrocyclic porphyrins perform their most
important biological functions: as Mg(II) complexes in the
chlorophylls, as Co(II) complexes in Vitamin B12, and as
Fe(II) or Fe(III) complexes in the hemoproteins. Once
coordinated, the metal ion is incorporated into the highly
resonant aromatic system of the porphyrin ring.

A number of monovalent cations(g;g., Na(I), K(I), and
Li(I)) form metal complexes with porphyrins in which one
of the two metal ions required to balance the charge of
the porphyrin ring is believed to lie above and the other
below the plane of the nucleus.

Numerous divalent metal ion complexes of porphyrins
have been prepared in which the metal ion is four-coordinate
and the resulting complex is square planar in geometry.
waever, some metallOporphyrins containing divalent ions,
such as Mg(II), Cd(II), and Zn(II), readily add an axial
ligand to their square planar structure to form five-
coordinate square pyramidal complexes. The addition of
two axial ligands to form six-coordinate complexes with
distorted octahedral geometry also occurs with divalent

metal chelates(e.g., Fe(II), Co(II), and Mn(II)) of

15
porphyrins. The latter metal ions are also capable of
undergoing oxidation to the trivalent state in which the
metalloporphyrin complexes carry a unit positive charge
and have an anionic counter-ligand associated with them,
often in the axial position.

The thermodynamic stabilities of the metalloporphyrins
as deduced from spectroscopic data as well as from
replacement and dissociation reactions follow the orderzlq
Pt(II)>de(II)> Ni(II)> Co(II)) Ag(II)> Cu(II)> Fe(II) >
Zn(II)> Mg(II) > Cd(II) > Sn(II)> Li2 >Na2> Ba(II)> K2 >
[Ag(I)]2. The order15

various metal ions into porphyrins in aqueous solutions

of the rates of incorporation of

does not correspond to the thermodynamic stability order,
the order of reactivity being Cu(II)) Zn(II)> Mn(II)> Co(II))
Fe(II)) Ni(II)> Cd(II).

Spectra. The electronic absorption spectra of the
porphyrins, metalloporphyrins, and their derivatives are
their most useful and most characteristic property. The
neutral tetrapyrrole macrocycles are characterized by four
absorption bands in the visible region(450 nm to 700 nm),
numbered sequentially I to IV from the red end of the
spectrum, and by a single intense absorption in the near
ultraviolet(~400 nm) known as the Soret band after
J. L. Soret who first discovered this band in hemoglobin
in 1883. These electronic absorptions have been assigned
to w-wf transitions between molecular orbitals delocalized

over the porphyrin framework.

 

16

The Soret band is observed for all tetrapyrroles in
which the nucleus is fully conjugated. It is the most
intense absorption band in the porphyrin compounds,
having molar extinction coefficients of the order of
2 to 5 x 105, which are 10 to 20 times as intense as the
strongest visible band. The Soret absorption can be easily
measured in absorption cells of 1-cm path length at
concentrations of the order of 10-5 and 10-6 M. The
positions of the Soret and visible absorptions are influenced
by changes in the w-electron density at the periphery of the
porphyrin nucleus. Factors which increase the w—electron
density cause bathochromic shifts in the absorption bands.

Chlorin and related dihydroporphyrins in which one
pyrrole double bond is reduced are characterized by an
intense absorption in the red region(band I) and Soret
absorptions less intense than those observed for the fully
oxidized porphyrins. Band I and the Soret band absorb at
higher energies in the chlorin dications.

Unlike the porphyrins and chlorins, the Soret band is
absent in the porphyrinogens and related hydrOporphyrins
since the conjugation of the porphyrin ring is destroyed
at the methene bridge carbons. The porphyrinogens also
lack absorption bands in the visible region, but an
absorption near 200 nm does occur in the ultraviolet region.
The porphomethene and porphodimethene are characterized by
an intense absorption near 500 nm, while phlorin compounds

have two main absorptions in the visible near 440 nm and

735 nm.

17

In the porphyrin dications, dianions, and chelates
with divalent metal ions, the porphyrin molecule more
closely approaches square symmetry, and the four visible
absorption bands are reduced to two main visible bands.

The Soret bands of the dication and dianion shift to longer
wavelength with respect to that observed for the neutral
porphyrin. A Soret and three visible absorptions are
observed for porphyrin monocations, with the Soret band
occurring at a shorter wavelength than that found in the
neutral porphyrin.

The square planar metalloporphyrins with divalent
ions are characterized by a Soret absorption in addition
to the two visible absorptions, usually called the a and B
bands, with the a band assigned to the absorption of
longest wavelength. In general, the coordination of either
one or two extra ligands to the square planar metallo-
porphyrins results in a bathochromic shift of the absorption
bands.

Adsorption on clay minerals. Because crude oils and
ancient sediments generally contain porphyrins believed to
be derived from chlorophyll, the interaction of porphyrins
with clay minerals has received recent attention. Hodgson

16'17 recently succeeded in generating a

and Casagrande
homologous series of porphyrins from a single porphyrin
under simulated geochemical conditions known to be important
in petroleum diagenesis. They also observed that in the

simulation experiments involving montmorillonite the

18

porphyrins were strongly adsorbed by the clay and were not
released even with ultrasonic treatment in the presence of
6 M.HCl and ether. Metallated porphyrins were also formed
when clays were present in the reaction mixtures. Hodgson18
has also shown that porphyrins are capable of extracting
Cu(II) ions from the exchange sites of montmorillonite.

Studies19 on the adsorption of commercial K-Cu
chlorophyllin by montmorillonite saturated with various
metal ions show that the porphyrin monoanion is adsorbed
only onto the external surfaces. Visible absorption
spectra of the porphyrin adsorbed on the clay suggest that
the cationic composition of the clay affects the state of
aggregation and relative orientation of the clay particles
which in turn affect the electronic arrangement of the

20'21 to intercalate proteins

porphyrin nucleus. Attempts
from blood serum into the crystal lattice of montmorillonite
resulted in the external surface adsorption of albumen,

22 with pure

hemoglobin, and hemin; however, experiments
hemin show that in addition to external adsorption the

. . + . . .
hemin cation 034H32N404Fe 1S intercalated into the
interlamellar region. While the hemin cation could not be
removed with boiling dioxane, the hemin cation was liberated

with pyridine, which can be intercalated as a cation and

swelling solvent.

 

19

2.3 mgggyTetraphenylporphyrin

Synthesis. mggg-Tetraphenylporphyrin(TPPH2)(Fig. 3a)
is a synthetic porphyrin often employed as a model for the
naturally occurring porphyrins. TPPH2 readily adds two
additional protons in acidic solutions to form the porphyrin
dication(TPPHZ+)(Fig. 3b) and can be transformed into a
metalloporphyrin(MTPP)(Fig. 30) by replacement of the two
inner protons by a monovalent, divalent, or trivalent
metal ion. TPPH2 of relatively high purity can be
conveniently synthesized from pyrrole and benzaldehyde in
highly acidic media such as propionic acid under reflux
conditions.23 The resulting purple crystals are contaminated
with approximately 3 per cent tetraphenylchlorin by weight.

Mechanisms of formation. Initial mechanistic investi-

 

gations of the Rothemund reaction24 for meso-tetra(g-substi—

tuted pheny1)porphyrins25

provided the proposed reaction
scheme shown in Figure 4 for the formation of mgsg—tetraaryl-
porphyrins. The initial condensation step between a
molecule of pyrrole and benzaldehyde produces a carbinol(I)
which undergoes oxidation to an aroylpyrrole(II). Condensa—
tion of the aroylpyrrole with another molecule of pyrrole
yields another carbinol(III), which undergoes dehydration
to a dipyrromethane(IV). Condensation with a further
molecule of aldehyde produces the carbinol(V) which
oxidizes to the aroyldipyrromethane(VI). Two molecules of
this aroyldipyrromethane condense to form an a,Y-dihydroxy-

porphyrinogen(VII). The loss of two water molecules yields

20

Figure 3. Molecular structures of (a) mesa-tetraphenyl-
porphyrin, (b) meso-tetraphenylporphyrin dication,
and (c) metal chelate of meso-tetraphenylporphyrin.

 

 

 

22

 

Figure 4. Mechanism proposed by Badger £3 £125 for the
Rothemund reaction.

23

u
(:3 I+ Mica” -—+' QZELFA"“*’ 4r§Le1w
n H on = 3

(I) (II)

1 a

ll ArOlI
‘ ‘(1;:cflo (IV) (III)

171%.»?

are 0|! (V) \
CW yI , M
0|!
Ar’c‘o (v1)

Ir l|
(VII)
Ir n u M
"‘Ar 4....— Ar.lr0‘"r ————> Ar Ar
“I H ‘7

(IX) (VIII) (X)

24
the porphodimethene intermediate(VIII) which can either
rearrange to a chlorin(IX) or undergo oxidation to the
porphyrin(X).
The initial condensation step in the mechanism

1 2

postulated by Adler 23.3; ’ for the synthesis of TPPH

2
also involves the formation of pyrrole-2-carbinol(l). The
next reaction step, which is acid-catalyzed, becomes the
condensation of two pyrrole-2-carbinols to form the
dipyrrylmethane carbinol(V), which may undergo oxidation to
a dipyrrylmethene carbinol. These carbinols then condense
with molecules of carbinol(I) to form chainlike polypyrryl
derivatives or polypyrryl carbinol derivatives. The
tetrapyrryl carbinols cyclize to form saturated ring
structures which undergo oxidation to phlorins, which
either rearrange to chlorine or oxidize to porphyrins. A
similar mechanism has been suggested for the reaction between

substituted benzaldehydes and pyrrole.3

More recently Dolphin“ undertook a detailed mechanistic
study of the formation of octamethyltetraphenylporphyrin
(GMTPP) from 3,4-dimethylpyrrole and benzaldehyde in acetic
acid. While the initial condensation steps are fast, the
subsequent oxidations to "planar intermediates" are slow and
allow the isolation and study of the intermediates. The
jprincipal intermediate isolated from the reaction was the
porphyrinogen(XI)(Fig. 5), a colorless solid which absorbs
Inear 200 nm. During the acid-catalyzed oxidation of

porphyrinogen to porphyrin, the zinc(II) chelate of the

porphodimethene(VIII) was isolated as an orange-red powder.

25

Figure 5. Summary of Dolphin'34 proposed mechanism for
meso-tetraarylporphyrin formation.

26

Ar ll
Ar
4 (1“ + 4 Ar-CHO ——->
N H H + 4 H20
II
Ar ll
(XI)
Ar
Ar Ar
Ar

 

(X)

27
Figure 5 summarizes Dolphin's proposed mechanism for the
formation of mgggysubstituted porphyrins from pyrroles
and benzaldehydes. A porphomethene(XII) and a porpho-
dimethene(VIII) are also believed to be intermediates in

uroporphyrin formation.26-28

2.4 Chemical Evolution of Porphyrins
Chemical evolution refers to the chemical events that
occurred approximately 4.5 to 3.5 billion years ago on the

primitive, prebiotic Earth and which culminated in the

 

formation of the first living cell. The concept of chemical
evolution is based on the hypothesis that, due to the
environmental conditions prevailing on the primitive Earth,
biologically important monomers and polymers were synthesized
from simple molecules which interacted with available
sources of energy and/or came in contact with catalytic
surfaces. The aggregation of these biomonomers and
biopolymers to form the first living organism marks the
start of biological evolution. It is believed that
«chemical and biological evolution continued simultaneously
Lnntil the advent of oxygen in the Earth's atmosphere. As
the Earth's atmOSphere became more oxidizing in nature,
chemical evolution presumably ceased and was completely
replaced by biological evolution.

Most authors who theorize on the origin of life
believe that at some stage of chemical evolution porphyrins
were synthesized from precursors available during this

29-36

Period.on the primitive Earth. This abiological

28
synthesis of porphyrins was crucial to the evolution of
living systems at two stages: the evolution of porphyrin-
dependent photosynthesis and the evolution of porphyrin-
dependent respiration. It is also of interest that
porphyrins may have been critical to the further progress
of chemical evolution while the primitive Earth was undergoing
the transition from a reducing atmosphere to an oxidizing
one. Lemmon37 has pointed out that the gradual accumulation
of oxygen would have led to the formation of hydrogen
peroxide which would have caused widespread oxidation and
therefore destruction of organic compounds. However, Calvin30
has shown that the incorporation of ferric ion into heme
porphyrin increases the catalytic ability of iron to
decompose hydrogen peroxide a thousand-fold.

Despite the prebiological and biological importance of
porphyrins, very little experimental data has been collected
on the mechanism of porphyrin abiosynthesis under simulated
primitive Earth conditions. Early work by Rothemund38 on
porphyrin synthesis showed that when a mixture of 5 M pyrrole
in methanol, 2 per cent formaldehyde in methanol, and
pyridine solvent was heated for 30 hours at 90-950 C in
sealed tubes, a 0.1 per cent yield of free base porphine
was obtained. Later Calvin 93 al39 added zinc salts to the
above mixture of reagents and acheived a more effective
condensation. However, neither of these experiments
simulated prebiotic chemistry.

In experiments designed to simulate conditions on the

29
prebiotiv Earth, Szutka £3 2140 exposed a mixture of
pyrrole, benzaldehyde, pyridine, and zinc acetate to
Y-radiation and obtained mggg—tetraphenylporphyrin as a
major product. More recently Szutka used a mixture of
pyrrole, benzaldehyde, and water exposed to ultraviolet
radiationl‘1 and aqueous solutions of a-aminolevulinic acid

42 to produce porphyrin-like

exposed to uv radiation
compounds. Krasnovsky and U'mrikhinal‘3 studied the effects
of atmospheric oxygen, water, catalysts, and electron

donors and acceptors on the production of porphyrin-like
material from the condensation of pyrrole and formaldehyde
in methanol. Atmospheric oxygen, water, and catalysts
increased the rate of porphyrin synthesis with catalysts
increasing the yield of the end product. Electron donors
and acceptors accelerated, inhibited, or had no effect on
the rate. Hodgson and Baker44 have reported the formation
of metal porphyrin complexes from aqueous pyrrole and
formaldehyde in the presence of metal cations and, under
simulated geochemical conditions, in the presence of aqueous
mineral suspensions; however, the absorption maxima and
minima of the reaction products were shifted 100 2 toward
shorter wavelengths. A satisfactory explanation for this
shift has not yet been found. Porphyrins have also been
generated from the action of electric discharges on a
gaseous mixture of methane, ammonia, and water.“5

Although the above experiments show that the synthesis

of porphyrins and/or porphyrin-like compounds occurs readily

30
under simulated prebiotic conditions, the yields of
product materials obtained have not been significant
enough to establish any one reaction as providing the
mechanism for the prebiological formation of porphyrins.
In addition, none of these experiments provides for the
storage and concentration of biologically relevant molecules
necessary for the formation of more complex molecules and,

ultimately, the first living organism.

2.5 Prebiotic Syntheses on Clay Minerals
46-48

J. D. Bernal was the first to suggest that very
fine clay deposits in marine and fresh water could have
concentrated simple organic molecules by adsorption,
catalyzed their condensation to more complex molecules,
and stored the condensation products during chemical
evolution. Recent research has shown that clay minerals,
such as montmorillonite and kaolinite, can act as hetero-

geneous catalysts for the formation of po1ypeptides’49-54

52,55 56

polynucleotides, and monosaccharides.

A significant amount of research has been accumulated

on the prebiotic synthesis of peptides on clay surfaces.
157

Pavlovskaya gt_ observed the formation of amino acids
during the exposure of solutions of formaldehyde and
ammonium nitrate to ultraviolet radiation in the presence
0f bentonite, kaolinite, and limonite. The formation of
amino acids from the simple gases CO2 and N113 in the

52

presence of silicate lattices has also been oberved.

31

Recently Degens and Matheja49’52’53 studied the
adsorption, polymerization, and catalysis of amino acids
on montmorillonite and kaolinite. They observed that
while montmorillonite was a more efficient adsorbent for
amino acids than kaolinite, kaolinite was a better catalyst.
Both minerals were found to promote polymerization of
amino acids under aqueous and dry conditions with peptides
and polyphenolic compounds as the principal condensation
products.

The amino acids are believed to be tightly bound to
the silicate surface by strong electrostatic interactions.
The amino groups are either hydrogen-bonded to the silica
oxygens or occur as positive ions electrostatically bound
to the silica oxygens. The carboxyl groups are generally
believed to be bound to the positively charged Al-oxy-
hydroxy sites via ionic bridges. This electrostatic
interaction results in the activation of the carboxyl
group, which is necessary for polymerization to occur.

It is postulated that a continuous flow of organic molecules
across the catalytically effective mineral surface is also
necessary for efficient polymerization.

Paecht-Horowitz gt 9150’51’54 showed that amino acid—
phosphate anhydrides(amino acid adenylates) concentrated on
clay surfaces polymerize to form polypeptides of different,
but discrete sizes rather than a continuous distribution of
molecular weights. Among the clays, illite and montmorillonite

showed a marked catalytic activity while appatite, kaolinite,

 

 

32
and permutite displayed no catalytic activity. The
dominant reaction in the heterogeneous polycondensation
of amino acid adenylates on clay surfaces is supposed to
involve the interaction between two active adenylates
resulting in a still higher peptide.

Harvey gt 2155 were able to synthesize the nucleic
acid bases uracil and cytosine from CO2 and NH3 in the
presence of kaolinite and water. The carbon dioxide is
believed to bind to the clay surface to form an activated
aluminum carbonate complex. The adsorption of nucleic
acid bases on clay minerals and their subsequent
polymerization to polynucleotides is determined by the
amounts of phosphates present during the reaction.52

There has been only one report to date on the formation
of sugars in the presence of clay minerals. Gabel and
Ponnamperuma56 refluxed an aqueous solution of formaldehyde
in the presence of alumina, kaolinite, and illite and
obtained a mixture of monosaccharides as condensation
products. They proposed that the basic sites on the crystal
lattices of the alumina and aluminosilicates were capable
of accepting protons from the formaldehyde and various
ithermediates adsorbed on the mineral surfaces, thereby
generating the necessary reactant molecules.

The foregoing experiments indicate that clay minerals
provide a catalytically active and efficient surface for
the heterogeneous catalysis of a variety of condensation

reactions relevant to the prebiotic synthesis of biologically

important molecules.

CHAPTER 3
EXPERIMENTAL SECTION

3.1 Preparation of Homoionic Montmorillonites

Mn+ montmorillonites. The montmorillonite used

 

throughout this research was a standard natural sample of
Wyoming bentonite(Vol clay) obtained from the American
Colloid Company, Skokie, Illinois. In order to remove
the more dense components, a slurry of the clay in ethanol
was prepared, and the clay particles were allowed to
sediment. The top colloidal fraction was removed, an excess
of the appropiate metal salt was added, and the suspension
was stirred for 24 hours to saturate the exchange sites with
the desired cation. The clay was removed by filtration,
washed with solvent until the AgNO3 test proved negative
for 01-, and dried overnight in air. The chloride salts of
Na(I), Mg(II), Co(II), Cu(II), Zn(II), and Fe(III) were used
to prepare the corresponding metal—saturated exchange forms
of montmorillonite.

Vanadium oxysulfate obtained from Alfa Inorganics was
used to prepare VO(II) montmorillonite. It should be
remarked that if freshly prepared VO(II) montmorillonite is

allowed to dry in air, the light blue color characteristic

33

34
of aqueous vanadyl ion, VO(H20)§+, is lost and a yellow-
green color develops. VO(II) montmorillonite was therefore
stored under methanol and filtered as needed.

TPA+ and H+ montmorillonites. Tetra(g-propyl)ammonium
(TPA+) montmorillonite was prepared according to the above
procedure with tetra(2—propyl)ammonium bromide obtained from
Eastman Kodak Company.

Hydrogen(H+) montmorillonite was prepared by using
Dowex 50W¥X8, 20-50 mesh ion exchange resin from J. T. Baker
Chemical Company. The hydrogen ion exchange form of the
resin was prepared by stirring 5.0 g of the resin for one
hour in 250 ml of a 1.0 M HCl solution, washing with distilled
water to remove excess 01‘, and drying overnight in air. The
resin was then added to a suspension of 2.0 g montmorillonite
in 300 ml ethanol. After an hour of being stirred, the mixture
was passed through a screen filter to remove the resin,
which was washed several times with ethanol to remove thy
clay particles which had adhered to the resin. The clay-
ethanol suspension was then filtered, and the clay was used
immediately after being dried. It should be remarked that it
is not possible to prepare a clay in which all the exchange
sites are occupied by H+ since Al3+ moves from the lattice to
the exchange sites before saturation with H+ becomes complete.
It is necessary to use the clay immediately after it is
dried since the movement of Al3+ from its lattice position
is facilitated during drying.

++

TPPH:+ montmorillonite. The TPPH,i exchange form of

 

 

35
montmorillonite was prepared by adding an excess of TPPH2
in glacial acetic acid to a slurry of montmorillonite in
ethanol. The mixture was stirred 24 hours, filtered, and
washed with ethanol to remove excess acetate ion. The clay
was bright green after being dried overnight in air. Its
solid state absorption spectra in the visible and infrared

regions are shown in Figures 6 and 7, respectively.

3.2 Synthesis of TPPH2

A modified synthesis of that described by Adler23

was
used to prepare the TPPH2 used in this research. Freshly
distilled pyrrole(3.6 ml, 52 mmoles) and freshly distilled
benzaldehyde(5.4 ml, 52 mmoles) were added to 250 ml
refluxing propionic acid. After one hour, the reaction
solution was transferred to a beaker and allowed to cool
overnight. Filtration yielded purple crystals which were
washed several times with methanol to remove impurities
and then were dried at 1100 C for two hours to remove
adsorbed propionic acid. The final product is contaminated

with 3 per cent tetraphenylchlorin by weight.

3.3 Adsorption of TPPH2 onto Montmorillonite
Metal ion-exchanged montmorillonite(500 mg) was added

to 10 mg(0.016 mmoles) of TPPH dissolved in 500 m1

2
reagent grade acetone at room temperature. Under these

conditions, the molar ratio of 142* to TPPH2 was 14:1. The

solution was stirred for 24 hours, and the adsorption of

36

 

Figure 6. Electronic absorption spegtra of TPPH++ in glacial

 

acetic acid( TPPH montmorillgnite( ----- ),
and Fe(III) montmorillonIte after adsorption of
TPPH2(-o-o- .

37

OOH
_

AECVIHOZme>§§
OO©
_

 

 

 
  

.2.
.._1
z
z.
z
.W
1.. .
1.1.1.1....

(is.

20.0mm HmmOm

 

/ / six
/. x. 7‘ /./1/
’. .._ I .z
,x N f
n” a x
,. .\
l .1

EONVSHOSSV

 

 

act;

 

Figure 7.

38

Infrared absorption spectra of (A) Sn(II)
montmorillonite dried 1 week at 110 C and

(B) Cu(II) montmorillonite after exchange with
TPPH and drying 4 weeks at 1300 C to remove
adsogbed acetone.

 

 

 

 

(%) BONVIIIWSNVHI

 

800

I600 |200

FREQUENCY (

3000 2000

cm)

39

 

 

(°/o) EDNVIIIWSNVHI

 

 

L

 

l600 I200 800
FREQUENCY (cm")

3000 2000

4000

40
TPPH2 was followed spectroscopically by observing any
changes in the intensities of the absorption peaks for TPPH2
in the visible region and by noting the growth of any new
visible absorption bands into the spectrum. The reaction
mixtures were then filtered, the clays were washed several

times with acetone and were then allowed to dry overnight in

air.

3.4 Reaction between Pyrrole and Benzaldehyde in the
Presence of Montmorillonite

Aerobic sygthesis. Freshly distilled pyrrole(0.31 ml,

4.5 mmoles) and freshly distilled benzaldehyde(0.46 ml,

4.5 mmoles) were added to 5.0 g of the metal cation

exchange form of montmorillonite suspended in 300 ml
distilled water. The amounts of pyrrole and benzaldehyde
were reduced to 0.19 ml(2.7 mmoles) and 0.28 ml(2.7 mmoles),
respectively, for the reaction with 5.0 g Fe(III) montmoril-
lonite. Under these conditions, the molar ratio of Mn+ to
pyrrole was 1:2.

The reaction was also carried out at a 1:4 molar ratio
by adding 0.12 ml(1.7 mmoles) pyrrole and 0.18 ml(1.7 mmoles)
benzaldehyde to 1.0 g metal cation—exchanged montmorillonite
suspended in 60 ml distilled water. One gram Fe(III)
montmorillonite required 0.08 ml(1.2 mmoles) pyrrole and
0.12 ml(1.2 mmoles) benzaldehyde.

Each reaction was allowed to proceed while it was stirred

continuously for 24 hours, during which time the colloidal

 

41
suspensions gradually changed from colorless to pale pink
to deep purple in color. The elapsed time before the
initial appearance of the pale pink color was dependent
upon the metal ion occupying the exchange sites of the
montmorillonite. The reaction between pyrrole and benzal-
dehyde progressed rapidly in the presence of Fe(III),
VO(II), Cu(II), and 11*, while the Zn(II), Co(II), and Mg(II)
exchange forms of montmorillonite required a considerably
longer period of time before a color change was observed.
While the reaction which produces a purple-colored product
occurred at the 1:2 molar ratio in the presence of Na(I)
montmorillonite, only a gray—colored product believed to be
a polymeric pyrrole compound similar to pyrrole black was
obtained at the 1:1 molar ratio. No observable reaction
between pyrrole and benzaldehyde occurred in the presence
of TPA+ montmorillonite at either molar ratio.

At the end of the reaction period, the clays and the
adsorbed products were freeze-dried, and their solid state
absorption spectra were recorded in the visible region.

Anaerobic sygthesis. Pyrrole and benzaldehyde were
distilled under argon and were stored under nitrogen in a
glove box. Slurries of 5.0 g Cu(II) and 5.0 g Zn(II)
montmorillonite in 300 ml distilled water were heated to
reflux temperature for one hour under nitrogen and then were
transferred to a glove box. Pyrrole(0.31 ml, 4.5 mmoles)
and benzaldehyde(0.46 ml, 4.5 mmoles) were added to each

slurry. Stirring the mixtures for 24 hours produced the

 

 

 

42
same gradual color changes as were observed during the
aerobic syntheses. Exposure to the atmosphere after
freeze-drying did not produce any changes in the color of
the clays or their visible absorption spectra, which were
similar to those obtained for the aerobic condensation

reactions.

3.5 Reaction between Pyrrole and Benzaldehyde in Water

To establish the catalytic role of the montmorillonite
surface during the condensation reaction, pyrrole and
benzaldehyde were allowed to react in the absence of any
clay surface. Pyrrole(0.06 ml, 0.87 mmoles) and benzal-
dehyde(0.09 ml, 0.87 mmoles) were added to 60 ml distilled
water which had been acidified to a pH of approximately 5.
The reaction was allowed to proceed while it was stirred
continuously for 24 hours after which time the solution
was transferred to a separatory funnel. The organic
products were extracted with 200 ml chloroform. The
organic layer was removed, dried over anhydrous sodium
sulfate, and filtered. The filtrate was diluted to 250 ml
in a volumetric flask, and the visible absorption spectrum
was recorded. The Soret absorbance indicated a yield of

less than one per cent porphyrin for the blank reaction.

3.6 Desorption of the Condensation Product
Method A. After the formation and adsorption of the

reaction product onto the clay surface, attempts were made

 

 

43
to desorb it with a large organic ion capable of competing
with it for the exchange sites on the clay. For this
reason TPA+ was selected as the exchange ion.

The general procedure for desorption was to add
72 mg(0.27 mmoles) [TPA] Br to a suspension of 150 mg
of reacted clay from the 1:4 molar reaction in 100 ml
distilled water. The mixture was allowed to stir for 24
hours, and then 200 ml benzene were added to extract any
desorbed product. After another hour of stirring, the
mixture was transferred to a separatory funnel, the aqueous
layer was removed, and the organic layer was dried over
anhydrous sodium sulfate. Filtration yielded a darkly
colored filtrate that was diluted to 500 ml in a volumetric
flask. Further dilution was often required to obtain an
absorbance value for the Soret band. The visible absorption
spectrum was recorded, and a per cent yield of TPPH2 was
calculated based on the absorbance of the Soret band(e =
1:49 x 10,3 for TPPH2 and e = 1:31 x 103 for TPPH?) and one
mole of TPPH2 per mole of metal ion. This desorption
method resulted in yields of only 2 per cent.

Method B. Approximately 100 mg of reacted clay from
the 1:2 molar reaction was placed in a Soxhlet extractor
'with 300 ml pyridine. Pyridine can be intercalated as the
:neutral molecule or the cation into the interlamellar
:region of montmorillonite. Its ability to remove hemin
cations from the intracrystal surfaces of montmorillonite

22

lhas already been noted. The extraction was allowed to

44
proceed overnight. The intensely colored extract was
diluted in a 500 ml volumetric flask and rediluted until
an absorbance value for the Soret band(e = 450 x 103 for
TPPH2) was obtained. Yields for the H" and Cu(II) exchange
forms of montmorillonite were 4 to 5 per cent.

Method 0. The Cu(II) exchange form of kaolinite was
prepared as described for the montmorillonites. Conducto-
metric titration yielded an exchange capacity of 4 meq/100 g
Georgia kaolinite, which was obtained from the ward's
Natural Science Establishment. The Cu(II) exchange form
(20 mg) and naturally occurring kaolinite(20 mg) were
suspended in 300 ml distilled water. Pyrrole(0.06 ml,

0.87 mmoles) and benzaldehyde(0.09 ml, 0.87 mmoles) were
added to each suspension. After being stirred for 24 hours,
the reaction mixtures were freeze-dried, and 2.0 g of the
resulting clay products were suspended in 200 ml chloroform
in which 214 mg(0.80 mmoles) [TPA] Br had been dissolved.
The mixtures were allowed to stir for 24 hours and were
then filtered. The filtrates were diluted in volumetric
flasks, and the visible absorption spectra were recorded.
‘Yields of 4 to 6 per cent were obtained(e = 453 x 103 for
TPPHZ). These yields were lower than expected since,
'unlike montmorillonite, kaolinite has an extremely low
cation exchange capacity with virtually all adsorption
occurring on the external surfaces.

Method D. This method attempted to desorb the

condensation product from the interlamellar region as it

45
was formed. Distilled pyrrole(0.06 ml, 0.87 mmoles) and
distilled benzaldehyde(0.09 ml, 0.87 mmoles) were added with
2.4 g(9.0 mmoles) [TPA] Br to 60 ml distilled water. The
mixture was allowed to stir for 5 min to bring all the
reagents into solution, and 1.0 g of clay was added.
Stirring was continued for 24 hours. The reaction mixture
was freeze-dried, and the clay product was resuspended in
200 ml chloroform. After 24 hours of continuous stirring,
the suspension was filtered. The filtrate was diluted to
500 ml in a volumetric flask and rediluted until a Soret
absorbance could be recorded. Yields for the H+, Cu(II),
and Fe(III) exchange forms of montmorillonite were 10 to 12
per cent porphyrin. Esr absorption spectra and elemental
analyses for Cu(II) montmorillonite indicate that the TPA+
cation undergoes ninety per cent exchange with the Cu(II) ions
in the interlamellar region during the desorption process.
However, total desorption of the product does not occur as
evidenced by the fact that the clay retains a light purple

color after the desorption process is complete.

3.7 Porphyrin Intermediates

Sygthesis. Distilled pyrrole(0.2 ml, 2.7 mmoles) and
distilled benzaldehyde(0.3 ml, 2.7 mmoles) were dissolved
in 250 m1 chloroform. One drop of concentrated HCl was
added as catalyst. The reaction solution was allowed to
stir for 2 hours and was then purged with argon for an

hour. The reaction solution was transferred to a glove box

 

 

46
containing a nitrogen atmosphere. The visible absorption
spectrum of the solution exhibited an intense absorption
at 477 nm(Fig. 8) which was assigned to the porphyrin
intermediate. If an aliquot of the reaction solution was
exposed to air overnight, the band at 477 nm disappeared
and was replaced by two Soret absorptions at 421 nm and
447 nm for the free base porphyrin and dication, respectively.
Attempts to isolate the intermediate either in crystalline
or powder form were unsuccessful.

Adsorption on montmorillonite. The reaction solution
was divided into two parts, and 1.0 g each of TPA+ and
Cu(II) montmorillonite was added to each part. Each
mixture was allowed to stir overnight in the glove box,
and filtration yielded purple-brown clays in both cases.
The TPA+ montmorillonite clay exhibited an intense absorption
at 485 nm while the Cu(II) montmorillonite clay had its
maximum absorption at 505 nm with shoulders at 475 nm and
447 nm(Fig. 9). The filtrate from the TPA+ montmorillonite
adsorption solution exhibited bands at 477 nm and 447 nm
assigned to the intermediate and dication, respectively.
The filtrate from the Cu(II) montmorillonite adsorption
solution exhibited CuTPP absorptions at 535 nm and 415 nm
and the intermediate absorption at 488 nm. Despite the
presence of Cu(II) in the filtrate, the 001 spacing for
the clay after adsorption was only 12.4 X, which indicates

that intercalation of the intermediate did not occur.

47

 

Figure 8. Electronic absorption spectra of an HCl-CHCl3
solution of pyrrole and benzaldehyde after 2
hours of reaction time( ) and after exposure
to air overnight( ----- ).

 

 

48

 

 

 

 

~--~-’

 

 

moz<mmomm<

500 600

WAVELENGTH (nm)

400

 

ll

49

 

 

Figure 9. Electronic absorption spectra of TPA+ montmoril-
lonite after the adsorption of the porphyrin
intermediate from HCl-CHCl solution( ) and

 

Cu(II) montmorillonite aftér adsorption of the
3

intermediate from HCl-CHCl solution( ----- ).

 

 

ABSORBANCE

50

 

 

 

 

 

l
400

500 600
WfiVELENGTH(nm)

51
3.8 Spectroscopy

Visible absorption. All electronic absorption Spectra
were recorded by use of a Cary 17 recording spectrophoto-
meter. Solid samples were recorded as Nujol mulls mounted
between two quartz discs. Solution spectra were obtained with
silica absorption cells having a path length of 1 cm.

Infgared absorption. Infrared Spectra were recorded
by use of a Perkin Elmer Model 237B grating infrared spectro-
photometer. Solid samples were recorded as thin self-
supporting films. A polystyrene film was employed as the
standard sample for calibration of peak positions.

Electron spin resonance. Electron spin resonance
spectra were recorded by use of a Varian E-4, X-band esr
spectrophotometer. The powdered clay samples were placed in
quartz esr tubes for measurement.

X-ray diffraction. The 001 basal Spacings for the
various montmorillonite clays were recorded by use of a
Phillips X-ray diffractometer using Cu K(a) radiation and a
N1 filter. The clay samples were deposited as slurries on
microscope slides and allowed to dry overnight before

measurements were obtained.

3.9 Chemical Analyses
Elemental analyses were performed on the clay samples
by Galbraith Laboratories, Inc. and by the Michigan State

University Soil Science Testing Laboratory.

CHAPTER 4
RESULTS AND DISCUSSION

4.1 Interaction of Egpp-Tetraphenylporphyrin with
Montmorillonite

VO(II) gpd Fe(III) montmorillonite. The four free
base porphyrin absorptions at 510 nm, 545 nm, 588 nm, and
645 nm rapidly decrease in intensity upon the addition of
the vanadyl or Fe(III) exchange forms of montmorillonite to
solutions of TPPH2 in acetone. The absorptions virtually
disappear from the Spectrum within a few hours. No new
absorption bands appear during the 24—hour reaction period.

Both clay exchange forms become bright green upon
addition to the reaction solution and retain this color
after filtration and air drying. The visible absorption
spectrum of the Fe(III) exchange form is shown in Figure 6.
The vanadyl exchange form exhibits a similar spectrum.
Comparison of this absorption spectrum with those for
TPPHZ+ in glacial acetic acid and for the TPPH:+ exchange
form of montmorillonite, both shown in Figure 6, indicates

that the meso-tetraphenylporphyrin dication is formed upon

 

adsorption of the free base porphyrin into the interlamellar

region of vanadyl and Fe(III) montmorillonite.

52

53

Inspection of Table 1 reveals that bathochromic shifts
of 10-15 nm occur in the Soret and band I absorptions for
the dication in the adsorbed state. This suggests that the
porphyrin molecule orients itself parallel to the silicate
layers of the montmorillonite flake with the layers
restricting the molecule to a more planar conformation in
the interlamellar region(Fig. 10). Since the porphyrin
molecule is a non-rigid aromatic system with low-energy
barriers to a variety of angular distortions from planarity,
the conformational change to a more planar structure is also

easily effected, especially where packing effects, which

 

often have a pronounced effect on porphyrin structures, are

absent. The planar conformation of TPPH:+ in the interlayer

space is evident from X-ray diffraction data. A 001

o
reflection of 14.2 A was obtained for a TPPHZ+-saturated

powder sample of montmorillonite. Because the basal spacing
is about 9.6 X when no interlamellar material is present,
the separation between the silicate sheets is 4.6 X, which
is the approximate thickness of the porphyrin nucleus.7 A
similar interlayer spacing of 14.6 X has been reported for

pure hemin intercalated into the interlamellar region of

2

montmorillonite as the monocation C H N 0 Fe+. 2 The

34 32 4 4
infrared spectrum of TPPH:+ exchanged onto montmorillonite

(Fig. 7) lends further support to the planar conformation

of TPPH:+ in the interlamellar region. The bands at

1 1

1483 cm-1, 1444 cm‘ , and 1239 cm-

++ 58
1* O

are similar to those

1

previously reported for TPPH The bands at 767 cm' ,

 

Table 1. Ultraviolet and Visible Absorption Spectra for
the Reaction of TPPH2 with Montmorillonite

 

 

Compound(solvent) Visible Bands(nm) Soret Band(nm)
TPPH2(acetone) 510 415
545
588
645
TPPHZ+(glacial 5552sh; 435
. 605 sh
acetic acid) 655
TPPHZ+ 55528h; 448
(montmorillonite) 22g Sh
TPPHZ+ ESSESh; 450
. 05 sh
(Fe(III) montmoril-
lonite) 668
TPPH++ SSSEsh; 450
(VO(II) montmoril- 3%? Sh
lonite)
CuTPP(acetone) 500(sh) 412
538
570(Sh)
CuTPP 545 416
(Cu(II) montmoril- 575(sh)
lonite)
ZnTPP(acetone) 515(sh) 421
555
593
CoTPP(acetone) 528 412
CoTPP 538 429
(Co(II) montmoril- 570(sh)

lonite

 

55

 

Figure 10. Schematic drawing showing the orientation of the
porphyrin skeleton between montmorillonite layers.

 

 

.OOQOSQ

56

 

 

 

A13+, Mg2+, Fe3+

Hydroxyls
Oxygens
Nitrogens
Carbons

4+

Si , occasionally Al3+

 

57

1 1

717 cm- , and 700 cm- are assigned to out-of—plane C—H

deformations characteristic of monosubstituted benzene.59’60

1 1

The 717 cm- and 700 cm- bands exhibit significant

pleochroism when an oriented film is rotated in the infrared

1

beam. The band at 700 cm- virtually disappears from the

spectrum while the band at 717 cm."1 gains in intensity.

CusIIl and Zn(II! montmorillonite. Figure 11 and
and Figure 12 show the Spectrophotometric changes in the
absorption spectrum of free base TPPH2 after the addition
of Cu(II) and Zn(II) montmorillonite, respectively. The
free base porphyrin absorption bands are gradually replaced
by an absorption band at 538 nm with shoulders at 500 nm
and 570 nm in the case of Cu(II) montmorillonite. The
Zn(II) montmorillonite reaction system behaves similarly
with bands at 515 nm, 553 nm, and 593 nm replacing the
TPPH2 bands. The Soret absorption shifts slightly from
415 nm to 412 nm for Cu(II) and to 421 nm for Zn(II). It
should be remarked that not all the absorption traces pass
through the isosbestic points at 474 nm, 523 nm, 555 nm,
577 nm, and 668 nm for Cu(II) and 551 nm, 584 nm, 615 nm,
and 668 nm for Zn(II) as a result of scattering caused by
finely divided clay particles which remain suspended in
the aliquot after centrifuging.

The new absorption bands which appear in the visible
region are similar to those reported in the literature for

61,62

meso-tetraphenylporphinatocopper(II)(CuTPP) and meso-

tetraphenylporphinatozinc(II)(ZnTPP)61 in benzene solution.

 

Figure 11.

_58

(A) Spectrophotometric changes after various
time intervals in the absorption spectrum of the
TPPH -acetone solution after the addition of
Cu(Ii) montmorillonite. (B) and (c) Visible and
Soret absorptions of the TPPH -acetone solution
before( ) and 24 hours afger( ----- ) the
addition of Cu(II) montmorillonite.

 

 

 

59

 

A B
n
mmmm 1
m 0 .I 4 u
0 0 8 .
0361 u
3)) -
1234 ”
(((( .
I. 1
s
.l 4
4 2 fin...” .....
A: 2 A l/
l 4 u I
I
-

 

 

 

 

 

 

 

 

 

SORET
REGION

 

 

 

woz<mmomm<

600 700
WflMELENGTH Own)

500

400

 

Figure 12.

(A) Spectrophotometric changes after various

time intervals in the absorption spectrum of the
TPPH —acetone solution after the addition of

Zn(I ) montmorillonite. (B) and (C) Visible and
Soret absorptions of the TPPH —acetone solution
before( ) and 24 hours afger( ----- ) the
addition of Zn(II) montmorillonite.

 

61

 

90 min
330 min
450 min

0min

E

1
2
3
4

 

 

 

 

 

SORET
REGION

 

 

 

 

 

mozqmmOmmd.

600 700
WAVELENGTH (nm)

500

400

 

 

62
The fact that the metal chelates are desorbed from the
montmorillonite surface probably results from the displace-
ment of the neutral complexes from the exchange sites by
the protons displaced from the porphyrin ring upon metal-
lation.

Cu(II) montmorillonite exhibits a bright green color
and Zn(II) montmorillonite a pale green color during the
initial stages of the reaction; however, the pale blue
and grayish white colors characteristic of the original
clays, respectively, are present at the end of the reaction.
If the amount of TPPH2 dissolved in solution is increased to
70 mg, 140 mg, or 280 mg, the clays retain a bright green
color at the completion of the reaction period. This
phenomenon is apparently the result of an equilibrium
established between the metal chelate and the dication on
the clay surface.

If 500 mg Cu(II) montmorillonite is suspended in
a concentrated acetone solution of TPPH2(«~-10_3 g) for 24
hours with continuous stirring, a light purple clay is
obtained upon filtration. eray diffraction data indicate
that the neutral porphyrin complex is adsorbed onto the
external surfaces with some organic material interstratified
in the interlamellar region. The small bathochromic shifts
observed in the absorption spectrum of CuTPP adsorbed on
montmorillonite also suggest that the adsorption is external:
the markedly more pronounced Spectral shifts which are

found when TPPH:+ is adsorbed into the interlamellar region

 

 

 

63
are absent.

Co(II) montmorillonite. The formation of a Co(II)
chelate of TPPH2 in solution is not observed Spectro-
scopically at room temperature. However, filtration
yields a yellow-tan clay whose absorption spectrum is
shown in Figure 13. Although the Soret and visible
absorptions are shifted 10-20 nm to the red from those
observed for CoTPP in solution, the species adsorbed on
montmorillonite is assigned to the Co(II) chelate of TPPH2.
The bathochromic shifts are probably the result of an
axial interaction between CoTPP and the silicate oxygens
or a CoTPP complex with free diatomic oxygen trapped on
the basal surfaces. A similar spectrum and bathochromic
shifts have been observed for a CoTPP complex of 02 in
toluene glass at 77 K.63 A 001 reflection of 12.6 3
indicates that only external adsorption of the CoTPP
complex occurs.

If the reaction between TPPH and Co(II) montmoril-

2
lonite is carried out in refluxing acetone, the formation
of CoTPP occurs as indicated by a gradual increase in
absorption at 528 nm(Fig. 14). Isosbestic points occur at
495 nm, 519 nm, 577 nm, and 668 nm.

Filtration yields the same clay as the room temperature

reaction. When the amount of TPPH is increased to 70 mg or

2
140 mg, the bright green clay characteristic of adsorbed

TPPHZ" is obtained.

Na(I) and Mg(II) montmorillonite. There are no

 

 

 

64

 

Figure 13. Electronic absorption Spectrum of CoTPP on Co(II)
montmorillonite.

 

 

 

ABSORBANCE

65

 

 

SORET
REGION

fl

 

 

 

 

l l

 

 

400

500 600
WAVELENGTH (nm)

700

 

 

 

Figure 14.

66

(A) SpectrOphotometric changes after various
time intervals in the absorption spectrum of the
TPPH -acetone solution after the addition of
Co(IE) montmorillonite at reflux temperature.
(B) and (C) Visible and Soret absorptions of
the TPP -acetone solution before( ) and 24
hours af er( ----- ) the addition of Co(II)

montmorillonite.

 

ABSORBANCE

67

 

I C I I A
1 0 min
SORET .
2 180 min
REGION H 270 min
| 4 570 min

 

 

 

 

 

 

 

 

 

 

 

~I

400 500 600
WAVELENGTH (nm)

 

68

significant decreases in the intensities of the free base
porphyrin absorptions after 24 hours when Na(I) or Mg(II)
montmorillonite is the adsorbent. Filtration, in both
cases, yields a pale green clay, which indicates that only
trace amounts of TPPH2 are adsorbed as the dication.

Mechanism of meso-tetraphenzlporphygin adsorption.
The formation of the gggg-tetraphenylporphyrin dication in
the interlamellar region of montmorillonite in the absence
of metalloporphyrin formation can be described by the
following set of equilibria:

 

 

“—9 2M(0H)(H2O)((:::;+ + 211 0+ (1)

n+
2M(H20)x + 2H 0 3

2 «E——-

 

 

 

 

 

 

TPPH2(soi'n) «e— TPPHz (2)
+ -—-%> ++
TPPH2 + 2330 E TPPH,‘ + 21120 (3) .

 

 

where represents surface or intercalated species.
The overall reaction for dication formation can then be

represented by

 

 

5 (n-1)+

2M(H20);+ + TPPE12(sol,n) <—— 2M(0H)(H2O)(x_1) + 11>]?ijr (4).

 

 

From the above set of reactions, it is evident that
the major factor governing the formation of the dication

upon adsorption of the free base porphyrin is the ability

69
of the exchangeable metal cation to transfer protons to
interlayer water molecules(Eq. 1). Hydrolysis constants

measure the extent to which the reaction

-1 +
Maze):+ + H20 jgé’ m(on)(H20)::_1; + 330* (5)

occurs in water and are an indication of the acidity of the
metal ion. Inspection of Table 2 reveals that TPPH:+
formation should occur most readily with the Fe(III)

exchange form of montmorillonite and should become
progressively more difficult as the pKh values of the metal
ions increase. This is observed experimentally as Fe(III)
and VO(II) exchanged montmorillonites quantitatively adsorb
TPPH2 as the dication while Na(I) and Mg(II) montmorillonites
convert only trace amounts of the free base porphyrin to the
dication.

The acidity of interlayer water can also be increased
by decreasing the water content in the interlamellar region
of montmorillonite. As the number of coordinated water
molecules decreases, the ability of the metal cation to
polarize the remaining water molecules increases, which
causes an increase in acidity. In experiments where the
adsorption of TPPH2 was performed in solvents immiscible
with water, such as methylene chloride and benzene, all the
metal ions listed in Table 2 became sufficiently acidic to
allow dication formation upon adsorption of the TPPH2

molecules as a result of the displacement of interlayer

water molecules.

 

 

70
Table 2. Hydrolysis Constants for Selected Metal'Ionsa

 

 

Metal Ion pKh
Fe(III) 2.19
VO(II) 4.77
Cu(II) 7.53
Co(II) 9.6
Zn(II) 9.60
Mg(II) 11.42
Na(I) 14.48

 

aValues of p are from J. E. Huheey, "Inorganic Chemistry:
Principles 0 Structure and Reactivity," Harper and Row,
New York, 1972 except for VO(II) which is from J. Bjerrum,
G. Schwarzenbach, and L. G. Sillen, Eds., "Stability
Constants of Metal-Ion Complexes: Part II, Inorganic
Ligands," The Chemical Society, London, 1958.

 

 

71
Cu(II), Zn(II), and Co(II) are the only exchangeable
ions which become incorporated into the porphyrin ring
during adsorption. The following set of equilibria

represent the metal chelation reaction:

 

—->

 

 

 

 

 

 

TPPH2( 8'01 ,n) a TPPH2 (6)
M(H 0)11+ TPPH “'"9' M(TPP)(n‘2)* 2H o+ x11 0 (7)
2 x + 2 <— + 3 + 2
(xi—2). ——> (II-2*
M(TPP) a M(TPP)(Sol,n) (8),

 

where the overall reaction is

 

(n-2)+

+
n) + 2H30 + XHZO (9).

n+ -—-a-
14(1120)x + TPPH2(sol,n) E M(TPP)(801,

 

The above set of reactions indicate that the chelation of
TPPH2 is dependent on the metal ion occupying the exchange
sites in the interlamellar region.

From Figures 11, 12, and 14 it can be seen that Cu(II)
is the most rapidly incorporated ion followed by Zn(II) and
Co(II), which reacts slowly even at reflux temperature.
This reactivity order, Cu(II))Zn(II)>>»Co(II), is similar
to that obtained for other porphyrins in aqueous environ-

15’64 The rapid incorporation of Cu(II) into

ments.
dimethyl protoporphyrin ester solubilized in aqueous

detergent solution has been attributed to the electrostatic

72
attraction of Cu(II) ions to the anionic porphyrin micelle-
water interface.65 It is postulated that the porphyrin is
mainly embedded in the hydrocarbon region of the micelle
complex with the nitrogen atoms in contact with the aqueous
phase. An analogous mechanism is proposed for metal chelate
formation in the interlamellar region. The metal ion is
electrostatically bonded to the negatively charged silicate
layers. When the dipolar acetone-porphyrin micelle enters
the interlamellar region, the positive end of the dipole is
attracted to the negatively charged layers and the negative
end is oriented towards the positive ions. Because acetone
is miscible with water, the micelle is able to penetrate the
hydrated coordination sphere of the metal ion so that metal
insertion into the ring can occur.

The brightly colored green clays which appear during
the course of the chelation reactions and are also obtained
when the concentration of TPPH2 in solution is increased
result from the reaction between the displaced ring protons
and the free base porphyrin.molecules as represented by the

‘reaction

 

 

+ “‘3‘ TPPH'H' + 2H

23 0 2(sol'n) <——— 4 2

3 + TPPH

0 (1o).

 

 

At Cu(II) to TPPH2 molar ratios of 14:1, 2:1, 1:1, and 1:2,

the amounts of free base porphyrin converted to the metal

chelate are approximately 95 %, 72 %, 71 %, and 50 %,

respectively. This indicates that approximately 30 % of

73
the free base porphyrin is involved in reaction 10 when
the molar ratios are 2:1 and 1:1 and accounts for the
brightly colored green clays obtained at the end of the
adsorption reaction.

The value of 50 % obtained for the 1:2 reaction
represents the incorporation of all the exchangeable Cu(II)
ions into the porphyrin ring. Spectros00pic measurements
indicate that 25 % of the free base porphyrin is adsorbed
as the dication. This figure corresponds to the complete
coverage of the internal surface area of montmorillonite by
porphyrin dications based on a theoretical surface area of
8 x 106 cm2/g for montmorillonite and an area of 300 32/
molecule of TPPH++. The calculated total internal surface
area per Cu(II) exchange position is 150 22, which indicates
that one TPPH:+ molecule occupies the area of two cation
exchange positions. Because this figure leaves half the cation
exchange positions charge deficient, the protons diSplaced
from the porphyrin ring during metallation must maintain the
conservation of charge in the interlamellar region.

Conclusion. Metal ion exchange forms of montmorillonite
adsorb free base ggggrtetraphenylporphyrin molecules and
undergo two distinct reactions with the adsorbed molecules:
dication formation and metalloporphyrin formation, as
represented by Equations 4 and 9, reSpectively.

Dication formation is found to be dependent on the

acidity of the exchangeable cation. The acidity of the

metal ion can be increased by performing the adsorption in a

74

solvent which is immiscible with water. Visible absorption
spectra, infrared pleochroic studies, and X-ray diffraction
data indicate that TPPH;+ assumes a more planar conformation
in the adsorbed state than in the crystalline state. This
suggests that the silicate layers of montmorillonite are
capable of inducing conformational changes in the structures
of porphyrins adsorbed into the interlamellar region.

Metalloporphyrin formation on clay surfaces also shows
a metal-ion dependence with Cu(II), Zn(II), and Co(II)
being the only exchangeable metal ions incorporated into
the porphyrin ring. Dication formation also occurs during
metalloporphyrin formation; however, the metallation reaction
predominates. It is interesting that the same order of
reactivity, Cu(II')>'Zn(II)>> Co(II), is found for metal ions
attached to clay surfaces as for other porphyrin model
systems.

The neutral porphyrin complexes CuTPP and CoTPP are
physically adsorbed only on the external surfaces of
montmorillonite. Intercalation is apparently restricted to

porphyrin cations such as TPPH;+ and cationic metallo—

22 This selective intercalation arises from the

porphyrins.
ability of these positively charged porphyrins to function
as exchangeable cations in the interlamellar region.
Bivalent transition metal complexes of TPPH2 are known
to act as heterogeneous catalysts for oxidative dehydrogen-
ation.66 The inability of neutral porphyrin complexes to

become intercalated into montmorillonite precludes the use

 

 

75
of swelling layer silicates as inorganic supports for these
porphyrin catalysts. However, the foregoing research does
suggest that cationic metalloporphyrin catalysts such as
FeTPP+, which is a homogeneous catalyst for the auto-

67

oxidation of methylene compounds, nuurbe intercalated

into clay minerals and function as heterogeneous catalysts.

4.2 Synthesis of Porphyrin Intermediates on the Intracrystal
Surfaces of Montmorillonite
Visible absorption spectra. Sufficient clay is
employed during the condensation reactions so that a single
monolayer of pyrrole and benzaldehyde can be formed in the

2+ and M3+ exchange

interlamellar region. In the case of M
ions, the amount of clay corresponds to a Mn+ to pyrrole
molar ratio of 1:2. Chemical analyses indicate that all the
pyrrole and benzaldehyde molecules are adsorbed for all
exchange forms of the clay.

Table 3 and Figure 15 indicate that the product of the
condensation reaction between equimolar amounts of pyrrole
and benzaldehyde under aqueous conditions at a concentration
of 1.5 x 10'"2 M_and in the presence of various cation
saturated montmorillonites exhibits an intense absorption
near 500 nm. The appearance of an intermediate absorbing
near 500 nm has been observed during the oxidation of
porphyrinogens in previous studies on porphyrin synthesis.

Mauzerall and Granickz6 assigned porphomethene and porpho-

dimethene structures to the intermediate absorbing at 500 nm

76

Table 3. Visible Absorption Spectra of the Pyrrole—
Benzaldehyde Condensation Product Adsorbed on
Various Montmorillonites

 

 

MpT Montmorillonite Amax(nm)
Fe(III) 518
Cu(II) 512
Zn(II) 511
Co(II) 506
Mg(II) 509
VO(II) 522
Na(I) 515a

H+ 515
TPA+ no reaction

 

aThis value is for the reaction in which the M+:pyrrole
ratio is 1:20

 

77

 

Figure 15. Electronic absorption spectra of the pyrrole-
benzaldehyde condensation product a sorbed on
Cu(II) montmorillonite( ) and H montmoril-
lonite( ----- ).

 

 

ABSORBANCE

 

 

 

 

 

’Q
I \\
\
\
I
I
’\ \
1’ ‘ \
I \ ‘
/ \
\ \
\ \
\\ \
\ ‘\
\ \
‘ \
\~ \
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\
\\
._ l J I
400 500 600

WAVELENGTH (nm)

 

79

during the chemical oxidation of uroporphyrinogen to
ur0porphyrin. Later, in a study on the chemical and

photoreduction of uroporphyrin, Mauzera1127'28

assigned
the tetrahydroporphyrin intermediate a porphomethene
structure. A second intermediate correSponding to a
dihydroporphyrin absorbed at 440 nm and 735 nm and was
assigned a phlorin structure. More recently Delphine was
able to isolate the zinc complex of the porphodimethene
during the oxidation of octamethyltetraphenylporphyrinogen
to OMTPP. The zinc chelate was found to absorb at 519 nm,
which was in agreement with the transient absorption which
appeared at 518 nm during the synthesis of OMTPP.

The formulation of the intermediate absorbing at 500 nm
as a porphodimethene or porphomethene during uroporphyrin
and OMTPP synthesis was based on its similarity to dipyrro-

methene. The spectral absorption preperties',l"26-28 nmr

4 26-28

chemical shifts, and the nature of the

4,26-28

pK'values,
zinc, sulfite, and dithionite complexes of the
intermediate were all found to be similar to those of the
dipyrromethene. The porphomethene contains one isolated
dipyrromethene unit while the porphodimethene consists of
two separate dipyrromethene chromophores. The high degree
of correlation between the preperties of the intermediate
and those of the dipyrromethene served to confirm the
dipyrromethene type structure for the intermediate as either
the porphomethene or porphodimethene.

A similar dipyrromethene chromophore(XIII) is proposed

 

 

 

80
for the condensation of pyrrole and benzaldehyde on
montmorillonite. A porphomethene or porphodimethene
intermediate can.then be represented by structures XIV

and.XV, respectively. Each of the species could exhibit

   

XIII XIV XV

the observed intense absorption near 500 nm. Although
the visible absorption spectrum.cannot distinguish between
the porphomethene and porphodimethene structures, the
intermediate on the montmorillonite surface is assigned
the pggprtetraphenylporphodimethene(TPPDM) structure based
on the similarity between the TPPH2 reaction and the OMTPP
reaction and the isolation of the porphodimethene interme-
diate from the OMTPP reaction as the zinc chelate. The
presence of an additional dipyrromethene unit would also
be expected to impart extra stability to the TTPDM structure
because of the gain in resonance energy.

The shift of the TPPDM absorption band from 480 nm

in acetic acid solutionl'dI

to longer wavelenghts when in
the adsorbed state suggests that the intermediate assumes a
more planar conformation in the interlamellar region than

is possible in solution. The ability of the silicate layers

 

 

 

 

81
to restrict porphyrin molecules to a more planar conformation
was noted previously for TPPH:+ molecules exchanged into the
interlamellar region of montmorillonite(pf., Section 4.1).

The 001 reflections for powdered samples of montmoril-
lonite containing the adsorbed intermediate were 14.7 X for
Cu(II) and 17.7 X for Fe(III) and 3* after drying at 200° c.
These basal spacings yield a separation between the silicate
sheets of 5.1 X and 8.1 X; however, broad, diffuse diffraction
bands were obtained at the higher 001 reflections which

indicates that interstratification of different layers was

present. An accurate determination of the interlayer

 

spacing due to TPPDM was therefore not possible at these
particular reflections. The lower 001 reflection exhibited
the sharp diffraction band indicative of a single organic
layer. If the thickness of a porphyrin molecule is taken to
be 4.7 2,7 then a basal Spacing of 5.1 X indicates that the
TPPDM molecules are oriented parallel to the silicate sheets
in a planar conformation.

Desorption studies. Further evidence for the existence
of a porphyrin precursor on the montmorillonite is provided
by the desorption experiments. Based on the yield of TPPH2
obtained from various extraction methods(p£,, Section 3.6),
Method D, in which 10 to 12 per cent TPPH2 can be obtained
by forming the intermediate in the presence of TPA+, provided
the most efficient means of displacing the adsorbed interme-
diate from the exchange sites of montmorillonite.

Figure 16 shows a typical absorption spectrum of the

 

 

 

 

 

Figure 16.

82

Electronic absorption spectra of the chloroform
extraction solution 2 hours and 24 hours
( ----- ) after the desorption of the pyrrole-
benzaldehyde condensation product from +Cu(II)
montmorillonite in the presence of TPA+

 

 

83

 

 

 

 

m02<mmomm<

500 600

WAVELENGTH (nm)

400

 

84
chloroform extract obtained by Method D. The absorption
band at 485 nm, whose intensity decreases with time as the
Soret absorptions increase, confirms the presence of the
desorbed porphyrin intermediate in solution. The similarity
in position of the intermediate absorption to that obtained
for the intermediate during the synthesis of TPPH2 in acetic

1-4

acid suggests that the same intermediate is formed in

both cases, namely TPPDM. The bands at 415 nm and 447 nm
are assigned to the Soret absorptions for TPPH2
respectively. The appearance of an absorbance for

and TPPHZ+,

diprotonated porphyrin is significant since an aprotic
extraction solvent was employed, which suggests that the
source of protons must be the clay surface itself. The
acidic nature of the montmorillonite surface therefore is
capable of maintaining the intermediate in the cationic form
needed to satisfy the exchange sites on the clay. The
presence of the Soret absorptions and the intermediate band
in solution suggests that oxidation of the intermediate
occurs after desorption from the montmorillonite surface.

Adsorption of theporpthin intermediate from solutigp.
The synthesis of the porphyrin intermediate in solution and
its subsequent adsorption onto the TPA+ and Cu(II) exchange
forms of montmorillonite provide additional evidence for
assigning the TPPDM structure to the condensation product on
the clay surface. Comparison of Figure 9 with Figure 15
shows the similarity in the shape and intensity of the

absorption bands, with the bands in Figure 15 absorbing at

 

 

 

 

 

85
longer wavelenghts. The bathochromic shift that occurs
when TPPDM is formed on the clay surface results from the
intercalation of the condensation product during its
synthesis while only external adsorption occurs during
the adsorption of the intermediate from solution. That
TTPDM is adsorbed from solution only onto external adsorption
sites is supported by the Xpray diffraction data, which
yielded 001 spacings of 12.4 X and 14.7 K for intercalated
Cu(II) ions and TPA+ ions, respectively.

The adsorption of the porphyrin intermediate from
acidic chloroform solution onto Cu(II) montmorillonite
results in the formation of CuTPP in solution(pf., Section
3.7). The Cu(II) ions which become complexed with TPPH2 in
the filtrate are evidently replaced in the interlamellar
region by the two protons which are displaced from the
porphyrin molecule during chelation rather than the
dipositive TPPDM intermediate. Since chloroform is a poor
swelling solvent, the inability of TPPDM to penetrate the
montmorillonite layers is not unexpected. Although the TPA+
ions maintain a separation of 5.1 X between the silicate
layers, TPPDM, because of its larger size, does not compete
favorably with TPA" for the exchange positions and interca-
lation does not occur.

The porphyrin intermediate when externally adsorbed
on Cu(II) montmorillonite absorbs at 505 nm as compared to
485 am when TPAI montmorillonite is the adsorbent. This

shift of 20 nm to longer wavelength is believed to be the

 

86

result of chelation of TPPDM with Cu(II) ions. The
shoulder at 475 nm is then assigned to the unchelated TPPDM
molecules. The shoulder at 447 nm results from the Soret
absorption for TPPH++.

MechaniSm.of meso-tetraphenzlporphodimethene formation.
The mechanism proposed for the formation of TPPDM on the
intracrystal surfaces of montmorillonite is an adaptation of
the pathway described by Dolphin“ for the synthesis of ppgpr
substituted porphyrins in acid solution. The following set
of equilibria represent the synthesis of TPPDM on the

montmorillonite surface:

 

 

11(1120);N + 1120 Z M(H20)x_1(0a)(n‘1)* + 1130+ (11)

 

 

4 Pyrrole(aq) + 4 Benzaldehyde(aq) EE

 

4 Pyrrole + 4 Benzaldehyde (12)

 

 

 

 

 

 

 

 

 

 

 

4 Pyrrole + 4 Benzaldehyde E; Porphyrinogen + 4H20 (13)
H 0”

Porphyrinogen ‘2: WIN" (14)
1130+

rpm" ->E TPPDM‘” (15) ,

 

 

The overall reaction for TPPDM formation can then be

 

 

 

 

 

 

_.__———-—'

 

 

87

represented by the equation

 

211 0+
4 Pyrrole(aq) + 4 Benzaldehyde(aq) E TPPDM++ + 4H20 (16).

 

The hydrolysis of the metal ion in the interlamellar region
as represented by Equation 11 provides the protons needed to
catalyze the autooxidation of the porphyrinogen to TPPH in
Equation 14 and the oxidation of TPPM to TPPDM in Equation 15.
The ability of the metal ion to hydrolyze, as measured by
the pKh values in Table 2, is an indication of the relative
rate of TPPDM synthesis on the clay surface. The synthesis
of TPPDM is found to proceed more rapidly in the presence of
the more acidic metal ions such as Fe(III), VO(II), and
Cu(II), which require only minutes to initiate the reaction,
while the other metal ions require a half hour or more to
produce the initial TPPDM condensation.

The oxidation of the porphyrinogen is believed to be
either free radical in nature with acid catalysis being a
function of protonation on a pyrrolic nitrogen or as a

4 The

hydride transfer to the hydr0peroxy(HD;) cation.
formation of TPPDM in the absence of oxygen suggests that
the acid catalysis of porphyrinogen oxidation follows a free
radical pathway. The removal of a hydrogen atom from.a
methene bridge carbon generates a resonance stabilized free
radical. Protonation of a pyrrolic nitrogen followed by the

loss of another hydrogen atom produces an oxidized dipyrro-

methene unit. This oxidation is repeated a second time to

 

88

 

Figure 17. Mechanism of meso-tetraphenylporphodimethene
formation on the intracrystal surfaces of
montmorillonite .

 

89

.0.H .¢.H
+H
H H
.¢.H ¢.H
. O +
n a H
.¢.H .0.H ¢H
H
H H H _
¢ o. ¢ ¢

 

 

 

 

90
yield TPPDM(Fig. 17).

From Figure 15 it is evident that only a small amount
of the TPPDM intermediate is oxidized to TPPH2 while in the
adsorbed state. This increased stability of TPPDM to air
and photolytic oxidation while in the adsorbed state is
believed to result from the ability of the negative charge
on the silicate layers to stabilize the resonant dipositive
charge on TPPDM. This electrostatic interaction is possible
because the dicationic structure of TPPDM permits it to
function as an exchangeable cation on the montmorillonite
surface. The counter anion effect of the silicate layers is
similar to that of the bromide ion used to stabilize and
isolate the pppprphenyl-3,3',4,4',5,5'-hexamethyl-2,2'-
dipyrromethene cation.“ It is significant that Dolphinq
reported the stability of the porphodimethene of OMTPP in
the absence of oxygen for several days while Mauzerallz8
observed that the porphomethene of ur0p0rphyrin was oxidized
rapidly by iodine but only very slowly by oxygen. The
slowness ofTPPDM to exidize in air was also observed during
its synthesis in a weakly acidic chloroform solution.
Several hours of reaction time were required before TPPHZ+
appeared in solution(g£., Section 5.7).

The formation of free base and diprotonated TPPH2
during desorption of TPPDM can be represented by the

following set of equilibria:

 

E ++

++
TPPDM E TPPDM(Sol,n) (17)

 

 

91

 

 

+

 

 

++ -€>
TPPDM + 21120 E TPPDM( 8 01,11) + 21130 (18)
so
2 ++
++ -—-t'
TPPDM(sol'n) E TPPH4(sol'n) + H20 (19)
tog
-——i»

The overall reaction may then be written as

0

++
++ -—-e-
2 TPPDM + 2H20* E TPPH4(sol'n) + TPPH2(sol'n) +

 

IO

 

 

+
21120 + 21130 (21) .

 

 

The above mechanism assumes that oxidation of TPPDM occurs
only after its desorption from the montmorillonite surface.
The trace amounts of TPPH2 formed on the clay during the
condensation reaction and the presence of TPPDM in the
extraction solution and its subsequent oxidation to porphyrin
would seem to support this assumption. It should be

remarked that a small percentage of TPPH:+ formed in solution

may result from the protonation reaction

 

+ ++
TPPH2(801,n) + 2H30 "">E TPPHMsolm) + 21120 (22).

 

 

The release of TPPDM from the interlamellar region removes
the silicate layers as counter anions and destabilizes the
intermediate toward air oxidation. This destabilization

results in the appearance of TPPH2 and TPPH:+ bands in the

 

92
extraction solution. The yield of porphyrin is determined
by the amount of TPPDM desorbed from the montmorillonite
surface, which is severely limited by the ability of
TPPDMI+ to function as an exchangeable cation in the
interlamellar region.

Conclusion. It is probable that the prebiotic
formation of porphyrins and porphyrin-like precursors
occurred on the surfaces of clay minerals such as montmoril-
lonite because of their ability to adsorb and concentrate
smaller organic molecules from the primitive ocean. The
presence of exchangeable metal cations and their ability to
hydrolyze in the interlamellar region enabled the clay
surface to function as a catalyst for the formation of
biological monomers.

The formation of TPPDM from pyrrole and benzaldehyde
in an aqueous environment is found to proceed quite easily
in the presence of various cation exchanged montmorillonites
under aerobic and anaerobic conditions. The negative charge
of the silicate layers is believed to stabilize diprotonated
TPPDM once it is formed on the clay surface. Appropriate
organic solvents are capable of desorbing TPPDM in the
presence of a suitable exchangeable cation with oxidation to
porphyrin occurring after desorption. A free radical, acid-
catalyzed mechanism which does not require the presence of
oxygen for oxidation is proposed for the prebiOtic formation
of porphodimethenes on the intracrystal surfaces of clay

minerals.

 

 

 

 

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