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(J 1‘ ,',x \ 1" n | O .1 .l', ‘4, 1 1 1" ’1, 1 1 1 1 .n 1‘ 1.11 1. 1 I 1 1111 1 . . .. - . . 1111 11111111111111 1 111 11 1" . 1 11 1 1' '1 1 11 11 1 ' -' ' ' '11 1 11,,1 '. ., .,.‘1' 1 l ,". 1 U 1 .0- 1, ‘1 1 .114 ‘ 4 1’1 11 quiz"? ‘ , 1 1.1 1 .. 1 I 4 1‘ 1 ,I‘ .1 .1 1 1 .' 1n 1 ' 1 . 1.. . 1" 1‘ . l ‘U 1 . ‘ II I . ‘ .I l1 ‘ I ‘ , 2 1 - 1 1 . , I r . 1111 1,1 111 1 .111 ,,,11,1 1 1 :mif" .r:?’ f. 1 :1; 11.11111,1 11 111,1 1' ' ~ “‘- v’z: «11111 1111111 111111 1111 I11 1111,1111! 1MW1M” THEE“ 5 This is to certify that the d. l’t t. to 1 THE SYNTHESIS listen hfl‘fl'c fifllzulou OF THE MOLYBDENUM-IRON—SULFUR CLUSTERS (SZMoSZFeL2)2', (32MoSZFeSZFeL2)3'. AND (32MoSZFeSZFeSZMoSZ)“‘ presented by Robert Hugo Tieckelmann has been accepted towards fulfillment of the requirements for PhD degree in Chemistry Major professor Date 1/17/91 MS U is an Affirmative Action/Equal Opportunity Institution 0-12771 L55” S “a"! Micki: 1:7; in: finite University ‘ ‘- flaw. {1 flag 9 V‘ ~33. II” ¥ “ 1 ‘ U” v w: 25¢ per day per item RETURNHG LIBRARY MATERIALS: Place in book retum to remove charge from circulation records THE SYNTHESIS AND CHARACTERIZATION OF THE MOLYBDENUM-IRON-SULFUR CLUSTERS [32Mos FeL2]2-, [32Mos FeS FeL233', AND 2 2 2 FeS FeS MOSZJH- [SZMOS2 2 2 By Robert Hugo Tieckelmann A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1981 ABSTRACT THE SYNTHESIS AND CHARACTERIZATION OF THE HOLYBDENUH-THoN—SULFUH CLUSTERS FeL213', AND [S2MoS FeL2]2-, [SZMOSZFeS 2 2 [S2MoS FeS FeS Mos2ju' 2 2 2 By Robert Hugo Tieckelmann The binuclear complexes, [FeMSu01212- (M = Mo,W), have been prepared by reaction of anhydrous ferrous chloride and tetrathiometallate. They can be converted to the [FeMSu(SAr)2]2- ions by reaction with either triethylamine/ arylthiol or sodium thiolate. The thiolate complexes can be converted back to [FeMSuCl2]2' by reaction with benzoyl chloride. The trinuclear complexes, [Fe2MS6(SAr)2]3- (M = Mo,W), have been prepared by reaction of either [FeuSu(SAr)u]2- or [Fe282(SAr)u]2' with tetrathiometallates in 1:2 and 1:1 ratios, respectively. They do not exhibit ligand exchange chemistry similar to the binuclear complexes. u- The tetranuclear complex, [Fe2Mo2S , has been pre- 101 pared in low yield by reaction of [FeuSu(SAr)u]2— and Robert Hugo Tieckelmann tetrathiometallate in a 1:“ ratio. The X-ray crystal structures of the tetraethylammonium salts of [FeMoSuCIZJZ- and [FezMoS6(S-EfTol)2]3- have been determined. The binuclear complex Contains the planar FeS2Mo subunit with terminal sulfides on molybdenum and chlorides on iron, resulting in essentially tetrahedral coordination about each metal atom. The trinuclear com- plex is a permutation of the dinuclear cluster with in- sertion of an FeS2 moiety into the FeS2Mo subunit to yield a linear FeSZFeS2Mo unit; terminal sulfides on the molyb- denum and thiophenolates on the iron complete the struc- ture. The tetranuclear complex is predicted to be an SZMOSZFeSZFeSZMoS2 linear unit based on its method of synthesis, magnetic susceptibility, and reactivity. Electronic spectral, magnetic susceptibility, and electrochemical data are reported for each complex. For the binuclear complexes, Mdssbauer studies suggest an Fe(II)-Mo(VI) description with slight charge delocaliza- tion from iron to molybdenum via bridging sulfur atoms. For the trinuclear complexes, Mdssbauer and ESR studies indicate an Fe(III)—Fe(II)-M0(VI) description. The tri- nuclear and tetranuclear complexes may be viewed as reduced and super-reduced forms of the [Fe282(SAr)u]2- ion, where each tetrathiometallate ligand stabilizes an adjacent Fe(II) Site. The major contributions to understanding the general Robert Hugo Tieckelmann physical and chemical behavior of Mo(W)-Fe-S systems are the stability of the FeS2Mo unit, the finding that the Fens“ core rearranges readily upon reaction with the tetrathiomolybdate(VI) anion, and the utility of these new complexes for calibrating the EXAFS spectra of the FeMo-cofactor. A variety of lines of evidence suggest that the FeSzMo unit will be an important structural frag- ment of the iron-molybdenum-cofactor of nitrogenase. To George and Betty for their continuing support, inspiration, and dedication to the education of their children; and to Stephanie 11 ACKNOWLEDGMENTS I would like to extend my thanks (again!) to Bruce Averill. His guidance on experimental details, particu- larly on interpretation of their results, his generous financial support, and his encouragement in meeting the requirements necessary for the completion of this degree are sincerely appreciated. I would also like to acknowledge the following in- dividuals for their contributions to my research: Dr. B. K. Teo and M. R. Antonio for the two crystal structures; Drs. E. Munck, T. A. Kent, and B. H. Huynh for obtaining the Mdssbauer spectra; J. C. Davis for obtaining the ESR speCtra; Dr. H. C. Silvis for various experimental details; and the remainder of Bruce Averill's Molecular WorkshOp for the teamwork necessary to maintain a viable research program. Many thanks to the Chemistry Department for not only providing a teaching assistantship, but also for providing the environment in which my research was performed. I would also like to thank my housemates, Ken Guyer and Scott Sandholm, for supporting the ideologies neces- sary to run our quasi-sane home and brewery. A special thanks to P. A. Warstler for typing this thesis and to "Jo" for preparing the graphics in record 111 time. Once again, I want to extend my heartfelt thanks to my parents and to Stephanie for their boundless support. iv TABLE OF CONTENTS Chapter Page LIST OF TABLES. . . . . . . . . . . . . . . . . . . ix LIST OF FIGURES . . . . . . . . . . . . . . . . . . xi LIST OF ABBREVIATIONS . . . . . . . . . . . . . . . xiv I. INTRODUCTION. . . . . . . . . . II. EXPERIMENTAL . . . . . . A. Materials and Methods . . . . . . . . . B. Preparation of (EtuN)2[FeMSuC12]. 1. (EtuN)2[FeMOSuClz]. . . . . 2. (EtuN)2[FeWSuC12] . . . . . . . . . C. Preparation of (EtuN)3[Fe2MS6(SAr)2]._. (EtuN)3[FeZMOS6(SPh)2]. Method A . (EtuN)3[Fe2MoS6(SPh)2]. Method B . (EtuN)3[Fe2MoS6(Sgp-Tol)2]. (EtuN)3[Fe2WS6(SPh)2] U'I-t'wm (EtuN)3[F82ws6(SfR-T01)2] \OCDGDCDNmmChU'IU'IUUWl-J D. (EtuN)u(Fe2M02$lO)o E. Interconversion Reactions . H O 1. (EtuN)2[FeMoSuClzj + (EtuN)2- [FeMoSu(SPh)2]. . . . . . . . . . . . . 10 2. (EtuN)2[FeMOSu(SPh)2] + (EtuN)2— [FeMoSuCl2] . . . . . . . . . . . . . . 10 Chapter F. Page Reactions of (EtuN)2[FeMoSuClz] with Thiols . . . . . . . . . . . . . . . . 10 l. EtSH, geBuSH, or BZSH and Et N. . . . . 10 3 2. BzSH and Et3N, Preparative Scale. . . . ll 3. Attempted Preparation of [FeMo- Su(Sz-gfxylyl)]2- . . . . . . . . . . . 12 A. Disodium g-Xylyl-a,u'-dithiolate. . . . 12 Preparation of Sulfides . . . . . . . . . . 13 1. Crown Complex of NaZS . . . . . . . . . l3 2. (EtuN)HS. . . . . . . . . . . . . . . . l3 Reactions of (EtuN)2[FeMoSuCl2] with SUlfideSo o o e o o o o o o o o o o o o o o 15 1. Crown Complex of NaZS . . . . . . . . . 15 2. (EtuN)HS. . . . . . . . . . . . . . . . 15 3. (EtuN)HS and Proton Sponge. . . ,,, . . l6 Reactions of (Q)2[FeMoSu(SAr)2] with Sulfides. . . . . . . . . . . . . . . . . . 16 l. NaHS. . . . . . . . . . . . . . . . . . l6 2. (EtuN)HS. . . . . . . . . . . . . . . . l7 3. (EtuN)HS and Proton Sponge. . . . . . . l7 Reactions of (EtuN)3[Fe2MoS6(SAr)2] . . . . 18 l. Benzoyl Chloride... . . . . . . . . . . l8 2. Mosfi' . . . . . . . . . . . . . . . . . 18 3. Sulfide and Mosfi’ . . . . . . . . . . . 19 2O Reactions of (EtuN)u[Fe2Mo S 2 10:I ° ° ' l. PhSH. . . . . . . . . . . . . . . . . . 20 vi Chapter III. 2. Dilute Acid . Reactions of Fens” Tetramers with (EtuN)2MOSu . o e . o e o o 1. (EtuN)2[FeuSu(SEt)u]. . . . . 2. (EtuN)2[FeuSu(S-thu)u] 3. (EtuN)2[FeuSu(S-peTol)u]. Growth of (EtuN)3[Fe2MoS6(S-p-Tol)2] Crystals. . . . . RESULTS AND DISCUSSION (EtuN)2[FeMSu012] (M==Mo,w) . 1. Synthesis and Reactivity. 2. Structure . . . . 3. Electronic Spectra. A. Magnetic Susceptibility . 5. Mbssbauer Spectroscopy. 6 Electrochemistry. 7. Summary . . . . . . . . . . (EtuN)3[Fe2MS6(SAr)2] (M = MO,W; Ar = Ph, p-Tol) . . . . . . . 1. Synthesis and Reactivity. 2. Structure . 3. Electronic Spectra. . . . . A. Magnetic Susceptibility 5. Mdssbauer Spectrosc0py. 6. Electrochemistry. . . vii Page 20 2O 2O 21 21 22 23 23 23 26 32 37 38 “3 AA “5 “5 “9 56 60 60 6A J ! '1'”- Chapter 7. Electron Paramagnetic Resonance Spectrosc0py. 8. Summary . C. (EtuN)2[FeZM02810 Synthesis and Reactivity. Electronic Spectrum . l. 2 3. Magnetic Susceptibility . A Electrochemis 5. Summary . IV. CONCLUSIONS. REFERENCES. 1. try. viii Page 67 70 71 71 7A 78 79 80 81 8h Table II III IV VI VII VIII LIST OF TABLES Page Interatomic Distances (A) with Esd's for (EtuN)2[FeMoSuC12]. . . . . . . . . . 28 Bond Angles (deg) with ESD's for (EtuN)2[FeMoSuCI2]. . . . . . . . . . . . 28 Selected Structural Parameters in the [52Mos*Ee012]2’(T),[012Fes*Fe012]2' 2 2 GIL and [Cl FeS*MoS*FeCl 32‘ all) —— 2 2 2 2 —— Complexes . . . . . . . . . . . . . . . . 30 Electronic Spectral Features and Magnetic Moments of (EtuN)2[FeMSu- 012] Complexes, M = Mo,w. . . . . . . . . 3A Mdssbauer Data for Various Binuclear Mo-Fe-S and w-Fe-S Systems. . . . . . . . A2 Selected Interatomic Distances (A) for (EtuN)3[Fe2MoS6(S-p-Tol)2]. . . . . . 51 Selected Bond Angles (deg) for Selected Structural Parameters 3- in the [SZMOS2FeS2Fe(S-p-Tol)2] 2- QXA [(ErTolS)2FeS2Fe(S-p-Tol)2] FeS M08233’ Q2) NTIL [SZMOS 2 2 ix Table VIII IX XI XII MOS FeC12J2- QIIL [SzMoS - 2 2 2 2- 2- FeCl2] I, [S2MoSZFe(SPh)2] (Z) [C12FeS Complexes Electronic Spectral Features and Magnetic Moments of (EtuN)3[Fe2- MoS6(S-pgTol)2] Complexes, M = Mo,W. . . . . Magnetic Susceptibility Data for the (EtuN)3[Fe2MS6(SAr)2] Complexes (M = Mo,w and Ar = Ph, p-Tol) and Selected Mo(W)-Fe-S Complexes . Electrochemical Data for the (EtuN)3— [Fe2M86(SAr)2] Series M = Mo,w and Ar = Ph, p—Tol. Selected Physical Data for the (EtAN)A[Fe2MO2310] Complex. Page 55 58 61 65 77 Figure LIST OF FIGURES Synthesis and Reactivity of [FeMSu— C1232” (M = Mo,W) in MeCN . . . Stereochemistry of the [$2MoS2— FeCl2J2- dianion. The crystal- lographic Ci-I site symmetry causes a two-fold disorder of the metal atoms (M = (M0 + Fe)/2) and the terminal ligands (X = (S + Cl)/2) . . Electronic spectra of (EtuN)2 [CleeS M082] and (EtuN)2[CleeS - 2 2 W82] in acetonitrile solution at 23° . . . . . . . . . . . . . . . Electronic spectra of (EtuN)2MoSu and (EtuN)2WSu in acetonitrile solution at 23°C. . . . . Correlation diagram relating isomer shift to formal oxidation state for various iron-sulfur clusters Massbauer spectrum of frozen DMF solution of (EtuN)2[FeMoSuClz] re- corded at A.2 K and with zero applied xi Page 25 27 33 36 39 Figure Page 6 field. Vertical bar indicates 1% absorption. . . . . . . . . . . . . . . A1 7 Synthesis and Reactivity of [Fe2- MS6(SR)2]3- (M = Mo,w; R = Ph,p-Tol) in MeCN . . . . . . . . . . . . . . . . 48 8 Stereochemistry of the [S2MOS2FG- S2Fe(S—p-Tol)2]3- trianion. Hydrogen atoms are omitted for clarity . . . . . 50 9 Schematic Detailing of Averaged Pairs of Terminal and Bridging Sulfurs in "Linear" Mo—Fe-S Systems . . . 53 10 Electronic Spectra of (EtuN)3- [(EeTolS)2FeS FeS M082] and (EtuN)3— 2 2 [p—TolS)2FeS2FeS2WS2] in aceto- nitrile solution at 23°C. . . . . . . . . 57 ll Mdssbauer spectrum of (EtuN)3- [S2MoS FeSZFe(SPh)2] recorded at 2 u.2 K and with 600 G applied field . . . . . . . . . . . . . . . . . . 62 2FeS2Fe- (SPh)2] in frozen acetonitrile solution. ESR spectra of (EtuN)3[S2MoS Conditions of ESR spectroscopy. Top spectrum: modulation amplitude, 5 G; magnetic field sweep rate, 120 s; time constant, 0.3 5. Bottom spectrum: xii Figure 12 13 1A modulation amplitude, l G; magnetic field sweep rate, 200 s; time constant, 0.2 s; instrument gain, 500 Synthesis and reactivity of u- [Fe2M02310] in MeCN . . . . . . . Electronic spectrum of (EtAN)A‘ [Fe2MO2SlO] in acetonitrile solu— tion at 23°C. 0 o e o e e o e xiii Page 69 75 76 Solvents: DMA = DMF = DMSO = EtOH = Et 0 = HMPA = MeCN = MeOH = NMF = Reagents: PhCOCl = Et3N = proton sponge CPOWI’I ’- Miscellaneous: Ar = BM = LIST OF ABBREVIATIONS N,N-dimethylacetamide N,N-dimethylformamide dimethylsulfoxide ethanol diethylether hexamethylphosphoramide acetonitrile methanol N-methylformamide benzoyl chloride triethylamine = N,N,N',N'-tetramethyl-1,8-naphthalenediamine l8-crown-6; l,A,7,10,13,16-hexaoxacyclo- octadecane. aryl Bohr magneton xiv Bz Et EtuN FeMoco IS Me MeuN Ph PhuAs PhuP prTol QS SCE Tol benzyl ethyl tetraethylammonium iron-molybdenum—cofactor isomer shift methyl tetramethylammonium phenyl tetraphenylarsonium tetraphenylphosphonium paraftolyl quadrupole splitting standard calomel electrode Toluene XV I. INTRODUCTION The 1977 report of the isolation1 and characteriza- tion2 of an iron-molybdenum cofactor (FeMoCo) from the 3 nitrogenase enzyme marked the beginning of a new area of inorganic chemistry. The FeMo-cofactor was reported to contain eight iron atoms and six acid labile sulfur atoms per molybdenum atom in a low molecular weight in- organic subunit that was extremely sensitive to oxygen and moisture. It was also reported not to contain, in readily removable form, the classic FeuSu clusters that were known to exist in the nitrogenase enzyme.“ Extended X-ray Absorption Fine Structure (EXAFS) results5 indicated a molybdenum atom surrounded by four sulfur atoms at an average distance of 2.35 A and by approximately three iron atOms at an average distance of about 2.71 A. This limited amount of data was the basis used by synthetic chemists attempting to synthesize new Mo-Fe-S compounds. The rush to synthesize and characterize Mo-Fe-S systems that might mimic or resemble the chemical and physical properties of the FeMazo has grown steadily. Although as of this writ- ing no synthetic Mo-Fe-S compounds closely approximate the reported stoichiometry and spectroscopic properties of the FeMoco, these contributions have significantly increased our understanding of basic Mo-Fe—S chemistry and thereby provided a firmer foundation for eventual understanding of the FeMoco. Two basic structural types of Mo-Fe-S compounds have been prepared. The first synthesized were the "cubane" Mo-Fe-S complexes6’7, [Fe6M0288(SR)9]3', [Fe6Mo289(SR)8]3- 3'3““- and [Fe7Mo288(SR) These interesting systems l2J have one specific structural component in common, the Fe3MoSu cubic subunit. The second structural type is the "linear" Mo-Fe-S complexes, which includes binuclear com- plexesa-lo, [FeMoSuR2J2- (R = SAr, OAr, Cl or R2 = SS); trinuclear complexesll'l3 , [Fe(MoSu)2]3-, [FezMoSuClu12-, and [Fe2MoS6(SAr)2]3-; and a (presumed) tetranuclear system, [Fe2Mo2SlO]u-. The structural component shared by all the linear systems is the planar FeS2Mo subunit. The research presented in this thesis describes the synthesis and characterization of the dinuclear compound, [FeMoSuC1232-; the trinuclear compounds, [Fe2MoS6(SAr)2]3-; and the only tetranuclear compound known, [Fe2Mo2SlO]u-. The synthesis and characterization of the tungsten ana- logues of the dinuclear and trinuclear compounds are also described. II. EXPERIMENTAL A. Materials and Methods All operations were performed in an atmosphere of di- nitrogen or argon purified by passage over hot BASF catalyst R 3-11 and supported P205 (Aquasorb). Solvents and reagents were degassed prior to use by repeated evacua— tion and flushing with purified dinitrogen or argon. Op- tical spectra of all reagents having well characterized spectra were checked for purity with a UV-VIS spectrometer prior to each use. Acetonitrile was either Aldrich Chemi- cal Company, Inc. Gold Label or distilled from CaH2. PrOpionitrile, toluene, tetrahydrofuran, and methanol were distilled from CaH2, sodium/benzophenone, and magnesium methoxide, respectively. Dimethylsulfoxide, N,N-dimethyl- acetamide, and N,N-dimethylformamide were Aldrich Gold Label and used without further purification. Triethyl- amine and benzoylchloride were stored over A A sieves and the latter was distilled to a trap submerged in liquid nitrogen prior to use. All mercaptans were obtained from Aldrich Chemical Company and used without further puri- fication. , 1A 15 gnglyl-a,a -dithiol , sodium hydrosulfide , 3,5- dimethylpyridiniumhexafluorOphosphate16, (EtuN)2Mosgl7, 18 and anhydrous FeCl2 were prepared as described. Large single crystals of Na2S-9H2O were quickly rinsed in distilled water and neat ethanollg before weighing prior to use. Salts of tetrathiomolybdate and tetrathiotungstate anions were prepared by the methods describedzoa-zoe. Tetraethylammonium perchlorate was prepared via reaction of tetraethylammonium hydroxide and perchloric acid in water, followed by recrystallization from acetonitrile/ diethylether. Sodium ggxylyl-a,a'-dithiolate was prepared from the free dithiol and 2 equivalents of sodium meth- oxide in methanol; subsequent removal of solvent yielded a fine white powder. The sodium salt of thiocresol was provided by H. C. Silvis; it was prepared by reaction of the free thiol with one equivalent of sodium methoxide in methanol, evaporation to dryness, evaporation twice with acetonitrile and drying in yagug. H. C. Silvis also pro- vided (EtuN)SPh25, prepared as noted. (EtuN)2[Fe282(82-9¢xy1y1)2]lu, (EtuN)2[Fe2Sé(SAr)u]lu, (EtuN)2[FeuSu01u]21, (PhuAs)2[FeuSuC1u]2l, (EtuN)2[FeuSu- (SR)u]22—2u, (PhuAs)2[FeuSu(SAr)u]2u, (BzEt3N)2[FeuSu- (SAr)u]2u, (EtuN)2[FeMoSu(SAr)2]8 and (PhuAs)2[FeMoSu- (S-peTol)2]8 were prepared by published methods. Optical spectra were obtained by use of either a Cary 17 or a Cary 219 spectrophotometer. 1H NMR spectra were ob- tained by use of a Varian T-60 spectrometer. Magnetic sus- ceptibility measurements were performed at room temperature by use of an Alpha Faraday balance by using Hg[C0(NCS)u] as calibrant. Electrochemical measurements were made with a - .__ PAR 17AA polarographic analyzer employing either dc polaro- graphy (dropping mercury electrode) or cyclic voltammetry (platinum electrode). Microanalyses were performed by Galbraith Laboratories, Inc., Knoxville,.Tenn. Melting points were obtained in sealed tubes in_zagug and are un- corrected. The MBssbauer experiments were performed by T. A. Kent and B. H. Huynh at the Gray Freshwater Biological Institute, University of Minnesota, Navarre, Minnesota. The Mossbauer spectrometer was of the constant accelera- tion type and has been described previously26. Electron paramagnetic resonance spectra were obtained on a Bruker ER-200D instrument equipped with an Oxford liquid helium flow cryostat. B. Preparation of (EtMN)2[FeMSugl2l 1. (EtuN)2[FeMoSuglel To a slurry of 0.29 g (2.28 mmol) of anhydrous FeCl2 in 10 mL of acetonitrile, a solution of 1.0 g (2.06 mmol) of (EtuN)2MoSu in 100 mL of acetonitrile was added and vigor- ously stirred. After 10 hours the solution was filtered and the filtrate reduced in vacuo to who mL. Cooling to -20°C gave the product as dark brown microcrystals; recrystal- lization from acetonitrile gave product as dark brown prisms (mp 217°C dec) in ~80% yield. Anal. Calcd for Cl6HuOCl2FeMoN28u: c, 31.u3; H, 6.59; Fe, 9.13; Mo, 15.69; N, “.58. Found: C, 31.63; H, 7.00; Fe, 9.16; Mo, 15.77; N, “.76. The same compound was also obtained in low yield from an acetonitrile solution containing 1 equiv of (EtuN)2MoSu and 2 equiv of (EtuN)2[FeuSuClu] after brief exposure to the atmosphere and subsequent anaerobic workup. 2° £§£ufllg£§§fl§uglgl To a slurry of 1.0 g (6.5 mmol) of anhydrous FeCl2 -+ in 50 mL of MeCN, a solution of 3.75 g (6.5 mmol) of (EtuN)2WSu in 250 mL of MeCN was added and vigorously stirred. After 10 hours the solution was filtered, and the volume of the filtrate was reduced in vacuo at “5°C until microcrystals appeared (150 mL final volume). Cooling to -20°C gave the product as red-brown crystals. One re- crystallization from warm MeCN (50°C) followed by cooling to ~20°C yielded the product as red-brown prisms (mp 21“- 216°C dec) in 65% yield. Anal. Calcd for 016HAOCl2FEWNZS“: C, 27.“8; H, 5.77; N, “.01; S, 18.3“. Found: C, 27.93; H, 5.91; N, “.32; S, 18.09. C. Preparation of (EtQN13L332M§6(SAr)2l 1. (Etufllq[Fe2MoSS(SPh)2]. Method A To a solution of 1.50 g (3.09 mmol) of (EtuN)2MoSu in 100 mL of MeCN was added a solution of 2.70 g (3.09 mmol) of (EtuN)2[Fe282(SPh)u] in “00 mL of MeCN. Prolonged re- action (3-“ weeks) at room temperature resulted in the disappearance of the Optical spectrum characteristic of MoSfi' and formation of product optical spectrum. The re- action mixture was filtered, and the solvent volume reduced at 50°C to ca. 200 mL. Cooling the solvent to -20°C afforded purple-black microcrystals which were collected while cold. Recrystallization was effected by dissolu- tion in a minimum amount of MeCN at 50°C, filtration, and slow cooling to -20°C. Product yield is typically “0%; subsequent solvent reductions of the mother liquor fol- lowed by recrystallization increase the yield to 80%, mp 190-19l°C (dec). Anal. Calcd for C 6H 3 7OFe2 C, “2.85; H, 7.00; Fe, 11.07; Mo, 9.51; N, “.16; S, 25.“2. MON3S83 Found: C, “2.97; H, 7.2“; Fe, 11.23; Mo, 8.97; N, “.21; S, 25.“5. 2. (EtuN13[Fe2MoSS(SPh)2]. Method B To a solution of 1.85 g (3.82 mmol) of (EtuN)2MoSu in 125 mL of MeCN at “5°C was added a solution of 2.0 g (1.91 mmol) of (EtuN)2[FeuSu(SPh)u] in 75 mL of MeCN also at “5°C. Prolonged reaction (5-7 days) at room tempera- ture resulted in the appearance of microcrystals; cooling to -20°C produced more crystals. The crystals were col- lected while cold and afforded the product in 60% yield (recrystallization as above). Further treatment of the mother liquor through solvent reduction at 50°C increased yields to 90%. 3 ° EQM3EF92M086(S:p-TOI)21 This compound was prepared by substituting (EtuN)2— [FeuSu(S-p7Tol)u] for the tetramer in Method B above and obtained as purple-black microcrystals (mp 189-190°C dec.). Anal. Calcd for 038H7uFe2MoN3SB: C, ““.01; H, 7.19; N, “.05. Found: C, ““.01; H, 7.09; N, “.23. u - flufl3L§l§2K§5__( SPh) 21 This compound was prepared by substituting (EtuN)2WSu for (EtuN)2MoSu in Methods A and B above. Reaction time for both methods was greater than those observed for the Mo analogues. For Method A reaction time was about three months, and reaction time for Method B was about 2-3 weeks. Product was recrystallized once from MeCN and obtained as red-black microcrystals (mp 181-182°C dec). Anal. Calcd for C36H Fe N 38w: C, 39.u2; H, 6.u3; N, 3.83. Found: 70 2 3 C. 39.85; H. 6.57; N, “.33. 5- LEEufll3LE§2E§5(S‘p‘T01)21 This compound was prepared by substituting (EtuN)2WSu for (EtuN)2MoSu in Method B above and obtained as red-black microcrystals in “0% yield following recrystallization from MeCN (mp 178-180°C dec). Anal. Calcd for C38H7uFe2- N388W; C, “0.57; H, 6.63; Fe, 9.93; N, 3.73; S, 22.80; W, 16.3“. Found: C, “0.85; H, 6.71; Fe, 9.32; N, 3.79; 8, 22.82; W, 15.92. D. (EtuN),!(Fe2Mg2§_lO_)_ To a solution of 3.88 g (8.0 mmol) of (EtuN)2MoSu in 250 mL of MeCN was added a solution of 2.2 g (2.0 mmol) of (EtuN)2[FeuSu(SPh)u] in 50 mL of MeCN. Prolonged re- action time (5-7 days) at room temperature resulted in the appearance of microcrystals; cooling to -20°C overnight produced more crystals. The crystals were collected; and spectroscopic Characterization indicated that they consisted of (EtuN)3[Fe2MoS6(SPh)2]. The mother liquor was reduced in volume until microcrystals formed, slowly cooled to -20°C, and stored at -20°C for “8 hours. The collection of crystals afforded a mixture of (Et“N)“' (FeZMOZSlO) and (EtuN)3tFe2MoS6(SPh)2]. Subsequent treat- ment (solvent reduction, cooling, filtration) of the mother liquor yielded more of the crystalline mixture. The desired product was isolated in pure form following three recrystal- lizations from a minimum of MeCN at 35°C and cooling to —20°C. The product was obtained as blue-black microcrystals in low (E 15%) yield (mp > 250°C dec). Anal. Calcd for CB2H80Fe2MO2N“SlO: C, 33.56; H, 7.0“; Fe, 9.75; Mo, 16.76; N, “.89; S, 28.00. Found: C, 33.80; H, 7.06; Fe, 9.88; Mo, 15.““; N, “.97; S, 27.67. 10 E. Interconversion Reactions 1. (EtuletFeMosnglzl + (EtuN)2[FeMoSQ(SPh)2l Treatment of a solution of 6.6 mg (11 umol) of (EtuN)2- [FeMoSuCl2] in 6.75 mL of acetonitrile with 3.3 uL (2“ umol) of triethylamine and 2.“ uL (2“ umol) of thiophenol resulted in an immediate color change from orange-brown to red-orange. The optical spectrum of the solution showed that [FeMoSu(SPh)2]2- was formed quantitatively. 2. .ggtuNl2EFeMoSu(SPh)2] + (Ethfligiflsfleéufilgl Treatment of a solution of 25 mg (0.33 umol) of (EtuN)2[FeMoSu(SPh)2] in 25 mL of acetonitrile with 9.0 DL (0.73 umol) of benzoyl chloride resulted in an immediate color change from red-orange to orange-brown. The optical spectrum of the solution was consistent with quantitative formation of [FeMoSu012J2-. F. Reactions of (Etth2EFeMoS“012] with Thiols 1. EtSH, t-BuSH, or BzSH and Et3N A stock solution of 0.190 g (0.31 mmol) of (EtuN)2- [FeMoSuClz] in 20 mL of MeCN was prepared and checked for purity with an optical spectrum. To a 10 mL vial stoppered with a serum cap was added 7.5 mL of MeCN, 22 DL (0.23 mmol) of Et N, 0.23 mmol of thiol [either 17 DL EtSH, 3 ll 26 DL t—BuSH, 27 DL BzSH, or 2“ uL PhSH (as a control)] and 2.5 mL (3.87 x 10-5 mol) of the stock solution. Each mixture was stirred 5-10 minutes; a color change was noted in each case. Optical spectra of each solution displayed a shift of absorbance features to lower energy (compared to features of starting material). These shifts are indica- tive of ligand substitution in the interconversion reac- tions mentioned above. 2. BzSH and Et3N, Preparative Scale A solution containing 0.330 mL (3.55 mmol) of Et3N, 0.“00 mL (3.“1 mmol) of BzSH, and 20 mL of MeCN was prepared. Addition of the above solution to 0.973 g (1.59 mmol) of (EtuN)2[FeMoSuC12] dissolved in 100 mL of MeCN resulted in a color change from brown to red-brown. Stirring the reaction mixture 2 hours, introduction of a 3-fold volume excess of THF, and overnight cooling to 0°C afforded a precipitate. The solid was collected, dried in vacuo, and recrystallized once from “5°C MeCN. Following over- night storage at -20°C, microcrystals were collected and redissolved in MeCN; their optical spectrum was that of the 583, 510, and “09 nm absorption featurescfi‘the [Fe- (MoSu)2]3- ionll. 12 3. Attempted Preparation of [FeMoSu(SZ-Q-xylyl)]2- To a stirred solution of 1.89 g (3.09 mmol) of (EtuN)2- [FeMoSuCl2] in 100 mL of MeCN was added a solution of 0.5“ g (3.17 mmol) of gexylyl-d,d'-dithiol in 25 mL of MeCN. Addition of 0.70 mL (6.8 mmol) of Et3N in 25 mL of MeCN resulted in a change in solution coloration from brown to red-brown. Overnight stirring yielded a copious pre- cipitate and a clear supernatant liquid. Upon filtration and dissolution in fresh MeCN an optical spectrum of the precipitate displayed only a continually rising absorbance spectrum with decreasing wavelength. “. Disodium g—Xylyl-d,d'-dithiolate To a slurry of O.““ g (2.06 mmol) of the dithiol salt in 10 mL of MeCN, 1.26 g (2.06 mmol) of (EtuN)é[FeMosuc12] dissolved in 60 mL of MeCN was added and vigorously stirred. The color of the solution changed quickly from brown to red-brown. Filtration removed sodium chloride, which was washed once with MeCN. The mother liquor was stored overnight at -20°C; subsequent inspection revealed a non-crystalline precipitate. After filtration, the mother liquor was reduced ~50 mL and 130 mL of THF intro- duced. This mixture was stored at -20°C overnight followed by collection of the precipitated solid. One recrystalliza- tion from a minimum of MeCN and cooling to -20°C afforded l3 microcrystals with an optical spectrum identical to that of the [Fe(MoSu)2]3- ionll. G. Preparation of Sulfides 1. Crown Complex of Na2§ A solution of 0.210 g (0.870 mmol) of NaZS-9H20 in 10 mL of MeOH was added to a solution of 0.“62 g (1.75 mmol) of 18-crown-6 in 10 mL of MeOH. The resulting clear solution was evaporated to dryness overnight leaving a fine white powder. 2. (EtuN HS Dissolution of 20 g (83.3 mmol) of Na28-9H20 in 350 mL of EtOH was followed by overnight stirring. ~The solu- tion was then filtered, and evaporated to dryness. The white powder that remained was redissolved in a minimum of EtOH ( 200 mL), and to this solution was added a solu- tion of 27.6 g (166.5 mmol) of EtuN01 in 125 mL of EtOH. The precipitate that formed (NaCl) was separated by filtra- tion, the solvent removed, and the resulting white powder dried in vacuo overnight. The powder was dissolved in a mini- mum of MeCN and the remaining NaCl removed via filtration. The MeCN was evaporated to dryness. Dissolution of this white solid in a minimum of 50°C MeCN and cooling to -20°C afforded large water-white crystals.. Concentration of the 1“ mother liquor and cooling to -20°C yielded additional crystalline solid. Recrystallization of the combined materials from a minimum of warm (50°C) MeCN and subse- quent cooling to ~20°C afforded water-white crystals in low (~20%) yield (mp > 250°C dec). Anal. Calcd for 08H21NS: C, 58.83; H, 12.96; N, 8.58; S, 19.63. Found: C, 58.“7; H, 12.93; N, 8.70; S, 19.02. To distinguish (EtuN)HS from (EtuN)282 (Calcd for C16HuON282: c, 59.20; H, 12.u2; N, 8.63; S, 19.75.), an iodimetric titration was performed. j Solutions were prepared and standardized according to ‘ Skoog and West27. A starch indicator was not employed; NaHS and Na28°9H20 were controls. Dissolution was ef- fected in either EtOH or MeOH to avoid loss of sulfide as H28, and performed anaerobically to limit formation of sulfite. The solutions of the "unknown" and the controls exhibited a yellow color (confirmed as 83-) immediately upon introduction of the standardized iodine solution to the sulfide solution. The hydrosulfide titrations ("unknown" and NaHS) had a distinct color change, at one- half the equivalence point, from the previously mentioned yellow solution to a clear solution. Addition of the io- dine titrant continued until a pale yellow color (con- firmed as I5) persisted for at least 30 3. These results established the "unknown" as best formulated as (EtuN)HS. 2- _ Confirmation of S2 and 13 was established by comparison of samples from the titration runs with optical spectra of standard solutions of the two ions28. 15 H. Reactions of (EtMN12EFeMoSu912J with Sulfides 1. Crown Complex of Na2§ To a solution of 0.532 g (0.870 mmol) of (EtuN)2- [FeMoSuC12] in 50 mL of MeCN was added a slurry of 0.528 g (0.870 mmol) of the sulfide-crown complex in 30 mL of MeCN. An optical spectrum indicated a distinct red shift of ab- sorbance features to lower energy. Storage at -20°C over— night was followed by an optical spectrum that displayed absorbance features corresponding to the [Fe2MOZSloju— anion. 2. _(__E_t_LIN)HS A solution of 0.73 g (“.“7 mmol) of (EtuN)HS in 100 mL of MeCN was slowly added to‘a solution of 1.55 g (2.5 mmol) of (EtuN)2[FeMoSuClz] in 100 mL of MeCN. A change in color from brown to red-brown occurred immediately; an optical spectrum of the reaction mixture indicated formation of [Fe(MoSu)2]3-. A 50 mL aliquot of the reaction mixture was removed and to it was added 0.10 g (0.60 mmol) of (EtuN)HS. An optical spectrum indicated formation of [Fe2M02310]u-. After 72 hours the 50 mL aliquot was fil- tered and the mother liquor reduced in solvent volume (to approximately 30 mL). This was followed by storage at —20°C for 5 months, at which time a microcrystalline solid was ap- parent. Collection of the crystals, drying lg vacuo, and 16 subsequent dissolution in MeCN yielded an optical spectrum whose absorbance features were indicative of [FeZMOZSlOJH- in good purity. 3. (EthN)HS and Proton Sponge Three stock solutions were prepared: 1) (EtuN)2- [FeMoSuC12], 0.37 s (0.60 mmol) in 20 mL of MeCN; 2) (EtuN)HS, 0.20 g (1.2 mmol) in 10 mL of MeCN; 3) proton sponge, 0.13 g (0.60 mmol) in 10 mL of MeCN. Samples each reagent [1.0 mL (0.03 mmol) of FeMoSu01§-, 0.25 mL (0.03 mmol) of (EtuN)HS, 0.50 mL (0.03 mmol) of proton sponge, 0.25 mL of MeCN] were mixed in a 5.0 mL gas tight syringe ac— cording to the following ratios: [FeMoSuC12]2-, (EtuN)HS, proton sponge, MeCN; 1,0,0,“; 1,1,0,3; 1,1,1,1; 1,“,“,0. The order of introduction for each experiment (sulfide, proton sponge, FeMoSuC1§-) was followed by two optical spectra at approximately 5-10 minutes and 20-25 minutes. Experiment “ was also checked with an optical spectrum after 18 hours. Experiments 2, 3, and “ had absorbance features that indicated formation of [Fe(MoSu)2]3-. I. Reactions of (Q)2[FeMoS“(SAr)2] with Sulfides l. NaHS To a stirred slurry of 11 mg (0.196 mmol) of NaHS in 5 mL of MeCN was added a solution of 100 mg (0.132 mmol) 17 of (EtuN)2[FeMoSu(SPh)2] in 10 mL of MeCN. After stirring for 20 minutes the solution darkened considerably. Optical spectra of the reaction indicated complete formation of [Fe(MoSu)2]3- after 2“ hours. 2. _(_E_t_uN)HS A solution of 0.065 g (0.398 mmol) of (EtuN)HS in 2.5 mL of MeCN was dripped into a solution of 0.20 g (0.158 mmol) of (PhuAs)2[FeMoSu(SPh)2] in 20 mL of MeCN. Optical spectra of the reaction mixture were obtained at 20 minutes, “0 minutes, 2“ hours, and “8 hours. Absorbance features indicative of [Fe(MoSu)2]3- were present in all spectra. 3. (EthN)HS and Proton Sponge Three stock solutions were prepared: 1) (PhuAs)2— [FeMoSu(S-peTol)2], 0.78 g (0.60 mmol) in 20 mL of MeCN; 2) (EtuN)HS, 0.37 g (1.2 mmol) in 10 mL of MeCN; 3) proton sponge, 0.13 g (0.60 mmol) in 10 mL of MeCN. Samples of each reagent [1.0 mL (0.03 mmol) of FeMoSu(S-pgTol)§-, 0.25 mL (0.03 mmol) of (EtuN)HS, 0.50 mL (0.03 mmol) of proton sponge, 0.25 mL of MeCN] were mixed in a 5.0 mL gas tight syringe according to the following ratios: [FeMoSu- (SepeTol)2]2, (EtuN)HS, proton sponge, MeCN; l,0,0,“; 1,1,0,3; 1,1,1,1; l,2,2,0; 1,“,“,0. Each experiment (order of mixing the solutions was sulfide, proton sponge, 18 [FeMoSu(S-peTol)2]2_) was monitored by two optical spectra at approximately 5-10 minutes and 20—25 minutes. Spectra of the last four experiments had absorbance features that indicated formation of [Fe(MoSu)2]3-. J. Reactions of (EtMN)3[Fe2MoS((SAr)2l 1. Benzpyl Chloride Dropwise introduction of a solution of 0.“82 mL (“.15 mmol) of benzoyl chloride in 50 mL MeCN to a stirred slurry of 1.68 g (1.66 mmol) of (EtuN)3[Fe2MoS6(SPh)2] in 50 mL MeCN resulted in the dissolution of solid and a change in the solution color from purple to brown. Following over- night storage at -20°C, “50 mL of anhydrous Et 0 was added 2 to the reaction mixture. After cooling to -20°C, the precipitated material was collected, dried in vacuo, re- crystallized from a minimum of MeCN and cooled to -20°C. Subsequent attempts at crystallization through reduction of the solvent volume failed because the solution de- posited films and slowly decomposed on standing. 2. Nos2 Dissolution of 0.11 g (0.11 mmol) of (EtuN)3[Fe2MoS6- (S-prol)2] and 0.32 g (0.66 mmol) of (EtuN)2MoSu in 50 mL of MeCN followed by maintenance at 55°C for approximately 2“ hours resulted in the appearance of optical features l9 attributable to [Fe2M02810]u'. Continued heating (“ days) and monitoring of the reaction mixture revealed decomposi- tion of all species except MoSE’. 3. Sulfide and MOSE: Three stock solutions were prepared: 1) (EtuN)3- [Fe2M086(SPh)2], 0.“69 g (0.“65 mmol) in 50 mL of MeCN; 2) (EtuN)2MoSu, 0.725 g (1.50 mmol) in 50 mL of MeCN; 3) (EtuN)HS, 0.60 g (3.67 mmol) in 20 mL of MeCN. Into a 13 x 100 mm test tube equipped with a serum cap were introduced via syringe samples (0.50 mL M033”, 1.60 mL Fe2MoS6(SPh)3-, 0.150 mL sulfide) of the above stock solu- tions according to the following ratios: [Fe2MoS6(SPh)2]3- Mosfi', (EtuN)HS; 1,0,0; 0,1,0; 2,2,1; 1,1,1; 1,2,1; 1,1,2; 1,0,1; 1,0,2. Each experiment (order of mixing was trimer, thiomolybdate, sulfide) was followed by 10 minutes of stir- ring and an optical spectrum was obtained. The mixture was then heated for 20 minutes at 60°C and another optical spectrum was obtained. Finally, the reaction mixtures were stored at room temperature for 7 hours culminating in an- other optical spectrum. All solutions containing sulfide were colored green and yielded optical spectra that had ab- sorption features similar to the [Fe2M02SlOJH- ion (which is normally blue in solution). Generation of the green solutions is independent of the presence of MoSz-. 20 K. Reactions of (EtuN)“[F32M92§1ol 1. PhSH Treatment of a stirred solution of 5.1 mg (“.“5 umol) of (EtuN)u[Fe2M02810] in 1.0 mL MeCN with “.5 EL (““.5 umol) of PhSH in 0.5 mL MeCN afforded an instantaneous color change from blue to red-orange. An optical spectrum indicated 90% conversion to (EtuN)2[FeMoSu(SPh)2]. 2. Dilute Acid A 5 to 10 mg (“.5 to 9.0 mmol) sample of (EtAN)“' [Fe2M02810] was dissolved in 2.0 mL MeCN. To this sample 0.5 mL of 2N HCl was added. Evolution of a gas resulted, and qualitative testing indicated the absence of odors characteristic of PhSH and p—TolSH. A strong odor of H28 was noted. L. Reactions of Fen§u Tetramers with (EtthzMoSM 1. gunmen“ (sewn A solution of 1.13 g (2.33 mmol) of (EtuN)2MoSu in 60 mL of MeCN was added to a stirred solution of 0.50 g (0.58“ mmol) of (EtuN)2[FeuSu(SEt)u] in 30 mL of MeCN. After 21 days storage at room temperature, reduction of the solvent volume (50%) at “5°C followed by storage at —15°C for 7 days yielded a microcrystalline precipitate. 21 The crystals were collected and dried in vacuo; dissolu- tion in MeCN gave a green solution whose optical spectrum indicated the presence of (EtuN)2MoSu and (EtuN)u[Fe2Mo S 2 10]' 2. (EtuN)2[FeuSu(S-37Bu)u] A solution of 1.33 g (1.37 mmol) of (EtuN)2[FeuSu- (S—pru)u] in 125 mL of MeCN, and a solution of 2.68 g (5.“9 mmol) of (EtuN)2MoSu in 200 mL of MeCN were warmed to 50°C and combined. The resulting solution was monitored via optical spectra for 21 days. There were no appre- ciable changes in optical features during this time period that indicated a reaction had occurred, however, reaction for another 21 days resulted in minute changes in the op- tical spectrum that indicated formation of [Fe2M02 10]“- in very low yield. 3- EEAHLzLEEAEMS-E-Tmul Introduction of a solution of 0.882 g (1.81 mmol) of (EtuN)2MoSu in “0 mL of MeCN to a solution of 0.250 g (0.226 mmol) of (EtuN)2[FeuSu(S-p§Tol)u] in 25 mL of MeCN was followed by maintenance of the final solution tempera- ture at “0°C for 30 hours. An Optical spectrum of the cooled solution indicated that a reaction had occurred, whereupon the solvent was evaporated to dryness. Introduction of a minimum of MeCN, cooling to -20°C, and collection of the 22 resulting crystalline material afforded a solid whose optical spectrum had features representative of both (EtuN)3[Fe2MoS6(S-pgTol)2] and (EtuN)u[Fe2M The 02810]. crystals were redissolved in the mother liquor with the aid of additional MeCN and the solution was maintained at 50°C for 1 week. Reduction of the solvent volume at 50°C until crystals formed, cooling to -20°C overnight, and subsequent collection of the crystals revealed M082— as the major product. Further workup of the mother liquor yielded crystalline material whose optical spectrum was dominated by the thiomolybdate anion. M. Growth of (EtuN)3[Fe9MoSK(S-p-Tol)9] Crystals This experiment was performed according to (EtuN)3- [FezMoS6(SAr)2] preparative method B above. Toga solution of 0.“91 g (0.“““ mmol) of (EtuN)2[FeuSu(S-p-Tol)ul in 32.5 mL of MeCN was added 0.“30 g (0.887 mmol) of (EtuN)2- Mosh in 3“.5 mL of MeCN. Both solutions were at room tem- perature. The flask containing the final reaction mix- ture was free of internal imperfections (scratches). The reaction mixture was submerged in a dewar containing room temperature H20 and stored in a closet free of light and drafts to insure thermal isolation. After 3 weeks reaction time inspection revealed many large crystals. The crystals were collected at room temperature, dried 12.22222: and not recrystallized. Optical spectra indicated that both the mother liquor and the crystals were of good purity. III. RESULTS AND DISCUSSIONS A. LEEAMQEFGMSHQIJ (M = Mo,w) 1. Synthesis and Reactivity The reaction of equimolar amounts of either tetra- thiomolybdate (VI) or tetrathiotungstate(VI) with anhydrous ferrous chloride in MeCN results in formation of the [S2MS2Fe012JZ- ions (M = Mo,W), isolated in high yield as tetraethylammonium salts. Reaction of the corresponding aryl thiolate derivatives, [SZMSZFe(SAr)2]2- (M No,w), with 2 equivalents of benzoyl chloride in MeCN also affords the [FeMSu01zl2- ions: MeCN [FeMSu(SAr)2]2- + 2PhCOC1 . * [FeMSuCLQ2- + 2PhCOSAr (where formation of the thioester is presumed by analogy to Holm's results2l). The reverse reactions (chloro derivative+ aryl thiolate derivative) can be accomplished by addition of either 2-3 equivalents of Et3N and ArSH or 2 equivalents of NaSAr (Ar = Ph, pgTol) in MeCN. The shift of the equilibrium to the desired product is aided in the chloro + thiolate conversions by excess Et3N/ArSH and by the formation of NaCl, respectively. Excess benzoyl chloride 23 2“ in the thiolate + chloro conversion causes decomposition and formation of an insoluble black solid (presumably through acylation of terminal sulfide ligands). A11 conversions occur rapidly and essentially quantitatively and can be followed spectrophotometrically by appearance and disap- pearance of key absorption features (see Figures 3 and “). While analogous behavior is observed for Fe2S2 and Pens“ clusterszl’zg, the FeMoSu cluster does not form the stable alkyl- and benzylthiolate compounds common to the dimeric and tetrameric iron salts. Early indications from spec— troscopic scale experiments led to large preparative scale reactions, but subsequent workup of these reaction mix- tures yielded insoluble solids or the [Fe(MoSu)2]3- ion as products. The Fe282 cluster was originally isolatedlu as [Fe2S2(82-pexyly1)2]2-; attempts to utilize the chelate effect and isolate the FeMoSu analogue starting from [FeMoSuC1212- were also unsuccessful. Two methods, iden- tical to these employed for aryl derivatives (grxylyl dithiol plus 2 Et N or the sodium dithiolate) resulted in 3 decomposition and the formation of [Fe(MoSu)2]3-, respec- tively. Garner has reported30 the structural characteriza- tion of the [FeMoSu(Sz-p-xy1yl)]2' ion, which was syn- thesized from MoSfi- and [Fe(S2-ggxylyl)2]2-. Attempts employing the dithiolate and [FeMoSu01ZJ2- presumably re- sult in reduction of the S2MoS2Fe core and subsequent de- composition in a reaction that is faster than simple dis- placement of halide by the dithiolate. Garner's method 25 2- 2- EeCl2 + MS” [FeMSu(SAr)2l PhCOCl 1. Et3N/ArSH 2. NaSR 2.. [FeMSuCl2l 1. Et3N/RSH, R = alkyl. 2. Na2(82)R', R' = ggxylyl. i [Fe(Msu)2]3' Figure 1. Synthesis and Reactivity of [FeMSuC12J2- (M = Mo,W) in MeCN. 26 effectively avoids this problem by initial coordination of the dithiolate to the iron. The reactions discussed are summarized in Figure l. 2. Structure The structure of the [FeMoSu01232' ion is depicted in Figure 2, and selected interatomic distances and bond angles are listed in Tables I and II. The crystal structure con- sists of discrete cations and anions in a “:2 ratio per unit cell with but one independent cation and one—half of the dianion per symmetric unit. Due to the location of the dianion at the crystallographic inversion center (I), there is a twofold disorder of the FeCl2 and MoS2 portions of [C12FeS MoS2J2—. This structure was successfully refined 2 as equivalent mixtures of Fe and Mo and Cl and S, yielding a crystallographically independent metal atom, M (M = (Fe + Mo)/2), and two independent terminal atoms, X2 and X3 (X = (S + Cl)/2). The structure is best described as two tetrahedral units Joined at a common edge. The six bond angles around the metal atom M range from 10“.00° to 111.69°; the average (109.“7°) is close to the ideal tetrahedral value of 109.“9°. The Mo-Fe distance is 2.786(1) A and, although the metal-metal distance may imply existence of a direct metal-metal interaction, detailed discussion of the electronic structure of the binuclear unit will be de- ferred to the Mdssbauer discussion. The bridging metal-sulfur Figure 2. 27 Stereochemistry of the [SZMOSZFeC1232- di- anion. The crystallographic Ci-I site symmetry causes a two—fold disorder of the metal atoms (M (x (Mo + Fe)/2) and the terminal ligands (S + C1)/2). 28 Table I. Interatomic Distances (A) with Esd's for (EtuN)2- [FeMoSuC12]. Bonded Distances Nonbdnded Distances M-M' 2.786(1) Sl...Sl' 3.566(3) M-Sl 2.268(2) Sl...X2 3.696(3) M-Sl' 2.258(2) Sl...x3 3.665(2) M-x2 2.199(2) Sl'...X2 3.67“(2) M-X3 2.200(2) Sl'...X3 3.669(2) X2...X3 3.583(3) Table II. Bond Angles (deg) with Esd's for (EtuN)2 [FeMoSuC12]. The Dianion M-Sl-M' 76.00(5) Sl'-M-X2 111.0u(7) Sl-M-Sl' 10“.00(5) Sl'-M-X3 110.77(7) Sl-M-X2 111.69(7) X2-M-X3 109.06(8) Sl-M-X3 llO.2“(7) 29 distances are longer (0.06 A) than the terminal metal ligand distances. This trend is consistent with data from the [(PhS)2FeS§M082]2- ion8 (the asterisk denotes bridge- ing sulfurs) where bridging bonds (Mo-S*-av 2.255(2) A, Fe-S* av 2.269(2) A) are substantially longer than the terminal bonds (Mo-S av 2.153(2) A). Averaging both sets of data from the thiolate analogue (M-S* av 2.26 A and M-X av 2.33 A (Fe-S(Ph) av 2.307(2) 3)) yields values that are close to the values crystallographically averaged for the [FeMoSuCl2l2' ion. Comparison of [FeMoSuCl2J2' (I) with other metal sul- 31,12 fide-chloride systems , [C12FeS FeC12J2- (II) and 2 [CleeSZMoS2FeCl2]2_ (III), reveals interesting stereo- chemical similarities and differences (pf. Table III). First, the Mo-Fe distances in I_and III are similar (2.786 (1) and 2.775(6) A, respectively),while the FeéFe distance in II is significantly shorter (2.716(1) A). The trend may be correlated to the larger covalent radius of molyb- denum (1.30 A for M0 vs 1.17 A for Fe)8. Second, the average metal bridging sulfur (M-S*) distances vary only slightly for the Mo-containing species (M-S*: I, 2.263(2) A) compared to the much smaller value for II, 2.201(1) A. Again this trend may be attributed to the larger covalent radius of molybdenum. Third, the metal-terminal ligand bonds (M-X: I, X = S, Cl; II and III, X = C1) exhibit wide variations. The crystallographically averaged value 30 Table III. Selected Structural Parameters in the 2- a. 2- if [SzMoSZFe012] Q),[C12FeSEFeC12] CIA and [C12FeS* FeCl2J2- CII)Complexes. N 2MoS 2 Parametera ;_ £231 ;;_}2 M-Feb 2.786(1) 2.716(1) 2.775(6) Mo-S* --- 2.204(5) 2.263(2) Fe-S* 2.201(1) 2.295(5) Mo-S --- _-_ 22.200(2) Fe-Cl 2.252(1) 2.225(10) s*...s* 3.566(3) 3.u63(1) c M-S*—Feb 76.00(5) 76.21(3) 76.05(l) S*-Mo-S* i ——— 109.5(1,9) 10u.00(5) S*-Fe-S* l 103.79(3) 100.9(2) S-Mo-S " -__ -__ 109.06(8) - Cl-Fe-Cl 105.37(“) 110.2(9) aDistances in angstroms (A) and angles in degrees (°). bM = M0 for I_and III, Fe for 1;, cNot reported. u Bridging sulfur atom. 31 for M-X of 2.200(2) A in l is smallest, followed by an average value of 2.225(10) A in III) and the largest aver— age value of 2.252(1) A determined for II, Since the thiolate analogue of I, [(PhS)2FeS§M082]2-, has an Mo-S bond average of 2.15 A and II_has an average of 2.25 A for the Fe-Cl bonds, one would predict an average M-X bond of 2.20 A for 3, based on the twofold disorder of the dianion. This is in accord with the observed value. Last, the average bridging metal-sulfur-metal angle in all four complexes (including the thiolate analogue of I) is acute and constant at approximately 76°. The tetraethylammonium cations of I_are crystallo- graphically ordered with normal bond lengths and angles, and will not be discussed further. Subsequent to the work on (EtuN)2[FeMoSuClz]8, Mfiller reported32’33 the structure of [PhuP][BzMe3N][FeMoSu012]. Detailing only the space group (P1), the cell parameters, and a few bond distances (Mo-Fe, 2.775 A; Mo-S*, 2.27 A; Mo-S, 2.18 A),his work, in essence, agrees with the data reported above. A single crystal x-ray structure of (EtuN)2[FeWSuCl2] 3“ 35 was not obtained. However, both Mfiller and Coucouvanis report work on this dianion, and indicate that the W- analogue is isostructural with the Mo-analogue. Unfor- tunately, Mfiller reports only the space group and cell parameters for [PhuPJEBzMe3N][FeWSuCl2]. Coucouvanis35 32 reports slightly more information: Fe-w, 2.821(2) A; M-S*, 2.203(8) A; M-S, 2.276(8) A; and M-S*-M, 76.“°(2). These data are not entirely consistent with the same param- eters obtained for [FeMoSu012l2'. While the M-M bond and the M-S*-M angle are not significantly larger than ex- pected, the values of the metal ligand bonds (terminal and bridging) seem to be reversed. 3. Electronic Spectra The electronic absorption spectra of the [FeMSu01212- ions (M = Mo,W) are shown in Figure 3; peak positions and molar absorptivities are presented in Table IV. The spectra of the chloro complexes are dominated by a pair of intense absorptions between 350 and “75 nm. They are thus similar to the spectra of the bis(tetrathiometallate) complexes of the first row transition metals36’37. The latter display an essentially symmetrical splitting of the lowest energy S + Mo and S + W charge transfer transitions of the original tetrathiometallates (MoSi‘, WSi’). This splitting is also observed in the [FeMSuC12J2- (M = Mo,W) 3“ 35 electronic spectra, and Mfiller and Coucouvanis agree the effect is due to the lowering of the symmetry of the tetra- thiometallate upon coordination of the ferrous chloride. Many other absorbance features are present in addition to those attributable to the tetrathiometallate transitions. Since low energy sulfide or chloride to iron charge transfer 33 l | . l [cz,.lr-'es,ws.,]2 (—) «15,000 I | [CleeSZMosz 12' (---) 70,000 | u‘ +2900 l - . l GOQigp') caftflfi) "QKKXD t-d 5,000 6.000 3.000 700 800 900 Figure 3. Electronic spectra of (EtuN)2[012FeSZM082] and (EtuN)2[012Fe82W82] in acetonitrile solu- tion at 23°. 3“ .mCOHpSDprcoo oapmcwmemfio pom popomnpoo .00mm on woman ofiaom 03p CH .AN-OH a H- n xm EoHIzV Ed cfifiov Ex .Oomm macapsaom omzo copmppcoocoo ca Unnammoe who: scan: .8: com A 4 pm mDLSpmom mpfimcopcfi 30H pom pdooxo .COHpSHow 20oz :Hm 2m H.m Em H.m nanoEoE ofipocwmz .Anhvsmm.AH.=vasm .Aoaavomm .Am.mflvzmm.flm.ovzsm .Aemavsfim.ngvmms .AH.ovmae.Annvoee .Aseoaos.ieavwmm also xaea .mossonoo .Aam.ovmmm.flo.flvomoa .Aonvzmm.Am.Hvomoa Hosoooam oesosoooam -mflmflo:mzosL -mmmaosmozomu agnoz ..l.l z .moondEoo flNHozmzmmH—NAZJumV .HO mpflmfioz OHpmflwmz GEM wmfifipmmm HGLPOTQW OHCOQDOmHm .>H mHDMB 35 transitions will also appear in this general region, specific assignments would be speculative. A discussion of general trends is, however, appropriate. Comparison of the optical spectrum of the [FeMoSuCl2l2' ion to that of the [FeWSuCl2J2- ion reveals a blue shift of the w + 3 electronic features relative to the Mo + S electronic features. This observation is consistent because the dif- ference in orbital energies will be greater for w + S transitions than for Mo + S transitions. These expecta- tions also hold for the WSfi' and MoSfi' optical spectra (Figure “). Each dinuclear complex also displays a low intensity band in the near—IR (NIR) region. Although features in this general region are usually ascribed to 38 5E + 5T2 transitions (see Holm and Coucouvanis39), the energies of the two bands are not sufficiently low to warrant such an assignment in this case. Coucouvanis35 and Silvis25 have suggested that the two bands are due to Fe + Mo(W) transitions. More specifically, Coucouvanis proposes a schematic energy level diagram where the Fe d orbitals are located between predominantly sulfur molecular orbitals and the empty Mo(VI) and W(VI) d orbitals, enabling facile low energy electronic transitions from filled pre- dominantly iron d orbitals to empty predominantly Mo, w d orbitals. Although identical, Silvis'assignment is based on the large half-widths and small extinction co- efficients of the absorptions. Regardless of present o.s-'I Gib- <14 02! .__._.ET:;._._.J.__ Figure.“. (Er, N)2MoS4 (—) (El4 N)2 WS4 ("') 500 600 700 Electronic spectra of (EtuN)2MoSu and (EtuN)2- WSu in acetonitrile solution at 23°C. 37 proposals, more detailed assignments must await more care- ful study of a significantly larger sample of bimetallic sulfur-containing systems. “. Magpetic susceptibility The Faraday method was used to obtain room tempera- ture magnetic susceptibilities of solid samples of (EtuN)2- [FeMoSuCl2], M = Mo,W. Both complexes yielded a value of 5.1 BM per formula unit, which is corrected for the di- amagnetic contributions of the ligands and cations through use of Pascal's constants. Each case corresponds to a system containing four unpaired electrons (S = 2). While the simplest explanation would be a monomeric high—spin Fe (II) complex“O (range “.9-5.5 BM), the data might accom- modate a strong intermolecular antiferromagnetic inter- 1 Mo(V). The action between high-spin d5 Fe(III) and d temperature dependence of the magnetic susceptibility was measured for the thiolate derivative8, (EtuN)2[FeMoSu- (SPh)u], to resolve this question, and to ascertain the possibility of intermolecular interactions. The results of the low temperature measurements on the solid and a magnetically dilute frozen DMF solution suggested that intermolecular interactions exist in the crystal due to long-range magnetic ordering. However, if an antiferro- magnetically coupled Fe(III)-Mo(V) formulation is assumed, the energy necessary to populate spin states greater than 38 S = 2 would be much greater than the thermal energy at 370 K (I2J|>>kT). The latter consideration is not con- sistent with the Mossbauer data. 5. M6ssbauer_§pectroscppy Application of Mossbauer spectroscopy to a chemical system that contains iron affords data that aid in under- standing the immediate environment of the iron nucleus. Massbauer spectroscopy was employed to assess the formal oxidation states of the metal centers in the [FeMoSuC1232- ions, M = Mo,W. Two parameters, the isomer shift (IS) and the quadrupole splitting (QS), have been used to establish a data base that allows comparisons between well characterized iron systems and "new" iron systems. A detailed theoretical discussion will not be presented here, because, the Massbauer effect is well known and sources that provide insight into its applications are numerous.“2 The main focus in this and subsequent dis- cussions will be the isomer shift. Correct interpretation of the [FeMoSuClZJZ- Mdssbauer results depends upon direct comparisons to established Fe-S systems, since Mo-Fe-S systems lack the abundance of physical data necessary to allow internal comparisons. In Figure 5, monomeric, dimeric, and tetrameric Fe—S systems are represented in a graph of IS vs formal oxidation state for iron in tetra- hedral sulfur sites.6a A distinct trend is evident and ISOMER SHIFT (mm/sec) vs. Fe METAL 39 0.7 '- O.6 - 0.5 - é - 0.4 - - 0.3 - - 0.2 ' " OI - - J. 1 I Fes’ Fez-5+ Fe2+ FORMAL OXIDATION STATE Figure 5. Correlation diagram relating isomer shift to formal oxidation state for various iron-sulfur clusters" “0 proves useful in determining the formal oxidation states of "new" iron environments. The [FeMoSuC12J2- ion exhibits IS (QS) values of 0.60 mm/s (2.12 mm/s) in a zero applied field (see Figure 6 and Table V). The values correspond well with analogous data for monomeric Fe(II) in a tetrahedral sulfur environment (IS (Q8), 0.6 mm/s (3.3 mm/s)). As a check, the [FeMoSuC1232- data was obtained in both solid form (diluted with boron nitride) and frozen solution (DMF) in order to avoid magnetic interactions in the solid state. The differences between IS (QS) values in both solid and solution form were negligible. Mossbauer data for other dinuclear Fe-M-S systems, including work on [FeMoSuC1212- publishedBLl’35 subsequent to our studies, is also presented in Table V. Despite slight differences in the environment of each iron nucleus and the temperature of each study, it is clear that the dinuclear MoSZFe core is best characterized as an Fe(II)-Mo(VI) system that has slight delocalization of electron density from iron to molybdenum through the sulfur bridges. A similar effect is evident for the tungsten analogues, but the delocalization through the sulfur bridges seems somewhat diminished. This last ob- servation is consistent with the electrochemical results of the [FeWSuCl2J2- ion. “1 -3 -2 J o 2 4 Velocity (mm/s) Mdssbauer spectrum of frozen DMF solution of Figure 6. (EtuN)2[FeMoSuC12] recorded at “.2 K and with zero applied field. Vertical bar indicates 1% absorption. “2 .UocfiEhopop pozm mz ofisofifl can .DLSpwhodEop Eoop p¢o +Adandvo .x mom u e .moann +mflA2mozanAa=sdeo .x m.: u a .moadn +Azaomvo .DMSQQADQEop .mpawm .m\ee mo moan: CH wcappfiadm paedoppmso Ame .m\EE mo moans ca uMHnm meomfi .me Aam.mvwz.o Amm.HV=:.o Azm.mvmm.o A:H.mvmm.o m.o.mm.am no .macw>coonoo Aom.mvmm.o Azm.avmm.o Awm.mv:m.o Awo.mvmz.o o.o.mm.am pm .mficm>coocoo m w Asm.mvmm.o Aoa.mvmz.o o.:m.ao so .soaaez m Amh.Hvss.o m Ama.mvom.o o.m .Hn no .nnmEHoxoofiB AmGVmH AmGVmH AmGVmH mhmava -mmmAcdmvzmzomL -mflmgnamvsmozomi -mmmaosmzoau nmmmfio:mozomu .mEopmmm mlomlz can mlomloz pmoaosnfim macanm> pom mono posmnmmwz .> manna “3 6. Electrochemistry The [FeMSu01232- ions (M = Mo,W) were studied by dc polarographic methods in approx. 1 mM solutions in Gold Label MeCN with 50 mM (Et“N)ClO“ as supporting electrolyte. Electrochemical scans over the range +1.0 to —2.0 V, versus the standard calomel electrode, revealed only one electrochemically irreversible reduction for each complex. The drOpping mercury electrode (0.5 s/drOp) was selected after initial experimental attempts with cyclic voltammetric methods employing platinum (flag), mercury (hanging drop), and carbon (glassy) electrodes. Absorption of the sulfur- rich compounds on the last three electrodes enabled observa- tion of only the cathodic wave in the cyclic voltammetric scan, since the anodic wave was often missing entirely (current flow severely restricted). This electrochemical behavior seems to be common for the general family of iron-molybdenum-sulfur complexes containing coordinated MoSZ-. The reductions for the molybdenum and tungsten ana- logues occurred at -1.23 and -l.35 V, respectively. The data are consistent with that of Mfiller3u. The lepeS of the log [i/id-i] vs. potential plot were “3 mV (Mo) and 100 mV (W); both values are far from the theoretically ideal value of 59 mV for a one electron reversible reduc- tion. The low potential slope value for the molybdenum analogue suggests a chemical reaction occurring faster ““ than the rate of diffusion of the reduced species from the electrode surface. Silvis noted25 that reduction of (EtuN)2[FeMoSu(SPh)2] results in formation of [Fe(MoSu)2]3- However, the value of the potential slope for the tungsten analogue indicates that this complex accepts less than one electron as the decomposition reaction occurs. The general difference in behavior was apparent during the actual electrochemical experiment; while the molybdenum system could be run without repeatedly cleaning the platinum counter electrode, the collection of reproducible data on the tungsten system required cleaning of the same counter electrode after each scan. Two other electrochemical studies on analogous systems qualitatively support the data presented here. Holm21 found that dimeric complexes of the type [Fe282xu]2_ (X = Cl, Br, I) exhibit irrever- sible reductions from -l.0 to -0.7 V without resolvable anodic processes, while Callahan and Pilieroul cite a difference in reduction potential for the series [Ni- 2- 2- 2- _ (Msu)2] 9 [Pd(MSu)2] a and [Pt(MSu)2] (M - Mo,W), where the tungsten compounds are reduced at more negative potentials than the molybdenum compounds. 7. Summary Preparation of the [FeMSuCl2J2- ions (M = Mo,W) from anhydrous FeCl2 and M83” is a facile one step operation. The thiolate and chloro derivatives can be interconverted “5 by using simple, readily available reagents. The two complexes are essentially isostructural and are best represented as two tetrahedra Joined at an edge. The optical spectra themselves are dominated by a splitting of features present in the electronic spectra of the parent thiometallate anions. Magnetic susceptibility, electro- chemical, and Mdssbauer results are consistent with a Fe(II)-M(VI) system with slight electron delocalization from Fe + M through the bridging sulfur atoms, but no direct metal-metal interaction. There is slightly less electron delocalization in the tungsten analogue. B. E21451 [Fe M86(SAr)2l (M = Mo,W; Ar = Ph, p—Tol). 3___.__ 1. Synthesis and Reactivity The [Fe2MS6(SAr)2]3- ions are formed in high yield as tetraethylammonium salts by two distinct reaction path- ways. First, reaction of equimolar amounts of either tetrathiomolybdate(VI) or tetrathiotungstate(VI) and the [Fe282(SPh)ul2- ion in MeCN for approx. 3 weeks (Mosfi') or “ months (WSfi') yields the trianion in good purity. Second, reaction of 1 equivalent of the [FeuSu(SAr)u]2- ions (Ar = Ph, pr01) with 2 equivalents of either tetra- thiomolybdate(VI) or tetrathiotungstate(VI) in MeCN for approx. 5 days (Mosfi') or 3 weeks (WSfi’) yields the tri- anion in good purity. The second pathway is more desirable “6 for the following reasons: 1) the reaction does not re— quire spectral monitoring because completion is signified by the less soluble product crystallizing from solution, 2) the reaction time is decreased by a factor of “. The latter may be due (in part) to the first reaction pathway requiring an oxidation-reduction step. The second reac- tion pathway only requires a cleavage of the [FeuSu(SAr)u]2- core into the desired product, 2_ MeCN [FeuSu(SAr)u]2- + 2MoS 2[Fe2MoS6(SAr)2l3-. In each case formation of the tungsten analogue requires prolonged reaction time (m“ times longer), which is pre- sumed to result from differences in the N-acceptor prop- erties of the wsfi' and Mosfi‘ ions. In the first reaction pathway the better N-acceptor properties of the MOSfi- ion apparently facilitate the oxidation-reduction step through stabilization of an intermediate similar to that found during the conversion of the [Fe282]2+ core to the 1“. In the second reaction pathway the [Fe“S“]2+ core better N-acceptor properties of the MoSfi‘ ion facilitate the simpler cleavage via stabilization of an intermediate required by the migration of an ArS- ion. The mechanism of the first reaction pathway also presumes the formation of disulfide as the oxidation half reaction. 2_ 2_ MeCN 3_ _ 2MoS“ +2[Fe2S2(SAr)u] '——_* 2[Fe2MoS6(SAr)2] +ArSSAr+2ArS “7 Attempts at synthesizing the [Fe2MoS6(SR)2]3— ions (R = Et, trBu) from alkyl-substituted Fe-S tetramers proved unsuccessful. This result is probably due to the instability of the alkylthiolate cluster intermediates com— pared to the arylthiolate analogues. A stable intermediate is necessary to facilitate ligand migration during the tetramer cleavage. Since alkylthiolates are more readily oxidized than arylthiolates, the observation that alkyl substituted Fe-S tetramers (where ligand is either ethane- thiolate or Egbutylthiolate) yield small amounts of the [FeZMOZSlolu- ion and do not yield the [Fe2MoS6(SR)2]3_ ion supports this hypothesis. After direct synthesis of an alkyl substituted tri- nuclear core was not successful, several attempts were made to synthesize the chloro derivative, [Fe2MOS6C1233-. Thesynthetic potential of this particular complex is 21 2- , [F3282C1u] obvious when compared to its counterparts and [Fe“S“Cl“]2-' Two methods were employed: 1) use of 2 equivalents of benzoyl chloride with 1 equivalent of [FezMoS6(SAr)2]3' produced oils and films that decomposed on standing; and 2) combination of the [Fe“S“Cl“]2- ion with tetrathiomolybdate(VI) afforded the [Fe(MoSu)2]3- ion in high yie1d25. The thermal stability in the range “5—65°C of the [Fe2MoS6(SAr)2]3' ions was studied in MeCN; the trinuclear anion readily decomposes when maintained at temperatures “8 2- 2- 2- 2- [Fe282(SR)u] +MS [FeuSu(SR)u] +2MS 3- [Fe2MS6(SR)2l M=Mo, PhCOCl [Fe2MoS6C12]3_ [FeuSu(SR)u]2-+2MoSi- (R = Et, peBu) [Feusu01uj2’+uMosz' l A or . 2- A + MOS ,4 V 3 [Fe(MoSu)2] v Decomposition Figure 7. Synthesis and Reactivity of [Fe2M86(SH)2]3' (M = Mo,W; R = Ph, prol) in MeCN. “9 greater than 55°C for periods longer than 2 hours. In addition, reaction of one mole of the [Fe2MoS6(S-p-Tol)2]3- ion with six moles of tetrathiomolybdate(VI) at a tempera- ture of 55° for time periods longer than 2 hours affords minute amounts of the [Fe2M028103u- anion, but the reaction mixture decomposes after 3-“ hours. These reactions are summarized in Figure 7. 2. Structure The structure of the [Fe2MoS6(S-p¢Tol)2]3' ion is de- picted in Figure 8, and selected interatomic distances and bond angles are listed in Tables VI and VII. The crystal structure consists of discrete cations and anions in a 6:2 ratio per unit cell. The structure is best described as containing three tetrahedral units: MoSu, Fesu, and S2- Fe(SAr)2 Joined at two common edges in a linear arrange- ment. The six bond angles around each metal atom [rangez 102.53°-ll6.89° (82Fe(SAr)2), 103.08-11“.““ (FeSu), and 105.25-lll.l“ (MoSu)] nearly average (109.50°) to the ideal tetrahedral value of 109.5“°. The MoFe distance is 2.778 A, while the FeFe distance is 2.691 A. Several Mo—Fe-S compounds have well documented crystal structures and selected data, pertinent to the [FezMoS6- (S-pgTol)2]3- ion, are listed in Table VIII. The [FeMoSu- (SPh)2]2- ion (V) is similar to the [Fe2MoS6(S-p-Tol)2]3- 50 Cl? ~ VS (:15 (13“ cm "I" \ 58 c: ' “‘2 cns AV 5‘ (A. ‘ “ ‘37 (403' oz 56 lib“ l a” a! 7 53 up C22 4." Figure 8. Stereochemistry of the [SzMoS FeSZFe(S-pe Tol)2]3- trianion. Hydrogen atoms are omitted 2 for clarity. 51 Table VI. Selected Interatomic Distances (A) for (EtuN)3- [Fe2MoS6(S-p7Tol)21. Bonded Distances Mo-S8 2.153 Fe2-S“ 2.188 Fel-S“ 2.228 Mo-S7 2.172 Fe2-S3 2.193 Fel-S3 2.235 Mo-S6 2.2“0 Fe2-Sl 2.315 Fel-SS 2.275 Mo-SS 2.2“8 Fe2-S2 2.326 Fel-S6 2.280 Non-bonded Distances Mo-Fel 2.778 Sl-S2 3.783 85-86 3.566 Fe2-Fel 2.691 S3-S“ 3.506 S7-S8 3.568 Table VII. Selected Bond Angles (deg) for (EtuN)3[Fe2Mo- S6(S-p-Tol)2]. SB-Mo—S7 111.1“ S“-Fe2-S3 106.31 S“-Fe1-S3 103.58 S8-Mo-S6 109.56 S“-Fe2-Sl 115.61 S“-Fe1-85 112.75 S8-Mo-SS 110.29 S“-Fe2-S2 106.62 S“-Fe1-S6 111.69 S7-Mo-S6 110.25 S“-Fe2-Sl 102.53 S3-Fe1-85 111.62 S7-Mo-SS 110.18 S3-Fe2-S2 116.89 S3-Fel-S6 11“.““ S6-Mo-SS 105.25 Sl-Fe2-S2 109.21 S5-Fe1-S6 103.08 Fe2-Fe1-Mo 176.29 Fe2-S“-Fe1 75.11 Mo-SS-Fel 75.77 Fe2-S3-Fe1 7“.86 Mo-S6-Fe1 75.83 52 ion (IV) in that both have Mo-S terminal bond distances 36 in that are smaller than the average Mo—S distance MoSi- of 2.17 A. The [Fe(MoSu)2]3- ion (VI) contains Mo-S terminal bonds distances that are similar, at 2.172 A, to the MoSfi- ion average. The bridging sulfur distances in IV_vary somewhat (the three sets are averaged in pairs in Figure 9) and the results indicate a trend that is unusual within the class of "linear" Mo-Fe-S compounds. As further discussion will attest, IV_is presumed to con- tain iron atoms in two distinct formal oxidation states13 and is unique among Mo-Fe-S complexes in this respect. The distances within the FeSZFe(SAr)2 subunit of IV_are on first approximation similar to those in the parent com- pound, (EtuN)2[Fe2S2(S-pgTol)2] (VII). The Fe-Fe distances are identical in ESL and 11 at 2.691 A, and the average Fe-S bridging and Fe-S(Ar) terminal bond distances differ only slightly («001 A) between Mano I_V_. This observation is indicative of only a minor disturbance in the FeS2Fe core of VII_on displacement of thiolate upon coordination of MoSfi’. However, a closer look shows that the Fe282 framework is distorted at two locations when IV_and VII_ are compared (Figures 8 and 9). The Fel-S* distances (nearest the Mo) are slightly decreased (0.0“3 A) while the other Fel-S* distances are increased a similar amount (0.030 A). This difference in bridging sulfur distances is clearly due to the coordinated MoS2 moiety. Although 53 .mEopmzm mlomloz znmocfiq= CH madmasm wcfiwofigm can HmcfiEpme mo mpfiwm cowwmo>< mo wcHHHmqu Ofiumsocom .m opswfim .MSMHSm mafiwpfipn monocoam m 71111. 11:11 N m m a . I m 0 0 @ Hooom.m mmmm.m *mmmm.m Zoom.m m -mm Ho m z a“ mH m till: 1111: m IIIII. |||||_m m m N s . III 0 m 0 Hommm.m oammm.m smeom.m anom.m *mmmm.moammm.m Ho -mfl A Ho my m :1 NH HH m IIIII. IIIII m_||||| .Inllum m m z . .II 0 m 0 O m mmsfi.m 20mm.m smsmm.m asmm.msmomm.m ENAH.N m .mm A m 2V EL HHH> m 711:: IIIII m m m s . I m 0 mmom.m amom.m_smmmm.m 2mmfi.m m -mfi Aedmv moZDEL m> m IIIII IIIII m 11:11 IIIIIN : I m m . III o 0 Id: mm mom mm Hoe mm o: mm mmAHoeumumvomozmomL .MH Hmm.m Hmfi.m a Hmm.m msm.m s =sm.m mmH.m um 5“ the effect is not as pronounced, the trend is visible in V and the iron nearest the M0 in IV_will be tentatively designated as "ferrous" based on this similarity. Com- parison of the Fe-Mo distance of 2.778 A in IVto the other systems in Table VIII yields some surprising results. Both III_and I_have Fe-Mo distances similar to IV_and V_and VI_ have distinctly different Fe-Mo distances. This first pair have the longer Fe-Mo distances (Table VIII) and are designated Fe(II)-Mo(VI) systems8’12 while the second pair have the shorter distances and their oxidation state assign— ments are split, V_is considered an Fe(II)-Mo(VI) complex8, but the assignment in VI is mixed and tentatively assumed to be Mo(VI)-Fe(II)-M0(V)ll. Obviously, no correlation between formal charges and the Mo-Fe distance exists. The average bridging metal-sulfur—metal angle in IV_ is 75.39° and compares favorably to average values in other Mo-Fe_Ssystems of 75.18 for V, 7“.77° for VI, and 76.1° for III, Compounds IV, VI, and III_are the only known "linear" trimeric Mo-Fe-S systems, yet all three complexes deviate from being truly linear. The angle described by the metal centers has its smallest deviation in III_with a value of l79.28°, IV and VI_have larger deviations with values of 176.29° and l72.6“°, respectively. Coucouvanis reasons that this apparent anomaly (in VI) is due to packing forces and the close proximity of several cation hydrogen atoms to two terminal sulfurs; a similar explantion might 55 .AEOCM ECCmonzHoEHXCWEoum ConH Comsponv Eoum CCMHCm wCHwUHCm ® .AmEOCm COCH Consponv Eoum CCMHCm wCHonCm V * .oHooHHnes cos h.omms Amva.ms Amvoo.os HHVH.os AmHvss.Hs mm.ms 2umu2: Asvmm.msH Hmvsm.msH mm.osH 2-2-2: Amvoom.m AOHmem.m Honom AmvmmH.m Hmvoom.m AmvmsH.m mmH.m omao2 Amvmmm.m “memom.m Amvaom.m Amvmmm.m :2m.m omuo2 Avams.m HHvomH.m Hovm22.m AHVozs.m mgs.m o2uoa Awesom.m AvaHm.m Hmm.m mica Hmvzmmd ASQNH mmm.m Rad ENH @muom AHVHom.m HHm.m smuom AmVHmm.m AHvam.m Hmm.m omuom 22 em NHHHH HHHH. HHHHH Hm N N HmEdmvoammo2m2 .8 HIHNHooC mo2m2 sale NH mHooC mo2 mmommmo2m mL CHsv NH :HHoarmimvoC mom mAmHoermvu m mos mmu DC» CH mCoumEmCmm Hopspospum nouooHom . moondEoo 8 IN IN mom MHB .Cle -mHM mo2 .A>u umH :Hoeuma vomm mom .HHH> oHnme 56 suffice for IV, In IV_only two cations in the unit cell have been crystallographically resolved, while the four re- maining cations remain disordered; meaningful discussion of the cations and their relationship to the angle of the metal centers is thus not possible. A single crystal x—ray diffraction study of the (EtuN)3[Fe2WS6(S-p-Tol)2] complex is in progress. 3. Electronic Spectra The electronic absorption spectra of the (EtuN)3- [FeZMS6(S-p-Tol)2] complexes (M = Mo,W) are shown in Figure 10; peak positions and molar absorptivities are presented in Table IX. The electronic spectra are similar to those of the (EtuN)2[FeMSu(S-pgTol)2] complexes8’25, in that the visible region (“00-600 nm) is dominated by intense absorp- tions, at least two of which clearly result from a splitting of the lowest energy sulfur + metal charge transfer transi- tions of the parent tetrathiometalates (MoSu', wsfi’). The specific S + M assignments for the [Fe2MoS6(S-p§Tol)2]3- ion (Mo-complex) and the [Fe2WS6(S-p¢Tol)2]3- ion (W- complex) are no clearer than the assignments were for the [FeMoSu(S-p§Tol)2]2- and [FeMoSuCl2JZ- ions. In the Mo- complex there is a triad of relatively sharp peaks in the “00-600 nm region and the assignment may be described as a splitting of the “67 nm absorption peak of the MoSfi- ion into two absorption peaks at ““2 nm and 510 nm. The 57 '. 30- s‘ 3 d 25,, . [(94092 FeSJ-‘es2 ““3213— 25 I, [(p-TblSleesteszwsz] an 20_ ‘ ~20 C X '03 “\ ‘ X '0‘3 (M"cm") X‘ - '5 (M-lcm-l) (-) '5 - “s 1’ ‘\ ("") l0 - ~\ '"0 \ \ \\ _ 5 \\ o I L i I “ - 5"? 260 ERK) 44C) 520 6CK) 79K) .00 Mun) Figure 10. Electronic spectra of (EtuN)3[(p-Tols)2FeS2- FeSzMoszl and (EtuN)3[(p¢Tols)2FeS 2FeSZWS2] in acetonitrile solution at 23°C. 58 .mCOHpCCHCpCoo OHCDCmemHU Com pmpooCCoo aoomm pm mumuw CHHOm 0:» CH9 KmE .HHOH w HIEoHI2V so sH Add H .oomm .soHosHon 2082 :Hs nmpCoEoE 22 :.m 22 m.m oHoosws2 Ao.mmvm~m.ACmvmmm omoCCumom .Asnvzmm.A2mVocm.22hvwmm Am.omv mHN.H2nvoom.Ao.smVHHm Hssoooon .Armvomz.fio.sHvoom.Annvmum Hm.mvaHH.Am.MHVOHm.Am.HvaHm oHsowooon ImhmAHoelmImvmmzmomu ImmmAHOBLMImemozmomg .3.02 n 2 .moonQEoo mmAHoermImv ImmzmomwmAZHCMV no mpCoEoz oHponmz oCm moCspmom Hmeoodm oHCOHuoon .xH mHan 59 W-complex, exhibiting an electronic spectrum similar to [FeWSu(S-peTol)2]2-, only has one broad peak and a host of poorly resolved features in the “00-600 nm region. Specific assignments for the electronic spectrum of the W-complex and the remainder of the electronic spectrum of the Mo-complex would be speculative, and only detailed theoretical calculations will allow definitive assign- ments. A discussion of general trends would, however, be appropriate. The Mo and w complexes exhibit the same blue shift of absorption features (relative to one another) that is characteristic of both the parent tetrathiometa- lates and their simplest Mo(W)—Fe-S derivatives, the [FeMSu(R)2]2- ions (M = Mo,W; R = Cl, S-Aryl)25. However neither complex (M0 or W) displays the low energy, low intensity bands in the near-IR (NIR) region that are characteristicyj’25 of the [FeMSuR2]2_ ions mentioned above and the [MoSu(FeC12)2]2- ion. A similar absorption 11 does not exist for the [Fe(MoSu)2]3- ion The only other electronic feature that [Fe(MoSu)2]3- and [FeMoS6— (S-prol)2]3- share is their "reduced" (trianionic) form as their most stable conformation. The electronic spectrum 11 of the [Fe(MoSu)2]3- ion has essentially the same triad of absorption peaks present in the electronic spectrum of the Fe Mo-complex, differing in the positions and the relative 2 magnitudes of the molar absorptivity values. The electronic spectrum of [Fe2MoS6(Sep-Tol)2]3- is unique and similarities 60 to the spectra of both the [FeMoSu(S-p-Tol)2]2- and the [Fe(MoSu)2]3- ions indicate that qualitative electronic assignments may not require detailed theoretical calculations on all three Mo-Fe—S systems. “. Magnetic Susceptibility Room temperature magnetic susceptibility data were obtained via the Faraday method on solid samples of (EtuN)3[Fe2MS6(SAr)2] (M = Mo,W; Ar = Ph, p-Tol). The four trinuclear complexes yielded values of 2.2-2.“ BM per formula unit (Table X); each value was corrected for the diamagnetic contributions of the ligands and cations by use of Pascal's constants. Each case corresponds to an Fe(II) (S = 5/2) system antiferromagnetically coupled to an Fe(III) (S = 2) system resulting in an S = 1/2 adduct. The distinction between an Fe(III)-Fe(II)-M0(VI) formula- tion and an Fe(III)-Fe(III)-M0(V) formulation cannot be based solely on the magnetic susceptibility results. The following M6ssbauer results lend more support to the Fe(III)-Fe(II)-Mo(Vl) assignment. 5. Mdssbauer Spectroscopy Figure 11 depicts the Mossbauer spectrum of (EtuN)3- [FeZMoS6(SPh)2] at “.2 K in a small (