Synthesis, Structure, and Characterization of the [Fe5S4(CO)12]2

Synthesis, Structure, and Reactivity of Clusters Containing the [Mo2O2(μ-E)2] (E = O, S) Core and Coordinated [Fe2S2(CO)6] Units. Daniel E. Barber, ...
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Organometallics 1995, 14, 3229-3237

3229

Synthesis, Structure, and Characterization of the [Fe&(CO)12]2-$-Ions and Studies on the Oxidative Conversion of the Dianion to [Fe&C0)12l2Daniel E. Barber,+Michal Sabat, Ekkehard Sinn,t and Bruce A. Averill*>$ Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Received March 8,1995@

The compounds ( P ~ ~ N ) ~ [ F ~ ~ S ((Pr4NhIW ~ ( C O ) I ~ and I ( P ~ ~ N ) [ F ~ ~ S ~ ( C O ) I ~((Pr4N)IW~THF V.O.BTHF, THF = tetrahydrofuran) have been prepared and structurally characterized. Crystals of (Pr4N)IV are monoclinic, space group C2/c (a= 34.16(1) b = 19.446(8) c = 17.902(8) p = 121.40(2)",R = 0.056, R, = 0.064,2368 reflections with I > 3 ~ 4 0 )crystals ; of (Pra)V.O.tiTHFare triclinic, space group P1 (a= 11.745(1)A, b = 11.807(1) c = 15.198(2) A, a = 68.58(1)",/3 = 85.99(1)",y = 75.46(1)",R = 0.023, R, = 0.038, 5296 reflections with I > 3a(I)). The two anions have similar structures, with a central Fe atom (Fe(I1) and Fe(1II) for IV and V, respectively) coordinated in a distorted tetrahedral geometry by two [Fe2S2(CO)6I2-ligands. Cluster IV undergoes an 02-specific oxidation to the known cluster [Fe6S6(C0)12I2-(VI)by a n undetermined mechanism. Cluster VI can be prepared cleanly by reaction of Fe&(C0)6 (I)with [Fe4S4(C0)12I2-(n),and a mechanism involving oxidative addition of I to I1 is postulated. (Pr4N)zVI has been obtained in crystalline form (orthorhombic, space group C m ~ 2a~=; 18.870(8) b = 17.307(13) c = 16.430(3) R = 0.057, R, = 0.076,2689reflections with I > 341)) and structurally characterized. Oxidative addition of I1 is also suggested as an important step in the formation of the previously reported cluster [M0OFe5S6(C0)12]~-,which insight has resulted in an improved synthesis for the latter cluster. This newly-discovered reactivity of [Fe4S4(C0)12I2-makes the compound a potentially useful reagent in preparing a variety of mixed M-Fe-S clusters having new stoichiometries and structures.

A,

A,

A,

A,

A,

Introduction The reductive chemistry of the butterfly cluster Fez&(CO)6 (I)lhas been well established for several years, primarily due to the efforts of Seyferth and co-worke ~ s . 2 ,Their ~ work and additional studies in our laboratory have shown that reduction of the neutral complex I with 1equiv of a reducing agent yields the disulfidebridged dimeric compound [Fe4S4(C0)12I2- (11),4and further reaction of I1 with a second 1 equiv of reductant yields the dianion [Fe&(CO)612- (IIIh2 The bridging sulfides of both I1 and 111possess nucleophilic character, as demonstrated by their reactions with alkyl halides to form S-alkylated product^^^,^ and, in the case of the fully reduced dianion 111,by reaction with a variety of metal halides to yield metal clusters in which the [??e2S2(CO)6]2-unit acts as a bidentate ligand coordinated via the bridging sulfide^.^

A,

A,

In our research, we have used the dianion I11 as an iron and sulfur source in reactions with molybdenum halides to yield molybdenum-iron-sulfur clusters with high iron and sulfur content. Examples of the unusual clusters prepared by this method include the [M0OFe&(C0)121~-,5~ [ M O F ~ ~ S ~ ( C O )and I ~ I[Mo~-,~~ Fe6S6(CO)1612- ion^.^^^^ Until recently,sbthe rearrangements involved in the formation of these clusters were poorly understood. In this paper, the preparation and characterization of the [Fe5S4(C0)12I2-(IV)and [Fe5S4(C0)121- (v> ions and studies of the oxidative conversion of IV to the known cluster [Fe6Ss(C0)12l2- (VIY are presented. During this work, novel reactivity of the [Fe4S4(C0)12I2-ion (11)was observed, and the chemistry of I1 has subsequently been shown to be important in many of the aforementioned cluster formation reactions. Experimental Section

Current address: Department of Chemistry, Oberlin College, 130 West Lorain Street, Oberlin, OH 44074. Current address: School of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, U.K. 8 Current address: E. C. Slater Instituut, Universiteit van Amsterdam, Plantage Muidergracht 12, 1018 TV Amsterdam, NL. @Abstractpublished in Advance ACS Abstracts, May 1, 1995. (1) (a) Hieber, W.; Gruber, J. 2.Anorg. ALZg. Chem. 1958, 296, 91. (b) Wei, C.-H.;Dahl, L. F. Inorg. Chem. 1965,4 , 1. (2)(a) Seyferth, D.; Henderson, R. S.; Song, L.-C. Organometallics 1982, I , 125. (b) Seyferth, D.; Kiwan, A. M.; Sinn, E. J. Organomet. Chem. 1985,281, 111. (3)(a) Cowie, M.; DeKock, R. L.; Wagenmaker, T.R.; Seyferth, D.; Henderson, R. S.; Gallagher, M. K. Organometallics 1989,8, 119. (b) Day, V. W.; Lesch, D. A,; Rauchfuss, T.B. J. Am. Chem. SOC.1982, 104, 1290. (c) Seyferth, D.; Henderson, R. S.; Gallagher, M. K. J. Oreanomet. Chem. 1980. 193. C75. (d)Boaan. L. E., Jr.; Lesch, D. A,; Rakhfuss, T. B. J . Organomet. Chem. l9k3, 250,429. (4) Bose, K. S.; Sinn, E.; Averill, B. A. Organometallics 1984,3,1126. +

*

All reactions were carried out under a dry nitrogen atmosphere (except where otherwise indicated) using standard Schlenk techniques. Solvents were distilled under Na; acetonitrile (MeCN) was distilled from CaHz, tetrahydrofuran (THF) was distilled from Na-benzophenone ketyl, methanol (MeOH) was distilled from magnesium methoxide, and 2-propanol (iPrOH) was distilled from Al(O-i-Pr)a. Hexane and diethyl ether were reagent grade and were used without further purification. Tetraalkylammonium halide salts were generally (5)(a) Bose, K. S.;Lamberty, P. C.; Kovacs, J. A.; Sinn, E.; Averill, B. A. Polyhedron 1986,5,393. (b)Eldredge, P. A.; Bose, K. S.; Barber, D. E.; Bryan, R. F.; Sinn, E.; Rheingold,A,; Averill, B. A. Inorg. Chem. 1991,30, 2365. ( c ) Eldredge, P. A,; Bryan, R. F.; Sinn, E.; Averill, B. A. J. Am. Chem. SOC.1988,110, 5573. (6) Lilley, G. L.; Sinn, E.; Averill, B. A. Inorg. Chem. 1986,25,1073.

a276-m3195123i4-3229$0~.0010 0 1995 American Chemical Society

Barber et al.

3230 Organometallics, Vol. 14, No. 7, 1995 dried for several hours under vacuum at 60-70 "C and were stored in a Vacuum Atmospheres drybox under argon. Iron pentacarbonyl, iron(I1) bromide, Superhydride (1 M LiEt3BH in THF), and sublimed sulfur were obtained from Aldrich and and were used as received. The compounds FezSz(C0)6 (I)3d [Fe2S2(C0)6]2-(111)2were prepared by published procedures; ( P ~ & & [ F ~ ~ S ~ ( C((Ph4Ad211) O ) ~ Z ] was prepared by the method of Bose et aL4 The peroxide compound DABCO.2Hz02 (DABCO = 1,4-diazabicyclo[2.2.2]octane) was prepared as de~cribed,~ and [Fe(phen)31[BF4I3was a gift from Phyllis Hunt (University of Virginia). Infrared spectra were recorded in an NaCl solution IR cell on a Mattson Cygnus 1000 FTIR spectrometer using EXPERT software. Electronic spectra were recorded on a Cary 219 UVvisible spectrophotometer. Electrochemical measurements were performed using a Princeton Applied Research Model 175 universal programmer and Model 173 potentiostat and a Yokogawa 3023 plotter; the working and auxiliary electrodes were Pt wire. All values are referenced to a saturated calomel electrode (SCE). Elemental analyses were obtained from Galbraith Laboratories, Inc. (Knoxville, TN). Preparation of ( P ~ ~ N ) ~ [ F ~ S S ~(Pr4N)dIVI. ( C O ) ~ ~ I , To a solution of 2 mmol of [FezS2(C0)6]2-,111,in ca. 40 mL of THF at -78 "C was added a room temperature (RT) suspension of 0.22g (1mmol) FeBr2 in ca. 5 mL of MeCN. The solution was allowed to warm to RT, during which time the deep green solution changed to dark greenish-brown color. The solvent was removed under vacuum, the residue was dissolved in ca. 3 mL of MeCN, and a solution of 0.80 g (3mmol) of (Pr4N)Br in ca. 30 mL of i-PrOH was added. The solution was filtered immediately to remove the microcrystalline product; larger X-ray-quality crystals separated from the filtrate upon standing (total isolated yield ca. 70%). IR (MeCN), vco (cm-'): 2021 (s), 1997 (vs), 1945 (s, sl br). Electronic spectrum (MeCN), , I (nm) ( E (M-l cm-'1): 700 (21261,480 (sh), 425 (4930,sh), 330 ( ' 2 0 000). Anal. Calcd (found) for C36Hd~Fe&&: c, 38.73 N, 2.51 (2.22); S, 11.49(12.36); Fe, 25.01 (38.01);H, 5.06(5.49); (24.49). Preparationof (PrkN)[FesS4(CO)121, (PqN)M.Method A. A solution of 1.12g (1mmol) of (Pr4N)21Vin ca. 20 mL of MeCN at RT was treated with 1 equiv of CpzFe+(prepared by reaction of ferrocene with NOBF4 (1equiv)); the color of the solution changed to deep bluish-green, and the conversion of IV to product was observed in the IR spectrum. The product was precipitated as X-ray-quality crystals (which contain l/2 equiv of THF solvent) in nearly quantitative yield (92%) by slow diffusion of hexane into a concentrated THF solution of V. IR (MeCN), vco (cm-I): 2046 (s), 2020 (vs), 1982 (s, br). Electronic spectrum (MeCN),, I(nm)( E (M-l cm-l)): 663 (4600), 486 (49001,330 (22300). Method B. A solution of 0.69 g (2mmol) of FezSz(C0)6,I, in ca. 20 mL of THF at -78 "C was treated with 3 mL (3mmol) of Superhydride. To this solution was added a RT suspension of 0.44 g (2 mmol) of FeBrz in ca. 10 mL of MeCN, and the solution was warmed to RT. The solvent was removed under vacuum, and the residue was redissolved in a minimum volume of MeCN. The product was precipitated by addition of 0.8 g (3 mmol) of (Pr4N)Br in i-PrOH and was shown to be identical to that obtained by method A (yield ca. 60%). Method C. To a solution of 2 mmol of I11 in ca. 30 mL of THF at -78 "C was added a solution of 0.30 g (1 mmol) of FeBrs in ca. 7 mL of MeCN at RT. The deep bluish-green solution was warmed t o RT, during which time the color changed to brownish-green. The solvent was removed under vacuum, and the residue was dissolved in MeCN. Solution IR spectra of the mother liquor showed YCO bands due t o V with small amounts of IV and [Fe6SdC0)1~]~(VI)as contaminants. Preparation of (Pr4N)2[FeaSs(C0)121, (Pr4N)2Ml. Method A. A solution of (Pr4N)ZIVin MeCN was prepared, and (7) Oswald, A. A,; Guertin, D. L. J . Org. Chem. 1963,28, 651.

Oz(g)was bubbled through the solution via syringe until the solution color had changed to deep purple and the IR spectrum showed no YCO bands due to IV. The solution volume was reduced, and i-PrOH was added to precipitate (Pr4NhVI in ca. 12% yield. X-ray-quality crystals were grown by slow cooling of saturated MeCNli-PrOH solutions of the cluster. IR (MeCN), YCO (cm-'1: 2042 (m), 2012 (vs), 1969 (ms, sl br). Method B. An equimolar solution of 0.17g (0.5mmol) of ( C O ) ~ ~ ] was , FezSz(C0)6, I, and ( P ~ ~ A S ) Z [ F ~ ~ S ~(Ph&s)zII, prepared in ca. 30 mL of MeCN at RT. The reaction was monitored by IR spectroscopy over ca. 3-4 h, during which time the solution changed from orange-green to deep purple. The product was spectroscopically identical to that obtained by method A. Preparation of (Ph&s)2[MoOFe&(CO)l2]. To a solution of 1.03g (3 mmol) of FezSz(C0)6 (I)in ca. 30 mL of THF at -78 "C was added 4 mL (4mmol) of Superhydride in 0.5 mL increments over 30 min. To the resulting deep green solution was added rapidly a solution of 0.38 g (1 mmol) of MoOCl3(THF)z in ca. 3 mL of MeCN. The initial deep orange reaction mixture turned brown upon warming to room temperature. The solvent was removed under vacuum, the residue was dissolved in ca. 2 mL of MeCN, and excess (Ph&)Cl in ca. 30 mL of i-PrOH was added, resulting in precipitation of a microcrystalline solid. The solid was filtered out, washed with i-PrOH, and dried under vacuum. The solution IR spectrum of this product (MeCN (cm-'): VCO, 2042 (m), 2008 (s), 1966 (ms); YM~-O, 930) was identical to that of the previously reported Yield: 0.93g (55%). X-ray Analyses. All X-ray measurements were carried out on a n Enraf-Nonius CAD4 diffractometer using Mo radiation. Unit cell dimensions were determined using the setting angles of 25 high-angle reflections. The intensities of three standard reflections were measured every 3 h of X-ray exposure and showed no significant fluctuations. Intensities were corrected for absorption by applying the program DIFABS.B Crystallographic calculations were performed by using the TEXSAN 5.0 software p a ~ k a g e .Crystal ~ data and collection and refinement parameters are summarized in Table 1 for (Pr4N)z[Fe&( C 0 ) l ~ l ((Pr4N)2IV), ( P ~ ~ N ) [ F ~ ~ S ~ ( C O ) I Z I ' .((Pr4N)V~THF O.BTHF),and (Pr8)2[Fe6&(C0)12] ((Pr4N)4vI). In the first two structures, the cations were crystallographically ordered with no unusual features. Tables of all positional and thermal parameters and selected distances and angles are available as supplementary material. (fi4N)2IV. MULTAN was used to locate 4 of the 5 iron atoms, and the other atoms were located via successive Fourier syntheses.1° All heavy atoms were refined isotropically using full-matrix least-squares refinement; no account was taken of H atoms. (Pr4N)V.0.5THFe Data collection and refinement were performed as for (Pr4N)2IV,except that structure solution was by direct methods (SIR-88)" and all heavy atoms were refined anisotropically. The THF molecule was found to be disordered between two positions on the center of symmetry, such that C atoms in the 2 and 4 positions of the rings in the two orientations were superimposed (C26 and C26*). The atoms of the THF were refined anisotropically in these two positions with occupancy factors of 0.5 for the nonoverlapped atoms and 1.0for the overlapped atoms. (Pr4N)aVI. Data collection and refinement were performed as for (Pr4N)V'.5THF, except that the heavy atoms of the cations were refined isotropically using the following model for disorder of one of the cations. While the mirror plane ~

~

~~

(8)Walker, N.;Stuart, D. Acta Crystallogr., Sect. A 1983,39,158. (9)T E X S M : Single Crystal Structure Analysis Software; Version 5.0; Molecular Structure Corp.: The Woodlands, TX 77381,1989. (10) Freyberg, D.P.;Mockler, G. M.; Sinn, E. J.Chem. SOC.,Dalton Trans. 1976, 447. (11)SIR88: Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Polidori, G.; Spagna, R.; Viterbo, D. J.Appl. Cystallogr. 1989,22, 389.

[Fe&CCO)ld2-~- Ions

Organometallics, Vol. 14, No. 7,1995 3231

Table 1. Summary of Crystal Data, Intensity Measurements, and Structure Solution and Refinement compd

(fi4N)ZN

(PrJW.0.5THF A. Crystal Data formula C ~ ~ H ~ ~ N Z O I Z S ~ F ~ ~C Z ~ H ~ O N O I Z . ~ S ~ F ~ ~ fw 1116 962 cryst color, habit black prism black prism cryst dimens (mm) 0.35 x 0.35 x 0.30 0.46 x 0.39 x 0.32 cryst system monoclinic triclinic 34.26 11.745(1) a (A) 19.446(8) 11.807(1) b (A) 17.902(8) 15.198(2) c (A) 90 68.58(1) a (de& 121.40(2) 85.99(1) /3 (de@ 90 75.46(1) y (deg) 10150 1899 v (A3) C2/c (No. 2) space group P1 (No. 1) z 8 2 1.46 1.69 dcalc(g/cm3) 1.64 21.32 p(Mo Ka) (cm-'1 B. Intensity Measurements Enraf-Nonius CAD4 Enraf-Nonius CAD4 diffractometer radiation Mo Ka Mo Ka 23 -120 T ("C) 0,-213 0,-28 scan type 50 48 2&,, (deg) total: 8153 total: 6287 no. of reflcns unique: 5957 (%"t = 0.010) corrs Lorentz-polarization absorption Lorentz-polarization absorption transm factors 0.80-1.25 C. Structure Solution and Refinement struct solution Multan direct methods (SIR88) ~. refinement full-matrix least squares full-matrix least squares function minimized ZW(lF0l - I F d 2 CW(lF0I - IFc1)2 2368 5296 no. observns with Z > 3dZ) 241 no. variables 432 reflcndparam 9.8 12.3 0.056; 0.054 0.023; 0.038 R;Rw 1.62 1.28 goodness of fit 0.72 ;ax peak (e/A3)in final diff map 0.28 symmetry of the ion requires two propyl groups to be located on the plane, the presence of several peaks in difference Fourier maps suggested that carbon atoms C11, C12, C14,and C 15 were disordered between two equally populated locations outside the symmetry plane. The final difference Fourier map showed a peak of 1.25 e/A3 intensity in the vicinity of the disordered propyl groups.

Results and Discussion Syntheses of Clusters. The cluster [Fe5S4(C0)1zI2-,

IV,see Chart 1,was originally observed as one of four products in the reaction of MoC15 with the dianion III.5a In addition, reaction of MoOCL(THF)z with 3 equiv of I11 to form [MoOFe5S6(CO)12l2-also yields significant amounts of cluster IV and, to a lesser extent, the known clustePc [ M O F ~ & ( C ~ ) I ~ ]A~ -direct . synthesis of IV was achieved by the straightforward reaction of FeBrz with 2 equiv of 111. Cluster IV can be isolated in ca. 70% yield as its P r a + salt and can be identified readily by characteristic vco bands a t 2027, 1989, and 1945 cm-l in its infrared spectrum. Solutions of the cluster are dark greenish-brown due to absorptions at 700 and 425 nm and an intense absorption a t ca. 330 nm. The crystal structure and physical properties of IV are consistent with the formulation of the cluster as a central Fe" chelated by two [ F ~ ~ S Z ( C O ligands ) ~ ~ ~(vide infra). Electrochemical measurements of IV (in MeCN, 0.1 M (Bu4NIC104) reveal a nearly reversible oxidation a t El12= -0.465 V vs SCE (a, = 120 mV, ipa/ipc= 1.02).

(fi4N)ZW

black prism 0.43 x 0.32 x 0.28 orthorhombic 18.870(8) 17.307(3) 16.430(6) 90 90 90 5366 Cmc2l (No. 36) 4 1.53 18.56 Enraf-Nonius CAD4 Mo Ka 23 w-28 50 total: 4145 Lorentz-polarization absorption 0.69-1.31 direct methods (SIR88) full-matrix least squares EW(IF0I - IFd* 2689 235 11.4 0.057; 0.076 2.22 1.25

The value of Eln for the oxidation process is at the upper limit of the range reported for the FeImI1redox couple of mononuclear four-coordinate iron complexes with thiolate ligands,12and therefore the existence of a stable Fe"' analog of IV was investigated. One-electronoxidation of IV by ferrocenium or [Fe(phen)3I3+(in MeCN at room temperature) resulted in quantitative conversion t o the monoanionic analog [FesS4(C0)1~1-,V. The conversion was monitored easily by observing the disappearance of the IR spectrum of the starting material and the appearance of the product's spectrum at 2047, 2021, and 1987 cm-l; in addition, the color of the solution changed to deep bluish-green, reflecting shifts in the cluster's visible absorptions to 660 and 488 nm. Cluster V can be isolated (also as its Pr4N+ salt) in >90% yield by precipitation from THFhexane. The cluster is somewhat less stable in solution than is the dianionic analog, and this property has hampered efforts to obtain analytically pure samples. The cluster is isostructural with cluster IV,and the physical properties of V indicate that the oxidation is localized on the central Fe atom (vide infra). Cluster V can also be synthesized in good yield by reaction of 1 equiv of FeBrz with a solution containing 1 equiv of I11 and 0.5 equiv of [Fe4S4(C0hzlZ-GI), prepared as described in the Experimental Section, or bv reaction of FeBrn with 2 eauiv of 111;thus the Fez+ (12)(a) Lane, R. W.; Ibers, J. A.; Frankel, R. B.; Holm, R. H. Proc. Natl. Acad. Sci. U S A . 1976,72,2868. (b)Millar, M.; Lee, J. F.; Koch, S. A,; Fikar, R. Inorg. Chem. 1982,21,4105. (c)Hagen, D.S.;Reynolds, J. G.; Holm, R. H. 3.Am. Chem. Soc. 1981,103,4054.

3232 Organometallics, Vol. 14, No. 7, 1995

Barber et al.

Chart 1. Schematic Drawings of the Clusters Referred to in This Work with Terminal CO Ligands Indicated by Straight Lines Attached to Fe

J

II

J

L

VI n

2-

Table 2. Selected Bond Distances (A)and Angles (deg) for [Fe~S4(C0)12I2(IV)and [Fe5S4(CO)121-

0”

Fe-S1 Fe-S2 Fe-S3 Fe-S4 Sl-Fe-S2 S3-Fe-S4 Sl-Fe-S3 S1-Fe- S4 S2-Fe- S3 S2- Fe-S4

IV

V

2.345(4) 2.338(5) 2.340(5) 2.345(4) 82.0(2) 81.9(2) 107.6(2) 149.0(2) 144.7(2) 107.6(2)

2.2674(8) 2.2691(8) 2.2689(8) 2.2673(8) 87.76(3) 87.84(3) 120.25(3) 120.48(3) 120.79(3) 123.55(3)

a Fe refers to the central Fe atom, Fe3 and Fel in IV and V, respectively.

ion acts as a one-electron reductant toward the [Fe&(C0)12l2-species present in solution. In both reactions the initial color of the solution was deep bluish-green, consistent with formation of exclusively V,but the color became more brownish-green upon warming to room temperature. M a r e d spectroscopy indicated that small amounts of N and [Fe6S6(C0)12]2-,VI (vide infra), were produced. Over several days the spectrum of V disappeared and bands at 2063,2018,1991, and 1929 cm-l appeared; these belong to as yet unidentified clusters, and their appearance reflects the relative instability of this cluster. Structural Characterization of New Clusters. Crystals of (Pr4N)2IV suitable for X-ray diffraction were obtained from saturated MeCNli-PrOH (ca. 193) solutions. The relevant crystal data are summarized in Table 1; the room-temperature intensity data were refined satisfactorily to final residuals of R = 5.6% and R, = 6.4%. The structure of the dianion IV,shown in Figure 1, consists of a central Fe atom coordinated by four S atoms from the two Fe2S2(CO)6ligands; Table 2 summarizes important bond distances and angles within the cluster. The average FeII-S distance is 2.343(5) A, which is slightly shorter than the corresponding value

Figure 1. Structure of the [FesS4(C0)12I2ion, IV (50% probability thermal ellipsoids). found for other Fe11-S4 complexes which contain thiolate-type ligands ([Fe(S2-o-xyl)2l2-, 2.37 A;13a[FeThe (SPh)4I2-, 2.36 A; [Fe(S2C402)2I2-, 2.39 geometry about the central Fe (Fe3)is severely distorted from tetrahedral with S-Fe3-S “bite” angles (due to the FenS2(CO)s ligands) of 82.0(2)” (average); the remaining S-Fe3-S angles are grouped as two smaller A13b9c).

(13)(a)Lane, R. W.;Ibers, J. A.; Frankel, R. B.; Papaefthymiou,G. C.; Holm, R. H. J. Am. Chem. Soc. 1977,99,84. (b) Coucouvanis, D.; Swenson, D.; Baenziger, N. C.; Holah, D. G.; Kostikas,A.; Simopoulos, A.; Petrouleas, V. J.Am. Chem. Soc. 1976,98,5721. (c) Coucouvanis, D.; Swenson,D.; Baenziger,N. C.; Murphy, C.; Holah, D. G.; Sfamas, N.; Simopoulos, A.; Kostikas, A. J. Am. Chem. Soc. 1981, 103, 3350.

[Fe5s4(CO)

1 f l - p -

Organometallics, Vol. 14, No. 7, 1995 3233

Ions

Table 3. Comparison of Structural Parameters of Chelated FezSz(CO)e Units in Selected Complexes bond length (av, Fe-Fe Fe-S

Fe -S- Fe 65.2 65.62 65.80 65.6

bond angle (av, deg) S-Fe-S Fe2S (dihedral) 83.7 85.98 83.6 95.3

104.9 108.61 104.99

(average 107.6(2)")and two larger (average 147")angles, with an overall average value of 127.3'. The latter observation is a result of the distortion of the central Fe from idealized Dw symmetry, as can also be seen by the dihedral angle of 128.4' between the Fe3-Sl-S2 and Fe3-S3-S4 planes (Dw symmetry requires that this angle be 90'). Similar distortions have not been observed in other iron(11)-thiolate complexes. l3 The two FezSz(CO)6units are identical within experimental error; the average Fe-Fe and Fe-S distances (2.48 and 2.30 A, respectively) and the Fe-S-Fe angle (average 65.3') are similar to the corresponding parameters in other structurally characterized complexes containing the Fe&(CO)6 unit (see Table 3). The S-Fe-S angle shows the most variability among the compounds listed; the value for IV is 83.7' (average) and results in a dihedral angle of 104.9' (average)between the two Fe2S planes. The structure of the monoanionic analog V has also been determined. Crystals of (Pr4N)V.0.5THF suitable for X-ray diffraction studies were obtained by slow diffusion of hexane into a THF solution of the cluster. Relevant crystal data are summarized in Table 1;the intensity data (-120 "C)were refined to residuals of R = 2.4% and R, = 3.9%. The structure of V is shown in Figure 2; selected bond distances and angles are comFigure 2. Structure of the [Fe&4(CO)121- ion, V (50% probability thermal ellipsoids). pared in Table 2. Again the geometry about the central Fe is distorted tetrahedral, with an S-Fel-S bite angle of 87.80(3)' (average); in this case the remaining S-Fel-S angles are all nearly equal, with an average value of 121.3'. This is because the symmetry about the central iron approaches idealized Dw, with a dihedral angle between the Sl-Fel-S2 and S3-FelS4 planes of 88.9". In addition, the Fel-S bond distance of 2.27(1) A (average) is nearly 0.1 A shorter than that in the dianion IV and is typical of other FelI1Sq bond d i ~ t a n c e s . l ~The ~ J ~FeaSa(CO)6units again are identical to each other and have Fe-S and Fe-Fe distances (2.31 A (average) and 2.50 A (average), respectively) and Fe-S-Fe angles (65.6' (average)) similar to those of other complexes containing this unit; Figure 3. Structure of the [FesS~j(C0)12]~ion, (50% the S-Fe-S angle (86.0' (average)) is more than 2 deg probability thermal ellipsoids). Important distances and larger than the corresponding angle in the reduced angles: Fe(1)-S( l)(av),2.210(4)A;Fe(1)-S(2,3)(av),2.304 cluster IV (see Table 3). (4) A; S(1)-Fe(1)-S(l'), 104.9" (see also Table 3). Although the structure of (BZE~~M~N)~[F~~S~(CO)~~I, most noteworthy differences are that the S3-Fel-S4 (BzEtzMeN)2VI, has been reported, the data only alangle is significantly smaller in the refined structure lowed for refinement to residuals of R = 9.9% and R , salt (83.7(1)' vs 86.8' (in BzEt2MeN)ZVI) of the Pra+ = 10.8% due to disorder in the cations.6 The current and that the average Fe-S distance in the FezSz(CO)6 work has provided crystals of (Pr4N)2VI that have ligands is more in keeping with the value observed in allowed significantly better refinement (R = 5.7%, R , other compounds that contain this moiety. The struc= 7.6%). The resulting structure, shown in Figure 3, tural parameters of the Fe&(CO)6 unit in VI are is essentially the same as that reported previously. The compared with those of other clusters in Table 3. Both the S-Fe-S angles (average 83.6") and the dihedral (14) Nametkin, N. S.; "yurin, V. D.; Aleksandrov, G. G.; Kuz'min, 0. V.; Nekhaev, A. I.; Andrianov, V. G.; Mavlonov, M.; Struchkov, Y. angle between the two Fe2S planes (105") are more T. Zzv. h a d . Nauk SSSR, Ser. Khim. 1979,6, 1353. similar to the corresponding values obtained in W, this (15) Koch, S. A.; Maelis, L. E.; Millar, M. J . A n . Chem. SOC.1983, 105,5944. average C-0 distance (1.14 A) is identical to that found UJ

Barber et al.

3234 Organometallics, Vol. 14, No. 7, 1995

in V. The Fel-S3,S4 average distance (2.304A) is near the average of the corresponding distances in clusters IV and V. Complete structural data are provided in the supplementary material. Reaction of [FeeS4(C0)12I2-(IV)with Oxidants. During routine manipulation of solutions of IV, it was observed that these solutions changed from dark greenA ish-brown to the characteristic deep purple color of the known compound [Fe6S6(C0)1212-,VI,6 upon exposure to air. More careful investigation of this phenomenon showed that the conversion could be effected by bubbling pure 02(g)via syringe through an MeCN solution of IV. Infrared spectroscopy indicated that VI was the only detectable CO-containing product of this reaction; no CO-containing intermediates were observed (see Figure 4). Similarly, reaction of the monoanion V with 0 2 resulted in the formation of a small amount of VI,but *=E 1 a large amount of Fe2Ss(CO)6, I, was also formed; this #-PI enhanced susceptibility t o oxidative decomposition is consistent with the observed lower stability of V in 0.20 (b) solution. Because the oxidative conversion of IV to VI must involve a novel rearrangement, this reaction was investigated in more detail. Oxidations of IV with ferrocenium and [Fe(phen)313+ were performed in order to determine whether the role of oxygen was as an outer-sphere oxidant. Cluster V was first produced during initial experiments with ferrocenium as oxidant; later experiments utilized the more powerfid oxidant [Fe(phen)3I3+.The latter experiments showed that, upon oxidation of V with [Fe(phen)3I3+,a significant amount of I was produced as 0.16 well as a mixture of several other unidentified clusters. At no time was any formation of VI detected either by IR spectroscopy or by changes in the color of the solution. Thus, 0 2 is not merely an outer-sphere oxidant but must play a more direct role in the conversion of IV to VI. A The reactions of both IV and V with superoxide and 0.08HzO2 were examined in order to determine whether oxidants related to 0 2 could also induce the conversion of these clusters to VI. Slow addition of 1equiv of KO2 0.04(solubilized in a 0.1 M DMSO solution of 18-cr0wn-6~~) to V in MeCN, a reaction which formally involves the same number of electrons as IV 02, resulted imr mediately in nearly quantitative conversion (by IR) of I V to IV,with a small amount of VI also formed; further 2200 2100 2000 1900 1800 1700 addition of KO2 resulted only in precipitation of an Wavenumber insoluble black solid. These observations indicate that Figure 4. Infrared spectra (YCO region, MeCN solution) superoxide acts as a reductant toward V,with formation of (a) IV,(b) IV + 0 2 (incomplete reaction), and (c) IV of VI due to reaction of the redox products IV and 0 2 , 0 2 (final product). Starred peaks in (b) correspond to W, and that reaction of superoxide with IV does not yield those indicated by # correspond to VI. VI but produces only insoluble decomposition products. Both IV and V reacted with HzO2 (as D A B C O ~ H Z O ~ ~ )cluster. The products from these reactions have not been further characterized; no evidence for formation to yield mixtures of compounds; in both cases a large of VI was noted, even upon exposure of the solutions to excess of H202 was required before significant reaction the atmosphere. equiv and >7 equiv for the two was observed ('€4 The stoichiometry of the reaction of EV with 0 2 was reactions, respectively). Although the mixtures in both evaluated by two separate experiments. Since the YCO reactions displayed infrared bands at 2040,2007,and infrared absorbances of VI (in MeCN) obeyed Beer's law 1964 cm-l, the relative intensities of these bands were in the concentration range 0-1 mM, the concentration different in each case; also, the mixture obtained from of VI present in solution could be determined using the reaction with IV was more complex, with additional infrared spectroscopy. Therefore, a solution containing lower-energy bands at 1930 and 1904 cm-'. No formaa known amount of IV was reacted slowly with 0 2 until tion of VI was observed in either case by IR spectroscopy IR absorbances of the starting material had disapthe or by detection of the characteristic purple color of the peared, and the amount of VI produced was determined (16)Valentine, J. S.; Curtis, A. B. J.Am. Chem. Soc. 1975,97, 224. by the sample's infrared absorbance at 2043 cm-'. This

0.25i

+

+

Organometallics, Vol. 14,No. 7,1995 3235

[Fe&4(CO)12-2- Ions

Table 4. Reactions of IV and V with Possible Oxidation Products

experiment indicated that 4 equiv of IV was required for the formation of 1 equiv of VI. The amount of 0 2 needed for this transformation was also determined by titration of a k n o w n amount of IV (in MeCN) with an 02-saturated acetone solution;17 IR spectroscopy indicated that the conversion was complete after addition of 5 equiv of 0 2 . These values should be considered as upper limits since slow oxidative decomposition of VI by 0 2 would result in the values being too high. Presumably several Fe-0 and/or Fe-S species are also present, but the nature of these products has not been determined. Reactions of Possible Oxidation Products. The possibility was considered that the formation of VI is a result of the rapid reaction of two transiently formed oxidation products of IV. The k n o w n clusters Fe2S2(COh (I), [Fe4S4(C0)1zI2- (111, and [Fe5S4(C0)121- (V) were considered as possible oxidation products of IV, and the reactions among these four compounds were examined to determine if this postulate was feasible. All reactions were performed in MeCN at room temperature, since these are the conditions under which the reactivity of IV with 0 2 is observed, and were monitored by IR spectroscopy. The results of these studies are summarized in Table 4 and are discussed in detail below. Reaction of an equimolar mixture of IV and I resulted immediately in a mixture of V and I1 in addition to the starting materials. Addition of an extra 1equiv of I t o this orange-green solution resulted in no spectroscopic changes other than an increase in the intensity of the bands associated with this cluster. This solution changed to the purplish color of VI over ca. 4 h; IR spectroscopy confirmed the presence of a large amount of VI as well as V and a small amount of I. Presumably I is acting as an oxidant toward IV in the initial step of this reaction, and VI is formed by a relatively slow reaction of some of the clusters present in solution. Similar results were obtained in the reaction of 1 equiv of [Fe5S4(C0)1~10 with 1 equiv of [Fe4S4(C0)1212(11). The initial reaction mixture again contained a mixture of all four clusters and gradually converted to a mixture of VI, V,and a small amount of IV. In this case, V acts as an oxidant toward I1 to produce IV and I. Further evidence for the observation that the two forms of [Fe&(C0)1zIn- (n = 1, 2) act as electrontransfer reagents comes from the absence of reactivity either between IV and I1 (the two more reduced species) or between V and I (the two more oxidized species) after several hours at room temperature. The final entry in Table 4 clearly indicates that the source of [Fe6S6(C0)12I2-(VI)in the preceding experiments is the reaction of Fe2S2(CO)6 (I) with [Fe4S4( C 0 ) d 2 - (11) at room temperature. An equimolar mixture of I and I1 cleanly reacted over ca. 4 h to yield (17) Ebsworth, E. A. V.; Connor, J. A.; Turner, J. J. Comprehensive Inorganic Chemistry;Pergamon Press Ltd.: Oxford, U.K., 1973; Vol. 2, p 709.

I

I

I

I

I

I

L

-1.5

-2.0

I

0.15mA

+OS

0

-0.5

-1.0 V

Figure 5. Cyclic voltammograms (0.1 M BmNC104 in MeCN, scan rate - 100 mV/s) of (a) [Fe4S4(CO)12l2-(II) and (b) FezSz(C0)e (I). VI, with only a small amount of either I1 or IV (produced by slow decomposition of I1 in solution) as a minor contaminant. This result indicates that the role of the [Fe&34(C0)12In- species is to act as a “redox buffer“ to maintain a supply of both I and I1 for the production of VI. A similar role for the “0-xylyl dithiolatedisulfide” mixture was suggested in the previously reported synthesis of VI.6 The electrochemical properties of I and I1 were studied by cyclic voltammetry. Figure 5a shows the cyclic voltammogram of I1 (MeCN, 0.1 M (BuN)C104); the initial potential sweep is anodic, with an initial potential of -1.0 V (vs SCE). The first cycle revealed an irreversible oxidation at E,, = -0.087 V, a somewhat broadened irreversible reduction a t E,, = -0.75 V, and an irreversible reduction at E,, = -1.84 V; subsequent cycles revealed an additional irreversible oxidation a t E,, = -0.86 V. More careful examination of this system revealed that the oxidation wave a t -0.86 V was not observed if the reduction at -1.84 V was not performed; similarly, the broad reduction at ca. -0.75 V was not observed if the oxidation at -0.087 V did not occur. These observations indicate that two additional electroactive species are produced upon reduction a t - 1.87

Barber et al.

3236 Organometallics, Vol. 14,No. 7,1995

V and oxidation at -0.087 V. The voltammogram can be interpreted in terms of the known chemistry of the FezSz(CO)6 species. The reduction a t -1.84 V is assumed t o correspond to the formation of [Fe2S2(C0)6I2(111) via eq 1, whereas the oxidation at -0.86 V cor-

[Fe4S4(C0)1212(11) + 2e- = 1 - 3 co

2[Fe2S2(C0)6]2- (111) (1)

Fe2S2(CO), (I)

+ 2e- = 1/2[Fe4S4(C0),,12-(11) (4)

responds t o re-oxidation of I11 and subsequent dimerization as shown in eq 2. Likewise, the oxidation wave at -0.087 V and the broad reduction wave at -0.75 V correspond to similar cleavage and dimerization reactions as shown in eqs 3 and 4, respectively. These four reactions are also assumed to explain the cyclic voltammogram of FezSz(C0)6 (Figure 5b), which displayed a broad, irreversible reduction at E,, % -1.0 V, a nearly irreversible reduction at E,, = -1.87 V,and an irreversible oxidation at -0.12 V that was absent if the first reduction at -1.0 V did not occur. By analogy to the cyclic voltammogram of 11, these electrode processes are assigned to eqs 4,1, and 3, respectively. The breadth of the reduction wave a t -1.0 V and the presence of a weak shoulder a t ca. -1.0 V in the oxidative sweep in the voltammogram of I (see Figure 5b) indicate that the chemistry at the electrode surface is complex; more detailed electrochemical studies would be required to allow meaningful statements to be made about the mechanisms of these electrode processes. The interpretation of this electrochemistry is supported both by the known chemistry of FezSz(CO)6 species and by the qualitative similarity of these voltammograms to that of diphenyl disulfide, which displays an irreversible reduction at -1.54 V and an irreversible oxidation a t -0.20 V assigned to the reactions shown in eqs 5 and 6,

+ 2e- = 2PhSPhS- = '/,PhSSPh + e-

PhSSPh

(5)

(6)

respectively.l8 Our electrochemical results demonstrate that 1-111 can be formed by purely electrochemical means and provide support for the idea that the role of the [FesS4(C0)121~-/~ couple in the reactions described in Table 4 is that of an electron-transfer reagent. The reaction of I with I1 to form VI provides a clear explanation of the reactions listed in Table 4 and of the published synthesis of VI from I and the 0-xylyl dithiolate-disulfide mixture.6 However, this reaction alone is far too slow to account for the production of VI by oxidation of IV. This reactivity could be responsible if the reaction between I and I1 were catalyzed by a third species in solution. Several postulated catalysts (H+(as (18)(a) Kriiger, H.-J.; Holm, R. H. Inorg. Chem. 1989,28,1148. (b) Bradbury, J . R.; Masters, A. F.; McDonell, A. C.; Brunette, A. A.; Bond, A. M.; Wedd, A. G. J . Am. Chem. SOC.1981,103, 1959.

1 - 3 co

Figure 6. Possible mechanism for formation of [Fe6s6(c0)1d2-, VI. CH3C02H), FeBr2, FeBr3, OH- (as (Me4N)OHin MeOH), OH-, and FeBr3 OH-) were added to FeBr2 equimolar solutions of I and 11, and the colors of the solutions were observed to determine qualitatively whether VI was formed. Both H+ and OH- were considered since the air and oxygen used in the previous reactions were not dried and perhaps contained traces of moisture; the Fe" and Fe"' salts were considered as possible side products in the degradation of IV, and FeOH- mixtures could conceivably result in the formation of catalytically active iron-oxo compounds. In no case was the immediate formation of VI noted upon addition of the potential catalyst. Upon addition of the Fe" solutions to the mixture an immediate bluish-green color was noted, consistent with the formation of V from Fe" and I1 (as described in the synthesis section). Thus, there is no evidence to suggest that the observed reaction of IV with 0 2 involves a catalyzed reaction between I and 11. Proposed Mechanisms for Cluster Formation. A straightforward mechanism can be proposed t o explain (VI)from FezSz(C0)6 the formation of [Fe6S~(C0)12]~(I) and [Fe4S4(C0)1212-(11). The mechanism, shown in Figure 6, features an initial oxidative decarbonylation of I1 by the disulfide bond of I; as mentioned in the Introduction, oxidative addition of I to metal carbonyls has been observed previ~usly.~ The intermediate thus formed then undergoes an intramolecular oxidative addition via the remaining disulfide bond to yield the final product. The first oxidative addition is most likely rate limiting since no intermediates can be detected in the reaction mixture. Intramolecular oxidative addition is a novel mode of reactivity recognized in our laboratory, and this reaction has been found to be important in the syntheses of several other Mo-Fe-S clusters.5b

+

+

Organometallics, Vol.14,No. 7,1995 3237

\I

-Fe-Fe-

+ YFe-FeI

I'

I/

I

'I'

-Fe -Fe 'I I\

(a)

2-

- Fe(CO -3

co

Figure 7. Possible mechanism for formation of [MoOFe&(C0)121~-. product argues for the importance of oxidative addition The observed importance of the oxidative addition in the cluster formation mechanism. chemistry of I1 led to a reexamination of the synthesis of [MoOFe&(C0)1~1~-.The reported synthesis of this Summary cluster5" involves reaction of 3 equiv of [FezSz(CO)6I2The novel clusters IV and V have been shown to be (111)with 1 equiv of the MoVspecies MoOC1dTHF)n and structurally analogous compounds related by a oneresults in the production of a significant amount of electron redox couple. Structural parameters suggest [Fe5S4(C0)12I2-(IV)as a byproduct. The desired prodthat the redox process is associated solely with the uct [MoOFe&(C0)1~1~-contains a central Fe"'(u-S)zFe"" couple of the central iron atom; the redox MoVOcore ligated by two FezSz(C0)6 ligands (see Chart potential of this couple (E1/2= -0.465 V vs NaCl SCE) 1). The presence of an [FezS~(CO)6lFe"'(u-s)~ unit is the highest yet reported for any Fe-S4 compound. (VI) strongly similar to that observed in [Fe6S6(C0)1~1~Clusters IV and V also represent one of the few suggests that an oxidative addition reaction is operative examples in which both the oxidized and reduced anain the formation of [MoOFe5S6(C0)12lz-. A plausible logs have been isolated and structurally characterized. mechanism for formation of this cluster is outlined in Cluster IV displays an unprecedented reactivity with Figure 7. The first two steps involve facile ligand 0 2 , which results in formation of VI. This transformadisplacement by [FezSz(C0)6I2- and [Fe4S4(C0)1zl2-to tion is not merely an outer-sphere oxidation but has yield intermediate b. Intramolecular oxidative addition been shown to be an On-specific process that must into Fe' then occurs, with the formal elimination of an volve a complicated rearrangement; therefore the lack [Fe(C0)31+species and formation of the observed prodof any observable intermediates is somewhat surprising. uct. Intermediate b is structurally similar to intermediThe clean reaction of I with I1 t o yield VI is too slow to ates proposed in the formation of [MoFe4S3(C0)1412-and be solely responsible for the observed reactivity of IV [ M O F ~ ~ S ~ ( C Ohowever, ) ~ ~ ] ~ in - ;those ~ ~ cases oxidative and is only a plausible mechanism if the reaction beaddition was proposed t o occur a t the low-valent Mo tween I and 11is catalyzed by some Fe-O/S species that centers. In this case, the intramolecular oxidative is present in the reaction mixture upon 0 2 oxidation. addition cannot occur at the high-valent MoVcenter and Finally, it is proposed that the species I1 plays a more so must occur at Fe as shown in Figure 7. significant role in reactions utilizing the dianion I11 The mechanism in Figure 7 requires 1equiv of I1 and than has previously been recognized. The proposed 1 equiv of I11 as opposed to the 3 equiv of I11 reported mechanisms for formation of VI and [MoOFe5S6(C0)1d2in the published synthesis. Therefore a solution of 3 demonstrate that oxidative addition may occur at either equiv of I was reacted with 4 equiv of Superhydride in Fe or Mo, dependent upon the oxidation states of the THF at -78 "C to produce formally the required mixture metal centers. This newly-discovered reactivity of of I1 and 111. Reaction of this mixture with 1equiv of [Fe&(C0)1~1~-makes the compound a potentially useMoOC13(THF)z resulted in the immediate formation of ful reagent in preparing a variety of mixed M-Fe-S a deep orange solution that gradually turned brown clusters having new stoichiometries and structures. upon warming to room temperature. The IR spectrum of the mother liquid in MeCN shows strong YCO bands Acknowledgment. We thank Dr. K. S. Bose and Dr. due to the expected product with additional very weak Steve Chmielewski for helpful discussions and advice. bands at 2080,2062,2016, and 2000 cm-'; no formation This research was supported by NSF Grant No. CHEof IV is observed, unlike the reported preparation 89-01474 to B.A.A. utilizing only the fully-reduced cluster 111. In addition, Supporting Information Available: Tables of positional because of the higher purity of the reaction mixture, the and thermal parameters and bond distances and angles for desired product can be isolated in >50%yield as opposed (Pr4N)21V,(Pr4N)V.0.5THF, and (PrJ92VI (21pages). Orderto routine yields of ca. 30% by the published procedure. ing information is given on any current masthead page. The success of the postulated mechanism in predicting OM950177F conditions that result in a higher yield of the desired