Inorg. Chem. 1981, 20. 2661-2676 carbonyls, the National Science Foundation for a grant in support of this research, and the University of California a t Berkeley Mass Spectrometric Facility for mass spectrometric data. We also thank the Miller Institute for Basic Research in Science for a grant in the form of a Miller Professorship (to E.L.M.). Registry NO.H2, 1333-74-0;CO, 630-08-0; O S ~ ( C O )15696-40-9; ~~,
2667
BBr3, 10294-33-4; BCI3, 10294-34-5; CH3Br, 74-83-9; C2HSBr,7496-4; o ~ ~ ( C o ) ~17731-88-3; Br~, O S ( C O ) ~ B ~ ~ ( N C C77590-16-0; H~), oS3(co)l&l2, 28109-18-4; os2(co)6cI4, 17633-40-8; CH4, 74-82-8; C2H4, 74-85-1; C2H6, 74-84-0; C3H8, 74-98-6; i-C4HIO,75-28-5; neo-C5HI2,463-82-1. Supplementary Material Available: Figures 1-3, the mass spectra of O S ~ ( C O ) ~os2(co)6c14, B~~, and O S ~ ( C O ) ~(3 ~C pages). I ~ Ordering information is given on any current masthead page.
Contribution from the Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545
Synthesis, Characterization, and Reactions of Iron-Sulfur Clusters Containing the S2 Ligand: [Cp2Fe2(S2)(SR)2]0%1+, [Cp4Fe4S5]0i1+*2+, and [Cp4Fe4S6] GREGORY J. KUBAS* and PHILLIP J. VERGAMINI Received September 2, 1980 Syntheses of the following disulfide-containing dinuclear and tetranuclear organoiron compounds are described: [Cp2Fe2(S2)(SR)2]0.'t,[Cp4Fe4Ss]0-1t*2t,and [Cp4Fe4S6](Cp = q5-C5HS).The compounds have been characterized by spectroscopic, electrochemical, and chemical properties. Temperature-dependent 'H NMR spectroscopy indicates fluxionality of the triply bridging disulfide ligand in Cp4Fe4Ssn(n = 0 and 2+). The cluster products resulting when these disulfide-containing complexes are chemically or electrochemically oxidized, including [CpFe(SR)(NCMe)] 22t, and adducts formed with other metals via the electron-rich disulfide ligands are discussed. Introduction
The extreme versatility of sulfur as a ligand has been well established. A variety of complexes containing elemental sulfur ( S , Sz, Ss,etc.) have been reported, and new modes of bonding have recently been characterized.' One reason for our interest in such systems is the potential for a rich and extensive redox chemistry and the differences in structure observable for pairs of a redox couple. Structural studies of the members of a reversible redox couple can lead to a better understanding of the bonding of these materials as well as their physical and chemical properties. Previous studies have shown that the structural features observed to vary upon changing the molecular oxidation state of transition-metal clusters usually involve metal-metal interactions.2 This has been demonstrated, for example, in the case of [CpzFe2(SEt)2(Sz)] ,3 which possesses an unprecendented planar FeSSFe geometry (Figure l ) . 4 There are several features which we have found interesting in this and other disulfide-containing species. Among these are the following: (1) they undergo several electron oxidation state changes without complete structural change; ( 2 ) the disulfur ligands are electron rich and have the ability to form donor-acceptor complexes; (3) an understanding of the reactivity of these coordinated ligands could lead to an understanding of the mechanisms of some processes involving metal clusters, such as catalysis. In part, a motivation behind these studies resides in the considerable interest in new modes of cluster configurations as potential models for the active sites in complex metalloenzymes, nature's catalysts. Obviously, a better understanding (1) (a) Vahrenkamp, H. Angew. Chem., In?. Ed. Engl. 1975.14, 322. (b) Muller, A. Inorg. Chem. 1979, 18, 2631. (c) Vergamini, P. J.; Kubas, G. J. Prog. Inorg. Chem. 1976, 21, 261. (2) Connelly, N. G.; Dahl, L. F. J. Am. Chem. SOC.1970, 92, 7472. (3) Vergamini, P. J.; Ryan, R. R.; Kubas, G. J. J . A m . Chem. SOC.1976, 98, 1980. (4) (a) Kubas, G. J.; Spiro, T. G . ;Terzis, A. J . Am. Chem. SOC.1973, 95, 273. (b) Terzis, A,; Rivest, R. Inorg. Chem. 1973, 12, 2132.
0020-1669/S1/1320-2667$01.25/0
of the relationship between the structure of the metal centers in complex biochemical systems and their function could lead to new materials for energy storage, energy transfer, and catalysis . Although we feel it is premature to attempt a direct comparison of the configurations demonstrated by the compounds reported here and specific biochemical species, we feel that disulfur-containing clusters should not be ruled out as being involved in some active sites of metalloenzymes. We have directed our efforts to the synthesis and characterization of dinuclear and tetranuclear organoiron-containing cluster compounds which feature bridging disulfur (S2)ligands. Resulting compounds reported here include the dinuclear species [Cp2Fe2(S2)(SR),]"[R = Me (la), Et (lb), Bzl (IC); n = 0, l+; Cp = a-C5H5] and the tetranuclear species [Cp4Fe4SS]"(5; n = 0, 1+, 2+) and [Cp4Fe4S,] ( 6 ) . Some chemical and electrochemical properties of these compounds are described as well. We also report variable-temperature 'H N M R evidence for fluxionality in the bonding of the triply bridging disulfide ligand in 5 and its corresponding dication [51z+. Experimental Section Materials and Methods. [CpFe(CO)2]2was obtained from Strem Chemical Co. or Pressure Chemical Co. and was recrystallized from CH2CI2-hexane under nitrogen before use. [CpFe(C0)2]2is nearly black when purified in this manner, and its solutions are slightly air sensitive. Preparations of [CpFe(CO)(SR)], were based on the literature method^.^ Except as noted, all commercially purchased solvents and chemicals were used as received. PF; and SbF, salts were obtained from the Ozark-Mahoning Co. The reactions described below have been carried out at an altitude of 7100 ft (atmospheric pressure -590 mmHg) except for those involving Et2&. The boiling points of solvents (hence reflux temperatures) are lower than the values at standard pressure. All reactions and manipulations of solutions were performed in a nitrogen atmosphere, except where noted. ( 5 ) (a) King, R. B.; Bisnette, M. B. Inorg. Chem. 1965,4,482. (b) Ahmad,
M.; Bruce, R.; Knox, G. R. J . Organomer. Chem. 1966, 4, 1 .
0 1981 American Chemical Society
2668 Inorganic Chemistry, Vol. 20, No. 8, 1981
Kubas and Vergamini
z
V
Figure 1. Molecular structure of l b , as reported in ref 4. Physical Measurements. Elemental analyses were performed by Galbraith Laboratories and are given in Table I along with conductivity data which were measured with a Barnstead Model PM 70CB conductivity bridge. Proton N M R and infrared spectra were recorded on Varian A-60, EM-360, and EM-390 and Perkin-Elmer 521 spectrometers and are given in Table 11. The UV-vis spectra were taken on a Cary 14 instrument, and the corresponding data are given in Table 111. Mass spectra were obtained by using a CED 21-1 10 instrument with the sample introduced via a heated direct-introduction probe a t 160 "C. Electrochemical experiments were carried out on a Princeton Applied Research Model 170 electrochemistry system with conventional three-electrode configurations. Prepration of Ethyl PolysuVdes. The method used by Bloomfield6 for preparing diethyl tetrasulfide was followed. A solution of 32 mL (0.4 mol) of sulfur monochloride (Research Organic Chemicals) in 250 mL of CS2 was slowly added to a solution of 62 mL (0.84 mol) of ethanethiol (Alrich Chemical Co.) in 550 mL of CS2. As soon as the brisk evolution of HCI had subsided, the solution was gently boiled for 30 min under reflux. The solvent was removed under reduced pressure, and the residue was vacuum distilled at approximately 0.2 torr. The first 6 mL of distillate was discarded, the next 35 mL were collected in the boiling range 62-72 "C, and a final 12-mL fraction boiled at 72-74 "C. Elemental analysis indicated that the 62-72 "C fraction (sp gr 1.15 g/mL) consisted of approximately 70% Et2& and 30% Et2S4,while the 72-74 "C fraction was presumably nearly pure EtzS4. These compounds possess strong, persistent stenches and should be manipulated entirely within a fume hood. Syntbeses. In many cases a given reaction sequence describes the isolation of one or more compounds which were later found to be produced by more convenient rational synthetic routes, resulting in higher yields. In some instances it was found that some of the compounds could be readily interconverted. These are noted where appropriate. Reaction of Et2& with [CpFe(CO)2]2.Formation and Isolation of Iron-Sulfur Clusters. When 8 mL (9.2 g) of the polysulfide mixture containing primarily Et2& [CpFe(C0)2]z(14 g, 41 mmol), and 170 mL of methylcyclohexane were refluxed for 11 h (or benzene for 23 h) with magnetic stirring, a dark green solution was produced, and a black precipitate formed. The reaction mixture was cooled and filtered. The solution was chromatographed on a 3 X 70 cm alumina column. Development with benzene saturated with nitrogen produced two closely spaced broad bands: a deep blue band containing Cp2Fe2(S2)(SEt), (lb) followed by a green band due to Cp3Fe3S2(SEt)(3). The compounds were isolated by the following sequence of operations: removal of solvent from the benzene eluates, dissolution of the resulting black residue in 20 mL of CS2, filtration, partial solvent removal, addition of 25 mL of hexane, and reduction of volume to 10 mL. The yields based on [CpFe(CO)2]2were 0.5 g (3%) for l b (mp 154-56 "C) and 0.4 g (3%) for 3 (mp 185 "C dec). These complexes are soluble in most organic solvents, especially CS2 and CH2CI2,and give green solutions which slowly decompose in the presence of air. They are air stable in the solid state, and large, well-formed single crystals of l b can easily be grown by slow cooling of CS2-hexane solutions under nitrogen. When the above-described reaction in benzene was stopped after 12 h, the reaction mixture was found to contain [CpFe(CO)(SEt)12as identified by infrared and NMRSbspectroscopy. The black precipitate that formed in a reaction of [CpFe(CO)z]2 (7 g) and ethyl polysulfide (16 mL) in refluxing benzene (70 mL) for 19 h was extracted with 50 mL of chloroform. Removal of solvent yielded a black solid (0.5 g, 8% yield), the infrared spectrum of which
s 3
m W
2
m
c)
E
n 3 0
-
(6) Bloomfield, G . F. J . Chem. SOC.1947, 1547.
I".
ICA
*J IW
Inorganic Chemistry, Vol, 20, No. 8, 1981 2669
Iron-Sulfur Clusters Containing the S2 Ligand Table 11. lnfrared and Proton NMR Data IR,“ cm-’ compd
S-S
lb
IC
other
S-R
1294d 124Y 1227e
513 507 516
la
NMR,~ CP 5.29 (s) 5.27 (s) 5.67 (s)
2a 2b
23OlR 2299R
4.62 (s) 4.62 (s)
2c
229V
4.68 (s)
3
1249e 5 25 484 497,509 470.483
5
5.2S0, 6 6.2S0,
5.45 (s), 6.30 (s) 5.12 (s), 5.57 (s) 1263, 1096, 536h 5.90 (s). 5.48 (s) 5.18 (s), 5.67 (s) 1250, 1090, 533h 5.15 (s), 5.65 (SI
R
re1 intensC
10.05 (s, CH,) 9.87 (m, C2H,) 3.02 (m), 3.44 (m, C6H5), 8.68 (s, CH,) 7.74 (s, CH,)I 7.51 (4, 7.3; CH,), 8.25 (t, 7.3, CH,)! 2.40 (m, C H,), 6:23 (s, C W f 6 9.0 (m, C,H,)
5:3 1:l 5:5:2 5:3 5:2:3 5:5:2 1:2:1 1:3 1:3 1: 1 1:l
1249e
1254e 2272K 2259
4.15 (s), 4.20 (s), 4.83 (s)
12S8e 2056’
3.93 (s)
2:l:l 6.55 (m, CH,), 8.23 (t, 7.3, CH,)
5:2:3
For Nujol mulls between CsBr plates. s = singlet, t = triplet, q = quartet, and m = multiplet; multiplicities, coupling constants (in hertz), and assignments are given in parentheses; reference Me,Si. Solvents: CS, for la-lc and 3;CDC1, for 5 , 6 , 5.2S02, and 6.2S02;CD,CN for the rest. ‘H NMR signal ratios. S-CH s mmetric deformation. e S-CH,wag. ‘H NMR signals due to coordinated CH,CN occur at 7 7.93, 7.90, and 7.80, respectively, for 2a-c., ;v(CN). v ( S 0 ) . v(C0). Table 111. Electronic Spectral Data compd
concqUM
la
6.7 X
lb
7.0 X
2b
4.1
[CpFe(CO)(SEt)I, [Cp,Fe,(S,)(SEt),] [PF,] [Cp,F~e,S,I [ P F J 2 Acetonitrile solution. sh = shoulder.
7.1 X 8.4 X 1.6 X
X
maxima,b nm
235 (25), 305 (111, 475 (2.4), 601 (2.0), 837 (3.5) low4 233 (28), 309 (121, 479 (2.8), 600 (2.1), 840 (4.4) lo-, 247 (24), 286 (lo), 337 (12), 402 (6.1) 319 (131, 460(2.4) sh lo-’ 660 (2.7) 10.’ 436 (4.4) sh, 660 (0.8) sh
Numbers in parentheses are e/lOOO;
displayed no carbonyl stretching frequencies. However, its NMR spectrum in CDC13showed four peaks in the cyclopentadienyl region, which were found to correspond to approximately equal amounts of Cp4Fe4S5( 5 ) and Cp4Fe4S6( 6 ) . Isolation of 6 was effected by extraction of the black solid with acetone in a Soxhlet extractor for 2 days. The N M R spectrum of the resulting precipitate exhibited two sharp peaks of equal intensity at T 5.18 and 5.67 due to 6 and weak peaks at T 5.12 and 5.57 due to 5. The composition of 6 was also substantiated by single-crystal X-ray diffraction The undissolved solid remaining after chloroform extraction was treated with about 100 mL of boiling bromobenzene. Upon being cooled to 0 “ C and allowed to stand for several days, the filtered bromobenzene solution deposited a black solid which was shown to be [CpFeS], (4) by infrared spectroscopy. Reaction of the polysulfide fraction containing primarily with [CpFe(CO),], in refluxing benzene for 23 h gave only barely detectable amounts of l b and 3. Preparation of 1 by Reaction of [CpFe(CO)(SR)I2 with SP A magnetically stirred mixture of [CpFe(CO)2]2(30 g, 84 mmol), Et2S2 (51 g, 320 mmol), and methylcyclohexane (350 mL) was refluxed for 16 h. The resulting solution of [CpFe(CO)(SEt)12was cooled, filtered to remove a black precipitate (pyrophoric when dry), and refluxed with ss (13.2 g, 51 mmol) for 3.5 h. After the reaction mixture was allowed to cool to about 40 “C. it was filtered to remove (7) Vergamini, P. J.; Ryan, R. R.; Kubas, G. J., to be submitted for pub-
lication.
a black precipitate (which was later found to contain small amounts of 4-6) and placed onto an alumina column (4 X 45 cm). Elution with deoxygenated benzene gave an initial yellow band of ferrocene, which was discarded, closely followed by the characteristic deep blue band of lb. No green band due to 3 was observed. The benzene eluate containing l b was collected, and the solvent was removed in vacuo. The residue was dissolved in about 30 mL of CS2 and filtered. After partial solvent removal, 40 mL of ethanol was added, and the volume of the solution was reduced to 30-40 mL. The black crystalline precipitate of l b was collected on a frit, washed with a small amount of hexane, and air-dried. This material weighed 2.7 g (8% yield based on [CpFe(CO),I2). la and IC were prepared in an identical fashion. IC was difficult to separate from unreacted Bz12S2,even with the use of chromatography. A pure sample of IC was obtained by oxidation of the crude material to [C~F~(NCCH,)(~BZ)]~[PF~]~ (2c) and reaction of the latter with aqueous polysulfide (see below). The products of the reactions of [CpFe(CO)(SPh)], and [CpFe(CO)(SeEt)]: with Sswere intractable. Improved Preparation of 1. Better yields of 1 were obtained by a procedure which utilizes 2 (see below) as starting material. The example given is for lb. A mixture of 20 mL of ammonium polysulfide solution (J. T. Baker, 20% “(NHJ2S”) and 20 mL of methanol was added dropwise to a vigorously stirred solution of 2b (7.36 g) in CH3CN (300 mL). The solution was stirred an additional 30 min, during which time it became gray-green and deposited a brown precipitate. After all solvent was removed in vacuo, the residue was extracted with 150 mL of CS2. Solvent removal from the filtered extract afforded 3.2 g of crude lb, which was purified by chromatography on alumina (4 X 20 cm). The compound, dissolved in a minimum amount of CS2, was placed onto the column and eluted with deoxygenated benzene. The yield of lb, which was isolated from the eluate as described above, was 2.7 g (63%). Aqueous sodium sulfide can be used instead of ammonium polysulfide in the preparation, but the resulting yield of 1 is lower (40%) unless the Na2S solution contains some dissolved sulfur (- ‘ / g mol of Ss/mol of Na2S). Addition of a 2:l molar ratio of “Na2Sp to 2 gave a 61% yield in the case of l b and a 44% yield in the case of IC. Reaction of 2 with aqueous Na2Se gave an intractable product. Preparation of [CpFe(NCCH3)(SR)]2[PF6]2 (2) from [CpFe(CO)(SR)I2. Examples are given here for a large-scale preparation of 2b and a small-scale preparation of 2a. A magnetically stirred mixture of 49.4 g of [ C P F ~ ( C O ) ~85 ] ~mL , of Et2S2,and 600 mL of (8) Rosenbuch, P.; Welcman, N. J. Chem. SOC.A 1972, 1963.
2670 Inorganic Chemistry, Vol. 20, No. 8, 1981 methylcyclohexane was refluxed for 16 h. The reaction mixture was cooled and filtered (Caution: the precipitate removed here is pyrophoric when dry), and all volatiles were removed in vacuo (0.1 torr). The residue (52.7 g), containing [CpFe(C0)(SEt)l2,was placed into a polyethylene container' along with 100 g of ",PF, (99.5%) and 1500 mL of CH3CN (Eastman,
