The influence of tertiary phosphines on the stability of FeS3P coordination units and the formation of iron−sulfur clusters has been investigated. Reaction of [Fe4S4Cl4]2- with a small excess of PR3 in acetonitrile/THF affords the cubane-type clusters [Fe4S4(PR3)4]1+ (R = Cy, But, Pri), one-electron reduced over the initial cluster and possessing an S = 1/2 ground state. These clusters may be electrochemically oxidized to [Fe4S4(PR3)4]2+ and reduced to [Fe4S4(PR3)4], which can also be generated in solution by chemical reduction. The neutral clusters upon standing in solution lose phosphine and aggregate to form dicubane ([Fe8S8(PCy3)6]) or tetracubane ([Fe16S16(PR3)8]; R = But, Pri) clusters. The [Fe8S8]0 dicubane core has two intercubane Fe−S bonds, defining an Fe2S2 rhomb and affording a structure of overall idealized C 2h symmetry. The tetracubane clusters consist of a cyclic array of four cubanes joined in four Fe2S2 rhombs in a structure of overall D 4 symmetry, and present a new structural motif in Fe−S cluster chemistry. Tertiary phosphines impose two significant features on this cluster chemistry. These ligands significantly stabilize the [Fe4S4]1+/0 core oxidation levels compared to the case of conventional [Fe4S4L4]3-,4- clusters (L = monoanion). Ligands with cone angles exceeding that of PEt3 (132°) favor tetrahedral FeS3P coordination sites. This has the effect of directing reactions away from the formation of Fe6S6 (four trigonal pyramidal) and Fe6S8 (six square pyramidal) clusters having the indicated sites which are disfavored by large cone angles. Structural principles governing polycubane clusters together with a brief enumeration of stereochemically feasible polycubanes are presented and discussed.

A new class of clusters, sulfide-bridged double cubanes containing the units MFe3(μ3-S)4 (M = Fe, Mo), has been investigated as possible synthetic precursors to the iron−molybdenum cofactor (FeMoco) of nitrogenase. Clusters containing the symmetric core structures [Fe4Q4-Q-Fe4Q4]2+ (11−13, Q = S, Se) have been prepared by the coupling of separate cubane clusters ([Fe4S4Cl4]2-, [Fe4Q4(LS3)Cl]2-, where LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzenate(3−)). Similarly, the cuboidal cluster [VFe4S6(PEt3)4Cl] was coupled to form [VFe4S6(PEt3)4]2S (14). The cluster [(Fe4S4Cl3)2S]4- (11) is the first structurally proven example of the class (Challen, P. R.; Koo, S.-M.; Dunham, W. R.; Coucouvanis, D. J. Am. Chem. Soc. 1990, 112, 2455). Bridged structures have also been established crystallographically for {[Fe4Se4(LS3)]2Se}4- (13) and 14. Other criteria for identification of this structure developed using 11−14 are coupled redox processes in cyclic voltammetry and the detection of intact double cubane ions by electrospray mass spectrometry. For the coupling of heterometal cubanes, the Mo site was protected by chelation with Meida (N-methylimidodiacetate(2−)) as in [(Meida)MoFe3S4Cl3]2- (6), thereby directing the bridging reaction to the Fe sites. The structures of two [VFe3S4]2+ cubane clusters containing the tricoordinate (Meida)V fragment are reported. The reaction system 6/Li2S afforded {[(Meida)MoFe3S4Cl2]2S}4- (15). The symmetrical sulfide-bridged double cubane structure of 15 has been established by the electrochemical and mass spectrometric criteria and by the existence of the four isomers consistent with this structure. The equimolar reaction system 6/[Fe4S4Cl4]2-/Li2S produced a mixture of {[(Meida)MoFe3S4Cl2]S(Fe4S4Cl3)}4- (17), 15, and 11. Cluster 17 is also formed in the system 15/[Fe4S4Cl4]2-. The unsymmetrical sulfide-bridged double cubane structure of 17 was established by mass spectrometry and detection of the two isomers consistent with this structure. In the first reaction system, the product mole ratio 17:15 ≈ 3:1 is explained in terms of differential steric hindrance of conformations arising from rotation around the Fe−S−Fe bridge. The core composition of double cubane 15 (Mo2Fe6S9) approaches that of FeMoco (MoFe7S9). The core composition of 17 is exactly the same as FeMoco; 17 is the first synthetic cluster with this property.

Treatment of oxidized clusters [(Cl4cat)(MeCN)MoFe3S4Cl3]2- (1) and [(Meida)MoFe3S4Cl3]2- (2) with tertiary phosphines in the presence of NaBPh4 in acetonitrile results in chloride substitution at the iron sites and the formation of clusters with the reduced [MoFe3S4]2+ core. Thus, 1 is a precursor to [(Cl4cat)(MeCN)MoFe3S4(PR3)3] (R = But (3), Pri (4)) and [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4] (5). Cluster 2 affords [[(Meida)MoFe3S4(PCy3)3]4Fe2(mu-Cl)L2]3+ (L = THF (6), MeCN (7)). The structures of 3-7 were established by X-ray analysis. Clusters 3 and 4 are single cubanes, centrosymmetric 5 (previously reported in a different space group: Demadis, K. D.; Campana, C. F.; Coucouvanis, D. J. Am. Chem. Soc. 1995, 117, 7832) is a double cubane with a rhomboidal Fe2S2 bridge, and 6 and 7 are tetracubanes. In the latter, four Meida oxygen atoms from different cubanes bind each of two central high-spin Fe(II) atoms in trans-Fe(mu-Cl)LO4 coordination. The topology of these clusters is not precedented. Zero-field Mössbauer parameters for all clusters are reported. Isomer shift considerations suggest the formulation [Mo3+Fe2+2Fe3+S4] for reduced clusters. Voltammetry of 3 and 4 reveals four-member electron transfer series encompassing the oxidation levels [MoFe3S4]4+,3+,2+,+ in the potential interval + 1.0 to -1.3 V vs SCE in dichloromethane. Compared to the clusters with monoanionic ligands at the iron sites, phosphine ligation shifts redox potentials to more positive values. This effect arises from reduction of cluster negative charge and the tendency of phosphines to stabilize lower oxidation states. The synthesis of reduced clusters 4 from 1 and of [Fe4S4(PPri3)4]+ from [Fe4S4Cl4]2- is accompanied by the formation of Pri3PS, detected by 31P NMR, indicating that the phosphine is the reductant. This result implies a similar function of tertiary phosphines in the synthesis of 3 and 5-7. (Cl4cat = tetrachlorocatecholate(2-); Meida = N-methyliminodiacetate(2-).)

No abstract

Crystallographic investigations of the MoFe proteins of nitrogenase have recently led to detailed structural information on their P-cluster and FeMo-cofactor cluster constituents.' Both clusters are octanuclear, the former consisting of two Fe& cubane cores linked via dual cysteinate bridges as well as by either an intercore S -S bondla.b or a commonly-shared core S atom,Ic and the latter of two cuboidal cores (one Fe& and one MoFe3S3) joined by three bridging pz-S atoms. In the interest of further understanding how these clusters are involved in the dinitrogen-fixing role of the enzymes, chemists are now presented with the task of preparing synthetic analogues, a formidable challenge in view of the unprecedented features inherent to the two bridged structures.2 The direct coupling of two iron-sulfur cubane type clusters has previously been achieved primarily in three different structural variations: Fed& cores connected by a single p2-S atom,3 MoFe3S4 cores connected by one p2-S atom linking Fe atoms and one p2-X ligand (X = S2-, OH-, CN-, N2&) linking octahedrally coordinated Mo atoms? and MFe3S4 cores (M = V, Nb, Mo, W, Re) connected by three pz-(SR) groups linking octahedrally coordinated M a t o m~.~f ' Herein, we describe two octanuclear clusters, [FesSs(PCy&] (Cy = cyclohexyl) and [Fe&2(CNt-Bu)12], each with a new mode of coupling between their respective tetranuclear core subunits, and each of potential utility as a precursor to the nitrogenase clusters.A mixture of [Fe(PEt&C12l7 (4.02 g, 11.0 "01) and NaSPh (2.88 g, 22.0 m o l ) was stirred in 50 mL of benzene for 2 h to give a deep red solution.8 Addition of benzyl trisulfide (4.60 g, 16.5 mmol), followed by PCy3 (7.00 g, 25.1 mmol), to the filtered solution induced an immediate color change to brown accompanied by formation of a black precipitate: Upon benzene removal, the residue was extracted with 80 mL of toluene, producing a dark brown solution: which was filtered, reduced in volume to 40 mL, and allowed to stand for up to 6 days. Crystalline [FesSs(PCy3)6]C7Hs (0.71 g, 21 %) was recovered (1) (a) Chan, M. K.; Kim, J.; Rees, D. [Fe(PEt3)2C12] was prepared by stoichiometric reaction of FeC12 and PEt3 in THF. 'H NMR (C6D6): 6 75.8 (CHz), 2.2 (CH3). (8) All reactions were performed under a pure dinitrogen atmosphere at room temperature. (9) The FAB mass spectrum of this solution exhibits a parent peak at m/z = 1473 corresponding to an unprecedented [Fe4S4(PCy3)4] cluster. Presumably, loss of one PCy3 upon standing facilitates dimerization and subsequent precipitation as the sparingly soluble dicubane product. 0002-786319511517-8863$09.00/0 Figure 1. Core structure (50% probability ellipsoids) and atom-labeling scheme of the dicubane cluster [Ft$s(PCy&]. The cluster resides on an inversion center located in the middle of the Fez& rhomb linking its two cubane components. Selected intercubane interatomic distances (8) and angles (deg): Fe-Fe 2.