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Density functional theory (DFT) calculations have been performed on the nitrogenase cofactor, FeMoco. Issues that have been addressed concern the nature of M-M interactions and the identity and origin of the central light atom, revealed in a recent crystallographic study of the FeMo protein of nitrogenase (Einsle, O.; et al. Science 2002, 297, 871). Introduction of Se in place of the S atoms in the cofactor and energy minimization results in an optimized structure very similar to that in the native enzyme. The nearly identical, short, lengths of the Fe-Fe distances in the Se and S analogues are interpreted in terms of M-M weak bonding interactions. DFT calculations with O or N as the central atoms in the FeMoco marginally support the assignment of the central atom as N rather than O. The assumption was made that the central atom is the N atom, and steps of a catalytic cycle were calculated starting with either of two possible states for the cofactor and maintaining the same charge throughout (by addition of equal numbers of H+ and e-) between steps. The states were [(CI)FeII 6FeIII MOIVS9(H+)3N3-(GI)(Im)]2-, [I-N-3H]2-, and ((Cl)FeII 4FeIII 3- MoIVS9(H+)3N3(GI)(Im)], [I-N-3H]0 (GI = deprotonated glycol; Im = imidazole). These are the triply protonated ENDOR/ESEEM [I-N]5- and Mõssbauer [I-N]3- models, respectively. [ABSTRACT FROM AUTHOR]

The reaction of the (Et4N)2[(Cl4-cat)Mo(MeCN)Fe3S4Cl3] cluster (I), with CO in the presence of Fe(PEt3)2Cl2 or PEt3–NaBPh4 in MeCN solution produced the new (Cl4-cat)Mo(PEt3)2(CO)2 complex (II), the (Cl4-cat)Mo(PEt3)Fe3S4(SPEt3)2Cl cluster (III), and the (Cl4-cat)2MoFe2S2(PEt3)2(CO)4 cluster (IV) as well as the known (Cl4-cat)Mo(O)Fe3S3(PEt3)3(CO)5 cluster. All compounds were successfully isolated as crystalline material by chromatographic separation and structurally characterized. The structural and Mo¨ssbauer features of the neutral cluster III with a [MoFe3S4]3+ core were very similar to those of the [MoFe3S4]2+ clusters. The unique S&z.dbnd6;PEt3 ligands of III seem to act as electron reservoirs through σ and π-interactions with the Fe atoms. An increase of electron density is evident around the formal [MoFe3S4]3+ core. Synthetic pathways that lead to the new compounds are also discussed. [Copyright &y& Elsevier]

New organometallic clusters with the MFe2(mu3-S)2 core (M = Mo or Fe) have been synthesized from inorganic [MoFe3S4] or [Fe4S4] clusters under high pressure CO. The reaction of (Cl4-cat)2Mo2Fe6S8(PR3)6[R = Et, (n)Pr] with high pressure CO produced the crystalline [MoFe2S2]4+ clusters, (Cl4-cat)Mo(O)Fe2S2(CO)(n)(PR3)6-n[n= 4, Et =I, (n)Pr =II; n = 5, Et =III] after flash column chromatography. The similar [MoFe2S2]4+ cluster, (Cl4-cat)2MoFe2S2(CO)2(depe)(2)(IV), also has been achieved by the reactions of (Cl4-cat)MoFe3S3(CO)6(PEt3)2 with depe by reductive decoupling of the cluster. For the [Fe3(mu3-S)2]4+ cluster, [Fe4S4(PcHex3)4](BPh4) was reacted with high pressure CO to produce a new Fe3S2(CO)7(PcHex)(2)(V) compound. These reactions generalized the preparation of organometallic compounds from inorganic clusters. All the compounds have been characterized by single crystal X-ray crystallography. A possible reaction pathway for the synthesis of the MFe2(mu3-S) clusters (M = Mo or Fe) has also been suggested.

Density functional theory calculations were carried out on the structurally characterized [(Cl(4)-cat)Mo(py)Fe(3)S(3) (CO)(4)(P(n)Pr(3))(3)], A, and (Cl(4)-cat)Mo(py)Fe(3)S(3)(CO)(6)(PEt(3))(2), B, and also on A(2)(-) and B(2+) clusters. The Fe-Fe distances in these molecules depend on the total number of valence electrons (60 e(-) in A and B(2)(+) and 62 e(-) in A(2)(-) and B) and undergo great structural changes upon addition or removal of electrons. The changes are consistent with known electron-counting rules in organometallic chemistry. The weak nature of the Fe-Fe bonding interactions in these clusters is apparent in the very similar energies of states with widely different Fe-Fe distances.