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Explorations of the reactivity of Fe/Mo/S clusters of some relevance to the FeMoco nitrogenase has led to new double-fused cubane clusters with the Mo2Fe6S8 core as derivatives of the known (Cl4-cat)2Mo2Fe6S8(PPr3)6 (l) fused double cubane. New clusters are obtained by substitution reactions of the PPr3 ligands with Cl(super -), BH4(super -) and N3(super -).

Research has been conducted on the uncharged bridged double cubane clusters. The synthesis and characterization of these clusters have been carried out and the details are described.

The synthesis of Mo/Fe/S clusters with the MoFe3S3 core in attempts to model the FeMo-cofactor in nitrogenase is detailed. The [MoFeS4](super 4) cuboidal subunits are reduced by one electron relative to the known [MoFe3S4](super 3) clusters and show shorter Fe-Fe and Fe-Mo distances, and the storage of reducing equivalents into M-M bonds and their use in the reduction of substrates may account for the presence of apparently extensive M-M bonding in the nitrogenase cofactor.

Research has been conducted on the high nuclearity Mo/Fe/S clusters. The synthesis of the heteronuclear MoFe (sub)3 S (sub)4 cubanes and their derivatives has been carried out and the results are reported.

A novel synthetic route to molecules containing cofacially oriented (Mo2O5)2+-bridged bis(catecholate) dianions is presented. The synthesis was particularly useful in the preparation of supermolecular molecules with catechol-functionalized, metalated macrocyclic ligands. Results indicate that the (Mo2O5(SALPHEN-catecholate)2)2- complexes have interesting reactivity properties which were unobserved in bis(porphyrin) complexes.

The behavior of the (L)MoFe3S4 cubanes as catalysts in the reduction of hydrazine to ammonia is investigated by inhibition measurements and x-ray structural analysis. Results indicate that the MoFe3S4 cores catalytically reduce N2H4 to NH3 with HCl as the proton source and Co(Cp) as the reducing agent. The data also suggest that reduction occurs at the Mo site, with carboxylate functions acting as local H+ delivery agents.

The study of the catalytic features of synthetic Fe/V/S clusters which are structurally identical to the Fe/V/S site of nitrogenase reveals that the (VFe3S4)2+ core is similar to that of the (MoFe3S4)3+ core. The (VFe3S4)2+ is an effective catalytic agent in the reduction of hydrazine to ammonia. The (VFe3S4)2+ also exhibits a decrease in the reduction efficiency when the liable sites are reduced.

The catalytic reduction of acetylene to ethylene and ethane are illustrated by using the (MoFe3S4)3+ synthetic cubanes. These synthetic cubanes are partial models for Mo/Fe cofactor of nitrogenase as they have the essential structural characteristics and reactivity features. The illustration reveals that the reduction reaction proceeds with first-order dependence on the proton concentration.

Syntheses of new single and double cubanes having the MoFe3S4 structural unit and Mo-bound polycarboxylate ligands involve reactions of various polycarboxylic acids with (Et4N)2(MoFe3S4Cl3(Cl4-cat)(CH3CN)) cluster. X-ray crystallography helps determine the structures of these compounds. Electrochemical and spectroscopic characteristics of these compounds reveal the similarity between these clusters with a Mo-coordination environment and those in the iron-Mo cofactor of nitrogenase.

The synthesis and structure of the first Mo/Fe/S/BH4 cluster is reported. A synopsis of the solution characterization as well as the reactivity of this cluster is also presented.

Replacement of Mo-bound ligands from the ((Cl4-cat)(CH3CN)MoFe3S4Cl3) by mercapto-carboxylic acids produces a new structural type of double cubanes. X-ray diffraction study helps examine the structure of a double cubane and the new cluster catalyzes the reduction of hydrazine. In these double cubanes two thiolate sulfur connects two MO atoms through mercapto-carboxylate anions serving as mu-eta(1)-eta(1) ligands.

Cu atoms can be structurally incorporated into Mo/Fe/S cluster, affording unique and unprecedented structures. It is demonstrated that the synthesis of heteropolynuclear clusters with different coordination geometries at different sites is possible by metal addition or metal substitution reactions.

The Fe nuclear resonance vibrational spectroscopy (NRVS) data for the FeCl(sub 4)(super -) ion in the solid state with NEt(sub 4)(super +) or P(C6H5)(sub 4)(super +) counterions and for the [Fe2S2Cl4](super 2-) cluster with NEt(sub 4)(super +) counterions and also their Raman and far-infrared (IR) spectra are described. The advantages of a combined NRVS, Raman, and IR approach to characterization of Fe-S complexes are discussed.

A systematic approach toward the synthesis of hetrometallic 'pentlandite-like' clusters is reported. It is concluded that specifically fictionalized MM'/S heterometallic clusters potentially can be building blocks for extended solids with unusual magnetic and electronics properties.

Interesting features of the nitrogenase cofactor, which includes short Fe-Fe distances in an electron deficient MoFe7S9 core, are presented. Density functional theory (DFT) calculations were carried out on model clusters to explore the Fe-Fe interactions in the cofactor.

Density functional theory (DFT) studies of the nitrogenase, to explore the origin and determine the nature of the central light atom and its importance as a structure-stabilizing feature are presented. The results of these studies and a possible mechanism of nitrogenase function are examined.

The reactions of [Mo(sub 2)Fe(sub 6)S(sub 8)](super 4+) with CO in dichloromethane (CH2Cl2) or tetrahydrofuran (THF) solution has led to the syntheses of a plethora of new clusters with the Roussin salt, MoFe3S3, incomplete cubane structure. The dissociation of the cubic subunits of [Mo(sub 2)Fe(sub 6)S(sub 8)](super 4+) make available the solvated [MoFeS2] and [Fe2S2] rhombic dimers and the association or reductive coupling of these dimers occurs to form the Fe4S4, Mo2Fe2S4 and MoFe3S4 cores.

Chemical research using X-ray crystal structure analysis has established the synthesis of (Cl4-cat)MoFe3S3(PEt3)2(CO)6 and (Cl4-cat)Mo(O)Fe3S3(PEt3)3(CO)5 clusters in the enzyme nitrogenase. The structure of each cluster is shown, together with interligand measurements. Structure to activity relationships are discussed.

The (Mo2O5)2+-bridged complexes bearing cofacially-oriented, catechol functionalized macrocyclic and SALPHEN ligands are synthesized at room temperature in oxygen-free CH2Cl2. Complexes (Bu4N)2 Mo2O5(NiTAD- O 2 )2 and (Bu4N)2 Mo2O5(Cu EtO2SALPHEN- O 2 )2 have cofacially oriented catecholate subunits and (Mo2O5)2+ bridging units in both crystal and molecular structures. The former has intramolecular Ni(1)-Ni(2) distance of 3.938 angstroms whereas the latter has Cu(1)-Cu(2) distance of 4.110 angstroms.

The mixed-ligand Fe4S4 cubanes of the type Fe4S4X(t-Bu3P)3 (X = Cl, 1; Br, 2; I, 3) are synthesized by adding Pt-Bu3 to an acetonitrile solution of the specific (Bu4N)2(Fe4S4X4) cluster, followed by the addition of NaBPh4. The molecules with the reduced (Fe4S4)+ core are the uncharged iron-sulfur cubanes and posses a C3v symmetry. The mean bond lengths of Fe-S and Fe-Fe are 2.297(7) and 2.768(10) degree Angstroms respectively. The molecules 1, 2 and 3 have S = 1/2 in the ground state and 1 exhibit antiferromagnetic coupling.

The EPR analysis of single and double cubane catalyzed reduction of hydrazine to ammonia reveals that the MoFe3S4 core in the cubanes remains intact after the reduction, implying that the core is directly involved in the catalysis. The MoFe3S4 clusters are structurally similar to the Fe/Mo/S nitrogenase center and easily catalyze N2H4 reduction, implying that the Mo atom is directly involved in Fe/Mo/S center-catalyzed reductions. The two single cubanes, ((Cl4-cat)MoFe3S4(Cl3.L))n- and ((citr)MoFe3S4(Cl3))3-, are the most active catalytic cubanes.

A catechol group that can aid other metalions as a bidentate ligand is found in the new binucleating ligand 2, 3-ethylene-5,6. The tetranuclear ((M(ETC)) (Mo(O)) (mu-S)2-MoC(O))2- or the trinuclear (M(ETC)) (MoC(O) (mu-S)2 Mo(O)(S2))2- complex is produced by the deprotonated catechol functionality of the M(II) complexes of the ligand, when the DMF ligands are displaced from either the ((DMF)3 MoC(O) (micro-S)2 MoC(O) (DMF)3)2+ or the ((DMF)3 MoC(O) (mu-S)2 MoC(O)(S2)) complex. The usefulness of ETC as a binucleating ligand is revealed by the crystal structure of the (E+N-4) (K) (Ni(ETC) (MoC(O) (micro-S)2 MoC(O))-(S2)) salt.