SYNTHESIS AND STRUCTURES OF THE [MOFE6S6(CO)16]2-, [MOFE4S3(CO)13(PET3)]2-, AND [MO2FE2S2(CO)12]2- IONS - HIGH-NUCLEARITY MO-FE-S CLUSTERS AS POTENTIAL PRECURSORS TO MODELS FOR THE FEMO-COFACTOR OF NITROGENASE

Reaction of [Mo(CO)4I3]- with 2-3 equiv of [Fe2S2(CO)6]2- in THF produces two new Mo-Fe-S-CO clusters, the [MoFe6S6(CO)16]2- (I) and [MoFe4S3(CO)14]2- (II) ions, both of which have been structurally characterized, the latter as its mono(triethylphosphine) substitution product, [MoFe4S3(CO)13(PEt3)]2- (II-P). Crystallographic data: (Ph4As)2(I), triclinic, P1BAR (No. 2), Z = 2, a = 12.473 (12) angstrom, b = 12.836 (13) angstrom, c = 22.360 (22) angstrom, alpha = 90.96 (2)degrees, beta = 97.58 (2)degrees, gamma = 99.52 (2)degrees, V = 3497 angstrom-3, R = 0.083, R(w) = 0.094, 9146 independent reflections with I > 2.5-sigma(I); (Et4N)2(I), monoclinic, P2(1)/c (No. 14), Z = 4, a = 19.275 (7) angstrom, b = 11.940 (3) angstrom, c = 20.975 (1) angstrom, beta = 90.38 (4)degrees, V = 4827 angstrom-3, R = 0.038, R(w) = 0.048, 3872 independent reflections with I > 3-sigma(I); (Ph4As)2(II-P), monoclinic, P2(1)/c (No. 14), Z = 4, a = 18.124 (4) angstrom, b = 15.630 (3) angstrom, c = 24.665 (5) angstrom, beta = 98.109 (2)degrees, V = 6917 angstrom-3, R = 0.063, R(w) = 0.063, 3874 independent reflections with I > 5-sigma(I). Cluster I consists of a low-symmetry (C1) arrangement of Fe atoms about a highly distorted trigonal-prismatic MoS6 core, in which all sulfur atoms bridge to two or three Fe atoms. In one direction along a pseudo-2-fold axis of the trigonal prism are two Fe atoms at 2.74 and 2.77 angstrom from Mo, while the opposite tetragonal face is spanned by a zigzag chain of four Fe atoms, one of which at 2.68 angstrom is also bonded to Mo. The four-Fe chain contains one relatively long (2.74 angstrom) Fe-Fe interaction and a single bridging CO ligand, with terminal CO's completing the ligation to each Fe. The structure of I is essentially identical in both salts examined, suggesting that it arises from electronic considerations rather than crystal packing effects. The MoFe6S6 core stoichiometry of I together with the presence of three Fe atoms 2.7 angstrom from Mo makes it the closest synthetic approximation yet to the core stoichiometry and structure of the FeMo-cofactor of nitrogenase, suggesting that it may serve as a precursor to models for the FeMo-cofactor via oxidative decarbonylation reactions. Cluster II-P also exhibits a low-symmetry structure in which three new Mo-Fe interactions have been formed within an approximately square-pyramidal MoS3(CO)2 coordination sphere derived from coordination of two [Fe2S2(CO)6]2- units to Mo with loss of one sulfur atom. The PEt3 ligand is coordinated to the only Fe atom not bonded to Mo. The average Mo-S distance in both I and II-P is 2.43 angstrom, consistent with a Mo(II) formulation. Reaction of [Mo(CO)5I]- with 1 equiv of [Fe2S2(CO)6]2- results in the formation of cluster II in addition to a new cluster, the [Mo2Fe2S2(CO)6]2- ion (III), which has been characterized as its Ph4As+ salt. Crystallographic data for (Ph4As)2(III): monoclinic, P2(1)/m (No. 11), Z = 4, a = 11.938 (20) angstrom, b = 16.729 (30) angstrom, c = 15.694 (25) angstrom, beta = 111.45 (50)degrees, V = 2918 angstrom-3, R = 0.062, R(w) = 0.058, 3182 independent reflections with I > 3-sigma(I). The structure of III consists of a centrosymmetric distorted octahedral Mo2Fe2S2 core containing a planar Mo2Fe2 unit, with each Mo coordinated by three terminal CO's, each Fe coordinated by two terminal CO's, and a single CO bridging the Mo and Fe. The average Mo-S distance is 2.54 angstrom, consistent with a Mo(0) formulation. Plausible schemes for formation of clusters I-III are presented: cluster III arises from reaction of [Fe2S2(CO)6]2- with two [Mo(CO)5I]- ions; cluster II results from reaction of [Mo(CO)4I3]- with the dimeric disulfide-containing species [Fe4S4(CO)12]2- produced in sity by oxidation of [Fe2S2(CO)6]2-; and cluster III requires the reaction of [Mo(CO)4I3]- with both [Fe2S2(CO)6]2- and [Fe4S4(CO)12]2-. The implications of clusters I-III for the structure of the FeMo-cofactor of nitrogenase and as potential precursors to detailed models for the FeMo-cofactor are discussed.