SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND PROPERTIES OF NEW SINGLE AND DOUBLE CUBANES CONTAINING THE MOFE3S4 STRUCTURAL UNIT AND MOLYBDENUM-BOUND POLYCARBOXYLATE LIGANDS - CLUSTERS WITH A MOLYBDENUM-COORDINATION ENVIRONMENT SIMILAR TO THAT IN THE IRON MOLYBDENUM COFACTOR OF NITROGENASE

The reaction of the previously characterized (Et4N)2[MoFe3S4Cl3(Cl4-cat)(Cl4-cat)(CH3CN)] (1) cluster with a variety of organic polycarboxylic acids, such as oxalic, citric, D-malic, R-citramalic, (methylimino)diacetic, or in some instances their salts ((Et4N)(citrate)), affords in good yields MoFe3S4 cubanes with a Mo-polycarboxylate moiety. More specifically, the reaction of 1 with oxalic acid yields (Et4N)4[MoFe3S4Cl4)2(mu-C2O4)], 3. Cluster 3 crystallizes in the monoclinic space group C2/c with a = 29.70(1) angstrom, b = 12.462(3) angstrom, c = 19.477(4) angstrom, beta = 155.76(2)-degrees, and Z = 4. The tetraanion consists of two MoFe3S4 cubane subunits linked by a bridging tetradentate oxalate through the Mo atoms. The Mo...Mo distance is found at 5.72 angstrom, and the Mo-Cl bond is 2.500(3) angstrom. The reaction of 3 with (Et4N)2(C2O4) in CH3CN gives (Et4N)3[(MoFe3S4Cl4)(C2O4)] (4) in moderate yields. 4 crystallizes in the monoclinic space group P2(1)/n with a = 10.379(3) angstrom, b = 22.682(8) angstrom, c = 18.735(7) angstrom, beta = 92.21(1)-degrees, and Z = 4. The trianion core consists of a MoFe3S4 cubane possessing a terminal oxalate bound to the Mo atom. Upon reaction with (Et4N)CN, 4 affords its Mo-bound cyano analog (Et4N)3[MoFe3S4Cl3)(C2O4)(CN)] (5), which crystallizes in monoclinic space group P2(1)/n with a = 10.331(2) angstrom, b = 22.580(6) angstrom, c = 19.083(4) angstrom, beta = 91.97(2) angstrom, and Z = 4. Metric features of 5 are similar to those of 4 with Mo-C and C-N bonds of 2.189(15) and 1.041(15) angstrom, respectively. Treatment of 4 with 0.5 equiv of (Et4N)CN followed by addition of (Et4N)SH affords the doubly-bridged double cubane (Et4N)5{[MoFe3S4Cl2(C2O4)]2(mu2-CN)(mu2-S)} (6). Cluster 6 crystallizes in the monoclinic space group P2(1)/n with a = 15.125(4) angstrom, b = 21.254(7) angstrom, c = 23.34(1) angstrom, beta = 94.62(3)-degrees, and Z = 4. The pentaanion consists of two MoFe3S4 cubane subunits having terminally Mo-bound bidentate oxalate and bridged by a mu2-CN through the Mo atoms and a mu2-S through two of the Fe atoms. Among the outstanding features in 6 are included long Mo...Mo and Fe...Fe(intercube) distances of 5.267(5) and 3.443(7) angstrom, respectively, and a Fe-mu2-S-Fe angle of 104.1(4)-degrees. The Mo-C, Mo-N, and C-N bond lengths are found at 2.25(3), 2.30-(3), and 1.02(4) angstrom, respectively. The Mo-C(N)-N(C) angle of 157(3)-degrees significantly diverges from linearity. Additionally, 1 reacts with (methylimino)diacetic acid to afford a cubane (11) with the same core structure and having the acid dianion coordinated to the Mo atom through its two carboxylate and imino N functions. Cluster 11 crystallizes in triclinic space group P1BAR with a = 10.356(3) angstrom, b = 11.506(4) angstrom, c = 15.474(7) angstrom, alpha = 98.14-(3)-degrees, beta = 100.64(3)-degrees, gamma = 92.02(3)-degrees, and Z = 2. The electrochemical properties, spectroscopic characteristics, and reactivity of the new clusters are discussed.