SYNTHESIS AND CHARACTERIZATION OF DOUBLE-CUBANE COMPLEXES CONTAINING MFE3SE4 CORES (M=MO, W)

The reaction system (Et4N)2MSe4 (M = Mo, W)/FeCl3/EtSH/Na in methanol or ethanol at ambient temperature has yielded three principal products: [M2Fe6Se8(SEt)9]3-, [M2Fe7Se8(SEt)12]3- (M = Mo, W); and [W2Fe7Se8(SEt)12]4- which are isolable as Et4N+ salts. The structure of (Et4N)3[Mo2Fe6Se8(SEt)9] has been determined by single-crystal X-ray diffraction. It crystallizes in the hexagonal space group P6(3)/m with a = 17.453(3), c = 16.575(2) angstrom and Z = 2. Reaction of [Mo2Fe7Se8(SEt)12]3- with 6 equiv of acetyl chloride or arylthiol afforded new clusters with substitution of the terminal ethanethiolate ligands: [Mo2Fe7Se8(SEt)6Cl6]3- and [Mo2Fe7Se8(SEt)6(SC6H4X)6]3-, X = H, Cl. The structure of (Et4N)3[Mo2Fe7Se8(SEt)6(SC6H4Cl)6 has been determined by single crystal X-ray diffraction. It crystallizes in the space group P1BAR (No. 2) with a = 11.401(2), b = 12.697(2), c = 19.897(3) angstrom, alpha = 87.91(1), beta = 74.93(1), gamma = 89.14(2)-degrees and Z = 1. The anions in both structures are of the 'double-cubane' type containing trigonally distorted MoFe3Se4 subclusters which are linked through the Mo atoms. In the former, linkage is through three mu-2-ethanethiolate ligands; whereas in the latter, the bridging is accomplished by a Fe(SEt)6 distorted trigonal anti-prismatic moiety. These complexes are structurally similar to the related sulfur double-cubanes. Incorporation of the larger selenide ion has a surprisingly small effect upon the M...Fe distances within the cubane subclusters. All products show the full stoichiometric amount of four selenium atoms per cubane unit, indicating that MSe42- can effectively deliver all four selenium atoms in the assembly process even though one is no longer covalently bound to M. All clusters exhibit isotropically shifted H-1 NMR spectra, which are the result of both contact and dipolar shift mechanisms and are larger in magnitude than the shifts observed in the sulfur analogs. This is consistent with the larger solution magnetic moments of these selenium double-cubanes as determined by the Evans method. Electrochemical reduction potentials of these selenium double-cubanes were nearly identical to their sulfur analogs as determined by cyclic voltammetry. Full tabulation of H-1 NMR isotropic shifts, electronic absorption maxima, electrochemical reduction potentials, room temperature solution magnetic susceptibilities, and representative cyclic voltammograms and H-1 NMR spectra are presented.