Hydrido copper clusters supported by dithiocarbamates: oxidative hydride removal and neutron diffraction analysis of [Cu7(H){S2C(aza-15-crown-5)}6]

Abstract

Reactions of Cu(I) salts with Na(S2CR) (R = NnPr 2, NEt2, aza-15-crown-5), and (Bu4N)(BH 4) in an 8:6:1 ratio in CH3CN solution at room temperature yield the monocationic hydride-centered octanuclear CuI clusters, [Cu8(H){S2CR}6](PF6) (R = N nPr2, 1H; NEt2, 2H; aza-15-crown-5, 3H). Further reactions of [Cu8(H){S 2CR}6](PF6) with 1 equiv of (Bu 4N)(BH4) produced neutral heptanuclear copper clusters, [Cu7(H){S2CR}6] (R = NnPr 2, 4H; NEt2, 5H; aza-15-crown-5, 6H) and clusters 4-6 can also be generated from the reaction of Cu(BF4)2, Na(S2CR), and (Bu4N) (BH4) in a 7:6:8 molar ratio in CH3CN. Reformation of cationic CuI8 clusters by adding 1 equiv of CuI salt to the neutral Cu7 clusters in solution is observed. Intriguingly, the central hydride in [Cu8(H){S2CN nPr2}6](PF6) can be oxidatively removed as H2 by Ce(NO3)62- to yield [CuII(S2CNnPr2)2] exploiting the redox-tolerant nature of dithiocarbamates. Regeneration of hydride-centered octanuclear copper clusters from the [CuII(S 2CNnPr2)2] can be achieved by reaction with Cu(I) ions and borohydride. The hydride release and regeneration of CuI8 was monitored by UV-visible titration experiments. To our knowledge, this is the first time that hydride encapsulated within a copper cluster can be released as H2 via chemical means. All complexes have been fully characterized by 1H NMR, FT-IR, UV-vis, and elemental analysis, and molecular structures of 1H, 2H, and 6H were clearly established by single-crystal X-ray diffraction. Both 1H and 2H exhibit a tetracapped tetrahedral Cu 8 skeleton, which is inscribed within a S12 icosahedron constituted by six dialkyl dithiocarbamate ligands in a tetrametallic- tetraconnective (μ2, μ2) bonding mode. The copper framework of 6H is a tricapped distorted tetrahedron in which the four-coordinate hydride is demonstrated to occupy the central site by single crystal neutron diffraction. Compounds 1-3 exhibit a yellow emission in both the solid state and in solution under UV irradiation at 77 K, and the structureless emission is assigned as a 3metal to ligand charge transfer (MLCT) excited state. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations on model compounds match the experimental structures and provide rationalization of their bonding and optical properties. © 2012 American Chemical Society.