Hydrido Copper Clusters Supported by Dithiocarbamates: Oxidative Hydride Removal and Neutron Diffraction Analysis of [Cu<sub>7</sub>(H){S<sub>2</sub>C(aza-15-crown-5)}<sub>6</sub>]
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Reactions of Cu(I) salts with Na(S2CR) (R = NnPr2, NEt2, aza-15-crown-5), and (Bu4N)(BH4) 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 = NnPr2, 1H; NEt2, 2H; aza-15-crown-5, 3H). Further reactions of [Cu8(H){S2CR}6](PF6) with 1 equiv of (Bu4N)(BH4) produced neutral heptanuclear copper clusters, [Cu7(H){S2CR}6] (R = NnPr2, 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){S2CNnPr2}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(S2CNnPr2)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 Cu8 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.
