Steric Effects of HN(CH2CH2PR2)2 on the Nuclearity of Copper Hydrides

Copper hydride clusters of the type (RPNHP) n Cu2n H2n (RPNHP = HN­(CH2CH2PR2)2; n = 2 and 3) have been synthesized from the reaction of (RPNHP)­CuBr with KO t Bu under H2 or in one pot from a 1:2:2 mixture of RPNHP, CuBr, and KO t Bu under H2. With medium-sized phosphorus substituents (R = i Pr and Cy), the phosphine ligands stabilize both hexanuclear and tetranuclear clusters; however, the smaller clusters are kinetic products and aggregate further over time. Use of a bulkier ligand tBuPNHP leads to the formation of only a tetranuclear cluster. Crystallographic studies reveal a distorted octahedral Cu6 unit in ( iPrPNHP)3Cu6H6 (2a) and (CyPNHP)3Cu6H6 (2b), while a tetrahedral Cu4 unit exists in (CyPNHP)2Cu4H4 (2b′) and ( tBuPNHP)2Cu4H4 (2c′), all furnished with face-capping hydrides and bridging RPNHP ligands. The aggregations are maintained in solution, although hydrides are fluxional. These copper clusters are capable of reducing aldehydes and ketones to the corresponding copper alkoxide species. Ranking their reactivity toward N-methyl-2-pyrrolecarboxaldehyde gives 2b′ > 2a, 2b ≫ 2c′, which correlates inversely with the order of thermal stability (against decomposition and cluster expansion).