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

Abstract

Copper hydride clusters of the type (^RPN^HP)_nCu_2nH_2n (^RPN^HP = HN(CH₂CH₂PR₂)₂; n = 2 and 3) have been synthesized from the reaction of (^RPN^HP)CuBr with KO^tBu under H₂ or in one pot from a 1:2:2 mixture of ^RPN^HP, CuBr, and KO^tBu under H₂. With medium-sized phosphorus substituents (R = ⁱPr 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 ^tBuPN^HP leads to the formation of only a tetranuclear cluster. Crystallographic studies reveal a distorted octahedral Cu₆ unit in (^iPrPN^HP)₃Cu₆H₆ (2a) and (^CyPN^HP)₃Cu₆H₆ (2b), while a tetrahedral Cu₄ unit exists in (^CyPN^HP)₂Cu₄H₄ (2b') and (^tBuPN^HP)₂Cu₄H₄ (2c'), all furnished with face-capping hydrides and bridging ^RPN^HP 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).