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

Abstract

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).

Copper–Ligand Cooperativity in H2 Activation Enables the Synthesis of Copper Hydride Complexes

“…Addition of benzylamine to a toluene solution of 1 in the presence of an excess of triethylamine led to the formation of the targeted aminophosphine 2 H , isolated as a white solid in 69% yield. Compound 2 H was characterized by X-ray diffraction (see the Supporting Information for details) and by multinuclear NMR experiments, exhibiting 31 P NMR shifts at 32.6 (dd, 3 J PP = 145, 149 Hz) and −12.6 ppm (d, 3 J PP = 147 Hz, average of two overlapping doublets) for the P−NH and the two Cy 2 P fragments, respectively. While the NH signal is masked by the cyclohexyl protons in benzene, it appears as a multiplet at 2.62 ppm in d 8 -THF.…”

“…The conversion never exceeded a 20% yield, but no side reaction was observed. Complex 6 was characterized in 31 47 Complex 6 was formed alternatively by the direct reaction of 3 with 1 equiv of potassium triethylborohydride for 1 h in toluene. After evaporation of the solvent and dissolution in Et 2 O, 6 could be isolated as a yellow powder.…”

“…In the case of copper, however, while a number of Cu–H intermediates have been proposed, for example in hydroelementation reactions as described by Buchwald, ,,, only a few have been fully characterized. , The synthesis of the hexameric complex [(Ph 3 P)­CuH] 6 by Osborn in 1971 led to the characterization of a number of polymetallic hydrido clusters or complexes, − mostly obtained through the reaction of copper halide or alkoxide complexes with borohydrides, boranes, or silanes. − The groups of Caulton, Norton, Appel, Guan, and Broere reported efficient procedures that involve the hydrogenolysis of Cu–OR bonds (Scheme ). − No terminal copper hydride has ever been isolated in the solid state, but Bertrand et al demonstrated that, by using a highly congested carbene ligand, a monomeric copper hydride could exist in solution, in equilibrium with its dimeric form . The same group used a cyclic alkylaminocarbene to stabilize a terminal copper hydride together with a Lewis acid, which was active in CO 2 hydrogenation .…”

Copper–Ligand Cooperativity in H2 Activation Enables the Synthesis of Copper Hydride Complexes

“…Addition of benzylamine to a toluene solution of 1 in the presence of an excess of triethylamine led to the formation of the targeted aminophosphine 2 H isolated as a white solid in 69 % yield. Compound 2 H was characterized by X-ray diffraction (see SI for details) and by multinuclear NMR experiments, exhibiting 31 P NMR shifts at 32.6 (dd, 3 JPP = 145, 149 Hz) and −12.6 ppm (d, 3 JPP = 147 Hz, average of two overlapping doublets) for the P-NH and the two Cy2P fragments, respectively. While the NH signal is masked by the cyclohexyl protons in benzene, it appears as a multiplet at 2.62 ppm in d8-THF.…”

“…[18][19][20][21][22][23][24] The groups of Caulton, Ruccolo, Appel, Guan and Broere reported efficient procedures that involve the hydrogenolysis of Cu-OR bonds (Scheme 1). [25][26][27][28][29][30][31] No terminal copper hydride has ever been isolated in the solid state but Bertrand et al demonstrated that, by using a highly congested carbene ligand, a monomeric copper hydride could exist in solution, in equilibrium with its dimeric form. 13 The same group used a cyclic alkylaminocarbene to stabilize a terminal copper hydride together with a Lewis acid, which was active in CO2 hydrogenation.…”