Incorporation of Pendant Bases into Rh(diphosphine)2 Complexes: Synthesis, Thermodynamic Studies, And Catalytic CO2 Hydrogenation Activity of [Rh(P2N2)

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• 作者：Alyssia M. Lilio ; Mark H. Reineke ; Curtis E. Moore ; Arnold L. Rheingold ; Michael K. Takase ; Clifford P. Kubiak
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：July 1, 2015
• 年：2015
• 卷：137
• 期：25
• 页码：8251-8260
• 全文大小：418K
• ISSN：1520-5126

文摘

A series of five [Rh(P₂N₂)₂]⁺ complexes (P₂N₂ = 1,5-diaza-3,7-diphosphacyclooctane) have been synthesized and characterized: [Rh(P^Ph₂N^Ph₂)₂]⁺ (1), [Rh(P^Ph₂N^Bn₂)₂]⁺ (2), [Rh(P^Ph₂N^PhOMe₂)₂]⁺ (3), [Rh(P^Cy₂N^Ph₂)₂]⁺ (4), and [Rh(P^Cy₂N^PhOMe₂)₂]⁺ (5). Complexes 1鈥?b>5 have been structurally characterized as square planar rhodium bis-diphosphine complexes with slight tetrahedral distortions. The corresponding hydride complexes 6鈥?b>10 have also been synthesized and characterized, and X-ray diffraction studies of HRh(P^Ph₂N^Bn₂)₂ (7), HRh(P^Ph₂N^PhOMe₂)₂ (8) and HRh(P^Cy₂N^Ph₂)₂ (9) show that the hydrides have distorted trigonal bipyramidal geometries. Equilibration of complexes 2鈥?b>5 with H₂ in the presence of 2,8,9-triisopropyl-2,5,8,9-tetraaza-1-phosphabicyclo[3,3,3]undecane (Verkade鈥檚 base) enabled the determination of the hydricities and estimated pK_a鈥檚 of the Rh(I) hydride complexes using the appropriate thermodynamic cycles. Complexes 1鈥?b>5 were active for CO₂ hydrogenation under mild conditions, and their relative rates were compared to that of [Rh(depe)₂]⁺, a nonpendant-amine-containing complex with a similar hydricity to the [Rh(P₂N₂)₂]⁺ complexes. It was determined that the added steric bulk of the amine groups on the P₂N₂ ligands hinders catalysis and that [Rh(depe)₂]⁺ was the most active catalyst for hydrogenation of CO₂ to formate.