Incorporation of Pendant Bases into Rh(diphosphine)_2 Complexes: Synthesis, Thermodynamic Studies, And Catalytic CO_2 Hydrogenation Activity of Rh(P_2N_2)_2^+ Complexes

摘要

A series of five [Rh(P_2N_2)_2]^+ complexes (P_2N_2 = 1,5-diaza-3,7-diphosphacyclooctane) have been synthesized and characterized: [Rh(P^(Ph)_2N^(Ph)_2)_2]^+ (1), [Rh(P^(Ph)_2N^(Bn)_2)_2]^+ (2), [Rh(P^(Ph)_2N^(PhOMe)_2)_2]^+ (3), [Rh(P^(Cy)_2N^(Ph)_2)_2]^+ (4), and [Rh(P^(Cy)_2N^(PhOMe)_2)_2]^+ (5). Complexes 1–5 have been structurally characterized as square planar rhodium bis-diphosphine complexes with slight tetrahedral distortions. The corresponding hydride complexes 6–10 have also been synthesized and characterized, and X-ray diffraction studies of HRh(P^(Ph)_2N^(Bn)_2)_2 (7), HRh(P^(Ph)_2N^(PhOMe)_2)_2 (8) and HRh(P^(Cy)_2N^(Ph)_2)_2 (9) show that the hydrides have distorted trigonal bipyramidal geometries. Equilibration of complexes 2–5 with H_2 in the presence of 2,8,9-triisopropyl-2,5,8,9-tetraaza-1-phosphabicyclo[3,3,3]undecane (Verkade’s base) enabled the determination of the hydricities and estimated pK_a’s of the Rh(I) hydride complexes using the appropriate thermodynamic cycles. Complexes 1–5 were active for CO_2 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(P_2N_2)_2]^+ complexes. It was determined that the added steric bulk of the amine groups on the P_2N_2 ligands hinders catalysis and that [Rh(depe)_2]^+ was the most active catalyst for hydrogenation of CO_2 to formate.