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

A series of five [Rh(P2N2)2](+) complexes (P2N2 = 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 H2 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 pKa's of the Rh(I) hydride complexes using the appropriate thermodynamic cycles. Complexes 1-5 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.