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(PPh2NPh2)2]+ (1), [Rh(PPh2NBn2)2]+ (2), [Rh(PPh2NPhOMe2)2]+ (3), [Rh(PCy2NPh2)2]+ (4), and [Rh(PCy2NPhOMe2)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(PPh2NBn2)2 (7), HRh(PPh2NPhOMe2)2 (8) and HRh(PCy2NPh2)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 w...