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 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.