A series of five [Rh(P2N2)2]+ complexes (P2N2 = 1,5-diaza-3,7-diphosphacyclooctane)
have been synthesized and characterized: [Rh(PPh
2NPh
2)2]+ (1), [Rh(PPh
2NBn
2)2]+ (2), [Rh(PPh
2NPhOMe
2)2]+ (3), [Rh(PCy
2NPh
2)2]+ (4), and [Rh(PCy
2NPhOMe
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(PPh
2NBn
2)2 (7), HRh(PPh
2NPhOMe
2)2 (8) and HRh(PCy
2NPh
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 pK
a’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.