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((P2N2Ph)-N-Ph)(2)](+) (<bold>1</bold>), [Rh(PPh2N2Bn)(2)](+) (<bold>2</bold>), [Rh((P2N2PhOMe)-N-Ph)(2)](+) (<bold>3</bold>), [Rh((P2N2Ph)-N-Cy)2](+) (<bold>4</bold>), and [Rh((P2N2PhOMe)-N-Cy)(2)](+) (<bold>5</bold>). Complexes <bold>15</bold> have been structurally characterized as square planar rhodium bis-diphosphine complexes with slight tetrahedral distortions. The corresponding hydride complexes <bold>610</bold> have also been synthesized and characterized, and X-ray diffraction studies of HRh(PPh2N2Bn)(2) (<bold>7</bold>), HRh((P2N2PhOMe)-N-Ph)(2) (<bold>8</bold>) and HRh((P2N2Ph)-N-Cy)(2) (<bold>9</bold>) show that the hydrides have distorted trigonal bipyramidal geometries. Equilibration of complexes <bold>2-5</bold> with H-2 in the presence of 2,8,9-triisopropyl-2,5,8,9-tetraaza-1-phosphabicyclo[3,3,3]undecane (Verkades base) enabled the determination of the hydricities and estimated pKas of the Rh(I) hydride complexes using the appropriate thermodynamic cycles. Complexes <bold>1-5</bold> 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.