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Published July 1, 2015 | Supplemental Material
Journal Article Open

Incorporation of Pendant Bases into Rh(diphosphine)_2 Complexes: Synthesis, Thermodynamic Studies, And Catalytic CO_2 Hydrogenation Activity of [Rh(P_2N_2)_2]^+ Complexes

Abstract

A series of five [Rh(P_2N_2)_2]^+ complexes (P_2N_2 = 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 H_2 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 CO_2 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(P_2N_2)_2]^+ complexes. It was determined that the added steric bulk of the amine groups on the P_2N_2 ligands hinders catalysis and that [Rh(depe)_2]^+ was the most active catalyst for hydrogenation of CO_2 to formate.

Additional Information

© 2015 American Chemical Society. Received: April 24, 2015; Published: June 4, 2015. The authors would like to acknowledge the Air Force Office of Scientific Research through the MURI program for funding under AFOSR Award No. FA9550-101-0572 and the Joint Center of Artificial Photosynthesis, a DOE Energy Innovation Hub, for funding under the Award DE-SC0004993.

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