A theoretical study of X ligand effect on catalytic activity of complexes RuHX(diamine)(PPh3)2 (X = NCMe, CO, Cl, OMe, OPh, CCMe and H) in H2-hydrogenation of ketones

Abstract

In this paper, the catalytic activities of RuHX(diamine)(PPh₃)₂ complexes with different X ligands (X = NCMe, CO, Cl, OMe, OPh, CCMe and H, corresponding catalytic processes are abbreviated in A, B, C, D, E, F and G systems, respectively) in the H₂-hydrogenation of ketones were investigated using density functional theory (DFT) method. Calculated results indicate that the rate-determining step in the whole catalytic cycle is hydrogen transfer (HT) for A–E but dihydrogen activation (DA) for F and G. The free energy barriers of the HT step for A–G are 36.1, 32.3, 21.2, 14.9, 21.9, 9.4 and 6.9 kcal mol⁻¹, respectively. The DA step consists of hydrogen coordination (HC) and hydrogen splitting (HS) steps if dihydrogen coordinates with the Ru center. The transition states (TSs) of H₂ coordinating with the Ru atom for A–G except B are located. The free energy barriers of DA for A–G are 17.8 (17.8, 2.6), 21.5, 12.8 (12.8, 3.8), 12.2 (11.2, 6.2), 13.6 (13.6, 4.1), 17.1 (9.7, 7.5) and 22.0 (10.4, 11.0) kcal mol⁻¹, respectively (the data in parentheses correspond to the barriers of HC and HS). HT barriers correlate well with the charges of hydride (H) in complex 1. HC barriers are closely related to the RuN¹ double bond in 4, and HS are in line with the proton-moved-distances (PMDs) from 5 to TS5-1. This study demonstrates that catalysts D, F and G show better catalytic activities than the others, which is in good agreement with experimental results.