The First Ru(η3-PCP) Complexes of the Electron-Rich Pincer Ligand 1,3-Bis((dicyclohexylphosphino)methyl)benzene:  Structure and Mechanism in Transfer Hydrogenation Catalysis

Mild routes to the first Ru(η3-pincer) derivatives of bis((dicyclohexylphosphino)methyl)benzene are reported. RuCl(η3-PCP)(PPh3) (3·PPh3; PCP = [2,6-(Cy2PCH2)2C6H3]) is formed quantitatively on reaction of the PC(H)P-arene ligand with RuCl2(PPh3)3 at 22 °C in the presence of base. Treatment of 3·PPh3 with KHB s Bu3 under nitrogen atmosphere generates RuH(η3-PCP)(PPh3)(N2), 4, as a mixture of conformational isomers in which the hydride and N2 ligands are trans or cis (4a or 4b, respectively). On exposure to H2, 4a/b undergo immediate, quantitative exchange of bound N2 for η2-H2, affording the corresponding isomers of RuH(η3-PCP)(PPh3)(H2), 5a/b. Complex 3·PPh3 resists displacement of PPh3 by H2 at 22 °C, but on exposure to CO, readily yields RuCl(η3-PCP)(CO)2, 6. Transfer hydrogenation of ketones is efficiently catalyzed by 3·PPh3 and 4, in a pathway involving loss of PPh3 and (for 3) exchange of chloride for hydride. The duration of pretreatment of 3·PPh3 with base (KOH, t BuOK) in refluxing 2-propanol is critical to activity, and indirect evidence supports formation of a catalytically active dihydride species during this process. X-ray crystal structures are reported for 3, 4a, 5a, and 6.