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(η 3 -pincer) derivatives of bis((dicyclohexylphosphino)methyl)benzene are reported. RuCl(η 3 -PCP)(PPh 3 ) ( 3 ·PPh 3; PCP = [2,6-(Cy 2 PCH 2 ) 2 C 6 H 3 ]) is formed quantitatively on reaction of the PC(H)P-arene ligand with RuCl 2 (PPh 3 ) 3 at 22 °C in the presence of base. Treatment of 3 ·PPh 3 with KHB s Bu 3 under nitrogen atmosphere generates RuH(η 3 -PCP)(PPh 3 )(N 2 ), 4, as a mixture of conformational isomers in which the hydride and N 2 ligands are trans or cis ( 4a or 4b, respectively). On exposure to H 2, 4a/b undergo immediate, quantitative exchange of bound N 2 for η 2 -H 2, affording the corresponding isomers of RuH(η 3 -PCP)(PPh 3 )(H 2 ), 5a/b . Complex 3 ·PPh 3 resists displacement of PPh 3 by H 2 at 22 °C, but on exposure to CO, readily yields RuCl(η 3 -PCP)(CO) 2, 6 . Transfer hydrogenation of ketones is efficiently catalyzed by 3 ·PPh 3 and 4, in a pathway involving loss of PPh 3 and (for 3 ) exchange of chloride for hydride. The duration of pretreatment of 3 ·PPh 3 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 .