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(eta(3)-pincer) derivatives of bis((dicyclohexylphosphino)methyl)benzene are reported. RuCl(eta(3)-PCP)(PPh3) (3.PPh3; PCP = [2,6-(Cy2PCH2)(2)C6H3]) is formed quantitatively on reaction of the PC(H)P-arene ligand with RuCl2(PPh3)(3) at 22degreesC in the presence of base. Treatment of 3.PPh3 with (KHBBu3)-Bu-s under nitrogen atmosphere generates RuH(eta(3)-PCP)(PPh3)(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 eta(2)-H-2, affording the corresponding isomers of RuH(eta(3)-PCP)(PPh3)(H-2), 5a/b. Complex 3.PPh3 resists displacement of PPh3 by H-2 at 22 degreesC, but on exposure to CO, readily yields RuCl(eta(3)-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, (BuOK)-Bu-t) 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.