Electronic Effects in PCP-Pincer Ru(II)-Based Hydrogen Transfer Catalysis

The synthesis and characterization of novel cyclometalated ruthenium(II) complexes [RuCl(PCPOMe)(PPh3)] and [RuCl(PCPCF3)(PPh3)] containing monoanionic, tridentate coordinating PCP-pincer ligands [C6H3{CH2P(p-MeOC6H4)2}2-2,6]- (PCPOMe) and [C6H3{CH2P(p-CF3C6H4)2}2-2,6]- (PCPCF3) are reported. These compounds have been tested as catalyst precursors in the hydrogen transfer reaction of cyclohexanone to cyclohexanol in 2-propanol using NaOH as a base. The initial rate of the hydrogen transfer reaction appeared to depend on the electronic character of the Ar2P- groups of the PCP-pincer ligand. Among the catalyst precursors studied, the complex [RuCl(PCPCF3)(PPh3)] was found to exhibit the highest activity and the initial TOF exceeded that observed for the Ph2P analogue [RuCl(PCPPh)(PPh3)]. Most importantly, catalysis performed with [RuCl(PCPCF3)(PPh3)] does not require pretreatment of the precursor in the absence of substrate. Conversely, a different catalytic profile and a low activity were observed when either the electron-poor [RuCl{C6H3(CH2P(C6F5)2)2-2,6}(PPh3)] ([RuCl(PCPF20)(PPh3)]) complex or its triflate analogue was used as catalyst precursors. NMR studies and ESI-MS measurements provided information concerning the catalytically active species formed during the pretreatment of the precursor complexes. The results indicate that during the pretreatment period a monoanionic, monohydride ruthenium(II) species, Na[Ru(H)(PCPiPr)(OiPr)(PPh3)], is selectively formed. The latter hydride complex was also obtained via an independent synthetic route. On the basis of both the present results and those previously reported in literature, a mechanism is proposed.