Ruthenium(II)-Arene Complexes of the Water-Soluble Ligand CAP as Catalysts for Homogeneous Transfer Hydrogenations in Aqueous Phase

The neutral Ru(II) complex kP-[RuCl2(eta6-p-cymene)(CAP)] (1), and the two ionic complexes kP-[RuCl(eta6-p-cymene)(MeCN)(CAP)]PF6 (2) and kP-[RuCl(eta6-p-cymene)(CAP)2]PF6 (3), containing the water-soluble phosphine 1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane (CAP), were tested as catalysts for homogeneous hydrogenation of benzylidene acetone, selectively producing the saturated ketone as product. The catalytic tests were carried out in aqueous phase under transfer hydrogenation conditions, at mild temperatures using sodium formate as hydrogen source. Complex 3, which showed the highest stability under the reaction conditions applied, was also tested for C=N bond reduction from selected cyclic imines. Preliminary NMR studies run under pseudo-catalytic conditions starting from 3 showed the formation of kP-[RuH(eta6-p-cymene)(CAP)2]PF6 (4) as the pivotal species in catalysis.