Influence of ion pairing on styrene hydrogenation using a cationic η6-arene β-diketiminato-ruthenium complex

A series of salts composed of the coordinatively unsatd. areneruthenium β-diketiminato cation and different anions, [(η6-C6H6)Ru(ArN:CMeCHCMe:NAr)][X] (1-4; Ar = 2,6-dimethylphenyl; X = OTf-, BF4-, PF6-, BPh4-) and [(η6-C6H6)Ru(ArN:CMeCHCMe:NAr)][BArF4] [5, ArF = 3,5-(CF3)2C6H3], were prepd. and characterized. The solid state structures of 1, 2, and 5 have also been detd. using single-crystal x-ray crystallog. Both soln. and solid state data reveal the presence of anion-cation interactions, the extent of which depends on the nature of the anion, which have been further rationalized via computed charge d. profiles using DFT energy optimized models. The catalytic activity of 1-5 in the hydrogenation of styrene was found to be highly dependent on the nature of the counteranion, as inferred from investigations based on high-pressure soln. NMR, pulsed gradient spin-echo (PGSE) NMR diffusion, and Overhauser NMR spectroscopy. A good correlation between catalytic activity and the extent and nature of ion pairing was found, and the structure of the active catalytic species is proposed.