Abstract
The isolable chelating bis(N-heterocyclic silylenyl)-substituted terphenyl ligand [Si(II)(Terp)Si(II)] as well as its bis(phosphine) analogue [P(III)(Terp)P(III)] have been synthesised and fully characterised. Their reaction with Ni(cod)(2) (cod = cycloocta-1,5-diene) affords the corresponding 16 VE nickel(0) complexes with an intramolecular η (2)-arene coordination of Ni, [E(Terp)E]Ni(η (2)-arene) (E = P(III), Si(II); arene = phenylene spacer). Due to a strong cooperativity of the Si and Ni sites in H(2) activation and H atom transfer, [Si(II)(Terp)Si(II)]Ni(η (2)-arene) mediates very effectively and chemoselectively the homogeneously catalysed hydrogenation of olefins bearing functional groups at 1 bar H(2) pressure and room temperature; in contrast, the bis(phosphine) analogous complex shows only poor activity. Catalytic and stoichiometric experiments revealed the important role of the η(2)-coordination of the Ni(0) site by the intramolecular phenylene with respect to the hydrogenation activity of [Si(II)(Terp)Si(II)]Ni(η (2)-arene). The mechanism has been established by kinetic measurements, including kinetic isotope effect (KIE) and Hammet-plot correlation. With this system, the currently highest performance of a homogeneous nickel-based hydrogenation catalyst of olefins (TON = 9800, TOF = 6800 h(-1)) could be realised.