Boosting homogeneous chemoselective hydrogenation of olefins mediated by a bis(silylenyl)terphenyl-nickel(0) pre-catalyst

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 eta(2)-arene coordination of Ni, [E(Terp)E]Ni(eta(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(eta(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 eta(2)-coordination of the Ni(0) site by the intramolecular phenylene with respect to the hydrogenation activity of [Si-II(Terp)Si-II]Ni(eta(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.

Automated summary

Two new ligands were synthesized and studied, one of which demonstrated high catalytic activity in the hydrogenation of olefins under mild conditions, achieving the highest performance of a homogeneous nickel-based catalyst. The coordination mode of the Ni(0) site by the intramolecular phenylene was found to play a crucial role in the catalytic activity, as revealed by kinetic measurements and correlation experiments.