A Silyl-Nickel Moiety as a Metal-Ligand Cooperative Site

Abstract

Nickel(II) complexes supported by a diphosphinosilyl ligand, [PhSi(2-(PPr2C6H4)-Pr-i)(2)](-) (PhSiP2), reveal unusual metal-ligand cooperativity (MLC). While (PhSiP2)-Ni(NHMes) (2a) was cleanly isolated at room temperature, a nickel triisopropylphenylamido species, (PhSiP2)Ni(NHTrip) (2b), slowly transformed into a nickel(II) phenyl species, [(ArNH)SiP2]Ni(Ph) (3b), where (ArNH)SiP2- = [(NHTrip)Si(2-(PPr2C6H4)-Pr-i)(2)](-). The X-ray crystallographic data of 3b exhibit a Si-N bond generated from Si-N coupling between the silyl moiety and amino group, along with cleavage of a Si-C bond. Because substoichiometric amounts of pi-acidic ligands, such as isocyanide, enhance the conversion rate of 2 to 3 (k(obs) = 0.28 vs 0.44 h(-1)), this reaction may involve reductive elimination (RE) and oxidative addition (OA) operating at the silyl-nickel moiety. This is further supported by the fact that the presence of excess it-acidic ligands results in the generation of demetalated ligands (ArNH)PhSiP2 (4) having both Si-N and Si-Ph bonds and the nickel(0) species. Theoretical evaluations also agree on such a pathway. Interestingly, the reaction of a nickel phenyl species (3) with gaseous carbon dioxide (CO2(g)) produces a nickel(II) carbamato complex, (PhSiP2)Ni(OC(O)NHAr) (6), which may involve a RE-OA process occurring at a nickel center. Although 2 and 3 might be in equilibrium, the reaction of 3 with CO2 does not follow this pathway. Instead, a CO2 interaction induces RE at the silyl-nickel moiety, followed by amide group transfer, to give a carbamato product, 6, based on our experimental and theoretical evaluations. These results highly support that group transfer involving MLC can be managed via a RE-OA pathway at the silyl-nickel(II) moiety.