Synthesis of [CNN] pincer nickel(ii) NHC chlorides and their catalytic effects on the hydrosilylation of aldehydes and ketones under mild conditions

Abstract

Eight [CNN] pincer nickel NHC chlorides, [^RC_carbeneN_amido^R′2N_amine–Ni–Cl] (R/R′2 = quinolinyl/ⁿBu (3a), quinolinyl/Me (3b), quinolinyl/PhCH₂ (3c), (CH₂)₄/ⁿBu (5a), (CH₂)₄/ⁱPr (5b), Me₂/ⁿBu (5c), Me₂/ⁱPr (5d), and Me₂/PhCH₂ (5e)), were prepared by the reactions of related asymmetric [CNN] NHC preligands with Ni(DME)Cl₂via N–H bond activation. The molecular structures of complexes 3a, 3b and 5a were investigated using single crystal XRD. The catalytic experiments showed that the nickel complexes could catalyze the hydrosilylation of unsaturated compounds like aldehydes and ketones, and complex 3b was found to exhibit the best catalytic activity at 30 °C. Using NaBHEt₃ as an additive, the catalytic activity of 3b was greatly improved. Furthermore, it was found that good to complete conversion of aldehydes could be achieved within 1.5 h at 30 °C after pre-reacting 3b and NaBHEt₃ with silane for 0.5 h. The substrate ketones could be converted to alcohols in good yields by prolonging the reaction time to 2.5 h at 30 °C. It is worth mentioning that all eight [CNN] pincer nickel NHC chlorides are stable in air, and this is rare in nickel catalysts for hydrosilylation of aldehydes and ketones. Moreover, the substrate universality of the catalytic system is also very good, with good functional group tolerance. The molecular structures of complexes 3a, 3b and 5a were investigated using single crystal XRD and a possible reaction mechanism was proposed.