Dual electronic effects achieving a high-performance Ni(II) pincer catalyst for CO2 photoreduction in a noble-metal-free system

A carbazolide-bis(NHC) Ni-II catalyst (1; NHC, N-heterocyclic carbene) for selective CO2 photoreduction was designed herein by a one-stone-two-birds strategy. The extended pi-conjugation and the strong sigma/pi electron-donation characteristics (two birds) of the carbazolide fragment (one stone) lead to significantly enhanced activity for photo-reduction of CO2 to CO. The turnover number (TON) and turnover frequency (TOF) of 1 were ninefold and eightfold higher than those of the reported pyridinol-bis(NHC) Ni-II complex at the same catalyst concentration using an identical Ir photosensitizer, respectively, with a selectivity of similar to 100%. More importantly, an organic dye was applied to displace the Ir photosensitizer to develop a noble-metal-free photocatalytic system, which maintained excellent performance and obtained an outstanding quantum yield of 11.2%. Detailed investigations combining experimental and computational studies revealed the catalytic mechanism, which highlights the potential of the one-stone-two-birds effect.