Molecular and Electronic Structures of [Ni(abpy)3]n (n = +2, +1, 0, -1; abpy = 2,2'-Azobis(pyridine)): A Catalyst for Photochemical Reduction of CO2 to CO Using [Ru(bpy)3]2+ as a Photosensitizer.

A redox-noninnocent 2,2'-azobis(pyridine) (abpy) ligand-based molecular catalyst, [NiII(abpy)3][PF6]2 (Ni-Cat) has been developed for efficient and selective photochemical reduction of CO2 to CO. The molecular and electronic structures of [Ni(abpy)3]n (n = +2, +1, 0, and -1) were confirmed by single-crystal X-ray crystallography and DFT calculations. Ni-Cat catalyzes the photochemical reduction of CO2 to CO in the presence of [Ru(bpy)3](PF6)2 (Ru-PS) as a photosensitizer (λex = 470 nm) producing HCOOH and H2 as minor products. After 6 h of irradiation, the turnover number, turnover frequency, and selectivity for CO formation in DMA-TEOH (4:1 v/v) medium (TEOH = triethanolamine) were TONCO = 2358, TOFCO = 393 h-1, and SCO = ∼99%. The cyclic voltammogram of Ni-Cat in DMA-H2O (9:1 v/v) under argon displays redox waves due to [NiIIL3]2+/[NiII(L●-)L2]1+, [NiII(L●-)L2]1+/[NiII(L●-)2L], and [NiII(L●-)2L]/[NiII(L●-)3]- redox couples (L = abpy), respectively. However, in the presence of CO2, Ni-Cat undergoes reduction by 2e only. [Ru-PS]+ that is realized after irradiation reduces Ni-Cat by 2e producing an azoanion radical (L●-) complex, [NiII(L●-)2L]. Subsequently, L●- of [NiII(L●-)2L] reacts with CO2 producing hydrazine carboxylic acid that forms an ester with TEOH. Homolysis of the ester evolves CO gas releasing back L that undergoes reduction again and forms a catalytic cycle.