Cis-[Co^sup II^(MPCA)X^sub 2^] (X = Cl or Br) complexes as catalyst exhibiting different activity for visible light induced photocatalytic CO^sub 2^-to-CO conversion

Two cobalt complexes, cis-[CoII(MPCA)Cl2] (C1) and cis-[CoII(MPCA)Br2] (C2) (MPCA = N, N’-dimethyl-N, N’-bis(2-pyridinylmethyl)cyclohexane-1,2-diamine), were synthesized as catalysts for visible light induced photocatalytic CO2 reduction. The two complexes adopt cis-octahedral coordination geometry with different halogen ligands at cis-coordination sites, which were characterized by X-ray crystal analysis. The two cobalt catalysts exhibited different activity on catalysing CO2-to-CO conversion in the presence of Ir(ppy)3 as photosensitizer and triethylamine as sacrificial electron donor in DMF under visible light irradiation. The rate of CO production of C2-system was faster than that of C1-system in the initial stage of irradiation. However, the better stability of C1-system than that of C2-system leading a total TONCO of 470 from C1-system compared with of 403 from C2-system were obtained after 48 h of irradiation. Mechanism studies revealed that the photoinduced electron transfer from Ir(ppy)3* to cobalt complex, C1 or C2, operates to generate active CoI species for CO2 binding. The C2-system possesses larger rate constant and driven force of the photoinduced electron transfer process than those of in C1-system.