Syntheses And Photocatalytic CO₂ Reduction Of Co(?) Complexes Based On Tripodal Organic Ligands

Using solar energy to convert carbon dioxide(CO₂)into high value-added fuels or chemicals is one of the effective ways to mitigate the climate problems caused by energy shortage and excessive CO₂ emissions.The promising strategy to obtain new energy sources is design efficient and stable photocatalytic CO₂ reduction catalysts.As a class of molecular catalysts,transition metal complexes have attracted extensive attention of researchers in photocatalytic CO₂ reduction,because their definite spatial structures facilitate structural modification.In this thesis,a series of Co complexes based on tripod ligands were designed,then the effects of coordination atoms and catalytic centers on the photocatalytic CO₂reduction performance were studied.Combined with structural characterization and theoretical calculations,the catalytic mechanism was initially revealed.This paper mainly includes the following two parts:1.Based on tripod ligands L1,L2 and L3{L1=N-(2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine,L2=N-(2-aminobenzyl)-N,N-bis(2-pyridylmethyl)amine,L3=N-(2-mercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine},three Co(?)homogeneous molecular catalysts Co L-O(1),Co L-NH(2)and Co L-S(3)were synthesized for photocatalytic CO₂ reduction.These three molecular catalysts have similar structures,with the coordination environment of Co(?)is Co N₃O,Co N₄ and Co N₃S respectivity.All of them showed excellent activity in the photocatalytic reduction of CO₂ to CO.Control experiments showed that 1 possess higher efficiency than 2 and 3 for the photocatalytic CO₂-to-CO conversion,with the corresponding TON is 5654,which are1.28 and 1.65 times higher than those of 2 and 3,respectively.Electrochemical studies and DFT theoretical calculations revealed that the excellent catalytic performance of 1can be ascribed to the influence of the coordination environment,which endows the reduction potential of the Co(?)center in 1 is more positive during the catalytic process,thus boosts the CO₂-to-CO conversion.2.Based on the bi-tripod ligand BPMAN(BPMAN=2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine),a dinuclear cobalt complex[Co₂BPMAN](short for Co₂L)was synthesized for photocatalytic CO_2.The results show that Co₂L is a stable homogeneous catalyst.Under visible-light irradiation,the performance of Co₂L(1?M)as a homogeneous photocatalyst for CO₂ reduction in aqueous media has been explored by using triethylolamine(TEOA)and BIH as sacrificial reductants.Co₂L shows high photocatalytic activity for CO₂-to-CO conversion,corresponding to the high TON of 12800 and high selectivity of 93.7%for CO formation.Mechanism study shows that in the photocatalytic process,the photosensitizer is excited to form an excited state photosensitizer(PS*)firstly,PS*accepts electrons from the sacrificial agent and undergoes reductive quenching to generate PS·^-,then PS·^-transfers electrons to the Co(?)center,the low-valence Co center reduces the adsorbed CO₂ to CO.