Synthesis Of Cobalt Complexes Based On Hydiazine And Salen-type Schiff Base Ligands For Visible-Light-Driven Photocatalytic Reduction Of CO₂

Photocatalytic CO₂ reduction is one of the effective ways to reduce CO₂emissions and achieve the carbon cycle,which is complex and the reduction products are variable.Therefore,the development of catalysts with high activity,high selectivity and high stability has become the frontier of photocatalytic CO₂ reduction.At present,noble metal based complexes have great limitations in terms of application due to its expensive price,low activity and selectivity in aqueous systems.Therefore,the development of efficient and low-cost non-noble metal based complex catalysts has become an urgent challenge to be addressed for photocatalytic CO₂ reduction.On this basis,a series of cobalt complex catalysts with good catalytic performance were designed and synthesized using ligand modulation strategy.The main works of this thesis are presented as follows:Chapter One:A review of homogeneous photocatalytic CO₂ reduction was conducted.The research background and the basic principles with performance evaluation criteria of photocatalytic CO₂ reduction are introduced.Meanwhile,the current research status of noble metal based complexes,non-noble metal complexes and supramolecular catalysts are highlighted.Chapter Two:Three complexes[Co（L¹）（Cl）₂]（1）,[Co（L²）（Cl）₂]（2）and[Co（L³）（Cl）₂]（3）were synthesized from the in situ reaction of 2,6-bis（（E）-1-hydrazinylidene）pyridine with salicylaldehyde,2-hydroxy-5-chlorobenzaldehyde or2-hydroxy-5-methylbenzalde-hyde,respectively.Under visible light condition,these mononuclear cobalt complexes were used as catalysts with[Ru（phen）3]（PF6）2 as the photosensitizer,TEOA as the sacrificial agent and H2O/CH3CN（v/v 1:4）as the mixed solvent.It was found that complex 1 with methyl substituent showed high activity for photocatalytic CO₂ reduction,with a catalytic conversion number（TON）of 3680 and a selectivity of 93%for CO generation,while complex 3 with chlorine substituent had a TON value of 940 and a CO selectivity of 83.2%.The electrochemical and fluorescence quenching experimental results showed that the catalytic system follows an oxidative quenching mode.Under CO₂atmosphere,the starting potential of complex 1 with methyl substituent is-0.9 V vs NHE which is more positive than the starting potential of complexes 2 and 3.This is more favorable for electrons transfer from photosensitizer to complex 1.Electrochemical properties are consistent with photocatalytic results.Chapter Three:O-phenylenediamine with the substituent of methyl,fluorine or nitro at position 4 and 2-hydroxy-5-methylbenzaldehyde were used to synthesize four N₂O₂-type Schiff base based mononuclear cobalt complexes[CoL⁴]（4）,[CoL⁵]（5）,[CoL⁶]（6）and[CoL⁷]（7）by in situ reaction.The results of photocatalytic experiments showed that the nitro-containing complex 7 exhibited excellent catalytic performance,with TON value of 5300 for CO generation.A TON value of 3380 was found for the CO generation catalyzed by complex 6 with fluorine as the substituent,while complex 4 with methyl substituent and complex 5 without substituent showed even poorer catalytic effects than CoCl₂.The electrochemical and fluorescence quenching experiments showed that complexes 6 and 7 with electron-withdrawing substituents of fluoro-and nitro have more positive redox potentials in comparison to complex 4 with electron-donating substituent.This is more favorable for electrons transfer from photosensitizer to complexes 6 and 7 with a stronger reducing power.DFT calculations was employed to explain the reaction mechanism of this photocatalytic system.The results showed that nitro-containing complex 7 exhibits lower potential barriers for both the critical CO₂coordination step and the C-O bond cleavage step than that of complex 5,which results in higher photocatalytic activity of complex 7 with stronger electron-withdrawing effect.Chapter Four:The mononuclear cobalt complex[CoL⁸]（8）and tetranuclear cobalt complex[Co₄（L⁸）₄]（9）were synthesized by using 2,2’-[1,2-ethanediylbisimino)]bis[6-（2-tert-butyl）-4-（4-pyridyl）phenol]（H₂L⁸）ligand and cobalt nitrate.Complexes 8 and 9 exhibited catalytic activity for visible light driven CO₂ reduction in water-containing system.The results showed that the TON and selectivity for CO production of complex 9 were 1810 and 80%,respectively.Besides,the TON and CO selectivity for complex 8 were 1070 and 51.4%.Notably,the catalytic activity and CO selectivity of complex 9 were better than those of complex 8.Electrochemical studies showed that the onset potential of complex 9 is more positive than that of complex 8,which is more favorable for electron transfer from photosensitizer to catalyst.Thus,its photocatalytic effect is better than that of complex 8.The fluorescence quenching experiment results showed that the catalytic system followed the oxidative quenching path and complex 9 had a larger quenching constant than that of 8,indicating higher electron transfer efficiency and photocatalytic activity.