Hydrogen Production Properties Of Metal-Organic Macrocycles Containing NADH Mimics

The problems of environmental pollution and the energy crisis have forced people to look for alternatives to fossil energy.Hydrogen is a green energy source with the characteristics of high energy density and low pollution which is one of the ideal substitutes for fossil energy.As we all know,hydrogenase is the most efficient system for catalyzing hydrogen production in nature,its main structure consists of coenzyme[4Fe4S]cluster and catalytic center[2Fe2S]cluster,and the coenzyme plays a role of transferring protons and electrons to improve the efficiency of hydrogen production.In addition,most of the highly efficient enzymatic reactions in nature depend on the coenzyme NADH that plays a role in transferring proton electrons in the reaction for efficient and stable catalytic reactions.In recent years,hydrogenase mimics photocatalytic hydrogen production is considered to be one of the most potential strategies for hydrogen production.The efficiency of hydrogen evolution usually relied on the photoinduced electron transfer（PET）process and the charge-hole separation state,and the lack of coenzyme may also be the key factor that prevents the enzyme mimics from reaching the enzyme level.The metal-organic macrocyclic structure is formed by organic functional ligands and metal ions through coordination self-assembly,which has a specific cavity size and shape that could encapsulate the guest molecules through weak interactions such asπ-πstacking,electrostatic interaction or hydrogen bonding,etc.The cavity provided confinement effect could reduce the distance between the host and the guest,resulting in the energy transfer between the host and the guest from intermolecular to intramolecular,and avoid unnecessary energy transfer for accelerating the rate of catalytic conversion,this process is the same as the enzyme-catalyzed process.The metal-organic macrocyclic with hydrophobic cavity often serve as one of the ideal mimic of enzymes due to the above characteristics.Therefore,the combination of metal-organic macrocycles and NADH mimics can expand the photocatalytic hydrogen production system and enhance the effect of photocatalytic hydrogen production,which is helpful for further understanding of the hydrogen production process of hydrogenase.In this paper,the NADH mimics benzothiazole and benzimidazole were used as the matrix,the positively charged cobalt-based metal-organic macrocyclic structures Co-TBC and Co-NBC with appropriate redox potentials were constructed by modifying classical NNO tridentate chelation sites on the ligand.metal-organic macrocycles were used as catalysts,while Ru（dcbpy）₃^4-（dcbpy=2,2’-bipyridine-4,4’-dicarboxylic acid）,a negatively charged ruthenium-based complex with excellent photoelectric properties,was selected as the photosensitizer and ascorbic acid（H₂A）as an electron sacrificial agent to study the effect of NADH mimics on the metal-organic macrocycle for the electron transfer rate and the effect of the host-guest interaction on the electron transfer pathway.In the photocatalytic hydrogen production experiment,the maximum hydrogen production TON value was 1710 after 24hours under irradiation,which confirms the efficiency of photocatalytic hydrogen production strategy of the metal-organic macrocyclic containing NADH mimics.In controlled experiments,the mononuclear complex Co-SL and the inhibitor Fe（dcbpy）₃^4-proved that negatively charged photosensitizers can form host-guest complexes through electrostatic interaction with positively charged metal-organic macrocycles.Further,the confinement effect closes the distance between the photosensitizer and the catalyst,leading the electron transfer changed from intermolecular to intramolecular,to accelerate the PET process.What is more,a metal-organic macrocyclic Co-MBC constructed with a ligand without the active center of the NADH mimic proved that the excellent proton electron transfer capability of the NADH mimic on the metal-organic macrocyclic structure can accelerate the LMCT（Ligand to Metal Charge Transfer）process,which further improves the photocatalytic production hydrogen rate,so as to realize the hydrogenase simulation process.