Phenanthridine-functionalized Metal-organic Macrocycles For Photocatalytic Hydrogenation

The active center of coenzyme NADH,as a carrier and electron donor of biological hydrogen,is the mediator of most redox reactions in organisms according to reported research.Because of high cost and consumption of NADH,the development and regeneration of NADH analogs for redox reaction have become a new direction for researchers.The metal-organic supramolecule constructed by coordination-driven self-assembly has the functions of recognition,separation,hydrogen production and catalysis,etc.Addittionally,it can act as an enzyme mimic to combine with NADH analogs to carry out a series of photocatalytic hydrogenation reaction for simulating the natural enzymatic catalysis process.In this paper,a metal-organic trinuclear macrocycle Co-DPD was successfully constructed by self-assembly of redox-active Co²⁺and the carefully designed organic ligand H₂DPD containing phenanthridine.The efficient photocatalytic hydrogenation of water could be achieved by applying Co-DPD as the photocatalytic hydrogenation catalyst,with Ru（bpy）₃²⁺as the photosensitizer and ascorbic acid（H₂A）as the electron donor.Inspired by enzyme catalysis,a biomimetic hydrogenated imine strategy is proposed,that is,the confined cavity of the supramolecular reactor is used to encapsulate the imine molecule effectively,for reducing the distance between the substrate and the active site of the coenzyme mimics to achieve catalytic hydrogenation.The coenzyme mimic phenanthridine was delicately assembled on the surface of the metal-organic trinuclear macrocycle to transfer the protons and electrons inside and outside the supramolecular reactor cavity.The continuous single electron and active hydrogen species outside the ring generated by light drive were regulated to transfer onto the three-membered ring for reducing phenanthridine to dihydrophenanthridine,through which the biomimetic hydrogenation reaction of imine molecules in the cavity was further realized.The cycle regeneration of coenzyme mimics and the catalytic hydrogenation of imines were efficiently and synchronously achieved through coordinating the two catalytic cycles inside and outside the metal-organic trinuclear macrocycle.This supramolecular reactor exhibited excellent performance in the catalytic hydrogenation reaction system,where imine molecules containing different substituent groups could achieve higher hydrogenation yield without exception.To further improve the efficiency for synthesizing secondary amine molecules,the chelating group 2-diphenylphosphinobenzaldehyde of the previous organic ligand H₂DPD was substituted into 2-pyridinecarboxaldehyde,to obtain the ligand H₂BPD.The N₂O tridentate chelating site of H₂BPD and cobalt ion（II）were self-assembled to form a metal-organic four-membered ring Co-BPD.Compared with that of Co-DPD,the potential difference between Ru^II/Ru^I of Ru（bpy）₃²⁺and Co^II/Co^I of Co-BPD increased,which improved the electron transfer efficiency and promoted the driving force of the entire catalytic reaction,consequently accelerating the efficiency of catalytic hydrogenation.