Tuning The O–O Bond Formation Pathways Of Molecular Water Oxidation Catalyst On Electrode Surface Via Second Coordination Sphere Engineering

Molecular WOCs have drawn much attention owing to the facile tunability of theirstructural and electronic properties,and the ability to mediate multiple proton-electron transfer reactions,which can facilitate comprehensive mechanistic studies.Up to now,the research on molecular WOCs mainly focuses on the coordination structure of them,however,it has not been fully studied that the catalytic efficiency of the relationship between the second coordination sphere of the molecular WOCs on its catalytic activity.In addition,the O–O bond formation mechanism of WOCs would be changed when they are introduced to the surface of electrodes,as the catalytic environment is totally different from the homogeneous environment.In order to study the effects of the second coordination sphere on immobilized WOCs on the surface of electrode,molecular [Ru(bda)]-type(bda = 2,2'-bipyridine-6,6'-dicarboxylate)water-oxidation catalyst with 4-vinylpyridine as the axial ligand(Complex 1)was immobilized,or co-immobilized with 1-(trifluoromethyl)-4-vinylbenzene(3F)or styrene(St)as the blocking unit on the surface of glassy carbon(GC)electrodes by electrochemical polymerization to prepare the functional electrodes poly-1@GC,poly-1+P3F@GC,and poly-1+PSt@GC,respectively.Kinetic studies of electrode surface reaction revealed that [Ru(bda)] triggers the O–O bond formation via the radical coupling interaction between the two metallo-oxyl radicals(I2M)in the homo-coupling polymer(poly-1),and the water nucleophilic attack(WNA)pathway in copolymers of poly-1+PSt,which means that the mechanism of O–O bond formation could be tuned by changing the distance between the center of WOCs.Although WNA is the major pathway for poly-1+P3F,the strong dipole moment of the 3F can interact with and stabilize these [Ru(bda)] intermediates with partial charges,resulting in negative shifts of the redox potentials of [Ru(bda)] in it,which could also suppress the role of the extra base on the solution APT process.Moreover,due to fluorine atoms of C(sp3)–F bonds can serve as hydrogen-bonding acceptor,the rate of the WNA pathway is finally accelerated.The comparison between these three electrodes revealed that the second coordination sphere surrounding the water oxidation catalysts plays vital roles in stabilizing their reactionintermediates,tuning the O–O bond formation pathways,and improving water-oxidation reaction kinetics without changing the first coordination structures.