Influence of pH on the Photochemical and Electrochemical Reduction of the Dinuclear Ruthenium Complex,[(phen)_2Ru(tatpp)Ru(phen)_2]Cl_4,in Water:Proton-Coupled Sequential and Concerted Multi-Electron Reduction

摘要

The dinuclear ruthenium complex[(phen)_2Ru(tatpp)Ru-(phen)_2]~(4+)(P;in which phen is 1,10-phenanthroline and tatpp is 9,11,20,22-tetraaza tetrapyrido[3,2-a:2'3'-c:3",2"-l:2'",3'"]-pentacene)undergoes a pho-todriven two-electron reduction in aqueous solution,thus storing light energy as chemical potential within its structure.The mechanism of this reduction is strongly influenced by the pH,in that basic conditions favor a sequential process involving two one-electron reductions and neutral or slightly acidic conditions favor a proton-coupled,bi-electronic process.In this complex,the central tatpp ligand is the site of electron storage and protonation of the central aza nitrogen atoms in the reduced products is observed as a function of the solution pH.The reduction mechanism and characterization of the rich array of products were determined by using a combination of cyclic and AC voltammetry along with UV-visible reflectance spectroelectrochemistry experiments.Both the reduction and protonation state of P could be followed as a function of pH and potential. From these data,estimates of the vari ous reduced species' pK_a values were obtained and the mechanism to form the doubly reduced,doubly protonated complex,[(phen)_2Ru(H_2tatpp)Ru-(phen)_2]~(4+)(H_2P)at low pH(<=7)could be shown to be a two-proton,two-electron process.Importantly,H_2P is also formed in the photochemical reaction with sacrificial reducing agents,albeit at reduced yields relative to those at higher pH.