Water-Assisted Concerted Proton-Electron Transfer at Co(II)-Aquo Sites in Polyoxotungstates With Photogenerated RuIII (bpy)33+ Oxidant

Abstract

The cobalt substituted polyoxotungstate [Co₆ (H₂ O)₂ (α-B-PW₉ O₃₄ )₂ (PW₆ O₂₆ )]^17- (Co6) displays fast electron transfer (ET) kinetics to photogenerated Ru^III (bpy)₃³⁺ , 4 to 5 orders of magnitude faster than the corresponding ET observed for cobalt oxide nanoparticles. Mechanistic evidence has been acquired indicating that: (i) the one-electron oxidation of Co6 involves Co(II) aquo or Co(II) hydroxo groups (abbreviated as Co6(II)-OH₂ and Co6(II)-OH, respectively, whose speciation in aqueous solution is associated to a pK_a of 7.6), and generates a Co(III)-OH moiety (Co6(III)-OH), as proven by transient absorption spectroscopy; (ii) at pH>pK_a , the Co6(II)-OH→Ru^III (bpy)₃³⁺ ET occurs via bimolecular kinetics, with a rate constant k close to the diffusion limit and dependent on the ionic strength of the medium, consistent with reaction between charged species; (iii) at pH <pK_a , the process involves Co6(II)-OH₂ →Co6(III)-OH transformation and proceeds via a multiple-site, concerted proton electron transfer (CPET) where water assists the transfer of the proton, as proven by the absence of effect of buffer base concentrations on the rate of the ET and by a H/D kinetic isotope in a range of 1.2-1.4. The reactivity of water is ascribed to its organization on the surface of the polyanionic scaffold through hydrogen bond networking involving the Co(II)-OH₂ group.

Keywords: Co-aquo moiety; cobalt polyoxometalate; flash photolysis; hydrogen bonding; proton coupled electron transfer.