Multielectron Photoreduction of a Bridged Ruthenium Dimer, [(phen)2Ru(tatpp)Ru(phen)2][PF6]4: Aqueous Reactivity and Chemical and Spectroelectrochemical Identification of the Photoproducts

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Abstract
The dinuclear ruthenium(II) complex [(phen)2Ru(tatpp)Ru(phen)2][PF6]4 (P) (where phen is 1,10-phenanthroline and tatpp is 9,11,20,22-tetraazatetrapyrido[3,2-a:2‘3‘-c:3‘ ‘,2‘ ‘-l:2‘ ‘‘,3‘ ‘‘]pentacene) is shown to accept up to four electrons and two protons on the central tatpp bridging ligand via a combination of stoichiometric chemical reductions and protonations and spectroelectrochemistry (SEC) in acetonitrile. The absorption spectra of seven distinct species related by reduction and/or protonation of the central tatpp ligand were obtained and the two sequential photoproducts obtained from visible irradiation of P in acetonitrile (with 0.25 M triethylamine (TEA)) thus identified as P- (singly reduced, nonprotonated P) and HP- (doubly reduced, monoprotonated P), respectively. Importantly, the photochemical activity is maintained in mixed water−acetonitrile (1:4) solutions under basic conditions, and the protonation state of the photoproducts is readily controlled by varying the solution pH between 8 and 12. Absorption spectra obtained by SEC under similar solvent conditions were virtually identical to those obtained photochemically, and thus the doubly reduced photoproducts were identified as P2- (pH 12), HP- (pH 10), and H 2 P (pH 8). This last photoproduct, H 2 P, is particularly promising with respect to solar hydrogen production in that it can be produced in the presence of water and its dehydrogenation under appropriate conditions could yield H2 and regenerate P. A qualitative MO diagram is presented as a framework for understanding the observed optical transitions as a function of oxidation and protonation state.

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