Synchronized Collective Proton-Assisted Electron Transfer in Solid State by Hydrogen-Bonding Ru(II)/Ru(III) Mixed-Valence Molecular Crystals

A proton-coupled electron transfer (PCET) reaction was widely studied with isolated organic molecules and metal complexes in solution in view of the biological catalytic reaction, while studying this reaction in the crystalline or solid-state phase, which has a novel example, would give insight into the rather internal environment of proteins without solvation and a creation of new molecular materials. We tried to crystallize a hydrogen-bonded (H-bonded) coordination polymer with one-dimensional nanoporous channels, formed from redox-active Ru^III complexes, [Ru^III(Hbim)₃] (Hbim^- = 2,2'-biimidazolate monoanion). As a result, a synchronized collective PCET phenomenon was observed for the molecular nanoporous crystal by novel solid-state cyclic voltammetry (CV), which could be measured by only setting some crystals on the electrode surface. The nanoporous crystals, {[Ru^III(Hbim)₃]}_n (1), are simultaneously induced to a synchronized collective Ru^IIRu^III mixed-valence state, {Ru^IIRu^III}_n, with alternating arrays of Ru^II and Ru^III complexes by PCET in a way of the reductive state of {Ru^IIRu^II}_n. Further, a new crystal with {Ru^IIRu^III}_n, {[Ru^II(H₂bim)(Hbim)₂][Ru^III(bim) (Hbim)₂][K(MeOBz)₆]}_n (2), was also prepared, and the solid-state CV revealed the same electrochemical behavior of {Ru^IIRu^III}_n with 1. The single crystal with {Ru^IIRu^III}_n of 2 was unusually a semiconductor with 5.12 × 10^-6 S/cm conductivity at 298 K by an impedance method (8.01 × 10^-6 S/cm by a direct-current method at 277 K). Thus, an unprecedented electron-hopping conductor driven by a low-barrier proton transfer through a PCET mechanism (E_a = 0.30 eV) was realized in the H-bonding molecular crystal with {Ru^IIRu^III}_n. Such studies on a PCET reaction in the crystalline state is not only worthwhile as a model of essential biological reactions without solvation, but also proposed to a new design of molecular materials to occur an electron transfer by using an intermolecular H-bond.