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

Abstract

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 complexes, [Ru(Hbim)] (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(Hbim)]} (1), are simultaneously induced to a synchronized collective RuRu mixed-valence state, {RuRu}, with alternating arrays of Ru and Ru complexes by PCET in a way of the reductive state of {RuRu}. Further, a new crystal with {RuRu}, {[Ru(Hbim)(Hbim)][Ru(bim) (Hbim)][K(MeOBz)]} (2), was also prepared, and the solid-state CV revealed the same electrochemical behavior of {RuRu} with 1. The single crystal with {RuRu} of 2 was unusually a semiconductor with 5.12 × 10 S/cm conductivity at 298 K by an impedance method (8.01 × 10 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 = 0.30 eV) was realized in the H-bonding molecular crystal with {RuRu}. 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.

A Novel Thermocell System Using Proton Solvation Entropy

“…1:1 and 2:1 solutions show redox peaks at ca. 0.22 V vs. Ag/AgCl at 297 K, which were assigned to the reversible redox peaks of Ru II/III [24] . With increasing the temperature, the peak potentials shift to a negative direction (Figure 3 c), which is consistent with the negative S e value ( .…”

“…[Ru II (Him) 6 ]Cl 2 and [Ru II (H 2 biim) 3 ](PF 6 ) 2 were prepared according to the previous reports [22, 23, 24] . [Ru II (Him) 6 ]Cl 2 was dissolved in a mixed solution of acetonitrile and aqueous buffer (1/1, v/v), and the pH was adjusted to 10.9 by the addition of NaOH.…”

Green Rust: The Simple Organizing ‘Seed’ of All Life?

“…Mackinawite is also known to be an excellent conductor, suggesting that electron-hopping along the iron layers in mackinawite could be coupled to proton transfer using intermolecular hydrogen-bonds through fluctuation and sequential H-bonding, as observed in peptides [ 21 , 231 , 233 ]. Such coupling of proton flux to electron flow, and vice-versa, would be comparable to the proton-coupled electron transfer (PCET) reduction of nitrite to NO reaction involving cytochrome cd 1 in nitrite reductase [ 79 , 176 , 234 , 235 ]. Indeed, we can imagine the aqueous interlayers within mackinawite acting as a proton wire with a rate of proton transfer over 25 nm ns −1 (about a 10th of the speed of sound) driven by a redox disequilibrium of ~1 eV [ 174 , 175 , 236 ].…”

Green Rust: The Simple Organizing &lsquo;Seed&rsquo; of All Life?&nbsp;&nbsp;<strong> </strong>

“…[19,203] Such coupling of proton flux to electron flow, and vice-versa, would be comparable to the proton-coupled electron transfer (PCET) reduction of nitrite to NO reaction involving cytochrome cd1 in nitrite reductase. [202,203,204,205,206] Indeed, we can imagine the aqueous interlayers within mackinawite acting as a proton wire with a rate of proton transfer over 25 nm ns −1 (about a tenth of the speed of sound) driven by a redox disequilibrium of ~1 eV . [117,200,207] Yet the hydroxyl ions (or proton holes) tend to be immobilized by hydrogen-bonding to sulfur and the dangling oxygen is partially hydrated, thus inhibiting the diffusion of hydroxyls through the galleries from the alkaline hydrothermal fluid, and thereby favoring proton transfer.…”