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 new H-bonded crystal [Ru III (Him) 3 (Im) 3 ] with three imidazole (Him) and three imidazolate (Im À ) groups was prepared to obtain a higher-temperature proton conductor than a Nafion membrane with water driving. The crystal is constructed by complementary NÀ H•••N H-bonds between the Ru III complexes and has a rare Icy-c* cubic network topology with a twofold interpenetration without crystal anisotropy. The crystals show a proton conductivity of 3.08 × 10 À 5 S cm À 1 at 450 K and a faster conductivity than those formed by only HIms. The high proton conductivity is attributed to not only molecular rotations and hopping motions of HIm frameworks that are activated at ~113 K, but also isotropic wholemolecule rotation of [Ru III (Him) 3 (Im) 3 ] at temperatures greater than 420 K. The latter rotation was confirmed by solid-state 2 H NMR spectroscopy; probable proton conduction routes were predicted and theoretically considered.
Invited for the cover of this issue is mainly the group of Makoto Tadokoro and co‐workers at Tokyo University of Science. Other co‐workers are Masaki Itoh, Ryota Nishimura, Kensuke Sekiguchi (TUS students), Dr. Norihisa Hoshino (Tohoku Univ.), Dr. Hajime Kamebuchi (Nihon Univ.), Dr. Jun Miyazaki (Tokyo Denki Univ.), Prof. Motohiro Mizuno (Kanazawa Univ.) and Prof. Tomoyuki Akutagawa (Tohoku Univ.). The image depicts on two mechanisms of proton transport rotations of the proton‐conductive starburst molecule [RuIII(HIm)3(Im)3]. Read the full text of the article at 10.1002/chem.202201397.
The crystals of [RuIII(HIm)3(Im)3](1) are one of the rapid proton‐transport materials to work as proton conductors at high temperatures. The proton flow is caused by the strong rotations of Im⋅⋅⋅H−Im H‐bonding imidazole groups from 200 K. In the upper left image, as the temperature increases to 400 K, six coordinated imidazole groups are strongly rotating to transport protons. In the lower right image, whole‐molecule rotations start becoming the main rotation above 400 K. More information can be found in the Research Article by M. Tadokoro et al. (DOI: 10.1002/chem.202201397).
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