To understand the effect of chemical modification on the stability and proton−electron coupling in neutral radical molecules with a proton−electron-transfer (PET) state, we investigate a nickel dithiolene complex with cyano-substituted pyrazine skeletons using experimental and theoretical methods. A Pourbaix diagram constructed from absorption spectroscopic and cyclic voltammetric measurements strongly suggests that the PET state of the complex is significantly more stable compared with that of the nonsubstituted complex. Theoretical calculations predicted that the introduction of electron-withdrawing groups leads to stabilization of the PET state mainly because of a greater delocalized electron distribution in the molecule. Crystallographic studies, with the support of theoretical calculations, revealed that the degree of coupling between protons and electrons varies depending on the Hammett σ value of the substituents; the electronic state of the nonsubstituted complex appears to be most sensitive to the protonated state mainly owing to the spatially confined π-electron system.

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Metal string complexes, linearly aligned transition metal arrays coordinated with the multidentate organic ligands, have gained much attention both in unique electronic/structural properties and in potential applications as conductive molecular nanowires. Here we report on a dimerized Ni II trinuclear complex, [Ni 6 Cl 2 (dpa) 8 ](I 5 ) 2 • 0.25I 2 (dpa − = 2,2′-dipyridylamide anion). X-ray structural analysis revealed that two trinuclear moieties are bridged by a Cl anion to form a dimerized string structure. This is the first example of two Ni string complexes that are connected. In the electronic absorption and Raman spectra, characteristic absorption bands and a vibration mode based on the dimer string structure were observed. In the solid state, dimer complexes align in one dimension in an MMMXMMMX (M = metal, X = halogen) manner, leading to the intra-and interdimer antiferromagnetic interactions.

Three mixed crystals, κ-(ET)2Ag2x Cu2(1–x)(CN)3 [ET is bis­(ethylenedithio)­tetrathiafulvalene; 0.24 < x < 0.71] with a κ-type packing motif of face-to-face ET dimers, were obtained by electrocrystallization. Regardless of the composition, each ET dimer fits into a hexagonal anionic opening (i.e., key-on-hole packing) similar to its parent spin liquid candidate, κ-(ET)2Cu2(CN)3. X-ray diffraction and energy dispersive spectroscopy analyses revealed that Cu and Ag atoms are statistically disordered with a fairly homogeneous distribution in a crystal. A structural variation depending on x is responsible for the change in the calculated band parameters related to intermolecular interactions, electron correlations, and frustrations. A salt with nearly equimolar amounts of Ag and Cu (x = 0.49) is semiconductive at ambient pressure and undergoes a Mott transition upon application of hydrostatic pressure. Along with the positive pressure dependence of the transition temperature, the temperature-independent amplitude of magnetic torque at low temperatures suggests that the insulating phase is a quantum spin liquid. Further application of pressure results in the appearance of a superconducting phase. Contrary to those of the parent salts, κ-(ET)2Cu2(CN)3 and κ-(ET)2Ag2(CN)3, the transition temperature increases as the pressure increases and eventually reaches 4.5 K at 1.65 GPa.

We synthesized a molecule-based proton–electron mixed conductor (PEMC), a Pt­(III) dithiolate complex with 1,4-naphthoquinone skeletons. The π-planar Pt complex involves a π-stacking column, which is connected by one-dimensional hydrogen bonding chains composed of water molecules. The room-temperature (RT) proton conductivity is 8.0 × 10–5 S cm–1 under ambient conditions, which is >2 orders of magnitude higher than that of the isomorphous Ni complex (7.2 × 10–7 S cm–1). The smaller activation energy (0.23 eV) compared to that of the Ni complex (0.42 eV) possibly originates from the less dense water, which promotes the reorientational dynamics, in the Pt complex with an expanded lattice, namely, negative chemical pressure upon substitution of Ni with the larger Pt. In addition, the Pt complex shows a relatively high RT electronic conductivity of 1.0 × 10–3 S cm–1 caused by the π-columns, approaching an ideal PEMC with comparable proton and electron conduction.