Role of <i>d</i>‑Elements in a Proton–Electron Coupling of <i>d</i>–π Hybridized Electron Systems

We first demonstrate the influence of d-elements on the electronic-state alternation of molecules coupled with proton transfer in d–π hybridized electron systems. Compact and planar metal complexes with protonated 2,3-pyrazinedithiolates (L), M­(HL)2 (M = Ni, Pd, and Pt), were synthesized and subsequently determined to be assembled by hydrogen bond (H-bond) interactions between pyrazine moieties. Structural and theoretical investigations revealed that these complexes are regarded as d–π hybridized electron systems based on a M­(S2C2)2 core, especially, significant d–π hybridization in the Pt­(S2C2)2 core was indicated. The pH-dependent optical and electrochemical measurements revealed that the Ni complex has a higher proton-accepting character and a stronger pH dependence for redox potential compared with the Pt complex. This indicates that the Ni complex has a larger amount of π-electron density on ligands than the Pt complex because the significant d–π hybridization in the Pt complex could reduce the amount of π-electron reconstructed by attaching/detaching proton. Cyclic voltammetry of Ni and Pt complexes that form an H-bonded multimer showed a potential splitting at the first redox wave (ΔE1/2 = 0.28 V for M = Ni and 0.17 V for M = Pt) corresponding to a mixed-valence state coupled with proton transfer. The ΔE1/2 values indicate that the change in electronic states by proton transfer is remarkable in the Ni complex, but moderate in the Pt complex. These experimental results lead that the d-element substitution plays a role in controlling the degree of proton–electron coupling in d–π hybridized electron systems.