Studies on Control of Proton－Electron Coupling and Functionalization Based on Metal-Organic Complexes

概要

Proton has been the subject of intense research on the electrochemical system, including ion transport and redox processes. Among them, proton conduction, which is a particular case of ionic conduction and has been widely used in the research of batteries and supercapacitors, can migrate through Lewis basic media such as water and ammonia via Grotthuss mechanism. In addition, the proton as a hard acid facilitates the formation of covalent bonds followed by the reconstruction of π-conjugated system to modulate the electronic state (so-called proton– electron coupling). To date, metal complexes have been extensively utilized for exploring and controlling the proton–electron coupling system because of the diverse selection of components, i.e., metal ions and ligand molecules. However, the metal ion serves as a redox-active center in most metal complexes, although the introduction of redox-active ligands is more promising to control the proton–electron coupling in metal complexes through their chemical modification. In addition, the geometric isomerization of metal complexes, which has been extensively studied, is also expected to affect the proton–electron coupling by controlling the electronic state through the isomeric transformation while keeping the components unchanged. This thesis aims to investigate the proton conduction behavior in surface-modified metal-organic frameworks (MOFs) and to achieve the control of proton–electron coupling by two approaches, i.e., liganddriven redox process and metal–ligand isomeric transformation.

Chapter 2. CO2 Mediated Proton Conductive Metal-Organic Frameworks
The author investigated the CO2 mediated proton conduction based on a diaminefunctionalized MOF, Mg2(dobpdc)-mmen (dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate; mmen = N,N′-dimethylethylenediamine), under high CO2 pressure condition. First, the humidity- and temperature-dependent proton conductivity was evaluated to determine the effect of the diamine modification. Electrochemical impedance measurements revealed that the proton conductivity of Mg2(dobpdc)-mmen (4.3 × 10−5 S cm−1) is four orders of magnitude higher than that of pristine Mg2(dobpdc) (2.9 × 10−9 S cm−1) at 298 K and 95% relative humidity (RH). Application of CO2 gas to Mg2(dobpdc)-mmen resulted in the increase in proton conductivity by an order of magnitude; from 1.2 × 10−12 S cm−1 under dry conditions (0.3 bar) to 2.2 × 10−11 S cm−1 under 10 bar CO2. This finding is firm evidence that the chemisorption of CO2 followed by the intermolecular proton transfer promote the production of proton responsible for conduction.

Chapter 3. Strong Proton–Electron Coupling in p-Planar Metal Complex with Redox-Active Ligands
The author selected a π-planar nickel complex with redox-active o-iminothiosemiquinonate (itsq1–) ligands to achieve strong proton-concerted electron transfer (PCET) system. The N,Sdonor ligand can promote the reconstruction of the π-conjugated system by protonation because of the expanded orbital on sulfur atoms. Furthermore, the higher σ and π donating ability of sulfur atom can stabilize the complexation with metal ions and improve the electron storage properties of the complex. The strong proton–electron coupling in the complex was demonstrated by combining experimental and theoretical approaches. Electrochemical measurements revealed that the shift in the acidity-dependent redox potential (ca. 63 mV) is twice larger than that of [Ni(dcpdt)2]2– (dcpdt2–: 5,6-dicyano-2,3-pyrazinedithiolate; ca. 29 mV) with redox-inactive ligands. On the other hand, theoretical calculations predicted that the proton–electron coupling in the complex, which was evaluated from the energetic shift of LUMO by protonation, is more pronounced than that in [Ni(dcpdt)2]2–. The stabilization of the LUMO in Ni(itsq)2 (4.2 eV per protonation step) is more pronounced than that of the HOMO in [Ni(dcpdt)2]2– (3.2 eV per protonation step). These findings indicate that metal complexes with redox-active ligands are promising platforms for developing novel PCET-type materials.

Chapter 4. Isomerization-controlled Proton–Electron Coupling in p-Planar Metal Complex
The author demonstrated the first example of the control of the proton–electron coupling by the metal–ligand isomeric transformations in a π-planar platinum complex, Pt(itsq)2. The trans isomer forms a herringbone-type packing structure, whereas the cis isomer is dimerized to cancel out the dipole in the crystal. Theoretical calculations revealed that the metal–ligand isomeric transformation has a significant effect on proton–electron coupling than the metal substitution. The trans and cis forms exhibit different energy stabilization of LUMO orbitals during different protonation process, which possibly arises from the different distributions of Pt atom to the Pt– S antibonding orbital. In addition, time-dependent 1H NMR spectroscopy demonstrated the isomeric transformation from cis to trans forms with different proton–electron coupling in organic solvents, as predicted by simulated potential energy curve. The faster transformation in C5D5N (k1 = 3.47×10–5 s–1) than those in other solvents (1.85×10–6 s–1 for CDCl3, 3.47×10–7 s–1 for CD2Cl2, and 3.24×10–7 s–1 for CD3CN) seems to be a consequence of the higher Lewis basicity of pyridine which stabilizes the intermediate state. These results indicate that the proton–electron coupling is controlled by external stimuli that induce the isomerization.