Rational Design of Proton–Electron-Transfer System Based on Nickel Dithiolene Complexes with Pyrazine Skeletons

Abstract

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.

Proton and Electron Transfer in the Formation of a Copper Dithiolene-Based Coordination Polymer

“…EPR and magnetic susceptibility measurements indicate that the protonated dithiolene complexes additionally possess unpaired electrons and must have been reduced during the formation of 3. The Xband cw EPR spectrum of a powder sample of 3 measured at 293 K (Figure 10) exhibits a typical anisotropic pattern for Cu 2+ (3d 9 , S = 1/2) in a magnetically nondiluted powder. 67,68 The 63,65 Cu hyperfine interaction between the electron spin S = 1/2 and the nuclear spin I Cu = 3/2 was not resolved due to the high metal atom concentration.…”

“…Often, dithiolene complexes can be isolated in a series with different total charges due to the ligands contributing to redox reactions. This behavior has been described as “non-innocence” and can be attributed to a similarity in energies of the metal and ligand frontier orbitals. − In N -heterocyclic dithiolene complexes, the electron transfer can further be coupled to the transfer of protons. − Metal dithiolene complexes find application in mainly two fields. First, especially N -heterocyclic dithiolene complexes serve in bioinorganic modeling studies, as the active sites of many metalloenzymeslike the molybdenum and tungsten-dependent oxidoreductasescontain metal dithiolene moieties. , Second, dithiolene metal complexes are widely investigated for their potential application in electronic devices.…”

Recent advances in the mechanisms of the hydrogen evolution reaction by non-innocent sulfur-coordinating metal complexes

“…The stability of the PCET state was computationally found to increase with the Hammett constant of the pyrazinedithiolate substituent group on a series of pyrazinedithiolate nickel complexes, attributed to increased charge delocalization. 152 Complexes 25 and 26, where the π-system of the pyrazine ring is extended, were also synthesized and their catalytic performances in aqueous solutions were evaluated and compared. 153 Indeed, complexes 25 and 26 achieve overpotential values of 0.17 V and 0.23 V, respectively, at pH 9.0, which indicates a large stabilization of the PCET state when compared to complex 23.…”

Data enhanced Hammett-equation: reaction barriers in chemical space

“…This model is compelling since it gives an intuitive concept of electron donating and electron withdrawing effects [3][4][5][6] in the context of free energy differences. The model quickly became quite successful and has been applied to problems ranging from its original purpose, quantifying substituent effects, 3 to redox potentials, 7 dipole moments, 8 orbital energies of metallorganic complexes, 9 aromaticity, [10][11][12][13][14][15][16][17][18][19][20][21] ion stabilization, 22 mechanicistic investigation, 23,24 catalyst activity of nanoparticles, 25 proton-electron coupling in radicals, 26 molecular conductance, 27 excited singlet state, 28 and even toxicities. 29 More recent approaches have also tried to apply the models to nonbenzyl systems.…”