Effect of Protonation upon Electronic Coupling in the Mixed Valence and Mixed Protonated Complex, [Ni(2,3-pyrazinedithiol)₂]

We demonstrate that protonation of a mixed valence molecule, generating a mixed valence mixed protonated (MVMP) state, results in a severe reduction in the electronic coupling intimately connected with electron transfer kinetics. This phenomenon is illustrated by synthesizing a mixed valence molecule, [Ni(2,3-pyrazinedithiol)₂], that can be asymmetrically protonated, rendering the MVMP state. We characterize the structural, electronic, vibrational, and magnetic properties of this complex in five different states, including the mixed valence and MVMP states, and then analyze the intervalence charge transfer (IVCT) band to demonstrate a five-fold reduction in electronic coupling upon protonation. We conclude that the reduction in electronic coupling is a result of the asymmetry of the electronic orbitals of the redox sites that results from the asymmetric protonation. This conclusion suggests that many systems designed to link electron and proton transfer will also exhibit a decrease in electronic coupling upon protonation as the strength of the interaction between redox and protonation sites is increased.