Mixed-Valency with Hydrogen-Bonded Bimetallic Systems

Electron-transfer (ET) is one of the simplest chemical events, yet it is a vital process for chemical reactivity.  Proton-coupled electron transfer (PCET) is the concerted transfer of protons and electrons which is fundamental in chemical and biological systems and processes in electrochemistry, photosynthesis, and enzymatic activity. Similarly, mixed-valence (MV) systems are pervasive in nature and provide a platform for multiple electron transfers systems in nature.  While both fields have been studied extensively, little work has been done on the intersection of the two.  This project explores the elegant use of MV bimetallic systems through hydrogen-bonded networks.  Using standard Schlenk and glovebox techniques, a variety of compounds have been synthesized using Mo₂(DAniF)₃(O₂CCH₃) (where DAniF is the anion of N,N’-di-p-anisylformamidine) as the starting material, and further reacted with four different bisbidentate ligands (2,5-dihydroxy-1,4-benzoquinone, 3,4-dihydroxycyclobut-3-ene-1,2-dione, N,N’-dimethyloxamide, ethanedioic acid). Using cyclic (CV) and differential pulse voltammetry (DPV) in THF and in THF with water, a comproportionation constant, K_c, of 1 x 10² is calculated which shows that there is a hydrogen-bonded dimer in solution. The rate of ET in the hydrogen-bonded system is several orders of magnitude lower compared to the covalently bridged dimer.  The electronic communication between the hydrogen-bonded dimers can be described as ‘semi-localized’ Class II based on the Robin-Day classification. Density Functional Theory calculations support these findings.