Independent Tuning of the p K a or the E 1/2 in a Family of Ruthenium Pyridine-Imidazole Complexes.

Two series of Ru II (acac) 2 (py-imH) complexes have been prepared, one with changes to the acac ligands and the other with substitutions to the imidazole. The proton-coupled electron transfer (PCET) thermochemistry of the complexes has been studied in acetonitrile, revealing that the acac substitutions almost exclusively affect the redox potentials of the complex (|Δ E 1/2 | ≫ |Δp K a |·0.059 V) while the changes to the imidazole primarily affect its acidity (|Δp K a |·0.059 V ≫ |Δ E 1/2 |). This decoupling is supported by DFT calculations, which show that the acac substitutions primarily affect the Ru-centered t 2g orbitals, while changes to the py-imH ligand primarily affect the ligand-centered π orbitals. More broadly, the decoupling stems from the physical separation of the electron and proton within the complex and highlights a clear design strategy to separately tune the redox and acid/base properties of H atom donor/acceptor molecules.