Independent Tuning of the pKa or the E1/2 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 (vertical bar Delta E-1/2 vertical bar >> vertical bar Delta pK(a)vertical bar center dot 0.059 V) while the changes to the imidazole primarily affect its acidity (vertical bar Delta pK(a)vertical bar center dot 0.059 V >>vertical bar Delta E-1/2 vertical bar). 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 pi 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.

Automated summary

Two series of Ru-II(acac)(2)(py-imH) complexes were synthesized, one with changes to acac ligands and the other with substitutions to imidazole. The proton-coupled electron transfer (PCET) thermochemistry of the complexes in acetonitrile was studied, and it was found that the acac substitutions mainly affected the redox potentials of the complex, while the changes to the imidazole primarily affected its acidity. DFT calculations supported this decoupling phenomenon, which has implications for separately tuning the redox and acid/base properties.