Structural, electronic and acid/base properties of [Ru(tpy)(tpyOH)] ²⁺ and [Ru(tpyOH)₂]²⁺ (tpy = 2,2′:6′,2″-terpyridine, tpyOH = 4′-hydroxy-2,2′: 6′,2″-terpyridine)

The development of pH switchable metal catalysts by choosing ligands with multiple protonation states would be a useful design principle. In order to better understand how such a system would work, interactions between the ligand and metal in the varying protonation states must be studied. In this work, we have synthesized para-hydroxyl-substituted terpyridine complexes of ruthenium, [Ru(tpy)(tpyOH)]²⁺ and [Ru(tpyOH)₂]²⁺ (tpy = 2,2′:6′,2″-terpyridine, tpyOH = 4′-hydroxy-2,2′: 6′,2″-terpyridine), to determine how the protonation state affects the electronic properties of the complexes using UV-Vis spectroscopy and computational techniques. In addition, we have studied the electrochemical and structural properties of these complexes. Both complexes yield standard Metal to Ligand Charge Transfer (MLCT) electronic transitions in the visible region when in the protonated state. However, when deprotonated new electronic transitions from a filled molecular orbital consisting of a mixture of metal d and deprotonated pyridinolate ligand orbitals to empty ligand orbitals are observed. These transitions have been termed mixed metal-ligand to ligand charge transfer (M_mixLCT). In deprotonated [Ru(tpy)(tpyO^-)]⁺, containing only one deprotonatable group, the ligand only mixes with one of the three degenerate metal d orbitals to give one new mixed metal-ligand orbital. However, in deprotonated [Ru(tpyO^-)₂], the two pyridinolate groups are orthogonal to each other and each mix with a separate metal d orbital to give two new mixed metal-ligand orbitals. These results lead to the potential design principle for catalytic systems in which specific orbital interactions based on protonation state and orientation of the ligand can be tuned.