Positional Effects of Hydroxy Groups on Catalytic Activity of Proton-Responsive Half-Sandwich Cp*Iridium(III) Complexes

Proton-responsive half-sandwich Cp*Ir­(III) complexes possessing a bipyridine ligand with two hydroxy groups at the 3,3′-, 4,4′-, 5,5′-, or 6,6′-positions (<b>3DHBP</b>, <b>4DHBP</b>, <b>5DHBP</b>, or <b>6DHBP</b>) were systematically investigated. UV–vis titration data provided average p<i>K</i>_a values of the hydroxy groups on the ligands. Both hydroxy groups were found to deprotonate in the pH 4.6–5.6 range for the <b>4</b>–<b>6DHBP</b> complexes. One of the hydroxy groups of the <b>3DHBP</b> complex exhibited a low p<i>K</i>_a value of <0.4 because the deprotonation is facilitated by the strong intramolecular hydrogen bond formed between the generated oxyanion and the remaining hydroxy group, which in turn leads to an elevated p<i>K</i>_a value of ∼13.6 for the second deprotonation step. The crystal structures of the <b>4</b>- and <b>6DHBP</b> complexes obtained from basic aqueous solutions revealed their deprotonated forms. The intramolecular hydrogen bond in the <b>3DHBP</b> complex was also observed in the crystal structures. The catalytic activities of these complexes in aqueous phase reactions, at appropriate pH, for hydrogenation of carbon dioxide (pH 8.5), dehydrogenation of formic acid (pH 1.8), and transfer hydrogenation reactions using formic acid/formate as a hydrogen source (pH 2.6 and 7.2) were investigated to compare the positional effects of the hydroxy groups. The <b>4</b>- and <b>6DHBP</b> complexes exhibited remarkably enhanced catalytic activities under basic conditions because of the resonance effect of the strong electron-donating oxyanions, whereas the <b>5DHBP</b> complex exhibited negligible activity despite the presence of electron-donating groups. The <b>3DHBP</b> complex exhibited relatively high catalytic activity at low pH owing to the one strong electron-donating oxyanion group stabilized by the intramolecular hydrogen bond. DFT calculations were employed to study the mechanism of CO₂ hydrogenation by the <b>4DHBP</b> and <b>6DHBP</b> complexes, and comparison of the activation free energies of the H₂ heterolysis and CO₂ insertion steps indicated that H₂ heterolysis is the rate-determining step for both complexes. The presence of a pendent base in the <b>6DHBP</b> complex was found to facilitate the rate-determining step and renders <b>6DHBP</b> a more effective catalyst for formate production