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 (3DHBP, 4DHBP, 5DHBP, or 6DHBP) were systematically investigated. UVvis titration data provided average pK(a) values of the hydroxy groups on the ligands. Both hydroxy groups were found to deprotonate in the pH 4.65.6 range for the 46DHBP complexes. One of the hydroxy groups of the 3DHBP complex exhibited a low pKa 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 pKa value of similar to 13.6 for the second deprotonation step. The crystal structures of the 4- and 6DHBP complexes obtained from basic aqueous solutions revealed their deprotonated forms. The intramolecular hydrogen bond in the 3DHBP 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 4- and 6DHBP complexes exhibited remarkably enhanced catalytic activities under basic conditions because of the resonance effect of the strong electron-donating oxyanions, whereas the 5DHBP complex exhibited negligible activity despite the presence of electron-donating groups. The 3DHBP 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 4DHBP and 6DHBP complexes, and comparison of the activation free energies of the H-2 heterolysis and CO2 insertion steps indicated that H-2 heterolysis is the rate-determining step for both complexes. The presence of a pendent base in the 6DHBP complex was found to facilitate the rate-determining step and renders 6DHBP a more effective catalyst for formate production.