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. UV–vis titration data provided average pKa 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 4–6DHBP 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 ∼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 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 6DHBP complex was found to facilitate
the rate-determining step and renders 6DHBP a more effective
catalyst for formate production.