Pyridyl–triazole ligands enable in situ generation of a highly active dihydride iridium(iii) complex for formic acid dehydrogenation

Abstract

The reaction of [Cp*IrCl₂]₂ with 1-pyridyl-1,2,3-triazoles La–Ld in the presence of sodium triflate yielded the complexes [Cp*Ir(k²-NN)(Cl)][OTf] (1a–1d) in which the k²-NN ligand coordinates to iridium through the pyridyl nitrogen and the N2 nitrogen of the triazole ring, leaving the more basic N3 atom free. These complexes serve as efficient catalytic precursors for the dehydrogenation of formic acid in the presence or absence of an external solvent, achieving turnover frequencies (TOF_max) of up to 10 703 h⁻¹ and producing a 1 : 1 mixture of hydrogen and carbon dioxide with no detectable carbon monoxide. The catalysts can be reused after a single reaction cycle, as they retain their activity. This retention of activity has allowed a cumulative turnover number (TON) of 26 876 to be reached after six reloading cycles of neat formic when using sodium formate as a base. Kinetic and ¹H NMR studies reveal that the active catalyst, [Cp*Ir(CO)H₂] (2), forms in situ from precursors 1a–1d under the reaction conditions. Thus, complexes 1a–1d function as stable precatalysts, simplifying the use of the otherwise unstable complex 2. The reaction mechanism is likely to involve mono- and dihydride species. The decarboxylation of the coordinated formate is the rate-determining step at high formic acid concentrations, according to the kinetic profiles and kinetic isotope effect (KIE) values obtained.