Dehydrogenation of formic acid by Ir-bisMETAMORPhos complexes: experimental and computational insight into the role of a cooperative ligand

The synthesis and tautomeric nature of three xanthene-based bisMETAMORPhos ligands (La-Lc) is reported. Coordination of these bis(sulfonamidophosphines) to Ir(acac)(cod) initially leads to the formation of Ir-I(L-H) species (1a), which convert via formal oxidative addition of the ligand to Ir-III(L) monohydride complexes 2a-c. The rate for this step strongly depends on the ligand employed. Ir-III complexes 2a-c were applied in the base-free dehydrogenation of formic acid, reaching turnover frequencies of 3090, 877 and 1791 h(-1), respectively. The dual role of the ligand in the mechanism of the dehydrogenation reaction was studied by H-1 and P-31 NMR spectroscopy and DFT calculations. Besides functioning as an internal base, bisMETAMORPhos also assists in the pre-assembly of formic acid within the Ir-coordination sphere and aids in stabilizing the rate-determining transition state through hydrogenbonding.