Iridium catalysts featuring amine-containing ligands for the dehydrogenation of formic acid

Abstract

The use of protic ligands to generate metal-ligand bifunctional catalysts has proved an excellent strategy to enhance the catalytic activity in formic acid dehydrogenation. We present here synthetic routes for complexes of general formula [Ir(H)2(IPr)(PR3)(CH3CN)2]BF4 (1a-c), [Ir (8-AQ)(H)2(IPr)(PR3)]BF4 (2a-c and 3), and [Ir(IPr)(C-N)(8-AQ)H]BF4 (5a-c) (8-AQ = 8-aminoquinoline; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; C-N = 1-phenylpiridine-1H, 1-phenylpirazol-1H or 2-(p-tolyl-1H)pyridine-1H). Complexes 2a-c, 3, 5a and c were evaluated as catalysts for the dehydrogenation of formic acid in water. The nature of the phosphane in complexes 2a-c and 3 affects drastically the catalytic activity, with more basic and less encumbered phosphanes bringing about better performances. DFT calculations suggest that the coordinated NH2 moiety of the 8-AQ ligand plays a crucial role throughout the catalytic cycle. Two key steps are the protonation of the hydride ligand directed by the NH2 moiety (H2 formation), and the hydride abstraction to regenerate the dihydride active species (CO2 formation). Complexes 5a and 5c do not improve the results obtained with 2a-c and 3, probably due to the fact that a preactivation step is required. This may be attributable to an isomerization step that places the NH2, initially trans to the hydride ligand, in cis position.