Dehydrogenation of formic acid using iridium-NSi species as catalyst precursors

Using a low loading of the iridium(III) complexes [Ir(CF₃SO₃)(κ²-NSi^iPr)₂] (1) (NSi^iPr = (4-methylpyridin-2-yloxy)diisopropylsilyl) and [{Ir(κ²-NSi^Me)₂}₂(μ-CF₃SO₃)₂] (2) (NSi^Me = (4-methylpyridin-2-yloxy)dimethylsilyl) in the presence of Et₃N, it has been possible to achieve the solventless selective dehydrogenation of formic acid. The best catalytic performance (TOF_{5 min} ≈ 2900 h^-1) has been achieved with 2 (0.1 mol%) and Et₃N (40 mol% to FA) at 373 K. Kinetic studies at variable temperatures show that the activation energy of the 2-catalyzed process at 353 K is 22.8 ± 0.8 kcal mol^-1. KIE values of 1.33, 2.86, and 3.33 were obtained for the 2-catalyzed dehydrogenation of HCOOD, DCOOH, and DCOOD, respectively, in the presence of 10 mol% of Et₃N at 353 K. These data show that the activation of the C-H bond of FA is the rate-determining step of the process. A DFT mechanistic study for the catalytic cycle involving hydride abstraction from the formate anion by the metal, assisted by a molecule of formic acid, and heterolytic H₂ formation has been performed. Moreover, the presence of Ir-formate intermediates was identified by means of NMR studies of the catalytic reactions in thf-d₈ at 323 K. In all the cases, the decomposition of the catalyst to give unactive crystalline iridium NPs was observed.