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

Using a low loading of the iridium(III) complexes [Ir(CF3SO3)(κ2-NSiiPr)2] (1) (NSiiPr = (4-methylpyridin-2-yloxy)diisopropylsilyl) and [{Ir(κ2-NSiMe)2}2(μ-CF3SO3)2] (2) (NSiMe = (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 (TOF5 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.The financial support from projects PGC2018-099383-B-I00 (MCIU/AEI/FEDER, UE), RTI2018-099136-A-I00 (MCIU/AEI/FEDER, UE), CSIC project 202080I024 and DGA/FSE project E42_20R is gratefully acknowledged. A. U. thankfully acknowledges the Spanish MECD for a FPU fellowship (FPU 2017/05417).Peer reviewe