Catalytic amine-borane dehydrogenation by a PCP-pincer Palladium complex: a combined experimental and DFT analysis of the reaction mechanism

Catalytic dehydrogenation of ammonia-borane (NH3oBH3, AB) and dimethylamine borane (NHMe2-BH3, DMAB) by the PdII complex [(tBuPCP)Pd(H2O)]PF6 [tBuPCP = 2,6-C6H3(CH2PtBu2)2] leads to oligomerization and formation of spent fuels of general formula cyclo-[BH2-NR2]n (n = 2,3; R = H, Me) as reaction byproducts, while one equivalent of H2 is released per amine-borane equivalent. The processes were followed through multinuclear (31P, 1H, 11B) variable temperature NMR spectroscopy; kinetic measurements on the hydrogen production rate and the relative rate constants were also carried out. One non-hydridic intermediate could be detected at low temperature, whose chemical nature was explored through a DFT modeling of the reaction mechanism, at the M06//6-31+G(d,p) computational level. The computational output was of help to propose a reliable mechanistic picture of the process.