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

Abstract

Catalytic dehydrogenation of ammonia-borane (NH₃·BH₃, AB) and dimethylamine borane (NHMe₂·BH₃, DMAB) by the Pd^II complex [(^tBuPCP)Pd(H₂O)]PF₆ [^tBuPCP = 2,6-C₆H₃(CH₂P^tBu₂)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.