Heterolytic Scission of Hydrogen Within a Crystalline Frustrated Lewis Pair

Abstract

We report the heterolysis of molecular hydrogen under ambient conditions by the crystalline frustrated Lewis pair (FLP) 1-{2-[bis(pentafluorophenyl)boryl]phenyl}-2,2,6,6tetramethylpiperidine (KCAT). The gas-solid reaction provides an approach to prepare the solvent-free, polycrystalline ion pair KCATH2 through a single crystal to single crystal transformation. The crystal lattice of KCATH2 increases in size relative to the parent KCAT by approximately 2%. Microscopy was used to follow the transformation of the highly colored red/orange KCAT to the colorless KCATH2 over a period of 2 hours at 300 K under a flow of H2 gas. There is no evidence of crystal decrepitation during hydrogen uptake. Inelastic neutron scattering employed over a temperature range from 4 -200 K did not provide evidence for the formation of polarized H2 in a pre-cursor complex within the crystal at low temperatures and high pressures. However, at 300 K the INS spectrum of KCAT transformed to the INS spectrum of KCATH2. Computational methods suggest that the driving force is more favorable in the solid state compared to the solution or gas phase, but the addition of H2 into the KCAT crystal is unfavorable. Ab Initio methods were used to calculate the INS spectra of KCAT, KCATH2 and a possible precursor complex of H2 in the pocket between the B and N of crystalline KCAT. Ex-situ NMR showed that the transformation from KCAT to KCATH2 is quantitative and our results suggest that the hydrogen heterolysis process occurs via H2 diffusion into the FLP crystal with a rate-limiting movement of H2 from inactive positions to reactive sites.

Activation of Molecular Hydrogen by Inter‐ and Intramolecular Al−N Lewis Pairs

“…(1-(2-(bis(penta-fluorophenyl)boryl) phenyl)-2,2,6,6-tetramethylpiperidine) was recently found to occur during a single-crystal to single-crystal transformation by Autrey and coworkers. [5] In contrast to these FLPs, the AlÀ N systems garnered far less attention. Such studies were pursued, e. g. by Uhl and coworkers, [6] and rare examples for reactions with chalcogendouble bonds (CO 2 , [7] ketones, [8] isocyanates [9] ), dehydrocoupling, [10] acryl and lactone polymerisation have been reported.…”

Polymorphism of dimethylaminoborane N(CH3)2-BH2

“…Dehydrogenation reactions based on H + and H À interactions are a promising strategy for reversible hydrogen release, storage and activation, e.g. in ammonia borane (Staubitz et al, 2010), amine-metal borohydrides (Jepsen, Ley, Filinchuk et al, 2015;Jepsen, Ley, Cerny et al, 2015;Castilla-Martinez et al, 2020), and solutions containing frustrated Lewis pairs (FLPs) (Welch et al, 2006;Stephan & Erker, 2010) and even solid FLPs (Bowden et al, 2020). Notably, aminoboranes offer high gravimetric hydrogen content, which may be released stepwise.…”