PNNL

Abstract

We report the heterolysis of molecular hydrogen under ambient conditions by the crystalline frustrated Lewis pair (FLP) 1-{2-[bis(pentafluorophenyl)boryl]phenyl}-2,2,6,6-tetramethylpiperidine (KCAT). The gas-solid reaction provides an approach to prepare the solvent-free, polycrystalline ion pair KCATH2 through an apparent 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 KAT 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 KAT 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 KAT to KATH2 is quantitative and our results suggest that the hydrogen heterolysis process to be diffusion controlled via H2 permeation into the FLP crystal. The authors (M.E.B., B.G., A.J.K., G.K.S., and T.A.) gratefully acknowledge support from U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences for providing the impetus for this research. S.M. was supported by DOE, Office of Science, Office of Workforce Development for Teachers and Scientists under the Science Undergraduate Laboratory Internships program. A portion of the research including some computation was performed using EMSL (grid.436923.9), a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. Further computing resources were provided by PNNL Research Computing. This research benefited from the use of the VISION beamline (IPTS- 12815) at the Spallation Neutron Source, Oak Ridge National Laboratory (ORNL), which is supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy (DOE). TOSCA experiments at the ISIS Neutron and Muon Source were supported by a beamtime allocation RB1510079 from the Science and Technology Facilities Council. K.C. and T.R. thank the Academy of Finland (project # 276586) for financial support.