Investigating Tetrel-Based Neutral Frustrated Lewis Pairs for Hydrogen Activation.

Abstract

Tetrel Lewis acids are a prospective alternative to commonly employed neutral boranes in frustrated Lewis pair (FLP) chemistry. While cationic tetrylium Lewis acids, being isolobal and iso(valence)electronic, are a natural replacement to boranes, neutral tetrel Lewis acids allude as less trivial options due to the absence of a formally empty p orbital on the acceptor atom. Recently, a series of intramolecular geminal FLPs (C2F5)3E-CH2-P(tBu)2 (E = Si, Ge, Sn) featuring neutral tetrel atoms as acceptor sites has been reported for activation of small molecules including H2. In this work, through density functional theory computations, we elucidate the general mechanistic picture of H2 activation by this family of FLPs. Our findings reveal that the acceptor atom derives the required Lewis acidity utilizing the antibonding orbitals of its adjacent bonds with the individual contributions depending on the identity of the acceptor and the donor atoms. By varying the identity of the Lewis acid and Lewis base sites and attached substituents, we unravel their interplay on the energetics of the H2 activation. We find that switching the donor site from P to N significantly affects the synchronous nature of the bond breaking/formations along the reaction pathway, and as a result, N-bearing FLPs have a more favorable H2 activation profile than those with P. Our results are quantitatively discussed in detail within the framework of the activation-strain model of reactivity along with the energy-decomposition analysis method. Finally, the reductive elimination decomposition route pertinent to the plausible extension of the H2 activation to catalytic hydrogenation by these FLPs is also examined.