Molecular Motion in Frustrated Lewis Pair Chemistry: insights from modelling

Sammanfattning: Mechanisms of reactions of the frustrated Lewis pairs (FLPs) with carbon dioxide (CO2) and hydrogen (H2) are studied by using quantum chemical modelling. FLPs are relatively novel chemical systems in which steric effects prevent a Lewis base (LB) from donating its electron pair to a Lewis acid (LA). From the main group of the periodic table, a variety of the electron pair donors and acceptors can create an FLP and the scope of the FLP chemistry is rapidly expanding at present. Representative intermolecular FLPs are phosphines and boranes with bulky electron-donating groups on phosphorus and bulky electron-withdrawing groups on boron – e.g., the tBu3P/B(C6F5)3 pair. The intramolecular FLPs feature linked LB and LA centers in one molecule.Investigations of the FLP reaction mechanisms were carried out using the transition state (TS) and the potential energy surface (PES) calculations plus the Born-Oppenheimer molecular dynamics (BOMD) as an efficient and robust implementation of general ab initio molecular dynamics scheme. In BOMD simulations, quantum and classical mechanics are combined. The electronic structure calculations are fully quantum via the density functional theory (DFT). Molecular motion at finite (non-zero) temperature is explicitly accounted for at non-quantized level via Newton’s equations. Due to recent advancements of computers and algorithms, one can treat fairly large macromolecular systems with BOMD and even include significant portion of the first solvation shell surrounding a large reacting complex in the molecular model.Main results are as follows. It is shown that dynamics is significant for understanding of FLP chemistry. The multiscale nature of motion – i.e., light molecules such as CO2 or H2 versus a pair of heavy LB and LA molecules – affects the evolution of interactions in the reacting complex. Motion which is perpendicular to the reaction coordinate was found to play a role in the transit of the activated complex through the TS-region. Regarding the heterolytic cleavage of H2 by tBu3P/B(C6F5)3 FLP simulated in gas phase and with explicit solvent, it was found that (i) the reaction path includes shallow quasi-minima “imbedded” in the TS-region, and (ii) tBu3P/B(C6F5)3 are almost stationary while proton- and hydride-like fragments of H2 move toward phosphorous and boron respectively. For binding of CO2 by tBu3P/B(C6F5)3 FLP, it was found that (i) the reacting complex can “wander” along the “potential energy wall” that temporarily blocks the path to the product, and (ii) the mechanism can combine the concerted and two-step reaction paths in solution. The discovered two-step binding of CO2 by tBu3P/B(C6F5)3 FLP involves solvent-stabilized phosphorus-carbon interactions (dative bonding). These and other presented results are corroborated and explained using TS and PES calculations. With computations of observable characteristics of reactions, it is pointed out how it could be possible to attain experimental proof of the results.