Activation of molecular hydrogen by a B/N frustrated Lewis pair.

The chemistry of frustrated Lewis pair (FLP) is widely explored in the activation of small molecules, the hydrogenation of CO2, and unsaturated organic species. A survey of several experimental works on the activation of small molecules by FLPs and the related mechanistic insights into their reactivity from electronic structure theory calculation are provided in the present review, along with the catalytic hydrogenation of CO2. The mechanistic insight into H2 activation is thoroughly discussed, which may provide a guideline to design more efficient FLP for H2 activation. FLPs can activate other small molecules like, CO, NO, CO2, SO2, N2O, alkenes, alkynes, etc. by cooperative action of the Lewis centers of FLPs, as revealed by several computational analyses. The activation barrier of H2 and other small molecules by the FLP can be decreased by utilizing the aromaticity criterion in the FLP as demonstrated by the nucleus independent chemical shift (NICS) analysis. The term boron-ligand cooperation (BLC), which is analogous to the metal-ligand cooperation (MLC), is invoked to describe a distinct class of reactivity of some specific FLPs towards H2 activation.

The efficacy of B borospherene to act as a host for noble gas atoms is explored via density functional theory based computations. Although the Ng@B complexes are thermochemically unstable with respect to dissociation into free Ng and B, it does not rule out their viability as all the systems possess a high activation free energy barrier (84.7-206.3 kcal mol). Therefore, once they are formed, it is hard to take out the Ng atom. Two Ng atoms can also be incorporated within B for the lighter Ng atoms (He and Ne). In fact, the destabilization offered by the encapsulation of one and two He atoms and one Ne atom inside B is significantly less than that in experimentally synthesized He@CH, highlighting their greater possibility for synthesis. Although Ar and Kr encapsulated B systems are very much destabilized by the repulsive interaction between Ng and B, an inspection of the bonding situation reveals that the confinement can even induce some degree of covalent interaction between two otherwise non-bonded Ng atoms. Ng atoms transfer electrons towards B which is smaller for lighter Ng atoms and gradually increases along He to Rn. Even if the electrostatic interaction between Ng and B is the most predominant term in these systems, the extent of the orbital interaction is also considerable. However, the very large Pauli repulsion counterbalances the attractive interaction, eventually turning the interaction repulsive in nature. Ng@B also shows dynamical behaviour involving continuous exchange between hexagonal and heptagonal holes, similar to the host cage, as understood from the very little variation in the activation barrier because of the Ng encapsulation. Furthermore, sandwich complexes like [(η-CMe)Fe(η-B)] and [(η-CMe)Fe(η-B)] are noted to be viable with the latter being slightly more stable than the former. The encapsulation of Xe slightly improves the dissociation energy associated with the decomposition into Xe@B and [Fe(η-CMe)] compared to that in the bare one.

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The cycloaddition of ethylene, cyanoethylene, and propylene to a five-membered P/B frustrated Lewis pair (FLP) is shown to be highly favorable under normal conditions, as confirmed by the computed thermodynamic and kinetic data. All of these cycloaddition reactions are concerted as highlighted by the intrinsic reaction coordinate (IRC) and Wiberg bond index calculations. Almost 70% of the reaction force is required for structural orientation to initiate electronic activity. The reactions are interpreted by the frontier molecular orbital (FMO) analysis and conceptual density functional theory (DFT)-based reactivity descriptors. It appears that ethylene and propylene will act as nucleophiles, while the FLP will act as an electrophile throughout the cycloaddition reaction, however, cyanoethylene will act as an electrophile and the FLP as a nucleophile. Regioselectivities of the cycloadditon of cyanoethylene and propylene to the FLP are further verified through philicity and dual descriptors. It is demonstrated that an FLP can be forced to act as an electrophile or a nucleophile by intelligently selecting its partner in a cycloaddition reaction. Even the P and B centers would behave differently within the same FLP. This strategy may be properly exploited by the experimentalists in designing a suitable reaction for the synthesis of any useful molecule possessing the desired property.