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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.

A DFT-based computational study has been performed on the hydrogenation of CO2, catalyzed by a bridged FLP. Formic acid might be formed in two possible pathways as revealed by this study. In one way, the Lewis basic center of the FLP activates H2, and the Lewis acidic center activates CO2 simultaneously in the first step of the reaction. Alternatively, the Lewis basic center of the FLP activates CO2, and the Lewis acidic center activates H2. The simultaneous activation of CO2 and H2 through single TS is confirmed by NBO analysis. Free energy profiles are also generated for both the possible pathways in solvent phase. It appears from these profiles that the first step, i.e., simultaneous activation of CO2 and H2, is the rate determining for both the reaction pathways. A significant amount of barrier height is reduced in comparison to that in the corresponding uncatalyzed reaction as observed in these profiles. The nature of donor-acceptor interactions present in the transition state geometries is further analyzed by energy decomposition analysis (EDA) methods. The EDA analysis shows that the HOMO of the FLP donates electron density to the LUMO of H2, the HOMO of H2 donates electron density to the LUMO of CO2, and several occupied MOs of CO2 donate electron density to the LUMO of FLP at the TS geometry.

The role of boron–ligand cooperation in activating molecular hydrogen by a set of six frustrated Lewis pair (FLP) systems is explored using density functional theory. The results obtained from thermochemical calculations show that all the studied FLP systems are capable of activating molecular hydrogen as the activation free energies are not too high (17.6–25.6 kcal mol−1). Sulphur based FLP 6 is the most promising one as it results in the smallest activation barrier among the studied sets. For a given FLP, the introduction of an electron donating –NMe2 group at the para position of the pyridine ring somewhat lowers the barrier and enhances the B–X (X = O, N, S) interaction. The B–X bond present within the FLPs plays a crucial role in facilitating the H2 activation process where it gets changed from the B+–X− type of interaction in the FLP to the B ← X dative bond upon H2 activation as understood from the energy decomposition analysis in combination with the natural orbital for chemical valence theory (EDA-NOCV). This mode of operation is termed as boron–ligand cooperation in analogy with the metal–ligand cooperation in transition metal complexes. The EDA-NOCV results obtained at the TS also support an electron transfer model where simultaneous electron transfer takes place from the Lewis basic center (N) of the FLP to σ*(H2) and from σ(H2) to the Lewis acidic center (B) of the FLP, resulting in a weakened H–H bond. The change in the aromaticity of the pyridine rings during the course of H2 activation is also monitored by nucleus independent chemical shift calculations. Finally, the ability of the studied FLP systems to act as hydrogenation catalysts is elucidated by studying the hydrogenation of CO2 to yield formic acid.

The mechanism of dihydrogen activation has been theoretically investigated by means of DFT calculation. An experimentally synthesized bridged P/B frustrated Lewis pair (FLP) and two designed FLPs are used for this purpose. The model FLPs 2 and 3 are more efficient than FLP 1 for H2 activation as revealed by the thermochemical and kinetic data. A significant amount of electron density is transferred from H2 molecule to the FLPs at the transition states (TSs) during the process of H2 activation, and this is greater at the corresponding TSs of FLPs 2 and 3 than that of FLP 1. The NICS(0) and NICS(1zz) of the boron heterocycle at the FLPs 2 and 3, and at the corresponding TSs and the product geometries of H2 activation demonstrate that the anti-aromatic character of the rings in the FLPs is remarkably reduced at the TSs and finally at the products and that is most likely responsible for enhanced activity of FLPs 2 and 3 by decreasing the activation barrier.

The activation of allene by a five-membered P/B frustrated Lewis pair (FLP) has been explored by DFT based computational study. It reveals the addition of allene to the FLP takes place via two thermodynamically favorable path. But, the attack of the P center of the FLP to the middle carbon of allene is kinetically more preferable. The reaction is further analysed by taking methyl-allene and cyano-allene and which produce four thermodynamically favorable regioisomers. However the attack of the P center of the FLP to the middle carbon is kinetically more preferable. The mechanism of the reactions are carefully analysed.

Department of Chemistry and Center for Theoretical Studies, Indian Institute of Technology Kharagpur, Kharagpur-721 302, West Bengal, India E-mail: pkc@chem.iitkgp.ac.in Manuscript received 23 July 2018, accepted 10 August 2018 Nitric oxide (NO) can be trapped by a “frustrated Lewis pair” (FLP) containing two ambiphilic boron centers as shown through the results from a DFT based computation. There are three possible ways in which NO can bind to the FLP. Among the three possible paths only two are thermodynamically favorable with respect to enthalpy as well as free energy changes. However, all the products are kinetically favorable as explained by lower values of free energy barriers (less than 15.0 kcal/mol). The favorable orbital interactions between the HOMO of FLP and LUMO of NO play a crucial role in giving these three possible products as revealed by molecular orbital analysis, which is further confirmed by NBO analysis, where the net gain of elec­tron density is shown to take place on the NO segment. On the other hand, the WBI values along with the AIM analysis re­veal that all the newly formed B-N and B-O bonds are partially covalent in nature. &nbsp