Can a decrease in anti-aromaticity increase the dihydrogen activation ability of a frustrated phosphorous/borane Lewis p
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Can a decrease in anti‑aromaticity increase the dihydrogen activation ability of a frustrated phosphorous/borane Lewis pair?: a DFT study Manas Ghara1 · Pratim Kumar Chattaraj1,2 Received: 29 September 2020 / Accepted: 11 November 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract 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 H 2 activation as revealed by the thermochemical and kinetic data. A significant amount of electron density is transferred from H 2 molecule to the FLPs at the transition states (TSs) during the process of H 2 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 H 2 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. Keywords Frustrated Lewis pair · Dihydrogen activation · Transition state · Aromaticity
1 Introduction In the last decade, the frustrated Lewis pair (FLP) has dramatically changed the chemistry of catalysis by replacing the transition metal based strategy in the activation of small molecules saving both economy and environment. When a Lewis acid (LA) and a Lewis base (LB) are unsuited to form a traditional donor–acceptor bond, a frustrated complex is formed as stated by Stephan [63]. As a result, the acidity and the basicity of the Lewis centers remain unquenched and they appeared as FLP with further reactivity. However, the frustrated Lewis centers may be present in the same molecule, and then, it is termed as an intramolecular FLP. An intramolecular boron/phosphorous FLP was reported by Stephan et al. [70, 71] in 2006 for the activation of molecular hydrogen in a reversible fashion. Following this discovery, several experimental [31, 34, 40, 44, 46, 59, 60, 69, 76] as well as theoretical studies [27, 28, 42, 45, 47, 50–52, * Pratim Kumar Chattaraj [email protected] 1
Department of Chemistry and Center for Theoretical Studies, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
2
56] have been made to find out the mechanism behind this activation process by the FLPs. From these studies, a reactivity model was prescribed, in which transfer of electron takes place simultaneously from the lone pair of the LB to the σ* orbital of H2 and from the σ orbital of H2 to the empty orbital of the LA in a push–pull mechanism. This promotes the weakening of the H–H bond as the reaction progresses and finally cleaves the bond h
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