Trioxatriangulenium (TOTA ⁺ ) as a robust carbon-based Lewis acid in frustrated Lewis pair chemistry

Abstract

We report the reactivity between the water stable Lewis acidic trioxatriangulenium ion (TOTA⁺) and a series of Lewis bases such as phosphines and N-heterocyclic carbene (NHC). The nature of the Lewis acid-base interaction was analyzed <i>via</i> variable temperature (VT) NMR spectroscopy, single-crystal X-ray diffraction, UV-visible spectroscopy, and DFT calculations. While small and strongly nucleophilic phosphines, such as PMe₃, led to the formation of a Lewis acid-base adduct, frustrated Lewis pairs (FLPs) were observed for sterically hindered bases such as P( ^<i>t</i> Bu)₃. The TOTA⁺-P( ^<i>t</i> Bu)₃ FLP was characterized as an encounter complex, and found to promote the heterolytic cleavage of disulfide bonds, formaldehyde fixation, dehydrogenation of 1,4-cyclohexadiene, heterolytic cleavage of the C-Br bonds, and interception of Staudinger reaction intermediates. Moreover, TOTA⁺ and NHC were found to first undergo single-electron transfer (SET) to form [TOTA]·[NHC]˙⁺, which was confirmed <i>via</i> electron paramagnetic resonance (EPR) spectroscopy, and subsequently form a [TOTA-NHC]⁺ adduct or a mixture of products depending the reaction conditions used.