Trioxatriangulenium (TOTA+) as a Robust Carbon-based Lewis Acid in Frustrated Lewis Pair Chemistry

We report the reactivity between the Lewis acidic trioxotriangulenium 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 via 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 stable Lewis acid-base adduct, frustrated Lewis pairs (FLPs) were observed for sterically hindered bases such as P(^tBu)₃. The TOTA⁺-P(^tBu)₃ FLP was characterized as an encounter complex, and was found to promote the heterolytic cleavage of disulfide bonds, formaldehyde fixation, dehydrogenation of 1,4-cyclohexadiene, heterolytic cleavage of C–Br bond, and interception of Staudinger reaction intermediate. Moreover, TOTA⁺ and NHC was found to first undergo single-electron transfer (SET) to form [TOTA]^•[NHC] ^•+, which was confirmed via electron paramagnetic resonance (EPR) spectroscopy, and subsequently form a [TOTA-NHC]⁺ adduct or a mixture of products depending the reaction condition used.