Electronic Properties and Reactivity of Triarylcarbenium Ions

Abstract

Carbocations are often presented as elusive species in traditional nucleophilic substitution or elimination reactions. In the 1950s, efforts to isolate carbocations were first reported in the literature. Since then, several attempts to stabilize carbocation centers have gained considerable attention, due to their unique physical and electronic properties. Based on these findings, this dissertation explores ways to synthesize novel carbocation species, namely triarylcarbenium ions, and study their reactivity, electronic properties, and applications in coordination chemistry and catalysis. Furthermore, this dissertation describes the syntheses of different triarylcarbenium ions and explores their Lewis acidic and electronic structure using density functional theory (DFT). Both quantitative and qualitative data were collected from each of the carbenium ions studied, including relative energies, molecular orbital (MO) diagrams, spin populations, and conformation analyses. Based on previous studies demonstrating the acidity and stability of trioxatriangulenium (TOTA+), its application as a Lewis acid (LA) for frustrated Lewis pair (FLP) chemistry was of considerable interest. DFT calculations demonstrate the partial localization of the cation on the central carbon in TOTA+ and the relative stabilization energies with different Lewis bases (LBs) to form FLPs. Data from these calculations support previous experimental findings and shed light on a proposed mechanistic pathway to activate phenyldisulfide. The activation of this disulfide bond occurs via π-stacking interaction with TOTA+, upon dissociation of the phosphine-based FLP. Thus, providing a robust, water-stable scaffold that can be used for activation of small molecules. Owing to the inherent acidity of the triarylcarbenium ions in this study, the synthesis of carbenium-based ambiphilic ligands that enable the coordination of electron-rich transition metal centers is presented. The interactions between the carbocation and the resultant metal complexes, containing [MCl3L]- centers (M = Co or Ni, L = pyridine), were calculated using DFT. Both gas phase and solvated models were used with and without Grimme’s empirical dispersion using D3 damping functions (GD3). GD3 functions support the presence of a weak electrostatic carbocation-[MCl3L]- interaction. Spin density and natural bond order analyses were used to understand the electronic profile of the one-electron reduction of a family of helical carbenium ions to form bench stable organic radicals. Spin population analyses demonstrated that the unpaired electron resided in the central carbon atom, containing about 30% of the radical character. Similarly, time dependent DFT studies were performed to model the electronic absorption profile of the helical carbon-based radical. Proposed uses for these radicals include photoredox applications, such as photo-induced reductive quenching reactions of organic substrates. DFT calculations supported experimental findings and shed light on the electronic profile and reactivity of different triarylcarbenium ions, including evidence for the formation of temperature dependent FLPs and their potential reactivity with other aryl-based substrates via LA-assisted π-stacking interactions. The calculated electronic profiles presented can be used to further study the reactivity of similar organic radicals.