Date of Award

Summer 2025

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

First Advisor

Dmitry V. Peryshkov

Abstract

Transition metal catalysis has been an integral part of synthetic chemistry for its ability to selectively activate strong substrate bonds, promoting numerous crucial transformations. Despite their utility, the precious metals that are often employed in these catalytic reactions suffer from several drawbacks (including high toxicity, low natural abundance, high costs, and low sustainability), increasing the motivation to design suitable metal-free alternatives. Towards this end, several ambiphilic main-group systems have been devised to show metal-like reactivity, such as carbenes, borylenes, and Frustrated Lewis Pairs. This manuscript will describe the ambiphilic reactivity of carborane-based phosphorus compounds, highlighting the unique metallomimetic reactivity that arises from tethering electron-donating phosphine arms onto an electron-accepting and redox-active carborane scaffold.

Carboranyl diphosphines have been shown to activate main-group hydrides, alcohols, and electron-deficient terminal alkynes in what can be described as overall oxidative addition to the molecular system. In these reactions, the carborane cluster becomes reduced by two electrons, with opening to a nido- form, and the phosphine arms are oxidized to phosphonium cations, with formation of new bonds to the substrate. In the reactions with alkynes, the products additionally feature novel carborane-containing alkenylphosphonium heterocycles. Insights into the mechanisms of these alkyne activations will be discussed, illuminating the origin of the resultant intramolecular B–H bond activation and cyclization. Carboranyl diphosphines have also been shown to activate the N–H bonds of ammonia, and the remarkable ability of this main-group system to affect the activation of both gaseous and aqueous ammonia to form air- and water-stable products will be highlighted. Subsequent hydrogen atom abstraction of the activated species with TEMPO represents the first example of ammonia oxidation via triple hydrogen atom abstraction facilitated by a metal-free system. Mechanistic investigations, including a computational examination of the unique electrophilic reactivity of the carborane-bound phosphines, will be described.

Lastly, the synthesis and characterization of numerous zwitterionic carboranyl diphosphoniums featuring phosphonium-bound -Br, -CN, -N3, -NCSe, and -NCS functionalities will be reviewed. The potential applications of these carboranyl diphosphoniums toward electrophilic group transfer will be discussed, and the reaction of a carboranyl diazidophosphonium with an external phosphine, resulting in the formation of P–N–P linkages, will be highlighted.

This work demonstrates the incredible synergy between the carborane backbone and exohedral phosphine substituents, enabling metallomimetic reactivity in this main-group system. The transformations and mechanistic details described in this manuscript represent a contribution to the chemistry of redox-active boron clusters and highlight the opportunities for carboranyl diphosphines in sustainable metal-free transformations.

Rights

© 2025, Amanda Lynn Humphries

Download

Share

COinS