Design and synthesis of phosphorus cations and exploring their reactivity

Abstract

Phosphorus cations are a compelling class of main-group species due to their high electrophilicity, potential for novel bonding, and ability to mimic some aspects of transition metal reactivity, especially in small-molecule activation and bond activation chemistry. Diimine ligands or related bidentate/ π-donating ligands can stabilize phosphorus cations through delocalization of positive charge and/ or by π-donation, enabling isolation of otherwise highly reactive or transient species while still retaining reactivity at the phosphorus center. However, diimine-stabilized phosphorus cations remain relatively underexplored in literature because their high electrophilic nature and tendency to undergo undesirable side reactions make them difficult to isolate and handle under normal conditions. Systems with constrained or planar phosphorus centers (often enforced by ligand frameworks) have been predicted and observed to show unusual reactivity: for example, enhanced Lewis acidity, small-molecule activation (e.g. N-H, C-H, halide abstraction), or insertions.. Previous works have shown that phosphorus dications with N-heterocyclic imine substituents are isolable. Although NHCs have been employed to stabilize phosphorus cations, their strong σ-donation often over-stabilizes the cation and suppresses electrophilicity. This leaves their reactivity less explored. Diimine ligands and aminotroponimine ligands, by contrast, provide charge delocalization while retaining reactivity, and their chelating nature can enforce unique geometries such as planar phosphorus centers. Thus, the phosphorus cations stabilized by these ligands, offer access to reactivity patterns that are less accessible with NHC stabilization. Motivated by these opportunities, my project aims to synthesize novel diimine and aminotroponimine stabilized phosphorus cations and systematically explore their reactivity.