Heterocyclic Phosphenium Cations and Their Divergent Coordination Chemistry

While they are often encountered as reaction intermediates, phosphenium cations are not commonly incorporated into π-conjugated systems. We report the synthesis and characterization of donor-stabilized phosphenium cations supported by pyridylhydrazonide ligands. The preparation of these cations relies on precise control of ligand E-Z isomerism. The heterocycles were treated with a variety of transition metals, with [Rh(COD)Cl]₂ yielding the only well-defined organometallic products. The optoelectronic properties of the phosphenium heterocycles and their transition-metal complexes were examined using UV-vis absorption spectroscopy, cyclic voltammetry, and modeling by density functional theory (DFT). Computations support the description of these compounds as phosphenium cations and corroborate our observation of a weak P-N_pyridine bond, which was manifested experimentally as the Rh adducts undergo selective insertion of Rh into the P-N_pyridine bond, depending on the substituent at phosphorus. The reported compounds provide a framework for further study of π-conjugated, N,N'-chelated phosphenium cations and their transition-metal adducts.