Heterocyclic Phosphenium Cations and Their Divergent Coordination Chemistry

Abstract

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.

Pnictogen‐Rich Heterocycles Derived from a Phosphadiazonium Cation

“…Lithium hydrazonide 1 a [11] (0.04 g, 2 μmol) was dissolved in benzene before [Mes*NP][OTf] (0.07 g, 2 μmol) was added and the reaction mixture was stirred for 6 h. The color of the solution changed from red to dark yellow during this time. [Li][OTf] was then removed via gravity filtration using a glass frit and the resulting solution was concentrated in vacuo to yield a brown solid crude product.…”

Access to ligand-stabilized PH-containing phosphenium complexes

“…In many cases, reactions were observed at the metal instead of the P centre and thus, to date, the class of donor-to-phosphenium complex adducts have been only rarely reported. 17–19 Very recently, Ragogna and Gilroy described the first N-donor-to-phosphenium complex adducts II where the N-donor centre was tethered as P -substituent (Fig. 1).…”

“…1). 18 The first non-stabilised P–H functional phosphenium complex was reported by Schrock and Churchill in 1982. 20 Afterwards, only a few further examples were described and sparsely investigated.…”

Kinetically Stabilized Diarylpnictogenium Ions

“…Unlike those, pnictogenium ions are also strong electrophiles, which can be attributed to the positive charge that attracts any nucleophile present. Consequently, the vast majority of phosphenium ions, [R 2 P] + , [2–33] arsenium ions, [R 2 As] + , [34–51] stibenium ions, [R 2 Sb] +[52–59] and bismuthenium ions, [R 2 Bi] + , [60–72] reported in the literature are either affected by ion pairing or stabilized by electron‐rich ligands or substituents, which inter‐ or intramolecularly fill the vacant p‐orbital. These electronically stabilized pnictogenium ions often have a dramatically reduced Lewis acidity and cannot be regarded as genuine 6 VE species as they fulfill or even exceed the octet rule.…”