An in situ redox method is employed to prepare N-heterocyclic bromophosphines in good yield and purity. Such bromophosphines may be treated with a variety of bromide-abstracting reagents to produce the corresponding N-heterocyclic phosphenium salts in excellent yield.Salts containing phosphenium cations have played an important role in the history and development of modern maingroup chemistry. In the most general definition, a phosphenium cation ( 1) is a cation that contains a dicoordinate phosphorus center bearing a total of six valence electrons and is the isovalent analogue of a carbene (2). 1,2 While numerous types of phosphenium cations have been prepared and studied, the most important class of phosphenium compounds is the relatively stable species in which the dicoordinate phosphorus center is supported by two adjacent amido substituents. Although such compounds are analogous to the now-ubiquitous N-heterocyclic carbenes (NHCs, 3) 3,4 and may be labeled N-heterocyclic phosphenium cations (NHPs, 4), it is worth noting that well-characterized NHPs were reported more than 35 years ago 5,6 and thus predate the first report of a stable NHC considerably. In fact, the structural characterization of a salt containing a 1,3,2-diazaphospholenium cation, an unsaturated NHP directly analogous to the most common type of "Arduengo" NHC, 7 was reported as early as 1990. 8

The synthesis and comprehensive characterization of a series of base-stabilized ChX(2) (Ch = Se, Te; X = Cl, Br) is reported using aryl-substituted diazabutadiene and 2,2'-bipyridine (bipy) as the ligands. In stark contrast to free ChX(2) the complexes display excellent thermal stability. Their use as viable ChX(2) reagents that may be stored for later use is demonstrated in principle. The syntheses are simple and high-yielding from commercially available or easily synthesized reagents. The bipy complexes are exceedingly rare examples of this ubiquitous ligand being utilized within Group 16 chemistry; the Se examples are the first to be characterized by X-ray crystallography, and the Te species are only the second.

A convenient preparative approach to low-valent phosphorus-rich oligomers is presented. Ligand substitution reactions involving anionic diphosphine ligands of the form [(PR2)2N](-) and [(PPh2)2C5H3](-) and a triphosphenium bromide P(I) precursor result in the formation of phosphorus(I)-containing heterocycles, several of which are of types that have never been prepared before. The methodology described also allows for the preparation of the known heterocycle cyclo-[P(PPh2)N(PPh2)]2 in better yields and purity than the synthetic approach reported previously. Preliminary reactivity studies demonstrate the viability of such zwitterionic oligomers as multidentate ligands for transition metals.