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

Go for the gold! Incorporating a borate anion into the backbone of a triphosphenium cation produces a unique zwitterionic phosphanide that can coordinate to one or two {AuCl} fragments depending on the steric bulk of the ligand (see picture; Au yellow, P purple, Cl green). Computational investigations show that in this μ-type ligand, the phosphorus atom behaves only as a σ,π donor.

Two unique systems based on low-coordinate main group elements that activate P4 are shown to quantitatively release the phosphorus cage upon short exposure to UV light. This reactivity marks the first reversible reactivity of P4, and the germanium system can be cycled 5 times without appreciable loss in activity. Theoretical calculations reveal that the LUMO is antibonding with respect to the main group element-phosphorus bonds and bonding with respect to reforming the P4 tetrahedron, providing a rationale for this unprecedented activity, and suggesting that the process is tunable based on the substituents.

Analytically pure chloride and bromide salts of two different cyclic triphosphenium cations are prepared by the reaction of PX3 (X=Cl, Br) in the presence of the halogen-scavenging reagent cyclohexene. For the brominated species, the neutral, volatile 1,2-dibromocyclohexane byproduct is readily removed under reduced pressure, and the desired salts are obtained in high yield. Reactions involving phosphorus trichloride are complicated by the formation of salts containing both chloride and hydrogen dichloride anions. Reactivity experiments on potential undesired halogenated diphosphine byproducts suggest that the formation of such species can be prevented by increasing the concentration of cyclohexene employed in the reaction.

A series of structurally differentiated cationic arsines containing imidazolium, cyclopropenium, formamidinium, and pyridinium substituents have been synthesized through short and scalable routes. Evaluation of the donor properties of these compounds by IR spectroscopy and DFT calculations reveals similar σ-electron-releasing abilities for all of them; however, their π-acceptor properties are strongly influenced by the nature of the positively charged group. We describe the coordination chemistry of the newly prepared α-cationic arsines toward different metal centers and their reactivity in the presence of strong oxidants to afford cationic As(V) species. Their unique electronic properties have been exploited in Pt(II) catalysis to develop a new catalyst with remarkable activity in the cycloisomerization of enynes to trisubstituted cyclopropanes. To the best of our knowledge, this is the first report on the use of α-cationic arsine ligands in catalysis