Polyphosphorus Cations Proliferate

Novel synthetic procedures developed by an international chemical team have added diversity to the difficult-to-prepare family of cyclic phosphino-phosphonium salts (J. Am. Chem. Soc., DOI: 10.1021/ja907693f). These catenated polyphosphorus cations containing organic substituents make up a unique class of compounds that potentially could be used in catalysis and materials development. Controlling the formation of complex architectures based on catenated phosphorus has been near impossible because P–P bonds are intrinsically weak, says Jan J. Weigand of Westfälische Wilhelms University, in Münster, Germany, who led the team with Neil Burford of Dalhousie University, in Halifax, Nova Scotia. Building on previous work from Burford’s group, the researchers show that overcoming that weakness is possible if a positive charge is controllably introduced into the system. For example, the team chlorinated cyclic tetracyclohexyltetraphosphine to form a phosphino-chlorophosphonium dication in which two phosphorus atoms of the P₄ ring host a chlorine in addition to a cyclohexyl (shown). Reacting this dication with trimethylphosphine and other organophosphines dissociates the P₄ ring to form various charged linear and heterocyclic carbon-phosphorus compounds, demonstrating the ability to further expand on polyphosphorus chemistry, Weigand says.