Structural alternatives for the formation of halogenophosphine-phosphenium complexes

NMR studies of reactions between some N-heterocyclic and acyclic diamino phosphenium ions (R2N)2P+ and P-chlorophosphines (R2N)2PCl suggest that the reactants interact via chloride scrambling rather than by formation of P-P bonded phosphenium-phosphine complexes. Computational studies of reactions between model ions (R'2N)2P+ and neutral phosphines (R'2N)2PX (X = F, Cl, Br) confirm that in the gas phase the formation of halide-bridged adducts is indeed preferred and only for the most electrophilic cation an alternative but energetically less favorable P-P bonded structure was found. The halide-bridged adducts feature nearly C2-symmetrical P...X...P arrays (for X = Cl, Br) or are loose molecular complexes arising from electrostatic interaction between nearly unperturbed fragments (for X = F). In the latter case, a P...F...P-bridged structure was located as a transition state of a fluoride transfer reaction. The formation of the adducts appears to be controlled by electrostatic rather than orbital interactions. Consideration of solvent effects by a polarizable continuum model indicates a destabilization of the adducts versus the isolated fragments and suggests that in solution extensive dissociation occurs. The computations further reveal a large solvent-induced lengthening of the P-Cl bonds in N-heterocyclic halogenophosphines which implies that the unusual P-Cl distances observed for these species are, to a large part, attributable to intermolecular influences.