Phosphenium Versus Pro-Phosphide Character of P-tert-butyl-dicyclopropeniophosphine: Zwitterionic Palladate Complexes of a Dicationic Phosphido Ligand.

With the view to enhancing the unique coordinating ability of the known phenyl-tetrakis(diisopropylamino)dicyclopropeniophosphine (Ph-DCP), replacement of the phenyl substituent by a tert-butyl substituent was envisaged. Both α-dicationic R-DCP phosphines, with R = Ph and Bu, were prepared in 54%-55% yield by substitution of RPCl with two equivalents of bis(diisopropylamino)-dicyclopropenylidene (BAC) and metathesis with NaBF. This method is implicitly consistent with the representation of R-DCPs as BAC-phosphenium adducts. The R-DCP salts were found to coordinate hard and soft Lewis acids such as a promoted oxygen atom (in the singlet spin state) in the corresponding R-DCP oxides, and electron-rich transition-metal centers in η-R-DCP complexes with AuCl, PtCl, or PdCl, respectively. Coordination of Ph-DCP with PdCl, which is a more electron-deficient Pd(II) center, leads to pentachlorinated dinuclear complexes [(Ph-DCP)PdCl]Cl, where the dicoordinate Cl bridge screens the repelling pairs of positive charges from each other. The same behavior is inferred for the Bu-DCP ligand, from which addition of an excess of (MeCN)PdCl was found to trigger a heterolytic cleavage of the DCP-Bu bond, releasing Bu and a dicationic phosphide, DCP: the latter is evidenced as a ligand in a tetranuclear complex ion [(μ-DCP)PdCl], which, upon HCl treatment, dissociates to a doubly zwitterionic dipalladate complex. All the complexes were isolated in 82%-97% yield, and five of them were characterized by X-ray crystallography.