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 tBu, were prepared in 54%–55% yield by substitution of RPCl2 with two equivalents of bis­(diisopropylamino)-dicyclopropenylidene (BAC) and metathesis with NaBF4. 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 η1-R-DCP complexes with AuCl, PtCl3–, or PdCl3–, respectively. Coordination of Ph-DCP with PdCl2, which is a more electron-deficient Pd­(II) center, leads to pentachlorinated dinuclear complexes [(Ph-DCP)­PdCl2]2Cl–, where the dicoordinate Cl– bridge screens the repelling pairs of positive charges from each other. The same behavior is inferred for the tBu-DCP ligand, from which addition of an excess of (MeCN)2PdCl2 was found to trigger a heterolytic cleavage of the DCP–tBu bond, releasing tBu+ and a dicationic phosphide, DCP–: the latter is evidenced as a ligand in a tetranuclear complex ion [(μ2-DCP–)­Pd2Cl4]2, 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.