Phosphenium Versus Pro-Phosphide Character of P-<i>tert-</i>butyl-dicyclopropeniophosphine: Zwitterionic Palladate Complexes of a Dicationic Phosphido Ligand

Abstract

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 <i>tert</i>-butyl substituent was envisaged. Both α-dicationic R-DCP phosphines, with R = Ph and ^<i>t</i>Bu, 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 ^<i>t</i>Bu-DCP ligand, from which addition of an excess of (MeCN)₂PdCl₂ was found to trigger a heterolytic cleavage of the DCP–^<i>t</i>Bu bond, releasing ^<i>t</i>Bu⁺ 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