Phosphenium
Versus Pro-Phosphide Character of P-<i>tert-</i>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 <i>tert</i>-butyl
substituent was envisaged. Both α-dicationic R-DCP phosphines,
with R = Ph and <sup><i>t</i></sup>Bu, were prepared in
54%–55% yield by substitution of RPCl<sub>2</sub> with two
equivalents of bis(diisopropylamino)-dicyclopropenylidene (BAC) and
metathesis with NaBF<sub>4</sub>. 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<sup>1</sup>-R-DCP complexes with AuCl, PtCl<sub>3</sub><sup>–</sup>, or PdCl<sub>3</sub><sup>–</sup>, respectively. Coordination
of Ph-DCP with PdCl<sub>2</sub>, which is a more electron-deficient
Pd(II) center, leads to pentachlorinated dinuclear complexes [(Ph-DCP)PdCl<sub>2</sub>]<sub>2</sub>Cl<sup>–</sup>, where the dicoordinate
Cl<sup>–</sup> bridge screens the repelling pairs of positive
charges from each other. The same behavior is inferred for the <sup><i>t</i></sup>Bu-DCP ligand, from which addition of an
excess of (MeCN)<sub>2</sub>PdCl<sub>2</sub> was found to trigger
a heterolytic cleavage of the DCP–<sup><i>t</i></sup>Bu bond, releasing <sup><i>t</i></sup>Bu<sup>+</sup> and
a dicationic phosphide, DCP<sup>–</sup>: the latter is evidenced
as a ligand in a tetranuclear complex ion [(μ<sup>2</sup>-DCP<sup>–</sup>)Pd<sub>2</sub>Cl<sub>4</sub>]<sub>2</sub>, 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