Phosphenium versus pro-phosphide character of P-tert-butyl-dicyclopropeniophosphine: zwitterionic palladate complexes of a dicationic phosphido ligand
Résumé
With the view to enhancing the unique coordinating ability of the known phenyl-tetrakis(diisopropylamino)-dicydopropeniophosphine (Ph-DCP), replacement of the phenyl substituent by a tert-butyl substituent was envisaged. Both alpha-dicationic R-DCP phosphines, with R = Ph and Bu-t, were prepared in 54%-55% yield by substitution of RPCl2 with two equivalents of bis(dlisopropylamino)-dicydopropenylidene (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 q1-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 [(PhD CP)PdCl2](2)Cl-, where the dicoordinate Cl-bridge screens the repelling pairs of positive charges from each other. The same behavior is inferred for the Bu-t-DCP ligand, from which addition of an excess of (MeCN)(2)PdCl2 was found to trigger a heterolytic cleavage of the DCP-Bu bond, releasing Bu-t(+) and a dicationic phosphide, DCP-: the latter is evidenced as a ligand in a tetranuclear complex ion [(mu(2)-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.