Transition metal–carbon bonds. Part XVI. Cationic allylic complexes of palladium(II). A cationic allylic complex of platinum(II)

Abstract

In aqueous acetone [Pd₂Cl₂(2-methylallyl)₂] reacts with PPh₃ to give the colourless cation [Pd(2-methylallyl)(PPh₃)₂]⁺, isolated as its tetraphenylborate salt. Similar complexes with PMe₂Ph and AsMe₂Ph were prepared. Treatment of [PdCl₂(PPh₃)₂] with 2-methylallylmagnesium chloride followed by hydrolysis gave the hydrated chloride [Pd(2-methylallyl)(PPh₃)₂]Cl·2H₂O. Attempts to prepare analogous complexes with other allylic ligands were unsuccessful. N.m.r. data show the 2-methylallyl ligands in these cations to be symmetrically π-bonded to the palladium. Conductimetric studies on systems of the type [Pd₂X₂ all₂]+x mol. of L have shown that the tendency to form ionic species decreases (1) for different ligands L in the order PMe₂Ph ∼ PEt₂Ph ∼ PEt₃ > PPh₃ > AsPh₃ > SbPh₃ > pyridine; (2) for different allyl ligands ‘all’ in the order 2-methylallyl > allyl > 1-methylallyl > 1,3-dimethylallyl > 1,1-dimethylallyl; and (3) for different halogens X in the order Cl > Br > I. Compounds of the type [Pd₂Cl₂ all₂] react rapidly with an excess of PPh₃ in aqueous acetone to give [Pd(PPh₃)₄] and [all PPh₃]⁺ Cl^–. Treatment of [Pd(PPh₃)₄] with an excess of the allylic chloride gives [PdCl all PPh₃].

Treatment of [PtCl₂(AsMe₂Ph)₂] with 2-methylallylmagnesium chloride followed by water and NaBPh₄ gave the salt [Pt(2-methylallyl)(AsMe₂Ph)₂]⁺ BPh₄^–.