Catalytic Hydroarylation of Olefins Promoted by Dicationic Platinum(II) and Palladium(II) Complexes. The Interplay of C−C Bond Formation and M−C Bond Cleavage

The coordinated olefin in dicationic platinum(II) and palladium(II) complexes [M(PNP)(olefin)](SbF6)2
(M = Pt, Pd; PNP = 2,6-bis(diphenylphosphinomethyl)pyridine; olefin = ethene, propene) is electrophilic
enough to react with benzene rings activated by methoxy substituents. If the proton released by the
aromatic ring is trapped by a base, stable σ-alkyl derivatives of the type [M(PNP)CH2CH(R)Ar](SbF6)
or [M(PNP)CH(R)CH2Ar](SbF6) (R = H, Me) are formed; otherwise the M−C σ-bond can be cleaved
by the proton, setting up a catalytic cycle that leads to the alkylated aromatic compound. The molecular
structure of the σ-alkyl derivative [(PNP)PtCH2CH2−C6H2(OMe)3](BF4) has been determined by X-ray
diffraction analysis, and the factors affecting the mechanism and the rates of the catalytic reaction have
been qualitatively investigated and rationalized, showing that the rates of C−C bond formation and M−C
bond cleavage are inversely correlated.