C-H activation and C-C coupling of arenes by cationic Pt(II) complexes

The synthesis and characterization of cationic platinum complexes of the type [(R2PC2H4PR2)-PtMe(OEt2)]BArF (R = Cy, Et) are reported. These electrophilic platinum cations are found to react quantitatively with arenes (benzene, toluene) at room temperature by undergoing intermolecular C-H activation with concomitant C-C coupling to generate complexes of the type [{Pt(R2PC2H4PR2)}(2)(mu-eta(3): eta(3)-biaryl)][BArF](2). The dianionic biaryl ligands in these compounds exhibit a rare mu-eta(3):eta(3)-bis-allyl bonding mode and can be removed from the complex with stoichiometric oxidants to generate the free biaryl and [(R2PC2H4PR2)Pt(mu-X)](2)[BArF](2) (R = Cy, Et; X = Cl, I). The cationic platinum complexes [(R2PC2H4PR2)-PtMe(OEt2)](3)BArF (R = Cy, Et) are also quite reactive with water, forming the bridging hydroxide complexes [(R2PC2H4PR2)Pt(mu-OH)](2)[BArF](2) (R = Cy, Et). A possible mechanism is proposed for the C-C coupling reaction based upon the structures of these bridging biphenyl complexes, which provides a new perspective for the related palladium-catalyzed oxidative coupling of arenes to form biaryls.