Mesoionic Complexes of Platinum(II) Derived from “Rollover” Cyclometalation: A Delicate Balance between Pt–C(sp³) and Pt–C(sp²) Bond Cleavage as a Result of Different Reaction Conditions

“Rollover” cyclometalation is a particular case of metal-mediated C–H bond activation, and the resulting complexes constitute an emerging class of cyclometalated compounds. In the case of 2,2′-bipyridine “rollover cyclometalation” has been used to synthesize the complexes [Pt­(bipy-H)­(Me)­(L)] (L = PPh₃, PCy₃, P­(OPh)₃, P­(<i>p</i>-tolyl)₃), whose protonation produces a series of stable corresponding pyridylenes [Pt­(bipy*)­(Me)­(L)]⁺. The unusual bipy* ligand may be described as an abnormal-remote heterocyclic chelated carbene or simply as a mesoionic cyclometalated ligand. These cationic species spontaneously convert in solution, through a retro-rollover reaction, to the corresponding isomers [Pt­(bipy)­(Me)­(L)]⁺, where the 2,2′-bipyridine is coordinated in the classical N,N bidentate mode. Isomerization is achieved at different rates (ranging over three orders of magnitude), depending on the nature of the phosphane ligand, the most basic (PCy₃) providing the fastest reaction. The mesoionic species [Pt­(bipy*)­(Me)­(L)]⁺ contain two Pt–C bonds: the balance between the Pt–C­(sp²) and Pt–C­(sp³) bond rupture is subtle, and competition is observed according to the reaction conditions. In the presence of an external neutral ligand L′ methane is released to give the cationic derivatives [Pt­(bipy-H)­(L)­(L′)]⁺, whereas reaction of the neutral [Pt­(bipy-H)­(Me)­(L)] with HCl may follow different routes depending on the nature of the neutral ligand L. Assuming all reactions take place through the formation of a hydride intermediate, quantum chemical calculations show that computed energy barriers are qualitatively consistent with observed reaction rates