Oxidation of a rollover cycloplatinated(ii) dimer by MeI: a kinetic study

Abstract

The known rollover cycloplatinated(II) complex [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, in which bpy-2H acts as a bridging rollover ligand, was reacted with MeI to give a new binuclear rollover cycloplatinated(IV) complex [Pt₂Me₄I₂(PPh₃)₂(μ-bpy-2H)], 2. The stereochemistry of 2 was fully identified by NMR spectroscopy (¹H and ³¹P) and confirmed by DFT calculations. To the best of our knowledge, 2 is the first example of a diplatinum(IV) complex having a bridging rollover bipyridine ligand. 1 has a 5d_π(Pt) → π*(bpy) MLCT band in the visible region which was used to easily follow the kinetics of its reaction with MeI; a double MeI oxidative addition was observed and classical S_N2 mechanism was suggested for both steps of the reaction. The large negative entropy of activation (ΔS^‡), found in each step, complies with an associative process. The rates are almost 3–5 times slower in the second step as compared to the first step, due the electronic effects transmitted through the rollover bpy ligand. The rates were also compared with that reported for the corresponding monomeric cyclometalated complex [PtMe(bpy-H)(PPh₃)] and found to be higher (in step 1) and usually lower (in step 2). Theoretical computations of the geometry of the possible reaction transition states and intermediates revealed that each step of the reaction takes place via a transition state with a nearly linear arrangement of the I–CH₃–Pt moiety. The computational results are in good agreement with the experimental findings, confirming the proposed mechanism.