Bis(diphenylphosphino)acetylene as bifunctional ligand in cycloplatinated complexes: Synthesis, characterization, crystal structures and mechanism of MeI oxidative addition

Abstract

Binuclear cycloplatinated(II) complexes with general formula of [Pt2Me2(CˆN)2(μ-dppac)], (1, CˆN = deprotonated 2-phenylpyridine (ppy); 2, CˆN = deprotonated benzo{h}quinoline (bhq)) in which dppac = 1,1′-bis(diphenylphosphino)acetylene, are synthesized by the reaction of [PtMe(SMe2)(CˆN)] with 0.5 equiv of dppac at room temperature. The complexes are fully characterized using multinuclear (1H, 31P and 195Pt) NMR spectroscopy and complex 2 is further identified by single crystal X-ray structure determination. Kinetics of the reaction of complexes 1 and 2 with MeI are investigated in CHCl3 and based on the UV–vis and 31P NMR data, a mechanism involving a double MeI oxidative addition is suggested. The classical SN2 mechanism is proposed for both steps and the involved intermediates are suggested. Although MeI in each step was trans oxidatively added to one of the platinum(II) centers, further trans to cis isomerizations of methyl and iodide ligands were also identified. The rates are almost four times slower in the second step as compared to the first step due to the electronic effects transmitted through the dppac ligand. Reaction rates concerning complex 2, having bhq ligand, are almost 1.3 times slower than those involving the related ppy complex 1. This is attributed to the stronger donor ability of the ppy ligand, as compared to that bhq ligand and is in agreement with the values of 1JPtP observed in the 31P NMR spectra of the complexes 1 and 2. The structure of the Pt(IV)–Pt(IV) dimeric complex [Pt2I2Me4(ppy)2(μ-dppac)], 3, produced by oxidative addition of complex 1 to MeI, is also determined using X-ray diffraction which is the first X-ray structural determination of a diplatinum complex containing two Pt(IV) centers bridged by one dppac ligand.