Uncommon Solvent Effect in Oxidative Addition of MeI to a New Dinuclear Platinum Complex Containing a Platina(II)cyclopentane Moiety

The reaction of the known complex cis,cis-[Me2Pta(μ-dppm)-(μ-SMe 2 )Ptb-CH 2 (CH 2 ) 2 CCH 2 (Pt b -C c )] [dppm = bis(diphenyl-phosphanyl)methane] with phthalazine (NN) proceeded by replacement of the labile bridging SMe 2 ligand with the bi-dentate N-donor ligand NN to give cis,cis-[Me 2 Pt a (μ-dppm)-(μ-NN)Pt b -CH 2 (CH 2 ) 2 CCH 2 (Ptb-C c )] (1) as a pale red solid in good yield. The complex was fully characterized by multinuclear ( 1 H, 31 P, 195 Pt) NMR spectroscopy. The subsequent reaction of complex 1 with excess Mel gave the colorless diplatinum(IV) complex [Me 3 Pt a (μ-dppm)(μ-I) 2 Pt b {CH 2 (CH 2 ) 2 C c H 2 -(Pt b -C c )}Me], in which the bridging NN ligand is replaced by bridging iodido ligands. The reddish color of complex 1, which is due to a metal-to-ligand charge transfer (MLCT) band in the visible region, was used to monitor its reaction with Mel in the solvents acetone, CH 2 Cl 2 , and benzene. The kinetic data revealed that the reactions in nonpolar benzene or slightly polar CH 2 Cl 2 proceeded in two steps, each following a common S N 2 mechanism. In the first step, Mel attacked the platina(II)cyclopentane center rather than the dimethyl-platinum(II) center, because the first center is more electron-rich than the second center. In the more polar acetone, the reaction proceeded similarly, with the exception that each step was accompanied by a solvolytic reaction, which was suggested to be responsible for the unusually slower reaction rate in acetone than in benzene or CH 2 Cl 2 . Consistently, the reaction rate in the highly polar solvent CH 3 CN was too slow for any meaningful measurement.