Computational Study of Multiple Pathways and Ion-Pairing in Oxidative Addition of Iodomethane to a Binuclear Organoplatinum(II) Complex containing Imine and Phosphine Bridging Ligands

A density functional theory (DFT) study of the reaction of [Me2Pt(μ-NN)(μ-dppm)PtMe2] (1) (NN = phthalazine, dppm = bis(diphenylphosphino)methane) with two equivalents of iodomethane in acetone (A) and benzene (B) reveals a mechanism in agreement with spectrocopic and kinetic data reported earlier by Rashidi and coworkers, for which computation permits additional insights. Following initial oxidation at one platinum(II) centre to form mixed valence outer-sphere ion-pairs containing a Pt^II→Pt^IV interaction, [Me3Pt^(+)(μ-NN)(μ-dppm)PtMe2·I^(-)] (6A, 7B), two competing mechanisms are found for the second oxidative addition at the remaining platinum(II) centre. In one mechanism (Path I), a rearrangement of intermediate 6A and 7B to form [Me3Pt(k^1-NN)(μ-dppm)(μ-I)PtMe2] (2aA, 2aB) occurs prior to oxidative addition giving, after subsequent steps, outer-sphere ion-pairs [Me3Pt(k^1-NN)(μ-dppm)(μ-I)PtMe3^(+)·I^(-)] (10A, 10B), followed by dissociation of phthalazine and formation of the product complex [Me3Pt(μ-dppm)(μ-I)2PtMe3] (4A, 4B) containing two Pt^IV centres.. In the other mechanism (Path 1I), oxidative addition occurs at the PtII centre of 7A and 7B, leading also to 10A and 10B. Paths I and II are competitive in acetone, but Path I is preferred in benzene. The first oxidative addition computes as having a lower barrier than the second, in accord with experiment, and we attribute this to the occurrence of a Pt^a···Pt^b interaction assisting the first oxidative addition at Pt^b.