Cyclopalladation of dimesityl selenide: synthesis, reactivity, structural characterization, isolation of an intermediate complex with C–H⋯Pd intra-molecular interaction and computational studies

Abstract

The reaction of dimesityl selenide (Mes2Se) with either PdCl2(PhCN)2 in toluene or PdCl2 in toluene-acetonitrile yields a chloro-bridged binuclear palladium complex, [Pd2Cl2(μ-Cl)2(Mes2Se)2] (1), whereas with Na2PdCl4 in refluxing ethanol, a cyclometallated palladium complex, [Pd2(μ-Cl)2{MesSeC6H2(Me2)CH2}2] (2) is afforded. 2 can also be obtained when 1 is refluxed in ethanol. On treatment with Pb(Epy)2 in dichloromethane, 2 afforded the Epy-bridged binuclear complexes, [Pd2(μ-Epy)2{MesSeC6H2(Me2)CH2}2] (3; E = S (3a) or Se (3b)). Treatment of 2 with PPh3 yields a bridge-cleaved monomeric complex, [PdCl{MesSeC6H2(Me2)CH2}(PPh3)]. The molecular structures of 1-3 were established by X-ray diffraction analyses. All the complexes are dimeric, with the palladium atoms acquiring a distorted square planar configuration. There are intra-molecular C-H···Pd interactions (d(M-H): 2.75 Å and <C-H···Pd: 111.23°) in 1 which facilitate the activation of the C-H (sp(3)) bond leading to metallation. The optimized geometry of 1 obtained using the DFT(B3LYP) computational method identified a C-H···Pd contact distance of 2.78 Å. There are two independent molecules of 2 in the unit cell, which differ slightly in bond lengths, bond angles and torsion angles. The mechanism of formation of the dimeric species 2 is examined using DFT (B3LYP) computations.