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 (sp³) 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.