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

Abstract

The reaction of dimesityl selenide (Mes(2)Se) with either PdCl2(PhCN)(2) in toluene or PdCl2 in toluene-acetonitrile yields a chloro-bridged binuclear palladium complex, [Pd2Cl2(mu-Cl)(2)(Mes(2)Se)(2)] (1), whereas with Na2PdCl4 in refluxing ethanol, a cyclometallated palladium complex, [Pd-2(mu-Cl)(2){MesSeC(6)H(2)(Me-2)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, [Pd-2(mu-Epy)(2){MesSeC(6)H(2)(Me-2)CH2} (2)] (3; E = S (3a) or Se (3b)). Treatment of 2 with PPh3 yields a bridge-cleaved monomeric complex, [PdCl{MesSeC(6)H(2)(Me-2)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 angstrom and <C-H center dot center dot center dot Pd: 111.23 degrees) 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 center dot center dot center dot Pd contact distance of 2.78 angstrom. 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.