The interaction between heteroditopic pyridine–nitrogen NHC with novel sulfur NHC ligands in palladium(0) derivatives: Synthesis and structural characterization of a bis-carbene palladium(0) olefin complex and formation in solution of an alkene–alkyne mixed intermediate as a consequence of the ligandshemilability

We have synthesized a new class of heteroditopic chelating ligands bearing the phenylthiomethyl group and variously substituted imidazole derivatives and the corresponding complexes [Pd(eta2-ma)(R-NHCCH2- SPh) (ma = maleic anhydride; R = methyl, mesityl, di-i-propylphenyl). We have compared their chemical characteristics and reactivity with those of similar palladium(0) complexes bearing the RNHC- CH2-Py ligands. The hemilability of the above-mentioned moieties is apparent when their palladium derivatives react with an excess of the same or different heteroditopic ligands. In these cases, we obtained complexes bearing maleic anhydride and two mono-coordinated heteroditopic ligands bound to the same metal center. However, at variance with the generally accepted opinion, precipitation of AgBr does not warrant the quantitative formation of the bis-heteroditopic Pd(0) derivative and in some cases, a reaction involving a heterogeneous equilibrium is observed. The reaction of the palladium(0) carbene–sulfur or carbene–nitrogen complexes with the alkyne dimethyl-2-butynedioate (dmbd) to give palladacyclometallate derivatives is not always warranted and formation of the intermediate [Pd(eta2-ma)(eta2-dmdb)(chi1-Mes-NHC-CH2-SPh)] is detected only in one case. Finally, we have carried out a diffractometric study on the solid-state structures of two derivatives and in particular we describe the configurations of the complex [Pd(eta2-ma)(j1-Me-NHC-CH2-SPh)2] and of [Pd(eta2-ma)(Me-NHC-CH2-SPh)].