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

摘要

We have synthesized a new class of heteroditopic chelating ligands bearing the phenylthiomethyl groupand 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 theirchemical 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 reactwith an excess of the same or different heteroditopic ligands. In these cases, we obtained complexes bearingmaleic 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 thequantitative formation of the bis-heteroditopic Pd(0) derivative and in some cases, a reaction involvinga heterogeneous equilibrium is observed.The reaction of the palladium(0) carbene–sulfur or carbene–nitrogen complexes with the alkynedimethyl-2-butynedioate (dmbd) to give palladacyclometallate derivatives is not always warrantedand formation of the intermediate [Pd(eta2-ma)(eta2-dmdb)(chi1-Mes-NHC-CH2-SPh)] is detected only inone case.Finally, we have carried out a diffractometric study on the solid-state structures of two derivatives andin 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)].