Synthesis and structural chemistry of N-{2-(arylthio/seleno)ethyl}morpholine/piperidine–palladium(II) complexes as potent catalysts for the Heck reaction

The reactions of in-situ generated PhS−, PyS− and PhSe− with 4-(2-chloroethyl)morpholine hydrochloride and 1-(2-chloroethyl)piperidine hydrochloride in N2 atmosphere have resulted in N-{2-(arylchalcogeno)ethyl}morpholine/piperidine ligands (L1−L6), which on reaction with Na2PdCl4 at room temperature result in complexes of composition [PdCl2(L)] (1–6; L = L1−L6). All the complexes except 2 have been characterized by X-ray crystallography. The geometry around Pd is nearly square planar in all the cases. The Pd−S and Pd−Se bond lengths are in the ranges 2.2598(10)−2.2795(14) and 2.3541(6)−2.3632(15) Å, respectively. In 77Se{1H} NMR spectra of Pd(II)-complexes with L5 and L6 (Se ligands), the signals appear at higher frequency (up to 187 ppm) with respect to those of corresponding free ligands. Pd(II)-complexes (2, 4 and 6) have been found promising for Heck reaction of aryl bromides with methyl acrylate. The TON values are up to 18 200 and TOF up to 758 h−1 (with complex 6 as catalyst). The Pd-complexes of piperidene derivatives show little higher catalytic efficiency than those of morpholine ones. The positive Hg-test has implied that nano-sized Pd(0) species probably catalyze Heck reactions.