CATALYSIS OF HYDRODEHALOGENATION RE­ACTION OF HALOARENES BY CARBENE PEPPSI-PALLADIUM COMPLEXES

Abstract

The synthesis of a number of carbene PEPPSI-complexes of palladium with various pyridine and carbene ligands was carried out by reactions of 1,3-bis-(2,6-dibenzhydryl-4-methyl­phenyl)imidazolium chloride IPr*.HCl [compounds 7a-c IPr*PdCl2L’, L’ = pyridine (а), 3-chloropyridine (b), 4-dimethylaminopyridine (с)], 1,3-bis-(2,6-diisopropylphenyl)­imida­zo­li­um perchlorate IPr.HClO4 [compounds 8a,b IPr.PdCl2L’, L’ = 3-chloropyridine (а), о-phenanthroline (b)], 1,3-diphenyl-4-(2,6-diisopropylphenyl)-1,2,4-triazolium perchlorate L.HClO4 (complex 9 LPdCl2L’, L’ = 3-chloropyridine) and 1,3-dicetyl­imidazolium bromide L.HBr (complex 11 LPdCl2L’, L’ = pyridine) with palladium chloride in pyridines (pyridine, 3-chloropyridine), or acetonitrile in the presence of potassium carbonate. Yields of compounds – from high (56–100%) to moderate (36 %). The structure of the compounds was confirmed by 1H and 13C NMR spectroscopy. Chemical shifts of carbene atoms in the 13C NMR spectra of complexes 7a-c. 8a, b 11 are in the region  151.0-156.2 ppm, for complex 9 - at  174.4 ppm A high catalytic effect of sterically shielded complexes 7a, b, 8a, b in the hydrodehalogenation reaction of p-dichlorobenzene and hexachlorobenzene under the action of potassium tert-butoxide in isopropanol was established. 1,3-Bis-(2,6-dibenzhydryl-4-methylphenyl)-imidazol-2-ylidene complexes 7a, b (quantitative conversions with p-dichlorobenzene are achieved with 0.013 mol% of catalyst) show the highest efficiency, but the compound with 4-dimethylaminopyridine ligand 7c has significantly lower efficiency (22% conversion under these conditions). Complexes with 1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene ligand 8a, b are close in efficiency to compounds 7a, b (for 7a quantitative conversion is achieved with 0.026 mol% of catalyst). Phenanthroline-containing complex 8b is less effective than complex 8a (87% conversion with 0.052 mol% of catalyst). Complex 9 is much less effective (even with 0.13 mol% of catalyst 13% conversion is achieved). Compound 11 catalyzes the reaction well only when the amount of catalyst is up to 1.3 mol% (98% conversion). Thus, compounds 7a, b, 8a are the best PEPPSI-catalysts for hydrodehalogenation of haloarenes promising for industrial decontamination of persistent orga­nic pollutants (hexachlorobenzene, DDT, di­oxins and polychlorinated biphenyls, etc.).