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Palladium carbene complexes – efficient catalysts for hydrodehalogenation of haloarenes

The work is aimed at solving the problem of neutralization of persistent organic pollutants by catalytic methods. In order to identify the relationship between electronic and steric factors of carbenes with catalytic efficiency in the reaction of hydrodehalogenation of haloarenes, the parameters for estimating the ligand influences are proposed (electronic - Ie indices, Ph philicities, ED electron donicities, and EA electron acceptabilities, steric - dimerization energies) using theoretical methods (DFT, B3LYP5, RHF). Synthesis of new heteroaromatic carbenes of the series of sterically shielded 1,3-diarylphenanthro-[9,10-d]imidazol-2-ylidene, 1,3-diaryl-2-methy­lene­phenan­thro-[9,10-d]imidazoline, tetrahydropyrimidin-2-ylidene, 1,3-diaryl­imidazol-2-ylidene, 1,3,4-triaryl-1,2,4-triazol-5-ylidenes, their carbene complexes with palladium iodide, as well as the carbene complex of the superbasic anionic carbene 1,3-bis-(4-oxidophenyl)-imidazol-2-ylidene with nickel(II) ion. 1H and 13C NMR spectroscopy, mass spectrometry and X-ray diffraction study methods were used to establish the structures of the synthesized compounds. A high catalytic effect of a number of sterically shielded carbene complexes (2.4b, 3.5, 4.6a) in the reaction of hydrodehalogenation of p-dichlorobenzene haloarenes with sodium methoxide in isopropanol at 80 °С (TON 98000–110000) was revealed that can be applied to solve the problem of neutralization of persistent organic pollutants. The efficiency of catalysis by sterically open compounds 4.6b and 6.7 appeared to be low (TON 74, 128). The high catalytic effect of sterically shielded compounds is explained by the increased stability of the complexes under the conditions of an alkaline medium caused by metal alkoxides.