Phenylene- and biphenylene-bridged bis-imidazolylidenes of palladium. Influence of the presence of pyrene tags on the catalytic activity of the complexes

摘要

A series of dimetallic complexes with N-heterocycliccarbene ligands with formally identical stereoelectronic propertieshave been obtained and fully characterized. The dimetallic complexeswere bridged by bis-imidazolylidenes, with different spacers (phenyleneand biphenylene). The N substituents were methyl ormethylpyrene groups. The related monometallic complexes werealso obtained. The catalytic properties of the complexes were tested inthe acylation of aryl halides with hydrocinnamaldehyde and in theSuzuki−Miyaura coupling between aryl halides and arylboronic acids.In general, the dimetallic complexes display better activities than themonometallic analogues. The results also indicate that thosecomplexes with pyrene groups at the N substituents display bettercatalytic activities than those with N-methyl groups. Theseobservations are interpreted as a consequence of the π stacking interaction between the substrates and the pyrene groups inthe pyrene-containing catalysts, which may provide some beneficial catalytic properties. The addition of a catalytic amount ofpyrene to the Suzuki−Miyaura coupling reactions results in a partial inhibition of the activities of the complexes with the pyrenesubstituents, while the activity of the complexes with the N-methyl groups is not affected.