Phenylene- and Biphenylene-Bridged Bis-Imidazolylidenes of Palladium. Influence of the Presence of Pyrene Tags on the Catalytic Activity of the Complexes

A series
of dimetallic complexes with N-heterocyclic carbene ligands
with formally identical stereoelectronic properties have been obtained
and fully characterized. The dimetallic complexes were bridged by
bis-imidazolylidenes, with different spacers (phenylene and biphenylene).
The N substituents were methyl or methylpyrene groups. The related
monometallic complexes were also obtained. The catalytic properties
of the complexes were tested in the acylation of aryl halides with
hydrocinnamaldehyde and in the Suzuki–Miyaura coupling between
aryl halides and arylboronic acids. In general, the dimetallic complexes
display better activities than the monometallic analogues. The results
also indicate that those complexes with pyrene groups at the N substituents
display better catalytic activities than those with N-methyl groups. These observations are interpreted as a consequence
of the π stacking interaction between the substrates and the
pyrene groups in the pyrene-containing catalysts, which may provide
some beneficial catalytic properties. The addition of a catalytic
amount of pyrene to the Suzuki–Miyaura coupling reactions results
in a partial inhibition of the activities of the complexes with the
pyrene substituents, while the activity of the complexes with the N-methyl groups is not affected.