Post-Modification of the Electronic Properties by Addition of π-Stacking Additives in N-Heterocyclic Carbene Complexes with Extended Polyaromatic Systems

A series of iridium complexes containing phenanthro[4,5-abc]phenazino[11,12-d]imidazol-2-ylidene and acetonaphtho[1,2-b]quinoxaline[11,12-d]imidazol-2-ylidene ligands have been obtained and fully characterized. These complexes display highly extended polyaromatic systems attached to the backbone of the N-heterocyclic carbene. The presence of this extended polyaromatic system makes the electron-donating character of these ligands sensitive to the presence of π-stacking additives, such as pyrene and hexafluorobenzene. The computational studies predict that the addition of pyrene affords an increase of the electron-donating character of the polyaromatic ligand (TEP decreases), while the addition of hexafluorobenzene has the opposite effect (TEP increases). This prediction is experimentally corroborated by IR spectroscopy, by measuring the shift of the CO stretching bands of a series of IrCl(NHC)(CO)2 complexes, where NHC is the N-heterocyclic carbene ligand with the polyaromatic system. Finally, the energy of the π-stacking interaction of one of the key Ir(I) complexes with pyrene and hexafluorobenzene has been estimated by using the Benesi-Hildebrand treat-ment, based on the δ-shifts observed by 1H NMR spectroscopy.MEC of Spain (CTQ2011-24055/BQU

Pyrene-Based Mono- and Di-N-Heterocyclic Carbene Ligand Complexes of Ruthenium for the Preparation of Mixed Arylated/Alkylated Arylpyridines

By using two pyrene-based mono- and di-N-heterocyclic carbene ligands, two ruthenium complexes (one monometallic and the other dimetallic) have been obtained and fully characterized. The molecular structure of the dimetallic complex has been determined by means of X-ray diffraction studies. The electrochemical studies reveal that the metal–metal communication in the dimetallic complex is weak. The catalytic activity of both complexes has been tested in the arylation of arylpyridines with aryl halides and in the hydroarylation of alkenes, where they showed similar activity. The sequential combination of these two catalytic processes (hydroarylation of alkenes followed by arylation of the resulting alkyl-substituted arylpyridine) allowed the preparation of mixed arylated/alkylated arylpyridines. In this tandem process, the dimetallic complex afforded activity higher than that of the monometallic complex. The activity was compared to that shown by the [RuCl2(p-cymene)]2 complex. This reaction constitutes an efficient method for reaching unsymmetrically substituted arylpyridines

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