Postmodification 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 ace-tonaphtho[1,2--b]quinoxaline [11,12--d]imidazol--2--ylidene ligands have been obtained and fully characterized.These com-plexes display highly extended polyaromatic systems attached to the backbone of the N--heterocyclic carbene.The pres-ence of this extended polyaromatic system makes the electron--donating character of these ligands sensitive to the pres-ence 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 1 H NMR spectroscopy.