Experimental and theoretical approaches to the influence of the addition of pyrene to a series of Pd and Ni NHC-based complexes: catalytic consequences.

A series of Ni and Pd complexes with three different N-heterocyclic carbene (NHC)-based ligands (imidazolylidene, benzimidazolylidene and pyrene-imidazolylidene) has been prepared and fully characterized. The influence of the addition of pyrene to solutions containing these complexes is studied by means of NMR and UV/Vis spectroscopies and by cyclic voltammetry. The addition of pyrene to the pyrene-NHC-containing Pd and Ni complexes gives rise to the formation of adducts by π-π stacking interactions between pyrene and the pyrene group of the NHC ligand. This interaction causes a modification of the electronic properties of the metal, as demonstrated by cyclic voltammetric studies of the Ni-NHC complexes. Theoretical calculations support this type of π-interactions, and justify the higher interactions observed with the pyrene-NHC containing complexes. The catalytic activities of the complexes were tested in the Suzuki-Miyaura C-C coupling and in the α-arylation of ketones. The addition of pyrene as an external π-stacking additive does not affect the activities of the complexes in the Suzuki-Miyaura coupling, but this observation may be justified due to the fact that the process is heterogeneously catalyzed, as indicated by the mercury-drop test. The addition of pyrene to the catalytic α-arylation of ketones results in a decrease in the activity of the reactions catalyzed by the pyrene-imidazolylidene palladium complex, whereas the other two catalysts do not modify their activity in the presence of this π-stacking additive.