Evaluating the electronic properties of ditopic and hetero-ditopic ligands derived from benzimidazole and pyrazole by 13C NMR spectroscopy

Huynh’s electronic parameter (HEP) was applied to distinguish the electronic properties of benzimidazole and pyrazole donors in N^N ditopic and N^N′ hetero-ditopic, dinucleating ligands by 13C NMR spectroscopy of their dipalladium complexes bearing terminal iPr2-bimy reporter ligands. The 13Ccarbene NMR resonances of the iPr2-bimy ligand (HEPs) indicate stronger donation of the symmetrical dibenzimidazole compared to that of the dipyrazole with a ΔHEP value of 1.54 ppm. Based on this benchmark value, the effect of spacer groups on the electron donation of the mixed benzimidazole-pyrazole ligands was investigated for the first time. The donicity gap between the benzimidazole and pyrazole donors increases with alkylene spacers, but shrinks with the 1,4-phenylene linker, suggesting electronic communication between the two different heterocycles via π-electron conjugation. Furthermore, these complex probes could also be used to catalyze the direct arylation of pentafluorobenzene, which showed that the unsymmetrically bridged, dinuclear complexes with alkylene linkers are more reactive.