Synthesis and a combined experimental/theoretical structural study of a comprehensive set of Pd/NHC complexes with o -, m -, and p -halogen-substituted aryl groups (X = F, Cl, Br, CF 3 ).

Pd/NHC complexes (NHCs - N -heterocyclic carbenes) with electron-withdrawing halogen groups were prepared by developing an optimized synthetic procedure to access imidazolium salts and the corresponding metal complexes. Structural X-ray analysis and computational studies have been carried out to evaluate the effect of halogen and CF 3 substituents on the Pd-NHC bond and have provided insight into the possible electronic effects on the molecular structure. The introduction of electron-withdrawing substituents changes the ratio of σ-/π-contributions to the Pd-NHC bond but does not affect the Pd-NHC bond energy. Here, we report the first optimized synthetic approach to access a comprehensive range of o -, m -, and p -XC 6 H 4 -substituted NHC ligands, including incorporation into Pd complexes (X = F, Cl, Br, CF 3 ). The catalytic activity of the obtained Pd/NHC complexes was compared in the Mizoroki-Heck reaction. For substitution with halogen atoms, the following relative trend was observed: X = Br > F > Cl, and for all halogen atoms, the catalytic activity changed in the following order: m -X, p -X > o -X. Evaluation of the relative catalytic activity showed a significant increase in the catalyst performance in the case of Br and CF 3 substituents compared to the unsubstituted Pd/NHC complex.