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

Pd/NHC complexes (NHCs - -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 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 -, -, and -XCH-substituted NHC ligands, including incorporation into Pd complexes (X = F, Cl, Br, CF). 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: -X, -X > -X. Evaluation of the relative catalytic activity showed a significant increase in the catalyst performance in the case of Br and CF substituents compared to the unsubstituted Pd/NHC complex.