Synthesis and a combined experimental/theoretical structural study of a comprehensive set of Pd/NHC complexes with<i>o</i>-,<i>m</i>-, and<i>p</i>-halogen-substituted aryl groups (X = F, Cl, Br, CF₃)

Abstract

Pd/NHC complexes (NHCs - <i>N</i>-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 CF₃ 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 <i>o</i>-, <i>m</i>-, and <i>p</i>-XC₆H₄-substituted NHC ligands, including incorporation into Pd complexes (X = F, Cl, Br, CF₃). 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: <i>m</i>-X, <i>p</i>-X > <i>o</i>-X. Evaluation of the relative catalytic activity showed a significant increase in the catalyst performance in the case of Br and CF₃ substituents compared to the unsubstituted Pd/NHC complex.