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, CF3)
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 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 ?-/?-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-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: 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 CF3 substituents compared to the unsubstituted Pd/NHC complex.
Reference: Dalton Trans., 2023, ASAP
