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 )

• Published in: Dalton transactions : an international journal of inorganic chemistry 2023-03, Vol.52 (13), p.4122-4135
• Main Authors: Pankov, Roman O, Prima, Darya O, Kostyukovich, Alexander Yu, Minyaev, Mikhail E, Ananikov, Valentine P
• Format: Article
• Language: English
• Subjects: Atomic properties; Bond energy; Catalytic activity; Coordination compounds; Molecular structure; Palladium; Substitution reactions; X ray analysis
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d2dt03665g

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 -XC H -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.