Synthesis and structure elucidation of allyl Pd(II) complexes of NHC ligands derived from substituted imidazo[1,5- a ]quinolin-1(2H)-ylidene

Abstract Nine Pd(II) complexes involving N -heterocyclic carbenes (NHCs) derived from 2-substituted and 2- and 7-substituted imidazo[1,5- a ]quinolin-1(2H)-ylidene with auxiliary allylic ligands were synthesized and characterized. The structure and configuration of the complexes were elucidated on the basis of combination of dynamic NMR and DFT studies. Conformational studies in respect of hindered rotation around Pd-C bond and η 3 -η 1 -η 3 pseudo allyl rotation were performed. The results from dynamic NMR and DFT studies confirmed the mechanism of selective η 3 -η 1 -η 3 isomerization, whose energy barriers are affected by steric hindrance of substituents at nitrogen atom. Energy barriers of isomerization (16.7–18.8 kcal/mol) are slightly influenced by the electronic nature of substituents at seventh position in imidazo[1,5- a ]quinolin-1(2H)-ylidene moiety. The results from DFT calculations were in good agreement with the experimental energy barriers.