ABSTRACT
A range of palladium complexes featuring electronically modified, imidazole-based abnormal N-heterocyclic carbene (aNHC) ligands have been prepared in the hopes of accessing a new class of cationic aNHC ligands electronically distinct from normal NHCs and aNHCs. These palladium complexes represent the first examples of transition metal-ligated aNHC complexes featuring a cationic moiety adjacent to the abnormal carbene center. It was anticipated that these design principles could facilitate electron transfer between the imidazolinylidene and the cationic heterocycle, thus reducing the electron density at the abnormal carbene center. However, this case study suggests that greater conformational restrictions that allow for heterocycle coplanarity are necessary to achieve significant electron transfer and enable access to a new class of cationic charge-appended aNHCs with unique electronic properties.
ABSTRACT
The CH2-linked bis(NHC) complexes [{(SMesIm)2CH2}PdBr2] and [{(SMesIm)2CH2}Pd(NCMe)2][PF6]2 are reported. These represent the first structurally characterized chelated, saturated bis(NHC) complexes. The complexes are subject to facile ligand fragmentation during their synthesis. Longer (CH2)n-linkers in the imidazolin-2-ylidene-based series of ligands afforded the pendant imidazolinium mono(NHC) complexes [{(SMesImH)(SMesIm)(CH2)n}PdBr3] by reaction of the diimidazolinium salts (n = 2, 3) with palladium acetate. These did not react to give the bis(NHC) complexes, as was the case for [{(SMesImH)(SMesIm)CH2}PdX3] (X = Br, I). Disilver(i) complexes [{(SMesIm)2(CH2)n}2Ag2][PF6]2n = 1 and 3, were also prepared.
