New Pyridine Dicarbene Pincer Ligands with Ring Expanded NHCs and their Nickel and Chromium Complexes

Abstract

The pincer complexes [NiIIBr(CNC)]Br (4), [CrIIIBr3(CNC)] (5a) and [CrIIIBr2.3Cl0.7(CNC)] (5b), where CNC = 3,3'‐(pyridine‐2,6‐diyl)bis(1‐mesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene), were obtained from the novel ligand CNC, generated in situ from the precursor (CHNCH)Br2 and [NiIIBr2(PPh3)2] or from [CrII{N(SiMe3)2}2(THF)2] and (CHNCH)Br2 by aminolysis, respectively. The tetrahedrally distorted square planar (τ4 ≅ 0.30) geometry and the singlet ground state of Ni in 4 were attributed to steric constraints of the CNC backbone. Computational methods highlighted the dependence of the coordination geometry and the singlet‐triplet energy difference on the size of the N‐substituent of the tetrahydropyrimidine wingtips and contrasted it to the situation in 5‐membered imidazolin‐2‐ylidene pincer analogues. The octahedral CrIII metal center in 5a and 5b is presumably formed after one electron oxidation from CH2Cl2. 4/MAO and 5a/MAO were catalysts of moderate activity for the oligomerization and polymerization of ethylene, respectively. The analogous (CH^N^CH)Br2 precursor, where (CH^N^CH) = 3,3'‐(pyridine‐2,6‐diylbis(methylene))bis(1‐mesityl‐3,4,5,6‐tetrahydropyrimidin‐1‐ium), was also prepared, however its coordination chemistry was not studied due to the inherent instability of the resulting free C^N^C ligand.