Structural characterization of Group 4 transition metal halide bis-Arduengo carbene complexes MCl4L2:: X-ray crystal structure analyses and DFT calculations

The Arduengo carbenes 1,3-diisopropyl-, 1-methyl-3-(1-methylpropyl)-, and 1-methyl-3-(2,4,6-trimethylbenzyl)imidazol-2-ylidene (3a–c) were reacted with MCl4(thf)2 (M=Zr, Hf) to yield the respective trans-(imidazol-2-ylidene)MCl4 complexes 4a–c (M=Zr) and 5a (M=Hf), respectively. These four Arduengo carbene-Group 4 metal halide complexes were characterized by X-ray diffraction. The pairs of carbene ligands are trans-positioned in a pseudo-octahedral coordination geometry at the Group 4 metals, and adopt a conformational orientation in the solid state where the two five-membered heterocycles are arranged coplanar and bisecting the ClMCl angle. DFT calculations have shown that the Arduengo carbenes serve as pure σ-donor ligands in these complexes and that the preferred conformational ligand orientation is based on steric reasons. The complexes 4a–c form moderately active ethene polymerization catalysts when activated with a large excess of methylalumoxane in toluene.