Phenolate‐ and Alcoholate‐Based Chromium N‐Heterocyclic Carbene Complexes: Structural Peculiarities, Magnetometric and Electron Paramagnetic Resonance Study, and Catalytic Ethylene Oligomerization

Although there have been advancements with other 3d transition metals, access to Cr(II/III)‐N‐heterocyclic carbene (NHC) complexes bearing phenolate or oalcoholate ligands is strongly limited. In an attempt to isolate the corresponding O ‐chelated dinuclear Cr(II)‐NHC complex, reaction of CrCl 2 with the bidentate NHC 1‐(mesityl)‐3‐(2‐O‐phenyl)−4,5‐dihydroimidazol‐2‐ylidene yields the corresponding mixed‐valence Cr‐complex [(1‐(mesityl)‐3‐(2‐μ‐O‐phenyl)−4,5‐dihydroimidazol‐2‐ylidene)Cr(I)‐(AcCN)(μ‐Cl) 2 (1‐(mesityl)‐3‐(2‐μ‐O‐phenyl)−4,5‐dihydroimidazol‐2‐ylidene)Cr(III)(Cl)] ( Cr1 ). Interestingly, reaction of CrCl 2 with the bidentate NHC 1‐(methyl)‐[3‐(2‐hydroxy‐2‐phenyl)eth‐1‐yl]‐imidazol‐2‐ylidene results in the unexpected formation of the bimetallic (Cr/Li) complex Cr2 . Both complexes were characterized by single‐crystal X‐ray analysis. The molecular structure of Cr1 features a binuclear complex comprising two inequivalent Cr‐centers with formal Cr(I) and Cr(III) oxidation states. The different oxidation states of both Cr‐centers in Cr1 were confirmed using SQUID magnetometry and high‐field electron paramagnetic resonance (EPR) measurements. The molecular structure of Cr2 features a bimetallic (Cr/Li) tetranuclear complex, again comprising two nonequivalent Cr‐centers. Upon activation with methylalumoxane (MAO), modified MAO (MMAO‐12), and ethylaluminum dichloride (EADC), respectively, complex Cr1 exhibited moderate catalytic activity (1.3–3.3 kg mol Cr −1 h −1 bar −1 ) in ethylene oligo‐/polymerization. A comparative study of the oligo‐/polymerization activities of the similar reported complexes is also presented. In most cases, Cr1 /MAO or Cr1 /EADC outperformed the structurally related chloride‐bridged Cr‐NHC‐complexes in ethylene oligomerization.