Chromium(II)/(III) Complexes Bearing Phosphino-<i>N</i>-Heterocyclic Carbene and <i>N</i>-Chelating <i>N</i>-Heterocyclic Carbene Ligands and Their Use in Ethylene Oligomerization

The bidentate Cr(III) phosphino-N-heterocyclic carbene (NHC) complex [(1-(2,6-diisopropylphen-1-yl)-3-(1-diphenylphosphinoeth-2-ylimidazol-2-ylidene)CrCl3(THF)] (Cr1) as well as the Cr(II) and Cr(III) complexes [(1-methyl-3-(2-amido-N-(2,6-diisopropylpheny-1-yl)phen-1-ylimidazol-2-ylidene)2Cr2-(μ-Cl)4] (Cr2), [(1-methyl-3-(2-amido-N-(2-biphen-1-yl)phen-1-ylimidazol-2-ylidene)2Cr2-(μ-Cl)4] (Cr3), [(1-methyl-3-(2-amido-N-(2,6-diisopropylpheny-1-yl)phen-1-ylimidazol-2-ylidene)2Cr2-(μ-Cl)4] (Cr4), [(1-methyl-3-(2-amido-N-(2,6-diisopropylpheny-1-yl)phen-1-ylimidazol-2-ylidene)Cr(benzyl)2(THF)] (Cr5), [(1-methyl-3-(2-amido-N-(2,6-diisopropylpheny-1-yl)phen-1-ylimidazol-2-ylidene)2Cr] (Cr7), [(1-methyl-3-(2-amido-N-(2,6-diisopropylpheny-1-yl)phen-1-ylimidazol-2-ylidene)2Cr2-(μ-Cl)2(Cl)2(THF)2] (Cr8), [(1-methyl-3-(2-amido-N-(2-biphen-1-yl)phen-1-ylimidazol-2-ylidene)2Cr] (Cr10), [(1-methyl-3-(2-amido-N-(2-biphen-1-yl)phen-1-ylimidazol-2-ylidene)Cr-(μ-Cl)2(1-methyl-3-(2-amido-N-(2-biphen-1-yl)phen-1-ylimidazol-2-ylidene)2Cr] (Cr11) containing a bidentate N-chelating NHC, as well as the Cr(II) cyclic alkylamino carbene (CAAC) complex [(CAAC)2CrCl2] (Cr6, CAAC = 1-(2,6-diisopropylpheny-1-yl)-3,3,5,5-tetramethyltetrahydropyrrol-2-ylidene) have been prepared. Cr1, Cr4, Cr7, Cr8, and Cr10–Cr12 were characterized by single-crystal X-ray analysis. The molecular structure of Cr4 features a binuclear complex comprising two Cr(II) metal centers where each metal center adopts a distorted square planar geometry, whereas Cr11 represents a noteworthy addition to the rare class of structurally characterized mixed-valence Cr-complexes composed of two nonequivalent Cr centers with formal Cr(II) and Cr(III) oxidation states. In the presence of methylalumoxane (MAO) as a cocatalyst, the novel chromium complexes display catalytic activity in ethylene oligomerization. Complex Cr2, with an N-diisopropylphenyl group, shows ca. 7 times higher polymerization activity (8.1 kg mol Cr–1 h–1 bar–1) than Cr3 (1.1 kg mol Cr–1 h–1 bar–1) bearing a bulky, yet flexible N-biphen-1-ylamine unit, indicating that less bulky ligands can enhance catalytic activity in chelated Cr complexes. The Cr(II)-CAAC complex Cr6 possesses increased activity compared to a structurally similar Cr(II)-NHC complex.