Indenyl Functionalized N-Heterocyclic Carbene Complexes of Chromium: Syntheses, Structures, and Reactivity Studies Relevant to Ethylene Oligomerization and Polymerization

The chromium­(III) complexes of the type L1CrXY (L1 = 1-(2,6-diisopropylphenyl)-3-[β-(4,7-dimethylindenyl)­ethyl]-imidazol-2-ylidene, X, Y = Cl, 2a-Cl; X = Cl, Y = Me, 2a-Me; X = Cl, Y = Ph, 2a-Ph; X, Y = benzyl, 2a-Bz) and L2CrCl2 (L2 = 1-(2,6-diisopropylphenyl)-3-[γ-(4,7-dimethylindenyl)­propyl]-imidazol-2-ylidene, 2b-Cl) have been prepared and characterized by analytical and diffraction methods. The salts [L1CrCl­(THF)]+[BArF4]− and [L2CrCl­(THF)]+[BArF4]− (ArF = 3,5-bis­(trifluoromethyl)­phenyl) were also characterized, having been obtained by abstraction of chloride, or indirectly a methyl, from 2a-Cl or 2a-Me, and 2b-Cl, by NaBArF4, respectively. Reaction of 2a-Bz with 1 equiv of [H­(Et2O)2]­[BArF4] in THF gave the thermally unstable salt [L1Cr­(Bz)­(THF)]+[BArF4], [2a-Bz]+[BArF4], which was also characterized crystallographically. The chromium­(II) analogue L1CrCl, 3a, was prepared from CrCl2 and L1K, but on prolonged standing in THF, it converted to the dimeric complex 4a. The complexes 2a-Cl, 2b-Cl, 2b-Me, and 2a-Bz in the presence of an Al activator were tested as ethylene polymerization and oligomerization catalysts, in general, showing activity toward polymer formation.本研究合成并通过分析测试与衍射方法表征了两类铬配合物：其一为通式L1CrXY的三价铬配合物，其中L1为1-(2,6-二异丙基苯基)-3-[β-(4,7-二甲基茚基)乙基]咪唑-2-卡宾，具体包括X、Y均为氯时的2a-Cl，X为氯、Y为甲基时的2a-Me，X为氯、Y为苯基时的2a-Ph，以及X、Y均为苄基（benzyl，Bz）时的2a-Bz；其二为L2CrCl2型配合物，其中L2为1-(2,6-二异丙基苯基)-3-[γ-(4,7-二甲基茚基)丙基]咪唑-2-卡宾，记为2b-Cl。 此外，本研究还表征了两类盐类配合物：分别通过NaBArF4（其中ArF为3,5-双(三氟甲基)苯基）从2a-Cl、2a-Me中攫取氯离子，或从2b-Cl中攫取甲基得到的[L1CrCl(四氢呋喃（THF）)]+[BArF4]−与[L2CrCl(THF)]+[BArF4]−。 将2a-Bz与1当量的[H(乙醚（Et2O）)2][BArF4]在THF中反应，得到热稳定性较差的盐类[L1Cr(Bz)(THF)]+[BArF4]，即[2a-Bz]+[BArF4]，该产物同样通过单晶衍射完成了表征。 由CrCl2与L1K制备得到了二价铬配合物L1CrCl（记为3a），但该配合物在THF中长时间静置后，会转化为二聚体配合物4a。 将2a-Cl、2b-Cl、2b-Me与2a-Bz作为催化剂，在铝活化剂存在下进行乙烯聚合与齐聚反应测试，结果显示此类配合物整体均表现出催化烯烃聚合的活性。