Phenylchromium(III) Chemistry Revisited 100 Years after Franz Hein (Part II): From LinCrPh3+ n(thf)x(n = 1, 2, 3) to Dimeric Triphenylchromate(II) Complexes

Fischer, Reinald,G?rls, Helmar,Suxdorf, Regina,Westerhausen, Matthias

, p. 3892 - 3905 (2020)

Polyphenylchromium(III) organometallics with various phenylation degrees and stabilized by diverse Lewis bases with various donor strengths and denticity were investigated in order to better understand the formation of (η6-arene)chromium complexes according to the procedure of Franz Hein (1892-1976) [ Organometallics 2019, 38, 498-511, DOI: 10.1021/acs.organomet.8b00811]. Part II focuses on hexa-, penta-, and tetraphenylchromates(III). Chromium(III) compounds with a lower phenylation degree will be discussed in a future part III. The numbering scheme of the complexes relates to the number of Cr-bound phenyl substituents. Hexaphenylchromate(III): The reaction of Ph3Cr(thf)3·0.25dx (3) (dx = 1,4-dioxane) with an ethereal solution of phenyllithium yields yellow-orange [Li3CrPh6(thf)2.3(OEt2)0.7] (6-thf-OEt2) which slowly degrades in contact with the reaction solution leading to emerald-green crystals of [{(Et2O)Li}2Ph3Cr(μ-O)]2 (3-Li2O). Pentaphenylchromate(III): Compound 6-thf-OEt2 reacts with 1 equiv of HCl-OEt2 solution to turquoise [{(thf)2Li}{(Et2O)Li}CrPh5] (5-thf-OEt2) that reacts with THF to the green contact ion pair [{(thf)2Li}2CrPh5] (5-thf) and with 12-crown-4 (12C4) to the light green solvent-separated ion pair [(12C4)Li(thf)]2 [CrPh5] (5-thf-12C4). Refluxing of 5-thf-OEt2 in diethyl ether leads to ether degradation and formation of 3-Li2O, whereas 5-thf-12C4 liberates biphenyl under similar reaction conditions. Tetraphenylchromate(III): The reaction of 3 with 1 equiv of phenyllithium in THF leads to a green reaction mixture. At -50 °C, red [(thf)4Li] [cis-(thf)2CrPh4]·2THF (4-thf) crystallizes which reversibly transforms into a green oil above -50 °C. Upon acidolysis of 5-thf-OEt2 with 1 equiv of HCl-OEt2 at -20 °C, the intermediately formed red complex is reduced to the dinuclear chromate(II) [{(thf)Li}CrPh3]2 (3-CrII-thf) (Cr-Cr 187.66(8) pm). Recrystallization of this product from THF yields solvent-separated [(thf)4Li]2 [(CrPh3)2] (3-CrII-thf4) with a Cr-Cr quadruple bond (Cr-Cr 183.7(2) pm) without contacts between the lithium ions and Cr-bound phenyl groups. Complex 3-CrII-thf reacts at room temperature in diethyl ether to the sandwich complexes bis(biphenyl)chromium(0) [(η6-Ph2)2Cr0] (π-4) and benzene-biphenylchromium(0) [(η6-C6H6)(η6-Ph2)Cr0] (π-3). Compounds in bold letters are authenticated by X-ray structure determinations.

Hein et al.

, p. 34 (1957)