American Chemical Society
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Phenylchromium(III) Chemistry Revisited 100 Years after Franz Hein (Part III): From (Ar)3–nCrCln(L)x (Ar = Ph, p‑F‑C6H4, C6D5; n = 0, 1, 2; L = thf, dme, tmeda, 12C4) to Bis(π-arene)chromium Complexes

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journal contribution
posted on 2022-04-07, 15:36 authored by Reinald Fischer, Helmar Görls, Regina Suxdorf, Matthias Westerhausen
One century ago, Franz Hein started the highly regarded arylchromium chemistry with his initial publication in January 1919. The formation of bis­(π-arene)chromium compounds according to Hein’s protocols has been performed with pure substrates of the type (Ar)3–nCrCln(L)x (n = 0, 1, 2; Ar = Ph, C6D5, p-F-C6H4, L = neutral Lewis bases with different base strength and denticity) as model compounds at magnesium halide-free conditions. Three main synthesis phases for the σ–π transfer of the phenyl groups have been recognized and simulated: Initial phase (excess of MgPh2): Ph3Cr­(thf)3·0.25 dx·(3-thf3, dx = 1,4-dioxane) eliminates tetrahydrofuran (THF) reversibly in C6D6, yielding bluish-green [Ph3Cr­(thf)2] (3-thf2) that reacts above 5 °C irreversibly to a black-brown solution of biphenyl, benzene, and “{(η6-Ph2)­CrI­(μ-η6-Ph)}2” (intermediate A). Ph2Mg reduces A to “(η6-Ph2)­Cr06-PhMgPh)” (intermediate B). Middle phase: Subsequent reactions of A: (i) Reduction to [(η6-C6H6)­(η6-Ph2)­Cr0] (π-3) by THF, (ii) disproportionation to [(η6-Ph2)2Cr0] (π-4) and “[(η6-C6H5)2CrII]n” that is reduced to “(η6-C6H5MgPh)2Cr0” by MgPh2, (iii) reductive coupling yielding [{(η6-Ph2)­Cr}2(μ-η66-Ph2)] ((π-3)2), or (iv) decomposition to biphenyl and chromium. Labeling experiments with (D5C6)3Cr­(thf)3·0.25 dx ([D15]­3-thf3) in C6D6 and [fac-(p-F-C6H4)3Cr­(thf)3] (3F-thf3) allow one to determine the origin of the hydrogen atom and the identification of the end products. Final phase: Deficit of Mg­(Ar)2 hinders formation of bis­(π-arene)chromium complexes. “Ph2CrCl­(thf)x” forms an equilibrium in THF solution with 3-thf3 and PhCrCl2(thf)x (1-Cl). During heating of a solution of 3-thf3, CrCl3(thf)3, and H2O (3:1:1) in THF o-metalation of a phenyl group leads to formation of mixed-valent tetranuclear [Ph4Cr44-OH)­(μ-Cl)23-Cl)­(μ3-C6H4)­(thf)4] (4-Cr4) besides biphenyl and benzene. The analogous reaction of [D15]­3-thf3, CrCl3(thf)3, and H2O yields [D5]­benzene, [D10]­biphenyl, and heptanuclear oxo-centered [Cr7II(μ-Cl)104-O)2(thf)6] (0-Cr7). Complexes of the type [(Ar)­CrCl2(thf)n] (Ar = Ph, Mes, p-F-C6H4) show homolytic Cr–C bond cleavage, leading to chromium­(II) chloride and biaryls.