Phenylchromium(III) Chemistry Revisited 100 Years after Franz Hein (Part III): From (Ar)_3–nCrCl_n(L)_x (Ar = Ph, p‑F‑C₆H₄, C₆D₅; n = 0, 1, 2; L = thf, dme, tmeda, 12C4) to Bis(π-arene)chromium Complexes

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–nCrCl_n(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)­Cr⁰(η⁶-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)₂Cr^II]_n” that is reduced to “(η⁶-C₆H₅MgPh)₂Cr⁰” by MgPh₂, (iii) reductive coupling yielding [{(η⁶-Ph₂)­Cr}₂(μ-η⁶:η⁶-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] (3_F-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 [Ph₄Cr₄(μ₄-OH)­(μ-Cl)₂(μ₃-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 [Cr₇^II(μ-Cl)₁₀(μ⁴-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.