Bis(benzene)chromium

In the late 1910s, Franz Hein started the investigation of "triphenylchromium" by reacting chromium trichloride with a Grignard reagent, phenyl magnesium bromide. Such a reaction gave a mixture of phenyl chromium, and Hein suggested that it contained a Cr(VI) species, "(C₆H₅)₅CrBr", generated via valence disproportionation.^[1][2]

5 C₆H₅MgBr + 4 CrCl₃ → (C₆H₅)₅CrBr + 2 MgBr₂ + 3 MgCl₂ + 3 CrCl₂

This event marked an advance in organochromium chemistry at the time. "(C₆H₅)₅CrBr" was described as having salt-like properties. However, the reported workup procedures for "(C₆H₅)₅CrBr" was challenging, and the yield was low.^[1][2] Zeiss and Tsutsui found that Hein's formulation of the chromium-containing products was flawed.^[1]

Ernst Otto Fischer proposed the synthesis of a chromium(0) complex with two benzene ligands, which would have a sandwich structure similar to that of ferrocene. In 1954, Walter Hafner, one of Fischer's PhD student put the idea into practice using the "reductive Friedel-Crafts reaction."^[3][4] Before their preparation of bis(benzene)chromium, it was known that heating chromium trichloride, aluminium trichloride, and aluminium powder in benzene under high pressures of carbon monoxide affords chromium hexacarbonyl. Under these conditions the aluminium trichloride abstracts chloride from the chromium, and the aluminium metal reduces Cr(III). Hafner repeated this procedure but omitted the CO, resulting in the formation of yellow [Cr(C₆H₆)₂]⁺:

6 C₆H₆ + 3 CrCl₃ + 2 Al + x AlCl₃ → 3 [(C₆H₆)₂Cr][AlCl₄]·(x−1)AlCl₃

Excess aluminium trichloride is needed to solubilize the product.^[5] The product [(C₆H₆)₂Cr]⁺ is not particularly air-sensitive. The cation is then reduced by sodium dithionite in aqueous sodium hydroxide. The resulting solid was bis(benzene)chromium:^[5][6]

2 [(C₆H₆)₂Cr]⁺ + S₂O2−4 + 4 OH⁻ → 2 (C₆H₆)₂Cr + 2 SO2−3 + 2 H₂O

The synthesis was intensely examined. Among the many results was the finding that the procedure was catalyzed by the presence of mesitylene. Heating [Cr(C₆H₆)₂]⁺ with aqueous base results in disproportionation.^[7]

Fischer and Seus soon prepared Hein's [Cr(C₆H₅−C₆H₅)₂]⁺ by an unambiguous route, thus confirming that Hein had unknowingly discovered sandwich complexes, a half-century ahead of the work on ferrocene.^[8][9] Illustrating the rapid pace of this research, the same issue of Chem. Ber. also describes the Mo(0) complex.^[10]

Using the technique of metal vapor synthesis, bis(benzene)chromium and many analogous compounds can be prepared by co-condensation of Cr vapor and arenes. In this way, the phosphabenzene complex [Cr(C₅H₅P)₂] can be prepared.^[11]

Bis(benzene)chromium is thermally stable under an inert gas atmosphere. It is diamagnetic. In 1956, Fischer and Weiss determined structure of bis(benzene)chromium by X-ray crystallography. It crystallizes with centrosymmetry in a cubic space group.^[12] The Cr-C distances are 214.1 and the C-C distances are 141.6 picometers.^[13] The C-C bonds are elongated from 139 pm for free benzene.

According to electrochemical studies, the half-wave potential (E_1/2) of the +1/0 couple is around -1.10 to -1.25 V versus Ferrocenium/Ferrocene at room temperature.^[14][15][16]

Theoretical chemical bonding of bis(benzene)chromium has been investigated since the discovery of this compound. The ground state configuration is (3e_2g)⁴(4a_1g)² (3e_2u)⁰. Analysis of the frontier orbitals suggested that the chromium-benzene interaction is largely contributed by the 𝝅 and/or 𝞭 interactions between the 3d metal orbitals and ligand 𝝅 orbitals.^[17][18] 3e_2g (HOMO-1) and 3e_1g (HOMO-2) molecular orbitals are 𝞭-bonding interactions between metal 3d𝞭 and ligand 𝝅 orbitals. The highest occupied molecular orbital (HOMO), 4a_1g, is the non-bonding metal d_z2 orbital. The lowest unoccupied molecular orbital (LUMO) is 3e_2u, which is purely ligand 𝝅 orbital. As for 4e_1g (LUMO+1) and 4e_2g (LUMO+2), they are composed of anti-bonding interaction between 3d𝝅 and ligand 𝝅 orbitals.

3d orbitals population of chromium(0) in bis(benzene)chromium was investigated, utilizing NBO analysis. While e_2g largely results from electron donation from the metal to the ligand, e_1g is mainly composed of the electrons donated from the benzene ligands.^[13]

In contrast to ferrocene, where 𝝅-interactions dominate the metal-ligand bonds, 𝞭-interactions play a significant role in bis(benzene)chromium.^[17][18]

The compound reacts with carboxylic acids to give chromium(II) carboxylates, such as chromium(II) acetate. Oxidation affords [Cr(C₆H₆)₂]⁺. Carbonylation gives (benzene)chromium tricarbonyl.

In late 1990s, Samuel and coworkers revealed that bis(benzene)chromium is an efficient organometallic radical scavenger. In contrast to cobaltocene, which traps radicals (R^•) to form 18-valence electron species (η⁵-C₅H₅)(η⁴-C₅H₅R)Co, bis(benzene)chromium reacts with radicals to form 17-valence electron species (η⁶-C₆H₆)(η⁵-C₆H₆R)Cr (R = H, D, isobutyronitrile).^[19]

Subsequently, bis(benzene)chromium was reported to catalyze hydrosilation of alcohols and aldehydes. Unlike late transition metal-catalyzed processes involving oxidative addition, the mechanism of this reaction is proposed to involve radicals and hydrogen atom abstraction.^[20]

The compound finds limited use in organic synthesis.^[21]