75 YEARS OF PI-COMPLEXES OF CHROMIUM(I)

Seventy-five years ago, by the reaction of chromium trichloride with phenylmagnesium bromide, Franz Hein first prepared pi-arene complexes of chromium(I). Taking into account the analytical data and mindful of the theoretical ideas of the chemistry of his day, he regarded these compounds as polyphenylchromium salts: (C6H5)5CrX (1), (C6H5)4CrX (2), and (C6H5)3 CrX (3). Independent of the assumed oxidation states, these salts had the same orange-red color and the same magnetic moment (1.73 mu(B)). Cathodic reduction yielded the species (C6H5)4Cr and (C6H5)3Cr. In later parallel studies Zeiss and Tsutsui and Fischer (1964-1956) succeeded in determining the correct structure of these polyphenylchromium compounds. Zeiss and Tsutsui reproduced Hein's procedure, and by reducing the products with LiAlH4, they isolated biphenyl, presumably a ligand in the case of 1 and 2, and biphenyl and benzene in the case of 3. On the basis of these results and on the knowledge of the structure of ferrocene, it was Onsager who expressed the idea of the pi-sandwich structure of Hein's salts: 1 = [Cr(pi-C6H5C6H5)2](OC6H5), 2 = [Cr(pi-C6H5C6H5)2]X, and 3 = [Cr(pi-C6H6C6H5)(pi-C6H6)]X. Fischer prepared 2 by an independent route, using his reductive Friedel-Crafts method. In 1956, the pi-arene structure of 1-3 was proved, but at about the same time (THF)3Cr(C6H5)3, the first (sigma-phenyl)chromium complex, was synthesized. Thermolysis of (THF)3Cr(C6H5)3 and subsequent methanolysis yielded 2, 3, [Cr(C6H6)2]X, biphenyl, and benzene as the main products. Considering these experimental results, Hahle and Stolze discussed the possible mechanism of this process, which is closely related to Hein's procedure, to prepare the complexes 1-3. Three interdependent reactions were proposed: disproportionation of Cr(C6H5)3, reductive elimination of biphenyl, and reductive elimination of benzene. The role of THF as a hydrogen donor in the last process is discussed.