A historical landmark: The discovery and re-discovery of the sandwich complexes


Helmut Werner

University of Würzburg, Germany


Sixty years ago, in the short period of time between December 1951 and February 1952, two papers appeared which laid the roots for what a few years later was called by Sir Ron Nyholm “The Renaissance of Inorganic Chemistry”. Two research groups, working in completely different fields, reported the isolation of a seemingly simple iron compound of the analytical composition FeC10H10 which quite soon became the flagship of a new chemical discipline. The structural elucidation of bis(cyclopentadienyl)iron, later called ferrocene, immediately initiated a research avalanche for which there was no precedent. In less than five years, the groups of the two pace makers, E. O. Fischer and G. Wilkinson, prepared bis- and tris(cyclopentadienyl) compounds of almost all of the transition metals. Moreover, it was shown that sandwich complexes structurally related to ferrocene, having two six- instead of two five-membered rings coordinated to the metal, could be prepared and that cationic derivatives of those bis(arene) metal compounds, originally assumed to possess a different structure, were prepared 35 years before.

In the meantime, the field of sandwich-type transition metal complexes has been expanded in various directions. Apart from compounds containing two cyclopentadienyl or two arene ligands, analogues with planar three-, four-, seven- and eight-membered ring systems were described. In the early 1970ies, we reported the preparation of the first triple-decker sandwiches, in the late 1980ies Scherer et al. obtained counterparts of ferrocene with P5 and As5 as ligands, and in the first decade of the 21st century Carmonas group isolated the dinuclear complex Zn2(C5Me5)2, in which a Zn2 unit is sandwiched by two pentamethylcyclopentadienyl rings. In the course of this breath-taking race, ferrocene – the star of the whole class of sandwich compounds – became a versatile building block for the synthesis of species with tailor-made properties. Recently, work in this field culminated in the preparation of a giant dendrimer with 243 ferrocenyl units at the periphery, which is one of the largest organometallic molecules known today and one of the highlights of Didier Astrucs work at the U.