[n]Borametalloarenophanes (n=1, 2): Strained Systems with Uncommon Reactivity Patterns

The chemistry of ansa-complexes is of increasing importance in organometallic chemistry and materials science, a development that is closely related to the discovery of the first strained [1]silaferrocenophane in 1975 and their high value as efficient monomers in the ring-opening polymerization to prepare high-molecular-weight metallopolymers. By contrast, interest in boron-bridged derivatives has sparked only recently, which is rather surprising given the anticipated high degree of molecular ring strain imposed by the small covalent radius of the boron nucleus. Thus, it was not until 1997 that boron was successfully incorporated into an ansa-bridge. Since then, the chemistry of strained [n]borametalloarenophanes has been studied systematically with respectto synthesis, electronic/structural properties and reactivity patterns. As a consequence, a rather large variety of [n]borametalloarenophanes based on different sandwich complex systems such as homoleptic [Fe(eta 5-C5H5)2] and [M(eta 6-C6H6)2] (M = V, Cr), as well as heteroleptic [Mn(eta 5-C5H5)(eta 6-C6H6)] and [M(eta 5-C5H5)(eta 7-C7H7)] (M = Ti, V, Cr) are now available. The structural and electronic consequences of the small mono- or diatomic boron bridges have been evaluated by different spectroscopic methods and X-ray diffraction studies. Most importantly, these systems were shown to possess an enhanced reactivity. Uncommon and exciting reaction pathways could be encountered, which are evidently driven by the release of molecular ring strain. This Microreview is intended to highlight the consequences of molecular ring strain on the properties of [n]borametalloarenophanes. We begin with a comprehensive overview on the experimental approaches applied to their synthesis, and consider their unique properties in great detail. Subsequently, we will recognize that the highly strained character of the [n]bora-metalloarenophanes enables unprecedented reaction pathways.