The evolution of the ansa -bridge and its effect on the scope of metallocene chemistry

摘要

As the field of metallocene chemistry was reaching maturity, the introduction of an ansa -bridge brought these complexes, once again, to the forefront of academic and industrial organometallic research. The body of work concerning the synthesis and application of ansa -metallocene complexes is so vast that it is impossible to mention all of the contributions that have been made to this chemistry in one review article. The examples presented here were selected to illustrate the multiple functions the ansa -bridge has served in the chemistry of metallocenes. These functions include: (a) fixing the symmetry of the metallocene complex by preventing free rotation of the rings, (b) controlling the stereochemistry of metallocene formation by directing the orientation of the rings upon metalation, (c) influencing the reactivity of the metal by enforcing a bent-sandwich geometry between the rings, (d) increasing the electrophilicity of the metal and increasing the access of substrates to the equatorial wedge of the complex by increasing the tilt of the rings on the metal, and (e) providing a reactive site at which ring opening polymerization chemistry, ligand substitution, reversible bridge formation, and reversible metal ion binding can occur. The ansa -metallocene ligand design has inspired the design of related bridged ligand frameworks. These include cyclopentadienyl-type ring systems tethered to amine, amide, alkoxide, ether, phosphine, phosphide, and arene ligands and interannular-bridged bis(η~6-arene), bis(dicarbollide), and bis(boratabenzene) transition metal complexes.