M.m]Metallocenophanes: Synthesis, Structure, and Properties

Abstract

AbstractMetallocenophanes are metallocenes in which the cyclopentadienyl ligands are linked intramolecularly through CmHn or CmHnX (X = heteroatom) chains. Two main classes of metallocenophanes are known: in [m]metallocenophanes the ligands of one metallocene are connected by a bridge (or by several bridges); in [m.n]metallocenophanes two (or more) metallocenes are brought together into one molecule by bridging groups. This article briefly describes the first class (mononuclear complexes) and then concentrates on the second (binuclear complexes). The [1.1]metallocenophanes, which are produced by linking two metallocene moieties through two C1, bridges, are of particular interest and are characterized by a variety of unusual and useful properties: they are very flexible conformationally; hydride abstraction from the CH2 bridges produces carbenium ions that are greatly stabilized by the adjacent metallocene moieties; deprotonation of one of the CH2 bridges is surprisingly easy and leads to the unusual situation that an intramolecular CHċC hydrogen bond stabilizes a carbanion; protonation of both metallocene moieties causes the liberation of one mole of hydrogen per mole of [1.1]metallocenophane, thereby allowing the use of these compounds as catalysts for the formation of hydrogen in acidic aqueous solutions.