Synthesis, structure, and dynamic behavior of ansa-ferrocenes with pyrazabole bridges

A variety of stereorigid ansa-ferrocenes 2 with o-phenylene-type bridges have been obtained by the reaction of 1,1′-diborylferrocenes (1,1′-Fc(BBrR)₂; Fc = ferrocenyl; R = CH₃, Br, OEt, NC₄H₈) with selected pyrazole derivatives. Subsequent intramolecular B-N adduct formation establishes the interannular bridges. The synthetic approach is based on the replacement of certain critical carbon-carbon links by the isoelectronic, self-assembling, dative boron-nitrogen bonds and thus follows the principles of noncovalent synthesis. The pyrazabole bridge does not seem to cause any significant distortion of the ferrocene core. Most substituents at boron or pyrazole lead to an ansa-ferrocene structure. However, the presence of both strong electron donors (i.e. pyrrolidine) at boron and strongly electron-withdrawing pyrazole substituents (i.e. ester groups) results in an open chain structure 2i with three-coordinate boron atoms. The tetravalent boron centers in ansa-1,1′-Fc[B(Br)(μ-pz)]₂, 2a, show an unusually high tendency to undergo nucleophilic substitution reactions, even though they are located at bridgehead positions. NMR spectroscopy, as well as chemical evidence argue against an unbridged open chain intermediate in the course of these reactions.