Electronic structure, geometry, and energetics of carbanion and carbocation of [1.1]ferrocenophane: An INDO study

Abstract

INDO-SCF calculations with constrained geometry optimization have been performed to determine the bridge geometries in [1.1]ferrocenophane and its carbocation and carbanion to address the question of possible C-H-C hydrogen bonding in the carbanion derivative. In the equilibrium geometry of the carbanion, the endo-hydrogen is bonded to one of the bridge carbon atoms and the calculated distance between the two bridge carbons seems too large to accommodate a stable C-H-C hydrogen bond. The results indicate that the observed proton NMR spectrum of carbanion should be interpreted in terms of rapid proton exchange between two bridge carbon atoms rather than a symmetric hydrogen bond. The ground state charge distributions show that the ionic bridges in both carbanion and carbocation are highly conjugated and most of the ionic charge in both molecules is distributed over the ferrocene ring system. The charge on the iron varies only slightly among the three molecules and the formal oxidation state of iron remains +2. The role of the iron seems to be that of a conduit for charge transfer between ferrocene rings upon conjugation.