Cofacial Metallocene Rings: Evidence for Interaction from Metal Atom Vibrational Anisotropies

Abstract

Temperature-dependent Mossbauer spectroscopy has been used to study the details of the metal atom motion in a number of face-to-face ferrocenyl moiety containing solids. While the hyperfine parameters for the 1,8-bis(ferrocenyl)naphthalene complexes are very similar to those observed for other ferrocenyl-group-containing organometallics, the face-to-face complexes show an unusual Gol'danskii-Karyagin asymmetry, which is not observed for the 1,5-bis(ferrocenyl) homologue. High-temperature data show that the difference in the mean-square-amplitude of vibration parallel and perpendicular to the symmetry axis running through the Fe atom is ∼9.3 x 10 - 1 9 cm 2 at 378 K when the ferrocenyl group is bonded directly to the naphthalene backbone and ∼4.6 x 10 - 1 9 cm 2 when there is an intervening carbon-carbon triple bond. One-electron oxidation of the 1,8-bis(ferrocenyl)-naphthalene complex leads to a cation in which there is no electron exchange between the two iron atoms on the Mossbauer time scale. The oxidized metal atoms evidence spin-lattice relaxation phenomena with very fast relaxation rates above T ≥ ∼200 K. The nonoxidized iron atom clearly exhibits anisotropic vibrations at T ≥ ∼270 K.