Bonding, motional anisotropy, and metal atom dynamics of iron in two ferrocene substituted boron ring compounds

Temperature-dependent 57Fe Mössbauer spectroscopy over the range 90≤T≤360 K has been used to elucidate the hyperfine interactions and dynamical behavior of the iron atom in two ferrocenyl substituted six-membered ring compounds. The root-mean-square amplitude of vibration (rmsav) of the iron atom in ferrocenyl boroxine I extracted from the Mössbauer measurements is in good agreement with the average Uij values observed in X-ray diffraction experiments, but is in less good agreement with the X-ray data reported for ferrocenyl borazine II. Moreover, the Mössbauer data lead to the conclusion that with respect to the iron atom motion, the boroxine compound I is a softer lattice than II. Significant motional anisotropy of the iron atom at 294 K is observed in I in agreement with the room temperature X-ray data reported earlier.