Solid State Dynamics of Tricarbonyl(η-1,5-cyclohexadienylium)iron Tetrafluoroborate and Tricarbonyl(η-1,5-cycloheptadienylium)iron Tetrafluoroborate

The dynamic behavior of [(C6H7)Fe(CO)3]BF4 (I) and [(C7H9)Fe(CO)3]BF4 (II) in the solid state has been investigated principally by NMR spectroscopy. High-resolution variable-temperature 1H and 13C NMR spectra indicate that both complexes have a solid state phase transition above which there is rapid reorientation of the cyclodienylium rings and fast exchange of the carbonyl groups. The transition occurs between 253 and 263 K for I and between 329 and 341 K for II. The presence of the phase transition is confirmed by differential scanning calorimetry (DSC). 57Fe Mossbauer spectroscopy supports the notion that complex I is highly mobile at room temperature, while II is relatively static. The activation energy for the cyclodienylium group rotation in the high-temperature phase of I is estimated from 1H spin−lattice relaxation time measurements to be 17.5 kJ mol-1. Static 13C NMR measurements of the solid complexes in the high-temperature phase indicate that the 13C chemical shift anisotropies are only 20−30 ...