| Abstract
| - The dynamic behavior of[(C6H7)Fe(CO)3]BF4and[(C7H9)Fe(CO)3]BF4in the solid state has been investigated by NMR and57Fe Mössbauer spectroscopy and also bycalorimetric (DSC) and crystallographic methods. Both complexeswere found to have a solid state phase transition below which they arerelatively static and above which there is whole-molecule tumbling.The transition occurs at 253−263 K for[(C6H7)Fe(CO)3]BF4,and at 329−341 K for[(C7H9)Fe(CO)3]BF4.
- The dynamic behavior of[(C6H7)Fe(CO)3]BF4(I) and[(C7H9)Fe(CO)3]BF4(II) in the solid state has beeninvestigated principally by NMR spectroscopy. High-resolutionvariable-temperature 1H and 13C NMRspectraindicate that both complexes have a solid state phase transition abovewhich there is rapid reorientation of thecyclodienylium rings and fast exchange of the carbonyl groups. Thetransition occurs between 253 and 263 Kfor I and between 329 and 341 K for II. Thepresence of the phase transition is confirmed by differentialscanningcalorimetry (DSC). 57Fe Mössbauerspectroscopy supports the notion that complex I is highlymobile at roomtemperature, while II is relatively static. Theactivation energy for the cyclodienylium group rotation inthehigh-temperature phase of I is estimated from 1Hspin−lattice relaxation time measurements to be 17.5 kJmol-1.Static 13C NMR measurements of the solid complexes inthe high-temperature phase indicate that the 13Cchemicalshift anisotropies are only 20−30 ppm. This is significantlyless than that expected to result from motion ofindividual groups and thus suggests that rotation of the whole moleculeis involved. A single-crystal X-raystructural determination of complex II, at 295 K, showedthat the complex is tetragonal (space group P41,a =10.610(1) Å, c = 21.761(3) Å, V =2449.7(5) Å3, ρcalc = 1.734 gcm-3), with eight cycloheptadienyl cationsandeight tetrafluoroborate anions per unit cell. In addition, powderX-ray diffraction studies of both I and IIconfirmthat at low temperatures both complexes have a tetragonal unit cell,which transforms to a cubic unit cell abovethe phase transition. The powder patterns, recorded above thephase transition, support the proposal that thecomplexes are undergoing whole-molecule tumbling in their dynamicregimes.
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