title: Solid State Dynamics of Tricarbonyl(eta-1,5-cyclohexadienylium)iron Tetrafluoroborate and Tricarbonyl(eta-1,5-cycloheptadienylium)iron Tetrafluoroborate.
abstract: The dynamic behavior of [(C(6)H(7))Fe(CO)(3)]BF(4) (I) and [(C(7)H(9))Fe(CO)(3)]BF(4) (II) in the solid state has been investigated principally by NMR spectroscopy. High-resolution variable-temperature (1)H and (13)C 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). (57)Fe Mössbauer 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 (1)H spin-lattice relaxation time measurements to be 17.5 kJ mol(-)(1). Static (13)C NMR measurements of the solid complexes in the high-temperature phase indicate that the (13)C chemical shift anisotropies are only 20-30 ppm. This is significantly less than that expected to result from motion of individual groups and thus suggests that rotation of the whole molecule is involved. A single-crystal X-ray structural determination of complex II, at 295 K, showed that the complex is tetragonal (space group P4(1), a = 10.610(1) Å, c = 21.761(3) Å, V = 2449.7(5) Å(3), rho(calc) = 1.734 g cm(-)(3)), with eight cycloheptadienyl cations and eight tetrafluoroborate anions per unit cell. In addition, powder X-ray diffraction studies of both I and II confirm that at low temperatures both complexes have a tetragonal unit cell, which transforms to a cubic unit cell above the phase transition. The powder patterns, recorded above the phase transition, support the proposal that the complexes are undergoing whole-molecule tumbling in their dynamic regimes.
authors.affiliation: Department of Physics, University of Nottingham, University Boulevard, Nottingham NG7 2RD, United Kingdom, Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom, Department of Chemistry, Queen Mary and Westfield College, Mile End Road, London E1 4NS, United Kingdom, Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland, and Department of Chemistry, University of Missouri-Rolla, Rolla, Missouri 65401-0249.
authors.forename: Dermot F.
authors.initials: DF
authors.lastname: Brougham
authors.forename: Patrick J.
authors.initials: PJ
authors.lastname: Barrie
authors.forename: Geoffrey E.
authors.initials: GE
authors.lastname: Hawkes
authors.forename: Isaac
authors.initials: I
authors.lastname: Abrahams
authors.forename: Majid
authors.initials: M
authors.lastname: Motevalli
authors.forename: David A.
authors.initials: DA
authors.lastname: Brown
authors.forename: Gary J.
authors.initials: GJ
authors.lastname: Long