title: A crystallographic, EPR and theoretical study of the Jahn-Teller distortion in [CuTp2] (Tp- = tris[pyrazol-1-yl]hydridoborate).
abstract: Crystals of the title compound (1) contain two independent, centrosymmetric half-molecules per asymmetric unit. While both of these show Jahn-Teller elongated six-coordinate geometries, the lengths of the elongated Cu-N bonds in the two molecules differ by 0.117(2) A at 30 K. The structure of one of these molecules (molecule A) does not vary with temperature below 350 K. The other molecule (molecule B) shows Cu-N bond lengths that are temperature-dependent between 225 and 375 K, but do not vary further at lower temperature. This indicates a fluxional axis of Jahn-Teller elongation in this molecule at these higher temperatures. Consideration of the thermal parameters in these structures implies that the fluxionality in molecule B is frozen out near 150 K. This conclusion is supported by a Q-band powder EPR study. The d-d transition energies of molecules A and B have been calculated by several density function (DF) methods, including a time-dependent DF calculation. The crystallographic data have been reproduced using the vibronic coupling model of Burgi and Hitchman. This has shown that the different fluxionality regimes for molecules A and B are not a consequence of their different static molecular structures, but rather reflect their different local environments in the crystal.
authors.affiliation: Department of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.
authors.forename: Colin A
authors.initials: CA
authors.lastname: Kilner
authors.forename: Eric J L
authors.initials: EJ
authors.lastname: McInnes
authors.forename: Michael A
authors.initials: MA
authors.lastname: Leech
authors.forename: Godfrey S
authors.initials: GS
authors.lastname: Beddard
authors.forename: Judith A K
authors.initials: JA
authors.lastname: Howard
authors.forename: Frank E
authors.initials: FE
authors.lastname: Mabbs
authors.forename: David
authors.initials: D
authors.lastname: Collison
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Bridgeman
authors.forename: Malcolm A
authors.initials: MA
authors.lastname: Halcrow