title: Unexpected Spin-Crossover and a Low-Pressure Phase Change in an Iron(II)/Dipyrazolylpyridine Complex Exhibiting a High-Spin Jahn-Teller Distortion.
abstract: The synthesis of 4-methyl-2,6-di(pyrazol-1-yl)pyridine (L) and four salts of [FeL2]X2 (X(-) = BF4(-), 1; X(-) = ClO4(-), 2; X(-) = PF6(-), 3; X(-) = CF3SO3(-), 4) are reported. Powder samples of 1 and 2 both exhibit abrupt, hysteretic spin-state transitions on cooling, with T1/2↓ = 204 and T1/2↑ = 209 K (1), and T1/2↓ = 175 and T1/2↑ = 193 K (2). The 18 K thermal hysteresis loop for 2 is unusually wide for a complex of this type. Single crystal structures of 2 show it to exhibit a Jahn-Teller-distorted six-coordinate geometry in its high-spin state, which would normally inhibit spin-crossover. Bulk samples of 1 and 2 are isostructural by X-ray powder diffraction, and undergo a crystallographic phase change during their spin-transitions. At temperatures below T1/2, exposing both compounds to 10(-5) Torr pressure inside the powder diffractometer causes a reversible transformation back to the high-temperature crystal phase. Consideration of thermodynamic data implies this cannot be accompanied by a low → high spin-state change, however. Both compounds also exhibit the LIESST effect, with 2 exhibiting an unusually high T(LIESST) of 112 K. The salts 3 and 4 are respectively high-spin and low-spin between 3 and 300 K, with crystalline 3 exhibiting a more pronounced version of the same Jahn-Teller distortion.
authors.affiliation: †School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom.
authors.forename: Laurence J
authors.initials: LJ
authors.lastname: Kershaw Cook
authors.affiliation: †School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom.
authors.forename: Flora L
authors.initials: FL
authors.lastname: Thorp-Greenwood
authors.affiliation: ‡Institute for Materials Research, School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom.
authors.forename: Tim P
authors.initials: TP
authors.lastname: Comyn
authors.affiliation: §School of Physics and Astronomy, University of Leeds, E. C. Stoner Building, Leeds LS2 9JT, United Kingdom.
authors.forename: Oscar
authors.initials: O
authors.lastname: Cespedes
authors.affiliation: ∥CNRS, Université de Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Chastanet
authors.affiliation: †School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom.
authors.forename: Malcolm A
authors.initials: MA
authors.lastname: Halcrow