No abstract

No abstract

di{5-methylpyrazol-3-yl}pyridine) adopts a 1 : 1 high : low spin state population, and can be converted into different high-spin anhydrous phases by recrystallisation (phase 1 A ) or by thermal dehydration (phase 1 B ). Upon cooling in vacuo, the latter undergoes a thermal spinstate transition centred near T 1/2 ¼ 205 K. The transition has a thermal hysteresis width of 65 K in freshly prepared samples, although this gradually narrows to 37 K on repeated scanning. X-Ray powder diffraction measurements performed in vacuo show that 1 B , initially formed at 375 K, exhibits two consecutive crystallographic phase changes near 300 and 270 K, before undergoing a third phase change concomitant with its spin-state transition. None of these new phases is isostructural with 1 A , which itself undergoes a thermal spin-crossover on cooling without a change in crystal symmetry.

Two salts of [FeL2](2+) (L = 2,6-bis[5-methyl-1H-pyrazol-3-yl]pyridine) are isostructural under ambient conditions but show different thermal spin-crossover behaviour, involving a variety of crystallographic phase changes.

Dedicated to George Christou on the occasion of his 60 th birthday. 2 TOC EntryFour different complexes of the type shown, [Fe(L R ) 2 ] 2+ , have been prepared, with R = methyl, allyl, benzyl and isopropyl. All the compounds are high-spin in acetone solution and the solid state at room temperature and below, except for one salt of [Fe(L Me ) 2 ] 2+ which is predominantly low-spin at 150 K in the crystal. This contrasts with the parent complex (R = H), which exhibits thermal spin-crossover just below room temperature. KeywordsIron; N-donor ligands; Crystal Structure; Magnetic Measurements; Spin-Crossover