Thermal and light-induced spin-transitions in iron(ii) complexes of 2,6-bis(4-halopyrazolyl)pyridines: the influence of polymorphism on a spin-crossover compound

Abstract

The syntheses of 2,6-bis(4-chloropyrazol-1-yl)pyridine (L¹), 2,6-bis(4-bromopyrazol-1-yl)pyridine (L²) and 2,6-bis(4-iodopyrazol-1-yl)pyridine (L³) by electrophilic halogenation of 2,6-bis(pyrazol-1-yl)pyridine are reported. The complex [Fe(L¹)₂][BF₄]₂ crystallises in two different solvent-free polymorphs. The tetragonal (α) form crystallises in a known version of the “terpyridine embrace” structure, and undergoes an abrupt spin-transition at 202 K. The orthorhombic (β) form exhibits a modified form of the same packing motif, containing two unique iron sites in a 2 : 1 ratio. One-third of the complex molecules in that material undergo a very gradual thermal spin-crossover centred at 137 K. Comparison of the two structures implies that spin-crossover cooperativity in the α-polymorph is transmitted in two dimensions within the extended lattice. [Fe(L²)₂][BF₄]₂ is isostructural with α-[Fe(L¹)₂][BF₄]₂ and exhibits a similarly abrupt spin-transition at 253 K. In contrast, [Fe(L³)₂][BF₄]₂ is low-spin as a powder at 360 K and below and can be crystallised as two different solvates from acetone solution. All three compounds exhibit the LIESST effect at 10 K, with photoconversions of 40–100%. Their LIESST relaxation temperatures obey the empirical T(LIESST) = T₀− 0.3T_1/2 (T₀ = 150 K) law that we have previously proposed for this class of compound.