The impact of whole-molecule disorder on spin-crossover in a family of isomorphous molecular crystals

Treatment of 2-(pyrazol-1-yl)-6-fluoropyridine with one equiv. of the appropriate 4-substituted 1H-pyrazole in the presence of sodium hydride gives moderate yields of 2-(pyrazol-1-yl)-6-(4-methylpyrazol-1-yl)pyridine (L^Me), 2-(pyrazol-1-yl)-6-(4-fluoropyrazol-1-yl)pyridine (L^F), 2-(pyrazol-1-yl)-6-(4-chloropyrazol-1-yl)pyridine (L^Cl) and 2-(pyrazol-1-yl)-6-(4-bromopyrazol-1-yl)pyridine (L^Br). Single crystals of [Fe(L^R)₂]Z₂ (R = Me, F or Br; Z^- = BF₄ ^- or ClO₄ ^-) are often well-formed, but are poor diffractors of X-rays. An analysis of [Fe(L^Me)₂][ClO₄]₂ showed non-statistical positional disorder of the methyl substituents, leading to whole molecule disorder in each residue of the asymmetric unit. Single crystals of [Fe(L^Br)₂][BF₄]₂ are isomorphous with the L^Me complex, but show less substituent disorder. All the complex salts are isomorphous by powder diffraction, and show thermal spin-transitions whose cooperativity differs from gradual (R = Me) to abrupt and hysteretic (R = Br). Some of the cooperative transitions exhibit irregular, closely spaced discontinuities which are not caused by crystallographic phase changes, and may reflect local heterogeneities associated with the cation disorder. No aspect of their crystal packing appears to correlate with their spin-transition cooperativity. However, weaker cooperativity may correlate with increased cation disorder in this system, which merits further investigation.