[Fe(bpp)₂][BF₄]₂ (bpp = 2,6-di{pyrazol-1-yl}pyridine) derivatives with a bent geometry of hexadec-1-ynyl or hexadecyl pyrazole substituents are isomorphous, and high-spin at room temperature. However, only the latter compound undergoes an abrupt, stepwise spin-transition on cooling. This may reflect the different conformational flexibilities of their long chain substituents.

Two phenyl ester and three benzyl ester derivatives have been synthesized from 2,6-di(pyrazol-1-yl)pyridine-4-carboxylic acid and the appropriate phenyl or benzyl alcohol using N,N′-dicyclohexylcarbodiimide as the coupling reagent. Complexation of the ligands with Fe[BF₄]₂·6H₂O in acetone yielded the corresponding [FeL₂][BF₄]₂ complex salts. Four of the new ligands and four of the complexes have been crystallographically characterised. Particularly noteworthy are two polymorphs of [Fe(L³)₂][BF₄]₂·2MeNO₂ (L³ = 3,4-dimethoxyphenyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), one of which is crystallographically characterised as high-spin while the other exhibits the onset of spin-crossover above room temperature. The other complexes are similarly low-spin at low temperature but exhibit gradual spin-crossover on heating, except for an acetone solvate of [Fe(L⁵)₂][BF₄]₂ (L⁵ = benzyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), which exhibits a more abrupt spin-transition at T_½ = 273 K with 9 K thermal hysteresis.

Solvated crystals of “[FeL ₂ ][BF ₄ ] ₂ ·H ₂ O” (L = 2-(1,2,4-triazol-1-yl)-6-(pyrazol-1-yl)pyridine) contain a 1D coordination polymer [Fe(μ-L) ₂ {Fe(OH ₂ ) ₂ L ₂ }][BF ₄ ] ₄ , with κ ¹ :κ ³ ,μ-L coordination.

2,6‐Di(4‐iodopyrazol‐1‐yl)pyridine is converted to 2,6‐di(4‐{alkyn‐1‐yl}pyrazol‐1‐yl)pyridine (L¹R; R’=C_nH_2n−3, n=12, 14, 16) derivatives by Sonogashira reactions. Hydrogenation of these intermediates yields the corresponding 2,6‐di(4‐alkylpyrazol‐1‐yl)pyridine (L²R; R=C_nH_2n+1) derivatives. Homoleptic Fe[BF₄]₂ complexes of all these ligands were prepared, bearing four long chain substituents in a discotic‐type array. Crystallographic data from two L¹R ligands imply the alkynyl substituents do not undergo intermolecular interdigitation in the solid state, which could explain the lower crystallinity and ill‐defined spin‐crossover in their iron complexes. Solid [Fe(L²R)₂][BF₄]₂ (n=12 and 14) exhibit an irreversible, abrupt low→high‐spin event near 350 K, which is not associated with solvent loss and may involve disordering of their alkyl chain conformations. No mesophase behavior was observed from any of these compounds.