2,6-Bis(pyrazol-1-yl)pyridine-4-carboxylate Esters with Alkyl Chain Substituents and Their Iron(II) Complexes

Two series of 4-(alkoxyphenyl) 2,6-bis{pyrazol-1-yl}pyridine-4-carboxyate (L³R) or alkyl 2,6-bis{pyrazol-1-yl}pyridine-4-carboxyate (L⁴R) esters have been synthesized and complexed to iron(II), where R = C _nH_{2 n+1} ( n = 6, 12, 14, 16, 18); two other derivatives related to L³R are also reported. While the solid [Fe(L⁴R)₂][BF₄]₂ compounds are isostructural by powder diffraction and show similar spin state behaviors, the [Fe(L³R)₂][BF₄]₂ series shows more varied structures and magnetic properties. This was confirmed by solvated crystal structures of [Fe(L³R)₂][BF₄]₂ with n = 6, 14, 16, which all adopt the P1̅ space group but show significantly different side-chain conformations and/or crystal packing. The solid complexes are mostly low spin at room temperature, with many exhibiting the onset of thermal spin crossover (SCO) upon warming. Heating the complexes with n ≥ 14 significantly above their SCO temperature transforms them irreversibly into a predominantly high spin state, which is accompanied by structure changes and loss of crystallinity by powder diffraction. These transformations do not coincide with lattice solvent loss and may reflect melting and refreezing of their alkyl chain conformations during the thermal cycle. Four of the complexes exhibit SCO in CD₃CN solution with T_1/2 = 273-277 K, which is apparently unaffected by their alkyl chain substituents.