Two new 4′,4′′-disubstituted dipyrazolylpyridine derivatives, and the structures and spin states of their iron(II) complexes

Reaction of 2 equiv of the sodium salt of ethyl pyrazole-4-carboxylate, with 1 equiv of 2,6-dibromopyridine, in diglyme at 130 °C for 5 days yields 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L1), with 2-bromo-6-[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L2) as a significant byproduct. Reduction of L1 with excess NaBH4 in thf affords 2,6-di[4-(hydroxymethyl)pyrazol-1-yl]pyridine (L3) in low yield. The crystalline complex [Fe(L1)2][BF4]2 · 2CF3CH2OH is low-spin at 150 K, while bulk samples with this formula are approximately 10% high-spin and 90% low-spin at room temperature. This ratio does not vary significantly on cooling from its magnetic susceptibility, suggesting that the material might be contaminated by a second, minor high-spin phase. Single crystals of [Fe(L3)2][BF4]2·1.4CH3CN have a mixed spin-state population, with the low-spin state predominating at 150 K. The [Fe(L3)2(BF4)]+ moieties in the lattice associate into 1-D chains through intermolecular O–H⋯O and O–H⋯F hydrogen bonding. Bulk samples of [Fe(L3)2][BF4]2 · H2O are fully low-spin below 200 K, but the magnetic data imply the onset of a gradual thermal spin-transition centred above room temperature. DSC and TGA measurements imply that this transition is centred at 322 K, and involves loss of lattice water. Both complexes undergo spin-crossover in (CD3)2CO solution, with transition midpoints near 250 K.