An iron(ii) spin-crossover metallacycle from a back-to-back bis-[dipyrazolylpyridine]

The syntheses of 4-mercapto-2,6-di(pyrazol-1-yl)pyridine (bppSH) and bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]disulfide (bppSSbpp) are reported. In contrast to previously published "back-to-back" bis-[2,6-di(pyrazol-1-yl)pyridine] derivatives, which form coordination polymers with transition ions that are usually insoluble, bppSSbpp yields soluble oligomeric complexes with iron(ii) and zinc(ii). Mass spectrometry and DOSY data show that [{Fe(μ-bppSSbpp)}_n]²ⁿ⁺ and [{Zn(μ-bppSSbpp)}_n]²ⁿ⁺ form tetranuclear metallacycles in nitromethane solution (n = 4), although ¹H NMR and conductivity measurements imply the iron compound may undergo more fragmentation than its zinc congener. Both [{Fe(bppSH)₂]²⁺ and [{Fe(μ-bppSSbpp)}_n]²ⁿ⁺ exhibit thermal spin-crossover in CD₃NO₂ solution, with midpoint temperatures near 245 K. The similarity of these equilibria implies there is little cooperativity between the iron centres in the metallacyclic structures.