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

Abstract

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 1 H 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 CD 3 NO 2 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.