Nonanuclear Spin-Crossover Complex Containing Iron(II) and Iron(III) Based on a 2,6-Bis(pyrazol-1-yl)pyridine Ligand Functionalized with a Carboxylate Group

The synthesis and magnetostructural characterization of [FeIII3(μ3-O)­(H2O)3[FeII(bppCOOH)­(bppCOO)]6]­(ClO4)13·(CH3)2CO)6·(solvate) (2) are reported. This compound is obtained as a secondary product during synthesis of the mononuclear complex [FeII(bppCOOH)2]­(ClO4)2 (1). The single-crystal X-ray diffraction structure of 2 shows that it contains the nonanuclear cluster of the formula [FeIII3(μ3-O)­(H2O)3[FeII(bppCOOH)­(bppCOO)]6]13+, which is formed by a central FeIII3O core coordinated to six partially deprotonated [FeII(bppCOOH)­(bppCOO)]+ complexes. Raman spectroscopy studies on single crystals of 1 and 2 have been performed to elucidate the spin and oxidation states of iron in 2. These studies and magnetic characterization indicate that most of the iron­(II) complexes of 2 remain in the low-spin (LS) state and present a gradual and incomplete spin crossover above 300 K. On the other hand, the FeIII trimer shows the expected antiferromagnetic behavior. From the structural point of view, 2 represents the first example in which bppCOO– acts as a bridging ligand, thus forming a polynuclear magnetic complex.