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 [Fe-3(III)(mu(3)-O) (H2O)(3)[Fe-II(bppCOOH) (bppCOO)](6)]-(ClO4)(13)center dot(CH3)(2)CO)(6).(solvate) (2) are reported. This compound is obtained as a secondary product during synthesis of the mononuclear complex [Fe-II(bppCOOH)(2)](ClO4)(2) (1). The single-crystal X-ray diffraction structure of 2 shows that it contains the nonanuclear cluster of the formula [Fe-3(III)(mu(3)-O)(H2O)(3)[Fe-II(bppCOOH)(bppCOO)](6)](13+), which is formed by a central (Fe3O)-O-III core coordinated to six partially deprotonated [Fe-II(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 Fe-III 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.