Spin State of Iron(II) Clathrochelate in the Cocrystallization Products with 2-Aminopyridine and 2-Hydroxypyridine

Abstract—The cocrystallization of the earlier described carboxy-containing iron(II) clathrochelate (I) with 2-hydroxypyridine and 2-aminopyridine affords two new crystalline products: the salt and cocrystal of this complex, respectively, formed by the acid–pyridine two-point heterosynthone, which is often met in multicomponent crystals of aromatic acids and ortho-substituted pyridines. The neutral nature of the cocrystallization product with 2-hydroxypyridine violates the rule of pK_a, which is fulfilled in the case of 2-aminopyridine, because of the equilibrium shift in the crystalline phase toward the keto form of 2-hydroxypyridine: pyridone-2. According to the X-ray diffraction (XRD) data, complex I in the salt and cocrystal exists in the low-spin state and undergoes no temperature-induced transition in a temperature range of 100–298 K.