A Stable Molecular Entity Derived from Rare Iron(II) Minerals: The Square-Planar High-Spin-d~6 Fe~(II)O4 Chromophore

摘要

Even under such disparate conditions as in a metalloenzyme active site or in the solid framework of a magnetic material, there are only a few parameters that determine the properties of a given metal center: the type of ligands and the metal oxidation state, coordination number, and spin state. For low-coordinate metals, the latter two parameters are highly interdependent and, in contrast to octahedral centers that exhibit both high- and low-spin states, tetracoordinate metals are almost invariably either high-spin tetrahedral or lower-spin non-tetrahedral. Exceptions to the rule are found in the rigid frameworks of solids that seem to force the metal atom into an environment considered unstable in a molecular analogue. Thus the rare mineral gillespite and the oxide ceramic SrFeO2 contain square-planar high-spin Fe~(II)O4 centers, which to date had never been found in a molecular entity. We have now synthesized the first coordination compounds of this type by employing a bis(bidentate) diolato environment. The origin of the planarization of the complexes is an unusually strong Jahn-Teller-initiated destabilization of the expected tetrahedral structure, which results in a high structural flexibility of the chromophore that is strongly correlated with its color.