Structural Characterization of Two Crystalline Forms of Bis[2-(1,5-dimethyltrazol-3-yl)-1,10-phenanthroline]-iron(II) Perchlorate—a Spin Crossover System

Abstract

At room temperature bis [2-(1,5-dimethyltriazol-3-yl)-1,10-phenanthroline]iron(II) perchlorate has magnetic and Mossbauer spectral properties which indicate that it contains both singlet and quintet state iron(II) in a ratio of approximately 1:2. Two polymorphs were identified in a small batch of crystals grown from acetone/ethanol for crystal structure determination. In polymorph 1 the average iron-nitrogen distance (1.97 Ǻ) is close to the value usually obtained for purely singlet state iron(II) in an [FeN₆]²⁺ system, whereas that for polymorph 2 (2.09 Ǻ) is consistent with the 1:2 ratio of singlet-to-quintet state species. The Mossbauer spectrum of the crystallographic sample is virtually identical to that of the bulk sample. There are small differences in the coordination sphere which could account for the different extent of stabilization of the singlet state in the two polymorphs. The crystal packing differs markedly for the two polymorphs but reveals no obvious rationalization for the spin-state differences. Bis [2-(1,5-dimethyltriazol-3-yl)-1,10-phenanthroline]iron(II) perchlorate , two-thirds water (polymorph 1): monoclinic, space group P 2₁/c, a 10.766(3), b 25.806(3), c 12.491(4) Ǻ, β 91.17(1)°, Z 4. Bis [2-(1,5-dimethyltriazol-3-yl)-1,10-phenanthroline]iron(II) perchlorate monohydrate (polymorph 2): orthorhombic, space group Pbca, a 16.687(4), b 12.272(2), c 34.305(8) Ǻ, Z 8.