A Spin Crossover Transition in a Mn(II) Chain Compound

Three new compounds have been synthesized and characterized with Fe(II), Co(II) and Mn(II), the polynitrile anionic ligand 1,1,3,3-tetracyano-2-(3-hydroxypropoxy)-propenide (tcnopr3OH ́) and the co-ligand 4,41-bipyridine (4,41-bpy). The Fe(II) compound, formulated as [FeII(tcnoprOH)2(H2O)2(4,41-bpy)2] (1), contains monomeric complexes where the Fe(II) ion is coordinated to two trans polynitrile ligands, two trans 4,41-bpy ligands and two trans water molecules. Compounds [MII(H2O)4(μ-4,41-bpy)][MII(tcnoprOH)4(μ-4,41-bpy)].3H2O, M = Mn (2) and Co (3), are isostructural and crystallize in segregated cationic and anionic chains that can be formulated as [MII(H2O)4(μ-4,41-bpy)]n2n+ and [MII(tcnoprOH)4(μ-4,41-bpy)]n2n ́, respectively with M = Mn (2) and Co (3). The magnetic properties of Compound 1 show the expected paramagnetic behavior for an isolated high spin S = 2 Fe(II) ion with a zero field splitting of |D| = 4.0(1) cm ́1. Compound 3 presents the expected behavior for isolated Co(II) centers, whereas Compound 2 shows an unexpected partial smooth spin crossover (SCO) transition in the anionic [MnII(tcnoprOH)4(μ-4,41-bpy)]n2n ́ chain together with a paramagnetic contribution of the cationic [MnII(H2O)4(μ-4,41-bpy)]n2n+ chain. This behavior has been confirmed with DSC measurements. This is one of the very few examples of SCO transition observed in a Mn(II) complex and the first one in a Mn(II) chain.