Structural and Electronic Modulation of Magnetic Properties in a Family of Chiral Iron Coordination Polymers

Abstract

The complexes FeL(2) [L = bidentate Schiff base ligands obtained from (R)-(+)-α-phenylethanamine and 4-substituted salicylaldehydes, substituent R = H, (t)Bu, NO(2), OMe, CN, OH] react with ditopic proligands 1,4-pyrazine (pz) or 4,4'-bipyridine (bpy), to give a family of optically pure Fe(II) polymeric chain complexes of formula {FeL(2)(μ-pz)}(∞) and {FeL(2)(μ-bpy)}(∞). Crystallographic studies show that a range of structures are formed including unidirectional and bidirectional linear polymers and canted zigzag chains. Interchain interactions via π-contacts and hydrogen bonding are also observed. SQuID magnetometry studies on all of the complexes reveal antiferromagnetic interactions, the magnitudes of which are rationalized on the basis of substituent electronic properties and bridging ligand identity. For complexes with bridging pz, the antiferromangnetic interaction is enhanced by electron-releasing substituents on the Fe units, and this is accompanied by a contraction in the intrachain distance. For complexes bridged with the longer bpy the intrachain antiferromagnetic couplings are much weaker as a result of the longer intrachain distance. The magnetic data for this series of chain complexes follow a Bonner-Fisher 1D chain model, alongside a zero field splitting (ZFS) model for Fe(II) (S = 2) as appropriate. The intrachain antiferromagnetic coupling J values, g-factors, and the axial ZFS parameter D were obtained.