Hydrogen bond and π stacking assisted formation of tris(oxalato)ferrate(III) based crystals of heteronuclear compounds

In this work we describe the synthesis, structure and characterization of two new heterometallic Co(III)–Fe(III) compounds derived from tris(oxalato)ferrate(III). This is the first report of isolated [Fe(ox)3]3− moieties involved in non-hybrid materials. They have been prepared by the reaction of K3[Fe(ox)3] (ox = oxalate) with [Co(NH3)5(imH)](ClO4)3 or fac-[Co(hmH)3](ClO4)3 (imH = imidazole and hmH = histamine), and are formulated as: [Co(NH3)5(imH)][Fe(ox)3]·6H2O (1) and fac-[Co(hmH)3][Fe(ox)3]·5.5H2O (2). IR and Vis spectra, thermal analysis and variable temperature magnetic susceptibilities have been investigated, revealing for both structures weak antiferromagnetic couplings transmitted by a network of hydrogen bonds and stacking interactions, with a lack of direct binding between the paramagnetic Fe(III) ions. Effective stacking interactions explain the stronger exchange observed in (2). In both structures, the complex ions are arranged in zigzags. In (2), despite the centrosymmetric space group, a chiral pattern of alternately positioned cations and anions is formed. Thus, adjacent zigzags reveal opposite chirality and form racemic layers with water molecules positioned at the border of the unit cell acting as spacers.

The newly synthesized oxalato-based heteronuclear compounds [Co(NH3)5(imidazole)][Fe(ox)3]·6H2O (1) and fac-[Co(histamine)3][Fe(ox)3]·5.5H2O (2) crystallize due to hydrogen bond and π stacking interactions, that are responsible for antiferromagnetic coupling of the Fe(III) ions. Complex 2 reveals an unusual packing with pseudochiral columns of alternately arranged cations and anions occurring in a centrosymmetrical space group.Figure optionsDownload as PowerPoint slideResearch highlights
► Co(NH3)5(imH)][Fe(ox)3]·6H2O(1), fac-[Co(hmH)3)][Fe(ox)3]·5.5H2O(2) are synthesized.
► The new oxalato-based heteronuclear compounds reveal weak antiferromagnetic coupling.
► Coupling between Fe(III) ions is transmitted by H-bonds and p stacking interactions.
► Complex 2 reveals pseudochiral columns of alternately arranged cations and anions.
► Such packing is quite unusual in oxalate anion chemistry.