Structural regulation of three Fe–Co cyanometallate complexes: reactant ratio issue†

Structural regulation of crystal structures to achieve specific structural architectures and/or desired functionalities is one of the main focuses in the field of molecular magnetism. The reaction of a trigonal tetradentate ligand, [TpFe^III(CN)₃]⁻ bridging unit and cobalt metal centres by the altering change of chemical stoichiometry afforded three different structures [{TpFe(CN)₃}₂Co(PyPz₃)]·4MeOH·5H₂O (1), [{TpFe(CN)₃}Co(PyPz₃)]₂(ClO₄)₂·2MeCN·2H₂O (2) and {(Tp)Fe(CN)₃Co(PyPz₃)}_n(BF₄)_n·2nMeOH (3) (PyPz₃ = 2-(di(1H-pyrazol-1-yl)methyl)-6-(1H-pyrazol-1-yl)pyridine and [TpFe^III(CN)₃]⁻ = tri(pyrazolyl)boratetricyanoiron(III)). Detailed structural studies reveal a gradual transformation among the complexes, from a trinuclear structure in 1, to a tetranuclear geometry in 2, and finally to a one-dimensional chain in 3. Magnetic susceptibility measurements revealed that all three complexes exhibit ferromagnetic interactions between the cyanide bridged Fe^III and Co^II centres, with exchange coupling constants of +7.89, +4.37, and +5.92 cm⁻¹, respectively. Our result demonstrates an effective strategy for targeted assembly of architectures by introducing coordinatively unsaturated ligands, complemented by the cyanide-bridged linkers [Tp^RFe^III(CN)₃]⁻.