Synthesis, crystal structures, and magnetic properties of cyano-bridged heterobimetallic chains based on [(Tp)Fe(CN)₃]^-

With the use of the tailored cyanometalate precursor, (Bu ₄N)[(Tp)Fe(CN)₃] (Tp = Tris(pyrazolyl)hydroborate) as the building block to react with fully solvated Cu^II, Co^II, and Ni^II cations, four one-dimensional (1D) heterobimetallic cyano-bridged chain complexes of squares, [(Tp)₂Fe^III ₂(CN)₆Cu(CH₃OH)·2CH₃OH] _n (1), [(Tp)₂Fe^III₂(CN) ₆Cu(DMF)· DMF]_n (2), [(Tp)₂Fe ^III₂(CN)₆M(CH₃OH) ₂·2CH₃OH]_n (M = Co (3) and Ni (4)), have been prepared. In complexes 1 and 2, the Cu^II ions are pentacoordinated in the form of a slightly distorted square-based pyramid, and they are linked by distorted octahedrons of [(Tp)Fe(CN)₃]^- to form 1D chains of squares. In complexes 3 and 4, both the central Co ^II and Ni^II ions have a slightly distorted octahedral coordination geometry, and they are bridged by [(Tp)Fe(CN)₃] ^- to form similar 1D chains of squares. There are weak interchain π-π stacking interactions through the pyrazolyl groups of the Tp ligands for complexes 3 and 4. The crystal structures and magnetic studies demonstrate that complexes 1 and 2 exhibit intrachain ferromagnetic coupling and single-chain magnets behavior, and the blocking temperature is ca. 6 K for complex 1 and ca. 3 K for complex 2. Complexes 3 and 4 show significant metamagnetic behavior, where the cyanides mediate the intrachain ferromagnetic coupling between Fe^III and Co^II or Ni^II ions and the interchain π-π stacking interactions lead to antiferromagnetic couplings. The field dependence of the magnetization measurements shows that the critical field is around 1 kOe for complex 3 and 0.8 kOe for complex 4 at 1.8 K.