Structural and magnetic studies on cyano-bridged rectangular Fe ₂M₂ (M = Cu, Ni) clusters

Using the tricyano precursor, (Bu₄N)[(Tp)Fe(CN)₃] (Tp = Tris(pyrazolyl) hydroborate) (1), four new tetranuclear clusters, [(Tp)Fe(CN)₃Cu(Tp)]₂·2H₂O (2), [(Tp)Fe(CN)₃Cu(bpca)]₂·4H₂O (3) (bpca = bis(2-pyridylcarbonyl)amidate anion), [(Tp)Fe(CN)₃Ni(tren)] ₂(ClO₄)₂·2H₂O (4) (tren = tris(2-amino)ethylamine), and [(Tp)Fe(CN)₃Ni(bipy)₂] ₂[(Tp)-Fe(CN)₃]₂·6H₂O (5) (bipy = 2,2′-bipyridine), have been synthesized and structurally characterized. The four clusters possess similar square structures, where Fe^III and M^II (M = Cu^II or Ni^II) ions alternate at the rectangle corners. There exist intermolecular π-π stacking interactions through pyrazolyl groups of Tp^- ligands in complexes 2 and 4, which lead to 1D chain structures. Complex 5 shows a 3D network structure through the coexistence of π-π stacking effects and hydrogen-bonding interactions. Magnetic studies show intramolecular ferromagnetic interactions in all four clusters. The exchange parameters are +11.91 and +1.38 cm^-1 for clusters 2 and 3, respectively, while uniaxial molecular anisotropy can be detected in complex 3 due to the distorted core in its molecular structure. Complex 4 has a ground state of S = 3 and shows SMM behavior with an effective energy barrier of U = 18.9 cm^-1. Unusual spin-glass-like dynamic relaxations are observed for complex 5.