Structural and magnetic studies on cyano-bridged rectangular Fe2M2 (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)(3)Cu(Tp)](2), 2H(2)O (2), [(Tp)Fe(CN)(3)Cu(bpca)](2).4H(2)O (3) (bpca) bis(2-pyridylcarbonyl) amidate anion), [(Tp)Fe(CN)(3)Ni(tren)](2)(ClO4)(2).2H(2)O (4) (tren) tris(2-amino) ethylamine), and [(Tp)Fe(CN)(3)Ni(bipy)(2)](2)[(Tp)Fe( CN)(3)](2).6H(2)O (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 pi-pi 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 pi-pi 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.