Cyano-Bridged Pentanuclear Fe^III₃M^II₂ (M = Ni, Co, Fe) Clusters: Synthesis, Structures, and Magnetic Properties

The reactions of [MII(TpmMe)(H2O)3]2+ (M = Ni, Co, Fe; TpmMe = tris(3,5-dimethyl-1-pyrazoyl)methane) with [Bu4N][(Tp)FeIII(CN)3] (Bu4N+ = tetrabutylammonium cation; Tp = tris(pyrazolyl)hydroborate) in MeCN−Et2O afford three pentanuclear cyano-bridged clusters, [(Tp)3(TpmMe)2FeIII3MII2(CN)9]ClO4·15H2O (M = Ni, 1; M = Co, 2) and [(Tp)3(TpmMe)2FeIII3FeII2(CN)9]BF4·15H2O (3). Single-crystal X-ray analyses reveal that they show the same trigonal bipyramidal structure featuring a D3h-symmetry core, in which two opposing TpmMe-ligated MII ions situated in the two apical positions are linked through cyanide bridges to an equatorial triangle of three Tp-ligated FeIII (S = 1/2) centers. Magnetic studies for complex 1 show ferromagnetic coupling giving an S = 7/2 ground state and an appreciable magnetic anisotropy with a negative D7/2 value equal to −0.79 cm-1. Complex 2 shows zero-field splitting parameters deducted from the magnetization data with D = −1.33 cm-1 and g = 2.81. Antiferromagnetic interaction was observed in complex 3.