Synthesis, structure and magnetic properties of trinuclear transition metal complexes based on pyridine-2-amidoxime

Four oximato-bridged trinuclear complexes [NiII2(Hpyaox)6MnIV]Cl·ClO4·6H2O (1), [NiII2(Hpyaox)6MnIV](ClO4)2·2DMF·4H2O (2), [NiII2(Hpyaox)6CrIII]ClO4·6H2O (3) and [NiII2(Hpyaox)6FeIII]ClO4·6H2O (4) based on pyridine-2-amidoxime (H2pyaox) were prepared in water–DMF solution. X-ray diffraction analysis reveals that they are isomorphous, and consist of linear triply oximato-bridged NiII2MnIV (1 and 2), NiII2CrIII (3) and NiII2FeIII (4) cations with MnIV, CrIII and FeIII ions at the centre. The Ni(II) ion is coordinated by three mono-deprotonated Hpyaox− anions, giving a [Ni(Hpyaox)3]− anion. Two [Ni(Hpyaox)3]− anions coordinate to the central M(III/IV) ion with six oximato oxygen atoms, yielding the trinuclear unit. The magnetic susceptibility measurements show that complexes 1–4 exhibit antiferromagnetic interaction between adjacent metal ions through the triple oximato bridges. By comparison, the unusual antiferromagnetic property of complexes 1–3 has been found to be related to the distortion of the NiN6 coordination configuration from octahedral to trigonal. The magnetic data were fitted by considering the magnetic coupling between adjacent ions, two terminal ions and nearest molecules, respectively.