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

Four oximato-bridged trinuclear complexes [Ni ^II ₂(Hpyaox) ₆Mn ^IV]Cl·ClO ₄·6H ₂O (1), [Ni ^II ₂(Hpyaox) ₆Mn ^IV](ClO ₄) ₂·2DMF· 4H ₂O (2), [Ni ^II ₂(Hpyaox) ₆Cr ^III]ClO ₄·6H ₂O (3) and [Ni ^II ₂(Hpyaox) ₆Fe ^III]ClO ₄·6H ₂O (4) based on pyridine-2-amidoxime (H ₂pyaox) were prepared in water-DMF solution. X-ray diffraction analysis reveals that they are isomorphous, and consist of linear triply oximato-bridged Ni ^II ₂Mn ^IV (1 and 2), Ni ^II ₂Cr ^III (3) and Ni ^II ₂Fe ^III (4) cations with Mn ^IV, Cr ^III and Fe ^III 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 NiN ₆ 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.