Molecular Iron(III) Phosphonates: Synthesis, Structure, Magnetism, and Mössbauer Studies

The reaction of Fe­(ClO₄)₂·6H₂O with t-BuPO₃H₂ or Cl₃CPO₃H₂ in the presence of an ancillary pyrazole phenolate as a coligand, H₂phpzH [H₂phpzH = 3(5)-(2-hydroxyphenyl)­pyrazole], afforded tetra- and pentanuclear Fe­(III) phosphonate complexes [Fe₄(t-BuPO₃)₄(HphpzH)₄]·5CH₃CN·5CH₂Cl₂ (1) and [HNEt₃]₂[Fe₅(μ₃-O)­(μ-OH)₂ (Cl₃CPO₃)₃­(HphpzH)₅­(μ-phpzH]·3CH₃CN·2H₂O (2). Single-crystal X-ray structural analysis reveals that 1 possesses a cubic double-4-ring (D4R) core similar to what is found in zeolites. The molecular structure of 2 reveals it to be pentanuclear. It crystallizes in the chiral P1 space group. Magnetic studies on 1 and 2 have also been carried out, which reveal that the bridging phosphonate ligands mediate weak antiferromagnetic interactions between the Fe­(III) ions. Magnetization dynamics of 1 and 2 have been corroborated by a Mössbauer spectroscopy analysis.