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

The reaction of Fe­(ClO<sub>4</sub>)<sub>2</sub>·6H<sub>2</sub>O with <i>t</i>-BuPO<sub>3</sub>H<sub>2</sub> or Cl<sub>3</sub>CPO<sub>3</sub>H<sub>2</sub> in the presence of an ancillary pyrazole phenolate as a coligand, H<sub>2</sub>phpzH [H<sub>2</sub>phpzH = 3(5)-(2-hydroxyphenyl)­pyrazole], afforded tetra- and pentanuclear Fe­(III) phosphonate complexes [Fe<sub>4</sub>(<i>t</i>-BuPO<sub>3</sub>)<sub>4</sub>(HphpzH)<sub>4</sub>]·5CH<sub>3</sub>CN·5CH<sub>2</sub>Cl<sub>2</sub> (<b>1</b>) and [HNEt<sub>3</sub>]<sub>2</sub>[Fe<sub>5</sub>(μ<sub>3</sub>-O)­(μ-OH)<sub>2</sub> (Cl<sub>3</sub>CPO<sub>3</sub>)<sub>3</sub>­(HphpzH)<sub>5</sub>­(μ-phpzH]·3CH<sub>3</sub>CN·2H<sub>2</sub>O (<b>2</b>). Single-crystal X-ray structural analysis reveals that <b>1</b> possesses a cubic double-4-ring (D4R) core similar to what is found in zeolites. The molecular structure of <b>2</b> reveals it to be pentanuclear. It crystallizes in the chiral <i>P</i>1 space group. Magnetic studies on <b>1</b> and <b>2</b> have also been carried out, which reveal that the bridging phosphonate ligands mediate weak antiferromagnetic interactions between the Fe­(III) ions. Magnetization dynamics of <b>1</b> and <b>2</b> have been corroborated by a Mössbauer spectroscopy analysis.