Synthesis and Characterization of High Nuclearity Iron(III) Phosphonate Molecular Clusters

Three new phosphonic acid ligands (4-tbutylphenyl phosphonic acid, 3,5-dimethylphenyl phosphonic acid, and diphenylmethylphosphonic acid) have been synthesized and employed in search of high molecularity iron(III) clusters. The cluster compounds are characterized by single crystal X-ray diffraction and magnetic measurements. The solvothermal reaction of FeCl3·6H2O with diphenylacetic acid and p-tbutylphenyl phosphonic acid resulted in an unprecedented dodecanuclear cluster [Fe12(μ2-O)4(μ3-O)4(O2CCHPh2)14(4-tbuPhPO3H)6](1) having a double butterfly like core structure. [Fe12(μ2-O)4(μ3-O)4(O2CPh)14(C10H17PO3H)6](2), another dodecanuclear cluster having core structure similar to 1, has been synthesized in a reaction between [Fe3O(O2CPh)6(H2O)3]Cl and camphylphosphonic acid in the presence of triethylamine at ambient condition. 3,5-Dimethylphenyl phosphonic acid on reacting solvothermally with an oxo-centered iron triangle [Fe3O(O2CCMe3)6(H2O)3]Cl gives a nonanuclear cluster [Fe9(μ3-O)4(O3PPh(Me)2)3(O2CCMe3)13](3) having icosahedral type core structure where three positions of the iron atoms have been replaced by phosphorus. Another nonanuclear [Fe9(O)3(OH)3(O3PCHPh2)6(O2CCMe3)6(H2O)9] (4), having a distorted cylindrical core structure, has been synthesized in a similar solvothermal reaction between [Fe3O(O2CCMe3)6(H2O)3]Cl and biphenylmethyl phosphonic acid. All compounds are characterized by IR spectra, elemental analysis, as well as single crystal X-ray analysis. Magnetic measurements for all the compounds reveal that there are antiferromagnetic interactions between the metal centers.