Hydrothermal synthesis, structural characterization and magnetic studies of the new pillared microporous ammonium Fe(III) carboxyethylphosphonate: [NH4][Fe2(OH){O3P(CH2)2CO2}2]

The preparation by hydrothermal reaction and the crystal structure of the iron(III) carboxyethylphosphonate of formula [NH4][Fe2(OH){O3P(CH2)2CO2}2] is reported. The green-yellow compound crystallizes in the monoclinic system, space group Pc(n.7), with the following unit-cell parameters: a=7.193(3)A, b=9.776(3)A, c=10.17(4)A and β=94.3(2) deg. It shows a typical layered hybrid organic-inorganic structure featuring an alternation of organic and inorganic layers along the a-axis of the unit cell. The bifunctional ligand [O3P(CH2)2CO2]3- is deprotonated and acts as a linker between adjacent inorganic layers, to form pillars along the a-axis. The inorganic layers are made up of dinuclear Fe(III) units, formed by coordination of the metal ions with the oxygen atoms originating from the [O3P-]2- end of the carboxyethylphosphonate molecules, the oxygen atoms of the [-CO2]- end group of a ligand belonging to the adjacent layer and the oxygen atom of the bridged OH group. Each Fe(III) ion is six-coordinated in a very distorted octahedral environment. Within the dimer the Fe-Fe separation is found to be 3.5A, and the angle inside the [Fe(1)-O(11)-Fe(2)] dimers is ∼124 deg. The resulting 3D framework contains micropores delimited by four adjacent dimers in the (bc) planes of the unit cell. These holes develop along the a-direction as tunnel-like pores and [NH4]+ cations are located there. The presence of the μ-hydroxo-bridged [Fe(1)-O(11)-Fe(2)] dimers in the lattice is also responsible for the magnetic behavior of the compound at low temperatures. The compound contains Fe3+ ions in the high-spin state and the two Fe(III) ions are antiferromagnetic coupled. The J/k value of -16.3K is similar to those found for other μ-hydroxo-bridged Fe(III) dimeric systems having the same geometry