Iron Lanthanide Phosphonate Clusters: {Fe₆Ln₆P₆} WellsDawson-like Structures with <i>D</i>_3<i>d</i> Symmetry

Abstract

Reaction of [Fe₃(μ₃-O)­(O₂C^<i>t</i>Bu)₆(HO₂C^<i>t</i>Bu)₃]­(O₂C^<i>t</i>Bu) and [Ln₂(O₂C^<i>t</i>Bu)₆(HO₂C^<i>t</i>Bu)₆] (Ln = lanthanide) with three different phosphonic acids produce a family of highly symmetrical {Fe₆Ln₆P₆} clusters with general formula [Fe₆Ln₆(μ₃-O)₂(CO₃)­(O₃PR)₆(O₂C^<i>t</i>Bu)₁₈], where R = methyl <b>1</b>, phenyl <b>2</b>, or <i>n</i>-hexyl <b>3</b>. All the clusters present an analogous metal frame to the previously reported {Ni₆Ln₆P₆} both being related to the well-known Wells–Dawson ion from polyoxometallate chemistry. These highly symmetrical clusters have, or approximate very closely to, <i>D</i>_3<i>d</i> point symmetry. Both Fe^III and Gd^III ions are magnetically isotropic and could thus exhibit promising magnetocaloric properties; hence we investigated the {Fe₆Gd₆P₆} compounds accordingly. Modeling the magnetic data of [Fe₆Gd₆(μ₃-O)₂(CO₃)­(O₃PPh)₆(O₂C^<i>t</i>Bu)₁₈] by the finite-temperature Lanczos method gave a strong antiferromagnetic Fe···Fe interaction (<i>J</i>_Fe–Fe = −30 cm^–1) and very weak Gd···Gd and Gd···Fe exchange interactions (|<i>J</i>| < 0.1 cm^–1). The strong antiferromagnetic Fe···Fe interaction could account for the relatively smaller −Δ<i>S</i>_m value observed, compared against the {Ni₆Gd₆P₆} analogues