Iron Lanthanide Phosphonate Clusters: {Fe6Ln6P6} Wells-Dawson-like Structures with D3d Symmetry

Reaction of [Fe-3(mu(3)-O)(O2C'Bu)(6)(HO2C'Bu)(3))](O2C'Bu) and [Ln(2)(O2C'Bu)(6)(HO2C'Bu)(6)] (Ln = lanthanide) with three different phosphonic acids produce a family of highly symmetrical {Fe(6)Ln(6)P(6)} clusters with general formula [Fe(6)Ln(6)(mu(3)-O)(2)(CO3)(O3PR)(6)(O2C'Bu)(18)], where R = methyl 1, phenyl 2, or n-hexyl 3. All the clusters present an analogous metal frame to the previously reported {Ni(6)Ln(6)P(6)} both being related to the well-known Wells-Dawson ion from polyoxometallate chemistry. These highly symmetrical clusters have, or approximate very closely to, D-3d 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(mu(3)-O)(2)(CO3)(O3PPh)(6)(O2C'Bu)(18)] by the finite-temperature Lanczos method gave a strong antiferromagnetic Fe center dot center dot center dot Fe interaction (J(Fe-Fe) = -30 cm(-1)) and very weak Gd center dot center dot center dot Gd and Gd center dot center dot center dot Fe exchange interactions (vertical bar J vertical bar < 0.1 cm(-1)). The strong antiferromagnetic Fe center dot center dot center dot Fe interaction could account for the relatively smaller -Delta S-m value observed, compared against the {Ni6Gd6P6} analogues.