Coming full circle: constructing a [Gd₆] wheel dimer by dimer and the importance of spin topology

The syntheses, structures, magnetic and thermodynamic properties of three related triethanolamine-based Gd^III complexes are described. The smallest, a dimer ([Gd₂]), can be viewed as the subunit from which the two larger complexes, a linear tetramer ([Gd₂]₂) and a cyclic hexamer ([Gd₂]₃), are composed by further deprotonation of the triethanolamine ligand. In all cases, nearest neighbour magnetic ions are weakly correlated by antiferromagnetic isotropic exchange, whose strength does not change significantly from one complex to another; J ranging from -0.10 to -0.13 cm^-1. Therefore, rather than the strength of the coupling, it is the spin topology that is the dominant factor in determining the differences between the physical properties - specifically, the nuclearity and the transition from open (dimer and tetramer) to cyclic (hexamer) boundary conditions. Indeed the hexanuclear wheel reaches the continuum limit of classical Heisenberg spin chains. In terms of the magnetocaloric properties, the smaller the nuclearity, the larger the magnetic entropy and adiabatic temperature changes.