High Relaxivity Confined to a Small Molecular Space: A Metallostar-Based, Potential MRI Contrast Agent

Good things come in small packages: For its moderate molecular weight, the metallostar complex [Fe{Gd2L(H2O)4}3]4− (see picture; L is a bipyridine–poly(aminocarboxylate) derivative) displays exceptionally high proton relaxivity, which is explained in terms of a rigid supramolecular structure, two inner-sphere water molecules with a near-optimal exchange rate, and six efficiently relaxing GdIII centers confined to a small molecular space.