In order to explore this reaction space more efficiently we initially turned to an automated robotic crystallisation platform which methodically explored the combination of multiple reactants under a range of conditions. Initial results indicated the presence of crystalline species containing up to 36 Fe III ions. Further manual examination revealed that the dissolution of FeBr3 in a 3,4-lutidine (lut)/acetonitrile solution, in the presence of hexamethylenetetramine (HMTA) results in the formation of (lutH)[Fe III 30(μ4-O)6(μ3-O)26(μ-OH)15Br9(lut)15]Br3 (1, Figure ; Table ). A similar reaction replacing the 3,4-lutidine with 4-ethylpyridine (Et-py) affords the isostructural species (Et-pyH)[Fe III 30(μ4-O)6(μ3-O)26(μ-OH)15Br9(Et-py)15]Br3 (1a). See the SI for details. For the sake of brevity, we restrict all further discussion to complex 1. This is surrounded by a partial [Fe supertetrahedron of octahedral Fe ions -in which three of the corners are truncated into three-membered rings, and one corner is truncated into a six membered ring. The [Fe cage is surrounded by a partial [Fe cuboctahedron of tetrahedral Fe ions. Thus, in total, there are twelve tetrahedral Fe III sites and eighteen octahedral Fe III sites. The Fe ions in the inner [Fe III 3O9] 18-triangle are linked to each other and to the metal ions in the [Fe unit through six µ4-and three µ3-O 2-ions. Twenty two µ3-O 2-ions then link the octahedral Fe ions in the [Fe III 18O22] 12+ unit to each other and to the tetrahedral Fe ions in the outer [Fe partial cuboctahedron. The remaining µ3-O 2-ions link three tetrahedral Fe ions in the [Fe partial cuboctahedron. The fifteen hydroxide ions are all located in the [Fe cage and are of two types: six are µ-bridging between Fe ions in the [Fe6] ring and the remaining nine are µ-bridging between Fe ions in the [Fe3] rings. The monodentate 3,4tutidine ligands are all bonded to octahedral Fe ions in the [Fe moiety, while the nine tetrahedral iron ions in the outer partial [Fe A structural comparison to the previously published molecular Fe oxides [Fe17] and [Fe34] (Figure ) and to the [Fe13] Keggin ion, shows some striking similarities. In each case the cluster is built up from alternate layers of tetrahedral and octahedral Fe ions. In [Fe13] and [Fe17] there is a single tetrahedral Fe ion in the centre, surrounded by a truncated tetrahedron of octahedral metal ions. [Fe17] has an additional outer shell of four tetrahedral metal ions arranged in a tetrahedron. [Fe34] contains a central tetrahedron of tetrahedral metal ions, surrounded by a truncated tetrahedron of octahedral metals ions, surrounded by a truncated tetrahedron of tetrahedral metal ions. [Fe30] has a central triangle of tetrahedral metal ions, rather than a tetrahedron, which clearly modifies the subsequent selfassembly process. The resultant changes in structure and symmetry have significant consequences for the magnetic behaviour (vide infra). The structural similarities across the [Fe13], [Fe17], [Fe30] and [Fe34] series, and to Fe mineral phases such as ferrihydrite, do therefore hint that the growth of these complexes occurs layer by layer through addition of polynuclear building blocks.