Directed Synthesis of {Mn18Cu6} Heterometallic Complexes**

The development of new synthetic strategies to assemble high-nuclearity transition metal complexes is a key target in modern coordination chemistry. One of the driving forces for this is their fascinating magnetic properties for example, single-molecule magnets or magnetic refrigerants and molecules with large spin ground states or large anisotropy barriers. The use of two, or more, different metal ions to assemble these clusters is an attractive synthetic target and controlling the bottom-up assembly of large heterometallic molecules is a considerable challenge. However, the potential rewards are significant, as there is a real possibility of control/design over the individual magnetic parameters that contribute to the overall molecular properties. Furthermore, new functionality can be added, such as the combination of magnetic and optical properties, or the production of catalysts or catalyst precursors with high activity and/or selectivity. Previously, polydentate ligands with specific binding sites/ donor atoms, linear linkers such as cyanide or rigid structure-directing ligands have been used to prepare heterometallic complexes. Herein, we describe a new stepby-step approach to synthesize large 3d–3d’ heterometallic oxo-bridged clusters. Firstly, we use a preformed Cu complex, which contains multiple, latent hydroxy binding sites, to target the trapping and encapsulation of an inner metal-oxo core. Secondly, the choice of Cu as the central ion increases the flexibility further, due to its range of typical coordination environments from [4] to [4+2]. We report two compounds that contain a striking “core-shell” {Mn18Cu6} complex as either a hexaor dication, where the Cu precursors encapsulate a hexacapped cuboctahedral manganese oxide {Mn12Mn II 6O14} nanocluster. The Cu center is enclosed using the bis-tris propane ligand {2,2’-(propane-1,3-diyldiimino)bis[2-(hydroxymethyl)propane-1,3-diol] (H6L, Scheme 1) forming the precursor