Polyoxometalates (POMs), anionic oxide clusters of the early transition metals, [1] represent a vast class of inorganic materials with a virtually unmatched range of properties applicable to biology, [2] magnetism, [3] materials science, [4] or catalysis. [5] This unique span of properties qualifies POMbased materials as prime candidates for the designed construction of electronically interesting materials. Polyoxometalates possess enormous diversity in both size and structure [1b, 6] and thereby provide access to a huge library of readily available and controllable fragments, that is, secondary building units (SBUs) [7] that can be interconnected by electrophiles.
The development of novel magnetic polyoxometalates [8] targets either the magnetic functionalization of the metal oxide fragment itself, which is mostly relevant for polyoxovanadates such as {V 15 As 6 }, [9] the interlinking of POM building blocks, as seen for {Mo 72 Fe 30 }, [10] [PMo 12 O 40 -(VO) 2 ] 5À , [11] or the use of lacunary POM fragments as multidentate ligands binding to polynuclear paramagnetic coordination clusters (e.g., {W 18 Cu 6 } [12] and {W 48 Cu 20 } [13] ). In particular, we reasoned that targeting the assembly of a mixed-valence manganese-based cluster [14][15] within a polyoxometalate ligand cage could offer many fantastic new possibilities for design and manipulation. For example, the POM "ligands" could be useful to "dilute" single-molecule magnets (SMMs) to remove unwanted dipolar interactions and also because of the intrinsic redox activity of the POM "ligands" that could allow additional routes to control [