**3. Conclusions**

The reaction between MnCl2·6H2O, H4TBC[4], H2bd, [NH4][ClO4] and NEt3 in CH3CN / dmf affords the dodecametallic, mixed-valent cluster [MnIII8MnII4(<sup>μ</sup>4-O)2(μ3-OH)6(μ-OH)4(μ-Cl)2(TBC[4])4(dmf)8(H2O)4(CH3CN)2]. The structure describes a central, near-planer Mn-oxo-hydroxo moiety, reminiscent of the Brucite lattice, encapsulated by [MnIII-TBC[4]]− moieties. Magnetic susceptibility and magnetization measurements reveal dominant and weak antiferromagnetic exchange interactions between the metal centres.

Somewhat surprisingly, 1,4-butanediol is a crucial component required for the formation of **1** despite the fact that it is not incorporated in the resulting cluster. Absence of this key reactant affords the previously reported mixed-valence TBC[4]-supported Mn-butterfly cluster, suggesting that this

chelate plays a crucial role in (possibly hindering) cluster formation and producing a significantly higher nuclearity species in the process; the TBC[4]-supported Mn-butterfly cluster (Figure 1b) can be formed rapidly in approximately 15 min, and quantitatively precipitates from solution within an hour. The development reported here suggests that the use of other potential coligands with similar chemical make-up may offer the possibility to access/isolate other large, discrete high-nuclearity species. Results from this study will be reported in due course and will ideally shed additional light on the important role of such non-innocent reactants.

**Author Contributions:** Conceptualization, S.J.D. and E.K.B.; methodology, M.A.P.; formal analysis, M.A.P., M.C.; writing, M.C., E.K.B. and S.J.D.; visualization, M.C., S.J.D.; funding acquisition, S.J.D., E.K.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the EPSRC through gran<sup>t</sup> reference numbers EP/I03255X/1 and EP/I031421/1. **Conflicts of Interest:** The authors declare no conflict of interest.
