**1. Introduction**

Manganese continues to play a prominent role in the chemistry of 3*d* transition metals, owing to its significance across a breadth of research areas, including bioinorganic [1] and biomedicinal chemistry [2], catalysis [3], nanomaterials [4], spectroscopy [5], and molecular magnetism [6]. In the latter category, the ability of the Mn ion to exist in a variety of stable oxidation states (II-IV) allows for the construction of polymetallic cluster compounds exhibiting a variety of interesting magnetic behaviours, including the stabilization of large spin ground states [7], the slow relaxation of magnetization [8], spin frustration [9], vibrational coherences [10], and enhanced magnetocaloric effects [11]. A key component in understanding the physical properties of all Mn-based molecular magnets is the construction of large families of related compounds so that structure-magnetism relationships can be quantitatively rationalised, and this requires the careful design and exploitation of specific organic bridging ligands.

We have been exploring the coordination chemistry of calix[*n*]arenes (C[*n*]s) with Mn (as well as other metals), as these molecules hold the potential to isolate coordination clusters in the solid state in various different ways, for example, by exploiting the wedge shape of *<sup>p</sup>*-*<sup>t</sup>*Bu-calix[4]arene (H4TBC[4]). As can be seen from the acetonitrile (CH3CN) solvate of H4TBC[4] (Figure 1a) [12], the shape of the building block exerts strong influence over assembly and typically results in antiparallel bilayer formation in the solid state. The hydrophobic cavities are offset in this case and are occupied by acetonitrile of crystallization, though the same phenomenon is also observed for other solvates such as dmf [13]. With respect to cluster formation, *<sup>p</sup>*-*<sup>t</sup>*Bu-calix[4]arene (H4TBC[4]) has proven to be a

particularly versatile platform for the synthesis of a wide range of different topologies, with nuclearities reaching up to fourteen [13–16]. A recurring structural theme we have noticed in this work is that the [MnIIITBC[4]]− moieties act as bridges to metal ions within the cluster through their phenolate groups (Figure 1b), but also as polyhedral capping units (Figure 1c) [17]. The latter suggests that these can be used to encapsulate small metal-oxo-hyroxo fragments growing in 2- or 3-dimensions. Thia, sulfonyl and sulfinyl calix[4]arenes have also been employed in this way, though they give access to markedly different topologies due to the presence of donor atoms at the bridge positions [18–23]. In all cases, new species isolated with methylene- or heteroatom-bridged calix[4]arenes would be of particular interest to magnetochemists researching topological spin frustration [24]. Herein, we discuss the synthesis, structure and magnetic behaviour of a mixed-valent [MnIII8MnII4] species built with TBC[4], the core metallic skeleton of which is related to the hydroxide-based mineral Brucite.

**Figure 1.** (**a**) Section of the extended structure found in the CH3CN solvate of H4TBC[4], showing the antiparallel bi-layer assembly and offset head-to-head arrangemen<sup>t</sup> of the host cavities [12]. (**b**) Partial single crystal X-ray structure of a mixed-valence TBC[4]-supported manganese cluster containing a butterfly-like [MnIII2MnII2(OH)2] core [13]. MnIII ions occupy the tetraphenolato pocket of the TBC[4] and act as capping units (shown as larger spheres in the inset diagram). (**c**) Partial single crystal X-ray structure of a C[4]-supported 3*d*-4*f* cluster topology that can be isolated with a range of lanthanides [14]. Analogous capping behaviour to that found in (**b**) is also observed here and is represented by the larger spheres in the inset diagram. Colour code: MnIII—purple; LnIII—green; O—red; C—grey; N—blue; ligated solvent—orange. *<sup>t</sup>*Bu groups, hydrogen atoms, and solvent of crystallization omitted for clarity.
