Reaction of CuCl2•2H2O with H4TC [4]A in a basic dmf/MeOH solution (see ESI for full details) leads to the formation of brown single crystals after 3 days, upon slow evaporation of the mother liquor. Crystals of [Cu24(µ4-TC[4]A)6(µ4-Cl)6(µ6-CO3)6(µ-OH)6(dmf)6]•22dmf (1•22dmf) were found to be in a triclinic cell and structure solution was performed in the P-1 space group (see SI; Fig. shows the PXRD). The asymmetric unit (ASU) contains two distinct half clusters (i.e. the unit cell contains two [Cu24] clusters) and symmetry expansion affords the cage shown in Fig. . The metallic skeleton (Fig. ) of 1 describes a [Cu II 24] truncated octahedron. The fully deprotonated TC [4]A ligands sit atop the six square faces, bonding to the four Cu ions through four µ-O atoms and four terminal S atoms, as expected. Below the square lies a µ4-Cl ion, creating the [Cu4(TC[4]A)Cl] building block. Six of these building blocks are self-assembled into the [Cu24] unit by a disordered combination of six µ6-CO3 2-ions and six µ-OH ions (see the CIF file/SI for full details), which lie in the hexagonal faces of the truncated octahedron. The carbonate anions originate from CO2 fixation. Perhaps counter-intuitively, the deliberate addition of CO3 2-ions to the reaction mixture, in the form of Na2CO3/NaHCO3, prevents crystal formation. We note that TC [4]A-stabilised truncated octahedra are known for Co II , Ni II and Mn II , 16 but 1 represents the first example obtained with Cu II . Interestingly a [Cu I 24] cage was recently synthesised using tetramercaptotetrathiacalix [4]arene, which suggests that control over metal oxidation state can be achieved through choice/variation of lower rim substituent, or through the use of different calix[n]arenes within the same reaction, routes not yet exploited. Examination of the extended structure of 2 reveals that symmetry equivalent clusters pack to form a bi-layer type structure (Fig. ). This type of self-assembled bi-layer structure is commonly observed in a variety of calixarene solvates and coordination compounds in which the ligands are not forced to pack in a parallel manner, e.g. as in the structure of 1. The result of this is that the closest crystallographically unique Cu•••Cu distance between neighbouring clusters is found to be significantly shorter than in 1, in this case at ~8.4 Å as shown in Fig. .