The single cluster has the formula [Hf6O4(OH)4]L24 and the double cluster [Hf12O8(OH)8(OH)6]L36 (or the equivalent [Hf6O4(OH)4]2(OH)6L36), where, in our BDC-free solutions, each terminating singly-coordinating L group could either be half of one bidentate formate anion (as formate takes up two coordination sites), or the combination of one metal-bound OH and one metal-bound H2O. The cluster-joining reaction could therefore, for example, take the form of 2[Hf6O4(OH)4]L18(OH)3(H2O)3 → [Hf12O8(OH)8(OH)6]L36 + 6H2O, i.e. three metalcoordinated terminating OH groups on each single cluster could form the 'belt' of six μ 2 -OH, releasing six metal-coordinated water molecules. The joining of clusters alone would give an entropy penalty when considering ΔG for the reaction -this release of water molecules could alleviate the entropy penalty or even make the cluster-joining entropically favourable. While this release of water may seem counterintuitive to our previous observation that water in the synthesis is required for the formation of hcp UiO frameworks, we note that this cluster joining reaction requires at least 3 L = (OH) and 3 L = (H2O)} groups on each single cluster (that is, a maximum of 18/24 L sites being coordinated by carboxylate groups); potentially, with less water in the synthesis, insufficient water is present, either in solution or as terminating groups on the clusters, to permit this cluster-joining.