In contrast with the 6-connected SBU found in the three-dimensional MOF-808, where the carboxylate ligands are found in triads at opposite faces of the SBU, the carboxylate ligands in GUF-14 are found in the inverse arrangement, in sites around the equator of the cluster, leading to the hexagonal topology. An analogous SBU has previously been observed in the two-dimensional [Hf6O4(OH)4(HCO2)6(BTB)2] material (BTB = benzene-1,3,5-tribenzoate), with Zr homologues reported (Figure ). In these examples, six formate ligands complete the coordination at the SBU -formate is well known to stabilise lower connectivity Zr6 SBUs through incorporation as a capping ligand -but in this work only a small quantity of formate in GUF-14 could be visualised by cRED. The occupancy of the formate carbon position in the asymmetric unit was freely refined to 0.28 (16); as the oxygens around the cluster are fully occupied, we can assume a disorder model encompassing the formate and a pair of hydroxide and water ligands, giving an overall formula for the cRED structure of GUF-14 of [Zr6(µ3-O)4(µ3-OH)4(OH)4.3(H2O)4.3(HCO2)1.7(EDB)3]. A structurally related material -CAU-26 -has been reported as a kinetic product in the reaction of zirconium acetate with benzene-1,4-dicarboxylic acid in neat acetic acid. The material, formulated as [Zr6O4(OH)4(OAc)6(BDC)3], exhibited low crystallinity and high defectivity, but electron diffraction data suggested a layered structure with hxl topology with eclipsed AA stacking and a larger interlayer distance. Similarly, the reaction of ZrOCl2 with 3,3′′,5,5′′-tetramercapto[1,1′:4′,1′′-terphenyl]-4,4′′-dicarboxylic acid in DMF and formic acid led to a material, Zr-TPDCS-2, which was inferred to have an isoreticular layered structure by PXRD analysis. In both cases, a structure was not refined from diffraction data. Synthetic optimisation (SI, Section S4) revealed that, for bulk syntheses, 125 equiv. of formic acid and 0.5% (v/v) water in DMF reliably yields as-synthesised samples of GUF-14, with powder X-ray diffractograms closely resembling those predicted from the structure elucidated by electron diffraction. Pawley refinement (Figure ) of the experimental powder diffraction data in the space group determined via cRED provided a good fit, with refined lattice parameters a = 21.466(4) Å and c = 16.666(4) Å consistent with those determined from cRED but with a longer c axis. We hypothesise that interlayer contraction along the c axis occurs as solvent is lost in the vacuum chamber of TEM, leading to this difference with the as-synthesised samples of GUF-14. An additional feature, consisting of a peak with an extremely asymmetric tail towards high angle, was present at low angle (2θ ~ 4.5º) but is not accounted for by the R3 ! m space group (marked with an asterisk in Figure ). The peak position corresponds to the (001) Bragg position, which would be systematically forbidden in R3 ! m, and the Warren like peak shape suggests that it arises from the presence of stacking faults between layers. Acid digestion of a sample of GUF-14 that had been washed with acetone and dried under turbopump vacuum at 393 K for 20 h allowed 1 H NMR spectroscopic analysis to determine the level of formate inclusion in the bulk structure. A fully formate capped Zr6 SBU would yield a MOF with ideal formula [Zr6(µ3-O)4(µ3-OH)4(HCO2)6(EDB)3] and an EDB 2-:HCOO -ratio of 1:2, but 1 H NMR spectra of acid-digested samples gave an EDB 2-:HCOO -ratio of 1:1.1, higher than that observed by cRED (1:0.3) but still short of a fully formate capped structure. As such, the bulk material likely corresponds to a partially formate-capped material with -assuming that water and hydroxide (in the absence of any other NMR spectroscopically visible organic capping ligands) comprise the residual capping species -approximate formula [Zr6(µ3-O)4(µ3-OH)4(HCO2)3.3(OH)2.7(OH2)2.7(EDB)3], but with particle-to-particle variations.