The capping formates of the Zr6 cluster and the alkyne spacer of the adjacent linker are separated by only 3.8 Å in the cRED structure. This short distance likely explains why formic acid modulation yields GUF-14 with 6-connected Zr6 SBUs and hxl topology, whilst identical syntheses using acetic acid as modulator yield GUF-12, with 18-connected Zr12 SBUs and hcp topology; modulator capping is required to yield the 6-connected SBU, but the close-packing arrangement precludes the bulkier acetic acid capping the SBU. Indeed, syntheses employing larger modulating agents such as trifluoroacetic acid, benzoic acid, or 3-fluorobenzoic acid (which has a pKa value close to formic acid) also did not yield GUF-14 (SI, Section S5). Similarly, attempts to prepare isoreticular versions of GUF-14 using biphenyl-4,4′-dicarboxylic acid linkers under the optimised conditions resulted in the formation of UiO-67 phases with fcu topology, highlighting the need for the alkyne spacer to facilitate this particular structure. Synthetic attempts using the shorter acetylenedicarboxylic acid linker were not successful, likely due to the thermal instability of the ligand. These results further imply that the minimal steric profiles of the alkyne spacer and the small formate capping units are essential to provide enough room for the formate caps of Zr6 clusters to sit between the layers and allow formation of the hxl phase. The low steric profile of the alkyne moiety has previously allowed isolation of other unexpected MOF phases linked by different metals. After removal of residual solvents, by heating, drying under vacuum, or a combination of both, changes in the powder X-ray diffractograms of GUF-14 were evident. For example, the diffractogram of a sample of GUF-14 that had been heated at 423 K for 16 h in a programmable oven shows shifting and broadening of the Bragg peak originally located around 7° 2θ -the (101) reflection -in the as-synthesised material.