The single crystal structure of 1-Br2 reveals that brominated MOF retains the same hexagonal space group P63/m as the parent MOF, 1as. The chemical transformation to 1-Br2 results in a small contraction along the c axis, from 13.958(4) Å to 13.538(1) Å, and a subsequent minor increase in unit cell volume from 2073.8(11) Å 3 to 2088.4(5) Å 3 . The bromine units are clearly visible, with two equally occupied linker conformations evident in the disorder model (Figure ). PXRD patterns of bulk samples prepared by vapour phase bromination also confirm that there is a clear transformation from 1np to 1-Br2. The PXRD pattern for 1-Br2 calculated from the crystal structure exhibits a close match to the experimental diffractogram (Figure ), and the structural transition is confirmed by Pawley refinement (SI, Figure ), indicating the bulk sample has been successfully brominated with high phase purity. The chemical transformation can also be monitored by Raman spectroscopy, where the alkene (C=C) band at 1670 cm -1 disappears upon full bromination of the structure, alongside appearance of a strong band at 641 cm -1 which is indicative of the C-Br stretch (Figure ). Quantitative conversion to 1-Br2 is also evident through 1 H and C NMR spectroscopy of acid-digested (D2SO4/DMSO-d6) samples. The disappearance of the resonance at δ = 6.6 ppm, assigned to the alkene protons of fumarate, and the emergence of a singlet at δ = 4.5 ppm in the 1 H NMR spectrum of 1-Br2 indicates that 1np has been fully and stereoselectively brominated to the expected meso product. Similarly, in the Even following activation for 20 hours at 150 °C under vacuum, the framework shows no crystallographic changes according to PXRD (Figure ). It is likely that the bulky bromine groups are sterically sufficient to prevent the framework from closing under ambient conditions. Despite the rigidity of 1-Br2, it would appear that the bromine units sterically block the pores, and so N2 uptake at Although both 1 and 2 are structurally independent, when 2 is subjected to the same bromination conditions it yields a material identical to 1-Br2 (SI, Section S8). For ease of analysis and interpretation, this product will be herein called 2-Br2, and displays an almost identical powder X-ray diffractogram to 1-Br2 (Figure ). The presence of the alkene bond in the fumaric acid linker remains the key target for reaction of the bromine vapours, however, the formate bridges are also unexpectedly lost. Both the chemisorption of Br2 across both the double bond of fumarate, and the loss of the formate bridges are evident in the 1 H NMR spectrum of a digested sample, whereby the resonances at δ = 6.6 and δ = 8.1 ppm, assigned to the fumarate alkene protons and the formate protons, respectively, disappear following exposure to bromine, leaving a singlet at δ = 4.5 ppm assigned to the now sp 3 hybridised linker C-H group (see SI, Figures and). Bromination is also confirmed using Raman spectroscopy (Figure ). The theoretical bromine content for this composition (43.0% wt) correlated well with the experimentally measured value (43.5% wt), which is indicative of bromine only reacting across the double bonds (expected: 43.0%), rather than bromide also replacing the formate as a coordinating counterion on the SBU (expected: 47.4%). The halogenation step has also impacted the gas sorption properties; 2 exhibits a BET surface area for N2 at 77 K of 959 m 2 g -1 but, like 1-Br2, 2-Br2 again shows negligible N2 uptake. The bromination of 1 and 2 therefore represents an unusual example whereby postsynthetic modification of two different MOFs results in the same final material.