Stereoselective Halogenation of Integral Unsaturated C‐C Bonds in Chemically and Mechanically Robust Zr and Hf MOFs

Abstract

Metal–organic frameworks (MOFs) containing ZrIV‐based secondary building units (SBUs), as in the UiO‐66 series, are receiving widespread research interest due to their enhanced chemical and mechanical stabilities. We report the synthesis and extensive characterisation, as both bulk microcrystalline and single crystal forms, of extended UiO‐66 (Zr and Hf) series MOFs containing integral unsaturated alkene, alkyne and butadiyne units, which serve as reactive sites for postsynthetic modification (PSM) by halogenation. The water stability of a Zr–stilbene MOF allows the dual insertion of both −OH and −Br groups in a single, aqueous bromohydrination step. Quantitative bromination of alkyne‐ and butadiyne‐containing MOFs is demonstrated to be stereoselective, as a consequence of the linker geometry when bound in the MOFs, while the inherent change in hybridisation and geometry of integral linker atoms is facilitated by the high mechanical stabilities of the MOFs, allowing bromination to be characterised in a single‐crystal to single‐crystal (SCSC) manner. The facile addition of bromine across the unsaturated C−C bonds in the MOFs in solution is extended to irreversible iodine sequestration in the vapour phase. A large‐pore interpenetrated Zr MOF demonstrates an I2 storage capacity of 279 % w/w, through a combination of chemisorption and physisorption, which is comparable to the highest reported capacities of benchmark iodine storage materials for radioactive I2 sequestration. We expect this facile PSM process to not only allow trapping of toxic vapours, but also modulate the mechanical properties of the MOFs.

Postsynthetic Modification of Zirconium Metal‐Organic Frameworks

“…Forgan et al have demonstrated that Zr MOFs possess the requisite mechanical stability[10d] to allow halogenation of integral unsaturated bonds of a series of materials, where the change in hybridisation of linker carbon atoms results in an overall mechanical contraction . UiO‐66 topology MOFs containing integral alkene, alkyne and butadiyne units (Figure a) were quantitatively brominated by immersing them in chloroform solutions containing Br 2 , with no decrease in porosity or crystallinity . As the linkers have restricted dynamic motion when bound within the MOFs, the transformations are stereoselective, yielding exclusively trans ‐dihaloalkene and meso ‐dihaloalkane units (Figure b–d).…”

“…Halogenation was extended to chemisorption of iodine from the vapour phase by addition across multiple bonds, which could have great significance in the sequestration of radioactive iodine from the nuclear industry. The interpenetrated Zr MOF [Zr 6 O 4 (OH) 4 (peb) 6 ] n , where peb is the bis‐alkyne ligand 4,4′‐[1,4‐phenylenebis(ethyne‐2,1‐diyl)]‐dibenzoate, can adsorb 279 % w/w of iodine by a combination of chemisorption across the alkyne units and physisorption in the large pores (Figure g) . The change in hybridisation of the carbon atoms in the linkers on postsynthetic halogenation also results in a change in mechanical compliance, which can be measured by single crystal nanoindentation.…”

“…Single‐crystal to single‐crystal transformation of e) [Zr 6 O 4 (OH) 4 (edb) 6 ] n to f) [Zr 6 O 4 (OH) 4 (edb‐Br 2 ) 6 ] n , redrawn from CCDC depositions 1062508 and 1062510. g) Iodine uptake of the MOFs described in part a). Reproduced with permission from ref . Copyright (2016) Wiley‐VCH.…”

Reversible Capture and Release of Cl2 and Br2 with a Redox-Active Metal–Organic Framework

“…The closest example is the irreversible bromination of an unsaturated organic ligand within a MOF and subsequent release of HBr upon treatment with base. 11 The lack of reports on reversible storage of the gaseous halogens (i.e., excluding I 2 ) in MOFs is staggering given the interest in the field, but it highlights the difficulty in isolating materials that can reversibly bind halogens and release them over at least several cycles.…”

Correlating Pressure‐Induced Emission Modulation with Linker Rotation in a Photoluminescent MOF

“…We have previously reported the synthesis and crystal structure of Hf-peb, [18] which crystallises in the cubic crystal system in space group Fd " 3m, and consists of Hf 6 O 4 (OH) 4 SBUs, as seen in the zirconium MOFs UiO-67(Zr) and UiOabdc(Zr). The longer linker gives rise to two-fold interpenetration (Figure 1 a); MOFs of the isoreticular Zr series were originally described as PIZOFs (porous interpenetrated zirconium organic frameworks).…”