Characterization of Porosity in Organic and Metal−Organic Macrocycles by Hyperpolarized ¹²⁹Xe NMR Spectroscopy

Abstract

Hyperpolarized (129)Xe NMR spectroscopy is used to establish the solid-state porosity of shape-persistent macrocycles with either an organic or metal-organic framework. These studies show that even upon removal of cocrystallized solvent molecules, the macrocycles maintain a porous or channeled structure. The technique can provide valuable information about systems for which X-ray crystallographic analysis is not feasible. [structure: see text]

Nanoporous molecular crystals

“…69 Another method of demonstrating nanoporosity via gas adsorption is hyperpolarised 129 Xe NMR, which can give valuable information on the binding sites within NMCs. [70][71][72][73][74][75][76][77] However, the mechanism of adsorption may vary for different adsorbates, for example, N 2 and Xe are relatively large probe molecules that are excluded from a number of porous zeolites and MOFs, 78,79 therefore, smaller gas probes (e.g. H 2 at 77 K or CO 2 at 273 K) may be more appropriate for the demonstration of rapid barrier-free adsorption within nanoporous crystals with very narrow access channels (<3 A).…”

“…These include the family of torus-shaped cucurbit[n]urils (n ¼ 5, 6 and 8), 130,131 for which gas adsorption studies of the crystal of cucurbit [6]uril 11 confirm nanoporosity, 98 a bis-urea macrocycle 12 shown to adsorb CO 2 , 99,100 macrocycles aligned via calcogen-calcogen interactions, 132 a macrocyclic dimer of a tetra-aryl-1,3-dioxolane-4,5-dimethanol (TADDOL) chiral ligand that in its unsolvated form can adsorb ether, 133 azacalixarenes that adsorb CO 2 selectively from air, 134,135 and a number of metal-organic macrocycles formed through coordination chemistry, some of which demonstrate gas adsorption subsequent to removal of included solvent. 76,[136][137][138][139][140][141][142][143] A very recent paper describes a remarkable gallium-based 'molecular wheel' which has been shown to possess nanoporosity by hyperpolarized 129 Xe NMR. 75…”

129Xenon NMR: Review of recent insights into porous materials

“…Porosity in solids composed of organic and metal-organic macrocycles has been investigated using HP Xenon NMR [194][195][196]. It was shown that the channel structure remains intact even upon removal of cocrystallized guest molecules.…”

NMR Hyperpolarization Techniques of Gases

“…[60] Since that work, HP xenon has been used to study diffusion in confined spaces or porous media [61] , [62] , [63] ; image such systems as a function of gas flow [64] or 129 Xe chemical shift [65] ; or spectroscopically probe single-crystal surfaces [66] , liquid crystals, [67] or combustion processes. [68] However, the greatest body of materials-related work has concerned the effort to probe void spaces and surfaces in microporous or nanoporous materials with HP 129 Xe, thereby providing information about pore size, pore shape, and gas dynamics in: nanochanneled organic, organometallic, and peptide-based molecular materials [69] (including in macroscopically oriented single crystals [70] ); multi-walled carbon nanotubes [71] ; gas hydrate clathrates [72] ; porous polymeric materials and aerogels [73] ; metalorganic frameworks [74] ; calixarene-based materials and nanoparticles [75] ; organo-clays [76] ; mesoporous silicas [77] ; and zeolites and related materials [78] -efforts that have been aided by computational studies of xenon in confined spaces (e.g., Refs. [79] ).…”