title: High-resolution solid-state 13C NMR spectroscopy of the paramagnetic metal-organic frameworks, STAM-1 and HKUST-1.
abstract: Solid-state (13)C magic-angle spinning (MAS) NMR spectroscopy is used to investigate the structure of the Cu(II)-based metal-organic frameworks (MOFs), HKUST-1 and STAM-1, and the structural changes occurring within these MOFs upon activation (dehydration). NMR spectroscopy is an attractive technique for the investigation of these materials, owing to its high sensitivity to local structure, without any requirement for longer-range order. However, interactions between nuclei and unpaired electrons in paramagnetic systems (e.g., Cu(II)-based MOFs) pose a considerable challenge, not only for spectral acquisition, but also in the assignment and interpretation of the spectral resonances. Here, we exploit the rapid T(1) relaxation of these materials to obtain (13)C NMR spectra using a spin-echo pulse sequence at natural abundance levels, and employ frequency-stepped acquisition to ensure uniform excitation of resonances over a wide frequency range. We then utilise selective (13)C isotopic labelling of the organic linker molecules to enable an unambiguous assignment of NMR spectra of both MOFs for the first time. We show that the monomethylated linker can be recovered from STAM-1 intact, demonstrating not only the interesting use of this MOF as a protecting group, but also the ability (for both STAM-1 and HKUST-1) to recover isotopically-enriched linkers, thereby reducing significantly the overall cost of the approach.
authors.affiliation: School of Chemistry and EaStCHEM, University of St Andrews, North Haugh, St Andrews KY16 9ST, UK.
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Dawson
authors.forename: Lauren E
authors.initials: LE
authors.lastname: Jamieson
authors.forename: M Infas H
authors.initials: MI
authors.lastname: Mohideen
authors.forename: Alistair C
authors.initials: AC
authors.lastname: McKinlay
authors.forename: Iain A
authors.initials: IA
authors.lastname: Smellie
authors.forename: Romain
authors.initials: R
authors.lastname: Cadou
authors.forename: Neil S
authors.initials: NS
authors.lastname: Keddie
authors.forename: Russell E
authors.initials: RE
authors.lastname: Morris
authors.forename: Sharon E
authors.initials: SE
authors.lastname: Ashbrook
mesh_terms: D002247:Carbon Isotopes / Q000737:chemistry; D018360:Crystallography, X-Ray; D007553:Isotope Labeling; D009682:Magnetic Resonance Spectroscopy; D008280:Magnetics; D000073396:Metal-Organic Frameworks; D008670:Metals / Q000737:chemistry*; D008968:Molecular Conformation; D009930:Organic Chemicals / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D013696:Temperature
chemical_list: D002247:Carbon Isotopes; D000073396:Metal-Organic Frameworks; D008670:Metals; D009930:Organic Chemicals; D009942:Organometallic Compounds; C581350:STAM-1; C539834:bis(1,3,5-benzenetricarboxylate)tricopper(II)