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.

Identification of Nonequivalent Framework Oxygen Species in Metal–Organic Frameworks by 17O Solid-State NMR

“…22 Its structure is built upon from the polyhedra containing eight-coordinated zirconium atoms, which are connected by 1,4-BDC (1,4-benzenedicarboxylate) linkers (Figure 1). The basic building unit, Zr 6 O 4 (OH) 4 (1,4-BDC) 6 , consists of an octahedral core formed by six Zr atoms. There are three chemically nonequivalent oxygen species associated with each Zr 6 O 4 (OH) 4 (1,4-BDC) 6 unit ( Figure 1): (1) the oxygen atoms from carboxylate group; (2) four μ 3 -O 2− anions capped alternatively on the four of the eight triangular faces of the octahedron; (3) four μ 3 -OH groups capped on the remaining triangular faces.…”

“…39 Since many MOFs are synthesized using nonaqueous solvents in the presence of a very small amount of water, such situation can also be exploited for making 17 O-enriched MOFs effectively. Indeed, we prepared MOF Zr-UiO-66, or Zr 6 O 4 (OH) 4 (BDC) 6 , by directly reacting solid ZrCl 4 and terephthalic acid in 25 mL of DMF with only 0.25 mL of 17 O-enriched water. 17 O-enriched microporous α-magnesium formate, α-Mg 3 (HCOO) 6 , was also prepared using the same approach.…”

Calculation and experimental measurement of paramagnetic NMR parameters of phenolic oximate Cu(ii) complexes

“…In recent years, paramagnetic NMR (or "pNMR") has developed greatly, with systems from metalloproteins 1 (dilute, isolated spins) to metal-organic frameworks [2][3][4] (denser networks of potentially coupled spins) and metal oxides 5 (very dense networks of highly coupled spins) being studied. For dilute spins, the NMR conditions can be selected such that the signal from nuclei near the paramagnetic centre is "invisible" owing to rapid relaxation and only the longer-range throughspace pseudocontact shifts are observed.…”

“…This assignment was confirmed using the relatively costly and timeconsuming approach of specific 13 C labelling in conjunction with 1 H-13 C cross polarisation (CP) NMR, which is generally inefficient for paramagnetic materials. 2 It would, therefore, be desirable to have a more general assignment method that does not rely on the development of bespoke synthetic pathways for efficient isotopic enrichment. Owing to the rapid MAS rates (necessitating the use of small rotors with, consequently, small sample volumes) required for highresolution pNMR spectra, sensitivity is inherently low and it would, therefore, also be advantageous to be able to predict shifts prior to the experimental measurement, particularly as resonances can be several hundred ppm away from their typical diamagnetic range.…”

“…5 However, in the intermediate regime, including materials such as catalytically-active transition metal complexes and metal-organic frameworks (MOFs), both the through-bond Fermi contact and through-space pseudocontact interactions affect the observed NMR spectrum and assignment can be both nontrivial and counterintuitive. 2,[6][7][8][9] For example, in the 13 C NMR spectrum of the MOF HKUST-1 (Cu3btc2, btc = benzene-1,3,5-tricarboxylate), 10 the broadest resonance, shifted most by paramagnetic interactions, is not the carboxylate C (separated from Cu 2+ by just two bonds), but rather the adjacent quaternary C (three bonds from Cu). This assignment was confirmed using the relatively costly and timeconsuming approach of specific 13 C labelling in conjunction with 1 H-13 C cross polarisation (CP) NMR, which is generally inefficient for paramagnetic materials.…”