13C pNMR of “crumple zone” Cu(II) isophthalate metal-organic frameworks

Abstract

NMR spectroscopy of paramagnetic materials (pNMR) has the potential to provide great structural insight, but many challenges remain in interpreting the spectra in detail. This work presents a study of a series of structurally analogous metal-organic frameworks (MOFs) based on 5-substituted isophthalate linkers and Cu(II) paddlewheel dimers, of interest owing to their "crumple zone" structural rearrangement on dehydration/rehydration. 13 C MAS NMR spectra of the MOFs reveal a wide variation in the observed resonance position for chemically similar C species in the different MOFs but, despite this, resonances are overlapped in several cases. However, by considering a combination of the integration of quantitative spectra, the resonance position as a function of temperature and T1 relaxation measurements, the spectra can be fully assigned. It is also demonstrated that the prototypical MOF in this series, STAM-1, displays a similar crumple zone rearrangement on dehydration to the well-characterised 5ethoxyisophthalate MOF (STAM-17-OEt) although, while the materials have similar local C environments, dehydrated STAM-1 exhibits less long-range order.

Structure Effects Induced by High Mechanical Compaction of STAM‐17‐OEt MOF Powders

“…In addition, there are two important obstacles to overcome before the employ of different MOFs in a systematic industrial-scale development: the typical limited chemical stability with respect to water and their largescale synthesis in powder form. [3][4][5][6] Surprisingly, the new MOF STAM-17-OEt (Cu(C 10 O 5 H 8 ) • 1.6 H 2 O) combines all these extraordinary properties: it is flexible (or better, hemilabile), [7][8][9] extremely water-stable, [7,8,10] and with this work we show that it is also very resistant to the mechanical pressures necessary for the industrialization of the material, and therefore easy to densify and commercialize even if originally synthesized in powder form. This last aspect is not at all trivial, because in literature many MOFs have been subjected to densification processes with or without the help of chemical binders, but in most cases a significant loss of crystallinity and surface area have been found, compromising the typical performances of the material, especially as adsorbent or catalyst.…”

“…[7] Its particular hemilability arises right from the removal (or addition, during hydration processes) of such coordinated water molecules, process that leads to a single-crystal to single-crystal transition. [7][8][9] The two Cu 2 + ions involved in the paddle-wheels confer to STAM-17-OEt (as well as to HKUST-1) peculiar magnetic properties: the two S = 1/2 spins establish an antiferromagnetic coupling characterized by a spin angular momentum with a singlet (S = 0) ground state and a triplet (S = 1) excited state. [22,23,[27][28][29][30] The latter state generates a characteristic Electron Paramagnetic Resonance (EPR) spectrum.…”

Proton-detected fast-magic-angle spinning NMR of paramagnetic inorganic solids

“…Despite notable proofs of concept highlighting the possibility of the detection of 1 H resonances, [16][17][18][19][20] the resulting spectra failed to provide a sufficient level of detail necessary for the identication of individual 1 H sites. The characterization of paramagnetic organometallic solids has therefore been mainly based on 13 C detection techniques, [21][22][23] for which sensitivity is optimal in larger MAS rotors (e.g. 1.9-2.5 mm) at intermediate spinning frequencies (e.g.…”

A Cu(ii)-MOF based on a propargyl carbamate-functionalized isophthalate ligand

“…690 m 2 g À1 in the original paper 12 and much higher than 1500 m 2 g À1 in the marketed materials) in which {Cu 2 } units are coordinated to four carboxylate groups to give the well-known paddle-wheel unit. 6,[13][14][15][16][17] Simpler ligands, like benzene-dicarboxylates (BDC), thanks to their structural rigidity (and simultaneous charge balancing effects of M(II) centers), have also been widely employed, as they can bear an additional, pendant and chemically tailored, functionality on the benzene ring. [18][19][20] In recent years, our research group has shown that a prop-…”