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

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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. 13C MAS NMR spectra 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 crumple zone rearrangement on dehydration, similar to the well-characterised 5-ethoxyisophthalate MOF (STAM-17-OEt) although, while the materials have similar local C environments, dehydrated STAM-1 exhibits less long-range order.

Introduction

Metal-organic frameworks (MOFs) are porous framework materials comprising an infinitely-connected network of metal ions or clusters connected by organic “linkers”. MOFs are typically of great interest owing to their high porosity and the wide range of chemical space that they can cover. In particular, their crystal-chemical stability allows for the concept of isoreticularity, whereby the same framework topology can form with multiple different linkers or metals. This provides great potential to tune the physical and chemical properties of a MOF for specific applications in fields such as catalysis, guest storage and drug delivery [[1], [2], [3]].

Many MOFs are unstable to hydrolysis, a phenomenon that Morris and co-workers have been seeking to address with hemilabile Cu(II)-based MOFs, containing copper “paddlewheel” dimers (Fig. 1a) connected by 5-substituted isophthalate linkers (Fig. 1b) [4,5]. The archetypal material in this series is STAM-1, with a carboxymethyl substituent on the linker (Fig. 1c) [4]. The crystal structure of STAM-1 is shown in Fig. 1d, and contains both hydrophobic pores (lined with the methyl ester groups) and hydrophilic pores (lined with Cu-bound water in the as-made material). Subsequently, McHugh et al. [5] prepared the isoreticular STAM-17-OEt, containing 5-ethoxyisophthalate linkers, and demonstrated that this material exhibited high hydrolytic stability, especially when compared to the more commonly used copper paddlewheel MOF, HKUST-1 [6], containing benzene 1,3,5-tricarboxylate linkers (although it should be noted that Giovine et al. [7] recently demonstrated that HKUST-1 is surprisingly stable to steaming at 150–200 °C). The origin of this high hydrothermal stability was shown to be a rearrangement of the coordination environment of one half of the paddlewheel dimers upon dehydration, whereby the carboxylate oxygens of one dimer interact with the vacated axial site on the copper atoms of dimers above and below. This interaction requires energy to break, revealing the water coordination site on the copper atoms, and was described as being similar to the crumple zone of a car, in that the otherwise destructive impact of the incoming water molecules is dissipated in returning the MOF to its original state, rather than resulting in hydrolysis [8].

The host-guest interactions in STAM-1 and STAM-17-OEt are fundamental to their rather unusual behaviour, and solid-state NMR spectroscopy has been demonstrated to be an excellent probe of such structural changes on the local level (even when the material may be disordered on the long-range scale to which Bragg diffraction is sensitive) [[9], [10], [11], [12]]. However, many MOFs pose a challenge for NMR spectroscopy owing to the presence of paramagnetic metal cations and, even when the metal is diamagnetic, acquiring NMR spectra of many of the metals used can be very challenging [11]. It is generally recognised that one of the simplest methods of enhancing resolution in NMR spectra of paramagnetic solids is to use very rapid magic angle spinning (MAS) [[13], [14], [15]], and this approach has previously been applied to obtain high-resolution 13C MAS NMR spectra of copper paddlewheel-based MOFs [[16], [17], [18]]. Dawson et al. carried out a detailed 13C NMR characterisation of as-made STAM-1 and observed the seven resonances expected by symmetry, albeit over the wide shift range of between ca. 850 and −50 ppm (Fig. 1e) [18]. By selective 13C labelling of the linker and comparison with HKUST-1 (which can be considered as structurally analogous on the local level) it was shown that the resonance at the highest shift corresponded not to the carbon site closest to the Cu paddlewheel dimers (as might have been expected), but actually to the adjacent quaternary carbon, C1. The complete spectral assignment is shown in Fig. 1e and further details can be found in Ref. [18].

This work presents the 13C MAS NMR spectra of a series of Cu(II) isophthalate MOFs [5,19,20] and investigates the sensitivity of 13C NMR spectroscopy to substitutions on the 5 position of the linker. It is also demonstrated that, although less ordered, the structure of dehydrated STAM-1 is very similar to that of dehydrated STAM-17-OEt on the local scale, confirming that this archetypal MOF also exhibits this interesting “crumple zone” response.

Section snippets

Synthesis

HKUST-1 was synthesised as previously reported [18]. Cu(NO3)2·3(H2O), (15.752 g, 66 mmol) and trimesic acid (9.262 g, 44 mmol) were dissolved in 264 mL EtOH/H2O (50:50), stirred to homogeneity at room temperature, and heated at 110 °C for 1 day in a Teflon-lined steel autoclave. The autoclave was cooled to room temperature and HKUST-1 was isolated as a bright blue powder by suction filtration.

STAM-1 was synthesised as previously reported [4,18]. Cu(NO3)2·3(H2O) (0.991 g, 4.1 mmol) and trimesic

Acquisition and assignment of 13C NMR spectra of STAM-17 analogues

The 13C MAS NMR spectra of STAM-1 (shown above in Fig. 1 for the as-made material) span the range from almost 1000 to −150 ppm [18,[23], [24], [25]]. However, to acquire the entire spectrum without distortion, frequency stepping of the transmitter is required, as well as extensive signal averaging. The only benefit of the lengthy frequency-stepping process is the observation of the C1 resonance between 700 and 900 ppm. This resonance is very broad relative to its temperature- and

Conclusions

This study has extended our initial work on Cu(II) paddlewheel crumple zone MOFs and investigates the effect of linker variation on their high-resolution 13C MAS NMR spectra. We have demonstrated that, by using rapid MAS, it is possible to record an essentially quantitative 13C NMR spectrum of these materials on a reasonable timescale, although this omits the C1 resonance, which is typically found at much higher shift and it much broader than the other resonances. The acquisition of

Acknowledgements

SEA thanks the Royal Society and Wolfson Foundation for a merit award. REM and LNM thank the EPSRC for support (EP/N50936X/1). The research data (and/or materials) supporting this publication can be accessed at DOI: 10.17630/0c27a0c9-aa47-47e4-b30a-a215976c3dbe [35].

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