Probing Interactions of N-Donor Molecules with Open Metal Sites within Paramagnetic Cr-MIL-101: A Solid-State NMR Spectroscopic and Density Functional Theory Study

Abstract

Understanding host-guest interactions is one of the key requirements for adjusting properties in metal-organic frameworks (MOFs). In particular, systems with coordinatively unsaturated Lewis acidic metal sites feature highly selective adsorption processes. This is attributed to strong interactions with Lewis basic guest molecules. Here we show that a combination of C MAS NMR spectroscopy with state-of-the-art density functional theory (DFT) calculations allows one to unravel the interactions of water, 2-aminopyridine, 3-aminopyridine, and diethylamine with the open metal sites in Cr-MIL-101. TheC MAS NMR spectra, obtained with ultrafast magic-angle spinning, are well resolved, with resonances distributed over 1000 ppm. They present a clear signature for each guest at the open metal sites. Based on competition experiments this leads to the following binding preference: water < diethylamine ≈ 2-aminopyridine < 3-aminopyridine. Assignments were done by exploiting distance sum relations derived from spin-lattice relaxation data and C{H} REDOR spectral editing. The experimental data were used to validate NMR shifts computed for the Cr-MIL-101 derivatives, which contain CrO clusters with magnetically coupled metal centers. While both approaches provide an unequivocal assignment and the arrangement of the guests at the open metal sites, the NMR data offer additional information about the guest and framework dynamics. We expect that our strategy has the potential for probing the binding situation of adsorbate mixtures at the open metal sites of MOFs in general and thus accesses the microscopic interaction mechanisms for this important material class, which is essential for deriving structure-property relationships.

NMR crystallography of molecular organics

“…Figure27 Agreement between experimental and calculated13 C NMR shift tensor principal values for 10 CSP structures of methyl β-D-xylopyranoside. (Top) calculation of shifts within the full crystal structure without optimisation did not allow the correct structure to be identified.…”

“…It is also worth noting that "pseudo-contact shifts" in paramagnetic systems are essentially dipolar in nature, and so can also be used to obtain distances between nuclei and paramagnetic metal centres. Such shifts have been used in NMR crystallography of, inter alia, lanthanide complexes [26], metal-organic frameworks [27], phthalocyanine polymorphs [28] and paramagnetic metalloproteins [29,30]. Paramagnetic systems are not discussed further in this review.…”

Impact of dehydration and mechanical amorphization on the magnetic properties of Ni(ii)-MOF-74

“…Only recently, first attempts of calculating such shifts in paramagnetic MOFs have emerged. [69][70][71] The 13 C MAS NMR spectra of Ni-MOF-74, deh-Ni-MOF-74 and am-Ni-MOF-74 are shown in Fig. 7.…”

Blind spheres of paramagnetic dopants in solid state NMR

“…The influence transition metal atoms exert on neighbouring NMR nuclei enables studies for example on battery materials [6][7][8] or metal organic framework compounds. 27 The computation of the influence on valence electrons is important and progress in the computation of paramagnetic shifts 28 has improved significantly in recent years. In contrast, lanthanides with few exceptions show hardly any influence from the valence shell and thus have been used for systematic experimental studies to identify contributions to the paramagnetic spin Hamiltonian.…”