Structural Chemistry, Monoclinic-to-Orthorhombic Phase Transition, and CO₂ Adsorption Behavior of the Small Pore Scandium Terephthalate, Sc₂(O₂CC₆H₄CO₂)₃, and Its Nitro- And Amino-Functionalized Derivatives

Abstract

The crystal structure of the small pore scandium terephthalate Sc(2)(O(2)CC(6)H(4)CO(2))(3) (hereafter Sc(2)BDC(3), BDC = 1,4-benzenedicarboxylate) has been investigated as a function of temperature and of functionalization, and its performance as an adsorbent for CO(2) has been examined. The structure of Sc(2)BDC(3) has been followed in vacuo over the temperature range 140 to 523 K by high resolution synchrotron X-ray powder diffraction, revealing a phase change at 225 K from monoclinic C2/c (low temperature) to Fddd (high temperature). The orthorhombic form shows negative thermal expansivity of 2.4 × 10(-5) K(-1): Rietveld analysis shows that this results largely from a decrease in the c axis, which is caused by carboxylate group rotation. (2)H wide-line and MAS NMR of deuterated Sc(2)BDC(3) indicates reorientation of phenyl groups via π flips at temperatures above 298 K. The same framework solid has also been prepared using monofunctionalized terephthalate linkers containing -NH(2) and -NO(2) groups. The structure of Sc(2)(NH(2)-BDC)(3) has been determined by Rietveld analysis of synchrotron powder diffraction at 100 and 298 K and found to be orthorhombic at both temperatures, whereas the structure of Sc(2)(NO(2)-BDC)(3) has been determined by single crystal diffraction at 298 K and Rietveld analysis of synchrotron powder diffraction at 100, 298, 373, and 473 K and is found to be monoclinic at all temperatures. Partial ordering of functional groups is observed in each structure. CO(2) adsorption at 196 and 273 K indicates that whereas Sc(2)BDC(3) has the largest capacity, Sc(2)(NH(2)-BDC)(3) shows the highest uptake at low partial pressure because of strong -NH(2)···CO(2) interactions. Remarkably, Sc(2)(NO(2)-BDC)(3) adsorbs 2.6 mmol CO(2) g(-1) at 196 K (P/P(0) = 0.5), suggesting that the -NO(2) groups are able to rotate to allow CO(2) molecules to diffuse along the narrow channels.

Solvent-switchable continuous-breathing behaviour in a diamondoid metal–organic framework and its influence on CO2 versus CH4 selectivity

“…Amino substituents on the terephthalate ligand are ordered, in contrast to the typical sitedisorder observed for ABDC ligands in other MOFs. 34,35 Ordering is consistent with the prominent role of the amino groups in linking the two networks via a 4-membered hydrogen-bonded ring motif (Figure 1c). The framework exhibits lozenge-shaped channels that lie along the a-axis, resembling those of aforementioned flexible MOFs such as MIL-53, although there is no pillar along the channel direction, but rather a helical arrangement of In(III) centres linked via ABDC ligands (Figure 1d).…”

Direct visualization of a guest-triggered crystal deformation based on a flexible ultramicroporous framework

“…Powder X-ray diffraction (PXRD) showed that 1 can retain framework integrity even after being heated at 673 K for 2 h (Supplementary Fig. S4), which is much higher than those for other organic and organic-inorganic hybrid solids showing large thermal expansion properties [4][5][6][7][8][9][10][11][12][13][14][15]21,22 . Gas sorption measurements showed that 1 cannot adsorb N 2 or O 2 at 77 K up to P/P 0 E1 but adsorb considerable CO 2 at 195 K, with an apparent Langmuir surface area of 188 m 2 g À 1 .…”

“…For example, a few flexible PCPs showed much larger thermal expansion than common solids because the open coordination frameworks are relatively flexible [4][5][6][7][8][9][10][11][12][13][14][15] . Whereas small positive thermal expansion (PTE, 0oao20 Â 10 À 6 K À 1 , a and b V for axial and volumetric thermal expansion coefficients, respectively) is an intrinsic property of common solids 16 , negative (NTE, ao0 K À 1 ) [17][18][19][20] , very large (|a|4100 Â 10 À 6 K À 1 ), tunable and other special thermal expansion behaviours are extremely rare and have received considerable theoretical and practical interest [21][22][23][24] , which may be rationally realized by PCPs.…”

Solid-State NMR Spectroscopy of Metal–Organic Framework Compounds (MOFs)

“…The MOF can be dissolved and the functional groups become then observable ex situ by liquid-state NMR spectroscopy as described in [127,128,129,130,131]. On the other hand, the functional groups can be detected in situ by MAS NMR spectroscopy [42,132,133]. The advantage of the in situ method is its non-destructive character.…”

“…The coordination environment can be detected using chemical shifts and/or quadrupolar coupling constants for metal atoms such as 27 Al [107,108,144], 71 Ga [44], 45 Sc [46,132], and 6 Li [145]. Furthermore, metal atoms outside the MOF lattice, i.e.…”