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

The crystal structure of the small pore scandium terephthalate Sc-2(O2CC6H4CO2)(3) (hereafter Sc2BDC3, BDC = 1,4-benzenedicarboxylate) has been investigated as a function of temperature and of functionalization, and its performance as an adsorbent for CO2 has been examined. The structure of Sc2BDC3 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 x 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. H-2 wide-line and MAS NMR of deuterated Sc2BDC3 indicates reorientation of phenyl groups via pi flips at temperatures above 298 K. The same framework solid has also been prepared using monofunctionalized terephthalate linkers containing -NH2 and -NO2 groups. The structure of Sc-2(NH2-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(NO2-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. CO2 adsorption at 196 and 273 K indicates that whereas Sc2BDC3 has the largest capacity, Sc-2(NH2-BDC)(3) shows the highest uptake at low partial pressure because of strong -NH2 center dot center dot center dot CO2 interactions. Remarkably, Sc-2(NO2-BDC)(3) adsorbs 2.6 mmol CO2 g(-1) at 196 K (P/P-0 = 0.5), suggesting that the -NO2 groups are able to rotate to allow CO2 molecules to diffuse along the narrow channels,