| Summary: | [Cu(4,4'-bipyridine)<sub>2</sub>(BF<sub>4</sub>)<sub>2</sub>] (ELM-11), an elastic layer-structured MOF (metal-organic framework), is expected to be a sophisticated CO<sub>2</sub> reservoir candidate because of its high capacity and recovery efficiency for CO<sub>2</sub> sorption. While ELM-11 shows a unique double-step gate sorption for CO<sub>2</sub> gas, the dynamics of the structural transition have not yet been clarified. In this study, the dynamics of the 4,4'-bipyridine linkers and the BF<sub>4</sub><sup>-</sup> anions were studied by determining <sup>1</sup>H spin-lattice relaxation times (<i>T</i><sub>1</sub>). The ELM-11 structural transition accompanying CO<sub>2</sub> sorption was also examined through the CO<sub>2</sub> uptake dependence of the <sup>1</sup>H spin–spin relaxation time (<i>T</i><sub>2</sub>), in addition to <i>T</i><sub>1</sub>. In its closed form, the temperature dependence of the <sup>1</sup>H <i>T</i><sub>1</sub> of ELM-11 was analyzed by considering the contributions of both paramagnetic and dipolar relaxations, which revealed the isotropic reorientation of BF<sub>4</sub><sup>-</sup> and the torsional flipping of the 4,4'-bipyridine moieties. The resultant activation energy of 32 kJ mol<sup>-1</sup> for the isotropic BF<sub>4</sub><sup>−</sup> reorientation is suggestive of strong (B-F...Cu<sup>2+</sup>) interactions between Cu(II) and the F atoms in BF<sub>4</sub><sup>−</sup>. Furthermore, the CO<sub>2</sub> uptake dependence of <i>T</i><sub>1</sub> was found to be dominated by competition between the increase in the longitudinal relaxation time of the electron spins and the decrease in the spin density in the unit cell.
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