Structural Dynamics of an ELM-11 Framework Transformation Accompanied with Double-Step CO2 Gate Sorption: An NMR Spin Relaxation Study

[Cu(4,4 '-bipyridine)(2)(BF4)(2)] (ELM-11), an elastic layer-structured MOF (metal-organic framework), is expected to be a sophisticated CO2 reservoir candidate because of its high capacity and recovery efficiency for CO2 sorption. While ELM-11 shows a unique double-step gate sorption for CO2 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 BF4- anions were studied by determining H-1 spin-lattice relaxation times (T-1). The ELM-11 structural transition accompanying CO2 sorption was also examined through the CO2 uptake dependence of the H-1 spin-spin relaxation time (T-2), in addition to T-1. In its closed form, the temperature dependence of the H-1 T-1 of ELM-11 was analyzed by considering the contributions of both paramagnetic and dipolar relaxations, which revealed the isotropic reorientation of BF4- and the torsional flipping of the 4,4 '-bipyridine moieties. The resultant activation energy of 32 kJ mol(-1) for the isotropic BF4- reorientation is suggestive of strong (B-F...Cu2+) interactions between Cu(II) and the F atoms in BF4-. Furthermore, the CO2 uptake dependence of T-1 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.