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

Abstract

[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 1H spin-lattice relaxation times (T1). The ELM-11 structural transition accompanying CO2 sorption was also examined through the CO2 uptake dependence of the 1H spin–spin relaxation time (T2), in addition to T1. In its closed form, the temperature dependence of the 1H T1 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 T1 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.