13C CP/MAS NMR study of accommodation of various organic molecules in (±)-[Co(en) 3]Cl 3 ionic crystal and their local structure

Abstract

Accommodation of various organic molecules into a one-dimensional nanochannel of tris(ethylenediamine) cobalt(III) chloride, [Co(en) 3]Cl 3, anhydrated racemic crystal was examined using weight increment measurements, thermogravimetric–differential thermal analysis (TG–DTA), powder X-ray diffraction (PXRD) and 13C cross-polarization/magic angle sample spinning (CP/MAS) NMR techniques. Results showed that the (±)-[Co(en) 3]Cl 3 ionic crystal accommodates n-alkanes, n-alkylalcohols, n-alkylamines, and acetonitrile in the 1D nanochannel without decomposition of the crystal lattice. The guest molecules are in all-trans conformation in the nanochannel. Furthermore, the activation energies of uni-axial molecular reorientation of n-alkanes and n-alkylalcohols in the nanochannel were determined using temperature dependence of the 1H spin–lattice relaxation time in the rotating frame ( T 1ρ). Results demonstrated that the activation energy per CH 2 unit (6.5 kJ mol −1) in n-alkylalcohols was twice that in n-alkanes (3.3 kJ mol −1), implying the possibility of dimer formation of n-alkylalcohols in the nanochannel by hydrogen bonding through OH group. These findings strongly suggest that the (±)-[Co(en) 3]Cl 3 ionic crystal has adsorption ability to various organic molecules. It will open new avenues to explore new types of zeolites that include ionic crystals.