Structure, adsorption and magnetic properties of chiral metal–organic frameworks bearing linear trinuclear secondary building blocks

Abstract

The reactions of new chiral organic ligands trimesoyltri(L-alanine) (L-TMTAH₃) or trimesoyltri(D-alanine) (D-TMTAH₃) with transition metal salts in the presence of an ancillary ligand of 4,4′-bipyridine gave two pairs of three dimensional frameworks [Co₃(L-TMTA)₂(4,4′-bpy)₄]·28H₂O (1), [Co₃(D-TMTA)₂(4,4′-bpy)₄]·28H₂O (2) [Ni₃(L-TMTA)₂(4,4′-bpy)₄]·2C₂H₅OH·14H₂O (3) and [Ni₃(D-TMTA)₂(4,4′-bpy)₄]·2C₂H₅OH·14H₂O (4). These compounds were characterized by elemental analysis, IR, and X-ray powder diffraction analysis and the structures of 1–3 were determined from X-ray single crystal diffraction analysis. Complexes 1–4 feature linear trinuclear secondary building blocks [M₃(COO)₄]²⁺ formed via the connection of three metal ions by four carboxylato groups from four TMTA³⁻ ligands. Every adjacent two linear trinuclear secondary building blocks are linked by one and three 4,4′-bipyridine molecules along the a and c axis, respectively, to form two-dimensional sheets, which are further connected by TMTA³⁻ ligands to construct a porous three dimensional framework with one-dimensional channels. Compound 3 was taken as an example to investigate the adsorption properties of compounds 1–4. It revealed a saturated hydrogen uptake of 216.6 cm³ g⁻¹ (2.0wt%) at 11.1 atm measured at 77 K, a maximum CO₂ uptake of 119.4 cm³ g⁻¹ (23.5wt%) at 19.5 atm measured at 298 K and a saturated CH₄ uptake of 77.8 cm³ g⁻¹ (5.6wt%) at 27.1 atm measured at 298 K. The magnetic studies of complexes 1 and 3 indicate the presence of antiferromagnetic interactions between the metal ions in the two compounds.