Functionalization of Microporous Lanthanide-Based Metal–Organic Frameworks by Dicarboxylate Ligands with Methyl-Substituted Thieno[2,3‑<i>b</i>]thiophene Groups: Sensing Activities and Magnetic Properties

From a methyl-substituted
thieno­[2,3-b]­thiophene dicarboxylate, three types
of three-dimensional (3-D) microporous lanthanide-based metal–organic
frameworks, {[Ln­(DMTDC)1.5­(H2O)2]·DEF}n (type I, Ln = Eu 1, Tb 2), {[Ln­(DMTDC)1.5­(H2O)2]·0.5DMF·0.5H2O}n (type II, Ln = Gd 3, Dy 4, Er 5), and {[Ln4(DMTDC)6­(DMF)2]·0.5DMF·1.5H2O}n (type III, Ln = Er 6) (H2DMTDC = 3,4-dimethylthieno­[2,3-b]­thiophene-2,5-dicarboxylic acid, DEF = N,N′-diethylformamide, DMF = N,N′-dimethylformamide), have been solventhermally
synthesized. Types I and II are isostructural,
which exhibit 1-D triangular channels constructed by double-stranded
rod-shaped {Ln­(CO2)2}n chains. Type III demonstrates an intriguing framework
with triple-stranded rod-shaped {Ln­(CO2)3}n chains arranged along the (1,1,0) and (1,–1,0)
axes and possesses two kinds of triangular channels along two axes,
respectively. Immobilization of the Lewis basic sites of thiophene
groups induced gas adsorption and sensing properties into these microporous
frameworks. Complexes 5­(Er) and 6­(Er) display
moderate adsorption properties toward N2 and CO2, and the Qst of CO2 are as
high as 36.3 and 34.8 kJ mol–1, respectively. Complexes 1­(Eu) and 2­(Tb) exhibit sensing properties toward
nitrobenzene, acetone, and the Cu2+ ion in both DMF and
aqueous solution. Complex 3­(Gd) shows a significant magnetocaloric
effect with ΔSm = 24.3 J·kg–1·K–1 at 3.0 K and 7 T. Complex 4­(Dy) exhibits slow magnetic relaxation with the energy barrier
Δ/kB of 48.29 K.