Transformation from a 2D stacked layer to 3D interpenetrated framework by changing the spacer functionality: synthesis, structure, adsorption, and magnetic properties

Abstract

Two novel coordination polymers of Cu(II), viz. [Cu(bipy)(1,4-napdc)(H₂O)₂]_n (1) and {[Cu(bpe)_1.5(1,4-napdc)](H₂O)}_n (2) (bipy = 4,4‘-bipyridine; bpe = 1,2-bis(4-pyridyl)ethane; 1,4-napdc^2- = 1,4-naphthalenedicarboxylate), have been synthesized and structurally characterized by changing only the pillar motifs. Both the compounds crystallize by slow evaporation from the ammoniacal solution of the as-synthesized solid. Framework 1 crystallizes in monoclinic crystal system, space group P2/n (No. 13), with a = 11.028(19) Å, b = 11.16(3) Å, c = 7.678(13) Å, β = 103.30(5)°, and Z = 2. Framework 2 crystallizes in triclinic system, space group, P1̄ (No. 2), a = 10.613(4) Å, b = 10.828(10) Å, c = 13.333(9) Å, α = 85.25(9)°, β = 82.59(6)°, γ = 60.37(5)°, and Z = 2. The structure determination reveals that 1 has a 2D network based on rectangular grids, where each Cu(II) is in 4 + 2 coordination mode. The 2D networks stacked in a staggered manner through the π−π interaction to form a 3D supramolecular network. In the case of 2, a {Cu(bpe)_1.5}_n ladder connected by 1,4-napdc^2- results a 2D cuboidal bilayer network and each bilayer network is interlocked by two adjacent identical network (upper and lower) forming 3-fold interpenetrated 3D framework with small channel along the c-axis, which accommodates two water molecules. The TGA and XRPD measurements reveal that both the frameworks are stable after dehydration. Adsorption measurements (N₂, CO₂, and different solvents, like H₂O, MeOH, etc.) were carried out for both frameworks. Framework 1 shows type-II sorption profile with N₂ in contrast to H₂O and MeOH, which are chemisorbed in the framework. In case of 2, only H₂O molecules can diffuse into the micropore, whereas N₂, CO₂, and MeOH cannot be adsorbed, as corroborated by the smaller channel aperture. The low-temperature (300−2 K) magnetic measurement of 1 and 2 reveals that both are weakly antiferromagnetically coupled (J = −1.85 cm^-1, g = 2.02; J = −0.153 cm^-1, g = 2.07), which is correlated by the magnetic pathway to the corresponding structure.