Contrasting interpenetration in two isoreticular MOFs with bridged layers: role of 2D layer packing density in 3D catenation.

Single-crystal X-ray diffraction analysis of two isoreticular MOFs, poly[[(μ-4,4'-diazenediyldibenzoato)[μ-1,3-bis(pyridin-4-yl)propane]cadmium(II)] dimethylformamide tetrasolvate], {[Cd(C14H8N2O4)(C13H14N2)]·4C3H7NO}n or {[Cd(adc)(bpp)]n·4DMF}n (1), and poly[aqua(μ-4,4'-diazenediyldibenzoato)[μ-1,3-bis(pyridin-4-yl)propane]cobalt(II)], [Co(C14H8N2O4)(C13H14N2)(H2O)]n or [Co(adc)(bpp)(H2O)]n (2), obtained via one-pot solvothermal synthesis, reveals a counterintuitive relationship in interpenetration behaviour [adc = azobenzene-4,4'-dicarboxylate or 4,4'-diazenediyldibenzoate, bpp = 1,3-bis(pyridin-4-yl)propane and DMF = N,N-dimethylformamide]. MOF 1 crystallizes in the orthorhombic system (Pccn, Z = 8) and adopts a non-interpenetrating 3D architecture with a calculated solvent-accessible void volume of 40.4%. The [Cd(adc)]n sublayers in 1 exhibit twofold interpenetration, resulting in closely packed layers that prevent further 3D interpenetration upon bridging by flexible bpp ligands. In contrast, MOF 2 crystallizes in the space group P21/c (Z = 4) and adopts a twofold interpenetrated 3D framework with a reduced solvent-accessible void volume of 4.4%. This architecture originates from non-interpenetrated yet loosely packed [Co(adc)]n layers. Photoluminescence studies corroborate this structural dichotomy. Dense intralayer packing in 1 induces concentration quenching, whereas rigid non-interpenetrated layers in 2 preserve linker-based emission. This comparative crystallographic study demonstrates that the packing density of 2D layers dictates the propensity for higher-order catenation, highlighting interpenetration as a geometric constraint rather than a synthetic flaw.