Syntheses of Metal−2-(Pyridin-4-yl)-1H-imidazole-4,5-dicarboxylate Networks with Topological Diversity: Gas Adsorption, Thermal Stability and Fluorescent Emission Properties

Six new metal−organic frameworks (MOFs), namely, {[Fe(HPIDC)(H₂O)]·2H₂O}_n (1), [Cd(HPIDC)(H₂O)]_n (2), [Zn(HPIDC)(H₂O)]_n (3), {[Eu₃(HPIDC)₄(H₂O)₈]·NO₃·4H₂O}_n (4), {[Tb₃(HPIDC)₄(H₂O)₈]·Cl·4H₂O}_n (5), and {[Y₃(HPIDC)₄(H₂O)₈]·Cl·4H₂O}_n (6), were hydrothermally synthesized and characterized (H₃PIDC = 2-(pyridin-4-yl)-1H-imidazole-4,5-dicarboxylic acid). Single crystal X-ray diffractions show ligands H₃PIDC are doubly deprotonated in all compounds, and the doubly deprotonated HPIDC²⁻ coordinates in the μ₃ or μ₄ mode to generate 2D or 3D MOFs, displaying four coordination modes. Both 1 and 2 are 3D MOFs, but different in structure topologies. In 1 the ligand HPIDC²⁻ and metal center Fe(II) act as 3-connected nodes to fabricate a (10,3) porous metal−organic net with 1D open channels where 1D zigzag water chains are reside through hydrogen-bonding interactions. This dehydrated porous material can absorb for N₂, with a determined BET surface area of 9.07 m² g⁻¹. In 2, however, ligand HPIDC²⁻ and metal center Cd(II) serve as 4-connected nodes to form a 3D honeycomb framework. Complexes 3−6 all are 2D MOFs built up from T-shaped 3-connected HPIDC²⁻ and 3- or 4-connected metal nodes. With the 3-connected metal nodes Zn(II), a stairway-like MOF with (4.8²) topology in 3 is created. As for 4-connected metal nodes Eu(III), Tb(III) or Y(III), isostructural MOFs with novel (4².6)₂(4.6²)₂(4².6³.8)₂(4².6².8²) nets in 4−6 form. Notably, ligands HPIDC²⁻ in 1, 3 and 4−6 are all T-shaped 3-connectors, but belong to different coordination modes, and the coordination lability also results in structural diversity, even with similar 3-connected nodes such as Fe(II) in 1 and Zn(II) in 3. The luminescent nature for 4 and 5 is the typical narrow emission bands of Eu(III) and Tb(III) ions, respectively. For 2, 3 and 6, the slightly blue-shifted emission bands are mainly due to intraligand emissions.