Stoichiometry, temperature, solvent, metal-directed syntheses of metal–organic frameworks based on flexible V-shaped methylenebis(3,5-dimethylpyrazole) and various aromatic dicarboxylate acids

Abstract

Seven new metal–organic frameworks (MOFs), namely, {[Cd₂(NDC)₂(H₂MDP)](H₂O)} (1), [Cd(NDC)(H₂MDP)] (2), {[Cd₂(NDC)(HNDC)₂(H₂MDP)₂](H₂O)_0.5} (3), [Cd(NH₂–BDC)(H₂MDP)₂] (4), [Cd(NH₂–BDC)(H₂MDP)₂] (5), [Cd(H₂MDP)(Br–BDC)_0.5(Br–HBDC)] (6), {[Co(H₂MDP)(Br–BDC)](H₂O)_0.5} (7) (H₂MDP = methylenebis(3,5-dimethylpyrazole), H₂NDC = 1,4-naphthalenedicarboxylic acid, NH₂–H₂BDC = 2-amino-1,4-benzenedicarboxylic acid, Br–H₂BDC = 2,5-dibromo-1,4-benzenedicarboxylic acid) have been controllably synthesized with H₂MDP and different aromatic dicarboxylic acids by tuning metal-to-ligand molar ratios, temperature, solvents and metal ions. Compound 1 is a 3D 2-fold interpenetrated pcu framework of 4¹²·6³ topology constructed from paddle-wheel SBUs and trans-conformation H₂MDP pillars, while compound 2 exhibits a 3D 3-fold interpenetrated diamondoid framework formed by single metal nodes via linear NDC²⁻ and trans-conformation H₂MDP ligands. Interestingly, compound 3 contains two different types of binuclear subunits formed by a pair of Cd(II) ions with partly deprotonated HNDC⁻ ligands, and these subunits are further linked by completely deprotonated NDC²⁻ and cis-conformation H₂MDP ligands to form a 2D (4,4) network. Compounds 4 and 5 are solvent-induced structural isomers based on 2D (4,4) layers, and therefore have different solvent-accessible voids. Compounds 6 and 7, synthesized with different metal salts under similar solvothermal conditions, are characteristic of a 3D hydrogen-bonded supramolecular network, and a 3D 3-fold interpenetrated diamondoid network with equal number of left- and right-handed helical {Cd(H₂MDP)}_n chains further bridged together by linear Br–BDC²⁻ ligands, respectively. In addition, the luminescent properties of solids 1–6 have been investigated at room temperature.