A Family of Metal–Organic Frameworks Based on Carboxylates and a Neutral, Long, and Rigid Ligand: Their Structural Revelation, Magnetic, and Luminescent Property Study

Four new two-dimensional/three-dimensional (2D/3D) bpmh-based metal organic frameworks, namely, {[Zn­(1,3-adaa)­(bpmh)]}_n (1), {[Cd­(1,3-adaa)­(bpmh)]}_n (2), {[Zn­(1,4-pdaa)­(bpmh)]}_n (3), and {[Co­(1,4-pdaa)­(bpmh)]}_n (4) (bpmh = N,N-bis-pyridin-4-ylmethylene-hydrazine, 1,3-adaa = 1,3-adamantane diacetic acid, 1,4-pdaa = 1,4-phenylene diacetic acid) have been synthesized through the slow diffusion technique. Structural determination reveals that compounds 1 and 2 have 2D layered architectures with similar framework topology, whereas 3 and 4 are isostuctural 3D frameworks. Both 1 and 2 perceives a common secondary building unit (SBU) [M₂(adaa)₄(bpmh)₄] [M = Zn­(1) and Cd­(2)]. In compound 1, 1,3-adaa exhibits both μ- 1,1 and μ- 1,2 bridging modes, whereas in 2 it shows both μ-1,1 and μ-1,1,2 bridging modes. The difference in the bridging mode of 1,3-adaa in 1 (Zn) and 2 (Cd) is responsible for the shorter M···M contacts in 2 (3.872 Å) than in 1 (4.13 Å) in the SBU. The 1,3-adaa ligands are sandwiched between the bpmh linkers in compounds 1 and 2. In compounds 3 and 4, 1,4-pdaa exhibits both μ-1 and μ-1,1 bridging modes and are isostructural in nature. The metal centers are arranged in a helical fashion around 2₁ screw axis in 3 and 4. In compounds 1–4, the used dicarboxylic acids act as pillars between the metal-bpmh layers. Solid-state photoluminescent properties of compounds 1–3 show ligand (n → π* and π → π*)-based florescence. The magnetic studies of compound 4 show presence of the antiferromagnetic exchange between the metal centers.