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.