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

Abstract

Four new two-dimensional/three-dimensional (2D/3D) bpmh-based metal organic frameworks, namely, {[Zn­(1,3-adaa)­(bpmh)]}_<i>n</i> (<b>1</b>), {[Cd­(1,3-adaa)­(bpmh)]}<i>_n</i> (<b>2</b>), {[Zn­(1,4-pdaa)­(bpmh)]}_<i>n</i> (<b>3</b>), and {[Co­(1,4-pdaa)­(bpmh)]}_<i>n</i> (<b>4</b>) (bpmh = <i>N</i>,<i>N</i>-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 <b>1</b> and <b>2</b> have 2D layered architectures with similar framework topology, whereas <b>3</b> and <b>4</b> are isostuctural 3D frameworks. Both <b>1</b> and <b>2</b> perceives a common secondary building unit (SBU) [M₂(adaa)₄(bpmh)₄] [M = Zn­(<b>1</b>) and Cd­(<b>2</b>)]. In compound <b>1</b>, 1,3-adaa exhibits both μ- 1,1 and μ- 1,2 bridging modes, whereas in <b>2</b> it shows both μ-1,1 and μ-1,1,2 bridging modes. The difference in the bridging mode of 1,3-adaa in <b>1 </b>(Zn) and <b>2 </b>(Cd) is responsible for the shorter M···M contacts in <b>2</b> (3.872 Å) than in <b>1</b> (4.13 Å) in the SBU. The 1,3-adaa ligands are sandwiched between the bpmh linkers in compounds <b>1</b> and <b>2.</b> In compounds <b>3</b> and <b>4</b>, 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 <b>3</b> and <b>4</b>. In compounds <b>1</b>–<b>4</b>, the used dicarboxylic acids act as pillars between the metal-bpmh layers. Solid-state photoluminescent properties of compounds <b>1</b>–<b>3</b> show ligand (n → π* and π → π*)-based florescence. The magnetic studies of compound <b>4</b> show presence of the antiferromagnetic exchange between the metal centers