Synthesis of a NbO Type Homochiral Cu(II) Metal–Organic Framework: Ferroelectric Behavior and Heterogeneous Catalysis of Three-Component Coupling and Pechmann Reactions

Abstract

A chiral tetracarboxylic acid ligand, H₄L, incorporating the (S)-(+)-2-methylpiperazine moiety in its middle, solvothermally forms a homochiral Cu(II) framework, {[Cu₂(L)(H₂O)₂]·(4DMF)(4H₂O)}_n (LCu). It forms a non-interpenetrated structure consisting of [Cu₂(COO)₄] paddle-wheel secondary bonding units (SBUs) with NbO topology. Interestingly, the framework LCu exhibits excellent ferroelectric properties. It shows a remnant polarization (Pr) of ∼3.5 μC cm^-2 and a coercive field (E_c) of ∼12 kV cm^-1 with a distinct electric hysteresis loop. Dielectric studies of LCu reveal almost frequency-independent behavior with a dielectric constant (ε_r) of ∼42 and a low dielectric loss (tan δ) of ∼0.04 up to 10⁶ Hz, for potential use in high-frequency applications. In addition, activated framework LCu' having uncoordinated metal sites acts as an efficient heterogeneous catalyst in the three-component coupling of amines, aldehydes, and alkynes, as well as in Pechmann reactions of phenols with β-ketoesters.