Ligand-Directed and pH-Controlled Assembly of Chiral 3d−3d Heterometallic Metal−Organic Frameworks

Use of unsymmetric ligand 1,2,4-benzenetricarboxylic acid (1,2,4-H3BTC) and controlling the reaction pH value enabled isolation of two novel chiral 3d-3d heterometallic complexes [Zn2Co(tib)3(H2O)5][Zn6(tib)2(1,2,4-BTC)6]·12.7H2O (1) and [ZnCo(tib)(1,2,4-BTC)(H2O)2]Cl·3H2O (2) [tib =1,3,5-tris(1-imidazolyl)benzene]. Comparative study revealed that the use of symmetric ligand 1,3,5-benzenetricarboxylic acid (1,3,5-H3BTC) resulted one achiral 3d-3d heterometallic complex [ZnCo(tib)(1,3,5-BTC)Cl] (3) with 3D structure. Complex 1 is a 2D network with cationic and anionic layers arranged alternately, while 2 is a 3D framework with rare 1D helical water chains. Furthermore, complex 1 displays ferroelectric behavior with a remnant electric polarization (Pr) of ∼0.177 μC/cm2 and an electric coercive field (Ec) of ∼17.68 kV/cm.