Structural diversities in the Zn(II), Mn(II) and Cd(II) coordination polymers induced by metal ions and/or anions

Under the same solvothermal conditions, nine novel coordination polymers (CPs), [Mn(L)(Cl)(H2O)] (1a), [Mn(L)(H2O)2]·(NO3) (2a), [MnL]·(HSO4)·MeCN·2H2O (3a), [Zn(L)2]·H2O (1b), [Zn(L)2] (2b and 3b), [Cd(L)]·Cl (1c), [Cd(L)(NO3)] (2c) and [CdL]·(HSO4)·2.5H2O (3c), were synthesized using a new ligand, namely 4-(2-chloroimidazo[1,2-a]pyridin-3-yl)-[2,4′-bipyridine]-6-carboxylic acid (HL). The Zn(II) CPs 1b-3b are similar 1D chains and their structures were mediated by the trigonal bipyramidal geometry of the Zn(II) cation, not by the counter-anions. However, when using Mn(II) and Cd(II) salts bearing various counter-anions (Cl−, NO3− or SO42−) as reagents, the Mn(II) (1a-3a) or Cd(II) (1c-3c) CPs do reveal different structures, which were induced by different anions acting as either a coordinated anion or a counter-anion for balancing the charge of 1a-3a or 1c-3c. When the larger NO3− anion replaced the Cl− anion, the 1D molecular box chain of 1a with a coordinated Cl− anion was changed to the 2D brickwall-type network of 2a with NO3− only as a counter-anion. However, the NO3− anion still could be coordinated to larger volume of the Cd(II) cation, leading to a 3D framework for 2c. Moreover, CPs 3a and 3c are isostructural, which could be induced by the much larger tetrahedral HSO4− counter-anion. Furthermore, the luminescent properties of the CPs have been investigated. The present work provides a promising approach to design and construct CPs or MOFs by adjusting metal salts bearing various counter-anions.