Reversible five-coordinate ⇆ six-coordinate transformation in cobalt(II) complexes

The heterocyclic ligands 2,6-bis(pyrazol-1-yl)pyridine (L-1) and 2,6-bis(benzimidazol-2-yl)pyridine (L-2) and their cobalt(II) complexes were synthesized. The blue five-coordinate complex [Co(L-1)Cl-2] isolated initially from the reaction mixture rapidly absorbed water vapour from the atmosphere to yield the pink six-coordinate complex [Co(L-1)(H2O)(3)]Cl-2. This change is reversible upon desiccation or transferring [Co(L-1)(H2O)(3)]Cl-2 into acetonitrile. The five coordinate complex [Co(L-2)Cl-2], however, remains stable under similar conditions. The structures of the complexes [Co(L-1)Cl-2], [Co(L-1)(H2O)(3)]Cl-2 and [Co(L-2)Cl-2] have been determined by x-ray crystallography. The magnetic susceptibilities and the electronic spectra for [Co(L-1)Cl-2], [Co(L-2)Cl-2] and [Co(L-1)(H2O)(3)]Cl-2 are presented. (C) 2017 Elsevier B.V. All rights reserved.