Second-sphere coordination in anion binding: Synthesis, characterization and X-ray structures of bis(diethylenetriamine)cobalt(III) complexes containing benzoates

•[Co(dien)2]3+ has been recognized as a molecular receptor for benzoates.•The isomeric identification of [Co(dien)2]3+ was done with the help of spectroscopic studies.•Binding constants with benzoates were determined by UV–vis titration method in aqueous medium.•X-ray study reveals the formation of chains and layers through second-sphere coordination network. New complexes of composition s-fac-[Co(dien)2]Cl2(Bz)·H2O (1), s-fac-[Co(dien)2]Cl(p-CBz)2·4.5H2O (2) and mer-[Co(dien)2](p-NBz)3·3H2O (3) were obtained by reacting aqueous solutions of bis(diethylenetriamine)cobalt(III) chloride and sodium salts of benzoates ((Bz=benzoate, CBz=p-chlorobenzoate, NBz=p-nitrobenzoate)) in 1:3 molar ratio. These complexes were characterized by TG analysis and spectroscopic studies (IR, NMR and UV–vis). IR and NMR studies were used for the isomeric identification of [Co(dien)2]3+ in new complexes. This cation, contains ligand diethylenetriamine (dien) bearing H-bond donors, capable of forming hydrogen bonds and its binding properties with benzoates have been studied using standard UV–vis spectroscopic titrations in aqueous medium (log k for Bz=2.11, p-CBz=3.64 and p-NBz=3.66). Single crystal X-ray study of complex 2 and 3 reveals that both the structures are dominantly stabilized by second-sphere coordination through H-bonding interactions of type-NH (dien)⋯O (benzoates) and H (water)⋯O (benzoates) in addition to the electrostatic forces of attractions. Further, the NH (dien)⋯Cl- (counter ion) and NH (dien)⋯O (water) types of interactions are also playing a dominant role to stabilize the crystal lattice in complex 2 and 3 respectively.