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

New complexes of composition s-fac-[Co(dien)(2)]Cl-2(Bz)-H2O (1), s-fac-[Co(dien)(2)]Cl(p-CBz)(2).4.5H(2)O (2) and mer-[Co(dien)(2)](p-NBz)(3).3H(2)O (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)center dot center dot center dot O (benzoates) and H (water)center dot center dot center dot O (benzoates) in addition to the electrostatic forces of attractions. Further, the NH (dien)center dot center dot center dot Cl- (counter ion) and NH (dien)center dot center dot center dot O (water) types of interactions are also playing a dominant role to stabilize the crystal lattice in complex 2 and 3 respectively. (C) 2014 Elsevier B.V. All rights reserved.