Determination of contact ion-pair formation in CoCl aqueous, methanol, and ethanol dilute solutions by UV-vis and X-ray absorption spectroscopies

• Published in: New journal of chemistry 2024-11, Vol.48 (46), p.19519-19529
• Main Authors: Tofoni, Alessandro, Busato, Matteo, Rigacci, Irene, Giustini, Mauro, D'Angelo, Paola
• Format: Article
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/d4nj03982c

A detailed investigation on the coordination environment of the Co 2+ ion has been carried out with the intent of quantifying the contact ion-pair formation in dilute (0.1 mol L −1 ) CoCl 2 aqueous, methanol (MeOH), and ethanol (EtOH) solutions. An effective approach has been employed combining UV-vis measurements, X-ray absorption spectroscopy, and density functional theory (DFT). The CoCl 2 metal salt is fully dissociated in aqueous solution with the Co 2+ cation first hydration shell formed by six water molecules arranged in an octahedral fashion. On the other hand, the chloride anion enters the Co 2+ coordination sphere giving rise to ionic pairs in MeOH and EtOH solution due to the weaker solvation ability of these solvents. The Co-Cl distances are 2.34(2) and 2.26(3) Å in MeOH and EtOH solutions, respectively, as determined by extended X-ray absorption fine structure data analysis. In MeOH solution the dominant species is the octahedral [CoCl(MeOH) 5 ] + complex, while for EtOH the spectral evidence can be interpreted with an equilibrium between different four-fold metal-chloro species. Structural distortions in the coordination clusters have been evidenced by the X-ray absorption near-edge structure analysis aided by DFT optimizations and allowed us to rationalize the spectroscopic outcome of the UV-vis measurements. The adopted combined approach provided an all-around structural picture of the coordination complexes formed when the CoCl 2 salt is dissolved in solvents with different coordinating properties. The inner sphere complex structures of CoCl 2 in diluted aqueous, methanol and ethanol solutions have been determined.