Infrared spectroscopic and computational studies of Co(ClO₄)₂ dissolved in N,N-dimethylformamide (DMF). Vibrations of DMF influenced by Co²⁺ or ClO₄⁻ or both

Infrared (IR) spectroscopy for N,N-dimethylformamide (DMF) shows that the OCN bend (δ_OCN) and the CO stretch (ν_CO) vibrations undergo an upshift and a downshift, respectively, on the dissolution of Co(ClO₄)₂. Quantum chemical calculations are performed for optimizing the structures and predicting the IR spectra of model complexes for solute species. The calculations reveal that Co²⁺ exerts a much larger influence than ClO₄⁻ on the vibrations of DMF. For Co²⁺(DMF)₆, in which each DMF molecule is coordinated to Co²⁺ via the O atom, the Co²⁺⋯DMF interaction upshifts the δ_OCN frequencies (+24 cm⁻¹ on average) while the dipole coupling gives rise to splitting (12 cm⁻¹) of the modes. On the other hand, the Co²⁺⋯DMF interaction downshifts the ν_CO frequencies (−15 cm⁻¹ on average) while the splitting of the modes amounts to 37 cm⁻¹. As a result, one of the ν_CO modes is located at an upshifted position (+13 cm⁻¹) despite the O-atom coordination. For six-coordinated isomers of Co²⁺(DMF)₇, the δ_OCN and ν_CO frequencies of the second-sphere DMF are close to those of bulk DMF in neat liquid. The calculations indicate that it is difficult to prove or exclude the formation of contact ion pairs [Co(DMF)₅ClO₄]⁺ and solvent-shared ion pairs [Co(DMF)₆ClO₄]⁺ by IR spectroscopy in the δ_OCN and ν_CO regions. However, asymmetric ClO stretches of the ClO₄⁻ moiety suggest that conceivable is the coexistence of solvent-shared ion pairs only.