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

Abstract

Infrared (IR) spectroscopy for N,N-dimethylformamide (DMF) shows that the OCN bend (δOCN) and the CO stretch (νCO) 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 δOCN 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 νCO frequencies (−15 cm−1 on average) while the splitting of the modes amounts to 37 cm−1. As a result, one of the νCO modes is located at an upshifted position (+13 cm−1) despite the O-atom coordination. For six-coordinated isomers of Co2+(DMF)7, the δOCN and νCO 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 δOCN and νCO regions. However, asymmetric ClO stretches of the ClO4− moiety suggest that conceivable is the coexistence of solvent-shared ion pairs only.