Individual solvation number of first-row transition metal(II) ions in solvent mixtures of N,N-dimethylformamide and N,N-dimethylacetamide—Solvation steric effect

Abstract

Raman spectra of solutions of manganese(II), nickel(II), copper(II) and zinc(II) perchlorate with varying molality were measured in N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA) and their mixtures by titration Raman spectroscopy at 298 K. The in-plane OC–N bending vibration at 660 cm⁻¹ of DMF and the stretching N–CH₃ vibration at 740 cm⁻¹ of DMA show an appreciable shift to a higher frequency upon coordination of the solvent molecules to the metal ion. These Raman spectra were deconvoluted and the bound solvent bands (ν_bound) to the metal ion were extracted from the free solvent band (ν_free). The number of solvent molecules bound to the metal ion or the individual solvation number n in a solvent mixture, i.e., n_DMF and n_DMA for DMF and DMA, respectively, were evaluated by analyzing the intensity decrease of the free solvent bands with increasing molality of the metal ion. It turned out that the manganese(II) ion is six-coordinated over the whole solvent composition range in the mixtures, i.e., n_DMF + n_DMA = 6, and the relationship n_DMF/n_DMA = x_DMF/x_DMA, where x denotes the mole fraction of the solvent, holds in the mixture. With regard to the zinc(II) ion, the total solvation number decreases from 6 with increasing x_DMA in the mixtures. In neat DMA, two bound solvent bands are extracted, indicating that two species, Zn(DMA)₆²⁺ and Zn(DMA)₄²⁺, coexist in equilibrium. With the six-coordinate zinc(II) solvated ion, the relationship n_DMF/n_DMA > x_DMF/x_DMA holds in the mixture. This shows that the solvation steric effect of DMA operates for the six-coordinate zinc(II) solvated ion, unlike for the manganese(II) one. The copper(II) ion is six-coordinated, although the coordination structure is strongly distorted owing to the Jahn–Teller effect. Indeed, two bands ascribable to the solvents bound at the equatorial and axial positions were extracted. In the mixtures, the relationship n_DMF/n_DMA > x_DMF/x_DMA holds for solvents bound at the equatorial position, i.e., the solvation steric effect operates among solvent molecules, while the relationship n_DMF/n_DMA ⩽ x_DMF/x_DMA holds for solvents bound at the axial position. This is expected because the Cu–O(solvent) bond length is longer for the solvent at the axial position, and the electron-pair donating ability is slightly stronger for DMA. The magnitude of the shift Δν (=ν_bound − ν_free) depends strongly on the metal ion. Indeed, the Δν values in neat DMF and DMA vary according to the relationship Δν = (ar)⁻ⁿ, a function of the ionic radius r of the metal ion, and the parameters a and n were evaluated. The shift Δν in the mixtures implies that the M–O(DMF) bond length in the M(DMF)_6−n(DMA)_n²⁺ solvated ion is elongated with increasing coordination number n_DMA of DMA.