Structure of solvent clusters and its effect on metal-ion reactivity in non-aqueous solvent mixtures

点击翻译按钮获取中文摘要

Solvation of the metal-ion and its complexation in some nonaqueous solvents and their mixtures have been studied by means of titration spectrophotometry, catorimetry and Roman spectrometry, all these are on-line controlled for the titration procedure and data acquisition. In the present project, we investigated solvation steric effect (SSE) in the initial and transition states of the metal-ion complexation in nonaqueous solvent mixtures. Solvents used are tetramethylurea (TMU), N,N-dimethylformanmide (DMF), N,N-dimethylacetamide (DMA), N,N-dimethylpropionamide (DMPA), etc. TMU and DMPA show a strong SSE, i.e., the solvation number of the metal ion is reduced, while DMF and DMA show a weak SSE, i.e., the solvation number is kept the same as that in water. It is found that DMPA has two conformational isomers. The two isomers coexist in equilibrium, and the equilibrium constant, and the corresponding enthalpy and entropy values were obtained. The conformational equilibrium largely changes when the solvent molecules simultaneously coordinate to the metal ion, which is ascribed to the conformational change arising from the strong solvation steric effect. This leads to a remarkable difference in the complexation behavior of the manganese(II) ion in TMU and DMPA, despite that the metal ion is five-solvated in both the solvents. The solvation energy of an activated species formed at the transition state of metal-ion complexation is also investigated from the view-point of solvation steric effect. The solvation enthalpy difference of an activated species is defined as the enthalpy of transfer of the activated species from solvent A to B, which is evaluated from the enthalpies of activated measured in the solvents and the enthalpy of transfer of reactants at the initial state from solvent A to B. We found that the solvation steric effect operates significantly at the transition state.