Bulky Solvent Research Articles

Solution equilibria of zinc(II) and cadmium(II) complexes with 2,2?-bipyridine in N,N-dimethylacetamide at 25�C

Complexation of zinc(II) and cadmium(II) ions with 2,2′-bipyridine (bpy) are studied in N,N-dimethylacetamide (DMA) by calorimetry. Formation constants, enthalpies, and entropies of five mononuclear complexes, [Zn(bpy)n]2+ (n=1–3) and [Cd(bpy)n]2+ (n=1,2), are determined, and compared with the corresponding values in an analogous but less bulky solvent, N,N-dimethylformamide (DMF). The zinc complexes are more stable and the formation is more exothermic in DMA than in DMF, whereas the solvent effect on the cadmium complexes are rather small. A largely positive value of the enthalpy of transfer of Zn2+ from DMF to DMA shows that the greater stability of the zinc complexes in DMA is due to the weaker solvation of the metal ion, which is caused by the steric hindrance of DMA molecules. The transfer enthalpies become smaller in the order Zn2+>[Zn(bpy)]2+>[Zn(bpy)2]2+>[Zn(bpy)3]2+ and dictate gradual relaxation of the steric effect in the complexes. On the other hand, the transfer enthalpies of Cd2+ and its complexes are all small, indicating that the hindrance is insignificant in the vicinity of this larger cation.

The influence of solvents on the basicity of dipolar amines

AbstractThe relative basicities of six monosubstituted dipolar trimethylamines (XCH2NMe2) were determined in nine different solvents from potentiometric titration. Solvation effects on basicity variations were analyzed by the solvatochromic parameters (π*, α and β). Non‐specific solvation of the substituted dimethylammonium ions is not reflected adequately by the solvent's dipolarity–polarizability constant, π*. A better representation of non‐specific interaction between solvents and the monosubstituted dipolar trimethylammonium ions is gained from the product of π* and the solvent dipole moment (μ). For these ions where the dipolar substituents are separated from the reaction center by one methylene unit, there is an interaction between the charge of the ammonium ion and the dipolar substituent. As a result, solvent molecules must not only solvate the dipolar substituents, but must also gain access between the lines of force of the dipolar substituent and the charge. Bulky solvents, such as nitrobenzene, cannot access adequately the region between the dipolar substituents and the reaction center to accomplish similar solvation as less bulky solvents.