Solvation and halogeno complexation of the manganese(II) ion in N-methyl-2-pyrolidone

Chloro and bromo complexation of the manganese(II) ion has been studied in N-methyl-2-pyrolidone (NMP) by titration calorimetry at 298 K. It is found that the manganese(II) ion forms a series of mononuclear MnCln(2−n)+ (n=1-4) and MnBrn(2−n)+ (n=1-3) complexes in NMP, and their formation constants, enthalpies and entropies have been obtained. The complexation in NMP is significantly weaker than that in N,N-dimethylpropionamide (DMPA), while it is almost to the same extent as that in N,N-dimethylacetamide (DMA), despite that these solvents coordinate to the metal ion through the carbonyl oxygen atom, and their electron-pair donating abilities are similar. This indicates that steric interaction among solvent molecules coordinated to the metal ion is significantly weaker in NMP than that in DMPA, while it is almost to the same extent in NMP and DMA. The manganese(II) ion is five-solvated in DMPA, and six-solvated in DMA. The complexation behavior in NMP implies that the manganese(II) ion is six-solvated in NMP, like in DMA. As NMP is a planar trans analogue of DMPA with respect to the propionyl group CH3CH2(CO)-, the magnitude of solvation steric effect around the metal ion for the planar trans DMPA may be similar to that for DMA. It is thus concluded that the local conformation, particularly around the coordinating carbonyl oxygen atom, plays a decisive role in the solvation steric effect of the metal ion in aprotic amide solvents.