Basicity of water in organic solvents (nitromethane, propane-1,2-diyl carbonate and acetone) using the 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecanenickel(II) cation as a probe of electron-pair acceptance

Abstract

The basicity of water in nitromethane (NM), propane-1,2-diyl carbonate (PDC) and acetone (AC) has been examined using spectral data for (1R,4S,8R,11S)-1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane-nickel(II){[Ni(TMC)]²⁺}. In these solvents water monomers are in equilibrium with their dimers: the water monomers are only weakly coordinated to the chelate in these solvents, whereas the dimers are more strongly coordinated. The equilibrium constant K_Ni,w=[Ni(TMC){(H₂O)₂}²⁺]/[Ni(TMC)²⁺][(H₂O)₂] where (H₂O)₂ is the water dimer, is 1.0 in NM, 1.8 in PDC and 5.0 in AC, showing that K_Ni,w increases with increasing electron-pair donicity of the bulk solvent molecules in the order NM < PDC < AC. These results emphasize that the basicity of water is strengthened by the polarization of O—H bonds due to dimerization or charge transfer through hydrogen bonding. The basicity of the dimer in each solvent has been evaluated using the relationship between the ¹H chemical shift of water and the donor number of the aprotic solvent.