Dimerisation of dimethyl sulphoxide in dipolar aprotic solvents using R,S,R,S-1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclo-tetradecanenickel(II) as a probe

Abstract

The dimerisation constants [K_dim in (mole fraction)^–1] of dimethyl sulphoxide (DMSO) in binary mixtures with the four diluents nitrobenzene (NB), propane-1,2-diol carbonate (PDC), N,N-dimethylacetamide (DMA) and pyridine (Py) have been determined using spectral changes due to the coordination of DMSO to the R,S,R,S-1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecanenickel(II). It was found that the fraction of the coordinated species of DMSO in NB and Py becomes maximum at ca. 0.5 DMSO mole fraction and decreases with increasing DMSO mole fraction, unlike other coordinating solvents such as acetonitrile (AN) and N,N-dimethylformamide (DMF). This is attributed to the self-association of DMSO. The K_dim values obtained were 14 in Py, 2.8 in NB, 1.9 in DMA and 0.6 in PDC. PDC tends to break up the self-association of DMSO because of the stronger dipole–dipole interactions between DMSO and PDC.