Second sphere coordination of phenols, anilines, and benzoic acids to the hexacyanocobaltate(III) anion

Abstract

Complexing between the hexacyanocobaltate(III) anion and ten substituted phenols, eight substituted anilines, and eight substituted benzoic acids has been studied in dimethyl sulfoxide solutions by means of 59Co nmr. For the phenols substantial changes in the chemical shifts and in the line widths are observed and are attributed to hydrogen bonding interactions. Effective equilibrium constants and chemical shifts for the hydrogen bonded species have been calculated. The equilibrium constants vary with the phenol substituent, being largest for electron withdrawing substituents and smallest for electron donating substituents. The chemical shifts are virtually independent of substituent. The data show that the lifetimes of the hydrogen bonded complexes are long compared to the rotational correlation time. The complex with para-nitrophenol has also been studied by measuring 1H and 13C spin–lattice relaxation times. These measurements allow the separation of the increase in 59Co line width due to increase in correlation time and that due to increase in quadrupole coupling constant. From the 13C measurements a value of the correlation time for the complex of 2.8 10−1 s is obtained together with a 59Co quadrupole coupling constant of 8.34 MHz. The equilibrium constants for complex formation with the anilines are small and only the products of the equilibrium constant and the chemical shift of the hydrogen bonded complex can be derived from the experimental data. The benzoic acids show evidence of dimerization in solution and this again prevents the calculation of absolute equilibrium constants. The data, however, do indicate that the ability to form hydrogen bond complexes does not vary dramatically with the acid strength of the hydrogen donor. With phenols and benzoic acids there is considerable line broadening on complex formation but with anilines only a small line broadening is observed. This is consistent with very short hydrogen bond lifetimes for the aniline complexes.