Thermodynamic Study of the Hydrogen-Bonding Ability of Tris(pentane-2,4-dionato)cobalt(III) and Tris(8-quinolinolato)-cobalt(III) with Chlorinated Phenols as Proton Donor

The hydrogen-bond formation of tris(pentane-2,4-dionato)cobalt(III) (Co(acac)3) and tris(8-quinolinolato)cobalt(III) (Co(q)3) with a series of chlorinated phenols has been studied by IR in chloroform and carbon tetrachloride at different temperatures ranging from 288 to 308 K. The IR spectra show clear evidence of hydrogen bonding between the chelates and the chlorinated phenols. From an equilibrium analysis, hydrogen-bonded complexes were found to be MB3•nHA, where MB3 and HA stand for the metal chelate and phenol, respectively, and n=1–3. The overall formation constants, βn (βn=[MB3•nHA]/[MB3][HA]n), were determined and considered in relation to acid-dissociation constants of the phenols. From the temperature dependence of the formation constants, the thermodynamic parameters were determined. Additionally, the validity of the obtained enthalpy was ascertained by calorimetric titration in the Co(q)3-3,5-dichlorophenol system. The negative D H0 and D S0 values obtained suggest the formation of a very strong type of hydrogen-bonded complexes, and that the thermodynamics of hydrogen-bond formation is dominated by D H0. Considering the formation constants and the thermodynamic parameters, Co(q)3 has a hydrogen-bonding ability stronger than that of Co(acac)3.