Analytical Sciences

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

Golam M. KHAN and Hisanori IMURA

Department of Environmental Sciences, Faculty of Science, Ibaraki University, Mito 310-8512, Japan

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 differenttemperatures ranging from 288 to 308 K. The IR spectra show clear evidence of hydrogen bonding between the chelatesand the chlorinated phenols. From an equilibrium analysis, hydrogen-bonded complexes were found to be MB₃nHA,where MB₃ and HA stand for the metal chelate and phenol, respectively, and n=1 - 3. The overall formation constants, beta_n(beta_n=[MB₃nHA]/[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 deltaH⁰ and deltaS⁰ values obtained suggest the formation of a very strong type ofhydrogen-bonded complexes, and that the thermodynamics of hydrogen-bond formation is dominated by deltaH⁰.Considering the formation constants and the thermodynamic parameters, Co(q)₃ has a hydrogen-bonding ability strongerthan that of Co(acac)₃.

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