Thermochromism of metal chelates with triphenylmethane complexons in aqueous solutions. III. Copper(II)-methylthymol blue and -methylxylenol blue systems.

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Reversible thermochromisms of copper(II) complexes with 3,3′-bis[bis(carboxymethyl)aminomethyl]thymol-sulfonphthalein (Methylthymol Blue; MTB) and 3,3′-bis[bis(carboxymethyl)aminomethyl]-p-xylenolsulfonphthalein (Methylxylenol Blue; MXB) were studied in aqueous solutions. The thermochromisms of these complexes were primarily ascribed to the temperature-dependence of the protolytic equilibria between a complex species AH having an uncoordinated free phenolic hydroxyl group and a complex species A having a coordinated phenolate group: AH\rightleftarrowsA+H. The equilibrium constants K of the reactions at 25 °C were determined to be p(K/mol dm−3)=4.66 for Cu(II)–MTB and 4.78 for Cu(II)–MXB. The thermodynamic parameters ΔH and ΔS for the reactions were calculated to be 26±2 kJ mol−1 and −6±3 J K−1 mol−1 (298 K) for both complexes, respectively. The rate constants of the proton dissociation of the phenolic hydroxyl group in the complexes AH were estimated to be 1.6×105 and 1.8×105 s−1 for Cu(II)–MTB and Cu(II)–MXB, respectively. The thermodynamic and kinetic data are discussed in terms of the effects of the substituents in the ligand molecules on the thermochromism.
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