Thermochromism of Metal Chelates with Triphenylmethane Complexons in Aqueous Solutions. I. Copper(II)–Xylenol Orange System

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Abstract
Reversible thermochromisms are found for about thirty kinds of metal complexes of triphenylmethane complexons in aqueous solutions. The thermochromism observed in the aqueous solution of copper(II) complex with 3,3′-bis[N,N-bis(carboxymethyl)aminomethyl]-o-cresolsulfonphthalein (Xylenol Orange, XO) was 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\ightleftharpoonsA+H. The equilibrium constant K of the reaction was determined to be p(K/mol dm−3)=4.40 at 25 °C. The thermodynamic parameters ΔH and ΔS for the reaction were calculated to be 26±2 kJ mol−1 and 1±3 J mol−1 K−1 (298 K), respectively. The rate constant of the proton dissociation of the phenolic hydroxyl group in the complex AH was estimated to be ca. 2±105s−1.