Synthesis and kinetics of photoisomerization processes of water soluble Pd(II) and Pt(II) compounds

Abstract

This study focuses on the synthesis and characterization of water-soluble square-planar Pd(II) and Pt(II) complexes containing azobenzene ligands, specifically 4-phenylazopyridine (Phpy) and 4-aminoazopyridine (Ampy), with the aim of studying the cis-trans isomerization mechanism of the azo ligand coordinated to the metal center. Once the indicated azo derivatives were synthesized and characterized, the corresponding Pd(II) derivatives [Pd(TMEDA)(Ampy)2](NO3)2 and [Pd(TMEDA)(Phpy)2](NO3)2 were obtained through substitution reactions. In the case of [Pd(TMEDA)(Ampy)2](NO3)2, an equilibrium is observed when dissolved in water, whereas for [Pd(TMEDA)(Phpy)2](NO3)2, exhibits this phenomenon in water and methanol. To prevent the observed de-coordination, Pt(II), a more inert metal center than Pd(II), was employed. However, when the square-planar complex [Pt(TMEDA)(NO3)2] was reacted with the Ampy ligand under the same conditions used for Pd(II), the desired Pt(II) diazo complex could not be obtained. Despite the de-coordination equilibrium, the kinetics of the cis-trans isomerization of the diazo complex [Pd(TMEDA)(Phpy)2](NO3)2 and the corresponding Phpy ligand were studied by UV/Vis spectroscopy and the apparent rate constants as well as the activation parameters ΔH‡ and ΔS‡ were determined. Unfortunately, for the [Pd(TMEDA)(Ampy)2](NO3)2 the cis-trans isomerization process resulted so fast that it could not be monitored with the equipment available in the laboratory