Reversible Structural Transformation and Catalytic Potential of Lanthanide-Azobenzenetetracarboxylates

Abstract

Inspired by the catalytic potential of lanthanide coordination polymers of 3,3′,5,5′-azobenzenetetracarboxylic acid (H4abtc), two new isostructural [Ln2III(Habtc)2(DMSO)4]·DMSO·H2O (LnIII= SmIII(I), EuIII= (II), DMSO = dimethyl sulfoxide) were synthesized and characterized. Their single-crystal structures were elucidated and described. Structural transformations of II in the solid state prompted by ligand substitution and thermal treatment were studied, from which genuine reversible transformation of II to [EuIII(Habtc)(H2O)4]·3H2O (II′) and [EuIII(Habtc)(H2O)2]·2H2O (II″) was revealed. This illustrates the rare case of reversible transformation in lanthanide coordination polymers. The transformation between II′ and II″ was also investigated. Structural transformations among these frameworks are discussed with regard to the coordination environment of EuIII, coordination modes of Habtc3-, and similarities and disparities in framework architecture and registration. In addition, the catalytic performance of II with and without the prior activation in CO2cycloaddition reaction with epichlorohydrin was studied in comparison with II′ and II″. The excellent performance of II disregarding the activation process has been demonstrated with the maximum turnover number and turnover frequency of 7682 and 1921 h-1, respectively, for the activated II and 7142 and 1786 h-1, respectively, for the nonactivated II. The maintenance of the catalytic efficiency over 10 cycles of the catalysis and the regeneration process is illustrated and discussed with respect to structural transformation.