Cerium(IV) Pyrasal Complexes: A pH-Dependent 8-to 10-Coordinate Cerium Chelate Switch

In this work, five cerium-(IV) complexes were synthesized, three of which were structural isomorphs from the same pyrasal ligand with the solid-state result identified by structural analysis dependent on the initial pH of the reaction solution and the temperature at which the reaction is performed. The ligands explored here are pyrasal ligands, which are Schiff-base ligands formed by the condensation of 2,3-diaminopyrazine and a salicylaldehyde derivative. Pyrasal ligands have weaker binding than other salophen-type ligands due to the electron-withdrawing effect of the nitrogen atoms contained within the pyrazine ring. The weaker binding leaves the ligand more susceptible to the changes in pH and temperature that alternate the chelating environment from 8- to 10-coordinate. This electron-withdrawing effect of the pyrazine backbone also deactivates the second amine after the first condensation addition of salicylaldehyde. Without a metal to template the complex formation reaction, even with extended reaction times and the addition of a large excess of ligand, the result is the addition of only one salicylaldehyde.

Automated summary

In this work, five cerium-(IV) complexes were synthesized, three of which were structural isomorphs from the same pyrasal ligand. The solid-state result was identified through structural analysis, which depended on the initial pH of the reaction solution and the temperature. The pyrasal ligands used in this study have weaker binding due to the electron-withdrawing effect of the nitrogen atoms within the pyrazine ring. The electron-withdrawing effect also deactivates the second amine after the first condensation with salicylaldehyde. Even with extended reaction times and excess ligand, without a metal template, only one salicylaldehyde is added.