Understanding the Coordination Chemistry and Structural and Photophysical Properties of Eu<sup>II</sup>- and Sm<sup>II</sup>-Containing Complexes of Hexamethylhexacyclen and Noncyclic Tetradentate Amines
Sergely Steephen Bokouende, Cassandra L. Ward, Matthew J. Allen
Abstract
Ligands play a crucial role in supporting or stabilizing the divalent oxidation state of lanthanide metals. To expand the range of ligands used to chelate divalent lanthanide ions, we synthesized and studied the structural and photophysical properties of complexes of Eu II and Sm II with hexamethylhexacyclen, 1,1,4,7,10,10-hexamethyltriethylenetetramine, tris[2-(dimethylamino)ethyl]amine, and tris[2-(isopropylamino)ethyl]amine as supporting ligands. Coordination of hexamethylhexacyclen, an analogue of 18-crown-6, generates sterically crowded complexes of Eu II and Sm II that are either seven or eight coordinate and adopt a range of geometries that differ from those of their 18-crown-6 counterparts and from those of lanthanide-containing complexes with the acyclic tetradente tertiary amine ligands included in this report. The emission spectra of Eu II (hexamethylhexacyclen) show a moderate sensitivity to counterion identity and are more red-shifted compared to those of complexes of Eu II with 18-crown-6 and the hexamethylated aza derivative of 2.2.2-cryptand. In addition, the morphology of hexamethylhexacyclen in [LnI(hexamethylhexacyclen)]I was found to resemble that of thermally stable alkalides of the form [M(hexamethylhexacyclen)]Na – (M = K + or Cs + ), suggesting that hexamethylhexacyclen could be an interesting ligand for strongly reducing lanthanide ions.