would reduce unfavorable steric interactions by moving the second
carboxylate group of these species away from the ancillary ligand.
For this series of spectra, the intensity decreases in the order 2,6-
DPA>2,4-DPA>3,5-DPA, which follows with a decrease in elec-
tron density at the chelating carboxylate as the functional groups
are redistributed evenly about the Pyr ring.
All attempts at crystallization of these species were unsuccess-
ful; the only reported Ln^3 þ crystal structures with these
dipicolinate derivatives are of polymeric species obtained under
hydrothermal conditions( 168 ), which cannot be directly related
to our solution results. Although more thorough analyses will be
required for the construction of accurate binding models, we have
established that the Pyr nitrogen in dipicolinate and related che-
lators can play an important part in dictating both lanthanide
FIG. 15 Structures of pyridine, picolinate, and three structural
isomers of dipicolinate, overlaid with an electron density map of the
highest occupied molecular orbital (HOMO) for each ligand. These
chromophores were explored to better understand the binding pro-
perties of DPA. Electron density maps generated using TitanÒ; higher
electron density is in black, lower in white.
34 MORGAN L. CABLEet al.