B. OXOPHILICITY
Most work on lanthanide chelation indicates a strong prefer-
ence for negatively charged or neutral donor groups with large
dipole moments ( 5 ). In particular, a significant oxophilic ten-
dency has been confirmed by the finding that rarely few lantha-
nide complexes with monodentate nitrogen donors exist.
However, evidence from complexes with aza-crown ligands
suggests that neutral nitrogen donors may be slightly preferred
over neutral oxygen donors ( 167 ).
Our work with dipicolinate derivatives also indicates that nitro-
gen donors can be preferred over oxygen donors in certain cases.
To better understand the binding behavior of dipicolinate, we
explored the coordination geometries of various DPA analogues
with Tb^3 þ. Three structural isomers were examined: pyridine-
2,4-dicarboxylic acid (2,4-DPA), pyridine-3,5-dicarboxylic acid
(3,5-DPA), and dipicolinate itself (pyridine-2,6-dicarboxylic acid,
DPA). Picolinic acid (Pic) and Pyr, which have one and both car-
boxyl arms eliminated, respectively, were also included. As DPA
usually coordinates in a tridentate fashion with the two
FIG. 12 Crystal structures of Tb(EDTA) NaOH 2H 2 O and TBA Tb
(DO2A)(DPA), showing trigonal and linear coordination sites, respec-
tively, available for dipicolinate binding (dark gray). Thermal ellipsoids
are at 50% probability; hydrogens and extraneous atoms omitted for
clarity. Tb(EDTA) NaOH 2H 2 O crystallized from deionized water
(18.2 MO-cm resistivity) at 4C.
LUMINESCENT LANTHANIDE SENSORS 31