Isostructural Series of Nine-Coordinate Chiral Lanthanide Complexes Based on Triazacyclononane

Nonadentate ligands based on triazacyclononane incorporating
pyridyl-2-phosphinate
groups form an isostructural series of complexes with Ln ions in the
solid state and in solution. The Ln ion is effectively shielded from
the solvent environment. Crystal structures reveal a rigid C3-symmetric tricapped trigonal-prismatic coordination
geometry that is maintained in solution for the methyl and phenylphosphinate
series, as shown by multinuclear NMR analysis. Variable-temperature
measurements of the field dependence of the water proton relaxivity
in gadolinium complexes indicate that these systems exclude solvent
from the primary coordination environment and minimize the second
sphere of solvation. The electronic relaxation time for the gadolinium
methylphosphinate complex has been estimated to be 550 (±150)
ps by EPR and NMR methods, compared to values of around 0.30–0.05
ps for the terbium–ytterbium series, deduced by analyzing the
field dependence (4.7–16.5 T) of the 31P NMR longitudinal
relaxation times. Values are compared with analogous azacarboxylate
ligand complexes, supporting a key role for donor atom polarizability
in determining the electronic relaxation. Spectral emission behavior
in solution of samarium, europium, terbium, and dysprosium complexes
is compared, and the resolved RRR-Λ and SSS-Δ complexes show strong circularly polarized luminescence.
The molecular quadratic hyperpolarizability ⟨βHLS⟩ has been measured in solution using hyper-Raleigh light-scattering
methods, for the whole series of lanthanide complexes of one ligand.
The values of ⟨βHLS⟩ reach a maximum
around the center of the series and are not simply dependent on the
number of f electrons, suggesting a dominant contribution from the
octupolar rather than the dipolar term.