Synthesis and Characterization of Nitrogen-Rich Macrocyclic Ligands and an Investigation of Their Coordination Chemistry with Lanthanum(III)

Derivatives
of the ligand 1,4,7,10-tetraazacyclododecane (cyclen)
containing pendant N-heterocyclic donors were prepared. The heterocycles
pyridine, pyridazine, pyrimidine, and pyrazine were conjugated to
cyclen to give 1,4,7,10-tetrakis­(pyridin-2-ylmethyl)-1,4,7,10-tetraazacyclododecane (Lpy), 1,4,7,10-tetrakis­(3-pyridazylmethyl)-1,4,7,10-tetraazacyclododecane
(Lpyd), 1,4,7,10-tetrakis­(4-pyrimidylmethyl)-1,4,7,10-tetraazacyclododecane
(Lpyr), and 1,4,7,10-tetrakis­(2-pyrazinylmethyl)-1,4,7,10-tetraazacyclododecane
(Lpz), respectively. The coordination chemistry of these
ligands was explored using the La3+ ion. Accordingly, complexes
of the general formula [La­(L)­(OTf)]­(OTf)2, where OTf =
trifluoromethanesulfonate and L = Lpy (1),
Lpyd (2), Lpyr (3),
and Lpz (4), were synthesized and characterized
by NMR spectroscopy. Crystal structures of 1 and 2 were also determined by X-ray diffraction studies, which
revealed 9-coordinate capped, twisted square-antiprismatic coordination
geometries for the central La3+ ion. The conformational
dynamics of 1–4 in solution were
investigated by variable-temperature NMR spectroscopy. Dynamic line-shape
and Eyring analyses enabled the determination of the activation parameters
for the interconversion of enantiomeric forms of the complexes. Unexpectedly,
the different pendant N-heterocycles of 1–4 give rise to varying values for the enthalpies and entropies
of activation for this process. Density functional theory calculations
were carried out to investigate the mechanism of this enantiomeric
interconversion. Computed activation parameters were consistent with
those experimentally determined for 1 but differed somewhat
from those of 2–4.