Homochiral, Helical Coordination Complexes of Lanthanides(III) and Mixed-Metal Lanthanides(III): Impact of the 1,8-Naphthalimide Supramolecular Tecton on Structure, Magnetic Properties, and Luminescence

The reactions of the lithium salt
of (S)-2-(1,8-naphthal­imido)-3-hydroxy­propanoate
(Lser–), an
enantio­pure carboxylate ligand containing a 1,8-naphthal­imide
π···π stacking supra­molecular tecton
and an alcohol functional group, with La­(NO3)3, Ce­(NO3)3, SmCl3, Eu­(NO3)3, Gd­(NO3)3, Tb­(NO3)3, and Dy­(NO3)3 under solvo­thermal
conditions (water/​ethanol) produced single crystals (characterized
by single-crystal X-ray crystallography) of [La3­(Lser)8­(OH)­(H2O)]·​(H2O,​EtOH)x (1), [Ce3­(Lser)8­(OH)­(H2O)]·​(H2O,​EtOH)x (2), [Sm3­(Lser)8­(OEt)]·​(H2O,​EtOH)x (3), [Eu3­(Lser)8­(OEt)]·​(H2O,​EtOH)x (4), [Gd3­(Lser)8­(OEt)]·​(H2O,​EtOH)x (5), [Tb3­(Lser)8­(OEt)]·​(H2O,​EtOH)x (6), and [Dy3­(Lser)8­(OEt)]·​(H2O,​EtOH)x (7), respectively. Mixed-metal complexes [Ce2.3Tb0.7­(Lser)8(OH)]·​(H2O,​EtOH)x (8), [Gd0.4Tb2.6­(Lser)8­(OEt)]·​(H2O,​EtOH)x (9), and
[Ce1.4Gd0.3Tb1.3­(Lser)8(OH)]·​(H2O,​EtOH)x (10) were prepared by using two or more types of lanthanides in the
solvo­thermal reactions (additional mixed-metal complexes were
prepared and characterized by ICP-MS). Single crystals of compounds 1–10 are isostructural: trinuclear, carboxylate-bonded
helicates organized by the non-covalent, π···π
stacking interactions of the 1,8-naphthal­imide groups into inter­twined M helices, with a pitch of 56 Å,
that are further arranged into a three-dimensional supra­molecular
framework by additional π···π stacking
interactions. Magnetic measurements of several compounds were as expected
for the metal(s) present, indicating no significant interactions between
metals within the helicates. The Ce complex 2 showed
weak anti­ferro­magnetic ordering below 50 K. All of the
complexes, with the exception of 2, showed luminescence
based on the 1,8-naphthal­imide group. Complex 2 has no emission, and complexes with mixed Ce/Tb ratios showed significant
quenching of the naphthal­imide-based luminescence, as quantitated
with solid-state, absolute quantum yield measurements of these mixed-metal
and the pure metal complexes. Lanthanide-based luminescence was only
observed for the Eu complex 4.