Lanthanide Podates with Predetermined Structural and Photophysical Properties: Strongly Luminescent Self-Assembled Heterodinuclear d−f Complexes with a Segmental Ligand Containing Heterocyclic Imines and Carboxamide Binding Units

The segmental ligand
2-{6-[N,N-diethylcarbamoyl]pyridin-2-yl}-1,1‘-dimethyl-5,5‘-methylene-2‘-(5-methylpyridin-2-yl)bis[1H-benzimidazole]
(L2) reacts with stoichiometric amounts of Ln(III) (Ln
= La−Nd, Sm−Tb, Tm−Lu, Y) and Zn(II) in acetonitrile to yield quantitatively
and selectively the heterodinuclear triple-helical
complexes [LnZn(L2)3]5+
under thermodynamic control. The crystal structure of
[EuZn(L2)3](ClO4)(CF3SO3)4(CH3CN)4 (13;
EuZnC111H111N25O19F12S4Cl,
monoclinic, C2/c, Z = 8) shows the
wrapping of the three ligands L2 about
a pseudo-C3 axis passing through the metal ions.
Zn(II) occupies the distorted pseudooctahedral capping
coordination
site defined by the three bidentate binding units while Eu(III)
lies in the resulting “facial” pseudotricapped
trigonal
prismatic site produced by the three remaining tridentate units as
exemplified by luminescence measurements using
the Eu(III) structural probe. The separation of contact and
pseudocontact contributions to the 1H-NMR
paramagnetic
shifts of the axial complexes
[LnZn(L2)3]5+ (Ln = Ce,
Pr, Nd, Sm, Eu, Tm, Yb) establishes that the triple
helical
structure is maintained in solution. Photophysical measurements
and quantum yields in acetonitrile indicate that the
terminal N,N-diethylcarbamoyl group in
L2 favors efficient intramolecular L2 →
Eu(III) energy transfers leading to
strong Eu-centered red luminescence. Improved resistance toward
hydrolysis also results from the use of carboxamide
groups, and no change in luminescence is observed for
[EuZn(L2)3]5+ in moist
acetonitrile. The preparation of the
segmental ligand L2 from the new asymmetric synthon
6-(N,N-diethylcarbamoyl)pyridine-2-carboxylic
acid is described
together with its crystal and molecular structure
(C33H33N7O, monoclinic,
P21/c, Z = 4). The
use of 3d metal ions
as a noncovalent tripodal spacer for lanthanide podates is discussed
together with the crucial role played by carboxamide
groups for the control of structural, electronic, and photophysical
properties.