Structural and Photophysical Properties of Lathanide(III) Complexes with a Novel Octadentate Iminophenolate Bibracchial Lariat Ether

We report here a structural and photophysical study of lanthanide complexes with the di-deprotonated form of the
bibracchial lariat ether N,N‘-bis(2-salicylaldiminobenzyl)-4,10-diaza-12-crown-4 (L3) (Ln = Ho(III)−Lu(III)). The X-ray
crystal structures of [Ho(L3−2H)](ClO4) (1) and [Er(L3−2H)](ClO4) (2) show the metal ion being eight-coordinate
and deeply buried in the cavity of the dianionic receptor. Both sidearms of L3 are on the same side of the crown
moiety, resulting in a syn conformation. Likewise, the lone pair of both pivotal nitrogen atoms is directed inward of
the receptor cavity in an endo−endo arrangement and the coordination polyhedron around the lanthanide ion may
be described as a distorted square antiprism that shows a deformation toward a square prism by ca. 11°. Attempts
to isolate complexes of the lightest members of the lanthanide series were unsuccessful, which suggests a certain
degree of selectivity of L3 toward the heaviest Ln(III) ions. This was evaluated and rationalized on the basis of
theoretical calculations performed in vacuo at the HF level, by using the 3-21G* basis set for the ligand atoms and
a 46+4fn effective core potential for lanthanides. For the [Ln(L3−2H)]+ systems, the calculated bond distances
between the metal ion and the coordinated donor atoms decrease along the lanthanide series, as usually observed
for Ln(III) complexes. However, for the related [Ln(L1−2H)]+ and [Ln(L2−2H)]+ systems our ab initio calculations
provide geometries in which some of the bond distances of the metal coordination environment increase across
the lanthanide series. Thus, thanks to the variation of the ionic radii of the lanthanide ions, receptors L1 (N,N‘-bis(2-salicylaldiminobenzyl)-4,13-diaza-18-crown-6) and L2 (N,N‘-bis(2-salicylaldiminobenzyl)-1,10-diaza-15-crown-5) are specially adapted for the complexation of the lighter lanthanide ions. On the other hand, the erbium and
ytterbium complexes of L3 have been shown to be emissive in the near-IR. Time-resolved studies of complexes
confirm that solvent is excluded from the inner coordination sphere in solution. The luminescence properties of the
complexes make them ideally suited for use as luminescent tags and suggest that q = 0 complexes of erbium
may, after all, be useful as luminescent tags in protic media.