Inorganic Chemistry
Article
induce the formation of s−f tetranuclear dimetallic triple-
ing by cations is possible. This templating depends on the “size
selectivity” between the template and the cavity of the helicate
(“lock-and-key principle”). Interestingly, for each cavity, the
distance from the center to the six ligand oxygen atoms is
about 3.0 Å, which is equal to the coordinate bond length of
K−O. Consequently, the cavity exhibits high selectivity and
affinity for potassium ions (Figure 1a).
stranded helicates and tetranuclear quadruple-stranded lantha-
+
nide helicates, respectively. More interestingly, the K and CO
2
templates successfully induced the circular helicates to
transform into either an s−f heterotetrametallic triple-stranded
helicate or a tetranuclear quadruple-stranded lanthanide
helicate, respectively, just like the intelligent molecular
transformers, which can be assembled into different config-
urations under different conditions, and can be reconstructed
under different external stimuli.
7
Complex 3 was prepared from ligand H L′ and cerium
2
chloride in a 1:1 ratio (Figure 2a), whereas compounds 4a and
4b were obtained from the ligand H L, KOH, and lanthanide
2
halide salts (1:2:1). KOH was added as a base and a template
RESULTS AND DISCUSSION
K -Induced Lanthanide Helicate Supermolecule Iden-
(
■
+
+
K
studies revealed that the crystals 4a and 4b have isostructural
relationships and belong to the monoclinic space group P2 /c.
tification Self-Assembly. A linear tetradentate ligand H L′,
2
is comprised of two binding strands anchored to a flexible
1
spacer (Scheme 1). This structure adopts a linear con-
To the best of our knowledge, complexes 4a and 4b are the
first examples of s−f tetranuclear dimetallic helicates.
3
+
The skeletal structure of complex 4a consists of two La
Scheme 1. Syntheses of H L′ and H L
2
2
ions, two K+ ions, three deprotonated ligands, and two
2
+
coordinated water molecules, forming [K La L (H O) ] .
2
2
3
2
2
−
the four metals and form the conformation of a tetranuclear
dimetallic triple-stranded helicate. The metal ions are
approximately aligned [∠K1···La1···La2, 167.250(4)°;
La2···La1···K1, 176.270(4)°], both K1···La1 and K2···La2
are regularly spaced by approximately 3.77 Å [K1···La1,
.7345(14) Å; K2···La2, 3.8027(15) Å], and the La1···La2
∠
3
+
distance is 7.1150(6) Å. Each K ion is coordinated by seven
coordination atoms, with three phenolic hydroxy and ethoxy
units from three different ligands and one solvent H O
2
formation favoring a dinuclear triple-stranded helicate
complex 3, Figure 1a). Structural analysis of complex 3
revealed that the two holes at both ends of the helicate are
formed by six oxygen atoms, respectively. This helicate
(
3
+
ment (Figure S7). In addition, each La ion is coordinated by
nine coordination atoms, arising from the coordination of
three tridentate salicylaldehyde−hydrazone domains from
three different ligands (La−N, 2.729−2.808 Å; La−O,
2
.381−2.613 Å), consequently three nitrogen atoms together
with six oxygen atoms establish the coordination geometry of
La , which can be best described as a distorted monocapped
square antiprism. Confirmation of the roles of K ions in the
3
+
+
self-assembly process is demonstrated by its inability to form
analogous helicate structures with other alkali- and alkaline-
earth-metal ions. As designed, the restricted geometric
constraints of the cavities provide high selectivity of the helical
+
+
+
recognizers toward K ions over other ions such as Li , Na ,
+
2+
2+
2+
2+
Rb , Mg , Ca , Sr , and Ba .
Additional experiments under identical conditions were
conducted to further investigate the ability of these lanthanide-
+
based helical complexes to selectivity bind K over other
alkaline and alkaline-earth metals. These methods included
UV−vis absorption, high-resolution electrospray ionization
1
mass spectrometry (HR-ESI-MS), and H NMR.
The UV−vis absorption spectrum of compound [L La ] was
3
2
recorded in an acetonitrile solution. Upon the addition of 0−
+
4
.0 equiv of K to the compound, the absorbance at 301 nm
slightly decreased while the absorption at 390 nm gradually
+
increased, suggesting the coordination of K with this
compound (Figures 1b and S13). Absorption titrations of
were conducted to examine the selectivity (Figure S14).
Changes in the spectra were not observed in the presence of
Figure 1. (a) Schematic representation of the lanthanide helicate
+
clusters for the recognition and fixation of K . (b) UV−vis spectral
+
changes for the compound [L La ] with K (0−4.0 equiv) in
3
2
−
5
+
acetonitrile. [L La ] = 0.67 × 10 M; [K ] = 0, 0.02, 0.04, 0.06, 0.08,
3
2
+
+
+
2+
2+
2+
2+
0
3
.10, 0.12, 0.14, 0.16, 0.18, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0,
.0, and 4.0 equiv.
other cations such as Li , Na , Rb , Mg , Ca , Sr , or Ba ,
+
indicating selective recognition in the presence of K . The
2
765
Inorg. Chem. 2021, 60, 2764−2770