Inorganic Chemistry
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internal metal ions. The cages are less prone to hydrolytic
degradation, and the vertices of the cages can be decorated in a
flexible fashion with different functional groups.
Scheme 1. Synthesis of the Complexes 1-Fe and 1-Zn
RESULTS AND DISCUSSION
Boronate ester-capped metal complexes of type A (Figure 2)
■
38
Figure 2. General structure of boronate ester-capped clathrochelates
(A) and monocapped pyridyloximato complexes (B).
39,40
complexes classify as “clathrochelates”
because the central
metal ion is completely surrounded by a macrobicyclic ligand
framework.
Boronate-ester capped clathrochelate complexes feature
remarkably inert B−O bonds. On the one hand, as a
consequence, they do not degrade in an aqueous solution,
Figure 3. Molecular structure of 1-Fe (a) and 1-Zn (b) in the crystal,
with views from the side and along the M···B axis. Triflate anions and
hydrogen atoms are not shown for clarity.
41
enabling biological applications. On the other hand, they are
not suited as constitutionally dynamic links in supramolecular
chemistry. However, clathrochelates with additional donor
groups in an apical position can be used as versatile
differences between the lengths of the Fe−N
pyridyl
and the Fe−
oximato
N
bonds, with the former being 0.1 Å shorter (1.89 vs
4
2,43
metalloligands.
1.99 Å). Another noteworthy difference between 1-Fe and
1-Zn is the coordination geometry of the metal ion. For 1-Zn,
one can observe a distorted trigonal prismatic geometry with
Complexes of type A are obtained by a metal-templated
condensation reaction of a 1,2-dioxime with a boronic
3
9,40
47
acid.
Using a pyridyloxime instead of a dioxime leads to
an average twist angle of φav = 14°. The coordination
44,45
II
monocapped, cationic complexes of type B (Figure 2).
geometry of the Fe complex, however, is better described as
The reported synthetic procedures indicate that the mono-
distorted octahedral with an average twist angle of φ = 40°.
av
capped complexes are not prone to hydrolytic degradation
The different coordination environments of the two complexes
also influences the boronate ester capping groups. Because of
the increased octahedral character of 1-Fe, the boron atom is
closer to the metal center than in 1-Zn (2.98 Å for 1-Fe vs 3.27
Å for 1-Zn).
II
(
e.g., CH Cl solutions of the Ni complex can be washed with
2
2
4
4
water).
We recently described first attempts to integrate compounds
of type B into more complex molecular architectures. In
particular, we have shown that dinuclear complexes with a
helicate-like structure can be obtained when ditopic bispyr-
idyloxime ligands were employed instead of simple pyridylox-
Next, we investigated the lability of 1-Zn by performing
exchange reactions. Solutions of the complex in methanol or
acetonitrile (13 mM) were mixed with 4-tert-butylphenylbor-
onic acid (1 equiv) or 2-acetylpyridine oxime (3 equiv)
(Scheme 2).
After an equilibration time of 15 min at 30 °C, the resulting
mixtures were investigated by mass spectrometry. For the
reactions with the boronic acid, we observed approximately
equally intense peaks for complexes with a phenylboronate
reaction with 2-acetylpyridine oxime, we observed peaks for
complexes containing zero, one, two, or three 2-acetylpyridine
oximato ligands. These experiments indicate that ligand
exchange reactions involving B−O bond rupture are possible
for monocapped pyridyloximato complexes.
4
6
imes. However, there was no information on whether the
helicates were formed under kinetic or (partial) thermody-
namic control.
Prior to targeting even more elaborate cage structures, we
decided to examine the structures and the dynamic behavior of
monocapped pyridyloximato complexes. For this purpose, we
synthesized the new complexes 1-Fe and 1-Zn by a reaction of
pyridine-2-aldoxime (3 equiv) with phenylboronic acid
(
1
1 equiv) and M(OTf) (1 equiv; M = Fe or Zn) (Scheme
). For the synthesis of 1-Fe, acetonitrile was used as thIeI
2
solvent (reflux, 90 min, 82% yield). The synthesis of the Zn
complex 1-Zn was accomplished in ethanol in the presence of
NaHCO (reflux, 3 h, 85%).
3
The complexes were characterized by NMR spectroscopy,
high-resolution mass spectrometry, and by single-crystal X-ray
diffraction (Figure 3). Whereas the Zn−N bonds in 1-Zn are
all of a similar length (2.13−2.19 Å), one can observe distinct
Encouraged by these results, we set out to explore if
boronate ester cages can be obtained with capped pyridylox-
imato complexes as links. For this purpose, we used the tritopic
ligand 2 (Scheme 3). This ligand was obtained from the
1
0874
Inorg. Chem. 2021, 60, 10873−10879
Fadaei-Tirani, Farzaneh
