Boronate Ester-Capped Helicates

Article abstract of DOI:10.1002/chem.202001392

Triple-stranded helicates were obtained by metal-templated multicomponent reactions of bispyridyloxime ligands with arylboronic acids. The helicates feature two hexa-coordinated M^II ions (M=Fe, Zn, or Mn), which are embedded in a macrobicyclic ligand framework, and two arylboronate ester capping groups. The latter can be used to introduce functional groups such as pyridines, aldehydes, nitriles, and carboxylic acids in apical position. The functionalized helicates have the potential to be used as nanoscale building blocks for more complex assemblies, as evidenced by the synthesis of a 3 nm-sized trianglimine.

Full text of DOI:10.1002/chem.202001392

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Supramolecular Chemistry |Hot Paper|
Boronate Ester-Capped Helicates
Erica Giraldi, Adrien B. Depallens, Daniel Ortiz, Farzaneh Fadaei-Tirani, Rosario Scopelliti, and
Kay Severin*[a]
When decorated with appropriate ‘sticky ends’, DNA rods can
be employed to build highly elaborate molecular nanostruc-
tures.[13] Linear helicates are much smaller than duplex DNA,
Abstract: Triple-stranded helicates were obtained by
metal-templated multicomponent reactions of bispyridyl-
but they still hold the potential to be used as nanoscale build-
oxime ligands with arylboronic acids. The helicates feature
two hexa-coordinated MII ions (M=Fe, Zn, or Mn), which
ing blocks for more complex assemblies. A key challenge in
this context is to synthesize helicates with ‘sticky ends’.
are embedded in a macrobicyclic ligand framework, and
In the following, we describe triple-stranded M2L3-type heli-
two arylboronate ester capping groups. The latter can be
cates, which are capped by arylboronate ester groups. The aryl
used to introduce functional groups such as pyridines, al-
groups can be decorated with different functional groups,
dehydes, nitriles, and carboxylic acids in apical position.
which could be used for the crosslinking of helicates via cova-
The functionalized helicates have the potential to be used
lent or non-covalent bonds.
as nanoscale building blocks for more complex assem-
Our work was inspired by two classes of compounds: triple
blies, as evidenced by the synthesis of a 3 nm-sized trian-
stranded, dinuclear helicates of type A, and boronate ester-
glimine.
capped pyridineoximato complexes of type B (Figure 1). The
helicates can be prepared by M2+-templated condensation re-
actions of bis-formylpyridine ligands with primary amines.[14,15]
Complexes of type B are obtained by M2+-templated conden-
sation reactions of phenylboronic acid with 2-acetylpyridineox-
Linear and cyclic helicates have been studied extensively for
more than 30 years.[1–3] The first comprehensive review article
ime.[16] We hypothesized that it should be possible to merge
was published by Piguet, Bernardinelli, and Hopfgartner in
1997.[3f] At that time, applications started to emerge. For exam-
the two synthetic approaches, allowing to access boronate
ple, helicates were used as selective hosts for cations or
anions, or as starting materials for the synthesis of molecular
knots.[3f] Since then, research on helicates has expanded and
diversified considerably. Developments include the utilization
of helicates as biomolecular probes,[4] as catalysts,[5] as molecu-
lar switches,[3a,6] and as precursors for topologically complex
molecular architectures.[7] Moreover, some helicates were
found to display spin-crossover behavior[8] or act as single mol-
ecule magnets.[9] Helicates have also been investigated in a
biological and medicinal context, leading to the discover of
complexes with cytotoxic, antiviral or antibacterial proper-
ties.[10]
ester-capped helicates of type C. The terminal arylboronate
ester groups could then be used to introduce functional
groups (’sticky ends’) in a defined orientation, as it was shown
for clathrochelate complexes.[17,18]
In order to test the feasibility of this approach, we first ex-
amined the reaction of p-tolylboronic acid (2 equiv), dioxime
The structural resemblance of double-stranded, linear heli-
cates and duplex DNA has been noted repeatedly,[1–3] with the
discussion focusing on the helical arrangement of the ligands
and the DNA strands, respectively. However, there is another
similarity between duplex DNA and linear helicates: both tend
to form rigid, rod-like structures.[11] Duplex DNA has a width of
around 2 nm and a persistence lengths of about 50 nm.[12]
[a] E. Giraldi, A. B. Depallens, Dr. D. Ortiz, Dr. F. Fadaei-Tirani, Dr. R. Scopelliti,
Prof. K. Severin
Institut des Sciences et Ingꢀnierie Chimiques
ꢁcole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
1015 Lausanne (Switzerland)
E-mail: kay.severin@epfl.ch
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202001392.
Figure 1. Triple-stranded helicates of type A (ref. [14]), boronate ester-
capped pyridineoximato complexes of type B (ref. [16]), and the boronate
ester-capped helicates C described in this work.
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ligand L1[19,20] (3 equiv), and Fe(OTf)2 (2 equiv) in acetonitrile.
When the suspension was heated to 658C, we could observe
slow dissolution of the starting materials and formation of a
dark red solution. Analysis of the latter by mass spectrometry
indicated the formation of the desired dinuclear complex 1-Fe.
Further studies showed that the reaction is more conveniently
performed with microwave heating (1008C, 90 min). Using this
procedure, we were able to isolate complex 1-Fe in 50% yield
(Scheme 1). Attempts to prepare zinc(II) and manganese(II)
complexes under similar conditions were not successful. How-
ever, using ethanol and a base instead of plain acetonitrile al-
lowed preparation of 1-Zn and 1-Mn in yields of 52% and
50%, respectively (Scheme 1). Attempts to increase the yields
by variation of the reaction conditions or the work-up proce-
dure were unfortunately not successful.
atoms (FeÀNav =1.95 ꢁ). With an average twist angle[21] of fav =
368, the coordination geometry around the Fe centers is in-be-
tween that of a trigonal prism (f=08) and that of an octahe-
dron (f=608). Iron complexes of type B show a similar geome-
try.[16] The strong distortion from the usual octahedral geome-
try of FeII complexes is likely the result of the boronate ester
capping group. It is worth noting that the combination of
linear pyridylimine ligands with FeII salts tends to give tetrahe-
dral assemblies with the general formula Fe4L6, and not dinu-
clear helicates.[14b,22] We assume that the low twist angle of the
Fe complex is a key factor in favoring a helicate structure in
the case 1-Fe.
Longer helicates can be obtained by using ligands L2, L3 or
L4 with C6H2R2 spacers between the pyridyloxime groups in
combination with 4-tert-butylphenylboronic acid and M(OTf)2
(M=Fe, Zn, or Mn) (Scheme 2). The corresponding com-
plexes 2–4 were prepared in yields between 51% and 56%
using similar reactions conditions as described for 1, even
though slightly longer reaction times were required for 4 (mi-
crowave heating for 4 h instead of 90 min). The presence of six
methyl or butoxy side chains results in an increased solubility
of the helicates 3 and 4, when compared to the moderately
soluble helicate 1 and the poorly soluble helicate 2.
The molecular structure of 3-Fe was analyzed by single-crys-
tal X-ray diffraction (Figure 3). The insertion of the C6H2Me2
spacers leads to an increase of the Fe···Fe distance from 7.7 ꢁ
Scheme 1. Synthesis of the helicates 1-Fe, 1-Zn, and 1-Mn. i. M=Fe, CH3CN,
MW, 1008C, 90 min; ii. M=Zn or Mn, EtOH, NaHCO3, MW, 1008C, 90 min.
The complexes are moderately soluble in CH3CN, CH3OH,
DMSO, DMF, or CH3NO2, but they display poor solubility in less
polar solvents such as CH2Cl2 or CHCl3. Analysis of solutions of
1-Fe in CD3CN by NMR spectroscopy showed well-resolved
spectra, indicating that the FeII ions are in a low spin configura-
tion. In contrast, the NMR data for 1-Mn point to a paramag-
netic complex. These findings are in line with what has been
reported for mononuclear complexes of type B.[16] Whereas 1-
Zn and 1-Mn are not sensitive to oxygen, 1-Fe needs to be
handled under inert atmosphere.
The molecular structure of 1-Fe in the crystal was deter-
mined by single crystal X-ray diffraction (Figure 2). As expect-
ed, the two FeII ions are each coordinated to six nitrogen
Scheme 2. Synthesis of the helicates 2–4 (M=Fe, Zn, Mn). i. M=Fe, CH3CN,
MW, 1008C, 90 min; ii. M=Zn or Mn, EtOH, NaHCO3, MW, 1008C, 90 min, for
4: 4 h.
Figure 2. Molecular structure of helicate 1-Fe in the crystal. Hydrogen
atoms, solvent molecules, and anions (2TfOÀ) are omitted for clarity. Color
code: C, grey; N, blue; O, red; B, yellow; Fe, orange.
Figure 3. Molecular structure of helicate 3-Fe in the crystal. Hydrogen
atoms, solvent molecules, and anions (2 TfOÀ) are omitted for clarity. Color
code: C, grey; N, blue; O, red; B, yellow; Fe, orange.
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in 1-Fe to 11.9 ꢁ in 3-Fe. The coordination geometry around
the Fe centers in 3-Fe is similar as it was observed for 1-Fe
(fav =388).
structures using dynamic covalent imine chemistry.[25] We have
synthesized the helicates 1-Fe-b and 3-Fe-b using 4-formyl-
phenylboronic acid. As in the other cases, the yields for these
complexes are around 50%. Furthermore, we have synthesized
helicates with cyano (1-Fe-c and 3-Fe-c), bromo (1-Fe-d and 3-
Fe-d), and carboxylic acid functions (1-Zn-e and 3-Zn-e) in
para position using the corresponding arylboronic acids
(Figure 4). These helicates represent potential metalloligands
for coordination polymers and metal organic frameworks.
To demonstrate that boronate ester-capped helicates can be
used as nanoscale building blocks, we have investigated the
polycondensation reaction of dialdehyde 3-Fe-b with (1R,2R)-
1,2-diaminocyclohexane. This reaction was expected to give a
’trianglimine’[26] via a [3+3] condensation. Trianglimines have
been studied extensively over the last years, but significantly
shorter dialdehydes are typically employed to build these mac-
rocycles.[26,27]
The redox properties of 3-Fe were investigated by cyclic vol-
tammetry. The measurements were performed in acetonitrile,
using the Fc/Fc+ redox couple as external standard and
TBAPF6 as electrolyte. The data showed a reversible single elec-
tron process at E1/2 =1.08 V, which can be attributed to the oxi-
dation of one metal center from FeII to FeIII (for details, see Fig-
ure S126). A second oxidation could not be observed due to
the limited window of the CD3CN/TBAPF6 system. CV measure-
ments in DMF indicated a reduced stability of the helicate, and
more detailed investigations were not performed.
Having established a protocol for the synthesis of boronate
ester-capped helicates, we next examined if we could use the
arylboronic acid to introduce functional groups. The potential
danger of this approach is an interference of the functional
group with the metal-templated condensation reactions (e.g.
by coordination of the functional group to the metal ion).
However, we found that such an interference is not an issue,
and different functional groups could be introduced without
compromising the yield.
A common solvent for the formation of trianglimines is di-
chloromethane, but helicate 3-Fe-b is poorly soluble in CH2Cl2.
Therefore, we focused on reactions in acetonitrile, in which
both, the dialdehyde and the diamine, are well soluble. Using
deuterated CD3CN, we were able to follow the reaction by
1H NMR spectroscopy. At room temperature, the disappearance
of the aldehyde CHO signal at d=10.08 ppm was observed
within few hours ([dialdehyde]=[diamine]=5.4 mm). After
24 hours, the NMR spectrum indicated that a defined new
product had formed in high yield (>95%), and the chemical
shifts matched what is expected for a trianglimine (Scheme 3).
We are not able to comment on the diastereoselectivity of the
macrocyclization. Since the starting materials (helicate and di-
amine) are both chiral, one can expect the formation of diaste-
reoisomers. However, these isomers are not necessarily re-
solved by 1H NMR spectroscopy. DOSY analysis showed that
the new structure is larger than the helicate 3-Fe-b (Dt (3-Fe-
b)=7.8ꢂ10À6 cm2 sÀ1, and Dt (5)=6.5ꢂ10À6 cm2 sÀ1, see Figures
S64 and S122). Additional evidence for the formation of a tri-
anglimine was obtained by high resolution ESI mass spectrom-
etry (for details, see Figure S123). Dominant peaks in the spec-
trum can be attributed to trianglimine 5 with a variable
First, we examined reactions of Fe(OTf)2 with 5-methoxypyri-
dine-3-boronic acid using the bispyridyloxime ligands L1 and
L3. The resulting helicates 1-Fe-a and 3-Fe-a were isolated in
55% and 60% yield, respectively (Figure 4). Based on the crys-
tallographic data for 1-Fe and 3-Fe, we estimate that the N-
donor groups in 3-Fe-a are approximately 2.1 nm (1-Fe) and
2.5 nm (3-Fe) apart from each other. The functionalized heli-
cates 1-Fe-a and 3-Fe-a therefore represents nanoscale metal-
loligands,[23] which might find applications in the synthesis of
large metallosupramolecular structures.[24] Building blocks with
aldehyde functions are of interest for the construction of nano-
number of OTfÀ anions: [5-OTf]5+, [5-2OTf]4+ and [5-3OTf]3+
.
To conclude: we have shown that boronate-ester capped
helicates can be obtained by metal-templated multicompo-
nent reactions of bispyridyloxime ligands with arylboronic
acids. The polycondensation reactions are compatible with
functionalized arylboronic acids. Consequently, it is possible to
prepare helicates with different functional groups in apical po-
sition, such as pyridines, nitriles, aldehydes, and carboxylic
acids. The functionalized helicates have the potential to be
used as nanoscale building blocks for more complex assem-
blies. As a proof-of-concept, we have synthesized a triangl-
imine by condensation of a 4-formylphenylboronate ester-
capped helicate with (1R,2R)-1,2-diaminocyclohexane. With a
size of approximately 3 nm, the resulting macrocycle is the
largest trianglimine reported to date.
Figure 4. Helicates with functionalized arylboronate ester caps.
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Scheme 3. Synthesis of trianglimine 5. The molecular structure of the macro-
cycle was obtained by molecular modelling using the Spartan software (H
atoms are not shown).[28] Color code: C, grey; N, violet; O, red; B, pink; Fe,
orange.
[11] There are examples of helicates with a high degree of conformational
freedom, for example, the “flexicate” assemblies described by Scott and
co-workers (ref. [10d]).
Acknowledgements
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Conflict of interest
The authors declare no conflict of interest.
Keywords: condensation reaction
·
dynamic covalent
chemistry · helicates · supramolecular chemistry · trianglimine
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[28] Wavefunction 2018, Spartan’18 (Wavefunction, Inc.).
Manuscript received: March 20, 2020
Version of record online: && &&, 0000
Chem. Eur. J. 2020, 26, 1 – 6
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&
These are not the final page numbers! ÞÞ

Communication
doi.org/10.1002/chem.202001392
Chemistry—A European Journal
COMMUNICATION
Helicates with caps: Metal-templated
multicomponent reactions of bispyridyl-
oxime ligands with arylboronic acids
give helicates with terminal arylboro-
nate ester groups. The latter can be
used to introduce functional groups
such as pyridines, aldehydes, nitriles,
and carboxylic acids in an apical posi-
tion (see figure).
&
Supramolecular Chemistry
E. Giraldi, A. B. Depallens, D. Ortiz,
F. Fadaei-Tirani, R. Scopelliti, K. Severin*
&& – &&
Boronate Ester-Capped Helicates
&
&
Chem. Eur. J. 2020, 26, 1 – 6
www.chemeurj.org
6
ꢀ 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!