Inflating face-capped Pd6L8 coordination cages

Article abstract of DOI:10.1039/c8cc04870c

Tritopic metalloligands were used to form two Pd₆L₈-type coordination cages. With molecular weights of more than 15 kDa and Pd?Pd distances of up to 4.2 nm, these complexes are among the largest palladium cages described to date.

Full text of DOI:10.1039/c8cc04870c

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Inflating face-capped Pd L coordination cages†
6
8
Suzanne M. Jansze,
Daniel Ortiz, Farzaneh Fadaei Tirani, Rosario Scopelliti,
Cite this:DOI: 10.1039/c8cc04870c
Laure Menin and Kay Severin
*
Received 18th June 2018,
Accepted 6th August 2018
DOI: 10.1039/c8cc04870c
rsc.li/chemcomm
6 8
3n 4n
Tritopic metalloligands were used to form two Pd L -type coordi- formula Pd L . The assembly with the lowest number of
nation cages. With molecular weights of more than 15 kDa and building blocks, Pd L , can only form if some of the ligands
3
4
2
+
PdÁ Á ÁPd distances of up to 4.2 nm, these complexes are among the act as chelates (two of the three N-donors bind to the same Pd
7
largest palladium cages described to date.
ion). If the N-donor position does not allow the formation of a
2
+
chelate, the resulting assembly will contain at least six Pd ions
8
Geometrical considerations are of central importance in syn- and eight ligands. Since it is straightforward to design tritopic
thetic metallosupramolecular chemistry. Metal complexes are ligands which cannot form chelates, Pd L complexes appear
1
3
n 4n
characterized by the number and the orientation of the available to be a good starting point if the aim is to create particularly
coordination sites, and ligands are categorized by the orientation large assemblies based on ligands with nanoscale dimensions.
of the coordinate vectors. Knowing these parameters, it is
possible to make a prediction of what kind of structure will (Scheme 1). These metalloligands contain three diamagnetic
For this study, we have used the tritopic ligands L1 and L2
9
II
form during the self-assembly process. Besides geometry, there Fe clathrochelate complexes. We have previously shown that
2
3
II
are other factors such as template effects or steric interactions,
which can influence the self-assembly process. However, geometrical supramolecular chemistry.
analyses have been remarkably successful in predicting and plexes is the ease of synthesis, because clathrochelates are
Fe clathrochelates are robust and versatile building blocks for
3
a,10
A key advantage of these com-
1
1
rationalizing the outcome of metal-based self-assembly.
formed in metal-templated multicomponent reactions. This
The absolute size of a ligand is irrelevant for a geometrical advantage is particularly evident for L1 and L2, which were
analysis. In principle, it should be possible to expand the size of prepared by one-pot reactions using readily accessible or com-
a metallosupramolecular structure by increasing the size of the mercially available starting materials. Specifically, the synthesis
ligands, while keeping the number and orientation of the coordinate
vectors constant. However, there is an obstacle. Larger ligands are
typically more flexible, which makes the orientation of the coordi-
nate vectors less defined. This flexibility could allow the formation of
other assemblies along with – or instead of – the targeted structure.
It should be noted that for some metal–ligand assemblies,
ligand flexibility is more of an issue than for others. For example, it
n
is possible to form Pd L2n-type coordination cages by combination
2+
of Pd ions with ditopic N-donor ligands. There are reports on
4
very large structures with n = 12, 24, or 30, but the assembly
process is very sensitive to the relative orientation of the coordinate
5
vectors of the two N-donors.
2
+
6
The combination of Pd ions with divergent, tritopic
N-donor ligands is expected to give assemblies of the general
Institut des Sciences et Ing ´e nierie Chimiques, Ecole Polytechnique F ´e d ´e rale de
Lausanne (EPFL), 1015 Lausanne, Switzerland. E-mail: kay.severin@epfl.ch
†
Electronic supplementary information (ESI) available: Experimental procedures,
characterisation and crystallographic data. CCDC 1849685–1849687. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c8cc04870c
Scheme 1 One-pot synthesis of the metalloligands L1 and L2. The yields
are calculated based on the triboronic acid as limiting reagent.
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was performed by heating a mixture of FeCl (4 eq.), nioxime
(
(
12 eq.), 3-pyridylboronic acid (5 eq.) and a triboronic acid
1 eq.) in a solvent mixture of chloroform, methanol, and
acetone (30 : 7 : 1) (Scheme 1). Substoichiometric amounts of
the triboronic acid were used to limit the formation of oligo-
mers. As a consequence, a mono clathrochelate complex with
two pyridylboronate ester caps was the major product for this
reaction. Since this complex is much smaller than L1 and L2,
it can be separated by size exclusion chromatography. The
yields of the desired tritopic ligands are 57% (L1) and 49%
(L2), respectively, when calculated based on the triboronic acid
as limiting reagent.
L1 and L2 were characterized by NMR spectroscopy and
high-resolution mass spectrometry (for details, see the ESI†). In
addition, both compounds were analysed by single crystal X-ray
diffraction. The results show that the N-donor atoms of L1
are approximately 2.2 nm apart (Fig. 1). The presence of the
additional phenyl spacers increases this value to 2.9 nm for
ligand L2.
Scheme 2 Synthesis of the coordination cages 1 and 2.
4a,5b,8b,12
1
viscous solvent.
Processing the data of the H DOSY
NMR also posed some problems related to the broad and over-
lapping peaks. Nonetheless, the DOSY spectra did confirm the
formation of complexes with a uniform diffusion coefficient.
The coordination cages 1 and 2 were prepared by combining
ligand L1 or L2 (4 eq.) with [Pd(CH CN) ](BF ) (3 eq.) in DMSO-d
6
6 8
For both reactions, clear evidence for the formation of Pd L -
3
4
4 2
type complexes was obtained by mass spectrometry. The MS
analyses were performed on a hybrid LTQ Orbitrap FTMS instru-
ment operated in the positive ionization mode. The HESI-II probe
in an Ion Max ion source was modified in order to perform cold-
spray ionization (CSI), a variant of electrospray ionization operated
6
(Scheme 2). Since the ligands are poorly soluble in DMSO-d ,
suspensions were initially obtained, which turned into clear
1
orange solutions after heating at 70 1C overnight. The H NMR
spectra of the solutions showed broad and not very well defined
peaks, which is not unusual for large cage complexes in a
13
at low temperature. CSI-MS can be used to prevent decomposi-
tion of labile supramolecular structures and was shown to be
crucial for such measurements. Using this methodology, the major
peaks observed in the 1200–2600 m/z mass range were attributed
12+
À
to [Pd
Fig. 2 and ESI,† Fig. S16).
Cage 1 was characterized by single crystal X-ray diffraction,
6
L
8
]
complexes with a variable number of BF
4
anions
(
1
2+
and graphic representations of the cationic [Pd (L1) ] complex
6
8
are depicted in Fig. 3 and 4. The location of most anions and
solvent molecules is ill defined in the electron density map, and
the solvent-masking program in OLEX2 was applied to treat this
1
4
disorder.
The six Pd ions are positioned at the vertices of an octa-
hedron, and the ligands panel the eight faces. The maximum
PdÁ Á ÁPd distance is 3.3 nm. This value is larger than what is
Fig. 1 Molecular structures of the ligands L1 (top) and L2 (bottom) as
determined by single crystal X-ray diffraction. Hydrogen atoms and solvent
molecules are omitted for clarity. Grey: C; blue: N; green: B; red: O; and
orange: Fe.
6 3
Fig. 2 CSI-HRMS of cage 2 in 10% DMSO-d in CH CN.
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Fig. 4 Space fill representation of cage 1 (left), as determined by X-ray
diffraction, and of cage 2 (right), as obtained by molecular modelling.
Solvent molecules are omitted for clarity. Grey: C; blue: N; green: B; red: O;
cyan: Pd; orange: Fe and white: H.
synthesized two large, tritopic N-donor ligands by using clathro-
chelate complexes as key structural elements. Upon combination
2+
6 8
with Pd ions, we were indeed able to form Pd L -type coordi-
nation cages. One cage was characterized by single crystal X-ray
diffraction. With a maximum PdÁÁÁPd distance of 3.3 nm, this
complex represents the largest M L cage (M = Pd, Pt) in the CCDC
6
n
database. The second cage is even larger (PdÁÁÁPd
B 4.2 nm),
max
but unfortunately we were not able to obtain diffraction data of
sufficient quality to solve the structure (despite using synchrotron
radiation). This failure is indicative of one key challenge when
working with metallosupramolecular structures of this size: the
structural characterization becomes exceedingly difficult. For the
present study, we have focused on structural aspects, but it is clear
that very large cages offer interesting opportunities in terms of
function (e.g. encapsulation of large guests). Investigations in this
direction are ongoing in our laboratory.
Fig. 3 Molecular structure of cage 1 as determined by single crystal X-ray
À
diffraction. Hydrogen atoms, BF
4
anions, and solvent molecules are omitted
for clarity. Grey: C; blue: N; green: B; red: O; cyan: Pd and orange: Fe.
found for other M
6
L
n
cages (M = Pd, Pt) reported in the
The work was supported by the Marie Curie Initial Training
Cambridge Crystallographic Data Center (CCDC) (for a more Network ‘‘ResMoSys’’, and by the Ecole Polytechnique F ´e d ´e rale
detailed analysis, see the ESI†). The cage features only small de Lausanne (EPFL).
openings, because the bulky side chains of the clathrochelate
complexes are in close proximity to each other (Fig. 4, left side).
3
3
The size of the cavity is estimated to be around 2.8 Â 10 Å , as Conflicts of interest
determined by the use of the VOIDOO software.
There are no conflicts to declare.
Single crystals for the larger cage 2 were also obtained.
Unfortunately, we were not able to obtain diffraction data of good
quality, despite testing different crystals and various experimental
Notes and references
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was possible to determine the size of the unit cell (B193 000 Å ,
see ESI†), but we were not able to solve the structure.
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