Superchiral Pd3L6 Coordination Complex and Its Reversible Structural Conversion into Pd3L3Cl6 Metallocycles

Article abstract of DOI:10.1002/anie.201506539

Large, non-symmetrical, inherently chiral bispyridyl ligand L derived from natural ursodeoxycholic bile acid was used for square-planar coordination of tetravalent Pd^II, yielding the cationic single enantiomer of superchiral coordination complex 1 Pd₃L₆ containing 60 well-defined chiral centers in its flower-like structure. Complex 1 can readily be transformed by addition of chloride into a smaller enantiomerically pure cyclic trimer 2 Pd₃L₃Cl₆ containing 30 chiral centers. This transformation is reversible and can be restored by the addition of silver cations. Furthermore, a mixture of two constitutional isomers of trimer, 2 and 2′, and dimer, 3 and 3′, can be obtained directly from L by its coordination to trans- or cis-N-pyridyl-coordinating Pd^II. These intriguing, water-resistant, stable supramolecular assemblies have been thoroughly described by ¹H DOSY NMR, mass spectrometry, circular dichroism, molecular modelling, and drift tube ion-mobility mass spectrometry.

Full text of DOI:10.1002/anie.201506539

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International Edition: DOI: 10.1002/anie.201506539
German Edition: DOI: 10.1002/ange.201506539
Chiral Complexes
Superchiral Pd L Coordination Complex and Its Reversible Structural
3
6
Conversion into Pd L Cl Metallocycles
3
3
6
Ond rˇ ej Jur cˇ ek,* Pia Bonakdarzadeh, Elina Kalenius,* Juha Matti Linnanto, Michael Groessl,
Richard Knochenmuss, Janne A. Ihalainen, and Kari Rissanen*
Abstract: Large, non-symmetrical, inherently chiral bispyridyl
ligand L derived from natural ursodeoxycholic bile acid was
used for square–planar coordination of tetravalent Pd ,
results in directed and stable interaction between the
subunits, is to utilize the coordination ability of organic
binding sites to metals, forming either distinct metallo-
supramolecular structures (capsules, cages, or other assem-
II
yielding the cationic single enantiomer of superchiral coordi-
[
1]
nation complex 1 Pd L6 containing 60 well-defined chiral
blies having cavities) or polymeric materials (metal–organic
3
[
2]
centers in its flower-like structure. Complex 1 can readily be
transformed by addition of chloride into a smaller enantio-
merically pure cyclic trimer 2 Pd L Cl containing 30 chiral
frameworks, or MOFs).
The coordination geometry of the metal ions and the
number of the binding sites of the ligands and overall
structure of the ligand define geometry of the coordination
3
3
6
centers. This transformation is reversible and can be restored
by the addition of silver cations. Furthermore, a mixture of two
constitutional isomers of trimer, 2 and 2’, and dimer, 3 and 3’,
can be obtained directly from L by its coordination to trans- or
II
assembly. The use of divalent or tetravalent Pd of cis-, trans-,
or square–planar binding geometry, together with various
rigid or flexible pyridyl ligands, has proven to be a successful
strategy in obtaining numerous supramolecular assemblies. In
general, the ligands used for their preparation are mostly
II
cis-N-pyridyl-coordinating Pd . These intriguing, water-resist-
ant, stable supramolecular assemblies have been thoroughly
1
[1]
described by H DOSY NMR, mass spectrometry, circular
symmetrical, relatively rigid, and very often achiral.
II
II
dichroism, molecular modelling, and drift tube ion-mobility
mass spectrometry.
To prepare discrete chiral species of Pd or Pt , two
pathways have been developed: i) utilization of chiral met-
allo-corners and achiral ligands, or ii) chelation of chiral
multidentate ligands to achiral metals. In the second case, the
chiral moiety might be appended to a side of the ligand, or
a simple chiral molecule can constitute the actual core of the
C
hirality is a common property of many naturally occurring
[
3]
compounds and an essential element of life. In the field of
supramolecular chemistry, there have been significant efforts
to mimic Nature in the preparation of chiral concave
supramolecular systems utilizing chiral natural products,
their derivatives, or various synthetic ligands. Chiral cavity-
containing structures are attractive for a variety of applica-
tions, from transport and recognition to catalysis and protec-
tion of biochemically active and inherently chiral compounds.
Supramolecular chemistry can utilize various reversible and
irreversible intermolecular interactions to obtain concave
assemblies. One of the most fascinating strategies, which
[4]
ligand.
Focusing on the chiral concave coordination assemblies
resembling the structures presented and starting from the
smallest ones, several examples of chiral metallo-supramolec-
ular rhomboids have been prepared by self-assembly of chiral
bis-pyridyl-substituted ditopic ligands with 908 cis-blocked
II
[5]
Pd , often with [(enPd)(NO ) ]. There are no chiral
3
2
supramolecular triangles described so far, but a large
[1a,c]
number of their achiral analogues.
And finally, the
preparation of large achiral molecular open boxes, either
trifacial or multi-facial, can be done through self-assembly of
di- or tetratopic ligands and 908 building blocks. The trifacial
box is the smallest and entropically the most preferred
[
*] Dr. O. Jur cˇ ek, P. Bonakdarzadeh, Dr. E. Kalenius, Prof. K. Rissanen
University of Jyvaskyla
Department of Chemistry, Nanoscience Center
P.O. Box 35, 40014 University of Jyvaskyla (Finland)
E-mail: jurcekondrej@gmail.com
isomer, which can be represented by either assembly M L or
6
3
M L . For the former assembly, the organic linker is often
3
6
[
6,7]
elina.o.kalenius@jyu.fi
kari.t.rissanen@jyu.fi
tetrapyridyl coordinating to 908 Pt or Pd metal corners.
The M L assembly is represented by work of Fujita et al.,
3
6
Dr. J. M. Linnanto
University of Tartu, Institute of Physics
Ravila 14c, 50411 Tartu (Estonia)
where 608 donor, 1,2-bis(ethynylpyridine)benzene, in combi-
II
nation with Pd , yielded solvato-controlled assemblies of
[8a]
Pd L and Pd L .
Clever et al. described an intertwined
3
6
4
8
Dr. M. Groessl, Dr. R. Knochenmuss
Tofwerk AG
Uttigenstrasse 22, 3600 Thun (Switzerland)
achiral Pd L cage built of two trefoil-knotted substructur-
3
6
[8b]
es. However, chiral concave structures of Pd L have not
3
6
yet been obtained.
Small terpenoids have been used on several occasions for
Prof. J. A. Ihalainen
University of Jyvaskyla, Nanoscience Center
Department of Biological and Environmental Science
P.O. Box 35, 40014 University of Jyvaskyla (Finland)
[9]
modification of coordination ligands, yet large triterpenoids,
such as bile acids, have not been utilized for preparation of the
ligand itself. Bile acids are inexpensive, natural, and enantio-
merically pure compounds (with over 9 chiral centers in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506539.
1
5462
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a single molecule). Their rigid tetracyclic steroid skeleton is
decorated by various numbers of readily reactive hydroxy
groups of defined stereochemistry, and by flexible alkyl chains
bearing carboxyl functional groups, which makes them
excellent building blocks for supramolecular and material
[10,11]
chemistry.
Several groups of covalent macrocycles built
of 2–5 bile acid units have been presented and studied for
their host–guest chemistry, for example, cyclocholates, chol-
[10,12]
aphanes, and cyclocholamides.
Nevertheless, large mac-
rocyclic bile acid-based systems have not been built by metal-
coordination. Out of available bile acids, only ursodeoxy-
cholic acid (UDCA) provides an optimal conformation of the
hydroxy groups (position 3R,7S in divergent manner) con-
taining 1108 angles, which can be further equipped by pyridyl
binding sites. For these reasons, UDCA was selected as the
starting building block for the preparation of our ligand when
aiming at the construction of superchiral coordination com-
plexes.
1
Figure 1. Formation of hexamer 1 from L as followed by H NMR
[D ]DMSO, 400 MHz, 298 K).
(
6
The ligand L was prepared in two steps from UDCA
(
Scheme 1). Complexation of L with [Pd(CH CN) (BF ) ] in
3 4 4 2
Figure 2. Optimized structures of 1 (CAM-B3LYP, hydrogens were
Scheme 1. Preparation of ligand L. Reaction conditions: a) potassium
omitted for clarity).
bicarbonate, DMF, benzyl bromide; b) triphosgene, pyridine, CH Cl .
2
2
2
:1 ratio in [D ]DMSO (558C, 15 min) provided supramolec-
6
1
ular assembly 1, Pd L . From the H NMR spectrum, clear
3
6
downfield chemical shifts of the carbamate -NH- protons and
pyridyl b-protons (H3’ and H3’’), and slight upfield shift of
pyridyl a-protons (H4’ and H4’’) were attributed to a success-
ful and quantitative metal–ligand complexation (Figure 1).
The number of signals for the complex corresponds to those
of the ligand, indicating that the supramolecular assembly is
formed spontaneously in symmetrical and precise order.
Based on the symmetry of 1, there are two options for the
ligand assembly around the Pd metal centers. The Pd L6
3
complex can be assembled by clockwise- and counterclock-
wise-directed three-membered ligand cycles aligned in 3!7
connectivity (barrel shape, numbering in Scheme 1; Support-
ing Information, Figure S5), or of two parallel clockwise
cycles aligned similarly 3!7 (flower shape, Figure 2). In both
cases the cycles are interconnected through coordination to
Figure 3. a) ESI-TOF MS spectrum from 1 (5 mm in CH CN). Inset
3
shows the fit of theoretical isotopic distribution for ion at m/z 776
6
+
which corresponds to molecular formula of [Pd L ] . b) IM-MS ion
3
6
II
6+
three atoms of square–planar Pd (PdN4 coordination
sphere). The comparison of their ground state energies of
mobilograms for ion [Pd
208C and 288C.
L
3
]
6
recorded at drift tube temperatures
1
optimized molecular models Eflower < Ebarrel (DE = 20 kcal.
À1
mol ) suggests preferential formation of the flower-like
complex (Figures 2, S5).
ESI-MS analysis of 1 shows an intense single peak
corresponding to ion [Pd L ] (Figure 3). The drift tube
closely related conformers of 1, which can be seen by the
different, yet close, drift times. Their distribution results from
variation of collision cross-sections (CCSs) for particular
6
+
3
6
+
6
2
IM-MS analysis for [Pd L ] shows the presence of several
conformers between 1106 and 1192 Š , corresponding to the
3
6
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maximum of 10 Š difference in CCS derived diameters (if
a spherical structure for the complex was assumed). The
conformational diversity is temperature-dependent and is
likely caused by the movements of the flexible alkyl side
chains bearing benzyl carboxylate group, which might resem-
ble “waving” of the complex (or possibly opening and closing
of the flower). Close to RT there are only two gas-phase
conformers observed, but at elevated temperature of 1208C at
least 10 distinct conformers can be seen (Figure 3).
Spatial movement of the benzyl groups can also be
13
observed from multiple C NMR signals corresponding to
benzyl carbons (Figures S7, S8b). Broadening and multiplicity
of the carbon signals in the upper side of the steroid skeleton
(
C11, C18, C19, and C21) can be assigned to the interaction
with the surrounding benzyl groups, or possible formation of
their self-inclusion complex (Figure S8a). Furthermore, the
H NMR spectrum of 1 at 1308C excludes the possibility that
the IM-MS profile observed could originate from different
isomers and not from different conformers as suggested
1
Figure 4. Comparison of experimental and simulated CD and UV/Vis
spectra of 1.
(
Figure S6). Unfortunately, the motion of benzyl groups
might also be the reason for the difficulties in obtaining an
1
3
X-ray quality single crystal of 1. Moreover, the C NMR
spectrum revealed four different signals for CH CN of the
3
palladium salt (Figures S7, S8a), which presents an initial
implication on the ability of 1 to operate in host–guest
chemistry.
1
H DOSY NMR measurements show that all the proton
signals originate from one species with diffusion coefficient
À11
2
À1
D = 6.73 ” 10 m s (logD = À10.17, [D ]DMSO, 303 K;
6
Figure S9). This value corresponds to the hydrodynamic
diameter of 3.6 nm, as calculated from the Stokes–Einstein
equation under the consideration of its roughly spherical
shape. This value is in good agreement with the calculated
3
.8 nm diameter obtained from the CCS acquired by IM-MS
Scheme 2. General preparation and interconversion of coordination
assemblies of L.
measurement and with the 3.9 nm diameter determined from
its molecular model.
Circular dichroism (CD) studies of L and 1 were per-
formed to examine their chirality. The CD spectra were
compared with those calculated from their optimized struc-
tures in methanol (Figure 4 for 1, Figure S36 for L).
Calculated spectra reproduce the overall structures of the
experimental CD spectra, indicating that structural models
are realistic. CD bands at around 240 nm are shifted to the
Scheme 2) by treatment with alkylammonium chlorides
(TMACl or TBACl). Six chloride anions replace three
molecules of L during this transformation, giving Pd complex
of the PdN Cl2 coordination sphere. After the saturation
2
point is reached, complex 2 remains stable even at higher
1
concentrations of TBACl, as shown by H NMR (Figure S12).
1
2
60 nm, owing to the formation of the coordination structure/
Observation of a single isomer 2 in H NMR spectrum is
bond. Similar shifts can be observed also from the UV/Vis
absorption spectra of L and 1 (Figure S33). The experimental
CD spectrum of 1 shows a negative shoulder in the low energy
side of the 270 nm band, and a very weak positive band at
about 330 nm. Calculations produce the weak 330 nm band
but underestimate the negative shoulder. For more details,
see the Supporting Information, Chapter 3.
further support for assembly 1 to be the flower-like assembly
(from two clockwise cycles connected through PdN , 1; by
4
À
addition of Cl we obtain one clockwise cycle of Ls
coordinating as trans-PdN Cl sphere, 2).
2
2
1
H DOSY NMR experiments show assembly with D =
7.15 ” 10 m s ([D ]DMSO, 303 K) for complex 2, which
À11
2
À1
6
corresponds to a complex that is larger than L yet smaller than
1 (for comparison, see Table 1). This could be assigned to two
possible assemblies Pd L Cl or Pd L Cl . The final product 2
Complex 1 was further studied for its stability in aqueous
solution by MS (1 measured from H O, Figure S11), and by
2
2
2
4
3
3
6
1
H NMR titration with D O (Figure S10). Both methods
cannot be observed directly by ESI-MS because of its zero net
charge. Instead, an MS experiment was designed to follow the
stepwise formation of 2 by measuring the reaction mixture
after partial additions of TMACl to the solution of 1. Indeed,
by this approach gradual and fast (within minutes) formation
2
showed 1 to be robust and water-stable. Interestingly, the
addition of D O into 10 mm [D ]DMSO solution in a 1:3 ratio
2
6
resulted in the formation of a gel.
Complex 1 can readily be transformed to a single constitu-
tional isomer of trimer Pd L Cl (reaction pathway 1, RP1;
4+
2+
of [Pd L Cl ] and [Pd L Cl ] ions, most likely turning in
3
3
6
3
5
2
3
4
4
1
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1
Table 1: Comparison of the results of IM-MS and H DOSY NMR of
determination of their size characteristics.
2
À11
2
À1
Complex
Ion
CCS [Š ]
D [”10 m s ]
L
3
2
–
276
679
879
688
–
21.9
8.38
–
–
7.15
6.73
4
+
+3’
[(enPd) L ]
[Pd L Cl ]
2
2
4
2
0
+
3
5
2
+
[
[
Pd L Cl ]
Pd L Cl ]
3
4
4
3
3
6
+
6
1
[Pd L ]
1165
3
6
0
the next step into [Pd L Cl ] , could be detected (Figures S13,
3
3
6
2+
S14). The formation of [Pd L Cl ] or a similar intermediate
that could lead to the complex [Pd L Cl ] were absent.
2
2
2
1
Figure 6. Comparison of H NMR spectra for coordination assemblies
Pd L , 3 and 3’ (enPd) L , and 2 and 2’ Pd L Cl (product obtained
either by the RP1 or RP2 pathways).
0
2
2
4
1
3
6
2
2
3
3
6
Interestingly, species having an odd number of chlorides were
not observed, thus the ligand leaves in one step from both
metal coordination sites, and is immediately replaced by
4+
a pair of chlorides. The IM-MS analysis for [Pd L Cl ] and
clear size differences (Table 1), which confirms the interpre-
tation of 2 to be the trimeric complex.
The trimeric assembly 2 can also be obtained by a second
reaction pathway (RP2), which proceeds directly from L by its
3
5
2
2+
[
Pd L Cl ] ions show a gradual decrease in their CCS values,
and cross sections of 879 Š and 688 Š are obtained for the
main conformers (Table 1, Figure S13).
3 4 4
2
2
II
The transformation of 1 into 2 is reversible and the
coordination to trans-Pd [(PhCN) PdCl ] in 1:1 ratio. Beside
2
2
+
1
original structure of 1 can be restored by adding Ag ions. The
the H NMR signals for 2 as the main product (the same
chemical shifts, as in the case of the RP1 product), there is
a very close second set of signals of a co-product 2’ (Figure 6).
addition of 6 equiv of AgBF to the sample of 2 obtained by
4
1
RP1 led to an immediate change in the H NMR spectra, and
1
further heating of the mixture (558C, 15 min) resulted in total
From the H DOSY NMR experiments, we can conclude
recovery of 1 (Figure 5). Respectively, addition of AgBF into
that 2’ and 2 are of the same size (Figure S21). Considering
the fact that the trans-Pd is thermodynamically more stable
4
II
RP1 ESI-MS sample led to reconstruction of 1 and appear-
6+
ance of the peak for [Pd L ] (Figure S15).
than its cis-isomer (and thus their interconversion is not
viable), two different constitutional isomers, 2 and 2’, of
Pd L Cl coordination complex can be presumed (Scheme 2).
3
6
3
3
6
Their ratio was determined as 2:2’ = 4:1 from the integration
1
of H NMR signals and our conclusions based on the isomeric
distribution was confirmed by comparison of the ground state
energies of their optimized molecular models, E < E (DE =
2
2’
À1
4
.4 kcal.mol ; Figure S18). Thus 2 should be present in
1
excess. Variable temperature (VT) H NMR experiments
were carried out to study the thermodynamic equilibrium
between 2 and 2’ (Figure 7). The thermodynamic data
obtained from the van ꢀt Hoff plot (Supporting Information,
Figure S25) are in agreement with those obtained by compu-
tational calculations and help explain the conversion (Sup-
porting Information, Section 2.5.2).
1
If the H NMR spectra of products obtained by RP1 and
RP2 are compared (Figure 6), in the case of RP1 there is only
one set of signals corresponding to trimeric structure 2 (3!7
connectivity). This uniform directional organization can
easily be explained as described above based on the
organization of the parental complex 1 (both cycles are of
the same directionality, thus giving the same directional single
trimeric cycle after the addition of chloride). On the contrary,
from the RP2 reaction pathway we can obtain both isomers 2
and 2’ as they are formed from free ligands coordinating to
1
Figure 5. H NMR spectra visualizing interconversion of 1 and 2 (from
down to top). The ligand L was added in slight excess to compare the
signals to a side-product of transformation during TBACl addition.
To provide further evidence supporting the interpretation
of 2 as Pd L Cl , and to fully exclude the possibility for 2 to be
a dimeric assembly, we used cis-blocked Pd [(enPd)(NO ) ]
for complexation with L in 1:1 ratio. As expected from the
coordination geometry of the metal, this procedure led to
a formation of two constitutional isomers of dimeric
3
3
6
II
3
2
II
trans-Pd in 3!7 or 7!3 connectivity.
Similar to RP1, the assemblies 2 and 2’ obtained by RP2
+
convert to 1 in the presence of Ag salt. Comparison on the
4+
[
(enPd) L ] complexes 3 and 3’ (Figure 6; see the Support-
ongoing chemistry of the reverse conversion for both cases
can be done (Scheme 3). In the case of RP2, two molecules of
trimer are needed for the formation of one molecule of 1, as
2
2
1
ing Information for more details). Comparison of H DOSY
NMR and CCS from IM-MS for assemblies 2 and 3 shows
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applications. The studies on their host–guest recognition
properties are currently ongoing in our lab.
Acknowledgements
We thank to Academy of Finland (KR: proj. no. 263256 and
2
65328, EK: no. 284562 and 278743), University of Jyväskylä,
and Estonian Research Council (JML: Grant IUT02-28) for
financial support. Johanna Lind and Esa Haapaniemi are
gratefully acknowledged for the help with MS and NMR.
Emer. Prof. Erkki Kolehmainen is gratefully acknowledged
for allowing OJ to carry out some preliminary experiments on
this topic during his Ph.D. studies. Dr. Ralf Troff is acknowl-
edged for initial MS measurement of a similar system.
Figure 7. Changes in distribution of isomer 2 and 2’ in dependence on
temperature as observed from VT H NMR measurement.
1
Keywords: bile acid · chirality · metallocycle · self-assembly ·
supramolecular chemistry
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15462–15467
Angew. Chem. 2015, 127, 15682–15687
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assemblies of L.
there are no free ligands present for the direct recovery as in
the case of RP1. This might suggest that the coordination
bonds between the ligands and metals in products of RP2 are
partially broken down first, giving the building blocks for re-
assembly into 1 in the second step. The same mode might also
be present in RP1 case.
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at the same time inherently chiral, ultimately forming an
unprecedented example of enantiomerically pure metallosu-
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II
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