Equipping metallo-supramolecular macrocycles with functional groups: Assemblies of pyridine-substituted urea ligands

Article abstract of DOI:10.1039/c2dt30190c

A series of di-(m-pyridyl)-urea ligands were prepared and characterized with respect to their conformations by NOESY experiments and crystallography. Methyl substitution in different positions of the pyridine rings provides control over the position of the pyridine N atoms relative to the urea carbonyl group. The ligands were used to self-assemble metallo-supramolecular M ₂L₂ and M₃L₃ macrocycles which are generated in a finely balanced equilibrium in DMSO and DMF according to DOSY NMR experiments and ESI FTICR mass spectrometry. Again, crystallography was used to characterize the assemblies. Methyl substitution in positions next to the pyridine nitrogen prevents coordination, while the other ligands form small metallo-supramolecular macrocycles. The incorporated urea carbonyl groups provide hydrogen bonding sites which converge towards the center of the assemblies. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt30190c

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PAPER
Equipping metallo-supramolecular macrocycles with functional groups:
assemblies of pyridine-substituted urea ligands†
a,b
c
c
c
b
d
Ralf W. Troffe, Rainer Hovorka, Torsten Weilandt, Arne Lützen, Mario Cetina, Martin Nieger,
b
a
Dieter Lentz, Kari Rissanen* and Christoph A. Schalley*
Received 25th January 2012, Accepted 1st May 2012
DOI: 10.1039/c2dt30190c
A series of di-(m-pyridyl)-urea ligands were prepared and characterized with respect to their
conformations by NOESY experiments and crystallography. Methyl substitution in different positions of
the pyridine rings provides control over the position of the pyridine N atoms relative to the urea carbonyl
group. The ligands were used to self-assemble metallo-supramolecular M L and M L macrocycles
2
2
3 3
which are generated in a finely balanced equilibrium in DMSO and DMF according to DOSY NMR
experiments and ESI FTICR mass spectrometry. Again, crystallography was used to characterize the
assemblies. Methyl substitution in positions next to the pyridine nitrogen prevents coordination, while the
other ligands form small metallo-supramolecular macrocycles. The incorporated urea carbonyl groups
provide hydrogen bonding sites which converge towards the center of the assemblies.
5,6
Introduction
sites. The formation of smaller complexes is normally entropi-
cally favoured because of the larger particle number if they can
form free of any enthalpically unfavorable strain. Nevertheless,
examples exist, wherein the larger complex is entropically
favoured over its corresponding smaller one presumable due to
Metallo-supramolecular self-assembly has been shown to be a
powerful synthetic strategy to form a large diversity of two- and
three-dimensional metallo-supramolecular architectures. In most
1
7
cases, one assembly is thermodynamically preferred and forms
large solvation entropies.
1
exclusively. Sometimes complexes form under kinetic control
Depending on the properties of the building blocks, highly
selective self-sorting processes were realized in metallo-supra-
and only slowly convert into the corresponding thermodynamic
2,3
8,9
products. The “error” correction processes that are mandatory
for self-assembly then occur on longer time scales.
molecular self-assembly. The properties of metallo-supramole-
cular complexes can be programmed by the introduction of
functionalities at the ligands used to induce such sorting
phenomena. Using this strategy, differently functionalized com-
plexes have been reported which contain for example crown
Besides such processes that have a single, well-defined end
product, equilibria between two or more metallo-supramolecular
4
species have been observed from time to time. Generally, the
1
0
11
equilibria between metallo-supramolecular complexes reflect a
delicate balance between enthalpy and entropy, which can be
influenced by various parameters, e.g. temperature, solvent and
structural parameters of the building blocks such as their flexi-
bility and geometry or the number of available coordination
ether moieties, pseudo-rotaxanes, or additional coordination
12
sites.
Several metallo-supramolecular complexes have been used as
host complexes for guest inclusion or are able to influence and/
1
3
14
or catalyze reactions within their inner cavities.
The dynamic complexity of many metallo-supramolecular
systems render their characterization sometimes difficult and
15
a
only the use of a number of complementary analytical methods
Institut für Chemie und Biochemie, Freie Universität Berlin,
16
Takusstraße 3, 14195 Berlin, Germany. E-mail: christoph@schalley-lab.de;
Fax: +49 30 838 55817; Tel: +49 30 838 52639
such as 1D and 2D NMR spectroscopy, DOSY NMR spec-
17
18
19
troscopy, mass spectrometry, and X-ray crystallography
provides a clear picture.
b
Department of Chemistry, Nanoscience Center, University of Jyväskylä,
P. O. Box 35, 40014 Jyväskylä, Finland. E-mail: kari.t.rissanen@jyu.fi;
Here, we present the self-assembly of several metallo-supra-
molecular M L complexes from different dipyridylurea and di-
Fax: +358-14-2602501; Tel: +358-50-5623721
c
Kekulé-Institut für Organische Chemie und Biochemie, Universität
2
2
Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
(
pyridylmethyl)-urea ligands and cis-blocked square-planar
d
Laboratory of Inorganic Chemistry, Department of Chemistry,
metal centers. Some of them exist in equilibrium with the corres-
P.O. Box 55, 00014 University of Helsinki, Finland
e
Institut für Chemie und Biochemie, Freie Universität Berlin,
ponding M L complexes which can be shifted significantly by
3 3
Fabeckstrasse 34–36, 14195 Berlin, Germany
temperature, concentration, and the choice of solvent. Despite
†
Electronic supplementary information (ESI) available: Experimental
details and additional information on the ligands and complexes. CCDC
64202–864209. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt30190c
the large number of M2L2 complexes containing cis-blocked
20
square-planar metal centers available in the literature, equili-
8
3,21,22
bria with M L complexes were rarely observed.
The urea
3
3
8410 | Dalton Trans., 2012, 41, 8410–8420
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Scheme 2 Three ligand conformers exemplarily shown for ligand 9.
“
syn” describes the pyridine N-atom converging with the carbonyl group
on the same side of the molecule, “anti” the situation in which both
diverge.
Scheme 1 Syntheses of the differently substituted N,N′-dipyridyl urea
and the two di-(pyridylmethyl)-urea ligands 9–15.
units implemented in the ligands enable the complexes to inter-
act with other molecules through hydrogen bonding or through
interactions with the urea dipole making the complexes interest-
ing for molecular recognition or hierarchical self-assembly.
Fig. 1 NOESY experiments with 10 clearly show the proximity of the
methyl group to the urea NH protons, while the pyridine p-proton is so
remote from the NH protons that only the pyridine m-proton signal is
seen. For a comparison with the NOESY experiments performed with 9,
Results and discussion
11, and 12, see the ESI.†
Synthesis of dipyridylurea ligands
The syntheses of all ligands prepared for this study are shown in
Scheme 1. Ligand 9 and some hydrogen bonded complexes
have been described and crystallographic work has been per-
and para to the pyridine nitrogen atom are the most polarized
positions, they are able to form intramolecular C–H⋯O con-
25
tacts with the carbonyl oxygen. These contacts not only flatten
2
3,24
formed with it earlier.
Also, ligand 15 was utilized pre-
the molecule, but considerably affect the opening angle between
two lines through the pyridine nitrogens and the pyridine
p-carbon atoms, and may even be important to establish a certain
orientation of the urea units connecting the two pyridine rings.
Thus, replacing one of these hydrogens by a methyl group is
expected to change this behaviour significantly. These changes
would likely also affect the formation of coordination complexes
with the dipyridylurea ligands incorporated in this study. Three
different conformers can be distinguished (Scheme 2).
viously for the preparation of an M L metallo-supramolecular
2
2
22
macrocycle. These compounds were re-synthesized here and
used for comparison with the other ligands and complexes.
All dipyridylurea ligands 9–15 were synthesized by the same
6
procedure (see ESI† for details and characterization data). The
corresponding amino- and aminomethyl pyridines 1–7 were
reacted with N,N′-carbonyldiimidazole (CDI, 8) in dry toluene
for 3.5 hours at 80 °C and further stirred for another 15 hours at
room temperature. The dipyridylurea ligands 9–15 precipitated
from the solutions in acceptable chemical yields. Their solubility
in most organic solvents is quite poor so that highly polar aprotic
solvents such as DMSO and DMF are required to dissolve suffi-
ciently high quantities of the ligands for their analytical charac-
terization and the formation of metallo-supramolecular
assemblies.
The conformational preferences of the ligands in solution
were established by 1D NOESY experiments in DMF-d . Ligand
7
1
0 (Fig. 1) may serve as an example. Due to the increased steric
hindrance introduced by the methyl group in the o-position of
the pyridine-nitrogen atoms, the anti–anti conformation is
strongly preferred. No NOE between the NH and the p-pyridine
CH is observed. Instead, strong NOEs are found between the
NH and the methyl group. Similarly, the other ligands can be
analyzed (ESI†): Ligand 9 slightly prefers the anti–anti isomer,
which exists in equilibrium with significant amounts of the other
two in solution. Ligand 11 clearly exhibits a syn–syn confor-
mation. Finally, 12 does not show a clear preference. Due to its
very low solubility in any of the tested solvents, no NOESY
NOESYexperiments: the preferred conformations of 9–13
To control the host–guest properties of the assemblies, it would
be desirable to control the ligand conformation and with it the
hosts’ structures. Since for ligands 9–13 the C–H bonds ortho
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spectra could be obtained for ligand 13. Since the methyl group
is in a position remote to the urea moiety, it is nevertheless likely
that it behaves in analogy to 9 and 12. For the latter two mole-
cules, the equilibrium between the three conformations is known
2
3,24
to be highly solvent dependent.
Therefore, it may change for
ligands 9, 12, and 13, when the solvents are changed. For
ligands 10 and 11 solvent effects are not expected to be as
strong, because the methyl group clearly prefers to reside on the
urea NH side.
Crystallographic characterization of dipyridylurea ligands
Several crystal structures of ligand 9 are known. In agreement
with the NOESY experiments, both the syn–syn and the anti–
anti isomer could be obtained depending on crystallization con-
23
ditions. The molecules are essentially planar because of the
C–H⋯OvC interactions that lock their conformations. The pyri-
dine rings are tilted only slightly with respect to the urea moiety
and dihedral angles between the urea and pyridine planes are
11.3(2)° and 11.6(2)°. In contrast, no structures of the corre-
sponding dimethyl analogues 10–13 have been reported so far.
Single crystals of ligands 10–12 suitable for X-ray crystallogra-
phy could be obtained by slow diffusion of diethyl ether into
saturated solutions in DMF. Crystals of 13 were obtained by
slow cooling of a hot DMSO solution. Interestingly, suitable
crystals of 11 were only obtained when it was crystallized in the
presence of imidazole and water as impurities.
Fig. 2 compares the different molecular structures in the crys-
tals. For 10 and 11, the conformations are anti–anti and syn–syn,
respectively, as expected from their solution preferences. Com-
pound 12 crystallized in a syn–anti conformation so that all three
conformations are realized in the crystal structures presented
here. The opening angles defined by the two N1–C4 and N1′–
C4′ lines clearly reflect the three conformations. They are close
to 90° for the two anti–anti structures of 10 and 13, much
smaller (23.2°) for syn–syn in 11 and much larger (164.1°) for
the syn–anti conformer of 12. The C –C –N –C dihedral
py
py
ur
ur
Fig. 2 Solid-state structures of 10–13 (top to bottom) containing the
atom-numbering scheme. The displacement ellipsoids for non-hydrogen
atoms are drawn at the 50% probability level. Intramolecular C–H⋯O
contacts are indicated by a dashed line; distances are H–O distances in
Å. Dihedral angles provide a measure of the tilt between the pyridine
and urea planes. The opening angle is defined as the angle between two
lines constructed through N1–C4 and N1′–C4′. See ESI† for more
details.
angles can be used as a measure of the tilt between the pyridine
and urea planes and are in the range of 158° to 175° for 11–13.
Thus, these ligands are not too far from planarity. They also
exhibit similar C–H⋯O distances in the range of 2.21 to 2.31 Å.
The solid-state structure of 10 differs significantly with longer
C–H⋯O distances of 2.579 and 2.638 Å and C –C –N –C
py
py
ur
ur
dihedral angles around 135°.
The significantly stronger deviation of 10 from planarity is
likely caused by a packing effect (Fig. 3). In the crystal structure
of 10, adjacent molecules are held together by two N–H⋯O
hydrogen bonds involving both urea NH moieties and the carbo-
nyl oxygen atom, so that an α-network is formed (see also ESI,†
Table S3). The hydrogen-bonded chains built of bifurcated
N–H⋯O hydrogen bonds run in opposite directions. This is
shown in the center of Fig. 3 (top), where two chains are shown
behind each other. Two C–H⋯π interactions participate also in
chain formation as methyl hydrogen atoms point to the pyridine
rings of neighbouring molecules (see also ESI,† Table S4).
Finally, these chains are mutually linked by C–H⋯N hydrogen
bonds forming double chains and hydrogen-bonded sheets
The supramolecular architecture of 11 in the crystal structure
is completely different from all the other compounds discussed
here, because water and imidazole molecules play a significant
role in the molecular packing (ESI,† Table S3). Both nitrogen
atoms of the urea moiety are linked to a neighbouring imidazole
molecule. In addition, the imidazole NH is further linked to the
oxygen atom of a water molecule. Both hydrogen atoms of the
water molecule form additional hydrogen bonds with pyridine
nitrogen atoms closing a hydrogen-bonded circle and forming a
two-dimensional network (Fig. 4). One C–H⋯O hydrogen bond
participates also in the network formation. Finally, the carbon-
centered hydrogen atom of the imidazole molecule is directed
(Fig. S2, ESI†).
8412 | Dalton Trans., 2012, 41, 8410–8420
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Fig. 3 A part of the crystal structure of 10, showing the α-network and
double chains formed by N–H⋯O and C–H⋯N hydrogen bonds.
Hydrogen bonds are indicated by dashed lines.
towards a pyridine ring of the neighbouring network, and thus, a
C–H⋯π interaction completes the three-dimensional hydrogen-
bonding network (ESI,† Table S4 and Fig. S3).
As in 10, the main hydrogen-bonding motif in 12 is an
α-network, but the molecules are disposed in a zig–zag manner
(
(
Fig. 5) and a chain is formed by two N–H⋯N hydrogen bonds
ESI,† Table S3). One C–H⋯O hydrogen bond links the neigh-
bouring chains into a two-dimensional network (ESI,† Fig. S4).
One C–H⋯π interaction and two π⋯π interactions also partici-
pate in the supramolecular assembly and thus generate a three-
dimensional network (ESI,† Table S4 and Fig. S5).
Finally, the molecules of 13 are linked by two N–H⋯N
hydrogen bonds between the nitrogen atoms of the urea moiety
and the pyridine rings in the crystal structure. Compared to 12,
two N–H bonds are directed to the pyridine rings of two differ-
ent molecules (Fig. 6). One C–H⋯N hydrogen bond connects
the molecules in such a way that they form a two-dimensional
network in a herringbone fashion (ESI,† Fig. S6). This network
is extended to a three-dimensional network by two C–H⋯π
interactions (ESI,† Table S4 and Fig. S7).
Fig. 4 Top: crystal packing diagram of 11, viewed along the crystallo-
graphic c axis, showing a two-dimensional network formed by N–H⋯N,
N–H⋯O and O–H⋯N hydrogen bonds between 11, imidazole and
water molecules. Hydrogen bonds are indicated by dashed lines.
Bottom: CPK plot of the crystal packing of 11.
Formation of metallo-supramolecular assemblies
The metallo-supramolecular assemblies under study have been
formed by mixing one of the ligands 9–15 with stoichiometric
amounts of the metal centers (en)Pd(NO ) 16, (dppp)Pd(OTf)
2
3
2
1
7, or (dppp)Pt(OTf) 18 (en: ethylenediamine, dppp: 1,3-bis-
2
(diphenylphosphino)propane) in water or in polar aprotic sol-
vents (DMSO or DMF). The resulting complexes are shown in
Scheme 3. If Fujita’s metal center 16 was used, only M L com-
2
2
plexes were observed. Instead, equilibria between M2L2 and
M L complexes were found, when Stang’s metal centers 17 and
3
3
1
8 were employed. Unfortunately, no well-defined assemblies
could be obtained from ligands 10 and 13.
The equilibria of the self-assembled Pd complexes are usually
reached within minutes, while the Pt complexes need several
Fig. 5 CPK plot of the crystal packing of ligand 12, showing the
zig–zag chains of the molecules.
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5
c,6
hours for error correction and equilibration.
Pd(II)-based
DMF, a wide temperature range for variable temperature NMR
experiments is accessible. However, neither of them is well-
suited as a spray solvent for electrospray mass spectrometric
experiments due to their low vapour pressures. The solutions for
mass spectrometric analysis were therefore prepared by mixing
and equilibrating metal center and ligand in DMSO followed by
dilution with acetonitrile and/or acetone to a building block con-
centration of ca. 200 μM.
assemblies were therefore stirred for one hour, and Pt(II)-based
complexes for one day at room temperature before characteriz-
ation in order to make sure that the equilibria are reached. For
NMR experiments, 1 : 1 mixtures of metal center 16, 17, or 18,
respectively, and ligands 9–15 were dissolved in 0.5 ml of deut-
erated water, DMSO, or DMF to prepare a solution with a build-
ing block concentration of 10 mM. Due to solubility reasons,
DMSO and DMF had to be used. Furthermore in the case of
NMR spectroscopy and ESI mass spectrometry
Coordination of the pyridine nitrogen atoms to the metal centers
can easily be determined from the chemical shifts of the protons
1
in H NMR spectroscopy. Especially the two o-protons of the
pyridine rings undergo small, but significant chemical shifts to
lower field upon coordination. A comparison of the experimental
1
H NMR data of the ligands with their corresponding metallo-
supramolecular assemblies revealed successful coordination for
all complexes shown in Scheme 3. Another reliable indicator of
Fig. 6 CPK plot of the packing of 13 in the crystal structure, showing
two N–H⋯N hydrogen bonds and the molecules arranged in a herring-
bone fashion.
195
complex formation for Pt complexes 29a/b–33a is the
Pt–P
coupling constant which is significantly smaller for the pyridine
Scheme 3 Self-assembly of complexes 19a–33a. The numbering scheme follows the three different metal complexes used for self-assembly as
given in the tables: for complexes 19a–23a, precursor complex 16 was used, for 24a/b–28a precursor 17 and for 29a/b–33a precursor 18. Index “a”
denotes M
L
2 2
, “b” M
L
3 3
complexes. When only “a” is given, no M
3
L
3
complexes were observed.
8414 | Dalton Trans., 2012, 41, 8410–8420
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Fig. 7d shows a mass spectrum of a DMF/acetonitrile solution of
2
4a/b. The M L complex appears in its +1 and +2 charge states
2 2
at m/z 1913 and m/z 882, respectively. In addition – and by no
3,5c,6,9,26
means unexpected
– a fragment corresponding to an
M L complex appears at m/z 1095 and signals for dimers of the
1
2
M L complexes are observed at m/z 1913 and m/z 1226. Such
2
2
M L dimers are typically observed as unspecific aggregates in
4
4
ESI mass spectrometry, when salts are sprayed into the ion
source. For entropic reasons (particle number), the most likely
second component would be an M L complex, which, however,
3
3
is not observed in the mass spectrum. Very likely, the reason for
this is that the sample needed to be diluted with acetone in order
to achieve good ionization conditions (see above). This shifts the
equilibrium towards the smaller complexes (see below). In order
to gain some evidence that the concentration of the M L com-
3
3
plexes in the sample solution indeed depends much on the
solvent, ESI-MS experiments were done with pure DMF as the
spray solvent and indeed, the M L complexes were observed as
3
3
a low intensity, but clearly visible signal (isotope pattern in
Fig. 7d). Similar results were obtained for assemblies 25a/
b–27a/b and 29a/b–32a/b. To gather more evidence for the pres-
17
ence of M L complexes in solution, DOSY NMR experiments
3
3
were considered as another option to not only separate the two
species in solution virtually by their diffusion coefficients, but
also to compare their hydrodynamic radii calculated using the
Stokes–Einstein equation (eqn (1); k : Boltzmann constant, T:
B
temperature, η: solvent viscosity) with radii calculated with the
2
7
MM2 force-field implemented in the CAChe program.
D ¼ kBT=6πηr
ð1Þ
Fig. 8 clearly shows that the two sets of signals can be
assigned to two complexes that diffuse at different rates. Indeed,
the experimental radii agree well with those obtained from the
force-field calculations for M2L and M3L complexes (for
1
Fig. 7 NMR spectra of assemblies synthesized from ligand 9: (a) H
1
1
NMR spectrum of 19a; (b) H NMR spectrum of 24a/b and (c)
H
3
1
2
3
NMR spectrum of 29a/b. The insets show the P NMR spectra, in
which the M and M complexes can clearly be seen as two separ-
ate singlet signals. (d) ESI-FTICR mass spectrum of 24a/b sprayed from
DMF/acetonitrile, the insets show the experimental isotopic pattern of
the peak at m/z 882 and the pattern calculated for [2 : 2 : 2] . The minor
deviations of both patterns are due to a small fraction of [1 : 1 : 1] con-
tributing to the experimental pattern. The peak assignments of the ions
observed in the mass spectrum denote [metal center : ligand :
details, see ESI,† Fig. S8 and Table S6). Therefore, we assign
the two complexes to the dimeric and trimeric assemblies.
Finally, we observed only one set of signals for metallo-supra-
molecular assemblies 28a and 33a which are formed from ligand
15. Without understanding the reason for this discrepancy yet,
this result is in contrast to data published by Ballester, Vilar, and
co-workers, who observed both dimeric and trimeric
2
L
2
3 3
L
2
+
+
charge
22
anion]
3 3
. Inset: isotope pattern of the doubly charged M L complex
assemblies.
sprayed from pure DMF (also see data in ESI†).
Crystallographic characterization of metallo-supramolecular
complexes
1
complexes ( J
= 3000–3036 Hz) than for the precursor
= 3657 Hz).
Pt–P
1
complex 18 ( J
Pt–P
Only one set of signals is observed for all assemblies contain-
ing 16 as the metal center, i.e. 19a–23a. The spectrum of 19a is
shown in Fig. 7a as a representative example. According to ESI
MS data, this set of signals can be clearly assigned to M2L2
complexes (for characterization data, see ESI†). In marked con-
trast, two different sets of signals are observed for 24a/b–27a/b
and 29a/b–32a/b which have been assembled from precursor
complexes 17 and 18, respectively (Fig. 7b,c). The presence of
two highly symmetrical complexes is confirmed by P NMR
spectra, which exhibit two singlets (Fig. 7b,c, insets). Unfortu-
nately, ESI mass spectrometry did not provide conclusive results
on the size of the second metallo-supramolecular complex.
Single crystals suitable for X-ray crystal structure analysis were
successfully obtained for 23a, 24a, 25a, and 28a. The crystals of
the M L complexes 24a, 25a and 28a were grown via diffusion
of diethyl ether or dichloromethane into a solution of the com-
plexes in DMF. Complex 23a crystallized from a solution in
water. For selected bond lengths and angles of the complexes see
Table S6 (ESI†).
2
2
Complex 24a (Fig. 9) crystallized with two independent cat-
ionic M L complexes and eight corresponding triflate anions in
3
1
2
2
the asymmetric unit, while the crystal structure of complex 25a
(Fig. 10) also includes three molecules of N,N-dimethylforma-
mide, one diethyl ether molecule and one crystal lattice water
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Fig. 9 Solid-state structure of one of the two independent cations of
complex 24a containing the atom-numbering scheme. The displacement
ellipsoids for non-hydrogen atoms are drawn at the 20% probability
level. The hydrogen atoms have been omitted for clarity.
1
Fig. 8 H DOSY NMR spectra of (a) 24a/b recorded at 243K in
DMF-d and (b) 29a/b recorded at 298 K in DMF-d
7
7
.
besides the M2L2 cationic complex and four triflate counter-
anions. The palladium atoms have a distorted square planar geo-
metry in both complexes. Two cis positions of the Pd(II) atoms
are occupied by two phosphorous atoms of the dppp ligand
while the remaining binding sites are coordinated by two pyridyl
nitrogens atoms from 1,3-bis(3-pyridyl)urea 9 and 11, respect-
ively. The N–Pd–N angles in the two independent cationic com-
plexes of 24a are narrowed and fall in the range of 85.7(3)–
Fig. 10 Solid-state structure of the cation of complex 25a containing
the atom-numbering scheme. The displacement ellipsoids for non-hydro-
gen atoms are drawn at the 20% probability level. The hydrogen atoms
have been omitted for clarity.
8
6.3(4)°. The deviation from the ideal square planar geometry is
probably caused by steric reasons. In 25a the N11–Pd1–N12
bond angle of 86.2(2)° is in the same range as in 24a, but the
N11′–Pd2–N12′ angle of 88.6(2)° is closer to that for an ideal
square planar geometry. The bidentate phosphine bite angles
range from 89.6(2)–90.5(1)° in complex 24a and are 89.4(1) and
methyl groups attached to the 4-position of the pyridine rings in
ligand 11 and the neighbouring phenyl rings of the dppp ligand.
In complex 28a, 1,3-bis(4-pyridylmethyl)urea 15 was used for
complex preparation (Fig. 11). A survey of the Cambridge Struc-
9
2.3(1)° in complex 25a while the distances between the Pd(II)
atoms are 8.81 Å in both cations of 24a and 8.83 Å in 25a. The
side view on a cation of 24a in Fig. 9 (right) reveals that both
ligands, dppp and 9, are almost perfectly overlapped. The dipyri-
dylurea moiety is slightly bent and the dihedral angles between
pyridyl rings are small ranging from 13.4(5) to 17.2(5)°.
Although the molecular structure of 25a is very similar to that of
28
tural Database revealed that a crystal structure of this com-
22
pound has already been published and that the compound
crystallizes in the orthorhombic space group P2 2 2 . However,
1
1 1
we obtained a monoclinic P2 /n polymorph of this compound,
1
which includes three dichloromethane molecules and a water
molecule in the asymmetric unit. The methylene spacers that
link the pyridyl units with the urea moiety cause a higher flexi-
bility of the ligands. Thus, a C-shape cation is observed
(Fig. 11). Methylene group incorporation resulted in a much
longer distance between the Pd atoms of 10.20 Å. The N–Pd–N
angles are 86.0(2) and 85.8(3)° and the P–Pd–P angles are
90.4(1) and 90.6 (1)°. This somewhat distorted square planar
2
4a, the ligands are not as well overlapped (Fig. 10). One dipyri-
dylurea moiety is almost flattened, with a dihedral angle between
the pyridyl rings of 7.8(3)° (N11–C61–N11′–C61′), and the
second one is strongly bent with an N12–C62–N12′–C62′ di-
hedral angle of 28.3(3)°. This deviation is probably a consequence
of the geometric arrangement of the ligands during complex for-
mation. Presumably, it is not caused by repulsion between the
8416 | Dalton Trans., 2012, 41, 8410–8420
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Fig. 13 Solid-state structure of the cation of complex 23a containing
the atom-numbering scheme. The displacement ellipsoids for non-hydro-
gen atoms are drawn at the 30% probability level. The hydrogen atoms
have been omitted for clarity.
Fig. 11 The solid-state structure of the cation of complex 28a contain-
ing the atom-numbering scheme. The displacement ellipsoids for non-
hydrogen atoms are drawn at the 20% probability level. The hydrogen
atoms have been omitted for clarity.
polar phenyl groups as those of the analogous (dppp) containing
Pd complexes (24a, 25a, and 28a). It does not show any
inclusion of anions or solvent molecules (Fig. 13 and Fig. S8,
ESI†).
Equilibrium studies: temperature and solvent
In order to better characterize the M L /M L equilibria, the
2
2
3 3
effects of temperature and solvent were examined for 24a/b as a
representative example. These complexes are particularly suited,
because they exist in almost a 1 : 1 ratio at room temperature in
DMF-d . This gives room for change in both directions. Temp-
7
1
31
erature-dependent H and P NMR spectra of 24a/b are shown
in Fig. 14. Starting from 233 K, two effects are observed with
increasing temperature: (a) A conversion of the M L complex
3
3
Fig. 12 Space-filling models of complex 24a (a) and complex 28a (b),
showing the position of one triflate counter-anion in the center of cation.
Other triflate counter-anions in 24a and 28a, as well as solvent mol-
ecules in complex 28a have been omitted for clarity. Only the major
component of the disordered triflate atoms in 28a is shown.
(
24b) into the M L complex (24a) with increasing temperature.
2 2
The assignment of the peaks to the two complexes is based on
the DOSY NMR spectra shown above. (b) The ligand exchange
between the two species becomes faster with increasing tempera-
ture. Between 333 K and 353 K it is too fast to clearly separate
1
31
the signals of the two species in the H and P NMR spectra
31
geometry of the palladium atoms is similar to the structures of
and coalescence of the two P NMR signals occurs at around Tc
31
2
4a and 25a.
≈ 340 K. The two P NMR signals can be used to extract quan-
titative data on the equilibrium between 24a and 24b from the
peak integrations and the initial concentration of the building
In all three structures discussed above, one of the triflate
counter-anions is located in the center of the pocket and is thus
positioned in the middle between the two cationic metal centers.
Nevertheless, their positions differ in the crystal structures. Thus,
in 24a (Fig. 12a) and 25a the triflate is located at the edge of the
M L cation. On the contrary, the longer ligand 15 in complex
29
blocks. Eqn (3) describes the equilibrium constant K for the
equilibrium in eqn (2). With a van’t Hoff plot of lnK over 1/T
(eqn (4)), the equilibrium enthalpy and entropy can be obtained
(Fig. S10, ESI†). The concentrations of the two complexes, the
equilibrium constants, the Gibbs energies at different tempera-
tures and the values obtained from the van’t Hoff plot are sum-
marized in Table 1.
2
2
2
8a enables encapsulation of one of the triflates inside the cavity
of the cation (Fig. 12b). Two other triflates in these complexes
are displaced at the sides of the metallo-macrocycles, each of
them interacting with one palladium center. However, in 28a,
one Pd atom interacts with a water oxygen atom instead of the
triflate with a Pd1⋯O3W distance of 3.035(7) Å.
The positive enthalpy (ΔHeq = 58.6 kJ mol− ) for the conver-
1
sion of two M L complexes 24b into three M L complexes
3
3
2 2
24a indicates the M L complex to be slightly strained, while the
2
2
The crystallization of ligand 12 with (en)Pd(NO ) 16 resulted
M3L3 complex can more easily adjust its geometry to avoid
strain upon cyclization. The rather large positive entropic term
3
2
in a dinuclear complex with a completely different overall geo-
metry. The en ligand in 16 is sterically much less demanding
than the dppp ligand in Pd complex 17. Thus, complex 23a
bears a much more open cavity that is not surrounded by non-
(ΔSeq = 174.0 J mol−
1
K
−1
; TΔSeq = 51.8 kJ mol ) reflects the
−1
increase in particle number when two M L complexes are con-
3
3
verted into three M L complexes. At higher temperatures the
2
2
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Dalton Trans., 2012, 41, 8410–8420 | 8417

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3
1
Fig. 15
P NMR spectra of 24a/b in different mixtures of (a) DMF-d
and (b) DMF-d and acetone-d
7
and acetonitrile-d
3
7
6
.
1
31
Fig. 14 Temperature-dependent H and P NMR spectra of 24a/b in
DMF-d
7
.
Table 1 Thermodynamic data for the 24a/b equilibrium (experimental
a
errors ca. 20%)
1
4 4 7
Fig. 16 Titration of 24a/b with NBu HSO in DMF-d . Left: H NMR
3
1
c (M
[mmol l
2
L
2
)
c (M
[mmol l
3
L
3
)
ΔGeq
spectra revealing a shift of the urea NH protons; right: P NMR spectra
indicating the formation of a host–guest complex and the complete shift
of the equilibrium to one species.
−
1
−1
K [mmol l−1
]
[kJ mol−1]
T [K]
]
]
2
2
2
2
3
3
33
53
73
93
03
13
5.1
11.2
19.1
26.2
29.2
33.2
41.6
35.6
27.6
20.5
17.5
13.5
0.075
1.1
9.2
43
81
201
18.1
14.6
11.1
7.6
5.9
4.1
respectively, at room temperature. Because of the low solubility,
the choice of solvents was quite narrow, but DMF–acetone and
DMF–acetonitrile mixtures turned out to be feasible. A drastic
influence of the solvent on the equilibrium is visible in Fig. 15.
While both complexes appear in about equal amounts in pure
DMF-d , M L complex 24a is clearly preferred in the presence
a
ΔHeq = 58.6 kJ mol−1; ΔSeq = 174.0 J mol−1 K−1
.
7
2 2
of acetone or acetonitrile. The signals assigned to M L complex
3
3
equilibrium is thus shifted towards the formation of M2L2
complexes.
2
4b nearly completely vanish when the amounts of acetone-d6
or acetonitrile-d are increased. This also explains why the M L
3 3
3
complexes were not observed in most of the ESI mass spectra
discussed above.
2
M3L3 , 3M2L2
ð2Þ
ð3Þ
3
½
M2L2ꢀ
M3L3ꢀ
K ¼
2
½
Anion binding
In order to test the ability of the metallo-supramolecular macro-
cycles to interact with anions, complexes 24a/b were titrated
with NBu HSO (Fig. 16) and NBu H PO (Fig. S11, ESI†) in
ΔGeq
ΔHeq ΔSeq
ln K ¼ ꢁ RT ¼ ꢁ
þ
ð4Þ
RT
T
4
4
4
2
4
To test the influence of the solvent on the M L /M L equili-
brium, P NMR spectra of 24a/b were recorded in different
DMF-d . In both titrations, similar results were obtained. In the
H NMR spectra, a clear shift of the urea NH signals is seen,
2
2
3
3
7
31
1
mixtures of DMF-d7 and acetone-d6 or acetonitrile-d3,
although the broadened signals make the interpretation of the
8418 | Dalton Trans., 2012, 41, 8410–8420
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3
1
NMR spectra somewhat difficult. The P NMR spectra also
reveal clear changes. Starting with an almost 1 : 1 mixture of the
M L and the M L complexes similar to the situation in Fig. 15
Notes and references
1
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2
2
3 3
(
(
top row), one new species is formed gradually upon the addition
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anion relative to the M L macrocycle, this species is more or
less the only one left. Although somewhat preliminary, we inter-
pret these results as indication that the anions bind through
hydrogen bonding to the urea units in a 1 : 1 ratio and completely
shift the M L /M L equilibrium to one side thus templating the
formation of only one host. As all attempts to record mass
spectra of this new species failed so far, it is not clear yet, which
of the two macrocycles is formed.
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In this work, we have shown that dipyridylurea ligands 9–15 can
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(
(
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2
2
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3. While just M L complexes (19a–23a) have been observed
2 2
with metal center 16, equilibria between M L and M L com-
2
2
3 3
plexes (24a/b–27a/b and 29a/b–32a/b) occur when metal
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2
(ab) S. De, K. Mahata and M. Schmittel, Chem. Soc. Rev., 2010, 39,
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1
31
combination of different analytical methods like H and
P
NMR spectroscopy, DOSY NMR spectroscopy, ESI mass spec-
trometry, and X-ray crystal structure analysis. The environment –
i.e. temperature and solvent – has significant effects on the equi-
libria which can be shifted almost completely from one side to
the other by changing one or more of these parameters. Never-
theless, a detailed explanation of these effects is quite difficult,
because the energy differences between the M L and M3L3
complexes are small with less than 10 kJ mol at room tempera-
ture and a superposition of several counterbalancing effects. A
limitation of our compounds is their low solubility, which
hinders their application in less competitive solvents with the
aim to investigate their host–guest properties in detail. Prelimi-
nary titration results, however, indicate anion binding, in which
hydrogen bonding is part of the attractive interactions as indi-
4
2
2
−
1
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(
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5
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1
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8420 | Dalton Trans., 2012, 41, 8410–8420
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