Synthesis, structural characterization and anion binding studies of palladium macrocycles with hydrogen-bonding ligands

Article abstract of DOI:10.1039/b703660d

The reactions between [Pd(P-P)(OTf)₂] (where P-P = dppp or dppf) and two different bipyridyl ligands (L¹ = 1,3-bis(4-pyridylmethyl) urea and L² = 1,3- bis(pyridinylmethyl)benzenedicarboxamide) containing hydrogen-bonding units have been studied. The X-ray crystal structures of three of these assemblies have been solved showing them to be the [2 + 2] metallo-macrocycles [Pd(P-P)(Lⁿ)]₂(OTf) ₄ [P-P = dppp, n = 1, (1); P-P = dppp, n = 2, (2); P-P = dppf, n = 1, (3)]. To confirm whether the dimeric assembly of one of these species (1) is retained in solution, several investigations have been carried out. ¹H NMR studies in DMSO and high resolution ESI mass spectrometry have shown that 1 is in equilibrium with a larger [3 + 3] metallo-macrocycle. The equilibrium between these two species can be modified by changing the temperature, concentration or solvent. Also, addition of certain anions (e.g. [H₂PO₄]^-) to the mixture shifts the equilibrium favoring the formation of the [2 + 2] metallo-macrocycle over the [3 + 3] (initially present in a larger proportion). The Royal Society of Chemistry.

Full text of DOI:10.1039/b703660d

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, structural characterization and anion binding studies of palladium
macrocycles with hydrogen-bonding ligands†‡
Pilar Diaz,a Jorge A. Tovilla,a,b Pablo Ballester,b,c Jordi Benet-Buchholzb and Ramo´n Vilar*a,b
Received 12th March 2007, Accepted 25th May 2007
First published as an Advance Article on the web 29th June 2007
DOI: 10.1039/b703660d
The reactions between [Pd(P–P)(OTf)2] (where P–P = dppp or dppf) and two different bipyridyl ligands
(L1 = 1,3-bis(4-pyridylmethyl)urea and L2 = 1,3- bis(pyridinylmethyl)benzenedicarboxamide)
containing hydrogen-bonding units have been studied. The X-ray crystal structures of three of these
assemblies have been solved showing them to be the [2 + 2] metallo-macrocycles [Pd(P–P)(Ln)]2(OTf)4
[P–P = dppp, n = 1, (1); P–P = dppp, n = 2, (2); P–P = dppf, n = 1, (3)]. To confirm whether the
dimeric assembly of one of these species (1) is retained in solution, several investigations have been
carried out. 1H NMR studies in DMSO and high resolution ESI mass spectrometry have shown that 1
is in equilibrium with a larger [3 + 3] metallo-macrocycle. The equilibrium between these two species
can be modified by changing the temperature, concentration or solvent. Also, addition of certain anions
(e.g. [H2PO4]−) to the mixture shifts the equilibrium favoring the formation of the [2 + 2]
metallo-macrocycle over the [3 + 3] (initially present in a larger proportion).
library (DCL) arises.25–28 Due to the dynamic equilibria established
Introduction
in a DCL, the stabilization of any given compound by addition
The synthesis of metallo-macrocycles and metallo-cages has
of an external species can lead to amplifying its formation.
received considerable attention over the past few years.1–8 This
Hence, the addition of a template to the library can shift the
type of assemblies are particularly attractive since the structural
equilibrium toward the formation of the compound that yields
and functional properties of metal centers can be combined with
the thermodynamically more stable host–template assembly. This
approach—pioneered by Sanders27 and Lehn26—has already been
used to successfully synthesise selective chemical receptors that
are amplified from a DCL.
the recognition abilities of organic ligands to yield species that can
act as hosts for specific guests. The selective recognition of guests
in the hollow cavities of the metallo-macrocycles and cages can in
turn have important potential applications in catalysis9 and in the
development of chemical sensors.10–15 One of the most successful
approaches to prepare metallo-assemblies is by self-assembly
where carefully designed building blocks are brought together by
coordination to metal centers with well defined geometries. This
approach has been successfully employed by Fujita,2,5,8 Stang,1,4
Raymond,16,17 Hump and Saalfrank18–20 amongst several other
groups to prepare a wide range of metallo-assemblies.
Although several reports have demonstrated that anions can be
successfully employed as templates for the formation of a wide
range of organic and metal–organic assemblies,29,30 to date there
are only a handful of examples where anions are used as templates
in DCLs.31–33
Herein we report the synthesis of a series of metallo-macrocycles
built from square planar [Pd(dppp)]2+ units and bis-pyridyl ligands
containing hydrogen bonding units (see Fig. 1).
Often, the assembly process can be improved (or modified)
by the presence of a template. Templating agents usually make
use of non-covalent interactions such as electrostatic forces, H-
bonding, p–p interactions and hydrophobic effects to pre-arrange
a group of building blocks in a suitable geometry to yield the
targeted assembly.21–24 When a specific set of building blocks can
be combined in different ways to yield different assemblies that can
continuously interconvert, the idea of a dynamic combinatorial
aDepartment of Chemistry, Imperial College London, London, UK SW7
2AZ. E-mail: r.vilar@imperial.ac.uk
bThe Institute of Chemical Research of Catalonia, Avgda. Pa¨ısos Catalans
16, 43007, Tarragona, Spain
cInstitucio´ Catalana de Recerca i Estudis Avanc¸ats (ICREA), Pg. Llu´ıs
Companys 23, 08010, Barcelona, Spain
Fig. 1 Schematic representation of ligands L1 and L2.
† CCDC reference numbers 624779–62478 and 624783-624784. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b703660d
‡ Electronic supplementary information (ESI) available: High resolution
ESI mass spectrum of 1 (Fig. S1–S3). See DOI: 10.1039/b703660d
Although a vast number of examples of metallo-macrocycles
based on palladium(II) and platinum(II) centres and bis-pyridyls
have been previously reported,10–13,34–46 in most cases the bis-
pyridyl bridge is rigid and does not contain further functionalities.
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We rationalized that an increased geometrical flexibility on the
bridging ligand—provided by CH2 groups between the pyridines
and the central spacer (see Fig. 1)—would potentially yield a wider
range of metallo-macrocycles. Furthermore, only a few examples
have been reported of metallo-macrocycles based on bis-pyridyl
bridges containing hydrogen bonding groups.47–50 Since we have
particular interest in the use of anions as templates to generate
selective receptors for such species,51–57 hydrogen bonding units
were introduced into ligands L1 and L2.
We have previously reported that the reaction of L1 with
different copper(II) salts yields a series of infinite coordination
networks whose structures are highly dependant on the coun-
teranions present in solution.58 Based on our previous results,
we hypothesized that these relatively flexible ligands could be
coordinated to metal complexes with restricted coordination sites
to yield metallo-macrocycles the structures of which could be
influenced by the presence of different anions. The results of
these investigations using cis-[Pd(P–P)]2+ (where P–P is a chelating
phosphine) are herein presented.
Fig. 2 Ortep-Plot (thermal ellipsoids shown at 50% probability level) of
1
=
L a (only molecule A is shown) showing the NH · · · O C hydrogen bond-
˚
˚
ing. Distances: O1AA · · · N1AB: 2.877(6) A (O1AA · · · H1AB: 2.09 A
˚
˚
uncorrected) and O1BA · · · N2BB: 2.868(6) A (O1BA · · · H2BB: 2.09 A
uncorrected).
Results and discussion
Syntheses of ligands and structural characterisation of L1
The bis-pyridyl compound L1 was prepared by modification of the
previously reported synthetic procedure,59 using 1,1ꢀ-bisimidazole
instead of diphenyl carbonate to generate the urea (see Ex-
perimental). Ligand L2 had also been previously reported and
was prepared using the literature procedure.60 Both compounds
were characterized spectroscopically and compared to the data
previously reported.
Fig. 3 Ortep-Plot (thermal ellipsoids shown at 50% probability level) of
1
=
L b showing the NH · · · O C hydrogen bonding. Distances: O1A · · · N1B:
˚
˚
2.866(6) A (O1A · · · H1B: 2.07 A uncorrected).
Single crystals of L1 suitable for X-ray crystallography were
obtained from CH2Cl2. Interestingly, two different crystal struc-
tures were obtained, a solvent free (L1a) and a monohydrate (L1b).
In both cases the structure shows L1 to have a cis-geometry
around the urea group. In the solvent-free crystal (L1a) the
structure contains two half independent molecules of L1 which
pack together in the centro-symmetrical space group Pccn (the
two independent molecules have a crystallographically imposed
Synthesis and crystal structures of metalla-macrocycles
[Pd(P–P)(Ln)]2[OTf]4
Once the ligands (L1 and L2) had been prepared and fully
characterized, their reactions with palladium(II) complexes were
investigated. We first studied the products resulting from the
reactions between [Pd(dppp)(OTf)2] (prepared from [Pd(dppp)Cl2]
and AgOTf) and the corresponding ligand (either L1 or L2). The
reactions were carried out in CH2Cl2 using a 1 : 1 palladium-
to-ligand ratio. Upon addition of the corresponding ligand to
the [Pd(dppp)(OTf)2] solution, a color change from yellow to
colorless and the appearance of a white precipitate was observed.
The reaction mixtures were stirred for several hours to ensure
that they had gone to completion. At this stage, the solvent was
removed under reduced pressure and the resulting solids washed
=
two-fold symmetry with the central C O bonds on two-fold
axes). On the other hand in L1b half molecules of L1 pack in the
chiral space group P21212; the molecule has crystallographically-
=
imposed twofold symmetry, with the central C O bond on the
twofold axes. The water molecule also lies on a twofold axis. In
1
=
both structures, the L molecules are connected via NH · · · O C
hydrogen bonds generating infinite chains (see Fig. 2 and Fig. 3).
In L1a the relative orientation of the inter-connected molecules
is dictated by a rotation of 35.5◦ of the planes defined by the
urea groups (see Fig. 2). The orientation of the pyridine rings
in neighboring molecules is inverted giving rise to a staggered
arrangement. In contrast, in L1b the hydrogen-bonding urea
groups of the connecting molecules are on the same plane defining
the relative orientation of the molecules. The pyridine rings from
neighboring molecules are perfectly stacked as shown in Fig. 3.
Although some previous structural studies on the co-
crystallization of L1 with other species (such as diiodopolyynes and
ureylene dicarboxylic acid)61,62 have been reported, this is the first
time that the crystal structure of a pure sample of L1 is reported.
1
with hexane. The 31P{ H} NMR spectra of both these solids
indicated that only one phosphorous-containing species had been
formed in each case. The product resulting from the reaction
with L1 had a d of 10.1 ppm while that obtained with L2 a d of
7.1 ppm. To explore whether the presence of a different anion in
the palladium(II) starting material would have an influence on the
outcome of the reaction, we also investigated the reaction between
[Pd(dppp)(PF6)2] and L1. The product obtained from this reaction,
had a very similar 31P NMR spectrum to the one resulting from the
reaction between [Pd(dppp)(OTf)2] and L1 (dP = 10.8 ppm plus a
signal corresponding to PF6). In order to establish unambiguously
the stoichiometry and molecular structure of these assemblies,
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single crystals of the three complexes were obtained and analyzed
by X-ray crystallography.
The molecular structures of these compounds in the solid state
showed them to be the [2 + 2] assemblies [Pd(dppp)(L1)]2(OTf)4
(1) and [Pd(dppp)(L2)]2(OTf)4 (2) (see Scheme 1).
Fig. 5 Molecular structure of 2 (atoms in Ortep-plot: thermal ellipsoids
shown at 50% probability level). The triflate counter-anions and non
relevant hydrogen atoms have been omitted for clarity. Selected bond
◦
˚
distances (A) and angles ( ): Pd1–N1: 2.095(6), Pd1–N8: 2.117(6), Pd1–P1:
2.273 (2), Pd1–P2: 2.264(2), N1–Pd1–N8: 86.2(2), Pd2–N4: 2.086(7),
Pd2–N5: 2.094(7), Pd2–P3: 2.269(2), Pd2–P4: 2.270(2) and N4–Pd2–N5:
86.1(3).
Scheme 1
In these two assemblies the palladium centers have a distorted
square planar geometry with two cis positions occupied by the
dppp ligand and the other two by the pyridyl groups of either L1
(in complex 1—see Fig. 4) or L2 (in complex 3—see Fig. 5).
form hydrogen bonds with two different triflate anions located
at the sides of the metallo-macrocycle. The distances between
the oxygen atoms of the triflate anions and the nitrogen atoms
˚
of the urea groups are in the region 2.8–2.9 A. A third triflate
anion is located in the middle of the assembly displaying weak
interactions to different adjacent aromatic rings. The fourth triflate
anion is located outside the “cavity” formed by the macrocycle at a
relatively short distance from Pd2 (shortest distance Pd2 · · · O1B:
˚
3.24 A). In the crystal packing intermolecular interactions between
urea groups cannot be observed.
The crystal structure of 2 (with ligand L2) shows one of the
triflate counter-anions located at the centre of the [2 + 2] assembly
while two more are found at the sides of the metallo-macrocycle
each of them interacting with one palladium center. The shortest
distances between the counter-anions and the palladium atoms
˚
˚
are: Pd1 · · · O2SA: 2.97 A and Pd2 · · · O2SB: 2.98 A. Further
intermolecular interactions are found in this structure, in the
crystal packing a combination of p–p and hydrogen bonding
interactions between ligands of neighboring molecules can be
found. These interactions lead to the formations of pairs of
molecules as is shown in Fig. 6.
Fig. 4 Molecular structure of 1 (atoms in Ortep-plot: thermal ellipsoids
shown at 50% probability level). The triflate counter-anions and non
relevant hydrogen atoms have been omitted for clarity. Selected bond
◦
˚
distances (A) and angles ( ): Pd1–N1: 2.108(4), Pd1–N8: 2.106(5),
Pd1–P1: 2.2710(14), Pd1–P2: 2.2791(13), N1–Pd1–N8: 85.78(19), Pd2–N4:
2.089(4), Pd2–N5: 2.110(5), Pd2–P3: 2.2745(14), Pd2–P4: 2.2740(13) and
N4–Pd2–N5: 86.07(17).
The N–Pd–N angles are contracted to 85.78(19)◦/86.07(17)◦ in
1 and 86.2(3)◦/86.1(3) in 2; this deviation from an ideal square
planar geometry is likely to be a consequence of the geometrical
arrangement of the corresponding ligands when the [2 + 2]
assembly forms. The CH2 groups that link the pyridyls to the
central urea or diamide groups in L1 and L2 respectively allow for
a good degree of flexibility in these ligands. As a consequence, the
bridging molecules are bent giving a bowl-like geometry to the
Pd2L2 cores in both 1 and 2 (see side view in Fig. 4, Fig. 5). This
is more pronounced in the case of 2 where the planes formed by
the coordination geometry around the two palladium centers are
63
˚
nearly parallel (with a Pd · · · Pd distance of approx. 10.5 A).
Fig. 6 Hydrogen bonding and p–p interactions between two molecules of
2. The triflate anions, solvent molecules and non relevant hydrogen atoms
have been omitted for clarity. Letter “A” in labelled atoms indicates that
these are at equivalent positions (−x,1−y,1−z).
In complex 1, the carbonyl groups of the ureas are pointing
to an isopropanol molecule which is located at the base of the
molecule between the urea groups. The NH groups of the ureas
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The distance of the observed hydrogen bond between the amides
2
˚
˚
of L is O3 · · · N7A: 2.836(10) A (O3 · · · H7A: 1.99 A uncorrected)
and the distance between the planes of the aromatics rings (C70–
C75) is ca. 3.7 A. The second L ligand (which is not engaged
2
˚
in hydrogen bonding) from each metallo-macrocycle, is packing
between the described pairs forming stacks of molecules. The p–
˚
p distance in this case is of 4.90 A too long to be considered
a strong intermolecular interaction. Due to the stacking of the
metallo-macrocycles, channels are formed in the crystal packing
(see Fig. 7). The channels contain triflate counter-anions and
partially disordered isopropanol molecules.
Fig. 8 Molecular structure of 3 (atoms in Ortep-plot: thermal ellip-
soids shown at 50% probability level) showing the interaction with the
triflate counter-anions. The fourth counter-anion, the solvent molecules
and non relevant hydrogen atoms have been omitted for clarity. Se-
lected bond distances and angles: Pd1–N1: 2.088(6), Pd1–N8: 2.096(6),
Pd1–P1: 2.3057(19), Pd1–P2: 2.3102(17), N1–Pd1–N8: 85.8(2)◦, Pd2–N4:
2.106(6), Pd2–N5: 2.104(6), Pd2–P3: 2.2984(18), Pd2–P4: 2.3030(19) and
N4–Pd2–N5: 84.7(3).
˚
˚
2.971(7) A (O2D · · · H7A: 2.10 A uncorrected). A third triflate
is interacting partially with the second urea group, O3A · · · N2:
˚
˚
3.117(7) A (O3A · · · H2B: 2.47 A uncorrected) and weakly with
˚
one of the palladium atoms (Pd1 · · · O2A: 4.16 A). The fourth
counter-anion is packing in the intermolecular area. Interactions
between urea groups were not observed in this case.
The X-ray crystal structures of these four metallo-macrocycles
indicate that, in the solid state, the geometrical constraints
imposed by both the bis-pyridyl ligands and by the cis-square
planar coordination around the palladium(II) centers favor the
formation of [2 + 2] assemblies. However, in solution it would not
be surprising that other species were present in equilibrium with
the [2 + 2] assemblies. In order to investigate this possibility, we
engaged in a detailed study of the behavior of 1 in solution.
Fig. 7 Crystal packing of 2 with view along the a-axes showing the
observed channel in the crystal array. The triflate counter-anions, solvent
molecules and hydrogen atoms have been omitted for clarity.
In order to investigate whether the nature of the chelating
phosphine could have an influence on the structure of the metallo-
macrocycle formed, the reaction between [Pd(dppf)(OTf)2]
(dppf = 1,1–bis(diphenylphosphino)ferrocene) and L1 was carried
out. As in the synthesis of 1 and 2, a solid was isolated
from the reaction mixture (see Experimental section) which,
based on its analytical and spectroscopic characterisation, was
formulated as [Pd(dppf)(L1)]n(OTf)2n. In the 31P NMR spectrum,
a singlet at 32.7 ppm was observed while the elemental analyses
were consistent with a Pd : dppf : L ratio of 1 : 1 : 1. To
determine the nuclearity of this assembly, single crystals suitable
for an X-ray crystallographic analysis were grown from an
acetone–isopropanol mixture. As for the previous molecules, this
compound was shown to be the [2 + 2] metallo-macrocycle
[Pd(dppf)(L1)]2(OTf)4 (3). The overall structure of 3 is very similar
to that of 1. The geometry around the palladium centers is also
a distorted square planar with an N–Pd–N angle of 85.8◦. As
in the other two structures, the bridging ligands in 3 are bent
giving a bowl-like structure with one of the triflates located at
the center of the assembly (see Fig. 8). A second triflate interacts
via hydrogen bonds with one of the urea groups (O1D · · · N6:
Solution studies of [Pd(dppp)(L1)]2[OTf]4 (1)
1
As indicated above, the 31P{ H} NMR spectrum of 1 showed
only one singlet at 10.1 ppm which is consistent with the X-ray
crystal structure of this compound. However, careful inspection
1
of the H NMR spectrum in different solvents suggested a more
complex behavior of this assembly in solution. When the 1H NMR
spectrum was recorded in d6-DMSO (see Fig. 9, spectrum at
the top) the signals were considerably broadened. Furthermore,
a set of small signals could be seen next to some of the main
resonances associated with the ligand. Interestingly, when the 1H
NMR spectrum of the same sample of 1 was recorded in d6-acetone
(see Fig. 9, spectrum at the bottom), sharp and well defined signals
were obtained, and no “extra” peaks were present.
It was hence of interest to investigate further the behavior of
the system in DMSO. First, a variable temperature 1H NMR
experiment between 300 and 350 K was carried out. As can be
seen in Fig. 10, upon increasing the temperature of 1 in d6-DMSO
˚
˚
2.907(7) A (O1D · · · H6C: 2.09 A uncorrected) and O2D · · · N7:
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To elucidate which one of the two phenomena outlined above
1
was responsible for the spectroscopic behavior observed, a H
NMR dilution experiment of 1 in d6-DMSO was carried out.65 As
can be seen in Fig. 11, upon reducing the concentration of 1 in
d6-DMSO no important shifts of the resonances were detected.
However, a clear change in the relative integration of the pairs of
signals associated to Ha and Hc can be seen (presumably a similar
change takes place for Hb however this signal and its “pair” are
too broad to be independently identified). This change is most
apparent if the first and last spectra are compared.
Fig. 9 Section of the 1H NMR spectra of complex 1 in d6-DMSO (top)
and d6-acetone (bottom). The structure of the ligand is shown to assign
labels (although the spectra are of the metallo-assembly 1).
Fig. 11 Section of the 1H NMR spectrum of 1 in d6-DMSO at different
concentrations ranging between 10.40 and 0.26 mM.
The fact that the relative integrations of the pairs of peaks
change at different concentrations indicates the presence of
assemblies with different stoichiometry rather than different con-
formations of the complex. As indicated above, a likely explanation
is that two or more assemblies with different nuclearities—
presumably the [2 + 2] and [3 + 3] metallo-macrocycles—
are present in DMSO. From an entropic point of view, the
smaller assembly (i.e. the [2 + 2] species) should form at higher
temperatures and at lower concentrations. With this in mind, we
have assigned in the 1H NMR spectra of Fig. 11 the proton signals
corresponding to the lower nuclearity assembly (i.e. the [2 + 2]) and
those for a higher nuclearity species (hypothetically the [3 + 3]).
To investigate whether the [3 + 3] metallo-assembly was indeed
present, a high resolution ESI mass spectrum of 1 was recorded. It
should be noted that the solvent employed to dissolve the sample
and record the mass spectrum was acetonitrile and not DMSO,
which was the solvent used in the NMR experiments described
above. The mass spectrum was dominated by a peak at 909.3 amu
which corresponds to [Pd(dppp)(L1)(OTf)]+. Closer inspection of
the isotopic pattern of this peak, allowed us to identify lower inten-
sity peaks of half mass-units which are indicative of the presence
of a doubly charged species, i.e. [Pd2(dppp)2(L1)2(OTf)2]2+. A low
intensity peak centered at 1439.19 amu with a complex isotopic
pattern (see Fig. 12) was also present in the spectrum, the m/z and
isotopic pattern of this peak are consistent with the formulation
[Pd3(dppp)3(L1)3(OTf)4]2+ which suggests that the [3 + 3] assembly
is indeed present in solution. The ESI mass spectrum did not
provide evidence for the presence of a [4 + 4] macrocyclic species
or any other higher nuclearity assemblies.
Fig. 10 Section of the 1H NMR spectrum of palladium complex 1 in
d6-DMSO at different temperatures ranging between 300 and 350 K.
two phenomena take place: sharpening of the signals and a shift of
some of the signals (those associated to Ha, Hb and Hc—see labels
in Fig. 9). To check that no decomposition of the assembly took
place, the mixture was cooled down to the original temperature
(300 K) and the 1H and 31P NMR spectra recorded again. Both of
these spectra showed the product to be identical to the one at the
beginning of the experiment.
The sharpening of the signals upon increasing the temperature
of the solution could be associated to different dynamic processes.
It could be possible that different conformations of the complex
are in intermediate exchange in DMSO solution giving rise to the
broadening and splitting of some of the proton signals (especially
those associated to the NH and CH2 groups). Upon increasing
the temperature the conformational exchange becomes fast in the
1
NMR timescale and produces a H-NMR spectrum with just a
single set of sharper signals. Another possible scenario is that
the chemical exchange is taking place between supramolecular
assemblies of different stoichiometry (e.g. a mixture of the [2 + 2]
and [3 + 3] metallo-macrocycles).64
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the anion) there are little changes in the chemical shifts of the
signals, however, an important change in the relative integration
between the signals assigned to the [2 + 2] and [3 + 3] assemblies
can be seen. This observation indicates that the initial addition
of [H2PO4]− modifies the equilibrium between the two assemblies
favoring the formation of the [2 + 2] macrocycle. This behavior
is probably due to the selective binding of the [H2PO4]− anion
with the [2 + 2] assembly with a concomitantly displacement
of the equilibrium between “free” assemblies. Interestingly, once
enough anion has been added (ca. 4 equivalents) to displace almost
completely the equilibria to the formation of [2 + 2] assembly, a
second phenomenon is observed. That is, the addition of more
than 4 equivalents of [H2PO4]− anion sharpen the proton signals
and the NH signal of the urea shifts slightly to lower field.
These observations are consistent with the elimination of the slow
chemical exchange previously operative between “free” assemblies
and responsible for the broadening of the signals together with the
complex formation between the remaining free [2 + 2] assembly,
at this point of the titration, and the anion being added.
Fig. 12 Peak associated to [Pd3(dppp)3(L1)3(OTf)4]2+ in the high resolu-
tion ESI mass spectrum of a solution of [Pd(dppp)(OTf)2] plus L1 (which
yields 1 in the solid state but a mixture of products in solution).
In summary, the mass spectrometric evidence together with the
1H NMR spectroscopic data, strongly suggest that in solution
there is a dynamic equilibrium between the [2 + 2] metallo-
macrocycle 1 (which is also the one that crystallizes) and the [3 + 3]
metallo-macrocycle [Pd3(dppp)3(L1)3](OTf)6—see Scheme 2). This
equilibrium is affected by the concentration, the solvent employed
and the temperature of the solution.
Interestingly, when the analogous titration experiment was
repeated with [NBu4][HSO4] a different behavior was observed (see
Fig. 14). Even after addition of ca. 12 equivalents of the anion,
no important changes were observed in the relative integration of
the peaks associated to the two different assemblies (i.e. the [2 + 2]
and [3 + 3] macrocycles) present in solution. However, a shift of
some of the signals can be clearly seen which suggests that the
added [HSO4]− binds selectively to the [3 + 3] metallo-host. In this
case, the extent of the binding has not displaced completely the
equilibrium towards the [3 + 3] assembly since the signals of the
[2 + 2] assembly are still visible in the 1H NMR spectra. Likewise,
the chemical equilibrium between free metallo-macrocycles is still
operative as can be inferred from the broadening of the proton
signals.
Scheme 2
Anion binding studies of 1
Our attention was then directed to establishing whether the
addition of different anions would modify the distribution of
products in the equilibrium described above. Therefore, a sample
of 1 was dissolved in d6-DMSO giving the expected mixture of
[2 + 2] and [3 + 3] assemblies (see previous section). Aliquots of
this mixture were taken to carry out 1H NMR titration experiments
with different anions. As is shown in Fig. 13, addition of increasing
amounts of [NBu4][H2PO4] to this solution caused interesting
changes in the spectrum. Initially (up to ca. 3 equivalents of
Fig. 14 Changes in the 1H NMR spectra of 1 in DMSO upon addition
of increasing amounts of [HSO4]− (between 0 and 11.41 equivalents).
Addition of di-carboxylates such as oxalate and malonate, had a
similar effect to the one observed with [H2PO4]− i.e. the equilibrium
is shifted to the formation of the [2 + 2] macrocycle.
Taken together, these results suggest the possibility of using
different anions to template the formation of metallo-macrocycle
of different sizes and stoichiometry (see Scheme 3). To confirm
Fig. 13 Changes in the 1H NMR spectra of 1 in DMSO upon addition
of increasing amounts of [H2PO4]− (between 0 and 11.41 equivalents).
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infusion). Capillary voltage 1.8 kV, cone 100 V and collision energy
set to 10 eV. [Pd(dppp)Cl2], [Pd(dppf)Cl2] and L2 were synthesized
following literature procedures.60,66,67
ꢀ
1
=
Synthesis of N,N -(C5H4N-p-CH2)2-NHC( O)NH (L )
A slight modification of a reported method by Fowler59 was
used: 1,1ꢀ-carbonyldiimidazole (0.689 g, 4.3 mmol) was dissolved
in CH2Cl2 (30 ml) under an argon atmosphere. 4-Picolylamine
(0.89 ml, 8.6 mmol) was added slowly into the solution and the
reaction was stirred at 40 ◦C for 20 h. The volume was then reduced
to ca. 10 ml under reduced pressure and ethyl acetate (50 ml) was
added to precipitate the crude product. The solid formed was
filtered, washed with ethyl acetate (2 × 10 ml) and air dried to
produce the final product as a white powder. Yield: 0.518 g, 50%.
Mp = 185–186 ◦C (mp reported by Fowler and co-workers = 186–
Scheme 3 Schematic representation of the effect of different anions in the
equilibrium between two metallo-macrocycles (square/triangle).
◦
187 C). IR (m (KBr)): 3335 (N–H), 3024 (aromatic C–H), 2934
1
=
(aliphatic C–H), 1628 (C O). H NMR (dmso-d6) d (in ppm) =
this hypothesis, several attempts to grow crystals from a mixture
containing [Pd(dppp)(OTf)2], L1 and [HSO4]− were carried out
with the hope to isolate in the solid state a [3 + 3] metallo-
macrocycle. Unfortunately, to date this has been unsuccessful
and further studies will be required to characterize the [3 + 3]
macrocycle in the solid state.
8.49 (dd, 4H, 2-PyH, 3JH–H = 4.4 Hz, 4JH–H = 1.7 Hz), 7.24 (dd, 4H,
3-PyH, 3JH–H = 4.4 Hz, 4JH–H = 1.7 Hz), 6.74 (t, 2H, NH, 3JNH–CH
=
6.1 Hz), 4.26 (d, 4H, SCH2, 3JNH–CH = 6.1 Hz). 13C NMR (dmso-d6)
=
d (in ppm) = 158.1 (C O), 149.4 (2-Py), 121.9 (3-Py), 42.0 (CH2).
Synthesis of [(dppp)Pd(l-L1)]2[SO3CF3]4 (1)
[Pd(dppp)Cl2] (0.089 g, 0.15 mmol) and AgSO3CF3 (0.077 g,
0.30 mmol) were dissolved in CH2Cl2 (20 ml) and stirred in an
inert atmosphere of N2 and in the absence of light for 72 h at room
temperature. After this time the reaction was filtered to remove
the precipitate of AgCl and the resulting yellowish solution of
[(dppp)Pd(CF3SO)2] was used in the following step without further
purification. To this solution ligand L1 (0.036 g, 0.15 mmol) was
added as a solid. Almost immediately the yellow solution became
colorless and a white precipitate started appearing. The mixture
was left stirring at room temperature overnight under N2. After
this time the mixture was filtered off recovering a white solid that
was washed with n-pentane and dried under reduced pressure at ca.
100 ◦C. Yield: 0.086 g, 54% (with respect to palladium). Elemental
Analysis Found: C, 47.36; H, 3.77; N, 5.14. Calculated for
C84H80F12N8O14P4Pd2S4: C, 47.62; H, 3.81; N, 5.29. IR (m (KBr)):
3395 (N–H), 3059 (aromatic C–H), 2925 (aliphatic C–H), 1665,
Conclusions
Three different [2 + 2] metallo-macrocycles (1–3) based on flexible
bis-pyridyl ligands containing hydrogen bonding groups have been
successfully prepared and structurally characterized. In the solid
state, these species have a bowl-type structure allowing, in some
cases, for the encapsulation of anionic species via a combination
1
of several weak supramolecular interactions. Detailed H NMR
spectroscopic solution studies for 1 have been carried out showing
that in DMSO this species is in a dynamic equilibrium with a
larger assembly (likely to be the [3 + 3] metallo-macrocycle). The
equilibrium between these two species can be modified by changing
the temperature, the concentration, the solvent or by addition of
different anions. Although at the moment the system gives rise to
only two different assemblies, it is expected that in the presence of
a mixture of ligands a larger library of metallo-macrocycles could
be obtained. Studies are currently underway to develop this type
of library and study whether the addition of different anions will
allow amplification of a specific metallo-assembly.
=
1618 (C O), 1571, 1437, 1256, 1161, 1030 (CF3SO3–). FAB(+)–
MS m/z (in amu): 1727 (2%), [Pd2(dppp)2(L1)(CF3SO3)3]+; 759
(2%), [Pd(dppp)(L1)]+; 667 (100%), [Pd(dppp)(CF3SO3)]+ (for the
results obtained using ESI(+)-MS see Results and Discussion
section). 31P NMR (acetone-d6) d = 10.1 ppm. 1H NMR (acetone-
Experimental
3
d6) d (in ppm) = 8.88 (d, 8H, 2-PyH, JH–H = 5.6 Hz), 7.79 (m,
16H, o-ArH(dppp)), 7.57 (m, 8H, p-ArH(dppp)), 7.49 (m, 16H, m-
ArH(dppp)), 7.03 (d, 8H, 3-PyH, 3JH–H = 5.6 Hz), 6.32 (broad, 4H,
General
All experimental procedures were carried out using standard high
vacuum and Schlenk-line techniques under an atmosphere of dry
nitrogen or argon. Glassware was dried in an oven at 150 ◦C
3
NH), 4.20 (d, 8H, CH2NH, JNH–CH = 5.7 Hz), 3.41 (broad, 8H,
CH2P), 2.41 (broad, 4H, CH2CH2P). 19F NMR (acetone-d6) d (in
ppm) = −78.8. Crystals suitable for X-ray analysis were obtained
by slow evaporation from an acetone–isopropanol solution.
1
prior to use. H, 13C and 31P NMR spectra were recorded on
a Bruker Avance 400 or Bruker Avance 500 spectrometers and
referenced to residual H and 13C signals of the solvents or 85%
1
Synthesis of [(dppp)Pd(l-L2)]2(SO3CF3)4 (2)
H3PO4 as an external standard (31P). The high resolution ESI mass
spectrometric results (see Fig. 12 and mass spectrum in the ESI‡)
were obtained at the University of London School of Pharmacy.
The sample was run in acetonitrile on a Waters QTOF Global
Ultima, positive ion mode, v-mode using borosilicate tips (direct
Pd(dppp)Cl2 (102.1 mg, 0.1466 mmol) was dissolved in ca. 20 mL
of CH2Cl2. To this solution, AgCF3SO3 (81.7 mg, 0.3180 mmol)
was added as a solid. The mixture was left stirring at room
temperature for 72 h with exclusion of light. After this time, the
3522 | Dalton Trans., 2007, 3516–3525
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reaction was filtered to remove AgCl and the yellowish solution
used in the following step without further purification. To this
yellow solution ligand L2 (52 mg, 0.1468 mmol) was added as a
solid. Almost immediately after addition, the yellow color of the
solution disappeared yielding a colorless solution and some white
precipitate. The reaction mixture was left stirring overnight after
which time it was filtered and the solvent was evaporated to dryness
under reduce pressure. This solid was washed with n-pentane and
dried under reduced pressure at ca. 100 ◦C. Yield: 125.1 mg, 73%
(with respect to palladium). Elemental Analysis Found: C, 50.50;
H, 3.76; N, 4.85. Calculated for C98H88N8O16S4F12P4Pd2: C, 50.52;
H, 3.78; N, 4.81. IR (m (KBr)): 3439 (N–H), 3057 (aromatic C–H),
−173 ◦C). Full-sphere data collection was used with x and u scans.
Programs used: Data collection Apex2 V. 1.0–22 (Bruker-Nonius
2004), data reduction Saint+ Version 6.22 (Bruker-Nonius 2001)
and absorption correction SADABS V. 2.10 (2003). The structure
solution and refinement was carried out using SHELXTL Version
6.10 (Sheldrick, 2000).68
.
CCDC reference numbers 624779–62478 and 624783-624784.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b703660d
Crystal data for L1a at 100 K. C13H14N4O1, 242.28 gmol−1,
˚
˚
orthorhombic, Pccn, a = 16.291(2) A, b = 16.300(2) A, c =
9.1949(12) A, V = 2441.7(6) A , Z = 8, qcalcd = 1.318 Mg m−3, R1 =
0.0634 (0.0700), wR2 = 0.1827 (0.1901), for 4197 reflections with
I > 2r(I) (for 4673 reflections [Rint: 0.0619] with a total measured
of 16 300 reflections), goodness-of-fit on F2 = 1.058, largest diff.
3
˚
˚
2926 (aliphatic C–H), 1668, 1618 (C O), 841 (PF6–). 31P NMR
=
(CD3CN) d = 7.1 ppm,. 1H NMR (acetone-d6) d (in ppm) = 8.45
3
(d, JHH = 6 Hz, 8H, py), 8.25 (broad singlet, 2H, Ph), 7.95 (dd,
3JHH = 6 Hz, 4JHH = 2 Hz, 4H, Ph), 7.75 (m, 43H, Ph), 7.60–7.28
(broad, 4H, NH), 7.13 (broad, 8H, py), 4.41 (broad, 8H, –CH2-
py), 3.05 (broad, 8H, –CH2CH2 dppp), 2.1 (broad, –CH2–dppp).
19F NMR (acetone-d6) d (in ppm) = −78.1 ppm. Crystals suitable
for X-ray crystallography were obtained from isopropanol.
−3
˚
peak (hole) = 0.488 (−0.278) e A .
Crystal data for L1b at 100 K. C13H14N4O1·H2O, 260.30 g
−1
˚
˚
mol , orthorhombic, P21212, a = 10.5821(18) A, b = 13.149(3) A,
c = 4.5942(8) A, V = 639.3(2) A , Z = 2, qcalcd = 1.352 Mg m−3, R1 =
0.0712 (0.0770), wR2 = 0.2021 (0.2063), for 2989 reflections with
I > 2r(I) (for 3252 reflections [Rint: 0.0292] with a total measured
of 11 047 reflections), goodness-of-fit on F2 = 1.175, largest diff.
3
˚
˚
Synthesis of [(dppf)Pd(l-L1)]2(SO3CF3)4 (3)
−3
˚
Pd(dppf)Cl2 (50 mg, 0.0612 mmol) was dissolved in ca. 20 mL of
dry and degassed CH2Cl2. To this solution, AgSO3CF3 (37 mg,
0.1440 mmol) was added as a solid. The mixture was left stirring
at room temperature for 72 h with exclusion of light. After this
time, the reaction was filtered to remove AgCl and the resulting
dark green solution was used in the following step without further
purification. To the solution containing Pd(dppf)(SO3CF3)2 ligand
L1 (15.1 mg, 0.0624 mmol) was added as a solid under vigorous
stirring. Almost immediately after addition the dark green color
changed to purple. The mixture was left stirring at room temper-
ature overnight after which time it was filtered to remove a white
peak (hole) = 0.419 (−0.894) e A .
Crystal data for 1 at 100 K. C80H80N8O2P4Pd2·4C1F3O3S1·
1·C3H8O1, 2196.60
g
mol−1, orthorhombic, P212121,
a
=
˚
˚
˚
18.6063(11) A, b = 21.5364(13) A, c = 27.1103(16) A, V =
3
10863.4(11) A , Z = 4, qcalcd = 1.343 Mg m−3, R1 = 0.0854
˚
(0.1067), wR2 = 0.2381 (0.2552), for 32 844 reflections with I >
2r(I) (for 41 344 reflections [Rint: 0.0501] with a total measured
of 160 178 reflections), goodness-of-fit on F2 = 1.091, largest diff.
−3
˚
peak (hole) = 3.535(−2.093) e A . This structure refines as a
racemic twin with ratio of 53 : 47. Crystals of this compound
are highly sensitive to losing partially its solvent molecules. After
refinement there are residual voids which are likely to contain
not-localised highly-disordered isopropanol molecules. In general
the triflate anions and specially the isopropanol molecules are
disordered. The detected highest peak and deepest hole of the
electron densities are localized close to one of the disordered
sulfur atoms belonging to a triflate anion and could not be
improved. Due to the high number of anions and localised solvent
molecules contained in the crystal packing, it was considered
better not to use SQUEEZE69 for data improvement. Distances
and thermal displacement parameters for these molecules were
partially restrained.
1
solid precipitate (which H NMR showed was some unreacted
ligand). The purple solution was concentrated to ca. 2 mL and
diethyl ether was added to yield a purple solid. The solid was
recovered and dried under reduced pressure overnight heating
at ca. 100 ◦C. Yield: 21.1 mg, 20% (with respect to palladium).
Elemental Analysis Found: C, 49.10; H, 3.59; N, 4.80. Calculated
for C98H84N8O14P4F12S4Fe2Pd2: C, 48.97; H, 3.50; N, 4.66. IR (m
(KBr)): 3396 (N–H), 3098 (aromatic C–H), 2925 (aliphatic C–
=
H), 1670, 1618 (C O), 1554, 1436, 1257, 1224, 1165, 1097, 1062,
1030 (CF3SO3–), 749, 696, 638, 556, 516 494, 467. 31P NMR
(CD2Cl2) d = 32.70 ppm. 19F NMR (CD2Cl2) d (in ppm) =
−80.8 ppm. Crystals suitable for X-ray analysis were obtained
from an acetone–isopropanol solution (by slow evaporation).
Crystal data for 2 at 100 K. The crystals measured are
of small dimensions and extremely sensitive to losing solvent.
C94H88N8O4P4Pd2·4C1F3O3S1·3/4C3H8O1·2H2O (some of the hy-
X-Ray structure determination
−1
¯
drogen atoms could not be localised), 2404.26 g mol , P1, a =
◦
˚
˚
˚
Crystals of 1, 2 and 3 were obtained by crystallisation in
isopropanol (1 and 2) and dichloromethane (3). Crystals were gen-
erally of poor quality and with the tendency of degradation due to
solvent losses. The measured crystals were quickly prepared under
inert conditions immersed in perfluoropolyether as protecting oil
for manipulation. Measurements were made on a Bruker-Nonius
diffractometer equipped with a APPEX 2 4K CCD area detector,
a FR591 rotating anode with Mo-Ka radiation, Montel mirrors
as monochromator and a Kryoflex low temperature device (T =
15.8443(7) A, b = 18.8383(9) A, c = 20.6983(8) A, a = 95.680(2) ,
b = 94.482(2)◦, c = 104.570(2)◦, V = 5915.8(4), Z = 2, qcalcd
=
1.350 Mg m−3, R1 = 0.0841 (0.1796), wR2 = 0.2237 (0.2690),
for 28010 reflections with I > 2r(I) (for 12 427 reflections [Rint:
0.1251] with a total measured of 74 819 reflections), goodness-
of-fit on F2 = 0.977, largest diff. peak (hole) = 2.424 (−1.143)
−3
˚
e A . Similar to compound 1, crystals of this compound are highly
sensitive to losing partially solvent molecules. The triflate anions
refined are partly disordered with large thermal ellipsoids. Also
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the isopropanol and the water molecules localised are disordered
and in some cases have partial assigned occupancies. Distances
and thermal displacement parameters for these anions/solvent
molecules are restrained. After final refinement there are residual
voids which probably contain highly disordered isopropanol and
water molecules which could not be clearly identified. The detected
highest peak and deepest hole of the residual electron densities
are localized close to one of the disordered solvent molecules
but could not be identified and refined as disordered solvent.
Applying SQUEEZE (Platon)69 did not give significant data
improvements resulting in similar R-values as modeling disordered
solvent molecules.
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Crystal data for 3 at 100 K. C94H84Fe2N8O2P4Pd2·4C1F3O3S1·
−1
¯
1C1H2Cl2·3H2O (without hydrogen atoms), 2535.28 g mol , P1,
◦
˚
˚
˚
a = 15.828(3) A, b = 16.901(3) A, c = 20.704(8) A, a = 71.936(4) ,
b = 86.141(4)◦, c = 87.923(4)◦, Z = 2, qcalcd = 1.603 Mg m−3, R1 =
0.0973 (0.1402), wR2 = 0.2504 (0.2967), for 25830 reflections with
I > 2r(I) (for 17 616 reflections [Rint: 0.0759] with a total measured
of 55 822 reflections), goodness-of-fit on F2 = 1.056, largest
−3
˚
diff. peak (hole) = 2.889 (−2.727) e A . The crystal contains
disordered water molecules with partial assigned occupancies. The
triflate counter-anions are also partly disordered and are showing
large thermal displacement parameters.
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macrocycle 1 were carried out by dissolving ca. 2 mg of the complex
1
in DMSO-D6, under Ar. H NMR spectra of this sample were
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recorded at increasing temperature at intervals of 10 K starting
from 300 K up to 350 K and back again to the initial temperature.
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integrity throughout the whole process.
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1H-NMR spectroscopic titrations of 1 with different anions
The following procedure is the one used for the titration with
[H2PO4]−. An analogous procedure was used for all the other
anions. All operations were carried out in an inert argon atmo-
sphere. A host stock solution was prepared by dissolving 1 (4.1 mg,
0.002 mmol) in d6-DMSO (2 ml). An aliquot of this solution (1.2
ml) was used to dissolve [NBu4][H2PO4] (6.6 mg, 0.02 mmol) to
prepare the guest stock solution employed for titrating the metallo-
macrocycle. An initial aliquot (0.5 ml) of the host stock solution
1
was transferred into an NMR tube and its H-NMR spectrum
determined. Increasing volumes of the guest stock solution were
added with a micro-syringe into the same sample to reach, at the
end, ca. 12 equivalents. The 1H-NMR spectra of the sample after
each addition were measured and the chemical shifts of the urea
and aromatic protons recorded. The 31P NMR spectra of selected
samples (including the initial and final points) were measured
to confirm the integrity of the complex throughout the whole
procedure.
38 M. Schweiger, S. R. Seidel, A. M. Arif and P. J. Stang, Angew. Chem.,
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