Tuning the Molecular Packing of Self-Assembled Amphiphilic PtII Complexes by Varying the Hydrophilic Side-Chain Length

Article abstract of DOI:10.1002/chem.202003445

Understanding the relationship between molecular design and packing modes constitutes one of the major challenges in self-assembly and is essential for the preparation of functional materials. Herein, we have achieved high precision control over the supramolecular packing of amphiphilic Pt^II complexes by systematic variation of the hydrophilic side-chain length. A novel approach of general applicability based on complementary X-ray diffraction and solid-state NMR spectroscopy has allowed us to establish a clear correlation between molecular features and supramolecular ordering. Systematically increasing the side-chain length gradually increases the steric demand and reduces the extent of aromatic interactions, thereby inducing a gradual shift in the molecular packing from parallel to a long-slipped organization. Notably, our findings highlight the necessity of advanced solid-state NMR techniques to gain structural information for supramolecular systems where single-crystal growth is not possible. Our work further demonstrates a new molecular design strategy to modulate aromatic interaction strengths and packing arrangements that could be useful for the engineering of functional materials based on Pt^II and aromatic molecules.

Full text of DOI:10.1002/chem.202003445

Accepted Article
Accepted Article
Title: Tuning the Molecular Packing of Self-Assembled Amphiphilic
Pt(II) Complexes by Variation of the Hydrophilic Side Chain
Length
Authors: Lorena Herkert, Philipp Selter, Constantin G. Daniliuc, Nils
Bäumer, Jasnamol P. Palakkal, Gustavo Fernandez, and
Michael Ryan Hansen
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202003445
Link to VoR: https://doi.org/10.1002/chem.202003445
01/2020

10.1002/chem.202003445
Chemistry - A European Journal
RESEARCH ARTICLE
Tuning the Molecular Packing of Self-Assembled Amphiphilic
Pt(II) Complexes by Variation of the Hydrophilic Side Chain Length
Lorena Herkert,[a]§ Philipp Selter,[b]§ Constantin G. Daniliuc,[a] Nils Bäumer,[a] Jasnamol P. Palakkal,[c]
Gustavo Fernández,*[a] and Michael Ryan Hansen*[b]
Abstract: Understanding the relationship between molecular design
and packing modes constitutes one of the major challenges in self-
assembly and is essential for the preparation of functional materials.
Herein, we have achieved high precision control over the
supramolecular packing of amphiphilic Pt(II) complexes by systematic
variation of the hydrophilic side chain length. A novel approach of
general applicability based on complementary X-ray diffraction and
solid-state NMR spectroscopy has allowed us to establish a clear
correlation between molecular features and supramolecular ordering:
systematically increasing the side chain length gradually increases
the steric demand and reduces the extent of aromatic interactions,
inducing a gradual shift in the molecular packing from parallel to a
long-slipped organization. Notably, our findings highlight the necessity
of advanced solid-state NMR techniques to gain structural information
for supramolecular systems where single-crystal growth is not
possible. Our work further demonstrates a new molecular design
strategy to modulate aromatic interaction strengths and packing
arrangements, which may be useful for the engineering of functional
materials based on Pt(II) and aromatic molecules.
enable additional applications in optoelectronics and
biomedicine.[3] Hence, control of the resulting structures through
molecular design is desirable.
As previously shown, tuning the length of peripheral alkyl chains
–which are commonly used to achieve solubility of the
compounds– can dramatically affect the self-assembly behavior
of Pt(II) complexes. For instance, we reported the self-assembly
of
two
oligophenyleneethynylene
(OPE)-based
bispyridyldichlorido Pt(II) complexes featuring terminal
dodecyloxy vs. methoxy chains.[4] We found that shortening the
side chains causes a drastic change from a slipped arrangement
of the molecules, both in solution and the solid state, to almost
parallel p-stacks in the solid state with enhanced aromatic
interactions and shortened Pt-Pt distances of » 4.4 Å. In another
elegant example, Yam and co-workers showed that the
hydrophobic tail length of amphiphilic Pt(II) complexes can alter
the morphologies and emission behavior of self-assemblies in
aqueous medium.[5] Additionally, for different luminescent Pt(II)
mesogens, it has been demonstrated that the variation of alkyl
chains can be used to modulate the mesophases with regards to
their stability and photophysical behavior.[6] Interestingly,
modification of the alkyl tail length for some of these compounds
has led to the emergence of multi-stimuli-responsive
polymorphism in the solid state,[6a,b] demonstrating the key impact
of small structural changes (for instance, C14 vs. C16 chain[6a]) on
the final properties of the systems.
Apart from nonpolar alkyl chains, polar oligo- or polyethylene
glycol chains (OEG, PEG) are commonly used as hydrophilic
counterparts for achieving solubility in aqueous media.[7]
However, the effect of EG chain length variation on the self-
assembly of Pt(II) complexes has been scarcely investigated.[8]
To the best of our knowledge, the only example of self-assembling
Pt(II) complexes featuring PEGn chains of different length was
reported by the group of Manners.[9] In this work, tetrazole-based
tridentate Pt(II) complexes with ancillary ligands based on PEGn
were observed to form 1D fibers (n = 16) or 2D platelets (n = 7) in
polar solvents, depending on the PEG chain length.
In this work, we demonstrate precise control over the
supramolecular packing of amphiphilic Pt(II) complexes by
systematic variation of the hydrophilic side chain length. This
understanding may contribute to establishing a correlation
between molecular design and packing modes, which remains
one of the greatest challenges in the field of self-assembly.[10] To
this end, we herein investigated the solid-state structures of a
series of rod-like bispyridyldichlorido Pt(II) complexes 1-4
featuring either three OEG chains on both termini, namely
triethyleneglycol (TEG, 1), diethyleneglycol (DEG, 2),
ethyleneglycol (EG, 3), or methoxy groups (4, Scheme 1). 1-4 are
smaller structural analogues of a previously reported Pt(II)
complex 5 that was found to exhibit a unique molecular
organization based on its crystal structure: the presence of
voluminous TEG chains hinders a parallel molecular arrangement
Introduction
Self-assembled structures of p-conjugated systems have long
been an area of active research in materials science, where
molecular assemblies in solution, in the solid state as well as in
the liquid crystalline state can be created.[1] It is widely known that
the functional properties of self-assembled structures are highly
dependent on the molecular structure of the monomers and that
minor changes in the molecular design can result in dramatic
differences in the features and functionalities of the resulting
materials.[2] Therefore, tuning the balance of attractive
noncovalent interactions (such as hydrogen bonding and
aromatic interactions) and steric effects by molecular design is
necessary to generate self-assembled materials with distinct
properties.[2] Besides purely organic compounds, the introduction
of a metal ion like Pt(II) to p-conjugated molecules can further
extend the range of possible intermolecular interactions and
[a]
[b]
L. Herkert, N. Bäumer, Dr. C. G. Daniliuc, Prof. Dr. G. Fernández
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstraße, 40, 48149 Münster (Germany)
E-mail: fernandg@uni-muenster.de
Dr. P. Selter, Prof. Dr. M. R. Hansen
Institut für Physikalische Chemie
Westfälische Wilhelms-Universität Münster
Corrensstraße, 28/30, 48149 Münster (Germany)
E-mail: mhansen@uni-muenster.de
[c]
§
Dr. J. P. Palakkal
Technische Universität Darmstadt, Department of Materials and
Earth Sciences, Alarich-Weiss-Straße 2, 64287 Darmstadt
(Germany)
Both authors contributed equally
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Scheme 1. Top left: synthesis of the Pt(II) complexes 1-4 and molecular structures of 1-5. Top right: representation of longitudinal and lateral shift of two stacked
molecules. Bottom: schematic depiction of the side chain length-dependent packing modes of 1-4 in the solid state.
and ultimately enables the formation of
a
characteristic
comparing the 1H and 13C MAS NMR results obtained from
complexes 1 to 3 with the crystallographic data. On this basis, we
are able to readily identify or exclude previously observed packing
structures for this specific series of OPE-based Pt(II) complexes,
filling in the missing structural information from crystallographic
data.
handshake motif, where the TEG groups of two adjacent Pt(II)
units are in close contact through multiple CH···O interactions
(see Scheme 1 and Fig. 1a).[11] To simplify our system, we
shortened the aromatic backbone of the ligand to two rings
(pyridine-ethynylphenylene) in order to reduce the amount of
potential interactions, highlighting the influence of the glycol
chains with identical p-surface. We envisioned that a gradual
decrease of the EG chain length while keeping the aromatic
surface of the molecules unchanged could progressively alleviate
the steric demand of the chains and, consequently, reinforce the
extent of p-overlap between the stacked molecules. This, in turn,
is expected to lead to a gradual transformation from a more
slipped to more parallel packing upon shortening the EG side
chains. Thus, in this report, we focus on the self-assembly
structures in the solid state, whereas investigations on the
processes in solution are still ongoing.
Usually, the solid-state structure and molecular organization are
determined by single-crystal X-ray diffraction, and we were able
to obtain suitable single crystals for complexes
Crystallization of complex 4, however, resulted in polycrystalline
powders unsuitable for X-ray analysis. Therefore,
complementary method was required for obtaining a systematic
comparison of all compounds. To this end, we employed solid-
state 1H and 13C Magic-Angle Spinning (MAS) NMR in
combination with two-dimensional, ‘through-space’ NMR
correlation techniques based on the direct magnetic dipole-dipole
(or dipolar) interaction between nuclear spins to elucidate the
aggregation of the methoxy-functionalized complex 4.[12] Our
approach consisted in identifying through-space 1H-13C
correlations characteristic for certain structural motifs by
Results and Discussion
Synthesis
Pt(II) complexes 1-4 were synthesized in moderate to good yields
by complexation reaction of the precursor salt [PtCl2(PhCN)2] with
the respective pyridine-based ligands in toluene at 100 °C (see
Scheme 1 and Supporting Information). Complexes 1-3 resulted
in crystalline solids upon concentration from Et2O or pentane-
containing DCM as well as EtOAc solutions, whereas all other
used solvents led to highly amorphous structures with soft texture.
Compound 4, however, could only be obtained as a powdery
solid, which is poorly soluble in most organic solvents due to the
short side chains. All complexes could be characterized by 1H/13C
NMR in solution, solid-state NMR, and gave good results for
elemental analyses.
1 to 3.
a
Crystal structure analysis
By slow vapor diffusion of Et2O into solutions of 1-3 in EtOAc, pale
yellow single crystals suitable for X-ray diffraction analysis could
be grown. 1 and 2 both were solved and refined in the triclinic
space group P-1, whereas 3 could be solved in the monoclinic
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space group P21/n (for details of crystal structure determinations
see the Supporting Information, Fig. S1-S6, Tables S1-S3). In the
following, the arrangements of the stacked molecules in the solid
state will be characterized in terms of a longitudinal shift along the
molecular axis as well as by a lateral offset as depicted in Scheme
1. Figure 1 shows the obtained molecular structures and packing
of complexes 1 to 3 in their crystal structures. For comparison, an
excerpt of the crystal structure of previously reported analog 5 is
also shown. Relevant close intermolecular contacts are
highlighted. In the crystal structure of 1 (Fig. 1b), the packing can
Figure 1. Excerpts of the packing diagrams from the crystal structures of (a) 5, (b,c) 1, (d) 2, and (e) 3 showing the stacking of molecules and change from (c)
handshake motif between adjacent amphiphilic molecules (1 and 5) to the more parallel stacking of molecules (2 and 3) caused by the gradual shortening of the
attached hydrophilic side chains from TEG (1), DEG (2) to EG (3).
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be viewed as an arrangement of longitudinally shifted molecular
units mainly stabilized by C-Harom.···O (2.31 Å, 2.58 Å) interactions
and a close Pt···H contact (2.91 Å) between the central Pt atom
of one molecule and the proton of the inner glycol chain of a
neighboring molecule. Such Pt···H interactions have been
observed in earlier reports and can be explained by donation of
available single crystal X-ray diffraction data for complex 4, a
complementary technique (solid-state NMR) was required to
elucidate the molecular arrangement in the solid state.
Insights into the molecular packing from solid-state NMR
2
electron density from the dz orbital of the Pt to the s* orbital of
Our approach consisted in identifying intermolecular contacts
characteristic for the packing motifs observed in 1-3 and 5. By
comparing these contacts with those potentially observed (or
alternatively not observed) for 4, a packing arrangement for this
complex can be derived. All samples used for solid-state NMR
measurements were powdered samples and were obtained under
identical conditions as those used for the preparation of the single
crystals discussed in the previous section (EtOAc/Et2O solutions).
We therefore assumed identical (or at the very least comparable)
arrangements of the molecules in the samples for solid-state NMR
studies and in the crystalline state. Additionally, we have
conducted powder XRD analysis of 1-4 to further probe this
assumption. A detailed summary and analysis of the XRD results
can be found in the Supporting Information. Compound 1 shows
the lowest crystallinity of all samples obtained by the precipitation
protocol (Fig. S18). While the positions of the reflexes are in
relatively good agreement with the predicted XRD pattern (based
on the corresponding single-crystal data), the overall intensity is
greatly diminished in particular for the range corresponding to the
long-range order. The sterically demanding TEG chains, which
adopt a highly ordered orientation in the single crystal, most likely
prevent this long-range order in the powder sample; nevertheless,
a similar short-range order could be confirmed. On the other hand,
the higher crystallinity of 2 is reflected in the sharp reflexes, which
match the predicted XRD pattern well (Fig. S19). Yet, the
observation of some minor discrepancies, in particular for the
immediate environment of the Pt(II) moiety, indicates that the high
level of interdigitation of the DEG chains may not be perfectly
transferred to the powdered sample used for the studies by solid-
state NMR. Nevertheless, based on the reasonably good
agreement between the experimental and predicted XRD
patterns, we infer that the orientation within the 1D stack is largely
preserved. For the sterically least demanding compound for which
single-crystal analysis was possible (3), the predicted XRD
pattern reproduces the experimental results without
discrepancies (Fig. S20). This result is logical considering the
absence of significant steric effects when the chains are reduced
to EG units. On this basis, we infer that the molecular orientation
in the bulk as well as in the single crystal matches. For compound
4 sharp reflexes could be appreciated as well, which suggests that
the sample has a highly crystalline nature, which matches the
observed trend for compounds 1-3 (Fig. S21).
the C-H.[13] Interestingly, no aromatic interactions are observed in
the crystal structure of 1 (for a top view of the p-system illustrating
the orientation of the aromatic rings see Fig. S2). Instead, the
structure formation is dominated by the TEG chains, which form
a large, bulky structure surrounding the neighboring aromatic
backbone in a so-called handshake motif (Fig. 1c), similarly to that
observed for complex 5 (Fig. 1a).[11] The lack of close p-p contacts
for 1 compared to the higher homolog 5 can be explained by the
considerably larger aromatic surface for the latter (six vs. four
aromatic rings), which enables a more efficient p-overlap of the
OPE backbone in the crystal. The relatively short p-system of 1
must orient in a twisted manner with a torsion angle of the two
aromatic rings (phenyl and pyridine) of 53.8° to form the present
structure. Stabilization of the handshake motif is achieved by
intermolecular C-Harom.···O interactions exhibiting distances in the
range of previously observed weak hydrogen bonding for such
systems (2.41-2.56 Å).[11] The closest Pt-Pt distance that can be
found in the packing of 1 is 9.854 Å, whereas the Pt-Pt distance
illustrating the longitudinal shift of two molecules is 18.120 Å (Fig.
1b).
In the single crystal structure of 2, a distinct change in the
arrangement compared to 1 is apparent (Fig. 1d). Although the
molecules still arrange in a slipped manner, the change from TEG
to DEG side chains facilitates a molecular stacking via p-p
interactions, leading to an assembly with a decreased longitudinal
molecular shift (inner stack Pt-Pt distance 8.126 Å vs. 18.120 Å in
1) and a slight lateral offset (see also Fig. S4). The parallel
displaced p-stacking of two pyridine units has a distance of
~3.2 Å. In contrast to the structure of 1, where the Pt(II)Cl2
fragment is surrounded by TEG chains, the Pt(II)Cl2 moiety in 2 is
located on top of the CºC bond of the next molecule in the stack.
The structure is further stabilized by C-Hglycol···O (2.44 Å) as well
as C-Hglycol···Carom. (2.90 Å) interactions. Thus, instead of a
handshake motif as in 1, the DEG side chains in 2 interdigitate
with neighboring side chains (Fig. S4). Furthermore, an
intermolecular weak C-Hglycol···Cl (2.84 Å) interaction is observed
within adjacent stacks (Fig. S4).
A shortening of the glycol chains to one unit (EG) in 3 results in a
similar molecular stacking to complex 2 (Fig. 1e) with a Pt-Pt
distance that is even slightly shorter for 3 (7.339 Å) compared to
2 (8.126 Å). Parallel displaced p-p interactions of the pyridine
rings with a distance of ~3.4 Å and C-Harom.···Cl (2.86 Å)
interactions stabilize the molecular packing. Additional
C-Hglycol···O (2.43 Å), C-Hglycol···Carom. (2.41 Å) and C-Hglycol···CºC
(2.87 Å) interactions of one stack with molecules of obliquely
arranged neighboring stacks are also present (Fig. S6).
In contrast to compounds 1-3, single crystals of 4 could not be
obtained. However, considering the trend followed by 1-3, we
would expect a more parallel arrangement of the molecules in 4
(shorter Pt-Pt distances) and enhanced aromatic interactions
compared to 1-3 due to the lack of any side chains. Due to no
Figure 2a summarizes the 1H MAS NMR spectra of 1-4. Overall,
1
broad H lines with few discernable details are observed due to
the strong 1H-1H dipolar couplings present in the solid state.
Nevertheless,
apart
from
the
broad
peak
around
δiso(1H) = 3.5 ppm assigned to the protons of the EG moieties, two
signals in the aromatic region are observed: while the signal at
higher ppm values (δiso(1H) = 9.1 to 8.1 ppm) can be assigned to
the aromatic protons adjacent to the pyridine nitrogen (Ha,
position Ha in Fig. 2), the 1H signal at lower values (δiso(1H) = 7.8
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Figure 2. (a) 1H MAS NMR spectra of complexes 1 to 4, acquired at 11.7 T (500 MHz) using 29.762 kHz MAS. The 1H chemical shifts of the discernable aromatic
protons (a-c) are highlighted in red and the dashed lines are guides to the eye. # indicate residual solvent and ‘Glycol/M’ denotes the position of the oligoglycol and
methoxy protons. 1H-1H DQ-SQ correlation spectrum of complex 4 at 29.762 kHz MAS using one (b) and four (c) rotor periods of DQ recoupling. The proposed
packing arrangement for 4 is shown in (d).
to 6.8 ppm) corresponds to the aromatic protons Hb (Hb-pyridyl)
and Hc. The exact position of the two peaks varies between
complexes
situated at δSQ = 3.5 ppm and δDQ = 7.0 ppm (attributed to the EG
units) with some smaller auto-correlation peaks visible at lower
ppm values most likely originating from the residual trapped
solvent. However, no auto-correlation signals in the aromatic
region (around δSQ = 7 ppm) are observed.
1
to 4, going from δiso(1Ha) = 9.1 ppm and
δiso(1Hb,c) = 7.8 ppm in complex 1, to δiso(1Ha) = 8.1 ppm and
δiso(1Hb,c) = 6.8 ppm in complexes 2 and 3. Since the moieties are
structurally identical, the difference can be solely attributed to
packing effects, in this case to aromatic ring current effects
caused by the neighboring aromatic backbone in 2 and 3, or the
lack thereof in 1. The shift to lower ppm values is often indicative
of p-p interactions as observed for example in p-conjugated
polymers and other conjugated systems.[14]
In analogy to the 1H MAS NMR spectrum, replacing the EG units
with methoxy groups leads to the appearance of three aromatic
1H signals in the DQ-SQ correlation spectra (Fig. 2b,c). While the
cross-correlation signals of protons Ha and Hc are readily
discernible at δSQ = 8.6 ppm and δSQ =5.9 ppm (δDQ = 14.6 ppm,
dark blue, marked ac and ca), the broad Gaussian peak
associated with protons Hb at around δSQ = 7 ppm is mostly
obscured. In addition to the cross-correlation between the
methoxy protons and aromatic proton Hc at δDQ = 9.4 ppm –which
is expected due to the close spatial proximity of the methoxy
groups on the same ring– a cross-correlation between the
methoxy protons and proton Ha is observed at δDQ = 12.2 ppm.
Interestingly, no cross-correlation between the aromatic proton
Hb and the methoxy protons is observable, though this could be
related to the broad nature and comparably low amplitude of the
peak at δSQ = 7.0 ppm. These results suggest that the methoxy
groups are placed in relatively close proximity to the complex
center, though whether this is from above/below the aromatic
backbone or from other neighboring complex is unclear at this
point. The most significant difference between the spectra of 4
and the other three samples is the appearance of several auto-
correlation peaks in the aromatic chemical shift range (marked
orange aa, blue-green bb, and yellow-green cc). While these are
barely discernible after one rotor period of DQ-excitation, Fig. 2b,
their amplitude increases when going to longer (four rotor periods)
DQ-excitation times, see Fig. 2c. This indicates a 1H-1H distance
of around 3.5 Å-4.0 Å, pointing to p-p stacking of the monomers
with a parallel arrangement of the molecular units (Fig. 2d).
Replacing the EG moieties with methoxy groups as in complex 4
leads to a markedly different 1H MAS NMR spectrum, cf. Fig. 2a.
The signal at δiso(1H) = 3.5 ppm is now significantly reduced in
intensity and width, and strong shifts of the 1H signals in the
aromatic region are observed. The upfield shift of proton c
suggests an even stronger participation of the outer ring in p-p
1
interactions compared to complexes 2 and 3. In contrast, the H
signal a is situated at a higher ppm value than that of 2 and 3,
which might lead to the wrong conclusion that the pyridyl ring of 4
undergoes weaker aromatic interactions than that of 2 and 3.
However, this effect is typical for bispyridyldichlorido Pt(II) and
Pd(II) complexes exhibiting a pseudo-parallel packing, and can
be attributed to C-Ha···Cl interactions between the a-pyridyl
protons of one molecule and the electron withdrawing Cl ligands
of an adjacent molecule.[4]
To further probe the molecular packing of complex 4, two-
dimensional (2D) 1H-1H double-quantum single-quantum
correlation (DQ-SQ) MAS NMR experiments were performed
1
(Fig. 2b,c and S9). This kind of experiment allows to identify H-
1H spin pairs in close spatial proximity to each other (up to ~3.5 Å
in rigid organic solids and longer distances if the systems are
flexible or proton diluted).[15] The 2D spectra of complexes 1 to 3
(Fig. S8-9) are characterized by a broad auto-correlation signal
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To summarize the results from H MAS and H-1H DQ-SQ NMR
correlation data, 1H MAS NMR revealed changes in chemical shift
of the aromatic protons associated with differences in aromatic
ring current effects, with 1 showing weaker and 2 and 3 showing
stronger ring current effects. Protons Ha and Hb in 4 show a
moderate effect of ~0.5 ppm, which would indicate a packing motif
where the protons are further away from the neighboring aromatic
ring center pointing towards the edges of the said ring, though
proximity of Ha to the electron-withdrawing Cl (Ha···Cl) could lead
to the shift to higher ppm. These observations fit well with a
packing model similar to the pseudo-parallel molecular
arrangement observed by Allampally et al. for a longer, methoxy-
functionalized OPE-based Pt(II) complex.[4]
complexes 1-3 is the splitting of the broad 13C signal at
δiso(13C) = 129 -122 ppm, assigned to the beta-carbon of the
pyridine moiety (Cb; position C2). This carbon is directly bonded
to proton Hb, which was previously attributed to a broad
Gaussian-shaped peak in the 1H MAS NMR spectra (vide supra).
Thus, the doublet character of this 13C signal indicates a more
complex 1H line shape, with the Gaussian distribution being
merely an approximation, which further explains the complex
cross-correlation peaks observed in the 1H-1H DQ-SQ NMR
spectrum of Fig. 2b,c. As the substitution of the EG groups occurs
on the outer phenyl ring, these changes observed for C2 and Hb
must be related to intermolecular packing effects. Another
interesting feature of Fig. 3 is the low-field shift of the alkyne
carbons C4 and C5 for complexes 2 and 3 compared to complex
1. This low-field shift is not observed for complex 4. Since the
packing mode switches from the handshake motif with a long-
shifted arrangement in complex 1 to a less shifted, staircase
pattern in complexes 2 and 3 with the Pt(II)Cl2 center being close
to the alkyne triple bond, we can attribute the low-field shift to a
change in packing.
Several of the 13C signals across all four complexes in Fig. 3 are
split into doublets, most notably the 13C signals assigned to
carbon C7 at δiso(13C) = ~109 ppm, as well as those for C3, C4,
and C8 in the case of complex 1. The splitting of carbons C7 and
C8 can be explained by two inequivalent carbon sites in the
asymmetric unit of the crystal structure. However, according to
the single-crystal XRD data, carbons C3 and C4 only occur once
in the asymmetric unit of complex 1. Therefore, it is likely that the
1
1
To confirm the results from 1H MAS and 1H-1H DQ-SQ NMR
correlation spectroscopy, 13C{1H} cross polarization (CP/MAS)
and 2D 13C{1H} heteronuclear correlation (HETCOR) NMR
experiments were performed (details are given in the Supporting
Information, Figs. S12-S17). Figure 3 summarizes the 13C{1H}
CP/MAS NMR spectra of complexes 1-4 along with the peak
assignment. Note that the assignment uses the numbering given
in Fig. 3, which is different from the numbering of the carbons in
the crystal structures. The assignment is based on a combination
of variable contact time CP measurements for complex 1 as well
as the corresponding 2D 13C{1H} HETCOR NMR data (Fig. S14).
To better visualize the changes in isotropic 13C chemical shift for
the different complexes, the 13C signals associated with the OPE
backbone are plotted in the lower part of Fig. 3. The most striking
difference between the spectrum of complex 4 and those of
Figure 3. 13C{1H} CP/MAS spectra of 1 to 4 acquired using a CP contact time of 2.0 ms at 9.4 T (2-4) and 7.05 T (1). The 13C assignment scheme (numbers) of the
OPE backbone is shown at the top. Note that signal 1 is severely broadened under these CP conditions and overlaps with position 8. The plot at the bottom shows
the variation of the 13C chemical shift between the complexes 1 to 4 highlighting changes in 13C chemical shift.
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sample used for MAS NMR has a slightly different packing and
additional disorder compared to the single crystal used for XRD
(cf. Figure S18-20). Figure 4 summarizes the 2D 13C{1H}
HETCOR spectra for 1, 2, and 4 acquired with 0.5 ms (green
contour lines) and 3.0 ms CP contact time (black contour lines) to
in a nearly parallel fashion as illustrated by the molecular
fragments in Fig. 4c.
Conclusions
1
differentiate between directly bonded H-13C and intermolecular
1H-13C contacts, respectively.
A comparison of these 2D
In summary, we have reported the synthesis and molecular
packing of four new amphiphilic Pt(II) based complexes 1-4 with
short aromatic OPE backbones and hydrophilic oligoethylene
glycol termini of different length. A systematic length variation was
achieved by gradually removing one (2), two (3) or three (4)
OCH2-CH2 units from the TEG-substituted derivative 1. Data from
single-crystal X-ray diffraction showed that complex 1 retained the
characteristic handshake motif previously described for complex
5, which features the same TEG side chains, but has a larger
aromatic surface than 1. The strong similarity between the
packing motifs of 1 and 5 indicate that the TEG groups with their
associated hydrophilic interactions are the primary forces
dictating the solid-state structure. However, in contrast to 5,
complex 1 shows no significant aromatic interactions due to the
reduced aromatic surface. Shortening the hydrophilic side chains
in complexes 2 and 3 resulted in structures with longitudinally
shifted molecules involving more pronounced p-p interactions,
though the bulky Pt(II)Cl2 moiety in the complex center prevents
efficient p-p stacking between neighboring complexes. Two-
dimensional solid-state 1H MAS and 13C MAS NMR experiments
for complexes 1, 2, 3, and 5 allowed us to identify 1H-13C through-
space correlations characteristic for each packing motif. These, in
turn, led us to propose a packing motif for the only complex (4) for
which single crystals could not be grown, emphasizing the
potential of solid-state NMR spectroscopy as a key tool for
structural determination in the solid state. Complex 4 was found
to exhibit a more parallel arrangement of the molecules stabilized
by p-p interactions, most likely with a short longitudinal and lateral
offset to accommodate the sterically demanding Pt(II)Cl2 moiety
at the complex center. On this basis, we conclude that the steric
demand of the central Pt(II)Cl2 moiety limits the effectiveness of
p-p interactions, which is supported by the fact that even short
monoethylene glycol termini appear to prevent an efficient p-
overlap of the ligands aromatic surface. A more pronounced p-
overlap is only possible after eliminating hydrophilic interactions
by replacing the ethylene glycol termini with methoxy groups.
Therefore, if p-p interactions are frustrated by steric demands,
hydrophilic interactions will become the dominant factor in the
self-assembly process leading to structural aggregation. Thus,
our investigations illustrate that even minor changes in the glycol
chain length can be used to modulate and control aromatic
interaction strengths and packing arrangements, which in turn
might be useful for the engineering of electronic devices based on
Pt(II) and aromatic molecules. Finally, in our view, the
combination of X-ray diffraction and solid-state NMR
HETCOR NMR datasets (green and black) allows us to identify
the packing structure for each complex as illustrated by the
structural fragments and corresponding assignments of 1H-13C
correlations below these spectra (see caption of Fig. 4 for details
on the assignment). The spectrum of complex 1 shows two 13C
signals for carbon C8 due to the existence of two inequivalent
positions in the crystallographic asymmetric unit. As for complex
5 (data shown in Fig. S12-13), this also results in different
intermolecular 1H-13C distances for the two positions, manifested
in the 2D HETCOR NMR spectra as correlations HbC8 and
HbC8’ labelled as b8 and b8’, respectively, in Fig. 4a (complex
1). The large longitudinal shift between two molecules in 1 is also
evident from the correlation HbC9, which links proton Hb (located
at the complex center) to the outer most OPE carbon C9, a
correlation that is only possible via intermolecular contacts. A
further indicator for the magnitude of the longitudinal shift is the
absence of potential intermolecular correlations HbC6 and HaC5,
which would be expected for an intermediate shift (these are
present in the case of complexes 2 and 3, see Fig. 4b).
Since the solid-state 2D HETCOR NMR spectra of 2 and 3 are
very similar, only those for 2 will be discussed in the following (Fig.
4b). As mentioned before, the crystal structures of 2 and 3 lack
the characteristic TEG handshake motif and feature a moderately
shifted aromatic p-p stacking arrangement. This packing can be
confirmed from the 2D HETCOR datasets in Figure 4b for 2 (see
Fig. S16 for 3), which shows numerous intermolecular
correlations, most notably HbC6, HaC5, HaC4, HbC9, HcC2, and
HcC1. While HbC9 and HbC8 were also observed for complex 1,
correlations HcC1, and HcC2 place the pyridine moiety close to
the second phenyl ring of the OPE backbone. Interestingly, the
correlations between C7 (bonded to proton Hc) and protons Ha
or Hb (attached to carbons C1 and C2) are not observed. In
addition, correlations HaC5 and HaC4 –which were not observed
for complex 1– are indicators for the Pt(II)Cl2 complex center
being in the proximity to the alkyne triple bond of the neighboring
molecule.
The 2D HETCOR spectra of 4 together with the proposed
molecular arrangement are shown in Fig. 4c. In contrast to the 2D
HETCOR spectra for 1 and 2, none of the intermolecular
correlations such as HbC9, HaC9, HaC6, HaC5, and HaC4 are
observed. This can only be achieved for a structural arrangement
with a parallel or nearly parallel p-p stacked arrangement of the
OPE backbones, similar to the one described by Allampally et al.
for
the
larger
OPE-based
complex
with
methoxy
functionalization.[4] Such
a
structural arrangement is also
spectroscopy represents
a general approach for detailed
supported by the 1H-1H DQ-SQ NMR correlation data, see Fig. 2.
Thus, through a rigorous comparison and careful assignment of
observed and non-observed 1H-13C correlations for complexes 1-
3 and 5 with known crystal structures, it can be concluded that the
aromatic interactions for the OPE backbone of complex 4 occur
structural elucidation that is potentially applicable to virtually all
types of molecular and macromolecular platforms.
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10.1002/chem.202003445
Chemistry - A European Journal
RESEARCH ARTICLE
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Keywords: Self-assembly • p-conjugated systems • weak non-
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10.1002/chem.202003445
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RESEARCH ARTICLE
Entry for the Table of Contents
Combined X-ray diffraction and advanced solid-state NMR studies have been introduced as a new, general tool for detailed structural
and supramolecular elucidation, using a series of amphiphilic Pt(II) complexes as model compounds. Our approach provides an
alternative for supramolecular systems where single-crystal growth is not possible, which can be potentially applied to multiple other
molecular and macromolecular platforms.
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