A Paramagnetic NMR Spectroscopy Toolbox for the Characterisation of Paramagnetic/Spin-Crossover Coordination Complexes and Metal–Organic Cages

Article abstract of DOI:10.1002/anie.202008439

The large paramagnetic shifts and short relaxation times resulting from the presence of a paramagnetic centre complicate NMR data acquisition and interpretation in solution. As a result, NMR analysis of paramagnetic complexes is limited in comparison to diamagnetic compounds and often relies on theoretical models. We report a toolbox of 1D (¹H, proton-coupled ¹³C, selective ¹H-decoupling ¹³C, steady-state NOE) and 2D (COSY, NOESY, HMQC) paramagnetic NMR methods that enables unprecedented structural characterisation and in some cases, provides more structural information than would be observable for a diamagnetic analogue. We demonstrate the toolbox's broad versatility for fields from coordination chemistry and spin-crossover complexes to supramolecular chemistry through the characterisation of Co^II and high-spin Fe^II mononuclear complexes as well as a Co₄L₆ cage.

Full text of DOI:10.1002/anie.202008439

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Accepted Article
Title: A Paramagnetic NMR Spectroscopy Toolbox for the
Characterisation of Paramagnetic/Spin-Crossover Coordination
Complexes and Metal-Organic Cages
Authors: Marc Lehr, Tobias Paschelke, Eicke Trumpf, Anna-Marlene
Vogt, Christian Näther, Frank Sönnichsen, and Anna
McConnell
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Angewandte Chemie International Edition
10.1002/anie.202008439
RESEARCH ARTICLE
A Paramagnetic NMR Spectroscopy Toolbox for the
Characterisation of Paramagnetic/Spin-Crossover Coordination
Complexes and Metal-Organic Cages
[
a]
[a]
[a]
[a]
[b]
Marc Lehr, Tobias Paschelke, Eicke Trumpf, Anna-Marlene Vogt, Christian Näther, Frank D.
[
a]
[a]
Sönnichsen, Anna J. McConnell*
[
a]
M. Lehr, T. Paschelke, E. Trumpf, A.-M. Vogt, Prof. Dr. F. D. Sönnichsen, Prof. Dr. A. J. McConnell*
Otto Diels Institute of Organic Chemistry
Christian-Albrechts-Universität zu Kiel
Otto-Hahn-Platz 4
Kiel 24098
Germany
E-mail: amcconnell@oc.uni-kiel.de
Prof. Dr. C. Näther
[
b]
Institute of Inorganic Chemistry
Christian-Albrechts-Universität zu Kiel
Max-Eyth-Straße 2
Kiel 24118
Germany
Supporting information for this article is given via a link at the end of the document.
Abstract: The large paramagnetic shifts and short relaxation times
resulting from the presence of a paramagnetic centre complicate
NMR data acquisition and interpretation in solution. As a result, NMR
analysis of paramagnetic complexes is limited in comparison to
diamagnetic compounds and often relies on theoretical models. We
pulse programs with long or multiple pulses are not suitable
since relaxation can occur before data acquisition takes place;[
uniform excitation is more difficult over the larger spectral range;
some signals may be lost completely in the case of very short
relaxation times and broad linewidths;[ structural information
6b]
6e]
1
13
[6e,
report
1
a
toolbox of 1D ( H, proton-coupled
C, selective
usually extracted from the chemical shift and J-coupling is lost.
13
9a, 10a]
H-decoupling C, steady-state NOE) and 2D (COSY, NOESY,
HMQC) paramagnetic NMR methods that enables unprecedented
structural characterisation and in some cases, provides more
structural information than would be observable for a diamagnetic
analogue. We demonstrate the toolbox’s broad versatility for fields
from coordination chemistry and spin-crossover complexes to
Furthermore, there are limitations to current methods for
spectral assignment; they often rely on the availability of
accurate theoretical models[
6b, 9b, 11]
or single crystal X-ray
structures for correlation of T
1
relaxation times to metal-proton
distances using the Solomon equation.[
4b, 10b, 10d]
As a result,
1
solution characterisation is, in many cases, limited to a H NMR
spectrum only where complete and unambiguous assignment
may not be possible.
II
supramolecular chemistry through the characterisation of Co and
II
high-spin Fe mononuclear complexes as well as a Co
L
4 6
cage.
Nevertheless, paramagnetic NMR spectroscopy also has
advantages compared to diamagnetic NMR spectroscopy: the
large paramagnetic shifts result in reduced likelihood of signal
overlap from dispersion of the NMR signals over a wider
chemical shift range;[10c] the fast relaxation times in comparison
to diamagnetic compounds enables reduction of the acquisition
Introduction
NMR spectroscopy is indispensable for the solution structural
characterisation of macromolecules from proteins[1] to
times and recycle delays, thereby significantly reducing the
demand on instrument time.[4b, 6b, 12] Alternatively, this can be
supramolecular
architectures,[2]
including
interlocked
structures,[ metal-organic cages[4] and topologically complex
molecules.[ With the standard suite of 1D and 2D NMR
methods, structural assignment of diamagnetic compounds and
complexes is straightforward but NMR spectroscopy in the
presence of paramagnetic centres is more difficult.
3]
exploited for a greater sensitivity of detection through extensive
scan averaging. In supramolecular chemistry this has allowed
5]
[4b, 10c]
detection of guest binding within paramagnetic cages,
even when the guest is present as a trace impurity.[4b]
Despite these advantages, the full potential of
paramagnetic NMR spectroscopy is still to be realised; in
comparison to the wealth of 1D and 2D NMR methods for
diamagnetic compounds, the number of methods suitable for
Paramagnetic NMR spectroscopy[ is central to many
fields from chemical and structural biology for studying the
structure, dynamics and interactions of proteins[1b, 1c, 7] to probing
spin-state populations in spin-crossover compounds[8] and the
structural characterisation of paramagnetic complexes and
supramolecular architectures.[4b,
acquisition and interpretation in the presence of a paramagnetic
centre presents a number of challenges due to the large
paramagnetic shifts, short relaxation times and broad linewidths:
6]
paramagnetic complexes is limited by fast relaxation although
paramagnetic DOSY has been recently reported.[6e, 13]
[
9]
10]
However, NMR data
[6c]
Furthermore, the data acquisition/interpretation difficulties still
need to be overcome to enable straightforward structural
characterisation by paramagnetic NMR spectroscopy.
1
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RESEARCH ARTICLE
We report a toolbox of 1D ( H, proton-coupled 13C,
selective H-decoupling 13C, steady-state NOE) and 2D (COSY,
NOESY, HMQC) paramagnetic NMR methods that have proven
particularly robust towards fast relaxation and enable
unprecedented in-depth structural analysis of paramagnetic
complexes in solution. We demonstrate the general applicability
of this selection of robust experiments by characterising
paramagnetic complexes from various fields of chemistry: CoII
mononuclear complexes 1a-7a (Figure 1a) as representative
examples of paramagnetic coordination complexes; FeII
mononuclear complex 1b (Figure 1a), whose perchlorate and
tetrafluoroborate salts are known to undergo spin-crossover in
1
different environments in the mer isomer (represented as black,
red and blue ligands, Figure 1a). The 2-pyridylquinoline (pq)
coordination motif was chosen to investigate the influence of
steric bulk on the fac/mer ratio since its coordination chemistry
with labile octahedral metal ions has been underexplored in
solution.[11b]
CoII complex 1a with the parent pq[15] ligand (SI, Section
2.1) was studied first rather than the literature-known FeII
complex 1b,[14] which could undergo spin-crossover complicating
NMR analysis. Complex 1a was prepared either in situ or as
1
crystals by mixing Co(BF
4 2 2
) •6H O and three equivalents of the
ligand in CD CN or EtOH, respectively. Single crystal X-ray
3
the solid state,[14] to represent the high-spin state of
spin-crossover complex; and metal-organic cage 8 (Figure 1b)
a
analysis revealed mer-1a crystallised (Figure 2 and SI, Section
‡
3.1.1.1). Like the analogous crystal structure of [Fe(pq)
3
](BF
4
)
2
(1b),[
14a]
adoption of the mer configuration is attributed to the
as an example of a paramagnetic supramolecular architecture.
ligands’ steric bulk and π-π stacking interactions are observed
between two of the ligands (black and red ligand environments
II
in Figure 1a) causing distortion of the N-Co -N angles from the
ideal 90° octahedral geometry (Table S3).
3 4 2
Figure 2. Single crystal X-ray structure of [Co(pq) ](BF ) (1a) with
counteranions omitted for clarity.
1
In solution, the H NMR spectra of the complex prepared
in situ and by redissolving the crystals in CD
3
CN were similar
1
(
Figure S32). The H signals were spread over a 250 ppm range
with 24 relatively sharp signals of equal intensity (linewidths up
to 70 Hz, Table S5) and 4 broader signals (Figure S34). This
suggests that the mer isomer is not only the solid-state structure
but also the only structure in solution; the deviation from the
expected 30 signals is attributed to either overlapping or broad
signals.
Assignment of the 1H spectrum was initially attempted
using the Solomon equation, which has been successfully
applied to assign the spectra of highly symmetric cages by
Figure 1. Paramagnetic a) mononuclear CoII and FeII complexes based on
correlating T
1
relaxation times to the metal-proton distances from
the single crystal X-ray structure.[
4b, 10b, 10d]
For complex 1a the T
1
sterically bulky 2-pyridylquinoline (pq) motifs and b) Co
L
4 6
cage for
characterisation by the toolbox of paramagnetic NMR techniques.
relaxation times varied from 0.7 to 80 ms (Table S4) but a
limitation of this currently available assignment method was
highlighted during analysis; only partial assignment was possible
since not only protons d and g (Figure 3a) but also, and more
Results and Discussion
importantly, the three different ligand environments cannot be
II
We initially investigated the mononuclear complexes to optimise
the toolbox and test its limits for spectral assignment. Since the
mononuclear complexes could form a mixture of meridional
distinguished on the basis of T
1
relaxation times/Co -proton
distances alone. Ward also encountered this limitation in the
assignment of
lower symmetry metal-organic cage.[10c]
a
(
mer) and facial (fac) isomers, up to four sets of NMR signals
Therefore, we sought to remove the reliance of assignment on
the Solomon equation by optimising a toolbox of paramagnetic
NMR experiments with broad applicability for the straightforward
characterisation of a variety of paramagnetic complexes. The
could, in principle, be observed based on symmetry
considerations: 1 set for the fac isomer (represented as green
ligands in Figure 1a) and 3 sets for the ligand in the three
2
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RESEARCH ARTICLE
Figure 3. a) 1H- H COSY NMR spectrum (600 MHz, CD
1
3 3 4 2
CN, 298 K) of mer-[Co(pq) ](BF ) (1a) where through-space (COSY) cross-peaks within the three ligand
environments are represented by the red, black and blue circles, respectively. Additional structural information in the form of exchange (EXSY) cross-peaks
orange boxes) and through-space (NOE) cross-peaks (purple boxes) is present due to chemical exchange and cross-correlation,[16] respectively. Note: the
(
numbers 1, 2, 3 on the proton labels represent the three sets of coupled protons within a single spin system (i.e. protons b-d). The absence of NOE cross-peaks
between protons h and j prevented assignment of these spin systems to a particular ligand environment and therefore, spin system j-l was arbitrarily labelled with
black, red and blue labels in decreasing chemical shift order of proton j to represent the three ligand environments. b) 1H- H NOESY NMR spectrum (600 MHz,
1
3 3 4 2
CD CN, 298 K) of mer-[Co(pq) ](BF ) (1a). Exchange (EXSY) cross-peaks are represented by the orange boxes and exchange cross-peaks between the
complex and excess ligand are shown by the green boxes.
following description of the structural assignment of complex 1a
and related mononuclear complexes is used to illustrate how the
paramagnetic NMR toolbox was optimised to overcome
commonly encountered data acquisition and interpretation
difficulties. An instruction manual for application of the toolbox to
the characterisation of other paramagnetic complexes and
cages is provided in Section 1.1.1 of the SI.
3 4 2
spectrum of [Co(bpy) ](BF ) as a reference complex showed
only the two expected cross-peaks arising from 3J coupling
(Figure S96). The origin of these additional cross-peaks was
[
16c,
investigated using NOESY since Wimperis and Bodenhausen
1
6d]
[16a, 16b]
as well as Bertini
have reported the presence of
additional relaxation-allowed cross-peaks in the COSY spectra
of paramagnetic complexes that result not from through-bond
coupling but rather cross-correlation from through-space (NOE)
coupling between the nuclei as well as between the nuclei and
the paramagnetic centre.
The standard NOESY pulse program was modified for
application to paramagnetic complexes (SI, Section 1.1.1.3). For
an initial experiment, a mixing time of 20 ms was chosen as a
compromise between the short relaxation times and
comparatively long NOE cross-relaxation rates. Pleasingly,
cross-peaks for the sharp signals (linewidth < 70 Hz) were
We initially turned our attention to COSY since this would
allow grouping of the resonances within the different ligand
environments. Encouraged by the observation of cross-peaks in
[
9a]
the COSY spectra of several paramagnetic complexes
and
supramolecular architectures[4b,
10a,
10b]
despite the broad
linewidths and short relaxation times, we tested and optimised
several COSY variants (SI, Section 1.1.1.2). Reduction of the
recycle delays and acquisition times due to the fast relaxation
times enabled the acquisition of more data in a shorter amount
of time using these optimised paramagnetic COSY parameters
1
observed (Figure 3b). Given the range of T relaxation times
(
typically 5.5 mins for 4 scans, Table S1) compared to the
within the complex, optimisation of the mixing time was
investigated. A series of NOESY spectra were measured varying
the mixing time from 1 ms to 20 ms and the cross-peaks were
integrated (Figure S36). For protons with T relaxation times
1
significantly longer than the mixing time, the exchange integral
approached a maximum as the mixing time increased. However,
standard COSY parameters (8 min for 1 scan, Table S1).
Cross-peaks were observed in spectra recorded using the
pulse programs cosygpqf (Figure 3a), cosyqf90 and cosygpmfqf
(
Figure S35), although intense diagonal peaks[6b] and commonly
observed artefacts, such as T noise streaks and anti-diagonal
1
peaks, were present to varying degrees dependent on the pulse
program. The availability of three suitable paramagnetic COSY
pulse programs will enable broad applicability to a variety of
paramagnetic complexes and facilitate the implementation of
paramagnetic COSY as a standard characterisation method.
The three COSY spectra of complex 1a display a large
number of cross-peaks (Figure 3a, Figure S35). Notably, COSY§
cross-peaks expected on the basis of 3J coupling were
observable facilitating identification of neighbouring protons and
thus, grouping of the proton resonances to one of the three
ligand environments (black, red and blue circles, Figure 3a).
Unexpectedly, additional cross-peaks were observed (orange
and purple boxes in Figure 3a, Figure S35), although the COSY
1
for protons with shorter T relaxation times (e.g. protons d and g),
the exchange integral reached a maximum before decreasing as
relaxation began competing with exchange when the mixing time
increased. A mixing time of 10 ms was found to be a good
compromise for maximising the exchange cross-peak of all
1
protons despite their differing T relaxation times.
Analysis of the NOESY spectrum revealed groups of three
cross-peaks (orange boxes, Figure 3b), corresponding to
chemical exchange between the three different ligand
environments of the mer isomer. Thus, the spectrum has no
NOE cross-peaks but is an EXSY spectrum since the mixing
time was so short. Furthermore, EXSY cross-peak intensities
can be close to 100%,[6c] whereas NOE intensities are, in
3
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Figure 4. 1H-13C HMQC NMR spectra (600 MHz/151 MHz, CD
3 3 4 2
CN, 298 K) of mer-[Co(pq) ](BF ) (1a). Two spectra with differing offsets (represented by the
dashed lines) were recorded since uniform excitation could not be achieved with standard square pulses over the large spectral range resulting in absent
cross-peaks or cross-peaks with reduced intensities (green circles) at the extremes of the spectral range.
general, small for small molecules and reduced even further by
the fast relaxation from coupling to the paramagnetic center.[6d]
Since some excess ligand was also present in the complex
solution, exchange cross-peaks were also observed between
the excess ligand and ligand in the complex (green boxes,
Figure 3b), enabling assignment of protons b-l in the complex
using the free ligand assignments (Table S6) despite broadening
and small shifts between free and excess ligand signals due to
the presence of CoII.
Steady-state NOE experiments, however, did allow assignment
of the broad signals above and below -22 ppm as protons a and
m, respectively (Figure S40). The complete proton assignment
1
of complex 1a was independently corroborated by T relaxation
measurements (Table S4) as well as exchange cross-peaks
between the excess ligand present in the sample and the
complex.
1
Having successfully assigned the H NMR spectrum of 1a,
we investigated assignment of the 13C NMR spectrum using
paramagnetic analogues of heteronuclear 1D and 2D techniques
(e.g. HSQC and HMBC). The proton-coupled 13C spectrum
contained signals over almost a 900 ppm range (Figure S41),
making uniform excitation over this very wide spectral range
difficult. Therefore, overlapping spectra of smaller spectral
widths were acquired to cover the entire range. The quaternary
carbons could be distinguished from the tertiary carbons on the
basis of the multiplicity and initially, the tertiary carbons were
assigned using selective 1H-decoupling C experiments where
one after another, each proton signal was selectively irradiated
during repeated acquisitions of the 13C NMR data (Figures S42-
S46). However, this method is time-consuming due to the
The COSY spectrum (Figure 3a) was then reanalysed with
the proton assignments to determine the origin of the additional
cross-peaks beyond the expected COSY cross-peaks (black,
red and blue circles). These additional cross-peaks correspond
to structural information that is not typically observable in the
COSY spectra of diamagnetic compounds; relaxation-allowed
through-space (NOE) cross-peaks between protons d and g
(
purple squares) were observed due to cross-correlation[16] and
13
EXSY cross-peaks (orange boxes) were observed due to
exchange between the three ligand environments, as confirmed
by the exchange cross-peaks in the NOESY spectrum (Figure
3
b).
Thus, almost complete assignment of the 1H spectrum of
a was possible using COSY and NOESY with the exceptions of
number of signals and the sensitivity of C NMR measurements
13
1
and therefore, we instead chose to investigate 2D heteronuclear
experiments.
the assignment of: i) spin systems g-h and l-k to a particular
ligand environment since relaxation-allowed (NOE) cross-peaks
were not observed between protons h and j in the COSY
spectrum; ii) the broad signals, which are proposed to be
protons a and m due to their close proximity to the paramagnetic
CoII centre. TOCSY (Figure S37) and steady-state NOE
experiments (Figures S38, S39) were carried out but exchange
cross-peaks rather than long-range coupling and NOE
cross-peaks, respectively, dominated these experiments.
Therefore, the unambiguous assignment of spin-system
containing protons j-k to a particular ligand environment was not
possible and the spin system was arbitrarily labelled with black,
red and blue labels according to decreasing chemical shift of
proton j to represent the three different ligand environments.
We focused on the HMQC pulse program as an alternative
to HSQC because of its simple four pulse sequence and
1
pleasingly, cross-peaks were observed for the sharp H signals
(linewidth < 70 Hz, Figures S47-S49). However, the acquisition
of two HMQC spectra was necessary to cover the large spectral
range in both dimensions as non-uniform excitation resulted in a
decrease in the intensity or complete loss of cross-peaks at the
extremes of the spectral range (Figure 4). Nevertheless, an
overlay of the two HMQC spectra confirmed the assignments
1
13
made using the selective H-decoupling
C experiments
(Figures S42-S46). HMBC spectra were also acquired in an
attempt to assign the quaternary carbons and the three spin
systems within a particular ligand environment; however, no
4
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RESEARCH ARTICLE
cross-peaks were observed due to the long delays resulting from
suitable TOCSY and HMBC pulse programs for paramagnetic
complexes.
3
the small magnitude of J coupling constants in contrast to the
large 1J coupling constants utilised in the HMQC experiments.
Therefore, the quaternary carbons and carbon a were tentatively
assigned through comparison to the reference complex
We then investigated the assignment of the species other
than the mer isomer in the spectra of 2a-7a. A set of sharp
signals consistent with only one ligand environment was visible
in the spectra of complexes 3a and 5a (Figures S93-S94), and
another set of broader signals, also consistent with only one
ligand environment, was visible for all complexes with the
exception of 6a (Figures S93-S94). We propose these two
species to be the fac isomer and a symmetric CoL -based
2
species. While these species cannot be distinguished on the
basis of the number of NMR signals, we attribute the set of
[
3 4 2
Co(bpy) ](BF ) (Figures S97-S99).
Thus, we successfully assigned the 1H and C NMR
13
1
spectra of complex mer-1a using a combination of 1D ( H,
proton-coupled 13C, selective 1H-decoupling 13C, steady-state
NOE) and 2D (COSY, NOESY, HMQC) NMR paramagnetic
experiments. We then sought to test the limits of this
paramagnetic NMR toolbox in complexes where proton coupling
within a spin system is interrupted by substitution as well as
complexes with broader linewidths. We prepared a series of
2
broader signals to a symmetric CoL -based species; this set of
broader signals was significant for the complex with pq-5’-Br yet
decreased in intensity upon addition of a fourth equivalent of
ligand while the mer-2a signals increased (Figure S51).
Furthermore, in two samples of the complex with pq-6-Ph, the
chemical shifts of the proposed symmetric CoL -based species
2
were sensitive to the differing water content of the samples
2
-pyridylquinoline ligands substituted in the 5’- or 6-positions
II
(
Figure 1a) and their corresponding Co complexes 2a-7a in situ
by mixing Co(BF
CD CN.
These complexes fulfil both criteria, as revealed by their
4 2 2
) •6H O and three equivalents of the ligand in
3
NMR spectra (Figures S93-S94, Table S5), and they also
allowed investigation of the influence of substitution on the
fac/mer isomerism. Based on comparison to the 1H NMR
spectrum of the parent complex 1a, it appeared that the mer
isomer also predominated for complexes 3a-7a but additional
species were also present (Figures S93-S94). In the case of the
complex with pq-5’-Br, a set of broader signals consistent with
only one ligand environment was also a significant species
within the mixture (Figure S51).
whereas those corresponding to mer-7a were not (Figure S86).
We attribute the observation of a symmetric CoL -based species
2
as well as mer-2a to the steric bulk of pq-5’-Br from not only the
quinoline ring but also the bromine substituent since this could
reduce the efficiency of π-π stacking interactions between two of
the ligands. In contrast, the predominance of mer-5a based on
the pq-6-Br ligand is likely due to the reduced steric influence of
the bromine substituent in the 6-position compared with the
5’-position in complex 2a.
Assignment of these species was more difficult than the
mer isomer since cross-peaks for these species were not
typically observable in the COSY spectra, most likely due to the
1
Initially, assignment of the proposed mer isomer in the H
and 13C NMR spectra of complexes 2a-7a was investigated. In
comparison to the parent complex 1a, the 1H signals of
complexes 2a-4a with 5’-substituted ligands had the largest
linewidths (>90 Hz, Table S5) followed by complexes 5a-7a
containing 6-substituted ligands (<130 Hz, Table S5).
Nevertheless, cross-peaks were still observable in the 2D
spectra (COSY, NOESY and HMQC) including exchange
cross-peaks in the COSY spectra to varying degrees for
2
broadness of the signals in the case of the CoL -based species
and low concentration in the case of the fac isomer (estimated to
constitute less than 1% of the complex mixture). However,
NOESY appears to be less sensitive to signal broadness than
COSY since exchange cross-peaks between the CoL -based
2
species and the mer complex were still observed (Figures S54,
complexes 3a-7a (Figures S59, S66, S72, S79, S87). The 1
H
S60, S67, S73). Furthermore, these cross-peaks were even
assignments for these complexes confirmed that in each case
the major species in solution is the mer isomer (Figures S93-
S94). 1H assignment of mer-2a was also possible despite the
absence of many cross-peaks in the COSY spectrum since
exchange cross-peaks were still observable in the NOESY
spectrum. We attribute the incompleteness of cross-peaks in the
COSY spectrum to the presence of broad linewidths (>200 Hz,
visible when the CoL
2
-based species was not detectable in the
1
H NMR spectrum due to signal broadness and/or the low
concentration of this species as seen in the spectrum of complex
6a (Figures S78, S80).
To further investigate the applicability of the paramagnetic
NMR toolbox we extended our studies to the characterisation of
a high spin/spin-crossover FeII complex. Complex 1b was
Table S5). Assignment of the 13C NMR spectra of complexes
prepared in a glovebox by mixing Fe(OTf)
of the pq ligand in dry CD
and three equivalents
CN. At room temperature the H NMR
2
1
1
2
a-7a was not as straightforward as the H spectra, most likely
3
1
3
due to the lower sensitivity of C NMR spectroscopy compared
to H NMR spectroscopy, the broadness of the signals and the
spectra of the complex contained broad signals for the complex
and therefore, detailed analysis was not possible. We attributed
the broad signals to fast ligand exchange processes and
therefore, variable temperature experiments were carried at
lower temperatures where ligand exchange would be slower.
Upon cooling the solutions from 298 K to 248 K, the
signals sharpened and displayed Curie-Weiss behaviour (Figure
S106-S109).[17] There was no evidence of spin-crossover over
this temperature range, consistent with previous studies on the
1
presence of multiple species. Thus, in some cases only partial
assignment was possible (Figures S62-S63, S68-S69, S74-S75,
S81-S83, S89-S91).
A comparison of the 1H assignments for mer-2a-7a to
those of the parent complex mer-1a showed that the signals of
the 2-pyridylquinoline backbone do not shift significantly upon
substitution and as expected, the 1H signals for the 5’- and
[
14b]
6
-positions are not present in the spectra of complexes 2a-4a
tetrafluoroborate salt of complex 1b in acetone.
At 248 K
1
13
and 5a-7a, respectively, due to substitution (Figures S93-S94).
The protons in these spin systems with substituents could be
assigned by the exchange peaks in the NOESY spectra but not
to a particular ligand environment, due to the disruption of proton
coupling by substitution in the COSY spectrum and absence of
almost complete assignment of the H and C NMR spectra was
possible for mer-1b since the signals were relatively sharp and
cross-peaks were observable in the COSY, NOESY and HMQC
spectra (Figures S101-S105). However, the carbon signals for d
and g could not be assigned on the basis of the HMQC
5
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RESEARCH ARTICLE
spectrum since the cross-peaks were absent or very weak,
be observed in a single 2D spectrum compared with multiple 1D
spectra for steady-state NOE experiments. For this reason,
steady-state NOE experiments may only be necessary in the
case of troubleshooting (Sections 1.1.1.3 and 1.1.1.4).
II
attributed to the increased influence of the paramagnetic Fe ion
on the relaxation times at lower temperature. In addition, at least
one other species was present at equilibrium as broader signals
1
The assignment of the 13C NMR spectrum has a similar
workflow beginning with acquisition of the 13C NMR spectrum
(toolbox experiment 4) followed by HMQC (toolbox experiment
were also seen in the H NMR spectrum but they could not be
assigned in the absence of cross-peaks in the 2D NMR spectra.
Given the large change in the linewidth of the 1H NMR
signals between 248 K and 298 K (Table S7), we carried out
temperature-dependent COSY, NOESY and HMQC experiments
to investigate whether cross-peaks were also observable at
higher temperatures (Figures S110-112). HMQC and COSY
appeared to be more sensitive to temperature than NOESY
since by 266 K most cross-peaks were no longer observable
1
13
5a) and/or selective H-decoupling C experiments (toolbox
experiment 5b) to identify the 1JCH coupling. Again, the 2D
HMQC experiment is preferable to
a series of selective
1
H-decoupling 13C experiments but these 1D experiments may
be useful in troubleshooting (Sections 1.1.1.6 and 1.1.1.7).
(
Figures S110, S112). Nevertheless, assignment of the 1H
signals was still possible exploiting the exchange cross-peaks in
the NOESY spectra (Figure S111) and the assignments from
lower temperatures to assign coupled protons within a single
spin system when COSY cross-peaks were not observable at
that temperature (Figures S110).
Finally, we applied the paramagnetic NMR toolbox to the
characterisation of paramagnetic metal-organic cages. The
rational design of lower symmetry metal-organic cages is
challenging and therefore, we prepared and characterised
instead the highly symmetric tetrahedral CoII
proof-of-principle for paramagnetic cage characterisation
2 2
Figures S113-S117). A solution of four equivalents of Co(NTf )
4 6
L cage 8 as
(
and six equivalents of ligand was heated at 50 °C in acetonitrile
and the cage was isolated by precipitation with diethyl ether.
1
The H NMR spectrum of the redissolved cage in CD
3
CN
contained 5 sharp and 2 broad signals, reflecting the presence
of one ligand environment due to fac coordination around the
metal centres. Only the expected through-bond cross-peaks
were observed in the COSY spectrum (Figure S114). Full and
unambiguous assignment of the 1H and 13C NMR spectra was
possible using the paramagnetic NMR toolbox, with the
exception of the quaternary carbons and the broad signals
corresponding to a and j (Figures S113, S115).
Figure 5. Proposed workflow including troubleshooting experiments (red
arrows) for the application of the paramagnetic NMR toolbox to the structural
characterisation of paramagnetic complexes and cages.
Following the characterisation of mononuclear complexes
1
a-7a, 1b and cage 8, we propose a workflow including
Conclusion
troubleshooting experiments for the application of the
paramagnetic NMR toolbox to the structural characterisation of
other paramagnetic complexes and cages (Figure 5). A more
detailed instruction manual for each toolbox experiment is
provided in the SI (Section 1.1.1). The workflow begins with the
We report a toolbox of 1D ( H, proton-coupled 13C, selective
1H-decoupling 13C, steady-state NOE) and 2D (COSY, NOESY,
HMQC) paramagnetic NMR methods that enables the
straightforward characterisation of paramagnetic complexes.
This toolbox overcomes the data acquisition challenges due to
the presence of paramagnetic centres, such as large
paramagnetic shifts and short relaxation times, and also
removes the reliance of data interpretation on theoretical
models[6b, 9b, 11] or the Solomon equation.[4b,
demonstrated the general applicability of this toolbox for fields
from coordination chemistry to spin-crossover complexes and
1
1
acquisition of a H NMR spectrum to establish the spectral width
as well as the linewidths of the signals since large spectral
widths often necessitate the acquisition of several 2D spectra
with smaller spectral widths and the observation of cross-peaks
in 2D spectra is, in many cases, dependent on the linewidth.
10b, 10d]
We
With broad linewidths (typically
> 100 Hz), variation and
optimisation of the temperature is recommended in an attempt to
reduce the linewidths and increase the likelihood of cross-peak
observation in 2D experiments.
COSY is recommended as the second toolbox experiment to
identify coupled protons within each spin system. However,
additional relaxation-allowed exchange and NOE cross-peaks
may be observable complicating assignment and therefore,
either NOESY (toolbox experiment 3a) or steady-state NOE
II
supramolecular chemistry through the characterisation of Co
and high-spin FeII mononuclear complexes as well as a Co
L
4 6
cage. Furthermore, we demonstrated the toolbox can be
successfully applied to structural characterisation in a variety of
situations: the assignment of complexes with multiple ligand
environments (e.g. mer complexes), complexes with a range of
signal linewidths (including broad signals in the case of the
(
toolbox experiment 3b) experiments are recommended to
1
CoL
fac-CoL
2
-based species) and mixtures of complexes (e.g. mer- and
isomers as well as CoL -based species).
complete the
H NMR spectrum assignment. These two
3
2
experiments provide similar structural information but NOESY
has the advantage that the exchange and NOE cross-peaks can
This study also shows the advantages of paramagnetic
versus diamagnetic NMR spectroscopy; the short relaxation
6
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Angewandte Chemie International Edition
10.1002/anie.202008439
RESEARCH ARTICLE
times of paramagnetic complexes enable reduction of the
repetition delays and acquisition times, thereby significantly
reducing the experiment times or enabling the acquisition of
more data in a similar time (Table S1). In addition to reduced
signal overlap and increased sensitivity, structural information
can be observed by paramagnetic NMR spectroscopy that would
not be observable in the diamagnetic analogue; in the COSY
Minameyer, S. Gsänger, B. Meyer, T. Drewello, D. M. Guldi, M. von
Delius, J. Am. Chem. Soc. 2018, 140, 13413-13420.
[
4]
a) M. Hardy, N. Struch, J. J. Holstein, G. Schnakenburg, N. Wagner, M.
Engeser, J. Beck, G. H. Clever, A. Lützen, Angew. Chem. Int. Ed. 2020,
59, 3195-3200; b) A. J. McConnell, C. M. Aitchison, A. B. Grommet, J.
R. Nitschke, J. Am. Chem. Soc. 2017, 139, 6294-6297; c) R. A. S.
Vasdev, J. A. Findlay, A. L. Garden, J. D. Crowley, Chem. Commun.
2019, 55, 7506-7509; d) A. J. Metherell, W. Cullen, N. H. Williams, M. D.
spectra of the CoII mer
mononuclear complexes,
Ward, Chem. Eur. J. 2018, 24, 1554-1560; e) S. M. Jansze, K. Severin,
J. Am. Chem. Soc. 2019, 141, 815-819.
relaxation-allowed through-space (NOE) cross-correlation peaks
and exchange (EXSY) cross-peaks were observed in addition to
the expected through-bond (COSY) cross-peaks. Furthermore,
the sensitivity of the exchange NOESY technique enabled the
identification of additional species present at equilibrium that
[
[
5]
6]
a) J. Zhong, L. Zhang, D. P. August, G. F. S. Whitehead, D. A. Leigh, J.
Am. Chem. Soc. 2019, 141, 14249-14256; b) Y. Inomata, T. Sawada, M.
Fujita, Chem 2020, 6, 294-303; c) H.-N. Zhang, W.-X. Gao, Y.-J. Lin,
G.-X. Jin, J. Am. Chem. Soc. 2019, 141, 16057-16063.
a) C. L. I. Bertini, Coord. Chem. Rev. 1996, 150, 1-28; b) A. J. Pell, G.
Pintacuda, C. P. Grey, Prog. Nucl. Magn. Reson. Spectrosc. 2019, 111,
1
were not visible in the H NMR spectra due to broad linewidths
and/or their low concentration.
1-271; c) C. L. I. Bertini, Coord. Chem. Rev. 1996, 150, 185-220; d) C.
While solution characterisation of paramagnetic complexes
and cages was previously typically limited to a 1H NMR
spectrum only, we demonstrate that in-depth structural analysis
comparable to that for diamagnetic compounds is now possible
using the paramagnetic NMR toolbox. We are now extending the
use of this toolbox to the characterisation of more complex as
well as mixtures of supramolecular architectures.
L. I. Bertini, Coord. Chem. Rev. 1996, 150, 163-184; e) M. P. Crockett,
H. Zhang, C. M. Thomas, J. A. Byers, Chem. Commun. 2019, 55,
14426-14429.
[
[
7]
8]
a) B.-B. Pan, F. Yang, Y. Ye, Q. Wu, C. Li, T. Huber, X.-C. Su, Chem.
Commun. 2016, 52, 10237-10240; b) A. Bahramzadeh, T. Huber, G.
Otting, Biochem. 2019, 58, 3243-3250.
a) R. W. Hogue, C. P. Lepper, G. B. Jameson, S. Brooker, Chem.
Commun. 2018, 54, 172-175; b) S. Rodríguez-Jiménez, M. Yang, I.
Stewart, A. L. Garden, S. Brooker, J. Am. Chem. Soc. 2017, 139,
18392-18396; c) D. F. Evans, J. Chem. Soc. 1959, 2003-2005; d) A.
Ferguson, M. A. Squire, D. Siretanu, D. Mitcov, C. Mathoniere, R.
Clerac, P. E. Kruger, Chem. Commun. 2013, 49, 1597-1599; e) R. A.
Bilbeisi, S. Zarra, H. L. C. Feltham, G. N. L. Jameson, J. K. Clegg, S.
Brooker, J. R. Nitschke, Chem. Eur. J. 2013, 19, 8058-8062; f) A. J.
McConnell, Supramol. Chem. 2018, 30, 858-868; g) A. A. Pavlov, G. L.
Denisov, M. A. Kiskin, Y. V. Nelyubina, V. V. Novikov, Inorg. Chem.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (DFG, project
number 413396832 (M.L.), project number 429518153 (T.P.)
and CRC677) for financial support and we also thank Prof. Dr
Thisbe K. Lindhorst and Prof. Dr Rainer Herges for bridging
financial support for M.L. We thank the spectroscopy department
and Dr Claus Bier for NMR and mass spectral data collection.
We thank Jan-Christian Carstensen, Felix Piontek, André
Petersen and Tjorge Neumann for preliminary studies on the
complexes and the synthesis of ligand precursors.
2017, 56, 14759-14762.
[
[
9]
a) H. S. Chow, E. C. Constable, C. E. Housecroft, K. J. Kulicke, Y. Tao,
Dalton Trans. 2005, 236-237; b) M. Kruck, D. C. Sauer, M. Enders, H.
Wadepohl, L. H. Gade, Dalton Trans. 2011, 40, 10406-10415.
10] a) C. Piguet, G. Bernardinelli, B. Bocquet, A. Quattropani, A. F.
Williams, J. Am. Chem. Soc. 1992, 114, 7440-7451; b) H. Amouri, L.
Mimassi, M. N. Rager, B. E. Mann, C. Guyard-Duhayon, L. Raehm,
Angew. Chem. Int. Ed. 2005, 44, 4543-4546; c) S. Turega, M.
Whitehead, B. R. Hall, A. J. H. M. Meijer, C. A. Hunter, M. D. Ward,
Inorg. Chem. 2013, 52, 1122-1132; d) I. S. Tidmarsh, B. F. Taylor, M. J.
Hardie, L. Russo, W. Clegg, M. D. Ward, New J. Chem. 2009, 33, 366-
Keywords: NMR spectroscopy • paramagnetic complex • metal-
organic cage • supramolecular chemistry • spin-crossover
375; e) M. J. Burke, G. S. Nichol, P. J. Lusby, J. Am. Chem. Soc. 2016,
‡
CCDC 1987819 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from the
1
38, 9308-9315.
[
[
11] a) W. C. Isley, S. Zarra, R. K. Carlson, R. A. Bilbeisi, T. K. Ronson, J. R.
Cambridge
http://www.ccdc.cam.ac.uk/data_request/cif.
It is likely that the cross-peaks attributed to through-bond correlations
also have contribution from through-space coupling due to
Crystallographic
Data
Centre
via
Nitschke, L. Gagliardi, C. J. Cramer, Phys. Chem. Chem. Phys. 2014,
16, 10620-10628; b) T. L. J. Huang, D. G. Brewer, Can. J. Chem. 1981,
59, 1689-1700; c) M. L. Wicholas, R. S. Drago, J. Am. Chem. Soc.
1968, 90, 6946-6950.
§
a
cross-correlation.[16]
12] a) S. Cai, C. Seu, Z. Kovacs, A. D. Sherry, Y. Chen, J. Am. Chem. Soc.
006, 128, 13474-13478; b) F. A. A. Mulder, L. Tenori, C. Luchinat,
Angew. Chem. Int. Ed. 2019, 58, 15283-15286.
2
[
[
1]
a) G. Wang, Z.-T. Zhang, B. Jiang, X. Zhang, C. Li, M. Liu, Anal.
Bioanal. Chem. 2014, 406, 2279-2288; b) W.-M. Liu, M. Overhand, M.
Ubbink, Coord. Chem. Rev. 2014, 273-274, 2-12; c) X.-C. Su, J.-L.
Chen, Acc. Chem. Res. 2019, 52, 1675-1686.
[13] S. Denis-Quanquin, F. Riobé, M.-A. Delsuc, O. Maury, N. Giraud,
Chem. Eur. J. 2016, 22, 18123-18131.
[14] a) I. S. Jahro, D. Onggo, Ismunandar, S. I. Rahayu, M. C. Muñoz, A. B.
Gaspar, M. Seredyuk, P. Gütlich, J. A. Real, Inorg. Chim. Acta 2008,
361, 4047-4054; b) D. Onggo, J. M. Hook, A. D. Rae, H. A. Goodwin,
Inorg. Chim. Acta 1990, 173, 19-30; c) C. Harris, S. Kokot, H. Patil, E.
Sinn, H. Wong, Aust. J. Chem. 1972, 25, 1631-1643.
2]
3]
a) A. Pastor, E. Martínez-Viviente, Coord. Chem. Rev. 2008, 252,
2314-2345; b) L. Avram, A. D. Wishard, B. C. Gibb, A. Bar-Shir, Angew.
Chem. Int. Ed. 2017, 56, 15314-15318; c) L. Avram, M. A. Iron, A. Bar-
Shir, Chem. Sci. 2016, 7, 6905-6909; d) Y. Cohen, S. Slovak, Org.
Chem. Front. 2019, 6, 1705-1718.
[15] R. Wang, H. Fan, W. Zhao, F. Li, Org. Lett. 2016, 18, 3558-3561.
[16] a) I. Bertini, C. Luchinat, D. Tarchi, Chem. Phys. Lett. 1993, 203, 445-
449; b) I. Bertini, M. Piccioli, D. Tarchi, C. Luchinat, Concepts Magn.
Reson. 1994, 6, 307-335; c) S. Wimperis, G. Bodenhausen, Mol. Phys.
1989, 66, 897-919; d) S. Wimperis, G. Bodenhausen, Chem. Phys. Lett.
1987, 140, 41-45.
[
a) M. Dumartin, M. C. Lipke, J. F. Stoddart, J. Am. Chem. Soc. 2019,
141, 18308-18317; b) H. Li, H. Zhang, A. D. Lammer, M. Wang, X. Li, V.
M. Lynch, J. L. Sessler, Nat. Chem. 2015, 7, 1003-1008; c) T.
Bunchuay, A. Docker, A. J. Martinez-Martinez, P. D. Beer, Angew.
Chem. Int. Ed. 2019, 58, 13823-13827; d) N. Pairault, H. Zhu, D.
Jansen, A. Huber, C. G. Daniliuc, S. Grimme, J. Niemeyer, Angew.
Chem. Int. Ed. 2020, 59, 5102-5107; e) Y. Xu, R. Kaur, B. Wang, M. B.
[17] C. P. Landee, M. M. Turnbull, J. Coord. Chem. 2014, 67, 375-439.
7
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