Calculation and experimental measurement of paramagnetic NMR parameters of phenolic oximate Cu(II) complexes

Article abstract of DOI:10.1039/c7cc05098d

We present a strategy for predicting the unusual ¹H and ¹³C shifts in NMR spectra of paramagnetic bisoximato copper(ii) complexes using DFT. We demonstrate good agreement with experimental measurements, although ¹H-¹³C correlation spectra show that a combined experimental and theoretical approach remains necessary for full assignment.

Full text of DOI:10.1039/c7cc05098d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Calculation and Experimental Measurement of Paramagnetic
NMR Parameters of Phenolic Oximate Cu(II) Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
Daniel M. Dawson, Zhipeng Ke, Frederick M. Mack, Rachel A. Doyle, Giulia P. M. Bignami,
Iain A. Smellie, Michael Bühl* and Sharon E. Ashbrook*
DOI: 10.1039/x0xx00000x
www.rsc.org/
1
We present a strategy for predicting the unusual H and 13C shifts consuming approach of specific 13C labelling in conjunction
in NMR spectra of paramagnetic bisoximato copper(II) complexes with 1H-13C cross polarisation (CP) NMR, which is generally
using DFT. We demonstrate good agreement with experimental inefficient for paramagnetic materials.2 It would, therefore, be
measurements, although 1H-13C correlation spectra show that a desirable to have a more general assignment method that
combined experimental and theoretical approach remains does not rely on the development of bespoke synthetic
necessary for full assignment.
pathways for efficient isotopic enrichment. Owing to the rapid
MAS rates (necessitating the use of small rotors with,
consequently, small sample volumes) required for high-
resolution pNMR spectra, sensitivity is inherently low and it
would, therefore, also be advantageous to be able to predict
shifts prior to the experimental measurement, particularly as
resonances can be several hundred ppm away from their
typical diamagnetic range.
In recent years, paramagnetic NMR (or “pNMR”) has
developed greatly, with systems from metalloproteins1 (dilute,
isolated spins) to metal-organic frameworks2-4 (denser
networks of potentially coupled spins) and metal oxides5 (very
dense networks of highly coupled spins) being studied. For
dilute spins, the NMR conditions can be selected such that the
signal from nuclei near the paramagnetic centre is “invisible”
owing to rapid relaxation and only the longer-range through-
space pseudocontact shifts are observed.1 These are generally
on the order of a few ppm and occur over distances such that
the unpaired electron can be treated as a point spin, resulting
in a simple 1/r3 relationship with the shift. For dense spin
networks such as transition metal oxides, it may be possible to
assign the NMR spectra by analysis of bonding pathways (via
oxygen).5 However, in the intermediate regime, including
materials such as catalytically-active transition metal
complexes and metal-organic frameworks (MOFs), both the
through-bond Fermi contact and through-space pseudocontact
interactions affect the observed NMR spectrum and
assignment can be both nontrivial and counterintuitive.2,6-9 For
example, in the 13C NMR spectrum of the MOF HKUST-1
(Cu3btc2, btc = benzene-1,3,5-tricarboxylate),10 the broadest
resonance, shifted most by paramagnetic interactions, is not
the carboxylate C (separated from Cu2+ by just two bonds), but
rather the adjacent quaternary C (three bonds from Cu). This
assignment was confirmed using the relatively costly and time-
Periodic density functional theory (DFT) calculations have
enjoyed great success in solid-state NMR, allowing the
optimisation of experimental structures to an energy minimum
and the subsequent calculation of highly accurate NMR
parameters11-13 However, pNMR DFT calculations are still in
their relative infancy, particularly for periodic solids. The field
is more advanced for molecular calculations, which have
successfully been used to assign the NMR spectra of a range of
complexes of interest.14-17 Recently, we reported the
calculated and experimental NMR parameters for
1 (see Fig.
1). This material provides a typical example of the non-intuitive
Fig. 1 The structures of bis(salycylaldoximato)copper(II) (
functionalised ligands considered, where compound
The numbering schemes used are indicated in red.
1
N has the formula Cu(LN)2.
= Cu(L1)2) and the
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nature of the paramagnetic shifts: at 298 K the 13C resonance the ligands leads to large spin density on atoms within 4 bonds
with the highest shift (1006 ppm) corresponds to C3, which is of Cu, but almost negligible spin density on the periphery of
bonded to the H with the lowest shift (–5.4 ppm) and the 13C the molecule. The partial s character of the HOMO on atoms
resonance with the second-highest shift (963 ppm) close to the Cu (see ESI) leads to large computed hyperfine
corresponds to C7, which is bonded to the H with the highest couplings for these nuclei, and Fig. 2c and 2d show the
shift (272 ppm). The calculated shifts for the complex (and predicted 1H and 13C peak positions for
2-9 at 298 K. While the
1
their variation with temperature) were in good agreement calculations indicate only minor substituent effects on the H
with experiment, particularly given that the calculations were shifts, larger effects are predicted for 13C, where some
carried out for a single gas-phase molecule rather than the resonances in the aromatic region can re-order.
periodic solid. Exploratory calculations suggested a negligible (J
0.3 cm–1) coupling between spins on adjacent molecules and contains the expected six resonances between 268 and –5.2
this, along with other long-range packing effects, led to only ppm. The spectrum was recorded in five steps (see ESI for
small shift differences of up to 4.7 ppm for the most shifted 1H details). Fig. 3b shows the 13C MAS NMR spectrum of
and,
Fig. 3a shows the 1H MAS NMR spectrum of
6, which
≈
6
and 56.9 ppm (~5% of the total shift range) for the most again, the expected 8 resonances are observed, between 1020
shifted 13C. By combining the predicted peak positions with and 54 ppm. The spectrum was recorded in four steps (see ESI
their calculated temperature dependence (assumed to be for details). As can be seen in Fig. 3c and d, the positions of
linear with 1/T, where a plot of δiso vs. 1/T yields a gradient many of the resonances in both spectra vary significantly with
dδ
iso/d(1/T) and a y intercept δiso∞), all resonances could be temperature, confirming large through-bond interactions with
assigned. As discussed above, the attraction of using DFT the unpaired electron. The range of observed shifts and their
calculations to assign the NMR spectra of paramagnetic
materials is that they can be carried out in advance of the
experiment and can, thus, be used to guide spectral
acquisition as well as assignment. Here we extend this
investigation to
with multidimensional multinuclear NMR spectra. To be able
to distinguish between positional isomers (cf. ) or to
account for subtle changes such as replacing a H atom with a
Me group (cf. or ) would greatly improve the predictive
2-9 (see Fig. 1), comparing DFT predictions
4, 6, 7
1/
2
4/9
power of the calculations and increase the potential of the
joint experimental/theoretical approach as a structural tool.
As described in the ESI, we employed our recently
introduced computational methodology6 to optimise
molecular complexes in the gas phase and calculate their
pNMR parameters. Fig. 2a shows the highest occupied
molecular orbital (HOMO) for the representative example of
which can be seen to arise from an interaction between the
d(x orbital of Cu and a -antibonding orbital of the ligands.
6,
2
2
σ
–y
)
As shown by the spin density isosurface of
6
(Fig 2b), the
framework of
presence of the unpaired electron within the
σ
Fig. 2 (a)
0.0004 a.u, respectively. Calculated positions of (c) 1H and (d) 13C resonances (at
α-HOMO and (b) spin density isosurfaces of 6, plotted at 0.02 and
Fig. 3 (a) 1H and (b) 13C (14.1 T, 298 K, 37.5 kHz MAS) NMR spectra of
in (c) 1H and (d) 13C peak positions of
as a function of 1/T.
6. Variation
298 K and neglecting the substituents) for
2
-
9.
6
2 | J. Name., 2012, 00, 1-3
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Table 1 Calculated (DFT) and experimental (exp.) pNMR parameters for 6.
temperature dependence are in good agreement with the
picture obtained from DFT calculations.
δiso at 298 K
dδiso/d(1/T) / 10–6
K
δiso∞ (ppm)
Partial spectral assignment was achieved using 1H-13C
heteronuclear correlation (HETCOR) experiments. Ideally,
these would select for magnetisation transfer through the
(ppm)
exp.
–5.2
24.0
5.9
DFT
–7.1
33.4
12.8
305
3.3
exp.
DFT
exp.
DFT
6.93
7.19
6.39
8.11
3.53
10.59
–2.80(3)×103
5.11(2)×103
1.6(2)×102
8.0(2)×104
–3.9(2)×102
2.1(4)×103
–4.19×103
7.80×103
1.90×103
8.85×104
–77.3
4.18(9)
6.85(7)
5.31(7)
–2(6)
H3
H4
H6
H7
H8
Hbr
C1
C2
C3
C4
C5
C6
C7
C8
1
scalar coupling with a mixing time selective for JCH. However,
this would be several ms, during which much of the
magnetisation would return to thermal equilibrium (T1
relaxation constants for C1-7 (not reported) were typically
<200 ms). Therefore, we transferred magnetisation via the
through-space dipolar coupling using cross polarisation (CP)
268
2.1
3.42(5)
15(1)
21.9
128
74
31.3
6.18×103
2.51×103
–2.55×104
3.19×105
2.84×104
7.60×103
–1.07×104
2.28×105
2.17×102
126.2 –3.916(6)×103
141.16(2) 117.8
118.14(9) 163.1
77.7
–1.303(3)×104
2.79(2)×105
3.24(3)×104
3.3(5)×102
–4.17(7)×103
2.53(2)×105
–5.8(12)×102
with a spin-lock duration of 100
distances similar to a C-H bond. The spectrum of
ꢀ
s to ensure transfer over
(shown in
1020 1198
82(5)
128.0
130.9
155.0
114.8
158.3
54.6
6
228
155
94
226.3
180.5
79.0
922
119.8(9)
154.2(1)
107.7(2)
111(5)
1
Fig. 4) was recorded with frequency stepping for both H and
13C (see ESI for details). The HETCOR spectrum allows
identification of the expected five CH pairs and three
960
54
1
quaternary C species. The H resonance at 21.9 ppm that does
not correlate to 13C corresponds to Hbr.
55.3
56.2(4)
Having identified the CH and C species from the HETCOR
spectra, assignment of all observed resonances was carried out
by comparison with the DFT calculations, with Table 1 giving
C3 and C7, despite similar proximity to the Cu centre. Although
somewhat surprising, this can be explained by the spin density
plot in Fig. 2b, which shows the interaction is highly
directional, and emphasises the need to combine accurate DFT
calculations with experiment to correctly assign all resonances.
Fig. 5a presents a plot of the calculated 13C δiso at 298 K
against the experimental values for the compounds in Fig. 1. It
can be seen that excellent agreement is observed for the
resonances with large paramagnetic shifts (C3, C4 and C7), but
that, where the shift is closer to the diamagnetic region (either
because the C is many bonds from Cu (as in C5) or because of
the final assignments for
computed data and final assignments for
several cases (for example, C1 and C6 in
6
(see ESI for all experimental and
and ). In
– see ESI),
2
-
5
7-9
4
assignment based only on the calculated position of the
resonances is incorrect, but the sign and magnitude of
∞
d
δiso/d(1/T) and the “diamagnetic” shift contribution, δiso
,
combined with the connectivity from the HETCOR spectrum,
allows a complete assignment to be made. It is clear from such
cases that measurement of the temperature dependence of
δ
iso is vital to enable complete assignment, and the dδiso/d(1/T)
∞
and δiso values presented here allow comparison of
experimental and computed spectra at any given temperature
(in the limit of the high-temperature regime model implicit in
the calculations). It is also noticeable that C1 and C2 have
much smaller paramagnetic shifts and narrower lines than
Fig. 5 Plots comparing calculated (DFT) and experimental (exp) (a) 13C
K), (b) 13C d iso/(1/T), (c) 1H iso (at 298 K) and (d) 1H d
iso/(1/T) for all complexes.
N.B. as discussed in the text, crystallographic inequivalence was observed for
Average experimental shifts for each C species are reported here – see ESI for
details. Only data for the thermodynamically-stable polymorph of are included.
δiso (at 298
Fig. 4 (14.1 T, 37.5 kHz MAS, 298 K) 1H-13C CP HETCOR spectrum of
with contact time of 100 s. Each region was recorded separately with
different acquisition conditions (see ESI) and contour levels are arbitrary.
6
recorded
δ
δ
δ
3.
a
ꢀ
7
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cancellation of positive and negative spin density, as in C1 and unexpectedly small paramagnetic shifts for the two C closest
C2), the error is greater. Fig. 5b shows a plot of calculated 13C (through space and through bond) to the Cu2+, however, DFT
dδiso/d(1/T) (which is proportional to the hyperfine coupling calculations are essential for enabling spectral assignment. Our
constant) against the experimental values for the same computational method appears generally applicable to these
compounds. Again, generally excellent agreement is achieved, complexes and we are now seeking to extend it to
although it can be seen from the inset in Fig. 5b that, for C1, multinuclear complexes more closely resembling the extended
there is often
a disagreement between calculation and structures of MOFs, in order to develop our joint
experiment on the sign of the hyperfine coupling constant computational/experimental NMR approach into a structural
when the latter becomes small. Fig. 5c and d show similar plots tool for these more demanding materials.
for the 1H resonances, with excellent agreement between
This work was supported by the EPSRC through the
experiment and calculation for most resonances. The sign of Collaborative Computational Project on NMR Crystallography
iso/d(1/T) for H5 is consistently miscalculated, but its (CCP-NC), via EP/M022501/1. SEA would also like to thank the
magnitude is essentially negligible in both cases. For Hbr, the Royal Society and Wolfson Foundation for a merit award. MB
magnitude of d iso/d(1/T) is calculated to be essentially would like to thank EaStCHEM and the School of Chemistry for
d
δ
δ
independent of substituents, but the experimental values support and access to a computer cluster maintained by Dr. H.
show a much wider variation, most likely reflecting that the Früchtl. ZK gratefully acknowledges a scholarship from the
static picture of the hydrogen-bonded species is incomplete China Scholarship Council. For research data supporting this
and motion along the O-H…O axis may lead to this discrepancy. publication see DOI: http://dx.doi.org/10.17630/9061ace0-
Initial exploratory calculations confirmed that the chemical 88fb-4a55-a1eb-05e020f369fd.
shift of this proton may indeed be more sensitive to vibrational
averaging than those of most other nuclei (see ESI), but a full
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Table of Contents Graphic
Unusual 1H and 13C NMR shifts for paramagnetic bisoximato copper(II)
complexes are understood by combining DFT calculations and
multidimensional experiments.