Unravelling the Complexities of Pseudocontact Shift Analysis in Lanthanide Coordination Complexes of Differing Symmetry

Article abstract of DOI:10.1002/anie.201906031

In two closely related series of eight-coordinate lanthanide complexes, a switch in the sign of the dominant ligand field parameter and striking variations in the sign, amplitude and orientation of the main component of the magnetic susceptibility tensor

Full text of DOI:10.1002/anie.201906031

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Unravelling the Complexities of Pseudocontact Shift Analysis in
Lanthanide Coordination Complexes of Differing Symmetry
Authors: David Parker, Elizaveta Suturina, Alice Harnden, Andrei
Batsanov, Mark Fox, Kevin Mason, Michele Vonci, Eric
McInnes, and nicholas chilton
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906031
Angew. Chem. 10.1002/ange.201906031
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http://dx.doi.org/10.1002/ange.201906031

Angewandte Chemie International Edition
10.1002/anie.201906031
COMMUNICATION
Unravelling the Complexities of Pseudocontact Shift Analysis in
Lanthanide Coordination Complexes of Differing Symmetry
[
a]
[b]
[a]
[a]
[a]
Alice C. Harnden, Elizaveta A. Suturina, Andrei S. Batsanov, Mark A. Fox, Kevin Mason,
[
c]
[c]
[c]
[a]
Michele Vonci, Eric J. L. McInnes, Nicholas F Chilton and David Parker*
Abstract: In two closely related series of eight-coordinate lanthanide
complexes, a switch in the sign of the dominant ligand field
parameter and striking variations in the sign, amplitude and
orientation of the main component of the magnetic susceptibility
aggregation for systems with a small ligand field (<kT), where no
[
9]
solvent is metal-bound. Indeed, such studies have emphasised
the need for very careful magneto-structural correlations, so that
the size, sign and orientation of the principal components of the
magnetic susceptibility tensor can be reliably determined, as
they vary for each different lanthanide complex of a common
ligand.
3
+
tensor as the Ln ion is permuted conspire to mask modest changes
in NMR paramagnetic shifts, but are evident in Yb EPR and Eu
emission spectra.
1
Pseudocontact shifts (PCS) dominate the experimental H NMR
paramagnetic shifts in 4f-block coordination complexes, as the
Lanthanide induced shift and paramagnetic relaxation
enhancement are subjects of continuous research driven by new
applications of lanthanide complexes as tags for biomolecular
unpaired electron spin populations on ligand protons are
[5a, 9b]
generally very small.
Such shifts can be described by the
[10]
McConnell equation:
[
1]
structure analysis,
resonance imaging (MRI). While Gd tags are employed in
double electron-electron resonance (DEER) distance
measurements in biomolecules, and Gd MRI contrast agents
and PARASHIFT probes for magnetic
[
2]
3+
PCS
1
2
2
⎡χ 3cos θ −1 + 3χ sin θ cos2ϕ⎤
δ
=
(
)
(1)
3
⎣
ax
rh
⎦
1
2πr
[
3]
3+
where r, θ and φ are nuclear polar coordinates, with respect to
the lanthanide ion, in the eigenframe of the magnetic
susceptibility tensor. The terms ꢀ!" and ꢀ!! represent the
axiality and rhombicity of the tensor, wherein ꢀ!! has a limit of
[
4]
are routinely used, non-Gd tags with pronounced magnetic
anisotropy continue to challenge existing theoretical models of
[
5]
[6]
paramagnetic shift and relaxation. Many of the conclusions
made in the early literature examining NMR behaviour in
paramagnetic lanthanide systems, for example, may have low
significance because of the limitations of Bleaney’s theory of
1
/3 of ꢀ!"
.
Here, we examine the spectroscopic behavior of selected C
1
2 +
-
1
and C
2
-symmetric lanthanide complexes [LnL ] and [LnL ] ,
[
7]
magnetic anisotropy. Often, unreasonable estimations of a
contact contribution were invoked in order to allow experimental
data to be fitted, and sometimes ‘distance only’ dependent
relaxation rate data were used to aid assignment,
notwithstanding the fact that the electron-nuclear dipolar
which are model systems for the temperature and pH dependent
[
2a, 2b]
PARASHIFT probes being developed in parallel.
Previous
work has shown that the magnetic susceptibility tensor can vary
3
+
3+
from nearly fully rhombic for Tm to almost fully axial for Tb
3
+
1a
and Dy in [LnL ] with the sign of axiality being negative for
3
+
3+
3+
3+ [5a, 9b]
interaction has been shown to have
a strong directional
Tb -Ho and positive for Er -Yb .
The corresponding
3
+
dependence, both in the dipolar and the Curie contributions to
diamagnetic Y complexes were used to examine in detail the
static and dynamic aspects of complex stereoisomerism, and
served as the starting point for computational studies and PCS
analysis, aided by the X-ray structural determination of the
[
6]
paramagnetic relaxation.
The theory of Bleaney assumed that ligand field (zero-field)
splitting of the ground J-multiplet is less than kT and that the
ligand-field parameters are invariant as the lanthanide ion is
permuted in an isostructural series. Neither assumption is
appropriate for most lanthanide complexes, and recent work has
highlighted the exquisite sensitivity of the ligand field to minor
3
+
1b
3+
2
neutral Yb complex of L , and the cationic Yb complex of L .
[
8]
structural perturbation, exemplified by the dramatic changes in
NMR pseudocontact shifts and Eu emission spectral form that
occur following a change of solvent or in the degree of
[a]
Dr A. C. Harnden, Dr A. S. Batsanov, Dr M. A. Fox, Dr K. Mason,
Prof D. Parker
Department of Chemistry
Scheme 1. Molecular structures of the complexes examined in this study.
Durham University
South Road, Durham, DH1 3LE, United Kingdom
david.parker@durham.ac.uk
Dr E. A. Suturina
1b
2
The lanthanide complexes of L and L were prepared in a
similar manner to the corresponding series based on the P-Me
[b]
1
a [11]
triphosphinate ligand, L , and were purified by reverse-phase
HPLC, or crystallized by diffusion of Et O into a methanol
Department of Chemistry
University of Bath
2
2
+
Claverton Down, Bath BA2 7AY, United Kingdom
Dr M. Vonci, Prof E. J. McInnes, Dr N. F. Chilton
School of Chemistry and Photon Science Institute,
The University of Manchester
solution (see SI). The cationic complexes, [LnL ] , were isolated
as their hexafluorophosphate salts. X-ray crystal structures at
[c]
1
20 K revealed the Ln(III) ions adopting a twisted square anti-
Oxford Road, Manchester, M13 9PL, United Kingdom
prismatic (TSAP) eight-coordinate geometry with no bound
solvent. All complexes crystallized in centrosymmetric space
Supporting information for this article is given via a link at the end of
the document.
1
b
2
1 6
groups, viz. P2 /c for [YbL ]·3.5MeOH, P1 for both [YbL ][PF ]
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Angewandte Chemie International Edition
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COMMUNICATION
2
·
3MeOH and (non-isomorphous) [TbL ][PF
6
]·6MeOH. Thus, in
Furthermore, the two possible TSAP isomers (RRR)/(RR)-
Λ−(λλλλ) and (RRR)/(RR)-Δ−(δδδδ) can be distinguished by
NOE spectroscopy. Inspection of the optimized structures of
each crystal structure both enantiomers were present equally,
with the configurations (RRR)-Λ-(λλλλ) or (SSS)-Δ-(δδδδ) for
1
b
2
1
1
[
YbL ], (RR)-Λ−(λλλλ) or (SS)-Δ-(δδδδ) for [LnL ][PF
6
] defining
each TSAP isomer showed that the H- H distance between the
benzylic CH and ring ‘cyclen’ protons should be about 2.1 Å
the chirality at phosphorus, the ring helicity and the twist of the
NCCN chelate rings respectively (Figure 1). Both phosphinate
2
(strong NOE) in (RRR)/(RR)-Δ−(δδδδ) whereas this distance is
ca 2.6 Å in the (RRR)/(RR) −Λ −(λλλλ) diastereoisomer. No
NOE was observed between these resonances, supporting the
presence of TSAP (RRR)/(RR) −Λ −(λλλλ) diastereoisomers in
2
+
1b
oxygen atoms in [YbL ] and two out of three in [YbL ] accepted
hydrogen bonds from methanol molecules of crystallization. The
average distances between the lanthanide ion and the hydrogen
t
atoms of the Bu NMR reporter group were similar and were
solution. Observation of a relatively weak NOE between the
2
+
1b
t
found to be 6.74 Å in [YbL ] and 6.62 Å in [YbL ]. In the twisted
square antiprismatic geometry (TSAP), the degree of ‘twist’
benzylic PCH
2
resonances and the proximate Bu and pyH
6
protons confirmed the solution assignment in each case,
consistent with the X-ray structural analyses. The rate of
between the macrocyclic N
4
plane and the N
2 2 3
O or NO planes
1
b
1a [11]
1b
was 26.4° for [YbL ], near identical to that found for [YbL ].
exchange between the minor and major isomer of [YL ] was
2
+
-1
-1
However, with [YbL ] this twist angle was reduced to 18.5°,
reflecting the impact of the additional pyridine chelate ring
moiety on the degree of twist, (Tables S1-4).
found to be 1.27 s . The reverse rate was slower (0.12 s ), and
the equilibrium constant calculated from these rates (Keq = 10.9)
was found to be in good agreement with that calculated by
3
1
integration of the fully relaxed P NMR spectrum (10:1). The
forward and reverse rates of the exchange of the isomers were
2
-1
-1
broadly similar in the case of [YL ]Cl (k = 0.87 – 1.28 s , k
=
-
1
0
.14 – 0.18 s ). Indeed, rates of these exchange processes are
1
b
1a
similar to those found for both [YL ] and [YL ].
In the interest of using these compounds as model systems for
1
PARASHIFT probes, the H NMR chemical shift of the tert-butyl
reporter group for selected lanthanide complexes (Tb-Yb) was
measured (Figure 2). A simple interpretation of these shifts
would suggest that similar magnetic anisotropies exist, with only
2
+
a slight reduction in the magnitude in the [LnL ] series.
1
b
Figure 1. (upper) Molecular structure of (SSS) - Δ - (δδδδ) [Yb.L ] viewed
from the top and side; H atoms are omitted for clarity; (lower) views of the
2
structure of (RR) -Λ − (λλλλ) – [Yb.L ]; CCDC: 1896115-116.
Density functional theory (DFT) geometry optimizations were
undertaken for Y(III) analogues where many possible
conformers within the various stereoisomers were analysed. At
the M06-2X/6-31G(d)/Stuttgart-ECP SMD-methanol model
[
11]
1b
chemistry, the TSAP (RRR)-Λ−(λλλλ) stereoisomer for [YL ]
-
1
was lowest in energy by ca 6.4 kJ mol and the TSAP (RR)-
2
+
-1
Λ−(λλλλ) stereoisomer for [YL ] by about 6.3 kJ mol (Figure
S1, Tables S5 and S6). These lowest energy conformers are
1
b
2 +
Figure 2. Schematic representation of the experimentally measured chemical
also observed in X-ray structures of [YbL ] and [YbL ] .
1b
2
shifts of the major tert-butyl signals in [LnL ] (top) and [LnL ]Cl (bottom)
(CD OD, 11.7 T, 295 K) (yellow - Tm, green - Er, magenta - Yb, black – Ho,
1
13
31
A full NMR assignment of the H, C and P resonances of
3
1
b
2 +
red - Dy, blue - Tb).
[
3
YL ] and [YL ] in CD OD was achieved using a combination of
1
1
1
1
1
13
31
two-dimensional H- H NOESY, H- H COSY, H- C/ P HMBC,
31
To delve further into the cause of these shifts, proton and
P
1
13
H- C HSQC and Pureshift NMR techniques (SI Figures S3 and
Table S10, for example, for the assignment of the 43 protons
3+
1b
2
NMR analyses of the Yb complexes of L and L were
undertaken over a range of magnetic fields (1 to 16.5T). For
1
b
[12]
1b
and 34 carbons in [YL ]).
observed in a 10:1 ratio, (cf. 5:1 in [YL ] ), most clearly
With [YL ], two isomers were
1b
2 +
[
Yb.L ], two isomers were observed (12:1) and with [YbL ]
three species could be discerned in ratio 10:2:0.5, with the major
species possessing time-averaged C -symmetry, (Figures S11-
S14). Using semi-automated combinational assignment
procedure in Spinach, the PCS data were assigned for every
1
a [11]
3
1
defined in the P NMR spectrum where each phosphorus
8
9
2
2
nucleus resonated as an Y coupled doublet, ( J = 6 Hz). For
a
2
+
[
YL ] , only one phosphorus resonance was observed for the
major stereoisomer (70%), consistent with the presence of time-
averaged C symmetry in solution. In each case, the major
species revealed the presence of only one strong NOE signal
between the exocyclic pyridyl CH and the macrocycle CH
resonances. The occurrence of this singular NOE correlation,
between the axial proton on the C carbon of the cyclen 12-N
ring and the ‘axial’ exocyclic CH pyridyl proton has been shown
to be consistent only with a TSAP coordination geometry.
[14]
1
H
resonance of the major species, (except the benzyl
2
3+
resonances), aided by the prior Y complex assignments.
1b
2 +
Both DFT optimized and XRD structures of [YbL ] and [YbL ]
have been used in the pNMR analysis (Tables S12 and S13). By
fitting equation (1) to the traceless part of the magnetic
2
2
2
4
2
susceptibility tensor, the high quality of the fits (R > 0.997 in
2
each case with DFT structures, see SI: Figure S15, Table S13)
[
11, 13]
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Angewandte Chemie International Edition
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allowed the corresponding pseudocontact shift fields to be
constructed (Figure 3). The ‘best-fit’ susceptibility tensors are
described by their axiality and rhombicity parameters, together
with the three Euler angles, with respect to the molecular frame,
1
a
1b
(
Table 1). Comparing data for [YbL ] and [YbL ], the structural
modification from methyl to benzylphosphinate groups has a
rather small impact on the amplitude, shape and orientation of
1
b
the PCS fields. However, comparing PCS fields for [YbL ] and
2
+
[
YbL ] , dramatic changes in the PCS fields were found, (Figure
). The magnetic anisotropy has changed sign, but the negative
3
PCS lobe is still oriented in the “equatorial plane” of the
molecule, because of a 90° switch in the orientation of the main
magnetic axis. Such behaviour is consistent with a change in the
!
!
sign of the second order ligand field parameter ꢀ .
2
+
3
Figure 4. Emission spectra (295K, CH OH, λex 270mm) of [EuL ] (upper) and
1
b
2
[
Eu.L ] (lower) highlighting splitting of the Δ J= 1 manifold, where Δ = 3ꢀ0
2
and ꢀ = −2 6 ꢀ2 in the spherical operator formalism.
Changes in the sign of ꢀ !! in closely related structures have
[
17]
been observed relatively rarely,
e.g. in systems that switch
Figure 3. Pseudocontact shift fields, reconstructed using the ‘best fit’
susceptibility tensor for [YbL ] (left) and [YbL ] (right); positive PCS is shown
in red (+200 ppm), negative in blue (-200 ppm) (see Table 1).
from a TSAP to a SAP coordination geometry, where the twist
angle changes by about +10°. Here, the change in angle is
1
b
2 +
1
b
2
around -8° for [YbL ] vs [YbL ]. The sensitivity of lanthanide
[
18]
ligand fields to distortion has been noted in theoretical work.
When the position of the principal magnetic axis changes
orientation, the ordering of the M sub-levels in a lanthanide
1
a
1b
Table 1. Magnetic susceptibility tensors (SI units) of [YbL ], [YbL ] and
2
+
[
YbL ] , obtained by fitting paramagnetic NMR chemical shift data (4.7 T,
J
[
15]
CD
3
OD, 295 K) and computed with SINGLE_ANISO
using the DFT
complex will alter as well. Such a change is most readily
observed by examining very low temperature EPR spectra, from
which information on the magnetic anisotropy of the ground
optimized structures (in parentheses), expressed in terms of axiality (ꢀ!"),
rhombicity (ꢀ!!) and Euler angles. See SI for the convention used.
[
5b-e]
3
state can be ascertained.
X-band continuous wave EPR
Complex
ꢀ!" (Å )
ꢀ!!/ꢀ!"
α(°)
β(°)
γ(°)
spectra were collected at 5 K for solid crystalline and frozen
1
a
0.11
0.14)
0.13
(0.11)
185
(204)
23
(30)
211
(21)
2
1b
[
[
YbL
YbL
]
]
2
solution (95% MeOH + 5% Et O) samples of [YbL ] and [YbL ].
(
The 5 K spectra can be simulated with the effective S = ½ model,
1
b
+0.13
0.16)
0.11
168
22
171
2
3+
which is appropriate for the Kramers
the lowest energy doublet is populated at 5 K, along with
F7/2 Yb ion where only
(
(0.08)
(239)
(25)
(345)
2
+
171
173
[
YbL ]
-0.09
0.10
70
90
269
(84)
hyperfine coupling to the Yb and Yb nuclei (I = 1/2 and 5/2,
(
-0.09)
(0.14)
(241)
(89)
respectively, with natural abundances of 14.2% and 16.1%).
[
19]
Simulations, using Easyspin, give “easy-axis-like” (5.25, 2.63,
!
Information on the sign and size of ꢀ is readily accessible by
2.08) and “easy-plane-like” (4.22, 3.60, 2.38) effective g-tensors
!
3
+
1b
2
analysis of the emission spectrum of the corresponding Eu
for [YbL ] and [YbL ], respectively (Figures 5 and S16; Tables
S16 and S17). The pairs of spectra in the solid state and
solution phases are very similar, with only very minor changes in
[
16]
!
!
complexes in methanol, Figure 4.
The values of ꢀ and ꢀ for
! !
2
+
-1
[
EuL ] were found to be +923 and -153 cm , contrasting with
-
1
1b
-1
values of -703 and -167 cm for [EuL ] and -596 and -228 cm
geff, strongly suggesting that no solvent coordination occurs. Ab
1
a
2 +
!
for [EuL ]. In water, the corresponding values were: [EuL ] , ꢀ .
initio CASSCF-SO calculations using both the XRD structures
!
-
1
!
!
-1
1b
!
!
-1
!
!
=
+735 cm , ꢀ = -220 cm ; [EuL ], ꢀ . = -650 cm , ꢀ =- 80
(including the two crystallographically independent molecules of
-
1
-1
1a [5a]
1b
cm and -660 and -122 cm for [EuL ],
;
ligand field
[YbL ]) and the DFT-optimized structures (see SI) have been
parameters are given here in the spherical tensor formalism.
performed to elucidate the electronic structure and magnetic
3
+
!
The confirmation of the change in sign of ꢀ for the complexes
anisotropy of the Yb complexes. The CASSCF-SO calculated
effective g-values for the ground Kramers doublets (Table S16)
agree very well with those obtained by EPR, confirming the
same “easy-axis-like” vs “easy-plane-like” anisotropies. The
calculations show that the main magnetic axis that corresponds
!
1
a
2
of L and L is supportive of the hypothesis that for these three
complexes, the cooperative effect of the change in size and
orientation of the major component of the magnetic susceptibility
tensor and the inversion of the sign of the ligand field conspire to
mask the complexity of changes in pseudocontact shift
behaviour.
2
to the smallest g-value in [YbL ] has a large tilt angle of ꢀ=74
1
b
degrees (Figure 5). In [YbL ] the orientation of the largest
effective g-value is tilted only by ꢀ = 35 degrees (Figure 5).
These results correlate with the differences in the orientation of
the high temperature magnetic susceptibility tensor (angle β in
Table 1) and the differences in luminescence spectra (Figure 4).ꢀ
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Angewandte Chemie International Edition
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COMMUNICATION
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b
2
Figure 5 CW X-band EPR spectra of [YbL ] (upper) and [YbL ] (lower) as a
frozen solution (95% MeOH, 5% Et O) at 5K. Experimental spectra are in
2
black and simulations are in red, each inset shows the orientation of the main
magnetic axis calculated by CASSCF-SO, based on the DFT optimized
structures (Yb – green, N – blue, O - red). Calculated and measured g-values
are in Table S15, simulation parameters in Table S16. Signals marked with an
asterisk (*) are spurious weak signals in the resonator and not intrinsic to the
sample.
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In conclusion, it has always been assumed – partly as a
consequence of dogged adherence to Bleaney theory – that the
sign of the ligand field parameter ꢀ !! governs the sense of
observed NMR pseudocontact shifts and its size determines
their magnitude. Here, we show for the first time that this
hypothesis may not hold. The crystal field splitting and the size,
sign and orientation of the major component of the magnetic
2
019, 150, 144101; g) L. Benda, J. Mareš, E. Ravera, G. Parigi, C.
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Parker, I. Kuprov, Phys. Chem. Chem. Phys. 2018, 20, 17676-17686;
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susceptibility tensor need to be considered when interpreting
[
[
7]
8]
B. Bleaney, J. Magn. Reson. (1969) 1972, 8, 91-100.
!
experimental pseudo-contact shift data. Here, ꢀ is positive for
!
a) M. Vonci, K. Mason, E. A. Suturina, A. T. Frawley, S. G. Worswick, I.
Kuprov, D. Parker, E. J. L. McInnes, N. F. Chilton, J. Am. Chem. Soc.
2017, 139, 14166-14172; b) M. Vonci, K. Mason, E. R. Neil, D. S. Yufit,
E. J. L. McInnes, D. Parker, N. F. Chilton, Inorg. Chem. 2019.
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W. Patrick, A. W. J. Poh, A. S. Batsanov, E. A. Suturina, M. Vonci, E. J.
L. McInnes, N. F. Chilton, D. Parker, Chem. Commun. 2018, 54, 8486-
3
+
2
Ln complexes of L while it is negative and of smaller
1
b
magnitude for complexes of L , and yet, counter-intuitively, the
pseudocontact shifts are both in the same direction and are
1
b
largest for complexes of L . Evidently, this study suggests a
need to exercise caution in the use of PCS data for structural
analyses, and indicates the need to understand better the link
[9]
3
+
J
between the ordering of the M sub-levels for a given Ln ion,
8
489.
their relative Boltzmann population and the overall magnetic
susceptibility size and anisotropy at a given temperature. Such
an understanding has parallels in the quest for high-temperature,
single molecule magnets, where the order and energy
[
[
10] H. M. McConnell, J. Chem. Phys. 1957, 27, 226-229.
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separation of the various M levels is of paramount importance.
Acknowledgements
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13] L. D. Bari, G. Pintacuda, P. Salvadori, R. S. Dickins, D. Parker, J. Am.
Chem. Soc. 2000, 122, 9257-9264.
We thank EPSRC for support (EP/N006909/1; L01212X;
N006895/1) including funding of the National EPR Facility, Bath
University for a Fellowship, and Dr. D.S. Yufit for assistance with
X-ray structural determinations. This paper is dedicated to the
one who is always there.
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Keywords: lanthanide • coordination • EPR • NMR • emission
[
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201906031
COMMUNICATION
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201906031
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
In two closely related series of
lanthanide complexes, a
switch in the sign of the
dominant ligand field
parameter and striking
variations in the sign,
Alice C. Harnden, Elizaveta A.
Suturina, Andrei S. Batsanov,
Mark A. Fox, Kevin Mason,
Michele Vonci, Eric J. L.
McInnes, Nicholas F Chilton and
David Parker*
amplitude and orientation of
the main component of the
magnetic susceptibility tensor
as the Ln ion is permuted
mask modest changes in
NMR paramagnetic shifts, but
are evident in Yb EPR and Eu
emission spectra.
Page No. – Page No.
Unravelling the Complexities
of Pseudocontact Shift
Analysis in Lanthanide
Coordination Complexes of
Differing Symmetry
3+
This article is protected by copyright. All rights reserved.