Electron density distribution studies as a tool to explore the behaviour of thiourea-based anion receptors

Article abstract of DOI:10.1039/c5ce00213c

Building on previous studies of anion-receptor complexes based on a urea scaffold substituted symmetrically with electron-withdrawing nitro groups, the electron density distribution in an analogous thiourea receptor complex and the related asymmetrically

Full text of DOI:10.1039/c5ce00213c

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ARTICLE TYPE
Electron density distribution studies as a tool to explore the behaviour of
thiourea-based anion receptors
Isabelle L. Kirby, a Mateusz B. Pitak,a Claire Wilson b, Philip A. Gale aand Simon J. Coles* a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Building on previous studies of anionꢀreceptor complexes based on a urea scaffold substituted
symmetrically with electronꢀwithdrawing nitro groups, the electron density distribution in an analogous
thiourea receptor complex and the related asymmetrically substituted urea and thiourea receptors are
described. On this basis it is possible to probe both the effect of changing the receptor core from a urea to
10 a thiourea moiety and that of asymmetrical substitution of the receptor molecule. These modifications are
shown to significantly alter the anion binding properties, solidꢀstate packing and electron density
distribution in the anionꢀreceptor complexes.
increase in hydrogen bond strength using this method.8 The
family of anionꢀreceptors studied has been extended to include
thiourea molecules and comparisons made between the electron
50 density distribution in anionꢀreceptor complexes of urea and
analogous thiourea receptors. In the previous study, the phenyl
rings of the urea receptor molecules were symmetrically
functionalised with nitro functional groups. These electronꢀ
withdrawing groups were included to increase the acidity of the
55 N−H bonds, which act as hydrogen bond donors and associate
with anions. The effect of altering the position of the electronꢀ
withdrawing group was investigated. However, there is also
interest in investigating the effect that asymmetric substitution of
the phenyl rings has on both the anion binding strength of the
60 receptors and on the charge density distribution across the
receptor molecule. These relatively simple anionꢀreceptors are
used as models to understand the more complex systems found in
anionꢀreceptor chemistry and to explain the effect of structural
modifications.
65 For this reason, receptor 1 (1ꢀ(4ꢀnitrophenyl)ꢀ3ꢀphenylurea) and 2
(1ꢀ(4ꢀnitrophenyl)ꢀ3ꢀphenylthiourea) shown in Figure 1 were
synthesised. The crystal structures of the chloride complexes of
both receptors were obtained, in addition to those of the receptor
molecules, and the electron density distribution in these
70 complexes modelled using the HansenꢀCoppens9 formalism (4)
and Invariom10 method (3). The chloride complex of the
symmetrical receptor,1,3ꢀbis(4ꢀnitrophenyl)thiourea 8, is also
included and comparison made with previously described8 ureaꢀ
based chloride complex 7.
Introduction
Anion receptor chemistry has a broad range of applications
15 including catalysis, sensing and transport.1 Ureas and thioureas
are frequently used because of their strong hydrogen bond donor
ability and easy synthetic modification.2 While the structural
properties of ureas in the solidꢀstate have been widely studied,
including their coꢀcrystal structures with hydrogen bond acceptor
20 molecules3, 4, the properties of analogous thioureas have been
much less explored5, 6. There are a number of differences between
ureas and thioureas that could be expected to alter the solidꢀstate
structures of their analogous compounds. Thiourea N−H bonds
are more acidic than those in ureas and hence thioureas are
25 stronger hydrogen bond donors. However sulfur is a less effective
hydrogen bond acceptor than oxygen. In the crystal structures of
urea containing compounds the most prominent motif is that of
the linear planar αꢀtape, while in analogous thioureas zigꢀzag
chains or dimer motifs are commonly observed. This is due to the
30 transꢀtrans conformation adopted by N,N’ꢀdisubstituted ureas
whereas the analogous thioureas exist in solution as a mixture of
three rotamers, the transꢀtrans, the transꢀcis and the cisꢀcis.
Custelcean et al., have shown that by varying the steric bulk of
the substituent in N, N’ꢀdisubstituted thioureas one can control
35 the hydrogen bonding motif present in the crystal structure.6
Recently we have studied the anion binding ability of a series of
N,N’ꢀdiphenyl substituted symmetrical urea receptors in both
solution and the solidꢀstate.7, 8 This systematic study investigates
the effect of altering the substituent position on the electron
40 density distribution across the anionꢀreceptor complexes, the
crystal packing arrangements and anion binding ability of the
receptors and attempts to correlate these properties. The strength
of the anion binding in the solidꢀstate was derived using the
methods associated with the Quantum Theory of Atoms of
45 Molecules (QTAIM). Increasing the basicity, by altering the
anion, was shown for the first time in the solidꢀstate to cause an
75 Here, a comparison between the crystal packing arrangements in
the receptor only and the complexed structures is discussed, along
with the anion binding abilities of the receptors. The electron
density distributions in the urea structures (3 and 7) are compared
to the thiourea structures (4 and 8) and symmetrical substitution
80 contrasted to asymmetric substitution (7 and 8 vs. 3 and 4).
The choice of counterion in these four anionꢀreceptor complexes
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is either the tetramethylammonium (TMA) cation found in 4 and
7 or the tetraethylammonium cation (TEA) present in 3 and 8 and
is based on solubility and crystallisation considerations.
5
Figure 1. Anionꢀreceptor molecules (1, 2, 5 and 6) and the chloride complexes of these receptors (3, 4, 7 and 8).
nitroaniline (0.20 g, 1.45 x10ꢀ3 moles, 1.5 eq) in pyridine (5 mL)
Experimental
was added. The clear orange solution was stirred overnight under
a nitrogen environment. The solvent was removed in vacuo and
45 the resulting solid dissolved in ethyl acetate and washed with 1M
HCl and the organic and aqueous layers separated. The organic
layer was washed with saturated sodium bicarbonate followed by
brine. The organic layer was then dried over MgSO4. After
filtration, the solvent was removed in vacuo and the solid purified
50 by recrystallisation from ethyl acetate/hexane. This resulted in a
yellow solid (0.096g, 3.02 x10ꢀ4 moles, 30%). MP: 193ꢀ196°C
(consistent with literature value of 195ꢀ196°C).15 1H NMR (300
MHz, d6ꢀDMSO, δ = ppm): 7.85 (d, J=8.8 Hz, 4H, CH), 8.24 (d,
J=8.8 Hz, 4H, CH), 10.77 (br. s, 2H, NH) (consistent with
55 literature values)16.
Synthesis
The synthesis of compound 5 has been previously reported7.
1ꢀ(4ꢀnitrophenyl)ꢀ3ꢀphenylurea (1):
10 The compound was synthesized according to the adapted
procedure of Miyahara.11 4ꢀ nitrophenylisocyanate (0.5 g, 3.05 x
10ꢀ3 moles, 1 eq) was dissolved in toluene (70 mL). To this was
added aniline (0.25 mL, 3.05 x10ꢀ3 moles, 1 eq). A white
precipitate formed. This was stirred overnight at room
15 temperature under a nitrogen atmosphere. The solid was filtered
and dried under vacuum (white solid, 0.73 g, 2.84 x10ꢀ3 moles, 94
%). MP: 212ꢀ214°C (consistent with literature value of 210ꢀ
211°C)12. 1H NMR (300 MHz, d6ꢀDMSO): 7.02 ppm (t, 0.75 Hz,
1H), 7.31ppm (t, 7.91 Hz, 2H), 7.47 ppm (d, 7.54 Hz, 2H), 7.69
20 (d, 9.42 Hz, 2H), 8.19 ppm (d, 9.04 Hz, 2H), 8.90 ppm (s, 1H),
9.42 (s, 1H) (consistent with literature reference)12.
Crystallisations
Compound 1 was suspended in methanol. TMA chloride was
added in methanol and led to full dissolution of the solution.
Crystals of receptor 1 then grew upon slow evaporation of the
60 solution. Compound 2 was dissolved in methanol. Vapour
diffusion of diethyl ether into the solution resulted in crystals.
Crystals of 3 were grown by the vapour diffusion of diethyl ether
in an acetonitrile solution of TEA chloride and 1ꢀ(4ꢀnitrophenyl)ꢀ
3ꢀphenylurea. Crystals of 4 were grown by vapour diffusion of
65 diethyl ether into a mixed methanol and ethanol solution of TMA
chloride and 1ꢀ(4ꢀnitrophenyl)ꢀ3ꢀphenylthiourea. 5 was dissolved
in acetonitrile and methanol. Crystals formed upon slow
evaporation of the solvent mixture. Crystals of 6 were grown by
dissolving 6 in acetonitrile and acetone with gently heating. The
70 undissolved solid was filtered and the filtrate was left for the
solvent to slowly evaporate. Crystals of 7 were grown as
described previously7 by vapour diffusion of diethyl ether into a
solution of TMA chloride and 1,3ꢀbis(4ꢀnitrophenyl)urea.
Crystals of 8 were grown by slow evaporation of an acetonitrile
1ꢀ(4ꢀnitrophenyl)ꢀ3ꢀphenylthiourea (2):
The compound was synthesized according to the adapted
procedure of Perveen et al.13 Aniline (0.23 mL, 2.78 x 10ꢀ3 moles,
25
1 eq) was dissolved in dichloromethane (35 mL). 4ꢀ
nitrophenylisothiocyanate (0.5 g, 2.78 x 10ꢀ3 moles) dissolved in
dichloromethane (35 mL) was added. The yellow solution was
left to stir overnight at room temperature under a nitrogen
atmosphere. The solvent was removed in vacuo, and a yellow
30 solid formed. This was recrystallised from ethanol. The solid was
filtered and washed and yielded a yellow solid (0.36 g, 1.32 x10ꢀ3
moles, 48 %). MP: 150ꢀ152°C (consistent with literature value of
145ꢀ146 °C).14 1H NMR (300 MHz, d6ꢀDMSO): 7.17 ppm (t, 7.54
Hz, 1H), 7.37 ppm (t, 7.54 Hz, 2H), 7.49 ppm (d, 7.54 Hz, 2H),
35 7.84 ppm (d, 9.2 Hz, 2H), 8.20 ppm (d, 9.04 Hz, 2H), 10.26 ppm
(br. s., 1H), 10.36 ppm (br. s., 1H) (consistent with literature
reference)14.
1,3ꢀbis(4ꢀnitrophenyl)thiourea (6):
The compound was synthesized according to the procedure of 75 solution of TEA chloride and 1,3ꢀbis(4ꢀnitrophenyl)thiourea.
40 Perveen et al.,13 4ꢀnitrophenylisothiocyanate (0.18 g, 1.00 x10ꢀ3
moles, 1.0 eq) was dissolved in pyridine (5 mL) and 4ꢀ
Discussion of the crystal structures of the receptors 1, 2, 5 and 6
.
is given in Section 5 of the ESI†.
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Table 1. Crystallographic details of structures anionꢀreceptor complexes
studied at high resolution (for 7 reported elsewhere8)
X-ray data collection
Structure
3
4
8
Xꢀray data for 1, 2, 4, 5, 6 and 8 were collected using methods
described previously on a Rigaku AFC 12 diffractometer,
mounted on a Rigaku FRꢀE+ Super Bright Very High Flux
rotating anode, equipped with a Saturn 724+ CCD detector.17 For
4 and 8 high quality high resolution data were collected according
to a strategy using two detector settings of 2θ which gave overall
redundancy of 5.8 and data completeness to sinθ/λ = 1.1 Åꢀ1 of
10 97.0% for 4; overall redundancy of 5.7 and data completeness to
sinθ/λ of 1.05 Åꢀ1 of 97.7% for 8. These resolution of data were
chosen as the cut off according to the rules of Herbstꢀ Irmer.18 Xꢀ
ray data for 3 was collected on the I19 beamline at the Diamond
Light Source19 using a Crystal Logic 4ꢀkappa diffractometer
15 equipped with a Rigaku Saturn 724+ CCD detector. Following
integration in CrystalClear20, all datasets were scaled and data
merged using SORTAV21.
Despite using synchrotron radiation, it was not possible to obtain
high resolution diffraction data for 3 of sufficient quality to allow
20 a full multipole model refinement. This dataset was subsequently
subjected to an Invariom refinement using the block matrix
technique outlined by Dittrich and coꢀworkers (see below)10.
Information for the data collection of 7 is given in the literature.8
The crystal structures were initially solved by direct methods and
25 refined using the independent atom model IAM (refinement
based on F2) using SHELXL22 in the WinGX suite23.
Crystallographic details of 3, 4, and 8 can be found in Table 1 as
this paper deals primarily with the high resolution crystal
structures. The full crystallographic details for the remaining
30 standard resolution crystal structures (1, 2, 5 and 6) are found
Table S5 of the ESI†.
Invariom High resolution High resolution
refinement
Independent Atom Model refinement
5
Molecular formula C21H31ClN4O3 C30H34ClN7O4S2 C34H40ClN9O8S2
Weight (g molꢀ1)
System
422.95
Monoclinic
656.21
Triclinic
P ꢀ1
802.32
Triclinic
P ꢀ1
8.968 (2)
15.649 (4)
15.899 (4)
113.061 (1)
103.685 (3)
101.701 (2)
1882.6 (8)
2
1.415
100 (2)
0.71073
0.276
840
Yellow
Prism
0.31 x 0.27 x
0.12
97.7
0.0476
ꢀ21 ≤ h ≤ 20
ꢀ38 ≤ k ≤ 34
ꢀ0 ≤ l ≤ 38
Space group
P 21/n
a (Å)
b (Å)
c (Å)
α (°)
β (°)
8.410 (6)
24.362 (18)
11.143 (8)
90
106.286 (7)
90
9.159 (1)
10.244 (2)
17.871 (3)
87.983 (6)
79.043 (5)
72.220 (2)
1566.9 (4)
2
γ (°)
V (Å)
Z
2191 (3)
4
D (g cmꢀ3)
Temp (K)
λ
1.282
1.391
100 (2)
0.68890
0.185
100 (2)
0.71073
0.303
ꢁ (mmꢀ1)
F(000)
Crystal
904
688
Colourless
Tablet
Yellow
Plate
Crystal size
(mm3)
0.07 x 0.04 x
0.02
0.80 x 0.20 x
0.10
Data completeness
Rmerge
99.9
0.0832
96.4
0.0407
Index ranges
ꢀ13 ≤ h ≤ 13
0 ≤ k ≤ 39
0 ≤ l ≤ 17
ꢀ22 ≤ h ≤ 22
ꢀ25 ≤ k ≤ 25
0 ≤ l ≤ 44
Tmin/Tmax
0.9859/ 0.9959 0.7935/ 0.9703 0.9194/ 0.9677
Final R indices [F2 >
R1 = 0.0572
wR2 = 0.1430 wR2 = 0.1456 wR2 = 0.1369
R1 = 0.0636 R1 = 0.0585 R1 = 0.0603
R1 = 0.0531
R1 = 0.0561
2σ(F2)]
R indices (all data)
wR2 = 0.1528 wR2 = 0.1506 wR2 = 0.1406
Data/ restraints/
parameters
9511/ 0/ 262 49621/ 0/ 397 55445/ 0/ 487
ꢀρ(r) (e Åꢀ3)
0.704/ ꢀ0.366 0.941/ ꢀ1.009
Aspherical Atom Model refinement
1.428/ ꢀ1.286
R(F)
R(F2)
GoF
0.0450
0.0927
1.8307
0.77
46.22
0.435/
ꢀ0.300
0.0348
0.0378
1.7461
1.10
27.89
0.650/
ꢀ0.557
0.0396
0.0544
2.5564
1.05
24.43
0.674/
ꢀ0.729
(sinθ/λ)max
Nref/Nvar
ꢀρ(r) (eÅꢀ3)
lengths for the appropriate functional group as determined from
neutron diffraction studies26. The anisotropic displacement
50 parameters for each hydrogen atom in the crystal structure were
then determined using the SHADE27 (Simple Hydrogen
Anisotropic Displacement Estimator) server. These were
imported into the multipole model and fixed throughout the
subsequent refinement steps. Next, multipole populations were
55 introduced, with the level of multipole gradually increased
from monopole up to the final level: hexadecapole for
heteroatoms while those of the carbon atoms were truncated at
the octopole level. For hydrogen atoms a single bond directed
dipole population was refined. For nonꢀhydrogen atoms
60 excluding sulfur an expansion (κ) parameter was refined while
κ’ was fixed as 1.00. Chemically equivalent atoms were
constrained to share the same expansion/ contraction
Multipole modelling of thiourea complexes 4 and 8
35 The IAM for 4 and 8 were transferred to the XD2006 suite24 for
full multipole refinement using the HansenꢀCoppens
formalism9. The core and valence scattering factors for the
atoms were derived from the ClementiꢀRoetti wave functions.25
The refinement was performed on F for all reflections with I >
40 3σ(I). Initially, only the scale factor was refined against the
whole resolution range of diffraction data. Subsequently the
positional and anisotropic displacement parameters of the nonꢀ
hydrogen atoms were refined against the reflections with
sin(θ)/λ > 0.7 Åꢀ1. The positions of the hydrogen atoms and
45 their isotropic displacement parameters were then refined
against the reflections with sin(θ)/λ < 0.7 Åꢀ1. The X−H
distances were then extended along the bond vector to standard
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parameters. For hydrogen the kappa values were fixed to values
of κ = κ’ = 1.20. A local coordinate axis system similar to that
previously reported was used.8 The correct modelling of sulfur
has been known for some time to be problematic and has been
discussed in detail in the charge density community9,28,29,30 and
therefore optimal radialꢀfunction parameters may not be the
defaults of the XD2006 programme suite.9 Following the work
of Espinosa et al.,28 and Dominiak and Coppens29 a series of
n(l) values were tested, however n(1, 2, 3, 4) = 4 4 4 4 gave the
60 were refined against all the reflections and subsequently the
positions and isotropic thermal displacement parameters for the
hydrogen atoms were refined against the low angle data. The
scale factor was then refined. This was followed by the
refinement of the multipole parameters, with the site symmetry
65 and chemical equivalence constraints maintained, while the
kappa parameters were kept fixed to those values obtained
from the Invariom database. The quality of the final refinement
is illustrated by the values of the R(F) and GoF given in Table
1.
5
10 best model for the electron density at sulfur and so was retained
in the refinement. Despite this, the residual density at the sulfur
atoms was shown to still be quite significant and anharmonic
motion modelling using Gram Charlier coefficients was
attempted. However this did not improve, and in some cases
15 increased, the residual density. Introduction of a κ’ parameter
did significantly lessen the residual density and was
incorporated into the model. The residual density was shown to
be a related to the resolution of the data, as revealed by
truncation at different levels (around 0.3/ ꢀ0.3 at 0.8 Åꢀ1 and
70
Quantitative analysis of the static electron density model of 3,
4, 7 and 8 was performed with the XDPROP module of the
XD2006 software suite.24
75 Theoretical Quantum Mechanics Studies
To test the validity of the Invariom modelling of 3, DFT
computational studies were performed on all four anion
receptorꢀcomplexes (3, 4, 7 and 8) using the Guassian98
80 software package37. The B3LYP38,39 /6ꢀ311++G**40 basis set
with diffuse functions was used to correctly model the
hydrogen atoms and hydrogen bonding in this system. The
resulting electron density distribution was analysed using the
AIM2000 suite41. Critical point analysis for both the
85 computationally and experimentally derived electron density
distribution can be found in the ESI†. The properties of the
electron density at the bond critical points from these
theoretical studies match fairly well with those of the
experimental electron density distribution, except for the C=S/
90 C=O bonds (see Birkedal et al.42 for explanation) and N—O
bonds of the nitro groups. This gives a good degree of
confidence in the use of the Invariom values used.
20 0.6/ ꢀ0.6 at 1.1 Åꢀ1 for crystal structure 4). The
κ’ value was
constrained as unconstrained multipole refinement led to
physically unreasonable values. In the final stages of the
refinement all parameters were refined together with the
exception of the κ and κ’ values, which were fixed. This led to
25 full convergence of the multipole model. The Xꢀray data used
in the refinement was truncated to an appropriate sin(θ)/λ limit
as outlined by HerbstꢀIrmer et al.19 As previously discussed,8
unconstrained refinement of the nitro groups produced a
chemically unreasonable range of ∇2ρ(rBCP) values and hence
30 mm2 symmetry constraints for the nitrogen atoms of these
groups were imposed and the two oxygen atoms in each nitro
group were constrained to be chemically identical. The quality
of the final model is indicated by the low R(F) and GoF value.
The Gaussian distribution of the residual electron density (see
35 fractal dimension distribution plots31 in the ESI†) suggests that
the residual density is noise and that the electron density has
been successfully fitted. The high data: parameter ratios
demonstrate that sufficient diffraction data has been collected
and that overfitting of the model has been avoided.
Results and Discussion
Solution state studies
95 Hydrogen bonding
The strength of the association of receptors 1, 2, 5 and 6 for
chloride was tested in solution in a 0.5% H2O d6ꢀDMSO
solvent mixture. To satisfy solubility requirements the chloride
was added as the tetrabutylammonium salt. The affinity of the
100 thiourea receptors was lower than for the urea receptors and
stronger affinity was observed for the symmetrically
substituted receptors (5 > 1 and 6 > 2). It suggests the number
of electronꢀwithdrawing substituents significantly impact on
the anion affinity of this receptor.
105 For each receptor the data fitted well to a 1:1 model (see fit
plots in Section 4 of the ESI†). The Ka shown in Table 2 for
receptors 1 and 2 were calculated by following the chemical
shift of the urea N−H hydrogen atom attached to the nitroꢀ
substituted phenyl ring. However, these values are consistent
110 with those for the urea N−H hydrogen atom attached to the
unsubstituted ring.
40
Invariom Refinement of crystal structure 3
Due to the rapid fallꢀoff of diffraction intensity at high
resolution for 3, despite the use of synchrotron radiation, an Xꢀ
45 ray diffraction data set of the required quality for
a
conventional aspherical atom refinement could not be
completed. However, several authors have reported the use of
databases of transferable multipole parameters to assist in
modelling the electron density distribution where data cannot
50 be obtained of sufficient quality.32ꢀ34 The approach of Dittrich
and coꢀworkers was used in this study.35 The model derived
from the independent atom model was transferred to the
XD2006 suite24. The preꢀprocessor programme InvariomTool36
was used to assign the multipole parameters and kappa values
55 for each atom which were incorporated into the model.
InvariomTool36 was used to provide the coordinate axes, along
with chemical equivalences and local site symmetries. A block
refinement was then conducted: positional and anisotropic
thermal displacement parameters for the nonꢀhydrogen atoms
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state thiourea complexes, Job plots (see ESI†) were constructed
to test the ratio of components in the complexes and how they
were interacting in solution. These suggested the presence of a
2:1 receptor: anion complex in the equilibrium mixture of the
thiourea receptors. Crystal packing effects may account for the
Table 2. Affinity constants (Ka) of the receptors with TBACl in a 0.5%
H2O d6ꢀDMSO solvent mixture
5
Receptor
TBACl
58 (9%)
32 (4%)
118 (2%)
50 (1%)
1
2
5
6
10 fact that this species is observed in the Xꢀray diffraction
studies. However, no fit could be made to a 2: 1 receptor: anion
ratio when modelling the titration data.
Due to the observation of a 2:1 receptor:anion ratio in the solidꢀ
3
4
7
8
Figure 2. Crystal structures 3, 4, 7 and 8 drawn as capped sticks with chloride anion as ball and stick. The cations have been omitted for clarity.
15 Description of atomic resolution structures
asymmetrically substituted receptors (1 and 2, see Figure 3) a
chelation of the acceptor part of the (thio)urea by the N−H
donor group is observed. However, in the urea structure (1) this
40 is a strong αꢀtape motif, with the D…A distance for each
hydrogen bond less than the combined van der Waals radii of
the donor and acceptor atoms (2.847(4) & 2.883(3) Å). In 2
(the thiourea receptor alone) the D…A distance is longer than
the combined Van der Waals radii (3.514(2) & 3.461(2) Å) and
45 this accounts for the zigꢀzag conformation observed.
Crystal structures 3 and 7 are both 1:1 anion:receptor
complexes, while in 4 and 8 the overall anion: receptor ratio is
1:2 as shown in Figure 2. The packing in 3 arises from
N2O…TEA, TEA…C=O, Cl…TEA and N2O…CH(phenyl)
20 contacts. Similar interactions are observed in 7, with TMA
replacing TEA. The thioureaꢀbased receptors display
differences in the crystal structure packing compared to the
urea structures. In 4 the chloride anion is complexed by two
ligands and therefore not involved in any other interactions.
25 There are S…NO2 and S…TMA contacts and Tꢀshaped type πꢀπ
interactions between the phenyl rings. In 8 S…TEA, TEA…NO2
contacts are accompanied by Cl…TEA contacts. It is interesting
to note that the sulfur atoms and each receptor molecule in both
thiourea complexes (4 and 8) are crystallographically
30 independent and distinct. As such it would be expected that the
electron density distribution will differ between these receptor
molecules.
Figure 3. Tape formation in the asymmetric receptor crystal
structures 1 (left) and 2 (right).
Hydrogen bonding
35 The hydrogen bonding in these systems has been carefully
analysed (see Table 3). In the receptor structures of the
50 It is interesting to note in the crystal structures of the
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Hirshfeld surface analysis
symmetrical
1,3ꢀbis(4ꢀnitrophenyl)urea
and
1,3ꢀbis(4ꢀ
The nature of the packing and the relative contributions of the
25 intermolecular interactions in each crystal were further
investigated by Hirshfeld surface analysis.44, 45 The Hirshfeld
surfaces and fingerprint plots of the asymmetric crystal
structures (3 and 4) are shown in Figure 4 while Figure 5
demonstrates the contribution of each interaction type to the
30 Hirshfeld surface. These show that, in each case, upon anion
binding the packing efficiency in the crystal structure
decreases, as the fingerprint plots of 3 and 4 (urea and thiourea
chloride complexes) are more diffuse at higher distances than
in 1 and 2. The shape of the fingerprints plots for 2 and 4
35 (thiourea receptor molecule and thiourea complex) are
relatively similar, while for the urea receptor the change in the
fingerprint plot upon anion binding is fairly dramatic with the
loss of the O…H tips, which correspond to the nitro…NH(urea)
contacts. From the plots of the relative contributions of each
40 interaction to the Hirshfeld surface, it is also observed that the
O…H interactions’ contribution markedly decreases upon anion
binding in both the urea (29.5% in 1 compared to 15.8% in 3)
and thiourea (18.8% in 3 in comparison to 11.8% in 4)
structures. This is accompanied by an increase in the
45 contribution of the H…H contacts (28.8% increases to 51.9% in
3 and 30% in 2 to 36.9% in 4). In the complexes the Hirshfeld
surfaces are mainly comprised of H…H and C…H interactions,
with smaller but valuable contributions from H…Cl and O…H
contacts. This behaviour is mirrored in the crystal structures of
50 the receptors (5 and 6) and anionꢀreceptor complexes (7 and 8)
of the symmetrical receptors (see ESI† figures S31, S32, S33
and S34 for the plots of these receptors).
nitrophenyl)thiourea receptors (5 and 6) that this tape motif is
replaced by N2O…NH(urea) interactions (this is graphically
displayed in the ESI† in figures S28, S29 and S30).43 This is
attributed to the effect of the symmetrical substitution of the
receptor.
In all four structures the chloride anion is chelated by hydrogen
bonds from the (thio)urea N—H hydrogen bond donor groups.
As described previously7 two hydrogen bonds are present in the
10 urea structures (3 and 7) and four hydrogen bonds in the
thiourea complexes (4 and 8). The asymmetric nature of the
receptor may have an effect on the hydrogen bond donor motif,
with the distances of the hydrogen bonds involving the N—Hs
attached to the nitro substituted phenyl ring shorter than those
15 involving the N—H bonds attached to the unsubstituted phenyl
ring in both 3 and 4. However, there is also variation in the
hydrogen bond distances in the symmetrically substituted
receptors (7 and 8) and therefore geometric arguments are not
enough to definitively state the full effect of asymmetric
20 substitution on hydrogen bond strength.
5
Table 3. Hydrogen bonding distances for 3, 4, 7 and 8
Crystal
Structure
3
H…A (Å)
D…A (Å)
∠ DHA (°)
2.37
2.41
2.31
2.43
2.33
2.39
2.37
2.33
2.50
2.42
2.28
2.38
3.223 (2)
3.246 (2)
3.178 (1)
3.273 (1)
3.168 (1)
3.246 (1)
3.206 (1)
3.160 (1)
3.344 (1)
3.276 (2)
3.149 (1)
3.223 (1)
161.8
157.9
169.6
160.7
158.9
165.1
158.1
158.0
162.2
165.5
167.7
159.5
4
7
8
C…H,
C…H,
H…
H
H…
H
O…
H
O…H H…Cl
H…
H
S…
H
C…H,
H…
S…
H
H
O…
H
H…Cl
H…
H
C…
H
H…
H
O…
H
55
Figure 4. Hirshfeld surface maps and fingerprint plots of receptors (1 and 2) and anionꢀreceptor complexes (3 and 4)
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Figure 5. Contribution of nonꢀcovalent interactions to the Hirshfeld surface in the ureaꢀbased structures (1 and 3) and thioureaꢀbased structures (2 and 4)
Electron density distribution
contrasted. For each structure full topological analysis of the
10 bonding was performed and Bader charges for all the atoms in
the structures calculated. (See ESI† Section 7 for tables of the
atomic charges and the properties of the electron density at the
BCPs for the covalent bonds in each structure.)
5
In order to further investigate the effects of the altered
substitution pattern and the urea versus thiourea comparison in
a quantitative manner the experimental electron density
distributions in all four crystal structures (3, 4, 7 and 8) were
15 Figure 6. Static deformation density plot of 3 in the plane of the urea molecule (left), ꢀ∇2ρ(r) map of 3 in the plane of the urea molecule (middle), and 4
(right). Positive density shown in red, negative density in blue. Zero contours are dashed. Contours are at 0.1 e Åꢀ3 for static deformation density plot and
in a logarithmic scale, e Åꢀ5 in the ꢀ∇2ρ(r) maps...
40 studies.8
As in the earlier study8, the carbon atoms of the phenyl rings to
General structural features of anionꢀreceptor complexes
20 Common features of each anionꢀreceptor complex were
expected to be similar for 3, 4, 7 and 8 and fit with the values
described in previous work on bisꢀsubstituted receptors (which
includes 8)8. Chemical intuition would lead one to predict
differences between the urea (3 and 7) and thiourea (4 and 8)
25 receptors and symmetric (7 and 8) and asymmetric (3 and 4)
receptors (vide infra). As expected, the tetraalkylammonium
cation in both asymmetric structures is positively charged (in 3
TEA has 0.268 e charge and in 4 TMA is 0.376 e) and
positively charged in 7 (0.367 e) and carries a slight negative
30 charge (ꢀ0.050 e) in 8. The chloride anion is negatively charged
in all four structures (ꢀ0.455 e in 3, ꢀ0.066 e in 4, ꢀ0.296 e in 7
and ꢀ0.163 e in 8). The electronic properties of the bonds in key
structural areas of the anion receptor complexes, the phenyl
rings and (thio)urea moieties were assessed. The average
35 values of the electron density (ρ(rBCP)) and the Laplacian of the
electron density (∇2ρ(rBCP)) at the BCPs (bond critical points)
of the bond paths for these covalent bonds are shown in Table 4
below. They are both comparable across the two types of
receptor and consistent with values obtained in previous related
which the (thio)urea is bonded carry a highly positive charge in
all four structures. Where the nitro group is attached to the
phenyl ring the carbon atom has a strong positive charge in all
45 cases (except one position in 8), while the analogous para
position on the unsubstituted phenyl ring has a much lower (as
in structure 3) or negative (the case in structure 4) charge. As
would be expected for the nitro groups, the oxygen atoms are
highly negative and the nitrogen atoms positively charged.
50
Hydrogen bonding
The presence of hydrogen bonding between the (thio)urea
groups and the chloride anion in each crystal structure was
determined by the presence of BCPs associated with each
55 hydrogen bonding interaction and these were analysed by
QTAIM46 (see ESI† Section 8). The energetic properties of the
hydrogen bonds; the local kinetic energy density (G(rBCP)), the
local potential energy density (V(rBCP)), the total energy density
(H(rBCP)) and the hydrogen bond energy (EHB) have been
60 derived (see Table 5)47 and demonstrate that the hydrogen
bonding in this set of structures is weak, closed shell and
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electrostatic in nature (|V(rBCP)|/G(rBCP) < 1, ∇2ρ(rBCP) and
H(rBCP) > 1).48
Table 4. Properties of the electron density at the BCPs for common
functional groups in 3, 4, 7 and 8.
∇2ρ(rBCP
(e Åꢀ5)
)
An interesting observation is that in both 3 and 4 the strength of
the hydrogen bonds are dependent on the substitution of the
phenyl ring to which the N—H hydrogen bond donor group is
attached. Where there is a nitro group present, the value of the
electron density and Laplacian of the electron density at the
bond critical point are increased when compared to those
values for the hydrogen bonds of the (thio)urea groups attached
10 to an unsubstituted phenyl ring. The bond path length (Rij) is
also elongated for the hydrogen bond donor group on the nonꢀ
substituted side of the receptor. This is a direct observation of
the hydrogen bond donor strengthening being caused by
electronꢀwithdrawing substituents. The fact that the chloride
15 anion is bound by two thiourea receptor molecules in 4
produces a total hydrogen bonding strength in 4 greater than
that in 3.
Bond type
ρ(rBCP
)
Rij (Å)
Bond Path length
(e Åꢀ3)
Urea Complex 3
3.0
2.2
5
C−O urea
Phenyl rings
nitro substituted
C−C bonds
Phenyl rings nonꢀ
nitro substituted
C−C bonds
Phenyl rings
nitro substituted
C−H bonds
Phenyl rings nonꢀ
nitro substituted
C−H bonds
ꢀ32.6
ꢀ18.4
1.2216
1.3957
2.1
1.8
1.8
ꢀ16.9
ꢀ17.6
ꢀ16.4
1.3985
1.0831
1.0830
Thiourea Complex 4
1.4
2.1
C−S thiourea
Phenyl rings
ꢀ3.4
ꢀ16.1
1.6797
1.3952
The values of the electron density and Laplacian of the electron
density at the BCPs do not vary so greatly in the symmetrically
20 substituted receptor complexes (7 and 8). However, in 8 (the
thiourea complex) there do appear to be differences in the
strength of the hydrogen bonding in each of the thiourea
molecules present in the crystal structure. This is most likely
caused by the distinct crystallographic environment of the
25 thiourea group in the structure.
nitro substituted
C−C bonds
Phenyl rings nonꢀ
nitro substituted
C−C bonds
Phenyl rings
nitro substituted
C−H bonds
2.2
1.8
1.8
ꢀ17.0
ꢀ16.4
ꢀ15.1
1.3963
1.0834
1.0835
Phenyl rings nonꢀ
nitro substituted
C−H bonds
Urea Complex 7
C−O urea
Phenyl rings
nitro substituted
C−C bonds
3.1
2.1
ꢀ39.1
ꢀ17.7
1.2237
1.3982
Phenyl rings
nitro substituted
C−H bonds
1.9
ꢀ17.6
1.0831
Thiourea Complex 8
C−S thiourea
Phenyl rings nitro
substituted C−C
bonds
1.5
2.1
ꢀ3.2
ꢀ16.8
1.6776
1.3943
Phenyl rings nitro
substituted C−H
bonds
1.9
ꢀ17.9
1.0835
Table 5. Topological parameters of the NH…Cl hydrogen bonding interactions in 3, 4, 7 and 8
D−H…A
H…A
ρ(rBCP
)
Rij H…A A−BCP
H−BCP
distance
(Å)
D−BCP G(rBCP
)
V(rBCP) |V(rBCP)|/
EHB
H(rBCP)
∇2ρ(rBCP
(e Åꢀ5)
)
(e Åꢀ3)
(Å)
distance
(Å)
distance
(Å)
(a.u)
(a.u) G(rBCP) (kJ molꢀ1) (kJ molꢀ1)
3. Unsymmetrical urea receptor with chloride
N(2)−H(2A)…CL(1) H(2A)…CL(1) 0.09(2) 1.974(9) 2.2804
N(3)−H(3A)…CL(1) H(3A)…CL(1) 0.07(2) 1.487(3) 2.4149
4. Unsymmetrical thiourea receptor with chloride
1.5291
1.5929
0.7514
0.8220
1.6934
1.6902
0.016 ꢀ0.011 0.6979 ꢀ14.41
0.012 ꢀ0.007 0.6628 ꢀ10.04
12.47
10.21
N(2)−H(2A)…CL(1) H(2A)…CL(1) 0.12(2) 2.107(7) 2.1947
N(3)−H(3A)…CL(1) H(3A)…CL(1) 0.09(1) 1.425(2) 2.3555
N(5)−H(5A)…CL(1) H(5A)…CL(1) 0.16(2) 2.042(5) 2.1941
N(6)−H(6A)…CL(1) H(6A)…CL(1) 0.10(2) 1.718(4) 2.2849
7. Symmetrical urea receptor with chloride
1.4331
1.4983
1.3953
1.4812
0.7616
0.8572
0.7988
0.8037
1.7473
1.7759
1.7763
1.7701
0.018 ꢀ0.014 0.779
0.012 ꢀ0.009 0.753
0.020 ꢀ0.018 0.914
0.015 ꢀ0.011 0.774
ꢀ18.32
ꢀ11.71
ꢀ23.42
ꢀ14.76
10.37
7.70
4.40
8.63
N(2)−H(2A)…CL(1) H(2A)…CL(1) 0.120(1) 1.949(4) 2.2423
N(3)−H(3A)…CL(1) H(3A)…CL(1) 0.125(2) 2.319(7) 2.1854
8. Symmetrical thiourea receptor with chloride
1.4656
1.4562
0.7767
0.7292
1.744
1.705
0.017 ꢀ0.014 0.808
0.020 ꢀ0.016 0.783
ꢀ17.98
ꢀ20.30
8.56
11.277
N(2)−H(2A)…CL(1) H(2A)…CL(1) 0.06(3) 1.585(5) 2.4302
N(3)−H(3A)…CL(1) H(3A)…CL(1) 0.06(2) 1.589(5) 2.4048
1.6135
1.5947
1.5057
1.5122
0.8167
0.8101
0.7012
0.7619
1.7382
1.7011
1.6475
1.7104
0.012 ꢀ0.007 0.623
0.012 ꢀ0.008 0.629
0.019 ꢀ0.013 0.684
0.018 ꢀ0.013 0.725
ꢀ9.76
ꢀ9.93
ꢀ16.69
ꢀ17.41
11.83
11.71
15.45
13.24
N(6)−H(6A)…CL(1) H(6A)…CL(1) 0.09(4) 2.36(2)
N(7)−H(7A)…CL(1) H(7A)…CL(1) 0.10(3) 2.25(1)
2.2070
2.2741
30
Comparison of urea and thiourea electron density distribution
A major difference in the urea and thiourea receptors is the
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properties of the bond critical points of the C=O/C=S bonds. In
position of the ring indicate that there is an interaction resulting
60 in the formation of a pseudo sixꢀmembered ring. These
interactions are slightly weaker in 4 than 3, possibly due to the
thiourea receptor molecules in 4 being twisted further out of the
plane of the phenyl rings than the urea group in 3. This is
evidenced by a longer bond path length for contacts involving
65 the thioureas (see Table 6). These are present in the crystal
structures of both the symmetrically substituted receptor
complexes (7 and 8), however in the thiourea (8) they are only
observed in one of the two independent receptor molecules in
the crystal structure.
the urea structures 3 and 7 (electron density and Laplacian of
the electron density are ~3.1 e Åꢀ3 and ꢀ36 e Åꢀ5) there is
agreement but these are different when compared to thioureas 4
and 8, where the electron density and Laplacian of the electron
density at the C−S BCP is substantially different (1.4 e Åꢀ3 and
ꢀ3.3 e Åꢀ5). This phenomenon is therefore a property of the
difference between urea and thiourea, rather than symmetrical
nitro substitution to unsymmetrical nitro substitution, arising
10 from the urea C=O bond being more polar in nature than the
C=S bond. This reflects the stronger difference in
electronegativity between carbon and oxygen as opposed to
carbon and sulfur.49
5
The differences in the electron density distribution across the
15 entire molecular ensemble between the urea (3, 7) and thiourea
(4, 8) based structures was probed by studying the atomic
charges (calculated using QTAIM theory and found tabulated
in Section 8 of the ESI†). Additionally the electrostatic
potential distributions in the crystal structures, visualised using
20 the MolIso program50 and shown for 3 in Figure 7 and for 4 in
Figure 8, were investigated. The electrostatic potential
distributions in 7 and 8 are also included in Section 9 of the
ESI† and follow the trends outlined below.
When comparing the two types of receptor (urea vs. thiourea),
25 there is a large variation in the electrostatic potential
distributions. The electrostatic potential distribution of both the
TEA/TMA groups and chloride anions in both 3 and 4 are
similar (as in 7 and 8). The electrostatic potential distribution
of the nitro groups varies significantly between 3 (less negative
30 electrostatic potential) and 4 (higher negative electrostatic
potential). This matches the trends in the atomic charges of the
nitrogen (higher positive charge in 3) and oxygen atoms
(higher negative charge in 4) in these groups. The most striking
difference is the electrostatic potential distributions around the
35 urea and thiourea. In 3 the oxygen has a slightly negative
electrostatic potential (matching the charge of this atom (ꢀ0.723
e)) while the sulfur atoms in 4 have a positive electrostatic
potential ꢀ again this matches their charges of 0.155 e (S1) and
0.651 e (S2). This is also detected in the electrostatic potential
40 distributions of 7 and 8, with the sulfur atoms of 8 carrying
charges of 1.166 e and ꢀ0.099 e and the oxygen atom of the
urea in 7 a charge of ꢀ1.047 e. Positive sulfur atoms have
previously been observed in the study of a thiadiazole crystal
structure, where the authors postulate the positive charge is
45 compensated by negatively charged nitrogen atoms. This is
also the case in the thiourea molecules presented herein.51
Differences in the electrostatic potential between the two
receptor molecules in structure 4 are clearly visible. This is
most marked between the two sulfur atoms, which have
50 considerably variable charges (vide supra) and electrostatic
potential distributions. This may be linked to the different
close contact environments in which the different sulfur atoms
are involved (see Table 6). In 4 S(1) participates in interactions
with the nitro groups of another thiourea group while S(2) has
55 interactions with the hydrogen atoms of a TMA group. Again,
this is also the case in the symmetrical thiourea 8.
70
Figure 7. Two views of the electrostatic potential distribution plot50
(units of e Åꢀ1) of crystal structure 3
In both 3 and 4 bond paths between the oxygen or sulfur atoms
of the (thio)urea moiety and hydrogen atoms in the ortho
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e compared to a charge of ꢀ1.047 e on the 1,3ꢀbis(4ꢀ
nitrophenyl)urea. Additionally, the two carbon atom charges
are 1.260 e in 3 and 1.506 e in 1,3ꢀbis(4ꢀnitrophenyl)urea. This
is due to the lower electronꢀwithdrawing effect exerted on the
20 urea of one nitro group (3) compared to two nitro groups in 7.
The nitrogen atoms of the urea were ꢀ1.260 e and ꢀ0.991 e in 3
and ꢀ1.098 e and ꢀ1.121 e in 1,3ꢀbis(4ꢀnitrophenyl)urea 7.
When considering the properties of the electron density at the
bond critical points for the phenyl rings C—C and C—H bonds
25 in the symmetrical and asymmetrical receptors, the values
agree within error. This suggests the effect of the loss of one
electronꢀwithdrawing group is hard to detect and that it is a less
dramatic change than altering the central anion binding group
from a urea to a thiourea (excluding the difference in charges
30 on the carbonyl carbon atoms). This is clearly an important
consideration for chemists when designing anionꢀreceptor
molecules. However, as has been demonstrated above, the
partial loss of an electronꢀwithdrawing unit can weaken
hydrogen bond donor abilities of N—H bonds at that portion of
35 the receptor structure.
Conclusions
Anion binding effects dramatic differences between these
related crystal structures, as illustrated by Hirshfeld surface
analysis. Charge density analysis is able to more thoroughly
40 account for these differences by systematically mapping the
electron density distribution across the series. The
asymmetrical substitution of the receptor molecules is shown to
alter the packing and intermolecular contacts in both the
receptor structures and the anionꢀreceptor complexes.
45 However, the variation between urea and thiourea substantially
changes the packing and anion binding properties (2:1 receptor:
anion in the thiourea complex versus 1:1 receptor:anion in the
urea complex). In addition, the electron density distribution in
the crystal structures significantly alters between thiourea and
50 urea receptors with dissimilarity in atomic charges, electrostatic
potential distributions and the properties of the electron density
at the BCPs of the (thio)urea groups. These considerable
differences help to account for the varying abilities and
applications of urea and thiourea molecules as anionꢀreceptors.
55 Hydrogen bond strength in the series of four chloride
complexes was determined and shown to be weak in nature and
dependent on the position of the hydrogen bond donor group on
the asymmetrically substituted receptor. This also differs
between thiourea receptor molecules involved in 2:1 receptor
60 anion binding interactions.
Figure 8. Two views of the electrostatic potential distribution plot50
(units of e Åꢀ1) of crystal structure 4
Table 6. Weaker nonꢀcovalent interactions in 3, 4 and 8
∇2ρ(rBCP
(e Åꢀ5)
)
Critical point ρ(rBCP
)
Rij
(Å)
d1
A−BCP
(Å)
d2
BCP−B
(Å)
ε
(e Åꢀ3)
Thiourea intermolecular interactions
…
0.049(1) 0.714(2) 3.4984
0.049(1) 0.714(2) 3.1779
0.020(7) 0.407(3) 2.9296
1.6849
1.6849
1.8328
1.8135 0.62
1.4930 0.62
1.0968 0.09
4. S(1) N(4) †
…
4. S(1) O(3) †
4. ‡
S(2)…H(72A)
0.018(7) 0.341(3) 2.9148
1.7884
1.1264 0.03
4. ‡
S(2)…H(73A)
…
0.035(2) 0.495(3) 3.2624
0.05(2) 0.654(5) 2.8488
1.6820
1.7969
1.5804 0.25
1.0519 0.04
8. S(1) N(7) §
8. δ
S(2)…H(96B)
0.03(2) 0.537(4) 2.8110
1.8312
0.9798 0.09
8.ξ
S(2)…H(93B)
Intramolecular interactions
3. O(1)…H(5) 0.115(2) 1.928(5) 2.1722
3.O(1)…H(13) 0.101(2) 1.473(4) 2.3485
4. S(1)…H(5) 0.094(3) 1.221(3) 2.6491
4. S(1)…H(13) 0.076(2) 0.972(2) 2.7812
4. S(2)…H(18) 0.084(3) 1.182(3) 2.6042
4. S(2)…H(26) 0.071(2) 0.947(3) 2.6512
1.2861
1.3276
1.6010
1.6262
1.5732
1.5679
0.8861 0.53
1.0208 0.33
1.0480 0.25
1.1549 0.60
1.0310 0.09
1.0833 0.35
Divergence in the electron density distribution, atomic charges
and electrostatic potential distribution across two examples
containing the same receptor molecule in the thiourea 2:1
complex, illustrates how properties of molecules can vary
65 greatly depending on the crystalline environment.
5
§Symmetry used to generate interaction 1ꢀx, 1ꢀy, 1ꢀz, δsymmetry used to
generate interaction ꢀ1+x, y, z, ξsymmetry used to generate interaction
2ꢀx, 1ꢀy, 2ꢀz, †symmetry used to generate interaction 1ꢀx, ꢀy, ꢀz ‡
symmetry used to generate interaction 1ꢀx, 1ꢀy, ꢀz.
10 Comparison of unsymmetrical and symmetrical complexes
A further key comparison is that of asymmetrical (3 and 4)
versus symmetrical substitution (7 and 8) of the receptor.
Comparing the urea portion of the symmetrical receptor 7 with
asymmetrical receptor 3, the difference between the atoms is
15 less extreme, with the oxygen atom carrying a charge of ꢀ0.723
Notes and references
a Chemistry, University of Southampton, Southampton, UK, SO17 1BJ.
Eꢀmail: s.j.coles@soton.ac.uk; philip.gale@soton.ac.uk
70 bDiamond Light Source, Diamond House, Harwell Science and
Innovation Campus, Didcot, Oxfordshire, UK, OX11 0DF. Current
10
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address: School of Chemistry, University of Glasgow, Joseph Black
Building, Glasgow, G12 8QQ.
26. F. H. Allen, O. Keenard, D. G. Watson, L. Brammer, A. G. Orpen
and R. Taylor, International Tables for Xꢀray Crystallography,
2006., Vol. C.
60
† Electronic Supplementary Information (ESI) available including
NMR titration data, QTAIM charges and theoretical and experimental
topological analyses of chemical bonding in the crystal structures.
Crystal structures deposited with the Cambridge Crystallographic Data
Centre with deposition numbers CCDC1042624ꢀ1042633. See
DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments
10 relevant to but not central to the matter under discussion, limited
experimental and spectral data, and crystallographic data.
5
27. A. Madsen, J. Appl. Crystallogr., 2006, 39, 757.
28. E. Espinosa, E. Molins and C. Lecomte, Phys. Rev. B., 1997, 56,
1820.
65 29. P. M. Dominiak and P. Coppens, Acta Crystallogr., Sect. A., 2006,
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