EXAFS Study on the Coordination Chemistry of the Solvated Copper(II) Ion in a Series of Oxygen Donor Solvents

Article abstract of DOI:10.1021/acs.inorgchem.0c00403

The structures of the solvated copper(II) ion in water and nine organic oxygen donor solvents with similar electron-pair donor ability, but with different space-demanding properties at coordination, have been studied by EXAFS. N,N′-Dimethylpropyleneurea and N,N,N′,N′-tetramethylurea are sufficiently space demanding at coordination to make the axial positions not accessible, resulting in square-planar copper(II) solvate complexes with an intense green color. The mean Cu-O bond distances in these two solvate complexes are 1.939(3) and 1.935(3) ?, respectively. The best fits of the remaining solvates, which are light blue in different hues, are obtained with a Jahn-Teller distorted-octahedral model consisting of four strongly bound solvent molecules in the equatorial positions at 1.96(2) ? and two in the axial positions but with different Cu-Oax bond distances: ca. 2.15 and 2.32 ?. This is in agreement with observations in solid-state structures of compounds containing hexaaquacopper(II) complexes crystallizing in noncentrosymmetric space groups and all reported crystal structures containing a [Cu(H2O)5(O-ligand)] complex with Jahn-Teller distortion. Such a structure is in agreement with previous EPR and EXAFS studies proving the hydrated copper(II) ion to be a noncentrosymmetric complex in aqueous solution. The refinements of the EXAFS data of the solids [Cu(H2O)6](ClO4)2, [Cu(H2O)6](BrO3)2, [Cu(H2O)6]SiF6, Cu(NO3)2·2.5H2O, and CuSO4·5H2O gave Cu-O bond distances significantly different from those reported in the crystallographic studies but similar to the configuration and bond distances in the hydrated copper(II) ion in aqueous solution. This may depend on whether the orientation of the axial positions is random in one or three dimensions, giving a mean structure of the solid with symmetry higher than that of the individual complexes. This study presents the very first experimental data from the new X-ray absorption spectroscopy beamline Balder at the MAX IV synchrotron radiation facility in Lund, Sweden, as well as the utilized properties of the beamline.

Full text of DOI:10.1021/acs.inorgchem.0c00403

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EXAFS Study on the Coordination Chemistry of the Solvated
Copper(II) Ion in a Series of Oxygen Donor Solvents
́
́
Ingmar Persson,* Daniel Lundberg, Eva G. Bajnoczi, Konstantin Klementiev, Justus Just,
and Kajsa G. V. Sigfridsson Clauss
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ABSTRACT: The structures of the solvated copper(II) ion in water and nine organic oxygen
donor solvents with similar electron-pair donor ability, but with different space-demanding
properties at coordination, have been studied by EXAFS. N,N′-Dimethylpropyleneurea and
N,N,N′,N′-tetramethylurea are sufficiently space demanding at coordination to make the axial
positions not accessible, resulting in square-planar copper(II) solvate complexes with an
intense green color. The mean Cu−O bond distances in these two solvate complexes are
1.939(3) and 1.935(3) Å, respectively. The best fits of the remaining solvates, which are light
blue in different hues, are obtained with a Jahn−Teller distorted-octahedral model consisting
of four strongly bound solvent molecules in the equatorial positions at 1.96(2) Å and two in
the axial positions but with different Cu−Oax bond distances: ca. 2.15 and 2.32 Å. This is in
agreement with observations in solid-state structures of compounds containing
hexaaquacopper(II) complexes crystallizing in noncentrosymmetric space groups and all
reported crystal structures containing a [Cu(H2O)5(O-ligand)] complex with Jahn−Teller
distortion. Such a structure is in agreement with previous EPR and EXAFS studies proving
the hydrated copper(II) ion to be a noncentrosymmetric complex in aqueous solution. The refinements of the EXAFS data of the
solids [Cu(H2O)6](ClO4)2, [Cu(H2O)6](BrO3)2, [Cu(H2O)6]SiF6, Cu(NO3)2·2.5H2O, and CuSO4·5H2O gave Cu−O bond
distances significantly different from those reported in the crystallographic studies but similar to the configuration and bond
distances in the hydrated copper(II) ion in aqueous solution. This may depend on whether the orientation of the axial positions is
random in one or three dimensions, giving a mean structure of the solid with symmetry higher than that of the individual complexes.
This study presents the very first experimental data from the new X-ray absorption spectroscopy beamline Balder at the MAX IV
synchrotron radiation facility in Lund, Sweden, as well as the utilized properties of the beamline.
ion in aqueous solution.10−25 A majority of these studies
INTRODUCTION
■
propose a tetragonally elongated square-pyramidal configu-
The most debated hydrated metal ion by far in recent years is
ration as the most likely or stable structure,9,10,13,15,16,25 but
copper(II). It was assumed for a long time that the hydrated
several studies have proposed dynamic equilibria among four-,
copper(II) ion is six-coordinate with a tetragonally elongated
five-, and/or six-coordinated hydrate complexes,14,19,20,22,23 as
octahedral configuration along a 4-fold axis, known as Jahn−
well as a six-coordinate Jahn−Teller distorted-octahedral
configuration.11,12,18,21 This shows that the discussion of the
structure of the hydrated copper(II) ion aqueous solution is
still a matter of debate.
Teller distortion,1−3 in aqueous solution. It can be concluded
that a majority of the solid compounds containing a hydrated
copper(II) ion have this configuration4,5 (Table S1). This has
led many to believe that the same structure is also present in
aqueous solution, even though a structure of a metal complex
in the solid state cannot be related to its structure in solution.6
Linear electric field EPR studies gave the first indications that
the hydrated copper(II) ion in aqueous solution was not
centrosymmetric, as required by a regular Jahn−Teller
distorted octahedron.7,8 A combined neutron scattering and
molecular dynamics study proposed that the hydrated
copper(II) ion is five-coordinate in aqueous solution.9 This
study was followed a by series of publications using EXAFS
and XANES spectroscopy, X-ray and neutron scattering on
liquids, and DFT and QM/MD simulations on different levels
to study the coordination chemistry of the hydrated copper(II)
The most extensive experimental studies have been
performed by Frank and co-workers, who in a series of studies
have applied XANES and high-k EXAFS spectroscopy on both
aqueous solutions and frozen samples of the hydrated
copper(II) ion.13,22,25 They have applied three basic models:
Received: February 11, 2020
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five-coordination in a tetragonally elongated square-pyramidal
configuration and six-coordination in Jahn−Teller distorted-
octahedral configurations with equal (centrosymmetric) and
different Cu−O bond distances in the axial positions
(noncentrosymmetric). The EXAFS data of an aqueous
copper(II) solution were fitted equally well with square-
pyramidal and noncentrosymmetric Jahn−Teller distorted-
octahedral models, with Cu−O bond distances of 1.97 and
2.21 Å and of 1.97, 2.19, and 2.33 Å, respectively.22 On the
other hand, in the refinement of XANES data using the MXAN
program, the best fit was obtained with a square-pyramidal
model with Cu−O bond distances of 1.95 and 2.23 Å.22
Studies on frozen samples of 1,3-propanediol and 1,5-
pentanediol aqueous glasses revealed asymmetric six-coordi-
nation with Cu−Oeq, Cu−Oax1, and Cu−Oax2 bond distances
of 4 × 1.94, 2.22, and 2.34 Å, respectively, from EXAFS data,
while MXAN data revealed copper sites with two kinds of six-
coordination, asymmetric Jahn−Teller distorted-octahedral (4
× 1.94, 1 × 2.14, and 2.28 Å) and capped-square-pyramidal
configurations (5 × 1.94 and 2.22 Å), and five-coordinated
sites in a square-pyramidal configuration (4 × 1.95 and 2.23
Å).25 D’Angelo and co-workers have studied the solvated
copper(II) ion methanol, dimethyl sulfoxide and acetontrile
solution by combined XANES and EXAFS, showing that
tetragonally elongated square-pyramidal coordination is
preferred in these solvents.26,27
It is important to emphasize that techniques such as EXAFS
and XANES spectroscopy cannot give an absolute answer
about the structure, as several models may fit the experimental
data equally or almost equally well and the starting model may
affect the outcome of the refinement. It is therefore important
to test as many reasonable structure models as possible.
Calculations have shown that the energy barrier between
centrosymmetric Jahn−Teller distorted [Cu(H2O)4(H2O)2]2+
and noncentrosymmetric [Cu(H2O)4(H2O)(H2O)]2+ is small,
−82 meV (−7.95 kJ mol−1).17 Independent QMCF/MD
molecular dynamics calculations did observe this delicate
energy balance, resulting in a mean equilibrium geometry with
Cu−O bond distances of 4 × 2.03, 2.15, and 2.30 Å and 4 ×
2.07 and 2 × 2.2 Å including and excluding the second
hydration shell into the QM zone, respectively, and a B3LYP/
MM simulation, 4 × 2.02 and 2 × 2.29 Å.11,12
In order to give another perspective on this debate, a survey
and analysis of reported solid-state structures containing four-,
five-, and six-coordinate copper(II) hydrates and other
copper(II) complexes with oxygen donor ligands are presented
in this study. Of these structures, a large majority have a Jahn−
Teller distorted-octahedral geometry around copper(II). Of
the 139 reported crystal structures containing a [Cu(H2O)6]2+
ion, 105 report a regular Jahn−Teller distorted configuration
with pairwise equal Cu−O bond distances due to symmetry
rules of the space groups. The mean Cu−Oeq and Cu−Oax
bond distances in these structures are 1.981 and 2.332 Å,
respectively (Table S1). Two structures containing a [Cu-
(H2O)6]2+ ion crystallize in a centrosymmetric space group,
but the copper in the [Cu(H2O)6]2+ ion is not in the center of
symmetry,28,29 and another five structures crystallize in
noncentrosymmetric space groups.30−33 In these seven
structures the axial Cu−O bond distances are significantly
different with mean distances of 2.298 and 2.362 Å (Table S1).
A limited number of the hexaaquacopper(II) complexes are
reported to have regular or slightly irregular octahedral
configurations: 7 and 21, respectively (Table S1).
To further check whether there are more examples showing
a significant difference in the Cu−Oax bond distances, crystal
structures containing complexes with the composition [Cu-
(H2O)5O′], where O′ is a monodentate oxygen donor ligand,
with reported Jahn−Teller distortion were summarized, as
such complexes cannot individually be centrosymmetric.
Indeed, all of these structures display significantly different
Cu−Oax bond distances (Table S2), and they all have similar
Cu−O bond distances in the equatorial positions independent
of whether the O′ ligand binds in an equatorial or an axial
position. Four of these structures have water molecules in the
axial positions, and the difference in Cu−Oax bond distance is
0.07−0.43 Å, and a similar pattern is seen in the compounds
with the O′ ligand in the axial position (Table S2).
A possible reason as to why a majority of the structures with
hexaaquacopper(II) ions crystallize in centrosymmetric space
groups is that the direction of possibly different Cu−Oax bond
distances is randomly distributed in the structure. The mean
structure, from a crystallographic point of view, becomes
centrosymmetric with identical Cu−Oax bond distances. This is
certainly also the case for the 28 crystal structures reporting an
apparent regular or a slightly distorted octahedral [Cu-
(H2O)6]2+ complex, but here the random orientation of the
axial positions is distributed in three dimensions (Table S1).
When the orientation of the axial positions is randomly
distributed in the structure, the mean structure has a higher
symmetry in comparison to the individual complexes, and the
observed space group is thereby allowed to have higher
symmetry in comparison to the individual complexes. By using
a lattice-independent structure method such as EXAFS the
distances to the absorbing atom are independent of orientation
in space, giving the actual distance distribution. A previous
EXAFS study has shown that the crystal structures of
[Cu(H2O)6](BrO3)2 and [Cu(H2O)6]SiF6, and some other
six-coordinate homoleptic copper(II) complexes displaying
regular octahedral geometry, actually have a distorted
geometry with two well-separated distances.34 This “too high
symmetry” effect has been also seen in e.g. the crystal
structures of [Sc(H2O)8.0](CF3SO3)3 and [Lu(H2O)8.2]-
(CF3SO3)3.35,36 In these two cases, a phase transition to
lower symmetry takes place on cooling and the true structure is
observed in the low-temperature phases. EXAFS studies have
shown that these complexes have the same structure
independent of the temperature of the solid or in aqueous
solution.35,36
The number of tetra- and pentaaquacopper(II) complexes
with no other ligands within the sum of the van der Waals
radii37 reported in crystallographic studies is low. Only four
structures, each with a tetra- or pentaaquacopper(II) ion, have
been reported (Table S3). Due to the limited number of such
reported structures, the literature search was extended to all
four-and five-coordinate copper(II) complexes with oxygen
donor ligands. The four-coordinate complexes have all a
square-planar coordination geometry with a mean Cu−O bond
distance of 1.928 Å (68 structures) (Table S3). Of the five-
coordinate copper(II) complexes with oxygen donor ligands all
except one display a tetragonally elongated square-pyramidal
geometry, with the last one being a trigonal bipyramid (Table
S3). The mean Cu−O bond distance in the square-pyramidal
copper(II) complexes is 1.955 Å in the equatorial plane and
2.258 Å to the ligand in the axial position (37 structures)
(Table S3).
B
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to precipitate copper(II) hydroxide, which was rinsed several times
with water. Anhydrous copper(II) trifluoromethanesulfonate (Cu-
(CF3SO3)2) was prepared by adding an excess of trifluoromethane-
sulfonic acid (CF3SO3H; Fluka) dropwise to an aqueous slurry of
copper(II) hydroxide until a clear solution was obtained. The
obtained solution was filtered, and water and excess acid were boiled
off at approximately 450 K in an oven. The resulting anhydrous
copper(II) trifluoromethanesulfonate was repeatedly ground using a
mortar and pestle with oven drying at 450 K in between until a fine,
dry, almost white powder was obtained. The dry salt was stored in the
oven at 450 K to avoid uptake of water. Hexaaquacopper(II)
perchlorate ([Cu(H2O)6](ClO4)2) was prepared by dissolving
copper(II) hydroxide in dilute perchloric acid (HClO4), the volume
was reduced, and precipitation took place after cooling in a
refrigerator. Copper(II) sulfate pentahydrate (CuSO4·5H2O) was
prepared in the same way as the perchlorate salt but using dilute
sulfuric acid, prepared from concentrated sulfuric acid (Merck,
analytical grade, 98%). The identities of the prepared solid
compounds were confirmed by determinations of the unit cell
parameters crystallographically.
Solutions and Crystals. The solutions for the EXAFS studies
were prepared by dissolving anhydrous copper(II) trifluoromethane-
sulfonate in the respective solvent to a concentration of 0.19 mol
dm−3 (Table S5). All solutions, except the dmpu and tmu solutions,
are light blue with somewhat different hues. The dmpu and tmu
solutions are dark green. In another experiment a saturated copper(II)
trifluoromethanesulfonate dmpu solution was stored in a refrigerator
for several years. Only low-quality single-crystals of tetrakis(N,N′-
dimethylpropyleneurea)copper(II) trifluoromethanesulfonate mono-
solvate ([Cu(dmpu)4](CF3SO3)2·dmpu; 1) were obtained. The
reported crystal structure represents the best of these. Repeated
attempts to prepare better crystals, using different common methods,
failed.
Balder Beamline at the MAX IV Synchrotron Light Facility,
Lund, Sweden. Copper K-edge X-ray absorption data were collected
in transmission mode at the Balder beamline at the MAX IV
synchrotron light facility, Lund University, Lund, Sweden. Balder is a
new high-flux wiggler beamline for X-ray absorption and emission
spectroscopy at the 3.0 GeV storage ring.39 High data acquisition
speed is crucial to be able to perform X-ray spectroscopy experiments
over 2000 eV without significant radiation damage. The double-
crystal fixed exit monochromator (DCM) (FMB Oxford) is equipped
with a direct drive motor on the Bragg axis, allowing for scan speeds
up to 0.5 s/1000 eV. In the current configuration of the data
acquisition a full EXAFS scan can be performed within acquisition
times down to 12 s. Spectra were acquired using a continuous scan
scheme, where the Bragg axis as well as the vertical beam offset are
moved simultaneously with constant velocity to fix the vertical
position of the beam. The Bragg axis is equipped with a rotary
encoder with a resulting resolution of 50 counts/μrad. Data
acquisition was performed using an Em# electrometer device,40
which is an MAX IV codevelopment together with ALBA of Spain.
The acquisition is (hardware) triggered at predefined positions of the
Bragg axis on the basis of a comparison of the encoder readout with a
look-up table of trigger positions at FPGA (field programmable gate
array) level utilizing the PandABox developed at Soleil in France, and
Diamond in the United Kingdom.41 The DCM is equipped with two
sets of crystals, Si(111) and Si(311), of which the Si(111) crystal set
was employed in this study. A vertically position sensitive ionization
chamber is coupled in closed-loop operation to a piezo drive on the
pitch axis of the second monochromator crystal for further
stabilization of the beam position on the sample during a scan.
Custom-developed ionization chambers were used for transmission
measurements. The ion chambers are 30 cm long; the first one, I0,
contained 1.5 bar of N2 (applied potential 2.0 kV) and the second
one, I1, contained 0.2 bar of Ar and 1.8 bar of N2 (applied potential
2.5 kV).
To summarize the present knowledge about the structure of
the hydrated copper(II) ion in aqueous solution, (1) there is
strong evidence that it is noncentrosymmetric, as shown by
linear electric field EPR6,7 and X-ray absorption spectroscopy
studies,13,22,25 (2) different kinds of theoretical simulations
have shown that [Cu(H2O)5]2+ and [Cu(H2O)5··H2O]2+ units
are more stable in aqueous solution in comparison to a regular
Jahn−Teller distorted [Cu(H2O)6]2+ ion,9−16,21−23,25 and (3)
solid-state structures containing [Cu(H2O)6]2+ ions crystallize
in noncentrosymmetric space groups or in centrosymmetric
space groups but the copper in [Cu(H2O)6]2+ is not in the
center of symmetry and [Cu(H2O)5(O′)] complexes display a
significant difference in the axial Cu−O bond distances as
discussed above.
The aim of this study is to collect high-quality EXAFS data
to high k values of solvated copper(II) ions in solution in series
of oxygen donor solvents and to test which of the three
different models, a tetragonally elongated square pyramid (5-
coordination) and tetragonally elongated octahedra with the
same or different axial Cu−O bond distances (6-coordination),
fit the data best and to make comparisons to previously
reported EXAFS and MXAN studies13,22,25 on the hydrated
copper(II) ion in aqueous solution. The studied solvents all
have similar physicochemical properties (Table S4) and
represent increasing spatial demand upon coordination in
order to study whether lower coordination numbers are
present for the solvents most space-demanding at coordina-
tion. For example, it has previously been reported that the
copper(II) ion is four-coordinate in hexamethylphosphoric
triamide, a space-demanding solvent at coordination.38
Another aim is to confirm or disprove whether the Cu−O
bond distances reported in the crystal structures of [Cu-
(H2O)6](ClO4)2, [Cu(H2O)6](BrO3)2, [Cu(H2O)6]SiF6, Cu-
(NO3)2·2.5H2O, and CuSO4·5H2O are in agreement with the
Cu−O bond distances obtained by EXAFS. If the crystallo-
graphic investigations of the reported structures cannot be
confirmed by EXAFS, it becomes evident that crystallographic
studies of compounds crystallizing in e.g. centrosymmetric
space groups may result in structural information with higher
symmetry in comparison to the individual units, e.g. metal
complexes, thereby leading to an incorrect description.
EXPERIMENTAL SECTION
■
Chemicals. Methanol (CH3OH; MeOH), N,N-dimethylforma-
mide ((CH3 )2 NCHO; dmf), N,N-diethylformamide
((CH3CH2)2NCHO; def), N,N-dimethylacetamide ((CH3)2NC-
(CH3)O; dma), N,N-diethylacetamide ((CH3CH2)2NC(CH3)O;
dea), N,N-dimethylpropionamide ((CH3)2NC(CH3CH2)O; dmp),
N,N-diethylpropionamide ((CH3CH2)2NC(CH3CH2)O; dep),
N,N,N′,N′-tetramethylurea ((CH3)2N)2CO; tmu), and N,N′-dime-
thylpropyleneurea ((CH2)3N(CH3)2CO; dmpu), all Sigma-Aldrich,
were used as purchased except for dmpu, which was purified by
distillation over calcium hydride (CaH2; Merck) under reduced
pressure and stored over 3 Å molecular sieves in a dark bottle.
Deionized water that was further purified by a Milli-Q Plus Ultrapure
water system, giving water with 18.2 MΩ cm resistance was used in
syntheses of the hydrated copper(II) salts and in the preparation of
aqueous solutions.
Copper powder (Aldrich, 99.9999% purity, lot #1297) was
dissolved in concentrated nitric acid (Merck, analytical grade). After
a part of the obtained solution was cooled in a refrigerator, solid
copper(II) nitrate hemipentahydrate (Cu(NO3)2·2.5H2O) precipi-
tated. After dilution of the remaining part of this solution with
deionized water, a 5 mol dm−3 sodium hydroxide solution (prepared
from sodium hydroxide (NaOH; Merck, analytical grade) was added
Extended X-ray Absorption Fine Structure (EXAFS). The
copper K-edge X-ray absorption data in this study were collected in
transmission mode. The liquid sample cells were made of a 3.0 mm
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Teflon spacer and Kapton foil windows held together with titanium
frames. The solid samples were measured as pressed pellets with
cellulose or polyethylene as the binding medium. The XAS spectra
were measured in continuous energy scanning at a speed of 50 s/full
EXAFS spectrum (kmax = 18 Å−1). The monochromatic flux at the
sample was ∼1012 photons/s with a spot size of approximately 0.10
(horizontal) × 2.0 (vertical) mm. For each sample, 20 repeats were
examined for possible radiation damage and afterward accumulated
into an average spectrum. Radiation damage, if it happens, is in solid
samples seen as sharp bends on the absorption coefficient curves and
as color variation of the sample visible live by a macro camera. In
general, only a single EXAFS scan was recorded on each sample
position. Immediately before the start of each repeat, the sample was
shifted by 0.20 mm horizontally into a fresh position to avoid
radiation dose accumulation. It was seen that aqueous solutions and
solid hydrates were the most prone to radiation damage, due to water
radiolysis as reported before.42 In such cases, when bubbles were seen
rapidly forming or abnormal changes in the EXAFS spectrum were
observed, the scanning speed was increased to 25 s/full EXAFS
spectrum. The energy axis was calibrated with repeated spectra of a
copper foil with the first inflection point assigned as 8980.3 eV.43 The
EXAFSPAK44 program package was used for data treatment. The
EXAFS oscillations were extracted using standard procedures for pre-
edge subtraction, spline removal, and data normalization.45 Model
fitting, including both single and multiple backscattering pathways,
was performed with theoretical phase and amplitude functions
calculated ab initio by means of the computer code FEFF7.46 The
k3-weighted EXAFS oscillation was analyzed using a nonlinear least-
squares fitting procedure.
Large-Angle X-ray Scattering (LAXS). The scattering from the
free surface of a saturated dmpu solution of copper(II) trifluor-
omethanesulfonate, (∼0.8 mol dm−3) was measured with a large-angle
θ−θ goniometer, as previously described.47,48 Intensity data were
collected in the range 1 < θ < 65° using Mo Kα radiation (λ = 0.7107
Å), following the same procedure as described elsewhere.49 The
experimental intensities were corrected for absorption and polar-
ization and normalized to a stoichiometric unit of volume
corresponding to one copper atom. After subtraction of the
structure-independent coherent and Compton scattering, the
remaining structure-dependent intensity function i(s) was Fourier-
transformed to obtain a modified radial distribution function (RDF),
expressed in the form D(r) − 4πr2r0. Minor erroneous peaks below
1.2 Å in the RDFs were removed by a Fourier back-transform
procedure to align the intensity function before fitting the model
functions. The KURVLR program50 was used for data treatment and
the STEPLR program51 for least-squares refinements of model
parameters by comparing calculated intensity contributions for
distinct interatomic interactions with the experimental reduced
intensities for values of s > 3.7 Å−1 (s = (4π/λ) sin θ).
Table 1. Crystallographic Data for 1
C38H72CuF6N12O12S2
mol wt
cryst syst
space group
1130.73
monoclinic
P21/c
a/Å
b/Å
c/Å
α/deg
12.629(6)
21.740(10)
10.050(5)
90
β/deg
γ/deg
V/Å3
T/K
Z
ρcalcd/g cm−3
μ/mm−1
103.046(8)
90
2688(2)
298(2)
2
1.397
0.570
cryst size/mm
θ range/deg
index ranges
no. of measd rflns
no. of unique rflns (Rint)
refinement method
final R1, wR2 (I > 2σ(I))
0.27 × 0.22 × 0.18
1.66−25.68
−15 ≤ h ≤ 15, −23 ≤ k ≤ 26, −11 ≤ l ≤ 12
13755
5094 (0.0511)
full-matrix least squares on F2
0.0727, 0.1960
0.1197, 0.2324
0.377
a
a
final R1, wR2 (all data)
max diff peak/e Å−3
max diff hole/e Å−3
−0.273
a
R values are defined as R1 = ∑||Fo| − |Fc||/∑|Fo| and wR2 =
[∑[w(Fo − Fc2)]/∑[w(Fo )2]]0.5.
2
2
RESULTS AND DISCUSSION
■
EXAFS Study of the Hydrated Copper(II) ion in
Aqueous Solution and the Solid State. In this study
EXAFS data on an aqueous solution of copper(II) trifluor-
omethanesulfonate and the solids [Cu(H2O)6](ClO4)2, Cu-
(NO3)2·2.5H2O, and CuSO4·5H2O have been collected at the
Balder beamline. They are compared with data on a 0.50 mol
dm−3 aqueous solution of copper(II) perchlorate and the solids
[Cu(H2O)6](BrO3)2, and [Cu(H2O)6]SiF6, collected at beam-
line 4-1 (old station) at the Stanford Synchrotron Radiation
Lightsource (SSRL), reported in ref 34. The EXAFS spectra of
the aqueous solutions and the solids [Cu(H2O)6](ClO4)2,
[Cu(H2O)6](BrO3)2, and [Cu(H2O)6]SiF6 almost super-
impose each other, showing that they have identical or at
least very similar structures (Figure 1). Also solid Cu(NO3)2·
2.5H2O almost superimposes with the aqueous copper(II)
solutions, implying a structural similarity even though
copper(II) binds nitrate oxygens in the equatorial and axial
positions53−55 (Figure 1). The data collected at SSRL have
been refitted with the three models used in this study. The
results of the refinements of the EXAFS data of the aqueous
solutions and these solids are summarized in Table 2. The best
fit of the hydrated copper(II) ion in aqueous solution is
obtained by a model with four Cu−Oeq bond distances in the
equatorial plane at 1.956(3) Å and two separate Cu−Oax bond
distances of 2.14(2) and 2.32(2) Å, which are in close
agreement with an EXAFS study reported by Frank et al.22 In
the solids [Cu(H2O)6](ClO4)2 and [Cu(H2O)6]SiF6, the Cu−
Oeq bond distances are within the limit of errors the same,
1.963(6) Å, the Cu−Oax bond distances are slightly longer
than those in aqueous solution, 2.18(1), 2.34(1) Å and
2.21(2), 2.32(3) Å, respectively. In solid [Cu(H2O)6](BrO3)2
Single-Crystal X-ray Diffraction. Data collection were per-
formed on a Bruker SMART platform equipped with a CCD area
detector and a graphite monochromator using Mo Kα (λ = 0.7107 Å)
radiation at ambient room temperature. A hemisphere of data with
1271 frames was collected using the ω scan method (0.3° frame
width). The crystal to detector distance was 50 mm. The first 50
frames were remeasured at the end of the data collection to monitor
crystal and instrument stability. The structure was solved by direct
methods in the SHELX 2016/6 program package52 and refined using
full-matrix least squares on F2.
Non-hydrogen atoms were treated anisotropically. Hydrogen atoms
were calculated in ideal positions riding on their respective carbon
atoms. Crystal and experimental data for 1 are summarized in Table 1.
UV−Vis Spectrophotometry. A PerkinElmer Lambda 2 double-
beam spectrophotometer was used to record the UV−vis spectra of
the copper(II) trifluoromethanesulfonate solutions studied by
EXAFS. The recorded spectra after appropriate dilution are shown
in Figure S1.
D
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The structure parameters obtained in the refinements applying
these models are given in Table 2 and Table S6. The fit of raw
EXAFS data and the Fourier transforms are shown in Figures 2
and 3, respectively.
Figure 1. Raw EXAFS spectra of a 0.20 mol dm−3 aqueous copper(II)
trifluoromethanesulfonate solution (black line), 0.50 mol dm−3
aqueous copper(II) perchlorate solution (reported in ref 34, purple
line, offset 4), solid [Cu(H2O)6](ClO4)2 (red line, offset 4), solid
Cu(NO3)2·2.5H2O (blue line, offset 6), solid [Cu(H2O)6](BrO4)2
(reported in ref 34, yellow line, offset 8), and [Cu(H2O)6]SiF6
(reported in ref 34, green line, offset 10).
Figure 2. Fit of raw EXAFS data of (a) solid copper(II) sulfate
pentahydrate (CuSO4·5H2O, offset 10), (b) solid hexaaquacopper(II)
hexafluorosilicate ([Cu(H2O)6]SiF6, offset 8), (c) solid
hexaaquacopper(II) bromate ([Cu(H2O)6](BrO3)2, offset 6), (d)
solid hexaaquacopper(II) nitrate (Cu(NO3)2·2.5H2O, offset 4), (e)
solid hexaaquacopper(II) perchlorate ([Cu(H2O)6](ClO4)2, offset 2),
and (f) aqueous solution of copper(II) trifluoromethanesulfonate (no
offset).
the Cu−Oeq and Cu−Oax bond distances are slightly longer,
1.967(5), 2.21(2) Å and 2.42(2) Å.
The mean Cu−Oeq and Cu−Oax bond distance in solid
Cu(NO3)2·2.5H2O have been refined to 1.959(3), 2.176(6),
and 2.39(1) Å, respectively (Table 2). The fitting using models
with copper binding four solvent molecules in the equatorial
plane and two in an axial position at the same Cu−Oax bond
distance (centrosymmetric Jahn−Teller distorted-octahedral
geometry) or only one solvent molecule in the axial position of
a tetragonally elongated square pyramid are slightly worse than
the model with different bond distances in the axial positions.
The three crystallographic studies on [Cu(H2O)6](ClO4)2
have reported very different Cu−O bond distance distribu-
tions, in spite of reporting the same space group, P21/c (No.
14) (Table S7).56−58 Two of these report very similar unit cell
Table 2. Summary of Refinements of EXAFS Data Using a Noncentrosymmetric Tetragonally Elongated Octahedral Geometry
of the Hydrated Copper(II) ion in Water (aq) and the Solid State for [Cu(H2O)6](ClO4)2 (A), Cu(NO3)2·2.5H2O (B), CuSO4·
a
5H2O (C), [Cu(H2O)6](BrO3)2 (D), and [Cu(H2O)6]SiF6 (E)
water
water*
A
B
C
D*
E*
d(Cu−Oeq)
σ2(Cu−Oeq)
n(Cu−Oeq)
1.957(3)
0.0055(3)
4
1.954(8)
0.0069(8)
4
1.963(3)
0.0060(3)
4
1.959(3)
0.0052(3)
4
1.959(3)
0.0051(3)
4
1.967(5)
0.0052(7)
4
1.962(6)
0.0050(8)
4
d(Cu−Oax1
)
)
2.14(2)
0.007(2)
1
2.15(5)
0.012(6)
1
2.184(8)
0.0065(9)
1
2.176(6)
0.0072(6)
1
2.218(9)
0.0066(7)
1
2.21(2)
0.010(2)
1
2.21(2)
0.011(4)
1
σ2(Cu−Oax1
n(Cu−Oax1
)
d(Cu−Oax2
)
)
2.32(2)
0.009(2)
1
3.914
0.012(3)
2.36(4)
0.014(6)
1
3.908
0.013(4)
2.343(8)
0.0076(8)
1
3.926
0.010(3)
2.394(8)
0.0084(8)
1
3.917
0.010(2)
2.376(9)
0.0074(9)
1
3.918
0.032(3)
3*4
3.58(2)
0.012(3)
1
2.42(2)
0.010(2)
1
3.934
0.010(4)
2.32(3)
0.012(3)
1
3.924
0.010(4)
σ2(Cu−Oax2
n(Cu−Oax2
)
b
b
b
b
b
b
b
d(MS(CuO4))
σ2(MS(CuO4))
n(MS(CuO4))
d(Cu−S1)
3*4
3*4
3*4
3*4
3*4
3*4
σ2(Cu−S1)
n(Cu−S1)
d(Cu−S2)
3.74(7)
0.024(8)
1
1.05(3)
8989.9(5)
12.85
σ2(Cu−S2)
n(Cu−S2)
2
S0
0.93(4)
8989.7(5)
18.14
1.03(4)
8988.7(6)
8.19
1.06(4)
8989.6(5)
16.36
0.95(3)
8989.6(3)
7.98
0.72(5)
8989.5(5)
8.19
0.84(5)
8989.9(5)
9.69
E0
F
a
2
Definitions: distance, d/Å; Debye-Waller coefficient, σ2/Å2; number of distances, n; amplitude reduction factor, S0 ; threshold energy, E0/eV;
weighted F factor, F. The asterisk denotes data collected at SSRL in 2001. Estimated errors within parentheses include the statistical errors from the
b
refinements and estimated experimental errors. Parameters without following parentheses have been fixed. The linear multiple scattering distance
with in the CuO4 equatorial plane has been linked to the Cu−O bond distance.
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and the other salts containing hydrated copper(II) ions (Table
2). It is interesting to note that the Cu−O bond distance at
2.39(1) Å fits perfectly, and the longest Cu−O/N distances fit
reasonably well with distances reported in the crystallographic
study of Cu(NO3)2·2.5H2O. However, these distances cannot
be refined without the presence of a relatively short Cu−Oax
bond distance, refined to 2.176(6) Å, which is absent in the
crystal structure. The best possible fit of the EXAFS data with
the reported Cu−O bond distances in ref 53 is shown in Figure
S2f and the corresponding FT in Figure S3d. In solid CuSO4·
5H2O, which crystallizes in the centrosymmetric space group
̅
P1 (No. 2), copper binds four water molecules in the
equatorial plane and two sulfate oxygens in the axial positions
in room-temperature studies.61−66 The mean Cu−Oeq bond
distances in CuSO4·5H2O obtained by EXAFS in this study,
1.959(3) Å (Table 2), is in very good agreement with those in
the reported crystallographic studies. However, for the axial
positions two different Cu−Oax bond distances are discerned,
2.22(1) and 2.38(1) Å (Table 2). The long Cu−Oax,SO4 bond
distance is in good agreement with that reported distances in
the crystallographic studies,61−66 while the short Cu−Oax,SO4
bond distance obtained in this EXAFS study is significantly
shorter. The fit of the raw EXAFS data and the Fourier
transforms of the studied solid copper(II) hydrates are shown
in Figures 2 and 3, respectively, and the refined structure
parameters are given in Table 2. Also in this case it is proven
that the crystallographically determined Cu−O bond distances
are not in full agreement with the experimental EXAFS data as
shown in Figure S2g and the corresponding FT data in Figure
S3e.
Figure 3. Fit of Fourier transforms of EXAFS data of (a) solid
copper(II) sulfate pentahydrate (CuSO4·5H2O, offset 1.0), (b) solid
hexaaquacopper(II) hexafluorosilicate ([Cu(H2O)6]SiF6, offset 0.8),
(c) solid hexaaquacopper(II) bromate ([Cu(H2O)6](BrO3)2, offset
0.6), (d) solid hexaaquacopper(II) nitrate ([Cu(H2O)6](NO3)2,
offset 0.4), (e) hexaaquacopper(II) perchlorate (Cu(NO3)2·2.5H2O,
offset 0.2), and (f) aqueous solution of copper(II) trifluoromethane-
sulfonate (no offset).
parameters,56,57 while the third study differs significantly58
(Table S7). In the first reported structure,56 the hydrated
copper(II) ion is six-coordinate with the expected Cu−Oeq
bond distances for a Jahn−Teller distorted complex. By a fit
the present EXAFS data with the reported Cu−O bond
distances fixed, the fit is reasonable but the axial distances are
clearly wrong with a very large Debye−Waller coefficient, σ2 =
0.05 Å2, and a remaining wave in the residual (Figure S2a); the
corresponding Fourier transform (FT) is shown in Figure S3a.
The other two reported structures57,58 propose pairwise Cu−O
bond distances at 2.090, 2.164, and 2.280 Å and 1.969, 2.084,
and 2.202 Å, respectively, which are in strong contradiction to
most other reported hexaaquacopper(II) complexes (Table
S1). The calculated EXAFS spectra based on the reported Cu−
O bond distances are almost completely out of phase in
comparison to the experimental EXAFS data (Figure S2b,c)
and the corresponding FTs in Figure 3a.
The crystallographic studies of [Cu(H2O)6](BrO3)2 and
[Cu(H2O)6]SiF6 report a regular octahedral structure of the
hexaaquacopper(II) ion with Cu−O bond distances of 2.079
and 2.074 Å,59,60 respectively, but they have been proven to
have a nonregular octahedral geometry as reported before.34
The calculated EXAFS functions on these structures are
completely out of phase with the experimental data (Figure
S2d,e) and the corresponding FTs in Figure S3b,c, showing
that the reported structures from crystallography cannot be
correct as discussed above and before.34
The crystal structure of Cu(NO3)2·2.5H2O has been
reported in three papers.53−55 Copper(II) binds two water
molecules and two nitrate oxygens in the equatorial positions
at Cu−O bond distances of 1.959 and 1.986 Å, respectively
and additionally three nitrate oxygens at longer distances, one
at 2.391 Å and two close to 2.66 Å53 (Table S7b). The other
two determinations report slightly different Cu−O bond
distances, 2.476 and two close to 2.62 Å (Table S7b).54,55
The fit of the EXAFS data gave a mean Cu−Oeq bond distance
of 1.959 Å and three longer Cu−Oax/Cu···N distances to
nitrate ions at 2.183(5), 2.398(5), and 2.734(8) Å. The Cu−
Oeq bond distance obtained in this EXAFS study is slightly
shorter (1.959(3) Å) than the mean distances in the reported
crystallographic investigations (1.972 and 1.990 Å)53,55 but is
in agreement with the Cu−Oeq distances in aqueous solution
These results show that the structure of the [Cu(H2O)6]2+
complex in solid [Cu(H2O)6(ClO4)2, [Cu(H2O)6](BrO3)2,
and [Cu(H2O)6]SiF6, the [Cu(OH2)2(ONO2)2]·unit in Cu-
(NO3)2·2.5H2O, and the [Cu(H2O)4(OSO3)2] unit in CuSO4·
5H2O reported from crystallographic studies cannot be
confirmed by the lattice-independent EXAFS method. All of
the analyzed solid-state structures were solved in centrosym-
metric space groups, which required pairwise equal Cu−O
bond distances or a regular octahedral geometry. It therefore
seems likely that the Cu−Oax bonds in these compounds also
may have different bond lengths, according to the EXAFS
results, and that they are randomly oriented in the solid state in
either one dimension, resulting in an apparent centrosym-
metric Jahn−Teller distorted-octahedral configuration, or in
three dimensions, resulting in regular or slightly distorted
octahedra. As a consequence, it seems likely that most of the
reported crystal structures containing a hexaaquacopper(II)
ion crystallizing in a centrosymmetric space group actually
have a similar bond distance distribution in the axial positions,
as observed in aqueous solution and the five solids analyzed in
this study. It is therefore important to complement crystallo-
graphic studies with a lattice-independent method such as
EXAFS to secure that a high-symmetry model indeed is the
correct one for complexes which may have a lower symmetry.
Coordination geometries where such a check is vital are those
for metal complexes with Jahn−Teller distorted-octahedral and
capped-trigonal-prismatic configurations.
Structure of the Solvated Copper(II) Ion in Methanol
and Six Amide Solvents Studied by EXAFS. The structure
of the solvated copper(II) ion in methanol, dmf, def, dma, dea,
dmp, and dep show the same pattern as in water with four
solvent molecules in the equatorial positions at ca. 1.96 Å and
two significantly different axial Cu−O bond distances. No
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Figure 4. Fit of raw EXAFS data of copper(II) trifluoromethanesulfonate in (a) N,N-dimethylpropylene urea (dmpu, offset +18), (b) N,N,N’N’-
tetramethylurea (tmu, offset +16), (c) N,N-diethylpropionamide (dep, offset +14), (d) N,N-dimethylpropionamide (dmp, offset +12), (e) N,N-
diethylacetamide (dea, offset +10), (f) N,N-dimethylacetamide (dma, offset +8), (g) N,N-diethylformamide (def, offset +6), (h) N,N-
dimethylformamide (dmf, offset +4), (i) methanol (offset +2), and (j) aqueous solution acidified with trifluoromethanesulfonic acid.
homoleptic copper(II) methanol solvate has been reported in
the solid state,4 and only four copper(II) complexes with four
bound methanol molecules have been reported.67−70 The
mean Cu−Oeq bond distance in the methanol-solvated
copper(II) ion in solution has been refined to 1.975(4) Å,
and the Cu−Oax bond distances have been refined to 2.202(8)
and 2.34(1) Å. The mean Cu···C distance and the Cu−Oeq−C
three-legged scattering path were refined to 3.03(2) and
3.18(4) Å, respectively, which corresponds to a Cu−O−C
bond angle of 128(2)°, assuming a O−C bond distance of 1.39
Å, which is in full agreement with the bond angles in the
reported solid solvates.67−70 Zitolo and D’Angelo have
reported an EXAFS and MXAN analysis of the methanol
solvated copper(II) ion in solution. They tested two models, a
tetragonally elongated square-pyramidal and a centrosymmet-
ric Jahn−Teller distorted-octahedral configuration, showing
that the tetragonally elongated square pyramid did fit best with
a mean Cu−Oeq bond distance of 1.98 Å, a Cu−Oax distance of
2.27 Å, and a mean Cu−Oeq−C bond angle of 129.5°.26 They
reported also that the dimethyl sulfoxide and acetonitrile
solvated copper(II) ions also have a tetragonally elongated
square-pyramidal configuration in solution.26,27
radii of copper and oxygen.37,77 The results of the EXAFS
studies of the def-, dma-, dea-, dmp-, and dep-solvated
copper(II) ion in the respective solvents are fully in line with
the results obtained in dmf. The fit of raw EXAFS data and
Fourier transforms are shown in Figures 4 and 5, respectively,
Six structures of solid compounds containing a hexakis-
(dmf)copper(II) ion have been reported, three of which
display a Jahn−Teller distorted-octahedral configuration,71−73
while the other three have an irregular octahedral geome-
try.74−76 The mean Cu−Oeq bond distance in the dmf-solvated
copper(II) ion in dmf solution has been refined to 1.963(3) Å,
and the Cu−O bond distances in the axial positions have been
refined to 2.178(8) and 2.33(1) Å. The Cu···C distance and
the Cu−O−C three-legged scattering path have been refined
to 2.855(7) and 3.03(2) Å, respectively, which corresponds to
a Cu−O−C bond angle of 124(1)°, assuming a OC bond
distance of 1.25 Å, which is in full agreement with the bond
angles in the reported structures containing hexakis(dmf)-
copper(II) ions.71−76
No structures of any hexacoordinate copper(II) complex
with the remaining amide solvents used in this study have been
reported.4 In the structure of tetrakis(dma)copper(II)
perchlorate, four dma molecules are bound in the equatorial
positions, and perchlorate oxygens are present in the axial
positions at long distance but shorter than the van der Waals
Figure 5. Fit of Fourier transforms of EXAFS data of (a) N,N-
dimethylpropylene urea (dmpu, offset +1.8), (b) N,N,N′,N′-
tetramethylurea (tmu, offset +1.6), (c) N,N-diethylpropionamide
(dep, offset +1.4), (d) N,N-dimethylpropionamide (dmp, offset +1.2),
(e) N,N-diethylacetamide (dea, offset +1.0), (f) N,N-dimethylaceta-
mide (dma, offset +0.8), (g) N,N-diethylformamide (def, offset +0.6),
(h) N,N-dimethylformamide (dmf, offset + 0.4), (i) methanol (offset
+0.2), and (j) aqueous solution acidified with trifluoromethanesul-
fonic acid.
and the refined structure parameters are given in Table 3. The
Cu−Oeq bond distances in the dmf- and def-solvated
copper(II) ions are slightly longer than in the dma, dea,
dmp, and dep copper(II) solvates (Table 3). The Cu−O−C
angles in the dmf and def copper(II) solvates, 124(1)°, are
significantly smaller than the dma, dea, dmp and dep angles,
130(1)° (Table 3). This pattern is also seen in the solid
structures of dmf- and dma-solvated copper(II) ions with mean
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Table 3. Summary of Refinements of EXAFS Data Using a Noncentrosymmetric Tetragonally Elongated Octahedral Geometry
of the Solvated Copper(II) ion in Water (aq), Methanol (M), N,N-Dimethylformamide (dmf), N,N-Dimethylacetamide (dma),
N,N-Dimethylpropionamide (dmp), N,N-Diethylformamide (def), N,N-Dimethylacetamide(dea), N,N-Dimethylpropionamide
a
(dep), N,N′-Dimethylpropyleneurea (dmpu), and N,N,N′,N′-Tetramethylurea (tmu)
water
methanol
dmf
def
dma
dea
dmp
dep
dmpu
tmu
d(Cu−Oeq)
σ2(Cu−Oeq)
n(Cu−Oeq)
1.957(3)
0.0055(3)
4
1.975(3)
0.0064(3)
4
1.963(3)
0.0061(3)
4
1.967(3)
0.0058(3)
4
1.956(3)
0.0060(3)
4
1.956(3)
0.0061(3)
4
1.952(3)
0.0062(3)
4
1.951(3)
0.0058(3)
4
1.939(3)
0.0057(3)
4
1.935(3)
0.0060(3)
4
d(Cu−Oax1
)
)
2.14(2)
0.007(2)
1
2.202(8)
0.0069(8)
1
2.178(8)
0.0068(8)
1
2.186(3)
0.007(1)
1
2.14(1)
0.007(2)
1
2.17(1)
0.007(2)
1
2.16(3)
0.011(2)
1
2.11(4)
0.014(4)
1
σ2(Cu−Oax1
n(Cu−Oax1
)
d(Cu−Oax2
)
)
2.32(2)
0.009(2)
1
2.34(1)
0.007(2)
1
2.33(1)
0.008(1)
1
2.34(1)
0.007(2)
1
2.32(2)
0.010(2)
1
2.34(2)
0.014(3)
1
2.30(4)
0.014(5)
1
2.27(4)
0.013(5)
1
σ2(Cu−Oax2
n(Cu−Oax2
)
d(MS(CuO4))
σ2(MS(CuO4))
n(MS(CuO4))
d(Cu−C)
3.914
0.012(3)
3*4
3.951
0.011(3)
3*4
3.03(2)
0.013(2)
4
3.926
0.012(2)
3*4
2.855(7)
0.009(4)
4
3.935
0.012(2)
3*4
2.859(9)
0.009(1)
4
3.912
0.009(3)
3*4
2.93(1)
0.011(1)
4
3.913
0.009(2)
3*4
2.92(1)
0.011(1)
4
3.904
0.011(4)
3*4
2.94(1)
0.012(1)
4
3.902
0.009(4)
3*4
2.94(1)
0.011(2)
4
3.878
0.009(3)
3*4
2.85(1)
0.014(2)
4
3.870
0.014(5)
3*4
2.91(2)
0.013(3)
4
σ2(Cu−C)
n(Cu−C)
d(Cu−O−C)
σ2(Cu−O−C)
n(Cu−O−C)
∠Cu−O−C
d(Cu−N)
3.18(4)
0.026(5)
8
3.03(2)
0.011(3)
8
123.9(3)
3.99(1)
0.009(4)
4
3.03(3)
0.013(2)
8
123.9(3)
3.99(2)
0.010(2)
4
3.07(2)
0.010(1)
8
130.9(4)
4.00(2)
0.010(2)
4
3.06(2)
0.011(2)
8
3.07(1)
0.010(1)
8
130.3(4)
4.01(2)
0.009(3)
4
3.08(1)
0.009(1)
8
132.2(6)
4.01(3)
0.010(3)
4
3.02(4)
0.009(2)
8
3.09(1)
0.011(1)
8
127.9(5)
131.8(6)
4.01(2)
0.010(2)
4
σ2(Cu−N)
n(Cu−N)
d(Cu−N−C)
σ2(Cu−N−C)
n(Cu−N−C)
S0
E0
F
4.27(4)
0.006(4)
8
1.10(4)
8989.8(5)
16.13
4.29(4)
0.006(4)
8
1.07(4)
8991.1(5)
19.73
4.24(5)
0.012(5)
8
1.09(4)
8989.9(5)
14.95
4.25(4)
0.012(4)
8
1.10(4)
8988.9(5)
12.91
4.24(7)
0.015(9)
8
1.10(4)
8989.5(5)
17.09
4.22(6)
0.012(7)
8
0.95(5)
8989.5(5)
2
0.93(4)
8989.7(5)
18.14
1.09(4)
8990.0(5)
18.26
0.92(3)
8988.2(5)
20.93
0.97(4)
8987.3(5)
23.85
16.80
a
2
Definitions: distance; d/Å, Debye−Waller coefficient, σ2/Å2; number of distances, n; amplitude reduction factor, S0 ; threshold energy, E0/eV;
weighted F factor, F. Estimated errors within parentheses includes the statistical errors from the refinements and estimated experimental errors.
Parameters without following parentheses have been fixed. The linear multiple scattering distance with in the CuO4 equatorial plane has been
b
linked to the Cu−O bond distance.
Cu−O−C bond angles of 124.5 and 129.6°, respectively.71−77
With an alkyl group binding to the amide carbon, the electron
density on the oxygen becomes slightly smaller and the binding
character slightly more electrostatic in comparison to
formamides with a hydrogen binding to the amide carbon.
The fits of raw EXAFS data and the Fourier transform of the
methanol and amide solvent copper(II) solvates are shown in
Figures 4 and 5, respectively, and the refined structure
parameters are given in Table 3.
Test of Different Models in the Refinements of EXAFS
Data. The refined structure parameters of the solvated
copper(II) ions using three models, a tetragonally elongated
square pyramid and centrosymmetric and noncentrosymmetric
tetragonally elongated octahedra, need to be compared. The
solid compounds have all a well-defined composition, which
suggests that a five-coordinate complex can be excluded. A
comparison of the two Jahn−Teller distorted models shows
that the mean Cu−Oax bond distance in the centrosymmetric
model becomes fairly close to the average of the two Cu−Oax
bond distances in the noncentrosymmetric model (Table 2 and
Table S6). Furthermore, the Debye−Waller coefficients, σ2,
become unreasonably large, strongly indicating that the
distance consists of at least two separated distances. Applying
the noncentrosymmetric model with two well-separated Cu−
Oax bond distances results in the refined Debye−Waller
coefficients are within the expected values for bond distances
of this kind, 0.005−0.009 Å−1. This strongly indicates that the
Cu−Oax bond distances in the studied solid hydrates and
solvates are different and are well separated with a difference of
0.15−0.20 Å (Tables 2 and 3).
For the studied solutions the tetragonally elongated square-
pyramidal model has to be included as well in this comparison.
Applying the centrosymmetric octahedral model results in very
large Debye−Waller coefficients for the Cu−Oax bond
distances as found in the solid hydrate samples discussed
above (Table S8a). When the square-pyramidal model is
applied, the Cu−Oax bond distance becomes close to the value
obtained by using the centrosymmetric octahedral model but
the Debye−Waller coefficients becomes significantly smaller,
0.009−0.014 Å2 (Table S8b), but still of an order indicating a
wide bond distance distribution for a single bond.
The pattern of bond distances and Debye−Waller
coefficients and the fact that the EXAFS spectra of solid
[Cu(H2O)6](ClO4)2, Cu(NO3)2·2.5H2O, [Cu(H2O)6]-
(BrO3)2, and [Cu(H2O)6]SiF6 and the aqueous solutions of
copper(II) superimpose (Figure 1) strongly indicate the
hydrated copper(II) ion and the solvated copper(II) ion in
methanol and the six amide solvents bind six solvent molecules
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in a noncentrosymmetric tetragonally elongated octahedral
geometry. This model is in full agreement with the observation
using EPR that the hydrated copper(II) ion is noncentrosym-
metric in aqueous solution6,7 and is in full agreement with the
EXAFS studies reported by Frank et al.13,22,25 The survey of
structure determinations of solid compounds containing a
hexaaquacopper(II) ion or a pentaaqua(O-ligand)copper(II)
complex described in the Introduction strongly indicates that
individual copper(II) complexes in fact are six-coordinate and
noncentrosymmetric. It is therefore very important to apply a
non-lattice-dependent method such as EXAFS to determine
the structures of such compounds.
Structure of the Solvated Copper(II) Ion in dmpu and
tmu Studied by EXAFS. In the solvents which are space
demanding upon coordination, dmpu and tmu, the axial
positions are protected by the ligands in the equatorial
positions (see below), resulting in square-planar complexes. It
was not possible to locate any more solvent molecules within a
reasonable bond distance. The Cu−O bond distances in the
dmpu- and tmu-solvated copper(II) ions are significantly
shorter, 1.939(3) and 1.935(3) Å, respectively, than the
shortest Cu−Oeq bond distances in the hexasolvated copper-
(II) ions, 1.954(3) Å, for acetamide and propionamide (Table
3) but are in agreement with four-coordinated copper(II)
complexes with oxygen donor ligands (Table S3). The
copper(II) trifluoromethanesulfonate solutions in dmpu and
tmu have an intense dark green color due to a strong
absorption band in the UV region and another at around 800
nm, both tailing into the visible region (Figure S1).
Structure of the dmpu-Solvated Copper(II) Ion in
dmpu Solution Studied by LAXS. The radial distribution
function of a LAXS study of a 0.8 mol dm−3 copper(II)
trifluoromethanesulfonate solution shows a peak just below 2
Å assigned to the Cu−O bond distance and refined to 1.93(2)
Å (Figure 6). The LAXS model used includes interatomic
interactions between the copper(II) ion and the solvating
ligands and interactions within the solvent molecules and
within the anions.
Crystal Structure of [Cu(dmpu)4](CF3SO3)2·dmpu (1).
The crystal structure of 1 consists of tetrakis(N,N’-
dimethylpropyleneurea)copper(II) and trifluoromethanesulfo-
nate ions, and with an additional noncoordinated N,N′-
dimethylpropyleneurea molecule in the lattice. The almost
planar dmpu molecules are stacked perpendicular to the
equatorial plane with two crystallographically independent
ligands having Cu−O bond distances of 1.900(3) and 1.902(3)
Å, with Cu−O−C angles of 127.3(3) and 129.5(3)°,
respectively, forming a regular square-planar coordination
(Figure 7). The low quality of the crystal made it necessary to
add additional loose structural restraints to obtain sound
results. For both of the ligand molecules, it was possible to
refine two alternate positions, C14A/B and C24A/B, for the
middle propylene carbon with partial occupancies at 73/27
and 53/47%, respectively, whereas the other ligand molecule
showed the typical half-chair formation partial distribution
refinement of its carbon, C14. The copper(II) methyl carbon
distances Cu−C are approximately 3.7(1) Å, at roughly 45°
from the flat CuO4 plane. Bond distances and angles in the
trifluoromethanesulfonate ions are in agreement with those in
previous studies, but due to disorder alternative positions were
added.4 The noncoordinated dmpu oxygen is 7.8 Å away from
the copper(II) ion. Selected crystallographic data are given in
Table 2, and selected bond distances and angles are given in
Figure 6. (Top) LAXS radial distribution curves for a 0.8 mol dm−3
copper(II) trifluoromethanesulfonate dmpu (top) separate model
contributions (offset 14) of the tetrakis(dmpu)copper(II) complex
(solid line), the trifluoromethanesulfonate ion (dashed line), and
dmpu solvent (dotted line); (middle) experimental RDF with D(r)
−4pr2ρ0 (solid line), sum of model contributions (dashed line), and
difference (dotted line); (bottom) reduced LAXS intensity functions
s.i(s) (thin line) and model s.icalc(s) (thick line).
Table 4. By a view perpendicular to the CuO4 plane of the
tetrakis(N,N’-dimethylpropyleneurea)copper(II) ion it is
clearly seen that the dmpu methyl groups effectively hinder
access of ligands to the axial positions. Furthermore, the
methyl groups near the axial position have no bonding
properties to the copper(II) ion.
Summary of the Coordination Chemistry of the
Hydrated and Solvated Copper(II) Ions. Even though a
large majority of the hexaaquacopper(II) complexes reported
in the solid state are found to be centrosymmetric (Table S1),
it has not been possible to confirm this with the lattice-
independent EXAFS method in this study. It therefore seems
very likely that the direction of the short and long axial Cu−O
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different space-demanding properties on coordination have
been studied by EXAFS spectroscopy. In the solvents with
large space-demanding properties on coordination, dmpu and
tmu, copper(II) is shown to have a square-planar configuration
with mean Cu−O bond distances of 1.93(2) Å. The best fits in
the other solvents are obtained with a noncentrosymmetric
tetragonally elongated octahedral model with different axial
Cu−O bond distances with mean Cu−Oeq, Cu−Oax1, and Cu−
Oax2 bond distances of ca. 1.96, 2.15, and 2.32 Å, respectively.
This model is in full agreement with that proposed by Frank et
al. for the hydrated copper(II) ion from EXAFS studies.13,22,25
However, from the point-of-view of EXAFS data other
configurations such as tetragonally elongated square pyramidal
cannot be ruled out. EPR studies have shown that the hydrated
copper(II) ion in aqueous solution is noncentrosymmetric.7,8
A large number of number of theoretical simulations have
concluded that [Cu(H2O)5]2+ and [Cu(H2O)5···H2O]2+
configurations are more stable in aqueous solution in
comparison to the centrosymmetric Jahn−Teller distorted-
octahedral configuration.11,12,15,16,21,23 An in-depth survey of
solid-state structures containing isolated hydrated copper(II)
ions has shown that hexaaquacopper(II) complexes crystalliz-
ing in noncentrosymmetric space groups as well as in
centrosymmetric space groups where the center of symmetry
is not on the copper in the [Cu(H2O)6]2+ ion display
significantly different Cu−O bond distances in the axial
positions (Table S1). Furthermore, all of the [Cu(H2O)5(O-
ligand)] complexes displaying Jahn−Teller distortion show the
same splitting in the Cu−Oax bond distances independent of
the position of the O-ligand (Table S2). EXAFS spectra of
aqueous copper(II) solutions and the solids [Cu(H2O)6]-
(ClO4)2, [Cu(H2O)6](BrO3)2, and [Cu(H2O)6]SiF6 are
superimposable, strongly indicating that the hydrated copper-
(II) ions in these salts have the same or very similar
configurations around copper as in aqueous solution. This is
in spite of the fact that the crystallographic studies report
centrosymmetric structures with a regular octahedral config-
uration for the last two complexes. This strongly indicates that
copper(II) complexes reported to be centrosymmetric in the
solid state are mean structures of noncentrosymmetric entities
with random orientation of the copper(II) complex in the
individual unit cells. A summary of the present information
from different methods and publications is as follows.
Figure 7. Structure of the tetrakis(dmpu)copper(II) unit in 1.
Ellipsoids for all non-hydrogen atoms are set at 40% probability.
Table 4. Selected Bond Distances (Å) and Angles (deg)
from the Crystal Structure Determination of Solid
[Cu(dmpu)4](CF3SO3)2·dmpu
bond distance
bond angle
Cu−O1
Cu−O2
1.900(3)
1.902(3)
O1−Cu−O2
O1−Cu−O2′
90.15(14)
89.86(14)
bond distances can be randomly distributed in solid
compounds, resulting in a mean structure with higher
symmetry in comparison to the symmetry of the individual
complexes. This seems also to be the case for hexaaquacopper-
(II) ions reported to have a regular or somewhat irregular
octahedral geometry, as some of these have been shown to
have the expected Jahn−Teller distortion (vide supra). Of the
complexes without a center of symmetry or with the center of
symmetry not on copper in the [Cu(H2O)6]2+ complex, only a
few display almost similar Cu−Oax distances, while the
remaining structures display significant different Cu−Oax
distances (Table S1). Furthermore, all reported [Cu-
(H2O)5(O-donor ligand)] complexes with Jahn−Teller
distortion display significantly different Cu−Oax distances
independent of whether the O-donor ligand is in equatorial
or axial position (Table S2). The high k range EXAFS data
collected in this study and by Frank et al.22 allows a high
resolution of distances. Using a model with two different axial
Cu−Oax bond distances resulted in stable refinements
independent of the system. This includes all solvents where
copper(II) is six-coordinate, all studied solvents except tmu
and dmpu, and the solids [Cu(H2O)6](ClO4)2, [Cu(H2O)6]-
(BrO3)2, [Cu(H2O)6]SiF6, Cu(NO3)2·2.5H2O, and CuSO4·
5H2O. The similar UV−vis spectra of the hydrated copper(II)
ion in aqueous solution and the solvated copper(II) ion in
methanol, dmf, def, dma, dea, dmp, and dep further support a
similar configuration (Figure S1).
• The hydrated copper(II) ion in aqueous solution is six-
coordinate with a noncentrosymmetric tetragonally
elongated octahedral geometry, and the difference in
the Cu−Oax bond distances is on the order of 0.2 Å,
• The solvated copper(II) ion in the seven organic oxygen
donor solvents without steric demands on coordination
have the same configuration in solution as that of the
hydrated copper(II) ion.
• The solvated copper(II) ion has a four-coordinate
square-planar configuration in space-demanding solvents
on coordination, as the axial positions become
inaccessible.
• It seems very likely that all six-coordinate copper(II)
complexes reported to be centrosymmetric in the solid
state have a noncentrosymmetric tetragonally elongated
octahedral configuration with a random orientation of at
least the axial ligands, but as crystallography gives the
mean of all unit cells in the solid, the mean structure
becomes centrosymmetric.
CONCLUSIONS
The structure of the solvated copper(II) ion in 10 oxygen
donor solvents with similar bonding characteristics but with
■
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00403.
Summary of reported solid-state structures with
hydrated copper(II) ions, complexes with the composi-
tion [Cu(H2O)5(O-ligand)], and four- and five-coor-
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AUTHOR INFORMATION
■
Corresponding Author
Ingmar Persson − Department of Molecular Sciences, Swedish
University of Agricultural Sciences, SE-750 07 Uppsala,
Sweden; orcid.org/0000-0002-1061-7536;
Email: ingmar.persson@slu.se
Authors
Daniel Lundberg − Department of Molecular Sciences, Swedish
University of Agricultural Sciences, SE-750 07 Uppsala, Sweden
́
́
Eva G. Bajnoczi − Department of Molecular Sciences, Swedish
University of Agricultural Sciences, SE-750 07 Uppsala,
Sweden; orcid.org/0000-0002-8469-5887
Konstantin Klementiev − MAX IV Laboratory, Lund
University, SE-221 00 Lund, Sweden
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A., III Hydration of copper(II): New insights from density functional
theory and the COSMO solvation model. J. Phys. Chem. A 2008, 112,
9104−9112.
Justus Just − MAX IV Laboratory, Lund University, SE-221 00
Lund, Sweden
Kajsa G. V. Sigfridsson Clauss − MAX IV Laboratory, Lund
University, SE-221 00 Lund, Sweden
(16) de Almeida, K. J.; Murugan, N. A.; Rinkevicius, Z.; Hugosson,
H. W.; Vahtras, O.; Agren, H.; Cesar, A. Conformations, structural
transitions and visible near-infrared absorption spectra of four-, five-
and six-coordinated Cu(II) aqua complexes. Phys. Chem. Chem. Phys.
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
(19) Bowron, D. T.; Amboage, M.; Boada, R.; Freeman, A.; Hayama,
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the Swedish Research Council for
financial support. The MAX IV laboratory, Lund University, is
acknowledged for allocation of beam time during the
commissioning period for these studies. Dr. Andrew Ken
Inge and Mr. Erik Svensson Grape, Department of Materials
and Environmental Chemistry, Stockholm University, are
acknowledged for the determination of unit cell parameters
of the solid compounds used in this study.
K
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