Oxido-pincer complexes of copper(II) - An EXAFS and EPR study of mono- and binuclear [(pydotH2)CuCl2]n (n = 1 or 2)

Article abstract of DOI:10.1016/j.poly.2011.10.023

The oxido-pincer ligand pydotH₂ (2,6-bis(1-hydroxy-1-o-tolyl- ethyl-η²O,O′)pyridine) forms two different Cu^II containing complexes when prepared from anhydrous CuCl₂. A combination of EPR spectroscopy and EXAFS allowed to structurally characterise the light-green dimer of the formula [(pydotH₂)CuCl(μ-Cl) ₂ClCu(pydotH₂)] and the penta-coordinate olive-green monomer [(pydotH₂)CuCl₂]. The molecular entities imply that the ligand remains protonated upon coordination. When dissolved in DMF both compounds form monomeric species [(pydotH₂)CuCl₂(DMF)] which could be characterised in detail by EPR, UV-Vis/NIR spectroscopy and electrochemical measurements. The assignments were supported by comparison with Cu^II complexes of the related ligands 2,6-bis(hydroxymethyl)pyridine (pydimH₂) and 2,6-bis(1-hydroxy-1-methyl)pyridine (pydipH ₂).

Full text of DOI:10.1016/j.poly.2011.10.023

Polyhedron 31 (2012) 649–656
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Polyhedron
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Oxido-pincer complexes of copper(II) – An EXAFS and EPR study of mono- and
binuclear [(pydotH2)CuCl2]n (n = 1 or 2)
Axel Klein a, , Katharina Butsch a,1, Sait Elmas a, Christian Biewer a, Dominikus Heift a,2, Sara Nitsche a,
⇑
Irene Schlipf b, Helmut Bertagnolli b
a Universität zu Köln, Department für Chemie, Institut für Anorganische Chemie, Greinstrasse 6, D 50939 Köln, Germany
b Universität Stuttgart, Institut für Physikalische Chemie, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
a r t i c l e i n f o
a b s t r a c t
The oxido-pincer ligand pydotH2 (2,6-bis(1-hydroxy-1-o-tolyl-ethyl-g2O,O0)pyridine) forms two differ-
Article history:
ent CuII containing complexes when prepared from anhydrous CuCl2. A combination of EPR spectroscopy
Received 11 January 2011
Accepted 19 October 2011
Available online 28 October 2011
and EXAFS allowed to structurally characterise the light-green dimer of the formula [(pydotH2)CuCl(l-
Cl)2ClCu(pydotH2)] and the penta-coordinate olive-green monomer [(pydotH2)CuCl2]. The molecular
entities imply that the ligand remains protonated upon coordination. When dissolved in DMF both com-
pounds form monomeric species [(pydotH2)CuCl2(DMF)] which could be characterised in detail by EPR,
UV–Vis/NIR spectroscopy and electrochemical measurements. The assignments were supported by com-
parison with CuII complexes of the related ligands 2,6-bis(hydroxymethyl)pyridine (pydimH2) and 2,6-
bis(1-hydroxy-1-methyl)pyridine (pydipH2).
Keywords:
Oxido-pincer ligand (O,N,O ligand)
Copper(II)
EPR spectroscopy
EXAFS
Coordination geometry
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
geometry and composition of the products is difficult and hindered
by the paramagnetic character of CuII complexes disabling rapid
Lying on the edge between the d block metals and the main
group elements, copper is one of the most studied transition metals.
One can trace back the high interest in this metal and the vast
number of applications to mainly two fundamental properties.
First, the element can easily shuttle between the two oxidation
states CuI and CuII, which explains e.g. the frequent use of copper
in redox-active metalloenzymes [1–4]. The reason for this is that,
although CuI with a d10 configuration is the intrinsically more stable
form, suitable ligand surrounding or even solvation can markedly
stabilise CuII (d9). This is combined with the second aspect, which
is the high flexibility of both copper oxidation states in coordination
number and geometry. While for CuI in unrestrained and non bio-
logical surrounding the major coordination geometry is tetrahedral,
the d9 configuration of CuII leads to the well-known Jahn-Teller dis-
tortion (or related distortions) and as a consequence numerous
coordination numbers (4–6) and geometries (square planar, square
pyramidal, tetragonally elongated or compressed octahedral, trigo-
nal bipyramidal) were observed for CuII complexes [4–8]. As a con-
sequence, when preparing new CuII complexes, prediction of the
structure elucidation by NMR spectroscopy. Frequently, this prob-
lem has been overcome by ligand design. E.g., the proper design
of multidentate ligands allows to predict (design) the coordination
geometry, provided the binding between the ligands donor atoms
and the metal is strong [3,5,6,9–11].
From the vast group of multidentate ligands, the so called O,N,O
oxido pincer ligands (Scheme 1) provide a scaffold to form CuII
complexes which is interesting for several reasons. Firstly,
although the oxido donor function might be varied to a large ex-
tend as depicted in Scheme 1, a relatively weak bonding of the pen-
dant (hard) oxido donor functions to late (soft) transition metals as
CuII can be expected. Secondly, these pincer ligands, which can
additionally be substituted in various positions, prefer a meridio-
nal binding.
Of special interest in this respect are the ligands based on
pyridine-2,6-dimethanol (pydimH2; Scheme 1) since they can bind
in their protonated or deprotonated form and thus provide an
interesting acid–base chemistry (including potential catalytic
applications) [12–15], as well as a variety of different coordination
possibilities. Fully or partly deprotonated pydimHꢀ or pydim2ꢀ
ligands were found to form oligomeric complexes as e.g.,
[Cu2(pydimH)2(pydimH2)2]2+ [16–18], [Cu2(g2 3-py-
,l-pydimH)2(g
⇑
Corresponding author. Tel.: +49 221 4704006; fax: +49 221 4705193.
E-mail address: axel.klein@uni-koeln.de (A. Klein).
Present address: Stanford University, Department of Chemistry, Stack Lab 333
dimH2)2]2+, [Cu(pydimH2)(pydimH)]+ [17] or clusters such as
[Cu3(O2CMe)2(pydim)2(MeOH)] and [Cu4(O2CMe)2(pydimH)4]2+
[19], and [Cu4(NO3)2(pydimH)4(H2O)(MeOH)] [20]. The latter are
interesting magnetic materials. Even in the fully protonated form,
1
N–S Axis Stauffer 2 107, Stanford, CA 94305-5080, United States.
2
Present address: ETH Hönggerberg, Laboratorium für Anorganische Chemie
Wolfgang-Pauli-Str. 10, HCI-H114 8093 Zürich, Switzerland.
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.10.023

650
A. Klein et al. / Polyhedron 31 (2012) 649–656
measured on Varian Cary50 Scan or Shimadzu UV-3600 photo spec-
R
R'
R
R'
trometers. EPR spectra were recorded in the X band on a Bruker Sys-
tem ELEXSYS 500E equipped with a Bruker Variable Temperature
Unit ER 4131VT. Usually, single spectra were recorded at low acqui-
sition times with a microwave power of 10 mW (or lower) and
modulation amplitudes ranging between one and four Gauss, g val-
ues were calibrated using a dpph sample. Spectral simulation was
carried out using the WINEPR Simfonia program from Bruker. Elec-
trochemical experiments were carried out in 0.1 M nBu4NPF6 solu-
O
O
R
R
N
N
N
OH
O
OH
O
O
OH
O
OH
pydimH2
pydik
pydicH2
R
R'
R
R'
O
R
O
R
R = alkyl or aryl
N
N
O
O
tions using
a three-electrode configuration (glassy carbon
pydicR2
pydimR2
R
R
electrode, Pt counter electrode, Ag/AgCl reference) and an Autolab
PGSTAT30 potentiostat and function generator. The ferrocene/ferro-
cenium (FeCp2/FeCp2+) couple served as internal reference.
Scheme 1. Various types of oxido pincer ligands providing
a O,N,O donor
set (pydicH2 = pyridine-2,6-dicarboxylic acid; pydik = pyridine-2,6-diketone;
pydimH2 = pyridine-2,6-dimethanol.
2.2. X-ray absorption spectroscopy
the combination of pydimH2 and CuII can lead to a number of dif-
ferent complexes, depending on the CuII/pydimH2 ratio and addi-
tional coligands. The homoleptic complex [Cu(pydim)2]2+ has
been reported with a number of counter anions such as Clꢀ,
The EXAFS and XANES measurements were performed at the
XAS beamline at the Ångströmquelle Karlsruhe (ANKA) under
ambient conditions at 20 °C. The synchrotron beam current was
ꢀ
ꢀ
ClO4 [17], NO3 [20], propionate [21], saccharinate (1,1-dioxo-
1,2-benzo[d]isothiazol-3-one-2-yl) [22], or niflumate (2-{[3-(tri-
fluoromethyl)phenyl]amino}nicotinate) [23]. In the presence of
further tridentate ligands such as pyridine-2,6-dicarboxylate octa-
hedral complexes were formed e.g., [(pydimH2)Cu(pydic)] [24].
Our group has recently reported that alternatively to the estab-
lished hexa-coordinate species [Cu(O,N,O)2]2+ penta-coordinate CuII
complexes [(O,N,O)CuCl2] (O,N,O = pydimH2, pydipH2 or pydotH2;
Scheme 2) can be obtained from reactions of the ligands and
CuCl2ꢁ2H2O. Furthermore, the penta-coordinate complexes were
found to undergo disproportionation reactions: 2½ðO; N; OÞCuCl2ꢂ¡
between 80 and 140 mA at 2.5 GeV storage ring energy.
A
Si(111) double crystal monochromator was used for measure-
ments at the Cu K-edge (8979 keV). The second monochromator
crystal was tilted for optimal harmonic rejection. The energy reso-
lution for the Cu K-edge energy is estimated to be 1.0 eV. The spec-
tra were recorded in transmission mode with ionisation chambers.
The first two ionisation chambers were filled with nitrogen gas, the
third one with a mixture of argon and nitrogen. The individual
pressures were adjusted to optimise the signal-to-noise ratio. En-
ergy calibration was performed with a copper metal foil. To avoid
mistakes in the XANES region arising from small changes in the en-
ergy calibration between two measurements, all spectra were cor-
rected to the theoretical edge energy of copper foil, which was
measured in every scan. The solid samples were embedded in a cel-
lulose matrix and pressed into pellets. The concentration was ad-
2ꢀ
½CuðO; N; OÞ2ꢂ2þ þ ½CuCl4ꢂ in polar solvents [25].
This stands in contrast to the complexes obtained from the phen-
oxy type of O,N,O oxido pincer ligands for which exclusively binu-
clear complexes [(O,N,O)CuCl2]2 were obtained from the reaction
of the ligands and CuCl2. When dissolved in DMF these complexes
transform into mononuclear species of the composition [(O,N,O)-
CuCl2(DMF)] while no disproportionation was observed [26].
Since copper complexes with oxido-pincer ligands are of vari-
ous interest e.g., as models for superoxide dismutases [27], as
Lewis acid catalysts [14,15] or as potential non-steroidal anti-
inflammatory drugs [28] the control over the nuclearity of corre-
sponding species in the solid and in solution is crucial.
Herein we report on a detailed study on the reaction products
obtained from the chiral ligand pydotH2 (Scheme 2) with anhy-
drous CuCl2. The two isolated products as well as species observed
in solution were examined by a combination of EPR, UV–Vis/NIR
spectroscopy, electrochemical measurements and EXAFS. Assign-
ments were further supported by comparison with related com-
plexes of the O,N,O ligands with established molecular structures
[25].
justed to yield an absorption jump of
D
ld ꢃ 1.3.
The data analysis of the measured XAFS spectra was performed
using program packages IFEFFIT (Athena, AUTOBK) and EXCURV98
[29–31]. Data analysis in k-space in the interval k listed in Table 1
was performed using the curved wave theory with XALPHA poten-
tials and phase shifts and the resulting EXAFS function was
weighted with k3. The amplitude reduction factor (AFAC) was
determined by fitting the reference foil and set at 0.9.
2.3. Materials and procedures
The ligands pydimH2, pydipH2 and pydotH2 and the CuII com-
plexes [(pydimH2)CuCl2], [(pydipH2)CuCl2][CuCl4] and [(pydipH2)2-
Cu]Cl2 were synthesised as previously reported [25].
[(pydimH2)CuCl2]: Synthesised from CuCl2ꢁ2H2O and pydimH2
and isolated as turquoise crystals. Yield: 202 mg (74%).
[(pydimH2)CuCl2]ꢁH2O): (Anal. Calc. for C7H11Cl2CuNO3: C, 28.83;
H, 3.80; N, 4.80. Found: C, 28.88; H, 3.80; N, 4.78%); Carefully
ground and dried samples of [(pydimH2)CuCl2] for EPR measure-
ments analysed to (Anal. Calc. for C7H9Cl2CuNO2: C, 30.73; H,
3.32; N, 5.12. Found: C, 30.73; H, 3.30; N, 5.13%).
2. Experimental
2.1. Instrumentation
Elemental analyses were carried out using a HEKAtech CHNS
EuroEA 3000 Analyzer. UV–Vis/NIR absorption spectra were
[Cu(pydipH2)2][CuCl4]: Synthesised from CuCl2ꢁ2H2O and py-
dipH2 and isolated as a blue powder. Yield: 306 mg (93%). (Anal.
CH3
OH
CH3
CH3
CH3
CH3
H3C
N
N
N
*
*
OH
OH
CH3
OH
OH
CH3
OH
pydipH2
pydotH2
pydimH2
Scheme 2. O,N,O oxido pincer ligands used in this study 2,6-bis(hydroxymethyl)pyridine (pydimH2), 2,6-bis(1-hydroxy-1-methyl)pyridine (pydipH2) and 2,6-bis(1-hydroxy-
1-o-tolyl-ethyl)pyridine (pydotH2).

A. Klein et al. / Polyhedron 31 (2012) 649–656
651
Table 1
Structural parameters of the solid Cu complexes determined from the Cu K-edge EXAFS spectra in comparison to XRD data.a
Compound
r (Å)
N
r
(Å)
E0 (eV)
Fit-index k-range (Åꢀ1
)
XRD data (Å)b
[(pydimH2)CuCl2]ꢁH2O
Cu–N
Cu–O
Cu–Cl
1.92(2)
2.02(2)
2.20(3)
2.68(3)
2.81(3)
1.91(2)
2.03(2)
2.17(2)
2.61(3)
2.77(3)
1.93(2)
2.01(2)
2.22(2)
2.55(3)
2.86(3)
3.37(3)
1.96(2)
2.15(2)
4.00(4)
2.86(3)
2.96(3)
1
2
1
1
4
1
2
1
1
4
1
2
2
1
4
1
2
4
2
4
4
0.032(3)
0.084(8)
0.063(6)
0.112(17)
0.077(12)
0.032(3)
0.071(7)
0.100(10)
0.112(17)
0.112(17)
0.039(4)
0.077(8)
0.110(11)
0.112(17)
0.095(14)
0.112(17)
0.059(6)
0.105(11)
0.092(14)
0.081(12)
0.087(13)
7.654
33.28
3.0–14.0
1.929(1)
1.997(2)/2.001(2)
2.2076(7)
2.6960(9)
2.942(2)/2.845(2)/2.832(2)/2.922(2)
Cuꢁ ꢁ ꢁC
Cu–N
Cu–O
Cu–Cl
[(pydotH2)CuCl2]
P1
6.113
6.542
30.34
3.0–13.0
Cuꢁ ꢁ ꢁC
Cu–N
Cu–O
Cu–Cl
[(pydotH2)CuCl(
l-Cl)2ClCu(pydotH2)]
31.90
3.0–13.5
P2
Cuꢁ ꢁ ꢁC
Cuꢁ ꢁ ꢁCu
Cu–N
[Cu(pydipH2)2]Cl2ꢁH2O
5.214
39.53
3.0–13.0
1.957(2)/1.958(2)
2.156(2)/2.173(2)/2.179(2)/2.196(2)
4.088(1)/4.121(1)
2.882(2)/2.871(2)/2.875(2)/2.885(2)
3.019(2)/3.026(2)/3.025(2)/3.039(2)
Cu–O
Cuꢁ ꢁ ꢁCl
Cuꢁ ꢁ ꢁC
Cuꢁ ꢁ ꢁC
a
Absorber-backscatterer distance r, coordination number N, Debye-Waller factor
r, shift of the threshold energy DE0 and the fit-index R for the fitted k-ranges. The
numbers in brackets are uncertainties in the last digits. The errors are estimated to be 1% for distances, 10% for Debye-Waller factors of shells with r < 2.5 Å and 15% for
Debye-Waller factors of shells with r > 2.5 Å. With respect to an unjustified expansion of the evaluation process, the coordination numbers were fixed for each evaluation and
varied manually in steps of one.
b
From ref. [25].
Calc. for C22H34Cl4Cu2N2O4: C, 40.07; H, 5.20; N, 4.25. Found: C,
40.06; H, 5.20; N, 4.23%).
[Cu(pydipH2)2]Cl2ꢁH2O: Synthesised from CuCl2ꢁ2H2O and py-
dipH2 (2 equiv.) and isolated as a turquoise powder. Yield:
500 mg (92%). (Anal. Calc. for C22H36Cl2CuN2O5: C, 48.66; H, 6.68;
N, 5.16. Found: C, 48.65; H, 6.65; N, 5.12%).
reaction solution, first a grass green product (P2) is precipitating
and can be filtered off. When no further grass green precipitate is
observed, the final olive green to brown solution is evaporated to
dryness giving an olive-green solid (P1). Elemental analysis con-
firms the identical elemental composition of the two complexes
and from these observations we assume that P1 and P2 have the
same chemical composition and stand in equilibrium with each
other. While P1 is readily soluble in organic solvents as CH2Cl2,
THF, acetone, DMF or MeCN, P2 dissolves only very slowly in
DMF or MeCN yielding olive green solutions (see absorption spec-
troscopy). From the different solubility we assume that the grass
green product P2 is a binuclear species, while the olive green deriv-
ative P1 is the corresponding monomer.
P2 is assumed to be a chlorido-bridged binuclear complex with
an ‘‘equatorial’’ coordination plane formed by the pyridine N
atoms, all four chlorido ligands and the two CuII ions. This confor-
mation is supposed from the rigidity of the O,N,O ligand, allowing
only equatorial tris-chelate coordination, as has been shown for re-
lated mononuclear complexes of 3d transition metals [17,25,32],
but has also been reported for various other complexes e.g. with
dialkylpyridine-2,6-dicarboxylate (O,N,O) [33] or bis(pyridin-2-
ylmethyl)amino]methyl}-2-phenolate (N3O) ligands [34]. For CuII
complexes of the ligands pydimH2 and pydipH2 (Scheme 2), which
were closely related to pydotH2, exclusively the P1 forms were ob-
served so far. This was unequivocally concluded from the high sol-
ubility in common organic solvents, the UV–Vis/NIR spectra and
from XRD of [(pydimH2)CuCl2]ꢁH2O [25]. Also for NiII, CoII, and ZnII
mononuclear complexes with structures similar as P1 have been
reported by us recently [25].
Synthesis of dichlorido-2,6-bis(1-hydroxy-1-methyl-ethyl-g2O,O0)
pyridinecopper(II) [(pydipH2)CuCl2]: An amount of 268.8 mg
(2 mmol) anhydrous CuCl2 and 390.5 mg (2 mmol) pydipH2 were
dissolved in 30 mL of abs. acetone (or methanol). The mixture
was stirred at room temperature for 2 h, then the solvent was re-
moved under reduced pressure and the residue recrystallised three
times from acetone and pentane to afford a blue powder. Yield:
310 mg (94%). (Anal. Calc. for C11H17Cl2CuNO2: C, 40.07; H, 5.20;
N, 4.25. Found: C, 40.12; H, 5.30; N, 4.26%).
Synthesis of dichloro-2,6-bis(1-hydroxy-1-o-tolyl-ethyl-g2O,O0)
pyridinecopper(II) [(pydotH2)CuCl2]n: An amount of 268.8 mg
(2 mmol) anhydrous CuCl2 and 694.4 mg (2 mmol) pydotH2 were
dissolved in 30 mL of abs. acetone. The mixture was stirred over
night at room temperature, after which an orange–yellow solution
has formed. Then the volume of the solvent was slowly reduced
under reduced pressure to about 10 mL yielding a bright green
solid (P2), while the solution has darkened. Evaporation of the
residual solution gave an olive-green powder, which was washed
with three 20 mL portions of n-pentane and dried (P1). Yield for
P1: 770 mg (80% per Cu atom). (Anal. Calc. for C23H25Cl2CuNO2:
C, 57.32; H, 5.23; N, 2.91. Found: C, 57.31; H, 5.25; N, 2.93%); Yield
for P2: 173 mg (18% per Cu atom). (Anal. Calc. for C23H25Cl2CuNO2:
C, 57.32; H, 5.23; N, 2.91. Found: C, 57.38; H, 5.28; N, 2.90%).
When reacting pydimH2 with anhydrous CuCl2 under the same
conditions as pydotH2 but in acetone or methanol solution, a green
powder is obtained which analyses to [(pydimH2)CuCl2]n. Although
the compound is virtually insoluble, from the EPR spectrum of the
isolated powder (see below) we can assign the compound to the
monomer (n = 1). The same compound has been recently obtained
from the reaction of the ligand with CuCl2ꢁ2H2O in aqueous ethanol
[25]. In contrast to this, the reaction of pydipH2 with anhydrous
CuCl2 either in acetone or in methanol leads to a blue compound,
which analysed correctly to be composed of the ligand and CuCl2
3. Results and discussion
3.1. Reaction of anhydrous CuCl2 with pydotH2 (2,6-bis(1-hydroxy-1-
o-tolyl-ethyl)pyridine)
The reaction of anhydrous CuCl2 with pydotH2 in dry acetone
(see Section 2) leads simultaneously to two different products P1
and P2 (Scheme 3). Upon evaporation of the yellow orange

652
A. Klein et al. / Polyhedron 31 (2012) 649–656
R = Methyl
R' o-Tolyl
R'
R'
R
R
N
R
OH
OH
R'
H
H
CuCl
+
2
O
R
O
R'
O
R'
O
in acetone
R
R
H
N
R'
Cl
Cl
N
N
Cu
Cu
Cl
R'
Cl
H
O
Cu
O
R
H
H
R'
Cl
[(pydotH2)CuCl2]
Cl
R
X2
P1
[(pydotH2)CuCl(µ-Cl)2CuCl(pydotH2)]
P2
Scheme 3. Reaction of the ligand pydotH2 with anhydrous CuCl2.
in a 1:1 ratio. The EPR spectrum of the isolated powder and absorp-
tion spectroscopic investigation clearly reveals that the product is
[Cu(pydipH2)2][CuCl4] (see below). The related compound [Cu(py-
dipH2)2]Cl2 has been recently obtained from the reaction of the li-
gand with CuCl2ꢁ2H2O in a 2:1 ratio in ethanol [25], which allowed
the unequivocal assignment.
In view of our previous results [25] we can summarise the results
on the preparation as follows. When ligand and CuCl2 are reacted in a
1:1 ratio the penta-coordinate complexes [(O,N,O)CuCl2] and/or the
products of the disproportionation reaction (1) were obtained.
2½CuðO; N; OÞ2ꢂ½CuCl4ꢂ¡½ðO; N; OÞCuCl2ꢂ
ð1Þ
Since the disproportionation reaction is not very fast, for py-
dimH2 exclusively the penta-coordinate complex [(pydimH2)-
CuCl2] is obtained which, due to its low solubility, precipitates
rapidly from the reaction mixture. For pydotH2 a mixture of mono-
nuclear and binuclear [(pydotH2)CuCl2]n (n = 1 or 2) is obtained,
since most of the initially formed penta-coordinate species imme-
diately precipitates from the reaction mixture in the form of the
(insoluble) dimer. This rapid precipitation leaves no time for the
slow disproportionation and at the end of the isolation process
small amounts of the monomeric complex can be obtained by ra-
pid work-up. For the pydipH2 ligand the solubility of all species
is high and after extended reaction time (over night) exclusively
the products of the disproportionation were obtained. Shortening
the reaction time markedly (2 h) allows to isolate the penta-coor-
dinate complex [(pydipH2)CuCl2]. Additionally, the type of solvent
or the water content of the reacted CuCl2 varies the solubility of the
products, while the rate of the disproportionation strongly de-
pends on the solvent polarity; in CH2Cl2 almost no reaction is ob-
served, even after hours, in acetone the reaction occurs
perceptively already after some minutes, while in DMF the reaction
is too fast to allow measurements on the penta-coordinate com-
plexes [(O,N,O)CuCl2].
Fig. 1. Comparison of the Cu-K edge XANES spectra of [(pydotH2)CuCl2] (solid line),
[(pydotH2)CuCl(
line) and [(pydimH2)CuCl2]ꢁH2O (dashed dotted line).
l
-Cl)2ClCu(pydotH2)] (dotted line), [(pydipH2)2Cu]Cl2ꢁH2O (dashed
about 8995–8998 eV. This signature can be assigned to 1s ? 4p
transitions and indicates that all compounds are in the oxidation
state CuII. This is supported by the absence of a well-defined pre-
peak at 8983–8984 eV which would be indicative for copper in
the oxidation state CuI [36]. Numeric XANES results are given in
Table S1 in the Supporting Information.
The 1s ? 4p resonance transition of the Cu K-edge is substan-
tially modified by the electronic environment such as ligand field,
the chemical bonding in the initial state and by the atomic valence
state [37]. Calculation of the shift of E0 in comparison to the pure me-
tal, DE0, and the edge width DEw, defined as the difference in energy
between the inflection point E0 and the main absorption maximum
of the edge [38], can provide information about the structure of
the studied complexes [39]. A semiquantitative correlation between
the edge width and the nature and number of the surrounding atoms
points to generally larger edge widths for lower coordination num-
From both P1 or P2 we could not obtain single crystals, there-
fore we embarked on an XAS study of both complexes, also includ-
ing the recently reported derivatives [(pydimH2)CuCl2]ꢁH2O and
[Cu(pydipH2)2]Cl2ꢁH2O. The two latter have been investigated by
single crystal XRD. Thus the EXAFS data can be compared with
XRD data for these two compounds.
bers, while
From our measurements we can state a smaller edge width
and an increased edge shift E0 for P1 in comparison to P2. The pen-
D
Ew is smaller for higher coordination polyhedra [40].
D
Ew
D
ta-coordinate complex [(pydimH2)CuCl2]ꢁH2O (structure estab-
lished by XRD) exhibits values very similar to those of P1, which
supports our assumption that P1 is mononuclear and presumably
3.2. XANES and EXAFS
The Cu K-edge X-ray absorption near edge spectra (XANES) of
penta-coordinate. The very different DEw and DE0 values for P2 indi-
[(pydotH2)CuCl2] (P1), [(pydotH2)CuCl(l-Cl)2ClCu(pydotH2)] (P2),
cate marked differences to the coordination geometry of P1, how-
ever the values obtained for the compound [Cu(pydipH2)2]Cl2,
which has a hexa-coordinate copper core (XRD), are very much un-
like those observed for P2. On the other hand, the CuII ion in the
[Cu(pydipH2)2]Cl2ꢁH2O and [(pydimH2)CuCl2]ꢁH2O, were recorded
(see Fig. 1). There is a scarcely observable signal at about 8976–
8979 eV due to the dipole forbidden 1s ? 3d transition [35], a
shoulder about 8985–8989 eV and an intense resonance peak at

A. Klein et al. / Polyhedron 31 (2012) 649–656
653
complex [Cu(pydipH2)2]2+ exhibits a tetragonally compressed octa-
hedral geometry (due to the two tight trans-oriented Cu–N bonds),
while the hexa-coordinate surrounding of the CuII ions in P2
(Scheme 2) is completely different from the presence of the Cl li-
gands. Considering that not only the coordination geometry but also
the electrostaticenvironmentis changedby theformation of a chlor-
ido-bridged binuclear complex, the variation of the edge width and
the edge shift of P1 in comparison to P2 is reasonable.
Fig. 2 shows the EXAFS spectra of P1 and P2, while the struc-
tural parameters of all investigated complexes determined by
curve fitting analyses of the EXAFS spectra at the Cu K-edge are
summarised and compared to XRD data in Table 1.
The fittings of the Cu K-edge EXAFS spectra (Fig. 2) of the com-
pound P1 and the reference complex [(pydimH2)CuCl2]ꢁH2O were
performed using a five shell model, wherein the first two coordina-
tion shells at about 2.0 Å consist of the two coordinating oxygen
atoms and one nitrogen of the pyridine-2,6-dimethanol ligands,
the third and fourth shell of the chlorine backscatterers at ꢄ2.2
and ꢄ2.7 Å and a fifth coordination shell at ꢄ2.8 Å of further back-
bone-carbon backscatterers of the chelating ligand. The fitting mod-
el for P2 was on one hand deduced from the parameters of P1 and an
additional chlorine backscatterer at about 2.2 Å. On the other hand
fit for P2 was obtained with a coordination sphere slightly different
from those of the two above mentioned complexes. The most strik-
ing difference is one long (2.55(3) Å) and two markedly shorter
(2.22(2) Å) Cu–Cl distances in P2, in contrast to the dialkylpyri-
dine-2,6-dicarboxylate complexes where all three Cu–Cl distances
are far more similar to each other (ranging from 2.25 to 2.35 Å). An-
other marked difference to the dialkylpyridine-2,6-dicarboxylate
complexes is that the Cu–O distances were short (about 2 Å) in P2,
while in the latter they range between 2.35 and 2.62 Å. Importantly,
in most so far structurally characterised complexes of the pyridine-
2,6-dimethanol ligands (as pydimH2 or pydotH2) the Cu–O bonds
were short [16–20,22,24,25] thus supporting our data for P2 from
EXAFS. The difference between the pyridine-2,6-dimethanol ligands
and the dialkylpyridine-2,6-dicarboxylates lies not only in the dif-
ferent O donor atoms (OH vs. O@C or OR) but also in the OC–
CNC–CO scaffold, which contains either short C@O or C–O bonds
for the latter ligands, making the O–Cu–O angles markedly smaller
and the Cu–O distances larger. Thus, the overall geometry of the
dialkyl-2-6-dicarboxylate complexes can be described as ‘‘fused’’
tetragonally elongated octahedra, while P2 resembles more a dimer
of two square pyramidal configured CuII complexes. Unfortunately,
from EXAFS we cannot say whether the long Cu–Cl bond is a termi-
nal or a bridging one, presumably the latter is the case.
the structures of the related complexes [(L)CuCl(l-Cl)2ClCu(L)]
(L = dialkylpyridine-2,6-dicarboxylate; R = Me [dmpc], Et [depc] or
R = iPr [dppc]) [33] were taken into account for the fitting. Based
on these three structures an additional Cu-shell was added to the
fit. Assuming a Cuꢁ ꢁ ꢁCu distance at about 3.4 Å improved the fitting
results significantly improved the fitting results. However, the best
3.3. EPR spectroscopy
From selected samples EPR spectra were recorded on finely di-
vided powders at ambient temperature or 110 K. Since in all cases
Fig. 2. Experimental (solid line) and calculated (dotted line)
[( Cl2){(pydotH2)CuCl}2] (P2, bottom) at the Cu K-edge.
k
3v(k) functions (left) and their Fourier transforms (right) of solid [(pydotH2)CuCl2] (P1, top) and
l

654
A. Klein et al. / Polyhedron 31 (2012) 649–656
(with the exception of [(pydipH2)CuCl2]) very broad lines were ob-
served with no hyperfine splitting (hfs), samples of P1 and P2 were
diamagnetically diluted using KAl(SO4)2. However, also for those
samples no hfs could be observed. Fig. 3 displays spectra of P1
and P2 underpinning their markedly different character, Table 2
lists essential EPR data of selected CuII complexes.
the lack of resolved hfs is indicative for ‘‘magnetically condensed
copper(II) compounds’’ [46]. Using the approximation of Eaton to
determine D from the spectral difference of the low-field edge of
the half-field signal and h
m
/2gb [47] we obtained D ꢄ 340 G (E is
assumed to be 0), which calculates to a Cuꢁ ꢁ ꢁCu distance of
4.44 Å. The result shows that the assumption of purely dipolar
interaction between the two unpaired electrons is not justified,
While P1 shows an almost isotropic rather narrow spectrum, P2
exhibits a complex spectral pattern and most importantly a half-
in line with the assumed structure of P2 (l
-Cl bridged CuII ions).
field signal (
D
MS = 2), which is very indicative for a binuclear
In order to obtain more information on the coordination of the CuII
ions in P2 we wanted to compare its EPR spectrum to those of
other binuclear chlorido-bridged CuII complexes (for details see
Supporting Information) and thus translated the three components
of the spectrum into three ‘‘observed’’ g values (Table 2). Rhombic
complex with ferromagnetic coupling of the unpaired spins of
two CuII ions in binuclear complexes resulting in an S = 1 ground
state [33,34,42–46]. Interestingly, there is almost no difference be-
tween spectra of P2 recorded at 298 K or at 110 K. The (DMS = 1)
spectrum of P2 consists of a narrow band centred at about 2300 G
and two further broad components at 2800 and 3500 G. Unfortu-
nately, due to the lack of hfs, we were not able to simulate the
spectrum of P2 and thus could not obtain the zero-field splitting
parameters D and E. This is a pity, since from these values the
Cuꢁ ꢁ ꢁCu distance could be calculated [46,47]. On the other hand
EPR spectra were reported for [(L)CuCl(l-Cl)2ClCu(L)] (L = dialkyl-
pyridine-2,6-dicarboxylate; alkyl = Et [depc] or R = iPr [dppc]) both
with two bridging and two terminally bound Cl ligands [33]. The
depc derivative exhibits a slightly distorted octahedral geom-
etry (Cl(equatorial,short)O2(axial,long)N(equatorial,short)l-Cl2(equatorial,short)),
around the copper atoms with a coplanar arrangement of N, Cl
and Cu atoms and exhibits a gav value very similar to P2. In contrast
to this, the bulkier dppc ligand forces a butterfly-like Cu2Cl2 core
with a strongly distorted octahedral geometry around the copper
atom. Consequently, the averaged g value is markedly smaller.
g1
The spectrum of the complex [Cu(H2py2th1as)(l-Cl)]2(ClO4)4
g2
(H2py2th1as = thiophen-2-yl)methylamino)methyl)-6-(bis((pyri-
din-2-yl)methyl)amino)-4-methylphenol) in the solid exhibits also
a relatively low averaged g value and axial symmetry, which is
probably due to the strong tetragonal distortion of the octahedral
ΔMS =
2
CuII
environment
(N3(equatorial,short)l-Cl(equatorial,short)O(axial,long)
l-Cl(axial,long) [34]. Axial EPR spectra were also found for related
chlorido-bridged binuclear complexes such as [Cu(HL)Cl]2
(H2L = [2-((E)-(2-hydroxyethylimino)methyl)-4-bromophenol])
2+
g3
[43], [Cu(apyhist)Cl]2
(apyhist = (4-imidazolyl)ethylene-2-ami-
no-1-ethylpyridine)[42] or [CuCl2(Mebta)]2 (Mebta = 1-methyl-
benzotriazole) [44] showing five-coordinated Cu(II). Thus, the
rhombic EPR spectrum of P2 is fully in line with a dimeric structure
Field (G)
1000 1500 2000 2500 3000 3500 4000
[(pydotH2)CuCl(
distorted octahedral coordination environment (O2(axial,short)
N(equatorial,short)Cl(terminal)
-Cl)2) of the CuII ions, in agreement with
the structure proposed from EXAFS. Unfortunately, the EPR data of
l-Cl)2ClCu(pydotH2)] with an only slightly
Fig. 3. X-band EPR spectra of [(pydotH2)CuCl2] (P1) (dotted) and [(pydotH2)CuCl2]2
(P2) (solid line), both measured as diluted powders (1:10 with KAl(SO4)2) at 110 K.
(l
Table 2
Selected EPR data of CuII complexes.a
gav
g1
g2
g3
Spectral symmetry
Molecular symmetryb
Mononuclear complexes
[(pydotH2)CuCl2] (P1)
[(pydimH2)CuCl2]
2.175
2.158
2.138
2.195
2.139
2.194
2.202
2.226
2.311
2.220
2.221
2.238
2.353f
2.405
2.180
2.094
2.220
2.190
2.139
2.120
2.100
2.120
2.068
1.973c
2.173
2.040
2.108
2.100
rhombic
rhombic
axial compressed
rhombic
rhombic
SPy
SPy
TBP
OC
OC
OE
[(pydipH2)CuCl2]
[(pydipH2)2Cu]Cl2
[(pydimH2)Cu(pydic)]d
[(pydotH2)CuCl2(DMF)]e
rhombic
axial elongated
2+
g
[Cu(H2O)6]
OE
Binuclear complexes
[(pydotH2)CuCl2]2 (P2)h
2.148
2.147
2.130
2.14
2.418
2.21
2.17
2.30
2.081
2.12
2.16
2.06
1.946
2.11
2.06
2.06
rhombic
rhombic
rhombic
axial elongated
OD
OD
OD
OD
i
[(
[(
l
l
-Cl)(depc)CuCl]2
-Cl)(dppc)CuCl]2
i
j
[Cu(H2py2th1as)(l-Cl)]2(ClO4)4
a
All samples measured as finely divided powders at X-band frequency, at 298 K; all g values from spectral simulation. gav = (g1 + g2 + g3)/3;
Symmetry assignment based on EPR spectroscopy (see text), SPy = square pyramidal, TBP = trigonal bipyramidal, OC = octahedral compressed, OD = octahedral dimeric,
D
g = g3 ꢀ g1.
b
OE = octahedral elongated.
c
A3 = 175 G.
From Ref. [24].
Measured at 110 K in glassy frozen DMF.
A1 = 107 G.
d
e
f
g
From Ref. [41].
h
g Values could not be obtained from simulation, g values are thus ‘‘observed values’’ as depicted in Fig. 3.
From Ref. [33].
Co-crystallised with 6 MeOH, from Ref. [34].
i
j

A. Klein et al. / Polyhedron 31 (2012) 649–656
655
P2 does not provide unequivocal information about the position of
the long Cu–Cl bond concluded from EXAFS (bridging or terminal).
Within the series of mononuclear penta-coordinate complexes
[(O,N,O)CuCl2] the EPR spectra of P1 and [(pydimH2)CuCl2] exhibit
rhombic symmetry, while [(pydipH2)CuCl2] shows an axially com-
pressed spectrum. The rhombic spectra are in line with the square
pyramidal structure found in the crystal of [(pydimH2)CuCl2] and
assumed for the EXAFS spectra of [(pydotH2)CuCl2]. The axially
compressed spectrum of [(pydipH2)CuCl2] showing hfs to 35,37Cu
(I = 3/2, 100% nat. abundance) on the g3 component (g||) points to
a more trigonal bipyramidal surrounding of the CuII ion [5,8,40].
The hexa-coordinate CuII complex [(pydipH2)2Cu]2+ in [(pydipH2)2-
Cu]Cl2 shows a rhombic spectrum very similar to the related com-
plex [(pydimH2)Cu(pydic)] [18]. These two examples represent the
small class of tetragonally compressed octahedral systems (better
described as pseudo Jahn-Teller systems) [8], which contrast to
the vast group of tetragonally elongated systems as [Cu(H2O)6]2+
[41], which normally exhibit spectra with axial symmetry [5,8,48].
When P1 or P2 were dissolved in DMF, identical spectra (isotro-
pic g = 2.193, no hfs), were obtained. At 110 K in glassy frozen solu-
tion, spectra exhibiting rhombic symmetry (gav = 2.194) and hfs on
the g1 component (A1 = 107 G) were recorded and the mononuclear
species [(pydotH2)CuCl2(DMF)] is assumed to be present in such
solutions [34,42,43,45]. The rhombic character of the signal is a
strong indication that the pydotH2 ligand is still completely coordi-
nated since it’s rigid scaffold does not allow a ‘‘relaxed’’ octahedral
coordination around CuII [5,48].
The compound P2 turned out to be insoluble in CH2Cl2 and solu-
tions in DMF (in which it dissolves slowly) showed identical
absorption spectra to solutions of P1 in DMF (see Table 3). The latter
spectra were assigned to the hexa-coordinate DMF complex [(pyd-
otH2)CuCl2(DMF)] in line with the results from EPR spectroscopy
and electrochemical measurements. The corresponding hexa-coor-
dinate solvent complex [(pydipH2)CuCl2(DMF)] is probably
observed in DMF solutions of [(pydipH2)CuCl2], while for sparingly
soluble [(pydimH2)CuCl2] in DMF the disproportionation (see Eq.
(1)) is too fast and only the disproportionation products were
observed as can be inferred from the strong LMCT band of [CuCl4]2ꢀ
at about 460 nm. However, absorption spectra of the penta-coordi-
nate species can be obtained in CH2Cl2 solution, in which the
solubility of the materials is sufficient, while the disproportionation
is hampered from the low polarity of the solvent. Furthermore,
coordination of CH2Cl2 is unlikely. A spectrum of P2 recorded from
a KBr pellet shows a long-wavelength absorption maximum at
rather low energy, comparable to the other hexa-coordinate com-
plexes. Interestingly, the absorption energies for the penta-coordi-
nate species are markedly higher than for the hexa-coordinate
complexes. Obviously the loss of the relatively strong chlorido li-
gands, when going from the penta-coordinate to the hexa-coordi-
nate species is not compensated by the coordination of a sixth
ligand, which is further evidence for a long bridging Cu–Cl bond.
3.5. Electrochemical measurements
From selected samples cyclovoltammetric (CV) measurements
were carried out using DMF as solvent and nBu4NPF6 as electrolyte.
Fig. 4 shows CVs of the hexa-coordinate [(pydipH2)2Cu]Cl2 (left)
and P1 (right). When P2 is dissolved in DMF identical plots were
obtained, which supports the above made assumption that the
binuclear complex dissociates in DMF yielding monomeric [(py-
dot)CuCl2(DMF)]. Corresponding measurements for P1 and P2
were carried out in acetone as solvent, again with identical results
for both compounds. In CH2Cl2 only P1 could be dissolved and gave
also very similar CVs.
All compounds exhibit a reversible and more or less broad
reduction wave around 0 V (vs. FeCp2/FeCp2+) which is assigned
to the CuII/CuI redox couple (data in the Supporting Information).
Since the reversibility and the shape of the waves depends on the
ligands ability to effectively (rapidly) reorganise the ligand sphere
after the redox step we can assign the obvious difference between
the hexa-coordinate complex (very broad wave) and the other sam-
ples (narrow waves) to the rather rigid hexa-coordination (com-
pressed octahedral) and the more flexible [(O,N,O)CuCl2(DMF)]
3.4. Absorption spectroscopy
Absorption spectra of the penta-coordinate complexes [(O,N,O)-
CuCl2] (including P1) were measured in CH2Cl2 or DMF solution.
Table 3
Long-wavelength absorption maxima of CuII complexes of O,N,O ligands.a
Compound
k
Solvent
[(pydotH2)CuCl2]
[(pydotH2)CuCl2]2
[(pydotH2)CuCl2(DMF)]
[(pydipH2)CuCl2(DMF)]
[(pydimH2)CuCl2]
786
890
948
898
775
798
892
905
CH2Cl2
KBr
DMF
DMF
CH2Cl2
CH2Cl2
DMF
[(pydipH2)CuCl2]
[Cu(pydimH2)2][CuCl4]
[Cu(pydipH2)2][CuCl4]
b
b
DMF
a
Wavelength k in nm; bands were assigned to d–d transition of the d9 system.
Further bands at 460 nm were assigned to LMCT bands originating from
b
[CuCl4]2ꢀ
.
10 µA
-1
10 µA
1
0
-2
-3
1
0
-1
-2
+
-3
+
E (V) vs FeCp / FeCp
E (V) vs FeCp / FeCp
2
2
2
2
Fig. 4. Cyclic voltammogramms of [(pydipH2)2Cu]Cl2 (left) and [(pydotH2)CuCl2], measured in DMF/nBu4PF6 at 298 K and 100 mV/s scan rate.

656
A. Klein et al. / Polyhedron 31 (2012) 649–656
coordination. Moreover, in the CV of [Cu(pydipH2)2]2+ further
reduction waves (missing for P1) point to partly decomposition
(presumably de-coordination of the O,N,O ligand and replacement
by DMF). Oxidation waves at about 0.6 V are irreversible in all cases
and can be either assigned to further oxidation of copper CuII/CuIII
or more likely to the oxidation of the chloride counter ions or the
chlorido ligands 2Clꢀ/Cl2.
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Cl)2ClCu(pydotH2)] and the penta-coordinate olive-green mono-
mer [(pydotH2)CuCl2]. The molecular entities imply that the ligand
remains protonated upon coordination. When dissolved in DMF
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a
monomeric complex [(pyd-
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UV–Vis/NIR spectroscopy and electrochemical measurements. All
assignments were supported by comparing CuII complexes of the
related ligands 2,6-bis(hydroxymethyl)pyridine (pydimH2) and
2,6-Bis(1-hydroxy-1-methyl)pyridine (pydipH2). Furthermore,
comparative preparative work using all three ligands has allowed
some insight into the formation reactions of the various species
[(O,N,O)CuCl2]n (n = 1 or 2), [(O,N,O)CuCl2(solv)] or [Cu(O,N,O)2]
[CuCl4] which can be obtained from a 1:1 reaction of the ligands
and CuCl2 by controlling the reaction conditions. Since the pydotH2
ligand is chiral, there might be some implication on specific spec-
troscopic properties as IR-frequencies or catalytic properties. How-
ever, the focus of this work lies exclusively on the structure
elucidation of the two complexes and none of the applied methods
is sensitive to the absolute structure.
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Ferreira, Inorg. Chem. Commun. 6 (2003) 294.
André Uthe and Andreas O. Schüren (University of Cologne)
were acknowledged for EPR measurements. We also would like
to thank the ‘‘Studienstiftung des Deutschen Volkes’’ (KB) for finan-
cial support.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2011.10.023.
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