Molecular structure, UV/vis spectra, and cyclic voltammograms of Mn(II), Co(II), and Zn(II) 5,10,15,20-tetraphenyl-21-oxaporphyrins

Article abstract of DOI:10.1021/ic302268h

The 5,10,15,20-tetraphenyl-21-oxaporphyrin complexes of Mn(II), Co(II), and Zn(II) have been crystallized and studied by X-ray diffraction, NMR and UV/vis spectroscopy, and mass spectrometry as well as cyclic voltammetry. The X-ray structure of the earlier described Cu(II) complex is also reported. All complex structures possess a five-coordinate, approximately square-pyramidal geometry with a slight deviation of the heteroaromatic moieties from planarity. The packing structures are characterized by parallel strands of complex molecules interacting by weak hydrogen bonds. In the case of Zn(II) an octahedral complex has also been isolated using a side-chain hydroxy functionalized oxaporphyrin ligand; the structure was verified by NMR and EXAFS spectroscopy. Cyclic voltammetry studies reveal that the reduction of the complex bound Mn(II), Co(II), and Zn(II) ions is a ligand-centered process whereas the first oxidation step depends on the metal ion present.

Full text of DOI:10.1021/ic302268h

Article
pubs.acs.org/IC
Molecular Structure, UV/Vis Spectra, and Cyclic Voltammograms of
Mn(II), Co(II), and Zn(II) 5,10,15,20-Tetraphenyl-21-oxaporphyrins
Silvio Stute,† Linda Gotzke,† Dirk Meyer,† Mohamed L. Merroun,‡ Peter Rapta,§,∥ Olga Kataeva,⊥
̈
Wilhelm Seichter,# Kerstin Gloe,† Lothar Dunsch,§ and Karsten Gloe*,†
†Department of Chemistry and Food Chemistry, TU Dresden, 01062 Dresden, Germany
‡Institute of Resource Ecology, Helmholtz Center Dresden-Rossendorf, 01314 Dresden, Germany
§Center of Spectroelectrochemistry, Leibniz Institute for Solid State and Materials Research, 01171 Dresden, Germany
∥Institute of Physical Chemistry and Chemical Physics, Slovak University of Technology, 81237 Bratislava, Slovak Republic
⊥A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan 420088, Russia
#Institute of Organic Chemistry, TU BA Freiberg, 09596 Freiberg, Germany
S
* Supporting Information
ABSTRACT: The 5,10,15,20-tetraphenyl-21-oxaporphyrin
complexes of Mn(II), Co(II), and Zn(II) have been
crystallized and studied by X-ray diffraction, NMR and UV/
vis spectroscopy, and mass spectrometry as well as cyclic
voltammetry. The X-ray structure of the earlier described
Cu(II) complex is also reported. All complex structures
possess a five-coordinate, approximately square-pyramidal
geometry with a slight deviation of the heteroaromatic
moieties from planarity. The packing structures are charac-
terized by parallel strands of complex molecules interacting by
weak hydrogen bonds. In the case of Zn(II) an octahedral
complex has also been isolated using a side-chain hydroxy
functionalized oxaporphyrin ligand; the structure was verified
by NMR and EXAFS spectroscopy. Cyclic voltammetry studies reveal that the reduction of the complex bound Mn(II), Co(II),
and Zn(II) ions is a ligand-centered process whereas the first oxidation step depends on the metal ion present.
Re(I)29 oxaporphyrin complexes have been published. It is
interesting to note that the spectral, magnetic, and electro-
INTRODUCTION
■
Due to its ability to form highly stable, noncharged complexes
chemical properties of Cu(II) oxaporphyrins have been
with divalent cations, the porphyrin core is one of the most
studied,16,26,27 but an X-ray structural characterization is
missing. Also the formation of Co(II) and Mn(II) complexes
has been shown only spectroscopically.16 Recently results on
Zn(II) complexes with a series of various meso-substitued
oxaporphyrins, including a crystal structure, have been
published.28
investigated ligand systems for metal complexation.1 Over the
years several modifications of porphyrins led to various new
structures enlarging the field of porphyrin chemistry.2
Specifically, replacement of one or more pyrrole nitrogen
atoms affects the electronic structure of the ring system. This
core modification can be achieved, for example, by inversion3,4
or substitution5−17 of one or more pyrrole rings. The
introduction of heterocycles other than pyrrole results in the
formation of the so-called heteroporphyrins, in which one or
more nitrogen atoms are replaced by phosphorus5−7 or an
element of group 16 (O, S, Se, Te).9−17 The insertion of
chalcogene atoms in the macrocyclic ring leads not only to the
alteration of the donor set and the ring size but also to the lack
of at least one core proton. These changes dramatically
influence characteristic properties such as aromaticity and metal
binding ability of the porphyrins.9,11,18 While thiophene and its
Se or Te containing analogues cause a decrease in the size of
the cavity, oxaporphyrins and regular porphyrins have almost
identical cavity.9,19 At present only a few papers on Fe(II/
III),20,21 Ni(I/II),16,22−26 Cu(I/II),16,26,27 Zn(II),16,28 and
To gain more insight into the binding pattern of 21-
oxaporphyrins, we report here the synthesis and character-
ization of Mn(II), Co(II), Cu(II), and Zn(II) complexes with
the 5,10,15,20-tetraaryl-21-oxaporphyrins 1−3.
RESULTS AND DISCUSSION
■
Symmetric 5,10,15,20-tetraaryl-21-oxaporphyrins (1, 2)
(Scheme 1) and the asymmetric oxaporphyrin bearing a side
chain with an additional donor function (3) (Scheme 2) were
used for metal binding experiments. According to the literature,
Received: October 16, 2012
Published: January 14, 2013
© 2013 American Chemical Society
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tion by chromatography and recrystallization, the desired
metalloporphyrins were obtained in almost quantitative yields.
In addition to the known Cu(II) complex26 we inserted Co(II)
(CoCl-1) and Mn(II) (MnCl-1) into ligand 1. In the case of
Zn(II) several complexes (ZnCl-1, ZnClO4-2, ZnCl-3) were
synthesized. The product metallooxaporphyrins are stable in
the solid and solution.
Scheme 1. Oxaporphyrins Studied
Crystallization of Mn(II), Co(II), Zn(II), and Cu(II) 21-
oxaporphyrins from chloroform/n-heptane or ethyl acetate
afforded crystals suitable for X-ray diffraction (Figure 1, Table
1). All complexes possess a five-coordinate, square pyramidal
geometry with chloride in the axial position. The metal atoms
are shifted out of the plane of the porphyrin donor atoms
toward the chloride. The metal−nitrogen distances are all
elongated compared to their four-coordinate31−34 and five-
coordinate31,35−38 N4-analogues. The furan ring coordinates in
an η1 fashion via the oxygen with M−O distances similar to
ether oxygen−metal bonds.39−42 In general, the exact assign-
ment of the heteroatoms is difficult due to almost identical
carbon−oxygen and carbon−nitrogen distances. However, the
problem could be solved on the basis of the relative peak
intensities of the heteroatoms as well as the metal−donor
distances. In case of ZnCl-1 the attempt to refine with
disordered oxygen and nitrogen atoms in positions 21 and 23
afforded no significant improvement of the structure quality,
and the anisotropic refinement failed. For the Mn(II) complex
the refinement of the structure with no disorder showed larger
vibrational parameters for the oxygen atom and smaller
vibrational parameters for one of the nitrogen atoms. Moreover,
additional peaks of different electron density appeared near this
N-atom, typical for wrong assignment of the atom types. When
the positions of the O and N atom were interchanged, the same
picture was observed. Thus two opposite atoms O and N were
refined with positional disorder approximately 60 to 40%. The
R-factor decreased by 0.004 for disordered structure in
comparison with nondisordered structure. The other two
nitrogen atoms showed no indication of disorder and in the
final structure U(eq.) were very similar for all four donor atoms.
the closure of the macrocycle was achieved by condensation of
a 2,5-bis(arylhydroxymethyl)-furan (1a−3a) with pyrrol and an
arylaldehyde.22 For the synthesis of the asymmetric oxapor-
phyrin 3 further steps were necessary to introduce the
additional hydroxyl donor function. The cyclization reaction
with the asymmetric 2-[(2-methoxy-5-methylphenyl)-
hydroxymethyl]-5-[(p-tolyl)hydroxymethyl]-furan (3a) af-
forded an oxaporphyrin containing a single methoxy group
(3b). As the separation of oxaporphyrins is often difficult
because of their high basicity,22 the formation of the Zn(II)
complex was used to improve the separation behavior in
chromatographic isolation. In contrast to the commonly used
chromatography employing the free base porphyrin on basic
aluminum oxide17,30 which results in broadened fractions and
significantly reduced yields, this procedure leads to a more
defined product fraction with an enhanced separation from
impurities. The conversion of the methoxy group into a
hydroxyl group with concomitant dissociation of the Zn(II)
complex was achieved under standard ether cleavage
conditions. Subsequent etherification with a 3-propanol
synthon and cleavage of the protecting group gave the desired
pentadentate oxaporphyrin 3.
The formation of all metal complexes was accomplished by
refluxing the oxaporphyrin and an appropriate metal chloride in
chloroform or a chloroform/methanol mixture. After purifica-
Scheme 2. Synthetic Route for the Synthesis of Oxaporphyrin 3
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Figure 1. Perspective (top) and side (bottom) views on the X-ray structures of the Mn(II), Co(II), Cu(II), and Zn(II) complexes of 1; phenyl rings
are omitted in the side views.
Table 1. Crystallographic Data for MnCl-1, CoCl-1, CuCl-1, and ZnCl-1
MnCl-1
CoCl-1
CuCl-1
ZnCl-1
formula
C45H29Cl4MnN3O
824.45
C44H28ClCoN3O
709.07
C45H29Cl4CuN3O
833.06
C44H28ClN3OZn
715.53
formula weight
T [K]
λ [Å]
space group
a [Å]
b [Å]
c [Å]
α
293
293
153
153
0.71073
P21/n (No. 14)
12.142(2)
22.300(1)
14.460(3)
90°
0.71073
P21/n (No. 14)
10.162(1)
15.936(1)
20.933(1)
90°
0.71073
P21/n (No. 14)
12.0352(3)
21.9674(6)
14.4403(4)
90°
0.71073
P21/n (No. 14)
9.9737(3)
15.8831(5)
20.9518(5)
90°
β
102.65(1)°
90°
3820.2(10)
4
90.61(1)
90°
3389.7(4)
4
102.788(1)°
90°
3723.06(17)
4
90.539(2)°
90°
3318.90(16)
4
γ
V [Å3]
Z
Dcalcd
1.433
1.390
1.486
1.432
μ [mm−1
]
0.666
0.626
0.915
0.862
a
R
0.0873
0.0461
0.0571
0.0450
b
Rw
0.2314
0.1152
0.1355
0.1081
a
b
2
2
R = ∑∥Fo| − |Fc∥/∑|Fo|. Rw = [∑w·(Fo − Fc2)/∑w(Fo )2]1/2
.
Similar disorders have already been reported.22,28 As this
precludes a precise assignment, the metal−O and metal−N
distances are only listed as metal−donor distances in Table 2.
All M−Cl distances are in the typical range22,28,43−45 with these
bonds almost perpendicular to the plane of the donor atoms.
The largest deviations from the vertical occur in MnCl-1 and
CuCl-1 with angles of 78.9° and 79.2°, respectively. While the
values of the bond lengths and angles in ZnCl-1 are in good
accordance with those in the Zn(II) oxaporphyrin complex
ZnCl-OTMPP46 having methoxycarbonyl functions in para-
position of meso-phenyl rings,28 the conformation of the
porphyrin core differs. In contrast to ZnCl-OTMPP adopting a
ruffled conformation, ZnCl-1 shows a “doming” with a
significant smaller deviation from the mean plane (average
atomic deviation of 0.05 Å). Despite the metal center being a
little closer to the plane of the donor atoms, CoCl-1 is
isostructural with ZnCl-1 and the porphyrin core shows a
doming distortion with the same average atomic deviation.
CuCl-1 and MnCl-1 are also isostructural and exhibit a saddle
distortion like the Ni(II) complex22 NiCl-ODPDTP.47 Each of
these three complexes contain a disordered solvent molecule
within the crystal, namely, chloroform (CuCl-1, MnCl-1) or
dichloromethane (NiCl-ODPDTP), that forms a weak hydro-
gen bond48 (2.40 Å, 2.34 Å, 2.87 Å) to the chloride ligand (M-
Cl···HCR3). The proximity of the bulky −CCl3 residue, which
is induced by this H-bond, might contribute to the greater
deviation of the chloride ligand from the vertical in CuCl-1 and
MnCl-1.
Table 2. Selected Distances (Å) in the Complexes of 1
Compared with Literature Data
a
b
c
M−D
M−Cl M plane X plane
MnCl-1
2.13, 2.14, 2.16, 2.20
2.05, 2.08, 2.10, 2.22
2.01, 2.04, 2.05, 2.14
2.07, 2.10, 2.15, 2.24
2.08, 2.09, 2.11, 2.19
2.00, 2.01, 2.06, 2.18
2.33
2.25
2.37
2.22
2.21
2.29
0.66
0.53
0.32
0.61
0.59
0.33
0.18
0.05
0.20
0.05
0.15
0.19
CoCl-1
CuCl-1
ZnCl-1
ZnCl-OTMPP28
NiCl-ODPDTP22
The crystal packing of CuCl-1 and MnCl-1 is characterized
by parallel strands of complex molecules along the c-axis
overlapping their inverted forms (Figure 2a). The chlorides of
the pairs of inverted molecules are pointing away from each
other, and the porphyrin plane distances are 3.77 Å and 3.85 Å,
respectively. In contrast, no overlapping pairs of porphyrin
a
Distance between metal center and donor atoms of the oxaporphyrin
b
(N3O). Distance between metal center and the plane defined by the
four inner core atoms (N3O). Average atomic deviation from the
c
mean plane defined by the 24 atoms of the porphyrin.
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Figure 3. Electronic absorption spectra of MnCl-1, CoCl-1, and
ZnCl-1 in CH2Cl2 including 5-fold magnification of the Q-bands.
Table 3. Electronic Spectral Data for the Metal Complexes of
1 in CH2Cl2
Soret bands λ [nm] (ε [103
M−1·cm−1])
Q-bands λ [nm] (ε [103
M−1·cm−1])
MnCl-1
CoCl-1
434 (101), 442 sh (97)
428 (108), 444 sh (87)
549 (10), 593 (7), 640 (3)
548 (15), 588 (13),
635 (7)
CuCl-126
ZnCl-1
416, 430
534, 572, 625
419 (180), 436 (154)
542 (18), 580 (17),
627 (8)
ZnCl-
426 (174), 442 (151)
547 (11), 588 (10),
636 (3)
OTMPP28
Figure 2. Molecular packing of (a) CuCl-1 including the disordered
chloroform molecules and (b) ZnCl-1. The gray and black molecules
are related by a 2-fold screw axis along the b-axis. Hydrogen atoms are
omitted for clarity.
rings can be found in CoCl-1 and ZnCl-1. The complex
molecules are stacked along the a-axis alternating with their
inverted form with the shortest porphyrin plane distances of
4.57 Å and 4.44 Å, respectively (Figure 2b). Besides the
previously mentioned hydrogen bonds with an attached solvent
molecule, the chloride interacts with hydrogen atoms on
neighboring molecules. So, in all four complex structures one or
two weak hydrogen bonds are formed with CH···Cl distances in
the range of 2.77−2.95 Å. In general, only a small number of
weak CH−π interactions are observed (distances between 2.64
and 3.00 Å).
The electronic spectra of MnCl-1, CoCl-1, and ZnCl-1
(Figure 3, Table 3) resemble the spectra of the five-coordinate
oxaporphyrin complexes with iron(II),20 copper(II),26 and
zinc(II).28 They show three Q-bands and a splitting of the
Soret band, which can be easily explained by the decreased
symmetry with respect to the metal complexes of the regular
porphyrins.49 Expectedly, the electronic spectrum of ZnCl-1
coincidences well with the spectrum of ZnCl-OTMPP28,46
having an average hypsochromic shift of 5−9 nm.
Figure 4. Cyclic voltammograms for ZnCl-1, MnCl-1, and CoCl-1 in
dichloromethane at a Pt electrode and a scan rate of 100 mV s−1. The
voltammetric curve of ligand 1 is not displayed.
Table 4. Electrochemical Data for the First Electron
Transitions of 1 and Its Metal Complexes
1
1
1
E1/2 (red.) [V] vs
1/2(dmFc)
E1/2 (ox.) [V] vs
1/2(dmFc)
ΔE1/2
E
E
[V]
1
−1.04(−1.11)
−0.84 (−0.91)
−0.82 (−0.89)
−0.82 (−0.89)
1.14 (1.07)
1.34 (1.27)
2.18
2.18
1.99
1.79
ZnCl-1
CoCl-1
MnCl-1
a
The redox chemistry of the present metal porphyrins was
followed by cyclic voltammetry (Figure 4, Table 4). It is known
that the E1/2 values depend on the number and the nature of
donor atoms in the porphyrin core.14,50 In general, the
reduction potentials are raised to more positive values on
replacement of nitrogen by oxygen or sulfur. However, the
1.17 (1.10)
0.97 (0.90)
a
Irreversible values in brackets calcd. vs SCE data obtained on Pt wire
working electrode under inert conditions and recorded at 100 mV s−1
scan speed in 0.2 M TBAPF6/CH2Cl2 with dmFc as internal standard.
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influence of the metal ions Mn(II), Co(II), and Zn(II) on
reduction potentials is not as distinct as that of the ligand.
In the literature the first reduction of the complexes is
discussed either as a metal-centered [M(II)−M(I)] or ligand-
centered one-electron transfer.20,26 The potential of the first
reduction of free base oxaporphyrin 1 is at −1.04 V. The half-
wave potentials of MnCl-1, CoCl-1, and ZnCl-1 are nearly
identical and are in accordance with the second half-wave
potential of FeCl2-120 (−0.895 V). Thus, a ligand-centered
reduction seems most likely because of the presence of strongly
different metal atoms in the respective complexes. In contrast,
the first cathodic peaks of CuCl-1 (−0.42 V) and NiCl-1
(−0.22 V) are visible at more positive potentials and can
therefore be assigned to a metal-centered reduction.26 Probably
this is caused by the stronger in-plane coordination of Ni(II)
and Cu(II) with a smaller deviation from the plane (0.32 Å)
compared to MnCl-1, CoCl-1, and ZnCl-1 (0.53−0.66 Å). The
quasireversible second reduction peaks of MnCl-1, CoCl-1, and
ZnCl-1 (not shown in Figure 4) appear at similar half wave
potentials of −1.3 V. Compared to the Fe(II)/Fe(III) redox
process in FeCl2-1 (0.595 V),20 the first oxidation potentials of
the investigated complexes occur at more positive potential
values and rise in the order Mn(II), Co(II), and Zn(II). ZnCl-1
and MnCl-1 undergo only one oxidation process while CoCl-1
exhibits two anodic peaks. The first irreversible oxidation step
of CoCl-1 points to a possible chemical follow-up reaction to
[Co(III)Cl-1]+ or a kinetically hindered electron transfer
process. The Co(II) complex of 5,10,15,20-tetraphenylporphyr-
in (H2TPP) features the same chemical behavior at 0.52 V in
the first oxidation step to that discussed for Co(II)/Co(III)
Figure 5. NOESY spectrum of ZnClO4-2 in CDCl3 at 298 K. Dark
spots display NOE signals and gray spots features proton exchange.
The inset shows the changes of the aryl ring signals associated with
temperature variation.
oxidation.51 For H2TPP (Eox = 1.07 V) it is known that the
1
oxidation potential of the corresponding Zn(II) complex
appears at lower values (Eox = 0.83 V).52 21-Oxaporphyrin 1
1
and its Zn(II) complex show the opposite behavior. The
estimated electrochemical gap for the complexes under study
increases in the order Mn(II), Co(II), Zn(II). The addition of
one electron to the LUMO becomes slightly easier in metal
complexes.
Similar to the reported 1H NMR spectra of Zn(II)
oxaporphyrins,28 ZnCl-1 and ZnClO4-2 show a downfield
shift of almost all proton signals compared to their free base
spectra. Due to the decreased symmetry, a splitting of the aryl
proton signals is observed, in particular for the protons in the
ortho-position. While the signals of the aryl rings adjacent to
the furan ring are close to each other, the signals of the aryl
rings adjacent to the pyrrole ring show a split of about 0.4 ppm
(see Figure 5). In accordance with the results of the single
crystal X-ray diffraction studies, this difference in the magnitude
of splitting can be assigned to anion coordination deviating
slightly from the vertical toward the pyrrole ring. Furthermore,
the broadened signals of the aryl rings adjacent to the pyrrole
become sharper upon cooling and coalesce upon heating,
indicating a lower rotational energy barrier.28,53,54
Figure 6. COSY spectrum of the aliphatic region of ZnCl-3 in CDCl3
at 298 K. The peaks marked by asterisks belong to ethyl acetate and
water. The gray highlighted signals are assigned to the side chain
interacting with Zn(II).
1
The H NMR spectrum of ZnCl-3 shows signals in the
phenylic region analogous to ZnCl-1 and ZnClO4-1. The
signals in the aliphatic region arising from the methyl and
methylene groups can be classified into two signal sets,
demonstrating the existence of two different complex species
(A and B) in the solution. While one signal set is fairly similar
to the free base spectrum, the other one shows strongly shifted
and partially split signals (Figure 6, Table 5). Thus, the latter
signal set can be assigned to a complex species in which the side
chain functionality interacts with the metal center (species A).
Especially the upfield shifting of the diastereotopic methylene
proton signals indicates that these protons are located above
the porphyrin core. The other signal set arises from a species
with no interaction between the fifth donor position and the
zinc(II) central atom (species B). Due to an integral ratio of
approximately 3:2 (A:B), species A can be assigned to be the
preferred species in solution.
To obtain more information about the binding condition of
Zn(II) in ZnCl-3, a Zn K-edge EXAFS spectrum of a solid
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1
Table 5. H NMR Chemical Shifts of the Aliphatic Proton
Table 6. EXAFS Structural Parameters of the Zn(II)
Complex of 3
Signals of 3 and its Zn(II) Complex
a
b
c
d
3
ZnCl-3
shell
N
R [Å]
model a
σ2 [Å2]
error
a
m-Ar-CH3
2.55
4.00
1.20
2.70
2.60/2.50
a
f
f
f
f
f
f
f
OCH2CH2CH2OH
OCH2CH2CH2OH
OCH2CH2CH2OH
OH
3.80/4.00
Zn−N/O
Zn−Cl
Zn−O
Zn−Calpha
Zn−Cmeso
Zn−Cbeta
Zn−C/Oalkyl chain
4
1
1
8
4
8
4
2.04
2.32
2.45
3.05
3.36
4.01
4.51
0.0031
0.0051
0.0046
0.014
0.056
0.056
0.056
0.056
0.056
0.056
0.056
1.00, 1.10/1.20
1.95, 2.15/2.55
a
a
not obsvd.
0.4/0.6
a
0.0027
0.012
Signals of the two species in the order coordinated/noncoordinated
side chain.
0.0065
model b
sample was measured (Figure 7). The theoretical phase and
amplitude function used in data analysis were calculated using
f
f
f
f
f
f
Zn−N/O
Zn−Cl
Zn−Calpha
Zn−Cmeso
Zn−Cbeta
4
1
8
4
8
4
2.04
2.30
3.04
3.36
4.01
4.52
0.0037
0.005
0.062
0.062
0.062
0.062
0.062
0.062
0.013
0.0036
0.012
Zn−C/Oalkyl chain
0.0072
a
Errors in coordination numbers are 25%, and standard deviations as
b
c
estimated by EXAFSPAK. Errors in distance are 0.02 Å. Debye−
d
Waller factor. Error is given as the normalized fit error ∑(χdata(k)k3 −
χfit(k)k3)2/(P − F) with P = number of data points, F = number of
f
variables. Fixed for calculation.
fall in a smaller range from 2.0 Å to 2.1 Å.55,56 But longer
distances of 2.47 Å or 2.71 Å57 have also been reported.
According to model b, this third peak has no structural origin
and could be due to a side-lobe peak, originating from a
truncation effect due to the limited reciprocal space integrated
in the Fourier transform. The presence of such nonphysical
shells using EXAFS spectroscopy is well documented, e.g., for
uranyl ions with chloride.58,59 Because both models are
statistically equivalent best-fit models, the origin of the peak
at 2.45 Å cannot be clarified unambiguously. However,
compared to the crystal structure of ZnCl-1 a reduction of
the Zn−N/O distances and an enlargement of the Zn−Cl
distance are observed. Both changes indicate that Zn(II) is
closer to the plane defined by the four donor atoms of the
porphyrin. This fact can be explained by the interaction with
the additional donor function. As model a is also supported by
Figure 7. EXAFS spectra (left panel) and corresponding Fourier
transforms (right panel) for models of the ZnCl-3 complex.
X-ray data for ZnCl-1. The four shells found in the Fourier
transform can be deconvoluted to either seven (model a, Figure
8) or six (model b) peaks (Table 6). According to model a, the
1
the H NMR studies, an octahedral coordination geometry as
shown in Figure 9 can be assumed for ZnCl-3.
Figure 9. Proposed structure of ZnCl-3.
Figure 8. Deconvolution of different peaks obtained in the EXAFS
spectrum and its corresponding Fourier transform of ZnCl-3
according to model a.
Preliminary experiments of metal extraction from a metal salt
mixture of Co(II), Ni(II), Cu(II), and Zn(II) perchlorates in
aqueous solution with H2TPP and 1 dissolved in toluene
showed a pronounced selectivity for Cu(II) and Zn(II). While
both of the porphyrins extract Cu(II) quantitatively, the
extraction of Zn(II) increases significantly on insertion of
oxygen in the porphyrin core, from 58% to 99%, respectively.
Compared to Cu(II) and Zn(II), the extraction of Co(II) and
Ni(II) is very low (9−11%).
first shell, which is closest to Zn(II), splits into three
components. The first two components can be assigned to
the four donor atoms of the porphyrin (N3O) and the chloride
with distances of 2.04 Å and 2.32 Å. The third peak at 2.45 Å
corresponds to one oxygen atom and is most likely attributed
to the OH group of the side chain. In general, Zn−O distances
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removal via spline fitting techniques and normalization using a
Victoreen function. The theoretical scattering phase and amplitude
functions used in data analysis were calculated using FEFF7.61 The
shift in threshold energy, ΔE0, was varied as a global parameter in the
fits. For extraction studies, H2TPP or 1 (5 × 10−3 M) in toluene and a
metal salt mixture of Co(ClO4)2, Ni(ClO4)2, Cu(ClO4)2, and
Zn(ClO4)2 (each 1 × 10−4 M) together with NaClO4 (5 × 10−3 M)
was used. For pH adjustment a HEPES/NaOH buffer (pH = 6.9) was
applied. The samples were stirred at 308 K for 24 h. The
concentrations of metal ions in aqueous solution were determined
by ICP-MS (ELAN-9000, Perkin-Elmer). CAUTION. Perchlorate
complexes are potentially explosive and appropriate caution should be
exercised in their handling.
CONCLUSIONS
■
Along with the synthesis of a pentadentate 21-oxaporphyrin, a
convenient isolation procedure for oxaporphyrins is reported
via the corresponding Zn(II) complex. A crucial feature of this
procedure is the larger retention time obtained in comparison
to the corresponding free base porphyrins, leading to improved
chromatographic separation and isolation for the 21-oxapor-
phyrins.
Novel complexes of 21-oxaporphyrins have also been
synthesized. Structural and electrochemical characterization of
the Mn(II), Co(II), and Zn(II) tetraphenyl-21-oxaporphyrins
(MnCl-1, CoCl-1, ZnCl-1) were performed, and the Cu(II)
complex (CuCl-1) was characterized by X-ray diffraction for
the first time. In all complexes investigated the metal center is
coordinated by the four donor atoms of the oxaporphyrin and a
chloride to give a square-pyramidal geometry. The electro-
chemical studies of Mn(II), Co(II), and Zn(II) oxaporphyrins
exhibit redox behavior different from that of the known Cu(II)
and Ni(II) complexes.
2,5-Bis[(4-bromophenyl)hydroxymethyl]-furan (2a). In a
three-necked, round-bottom flask with rubber septum and gas inlet
tube tetramethylethylenediamine (1.9 mL, 0.01 mol) and furan (0.4
mL, 0.01 mol) were dissolved in 10 mL of dry hexane. After addition
of an n-butyllithium solution (25 mL, 1.60 M in n-hexane) and
refluxing for 1 h, the reaction mixture was cooled to 0 °C. A solution
of 4-bromobenzaldehyde (1.85 g, 0.01 mol) in 30 mL of THF was
added, and the mixture was stirred for another 10 min at room
temperature. The reaction was quenched with saturated NH4Cl
solution, and the aqueous layer was extracted with ethyl acetate. The
combined organic layer was dried over Na2SO4, and the solvent was
removed. The crude product was purified by column chromatography
on silica gel using ethyl acetate/heptane (3:7) as eluent. The desired
By introduction of a flexible side chain bearing a potential
donor atom, the preference for octahedral coordination of
Zn(II) could be verified via NMR and EXAFS studies.
However, there is a chemical equilibrium of the two complex
species with and without participation of the additional donor
function in solution.
1
diol 2a was obtained as the third fraction (1.07 g, 49%). H NMR
(CDCl3, δ in ppm) 3.0 (s, 2H, OH), 5.5 (s, 2H, CH), 5.9 (d, 2H, β-
furan-H), 7.2 (d, 4H, o-Ph-H), 7.4 (d, 4H, m-Ph-H).
2-[(2-Methoxy-5-methylphenyl)hydroxymethyl]-5-[(p-tolyl)-
hydroxymethyl]-furan (3a). Furan (3.6 mL, 0.05 mol) was
dissolved in 10 mL of dry THF in a three-necked, round-bottom
flask with rubber septum and gas inlet tube. An n-butyllithium solution
(14.4 mL, 1.67 M in n-hexane) was added slowly, and the reaction
mixture was refluxed for 1 h under slow argon purging. After cooling
to 0 °C an ice cold solution of 2.0 mL (0.02 mol) of 4-
methylbenzaldehyde in 10 mL of THF was added. The solution was
stirred for 10 min at room temperature followed by quenching of the
reaction with saturated NH4Cl solution. The organic layer was dried
over Na2SO4, and the solvent was removed. The crude product was
purified by column chromatography on silica gel using ethyl acetate/
heptane (3:7) as eluent. The mono-ol, 2-[(4-methylphenyl)-
hydroxymethyl]-furan, was obtained as the second fraction in almost
quantitative yield. 1H NMR (CDCl3, δ in ppm) 2.35 (s, 3H, CH3), 2.4
(s, 1H, OH), 5.8 (d, 1H, CH), 6.1 (d, 1H, β-furan-H), 6.3 (dd, 1H, β-
furan-H), 7.2 (d, 2H, m-Ar-H), 7.3 (d, 1H, o-Ar-H), 7.4 (d, 1H, β-
furan-H).
The mono-ol (3.00 g, 0.02 mol) was dissolved in 10 mL of dry THF
in a three-necked, round-bottom flask with rubber septum and gas
inlet. After cooling to 0 °C, n-butyllithium (28.6 mL, 1.67 M in n-
hexane) was dropped slowly into the solution, and it was stirred for 1 h
at 40 °C. The reaction mixture was cooled again to 0 °C and 2-
methoxy-5-methyl-benzaldehyde in 10 mL of THF was added. After
stirring for 10 min at room temperature and quenching with saturated
NH4Cl solution, the organic layer was dried over Na2SO4 and the
solvent was removed. The crude product was purified by column
chromatography on silica gel using ethyl acetate/heptane (3:7) as
eluent. The desired diol 3a was obtained as the third fraction (1.14 g,
21%). Due to the lability of the product, no NMR spectra could be
recorded; the product was directly used for a cyclization reaction.
5,10,15,20-Tetrakis(4-bromophenyl)-21-oxaporphyrin (2). A
solution of 2a (1.34 g, 0.003 mol), 4-bromobenzaldehyde (1.10 g,
0.006 mol) and pyrrole (0.6 mL, 0.009 mol) in 1 L of dry
dichloromethane was degassed and purged with argon. To start the
condensation reaction, BF3 diethyl etherate (0.04 mL, 0.3 mmol) was
added, and the reaction mixture was stirred for 2 h in the dark under
argon atmosphere at room temperature. After addition of 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ) (1.36 g, 0.006 mol) and stirring
on air for 10 h, the solvent was removed. The product 2 was purified
by column chromatography on basic aluminum oxide (chloroform)
EXPERIMENTAL SECTION
■
Materials. Column chromatography was carried out with silica gel
60 (0.040−0.063 mm Merck) or aluminum oxide 90 active basic
(0.063−0.2 mm Merck). 2,5-Bis(phenylhydroxymethyl)furan (1a) and
5,10,15,20-tetraphenyl-21-oxaporphyrin (1) were synthesized as
reported previously.17 Crystals for X-ray diffraction were prepared
by slow evaporation of the solvent from the metal complex solution in
chloroform/n-heptane (2:1) for CoCl-1, MnCl-1 or in chloroform/
ethyl acetate (1:1) for CuCl-1, ZnCl-1.
1
Instrumentation. H NMR spectra were measured on a Bruker
DRX-500 at 500 MHz. The signals are described as s: singlet, d:
doublet, m: multiplet, dd: double doublet, b: broad. UV/vis spectra
were recorded on a Perkin-Elmer Lambda 2. The mass spectra were
obtained on an Esquire Hewlett & Packard ESI-MS and a Shimadzu
Kratos Kompact MALDI II mass spectrometer. The cyclic voltammo-
grams were obtained with a PAR 273 potentiostat (EG&G, USA) in a
three-electrode system using platinum wires as working and counter
electrodes and a silver wire as pseudoreference electrode. Deca-
methylferrocene (dmFc) was added as internal standard. Dichloro-
methane (dried; Fluka), acetonitrile (puriss. absolute; Fluka), and
dmFc (p.a.; Merck) were used as received. Tetrabutylammonium
hexafluorophosphate (TBAPF6, Fluka, dried under reduced pressure at
340 K for 24 h prior to use; Fluka) was used as the supporting
electrolyte. Elemental analysis was carried out with a CHNS element
analyzer Hecatech EA 3000 Euro Vector. IR spectra were recorded on
a Nicolet ATR Avatar 360. X-ray data for CuCl-1 and ZnCl-1 were
collected at a Kappa APEX 2 diffractometer (Bruker-AXS). Solution
and structure refinement calculations were performed by SHELXS-97
and SHELXL-97.60 The C−Cl bond distances of the included solvent
molecule in CuCl-1 have been restrained to a target value of 1.70(1)
Å. X-ray data for MnCl-1 and CoCl-1 were collected at a Kappa CCD
diffractometer (Bruker-AXS). Zinc K-edge X-ray absorption spectra
were collected from a solid sample in a KBr pellet at the Hamburger
Synchrotronstrahlungslabor (HASYLAB Hamburg, Germany), Beam-
line E4. Measurements were performed by using a Si(111) double-
crystal monochromator and Si-coated mirrors for focusing and
rejection of higher harmonics. The data were collected in transmission
mode using an Ar-flushed ionization chamber. The EXAFS oscillations
were isolated from the raw, averaged data by removal of the pre-edge
background, approximated by a first-order polynomial, followed by μ0
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1
and obtained as a purple solid (0.42 g, 15%). H NMR (CDCl3, δ in
ppm) −1.7 (s, 1H, NH), 7.9 (2 d, 8H, m-Ar-H), 8.1 (2 d, 8H, o-Ar-
H), 8.6 (dd, 4H, β-pyrrolenine-H), 8.9 (s, 2H, β-pyrrole-H), 9.2 (s,
2H, β-furan-H); MALDI-MS obsd. mass 934, calcd. mass 930.9
(100%).
νCH), 2856 (m, νCH). MALDI-MS obsd. mass 748, calcd. mass
745.3.
Chloro-(5,10,15,20-tetraphenyl-21-oxaporphyrinato)-
manganese(II) (MnCl-1). A solution of 1 (0.05 g, 0.08 mmol) and
MnCl2·2H2O (0.12 g, 0.80 mmol) in 20 mL of 1:1 chloroform/
methanol was refluxed for 72 h. Product MnCl-1 was purified by
column chromatography on silica gel (ethyl acetate) and obtained as
brown purple solid (0.06 g, quant.). ESI-MS obsd. mass 669.1 (M −
Cl, 100%), calcd. mass 669.2 (M − Cl, 100%); Anal. Calcd C, 74.95;
H, 4.00; N, 5.96. Found C, 73.10; H, 4.05; N, 5.58.
Chloro-(5,10,15,20-tetraphenyl-21-oxaporphyrinato)cobalt-
(II) (CoCl-1). A solution of 1 (0.05 g, 0.08 mmol) and CoCl2·2H2O
(0.07 g, 0.40 mmol) in 20 mL of 1:1 chloroform/methanol was
refluxed for 24 h. Product CoCl-1 was purified by column
chromatography on silica gel (ethyl acetate) and obtained as a
brown purple solid (0.05 g, 85%). ESI-MS obsd. mass 673.1 (M − Cl,
100%), calcd. mass 673.2 (M − Cl, 100%); Anal. Calcd C, 74.53; H,
3.98; N, 5.93. Found C, 73.35; H, 4.10; N, 5.80.
Chloro-(5,10,15,20-tetraphenyl-21-oxaporphyrinato)-
copper(II) (CuCl-1). A solution of 1 (0.06 g, 0.09 mmol) and
CuCl2·2H2O (0.15 g, 0.90 mmol) in 20 mL of 1:1 chloroform/
methanol was refluxed for 24 h. Product CuCl-1 was purified by
column chromatography on silica gel (ethyl acetate) and obtained as a
purple solid (0.06 g, quant.). ESI-MS obsd. mass 677.1 (M − Cl,
100%), calcd. mass 677.2 (M − Cl, 100%); Anal. Calcd C, 74.05; H,
3.95; N, 5.98. Found C, 73.00; H, 4.10; N, 5.66.
5-(2-Methoxy-5-methylphenyl)-10,15,20-tri(p-tolyl)-21-oxa-
porphyrin (3b). The oxaporphyrin 3b was prepared under the same
experimental conditions as mentioned for 2. A solution of 3a (3.15 g,
0.009 mol), 4-methylbenzaldehyde (2.2 mL, 0.019 mol), and pyrrole
(1.9 mL, 0.028 mol) in 2 L of dry dichloromethane and BF3 diethyl
etherate (0.35 mL, 0.003 mol) was used. The oxidation was
accomplished by addition of DDQ (2.11 g, 0.009 mol) and stirring
in air for 10 h. For simplified separation the crude product was reacted
to the zinc(II) complex by adding 10 equiv of ZnCl2 and brief heating.
ZnCl-3b was obtained as a purple solid after column chromatography
on silica gel (0.70 g, 0.81 mmol). Demetalation was achieved by
treating ZnCl-3b in dichloromethane with conc. HBr solution (3 mL).
The organic layer was washed with water and NaHCO3 solution.
Column chromatography on basic aluminum oxide (dichloromethane/
ethyl acetate 5:1) afforded the free base porphyrin 3b as purple solid
1
(0.55 g, 9%). H NMR (CDCl3, δ in ppm) −1.4 (s, 1H, NH), 2.6 (s,
3H, m-Ar−CH3), 2.7 (s, 9H, p-Ar−CH3), 3.6 (s, 3H, OCH3), 7.3 (d,
1H, m−Ar-H), 7.6 (m, 7H, m/p−Ar-H), 7.8 (d, 1H, o−Ar-H), 8.1 (m,
6H, o−Ar-H), 8.6 (2 dd, 4H, β-pyrrolenine-H), 8.9 (2 d, 2H, β-
pyrrole-H), 9.2 (2 d, 2H, β-furan-H); MALDI-MS obsd. mass 704,
calcd. mass 701.3.
5-[2-(3-Hydroxypropoxy)-5-methylphenyl]-10,15,20-tri(p-
tolyl)-21-oxaporphyrin (3). A solution of ZnCl-3b (0.21 g, 0.30
mmol) and BBr3 (0.13 mL, 1.30 mmol) in 10 mL of dry
dichloromethane was stirred for 24 h at room temperature. After
cooling to 0 °C, 50 mL of water was added slowly to quench the
reaction. The aqueous layer was adjusted to pH 6 using a NaOH
solution (1 M) and extracted with ethyl acetate. The combined organic
phases were dried over Na2SO4, and the solvent was removed. The
hydroxyl functionalized porphyrin 3c was purified by column
chromatography on basic aluminum oxide (chloroform/ethyl acetate
20:1) and obtained as a purple solid (0.14 g, 79%). 1H NMR (CDCl3,
δ in ppm) 2.6 (s, 3H, m-Ar-CH3), 2.7 (3 s, 9H, p-Ar-CH3), 7.3 (d, 1H,
m-Ar-H), 7.6 (m, 7H, m/p-Ar-H), 7.8 (s, 1H, o-Ar-H), 8.1 (m, 6H, o-
Ar-H), 8.7 (m, 4H, β-pyrrolenine-H), 8.9 (s, 2H, β-pyrrole-H), 9.3 (d,
2H, β-furan-H); MALDI-MS obsd. mass 689, calcd. mass 687.3.
A solution of 3c (0.12 g, 1.80 mmol), 2-(3-bromopropoxy)-
tetrahydro-2H-pyran (0.06 mL, 0.53 mmol), K2CO3 (0.07 g, 0.53
mmol), and 18-crown-6 (0.001 g, 0.04 mmol) in dry acetonitrile was
refluxed for 48 h. After removal of the solvent, the crude product was
dissolved in dichloromethane. The solution was washed with water
and dried over Na2SO4. The tetrahydro-2H-pyran (THP) protected
porphyrin 3d was purified by column chromatography on basic
aluminum oxide (chloroform/ethyl acetate 9:1) and obtained as
purple solid (0.13 g, 86%). 1H NMR (CDCl3, δ in ppm) −1.5 (s, 1H,
NH), 1.5−1.9 (m, 8H, OCH2CH2CH2O/THP-CH2), 2.5 (s, 3H, m-
Ar-CH3), 2.7 (3 s, 9H, p-Ar-CH3), 3.1 (m, 2H, OCH2), 3.5 (m, 2H,
THP-OCH2), 4.0 (m, 3H, OCH2/THP-CH), 7.3 (d, 1H, m-Ar-H),
7.6 (m, 7H, m/p-Ar-H), 7.7 (s, 1H, o-Ar-H), 8.1 (m, 6H, o-Ar-H), 8.6
(m, 4H, β-pyrrolenine-H), 8.9 (s, 2H, β-pyrrole-H), 9.2 (d, 2H, β-
furan-H); MALDI-MS obsd. mass 831, calcd. mass 829.4.
Chloro-(5,10,15,20-tetraphenyl-21-oxaporphyrinato)zinc(II)
(ZnCl-1). A solution of 1 (0.15 g, 0.24 mmol) and ZnCl2 (0.17 g, 1.20
mmol) in 20 mL of 1:1 chloroform/methanol was refluxed for 24 h.
Product ZnCl-1 was purified by column chromatography on silica gel
1
(ethyl acetate) and obtained as a purple solid (0.17 g, quant.). H
NMR (CDCl3, δ in ppm) 7.7 (bs, 2H, para-H), 7.8 (m, 10H, m/p-Ph-
H), 8.05 (bd, 2H, o-Ph-H), 8.2 (d, 2H, o-Ph-H), 8.3 (d, 2H, o-Ph-H),
8.35 (bs, 2H, o-Ph-H), 8.85 (m, 4H, β-pyrrolenine/pyrrole-H), 9.0 (d,
2H, β-pyrrolenine-H), 9.35 (s, 2H, β-furan-H); ESI-MS obsd. mass
678.1 (M − Cl, 100%), calcd. mass 678.2 (M − Cl, 100%); Anal.
Calcd C, 73.86; H, 3.94; N, 5.87. Found C, 72.15; H, 4.30; N, 5.20.
Perchlorato-[5,10,15,20-tetrakis(4-bromophenyl)-21-
oxaporphyrinato]zinc(II) (ZnClO4-2). A solution of 2 (0.05 g, 0.05
mmol) and Zn(ClO4)2 (0.06 g, 0.16 mmol) in 70 mL of 2:5
chloroform/methanol was refluxed for 24 h. Product ZnClO4-2 was
purified by column chromatography on silica gel (ethyl acetate) and
1
obtained as a purple solid (0.06 g, quant.). H NMR (CDCl3, δ in
ppm) 7.9 (bm, 4H, o/m-Ph-H), 7.95 (m, 6H, m-Ph-H), 8.1 (d, 2H, o-
Ph-H), 8.15 (d, 2H, o-Ph-H), 8.22 (bd, 2H, o-Ph-H), 8.85 (m, 4H, β-
pyrrolenine/pyrrole-H), 9.0 (d, 2H, β-pyrrolenine-H), 9.4 (s, 2H, β-
furan-H); ESI-MS obsd. mass 995.8 (M − Cl, 100%), calcd. mass
995.8 (M − Cl, 100%).
Chloro-{5-[2-(3-hydroxypropoxy)-5-methylphenyl]-
10,15,20-tri(p-tolyl)-21-oxaporphyrinato}zinc(II) (ZnCl-3). A
solution of 3 (0.09 g, 0.12 mmol) and ZnCl2 (0.16 g, 1.2 mmol) in
20 mL of 1:1 chloroform/methanol was refluxed for 24 h. Product
ZnCl-3 was purified by column chromatography on silica gel (ethyl
1
acetate) and obtained as a purple solid (0.10 g, quant.). H NMR
Deprotection of the THP ether was done by treating a solution of
3d (0.13 g, 0.15 mmol) in 20 mL of chloroform/methanol (1:1) with
p-toluenesulfonic acid monohydrate (0.11 g, 0.60 mmol) and a drop of
water. The solution was stirred for 24 h at 40 °C. The solvent was
removed; the crude product was dissolved in chloroform and washed
with water. Product 3 was isolated after column chromatography on
silica gel (chloroform/methanol 9:1) as a purple solid (0.09 g, 80%).
1H NMR (CDCl3, δ in ppm) 1.20 (m, 2H, OCH2CH2CH2OH), 2.55
(s, 3H, m-Ar-CH3), 2.70 (m, 2H, OCH2CH2CH2OH), 2.80 (3 s, 9H,
p-Ar-CH3), 4.00 (m, 2H, OCH2CH2CH2OH), 7.25 (d, 1H, m−-Ar-
H), 7.60 (m, 7H, m/p-Ar-H), 7.80 (s, 1H, o-Ar-H), 8.10 (m, 6H, o-Ar-
H), 8.55 (m, 4H, β-pyrrolenine-H), 8.90 (s, 2H, β-pyrrole-H), 9.20 (d,
2H, β-furan-H); IR 3500 (w, br, νOH), 3233 (w, νNH), 2920 (m,
(CDCl3, δ in ppm) 0.4 (bs, OH), 0.6 (bs, OH′), 1.00−1.10 (m, 1.4H,
OCH2CH2CH2OH), 1.20 (m, 0.6H, OCH2CH2CH2OH′), 1.95 and
2.15 (2 m, 2H, OCH2CH2CH2OH), 2.50 (s, 1H, m-Ar-CH3′), 2.55
(m, 1H, OCH2CH2CH2OH′), 2.60 (s, 2H, m-Ar-CH3), 2.70 (m, 9H,
p-Ar-CH3), 3.80 (t, 1.2H, OCH2CH2CH2OH), 4.00 (m, 0.8H,
OCH2CH2CH2OH′), 7.20 (s, 1H, m-Ar-H), 7.50−7.65 (m, 7H, m/
p-Ar-H), 7.90 (bs, 2H, o-Ar-H), 7.95 (s, 1H, p-Ar-H), 8.10 (m, 1H, o-
Ar-H), 8.20 (m, 1H, o-Ar-H), 8.25−8.30 (bm, 2H, o-Ar-H), 8.85 (m,
4H, β-pyrrolenine/pyrrole-H), 9.00 (d, 2H, β-pyrrolenine-H), 9.35 (s,
2H, β-furan-H); MALDI-MS obsd. mass 810 (M − Cl, 100%), calcd.
mass 808.3 (M − Cl, 100%); IR 3430 (w, νOH), 3022 (w, νCH),
2920 (m, νCH).
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ASSOCIATED CONTENT
■
S
* Supporting Information
ski, L.; Olmstead, M. M.; Balch,
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Crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org. These data can also be
obtained free of charge from the CCDC (numbers 916565−
916568) via www.ccdc.cam.ac.uk/data_request/cif, by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Center, 12, Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: karsten.gloe@chemie.tu-dresden.de.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank Steffen Seifert and the
HASYLAB, Hamburg, Germany, for recording the EXAFS
spectrum. Furthermore we are grateful for the determination of
metal concentrations in aqueous solution by ICP-MS measure-
ments at the Helmholtz Center Dresden-Rossendorf, Germany.
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