One-step synthesis of 4,4′-dicyano-2,2′-bipyridine and its bis(4,4′-di-tert-butyl-2,2′-bipyridine)ruthenium(II) complex

Article abstract of DOI:10.1002/ejic.200701304

This report describes a new route for the fast, economical and effective one-step synthesis and facile workup procedure of 4,4′-dicyano-2,2′- bipyridine (dnbpy) and its corresponding ruthenium complexes. The complex [(tbbpy)₂Ru(dnbpy)]-PF₆)₂ was prepared for the first time (tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) and the solid-state molecular structure was investigated with the help of a single-crystal X-ray analysis. The influence of the dnbpy ligand on the metal-to-ligand charge-transfer (MLCT) processes of the complex was studied. The compound shows two absorption maxima in the MLCT region of the UV/Vis spectrum at 418 and 510 nm and the emission is 116 nm redshifted in comparison to that of [(tbbpy)₃Ru](PF₆)₂. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701304

FULL PAPER
DOI: 10.1002/ejic.200701304
One-Step Synthesis of 4,4Ј-Dicyano-2,2Ј-bipyridine and Its Bis(4,4Ј-di-tert-
butyl-2,2Ј-bipyridine)ruthenium(II) Complex
Sebastian Losse,[a] Helmar Görls,[a] Robert Groarke,[b] Johannes G. Vos,[b] and Sven Rau,*[a]
Keywords: Nitrogen heterocycles / Bipyridine / Ruthenium / Coordination chemistry
This report describes a new route for the fast, economical and
effective one-step synthesis and facile workup procedure of
4,4Ј-dicyano-2,2Ј-bipyridine (dnbpy) and its corresponding
ruthenium complexes. The complex [(tbbpy)2Ru(dnbpy)]-
(PF6)2 was prepared for the first time (tbbpy = 4,4Ј-di-tert-
butyl-2,2Ј-bipyridine) and the solid-state molecular structure
was investigated with the help of a single-crystal X-ray
analysis. The influence of the dnbpy ligand on the metal-to-
ligand charge-transfer (MLCT) processes of the complex was
studied. The compound shows two absorption maxima in the
MLCT region of the UV/Vis spectrum at 418 and 510 nm and
the emission is 116 nm redshifted in comparison to that of
[(tbbpy)3Ru](PF6)2.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
route, a one-step synthesis of the ligand, presented here is
a fast, economical and effective approach to this synthon.
In order to investigate the effect of the cyano substituent
on the reactivity of the bipyridine ligand, we prepared the
corresponding ruthenium(II) complex [(tbbpy)2Ru-
(dnbpy)](PF6)2 (1) (tbbpy is 4,4Ј-di-tert-butyl-2,2Ј-bipyr-
idine). The formation of 1 has been verified by NMR spec-
troscopy and high-resolution mass spectrometry. Moreover,
for the first time the solid-state molecular structure of a
dnbpy-containing compound has been investigated by sin-
gle-crystal X-ray analysis. The absorption and emission
measurements of 1 were recorded to investigate MLCT pro-
cesses which are correlated with the homoleptic rutheni-
um(II) complex [(tbbpy)3Ru](PF6)2 (2).
Introduction
The preparation of 4,4Ј-dicyano-2,2Ј-bipyridine (dnbpy)
is a known multistep synthesis with a low overall yield.[1,2]
The compound, which is used as a functional ligand for
transition metal complexes,[3] is an important intermediate
for the preparation of potential anticancer agents[2] and acts
as a precursor in ligand design for several luminescent ru-
thenium(II) bipyridine-calix[4]arene complexes.[4] The cy-
ano function itself can be used for several reactions, e.g. to
synthesise carboxylic acids, amines or amidines. Moreover,
this function can also be utilised for the synthesis of com-
pounds like 2-aryl-4,6-bis(2-pyridyl)-s-triazine.[5] It has also
been shown that the cyano function may serve as a donor
site for the formation of heterooligonuclear metal com-
plexes.[6] Cyano moieties have been introduced by modifica-
tion of appropriately substituted bipyridines with the help
of Pd-catalysed reactions.[7,8] Considering the possible ap-
plications and the potential of further reactions at the cy-
ano group, the multistep low-yield synthesis represents a
serious drawback for further investigations. The electronic
structure of the 4,4Ј-dicyano-2,2Ј-bipyridine will most likely
differ from that of the normal bipyridine as the substituents
will lower the energy of the LUMO (π*). This fact might
be exploited for the tuning of photophysical properties in
metal complexes featuring metal-to-ligand charge-transfer
(MLCT) excitation and emission. The simplified synthetic
Results and Discussion
Synthesis of dnbpy and [(tbbpy)2Ru(dnbpy)](PF6)2
The coupling reaction of 4-cyanopyridine in the presence
of 10% Pd/C as a catalyst under reflux conditions lead to
the formation of dnbpy (Scheme 1). Working under ex-
clusion of oxygen was essential in this reaction.
[a] Friedrich-Schiller-Universität Jena, Institut für Anorganische
und Analytische Chemie,
August-Bebel-Straße 2, 07743 Jena, Germany
Fax: +49-3641-948102
E-mail: sven.rau@uni-jena.de
[b] Dublin City University, SFI Strategic Research Cluster for
Solar Energy Conversion, School of Chemical Sciences,
Dublin 9, Ireland
Scheme 1. Coupling reaction of 4-cyanopyridine.
4448
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 4448–4452

Synthesis of 4,4Ј-Dicyano-2,2Ј-bipyridine and Its Ruthenium(II) Complex
Scheme 2. Complexation reaction for the synthesis of 1, numbering scheme is the same as that used in Figure 1.
When the reaction was performed under aerobic condi-
tions no conversion was observed. The modest yield of 20%
was more than compensated by a facile workup procedure
which consists of a simple recrystallisation from chloro-
form. In addition, the starting materials, especially the cata-
lyst, could be reused in further preparations. In contrast
to the reaction of tert-butylpyridine with Pd/C where large
quantities of the corresponding 4,4Ј,4ЈЈ-tri-tert-butyl-
2,2Ј:6Ј,2ЈЈ-terpyridine derivative may be obtained,[9] no cy-
ano-substituted terpyridine could be observed under these
conditions. The reaction of the dnbpy ligand with a suitable
ruthenium precursor, (tbbpy)2Ru(Cl)2, was performed in or-
der to assess the reactivity of this ligand in the formation of
transition metal complexes. Complex 1 could be obtained in
the presence of AgNO3 in dried dmf and under microwave
Figure 2. Solid-state molecular structure of the ruthenium(II) com-
irradiation in more than 70% yield (Scheme 2). However, it
could not be isolated when the reaction was attempted un-
der standard conditions,[10] i.e. reflux in ethanol–water mix-
tures. These observations suggest that the chelating nitrogen
functionalities are deactivated because of the strong elec-
tron-withdrawing effect of the cyano groups. The 1H NMR
spectrum of 1 shows the typical signals for a ruthenium(II)
bipyridine complex (Figure 1). The singlet at δ = 8.79 ppm
(H3) and the two doublets at δ = 7.95 and 7.70 ppm (H6
and H5, respectively) are low-field shifted compared to the
tbbpy signals and can be assigned to dnbpy on the basis of
two-dimensional NMR experiments. The number and posi-
tion of all signals in the NMR spectrum suggest a symmet-
rical binding of the dnbpy to the ruthenium centre.
plex 1 containing dnbpy.
The ruthenium centre is surrounded by the three bipyr-
idine ligands in a distorted octahedral coordination geome-
try. The six Ru–N distances fall in the range of 2.044(5)–
2.054(5) Å with bite angles of the ligands of 78.24(18)° for
tbbpy and 79.3(3)° for dnbpy. These values are similar to
those reported for the [(tbbpy)3Ru]2+ complex.[11]
Complex 1 exhibits a horizontal mirror plane which ex-
plains the symmetrical NMR spectra observed. Selected
bond lengths and angles are reported in Table 1. Complex
2 was prepared under standard conditions as described in
the literature.[10,11]
Table 1. Selected bond lengths [Å] and angles [°] for 1.
Ru–N1
Ru–N4
Ru–N3A
N2–C6
N1–Ru–N1A 79.3(3)
N4A–Ru–N3 78.24(18)
N3A–Ru–N3 88.5(3)
2.044(5)
2.045(5)
2.054(5)
1.151(11)
Ru–N1A
Ru–N3
Ru–N4A
C3–C6
N4–Ru–N3
N1A–Ru–N4A
N1–Ru–N4
2.044(5)
2.054(5)
2.045(5)
1.440(11)
78.25(18)
87.95(18)
87.95(18)
1
Figure 1. 400 MHz H NMR spectrum of 1 in CD3CN; aromatic
region, dnbpy signals are labelled, all other signals can be assigned
to the two tbbpy ligands.
Absorption and Emission Spectra
The UV/Vis and emission data of 1 and 2 in acetonitrile
The recording of a high-resolution mass spectrum (see and dichloromethane are presented in Table 2. The excited-
Experimental Section) confirms the formation of 1 and the state lifetimes of 1 and 2 at different temperatures and in
molecular structure was further verified by a single-crystal different solvents are given in Table 3. The absorption spec-
X-ray analysis (Figure 2).
tra of both complexes 1 and 2 in acetonitrile exhibit UV
Eur. J. Inorg. Chem. 2008, 4448–4452
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4449

S. Losse, H. Görls, R. Groarke, J. G. Vos, S. Rau
FULL PAPER
transitions corresponding to intraligand transitions at
210 nm and 288 nm. For the homoleptic tbbpy complex 2
a single maximum at 460 nm was observed in the visible
range (Figure 3) and has been assigned as a 1MLCT transi-
tion. The ruthenium complex 1 exhibits two maxima at
418 nm and 510 nm which are indicative of two different
1
1MLCT transitions. Relative to the energy of the MLCT
1
band of 2, the lowest-energy MLCT band is redshifted by
50 nm and the high-energy MLCT band, presumably Ru–
tbbpy-based, is hypsochromically shifted by 42 nm (Fig-
ure 3). These observations are similar to the values ob-
served for the heteroleptic ruthenium complexes with dicar-
boxylic esters of 2,2Ј-bipyridine.[11,12] Recently, we synthe-
sised a ruthenium complex with carboxylic methylester
groups [(tbbpy)Ru(dmcb)]2+ whose absorption spectrum
shows two MLCT maxima as well (dmcb = 4,4Ј-dimeth-
oxycarbonyl-2,2Ј-bipyridine). In an acetonitrile solution the
first MLCT band occurs at 426 nm and the second at
492 nm, which implies a hypsochromic shift of 34 nm and
a bathochromic shift of 32 nm compared with 2. In this
compound we have assigned the transition at 492 nm to the
Ru–dmcb 1MLCT using resonance Raman spectroscopy.
These comparisons support the assumption that the dicya-
noligand is a stronger π-acceptor than tbbpy because of a
stronger –M-effect. So the two distinct maxima of the hetero-
leptic complex 1 may be assigned as d–π* (dnbpy) for the
low-energy maximum at 510 nm and d–π* (tbbpy) for the
high-energy maximum at 418 nm. This interpretation is fur-
ther supported by electrochemical data. For 1 in acetonitrile
the metal-based oxidation is observed at 1.34 V vs. SCE,
while three quasi-reversible ligand-based reductions are ob-
served at –0.86, –1.44 and –1.73 V. The rather positive first
reduction process, about 400 mV more positive than ob-
served for 2, can be explained by the strong π-acceptor
properties of the dnbpy ligand.
Figure 3. Absorption spectra in acetonitrile for [(tbbpy)2Ru-
(dnbpy)]2+ (1) (solid) and [(tbbpy)3Ru]2+ (2) (dashed).
shows an emission wavelength of 691 nm in acetonitrile
which is 77 nm redshifted compared to 2. On the basis of
the absorption and electrochemical data we propose that
the emission of 1 occurs exclusively from the lowest-energy
3MLCT state localised on the dnbpy ligand. Ruthenium
complexes emitting in the red are known in the litera-
ture.[13,14] Most of these compounds are difficult to synthe-
sise because large π-systems are used to generate the red
emission which makes it a complicated and often multistep
preparation. The presented dnbpy-containing ruthenium
complex is emissive in the red at room temperature and in
fluids without further substituents and can be obtained eas-
ily. The lifetimes obtained at lower temperature correlate
well with the energy gap law with lower lifetimes for 1
(emission at 730 nm) in comparison with 2 (emission at
613 nm).
Table 2. UV/Vis and emission data of 1 in acetonitrile and dichloro-
methane.
Sample
Solvent
λmax [nm]
Emission [nm]
1
1
CH3CN
CH2Cl2
285, 312, sh. 395, 418, 510 730
285, 312, sh. 395, 418, 512 730
Table 3. Excited-state liftetime of 1 and 2 in aerated dichlorometh-
ane (DCM) and acetonitrile (ACN) at room temperature and in a
4:5 mixture of propionitrile (PCN)/butyronitrile (BuCN) at 77 K.
Figure 4. Emission spectra in acetonitrile of [(tbbpy)2Ru(dnbpy)]2+
(1) with an excitation wavelength of 280 nm (solid), 410 nm
(dashed) and 510 nm (dotted); [(tbbpy)3Ru]2+ (2) at 460 nm
(dashed-dotted); uncorrected at room temperature.
Sample
Solvent
Temperature
Lifetime [ns]
1
1
1
2
2
2
DCM
ACN
4:5 PCN/BuCN 77 K
DCM
ACN
4:5 PCN/BuCN 77 K
room temperature
room temperature
200
134
3580
264
105
room temperature
room temperature
Conclusions
5697
The ligand dnbpy was prepared in a one-step synthesis
for the first time. The bipyridine derivative can easily be
While the emission spectrum of 2 has a maximum at obtained by a fast and facile workup procedure. The com-
614 nm in acetonitrile (Figure 4), for 1 a maximum is ob- plexation reaction of dnbpy on the ruthenium(II) fragment
served at 730 nm. This represents a bathochromic shift of (tbbpy)2Ru(Cl)2 resulting in the corresponding complex 1 is
116 nm compared to 2. The complex [(tbbpy)Ru(dmcb)]2+ only possible under microwave irradiation in DMF with the
4450
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 4448–4452

Synthesis of 4,4Ј-Dicyano-2,2Ј-bipyridine and Its Ruthenium(II) Complex
The pale yellow precipitate was collected on a frit, washed with
addition of AgNO3. All analytical data confirm the forma-
tion of 1. The solid-state structure of 1 has been investi-
gated and the obtained bond lengths and angles lie in the
expected range. The electrochemical, absorption and emis-
sion data of the ruthenium complex 1 lead to the conclusion
that dnbpy has a lower-energy π*-state than tbbpy and
dmcbpy. With the now established one-step synthesis of
dnbpy, larger amounts of this important building block can
be isolated faster and much more economically. The appli-
cation for the manipulation of MLCT processes in rutheni-
um(II) complexes was demonstrated.
ethanol and dried in vacuo. Yield: 6.2 g (20%). MS (DE): m/z (%)
1
= 206 (100), 179 (20), 103 (20). H NMR (400 MHz, CDCl3): δ =
8.85 (d, J = 5.2 Hz, 2 H), 8.69 (s, 2 H), 7.58 (d, J = 4.8 Hz, 2 H)
ppm. 13C NMR (50 MHz, CDCl3): δ = 116.32, 121.82, 123.1, 125.8,
150.3, 155.4 ppm.
[(tbbpy)2Ru(dnbpy)](PF6)2 (1): Ru(tbbpy)2Cl2 (200 mg, 0.28 mmol),
dnbpy (58.2 mg, 0.28 mmol) and AgNO3 (96 mg, 0.56 mmol) were
dissolved in DMF (100 mL). The reaction mixture was heated to
reflux under microwave irradiation for 2 h at 200 W. After cooling
to room temperature, filtration and evaporation of the solvent, eth-
anol was added to give a clear solution from which the product
could be precipitated by adding an aqueous solution of NH4PF6.
The solid was isolated, washed with diethyl ether and dried in
vacuo. The crude product was purified by column chromatography
on silica with acetonitrile as eluent. Crystals suitable for the X-ray
diffraction were obtained from a methanol/water solution. Yield:
236 mg (74%). MS (ESI in acetonitrile and methanol): m/z (%) =
989 (100) [M – PF6]+, 422 (10) [[M – 2PF6)/2]2+. MS (HR-ESI in
methanol): m/z = 983.3189 [M – PF6]+, calcd. (C48H54F6N8P6Ru)
Experimental Section
General: 1H and 13C NMR spectra were recorded with a Bruker
400 MHz/200 MHz spectrophotometer, and UV/Vis spectra were
obtained with a Analytic Jena SPECORD® S 600. The mass spec-
tra were recorded with a SSQ 170, Finnigan Mat spectrometer.
Electrospray mass spectra were recorded with a Finnnigan MAT,
MAT 95 XL. The high-resolution mass spectrum peak for 1 is given
for the 96Ru isotope. All other values quoted for m/z are for the
most-intense peak of the isotope pattern. Emission spectra were
recorded with a Perkin–Elmer LS50B spectrometer equipped with
a Hamamatsu R928 red-sensitive detector. Electrochemical experi-
ments were carried out using a CHI750C electrochemical bipo-
tentiostat. Cyclic voltammograms and differential pulse voltammo-
grams were recorded against a saturated calomel reference elec-
trode (SCE). Glassy carbon (GC) and platinum (Pt) macro elec-
trodes were used as the working electrodes and a platinum wire was
employed as the counter electrode. A solution of 0.1  TBA PF6
(Fluka, electrochemical grade) in acetonitrile (Aldrich, anhydrous,
99.8%) was used. Luminescence lifetime measurements were ob-
tained using an Edinburgh Analytical Instruments (EAI) time-
correlated single-photon-counting apparatus (TCSPC) comprised
of two model J-yA monochromators (emission and excitation), a
single-photon photomultiplier detection system model 5300 and a
F900 nanosecond flashlamp (nitrogen-filled at 1.1 atm pressure,
40 kHz or 0.3 atm pressure, 20 kHz) interfaced with a personal
computer by a Norland MCA card. A 410 nm cut-off filter was
used in emission to attenuate scatter of the excitation light
(337 nm); luminescence was monitored at the λmax of the emission.
1
983.3189. H NMR (400 MHz, CD3CN): δ = 8.79 (s, J = 1.2 Hz,
2 H), 8.47 (d, J = 2 Hz, 2 H), 8.45 (d, J = 2.0 Hz, 2 H), 7.95 (d, J
= 6.0 Hz, 2 H), 7.7 (dd, J = 1.6 and 6.0 Hz, 2 H), 7.47 (m, 4 H),
7.41 (dd, J = 2.0 and 10 Hz, 2 H), 7.36 (dd, J = 2.0 and 10 Hz, 2
H), 1.39 (s, 36 H) ppm. 13C NMR (100 MHz, CD3CN): δ = 30.4,
36.4, 116.5, 120.9, 122.7, 125.8, 126.0, 127.9, 130.3, 151.6, 152.2,
154.0, 157.2, 157.4, 158.5, 164.4, 164.5 ppm.
Crystal Structure Determination: The intensity data for the com-
pound was collected on a Nonius Kappa CCD diffractometer using
graphite-monochromated Mo-Kα radiation. Data were corrected
for Lorentz-polarisation effects, but not for absorption effects.[15–16]
The structure was solved by direct methods (SHELXS[17]) and re-
2
fined by full-matrix least-squares techniques against Fo
(SHELXL-97).[18] The hydrogen atoms were included at calculated
positions with fixed thermal parameters. All non-hydrogen atoms
were refined anisotropically.[18] XP (SIEMENS Analytical X-ray
Instruments, Inc.) was used for structure representations.
CCDC-667619 (for 1) contains the supplementary crystallographic
data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/datarequest/cif.
Data correlation and manipulation was carried out using EAI F900
software version 6.24. Emission lifetimes were calculated using a
single-exponential fitting function, Levenberg–Marquardt algo-
rithm with iterative deconvolution (Edinburgh instruments F900
software). The reduced v2 and residual plots were used to judge
the quality of the fits. Lifetimes are Ϯ5%.
Acknowledgments
We thank the Foundation for the technology, innovation and re-
search in Thuringia (STIFT), the Deutsche Forschungsgemein-
schaft (DFG) and the University of Jena for financial support. S. L.
thanks the scholarship programme of the German Federal Envi-
ronmental Foundation (DBU) for financial support. J. G. V thanks
Science Foundation Ireland for supporting this work.
4-Cyanopyridine and 10% Pd/C were used as purchased from
Fluka and [(tbbpy)3Ru](PF6)2 (2) was prepared according to a lit-
erature procedure.[10,11]
Preparations
[1] G. Maerker, F. H. Case, J. Am. Chem. Soc. 1958, 80, 2745–
2748.
[2] J. Stanek, G. Caravatti, H.-G. Capraro, P. Furet, H. Mett, P.
Schneider, U. Regenass, J. Med. Chem. 1993, 36, 46–54.
[3] P. N. W. Baxter, J. A. Connor, J. Organomet. Chem. 1995, 486,
115–121.
[4] P. D. Beer, F. Szemes, P. Passaniti, M. Maestri, Inorg. Chem.
2004, 43, 3965–3975.
[5] M. I. J. Polson, E. A. Medlycott, G. S. Hanan, L. Mikelsons,
N. J. Taylor, M. Watanabe, Y. Tanaka, F. Loiseau, R. Passalac-
qua, S. Campagna, Chem. Eur. J. 2004, 10, 3640–3648.
4,4Ј-Dicyano-2,2Ј-bipyridine (dnbpy): The reaction was carried out
under argon to prevent oxygen at all times. 4-Cyanopyridine (30 g,
0.29 mol) and 10% Pd/C (900 mg) were placed into a 500 mL
round-bottomed flask and heated to reflux for 24 h. After cooling
to room temperature, chloroform was added (250 mL). The dark
black suspension was filtered through a frit to give a pale yellow
solution under removal of the catalyst Pd/C. Chloroform was evap-
orated until the product started to crystallise. Pentane (50 mL) was
added and the concentrated solution was stored in a fridge for 5 h.
Eur. J. Inorg. Chem. 2008, 4448–4452
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4451

S. Losse, H. Görls, R. Groarke, J. G. Vos, S. Rau
FULL PAPER
[6]
[7]
[8]
[13] E. Ioachim, E. A. Medlycott, G. S. Hanan, F. Loiseau, S. Cam-
pagna, Inorg. Chim. Acta 2006, 359, 766–774.
[14] E. Ioachim, E. A. Medlycott, G. S. Hanan, F. Loiseau, V. Rice-
vuto, S. Campagna, Inorg. Chem. Commun. 2005, 8, 559–563.
[15] COLLECT, Data Collection Software; Nonius B. V., Nether-
lands, 1998.
[16] Z. Otwinowski, W. Minor, “Processing of X-ray Diffraction
Data Collected in Oscillation Mode”, in Methods in Enzy-
mology, vol. 276, Macromolecular Crystallography, Part A.
(Eds.: C. W. Carter, R. M. Sweet), Academic Press, 1997, pp.
307–326.
[17] G. M. Sheldrick, Acta Crystallogr., Sect. A 1990, 46, 467.
[18] G. M. Sheldrick, SHELXL-97 (release 97–2), University of
Göttingen, Germany, 1997.
P. N. W. Baxter, J. A. Connor, J. Organomet. Chem. 1988, 355,
193–196.
J. Wang, Y.-Q. Fang, G. S. Hanan, F. Loiseau, S. Campagna,
Inorg. Chem. 2005, 44, 5–7.
M. Borgström, S. Ott, R. Lomoth, J. Bergquist, L. Ham-
marström, O. Johansson, Inorg. Chem. 2006, 45, 4820–4829.
[9] T. B. Hadda, H. Le Bozec, Polyhedron 1988, 7, 575–577.
[10] S. Rau, B. Schäfer, A. Grüßing, S. Schebesta, K. Lamm, J.
Vieth, H. Görls, D. Walther, M. Rudolph, U. W. Grummt, E.
Birkner, Inorg. Chim. Acta 2004, 357, 4496–4503.
[11] M. Schwalbe, B. Schäfer, H. Görls, S. Rau, S. Tschierlei, M.
Schmitt, J. Popp, G. Vaughan, W. Henry, J. G. Vos, Eur. J. In-
org. Chem. 2008, 21, 3310–3319.
[12] W. F. Wacholtz, R. A. Auerbach, R. H. Schmehl, Inorg. Chem.
1986, 25, 227–234.
Received: December 3, 2007; revised June 6, 2008
Published Online: August 26, 2008
4452
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 4448–4452