Pirouetting copper(I)-assembled pseudo-rotaxanes: Strong influence of the axle structure on the motion rate

Article abstract of DOI:10.1002/ejic.200700215

Based on the gathering and threading effect of copper(I), three different pseudo-rotaxanes have been prepared and characterised in solution. They consist of the same two-chelate ring and various coordinating fragments threaded through the ring. The ring-incorporated chelates are a 2,2′,6′,6″- terpyridine (terpy) unit and a 2,9-diphenyl-1,10-phenanthroline (dpp) motif. The threaded chelates are a 2,9-bis(p-anisyl)-1,10-phenanthroline (dap), a 6,6′-bis(p-anisyl)-2,2′-bipyridine (dabipy) or a 8,8′-diphenyl-3,3′-biisoquinoline (dpbiiq) derivative. Using electrochemical techniques, the pirouetting motions of the axle within the ring through which it is threaded have been investigated. In dichloromethane/ acetonitrile, a strong dependence of the motion rate on the structure of the threaded chelate has been observed, evidencing that rigidity of the thread-contained chelate and steric hindrance lead to slow moving molecular machines. Pirouetting is the main dynamic process observed but unthreading of the string-like organic component represents another minor electrochemically induced reaction. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700215

FULL PAPER
DOI: 10.1002/ejic.200700215
Pirouetting Copper(I)-Assembled Pseudo-Rotaxanes: Strong Influence of the
Axle Structure on the Motion Rate
Jean-Paul Collin,[a] Fabien Durola,[a] Pierre Mobian,[a] and Jean-Pierre Sauvage*[a]
Keywords: Pseudo-rotaxanes / Diimine ligands / Copper complexes / Coordination modes / Pirouetting motion
Based on the gathering and threading effect of copper(I),
three different pseudo-rotaxanes have been prepared and
characterised in solution. They consist of the same two-che-
late ring and various coordinating fragments threaded
through the ring. The ring-incorporated chelates are a
2,2Ј,6Ј,6ЈЈ-terpyridine (terpy) unit and a 2,9-diphenyl-1,10-
phenanthroline (dpp) motif. The threaded chelates are a 2,9-
bis(p-anisyl)-1,10-phenanthroline (dap), a 6,6Ј-bis(p-anisyl)-
2,2Ј-bipyridine (dabipy) or a 8,8Ј-diphenyl-3,3Ј-biisoquino-
line (dpbiiq) derivative. Using electrochemical techniques,
the pirouetting motions of the axle within the ring through
which it is threaded have been investigated. In dichloro-
methane/acetonitrile, a strong dependence of the motion rate
on the structure of the threaded chelate has been observed,
evidencing that rigidity of the thread-contained chelate and
steric hindrance lead to slow moving molecular machines.
Pirouetting is the main dynamic process observed but
unthreading of the string-like organic component represents
another minor electrochemically induced reaction.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
ties of pseudo-rotaxanes. The first one is particularly sig-
nificant since it triggered the interest of the scientific com-
munity for interlocking compounds-based molecular ma-
chines.[9] Later on, our group has made and investigated a
shuttle-like pseudo-rotaxane[10] and its corresponding rot-
axane.[11] Although shuttling of the pseudo-rotaxane was
clearly evidenced, we could also identify another process,
the unthreading reaction, which interfered with the ring
translation motion. Obviously, the rates of both processes
must be related since some of the rearrangement steps for
these distinct reactions are identical.
Introduction
The field of synthetic molecular machines and motors is
receiving much attention,[1] either in relation to catenanes
and rotaxanes[2] or based on non-interlocking molecules.[3]
In this former family of compounds, transition metal-con-
taining systems have been much investigated.[4] As far as
interlocking molecular assemblies are concerned, transition
metals play an equally important role. Our group has pro-
posed several types of dynamic systems, the mobile compo-
nents of the machine being set in motion either by an elec-
trochemical signal,[5] a photochemical stimulus[6] or a chem-
ical reaction.[7] Copper-containing pirouetting rotaxanes
represent an interesting subclass of such dynamic systems.
They can be regarded as simple models of future rotary
motors, whose elaboration seems today to be highly com-
plex when interlocking compounds are used.[8] They can
also be of interest by themselves as bistable species whose
states display very different geometrical properties.
In this article we would like to report on the synthesis
and the dynamic properties of three copper-complexed
pseudo-rotaxanes whose motions are triggered by a redox
process (CuII/CuI). By comparing the electrochemical be-
haviour of these three compounds, important information
on the influence of the structural parameters on the motion
rate could be obtained. As far as we are aware, limited stud-
ies have been carried out on the controlled dynamic proper-
Results and Discussions
The pirouetting rearrangement described herein is
sketched in Scheme 1. The three threaded species have been
prepared according to the classical procedure from our
group, based on coordination of two bidentate chelates on
a copper(I) centre.[12] Obviously, the cyclic nature of 4 pre-
vents formation of tetrahedral copper(I) complexes con-
sisting of two such dpp-containing ligands. This simple
principle has important consequences since the coordina-
tion reaction will necessarily lead to the formation of a thre-
aded species provided the correct copper(I)-two-bidentate
ligand stoichiometry is respected.[13] The copper(I)-induced
gathering and threading reaction is represented in Figure 1
along with the chemical formulae of the various precursors
and the products.
[a] Laboratoire de Chimie Organo-Minerale, UMR 7177 du CNRS
Institut de Chimie, Université Louis Pasteur,
1 rue Blaise Pascal, 67000 Strasbourg Cedex, France
Fax: +33-390-241-368
Compound 4 is a 33-membered ring that has been used
in several studies carried out in our group on controlled
dynamic catenanes and rotaxanes. The presence of the terpy
E-mail: sauvage@chimie.u-strasbg.fr
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Pirouetting Copper(I)-Assembled Pseudo-Rotaxanes
FULL PAPER
markedly different chelates. 5 incorporates a dpp unit. It is
thus a highly rigid block due to the presence of the 1,10-
phenanthroline core. At the same time it is rather bulky due
to the two phenyl groups close to the coordination site of
the ligand. 6 is equally sterically hindering but the pyridine
nuclei of the 2,2Ј-bipy fragment can easily rotate with re-
spect to one another about the C2–C2Ј bond. Finally, the
dpbiiq fragment of 7 is such that steric hindrance around
the complexing site of the ligand is limited, due to the large
separation between the two lateral phenyl rings borne by
the 3,3Ј-biisoquinoline nucleus. In addition, free rotation
about the C3–C3Ј bond allows this ligand to rearrange eas-
ily. Ligand 7 could be obtained in high yield via a two-step
procedure as depicted in Figure 2. The first step concerns
the formation of diphenol compound 9 from 8,8Ј-dianisyl-
3,3Ј-biisoquinoline (8) using a classical ether deprotection
method. The resulting diphenol 9 was then converted into
the target ligand 7 following a classical Williamson method-
ology (Cs2CO3, DMF, 55 °C) by reaction with 2 equiv. of
commercially available 2-(6-bromohexyloxy)-tetrahydro-
2H-pyran.
Scheme 1. Pirouetting principle of copper complexed pseudo-rotax-
anes. The U-shaped symbols represent bidentate chelates (dpp for
the ring-comprised chelate and dap, dabipy or dpbiiq for the che-
lates belonging to the thread). The M-shaped symbol designates
the tridentate terpy coordinating motif.
+
+
site in its structure does not interfere with the threading
Three complexes 14+, 24 and 34 were synthesized as
reaction since copper(I) interacts preferably with bidentate shown in Figure 1. The synthesis of these copper complexes
chelates such as dpp. This behaviour has been unambigu- were performed using a methodology usually employed in
1
ously demonstrated by H-NMR analysis of the concerned our group. In a typical procedure, the macrocycle 4 was
pseudo-rotaxanes. The three string-like fragments contain dissolved in a degassed solution of dichloromethane and
+
+
Figure 1. Copper(I)-directed synthesis of the three pseudo-rotaxanes 14+, 24 and 34 described in this article. The subscript 4 refers to
the coordination number of the copper centre. In the copper(I) complexes of the graphic, the coordination number is equal to 4 since
the metal is coordinated to a dpp fragment and another bidentate chelate belonging to the thread.
Eur. J. Inorg. Chem. 2007, 2420–2425
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J.-P. Collin, F. Durola, P. Mobian, J.-P. Sauvage
FULL PAPER
characteristic of copper(I) complexes of aromatic diimine
ligands, indicates that the complexation process occurred
efficiently. After, the solution was stirred for two hours un-
der argon at room temperature, and the solvents were evap-
orated. The complexation process occurred quasi-quantita-
vely as evidence by 1H-NMR spectroscopy. Due to their
+
instability on chromatographic supports, complexes 14+, 24
or 34+ were not further purified and were directly submitted
to the electrochemical measurements.
Electrochemically-Driven Molecular Motions
The cyclic voltammograms of Scheme 2 have been ob-
tained by scanning first from –0.4 V to +1.0 V, with a silver
wire as a pseudo-reference electrode, the return scan being
carried out immediately after the potential of +1.0 V was
reached. The oxidation potential for the reaction
Figure 2. Preparation of ligand 7 from 8,8Ј-bis(p-anisyl)-3,3Ј-biiso-
quinoline (8). The demethylation reaction is quantitative. Com-
pound 7 can be obtained in good yield from 9 after chromato-
graphic separation.
acetonitrile (1:1). Upon addition of the copper salt
(1 equiv.), the solution turned immediately to dark orange.
4-coordinate CuI complex – e– Ǟ 4-coordinate CuII complex
The reaction mixture was stirred for 20 min and the diimine is thus easily read from the cyclic voltammograms. Their
ligand (respectively 5, 6 and 7) was then added. A dark red values at a scan rate of 800 mV/s are indicated on the
solution was instantaneously obtained. This color, which is Scheme 2.
Scheme 2. Cyclic voltammograms of complexes 14+, 24+ and 34+ recorded with a Pt working electrode in CH2Cl2/CH3CN (1:9) with 0.1 
Bu4NBF4 at four different scan rates (20, 100, 400 and 800 mV/s). The electrochemical responses of the four-coordinate copper(II)
complexes are found around +0.7 V (for 12+ and 22+) or around +0.5 V (for 32+), whereas reduction of five-coordinate copper(II) com-
2+
2+
plexes 152+, 25 and 35 occurs around –0.1 V.
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Pirouetting Copper(I)-Assembled Pseudo-Rotaxanes
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At a high scan rate, the return wave corresponding to the motion, which does not exclude that a certain amount led
following process
4-coordinate CuII complex + e– Ǟ 4-coordinate CuI complex
to unthreading and dissociation.
By comparing 1n+ and 2n+, especially at 100 mV/s, it is
2+
obvious that 14 is much less mobile than 242+. This is
is easily observed, which allows to determine E1/2 and ∆Ep
for the three studied complexes:
142+/14+: E1/2 = +0.76 V
242+/24+: E1/2 = +0.77 V
342+/34+: E1/2 = +0.49 V
2+
confirmed at 400 mV/s since at this scan rate, 14 does not
seem to rearrange significantly whereas the major part of
242+ is converted into the rearranged complex 252+. The dif-
ference between 1n+ and 2n+ holds to the presence of a 1,10-
phenanthroline (phen) unit in the thread of the former com-
plex and 2,2Ј-bipyridine (bipy) chelate for the latter com-
∆Ep varies between 80 mV and 120 mV within the series plex. The comparison between their electrochemical proper-
of three complexes. At a lower scan rate, rearrangement of ties allows to estimate the effect of having a rigid block
the thermodynamically unstable four-coordinate copper(II) (phen) vs. a potentially flexible chelate (bipy) with rotation
takes place, at least in part, before this complex is reduced around the C–C bond connecting the two pyridine nuclei.
back to its monovalent corresponding state. This is clearly It should be noticed that both threads (in 1n+ and 2n+) are
seen by the presence of a peak between –0.2 V and 0 V.
From the E1/2 values of the four-coordinate complexes, number of phenyl rings at identical positions.
it is clear that the four-coordinate copper(I) state is more Very spectacular is the effect of the biisoquinoline ligand,
stabilised in 14 and 24 than in complex 34+. This can eas- as demonstrated by the electrochemical behaviour of 3n+
ily be understood by considering the steric arrangements Even at 800 mV/s reversibility is almost completely lost,
equally sterically hindering since they contain the same
+
+
.
+
+
of the compounds. Whereas 14 and 24 contain a highly with the appearance of a relatively intense wave at ≈ –0.1 V
shielding set of ligands (both chelates of each complex bear corresponding to the reduction process of the rearranged
phenyl rings attached on the ortho positions to the N complex 352+. This observation is directly related to the non
+
atoms), the thread of 34 is less encumbering.
sterically hindering nature of 7 which facilitates ligand ex-
As far as the five-coordinate copper(I) or copper(II) change in the coordination sphere of the metal. The pres-
states are concerned, important information can be ob- ence of two elongating groups attached on the phenolic
tained at low scan rate. By analogy with the oxidation po- oxygen atoms of 7 may also increase the chemical reversibil-
tentials of the four-coordinate copper(I) complexe, the re- ity of the pirouetting process, inhibiting at least in part the
duction potentials of the corresponding five-coordinate unthreading reaction.
copper(II) complexes can easily be estimated from the cyclic
A rough estimation of the rate constant for the re-
voltammograms. They are also indicated on Scheme 2, for arrangement of the four-coordinate copper(II) complex can
a scan rate of 20 mV/s. It is noteworthy that the reduction be obtained from electrochemical measurements and, in
potentials (CuII/CuI) for the three studied complexes are particular, from Scheme 2:
almost identical (Ered ≈ –0.07 V). This observation tends to
2+
14 Ǟ 152+, k ≈ 0.1 s–1
indicate that, as expected, the five-coordinate state is less
24 Ǟ 252+, k ≈ 1 s–1
2+
sensitive to steric hindrance than the tetrahedral or pseudo-
tetrahedral geometry.
2+
34 Ǟ 352+, k ≈ 5 s–1
The chemical reversibility of the redox processes depends
much on the nature of the complex and on the scan rate.
Concerning the pirouetting motion, pseudo-rotaxane
3
n+, incorporating a dpbiiq fragment in its thread, is about
As already observed with pirouetting rotaxanes,[5] as the
scan rate is increased, the reversibility of the four-coordi-
nate copper(I) complex oxidation reaction increases. By
scanning at a lower and lower rate, rearrangement of the
system becomes more and more noticeable, as evidenced by
the increase of the wave corresponding to the reduction of
the five-coordinate copper(II) complex. In parallel, the re-
duction wave for the four-coordinate complex decreases.
Generally speaking, if this latter one is intense, this implies
50 times more efficient than pseudo-rotaxane 1n+, which
threaded ligand (dap) is more rigid and more sterically hin-
dering.
As far as the rearrangement of the five-coordinate cop-
per(I) complexes is concerned, the reaction was shown to
be too fast for the presently used electrochemical tech-
niques. This is in good agreement with previous studies.[5]
that the compound has no time to rearrange substantially.
Conclusion
2+
By contrast, if the reduction wave around +0.42 V (for 34
)
is small, the complex undergoes a complete rearrangement
to a large proportion.
Three copper(I)-assembled pseudo-rotaxanes were syn-
thesized. These molecular machine precursors contain the
If the scan rate is sufficently slow (Ͻ 20 mV/s), the three same two-chelate ring and three different diimine ligands
complexes have time to rearrange as shown by the absence threaded through the macrocycle. The rearrangement of
of any reduction wave at +0.4 V to +0.8 V. On the other these complexes from the four-coordinate CuII to the five-
hand, the presence of a reduction wave at a slightly negative coordinate CuII species has been investigated by cyclic vol-
potential may indicate that a certain proportion of the four- tammetry. The electrochemical responses clearly show a
coordinate copper(II) complex underwent a pirouetting great difference of dynamic behaviour between these three
Eur. J. Inorg. Chem. 2007, 2420–2425
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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J.-P. Collin, F. Durola, P. Mobian, J.-P. Sauvage
FULL PAPER
3.40 (m, 4 H), 1.93–1.42 (m, 28 H) ppm. ES-MS: m/z = 809.4720
prepared pseudo-rotaxanes. An estimation of the rate of li-
gand exchange was possible. These results highlight that the
flexibility of the threaded chelate and low steric hindrance
of its coordination site give rise to fast-moving molecular
machines.
(calculated 809.4530 for C52H60N2O6 + H+).
+
+
+
CuI Complexes 14
, 24 and 34 : In a typical procedure,
Cu(CH3CN)4PF6 was added to a stirred and degassed solution of
macrocycle 4 dissolved in CH2Cl2/CH3CN (50:50). A deep orange
coloration appeared instantly. After 30 min at room temperature,
the diimine ligand (respectively 5, 6 or 7) was added as a solid to
the solution, which immediately turned dark red. After the solution
was stirred for two hours under argon at room temperature, the
solvents were removed under high vacuum. A dark red solid was
obtained quasi-quantitavely (NMR control). Due to its instability
on alumina and on silica gel, complexes were not further purified
and were directly submitted to the electrochemical measurements.
Experimental Section
The following chemicals were obtained commercially and were used
without further purification: 2-(6-bromohexyloxy)-tetrahydro-2H-
pyran (Aldrich), cesium carbonate (Aldrich).
Dry solvents were obtained with suitable dessicants: N,N-dimethyl-
formamide was distilled from anhydrous aluminum oxide, dichloro-
methane from calcium hydride. Fluka aluminum oxide (no. 17994)
was used for alumina columns and filtrations.
1
14+: H NMR (CD2Cl2, 300 MHz): δ = 8.76 (d, J = 7.2 Hz, 2 H),
8.68 (br., 2 H), 8.49 (d, J = 8.4 Hz, 2 H), 8.45 (d, J = 8.7 Hz, 2 H),
8.41 (br., 2 H), 8.10 (t, J = 7.2 Hz, 1 H), 8.04 (s, 2 H), 7.99 (s, 2
H), 7.89 (d, J = 8.4 Hz, 2 H), 7.85 (d, J = 8.7 Hz, 2 H), 7.75 (br.,
2 H), 7.48 (d, J = 8.7 Hz, 4 H), 7.40–7.34 (m, 4 H), 6.11–6.00 (m,
8 H), 3.49 (s, 6 H), 3.25 (br., 4 H), 2.95 (br., 4 H), 2.14 (br., 4 H)
1H NMR spectra were recorded with a Bruker AVANCE 300
(300 MHz, 1H) spectrometer using deuterated solvents. The spectra
were collected at 25 °C and the chemical shifts were referenced to
ppm. ES-MS: m/z
= 1132.3584 (calculated 1132.3606 for
1
residual solvent protons as internal standards. H: [D6]DMSO δ =
C71H55CuN7O4+).
2.50 ppm, CD2Cl2 δ = 5.32 ppm, CD3CN δ = 1.93 ppm. Mass spec-
tra were obtained with a VG ZAB-HF spectrometer (FAB) and a
VG-BIOQ triple quadrupole in positive or negative mode (ES-MS).
1
24+: H NMR (CD2Cl2, 300 MHz): δ = 8.84 (d, J = 7.8 Hz, 2 H),
8.75 (br. s, 2 H), 8.51 (d, J = 7.8 Hz, 2 H), 8.40 (d, J = 8.4 Hz, 2
H), 8.05 (t, J = 7.5 Hz, 1 H), 7.92 (s, 2 H), 7.84 (d, J = 8.4 Hz, 4
H), 7.74–7.67 (m, 4 H), 7.57 (d, J = 8.7 Hz, 4 H), 7.35 (d, J =
6.9 Hz, 2 H), 7.21 (d, J = 8.7 Hz, 4 H), 6.36 (d, J = 8.7 Hz, 4 H),
5.95 (d, J = 8.7 Hz, 4 H), 3.69 (br., 4 H), 3.35 (s, 6 H), 3.06 (br., 4
H), 2.29 (br., 4 H) ppm. ES-MS: m/z = 1108.3585 (calculated
1108.3606 for C69H55CuN7O4+).
Electrochemical measurements were performed with a three-elec-
trode system consisting of a platinum working electrode, a platinum
wire counter electrode, and a silver wire as a pseudo-reference elec-
trode. All measurements were carried out at room temperature un-
der Ar, in degassed spectroscopic grade solvents, using 0.1 
nBu4NBF4 solutions in CH2Cl2/CH3CN (1:9) as supporting elec-
trolyte. An EG&G Princeton Applied Research model 273A po-
tentiostat connected to a computer was used (software from Elec-
trochemistry Research).
34+: H NMR (CD3CN, 300 MHz): δ = 9.03 (s, 2 H), 8.71 (br. s, 2
1
H), 8.59 (d, J = 8.4 Hz, 2 H), 8.55 (s, 2 H), 8.50 (d, J = 7.8 Hz, 2
H), 8.32 (s, 2 H), 8.12 (t, J = 7.8 Hz, 1 H), 8.04 (d, J = 8.4 Hz, 2
H), 8.01 (s, 2 H), 7.80–7.74 (m, 6 H), 7.56–7.53 (m, 6 H), 7.15 (d,
J = 8.7 Hz, 4 H), 6.86 (d, J = 8.4 Hz, 4 H), 6.14 (d, J = 8.7 Hz, 4
H), 4.48 (t, J = 4.5 Hz, 2 H), 3.94–3.90 (m, 4 H), 3.74–3.60 (m, 4
H), 3.35 (br., 8 H), 2.90 (br., 4 H), 2.03 (br., 4 H), 1.70–1.38 (m,
28 H) ppm. ES-MS: m/z = 1548.6520 (calculated 1548.6533 for
C97H95CuN7O8+).
8,8Ј-Dihydroxyphenyl-3,3Ј-biisoquinoline (9): 8,8Ј-Dianisyl-3,3Ј-bi-
isoquinoline (8) (1.00 g, 2.13 mmol) and pyridinium chloride
(about 10 equiv., 2.5 g) are mixed into a little flask and heated to
reflux in an adapted microwave oven for 10 min. Twice, the same
quantity of pyridinium chloride is added and the mixture heated
to reflux again for 10 min. The mixture is dissolved in distilled
water (1 L) and then neutralised with sodium hydroxide to give a
suspension of a pale yellow precipitate. After filtration, the given
compound is quantitatively obtained without further purification
Acknowledgments
1
(pale yellow solid, 94 mg, 100%). M.p. Ͼ 300 °C. H NMR ([D6]-
Funding from the following institutions is gratefully acknowledged:
the Centre National de la Recherche Scientifique (CNRS) and the
Région Alsace (fellowship to F. D.), the European Commission
(MOLDYNLOGIC) and the European Communities (BIOMACH
NMP-2002-3.4.1.1-3, contract 505487-1; financial support and fel-
lowship to P. M.).
DMSO, 300 MHz): δ = 9.35 (s, 2 H), 9.09 (s, 2 H), 8.15 (d, J =
8.2 Hz, 2 H), 7.94 (t, J = 7.7 Hz, 2 H), 7.65 (d, J = 7.0 Hz, 2 H),
7.45 (d, J = 8.4 Hz, 4 H), 7.00 (d, J = 8.4 Hz, 4 H) ppm. ES-MS:
m/z = 441.1559 (calculated 441.1598 for C30H20N2O2 + H+).
Ligand 7: 2-(6-Bromohexyloxy)-tetrahydro-2H-pyran (180 mg,
0.68 mmol), cesium carbonate (150 mg, 0.45 mmol) and 8,8Ј-dihy-
droxyphenyl-3,3Ј-biisoquinoline 9 (50 mg, 0.11 mmol) were mixed
in dry DMF (10 mL) and stirred under Ar at 55 °C overnight. The
solvent was removed and the residue was taken up with CH2Cl2/
H2O. The organic phase was separated and the aqueous phase ex-
tracted twice with dichloromethane. The combined organic phases
were washed first with brine, then with distilled water. The solvent
was removed and the residue purified by chromatography over alu-
minium oxide by using dichloromethane/pentane (1:4) as the eluent
to give the title compound (white solid, 60 mg, 65%). 1H NMR
(CD2Cl2, 300 MHz): δ = 9.47 (s, 2 H), 8.98 (s, 2 H), 8.00 (d, J =
8.4 Hz, 2 H), 7.78 (dd, J = 8.1, 8.4 Hz, 2 H), 7.55 (d, J = 8.1 Hz,
2 H), 7.54 (d, J = 8.7 Hz, 4 H), 7.12 (d, J = 8.7 Hz, 4 H), 4.60 (t,
J = 3.7 Hz, 2 H), 4.12 (t, J = 6.6 Hz, 4 H), 3.92–3.74 (m, 4 H), 3.54–
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Received: February 16, 2007
Published Online: April 26, 2007
Eur. J. Inorg. Chem. 2007, 2420–2425
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