Assembly and stepwise oxidation of interpenetrated coordination cages based on phenothiazine

Article abstract of DOI:10.1002/anie.201302536

A breath of fresh air is sufficient for the eightfold S-monooxygenation of an interpenetrated double cage based on eight phenothiazine ligands and four square-planar-coordinated Pd^II cations. Besides these two cages, which were both characterized by X-ray crystallography, an eightfold S-dioxygenated double-cage was obtained under harsher oxidation conditions.

Full text of DOI:10.1002/anie.201302536

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Communications
Coordination Cages
A breath of fresh air is sufficient for the
eightfold S-monooxygenation of an inter-
penetrated double cage based on eight
phenothiazine ligands and four square-
planar-coordinated PdII cations. Besides
these two cages, which were both char-
acterized by X-ray crystallography, an
eightfold S-dioxygenated double-cage
was obtained under harsher oxidation
conditions.
M. Frank, J. Hey, I. Balcioglu, Y.-S. Chen,
D. Stalke, T. Suenobu, S. Fukuzumi,
H. Frauendorf,
G. H. Clever*
&&&& — &&&&
Assembly and Stepwise Oxidation of
Interpenetrated Coordination Cages
Based on Phenothiazine
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DOI: 10.1002/anie.201302536
Coordination Cages
Assembly and Stepwise Oxidation of Interpenetrated Coordination
Cages Based on Phenothiazine**
Marina Frank, Jakob Hey, Ilker Balcioglu, Yu-Sheng Chen, Dietmar Stalke,
Tomoyoshi Suenobu, Shunichi Fukuzumi, Holm Frauendorf, and Guido H. Clever*
Research on coordination cages[1] which self-assemble from
organic ligands and metal ions is shifting more and more from
a focus on structural aspects towards the implementation of
function.[2] In this sense, coordination cages have found
application in fields such as selective anion recognition,[3]
sequestering of hazardous substances,[4] drug delivery,[5]
stabilization of reactive reagents and intermediates,[6] catal-
ysis,[7] material science, and photophysical devices.[8]
have been widely used as electron-donor components in
systems capable of photoinduced charge separation,[16] as
charge carriers in light-powered molecular machines,[17] and
as redox shuttles in lithium-ion batteries.[18] To enhance the
electro- and photochemical properties and to be able to study
cooperative phenomena, Mꢀller and co-workers have pre-
pared phenothiazine-based conjugated oligomers[19] and
rings[20] by using a covalent approach based on C C cross-
À
For the area of non-silicon photovoltaics[9] as well as for
the related topic of molecular electronics,[10] discrete redox-
active (in)organic compounds show potential for technolog-
ical application[11] because of their tuneable optical and
electronic properties, monodispersity, and bottom-up inte-
gration into complex aggregates of defined morphology.[12]
The pharmaceutically important heterocycle phenothiazine
(1)[13] serves as a promising candidate for the preparation of
such materials because of its beneficial redox behavior of
readily undergoing a reversible one-electron oxidation to its
radical cation (E0 =+ 0.73 V in acetonitrile versus SCE)[14]
with the possibility of further oxidation or disproportionation
processes.[15] Consequently, monomeric phenothiazine species
coupling reactions.[21] Although this route was shown to
deliver a variety of multimeric phenothiazine derivatives, it
suffers from the typical material losses associated with
multistep synthesis and the need for tedious purification
protocols.
Research on the rational self-assembly of metal–organic
architectures such as rings, cages,[1] and knots[22] has shown,
however, that the synthetic limitations in the preparation of
discrete oligomers can be overcome by applying supramolec-
ular coordination chemistry.[23]
Here, we report the spontaneous and quantitative cluster-
ing of eight phenothiazine units into an interpenetrated
double cage by self-assembly as well as its eightfold mono-
and dioxygenation. The design is based on our previously
reported interpenetrated double cages comprising dibenzo-
[*] M. Frank, J. Hey, I. Balcioglu, Prof. Dr. D. Stalke,
Prof. Dr. G. H. Clever
suberone
backbones
and
square-planar-coordinated
PdII ions.[24,25] Our current and previous work, as well as the
benzophenone-based interpenetrated double cage reported
by Kuroda and co-workers[26] show that the [Pd4L8] design
principle tolerates the incorporation of a variety of backbone
structures, including the electrochemically interesting com-
pound phenothiazine.
Institute for Inorganic Chemistry
Georg-August University Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: gclever@gwdg.de
Homepage: http://www.clever-lab.de
Dr. H. Frauendorf
Institute for Organic and Biomolecular Chemistry
Georg-August University Gçttingen
Tammannstrasse 2, 37077 Gçttingen (Germany)
Ligand 3 was synthesized in three steps starting from
commercially available 10H-phenothiazine (1) by N alkyla-
tion with n-hexylbromide, a subsequent selective bromination
at the 3,7-positions to give compound 2,[19a] followed by
Sonogashira cross-coupling with 2 equiv of 3-ethinylpyridine
(Scheme 1). Heating a 2:1 mixture of ligand 3 and [Pd-
(CH3CN)4](BF4)2 at 708C for 6 h in acetonitrile resulted in the
quantitative formation of the interpenetrated coordination
Dr. Y.-S. Chen
Center for Advanced Radiation Source (ChemMatCARS)
The University of Chicago c/o APS/ANL (USA)
Prof. Dr. T. Suenobu, Prof. Dr. S. Fukuzumi
Department of Material and Life Science, Graduate School of
Engineering, Osaka University, ALCA, Japan Science and Technology
Agency (JST), Suita, Osaka 565-0871 (Japan)
1
compound 6, as confirmed by H NMR spectroscopy (Fig-
Prof. Dr. S. Fukuzumi
Department of Bioinspired Science, Ewha Womans University
Seoul 120-750 (Korea)
ure 1a) and high-resolution ESI mass spectrometry (Fig-
ure 2a). Upon conversion of ligand 3 into the highly
1
symmetric double cage 6, all the H NMR signals split into
[**] M.F. thanks the Evonik Foundation and J.H. the CaSuS program of
Lower Saxony for PhD fellowships. We thank the DFG (CL 489/2-1),
the FCI, and the HeKKSaGOn consortium for support, and Dr. M.
John for NMR measurements. ChemMatCARS and the Advanced
Photon Source are supported by the NSF Department of Energy
(NSF/CHE-0822838, DE-AC02-06CH11357).
two sets of equal intensity, in full accordance with our
previous findings with the dibenzosuberone-based sys-
tems.[24,25] The 1H NMR signals of the pyridine moiety
undergo a considerable downfield shift, which indicates the
binding to the palladium(II) cations. The signals of the
phenothiazine backbone protons and the N-CH2 group of the
hexyl residue show an upfield shift upon cage formation
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201302536.
2
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Figure 2. ESI mass spectra of the double cages a) 6, b) 7, and c) 8
(* denotes ClÀ-containing species [2Cl+BF4@6+nBF4](5Àn)+; n=0–2).
Scheme 1. Synthesis and assembly of phenothiazine-based coordina-
tion cage 6 and oxidation to cages 7 and 8. a) n-Hexylbromide, KOtBu,
THF; b) Br2, HOAc; c) 3-ethinylpyridine, CuI, [PdCl2(PPh3)2], NEt3;
d) Cu(NO3)2·3H2O, CH2Cl2, ultrasound; e) m-CPBA, CH2Cl2; f) Fe,
HOAc; g) [Pd(CH3CN)4](BF4)2, CD3CN; h) two months exposure to air.
Nevertheless, the resonances are found in the ppm range
expected for a double-cage species (see the Supporting
Information). We assume that ligand 4 exists as a quickly
equilibrating mixture of two ring-flip isomers with the oxygen
substituent in either the favored pseudo-axial or the pseudo-
equatorial position[28] which leads to a mixture of several
diastereomeric double cages upon reaction with PdII.[29]
Next, we performed the oxidation of ligand 3 under
harsher conditions by using meta-chloroperbenzoic acid
(m-CPBA) as the oxidizing reagent, which resulted in the
formation of an S-dioxygenated derivative with N-oxidized
pyridine residues.[30] Reductive recovery of the free pyridine
rings with iron powder in acetic acid yielded sulfone ligand 5.
1
The H NMR spectrum of 5 is similar to that of derivative 4,
with the main difference being a further downfield shift of the
proton next to the sulfone group (proton a in Figure 1d) and
an upfield shift of its N-CH2 proton signal.
Heating a 2:1 mixture of ligand 5 and [Pd(CH3CN)4]-
(BF4)2 in acetonitrile leads again to the formation of an
interpenetrated double cage 8, as can be rationalized from the
ESI mass spectrometric analysis (Figure 2c) and the corre-
Figure 1. a–e) 1H NMR spectra (300 MHz, CD3CN, 293 K) of ligands 3,
4, and 5 (2.80 mm) and cages 6 and 8 (0.35 mm; 3, 6: X = S; 4: X =
1
sponding H NMR spectrum (Figure 1e). All proton signals
of 8 are relatively sharp, which is similar to the spectrum of
nonoxygenated double cage 6, but different to the spectrum
of monooxygenated double cage 7.
=
=
S O: 5, 8: X = S( O)2).
(Figure 1a). The ESI mass spectrum of double cage 6 shows
a series of species [3BF4@6 + nBF4](5Àn)+ (n = 0–2) containing
a variable number of BF4À counterions (Figure 2a).
Further characterization of the different ligands and
corresponding cages was carried out by UV/Vis and fluores-
cence spectroscopy (Figure 3). Ligand 3 shows two absorption
bands with maxima at 287 nm and 368 nm. The spectra of
ligands 4 and 5 are similar to each other (with absorption
maxima at 359 nm and 362 nm, respectively), but quite
distinct from that of the nonoxidized derivative 3 (Figure 3a).
The UV/Vis spectrum of double cage 6 shows two absorptions
(Figure 3b) which are shifted to longer wavelengths com-
pared to the spectrum of 3. Remarkably, the spectrum of
double cage 7 shows two intensive absorption maxima,
whereas the parental ligand 4 shows only one. The absorption
maximum of 8 is only slightly shifted compared to ligand 5.
Nevertheless, both spectra are clearly distinguishable. All
Subsequently, we converted ligand 3 into the S-mono-
oxygenated ligand 4 by a two-electron oxidation process with
Cu(NO3)2·3H2O in CH2Cl2 as a mild oxidizing reagent.[27]
Compared to 3, the backbone signals in the 1H NMR
spectrum of 4 show a significant downfield shift as a result
of the oxidation of the sulfur atom (Figure 1c). Like its
precursor, sulfoxide 4 can be converted into an interpene-
trated double cage 7. Whereas the ESI mass spectrum
confirms a clean and quantitative formation of 7 in solution
(Figure 2b), the interpretation of the 1H NMR spectra
between 238 and 298 K is encumbered by signal broadening.
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Figure 3. a) UV/Vis spectra of ligands 3, 4, and 5 (0.35 mm) and
b) cages 6, 7, and 8 (0.04 mm). c) Fluorescence spectra of ligands 3, 4,
and 5 (2.80 mm; intensities normalized, excitation at the absorbance
maxima). d) Photographs of solutions of the ligands in acetonitrile
irradiated at 365 nm (left) and the cages under white light (right).
three ligand derivatives are highly luminescent, as shown by
their fluorescence spectra (Figure 3c) and photographs of
their solutions in acetonitrile illuminated at 365 nm (Fig-
ure 3d). Ligand 3 shows a strong yellow-green fluorescence,
while the emissions of 4 and 5 are blue-shifted. In contrast to
the ligands, no emission of the corresponding double cages
was observed, most probably because of quenching by the
metal cations.
Single crystals of nonoxidized double cage 6 suitable for
X-ray structure determination were grown by slow diffusion
of diethyl ether into a solution of 6 in acetonitrile. The
structure reveals that the D4-symmetric double cage features
three pockets, all filled with a BF4À ion (Figure 4a). Two
further BF4À ions occupy positions near the outer faces of the
À
two terminal palladium cations and the remaining three BF4
Figure 4. X-ray single-crystal structures of double cages a) 6 and b) 7.
c) Partial overlay of structures of 6 (gray) and 7 (red) indicating the
changes in the ligand structure and Pd–Pd distances. d) Space-filling
top view of one of the inner Pd(pyridine)4 planes of 6 (top) and 7
(bottom). (C: gray, N: blue, O: red, S: yellow, B: brown, F: green, Pd:
purple.)
ions are found in the channels spanning the solid-state
structure (see the Supporting Information). The observed
double-cage structure is comparable to the structure of the
previously reported dibenzosuberone double cage.[24]
Surprisingly, we discovered that exposure of the single
crystals of compound 6 to air for two months resulted in
monooxygenation of all eight phenothiazine backbones to
give crystals of compound 7, presumably in a crystal-to-crystal
conversion process (Figure 4b). ESI mass spectrometric
analysis of an acetonitrile solution of the crystalline sample
of 7 confirmed it had the same molecular composition as the
sample obtained starting from ligand 4.
Although we assume the presence of a mixture of
diastereomers in the sample of 7 obtained by solution
synthesis starting from ligand 4, the solid-state-oxidized
crystalline sample of 7 consists of only one, highly symmetric
isomer, as shown by the X-ray structure determination
depicted in Figure 4b. In this structure, all eight phenothia-
zines carry one oxygen substituent in a pseudo-axial position,
thus pointing away from the central Pd4 axis. The rotational
sense of the oxygen substituents is clockwise in one [Pd244]
subunit and counterclockwise in the other, when viewed along
the Pd4 axis. Most intriguingly, the molecular structures of 6
and 7 are quite distinct from each other. The ring fold of the
phenothiazine system (angle between the two benzene ring
planes) decreases from 1558 to 1508 upon monooxygenation,
which results in a 5% decrease in the Pd–Pd distance in the
[Pd2L4] subunits (from 17.62 ꢁ in [Pd234] to 16.79 ꢁ in [Pd244];
Figure 4c). In the context of the interpenetrated double-cage
structure, this shrinking particularly affects the outer pockets,
with the distance between the inner and the neighboring outer
Pd cation decreasing by 7% from 8.77 ꢁ in 6 to 8.16 ꢁ 7
(Figure 4c). It is further interesting to compare the position of
the interpenetrating ligand backbones near the plane of the
inner Pd(pyridine)4 complexes (Figure 4d). In both struc-
tures, the four pyridine rings are almost perpendicular to the
square-planar Pd(N)4 coordination sphere. The space-filling
top view shows that the phenothiazine moieties of the
interdigitated ligands pass through the gaps between the
pyridine rings. In both cases, the sulfur atoms point towards
4
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a pyridine ring (distance between the sulfur and the closest
ring plane is 3.44 ꢁ in 6 and 3.42 ꢁ in 7). Since the oxygen
substituents on the sulfur atoms in 7 find enough space to
occupy the pseudo-axial positions of the phenothiazine
systems, the structures of 6 and 7 are surprisingly similar
around the inner Pd(pyridine)4 planes. This must be different
for the dioxygenated cage derivative 8, since every sulfur
atom has to accommodate an additional oxygen substituent.
A DFT geometry optimization of 8 was conducted to support
this assumption (see the Supporting Information).
Subsequently, we tried to oxidize double cage 6 by the
same pathway as chosen for ligand 4: by using a suspension of
Cu(NO3)2·3H2O in CH2Cl2. Monitoring the oxidation process
by ESI mass spectrometry revealed a stepwise oxidation,
thereby resulting in the formation of 7 as its nitrate salt after
a reaction time of about 2 h. The oxidation does not stop at
the eightfold monooxygenated product 7, but proceeds
further to yield double cages carrying up to 16 oxygen
substituents (8; see the Supporting Information).
of an intramolecular disproportionation pathway. The use of
H218O confirmed water as the oxygen source (see the
Supporting Information).
Herein, we reported a series of three highly symmetric,
interpenetrated coordination cages based on phenothiazine
and its oxidation products. All cages were synthesized by
quantitative, metal-mediated self-assembly from the respec-
tive ligands. Furthermore, we could show that the oxygenated
cage derivatives 7 and 8 are also accessible by direct oxidation
of the parental cage 6 in the presence of water. In comparison
to ligand 3, the hydrolysis kinetics of the oxidized species
[6]8C+ suggest a communication of the redox centers within the
double cage which results in an accelerated intramolecular
disproportionation. We believe that this simple self-assembly
strategy for the clustering of redox-active compounds will
influence the development of novel functional materials with
applications in charge transport and storage. Currently we are
further elucidating the spectroscopic and electrochemical
properties in the presence and absence of water, as well as the
anion-binding capabilities of this new family of interpene-
trated coordination cages.
The nontrivial signal patterns observed in the ESI mass
spectraof these reactions (cooperative formation of 7 and
further oxygenation to 8 in double steps; see the Supporting
Information) indicate that the processes most likely follow
a mechanism that strongly depends on the structural and Experimental Section
The synthesis of ligands 3, 4, and 5 from 3,7-dibromo-N-hexylphe-
electronic communication between the eight phenothiazine
units within the densely packed double-cage architecture. To
gain more insight into the different redox behavior of ligand 3
compared to double cage 6, we first titrated their acetonitrile
solutions with the oxidant [FeIII(bipyridine)3]3+ to generate
the radical cations [3]C+ and [6]8C+, respectively, followed by
the reaction with water. As shown in Figure 5, [6]8C+ hydrol-
nothiazine (2) is described in the Supporting Information. Cages 6, 7,
and 8 were obtained from the respective ligands by addition of
0.5 equiv of [Pd(CH3CN)4](BF4)2 in acetonitrile. The oxidation of
cage
6 was performed using Cu(NO3)2·3H2O in CH2Cl2 or
[FeIII(bipyridine)3](PF6)3 in CD3CN. Single crystals suitable for X-
ray crystal-structure analysis were grown by slow diffusion of diethyl
ether into a solution of 6 in acetonitrile. The crystals of 7 were
obtained by exposure of a sample of 6 to air for two months.
CCDC 904004 and 904005 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Received: March 26, 2013
Published online: && &&, &&&&
Keywords: cage compounds · interpenetration ·
.
phenothiazines · redox chemistry · supramolecular chemistry
Figure 5. Temporal evolution of the radical cation absorption band at
680 nm of a) [3]C+ and b) [6]8C+ in the presence of different amounts of
water, and proposed mechanisms explaining the observed rate differ-
ences (298 K, N2 atmosphere, concentration: 0.08 mm with respect to
3, solvent: acetonitrile, oxidant: [FeIII(bipyridine)3](PF6)3, oxidant addi-
tion at t = 0 s, water addition at t = 100 s; *presumably paralleled by
further radical decay pathways, # formal fourfold intramolecular dispro-
portionation product).
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