Transition metal complexes of axially chiral tetrathioether bay-substituted perylene bisimide dyes

Article abstract of DOI:10.1039/c3cc45351k

Nucleophilic substitution of 1,6,7,12-tetrachloro perylene bisimide (PBI) with n-butanethiol provided a novel, highly twisted PBI bearing four sulphur coordination sites at the bay positions, from which silver and palladium complexes were prepared.

Full text of DOI:10.1039/c3cc45351k

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Transition metal complexes of axially chiral
tetrathioether bay-substituted perylene
Cite this: DOI: 10.1039/c3cc45351k
bisimide dyes†‡
Received 15th July 2013,
Mei-Jin Lin,a Marcus Schulze,a Krzysztof Radackib and Frank Wurthner*a
Accepted 10th August 2013
¨
DOI: 10.1039/c3cc45351k
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Nucleophilic substitution of 1,6,7,12-tetrachloro perylene bisimide
Accordingly, our first approach towards atropo-enantiomers
(PBI) with n-butanethiol provided a novel, highly twisted PBI and diastereomers for this class of dyes was to append covalent
bearing four sulphur coordination sites at the bay positions, from tethers between 1,7- and 6,12-phenoxy substituents.8 Only in
which silver and palladium complexes were prepared.
2007, we realized that indeed the atropo-enantiomers (P and M) of
the most common synthetic precursors, i.e. 1,6,7,12-tetrachloro-
There are only a few molecules that had a comparable impact PBIs,5 could be resolved under ambient conditions on chiral
on the development of chemistry as did the 2,20-substituted HPLC phases due to a sufficiently high racemization barrier of
1,10-binaphthyls, in particular 1,10-binaphthyl-2,20-diol (BINOL)1 98 kJ molꢀ1 9
. In the same paper, we theoretically analysed a
and 2,20-bis(diphenylphosphanyl)-1,10-binaphthyl (BINAP)2 which, range of potential bay substituents, including sulphur and
after coordination to metal ions, afforded a plethora of catalysts phosphorus for which we predicted significantly improved
for asymmetric syntheses.1,2 As pointed out by Noyori in his configurational stabilities owing to their larger van-der-Waals
recent reflections,3 however, beyond the rational arguments radii. Herein, we report for the first time on a 1,6,7,12-tetra-
for the exploration of these C2-symmetric molecules in asym- (n-butylthio)-functionalized PBI and its successful coordina-
metric catalysis it was also the structural beauty of the axially tion to transition metal ions to afford configurationally stable
dissymmetric molecular scaffold that attracted his attention C2-symmetric organometallic scaffolds composed of two metal
in the 1970’s. In a similar way, we were attracted by 1,6,7,12- ions linked by the photo- and redox-active PBI dye. Whilst such
bay-substituted perylene bisimides (PBIs).4 On the one hand, bis-metalated chiral PBI complexes are entirely unprecedented,
these dyes were known for their outstanding, brilliant red we like to point out that a few transition metal complexes of
fluorescence5 and, on the other hand, the distortion imparted PBIs bearing either s-bonded Pd(II) subunits10 or acetylene-
by the bay substituents afforded a chiral backbone quite similar tethered metal centres,11 resp., in bay positions have been
to 1,10-binaphthyls.6 In contrast to the 1,10-binaphthyl scaffold reported already by Rybtchinski, Castellano and Wasielewski
which is characterized by conformational flexibility combined and their co-workers. In contrast to these examples that are more
with a high racemization barrier (B100 kJ molꢀ1 for the parent closely related to our work there are plenty of more conventional
scaffold and B155 kJ molꢀ1 for BINOL)7 the 1,6,7,12-substituted PBI–metal coordination complexes derived from PBIs equipped
PBIs such as the tetraphenoxy derivatives are more rigid chiral with established ligands such as bipyridine and terpyridine
scaffolds (defined by dihedral angles between the two naphthalene attached to the PBI imide positions.12
p-planes in the range of 01 and 351 depending on the nature of
Whilst bay-substituted PBIs bearing two alkylthio chains
bay substituents)4 combined with less pronounced configura- have been obtained from the 1,7-dibromo derivatives by nucleo-
tional stability.
philic displacement reactions,13 several attempts in our laboratory
to replace the four chloride substituents of core-tetrachlorinated
PBI 14 were unsuccessful under the reported reaction conditions.
Accordingly, in order to accelerate the reaction by increased
concentration of n-butanethiolate in N-methyl-2-pyrrolidone
(NMP), the phase transfer catalyst (n-Bu)4NBF4 was added. This
modified synthetic route afforded the target purple PBI 2 bearing
four n-butylthio groups at the bay areas in 45% isolated yield
(Scheme 1, for details see ESI‡).
a
¨
¨
¨
Universitat Wurzburg, Institut fur Organische Chemie & Center for Nanosystems
¨
Chemistry, Am Hubland, 97074 Wurzburg, Germany.
E-mail: wuerthner@chemie.uni-wuerzburg.de; Fax: +49 931 3184756;
Tel: +49 931 3185340
b
¨
¨
¨
Universitat Wurzburg, Institut fur Anorganische Chemie, Am Hubland,
¨
97074 Wurzburg, Germany
† This work is dedicated to Professor Manfred T. Reetz on the occasion of his
70th birthday.
The optical properties of the tetra-mercapto substituted PBI 2
‡ Electronic supplementary information (ESI) available: CCDC 946435. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc45351k were examined by UV-Vis absorption and fluorescence spectroscopy
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Scheme 1 Synthesis of tetra-(n-butylthio)-substituted PBI 2 and optical resolution
by HPLC on the chiral stationary phase. R = cyclohexyl.
in dichloromethane (DCM) (see Fig. S3, ESI‡). At room tem-
perature, the UV-Vis spectrum of PBI 2 showed a broad band
with an absorption maximum at 565 nm, which is shifted
Fig. 1 UV-Vis absorption spectra of PBI 2 (violet line) and its complexes upon
addition of two equivalents of AgOTf (magenta line) and PdCl2(MeCN)2 (green line)
in DCM, respectively. The inset shows the color changes of PBI 2 in DCM upon
addition of AgOTf and PdCl2(MeCN)2, respectively.
bathochromically by 38 nm in comparison to that of the parent
abs
max
PBI without bay substituents (l
at B527 nm).14 The most
remarkable feature of the absorption spectrum is, however, the
loss of vibronic fine structure and the quite intense absorption
throughout the whole visible range for PBI 2. The fluorescence
with the PBI 2 ligand. Upon stepwise addition of silver triflate
(AgOTf) or bis(acetonitrile)dichloropalladium [PdCl2(MeCN)2]
to a DCM–MeOH (9 : 1) solution of PBI 2 (for UV-Vis titration
data of [2ꢁPd2] see Fig. S11 and S12, ESI‡) the color of the
solutions changed from violet to magenta or green, respectively
(Fig. 1, inset). UV-Vis spectroscopy revealed that the addition of
two equivalents of Ag+ ions results in a hypsochromic shift of
the absorption maximum whilst the addition of two equivalents
of Pd2+ ions leads to a bathochromic shift in comparison to
that of PBI 2 (Fig. 1). For the latter case we also note the appearance
of an entirely new band on the short-wavelength side with an
absorption maximum at 438 nm which might have originated
from a metal-to-ligand charge transfer (MLCT).17 These spectral
features are obviously related to the coordination of the sulphur
substituents in the PBI 2 bay areas by Ag+ and Pd2+ ions to give the
complexes [2ꢁAg2] and [2ꢁPd2], respectively (Fig. 2).
spectrum of PBI 2 likewise lacks vibronic fine structure and
em
max
reveals a rather large Stokes shift of 169 nm (l
at 696 nm)
and a fluorescence quantum yield of only 0.5%. Besides the
heavy atom effect which may account for increased spin–orbit
coupling and concomitant quenching of the fluorescence, these
spectral changes may be attributed to the distortion of the
perylene scaffold by the bulky sulphur substituents. Already for
four oxygen substituents (covalent radius: 0.73 Å; van-der-Waals
radius: 1.52 Å)9,15 at the bay positions, propeller-like distortions
of the central six-membered ring of the PBI scaffold by B251
and racemization barriers of B60 kJ molꢀ1 were observed.4,9
Accordingly, in the case of four sulphur substituents (covalent
radius: 1.02 Å; van-der-Waals radius: 1.80 Å),9,15 both the
distortion angle and the racemization barrier are expected to
increase considerably.
In agreement with this prediction, we were able to separate
the M and P atropo-enantiomers of PBI 2 by HPLC using a
semipreparative chiral column at room temperature (Scheme 1;
for details see ESI‡). The configuration of these atropo-enantiomers
proved to be stable at room temperature and temperatures >80 1C
were required to observe the racemization process. Thus, according
to time-dependent circular dichroism (CD) experiments, PBI 2
has a half-life of B2 h at 373 K and B4 h at 368 K in 1,1,2,2-
tetrachloroethane (see Table S1, Fig. S8 and S9, ESI‡). Further-
more, from the time-dependent CD data in the temperature
range from 363 to 373 K, we derived the activation parameters
of the racemization process of 2 using the Eyring equation
(see Table S1, ESI‡), providing a free energy of the transition
state DG‡ = 119 kJ molꢀ1. This value exceeds those found for
The two transition metal complexes of PBI 2 could also be
prepared on a preparative scale by the abovementioned method
(see ESI‡) and characterized by MALDI-TOF mass spectrometry
1
and H NMR spectroscopy. The MALDI-TOF spectra show the
expected ion peaks for [2ꢁAg2] with one coordinated triflate
anion and [2ꢁPd2] with all four coordinated chloride anions at
m/z values of 1272.1 ([M-OTf ]+) and 1261.6 ([M]ꢀ), respectively
(Fig. S4 and S5, ESI‡). In the NMR spectra of 2 in deuterated
DCM solution (Fig. S1, ESI‡) the proton signal for the PBI
core is downfield shifted from 8.60 ppm (PBI 2) to 9.06 ppm,
tetraphenoxy-PBIs (60 kJ molꢀ1) and tetrachloro-PBIs (98 kJ molꢀ1
)
and matches the value of the highly reactive tetrabromo-
PBIs (118 kJ molꢀ1).9 Accordingly, our result complies with
our expectations based on geometrical parameters (covalent
and van-der-Waals radii) of the bay substituents.
Thioethers are soft Lewis base ligands in coordination chemistry
and according to the hard–soft acid–base (HSAB) theory they
prefer to coordinate to soft metals such as Ag+, Pd2+ and Pt2+
Fig. 2 Proposed structures of the generated complexes [2ꢁAg2] (a) and [2ꢁPd2] (b).
ions.16 Here we chose Ag+ and Pd2+ for coordination reactions R = cyclohexyl.
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weak Pd–Pd interactions (dPd–Pd = 3.39 and 3.56 Å) to give a
racemic 1-D coordination network composed of alternating P
and M atropo-enantiomers of PBI 2 (Fig. 3c). Because Pd–Pd
bonds will be broken upon dissolution to give 16 electron Pd
species, these Pd–PBI complexes appear to be quite promising
for a new generation of catalysts whose catalytic activity is
driven by the photo- and redox-active PBI dyes.21
In conclusion, we have shown for the first time that axially
chiral perylene bisimide scaffolds bearing sulphur substituents
in the four PBI bay positions can form coordination complexes
with late transition metals. We expect a plethora of possible
applications of these and related multifunctional organometallic
complexes that combine chirality, a photo- and redox-functional
dye scaffold and catalytically active metal ions.
Financial support of our work from the DFG in the frame-
work of the priority programme SPP 1243 on Quantum transport
at the molecular scale is acknowledged.
Fig. 3 Side view (a) and top view (b) along the N–N direction of [2ꢁPd2] as well
as its 1-D molecular network highlighting weak Pd–Pd interactions (c). For clarity,
all protons are omitted in both views and cyclohexane units are omitted in side
views. The n-butylthio groups are colored in cyan for better visibility in (a) and (b),
but omitted in (c).
Notes and references
suggesting the withdrawal of electron density from the sulphur by
the donation to the Ag+ ion. Similar spectral shifts were observed
in the 1H NMR spectrum of the [2ꢁPd2] complex (Fig. S1, ESI‡).
The generation and composition of the complexes [2ꢁAg2] and
[2ꢁPd2] could be deduced from 1H NMR, UV-Vis and MALDI-TOF
experiments, revealing that each PBI molecule coordinates
(up to) two AgOTf or PdCl2 moieties, presumably by utilization
of the chelating ability of its two sulphur-decorated bay
areas. Based on other reported silver and palladium thioether
complexes,18 two finite PBI complexes with triflate and chloride
anions as terminal ligands are proposed for the [2ꢁAg2] and
[2ꢁPd2] structures, respectively, as depicted in Fig. 2a and b.
Unfortunately, all of our attempts to crystallize [2ꢁAg2] failed,
whilst single crystals of racemic [2ꢁPd2] were obtained rather
easily by dissolving PBI 2 with two equivalents of PdCl2(MeCN)2
in a CH2Cl2–MeOH = 90 : 10 solvent mixture, followed by slow
evaporation of the solvent. Hence, the first example of a bis-
metalated chiral PBI core structure was unambiguously con-
firmed by single-crystal X-ray analysis.
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by 311 which provides just the right bite angle to the sulphur
atoms for chelation of the Pd2+ metal ions. Accordingly, the
S–Pd–S angle is 87.81 and both Pd–S bonds are 2.26 Å and 2.29 Å
long, respectively. Well separated by the alkylthio chains, no
p–p stacking of perylene cores is observed in the crystal of the
[2ꢁPd2] complex where the chiral self-discrimination of P and M
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c
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