A highly stable, Au/Ru heterobimetallic photoredox catalyst with a [2.2]paracyclophane backbone

Article abstract of DOI:10.1039/c9dt04366g

We report the synthesis and catalytic application of a highly stable distance-defined Au/Ru heterobimetallic complex. [2.2]Paracyclophane serves as a backbone, holding the two metal centers in a spatial orientation and metal-metal fixed distance. The Au/Ru heterobimetallic complex is highly stable, easily accessible and exhibits promising catalytic activity in a visible-light mediated dual Au/Ru Meyer-Schuster rearrangement.

Full text of DOI:10.1039/c9dt04366g

Showcasing research from Professor Stefan Bräse's laboratory,
Institute of Chemistry, Karlsruhe Institute of Technology,
Karlsruhe, Germany.
As featured in:
Volume 48 Number 48 28 December 2019 Pages 17693–17954
Dalton
A highly stable, Au/Ru heterobimetallic photoredox catalyst with
a [2.2]paracyclophane backbone
Transactions
An international journal of inorganic chemistry
rsc.li/dalton
The unique “bent and battered” structure of [2.2]paracyclophane
features planar chirality, high rigidity and enables transannular
electronic communication which makes this scaffold an ideal
playground for the investigation of cooperative effects in
distance-defined heterobimetallic complexes. In the featured
heterobimetallic Au/Ru complex, [2.2]paracyclophane acts as
a backbone, holding the two metal centers in a fixed spacial
orientation and distance. The Au/Ru complex is easily accessible,
exhibits high stability and shows promising properties as
photoredox catalyst in a visible-light mediated arylative
Meyer–Schuster rearrangement.
ISSN 1477-9226
PAPER
Masaki Matsuda et al.
Intermolecular interactions of tetrabenzoporphyrin- and
phthalocyanine-based charge-transfer complexes
See Stefan Bräse et al., Dalton Trans.,
2019, 48, 17704.
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A highly stable, Au/Ru heterobimetallic photoredox
catalyst with a [2.2]paracyclophane backbone†
Cite this: Dalton Trans., 2019, 48,
17704
Daniel M. Knoll, a Christoph Zippel, a Zahid Hassan, a Martin Nieger,
b
Received 11th November 2019,
Accepted 14th November 2019
a,e
Patrick Weis, d Manfred M. Kappes c,d and Stefan Bräse
*
DOI: 10.1039/c9dt04366g
rsc.li/dalton
We report the synthesis and catalytic application of
a
Enabling chemical synthesis with low-energy visible light
highly stable distance-defined Au/Ru heterobimetallic complex. can be used to promote photoredox reactions. One of the
[2.2]Paracyclophane serves as a backbone, holding the two metal highest achievements of chemistry as a field is the use of sus-
centers in a spatial orientation and metal–metal fixed distance. tainable practices employing low-cost and nature abundant
The Au/Ru heterobimetallic complex is highly stable, easily acces- visible-light energy to drive reactions instead of thermal, electri-
sible and exhibits promising catalytic activity in a visible-light cal or mechanical energy. Visible-light mediated photoredox
mediated dual Au/Ru Meyer–Schuster rearrangement.
catalysis has seen a rapid growth over the past 20 years.17–22
Similarly, gold catalysis has gained new prominence for driving
efficient and selective processes under mild conditions,23–25
especially in combination with photoredox catalysis.26
Catalysis with mononuclear complexes employs sophisticated
ligands to fine-tune the coordination sphere of the metal
centre to achieve the desired reactivity.1–5 An elegant way to
introduce electronic flexibility into the catalytic cycle and
additionally enable cooperative or tandem catalysis is found in
heterobimetallic complexes.6–8 Here, two possibilities arise. In
one mode, one metal fulfils the classic activation of the sub-
strate, whereas its catalytic activity is enhanced by the second
metal centre. Alternatively, both metal centers can work
together in a truly cooperative fashion, as has been demon-
strated for metallosalens.9,10 This latter concept is widely
found in enzyme bio-catalysis, which rationalizes their out-
standing activity and selectivity.11–13 Despite the vast number
of examples for enzymatic catalysis and even their diverse
applications in industry, only a few examples for defined
binuclear organometallic complexes which exhibit cooperative
effects are known.14–16
However, to date, heterobimetallic complexes with mole-
cular architectures of spatial orientation and metal–metal
fixed bond distance have seldom been explored. Exploring dis-
tance-defined visible light absorbing coordination complexes
based on PCP as a new platform might allow to study the
effect of metal–metal distance and their electronic communi-
cation on cooperativity. The unique “bent and battered” struc-
ture of [2.2]paracyclophane (PCP) features planar chirality,
high rigidity and transannular electronic communication which
makes this scaffold an ideal playground for the synthesis of
distance-defined bimetallic complexes.38 Disubstituted PCPs
with two substituents located on different decks display four
different regioisomers that are named pseudo-geminal, -ortho,
-meta and -para shown in Fig. 1. The short distance between
the two decks of the [2.2]paracyclophane causes an overlap of
the π-system and leads to
a
transannular electronic
communication.27–29 The through-space conjugation effect is
aInstitute of Organic Chemistry, Karlsruhe Institute of Technology (KIT),
Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany. E-mail: braese@kit.edu
bDepartment of Chemistry, University of Helsinki, P.O. Box 55 A.I. Virtasen aukio 1,
00014 University of Helsinki, Finland
cInstitute of Nanotechnology, Karlsruhe Institute of Technology,
Herman-vonHelmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
dInstitute of Physical Chemistry, Karlsruhe Institute of Technology (KIT),
Fritz-HaberWeg 2, 76131 Karlsruhe, Germany
eInstitute of Toxicology and Genetics, Karlsruhe Institute of Technology (KIT),
Herman-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, analytical data, crystallographic data and kinetic data. CCDC 1958713
(5). For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9dt04366g
Fig. 1 Design of heterobimetallic PCPs with increasing M–M distance.
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especially interesting in the case of the pseudo-para and -meta
isomers. In these isomers, no direct metal–metal interaction is
possible and the electronic communication via PCP between
the two metal centres can be studied selectively.
To evaluate the catalytic performance of the newly syn-
thesized heterobimetallic PCP Au/Ru complexes, skeletal
rearrangements are good probes and the visible-light mediated
arylative Meyer–Schuster rearrangement also is known to be a
good “acid test” to examine stability and catalytic potential.
We investigated a well-studied reaction in which both metals
participate in the mechanism instead of one metal only being
the “fine-tuning” for the other.
Results and discussion
Inspired by recent investigations on the visible-light mediated
arylative Meyer–Schuster rearrangement by the groups of
Glorius, Luna and Shin,30–32 we focused on a new heterobime-
tallic version of the photoredox catalyst system employed. The
gold(I) catalyst, PPh3AuCl can be incorporated by replacing one
2+
of the phenyl rings with PCP. We changed the Ru(bpy)3
cation for Ru(bpy)2ppy+ because of easier synthetic access,5 a
wider spectral excitation window and its suitability as a photo-
sensitizer.33 Furthermore, the reaction studied in detail entails
oxidative quenching of the excited state of the photocatalyst,
which is facilitated for cyclometalated ruthenium complexes
when compared to their non-cyclometalated counterparts.20
To be able to compare the influence of PCP on the outcome
of catalysis, we first set out to prepare the respective PCP-Au
and PCP-Ru, resembling the coordination environments found
in the later heterobimetallic complex (Scheme 1).
Starting from PCP dibromide 1, selective monolithiation5
and reaction with chlorodiphenylphosphine gave 4-bromo-16-
diphenylphosphoryl[2.2]paracyclophane (2). The high selecti-
vity of the monolithiation allows for a further functionalization
at the 4-position via a cross-coupling reaction. Under opti-
mized Suzuki–Miyaura reaction conditions for PCP,5 a new
C–C bond to 2-phenylpyridine was formed. After two steps,
compound 3 was obtained which bears the (oxidized) phos-
phine as a selective binding site for Au(I) and the 2-phenylpyri-
dine moiety for the incorporation of Ru(II). Reduction of the
phosphoryl moiety with trichlorosilane to the free phosphine
4 34 directly followed by auration provided us with the Au(I)
complex 5. Initial efforts to introduce ruthenium into the
Scheme 2 Synthetic access to target complex pAuRu.
2-phenylpyridine moiety were met with failure, presumably
due to the use of commonly employed AgBF4.33 However, a
milder and silver-free method35 was successfully applied to
obtain the cationic heterobimetallic complex pAuRu in 27%
yield over 5 steps (Scheme 2). Due to the chiral-at-metal centre
and the inherent planar chirality of heterodisubstituted PCP,
pAuRu was obtained as a mixture of diastereomers. Despite
our efforts, we did not succeed in separation of the diastereo-
meric mixture by HPLC.
Absorption spectra of the complexes PPh3AuCl (red),
[Ru(bpy)2(ppy)]PF6 (blue), PCP-Au (black), PCP-Ru (orange)
and pAuRu (green) were obtained in MeOH/MeCN, 3 : 1 at 298 K
(Fig. 2). Absorption maxima in the UV-vis spectra and molar
extinction coefficients are summarized in Table 1.
The absorption bands in the ultraviolet region below
350 nm arise from spin allowed ligand centered (LC) π → π*
transition. As the only feature, PCP–Ru and pAuRu exhibit
an additional LC band at 320 nm which arises from the PCP
Scheme 1 Monometallic complexes PCP-Au and PCP-Ru.
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Excitation of the pAuRu complex at wavelengths defined by
the absorption maxima at 485 nm and 548 nm produces emis-
sion bands that are substantially shifted to longer wavelengths.
At both excitation wavelengths, strong fluorescence at 795 nm
is observed. Upon excitation at 485 nm, the compound shows
additional emission at 543 nm and 507 nm (Fig. 3).
No single crystals of pAuRu suitable for X-ray diffraction were
obtained. However, the direct precursor 5 did provide us with
suitable single crystals and confirmed the molecular structure as
shown in Fig. 4. The phenylpyridine moiety (plane through C17–
C22) is twisted out of the PCP deck plane (C12–C16) by 41.8°.
This angle is significantly larger than comparable PCP pyridyl
scaffolds.36 The P–Au and Au–Cl distances are in the expected
range with bond lengths of 2.236 Å and 2.292 Å respectively.37
The gold atom is almost linearly coordinated, displaying a gold(I)
typical twofold coordination mode [P–Au–Cl: 176.2°]. The P–Au–
Cl “tail” is turned towards the PCP scaffold rather than the
phenyl substituents to form a 98.6° torsion angle (C5–C4, P–Au).
Fig. 2 Absorption spectra in MeOH/MeCN, 3 : 1 at 298 K.
Table 1 Absorption spectral properties of metal complexesa
abs
λ
[nm]
max
Entry
1
Compound
(ε [104 M−1 cm−1])
[Ru(bpy)2(ppy)]PF6
371 (0.99)
410 (0.91)
490 (0.71)
552 (0.81)
366 (1.21)
414 (0.89)
490 (0.83)
552 (0.89)
366 (0.87)
414 (0.62)
490 (0.60)
552 (0.63)
2
3
PCP-Ru
pAuRu
a MeOH/MeCN, 3 : 1, 298 K.
backbone. Examination of the lower energy absorption bands
reveals a large spectral envelope which arises from the cyclo-
metalated phenylpyridine ligand. This also causes additional
features below 300 nm which can be assigned to intraligand
π → π* transitions. In the spectra, two bands which are consist-
ent with metal-to-ligand charge transfer (MLCT) are found in
the region between 350–450 nm and 450–600 nm.
Fig. 3 Emission spectra at absorption maxima 485 nm (black) and at
548 nm (red) for pAuRu in MeOH/MeCN, 3 : 1 at 298 K.
The first band can be assigned to an MLCT from the ruthe-
nium center to the cyclometalating phenylpyridyl ligand,
whereas the band at higher wavelengths is primarily localized
on the polypyridyl ligands.
The absorption of PCP-Ru and pAuRu is slightly red-shifted
compared to [Ru(bpy)2(ppy)]PF6. The heterobimetallic complex
pAuRu also shows lower molar extinction coefficients than the
monometallic counterparts. This trend can be observed already
when going from [Ru(bpy)2(ppy)]PF6 to PCP-Ru. Therefore, we
postulate that this decrease in molar exctinction coefficient
as well as the red-shift is due to the increase in electron density
by the introduction of the electron-rich PCP moiety and the
additional electron-rich phosphine respectively.
Fig. 4 Molecular structure of complex 5 (orange – P, yellow – Au,
green – Cl, blue – N, displacement parameters are drawn at 30% prob-
ability level).
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Catalysis application
ment and afforded the arylated enone 11 in 63% yield with a
reaction time of 25 minutes (see ESI† for kinetics) (entry 5).
A comparison of reaction kinetics for the monometallic
PCP-Au and PCP-Ru with those measured for pAuRu show no
significant difference in induction period or conversion rate.
While no mechanistic investigations were performed, this
suggests that the reaction follows the mechanistic proposals in
literature.30–32
Since both steric and electronic influences that may result
from the slightly different coordination environment through
the PCP backbone can be excluded, the observed drop in yield
when using pAuRu could suggest a through-space metal–metal
communication and deactivation. This communication would
be enabled by the π-conjugated PCP backbone. However, other
factors (e.g. reduced extinction coefficient) could not be
excluded and might influence the outcome of this reaction
as well.
The lack of understanding about how heterobimetallic
complexes are to be designed for applications in cooperative
catalysis demands new frameworks on which to study these
complexes systematically. In this work we demonstrated
the synthesis and catalytic viability of the heterobimetallic
complex pAuRu featuring [2.2]paracyclophane as a rigid
backbone. The performance of pAuRu complex in photoredox
catalysis was evaluated and compared to the monometallic
approach.
The remarkable stability, high yielding and facile synthetic
access of pAuRu make it an invaluable first-of-its-kind PCP
heterobimetallic complex. The investigated arylative Meyer–
Schuster rearrangement reaction is successfully catalyzed in
63% yield with pAuRu. Compared with the respective mono-
metallic complexes, a drop in yield was observed, suggesting
metal-to-metal interaction by through-space transannular
communication.
The model reaction to test pAuRu was chosen to be the visible-
light mediated arylative Meyer–Schuster rearrangement (MSR)
which was recently published by three groups: Glorius, Luna
and Shin.30–32 The Shin group reported a Au(I) to Ru(II) ratio of
1 : 1 and was therefore most fit for comparison with our hetero-
bimetallic complex (Scheme 3).
With our setup, the conditions reported by Shin et al.,
afforded the arylated enone 11 in 76% yield (Table 2, entry 1).
During our investigations, we found that a mixture of metha-
nol and acetonitrile provided overall more consistent results
(entry 2). We attribute this to the complete solution of the di-
azonium salt 10 which is added in four-fold excess and is not
well-soluble in methanol.
In order to exclude possible electronic effects induced by
the different coordination sphere that arises from the 2-phenyl-
pyridine unit within the pAuRu, the literature procedure by
Shin et al. was additionally conducted with the monocationic
Ru(bpy)2(ppy)PF6 instead of the dicationic Ru(bpy)3Cl2 (entry 3).
Furthermore, we investigated the impact of the sterically
demanding PCP backbone on the overall reaction. The corres-
ponding monometallic PCP derivatives PCP-Au and PCP-Ru
were applied in a test reaction (entry 4). All of those modifi-
cations on the catalytic system consistently provided the same
results which leads to the conclusion that neither the different
coordination sphere that results from exchanging a bpy ligand
with a ppy ligand, nor the PCP backbone influence the overall
reactivity.
The single-molecule, heterobimetallic pAuRu complex
could successfully catalyze the Meyer–Schuster rearrange-
We believe that pAuRu is the first in a line of distance-
modulated bimetallic complexes exploiting the isomeric geo-
metry features of PCP. We hope this will aid in the design and
the understanding of metal–metal interactions in bimetallic
complexes in catalysis applications. Synthetic routes towards
further isomers and PCP-based other heterobimetallic com-
plexes are underway in our laboratories.
Scheme 3 Literature reaction conditions reported by Shin et al.
Table 2 Results of the Meyer–Schuster rearrangement with pAuRu and
the monometallic PCP complexes PCP-Au and PCP-Ru
Conflicts of interest
No conflicts of interest to declare.
Notes and references
Yielda
[%]
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Entry
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2
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PPh3AuCl, Ru(bpy)3Cl2
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PPh3AuCl, Ru(bpy)2(ppy)PF6
PCP-Au, PCP-Ru
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MeCN/MeOH 3 : 1
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a Isolated yields. b NMR yields.
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