Simple Synthetic Routes to Carbene-M-Amido (M=Cu, Ag, Au) Complexes for Luminescence and Photocatalysis Applications

Article abstract of DOI:10.1002/chem.202101476

The development of novel and operationally simple synthetic routes to carbene-metal-amido (CMA) complexes of copper, silver and gold relevant for photonic applications are reported. A mild base and sustainable solvents allow all reactions to be conducted in air and at room temperature, leading to high yields of the targeted compounds even on multigram scales. The effect of various mild bases on the N?H metallation was studied in silico and experimentally, while a mechanochemical, solvent-free synthetic approach was also developed. Our photophysical studies on [M(NHC)(Cbz)] (Cbz=carbazolyl) indicate that the occurrence of fluorescent or phosphorescent states is determined primarily by the metal, providing control over the excited state properties. Consequently, we demonstrate the potential of the new CMAs beyond luminescence applications by employing a selected CMA as a photocatalyst. The exemplified synthetic ease is expected to accelerate the applications of CMAs in photocatalysis and materials chemistry.

Full text of DOI:10.1002/chem.202101476

Accepted Article
Accepted Article
Title: Simple Synthetic Routes to Carbene-M-Amido (M = Cu, Ag, Au)
Complexes for Luminescence and Photocatalysis Applications
Authors: Steven Patrick Nolan, Nikolaos V. Tzouras, Ekaterina A.
Martynova, Xinyuan Ma, Thomas Scattolin, Benjamin Hupp,
Hendrik Busen, Marina Saab, Ziyun Zhang, Laura Falivene,
Gianmarco Pisanò, Kristof Van Hecke, Luigi Cavallo,
Catherine S. J. Cazin, and Andreas Steffen
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202101476
Link to VoR: https://doi.org/10.1002/chem.202101476
01/2020

10.1002/chem.202101476
Chemistry - A European Journal
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Simple Synthetic Routes to Carbene-M-Amido (M = Cu, Ag, Au)
Complexes for Luminescence and Photocatalysis Applications
Nikolaos V. Tzouras,[a] Ekaterina A. Martynova,[a] Xinyuan Ma,[a] Thomas Scattolin,[a] Benjamin Hupp,[b]
Hendrik Busen,[b] Marina Saab,[a] Ziyun Zhang,[c] Laura Falivene,[c] Gianmarco Pisanò,[a] Kristof Van
Hecke,[a] Luigi Cavallo,[c] Catherine S. J. Cazin,[a] Andreas Steffen*[b] and Steven P. Nolan*[a]
[a]
[b]
[c]
N. V. Tzouras, E. A. Martynova, X. Ma, Dr. T. Scatollin, M. Saab, G. Pisanò, Prof. Dr. K. Van Hecke, Prof. Dr. C. S. J. Cazin and Prof. Dr. S. P. Nolan
Department of Chemistry and Centre for Sustainable Chemistry
Ghent University
Krijgslaan 281,S-3, 9000 Ghent, Belgium
E-mail: steven.nolan@ugent.be
H. Busen, Dr. B. Hupp, Prof. Dr. A. Steffen
Faculty of Chemistry and Chemical Biology,
TU Dortmund University
Otto-Hahn-Str. 6, 44227 Dortmund, Germany
E-mail: andreas.steffen@tu-dortmund.de
Z. Zhang, Dr. L. Falivene, Prof. Dr. L. Cavallo
KAUST Catalysis Center, Physical Sciences and Engineering Division
King Abdullah University of Science and Technology
Thuwal 23955-6900, Saudi Arabia
Supporting information for this article is given via a link at the end of the document.
Abstract: The development of novel and operationally simple
examples bearing the more common (benz)imidazolylidene
synthetic routes to Carbene-Metal-Amido (CMA) complexes of copper, framework and a mesoionic carbene have been reported only
silver and gold relevant for photonic applications are reported. A mild
base and sustainable solvents allow all reactions to be conducted in
air and at room temperature, leading to high yields of the targeted
compounds even on multigram scales. The effect of various mild
bases on the N-H metallation was studied in silico and experimentally,
while a mechanochemical, solvent-free synthetic approach was also
developed. Our photophysical studies on [M(NHC)(Cbz)] (Cbz =
carbazolyl) indicate that the occurrence of fluorescent or
phosphorescent states is determined primarily by the metal, providing
control over the excited state properties. Consequently, we
demonstrate the potential of the new CMAs beyond luminescence
applications by employing a selected CMA as a photocatalyst. The
exemplified synthetic ease is expected to accelerate the applications
of CMAs in photocatalysis and materials chemistry.
very recently.[19,20] The peculiar combination of electronic
properties of the single components enables the population of
ligand-to-ligand (LL)CT states with a small energy gap between
the singlet and triplet excited states, which is highly beneficial for
emission via Thermally Activated Delayed Fluoresence (TADF)
[3-14,17-19,21]
with extraordinarily high radiative rate constants kr.
However, variations of the general formula, specifically to make it
more suitable for other photophysical, -chemical or -catalytic
applications, has been of comparatively lower priority. A rare
exception is a recent study by Li et al., where the Cu-carbazolyl
moiety was combined with common imidazolylidenes as weak π-
acceptors.[19a] The authors postulate that the balance of LLCT and
ligand centered (LC) ππ* states localized at the carbazolyl was
tipped towards the latter, resulting in dual fluorescence and
remarkably long-lived phosphorescence on the millisecond
timescale in the solid state, at room temperature. Although this
study was confined to CMAs of Cu and is, from our point of view,
not fully consistent in the interpretation of the photophysical
properties (see below), there is clear indication of great potential
of this class of photoactive compounds beyond TADF. Importantly,
while the origins of their photochemical behaviour are rigorously
being studied,[15-18] an aspect of the chemistry of CMAs (and of
metal-amido complexes in general) which has remained largely
stagnant deals with their synthesis.
The fundamental point of focus in the case of metal-amido
complexes, regardless of the ancillary ligand employed, is the
formation of the M-N bond. Synthetic routes leading to the vast
majority of CMAs reported thus far make use of the well-defined
[MCl(L)] complexes in combination with carbazole and a strong
base such as KOtBu, NaOtBu, or KHMDS in THF as the solvent,
in order to achieve N-H metalation.[3-14,19] Other approaches
include the early use of NaOH and NBu4Cl as a phase transfer
catalyst in a biphasic DCM/H2O solvent system,[22] the use of
lithium amides and reaction of the latter with a [AuCl(L)] complex
or a cationic gold precursor,[23] as well as the use of the well-
Introduction
In the course of the last five years, Carbene-Metal-Amido (CMA)
complexes of the coinage metals have emerged as highly
promising candidates for application in organic light-emitting
diodes (OLEDS) and other next-generation photonic
applications.[1-14] Some examples have already been
demonstrated in highly luminescent OLEDS.[4,6,7-9,11-13] The
excited state properties of these donor-bridge-acceptor
organometallic emitters have been investigated thoroughly and
the structural features which modulate those properties have
been largely established.[15-18] The key structural components
needed are: i) a simple amide donor such as the carbazole
framework, ii) a linear, two-coordinate d10 coinage metal (Cu, Ag,
Au), and iii) an acceptor ligand of the carbene type.[17] For the
latter, the most frequently encountered ones are those of the
cyclic (alkyl)(amino) carbene and monoamido- or diamido-
carbene (MAC and DAC, respectively) families,[3-14] while
1
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10.1002/chem.202101476
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defined [Au(OH)(IPr)] synthon and more recently
a new,
commercially available Au-Aryl synthon for N-H auration in THF
and benzene, respectively.[24] In the case of Cu, amido complexes
bearing phosphines have been synthesized using lithium amides
or by using KHMDS as the base,[25] while NHC-bearing
complexes have been synthesized by addition of amines to a
[CuMe(NHC)] complex (Me = methyl) and later by using CsOH.[26]
In the case of Ag, the amine lithiation approach was recently
described for the synthesis of phosphine-stabilized Ag-amido
complexes.[27] All of these synthetic routes have been described
in the literature as straightforward and practical on a small scale,
yet in reality these have important flaws with regard to
sustainability, atom- and step-economy and operational simplicity.
For example, the use of strong bases requires strictly inert
conditions, excluding air and moisture, regardless of the stability
of the initial or final organometallic materials. Furthermore, many
protocols are carried out in toxic solvents and require either two
synthetic steps or the presence of a phase transfer catalyst and
long reaction times, without being necessarily applicable to all
ligand types and all coinage metals.
Therefore, we were intrigued by the possibility of providing a
simplified and general synthetic protocol to access photoactive
CMAs of Cu, Ag and Au bearing various carbene-type ligands,
studying the excited state properties of new complexes and
showcasing their application in photocatalysis. As a starting point,
the use of a mild, cost-efficient and robust base should be sought
to render the synthesis amenable to operationally simpler and
milder conditions, as we have demonstrated in a number of recent
reports.[24b,28,29]
Table 1. Optimization of the synthetic protocols.
Entry/Metal[a]
Solvent
Base
Temp (oC)
Conversion/
Yield (%)
1/Au
2/Au
3/Au
4/Au
5/Au
6/Au
EtOH
K2CO3
K2CO3
K2CO3
NEt3
40
85
EtOH
60
100
100/88
26
Acetone
Acetone
Acetone
RT[b]
40
NEt3
60
30[c]
80
EtOAc/H2O
1:1
K2CO3
60
7/Ag
8/Ag
EtOH
K2CO3
K2CO3
40
40
100/70[d]
100/72[d]
THF/EtOH
4:1
9/Ag
Acetone
THF
K2CO3
KOtBu
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NEt3
RT[b]
RT
100/79
100/72[e]
100/87
93
10/Ag
11/Cu
12/Cu
13/Cu
14/Cu
15/Cu
16/Cu
EtOH
RT[b]
40
Results and Discussion
Acetone
Acetone
Acetone
iPrOH
We began our studies by utilising the optimal reaction conditions
developed for the transmetallation of organoboranes to gold,[24b]
[MCl(NHC)] complexes 1a-3a and carbazole as the most
frequently encountered amido framework (Entries 1, 7 and 11,
Table 1). In these initial experiments, full conversion was not
achieved in the case of gold, while in the case of silver the reaction
also led to unidentified species which could only be removed from
the final material by filtration through basic alumina. Increasing
the temperature in the case of gold led to full conversion, while
using acetone as the solvent proved to be the optimal choice as
it leads to full conversion at ambient temperature. Using NEt3 as
the base, or using a biphasic system, led to incomplete
conversion (Entries 4-6, Table 1). DFT calculations were used to
investigate the effect of different weak bases on the metallation
step and a concerted N-H deprotonation/metalation with K2CO3
has the lowest kinetic barrier and was more thermodynamically
favoured, thus supporting the fact that this base was
experimentally optimal (see ESI for details). In the case of silver,
acetone also proved to be the superior solvent, leading to full
conversion (Entry 9, Table 1). To provide a comparative test, the
synthesis of 5a was performed in a glovebox using a strong base
and led to a slightly lower isolated yield (Entry 10, Table 1). Finally,
in the case of copper, the use of ethanol (Entry 11, Table 1)
proved best while other sets of conditions proved inferior (Entries
12-16, Table 1). Note here that solvents used are reagent grade
solvents and used as received.
40
93[c]
60
100[f]
100
80
Acetone
40-60
0[c]
[a] Conditions unless otherwise noted: [MCl(IPr)] (50 mg), carbazole (1.1
equiv.), base (3.0 equiv.), 0.5 mL of solvent, 24 hours. [b] Room temperature
(RT) under operating conditions can range from 25 to 40 oC. [c] 48 h. [d]
Impurities in the product spectra. [e] Inside a glovebox. [f] 3.5 equiv. of base.
With the optimal conditions for all three coinage metals in hand,
we began to explore the ligand scope of the synthetic method
(Scheme 1). While investigating the scope, we found that the
synthesis of each specific complex may benefit by slight variations
of the optimal conditions shown in Table 1, such as alternating
between ethanol and acetone as solvents. In the case of gold, a
saturated backbone on the NHC did not affect the reaction and
product 4b was obtained in high yield. When a bulkier ligand is
present, unsurprisingly the reaction is slower and requires more
than 24 hours to reach full conversion (see ESI for detailed
description). Substitution on the NHC backbone did not affect the
reaction, with complex 4d obtained under the standard conditions.
Complexes bearing N-alkyl substituted NHCs were also
amenable to these conditions and the CMAs 4e-4g were
synthesized in high yields in only 13 hours. In the case of silver,
2
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compounds 5b and 5c were successfully synthesized in high
yields, and require longer reaction time, because of the increased
bulk of the ligands. Interestingly, the X-ray molecular structure of
5c shows that the plane of the carbazolyl fragment is forced to
adopt a parallel orientation with respect to the plane of the
imidazolylidene moiety, which is a unique feature among
complexes with N-aryl substituted NHC ligands reported
here.This feature of 5c in the solid state is attributed to the steric
profile of the specific NHC ligand. In the case of copper, complex
6b bearing a less bulky NHC was also synthesized but requires
48 hours of reaction time. Finally, in order to showcase the
versatility of the route, complexes beyond imidazol(in)ylidenes
ligands were targeted. Compound 6c, bearing
a cyclic
(alkyl)(amino) carbene ligand was successfully synthesized using
the very mild standard conditions.
Scheme 1. Scope of the CMA synthetic route. The X-ray molecular structure of complexes 4b, 4e, 4f, 5c and 6c are presented, showing thermal displacement
ellipsoids at the 50% probability level and hydrogen atoms omitted for clarity (see ESI for more detailed structural information).[30]
3
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In order to establish whether this methodology is applicable to
alternative amines, we targeted the synthesis of other gold-NHC
amido complexes (Scheme 2). Amines with significantly higher
pKa values than carbazole were also metallated efficiently (4h-4j).
In the case of 2-aminopyridine, we found that use of other weak
bases led to no conversion (see ESI for details). The calculated
energy profiles of the reaction, suggest that the kinetic barrier for
a concerted deprotonation/metallation are similar for all other
weak bases and theoretically should allow all transformations,
however the case of K2CO3 is significantly more favoured
thermodynamically (see ESI for details) and it appears to hold a
privileged position in enabling this methodology. Interestingly, the
alternative use of ethanol instead of acetone proved to be
essential in order to achieve full conversion in the case of complex
4j. This complex was synthesized on a gram scale, thus
highlighting the scalability and versatility of the synthetic route.
Scheme 3. Large scale synthesis, one-pot synthesis and mechanochemical
synthesis of CMA complexes.
The authors indicated that intermolecular forces in the single-
crystals may be responsible for the long-lived emission, but do not
provide further details. It is important to note that the observed
phosphorescence is bathochromically shifted to that reported for
HCbz, KCbz and their derivatives (ca. 400-540 nm),[7,31,32] which
excludes the interpretation of simple 3Cbz emission. Thompson et
al. previously reported that some structurally related
[Cu(CAAC)(Cbz)] complexes exhibit in the microcrystalline solid
state 3Cbz emission (ca. 430-550 nm) and in addition, due to the
formation of aggregates, the same vibrationally resolved
phosphorescence as reported for [Cu(NHC)(Cbz)].[see ESI of reference
Scheme 2. Scope of amines in the case of gold.
Scalability is an important aspect of any synthetic method. As
shown in Scheme 3, the methods described here can be carried
out on multigram scale, leading to products in high purity and
yields, using equimolar amounts of starting materials.
Furthermore, in the case of gold and copper, one-pot syntheses
of the desired CMA compounds were successfully carried out
directly from the imidazolium salt and metal sources. In order to
showcase potential further improvements and inspired by
previous advances in mechanochemical synthesis of Cu-NHC
complexes,[29] we also performed a mechanochemical, solvent-
free, high-yielding synthesis of 6a in a planetary ball-mill, with a
reaction time of 30 minutes (Scheme 3) to demonstrate an initial
proof-of-concept using solvent-free methods. This is currently
being further investigated in our laboratories.
7]
Although the nature of this particular aggregate state has not
been further clarified, it is feasible that its emission is due to
exciton diffusion in the microcrystals, which may explain the very
long lifetimes as also found for HCbz.[32] However, the question
remains whether persistent long-lived triplet states of
[M(NHC)(Cbz)] can be realized in solution and, more
fundamentally, whether higher homologues of Cu could exert any
influence on the excited state properties as found for coinage
metal CAAC carbazolyl complexes.[9]
For the photophysical studies, the gold and silver
complexes bearing IPr (4a, 5a) and SIPr (4b, 5b) were chosen as
With a variety of imidazolylidene-based CMA complexes in
hand, we wanted to take a closer look at their photophysical
properties and applicability in photocatalysis, for which long-lived
excited states are beneficial. It has previously been shown that
they provide the best comparison with the [Cu(NHC)(Cbz)] room
[19a]
temperature phosphorescence (RTPs)
and TADF emitters
bearing benzimidazolylidenes.[19b] An overview of all spectra and
photophysical data is presented in the ESI, Table S4 and Figures
S5-S12.
[Au(MeIm)(Cbz)]
exhibits
dual
vibronically
resolved
phosphorescence (λmax = 404 and 584 nm) in the solid state,
where it forms π-stacked aggregates and intermolecular H-Au
bonds.[22a] Recently, [Cu(NHC)(Cbz)] complexes were reported
to emit from the S1 state in solution, turning into dual fluorescence
at 400-550 nm and remarkably ultralong phosphorescence at
580-750 nm on the millisecond timescale at RT in the solid state,
The absorption spectra of CMA complexes 4a,b and 5a,b
are very similar with multiple overlapping bands and shoulders
between 235-380 nm (Figure 1). The high-energy bands with
maxima at 239, 278 and 311 nm are assigned to LC transitions of
the carbene and carbazole ligands as the metal ions neither
influence the extinction coefficients nor the energies significantly.
3
which was attributed to the population of both 1ππ* and ππ*
states localized on the carbazolyl ligand.[19a]
4
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exceptional long lifetimes of τ = 332 and 266 µs, respectively. The
stronger spin orbital coupling (SOC) of Au apparently facilitates
ISC S1Tn with much higher efficiency, i.e. kISC > 1010 s-1,
quenching any prompt fluorescence. Interestingly, the emissive
T1 state is not well coupled to the ground state, resulting in very
small oscillator strength and low kphos ≈ 103 s-1, which is beneficial
for photocatalytic applications (see below). We also note that the
nature of the carbene ligand does not seem to influence the
luminescence properties, although our TD-DFT calculations
predict the 3LLCT state to be lowest in energy for the SIPr
complexes 4b/5b, while the weaker π-acceptor IPr leads to a T1
state of 3LC character localized at the Cbz ligand for 4a/5a (ESI).
Linearly coordinated coinage metal carbazolyl complexes are
very flexible with regard to the dihedral angle between the ligand
planes, which has an enormous influence on the luminescence
properties.[4b,7,14,35] Furthermore, the ordering of the excited states
highly depends on the environment, i.e. polarity and specific
solvent interactions. Therefore, we conclude that benzimidazole-
based carbenes present a borderline in terms of π-acceptor
strength to facilitate S1/T1 states of LLCT nature for TADF,[7,19b]
while weaker acceptors provide access to longer-lived triplet
states.
Figure 1. Absorption spectra of 4a,b and 5a,b in THF solution at room
temperature.
The absorption band between 320-350 nm is considerably
broadened and the corresponding extinction coefficients of the
gold complexes 4a,b of ca. e = 11,000 M-1 cm-1 surpass those of
their silver congeners 5a,b (e ≈ 4,000 M-1 cm-1) by a factor of 2-3.
Furthermore, this absorption band shows a minor bathochromic
shift from the IPr (4a, 5a) to the SIPr complexes (4b, 5b) that most
likely derives from the differences in acceptor strength between
the NHC ligands. These features would be in line with the
conclusion that it is a Cbz → NHC charge transfer (LLCT) with
significant contribution from metal-centered orbitals. The lowest
energy absorption at ca. 370 nm is only weakly allowed and
indepdenent of the nature of the carbenes, but shifts with the
metal ion. Thus, it is most likely a symmetry forbidden MLCT to
the Cbz. Our DFT and TD-DFT calculations generally support
these assignments (see ESI). However, we note that since
rotation of the Cbz ligand around the M-C bond can occur with a
very low energy barrier, the absorption spectrum in solution is the
sum of all rotamers, each of which has different Franck-Condon-
factors for the respective transitions, which can also shift in
energy.[4b, 33, 34]
In the solid state, we find that aggregation influences the
luminescence properties of both the silver and gold complexes.
Due to insufficient SOC, 5a,b show significantly broadened dual
fluorescence from the LC 1Cbz state at λfl ≈ 390 nm with
nanosecond lifetimes, indicative for kISC
<
109 s-1, and
phosphorescence between λphos ≈ 480-700 nm (τ = 45 and 335 µs
for 5a and 5b, respectively). The triplet state emission is
vibrationally not well resolved and the excitation spectra display
low-energy transitions between 400-480 nm distinctly different
from the molecular absorption spectrum in solution (Figures S11
and S13). Gold complexes 4a,b retain the 3Cbz emission (λphos
=
424 nm) already observed in solution. However, their respective
lifetimes are significantly decreased to 74 (4a) and 38 (4b) µs,
and
a second, vibrationally resolved phosphorescence is
detected with τ = 885 and 374 µs, respectively (Figures 2 and S6).
The excitation spectra of the two simultaneously phosphorescent
states are very similar and in line with the absorption spectra in
solution, although the longer-lived emission exhibits additional
excitation bands between 400-440 nm. This leads us to conclude
that besides direct excitation of the aggregate state, its emission
When investigating the emission properties, stability issues
were encountered (see ESI), in particular for the silver complexes
5a and 5b. In THF solution, 5a displays multiple emission bands
with distinct vibrational progression and an overall quantum yield
of φ = 0.08. The fluorescence at λem = 343 nm (τ = 13.5 ns)
appears to stem from free carbazole as a photodecomposition
product because the excitation spectrum of that band differs
significantly from the absorption (Figure S10). Between 380-440
3
is also triggered by triplet energy transfer from the Cbz excited
state of the monomers, explaining also the change in lifetime
mentioned above. The implication of these findings is that the
aggregate is already formed in the ground state, and its emission
not merely the result of excimer formation in the solid state.
Although aggregation induced emission occurs in our Ag
and Au analogs in the solid state, and despite the intermolecular
interactions of 4b in the single crystals being very similar to those
reported for [Cu(IMes/IPr)(Cbz)], we do not observe such
remarkably long-lived RTP as reported for those Cu
complexes.[19a] But bearing in mind that it has also been reported
for [Au(MeIm)(Cbz)] and some [Cu(CAAC)(Cbz)] complexes,[7,22a]
and that the aggregate emission of 4a,b and 5a,b is energetically
different, it is possible that the specific crystalline environment or
defect sites are responsible for the formation of very long-lived
charge-separated excitons as recently reported for HCbz.[32]
Complex [Au(SIPr)(Cbz)] (4b) demonstrated the highest
photostability, high-energy phosphorescence in the blue-green
range with a remarkable long lifetime in solution, and showed an
1
nm, fluorescence from the Cbz LC state occurs as a result of
insufficient spin-orbit coupling mediated by the Ag ion, leading to
very slow intersystem-crossing to the triplet moieties. In addition,
very weak phosphorescence from the 3Cbz state is hidden
underneath the fluorescence band at λem = 437 nm with a
significantly longer lifetime of at least 1-10 µs. A similar solution
3
behaviour is observed for 5b, with a more pronounced Cbz LC
emission with τ = 332 µs, although we note that concentration and
time influence the photolytic stability to a much greater extent
(Figure S12). However, 4a/b are much more stable under
photolytic conditions and, in contrast to their Cu[19] and Ag
congeners 5a,b, emit solely via phosphorescence in THF solution
3
from a high-energy Cbz state at λem = 430 nm (φ = 0.32) with
5
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indication to undergo energy transfer, and was thus chosen as
triplet state sensitizer for a proof-of-concept photocatalysis
reaction. We selected (E,E’)-dicinnamyl ether as the substrate for
a [2+2] cycloaddition via triplet energy transfer[36] upon irradiation
at 365 nm with a 18W LED, where 4b still offers a reasonable
extinction coefficient and absorption by common borosilicate
glass does not appear to be a major issue. Both catalyst loading
and irradiation time were varied to screen the performance limit of
4b (Table 2, entries 1-5) in this intial photocatalytic study.
Full conversion to the desired cycloaddition product was only
achieved with 4b acting as a photocatalyst. While some
conversion (12%) was observed in the absence of 4b after
irradiation for 21 h (entry 6), most likely the result of direct
excitation of the triplet state of (E,E’)-dicinnamyl ether, the
reaction was completely inhibited when kept in the dark. Overall,
4b exhibits remarkable efficiency for
a
non-optimized
photocatalyst in this chosen [2+2] cycloaddition reaction. Further
work is ongoing to test the potential of 4b and related complexes
as photocatalysts.
Conclusion
We have demonstrated for the first time that CMAs derived
from copper-, silver- and gold-NHC complexes can be accessed
via simple and sustainable synthetic routes. This methodology
makes use of a mild base, K2CO3, which was demonstrated
superior to other mild bases both experimentally and
computationally. The issue of solvent use was addressed in the
deployment of green solvents (acetone and ethanol) in reactions
performed in classical batch mode and under solventless
conditions using a mechanochemical approach, which is also
showcased for the first time for CMA synthesis. The synthesis of
CMAs displays a wide scope with regard to both the NHC ligand
and the amine used, while it is scalable and does not require
elaborate experimental setups and/or handling. The potential of
this advance is evident not only by the simplicity and mild
conditions used, but also by the examples of CMA synthesis
directly from imidazolium salts, metal sources and carbazole in a
one-pot fashion. The combination of desirable reaction solvents,
mild reagents, open-to-air conditions and wide scope are
expected to significantly aid in the construction of diverse CMA
libararies for photochemical and other applications. Considering
the original aim of the photophysical and photocatalytic survey,
two important insights are provided by this study: 1) the influence
of the transition metal on the emission properties of NHC-M-Cbz
is perceivable (enhanced SOC to facilitate ISC, ultimately giving
pure phosphorescence for Au), but within the boundaries of this
particular ligand system, highly efficient emitters rivalling
analogous CAAC or MAC complexes are not obtainable solely by
changing the metal and 2) minor variations of the carbene ligand
Figure 2. Normalized emission (blue) and excitation (black for λem = 430 nm,
grey for λem = 540 nm) spectra of 4b in THF solution and in the solid state (inset)
at room temperature.
Table 2. Optimization and control studies for the photocatalytic [2+2]
cycloaddtion.
are far more consequential, as
a comparison with the
Catalyst Load[a]
(mol%)
Time (h)
Conversion[b] (%)
corresponding benzimidazole-based carbene complexes
reported by Thompson and co-workers[19b] illustrates, however,
the introduction of the 4d and, particularly, 5d TM appears to be
highly beneficial to fine-tune photophysical properties. This was
confirmed by the impressive performance of the fairly simple gold
complex [Au(SIPr)(Cbz)] (4b) in a proof-of-concept photocatalysis
reaction, where complete conversion was achieved even at
relatively low catalyst loadings ( 5 mol%) and short reaction times
(4 hours). This finding may provide a new perspective on the
wide-ranging applicability of CMAs, especially with highly
versatile carbenes such as 5-membered imidazole-based NHCs,
that have attracted much less attention in studies of CMA
complexes due to their lackluster performance in OLED emitter
materials.
Entry
1
2
50
10
5
21
21
21
4
>99
>99
>99
>99
>99
12
3
4
10
5
5
4
6
none
10
21
21
7[c]
0
[a] Catalyst loading for entry 1 was simply weighed, for entries 3-7, a stock
solution in THF was used. [b] Conversions were determined from the ratio
of the product and reactant signals in the 1H NMR spectrum. [c] Control
reaction performed in the dark.
6
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Acknowledgements
We gratefully acknowledge VLAIO (SBO project CO2PERATE),
the Special Research Fund (BOF) of Ghent University is also
acknowledged for Doctoral Scholarship (01D14919) to NVT,
starting and project grants to SPN and CSJC, and project grant
(01N03217) to KVH and MS. XM thanks the China Scholarsip
Counsil (CSC) for a PhD scholarship. The Research Foundation
– Flanders (FWO) is also gratefully acknowledged for a
Fundamental Research PhD fellowship to NVT (11I6921N) and
project AUGE/11/029 to KVH. AS thanks the DFG (STE1834/4-
2). This work was also supported by Deutsche
Forschungsgemeinschaft [DFG, Priority Program SPP 2102
“Light-controlled reactivity of metal complexes” (1834/7-1).
Umicore AG is thankfully acknowledged for generous gifts of
materials.
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Keywords: Carbene-metal-amides • Coinage metal-N-
Heterocyclic carbene complexes • Mechanochemistry •
Photocatalysis • Synthetic methods
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8
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Entry for the Table of Contents
General synthetic route to CMA
L = NHC, CAAC
L
= Cu, Ag and Au
K2CO3 (3.0-3.5 eq.),
M
M
Cl
+ 1.1 eq HNR2
Acetone or EtOH, RT or 40 oC
N
v
Sustainable/Scalable v In air
v
Various NHC & amines v Mechanism
Let there be light!: Interest in Carbene-Metal-Amido (CMA) complexes of the coinage metals has inspired comprehensive efforts
towards simplifying their synthesis. We now showcase simple and sustainable routes to various CMAs, using mild conditions for the N-
H metalation of carbazole and other amines. The action of a mild base is studied computationally and the photophysical properties of
some new CMA complexes are examined.
9
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