Coumaraz-2-on-4-ylidene: Ambiphilic N-Heterocyclic Carbenes with a Tunable Electronic Structure

Article abstract of DOI:10.1002/anie.201804249

Herein, a coumaraz-2-on-4-ylidene (1) as a new example of an ambiphilic N-heterocyclic carbene, having electronic properties that can be fine-tuned, is reported. The N-carbamic and aryl groups on the carbene carbon center provide exceptionally high electrophilicity and nucleophilicity simultaneously to the carbene center, as evidenced by the ⁷⁷Se NMR chemical shifts of their selenoketone derivatives and the CO stretching strengths of their rhodium carbonyl complexes. Since the precursors of 1 could be synthesized from various functionalized Schiff bases in a practical and scalable manner, the electronic properties of 1 can be fine-tuned in a quantitative and predictable way by using the Hammett σ constant of the functional groups on aryl ring. The facile electronic tuning capability of 1 may be applicable to eliciting novel properties in main-group and transition-metal chemistry.

Full text of DOI:10.1002/anie.201804249

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Accepted Article
Title: Coumaraz-2-on-4-ylidene: Ambiphilic N-heterocyclic Carbenes
with a Fine-Tunable Electronic Structure
Authors: Hayoung Song, Hyunho Kim, and Eunsung Lee
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201804249
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10.1002/anie.201804249
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Coumaraz-2-on-4-ylidene: Ambiphilic N-heterocyclic Carbenes
with a Fine-Tunable Electronic Structure
Hayoung Song,*[a, b] Hyunho Kim,[a, b] and Eunsung Lee*[a, b, c]
Abstract: Herein, a coumaraz-2-on-4-ylidene (1) as a new example
of ambiphilic N-heterocyclic carbenes with fine tunable electronic
properties is reported. The N-carbamic and aryl groups on carbene
carbon provide exceptionally high electrophilicity and nucleophilicity
simultaneously to the carbene center, as evidenced by the 77Se NMR
chemical shifts of their selenoketone derivatives and the CO
stretching strengths of their rhodium carbonyl complexes. Since the
precursors of 1 could be synthesized from various functionalized
Schiff bases in a practical and scalable manner, the electronic
properties of 1 can be fine-tuned in quantitative and predictable way
using the Hammett σ constant of the functional groups on aryl ring.
The facile electronic tuning capability of 1 may be further applicable
to eliciting novel properties in main-group and transition metal
chemistry.
NHCs.[14],[15],[18] Similarly, introducing less π-basic aryl[16] or
amide[11],[12],[13] groups on carbene carbon decreases the LUMO
energy level of NHCs. Combining both approaches, there have
been efforts to develop NHCs with high HOMO energy and low
LUMO energy levels, which can be considered ambiphilic NHCs.
Utilizing these approaches, the electronic parameters of several
NHCs have been successfully modified (Figure 1). Although the
electronic properties of the NHCs were adjusted qualitatively,
quantitative fine-tuning the electronic parameters of the NHCs
were not demonstrated to date, possibly due to the difficulty of
structural modification or synthetic limitation of the NHCs.
N-heterocyclic carbenes (NHCs)[1] are enjoying the spotlight as
versatile ligands that stabilize intricate chemical structures of
transition metal complexes,[2] nanoparticles,[3] and main-group
elements.[4] Such NHC-supported systems often exhibit unique
properties and stand out as, for example, unprecedented
catalysts[5], [6] and highly emissive materials.[7] These important
applications of NHCs have motivated various structural
modifications to attain desired electronic[8] and steric properties.[9]
In particular, as interest in transition metal complexes of NHCs
has increased for the development of more efficient catalysts or
light emitting devices, the control of their electronic parameters
has also become important. In an effort to systematically modify
these electronic parameters, groups as diverse as amino,[10]
amido,[11],[12],[13] alkyl,[14],[15] aryl,[16] and even entirely inorganic
groups[17] have been installed in the NHC backbone.
Figure 1. HOMO and LUMO energy levels (eV), and singlet-triplet transition
energy (∆EST, kcal/mol) of selected NHCs calculated at the B3LYP/def2-TZVP
level of theory.
A new class of ambiphilic NHCs with fine-tunable electronic
parameters could be available from coumaraz-2-on-4-ylidene (1)
(Figure 1). Note that salicylaldehyde, a starting material of 1 has
the great advantage that it offers the chance to introduction a
broad range of substituents on aryl ring. This synthetic advantage
allows the quantitative and predictable fine-tuning of the
electronic parameter of 1 according to the Hammett σ constant of
the substituent.
The control of the electronic parameters of NHCs is strongly
associated with the electronegativity and π-basicity of neighboring
functional groups of the carbene carbon. For example, replacing
the nitrogen atom adjacent to carbene center with less
electronegative carbon atom increases HOMO energy level of
The less π-basic carbamic and aryl group in 1 expectedly drives
its LUMO down in energy to –2.43 eV, and so affords an
exceptionally electrophilic carbene center in 1 – one even more
so than that in the six-membered diamidocarbene (6-DAC, –2.20
eV) (Figure 1). At the same time, the HOMO of 1 (–5.96 eV)
remains considerably higher than that of 6-DAC (–6.23 eV), and
similar to that of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene
(IPr, –5.96 eV), which is generally regarded as a potent
nucleophile. These computed electronic characteristics of 1,
together with the narrowest HOMO-LUMO gap and smallest ∆EST
among the NHCs synthesized so far, suggest that this new type
of NHC might show unique ligand properties in transition metal
complexes. Here we report the synthesis and characterization of
precursors of 1 and their rhodium complexes in addition to their
ambiphilic electronic properties demonstrated by 77Se NMR
chemical shifts and IR data.
[a]
H. Song, H. Kim, Prof. E. Lee
Center for Self–assembly and Complexity
Institute for Basic Science (IBS)
Pohang, 37673, Republic of Korea
E-mail: shy0714@postech.ac.kr (H. Song)
E-mail: eslee@postech.ac.kr (E. Lee)
H. Song, H. Kim, Prof. E. Lee
Department of Chemistry
Pohang University of Science and Technology (POSTECH)
Pohang, 37673, Republic of Korea
[b]
[c]
Prof. E. Lee
Division of Advanced Materials Science
Pohang University of Science and Technology (POSTECH)
Pohang, 37673, Republic of Korea
Supporting information for this article is given via a link at the end of
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10.1002/anie.201804249
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Scheme 2. Attempted dehydrohalogenation of 3 and 4
To access
a
free carbene 1, we attempted the
dehydrohalogenation of the precursors using sterically bulky
strong bases (LiHMDS, KHMDS) (Scheme 2). However, for the
N-aryl precursors (3a, 3b), only dimerized products 5 were
observed, and were fully characterized (see SI). The dimerization
of NHC is facile for free carbenes with a low ∆EST[20], such as 5-
DAC reported by Ganter.[12c, 12d] The N-alkyl precursors 3c, 3d
gave only the Cl¯ metathesis products 6, analogous to those
reported by Lassaletta[21] and Bertrand[22] groups for their NHCs,
but considerably more stable to air and temperature. To prepare
more acidic precursors, the Cl- in 2 was metathesized for OTf¯ to
give ionic coumaraz-2-onium trflate 4a and 4c. Interestingly, the
more acidic precursor 4a reacted by nucleophilic addition of the
base at the 7-position of coumaraz-2-onium, to form the
dearomatized product 7a which was isolated and characterized
by X-ray crystallography (see SI). It is notable that a similar result
was previously reported by the Bertrand group for their NHCs.[22]
Scheme 1. Synthesis of substituted carbene precursor 1
The synthesis of 4-chloro-coumaraz-2-one (3), a precursor to 1,
was carried out with a good isolated yield by a two-step procedure
(Scheme 1), in which imine condensation of salicylaldehyde to
salicylimine, also known as Schiff base (2), was followed by
carbonylation with triphosgene and trimethylamine to give 3 after
a simple purification. The procedure allows multigram scale
synthesis, and accommodates well a number of substituents on
the aryl and the carbamic N-group.
Owing to the electron-deficient nature of the coumaraz-2-one
heterocycle, precursors 3 were isolated as neutral 4-chloro
adducts – the form in which the Bielawski[12a, 12b] and Ganter[12c, 12d]
groups reported their amidocarbene precursors. The structure of
4-chloro-coumaraz-2-one (3a) was elucidated by X-ray
crystallographic analysis (see SI). The C–Cl bond distance in 3a
(1.854(1) Å) is closer to the average sp3 C–Cl bond distance (ca.
1.76 Å)[19] than the C–Cl in 6-DAC•HCl (1.882(2) Å).[12b]
Consistent with the LUMO energies computed for the free
carbenes 1 and 6-DAC in Figure 1, the experimental C–Cl
distances suggest that the iminium precursor 1•H+ is likewise
more electrophilic than 6-DAC•H+, and so binds the chloride at the
4-position of 1•H+ more tightly. The presence of electron–donating
groups in the para- aromatic positions of 3i’ and 3j’ expectedly
reduces the electrophilicity of coumaraz-2-onium heterocycle,
and permits dissociation of the chloride – at least in a dynamic,
reversible process – as evidenced by the symmetric 1H NMR
spectra of 3i’ and 3j’ that feature only two methyl group
i
resonances for the Dipp Pr groups in CDCl3. In contrast, the
asymmetry of these iPr 1H NMR resonances seen for 3a, 3e – 3h
shows them to adopt neutral, 4-chloro-coumaraz-2-ones forms in
CDCl3 and C6D6 solutions.
Scheme 3. Trapping of [1] in situ (R = Dipp)
Although our attempts to produce free 1 in at least an observable
form have yet to succeed, trapping experiments suggested that 1
might be forming in situ as
a
free NHC from the
dehydrohalogenation of 3. When treatment of 3 with LiHMDS at –
78 °C was followed by addition of addition of either isocyanate,
elemental sulfur, or styrene, products 8, 9, or 10 were isolated in
good yields (Scheme 3), and were also characterized by X-ray
crystallography (see SI). However, the less sterically bulky N-alkyl
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10.1002/anie.201804249
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precursors 3c could not be trapped by the same method, giving
instead the base adduct 6c. This result stresses the importance
of steric protection of 4-position in 3 to in situ generate free
carbene 1.
practice, both predictably and considerably, thanks to the ready
selection of the aromatic ring substituents R in the synthesis of
precursors 3 (Scheme 4 and Figure 3). However, contrary to the
previous assertion of a link between the 1JC–Se coupling constants
in NHC=Se and the HOMO energy levels of the free NHCs, [23b]
-1
1
we observed that the JC–Se values of 11 were all effectively
constant (Scheme 4 and Figure S1).
y = -0.6822x - 2.3711
R² = 0.9582
-2
1.5
1
-3
0.5
-4
R² = 0.8157
y = -0.4752x - 5.9062
0
-0.5
-1
R² = 0.9362
-5
-6
-7
-1.5
-2
R² = 0.8909
-2.5
-1
-0.5
0
0.5
-3
σ
-3.5
Figure 2. HOMO (in orange) and LUMO (in blue) energy levels of 1 vs the
Hammett constant σ of the aromatic substituents R in 1 (calculated at
B3LYP/def2-TZVP).
0
500
δ(77Se) (ppm)
1000
1500
Figure 3. δ(77Se) (ppm, in d6-acetone) of NHC=Se (data for 11 are in orange,
all typical NHC=Se including 11 are in blue) vs LUMO of the free NHCs
(calculated at B3LYP/def2-TZVP).
The extent of the substituent effect in the correlation of the
HOMO and LUMO energy levels of 1 with their Hammett σ
constants is presented in Figure 2. A linear relationship between
frontier orbital energy levels of 1 and the σ constants of each
substituent was successfully established.
To confirm the potential for the ligand application of transition
metal complexes, the rhodium–1 complexes (12) were
successfully synthesized by the reaction of in situ generated free
carbene 1 with [Rh(cod)Cl]2 (Scheme 5). The complexes were
stable enough to be isolated by column chromatography.
Complexes 12a and 12j exhibit 13C-NMR peaks at 273.88 and
258.40 ppm in CDCl3, whichare assigned as the electron deficient
carbene carbons. The single crystal structures of 12a and 12j
were also elucidated by X-ray crystallographic analysis (see SI).
Scheme 4. Synthesis of selenoketone 11, and their 77Se NMR chemical shifts
and 1JC–Se coupling constants
Furthermore, use of elemental Se to trap 1 in situ gave the
respective selenoketone 11 (Scheme 4), whose 77Se-NMR data
offer useful experimental probes of the electronic characteristics
of their parent free NHCs.[23] In particular, 77Se-NMR chemical
Scheme 5. Synthesis of their (1,5-cyclooctadiene)rhodium chloride (12) and
dicarbonyl complexes(13)
1
shifts were found to correlate with the LUMO,[23a] and JC–Se
coupling constants with the HOMO energies computed for the
parent free NHCs.[23b] All of the selenoketone 11 showed highly
deshielded 77Se NMR resonances, to an extent exceeding that in
selenoketones of 5- and 6-DAC (850 ppm, vs 1103 ppm for 11a,
in d6-acetone),[23b] and rivaling that of the selenoketone of a cyclic
(alkyl)(amido)carbene (1179 ppm), which was known to be the
most π-acidic NHC to date.[13c] As the shielding of Se in NHC=Se
trends directly with the LUMO energy level computed for the
parent free NHC (Figure 3 and Table S1), the 77Se NMR data for
11 suggest their parent 1 to exhibit exceptionally high π-acidity –
as gauged by its computed LUMO energy level, and to rival the
highest π-acidity metrics found yet for NHCs. [13c] Better still, the
π-acidity of NHCs based on 1 should be possible to vary in
To investigate the electron donating effect of 1 using the average
IR frequency (νav
) of rhodium carbonyl complexes, the
CO
complexes 13 were also prepared via carbon monoxide bubbling
of 12. The IR spectrum of 13a exhibited two IR absorptions at
1992.9 and 2081.6 cm-1 (ATR). The νav of 13a is 2037.3 cm-1,
CO
which is larger than the value of cAAC (2035.5 cm-1), and smaller
than that of regular NHCs (2040.5 cm-1).[24] Interestingly, the νav
CO
of 13j is significantly reduced at 2035.2 cm-1 (1993.6 and 2076.8
cm-1 (ATR)) compared to the νavCO of 13a, and even closer to the
νavCO of cAAC. The result shows that the introduction of functional
groups on the aryl ring can significantly change the ligand
properties of carbenes. Interestingly, the νavCO of 13 is much lower
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10.1002/anie.201804249
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and 2045 cm-1) even though their electrophilicity is similar. The
result shows that the high nucleophilicity of 1 engenders an even
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In summary, we have successfully designed and synthesized
coumaraz-2-on-4-ylidene (1) precursors and their metal
complexes. Furthermore, their electronic properties could be fully
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CO
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This work was supported by Institute for Basic Science (IBS)
[IBSR007-D1] and a National Research Foundation of Korea
(NRF) grant funded by the Korean government [Ministry of
Science, ICT and Future Planning (MSIP)] (No. NRF-
2016H1A2A1907122 – Global Ph.D. Fellowship Program). The X-
ray crystallography analysis with synchrotron radiation was
performed at the Pohang Accelerator Laboratory (PLS-II BL2D
SMC beamline). We thank Dr. Gregory B. Boursalian and Dr.
Dmitry V. Yandulov for helpful discussions.
Keywords: Carbene ligand • Ligand design • Main group
elements • N-heterocyclic carbene • Transition metal complex
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Entry for the Table of Contents (Please choose one layout)
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Hayoung Song,* Hyunho Kim, and
Eunsung Lee*,
EDG
Coumaraz-2-on-4-ylidene
 Practical synthetic method
 Strong σ-basicity and π-acidity
 Tunable electronic structure
Page No. – Page No.
Coumaraz-2-on-4-ylidene: Ambiphilic
N-heterocyclic Carbenes with a Fine-
Tunable Electronic Structure
EWG
Coumaraz-2-on-4-ylidene, as a new ambiphilic N-heterocyclic carbene platform, is
reported. The precursors were synthesized from various functionalized Schiff bases
in a practical and scalable manner. Due to the synthetic advantage, electronic
properties of coumaraz-2-on-4-ylidene can be also easily fine-tuned in quantitative
and predictable way, as proved by the Hammett σ constants of the functional groups
on aryl ring.
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