Azobenzene Isomerization-Induced Photomodulation of Electronic Properties of N-Heterocyclic Carbenes

Article abstract of DOI:10.1002/chem.201905161

Azobenzene-based protonated N-heterocyclic carbenes (NHCs), N,N’-bis(azobenzene)imidazolium chlorides (IAz-X?HCl; X=OMe, Br, H) were successfully synthesized and switching abilities of the attached azobenzene units along with the concomitant photoinduced

Full text of DOI:10.1002/chem.201905161

A Journal of
Accepted Article
Title: Azobenzene-Isomerization Induced Photomodulation of
Electronic Properties of N-Heterocyclic Carbene
Authors: Aminul Islam SK, Kshama Kundu, and Pintu K. Kundu
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Chemistry - A European Journal
10.1002/chem.201905161
Azobenzene-Isomerization Induced Photomodulation of
Electronic Properties of N-Heterocyclic Carbene
[
a]
[b]
[a]
Aminul Islam SK, Kshama Kundu, and Pintu K. Kundu*
[
a]
Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical
Technology, N. P. Marg, Matunga, Mumbai 400019, India, E-mail: pk.kundu@ictmumbai.edu.in
[
b]
Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
Abstract:
Azobenzene-based
protonated
N-heterocyclic
carbenes
(NHCs),
N,N'-
bis(azobenzene)imidazolium chlorides (IAz-X.HCl; X = OMe, Br, H) were successfully
synthesized and switching abilities of the attached azobenzene units along with the concomitant
photoinduced generation of geometric isomers were studied. Upon irradiation with 365 nm UV
light, a p-methoxy-azobenzene derivative get transformed from all-trans isomer to nearly all-cis
isomer at the photostationary state. The extent of photomodulation of electronic properties in the
NHC’s-ring of the p-methoxy-azobenzene derivative is determined via Tolman Electronic
Parameter (TEP). Iridium complex, [(IAz-OMe)IrCl(CO) ] was synthesized and infrared spectra
2
were recorded in dichloromethane solution via making film into NaCl crystals and by drop casting
in an ATR crystal. Comparison in averaged carbonyl stretching frequency between all-trans isomer
tt
cc
tt–cc
-1
(
ῦ
av ) and all-cis isomer (ῦav ) indicates a significant decrement of Δ
ῦ
av = 2.7 cm (film) and 3.8
cm (ATR). Therefore, moderate to excellent enhancement of electron density (Δ TEP = 2.3 cm-
-
1
tt–cc
1
-1
[
film] and 3.2 cm [ATR]) at the C-2 position of the NHC is achieved via trans → cis
isomerization of the remotely placed azobenzene units. This fine phototuning of electron donating
ability at the catalytic centre would pave the way to control NHC/NHC-metal catalyzed organic
transformations via external stimuli.
1
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Taking control over organic transformations via external non-invasive stimuli, such as light
minimizes the need of synthesizing differentially substituted catalysts having varied electronic and
/
or steric properties. Alteration of the intrinsic properties of a catalyst to tune its reactivity is thus of
huge interest among scientists, especially those who are working in the field of artificial switchable
1
catalysis. As early as in the seventies, light-controlled catalysis using photoswitchable enzymes
2
drew attention to a range of research groups. Inspired by photocontrolled enzymatic activity, Ueno
and Osa developed azobenzene capped β-cyclodextrin to regulate the hydrolysis of p-nitrophenyl
3
4
acetate. Whereas, Hecht’s group have achieved a remote photocontrol over piperidine’s basicity
5
through brilliant incorporation of bulky azobenzene moiety into its structure. Cacciapaglia’s group
described a phototunable bis-barium complex of azobis(benzo-18-crown-6), that controls the rate of
ethanolysis of anilides. Of late, trans-form of an azobenzene-based thiourea was shown by Pericàs
et al. as an efficient catalyst over the cis-form in Michael addition of acetylactone to m-
6
bromonitrostyrene. On the other hand, photoswitchable catalysis using metal nanoparticles-
7
modified with photochromic organic ligands, such as azobenzenes have also been explored.
However, all the above discussed modification in catalytic activities was achieved mostly via steric,
geometric or polarity changes and not via electronic changes between the isomeric states of the
azobenzenes.
N-heterocyclic carbenes (NHCs) are electron-rich, neutral σ-donor ligands, which along
with their transition metal complexes have been studied by many research groups worldwide due to
8
9
10
their enormous utilities. Pioneering work by Öfele, Wanzlick and Arduengo (first stable and
isolable NHC) have promoted the chemistry of NHCs to its high and the field no more remains a
mere laboratory curiosity. A detail understanding and various aspects of varied kinds of NHCs have
1
1
been documented in a number of reviews. Catalytic activities of NHCs and their metal complexes
are known to be governed by the electron donating abilities of the corresponding NHCs. Alterations
in electronic properties of NHCs have been achieved mainly via chemical modifications such as
varying the ring substituents and changing the heterocyclic backbone structure. Based on a fixed
imidazole-2-ylidenes ring structure, Plenio and co-workers have investigated the alteration in
1
2
electronic properties of NHCs with varied aryl groups. In these cases, nitrogen (Fig. S1a) of the
imidazolium ring were directly modified by attaching aromatic moieties having varied p-
substituents. The electron withdrawing substituents, such as –SO
electron donating capacity of the NHCs whereas, the electron donating groups (e.g. –NEt
2
Ar were proved to reduce the
) have
2
been shown to significantly enhance the electron donating properties of the NHCs. However,
photomodulation of electronic properties of NHCs were first reported by Bielawski and co-
1
3
workers. Here, the backbone of NHC precursors were modified with diarylethene (DAE) moiety
4
5
in such a way that the double bond between C -C carbon of NHCs become saturated (ring-closed
form) upon photoinduced (λ = 315 nm) electrocyclization reactions (Fig. S1b). In a report by
1
4
Monkowius, azobenzene functionality was unsymmetrically tagged to imidazolium ring to make
azobenzene-containing (NHC)AuCl complexes, however, they have not described any study on
electronic photomodulation. Hence, there is still an urgency to discover novel photoswitchable
organocatalysts that would alter their electronic distribution over the catalytic site via external
stimuli such as light. To the best of our knowledge, successful utilization of azobenzene molecular
switches to alter electronic environment on the active site of a catalyst is rare.
2
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Towards these objectives, a novel family of photoswitchable-NHC precursors, N,N'-
bis(azobenzene)imidazolium chlorides (IAz-X.HCl-3a-c, Scheme 1) were synthesized, where the
substituted azobenzene-derivatives are symmetrically bonded to both the nitrogen of the protonated
N-heterocyclic carbenes. A series of conventional synthetic procedures were modified and utilized
to produce our desired products, IAz-OMe.HCl (3a), IAz-Br.HCl (3b) and IAz-H.HCl (3c) in
moderate to good isolated yields (SI-3).
Scheme 1. Synthesis of N,N'-bis(azobenzene)imidazolium chlorides (IAz-X.HCl-3a-c).
The methoxy derivative, IAz-OMe.HCl-3a was first utilized to study the photoswitching
ability of the attached azobenzene units with light irradiations (Fig. 1a-e). A solution of 3a in
DMSO absorbs at λmax ~ 360 nm (Fig. 1b), which is the characteristic absorption band due to π →
trans
π* transition of trans-azobenzene units ( Az-OMe) that are attached to the imidazolium moieties.
UV-visible spectroscopic studies with varied concentration of 3a provides molar absorptivity of
trans
-1
-1
Az-OMe at λ = 360 nm is ε ~ 28400 L mol cm (Fig. S2a-b, e). A series of UV-visible
-5
absorption spectra were then recorded after sequential irradiations of a 2.5 × 10 M solution of 3a in
-2
DMSO with 365 nm UV light (I ~ 1.2 mW cm , Fig. 1b). It is evidenced from the spectral profile
that 80 s of UV exposure is sufficient to reach to the photostationary state (PSS under UV), where
absorption at ~360 nm nearly vanishes due to successful trans → cis isomerization. And a new
comparatively weaker π → π* absorption band of the corresponding cis-isomers centred at λmax
cis
-1
-1
~310 nm ( Az-OMe; ε ~ 7600 L mol cm ; Fig. S2c-e) arises along with another absorption band
cis
-1
-1
(n → π* transition of Az-OMe units) in the visible region at λmax ~440 nm (ε ~ 1700 L mol cm ;
Fig. S2c-e). Upon sequential irradiation with blue light (Fig. 1c), the solution of 3a under PSS @
UV gradually transforms from cis- to the trans-isomers and finally a steady state is reached after 10
seconds of blue light exposure (PSS under blue light). However, a complete back isomerization to
the trans-isomers requires further ~5 minutes of warming at around 110 ℃. Noteworthy, the thermal
cis
stability of cis-isomers ( Az-OMe in 3a) is found to be excellent at dark and the half-life is
3
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10.1002/chem.201905161
0
calculated to be ~18 h in DMSO at 26 C (Fig. 1d and Fig. S3). The photoswitches were then tested
for their recyclability as depicted in Fig. 1e. Greatly, the solution containing 3a is highly
photostable and fully reversible: even after 50 repeated cycles of exposures, where each cycle
consists of 80 s UV followed by 20 s blue light irradiations, the system does not show any
detectable fatigue. Effective photoswitchablity of 3a were also observed in a variety of other
solvents, including polar protic solvents like, water, methanol, ethanol, isopropanol (Fig. S4a-d),
and aprotic solvents such as acetonitrile, tetrahydrofuran, chloroform and dichloromethane (Fig.
S4e-h). Similarly, a detail UV-visible spectroscopic studies (SI-1, Fig. S5-10) of trans ↔ cis
isomerization of azobenzene units in IAz-Br.HCl-3b and IAz-H.HCl-3c were also performed.
Figure 1. (a) Isomerization of azobenzene units (Az-X) in 3. UV-visible spectra representing trans ↔ cis isomerization
of Az-OMe in 3a upon UV (b) and blue light irradiation (c). (d) Thermal stability (room temperature) of
cis
trans
cis
-5
photoisomerized-3a ( Az-OMe) at dark. (e) Recyclability between
Az-OMe ↔ Az-OMe. [3a] = 2.5 × 10 M in
DMSO.
Photoinduced generation of geometric isomers and concomitant photomodulation over
1
imidazolium moieties in N,N'-bis(azobenzene)imidazolium chloride-3a-c were then studied via H-
NMR spectroscopy. Nuclear magnetic resonance (NMR) spectra of 3a-c (Fig. S27o-t) show us the
presence of single isomeric form in each salt under ambient conditions. And UV-visible
spectroscopic studies also confirms the presence of trans-isomers in the solution. Thus the
trans-trans
thermodynamically stable forms of imidazolium chlorides-3 are denoted as, 3
(Scheme 2).
Photoisomerization of 3 would generate a total of three geometric isomers, 3trans-trans, 3
trans-cis
and
cis-cis
1
trans-trans
-3
3
. Figure 2a describes partial H-NMR spectra of 3a
in DMSO-d
6
(7.4 × 10 M);
2
4
5
showing important regions, where protons attached to the imidazolium ring ( C-H and C/ C-H) and
4
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Scheme 2. Photoisomerization of azobenzenes in IAz-X.HCl-3a-c (3trans-trans) to produce 3trans-cis and 3cis-cis isomers.
the azobenzene moieties resonate in a 500 MHz instrument. Aromatic protons of
trans
thermodynamically stable trans-azobenzenes ( Az-OMe) are shown at the right panel (Fig. 2a,
2
tt
4
5
tt
right panel), whereas, the corresponding imidazolium ring protons, denoted as C-H and C/ C-H
are shown in the left and the middle panel of Fig. 2a respectively. Upon sequential UV-irradiations
cis
(
Fig. 2b-d), evolution of peaks due to cis-azobenzenes ( Az-OMe) appear gradually and proton
trans
peaks due to the
Az-OMe started reducing accordingly. And after 2 h exposure with UV light
the photostationary state is reached where peaks due to the trans-azobenzenes nearly disappear.
Integration of azobenzenes protons at PSS under UV provide ~96% cis-isomers (Fig. 2d, Fig S12).
As seen from the spectra that upon trans → cis isomerization, the intensity of proton resonance
2
tt
peak at 9.85 ppm ( C-H ) gets reduced and a couple of new peaks arises at 9.76 and 9.67 ppm. And
overall integration of the three mentioned peaks count a single proton. We assign these peaks at
2
tc
trans-cis
2
cc
cis-cis
9
.76 ppm as C-H (3a
) and at 9.67 ppm as C-H (3a
). Concomitantly, other
4
5
4
5
tt
imidazolium ring protons ( C/ C-H) also get shifted from 8.44 ppm ( C/ C-H ) to 8.38, 8.35 ppm
for C/ C-H and 8.29 ppm for C/ C-H . Noteworthy, the 3a
indication of C-H peak) at the photostationary state and the ratio of 3a
:92 (Fig. S12). Therefore, 2 h of UV-irradiations transforms 3a
isomers even at the concentration as high as ~10 M in DMSO. NMR study on the back
isomerization reactions reveals a complete back reaction after irradiation of the 3a
4
5
tc
4
5
cc
trans-trans
isomer is fully absent (no
trans-cis
2
tt
cis-cis
and 3a
is about
trans-trans
cis-cis
8
isomers to nearly 3a
-2
cis-cis
with 30
minutes of blue light followed by 1 h of warming at ~110 ℃ (Fig. 2e and 2f, respectively).
However, blue light irradiations generates another photostationary state with overall 72% trans-
trans-trans
trans-cis
cis-cis
isomers and the relative ratio of the three geometrical isomers of 3a is 3a
5
:3a
:3a
=
1
2:40:8 (Fig. S13). In contrast, a detail H-NMR spectroscopic studies (SI-2, Fig. S14-20) of IAz-
Br.HCl-3b and IAz-H.HCl-3c revealed that trans → cis isomerization in both the imidazolium salts
cis
cis
remains incomplete (61% Az-Br and 49% Az-H at the PSS under UV).
5
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1
Figure 2. Partial H-NMR (500 MHz, DMSO-d
6
) spectra of IAz-OMe.HCl-3a: (a) at ambient conditions, (b-d) after
UV irradiations, (e) followed by blue light irradiation and (f) followed by heating. [For the full spectra see Fig. S11].
The extent of trans → cis isomerization being nearly complete in case of 3a, we have made
use of this imidazolium salt to determine the extent of photoinduced electronic modification on its
1
5
carbene carbon centre. A recent review by H. V. Huynh on determination of electronic properties
of various carbenes revealed that one of the best ways in determining electron densities on the
carbene carbon of NHCs is via determining Tolman Electronic Parameter16 (TEP). Nickel
complexes of the type [(NHCs)Ni(CO)
measuring stretching frequencies of the carbonyl (CO) functionality. However, nowadays less toxic
complexes cis-[(NHCs)RhCl(CO) ] and cis-[(NHCs)IrCl(CO) ] are mostly preferred. The averaged
3
] were initially been used for the calculation of TEP via
2
2
CO stretching frequencies (ῦav) of a fixed metal (Rh/Ir) complex could simply be used to determine
the changes in electron-donating ability of the NHCs: a smaller averaged wavenumber indicates a
stronger net donor. To correlate the TEP values obtained from different complexes a number of
mathematical formulae have also been derived.17 To this end, we have synthesized iridium
1
8
complexes of imidazolium salt-3a following the modified Herrmann’s procedures. Towards this,
solid powder of 3a was added to a suspension of [IrCl(cod)] (cod = cyclooctadiene) and potassium
2
tert-butoxide in dry tetrahydrofuran under inert atmosphere and stirred for about 5 h to yield [(IAz-
OMe)IrCl(cod)] (4, Fig. 3a). A solution of 4 in dry dichloromethane was then reacted with excess
trans-trans
carbon monoxide gas to produce our desired product, [(IAz-OMe)IrCl(CO)
2
] (5
) – as
1
13
characterized by H-, C-NMR and high-resolution mass spectra (Fig. S27w, S27x and S28,
respectively).
6
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Figure 3. (a) Synthesis of iridium complex-5, [(IAz-OMe)IrCl(CO)
2
] from 3a. (b) Isomerization of azobenzene units
attached to the iridium complex-5 in chloroform ([5] = 2.5 × 10-5 M). (c) Schematic representation of isomerization of 5
and the corresponding IR spectrum (film) of carbonyl functionalities. (d) Equation for calculating TEP.
Isomerization of the azobenzene units attached to the NHC-iridium-complex (5) was then
studied (Fig. 3b-c). According to our expectations, an excellent trans → cis isomerization is
accomplished via UV-visible spectroscopy in solvents like chloroform (Fig. 3b) and
1
dichloromethane (Fig. S21). The extent of isomerization is obtained by H-NMR spectroscopy (Fig.
S22). It is evidenced from the spectra that the resonance peak due to ortho-methyl groups of the
azobenzene units remains well resolved among all the three possible geometric isomers. The
trans-trans
relative ratio of the three geometric isomers at the PSS under UV is thus calculated to be, 5
:
trans-cis cis-cis
5
: 5
= 0:12:88 (trans:cis = 6:94). Infrared spectra were then recorded in dichloromethane
to monitor the carbonyl stretching frequencies in those iridium-NHC complexes. To be noted here,
the percentage isomerization (trans → cis) of the azobenzene units in 5 is in fact same in
chloroform and dichloromethane (Fig. S21e). To this end, a stock solution of 5 in dichloromethane
-
3
cis-
(
5 ×10 M) was divided into two parts; one part was kept under UV illumination for about 2 h (5
)
cis
while the other part was kept at dark (5trans-trans). Infrared spectra were then recorded individually
under a fixed instrumental conditions via making film - by placing dichloromethane solution
trans-trans
between two NaCl crystal plates. Carbonyl stretching frequencies of 5
are observed at
-1
tt
-1
2
068.5 and 1981.5 cm (Fig. 3c, left and Fig. S23) with an average, ῦav (CO) = 2025 cm .
cis-cis
(mixed with ~12% 5trans-cis) provides carbonyl
Interestingly, the UV-irradiated sample, 5
-1
stretching frequencies at 2064.7 and 1979.8 cm (Fig. 3c, right and Fig. S24) with an average value
cc
-1
tt-cc
of ῦav (CO) = 2022.3 cm . Therefore, the reduction in averaged frequency is Δ
ῦ
av (CO) = 2.7
-1
cis-
cm , which indicates moderate to high enhancement of net-electron donating ability of NHC in 5
cis
trans-trans
as compared to 5
. UV-induced alteration in electron donating properties of the carbene
7
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carbon of the corresponding imidazolium salt-3a is then obtained via calculating TEP values using
1
9
trans-trans
the equation (Fig. 3d) by Nolan, one of the pioneers in this field. TEP values of 5
is
-1
cis-cis
-1
calculated to be 2052.4 cm whereas the same for the 5
as 2.3 cm reduction in TEP values (Δ TEP = 2.3 cm ) simply via trans → cis isomerization of
is 2050.1 cm ; which account as high
-1
tt-cc
-1
the attached azobenzene (Az-OMe) units to the NHC ring.
In order to reconfirm the alteration of TEP values, IR spectra using ATR technology were
also recorded. Sample preparation was kept constant in dichloromethane as mentioned above. A
droplet of solution (~10 μL) of complex-5 was placed at the ATR crystal and scanned till solvent
gets evaporated (~20 scans). Sharp peaks of CO functionality were recorded in both the geometric
trans-trans
-1
tt
isomers of the complex. Whereas, 5
shows peaks at 2066.5 and 1975.7 cm (Fig. S25, ῦav
-1
tt
cis-cis
(mixed with ~12% 5trans-cis) appears at
(
CO) = 2021.1 cm and TEP = 2049.1), peaks in 5
-1
cc
-1
cc
2
059.3 and 1975.2 cm (Fig. S26, ῦav (CO) = 2017.3 cm and TEP = 2045.9). Therefore, the
extent of alteration in the averaged carbonyl frequency upon trans → cis isomerization in ATR
tt-cc
-1
tt-
technology is as high as, Δ
ῦ
av (CO) = 3.8 cm and the corresponding decrement in TEP is Δ
cc
-1
TEP = 3.2 cm . We have then compared this extent of photomodulation with the extent of
chemically modified derivatives of similar heterocyclic ring system. Metal complexes of NHCs
having a fixed imidazolin-2-ylidene ring that differs in N-alkyl substituents, such as tert-butyl,
isopropyl, cyclohexyl and cyclododecane, apparently have no difference in their averaged CO
1
5
stretching frequencies. However, NHCs, that differs in N-aryl substituents, such as [N,N'-
bis(aryl)imidazole]-2-ylidene (varied para-substituted aryl group, Fig. S1a) have dissimilar electron
donating capacity. The most electron-rich NHC in the mentioned N-aryl series (R = –NEt
2
, Fig.
S1a) was reported to have ῦav (CO) of the corresponding iridium complex that is diminished by 2
-1
cm as compared to one of the most common and conventional mesityl derivative (R= Me, Fig.
1
2
S1a). Therefore, the most popular and verified TEP based methodology to quantify electronic
properties of NHCs unambiguously proves that our azobenzene-based NHC and its metal complex
have the potentiality to alter reaction kinetics via remote photomodulation. Secondly, this proof-of-
concept could be exploited to procure even better alteration in electronic properties by varying the
attached azobenzene derivatives in our novel class of photoswitchable-NHC. Nonetheless, the
switching of photoinduced steric influences to the metal center (percent volume buried; %Vbur
)
could be an interesting additional investigation to better understand the overall alteration of the
stereoelectronic effect on these NHCs.
In summary, fine tuning of intrinsic electron-donating properties of a novel class of
azobenzene-based photoswitchable-NHC is achieved via external stimuli. Towards this, three
derivatives of N,N'-bis(azobenzene)imidazolium chlorides were synthesized, where azobenzene-
derivatives differ at the 4'-substitutions. Trans → cis isomerization of the N-attached azobenzenes
1
were studied via UV-visible spectroscopy as well as H-NMR spectroscopy. Successful synthesis of
the iridium complex ([(IAz-OMe)IrCl(CO) ]) provided us the percentage isomerization of the
2
attached azobenzenes in the complex at the photostationary state under UV (94% cis) is virtually
same as that of the NHC salt. Electron-donating ability of the photoswitchable-NHC before and
after UV irradiation is measured via averaged carbonyl stretching frequency (ῦav (CO)) and is
expressed in terms of TEP values. Upon isomerization, ῦav (CO) get diminished by at least 2.7 cm-1
-1
(
3.8 cm in ATR technology) in dichloromethane, which accounts for the reduction in TEP values
8
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Chemistry - A European Journal
10.1002/chem.201905161
of Δtt-ccTEP = 2.3 cm or more (3.2 cm in ATR technology). Therefore, trans → cis isomerization
of the azobenzene units significantly enhances the electron donating ability of our novel
azobenzene-based NHC. Utilization of such phototuning of electron density at the active site of a
catalyst to alter reaction kinetics of organic transformations is currently under progress.
-1
-1
Acknowledgements: This work was supported by the Science and Engineering Research Board,
statutory body under the Department of Science and Technology, Government of India (File No.
ECR/2016/000829). The corresponding author is thankful to the ‘UGC-Faculty Recharge
Programme’.
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Table of Contents:
Photoswitches: The photoinduced trans → cis isomerization can alter the electron-donating
abilities of azobenzene-based N-heterocyclic carbene. Appropriate iridium-complex (see figure)
was synthesized and utilized to calculate Tolman Electronic Parameter (TEP) to provide the extent
of alteration.
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