Synthesis and study of redox-active acyclic triazenes: Toward electrochromic applications

Article abstract of DOI:10.1021/jo200139f

Coupling of various 4-substituted phenyl azides with two distinct quinone-containing N-heterocyclic carbenes (NHCs) afforded the respective mono- and ditopic 1,3-disubstituted acyclic triazenes in moderate to excellent yields (38-92%). Depending on their pendant substituents (derived from the azides), the acyclic triazenes exhibited intense absorptions in the visible spectrum (359-428 nm), which were bathochromically shifted by up to Δλ = 68 nm upon reduction of the quinone moiety on the component derived from the NHC. Cyclic voltammetry confirmed that the aforementioned redox processes were reversible, and a related set of UV-vis spectroelectrochemical experiments revealed that bulk electrolysis may also be used to switch reversibly the colors exhibited by these triazenes.

Full text of DOI:10.1021/jo200139f

ARTICLE
pubs.acs.org/joc
Synthesis and Study of Redox-Active Acyclic Triazenes: Toward
Electrochromic Applications
Robert J. Ono,† Yasuo Suzuki,‡ Dimitri M. Khramov,† Mitsuru Ueda,‡ Jonathan L. Sessler,†,§ and
Christopher W. Bielawski*,†
†Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, United States
‡Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152-8552, Japan
§Department of Chemistry, Yonsei University, Seoul 120-749, Korea
S Supporting Information
b
ABSTRACT: Coupling of various 4-substituted phenyl azides
with two distinct quinone-containing N-heterocyclic carbenes
(NHCs) afforded the respective mono- and ditopic 1,3-disub-
stituted acyclic triazenes in moderate to excellent yields (38-
92%). Depending on their pendant substituents (derived from
the azides), the acyclic triazenes exhibited intense absorptions
in the visible spectrum (359-428 nm), which were bath-
ochromically shifted by up to Δλ = 68 nm upon reduction of
the quinone moiety on the component derived from the NHC.
Cyclic voltammetry confirmed that the aforementioned redox
processes were reversible, and a related set of UV-vis spectro-
electrochemical experiments revealed that bulk electrolysis may also be used to switch reversibly the colors exhibited by these
triazenes.
’ INTRODUCTION
complementary ditopic organic azides.16 The aforementioned
polymerization process utilizes the NHC/azide coupling reac-
tion to afford acyclic triazenes (see Figure 1). Attractive features
of this reaction include good yields of product, perfect atom
economy, and high functional group tolerance.17 Moreover, the
acyclic triazene products have been found to exhibit high thermal
stabilities and intense absorption bands in the visible spectrum
(λmax 364-450 nm; ε ≈ 104 L mol-1 cm-1).18a It was also
demonstrated by experiment19 and later confirmed by theory20
that the electronic delocalization across the triazene linkage in
these substrates was efficient and that their absorption character-
istics could be tuned by installing functional groups onto the
NHC and/or azide components.
In parallel with the study of the NHC/azide coupling reaction,
we have also recently reported a series of quinone-annulated
NHCs, NqMes (1)21 and QBI (2),22 featuring naphthoquinone
and quinone functional groups, respectively (Figure 2). These
electroactive NHCs were found to form stable complexes with a
number of transition metals and enabled redox-switchable elec-
tronic communication between the two components.23,24 We
have since demonstrated that these characteristics may be
harnessed to tune the catalytic properties of the coordinated
metals25 and to support ion-mediated electron transfer reactions
within supramolecular host-guest ensembles.26
Electrochromic materials—materials which reversibly change
their colors in response to electrochemical switching1—are
currently of great interest for their potential utility in applications
ranging from color-changing displays and windows to protective
eyewear and data storage devices. While a majority of the systems
in use are based on inorganic materials,2 which inherently present
challenges in processing and modification, it has been proposed
that organic analogues may overcome many of these issues and
enable the realization of viable alternatives.1,3 Numerous electro-
chromic organic molecules have been reported to date; well-
known examples include members of the viologen family,1,4
polyanilines,5,6 poly(3,4-ethylenedioxythiophene),7-9 and ap-
propriately functionalized bipyrrolic species.10,11 However, many
of these systems suffer from various drawbacks, including com-
plicated and tedious syntheses and, in some cases, limited
electrochemical stability. Hence, there remains a demand for
new classes of electrochromic organic motifs that are readily
accessible, feature tunable electronic and/or structural compo-
nents, and exhibit reversible electrochemical behavior.
With the above needs in mind, we have focused recently on the
development of new electrochromic systems derived from
N-heterocyclic carbenes (NHCs).12,13 This interest was moti-
vated in part by our general interest in using NHCs to address
outstanding challenges in the field of materials chemistry.14,15
For example, we discovered that new classes of highly fluorescent
polymers may be prepared by combining bis(NHC)s with
Received: January 20, 2011
Published: March 22, 2011
r
2011 American Chemical Society
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Building on the features intrinsic to the aforementioned systems
(i.e., variable optical properties and reversible electrochemistry),
we reasoned that redox-active triazenes prepared through NHC/
azide coupling chemistry involving NqMes or QBI could find
use in the preparation of electrochromic materials. Moreover,
we expected that the high nitrogen content of the triazenes
produced from these reactions would enhance their molar
absorptivities18 and that materials incorporating subunits such
as 1 and 2 would prove electroactive. Herein, we describe the
synthesis of a series of acyclic triazenes derived from two different
redox-active NHC scaffolds and provide details of their thermal,
optical, and electrochemical properties. On the basis of the
results presented in this report, we conclude that these materials
hold potential for application in a wide range of electrochromic
devices.
deficient and electron-rich para-substituted phenyl azides were
selected as coupling partners for the quinone-containing NHCs
mentioned above.18 Treatment of 1 or 2 with an excess of various
para-substituted aryl azides 3a-f in THF afforded triazenes 4a-
f and 5a-c in moderate to excellent yields (38-92%, Scheme 1).
The NMR spectroscopic data recorded for 4 and 5 were consistent
with those reported for other ylidene-triazenes.18,20,22 Key
spectral features include diagnostic upfield shifts in the 13C
NMR signals assigned to the carbene carbon atoms at the
2- (or 2,20-) position(s) (e.g., 4, δ 174.3-174.4; 5, δ 165.2-
165.7; CD2Cl2) relative to those recorded for 1 or 2, respectively
(i.e., 1, δ 231.7, C6D6; 2, δ 232.6, THF-d8). Collectively, these
findings are consistent with the loss of carbenoid character upon
NHC/azide coupling.
With the aforementioned triazenes in hand, their thermal
stabilities were evaluated by thermogravimetric analysis (TGA).
Despite their relatively high nitrogen content, triazenes 4 and 5
exhibited high thermal stabilities, with decomposition tempera-
tures (defined as the temperature at which a 5% loss in mass
was observed) ranging from 257 up to 302 °C. It has previously
been shown that other NHC-containing triazenes with bulky N
substituents (e.g., mesityl) also show high thermal stabilities and
that their decomposition proceeds through loss of dinitrogen to
afford the respective guanidines.16,18 However, the initial mass loss
observed in 4 and 5 upon heating did not correspond to loss of N2,
leading us to suggest that competing decomposition processes
may be operative at elevated temperatures in the case of these
compounds.
’ RESULTS AND DISCUSSION
To explore the influence of aryl substitution on the absorption
properties of the corresponding triazenes, a variety of electron-
Figure 1. Triazene formation via coupling of an NHC with an azide.
The optical properties of 4 and 5 were next evaluated using
UV-vis spectroscopy. In general, the triazenes exhibited intense
absorptions (ε = 3.2 ꢀ 104-6.9 ꢀ 104 L mol-1 cm-1) in the
300-500 nm spectral range with minor features observed between
500 and 600 nm (Table 1). Weaker, lower-energy absorptions
were also observed around 550 nm and were attributed to the
formation of charge transfer complexes. Dissolution of the
triazenes in CH2Cl2 generally afforded solutions that were dark
red, although solutions derived from relatively electron-rich
Figure 2. Redox-active N-heterocyclic carbenes. Mes
trimethylphenyl.
= 2,4,6-
Scheme 1. Synthesis of Triazenes 4a-f and 5a-ca
a Legend: (i) THF, 23 °C, 12 h.
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Figure 3. Selected UV-vis absorption spectra of 4 and 5 in CH2Cl2 (10 μM) before (solid shapes) and after (open shapes) reduction with CoCp2 (2 equiv).
Table 1. Summary of UV-Vis Absorption Data for Triazenes
The resonance contribution enabled in 4a (but not in 4e) is
believed to lower its absorption energy, leading to a red-shifted
absorption maxima in comparison to 4e.
4 and 5
R
λmax (nm)a λmax(reduced) (nm)b Q(Δλ) (nm)c log εd
The UV-vis absorption profiles of 4and 5also enabled qualitative
comparisons between the two series of mono- and ditopic
triazenes studied. In general, QBI (2) appeared to be more
electron rich than its monotopic congener, NqMes (1). This
conclusion, ascribed to the presence of four electron-donating
nitrogen groups in the former system (versus two in NqMes),
was reflected in the analogous observation that triazenes 5 generally
exhibited lower energy absorption energies than their respective
monotopic analogues (4) possessing the same substituents.
Furthermore, the extinction coefficients measured for 5 were
roughly twice as large as those measured for 4 (see Figure 3), a
result that suggested to us that the triazene linkage—two of
which are present for 5, while 4 possesses only one—is the
primary chromophore responsible for the intense coloration
exhibited by these systems. On the basis of the above spectro-
scopic findings, we recommend that triazenes 4 and 5 be viewed
as donor-acceptor systems, wherein the NHC fragment is a
donor and the component derived from phenyl azide acts as the
acceptor.18
Recently, we reported that methylation of a triazene derived
from N,N0-dimesitylimidazolylidene and p-nitrophenyl azide
afforded a cationic species that exhibits a hypsochromically
shifted λmax value compared to that of its unmethylated precursor
(Δλ = 65 nm).20 Considering that triazenes 4 and 5 contain
quinone moieties, subunits that may undergo one-electron
reductions to form ionic species, we envisioned that reduction,
being a charge-building process, would increase the electron
density of the system and effectively render the NHC component
a stronger donor (Scheme 2). Hence, we expected to see a
measurable spectroscopic response upon reduction of 4 and 5.
To explore their redox-induced colorimetric response, solutions
of 4 and 5 in CH2Cl2 were independently treated with CoCp2 (2
equiv)—a sufficiently strong reagent for reducing quinones25—
and the UV-vis spectra of the resulting reaction mixtures were
recorded. The λmax values measured for the reduced triazenes
were bathochromically shifted up to Δλ = 68 nm, a finding that
is consistent with an increase in electron density arising from
a reduction process. As summarized in Table 2, the magnitude of
4a NO2
408
359
368
381
366
367
428
377
381
472
360
368
408
380
376
496
412
416
64
1
4.51
4.61
4.54
4.56
4.56
4.55
4.80
4.84
4.77
4b
H
4c OMe
4d CN
4e CF3
4f Cl
0
27
14
9
5a NO2
68
35
35
5b
H
5c OMe
a Recorded using 10 μM solutions of analyte in CH2Cl2 at 23 °C.
b Spectra were obtained after the addition of 2 equiv of CoCp2 to the
analyte solution. c Δλ = λmax(reduced) - λmax. d Molar absorptivity (ε)
expressed in units of L mol-1 cm-1
.
azides (i.e., 4b,c,f) were characterized by a deep violet color due,
presumably, to the higher transmittance in the blue region of the
visible spectrum (∼460 nm).
As shown in Figure 3, the absorption maxima of the triazenes
were influenced by the substituents derived from the phenyl
azide component. In general, triazenes possessing strongly
electron-withdrawing groups absorbed radiation at relatively
low energies compared to analogues possessing electron-rich
arenes. For example, nitro-substituted 4a and 5a exhibited the
lowest energy absorptions in their respective classes (λmax = 408
and 428 nm, respectively). However, this trend did not hold for
the relatively electron rich species 4c and 5c. Due to their
electron-donating methoxy subsituents, compounds 4c and 5c
were expected to absorb at the shortest wavelengths among the
triazenes studied; conversely, their absorption maxima were red-
shifted beyond what was seen even for the unsubstituted systems
4b and 5b (R = H), respectively. Presumably, 4c and 5c enable
extended electronic delocalization with the pendant methoxy
groups, an interaction that lowers their absorption energies in
comparison to those of their unsubstituted analogues. A similar
discrepancy was also observed for 4a (R = NO2) and 4e (R = CF3),
which both feature strongly electron withdrawing substituents.
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Scheme 2. Example of the Reversible Reduction of 4
Table 2. Summary of Various Redox Potentials and Carbonyl
Stretching Frequencies
b
E1/2 (V)a
νCdO (cm-1 b
)
E1/2 (V)a
νCdO (cm-1
)
4a
4b
4c
4d
4e
4f
-0.60
-0.65
-0.64
-0.64
-0.61
-0.65
1668
1663
1664
1668
1668
1664
5a
5b
5c
-0.20
-0.34
-0.38
1671
1665
1664
Figure 4. Normalized difference UV-vis absorption spectra recorded
at 60 s intervals showing the shift in λmax upon (A) reduction of 5a
(Eapp = -0.5 V) and (B) subsequent reoxidation of 5a (Eapp = 0.4 V).
Downward arrows indicate the consumption (A) of the neutral com-
pound and (B) of the reduced compound; upward arrows indicate the
formation (A) of the reduced compound and (B) of the neutral
compound over time.
a Measurements performed using 1 mM analyte and 0.1 M nBu4NPF6 in
CH2Cl2. b The quinone stretching energies νCdO were measured in KBr
matrices.
the shift correlated with the λmax of the neutral species, where
triazenes exhibiting low-energy absorptions exhibited the largest
spectral shifts. Hence, triazenes featuring strong donor groups
(e.g., QBI moiety 2) and/or strong acceptor groups (e.g., NO2 or
CN) produced the largest spectral shifts (Δλ = 14-68 nm),
whereas weaker donor/acceptor combinations (e.g., 4b,c) gave
rise to no discernible change in λmax upon reduction. By compar-
ison, 5b,c, which featured the same para substituents as 4b,c,
respectively, both exhibited a Δλ value of 35 nm. The discre-
pancy is consistent with QBI (2) being more electron-rich than
NqMes (1), rendering the former a relatively strong donor.
Collectively, these results suggested to us that the optical proper-
ties of the triazenes may be tuned by adjusting the donor strength
of the NHC or through the incorporation of pendant functional
groups via changing the azide component employed during their
synthesis.
A series of electrochemical measurements were subsequently
performed to assess the reversibility of the aforementioned
reduction processes. Cyclic voltammetry (CV) (using nBu4NPF6
as the electrolyte) revealed reversible one-electron-reduction
features for 4 and 5 in CH2Cl2 (see the Supporting Information)
that were consistent with the conversion of a p-quinone moiety
to its respective semiquinone radical anion (see Scheme 2).22 As
summarized in Table 2, the half-wave potentials (E1/2) of 4a-f
were dependent on the substituent derived from their respective
aryl azides. Substrates containing relatively electron rich triazene
fragments, such as 4b,c, exhibited the highest quinone reduction
potentials (-0.65 and -0.64 V vs SCE, respectively); conver-
sely, electron-deficient species, such as 4a, exhibited the lowest
measured E1/2 (-0.60 V) within the series of analogues included
in this study. Similar trends were observed for 5a-c, where the
electron-donating or -withdrawing ability of the substituent
derived from the aryl azide influenced the potential of the
respective triazene redox couple.
character of the triazenes. The strength of the quinone CdO
bonds, and consequently the carbonyl stretching frequency
(νCdO), should increase with the electron-deficient character
of the triazene. As seen in Table 2, the highest νCdO values were
observed for the most electron-deficient complexes in each
respective series of triazenes studied (i.e., 4a and 5a, re-
spectively). As expected, the νCdO values steadily decreased
with increasing electron-donating ability of the pendant substit-
uents, although the differences were less pronounced in the case
of 4 than in the case of 5. Moreover, a correlation was observed
between the E1/2 values of 4 and 5 and their respective νCdO
values, wherein an increasing νCdO value corresponded with a
decreasing E1/2 value (Table 2). Qualitatively, these results
suggested to us that electron-deficient species (i.e., 4a) undergo
reduction at lower potentials than do relatively electron rich
analogues (i.e., 4c). Such a conclusion was consistent with the
absorption characteristics of the triazenes discussed above.
Finally, having confirmed both the chemochromic behavior
and electrochemical reversibility, we explored whether 4 or 5
displayed electrochromic characteristics. A series of UV-vis
spectroelectrochemical experiments were devised: here, a solu-
tion of the analyte in CH2Cl2 was taken as the background, and
the absorption spectra were recorded at 60 s intervals while
applying a suitably cathodic potential (Eapp = -0.5 V) to a high-
surface-area Pt mesh immersed in the solution. Consistent with
the UV-vis data discussed above, we observed that the signals
attributable to the (neutral) triazene diminished upon bulk
electrolysis, while signals associated with the reduced compound
intensified (cf. Table 2). For example, the time-resolved UV-vis
spectra of 5a shown in Figure 4A revealed a diminishing signal at
424 nm upon electrochemical reduction, as the signal at 508 nm
concomitantly increased. These data compared well with the
absorption profiles of both neutral and chemically reduced 5a, for
which λmax values of 428 and 496 nm were observed, respectively
IR spectroscopic analysis was also performed to evaluate the
effects of the pendant functional groups on the overall electronic
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(Table 2). To determine if the aforementioned reduction could
be reversed, bulk electrolysis was performed at a sufficiently anodic
potential (Eapp = 0.4 V) following reduction, while a series of
optical difference spectra were recorded. As shown in Figure 4B,
signals attributable to the reduced compound diminished at this
potential, while those associated with the neutral compound
intensified. Similar results were observed for the other triazenes
investigated (see the Supporting Information). Taken in concert,
these studies served to confirm that the colors exhibited by 4 and
5 in solution could be switched using electrochemical techniques.
an atmosphere of dry nitrogen. The cell was equipped with gold working,
tungsten counter, and silver quasi-reference electrodes. Measurements
were performed in dry CH2Cl2 with 0.1 M [tetra-n-butylammonium]-
[PF6] (TBAP) as the electrolyte and decamethylferrocene (Fc*) as the
internal standard. Unless otherwise noted, all potentials were deter-
mined at 100 mV s-1 scan rates and referenced to the saturated calomel
electrode (SCE) by shifting (Fc*)0/þ to -0.057 V.29 UV-vis spectro-
electrochemical measurements were performed using a custom-made
quartz cuvette with 0.2 cm path length equipped with custom-made
platinum working, tungsten counter, and silver quasi-reference electro-
des. All measurements were made in dry CH2Cl2 using 50 μM analyte
with 0.1 M TBAP as the electrolyte.
’ CONCLUSION
Synthesis of 4a. A solution of free carbene 1 (87.1 mg, 0.20 mmol)
and 4-nitrophenyl azide (32.8 mg, 0.40 mmol) in dry THF (2 mL) was
stirred at 23 °C for 12 h. The residual solvent was then removed under
reduced pressure, and the crude product was purified by column
chromatography (SiO2; 1/3 v/v hexanes/CH2Cl2) to afford the desired
product as a dark red solid (80.0 mg, 67% yield). Mp: 195-198 °C. Td =
276 °C. 1H NMR (300 MHz, CDCl3): δ 8.05 (dd, J = 2.4, 8.8 Hz, 2H),
7.93 (d, J = 8.8 Hz, 2H), 7.71 (dd, J = 2.4, 8.8 Hz, 2H), 7.06 (s, 4H), 6.72
(d, J = 8.8 Hz, 2H), 2.41 (s, 6H), 2.16 (s, 12H). 1H NMR (400 MHz,
CD2Cl2): δ 8.01 (dd, J = 2.4, 8.8 Hz, 2H), 7.95 (d, J = 8.8 Hz, 2H), 7.75
(dd, J = 2.4, 8.8 Hz, 2H), 7.12 (s, 4H), 6.71 (d, J = 8.8 Hz, 2H), 2.46 (s,
6H), 2.17 (s, 12H). 13C NMR (100 MHz, CD2Cl2): δ 174.3, 155.7,
153.1, 146.4, 140.1, 135.0, 134.6, 132.3, 131.9, 129.7, 129.1, 126.8, 124.3,
122.5, 21.3, 18.0. HRMS: m/z calcd for C35H31N6O4 [M þ H]þ
599.2392, found 599.2401. FT-IR (KBr): νCO 1668 cm-1. Anal. Calcd
for C35H30N6O4: C, 70.22; H, 5.05; N, 14.04. Found: C, 70.48; H, 4.92;
N, 13.83.
Synthesis of 4b. To a Schlenk flask charged with a solution of free
carbene 1 (87.1 mg, 0.20 mmol) in dry THF (2 mL) was added in one
portion a solution of phenyl azide (0.4 mmol) in dry THF (2 mL). The
resulting solution was then stirred at 23 °C for 12 h. After the residual
solvent was removed under reduced pressure, the crude product was
purified by column chromatography (SiO2; 1/2 v/v hexanes/CH2Cl2)
to afford the desired product as a dark violet solid (94.5 mg, 85% yield).
Mp: 242-244 °C. Td = 302 °C. 1H NMR (300 MHz, CDCl3): δ 7.98
(dd, J = 2.4, 8.8 Hz, 2H), 7.68 (dd, J = 2.4, 9.2 Hz, 2H), 7.01 (m, 3H),
7.05 (s, 4H), 6.70 (dd, 2H), 2.40 (s, 6H), 2.19 (s, 12H). 1H NMR (400
MHz, CD2Cl2): δ 8.00 (dd, J = 2.4, 8.8 Hz, 2H), 7.73 (dd, J = 2.4, 9.2 Hz,
2H), 7.14-7.11 (m, 7H), 6.65 (dd, 4H), 2.46 (s, 6H), 2.19 (s, 12H).
13C NMR (100 MHz, CD2Cl2): δ 174.4, 162.6, 150.8, 139.7, 135.2, 134.3,
132.7, 132.0, 129.6, 129.0, 128.6, 127.8, 126.6, 122.3, 21.3, 18.1. HRMS:
m/z calcd for C35H32N5O2 [M þ H]þ 554.2560, found 554.2550. FT-IR
(KBr): νCO 1663 cm-1. Anal. Calcd for C35H31N5O2: C, 75.93; H, 5.64;
N, 12.65. Found: C, 75.32; H, 5.66; N, 12.20.
In summary, a series of mono- and ditopic redox-active 1,3-
disubstituted triazenes were synthesized by coupling two distinct
quinone-containing NHCs with various aryl azides. The coupling
reactions were performed using the respective free NHCs, and
the resulting products were obtained in modest to excellent
yields (38-92%). The triazenes were thermally stable up to
302 °C, and their absorption spectra were consistent with
efficient electronic delocalization through the triazene linkages.
In general, the λmax values of the triazenes underwent batho-
chromic shifts as the aryl azide moiety became more electron
deficient or when the NHC moiety became more electron rich
(i.e., upon reductionofthe quinone). Cyclicvoltammetric analyses
of the triazenes revealed that these compounds were capable of
undergoing reversible reduction and provided support for the
proposal that there is electronic communication between the
NHC and the aryl azide moieties, as evidenced by a correlation
between reduction potential and the electron-withdrawing ability
of the para substituent on the phenyl group. Finally, the electro-
chromic properties of the triazenes were verified by UV-vis
spectroelectrochemical analyses, which confirmed that reversible
color changes could be induced using electrochemical methods.
Collectively, these results highlight the versatility of NHC-
triazene coupling chemistry for the preparation of tunable and
configurable optical materials. The robust nature of the products,
considered in conjunction with their generally well-behaved
electrochemistry, leads us to propose that triazene-based systems,
such as those described herein, are poised for use in a variety of
applications requiring organic electrochromic materials.
’ EXPERIMENTAL SECTION
General Considerations. Unless otherwise noted, all compounds
were prepared under an atmosphere of nitrogen using drybox or Schlenk
techniques. CH2Cl2 and THF were distilled from CaH2 or sodium
benzophenone ketyl under a nitrogen atmosphere prior to use. Solvents
were degassed by three consecutive freeze-pump-thaw cycles. All
other chemicals were used as received without further purification.
NqMes (1),21 QBI (2),22 and aryl azides 327,28 were prepared as previously
described. 1H and 13C NMR spectra were recorded using a 300, 400, or
500 MHz spectrometer. Chemical shifts (δ) areexpressedinppmdownfield
from tetramethylsilane using the residual nondeuterated solvent as an
internal standard. For 1H NMR: CDCl3, δ 7.24 ppm; CD2Cl2, δ 5.32
ppm. For 13C NMR: CD2Cl2, δ 53.8 ppm. UV-vis spectroscopic measure-
ments were made with matched quartz cuvettes with 1 cm path lengths
and 3.0 mL sample solution volumes. Beer’s law measurements were
performed using 1, 2.5, 5, and 10 μM sample concentrations. All UV-vis
spectroscopic measurements were performed in dry CH2Cl2 under an
atmosphere of nitrogen. Thermogravimetric analyses were performed
under an atmosphere of nitrogen at a scan rate of 20 °C min-1. Cyclic
voltammetry was conducted using a gastight, three-electrode cell under
Synthesis of 4c. To a Schlenk flask charged with a solution of free
carbene 1 (87.1 mg, 0.20 mmol) in dry THF was added in one portion a
solution of 4-methoxyphenyl azide (0.4 mmol) in dry THF (2 mL). The
resulting solution was then stirred at 23 °C for 12 h. After the residual
solvent was removed under reduced pressure, the crude product was
purified by column chromatography (SiO2; 1/3 v/v hexanes/CH2Cl2)
to afford the desired product as a black solid (44.4 mg, 38% yield). Mp:
208-212 °C. Td = 294 °C. 1H NMR (300 MHz, CDCl3): δ 7.96 (dd, J =
2.4, 8.8 Hz, 2H), 7.66 (dd, J = 2.8, 8.8, 2H), 7.03 (s, 3H), 6.65 (d, J = 6.4
Hz, 2H), 6.60 (d, 2H), 3.75 (s, 3H), 2.36 (s, 6H), 2.17 (s, 12H).
1H NMR (400 MHz, CD2Cl2): δ 8.00 (dd, J = 2.4, 8.8 Hz, 2H), 7.72 (dd,
J = 2.8, 8.8, 2H), 7.11 (s, 4H), 6.64 (dd, J = 6.4 Hz, 4H), 3.76 (s, 3H),
2.45 (s, 6H), 2.19 (s, 12H). 13C NMR (100 MHz, CD2Cl2): δ 174.3,
159.7, 152.2, 144.6, 139.6, 135.3, 132.8, 132.0, 130.0, 129.0, 126.5, 123.7,
113.7, 55.6, 21.3, 18.1. HRMS: m/z calcd for C36H34N5O3 [M þ H]þ
584.2664, found 584.2656. FT-IR (KBr): νCO 1664 cm-1. Anal. Calcd
for C36H33N5O3: C, 74.08; H, 5.70; N, 11.92. Found: C, 73.65; H, 5.72;
N, 11.92.
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Synthesis of 4d. A solution of free carbene 1 (87.1 mg, 0.20 mmol)
and 4-cyanophenyl azide (34.6 mg, 0.24 mmol) in dry THF (2 mL) was
stirred at 23 °C for 12 h. The residual solvent was then removed under
reduced pressure, and the crude product was purified by column
chromatography (SiO2; CH2Cl2) to afford the desired product as a
dark red solid (84.8 mg, 73% yield). Mp: 248-250 °C. Td = 277 °C. 1H
NMR (300 MHz, CDCl3): δ 7.99 (dd, J = 2.4, 8.8 Hz, 2H), 7.70 (dd,
J = 2.4, 9.2 Hz, 2H), 7.34 (d, J = 8.4 Hz, 2H), 7.05 (s, 4H), 6.68 (d, J = 8.8
Hz, 2H), 2.40 (s, 6H), 2.15 (s, 12H). 1H NMR (400 MHz, CD2Cl2): δ
8.02 (dd, J = 2.4, 8.8 Hz, 2H), 7.76 (dd, J = 2.4, 9.2 Hz, 2H), 7.41 (d,
J = 8.4 Hz, 2H), 7.13 (s, 4H), 6.69 (d, J = 8.8 Hz, 2H), 2.47 (s, 6H), 2.18
(s, 12H). 13C NMR (100 MHz, CD2Cl2): δ 174.3, 154.0, 153.0, 140.0,
135.0, 134.5, 132.7, 132.4, 131.9, 129.7, 129.0, 126.7, 122.7, 119.4, 110.2,
21.3, 18.0. HRMS: m/z calcd for C36H31N6O2 [M þ H]þ 579.2514,
found 579.2503. FT-IR (KBr): νCO 1668 cm-1. Anal. Calcd for
C36H30N6O2: C, 74.72; H, 5.23; N, 14.52. Found: C, 74.48; H, 5.24;
N, 14.24.
Synthesis of 4e. To a Schlenk flask charged with a solution of free
carbene 1 (87.1 mg, 0.20 mmol) in dry THF (2 mL) was added in one
portion a solution of 4-(trifluoromethyl)phenyl azide (0.4 mmol) in dry
THF (2 mL). The resulting solution was then stirred at 23 °C for 4 h.
After the residual solvent was removed under reduced pressure, the
crude product was purified by column chromatography (SiO2; 1/2 v/v
hexanes/CH2Cl2) to afford the desired product as a dark red solid
(104.4 mg, 84% yield). Mp: 245-246 °C. Td = 257 °C. 1H NMR (300
MHz, CDCl3): δ 7.99 (dd, J = 2.4, 9.2 Hz, 2H), 7.69 (dd, J = 2.4, 9.2 Hz,
2H), 7.31 (d, J = 9.2 Hz, 2H), 7.05 (s, 4H), 6.73 (d, J = 8.8 Hz, 2H), 2.40
(s, 6H), 2.17 (s, 12H). 1H NMR (400 MHz, CD2Cl2): δ 8.03 (dd, J =
2.4, 9.2 Hz, 2H), 7.75 (dd, J = 2.4, 9.2 Hz, 2H), 7.95 (d, J = 9.2 Hz, 2H),
7.14 (s, 4H), 6.75 (d, J = 8.8 Hz, 2H), 2.47 (s, 6H), 2.20 (s, 12H). 13C
NMR (100 MHz, CD2Cl2): δ 174.3, 153.6, 153.0, 139.9, 135.1, 134.5,
132.5, 131.9, 129.7, 129.0, 128.9, 128.6, 126.7, 126.1, 125.7, 125.7, 123.4,
122.4, 21.3, 18.1. HRMS: m/z calcd for C36H31F3N5O2 [M þ H]þ
622.2420, found 622.2424. FT-IR (KBr): νCO 1668 cm-1. Anal. Calcd
for C36H30F3N5O2: C, 69.55; H, 4.86; N, 11.27. Found: C, 69.28; H,
4.87; N, 11.16.
(d, J = 8.8 Hz, 4H), 2.41 (s, 12H), 2.15 (s, 24H). 13C NMR (100 MHz,
CD2Cl2): δ 165.2, 155.5, 152.9, 146.5, 140.4, 134.8, 131.6, 129.9, 125.7,
124.3, 122.5, 21.3, 18.2. HRMS: m/z calcd for C56H53N12O6 [M þ H]þ
989.4199, found 989.4206. FT-IR (KBr): νCO 1671 cm-1. Anal. Calcd
for C56H52N12O6: C, 68.00; H, 5.30; N, 16.99. Found: C, 68.16; H, 5.43;
N, 16.48.
Synthesis of 5b. To a Schlenk flask charged with a solution of free
carbene 2 (50.0 mg, 0.076 mmol) in dry THF (2 mL) was added in one
portion a solution of phenyl azide (0.22 mmol) in dry THF (2 mL). The
resulting solution was then stirred at 23 °C for 12 h. After the solvent was
removed under reduced pressure, the crude product was purified by
column chromatography (SiO2; 3/1 v/v hexanes/EtOAc) to afford
the desired product as a red-brown solid (60 mg, 88% yield). Mp:
1
244-246 °C. Td = 268 °C. H NMR (300 MHz, CDCl3): δ 7.06
(m, 6H), 6.91 (s, 8H), 6.64 (m, 4H), 2.29 (s, 12H), 2.12 (s, 24H). 1HNMR
(400 MHz, CD2Cl2): δ 7.11-7.09 (m, 6H), 7.05 (s, 8H), 6.59 (dd, 4H),
2.40 (s, 12H), 2.16 (s, 24H). 13C NMR (100 MHz, CD2Cl2): δ 165.6,
152.4, 150.7, 139.9, 135.0, 132.1, 129.7, 128.6, 127.9, 125.4, 122.3, 21.2,
18.2. HRMS: m/z calcd for C56H55N10O2 [M þ H]þ 899.4502, found
899.4504. FT-IR (KBr): νCO 1665 cm-1. Anal. Calcd for C56H54N10O2: C,
74.81; H, 6.05; N, 15.58. Found: C, 75.03; H, 6.06; N, 15.51.
Synthesis of 5c. To a Schlenk flask charged with a solution of free
carbene 2 (35.0 mg, 0.053 mmol) in dry THF (2 mL) was added in one
portion a solution of 4-methoxyphenyl azide (0.28 mmol) in dry THF
(2 mL). The resulting solution was then stirred at 23 °C for 12 h. After
the solvent was removed under reduced pressure, the crude product was
purified by column chromatography (SiO2; first 3/1 v/v hexanes/
EtOAc to remove impurities then 3/2/1 v/v/v hexanes/EtOAc/
CH2Cl2) to afford the desired product as a brown solid (21 mg, 41%
yield). Mp: 260-262 °C. Td = 237 °C. 1H NMR (300 MHz, CDCl3): δ
6.90 (s, 8H), 6.60-6.58 (m, 8H), 3.73 (s, 6H), 2.29 (s, 12H), 2.11 (s,
24H). 1H NMR (400 MHz, CD2Cl2): δ 7.04 (s, 8H), 6.61 (m, 4H), 6.56
(m, 4H), 3.74 (s, 3H), 2.39 (s, 12H), 2.15 (s, 24H). 13C NMR (100
MHz, CD2Cl2): δ 165.7, 159.7, 151.9, 144.5, 139.8, 135.1, 132.0, 129.6,
125.3, 123.6, 113.7, 55.6, 21.2, 18.2. HRMS: m/z calcd for C58H59N10O4
Synthesis of 4f. To a Schlenk flask charged with a solution of free
carbene 1 (87.1 mg, 0.20 mmol) in dry THF (2 mL) was added in one
portion a solution of 4-chlorophenyl azide (0.4 mmol) in dry THF
(2 mL). The resulting solution was then stirred at 23 °C for 12 h. After
the residual solvent was removed under reduced pressure, the crude
product was purified by column chromatography (SiO2; CH2Cl2) to
afford the desired product as a dark violet solid (108.4 mg, 92% yield).
Mp: 239-242 °C. Td = 267 °C. 1H NMR (300 MHz, CDCl3): δ 8.02
(dd, J = 2.4, 9.2 Hz, 2H), 7.73 (dd, J = 2.4, 8.8 Hz, 2H), 7.08 (s, 4H), 7.07
(d, J = 8.8 Hz, 2H), 6.63 (d, J = 8.8 Hz, 2H), 2.44 (s, 6H), 2.21 (s, 12H).
1H NMR (400 MHz, CD2Cl2): δ 8.00 (dd, J = 2.4, 9.2 Hz, 2H), 7.73 (dd,
J = 2.4, 8.8 Hz, 2H), 7.11 (s, 4H), 7.09 (d, J = 8.8 Hz, 2H), 6.58 (d, J = 8.8
Hz, 2H), 2.46 (s, 6H), 2.18 (s, 12H). 13C NMR (100 MHz, CD2Cl2): δ
174.3, 152.7, 149.5, 139.8, 135.2, 134.4, 133.0, 132.6, 131.9, 129.6, 129.0,
128.7, 126.6, 123.6, 21.3, 18.1. HRMS: m/z calcd for C35H31N5O2Cl
[M þ H]þ 959.4711, found 959.4715. FT-IR (KBr): νCO 1664 cm-1
.
Anal. Calcd for C58H58N10O4: C, 72.63; H, 6.10; N, 14.60. Found: C,
72.34; H, 6.21; N, 14.08.
’ ASSOCIATED CONTENT
S
Supporting Information. Figures giving cyclic voltam-
b
mograms, UV-vis spectroelectrochemical data, and NMR spec-
tra. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: bielawski@cm.utexas.edu.
[M þ H]þ 588.2170, found 588.2161. FT-IR (KBr): νCO 1664 cm-1
.
Anal. Calcd for C35H30N5O2Cl: C, 71.48; H, 5.14; N, 11.91. Found: C,
71.46; H, 5.06; N, 11.67.
’ ACKNOWLEDGMENT
Synthesis of 5a. Free carbene 2 (50.0 mg, 0.076 mmol),
4-nitrophenyl azide (29.0 mg, 0.17 mmol), and dry THF (2 mL) were
added to an oven-dried Schlenk flask. The resulting solution was then
stirred at 23 °C for 12 h. The residual solvent was then removed under
reduced pressure, and the crude product was triturated with hexanes
(3 ꢀ 25 mL) and Et2O (3 ꢀ 25 mL) to afford the desired product as a
dark red orange solid (39 mg, 52% yield). Mp: 252-253 °C. Td =
294 °C. 1H NMR (300 MHz, CDCl3): δ 7.91 (d, J = 8.8 Hz, 4H), 6.95
This work was made possible by the generous financial
support of the National Science Foundation (Grant Nos.
CHE-0645563 and CHE-0749571 to C.W.B. and J.L.S., re-
spectively) and the Robert A. Welch Foundation (Grant Nos.
F-1621 and F-1018 to C.W.B. and J.L.S., respectively). Support
from the Korean WCU program (grant R32-2008-000-10217-0)
is also acknowledged. We thank Prof. A. G. Tennyson for
assistance with the spectroelectrochemical experiments and for
insightful discussions.
1
(s, 8H), 6.68 (d, J = 8.8 Hz, 4H), 2.32 (s, 12H), 2.11 (s, 24H). H
NMR (400 MHz, CD2Cl2): δ 7.93(d, J = 8.8 Hz, 4H), 7.07 (s, 8H), 6.66
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ARTICLE
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