Influence of Annulation on the Electron Spin within the 1,2,3-Triazole Ring in Annulenotriazole Anion Radicals

Article abstract of DOI:10.1021/acs.joc.7b03245

The low temperature (-100 °C) single electron reduction of N1-phenylbenzotriazole in liquid ammonia, and the room temperature reduction of N1-phenylcyclooctatetraenotriazole in hexamethylphosphoramide, yields stable solutions of both anion radicals, which

Full text of DOI:10.1021/acs.joc.7b03245

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Influence of Annulation on the Electron Spin within the
1,2,3-Triazole Ring in Annulenotriazole Anion Radicals
Steven J. Peters, and Donald L Maybell
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b03245 • Publication Date (Web): 05 Mar 2018
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Influence of Annulation on the Electron Spin within the 1,2,3-Triazole Ring in
Annulenotriazole Anion Radicals
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Steven J. Peters* and Donald L. Maybell III
Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160
sjpeter@illinoisstate.edu
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ABSTRACT
The low temperature (-100oC) single electron reduction of N1-phenyl benzotriazole in liquid
ammonia, and the room temperature reduction of N1-phenyl cyclooctatetraenotriazole in
hexamethylphosphoramide, yields stable solutions of both anion radicals, which were studied via
EPR spectroscopy. The amount of electron spin localized within the triazole ring, and how spin
is distributed within this ring, is greatly influenced by the size of the annulene ring attached.
UB3LYP/6-31++G(d,p) geometry optimizations using DFT methods were carried out for both
anion radicals, and the calculated coupling constants (and electron spin densities) are in good
agreement with the EPR spectroscopic results. Both theory and experiment show that much of
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the unpaired electron spin in the N1-phenyl benzotriazole anion radical is delocalized over the
entire π system of benzotriazole ring including the phenyl ring attached, but that a significant
percentage of total spin is found to reside within triazole ring with much of it centered on the
second nitrogen (N2). With the N1-phenyl cyclooctatetraenotriazole anion radical, the majority
of spin is localized over the π system of the COT ring, however a relatively small amount of total
spin, found within the triazole moiety, is largely concentrated on two of the nitrogens (N1 and
N3) within the ring.
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INTRODUCTION
Aromatic N1-Benzotriazole (Bt) has been extensively studied for its broad range of synthetic
applications and biological/medicinal activity.1-12 For example, in the area of pharmacology, Bt
derivatives have shown useful activity as antimicrobial, antitumor and cholesterol lowering
agents.1-5 The aromatic character of the Bt fused ring in these molecular species is key to their
stability in drugs. In addition, the hydrogen bonding capability and the dipole moment
associated with the Bt ring are important factors related to its biological utility.4 For example,
some N1-substituted benzotriazoles have shown promise as biological inhibitors of certain
dioxygenases,3,5 which are an important target for the treatment of cancer, as well as inhibition of
phosphatases and intercellular ion exchangers.1,2
The application of Bt derivatives to organic synthesis has been realized for decades, where
they have been uses as synthetic auxiliaries for the introduction of diverse functional groups into
molecules. 6-12 Katritzky research group has investigated the chemistry involving Bt derivatives
and has published much in this area.7-11 There are a number of important functional roles that
benzotriazole has in organic transformations, and two of these include its ability to function as an
electron-donor or an electron-acceptor under certain conditions.11 As an electron acceptor, Bz
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becomes a good leaving group in elimination reactions, with the formation of a stable
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–
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benzotriazolate (Bt ) anion. For example, Scheme 1 depicts such an elimination of Bt after the
one electron reduction of N1-benzotriazolyl-alkylamines with Samarium (II) iodide (SmI2).8,9,11
The resulting alkylamine
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Scheme 1. Synthesis of pyrrolidine via the one electron reduction of N1-benzotriazolyl-
alkylamine with SmI2.
radical intermediate will undergo a rapid intramolecular cyclization to generate a pyrrolidine
ring.9 The use of benzotriazole derivatives in such radical cyclization reactions has been shown
to be a viable method to generate other nitrogen containing heterocycles, which are key
ingredients in alkaloids.9,10,11
Given the myriad chemical studies performed on N1-benzotriazole derivatives, we find it
interesting that studies involving N1-substituted derivatives of other annulenotriazole systems
have largely been unexplored. The only other known system is that of the N1-phenylcyclo-
octatetraenotriazole (COTt-Ph), which was first synthesized by Krebs in 1967.13 However,
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the chemical properties of this non-aromatic (with respect to the [8]annulene ring) triazole are
largely unknown. Having a viable method for generating COTt-Ph, we have chosen to compare
the one electron reduction chemistry of this system with the fully aromatic N1-
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phenylbenzotriazole (Bt-Ph). Our interest lies in exploring how different annulene rings will
influence electron spin distribution within the triazole ring (e.g., within the three bonded
nitrogens) attached. Since these annulene rings are quantum mechanically different from one
another (e.g., [6]annulene is fully aromatic (4n+2 πe-) while [8]annulene is non-aromatic (4n πe-
), the difference in spin distributions should be profound. Upon reduction of COTt-Ph, we
anticipate that much of electron density will be drawn into the COT ring. This is a consequence
of the expected high electron affinity (EA) of the [8]annulenyl moiety in COTt-Ph.14,15
However, it is unclear to what degree the fused triazole will attenuate the spin distribution
throughout this system. On the other hand, benzene has a negative electron affinity due to loss
of aromatic character upon addition of an electron,16 therefore we anticipate the triazole moiety
will have a considerable influence on spin distribution within the anion radical of Bt-Ph.
To the best of our knowledge, no spectroscopic investigations involving the anion radical of
N1-substituted benzotriazoles have been published prior to these studies. We have successfully
generated, and detected via EPR spectroscopy, the anion radicals of both Bt-Ph and COTt-Ph,
and a complete assessment of spin density has been performed on both of these annulenotriazole
systems. These studies will help with the understanding of differences in the electronic
structures of these anion radicals as compared to those of the unreduced species, and will add to
the important understanding of the structure and reactivity of annulenotriazole compounds.
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RESULTS AND DISCUSSION
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The one electron reduction of Bt-Ph was carried out by exposing a liquid ammonia solution
to a freshly distilled sodium metal surface under low temperature (-100oC) and high vacuum
conditions. The resulting light blue solutions exhibited a strong, somewhat broad, EPR signal
(Figure 1). The solution must be kept cold or the blue color fades rapidly with loss of the
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N
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Magnetic Field (Gauss)
Figure 1. (Upper) X-band EPR spectrum of a liquid ammonia solution containing the N1-
·–
phenylbenzotriazole anion radical (Bt-Ph ) recorded at a temperature of 190 K. (Lower)
Computer generated EPR spectrum using aH’s for nine unique protons: 4.30 G, 3.80 G, 3.55 G,
1.05 G, 0.83 G, 0.63 G, 0.55 G, 0.30 G, and 0.25 G and aN’s for three nitrogen-14 atoms: 8.00 G,
1.36 G and 0.90 G. The peak-to-peak linewidth (ꢀwpp) is 0.35 G.
EPR signal, signifying loss of the anion radical. The rapid loss of anion radical upon warming or
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within a few hours of it formation may explain why others were unable to detect Bt-Ph .
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Attempts were made to reduce the observed peak-to-peak linewidth in the spectral data by
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lowering the solution concentration of Bt-Ph . These experiments did show some
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improvement in linewidth although the spectrum was still broad. These low concentration
experiments resulted in poor EPR signal strength for meaningful analysis. We conclude that the
broadening in likely due to rapid spin-spin exchange due to the planarity of the Bt-Ph system.
The computer simulation of this spectrum reveals that the odd electron exhibits coupling to nine
unique protons and three nitrogen atoms, see Figure 1. The magnitude of the observed coupling
to the three nitrogens reveals that a substantial amount of spin is localized within the triazole
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·–
ring. While these EPR results suggest the presence of Bt-Ph in solution, theoretical
calculations were needed to correctly assign the hyperfine coupling constants (hfccs) to the
respective nuclei throughout the system.
Density Functional Theory (DFT) methodologies have been shown to predict isotropic hfccs
exceptionally well for annulene anion radicals,18-20 and have also proven to adequately predict
coupling constants observed in organic radicals that contain nitrogen nuclei.21,22 The geometry
·–
for Bt-Ph was optimized using the UB3LYP/6-31++G(d,p) level of theory along with the
theoretical hfccs.23 Table 1 gives these results along with the experimentally determined hfcc
values. Although these calculations were performed for the gas-phase Bz-Ph anion radical, we
find that there is remarkably good agreement between theory and experiment especially for the
aHs and aNs associated with the benzotriazole ring. Additional calculations were performed that
included the polarizable continuum model (PCM) and mimic the solvation properties of liquid
ammonia. In these calculations, the solvent choice was ethanol since the dielectric constant of
this solvent is nearly the same as that for liquid ammonia at -74oC.23 Table 1S in Supporting
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Table 1. UB3LYP/6-31++G(d,p) Calculated and Experimental Coupling Constants (Gauss)
·–
with Mulliken Calculated Spin Densities (ρN and ρC) for the Gas-Phase Bt-Ph
Positiona
Exptb
0.90
8.00
1.36
4.30
1.05
3.80
0.83
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Calc
-0.72
7.46
2.38
-4.45
1.37
-4.46
-1.03
-
ρN and ρC
-0.0084
0.40
1(N)
2(N)
3(N)
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0.11
0.20
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0.19
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0.021
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0.024
-0.010
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0.30
0.63
-0.82
0.043
-2.26
0.27
-1.32
0.013
0.028
aPosition assignments are shown in Figure 1. The sign for each of the measured coupling
b
constants was not determined, but is assumed to be the same as the calculated values.
Information gives the calculated values for aHs and aNs. We find that these values are close to
those obtained for the gas-phase system. The one noticeable difference is with the coupling
constant determined for N1 (aN1 = -0.72 G for the calculated gas-phase anion radical while the
PCM model gives a value that is an order of magnitude smaller, aN1 = 0.073 G). Comparison
with the experimentally determined values suggest that the calculated gas-phase values are in
better agreement with the measured values.
Included in Table 1 are the calculated Mulliken spin densities throughout the π system of Bt-
·–
Ph . The predicted carbon pz spin densities (ρC) are given, since it is known that aH is
proportional to the spin density on the adjacent sp2 carbon.24 We see from both the calculated
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spin densities, and magnitude of the experimentally determined hfccs, that the majority of
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electron spin in Bt-Ph is concentrated within the fused benzotriazole ring, and that a relatively
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small amount of spin resides in the phenyl ring attached to N1. We also find a substantial
amount of electron spin residing on the 3 Ns (total ρN > 0.50) with the largest density on N2 (ρN2
= 0.40). This localization of spin on N2 is supported by the large coupling observed in the EPR
spectrum where aN2 = 8.00 G. With over half of the total spin residing in this portion of the Bt
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ring, the strong electron withdrawing nature of the fused triazole ring is realized. The ability of
the 3 Ns to pull much unpaired electron density away from an annulene ring, and into the
adjacent triazole moiety, can be further tested through exploring the one electron reduction of
COTt-Ph.
The synthesis of COTt-Ph was necessary before reduction studies could be performed, and
was carried out using the method developed by Krebs.13 The preparation involves the reaction of
monobromocyclooctatetraene (C8H7Br) with potassium tert-butoxide in the presence of phenyl
azide, reaction 1. The dehydrohalogenation of C8H7Br generates the reactive cyclooctatrienyne
([8]annulyne), which undergoes a [2+3] cycloaddition with the azide resulting in the formation
of a stable triazole ring. The formation of COTt-Ph was confirmed via mass spectrometry (ESI-
MS) and NMR spectroscopy (see Supporting Information). The 1H-NMR analysis suggests that
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the COT ring retains its tub shaped geometry with D2h symmetry upon formation of COTt-Ph.
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(The protons on the eight-membered ring appear from 6.4-5.7 ppm, which is close to the
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chemical shift for the protons on [8]annulene (
= 5.68 ppm).25)
δ
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The one electron reduction of COTt-Ph was carried out with sodium metal under vacuum
conditions at room temperature in hexamethylphosphoramide (HMPA). The resulting green
solution exhibits a strong EPR signal; however, attempts to simulate this very complex spectrum
proved to be difficult due to the many unique couplings observed from the interaction of the free
electron with many magnetically non-equivalent nuclei. (There are at least 11 unique proton and
3 unique nitrogen couplings possible; see Supporting Information for the entire EPR spectrum.26)
We expect that no hyperfine coupling from the counter ion (Na+) will be observed since ion
association is typically not detected when HMPA is the solvent.27,28 We surmised that the
smallest couplings observed come from the small amount of unpaired electron spin residing in
the π system of the phenyl ring attached to N1. Therefore, efforts were undertaken to simplify
the EPR spectrum through replacement of this phenyl ring with the perdeuteriated analog.
It was anticipated that the EPR signal from the anion radical of this perdeuteriated isotopamer
·–
(e.g, COTt-PhD5 ) would not exhibit any hyperfine coupling associated with the phenyl ring
since the ratio of gyromagnetic ratios is, γD/γH = 0.154.24 Any hyperfine coupling associated
with these deuterium atoms will be lost in the spectral linewidth and the spectrum obtained will
be less complex and easier to simulate. In addition, any electron-hydrogen coupling now
·–
observed for COTt-PhD5 must come from the 6 Hs on the COT ring.
The formation of COTt-PhD5 first required the synthesis of the perdeuterophenyl azide
(C6D5N3) precursor. This was carried out by reacting deuteroaniline (C6D5NH2) with tert-butyl
nitrite (t-BuONO) and azidotrimethylsilane (TMSN3) in acetonitrile, reaction 2.29 Upon
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purification of C6D5N3, its structure was confirmed via 13C-NMR spectroscopy (All Cs bonded to
deuterium are now split into triplets). With the C6D5N3 in hand, the synthesis of COTt-PhD5
was carried out in the same manner as already described, reaction 1. The 1H-NMR spectrum
exhibits resonances only associated with the COT ring protons (all resonances associated with
the phenyl ring hydrogens are absent) and the 13C-NMR spectrum shows coupling to the
deuterium atoms attached to the five phenyl ring carbons; see Supporting Information.
The reduction of COTt-PhD5 was performed using the same conditions as for the
perprotiated analog, and the resulting EPR spectrum was indeed considerably less complex, with
significantly less coupling observed (Figure 2). A computer simulation was generated using
hyperfine couplings associated with six protons and three nitrogens, which again must be
·–
associated with only the COTt ring in COTt-PhD5 (see Figure 2). As hoped, the coupling
between the unpaired electron and the deuterium atoms on the phenyl ring is buried in the EPR
linewidth. (A small aD = 0.06 G was used to improve line-shape.) With these electron-nuclear
·–
couplings now determined, the simulations of the complex spectrum obtained for COTt-Ph
becomes achievable, since all remaining hyperfine coupling observed must come from the
phenyl ring protons. Incorporation of these five aHs produced a simulation that is remarkably
similar to that of the recorded spectrum, Figure 3. These results reveal that the odd electron is no
·–
longer concentrated within the triazole moiety, as in the case of Bt-Ph , but resides mostly
within the [8]annulenyl ring where the three nitrogens now simply attenuate the spin distribution.
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D
D
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Magnetic Field (Gauss)
Figure 2. (Upper) X-band EPR spectrum of a HMPA solution containing the N1-phenyl(D5)-
·–
cyclooctatetraenotriazole anion radical (COTt-PhD5 ) recorded at 295 K. (Lower) Computer
generated EPR spectrum using aH’s for six protons: 5.26 G, 5.16 G, 4.12 G, 2.87 G, 1.61 G, and
0.72 G and aN’s for three nitrogen-14 atoms: 2.02 G, 0.32 G and 0.10 G and a small aD = 0.06 G
(2H). The peak-to-peak linewidth (ꢀwpp) is 0.11 G.
·–
As with the Bt-Ph system, DFT calculations were performed to assign the hyperfine
·–
couplings to the respective hydrogens and nitrogens in COTt-Ph . The geometry optimization
and the gas-phase theoretical hyperfine couplings and Mulliken spin densities (ρN and ρC) were
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Magnetic Field (Gauss)
Figure 3. (Upper) X-band EPR spectrum (downfield half) of a HMPA solution containing the
·–
N1-phenylcyclooctatetraenotriazole anion radical (COTt-Ph ) recorded at 295 K. (Lower)
Computer generated EPR spectrum using aH’s for 11 protons: 5.24 G, 5.14 G, 4.10 G, 2.84 G,
1.60 G, 0.73 G, 0.325 G, 0.315 G, 0.17 G, 0.12 G and 0.030 G and aN’s for three nitrogen-14
atoms: 2.00 G, 0.315 G and 0.095 G. The peak-to-peak linewidth (ꢀwpp) is 0.05 G.
calculated using the same DFT protocol. These calculated results along with the measured
coupling constants are shown in Table 2. Calculations on the neutral COTt-Ph predict that the
COT ring has a tub (D2h) conformation (in agreement with our experimental results), but for
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COTt-Ph the ring is planar with the expected D4h geometry. Comparison between the
·–
theoretical and experimental aHs and aNs indicate that there is good agreement especially for
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Table 2. UB3LYP/6-31++G(d,p) Calculated and Experimental Coupling Constants (Gauss)
·–
and Mulliken Calculated Spin Densities (ρN and ρC) for the Gas-Phase COTt-Ph .
Positiona
Exptb
2.00
0.095
0.315
5.24
0.73
5.15
1.60
4.10
2.84
-
Calc
1.34
0.064
0.080
-5.97
-0.15
-5.88
-0.75
-4.98
2.02
-
ρN and ρC
0.056
0.002
-0.002
0.25
N(1)
N(2)
N(3)
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-0.036
0.24
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-0.0015
0.22
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0.036
0.16
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-
0.013
-0.046
0.051
-0.006
0.044
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0.037
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-
0.17
0.12
0.325
0.030
0.315
-0.69
0.37
-1.01
0.28
-0.74
b
aPosition assignments are shown in Figure 3. The sign for each of the measured coupling
constants was not determined, but is assumed to be the same as the calculated values.
those coupling constants associated with the COT ring and with aN1 (2.00 G vs 1.34 G).
However, there is less agreement between the calculated and measured aN2 and aN3 values.
Theory predicts similar electron coupling to both these nitrogens, although with aN2 slightly
smaller than aN3, while the experimental results reveal that the couplings are quite different. (aN3
is nearly four times larger than that predicted by theory.) The DFT calculation also predict larger
aH’s for the phenyl ring protons than those found experimentally (see Table 2). Additional
·–
calculations were performed on COTt-Ph that including PCM modeling. As with the Bt-Ph
·–
system, the solvent chosen was ethanol and the results of these calculations can be found in
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Table 2S in Supporting Information. Interestingly, these calculations do show better agreement
with the experimental values for N1 and N3 (theory gives aN1 = 1.91 G and aN3 = 0.44 G and the
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experimental values are aN1 = 2.00 G and aN3 = 0.315 G). The PCM predicted value for N2 (aN2
= -0.27 G) however is in worse agreement than that for the gas-phase value (aN2 = 0.064 G)
while the experimental value for N2 is aN2 = 0.095 G. The EPR results reveal that far less
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electron spin resides within the π system of this ring.
The two annulenotriazole anion radicals we have investigated here exhibit considerable
differences in the location of the unpaired electron within the triazole ring (see Tables 1 and 2).
·–
The results indicate that the electron spin residing in the heterocyclic portion of COTt-Ph is
concentrated almost entirely on N1 and N3 (aN1 and aN3 >> aN2) whereas the largest portion of
·–
spin in Bt-Ph is clearly found on N2. These findings suggest a significant difference in the
electronic structure of the triazole moiety, depending on whether COT or benzene is present in
these annulenotriazole systems. These differences can be visualized by first comparing the
changes to the molecular orbital plots associated with the neutral 1,2,3-triazole and
annulenotriazoles (e.g., HOMOs) and then comparing these HOMO plots with the spin density
plots for the respective annulenotriazole anion radicals.
Neutral 1,2,3-triazole (Trz) is known to undergo a significant change to its HOMO π
electronic structure upon benzannulation.31 As seen in Figure 4, the B3LYP generated HOMO
plots of both Trz and 1H-benzotriazole (Bt-H) show the extent of this change and the quantum
mechanical “location” of π electrons.31,32 Notably, the charge density in Trz is concentrated on
N2 while there is a node that persists on N2 in the benzotriazole system (see Figure 4). It
becomes clear from these molecular orbital plots that the fused benzene ring has a profound
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effect on the location of the π electrons within the triazole moiety. We also see for the latter
system that much of the charge density now populates the N1 and N3 sites.31 Theory also
reveals that annulation of the triazole ring with COT (e.g., COTt-H) generates a HOMO π
electronic structure that is qualitatively similar to Trz in that much of the charge density is
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Figure 4. B3LYP HOMO π-electron charge density plots Trz (left) and Bt-H and COTt-H
·–
(center) and unpaired electron spin density plots for Bt-H and COTt-H (right).
·–
present on N2, see Figure 4.33 The similarity is somewhat expected, because the tub
conformation of the COT ring results in a molecular system that is essentially olefinic,25 and
therefore the π electrons within the ethylene bond shared by the COT and the triazole moiety can
contribute in the same way to the aromatic character of the heterocycle.34
Figure 4 also contains the B3LYP calculated spin density plots for the anion radicals of Bt-H
·–
and COTt-H. (Replacement of the phenyl ring on N1 in Bt-Ph and COTt-Ph with a
·–
·–
hydrogen atom (e.g., Bt-H and COTt-H ) shows no major change to the spin density
·–
distribution and gives a plot that still agrees qualitatively with the magnitude of the aN’s
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observed for the nitrogens within the triazole moiety, see Tables 1 and 2. The calculated
Mulliken charges have also been determined for these anion radicals and these results can be
found in Supporting Information. We find that a considerable about of negative charge does
reside within the triazole ring for both anion radical systems with the majority of charge residing
on N2 and N3 in both systems.
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The unpaired electron density plots obtained are indicative of the SOMO π electronic
structures (see Supporting Information) and reveal how different these molecular orbitals are
from the HOMO plots associated with the neutral annulenotriazole. Remarkably, we see that the
portion of the spin density plot (and therefore the SOMO) associated with the triazole moiety in
·–
Bt-H is reminiscent of the HOMO plot for COTt-H (and Trz) in that a node is no longer
·–
present on N2, whereas the electron density plot for COTt-H and the HOMO plot for Bt-H are
alike in that the electron density (spin and charge, respectively) tends to populate the N1 and N3
sites at the expense of N2 site. The similarity in these electronic structures may result from
·–
COTt-H exhibiting more aromatic character with the addition of the single electron, whereas
·–
Bt-H exhibits more anti-aromatic character with the addition of an electron. These two (Bt-
·–
·–
Ph and COTt-Ph ) represent the only N1-annulenotriazole anion radicals reported, but there
are many other annulenoazoles that await further study as anion radicals. We are currently
exploring the reduction chemistry of these interesting heterocycles.
EXPERIMENTAL SECTION
Materials. Deuteriated analine (C6D5NH2) was purchased from Cambridge Isotopes, Inc. and
used in the synthesis of deuteriated phenyl azide (C6D5N3).
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Synthesis of N1-Phenyl-[8]annulenotriazole (COTt-Ph and COTt-PhD5). The synthesis
of monobromocyclooctatetraene (BrCOT) was first carried out based on Kreb’s preparation, 13,35
and is already described elsewhere.36 Once the BrCOT was generated, the synthesis of COTt-
Ph was carried out using the procedure also described by Krebs.13 An ether solution (20 mL)
charged with 0.40 g BrCOT (2.4 mmol), 0.3 g phenyl azide (2.5 mmol) and 0.3 g potassium tert-
butoxide (2.3 mmol) was stirred for 72 hrs under ambient conditions. After this time, a dilute
aqueous solution of acetic acid was added to the orange/brown turbid solution. The ether layer
was dried with anhydrous MgSO4 and filtered. The ether was removed leaving behind the crude
solid product. Purification of COTt-Ph was performed via flash chromatography using silica gel
as the stationary phase. Chloroform was used first as the mobile phase to remove all unwanted
side products from the synthesis, followed by a 1:1 mixture of chloroform and ethyl acetate,
which was used to remove the desired product from the column. Removal of the solvent mixture
left behind a yellow solid, and NMR and ESI-MS analysis confirm the formation of COTt-Ph
(see Supporting Information for spectral data).
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Synthesis of perdeuteriated phenyl azide (C6D5N3) from deuteriated aniline C6D5NH2. The
synthesis of C6D5N3 was carried out using the procedure developed by Barral et al.29 A solution
containing 1.0 g C6D5NH2 (10 mmol) in 20 mL CH3CN was cooled to 0oC, after which was
added 1.70 g t-BuONO (16.5 mmol) followed by 1.50 g TMSN3 (13 mmol) dropwise. The
resulting solution was warmed slowly to room temperature and stirred for 1 hr. The CH3CN was
removed under vacuum and the crude product was purified via silica gel flash chromatography
with hexanes as the mobile phase. 13C-NMR analysis confirmed that C6D5N3 was generated.
This azide was then used in the synthesis of COTt-PhD5. See Supporting Information for 1H-
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NMR and for 13C-NMR spectral data. All 1H-NMR and 13C{1H}-NMR spectroscopic data were
collected using a Bruker Avance III 400 MHz NMR spectrometer.
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Reduction of N1-Phenyl Benzotriazole in Liquid Ammonia for EPR Experiments.
A Pyrex glass apparatus (see Supporting Information for schematic) was charged with 0.084
mmol of Bt-Ph in bulb A. A small amount of sodium metal was placed into bulb B, which was
then sealed at point C, and the entire apparatus was evacuated. The sodium metal was distilled
into bulb D to form a pristine metal mirror, after this, bulb B was sealed at point E.
Approximately 2.0 mL of anhydrous liquid ammonia (dried with a small piece of potassium
metal) was distilled through the vacuum manifold directly into bulb A using a liquid
nitrogen/acetone slurry. The evacuated apparatus was sealed from the vacuum line at point F.
The Bt-Ph was mildly soluble in the liquid ammonia, but upon addition of metal, it became clear
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o
that the Bt-Ph dissolved nicely into solution. All reductions were carried out at -100 C. A
·–
series of EPR tubes were harvested from the apparatus as the solution was exposed to more and
more metal. Each sample was interrogated via EPR spectroscopy until the sample with the best
signal-to-noise ratio was found for the anion radical in solution. All spectroscopic data were
collected at -100oC.
Reductions of N1-Phenyl Cyclooctatetraenotriazole (COTt-Ph and COTt-PhD5) in HMPA.
The reductions were carried out by allowing a HMPA solution of either COTt-Ph or COTt-
PhD5 to come into contact with a sodium metal mirror at room temperature in vacuo as
previously described.37
All spectra were recorded using a Bruker EMX X-band EPR spectrometer equipped with a
variable-temperature unit. Data was collected with the following EPR parameters: microwave
power = 25 dB, modulation amplitude = 0.1 G, time constant and conversion times = 655.36 ms.
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In the case of spectra collected for the Bt-Ph anion radical, once it was determined that the peak-
to-peak linewidth was greater than 0.3 G, the spectra were then re-collected with a modulation
amplitude of 0.25 G for the purpose of improving the signal strength.
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·–
·–
Computational Methodology. Both Bt-Ph and COTt-Ph geometry optimizations were
carried out via DFT calculations at the UB3LYP/6-31++G(d,p) level of theory as were the
theoretically determined coupling constants and spin densities. All calculations were
implemented in Gaussian 09, Revision D01,23 and were done for the gas-phase molecules. In the
case of the PCM modelling, the solvent used was ethanol since it has a dielectric constant similar
to that for ammonia.23 The optimized gas-phase structures were used and a single-point
UB3LYP/6-31++G(d,p) calculation was performed to determine the coupling constants and spin
densities. The calculated Mulliken spin densities were used and are the difference between the
alpha and beta spin occupancies. All optimizations did converge and stability in the calculated
structures was determined by performing a vibrational analysis on all structures; no imaginary
frequencies were found.
ACKNOWLEDGMENTS
Acknowledgment is made to the donors of the American Chemical Society Petroleum Research
Fund for support of this research (ACS-PRF 51677-UR4).
SUPPORTING INFORMATION
EPR spectrum obtained from the reduction of COTt-Ph and the 1H- and 13C-NMR spectra and
ESI-MS from the synthesis of COTt-Ph. Further information regarding the DFT calculations for
·–
Bt-Ph and COTt-Ph . This material is available free of charge via the Internet at
·–
http://pubs.acs.org.
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REFERENCES
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C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.;
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dielectric constant for ethanol at 25oC is 24.3 and for liquid ammonia at -77oC is 25, see
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25. Fray, G. I.; Saxton, R. G. The Chemistry of Cyclooctatetraene and its Derivatives;
Cambridge University Press: Cambridge, 1978.
·–
26. There are no magnetically equivalent hydrogens in Ct-Ph , because of the low
molecular symmetry.
27. Levin, G.; Jagur-Grodzinski, J.; Szwarc, M. J. Am. Chem. Soc. 1970, 92, 2268-2275.
28. Stevenson, C. D.; Echegoyen, L.; Lizardi, L. R. J. Phys. Chem. 1972, 76, 1439.
29. Barral, K.; Moorhouse, A. D.; Moses, J. E. Org. Lett. 2007, 9, 1809-1811.
30. Mono- and di-substituted COT anion radicals are known to undergo a Jahn-Teller
distortion where the ring retains a D4h symmetry with alternating long and short C-C
bonds, see references 19-20.
31. Tomas, F; Abboud, J. M.; Laynez, J.; Notario, R.; Santos, L.; Nilsson, S. O.; Catalan, J.;
Claramunt, R. M.; Elguero, J. J. Am. Chem. Soc. 1989, 111, 7348-7353.
32. The HOMO density maps shown in Figure 4 agrees with the results published in
reference 31.
33. DFT calculations were performed on the neutral COTt-H using the same B3LYP
protocol.
34. 1,2,3-Triazole heterocycles are considered aromatic, see Balaban, A. T.; Oniciu, D. C.;
Katritzky, A. R. Chem. Rev. 2004, 104, 2777-2812.
35. Krebs, A. Angew. Chem. 1965, 77, 966.
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37. Servos, M. A.; Smart, N. C.; Kassabaum, M. E.; Scholtens, C. A.; Peters, S. J. J. Org.
Chem. 2013, 78, 3908−3917.
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