Fluorine substituted non-symmetric phenazines: A new synthetic protocol from polyfluorinated azobenzenes

Article abstract of DOI:10.24820/ark.5550190.p010.940

A high ortho regioselectivity in nucleophilic aromatic substitution in low polar solvents is reported in the reaction of polyfluorinated azobenzenes with anilines. o-Anilino substituted derivatives could be easily transformed into non-symmetric phenazines

Full text of DOI:10.24820/ark.5550190.p010.940

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Arkivoc 2019, part v, 0-0
Fluorine substituted non-symmetric phenazines: a new synthetic protocol from
polyfluorinated azobenzenes
Luca Vaghi,* Mattia Coletta, Paolo Coghi, Ivan Andreosso, Luca Beverina, Riccardo Ruffo, and Antonio
Papagni
Department of Materials Science, University of Milano-Bicocca, via R. Cozzi 55, 20125 Milano, Italy
Email: luca.vaghi@unimib.it
Received 03-27-2019
Accepted 06-04-2019
Published on line 07-03-2019
Abstract
A high ortho regioselectivity in nucleophilic aromatic substitution in low polar solvents is reported in the
reaction of polyfluorinated azobenzenes with anilines. o-Anilino substituted derivatives could be easily
transformed into non-symmetric phenazines by acid catalyzed electrocyclization. Phenazines are interesting
compounds for their important properties as bioactive molecules. In addition, these fluorinated phenazines
manifest interesting spectroscopic and electronic properties that make them candidates for n-type organic
semiconductor. Furthermore, the prepared phenazines show reversible reduction with the LUMO and HOMO
energy level controlled by the electronic nature of the substituent on the non-fluorinated ring of the system.
Keywords: Polyfluorinated compounds, azobenzenes, phenazines, aromatic nucleophilic substitution,
nitrogen-containing heterocycles
DOI: https://doi.org/10.24820/ark.5550190.p010.940
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Introduction
Phenazines, known since the beginning of 19th century, represent an important class of heterocycles showing
a great potential as bioactive molecules and as organic-based semiconducting materials. As an example,
phenazines, isolated from Pseudomonas, Streptomyces and from soil or marine organisms manifest important
biological properties including antibiotic, antitumor, antimalaria and antiparasitic activities1-3 and recently, it is
also reported their biofilm eradication properties in the treatment of antibiotic resistant bacteria.4 In addition,
fluorinated phenazines have received particular attention since they present interesting HOMO and LUMO
energy levels respect to the hydrogenated counterpart5,6 and have been proposed as n-type semiconductors
potentially exploitable for developing new organic light emitting diode (OLED) and organic field effect
transistor (OFET) devices.7,8 Furthermore, fully fluorinated phenazine has been proposed as system for
photochemical oxidation of water with a potential application in light induced water splitting processes.9
Despite these peculiar properties, a limited number of phenazines are known or commercially available
and this is basically due to the lack of general protocols for their synthesis, limited library of starting materials,
harsh experimental conditions and low yields. The reported synthetic strategies can be divided meanly in two
categories: (1) strategies for symmetrical substituted phenazines and (2) strategies for unsymmetrical
substituted phenazine (Scheme 1). Within the protocols of the first type deserve to be mentioned: the
dimerization of nitroso-arenes by Bamberger-Ham reaction,10 the dimerization of anilines via Pd-Ag C-H
activation11 and the homocoupling of 2-iodoanilines catalysed by sulfonato-Cu(salen) complex.12 Within those
of second type, the reported synthetic approaches are: the historical Wohl-Aue by condensation of nitro
aromatics and anilines,13,14 the reductive cyclization of 2-nitrodiphenylamines by sodium borohydride in
ethanolic sodium ethoxide solution,15 the combination of 2-bromo-anilines with 2-bromo-nitrobenzenes via
sequential Buchwald-reduction-Buchwald,16 the reaction of 2-bromo-nitrobenzenes with anilines via
Buchwald-Hartwig/reductive cyclization,17 the condensation between
a
quinone and ortho-
phenylenediamine2,18 and the Rh-catalysed ortho-amination of azobenzenes with aryl azides.19
Scheme 1. Synthetic strategies to symmetric and non-symmetric substituted phenazines.
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In the Rh-catalyzed phenazine synthesis from azobenzenes and azides developed by Ellman,19 an ortho-
anilino substituted azobenzene is proposed as key intermediate in the catalytic cycle. To obtain reasonable
chemical yield in the corresponding phenazines the use glacial acetic acid as solvent is a crucial factor. Indeed,
acetic acid facilitates the [3,3]-sigmatropic rearrangement by favoring the ortho-quinoid hydrazinic form (in
prototropic equilibrium with the ortho-anilino substituted azobenzene) as well as the ejection and
sequestration of the aniline during the aromatization process. The latter aspect is important since the aniline
can inhibit the catalyst. The use of non-symmetric azo-derivatives is suggested to drive the ortho-metalation
only on the less hindered aromatic ring bound to the azo function.
Symmetric polyfluorophenazines are obtained as by-products in the chemical20 and electrochemical21
oxidation of polyfluorinated anilines to the corresponding azobenzene, while very limited reports describe
protocols for the preparation of non-symmetric polyfluorinated phenazines. Indeed, 2,3-difluorophenazine
derivatives are prepared by reacting the corresponding 3,4-difluoro-phenylendiamine with 2,5-dihydroxy-1,4-
benzoquinone,22 1,2,3,4-tetrafluorophenazines are reported to be isolated as by-product from the thermal
decomposition of aryl azides in the presence of pentafluoroaniline23 and by heating the respective quinoxaline
derivative of 7,8-dioxo-3-methyl-3-phenyl-1,4,5,6-tetrafluorobicyclo[2.2.2]oct-5-ene (Scheme 2).24
Scheme 2. Synthetic pathways to polyfluorophenazines.
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Results and Discussion
Herein we report a new protocol for the synthesis of 1,2,3,4-tetrafluoro and 1,3,4-triflurophenazines starting
from polyfluoroazobenzenes and anilines. The synthetic strategy is based on our methodology developed in
the past for synthesizing 1,2,3,4-tetrafluoroacridines.25 In this methodology, the decafluorobenzophenone
undergoes ortho nucleophilic aromatic substitution (SNAr) by reacting with anilines, with the ortho/para
regiochemistry controlled by the solvent basicity (expressed by its Donor Number, DN26). Indeed, thanks to the
hydrogen bonding between the N-H of the aniline and the lone pair of the carbonyl group, a high ortho/para
regioselectivity is observed solvents of low donicity. The resulting intermediate is subsequently treated under
strong acidic conditions to afford the target acridines via 6π electrocyclization. Acridines and phenazines are
structurally related, thus we supposed that a similar protocol could be apply for the preparation of fluorinated
phenazines (Scheme 3). We identify polyfluoroazobenzenes as ideal starting material for this strategy,
obviously high level of fluorination is required to achieve the SNAr and at least one fluorine must be present in
the ortho position. In similar experimental conditions (low DN solvent), a hydrogen bonding is also expected
between the lone pair of the nitrogen of the azo group and the N-H hydrogen of aromatic amines.
Scheme 3. Phenazines versus acridines synthetic strategies.
Compared to polyfluorobenzophenone it is expected that polyfluoroazobenzene could exploit a similar
reactivity towards nucleophiles since, thanks to the conjugation capability of azo group, both the two
polyfluoroaromatic rings should show a more electron-poor character. This is also supported by the known
reactivity of decafluoroazobenzene towards oxygen- and nitrogen-nucleophiles.27 In our strategy the
preparation ortho-anilino substituted intermediates could be carried out by fluorine SNAr with anilines on the
suitable polyfluorinated azobenzene, avoiding to the use of expensive catalysts and aromatic azides required
in the Ellman protocol.19 This latter, as additional drawback, requires azobenzenes bearing at least a hydrogen
ortho to the azo function.
The synthesis of symmetrical azobenzenes has been reported via the oxidation of primary aromatic
amines with different oxidants,28 but in the case of highly fluorinated anilines the oxidation of amino group
20
requires an excess of strong oxidants such Pb(OAc)4 to afford reproducible and satisfactory results when
gram quantity of products are needed. Recently, we develop a mechanochemical methodology to afford
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polyfluoro symmetric azobenzenes from the corresponding anilines.29 When applicable, the
mechano-chemistry represents the optimal experimental condition, since solvents are avoided and the
reagents are at their maximum concentration, thereby resulting in shorter reaction times and simple work up
procedures. By this technique, starting from pentafluoroaniline 1a and tetrafluoroaniline 1b, gram scale of
decafluoroazobenzene 2a and octafluoroazobenzene 2b, respectively, were easily prepared (Scheme 4).
Scheme 4. Mechanochemical oxidation of fluorinated anilines.
2a and 2b were then subjected to the SNAr reaction with three aromatic amines with decreasing electron
richness, i.e. 4-methoxyaniline 3a, aniline 3b and 4-bromoaniline 3c (Scheme 5). In the case of 2a, to drive the
reaction in favor to the ortho-substitution products, it is mandatory to operate in low polar solvents (i.e. DN =
0), otherwise the para position will be the most reactive. Conversely, the reaction with 2b can be carried out in
solvent with higher DN since para position is no more reactive by the presence of hydrogens.
Scheme 5. Regioselective ortho-SNAr of 2a and 2b with anilines.
Thus, 1,2-dichloroethane (DCE) and decalin (DN=0) were chosen as solvent to test the SNAr reaction on 2a,
2 equiv of aniline 3a were used, 1 equiv serves as HF scavenger (Table 1, Entries 1 and 2), a sacrificial base
such as triethylamine could interfere in the hydrogen bonding system. Under these conditions, we were able
to isolate 43% of 4aa in refluxing DCE and 15% in decalin at 170 °C. The higher boiling point of decalin allowed
the reaction to be complete in 2 h respect to 6 h for DCE. Polysubstitution by-products were heavily present,
especially when the reaction was performed in decalin, indicating that at the temperature of 170 °C in such
low polar environment the HF formed moves away on gaseous form. Then, focusing at decalin, we lowered
the 2a:3a ratio to 1:1.5 (Table 1, Entry 3), increased yield (35%) and less polysubstitution by-products were
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observed. We finally used 1:1.1 2a:3a ratio with a slow flow of nitrogen and an aqueous K2CO3 trap at the end
to capture the HF (Table 1, Entry 4), with this setup a satisfactory 57% isolated yield was achieved. The
procedure was repeated with 3b and 3c (Table 1, Entries 5 and 6) affording 54% of 4ab and 61% of 4ac,
respectively. Finally, we performed the ortho-substitution on 2b. The absence of selectivity issues allowed the
use of standard conditions for SNAr, namely dimethyl sulfoxide (DMSO) (DN=29.8) as solvent, Et3N as sacrificial
base at room temperature, affording the desired 4ba, 4bb and 4bc in 68%, 55% and 63% yield, respectively
(Table 1, Entries 7, 8 and 9).
Table 1. Outputs of the nucleophilic aromatic substitution of 2a and 2b with anilines
Entry
1
2
Substrate
Aniline
3a
Ratio 2:3
1:2
Solvent
DCE
T °C
84
Time h
Yield %
43
2a
2a
2a
2a
2a
2a
2b
2b
2b
6
2
2
2
4
4
3
5
5
3a
3a
3a
3b
3c
3a
3b
3c
1:2
decalin
decalin
decalin
decalin
decalin
DMSO
DMSO
DMSO
170
170
170
170
170
20
15
35
57
54
61
68
55
63
3
1:1.5
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
4a
5a
6a
7b
8b
9b
20
20
a N2 flow and aqueous K2CO3 trap. b Et3N (1 equiv).
The ortho-substituted products 4 have been transformed into the corresponding phenazines 5 by
acid-catalyzed electrocyclic reaction (Scheme 6). As previously described for the synthesis of acridines,25,30
trifluoroacetic acid (TFA) has been selected to be used both as catalyst and as reaction solvent. The
electrocyclic reactions occur in very mild condition by simple stirring at room temperature overnight affording
the desired products in satisfactory yields ranging from 50% to 74%.
Scheme 6. Acid catalyzed electrocyclization-aromatization of derivatives 4.
In analogy to ortho-anilino substituted nonaflurorobenzophenones,25 a similar mechanism for the
transformation of derivatives 4 into 5 can be proposed (Scheme 7). Thus, it is reasonable that the cyclization
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proceeds via 6π-electrocyclization, involving the hydrazone form of the azo group favored by a prototropic
process in the strong acid media, to give the target phenazine with ejection of a molecule of fluoroaniline. A
similar mechanism has been proposed by Ellman,19 where the cyclization is driven by the acidic conditions of
the reaction media.
Scheme 7. Mechanism of the acid-catalyzed electrocyclization-aromatization.
The tetrafluoro phenazines 5aa, 5ab and 5ac were spectroscopically and electrochemically characterized
(Table 2). Absorption maxima are at 360 nm for 5aa, 370 nm for 5ab and 374 for 5ac. The emission is in the
visible region with an energy modulated by the electron-donating nature of the substituent Y, ranging from
435 nm to 484 nm with Stoke shifts up to 120 nm. The emission is also observed in the solid state. The
replacement of hydrogens with electronegative fluorine atoms strongly influences the electrochemical
properties and HOMO-LUMO energy level. Indeed, a significant lowering of both HOMO and LUMO energies
has been observed in phenazines 5 with differences greater than 0.3 eV when compared to the
non-fluorinated phenazine (LUMO = -3.4 eV).31 The fluorination also makes phenazines particularly stable to
oxidation with HOMO orbital energy almost in all cases below -6 eV. As expected, the presence of electron-
donating groups has a greater effect on the energy of HOMO orbitals than on the corresponding LUMOs. The
reduction potential is in line with the effect exerted by substituents. Thanks to their HOMO and LUMO energy
levels and their low band gaps (<3 eV) Phenazines 5aa, 5ab and 5ac possess electronic properties that make
them very attractive for applications in photovoltaic devices. In addition, the energy level of the HOMO is
lower than the air oxidation threshold (-5.2 eV), and this is important since a significant stability against
oxidative degradation is expected.
Table 2. Spectroscopic and electrochemical characteristics of phenazines 5aa, 5ab and 5ac
Phenazine A max (nm) E max (nm) E0 red (V) vs.
Optical band
gap (eV)
2.58
2.76
2.58
HOMO (eV)a LUMO (eV)b
Fc+/Fc
5aa
5ab
5ac
360, 404
370, 405
374, 410
480
435
484
-1.44
-1.34
-1.19
-6.33
-6.61
-6.58
-3.75
-3.85
-4.00
aCalculated using the LUMO level and the optical band gap
ban electron vacuum level of 5.19 V was considered for the Fc+/Fc couple.
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Conclusions
The synthesis of non-symmetrical fluorinated phenazines has been achieved via nucleophilic aromatic
substitution and subsequent acid-promoted electrocyclization and aromatization. The use of decalin as solvent
allowed the ortho regioselectivity in the SNAr of anilines on decafluoroazobenzene, affording the desired
ortho-substituted azobenzene in satisfactory yields, while dimethyl sulfoxide at room temperature afforded
the ortho-substitution products of octafluoroazobenzene in good yields and short reaction times. The ortho-
anilino substituted azobenzenes prepared were treated with TFA to promote an acid electrocyclic
rearrangement to the target non-symmetric fluorinated phenazines in good chemical yields. The tetrafluoro
phenazines were analyzed by means of optical and electrochemical properties indicating that the fluorination
of the phenazine nucleus strongly influences the electrooptical properties of these heterocycles and makes
them interesting systems for photovoltaic and electroluminescent devices. Furthermore, the phenazine LUMO
energy levels are higher than that of PCBM (-3.75 eV) thus, they should be interesting acceptor units in bulk
heterojunction (BHJ) photovoltaic cells.
Experimental Section
General. All materials were used as purchased (from Fluorochem Ltd, Tokyo Chemical Industry Co. or Sigma-
Aldrich Chemical Co.). Chromatographic purifications were performed using Merck 9385 silica gel, pore size 60
Å (230–400 mesh). Melting points were measured with a Stanford Research Systems Optimelt apparatus. IR
spectra were recorded with a Perkin Elmer Spectrum 100 FT-IR spectrometer equipped with a universal ATR
1
13
19
sampling accessory. H, C and F NMR were recorded with a Bruker AVANCE III HD 400 MHz spectrometer
(1H: 400 MHz, 13C: 101 MHz, 19F: 376 MHz) Chemical shifts (δ) are expressed in parts per million (ppm), and
coupling constants are given in Hz. Splitting patterns are indicated as follows: s = singlet, d = doublet, t =
triplet, q = quartet, m = multiplet, dd = doublet of doublets. Mass analyses were performed using a VG 7070
EQ-HF instrument. Elemental analyses were obtained with an Elementar vario MICRO cube instrument.
General procedure for the synthesis of compounds 4aa, 4ab and 4ac. A mixture of decafluoroazobenzene
(2a) (1 mmol), aniline (3a-c) (1.1 mmol) and decalin (10 mL) was heated at 170 °C with a N2 flux ended in an
aqueous K2CO3 trap for the time stated in the text. The mixture was then put over a chromatographic column
(SiO2), elution first with n-hexane then with n-hexane/CH2Cl2 8:2 afforded the desired compound in a pure
form.
(4-methoxyphenyl){2,3,4,5-tetrafluoro-6-[(pentafluorophenyl)diazenyl]phenyl}amine (4aa). Red-violet solid;
mp 126 °C. 1H NMR (400 MHz, CDCl3), δ (ppm): 10.75 (s, 1H), 7.02 (dd, J 8.9, 2.5 Hz, 2H), 6.87 (d, J 8.9 Hz, 2H),
19
3.81 (s, 3H). F NMR (376 MHz, CDCl3), δ (ppm): -144.27 – -144.39 (m, 1F), -146.71 – -144.80 (m, 1F), -148.42
– -148.49 (m, 1F), -149.91 – -149.98 (m, 2F), -151.61 – -151.70 (m, 1F), -161.82 – -161.99 (m, 2F), -171.64 – -
171.70 (m, 1F). IR (ATR): 3171, 2922, 2852, 1647, 1594, 1491, 1432, 1406, 1322, 1267, 1160, 1129, 1079, 1052,
1010, 992, 967, 858, 822, 794, 761, 742, 707, 695 cm-1. Anal. Calcd. for C19H8F9N3O: C, 49.05; H, 1.73; N, 9.03.
Found: C, 49.15; H, 1.70; N, 9.11.
1
2,3,4,5-tetrafluoro-6-[(pentafluorophenyl)diazenyl]-N-phenylaniline (4ab). Red solid; mp 140 °C. H NMR
(400 MHz, CDCl3), δ (ppm): 10.52 (s, 1H), 7.35 – 7.31 (m, 2H), 7.14 (t, J 7.8 Hz, 1H), 7.03 (dd, J 7.7, 3.1 Hz, 2H).
19F NMR (376 MHz, CDCl3), δ (ppm): -143.62 - -143.71 (m, 1F), -145.26 – -145.34 (m, 1F), -145.95 – -147.10 (m,
1F), -149.15 – -149.22 (m, 2F), -150.21 – -151.36 (m, 1F), -160.54 – -162.26 (m, 2F), -169.11 – -170.14 (m, 1F).
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IR (ATR): 3203, 2922, 2851, 1646, 1593, 1503, 1432, 1405, 1322, 1266, 1162, 1128, 1078, 991, 967, 761, 694
cm-1. Anal. Calcd. for C18H6F9N3: C, 49.67; H, 1.39; N, 9.65. Found: C, 49.44; H, 1.41; N, 9.85.
N-(4-Bromophenyl)-2,3,4,5-tetrafluoro-6-[(pentafluorophenyl)diazenyl]aniline (4ac). Red solid; mp 125 °C. 1H
19
NMR (400 MHz, CDCl3), δ (ppm): 10.37 (s, 1H), 7.47 – 7.40 (m, 2H), 6.88 (dd, J 8.7, 3.3 Hz, 2H). F NMR (376
MHz, CDCl3), δ (ppm): -143.09 – -143.21 (m, 1F), -145.17 (ddd, J 19.5, 8.4, 3.9 Hz, 1F), -146.00 (td, J 20.7, 6.6
Hz, 1F), -148.99 – -149.14 (m, 1F), -150.08 (t, J 20.9 Hz, 1F), -161.09 – -161.27 (m, 1F), -168.48 – -168.70 (m,
1F). IR (ATR): 3163, 2958, 2850, 1647, 1588, 1500, 1427, 1404, 1322, 1264, 1242, 1163, 1141, 1072, 1050,
1007, 989, 971, 863, 807 cm-1. Anal. Calcd. for C19H5BrF9N3: C, 42.05; H, 0.98; N, 8.17. Found: C, 42.35; H, 1.03;
N, 8.19.
General procedure for the synthesis of compounds 4ba,4bb and 4bc. Aniline (3a-c) (1.1 mmol) was added to
a solution of octafluoroazobenzene (2b) (1 mmol) and Et3N (1 mmol) in dry DMSO under N2 atmosphere. The
mixture was stirred at r.t. for the time stated in the text. An aqueous solution of KHSO4 was then added, the
aqueous phase was extracted with Et2O, the organic layer was dried with Na2SO4 and the solvent removed
under reduced pressure. Chromatography (SiO2, n-hexane/CH2Cl2 8:2) afforded the desired compound in a
pure form.
(4-Methoxyphenyl){3,4,6-trifluoro-2-[(2,3,5,6-tetrafluorophenyl)diazenyl]phenyl}amine (4ba). Violet solid;
1
mp 137 °C. H NMR (400 MHz, CDCl3), δ (ppm): 10.52 (s, 1H), 7.08 – 7.17 (m, 2H), 6.97 (dd, J 8.9, 3.0 Hz, 2H),
6.85 (dd, J 8.9, 2.8 Hz, 2H), 3.80 (s, 3H). 19F NMR (376 MHz, CDCl3), δ (ppm): -135.49 – -136.86 (m, 3F), -142.33
– -144.52 (m, 2F), -150.15 – -152.31 (m, 2F). IR (ATR): 3052, 2961, 2923, 2848, 1738, 1495, 1379, 1256, 1227,
1052, 1025, 956, 868, 795, 699 cm-1. Anal. Calcd. for C19H10F7N3O: C, 53.16; H, 2.35; N, 9.79. Found: C, 53.06;
H, 2.45; N, 9.69.
1
3,4,6-Trifluoro-N-phenyl-2-[(2,3,5,6-tetrafluorophenyl)diazenyl]aniline (4bb). Violet solid; mp 106 °C. H
NMR (400 MHz, CDCl3), δ (ppm): 10.18 (s, 1H), 7.29 (t, J 8.9Hz, 2H), 7.12-7.20 (m, 2H), 7.07 (t, J 8.9Hz, 1H), 6.98
(dd, J 8.9, 3.2 Hz, 2H). 19F NMR (376 MHz, CDCl3), δ (ppm): -137.93 – -138.46 (m, 3F), -145.49 – -146.81 (m, 2F),
-149.52 – -150.10 (m, 2F). IR (ATR): 3065, 2922, 1591, 1498, 1427, 1379, 1278, 1258, 1176, 1050, 953, 854,
799, 745, 693 cm-1. Anal. Calcd. for C18H8F7N3: C, 54.15; H, 2.02; N,10.52. Found: C, 54.25; H, 2.15; N, 10.44.
N-(4-Bromophenyl)-3,4,6-trifluoro-2-[(2,3,5,6-tetrafluorophenyl)diazenyl]aniline (4bc). Violet solid; mp 91
°C. 1H NMR (400 MHz, CDCl3), δ (ppm): 10.03 (s, 1H), 7.42 – 7.36 (m, 1H), 7.29 – 7.12 (m, 4H), 6.86 – 6.80 (m,
1H). 19F NMR (376 MHz, CDCl3), δ (ppm); -137.96 – -138.16 (m, 3F), -149.53 – -150.03 (m, 4F). IR (ATR): 3163,
2958-2850, 1647, 1588, 1500, 1427, 1404, 1322, 1264, 1242, 1163, 1141, 1072, 1050, 1007, 989, 971, 863, 807
cm-1. Anal. Calcd. for C18H7BrF7N3: C, 45.21; H, 1.48; N, 8.79. Found: C, 45.35; H, 1.33; N, 8.77.
General procedure for the synthesis of phenazines 5. A solution of compound 4 (0.5 mmol) in TFA (5mL) was
stirred at r.t. for 12h, under N2 atmosphere. The solution was dropped into a saturated aqueous NaHCO3
solution, the resulting aqueous phase was extracted with CH2Cl2, the organic layer was dried with Na2SO4 and
the solvent removed under reduced pressure. Chromatography (SiO2, n-hexane/CH2Cl2 1:1) afforded the
desired phenazine in a pure form.
1
1,2,3,4-Tetrafluoro-7-methoxyphenazine (5aa). Yellow solid; mp 180 °C. H NMR (400 MHz, CDCl3), δ: 8.18 (d,
J 9.5 Hz, 1H), 7.61 (dd, J 9.5, 2.7 Hz, 1H), 7.49 (d, J 2.7 Hz, 1H), 4.05 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 162.8
(s), 145.1 (s), 142.6 – 140.7 (m), 139.9 – 138.5 (m), 131.1 – 130.8 (m), 130.7 (s), 129.3 – 129.0 (m), 128.5 (s),
104.5 (s), 56.2 (s). 19F NMR (376 MHz, CDCl3) δ -151.61 – -151.72 (m, 1F), -152.21 – -152.32 (m, 1F), -153.28 – -
153.40 (m, 1F), -154.00 – -154.13 (m, 1F). IR (ATR): 2962, 1675, 1619, 1531, 1486, 1455, 1413, 1374, 1336,
1249, 1224, 1189, 1128, 1017, 831 cm-1. Anal. Calcd. for C13H6F4N2O: C, 55.33; H, 2.14; N, 9.93. Found: C,
55.45; H, 2.23; N, 9.91.
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1,2,3,4-Tetrafluorophenazine (5ab).24 Yellow solid; mp 232 °C. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.38 – 8.32
(m, 2H), 8.00 – 7.93 (m, 2H). 13C NMR (101 MHz, CDCl3), δ: 143.2 (s), 142.5 – 142.0 (m), 139.8 – 139.4 (m),
132.4 (s), 131.2 – 131.0 (m), 129.7 (s). 19F NMR (376 MHz, CDCl3), δ (ppm): -151.01 – -151.08 (m, 2F), -151.78
– -151.84 (m, 2F). IR (ATR): 3096, 3005, 1679, 1594, 1533, 1477, 1389, 1330, 1204, 1140, 1020, 770 cm-1. Anal.
Calcd. for C12H4F4N2: C, 57.16; H, 1.60; N, 11.11. Found: C, 57.15; H, 1.81; N, 11.33.
1
7-Bromo-1,2,3,4-tetrafluorophenazine (5ac). Yellow solid, mp 191 °C. H NMR (400 MHz, CDCl3), δ: 8.54 (d, J
2.1 Hz, 1H), 8.20 (d, J 9.3 Hz, 1H), 8.00 (dd, J 9.3, 2.1 Hz, 1H). 13C NMR (101 MHz, CDCl3), δ 143.3 (s), 142.9 –
142.0 (m), 141.8 (s), 140.4 – 139.4 (m), 136.2 (s), 131.7 (s), 131.5 – 131.3 (m), 131.2 – 130.9 (m), 130.8 (s),
127.4 (s). 19F NMR (376 MHz, CDCl3), δ: -149.51 – -149.66 (m, 1F), -150.05 – -150.18 (m, 1F), -151.27 – -151.38
(m, 2F). IR (ATR): 3067, 1676, 1596, 1532, 1480, 1443, 1368, 1330, 1016, 933, 829 cm-1. Anal. Calcd. for
C12H3BrF4N2: C, 43.54; H, 0.91; N, 8.46. Found: C, 43.48; H, 0.88; N, 8.45.
1
1,2,4-Trifluoro-7-methoxyphenazine (5ba). Yellow solid; mp 229 °C. H NMR (400 MHz, CDCl3), δ (ppm): 8.17
(d, J 9.5 Hz, 1H), 7.57 (dd, J 9.5, 2.7 Hz, 1H), 7.50 (d, J 2.7 Hz ,1H), 7.36-7.41 (m, 1H), 4.05 (s, 3H). 13C NMR (101
MHz, CDCl3) δ (ppm): 163.1 (s), 153.3 (ddd, J 262.4, 12.0, 4.5 Hz), 147.9 (dt, J 254.0, 12.3 Hz), 145.6 (s), 140.1
(t, J 2.4 Hz), 139.8 (ddd, J 256.8, 12.9, 6.2 Hz), 134.7 (dd, J 10.5, 6.2 Hz), 131.1 (s), 129.9 (d, J 14.3 Hz), 128.0 (s),
19
104.4 (s), 104.6 – 103.9 (m), 56.2 (s). F NMR (376 MHz, CDCl3) δ (ppm): -124.57 – -124.70 (m, 1F), -131.29
(dd,.J.16.7, 2.4 Hz, 1F), -155.65 – -155.82 (m, 1F). IR (ATR): 3040, 2976, 1653, 1620, 1500, 1457, 1422, 1369,
1309, 1251, 1266, 1171, 1127, 1100, 1014, 993, 951, 867, 847, 756, 725 cm-1. Anal. Calcd. for C13H7F3N2O: C,
59.10; H, 2.67; N, 10.60. Found: C, 59.28; H, 2.77; N, 10.48.
1
1,2,4-Trifluorophenazine (5bb). Yellow solid; mp 231 °C. H NMR (400 MHz, CDCl3), δ (ppm): 8.41 – 8.29 (m,
13
2H), 8.01 – 7.88 (m, 2H), 7.48 (ddd, J 10.2, 9.5, 6.3 Hz, 1H). C NMR (101 MHz, CDCl3) δ (ppm): 154.8 – 153.8
(m), 152.1 – 151.4 (m), 147.6 (dt, J 254.9, 12.1 Hz), 143.2 (dd, J 105.1, 2.4 Hz), 141.5 – 141.0 (m), 139.0 – 138.4
19
(m), 132.7 (s), 131.8 (s), 131.7 (s), 130.1 (s), 129.6 (s), 105.9 (dd, J 27.3, 24.3 Hz). F NMR (376 MHz, CDCl3), δ
(ppm): -123.95 (dd, J 20.6, 2.0 Hz, 1F), -130.88 (d, J 14.4 Hz, 1F), -154.42 (dd, J 20.6, 16.4 Hz, 1F). IR (ATR):
3041, 2922, 2853, 1658, 1614, 1529, 1490, 1467, 1423, 1403, 1385, 1361, 1328, 1318, 1289, 1248, 1218, 1187,
1138, 1100, 989, 972, 908, 864, 834, 784, 764, 730, 700, 656, 643, 605, 596, 582 cm-1. Anal. Calcd. for
C12H5F3N2: C, 61.55; H, 2.15; N, 11.96. Found: C, 61.50; H, 2.16; N, 11.99.
7-Bromo-1,2,4-trifluorophenazine (5bc). Yellow solid; mp 182 °C. 1H NMR (400 MHz, CDCl3), δ (ppm): 8.52 (d, J
13
2.0 Hz, 1H), 8.18 (d, J 9.3 Hz, 1H), 7.96 (dd, J 9.3, 2.1 Hz, 1H), 7.49 (ddd, J 10.1, 9.5, 6.3 Hz, 1H). C NMR (101
MHz, CDCl3), δ (ppm): 153.1 (ddd, J 264.4, 11.8, 4.7 Hz), 148.1 (dt, J 256.2, 12.3 Hz), 143.7 (d, J 2.1 Hz), 141.2
(s), 140.0 (ddd, J 19.5, 15.2, 6.5 Hz), 135.7 (s), 135.3 – 135.0 (m), 131.8 – 131.6 (m), 131.6 (d, J 6.1 Hz), 131.1
(s), 127.7 (s), 106.3 (dd, J 27.4, 24.2 Hz). 19F NMR (376 MHz, CDCl3), δ (ppm): -123.49 (d, J 20.8 Hz, 1F), -129.38
(d, J 17.5 Hz, 1F), -153.98 (dd, J 20.6, 15.9 Hz, 1F). IR (ATR): 3067, 3034, 2922, 2852, 1745, 1652, 1599, 1530,
1484, 1452, 1419, 1364, 1316, 1277, 1240, 1207, 1173, 1140, 1100, 1049, 1027, 993, 968, 922, 870, 827, 805,
796, 740, 710, 645, 628, 596 cm-1. Anal. Calcd. for C12H4BrF3N2: C, 46.04; H, 1.29; N 8.95. Found: C, 46.00; H,
1.25; N, 8.91.
Electrochemistry
Solutions of 5aa, 5ab, and 5ac were prepared dissolving the molecules in 0.1 M Tetrabutylammonium
perchlorate in 2:1 vol ratio CH3CN:CH2Cl2. The concentration of electroactive species was around 5x10-4 M.
Cyclic Voltammetries (CV) and Differential Pulse Voltammetry (DPV) were carried out at scan rate of 50 mV s-1
and 20 mV s-1, respectively, using an Eg&G PARSTAT 2273 potentiostat/galvanostat in a two compartment,
three electrode electrochemical cell in a glove box filled with Ar ([O2] and [H2O] ≤ 1 ppm). The working,
counter, and the pseudo-reference electrodes were a glassy carbon pin, a Pt flag and an Ag/AgCl wire,
respectively. The working electrodes discs were well polished with alumina 0.1 µm suspension, sonicated for
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15 min in deionized water, and then washed with 2-propanol. The Ag/AgCl pseudo-reference electrode was
calibrated, by adding ferrocene (5x10-5 M) to the test solution before and after each measurement.
Acknowledgements
We thank Dr. Giorgio Patriarca for the NMR characterization, Dr. Matteo Tironi for the preparation of one
sample and Prof. Giovanni Palmisano (Dipartimento di Scienza e Alta Tecnologia, Università degli Studi
dell’Insubria) for his fine suggestions.
Supplementary Material
1
13
19
CV traces and DPVs of the derivates 5aa, 5ab, and 5ab used to calculate the values of table 2; H, C and F
NMR spectra of phenazines 5. This material can be found using the link "Supplementary Material" in the
journal issue contents page.
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