Electronically Stabilized Nonplanar Phenalenyl Radical and Its Planar Isomer

Article abstract of DOI:10.1021/jacs.5b07959

Stable phenalenyl radicals have great potential as the basis for new materials for applications in the field of molecular electronics. In particular, electronically stabilized phenalenyl species that do not require steric shielding are molecules of fundamental interest, but are notoriously difficult to synthesize. Herein, the synthesis and characterization of two phenalenyl-type cations is reported: planar benzo[i]naphtho[2,1,8-mna]xanthenium (8⁺) and helical benzo[a]naphtho[8,1,2-jkl]xanthenium (9⁺), which can be reduced to the corresponding radicals. Radical 9 represents the first stable, helical phenalenyl radical which does not rely on bulky substituents to ensure its stability. Both cations are water-soluble, and the radicals are stable for weeks at room temperature under air. These compounds were characterized crystallographically, and also by NMR, EPR, electrochemistry, and electronic spectra. The synthesis of the previously reported compound benzo[5,6]naphthaceno[1,12,11,10-jklmna]xanthylium (5⁺), the largest oxygen-containing polycyclic hydrocarbon, was undertaken for comparison with 8⁺ and 9⁺, allowing us to report its crystal structure here for the first time. The different properties of these compounds and their radicals are explained by considering their differing aromaticities using in-depth computational methods.

Full text of DOI:10.1021/jacs.5b07959

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Article
An Electronically-Stabilized Non-Planar
Phenalenyl Radical and Its Planar Isomer
Ommid Anamimoghadam, Mark D Symes, De-Liang Long, Stephen Sproules, Leroy Cronin, and Goetz Bucher
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b07959 • Publication Date (Web): 02 Nov 2015
Downloaded from http://pubs.acs.org on November 3, 2015
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An Electronically-Stabilized Non-Planar Phenalenyl Radical and
Its Planar Isomer
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Ommid Anamimoghadam, Mark D. Symes, DeꢀLiang Long, Stephen Sproules, Leroy Cronin, Götz
Bucher*
WestChem, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, United Kingdom.
ABSTRACT: Stable phenalenyl radicals have great potential as the basis for new materials for applications in the field of molecuꢀ
lar electronics. In particular, electronically stabilized phenalenyl species that do not require steric shielding are molecules of funꢀ
damental interest, but are notoriously difficult to synthesize. Herein, the synthesis and characterization of two phenalenyl type
cations is reported: planar benzo[i]naphtho[2,1,8ꢀmna]xanthenium (8+) and helical benzo[a]naphthaꢀ[8,1,2ꢀjkl]xanthenium (9+)
which can be reduced to the corresponding radicals. Radical 9 represents the first stable, helical phenalenyl radical which does not
rely on bulky substituents to ensure its stability. Both cations are waterꢀsoluble and the radicals are stable for weeks at room temꢀ
perature under air. These compounds were characterized crystallographically, and also by NMR, EPR, electrochemistry and elecꢀ
tronic spectra. The synthesis of the previously reported compound benzo[5,6]naphthaceno[1,12,11,10ꢀjklmna]xanthylium (5+), the
largest oxygenꢀcontaining polycyclic hydrocarbon, was undertaken for comparison with 8+ and 9+, allowing us to report its crystal
structure here for the first time. The different properties of these compounds and their radicals are explained by considering their
differing aromaticities using inꢀdepth computational methods.
paired electron is not prone to distribute across the phenyl
INTRODUCTION
rings. As a result, the oneꢀelectron reduction of 1+ does not
The quest to identify stable πꢀconjugated cations and radiꢀ
cals that do not require steric shielding has fascinated chemꢀ
ists for decades.1ꢀ13 These compounds are of great fundamenꢀ
tal interest and yet they also have a range of potential appliꢀ
cations in photochemistry,14,15 synthesis,16,17 materials sciꢀ
ence,18ꢀ21 spintronics22ꢀ24 and biology.25,26 In particular, openꢀ
shell polycyclic aromatic hydrocarbons have been shown to
be promising molecular conductors and spin carriers in the
emerging field of organic spin chemistry.27
result in stable openꢀshell species but in shortꢀlived radicals
that undergo rapid sigmaꢀdimerization between the central
carbon atoms.31,32 The first helical structural analog of 1+ that
can form a stable radical has recently been reported (comꢀ
pound 2), wherein the radical is protected by stericallyꢀ
hindering groups.33,34
On account of the reactivity of openꢀshell species, however,
it is often necessary to strike a compromise between stability
and functionality. For example, in spintronics applications
there must be excellent communication between spins, which
means that relatively exposed radical centers are desirable;2
however, this also favors radical dimerization and hence spin
inactivation. Incorporation of bulky substituents to prevent
dimerization can stabilize such radicals but also isolates the
spins, reducing any desirable interactions.
An alternative strategy involves electronic stabilization
through incorporation of heteroatoms into the polycyclic
aromatic hydrocarbon in order to decrease the spin densities,
eliminating the need for bulky groups.28,29 However, this
strategy may not always be favourable for the design of
stable πꢀradicals, for the nature of the polycyclic framework
and the position of the incorporated heteroatom(s) are critical
for the spin distribution in the molecule. Hence, it is imꢀ
portant to consider substructures that are conducive to effiꢀ
cient electronic stabilization such as the phenalenyl moiety,
which affords good delocalization and spinꢀactive πꢀ
dimerization.30 Trioxatriangelenum 1+ (see Figure 1) is a
case in point: it is as a remarkably stable pyryliumꢀbased
carbenium ion, but due to its polycyclic framework (which
incorporates three oxygen atoms), the spin density when 1+ is
reduced by one electron is weakly delocalized, i.e. the unꢀ
Figure 1 Molecular structures of 1+, 2, 3, 4+, 4 and 5+.
Pioneering work in this field has been performed by Haddon
et al. leading to the synthesis of 1,9ꢀdithiophenalenyl 3 stabiꢀ
lized via a disulfide bridge.28,29 Meanwhile, our groups preꢀ
viously reported the first example of electronic stabilization
by an oxygen atom in the phenalenyl species naphtho[2,1,8ꢀ
mna]xanthenyl (4), which was generated by reduction of the
parent cation.35 Further planar stable openꢀshell species have
been prepared by Kubo et al. which are stabilised through
delocalization in a large polyaromatic framework. Also
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bulky substituents were introduced in order to obtain crystals
phenalenꢀ1ꢀone (1 eq) and subsequent oxidation with DDQ,
9ꢀ(3ꢀmethoxynaphthalenyl)ꢀ 1Hꢀphenalenꢀ1ꢀone was obꢀ
for Xꢀray structure analysis.36
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Herein, we describe the synthesis and characterization of
two unprecedented phenalenyl species with intriguing propꢀ
erties (benzo[i]naphtho[2,1,8ꢀmna]xanthenium (8+) and
benzo[a]naphthaꢀ[8,1,2ꢀjkl]xanthenium (9+), which we comꢀ
pare with 5+, the largest oxygenꢀincorporating polycyclic
hydrocarbon.37, Interestingly, whilst cation 8+ is planar, catiꢀ
on 9+ is helical. The radicals of both 8 and 9 could be proꢀ
duced on a preparative scale via bulk electrolysis of the
corresponding solutions of the cations, and their properties
were investigated by EPR spectroscopy and computational
methods. Remarkably, the radicals can be stored under an air
atmosphere for weeks and both 8 and 9 show astonishingly
stable redox behavior under air. To our knowledge, radical 9
represents the first electronicallyꢀstabilized nonꢀplanar
phenalenylꢀtype radical without sterically hindering groups
yet reported. Using EPR spectroscopy analysis supported by
simulated spectra, we can confirm distinctly lower spin
densities at the αꢀpositions of the phenalenyl skeletons of 8
and 9 compared with 1,9ꢀdithiophenalenyl 3.28,29 This deꢀ
crease of the spin densities suggests a lower tendency for
sigmaꢀdimerization of 8 and 9 compared to 3.
tained as a yellow powder in a yield of 36% (0.68 g; 2
mmol). HꢀNMR (400 MHz; CDCl3) δ 3.79 (s, OMe), 6.58
1
(d, J = 9.7 Hz, 1H), 7.22 (s, 1H), 7.33 (ddd, J = 8.1, 6.9, 1.2
Hz, 1H), 7.44 (ddd, J = 8.2, 6.9, 1.3 Hz, 1H), 7.64 – 7.60 (m,
2H), 7.66 (d, J = 8.2 Hz, 1H), 7.69 (d, J = 9.8 Hz, 1H), 7.77
(t, J = 7.6 Hz, 1H), 7.80 (dd, J = 8.2, 0.5 Hz, 1H), 8.06 (dd, J
= 8.2, 1.0 Hz, 1H), 8.22 (d, J = 8.3 Hz, 1H).13CꢀNMR (100
MHz; CDCl3) δ 55.78 (OMe), 105.49 (Cq), 123.73 (CH),
126.28 (CH), 126.24 (CH), 126.84 (CH), 127.47 (CH),
127.87 (2CH), 128.36 (Cq), 128.86 (Cq), 129.54 (Cq),
130.35 (CH), 131.20 (2CH), 131.73 (CH), 131.84 (CH),
132.28 (Cq), 133.94 (CH), 134.64 (Cq), 134.85 (Cq), 140.35
(Cq), 155.79 (Cq), 185.49 (C=O). UV (in MeCN) – λmax
[nm] (log ε): 305 (3.73), 320 (3.75), 360 (3.97). IR (ATIR)
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ν
max [cmꢀ1] 3010ꢀ2827 (CHaromatic, weak), 1636 (C=O, sharp
& intense), 1622, 1598, 1579, 1555, 1503, 1497, 1476, 1458,
1427, 1390, 1385, 1352, 1327, 1248, 1244, 1241 (CꢀO,
sharp), 1198, 1171, 1123, 1107, 1076, 1040. MS (EI+) m/z
(rel. intensity): 336 (M+, 23), 320 (7), 306 (80), 305 (100),
292 (14), 276 (27), 263 (30), 237 (7), 160 (18), 153 (23), 146
(15), 132 (11), 86 (12), 84 (18), 49 (12), 44 (8). HRMS calcd
for C24H16O: 336.1150, found: 336.1146. Melting point: 194ꢀ
196 °C (recrystallization from CH2Cl2).
In spite of their large size, both 8+ and 9+ are soluble in
aqueous media (with the correct choice of counterion), which
may serve to make them suitable as dyes or prospective
DNA intercalators.25 Crystal structures were obtained for
both 8+ and 9+, and these cations exhibit interesting photoꢀ
chemical and redox behavior. We also prepared the previousꢀ
ly reported cation of benzo[5,6]naphthacenoꢀ[1,12,11,10ꢀ
jklmna]xanthylium (5+) ‒ the largest polycyclic hydrocarbon
containing a pyrylium moiety yet synthesized ‒ in order to
compare the differences in its structural and electronic propꢀ
erties with those of 8+ and 9+. Our investigations of 5+ have
allowed us to obtain the crystal structure of this cation for the
first time.
Synthesis of 9ꢀ(2ꢀmethoxynaphthalenyl)ꢀ1Hꢀphenalenꢀ1ꢀ
one (7). Following the general procedure, the Grignard reaꢀ
gent was generated from 0.23 g (9.4 mmol; 1.7 eq) Mg and
1.97 g (8.32 mmol; 1.5 eq) 1ꢀbromoꢀ2ꢀmethoxynaphthalene
dissolved in 5 mL THF at 40ꢀ50 °C. After reflux with 1Hꢀ
phenalenꢀ1ꢀone (1 eq) and subsequent oxidation with DDQ
9ꢀ(2ꢀmethoxynaphthalenyl)ꢀ1Hꢀphenalenꢀ1ꢀone was obꢀ
tained as an orange powder in a yield of 70% (1.30 g; 3.86
1
mmol). HꢀNMR (400 MHz; CDCl3) δ 3.79 (s, OMe), 6.50
(d, J = 9.7 Hz, 1H), 7.10 (d, J = 8.5 Hz, 1H), 7.20 (ddd, J =
8.1, 6.7, 1.3 Hz, 1H), 7.30 (ddd, J = 8.1, 6.7, 1.2 Hz, 1H),
7.42 (d, J = 9.0 Hz, 1H), 7.59 (d, J = 8.3 Hz, 1H), 7.66 (dd, J
= 8.2, 7.1 Hz, 1H), 7.70 (d, J = 9.7 Hz, 1H), 7.80 (d, J = 6.3
Hz, 1H), 7.85 (d, J = 8.1 Hz, 1H), 7.94 (d, J = 9.0 Hz, 1H),
8.10 (d, J = 8.2 Hz, 1H), 8.27 (d, J = 8.3 Hz, 1H). 13CꢀNMR
(100 MHz; CDCl3) δ 56.85 (OMe), 77.36 (Cq), 113.95 (CH),
123.58 (CH), 124.16 (CH), 126.23 (Cq), 126.35 (CH),
126.54 (CH), 127.97 (Cq), 128.29 (CH), 128.66 (Cq), 128.70
(Cq), 128.94 (CH), 129.44 (Cq), 130.33 (CH), 131.15 (CH),
131.84 (CH), 132.20 (Cq), 132.56 (CH), 134.21 (CH),
140.41 (CH), 142.38 (Cq), 153.06 (Cq), 185.39 (C=O). UV
(in MeCN) – λmax [nm] (log ε): 312 (3.75), 341 (3.91), 358
(4.03). IR (ATIR) νmax [cmꢀ1] 3032ꢀ2830 (CHaromatic, weak),
1630 (C=O, sharp & intense), 1609, 1591, 1545, 1510, 1447,
1380, 1384, 1329, 1253 (CꢀO, sharp & intense), 1236, 1183,
1176, 1145, 1120, 1084, 1065, 1064. MS (EI+) m/z (rel.
intensity): 336 (M+, 9), 305 (93), 263 (7), 153 (8), 86 (65),
84 (100), 51 (22), 49 (68), 47 (18). HRMS calcd for
C24H16O: 336.1150, found: 336.1145. Melting point: 203ꢀ
205 °C (recrystallization from CH2Cl2).
General procedure for the synthesis of benzonaphthoxanꢀ
thenium cations. 0.100 g 9ꢀ(2ꢀMethoxyaryl)ꢀ1Hꢀphenalenꢀ
1ꢀone (0.35 mmol) was dissolved in 40 mL CH2Cl2 in a
round bottom flask placed in an NaCl/ice bath. Argon was
purged through for 5 minutes and the flask was sealed with a
septum and supplied with an argonꢀfilled balloon. While
stirring, 1.2 mL BBr3 in heptane (1 M) was slowly added by
syringe. The color of the solution changed from orange to
dark brown. After 10 minutes, the NaCl/iceꢀbath was reꢀ
EXPERIMENTAL SECTION
General procedure for the synthesis of 9ꢀ
(methoxynaphthalenyl)ꢀ1Hꢀphenalenꢀ1ꢀones. In a twoꢀ
neck round bottom flask, Mg in 5 mL of THF was activated
with 0.1 mL 1,2ꢀdibromoethane under argon. Then the broꢀ
minated aryl compound was added to the reaction mixture
dropwise in THF by means of a syringe. With slow stirring
and heating, the Grignard reagent began to form. After 45
min, 1.00 g (5.5 mmol, 1 eq) of 1Hꢀphenalenꢀ1ꢀone disꢀ
solved in 10 mL of dry THF was gradually added to the grey
solution. The mixture was then refluxed for 4 h. After coolꢀ
ing down to room temperature, the reaction was quenched
with 5 mL sat. NH4Cl solution. Then 20 mL H2O was added
and the product was extracted with EtOAc (3 x 30 mL) and
washed with 20 mL brine and dried over MgSO4. When the
solvent was evaporated to dryness, the crude product and
1.30 g DDQ (5.6 mmol; 1 eq) were mixed in CH2Cl2 and
refluxed overnight at 55ꢀ60 °C. The next day, the solvent
was evaporated by rotary evaporator inside a fumehood. By
flash column chromatography using silica and CH2Cl2 as the
eluent, the desired product was obtained.
Synthesis of 9ꢀ(3ꢀmethoxynaphthalenyl)ꢀ1Hꢀphenalenꢀ1ꢀ
one (6). Following the general procedure, the Grignard reaꢀ
gent was generated from 0.23 g (9.4 mmol; 1.7 eq) Mg and
1.97 g (8.32 mmol; 1.5 eq) 2ꢀbromoꢀ3ꢀmethoxynaphthalene
dissolved in 5 mL THF at 40ꢀ50 °C. After reflux with 1Hꢀ
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moved and the solution was stirred for 4 hours. After this recrystallized from acetic acid resulting in 14ꢀ
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time, adding 30 mL of H2O quenched the reaction and disꢀ
solved the precipitate giving an orange solution. The aqueous
layer was separated and the organic salt in the organic layer
was extracted with distilled H2O (2 x 10 mL). After the
aqueous layers were combined and filtered, 5 mL HBF4
solution was added, yielding a precipitate which was sepaꢀ
rated by vacuum filtration.
phenyldibenzo[a,j]xanthenium bromide as redꢀorange crysꢀ
tals with a golden lustre in a yield of 59% (216 mg; 0.5
mmol; lit.: 83%37). Without further purification, the batch of
14ꢀphenyldibenzo[a,j]xanthenium bromide was dissolved in
60 mL of MeCN and 5 mL of HBF4 solution (48 wt. % in
H2O). After UV irradiation overnight, black needles of 5+
were obtained in a yield of 30% over two steps (112 mg; 0.3
mmol). This compound analyzed as per the analogous broꢀ
mide salt previously reported.37 IR (ATIR) νmax [cmꢀ1] 3055
(CHaromatics, weak), 1617, 1605, 1582 (C=O+, intense), 1464,
1431, 1395, 1353, 1335, 1314, 1264, 1251, 1241, 1210,
1199, 1144, 1077, 1022 (BꢀF, intense). MS (FAB+) m/z (rel.
intensity): 353 (36), 121 (30), 91 (10), 77 (10), 44 (100).
Melting Point: > 360 °C (recrystallization from acetonitrile).
HRMS calcd for C27H13O: 353.0966, found: 353.0968.
Computational methods: All calculations were performed
using the Gaussian09 suite of programmes.38 Optimizations
were performed using the B3LYP39 and M05ꢀ2X40 density
functional methods, in combination with 6ꢀ31G** and 6ꢀ
31+G* basis sets.41 All minima were characterized as such
by performing a vibrational analysis.
Synthesis of benzo[i]naphtho[2,1,8ꢀmna]xanthenium
tetrafluoroborate (8+). Following the general procedure, 8+
was obtained as a brown powder in a yield of 64%. 1HꢀNMR
(400 MHz; CD3CN) δ 7.71 (t, J = 7.5 Hz, 1H), 7.79 (t, J =
7.1 Hz, 1H), 8.15 (d, J = 8.4 Hz, 1H), 8.25 (d, J = 8.5 Hz,
1H), 8.39 (d, J = 9.1 Hz, 1H), 8.43 (t, J = 7.7 Hz, 1H), 8.61
(s, 1H), 9.05 (m, 2H), 9.20 (d, J = 8.7 Hz, 1H), 9.29 (d, J =
9.1 Hz, 1H), 9.36 (d, J = 8.7 Hz, 1H), 9.49 (s, 1H). 13CꢀNMR
(100 MHz; CD3CN) δ 115.63 (Cq), 116.82 (CH), 118.66
(Cq), 120.92 (CH), 122.33 (Cq), 123.45 (CH), 128.40 (CH),
128.48 (CH), 128.63 (Cq), 128.96 (CH), 130.21 (CH),
131.20 (Cq), 131.45 (CH), 131.86 (CH), 132.33 (Cq), 137.30
(Cq), 141.83 (CH), 142.75 (CH), 145.96 (Cq), 148.40 (CH),
149.11 (Cq), 150.36 (CH), 166.07 (C=O+). UV (in MeCN) –
λmax [nm] (log ε): 322 (4.15), 363 (3.82), 469 (4.68). IR
(ATIR) νmax [cmꢀ1] 3074 (CHaromatic, weak), 1603, 1597,
1591, 1566 (C=O+, sharp & intense), 1507, 1472, 1445,
1421, 1360, 1334, 1288, 1239, 1211, 1199, 1163, 1144,
1136, 1027 (BꢀF, sharp & intense). MS(EI+) m/z (rel. intenꢀ
sity): 305 (100), 276 (21), 274 (9), 231 (5), 219 (5), 187 (8),
181 (7), 169 (8), 153 (13), 131 (10), 119 (8), 85 (6), 69 (16),
44 (83), 40 (15), 36 (22). HRMS calcd for C23H13O+:
305.0966, found: 336.0967. Melting point: 276ꢀ280 °C (reꢀ
crystallization from acetone).
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RESULTS AND DISCUSSION
Our synthetic methodology is shown in Scheme 1, and is
based on a 4ꢀstep synthesis via the two new precursors 9ꢀ(3ꢀ
methoxynaphthalenyl)ꢀ1Hꢀphenalenꢀ1ꢀone 6 (which gives
8+ ) and 9ꢀ(2ꢀmethoxynaphthalenyl)ꢀ1Hꢀphenalenꢀ1ꢀone 7
(which leads to 9+).
Synthesis of benzo[a]naphtho[8,1,2ꢀjkl]xanthenium tetraꢀ
fluoroborate (9+). Following the general procedure, 9+ was
obtained as a brown powder in a yield of 67%.1HꢀNMR (400
MHz; CD3CN) δ 7.95 (ddd, J = 8.0, 7.1, 1.0 Hz, 1H), 8.07
(ddd, J = 8.5, 7.1, 1.4 Hz, 1H), 8.26 (d, J = 9.1 Hz, 1H), 8.33
(dd, J = 8.0, 1.2 Hz, 1H), 8.58 (t, J = 7.7 Hz, 1H), 8.60 (d, J
= 9.1 Hz, 1H), 8.69 (d, J = 9.1 Hz, 1H), 9.18 – 9.10 (m, 2H),
9.28 (d, J = 8.5 Hz, 1H), 9.34 (d, J = 9.1 Hz, 1H), 9.39 (d, J
= 9.1 Hz, 1H), 9.61 (d, J = 9.1 Hz, 1H). 13CꢀNMR (100
MHz; CD3CN) δ 117.00 (Cq), 117.64 (Cq), 118.65 (CH),
120.11 (CH), 121.84 (Cq), 126.27 (CH), 128.26 (CH),
129.20 (Cq), 129.24 (Cq), 129.97 (CH), 130.57 (Cq), 131.39
(CH), 131.43 (CH), 131.45 (CH), 133.17 (Cq), 139.27 (CH),
140.42 (CH), 140.95 (CH), 144.32 (Cq), 146.73 (CH),
146.88 (CH), 155.76 (Cq), 161.38 (C=O+). UV (in MeCN) –
λmax [nm] (log ε): 311 (4.91), 505 (5.12). IR (ATIR) νmax
[cmꢀ1] 3061, 1617, 1602, 1577, 1564, 1545, 1518, 1495,
1472, 1431, 1412, 1382, 1350, 1324, 1279, 1246, 1236,
1224, 1192, 1176, 1147, 1137, 1115, 1091, 1032, 983.
MS(EI+) m/z (rel. intensity): 305 (100), 276 (28), 274 (10),
153 (9), 152 (8), 137 (7), 69 (3), 44 (4), 40 (3). HRMS calcd
for C23H13O+: 305.0966, found: 305.0972. Melting point:
273ꢀ275 °C (recrystallization from acetone).
Scheme 1 Synthesis of phenalenyl cations and radicals; a) i. ((3ꢀ
methoxynaphthalenꢀ2ꢀyl)magnesium bromide, THF, rt; ii.) DDQ,
CH2Cl2, reflux. b) i. (2ꢀmethoxynaphthalenꢀ1ꢀyl)magnesium bromide,
THF, rt; ii. DDQ, CH2Cl2, reflux. c) i. BBr3, CH2Cl2, 0°ꢀrt; ii. HBF4 (48
wt. % in H2O).
Both precursors were synthesized by a Michaelꢀtype Griꢀ
gnard addition to the 9ꢀC position of 1Hꢀphenalenone, with
subsequent oxidation using DDQ.42 This procedure gave 6
and 7 in yields of 36% and 70% respectively over two steps.
Subsequently, demethylation using BBr3 in CH2Cl2 resulted
in the corresponding 9ꢀ(hydroxynaphthalenyl)ꢀ1Hꢀphenalenꢀ
1ꢀones which underwent rapid cyclization in presence of
water to give 8+ and 9+ with bromide as the counterion. Theꢀ
se salts were soluble in water and were isolated in pure form
Synthesis
of
benzo[5,6]naphthaceno[1,12,11,10ꢀ
jklmna]xanthylium tetrafluoroborate (5+). In a twoꢀneck
round bottom flask, 300 mg (0.8 mmol; 1 eq) of 14ꢀphenylꢀ
14Hꢀdibenzo[a,j]xanthene was dissolved in 25 mL of glacial
acetic acid under reflux at 100 °C. Then 0.05 mL (1 mmol;
1.2 eq) of Br2 mixed with 5 mL acetic acid were added
dropwise to the solution and the reaction was refluxed for 30
minutes. After cooling down the red solution, the precipitate
was separated by vacuum filtration. The crude product was
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by extraction into aqueous solution followed by precipitation
assembly of 5+, 8+ and 9+ obtained from Xꢀray structure
analysis are displayed below in Figure 2.
as the tetrafluoroborate salts, yielding 8+ (64%) and 9+
(67%).
1
2
In terms of electronic properties, cation 8+ shows a red
shift of 13 nm in its absorption spectrum compared with the
previously reported cation 4+ on account of the more extendꢀ
ed polyaromatic framework in 8+ (Figure 3). The same trend
is also evident with 9+, but to an even greater extent. 9+
shows a broader absorption than 8+ with a maximum at 505
nm (a bathochromic shift of 36 nm compared with 8+). This
result is unexpected and intriguing, given that the benzeneꢀ
fused skeleton is less linear in 9+ than it is in 8+. According
to Clar’s approach, 9+ comprises more Robinson rings than
8+ and hence 9+ should exhibit a higher aromaticity when
compared to 8+. This in turn would imply that 9+ should
absorb shorter wavelengths of light than 8+. However, as this
is clearly not the case, we propose that the geometry and the
position of the pyrylium ring in 9+ are disrupting this aromaꢀ
ticity and causing a bathochromic shift to manifest instead.
The different intensities of fluorescence emission from 4+,
8+ and 9+ could be observed with the naked eye (Figure 3,
inset). The fluorescence emission spectra (Figure 3) show the
emission of 4+ (2.8 × 10ꢀ5 mol/L), 8+ (1 × 10ꢀ5 mol/L) and 9+
(3.1 × 10ꢀ6 mol/L) upon excitation at 435 nm, 465 nm and
505 nm, respectively. 4+ emits between 450 and 600 nm with
a maximum at 471 nm giving a Stokes shift of 15 nm. 8+ also
emits up to 600 nm, however, with a considerably lower
intensity. The maximum peak lies at 516 nm, which correꢀ
sponds to a Stokes shift of 47 nm. A broad emission between
510 nm and 690 nm can be observed for 9+ with a maximum
peak at 562 nm, indicating a red shift (and a higher intensity)
compared with 8+. Cation 8+ displayed a relatively low intenꢀ
sity compared to 4+ and 9+. This can be attributed to colliꢀ
sional quenching by πꢀπ stacking interactions. In the case of
9+, the helical structure results in the minimization of πꢀπ
interactions and hence a greater emission intensity.
3
4
5
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8
9
Cation 8+ crystallized from acetone in a triclinic system
with space group Pꢀ1 (see, Table S4 and Figure S6 in the
Supporting Information). The molecules in this structure are
arranged in πꢀπ stacking arrays separated by distances of
between 3.286ꢀ3.389 Å. 8+ can be considered planar with a
negligibly small torsion angle of 1.068° in the bay region.
Cation 9+ also crystallized from acetone in a triclinic system
with space group Pꢀ1 (see Table S5 and Figure S7 in Supꢀ
porting Information) as a racemic mixture of the possible
enantiomers, P and M. Due to its fjord region, 9+ exhibits a
helical geometry with a torsion angle of 28.579°.
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Figure 3 UVꢀvisible and fluorescence spectra of 4+, 8+ and 9+ in MeCN.
Figure 2 Illustration of πꢀπ stacking arrangements of compounds 5+, 8+
and 9+ obtained from Xꢀray structure analysis. Counterions are not
displayed for clarity.
The cyclic voltammogram (CV) of 8+ is shown as the black
dashed line in Figure 4 (see Supporting Information for
electrochemical methods). Two reversible redox waves are
evident centered around ‒0.43 V and ‒1.53 V vs. ferroꢀ
cene/ferrocenium. The least negative of these waves correꢀ
sponds to the formation of the neutral radical species, and the
second to the formation of the 8‒ monoꢀanion. A CV of 9+
under the same conditions is shown for comparison as the
solid red line in Figure 4. Again, two wellꢀdefined reversible
waves are evident in the absence of oxygen, occurring at
roughly ‒0.59 V and ‒1.67 V vs. ferrocene/ferrocenium.
Compared to 4+(35), these redox process for 8+ are both
somewhat less negative. In the case of 4+, Ecation/radical = ‒0.52
V for the formation of the neutral radical from the cation,
whilst the reduction of this radical to give the anion occurred
at Eradical/anion = ‒1.66 V. This indicates that 8+ is easier to
Meanwhile, the synthesis of 5+ was achieved using a slightly
modified literature procedure (see Experimental Section for
details),37 resulting in crystals of 5+ as the tetrafluoroborate
salt that proved suitable for Xꢀray diffraction. Analysis of the
resulting Xꢀray structure of 5+ (see Table S6 and Figure S5
in Supporting Information) shows an arrangement in a triꢀ
clinic system, with antiplanar πꢀstacking interactions giving
rise to two parallel columns in the unit cell. Between these
columns antiplanar πꢀπ interacting pairs of 5+ are arranged
with a separation of 3.367ꢀ3.393 Å. The molecule can be
considered planar with negligibly small torsion angles of
1.067° and 0.525° in the bay regions. Illustrations of the
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reduce than 4+, both in the cationic and neutral radical forms,
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possibly on account of the greater degree of delocalization in
8+. For 9+, formation of the neutral radical species is harder
than in the case of 4+, but formation of the cation occurs at
roughly the same potential. In contrast, compound 5+ showed
significantly more cathodic shifts for both its corresponding
waves with Ecation/radical = ‒0.96 V and Eradical/anion = ‒1.80 V
(see Figure S1 in the Supporting Information).
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Figure 5 Cyclic voltammograms of compounds 8+ and 9+ at room
temperature and under an air atmosphere at a scan rate of 100 mV sꢀ1 on
a glassy carbon electrode of area 0.071 cm2. Peaks are referenced to the
Fc/Fc+ couple. Both compounds were cycled 2000.
The radicals 8 and 9 were formed electrochemically by bulk
electrolysis of solutions of the corresponding cations at –1.0
V vs. ferrocene/ferrocenium with 0.1 M N(Bu)4BF4 as the
supporting electrolyte. Upon complete reduction to the radiꢀ
cals, the colour of the solutions had turned to yellow and
dark red for 8 and 9, respectively. The UV/Vis spectra
showed the full depletion of the absorption peaks indicative
of the cations and suggested the presence of the correspondꢀ
ing radicals (see Figure S2 in the Supporting Information). In
the case of 8, three maxima were detected with values at 403
nm, 441 nm and 465 nm including a shoulder at 494 nm.
Further absorption peaks were evident between 300 nm and
380 nm with peaks at 320 nm, 335 nm and 362 nm which
appear to be similar to the absorption of the corresponding
cation. The UV spectrum of 9 shows strong absorption beꢀ
tween 320 nm and 450 nm with a clear maximum peak at
360 nm. Furthermore a broad shoulder with moderate abꢀ
sorption can be found between 455 nm and 565 nm. It is
noteworthy that the absorption spectrum of radical 9 is very
similar to the absorption spectrum of radical 4 published
previously.35
Product solutions from completed bulk electrolyses were
investigated by EPR spectroscopy, which confirmed that
radicals had been formed. Signals with gꢀvalues of 2.00258
were obtained for 8 and 2.0028 for 9, which are close to that
of a free electron (g = 2.002319). Figure 6 shows the couꢀ
pling patterns in which the total number of lines, hypothetiꢀ
cally made up of (2 × 1 × ½ + 1)13 = 8192 lines, cannot be
observed due to the limited resolution. Nonetheless, by comꢀ
putational methods Mulliken spin densities (calculated at the
M05ꢀ2X/6ꢀ31G** level of theory ‒ see Tables S2 & S3 in
the Supporting Information) could be assigned to the radical
centers. These spin densities correlate with the hyperfine
coupling constants, allowing spectra which match with the
experimental spectra to be simulated (see Figure 6 including
Figure S3 & S4 in the Supporting Information). It is noteꢀ
worthy that the spin densities at the αꢀposition of the
phenalenyl skeleton are below the corresponding spin densiꢀ
ties of 1,9ꢀdithiophenalenyl 3 (0.51 mT & 0.54 mT)28,29
indicative of a higher stabilization of 8 and 9 compared with
3.
Figure 4 Cyclic voltammograms of 8+ and 9+ at room temperature
obtained at scan rates of 100 mV sꢀ1 on a glassy carbon working elecꢀ
trode of area 0.071 cmꢀ2. Peaks are referenced to the Fc/Fc+ couple
(wave not shown).
When cyclic voltammograms of 8+ and 9+ were recorded
under an air atmosphere, the first redox waves for both comꢀ
pounds (corresponding to the cation/radical redox processes)
showed very good reversibility, which is indicative of a lack
of side reactions. After 2000 cycles at a scan rate of 100
mV/s, no depletion of the first redox wave for either 8+ or 9+
was observed (Figure 5). These results could be reproduced
using the same samples after one week. Hence radicals 8 and
9 may be capable of acting as stable paramagnetic redox
partners without the need for an inert atmosphere. Hence
these radicals are stable to redox cycling in air, with no sigꢀ
nificant degradation evident by CV over the course of severꢀ
al hours. Other radicals (e.g. DPPH, TEMPO and verdazyl
radicals) are also stable over long periods of time. However,
in these cases the spin density is concentrated on heteroaꢀ
toms (O and N) and bulky substituents are required in order
to prevent deleterious side reactions occurring. This is in
contrast to the carbonꢀbased, delocalized radicals stabilised
through electronics that we report herein. The second redox
waves for both 8 and 9 (corresponding to the radical/anion
redox couples) were found to be irreversible when probed
under air.
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Figure 6 Xꢀband EPR spectra of 8 and 9 in DMSO at 293 K (experiꢀ
mental conditions for 8: frequency, 9.4481 GHz; power, 0.63 mW;
modulation, 0.02 mT; experimental conditions for 9: frequency, 9.8812
GHz; power, 0.63 mW; modulation, 0.02 mT). Experimental data are
shown by the black traces and the simulations as grey lines.
Figure 7 Molecular structures of 5+, 8+ and 9+ showing CꢀO distances
and torsion angles in the bay and fjord regions. HOMA values are
calculated using bond lengths from Xꢀray data analysis. NICS values are
obtained from calculation based on the B3LYP level of theory using a
basis set 6ꢀ31+G*. In the case of 9+, the NICS values for both sides of
the molecule were computed using dummy atoms which are 1 Å distant
from the center of each ring.
Using the bond lengths obtained from the Xꢀray data, Harꢀ
monic Oscillating Model of Aromaticity (HOMA)43 indices
were calculated using the HOMA model formula.44 In addiꢀ
tion, Nucleus Independent Chemical Shift (NICS)45 values of
dummy atoms centered 1 Å above the plane of benzenoid
rings were computed based on the B3LYP level of theory
using a basis set of 6ꢀ31+G*. For compound 5+, the pyrylium
ring exhibits the lowest HOMA and NICS values of 0.42 and
–4.21 due to the perturbation caused by the oxygen atom (see
Figure 7). The bond lengths of CxꢀO and CyꢀO are identical
(1.362 Å). The degrees of aromaticity for the inner rings B
and F are distinctly lower compared to those of the outer
rings (see Table S1 in Supporting Information). Rings A, E
and G exhibit the highest degrees of aromaticity. The inner
rings are less aromatic than rings C and D, hence 5+ appears
to comprise two naphthalene substructures at the outer
frames (AꢀC and DꢀE) characterized by their electronic
structure. Thus there appears to be less phenalenyl character
in 5+ than in 8+ or 9+, which could explain the more cathodic
reduction potentials of 5+ relative to 8+ and 9+ (less
phenalenyl character and so less stabilization through deloꢀ
calization).
As 9+ is nonꢀplanar, a statement about the aromaticity based
on the electromagnetic parameters using NICS values is
considered for both sides of the plane at a distance of 1 Å
from the center of each 6ꢀmembered ring. The pyrylium ring
exhibits a slight enhancement of aromaticity compared with
the ones for 8+ and 5+ which is observed by considering the
HOMA index and NICS values of 0.52 and –5.93/–7.22,
respectively. Contrary to 8+, the benzene ring E in 9+ is more
aromatic than ring D adjacent to the pyrylium ring. Comparꢀ
ing the planarities of the pyrylium rings in all the presented
molecules, the root mean square deviations of fitted atoms in
these rings are below 0.01 Å, indicating a negligible geometꢀ
rical effect on the degrees of aromaticity.
We propose that a more pronounced oxoniumꢀcarbenium
resonance structure is found on the side of the phenalenyl
unit in cations 8+ and 9+ (when compared to cation 5+) repreꢀ
sented by the bond lengths of 1.337 and 1.332 Å, respectiveꢀ
ly. Hence, it appears that cations 8+ and 9+ have the propensiꢀ
ty to resemble the electronic structure of the chromophore
compound phenalenone.46 In contrast, 5+ shows very low
degrees of aromaticity in the inner rings C and F. Thus a
delocalization pattern similar to a phenalenyl unit comparaꢀ
ble with 8+ and 9+ is nonꢀexistent, i.e. 5+ can be classified as
a nonꢀphenalenyl type cation. This aromaticity evaluation
may indicate that compounds 8+ and 9+ do not represent
typical electronic structures of xanthenyl species due to the
presence of the phenalenyl skeleton. In contrast with xanꢀ
thenyl, 8+ and 9+ are not prone to undergo nuclueophilic
attack on the 4ꢀ position of the pyrylium ring. Hence, we
demonstrate that a phenalenyl unit combined with pyrylium
derivatives can result in complementary electronic systems,
where the two αꢀpositions of the phenalenyl moiety represent
the commonly reactive sites of the pyrylium ring (the 2´ and
the 4´ positions). Such a situation was previously suggested
by Haddon and coꢀworkers as a result of their elegant work
with 1,9ꢀdithiophenalenyl 3, which incorporates a 1,2ꢀ
dithiolyl ring.28,29 However, we note that the oxidized forms
of the phenalenyl radicals presented herein also represent
fascinating dyes and hence may also open up new avenues of
research in the area of phenalenyl chemistry.
The evaluation of the aromaticity for 8+ shows that the
three rings A, B and C resemble rather more the electronic
structure of phenalenyl than it is the case for 5+. The naphꢀ
thalene unit annulated with the pyrylium ring has a slightly
higher degree of aromaticity in ring D compared to ring E. It
is noteworthy that the CxꢀO bond is distinctly shorter than the
CyꢀO bond indicating that a nucleophile (e.g. hydroxide)
would not be prone to attack the ring at the Cy position. This
is consistent with Clar’s rule, as otherwise a loss of aromaticꢀ
ity would result, which can be depicted by depletion of one
Robinson ring. The tendency for a nucleophilic attack at the
Cx and Cb positions is additionally decreased due to the conꢀ
jugation provided by the phenalenyl unit, resulting in an
optimal stabilization of the whole molecule.
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CONCLUSIONS
In conclusion, we have demonstrated that our synthetic
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protocol gives access to a new series of πꢀamphoteric species
of interest in the field of phenalenyl chemistry, including the
first example of a helical phenalenyl radical stabilized only
by the incorporation of a heteroatom ever reported. Our
results highlight the remarkable stability of these electroniꢀ
callyꢀstabilized radicals which can be stored under air for
weeks without noticeable degradation. It is noteworthy that
phenalenyl and pyrylium species on their own give shortꢀ
lived radicals upon oneꢀelectron reduction under air, which
subsequently undergo sideꢀreactions such as sigmaꢀ
dimerization. Hence, the high reversibility of the cationꢀ
radical redox processes under air reported herein suggests an
intriguing complementary stabilization between the
phenalenyl and pyrylium substructures affording novel,
stable phenalenylꢀtype species which may act as dyes or
stable openꢀshell species depending on their redox states.
Although 8+ and 9+ have an electronic structure based on
phenalenyl (in contrast to 5+), the overall polyaromatic
frameworks of these two compounds represent two distinct
scaffolds due to their differing shapes. Interestingly the
absorption of 9+ is distinctly red shifted compared with that
of 8+ despite its less linear benzeneꢀfused skeleton. The nonꢀ
planar topology of 9+ prevents efficient fluorescence quenchꢀ
ing by πꢀπ interactions compared with its isomer 8+. This, in
combination with its water solubility, makes 9+ a promising
candidate as a dye for DNA intercalation studies. Future
work in our laboratory will endeavour to focus on the potenꢀ
tial applications of these compounds and on preparing even
more complex and topologically interesting cations and
radicals of this sort.
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ASSOCIATED CONTENT
Supporting Information
Detailed experimental protocols, additional electrochemical,
electronic, EPR, computational and crystallographic data, as
well as characterisation of all new compounds reported herein.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* goebu@chem.gla.ac.uk
Author Contributions
The manuscript was written through contributions of the auꢀ
thors.
ACKNOWLEDGMENTS
MDS thanks the University of Glasgow for a Kelvin Smith
Research Fellowship. This work was supported by the Universiꢀ
ty of Glasgow and the EPSRC. LC thanks the Royal Society for
a Wolfson Merit Award.
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