Naphthoxanthenyl, a new stable phenalenyl type radical stabilized by electronic effects

Article abstract of DOI:10.1021/ol401117g

Naphthoxanthenyl 1 is a new stable phenalenyl-type radical. Electrochemical studies indicate that 1 has two reversible redox processes that occur on comparatively short time scales. Crystals containing 1 can be grown by electrocrystallization, suggesting that they are conductive.

Full text of DOI:10.1021/ol401117g

ORGANIC
LETTERS
2013
Vol. 15, No. 12
2970–2973
Naphthoxanthenyl, a New Stable
Phenalenyl Type Radical Stabilized
by Electronic Effects
Ommid Anamimoghadam,† Mark D. Symes,† Christoph Busche,† De-Liang Long,†
Stuart T. Caldwell,† Cristina Flors,‡,§ Santi Nonell,‡ Leroy Cronin,*,† and
,†
€
Gotz Bucher*
WestCHEM, School of Chemistry, University of Glasgow, Joseph-Black-Building,
University Avenue, Glasgow G12 8QQ, United Kingdom, and Grup d’Enginyeria
´
ꢀ
Molecular, Institut Quımic de Sarria, Universitat Ramon Llull, E-08017 Barcelona, Spain
goebu@chem.gla.ac.uk
Received April 22, 2013
ABSTRACT
Naphthoxanthenyl 1 is a new stable phenalenyl-type radical. Electrochemical studies indicate that 1 has two reversible redox processes that occur
on comparatively short time scales. Crystals containing 1 can be grown by electrocrystallization, suggesting that they are conductive.
Stable π-conjugated radicals constitute promising build-
ing blocks for spin carriers in next generation functional
materials involving magnetic systems.1ꢀ3 Effective radical
spin carriers must show high stability and persistence, but
for applied aspects, communication between spins must
also be possible.4 In this regard, delocalized radicals based
on phenalenyl (PLY) hold a significant advantage over
both metal ions and nitroxyl radicals, which are currently
the most widely used spin carriers.5 The ability of PLY to
exhibit such metal-like behavior was first suggested by
Haddon,6 who also pointed out PLY’s potential for deri-
vatization and its ability to form stable radicals, cations,
and anions with invariant bond orders.
In order to design stable PLY-type radicals, it is neces-
sary to prevent the formation of σ-dimers.7 PLY itself
undergoes dimerization between the R-positions and sub-
sequent oxidative cyclization to form dibenzoperylene
in the solid state.8 PLY-type radicals substituted with
sterically hindering groups as such as tert-butylated PLY
form paramagnetic π-dimers that are predicted to show
aromaticity between the π-planes.9,10 Alternatively, PLY
radicals can be stabilized electronically via incorporating
† University of Glasgow.
‡ Universitat Ramon Llull.
§ Current address: IMDEA Nanociencia, C/Faraday 9, E-28049 Madrid,
Spain.
€
(1) Mullegger, S.; Rashidi, M.; Fattinger, M.; Koch, R. J. Phys.
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(5) Morita, Y.; Nishida, S. Phenalenyls, Cyclopentadienyls, and
Other Carbon-Centered Radicals. In Stable Radicals: Fundamental
and Applied Aspects of Odd-Electron Compounds; Hicks, R. G., Ed.; John
Wiley & Sons: Chichester, 2010; p 81.
(9) Goto, K.; Kubo, T.; Yamamoto, K.; Nakasuji, K.; Sato, K.;
Shiomi, D.; Takui, T.; Kubota, M.; Kobayashi, T.; Yakusi, K.; Ouyang,
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r
10.1021/ol401117g
Published on Web 06/03/2013
2013 American Chemical Society

heteroatoms to decrease the spin densities at the R-posi-
tions. 1,9-Dithiophenalenyl as well as 1,6,7,9-tetrathio-
phenalenyl represent the pioneering and only examples
of stable PLY species stabilized solely through electronic
effects.11,12
We now wish to report on the synthesis and electro-
chemical studies of a new radical, naphthoxanthenyl (1),
which incorporates a PLY core, and is further stabilized
via a xanthene substructure. 1 is the first example of a
PLY radical that is stabilized solely through the elec-
tronic effects of a single oxygen atom and a Clar sextet,
and furthermore this stabilization is achieved without the
use of any sterically hindering substituents. This makes 1
an excellent candidate for use as a spin carrier in molecular
electronics, as it lacks bulky substituents which could
inhibit interspin communication.13
It was also possible to prepare 1 in a four-step synthe-
sis (Scheme 2), starting from phenalenone. The well-
established Michael-type Grignard addition23 of 2-methoxy-
phenylmagnesium bromide to the 9-C position of
phenalenone, followed by oxidation with DDQ, gave
9-(2-methoxyphenyl)-phenalenone (4) (X-ray data included
in the Supporting Information (SI)). Demethylation of
4 using BBr3 in dichloromethane resulted in cyclization
to naphthoxanthenium bromide. Naphthoxanthenium
bromide is very soluble in water and can be isolated through
extraction and subsequent precipitation by means of coun-
terion exchange with HBF4 to yield naphthoxanthenium
tetrafluoroborate (5) as orange crystals. The yield over four
steps was 36%, and the neutral radical could be generated
by one-electron reduction of 5, e.g. using sodium iodide in
acetonitrile or DMSO.
Radical 1 was synthesized using two different routes.
Photolysis of a solution of 9-phenylphenalenone (2)14
in benzene in the presence of tetracyanoethylene (TCNE)
yielded 1 (Scheme 1), via transient 1H-2-oxa-benzpyrene
(3)15 with a weak C(sp3)ꢀH bond (UM05-2X/6-31G(d):
BDEcalcd = 28.8 kcal molꢀ1).16,17 This is distinctly lower
than the C(sp3)ꢀH bond dissociation enthalpy in phena-
lene (64 kcal molꢀ1).18 The intramolecular addition of a
triplet ketone to a β-phenyl ring is known as β-phenyl
quenching (BPQ).19ꢀ22 In the absence of added quenchers,
intermediate3 rapidlydecays byelectrocyclic ringopening,
yielding 2. In the presence of TCNE, the labile C(sp3)-
bound hydrogen atom of 3 is transferred to the TCNE
molecule, yielding 1.15 The formation of 1 was confirmed
by ESR spectroscopy (vide infra).
Single crystals of 5 were obtained by recrystalliza-
tion from acetone (see SI). The X-ray analysis indicates
interplanar distances between 3.294 and 3.372 A, indicat-
ing a π-stacked structure with considerable interaction
between the π-planes (see SI).
Scheme 2. Synthesis of 5 and Reduction to 1
Scheme 1. Photochemical Approach to 1 via β-Phenyl
Quenching
Solutions of 5 in acetone or acetonitrile are easily
reduced by adding NaI, yielding brown-greenish solutions
(15) The transient species derived from the photolysis of 2 has
been previously ascribed to an intramolecular charge transfer inter-
mediate ( Flors, C.; Ogilby, P. R.; Luis, J. G.; Grillo, T. A.; Izquierdo,
L. R.; Gentili, P. L.; Bussotti, L.; Nonell, S. Photochem. Photobiol.
2006, 82, 95). Our results now suggest that this species in fact is the
oxabenzpyrene 3.
(16) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Theory
Comput. 2006, 2, 364.
(17) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724.
(18) Bausch, M. J.; Gostowski, R.; Jirka, G.; Selmarten, D.; Winter,
G. J. Org. Chem. 1990, 55, 5805.
(19) Wismontski-Knittel, T.; Kilp, T. J. Phys. Chem. 1984, 88, 110.
(20) Samanta, S.; Mishra, B. K.; Pace, T. C. S.; Sathyamurthy, N.;
Bohne, C.; Moorthy, J. N. J. Org. Chem. 2006, 71, 4453.
(21) Bucher, G. J. Phys. Chem. A 2008, 112, 5411.
(10) Suzuki, S.; Morita, Y.; Fukui, K.; Sato, K.; Shiomi, D.; Takui,
T.; Nakasuji, K. J. Am. Chem. Soc. 2006, 128, 2530.
(11) Beer, L.; Mandal, S. K.; Reed, R. W.; Oakley, R. T.; Tham, F. S.;
Donnadieu, B.; Haddon, R. C. Cryst. Growth Des. 2007, 7, 802.
(12) Haddon, R. C.; Wudl, F.; Kaplan, M. L.; Marshall, J. H.; Cais,
R. E.; Bramwell, F. B. J. Am. Chem. Soc. 1978, 100, 7629.
(13) Shun, Z.; Wu, J. J. Mater. Chem. 2012, 22, 4151.
(22) Smith, M. J.; Bucher, G. J. Phys. Chem. A 2010, 114, 10712.
(23) Koelsch, C. F.; Anthes, J. A. J. Org. Chem. 1941, 6, 558.
~
(14) Quinones, W.; Escobar, G.; Echeverri, F.; Torres, F.; Rosero,
Y.; Arango, V.; Cardona, G.; Gallego, A. Molecules 2000, 5, 974.
Org. Lett., Vol. 15, No. 12, 2013
2971

containing radical 1. Upon evaporation of the solvent,
a black solid with a bronze luster is obtained. Both the
solution and the solid are paramagnetic and ESR active.
Figure 1 shows a highly resolved ESR spectrum obtained
using DMSO as solvent which shows hyperfine couplings.
The signal has a g-valueof2.0027anda totalwidthof32G.
The complexity is due to hyperfine coupling with the 11
nonequivalent protons of the NX molecule. However,
solutions of 1, generated either photochemically from 2
or by reduction of 5 with NaI in DMSO, are brown, not
green.24 The UV/vis spectrum of a sample of 1 generated
by NaI reduction of 5 in DMSO is shown in Figure 2.
Cation 5 has a sharp λmax at 457 nm, which is no longer
visible in the reduced sample. Instead, λmax values at 372,
395, and 412 nm were detected. Also, moderate absorption
is shown between 440 and 520 nm, and low absorption, up
to 700 nm.25 Given the absence of bands due to cation 5,
the synthesis of 1 from 5 using NaI appears essentially
quantitative.
Figure 2. UV/vis spectrum of 1 in DMSO at room temperature
showing a complete depletion of 5 after reduction using NaI at
40 °C after 3 h.
Investigation of 5 with cyclic voltammetry (CV) under
Ar revealed two reversible reduction/oxidation steps
(Figure 3). The potential for the reduction of the cation
resulting in the radical was E1/2 (1) = ꢀ0.52 V, and for the
formation of the anion resulting from the reduction
of the radical it was E1/2 (2) = ꢀ1.66 V (all potentials vs
the ferrocene/ferrocenium couple, Fc/Fcþ). For open-shell
polyaromatic species, the difference in redox potentials
between the cation/radical couple and the radical/anion
couple can be taken as a measure of the pairing energy
required to place a second electron in the SOMO of the
radical. In our case, we find that this value equates to 1.14
V, whichis in a rangetypical for phenalenyl-type radicals.29
Moreover, this magnitude represents an approximation
for the on-site Coulombic repulsion energy U, a parameter
which is important for molecular conductors in the solid
state.30
In contrast to the situation under Ar, when the CV
was recorded under an atmosphere of air, the redox wave
centered around ꢀ0.52 V remained unchanged and fully
reversible, but the second reduction at ꢀ1.66 V became
irreversible and multiple new peaks appeared below ꢀ1 V
vs ferrocene/ferrocenium. This suggests that (at least on
the time scale of the CV) radical 1 is stable under ambient
conditions, whereas the corresponding anion is prone to
rapid aerial degradation.
The kinetics of the redox processes shown in Figure 3
were probed by examining the scan-rate dependence
of CVs of 5 at a Pt microelectrode under an atmosphere
of Ar. As the scan rate was increased, the splitting of the
anodic and cathodic peak potentials increased only mar-
ginally for both reduction processes (cation þ electron f
neutral compound and then neutral compound þ electron
f anion), even at scan rates as high as 2000 V sꢀ1. The
resulting “trumpet plots” for these reduction processes
(see SI)31,32 allowed an electrochemical rate constant, k,
to be obtained for both reduction events of 5 by fitting the
Figure 1. ESR spectrum of 1 in degassed DMSO solution, ambient
temperature.
In the solid state, radical 1, generated by reduction
of 5, is remarkably stable, and the black material can be
heated under air to T = 495 K (melting point: T = 422 K)
with an increase of the ESR intensity.26 ESR spectra of
solid samples containing 1, thus generated, do not show
hyperfine coupling. The ESR spectrum of 1 obtained by
reduction of5 (shown inFigure1) and a spectrum obtained
by photolysis of 2 in benzene in the presence of TCNE
are identical. The ESR spectrum can be approximately
simulated using four different hyperfine coupling constants
of 5.9 (4H), 2.3 (2H), 1.9 (1H), and 1.5 G (2H), which
agree well with predictions from B3LYP/eprII calcula-
tions (see SI).27,28
(24) The green color of samples obtained in acetone or acetonitrile is
likely due to CT complexes between cation 5 and radical 1.
(25) We note that no σ-dimer formed from 1 would be expected to
absorb at λ = 700 nm.
(26) This might indicate some formation of dimers in the crystal
(π or less likely σ), which for entropic reasons is broken up at high T.
(27) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(29) Nakasuji, K.; Kubo, T. Bull. Chem. Soc. Jpn. 2004, 77, 1791.
(30) Torrance, J. B. Acc. Chem. Res. 1979, 12 (3), 79.
(31) Laviron, E. J. Electroanal. Chem. 1979, 101, 19.
(28) Barone, V. Structure, Magnetic Properties and Reactivities of
Open-Shell Species from Density Functional and Self-Consistent Hy-
brid Methods. In Recent Advances in Density Functional Methods: Part I;
Chong, D. P., Ed.; World Scientific Publishing Co. Pte. Ltd.: Singapore, 1996;
Vol. 1, p 287.
ꢁ
(32) Costentin, C.; Robert, M.; Saveant, J.-M.; Teillout, A.-L. Proc.
Natl. Acad. Sci. U.S.A. 2009, 106, 11829.
2972
Org. Lett., Vol. 15, No. 12, 2013

Electrocrystallization has proved to be a valuable tech-
niquefor studying so-called organic metals,36 suchasPLY.
During the process of electrocrystallization, ordered mo-
lecular assemblies are typically formed from mixed valence
salts, giving extended crystalline structures that continue
to grow on the electrode. Such materials are naturally
conductive and could even possess superconductive prop-
erties.37 When 5 was taken in an acetonitrile solution of
(NBu4)BF4 and subjected to galvanostatic electrolysis,
greenish-brown thread-like microcrystalline material was
observed growing on the Pt cathode, reminiscent of sam-
ples of 1 obtained by chemical reduction of 5 (see SI). The
similarity in appearance to 1 and deposition of the material
on the cathode suggest that reduction of 5 to a species
containing radical 1 is a key step in the formation of these
conductive crystals. Hence, although we were unable to
obtain crystals of sufficient quality to conduct an X-ray
structure analysis, the very fact that compound 5 is amen-
able to reductive electrocrystallization indicates that con-
ductive assemblies containing 1 can be readily obtained.
This bodes well for potential applications for compound 1
and its derivatives in molecular electronics.
Figure 3. Cyclic voltammogram of 5 at room temperature
obtained at a scan rate of 20 mV sꢀ1 on a glassy carbon working
electrode of area 0.071 cm2. Peaks are referenced to the Fc/Fcþ
couple (wave not shown).
experimental values to simulated CVs. Using the experi-
mentally determined diffusion coefficient for 5 (Do
=
(3.7 ( 0.6) ꢁ 10ꢀ6 cm2 sꢀ1 for the radical cation and
Do = (1.1 ( 0.1) ꢁ 10ꢀ5 cm2 sꢀ1 for the neutral compound,
see SI), and assuming a transfer coefficient of R = 0.5,
values of k were obtained by simulating CVs to produce
the curves shown in the SI, which best fit the experimental
data points.33 This analysis yielded a lower limit value
of k ≈ 0.3 cm sꢀ1 for the first redox event (at ꢀ0.52 V vs.
Fc/Fcþ) and k ≈ 1 cm sꢀ1 for the second reduction process.
The actual rates may in fact be faster than these lower
bounds; however, we were unable to scan at faster sweep
rates due to uncompensated resistance in the electroche-
mical cell, which acts to produce an artificial apparent
slowing of k.34 These data suggest that electron transfer
between both 5 and 1 on the one hand and the electrode
on the other is comparatively fast for an organic molecule,
at least in part due to extensive charge delocalization which
acts to minimize solvent reorganization factors during
electron transfer.35 This is turn suggests that, in solution
at least, the conversion between 5 and 1 at a Pt electrode is
both reversible and rapid.
In summary, naphthoxanthenyl (1) represents the first
example of a new class of stable oxygen-functionalized
phenalenyl-type free radicals. The synthetic approach de-
tailed herein provides great potential to prepare a plethora
of promising analogs or functionalized derivatives of 1 for a
variety of potential applications.
Acknowledgment. Part of this research was conducted
within the Glasgow Centre for Physical Organic Chemistry
in WestCHEM, as funded by EPSRC. Funding by the
Spanish Ministerio de Economıa y Competitividad (RyC-
´
2011-07637) is acknowledged. We wish to express our
gratitude to Micha Kunze and Dr. Chris Kay (University
College, London), for pointing out DMSO as solvent
for recording the ESR spectrum of 1. We thank Dr. Dirk
€
Grote (Ruhr-Universitat Bochum) for the simulation of
the ESR spectrum of 1.
Supporting Information Available. Available detailed
synthetic protocols for the syntheses of 4, 5, and 1; spectro-
scopic data and crystallographic data for 5. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
(33) Symes, M. D.; Surendranath, Y.; Lutterman, D. A.; Nocera,
D. G. J. Am. Chem. Soc. 2011, 133, 5174.
(34) Roullier, L.; Laviron, E. J. Electroanal. Chem. 1983, 157, 193.
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(35) Saveant, J.-M.; Tessier, D. Faraday Discuss. Chem. Soc. 1982,
74, 57.
(37) Coronado, E.; Day, P. Chem. Rev. 2004, 104, 5419.
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(36) Batail, P.; Boubekeur, K.; Fourmigue, M.; Gabriel, J.-C. P.
Chem. Mater. 1998, 10, 3005.
The authors declare no competing financial interest.
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