Fluorescent hexaaryl- and hexa-heteroaryl[3]-radialenes: Synthesis, structures, and properties

Article abstract of DOI:10.3762/bjoc.8.7

The syntheses of three new [3]radialenes - hexakis(3, 5-dimethylpyrazolyl)- , hexakis(3-cyanophenyl)-, and hexakis(3, 4-dicyanophenyl)[3]radialene (1-3) - are reported. Compound 3 is obtained in five steps with an excellent yield of 76% in the key step. C

Full text of DOI:10.3762/bjoc.8.7

Fluorescent hexaaryl- and hexa-heteroaryl[3]-
radialenes: Synthesis, structures,
and properties
Antonio Avellaneda, Courtney A. Hollis, Xin He and Christopher J. Sumby*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 71–80.
School of Chemistry & Physics, The University of Adelaide, Adelaide,
SA 5005, Australia. Phone: +61 8 8303 7406. Fax: +61 8 8303 4358
doi:10.3762/bjoc.8.7
Received: 25 October 2011
Accepted: 16 December 2011
Published: 11 January 2012
Email:
Christopher J. Sumby* - christopher.sumby@adelaide.edu.au
* Corresponding author
Associate Editor: H. Ritter
Keywords:
© 2012 Avellaneda et al; licensee Beilstein-Institut.
License and terms: see end of document.
anion–π interactions; cross-conjugated compounds; electron deficient
compounds; fluorescence; radialenes
Abstract
The syntheses of three new [3]radialenes – hexakis(3,5-dimethylpyrazolyl)-, hexakis(3-cyanophenyl)-, and hexakis(3,4-
dicyanophenyl)[3]radialene (1–3) – are reported. Compound 3 is obtained in five steps with an excellent yield of 76% in the key
step. Compared to that, the respective steps of the syntheses of 1 and 2 result in lower yields. All compounds adopt a double bladed
propeller conformation in solution. Compound 3 is considerably more electron deficient than previously reported hexaaryl[3]radi-
alenes, with reduction potentials of −0.06 and −0.45 V in CH2Cl2. The compounds mostly display red fluorescence with large
Stokes shifts.
Introduction
Cross-conjugated compounds are those which “contain three In turn, radialenes are cyclic cross-conjugated polyenes which
unsaturated groups, two of which though conjugated to a third exhibit a general formula of C2nH2n and contain n ring atoms
unsaturated centre are not conjugated to each other” [1,2]. Such and n exocyclic double bonds (Figure 1b) [9]. Like their acyclic
compounds display interesting physical properties and chem- dendralene analogues, radialenes have proven to be a signifi-
ical reactivity [1,2]. The parent dendralenes are acyclic cross- cant synthetic challenge. The parent compounds [3]- and
conjugated polyenes (Figure 1a) whose relative instability had [4]radialene were initially prepared in the 1960’s, however,
prevented them from being successfully produced in large quan- progress since then has been protracted [10-12]. Derivatives of
tities [3]. Recently, Sherburn reported the synthesis of the first [3]radialene containing aryl and heteroaryl moieties have
six members of the parent dendralene series on gram scales gained more interest than the parent compound itself due to
[4-8]. Initial studies have shown that the odd-numbered their increased stability. The preparation of hexaaryl[3]radi-
dendralenes are much more chemically reactive than the even- alenes, using Fukunaga’s method of reacting stabilised carban-
numbered dendralenes.
ions with tetrachlorocyclopropene [13,14], was originally
71

Beilstein J. Org. Chem. 2012, 8, 71–80.
π-system and a fluoride anion in its hexanuclear silver(I) cage
structure [18]. Hexakis(4-cyanophenyl)[3]radialene also shows
anion–π interactions involving the PF6− and ClO4− anions in the
solid-state [21]. Cyclic voltammetry experiments show that the
most electron deficient hexaaryl[3]radialene synthesised thus
far is hexakis(4-cyanophenyl)[3]radialene [17].
The focus of our current work is the synthesis of new hexaaryl-
and hexa-heteroaryl[3]radialenes which are more electron defi-
cient and, therefore, more disposed toward forming anion–π
interactions. Ultimately, the most desirable of these compounds
will be able to form capsular metallo-supramolecular assem-
blies. Herein, we outline the synthesis of three new [3]radialene
derivatives (1–3), including the most electron deficient hexa-
aryl[3]radialene observed thus far, and studies of their electro-
chemical and photophysical properties.
Results and Discussion
The first new [3]radialene compound targeted was
hexa(pyrazol-1-yl)[3]radialene. However, the required starting
material, 1-[(1H-pyrazol-1-yl)methyl]-1H-pyrazole, undergoes
non-specific lithiation with n-butyllithium [27]. As selective
lithiation at the methane position is required, bis(3,5-
dimethylpyrazol-1-yl)methane [28] – where the 3- and 5-posi-
tions are blocked with methyl groups – was used to give
hexakis(3,5-dimethylpyrazol-1-yl)[3]radialene (1) via Fuku-
Figure 1: The structures of a) the parent [3]-, [4]-, [5]-, and
[6]dendralenes and b) the corresponding radialenes. The structure of
c) hexakis(3,5-dimethylpyrazolyl)[3]radialene (1) and d) hexakis(3-
cyanophenyl)[3]radialene (2) and hexakis(3,4-dicyanophenyl)[3]radia-
lene (3).
reported by Oda [15,16]. Hexapyridyl[3]radialenes were naga’s method in 14% yield. The poor yield for this reaction
obtained shortly afterwards with the synthesis of hexa(2- was attributed to the considerable steric hindrance deriving
pyridyl)[3]radialene being reported concurrently by Oda and from the 12 methyl substituents and, despite considerable effort,
Steel [17,18].
could not be optimised further. The compound was readily iden-
tified by its intense yellow–green solutions and the consider-
The coordination chemistry of hexaaryl- and hexa- able upfield shifts of the pyrazole H4 proton (5.69 ppm) and the
pyridyl[3]radialenes has been studied to a limited extent [18- methyl hydrogen atoms (1.80 and 2.07 ppm) arising from the
21]. Three coordination modes were observed for hexa(2- double-bladed propeller conformation adopted by 1.
pyridyl)[3]radialene with Ag(I): A discrete M6L2 cage, a 1-D
coordination polymer composed of M3L2 cages bridged by In addition to the study of [3]radialenes with heterocyclic
linear silver atoms, and a second 1-D coordination polymer donors [18,19], we have devoted substantial attention to the
where the radialene ligand acts as a tetradentate bridge [18,19]. study of [3]radialenes with nitrile donors in the 4-position of the
Hexa(4-pyridyl)[3]radialene also acts as a bridging ligand in a aryl ring [21] and thus, undertook the synthesis of the isomeric
3-D coordination polymer with AgClO4 [20], while isomor- compound, hexakis(3-cyanophenyl)[3]radialene (2). The
phous 6,3-connected 2-D coordination polymers were obtained precursor 6 was synthesised from 3,3’-diaminodiphenyl-
when hexakis(4-cyanophenyl)[3]radialene was reacted with methane (4) [29-31] in two steps: diazotisation and reaction
AgPF6 and AgClO4 [21].
with potassium iodide to afford 3,3’-diiododiphenylmethane (5)
[32] followed by treatment with copper cyanide to give 3,3’-
Anion–π interactions have recently received much attention dicyanodiphenylmethane (6) rather than the literature route via
[22-26], where the π-system is typically an electron deficient diphenylmethane-3,3’-dicarboxamide [33]. [3]Radialene 2 was
heterocyclic system. The electron deficient nature of the hexa- obtained from 6 in 16% yield (Scheme 1a). The marked differ-
aryl[3]radialenes is borne out in structures of these compounds ence in yields between 2 and hexakis(4-cyanophenyl)[3]radia-
which also show anion–π interactions and CH···Xanion hydrogen lene, 73% reported by Oda [15], is a consequence of the rela-
bonding in the solid-state [17,21]. Hexa(2-pyridyl)[3]radialene tive stability of the carbanions of the two dicyanodiphenyl-
exhibits a very short contact (2.67 Å) between the radialene methane precursors.
72

Beilstein J. Org. Chem. 2012, 8, 71–80.
Scheme 1: Synthesis of (a) hexakis(3-cyanophenyl)[3]radialene (2) and (b) hexakis(3,4-dicyanophenyl)[3]radialene (3).
To maintain a high yielding synthesis whilst investigating the Small red crystals of compound 3 [37], that are suitable for
substitution in the 3-position, a new strategy was developed X-ray crystallography, were obtained by slow evaporation in
which involved the retention of an electron withdrawing group acetonitrile. The ligand crystallises in the monoclinic space
in the 4-position while incorporating new substitutents at the group P21/c and the asymmetric unit contains one molecule of
3-position. The simplest demonstration of this approach is the the radialene ligand and an acetonitrile solvate molecule
preparation of hexakis(3,4-dicyanophenyl)[3]radialene (3), (Figure 2). The small crystals were weakly diffracting but the
which was synthesised in excellent yield (76%, Scheme 1b). structure refined to R1 6.82% with no significant disorder prob-
The precursor methane 11 was obtained from 7 in four steps: lems. As expected, the [3]radialene core is planar and the
hydrogenation on 5% Pd/C to 8 [34], followed by reaction with “arms” extend in a double-bladed propeller conformation with
formamide to give the corresponding diimide 9, treatment with torsion angles of ca. 39° on average which is common for hexa-
aqueous ammonia solution to yield the tetraamide 10 and finally aryl[3]radialenes [18-20]. The nitrile substituents in the 4-pos-
dehydration to the tetranitrile 11 using thionyl chloride [35,36]. ition extend directly out from the structure in approximately the
Initial attempts to synthesise 3 using the standard procedure same plane as the core, whereas those in the 3-position are situ-
[15,16] were unsuccessful. In fact, the far greater acidity of 11 ated above and below the plane. In the conformation observed,
(compared to 6 or 4,4’-dicyanodiphenylmethane) allowed the four are up and two are directed down. This type of con-
use of sodium hydride instead of n-butyllithium as a base in the formation is less commonly encountered as symmetrical
synthesis of 3. Following aerial oxidation of the dianion, 3 was arrangements of multi-armed compounds, those with no net
isolated by precipitation in saturated ammonium chloride solu- dipole, are more favoured [38]. Bond lengths and angles about
tion.
the central core are consistent with a [3]radialene derivative.
73

Beilstein J. Org. Chem. 2012, 8, 71–80.
by reduction. Up to now, the most electron deficient
hexaaryl[3]radialene reported, hexakis(4-cyano-phenyl)[3]radi-
alene, recorded reduction potentials of −0.63 and −1.03 V in
dichloromethane (Table 1) [17]. Isomer 2 is slightly harder to
reduce than hexakis(4-cyanophenyl)[3]radialene, with poten-
tials of −0.80 and −1.32 V in dichloromethane. This is due to
the inability of the nitrile groups in the 3-position to stabilise
the anion by resonance delocalisation. A similar effect on the
reduction potentials is observed for the pyridyl series
[18,20,41], whereby the change from a 2-substituted pyridine to
a 3-substituted azine ring system results in a compound that is
0.10 V more difficult to reduce. Conversely, compound 3 is
very easy to reduce. This is shown by its reduction potentials of
−0.06 and −0.45 V, respectively, and is attributed to the
combined electron-withdrawing effect of the twelve nitrile
substituents. While compound 3 is considerably more electron
deficient than other hexaaryl[3]radialenes, it still exists in its
neutral form. In contrast, other related [3]radialene compounds
– such as hexacyano[3]radialene [13] – exist as stable dianions.
Among the [3]radialene compounds we have been studying, this
Figure 2: A perspective view of the asymmetric unit of 3.
Each [3]radialene forms three lone pair-nitrile to electron defi- attributes a ‘goldilocks’-type electron deficiency to compound 3
cient [3]radialene core carbon contacts [22-26] within the struc- making it suitable for investigating anion–π interactions. The
ture, with distances of 3.205, 3.219 and 3.267 Å.
compound is considerably electron deficient, however, it is still
maintained in its neutral form. This will allow us to further
[3]Radialene compounds have been shown to undergo two one- study anion–π interactions with these systems [21]. The oppo-
electron reductions to yield the radical anion and dianion site extreme exists for compound 1 because the electron rich
species [1]. In this context, in materials science [1,39,40], there azole rings and methyl substituents make it particularly hard to
is considerable interest in the use of the carbanions obtained reduce. Its first reduction potential is −1.21 V and its second
Table 1: Electrochemical potentials for the synthesised [3]radialenes 1–3 and related compounds.
compound – [3]radialene
E(1)
E(2)
reference
1
−1.21a,b
−1.29c
—
this work
[17]
hexakis(4-bromophenyl)-
hexakis(4-carbomethoxyphenyl)-
−1.77c
−1.03c
−1.33c
[17]
−0.93a,b
−1.15c
−1.29a,b
−1.55c
[41]
hexa(2-pyridyl)-
hexa(3-pyridyl)-
[17]
−1.03a,b
−1.17c
−1.48a,b,d
−1.64c
[41]
[17]
hexa(4-pyridyl)-
−1.02a,b
−0.80a,b
−0.63a,b
−0.86d
−1.33a,b,d
−1.32a,b,d
−1.03a,b
−1.11d
−0.45a,b
+0.34e
[41]
2
this work
this work
[17]
hexakis(4-cyanophenyl)-
3
−0.06a,b
+1.13e
this work
[13]
hexacyano-
aPotentials (V) measured in CH2Cl2/0.1 mol·L−1 [(n-C4H9)4]NPF6 (the ferrocene/ferrocenium couple occurred at +0.46 V vs Ag/Ag+).
bUncertainty in E1/2 values ca. ± 0.02 V.
cPotentials (V) measured in DMF/0.1 mol·L−1 [(n-C4H9)4]NClO4 (the ferrocene/ferrocenium couple occurred at +0.16 V vs Ag/Ag+).
dIrreversible (approximate value estimated from anodic half-scan).
ePotentials (V) measured in CH3CN/0.1 mol·L−1 (various electrodes and supporting electrolytes used).
74

Beilstein J. Org. Chem. 2012, 8, 71–80.
was not observed as it occurs outside the solvent window for The fluorescence of hexaaryl[3]radialenes has not been widely
dichloromethane.
studied [42]. Solutions of compound 1 visibly fluoresce bright
blue; its fluorescence maximum being 467 nm in
Most hexaaryl[3]radialenes are orange or red in colour with dichloromethane with a Stokes shift of 124 nm (Table 2). The
UV–visible absorption maxima in the range of 460–490 nm in absorption and fluorescence properties of compound 3 had to be
dichloromethane (Table 2, Figure 3a), with 2 and 3 being measured in acetone due to its limited solubility in
consistent with this observation. Compared to that, compound 1 dichloromethane (Figure 3). Hexakis(4-cyanophenyl)[3]radia-
is a yellow–brown solid with a λmax of 443 nm which dissolves lene and compounds 2 and 3 also exhibit large Stokes shifts, of
to produce yellow–green solutions. In dichloromethane the at least 130 nm, for their fluorescence maxima in acetone. The
absorbance maxima for 1 is around 443 nm (logε = 4.33), with a large Stokes shifts are consistent with the HOMO of the [3]radi-
shoulder around 415 nm. In contrast to that, the UV–visible alene being located predominantly on the exocyclic double
spectra in acetone shows two absorption peaks at around 440 bonds [40]. As a consequence, the electronic structure of the
and 475 nm, as well as a broad tail that extends out to approxi- excited state is considerably more polar than that of the ground
mately 600 nm. The fact that the concentrations of both solu- state.[42] This is consistent with the Stokes shift being
tions of 1 are similar (~0.04 mM) indicates that 1 – which has 10–20 nm larger for the spectra measured in acetone and with
12 methyl substituents arranged above and below the [3]radia- calculations that show intramolecular charge transfer character
lene core – aggregates in the more polar acetone solvent in the lowest excited states for hexaaryl[3]radialene derivatives
(polarity index for dichloromethane 3.1; acetone 5.1). The less [42]. In the excited state, rotation about the exocyclic bonds of
hydrophobic [3]radialenes, 2 and 3, do not show the same the cyclopropane ring is also possible. The fluorescence emis-
behaviour. The more electron deficient derivatives, 2 and 3, sion of the hexaaryl[3]radialenes, coupled with the large Stokes
along with the previously synthesised compounds hexa(2- shifts, suggests that these compounds would be useful as
pyridyl)[3]radialene, hexa(3-pyridyl)[3]radialene, and sensor components. Indeed, our efforts to investigate anion–π
hexakis(4-cyanophenyl)[3]radialene, show a weak absorption in interactions [43] are being undertaken with a view to utilise
the near infrared (around 800 nm). This is consistent with [3]radialenes as a building block for the synthesis of anion
spectra observed for the radical anions [17].
sensors.
Table 2: Visible absorption maxima, molar extinction coefficient ε and fluorescence emission maxima for various hexaaryl[3]radialene compounds.
compound – [3]radialene
hexa(2-pyridyl)-
λmax (nm)
logε
fluorescence max (nm)a
reference
464a
463b
465a
464b
463a
443a
440b
461a
461b
489a
487b
493b
467a
483a
485a
492a
4.53
4.25
4.48
4.16
4.62
4.33
4.13
4.21
4.24
4.42
4.37
4.39
4.42
4.68
4.64
4.73
555a
564b
this work
this work
this work
this work
[41]
585a
hexa(3-pyridyl)-
hexa(4-pyridyl)-
1
598b
not reported
467a
this work
this work
this work
this work
this work
this work
this work
[42]
468b
576a
2
595b
620a
hexakis(4-cyanophenyl)-
626b
3
625b
hexaphenyl-
617a
hexakis(4-chlorophenyl)-
hexakis(4-bromophenyl)-
hexakis(4-iodophenyl)-
606a
[42]
not reported
not reported
[15]
[15]
aUV–visible and fluorescence spectra measured in dichloromethane.
bUV–visible and fluorescence spectra measured in acetone.
75

Beilstein J. Org. Chem. 2012, 8, 71–80.
Figure 3: (a) UV–visible (bold line) and fluorescence (dashed line) spectra of 1, 2, hexa(2-pyridyl)[3]radialene, hexa(3-pyridyl)[3]radialene, and
hexakis(4-cyanophenyl)[3]radialene in dichloromethane. (b) UV–visible (bold line) and fluorescence (dashed line) spectra of 1, 2, 3, hexa(2-
pyridyl)[3]radialene, hexa(3-pyridyl)[3]radialene, and hexakis(4-cyanophenyl)[3]radialene in acetone.
Conclusion
steric hindrance, compound 3 is obtained with an excellent yield
In summary, this work has shown that the range of the avail- of 76% in the key step. In contrast to that, the final steps of the
able [3]radialene derivatives can be extended. However, these syntheses of 1 and 2 are more difficult because the precursors
extensions are limited depending on the nature and position of have greater steric bulk and do not form carbanions with the
the substituents. It is necessary to maintain an electron-with- requisite stability, respectively. In this regard, precursor active
drawing group in the 4-position of the aryl ring and to limit the methylene compounds with functional groups that are able to
steric bulk about the core. As the precursor methane forms a coordinate transition metals in the 3-position and a nitro group
very stable carbanion and the substituents provide minimal para to the methylene are under investigation in our laboratory.
76

Beilstein J. Org. Chem. 2012, 8, 71–80.
Compounds 2 and 3 are electron deficient, 3 noticeably so, and platinum wire auxiliary and pseudo-reference electrodes.
undergo facile reductions to their radical anion and dianion Ferrocene was added as an internal standard on completion of
species. We expect that the extension of these compounds in the each experiment and tabulated potentials are given vs the satu-
3-position will lead to the formation of new cage structures [18] rated calomel electrode [E0(Fc/Fc+) = 460 mV vs SCE
which will utilise the electron deficient nature of the [3]radia- (dichloromethane)]. Cyclic voltammetry was performed with a
lene core to act as anion receptors. The useful electrochemical sweep rate of 100 mVs−1.
and fluorescence properties of these compounds may allow
them to be employed as building blocks of anion sensors.
X-ray crystallography
Crystals were mounted under oil on a loop and X-ray diffrac-
tion data were collected at 150(2) K with Mo Kα radiation (λ =
0.71073 Å) using an Oxford Diffraction X-Calibur Diffrac-
Experimental
General experimental
Melting points were determined using a Gallenkamp variable tometer fitted with an Eos CCD detector. The data set was
heat melting point apparatus and are uncorrected. UV–visible corrected for absorption using a multi-scan method. Structures
absorption spectra were recorded on a Varian CARY 5000 were solved by direct methods using SHELXS-97 [45] and
spectrophotometer. Samples were dissolved in dichloromethane refined by full-matrix least squares on F2 by SHELXL-97 [46],
or acetone at a concentration of approximately 0.03 mM. Fluo- interfaced through the program X-Seed [47]. In general, all non-
rescence spectra were recorded on a Varian CARY eclipse hydrogen atoms were refined anisotropically and hydrogen
spectrophotometer. Samples were dissolved in dichloromethane atoms were included as invariants at geometrically estimated
or acetone at a concentration of approximately 0.01 mM. positions. CCDC 824692 contains the supplementary crystallo-
Infrared spectra were recorded using a Perkin Elmer Spectrum graphic data for this structure. These data can be obtained free
100 FTIR spectrometer with universal ATR sampling acces- of charge from The Cambridge Crystallographic Data Centre
sory. The Campbell microanalytical laboratory at the Univer- via http://www.ccdc.cam.ac.uk/data_request/cif.
sity of Otago performed the elemental analyses.
Synthetic procedures
Low resolution electrospray ionisation mass spectra (ESIMS) Hexakis(3,5-dimethylpyrazolyl)[3]radialene (1). Bis(3,5-
were recorded on a Finnigan LCQ mass spectrometer. Samples dimethylpyrazol-1yl)methane (2.00 g, 9.8 mmol) was placed in
were dissolved in HPLC grade methanol or acetonitrile with a a dry two-necked flask under nitrogen. Dry tetrahydrofuran
concentration of 0.01 mg/cm3. High resolution electrospray (40 mL) was added and the solution was cooled to −78 °C.
ionisation mass spectroscopy (ESI-HRMS) was performed by n-Butyllithium (4.0 mL of a 2.5 M solution in hexane) was
the Adelaide Proteomics Centre using an LTQ Orbitrap XL added slowly and the reaction mixture was stirred for 30 min.
ETD spectrometer. 1H NMR spectra were recorded on a Varian Tetrachlorocyclopropene (0.2 mL 1.6 mmol) was added and the
Gemini 300 MHz spectrometer (75 MHz for 13C NMR) or a solution was stirred at −78 °C for 1 h, at 0 °C for 30 min and at
Varian Inova 600 MHz spectrometer (150 MHz for 13C NMR). room temperature for 30 min. The mixture was again cooled to
1H NMR spectra recorded in CDCl3 were referenced to the 0 °C and oxygen bubbled through it for 30 min at 0 °C and then
internal standard Me4Si (0 ppm). 1H NMR spectra recorded in for 1 h at room temperature. The resultant brown solution
DMSO-d6 and acetone-d6 were referenced to the solvent peaks was quenched with water (40 mL) and extracted with
2.50 ppm and 2.05 ppm, respectively. 13C NMR spectra dichloromethane (50 mL) followed by further dichloromethane
recorded in CDCl3, DMSO-d6 and acetone-d6 were referenced (5 × 25 mL). The organic extracts were combined, dried over
to the solvent peaks 77.0 ppm, 39.5 ppm and 29.9 ppm, respect- anhydrous magnesium sulfate, and the solvent was evaporated
ively. Unless otherwise stated, reagents were obtained from to yield a red–brown oil. Purification via alumina chromatog-
commercial sources and used as received. Bis(3,5- raphy eluting with 9:1 CH2Cl2:MeOH, followed by silica chro-
dimethylpyrazol-1-yl)methane [28] and 3,3’-diaminodiphenyl- matography eluting with 9:1 CHCl3:MeOH yielded 1 as a
methane [29-31] were synthesised according to literature pro- yellow–brown solid (150 mg, 14%). Mp 220 °C dec; 1H NMR
cedures. Solvents were dried by literature procedures [44] and (300 MHz, CDCl3) δ 1.80 (s, 18H, CH3), 2.07 (s, 18H, CH3),
freshly distilled as required.
5.69 (s, 6H, H4); 13C NMR (75 MHz, CDCl3) δ 10.6, 13.6,
102.6, 107.6, 118.2, 141.5, 150.4; HRMS (m/z): [M + H+] calcd
for C36H43N12, 643.37282; found, 643.37632.
Cyclic voltammetry
Cyclic voltammetry measurements were performed on a PAR
Model 263A potentiostat under nitrogen. Measurements were 3,3’-Diiododiphenylmethane (5). Based on related procedures
recorded on 1 mM solutions in dichloromethane/0.1 M [(n- [21,32], 3,3’-diaminodiphenylmethane (0.62 g, 3.1 mmol) in
C4H9)4]NPF6] solution using a platinum working electrode, concentrated sulfuric acid (10 mL) was stirred at 0 °C. Sodium
77

Beilstein J. Org. Chem. 2012, 8, 71–80.
nitrite (0.62 g, 9.0 mmol) in water (6 mL) was added dropwise solid (40 mg, 16%). Mp 321 °C dec; 1H NMR (300 MHz,
over a period of 10 min. The resultant solution was stirred at CDCl3) δ 6.96 (s, 6H, H2), 7.23 (d, J = 7.9 Hz, 6H, H6), 7.30 (t,
0 °C for 30 min followed by the addition of potassium iodide J = 7.9 Hz, 6H, H5), 7.59 (d, J = 7.9 Hz, 6H, H4); 13C NMR
(3.62 g, 21.8 mmol) in water (40 mL). The reaction mixture was (75 MHz, CDCl3): δ 112.5, 117.9, 118.9, 123.0, 129.5, 132.3,
heated at 50 °C for 1 h, then cooled to room temperature and 134.0, 140.6; HRMS (m/z): [M + H+] calcd for C48H25N6,
neutralized with aqueous sodium hydroxide solution (50% w/v, 685.21352; found, 685.21196; FTIR vmax/cm−1: 2228 (C≡N);
5 mL). The mixture was extracted with dichloromethane (6 × 10 Anal. calcd for C48H24N6·H2O C, 82.03; H, 3.74; N, 11.96%;
mL). The organic fractions were combined and washed with found for C, 81.54; H, 4.31; N, 10.90.
1 M hydrochloric acid solution (20 mL), 1 M sodium thiosul-
fate solution (20 mL), dried over anhydrous magnesium sulfate 4,4’-Methyldiphthalic acid (8). Based on the procedure
and the solvent was evaporated under vacuum to yield a brown outlined [34], 4,4’-carbonyldiphthalic acid (4.4 g, 12.3 mmol)
solid. Purification via silica chromatography eluting with was dissolved in ethanol (100 mL). 5% Pd/C (0.88 g) was
hexane yielded 3,3’-diiododiphenylmethane as white needles added and the resultant mixture was heated at reflux under a
(0.73 g, 55%). Mp 63–65 °C; 1H NMR (300 MHz, CDCl3) δ hydrogen atmosphere for 1 week. After cooling the mixture was
3.85 (s, 2H, CH2), 7.03 (t, J = 7.8 Hz, 2H, H5), 7.12 (d, J = 7.8 filtered and the filtrate was concentrated under reduced pres-
Hz, 2H, H6), 7.53–7.70 (overlapped d and s, 4H, H2, H4); 13C sure. The residue was dissolved in water (30 mL), basified with
NMR (75 MHz, CDCl3) δ 40.9, 94.7, 128.2, 130.3, 135.5, sodium hydroxide (4.0 g) and heated at reflux for 1 h. Upon
137.8, 142.6; ESIMS (m/z): [M + H+] 420.1.
cooling the solution was acidified with dilute sulfuric acid and
concentrated to 30 mL under reduced pressure which resulted in
3,3’-Dicyanodiphenylmethane (6). A mixture of 3,3’- the precipitation of a white solid. Filtration and subsequent
diidodiphenylmethane (0.71 g, 1.7 mmol) and copper cyanide drying in a desiccator overnight yielded 8 as a white solid (4.06
(0.47 g, 4.1 mmol) in dry DMF (15 mL) was heated at 100 °C g, 95%). Mp 255 °C (lit. 250 °C [21]); 1H NMR (300 MHz,
for 2 d. The cooled reaction mixture was diluted with ethyl DMSO-d6) δ 4.09 (s, 2H, CH2), 7.39 (d, J = 8.0 Hz, 2H, H5),
acetate (40 mL) and the resultant solution was washed with 7.81 (s, 2H, H3), 7.91 (d, J = 8.0 Hz, 2H, H6); 13C NMR (75
concentrated ammonia solution (30 mL), water (20 mL) and MHz, DMSO-d6) 130.7, 131.0, 131.4, 131.8, 134.6, 143.3,
brine (20 mL). Then, it was dried over anhydrous magnesium 168.0, 168.3, DMSO peak obscures CH2 carbon; ESIMS (-ve
sulfate and the solvent evaporated under vacuum to yield 6 as mode) (m/z): 343.0 ([M − H]−); FTIR vmax/cm−1: 3435 (O–H),
an off-white solid (0.33 g, 90%). Mp 151–153 °C (lit. 153–154 1656 (C=O).
°C [20]); 1H NMR (300 MHz, CDCl3) δ 4.05 (s, 2H, CH2),
7.41–7.48 (m, 6H, H2, H4, H5), 7.56 (d, J = 7.8 Hz, 2H, H6); 4,4’-Methyldiphthalimide (9) [48]. 4,4’-Methyldiphthalic acid
13C NMR (75 MHz, CDCl3) δ 40.8, 112.9, 118.5, 129.6, 130.5, (4.0 g, 11.6 mmol) was suspended in formamide (35 mL) and
132.3, 133.3, 140.9; ESIMS (m/z): [M + H+] 218.2; FTIR vmax/ stirred at 190 °C for 2 h, then at 150 °C for 1 h, before being
cm−1: 2224 (C≡N).
cooled to room temperature. The resultant precipitate was
collected via filtration and washed thoroughly with water.
Hexakis(3-cyanophenyl)[3]radialene (2). 3,3’-Dicyan- Drying in a desiccator overnight yielded 9 as a white powder
odiphenylmethane (0.5 g, 2.28 mmol) was placed in a dry two- (3.01 g, 85%). Mp >280 °C. 1H NMR (300 MHz, DMSO-d6) δ
necked flask under argon. Dry tetrahydrofuran (20 mL) was 4.33 (s, 2H, CH2), 7.77–7.80 (m, 6H), 11.27 (s, 2H, NH), 13C
added and the solution was cooled to −78 °C. n-Butyllithium NMR (75 MHz, DMSO-d6) 40.6, 123.2, 123.3, 130.8, 133.3,
(1.04 mL of a 2.2 M solution in hexane) was added slowly and 134.7, 147.4, 169.0, 169.1; ESIMS (m/z): [M + H+] 307.4;
the reaction mixture was stirred for 30 min. Tetrachlorocyclo- FTIR: vmax/cm−1: 3185 (N–H), 1770 and 1712 (C=O).
propene (46 μL, 0.38 mmol) was added and the solution was
stirred at −78 °C for 1 h, at 0 °C for 30 min and at room 4,4’-Methyldiphthalamide (10) [48]. 4,4’-Methyldiphthal-
temperature for 30 min. The mixture was again cooled to 0 °C imide (2.98 g, 9.7 mmol) was finely crushed and suspended in
and oxygen bubbled through it for 30 min at 0 °C and then for 28% concentrated ammonia solution (40 mL). The flask was
1 h at room temperature. The resultant brown solution stoppered and the mixture stirred 2 d at room temperature. The
was quenched with water (40 mL) and extracted with resultant mixture was filtered and the precipitate was washed
dichloromethane (30 mL) followed by further dichloromethane thoroughly with water. Drying in a desiccator overnight yielded
(5 × 10 mL). The organic extracts were combined and dried 10 as a white powder (2.82 g, 85%). Mp > 280 °C; 1H NMR
over anhydrous magnesium sulfate, then, the solvent was evap- (300 MHz, DMSO-d6) δ 4.01 (s, 2H, CH2), 7.29–7.43 (m,
orated to yield a brown oil. Purification via silica chromatog- 10H), 7.67–7.69 (m, 4H); 13C NMR (75 MHz, DMSO-d6)
raphy eluting with 1:2 EtOAc/hexane yielded 2 as an orange 127.9, 129.3, 133.8, 136.7, 142.0, 169.9, 170.2, DMSO peak
78

Beilstein J. Org. Chem. 2012, 8, 71–80.
obscures CH2 carbon; ESIMS (-ve mode) (m/z): 3 ([M − H+]−)
40.8; FTIR: vmax/cm−1: 3329 and 3176 (N–H), 1693 and 1649
(C=O).
Supporting Information
Supporting Information File 1
1H and 13C NMR spectra of all compounds, cyclic
voltammograms of hexaaryl[3]radialenes, crystal data and
structure refinement for 3.
4,4’-Methyldiphthalonitrile (11). 4,4’-Methyldiphthalamide
(2.78 g, 8.2 mmol) was suspended in DMF (40 mL) and the
mixture was cooled to −20 °C. Sulfonyl chloride (20 mL) was
added dropwise ensuring the temperature remained below 0 °C.
The resultant mixture was stirred at 0 °C for 2 h and then
allowed to warm to room temperature overnight. The reaction
mixture was slowly poured onto crushed ice (400 g) and stirred
until the ice was completely melted. The resultant precipitate
was collected via filtration, washed thoroughly with water and
dried in a desiccator. Purification via silica chromatography
eluting with dichloromethane yielded 11 as a white powder
(1.96 g, 89%). Mp 205 °C; 1H NMR (300 MHz, acetone-d6) δ
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-7-S1.pdf]
Supporting Information File 2
Crystallographic information file for acetonitrile solvate of
hexakis(3,4-dicyanophenyl)[3]radialene.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-7-S2.cif]
4.45 (s, 2H, CH2), 7.92 (m, 2H), 8.04 (m, 4H); 13C NMR (75 Acknowledgements
MHz, acetone-d6) 41.5, 114.7, 116.4 (two signals overlapped), C. J. S. thanks the Australian Research Council for a Future
116.8, 135.1, 135.4 (two signals overlapped), 146.9; ESIMS (m/ Fellowship (FT0991910) and for supporting this research
z): ([M − H]−) 267.1; FTIR: vmax/cm−1: 2233 (C≡N); Anal. (DP0773011). A. A. thanks the University of Adelaide for a
calcd for C17H8N4 C, 76.10; H, 3.01; N, 20.89%; found C, visiting research fellowship.
75.82; H, 3.01; N, 21.10.
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License and Terms
27.Diez-Barra, E.; de la Hoz, A.; Sánchez-Migallón, A.; Tejeda, J.
J. Chem. Soc., Perkin Trans. 1 1993, 1079–1083.
doi:10.1039/P19930001079
This is an Open Access article under the terms of the
Creative Commons Attribution License
28.Hill, M. S.; Mahon, M. F.; McGinley, J. M. G.; Molloy, K. C. Polyhedron
2001, 20, 1995–2002. doi:10.1016/S0277-5387(01)00799-9
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Bioorg. Med. Chem. 2004, 12, 2759–2772.
(http://creativecommons.org/licenses/by/2.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
doi:10.1016/j.bmc.2004.02.040
30.Preston, P. N.; Jigaginni, V. B.; Soutar, I.; Woodfine, B.; Stewart, N. J.;
Hay, J. N. Polymer 1994, 35, 2378–2384.
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
doi:10.1016/0032-3861(94)90776-5
(http://www.beilstein-journals.org/bjoc)
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The definitive version of this article is the electronic one
which can be found at:
doi:10.3762/bjoc.8.7
33.Bartlett, R. K.; O’Neill, G.; Savill, N. G.; Thomas, S. L. S.; Wall, W. F. B.
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37.Structure 3: C56H21N13, FW 875.86, monoclinic, P21/c, a =
13.7345(13), b = 12.6611(9), c = 25.128(2) Å, b = 93.942(8)°, V =
4359.3(6) Å−3, Z = 4, D(calc) = 1.335 Mg/m3, µ = 0.084 mm−1, F(000) =
1792, red block, 0.150 × 0.128 × 0.040 mm3, q range 2.67 to 23.26°,
reflections collected 28680, independent reflections 6241 [Rint =
0.1149], completeness 99.9%, parameters 623, GoF 1.025, R1 0.0682,
wR2 = 0.1608.
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80