Synthesis and characterization of 2,7-bis(2-pyridyl)-1,8-diazaanthraquinone - A redox-active ligand designed for the construction of supramolecular grids

Article abstract of DOI:10.1139/V06-094

Double condensation of 2-acetylpyridine with 1,3-diaminobenzene-4,6- dicarboxaldehyde affords 2,7-bis(2-pyridyl)-1,8-diazaanthracene, which was subsequently oxidized to the corresponding quinone. Electrochemical studies indicate two reversible reduction processes corresponding to semiquinone and hydroquinonate formation. Electron-withdrawing pyridine groups and the nitrogen atoms make this somewhat more easily reduced than anthraquinone. This compound is redox-active and can be reduced to its radical anion, a potential spin-bearing ligand for the construction of [2 x 2] metallo-grid structures.

Full text of DOI:10.1139/V06-094

1263
Synthesis and characterization of 2,7-bis(2-pyridyl)-
1,8-diazaanthraquinone — A redox-active ligand
designed for the construction of supramolecular
grids1
Rajsapan Jain, Sharon L. Caldwell, Anika S. Louie, and Robin G. Hicks
Abstract: Double condensation of 2-acetylpyridine with 1,3-diaminobenzene-4,6-dicarboxaldehyde affords 2,7-bis(2-
pyridyl)-1,8-diazaanthracene, which was subsequently oxidized to the corresponding quinone. Electrochemical studies
indicate two reversible reduction processes corresponding to semiquinone and hydroquinonate formation. Electron-
withdrawing pyridine groups and the nitrogen atoms make this somewhat more easily reduced than anthraquinone. This
compound is redox-active and can be reduced to its radical anion, a potential spin-bearing ligand for the construction
of [2 × 2] metallo-grid structures.
Key words: quinone, grid, supramolecular, bistridentate, electrochemistry, metallosupramolecular chemistry.
Résumé : La double condensation de la 2-acétylpyridine avec du 1,3-diaminobenzène-4,6-dicarboxaldéhyde conduit à
la formation de 2,7-bis(2-pyridyl)-1,8-diazaanthrène qui s’oxyde subséquemment en quinone correspondante. Des études
électrochimiques indiquent qu’il existe deux processus de réductions réversibles qui correspondent à la formation de la
semiquinone et de l’hydroquinonate. Par comparaison avec l’anthraquinone, les groupes pyridines et les atomes d’azote
électroattracteurs rendent cette réduction plus facile. Ce composé est actif dans un système redox et il peut être réduit
en anion radical, un ligand qui pourrait s’avérer utile comme porteur de spin dans la construction de structures en grilles
métalliques [2 × 2].
Mots clés : quinone, grille, supramoléculaire, bistridentate, électrochimie, chimie métallosupramoléculaire.
[Traduit par la Rédaction] Jain et al. 1267
Introduction
multiple polydentate binding sites situated so as to enforce a
regular m × n array of octahedrally or tetrahedrally coordi-
nated metal ions. Figure 1 schematically depicts the assem-
bly of a 2 × 2 grid based on bisbidentate or bistridentate
ligands in conjunction with tetrahedrally or octahedrally co-
ordinated metal ions, respectively. A huge number of grids
have been prepared, many of which form in high yield simply
from reactions of the free ligand and a metal ion source. The
structural diversity is impressive and includes larger grids (n
× n, n ≥ 3) (4), rectangular (n × m, n ≠ m) grids, grids with
incompletely filled metal sites (5), and even grids with ex-
traneous (noncoordinated) ligands intercalated into the grid (6).
Beyond their attractive structural features, the properties
of grid molecules have been heavily studied. The incorpora-
tion of redox-active metals into grid structures can lead to a
rich redox chemistry (7), and the magnetic (8) properties of
these materials have also been investigated. In all these cases
the redox- or spin-active component of the grids are the
metal ions; the ligands play an important structural role but
can be described as only passive contributors to the electronic
functionality of the grids. No examples of metallosupra-
molecular grids based on redox-active or spin-containing
ligands have yet been reported, although we have described
the synthesis of a stable radical containing an analogue of 2
designed for such purposes (9).
The designed assembly of metal–ligand architectures —
metallosupramolecular chemistry — is an immensely popu-
lar contemporary facet of coordination chemistry. Judicious
ligand design, coupled with the thermodynamic control that
spurs the self-assembly of metal ions and multidentate recep-
tor ligands, has led to a plethora of well-defined geometric
shapes (1) as well as coordination polymers (2) and net-
works. The ultimate product is contingent on ligand topol-
ogy in addition to its compatibility with different metal ions.
This field of research is spurred by the promise of applica-
tions based on the potential function (magnetic, optical, cat-
alytic, etc.) of these materials.
One of the more intensively studied classes of discrete
metallosupramolecular structures are the so-called “grids”
(3). This architectural class is characterized by ligands with
Received 22 November 2005. Accepted 12 January 2006.
Published on the NRC Research Press Web site at
http://canjchem.nrc.ca on 20 July 2006.
R. Jain, S.L. Caldwell, A.S. Louie, and R.G. Hicks.2
Department of Chemistry, University of Victoria,
P.O. Box 3065 STN CSC, Victoria, BC V8W 3V6, Canada.
1This article is part of a Special Issue dedicated to Dr. Alfred
Bader.
Herein we present the design and synthesis of a new
bistridentate ligand (5), which has been designed specifically
2Corresponding author (e-mail: rhicks@uvic.ca).
Can. J. Chem. 84: 1263–1267 (2006)
doi:10.1139/V06-094
© 2006 NRC Canada

1264
Can. J. Chem. Vol. 84, 2006
Fig. 1. Schematic of the assembly of a 2 × 2 grid with selected examples of L.
Scheme 1.
O
O
O
R
R
R
+
N
Br
Br
N
N
NMe2
O
R = H, Me
O
O
N
N
N
O
N
NMe2
N
N
N
6
7
Scheme 2.
for 2 × 2 grid assembly and has the potential to play an ac-
tive role in electronic (redox, magnetic, optical, etc.) proper-
ties of the assembled grid. The metal-binding sites present in
5 are analogous to those found in 3, i.e., suited for 2 × 2
grid assembly based on octahedrally coordinated metals.
However, 5 is distinguished by the presence of the p-
(anthra)quinone functional unit embedded in the structure.
Quinones are well-known to be redox active and form stable
radical anions and dianions. Moreover, metal complexes of
o-quinones possess a number of very interesting physico-
chemical properties such as strong magnetic exchange be-
tween metal- and semiquinone-based spins and molecular
bistability (valence tautomerism) (10). The incorporation of
quinone-type functional units into supramolecular assem-
blies is quite rare (11). The molecular structure of 5 has an
appropriate topology of metal binding sites for the formation
of 2 × 2 grid structures, and the presence of the p-quinone
component offers the possibility of magnetically coupled
metal–ligand arrays or even grid structures capable of exhib-
iting valence tautomerism.
NNMe2
H
NNMe2
Cl
NCS
N
N
6
8
9
tial attempts to construct 5 were based on this strategy, for
which the azadiene 6 was identified as a key intermediate.
However, attempts to prepare this substrate from the reac-
tion of (2-pyridyl) vinyl ketone (7), prepared by oxidation
of the corresponding secondary alcohol (13) with N,N-
dimethylhydrazine, failed. We then turned to a different strat-
egy to prepare 6, outlined in Scheme 2. 2-Pyridinecarbox-
aldehyde-N,N-dimethylhydrazone (14) was chlorinated with
NCS to give 9. Attempts to convert 9 to azadiene 6 through
reactions with vinyl nucleophiles (Grignard, lithium reagents)
led to no reaction or extensive substrate decomposition. Ad-
dition of late metal catalysts did not improve these reactions.
Therefore, a different route was sought.
O
The successful synthesis of 5 is outlined in Schemes 3 and 4.
The key intermediate is 4,6-diamino-1,3-benzenedicar-
boxaldehyde (14), which was made by slight modification of
literature procedures (Scheme 3) (15). m-Xylene was bisni-
trated and subsequently refluxed with I2 for 16 h in pyridine
to give the air-sensitive bis(pyridinium)diiodide salt (10).
This salt was reacted with p-nitrosodimethylaniline hydro-
chloride (11) (16) to give the bis(imine) oxide (12), which
was deprotected to give 4,6-dinitro-1,3-benzaldehyde (13).
Attempts to reduce 13 to the diamine using the literature
method (15) failed in our hands. This problem was solved by
adopting the protocol described for the reduction of 6-nitro-
N
N
N
O
N
5
Synthesis
Diaza-9,10-anthraquinones are commonly prepared
through hetero-Diels–Alder reactions of 1,4-benzoquinones
or quinoline–diones and azadienes (Scheme 1) (12). Our ini-
© 2006 NRC Canada

Jain et al.
1265
Scheme 3. Reagents and conditions: (i) HNO3 (concd.) O °C to rt; (ii) pyridine–I2, 95 °C; (iii) 2HCl, NaNO2; (iv) 11, EtOH – 10%
NaOH (–5 to 5 °C); (v) H2SO4 (6 N), toluene, 65 °C; (vi) FeSO4·7H2O, EtOH–H2O–NH4OH, 80 °C.
N
N
Me
Me
Me
Me
(ii)
(i)
2I
NO2
86%
99%
O2N
NO2
O2N
10
.HCl
NMe2
(iii)
95%
NMe2
ON
(iv)
11
55%
Me2N
NMe2
O
O
OHC
H2N
CHO
OHC
O2N
CHO
NO2
N
N
(v)
54%
(vi)
67%
NH2
O2N
NO2
14
13
12
Scheme 4. Reagents and conditions: (i) 2-acetylpyridine
(20 equiv.), 10% KOH, EtOH, 100 °C; (ii) CH3COOH, CrO3–
H2O, ∆.
Fig. 2. Cyclic voltammogram of 2,7-dipyridyl-1,8-diaza-9,10-
anthraquinone. The redox process at 0 mV is the ferrocene/
ferrocenium redox couple, added as an internal reference.
(i)
14
N
N
82%
N
N
15
(ii)
84%
O
N
N
N
O
N
5
piperinal (17), i.e., aq. FeSO4·7H2O in EtOH–H2O followed
by addition of 30% NH4OH at 80 °C. This facilitated forma-
tion of 14 in 67% yield.
1662 cm–1. The UV–vis spectrum showed four intense ab-
sorptions at 230, 260, 285, and 315 nm. Other spectroscopic
and analytical tools confirmed the structure and purity of 5.
The redox properties of 5 were probed electrochemically.
The cyclic voltammogram of 9 (Fig. 2) has two reversible
Dialdehyde 14 then underwent a double condensation re-
action with an excess of 2-acetylpyridine to give 2,7-bis(2-
pyridyl)-1,8-diazaanthracene (15) in excellent yield. We note
the potential use of this compound as a bisbidendate ligand
in its own right. The oxidation of 15 to target quinone 5 was
initially attempted using V2O5 and NaClO3, similar to the
oxidation of anthracene to anthraquinone (16), but this
method was plagued with dismal conversions and recovery
of mostly starting material. Aqueous chromium trioxide in
glacial acetic acid proved to be a more effective alternative.
Upon cooling the reaction after refluxing for 30 min,
quinone 5 precipitated and was isolated in 84% yield. The
IR spectrum of 5 shows two carbonyl stretches at 1703 and
°
°
=
reduction waves at potentials of E1 = –1.13 V and E2
−1.67 V vs. the ferrocene/ferrocenium redox couple. Table 1
presents the reduction potentials of this compound along
with corresponding data for several other compounds. 9,10-
Anthraquinone is considerably more difficult to reduce
°
°
°
(E1 = –1.39 V, E2 = –1.86 V) than 1,4-benzoquinone (E1 =
–0.96 V, E2 = –1.56 V) because of the effects of annelation.
°
The introduction of two nitrogen atoms at the 1,8 positions
(i.e., 1,8-diazaanthraquinone (16); made according to literature
methods (12b)) renders this compound slightly more easily
© 2006 NRC Canada

1266
Can. J. Chem. Vol. 84, 2006
Table 1. Reduction potentials of p-quinones in
CH2Cl2.
with N-chlorosuccinimide in small portions. After stirring
overnight, the solvent was removed under reduced pressure
and hexanes were added. The succinimide by-product pre-
cipitated and was filtered. The yellow-orange filtrate was
O
1
Me
Me
cooled to afford crystals of 9. Yield: 1.67 g (67%). H NMR
(CDCl3, ppm) δ: 8.6 (d, 1H), 7.9 (d, 1H), 7.6 (dd, 1H), 7.2
(dd, 1H), 2.9 (s, 6H). 13C NMR (CDCl3, ppm) δ: 152.1,
149.0, 136.3, 131.9, 124.1, 122.3, 46.7.
N
N
O
16
Synthesis of 1,3-diamino-4,6-benzenedicarboxaldehyde (14)
A solution of 4,6-dinitro-1,3-benzaldehyde (13) (1.6 g,
7.1 mmol) in EtOH–H2O (50:50, 80 mL) was brought to
reflux and then a solution of ferrous sulfate heptahydrate
(36.8 g, 0.132 mol) in H2O (80 mL) was added to the vigor-
ously stirred reaction mixture, followed by ammonium hy-
droxide (36 mL) while refluxing. The thick black slurry was
refluxed for an additional hour. Thereafter, the hot solution
was suction filtered using a large Büchner funnel. Finally,
hot ethanol (approx. 50 mL) was used to wash the residue to
make sure all the product was filtered through. The filtrate
was then brought to a boil and refiltered to remove any me-
tallic black residue that may not have been eliminated the
first time. The resultant yellow filtrate was boiled to reduce
the volume (to approximately one-third) and on cooling,
crude 14 (light brown) precipitated, which was isolated by
another filtration. The product was recrystallized using a
H2O–EtOH (5:1) mixture. Yield: 0.780 g (67%). Character-
ization data (1H and 13C NMR) of 14 was the same as
reported previously (15).
°
°
Compound
1,4-Benzoquinone
9,10-Anthraquinone
16
5
E1 (V)
E2 (V)
–0.96
–1.39
–1.24
–1.13
–1.56
–1.86
–1.77
–1.67
°
°
reduced (E1 = –1.24 V, E2 = –1.77 V) and the title
°
compound has yet still higher reduction potentials (E1 = –
°
1.13 V, E2 = –1.67 V) because of the electron-withdrawing
2-pyridyl substituents. Based on this, reduction of 5 should
be chemically possible by reaction with cobaltocene (oxida-
tion potential, E1 = –1.33 V). This was found to be the case
as evidenced by a striking color change from pale yellow to
deep red on adding cobaltocene to a solution of 5. However,
the presumed cobaltocenium salt of the radical anion of 5
was exceptionally reactive and could not be isolated, as is
often the case in p-quinones with low reduction potentials.
Despite this shortcoming, we anticipate the radical anion of
5 to gain stability upon coordination as observed in other p-
dioxolene systems (18).
°
Synthesis of 2,7-dipyridyl-1,8-diazaanthracene (15)
KOH (10%, 0.4 mL) was added to a solution of
1,3-diamino-4,6-benzenedicarboxaldehyde (14, 0.300 g
1.83 mmol) and of 2-acetylpyridine (4.432 g, 36.5 mmol) at
100–110 °C. The solution was stirred for an additional
15 min. Thereafter, 15 mL of ethanol was added to the mix-
ture and boiled for a short time (-2 min). The reaction mix-
ture was cooled, filtered, and the yellow residue washed with
EtOH and dried in vacuo to give 15, which was
recrystallized from hot EtOH. Yield: 0.500 g (82%); mp 239
to 240 °C. 1H NMR (CDCl3, 500 MHz) δ: 7.40 (m, 2H, H18,
H23), 7.92 (m, 2H, H17, H22), 8.37 (s, 1H, H10), 8.44 (d,
2H, H4, H5, 3J = 8.8 Hz), 8.61 (d, 2H, H3, H6, 3J = 8.8 Hz),
8.76–8.80 (m, 4H, H16, H19, H21, H24), 9.07 (s, 1H, H9).
13C NMR (CDCl3, 500 MHz) δ: 119.2 (C3, C6), 122.3 (C18,
C23), 124.4 (C16, C21), 126.4 (C10), 127.0 (C11, C14),
129.3 (C9), 136.9, 137.0 (C4, C5, C17, C22), 146.7 (C12,
C13), 149.1 (C19, C24), 155.9 (C15, C20), 157.5 (C2, C7).
LSIMS: 335 [M + H]+. Anal. calcd. for C22H14N4 (%): C
79.02, H 4.22, N 16.76; found: C 78.73, H 4.23, N 16.74.
Experimental
General considerations
All reagents were purchased from Sigma-Aldrich and
used as received. All solvents were dried and distilled under
argon prior to use (ether, Na–benzophenone, CH2Cl2, CaH2).
Infrared spectra were recorded on a PerkinElmer FT-IR
spectrometer using KBr pellets. NMR spectra were acquired
on a Bruker AMX500 spectrometer in CDCl3. Mass spectra
were recorded on a Kratos Concept IH mass spectrometer
system. UV–vis data were collected on a Cary 100 SCAN
spectrophotometer. The electrochemical experiments were
performed using a CV-50W voltammetric analyzer (BAS) at
room temperature (22 2 °C). Cyclic voltammetry experi-
ments were performed in CH2Cl2 containing 0.1 mol/L of n-
Bu4NBF4 under argon. A carbon electrode (BAS, diameter
3.0 mm) was used as the working electrode. Platinum and
silver wires were used as the auxiliary and reference elec-
trodes, respectively. The working electrode was polished on
alumina before use. iR compensations were applied for all
experiments. All peak potentials are reported vs. an internal
reference (ferrocene/ferrocenium). Elemental analyses were
performed by Canadian Microanalytical Services Ltd., Van-
couver, British Columbia.
Synthesis of 2,7-dipyridyl-1,8-diaza-9,10-anthraquinone
(2,7-DIP-DAAQ) (5)
To 15 (0.600 g, 1.80 mmol) in a two-necked flask,
mounted by a reflux condenser, was added glacial acetic
acid (6 mL), then the flask stoppered. The reaction mixture
was brought to reflux, and while that was underway, CrO3
(4 equiv. mol) were dissolved in H2O (0.6 mL), followed by
glacial acetic acid (3 mL). The heat source was removed
from the reaction mixture and the mixture was dropped into
the stirring reaction mixture over a course of -10 min. The
Synthesis of 2-pyridyl-N,N-dimethylaminochloroimine (9)
A CH2Cl2 solution (35 mL) of 2-pyridinecarboxaldehyde
dimethylhydrazone (2.03 g, 13.6 mmol) was slowly treated
© 2006 NRC Canada

Jain et al.
1267
6. S. Toyota, C.R. Woods, M. Benaglia, R. Haldimann, K.
Wärnmark, K. Hardcastle, and J.S. Siegel. Angew. Chem. Int.
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reaction was then refluxed for a further 10–15 min. Pure
2,7-DIP-DAAQ usually precipitates on cooling, but if it did
not, the reaction solution at room temperature was precipi-
tated into ice-cold water (30 mL) with continued stirring for
another 10–15 min (cold conditions must be maintained).
Filtration afforded a crude yellow residue that was washed
thoroughly with hot water, cold water, and diethyl ether.
Recrystallization from glacial acetic acid afforded 5. Yield:
0.550 g (84%); mp 316 °C. UV–vis (CH2Cl2, nm) λmax (ε):
230 (20 100 (mol/L)–1 cm–1), 260 (22 100 (mol/L)–1 cm–1),
285 (23 100 (mol/L)–1 cm–1), 315 (27 500 (mol/L)–1 cm–1).
Pertinent IR stetches (cm–1): 1703 (m), 1662 (s), 1589 (s),
7. M.T. Youinou, N. Rahmouni, J. Fischer, and J.A. Osborn.
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Volkmer, U.S. Schubert, and J.-M. Lehn. Phys. Rev. Lett. 78,
3390 (1997); O. Waldmann, J. Hassmann, P. Müller, D.
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Waldmann, and P. Müller. Eur. J. Inorg. Chem. 517 (1999); Z.
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73 (2000); O. Waldmann, L. Zhao, and L.K. Thompson. Phys.
Rev. Lett. 88, 066401 (2002); M. Ruben, E. Breuning, J.-M.
Lehn, V. Ksenofontov, F. Renz, and P. Gütlich. Chem. Eur. J.
9, 4422 (2003); L.K. Thompson, T.L. Kelly, L.N. Dawe, H.
Grove, M.T. Lemaire, J.A.K. Howard, E.C. Spencer, C.J.
Matthews, S.T. Onions, S.J. Coles, P.N. Horton, M.B.
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1
1330 (s). H NMR (CDCl3, 500 MHz) δ: 7.46 (m, 2H, H18,
H23), 7.96 (m, 2H, H17, H22), 8.76–8.79 (m, 4H, H3–H6),
8.85 (d, 2H, H16, H21, 3J = 8.1 Hz), 8.97 (d, 2H, H19, H24,
3J = 8.1 Hz). 13C NMR (CDCl3, 500 MHz) δ: 123.2 (C18,
C23), 125.4 (C3, C6, C16, C21), 129.8 (C11, C14), 136.4
(C17, C22), 137.4 (C4, C5), 148.8 (C12, C13), 149.4 (C19,
C24), 154.1 (C15, C20), 161.2 (C2, C7), 180.1, 182.0 (C9,
C10). HRMS calcd. for C22H12N4O2: 364.10; found:
364.0955
0.0016. Anal. calcd. for C22H12N4O2 (%): C
72.52, H 3.32, N 15.38: found: C 72.77, H 3.38, N 15.37.
Acknowledgements
We thank the University of Victoria and the Natural Sci-
ences and Engineering Research Council of Canada
(NSERC) for support of this research.
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