Synthesis of a new polypyridinic highly conjugated ligand with electron-acceptor properties

Article abstract of DOI:10.1016/S0040-4039(01)01289-8

A new acceptor polypyridinic ligand functionalized with a quinone fragment is reported. The ligand, dipyrido[3,2-a:2′,3′-c]-benzo[3,4]-phenazine-11,16-quinone, Nqphen, was synthesized by condensation of 1,10-phenanthroline-5,6-dione and 2,3-diamino-1,4-naphthoquinone. The syntheses of two rhenium complexes with this ligand are also reported.

Full text of DOI:10.1016/S0040-4039(01)01289-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 6463–6467
Synthesis of a new polypyridinic highly conjugated ligand with
electron-acceptor properties
Ramiro D´ıaz,a,* Oscar Reyes,a Ange´lica Francois,a Ana Mar´ıa Leivab and Ba´rbara Loebb,*
aFacultad de Ciencias, Universidad Cato´lica de Temuco, Casilla 15-D, Temuco, Chile
bFacultad de Qu´ımica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago, Chile
Received 31 May 2001; accepted 10 July 2001
Abstract—A new acceptor polypyridinic ligand functionalized with a quinone fragment is reported. The ligand, dipyrido[3,2-
a:2%,3%-c]-benzo[3,4]-phenazine-11,16-quinone, Nqphen, was synthesized by condensation of 1,10-phenanthroline-5,6-dione and
2,3-diamino-1,4-naphthoquinone. The syntheses of two rhenium complexes with this ligand are also reported. © 2001 Elsevier
Science Ltd. All rights reserved.
1. Introduction
The development of artificial systems able to transform
solar energy in a form of usable energy is one of the
present big challenges of many research groups.1 In this
context, d6 transition metal complexes with bidentate
(N,N) type ligands as 2,2%-bipyridine (bpy) or 1,10-
The quinone moiety had also been incorporated to
previously reported ligands as Aqphen5,6 and bpy-AQ,7
although Nqphen has the advantage of being more
rigid and linear, favoring in this way the electron
transfer to the quinone.
phenanthroline (phen), and monodentate ligands as
pyridine (py) or their derivatives, have been amply
studied. The interest in these types of complexes relies
on their capacity to absorb visible light and thereby to
access to excited states by means of metal-to-ligand
charge-transfer (MLCT) processes.2 The electron pro-
moted to a ligand orbital in the excited state can be
injected into the conducting band of the electrode
oxide, starting in this way the conversion of light into
electricity. Similar studies with porphirine as ligand
have been reported.3 In order to favor photoinduced
electron transfer, carotenoid units were covalently
linked to the porphirinic entity, as well as quinonic
residues. This latter type of function plays an important
role as acceptor moieties in natural photosynthetic
systems.4
2. Results and discussion
The synthesis of Nqphen, I, was conducted by the
condensation of 1,10-phenanthroline-5,6-dione (phen-
dione) and 2,3-diamino-1,4-naphthoquinone chlorhi-
drate (V).
This latter reagent was synthesized starting from 2,3-
dicloro-1,4-naphthoquinone (Aldrich), by successive
substitutions of the chlorine atoms by amino groups, as
shown in Scheme 1. The synthesis of the intermediate
compounds II, III, and IV was made according to
published procedures.8,9 Compound V was synthesized
by acid hydrolysis of IV in methanol.
In an attempt to combine these two properties—the
ability to generate MLCT by (N,N) coordination to
metal and the presence of a quinonic acceptor group—
the ligand dipyrido[3,2-a:2%,3%-c]-benzo[3,4]-phenazine-
11,16-quinone, Nqphen (I), was designed and prepared.
In regard to the characterization of the new ligand
Nqphen I, an important feature in its IR spectrum is
the disappearance of the (NH) stretching of 2,3-
diamino-1,4-naphthoquinone (V), (3477 and 3364
cm−1), verifying in this way the condensation reaction
to form I. The w (CꢀO) of the quinonic fragment of this
* Corresponding authors.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01289-8

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R. D´ıaz et al. / Tetrahedron Letters 42 (2001) 6463–6467
Scheme 1.
ligand appears at 1686 cm−1, slightly displaced to higher
energy in regard to the corresponding position in pre-
cursor V (1682 cm−1).
(OTf=trifluoromethanesulfonate anion). These com-
plexes are more soluble than the free ligand. Fig. 1B
1
shows the H NMR pattern for VII in CD3CN. The
simplicity of the pattern spectra denotes the magnetic
equivalence of the phenanthroline protons fragment as
well as the quinonic protons fragment due to ring
planarity and molecular symmetry. This is also reflected
by an integral relation 1:1 in both protons fragment
and 2:1 between phenanthroline protons and quinonic
protons. The main effect observed due to coordination
Ligand I showed low solubility in most common sol-
vents. The better solubility in CDCl3 permits to record
the 1H NMR spectrum (Fig. 1A). The effect on the
NMR pattern of metal coordination to the ligand was
checked by synthesizing the complexes (Nqphen)Re-
(CO)3Cl (VI) and (Nqphen)Re(CO)3(OTf) (VII),
Figure 1. 1H NMR for (A) Nqphen in CDCl3, and (B) (Nqphen)Re(CO)3(OTf) in CD3CN.

R. D´ıaz et al. / Tetrahedron Letters 42 (2001) 6463–6467
6465
Figure 2. 1H–13C-hetero COSY for VII in CD3CN.
spectrum for VII in acetonitrile. The most deshielded
C(1) belongs to the phenanthroline fragment, adjacent
to the nitrogen atom that lost charge density by its
coordination to the metal. This carbon atom is bound
to Ha,a% (9.63 ppm.). The protons in para-position Hc,c%
in addition to being affected by the coordination are
also affected by the anisotropie of the pyrazine nitro-
gen. That is the cause of a great chemical shift to low
field (9.94 ppm). Table 1 summarizes the final assign-
ment of the 1H NMR spectra, done by using the
1H–13C-hetero COSY information.
of Nqphen to Re(I) is the enhanced deshielding of the
protons of the phenanthrolinic fragment of the ligand,
which are the ones most affected by the presence of the
metal.
13C NMR† and 1H–13C-hetero COSY spectra were
recorded in order to confirm the proton assignment for
1
1
the H NMR. Fig. 2 shows the H–13C-hetero COSY
† 13C NMR spectra data for VII (in CD3CN): C(1), 157.10; C(2),
128.36; C(3), 137.57; C(4), 122.81; C(5), 129.66; C(6), 133.79; C(7),
145.34; C(8), 149.96; C(9), 141.61; C(10), 127.65; C(11), 135.64;
C(12), 189.98; C(13), 180.49; C(14), 193.18.
The UV–vis spectra of I recorded in chloroform pre-
sents three bands at 241 (m=8.3×104), 274 (m=6.5×104)

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Table 1. 1H NMR signals for ligand I, and compounds VI and VII
Compound
Chemical shifts (in ppm) (see Fig. 1)
H(a), H(a%)
H(b), H(b%)
H(c), H(c%)
H(d), H(d%)
H(e), H(e%)
Nqphena
9.41 (d)
7.92 (dd)
9.80 (d)
8.54 (dd)
7.98 (dd)
Jab=4.18
J
J
ba=4.18
bc=8.12
J
cb=8.12
J
de=5.66
Jed=5.66
Jed%=3.44
Jde%=3.44
[Nqphen Re(CO)3Cl]a
[Nqphen Re(CO)3(OTf)]b
9.58 (dd)
8.14 (dd)
10.00 (dd)
8.58 (dd)
8.03 (dd)
Jed=5.91
Jed%=3.45
J
J
ab=5.16
ac=1.47
J
J
ba=5.16
bc=8.23
J
J
cb=8.12
ca=1.47
J
de=5.91
Jde%=3.45
9.59 (dd)
8.29 (dd)
9.94 (dd)
8.47 (dd)
8.05 (dd)
J
J
ab=5.41
ac=1.23
J
J
ba=5.41
bc=8.37
J
J
cb=8.37
ca=1.23
J
de=5.91
J
ed=5.91
Jde%=3.44
Jed%=3.44
a In CDCl3.
b In CD3CN.
and 317 (m=5.9×104) nm. The first two bands are
assigned to p–p* intraligand transitions and the third to
an n–p* transition. Compounds VI and VII present the
same pattern in the UV region, with some displacement
of the bands due to coordination. An additional band,
which is assigned to MLCT, appears as a shoulder in
the 360–400 nm (m:1×104) region.
obtained (yield 77%). Elemental analysis (%) calcd for
C10H8N2O2·HCl: C, 53.47; H, 4.04; N, 12.47. Found: C,
1
51.81; H, 4.09; N, 12.59. H NMR (CDCl3): 8.08 (dd,
H(a), H(a%)); 7.52 (dd, H(b), H(b%)); 5.49 (s, NH2). IR
(cm−1) 1682 (w CꢀO), 3477.2, 3363.6 (w NꢁH), 1645
(lNꢁH). UV–vis (nm, acetonitrile): 246 (m=1.2×104),
283 (m=1.3×104), 510 (m=1.02×103).
3.3. Synthesis of dipyrido[3,2-a:2%,3%-c]-benzo[3,4]-phen-
The electron-acceptor capacity of Nqphen was checked
by cyclic voltammetry, where three irreversible reduc-
tion peaks appear: Ep=−0.708; −1.340; −1.585 V (ver-
sus SCE in CH3CN). The first is assigned to the
semiquinone and dianion formation, the second to the
pyrazine fragment and the third to the phenanthroline
part of Nqphen.
azine-11,16-quinone (Nqphen) I
Phendione (0.618 g, 2.94 mmol) and V (0.660 g, 2.94
mmol) were suspended in dry ethanol (40 ml). The
mixture was refluxed for 1.5 h under a N2 atmosphere.
A green solid was formed. The mixture was cooled to rt
and the solid was filtered, washed with ethanol and
diethyl ether. After drying under high vacuum, 0.926 g
of Nqphen were obtained as a green powder. Yield
87%; mp 456.25°C. Elemental analysis (%) calcd for
C22H10N4O2: C, 72.93; H, 4.01; N, 12.47. Found: C,
73.21; H, 3.89; N, 12.01.
According to the results discussed above, it can be
concluded that the Nqphen ligand was obtained in
good yield and purity. It shows acceptable coordination
capacity, and strong acceptor properties. This makes it
a good candidate for the preparation of excited state
charge separate systems.
3.4. Synthesis of (Nqphen)Re(CO)3Cl (VI)
Compound I (0.60 g, 1.66 mmol) and Re(CO)5Cl‡ (0.60
g, 1.66 mmol) were suspended in 50 ml of dry toluene.
The mixture was stirred and refluxed for 2 h under N2.
A yellow solid was formed. The reaction mixture was
cooled to rt and the solid was filtered, washed with
petroleum ether and diethyl ether. After drying under
high vacuum, 1.20 g of product was obtained as a
yellow powder. Yield 94%; mp 381°C (dec.) Elemental
analysis (%) calcd for C25H10ClN4O5Re: C, 44.95; H,
1.51; N, 8.39. Found: C, 43.85; H, 1.48; N, 8.02.
3. Experimental
3.1. Synthesis of 1,10-phenanthroline-5,6-dione
(phendione)
This syntheses was carried out according to published
procedures.5,10
3.2. Synthesis of 2,3-diamino-1,4-naphthoquinone hydro-
chloride (V)
3.5. Synthesis of (Nqphen)Re(CO)3(OTf) (VII)
Compound IV (3 g, 11.62 mmol) was suspended in dry
methanol (40 ml). This mixture was then cooled in an
ice bath and a current of dry hydrogen chloride was
passed through it for 2 h. During this time, a yellow
solid was formed. The mixture was heated at 60°C for
1 h and then refluxed for 4 h. Yellow needles were
obtained by cooling to rt. These were filtered, washed
with methanol and diethyl ether. After drying under
high vacuum, 2.1 g of dark-yellow product were
Compound VI (0.60 g, 0.90 mmol) and silver tri-
fluoromethanesulfonate (AgOTf) (0.231 g, 0.90 mmol)
were suspended in 40 ml of dry THF. The mixture was
stirred and refluxed for 1 h under N2 in the dark. The
‡ Re(CO)5Cl was kindly supplied by Dr. H. Klahn: Klahn, A. H.;
Oelckers, B.; Toro, A.; Godoy, F. J. Organomet. Chem. 1997, 548,
121.

R. D´ıaz et al. / Tetrahedron Letters 42 (2001) 6463–6467
6467
solution was filtered hot to remove the AgCl formed.
The filtrate was evaporated to dryness and the yellow
residue was dissolved in acetonitrile. The product was
reprecipitated by addition of diethyl ether, filtered and
finally purified by chromatography on alumina with
chloroform/acetonitrile 1:1 (v/v) as eluent. (Nqphen)-
Re(CO)3(OTf) was obtained as a yellow powder (0.49
g). Yield 70%; mp 385.56°C (dec.). Elemental analysis
(%) calcd for C26H10F3N4O8ReS: C, 39.95; H, 1.29; N,
7.16; S, 4.09. Found: C, 38.25; H, 1.09; N, 6.99; S, 3.98.
5. Lo´pez, R.; Loeb, B.; Boussie, T.; Meyer, T. J. Tetra-
hedron Lett. 1996, 37, 5437.
6. Lo´pez, R.; Leiva, A. M.; Zuloaga, F.; Loeb, B.; Noram-
buena, E.; Omberg, K. M.; Schoonover, J. R.; Striplin,
D.; Devenney, M.; Meyer, T. J. Inorg. Chem. 1999, 38,
2924.
7. (a) Opperman, K. A.; Mecklenburg, S. L.; Meyer, T. J.
Inorg. Chem. 1994, 33, 5295; (b) Mecklenburg, S. L.;
McCafferty, D. G.; Schoonover, J. R.; Peek, B. M.;
Ericson, B. W.; Meyer, T. J. Inorg. Chem. 1994, 33, 2974.
8. (a) Hoover, J. R.; Day, A. E. J. Am. Chem. Soc. 1954, 76,
4148; (b) Lien, J.; Huang, L.; Wang, J.; Teng, C.; Lee,
K.; Kuo, S. Chem. Pharm. Bull. 1996, 1181; (c) Anacona,
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Acknowledgements
9. Data for compounds II, III, and IV synthesized in this
work.
Fondecyt support through Project 8980007 (L´ıneas
Complementarias) is gratefully acknowledged.
2-Amino-3-chloro-1,4-naphthoquinone (II): Yield 74%. Ele-
mental analysis (%): Calcd for C10H6NO2Cl; C, 57.86; N,
6.75; H, 2.89. Found: C, 57.25; N, 6.58; H, 2.69. 1H
NMR (CDCl3): 8.17 (dd, H(a)); 8.07 (dd, H(a%)); 7.75 (td,
H(b)); 7.66 (td, H(b%)); 5.50 (s, NH2). IR (cm−1): w (CꢀO),
1686; w (NꢁH), 3412, 3300. UV–vis (nm): 212, 265, 435.
2-Acetamido-3-chloro-1,4-naphthoquinone (III): Yield
81%. Elemental analysis (%): Calcd for C12H8NO3Cl; C,
57.74; N, 5.61; H, 3.21. Found: C, 56.91; N, 5.47; H,
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1
3.05. H NMR (CDCl3): 8.17 (dd, H(a)); 8.06 (dd, H(a%));
7.75 (td, H(b)); 7.65 (td, H(b%)); 2.40 (s, CH3). IR (cm−1):
w (CꢀO, quinone), 1686; w (CꢀO, amide), 1715; w (NꢁH),
3310. UV–vis (nm): 346.
2-Acetamido-3-amino-1,4-naphthoquinone (IV): Yield 73%.
Elemental analysis (%): Calcd for C12H10N2O3; C, 62.60;
1
N, 12.20; H, 4.03. Found: C, 62.10; N, 12.09; H, 3.99. H
NMR (in ppm): 8.07 (dd, H(a)); 7.95 (dd, H(a%)); 7.65 (td,
H(b)); 7.55 (td, H(b%)); 5.52 (s, NH2); 2.24 (s, CH3). IR
(cm−1): w (CꢀO, quinone), 1657; w (CꢀO, amide), 1685; w
(NꢁH), 3386 and 3296. UV–vis (nm): 254, 278, 461.
10. Yamada, M.; Tanaka, Y.; Yoshimoto, Y.; Furoda, S.;
Shimao, I. Bull. Chem. Soc. Jpn. 1992, 65, 1006.