Isolation and characterization of an oxidative degradation product of a polypyridine ligand

Article abstract of DOI:10.1039/a803255f

The origin of the fragility of a polypyridine ligand under oxidizing conditions is discussed.

Full text of DOI:10.1039/a803255f

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Isolation and characterization of an oxidative degradation product of a
polypyridine ligand
Michael Renz, Catherine Hemmert, Bruno Donnadieu and Bernard Meunier*†
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse cedex 4, France
The origin of the fragility of a polypyridine ligand under
oxidizing conditions is discussed.
membered ring is around 4 pm longer than the N(1)–C(1) bond,
as expected for a C–N+ bond of a pyridinium entity (Fig. 1). The
angle between the two planes formed by the imidazopyridinium
and the pyridine connected to the C(7) carbon atom (Fig. 1) is
only 5°, strongly suggesting that the conjugation is extended to
this pyridine.
The catalytic activity of non-heme complexes has been recently
investigated in relation to non-heme containing enzymes having
one or two iron centers.1 Mononuclear iron complexes with
polypyridine ligands have been studied as potential oxidation
catalysts.2 In order to compare the activity of a mononuclear
polypyridine–iron complex with porphyrin or phthalocyanine
iron catalysts in the oxidative degradation of chlorophenols,3
we synthesized a new symmetric tetrapyridine ligand, bis[bis(2-
pyridyl)methyl]amine (BDPMA, Scheme 1).4 The iron(iii)
complex of this ligand, Fe(BDPMA)(NO3)3, is able to catalyze
the oxidative degradation of aromatic pollutants such as
chlorinated phenols in the presence of KHSO5.5 Unfortunately,
the oxidation stopped at the quinone level (without formation of
ring cleavage products which are observed with an iron–
phthalocyanine catalyst)3 and the catalyst seemed to be short-
lived. In attempts to crystallize the iron(iii) complex of this
tetrapyridyl ligand, we found the reason for the low catalytic
activity that we observed in chlorophenol oxidations. Instead of
a metal complex, the cationic compound 1,3,3-tris(2-pyridyl)-
3H-imidazo[1,5-a]pyridin-4-ium (TPIP) was surprisingly ob-
tained.‡ This degradation product of the BDPMA ligand was
The formation of TPIP can be explained by the oxidation of
BDPMA in the presence of FeIII species (in the absence of metal
traces BDPMA is stable and has been completely charac-
terized). The first step consists of a two-electron oxidation of
BDPMA by 2 equiv. of iron(iii). The resulting imine can be
activated by protonation and then attacked by a pyridine
nitrogen acting as internal nucleophile to generate a five-
membered ring (Scheme 1). This reaction is already known for
pyridyl substituted imines, which after cyclization, are further
oxidized to imidazo[1,5-a]pyridines.6 This is not possible in the
present case because a quaternary carbon is created during the
cyclization (C3, Scheme 1). Consequently, only one C–N
double bond can be obtained by dehydrogenation of the five-
membered ring (step 3, Scheme 1), and the pyridinium nitrogen
remains positively charged. The cationic TPIP was obtained
with nitrate as counterion (Scheme 1).
1
completely characterized by H NMR, FAB-MS spectrometry
and X-ray analysis (Fig. 1).§ The key feature of this structure is
the presence of a pyridinium entity with the three other pyridine
substituents still intact. Formally, this modified ligand TPIP can
be obtained from BDPMA by abstraction of four electrons and
three protons. In the crystal structure, the bond lengths of the
pyridinium moiety are as expected for such rings and the
shortest bond length [N(1)–C(7)] of the imidazole moiety is in
agreement with a double bond. The N(2)–C(1) bond of the five-
H
NH
H
N
N
N
N
N
N
N
N
N
Zn, MeCO2H
2 FeIII 2 FeII + 2 H+
H
BDPMA
+ H+
MnO2, MeCO2H
13
14
9
12
11
8
H
7
8a
1
N10
N
6
+N
N
N 2
NH
3
4
+
5
–2 e– –2 H+
N16
N
15
N
N
17
Fig. 1 Crystal structure of TPIP (atom numbering is differing from that
presented in Scheme 1).§ Selected bond lengths (Å): N(1)–C(1) 1.457(4),
N(1)–C(7) 1.293(5), N(2)–C(1) 1.498(4), N(2)–C(2) 1.336(4), N(2)–C(6)
1.364(4), C(2)–C(3) 1.374(5), C(3)–C(4) 1.383(6), C(4)–C(5) 1.388(5),
C(5)–C(6) 1.377(5), C(6)–C(7) 1.471(5).
20
18
19
TPIP
Scheme 1 Mechanism of the oxidative cyclization of BDPMA
Chem. Commun., 1998
1635

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14-H], 9.08 [d, 3J(HH) = 4.8 Hz, 1 H, 11-H], 9.13 [td, 3J(HH) 7.7, 4J(HH)
1.0 Hz, 1 H, 7-H], 9.60 [d, 3J(HH) 8.0 Hz, 1 H, 8-H], 10.18 [d, 3J(HH) 6.2
Hz, 1 H, 5-H]; FAB-MS (meta-nitrobenzyl alcohol): m/z (%): 350 (100).
TPIP (preparative synthesis): 300 mg (1.63 mmol) of bis(2-pyridyl)-
ketone and 300 mg (1.63 mmol) of bis(2-pyridyl)methylamine4 were
dissolved in 10 ml of absolute isopropanol and dried over molecular sieves
3 Å for 1 h at room temperature. 350 ml (5.5 mmol) of glacial acetic acid was
added under N2 atmosphere and the reaction mixture refluxed for 5 h. 1.42
g (16.3 mmol) of MnO2 was added (the oil bath was removed for the
addition). After 1 h heating was switched off and the reaction mixture
allowed to cool slowly. After 16 h, solids were removed by filtration. The
solvent was evaporated (50 °C, 30 Torr) and the residue dissolved in 10 ml
CH2Cl2 and washed with brine (2 3 5 ml). The crude reaction mixture was
dissolved in MeOH and exposed to a MTBE atmosphere. 498 mg (1.19
mmol, 73%) of yellow crystals were obtained.
To prove this mechanism, we synthesized TPIP on a large
scale via the imine. The latter is already known as precursor of
BDPMA and has been reduced by Zn to obtain BDPMA.4 For
the oxidation, MnO2 has been employed and TPIP was isolated
in 73% yield.‡ A slow oxidation leading to the C–N double
bond was observed in the presence of molecular oxygen, but we
found that MnO2 was the suitable oxidant to efficiently prepare
TPIP.
The isolation and characterization of this degradation product
is a significant illustration of the denaturation of a polypyridine
ligand in oxidative conditions. The mechanism of the degrada-
tion was confirmed by the synthesis of the obtained product
from the intermediary imine. The present work shows that
ligands having easily oxidizable C–H bonds, i.e. activated in
benzylic position or in a-position of a hetero atom, have to be
avoided for stability reasons in the design of effective and long-
lived oxidation catalysts.
§ Crystal data for C22H16N6O3 (TPIP): M = 412.41, yellow pale crystal
(0.20 3 0.20 3 0.10 mm), monoclinic, space group I2/a, a = 22.665(3),
b = 8.4832(8), c = 23.415(3) Å, b = 117.83(2)°, U = 3982 Å3, Z = 8,
Dc = 1.37 g cm23, l = 0.71073 Å, T = 160(2) K, m(Mo-Ka) = 0.89 cm21
,
We are grateful to the CNRS for financial support, especially
M. R. for a postdoctoral fellowship.
25822 reflections (3166 independent) were collected on a STOE-IPDS
diffractometer, 281 parameters were refined using the least-squares method
on F7, R = 0.051 and Rw = 0.055. CCDC: 182/925.
Notes and References
† E-mail: bmeunier@lcc-toulouse.fr
1 B. J. Wallar and J. D. Lipscomb, Chem. Rev., 1996, 96, 2625; L. Que
Jr. and R. Y. N. Ho, Chem. Rev., 1996, 96, 2607.
2 P. A. MacFaul, I. W. C. E. Arends, K. U. Ingold and D. D. M. Wayner,
J. Chem. Soc., Perkin Trans. 2, 1997, 135; M. Lubben, A. Meetsma,
E. C. Wilkinson, B. Feringa and L. Que Jr., Angew. Chem., Int. Ed. Engl.,
1995, 34, 1512.
3 A. Hadasch, A. Sorokin, A. Rabion and B. Meunier, New J. Chem., 1998,
22, 45; B. Meunier and A. Sorokin, Acc. Chem. Res., 1997, 30, 470;
A. Sorokin, J. L. Se´ris and B. Meunier, Science, 1995, 268, 1163.
4 M. Renz, C. Hemmert and B. Meunier, Eur. J. Org. Chem., 1998, in
press.
5 C. Hemmert, M. Renz and B. Meunier, J. Mol. Catal. A, 1998, in
press.
6 A. P. Krapcho and J. R. Powell, Tetrahedron Lett., 1986, 27, 3713.
7 D. J. Watkin, C. K. Prout and L. J. Pearce, CAMERON, Chemical
Crystallography Laboratory, University of Oxford, 1996.
‡ Preparations: TPIP [oxidation of BDPMA with Fe(NO3)3]: BDPMA (100
mg, 0.283 mmol) in methanol (2 ml) was added to a solution of
Fe(NO3)3·9H2O (105 mg, 0.260 mmol) in methanol (2 ml) and the mixture
allowed to stand for 1 h. The solvent was evaporated and the residue dried
under oil-pump vacuum for 3 h at 60 °C and ca. 10 mg of the obtained
powder were dissolved in acetone. The solution was allowed to stand in an
atmosphere of diethyl ether for two weeks. The solution became colorless,
brown drops condensed and a large branched crystal was obtained, on which
an X-ray analysis could be carried out. A yellow crystal was also obtained
from an ethanolic solution in a methyl tert-butyl ether (MTBE) atmosphere.
A part of this was cut and used for the presented X-ray structure. The
remaining crystal was dissolved in dry (CD3)2SO for 1H NMR analysis and
mass spectrometry. 1H NMR [250 MHz, (CD3)2SO, 25 °C] d 7.69 [dd,
3J(HH) 7.7, 3J(HH) 4.8 Hz, 2 H, 18-H], 7.91 [d, 3J(HH) 7.9 Hz, 2 H, 20-H],
7.93 [ddd, 3J(HH) 7.8, 3J(HH) 4.8, 4J(HH) 1.0 Hz, 1 H, 12-H], 8.14 [td,
3J(HH) 7.7, 4J(HH) 1.7 Hz, 2-H, 19-H], 8.32 [td, 3J(HH) = 7.8 Hz, 4J(HH)
1.7 Hz, 1 H, 13-H], 8.51 [ddd, 3J(HH) 7.7, 3J(HH) 6.2, 4J(HH) 1.1 Hz, 1 H,
6-H], 8.75 [d, 3J(HH) 4.8 Hz, 2 H, 17-H), 8.76 [d, 3J(HH) 7.8 Hz, 1 H,
Received in Basel, Switzerland, 30th April 1998; 8/03255F
1636
Chem. Commun., 1998