A unique series of dinuclear transition metal-polyradical complexes with a m-phenylenediamine spacer and their catalytic reactivity

Article abstract of DOI:10.1039/b300792h

A series of dinuclear transition metal complexes with either six or four iminosemiquinone radicals, in which the metal centres are separated by a distance of ～6.8 A, together with their catalytic reactivity is reported.

Full text of DOI:10.1039/b300792h

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A unique series of dinuclear transition metal–polyradical complexes with
a m-phenylenediamine spacer and their catalytic reactivity†
Soumen Mukherjee, Eva Rentschler, Thomas Weyhermüller, Karl Wieghardt and Phalguni
Chaudhuri*
Max-Planck-Institut für Bioanorganische Chemie (formerly Max-Planck-Institut für Strahlenchemie),
Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany. E-mail: chaudh@mpi-muelheim.mpg.de
Received (in Cambridge, UK) 20th January 2003, Accepted 4th June 2003
First published as an Advance Article on the web 20th June 2003
A series of dinuclear transition metal complexes with either
six or four iminosemiquinone radicals, in which the metal
centres are separated by a distance of ~ 6.8 Å, together with
their catalytic reactivity is reported.
tion geometry around the nitrogen donor is planar indicating
that this nitrogen is three-coordinated (sp2 hybridization) and
not protonated; this renders the ligands o-iminobenzosemiqui-
nonates, [L··]22, and consequently, the copper ions are assigned
to the oxidation state +II. Thus 1 is the dimer containing m-
phenylene bridges of the analogous mononuclear Cu(II) com-
plex with the ligand 2-anilino-4,6-di-tert-butylphenol.4b
The neutral molecule of 3 or 4 contains two M(N,O)3-units
separated by the m-phenylene spacer. The metal centers are
hexacoordinated and the geometrical parameters are identical
within the experimental error of the analogous mononuclear
Fe(III) and Co(III) complexes.4
In the last few years there has been a profusion of studies for the
development of molecular magnetic materials1 based on the
hybridization of organic–inorganic molecules in which para-
magnetic ions are coordinated to organic open-shell radical
ligands.1,2 This surge of interest has been also due partly to the
relevance of such molecules to biological electron-transfer
processes.3
Recently we have described a non-innocent o-iminobenzose-
miquinone radical ligand based on aniline.4 As an obvious and
natural progression of our interest in iminosemiquinone ligands
we have synthesized a dinucleating ligand H4L, based on m-
phenylenediamine.
Complex 3 contains two high-spin d5 ferric ions and,
consequently the Fe–N and Fe–O bond lengths are the longest
of the whole series of complexes with Fe–N and Fe–O bond
distances at 2.105 ± 0.035 Å and 2.025 ± 0.21 Å. That 3 contains
octahedral high-spin ferric ions (d5) is clearly established by its
zero-field Mössbauer spectrum at 80 K: d = 0.56 mm s21 and
DEQ = 1.011 mm s21
.
The six equidistant, short Co–O distances at 1.899 ± 0.004 Å
together with short Co–N distances at 1.930 ± 0.009 Å in 4
containing two fac-CoO3N3 units are compatible with the low-
spin d6 configuration of the Co(III) ions coordinated to six
iminosemiquinone radicals.
Fig. 2 shows the magnetic behaviour (meff vs. T) together with
the model used for the simulation of the experimental data for 4.
We note that the model used is a simplified one. As a
preliminary result the parameters obtained from the simulation
are: J1 = 29.7 cm21, J2 = +13.0 cm21, g = 2.0 (fixed). The
solid line in Fig. 2 represents the simulation using the
Hamiltonian given as the inset. Complexes 2 and 5 are
The ligand H4L reacts with different metal ions‡ in the
presence of a base and air to yield MII (L··)2 [M = Cu 1, Ni 2],
2
MIII2(L··)3 [M = Fe 3, Co 4, Cr 5] or MnIV2(LA·)2(L··) 6, in
which either four or six iminosemiquinone radicals are present.
The structures of the compounds except 2 and 5 have been
determined by single crystal X-ray crystallography at 100 K.
Fig. 1 shows the structures of CuII (L··)2 1 (left) and CoIII2(L··)3
2
4 (right) as examples; the structures of FeIII2(L··)3 3 and
MnIV2(LA·)2(L··) 6 together with their selected metrical parame-
ters are provided as supplementary information.† Complex 1
contains two distorted square-planar copper ions. The coordina-
diamagnetic, whereas complex 1 contains two uncoupled Cu(II
)
centers. The magnetic behaviour of 3 is provided in Figure S1
(supplementary material).†
A novel series of dinuclear (iminosemiquinone)metal com-
plexes is described that provides a suitable basis for further
research in a systematic way, especially on the metal–radical
interactions and their impact on the oxidative catalytic reac-
tions. Indeed, 1 and 6 can catalyze the aerial oxidation of
Fig. 1 Structures of 1 and 4. tert-Butyl groups in 4 are omitted for clarity.
Cu(1)…Cu(2) 6.697 Å, Co(1)…Co(2) 6.719 Å. Average bond distances:
Cu(1)–O 1.923(9), Cu(1)–N 1.948(3), Cu(2)–O 1.915(13), Cu(2)–N
1.942(7), Co(1)–O 1.899, Co(1)–N 1.925, Co(2)–O 1.899, Co(2)–N 1.934
Å.
† Electronic Supplementary Information (ESI) available: experimental
details and crystal data for 1, 3, 4 and 6. See http://www.rsc.org/suppdata/
cc/b3/b300792h/
Fig. 2 The magnetic moment (meff) vs. temperature (T) plot for 4 together
with the model and the Hamiltonian used. The solid line represents the best
fit.
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CHEM. COMMUN., 2003, 1828–1829
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dissolved under argon in CH3CN–CH3OH (1 : 1) and the resulting solution
was refluxed for 1 h and filtered in the air. Slow evaporation of the filtrate
afforded green microcrystals of 1. Recrystallization from THF–CH3OH (4
: 1) afforded X-ray quality crystals. Yield: 0.40 g ( ~ 53%).
catechol to quinone; the dimanganese(IV) compound, 6, is a
better catalyst than the dicopper(II) complex, 1, for the aerial
oxidation of catechol to quinone, thus mimicking the function of
catechol oxidase.5
Crystal data for 1·1.5(C4H8O): C68H88O4N4Cu2·C6H12O1.5. Mr
=
1260.66, monoclinic, space group P21/c, a = 15.2345(12), b = 18.903(2),
c = 24.840(3) Å, b = 90.51(2), V = 7153.1(13) Å3, Z = 4, T = 100(2) K,
l(MoKa) = 0.71073 Å, 46103 reflections collected, 19375 independent
reflections (Rint = 0.0438), data/restraints/parameters = 19318/0/808,
ShelXTL software package, F2 refinement, R1 = 0.0472, R2 = 0.1307 (all
data).
For 3: The ligand H4L (0.3 g, 0.6 mmol), FeCl2·4H2O (0.08 g, 0.44
mmol) and NEt3 (0.4 cm3) were dissolved in a solvent mixture (40 cm3) of
CH2Cl2–CH3CN (4 : 5) and the resulting solution was refluxed for 0.5 h and
filtered. Slow evaporation of the filtrate afforded green crystals of 3.
Recrystallization from acetone afforded X-ray quality crystals. Yield: 0.18
g ( ~ 54%).
Oxidative catalytic reactions were investigated by using 2 3
1027 mole of 6 in dichloromethane (25 cm3), in which different
amounts of the substrate 3,5-di-tert-butylcatechol (2 3 1026 to
20 3 1026 mole) was added to be stirred in air at ambient
temperature and the progress of the reaction was monitored with
time by liquid chromatography and UV–vis spectroscopy. To
unequivocally establish the identity of the quinone the retention
time and spectral data were compared to those of the
commercially available compound. The rate law observed is
first order in both substrate and catalyst. No product other than
the corresponding quinone was detected and 6 has been proved
to be a very good catalyst for the aerial oxidation of catechol
with 100% conversion and a turnover number expressed in mole
product per mole catalyst of 500 after 24 h. Presumably an
outer-sphere electron-transfer mechanism is operative.
Crystal data for 3·0.5CH3COCH3: C102H132Fe2N6O6·0.5C3H6O. Mf
=
1678.88, monoclinic, space group P21/n, a = 25.374(2), b = 15.7754(12),
c = 26.287(2) Å, b = 106.40(1)°, V = 10094.2(13) Å3, Z = 4, T = 100(2)
K, l(MoKa) = 0.71073 Å, 33764 reflections measured at an intensity
threshold of 2s(I), 14847 independent reflections (Rint = 0.0768) analyzed,
data/restraints/parameters = 14670/0/1081, ShelXTL software package, F2
refinement, R1 = 0.0658, R2 = 0.1303 (all data).
For 4: The same protocol as that for 3 using Co(ClO4)2·6H2O (0.16 g, 0.4
mmol) yielded dark brown crystals. Yield: 0.27 g (80%).
Crystal data for 4·3CH2Cl2: C102H132Co2N6O6·3CH2Cl2, Mf = 1910.77,
¯
triclinic, space group P1, a = 15.5627(8), b = 16.2417(12), C = 23.59482)
Å, a = 74.09(1)°, b = 76.48(1)°, g = 66.45(1)°, V = 5205.1(6) Å3, Z =
2, T = 100(2) K, l(MoKa) = 0.71073 Å, 47876 reflections measured at an
intensity threshold of 2s(I), 29002 independent reflections (Rint = 0.0441)
analyzed, data/restraints/parameters = 26325/0/1105. Residual electron
density peaks and holes are located at Cl. ShelXTL software package, F2
refinement, R1 = 0.0881, R2 = 0.1275 (all data).
6: To a solution of the ligand (0.52 g, 1 mmol) in CH3OH (25 cm3)
containing [Bu4N]OCH3 (0.9 cm3, 2.5 mmol) “manganese(III) acetate”
(0.13 g, 0.2 mmol) was added to produce a brown solution, which was
refluxed in air for 0.5 h and filtered to remove any solid particles. The deep
brown microcrystalline solid separated after cooling was recrystallized from
CH2Cl2–CH3CN (1 : 1). Yield: 0.32 g (60%).
Although this mechanism is in accord with the electro-
chemical data† for 6, we could not detect the fate of O2 involved
in the catalytic cycle. Probably the catalase-like activity of 6
disproportionates hydrogen peroxide produced during the
reaction.
In summary, we have reported a rational synthesis of
dinuclear transition metal–iminosemiquinone radical com-
plexes as an obvious progression of our earlier report4 and
demonstrated that the Mn(IV)-radical complex 6 can catalyze
the oxidation of catechol with molecular oxygen as the sole
oxidant to afford quinone in excellent yield under mild
conditions to mimic the function of the copper-containing
enzyme catechol oxidase.
Financial support from the DFG (Priority Program Ch111/
2–1) is gratefully acknowledged. Thanks are due to Mrs H.
Schucht, Mrs P. Höfer, Mr U. Pieper, Mr A. Göbels, Mr B.
Mienert and Mrs U. Westhoff for skilful technical assistance.
We thank Dr U. Schatzschneider for help in simulating the
magnetic data for 4.
Note added to proof: We reported the metal complexes of the
present ligand H4L first in ICCC 35 (Heidelberg) and then in
ICMM’2002 (Valencia). Recently the same ligand has also been
described in Inorg. Chem., 2003, 42, 701, but unfortunately
without giving any prior reference.
Crystal data for 6·0.5CH2Cl2: C102H132Mn2N6O6·0.5CH2Cl2. Mr
=
1690.48, monoclinic, space group P21/n, a = 25.0716(9), b = 15.7152(6),
c = 26.2842(12) Å, b = 105.43(1)°, V = 9982.8(7) Å3, Z = 4, T = 100(2)
K, l(MoKa) = 0.71073 Å, 56918 reflections collected, 13020 independent
reflections (Rint = 0.0922), data/restraints/parameters = 12926/0/1072,
ShelXTL software package, F2 refinement, R1 = 0.0592, R2 = 0.1444 (all
data). CCDC reference numbers: 207577, 199328–199330 for 1, 3, 4 and 6,
respectively. See http://www.rsc.org/suppdata/cc/b3/b300792h/ for crys-
tallographic data in .cif format.
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Dekker, New York, 1999; (b) Molecular Magnetism: New Magnetic
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Notes and references
‡
1: The ligand H4L (0.3 g, 0.6 mmol), prepared in an analogous manner
5 C. Gerdemann, C. Eicken and B. Krebs, Acc. Chem. Res., 2002, 35,
183.
described earlier by us,4 CuCl (0.06 g, 0.6 mmol) and NEt3 (0.4 cm3) were
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