Dinuclear and mononuclear manganese(IV)-radical complexes and their catalytic catecholase activity

Article abstract of DOI:10.1039/b410842f

Seven o-aminophenol ligands based on aniline and m-phenylenediamine, which act as mono- and di-nucleating non-innocent ligands, respectively, together with their seven Mn(IV)-complexes, 1-7 are described. One of them, 1, is dinuclear and 2-7 are mononuclear, in which the ligands are coordinated in their oxidized o-iminobenzosemiquinone radical forms. The crystal structures of 1 and 3 were determined by X-ray diffraction and the electronic structures were established by various physical methods including EPR and variable-temperature (2-290 K) susceptibility measurements. Electrochemical measurements (CV and SQW) indicate primarily ligand-centered redox processes. The Mn(IV)-radical complexes, 1-7, catalyze the oxidation of 3,5-di-tert-butylcatechol with molecular oxygen as the sole oxidant to afford 3,5-di-tert-butylquinone quantitatively under mild conditions to mimic the function of the copper-containing enzyme catechol oxidase. An "on-off" mechanism of the radicals without redox participation of the metal center is proposed for the catalytic oxidation process. Complex 1 is found to be a good catalyst for oxidative C-C coupling of hindered phenols to diphenoquinones.

Full text of DOI:10.1039/b410842f

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F U L L P A P E R
Dinuclear and mononuclear manganese(IV)-radical complexes and
their catalytic catecholase activity†
Soumen Mukherjee,a Thomas Weyhermu¨ller,a Eberhard Bothe,a Karl Wieghardta and Phalguni
Chaudhuri*b
a Max-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mu¨lheim an
der Ruhr, Germany
b Max-Planck-Institut fu¨r Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mu¨lheim an
der Ruhr, Germany. E-mail: Chaudh@mpi-muelheim-mpg.de
Received 15th July 2004, Accepted 29th September 2004
First published as an Advance Article on the web 19th October 2004
Seven o-aminophenol ligands based on aniline and m-phenylenediamine, which act as mono- and di-nucleating
non-innocent ligands, respectively, together with their seven Mn(IV)-complexes, 1–7 are described. One of them, 1, is
dinuclear and 2–7 are mononuclear, in which the ligands are coordinated in their oxidized o-iminobenzosemiquinone
radical forms. The crystal structures of 1 and 3 were determined by X-ray diffraction and the electronic structures
were established by various physical methods including EPR and variable-temperature (2–290 K) susceptibility
measurements. Electrochemical measurements (CV and SQW) indicate primarily ligand-centered redox processes.
The Mn(IV)-radical complexes, 1–7, catalyze the oxidation of 3,5-di-tert-butylcatechol with molecular oxygen as the
sole oxidant to afford 3,5-di-tert-butylquinone quantitatively under mild conditions to mimic the function of the
copper-containing enzyme catechol oxidase. An “on–off” mechanism of the radicals without redox participation of
the metal center is proposed for the catalytic oxidation process. Complex 1 is found to be a good catalyst for oxidative
C–C coupling of hindered phenols to diphenoquinones.
each copper center is coordinated to three histidine residues
and a bridging probably hydroxide ligand, thus resulting in
Introduction
Widespread occurrence of tyrosine radicals in metalloproteins1
a coordination number of four for each cupric ion with a
involved in oxygen-dependent enzymatic radical catalysis has
sparked and fuelled the interest of bioinorganic chemists in
using phenol-containing ligands2 to synthesize model com-
pounds that contain coordinated phenolate and its one-electron
oxidized phenoxyl radical. Additionally, in the field of molecu-
lar magnetism3 redox-active paramagnetic ligands are gaining
increasing attention for the generation of molecular high-
spin materials based on the hybridization of organic–inorganic
molecules in which paramagnetic metal ions are coordinated
to organic open-shell radical ligands.4 This surge of interest
has been also due partly to the relevance of such molecules
to biological electron-transfer processes.
˚
Cu(II) · · · Cu(II) distance of 2.9 A. Consequently, the search for
complexes capable of mimicking the function of catechol oxidase
is primarily involved with dicopper(II) complexes,7 although
complexes of other transition metal ions, particularly Mn,8 are
also known to exhibit catechol oxidase activity.
Recently we have described two 2-aminophenol ligands,
based on aniline9 and m-phenylenediamine,10 which act as
mononucleating (H2L2) and dinucleating (H4L1) non-innocent
ligands, respectively.
1b,5
The ubiquitous enzymes catechol oxidases, found in bacte-
ria, fungi and plants, belong to the class of type 3 copper proteins
and are also known as o-diphenol oxidases or polyphenol
oxidases. In contrast to tyrosinase, also a type 3 dicopper
protein, catechol oxidases catalyze exclusively the two-electron
oxidation of o-diphenols (i.e. catechols) to the corresponding o-
quinones (called catecholase activity) coupled with the reduction
of dioxygen to water. The resulting highly reactive quinones
autopolymerize to form brown polyphenolic catechol melanins,
a process that is thought to be utilized by the plants for their
defense against the possible damage caused by pathogens or
insects. This enzyme contains an EPR-silent, i.e. a strongly
antiferromagnetically coupled, dicopper(II) center in the resting
oxidized state. Three dimensional X-ray crystal structural anal-
ysis of catechol oxidase from sweet potato in the resting dicupric
These ligands can coordinate not only in their different
deprotonated forms but also in their oxidized o-imonobenzo-
semiquinonate radical forms. The different forms which have
been characterized unambiguously are depicted in Scheme 1.
Cu(II)Cu(II) state, in the reduced Cu(I)Cu(I) form and in complex
5c,6
with the inhibitor phenylthiourea, have been reported.
In
the deoxy or reduced state the two copper ions are in the +I
˚
oxidation state at a distance of 4.4 A. In the met oxidized form
† Electronic supplementary information (ESI) available: Figs. 10 and
11. Analytical IR, NMR and mass spectrometric data for H2LCH
,
3
H2LCl, H2LOCH and corresponding data for complexes 4–7. See
3
http://www.rsc.org/suppdata/dt/b4/b410842f/
Scheme 1
3 8 4 2
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
©

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Scheme 2
These two ligands react with different metal ions in the
presence of a base and air to yield radical complexes, in
which in some cases as many as six iminosemiquinone radicals
are present. As a preliminary communication10 we have also
Molecule-ion peaks for complexes were observed in EI-
MS. For example, m/z = 1646 was obtained for complex 1,
showing clearly the composition to be [Mn2L1 ], thus indicating
3
that all of the ligands cannot be present in their “simple”
innocent form. Similarly, the metal : ligand ratio of 1 : 3
was ascertained from the MS-data for other complexes. Mass
spectrometric data for all complexes are summarized in the
Experimental section.
The electronic spectrum of 1 in dichloromethane solution
shown in Fig. 1 exhibits above 450 nm several maxima at 550
(e = 1880), 648 (e = 11500), 807 nm (e = 10700 M−1 cm−1) and
a shoulder at 953 nm (e ∼ 8430 M−1 cm−1) which are assigned to
intraligand p–p* transitions of the iminosemiquinone ligands.
In contrast, the spectra of 3–7 resemble closely that reported for
the Mn(IV)-compound12 2 with the ligand H2L2.
reported that the Mn(IV)-complex of the dinucleating ligand
•
H4L1, Mn2IV(L1 )2(L1••), containing four radicals, can catalyze
A
the oxidation of catechol with molecular oxygen as the sole
oxidant to afford quinone quantitatively under mild conditions
to mimic the function of the copper-containing enzyme catechol
oxidase. Interestingly, the corresponding dicopper(II) complex,
CuII2(L1••)2, containing four radicals exhibits much less catechol
oxidase activity than that of the corresponding dimanganese(IV)
complex. In this work we extend our previous study to similar
substituted aniline-containing ligands together with the cat-
echolase activity of manganese(IV) complexes containing the
ligands L1–L7 in detail (Scheme 2).
Moreover, we describe here some preliminary results of typical
radical mediated reaction, viz. oxidative coupling of 2,6-di-
tert-butylphenol catalytically by the dimanganese(IV) complex
•
MnIV2(L1 A)2(L1••) in the presence of air.
Results and discussion
The ligands H4L1 and H2L2–H2L7 are readily available11 in
good yields from the reaction of 3,5-di-tert-butylcatechol and
m-phenylenediamine (2 : 1) (for H4L1) or differently substituted
aniline (1 : 1) for H2L2–H2L7 in n-heptane in the presence of air
and the base triethylamine.
In the IR the ligands show characteristic strong peaks at 3392–
3370, 3350–3330 and 2960–2860 cm−1 attributable to m(OH),
m(NH) and m(CH) of the tert-butyl groups, respectively. Selected
IR peaks are listed in the Experimental section. The ligands were
characterized also by different other spectroscopic techniques
viz.1H NMR, MS and melting points. They exhibit “normal”
behaviour (see Experimental section) and do not warrant any
special discussion.
Schemes 1 and 2 show the ligands in different oxidation
states observed and the complexes prepared with their labels,
respectively. When a reaction mixture of the ligand either
H4L1 or H2L2–H2L7, [Bu4N][OCH3] and manganese(III) acetate
[MnIII3(l3-O)(CH3COO)6]CH3COO, in methanol was heated to
reflux in the presence of air, deep brown microcrystalline crystals
of complexes 1–7 were obtained.
•
••
Fig. 1 Electronic spectra of complex 1, [MnIV2(L1 A)2(L1 )], and its two
electrochemically oxidized forms in CH2Cl2.
Description of the structures
The crystal structures of complexes 1 and 3 have been de-
termined by single-crystal X-ray crystallography at 100(2) K.
Fig. 2 shows the structure of a neutral molecule in crystals of 1.
Selected bond lengths and angles for complex 1 are summarized
in Table 1.
Complex 1 crystallizes in the monoclinic space group P21/n
••
(Z = 4), the same as that for the corresponding [FeIII2(L1 )3],10
thus rendering them isomorphous. It is misleading to infer
from this fact that complex 1 and [FeIII2(L1••)3] are isostruc-
tural, and in particular, that they contain the same charge
distributions among the metal ions and ligands, i.e. three bis(o-
iminosemiquinonato(−) radicals) and two trivalent metal ions.
That manganese centers, Mn(1) and Mn(2), cannot be ascribed
to an oxidation state of +III is clearly borne out by the facts
The most salient features observed for all complexes in the
IR are the absence of the frequencies attributable to m(NH)
and m(OH) stretching. A medium intense band at ∼1570 cm−1
assigned to m(C–N) and a sharp peak at ∼1440 cm−1 due to m(C-
O) appear for all complexes. Details of the IR peaks are given in
the Experimental section.
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3
3 8 4 3

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˚
Table 1 Selected bond distances (A) for complex 1
Mn(1)–O(1)
Mn(2)–O(20)
Mn(1)–N(7)
Mn(2)–N(14)
C(1)–O(1)
C(6)–N(7)
C(8)–N(7)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
1.884(3)
1.921(3)
1.936(3)
1.975(3)
1.323(5)
1.381(5)
1.427(5)
1.415(5)
1.395(6)
1.410(6)
1.385(6)
1.397(6)
1.423(6)
1.318(5)
1.359(5)
1.436(5)
1.410(6)
1.382(6)
1.427(6)
1.368(6)
1.406(6)
1.426(6)
Mn(1)–O(41)
Mn(2)–O(60)
Mn(1)–N(47)
Mn(2)–N(54)
C(41)–O(41)
C(46)–N(47)
C(48)–N(47)
C(41)–C(42)
C(42)–C(43)
C(43)–C(44)
C(44)–C(45)
C(45)–C(46)
C(46)–C(41)
C(60)–O(60)
C(55)–N(54)
C(52)–N(54)
C(60)–C(59)
C(59)–C(58)
C(58)–C(57)
C(57)–C(56)
C(56)–C(55)
C(55)–C(60)
Mn(1) · · · Mn(2)
1.933(3)
1.999(3)
1.959(3)
1.988(3)
1.310(5)
1.360(5)
1.438(5)
1.414(6)
1.383(5)
1.418(6)
1.369(6)
1.414(6)
1.427(6)
1.294(4)
1.343(5)
1.440(5)
1.421(6)
1.367(6)
1.429(5)
1.355(5)
1.415(5)
1.445(5)
6.762(2)
Mn(1)–O(81)
Mn(2)–O(100)
Mn(1)–N(87)
Mn(2)–N(94)
C(81)–O(81)
C(86)–N(87)
C(88)–N(87)
C(81)–C(82)
C(82)–C(83)
C(83)–C(84)
C(84)–C(85)
C(85)–C(86)
C(86)–C(81)
C(100)–O(100)
C(95)–N(94)
C(92)–N(94)
C(100)–C(99)
C(99)–C(98)
C(98)–C(97)
C(97)–C(96)
C(96)–C(95)
C(95)–C(100)
1.967(3)
1.905(3)
1.972(3)
1.933(3)
1.297(4)
1.342(5)
1.425(5)
1.428(6)
1.368(5)
1.434(5)
1.369(5)
1.420(5)
1.427(5)
1.330(5)
1.383(5)
1.414(5)
1.417(5)
1.381(6)
1.407(6)
1.377(6)
1.405(6)
1.410(5)
C(5)–C(6)
C(6)–C(1)
C(20)–O(20)
C(15)–N(14)
C(12)–N(14)
C(20)–C(19)
C(19)–C(18)
C(18)–C(17)
C(16)–C(17)
C(16)–C(15)
C(15)–C(20)
donor atoms O(1)/N(7) and O(20)/N(14) has a mixed-valence
•
character [L1 A]3−, one part with an iminobenzoquinonate(1−)
radical and the other part displaying the characteristic features
of an O,N-coordinated aromatic amidophenolate dianion with
˚
˚
C(1)–O(1) 1.323 A, C(6)–N(7) 1.381 A and six equidistant
˚
C–C at 1.405 ( 0.020) A of the phenyl ring containing the
tert-butyl substituents; both nitrogen atoms are sp2 hybridized.
Thus the ligand contributes three negative charges towards elec-
troneutrality. The third ligand with O(81), N(87), O(100), N(84)
exhibits features which are very similar to the second one just
described; one part consists of the amidophenolate(2−) ring and
the other, the iminobenzosemiquinone(1−) form. Thus complex
1 consists of four iminobenzosemiquinone(1−) p-radicals, and
two O,N-coordinated aromatic amidophenolatedianion(2−).
Hence each manganese ion must be in +4 oxidation state and
is coordinated to (i) two phenolate oxygens and two imine-
nitrogens from two o-iminobenzosemiquinonate(1−) radicals,
and (ii) one phenolate oxygen and one amide-nitrogen from
the amidophenolate dianion(2−), thus rendering the complex
to be neutral. Summarily, the crystal structure of 1 and its
physical properties (loc. cit.) are in excellent agreement with a
dimanganese(IV)-formulation with four radicals.
Dark brown crystals of complex 3 afforded by diethyl
ether–methanol solution was subjected to single-crystal X-ray
diffraction study at 100(2) K. Selected bond lengths and
angles are summarized in Table 2. The neutral molecule in
crystals of 3, shown in Fig. 3, contains two O,N-coordinated
o-iminobenzosemiquinonate(1−) p radical ligands, as is clearly
borne out by the observation that (i) both nitrogens, N(7)
and N(67), are sp2 hybridized and not protonated, (ii) the six-
membered ring of the iminobenzosemiquinonate part displays
the typical quinoid distortions, comprising a short, a long, and
another short C–C bond followed by three long ones, (iii) the
C–O and C–N bond lengths are short, approaching double
bonds. The third ligand, in contrast, is N-deprotonated; the
corresponding nitrogen N(37) is sp2 hybridized (the sum of three
bond angles C36–N37–C38 118.2(2), C35–N37–Mn(1) 113.0(1)
Fig. 2 Molecular structure of the neutral complex 1.
that (i) none of the manganese centers show any Jahn–Teller
effect, as is expected for a d4 system, and (ii) the observed Mn–
˚
N bond distances, av. 1.961 A are too short in comparison to the
corresponding bond lengths reported for Mn(III)-complexes.13
As will be shown below, each manganese center must be ascribed
to a +IV (d3) oxidation level.
The charge distributions of three ligands, acting each of them
as expected (as a bis(O,N-bidentate)) bridging ligand with a m-
phenylene group as a spacer, are different (see Table 1). The
ligand containing the donor atoms O(41), N(47) on one side
of the m-phenylene spacer and O(60), N(54) on the other side
exhibits typical characteristics of the charge distribution for two
••
o-iminobenzosemiquinonate(1−) p-radical ligands, [L1 ]2−: two
short C–N and C–O bond distances, approaching double bonds,
together with the loss of aromaticity at the phenyl rings con-
taining the tert-butyl substituents. The second ligand with the
3 8 4 4
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3

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◦
˚
Table 2 Selected bond distances (A) and angles ( ) for complex 3
Mn(1)–O(31)
Mn(1)–O(1)
Mn(1)–N(37)
Mn(1)–O(61)
Mn(1)–N(7)
Mn(1)–N(67)
C(1)–O(1)
C(1)–C(6)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–N(7)
N(7)–C(8)
O(31)–C(31)
C(31)–C(32)
C(31)–C(36)
C(32)–C(33)
C(33)–C(34)
C(34)–C(35)
C(35)–C(36)
C(36)–N(37)
N(37)–C(38)
1.8676(13)
1.9050(13)
1.9189(16)
1.9394(13)
1.9522(16)
1.9664(17)
1.303(2)
1.428(3)
1.428(3)
1.375(3)
1.431(3)
1.368(3)
1.418(3)
1.348(2)
1.425(2)
1.330(2)
1.412(3)
1.413(3)
1.390(3)
1.411(3)
1.385(3)
1.404(3)
1.384(2)
1.429(2)
O(31)–Mn(1)–O(1)
O(31)–Mn(1)–N(37)
O(1)–Mn(1)–N(37)
O(31)–Mn(1)–O(61)
O(1)–Mn(1)–O(61)
N(37)–Mn(1)–O(61)
178.12(6)
83.45(6)
96.27(6)
89.43(6)
90.87(6)
172.84(6)
O(31)–Mn(1)–N(7)
O(1)–Mn(1)–N(7)
N(37)–Mn(1)–N(7)
O(61)–Mn(1)–N(7)
O(31)–Mn(1)–N(67)
O(1)–Mn(1)–N(67)
N(37)–Mn(1)–N(67)
O(61)–Mn(1)–N(67)
N(7)–Mn(1)–N(67)
C(6)–N(7)–C(8)
C(6)–N(7)–Mn(1)
C(8)–N(7)–Mn(1)
C(1)–O(1)–Mn(1)
C(31)–O(31)–Mn(1)
C(36)–N(37)–C(38)
C(36)–N(37)–Mn(1)
C(38)–N(37)–Mn(1)
C(61)–O(61)–Mn(1)
C(66)–N(67)–C(68)
C(66)–N(67)–Mn(1)
C(68)–N(67)–Mn(1)
96.61(6)
81.56(6)
96.08(7)
85.44(6)
96.44(6)
85.44(6)
98.95(7)
81.07(6)
161.09(7)
120.90(16)
113.47(13)
124.52(12)
115.18(12)
115.33(12)
118.18(16)
113.00(12)
126.78(12)
114.57(12)
121.10(17)
113.12(12)
124.12(13)
C(61)–O(61)
C(61)–C(62)
C(61)–C(66)
C(62)–C(63)
C(63)–C(64)
C(64)–C(65)
C(65)–C(66)
C(66)–N(67)
N(67)–C(68)
1.297(2)
1.426(3)
1.441(3)
1.379(3)
1.424(3)
1.373(3)
1.414(3)
1.353(2)
1.422(2)
and C38–N37–Mn(1) 126.8(1) is 358.0◦), thus providing
an o-amidophenolate dianion(2−) ligand. The six C–C bonds
˚
of the amidophenolate ring are equidistant at 1.40 0.015 A
and C(31)–O(31) and C(36)–N(37) distances at 1.330(2) and
˚
1.384(2) A, respectively are long. Thus, the central Mn(1)
must be ascribed to +IV (d3) oxidation level. The observed
Mn–O and Mn–N bond distances, av. 1.90 ( 0.035) and 1.94
˚
( 0.025) A, respectively, support this view; they are shorter
than the comparable reported values for Mn(III)-complexes13
2j
and similar to those for Mn(IV)-complexes. Additionally, the
pseudo-octahedral O3N3Mn polyhedron in complex 3 does
not show any Jahn–Teller distortion. The crystal structure of
complex 3 is in excellent agreement with the charge distribution
•
of [MnIV(L3A)(L3 )2] and is identical as expected with that for
the Mn(IV)-complex with the ligand H2L2, complex 2, reported
earlier by us.12
It is obvious that the structures of other complexes, 4–7, are
identical to those of 2 and 3 and hence, we have refrained from
determining the crystal structures of them.
Magnetic susceptibility and EPR data
In order to establish the electronic ground states for complexes
Fig. 3 Perspective view of the neutral molecule in 3.
1–7, variable–temperature (2–290 K) magnetic susceptibility
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3
3 8 4 5

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measurements on powdered samples of 1 and 3 as a represen-
tative of the mononuclear complexes 2–7 were performed by
using a SQUID magnetometer in an external magnetic field of
1.0 T.
Fig. 4 shows the temperature dependence of the magnetic
moment, leff, and of the susceptibility vM for compound 1. The
leff-value of 2.72 lB (vMT = 0.924 cm3 mol−1 K) at 290 K
does not change drastically till 30 K to reach a value of 2.44
lB (vMT = 0.744 cm3 mol−1 K). These values are very close
to 2.45 lB, expected for two uncoupled S = 1/2 states and
indicate clearly strong antiferromagnetic interaction operative
between the paramagnetic Mn(IV) (S = 3/2) and the radical
(S = 1/2) centers. On each side of the m-phenylene-spacer
there are two radicals and one Mn(IV) center, which are strongly
antiferromagnetically coupled yielding thus two S = 1/2 residual
fictive spins for the whole molecule. This qualitative picture is
schematically shown in Scheme 3.
Fig. 5 Temperature dependence of the magnetic moments leff of
complex 3. The solid line represents the best fit of the experimental
data using the parameters given in the text.
coupled S = 1/2 residual spins, as depicted in Scheme 3, and
have reported a detailed analysis of the magnetic properties
of complex 1; hence, we are refraining from repeating the
description.
Fig. 5 shows the temperature dependence of the effective
magnetic moment leff for complex 3. On lowering the tem-
perature leff decreases only from 1.80 at 290 K to 1.70 lB at
15 K. This behaviour indicates a ground state of St = 1/2 for
3. As the overall magnetic behaviour is dominated by much
stronger antiferromagnetic interactions between ligand radicals
and metal-based d-electrons, the decrease of leff on lowering the
temperature could well be fitted by a single coupling constant
J = J12 = J23 [coupling between the radical centers (S = 1/2)
and Mn(IV) center (S = 3/2)] with J13 [coupling between the
radical centers (S = 1/2)] set to zero. Using the Hamiltonian:
Fig. 4 Plots of leff and vM vs. T for complex 1. The solid lines represent
the simulations using the parameters given in the text.
H = 2J(S1·S2 + S2S3) − 2J13(S1·S3)
the following parameters were obtained from the simulation of
the experimental data, shown as the solid line in Fig. 5: S1 =
S3 = 1/2, S2 = 3/2, g1 = g3 = 2.0 (fixed), g2 = 1.97 (fixed), J13
=
0 (fixed), J = −292 cm−1. Hence, a strong antiferromagnetic
coupling exists between the iminosemiquinone-ligands and the
Mn(IV) center, similar to that observed for the ligand H2L2.12
The manganese-based St = 1/2 ground state for 3 has also
been established by X-band EPR spectroscopy. Fig. 6 shows the
EPR spectrum of 3 in CH2Cl2 solution at 298 K. The spectrum
displays an isotropic signal at giso = 2.009 with hyperfine
coupling to the 55Mn nucleus (I = 5/2) of Aiso = 106.6 G,
and in addition, superhyperfine coupling to three 14N (I = 1)
donor atoms and two protons (I = 1/2) was clearly detected,
A(14N) = 4.29 G, A(1H) = 5.15 G, A(1H) = 2.83 G. This spectrum
Scheme 3
We have performed susceptibility measurements by collecting
24 data points at an interval of 1 K in the temperature range of
2.0–25.0 K and 27 data points in the range 30.0–290.0 K, which
are shown as a plot of vM vs. T, in which a maximum at 5 K with
leff = 1.58 lB (vMT = 0.31 cm3 mol−1 K) is observed. This is in
accord with the qualitative picture (Scheme 3) and represents the
weak antiferromagnetic coupling yielding an St = 0 ground state
resulting from the two fictive S = 1/2 states. The susceptibilities
of 1 were calculated by using the spin-Hamiltonian:
•
•
resembles closely to that reported for [MnIV(L2 A)(L2 )2].12
The EPR-spectrum of 1 at 10 K is described later under the
Catalytic catecholase activity section.
Electrochemistry and spectroelectrochemistry
Cyclic voltammograms (CV) and square wave voltammograms
••
••
(SQW) of complex 1 and the analogous CuII2(L1 )2, FeIII2(L1
)
3
ˆ
ꢀ
ꢀ
H = −2JS1·S2
••
and CoIII2(L1 )3, whose structures have been reported earlier,10
and which have been included here to support the assignment
of the Mn complexes, have been recorded in CH2Cl2 solutions
containing 0.1 M TBAPF6 as supporting electrolyte at a glassy
carbon working electrode and using a Ag/AgNO3 reference
electrode. Ferrocene was added as an internal standard after
completion of a set of experiments, and potentials are referenced
vs. the ferrocenium/ferrocene couple (Fc+/Fc). Coulometric ex-
periments were performed at appropriate potentials (at −25 ◦C)
to determine the number of electrons/molecule transferred
in the redox process under investigation. During coulometry
electronic spectra were recorded in order to obtain information
with S1 = S2 = 1/2 and a very satisfactory fit, particularly
in the low-temperature region, only with the parameters J =
−3.25 cm−1, g1 = g2 = 1.98 is shown as a solid line in Fig. 4.
The agreement between the calculated and observed magnetic
moments is reasonable and the simulated curve is also shown
as a solid line in Fig. 4. The best fit parameters are J =
−3.95 cm−1, g1 = g2 = 2.12. No paramagnetic impurity or TIP
was used for the two simulations shown in Fig. 4. A very similar
observation has been reported recently by Gatteschi et al.14 for
the same compound. These authors evaluated a singlet–triplet
gap of ∼7 cm−1, arising from two weakly antiferromagnetically
3 8 4 6
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3

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Table 3 Formal electrode potentials for oxidation and reduction (in V vs. Fc+/Fc) of the dimeric complexes measured at ambient temperatures in
CH2Cl2 solutions containing 0.1 M TBAPF6.
Complex
E1/22(ox)
E1/21(ox)
E1/21(red)
E1/22(red)
−1.56
••
CuII2(L1
)
)
)
+0.44a
−0.120
−0.277
−0.108
−0.120
−0.210
−0.431
−0.288
−0.341
−0.960
−1.24irr
−1.114
−1.019
−1.13
2
••
CoIII2(L1
+0.247
+0.481
+0.575
+0.174
+0.382
+0.101irr
3
••
FeIII2(L1
−1.278
−1.557irr
3•
••
1 MnIV2(L1 A)2(L1
)
−1.234
−1.632irr
irr Redox process is irreversible on the time scale of voltammetric experiments (e.g. in CV at scan rates up to 0.8 V/s); peak potentials are given. a Two
electron processes.
Fig. 7 Cyclic voltammograms (bold) and square-wave voltammograms
(dotted) of 1 in CH2Cl2 at a scan rate of 50 mV s−1 and at a frequency
of 20 Hz, respectively.
Fig. 6 X-Band EPR spectrum of 3 in CH2Cl2 at 298 K. Conditions:
frequency 9.44 GHz; power 10 mW; modulation amplitude 8 G.
about the mechanism of the redox process. The CV and SQW
voltammograms recorded revealed that in each complex a
number of oxidations and reductions are discernible, the redox
potentials of which are compiled in Table 3. Figs. 7 and 8 show
CV and SQW voltammograms for the MnIV (1) and FeIII
-
2
2
complexes, respectively.
It is seen from column E1/21(ox) of the table that the dimeric
complexes, can be twice reversibly oxidized at two low potentials
of close vicinity (E1/21(ox); range −0.12 to −0.43 V vs. Fc+/Fc).
A second set of pairwise, reversible oxidations occur with
the iron and cobalt complexes at somewhat higher potentials
(E1/22(ox); range +0.2 to +0.5 V), with the same pattern as those
at E1/21(ox). Also, at negative potentials (E1/21(red); at −0.96 to
−1.3 V) similar reversible reduction couples occur with copper,
iron and manganese (complex 1) complexes.
Coulometric experiments with the oxidations show that a
charge of two Faradays/mol is transferred for each couple,
i.e. each single peak corresponds to an one electron transfer
process. We can assign all these couples firmly to ligand-centered
oxidations of two radicals to the corresponding quinoid forms
of the ligand as (i) the redox potentials as well as the electronic
spectra (vide infra), of the oxidized species are similar for all
Fig. 8 (a) Cyclic voltammograms (bold) and square-wave voltammo-
••
grams (dotted) of FeIII2(L1 )3 in CH2Cl2; (b) electronic spectral changes
••
during the coulometric two-electron oxidation of FeIII2(L1 )3.
complexes irrespective of the nature of the central metal ion,
and (ii) with the respective analogous monomeric complexes
11d
it was demonstrated earlier that their oxidations are ligand-
centered, and they occur at potentials which are very similar to
those of the present dimeric forms.
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3
3 8 4 7

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The close vicinity of the redox potentials for each couple
shows that there is only little interaction between the oxidation
sites, i.e. the two radicals, which conforms with the magnetic
measurements. Therefore we assign the two oxidation sites of a
couple to two radicals at the same ligand, i.e. each is coordinated
to two metal ions.
With the dicopper(II) compound, a single broad CV wave
is observed in the range of E1/22(ox), with a peak separation
between oxidation and re-reduction peaks which became in-
creasingly higher with higher scan rates. This behaviour is typical
for sluggish heterogeneous electron transfer kinetics, and under
these conditions two steps cannot be distinguished. During
coulometry, the oxidized product formed at E1/22(ox) was found
to be not fully stable, but about two electrons per complex
molecule passed. Therefore we assign the same mechanism to
the oxidation processes for the dimeric complexes. With complex
1 (MnIV2), the oxidative wave in the second range is only very
poorly developed and irreversible, but in view of the similarity
of the structure of 1, and the potential at which the oxidation
takes place, it might occur at the same place.
meta positions amount to 0.56 V for the reversible reductions
and 0.49 V for the reversible oxidations, which correspond to
alterations of the free energy by 54 and 47 kJ mol−1, respectively.
This difference is not reflected in the rate constant ksub for the
catecholase activity described later; there is no correlation visible
between the rate constants and the electronic properties of the
substituents (or the reduction potential). Therefore we conclude
that the rate limiting step in the reaction sequence of the catalysis
is not the electron transfer.
Catalytic catecholase activity
We have examined the catalytic activity of complexes 1–7 for
oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) by O2 to o-
quinone(3,5-DTBQ) in air-saturated dichloromethane accord-
ing to the reaction:
Reductions, E1/21(red), take place at around −1 V, again
pairwise with Cu(II), Fe(III) and Mn(IV). The interpretation is
analogous to the oxidations: reduction of two radicals of the
same ligand to its dianionic form. As to the number of electrons,
with the reductions we only can compare their peak heights
in the voltammograms with the peak heights for oxidation,
because none of the products of E1/21(red) (and a forteriori
of E1/22(red)) was sufficiently stable to allow the collection
of reliable coulometric data. The reductions at E1/22(red) are
irreversible in the time scale of the voltammetric experiments
(with exception of a single reversible wave at −1.56 V with
dicopper(II)) and do not warrant further discussion.
The reactivity studies were performed in dichloromethane
solution because of the good solubility of the complexes as well
as of the substrate and of its product. No base was added to the
solutions to avoid the autooxidation of the substrate by air in
presence of base. The above catalytic oxidation does not proceed
in the absence of the catalysts, the manganese(IV) complexes 1–7.
Prior to a detailed kinetic study we evaluated quantitatively
the catalytic activity of the complexes. For this purpose, 5 ×
10−6 mol of a complex (catalyst) in 25 ml of dichloromethane
was treated with 50 equivalents of 3,5-DTBC and stirred in air
at ambient temperature. The products were analyzed by liquid
chromatography (LC). The retention time in LC for 3,5-DTBC
and 3,5-DTBQ was found to be 8.0 and 10.5 min, respectively
(column: Luna-5-phenylhexyl, eluent: methanol–water (3 : 1),
rate 0.8 ml min−1) as observed for commercially available pure
compound. It was found that for all the oxidative reactions
involving 3,5-DTBC as the substrate, 3,5-DTBQ was the only
oxidized product.
The UV/VIS spectral changes recorded (Fig. 1 for complex
1 and Fig. 8 for the analogous FeIII2(L1••)3) during coulometry
showed very similar features with all oxidations of all complexes:
the appearance of an broad absorption band extending from
400–600 nm with a maximum at 500 nm. With CoIII2- and FeIII
-
2
complexes, which could be four times reversibly oxidized, the
spectral changes maintain almost isosbestic behaviour over all
four oxidations. This confirms that the underlying oxidation
process is the same for all oxidations, namely formation of
The dinuclear copper(II) complex of the ligand H4L1,
11d
quinoid structures. It was shown earlier that generation of
••
CuII2(L1 )2, described earlier by us,10 was used also as a catalyst.
such structures is accompanied by the formation of a new
absorption band in the above wavelength range.
Stirring in air a dichloromethane solution of CuII2(L1••)2 and
3,5-DTBC in the ratio of 1 : 50 yielded at the end of 24 h only
∼16% 3,5-DTBQ and other peaks in LC, which are assigned to
the decomposition products of the catalyst CuII2(L1••)2 by LC-
MS coupling measurements. Thus the turnover number turned
out to be only eight indicating that the dicopper complex is
not a good catalyst for such 2e-oxidation of 3,5-DTBC to 3,5-
DTBQ. This result might be indicative of non-involvement of
any radical mechanism and is in line with the presently accepted
mechanism for the dicopper enzyme catechol oxidase.5
CV and SQW experiments were also carried out with the
monomeric, two radical containing MnIV complexes 2–7, which
differ only in the meta-substituents at the aniline moieties. In
analogy to the dimeric complexes we expect now two ligand-
centered oxidations and two ligand-centered reductions for
oxidation and reduction of the two radicals, respectively. The
results listed in Table 4 show that this is indeed the case. It is seen
that the nature of the substituents causes a shift of the reversible
redox potentials in the expected manner: electron-withdrawing
groups facilitate the reductions and impede the oxidations and
vice versa. The total shifts introduced by the substituents in the
•
When complex 1, MnIV2(L1 A)2(L1••), and 3,5-DTBC in the
ratio 1 : 50 was dissolved in CH2Cl2 in the presence of air,
Table 4 Formal electrode potentials for oxidation and reduction (in V vs. Fc+/Fc) of the monomeric 3,5-disubstituted anilino complexes 2–7
Complex
Substituent
E1/22(ox)a
E1/21(ox)b
E1/21(red)b
E1/22(red)b
2
3
4
5
6
7
H
+0.46
+0.52
n.d.
+0.50
+0.75
+0.48
+0.41
−0.48
+0.01
−0.48
−0.13
−0.40
−1.05
−1.24
−0.68
−1.11
−0.76
−1.03
−1.35
−1.84
−1.12
−1.38
−1.56
−1.35
C(CH3)3
CF3
CH3
Cl
OCH3
a The redox processes described by E1/22(ox) and E1/22(red) are all irreversible on the time scale of the voltammetric experiments and peak potentials
at a scan rate of 0.2 V s−1 are given. b The values of E1/21(ox) and E1/21(red) refer to reversible processes, with the exception of -H, (complex 2) where
already the first reduction is irreversible.
3 8 4 8
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3

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the dark brown color changes slowly to yellow and then finally
to red. Analysis of the resulting solution after 24 h shows a
100% conversion of 3,5-DTBC to 3,5-DTBQ. On increasing the
amount of 3,5-DTBC the turnover number (TON) increases and
the maximum turnover reached after 24 h is 500, indicating that
complex 1 is a good catalyst for the two-electron oxidation of
3,5-DTBC to 3,5-DTBQ and thus mimicking the function of the
dicopper-protein catechol oxidase.
Reaction kinetics were performed by observing the change
in absorbance at 408 nm, which is characteristic of 3,5-DTBQ
in CH2Cl2. In a typical reaction the catalyst was dissolved
in CH2Cl2 and the substrate, 3,5-DTBC was added, and the
resulting solution was stirred in presence of air; the resultant
spectral change as a function of time was measured. For
calculation of rate constants, initial rate method was used and
the velocity of the reaction was obtained from the slope of the
tangent to the absorbance vs. time curve. This procedure was
used for all kinetic measurements.
The kinetics of the oxidative reaction at a constant catalyst
concentration ([complex 1] = 2 × 10−7 mol per 10 ml) in
dichloromethane (10 ml) were investigated with variation of the
substrate concentration, (2–20) × 10−6 mol per 10 ml. As an
example, the electronic spectra for [cat]/[subs] = 1/30 shows an
increase in the intensity of the peak at 408 nm with time and are
depicted in Fig. 9(a). The difference in absorbance DA at 408 nm,
was plotted against time to obtain the velocity (ro) for that
particular catalyst to substrate concentration ratio (Fig. 9(b)).
From variation of the velocity as a function of substrate
concentration, the rate of the reaction with respect to substrate
was found to be first order with ksub = 9.13 × 103 mol−1 min−1.
Similar procedures with varying catalyst concentrations keeping
a constant substrate concentration (5 × 10−6 mol per 10 ml)
yielded different velocities (r’o), thus leading to the result for the
order of the reaction with respect to the catalyst.
A first-order dependence with kcat = 2.26 × 105 mol−1 min−1
was obtained, resulting in an overall rate-law:
Rate = k[catalyst][substrate]
Similar kinetic experiments were performed with other
Mn(IV)-radical complexes, 2–7, and the same rate-law was
observed. The kinetic data together with the relevant reduction
potentials Ered are summarized in Table 5.
To throw more light into this two-electron oxidation pro-
cess, we performed additional experiments to demonstrate
the following salient points: (i) the radicals coordinated to
the manganese(IV) centers are participating in the conversion
process; (ii) oxygen of air is essential for the catalysis, and (iii)
the stoichiometry and fate of oxygen involved.
One equivalent of the dinuclear complex 1 and 10 equivalents
of the substrate 3,5-DTBC in CH2Cl2 were allowed to react for
1 h under argon in a glove box.
An aliquot of this reaction solution was divided into two
parts, (i) to determine by LC the amount of 3,5-DTBQ formed
in absence of air, and (ii) to measure an EPR spectrum for
comparison with that of 1 in CH2Cl2 at 10 K.
From the quantitative LC-study, also under argon, formation
of only 2 equivalents of 3,5-DTBQ could be observed, indicating
that four radical electrons of 1 (see EPR) are involved.
The X-band EPR spectra at 10 K of 1 in CH2Cl2 and of the
frozen solution after the reaction with 3,5-DTBC in glove box
are shown in Fig. 10 (ESI†). The spectrum of only 1 consists
of a broad signal centered at g ≈ 2.0 and a weak 11-line
hyperfine signal at g ≈ 4.05. The assignment of the spectrum
is certainly not straightforward and needs further study. A more
complicated EPR spectrum was observed when 1 was reacted
with two equivalents of DTBC under argon. The spectrum shows
broad peaks between g ≈ 20 and g ≈ 4 along with hyperfine
signals centered at g ∼ 2.0 presumably due to the formation of
two S = 3/2 states at each manganese center of the dimer.
The remaining reaction solution containing 1 from the glove
box was subjected to a quantitative LC-study after long exposure
to air to ascertain that 10 equivalents of 3,5-DTBQ have been
formed, indicating the essentiality of aerial oxygen for the
catalytic process.
The results of the same type of experiments with complex
2, a mononuclear Mn(IV) (S = 3/2) with two radicals, are the
following: (i) one equivalent of 3,5-DTBQ is formed in glove box
in absence of aerial oxygen, i.e. involvement of only two radical
electrons; (ii) the EPR spectrum of the solution containing 2 and
3,5-DTBC from the glove-box exhibits peak centered at g ∼ 3.15
and a low intensity peak at g ∼ 2.0, which could be assigned to
an S = 3/2 system due to the Mn(IV)-center, (iii) long exposure
to air of the reaction solution results in quantitative oxidation
of added 3,5-DTBC to 3,5-DTBQ unambiguously showing the
necessity of aerial oxygen for the catalysis.
Oxygen-uptake measurements
As complex 3, characterized by most of the methods available to
us, exhibits highest catalytic activity (Table 5), we choose 3 for
oxygen uptake measurements which were performed at −25 ◦C
and at −10 ◦C. These low-temperatures were selected consid-
ering the high vapor pressure of the solvent dichloromethane
and to reduce, as much as possible, decomposition of hydrogen
peroxide, which might be expected as a product of the 2e-
oxidation of 3,5-DTBC to 3,5-DTBQ by oxygen.
At −25 ◦C, one equivalent of oxygen was needed to convert all
(0.5 mmol) 3,5-DTBC to 3,5-DTBQ; the reaction was followed
by using a gas-burette method for 320 min till saturation of
O2-uptake occurred. The presence of hydrogen peroxide15 in the
catalytic solution was confirmed both by detection of titanyl(IV)–
peroxo complex and by liberation of iodine from an acidic
potassium iodide solution. Therefore, O2-uptake measurements
Fig. 9 (a) Electronic spectral change of a solution of 1 (2 × 10−7 mol)
and 3,5-DTBC (6 × 10−6 mol) in CH2Cl2 (10 ml); (b) a plot of the
difference in absorbance (DA) vs time to evaluate the velocity of the
catalysis.
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3
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at −25 ◦C results in the following stoichiometry for the reaction:
low of 0.247 lmol substrate (mg catalyst min)−1 for the ditri-
fluoromethyl-substituted ligand, complex 4. It is surprising that
the value of 0.210 is so low for the dinuclear complex 1,
considering that it contains two additional radicals. However,
as a general rule-of-thumb it can be concluded that electron-
withdrawing substituents reduce the activity, whereas electron-
pushing groups make the catalyst more active by increasing
the electron density on the aniline-nitrogen, which presumably
thus positively influences the hydrogen bond-formation with the
substrate, as has been proposed in the pre-equilibrium step prior
to oxidation (Scheme 4). Apparently, the polarization effect
resulting from the effect on the resonance interaction of the
highly polarizing meta-substituents, R, with the aniline ring is
operative.17 Hydrogen-bond formation between the catalyst and
the substrate seems to be the driving force for the equilibrium
shown in Scheme 4.
However, it is clear from Table 5 that there is a complex
interplay between the structural and electronic properties (as evi-
denced by their electrochemical properties) of the manganese(IV)
complexes in determining the hydrogen bond-formation in the
equilibrium described in Scheme 4, essential for the oxidative
process. For this series of complexes, it appears that the
redox properties are secondary in importance to the hydrogen
bond-formation in determining the catalytic properties of the
complexes.
CH Cl
2
2
3,5-DTBC + O2 −−−−−−→ 3,5-DTBQ + H2O2
Cat
At −25 ◦C, catalase-activity of 3 is very slow and H2O2 formed
did not undergo appreciable decomposition to H2O and O2.
However, at −10 ◦C, 0.5 equivalent of oxygen was required
in order to convert all 3,5-DTBC (0.5 mmol) to 3,5-DTBQ;
saturation of O2-uptake occurred after 152 min reaction-time.
At −10 ◦C formed hydrogen peroxide decomposed to H2O and
O2, and the latter was also taken up by the substrate to reduce
the total amount of O2 consumed. At −10 ◦C and hence at room
temperature the stoichiometry changes to:
2(3,5-DTBC) + 2O2 → 2(3,5-DTBQ) + 2H2O2
2H2O2 → 2H2O + O2
2(3,5-DTBC) + O2 → 2(3,5-DTBQ) + 2H2O
due to incipient decomposition of H2O2.
Mechanism
It is tempting to propose a mechanism based on the electro-
chemical data, O2-uptake measurements and the experiments
described earlier. Additionally, the EPR data show that the
intermediate could be the two-electron reduced form, i.e.
involvement of both radicals in the oxidation, of the Mn(IV)-
complex. With incoming oxygen of air the reduced ligands get
re-oxidized to the radical form of the complex which carries out
the next turnover.
Oxidative C-C coupling of hindered phenols to diphenoquinones:
conversion of 2,6-di-tert-butylphenol to 3,3ꢀ-5,5ꢀ-tetra-tert-
butyldiphenoquinone
The proton transfer followed by the electron transfer as
implied in Scheme 4 may occur also as hydrogen atom transfer.
However, the proposed “on–off” mechanism of the radicals
without redox-participation of the metal center seems to be
very common for the catalysis involving such metal-radical
complexes.16
As the oxidative carbon–carbon coupling of hindered phenols
leading to diphenoquinones is radical-mediated reactions, we
have studied complex 1 containing radicals for oxidative stud-
ies with 2,6-di-tert-butylphenol. When 2,6-di-tert-butylphenol
(DTBP) (2 mmol) is added to complex 1 (0.01 mmol) in
dichloromethane–methanol (1 : 1) solvent mix (50 ml) the dark
brown colour slowly turns to red in presence of air. It was
found that after 48 h 100% of the phenol was converted to 3,3ꢁ-
5,5ꢁ-tetra-tert-butyldiphenolquinone (TTBD), thus resulting in
a turnover number of 200.
Scheme 4
The results of the catalytic oxidations are presented in Table 5.
Despite the fact that no clear correlation between the redox
potentials of the catalysts, the Mn(IV)-complexes, and the
velocity of the catecholase activity was revealed for 1–7, a rough
correlation is envisagable between the catalytic activity and the
substituents on the aniline ring. Leaving aside complex 1 because
of its dinuclear nature, the catalytic activities of the complexes
range from a high 0.920 lmol substrate (mg catalyst min)−1
for the di-tert-butyl substituted aniline ring, complex 3, to a
For the reaction a maximum turnover number of 1284/48 h
was observed. The course of the reaction was followed by
optical spectroscopy, as TTBD shows a characteristic peak in
the electronic spectrum at 425 nm. Such changes in electronic
spectra with different time intervals were monitored by removing
the catalyst from the reaction solution using a column of
Amberlyst-resin and are depicted in Fig. 11 (ESI†). The resultant
plot of absorbance vs. time indicates a complicated nature of the
kinetics and warrants a more detailed study.
Table 5 Kinetic and relevant reduction potential data for complexes 1–7 as potential functional mimics of catechol oxidase
Complex
Substituent R
E
Red /V (vs. Fc+/Fc)
10−4
k
subs/mol−1 min−1
Maximum TONa/24 h
Catalytic activitya/min−1
•
••
1 MnIV2(L1 A)2(L1
)
—
H
C(CH3)3
CF3
CH3
Cl
−1.02, 1.23
0.9
1.5
2.8
4
4.6
5.3
9.1
500
48
169
48
84
45
0.210
0.355
0.920
0.247
0.569
0.272
0.260
•
2 MnIV(L2A)(L2 )2
−1.05, −1.35
−1.24, −1.84
−0.68, −1.12
−1.11, −1.38
−0.76, −1.56
−1.03, −1.35
•
3 MnIV(L3A)(L3 )2
•
4 MnIV(L4A)(L4 )2
•
5 MnIV(L5A)(L5 )2
•
6 MnIV(L6A)(L6 )2
•
7 MnIV(L7A)(L7 )2
OCH3
40
a Turnover number = [number of mol of product]/[number of mol of catalyst]. b Activity expressed as lmol substrate oxidized per mg catalyst.
3 8 5 0
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3

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or ESI (in CH2Cl2) mode with a FinniganMAT 95 or 8200
spectrometer. Magnetic susceptibilities of the polycrystalline
samples were recorded on a SQUID magnetometer (MPMS,
Quantum Design) in the temperature range 2–290 K with an
applied field of 1 T. Diamagnetic contributions were estimated
for each compound by using Pascal’s constants. X-Band EPR
spectra were recorded on a Bruker ESP 300E spectrometer
equipped with a helium flow cryostat (Oxford Instruments ESR
910), an NMR field probe (Bruker 035M), and a microwave
frequency counter HP5352B. Spin-Hamiltonian simulations of
the EPR spectra were performed by iteration of the (an)isotropic
g values, hyperfine coupling constants, and line widths. GC of
the organic products during catalysis were performed either on
HP 5890 II or HP 6890 instruments using RTX-1701 15 m S-
41 columns. GC-MS was performed using the above columns
coupled with a HP 5973 mass spectrometer with mass selective
detector. NMR spectra were measured using Bruker ARX 250
(250 and 63 MHz for 1H and 13C NMR, respectively).
Concluding remarks
To conclude, the following points of this study deserve particular
attention:
As an obvious progression of our earlier reports9–12 dinuclear
transition metal–iminosemiquinone complexes containing as
high as six radicals can be rationally synthesized. A diman-
ganese(IV) complex containing four radicals has been struc-
turally and spectroscopically characterized. On each side of the
m-phenylene spacer in 1 there are two radicals (S = 1/2) and one
Mn(IV) center (SMn = 3/2), which are strong antiferromagneti-
cally coupled yielding thus two S = 1/2 residual fictive spins even
at room temperature for the whole molecule. Antiferromagnetic
exchange interaction between these two fictive residual spins
is weak with a singlet–triplet gap of ∼7 cm−1. The strong
antiferromagnetic coupling between the radical ligands and the
Mn(IV) ions can be ascribed to the large covalency of the Mn–N
bonds in 1.
A series of disubstituted 2-anilino-4,6-di-tert-butylphenol lig-
ands has been prepared to synthesize six new radical-containing
Mn(IV) compounds. As expected, these Mn(IV) compounds, 3–7,
exhibit strong antiferromagnetic coupling yielding an St = 1/2
ground state, like that for 2.12
Cyclic voltammograms, square-wave voltammograms, and
coulometric experiments were performed using an EG&G
potentiostat/galvanostat. Simulations of the cyclic voltammo-
grams were obtained using the program DigiSim 3.0 (Bioana-
lytical Systems, Inc., West Lafayette, IN).
Electrochemical measurements indicate redox processes
which are mainly ligand-centered. Thus for each complex a
number of oxidations and reductions, resulting in quinoid and
phenolate forms of the ligands, respectively, are discernible.
The investigation emphasizes that the Mn(IV)–radical com-
plexes 1–7 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.
LC analysis of the reaction solutions during catalysis were
performed on HPLC instrumentation using a Gilson M305
pump, and the Diode-Array-Detector (DAD) SPDM 10 AV
(Shimadzu Corporation). MeOH and water in the ratio 3 : 1
with flow velocity 0.8 ml min−1 was used as eluent through a
Luna-5 phenylhexyl column.
Catalytic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC)
In dichloromethane (25 ml), the Mn(IV) complexes 1–7, were
dissolved and 3,5-DTBC was added and the solution was stirred
in room temperature for 24 h. The solution was then subjected
to liquid chromatographic studies.
For kinetic measurements, 1 ml from the reaction solution
was diluted to 10 ml with dichloromethane. The change in the
concentration of the product, 3,5-di-tert-butyl-o-benzoquinone,
was monitored by electronic spectroscopy.
1−7 in CH Cl
2
Catechol −−−−−−→ o-Quinone
Air
Although the nature of the substituents on the aniline moiety
of the ligand causes a shift of the reversible redox potentials in
the expected manner, i.e. electron-withdrawing groups facilitate
the reductions and impede the oxidations and vice versa, this
change in potentials is not reflected in the rate constant for the
catecholase activity, thus indicating that the rate-determining
step in the reaction sequence of the catalysis (Scheme 4) probably
is not the electron transfer.
Catalytic oxidation of 2,6-di-tert-butylphenol
An “on–off” mechanism of the radicals without redox-
participation of the metal center manganese has been proposed
for the catalytic process, in which hydrogen-bond formation
between the catalyst and the substrate catechol seems to
be the driving force for the oxidation. It appears that the
redox properties are secondary in importance to the hydrogen-
bond formation in determining the catalytic properties of the
complexes.
Complex 1 has been found to be a good catalyst for oxidative
C–C coupling of hindered phenols to diphenoquinones. Thus,
2,6-di-tert-butylphenol (DTBP) was oxidized by aerial oxygen
quantitatively to 3,3ꢁ-5,5ꢁ-tetra-tert-butyl-4,4ꢁ-diphenoquinone
(TTBD) by using complex 1 as a catalyst.
In a dichloromethane/methanol solvent mixture (1 : 1; 50 ml)
complex 1 was dissolved. To it 2,6-di-tert-butylphenol was
added and the solution was stirred for 48 h. The solution
was then filtered and the filtrate was evaporated to dryness.
5 ml of methanol were added to dissolve the excess 2,6-di-tert-
butylphenol and the methanolic solution was subjected to GC
studies with the presence of hexadecane (C16) as standard.
For measuring the progress of the reaction, 50 ll from
the aliquot was passed through an Amberlyst cationic ion-
exchanger and washed with 10 ml (2 × 5) dichloromethane.
The change in the concentration of product 3,3’-5,5’-tetra-tert-
butyldiphenoquinone, was monitored by optical spectroscopy.
Complex 1
X-Ray crystallographic data collection and refinement of the
structures
DTBP −−−−−−→ TTBD
Air
Thus, as is expected, a radical-mediated reaction is catalyzed
by 1.
Dark brown single crystals of 1 and 3 were coated with
perfluoropolyether, picked up with glass fibers, and mounted on
a Nonius Kappa-CCD diffractometer equipped with a cryogenic
nitrogen cold stream at 100 K. Graphite-monochromated Mo-
Experimental
˚
Ka radiation (k = 0.71073 A) was used. Intensity data were
Materials and physical measurements
corrected for Lorentz and polarization effects. The intensity
data were not corrected for absorption due to a low absorption
coefficient. The SHELXTL software package18 was used for
solution, refinement, and artwork of the structure, and neutral
atom scattering factors of the program were used. All structures
were solved and refined by direct methods and difference Fourier
techniques. Non-hydrogen atoms were refined anisotropically,
Commercial grade chemicals were used for the synthetic pur-
poses and solvents were distilled and dried before use. Fourier
transform IR spectroscopy on KBr pellets was performed on
a Perkin-Elmer 2000 FT-IR instrument. Solution electronic
spectra were measured on a Perkin-Elmer Lambda 19 spec-
trophotometer. Mass spectra were recorded either in the EI
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3
3 8 5 1

View Article Online
Table 6 Crystallographic data for 1 and 3
1
3
Empirical formula
C102H132Mn2N6O6·0.5CH2Cl2
C84H123MnN3O3
1277.79
100(2)
Mr
1690.48
100(2)
T/K
˚
k/A
0.71073
Monoclinic
P21/n
0.71073
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/c
˚
˚
˚
˚
a = 25.0716(9) A
a = 16.6726(4) A
b = 15.7152(6) A
b = 14.5993(4) A
˚
˚
c = 26.2842(12) A
c = 33.4362(12) A
b = 97.20(1)◦
8074.5(4), 4
b = 105.43(1)◦
3
˚
V/A , Z
9982.8(7), 4
Dc/Mg m−3
1.125
0.332
1.051
0.209
l/mm−1
F(000)
3620
2788
Crystal size/mm
h range for data collection/◦
Index ranges, hkl
Reflections collected
0.25 × 0.20 × 0.16
0.22 × 0.14 × 0.14
2.06 to 22.50
2.96 to 27.50
−27 to 27, −16 to 17, −29 to 23
56918
−21 to 21, −18 to 18, −43 to 43
35212
Independent reflections (Rint
Data/restraints/param.
Goodness-of-fit on F2
)
13020 (0.0922)
12926/0/1072
1.027
R1 = 0.0592, wR2 = 0.1236
R1 = 0.1005, wR2 = 0.1444
0.822, −0.586
18437 (0.0410)
18437/0/856
1.062
R1 = 0.0590, wR2 = 0.1285
R1 = 0.0830, wR2 = 0.1394
1.247, −0.570
Final R indices [I >2r(I)]
R indices (all data)
Largest diff. peak, hole/e A−3
and hydrogen atoms were placed at calculated positions and
refined as riding atoms with isotropic displacement parameters.
A dichloromethane solvent molecule in 1 was found to be
not fully occupied. The C and Cl atoms were anisotropically
refined with a given occupation factor of 0.5. Details of the data
collection and structure refinements are summarized in Table 6.
CCDC reference numbers 199330 for 1 and 244678 for 3.
See http://www.rsc.org/suppdata/dt/b4/b410842f/ for crys-
tallographic data in CIF or other electronic format.
H2L3 (R = C(CH3)3). Mp 180 ◦C. IR (KBr, cm−1): 3349br s,
1
2964s, 1599s, 1479m, 1360m, 1311m, 1234m, 852m, 774m. H
NMR (CDCl3): d 1.26 (s, 27H), 1.44 (s, 9H), 5.02 (s, 1H), 6.23 (s,
1H), 6.54 (s, 2H), 6.91–6.93 (m, 1H), 7.06–7.07 (m, 1H), 7.17–
7.18 (m, 1H). 13C NMR (CDCl3): d 29.48, 31.34, 31.58, 34.35,
34.82, 34.94, 109.78, 114.18, 120.77, 121.10, 128.21, 135.16,
141.98, 145.50, 148.62, 151.90. 13C NMR (CH3OH): d 30.21,
31.91, 32.13, 35.65, 35.95, 111.15, 114.17, 120.32, 142.87, 152.63.
EI-MS: m/z (relative intensity %) 411 (5), 410 (35), 409 (100),
57 (17). Anal. Calc. for C28H43NO. Found: C, 82.09; H, 10.58;
N, 3.42. Found: C, 81.1; H, 10.5; N, 3.3%.
Preparation of the ligands H4L1, H2L2–H2L7
H2L4 (R = CF3). Mp 158 ◦C. IR (KBr, cm−1): 3459s, 3339s,
2962s, 1621s, 1479s, 1384m, 1282vs, 1169vs, 1140vs, 1001m, 875s,
683m. 1H NMR (CDCl3): d 1.26 (s, 9H), 1.43 (s, 9H), 5.44 (s, 1H),
5.94 (br, 1H), 7.00–7.05 (m, 3H), 7.26–7.30 (m, 2H). 13C NMR
(CDCl3): d 29.47, 31.51, 34.35, 35.01, 94.93, 95.35, 97.66, 97.94,
98.11, 121.63, 122.90, 126.08, 135.87, 142.66, 149.13, 149.47,
162.20, 165.86. 13C NMR (CH3OH): d 30.08, 31.99, 35.19, 36.09,
110.99, 114.69, 121.74, 122.56, 128.59, 132.99, 138.27, 143.42,
150.28. EI-MS: m/z (relative intensity %): 434 (25), 433 (100),
419 (22), 418 (92), 57 (51), 41(11). Anal. Calc. for C22H25F6NO:
C, 60,97; H, 5.81; N, 3.23. Found: C, 60.9; H, 5.8; N, 3.2%.
Mp, IR, 1H NMR, 13C NMR, EI-MS and microanalysis data
The ligands were prepared according to a very similar procedure
described in ref. 11.
1,3-Bis(4,6-di-tert-butyl-2-iminophenol)benzene, H4L1. To
150 ml of n-heptane, 3,5-di-tert-butylcatechol (14.4 g, 65 mmol)
was added along with 2 ml of NEt3. 1,3-Phenylenediamine
(3.24 g, 30 mmol) was added and the solution was refluxed
with stirring for 3 h, which was then cooled and the stirring
was continued in a closed vessel for another 2 days. A light
gray solid precipitated which was filtered and washed carefully
with cold n-pentane. Yield: 15 g (96%). Mp > 200 ◦C. IR
(KBr, cm−1) 3392s, 3349s, 2959s, 2907s, 2868s, 1614s, 1420s.
1H NMR (CDCl3): d 1.22 (s, 18H), 1.37 (s, 18H), 6.12 (s, 1H),
6.25 (d, 2H), 7.02 (d, 3H), 7.18 (d, 2H); 13C NMR (CDCl3): d
29.28, 31.33, 34.12, 34.74, 121.27, 122.20, 126.86, 141.1, 148.83;
EI-MS: m/z = 516 [M+]. Anal. Calc. for C34H48N2O2: C, 79.02;
H, 9.36; N, 5.42. Found: C, 79.0; H, 9.4; N, 5.3%.
Cl
OCH
3
for H2LCH , H2L , H2L
are given as ESI.†
3
Preparation of complexes
•
MnIV2(L1 A)2(L1••) 1. To a solution of the ligand H4L1 (0.52 g,
1 mmol) in CH3OH (25 ml) containing [Bu4N]OCH3 (0.9 ml,
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%). Anal. Calc.
for C102H132Mn2N6O6: C, 74.34; H, 8.07; N, 5.10; Mn, 6.67.
Found: C, 74.3; H, 8.0; N, 5.0; Mn, 6.5%. ESI(+)-MS: m/z 1646
[Mn2L1 ]+, 1134 [Mn2L1 ]+, 823 [Mn2L1 ]2+. IR (KBr/cm−1):
The ligand H2L2, 2-anilino-4,6-di-tert-butylphenol, has been
11d
described earlier.
H2L3–H2L7, 2-(3,5-disubstituted anilino)-4,6-di-tert-butylphenol
As the 3,5-disubstituted anilino-ligands were prepared by a
similar protocol, a representative method is only described. A
solution containing 3,5-di-tert-butylcatechol (4.44 g, 20 mmol),
the disubstituted aniline (20 mmol) and triethylamine (0.2 ml)
in n-heptane (40 ml) was stirred at ambient temperature for 24 h
to yield a brown suspension. The brown solid was collected by
filtration and washed with cold n-heptane and air-dried. The
compounds were recrystallized from n-hexane yielding colorless
crystalline materials. The yields were in the range 60–80% based
on the starting catechol.
3
2
3
=
1578s m(C N), 1470s m(C–O), and other peaks 1520m, 1360s,
1268s, 1021m, 703w. UV-VIS (CH2Cl2): k/nm (e/M−1 cm−1):
550 (11880), 648 (11500), 807 (10700).
MnIV(LA)(L•)2 (2–7). Complexes 2–7 were prepared by a
same protocol. The ligand H2LX (2 mmol) and “manganese(III)
acetate” (0.13 g, 0.2 mmol) were dissolved in methanol (25 ml),
3 8 5 2
D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3

View Article Online
containing [Bu4N]OCH3 (0.9 ml, 2.5 mmol), whereupon a deep
red solution was produced. The solution was heated to reflux for
1 h in the presence of air. From the cooled and filtered solution
black microcrystals crystallized upon slow evaporation of the
solvent. The material was recrystallized from a ether–methanol
solvent mixture; yield ∼60–70%.
5 (a) E. I. Solomon, U. M. Sundaram and T. E. Machonkin, Chem.
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6 T. Klabunde, C. Eicken, J. C. Sacchettini and B. Krebs, Nat. Struct.
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10 S. Mukherjee, E. Rentschler, T. Weyhermu¨ller, K. Wieghardt and P.
Chaudhuri, Chem. Commun., 2003, 1828.
11 (a) FRG Pat., 110 4522, p. 1959; (b) Chem. Abstr., 1962, 56, p. 5887;
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Complex 2. The complex has been reported in ref. 12.
Complex 3. Anal. Calc. for C84H123MnN3O3: C, 78.95; H,
9.70; N, 3.29; Mn, 4.30. Found: C, 78.8; H, 9.8; N, 3.2; Mn, 4.3%.
IR (KBr/cm−1): 2960s, 2904m, 2867m, 1592m, 1578m, 1478m,
1462m, 1360m, 756m, 706m. ESI(+)-MS (CH2Cl2): m/z 1277
[MnL3 ]+. UV-Vis (CH2Cl2) [kmax/nm, e/M−1 cm−1)], 730 (4000),
3
515 (16900), 500sh. Magnetic measurements: leff/lB (T/K) =
1.22 (2), 1.69 (15), 1.70 (100), 1.71 (200), 1.80 (290).
For complexes 4–7 the analytical data, ESI-MS (positive) and
the experimental magnetic moments (2–290 K) together with
their simulations are given as ESI.†
Acknowledgements
Financial support from the German Research Council (DFG) is
gratefully acknowledged (Project: Priority Program Ch111/2-2).
Skilful technical assistance of Mrs H. Schucht, Mrs R. Wagner,
Mrs U. Westhoff, Mr A. Go¨bels and Mr U. Pieper is thankfully
acknowledged.
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D a l t o n T r a n s . , 2 0 0 4 , 3 8 4 2 – 3 8 5 3
3 8 5 3