Synthesis and characterization of oligonuclear Ru, Co and Cu oxidation catalysts

Article abstract of DOI:10.1002/ejic.201000758

In this work, we report the preparation and crystal structures of three new oligonuclear complexes, Ru₂(bbpmp)(μ-OAc)₃ (4), [Co₂(bbpmp)(μ-OAc)(μ-OMe)](PF₆) (5), [Cu ₄(Hbbpmp)₂(μ-OAc)(H2O)₂](OAc)(PF ₆)₂ (6) {H₃bbpmp = 2,6-bis[(2-hydroxybenzyl)- (2-pyridylmethyl)aminomethyl]-4-methyl-phenol (3)}. The structures of the complexes were determined by single-crystal X-ray diffraction. The oxidation states of ruthenium, cobalt and copper in the complexes are +3, +3 and +2, respectively. In 4 and 5, Ru^III and Co^III are coordinated to four oxygen and two nitrogen atoms in an octahedral geometry, while in 6, Cu^II adopts both octahedral (CuN₂O₄) and square-pyramidal (CuN₂O₃) geometry. The potential of the three complexes as oxidation catalysts has been investigated. Copyright

Full text of DOI:10.1002/ejic.201000758

FULL PAPER
DOI: 10.1002/ejic.201000758
Synthesis and Characterization of Oligonuclear Ru, Co and Cu Oxidation
Catalysts
Bao-Lin Lee,[a] Markus D. Kärkäs,[a] Eric V. Johnston,[a] Andrew K. Inge,[b]
Lien-Hoa Tran,[a] Yunhua Xu,[c] Örjan Hansson,[d] Xiaodong Zou,[b] and Björn Åkermark*[a]
Keywords: O-O activation / Homogeneous catalysis / N,O ligands / Oxidation
In this work, we report the preparation and crystal structures
of three new oligonuclear complexes, Ru2(bbpmp)(μ-OAc)3
(4), [Co2(bbpmp)(μ-OAc)(μ-OMe)](PF6) (5), [Cu4(Hbbpmp)2-
states of ruthenium, cobalt and copper in the complexes are
+3, +3 and +2, respectively. In 4 and 5, RuIII and CoIII are
coordinated to four oxygen and two nitrogen atoms in an oc-
tahedral geometry, while in 6, CuII adopts both octahedral
(CuN2O4) and square-pyramidal (CuN2O3) geometry. The
(μ-OAc)(H2O)2](OAc)(PF6)2 (6) {H3bbpmp
=
2,6-bis[(2-
hydroxybenzyl)-(2-pyridylmethyl)aminomethyl]-4-methyl-
phenol (3)}. The structures of the complexes were deter- potential of the three complexes as oxidation catalysts has
mined by single-crystal X-ray diffraction. The oxidation
been investigated.
seemed interesting to study if 4 and the related cobalt and
copper complexes 5 and 6 could be used as catalysts in
Introduction
Efficient catalysts for water oxidation are essential for “green” oxidation of organic substrates by molecular oxy-
construction of a device that can mimic photosystem II gen.
(PSII) in the natural photosynthetic apparatus. The water
oxidation catalyst of PSII consists of a tetranuclear manga-
nese cluster, the oxygen evolving complex (OEC).[1] Al-
though a fair number manganese-based OEC mimics have
been prepared, none is an efficient water oxidation cata-
lyst.[2] By contrast, several ruthenium complexes, starting
with Meyers blue dimer,[3] have been shown to be efficient
catalysts.[4]
Results and Discussion
Syntheses and Crystal Structures of 4–6
The ligand 3 was synthesized by using a slightly modified
published procedure,[6] (Scheme 1). The bis(chloromethyl)
compound 1 was first prepared from 2,6-bis(hydroxymeth-
yl)-4-methylphenol and then reacted with amine 2, which
was in turn prepared by reductive amination of salicylalde-
hyde with picolylamine.
A dinuclear Mn complex of the anion of ligand 3
(bbpmp) has been studied earlier,[5] but exploratory
attempts to use this for catalytic water oxidation have failed.
We therefore decided to prepare the related Ru complex 4
and test this. Unfortunately, also complex 4 failed to cata-
lyze water oxidation, probably because of oxidative degra-
dation of the ligand. However, there is a demand for oxi-
dation processes, which use a catalyst and oxygen as ter-
minal oxidant. In these processes oxygen is converted to
water, which is the reverse of water oxidation. It therefore
[a] Department of Organic Chemistry, Stockholm University,
10691 Stockholm, Sweden
E-mail: bjorn.akermark@organ.su.se
[b] Structural Chemistry, Department of Materials and
Environmental Chemistry, Stockholm University,
10691 Stockholm, Sweden
[c] Department of Chemistry, School of Chemical Science and
Engineering, Royal Institute of Technology (KTH),
10044 Stockholm, Sweden
[d] Biophysics Group, Department of Chemistry, University of
Gothenburg,
P. O. Box 462, 40530 Gothenburg, Sweden
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201000758.
Scheme 1. Syntheses of complexes 4–6.
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2010, 5462–5470
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Oligonuclear Ru, Co and Cu Oxidation Catalysts
The ruthenium complex 4 was prepared by adding
RuCl3·xH2O and NaOAc to a suspension of ligand 3 in
MeOH. The reaction was heated at reflux for 48 h and puri-
fied by column chromatography to afford the dark-blue Ru
complex.
Complexes 5 and 6 were formed in a similar way by add-
ing NaOAc and Co(OAc)2·4H2O or Cu(OAc)2·4H2O,
respectively, to a suspension of ligand 3 in MeOH. The re-
action mixture was heated at reflux overnight, then cooled
and filtered, and finally KPF6 was added. The copper com-
plex 6 precipitated directly and could be isolated as a green
powder, while the cobalt complex 5 required a couple of
days to precipitate as a brown solid.
The crystal structures of the metal complexes 4, 5 and 6
were determined from single-crystal X-ray diffraction data.
The crystal data and structure refinement parameters are
given in Table 3. Selected bond lengths of 4, 5, and 6 are
listed in Table S1.
Figure 1. X-ray structure of the cation [Ru2(bbpmp)(μ-OAc)2]+
from 4 at a 30% probability level. Hydrogen atoms and solvent
molecules are omitted for clarity.
the coordination of the alcohols, which could explain the
relatively inefficient oxidation of secondary alcohols to
ketones.
The crystal structure of
4 contains one unique
[Ru2(bbpmp)(μ-OAc)2]+ complex and seven unique water
molecules. The two ruthenium atoms have similar octahe-
dral coordination environments, each coordinated to one
pyridyl and one tertiary amine nitrogen atom, two phenol-
ate oxygen atoms, and two oxygen atoms from two different
acetates. The ruthenium atoms are bridged by one oxygen
atom of the central phenolate of the bbpmp and two biden-
tate acetates (Figure 1). The dinuclear Ru complex adopts
a pseudo-twofold symmetry through the C–O bond of the
bridging phenolate, which is similar to that in
[Fe2(bbpmp)(μ-OAc2)]ClO4·H2O.[7] The bond valence sum
(BVS) for each ruthenium centre is 3.25, which suggests
that the oxidation state of both ruthenium atoms is 3. This
is also supported by mass spectrometry (MS), UV/Vis and
EPR spectroscopy, and cyclic voltammetry (CV). Seven
unique oxygen positions, which form hydrogen bonds to the
two terminal phenolate oxygen atoms and one acetate oxy-
gen atom, were located in the structure refinement. How-
ever, the third acetate counterion, which is suggested by ele-
mental analysis, could not be located, presumably because
of disorder in the crystal. The actetate counterion is
The crystal structure of
5 contains one unique
[Co2(bbpmp)(μ-OAc)(μ-OMe)]+ complex and one unique
hexafluorophosphate anion. As in complex 4, the two co-
balt atoms are octahedrally coordinated (Figure 2), each to
two nitrogen atoms and two oxygen atoms of the bbpmp
ligand, one oxygen atom of the bridging acetate, and the
oxygen atom of the bridging methoxy group. The dinuclear
CoIII complex adopts a pseudomirror symmetry with the
mirror plane perpendicular to Co(1)–O–Co(2). BVS calcu-
lations indicate that all oxygen atoms of the ligands are de-
protonated, which suggests that both cobalt atoms have a
charge of +3. This is in accord with an earlier report, which
shows that cobalt complexes prepared from CoII and phen-
olic ligands are readily converted to CoIII complexes.[13]
strongly suggested by the fact that the complex has a
III/III
Ru2
oxidation state.
There seem to be few X-ray crystal structures known for
complexes related to 4, but it is interesting to compare the
Ru-phenolate bond length of 4 to those of other phenolate
complexes of ruthenium of different oxidation states. The
Ru–O bond to the terminal phenolate is ca 1.95 Å in 4,
which seems to be unusually short. In two formally RuIII
diamide-phenolate complexes, the Ru–O bond is distinctly
longer, ca. 2.03 Å.[8] Even longer Ru–O phenolate distances
were observed for a series of mononuclear RuII complexes
Figure 2. X-ray structure of the cation [Co2(bbpmp)(μ-OAc)(μ-
OMe)]+ from 5 at a 30% probability level. Hydrogen atoms and
solvent molecules are omitted for clarity.
Somewhat surprisingly, the copper complex 6 crystallizes
with a bidentate N–O(phenolate) ligand (2.03–2.10 Å),[9–11] as a linear tetramer. The crystal structure contains one
and in a formally RuIII biscarbene complex, the bond unique [Cu4(Hbbpmp)2(μ-OAc)(H2O)2]3+ complex. The +3
length is 2.27 Å.[12] The significance of the short Ru–O charge is balanced by two unique hexafluorophosphate
bond in 4 is not clear, but perhaps it is the result of an anions and one unique hydrogen-bonded acetate group. The
unusually electrophilic RuIII centre. The ligand in the com- two central copper atoms of the tetranuclear complex adopt
plex should be fairly rigid, as indicated by the structure of a distorted octahedral coordination, while the two terminal
the complex 4 (Figure 1). This could lead to steric effects in copper atoms adopt a square-pyramidal coordination (Fig-
Eur. J. Inorg. Chem. 2010, 5462–5470
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B. Åkermark et al.
FULL PAPER
ure 3). As in the metal centres in 4 and 5, each copper ion is should rearrange and dissociate to dinuclear complexes
coordinated to one pyridyl and one tertiary amine nitrogen fairly readily. The fact that the cobalt atoms in 5 and the
atom. In addition, each central copper atom is coordinated central copper atoms in 6 have two M–O–M connections
to three phenolate groups and one acetate oxygen atom, results in much smaller M–O–M (93.7°–99.2°) and O–M–
while each terminal copper atom is coordinated to one O (80.5°–82.0°) angles than the corresponding angles when
phenolate, one phenol and one water oxygen atom. The cen- there is only one M–O–M connection: 110.0°–119.6° and
tral copper atoms are bridged through one acetate and two ca. 90°, as in 4 and between the terminal and central copper
phenolate groups from two different bbpmp ligands, while atoms in 6, respectively (see Table S1).
the terminal and central copper atoms are bridged through
one phenolate only.
The Co–Co distance (2.90 Å) in 5 is much smaller than
the Ru–Ru distance (3.50 Å) in 4, partly because of the
double Co–O–Co bridges. However, the central copper
atoms also have double bridges, yet the Cu–Cu distance is
fairly long (3.28 Å). This is due to the longer distances from
copper to the axial oxygen atoms, caused by the Jahn–Teller
effect. This effect is shown even more clearly by the long
Cu–Cu distance between the terminal and central copper
atoms (3.49–3.70 Å).
The octahedra in 5 are more distorted than those in 4,
because of the double M–O–M bridges and the octahedra
in 6 are even more so because of both the double Cu–O–
Cu bridges (Cu central) and the Jahn–Teller effect. In all
complexes, the amine and pyridyl groups from the same
bbpmp ligand are coordinated in a cis fashion to the metal
ions, with an N–M–N angle between 81.8° and 84.4°, which
indicates a geometrical strain in the ligand. The phenolate
groups from the same bbpmp ligand are coordinated in a
trans manner to the metal ions in 4 and 5, but cis to the
metal ions in 6.
Figure 3. X-ray structure of the trication [Cu4(Hbbpmp)2(μ-
OAc)(H2O)2]3+ from 6 at a 30% probability level. Hydrogen atoms
and solvent molecules are omitted for clarity.
The complexes 4 and 6 have pseudo-twofold symmetry,
The tetranuclear copper complex adopts a pseudo-two- while 5 has a pseudomirror symmetry. In 4 and 6, there are
fold symmetry along the C–C bond of the bridging acetate π–π interactions between the phenolate and pyridyl rings,
between the central copper atoms Cu2 and Cu3. The dis- but not in 5 because of the short Co–Co distance. Addition-
torted coordination at the axial positions is a result of the ally, in complex 6, the terminal phenol and pyridyl rings,
Jahn–Teller effect, common for CuII complexes. The ter- which are coordinated to the same terminal copper atom,
minal copper atoms, Cu1 and Cu4, have identical coordina- can also interact as a result of the longer axial coordination
tion environments, in which amine and pyridyl nitrogen to the phenol oxygen atom. Finally, both bridging acetate
atoms of the bbpmp ligand are coordinated cis relative to groups in 4 are coordinated in a trans fashion to the amine
one another, and one oxygen atom of a central phenolate and pyridine groups, while in 5, the acetate is coordinated
group and one water molecule complete coordination in the in a trans manner only to the pyridine groups, and in 6,
equatorial plane. The terminal phenol groups in the axial only to the amine groups.
position have considerably longer bonds to the CuII atoms
(2.41 and 2.44 Å) than the bridging phenolate groups (1.89
and 1.91 Å). The low bond valence with an average value
of 0.135 between Cu and the terminal phenol oxygen atoms
Electrochemistry
O2 and O7 and the long bonds suggest that these phenol
The cyclic voltammograms of ligand 3 and complexes 4–
groups are protonated. Similarly, the phenol groups coordi- 6 were recorded in 1 mm solution in acetonitrile (MeCN)
nated axially to CuII in the alkoxide-bridged complex with Bu4NPF6 as the supporting electrolyte, a glassy carbon
[Cu2(H2bbppnol)(OAc)(H2O)2]Cl2·2H2O have been re- disk as the working electrode and Ag/AgNO3 as the refer-
ported to have protonated oxygen atoms.[14]
ence electrode in the potential range of –2.0 to +2.0 V vs.
A comparison of the three complexes shows that the Ru– Ag/Ag+. The potentials were measured vs. Fc+/Fc0 and
Ru atoms in 4, the Co–Co atoms in 5 and the central Cu– converted to potentials vs. NHE by adding +0.63 V. The
Cu atoms in 6 are bridged by three molecules: one phenol- cyclic voltammograms of 3 and complexes 4 and 5 are
ate and two acetate groups in 4, one phenolate, one acetate shown in Figure 4. The copper complex 6 did not give a
and one methoxy group in 5, and two phenolate groups and proper CV but there are indications of redox peaks at –1.17
one acetate group in 6. By contrast, the terminal and cen- and –0.81 V vs. NHE that can perhaps be ascribed to
tral copper atoms in 6 are bridged through only one phenol- CuIICuII Ǟ CuIICuI and CuIICuI Ǟ CuICuI processes in
ate group. This together with the fact that the terminal cop- accordance with a previous report on a related alkoxide-
per atoms are only five-coordinate suggest that the complex bridged complex.[15]
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Oligonuclear Ru, Co and Cu Oxidation Catalysts
Figure 5. UV/Vis absorption spectra of ligand 3 and complexes 4–
6 in dry acetonitrile (c = 0.1 mm).
Figure 4. Cyclic voltammograms of ligand 3 and complexes 4 and
5. Conditions: 1.0 mm of 3, 4 or 5 in MeCN with 0.1 m Bu4NPF6
as supporting electrolyte.
By contrast, the Ru complex 4 displays a strong and
broad visible absorption with a peak at 634 nm (ε =
6820 m–1 cm–1), which can be assigned to a LMCT transi-
tion, and a band at 331 nm (ε = 8150 m–1 cm–1). This spec-
trum closely resembles that of the dinuclear RuIII/III com-
plex containing the ligand 2,6-bis[bis(2-pyridylmethyl)-
aminomethyl]-4-methylphenol and supports the predicted
oxidation state of complex 4.[16,18]
The electrochemistry of the Ru complex 4 exhibits two
reversible one-electron redox couples with E1/2 values of
–0.76 and –0.25 V vs. NHE, which are assigned to the re-
ductions RuIIIRuII Ǟ RuIIRuII and RuIIIRuIII Ǟ RuIIIRuII,
respectively, and a irreversible peak at Epa = +0.94 V vs.
NHE, which is assigned to the oxidation RuIIIRuIII Ǟ RuIV-
RuIII. This may be compared with the values of RuIIIRuII
Ǟ RuIIRuII and RuIIIRuIII Ǟ RuIIIRuII for the related dinu-
clear Ru complex containing the monoanionic ligand 2,6-
bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol, with
E1/2 values of +0.02 and +0.72 V vs. NHE.[16] These metal-
EPR Spectroscopy
The Ru complex 4 was also studied with X-band EPR
centred oxidations for complex 4 are thus lowered by 0.78 spectroscopy at 77 K. Frozen solutions of 4 at a concentra-
and 0.97 V, respectively, which is close to the values ob- tion of 1 mm in either dcm, chloroform or thf gave only
served earlier for tris(bipyridine)-type complexes, where the weak signals with g values of approximately 2.39, 2.28, and
oxidation potential decreases by ca 0.4 V per added nega- 1.72. These are probably due to the small amount of mixed
II/III
tive substituent, in this case, the carboxylate groups.[17] Two valence species (Ru2
or Ru2III/IV),[16] while the major
III/III
irreversible oxidation peaks are also observed at ca. 1.24 part of the complexes resides in the EPR-silent Ru2
and 1.46 V vs. NHE, tentatively assigned to ligand oxi- state.
dations. The values for 4 are also close to those found for
a similar complex, where the terminal phenol groups of the
ligand are di-tert-butylphenol instead of a simple phenol
Catalytic Activity
(–0.85, –0.62 and +0.75 V vs. NHE).[18] The Co complex 5
shows one reversible redox couple with an E1/2 value of
The oxidation of alcohols to ketones and aldehydes with
+0.72 V vs. NHE, one quasireversible reduction peak at
ruthenium catalysts has been studied extensively earlier.[19a]
–0.70 V vs. NHE and one irreversible oxidation peak at
Both RuCl3 and RuCl2(Ph3P)2,[19b] and trinuclear ruthe-
+1.25 V vs. NHE.
nium carboxylates[19c] have been shown to be viable cata-
lysts for the aerobic oxidation of alcohols to aldehydes and
ketones.
UV/Vis Spectroscopy
In order to study the catalytic properties of complex 4,
The UV/Vis spectra of ligand 3 and complexes 4–6 were we first tried the oxidation of benzylic alcohols by using
recorded in acetonitrile and are shown in Figure 5. The oxygen as the oxidant. The results proved to be difficult to
spectra of the Co and Cu complexes 5 and 6, respectively, reproduce, which suggested that the aromatic nucleus was
are quite similar, and lack intense absorption bands in the oxidized. Hydrogen peroxide and tert-butyl hydroperoxide
region above 550 nm. The Co complex 5 has a broad ab- were also evaluated as oxidants, and benzyl alcohol was the
sorption band at 406 nm (ε = 3128 m–1 cm–1) with a shoul- model substrate. Although they gave good conversion, the
der at 480 nm (ε = 2470 m–1 cm–1). The spectrum of the Cu selectivity was poor (aldehyde/carboxylic acid 1:1). How-
complex 6 shows a broad absorption band at 438 nm (ε = ever, with diacetoxyiodobenzene as oxidant, which should
2840 m–1 cm–1) and
2230 m–1 cm–1).
a
shoulder at 470 nm (ε
=
give a Ru-oxo intermediate, the aldehyde was obtained in
excellent yield and selectivity (Scheme 2, Table 1).
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Scheme 2. Oxidation of alcohols with Ru complex 4 as catalyst.
Table 1. Oxidation of alcohols with Ru complex 4 as catalyst.[a]
Scheme 3. Coupled aerobic oxidation, where complexes 5 and 6
were evaluated as oxygen-activating catalysts.
The cobalt complex 5 was tested as an oxygen-activating
catalyst in allylic acetoxylation of alkenes (Scheme 4).[21,22]
Three different olefins were studied, cyclohexene, allyl-
benzene and 1-octene. The corresponding allylic acetates
were formed in moderate yields (Table 2). However, the re-
action was not optimized since it was only used for demon-
strating the properties of complex 5, and complete conver-
sion of the alkenes was in fact observed. Complex 5 thus
appears to compare favourably with common oxygen-acti-
vating catalysts, such as cobalt salen complexes[22] and
phthalocyanins.[21b,23]
.
[a] Complex 4 (0.003 mmol) and dodecane (0.15 mmol) were dis-
solved in dry MeCN/THF (1 mL, 1:1). The substrate (0.15 mmol)
and iodobenzene diacetate (0.6 mmol) were added. The mixture
was degassed and flushed with argon and heated to 40 °C.
[b] Determined by GC, with dodecane as internal standard {yield
(%) = [nprod.]/[nsubs.(t=0)]ϫ100; conversion (%) = (1 – [nsubs.]/
[nsubs.(t=0)])ϫ100}. [c] Ͻ1% of the corresponding carboxylic acid
was observed. [d] 73% after 20 h. [e] Decane was used as internal
standard.
Scheme 4. Aerobic acetoxylation, where Co complex 5 was used as
an oxygen-activating catalyst.
Table 2. Biomimetic aerobic acetoxylation of olefins with Co com-
plex 5 as oxygen-activating catalyst.[a]
Both benzylic and aliphatic primary alcohols could be
converted into the corresponding aldehydes in excellent
yield and selectivity (only traces of the carboxylic acid were
observed). The oxidation of secondary alcohols was less ef-
ficient (see Table 1, Entries 3, 4 and 7), with the exception
of the cyclic alcohol, indanol (Entry 2). The result could
perhaps be due to steric hindrance, caused by the fairly tight
structure of complex 4.
Control experiments, without catalyst 4, were performed
and resulted in ca. 10% conversion.
[a]
Pd(OAc)2
(0.034 mmol),
hydroquinone
(0.14 mmol),
LiOAc·2H2O (0.14 mmol) and complex 5 (0.014 mmol) were dis-
solved in AcOH (2 mL). The mixture was stirred under air for
20 min, and the flask was then sealed, degassed and purged with
oxygen. Finally, the substrate (0.68 mmol) was added, and the reac-
tion mixture was heated to 50 °C. [b] Isolated yield.
The introduction of electron-donating groups, expected
to give more electron-rich substrates, was expected to facili-
tate the oxidation of alcohol to ketone. However, no such
effect could be observed (Entries 3 and 4). The introduction
of a methyl group at the ortho position on the aromatic ring
was expected to cause steric hindrance and slow down the
The Cu complex 6 shows no activity as an oxygen-activa-
reaction. 2-Methylbenzyl alcohol was chosen as the sub- ting catalyst in the aerobic acetoxylation of olefins, presum-
strate, but the reaction gave essentially the same high yield ably because the oxidation of palladium(0) is too slow to
as the parent benzyl alcohol (see Table 1, Entries 1 and 5). compete with precipitation of metallic palladium. However,
It has been demonstrated earlier that Co and Cu com- like the related dinuclear Cu complex with a bridging alk-
plexes are effective catalysts for the activation of molecular oxide group,[15] it did function as a catalyst for the aerobic
oxygen.[20] We therefore decided to evaluate the potential of oxidation of 3,5-di-tert-butylcatechol. It is thus a cate-
our newly synthesized catalysts in coupled aerobic oxi- cholase mimic, and the rate of oxidation is comparable to
dation reactions (Scheme 3).
that of the alkoxide-bridged complex.[15]
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Oligonuclear Ru, Co and Cu Oxidation Catalysts
discs. The UV/Vis absorption spectra were measured on a CARY
300 Bio UV/Vis spectrophotometer.
Conclusions
The ruthenium, cobalt and copper complexes are ef-
ficient catalysts for different reactions such as the oxidation
of alcohols to aldehydes and ketones, allylic oxidation and
the oxidation of 3,5-di-tert-butylcatechol to the correspond-
ing ortho-quinone. This suggests that the configuration of
ligand 3, in which there are three phenolate groups and two
amine functions that can bind at least two metal ions, is
a favourable arrangement for producing efficient oxidation
catalysts. Although all complexes efficiently activate oxy-
EPR spectra were recorded at 77 K on a Varian E9 spectrometer
equipped with a quartz insert for liquid nitrogen. Measurements
were made with a microwave frequency of 9.12 GHz, a microwave
power of 2 mW and a modulation amplitude of 2 mT.
Single crystal X-ray diffraction data of 4 (dark-blue platelike crys-
tal), 5 (brown plate crystal) and 6 (green platelike crystal) were
collected on a MarCCD at 100 K by using synchrotron radiation
(λ = 0.9077 Å) at the beam line I-911–2, Max Lab, Sweden. Twin-
solve was used for indexing, data reduction, and numerical absorp-
gen, there are substantial differences in their catalytic ac- tion correction on the data. The structures were solved by direct
methods with SHELXS97.[24] All non-hydrogen atoms of the com-
tivity. Since the oxidation of alcohols probably proceeds
through β-elimination, one can perhaps conclude that more
plexes were refined with anisotropic thermal parameters by using
a full-matrix least-squares technique on F2 with SHELXL97.[24]
facile β-elimination with ruthenium is the reason that this
Hydrogen atoms were geometrically positioned. The thermal dis-
catalyst is more efficient than the others. None of the com-
placement parameters of hydrogen atoms were set to 1.2 times the
plexes is capable of catalyzing water oxidation, probably be-
thermal parameter of the bonded aromatic carbon or 1.5 times the
cause the ligand is oxidized at the required high oxidation
thermal parameter of the bonded methyl carbon. Intensity data
potentials. However, the copper complex 6 is very interest-
was truncated at a resolution of 0.86 Å for 6. One of the toluene
ing in relation to water oxidation, since it is tetranuclear, as
molecules in 6 was disordered into two positions with an occu-
pancy of 0.5 each. Crystallographic details of 4, 5 and 6 are pro-
is the manganese complex in the OEC of PSII.[1] Because
the oxidation of catechol is not cleanly first order, it is prob- vided in Table 3. SQUEEZE was applied by using PLATON to
manage solvent accessible voids.[25] BVS values were obtained from
able that 6 is partly dissociated in solution. However, it is
interesting to compare it with the dinuclear alkoxide-
Bond Valence Calculator.[26] CCDC-783630 (for 4), -783631 (for 5)
and -783632 (for 6) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
diate is in fact tetranuclear. This ease of formation of a
ac.uk/data_request/cif.
bridged complex, which reacts with catechol in a second
order reaction.[15] This suggestsg that the reactive interme-
tetranuclear complex suggests that also other metals, e.g.
manganese, might form tetranuclear complexes as interme-
diates with ligands related to compound 3. On the basis of
these results with complexes of ligand 3, we are now focus-
ing on the synthesis of such ligands, which are more stable
towards oxidative degradation.
Synthesis of 2,6-Bis(chloromethyl)-4-methylphenol (1): 2,6-Bis-
(hydroxymethy1)-4-methylphenol (2.50 g, 14.9 mmol) was added to
SOCl2 (20 mL) at 0 °C. After warming to ambient temperature,
the reaction was stirred for 12 h and SOCl2 was evaporated. The
remaining residue was dissolved in CH2Cl2 and washed with water
(3ϫ50 mL) and dried with anhydrous MgSO4. The dried solution
was partly evaporated at reduced pressure. Pentane was layered on
top, and the flask was put in the refrigerator to give crystals, which
Experimental Section
were filtered and washed with pentane to give the title compound
as yellow–grey needles (1.98 g, 65%). 1H NMR (400 MHz, CDCl3,
Chemicals were purchased from commercial suppliers at reagent
grade purity or better and used without further purification unless
25 °C): δ = 7.09 (s, 2 H, Ar-H), 4.66 (s, 4 H, 2 CH2), 2.28 (s, 3 H,
CH3) ppm. 13C NMR (100 MHz, CDCl3, 25 °C): δ = 150.99,
otherwise stated. All solvents were dried by standard methods
131.61, 130.55, 124.63, 42.48, 20.32 ppm. ESI-HRMS calcd. for
when needed. 1H NMR and 13C NMR were recorded on a
C9H10Cl3O [M + Cl–] 238.9803; found 238.9809.
400 MHz spectrometer using [D1]chloroform (δ =7.26 ppm 1H,
77 ppm 13C) as internal standard. Flash chromatography was car-
ried out with a 60-Å (particle size 35–70 μm) normal-phase silica
gel. High resolution mass spectra measurements were recorded on
a Bruker Daltonics microTOF spectrometer with an electrospray
ionizer. Cyclic voltammetry measurements were carried out with an
Autolab potentiostat with a GPES electrochemical interface (Eco
Chemie), by using a glassy carbon disk (diameter 3 mm) as the
working electrode, a platinum wire as counter electrode. The refer-
ence electrode was a non-aqueous Ag/Ag+ electrode (0.1 m AgNO3
in acetonitrile). The supporting electrolyte used was 0.1 m Bu4NPF6
in acetonitrile. The reference electrode had a potential of –0.055 vs.
the ferrocenium/ferrocene (Fc+/0) couple in acetonitrile as an exter-
Synthesis of (2-Hydroxybenzyl)(2-pyridylmethyl)amine (2): To a
solution of salicylaldehyde (0.61 g, 5.0 mmol) in MeOH (10 mL)
was added 2-(aminomethyl)pyridine (0.54 g, 5.0 mmol). The clear
yellow solution was stirred at ambient temperature for 20 min. At
ambient temperature, NaBH4 (0.19 g, 5.00 mmol) was added in
small portions, which led to the development of a gas. The reaction
mixture became colourless. After stirring at ambient temperature
for 1 h, the solvent was evaporated. H2O (10 mL) was added to the
residue, and the pH was adjusted to 7 by 2 m HCl. The solution
was extracted with CH2Cl2 (3ϫ6 mL). The organic phases were
combined and dried with MgSO4. Evaporation of the solvent gave
light-brown crystals. Purification by silica flash chromatography
(EtOAc) gave the desired product as pale-yellow crystals (0.76 g,
70%). 1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.56–8.59 (m, 1 H,
nal standard. All potentials reported were determined vs. Fc+/0
.
They are reported vs. NHE by adding 0.63 V to the E1/2 value of
Fc+/0. Half-wave potentials (E1/2) were determined by CV as the
average of the anodic and cathodic peak potentials [E1/2 = (Epa
+
Epc)/2]. Reversibility was confirmed by peak splits of ΔE = 60–
70 mV and ipa/ipc = 1. IR spectra were recorded on a Perkin–Elmer
Spectrum One spectrometer by using samples prepared as KBr
Eur. J. Inorg. Chem. 2010, 5462–5470
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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B. Åkermark et al.
FULL PAPER
Table 3. Crystal data and structure refinement for complexes 4, 5 and 6.
4·7H2O
5
6·2(toluene)
[a]
Empirical formula
Fw
T (K)
C41H56N4O16Ru2
1063.04 [a]
100
C38H39Co2F6N4O6P
910.56
100
C88H94Cu4F12N8O12P2
1999.81
100
Wavelength (Å)
Crystal system
Space group
0.9077
orthorhombic
Pbca
0.9077
monoclinic
C2/c
0.9077
monoclinic
P21/n
a (Å)
b (Å)
c (Å)
13.476(3)
20.816(4)
30.892(6)
–
8665(3)
8
1.517
0.765
4008
30.779(6)
18.618(4)
15.661(3)
109.61(3)
8454(3)
8
1.431
0.897
14.922(3)
25.504(5)
24.265(5)
103.87(3)
8965(3)
4
1.482
1.061
β (°)
V (Å3)
Z
Dcalcd. (g/cm3)
Absorption coefficient (mm–1)
F(000)
3728
4112
Crystal size (mm)
θ range for data collection (°)
Index ranges
0.021ϫ0.086ϫ0.097
3.27 to 32.27
–15 Յ h Յ 15, –24 Յ k Յ 24,
–36 Յ l Յ 36
0.028ϫ0.071ϫ0.117
3.53 to 33.45
–37 Յ h Յ 37, –20 Յ k Յ 20,
–18 Յ l Յ 18
53836
0.042ϫ0.080ϫ0.226
3.25 to 31.85
–17 Յ h Յ 17, –29 Յ k Յ 29,
–27 Յ l Յ 27
103476
Reflections collected
102439
Independent reflections
Completeness to θmax (%)
Absorption correction
Max. and min. transmission
Refinement method
7771 [R(int) = 0.1130]
0.997 to θ = 32.27°
empirical
1.000 and 0.9823
full-matrix least squares on F2
7365/0/369
7422 [R(int) = 0.0572]
0.935 to θ = 33.45°
empirical
1.000 and 0.9734
full-matrix least squares on F2
7422/0/390
13866 [R(int) = 0.0779]
0.939 to θ = 31.85°
empirical
1.000 and 0.9829
full-matrix least squares on F2
13866/5/721
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
R indices (all data)
1.001
1.051
1.058
R1 = 0.0940, wR2 = 0.2537
R1 = 0.1174, wR2 = 0.2704
R1 = 0.0670, wR2 = 0.1869
R1 = 0.0839, wR2 = 0.1978
0.539 and –0.586
R1 = 0.0905, wR2 = 0.2424
R1 = 0.1159, wR2 = 0.2615
1.011 and –1.275
Largest diff. peak and hole (e/Å3) 1.699 and –0.916
[a] Empirical formula includes acetate and hydrogen atoms of crystalline water molecules that were excluded in the structure refinement.
pyr-H), 7.65 (td, J = 7.61, 1.73 Hz, 1 H, pyr-H), 7.18–7.22 (m, 2 7.73 Hz, 2 H, Ar-H), 3.85 (s, 4 H, 2 CH2), 3.81 (s, 4 H, 2 CH2),
H, pyr-H), 7.16 (m, 1 H, Ar-H), 6.96 (m, 1 H, Ar-H), 6.86 (dd, J
= 8.08, 1.18 Hz, 1 H, Ar-H), 6.78 (td, J = 7.40, 1.30 Hz, 1 H, Ar-
H), 4.00 (s, 2 H, CH2), 3.91 (s, 2 H, CH2) ppm. 13C NMR
3.77 (s, 4 H, 2 CH2), 2.21 (s, 3 H, CH3) ppm. 13C NMR (100 MHz,
CDCl3, 25 °C): δ = 157.55, 157.22, 153.75, 148.47, 137.00, 131.11,
129.90, 128.93, 127.74, 123.55, 122.80, 122.31, 118.96, 116.52,
(100 MHz, CDCl3, 25 °C): δ = 158.24, 157.79, 149.47, 136.68, 57.57, 56.68, 54.55, 20.45 ppm. IR (KBr disc): ν = 3600–2400 (br.),
˜
128.79, 128.60, 122.69, 122.42, 119.03, 116.44, 53.09, 51.91 ppm.
3406, 3041, 3010, 2913, 2835, 2814, 2700, 1600, 1592, 1488, 1430,
ESI-HRMS calcd. for C13H15N2O [M + H+] 215.1179; found 1368, 1304, 1265, 1253, 1228, 1096, 968, 865, 757 cm–1. ESI-HRMS
215.1189.
calcd. for C35H37N4O3 [M + H+] 561.2860; found 561.2879.
Synthesis of 2,6-Bis[(2-hydroxybenzyl)(2-pyridylmethyl)aminometh-
yl]-4-methylphenol (3): (2-Hydroxybenzyl)(2-pyridylmethyl)amine
(5.22 g, 24.4 mmol) was added to a solution of 2,6-bis(chlorometh-
yl)-4-methylphenol (2.50 g, 12.2 mmol) in CH2Cl2 (40 mL). The re-
action mixture turned brown and cloudy. When Et3N (8.5 mL,
61 mmol) was added, the reaction mixture turned light brown, and
a precipitate formed. After stirring at ambient temperature over-
night, the reaction mixture was diluted with CH2Cl2 (20 mL),
washed with brine (4ϫ50 mL), dried with MgSO4 and evaporated
to give a brown oil. Recrystallization from CH2Cl2/2-propanol gave
a white solid, which was filtered and washed with 2-propanol to
give 3 (5.58 g, 82%). 1H NMR (400 MHz, CDCl3, 25 °C): δ = 8.60–
8.64 (d, J = 4.74 Hz, 2 H, pyr-H), 7.62–7.69 (td, J = 7.97, 1.74 Hz,
2 H, pyr-H), 7.11–7.25 (m, 4 H, pyr-H), 7.11–7.17 (t, J = 7.97 Hz,
Synthesis of Ru2(bbpmp)(OAc)3 (4): RuCl3·xH2O (0.50 g, 1.9 mmol)
and NaOAc (0.88 g, 10.7 mmol) was added to a suspension of li-
gand 3 (0.50 g, 0.89 mmol) in MeOH (25 mL). The dark solution
was heated at reflux for 48 h and then cooled to room temperature.
After the solvent was evaporated, acetone was added, and the insol-
uble salts were filtered. Evaporation of the filtrate gave a dark solid,
which was purified by silica flash chromatography (MeCN/H2O/
NEt3, 90:10:5) to give 4 as a dark-blue solid (0.31 g, 39%). IR (KBr
disc): ν = 3400(br.), 2913, 2844, 1609, 1555, 1477, 1447, 1412, 1381,
˜
1262, 1153, 1108, 1031, 883, 761 cm–1. ESI-HRMS calcd. for
C39H39N4O7Ru2 [Ru2(bbpmp)(OAc)2+] 879.0900: found 879.0902.
C43H51N5O12Ru2 [Ru2(bbpmp)(OAc)3·CH3CN·3H2O] (1032.04):
calcd. C 50.04, H 4.98, N 6.79; found C 49.95, H 4.85, N 6.60.
2 H, Ar-H), 7.00–7.05 (d, J = 7.73 Hz, 2 H, Ar-H), 6.88 (s, 2 H, Single crystals suitable for X-ray crystallography were obtained by
Ar-H), 6.79–6.85 (d, J = 7.97 Hz, 2 H, Ar-H), 6.73–6.78 (t, J = vapour diffusion of toluene into an acetonitrile solution of 4.
5468
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Eur. J. Inorg. Chem. 2010, 5462–5470

Oligonuclear Ru, Co and Cu Oxidation Catalysts
Synthesis of Co2(bbpmp)(OAc)(OMe)(PF6) (5): Ligand 3 (0.50 g,
0.89 mmol) was suspended in MeOH (15 mL). Co(OAc)2·4H2O
(0.46 g, 1.8 mmol) was added, and the colour turned brown. After
addition of NaOAc (0.22 g, 2.7 mmol), the reaction mixture was
heated at reflux overnight and then centrifuged. A pink insoluble
solid was separated from the brown solution. To this brown solu-
tion was added a saturated aqueous solution of KPF6 (5 mL), and
the solution was put in a refrigerator. After a couple of days a
brown precipitate had formed, which was filtered, washed with
absorption band of the product 3,5-di-tert-butyl-o-benzoquinone at
406 nm.
Supporting Information (see footnote on the first page of this arti-
cle): X-ray structures and selected bond lengths and angles of 4, 5,
and 6 are presented.
Acknowledgments
We thank Dr. Junliang Sun and Professors Licheng Sun and Jan-
E. Bäckvall for discussions and The Knut & Alice Wallenberg
Foundation, The Swedish Research Council, and The Göran Gus-
tafsson Foundation for financial support.
H O and Et O and then dried (0.35 g, 43%). IR (KBr disc): ν =
˜
2
2
3413 (br.), 2918, 2842, 1560, 1477, 1412, 1266, 1017, 758 cm–1. ESI-
HRMS calcd. for C38H39Co2N4O6 [Co2(bbpmp)(OAc)(OMe)+]
765.1528; found 765.1523. C42H49Co2F6N4O7P [Co2(bbpmp)-
(OAc)(OMe)(PF6)·Et2O] (984.70): calcd. C 51.23, H 5.02, N 5.69;
found C 52.70, H 5.00, N 5.60. Single crystals suitable for X-ray
crystallography were obtained by vapour diffusion of toluene into
an acetonitrile solution of 5.
[1] a) W. Rüttinger, G. C. Dismukes, Chem. Rev. 1997, 97, 1–24; b)
L. Sun, L. Hammarström, B. Åkermark, S. Styring, Chem. Soc.
Rev. 2001, 30, 36–49; c) M. Yagi, M. Kaneko, Chem. Rev. 2001,
101, 21–36; d) B. Loll, J. Kern, W. Saenger, A. Zouni, J. Biesi-
adka, Nature 2005, 438, 1040–1044; e) K. N. Ferreira, T. M.
Iverson, K. Maghlaoui, J. Barber, S. Iwata, Science 2004, 303,
1831–1838.
[2] a) J. Limburg, J. S. Vrettos, H. Y. Chen, J. C. de Paula, R. H.
Crabtree, G. W. Brudvig, J. Am. Chem. Soc. 2001, 123, 423–
430; b) J. Limburg, G. W. Brudvig, R. H. Crabtree, J. Am.
Chem. Soc. 1997, 119, 2761–2762; c) W. Rüttinger, M. Yagi, K.
Wolf, S. Bernasek, G. C. Dismukes, J. Am. Chem. Soc. 2000,
122, 10353–10357; d) A. K. Poulsen, A. Rompel, C. J. McKen-
zie, Angew. Chem. Int. Ed. 2005, 44, 6916–6920; e) Y. Naruta,
M.-A. Sasayama, T. Sasaki, Angew. Chem. Int. Ed. Engl. 1994,
33, 1839–1841; f) Y. Gao, J. Liu, M. Wang, Y. Na, B. Åkerm-
ark, L. Sun, Tetrahedron 2007, 63, 1987–1994; g) Y. Gao, T.
Åkermark, J. Liu, L. Sun, B. Åkermark, J. Am. Chem. Soc.
2009, 131, 8726–8727; h) C. W. Cady, R. H. Crabtree, G. W.
Brudvig, Coord. Chem. Rev. 2008, 252, 444–455; i) G. Berggren,
Dissertation, Uppsala University, 2009, pp. 22–28; j) S. Mukh-
opadhyay, S. K. Mandal, S. Bhaduri, W. H. Armstrong, Chem.
Rev. 2004, 104, 3981–4026.
[3] a) J. A. Gilbert, D. S. Eggleston, W. R. Murphy, D. A. Gese-
lowitz, S. W. Gersten, D. G. Hodgson, T. J. Meyer, J. Am.
Chem. Soc. 1985, 107, 3855–3864; b) S. W. Gersten, G. J. Sam-
uels, T. J. Meyer, J. Am. Chem. Soc. 1982, 104, 4029–4030; c)
T. J. Meyer, Acc. Chem. Res. 1989, 22, 163–170.
[4] a) F. Liu, J. J. Concepción, J. W. Jurss, T. Cardolaccia, J. L.
Tempelton, T. J. Meyer, Inorg. Chem. 2008, 47, 1727–1752; b)
X. Sala, I. Romero, L. Escriche, A. Llobet, Angew. Chem. Int.
Ed. 2009, 48, 2842–2852; c) Z. Deng, H.-W. Tseng, R. Zong,
D. Wang, R. Thummel, Inorg. Chem. 2008, 47, 1835–1848; d)
Y. Xu, T. Åkermark, V. Gyollai, D. Zou, L. Eriksson, L. Duan,
R. Zhang, B. Åkermark, L. Sun, Inorg. Chem. 2009, 48, 2717–
2719; e) L. Duan, A. Fischer, Y. Xu, L. Sun, J. Am. Chem. Soc.
2009, 131, 10397–10399.
Synthesis of Cu4(Hbbpmp)2(OAc)2(H2O)2(PF6)2 (6): Ligand
3
(0.45 g, 0.80 mmol) was suspended in MeOH (15 mL). When
Cu(OAc)2·4H2O (0.46 g, 1.8 mmol) was added, the colour turned
green. After heating at reflux overnight, a saturated aqueous solu-
tion of KPF6 (12 mL) was added, and a green precipitate was
formed. The mixture was put in the refrigerator overnight for full
precipitation. The solution was centrifuged, and the green solid was
washed with H2O followed with Et2O, and finally dried (0.64 g,
90%). IR (KBr disc): ν = 3600, 3400 (br.), 2908, 2950, 1611, 1565,
˜
1476, 1446, 1380, 1268, 1112, 1083, 1029, 841, 758 cm–1. ESI-
HRMS calcd. for C35H33Cu2N4O3 [Cu2(bbpmp)+] 683.1139; found
683.1140. C74H86Cu4F12N8O16P2 [Cu4(Hbbpmp)2(OAc)2(H2O)2-
(PF6)2·4H2O] (1887.64): calcd. C 47.09, H 4.59, N 5.94; found C
46.20, H 4.10, N 5.95. Plate-shaped crystals suitable for X-ray crys-
tallography were obtained by vapour diffusion of toluene into an
acetonitrile solution of 6.
General Procedure for the Catalytic Oxidations of Alcohols with the
Ru dimer 4: To a solution of the catalyst (3.0 mg, 0.003 mmol) and
dodecane (33.3 μL, 0.15 mmol) in a mixture of freshly distilled thf
and acetonitrile (1:1, 1.0 mL) was added the substrate (0.15 mmol)
followed by the oxidant iodobenzene diacetate (0.193 g,
0.60 mmol). Finally, the mixture was flushed with argon and stirred
at 40 °C. Aliquots were taken and filtered through a silica plug and
analyzed by GC.
General Procedure for the Biomimetic Acetoxylation with the Co
dimer 5: Pd(OAc)2 (7.6 mg, 0.034 mmol), the Co dimer 5 (12.5 mg,
0.014 mmol),
hydroquinone
(15.0 mg,
0.14 mmol)
and
LiOAc·2H2O (22.5 mg, 0.14 mmol) were dissolved in AcOH (2 mL)
and stirred in air for 20 min. The reaction flask was sealed, de-
gassed and purged with oxygen, and finally, the substrate
(0.68 mmol) was added. After stirring at 50 °C for 6–24 h, the reac-
tion mixture was diluted with water and extracted with pentane or
dcm. The organic phases were combined, washed with water, satu-
rated aqueous solution of NaHCO3 and dried with MgSO4. The
solution was filtered and carefully evaporated to yield the product.
[5] M. F. Anderlund, J. Zheng, M. Ghiladi, M. Kritikos, E. Ri-
vière, L. Sun, J.-J. Girerd, B. Åkermark, Inorg. Chem. Commun.
2006, 9, 1195–1198.
[6] B. Krebs, K. Schepers, B. Bremer, G. Henkel, E. Althaus, W.
Müller-Warmuth, K. Griesar, W. Haase, Inorg. Chem. 1994, 33,
1907–1914.
[7] A. Neves, M. A. de Brito, I. Vencato, V. Drago, K. Griesar, W.
Haase, Inorg. Chem. 1996, 35, 2360–2368.
[8] N. L. Fry, M. J. Rose, D. L. Rogow, C. Nyitray, M. Kaur, P. K.
Mascharak, Inorg. Chem. 2010, 49, 1487–1495.
[9] L. Gong, S. P. Mulcahy, K. Harms, E. Meggers, J. Am. Chem.
Soc. 2009, 131, 9602–9603.
[10] C.-F. Leung, S. M. Ng, J. Xiang, W.-Y. Wong, M. H.-W. Lam,
C.-C. Ko, T.-C. Lau, Organometallics 2009, 28, 5709–5714.
[11] Z.-J. Xu, R. Fang, C. Zhao, J.-S. Huang, G.-Y. Li, N. Zhu, C.-
M. Che, J. Am. Chem. Soc. 2009, 131, 4405–4417.
[12] M. Gatti, L. Vielle-Petit, X. Luan, R. Maritz, E. Dinkel, A.
Linden, R. Dorta, J. Am. Chem. Soc. 2009, 131, 9498–9499.
Aerobic Oxidation of 3,5-Di-tert-butylcatechol with Complex 6:
Stock solutions of catalyst 6 (0.5 mm) and 3,5-di-tert-butylcatechol
(0.18 m) in MeOH were prepared. A cuvette with a stirring bar
was charged with MeOH (2.56 mL) saturated with O2, followed by
aqueous tris buffer pH 8.0 (200 μL, 2.7 m) and a solution of 3,5-
di-tert-butylcatechol (100 μL) ([3,5-dtbc]0 ≈ 6.0 mm). Finally, the
reaction was initiated by the addition of a solution of complex 6
(144 μL, [6]0 ≈ 24 μm) at ambient temperature. Oxidation was ob-
served spectrophotometrically by an increase in the characteristic
Eur. J. Inorg. Chem. 2010, 5462–5470
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B. Åkermark et al.
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[13]
[14]
[15]
[20] J. Piera, J.-E. Bäckvall, Angew. Chem. Int. Ed. 2008, 47, 3506–
3523.
[21] a) S. E. Byström, E. M. Larsson, B. Åkermark, J. Org. Chem.
1990, 55, 5674–5675; b) J.-E. Bäckvall, R. B. Hopkins, H.
Grennberg, M. Mader, A. K. Awasthi, J. Am. Chem. Soc. 1990,
112, 5160.
[22] a) J.-E. Bäckvall, R. L. Chowdhury, U. Karlsson, J. Chem.
Soc., Chem. Commun. 1991, 473; b) G.-Z. Wang, U. Andreas-
son, J.-E. Bäckvall, J. Chem. Soc., Chem. Commun. 1994, 1037;
c) G. Csjernyik, A. H. Éll, L. Fadini, B. Pugin, J.-E. Bäckvall,
J. Org. Chem. 2002, 67, 1657–1662.
[23] J.-E. Bäckvall, R. B. Hopkins, Tetrahedron Lett. 1988, 29,
2885–2888.
A. R. Paital, M. Sarkar, M. Mikuiya, D. Ray, Eur. J. Inorg.
Chem. 2007, 4762–4769.
A. Neves, L. R. Rossi, I. Venkato, V. Drago, W. Haase, R. Wer-
ner, Inorg. Chim. Acta 1998, 281, 111–115.
A. Neves, L. M. Rossi, A. J. Bortoluzzi, B. Szpoganicz, C.
Wiezbicki, E. Schwingel, S. Ostrovsky, W. Haase, Inorg. Chem.
2002, 41, 1788–1794.
[16] R. Lomoth, A. Magnuson, Y. Xu, L. Sun, J. Phys. Chem. A
2003, 107, 4373–4380.
[17] T. Norrby, A. Börje, B. Åkermark, L. Hammarström, J. Alsins,
K. Lashgari, R. Norrestam, J. Mårtensson, G. Stenhagen, In-
org. Chem. 1997, 36, 5850–5858.
[18] Y. Xu, unpublished results.
[24] G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
[25] A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7–13.
[26] C. Hormillosa, VALENCE, Version 2.00, Institute for Materi-
als Research, McMaster University, 1993.
[19] a) For a recent overview, see: W. C. Arends, R. A. Sheldon in
Modern Oxidation Methods (Ed.: J.-E. Bäckvall), Wiley VCH
Verlag, Weinheim, 2004, p. 83–118; b) F. H. Jardine, Prog. In-
org. Chem. 1984, 31, 265–370; c) C. Bilgrien, S. Davis, R. S.
Drago, J. Am. Chem. Soc. 1987, 109, 3786–3787.
Received: July 12, 2010
Published Online: October 22, 2010
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Eur. J. Inorg. Chem. 2010, 5462–5470