Catechol Oxidation with Dinuclear Copper Complexes of Aliphatic Tripodal Amino Alcohols

Article abstract of DOI:10.1002/ejic.200500554

A set of unsymmetrical tripodal (aminoalkyl)bis(hydoxyalkyl)-amine ligands H₄-2 to H₄-6 with either ethyl or propyl ligand arms has been prepared and characterized. These ligands react with copper hydroxide and ammonium hexafluorophosphate, copper(II) bromide or tris(triphenylphosphane) copper(I) bromide to give di- and polynuclear complexes. The dinuclear copper complexes 9a-f catalyze the oxidation of 3,5-di-tert-butylcatechol (DTBC). The coordination geometry at the copper center is influenced by the length of the side arms resulting in a different reactivity of complexes 9a-f in the DTBC oxidation. Alternative preparation procedures lead in selected cases to the formation of coordination polymers 10a and 10b. The reversible formation of the poly(dinuclear) cations in 10a illustrates that the apical alcohol groups are labile. The solid-state structures are neither necessarily identical with solution structure nor do they represent the catalytically active species. Electronic spectra, however, agree with a dinuclear structures in solution as indicated by the characteristics of the oxygen-copper-CT. The observation of comparable catalytic activities for complexes 9a-9d renders the bridging coordination of the catechol by both copper ions of a dinuclear complex moiety unlikely. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200500554

FULL PAPER
DOI: 10.1002/ejic.200500554
Catechol Oxidation with Dinuclear Copper Complexes of Aliphatic Tripodal
Amino Alcohols
[a]
[a]
[a]
[b]
[b]
Christoph Jocher, Tania Pape, Wolfram W. Seidel, Patrick Gamez, Jan Reedijk,
and F. Ekkehardt Hahn*[
a]
Keywords: Tripodal ligands / Amino alcohols / Copper / Catechol oxidase
A set of unsymmetrical tripodal (aminoalkyl)bis(hydoxyalkyl)-
amine ligands H -2 to H -6 with either ethyl or propyl ligand
arms has been prepared and characterized. These ligands
react with copper hydroxide and ammonium hexafluorophos-
phate, copper(II) bromide or tris(triphenylphosphane)cop-
10a and 10b. The reversible formation of the poly(dinuclear)
cations in 10a illustrates that the apical alcohol groups are
labile. The solid-state structures are neither necessarily iden-
tical with solution structure nor do they represent the catalyt-
ically active species. Electronic spectra, however, agree with
a dinuclear structures in solution as indicated by the charac-
teristics of the oxygen–copper-CT. The observation of com-
parable catalytic activities for complexes 9a–9d renders the
bridging coordination of the catechol by both copper ions of
a dinuclear complex moiety unlikely.
4
4
per(
dinuclear copper complexes 9a–f catalyze the oxidation of
,5-di-tert-butylcatechol (DTBC). The coordination geometry
I) bromide to give di- and polynuclear complexes. The
3
at the copper center is influenced by the length of the side
arms resulting in a different reactivity of complexes 9a–f in
the DTBC oxidation. Alternative preparation procedures lead
in selected cases to the formation of coordination polymers
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
The diverse applications of tripodal tetraamine ligands
[
1]
in coordination chemistry have been reviewed recently. Se-
veral transition metal complexes of triethanolamine illus-
trate the application of aliphatic tripodal ligands in supra-
molecular chemistry.[ Furthermore, unsymmetrical tripo-
dal ligands are used in modeling active metal sites in en-
2]
zymes. For example, a variety of aromatic tripodal NN O
2
and NNO ligands have been prepared to model the metal
2
site in the copper enzyme galactose oxidase.[ The field of
aliphatic tripodal ligands with unsymmetrical donor func-
tions developed more recently. Some complexes of the un-
3]
[
4]
symmetrical aliphatic tripodal ligands NN O-222 (H -A),
NN O-332 (H -1), NN O-333 (H -B), NNO -222 (H -
2
5
[
5]
2
4c]
5
2
[3i]
5
2
4
[
[6]
2
3
), NNO -233 (H -4), NNO -223 (H -3), and NNO2-
2 4 2 4
[
7]
22 (H -5) have been prepared (Figure 1). Complexes of
4
sulfur containing unsymmetrical tripodal ligands have also
[
8]
been reported.
Figure 1. Aliphatic tripodal amino alcohols.
[
a] Institut für Anorganische und Analytische Chemie and NRW
Graduate School of Chemistry, Westfälische Wilhelms-Uni-
versität Münster,
Others and we have studied the correlation between the
topology of tripodal ligands and the reactivity of their com-
plexes. A remarkable example for the influence of ligand
topology on complex reactivity is the phosphate ester cleav-
age promoted by tris(3-aminopropyl)amine (trpn) com-
Wilhelm Klemm Straße 8, 48149 Münster, Germany
Fax: +49-251-8333108
E-mail: fehahn@uni-muenster.de
[
b] Leiden Institute of Chemistry, Universiteit Leiden,
P. O. Box 9502, 2300 RA Leiden, The Netherlands
E-mail: reedijk@chem.leidenuniv.nl
4914
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2005, 4914–4923

Catechol Oxidation with Dinuclear Copper Complexe
FULL PAPER
plexes of cobalt while the tris(2-aminoethyl)amine (tren) cedures described for the preparation of ligands NNO -222
2
complexes are 300 times less active.[ Other studies found (H -2),
9]
[19]
NN O-222 (H -A),
[20]
NN O-332 (H -1)
[21]
were
4
2
5
2
5
interrelations between the ligand topology and complex found to be less efficient in our hands.
properties for copper()[ and nickel()
10]
[11]
complexes of
The preparation of the diamino-dihydroxy ligand NNO2-
the tetraamine ligands tren, (3-aminopropyl)bis(2-amino- 323 (H -6) (Scheme 1) starts with the Michael addition of
4
[
8a]
ethyl)amine (baep), (2-aminoethyl)bis(3-aminopropyl)- one equivalent ethyl acrylate to ethanolamine.
A small
amine (abap), and trpn. amount of methanol and excess acrylonitrile were added to
In this contribution, we present a study on copper com- the wax-like solid 7. The mixture was heated to 60 °C for
plexes of a set of NNO ligands and extend the relation 24 h. After removal of volatiles compound 8 was isolated
2
between ligand shape and complex properties to catalysis. and the ester and nitrile group were reduced with aluminum
To allow for a fine-tuning of the geometric strain the flexi- hydride at temperatures increasing from 0 °C to ambient
ble aliphatic ligands H -2 to H -6 (Figure 1) and their cop- temperature. The reaction mixture was carefully hydrolyzed
4
4
per() complexes were studied. The copper complexes of after eight hours and H -6 was obtained by Soxhlet extrac-
4
ligands H -1 to H -6 (9a–f) may serve as models of the me- tion with methanol for 24 h. The extract was dried with
5
4
®
tal site in type 3 copper proteins. One member of this group sodium sulfate, filtered through celite and volatiles were
is the ubiquitous plant enzyme catechol oxidase (CO, removed. Bulb to bulb distillation affords ligand NNO -323
2
E.C.1.10.3.1) that converts catechols under consumption of (H -6) as colorless viscous oil.
4
molecular oxygen into o-quinones, which auto-polymerize
to protect the plant from damage.[ Many models for the
12]
active center in CO have been prepared,[13] but so far con-
Solid State Structure of Dinuclear Copper Complexes
version rates of the model compounds are lower by four
orders of magnitude compared to the enzyme.[ In our
study 3,5-di-tert-butylcatechol (DTBC) was chosen as
model substrate (Figure 2). To date different mechanisms
for the oxidation of this substrate are discussed controver-
Copper complexes of the tripodal amino alcohols were
14]
prepared as described for complex [Cu(H -4)] (PF ) ·
3
2
6 2
[2e,3i]
2
MeOH 9c·2MeOH.
For the preparation of the copper
complexes, one equivalent of ligand was added to an equi-
molar suspension of copper() hydroxide in deionized
[
15]
sially based on measurements on model compounds.
water. After stirring for one hour two equivalents NH PF6
4
were added. The solutions were concentrated to approxi-
mately 1 mol/L by warming to 40 °C. Blue solids precipi-
tated upon cooling of these solutions to 5 °C. Diffusion of
diethyl ether into an ethanol (9a), acetonitrile (9b, 9e, 9f),
or methanol (9d) solutions of these residues gave crystals of
[
Cu(H -2) ] (PF ) ·0.5EtOH·0.5H O (9a·0.5EtOH·0.5H O),
3 2 2 6 4 2 2
[
Cu(H -3)] (PF ) ·2H O (9b·2H O), [Cu(H -5)] (PF ) ·
3
2
6 2
2
2
3
2
6 2
CH OH (9d·CH OH), [Cu(H -6)] (PF ) (9e), and [Cu(H -
3
3
3
2
6 2
4
Figure 2. Oxidation of DTBC to 3,5-di-tert-butylquinone (DTBQ). 1)] (PF ) (9f). The molecular structures of these complexes
2
6 2
were determined by X-ray diffraction (Figure 2 and Fig-
ure 3). The dinuclear complexes of type 9 were the domi-
nant species under the reaction conditions selected. All
attempts to isolate and characterize mononuclear com-
plexes by adding more NH PF or NaPF failed.
Results and Discussion
4
6
6
Ligand Synthesis
The structure determinations on 9a, 9b, 9d, 9e, and 9f in
Ethanolamine, propanolamine and diethanolamine, the solid state show dinuclear dicationic complexes
2+
respectively are employed as starting materials for the syn- [Cu L ] as were previously observed for 9c, in which one
2
2
[
16]
thesis of ligands H -1 to H -6. A Strecker synthesis
or deprotonated alcohol group of each ligand serves as bridg-
5
4
Michael addition of acrylic acid derivatives[ is applied to ing donor to the second copper ion in a µ -fashion (Fig-
17]
3
assemble the tripodal backbone. The ester or nitrile func- ure 3 and Figure 4). The second hydroxyl arm of each li-
tions of the protected ligands are reduced with aluminum gand remains protonated as indicated by much longer cop-
hydride prepared in situ.[
18]
Alternative synthetic pro- per–oxygen distances relative to the bridging alkoxide (com-
2 4
Scheme 1. Synthesis of the ligand NNO -323 (H -6).
Eur. J. Inorg. Chem. 2005, 4914–4923
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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C. Jocher, T. Pape, W. W. Seidel, P. Gamez, J. Reedijk, F. E. Hahn
FULL PAPER
Figure 3. Molecular structures of the dinuclear cations of 9b and 9d–f.
pare Cu–O1 and Cu–O2 in Table 1). Comparison of the unit cell of 9d contains two not related halves of the dinu-
2+
various structures reveals that propylalkoxide ligand arms, clear complex ion [9d] . Complex 9a crystallizes in a polar
if available, preferably act as bridging donors. Complexes space group (P2 ) and the unit cell contains four different
1
2+
9
b, 9e, and 9f form centrosymmetric complex ions with a copper atoms of two dinuclear complex dications [9a] .
planar [Cu (OR) ] moiety and with trans-arrangement of The observation of the polar space group P2 for 9a can be
2
+
2
2
1
the apical alcohol functions.
rationalized by the helicity of the dinuclear units and by the
fact that the diamino chelate rings adopt different configu-
rations in the two dinuclear complexes (λλ and λδ, respec-
tively) (Figure 4).
Bond lengths and angles differ between the two dinuclear
units in crystals of 9a. Consequently, four different τ-val-
[
22]
ues
(0.21, 0.32, 0.51, 0.36) were observed for the four
copper atoms in the asymmetric unit, indicating a distorted
tetragonal-pyramidal coordination environment for the me-
tal ions. The Cu–O distances range from 2.177(4) Å to
2.265 Å for the alcohol functions in the apical position.
Within the Cu(µ-O) Cu core the Cu–O2 distances in trans-
2
position to the tertiary nitrogen donor are distinctively
shorter [1.939(3) Å to 1.956(4) Å] than the Cu–O2 bond
lengths trans to the primary nitrogen donor [1.998(4) Å to
2.075(4) Å, Table 1].
While 9d exhibits an almost ideal tetragonal-pyramidal
copper environment (τ = 0.03), 9e and 9f are also close to
such a coordination geometry (τ = 0.10 and τ = 0.12,
respectively). The highest τ-value is observed for 9a and 9b
Figure 4. Molecular structure of the two dinuclear isomeric dicat-
ions in 9a.
Complexes 9a and 9d with ligands H -2 and H -5, (τ = 0.3) which can be attributed to the shorter ethyl ligand
4
4
respectively, adopt a rather bowl-shaped geometry with a arms leading to a distortion towards a trigonal-bipyramidal
2
+
butterfly-like distorted [Cu (OR) ] core and cis-consti- coordination geometry.
2
2
tution of the coordinated alcohol groups. The short ligand
An interesting structural feature of these dinuclear com-
arms of the ethyl alcohol lead to geometric strain and a plexes is the angle at the bridging oxygen atoms. Often, the
II
distortion of the angles at the copper atom (Table 1). magnetic coupling between two paramagnetic Cu centers
Hence, metal ions with a different coordination environ- is antiferromagnetic for angles larger than 96° while a ferro-
ment are found in the crystal structures of 9a and 9d. The magnetic interaction is observed for smaller angles.[
23a,23b]
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Catechol Oxidation with Dinuclear Copper Complexe
FULL PAPER
Table 1. Selected bond lengths [Å] and angles [°] for 9a·0.5EtOH·0.5H
2
2 3
O, 9b·2H O, 9d·2CH OH, 9e, 9f, and 10a.
9 [
a
a]
9b
9c[b]
9d[c]
9e
9f
10a
Cu–O1
Cu–O2
Cu–O2*
Cu–N1
Cu–N2
2.220(4), 2.255(4)
.177(4), 2.265(4)
1.998(4), 2.005(3)
.075(4), 2.019(4)
1.956(4), 1.947(4)
.939(3), 1.945(3)
2.009(5), 2.028(5)
.000(5), 2.018(4)
1.996(4), 1.994(5)
2.337(2)
2.338(4)
2.279(5) 2.283(4)
1.957(3) 1.955(3)
1.966(3) 1.960(3)
2.036(4) 2.026(4)
1.971(5) 1.978(4)
2.358(2)
–
2.500(1)
2
1.9423(15)
1.9357(14)
2.018(2)
1.997(2)
–
1.919(3)
1.941(3)
2.030(4)
1.990(4)
1.9231(12)
1.9525(12)
2.0410(14)
1.9859(15)
1.965(2)
1.952(2)
2.058(2)
2.009(2)
1.927(3)
1.940(3)
2.024(4)
1.987(4)
2
1
2
2.025(4), 2.005(5)
Cu–N3
Cu–Cu
–
–
–
–
2.171(3)
2.9692(7)
–
2.8929(9) 2.9060(9) 2.9368(5)
3.0009(11)
2.9278(10)
3.0129(4)
2.9821(12)
2
.9364(11)
O2*–Cu–N1 O2*–Cu–N1 O2–Cu–N2
175.38(6) 172.77(14) 168.0(2) 168.6(2)
Largest angle
2nd largest angle
Cu–O2–Cu*
O2*–Cu–N1
O2*–Cu–N1 O––Cu––N2 O2–Cu–N2
171.21(5) 161.84(10) 174.9(2)
1
1
66.7(2), 168.3(2)
68.3(2), 169.5(2)
O2–Cu–N2
O2–Cu–N2 O2–Cu–N2 O2*–Cu–N1
O2–Cu–N2 O*–Cu––N1 O2*–Cu–N1
1
1
54.0(2) 149.3(2)
37.7(2), 147.9(2)
157.15(8)
162.0(2)
166.2(2) 166.57(14) 164.93(7)
154.85(8)
169.93(15)
94.30(15), 93.80(15) 98.45(6)
4.45(15), 92.55(14)
102.01(13)
96.91(13) 96.83(13) 102.05(5)
98.57(8)
100.91(15)
9
[
a] The asymmetric unit contains for copper atoms in two dinuclear complexes 9a. [b] Published data.[3i] [c] Atoms labeled with an asterisk
denote atoms from different ligands not necessarily related by a symmetry element.
The bridging alkoxide groups lead to angles below this
value for complex 9a, while the Cu–O–Cu angles fall in the
range 98° and 102° (Table 1) for the other complexes. How-
ever, temperature dependent measurements of the magnetic
susceptibility for 9b–f indicate that the coupling between
the copper centers is antiferromagnetic in all cases.
Electronic Spectra
The electronic spectra of the dinuclear copper complexes
9
a–f show three transitions (Table 2, Figure 5 for 9f). The
Figure 5. Electronic spectrum of 9f in acetonitrile solution.
–1
most intense absorption is observed between 36 500 cm
–
1
(9a) and 39 400 (9e) cm with extinction coefficients be-
tween 4900 L/mol·cm (9c) to 15700 L/mol·cm (9f). This ab-
The second absorption is observed between 27300 cm–1
sorption is assigned to a nitrogen-centered ligand-to-metal (9c, 9e, 9f) and 27800 cm–1 (9a) with extinction coefficients
charge transfer (LMCT) by comparison with a variety of around 2000 L/mol·cm (Table 2). As the intensity of this
copper amine complexes.[
23c]
Hence, a distinct difference in band is similar in all complexes and as the common feature
intensity appears between the complex with the triamine of complexes of type 9 is the dialkoxy bridge, this band is
NN O ligand (9f) and the complexes with the diamine associated with a charge transfer from the alkoxy oxygen
2
[
24]
NNO ligands (9a–9e).
atoms to the copper centers.
This assignment is sup-
2
Table 2. Listing of τ-values, parameters of the electronic spectra, redox potentials, DTBQ yields and turnover numbers upon catalytic
oxidation with complexes 9a–f.
9a
9b
9c
9d
9e
9f
τ-value[22]
UV/Vis [cm ]
ε [L/molcm])
0.21, 0.32, 0.51, 0.36
36500
(6700)
2
(
1
(
0.30
0.18
0.03
0.10
0.12
–1
37300
(7900)
27400
(1800)
16580
(160)
1.01
–1.42
83
216
37000
(4900)
27300
(2000)
17090
(120)
0.90
–1.16
99
227
38800
(7100)
27500
(1000)
16100
(150)
1.05
–1.07
80
94
39400
(8700)
27300
(2300)
16500
(180)
1.42
–1.33
50
140
38230
(15700)
27300
(2080)
15150
(580)
0.87/1.50
–1.78
36
(
7800
540)
5250
170)
1.15/1.27
Reduction vs. Fc/Fc+ [V] –1.43/–1.31 –1.10/–1.00
Oxidation vs. Fc/Fc+ [V]
Yield DTBC [%]
15
180
–
1
TON [h ]
504
Eur. J. Inorg. Chem. 2005, 4914–4923
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C. Jocher, T. Pape, W. W. Seidel, P. Gamez, J. Reedijk, F. E. Hahn
FULL PAPER
ported by the intensity decrease observed for complexes 9a
and 9d. Within these two complexes the Cu(µ-O) Cu rings
2
are distorted, because five-membered chelate rings are in-
volved in the formation of the bridging four-membered ring
(Figure 4). Apparently, the planarity of this four-membered
ring is responsible for an optimum absorption. The ob-
served difference of the extinction coefficient for the band
–
1
around 27000 cm between the cis structures 9a and 9d and
the trans structures 9b, 9c, and 9e support retention of the
dinuclear structures in solution.
The absorptions with the lowest intensity and energy re-
sult from dǞd transitions. They fall in the range between
Figure 6. Irreversible reduction and oxidation peaks for 9e. CV
–
1
–1
data were recorded in a 0.1  solution of Bu
4
NPF
6
in acetonitrile
15150 cm (9f) and 17090 (9c) cm with extinction coeffi-
+
against Fc/Fc .
cients from 120 L/mol·cm (9c) to 580 L/mol·cm (9f). The
increased extinction coefficient in 9f is most likely associ-
ated with the extra nitrogen atom present which causes a
stronger mixing of ligand orbitals and metal d-orbitals in
functions. A sharp reoxidation peak is observed around
0.4 V resulting from the oxidation of metallic copper to
–
9f.
II
Cu in the presence of ligand. A limited number of stable
The energy of the dǞd bands is consistent with either a
I
[27]
I
Cu –OR species is known. While the Cu alkoxide can be
stabilized in the absence of air or water either sterically or
by conjugated π–systems in phenolates or enolates,
trigonal-bipyramidal or square-pyramidal geometry
a
around the copper atoms. The coordination geometry at the
copper center has been derived from the energy of the dǞd
I
Cu –OMe species decomposes vigorously into copper me-
[
10b]
absorption band for CuN ,
CuN and CuN complex
4
5 6
tal, formaldehyde and methanol.
fragments or complexes.[ However, with compounds 9a–
25]
The irreversible nature of the oxidation process suggests
9e no significant correlation was observed between τ-value
III
that hypothetical Cu species are highly reactive and might
and energy of the low energy transition. A comparison of
different methods to determine the distortion of the trigo-
nal-bipyramidal towards the tetragonal-pyramidal geome-
try showed the τ-value to be a sufficient criteria to distin-
lead to ligand degradation. Within the functional groups
present, the bridging alkoxide functions are most suscepti-
ble for oxidation, which can produce aldehydes, carboxylic
acid derivatives or CC coupled products.
guish between the two geometries for CuN chromo-
5
[
26]
phores. In general, the dǞd transitions of complexes of
type 9 shift hypsochromic with an increasing number of six-
membered chelate rings as well as for a change in the donor
set from NNO to NN O ligands (Table 2).
DTBC Oxidation
Acetonitrile solutions of complexes of type 9 and DTBC
2
2
(30 equiv.) were mixed in a shaking device under an oxygen
atmosphere. The amount of oxygen consumed was moni-
tored by an experimental set up described by one of us pre-
viously for similar experiments. Typically, green solutions
Cyclic Voltammetry
[
28]
Cyclic voltammetry experiments show one irreversible re- were obtained upon mixing of the catalyst and the DTBC
duction and one irreversible oxidation peak for the copper solution. The addition of one equivalent sodium methoxide
complexes 9b–9e in acetonitrile solution (Table 2). The initiated oxygen consumption, otherwise complexes of type
CVЈs of complex 9e are shown as an example in Figure 6. 9 were found catalytically inactive. The order of mixing of
The presence of four reduction peaks for 9a can be attrib- the solutions influenced the yield of the oxidation product
uted to four different copper centers displaying different τ- DTBQ. For example, mixing of 9b and DTBC followed by
values as found in the solid-state structure. For the oxi- addition of base gave lower yields (25%–30%) than mixing
dation process two peaks are observed for compounds 9a 9b and base followed by DTBC addition (83% yield,
(1.15 V and 1.27 V) and 9f (0.87 V and 1.50 V). In general, Table 2). In general, complexes 9a–f show a variation in
reduction of complexes of type 9 occurs between –1.78 V catalytic activity giving yields from 15 to 99% with a TON
and –1.07 V while oxidations are observed between 0.87 from 90 to 500 h–1 (Table 2). A plot of the oxygen consump-
+
and 1.50 V vs. Fc/Fc .
tion during the oxidation of DTBC with complex 9f as the
Compound 9f with an NN O ligand exhibits the strong- catalyst is depicted in Figure 7. The experimental data con-
2
est ligand field, which is reflected by the lowest reduction firm our assumption that a nitrogen (amine) rich coordina-
potential (–1.78 V) in the series of complexes of type 9. The tion environment improves the catalytic activity of the cop-
precipitation of copper metal during this reduction suggests per center for the DTBC oxidation. Unfortunately, com-
II
I
that Cu is reduced to Cu , whose complexes are unstable. pound 9f is less stable than 9a–e under the required basic
The irreversibility of the reduction process with complexes conditions. The coordination geometry described by the τ-
I
of type 9 is thus explained with the instability of Cu within value and catalytic activity do not correlate. Reaction rates
a strong ligand field generated by the alkoxide and amine and yields are best in the presence of six-membered chelate
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Catechol Oxidation with Dinuclear Copper Complexe
FULL PAPER
rings for the alcohol functions and five-membered chelate (apical) alcohol function at the copper center. Apparently,
rings for the terminal amine functions.
this apical alcohol function is the most labile in the complex
and can be substituted for other ligands. Furthermore, 10b
can be reconverted to 9c in the absence of competing li-
gands such as methanol or TMP. Dissolving the purple
complex [Cu(H -4)(TMP)] (PF ) (10b) in acetonitrile gives
3
2
6 2
a blue solution. Addition of diethyl ether to this solution
leads to precipitation of the dark blue dinuclear complex
9c.
In conclusion, the energy difference between the two so-
lid-state structures of type 9 and type 10 is small enough to
allow interconversion. Apparently, the apical alcohol li-
gands in the dinuclear copper complexes of type 9 are labile
and can be substituted for the catechol substrate without
breaking up the dinuclear units immediately. The Cu–Cu
distances in complexes of type 9 fall in the range between
2.89 Å (9a) and 3.01 Å (9e). However, the observation of
2
Figure 7. Consumption of O (mL) vs. time (s) for the catalytic
comparable catalytic activity for both the cis complexes 9a
and 9d and the trans complexes 9b and 9c renders the bridg-
ing coordination of the catechol by both copper ions of a
dinuclear complex moiety unlikely. We therefore assume
that during the catalytic cycle catechol substitutes one of
oxidation of DTBC (3 mmol) with 9f in acetonitrile (maximum
oxygen uptake: 33.6 mL).
Additional Copper Complexes
the apical alcohol groups in the dinuclear copper complexes
1
The stability of the copper() complexes of type 9 was and eventually η -coordinated catechol replaces one of the
II
corroborated by the formation of a purple Cu species bridging alkoxide arms with the second oxygen donor
I
[29]
upon reaction of [Cu (PPh ) Br] with ligand NNO -233 atom. This process is aided by addition of a base which
3
3
2
(
H -4). Equimolar amounts of the copper() compound and deprotonates the catechol ligand thereby enhancing its ba-
4
the ligand were mixed at air in a dichloromethane solution. sicity. The addition of bases to the dinuclear complexes of
The initially colorless solution turned blue rapidly and a type 9 in the absence of catechol leads to decomposition of
purple solid 10a precipitated. The elemental analysis sug- the dinuclear complexes to a brown insoluble substance.
gested a ratio of Cu/Br/ligand of 1:1:1. The purple color of
10a differs from the blue color of the dinuclear hexafluoro-
phosphate salts 9a–9e. Compound 10a can be obtained di- Conclusions
rectly by reaction of anhydrous copper() bromide and H4-
The rates of catalytic DTBC oxidation by the model
compounds 9a–f are far from modeling the enzyme catechol
oxidase. However, 9f shows an activity which is comparable
4
in methanol. The X-ray structure analysis revealed that a
polymer chain consisting of dinuclear dicationic units,
which are bridged by alkoxy functions of the singly depro-
tonated ligand had formed (Figure 8). The remaining
alcohol function of the ligand binds to a copper atom of
an adjacent dinuclear unit in the apical position. The coor-
dination geometry around the copper atoms is square-py-
ramidal with τ = 0 (Table 1).
[
13b,13d]
to some other model compounds.
The aim of this
study was to probe the relation between the particular li-
gand design of basically similar ligands and the catalytic
II
activity of its Cu complexes. Complexes 9a to 9e show
indeed a different catalytic activity manifested in a variation
of the yield in DTBC oxidation (15% to 99%) and different
–1
TON (94 to 227 h ). Since complexes 9a to 9e contain
identical donor sets all differences in the activity can be
attributed to different arm length at the tripodal ligands.
However, the derivation of linear correlations between
structural factors and catalytic activity turns out to be diffi-
cult.
Figure 8. Fraction of the poly(dinuclear) coordination polymer
2+
Experimental Section
built from [Cu
2
(H
3
-4)
2
]
cations of complex 10a.
Materials and Methods: All manipulations were carried out in air
unless noted otherwise. Solvents were purified by standard methods
and freshly distilled prior use. Infrared spectra were recorded in
KBr (pellets or thin films) with a Bruker Vector 22 FT spectrome-
In an attempt to exchange the apical protonated alcohol
function in the dinuclear complex [Cu(H -4)] (PF ) (9c)
against 2,4,6-trimethylphenol (TMP), purple [Cu(H3-
3
2
6 2
1
13
4
)(TMP)] (PF ) (10b) precipitated from a green solution. ter. H and C NMR spectra were recorded on Bruker AC 200 or
2 6 2
We assume that the TMP ligand substituted the protonated Bruker ARX 300 spectrometers. Elemantal analyses (C,H,N) were
4919
Eur. J. Inorg. Chem. 2005, 4914–4923
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org

C. Jocher, T. Pape, W. W. Seidel, P. Gamez, J. Reedijk, F. E. Hahn
FULL PAPER
performed with a Vario EL III Elemental Analyzer at the Institut
für Anorganische und Analytische Chemie, University of Münster.
MALDI, ESI and EI mass spectra were measured with Bruker Re-
flex IV, Micromass Quattro LC and Varian MAT 212 instruments,
respectively. UV/Vis spectra were recorded in acetonitrile with a
Varian Cary 50 spectrophotometer. Cyclic voltammetry experi-
ments were carried out with a Eco-Chemie PGSTAT 30 potentios-
tat using a three-electrode cell configuration (working electrode:
glassy carbon, auxiliary electrode: platinum, reference electrode:
Ag/AgCl/3  KCl). The experiments were performed in 0.1  ace-
H, NCH
NMR (50.3 MHz, CDCl
(NCH CH OH), 55.6 (NCH
51.8 (NCH
(NCH CH CH
= 3358 (s, NH), 3285 (s, NH), 2949 (s, CH), 2871 (s, CH), 2829 (s,
CH), 1662 (w), 1602 (w, NH), 1464 (m, CH), 1375 (w, CH), 1299
(w), 1060 (s, CO), 910 (m), 870 (w), 750 (w). MALDI-MS
2
CH
2
CH
2
OH), 1.55 (qi, 2 H, NCH
): δ = 62.2 (NCH
CH OH), 53.3 (NCH
), 39.8 (NCH CH CH
), 28.7 (NCH CH CH OH). IR (KBr film): ν
2
CH
2
CH
CH
CH
NH
2
NH
OH), 59.4
CH OH),
), 29.6
2
). 13C
3
2
CH
2
2
2
2
2
2
2
2
2
2
CH
2
CH
2
NH
2
2
2
2
2
2
2
2
NH
2
2
2
2
+
(337.0 nm, 3 ns) m/z: 177.0 [MH] .
Synthesis of Copper Complexes. General Procedure: Copper hydrox-
ide was prepared from copper chloride and sodium hydroxide in
the presence of a small amount of polyethylene glycol ether at 0 °C.
The turquoise solid can be stored at –30 °C for several months
without darkening. The preparation of copper complexes of type 9
was carried out according to a published procedure described for
the preparation of complex 9c.[ Copper hydroxide was suspended
in deionized water (10–20 mL) and the ligand dissolved in water
(10 to 20 mL) was added. The reaction mixture was stirred at 40 °C
for 2 h. Solid NH PF was added and the volume of the solutions
tonitrile solutions of Bu
rates of 100 mV/s. A computer-controlled apparatus
4 6
NPF as supporting electrolyte with scan
[28]
was used
for oxygen uptake measurements in oxidation experiments with
DTBC (Aldrich). Solutions of DTBC were freshly prepared. 30
equivalents of DTBC were mixed with one equivalent catalyst and
one equivalent sodium methoxide in acetonitrile at 25 °C.
3i]
-2),[4c] NNO
[7]
Ligand Syntheses: Ligands NNO
2
-222 (H -223 (H
4
2
4
-
[
6b]
[3i]
3),
NNO
2
-233 (H
4
-4), NNO
2
-322 (H -5), and NN O-332
4 2
-1)[5] were prepared according to previously published pro-
(H
5
4
6
cedures.
was reduced to reach a concentration of approximately 1 . Crys-
talline material precipitated upon storage at 5 °C. For further puri-
fication, the raw material was recrystallized from ethanol
Ethyl 3-[(2-Cyanoethyl)(2-hydroxyethyl)amino]propanoate (8): 2.0
equivalents of acrylonitrile (5.46 g, 0.10 mol) were added to a solu-
tion of ethyl 3-[(2-hydroxyethyl)amino]propanoate (7)[ (8.29 g,
(
(
9a·0.5EtOH·0.5H
9b·2H O, 9e, 9f) or dichloromethane (10a). Yields and selected
O, 9b·2H O,
2
O),
methanol
(9d·MeOH),
acetonitrile
8a]
2
0.0514 mol) in methanol (5 mL). The reaction mixture was heated
analytical data for complexes 9a·0.5EtOH·0.5H
d·MeOH, 9e, 9f and 10a are given below.
2
2
to 60 °C for 48 h. Excess acrylonitrile and methanol were removed
under reduced pressure and 8 remained as a colorless liquid. Yield:
9
1
[Cu (H -2) (PF ·0.5EtOH·0.5H
Cu(OH) (425 mg), 4.36 mmol, ligand H
and NH PF (711 mg, 4.36 mmol) were used in the procedure de-
scribed for the preparation of 9c. Yield: 340 mg (0.457 mmol,
2
3
2
]
2
6
)
4
2
O
(9a·0.5EtOH·0.5H
2
O):
1
2
0.6 g (0.049 mol, 96%). H NMR (200 MHz, CDCl
3
): δ = 4.07 (q,
OH), 2.85–2.70 (m, 4 H,
2
OH), 2.47–2.33 (m, 4 H,
2
4
-2 (647 mg, 4.36 mmol)
H, OCH
2
CH
), 2.59 (t, 2 H, NCH
CH ), 1.18 (t, 3 H, OCH
): δ = 172.4 [C(O)OEt], 118.7 (CN), 60.6 (CH
O), 55.6 (NCH CH OH), 49.6 (NCH ), 48.8 (NCH
NCH CH CO Et), 16.6 (NCH CH CN), 13.9 (OCH CH
3
), 3.52 (t, 2 H, CH
CH
CH
3 2
CH
NCH
NCH
CDCl
2
2
4
6
[3i]
1
3
2
2
2
3
). C NMR (50.3 MHz,
O), 59.4
), 32.7
).
–
1
2
2
(
1%). IR (KBr): ν˜ = 3364 cm (s), 3312 (s), 3208 (s, br), 2990 (s),
3
2
927 (s), 2870 (s), 1597 (m), 1473 (m), 1371 (w), 1269 (w), 1134
m). MALDI-MS (337.0 nm, 3 ns) m/z: 482 (45, [Cu
(CH
2
2
2
2
2
+
3
+
(H
-2) ] ).
3 2
3
-2)
2
] ), 416
(
2
2
2
2
2
2
3
+
(
75, [Cu
(H -3)
.00 mmol), ligand
2
(H
3
-2)
2
(CH
3
CN)] ), 419 (100, [Cu
2
(H
Cu(OH)
-3 (330 mg, 2.00 mmol) and NH
N-(2-Hydroxyethyl)-N-(3-hydroxypropyl)propylene-1,3-diamine
NNO -323 (H -6): Dry lithium aluminum hydride (8.36 g,
.209 mol) was suspended in THF (200 mL). After stirring for 60
minutes and cooling to –5 °C concentrated sulfuric acid (4.80 g,
.047 mol) was added carefully dropwise (Caution: this reaction is
[Cu
2
3
2
](PF
6
)
2
·2H
2
O
(9b·2H
2
O):
2
(195 mg,
PF
2
4
2
H
4
4
6
0
(
530 mg, 4.00 mmol) were used. Yield: 308 mg (0.397 mmol, 40%).
9
b·2H
2 38 4 2 12 6 2
O C14H N Cu F O P (775.52): calcd. C 21.68, H 4.94, N
0
1
9
7.22; found C 21.59, H 4.35, N 6.91. F NMR (188.3 MHz,
extremely exothermic, hydrogen is evolved). The suspension was
stirred for 60 min and ethyl 3-[(2-cyanoethyl)(2-hydroxyethyl)
amino]propanoate (8) (15.0 g, 0.020 mol) dissolved in THF
1
CD
327 (s), 3128 (m), 2919 (m), 2861 (m), 2821 (m), 1594 (m), 1472
m), 1320 (m), 1120 (s), 1084 (s), 1062 (s), 830 (s, PF ). MALDI-
3
CN): δ = –66.8 ( JFP = 710 Hz). IR (KBr): ν˜ = 3627 cm–1 (m),
3
(
6
(30 mL) was slowly added. The reaction mixture should not be al-
+
MS (337.0 nm, 3 ns) m/z: 447 (100, [Cu
2
(H
3
-3)
2
] ), 592.9 (30,
lowed to boil during the addition of 8. The suspension was stirred
at room temperature for 8 h and then quenched by slow addition
of degassed water (15.1 g, 0.84 mol) at 0 °C. The reaction mixture
+
[{Cu
2
(H
3
-3)
2
}PF
6
] ).
[Cu (H -5) ](PF ) ·CH OH (9d·CH OH): Cu(OH)2 (292 mg,
2
3
2
6 2
3
3
was filtered under an argon atmosphere. The filtrate contained H
4
-
4 4 6
3.00 mmol), ligand H -5 (487 mg, 3.00 mmol) and NH PF
6
and side products, but most of the ligand was found in the solid
(980 mg, 6.00 mmol) were used. Yield: 550 mg (0.713 mmol, 48%).
–
1
residue. It was isolated by continuous extraction with methanol for
one day. An immediate work-up after hydrolysis increased the yield
of ligand H -6 because otherwise aluminum complexes form. The
4
extract and the filtrate were united and dried with sodium sulfate.
Removal of the solvent gave a colorless to yellow residue, which
was suspended in dichloromethane (200 mL) and dried with so-
dium sulfate. The drying agent and precipitated lithium salts were
separated by filtration through Celite . Distillation under reduced
pressure (180 °C, 0.05 mbar) afforded H
3
1
IR (KBr): ν˜ = 3666 cm (s), 3448 (s), 3343 (s), 3297 (s), 3131 (s),
2951 (m), 2921 (m), 2880 (m), 2765 (m), 1604 (s), 1463 (m), 1333
(m), 1286 (m), 1265 (m), 1161 (s), 1072 (s), 1044 (s), 1015 (s), 833
(s, PF6).
[
6
Cu
(383 mg, 2.18 mmol) and NH
used. Yield: 350 mg (0.456 mmol, 42%). IR (KBr): ν˜ = 3591 cm
2
(H
3
-6)
2
](PF
6
)
2
(9e): Cu(OH)
2
(211 mg, 2.18 mmol), ligand H
4
-
4
PF
6
(711 mg, 4.36 mmol) were
–
1
®
(
(
s), 3358 (s), 3305 (s), 2951 (m), 2871 (m), 2803 (w), 2712 (w), 1602
m), 1463 (m), 1311 (w), 1089 (s), 1048 (s), 839 (s, PF ). MALDI-
4
-6 as colorless oil. Yield:
6
+
.37 g (19.1 mmol, 27%). C
1.44, N 15.89; found C 53.32, H 11.43, N 15.12. H NMR
): δ = 3.62 (t, 2 H, NCH CH CH OH), 3.54
OH), 3.29 (s br, 4 H, OH + NH ), 2.68 (t, 2 H,
OH), 2.48 (t, [Cu
NH ), 1.63 (qi, 2 1 (701 mg, 4.00 mmol) and NH
8 20 2 2
H N O (176.26): calcd. C 54.52, H
MS (337.0 nm, 3 ns) m/z: 475 (100, [Cu
2
(H
3 2
-6) ] ), 659 (10,
1
+
[{Cu
2
(H
3
-6)
2
}PF
6
CH
3
CN] ), negative ions m/z: 910 ([{Cu
2
-
(
(
200.1 MHz, CDCl
t, 2 H, NCH CH
CH CH NH
CH OH), 2.46 (t, 2 H, NCH
3
2
2
2
–
(
H
3
-6)
2
}(PF ) ] ).
6 3
2
2
2
NCH
2
2
2
2
2
), 2.54 (t, 2 H, NCH
2
CH
2
CH
2
2
(H
4
-1) ](PF
2
6
)
2
(9f): Cu(OH)
2
(390 mg, 4.00 mmol), ligand H
5
-
H, NCH
2
2
2
CH
2
CH
2
2
4
PF
6
(1.30 g, 8.00 mmol) were used.
4920
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2005, 4914–4923

Catechol Oxidation with Dinuclear Copper Complexe
FULL PAPER
Preparation Starting from Cu : A solution of H -4 (353 mg,
4
Yield: 330 mg (0.431 mmol, 22%). IR (KBr): ν˜ = 3373 cm–1 (m),
II
3
(
1
355 (m), 3324 (m), 3304 (m), 2974 (m), 2946 (m), 2933 (m), 2892 2.0 mmol) in methanol (10 mL) was added to a brown solution of
m), 2857 (m), 1603 (m), 1487 (w), 1457 (w), 1244 (w), 1150 (m), copper bromide (447 mg, 2.0 mmol) in methanol (10 mL). Immedi-
056 (m), 1022 (m), 835 (s, PF ). MALDI-MS (337.0 nm, 3 ns) ately upon mixing the color of the solution changed to blue. Purple
6
+
+
m/z: 238 (24, [Cu(H
6
5
-1)] ), 422 (24, [Cu(H
6 2 3 2
06 (100, [Cu(H -1)(PF ) (CH CN) ] ).
5
-1)(PF
6
)(CH
3
CN)] ), crystals formed upon standing in air. More crystals were precipi-
tated by diffusion of diethyl ether into this solution. Yield: 227 mg
+
5
I
(0.356 mmol, 36%). C
7 2 2 2
H18Br CuN O (318.66): calcd. C 30.15, H
[
H
Cu (H -4) ]Br
2
3
2
2
(10a): Preparation starting from Cu : A solution of
6.01, N 8.79; found C 30.19, H 6.01, N 8.72%.
4
-4 (162 mg, 1.0 mmol) in dichloromethane (10 mL) was added
I
to a colorless solution of [Cu (PPh
3
)
3
Br] (930 mg, 1.0 mmol) in
dichloromethane (20 mL). Within several minutes the solution
turned blue upon stirring under air. Purple crystals precipitated,
Crystal Structure Analyses: Blue crystals of 9a·0.5EtOH·0.5H
9b·2H O, 9d·MeOH, 9e and 9f and purple crystals of 10a were
obtained by recrystallization from ethanol (9a·0.5EtOH·0.5H O),
O, 9e, 9f) or dichloro-
(318.66): methane (10a) at 5 °C. Diffraction data were measured at low tem-
calcd. C 30.15, H 6.01, N 8.79; found C 30.78, H 6.04, N 8.90%. perature (Table 3 and Table 4) on a Bruker APEX diffractometer
2
O,
2
2
which were suitable for X-ray diffraction analysis. Yield: 65 mg methanol (9d·MeOH), acetonitrile (9b·2H
0.10 mmol, 20%). [Cu (H -4) ]Br 10a C 18BrCuN
2
(
2
3
2
2
7
H
2 2
O
Table 3. Summary of crystallographic data for 9a·0.5EtOH·0.5H
a·0.5EtOH·0.5H
0.18×0.05×0.4
2
O, 9b·2H
2
O, and9d·2CH
9b·2H
0.26×0.21×0.18
3
OH.
9
2
O
2
O
9d·MeOH
Crystal size [mm]
Formula
fw, amu
a [Å]
0.21×0.13×0.10
C H38Cu F N O P
15 2 12 4 5 2
771.51
11.6574(5)
19.2777(8)
26.3811(11)
90
C
13
H34Cu
2
F
12
N
4
O
5
P
2
C
14
H38Cu
2
F
12
N
4
O
6
P
2
743.36
775.50
7.4536(7)
10.6827(10)
17.0136(16)
90
12.9766(13)
15.2839(15)
13.5796(14)
90
b [Å]
c [Å]
α [deg]
β [deg]
96.874(2)
90
2673.9(5)
91.398(1)
90
1354.3(2)
90
90
5928.6(4)
Pccn
γ [deg]
3
V [Å ]
Space group
P2
1
1
P2 /c
Z
4
2
8
–3
dcalcd. [g cm ]
1.847
1.826
173(2)
9414
8217
0.0420
0.0933
1.004
3.0–50.1
0.751/–0.308
1.902
1.810
153(2)
2375
2254
0.0246
0.0596
1.048
4.5–50.0
0.774/–0.241
1.729
1.651
173(2)
5233
–1
µ [mm ]
T [K]
Unique data
Observed data [I Ն 2σ(I)]
R (observed data)
wR (observed data)
GOF
4247
0.0553
0.1386
1.043
3.7–50.1
1.395/–1.042
2
θ-range [deg]
3
Residiual electron density [e/Å ]
Table 4. Summary of crystallographic data for 9e, 9f, and 10a.
9
e
9f
10a
0.07×0.04×0.03
2 4 4
Cu N O
Crystal size [mm]
Formula
fw, amu
a [Å]
b [Å]
c [Å]
0.10×0.08×0.02
C
0.20×0.07×0.02
16
H
38Cu
2
F
12
N
4
O
4
P
2
C
16
H
40Cu
2
F
12
N
6
O
2
P
2
C
16
H
38Br
2
767.52
7.9889(5)
8.3591(5)
765.56
7.7147(8)
637.40
8.316(2)
16.212(3)
8.671(2)
90
91.179(4)
90
9.6178(10)
9.6401(10)
77.686(2)
85.577(2)
89.414(2)
696.73(13)
11.0443(7)
α [deg]
β [deg]
76.1580(10)
76.2270(10)
79.0790(10)
688.63(7)
γ [deg]
3
V [Å ]
1168.7(4)
P2 /n
1
¯
P1
¯
P1
Space group
Z
1
1
2
–3
dcalcd. [g cm ]
1.851
1.774
153(2)
3139
1.825
1.750
173(2)
4002
1.811
5.267
123(2)
2675
2159
0.0573
0.1108
1.107
5.0–50.0
1.265/–0.907
–1
µ [mm ]
T [K]
Unique data
Observed data [I Ն 2σ(I)]
R (observed data)
wR (observed data)
GOF
2859
3087
0.0265
0.0685
1.019
3.9–55.0
0.0477
0.0886
0.969
4.3–60.2
2
θ-range [deg]
3
Residiual electron density [e/Å ] 0.350/–0.336
0.605/–0.501
Eur. J. Inorg. Chem. 2005, 4914–4923
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4921

C. Jocher, T. Pape, W. W. Seidel, P. Gamez, J. Reedijk, F. E. Hahn
FULL PAPER
equipped with a rotatingmolybdenum anode (λ = 0.71073 Å) and
a CCD area detector. Empirical absorption corrections were ap-
[8] a) C. Ochs, F. E. Hahn, R. Fröhlich, Chem. Eur. J. 2000, 6,
2
193–2199; b) F. E. Hahn, C. Ochs, T. Lügger, R. Fröhlich,
Inorg. Chem. 2004, 43, 6101–6107.
plied to all data sets. The structures were solved and refined by
standard Patterson- and Fourier-techniques.[
30]
[9] a) J. A. Connolly, J. H. Kim, M. Banaszczyk, M. Drouin, J.
Chin, Inorg. Chem. 1995, 34, 1094–1099; b) J. A. Connolly, M.
Banaszczyk, R. C. Hynes, J. Chin, Inorg. Chem. 1994, 33, 665–
All non-hydrogen
atoms were refined with anisotropic displacement parameters, and
hydrogen atoms were added to the structure model on calculated
positions. Selected crystal, data collection and refinement details
are listed in Table 3 and Table 4.
6
4
69; c) J. Chin, M. Banaszczyk, J. Am. Chem. Soc. 1989, 111,
103–4105; d) J. Chin, M. Banaszczyk, V. Jubian, X. Zou, J.
Am. Chem. Soc. 1989, 111, 186–190.
[
10] a) A. M. Dittler-Klingemann, F. E. Hahn, Inorg. Chem. 1996,
Supplementary Material: CCDC-276044 (for 9a·0.5EtOH·
3
5, 1996–1999; b) A. M. Dittler-Klingemann, C. Orvig, F. E.
2 2
0.5H O), -276045 (for 9b·2H O), -276046 (for 9d·MeOH), -276047
Hahn, F. Thaler, C. D. Hubbard, R. van Eldik, S. Schindler, I.
Fabian, Inorg. Chem. 1996, 35, 7798–7803.
11] C. Ochs, F. E. Hahn, T. Lügger, Eur. J. Inorg. Chem. 2001,
1279–1285.
(for 9e), -276048 (for 9f), and -276049 (for 10a) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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