Dinuclear copper complexes based on parallel β-diiminato binding sites and their reactions with O2: Evidence for a Cu-O-Cu entity

Article abstract of DOI:10.1021/ic101249k

Investigations concerning the system β-diketiminato-Cu ^I/O₂ have revealed valuable insights that may be discussed in terms of the behavior of mononuclear oxygenases containing copper. On the other hand nature also employs dinuclear C

Full text of DOI:10.1021/ic101249k

ARTICLE
pubs.acs.org/IC
Dinuclear Copper Complexes Based on Parallel β-Diiminato Binding
Sites and their Reactions with O2: Evidence for a Cu-O-Cu Entity†
Peter Haack,‡ Christian Limberg,*,‡ Kallol Ray,‡ Beatrice Braun,‡ Uwe Kuhlmann,§ Peter Hildebrandt,§ and
Christian Herwig‡
‡Humboldt-Universit€at zu Berlin, Institut f€ur Chemie, Brook-Taylor-Strasse 2, 12489 Berlin, Germany
§Technische Universit€at Berlin, Institut f€ur Chemie, Sekr. PC14, Strasse des 17 Juni 135, D-10623 Berlin, Germany
S Supporting Information
b
ABSTRACT:
Investigations concerning the system β-diketiminato-CuI/O2 have revealed valuable insights that may be discussed in terms of the
behavior of mononuclear oxygenases containing copper. On the other hand nature also employs dinuclear Cu enzymes for the
activation of O2. With this background the ligand system [Me C H Xanthdim]2- containing two parallel β-diiminato binding sites
2
6
3
linked by a xanthene backbone with 2,3-dimethylphenyl residues at the diiminato units was investigated with respect to its copper
coordination chemistry. The diimine [Me C H Xanthdim]H2 was treated with CuOtBu in the presence of acetonitrile, PPh3, and
2
6
3
PMe3 to yield the corresponding complexes [Me C H Xanthdim](Cu(L))2 (L = CH3CN, 1, PPh3, 2, and PMe3, 3) that proved to be
stable and were fully characterized. Single crystal X-ray diffraction analyses performed for the three complexes showed that
considerable steric crowding within the binding pockets of 2 leads to a very long Cu-Cu distance while the structures of 1 and 3 are
relaxed. Compounds 2 and 3 are relatively robust toward air, whereas 1 is very sensitive and quantitatively reacts with O2 at room
temperature (r.t.) within less than 2 min to give intractable compounds. At low temperatures the formation of a green intermediate
was observed that was identified as a CuII-O-CuII species spectroscopically and chemically. This finding is of relevance also in the
context of the results obtained testing 1 as a catalyst for phenol oxidation using O2: 1 efficiently catalyzes phenol coupling, while
there was no evidence for any oxygenation reactions occurring.
2
6
3
’ INTRODUCTION
catalysts6 has been investigated. Furthermore, corresponding
iron(II) complexes were prepared to study their behavior in
the presence of O2.7 It seemed only natural to extend this work
also to copper chemistry: Both the discovery of various di- or
polynuclear O2-activating copper enzymes such as the particulate
methane monooxygenase (pMMO)8 as well as reports on the
selective oxidation of methane to methanol at dinuclear copper
sites embedded in zeolites,9 stimulate research on dinuclear
copper systems.
The chemistry of mononuclear β-diketiminato copper com-
plexes has been mainly investigated by Tolman et al., often with
the aim of gaining insights into the binding and activation of O2
in mononuclear copper enzymes, such as the neuroenzymes
dopamine β-monooxygenase and peptidylglycine R-hydroxylat-
ing monooxygenase.3,10,11 It has been found that CuI complexes
The β-diiminato ligand system is very versatile and has been
shown to display a very rich coordination chemistry stabilizing
high as well as low oxidation states.1 This of course makes it also
ideal as a ligand in transition metal complexes that activate small
substrates, such as O2, N2, and CO2, and recent years have
revealed several impressive precedent cases.2,3 Many of the
corresponding systems have been shown to work via a mechan-
ism where two metal centers cooperate in the activation process,2
a principle that is also common in many metalloenzymes. This
has recently stimulated a strong interest in the design of ligands
containing two β-diiminato moieties in close proximity.4 We
have focused in particular on a ligand where two β-diiminato
units are oriented parallel to each other, and recently we
described the development of a corresponding ligand precursor,
Me2C6H3
[
Xanthdim]H2 (Chart 1).5 It has been successfully
employed for the synthesis of zinc as well as of magnesium
Received: June 23, 2010
complexes, whose potential as epoxide/CO2 copolymerization
Published: February 22, 2011
r
2011 American Chemical Society
2133
dx.doi.org/10.1021/ic101249k Inorg. Chem. 2011, 50, 2133–2142
|

Inorganic Chemistry
ARTICLE
Chart 1
was prepared by applying a discharge to a 1:1 mixture of 16O2 and 18O2
in a flask sealed with a Young tap (at 125 mbar) as evidenced by EI mass
spectrometry. Subsequently, the flask was filled up with argon until a
pressure of 1 bar was reached. The gas mixture (20-40 mL) was
removed via a syringe and bubbled at -80 °C through the solution. Pure
copper(I)-tert-butoxide, CuOtBu, was prepared according to the litera-
ture procedure.14 [Na(thf)O(2,4-tBu)C6H3], A0, was obtained by add-
ing an excess of NaH to a solution of 2,4-di-tert-butylphenol, A, in
tetrahydrofuran (thf). The resulting suspension was filtered after stirring
for 4 h at r.t. Precipitation of the product was achieved by reducing the
volume of the filtrate to a quarter under reduced pressure and adding a
3-fold excess of hexane. Filtration and drying in vacuo gave a white solid,
1
A0, in 65% yield. Purity of A0 was ensured by H NMR spectroscopy.
Compound 4, [(N(2,6-Me2C6H3)C(CH3))2CH]Cu(MeCN), is al-
ready known,18 but was synthesized via an alternative route: After
stirring a solution of β-diketimine [(N(2,6-Me2C6H3)C(CH3))2CH]H
and CuOtBu in a benzene/acetonitrile solvent mixture (2:1) for 18 h at
r.t., all volatiles were removed. The product was extracted with hexane,
and reduction of the volume caused the precipitation of a yellow solid. A
filtration and subsequent drying in vacuo led to 4 in 38% yield. The
NMR data and the chemical behavior of 4 are in agreement with those
containing sufficiently bulky β-diketiminato ligands allow for the
isolation and characterization of 1:1 adducts of O2, while
sterically less demanding ligands lead to dinuclear complexes
with Cu(μ-O)2Cu diamond core structures (Chart 2).3 More-
over it was shown that within the 1:1 adducts the resulting
superoxide/peroxide ligands can be further activated by
the presence of an additional copper(I) center so that the
β-diketiminato ligand gets oxidized to yield a Cu(II)-o-iminose-
miquinone species.12
reported previously.18
Me2C6H3
[
Xanthdim](Cu(NCCH3))2, 1. Addition of 10 mL of ben-
2 6 3
On this background we therefore set out exploring the
zene to a suspension of 400 mg (0.46 mmol) of [Me C H Xanthdim]H2
and 125 mg (0.92 mmol) of CuOtBu in 5 mL of acetoniltrile led to a
clear yellow solution which was stirred for 18 h at r.t. The resulting
cloudy solution was treated with 20 mL of acetonitrile. A yellow solid
precipitated, which was filtered off after the reaction mixture had been
stirred for further 4 h at r.t. Drying of the residue under vacuum yielded
348 mg (0.32 mmol, 70%) 1.
2-
capability of the novel ligand system [Me C H Xanthdim] to
2
6
3
form stable, dinuclear CuI complexes.
This appeared to be a challenging task, since CuI complexes
are often unstable because of a marked intrinsic tendency to
disproportionate to CuII and Cu metal, and this problem occurs
all the more in dinuclear complexes. In case of success, an
investigation seemed to be worthwhile not only concerning the
cooperativity of the two CuI centers in a potential O2 activation
process but also to examine what kind of difference the parallel
arrangement of the diiminato donor functions makes in compar-
ison to the vis-ꢀa-vis orientation that is preferred if mononuclear
complexes are employed (for instance a cis Cu-O-O-Cu unit
could be imagined13).
1H NMR (CD2Cl2, 297 K): δ 1.33 (s, 18H, C(CH3)3), 1.69 (s, 6H,
C(CH3)2), 1.99 (s, 6H, CH3CN), 2.19 (s, 12H, CH3), 2.36 (s, 12H,
CH3), 6.73 (m, 8H, CH), 6.85 (ps-t, 4H, CH), 7.07 (d, 4J(H, H) = 2.0
Hz, 2H, CH), 7.24 (d, 4J(H, H) = 2.0 Hz, 2H, CH), 7.72 (s, 4H, NCH);
13C NMR (CD2Cl2, 297 K): δ 2.6 (CH3CN), 14.9 (CH3), 20.5 (CH3),
31.4 (C(CH3)3), 33.3 (C(CH3)2), 34.3 (C(CH3)3), 34.8 (C(CH3)2),
102.7 (C), 116.7 (CAr), 118.6 (CHAr), 119.5 (CHAr), 124.0 (CHAr),
124.1 (CHAr), 126.8 (CHAr), 128.4 (CAr), 129.2 (CAr), 131.3 (CAr),
136.8 (CAr), 144.3 (CAr), 145.9 (CAr), 154.5 (CAr), 158.9 (NCH); Anal.
found: C 71.80, H 6.75, N: 8.13; Calcd for C65H74N6Cu2O: C 72.13, H
6.89, N 7.76%; IR (KBr, cm-1): 3057 (w), 2867 (m), 2257 (w, CꢀN),
1588 (s), 1573 (s), 1452 (vs), 1371 (m), 1323 (vs), 1244 (vs), 1219 (s),
1186 (w), 1165 (w), 1059 (m), 1017 (m), 985 (w), 877 (w), 857 (w),
’ EXPERIMENTAL SECTION
General Procedures. All manipulations were carried out in a
glovebox, or else by means of Schlenk-type techniques involving the use
1
of a dry argon atmosphere. The H, 13C, and 31P NMR spectra were
recorded on a Bruker DPX 300 NMR spectrometer (1H 300.1 MHz, 13
C
808 (w), 782 (s), 748 (w), 726 (w), 682 (w), 654 (w).
Me2C6H3
75.5 MHz, 31P 121.5 MHz) with CH2Cl2 or CDCl3 as solvent at 20 °C.
[
Xanthdim](Cu(PPh3))2, 2. After stirring a yellow solution
2 6 3
of 300 mg (0.34 mmol) of [Me C H Xanthdim]H2, 94 mg (0.69 mmol)
of CuOtBu and 180 mg (0.69 mmol) of PPh3 in 3 mL of benzene for 19 h
at r.t., it was concentrated to a volume of 1.5 mL. Addition of 10 mL of
acetonitrile caused the formation of a yellow precipitate which was
filtered off after stirring the suspension for additional 3 h. Separation of
all volatile components under vacuum led to 394 mg (0.26 mmol, 75%)
of 2.
1
The H NMR spectra were calibrated against the residual proton, the
13C NMR spectra against natural abundance 13C resonances of the
deuterated solvents (CD2Cl2 δH 5.32 ppm and δC 53.5 ppm, CDCl3 δH
7.26 ppm and δC 77.0 ppm, respectively), and the 31P NMR spectra
against H3PO4 as an external standard. Microanalyses were performed
on a Leco CHNS-932 elemental analyzer. Infrared (IR) spectra were
recorded using samples prepared as KBr pellets with a Shimadzu FTIR-
8400S-spectrometer. UV/vis spectra were obtained at variable tempera-
tures on an Agilent 8453 UV-visible Spectrophotometer equipped with
a Unisoku USP-203-A cryostat. When necessary baseline drifting caused
by minimal frosting of the cuvettes under low temperature conditions
were corrected by subtracting an average value of a region with no
absorbance (1080-1100 nm). Raman spectra at low temperature,
controlled via a Bruker heating control unit, were acquired using a
Bruker RAM II FT-Raman Module (1064-nm excitation; Nd:YAG
laser) and a Horiba Jobin-Yvon LabRAM HR800 confocal Raman
Spectrometer (647-nm excitation, Kr ion laser).
1H NMR (CD2Cl2, 297 K): δ 1.32 (s, 18H, C(CH3)3), 1.68 (s, 6H,
C(CH3)2), 1.88 (s, 12H, CH3), 2.01 (s, 12H, CH3), 6.70 -7.25 (m, br,
46H, CH), 7.92 (s, 4H, HCN); 13C NMR (CD2Cl2, 297 K): δ 15.3
(C(CH3)2), 20.4 (CH3), 31.4 (C(CH3)3), 34.2 (C(CH3)2), 34.3
(CH3), 34.7 (C(CH3)3), 103.2 (C), 119.7 (CHAr), 121.8 (CHAr),
124.4 (CHAr), 125.1 (CHAr), 126.7 (CHAr), 128.3 (d, J(C, P) = 9.5
Hz, CHAr), 129.0 (J(C, P) = 10.1 Hz, CAr), 129.4 (CHAr), 131.0 (CAr),
132.7 (CAr), 133.3 (d, J(C, P) = 14.8 Hz, CHAr), 137.1 (CAr), 144.2
(CAr), 144.6 (CAr), 156.1 (CAr), 159.2 (NCH); 31P NMR (CD2Cl2, 297
K): δ 3.52 (s); Anal. found: C 76.14, H 6.74, N: 3.41; Calcd for
C97H98N4Cu2OP2: C 76.40, H 6.48, N 3.67%; IR (KBr, cm-1): 3054
(m), 2863 (m), 1588 (s), 1577 (s), 1459 (vs), 1437 (vs), 1364 (m), 1317
Materials. Solvents were purified, dried, degassed, and stored over
molecular sieves prior to use. A mixture of 18O2, 16O18O, 16O2 (1:2:1)
2134
dx.doi.org/10.1021/ic101249k |Inorg. Chem. 2011, 50, 2133–2142

Inorganic Chemistry
ARTICLE
1
Chart 2
B: H NMR (CDCl3, 297 K): δ 1.32 (s, 18H, C(CH3)3), 1.45 (s,
18H, C(CH3)3), 7.12 (d, 4J(H, H) = 2.5 Hz, 2H, CH), 7.39 (d, 4J(H,
H) = 2.5 Hz, 2H, CH); 13C NMR (CDCl3, 297 K): δ 29.6 (C(CH3)3),
31.6 (C(CH3)3), 34.4 (C(CH3)3), 35.1 (C(CH3)3), 122.4 (CAr), 124.7
(CHAr), 125.2 (CHAr), 136.2 (CAr), 142.9 (CAr), 149.7 (CAr).
Low Temperature Experiments Concerning the Reaction
of 1 with O2. (a) UV/vis Spectroscopy. A 1-cm-path-length quartz
cuvette filled with 2 mL of a solution of 1 in thf (1 mM) was sealed with a
septum and cooled to -80 °C. Dry O2 was bubbled through it for 10 s
and the spectra were recorded in an interval of 2 s. Spectroscopic
changes (after 10, 30, 50, 110, 210, 610 s) are summarized in Figure 4.
Either addition of 0.1 mL of a 400 mM solution of PPh3 in thf or
annealing of the sample caused decrease of the new band.
The same procedure was applied when 0.1 mL of a 2-(tert-butylsul-
fonyl)iodosylbenzene solution in CH2Cl2 (20 mM) or 1.8 mg of
Me3NO in 0.2 mL of thf were added instead of O2 at -70 °C.
ε of the Cu-O-Cu intermediate was determined assuming a
complete conversion of 1; dilution effects upon substrate addition were
considered.
(b) Spectrophotometric O2 Titrations. A 1-cm-path-length quartz
cuvette filled with 2 mL of a solution of 1 in thf (1 mM) was sealed with a
septum and cooled to -80 °C. In one experiment, spectra were taken for
more than 30 min, monitoring the absorbance at 640 and 1080 nm, to
ensure that no oxygen from air diffused into the sample over a longer
period of time.
Addition of 0.5 equiv of O2 or 1 equiv of O2 was performed by
injecting a O2-saturated thf solution (10 mM), prepared by bubbling O2
through argon-saturated thf in a Schlenk tube for 25 min at 25 °C.15 In
case of the experiment involving 1 equiv of O2, 200 μL of the O2-
saturated thf solution were injected into the cuvette, whereas 0.5 equiv of
O2 were added by injecting 100 μL of the O2-saturated thf solution
followed by 100 μL of an argon-saturated thf solution to keep the level of
dilution constant.
(vs), 1244 (m), 1217 (m), 1065 (m), 909 (w), 853 (w), 781 (m), 744
(m), 694 (s), 528 (s), 503 (m), 493 (m).
Me2C6H3
[
Xanthdim](Cu(PMe3))2, 3. A yellow solution of 300 mg
(0.34 mmol) of [Me C H Xanthdim]H2 and 94 mg (0.69 mmol) of
CuOtBu in 5 mL of benzene was rapidly cooled to -30 °C. On top of
the resulting frozen mixture approximately 0.5 mL (369 mg, 4.73 mmol)
of PMe3 were condensed. Slowly warming to r.t. led to an orange
solution, which was stirred for 17 h. After the removal of all volatiles the
remaining orange crude product was redissolved in 2 mL of benzene
followed by an addition of 10 mL of acetonitrile. The resulting yellow
suspension was stirred for further 16 h at r.t. Filtration and drying of the
residue under vacuum gave 264 mg of (0.23 mmol, 67%) 3 as a
yellow solid.
2 6 3
1H NMR (CD2Cl2, 297 K): δ 0.81 (d, 2J(H, P) = 6.3 Hz), 18H, P-
CH3), 1.33 (s, 18H, C(CH3)3), 1.68 (s, 6H, C(CH3)2), 2.22 (s, 12H,
CH3), 2.33 (s, 12H, CH3), 6.75-6.91 (m, 12H, CH), 7.07 (d, 4J(H, H) =
2.1 Hz, 2H, CH), 7.21 (d, 4J(H, H) = 2.4 Hz, 2H, CH), 7.82 (d, J(H, P) =
2.1 Hz, 4H, NCH); 13C NMR (CD2Cl2, 297 K): δ 15.2 (d, 1J(C, P) =
22.6 Hz, CH3), 15.2 (d, J(C, P) = 3.1 Hz, CH3), 20.5 (CH3), 31.4
(C(CH3)3), 34.0 (C(CH3)2), 34.2 (C(CH3)3), 34.7 (C(CH3)2), 103.1
(d, J(C, P) = 1.4 Hz, (C)), 119.6 (CHAr), 120.8 (CHAr), 124.0 (CHAr),
125.9 (CHAr), 126.7 (CHAr), 128.3 (CAr), 129.0 (CAr), 131.1 (CAr),
The reaction was followed by monitoring the absorption band at 640
nm until no further increase was observed, which took not longer than
15 min.
136.6 (CAr), 144.2 (CAr), 145.0 (CAr), 156.3 (CAr), 158.2 (NCHAr); 31
P
The investigation of the oxygen atom-uptake from sPhIO was carried
out as described above: addition of the first equiv of sPhIO was followed
by injection of an excess of O2 or a second equiv of sPhIO after
equilibration with stirring.
(c) Raman Spectroscopy. A screw-cap NMR tube equipped with
septum was charged with 0.5 mL of a solution of 1 in thf (46 mM or 9
mM) and cooled to -80 °C. Dry 16O2 and 18O2, respectively, were
bubbled through the solutions for 10 s prior to the spectroscopic
measurements, carried out at -80 °C.
(d) PPh3 Oxidation. Under strictly anaerobic conditions 20 mg of
(18.5 μmol) 1 were dissolved in 8 mL of thf, cooled to -80 °C and
exposed to an excess of dry dioxygen under vigorous stirring. After 20
min dioxygen was removed by several vacuum/argon cycles, and a
solution of 48.4 mg of (0.185 mmol) PPh3 in 1 mL of thf was added. The
reaction mixture was stirred for further 45 min at -80 °C and
subsequently annealed to room temperature (r.t.). Removal of all
volatiles led to a brown solid, to which 14.3 mg of (36.9 μmol)
[nBu4N]PF6 were added as an internal standard, and the conversion
was determined by 31P NMR spectroscopy. A control experiment in the
absence of 1 did not lead to phosphane oxidation.
NMR (CD2Cl2, 297 K): δ -47.72 (s); Anal. found: C 70.06, H 7.33, N:
4.81; Calcd for C67H86N4Cu2OP2: C 69.83, H 7.52, N 4.86%; IR (KBr,
cm-1): 3058 (w), 2901 (m), 2865 (m), 1588 (s), 1577 (s), 1459 (vs),
1445 (vs), 1380 (m), 1362 (vs), 1322 (s), 1284 (m), 1273 (m), 1246
(m), 1219 (s), 1187 (w), 1063 (m), 986 (w), 958 (m), 856 (w), 780 (m),
721 (m), 671 (w).
Catalytic Oxidative Coupling of 2,4-Di-tert-butylphenol A
in Presence of 1 to 3,30,5,50-Tetra-tert-butyl 2,20-biphenol
B. Under strictly anaerobic conditions 25 mg (23 μmol) of 1 and 47.6
mg (0.23 mmol) of A were dissolved in 4 mL of thf and toluene,
respectively. Optionally molecular sieves were added. After exposure to
an excess of dry dioxygen the reaction mixture was stirred vigorously for
6 h at r.t. The reaction was terminated by removal of all volatiles,
redissolving in 30 mL of diethyl ether, and treatment of the organic
phase with 20 mL of diluted HCl (4 M). Any molecular sieves employed
were removed by filtering off the ether solution through a plug of Celite
prior to the hydrochloric acid addition. After separating the phases
the aqueous one was extracted twice with 20 mL of diethyl ether. The
combined organic phases were dried with MgSO4, and after filtration the
solvent was removed under reduced pressure. To the resulting solid 17.9
μL (0.23 mmol) dimethylformamide were added as an internal standard,
Crystal Structure Determinations. Crystals of 1 ꢁ 4(CH3CN)
and 3 suitable for single crystal analysis were obtained from a 5:1 mixture
of acetonitrile and benzene. Crystals of 2 ꢁ 6(C7H8) were obtained by
storing a concentrated solution in toluene at 4 °C. Data collections were
performed at 100 K on a Stoe IPDS 2T diffractometer using Mo-KR
radiation (λ = 0.71073 Å); radiation source was a sealed tube generator
with graphite monochromator. Numerical absorption correction was
applied for 1 and multiscan (PLATON16) absorption correction for 2
1
and the conversion was determined by H NMR spectroscopy. Yields
were determined from at least two replicate runs and are given with an
accuracy of (1%. Alternatively, the sodium salt of A, [Na(thf)O(2,4-
tBu)C6H3], A0 (69.3 mg 0.23 mmol) was employed. Because of the
mononuclear nature of the complex [(N(2,6-Me2C6H3)C-
(CH3))2CH]Cu(MeCN) 4, in comparison to 1 the double amount of
equivalents (0.46 mmol) was used.
2135
dx.doi.org/10.1021/ic101249k |Inorg. Chem. 2011, 50, 2133–2142

Inorganic Chemistry
ARTICLE
Table 1. Crystal Data and Experimental Parameters for the Crystal Structure Analyses of 1, 2, and 3
1 ꢁ 4(CH3CN)
73H86Cu2N10
2 ꢁ 6(C7H8)
3
formula
C
O
C139H146Cu2N4OP2
2077.62
C67H86N4Cu2OP2
1152.42
weight, g mol-1
crystal system
space group
a, Å
1246.60
triclinic
monoclinic
P2/c
monoclinic
P21/c
P1
16.0571(8)
16.0722(9)
16.1381(9)
98.674(4)
105.632(4)
119.080(4)
3302.9(3)
2
16.8182(9)
15.6847(9)
27.1407(9)
90
13.6624(4)
28.0073(9)
17.2399(5)
90
b, Å
c, Å
R, deg
β, deg
127.083(2)
90
107.578(2)
90
γ, deg
V, Å3
5711.5(5)
2
6288.8(3)
4
Z
density, g cm-3
μ(MoKR), mm-1
F(000)
1.253
1.208
1.217
0.695
0.455
0.771
1320
2208
2448
GoF
0.992
1.018
1.070
Rind[I > 2σ(I)]
R1 = 0.0752
wR2 = 0.1596
R1 = 0.1369
wR2 = 0.1829
-1.11/1.36
R1 = 0.0362
wR2 = 0.0907
R1 = 0.0428
wR2 = 0.0937
-0.62/0.63
R1 = 0.0833
wR2 = 0.1868
R1 = 0.1082
wR2 = 0.2001
-1.26/1.23
Rind (all data)
ΔFmin/ΔFmax, e Å-3
and 3. The structures (Table 1) were solved by direct methods
(SHELXS-97)17a and refined by full matrix least-squares procedures
based on F2 with all measured reflections (SHELXL-97).17b All non-
hydrogen atoms were refined anisotropically. H atoms were introduced
in their idealized positions and refined as riding.
Scheme 1
’ RESULTS AND DISCUSSION
Complex Synthesis. First experiments on the system
Me2C6H3
[
Xanthdim]2-/CuI confirmed the suspicions with re-
spect to disproportionation. While mononuclear diketiminato-
CuI complexes were successfully synthesized in the past18 by reac-
tions of the corresponding lithium diketiminates with [Cu-
(NCCH3)4]SO3CF3, all attempts to react [Me C H Xanthdim]Li2
with [Cu(NCCH3)4]SO3CF3 led to immediate disproportiona-
tion. Lee et al. successfully prepared a CuI complex of a
macrocyclic ligand containing two diketiminato units4d in the
absence of strongly coordinating solvents like acetonitrile by
reacting the protonated form of the ligand in toluene with
copper(I)-tert-butoxide. Isolation was possibly aided by the
insolubility of the product. However, applying the same proce-
2
6
3
was characterized by IR spectroscopy, elemental analysis, and
NMR spectroscopy.
After dissolution of the ligand precursor, the protons bound
inside the asymmetric diimine units rapidly flip from one N atom
to the other; hence, the protons belonging to the aldimine
backbone are equivalent and split to a doublet by coupling with
the N-H proton in the 1H NMR spectrum. Consequently, the
observation of a singlet signal for the diiminato unit is a first
indication of a successful deprotonation, and a second one is the
absence of a triplet originating from the N-H unit at around
12 ppm. The 1H NMR spectrum of 1 in CD2Cl2 matched these
criteria and thus revealed the quantitative replacement of the
N-H protons by copper ions. At the same time it pointed to a
highly symmetric structure. Two singlets are observed for all
methyl groups bonded in 2- and 3-positions of the aryl rings,
shifted to lower field in comparison to the corresponding signals
dure to [Me C H Xanthdim]H2 again only led to disproportiona-
tion, even when thf was chosen as the solvent to provide a
stronger potential coligand for the stabilization of complexed CuI
centers (toluene molecules served as coligands in the system of
Lee). A successful synthesis was finally achieved by addition of
acetonitrile.
2
6
3
Treatment of [Me C H Xanthdim]H2 with 2 equiv of copper-
(I)-tert-butoxide in a mixture of benzene and acetonitrile (2:1) at
r.t. immediately led to a clear yellow solution, and subsequent
addition of excessive acetonitrile led to the precipitation of the
2
6
3
in [Me C H Xanthdim]H2. The acetonitrile ligands at the copper
centers also show only one singlet. Addition of a large excess of
degassed water to such an NMR sample (CD2Cl2 as the solvent)
revealed also water-sensitivity of 1, albeit not to an extent that
could have been expected in view of the rather basic character of
2
6
3
product [Me C H Xanthdim](Cu(NCCH3))2, 1 (Scheme 1). 1
could be isolated in 70% yield after work up, and it is readily
soluble in benzene, toluene, and thf. As expected, the complex is
very sensitive to O2, both in the solid state and in solution, and it
2
6
3
2136
dx.doi.org/10.1021/ic101249k |Inorg. Chem. 2011, 50, 2133–2142

Inorganic Chemistry
ARTICLE
Scheme 2
Figure 1. Molecular structure of 1 co-crystallized with four molecules of
acetonitrile. All hydrogen atoms and solvent molecules have been
omitted for clarity. Selected bond lengths [Å] and angles [deg]:
Cu1-Cu2 4.7739(10), Cu1-N1 1.952(4), Cu1-N2 1.990(4),
Cu2-N3 1.953(4), Cu2-N4 1.979(4), Cu1-N6 1.882(5), N6-C64
1.141(7), Cu2-N5 1.867(2), N5-C35 1.159(8), N1-C9 1.338(6),
N2-C11 1.315(6), C9-C10 1.393(7), C10-C11 1.423(7), N3-C25
1.333(6), N4-C34 1.328(6), C25-C24 1.399(7), C24-C34 1.402(8),
N1-Cu1-N2 96.87(17), N1-Cu1-N6 142.6(2), Cu1-N6-C64
164.6(5), N3-Cu2-N4 97.30(18), N3-Cu2-N5 136.7(2), Cu2-
N5-C35 165.5(5), N1-C9-C10 127.5(4), C9-C10-C11 125.3(4),
N2-C11-C10 126.2(5), N3-C25-C24 128.4(5), C25-C24-C34
124.7(5), N4-C34-C24 126.7(5), C9-C10-C20-C53 -44.8(7),
C34-C24-C23-C22 62.8(8), C10-C9-N1-Cu1 11.1(8), C24-
C25-N3-Cu2 9.7(8), C9-N1-Cu1-N6 150.5(5), C34-N4-
Cu2-N5 -149.6(4), N1-Cu1-N2-C11 21.4(5), N3-Cu2-N4-
C34 22.8(5).
Figure 2. Molecular structure of 2 co-crystallized with six molecules of
toluene. All hydrogen atoms and solvent molecules have been omitted
for clarity. Selected bond lengths [Å] and angles [deg]: Cu1-Cu10
6.6303(4), Cu1-N1 1.9633(12), Cu1-N2 1.9699(12), Cu1-P1
2.1748(4), N1-C27 1.3251(19), N2-C29 1.3227(19), C27-C28
1.407(2), C28-C29 1.402(2), N1-Cu1-N2 96.65(5), N1-Cu1-
P1 131.05(4), N1-C27-C28 128.17(13), C27-C28-C29
124.89(13), N2-C29-C28 126.15(13), C45-C38-C28-C29
127.2(2), C28-C27-N1-Cu1 -4.6(3), C27-N1-Cu1-P1 -
173.26(12), N1-Cu1-N2-C29 2.33(16).
the β-diiminato ligands: After 30 min the 1H NMR spectrum shows
signals for three different species: 1, the mononuclear complex
Me2C6H3
[
Xanthdim]H(Cu(NCCH3)), and [Me2C6H3Xanthdim]H2
in an approximate ratio of 1:2:3. Complete hydrolysis of 1 occurs
within 48 h, and it is accompanied by the formation of an orange
colored precipitate. Single crystals of 1 could be grown by storing
a solution of 1 in a 5:1 mixture of acetonitrile and benzene at r.t.,
and the result of the X-ray diffraction analysis is shown in
Figure 1. The two copper ions show a distorted trigonal planar
coordination geometry with chelating binding angles of
96.87(17)° (N1-Cu1-N2) and 97.30(18)° (N3-Cu2-N4).
The Cu-Cu separation of 4.7739(10) Å is comparable to the
employed as precursors for oxidation chemistry, too. Hence,
our investigations were extended to PPh3 and PMe3 ligands.
When the reaction shown in Scheme 1 was performed in the
absence of acetonitrile but in the presence of 2 equiv of
triphenylphosphane, again a yellow solution was formed, from
which yellow [Me C H Xanthdim](Cu(PPh3))2, 2, could be pre-
cipitated (Scheme 2) via addition of acetonitrile (75% yield after
workup).
2
6
3
one observed in an analogous (three-coordinated) zinc complex
Me2C6H3
[
Xanthdim](ZnEt)2 (4.8654(7) Å).6 Furthermore, it can
Compound 2 shows similar solubilities as 1 but it is signifi-
cantly more stable against air: a solution changes color from
yellow to brown only after several minutes, and the solid is stable
in air for hours. 2 has been characterized by NMR spectroscopy,
IR spectroscopy and elemental analysis. Naturally, the 1H NMR
spectrum obtained for 2 only shows slight differences in compar-
ison to the one recorded for 1. The phosphane ligands give rise to
a singlet resonance at 3.52 ppm in the 31P NMR spectrum, which
thus compares well to the signals found for other diketiminato-
Cu-phosphane complexes.19 Single crystals suitable for X-ray
diffraction could be grown by storing a concentrated solution of 2
in toluene at 4 °C, and Figure 2 shows the molecular structure.
Complex 2 crystallizes as a toluene solvate in the centrosym-
metric space group P2/c with the complex molecule lying on a
be seen that the binding pockets are indeed oriented parallel to
each other without any distortion because of the quite low steric
demand of the coordinating acetonitrile ligands.
As for a large number of the diketiminato-copper chemistry
reported so far, mononuclear CuI-acetonitrile complexes were
employed (since the acetonitrile ligand is readily replaced),3,18
1 can be envisaged as a valuable precursor for novel multicopper
chemistry and O2 activation studies (also compare below). In this
context it seemed worthwhile to examine the influence of the
coligands L in [Me C H Xanthdim](CuL)2 on the stability and
O2-reactivity. Phosphane ligands have been shown to replace
acetonitrile ligands at diketiminato-CuI units leading to com-
plexes with retarded reactivity, which, however, were often
2
6
3
2137
dx.doi.org/10.1021/ic101249k |Inorg. Chem. 2011, 50, 2133–2142

Inorganic Chemistry
ARTICLE
the reaction vessel was allowed to anneal. This resulted in a
bright orange colored solution, from which all volatiles were
removed subsequently. Redissolution in benzene and treatment
with excessive acetonitrile then gave a yellow precipitate
Me2C6H3
[
Xanthdim](Cu(PMe3))2, 3, that could be isolated in
67% yield (Scheme 2). Compound 3 was characterized by means
of NMR and IR spectroscopy as well as by elemental analysis.
The 1H NMR spectrum of 3 is differing from the one observed
for 2 principally only in two aspects: instead of the resonances
belonging to the triphenylphosphane ligands a doublet is ob-
served at 0.81 ppm for the trimethylphosphane ligand, and the
protons belonging to the aldimine units cause a doublet signal
(because of the long-range coupling with the P atoms, that does
not occur in case of 2).
Storing a solution of 3 in a 5:1 mixture of acetonitrile and
benzene at r.t. led to the formation of single crystals, which were
examined by X-ray diffraction, and the result of the correspond-
ing structure analysis is shown in Figure 3.
Figure 3. Molecular structure of 3. All hydrogen atoms have been
omitted for clarity. Selected bond lengths [Å] and angles [deg]: Cu1-
Cu2 5.6670(11) Cu1-N1 1.967(4), Cu1-N2 1.959(4), Cu2-N3
1.967(5), Cu2-N4 1.934(5), Cu1-P1 2.1499(17), Cu2-P2
2.1437(19), N1-C12 1.314(6), N2-C14 1.330(7), C12-C13
1.418(7), C13-C14 1.398(7), N3-C28 1.313(7), N4-C32
1.331(7), C27-C28 1.410(7), C27-C32 1.405(8), N1-Cu1-N2
95.96(17), N1-Cu1-P1 122.61(13), N3-Cu2-N4 96.96(18),
N3-Cu2-P2 126.45(15), N1-C12-C13 125.9(5), C12-C13-
C14 124.2(4), N2-C14-C13 128.3(5), N3-C28-C27 126.0(5),
C32-C27-C28 123.9(5), N4-C32-C27 128.4(5), C57-C23-
C13-C14 -36.7(7), C25-C26-C27-C28 -50.7(8), C13-C12-
N1-Cu1 -10.5(8), C27-C28-N3-Cu2 3.5(8), C12-N1-Cu1-
P1 -154.2(4) C32-N4-Cu2-P2 -165.0(4), N1-Cu1-N2-C14
-17.0(4), N3-Cu2-N4-C32 11.7(5).
As expected the Cu-Cu distance is significantly shorter
(5.6670(11) Å) than the one in 2 as the trimethylphosphane
ligands are less bulky and thus allow a closer approach of the two
Cu centers. Accordingly, also the Cu-P distances are somewhat
shorter for 3 (2.1499(17) and 2.1437(19) Å) than for 2 ꢁ
6(C7H8) (2.1748(4) Å), as there are less steric clashes within the
binding pocket. Otherwise the structural data of the diiminato-
Cu moieties correspond well to those observed for 2 ꢁ 6(C7H8).
As in that case, the Cu atoms are not located within the plane
defined by the diiminato framework ((C13-C12-N1-Cu1) =
-10.5(8)° and (C27-C28-N3-Cu2) = 3.5(8)°). However,
while the Cu atoms turn away from each other in 2 they are
oriented in the same direction within 3.
Like 2, complex 3 is only slightly sensitive to air. Thus, the
stability of 2 and 3 may mainly originate from electronic rather
than steric effects.
2-fold axis. Both copper ions display a trigonal planar coordina-
tion sphere, which is distorted because of the acute angle N1-
Cu1-N2 of 96.65(5)°. The Cu-Cu distance amounts to
6.6303(4) Å, which is considerably longer than the distance
observed for 1 ꢁ 4(CH3CN) (4.7739(10) Å). This is due to the
steric bulk of the triphenylphosphane ligands that obviously push
the Cu centers (including the corresponding binding pockets)
apart. For the same reason, the Cu ions are positioned somewhat
outside the planes defined by the diiminato binding pockets
((C28-C27-N1-Cu1) = -4.6(3)°. As already derived from
the NMR data, these diiminato units are quite symmetric
showing equally long C-C and C-N bonds, respectively. The
Cu-N and Cu-P bond distances are comparable to those
observed in diketiminato-Cu-phosphane complexes.19
Phosphane ligands are stronger σ-donors and also better
π-acceptors than acetonitrile, and this might account for the
increased stability of 2 against air. In fact, phosphane ligands have
been shown to replace O2 coordinated to a mononuclear
diketiminato-copper complex, while O2 replaces acetonitrile.12
On the other hand, the Cu centers are well protected within 2,
and this might contribute, too.
Activity of 1 As a Catalyst for the Oxidative Coupling of
Phenols. The coupling of arenes is of great importance for
general organic synthesis, and the copper-catalyzed phenol
coupling to arrive at chiral biphenol derivatives is used exten-
sively as a test reaction for the catalytic activity of new copper
complexes.20 These reactions usually start from CuI compounds
and are assumed to involve an initial O2 activation to yield CuII
centers, which oxidize either free phenols via a proton-coupled
(outer-sphere) electron-transfer21 or by formation of a dinuclear
CuII phenoxo complex.22 For the subsequent coupling both a
free radical type reaction and an alternative pathway including
the decomposition of binuclear CuII phenoxo species are dis-
cussed (Chart 3).21,22 Assuming that indeed the decisive cou-
pling step requires the contact of two such units, it is also
reasonable to assume that dinuclear catalysts may have advan-
tages in this context, and in fact dicopper(I) complexes, speci-
fically synthesized by employing dinucleating ligands, were
shown to couple 2,4-di-tert-butylphenol, A, to the corresponding
bisphenol derivative B (Scheme 3) in most of the cases reported
so far.23
To clarify the effect of the steric bulk, trimethylphosphane was
also tested as a coligand, as it is far less sterically demanding and
at the same time a better σ-donor than triphenylphosphane. To
synthesize a corresponding PMe3 complex, benzene was added
It was thus of particular interest to test the potential of 1 with
respect to this kind of reactivity. Since the reaction shown in
Scheme 3 could be expected to produce also water (if not H2O2 is
formed22), and 1 is not stable against water over longer periods of
time, we have initially used the sodium salt of A, sodium 2,4-di-
tert-butylphenolate A0.
to copper(I)-tert-butoxide and [Me C H Xanthdim]H2, and the
resulting solution, which lacks of proper σ-donor molecules, was
quickly frozen to avoid disproportionation of the unstable
dinuclear complex initially formed. Excessive trimethylpho-
sphane was condensed on top of the frozen mixture, and
2
6
3
Accordingly, a solution of 1 and 10 equiv of sodium 2,4-di-tert-
butylphenolate in 4 mL of thf was stirred for 6 h at r.t. under an
2138
dx.doi.org/10.1021/ic101249k |Inorg. Chem. 2011, 50, 2133–2142

Inorganic Chemistry
ARTICLE
Chart 3
Scheme 3
Figure 4. UV/vis spectra monitoring the reaction of 1 in thf (1 mM)
with excessive O2 at -80 °C. The various traces in the direction of the
arrow represent the spectra recorded before O2 addition and 10, 30, 50,
110, 210, and 610 s subsequent to O2 addition.
7.4 is somewhat higher than those reached by other dinuclear CuI
systems.23 Comparison with the coupling yields/efficiencies of
more simple copper based systems employed in organic synth-
esis is difficult because of the quite different reaction
conditions.24 However, it becomes obvious that their TOFs are
lower while their robustness is higher, so that higher maximum
TONs can be reached. For further comparison we also carried
out this reaction with the known β-diketiminato-CuI complex,
[(N(2,6-Me2C6H3)C(CH3))2CH]Cu(MeCN) 4, as a catalyst.18
To keep the quantity of copper equivalents constant, the molar
percentage was doubled (with respect to the one employed for
1). Toluene was chosen as the solvent, and molecular sieves were
added such that the yield of 62% (line 6) has to be compared with
line 5. The comparison shows that the prearranged dinuclear
species brings out only a small advantage.
Identification of the Primary Product in Contact with
O2. In principle, the system 1/A/O2 also could have led to
ortho-hydroxylations, which, however, were not observed. Such
tyrosinase activity requires the formation of a CuO2Cu inter-
mediate and an accessible coordination site for phenolate
binding.21,25 To investigate whether the reaction of 1 with O2
leads to a CuO2Cu core, 1 was dissolved in thf and treated with
O2 at -80 °C. Notably, the formation of a green solution was
observed, which changed color to brown upon annealing to r.t.
Low-temperature UV/vis measurements (Figure 4) showed that
during the reaction of 1 with O2 at -80 °C a new band at about
640 nm (ε = 1000 M-1 cm-1) evolves.
A band at this wavelength cannot arise from a Cu(μ-O)2Cu
entity, as corresponding complexes usually absorb around 420
nm. The extinction coefficient points either to a CuII d-d
transition of a Cu(μ-O)Cu species or to a ligand-to-metal charge
transfer (LMCT) band that should be expected for CuII peroxo
units.3b,18,26,27 Copper peroxide complexes should be character-
ized by further bands (e.g., at ca. 350 nm for Cu(μ-η2:η2-O2)Cu
units or at ca. 520 nm for Cu(μ-η1:η1-O2)Cu moieties),27,28 but
the corresponding regions of the spectrum of 1 are masked by
ligand absorptions, so that this did not aid the assignments.
Spectrophotometric O2 titration experiments indicate that com-
plete conversion of 1 to the intermediate requires 0.5-1 equiv of
O2 per dicopper unit (after addition of 1 equiv excessive O2 does
not lead to a further increase of the 640 nm band, see Supporting
Information, Figures S1-S3). Hence, this finding does not hint
unequivocally into one or the other direction; it might be due to
Table 2. Oxidative Coupling of 2,4-Di-tert-butylphenol, A,
Catalyzed by 1 (10 mol %)c
line
solvent
molecular sieves
yield [%]
turnover number
1a
2
thf
no
no
yes
no
yes
yes
39
27
31
53
74
62
3.9
2.7
3.1
5.3
7.4
6.2
thf
3
thf
4
toluene
toluene
toluene
5
6b
a Sodium 2,4-di-tert-butylphenolate, A0, was used instead of 2,4-di-tert-
butylphenol, A, as a substrate. b The mononuclear catalyst 4 was
c
employed instead of 1 with a doubled concentration. The data refer
to a reaction time of 6 h at r.t.
atmosphere of dioxygen. After workup, B was identified as the
1
reaction product by means of H and 13C NMR spectroscopy
with a yield of 39% (based on A0, Table 2, line 1).
Encouraged by these results we have also used the phenol A
itself for comparison. Under the same conditions as described
above, B was formed in 27% yield (line 2). Hence, the conversion
decreases somewhat because of the deleterious effect of the water
produced concomitantly, albeit not to an extent that could have
been envisaged. This finding can be explained assuming that the
CuII intermediates formed during catalysis are somewhat more
resistive toward hydrolysis than 1. To test whether hydrolysis can
be avoided completely, in the next step the catalytic oxidations
were performed in the presence of molecular sieves, which were
supposed to remove the water formed during the reaction. The
coupling of A proceeded slightly more effectively then with a 31%
yield of B (line 3) which, however, is still less than that achieved
with A0 (vide supra).
Variation of the solvent showed that performance of the
coupling in a less strongly coordinating solvent allows for
considerably higher conversions: in toluene 53% B (line 4) is
obtained and the coupling activity of 1 can be further increased
by addition of molecular sieves leading to a maximum value of
74% yield (line 5). The resulting turnover number (TON) of
2139
dx.doi.org/10.1021/ic101249k |Inorg. Chem. 2011, 50, 2133–2142

Inorganic Chemistry
ARTICLE
partial decay of initial O2 adducts like those depicted in Scheme 4
or to equilibria, for instance, between 1/O2 and the Cu(μ-O)Cu
species. However, treatment of 1 at -70 °C with 1 equiv of a
soluble derivative of iodosobenzene, 2-(tert-butylsulfo-
nyl)iodosylbenzene (sPhIO),29 caused the development of the
same UV/vis band at 640 nm as observed after O2 addition,
which argues in favor of a Cu(μ-O)Cu species (Scheme 4). This
reaction proceeds very slowly, though, so that it could not be
brought to completion within 6 h as indicated by the sudden
increase of the characteristic absorption band when O2 was
added after that time. Almost quantitative conversion was
possible when an excess of sPhIO (10 equiv) was employed,
and the final spectrum showed no additional features as com-
pared to the one recorded on treatment with 1 equiv, which yet
again supports the formation of a Cu(μ-O)Cu complex. The
same observations were made employing Me3NO as the OAT
reagent.
Scheme 4. Possible Routes to a CuII-O-CuII Intermediate
Comparison with the spectral data reported for the few known
examples of Cu(μ-O)Cu complexes, as summarized in Table 3,
also confirms the assignment of the 640 nm band to a
Cu(μ-O)Cu species.
Further support is derived from low-temperature Raman and
resonance Raman spectroscopy (-80 °C, thf). After reaction of 1
with 16O2 a new band at 593 cm-1 was observed in the Raman
spectrum (1064 nm excitation) that decays upon annealing at
ambient temperature. In addition, the band shifts down to 568
cm-1 when the reaction is carried out with 18O2 (Figure 5). No
other isotopically sensitive signals could be detected. Raman
bands at around 600 cm-1 were reported for Cu(μ-O)2Cu
compounds, but, as outlined above, these complexes do not
exhibit UV-vis absorption bands between 600 and 700
nm.18,5,27 To distinguish between Cu(μ-O)2Cu and Cu(μ-O)Cu
species we have performed an additional experiment with a 1:2:1
mixture of 18O2:16O18O:16O2 (12.5% in argon). Because of the
dilution by argon, the signal-to-noise ratio of the spectra obtained
from these experiments was distinct as compared to those in
Figure 5, but sufficient to identify the two bands at 568 and 593
cm-1. Some additional Raman activity was noted between these
bands; however, a 1:2:1 intensity distribution for a triplet with
band components at 568, ∼580, and 593 cm-1, as expected for a
Cu(μ-O)2Cu moiety, could safely be ruled out.
Figure 5. Raman spectra recorded after oxygenation of 1 (λex = 1064
nm, thf, 46 mM, -80 °C) with 16O2 (gray line) and 18O2 (black line).
The conjugate bands at 593 (16O) and 568 cm-1 (18O) are
readily detected, but there is not significant enhancement for
these modes. However, 647 nm excitation allows detecting two
additional bands at 612 and 640 cm-1 which, upon reaction with
18O2, shift down to 598 and 622 cm-1, respectively (see
Supporting Information, Figure S4). The quite similar frequen-
cies of these band pairs suggest structurally similar complexes and
thus a heterogeneity of the intermediate, presumably originating
Unfortunately, no vibrational spectra have been reported in
the literature for the Cu-O-Cu compounds in Table 3 that
might further back the assignment. The only literature data
including vibrational frequencies refer to oxygen-activated Cu-
ZSM-5 that has been shown to selectively oxidize methane to
methanol. Here, a [Cu2O]2þ core was identified as the active site,
which gives rise to an electronic transition at 441 nm, and to 18O-
sensitive resonance Raman bands at 456 and 870 cm-1, assigned
to the symmetric and asymmetric (Cu-O-Cu) stretching
mode, respectively (Table 3).9a We like to point out that Raman
spectra of oxygenation product of 1, obtained with 1064- or 647-
nm excitation, do not display 18O-sensitive bands in the regions
from a distribution of 1 O aggregates, although the involvement
3
of Cu(μ-O)2Cu moiety cannot rigorously be ruled out.
There are several options how a Cu-O-Cu complex can be
formed from 1 and O2. A transient 1 O2 species may be formed
3
that transfers one O atom onto an unreacted molecule of 1. On
the other hand O2 activation could also proceed between two
around 450 or 850 cm-1
.
different molecules of 1 to give a transient 12 O2 species, whose
3
CuO2Cu moiety gets reduced in an intramolecular process by the
Thus, both the vibrational spectra and the electronic spectra of
Cu-ZSM-5 differ substantially from the intermediate state ana-
lyzed in this work, and evidently reflect the differences in electron
density distribution and geometry of a Cu-O-Cu unit as a part
of a zeolite on the one hand and within the N-donor sphere of a
coordination compound on the other hand.
two pendant CuI centers to finally yield in two molecules of 1 O.
3
Moreover we cannot rule out that the Cu-O-Cu species
contains Cu centers belonging to two different molecules of 1,
in which case it would correspond to a tetranuclear 1(O)21
complex (Scheme 4).
In any case, according to the observations described above the
Cu-O-Cu species generated from 1 is unstable and
Further Raman experiments were carried out with 647 nm
excitation, that is, in resonance with the 640 nm absorption band.
2140
dx.doi.org/10.1021/ic101249k |Inorg. Chem. 2011, 50, 2133–2142

Inorganic Chemistry
ARTICLE
Table 3. Summary of UV/vis and Raman Spectral Data of Known Cu(μ-O)Cu Species
complex
solvent
λmax (nm) (ε (M-1 cm-1))
υ(Cu-16O) (cm-1) (υ(Cu-18O) (cm-1))
ref.
[((Bpy2)Cu)2O]
CH2Cl2
CH2Cl2
PhNO2
PhNO2
PhNO2
PhNO2
CH2Cl2
thf
660 (760)
650 (180)
600 (130)
640 (140)
625 (80)
30b
[((Bpi)Cu)2O]
30c
[((TMEDA)Cu)2O]
[((TEEDA)Cu)2O]
[((TPEDA)Cu)2O]
[((TMPDA)Cu)2O]
[((HB(3,5-Me2pz)3)Cu)2O]
[((MePY2)Cu)2O]
1 þ O2 at -80 °C
30d
30d
30d
650 (240)
660 (105)
680 (100)
640 (1000)
30d
30e
30f
thf
593 (568)
612 (598),a 640 (622)a
this work
Cu-ZSM-5 after O2 activation
441
456 (448), 870 (830)
9a,9d
a Only observed with 647 nm excitation, (Bpy2) = N-benzyl-2-(pyridin-2-yl)-N-(2-(pyridin-2-yl)ethyl)ethanamine, Bpi = biphenyl-2,20-di(carb(2-(2-
pyridyl)ethylimine, TMEDA = N,N,N0,N0-tetramethylethylendiamine, TEEDA = N,N,N0,N0-tetraethylethylendiamine, TPEDA = N,N,N0,N0-tetra-
propylethylendiamine, TMPDA = N,N,N0,N0-tetramethyl-1,3-propanediamine, MePY2 = N-methyl-N,N0-bis[2-(2-pyridyl)ethyl]amine.
decomposes at ambient conditions to give unknown products.
This finding is not surprising considering that some of the few
Cu-O-Cu compounds isolated so far also decompose very
easily.30a,e The chemical behavior of this Cu-O-Cu intermedi-
ate is also in good agreement with corresponding reports in the
literature since, for example, it was found to show O atom
transfer reactivity: Treatment with 10 equiv of PPh3 for 30 min at
-80 °C followed by annealing to r.t. gave 2 and O=PPh3 (33%,
referenced to the amount of 1 employed) as evidenced by UV/vis
and NMR spectroscopy.
Formation of a Cu-O-Cu species as the primary product
instead of Cu-O-O-Cu or Cu(μ-O)2Cu intermediates could
also explain why no tyrosinase activity has been observed for 1.
Nevertheless, the findings reported here are still of substantial
bioinorganic interest. In the context of the Cu-ZSM-5/O2 and
pMMO work described above it was suggested that “future
research should focus on how the CuII-O-CuII species possibly
translates to the context of protein environments; the structure
will likely become a focus of a bioinorganic field invigorated by
the proposal.”9b Furthermore it was noted: “The mixed-valent
oxodicopper(II/III) or simple oxodicopper(II) centers are both
candidates for the enzyme active site methane oxidizer. The
former has not yet been identified or isolated among the known
crop of Cu2O2 synthetic model compounds; the latter has some
precedent, but does not have a developed chemistry.”8b The
work presented here makes a contribution to the, as can be
judged from these quotations, important but scarcely investi-
gated field of Cu-O-Cu compounds.
low temperature showed that an unstable Cu-O-Cu complex is
formed during the initial steps. Future work will now focus on the
utilization of complexes like 1 as building-blocks for the modeling
of multicopper enzymes.
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data in
b
cif format. Further details are given in Figures S1-S4. This
material is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: christian.limberg@chemie.hu-berlin.de.
’ ACKNOWLEDGMENT
We are grateful to the Fonds der Chemischen Industrie, the
BMBF, the Dr. Otto R€ohm Ged€achtnisstiftung, and the DFG
(Cluster of excellence “Unifying concepts in catalysis”) for
financial support as well as to Bayer Services GmbH & Co.
OHG, BASF AG and Sasol GmbH for the supply of chemicals.
We also would like to thank M. F. Pilz for pioneering work,
S. Hinze and A. K€argel for the preparation of starting materials,
and P. Neubauer for X-ray diffraction analysis.
DEDICATION
†Dedicated to Prof. Erhard Kemnitz on the occasion of his 60th
birthday.
’ CONCLUSIONS
A procedure has been developed that allows for the synthesis
and isolation of novel, dinuclear CuI complexes based on
’ REFERENCES
(1) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002,
102, 3031.
the bis(diiminato) framework provided by the ligand
Me2C6H3
[
Xanthdim]2-. Despite the pronounced tendency of CuI
compounds to undergo disproportionation reactions (especially in
case of dinuclear cores) complexes [Me C H Xanthdim](Cu(L))2
(L = CH3CN, 1, PPh3, 2, PMe3, 3), once synthesized, proved stable
against corresponding decomposition reactions. As envisaged, 1
shows a high reactivity toward dioxygen that is retarded by
replacement of the acetonitrile coligands by phosphane ligands as
in 2 and 3 because of electronic reasons. 1 proved an efficient
catalyst for the oxidative coupling of phenols in the presence of O2,
and a closer inspection of the reactivity of 1 in contact with O2 at
2
6
3
(2) Compare for instance: Smith, J. M.; Lachicotte, R. L.; Pittard,
K. A.; Cundari, T. R.; Lukat-Rodgers, G.; Rodgers, K. R.; Holland, P. L. J.
Am. Chem. Soc. 2001, 123, 9222. Coates, G. W.; Moore, D. R. Angew.
Chem., Int. Ed. 2004, 43, 6618. Tsai, Y.-C.; Wang, P.-Y.; Chen, S.-A.;
Chen, J.-M. J. Am. Chem. Soc. 2007, 129, 8066. Monillas, W. H.; Yap,
G. P. A.; MacAdams, L. A.; Theopold, K. H. J. Am. Chem. Soc. 2007,
129, 8090. Pfirrmann, S.; Limberg, C.; Herwig, C.; St€osser, R.; Ziemer, B.
Angew. Chem., Int. Ed. 2009, 48, 3357. Pfirrmann, S.; Yao, S.; Ziemer, B.;
St€osser, R.; Driess, M.; Limberg, C. Organometallics 2009, 28, 6855. Yao,
2141
dx.doi.org/10.1021/ic101249k |Inorg. Chem. 2011, 50, 2133–2142

Inorganic Chemistry
ARTICLE
S.; Xiong, Y.; Vogt, M.; Gr€utzmacher, H.; Herwig, C.; Limberg, C.;
Driess, M. Angew. Chem., Int. Ed. 2009, 48, 8107. Yao, S.; Herwig, C.;
Xiong, Y.; Company, A.; Bill, E.; Limberg, C.; Driess, M. Angew. Chem.,
Int. Ed. 2010, 49, 7054.
Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement;
University of G€ottingen: G€ottingen, Germany, 1997.
(18) Spencer, D. J. E.; Reynolds, A. M.; Holland, P. L.; Jazdzewski,
B. A.; Duboc-Toia, C.; Le Pape, L.; Yokota, S.; Tachi, Y.; Itoh, S.;
Tolman, W. B. Inorg. Chem. 2002, 41, 6307.
(19) (a) Badiei, Y. M.; Warren, T. H. J. Organomet. Chem. 2005,
690, 5989. (b) York, J. T.; Young, V. G.; Tolman, W. B. Inorg. Chem.
2006, 45, 4191.
(3) (a) Aboelella, N. W.; Lewis, E. A.; Reynolds, A. M.; Brennessel,
W. W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 10660.
(b) Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.; Holland, P. L.;
Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108. (c) Aboelella, N. W.;
Kryatov, S. V.; Gherman, B. F.; Brennessel, W. W.; Young, V. G., Jr.;
Sarangi, R.; Rybak-Akimova, E. V.; Hodgson, K. O.; Hedman, B.;
Solomon, E. I.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2004,
126, 16896. (d) Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047.
(e) Reynolds, A. M.; Gherman, B. F.; Cramer, C. J.; Tolman, W. B. Inorg.
Chem. 2005, 44, 6989. (f) Hong, S.; Hill, L. M. R.; Gupta, A. K.; Naab,
B. D.; Gilroy, J. B.; Hicks, R. G.; Cramer, C. J.; Tolman, W. B. Inorg.
Chem. 2009, 48, 4514. (g) Hong, S.; Gupta, A. K.; Tolman, W. B. Inorg.
Chem. 2009, 48, 6323.
(20) (a) v. d. Vlugt, J. I.; Meyer, F. Top. Organomet. Chem. 2007,
ꢁ
22, 191. (b) Prokofieva, A.; Prikhodko, A. I.; Dechert, S.; Meyer, F.
Chem. Commun. 2008, 1005. (c) Mulrooney, C. A.; Li, X.; DiVirgilio,
E. S.; Kozlowski, M. C. J. Am. Chem. Soc. 2003, 125, 6856. (d) Tsubaki,
K.; Miura, M.; Morikawa, H.; Tanaka, H.; Kawabata, T.; Furuta, T.;
Tanaka, K.; Fuji, K. J. Am. Chem. Soc. 2003, 125, 16200.
(21) Herres-Pawlis, S.; Verma, P.; Haase, R.; Kang, P.; Lyons, C. T.;
Wasinger, E. C.; Fl€orke, U.; Henkel, G.; Stack, T. D. P. J. Am. Chem. Soc.
2009, 131, 1154.
(4) (a) Bourget-Merle, L.; Hitchcock, P. B.; Lappert, M. F. J.
Organomet. Chem. 2004, 689, 4357. (b) Vitanova, D. V.; Hampel, F.;
Hultzsch, K. C. J. Organomet. Chem. 2005, 690, 5182. (c) Curtis, N. F.
Comprehensive Coordination Chemistry II; McCleverty, J. A.; Meyer, T. J.,
Eds.; Elsevier: Oxford, 2004; Vol. 1, Chapter 1.20. (d) Lee, S. Y.; Na,
S. J.; Kwon, H. Y.; Lee, B. Y.; Kang, S. O. Organometallics 2004, 23, 5382.
(e) Lee, B. Y.; Na, S. J.; Kwon, H. Y.; Lee, S. Y.; Na, S. J.; Han, S.; Yun, H.;
Lee, H.; Park, Y.-W. J. Am. Chem. Soc. 2005, 127, 3031. (f) Vela, J.; Zhu,
L.; Flaschenriem, C. J.; Brennessel, W. W.; Lachicotte, R. J.; Holland,
P. L. Organometallics 2007, 26, 3416. Also compare: Allen, S. D.; Moore,
D. R.; Lobkovsky, E. B.; Coates, G. W. J. Organomet. Chem. 2003,
683, 137 for an alternative approach, involving, however, oxo
imido units. Piesik, D. F.-J.; Range, S.; Harder, S. Organometallics
2008, 27, 6178. Range, S.; Piesik, D. F.-J.; Harder, S. Eur. J. Inorg. Chem.
2008, 3442.
(22) Kitajima, N.; Koda, T.; Iwata, Y.; Moro-oka, Y. J. Am. Chem. Soc.
1990, 112, 8833.
(23) (a) Temma, T.; Hatano, B.; Habaue, S. Polymer 2006, 47, 1845.
(b) Sharma, V. B.; Jain, S. L.; Sain, B. J. Mol. Catal. A: Chem. 2004,
219, 61. (c) Gao, J.; Reibenspies, J. H. Angew. Chem., Int. Ed. 2003,
42, 6008. (d) Martell, A. E.; Mulrooney, C. A.; Li, X.; Di Virgilio, E. S.;
Kozlowski, M. C. J. Am. Chem. Soc. 2003, 125, 6856. (e) Gupta, R.;
Mukherjee, R. Tetrahedron Lett. 2000, 41, 7763.
(24) Buisman, G. J. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.
Tetrahedron: Asymmetry 1993, 4, 1625. W€unnemann, S.; Fr€ohlich, R.;
Hoppe, D. Eur. J. Org. Chem. 2008, 684.
(25) (a) Mirica, L. M.; Vance, M.; Rudd, D. J.; Hedman, B.;
Hodgson, K. O.; Solomon, E. I.; Stack, T. P. D. Science 2005,
308, 1890. (b) Op’t Holt, B. T.; Vance, M. A.; Mirica, L. M.; Heppner,
D. E.; Stack, T. P. D.; Solomon, E. I. J. Am. Chem. Soc. 2009, 131, 6421.
(26) Kajita, Y.; Arii, H.; Saito, T.; Saito, Y.; Nagatomo, S.; Kitagawa,
T.; Funahashi, Y.; Ozawa, T.; Masuda, H. Inorg. Chem. 2007, 46, 3322.
(27) Maiti, D.; Woertink, J. S.; Narducci Sarjeant, A. A.; Solomon,
E. I.; Karlin, K. D. Inorg. Chem. 2008, 47, 3787.
(5) Pilz, M. F.; Limberg, C.; Ziemer, B. J. Org. Chem. 2006, 71, 4559.
(6) Pilz, M. F.; Limberg, C.; Lazarov, B. B.; Hultzsch, K. C.; Ziemer,
B. Organometallics 2007, 26, 3668. Piesik, D. F.-J.; Haack, P.; Harder, S.;
Limberg, C. Inorg. Chem. 2009, 48, 11259.
(7) Pilz, M. F.; Limberg, C.; Demeshko, S.; Meyer, F.; Ziemer, B.
Dalton Trans. 2008, 1917.
(28) (a) Mirica, L. M.; Rudd, D. J.; Vance, M. A.; Solomon, E. I.;
Hodgson, K. O.; Hedman, B.; Stack, T. P. D. J. Am. Chem. Soc. 2006,
128, 2654. (b) Ottenwaelder, X.; Rudd, D. J.; Corbett, M. C.; Hodgson,
K. O.; Hedman, B.; Stack, T. P. D. J. Am. Chem. Soc. 2006, 128, 9268.
(29) Macikenas, D.; Skrzypczak-Jankun, E.; Protasiewicz, J. D. J. Am.
Chem. Soc. 1999, 121, 7164.
(30) (a) Pavlova, S. V.; Chen, K. H.-C.; Chan, S. I. Dalton Trans.
2004, 3261. (b) Karlin, K. D.; Gulneth, Y.; Hayes, J. C.; Zubieta, J. Inorg.
Chem. 1984, 23, 519. (c) Reglier, M.; Jorand, C.; Waegell, B. J. Chem.
Soc., Chem. Commun. 1990, 1752. (d) El-Sayed, M. A.; Davies, G.;
Kasem, T. S. Inorg. Chem. 1990, 29, 4730. (e) Kitajima, N.; Koda, T.;
Moro-oka, Y. Chem. Lett. 1988, 347. Kitajima, N.; Koda, T.; Hashimoto,
S.; Kitagawa, T.; Moro-oka, Y. J. Am. Chem. Soc. 1991, 113, 5664. (f)
Obias, H. V.; Lin, Y.; Murthy, N. N.; Pidcock, E.; Solomon, E. I.; Ralle,
M.; Blackburn, N. J.; Neuhold, Y.-M.; Zuberb€uhler, A. D.; Karlin, K. D. J.
Am. Chem. Soc. 1998, 120, 12960. (g) Davies, G.; El-Sayed, M. A. Inorg.
Chem. 1983, 22, 1257.
(8) (a) Balasubramanian, R.; Smith, S. M.; Rawat, S.; Yatsunyk, L. A.;
Stemmler, T. L.; Rosenzweig, A. C. Nature 2010, 465, 115. (b) Himes,
R. A.; Barnese, K.; Karlin, K. D. Angew. Chem., Int. Ed. 2010, 49, 6714.
(9) (a) Woertink, J. S.; Smeets, P. J.; Groothaert, M. H.; Vance,
M. A.; Sels, B. F.; Schoonheydt, R. A.; Solomon, E. I. Proc. Natl. Acad. Sci.
U.S.A. 2009, 106, 18908. (b) Himes, R. A.; Karlin, K. D. Proc. Natl. Acad.
Sci. U.S.A. 2009, 106, 18877. (c) Smeets, P. J.; Hadt, R. G.; Woertink,
J. S.; Vanelderen, P.; Schoonheydt, R. A.; Sels, B. F.; Solomon, E. I. J. Am.
Chem. Soc. 2010, 132, 14736. (d) Groothaert, M. H.; Smeets, P. J.; Sels,
B. F.; Jacobs, P. A.; Schoonheydt, R. A. J. Am. Chem. Soc. 2005, 127, 1394.
(10) Klinman, J. P. Chem. Rev. 1996, 96, 2541. Prigge, S. T.; Eipper,
B. A.; Mains, R. E.; Amzel, L. M. Science 2004, 304, 864. Rolff, M.;
Tuczek, F. Angew. Chem., Int. Ed. 2008, 13, 2344.
(11) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998. Laitar,
D. S.; Mathison, C. J. N.; Davis, W. M.; Sadighi, J. P. Inorg. Chem. 2003,
42, 7354. Shimokawa, C.; Teraoka, J.; Tachi, Y.; Itoh, S. J. Inorg. Biochem.
2006, 100, 1118.
(12) Reynolds, A. M.; Lewis, E. A.; Aboelella, N. W.; Tolman, W. B.
Chem. Commun. 2005, 2014.
(13) (a) Blackburn, N. J.; Strange, R. W.; Cruse, R. W.; Karlin, K. D.
J. Am. Chem. Soc. 1987, 109, 1235. (b) Pavlova, S. V.; To, H. L.; Chan, E.
S. H; Li, H.-W.; Mak, T. C. B.; Lee, H. K.; Chan, S. I. Dalton Trans.
2006, 2232.
(14) Tsuda, T.; Hashimoto, T.; Saegusa, T. J. Am. Chem. Soc. 1972,
94, 658.
(15) Battino, R. Oxygen and Ozone; Battino, R., Ed.; Pergamon
Press: New York, 1981; Vol. 7.
(16) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
(17) (a) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure
Solution; University of G€ottingen: G€ottingen, Germany, 1997; (b)
2142
dx.doi.org/10.1021/ic101249k |Inorg. Chem. 2011, 50, 2133–2142