Copper complexes with non-innocent ligands: Probing CuII/catecholato-CuI/o-semiquinonato redox isomer equilibria with EPR spectroscopy

Article abstract of DOI:10.1016/S0020-1693(02)01081-2

The temperature-dependent intramolecular metal/ligand electron transfer equilibrium (L)Cu^II(Q^2-)?(L)Cu^I(Q ^·-) has been quantitatively analyzed by EPR as a function of the non-innocent o-quinonoid ligand Q and of the co-ligand L. Suitable co-ligands were 1-methyl-(2-methylthiomethyl)-1H-benzimidazole and 1-diphenylphosphino-2-dimethylaminoethane (dde) which can tolerate both copper oxidation states. EPR hyperfine data were extracted for a variety of alkyl- and methoxy-substituted o-semiquinones and their copper(I) complexes in solution. A close match of Q and Cu redox orbitals is essential for observing the redox isomer equilibrium which has been similarly reported for aminoxidase enzymes. Efforts to isolate the mixed-ligand compounds from solution failed due to coordination disproportionation. The new [Cu(dde)₂](ClO₄) was structurally characterized with comparable Cu-N and Cu-P distances of 2.24-2.27 A? and a P-Cu-P angle of 142.41(5)°, the '2+2' coordination pattern could be reproduced by DFT calculations.

Full text of DOI:10.1016/S0020-1693(02)01081-2

Inorganica Chimica Acta 337 (2002) 163ꢁ172
/
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Copper complexes with non-innocent ligands:
probing CuII/catecholato-CuI/o-semiquinonato redox isomer
equilibria with EPR spectroscopy
Wolfgang Kaim a,*, Matthias Wanner a, Axel Knodler a, Stanislav Za´lisˇ b
¨
a Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany
¨
¨
b J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejsˇkova 3, CZ-18223 Prague, Czech Republic
Received 1 March 2002; accepted 17 May 2002
Dedicated to Professor Karl Wieghardt
Abstract
The temperature-dependent intramolecular metal/ligand electron transfer equilibrium (L)CuII(Q2ꢀ)X
/
(L)CuI(Q+ꢀ) has been
quantitatively analyzed by EPR as a function of the non-innocent o-quinonoid ligand Q and of the co-ligand L. Suitable co-ligands
were 1-methyl-(2-methylthiomethyl)-1H-benzimidazole and 1-diphenylphosphino-2-dimethylaminoethane (dde) which can tolerate
both copper oxidation states. EPR hyperfine data were extracted for a variety of alkyl- and methoxy-substituted o-semiquinones
and their copper(I) complexes in solution. A close match of Q and Cu redox orbitals is essential for observing the redox isomer
equilibrium which has been similarly reported for aminoxidase enzymes. Efforts to isolate the mixed-ligand compounds from
solution failed due to coordination disproportionation. The new [Cu(dde)2](ClO4) was structurally characterized with comparable
˚
CuÃ
/
N and CuÃ
/
P distances of 2.24ꢁ
/
2.27 A and a PÃ
/
CuÃ
/
P angle of 142.41(5)o, the ‘2ꢂ
2’ coordination pattern could be reproduced
/
by DFT calculations.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Copper compounds; EPR spectroscopy; Quinone ligands; Redox isomerism; Structure
1. Introduction
isomerism [6]. This unusual phenomenon has been
observed mainly for low-spin CoIII(Q2ꢀ)/high-spin
ꢀ
Jørgensen’s concept of non-innocent ligands [1] in
coordination chemistry has been usually employed in a
static sense, e.g. questioning whether e.g. 2,2?-bipyridine
[2], a dithiolene [3] or an o-quinone ligand [4] coordi-
nates as a neutral species (with the metal in a low
oxidation state) or as a negatively charged species with
high-valent metal. Recently, Wieghardt’s group has
extended this approach to imino- and thio-o-quinones
[5]. However, there is also a dynamic aspect to ‘non-
innocence’: Intramolecular metal-to-ligand electron
transfer equilibria have been observed in complexes
and were circumscribed as valence tautomerism or redox
CoII(Q+
)
systems [6,7] and for MnIV(Q2ꢀ)/
MnIII(Q+ꢀ) equilibria [8]; the temperature-dependence
of Eq. (1) involving copper has been found in enzymes
[9] (where it may contribute to the O2 activation by
amine oxidases [10]) and, more recently, in small systems
by us [11] and others [12].
(Q2ꢀ)CuIIL?(Q+ꢀ)CuIL
(1)
A first recent report on the variation of Q [11b] has
indicated the very narrow range where such an equili-
brium can be observed (Scheme 1).
Herein we present detailed EPR studies on the redox
isomer equilibrium (1) as a function of L and Q,
analyzing the position of the equilibrium and the EPR
characteristics, including the EPR hyperfine structure of
the semiquinones and of their CuI complexes. In
addition to a more detailed EPR analysis of the
* Corresponding author. Tel.: ꢂ
4165
E-mail address: kaim@iac.uni-stuttgart.de (W. Kaim).
/
49-711-685 4170; fax: ꢂ49-711-685
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 0 8 1 - 2

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W. Kaim et al. / Inorganica Chimica Acta 337 (2002) 163ꢁ172
/
The following results were obtained.
2.1.1. Qꢃ
/
PQQTME, Lꢃmmb
/
PQQTME, a rather electron-poor quinone [15b]
(Table 1), yields only the copper(II)/catecholate form
(mmb)CuII(PQQTME2ꢀ) according to EPR spectro-
scopy between 110 and 320 K in toluene or THF
solution. The experiment in THF gave poorly resolved
spectra due to a temperature-dependent equilibrium
between two copper(II) species, which we tentatively
attribute to coordination mode changes of the isoam-
bialterdentate ligand PQQTME [14,15] in the presence
of potentially coordinating THF. The EPR data are
summarized in Table 2.
Scheme 1.
combination with Lꢃ
/
mmbꢃ
/
1-methyl-2-(methylthio-
Q1ꢁQ6, we now
The comparatively less negative potential of
ꢀ
methyl)-1H-benzimidazole and Qꢃ
/
/
PQQTME8/+ at ꢀ
0.80 V versus ferrocenium/ferro-
cene (Table 1) is responsible for this result, i.e. the
absence of EPR-detectable redox isomerism.
/
studied a wider variety of components in order to probe
the conditions for such redox isomer equilibria (Scheme
2).
In one approach, the o-quinone/catechol ligand
system was replaced by the imino-o-quinone/o-amino-
phenol pair QA/H2QA while keeping mmb as auxiliary,
CuI- and CuII-tolerant [13] ligand. We also used an
electron-poor biological o-quinone, pyrroloquinoline-
quinone (PQQ) [14] as the trimethyl ester PQQTME
[15], in combination with copper and mmb. In yet
another variation we combined the standard 3,5-di-tert-
butyl-o-quinone Q3 with 1-diphenylphosphino-2-di-
methylaminoethane (dde) [16]. As in mmb [11], the
mixed donor set of dde was introduced to stabilize both
CuII (through N) and CuI (through P), thereby allowing
us to detect both states of equilibrium (1) with their very
different EPR signature [17].
2.1.2. Qꢃ
The standard o-quinone, 3,5-di-tert-butyl-o-quinone
Q3, was reacted with copper and the mixed NÃ
P donor
bidentate ligand dde. Similarly as mmb [11], dde
combines a CuII-preferring nitrogen donor atom (here
trialkylamine N) and a CuI-preferring ‘soft’ donor
center, here phosphane P instead of thioether S. As
will be shown later through the structure of
[Cu(dde)2]ꢂ, dde can coordinate to CuI to form a five-
/
Q3, Lꢃ
/
dde
/
membered chelate ring with relatively stronger CuÃ
/P
than CuÃN bonds.
/
The product of this reaction is a redox isomer
equilibrium (1) as evident from temperature-dependent
EPR spectroscopy (Fig. 1)
At the low- and high-temperature limits the EPR
spectra could be analyzed, however, the gÞ region of the
CuII was poorly resolved. As previously noted for the
amine oxidase enzyme [9] and for synthetic systems (1)
In the course of attempts to isolate corresponding
mixed-ligand compounds we also obtained a structure of
[Cu(dde)2](ClO4), which was reproduced by DFT calcu-
lations.
with Lꢃmmb [11b], the copper(II) form is the more
/
2. Results and discussion
stable, the low-temperature form; the semiquinone EPR
spectrum of the high-temperature species (dde)CuI-
[(Q3)+ꢀ] is shown in Fig. 2.
The EPR data, g-factors and hyperfine information,
are listed in Tables 2 and 3.
The equilibrium (1) has been quantitatively analyzed
as described previously [11b]: the relative amounts Ik of
the CuI/o-semiquinonato (II) and CuII/catecholato (III)
forms were approximated at various temperatures
through double integration of the EPR spectrum.
Parameters A and B of the linear Eq. (2) are summar-
ized in Table 1.
2.1. Redox isomer equilibrium analysis
The equilibrium (1) can be produced by combining
the components L, Qnꢀ and Cunꢂ (nꢃ
/
0, 1 or 2) [11]. A
most convienient method, if the quinone state (nꢃ0) is
available, is to use activated copper metal [11b]. The
new systems with QꢃPQQTME and Lꢃmmb or Qꢃ
Q3 and Lꢃ
dde have been produced accordingly in THF
or toluene solution. As an alternative, a copper(II) salt
2) in combination with a dihydro form
/
/
/
/
/
may be used (nꢃ
/
H2Q in the presence of a base. o-Aminophenol H2QA,
the related o-aminothiophenol H2QB and o-phenylene-
diamine H2QC have thus been reacted in dry THF with
anhydrous CuCl2, the H2Q compounds serving simulta-
neously as deprotonating base.
ln(II=III)ꢃA=T ꢂB
(2)
For the (dde)Cu(Q3) system the parameters A and B
are rather small, indicating a relatively low activation
barrier (reorganization energy) for equilibrium (1) in

W. Kaim et al. / Inorganica Chimica Acta 337 (2002) 163ꢁ
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165
Scheme 2.
comparison to the sterically less encumbered systems
with mmb.
nating base. Tables 1 and 2 contain EPR and equili-
brium parameters, the EPR spectrum of the
semiquinone form of (mmb)Cu(QA) could not be
analyzed due to the low symmetry and coupling from
several non-equivalent nuclei.
Reaction under identical conditions with o-ami-
nothiophenol H2QB did not produce paramagnetic
species while o-phenylenediamine H2QC gave poorly
resolved EPR signals attributable to copper(II) species.
2.1.3. Qꢃ
/
QA ꢁ
/
QC, Lꢃ
mmb
/
The reaction of o-aminophenol H2QA with anhy-
drous CuCl2 in dry THF in the presence of mmb also
produced temperature-dependent EPR signals with the
CuII form dominating at low temperature and a CuI/
imino-o-semiquinone species at high temperature. No
extra supporting base was necessary, obviously, H2QA
can serve simultaneously as a ligand and as a deproto-

166
W. Kaim et al. / Inorganica Chimica Acta 337 (2002) 163ꢁ172
/
Table 1
Redox isomer equilibrium data for complexes (L)Cunꢂ(Qnꢀ) in THF solution
a
b
c
c
Quinone
E
L
A
B
R2
Reference
PQQ
Q3
ꢀ0.80
ꢀ1.09
n.a.
mmb only CuII (Q2ꢀ) forms observed
dde ꢀ308
mmb ꢀ1107
mmb ꢀ1120
mmb ꢀ2639
only CuII (Q2ꢀ) forms observed
only CuII (Q2ꢀ) forms observed
0.990
this work
this work
this work
[11b]
ꢀ0.99
ꢂ0.85
ꢂ1.22
ꢂ4.07
QA
Q1
0.919
0.986
0.975
d
ꢀ0.83
ꢀ1.01
ꢀ1.09
ꢀ1.11
ꢀ1.14
ꢀ1.21
Q2
[11b]
[11b]
Q3
mmb ꢀ2903
ꢂ6.15
0.995
Q4
mmb only CuI (Q+ꢀ) forms observed
mmb only CuI (Q+ꢀ) forms observed
mmb only CuI (Q+ꢀ) forms observed
only CuI (Q+ꢀ) forms observed
only CuI (Q+ꢀ) forms observed
only CuI (Q+ꢀ) forms observed
only CuI (Q+ꢀ) forms observed
only CuI (Q+ꢀ) forms observed
only CuI (Q+ꢀ) forms observed
[11b]
[11b]
Q5
Q6
[11b]
a
Parameters A and B from Eq (2).
ꢀ
b
c
Redox potential for the Q/Q+ transition vs. ferrocenium/ferrocene.
A95, B90.2.
d
Irreversible; cathodic peak potential given at 100 mV sꢀ1 scan rate.
Table 2
EPR data of complexes (L)Cu(Q) in frozen solution at 110 K
a
Q
L
g
½½
GÞ
A
(
63Cu/65Cu)
jj
AÞ (Cu)
Medium
PQQTME
mmb
dde
2.250
2.26
2.057
c
20.0/18.6
b
2.1
c
toluene
THF
THF
Q3
QA
Q1
Q2
Q3
Q5
Q6
17
16.5
b
c
mmb
mmb
mmb
mmb
mmb
mmb
2.253
2.263
2.244
2.248
2.035
2.035
2.058
2.059
2.048
2.058
2.0045
2.0048
19.7/18.3
19.6/18.3
20.0/18.6
3.3
3.3
toluene
THF
2.7
c
toluene
toluene
toluene
b
3.5
3.5
b
c
a
Coupling constants A in mT.
63Cu and 65Cu hyperfine coupling not resolved.
gÞ-region not suffiently resolved.
b
c
2.1.4. Qꢃ
As previously described [11], the combination of
copper, mmb and the six o-quinone ligands Q1ꢁQ6
produces EPR-detectable redox isomer equilibria (1) for
the first three members of the series whereas Q4ꢁQ6
/
Q1ꢁ
/
Q6, Lꢃ
/
mmb
2.2.2. EPR spectra of o-semiquinones
In view of the fragmentary EPR information on the
semiquinones of compounds Q1ꢁQ6 [19ꢁ
21], we studied
/
/
/
all these radical anions under comparable conditions by
/
produce only copper(I)/o-semiquinone complexes (Ta-
ble 1). However, temperature variation now showed the
formation of at last two different such species in the
latter case. We thus studied the EPR spectra, especially
the hyperfine splitting of the copper(I)/o-semiquinonato
complexes, in more detail.
2.2. EPR analysis
2.2.1. EPR spectra in frozen solution
The parameters of the low-temperature EPR spectra
from frozen solution in Table 2 illustrate the stark
difference between copper(II) and copper(II)/radical
systems. The latter exhibit lower g anisotropy and
much lower metal hyperfine coupling constants [17].
Within the series of CuII/catecholate species there is little
spectral difference, confirming rather invariant struc-
tures [11b,18].
Fig. 1. Temperature dependent EPR spectra of the (dde)Cu(Q3)
system in THF solution.

W. Kaim et al. / Inorganica Chimica Acta 337 (2002) 163ꢁ
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167
Table 4
EPR data of o-semiquinones Q+ in THF solution
ꢀ
a
b
Q
g
a(H4)
a(CH3), a(OCH3)
a(13C)
T (K)
Q1 2.0049 0.087
Q2 2.0047 0.316
Q3 2.0047 0.300
Q4 2.0048 0.368
Q5 2.0047 0.355
Q6 2.0047
0.580
n.o.
n.o.
n.o.
250
305
320
290
280
305
c
0.185
n.o.
n.o.
0.089
0.031
B0.012
n.o.
0.370
a
Coupling constants in mT.
From 4 equiv. C.
H4 and H5.
b
c
EPR. A typical spectrum is shown in Fig. 3, the data are
summarized in Table 4.
Fig. 2. EPR spectrum of (dde)CuI[(Q3)+ꢀ] in THF at 315 K with
computer simulation.
o-Benzosemiquinones typically have the a b1-orbital
(Scheme 3) as singly occupied molecular orbital
(SOMO) with little spin density in the 3,6-positions
[4,17,20,21]. Accordingly, the substituents in those
positions exhibit only small, frequently undetectable
hyperfine coupling whereas the groups in positions 4
and 5 contribute largely to the observed EPR splitting
patterns.
ꢀ
Whereas the high-symmetry species (Q2)+ exhibits
EPR-detectable 13C isotope coupling (Table 4) [20,22],
ꢀ
the unsymmetrical (Q1)+ shows larger coupling from
the 5-methyl b-protons than from the a-proton in 4-
position (Fig. 3). g-Protons from tert-butyl or methoxy
substituents have generally small coupling constants, the
largest such value was determined for the 5-methoxy
ꢀ
group in (Q4)+ (Table 4).
2.2.3. EPR spectra of copper(I)/o-semiquinonato
complexes
The isotropic g-values of the copper(I)/o-semiquino-
nato complexes described here (Table 3) do not differ
much from the values around 2.0046 of the free radical
ꢀ
Fig. 3. EPR spectrum of (Q1)+ in THF at 250 K with computer
simulation.
Table 3
EPR data of copper(I)/o-semiquinonato complexes (L)CuI(Q+
ꢀ
a
)
Q
L
medium (T (K))
A(63Cu/65Cu)
a(Ha)
a(other)
g
Q1
Q2
Q3
Q3
Q4
mmb
mmb
mmb
dde
toluene (370)
toluene (320)
toluol (380)
THF (315)
THF (340)
THF (240)
toluene (290)
toluene (230)
THF (330)
THF (210)
toluene (330)
THF (320)
THF (190)
toluene (310)
0.51/0.54
0.53/0.56
0.51/0.54
0.56/0.60
0.40/0.43
0.92/0.98
0.40/0.43
0.92/0.98
0.46/0.50
0.77/0.82
0.41/0.44
0.44/0.47
0.75/0.81
0.39/0.42
0.34
0.33
0.30
0.35
0.40
0.38
0.40
0.37
0.36
0.36
0.36
0.06 (MeÃH)
n.o.
2.0048
2.0055
2.0052
2.0039
2.0046
2.0044
2.0050
2.0045
2.0053
2.0046
2.0047
2.0048
2.0046
2.0047
n.o.
n.o.
mmb
mmb
mmb
mmb
mmb
mmb
mmb
mmb
mmb
mmb
0.11 (OMeÃH)
0.11 (OMeÃH)
n.o.
0.10 (OMeÃH)
n.o.
Q5
Q6
n.o.
n.o.
n.o.
n.o.
0.21 (2ꢄ13C)
a
Hyperfine coupling in mT.

168
W. Kaim et al. / Inorganica Chimica Acta 337 (2002) 163ꢁ172
/
Iꢃ
moment, lie between 0.5 and 0.6 mT for the mmbꢁ
or ddeꢁ
CuI complexes with the alkyl-substituted o-
semiquinones (Q1)+ꢀ ꢁ(Q3)+ꢀ. In contrast, complexes
(L2)CuI[(Q3)+ꢀ] with Lꢃ
PR3 or SR2 have distinctly
higher coupling constants a(63,65Cu)ꢀ
1 mT whereas
those with Lꢃcarbon donors have copper splittings
/
3/2 but 65Cu with 7% higher nuclear magnetic
CuI
/
/
/
/
/
/
between 0.6 and 0.7 mT [17]. This comparison suggests
that the coordination at the metal is unsymmetrical
(non-tetrahedral), perhaps in the direction of a trigonal
Scheme 3.
ꢀ
ligands (Table 4). For the compounds (mmb)CuI(Q+
)
pyramid which would be typical for (3ꢂ
copper(I) [23]. We thus propose that the larger 3p-
element (EꢃS or P) adopts a qualitatively different
position from the nitrogen donor atom in complexes
(L)CuI(Q+ꢀ), Lꢃ
mmb or dde (Scheme 4). If only 3p-
/
1)-coordinated
ꢀ
they are marginally higher, the giso for (dde)CuI[(Q3)+
]
is slightly lower than for (Q3)+ꢀ, in agreement with the
presence of low-lying unoccupied orbitals on the phos-
phorus atom. This result stands in contrast to the
copper(I)/o-semiquinonato complexes of chelating bis-
or tris(thioethers) which exhibit higher values between
/
/
donor elements E are available, this differentiation is not
possible.
Support for this model comes from the observation of
temperature-dependent EPR spectra for the o-semiqui-
2.0058 and 2.0093, in agreement with some CuÃ
/
S bond
covalency [17].
nonato complexes (mmb)CuI(Q+ꢀ), Qꢃ
/
Q4ꢁQ6 (Fig. 5,
/
Correspondingly, the semiquinone hyperfine para-
meters from the EPR spectra (Fig. 4) are little changed
relative to the slightly smaller values of the free ligands,
except for (mmb)CuI[(Q1)+ꢀ] which exhibits a typical
Table 3).
Generally, the low-temperature forms have higher
metal coupling constants (0.7ꢁ1.0 mT) and slightly
higher g-factors than the high-temperature species
(0.4ꢁ0.5 mT), in agreement with the observations stated
above. Based on the conspicuous absence of EPR-
/
large 4-H and small 5-methyl coupling constant, in
with its opposite splitting
/
contrast to free (Q1)+
pattern (Table 4). Apparently, coordination of copper(I)
ꢀ
31
ꢀ
detectable P hyperfine coupling for (dde)CuI[(Q3)+
]
to (Q1)+ reverses the unsymmetry of the b1 SOMO,
ꢀ
(Fig. 2) we tentatively associate the high-temperature
form with the lower metal hyperfine value to a config-
possibly through differences in the metal bonding to the
semiquinone oxygen atoms.
uration with a largely coplanar semiquinoneꢁ
arrangement (Scheme 4: B). Small metal and donor
E bond lies in
/
Cuꢁ/E
Fig. 4 illustrates the asymmetric line broadening due
to insufficiently averaged contributions from the A and
g-tensors in the radical complexes.
The metal coupling constants from the 63Cu (69.2%
nat. abundance) and 65Cu (30.8%) isotopes, both with
atom coupling is expected when the CuÃ
/
the nodal plane of the spin-bearing p-system [24].
2.3. Crystal structure analysis of [Cu(dde)2](ClO4)
All attempts to isolate mixed-ligand complexes
(L)Cunꢂ(Qnꢀ) from EPR-active solutions for crystal
structure analysis have failed so far. Slow evaporation
typically produces the well-known homoleptic species
CuII(Q+ꢀ)2 [25]. For the approach to use EXAFS in
structure determination [26], we have set out to structu-
rally characterize also the corresponding homoleptic
compounds [Cu(L)2]nꢂ, reports on the systems with
Lꢃ
structure
/
mmb and nꢃ
/
1 or 2 have appeared [13a], as has a
(Me3TACN)CuII[(Q3)2ꢀ],
analysis
of
Fig. 4. EPR spectrum of (mmb)CuI[(Q2)+ꢀ] in THF at 320 K with
computer simulation.
Scheme 4.

W. Kaim et al. / Inorganica Chimica Acta 337 (2002) 163ꢁ
/172
169
Fig. 6. Molecular structure of the cation in the crystal of [Cu-
(dde)2](ClO4).
discrepancy to a shallow energy minimum for this
‘secondary’ bond. Both the similarity of CuÃ
CuÃN bond distances and the wide PÃCuÃP angle of
about 1428 suggest a ‘2ꢂ2’ coordination pattern with
/P and
/
/
/
/
the P-atoms as primary donors and the trialkylamine
nitrogen atoms as secondary donors. A related effect
had been reported for [Cu(mmb)2]ꢂ where the imine N-
˚
ꢃ1.920 A) with a
Fig. 5. Temperature dependent EPR spectra of (mmb)CuI[(Q5)+ꢀ] in
THF solution.
atoms occupy the primary sites (dCuÃN
/
close to linear angle NÃ
/
CuÃN of 169.88 whereas the
/
Me3TACNꢃ1,4,7-trimethyl-1,4,7-triazacyclononane
/
thioether S-atoms are in a secondary position with long
˚
[11b]. Herein we have used the dde ligand in connection
with copper and have obtained a crystalline copper(I)
complex [Cu(dde)2](ClO4) from the reaction of excess
bonds of ꢀ
copper(I) prefers imineꢁ
phaneꢁP over amineꢁN.
The comparison with other copper complexes of N,P-
donor ligands AÃC in Table 6 confirms the special
configuration in [Cu(dde)2]ꢂ with unusually long CuÃ
bonds and rather large PÃCuÃP and NÃCuÃN angles,
illustrating a typical 2ꢂ2 coordination pattern.
/
2.6 A to the metal [13a]. Obviously,
/
N over thioetherꢁS but phos-
/
/
/
dde with Cu(ClO4)2×/6H2O in THF under argon. The
function of triorganophosphanes as reductants towards
copper(II) is not uncommon [27].
/
/N
Fig. 6 shows the molecular cation of [Cu(dde)2](ClO4)
in the crystal, Tables 5 and 6 list the crystallographic
and essential bond parameter information.
/
/
/
/
/
Table 6 also includes the results from a DFT
calculation of the cation [Cu(dde)2]ꢂ and bond para-
2.4. Summary
meters of comparable copper complexes with PÃ
/
N
This study has confirmed that an o-quinone/copper-
based redox isomer equilibrium (1) can be detected
through EPR only in a rather narrow range of matching
copper-dp and quinone p*-orbitals (Scheme 1). With the
donor ligands AÃ
/
C [28ꢁ30] (Scheme 5).
/
The correspondence between DFT calculation results
and experiment is very good except for the unusually
extended CuÃ
they actually are. Apart from the not uncommon
overestimation of CuÃN bonds [13a], we attribute this
/N bonds which are calculated longer than
PÃ
/
N mixed donor ligand dde there is a larger tempera-
ture range for the simultaneous observation of indivi-
dual copper(I)/o-semiquinonato and copper(II)/
/

170
W. Kaim et al. / Inorganica Chimica Acta 337 (2002) 163ꢁ172
/
Table 5
Crystal data and structure refinement information for [Cu(dde)2]-
(ClO4)
EPR and other spectroscopic techniques [31,32]. Both
the smaller conformational change and thus smaller
reorganization energy of the d6/d5 transition as well
as stronger p-interaction from the second row transi-
tion metal may be responsible for this result.
On the other hand, the smaller p-interaction from
copper complex fragments and the larger conforma-
tional change expected for the d10/d9 transition causes
typically dichotomous states, either copper(II) species
or copper(I) radical forms [17,33]. Only under special
conditions are both forms detectable simultaneously,
we shall, therefore, devote further experimental and
computational efforts to investigate this remarkable
dichotomy.
Empirical formula
Formula weight (g molꢀ1
Temperature (K)
C32H40ClCuN2O4P2
677.59
173(2)
)
ꢁ
/
˚
Wavelength (A)
0.71073
hexagonal
P3121
Crystal system
Space group
Unit cell dimensions
˚
a (A)
10.0961(14)
10.0961(14)
27.304(6)
90
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
90
120
3
˚
V (A )
2410.3(7)
3
1.400
Z
Dcalc (mg mꢀ3
)
3. Experimental
m (mmꢀ1
F(000)
Crystal size (mm)
)
0.901
1062
3.1. Instrumentation
0.6ꢄ0.4ꢄ0.2
uRange (8)
Index ranges
2.24ꢁ29.04
/
ꢀ35h 513, ꢀ135k 511,
ꢀ365l 537
13 480
EPR spectra were recorded in the X-band on a Bruker
System ESP 300 equipped with a Bruker ER035M
gaussmeter and a HP 5350B microwave counter. 1H
NMR spectra were taken on a Bruker AC 250 spectro-
meter. Cyclic voltammetry was carried out at 100 mV
sꢀ1 scan rate in THF/0.1 M Bu4NPF6 using a three-
electrode configuration (glassy carbon working elec-
trode, Pt counter electrode, Ag/AgCl reference) and a
PAR 273 potentiostat and function generator. The
ferrocene/ferrocenium couple served as internal refer-
ence.
Number of reflections
Number of independent reflections
Data/restraints/parameters
Goodness-of-fit in F2
R [I ꢀ2s(I)]
R (all data)
Residual electron density (greatest
4294 (Rintꢃ0.0737)
4294/0/191
1.106
R1ꢃ0.0487, wR2ꢃ0.1060
R1ꢃ0.0668, wR2ꢃ0.1151
0.710 and ꢀ0.385
ꢀ3
˚
maximum and minimum) (e A
)
catecholate forms, however, all spectroscopic results
indicate negligible covalency and metalꢁligand orbital
mixing.
/
3.2. Radical generation
There are thus two quite different manifestations of
the ‘non-innocence’ of quinonoid ligands.
o-Semiquinone radicals were prepared for EPR
spectroscopy in sealed tubes using freshly distilled
potassium as reductant in THF solution. The paramag-
netic copper complexes (L)Cu(Q) were typically ob-
tained in such sealed apparatuses by combining L, Q
and activated Cu powder (the latter in excess) with the
condensed solvent. As a variation, Q and Cu can be
ꢁ
/
Strongly interacting metal centers such as rutheniu-
m(II) produce complexes with highly mixed states,
where e.g. both RuIII(Q2ꢀ) and RuII(Q+ꢀ) or
RuII(m-Q+ꢀ)RuII and RuII(m-Q2ꢀ)RuIII ‘resonance’
forms contribute to one delocalized state according to
Table 6
o
˚
Selected bond distances (A) and angles ( ) in copper complexes with PÃN ligands
a
˚
Bond distances (A) and bond angles (8)
[Cu(dde)2](ClO4)
exp. DFT
2.250(3)
[Cu(A)2](ClO4)×C2H5OH
[Cu(B)](BF4) [Cu(C)](ClO4)2×C6H6
CuÃN
CuÃP
NÃCuÃN(0A)
NÃCuÃP
PÃCuÃP(0A)
NÃCuÃP(0A)
ref.
2.340
2.258
2.177(3)
2.258(1)
111.7(2)
82.7(1)
2.169(8)
2.197(2)
85.7(4)
87.0(2)
132.8(1)
126.2(1)
[29]
2.06(1)
2.325(4)
86.3(4)
2.2630(8)
114.67(15)
86.78(8)
113.1
b
86.6
142.9
114.3
84.9(3)
101.8(1)
142.40(5)
113.83(8)
this work
131.3(1)
129.7(2)
b
166.3(2)
[30]
this work [28]
a
Copper(II) complex.
Average value.
b

W. Kaim et al. / Inorganica Chimica Acta 337 (2002) 163ꢁ
/172
171
Scheme 5.
reacted to Cu(Q)2 first and then combined with L in a
controlled fashion. The reactions of H2Q species were
carried out with anhydrous CuCl2 in THF.
Slater type orbital (STO) basis sets of double-z quality
with polarization functions for H and C atoms within
phenyl ligands and triple-z quality with polarization
functions for the remaining atoms of the system were
employed. Inner shells were represented by the frozen
core approximation (1s for C, N; 1s-2p for P; 1s-3s for
Cu). Within ADF the functional including Becke’s
gradient correction [36] to the local exchange expression
in conjunction with Perdew’s gradient correction [37] to
the local density approximation (LDA) with VWN
parametrization of electron gas data (ADF-BP) was
used.
3.3. Syntheses
3.3.1. Starting materials
The quinones were obtained commercially or were
prepared as described before [11b]. The ligands dde [16]
and PQQTME [15] were obtained according to the
literature.
The geometry optimization was performed without
symmetry constrains.
3.3.2. [Cu(dde)2](ClO4)
Addition of 57.7 mg (0.155 mmol) Cu(ClO4)2×
/6H2O
to a THF solution (5 ml) of 80.2 mg (0.311 mmol) dde
resulted in a discoloring after 5 min of stirring. Reduc-
4. Supplementary material
tion to about 1/3 of the volume and cooling to ꢀ17 8C
/
produced 35 mg (30%) of a colorless material which
contained a few single crystals suitable for X-ray
diffraction. Anal. Found: C, 51.74; H, 6.00; N, 3.31.
Calc. for C32H48ClCuN2P2O8 (749.7): C, 51.27; H, 6.43;
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC No. 179337. Copies of the infor-
mation may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2
N, 3.72%. 1H NMR (CD2Cl2): dꢃ
2.42ꢁ2.53 (m, 4H, CH2), 2.62ꢁ2.71 (m, 4H, CH2), 7.32ꢁ
7.57 (m, 20H, PPh2).
/2.30 (s, 12H, NMe2),
/
/
/
1EZ, UK (fax: ꢂ44-1223-336-033; e-mail: deposit@
/
ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
3.4. Crystallography
Acknowledgements
Single crystals of [Cu(dde)2](ClO4) were obtained
from the raction solution. X-ray diffraction was carried
This work has been supported by the Deutsche
Forschungsgemeinschaft (project KA 618/18) and by
the Fonds der Chemischen Industrie. We thank Dr. W.
Schwarz for the crystallographic data collection and Dr.
K.-W. Klinkhammer for assistance in the space group
identification.
out with a Siemens P4 instruement using graphite
˚
0.71073 A).
monochromated Mo Ka radiation (lꢃ
/
The data were corrected for Lorentz and polarization
effects. The structure was solved by direct methods
using the SHELXTL program package [34]. All non-
hydrogen atoms were refined anisotropically, and hy-
drogen atoms were introduced at calculated positions
with coupled isotropic temperature factors. Further
information is summarized in Table 5.
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