A Copper(I) Complex with Two Unpaired Electrons, Synthesised by Oxidation of a Copper(II) Complex with Two Redox-Active Ligands

Article abstract of DOI:10.1002/anie.202109367

Two homoleptic copper(II) complexes [Cu(L1)₂] and [Cu(L2)₂] with anionic redox-active ligands were synthesised, one with urea azine (L1) and the other with thio-urea azine (L2) ligands. One-electron oxidation of the complexes initiates an unprecedented redox-induced electron transfer process, leading to monocationic copper(I) complexes [Cu(L1)₂]⁺ and [Cu(L2)₂]⁺ with two oxidised ligands. While [Cu(L1)₂]⁺ is best described as a Cu^I complex with two neutral radical ligands that couple antiferromagnetically, [Cu(L2)₂]⁺ is a Cu^I complex with two clearly different ligand units in the solid state and with a magnetic susceptibility close to a diamagnetic compound. Further one-electron oxidation of the complex with L1 ligands results in a dication [Cu(L1)₂]²⁺, best described as a Cu^I complex with a twofold oxidised, monocationic ligand and a neutral radical ligand. The stability in at least three redox states, the accumulation of spin density at the ligands and the facile ligand-metal electron transfer make these complexes highly attractive for a variety of applications; here the catalytic aerobic oxidation of alcohols to aldehydes is tested.

Full text of DOI:10.1002/anie.202109367

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Accepted Article
Title: A Copper(I) Complex with Two Unpaired Electrons, Synthesised
by Oxidation of a Copper(II) Complex with Two Redox-Active
Ligands
Authors: Hans-Jörg Himmel, Marco Werr, Elisabeth Kaifer, Markus
Enders, Andika Asyuda, and Michael Zharnikov
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10.1002/anie.202109367
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A Copper(I) Complex with Two Unpaired Electrons, Synthesised by
Oxidation of a Copper(II) Complex with Two Redox-Active Ligands
Marco Werr,a Elisabeth Kaifer,a Markus Enders,a Andika Asyuda,b Michael Zharnikov,b
Hans-Jörg Himmel a,*
Abstract. Two homoleptic copper(II) complexes [Cu(L1)2] and [Cu(L2)2] with anionic
redox-active ligands were synthesised, one with urea azine (L1) and the other with
thio-urea azine (L2) ligands. One-electron oxidation of the complexes initiates an
unprecedented redox-induced electron transfer process, leading to monocationic
copper(I) complexes [Cu(L1)2]+ and [Cu(L2)2]+ with two oxidised ligands. While
[Cu(L1)2]+ is best described as a CuI complex with two neutral radical ligands that
couple antiferromagnetically, [Cu(L2)2]+ is a CuI complex with two clearly different
ligand units in the solid state and with a magnetic susceptibility close to a diamagnetic
compound. Further one-electron oxidation of the complex with L1 ligands results in a
dication [Cu(L1)2]2+, best described as a CuI complex with a twofold oxidised,
monocationic ligand and a neutral radical ligand. The stability in at least three redox
states, the accumulation of spin density at the ligands and the facile ligand-metal
electron transfer makes these complexes highly attractive for a variety of applications;
here the catalytic aerobic oxidation of alcohols to aldehydes is tested.
*
a) M. Werr, Dr. E. Kaifer, Prof. M. Enders, Prof. H.-J. Himmel,
Anorganisch-Chemisches Institut, Ruprecht-Karls Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
b) A. Asyuda, Prof. M. Zharnikov
Angewandte Physikalische Chemie, Ruprecht-Karls Universität Heidelberg
Im Neuenheimer Feld 253, 69120 Heidelberg, Germany
1
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Introduction
The integration of redox-active ligands in molecular coordination compounds leads to
intriguing electronic structures and opens up new possibilities for applications in
catalysis and materials science.[1-5] Intramolecular ligand-metal electron transfer (IET)
processes in these complexes could be used for the activation of substrates in catalytic
cycles, with the redox-active ligand acting as electron reservoir; moreover, radical
ligands generated by IET generally induce a special ligand-centered reactivity.[6,7,8] In
redox-induced intramolecular electron transfer (RIET) processes, the metal is reduced
in an overall oxidation or oxidised in an overall reduction of a coordination compound.
The first examples for RIET were reported by Miller et al. for dinuclear cobalt
complexes with bridging tetraoxolene ligands.[9,10,11] With relevance to this work, our
group recently reported examples of RIET processes for copper and cobalt complexes
with redox-active guanidine ligands.[12,13]
Copper complexes with redox-active ligands were intensively studied.[14-17] A variety of
stable CuII complexes with radical ligands was reported, and phenoxyl radical
complexes of salen-type ligands established as mixed-valent galactose oxidase
models.[18-26] The mononuclear complex CuII(SQ)2 (SQ denotes an o-imino-
semiquinonato ligand) features two radical ligands. Oxidative addition of bromine leads
to a bis(o-iminobenzoquinone) complex;[27] the two radical ligands provide the required
electrons. Desage-El Murr and Fensterbank et al. reported trifluoromethylation of
CuII(SQ)2,[28] and application of the resulting unstable complex as a source of CF3
radicals.[29] Stoichiometric reactions with aryl boronic acids gave N-arylation
products.[30]
We already studied the redox and coordination chemistry of neutral redox-active urea
azine ligands.[31,32,33] CuII complexes with a neutral, peralkylated urea azine ligand
were found to be stable in the solid state, but amenable to IET induced demetallation
and decomposition in solution (Scheme 1a).[33] To avoid decomposition, we decided to
strengthen the metal-ligand bonding by using anionic urea azine ligands, generated by
deprotonation of urea azines with partially alkylated substituents and N-H groups. In
this context we note that Panda, Lahiri et al. reported ruthenium complexes of the
twice-deprotonated, symmetric thio-urea azine 2,2‘-azobis(benzothiazole) (abbt) and
found valence tautomerism (VT) between [{Ru2.5(acac)2}2(-abbt·−)] and
[{RuIII(acac)2}2(-abbt2−)].[34]
2
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Here we report the redox chemistry of the first mononuclear CuII complexes with two
monoanionic, deprotonated urea azine or thio-urea azine ligands, L1 and L2 (Scheme
1b). The formation of complexes with bridging redox-active ligands is avoided by use
of asymmetric urea azines or thio-urea azines with only one N-H function. The one-
electron oxidation of the initially formed neutral CuII complexes is accompanied by a
RIET process to give unprecedented monocationic CuI complexes with oxidised
ligands. Further one-electron oxidation is again ligand-centered, leading to a stable
dicationic CuI complex.
Scheme 1. a) Demetallation of a complex with urea azine ligand initiated by
intramolecular electron transfer (IET) from the ligand to the metal (X = Cl, Br). b)
Synthesis of asymmetric azines HL1 and HL2.
Results and Discussion
The synthesis of new HL1 and HL2 (protonated versions of the ligands) is sketched in
Scheme 1b, following previously established synthetic strategies by our group.[31,32]
Both molecules are built of the same aliphatic guanidinyl group attached to a different
aromatic guanidinyl (HL1) or thio-guanidinyl group (HL2). In crystals of HL1, grown by
3
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layering a 1,2-difluorobenzene solution with n-pentane, dimeric units are formed, in
which two molecules are bound by two N-H···N hydrogen bonds (see Supporting
Information). B3LYP/def2-TZVP calculations predict a significant binding energy (E
including zero-point energy corrections) of 85 kJ mol−1 for the dimer. The unit cell
contains four different, almost planar molecules. With average values of 1.424 and
1.295 Å, the central N-N single and the imino C=N double bond lengths are similar to
those in other neutral urea azines.[31,32,35]
The redox activity of the protonated ligands was studied by cyclic voltammetry (see
Supporting Information). The curve recorded for HL1 contains a relatively sharp
oxidation/reduction wave at –0.442 V/–0.545 V and a broad one at 0.102 V/–0.127 V.
Similarly, for HL2 a relatively sharp oxidation/reduction wave at –0.235 V/–0.366 V and
a broad one at 0.508 V/0.278 V was observed. The underlying two one-electron
processes involve a change in the hydrogen bonding or even proton transfer, since
hydrogen bonding is strengthened upon oxidation of the hydrogen-bond donor
component. The deprotonated versions of the two ligands could adopt three different
redox states (anionic, radical neutral or cationic, see Lewis structures in Scheme 2a).
Scheme 2. a) Possible redox states of the new ligands. b) Synthesis of [Cu(L1)2] and
[Cu(L2)2] (X = NMe (L1) or S (L2)).
The two homoleptic CuII complexes [Cu(L1)2] and [Cu(L2)2] were obtained by reaction
of CuII acetate with HL1 and HL2, respectively, in the presence of KOtBu (Scheme 2b).
[Cu(L1)2] is more sensitive to dioxygen than [Cu(L2)2]. Structural characterisation in
the solid state by SCXRD (Table 1 and Figure 1) found a copper coordination mode in
between tetrahedral and square planar with dihedral angles between the two CuN2
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planes of 51.5° for [Cu(L1)2] and 54.0° for [Cu(L2)2]. As discussed previously, this
special structure is due to - and -contributions of the metal-guanidine bonding.[17]
Due to the asymmetric design of the azine ligands, cis- or trans-type ligand
conformations are possible; [Cu(L1)2] crystallises in the cis-type and [Cu(L2)2] in the
trans-type form. The bond parameters (e.g. average Cu-N distance of 1.962 Å in
[Cu(L1)2] and 1.965 Å in [Cu(L2)2], and central N-N ligand bond length of 1.426(2) Å in
[Cu(L1)2] and 1.420(2) Å in [Cu(L2)2]) are similar for both complexes, indicating CuII
with two monoanionic ligands. We also prepared the zinc complex [Zn(L1)2]. Here, the
dihedral angle between the two ZnN2 planes is 79.4°, being closer to the 90° expected
for tetrahedral coordination, and the N-N bond length measures 1.432(1) Å.
Table 1. Structure parameters (bond lengths in Å, angles in °) for all complexes in the
solid state from SCXRD.[36]
[Zn(L1)2]
[Cu(L1)2]
[Cu(L2)2] [Cu(L1)2](SbF6) [Cu(L2)2](SbF6)
a
1.387(1) 1.381(2)/1.381(2) 1.765(3)
1.365(1) 1.371(2)/1.369(2) 1.365(3)
1.307(1) 1.305(2)/1.307(2) 1.304(3)
1.432(1) 1.426(2)/1.426(2) 1.420(2)
1.307(1) 1.313(2)/1.313(2) 1.326(3)
1.376(1) 1.362(2)/1.359(2) 1.365(3)
1.381(1) 1.381(2)/1.381(2) 1.351(3)
1.958(1) 1.924(2)/1.942(2) 1.944(2)
2.052(1) 2.007(2)/1.976(2) 1.985(2)
1.376(5)
1.343(5)
1.340(5)
1.369(4)
1.341(5)
1.342(5)
1.368(5)
1.933(3)
2.095(3)
65.4°
1.758(3)/1.746(3)
1.367(4)/1.351(4)
1.310(4)/1.327(4)
1.405(4)/1.371(4)
1.340(4)/1.362(4)
1.345(4)/1.338(4)
1.342(4)/1.336(5)
1.909(3)/1.922(3)
1.962(3)/1.986(3)
60.9°
b
c
d
e
f
g
h
i
a
∠ ML2
79.4°
51.5°
54.0°
[Cu(L1)2](PF6)2
a
b
c
d
e
f
1.348(12)/1.361(11)
1.348(12)/1.345(12)
1.372(12)/1.355(12)
1.348(10)/1.369(10)
1.376(12)/1.379(12)
1.346(12)/1.325(13)
1.332(12)/1.337(12)
1.910(8)/1.917(8)
1.974(8)/1.981(8)
54°
g
h
i
a
∠ ML2
a ML2 denotes the smallest dihedral angle between the two MN2 planes.
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Figure 1. Illustration of the solid-state structures of [Cu(L1)2] (a), [Cu(L2)2] (b) and
[Zn(L1)2] (c) from two perspectives. Displacement ellipsoids drawn at the 50%
probability level. Hydrogen atoms omitted. Colour code: N-blue, C-grey, S-yellow,
Cu-red, Zn-silver.
The UV-vis spectra of the two neutral complexes (Figure 2 and Supporting Information)
are dominated by strong absorptions in the UV region (338 nm, 243 nm for [Cu(L1)2]
and 324 nm, 229 nm for [Cu(L2)2]). Only weak bands appear in the visible region
(~450 nm and ~650 nm for both complexes), being responsible for their green colour.
In addition, weak, broad bands around 1900 nm were tentatively assigned to
ligand → metal charge transfer excitations. By contrast, the optical spectra for
[Zn(L1)2] (see Supporting Information) showed strong absorption bands only in the UV
6
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region (at 337, 284, and 235 nm). In summary, the electron spectra give no hint for the
presence of oxidised ligands, in line with the description as CuII complexes with anionic
ligands.
a)
b)
30
25
20
15
10
5
15
10
Cu(L1)2
Cu(L1)2PF6
Cu(L1)2(SbF6)2
0
0.6
300
400
500
l / nm
600
700
800
5
0
c)
0.4
0.2
0.0
.
L1
300
400
500
600
700
800 500 1000 1500 2000 2500
l / nm
l / nm
Figure 2. a) UV-vis spectra of [Cu(L1)2], [Cu(L1)2]+ and [Cu(L1)2]2+ in CH2Cl2. b) Zoom
into the visible and near-IR regions. c) Simulation of the electron spectrum of the free
radical L1· from TD-DFT (B3LYP+D3/def2-TZVP) calculations.
Four one-electron redox processes were found in the cyclic voltammograms of both
complexes (Figure 3). The redox potentials are significantly higher (less negative) for
[Cu(L2)2] than for [Cu(L1)2]. The redox process with the lowest potential (E1/2 = −1.40 V
for the redox couple [Cu(L1)2]/[Cu(L1)2]− and −1.29 V for [Cu(L2)2]/[Cu(L2)2]−) is
assigned to a metal-centered process (CuII → CuI). The potential is too low for a ligand-
centered redox event. The low potential of the CuII/CuI redox couple for both neutral
complexes is in line with the description as CuII complexes. Then, the second redox
process at −0.71 V for [Cu(L1)2]+/[Cu(L1)2] and −0.44 V for [Cu(L2)2]+/[Cu(L2)2]
belongs to a ligand-centered redox event ((L1/L2)− → (L1/L2 ·0). Two further quasi-
reversible, one-electron redox events were observed at E1/2 = −0.27 and +0.09 V for
7
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[Cu(L1)2] and at −0.03 and +0.31 V for [Cu(L2)2]. Hence the CV data indicate stability
of the complexes in several redox states. However, for [Cu(L2)2] the waves broaden
with increasing scan speed, especially in direction of reduction, arguing for some
redox-induced structural and/or electronic changes.
30
20
10
0
Cu(L1)2
-10
-20
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
40
30
20
10
0
Cu(L2)2
-10
-20
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
E / V
Figure 3. Cyclic voltammograms for [Cu(L1)2] and [Cu(L2)2] in CH2Cl2 (100 mV s−1
scan rate, 0.1 M nBu4NPF6 as supporting electrolyte, potentials given vs. Fc+/Fc).
Chemical oxidation of the two complexes with one equivalent of FcPF6 or AgSbF6
(Figure 4a) leads cleanly to the deep-purple salts [Cu(L1)2](PF6/SbF6) and
[Cu(L2)2](PF6/SbF6) (83% and 75% isolated yield, respectively). Figure 4b sketches
the Lewis structures representing possible descriptions of the charge distributions in
[Cu(L1)2]+ and [Cu(L2)2]+. Structure A+ is a CuII complex with neutral radical and
monoanionic ligands. Of course, the unpaired electron could also be delocalised over
both ligand units. Structures B+ and C+ are CuI complexes. In B+, both ligand units are
neutral radicals; for C+ a closed-shell singlet state results with one monocationic and
one monoanionic ligand unit. It will be shown in the following that none of the structures
alone fits to the analytic data. Structures B+ and C+ both contribute to an adequate
description of the electronic structure of the monocationic complexes, but the weighting
of the two structures is very different for [Cu(L1)2]+ and [Cu(L2)2]+. In both cases,
oxidation leads to a CuI complex; overall oxidation induces copper reduction
(CuII → CuI) in a redox-induced electron transfer (RIET) process.
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Figure 4. a) One-electron oxidation of the neutral complexes. b) Possible descriptions
of the electronic distribution in [Cu(L1)2]+ and [Cu(L2)2]+. Structures B+ and C+ describe
best the electronic structures. c) Illustration of the solid-state structures of
[Cu(L1)2](SbF6) (left) and [Cu(L2)2](SbF6) (right). Displacement ellipsoids drawn at the
50% probability level. Hydrogen atoms omitted. Colour code: N-blue, C-grey, S-yellow,
Cu-red, Sb-mauve, F-green.
−
SCXRD of both complexes with SbF6 counter-ions (crystals obtained by layering
saturated 1,2-difluorobenzene solutions with n-pentane, Figure 4c and Table 1),
showed that the two ligands are equal in [Cu(L1)2]+, but significantly different in
[Cu(L2)2]+. In both cases, the central N-N bond is shortened upon oxidation, from
1.426(2) Å in [Cu(L1)2] to 1.369(4) Å in [Cu(L1)2]+, and from 1.420(2) Å in [Cu(L2)2] to
1.405(4)/1.371(4) Å in [Cu(L2)2]+. By contrast, the two imino N=C bonds in the neutral
complexes (1.305(2)/1.307(2)/1.313(2)/1.313(2) Å and 1.304(3)/1.326(3) Å in
9
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[Cu(L1)2] and [Cu(L2)2], respectively), are elongated (to 1.340(5)/1.341(5) Å and
1.310(4)/1.327(4)/1.340(4)/1.362(4) Å in [Cu(L1)2]+ and [Cu(L2)2]+, respectively). The
average Cu-N bond length and the dihedral angle increase upon oxidation of [Cu(L1)2],
clearly signalling formation of a CuI complex with two neutral radical ligands (B+ in
Figure 4b). In the case of [Cu(L2)2]+, the differences in the ligand structures are far too
large to be explained by crystal packing effects, pointing to different ligand redox
states. With 1.405(4) and 1.371(4) Å, the N-N bond lengths are both shorter than in
neutral [Cu(L2)2] (1.420(2) Å). The Cu-N bond distances vary significantly (1.909(3) -
1.986(3) Å) and oxidation leads to an increase of the dihedral angle at the copper atom
(from 54.0° in [Cu(L2)2] to 60.9° in [Cu(L2)2]+), pointing again to CuI. The structure
indicates a significant contribution of C+. Structure A+ cannot be completely discarded
at this point of the discussion, but is not in line with the results of the other analytic data
(see below).
The UV-vis spectra of [Cu(L1)2]+ and [Cu(L2)2]+ show intense absorptions in the vis
region, but at different wavelengths (505 and 555 nm (shoulder) for [Cu(L1)2]+, and
485 and 720 nm (shoulder) for [Cu(L2)2]+ (Figure 2a and Supporting Information),
pointing to the presence of oxidised ligands. The simulated electron spectrum for the
isolated neutral radical L1· based on TD-DFT calculations (B3LYP+D3/def2-TZVP,
Figure 2c) closely matches the experimental spectrum of [Cu(L1)2]+ in the visible
region, supporting description B+ for [Cu(L1)2]+. The differences in the electron spectra
of the two complexes point to different ligand redox states, in line with a significant
contribution from C+ in the case of [Cu(L2)2]+.
Background-corrected Cu 2p X-ray photoelectron (XP) spectra of the complexes
[Cu(L1)2]PF6 and [Cu(L2)2]PF6 are shown in Figure 5; complementary C 1s, N s, and
S 2p XP data are presented in the Supporting Information. Note that the wide scan
spectra of both complexes (not shown) exhibit a pronounced F 1s peak, manifesting
the expected presence of fluorine in the counter ions. The Cu 2p XP spectra in Figure 5
exhibit the Cu 2p3/2 and Cu 2p1/2 components at binding energies of 932.8 eV and
952.6 eV for complex [Cu(L1)2]PF6 and 932.6 and 952.8 eV for [Cu(L2)2]PF6. These
energies are characteristic of copper in the oxidation state +1,[37] corresponding to the
electronic configuration 3d10.[38] The Cu 2p3/2 and Cu 2p1/2 components represent well-
defined singular peaks, with no traces of the analogous components characteristic of
copper in the oxidation state +2, expected at a binding energy of 934.3-934.7 eV for
Cu 2p3/2 (3d9 configuration).[39] The spectrum of the latter species is also characterized
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by the intense satellite structure in the post-peak region,[39,40] fully absent in the spectra
presented in Figure 5. Hence the XPS data clearly show that [Cu(L1)2]+ and [Cu(L2)2]+
are CuI complexes.
XPS: Cu 2p
hn = 1253.6 eV
[Cu(L2)2](PF6)
[Cu(L1)2](PF6)
960
950
940
930
Binding energy / eV
Figure 5. Background-corrected Cu 2p XP spectra of [Cu(L1)2]PF6 and [Cu(L2)2]PF6
(open circles). The Cu 2p3/2 and Cu 2p1/2 components are tentatively fitted by
symmetric Voigt peaks (solid red and blue lines, respectively).
Magnetometric (SQUID) measurements on a solid powder sample of [Cu(L1)2](SbF6)
confirmed the presence of two unpaired electrons (Figure 6), in accordance with B+
(Figure 4b). A curve typical of a weakly antiferromagnetically coupled two-spin system
was obtained, and an isotropic J value of −3.5 cm−1 (spin Hamiltonian H = −J S1 S2)
and a g factor of 1.9947 followed from a Bleaney-Bowers curve fit. The magnetometric
measurements on [Cu(L2)2]SbF6 gave different results. Here the T value is small,
arguing for structure B+ with an extremely strong antiferromagnetic coupling between
the two ligand-centered unpaired electrons or (more likely) a dominating contribution
from the closed-shell structure C+ in the solid state, the latter being in line with the
presence of two different ligands in the XRD structure.
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0.8
0.6
0.4
0.2
0.0
0
50
100
150
200
250
300
T / K
Figure 6. Plot of the magnetometric (SQUID) data for [Cu(L1)2](SbF6) (black filled
circles) and [Cu(L2)2](PF6) (blue open circles), measured at 50 mT. The dashed red
curve results from a Bleaney-Bowers fit for [Cu(L1)2]SbF6.
Paramagnetic NMR spectroscopy has been established as a powerful tool for the
analysis of paramagnetic molecules in solution;[41,42] we applied this technique
1
previously for complexes with guanidine ligands.[43-45] Figure 7 shows the H NMR
spectra for [Cu(L1)2]+ and [Cu(L2)2]+ at various temperatures (see 13C NMR spectra in
the Supporting Information). The signal dispersion is much larger for [Cu(L1)2]+,
indicating more spin density on the ligands. Seven signals are observed at high
temperature corresponding to four aromatic, two methyl and one methylene hydrogen
signal. The latter corresponds to four inequivalent H atoms which are in a fast chemical
exchange (diastereotopic H atoms of each CH2 group and inequivalence of the two
CH2 groups due to restricted rotation around the C=N double bond). At lower
temperatures, this signal splits into two and then into four signals. The equivalence of
the hydrogen atoms of the ethylene bridge stems from two dynamic processes. One
process interconverts the carbon atoms by a rotation around the N=C double bond; a
second dynamic process exchanges the diastereotopic H atoms of each CH2 moiety
(see Supporting information).
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1
Figure 7. Paramagnetic H NMR spectra of a) [Cu(L1)2]PF6 and b) [Cu(L2)2]PF6 at
various temperatures in CD2Cl2 (399.89 MHz).
The 1H NMR spectrum of [Cu(L2)2]+ could be assigned similar to that of [Cu(L1)2]+, but
the smaller signal dispersion indicates less spin density on the ligands. The aromatic
protons give rise to the two downfield shifted signals at  = 18.12 and 11.54 ppm as
well as to the two upfield shifted signals at  = −16.23 and −24.84 ppm. Also, the NCH3
groups at the aliphatic guanidinyl moiety are chemically equivalent in the NMR
spectrum and likely belong to the downfield shifted signal at  = 21.80 ppm. In
difference to the spectra of [Cu(L1)2]+, the CH2 protons give rise to two signals at room
13
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temperature, however 2D NOESY experiments show that they are in a chemical
exchange.
The temperature-dependence of the NMR signals deviates for both complexes from
the Curie behaviour (1/T dependence) which is expected for the Fermi contact shift for
non-coupled systems; the signals for both complexes tend to shift into the diamagnetic
region with decreasing temperature. It was not possible to fit adequately the NMR data
of [Cu(L1)2]+ (see Supporting Information) with the standard formula for a magnetically-
coupled system with two spin centers SA = Sb = 1/2.[46] The data at low and high
temperatures seem to follow different functions. On the other hand, fits of the
temperature-dependence of the NMR signals of [Cu(L2)2]+ were satisfactory, but only
with extraordinary large hyperfine coupling constants A and extremely large
antiferromagnetic coupling constant J as fit parameters (see Supporting Information).
The NMR data are in line with a mixture of two different descriptions of the charge
distribution, with a larger contribution of C+ to the electronic structure in the case of
[Cu(L2)2]+.
In summary, all results (especially SCXRD, UV-vis and XP spectroscopy, and also
EPR spectroscopy (main signal contribution with g factor near the free-electron value,
only weak copper hyperfine structure visible due to small amount of copper-centered
spin density (see Supporting Information))) indicate that [Cu(L1)2]+ and [Cu(L2)2]+ are
both CuI complexes with two oxidised ligand units. For [Cu(L1)2]+, the electronic
structure is best described by B+ in Figure 4b (being in pleasing agreement with the
SCXRD and SQUID results), but the difficulties in fitting the temperature-dependence
of the NMR signals argues for a (temperature-dependent) contribution of C+ or
alternatively to a sensitivity of the electronic structure to the discussed dynamic effects
in solution. In the case of [Cu(L2)2]+, the inequivalence of the two ligand units in the
solid-state structure and the low T value clearly point to a larger contribution from
structure C+ with respect to [Cu(L1)2]+. However, the NMR data show some
paramagnetic character, in line with a significant contribution from structure B+. A
conversion from B+ to C+ could be described as a ligand disproportionation.
Interestingly, Chaudhuri, Wieghardt et al. reported a Zn complex with two redox-active
ligands for which the presence of two isomers related by such a ligand
disproportionation was indeed evidenced.[47] The green diamagnetic form of this
complex is thermodynamically more stable than the red paramagnetic isomer. The
authors demonstrate that isomerisation from the red, paramagnetic form to the green,
14
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diamagnetic isomer could be initiated by redox cycling or changes in the solvent or
experimental conditions. In the case of [Cu(L2)2]+, the electronic structure certainly
differs in solution and in the solid state. However, we have no indication for the co-
existence of two isomers (one being closer to structure C+ and the other one closer to
B+) or the possibility to initiate isomerisation by change of solvent or reaction
conditions.
The RIET process could be roughly rationalized as follows. The +II oxidation state of
the copper atom in neutral [Cu(L1)2] and [Cu(L2)2] is certainly favoured by a significant
ionic contribution to bonding between the anionic ligands and CuII. Ligand-centered
one-electron oxidation generating a neutral radical ligand decreases this ionic
contribution, thereby destabilising CuII with respect to CuI. In the case of [Cu(L1)2]+, an
electron is transferred from the second ligand to copper, leading eventually to a CuI
complex with two neutral radical ligands (structure B+). For [Cu(L2)2]+, the electron
needed for copper reduction stems in part from the neutral radical ligand. It should be
noted that CuI complexes with two radical ligand units like [Cu(L1)2]+ are extremely rare
and almost exclusively restricted to a few complexes with two verdazyl radical
ligands.[48-53] Interestingly, the singlet-triplet energy difference in these systems could
be tuned and strongly depends on the coordination mode.
Further one-electron oxidation of [Cu(L1)2](SbF6) with a second equivalent of AgSbF6
produced the salt of the dicationic complex, [Cu(L1)2](SbF6)2 in 58% isolated yield
(Figure 8a). This complex could either be described as a CuII complex with two neutral
radical ligands (A2+), summing up to three unpaired electrons, as a CuI complex with a
cationic ligand and a neutral radical ligand (B2+) or a CuII complex with one anionic
ligand and one cationic ligand (C2+), resulting in one unpaired electron for the latter two
(Figure 8b). Obviously, the electronic structure could also be in between these three
descriptions. XRD of crystals of [Cu(L1)2](PF6)2, obtained from gas-phase diffusion of
n-pentane into a saturated acetone solution (Table 1 and Figure 8c), showed that the
two ligand units are different, and the Cu-N bond distances vary significantly
(1.910(8) - 1.981(8) Å), being more compatible with B2+ and C2+. The N-N bonds of the
ligands measure 1.348(10) and 1.369(10) Å.
In the UV-vis spectrum of [Cu(L1)2]2+ (see Figure 2), strong absorptions appear in the
visible region (at 568 nm (sh) and 471 nm), being close to the absorptions in the
monocationic complex [Cu(L1)2]+ (555 nm (sh) and 505 nm). In addition, a quite strong
band appears in the NIR region near 1200 nm, tentatively assigned to a metal → ligand
15
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10.1002/anie.202109367
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charge-transfer transition. These bands strongly argue for a neutral radical ligand unit,
as present in B2+.
Magnetometric (SQUID) measurements on a solid powder sample of [Cu(L1)2](SbF6)2
(see Supporting Information) show that with increasing temperature the T value
approaches the expectation value for one unpaired electron (0.375 cm3 K mol−1). It
drops down at very low temperatures (ca. 25 K), most likely due to intermolecular
antiferromagnetic coupling. Hence these data strongly support structure B2+. They are
clearly not compatible with A2+.
EPR spectra in CH2Cl2 (see Supporting Information) at room-temperature show a
single signal with a g value of 2.004, compatible with a ligand-centered radical but not
with a copper-centered radical (in the latter case the g value should be higher). Hence,
all results indicate that B2+ describes the electronic structure most adequately.
Structure B2+ directly follows from ligand-centered one-electron oxidation of [Cu(L1)2]+
(CuI complex with two neutral radical ligands).
16
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10.1002/anie.202109367
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Figure 8. a) Two-electron oxidation of [Cu(L1)2]. b) Possible electronic structures of
[Cu(L1)2]2+. Structure B2+ in the box describes best the electronic structure. c)
Illustration of the solid-state structure of [Cu(L1)2](PF6)2 (without co-crystallised
acetone). Displacement ellipsoids drawn at the 50% probability level. Hydrogen atoms
omitted. Colour code: N-blue, C-grey, Cu-red, P-orange, F-green.
Finally, we tested the catalytic performance of the new complexes in the aerobic
alcohol oxidation to aldehydes. Benzyl alcohol (BzOH) and cinnamyl alcohol (CinnOH)
were used as model substrates. The best results with BzOH (Table 2) were obtained
with 5 mol% of the catalyst and 2 eq. of Cs2CO3 (93% conversion for [Cu(L1)2] as
catalyst in C7D8, and 99% for [Cu(L2)2] as catalyst in CD2Cl2). By contrast, [Zn(L1)2]
was only poorly active (entry 15). Oxidation of CinnOH with the optimized conditions
yielded 72% conversion. On the other hand, the oxidation of the primary alcohol
nOctOH failed (entry 14). Signals due to the urea 1,3-dimethyl-2-imidazolidinone were
observed in the NMR spectra after 22 h reaction time, and traces of hydrazone in the
reactions with [Cu(L2)2] as catalyst. The slow catalyst deactivation by decomposition
17
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with increasing reaction time explains the reduced conversion with a lower catalyst
loading of 1 mol% (entry 2). In this respect, the new catalysts behave similar to
previously reported catalysts.[19,20,24,25,54] Additional experiments (see Supporting
Information) indicate that the catalytic activity (conversion/time) of the two complexes
is comparable within the first 5 h of reaction. Interestingly, a higher conversion is
eventually achieved with [Cu(L2)2], although L1 is a better electron-donor than L2
(Table 2). Complex [Cu(L1)2] is more rapidly oxidised by dioxygen and seems to
de ompose more r pidly in high oxid tion st tes. Moreover, "overoxid tion“ of this
complex (removal of three electrons) due to the lower redox potentials for all three one-
electron oxidation steps (see CV curves in Figure 3) possibly leads to an increased
loss of the catalyst. Further analysis of the catalytic mechanism is the subject of
ongoing research in our laboratory.
Table 2. Summary of the results of the catalytic experiments.
Entry Catalyst Cat. mol%
Base
Base eq Solvent Substrate Conversion/%
1 [Cu(L1)2]
2 [Cu(L1)2]
3 [Cu(L1)2]
4 [Cu(L1)2]
5 [Cu(L1)2]
6 [Cu(L1)2]
7 [Cu(L1)2]
8 [Cu(L1)2]
9 [Cu(L2)2]
10 [Cu(L2)2]
11 [Cu(L2)2]
12 [Cu(L2)2]
13 [Cu(L2)2]
14 [Cu(L2)2]
15 [Zn(L1)2]
5
1
5
5
5
5
5
5
5
2
1
5
5
5
5
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
no Base
KOH
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
2
2
2
2
1
0.1
0
2
2
2
CD2Cl2 BzOH
CD2Cl2 BzOH
Tol-d8 BzOH
CD3CN BzOH
CD2Cl2 BzOH
CD2Cl2 BzOH
CD2Cl2 BzOH
CD2Cl2 BzOH
CD2Cl2 BzOH
CD2Cl2 BzOH
CD2Cl2 BzOH
Tol-d8 BzOH
CD2Cl2 CinnOH
CD2Cl2 nOctOH
Tol-d8 BzOH
87
62
93
80
65
29
29
10
99
98
54
93
72
2
2
2
2
2
2
17
*NMR conversion determined from substrate/product ratio.
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Conclusions
New homoleptic CuII complexes [Cu(L1)2] and [Cu(L2)2] with monoanionic, redox-
active and asymmetric ligands (urea azines (L1) or thio-urea azines (L2)) were
synthesised. The ligand design relied on previously established procedures by our
group for the directed synthesis of asymmetric redox-active urea azines and related
thio-urea azines. Only by virtue of their asymmetric structure, these ligands could be
used after deprotonation as chelating, anionic redox-active ligands for mononuclear
complexes. In chemical redox reactions, the monocationic complexes [Cu(L1)2]+ and
[Cu(L2)2]+ as well as the dicationic complex [Cu(L1)2]2+ were synthesised and fully
characterized. Fortunately, the electronic structure elucidation, relying on a variety of
different analytical methods, is facilitated by structural characterisation in three
different redox states, being extremely rare in the field of complexes with redox-active
ligands.
One-electron oxidation of [Cu(L1)2] and [Cu(L2)2] is accompanied by an
unprecedented RIET process leading to metal reduction (CuII – CuI) upon overall one-
electron oxidation. In [Cu(L1)2]+, the CuI center is coordinated by two equivalent,
neutral radical ligands, that couple antiferromagnetically. In [Cu(L2)2]+, the CuI atom is
coordinated by two different ligands, resulting in a complex with a magnetic
susceptibility close to a diamagnetic compound. Hence slight variations within the
ligand design lead to significant changes in the electronic structure and properties.
The dication [Cu(L1)2]2+ was synthesised by two-electron oxidation of [Cu(L1)2]. In the
solid-state structure, the two ligand units are different, and a detailed analysis indicates
that the electronic structure is best described in terms of a CuI complex with one
twofold-oxidised monocationic ligand unit and one singly-oxidised neutral radical
ligand.
Their exceptional stability in several redox states makes complexes of redox-active
asymmetric azine ligands highly interesting for applications. Within a representative
test reaction, their use for the catalytic aerobic oxidation of alcohols to aldehydes was
demonstrated. In ongoing work, we explore further catalysis with copper, and also zinc
and cobalt complexes of these novel ligand platforms.
19
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Acknowledgements
The authors gratefully acknowledge continuous financial support by the German
research foundation (DFG). We thank the state of Baden-Württemberg through
bwHPC and the German Research Foundation (DFG) through grant no INST 40/575-
1 FUGG (JUSTUS 2 cluster).
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TOC entry
A Copper(I) Complex with
Two Unpaired Electrons,
Synthesised by Oxidation
of a Copper(II) Complex
with Two Redox-Active
Ligands
M. Werr, E. Kaifer, M.
Enders, A. Asyuda, M.
Zharnikov, H.-J. Himmel*
Oxidation-induced reduction: The oxidation of mononuclear CuII complexes with novel
unsymmetric redox-active urea azine or thio-urea azine ligands leads to metal
reduction and formation of complexes with intriguing electronic structures. The stability
in several redox states is the basis for their first application in catalytic aerobic alcohol
oxidation.
Key-words: alcohol oxidation, copper, radicals, redox-induced electron transfer, urea
azines
24
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