Isolation of neutral mononuclear copper complexes stabilized by two cyclic (Alkyl)(amino)carbenes

Article abstract of DOI:10.1021/ja502521b

Two (cAAC)₂Cu complexes, featuring a two-coordinate copper atom in the formal oxidation state zero, were prepared by reducing (Et ₂-cAAC)₂Cu⁺I^- with metallic sodium in THF, and by a one-pot synthesis using Me₂-cAAC, Cu(II)Cl ₂, and KC₈ in toluene in a molar ratio of 2:1:2, respectively. Both complexes are highly air and moisture sensitive but can be stored in the solid state for a month at room temperature. DFT calculations showed that in these complexes the copper center has a d¹⁰ electronic configuration and the unpaired electron is delocalized over two carbene carbon atoms. This was further confirmed by the EPR spectra, which exhibit multiple hyperfine lines due to the coupling of the unpaired electron with ^63,65Cu isotopes, ¹⁴N, and ¹H nuclei.

Full text of DOI:10.1021/ja502521b

Communication
pubs.acs.org/JACS
Isolation of Neutral Mononuclear Copper Complexes Stabilized by
Two Cyclic (Alkyl)(amino)carbenes
David S. Weinberger,# Nurul Amin SK,† Kartik Chandra Mondal,† Mohand Melaimi,# Guy Bertrand,*,#
A. Claudia Stuckl,† Herbert W. Roesky,*,† Birger Dittrich,§,† Serhiy Demeshko,† Brigitte Schwederski,‡
̈
Wolfgang Kaim,*,‡ Paul Jerabek,⊥ and Gernot Frenking*,⊥
#UCSD-CNRS Joint Research Chemistry Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of
California−San Diego, La Jolla, California 92093-0343, United States
†Institut fur Anorganische Chemie, Georg-August-Universitat, Tammannstraße 4, 37077 Gottingen, Germany
̈
̈
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§Institut fur Anorganische und Angewandte Chemie, Universitat Hamburg, Martin-Luther-King-Platz 6, Raum AC 15c (Erdgeschoss),
̈
̈
20146 Hamburg, Germany
‡Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
⊥Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany
̈
̈
̈
S
* Supporting Information
as a π-acceptor, copper(0) was still precipitated and absorbed at
ABSTRACT: Two (cAAC)2Cu complexes, featuring a
two-coordinate copper atom in the formal oxidation state
zero, were prepared by reducing (Et2-cAAC)2Cu+I− with
metallic sodium in THF, and by a one-pot synthesis using
Me2-cAAC, Cu(II)Cl2, and KC8 in toluene in a molar ratio
of 2:1:2, respectively. Both complexes are highly air and
moisture sensitive but can be stored in the solid state for a
month at room temperature. DFT calculations showed
that in these complexes the copper center has a d10
electronic configuration and the unpaired electron is
delocalized over two carbene carbon atoms. This was
further confirmed by the EPR spectra, which exhibit
multiple hyperfine lines due to the coupling of the
unpaired electron with 63,65Cu isotopes, 14N, and 1H
nuclei.
the electrode surface. On the other hand, the electrogenerated
[Cu(phen)2] neutral species have been clearly identified by
electron paramagnetic resonance (EPR) spectroscopy as CuI
complexes with partially reduced non-innocent phenanthroline
ligands.6b
There is some evidence presented by Bowmaker for the
existence of [Cu(PH3)n]0.7 These complexes were claimed to
be prepared by co-deposition of copper vapor with neat
phosphine at 60 K. Subsequent calculations show the HOMO
for [Cu(PH3)2]0, [Cu(PH3)3]0, and [Cu(PH3)4]0 to be
bonding orbitals with a combination of the Cu 4p and the
ligand orbitals of π-symmetry.8 While the calculations show the
opportunity to isolate such complexes, they have only been
characterized at low temperatures by infrared (IR) spectrosco-
py.
Additionally, Atkins and Timms reported that when PN
was condensed with copper at low temperatures in a krypton
matrix, the IR spectrum produced was different from that of the
starting material.9 It was postulated that the resulting complex,
which decomposed upon warming, was NP→Cu←PN.
We reasoned, that in order to stabilize an electron-rich
copper(0) and prevent it from precipitating as metal, a ligand
that forms very strong bonds to copper and is capable of
accepting electron density would be required. We recently
demonstrated that cyclic (alkyl)(amino)carbenes (cAACs)10,11
can stabilize transition metals such as gold(0),12 manga-
nese(0),13 and zinc(0)14 along with a wide variety of
phosphorus-, boron-, silicon-, and even carbon-based radicals.15
In this Communication we report the synthesis, single-crystal
X-ray diffraction and EPR studies, and density functional theory
(DFT) calculations of two-coordinate copper complexes
featuring the metal in the formal oxidation state zero.
Complex (Et2-cAAC)2Cu+I− (1) was synthesized by reacting
2 equivalents of free carbene and copper(I) iodide (Scheme 1).
opper has been shown to be prevalent in redox-active
C
metalloproteins with its most common oxidation states
being +I and +II.1,2 While this has motivated research in the
study of copper complexes that show reduction possibilities, to
date, no stable complexes have been fully characterized in
which the formal charge on the copper atom is zero. The
closest copper complexes to be isolated in the zero oxidation
state are thin layers and small clusters of copper deposited on a
metal oxide surface. These have been shown to be active
catalysts for the water−gas shift reaction and the hydrogenation
of CO2 to methanol;3 however, it has been established, both
experimentally and computationally, that the overall charge on
the copper is between 0 and 1.4
Excluding elemental copper, mononuclear ligated copper(0)
complexes have been reported as intermediates in copper-
catalyzed arylation of nitrogen and oxygen and have only been
detected by cyclic voltammetry (CV) at fast scan rates.5
Addition of an aryl halide to the CV cell shows higher
concentration of the proposed copper(0) species due to the
suggested outer-sphere one-electron reduction of the aryl
halide.6 Furthermore, even with the addition of phenanthroline
Received: March 12, 2014
Published: April 14, 2014
© 2014 American Chemical Society
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Scheme 1. Synthetic Strategy for Carbene−Copper
Complexes
The cyclic voltammogram of 1 in THF, containing 0.1 M
nBu4NPF6 as electrolyte (see Supporting Information (SI)),
showed a quasi-reversible one-electron reduction at E1/2
=
−1.36 V versus Cp*2Fe+/Cp*2Fe (Cp*2Fe = decamethylferro-
cene). Therefore, we attempted the reduction of complex 1
with sodium metal (Scheme 1). In a glovebox a THF solution
of 1 was stirred with a fresh piece of sodium metal for 2 h in a
scintillation vial. The solution was filtered and evaporated. The
solid was then extracted with n-hexane and the solvent removed
to yield 2 (35% yield) as a highly water- and air-sensitive
reddish-brown solid. Crystals of 2 were obtained from a
concentrated solution of hexanes at −30 °C.
Similarly, a one-pot reaction of Me2-cAAC, CuCl2, and KC8
in a molar ratio of 2:1:2 in toluene produced a green solution
from which dark plates of the desired complex (Me2-cAAC)2Cu
(3) were isolated in 7% yield (Scheme 1). Single crystals were
obtained from a toluene solution at −32 °C. 3 is stable under
an inert atmosphere and decomposes above 142 °C. 3 is not
stable in benzene and n-hexane, in which it decomposes to
produce a light yellow solution. It has been observed that the
toluene solution of 3 is stable and retains its color under an
inert atmosphere. The crystals of 3 can be stored in a glovebox
for a month at room temperature, whereas on exposure to air
they slowly lose their color to produce a light yellow solid after
6 h. The UV−vis−near-IR spectrum of the toluene solution of
3 showed absorption bands at 330, 435, 482, 595, 655, 755, and
1214 nm (see SI).
Figure 1. Molecular structures of compounds (Et2-cAAC)2Cu (2) (a)
and (Me2-cAAC)2Cu (3) (b). H atoms are omitted for clarity. Selected
experimental [calculated at M06/def2-SVP for 3] bond lengths [Å]
and angles [°] of 2/3: Cu1−C1 1.8636(15)/1.8674(10) [1.887], N1−
C1 1.3492(19)/1.3510(13) [1.353], N1−C9/1.4393(13) [1.430],
C1−C2/1.5231(14) [1.518]; C1−Cu1−C1i 180/180[180.0], N1−
C1−C2/107.29(8) [107.5], N1−C1−Cu1/122.98(7) [122.3], C2−
C1−Cu1/129.72(7) [130.2].
much larger coefficients at C1/C1′ (0.34) than at N1/N1′
(0.09) (Figure 2).
Both complexes 2 and 3 crystallize in the triclinic space
group P1. The central copper atom resides on the center of
̅
symmetry and adopts a two-coordinate linear geometry (Figure
1). The C−Cu bond distances are experimentally found to be
1.8636(15) and 1.8674(10) Å for 2 and 3, respectively. The C−
N bond distances are 1.3492(19) Å in 2 and 1.3510(13) Å in 3.
The Cu−C and C−N bond distances of 1 range from 1.920(5)
to 1.946(5) Å and from 1.299(6) to 1.310(6) Å, respectively
(see SI), while the C−Cu−C bond angle in 1 is 180.0(2)/
168.4(2)°, which is close to that of 180° in 2/3.
Quantum chemical calculations using DFT at the M06/def2-
SVP level give insight into the bonding situation. The
calculated structure of 3, which is shown in Figure S6, is in
excellent agreement with the experiment. The theoretical C1−
Cu bond length in the cation 3+ (1.935 Å, see Figure S9) is
somewhat longer than in the neutral parent molecule, which
concurs with the experimental data. The calculated spin density
of the unpaired electron in 3 shows that it is delocalized over
the p(π) orbitals of the N1−C1−Cu−C1′−N1′ moiety, with
Figure 2. Spin density of (Me2-cAAC)2Cu (3) (M06/def2-SVP). This
singly occupied π orbital entails partial C1−Cu−C1′ π bonding, which
explains why the bonds are shorter than in the cation 3+. The bonding
situation in the copper compound 3 is very similar to that of the
analogous gold species Au(cAAC)2.12
Complexes 2 and 3 are EPR active (see SI). The room-
temperature EPR spectrum of 2 shows a signal at g = 1.997,
with partially resolved hyperfine coupling of about 7 G,
involving the copper isotopes 63Cu (69.2%, I = 3/2) and 65Cu
(30.8%, I = 3/2; ratio a(65Cu)/a(63Cu) = 1.07). A similar, well-
resolved spectrum was obtained for 3 in toluene at room
temperature (g = 1.9996; Figure 3). It could be simulated with
a(63Cu) = 7.95 G, a(65Cu) = 8.51 G, a(14N) = 2.5 G (2N), and
a(1H) = 0.9 G (12H). The latter coupling can be tentatively
attributed to the four methyl groups (CMe2) of the ligand in
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Communication
reversible one-electron process at E1/2 = −1.35 V versus
Cp*2Fe+/ Cp*2Fe (see SI), suggesting the formation of 3−.
In conclusion, we have shown that by utilizing cAACs, which
are both excellent σ-donor and π-acceptor ligands, the first
neutral two-coordinate Cu complexes could be prepared,
isolated, and fully characterized by X-ray crystallography and
EPR. Accessing what previously had only been a hypothesized
intermediate in copper catalysis will allow now for reactivity
studies.
Synthesis of Complexes 1−3. General. All manipulations
were performed under an inert atmosphere of dry nitrogen,
using standard Schlenk techniques. Dry, oxygen-free solvents
were employed. EPR spectra were recorded on a Bruker EMX
instrument. 1H and 13C NMR spectra were recorded on Varian
1
Inova 500 spectrometers (500.1 MHz for H, 125.8 MHz for
13C). All spectra were obtained in the solvent indicated at 25
Figure 3. Experimental (black) and simulated (red) EPR spectra of
(Me2-cAAC)2Cu (3).
°C. Chemical shifts are given in ppm and are referenced to
SiMe4 (1H, 13C). Melting points were measured with a Buchi
̈
melting point apparatus system.
the position β to the carbene carbon, whose hyperfine coupling
(Et2-cAAC)2Cu+I− (1). In a glovebox, a Schlenk flask was
loaded with Et2-cAAC (1.53 g, 4.88 mmol) and CuI (0.45
equiv, 0.418 g, 2.19 mmol). At −78 °C, 60 mL of THF was
added, and the mixture was warmed to room temperature over
14 h. The THF solution was evaporated to dryness and
subsequently washed with 20 mL of n-hexane three times. 1
was extracted with 3 × 10 mL of CH2Cl2, and after evaporation
of the solvent under vacuum, 1 was obtained as a white powder
could not be determined due to the low natural abundance of
1
13C. An alternative for the H coupling would be the four
inside-oriented methyl groups of the N-aryl substituents, as
suggested by the spin density plot (Figure 2). In agreement
with the calculated spin density distribution, the nitrogen
hyperfine coupling is relatively small, as is the metal hyperfine
coupling in comparison to that of paramagnetic compounds of
CuI or CuII,16 signifying little copper participation at the singly
occupied molecular orbital (SOMO). In the same vein, the
isotropic g factors close to the free electron value of 2.0023
confirm marginal contributions from the transition element to
the SOMO, suggesting a three-center carbene/copper/carbene
description with predominantly carbene radical character.
The magnetic properties of 3 were further investigated by
temperature-dependent susceptibility measurements (Figure 4).
1
(1.35 g; 75% yield): mp 203 °C (dec.); H NMR (C6D6, 400
MHz) δ = 7.35 (t, J = 7.2 Hz, 1H), 7.14 (d, J = 7.2 Hz, 2H),
2.62 (br, 2H), 1.95 (s, 2H), 1.71 (br, 2H), 1.56 (br, 2H), 1.26
(br, 6H), 1.20 (br, 6H), 1.18 (d, J = 6 Hz, 6H), 0.94 (br, 6H),
0.7 (br, 6H); 13C NMR (C6D6, 75 MHz) δ = 249.33 (C), 144.3
(C), 134.1 (C), 129.9 (CH), 124.1 (CH), 83.0 (C), 64.0 (C),
39.9 (CH2), 31.5 (CH2), 29.1 (CH3), 28.8 (CH3), 27.2 (CH),
21.8 (CH3), 9.6 (CH3) ppm; HRMS (ESI-TOFMS) m/z
calculated for [C44H70CuN2]+ 689.4835, found 689.4843.
(Et2-cAAC)2Cu (2). To a solution of 1 (125 mg, 0.153 mmol)
in 20 mL of THF was added a clean piece of Na metal (200
mg). After 15 min at room temperature, the solution turned to
a red/brown color. The solution was allowed to stir for an
additional hour and subsequently filtered. The filtrate was
evaporated under vacuum. The solid was then extracted with 4
× 20 mL of n-hexanes, and the solvent was evaporated under
vacuum to yield 2 as a reddish-brown solid. Crystals suitable for
X-ray analysis were grown at −30 °C from a concentrated
solution of n-hexanes (37 mg; 35% yield); 2 decomposes above
131 °C.
Figure 4. Magnetic moment vs temperature for 3. The solid line
(Me2-cAAC)2Cu (3). A 1:2:2 molar mixture of CuCl2 (76 mg,
0.562 mmol), Me2-cAAC (322 mg, 1.13 mmol), and KC8 (152
mg, 1.12 mmol) and toluene (70 mL) were separately cooled to
−78 °C, and then the toluene was added to the mixture
through a cannula. The reaction solution was then slowly
warmed to room temperature and stirred for 12 h. During this
period the color of the solution changed to dark green, and it
was subsequently filtered. The filtrate was stored at room
temperature for 6 h. Then the solution was filtered again to
remove colorless byproducts and stored at −32 °C in a freezer
overnight. Dark plates of 3 were formed (23 mg, 7% yield)
which were separated by filtration. C, H, and N analysis (%)
found (calcd) for C40H62N2Cu; C, 75.70 (75.72); H, 9.65
(9.84); N, 4.45 (4.41). 3 decomposes above 142 °C and turns
to a black liquid. UV−visible bands are observed at 330, 435,
482, 595, 655, 755, and 1214 nm.
represents the calculated curve.
The experimental magnetic moment of 3 is 1.74 μB at 155 K,
which is very close to the spin-only value of 1.73 μB for one
unpaired electron. Magnetic moment remains nearly constant
down to 7 K. Some decrease below this temperature can be
attributed to the weak intermolecular antiferromagnetic
interaction. Analysis of the experimental data (see SI for
more details) led to the values g = 2.00 and Weiss temperature
Θ = −0.9 K (or zJ = −2.5 cm−1, where J is the intermolecular
interaction parameter between two nearest-neighbor magnetic
species and z is the number of nearest neighbors;
intermolecular CcAAC−CH3 distance is 5.25 Å).
The cyclic voltammogram of (Me2-cAAC)2Cu (3) in DMF,
containing 0.1 M nBu4NPF6 as electrolyte, exhibited a quasi-
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(13) Samuel, P. P.; Mondal, K. C.; Roesky, H. W.; Hermann, M.;
EPR. Simulation for the EPR spectrum was done using
EasySpin.17
Frenking, G.; Demeshko, S.; Meyer, F.; Stuckl, A. C.; Christian, J. H.;
̈
Dalal, N. S.; Ungur, L.; Chibotaru, L. F.; Propper, K.; Meents, A.;
̈
Dittrich, B. Angew. Chem., Int. Ed. 2013, 52, 11817−11821; Angew.
ASSOCIATED CONTENT
* Supporting Information
■
Chem. 2013, 125, 12033−12037.
S
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Frenking, G.; Sidhu, N. S.; Dittrich, B. J. Am. Chem. Soc. 2013, 135,
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General syntheses; UV−vis−NIR, EPR, crystal structure
determination, and magnetic measurements; and theoretical
details. This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Authors
guybertrand@ucsd.edu
hroesky@gwdg.de
kaim@iac.uni-stuttgart.de
frenking@chemie.uni-marburg.de
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
G.; Niepotter, B.; Wolf, H.; Herbst-Irmer, R.; Stalke, D. Angew. Chem.,
̈
H.W.R. and G.B. thank the Deutsche Forschungsgemeinschaft
(DFG RO224/60-I) and the DOE (DE-FG02-13ER16370),
respectively, for financial support. We thank S. Neudeck and
Prof. Dr. F. Meyer for CV and UV−vis−near-IR measurements.
Int. Ed. 2013, 52, 2963−2967; Angew. Chem. 2013, 125, 3036−3040.
(j) Mondal, K. C.; Samuel, P. P.; Tretiakov, M.; Singh, A. P.; Roesky,
H. W.; Stuckl, A. C.; Niepotter, B.; Carl, E.; Wolf, H.; Herbst-Irmer,
̈
̈
R.; Stalke, D. Inorg. Chem. 2013, 52, 4736−4743.
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We thank J. Lubben for checking unit cell parameters of 3. We
̈
thank F. Engelhardt and Prof. Dr. D. Stalke for providing the
crystallographic information on 3+OTf‑. Dedicated to Prof.
Joachim Sauer on the occasion of his 65th birthday.
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