Stabilization of a cobalt-cobalt bond by two cyclic alkyl amino carbenes

Article abstract of DOI:10.1021/ja4123285

(Me₂-cAAC:)₂Co₂ (2, where Me ₂-cAAC: = cyclic alkyl amino carbene,:C(CH₂)(CMe ₂)₂N-2,6-iPr₂C₆H₃)) was synthesized via the reduction of precursor (Me₂-cAAC:Co ^II(μ-Cl)Cl)₂ (1) with KC₈. 2 contains two cobalt atoms in the formal oxidation state zero. Magnetic measurement revealed that 2 has a singlet spin ground state S = 0. The cyclic voltammogram of 2 exhibits both one-electron oxidation and reduction, indicating the possible synthesis of stable species containing 2^?- and 2 ^?+ ions. The latter was synthesized via reduction of 1 with required equivalents of KC₈ and characterized as [(Me ₂-cAAC:)₂Co₂]^?+OTf^- (2^?+OTf^-). Electron paramagnetic resonance spectroscopy of 2^?+ reveals the coupling of the electron spin with 2 equiv ⁵⁹Co isotopes, leading to a (Co^0.5) ₂ state. The experimental Co1-Co2 bond distances are 2.6550(6) and 2.4610(6) A for 2 and 2^?+OTf^-, respectively. Theoretical investigation revealed that both 2 and 2^?+OTf ^- possess a Co-Co bond with an average value of 2.585 A. A slight increase of the Co-Co bond length in 2 is more likely to be caused by the strong π-accepting property of cAAC. 2^?+ is only 0.8 kcal/mol higher in energy than the energy minimum. The shortening of the Co-Co bond of 2^?+ is caused by intermolecular interactions.

Full text of DOI:10.1021/ja4123285

Communication
pubs.acs.org/JACS
Stabilization of a Cobalt−Cobalt Bond by Two Cyclic Alkyl Amino
Carbenes
Kartik Chandra Mondal,† Prinson P. Samuel,† Herbert W. Roesky,*,† Elena Carl,† Regine Herbst-Irmer,†
Dietmar Stalke,*,† Brigitte Schwederski,‡ Wolfgang Kaim,‡ Liviu Ungur,§ Liviu F. Chibotaru,§
Markus Hermann,⊥ and Gernot Frenking*,⊥
†Institut fur Anorganische Chemie, Georg-August-Universitat, Tammannstraße 4, 37077 Gottingen, Germany
̈
̈
̈
̈
‡Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
̈
§DQPC and INPAC, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium
⊥Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Strasse, 35032 Marburg, Germany
̈
S
* Supporting Information
Co(I) was synthesized by reacting (Ph3P:)2CoICl with 2 equiv
of NHC:.6b Further reduction of (NHC:)2CoICl with metallic
sodium or sodium amalgam leads to the C−H bond activation of
NHCs, which finally results in the formation of
(NHC:)2CoIIR2.6e To our surprise, so far, two-valent Co(0)
species have not been stabilized by NHC:. A compound
containing one Co(0) atom should have one unpaired6c electron,
which might be why it initiates C−H bond activation.6a,b
ABSTRACT: (Me2-cAAC:)2Co2 (2, where Me2-cAAC: =
cyclic alkyl amino carbene, :C(CH2)(CMe2)2N-2,6-
iPr2C6H3)) was synthesized via the reduction of precursor
(Me2-cAAC:CoII(μ-Cl)Cl)2 (1) with KC8. 2 contains two
cobalt atoms in the formal oxidation state zero. Magnetic
measurement revealed that 2 has a singlet spin ground
state S = 0. The cyclic voltammogram of 2 exhibits both
one-electron oxidation and reduction, indicating the
possible synthesis of stable species containing 2•− and
2•+ ions. The latter was synthesized via reduction of 1 with
required equivalents of KC8 and characterized as [(Me2-
cAAC:)2Co2]•+OTf− (2•+OTf−). Electron paramagnetic
resonance spectroscopy of 2•+ reveals the coupling of the
electron spin with 2 equiv 59Co isotopes, leading to a
(Co0.5)2 state. The experimental Co1−Co2 bond distances
are 2.6550(6) and 2.4610(6) Å for 2 and 2•+OTf−,
respectively. Theoretical investigation revealed that both 2
and 2•+OTf− possess a Co−Co bond with an average
value of 2.585 Å. A slight increase of the Co−Co bond
length in 2 is more likely to be caused by the strong π-
accepting property of cAAC. 2•+ is only 0.8 kcal/mol
higher in energy than the energy minimum. The
shortening of the Co−Co bond of 2•+ is caused by
intermolecular interactions.
However, [(NHC:)2Co]+BPh4 , which contains a two-coordi-
−
nated Co(I), can be synthesized from (NHC:)2CoICl with three-
coordinated Co(I).6a Recently, square planar [(NHC:)4-
6d
Co]+BPh4
was prepared by reacting (Ph3P:)2CoICl with a
−
less bulky NHC in a 1:4 molar ratio in the presence of NaBPh4.
It is well known that NHCs have an enormous impact on the
production of compounds with low-valent elements through the
formation of donor−acceptor coordinate bonds.7 The carbene
carbon atom of NHC: is bound to two σ-withdrawing and two π-
donating N-atoms. When the cyclic alkyl amino carbene
(cAAC:) is used instead of the NHC:, one σ-withdrawing and
π-donating N-atom is replaced by one σ-donating quarternary C-
atom. The cAAC: is becoming more nucleophilic but also more
electrophilic when compared with NHC:.8−10 Hence, several
radical species of main-group elements were stabilized by
cAAC:.9
Keeping such crucial differences in mind, we explored the
synthesis, structure, and theoretical analysis of (Me2-
cAAC:)2Co2 (2, where Me2-cAAC: = :C(CH2)(CMe2)2N-2,6-
iPr2C6H3), with two cobalt atoms in the formal oxidation state
zero, and its radical cationic analogue, [(Me2-cAAC:)2Co2]•+-
OTf− (2•+OTf−) (Scheme 1). Furthermore, the nature of the
Co−Co bond11−13 is discussed, which is still an interesting topic
in terms of the existence of metal−metal bonds.
obalt compounds are often used as efficient catalysts for
1
C−F bond activations,2 and
Corganic transformations,
initiators for polymerization reactions.3 The N-heterocyclic
carbene (NHC: = :C[N(2,6-iPr2C6H3)CH]2)) was found to be
very advantageous toward the stabilization of several metal-
containing species that show a broad range of important catalytic
properties.1 They were utilized to stabilize monoatomic metals
such as Ni4a,b and Pd4c in their formal zero oxidation state.
Besides transition metals, molecular species (Si2, Ge2, Sn2, P2,
As2, and B2)5 have also been prepared in the presence of various
NHCs.
(Me2-cAAC:CoII(μ-Cl)Cl)2 (1), an analogue of
(NHC:CoII(μ-Cl)Cl)2,6a,b was synthesized in 80% yield when
Me2-cAAC: and CoCl2 were reacted in a 1:1 molar ratio in THF.
The product was isolated as a light violet-blue crystalline powder.
1 is insoluble in toluene and benzene; when we attempted to
crystallize it from THF, initially a dark blue solution was formed,
which slowly turned to a green-blue color, indicating
To date, several NHC-stabilized cobalt compounds have been
reported. (NHC:CoII(μ-Cl)Cl)2 and (NHC:)2CoIICl2 are
formed when NHC is reacted with CoCl2 in 1:1 and 2:1 molar
ratio, respectively.6 (NHC:)2CoICl with three-coordinated
Received: December 4, 2013
Published: January 17, 2014
© 2014 American Chemical Society
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Scheme 1. Syntheses of the Chemical Species 2 and 2•+ from 1
under KC8 Reduction
decomposition. A comparable decomposition was reported for
(NHC:CoII(μ-Cl)Cl)2.6a
The dark blue THF solution of 1 was further reduced to 2
utilizing 4 equiv of KC8 (Scheme 1). Compound 2 crystallizes as
large black shiny needles in 65% yield from the dark brown
solution stored at −32 °C. 2 is soluble in THF, toluene, and
benzene. Precursor 1 decomposes above 293 °C, while 2 melts in
the range of 249−250 °C. The 1H and 13C resonances recorded
in C6D6 show line broadening; in contrast, the reported
Co2(CO)6(HCCC6H10−OH) exhibits no line broadening in
the NMR spectra.11 All resonances of 2 are upfield shifted when
compared with those of free Me2-cAAC:. Compound 2 was also
characterized by ESI-MS (m/z [M+] (%): 688 (100))
(Supporting Information (SI)).
Compound 2·toluene crystallizes in the orthorhombic space
group Pna21. 2 contains two Co(0) atoms, each coordinated by
one Me2-cAAC: carbene carbon and additionally η6-bound to the
carbon perimeter of the diisopropylaniline part of another
carbene molecule (Figure 1). Each of the Co(0) atoms adopts
the coordination environment of a half-sandwich complex. The
Co1−Co2 bond distance of 2 (2.6550(6) Å) is close to the values
quoted in the literature for weak Co−Co closed-shell
interactions.12 For comparison, the Co−Co single bond length
in (CO)4Co−Co(CO)4 is 2.52 Å.13 The Co−Caromatic bond
distances are in the range of 2.075(2)−2.187(2) Å, close to the
values reported for cobalt half-sandwich complexes. The Co1−
C1 and Co2−C21 bond lengths of 2 are 1.854(2) and 1.853(2)
Å, which are shortened by ∼0.2 Å when compared with those of
1a (2.038(4)−2.042(4) Å). The reported Co−CNHC bond
lengths range from 1.87 to 2.09 Å.6,14
It has been found that few carbenes can stabilize the radical
center on carbene carbon atoms and form M−C single bonds.15
On one hand, the CcAAC−N bond length becomes longer due to
the stabilization of the radical electron on the carbene carbon.16
On the other hand, when the cAAC: carbene forms a coordinate
bond, the CcAAC−N bond becomes shorter (1.298(5)−1.301(5)
Å in 1a, see SI).17 The Ccarbene−N bond lengths in 2 are
comparably short (1.369(3) Å for C1−N1 and 1.367(3) Å for
C21−N2), which might be due to strong electronic back-
donation from cobalt to the carbene carbon atom.10
The magnetic susceptibility measurement on 2 confirms the
diamagnetic spin ground state (S = 0), similar to that of
Co2(CO)6(HCCC6H10−OH) with a Co−Co bond length of
2.46482(13) Å.11 On the basis of theoretical charge density
studies and DFT calculation, it has been concluded that
Figure 1. Molecular structures of (a) 2 and (b) 2•+. H-atoms are omitted
for clarity. Selected experimental [calculated at B3LYP/def2-SVP for the
ground state] bond lengths [Å] and angles [°] of 2/2•+: Co1−C1
1.854(2)/1.891(2) [1.854/1.975], Co2−C21 1.853(2)/ 1.920(2)
[1.853/1.891], Co1−Co2 2.6550(6)/2.4610(6) [2.586/2.596], C1−
N1 1.369(3)/1.338(3) [1.364/1.326], C21−N2 1.367(3)/1.328(3)
[1.364/1.367]; N1−C1−Co1 125.52(17)/123.59(18) [124.9/122.5],
C2−C1−Co1 129.38(17)/129.87(17) [129.2/129.2], N1−C1−C2
104.9(19)/ 106.4(2) [105.7/108.4], N2−C21−Co2 124.92(17)/
123.22(18) [124.9/126.2], C22−C21−Co2 129.45(16)/129.96(18)
[129.2/126.9], N2−C21−C22 105.50(19)/106.47(2) [105.7/106.6].
Co2(CO)6(HCCC6H10−OH) does not possess a direct
Co−Co bond, as expected on the basis of obeying the
conventional 18-electron rule rather a singlet diradical character
as predicted by DFT calculation.11a
The cyclic voltammogram of a dichloromethane solution of 2,
containing 0.1 M n-Bu4NPF6 as an electrolyte, shows a one-
electron reversible reduction of E1/2 = −0.80 V and a one-
electron reversible oxidation at E1/2 = +0.45 V versus (Cp2Fe)/
(Cp2Fe)+, indicating the formation of both radical anion 2•− and
radical cation 2•+ (Figure 2). Indeed, the radical cation 2•+ can be
synthesized as 2•+OTf− in 90% yield when 1 is reduced with 3
equiv of KC8 in the presence of LiOTf (Scheme 1). Black blocks
of 2•+OTf− are formed when a THF/toluene solution (5:2) is
stored at −32 °C for 1 week in a refrigerator. The dark blocks of
2•+OTf− do not change or decompose when they are heated to
325 °C.
Compound 2•+OTf− crystallizes in the monoclinic space
group P21/n. The molecular unit of 2•+OTf− is similar (Figure 1)
to that of 2. To balance the charge, it contains one disordered
triflate anion (OTf−). The Co−Co distance is decreased to
2.4610(6) Å in 2•+OTf− when compared with that of 2
(2.6550(6) Å). The C−N bond lengths slightly decrease
(1.328(3)−1.338(3) Å), while the Co−CcAAC bond distances
increase (1.891(2)−1.920(2) Å), compared with those of 2. This
is in line with the decrease of back-bonding from dicobalt to both
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Figure 2. Cyclic voltammogram of a dichloromethane solution of 2,
containing 0.1 M n-Bu4NPF6 as an electrolyte.
of the carbenes due to the decrease of electron density on the
dicobalt unit of 2•+OTf−. A superposition image and tables of
bond lengths and angles of 2 and 2•+OTf− are given in the SI.
At room temperature, the paramagnetic THF solutions of
2•+OTf− exhibit an unresolved EPR signal, about 1200 G wide.
At 115 K in frozen solution, the spectrum revealed separate g
tensor components at g1 = 2.346, g2 = 2.07, and g3 = 2.008. The
most revealing feature is the g1 component, which shows a clear
splitting by 2 equiv 59Co (I = 7/2) nuclei with the proper
intensity distribution 1:2:3:4:5:6:7:8:7:6:5:4:3:2:1 for coupling
of one electron spin with two Co nuclei (Figure S4). The
coupling constant of only 54 G is also compatible with metal−
metal spin delocalization and thus with valence averaging to a
new (Co0.5)2 species (class III mixed-valency according to the
Robin and Day definition). Hyperfine structure for g2 is not
resolved at X band frequency, and g3 shows an 8-line splitting
from 59Co with a coupling constant of 33 G.
We optimized the geometry of 2 using density functional
theory (DFT) at the B3LYP/def2-SVP level.18 The calculated
structure is in good agreement with the experimental values (see
Figure 1 and SI). We analyzed the bonding situation in the
molecule with the Energy Decomposition Analysis using Natural
Orbitals for Chemical Valency method (EDA-NOCV)19 at the
BP86/TZVP+ level using the B3LYP/def2-SVP optimized
geometry. The full set of numerical results is shown in Table
S13. The most important question concerns the nature of the
Co−Co and Me2-cAAC−Co interactions; it can be answered by
inspection of the deformation densities, which come from the
EDA-NOCV calculations, and the associated stabilization
energies, which come from the pairwise orbital interactions
between the two fragments that are shown at the top of Figure 3.
There are four pairs of orbital interactions that dominate the
total ΔEorb term. The largest contribution, ΔEorb(1) = −71.8
kcal/mol, comes from the σ donation of the Me2-cAAC donor
orbitals into a Co−Co bonding orbital. The second largest
contribution, ΔEorb(2) = −47.7 kcal/mol, comes also from σ
donation Me2-cAAC→CoCo←cAAC-Me2, but the shape of the
deformation density, Δρ(2), shows that the acceptor orbital of
the Co2 fragment has antibonding character (Figure 3b). Since
ΔEorb(1) is significantly larger than ΔEorb(2), it can be concluded
that there is a Co−Co bond in 2.
Figure 3. Results of the EDA-NOCV calculations of 2: deformation
densities Δρ of the four most important pairs of interacting orbitals of
the two fragments, which are shown at the top of the figure, and
associated stabilization energies ΔEorb. The charge flow of the
deformation densities is red→blue.
Figure 4. Laplacian distribution of 2 in the Me2-cAAC−Co−Co plane.
point of the Co−Co bond. The other two orbital contributions,
ΔEorb(3) = −44.0 kcal/mol and ΔEorb(4) = −40.4 kcal/mol, are
easily identified as components of the Me2-cAAC←CoCo→
cAAC-Me2 π back-donation. Thus, 2 has a Co−Co bond, and it
possesses donor−acceptor bonds between the Me2-cAAC
ligands and the Co2 moiety, which has significant π back-
donation.
This is supported by a Quantum Theory of Atoms in
Molecules (QTAIM)20 calculation, which shows a Co−Co bond
path. Even more conclusive is the shape of the Laplacian
distribution in the Me2-cAAC−Co−Co plane (Figure 4). There
is clearly a small area of charge concentration at the bond critical
We also optimized the geometry of the radical cation 2•+
without the counterion. The calculated bond lengths and angles
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are in good agreement with experiment (Figure 1), except for the
Co−Co distance. X-ray structure analysis shows that the Co−Co
bond in the cation is significantly shorter (2.4610 Å) than that in
the neutral species (2.6551 Å), while the calculations give nearly
the same distances for 2 and 2•+. We optimized the geometry of
the cation with a frozen Co−Co bond length of 2.4610 Å, which
gave a structure that is only 0.8 kcal/mol higher in energy than
the energy minimum. It is conceivable that the Co−Co bond
shortening in 2•+ comes from intermolecular interactions.
One cobalt atom has nine valence electrons. Since 2 has a
singlet ground state, the cobalt atoms should have at least one
electron-sharing bond. Molecular orbital analysis shows that the
HOMO−1 (Figure S8) is clearly a Co−Co bond which comes
from a hybridization of d(z2), s, and p(z). The NBO calculations
also give a Co−Co bond with an occupation of 1.93e that has
15% s, 6% p, and 75% d character and a Wiberg bond order of
0.87. This leads to a straightforward explanation of the bonding
situation where each Co atom has an 18-electron configuration.
This means that the Co−Co bond is not just a weak, closed-shell
interaction but a real Co−Co single bond, which has a bond
length of 2.66 Å. This is somewhat longer than a typical single
bond (2.52 Å),13,21 which could be caused by the strong π-
accepting property of cAAC.10 The phenyl groups and the cAAC
ligands donate altogether eight electrons to each Co atom of the
Co2 fragment, which therefore attains an 18e configuration. The
ionization takes place from the HOMO, which is a Co−ligand
orbital. The large stabilization which comes from the orbital
interactions, ΔEorb(1) (Figure 3a), is, according to this,
formation of the Co−Co bond between the unpaired electrons
of the fragments.
In conclusion, we have synthesized (Me2-cAAC:)2Co2(0) (2)
with two cobalt atoms in the formal oxidation state zero,
stabilized by two cyclic alkyl amino carbenes. 2 possesses a
diamagnetic spin ground state. Cyclic voltammetry shows that 2
can be reversibly reduced or oxidized by one electron to generate
the radical anion 2•− or cation 2•+. The Co−Co bond length is
shortened by ∼0.2 Å in 2•+OTf− when 2 undergoes one-electron
oxidation to produce 2•+. Low-temperature electron para-
magnetic resonance spectroscopy of 2•+ reveals the coupling of
the electron spin with 2 equiv 59Co isotopes, leading to a (Co0.5)2
state. Both 2 and 2•+OTf− can be synthesized from their
precursor, (Me2-cAAC:CoII(μ-Cl)Cl)2 (1), under KC8 reduc-
tion. Moreover, (Me2-cAAC:)2CoICl (3), with three-coordinate
Co(I), was prepared by the reaction of 1, Me2-cAAC:, and KC8 in
a molar ratio of 1:2:2 (SI).
ACKNOWLEDGMENTS
■
Dedicated to Prof. C. N. R. Rao on the occasion of his 80th
birthday. H.W.R. thanks the Deutsche Forschungsgemeinschaft
(DFG RO 224/60-I) for financial support. We thank S. Neudeck,
Dr. S. Demeshko, and Prof. Dr. F. Meyer for magnetic studies,
CV, and UV−vis measurements.
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ASSOCIATED CONTENT
■
S
* Supporting Information
(16) Mondal, K. C.; Samuel, P. P.; Tretiakov, M.; Singh, A. P.; Roesky,
Synthesis of 1, 1a, 1b, 1c, 2•+OTf−, and 3; UV, EPR, magnetism,
crystal structure determination, and theoretical details. This
material is available free of charge via the Internet at http://pubs.
acs.org.
H. W.; Stuckl, A. C.; Niepotter, B.; Carl, E.; Wolf, H.; Herbst-Irmer, R.;
̈
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Stalke, D. Inorg. Chem. 2013, 52, 4736.
(17) See Supporting Information of Li, Y.; Mondal, K. C.; Roesky, H.
W.; Zhu, H.; Stollberg, P.; Herbst-Irmer, R.; Stalke, D.; Andrada, D. M. J.
Am. Chem. Soc. 2013, 135, 12422.
(18) The description of the theoretical methods is given in the SI.
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AUTHOR INFORMATION
■
Corresponding Authors
hroesky@gwdg.de
dstalke@chemie.uni-goettingen
frenking@chemie.uni-marburg.de
(21) Pyykko, P.; Atsumi, M. Chem.Eur. J. 2009, 15, 186.
̈
Notes
The authors declare no competing financial interest.
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