Stabilization of a two-coordinate mononuclear cobalt(0) compound

Article abstract of DOI:10.1002/chem.201403525

Compound (Me₂-cAAC:)₂Co⁰ (2; Me ₂-cAAC:=cyclic (alkyl) amino carbene;:C(CH₂)(CMe ₂)₂N-2,6-iPr₂C₆H₃) was synthesized by the reduction of the precursor (Me₂-cAAC:) ₂Co^ICl (1) with KC₈ in THF. The cyclic voltammogram of 1 exhibited one-electron reduction, which suggests that synthesis of a bent 2-metallaallene (2) from 1 should be possible. Compound 2 contains one cobalt atom in the formal oxidation state zero, which is stabilized by two Me₂-cAAC: ligands. Bond lengths from X-ray diffraction are 1.871(2) and 1.877(2)? with a C-Co-C bond angle of 170.12(8)?. The EPR spectrum of 2 exhibited a broad resonance attributed to the unique quasi-linear structure, which favors near degeneracy and gives rise to very rapid relaxation conditions. The cAAC-Co bond in 2 can be considered as a typical Dewar-Chatt-Duncanson type of bonding, which in turn retains 2.5electron pairs on the Co atom as nonbonding electrons.

Full text of DOI:10.1002/chem.201403525

DOI: 10.1002/chem.201403525
Communication
&
EPR Spectroscopy
Stabilization of a Two-Coordinate Mononuclear Cobalt(0)
Compound
Kartik Chandra Mondal,[a] Sudipta Roy,[a] Susmita De,[b] Pattiyil Parameswaran,*[b]
Birger Dittrich,*[c] Fabian Ehret,[d] Wolfgang Kaim,*[d] and Herbert W. Roesky*[a]
In memory of Mike Lappert
[(CO)4CoꢀCo(CO)4] along with C60.[2] Substituted phosphine li-
gands can stabilize mononuclear Co0 species with the general
formula [Co0(:PR3)4] (R = Me, Ph), which have a d9 electronic
configuration with tetrahedral symmetry.[3] N-Heterocyclic car-
bene (NHC:) can form stable mono (NHC:CoII(m-Cl)Cl)2 or bis
adducts (NHC:)2CoIICl2 when it reacts with CoCl2 in 1:1 or 2:1
molar ratio, respectively.[4] (NHC:)2CoICl with three-coordinate
cobalt(I) was prepared by reacting (Ph3P:)2CoICl with two
equivalents of NHC:.[4b] Two-coordinate stable and isolable
cobalt(0) species have not been characterized neither with
phosphine nor with NHC: as ligands. The synthesis of
(NHC:)2Co0 from (NHC:)2CoICl was not successful.[4d] The reduc-
tion of (NHC:)2CoICl with metallic sodium or sodium amalgam
leads to the CꢀH bond activation of NHCs, which finally results
in the formation of (NHC:)2CoIIR2.[4d] A two-coordinate Co0 com-
pound should have an unpaired[3,4c] electron, which might be
Abstract: Compound (Me2-cAAC:)2Co0 (2; Me2-cAAC:=
cyclic (alkyl) amino carbene; :C(CH2)(CMe2)2N-2,6-iPr2C6H3)
was synthesized by the reduction of the precursor (Me2-
cAAC:)2CoICl (1) with KC8 in THF. The cyclic voltammogram
of 1 exhibited one-electron reduction, which suggests
that synthesis of a bent 2-metallaallene (2) from 1 should
be possible. Compound 2 contains one cobalt atom in the
formal oxidation state zero, which is stabilized by two
Me2-cAAC: ligands. Bond lengths from X-ray diffraction are
1.871(2) and 1.877(2) ꢀ with a C-Co-C bond angle of
170.12(8)8. The EPR spectrum of 2 exhibited a broad reso-
nance attributed to the unique quasi-linear structure,
which favors near degeneracy and gives rise to very rapid
relaxation conditions. The cAACꢀCo bond in 2 can be con-
sidered as a typical Dewar–Chatt–Duncanson type of
bonding, which in turn retains 2.5 electron pairs on the
Co atom as nonbonding electrons.
the reason why it initiates CꢀH bond activation.[4e]
+
¯
[(NHC:)2Co] BPh4 with a two-coordinate cobalt(I) was synthe-
sized from a three-coordinate cobalt(I) species (NHC:)2CoICl by
ꢀ [4a]
removing the chloride ion and replacing it by BPh4
.
Recent-
ꢀ [4e]
ly, square-planar [(NHC:)4Co]+[BPh4 ] was synthesized by re-
acting (Ph3P:)2CoICl with a less bulky NHC: in a 1:4 molar ratio
in the presence of NaBPh4. Herein, we report the synthesis,
characterization, EPR, and theoretical calculations of two-coor-
dinate mononuclear (Me2-cAAC:)2Co0 (2).
Over the last few decades, several fascinating Co0 species were
reported.[1] They can mediate several unusual unusual transfor-
mations in organic chemistry and have received great impor-
tance as active catalysts. Carbon monoxide stabilizes [Co2(CO)8]
species, which mainly exists in
a CO-bridged dimeric
Since the cAAC carbene is a better s donor and p accept-
or,[5] we choose the (Me2-cAAC:)2CoICl (1)[6] as a precursor for
the synthesis of 2. The cyclic voltammogram of a dimethylfor-
[(CO)3Co(m-CO)2Co(CO)3] form, but can be cocrystallized as
[a] Dr. K. C. Mondal, Dr. S. Roy, Prof. Dr. H. W. Roesky
Institut fꢀr Anorganische Chemie, Universitꢁt Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: hroesky@gwdg.de
[b] Dr. S. De, Dr. P. Parameswaran
Dept. of Chemistry, National Institute of Technology Calicut
NIT Campus P.O., Kozhikhode - 673 601, Kerala (India)
E-mail: param@nitc.ac.in
[c] Dr. B. Dittrich
Universitꢁt Hamburg
Martin-Luther-King-Platz 6, 20146-Hamburg (Germany)
E-mail: birger.dittrich@chemie.uni-hamburg.de
[d] Dr. F. Ehret, Prof. Dr. W. Kaim
Universitꢁt Stuttgart, Institut fꢀr Anorganische Chemie
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
E-mail: kaim@iac.uni-stuttgart.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403525. It contains the synthesis, X-ray
single-crystal diffraction, UV/Vis, catalysis, and theoretical calculations
data.
Figure 1. Cyclic voltammogram of a dimethylformamide solution of 1, con-
taining 0.1m nBu4NPF6 as an electrolyte. The potentials refer to the decame-
thylferrocene/ferrocenium system as a standard.
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Communication
calculated equilibrium geometries of the doublet
state (2m) and the quartet state (2m) of CoL2 sug-
gest that the doublet state 2m resembles more
closely to the crystal structure of 2. Note that the
quartet state 2m’ is 16.9 kcalmolꢀ1 higher in energy
than the doublet state 2m (Figure S3 in the Sup-
porting Information). Thus, the geometrical data and
relative energy indicate the doublet 2m as the
ground state. The Co center in 2m exists in a bent
geometry with the bending angle of 168.28. The di-
hedral angle between the planes of the two cAAC: ligands in
2m (78.88) indicates that the two cAAC: ligands are almost
perpendicular to each other. The CoꢀCcAAC bond length is
slightly longer, and the CcAACꢀN bond lengths are slightly short-
er in 2m than the corresponding bond lengths in the previ-
ously reported (Me2ꢀcAAC:)2Co2.[11] Note also that the CcAACꢀN
bond length is significantly longer than the CcAACꢀN bond
length in free cAAC:, which signifies Co!CꢀN p*-back dona-
tion. Hence, these geometrical data suggest that 2m can be
considered as a bent 2-metallaallene.[12]
Scheme 1. Syntheses of (Me2-cAAC:)2Co0 (2) from (cAAC:)2CoICl (1).
mamide solution of 1, containing 0.1m nBu4NPF6 as electrolyte,
showed a one-electron quasi-reversible reduction at E1/2
ꢀ0.57 V versus (Cp*2Fe)/(Cp*2Fe)+ indicating the possible for-
mation of 2 (Figure 1). Indeed, compound 2 was chemically
synthesized from 1 (Scheme 1).
=
The 1:1 molar mixture of (Me2-cAAC:)2CoICl (1) and KC8 react-
ed in THF to give a dark blue solution, which was filtered
(Scheme 1). This solution was slowly concentrated under
vacuum to form dark shiny needles of (Me2-cAAC:)2Co0 (2),
which are separated by filtration and dried in vacuum, giving
98% yield. The volume of the solvent should be maintained to
carry out a cleaner reaction to obtain such high yields (see the
Supporting Information). However, it is important to mention
that the yield of 2 dropped upon decreasing the volume of
the solvent. Compound 2 can also be synthesized within fif-
teen minutes by the reduction of precursor 1 at room temper-
ature in a similar yield when 2.2 equivalents of LiNiPr2 (LDA)
were employed as a reducing agent, instead of one equivalent
of KC8 (see the Supporting Information). The detailed synthesis,
purification, and characterization of compound 2 are given in
the Supporting Information. Compound 2 is stable in an inert
atmosphere and decomposes above 1348C. It produces a blue
color when dissolved in organic solvents, such as THF, toluene,
The natural bond orbital (NBO) charge analysis revealed that
the Co center in 2m is positively charged (0.122 e), and hence
the CcAAC!Co s donation can be considered as being weaker
!
than the CcAAC Co p-back donation.[10] This is reflected in the
smaller Wiberg bond index of the CꢀN bond (1.31) compared
with the corresponding value in free cAAC: (1.50). On the one
hand, the NBO spin density in 2m showed significant accumu-
lation of a spin on the Co atom (1.526). On the other hand,
the b-spin density is delocalized over the cAAC: ligand with
major coefficient on the carbene C and the N atoms (0.253;
Figure 3a). This results in one unpaired electron of 2m. The
Mulliken charge and spin density analysis also give similar re-
sults (Table S3 in the Supporting Information). Such accumula-
tion of excess a-spin density at the metal center and delocali-
zation of the b-spin over the cAAC: ligands was also observed
for the recently reported (cAAC:)2Mn complex.[13] The bonding
1
and benzene. H NMR spectrum of 2 showed broad resonances
ranging from d=ꢀ2.5 to 10 ppm indicating the paramagnet-
ic[7–9] nature of the solution, as was expected. Compound 2
can be further characterized by MS (EI) spectrometry (m/z
(100%): 629.4; see the Supporting Information). The reaction
energy for the formation of 2 from 1 was calculated (at the
M06/def2-TZVPP//BP86/def2-SVP level of theory) to be exo-
thermic by ꢀ17.2 kcalmolꢀ1 (KC8 as a reducing agent), which
also indicates the feasibility of the reaction (Scheme 1).
¯
Compound 2 crystallizes in the triclinic space group P1. The
central cobalt atom is bound to two carbene carbon atoms
adopting a two-coordinate bent geometry (Figure 2). The C-
Co-C bond angle is 170.12(8)8, which is far wider when com-
pared with that of precursor 1 (122.33(10)8). The CoꢀCcAAC and
NꢀCcAAC bond lengths of 2 are 1.871(2)/1.877(2) and 1.343(2)/
1.349(2) ꢀ, respectively, which are close to the values of
1.920(2)/1.932(2) and 1.335(3)/1.333(3) found in 1.[6] The slight
shortening of the CoꢀCcAAC bond lengths and increasing of
CcAACꢀN bond lengths might be due to the Co!CcAAC p-back
donation.
Figure 2. Molecular structure of compound 2. Hydrogen atoms are omitted
for clarity. Selected experimental [calculated ones at the BP86/def2-SVP level
for the ground state] bond lengths [ꢀ] and angles [8] from single-crystal X-
ray diffraction: Co1ꢀC1 1.871(2) [1.883], Co1ꢀC21 1.877(2) [1.884], C1ꢀN1
1.343(2) [1.362], C21ꢀN2 1.349(2) [1.362]; C1-Co1-C21 170.12(8) [168.2], C2-
C1-N1 106.67(16) [106.3], N2-C21-C22 106.81(15)[ 106.3], C2-C1-Co1
122.51(14) [121.9], N1-C1-Co1 130.69(13) [131.4].
We have performed theoretical calculations at the M06/def2-
TZVPP//BP86/def2-SVP level of theory for the different spin
states of the two-coordinate cobalt model compound CoL2
(L=Me2-cAAC:) (2) using Gaussian 09 program package.[10] The
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Communication
in the two-coordinate 2m is described by the follow-
ing molecular orbital (MO) and the EDA-NOCV analy-
sis (see the Supporting Information).[10]
The LꢀCoꢀL bonding in CoL2 (2m) (L=Me2-cAAC:)
can be explained as follows (Figure S4 in the Sup-
porting Information). Thirteen valence electrons are
available for the
CcAAC-Co-CcAAC bond formation,
which are comprised of nine valence electrons from
Co (4s23d7) and one s lone pair from each of the
cAAC: ligands. The molecular orbital (MO) analysis re-
vealed that four of the 3d orbitals on Co are doubly
occupied and the fifth d orbital is singly occupied.
This fifth d orbital is significantly mixed with the 4s
orbital on the Co center. This leads to two hybrid or-
bitals on the Co atom, namely, one empty and anoth-
er with an unpaired electron (Figure 3b). Thus, the
vacant orbital on Co can accept electrons from the
bonding combination of the s-lone pairs of electrons
of the two cAAC: ligands (Figure 3c). However, there
is no proper symmetric d orbital on the Co that can
interact with the antibonding combination of the
two s-lone pairs of the two cAAC: ligands (Fig-
ure 3d). Bending of the CcAAC-Co-CcAAC results in the
mixing of the s-lone pair and the CꢀN p*-MO of the
cAAC: ligand. The resultant orbitals in turn could
overlap more effectively with the 3d orbital on the
Co atom (Figure 3e, f). This interaction can be consid-
ered as the donation of the 3d electrons from Co to
the vacant CꢀN p*-MO, which leads to the CꢀN bond
elongation, as well as reduction of the CꢀN Wiberg
bond indices. The remaining four electrons on Co
Figure 3. Plot of a) NBO spin density, where the blue color corresponds to a-spin density
and the green color corresponds to b-spin density b)–f) important occupied molecular
orbitals of CoL2, L=Me2-cAAC (2m) at the M06/def2-TZVPP//BP86/def2-SVP level of
theory.
atom are retained as two lone pairs (Figure S4 in the Support-
ing Information). The molecular orbitals analysis suggests that
the bonding in bent 2-metallaallene (2m) can be best de-
scribed as a slightly modified bonding situation similar to that
of carbones.[14] Herein, the CoꢀCcAAC bonding can be consid-
ered as that of typical Dewar–Chatt–Duncanson type of bond-
ing.[15] This results in two CoꢀCcAAC partial double bonds
and 2.5 electron pairs on Co as nonbonding electrons.
This bonding interaction can be further analyzed in a quan-
titative manner by EDA-NOCV approach implemented in
ADF 2013.01.[10]
As a d9 transition-metal compound with a calculated d(z2)1
configuration, compound 2 should exhibit EPR activity. Indeed,
an extremely broad but unresolved (g components, 59Co hy-
perfine coupling) resonance was observed in the X-band ex-
periment at 4 K in the solid and in THF solution (Figure 4).
However, no EPR resonance was observed at room tempera-
Figure 4. X-band EPR spectrum of solid sample of 2 at 4 K. The resonance is
unresolved due to the 59Co hyperfine coupling.
ture. This unusual behavior for a S=1= species is attributed to
2
the unique quasi-linear structure, which favors near degenera-
cy and gives rise to very rapid relaxation conditions.
smoothly under different conditions in the presence of 2 as
catalyst (see the Supporting Information). Yoshikai and co-
workers have developed a synthetic route of alkenylation of N-
methylpyrrolidinone by using CoBr2 and pyphos (pyphos=
Ph2P(CH2)2-C5H4N) as a ligand in the presence of 60 mol% of
tBuCH2MgBr at room temperature.[18] However, the actual cata-
lyst might be an intermediate species, which has never been
isolated and characterized. Most often, cobalt-catalyzed organ-
Cobalt compounds are often utilized as efficient catalysts for
transformations in organic chemistry.[16–18] The successful syn-
thesis of compound 2, having a cobalt(0) atom with a low co-
ordination geometry, inspired us to examine its catalytic activi-
ty. C2-Alkenylation of N-pyrimidylindole with diphenylacetylene
and cyclotrimerization of diphenylacetylene proceeded
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Communication
ic transformations (such as arylation,[18a] alkylation,[18b] and alke-
nylation[18c]) involve the use of Grignard reagents (RMgX) as
common ingredients, where the role of RMgX is not well
known. Herein, we showed that well characterized (Me2-
cAAC:)2Co0 (2) can catalyze an alkenylation process without
using any Grignard reagent (Table S6 in the Supporting Infor-
mation).
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In conclusion, a mononuclear cobalt(0) species was stabi-
lized by two cyclic alkyl(amino) carbenes, which is formulated
as (Me2-cAAC:)2Co0 (2). It was synthesized from the precursor
(Me2-cAAC:)2CoICl (1) in 98% yield. Compound 2 is stable in an
inert atmosphere for three months and decomposes above
1348C. It was characterized by X-ray single-crystal diffraction
and further studied by EPR and theoretical calculations. The
cobalt atom of 2 adopts a two-coordinate bent geometry with
a C-Co-C bond angle of 1708. A broad EPR resonance was ob-
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[10] See the Supporting Information for details of computational methodol-
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served, as was expected for a d9 system (S=1= ) with unique
2
[11] See the main text of Ref. [6].
quasi-linear structure favoring a near-degenerate electronic
state which is undergoing very rapid relaxation. The CcAACꢀCo
bond in 2 can be considered as a typical Dewar–Chatt–Dun-
canson type of bonding with the retention of 2.5 nonbonding
pairs of electrons on the Co atom. The geometry and bonding
analysis indicates that it can likewise be considered as 2-cobal-
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(see the Supporting Information).[19]
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Keywords: carbenes · cobalt · density functional calculations ·
EPR spectroscopy · synthetic methods
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Published online on July 22, 2014
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11649
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