Square-planar cobalt complexes with monodentate N-heterocyclic carbene ligation: Synthesis, structure, and catalytic application

Article abstract of DOI:10.1021/om200527y

The unique cobalt(I)-NHC complex [Co(IEt)₄][BPh₄] (1) (IEt = 1,3-diethyl-4,5-dimethylimidazole-2-ylidene) and its analogues employing other carbene ligands were prepared by the reactions of [Co(PPh ₃)₃Cl] with free carbene ligands followed by anion exchange. X-ray diffraction revealed 1 features a homoleptic cation [Co(IEt)₄]⁺ with square-planar geometry. Electrochemical studies showed the square-planar complexes support the redox series [Co(IEt)₄]^+/2+/3+, of which the divalent cation has been synthesized and structurally characterized as in [Co(IEt)₄][BF ₄]₂ (2). Both EPR and DFT studies indicated 2 has a low-spin Co(II) center. 1 can efficiently catalyze the oxidative homocoupling reactions of aryl Grignard reagents. Investigations on the stoichiometric reactions of 1 with organic halides and 2 with aryl Grignard reagents established an interesting radical mechanism involving the [Co(IEt) ₄]^+/2+ redox shuttle for these cobalt-NHC complex catalyzed homocoupling reactions.

Full text of DOI:10.1021/om200527y

ARTICLE
pubs.acs.org/Organometallics
Square-Planar Cobalt Complexes with Monodentate N-Heterocyclic
Carbene Ligation: Synthesis, Structure, and Catalytic Application
Zhenbo Mo,† Yuxue Li,† Hung Kay Lee,‡ and Liang Deng*,†
†State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai, People's Republic of China, 200032
‡Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, People's Republic of China
S Supporting Information
b
ABSTRACT: The unique cobalt(I)-NHC complex [Co(IEt)4]
[BPh4] (1) (IEt = 1,3-diethyl-4,5-dimethylimidazole-2-ylidene)
and its analogues employing other carbene ligands were pre-
pared by the reactions of [Co(PPh3)3Cl] with free carbene
ligands followed by anion exchange. X-ray diffraction revealed 1
features a homoleptic cation [Co(IEt)4]+ with square-planar
geometry. Electrochemical studies showed the square-planar
complexes support the redox series [Co(IEt)4]+/2+/3+, of
which the divalent cation has been synthesized and structurally characterized as in [Co(IEt)4][BF4]2 (2). Both EPR and DFT
studies indicated 2 has a low-spin Co(II) center. 1 can efficiently catalyze the oxidative homocoupling reactions of aryl Grignard
reagents. Investigations on the stoichiometric reactions of 1 with organic halides and 2 with aryl Grignard reagents established an
interesting radical mechanism involving the [Co(IEt)4]+/2+ redox shuttle for these cobalt-NHC complex catalyzed homocoupling
reactions.
’ INTRODUCTION
importantly, they are efficient catalysts to mediate the homo-
coupling reactions of aryl Grignard reagents, in which a unique
radical-type mechanism based on a Co(I)/Co(II) redox shuttle
has been disclosed.
The cobalt-catalyzed coupling reaction is one of the traditional
yet very efficient methods for CꢀC bond construction.1 Since the
pioneer work by Kharasch,2 continuing efforts in this field by
Cahiez, Gosmini, Oshima, et al. have led to the emergence of ver-
satile coupling methods,1,3 in which simple cobalt salts in combina-
tion with amine or phosphine ligands are generally applied as the
catalysts. In addition to these catalyst systems, recently,
N-heterocyclic carbenes (NHCs) have been introduced to cobalt
catalysis.4 Although still in its infancy, it has been well established
that the utilization of NHCs could dramatically influence both
the reactivity and selectivity of the catalyzed reactions.4h Success-
ful applications of NHCs in cobalt-catalyzed reactions include
the cross-coupling reaction of aryl halide with aryl Grignard
reagent,4a dehydrohalogenation of alkyl halides with Grignard
reagent,4c sequential cyclization/cross-coupling of 6-halo-1-hex-
ene derivatives with Grignard reagent,4e intramolecular [2+2+2]
cycloaddition of enediynes,4b and so on.
In view of the fine performance of NHC-promoted cobalt
catalysis and in association with our interest in late 3d transition
metal-NHC chemistry,5 we are curious about the chemistry of
cobalt-NHC complexes. Cobalt complexes bearing NHC ligands
are known, but most of the previous studies were focused on
complexes with complicated coordinationenvironments.6 Distinct
from those studies, we report here the synthesis, structure, and
catalytic application of a new category of cobalt-NHC complexes
with exclusively monodentate NHC ligation. Not only do these
cobalt-NHC complexes represent rare examples of homoleptic
cobalt compounds having square-planar geometry,7 but more
’ RESULTS AND DISCUSSION
Homoleptic Cobalt(I)-NHC Complex. The unique cobalt(I)-
NHC complex [Co(IEt)4][BPh4] (1) (IEt = 1,3-diethyl-4,5-
dimethylimidazole-2-ylidene) was prepared as a blue crystalline
solid in 80% yield from the reaction of [Co(PPh3)3Cl]8 with
four equivalents of IEt9 and one equivalent of NaBPh4 in THF
(Scheme 1).10 1 is air- and moisture-sensitive and soluble in polar
solvents such as THF and acetonitrile. Its 1H NMR spectrum in
d8-THF displays seven sets of peaks, at 0.39, 1.98, 3.74, 5.50, 6.65,
6.85, and 7.28 ppm, with the corresponding integral ratio of
6:6:2:2:1:2:2, suggesting a 4:1 ratio of the carbene ligands to the
borate anion [BPh4]ꢀ. The resonances at 3.74 and 5.50 ppm
are corresponding to the two methylene protons on the same
ethyl group, indicative of the restricted rotation of ethyl groups. 1
forms a green solution in THF. Its UVꢀvis spectrum shows three
absorption bands around 406 (ε = 3823 Mꢀ1 cmꢀ1), 586 (ε =
8924 Mꢀ1 cmꢀ1), and 650 (ε = 3710 Mꢀ1 cmꢀ1) nm, which are
typical of many square-planar d8 complexes (Figure 1).7a,11
The molecular structure of 1 has been confirmed by single-
crystal X-ray diffraction study. As shown in Figure 2, the cation
Received: June 20, 2011
Published: August 18, 2011
r
2011 American Chemical Society
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[Co(IEt)4]+ has pseudo-D4 symmetry in which the cobalt center
is bound to four carbene ligands in a square-planar geometry with
the CoꢀC(carbene) bond distances ranging from 1.923(3) to
1.936(3) Å (Table 1). These CoꢀC(carbene) separations are
close to those observed in the pincer complexes [(CNC)CoX]
(X = Me, Br),6e but shorter than the ones in the tetrahedral
complex [(TIMEN)Co(CO)]Cl.6f Interestingly, due to steric
repulsion among the ethyl groups, the four imidazole planes are
not perpendicular to the idealized coordinate plane (Co(1)ꢀ
C(1)ꢀC(2)ꢀC(3)ꢀC(4)) but have dihedral angles from 60.6ꢀ
to 63.3ꢀ. It is worth mentioning that, although cobalt(I) com-
plexes are not scarce, 1 represents one of the few examples of
homoleptic ones adopting square-planar geometry.7aꢀc
Scheme 1. Preparation of Homoleptic Co(I)-NHC
Complexes
The redox property of 1 has been examined by cyclic
voltammetry study. Its voltammogram shows three oxidation
processes, two quasi-reversible ones with E1/2+/2+ = ꢀ1.26 V and
E1/22+/3+ = 1.37 V corresponding to the redox processes of the
cation, and an irreversible one near 0.90 V correlated to the
oxidation of the borate anion,12 and no reduction out to ꢀ2.0 V
was observed (Figure 3). The potential for the [Co(IEt)4]+/2+
couple is lower than that for the Co(I)/Co(II) couple of vitamin
B12,13 but close to those observed for other square-planar cobalt
macrocycles.14
Homoleptic Cobalt(II)-NHC Complex. In light of the stability
of the divalent species [Co(IEt)4]2+ over a broad potential
window, the divalent complex [Co(IEt)4][BF4]2 (2) has been
synthesized via either the oxidation reaction or direct interac-
tion of CoCl2 with IEt and NaBF4 (Scheme 2). 2 has been fully
1
characterized by various spectroscopic methods. Its H NMR
Table 1. Selected Structural Parameters of [Co(IEt)4]+, 2+
from X-ray Diffraction and Computational Studies
[Co(IEt)4]+
[Co(IEt)4]2+
X-ray
calcd
1.991
X-ray
calcd
2.028
CoꢀC distance (Å) a
1.935(3)
1.936(3)
1.928(3)
1.923(3)
60.6 to 63.3
1.947(7)
1.964(7)
1.981(7)
1.969(6)
60.4 to 61.7
R (deg) b
61.5
62.7
Figure 1. UVꢀvis absorption spectra in THF of [Co(IEt)4][BPh4]
(1, black) and [Co(IEt)4][BF4]2 (2, red) recorded at room temperature.
a CoꢀC(carbene) distances. b Dihedral angles between the CoꢀC(1)ꢀ
C(2)ꢀC(3)ꢀC(4) plane and the four imidazolylidene planes.
Figure 2. Structures of cations [Co(IEt)4]+,2+ in 1 and 2, showing 30% probability ellipsoids and the partial atom-numbering scheme.
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Figure 4. EPR spectrum of [Co(IEt)4][BF4]2 (2) recorded in a frozen
acetonitrile solution at 4 K.
Figure 3. Cyclic voltammogram (50 mV/s) of ca. 1 mM THF solution
prepared from [Co(IEt)4][BPh4] (1). Waves marked by asterisks are
caused by the redox process of tetraphenylborate anion.
nonbonding 3dz2 orbital of the univalent cation (vide supra),
and steric repulsion between those ethyl groups might prevent
further shortening of the CoꢀC(carbene) bonds. Recently,
Spicer and Murphy reported a closely related tetrahedral cobalt-
(II) complex bearing a cyclic tetradentate carbene ligand.6j
Because the ligand field stabilization energies for these two
tetracarbene ligand sets should be approximately the same, steric
factors might account for the distinct coordination geometries. It
is worth mentioning that four-coordinate Co(II) complexes
under a monodentate ligand environment usually are tetrahedral,
but 2 is among the few having square-planar structure.7d,17
The electronic structure of [Co(IEt)4]2+ was investigated by
EPR spectroscopy. The EPR spectrum of [Co(IEt)4][BF4]2 (2)
recorded at 4 K in frozen CH3CN glass is presented in Figure 4.
The spectrum exhibits axial symmetry with well-resolved eight-
line patterns with g^ = 2.95, A^ = 160 G and g = 2.02, A = 249 G,
similar to those found for a variety of low-spin Co(II)
complexes.18 The eight-line splitting pattern of the EPR spec-
trum, arisingfrominteraction of an unpaired electron with a single
59Co (I = 7/2) nucleus, clearly proves the presence of a low-spin
d7-Co(II) center. The presence of a Co(II) center in [Co(IEt)4]2+
was further supported by computational study. The optimized
structures show good agreement with crystallography studies
(Table 1).
Scheme 2. Preparation of Homoleptic Co(II)-NHC
Complex
Theoretical calculations indicated the singlet cation [Co(IEt)4]+
is a formal Co(I) complex, and its LUMO and HOMO are
composed mainly by the 4pz and 3dz2 orbitals of the metal center,
respectively (Figure 5). The one-electron oxidation of [Co(IEt)4]+
generates the doublet dication [Co(IEt)4]2+, which holds a Co(II)
center with its spin density residing primarily on the metal’s 3dz2
orbital(Figure6), beingin strongcontrast to its isoelectronic nickel
complex [Ni(crown-carbene)]3+, which was predicted to have a
formal Ni(II) center bonded with a tetradentate crown carbene
radical cation.19
Homoleptic Cobalt-NHC Complex Catalyzed Homocou-
pling Reactions. Cobalt(I) species are prominent in electron
transfer chemistry, which has been well documented by the redox
chemistry of coenzyme B12, cobalt-corrins, and their derivatives.20
However these studies were generally performed on Co(I)
species generated in situ. The well-defined square-planar Co(I)
spectrum in CD3CN displays four broad peaks in the range
ꢀ1.62 to 8.10 ppm, revealing its paramagnetism. The complex
has its solution magnetic moment μeff = 2.4 μB at room
temperature,15 characteristic for four-coordinate low-spin Co(II)
complexes.7d,16 Differing from that of the d8 complex
[Co(IEt)4]+, the absorption spectrum of 2 shows no absorption
band in the visible region (Figure 1), suggestive of its low-spin d7
metal center.
X-ray crystallography study revealed the dication [Co(IEt)4]2+
in 2 retains the pseudo-D4 symmetry (Figure 2) with dimen-
sions similar to those of the univalent cation in 1 (Table 1). The
geometrical resemblance might result from the joint effects of
electronic and steric factors since the oxidation from [Co(IEt)4]+
to [Co(IEt)4]2+ should remove only an electron from the
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Scheme 3. Redox Conversions between [Co(IEt)4]+ and
[Co(IEt)4]2+
unique features for this reaction are worth noting. First, the
reaction can proceed efficiently with a very low catalyst load of
1%. Second, the catalysis is applicable to aryl Grignard reagents
with both electron-donating and electron-withdrawing groups in
good yields. What’s more, high-yield conversions of ortho-sub-
stituted aryl Grignard reagents can be achieved (entries 3, 4, and 6),
which normally gave coupling products in low yields under other
catalyst systems.22
Based on these understandings, a radical-type mechanism invol-
ving the redox shuttle between [Co(IEt)4]+ and [Co(IEt)4]2+ has
been proposed for these cobalt-catalyzed coupling reactions. As
rationalized in Scheme 4, the catalytic cycle might start from
the electron transfer (ET) reaction between the univalent cation
[Co(IEt)4]+ and the organic halide (RX), through which the
divalent speceis [Co(IEt)4]2+ and an alkyl radical (R ) will be
3
Figure 5. HOMO and LUMO orbitals of [Co(IEt)4]+, depicted using
isodensity at 0.03 au.
generated. The alkyl radical might subsequently be deactivated via
either H-atom abstraction or dimerization, while the Co(II)
complex could further interact with the Grignard reagent to yield
an aryl radical (Ar ) and the univalent cation [Co(IEt)4]+. Further
3
dimerization of the aryl radical will furnish the biaryl (ArꢀAr).
Meanwhile, the regenerated cation [Co(IEt)4]+ will join a new
cycle upon its interaction with another organic halide molecule. It
should be mentioned that due to steric repulsion the involvement
of five-coordinateintermediate[ArCo(IEt)4]+ or [ClCo(IEt)4]+ in
this catalytic cycle is very unlikely. Hence, these probable transi-
tional species are better to be viewed as loosely bound ion pairs
[Ar Co(IEt)4]+ and [Cl Co(IEt)4]+.23 Notably, the radical-
3 3 3
3 3 3
type pathway with a Co(I)/Co(II) redox shuttle disclosed here is
distinct from the traditional organometallic mechanisms,1,2,22 but
an unambiguous understanding on the mechanism of carbonꢀ
halogen bond cleavage and carbonꢀcarbon bond formation steps
needs further investigation.
Figure 6. Spin density population (Rꢀβ) for [Co(IEt)4]2+ (S = 1/2)
by DFT(B3LYP/6-311+G**/cc-pVTZ) calculation.
’ CONCLUSION
complex 1 provides an excellent opportunity to study its redox
reactions. As illustrated in eq 1 (Scheme 3), the univalent cation
[Co(IEt)4]+ is prone to oxidation, as it can be readily oxidized by
common organic halides, such as 3,5-dimethylphenyl iodide,
benzyl bromide, and 2-methyl-1,2-dichloropropane, to yield the
dication [Co(IEt)4]2+ and the corresponding organic radicals
that subsequently undergo dimerization or H-abstraction reac-
tions. On the other hand, unexpected redox reaction of the
divalent cation [Co(IEt)4]2+ with aryl Grignard reagent (p-
tolylmagnesium bromide) was observed, which gave the univalent
cation [Co(IEt)4]+ and biaryl in high yield (eq 2 in Scheme 3). No
reaction was noticed when treating either [Co(IEt)4]+ with aryl
Grignard reagent or [Co(IEt)4]2+ with the organic halides. These
stoichiometric reactions demonstrated that square-planar com-
plexes are adept at one-electron redox reactions.
We have demonstrated that the ligation of monodentate
N-heterocyclic carbene ligands to a cobalt center can lead to
the formation of unprecedented homoleptic cobalt complexes
[Co(NHC)4]+,2+ with square-planar geometry. These homolep-
tic cobalt-NHC complexes are adept at electron transfer reactions
and can be used as catalysts to mediate the oxidative homocou-
pling reactions of various aryl Grignard reagents, demonstrating a
unique radical-type mechanism via a Co(I)/Co(II) redox shuttle.
’ EXPERIMENTAL SECTION
General Procedures. All experiments were performed under an
atmosphere of dry dinitrogen with the rigid exclusion of air and moisture
using standard Schlenk or cannula techniques, or in a glovebox. All
organic solvents were freshly distilled from sodium benzophenone ketyl
immediately prior to use. [(PPh3)3CoCl],8 2,5-diisopropyl-3,4-dimethy-
limidazol-1-ylidene (IPr),9 2,5-diethyl-3,4-dimethylimidazol-1-ylidene
(IEt),9 and 2,3,4,5-tetramethylimidazol-1-ylidene (IMe)9 were prepared
according to literature methods. All chemicals were purchased from
According to these, a catalytic method employing the homo-
leptic cobalt-NHC complexes as catalysts for biaryl synthesis was
developed. As depicted in Table 2, 1 is very efficient in catalyzing
the homocoupling reactions of various aryl Grignard reagents
with 2-methyl-1,2-dichloropropane as the oxidant.21 Several
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Table 2. [Co(IEt)4][BPh4]-Catalyzed Oxidative Homocou-
pling of Aryl Grignard Reagentsa,b,c
Scheme 4. Possible Mechanism for [Co(NHC)4]+-Catalyzed
Homocoupling Reaction of Aryl Grignard Reagent with
Organic Halides As Oxidant
assigned unambiguously by analysis of symmetry and systematic absences
determined by XPREP. All structures were solved and refined using
SHELXTL.25 Metal and first coordination sphere atoms were located
from direct-methods E-maps; other non-hydrogen atoms were found in
alternating difference Fourier synthesis and least-squares refinement
cycles, and during final cycles were refined anisotropically. Hydrogen
atoms were placed in calculated positions employing a riding model.
Final crystal parameters and agreement factors are reported in Table 3.
Preparation of [Co(IEt)4][BPh4] (1). To a THF (15 mL) solution
of 1,3-diethyl-4,5-dimethylimidazolin-2-ylidene (0.608 g, 4.0 mmol) was
slowly added Co(PPh3)3Cl (0.881 g, 1.0 mmol) at room temperature.
The reaction mixture was stirred for half an hour, and NaBPh4 (0.342 g,
1.0 mmol) was then added. After stirring overnight and removal of
the solvent, the residue was washed with n-hexane (5 mL) and extracted
with THF (5 mL). The blue extraction was filtrated and added to a
small portion of toluene (1 mL). Slow evaporation of THF afforded
1 0.5THF toluene as a bluish-green crystalline solid (0.816 g, 80%).
a All reactions were carried out using aryl Grignard reagents (1.0 mmol),
1,2-dichloroisobutane (0.50 mmol), and a catalytic amount of
[Co(IEt)4][BPh4] (1% mol) in THF (10 mL). b Isolated yield after
column chromatography. c 5% mol [Co(IEt)4][BPh4] was used.
either Strem or J&K Chemical Co. and used as received unless otherwise
1
noted. H and 13C NMR spectra were recorded on a Varian Mercury
300 or 400 MHz spectrometer. All chemical shifts were reported in δ
units with references to the residual protons of the deuterated solvents
for proton chemical shifts and the 13C of deuterated solvents for carbon
chemical shifts. Mass spectra were recorded with a HP-5989 instrument.
GC/MS was performed on a Shimadzu GCMS-QP2010 Plus spectro-
meter. Elemental analysis was performed by the Analytical Laboratory of
Shanghai Institute of Organic Chemistry (CAS). Magnetic moments
were measured at 23 ꢀC by the method originally described by Evans
with stock and experimental solutions containing a known amount of a
(CH3)3SiOSi(CH3)3 standard.15 Absorption spectra were recorded
with a Hitachi U-3310 UVꢀvis spectrophotometer. Cyclic voltammetry
measurements were made with a CHI 600D potentiostation in THF
solutionsusing a sweep rate of 50 mV/s, a glassy carbon workingelectrode,
0.1 M (Bu4N)(PF6) supporting electrolyte, and a SCE reference elec-
trode. Under these conditions, E1/2 = 0.55 V for the [Cp2Fe]0,+ couple.
X-Band EPR spectra were recorded on a Bruker EMX EPR spectrometer
equipped with a variable-temperature helium flow cryostat system
(Oxford Instruments).
3
3
1H NMR (300 MHz, d8-THF): δ0.39 (t, 3J(H,H) = 7.2 Hz, 24H, NCH2CH3),
1.98 (s, 24H, NCCH3), 3.74 (m, 8H, NCH2CH3), 5.50 (m, 8H,
NCH2CH3), 6.65 (m, 4H, p-C6H5), 6.85 (t, J = 7.2 Hz, 8H, m-C6H5),
7.28 (m, 8H, o-C6H5). 13C NMR (75 MHz, d8-THF): δ 8.32
(NCH2CH3), 13.31 (NCCH3), 42.56 (NCH2CH3), 121.0 (C6H5),
123.9 (NCCH3), 124.8 (C6H5), 136.5 (C6H5), 164.7 (C6H5), 198.6
(br, C-Co carbene). Anal. Calcd for C62H88BCoN8O0.5 (1 + 0.5THF):
C, 72.78; H, 8.67; N, 10.95. Found: C, 72.40; H, 8.54; N, 10.37.
Preparation of [Co(IMe)4][BPh4]. This complex was obtained
(0.740 g, 85%) as a bluish-green crystalline solid by the reaction of 1,3-
dimethyl-4,5-dimethyl imidazolin-2-ylidene (0.500 g, 4.0 mmol) with
Co(PPh3)3Cl (0.881 g, 1.0 mmol) and NaBPh4 (0.342 g, 1.0 mmol) in
1
THF (15 mL) using a procedure similar to that for 1. H NMR (300
MHz, d8-THF): δ 1.91 (s, 24H, NCCH3), 3.38 (s, 24H, NCH3), 6.68
(m, 4H, p-C6H5), 6.82 (t, J = 6.9 Hz, 8H, m-C6H5), 7.27 (m, 8H, o-
C6H5). 13C NMR (100 MHz, d8-THF): δ 9.27 (NCCH3), 34.07
(NCH3), 121.9 (C6H5), 123.9 (NCCH3), 125.8 (C6H5), 137.3 (C6H5),
165.4 (C6H5), 201.3 (br, C-Co carbene). Anal. Calcd for C52H68BCoN8:
C, 71.39; H, 7.83; N, 12.81. Found: C, 71.73; H, 7.83; N, 12.30.
X-ray Structure Determinations. The structures of the four
compoundsinTableS1weredetermined. Diffraction-quality crystalswere
obtained as 1 0.5THF toluene, [Co(IMe)4][BPh4], [Co(IPr)4][BPh4]
3
3
in THF/toluene, and 2 THF in THF. Crystallizations were performed
3
at room temperature. Crystals were coated with Paratone-N oil and
mounted on a Bruker APEX CCD-based diffractometer equipped with
an Oxford low-temperature apparatus. Data were collected with scans of
0.3 s/frame for 30 s. Cell parameters were retrieved with SMART software and
refined using SAINT software on all reflections. Data integration was
performed with SAINT, which corrects for Lorentz polarization and decay.
Absorption corrections were applied using SADABS.24 Space groups were
Preparation of [Co(IPr)4][BPh4]. This complex was obtained
(0.836 g, 76%) as a bluish-green crystalline solid by the reaction of 1,3-
diisopropyl-4,5-dimethyl imidazolin-2-ylidene (0.720 g, 4 mmol) with
Co(PPh3)3Cl (0.881 g, 1.0 mmol) and NaBPh4 (0.342 g, 1.0 mmol)
1
in THF (15 mL) using a procedure similar to that for 1. H NMR
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Table 3. Crystal Data and Summary of Data Collection and Refinement for 1 0.5THF toluene, 2 THF, [Co(IMe)4][BPh4], and
ARTICLE
3
3
3
[Co(IPr)4][BPh4]
1 0.5THF toluene
2 THF
[Co(IMe)4][BPh4]
[Co(IPr)4][BPh4]
3
3
3
formula
C69H95.5BCoN8O0.5
0.30 ꢁ 0.25 ꢁ 0.20
1114.8
C40H72B2CoF8N8O
C52H68BCoN8
0.30 ꢁ 0.25 ꢁ 0.20
874.88
C68H100BCoN8
0.32 ꢁ 0.28 ꢁ 0.24
1099.3
cryst size (mm)
fw
0.21 ꢁ 0.13 ꢁ 0.10
913.6
cryst syst
space group
a, Å
triclinic
trigonal
P3(2)
triclinic
monoclinic
P21/c
P1
P1
11.5698(3)
16.3321(4)
17.6166(4)
89.199(1)
84.057(1)
82.613(1)
3283.5(1)
2
11.4224(4)
11.4224(4)
32.1421(2)
90
11.5569(3)
18.8089(5)
23.9386(6)
103.176(1)
97.061(1)
92.145(1)
5016.6(2)
4
19.043(2)
19.781(2)
19.401(2)
90
b, Å
c, Å
R, deg
β, deg
90
116.133(1)
90
γ, deg
V, Å3
120
3631.8(3)
3
6560.9(9)
4
Z
Dcalcd, Mg/m3
radiation (λ), Å
2θ range, deg
μ, mmꢀ1
F(000)
1.128
1.253
1.158
1.113
Mo KR (0.71073)
2.4 to 50.1
0.307
Mo KR (0.71073)
3.8 to 50.9
0.424
Mo KR (0.71073)
1.9 to 50.1
0.384
Mo KR (0.71073)
2.4 to 52.0
0.306
1203
1455
1872
2384
no. of obsd reflns
no. of params refnd
goodness of fit
R1
11 587
8964
17 671
12 755
788
550
1303
727
0.971
0.792
0.998
1.008
0.052
0.057
0.056
0.047
wR2
0.144
0.099
0.108
0.133
(300 MHz, d8-THF): δ 0.51 (d, 3J(H,H) = 6.9 Hz, 24H, NCH(CH3)2),
1.52 (d, 3J(H,H) = 6.9 Hz, 24H, NCH(CH3)2), 2.09 (s, 24H, NCCH3),
6.15 (m, 8H, NCH(CH3)2), 6.68 (m, 4H, p-C6H5), 6.82 (t, J = 6.6 Hz,
8H, m-C6H5), 7.25 (m, 8H, o-C6H5). 13C NMR (75 MHz, d8-THF): δ
10.80 (NCCH3), 20.88 (NCH(CH3)2), 21.31 (NCH(CH3)2), 52.99
(NCH(CH3)2), 121.8 (C6H5), 125.7 (NCCH3), 126.2 (C6H5), 137.3
(C6H5), 165.4 (C6H5), 199.3 (br, C-Co carbene). Anal. Calcd for C68-
H100BCoN8: C, 74.29; H, 9.17; N, 10.19. Found: C, 74.38; H, 9.34; N, 10.44.
Preparation of [Co(IEt)4][BF4]2 (2). Method A. To a THF
(15 mL) solution of 1,3-diethyl-4,5-dimethylimidazolin-2-ylidene (0.608
g, 4.0 mmol) were slowly added Co(PPh3)3Cl (0.881 g, 1.0 mmol) and
NaBF4 (0.109 g, 1.0 mmol). After stirring overnight, [Cp2Fe][BF4]
(0.300 g, 1.1 mmol) was added, and the reaction mixture was further
stirred for 24 h. After filtration and concentration, n-hexane (10 mL) was
added to the concentrated filtrate (ca. 6 mL), which led to the
precipitation of a pale blue solid. Recrystallization of this solid in THF
mixture was warmed to room temperature and stirred for 6 h. Addition
of n-hexane (10 mL) to the reaction mixture led to the precipitation of a
pale blue solid. After filtration, the filtrate was quenched with a 1.0 M
aqueous HCl solution (0.5 mL). The organic phase was separated and
analyzed by GC-MS with hexamethylbenzene (8.1 mg, 0.05 mmol) as an
internal standard. The reaction yielded (determined by GC) (p-CH3-
PhCH2)2 (90%) and p-xylene (10%). The 1H NMR spectrum of the pale
1
blue solid in CD3CN shows all the characteristic H NMR peaks of
[Co(IEt)4]2+ in 2.
Reaction of 1 with 1,3-Dimethyl-5-iodobenzene. To a
solution of [Co(IEt)4][BPh4] (98.7 mg, 0.1 mmol) in THF (5 mL)
was slowly added 1,3-dimethyl-5-iodobenzene (23.2 mg, 0.1 mmol)
at ꢀ116 ꢀC. The mixture was warmed to room temperature and stirred
for 6 h. Addition of n-hexane (10 mL) to the reaction mixture led to the
precipitation of a pale blue solid. After filtration, the filtrate was
quenched with a 1.0 M aqueous HCl solution (0.5 mL). The organic
phase was separated and analyzed by GC-MS with hexamethylbenzene
(8.1 mg, 0.05 mmol) as an internal standard. The reaction yielded 1,
(5 mL) afforded 2 THF as an off-white crystalline solid (0.476 g, 52%).
3
1
The H NMR spectrum of this paramagnetic complex displayed four
1
1
characteristic peaks in the range ꢀ1.62 to 8.10 ppm. H NMR (CD3-
3-dimethylbenzene quantitatively. The H NMR spectrum of the pale
1
CN): δ ꢀ1.62, 1.96, 3.40, 8.10. Anal. Calcd for C40H72B2CoF8N8O (2 +
THF): C, 52.59; H, 7.94; N, 12.27. Found: C, 52.76; H, 8.09; N, 11.63.
Method B. To a THF (15 mL) solution of 1,3-diethyl-4,5-dimethylimi-
dazolin-2-ylidene (0.608 g, 4.0 mmol) was slowly added CoCl2 (0.130 g,
1.0 mmol) and NaBF4 (0.218 g, 2.0 mmol) at room temperature. The
reaction mixture was further stirred for 12 h. After filtration and
concentration, n-hexane (10 mL) was added to the concentrated filtrate
(ca. 6 mL), which led to the precipitation of a pale blue solid.
blue solid in CD3CN shows all the characteristic H NMR peaks of
[Co(IEt)4]2+ in 2.
Reaction of 1 with 2-Methyl-1,2-dichloropropane. To a
solution of [Co(IEt)4][BPh4] (197.4 mg, 0.2 mmol) in THF (5 mL) was
slowly added 2-methyl-1,2-dichloropropane (11.6 μL, 0.1 mmol)
atꢀ116 ꢀC. The mixture waswarmedtoroomtemperature and stirred for
24 h. Addition of n-hexane (10 mL) to the reaction mixture led to the
precipitation of pale blue solid. The 1H NMR spectrum of the pale blue
solid in CD3CN shows all the characteristic 1HNMRpeaksof[Co(IEt)4]2+
in 2. 2-Methylpropene was presumably formed as the byproduct similar to
some iron-catalyzed oxidative coupling reactions.26
Recrystallization of the solid in THF (5 mL) afforded 2 THF as an
3
off-white crystalline solid (0.682 g, 75%). The 1H NMR spectrum of this
material was identical with that of the product from method A.
Reaction of 1 with 4-Methylbenzyl Bromide. To a solution of
[Co(IEt)4][BPh4] (98.7 mg, 0.1 mmol) in THF (5 mL) was slowly
added 4-methylbenzyl bromide (18.5 mg, 0.1 mmol) at ꢀ116 ꢀC. The
Reaction of 2 with p-Me-PhMgBr. To a solution of [Co-
(IEt)4][BF4]2 (168 mg, 0.2 mmol) in THF (5 mL) was slowly added
p-Me-PhMgBr (0.2 mL, 1.0 M in THF) at ꢀ116 ꢀC. The mixture was
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Organometallics
ARTICLE
then warmed to room temperature and stirred for 12 h. After filtration, part
of the filtrate (ca. 2 mL) was quenched with a 1.0 M aqueous HCl solution
(0.5 mL). The organic phase was separated and analyzed by GC-MS using
hexamethylbenzene as an internal standard, which indicated the quantitative
formation of 4,40-dimethylbiphenyl. The other portion of the filtrate (ca.
3 mL) was concentrated, and a small portion of toluene (0.5 mL) was added.
Slow evaporation of THF afforded a bluish-green crystalline solid, which
shows identical 1H NMR spectrum to that of [Co(IEt)4]+ in 1.
and the National Natural Science Foundation of China (Nos.
21002114 and 20872168).
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General Procedure for Cobalt-NHC Complex-Catalyzed
HomocouplingofGrignard Reagents. Toafrozen solutionof[Co-
(IEt)4][BPh4] (9.9 mg, 0.01 mmol) in THF (10mL) was added Grignard
reagent27 (1.0 mmol) and 2-methyl-1,2-dichloropropane (58 μL,
0.5 mmol) at ꢀ116 ꢀC. The mixture was then warmed to room tem-
perature or heated to 50 ꢀC (Table 2) and stirred for 6 h. After quenching
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Et2O (5 mL ꢁ 3), and the combined organic portions were dried over
anhydrous Na2SO4. After removal of the solvent, the resulting residue was
subjected to column chromatographic separation (SiO2, 300ꢀ400 mesh)
to give the following biaryls as colorless solids. All the isolated yields are
given with respectto the Grignard reagents. The 1H NMR spectra ofthese
biaryls are consistent with those reported in the literature.27
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Biphenyl. Yield: 86%. Colorless solid. 1H NMR (300 MHz, CDCl3):
δ 7.61 (d, J = 7.5 Hz, 4H, o-C6H5), 7.44 (t, J = 7.2 Hz, 4H, m-C6H5), 7.34
(t, J = 7.2 Hz, 2H, p-C6H5).
1
4,40-Dimethlbiphenyl. Yield: 83%. Colorless solid. H NMR (300
MHz, CDCl3): δ 7.47 (d, J = 7.5 Hz, 4H, C6H4), 7.23 (d, J = 7.5 Hz, 4H,
C6H4), 2.38 (s, 6H, CH3).
2,20-Dimethlbiphenyl. Yield: 81%. Colorless solid. 1H NMR (300 MHz,
CDCl3): δ 7.18 (m, 6H, C6H4), 7.03 (m, 4H, C6H4), 1.98 (s, 6H, CH3).
2,20-4,40-6,60-Hexamethlbiphenyl. Yield: 75%. Colorless solid. 1H
NMR (300 MHz, CDCl3): δ 6.85 (s, 4H, C6H2), 2.25 (s, 6H, p-CH3),
1.78 (s, 12H, m-CH3).
1
4,40-Dichlorobiphenyl. Yield: 74%. Colorless solid. H NMR (300
MHz, CDCl3): δ 7.39ꢀ7.49 (m, 8H, C6H4).
2,20-Dimethoxybiphenyl. Yield: 74%. Colorless solid. 1H NMR (300
MHz, CDCl3): δ 7.26 (t, J = 7.8 Hz, 2H, C6H4), 7.17 (d, J = 7.2 Hz, 2H,
C6H4), 6.93 (m, 4H, C6H4), 3.70 (s, 6H, OCH3).
4,40-Dimethoxybiphenyl. Yield: 84%. Colorless solid. 1H NMR (300
MHz, CDCl3): δ 7.41 (d, J = 8.7 Hz, 4H, C6H4), 6.89 (d, J = 8.7 Hz, 4H,
C6H4), 3.77 (s, 6H, OCH3).
Calculation Details. To haveabetterunderstandingofthestructure
ofthe Cocomplexes, density functionaltheory (DFT)28 studies have been
performed with the Gaussian0929 program using the BS-B3LYP30 meth-
od. The 6-311+G** basis set was used for the C, H, and N atoms, and the
cc-PVTZ basis set was used for the Co atom. The structures of [Co-
(IEt)4]+ and [Co(IEt)4]2+ with D4 symmetry were fully optimized.
Calculations were performed assuming a singlet electronic ground state
for [Co(IEt)4]+ and a doublet state for [Co(IEt)4]2+.
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’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic files in
b
(9) Kuhn, N.; Kratz, T. Synthesis 1993, 561–562.
CIF format, cyclic voltammogram of the cobalt complexes, and
computational details. This material is available free of charge via
the Internet at http://pubs.acs.org.
(10) AnaloguesCo(I)-NHCcomplexes[Co(NHC)4][BPh4] employ-
ing other N-heterocyclic carbenes (NHC = 1,3-dimethyl-4,5-dimethylimi-
dazole-2-ylidene (IMe), and 1,3-diisopropyl-4,5-dimethylimidazole-2-
ylidene (IPr)) have also been prepared and structurally characterized.
For detailed information, see Experimental Section.
’ AUTHOR INFORMATION
(11) (a) Mason, W. R.; Gray, H. B. J. Am. Chem. Soc. 1968,
90, 5721–5729. (b) Gray, H. B.; Pree, J. R. J. Am. Chem. Soc. 1970, 92,
7306–7312.
Corresponding Author
*E-mail: deng@sioc.ac.cn.
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1127–1134.
’ ACKNOWLEDGMENT
This research was financially supported by the National Basic
Research Program of China (973 Program, No. 2011CB808705)
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