Facile routes to abnormal-NHC-cobalt(II) complexes

Article abstract of DOI:10.1039/c7dt01778b

Deprotonation of [IPr^Ph]I (1) with Co{N(SiMe₃)₂}₂ readily affords the abnormal N-heterocyclic carbene (aNHC) complex (aIPr^Ph)₂CoI₂ (2) (aIPr^Ph = 1,3-bis(2,6-iPr₂C₆H₃)-2-phenyl-imidazol-4-ylidene). Treatment of 1 with NaHBEt₃ yields (aIPr^Ph)BEt₃ (3) that serves as an aNHC-transfer agent and yields (aIPr^Ph)Co{N(SiMe₃)₂}₂ (4) on reaction with Co{N(SiMe₃)₂}₂.

Full text of DOI:10.1039/c7dt01778b

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DOI: 10.1039/C7DT01778B
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Facile Routes to Abnormal-NHC-Cobalt(II) Complexes†
Rajendra S. Ghadwal,*,a,b Jan-Hendrik Lamm,a,b Dennis Rottschäfer,a,b Christian J. Schürmann,c and
Serhiy Demeshkoc
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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Chart 1. Normal- (A) and abnormal- (B) NHCs. The first aNHC-compound (I) and the
first metal-free stable aNHC (II).
Deprotonation of [IPrPh]I (1) with Co{N(SiMe3)2}2 readily affords
the abnormal N-heterocyclic carbene (aNHC) complex (aIPrPh)2CoI2
(2) (aIPrPh
= 1,3-bis(2,6-iPr2C6H3)-2-phenyl-imidazol-4-ylidene).
Treatment of 1 with NaHBEt3 yields (aIPrPh)BEt3 (3) that serves as
an aNHC-transfer agent and yields (aIPrPh)Co{N(SiMe3)2}2 (4) on re-
action with Co{N(SiMe3)2}2.
N-Heterocyclic carbenes (NHCs) (A, Chart 1) are very versatile
neutral carbon-donor ligands in transition metal1 as well as in
main group chemistry.2 They bind to metals at the C2 carbon
atom and are known as normal NHCs.3 The success of NHCs in
catalysis and beyond has further prompted interest in new
types of carbon-donor ligands.4 In recent years, a different
sets of ligands is at the forefront of contemporary research.
Despite the high potential of aNHCs, only a few structurally
characterized aNHC-complexes featuring a base metal (Fe and
Co in particular) are known.9 This emphasizes the lack of their
facile synthetic methods. We recently reported a catalytic
method to C2-arylated imidazolium salts using an NHC, which
are excellent synthons to aNHC-metal complexes.7f, 7i, 10
type of NHCs (B, Chart 1) that bind to metals at the imidazol-
backbone (C4- or C5-) instead of the C2-carbon atom have
emerged as very appealing ligands.3 They are generally
regarded as abnormal NHCs (aNHC) (
al. described the first aNHC-metal complex (
B
).5 In 2001, Crabtree et
), whereas
I
Bertrand and co-workers reported the first metal free stable
aNHC (II) in 2009.6 Experimental and theoretical data5 suggest
that aNHCs (B) are stronger σ-donors than NHCs (A) and CAACs
Herein, we report two synthetic strategies to aNHC-cobalt
complexes (aIPrPh)2CoI2 (
2
) and (aIPrPh)Co{N(SiMe3)2}2 (
3
) [1,3-
bis(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene] by the
utilization of the C2-arylated imidazolium salt (IPrPh)I ( ) [IPrPh
= 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazolium].
Reaction of with Co{N(SiMe3)2}2 in toluene leads to the
formation of complex as a turquoise crystalline solid in 79%
(cyclic alkyl amino carbenes), and hence offer exciting
perspectives for their applications.3c, 7 Nevertheless, the major-
ity of aNHCs-complexes reported so far are serendipitous
products.3a, 3c Therefore, the development of rational synthetic
methods to aNHC-compounds is important and highly desired
to facilitate their exploration.
Earth abundant base metal complexes are appealing candi-
dates in sustainable catalysis as precious heavy metals
surrogates.8 The investigation of 3d-metal complexes with new
1
1
2
yield (Scheme 1). NHC-coinage metal compounds are impor-
tant carbene transfer agents.1e, 11 Owing to the facile synthetic
accessibility, stability and solubility in common organic
solvents, NHC-main group compounds may be more appealing
candidates for this purpose. In addition, the liberated main
group species can be easily separated from the products. To
the best of our knowledge, such approaches remained so far
unexplored. A weak Lewis acid such as BEt3 may be an inter-
esting reagent due to its high volatility and commercial
a.Anorganische Molekülchemie und Katalyse, Am Lehrstuhl für Anorganische
Chemie und Strukturchemie, Fakultät für Chemie, Universität Bielefeld,
Universitätsstr. 25, D-33615, Bielefeld, Germany.
Email: rghadwal@uni-bielefeld.de
b.Centrum für Molekulare Materialien, Fakultät für Chemie, Universität Bielefeld,
Universitätsstr. 25, 33615, Bielefeld, Germany.
availability.12 Reaction
compound (aIPrPh)BEt3
1
with Na{HBEt3) affords the desired
) in 75% yield. With in hand, we
c. Institut für Anorganische Chemie, Georg-August-Universität Göttingen,
Tammannstrasse 4, D-37077 Gꢀttingen, Germany.
(3
3
decided to probe its suitability as carbene transfer agent.
Indeed, acts as an aNHC-transfer agent and affords
(aIPrPh)Co{N(SiMe3)2}2 (
† aNHC (Abnormal N-heterocyclic carbene).
Electronic Supplementary Information (ESI) available: Synthesis, NMR spectra, and
X-ray crystallographic details of 2, 3, and 4. See DOI: 10.1039/x0xx00000x
3
4
) on treatment with Co{N(SiMe3)2}2.
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Scheme 1. Synthesis of compounds 2 3, and 4.
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DOI: 10.1039/C7DT01778B
Figure 2. Molecular structure of cobalt complex (aIPrPh)Co{N(SiMe3)2}2 (4). Anisotropic
displacement parameters are depicted at the 50% probability level. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and bond angles (°): Co1–C1:
2.0748(16); Co1–N1: 1.9484(15); Co1–N2: 1.9342(14); C1–C2 1.366(2); N4–C1–C2:
102.38(13); C1–Co1–N1: 109.94(6); C1–Co1–N2: 127.70(6); N1–Co1–N2: 121.81(6).
Compounds
are stable under an inert gas atmosphere. Compound
are paramagnetic, each showing non-overlapping 1H NMR
signals, respectively in the – 3.35 to 26.51 and –26.10 to
2
–
4
are soluble in common organics solvents and
2
and
4

However, they are comparable with Layfield’s aNHC-cobalt
compound [2.059(2) Å].9b The angle C–Co–C [125.01(10)°] is
the largest bond angle at the cobalt atom, whereas the C4–
44.83 ppm region. The C5-hydrogen atom of the imidazole-ring
exhibits a broad 1H NMR signal at

4
26.51 (for
shows resonances for the
exhibits
–19.08 ppm for the N(SiMe3)2 groups which is
consistent with Layfield’s cobalt amide derivative.9b 1H and 13C
NMR spectra of display expected resonances for carbene
ligand and ethyl groups. The 11B NMR spectrum of
shows a
signal at – 14.36 ppm.
Solid-state molecular structures of
and (Figure 2) have been determined by single-crystal X-ray
diffraction studies. Complex features a distorted-tetrahedral
2) or 35.60 ppm
1
(for
4
). The H NMR spectrum of
Co1–I3 bond angle is 103.72(7)°. The molecular structure of
4
carbene and amide ligands. The 1H NMR spectrum of
4
(Figure 2) features a distorted trigonal-planar coordination
geometry at the three-fold coordinated Co(II) center. The Co–
C(carbene) bond length [2.0748(16) Å] is comparable with that of
the reported aNHC-complex [2.059(2) Å],9b but shorter than
those of the related (NHC)Co{N(SiMe3)2}2 complexes.14 The
a signal at

3
3

Co–N bond lengths of
4
are consistent with those of the
2
(Figure 1), 3 (Figure S11),
14
reported Co(II) amide complexes.9b, Both species
2 and 4
4
feature the imidazol-ring C–C bond length of av. 1.37 Å and the
C–C(carbene)–N bond angle of av. 102.2°, clearly indicating the
carbene nature of the cobalt-bound carbon atom.5 Compound
2
geometry at the four-fold coordinated cobalt atom that binds
to two aNHCs and two iodide ligands. The Co–I [2.6045(5) and
2.6254(6) Å] bond lengths are comparable with other NHC-
cobalt complexes,13 however the Co–C(carbene) [2.026(2) and
2
exhibits a

MT value of 2.05 cm3mol–1K (corresponds to a
B) in the solid state at 200 K (Figure
magnetic moment of 4.05

S9), which is consistent with other high-spin (S = 3/2)
tetrahedral cobalt(II) complexes with some second-order
orbital angular contribution to the moment (3.89 B for spin-
only d7 configuration).
2.035(2) Å] bond lengths of 2
are remarkably shorter.9b
Preliminary catalytic activity of
2 has been investigated in
Kumada cross coupling reactions using various aryl halides and
phenylmagnesium bromide (Table 1). Biaryls (Ar–Ph) can be
prepared up to 79% (entry 4) yield with aryl bromides,
however, dehalohydrogenation that yields arenes Ar–H
remains a competing reaction.13 4-Iodotoluene (entry 3)
exclusively yields toluene whereas 4-chlorotoluene (entry 2)
reacts rather sluggishly. Aryl bromide bearing sterically
demanding substituents (entry 6) or an electron releasing
group (entry 9) give arenes as major products (see ESI for
further details).
In summary, rational synthesis protocols to aNHC-cobalt(II)
Figure 1. Molecular structure of cobalt complex (aIPrPh)2CoI2 (2). Anisotropic
displacement parameters are depicted at the 50% probability level. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and bond angles (°): Co1–C4:
2.021(2); Co1–C39: 2.037(2); Co1–I2: 2.6067(5); Co1–I3: 2.6259(6); C4–C38: 1.369(3);
N5–C4–C38: 102.0(2); C4–Co1–C39: 125.01(10); I2–Co1–I3: 111.722(14); C4–Co1–I2:
111.95(7); C4–Co1–I3: 103.70(7).
complexes
imidazolium salt
yield accessibility and the presence easy leaving iodides make
2
and
4
are reported. Deprotonation of the
1
with Co{N(SiMe3)2 affords
2
in 79%. The high
2
a promising candidate for the synthesis of other, low-valent
2 | J. Name., 2012, 00, 1-3
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2
3
4
5
6
7
100.0 / – / –
79.6 / – / 20.4
8
9
28.2 / 58.8 / 13.0
78.9 / – / 21.1
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aafter 20 h, determined by GC/MS.
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DOI: 10.1039/C7DT01778B
TOC:
Deprotonation of 1 with Co{N(SiMe3)2}2 affords aNHC-Co(II) complex 2, whereas carbene transfer from 3
to Co{N(SiMe3)2}2 enables access to complex 4.