Half-sandwich nickel complexes with ring-expanded NHC ligands-synthesis, structure and catalytic activity in Kumada-Tamao-Corriu coupling

Article abstract of DOI:10.1039/c5dt04663g

The general synthesis of [Ni(Cp)(X)(NHC)] complexes from a nickel halide, CpLi, and a carbene solution is reported. This procedure yields unprecedented complexes with ring-expanded NHC ligands (RE-NHC) of six- (1a, 1b), seven- (1c), and eight-membered (1d) heterocycles. The NMR spectra of 1a-1d are consistent with the hindered rotation of Ni-C_carbene and N-C_Mes bonds, while X-ray analyses of 1b, 1c, and 1d reveal a pronounced trans influence of the RE-NHC ligands. Complexes 1a-1e are efficient pre-catalysts in Kumada-Tamao-Corriu coupling with the maximum efficiency observed for complexes bearing the six-membered NHC.

Full text of DOI:10.1039/c5dt04663g

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Half-sandwich nickel complexes with ring-expanded NHC ligands –
synthesis, structure and catalytic activity in Kumada-Tamao-Corriu
coupling
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
DOI: 10.1039/x0xx00000x
Ł. Banach, P. A. Guńka and W. Buchowicz*
www.rsc.org/
A general synthesis of complexes [Ni(Cp)(X)(NHC)] from a nickel
halide, CpLi, and a carbene solution is reported. This procedure
yields unprecedented complexes with ring-expanded NHC ligands
(RE-NHC) of six- (1a, 1b), seven- (1c), and eight-membered (1d)
heterocycles. The NMR spectra of 1a-1d are consistent with
hindered rotation of Ni-Ccarbene and N-CMes bonds, while X-ray
analyses of 1b, 1c, and 1d reveal a pronounced trans influence of
the RE-NHC ligands. Complexes 1a-1e are efficient pre-catalysts in
Kumada-Tamao-Corriucoupling
with
maximum
efficiency
observed for complexes bearing the six-membered NHC.
5
Half-sandwich nickel complexes of the general formula [Ni(ƞ -
C H R)(X)(NHC)] (R = H, alkyl, alkenyl; X = Cl, Br, I; NHC = N-
5
4
heterocyclic carbene) are known only for five-membered
NHCs. These complexes are usually synthesized in reactions of
1
nickelocene with imidazolium halides (Scheme 1a). However,
for compounds with bulky substituents on the Scheme 1. Reported synthetic routes to complexes [Ni(ƞ -C H R5-n)(X)(NHC)].
5
5
n
cyclopentadienyl (Cp) ligand, another protocol has to be
utilized (Scheme 1b).
2
complexes by facilitating formation of the catalytically active
intermediate and increasing its stability. On the other hand,
ring-expanded NHC (RE-NHC) may enable an intramolecular C-
Owing to their catalytic activity in many organic
transformations, these air- and moisture-stable complexes are
2-5
H
activation, which can be detrimental for catalytic
currently the object of intensive research. Efforts on tuning
properties of [Ni(Cp)(X)(NHC)] are focused on the appropriate
ligand selection within the scope of synthetic methodologies
presented on Scheme 1, i.e. for five-membered NHCs only.
7
performance.
Although RE-NHC nickel complexes are known, these studies
7,8
are limited to only a few papers. To the best of our
knowledge, up to date there have been no reports on
1a,3b,3c,3d,4
5
Since the halide
or Cp ligands can be readily
[
[
Ni(Cp)(X)(RE-NHC)]. Driven by scientific curiosity to see if
Ni(Cp)(X)(RE-NHC)] complexes are of greater catalytic activity
displaced from the title complexes, NHC is the ligand of choice
for modifications aimed at enhancing their catalytic activity.
Recently, it has been shown that increasing the steric demand
of NHCs by means of bulky substituents on the nitrogen atoms
improved efficiency of the arylamination of aromatic halides
than their congeners with five-membered “classical” NHCs, we
decided to develop the synthesis of such complexes.
Our initial attempts to prepare the desired RE-NHC nickel
complexes according to the procedure shown in Scheme 1a,
i.e. from nickelocene and the appropriate carbene precursor,
3i
catalysed by [Ni(Cp)(X)(NHC)]. Another way to enhance
spatial requirements as well as electron-donating strength of
6
failed. Reactions involving the use of [Ni(acac) ] as the starting
NHC ligands is expansion of the carbenic heterocycle. Both of
2
material were also unsuccessful (Scheme 1b). A probable
reason for this lack of reactivity is the weak acidity of carbene
precursors and steric congestion around the reacting centre.
these properties are presumed to influence reactivity of
9
6
a.
Faculty of Chemistry Warsaw University of Technology, Noakowskiego 3, 00-664
Inspired by the synthesis of related complexes with Cp*-
Warsaw, Poland
e-mail: wbuch@ch.pw.edu.pl.
Electronic Supplementary Information (ESI) available: DOI: 10.1039/x0xx00000x
3g
functionalized NHC ligands, we decided to test a similar
approach involving addition of the two ligands (Cp and RE-
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Scheme 2. Synthesis of complexes [Ni(Cp)(X)(NHC)]
NHC) to a nickel halide. Despite plausible formation of strongly broadened. We tentatively interpret these
4f,12
different products, e.g. nickelocene and [Ni(X) (RE-NHC) ], the phenomena in terms of spin equilibrium,
i.e. increasing
2
2
desired [Ni(Cp)(X)(RE-NHC)] complexes were obtained in content of the high-spin form of the complex which is
remarkably good yields as the only isolable products. More correlated to the NHC’s ring size.
specifically, [Ni(DME)(X) ] (X = Br or Cl, DME = 1,2- Crystal structure analysis of complexes 1b-1d revealed that
2
dimethoxyethane) was treated first with a CpLi solution in THF they crystallize in the monoclinic crystal system and their
to form what we believe is a dinuclear complex [Ni(Cp)(µ- molecular geometries are typical for nickel(II) NHC complexes
10
13
X)]2. Subsequently, a freshly prepared RE-NHC solution, (Figures 1-3). The Ni−CCp bond lengths exhibit a pronounced
obtained by deprotonation of the carbene precursor with BuLi, trans influence of the RE-NHC ligands while the C-C bond
was injected. After stirring overnight and conventional lengths in Cp (see Figure captions for selected bond lengths)
purification complexes 1a-1d were obtained in 32-41% yields show variations characteristic of the so-called “diene
14,15
(
two steps from the Ni halide) as deep purple solids distortion”.
1
1
(Scheme2).
Moreover, to further expand the scope of our method, we
have applied it to prepare a complex with a five-membered
3
c
NHC Thus, [Ni(Cp)(Br)(SIMes)]
(SIMes
=
1,3-bis(2,4,6-
1e was
trimethyl-phenyl)-4,5-dihydroimidazol-2-ylidene)
(
)
obtained via this simple procedure in 61% yield (two steps
from [Ni(DME)Br ]).
2
The NMR spectra of complexes 1a-1d were conventionally
recorded in CDCl at ambient temperature. Signals originating
3
from NHC ligands imply restricted Ni-Ccarbene and N-CMes bond
rotation. Owing to the inequivalence of hydrogen atoms in
1
each NCH group their signals appear in the H NMR spectra as
2
unresolved multiplets. Moreover, as a result of the hindered
rotation, all complexes 1a-1d have characteristic set of signals
for mesityl wingtips, showing two separated equal singlets (2:2
ratio) for meta hydrogens from phenyl rings and three singlets
(6:6:6 ratio) for methyl groups, i.e. ortho methyls and meta
hydrogens are nonequivalent within each mesityl moiety.
1
Another interesting feature of H NMR spectra concerns the Cp
group. While for [Ni(Cp)(X)(NHC)] with five-membered NHCs
the Cp singlet appears from 5.2 to 4.6 ppm, its chemical shift
Figure 1. Molecular structure of complex 1b. Since there are no significant differences
in the conformations of four molecules present in the asymmetric unit only one is
shown. Thermal ellipsoids are drawn at the 50% probability level. Selected bond
for 1a
-1d is unusual and strongly influenced by size of the NHC lengths in Å: Ni1−Br1 2.3371(5), Ni1−C1 2.054(3), Ni1−C2 2.193(3), Ni1−C3 2.197(3),
Ni1−C4 2.165(3), Ni1−C5 2.151(3), Ni1−C6 1.905(3), C1−C2 1.429(5), C1−C5 1.433(5),
C2−C3 1.379(4), C3−C4 1.440(4), C4−C5 1.387(5).
as it varies from 4.21 ppm for 1b to 3.68 ppm for 1d
Moreover, all signals in the H NMR spectrum of 1d are
.
1
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appeared to have no influence on the catalytic performance
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DOI: 10.1039/C5DT04663G
(Table 1, entries 2 and 3). Further expansion of NHC ligand to
seven- or eight-membered rings resulted in diminished
activity. However, high conversions could be achieved at
longer reaction times (Table 1, entries 4-7). Selectivity is
another parameter affected by ring expansion beyond the six-
membered system (Table 1, entries 4, 5, 7).
Table 1. Kumada-Tamao-Corriu coupling of PhMgBr with halotoluenes
a
catalysed by 1a-1e.
Figure 2. Molecular structure of complex 1c. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths in Å: Ni1−Br1 2.3281(3), Ni1−C1 2.0529(17),
Ni1−C2 2.1914(17), Ni1−C3 2.1847(17), Ni1−C4 2.1722(18), Ni1−C5 2.1422(17), Ni1−C6
[
b]
[b]
Entry
Catalyst
1e
n
0
1
1
2
2
3
3
1
1
0
X′
Conversion (%)
Ratio A:B:C
91:6:3
92:6:2
92:7:1
89:11:0
85:10:5
-
1.8972(15), C1−C2 1.431(3), C1−C5 1.436(3), C2−C3 1.384(3), C3−C4 1.444(2), C4−C5
1
2
3
4
3-Br
3-Br
3-Br
3-Br
3-Br
3-Br
3-Br
4-Cl
4-Cl
4-Cl
78
1.389(3).
1a
97
1b
98
1c
36
[
[
[
c]
c]
5
6
7
8
9
1
1c
86
1d
traces
97
1d
70:20:10
-
1a
traces
68
c]
[
1a
65:22:13
54:36:10
c]
0
1e
42
[
(
1
a] All reactions in duplicate; reaction conditions: THF (2.8 mL), halotoluene
0.66 mmol), phenylmagnesium bromide solution in THF (0.81 mL, 1.00 mmol,
.24 mol/L), nickel complex (6.7 μmol, 1 mol%), room temperature, 90 min;
See the Supporting Information for details. [b] Determined with GC. [c]
Reaction time = 21 h.
Figure 3. Molecular structure of complex 1d. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths in Å: Ni1−Br1 2.3466(4), Ni1−C1 2.074(2),
Ni1−C2 2.195(2), Ni1−C3 2.178(2), Ni1−C4 2.150(2), Ni1−C5 2.161(2), Ni1−C6 1.918(2),
C1−C2 1.438(3), C1−C5 1.425(4), C2−C3 1.380(3), C3−C4 1.439(3), C4−C5 1.393(4).
Conclusions
We report here a new general synthesis of complexes
Complex [Ni(Cp)(Cl)(IMes)] (IMes = 1,3-bis(2,4,6-trimethyl-
phenyl)imidazol-2-ylidene) was reported as an active catalyst
in Kumada-Tamao-Corriu (KTC) coupling between p-
[
Ni(Cp)(X)(NHC)] that is applicable to ring-expanded NHC
ligands. Complexes with the six-membered NHC provided the
best efficiency in KTC coupling. Further studies will focus on
application of [Ni(Cp)(X)(RE-NHC)] in other catalytic reactions,
including arylamination of aromatic halides.
3a
tolylmagnesium bromide and phenyl O-sulfamate. To our
surprise, further application of [Ni(Cp)(X)(NHC)] complexes in
3
l,3p,16
this reaction are limited.
influence of NHC ring expansion on the activity of
Ni(Cp)(X)(NHC)] in KTC coupling, we tested complexes 1a 1e
In order to get insight into the
[
-
Acknowledgements
The authors thank Warsaw University of Technology for financial
support.
in reactions of PhMgBr with halotoluenes (3-bromotoluene, 4-
chlorotoluene). Expansion of the NHC ligand from a five- to six-
membered ring proved to be beneficial for the catalytic activity
(Table 1, entries 1-3, 9,10). Catalyst loading of only 1%mol of 1a
or 1b is sufficient to achieve conversions over 95% with very
good selectivity at room temperature in 90 minutes. Also,
the nature of the halogen ligand in the nickel complex
Notes and references
17
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