C-Br activation of aryl bromides at Ni0(NHC)2: Stoichiometric reactions, catalytic application in Suzuki-Miyaura cross-coupling, and catalyst degradation

Article abstract of DOI:10.1021/om300399j

Complex [Ni₂(ⁱPr₂Im)₄(COD)] (1) (ⁱPr₂Im = 1,3-diisopropylimidazolin-2-ylidene) is a very efficient catalyst for the Suzuki-Miyaura cross-coupling reaction of 4-bromotoluene with phenylboronic acid and also mediates the Ullmann-type homo-cross-coupling reaction of bromobenzene with a moderate efficiency. Stoichiometric reactions of complex 1 with aryl bromides (ArBr) at room temperature lead to mixtures of aryl bromo complexes of the type trans-[Ni( ⁱPr₂Im)₂(Br)(Ar)] and the bis(bromo) complex trans-[Ni(ⁱPr₂Im)₂(Br)₂] 2. The complexes trans-[Ni(ⁱPr₂Im)₂(Br)(Ar)] (for Ar = Ph 3, 4-MeC₆H₄4, 4-Me(O)CC₆H₄5, 4-MeOC₆H₄6, 4-MeSC₆H₄7, 4-Me ₂NC₆H₄8, 2-C₅NH₄9) can be selectively synthesized by working at low temperatures and using a high dilution of the starting materials. A major deactivation pathway for trans-[Ni(ⁱPr₂Im)₂(Br)(Ar)] was identified in the presence of aryl bromides. This deactivation process includes (i) the formation of trans-[Ni(ⁱPr₂Im)₂(Br) ₂] from trans-[Ni(ⁱPr₂Im)₂(Br)(Ar)] (2) and ArBr and (ii) the formation of an imidazolium salt of the type 2[ ⁱPr₂Im-Ar]⁺[NiBr₄]^2- from trans-[Ni(ⁱPr₂Im)₂(Br)₂] (2) and ArBr. The reactions of complex 2 with a series of aryl halides at higher temperatures lead to the decomposition of the bis(carbene) nickel moiety with formation of the imidazolium salts 2[ⁱPr₂Im-Ar] ⁺[NiBr₂X₂]^2- (for X = I, Ar = Ph 10 and X = Br, Ar = Ph 11, 4-MeC₆H₄12, 4-FC₆H ₄13, 4-OSi(CH₃)₃-C₆H₄14) in high yields.

Full text of DOI:10.1021/om300399j

Article
pubs.acs.org/Organometallics
C−Br Activation of Aryl Bromides at Ni0(NHC)2: Stoichiometric
Reactions, Catalytic Application in Suzuki−Miyaura Cross-Coupling,
and Catalyst Degradation
Thomas Zell, Peter Fischer, David Schmidt, and Udo Radius*
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: Complex [Ni2(iPr2Im)4(COD)] (1) (iPr2Im = 1,3-
diisopropylimidazolin-2-ylidene) is a very efficient catalyst for the
Suzuki−Miyaura cross-coupling reaction of 4-bromotoluene with
phenylboronic acid and also mediates the Ullmann-type homo-
cross-coupling reaction of bromobenzene with a moderate
efficiency. Stoichiometric reactions of complex 1 with aryl bromides (ArBr) at room temperature lead to mixtures of aryl
bromo complexes of the type trans-[Ni(iPr2Im)2(Br)(Ar)] and the bis(bromo) complex trans-[Ni(iPr2Im)2(Br)2] 2. The
complexes trans-[Ni(iPr2Im)2(Br)(Ar)] (for Ar = Ph 3, 4-MeC6H4 4, 4-Me(O)CC6H4 5, 4-MeOC6H4 6, 4-MeSC6H4 7, 4-
Me2NC6H4 8, 2-C5NH4 9) can be selectively synthesized by working at low temperatures and using a high dilution of the starting
materials. A major deactivation pathway for trans-[Ni(iPr2Im)2(Br)(Ar)] was identified in the presence of aryl bromides. This
deactivation process includes (i) the formation of trans-[Ni(iPr2Im)2(Br)2] from trans-[Ni(iPr2Im)2(Br)(Ar)] (2) and ArBr and
(ii) the formation of an imidazolium salt of the type 2[iPr2Im-Ar]+[NiBr4]2− from trans-[Ni(iPr2Im)2(Br)2] (2) and ArBr. The
reactions of complex 2 with a series of aryl halides at higher temperatures lead to the decomposition of the bis(carbene) nickel
moiety with formation of the imidazolium salts 2[iPr2Im-Ar]+[NiBr2X2]2− (for X = I, Ar = Ph 10 and X = Br, Ar = Ph 11, 4-
MeC6H4 12, 4-FC6H4 13, 4-OSi(CH3)3-C6H4 14) in high yields.
Matsubara and co-workers6a reported recently the synthesis
INTRODUCTION
■
of a three-coordinated, T-shaped nickel(I) chloride complex,
[NiI(Dip2Im)2(Cl)] (Dip2Im = 1,3-bis(2,6-diisopropylphenyl)-
imidazolin-2-ylidene), stabilized by two bulky NHC ligands
from the reaction of [Ni(Dip2Im)2] with aryl chlorides. This
complex was isolated in a yield of 34% and has been
characterized using crystal structure determination and proton
NMR spectroscopy. Matsubara et al. demonstrated that this
nickel(I) complex catalyzes Kumada−Corriu cross-coupling
reactions of aryl bromides with phenyl magnesium chloride,
and a Ni(I)/Ni(III) cycle was proposed for this reaction.
Similarly, Louie et al. reported the synthesis and structural
characterization of the paramagnetic T-shaped complexes
[NiI(Mes2Im)2(X)] (X = Cl, Br, I; Mes2Im = 1,3-bis(2,4,6-
trimethylphenyl)imidazolin-2-ylidene).6b These complexes
have also been prepared from the reaction of [Ni(Mes2Im)2]
with aryl halides Ar−X, and the potential products trans-
[NiII(Mes2Im)2(X)(Ar)], which would arise from oxidative
addition, have not been observed. These NiI complexes have
been employed as efficient catalysts in Kumada−Corriu and
Suzuki−Miyaura cross-coupling reactions. Most interestingly,
reinvestigating Cavell’s system, these authors found complexes
of the type trans-[Ni(Me2ImMe2)2(Br)(Ar)] (Ar = 4-
MeOC6H4, 4-MeC6H4, 4-CF3C6H4) to be unstable, even in
the solid state under an N2 atmosphere.
Transition metal mediated cross-coupling reactions are among
the most powerful tools in the area of catalytic C−C bond
formation.1 We2 and others3−6 have shown that NHC-
stabilized nickel complexes are efficient catalysts for Kuma-
da−Corriu and Suzuki−Miyaura cross-coupling reactions of
aryl fluorides and aryl chlorides. In earlier studies, we have
shown that dinuclear [Ni2(iPr2Im)4(COD)] (1) (iPr2Im = 1,3-
diisopropylimidazolin-2-ylidene), which acts as a source for the
electron-rich {Ni(iPr2Im)2} complex fragment in stoichiometric
and catalytic reactions,2,7 readily activates Ar−F and Ar−Cl
bonds and catalyzes Suzuki−Miyaura cross-coupling reactions
of perfluorinated arenes and unactivated aryl chlorides with aryl
boronic acids. In stoichiometric reactions we have demon-
strated that oxidative addition of poly- and perfluorinated
arenes (ArF−F) affords four-coordinated square-planar nickel-
(II) complexes of the type trans-[Ni(iPr2Im)2(F)(ArF)].
Oxidative addition reactions of different aryl chlorides to
complex 1 revealed high selectivity for a broad range of
substrates, ranging from activated to deactivated aryl chlorides
to give complexes of the type trans-[Ni(iPr2Im)2(Cl)(Ar)]. In
the case of alkyl and aryl bromides and iodides, Cavell et al.
reported the synthesis of the complexes trans-[Ni-
(Me2 ImMe2 )2 (Br)(2-MeC6 H4 )] and trans-[Ni-
(Me2ImMe2)2(I)(Me)] (Me2ImMe2 = 1,3,4,5-tetramethylimi-
dazolin-2-ylidene) from the oxidative addition of the
corresponding organic halides to the in situ generated
{Ni(Me2ImMe2)2} fragment prepared from the reaction of
[Ni(COD)2] with 2 equivalents of the ligand.4a
Received: May 11, 2012
Published: July 10, 2012
© 2012 American Chemical Society
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All these results suggest that the generally accepted
mechanism for the cross-coupling reactions via oxidative
addition, transmetalation, and reductive elimination may not
be applicable to all NHC/Ni-based systems. At least the steric
demand of the NHC and presumably also its donor strength
and the nature of the substrate Ar−X have an influence on the
catalytic pathway. This implies the question of the formation of
the (expected) nickel(II) aryl halide complexes trans-
[NiII(R2Im)2(X)(Ar)] or the (rather unexpected) NiI aryl
halide complexes [NiI(R2Im)2(Ar)]/[NiI(R2Im)2(X)] obtained
from the reaction of a {Ni(R2Im)2} source and Ar−X. In
continuation of our earlier work on Car−F and Car−Cl bond
activation we report here the reactivity of 1 with respect to aryl
bromides and aryl iodides as well as on deactivation pathways
of Ni-catalyzed cross-coupling reactions.
ligated complexes trans-[Ni(Me2ImMe2)2(Br)(Ar)] (Ar = 4-
MeOC6H4, 4-MeC6H4, 4-CF3C6H4), we have found that the
isolated compounds 3−9 are stable in solution and in the solid
state for several months under an inert atmosphere. Once
formed, the complexes trans-[Ni(iPr2Im)2(Br)(Ar)] seem to be
very robust towards decomposition. Thus, heating a sample of
trans-[Ni(iPr2Im)2(Br)(C6H5)] (3) in deuterobenzene for
several days to 80 °C does not lead to decomposition or to
ligand scrambling (vide infra). However, heating a sample of
trans-[Ni(iPr2Im)2(Br)(C6H5)] (3) in the presence of addi-
tional phenyl bromide in deuterobenzene leads among other
products (vide infra) to minor amounts of trans-[Ni-
(iPr2Im)2(Br)2] (2) and biphenyl (detected by GC-MS). This
finding may allow the assumption that the formation of the side
products observed for the reaction of 1 and Ar−Br at room
temperature originates from the reaction of the aryl bromide
complexes with additional aryl bromide.
RESULTS AND DISCUSSIONS
■
The compounds 3−9 were characterized by elemental
analysis, EI-MS, IR and multinuclear NMR spectroscopy. The
results of the CHN analyses are in a good agreement with the
proposal of the reaction product as given in Scheme 1. The 1H
and 13C{1H} NMR spectra of the products 3−9 clearly reveal a
trans-configuration of the NHC ligands in solution, and the
integrations of the NHC and aryl ligand protons confirm the
formation of trans-[Ni(iPr2Im)2(Br)(Ar)]. The compounds
display inequivalent methyl groups of the NHCs’ isopropyl
substituents, represented by two distinct sets of resonances in
Stoichiometric Car−Br Activation. Although complex 1
readily reacts with poly- and perfluorinated arenes (ArF−F)
with formation of products of an oxidative addition,2 no
transformation was observed in stoichiometric reactions with
fluorobenzene, even under elevated temperatures or photolytic
conditions. Similarly to previously reported deactivation
reactions for NHC complexes of group 10 metals,8 the reaction
of complex 1 with iodobenzene resulted in the formation of
elemental nickel. Chlorobenzene2e and bromobenzene react
with compound 1 with formation of the corresponding aryl
halide complex. The room-temperature reactions of
[Ni2(iPr2Im)4(COD)] (1) with stoichiometric amounts of
different aryl bromides Ar−Br result in the formation of a
mixture of diamagnetic products. These mixtures include the
NiII aryl bromide complex trans-[Ni(iPr2Im)2(Br)(Ar)], the NiII
bis(bromide) complex trans-[Ni(iPr2Im)2(Br)2] (2), and
another species, presumably the NiII bis(aryl) complex
[Ni(iPr2Im)2(Ar)2], in various amounts. Depending on the
nature of the aryl bromide and on the conditions employed, the
amount on the secondary reaction products [Ni(iPr2Im)2(Br)2]
and [Ni(iPr2Im)2(Ar)2] can increase to more than 50%.
Although radical intermediates cannot be ruled out, we did
not observe any NiI intermediates in these reactions.
1
the H and 13C{1H} NMR spectra. The spectra reveal sets of
equivalent methine resonances of the isopropyl groups and
equivalent backbone and carbene carbon resonances. A
1
summary of the H NMR and 13C{1H} NMR resonances of
the NHC ligands in the complexes 3−9 is given in Tables 1 and
2.
1
Table 1. H NMR Chemical Shifts (C6D6) of the NHC
Ligands in Compounds 3−9
i
d, 12 H,
CH3
d, 12 H,
CH3
s, 4 H,
CHCH
sept, 4 H, Pr-
compound
CH
3
4
5
6
7
8
9
1.18
1.20
1.16
1.19
1.18
1.22
1.28
1.56
1.57
1.54
1.56
1.56
1.58
1.63
6.22
6.23
6.19
6.25
6.23
6.27
6.23
6.55
6.58
6.47
6.55
6.51
6.58
6.81
The selective synthesis of the complexes trans-[Ni-
(iPr2Im)2(Br)(Ar)] (Ar = Ph 3, 4-MeC6H4 4, 4-Me(O)CC6H4
5, 4-MeOC6H4 6, 4-MeSC6H4 7, 4-Me2NC6H4 8, 2-C5NH4 9)
(see Scheme 1) can be achieved by performing the reaction at
Scheme 1. Synthesis of the Activation Products of Aryl
Bromides of the Type trans-[Ni(iPr2Im)2(Br)(Ar)] for Ar =
Ph 3, 4-MeC6H4 4, 4-Me(O)CC6H4 5, 4-MeOC6H4 6, 4-
MeSC6H4 7, 4-Me2NC6H4 8, and 2-C5NH4 9
The reactions of differently substituted aryl bromides,
ranging from electron-rich to electron-poor substrates, show a
high chemoselectivity toward Car−Br activation. The reaction
of 1 with 4-bromoacetophenone affords the complex trans-
Table 2. 13C NMR Chemical Shifts (C6D6) of the NHC
Ligands in Compounds 3−9
compound
CH3
CH3
iPr-CH
CHCH
NCN
3
4
5
6
7
8
9
22.9
22.9
22.9
22.9
22.9
22.9
23.2
23.9
24.0
23.9
23.9
23.9
24.0
23.9
52.0
52.0
52.1
52.0
52.0
52.0
51.8
115.9
115.8
116.1
115.9
116.0
115.8
115.8
184.0
184.4
182.7
184.5
183.6
185.1
183.8
low temperature (−78 °C) and in high dilution of the reactants.
The complexes 3−9 were isolated in more than 80% yield, but
small amounts (<5%) of the bis(bromo) complex trans-
[Ni(iPr2Im)2(Br)2] (2) were observed occasionally in the
crude reaction mixture. In contrast to the observations made by
Louie et al. for the 1,3,4,5-tetramethylimidazolin-2-ylidene-
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[Ni(iPr2Im)2(Br)(4-Me(O)CC6H4)] (5) in a high yield and
does not lead to a likely CO side-on coordinated nickel
ketone complex. The IR spectrum of compound 5 clearly
reveals the CO stretch at 1660 cm−1, and the 13C{1H} NMR
spectrum shows a resonance for the carbonyl carbon atom at
196.7 ppm. Furthermore, we have previously shown that
complex 1 reacts with thioethers delivering Car−S bond
activation products,7d but the reaction of 1 with 4-MeSC6H4-
Br leads exclusively to the oxidative addition of the C−Br bond.
Single crystals of trans-[Ni(iPr2Im)2(Br)(4-Me2NC6H4)] (8)
have been obtained from a saturated benzene solution at room
temperature. The result of the X-ray diffraction analysis (see
Figure 1) clearly confirms the formation of a reaction product
Scheme 2. Catalytic C−Br Activation: (i)
[Ni2(iPr2Im)4(COD)] (1)-Catalyzed Suzuki−Miyaura Cross-
Coupling Reaction of 4-Bromotoluene with Phenylboronic
a
Acid ; (ii) [Ni2(iPr2Im)4(COD)] (1)-Catalyzed Reductive
b
Homocoupling Reaction of Bromobenzene
a
Reaction conditions for quantitative conversion of the aryl bromide:
0.61 mol % catalyst 1, 1.1 equiv of phenyboronic acid, 16 h in boiling
b
toluene, 3.0 equiv of KOtBu. Reaction conditions: 0.80 mol % catalyst
n
1, 1.6 equiv of zinc, 16 h, 125 °C, Bu4NBr, 46% yield.
since the temperatures, additives, solvents, and most
importantly the substrates used vary. However, it might be
stated that complex 1 reveals a significantly higher catalytic
efficiency in this type of transformation compared to other
reported nickel-based catalysts. Leadbeater et al. obtained 80%
yield of the cross-coupling product in the reaction of 4-
bromotoluene with phenylboronic acid in the presence of 3.0
mol % trans-[Ni(NEt3)2(Cl)2] and 3.0 equivalents of Na3PO4
in dioxane at 95 °C.9a Furthermore they demonstrated in this
report that the analogue 2,2′-bipyridine complex [Ni(bipy)-
(Cl)2] is less active. In another publication, the Leadbeater
group studied cross-coupling reactions of aryl bromides
catalyzed by in situ generated phosphine-ligated nickel
complexes [Ni(dppf)(Cl)2] (with dppf = 1,10-bis-
(diphenylphosphino)ferrocene) and [Ni{P(OMe)3}2(Cl)2].9b
Perec et al. applied phosphine complexes of the type
[Ni(L)2(Cl)2] in the presence of additional ligand L (with
(L)2 = (PPh3)2 and dppe = 1,2-bis(diphenylphosphino)ethane)
in cross-coupling reactions of electron-rich and electron-poor
aryl bromides with phenylboronic acid.9d The dppe-chelated
complex revealed a higher activity. A reaction of methyl-4-
bromobenzoate (activated substrate) with 1.2 equivalents of
phenylboronic acid and 3.0 equivalents of K3PO4 at a catalyst
loading of 1.0 mol % [Ni(dppe)(Cl)2] and the addition of 1.0
mol % dppe led to the formation of 88% of the cross-coupling
product after heating the reaction for 14 h to 80 °C in toluene.
Lee and co-workers used nickel(II) complexes of bidentate
NHC/phoshine ligands as catalysts for cross-coupling reactions
of aryl bromides and chlorides with phenylboronic acid.9e The
catalytic coupling of bromobenzene with 1.3 equivalents of
phenylboronic acid in the presence of 2.6 equivalents of base
(K3PO4) and 3.0 mol % of the complex led to the formation of
95% biphenyl after heating the reaction mixture for 12 h in
toluene to 80 °C. Full conversion of the aryl bromide was
achieved under the same conditions with the addition of 6 mol
% PPh3. Similarly, Chen et al. observed a poor catalytic activity
of nickel(II) complexes of the type [Ni(L)](PF6)2 with L =
b i s ( N - p y r i d y l i m i d a z o l ) m e t h a n e a n d b i s ( N -
picolylbenzimidazol)methane. Using this catalyst for the
reaction of 4-bromotoluene with 1.2 equivalents of phenyl-
boronic acid (2.4 equiv of K3PO4·3H2O, 3.0 mol % catalyst, 80
°C, 10 h), the yield of cross-coupling product was raised from
21% up to 100% upon addition of 6.0 mol % PPh3.
Figure 1. ORTEP diagram of the molecular structure of trans-
[Ni(iPr2Im)2(Br)(4-Me2NC6H4)] (8) in the solid state (ellipsoids set
at the 50% probability level). The hydrogen atoms have been omitted
for clarity. Bond lengths (Å) and angles (deg): Ni−C1 1.909(2), Ni−
C11 1.907(2), Ni−C21 1.910(2), Ni−Br 2.4064(6), C21−Ni−C1
90.0(4), C21−Ni−C11 89.4(4), C1−Ni−C11 177.7(4), C21−Ni−Br
174.7(3), C1−Ni−Br 90.0(3), C11−Ni−Br 90.8(3).
of C−Br oxidative addition to give the square-planar, four-
coordinate nickel(II) complex trans-[Ni(iPr2Im)2(Br)(4-
Me2NC6H4)] (8). The distances and angles observed are
unexceptional.
Catalytic Car−Br Activation: Suzuki−Miyaura Cross-
Coupling and Reductive Homocoupling. We reported
earlier that complex 1 is an efficient catalyst for Suzuki−
Miyaura cross-coupling reactions of aryl chlorides with
phenylboronic acid.2e Variation of the reaction conditions
such as catalyst loading, temperature, and kind and
stoichiometric amount of the additive led to quantitative
conversion of chlorobenzene and 4-chlorotoluene to the
corresponding biaryl in the cross-coupling reaction with
phenylboronic acid. The optimized catalytic reactions were
performed in the presence of 3.0 equivalents of KOtBu in
boiling toluene with a catalyst loading of 2.4 mol % for 16 h.
Catalyst 1 shows under the same conditions a significantly
higher catalytic efficiency using aryl bromides. Quantitative
conversion of 4-bromotoluene to 4-methylbiphenyl was
observed with a catalyst loading of 0.61 mol % (see Scheme
2). Minor amounts of 4,4′-dimethyl-1,1′-biphenyl (1.1% with
respect to 4-methylbiphenyl) and biphenyl (4.3% with respect
to 4-methylbiphenyl) were detected due to homocoupling of
the aryl bromide and the boronic acid, respectively.
Different NHC-, phosphine-, and amine-ligated nickel
complexes have been reported as active catalysts for the
Suzuki−Miyaura cross-coupling reactions of aryl bromides so
far,9 and the substrates tested ranged from activated to
deactivated aryl bromides. It is unfeasible to compare catalyst
activities in cross-coupling reactions between different studies,
Complex [Ni2(iPr2Im)4(COD)] (1) also catalyzes the
reductive homocoupling reaction of bromobenzene in the
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presence of zinc metal (see Scheme 2). First attempts to use 1
as a catalyst in boiling toluene as a solvent and potassium tert-
butoxide or cesium carbonate as a base were not successful,
since the reaction in the presence of 3.0 equivalents of the
corresponding base, 1.6 equivalents of zinc powder, and 1.2 mol
% 1 led to the formation of biphenyl in only 9% and 18% yield,
respectively (see Experimental Section, Table 3). A higher
deactivation products from the complexes trans-[Ni-
(iPr2Im)2(Br)(Ar)] in the presence of Ar−X. Heating a sample
of trans-[Ni(iPr2Im)2(Br)(C6H5)] (3) with 8 equivalents of
phenyl bromide in deuterobenzene for 16 h leads to the
formation of 2[iPr2Im-C6H5]+[NiBr4]2− (11) in high yields and
to minor amounts of trans-[Ni(iPr2Im)2(Br)2] (2). Biphenyl
was detected from this mixture via GC-MS analysis. The
formation of 11 was also observed in an almost quantitative
yield starting from 1 and an excess of phenyl bromide at higher
temperatures. In principle, similar reactions (quarternization of
phosphines) have been known for phosphine systems for a very
long time. Over 50 years ago, Horner et al. reported the
reaction of tertiary phosphines with aryl halides to afford
quaternary phosphonium salts in the presence of 3d transition
metal halides,12 and also identified some intermediates (A, B,
and C in Scheme 3).
Similar reactions of NHC ligands have been observed in the
case of group 8 metals by Cavell et al. at [Pd(Me2ImMe2)2].8 If
iodobenzene is used as a coupling substrate, 2-phenyl-1,3,4,5-
tetramethylimidazolium iodide salt was isolated, which was
formed by reductive elimination from [Pd(Me2ImMe2)2(I)-
(C6H5)]. Similarly, in the case of nickel, 1,2,3,4,5-penta-
methylimidazolium ion formation was observed from the
reaction of “[Ni(Me2ImMe2)2]” with MeI, even when the
reaction and workup were performed at temperatures as low as
−50 °C.
Table 3. Isolated Yields of Biphenyl from the Reductive
a
Homocoupling Reactions of Bromobenzene
base
Et3N
Cs2CO3
yield (%)
mol % 1
yield (%)
b
b
c
c
c
1
2
9
3
4
5
0.8
2.5
7.5
46
54
73
18
a
Variation of reaction conditions (base, catalyst loading). The reaction
b
time was 16 h and 1.6 equiv of zinc was used. 3.0 equiv of base,
refluxing toluene, 1.2 mol % catalyst. 2.2 equiv of NBu4Br, 125 °C.
c
catalytic activity was observed for the reactions performed in
the ionic liquid tetra-n-butylammonium bromide at 125 °C.
With a lower catalyst loading of 0.8 mol % the homocoupling
product was formed in a higher yield of 46%. An increase in
catalyst loading up to 7.5 mol % led to a rise of the yield up to
73% under the same conditions.
Deactivation Pathways. NHCs are currently widely
employed as strong donor ligands for the synthesis of metal
complexes and in catalysis. It is generally accepted that these
ligands should in principle provide stable catalyst systems, and
in some cases NHC complexes have been used at very high
temperatures. One of the major problems using NHCs as
ligands in catalysis is thatdespite strong transition metal−
NHC bindingNHCs are susceptible to loss by reductive
elimination reaction from the metal center, leading to
deactivation of the catalyst. The reductive elimination of
imidazolium salts represents a major catalyst deactivation
pathway of NHC complexes.8,11 In contrast to transition metal
phosphine complexes, these reactions are generally irreversible
for transition metal NHC complexes.
The formation of imidazolium salts of the type 2[NHC-
Ar]+[NiX4]2− seems to be the final product of the deactivation
using 1 as a catalyst. Since the major deactivation path
identified includes (i) formation of trans-[Ni(iPr2Im)2(Br)2]
from trans-[Ni(iPr2Im)2(Br)(Ar)] and ArBr and (ii) formation
of 2[iPr2Im-Ar]+[NiX2Br2]2− from trans-[Ni(iPr2Im)2(Br)2]
and ArBr, we examined the reaction of the Ni(II) complex
trans-[Ni(iPr2Im)2(Br)2] (2)13 with an excess of different aryl
halides Ar−X (see Scheme 4). Whereas fluorobenzene and
Scheme 4. Reactions of trans-[Ni(iPr2Im)2(Br)2] (2) with
Aryl Halides (for X = I, R = H 10 and for X = Br, R = H 11, R
= CH3 12, R = F 13, R = OSi(CH3)3 14)
During our work on catalytic cross-coupling reactions using 1
as a catalyst, we became aware of two major decomposition
products that arise from reactions of 1 with organic halides,
especially in the absence of coupling reagents. One of the
products is the dihalide complex trans-[Ni(iPr2Im)2(X)2], and
the other the imidazolium salt 2[iPr2Im-C6H5]+[NiX4]2− (see
Scheme 3). As mentioned above, heating a sample of trans-
[Ni(iPr2Im)2(Br)(C6H5)] (3) in deuterobenzene for several
days to 80 °C does not lead to decomposition or ligand
scrambling. Thus, we speculated on the formation of
chlorobenzene are unreactive, the reactions of bromobenzene,
iodobenzene, and some bromo benzene derivates with 2 lead to
the formation of ionic compounds of the type 2[iPr2Im-
Ar]+[NiBr2X2]2− (for X = I, R = H 10 and for X = Br, R = H
11, R = CH3 12, R = F 13, R = OSi(CH3)3 14). However, the
reaction conditions to drive the stoichiometric reaction to
completeness are rather vigorous. In order to obtain good
yields of the resulting blue- to green-colored products, the
starting materials needed to be stirred for several days at high
temperatures (>100 °C) in toluene. A stoichiometric use of the
starting material does not lead to full conversion under these
conditions, and in all cases some trans-[Ni(iPr2Im)2(Br)2] (2)
remains in the red-colored solution. The imidazolium salts
show the characteristic color of the particular anionic nickel
component [NiBr2I2]2− (green) and [NiBr4]2− (blue).14 All
Scheme 3. Catalyst Deactivation Reactions: (i) Formation of
Aryl Imidazolium Salts; (ii) Quaternization of Tertiary
Phosphines as Reported by Horner et al. for M = Ni, Co, Zn;
X = Br, Cl
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isolated imidazolium salts have been characterized by elemental
[Ni(iPr2Im)2(Br)(Ar)] with aryl bromide to give the bis-
(bromo) complex trans-[Ni(iPr2Im)2(Br)2] (2), followed by
formation of an aryl-imidazolium salt of the type [iPr2Im-
Ar]+[NiBr4]2−.
1
analysis and IR and NMR spectroscopy. The H and 13C{1H}
NMR spectra of these compounds show in addition to the
signals for the aryl substituent a resonance for the isopropyl
methyl group, for the isopropyl methine group, and for the
backbone unit, indicating their NMR spectroscopic equiv-
alence. The resonances of the aryl-binding imdiazolium carbon
atoms were detected in a close range between 144.0 (13) and
144.9 ppm (14), a region typically observed for an aryl-
substituted imidazolium salt of the type [R2Im-Ar]+.15
The formation of the imidazolium salts was confirmed by X-
ray crystallography. Single crystals of 2[iPr2Im-C6H5]+[NiBr4]2−
(11) have been obtained from a saturated acetone solution at
room temperature. The molecular structure of the imidazolium
cation is shown in Figure 2. The bond distances and angles
EXPERIMENTAL SECTION
■
General Considerations. All reactions and subsequent manipu-
lations involving organometallic reagents were performed under a
nitrogen or argon atmosphere using standard Schlenk techniques as
reported previously.16 Elemental analyses were performed in the
microanalytical laboratory of the Inorganic Chemistry Department of
the University of Wurzburg. The EI mass spectra were recorded with a
̈
Varian MAT 3830 (70 eV). NMR spectra were recorded with Bruker
Avance 400 and Avance 500 spectrometers at 296 K in C6D6 or
acetone-d6. NMR spectroscopic data are listed in parts per million
(ppm) and are reported relative to tetramethylsilane (1H, 13C, 29Si)
and tri(chloro)fluoromethane (19F) as external standard. Coupling
constants are quoted in hertz (Hz). The 13C NMR spectra are broad-
band proton-decoupled and internally referenced to the natural-
abundance carbon resonances (13C: C6D6 128.02 ppm and acetone-d6
29.87 ppm). The assignments of the 13C{1H} NMR resonances are
supported by DEPT 135 NMR experiments. The 1H NMR spectra are
referenced internally to residual protio-solvent resonances (1H:
C6D5H 7.15 ppm and acetone-d5 2.05 ppm). Infrared spectra were
recorded with a Bruker Vertex 70 FT-IR spectrometer or with a
Nicolet 380 FT-IR from solid samples using attenuated total
reflectance (ATR) or as KBr pellets. All IR data are quoted in
wavenumbers (cm−1). GC-MS samples of the cross-coupling reactions
were diluted with diethyl ether, and their analyses were performed
using a Varian 320-MS (70 eV) equipped with a Varian 450-GC.
Compound 12a,7a and trans-[Ni(iPr2Im)2(Br)2] (2)13 were prepared
according to literature procedures; all other compounds have been
obtained from commercial sources and were used without further
purification.
Figure 2. ORTEP diagram of the molecular structure of the
imidazolium cation [iPr2Im-C6H5]+ of 11 in the solid state (ellipsoids
set at the 50% probability level). The hydrogen atoms have been
omitted for clarity. Bond lengths (Å) and angles (deg): C1−N1
1.329(12), C1−N2 1.339(10), C1−C10 1.483(12); N1−C1−N2
108.6(8), N1−C1−C10 126.0(8), N2−C1−C10 125.3(8), C1−C10−
C11 119.5(8), C1−C10−C15 119.7(8), C11−C10−C15 120.7(9).
Synthesis. General Method for the Synthesis of the Complexes
trans-[Ni(iPr2Im)2(Br)(Ar)]. [Ni2(iPr2Im)4(COD)] (1) was suspended
in n-hexane, and the mixture was cooled to −78 °C using a dry-ice/
isopropyl-alcohol bath. The corresponding aryl bromide was dissolved
in n-hexane and added dropwise to the mixture. The reaction mixture
was allowed to reach room temperature overnight, and all volatiles
were removed in vacuo. The residue was suspended in n-hexane, and
the product was filtered off, washed with n-hexane, and dried in vacuo.
Synthesis of trans-[Ni(iPr2Im)2(Br)(Ph)], 3. A 336 mg amount of
[Ni2(iPr2Im)4(COD)] (1) (0.40 mmol) was suspendend in 120 mL of
n-hexane, and 85 μL of phenyl bromide (0.80 mmol) in 50 mL of n-
hexane was added. Workup: 20 mL n-hexane for suspending and 10
mL of n-hexane for washing. Yield: 383 mg (92%) of a brownish-
yellow solid. Anal. Calcd (found) for C24H37BrN4Ni [520.17 g/mol]:
C 55.42 (55.90), H 7.17 (7.13), 10.77 (10.86). EI-MS m/z (%): 520
(1.4) [M]+, 443 (16.5) [Ni(iPr2Im)2(Br)]+, 439 (3.7) [Ni-
observed are as expected, and the dihedral angle (between the
planes N2−N1−C1 and C10−C11−C15) along the C−C
bond between the imidazolium ring and the phenyl ring of
96.7(5)° is close to perpendicular. For the reaction of aryl
bromides with trans-[Ni(iPr2Im)2(Br)2] (2) we assume a
stepwise deactivation via intermediates of the type [iPr2Im-
Ar]+[Ni(iPr2Im)Br3]−.
CONCLUSION
■
In summary, we have shown that the compound
[Ni2(iPr2Im)4(COD)] (1) is an excellent catalyst for the
Suzuki−Miyaura cross-coupling of aryl bromides with phenyl
boronic acid. Its activity is higher than reported for other
nickel-based catalysts. Recent reports in the literature describe
the formation of Ni(I) radicals of the type [NiI(NHC)2(X)] in
reactions of [Ni(NHC)2] with aryl halides if sterically
demanding NHCs are employed. In contrast to these findings
we have demonstrated that stoichiometric reactions of 1 with a
variety of aryl bromides lead to oxidative addition and the
formation of stable, four-coordinated, diamagnetic nickel(II)
aryl bromide complexes of the type trans-[Ni(iPr2Im)2(Br)-
(Ar)]. Furthermore, we present some insights into NHC
catalyst deactivation. On the basis of observations in
stoichiometric reactions we revealed a deactivation pathway
of {Ni(NHC)2} by reactions with aryl bromides at elevated
temperatures. This catalyst degradation might be responsible
for the relatively poor activity of 1 as a catalyst for the reductive
homocoupling of bromobenzene to biphenyl. The process
involves the reaction of the aryl bromo complexes trans-
1
(iPr2Im)2(Ph)]+. H NMR (500 MHz, C6D6, 23 °C): δ/ppm 1.18
3
i
3
i
(d, 12 H, JHH = 6.8 Hz, Pr-CH3), 1.56 (d, 12 H, JHH = 6.8 Hz, Pr-
CH3), 6.22 (s, 4 H, CHCH), 6.55 (sept, 4 H, 3JHH = 6.8 Hz, iPr-CH),
6.77 (m, 1 H, aryl-Hp), 7.64 (m, 2 H, aryl-Htert), 7.55 (m, 2 H, aryl-
Htert). 13C{1H} NMR (100 MHz, C6D6, 23 °C): δ/ppm 22.9 (iPr-
CH3), 23.9 (iPr-CH3), 52.0 (iPr-CH), 115.9 (NCCN), 121.1 (aryl-Cp),
126.0 (aryl-Ctert), 138.2 (aryl-Ctert), 158.3 (aryl-Ci), 184.0 (NCN). IR
(ATR, [cm−1]): 673 (w), 704 (s, Ph-δCH,oop), 734 (m, Ph-δCH,oop), 755
(vw), 799 (vw), 831 (vw), 881 (vw), 998 (vw), 1018 (vw), 1028 (vw),
1053 (vw), 1080 (vw), 1133 (w), 1177 (vs), 1215 (s, NHC-γCH), 1247
(w), 1302 (w), 1371 (m), 1394 (m), 1412 (m), 1425 (m), 1445 (w),
1465 (w), 1513 (vw), 1530 (vw), 1562 (w), 1630 (w), 1659 (m), 2876
(vw, νCH), 2934 (vw, νCH), 2974 (w, νCH), 3083 (vw, νCH). trans-
[Ni(iPr2Im)2(Br)2] (2). If the same reaction is performed at room
temperature in a reduced volume of solvent (hexane, thf, diethyl ether,
benzene, or toluene), a mixture of trans-[Ni(iPr2Im)2(Br)(Ph)] (3),
trans-[Ni(iPr2Im)2(Br)2] (2), and a third NHC-Ni-containing
component, presumably trans-[Ni(iPr2Im)2(Ph)2], can be isolated.
The exact amount of each product formed in the mixture varies
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strongly and leads to mixtures containing up to 50% of compound 2.
Single crystals of compound 2 were obtained by storing a saturated
toluene solution of the reaction mixture obtained from the reaction of
1 with PhBr at room temperature at −30 °C. X-ray analysis of these
crystals led to an identical structure to that reported in the literature.5n
Anal. Calcd (found) for C18H32Br2N4Ni [522.97 g/mol]: C 41.49
(41.34), H 6.52 (6.17), 10.24 (10.71). 1H NMR (400 MHz, C6D6, 25
and 10 mL of n-hexane for washing. Yield: 357 mg (81%) of a light
brown solid. Anal. Calcd (found) for C25H39BrN4NiO [550.20 g/
mol]: C 54.57 (54.41), H 7.14 (7.18), 10.18 (10.13). EI-MS m/z (%):
550 (3.8) [M]+, 259 (3.3) [Ni(CH2CHHIm)(C6H4OMe)]+. 1H NMR
3
i
(400 MHz, C6D6, 23 °C): δ/ppm 1.19 (d, 12 H, JHH = 6.8 Hz, Pr-
3
i
CH3), 1.56 (d, 12 H, JHH = 6.8 Hz, Pr-CH3), 3.33 (s, 3 H, OCH3),
3
i
6.25 (s, 4 H, CHCH), 6.55 (sept, 4 H, JHH = 6.8 Hz, Pr-CH), 6.66
(m, 2 H, aryl-H), 7.26 (m, 2 H, aryl-H). 13C{1H} NMR (100 MHz,
C6D6, 23 °C): δ/ppm 22.9 (iPr-CH3), 23.9 (iPr-CH3), 52.0 (iPr-CH),
54.4 (OCH3), 112.7 (aryl-Ctert), 115.9 (NCCN), 138.2 (aryl-Ctert),
143.6 (aryl-Cquart), 155.9 (aryl-Cquart), 184.5 (NCN). IR (ATR,
[cm−1]): 630 (s), 675 (s), 691 (vs), 704 (vs), 732 (m), 811 (m, Ph-
δCH,oop), 846 (m), 880 (m), 926 (w), 1005 (m), 1025 (m), 1080 (w),
1132 (m), 1174 (m), 1216 (vs, NHC-γCH), 1258 (w), 1300 (m), 1369
(s), 1391 (s), 1405 (s), 1424 (s), 1465 (m), 1518 (w), 1595 (w), 1660
(vw), 2870 (vw, νCH), 2932 (w, νCH), 2969 (w, νCH), 3135 (vw,
3
i
°C): δ/ppm 1.51 (d, 24 H, JHH = 6.8 Hz, Pr-CH3), 6.28 (s, 4 H,
CHCH), 6.93 (sept, 4 H, 3JHH = 6.8 Hz, iPr-CH). 13C{1H} NMR (100
MHz, C6D6, 25 °C): δ/ppm 23.1 (iPr-CH3), 52.4 (iPr-CH), 116.6
(NCCN). Due to the low solubility of the compound in benzene, the
resonance of the carbene carbon atom was not detected in benzene,
1
but in acetone-d6. H NMR (500 MHz, acetone-d6, 23 °C): δ/ppm
1.65 (d, 24 H, 3JHH = 6.8 Hz, iPr-CH3), 6.73 (sept, 4 H, 3JHH = 6.8 Hz,
iPr-CH), 7.15 (s, 4 H, CHCH). 13C{1H} NMR (125 MHz, acetone-d6,
23 °C): δ/ppm 24.1 (iPr-CH3), 53.9 (iPr-CH), 119.0 (NCCN), 170.4
(NCN). IR (ATR, [cm−1]): 709 (m), 735 (w), 833 (vw), 1005 (vw),
1032 (vw), 1135 (w), 1176 (w), 1213 (vs, NHC-γCH), 1309 (w), 1340
(vw), 1370 (m), 1393 (m), 1416 (m), 1429 (s), 1431 (s), 1448 (w),
1457 (w), 1465 (w), 1472 (w), 1476 (vw), 2875 (w, νCH), 2931 (w,
νCH), 2972 (m, νCH), 3093 (vw, νCH), 3122 (w, νCH), 3142 (vw),
3154 (vw).
ν
CH).
Synthesis of trans-[Ni(iPr2Im)2(Br)(4-MeSC6H4)], 7. A 168 mg
sample of [Ni2(iPr2Im)4(COD)] (1) (0.20 mmol) was suspendend in
50 mL of n-hexane, and 85 mg of 4-bromothioanisol (0.42 mmol) in
20 mL of n-hexane was added. Workup: 10 mL n-hexane for
suspending and 10 mL of n-hexane for washing. Yield: 171 mg (76%)
of a light brown solid. Anal. Calcd (found) for C25H39BrN4NiS
[566.26 g/mol]: C 53.03 (53.43), H 6.94 (7.07), 9.89 (9.68). EI-MS
m/z (%): 443 (14.1) [Ni(iPr2Im)2(Br)]+, 275 (82.2) [Ni-
(CH2CHHIm)(C6H5-S-CH3)]+, 233 (12.7) [Ni(CH2CHHIm)(Br)]+.
Synthesis of trans-[Ni(iPr2Im)2(Br)(4-MeC6H4)], 4. A 336 mg sample
of [Ni2(iPr2Im)4(COD)] (1) (0.40 mmol) was suspendend in 100 mL
of n-hexane, and 100 μL of 4-bromotoluene (0.80 mmol) in 75 mL of
n-hexane was added. Workup: 20 mL n-hexane for suspending and 10
mL of n-hexane for washing. Yield: 364 mg (83%) of a yellow solid.
Anal. Calcd (found) for C25H39BrN4Ni [534.20 g/mol]: C 56.21
(56.27), H 7.36 (7.31), 10.49 (10.51). EI-MS m/z (%): 443 (5.5)
[Ni(iPr2Im)2(Br)]+, 243 (100.0) [Ni(CH2CHHIm)(C6H4Me)]+, 233
1H NMR (400 MHz, C6D6, 25 °C): δ/ppm 1.18 (d, 12 H, JHH = 6.8
3
i
3
i
Hz, Pr-CH3), 1.56 (d, 12 H, JHH = 6.8 Hz, Pr-CH3), 2.04 (s, 3
3
i
H,SCH3), 6.23 (s, 4 H, CHCH), 6.51 (sept, 4 H, JHH = 6.8 Hz, Pr-
3
3
CH), 7.01 (d, 2 H, JHH = 8.1 Hz, aryl-Hp), 7.46 (d, 2 H, JHH = 8.1
Hz, aryl-Ho). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ/ppm 17.2
(SCH3), 22.9 (iPr-CH3), 23.9 (iPr-CH3), 52.0 (iPr-CH), 116.0
(NCCN), 126.7 (aryl-Cquart), 129.3 (aryl-Ctert), 138.5 (aryl-Ctert),
155.6 (aryl-Cquart), 183.6 (NCN). IR (ATR, [cm−1]): 455 (w), 466
(w), 511 (vw), 579 (w), 698 (vs), 799 (m, Ph-δCH,oop), 880 (w), 1027
(m), 1096 (m), 1132 (w), 1173 (w), 1217 (vs, NHC-γCH), 1262 (w),
1300 (w), 1370 (m), 1391 (w), 1407 (m), 1424 (m), 1467 (w), 1562
(vw), 2871 (vw), 2932 (w, νCH), 2969 (w, νCH), 3059 (vw, νCH).
Synthesis of trans-[Ni(iPr2Im)2(Br)(4-Me2NC6H4)], 8. A 336 mg
portion of [Ni2(iPr2Im)4(COD)] (1) (0.40 mmol) was suspendend in
100 mL of n-hexane, and 161 mg of 4-bromo-N,N-dimethylaniline
(0.42 mmol) in 20 mL of n-hexane was added. Workup: 20 mL n-
hexane for suspending and 10 mL of n-hexane for washing. Yield: 388
mg (86%) of a light brown solid. Single crystals suitable for X-ray
diffraction were obtained from recrystallization in benzene. Anal. Calcd
(found) for C26H42BrN5Ni [563.24 g/mol]: C 55.44 (55.08), H 7.52
(7.51), 12.43 (12.34). EI-MS m/z (%): 443 (2.7) [Ni(iPr2Im)2(Br)]+,
362 (33.5) [Ni(iPr2Im)2]+, 272 (11.1) [Ni(CH2CHHIm)-
1
(18.3) [Ni(CH2CHHIm)(Br)]+. H NMR (400 MHz, C6D6, 23 °C):
3
i
3
δ/ppm 1.20 (d, 12 H, JHH = 6.8 Hz, Pr-CH3), 1.57 (d, 12 H, JHH
=
6.8 Hz, iPr-CH3), 2.10 (s, 3 H, aryl-CH3), 6.23 (s, 4 H, CHCH), 6.58
(sept, 4 H, 3JHH = 6.8 Hz, iPr-CH), 6.78 (m, 2 H, aryl-H), 7.44 (d, 2 H,
aryl-H). 13C{1H} NMR (125 MHz, C6D6, 23 °C): δ/ppm 20.9 (aryl-
CH3), 22.9 (iPr-CH3), 24.0 (iPr-CH3), 52.0 (iPr-CH), 115.8 (NCCN),
127.2 (aryl-Ctert), 129.4 (aryl-Cquart), 137.9 (aryl-Ctert), 152.6 (aryl-
Cquart), 184.4 (NCN). IR (ATR, [cm−1]): 609 (w), 675 (s), 688 (vs),
708 (s), 735 (w), 791 (m, Ph-δCH,oop), 816 (w), 831 (w), 881 (w), 930
(vw), 1013 (w), 1028 (w), 1045 (w), 1082 (w), 1114 (w), 1133 (m),
1185 (w), 1215 (vs, NHC-γCH), 1262 (vw), 1301 (w), 1369 (m), 1391
(m), 1406 (m), 1425 (m), 1466 (w), 1478 (w), 1561 (vw), 1615 (vw),
1663 (vw), 2870 (vw, νCH), 2933 (w, νCH), 2968 (w, νCH).
Synthesis of trans-[Ni(iPr2Im)2(Br)(4-Me(O)CC6H4)], 5. A 336 mg
portion of [Ni2(iPr2Im)4(COD)] (1) (0.40 mmol) was suspendend in
100 mL of nhexane. A 160 mg sample of 4-bromoacetophenone (0.80
mmol) in 35 mL of n-hexane was added. Workup: 20 mL n-hexane for
suspending and 10 mL of n-hexane for washing. Yield: 405 mg (90%)
of a yellow solid. Anal. Calcd (found) for C26H39BrN4NiO [562.21 g/
mol]: C 55.54 (55.21), H 6.99 (7.03), 9.97 (9.98). EI-MS m/z (%):
520 (3.0) [Ni(iPr2Im)2(Br)(C6H5)+, 443 (23.3) [Ni(iPr2Im)2(Br)]+,
271 (100.0) [Ni(CH2CHHIm)(CH3(O)CC6H4)]+, 233 (24.2) [Ni-
1
(C6H4NMe2)]+, 233 (9.3) [Ni(CH2CHHIm)(Br)]+. H NMR (500
3
i
MHz, C6D6, 23 °C): δ/ppm 1.22 (d, 12 H, JHH = 6.8 Hz, Pr-CH3),
1.58 (d, 12 H, 3JHH = 6.8 Hz, iPr-CH3), 2.52 (s, 6 H, NCH3), 6.27 (s, 4
H, CHCH), 6.52 (m, 2 H, aryl-Hm), 6.58 (sept, 4 H, JHH = 6.8 Hz,
3
1
(CH2CHHIm)(Br)]+. H NMR (400 MHz, C6D6, 23 °C): δ/ppm
iPr−CH), 7.19 (d, 2 H, aryl-Ho). 13C{1H} NMR (125 MHz, C6D6, 23
°C): δ/ppm 22.9 (iPr-CH3), 24.0 (iPr-CH3), 41.2 (NCH3), 52.0 (iPr-
CH), 113.4 (aryl-Ctert), 115.8 (NCCN), 138.5 (aryl-Ctert), 139.2 (aryl-
Cquart), 146.8 (aryl-Cquart), 185.1 (NCN). IR (ATR, [cm−1]): 630 (w),
675 (m), 697 (vs), 729 (w), 746 (w), 794 (s, Ph-δCH,oop), 816 (w), 827
(w), 880 (vw), 941 (w), 1001 (w), 1028 (w), 1054 (w), 1080 (w),
1116 (w), 1133 (m), 1158 (w), 1172 (m), 1218 (vs, NHC-γCH), 1302
(m), 1326 (w), 1369 (m), 1388 (m), 1406 (m), 1425 (w), 1460 (m),
1484 (vw), 1538 (vw), 1580 (vw), 1612 (w), 2785 (vw), 2784 (vw),
2870 (vw, νCH), 2932 (vw, νCH), 2967 (w, νCH), 3059 (vw, νCH),
3094 (vw, νCH), 3156 (vw).
3
1.16 (d, 12 H, 3JHH = 6.8 Hz, iPr-CH3), 1.54 (d, 12 H, JHH = 6.8 Hz,
iPr-CH3), 2.12 (s, 3 H, C(O)CH3), 6.19 (s, 4 H, CHCH), 6.47 (sept, 4
H, 3JHH = 6.8 Hz, iPr-CH), 7.56 (m, 2 H, aryl-Hm), 7.74 (d, 2 H, aryl-
Ho). 13C{1H} NMR (125 MHz, C6D6, 23 °C): δ/ppm 22.9 (iPr-CH3),
23.9 (iPr-CH3), 25.8 (C(O)CH3), 52.1 (iPr-CH), 116.1 (NCCN),
124.9 (aryl-Ctert), 131.7 (aryl-Cquart), 137.5 (aryl-Ctert), 174.4 (aryl-
Cquart), 182.7 (NCN), 196.7 (C(O)CH3). IR (ATR, [cm−1]): 631
(vw), 645 (vw), 676 (w), 689 (m), 702 (vs), 733 (w), 809 (m, Ph-
δCH,oop), 839 (vw), 881 (vw), 951 (w), 1009 (w), 1027 (w), 1039 (w),
1078 (w), 1134 (w), 1181 (m), 1215 (vs, NHC-γCH), 1274 (s), 1301
(w), 1357 (m), 1370 (m), 1391 (w), 1408 (m), 1425 (m), 1467 (vw),
1534 (w), 1567 (vs), 1660 (vs, νCO), 2874 (vw, νCH), 2933 (vw, νCH),
2971 (w, νCH), 3086 (vw, νCH), 3116 (vw, νCH).
Synthesis of trans-[Ni(iPr2Im)2(Br)(2-C5NH4)], 9. A 336 mg amount
of [Ni2(iPr2Im)4(COD)] 1 (0.40 mmol) was suspendend in 100 mL of
n-hexane, and 77 μL of 4-bromopyridine (0.80 mmol) in 75 mL of n-
hexane was added. Workup: 20 mL n-hexane for suspending and 10
mL of n-hexane for washing. Yield: 363 mg (87%) of a light brown
solid. Anal. Calcd (found) for C23H36BrN5Ni [521.16 g/mol]: C 53.01
(52.74), H 6.96 (6.96), 13.44 (12.99). EI-MS m/z (%): 521 (1.3)
Synthesis of trans-[Ni(iPr2Im)2(Br)(4-MeOC6H4)], 6. A 336 mg
amount of [Ni2(iPr2Im)4(COD)] (1) (0.40 mmol) was suspendend in
100 mL of n-hexane, and 100 μL of 4-bromoanisole (0.80 mmol) in 75
mL of n-hexane was added. Workup: 20 mL n-hexane for suspending
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[M]+, 520 (6.6) [Ni(iPr2Im)2(Br)(2-C5NH3)]+, 443 (10.7) [Ni-
(iPr2Im)2Br)]+, 233 (57.2) [Ni(CH2CHHIm)(Br)]+, 230 (28.0)
vacuo to afford 88 mg (94.5 μmol, 98%) of 2[iPr2Im-C6H5]+[NiBr4]2−
(11) (see below) as a blue solid.
1
[Ni(CH2CHHIm)(C5NH4)]+. H NMR (400 MHz, C6D6, 23 °C):
Method B: 632 μL (6.00 mmol, 50 equiv) of bromobenzene was
added to a suspension of 100 mg (120 μmol) of [Ni2(iPr2Im)4(COD)]
(1) in 5 mL of toluene. The mixture was stirred for 16 h at 100 °C. All
volatiles were removed in vacuo to afford 111 mg (119 μmol, 99%) of
2[iPr2Im-C6H5]+[NiBr4]2− (11) (see below) as a blue solid.
In both cases biphenyl could be detected in the GC-MS of the
reaction mixture.
3
i
3
δ/ppm 1.28 (d, 12 H, JHH = 6.8 Hz, Pr-CH3), 1.63 (d, 12 H, JHH
=
i
6.8 Hz, Pr-CH3), 6.20 (m, 1 H, aryl-H), 6.23 (s, 4 H, CHCH), 6.62
(m, 1 H, aryl-H), 6.81 (sept, 4 H, 3JHH = 6.8 Hz, iPr-CH3), 7.56 (m, 2
H, aryl-H), 8.43 (m, 1 H, aryl-H). 13C{1H} NMR (100 MHz, C6D6, 23
°C): δ/ppm 23.2 (iPr-CH3), 23.9 (iPr-CH3), 51.8 (iPr-CH), 115.2
(aryl-Ctert), 115.8 (NCCN), 129.4 (aryl-Ctert), 132.5 (aryl-Ctert), 147.6
(aryl-Ctert), 183.8 (NCN), 192.2 (aryl-Cquart). IR (ATR, [cm−1]): 673
(m), 695 (vs), 741 (w), 759 (m), 798 (vw), 818 (vw), 931 (vw), 972
(vw), 1001 (vw), 1027 (w), 1041 (vw), 1081 (w), 1107 (vw), 1133
(w), 1175 (vw), 1213 (vs, NHC-γCH), 1254 (vw), 1271 (vw), 1305
(w), 1324 (vw), 1370 (m), 1395 (m), 1407 (m), 1423 (m), 1436 (m),
1465 (w), 1515 (vw), 1538 (w), 1562 (w), 1605 (vw), 1663 (w), 1980
(vw), 2163 (vw), 2872 (vw, νCH), 2935 (vw, νCH), 2971 (w, νCH),
3057 (vw, νCH), 3127 (vw, νCH).
Synthesis of 2[iPr2Im-C6H5]+[NiBr2I2]2−, 10. A 112 μL (1.00 mmol)
amount of iodobenzene was added to a suspension of 260 mg (0.50
mmol) of trans-[Ni(iPr2Im)2(Br)2] in 15 mL of toluene. The red
mixture was stirred for 70 h at 120 °C. A brown precipitate that
formed during this time was filtered off from the solution while hot.
The product was washed with 5 mL of toluene and was dried in vacuo
to give 250 mg (0.23 mmol, 46%) of a brown solid. Anal. Calcd
(found) for C30H42Br2I2N4Ni [930.99 g/mol]: C 38.70 (38.88), H
1
4.55 (4.88), N 6.02 (5.98). H NMR (400 MHz, acetone-d6): δ/ppm
Cross-Coupling Reaction of 4-Bromotoluene with Phenylboronic
Acid. A 656 mg sample of phenylboronic acid (5.37 mmol, 1.1 equiv),
1.64 g of KOtBu (14.64 mmol, 3.0 eqiuv), and 25 mg of
[Ni2(iPr2Im)4(COD)] (29.8 μmol, 0.61 mol %) (1) were suspended
in 10 mL of toluene. After adding 600 μL (4.88 mmol, 1.0 equiv) of 4-
bromotoluene the reaction mixture was refluxed for 16 h. After cooling
the mixture to room temperature the mixture was analyzed by GC-MS
1.62 (d, 12 H, 3JHH = 6.5 Hz, iPr-CH3), 4.54 (sept, 2 H, 3JHH = 6.5 Hz,
iPr-CH), 7.86 (m, 3 H, aryl-Hm + aryl-Hp), 7.97 (d, 2 H, 3JHH = 6.8 Hz,
aryl-Ho), 8.21 (s, 2 H, NCHCHN). 13C{1H} NMR (100 MHz,
acetone-d6): δ/ppm 25.5 (iPr-CH3), 53.1 (iPr-CH), 121.9 (NCCN),
123.6 (aryl-Ci), 131.8 (aryl-Ctert), 133.1 (aryl-Ctert), 134.4 (aryl-Cp),
144.7 (NCN). IR (ATR, [cm−1]): 707 (s), 733 (s), 775 (s), 1080 (w),
1114 (vw), 1139 (w), 1185 (w), 1215 (vs, NHC-γCH), 1377 (w), 1399
(vw), 1419 (vw), 1443 (s), 1453 (m), 1461 (m), 1485 (vs), 1488 (m),
1572 (w), 1605 (vw), 2879 (vw, νCH), 2935 (w, νCH), 2981 (s, νCH),
3075 (m, νCH), 3112 (m, νCH).
1
and H NMR spectroscopy. 4-Methyl-1,1′-biphenyl: EI-MS m/z (%):
168 (100) [M]+.
General Procedure of the Reductive Homocoupling Reactions. In
Toluene. A 500 μL amount of bromobenzene (4.78 mmol) was added
to a suspension of 48 mg of [Ni2(iPr2Im)4(COD)] (1) (57.1 μmol,
1.20 mol %), 500 mg of zinc (7.64 mmol, 1.6 equiv), and the
corresponding base in 10 mL of toluene. The reaction mixture was
refluxed for 16 h. After cooling the mixture to room temperature all
volatiles were removed in vacuo. Afterward the residue was dissolved in
60 mL of hexane, and 50 mL of half-concentrated aqueous potassium
hydroxide solution was added. The organic layer was separated and the
organic phase was extracted three times with 50 mL of half-
concentrated aqueous potassium hydroxide solution. The combined
aqueous phases were extracted five times with 50 mL of hexane and
three times with 50 mL of dichloromethane. All organic phases were
combined, dried over sodium sulfate, and filtered. All volatiles were
removed in vacuo. For the low yield conversions the product was
chromatographically purified over silica gel with hexane as solvent.
Yields are given in Table 3.
Synthesis of 2[iPr2Im-C6H5]+[NiBr4]2−, 11. A 105 μL (1.00 mmol)
sample of bromobenzene was added to a suspension of 260 mg (0.50
mmol) of trans-[Ni(iPr2Im)2(Br)2] in 15 mL of toluene. The red
mixture was stirred for 70 h at 120 °C. A blue precipitate that formed
during this time was filtered off from the solution while hot. The
product was washed with 10 mL of toluene and was dried in vacuo to
give 340 mg (0.41 mmol, 81%) of a blue solid. Anal. Calcd (found) for
C30H42Br4N41Ni [836.99 g/mol]: C 43.05 (42.25), H 5.06 (5.33), N
6.69 (6.48). H NMR (400 MHz, acetone-d6): δ/ppm 1.62 (d, 12 H,
3JHH = 6.6 Hz, iPr-CH3), 4.58 (sept, 2 H, 3JHH = 6.5 Hz, iPr-CH), 7.84
(m, 3 H, aryl-Hm + aryl-Hp), 7.96 (d, 2 H, 3JHH = 6.8 Hz, aryl-Ho), 8.11
(s, 2 H, NCHCHN). 13C{1H} NMR (100 MHz, acetone-d6): δ/ppm
26.5 (iPr-CH3), 53.3 (iPr-CH), 121.6 (NCCN), 123.9 (aryl-Ci) 132.0
(aryl-Ctert), 133.3 (aryl-Ctert), 134.5 (aryl-Cp), 144.8 (NCN). IR (ATR,
[cm−1]): 712 (s), 733 (s), 776 (vs), 1011 (vw), 1081 (vw), 1115 (vw),
1140 (w), 1181 (w), 1213 (vs, NHC-γCH), 1295 (vw), 1377 (w), 1399
(vw), 1419 (vw), 1445 (m), 1462 (m), 1466 (m), 1485 (s), 1488 (m),
1572 (w), 1606 (vw), 1640 (vw), 2936 (w, νCH), 2984 (s, νCH), 3110
(s, νCH).
In Tetra-n-butylammonium Bromide. A 5.00 g portion of tetra-n-
butylammonium bromide (15.51 mmol) was dried in vacuo at 100 °C
for 5 h. After cooling the ionic liquid to room temperature the salt was
added to the corresponding amount of [Ni2(iPr2Im)4(COD)] (1) and
500 mg of zinc (7.64 mmol, 1.6 equiv). After adding 500 μL of
bromobenzene (4.78 mmol) the mixture was heated to 125 °C for 16
h. After cooling the reaction mixture to room temperature all volatiles
were removed in vacuo, and the residue was solved in 50 mL of water.
The water was extracted eight times with 50 mL of diethyl ether. The
combined organic layers were dried over sodium sulfate and filtered.
The diethyl ether was removed in vacuo. Yields are given in Table 1.
Anal. Calcd (found) for C12H10 [154.21 g/mol]: C 93.46 (93.28), H
Synthesis of 2[iPr2Im-C6H4-CH3]+[NiBr4]2−, 12. 121 μL (1.00
mmol) 4-Bromotoluene were added to a suspension of 260 mg
(0.50 mmol) trans-[Ni(iPr2Im)2(Br)2] in 15 mL toluene. The red
mixture was stirred for 90 h at 100 °C. A blue precipitate which
formed during this time was filtered off from the solution while hot.
The product was washed with 25 mL toluene and was dried in vacuo to
give 200 mg (0.23 mmol, 46%) of a light blue solid. Anal. Calcd
(found) for C32H46Br4N4Ni [865.04 g/mol]: C 44.43 (44.44), H 5.36
(5.42), N 6.48 (6.49). 1H NMR (400 MHz, acetone-d6): δ/ppm =1.63
1
6.54 (6.40). EI-MS m/z (%): 154 (100) [M]+. H NMR (400 MHz,
3
i
acetone-d6, 23 °C): δ/ppm 7.31 (m, 2 H, aryl-Hp), 7.46 (m, 4 H, aryl-
Htert), 7.65 (m, 4 H, aryl-Htert). 13C{1H} NMR (100 MHz, acetone-d6,
23 °C): δ/ppm 128.7 (aryl-Cp), 128.7 (aryl-Ctert), 130.7 (aryl-Ctert),
142.9 (aryl-Ci). IR (ATR, [cm−1]): 693 (vs), 725 (vs), 834 (vw), 903
(w), 1006 (w), 1041 (vw), 1091 (vw), 1112 (vw), 1170 (w), 1182
(vw), 1308 (vw), 1345 (w), 1385 (vw), 1429 (m), 1477 (m), 1516
(vw), 1570 (vw), 1597 (vw), 1650 (vw), 1691 (vw), 1748 (vw), 1797
(vw), 1878 (vw), 1952 (vw), 3034 (w), 3062 (vw), 3089 (vw), 3111
(vw).
Reaction of 3 with an Excess of Bromobenzene at Higher
Temperatures. Method A: 81.7 μL (775 μmol, 8 equiv) of
bromobenzene was added to a suspension of 50 mg (96.9 μmol) of
trans-[Ni(iPr2Im)2(Br)(C6H5)] (3) in 5 mL of toluene. The colorless
mixture was stirred for 16 h at 100 °C. All volatiles were removed in
(d, 12 H, JHH = 6.7 Hz, Pr−CH3), 2.54 (s, 3 H, CH3), 4.60 (sept, 2
H, 3JHH = 6.6 Hz, iPr−CH), 7.67 (d, 2 H, 3JHH = 7.6 Hz, aryl-Hm), 7.83
(d, 2 H, JHH = 7.9 Hz, aryl-Ho), 8.16 (s, 2 H, NCHCHN). 13C{1H}
3
NMR (100 MHz, acetone-d6): δ/ppm =22.6 (CH3), 27.5 (iPr-CH3),
52.5 (iPr-CH), 120.1 (aryl-Ci), 122.0 (NCCN), 132.3 (aryl-Ctert), 133.2
(aryl-Ctert), 144.2 (aryl-Cp), 144.4 (NCN). IR (ATR, [cm−1]): 725
(m), 764 (vw), 819 (s), 831 (s), 1007 (vw), 1082 (w), 1093 (vw),
1133 (w), 1185 (m), 1207 (vs, NHC-γCH), 1221 (m), 1319 (vw),
1380 (m), 1394 (vw), 1411 (w), 1419(w), 1437 (w), 1463 (m), 1469
(m), 1475 (m), 1479 (s), 1488 (m), 1526 (vw), 1539 (vw), 1574
(vw), 1615 (w), 1618 (w), 2879 (w, νCH), 2978 (s, νCH), 3026 (m,
ν
CH), 3061 (m, νCH), 3113 (m, νCH), 3172 (w).
Synthesis of 2[iPr2Im-C6H4-F]+[NiBr4]2−, 13. A 109 μL (1.00 mmol)
portion of 4-bromofluorobenzene was added to a suspension of 260
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dx.doi.org/10.1021/om300399j | Organometallics 2012, 31, 5065−5073

Organometallics
Article
mg (0.50 mmol) of trans-[Ni(iPr2Im)2(Br)2] in 15 mL of toluene. The
red mixture was stirred for 70 h at 120 °C. A blue precipitate that
formed during this time was filtered off from the solution while hot.
The product was washed with 5 mL of toluene and was dried in vacuo
to give 300 mg (0.34 mmol, 68%) of a blue solid. Anal. Calcd (found)
for C30H40Br4F2N4Ni [872.97 g/mol]: C 41.28 (40.74), H 4.62 (4.69),
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Funding
1
This work was financed by the University of Karlsruhe (TH),
the Julius-Maximilians-University Wurzburg, the DFG (Deut-
sche Forschungsgemeinschaft), and the Landesgraduiertenfor-
N 6.42 (6.27). H NMR (400 MHz, acetone-d6): δ/ppm 1.66 (d, 12
3
i
3
i
H, JHH = 6.6 Hz, Pr-CH3), 4.57 (sept, 2 H, JHH = 6.5 Hz, Pr-CH),
7.58 (t, 2 H, 3JHH = 8.3 Hz, aryl-Hm), 8.16 (m, 4 H, NCHCHN + aryl-
Ho). 13C{1H} NMR (100 MHz, acetone-d6): δ/ppm 27.4 (iPr-CH3),
53.4 (iPr-CH), 119.4 (aryl-Ci), 119.6 (aryl-Cm), 122.1 (NCCN), 135.5
(aryl-Co), 144.0 (NCN), 165.5 (aryl-Cp). 19F NMR (376 MHz,
acetone-d6): δ/ppm −107.9 (s, F). IR (ATR, [cm−1]): 740 (m), 789
(w), 844 (s), 1081 (vw), 1137 (vw), 1163 (w), 1182 (w), 1217 (vs,
NHC-γCH), 1242 (w), 1378 (w), 1414 (vw), 1437 (vw), 1462 (s),
1484 (s), 1526 (m), 1613 (m), 2984 (m, νCH), 3104 (s, νCH), 3121
(s, νCH).
̈
̈
derung Baden-Wurttemberg.
̈
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for financial support of the University of
Karlsruhe (TH), the Julius-Maximilians-University Wurzburg,
■
̈
Synthesis of 2[iPr2Im-C6H4-OSi(CH3)3]+[NiBr4]2−, 14. A 266 μL
(1.00 mmol) amount of 4-(trimethylsilyloxy)bromobenzene was
added to a suspension of 260 mg (0.50 mmol) of trans-
[Ni(iPr2Im)2(Br)2] in 15 mL of toluene. The red mixture was stirred
for 70 h at 120 °C. A blue precipitate that formed during this time was
filtered off from the solution while hot. The product was washed with
10 mL of toluene and was dried in vacuo to give 400 mg (0.40 mmol,
79%) of a blue solid. Anal. Calcd (found) for C36H58Br4N4NiO21Si2
[1013.35 g/mol]: C 42.67 (42.31), H 5.77 (5.67), N 5.53 (5.55). H
NMR (400 MHz, acetone-d6): δ/ppm 0.36 (s, 9 H, Si-CH3), 1.61 (d,
and the DFG. T.Z. acknowledges a scholarship from the
“Landesgraduiertenforderung Baden-Wurttemberg”.
̈
̈
DEDICATION
■
Dedicated to Professor David Milstein on the occasion of his
65th birthday.
REFERENCES
■
3
i
3
i
(1) (a) Stang, P. J., Diederich, F., Eds. Metal-Catalyzed Cross-Coupling
Reactions; Wiley-VCH: Weinheim, Germany, 1998. (b) Miyaura, N.,
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12 H, JHH = 6.7 Hz, Pr-CH3), 4.57 (sept, 2 H, JHH = 6.6 Hz, Pr-
3
3
CH), 7.26 (d, 2 H, JHH = 8.3 Hz, aryl-Hm), 7.86 (d, 2 H, JHH = 8.5
Hz, aryl-Ho), 8.15 (s, 2 H, NCHCHN). 13C{1H} NMR (100 MHz,
acetone-d6): δ/ppm 1.7 (Si-CH3), 27.9 (CH3), 53.1 (iPr-CH), 116.3
(aryl-Ci), 121.8 (NCCN), 123.5 (aryl-Cm), 135.2 (aryl-Co), 144.9
(NCN), 160.6 (aryl-Cp). 29Si NMR (79 MHz, acetone-d6): δ/ppm
21.7 (m, Si). IR (ATR, [cm−1]): 738 (w), 766 (w), 823 (m), 844 (vs),
910 (s), 1077 (vw), 1106 (w), 1136 (w), 1179 (m), 1211 (s, NHC-
γCH), 1253 (s), 1280 (vs), 1375 (w), 1394 (vw), 1419 (w), 1437 (w),
1462 (m) 1469 (s), 1475 (m), 1519 (w), 1524 (m), 1526 (m), 1608
(s), 2983 (w, νCH), 3105 (w, νCH), 3151 (vw), 3322 (m).
Crystallography. Crystals were immersed in a film of perfluor-
opolyether oil on a glass fiber and transferred to a Stoe-IPDS II image
plate diffractometer (Mo Kα, 200 K). The structures were solved by
direct methods and refined with the SHELXTL software package.17 All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were assigned to idealized positions and were included in structure
factor calculations. Extinction corrections were applied as required.
Crystal Data for [8·2(C6H6)]: C26H42BrN5Ni, Mr = 719.48, orange-
colored block, 0.21 × 0.19 × 0.15 mm, monoclinic, space group P21/c,
a = 19.215(4) Å, b = 9.0535(18) Å, c = 23.233(5) Å, β = 108.52(3)°, V
= 3832.4(13) Å3, Z = 4, ρcalcd = 1.247 g·cm−3, μ = 1.580 mm−1, F(000)
= 1520, T = 200(2) K, all data: R1 = 0.0647 and wR2 = 0.1169, I >
2σ(I): R1 = 0.0492 and wR2 = 0.1104, Goof 1.091, 9130 independent
reflections [2θ ≤ 61.7°] and 416 parameters, 0 restraints. Crystal Data
for 11: C30H42Br4N4Ni, Mr = 836.97, blue block 0.25 × 0.20 × 0.14
mm, monoclinic, space group P21/c, a = 21.236(4) Å, b = 20.872(4) Å,
c = 16.673(3) Å, β = 106.39(3)°, V = 7090(2) Å3, Z = 8, ρcalcd = 1.568
g·cm−3, μ = 5.080 mm−1, F(000) = 3344, T = 200(2) K, all data: R1 =
0.1020 and wR2 = 0.2370, I > 2σ(I): R1 = 0.0816 and wR2 = 0.2208,
Goof 1.068, 14 15 independent reflections [2θ ≤ 53.4°] and 719
parameters, 0 restraints.
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ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystal data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: u.radius@uni-wuerzburg.de.
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̈
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̈
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̈
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