Synthesis of Ni(II) complexes with unsymmetric [O,N,O′]-pincer ligands and their use as precatalysts in carbon-carbon bond formations to access diarylmethanes

Article abstract of DOI:10.1016/j.ica.2014.05.023

1-Acetyl-[1a,3,5-CF₃,1-C(=O)CH₃] and 1-benzoyl-5-hydroxypyrazolines [1b, 3,5-CF₃, 1-C(=O)C ₆H₅] have been synthesized and reacted with Ni(OAc) ₂·4H₂O in the presence of PPh₃ to form square planar nickel complexes, which revealed a O,N,O′-coordination. An additional coordination site on the nickel center is occupied by one triphenylphosphane ligand. After having investigated the properties of the complexes, catalytic experiments have been carried out to synthesize diarylmethanes. Applying the complexes in the nickel-catalyzed carbon-carbon cross coupling of aryl halides with benzylzinc bromide excellent yields and selectivities of the corresponding diarylmethane have been obtained. Moreover, various experiments have been performed to shed light on the underlying reaction mechanism.

Full text of DOI:10.1016/j.ica.2014.05.023

Accepted Manuscript
Synthesis of Ni(II) complexes with unsymmetric [O,N,O´]-pincer ligands and
their use as precatalysts in carbon–carbon bond formations to access diaryl
methanes
Chika I. Someya, Elisabeth Irran, Stephan Enthaler
PII:
S0020-1693(14)00312-0
http://dx.doi.org/10.1016/j.ica.2014.05.023
ICA 16005
DOI:
Reference:
Inorganica Chimica Acta
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9 April 2014
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20 May 2014
Please cite this article as: C.I. Someya, E. Irran, S. Enthaler, Synthesis of Ni(II) complexes with unsymmetric [O,N,O
´]-pincer ligands and their use as precatalysts in carbon–carbon bond formations to access diaryl methanes,
Inorganica Chimica Acta (2014), doi: http://dx.doi.org/10.1016/j.ica.2014.05.023
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Synthesis of Ni(II) complexes with unsymmetric [O,N,O´]-pincer ligands and their use
as precatalysts in carbon–carbon bond formations to access diaryl methanes
Chika I. Someya,[a] Elisabeth Irran,[b] and Stephan Enthaler[a]*
[a] Technische Universität Berlin, Institute of Chemistry: Cluster of Excellence “Unifying
Concepts in Catalysis”, Straße des 17. Juni 115/C2, 10623 Berlin, Germany
Fax: +493031429732, E-mail: stephan.enthaler@tu-berlin.de
[b] Technische Universität Berlin, Institute of Chemistry: Metalorganics and Inorganic
Materials, , Straße des 17. Juni 135/C2, D-10623 Berlin, Germany
Abstract:
1-Acetyl- [1a, 3,5-CF3, 1-C(=O)CH3] and 1-benzoyl-5-hydroxypyrazolines [1b, 3,5-CF3, 1-
C(=O)C6H5] have been synthesized and reacted with Ni(OAc)2•4H2O in the presence of PPh3
to form square planar nickel complexes, which revealed a O,N,O´–coordination. An
additional coordination site on the nickel centre is occupied by one triphenylphosphane
ligand. After having investigated the properties of the complexes, catalytic experiments have
been carried out to synthesize diarylmethanes. Applying the complexes in the nickel-
catalyzed carbon–carbon cross coupling of aryl halides with benzylzinc bromide excellent
yields and selectivities of the corresponding diarylmethane have been obtained. Moreover,
various experiments have been performed to shed light on the underlying reaction mechanism.
Keywords: catalysis / cross coupling / nickel / tridentate ligands / zinc reagents
Graphical Abstract:
1

1. Introduction
Nowadays metal-catalyzed transformations are widely used for carbon–carbon bond
formations.1 During the last decades remarkable improvements have been reported for the
coupling of sp-, sp2-, and sp3-hybridized carbon nucleophiles with sp-, sp2-, and sp3-
hybridized carbon electrophiles.2 Based on that, in recent times coupling reactions have
become apparent as a methodology of choice with applicability in the production of fine
chemicals, pharmaceuticals, agricultural chemicals and natural products.3 As catalyst core
palladium or nickel occupy a central role, performing the coupling reactions with excellent
activities, selectivities and high functional-group tolerance. 4 In this regard, the metal-
catalyzed C-C bond formation can be an option to access diarylmethanes, which are attractive
motifs in pharmaceuticals and biologically active compounds e.g., Beclobrate or Piritrexim
(Scheme 1). 5 , 6 Among the different approaches the application of organometallic zinc
reagents as coupling partner allow access to a broad range of products under mild conditions
and functional group tolerance.6a,6b,7 In more detail, the necessity for the design and fine-
tuning of the catalyst by ligands was crucial to obtain products in high yields and selectivities.
Hence, the development of novel ligands is important for improving the catalyst performance,
taking furthermore into account availability, easy synthesis, high tunability, flexibility and
stability of the ligands.8
Scheme 1. Synthetic strategy to access diarylmethanes via cross coupling reactions and
biologically active compounds containing the diarylmethane motif.
Recently we studied the abilities of complexes containing [O,N,O´]-pincer ligands in
coordination chemistry (Scheme 2).9,10 The octahedral nickel complexes were successfully
2

applied as pre-catalyst in cross coupling reactions. Interestingly, the coordination geometry
can be easily switched from an octahedral to a square planar geometry by changing the kind
of co-ligands from 4-dimethylaminopyridine to phosphanes.11 In this regard, we became
interested in the catalytic abilities of such square planar complexes in cross coupling
reactions. Hence, we report herein our ongoing research on the synthesis and characterization
of square planar nickel complexes modified by tridentate O,N,O´–ligands and a
triphenylphosphane as the co-ligand and the application in carbon–carbon cross coupling
reactions of aryl halides with benzylzinc bromide to access the diarylmethane motif.
Scheme 2. Nickel complexes modified by O,N,O´–ligands (dmap
=
4-
dimethylaminopyridine).
2. Results and Discussion
2.1. Complex Synthesis, Characterization and Properties
The 1-acetyl- and 1-benzoyl-5-hydroxypyrazoline 1 were synthesized in accordance to the
procedure reported in the literature (Scheme 3).12 Acetohydrazide (3a) and benzohydrazide
(3b) were reacted with equimolar amounts of hexafluoroacetyl acetone (4) and refluxed for
24 hours in ethanol. After work-up the desired ligands 1 were obtained as colourless
crystalline compounds (83–87%).9-12
3

Scheme 3. Synthesis of 1-acetyl- and 1-benzoyl-5-hydroxypyrazoline 1.
With these chemicals in hand we examined the coordination to nickel salts (Scheme 4). In
agreement to our previously established protocol a methanol solution of 1 and an excess of
triphenylphosphane (3.0 equiv.) was added to a solution of Ni(OAc)2•4H2O in methanol at
room temperature.11,13After stirring overnight, all volatiles were removed to obtain brown
powders, which were extracted with ethanol and purified by crystallization to obtain red–
brown crystals. Crystals suitable for X–ray measurements were grown from ethanol by slow
evaporation of the solvent at room temperature.
Scheme 4. Synthesis of nickel complexes 2a and 2b.
The solid–state structures of complexes 2a and 2b have been characterized by single–crystal
X–ray diffraction analysis. Thermal ellipsoid plots are shown in Figure 1 and selected bond
lengths and angles are listed in Table 1. The tridentate ligand is coordinated in a O,N,O´–
mode creating a five–membered as well as a six–membered ring system and therefore
shielding one side of the metal. The triphenylphosphane ligand is cis–positioned to the
oxygen donors, while the nitrogen of the ligand is connected to the nickel centre in the trans-
position. A similar square planar motif was observed by Joshi and co-workers13a and more
recently by us when applying ammonia instead of triphenylphosphane as the co-ligand, while
with dmap an octahedral structure was achieved.10a,11 The Ni–N1, Ni–O1, Ni–O2 and Ni–
P1bond lengths are found in a similar range of those observed in nickel(II) complexes having
the same coordinating atoms.11
4

Figure 1. Molecular structures of 2a (Figure 1a) and 2b (Figure 1b). Thermal ellipsoids are
drawn at the 50% probability level.
Table 1. Selected bond lengths and angles of 2a and 2b.
2a
2b
Ni-N1 [Å]
Ni-O1 [Å]
Ni-O2 [Å]
1.863(3)
1.810(3)
1.823(3)
1.8602(19)
1.8154(14)
1.8355(14)
5

Ni-P1 [Å]
2.2252(10)
179.26(12)
172.15(11)
95.98(13)
83.69(11)
91.68(8)
2.2313(7)
179.00(7)
172.70(6)
95.76(7)
83.40(7)
81.53(5)
89.30(5)
O1-Ni-O2 [º]
N1-Ni-P1 [º]
N1-Ni-O1 [º]
N1-Ni-O2 [º]
O1-Ni-P1 [º]
O2-Ni-P1 [º]
88.64(8)
In contrast to our recently reported octahedral paramagnetic complexes the observed square
planar structures (diamagnetic properties) allow for NMR investigations. The complexes 2
were characterized by 1H NMR spectroscopy applying CDCl3 as solvent, showing the C–H
proton in the six–membered ring at 6.05 ppm for 2a and 6.13 ppm for 2b, respectively (Table
2). In the 19F NMR spectrum two sharp signals for the two CF3 groups at -65.5 ppm and -71.7
ppm for 2a, and -65.4 ppm, -71.6 ppm for 2b were observed, which are as expected different
from those of ligand 1. The coordination of the triphenylphosphane was confirmed by
31P{1H} NMR; here a signal at 14.0 ppm for 2a and 14.8 ppm for 2b, respectively were
detected, while the uncoordinated triphenylphosphane has a chemical shift of -4.75 ppm.
Furthermore, the coordination mode of the acetyl and benzoyl functionality was investigated
by IR measurements. The absence of signals in the range of 1610–1725 cm-1 excludes the
possibility for an uncoordinated C=O functionality of the ligand. Strong bands were
monitored in the region of 1533–1603 cm-1, which can be attributed to the C=N and C=C
functionalities.
Table 2. Analytical properties of 1a, 1b, 2a and 2b.
1a
1b
2a
2b
1H NMR
4-CH(H)
3.30–3.80
3.00–3.66
6.05
6.13
[ppm][a]
19F NMR [ppm][a]
-67.7, -81.4
-
-67.4, -80.5
-
-65.5, -71.7
14.0
-65.4, -71.6
14.8
31P{1H}
[ppm][a]
NMR
IR ν (C=O)
[cm-1][b]
1699 (s)
1678 (s)
-
-
6

IR ν (C=N/C=C)
[cm-1][b]
1645 (s)
-
1639 (s)
-
1603 (s)
1543 (s)
1596 (s)
1533 (s)
[a] The chemical shift was measured in CDCl3 at 25 °C. [b] Measured in KBr at 25 °C.
2.2. Catalytic investigations
After studying the properties of the complexes, the catalytic abilities of 2 were investigated in
the coupling of aryl halides and organometallic zinc reagents to produce diarylmethanes
(Table 3).6b, 14 To find suitable reaction conditions, the coupling of 4-iodoanisole (5) with
benzylzinc bromide (6) was performed in the presence of 2 mol% of 2b as model reaction
(Table 3, entries 1 and 2). First the reaction outcome was examined at different reaction
temperatures. Only in the case of elevated temperature (70 °C) the desired coupling product
was observed in good yields (Table 3, entry 1). A similar result (yield: >99%) was achieved
with the pre-catalyst 2a (Table 3, entry 4). Noteworthy, the nickel complexes did operate as
the pre-catalyst, whilst in its absence no reaction took place (Table 3, entry 3). Furthermore,
the catalyst loading was reduced (Table 3, entries 5-8). The cross coupling reaction in the
presence of 1.0 mol% or 0.1 mol% of the pre-catalyst 2b resulted in the formation of 5a in
46% and 29% yield after one hour (Table 3, entries 7 and 8). However, after 24 hours in both
cases excellent yields of the coupling product were observed (Table 3, entries 5 and 6). Note,
the obtained result for 0.1 mol% catalyst loading corresponds to a turnover frequency of 290
h-1.
Table 3. Nickel–catalyzed C–C bond formation of benzylzinc bromide with 4-iodoanisole –
optimization of the reaction conditions.[a]
Entry
pre-catalyst (mol%)
T [°C ]
70
Yield [%][b]
1
2
3
4
2b (2)
2b (2)
-
>99
<1
r.t.
70
<1
2a (2)
70
>99
7

5
6
7
8
9
2b (1)
2b (0.1)
2b (1)
70
70
70
70
70
>99
>99
46[c]
29[c]
>99[d]
2b (0.1)
2b (2)
[a] Reaction conditions: 4-iodoanisole (0.77 mmol), benzylzinc bromide (1.15 mmol, 0.5 M
in THF), pre-catalyst (2.0 mol%), 70 °C, 24 h. [b] Yield determined by GC-MS. [c] Reaction
time: 1 hour. [d] 3 h.
Moreover, the complex 2b was applied as pre-catalyst in the reaction of benzylzinc bromide
and different aryl halides (Table 4). Excellent yields (>99%) were observed for various aryl
bromides in the presence of 2.0 mol% pre-catalyst at 70 °C within 24 hours (Table 4, entries
1, 3, 5 and 6).15 Along with aryl bromides aryl chlorides were investigated. However, a
lower activity of the catalyst was found (Table 4, entries 2 and 4). In contrast, aryl fluorides
were not converted to the corresponding cross coupling product, probably due to the high
bond dissociation energy of the C-F bond.16 The reaction of aryl dihalides exhibited a higher
reactivity and excellent chemoselectivity towards the bromo functionality (Table 4, entries 8
and 9). On the other hand, for 2-bromomesitylene (13) a poor reactivity was detected for
complex 2b, probably because of steric hindrance (Table 4, entry 7). In the case of the
challenging electron withdrawing trifluoromethyl or dimethyl amino group, excellent yields
and selectivities for the coupling products were monitored (Table 4, entries 10-12).
Furthermore, the reaction of heteroarenes with benzylzinc bromide resulted in good to
excellent formation of the desired products (Table 4, entries 18-20). Noteworthy, in case of
2,6-dibromopyridine an excellent selectivity was found for the mono-benzylation (Table 4,
entry 19).
Table 4. Nickel–catalyzed C–C bond formation of aryl halides with benzylzinc bromide –
scope and limitations.[a]
8

Entry
1
Substrate
Product
Yield [%][b]
>99
7a
7
2
3
4
5
6
35
>99
79
7a
5a
8
Br
MeO
9
5a
10
11
>99
>99
11a
12a
12
7
5
13a
14a
15a
16a
17a
18a
19a
13
8
>99
>99
>99
>99
>99
70
14
15
16
17
18
19
9
10
11
12
13
14
15
20a
21a
<1
20
21
>99
16
17
18
>99
28
22a
23a
24a
22
23
>99
24
9

19[c]
20
48
79
25a
26a
25
26
21
>99
27a
27
[a] Reaction conditions: substrate (0.77 mmol), benzylzinc bromide (1.15 mmol, 0.5 M in
1
THF), pre-catalyst (2.0 mol%), 70 °C, 24 h. [b] Yield determined by GC-MS and H NMR.
[c] 1.5 equiv. of benzylzinc bromide was used. 2-Benzyl-6-bromopyridine was observed as
product.
Moreover, the influence of the organometallic zinc reagent was studied (Table 5). First, the
coupling reaction of dimethyl zinc, diethyl zinc, di-iso-propyl zinc and di-tert-butyl zinc was
examined (Table 5, entries 1-3, 6-7). A good yield was observed for the production of p-
methylanisole (74%), while for sterically demanding alkyl functionalities a decreased activity
was monitored. For studying the effect of β-hydride elimination 4-iodoanisole was reacted
with α-methylbenzylzinc bromide (Table 5, entry 4). The desired coupling product was
obtained in a yield of 59%. However, non-negligible amount of the homocoupling products
4,4'-dimethoxybiphenyl (17%) and 2,3-diphenylbutane (15%). Furthermore, styrene (10%)
was monitored as the β-hydride elimination product. Interestingly, the complex 2b was
capable to transform a heteroaryl organometalic zinc reagent to the desired 2-aryl pyridine in
good yield (Table 5, entry 5). On the other hand changing the source of halide source from
aryl bromide to alkyl bromide, revealed not the formation of the desired product (Table 5,
entries 8-9). Moreover, the set-up of the reaction was modified by in situ pre-generation of
the organometallic zinc reagent. Based on that, the corresponding benzyl magnesium bromide
reagents 36a or 36b were formed by reacting benzyl bromide 35a or benzyl bromide-d2 35b
and magnesium in THF at room temperature for 2 hours.17 To the stirred solution of 36a or
36b was added ZnBr2 at room temperature and after stirring for 1 hour benzylzinc bromide 6
or 37 was generated.18 The reaction mixture of 6 or 37 was filtered and the solution was used
in the cross coupling reaction of 4-iodoanisole and 2b as pre-catalyst. The coupling product
5a was obtained in a yield of 86%, and 37a was obtained in 84% yield.
Table 5. Nickel–catalyzed C–C bond formation of aryl halides with organometallic zinc
compounds – scope and limitations.[a]
10

Entry
1
Substrate
Zn-source
Yield [%][b]
74
ZnMe2 28
9
Br
Br
2
3
4
ZnEt2 29
23
9
MeO
MeO
9
9
ZntBu2 30
Br
59[c]
ZnBr
MeO
9
9
9
9
31
Br
Br
Br
5
6
75
<1
15
<1
<1
86
ZnBr
MeO
MeO
MeO
N
32
ZnBr 33
7[d]
8
ZnBr 33
ZnBr
6
34
ZnBr
ZnBr
9[d]
10[e]
6
6
34
Br
MeO
MeO
9
Br
11
84
9
37
[a] Reaction conditions: substrate (0.77 mmol), zinc source (1.15 mmol), pre-catalyst (2.0
1
mol%), 70 °C, 24 h. [b] Yield determined by GC-MS and H NMR. [c] 15% of 2,3-
diphenylbutane and 17% of 4,4'-dimethoxybiphenyl were observed as homocoupling
products. 10% of styrene was detected. [d] Addition of Bu4NI (2.31 mmol). See Ref. 19. [e]
in situ pre-generation of the organometallic zinc reagent.
Moreover, the reaction in the course of time applying the cross coupling of 4-iodoanisole (5)
and benzylzinc bromide (6) as model reaction was examined (Figure 2). The abilities of the
pre-catalyst 2b in THF were studied at different reaction temperatures (55 °C, 60 °C, 65 °C
11

and 70 °C). By comparison the time to reach 50% yield, e.g., 80 min (70 °C), 120 min
(65 °C), 190 min (60 °C), and 310 min (55 °C), a decrease in activity was observed with
reduced reaction temperature. However, at the investigated temperatures full conversion was
reached within 150-480 min. Worth of note is that at each temperature an induction period of
several minutes was detected, which is probably required to produce the active catalyst. To
enhance the formation of the catalyst the pre-catalyst 2b was pretreated on the one hand with
benzylzinc bromide and on the other hand with 4-iodoanisole 5 for 10 minutes at 70 °C
(Figure 3). Subsequently, 4-iodoanisole or benzylzinc bromide, respectively, was added and
the reaction proceeding was investigated. In case of the pretreatment of 2b with benzyl zinc
bromide no induction period was observed, whereas for treatment of 2b with 5 an induction
period was noticed. On the other hand, the time to reach 50% yield showed 25 min (a)
pretreatment with benzylzinc bromide, 80 min (b) without any pretreatment, and 180 min (c)
with pretreatment with 5. A comparison of the rate constants showed an increased catalytic
activity for the pretreatment with benzylzinc bromide.
Figure 2. Reaction process of the coupling reactions of 4-iodoanisole (5) with benzylzinc
bromide (6) using the pre-catalyst 2b (2.0 mol%) in THF at different temperature (55 °C,
60 °C, 65 °C and 70 °C).
12

Figure 3. Time courses of the coupling reactions of a) after pretreatment of 2b with
benzylzinc bromide, b) Reaction conditions: substrate (0.77 mmol), benzylzinc bromide (1.15
mmol, 0.5 M in THF), pre-catalyst (2.0 mol%), 70 °C, and c) after pretreatment of 2b with
4-iodoanisole.
In addition, the established cross coupling protocol was embedded as key reaction in the
synthesis of diaryl ketones, which are attractive motifs for pharmaceutical and material-
science (Scheme 6).20 In more detail, the cross coupling of 5 and 6 was carried out in the
presence of 2b as precatalyst resulting in full conversion after 24 hours. Subsequently tert-
butyl peroxide was added to the solution and heating was continued for 24 hours. After work-
up the ketone 5b was obtained in 40% yield.
Scheme 6. Synthesis of diaryl ketones via nickel-catalyzed cross coupling reaction and
subsequent oxidation.
After having studied the scope and limitation of the nickel complexes 2 in cross coupling
reactions, additional investigations were carried out to gain insight into the reaction
mechanism. First, the behaviour of the complex 2b was investigated in the coordinating
13

solvent THF by NMR spectroscopy, since THF was applied as reaction medium for the
19
catalytic reactions (Figure 4). The F NMR spectrum of 2b in [D8]THF revealed two sets of
broad signals, at -67.6 and -73.8 ppm, which can probably be assigned to the complex 2b,
and at -6.3 and -41.7 ppm, which can probably be assigned to complex 2b-1. Moreover, in the
31P{1H} NMR spectrum one signal with a chemical shift of 14.8 ppm was observed in CDCl3,
whereas in THF a broad signal at -8.1 ppm was detected. These broad NMR signals could be
due to the coordination of THF to the nickel consequently by this creating an octahedral
31
paramagnetic species. Furthermore, low temperature P{1H} NMR analysis in [D8]THF at
194 K showed a sharp signal at -10.5 ppm, which can probably be assigned to free
triphenylphosphane, and a minor signal at 12.7 ppm, which can be assigned as coordinated
triphenylphosphane in complex 2b. Noteworthy, an addition of an excess of
triphenylphosphane (5 equiv.) to the dissolved complex 2b in [D8]THF showed an increase of
the signal at -10.5 ppm, which further proves the presence of free triphenylphosphane on the
NMR time scale. The broadness of the signals at room temperature can be either explained by
presence of the paramagnetic species or by exchange reactions between coordination and
dissociation processes. Based on that, we speculate a dissociation of PPh3 and the formation
of an octahedral nickel THF complex. Remarkably, the straightforward exchange of the
triphenylphosphane can be a useful tool for catalysis, due to easy creation of an active site for
metal-substrate interactions. Moreover, the difference of the structure of 2b depending on the
solvent was also confirmed by UV-Vis spectroscopy. The spectra of 2b in dichloromethane
(325.5 nm, 370.5 nm, 399.0 nm) and THF (380.0 nm, 397.0 nm, 417.5 nm) showed different
absorption maxima.
14

19
19
Figure 4. a) F NMR spectrum in CDCl3, b) F NMR spectrum in [D8]THF, c) 31P{1H}
NMR spectrum in CDCl3, and d) 31P{1H} NMR spectrum at different temperature in
[D8]THF.
To further verify the exchange of triphenylphosphane by other donor systems, we
investigated the stability of the complex 2b in the presence of phosphane ligands in a 1:1
15

ratio (Figure 5).21 First, the reaction of 2b and the phosphane P(2,6-(MeO)2Ph)3 38 lead to the
formation of the new complex 38a, as observed by NMR techniques.11d For instance in the
19F NMR spectrum a new set of signals were detected along with the signals of 2b (2b:38a =
2:1), which were assigned to the complex 38a, in which triphenylphosphane is replaced by 38.
Moreover, phosphanes 39 and 40 were added to complex 2b, and revealed that
triphenylphosphane tend to exchange with more basic phosphanes, like P(2,6-(MeO)2Ph)3 38
or n-tributylphosphane (39), while no exchange was observed for the reaction with
tris(phenylethynyl)phosphane (40), which has a weaker basicity than triphenylphosphane.
Figure 5. 19F NMR spectra of the reaction mixture of 2b with phosphanes 38-40 in CDCl3.
To rule out the existence of nickel particles as the active catalyst, a large excess of Hg(0) was
added to the stirred reaction mixture of benzylzinc bromide (6) and 4-iodoanisole (5) after the
reaction had proceeded for 3 minutes to allow the generation of the “real” catalyst (Scheme
7).22 The reaction was continued and after 3 hours a sample was taken. In case of a
heterogeneous catalyst the addition of Hg will hamper the proceeding of the reaction.
However no significant suppression of the reaction rate in the coupling reaction was observed
(>99% yield after 3 h). This observation indicates the presence of a homogeneous catalyst.
Furthermore, the formation of a heterogeneous catalyst could be excluded by the addition of a
large excess of triphenylphosphane, since no negative effect on the reaction outcome was
16

noticed and full conversion was reached within 3 hours. Noteworthy, the tridentate O,N,O´-
ligand was stable under cross coupling reactions and was not effected by the addition of an
excess of triphenylphosphane, for instance by formation of nickel-phosphane species.
Scheme 7. Evaluation for homogeneous vs. heterogeneous nickel catalysis.
Based on the performed experiments we proposed a reaction mechanism as stated in Scheme
8.11 First the triphenylphosphane ligand dissociates from complexes 2b to create open
coordination sites. In the NMR study the dissociation was monitored by 31P{1H} NMR and
the free coordination sites were occupied by the solvent THF to form the complex A. The
complex A reacts with the zinc reagents to create complex 2b-2, 2b-3, 2b-4 or 2b-5 which
contains a 4-membered ring system. 23 Afterwards the organic halide approaches the
coordination sphere of the nickel and mediates a single electron transfer (SET) to oxidize the
complex and produce a radical. The one-electron oxidized complex can on the one hand
oxidized at the metal center (2b-6 or 2b-7) or on the other hand the electron can be stored in
the ligand (non-inocent ligand, 2b-8 or 2b-9). Subsequently, ZnBrX is eliminated and the
radical recombines with the nickel species to form the species 2b-10. Finally, an elimination
of the organic product occurs and complex 2b-1 is regenerated.
17

Scheme 8. Proposed reaction mechanism for the nickel-catalyzed cross coupling.
3. Conclusion
In summary, we have synthesized and characterized nickel complexes with the highly
flexible [O,N,O´]-pincer ligands (1a-2H and 1b-2H). As co–ligand triphenylphosphane is
coordinated to the nickel. Moreover, the properties of the square planar complexes were
examined with various techniques. The complexes were applied as pre-catalysts in carbon–
carbon bond formations demonstrating an excellent performance to access diaryl methanes.
For a better understanding of the system various investigations were carried to understand the
underlying reaction mechanism.
4. Experimental Section
Synthesis of 2a: To a mixture of 1a (1.86 mmol) and triphenylphosphane (5.57 mmol) in
methanol (10 mL) was added a solution of Ni(OAc)2·4H2O (1.86 mmol) at room temperature.
The solution was stirred over night. After stirring overnight, the volatiles were removed in
vacuo to obtain brown powders, which were extracted with ethanol and purified by
18

crystallization to obtain crystals. Crystals suitable for single-crystal X-ray diffraction analysis
were obtained from ethanol solution at room temperature. Yield = 450 mg (42%, red crystals).
Mp: 150–151 °C. 1H NMR (200 MHz, CDCl3, 25 °C) δ = 7.44–7.62 (m, 15H, Ar), 6.05 (s, br,
1H, CH), 1.88 (s, 3H, C(=O)CH3) ppm. 13C{1H} NMR (100 MHz, CDCl3, 25 °C) δ = 176.9,
157.0 (q, 2JC-F = 33.8 Hz), 141.6 (q, 2JC-F = 30.2 Hz), 134.4, 132.2 (q, 1JC-P = 9.4 Hz), 131.1,
128.7, 118.6 (q, 1JC-F = 280.8 Hz), 117.9 (q, 1JC-F = 279.6 Hz), 90.5, 16.5 ppm. 19F NMR (188
31
MHz, CDCl3, 25 °C) δ = -65.5, -71.7 ppm. P{1H} NMR (81 MHz, CDCl3, 25°C) δ = 14.8
ppm. IR (KBr): ν = 3417 (br), 3048 (w), 1603 (m), 1580 (w), 1543 (m), 1522 (m), 1482 (w),
1460 (w), 1433 (m), 1384 (w), 1349 (m), 1342 (m), 1311 (w), 1267 (s), 1204 (s), 1180 (s),
1163 (s), 1152 (s), 1125 (s), 1095 (m), 1063 (m), 1026 (w), 1000 (w), 919 (w), 819 (w), 792
(m), 752 (m), 744 (m), 728 (w), 711 (w), 693 (s), 646 (w) cm-1. HRMS calc. for
C25H19F6N2O2NiP+H: 583.05201, found: 583.05149. Anal. Calcd. for C25H19F6N2O2NiP: C,
51.50; H, 3.28; N, 4.80. Found: C, 52.14; H, 3.21; N, 4.84. UV-Vis [nm, (ε, M-1cm-1)]: 316.5
(1900), 352.5 (1500) nm in dichloromethane. 370.0 (1400), 384.5 (1180) nm in THF.
Synthesis of 2b: To a methanol solution of Ni(OAc)2·4H2O (3.05 mmol) was added the
methanol solution (10 mL) of 1b (3.05 mmol) and triphenylphosphane (9.15 mmol). The
solution was stirred over night. After stirring overnight, the volatiles were removed in vacuo
to obtain brown powders, which were extracted with ethanol and purified by crystallization to
obtain crystals. Crystals suitable for single-crystal X-ray diffraction analysis were obtained
from ethanol solution at room temperature. Yield = 1.0 g (51%, red crystals). Mp: 141–
1
142 °C. H NMR (200 MHz, CDCl3, 25 °C) δ = 7.16–7.68 (m, 20H, Ar), 6.13 (s, 1H, CH)
ppm. 13C{1H} NMR (100 MHz, CDCl3, 25 °C) δ = 172.9, 157.2 (q, 2JC-F = 34.4 Hz), 142.3 (q,
1
2JC-F = 30.6 Hz), 134.4, 134.3, 132.3 (q, JC-P = 10.0 Hz), 130.9, 130.6, 128.8, 128.7, 128.1,
118.9 (q, 1JC-F = 281.1 Hz), 118.0 (q, 1JC-F = 279.6 Hz), 91.0 ppm. 19F NMR (188 MHz, C6D6,
25 °C) δ = -65.4, -71.6 ppm. 31P{1H} NMR (81 MHz, CDCl3, 25°C) δ = 14.5 ppm. IR (KBr):
ν = 3432 (br), 3049 (w), 2965 (w), 1596 (m), 1533 (m), 1512 (m), 1493 (w), 1480 (w), 1453
(w), 1437 (m), 1358 (m), 1346 (m), 1268 (s), 1198 (s), 1177 (s), 1155 (m), 1133 (m), 1116
(w), 1097 (m), 1065 (m), 1030 (w), 999 (w), 890 (w), 853 (w), 806 (m), 787 (w), 751 (m),
710 (m), 704 (m), 693 (m), 647 (w), 619 (w), 586 (w), 532 (m), 508 (m) cm-1. HRMS calc.
for C30H21F6N2O2NiP+H: 645,06766, found 645,06706. Anal. Calcd. for C30H21F6N2O2NiP:
C, 55.85; H, 3.28; N, 4.34. Found: C, 56.11; H, 3.30; N, 4.33. UV-Vis [nm, (ε, M-1cm-1)]:
19

325.5 (2800), 370.5 (3000), 399.0 (2800) nm in dichloromethane. 380.0 (2700), 397.0 (3500),
417.5 (2800) nm in THF.
Single-crystal X-ray structure determination24: Crystals were each mounted on a glass
capillary in perfluorinated oil and measured in a cold N2 flow. The data were collected using
an Oxford Diffraction Xcalibur S Sapphire at 150(2) K (MoKα radiation, λ = 0.71073 Å). The
structures were solved by direct methods and refined on F2 with the SHELX-97 software
package. The positions of the hydrogen atoms were calculated and considered isotropically
according to a riding model.
CCDC-872012 (for 2a), and -872013 (for 2b) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
General procedure for the catalytic C-C coupling reaction: A Schlenk flask was charged
with an appropriate amount of complex 2b (0.016 mmol, 2.0 mol%) and the corresponding
bromo or iodo arene (0.77 mmol). The flask was cycled with nitrogen and vacuum.
Afterwards a THF solution of benzylzinc bromide (2.2 mL, 1.1 mmol, 0.5 M in THF) was
added. The flask was sealed and heated at 70 °C for 24 hours. After that time, the mixture
was cooled, and dichloromethane and water were added. The aqueous layer was extracted
with dichloromethane and the collected organic layers were washed with water and dried
with Na2SO4. The coupling product was confirmed by GC-MS (with n-dodecane as internal
standard) and NMR analysis. The analytical properties of the products are in agreement with
literature data.6a, 25
5. Acknowledgements
Financial support from the Cluster of Excellence “Unifying Concepts in Catalysis” (EXC
314/1; www.unicat.tu-berlin.de; funded by the Deutsche Forschungsgemeinschaft and
administered by the Technische Universität Berlin) is gratefully acknowledged. Chika I.
Someya thanks the Berlin International Graduate School of Natural Sciences and Engineering
(BIG NSE) for ideational support. Moreover, Marianna Tsaroucha (Technische Universität
Berlin) is thanked for technical support.
20

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24

Highlights
ꢀ
Ni(II) complexes with unsymmetric [O,N,O´]-pincer ligands have been studied
ꢀ
A square planar geometry was observed with triphenylphosphane as co-ligand have
been observed
ꢀ
The Ni(II) complexes were applied as precatalysts in cross-coupling reactions to
access diaryl methanes
25