Modulating sonogashira cross-coupling reactivity in four-coordinate nickel complexes by using geometric control

Article abstract of DOI:10.1002/ejic.201500148

Herein, we present the synthesis of nickel complexes with tripodal phosphine ligands, CH₃Si(CH₂PPh₂)₃ and CH₃C(CH₂PPh₂)₃, and their application as catalysts in Sonogas

Full text of DOI:10.1002/ejic.201500148

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DOI:10.1002/ejic.201500148
Modulating Sonogashira Cross-Coupling Reactivity in
Four-Coordinate Nickel Complexes by Using Geometric
Control
Anette Petuker,[a] Christian Merten,*[b] and Ulf-Peter Apfel*[a]
Keywords: Cross-coupling / Nickel / Tripodal ligands / Silicon / Alkynes
Herein, we present the synthesis of nickel complexes with
tripodal phosphine ligands, CH3Si(CH2PPh2)3 and CH3C-
(CH2PPh2)3, and their application as catalysts in Sonogashira
cross-coupling reactions in water. Although both types of
nickel complexes are based on similar tripodal ligands, the
Si-derived compounds adopt stable tetrahedral coordination
geometries, whereas the C-derived counterparts adopt a
square-planar coordination environment. This structural and
electronic difference has an important effect on the catalytic
properties of the complexes. Our study demonstrates that C-
derived complexes are catalytically inactive, whereas the
complexes [CH3Si(CH2PPh2)3NiX2] (X = Cl–, Br–) are compe-
tent catalysts for cross-coupling reactions of aryl halides with
phenylacetylenes. This investigation reveals the importance
of structural tuning on catalysis and strongly supports the
theory that tetrahedral (PR3)2NiCl2 complexes are the active
species in Sonogashira cross-coupling reactions.
pincer complex 1 (Scheme 1).[10] The participation of a NiIV
species in the catalytic cycle was suggested,[14] and struc-
tural as well as spectroscopic evidence for a transmetalla-
tion process of a NiII species as the first step within this
coupling process was provided.[11] These results were fur-
ther supported by recent findings of the groups of Hartwig
and Driess, who used the [ECE] pincer complexes 2
(Scheme 1).[14] Liu et al. described the nickel-catalyzed
Sonogashira coupling of non-activated secondary alkyl
bromides and iodides with bis(oxazoline) 3 in the presence
of nickel bromide.[15]
Introduction
Sonogashira cross-coupling reactions are a convenient
method to incorporate alkynes into arenes, which are struc-
tures commonly found in biologically active molecules, con-
jugated polymers, and organic photosensitizers.[1–3] Palla-
dium complexes, such as Pd(PPh3)4, Pd(PPh3)2Cl2,
Pd(dppe)Cl2, and Pd(dppp)Cl2, are frequently used for C–
C-bond-formation chemistry and are compatible with a
wide variety of alkynes as well as aryl iodides, bromides,
and triflates.[4–7] Recently, the groups of Fu,[8] Glorius,[9]
and Hu[10,11] extended the substrate scope of Pd-catalyzed
coupling reactions to work with non-activated alkyl halides
by utilizing N-heterocyclic carbene ligands. However, be-
cause of the high cost and possible toxicity of palladium
in industrial synthesis, the use of nickel as a cheap, widely
available, and less toxic metal is more desirable.[12,13]
In contrast to palladium catalysis, examples of nickel-
catalyzed Sonogashira reactions are still rare, and their
mechanisms are poorly understood. General structural and
electronic requirements for Ni-based catalysts are un-
known. In the few studies available, Hu et al. described the
coupling of non-activated alkyl halides, utilizing the NiII
[a] Anorganische Chemie I, Ruhr Universität Bochum,
Universitätsstraße 150, 44801 Bochum, Germany
E-mail: ulf.apfel@rub.de
http://www.ruhr-uni-bochum.de/smallmolecules
[b] Organische Chemie II, Ruhr Universität Bochum,
Universitätsstraße 150, 44801 Bochum, Germany
E-mail: christian.merten@rub.de
http://www.ruhr-unibochum.de/chirality
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500148.
Scheme 1. Literature-known nickel catalysts for the Sonogashira
coupling.[10,11,14,16]
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Although (PR3)2PdCl2 complexes are effective catalysts
for Sonogashira couplings,[5–7] and although (PR3)2NiIIX2
complexes (X = Cl–, Br–; R = alkyl, aryl) are commonly
used pre-catalysts,[17–21] only Ni(PPh3)2Cl2 (4) allowed for
an efficient synthesis of tolanes through Sonogashira cou-
pling.[16,22] Unlike in the case of palladium, nickel com-
plexes of bidentate ligands as well as electron-rich mono-
dentate ligands did not afford the respective tolanes. This
exemplifies the limitations of the investigated nickel phos-
phine complexes for this process.[16] It was suggested that
the strong coordination of nickel to alkynes is a key prob-
lem associated with Ni-based Sonogashira catalysis.[23]
Because (PPh3)2NiCl2 can exist in either square-planar
or tetrahedral geometry,[24,25] it is unclear whether or not
both conformational isomers are catalytically active. In a
report on the stereoselective heterodimerization of styrene
and propene catalyzed by tripodal NiII complexes,[26] it was
shown that tetrahedral Ni complexes were able to perform
catalytic heterodimerization reactions, whereas the square-
planar analogues could not. Inspired by these results, we
became interested in using tripodal phosphine ligands in
Sonogashira reactions. We take advantage of the fact that
tripodal phosphine ligands can enforce either square-planar
or tetrahedral geometry, which depends on the identity of
the central atom (i.e., carbon or silicon). To the best of our
knowledge, this work presents the first evidence that tetra-
hedral (PR3)2NiIIX2 moieties are an essential prerequisite
for successful Ni–phosphine-based Sonogashira cross-cou-
pling reactions.
Scheme 2. Synthesis of tripodal phosphines and their Ni complexes.
(i) nBuLi, THF, HPPh2, –78 °C; (ii) NiX2 (X = Cl–, Br–), DMF/
THF (1:1).
Figure 1. Molecular structures of compounds 7 (left) and 9 (right)
with thermal ellipsoids drawn at the 50% probability level
(hydrogen atoms and thermal ellipsoids of the phenyl groups are
omitted for clarity).
A detailed bonding analysis of 7 and 9 shows that the
Ni–P distances significantly decrease [from ca. 2.296 (for 7)
to 2.179 Å (for 9)] upon substitution of the silicon atom by
a carbon atom. The bonding angles also decrease from
99.25 (for 7) to 95.26° (for 9) in the case of P–Ni–P and
Results and Discussion
By following established synthesis routes, diphenyl- from 126.92 (for 7) to 91.21° (for 9) in the case of Cl–Ni–
phosphine was deprotonated and allowed to react with 1,3- Cl. The structural features suggest that the substitution of
dichloro-2-(chloromethyl)-2-methylpropane to afford the the quaternary carbon atom by a silicon atom allows for
tripodal phosphine Triphos (compound 5).[27] Likewise, altering the donating ability of the ligand without altering
tris(chloromethyl)methylsilane[28] reacts with LiPPh2 to af- the bulkiness of the donor atoms.
ford TriphosSi (compound 6).
To test whether complexes 7 and 9 can interconvert be-
gives tween square-planar and tetrahedral geometry, as reported
[29]
The reaction of 6 with NiCl2 or (PPh3)2NiCl2
the complex [(TriphosSi)NiCl2] (7) in approximately 75% for 4,[24,25] we investigated the temperature dependence of
yield as a dark red, crystalline solid. Similarly, when 6 is the conformational preferences of complexes 7 and 9 by
treated with NiBr2, the complex [(TriphosSi)NiBr2] (8) can measuring their absorption spectra in acetonitrile at dif-
be obtained in 55% yield (Scheme 2). The molecular struc- ferent temperatures (20–70 °C). At room temperature, both
tures of 7 (Figure 1) and 8 (Figure S1) comprise tetrahedral complexes show different UV/Vis spectra (Figure 2). Com-
NiII centers bearing two phosphine and two chlorido li- plex 7 displays bands at 830, 487, and 383 nm, which are
gands. Ligand 6 coordinates in a bidentate mode, which characteristic for tetrahedral nickel complexes, whereas
leaves one free diphenylphosphine group. The Coordination compound 9 shows a single band at 466 nm, which is in-
of CH3Si(CH2PPh2)2 to Ni leads to a six-membered ring dicative of a square-planar coordination environment
chelate that adopts a chair conformation. This coordination around the Ni center. Increasing or decreasing the tempera-
mode is surprising, as it differs from a square-planar (PR3)2- ture does not alter the spectral features of both complexes,
NiIIX2 species obtained with the carbon-derivative 9 as a which implies that their conformations are stable and do
ligand, as reported by McGrady et al.[30] Contrary to com- not interconvert in solution (Figures S2 and S3). This ob-
plexes 7 and 8, the reaction of NiX2 with ligand 5 afforded servation was further supported by the magnetic moments
the square-planar complexes [(Triphos)NiCl2] (9) and determined by using the Evans method in acetonitrile. Para-
[(Triphos)NiBr2] (10) in the solid state (Scheme 2).
magnetic 7 has a magnetic moment of 3.13 μB, which is con-
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sistent with the theoretical value of 2.83 μB expected for a planar geometry. Because this is energetically less favorable,
tetrahedral NiII center, whereas 9 does not exhibit a mag- α(C–E–C) remains close the ideal angle. Instead, an elong-
netic moment, because it is diamagnetic, as expected for a ation of the C–E bond makes it more similar in length com-
low-spin square-planar NiII complex.
pared to the Ni–P bonds on the opposite side of the six-
membered chelate ring. This allows the ring to convert to a
more favorable chair conformation. This, in turn, decreases
α(C–P–Ni), thus improving the σ donation from the p lone
pairs and forcing the nickel center into a high-spin state.
Hence, it is the longer C–Si bonds that decrease the steric
tension in the six-membered chelate ring and allow 7 to
adopt a chair conformation and a high-spin state.
Table 1. Selected bond lengths d [Å], bond angles α [°], and torsion
angles τ [°] of the optimized geometries[a] of low-spin square-planar
(sqpl) and high-spin tetrahedral (tetrah) forms of complexes 7 and
9 [PBEPBE/6-31G(2d,p)].
sqpl 9
tetrah 9[b]
sqpl 7[b]
tetrah 7
d(P–C)
d(C–E)
d(Ni–P)
1.87
1.86
1.54
1.86
1.85
1.55
1.86
1.85
1.90
1.89
2.12
2.13
1.83
1.90
2.20
2.11
2.19
Figure 2. UV/Vis spectra of compounds 7 and 9 in acetonitrile.
α(P–Ni–P)
α(C–E–C)
α(C–P–Ni)
99.2
98.9
103.8
101.1
121.5
121.6
39.4
104.4
108.6
111.2
111.1
57.8
108.7
118.7
119.7
41.4
113.2
109.6
109.9
58.5
To better understand the origin of the structural prefer-
ences induced by the two tripodal ligands, DFT calculations
were performed. Starting from the crystal structures of 7
and 9, we optimized the geometries of a tetrahedral and a
square-planar complex for both ligands. A survey of a
variety of functionals (B3LYP, PBEPBE, PBE1PBE,
ωB97XD, M06–2X) revealed that only PBEPBE correctly
predicts the experimentally observed preferences of a high-
τ(Ni–P–C–E)
–37.1
–55.1
–37.8
–58.4
[a] Some values differ for the two binding arms, because the third
unbound arm breaks the symmetry of the six-membered ring. [b]
Hypothetical structures.
However, because steric tension appears to be the key
spin tetrahedral complex 7 (ΔE = 3.1 kcal/mol) and a low- factor determining the spin-state change of the nickel cen-
spin square-planar complex 9 (ΔE = 4.2 kcal/mol). All ter, it is not surprising that the complex mixture of disper-
other evaluated functionals predicted both ligands to stabi- sion interactions and electronic configurations at the metal
lize the high-spin configuration (Table S2).
center leads to false results in the DFT-based predictions.
Because of this uncertainty in the prediction of energetic
Beletskaya et al. reported that compound 4 is an excel-
preferences, we decided to focus on a comparison of struc- lent catalyst for the CuI-assisted coupling of 4-iodotoluene
tural parameters of the optimized geometries. Table 1 and phenyl acetylene in dioxane/water mixtures with 5mol-
summarizes some selected geometric parameters of the six- % catalyst loading.[16] In our hands, however, yields were
membered chelate ring for the four optimized structures considerably lower. We noticed that when CuII was replaced
(PBEPBE level). A close examination of the data sets shows with [Cu(CH3CN)4]BF4, the yields of 1-methyl-4-(phenyl-
that there are several parameters that mostly depend on ethynyl)benzene could be enhanced. Under these condi-
either the configuration at the metal center [d(Ni–P)
α(C–P–Ni), τ(Ni–P–C–E)] or the nature of the atom E [d(C–E)
,
,
tions, we tested the capability of the different nickel com-
plexes in catalyzing the Sonogashira coupling of 4-iodo-
α(P–Ni–P)]. The angle α(C–E–C) increases by approximately 5– toluene and phenyl acetylene in water (Table 2). Whereas
7° upon going from a square-planar to a tetrahedral config- the reaction catalyzed by 4 gave 1-methyl-4-(phenylethynyl)-
uration at the metal center. However, it is important to note benzene in 57% yield, reactions in the presence of 9 or 10
that this angle is overall larger for the complexes of ligand did not afford any coupling product. This result is consis-
5 than for those of ligand 6. In fact, for the configurations tent with previous reports that bidentate phosphine ligands
found experimentally, square-planar 9 and tetrahedral 7, reduce the catalytic activity of nickel-containing cata-
the calculated angles are much closer to the ideal angle lysts.[16] A striking observation was made when compound
(109.4°) of a tetrahedral ER4 unit than for the respective 7 was tested as a catalyst under the same aqueous reaction
hypothetical structures. Therefore, α(C–E–C), which is closely conditions. In this case, 1-methyl-4-(phenylethynyl)benzene
related to the distance d(C–E), is assumed to be the impor- was obtained in 29% yield. Because both nickel complexes
tant factor determining the bonding geometry. Upon sub- 7 and 9 possess comparable steric bulk at the phosphorus
stitution of the central carbon by
a silicon atom, atoms and differ only in their coordination geometry, we
d(C–E) increases from 1.55 to 1.90 Å. Consequently, the an- believe that the tetrahedral coordination environment
gle α(C–E–C) would need to sharpen to maintain a square- around nickel is an important requirement in nickel-cata-
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lyzed Sonogashira coupling reactions involving chelating
phosphine ligands.
Table 2. Catalytic Sonogashira coupling with 5mol-% Ni complex.
Entry
Compound Solvent
Base
Yield
[%][a]
1
2
3
4
4
9
dioxane/H2O
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
K2CO3
57
0
0
dioxane/H2O
dioxane/H2O
dioxane/H2O
dioxane
piperidine
CH3CN
THF
dioxane
dioxane
dioxane/H2O
10
7
29
30
16
25
0
traces
29
17
Scheme 3. Proposed mechanism for the Ni-based Sonogashira cou-
pling.
5
7
6[b]
7
7
7
8
7
behavior is quite similar to that observed for compound 9
(ca. +0.60 V). Therefore, an oxidative addition and stabili-
zation of NiIV intermediates does not seem plausible as the
first reaction step.
9[c]
10
11
7
7
8
[a] Isolated yield, determined in triplicates. [b] Room temperature.
[c] 1mol-% Ni complex.
2) In the absence of Cu+, no coupling product could be
obtained.
Next, we were interested in the solvent dependence of
this coupling process in the presence of 7 (Table 2). It is
notable that in polar solvents, such as dioxane/water mix-
tures, pure dioxane, or acetonitrile, similar yields of 1-
methyl-4-(phenylethynyl)benzene were obtained. Remarka-
bly, whereas the reactions described above require high tem-
perature, reactions in piperidine readily proceed at room
temperature (but at the cost of lower yields). In less polar
solvents, no coupling products were observed. To investi-
gate the catalytic efficiency of compound 7, we explored
the effect of catalyst loading. Unfortunately, lowering the
catalyst loading to 1mol-% lead to a trace-amount yield of
the coupling product. When 7 is replaced by bromide 8,
a significantly lower yield (17%) of 1-methyl-4-(phenyl-
ethynyl)benzene was observed. This result can most likely
be explained by the stronger trans effect of Br– compared
to Cl–, which renders the former a worse leaving group.
Whereas 7 catalyzes the coupling of 4-iodotoluene and
phenyl acetylene under the conditions described herein, no
reaction was observed when the coupling of alkyl halides,
for example, iodocyclohexane, and phenylacetylene was at-
tempted. Likewise, when inactivated iodotoluene derivatives
were used, such as 4-iodoanisol, only low yields of the de-
sired alkynes were obtained (Scheme S1). However, this be-
havior can be beneficial when several functional groups are
present within the molecule. For example, the coupling of
1-bromo-4-(2-bromoethyl)benzene afforded 1-(2-bromo-
ethyl)-4-(phenylethynyl)benzene (Scheme S1) as the sole
coupling product.
3) Complex 7 and 9 react readily with Cu–phenylacetyl-
ene, which suggests that transmetallation is the initial step.
However, the acetylene complexes are of different nature as
revealed by the different UV/Vis spectra (Figure S7).
4) It seems that only a tetrahedral (PR3)2NiX2 (X = Cl–,
Br–) complex can initiate the catalytic cycle. A similar be-
havior was observed by Huttner et al. for the stereoselective
heterodimerization of styrene and propene.[26]
Conclusions
In conclusion, we could show that the substitution of a
carbon atom by
a silicon atom within the CH3E-
(CH2PPh2)3 ligand system (E = C or Si) has a major in-
fluence on the structure of the resulting Ni complexes. The
different coordination environments were clearly shown by
single-crystal X-ray diffraction analyses. Furthermore, UV/
Vis as well as NMR spectroscopic investigations suggested
that both complexes are locked in their conformation. This
feature allowed us to investigate the lack of catalytic activity
in square-planar nickel–phosphine complexes for the
Sonogashira cross-coupling reaction. As an important re-
sult, it was found that a tetrahedral (PR3)2NiCl2 moiety is
required to allow for Sonogashira cross-coupling reactions.
The herein presented result might help to find and design
phosphine-based Ni complexes as cheap, water-soluble, and
robust catalysts for this important process.
Although the couplings were performed under non-opti-
mized conditions, we gathered significant mechanistic in-
sights. We believe that a mechanism (Scheme 3) comparable
to that reported by Nakamura et al. is operative.[31] The
following points support this theory:
Experimental Section
General: All reactions were performed under a dry N2 or Ar atmo-
sphere by using standard Schlenk techniques or by working in a
Glovebox. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded
with a Bruker DPX-200 NMR or a Bruker DPX-250 NMR spec-
1) A one-electron oxidation can be observed at positive
potentials (ca. +0.65 V) for Ni complex 7. This oxidation trometer at room temperature. Peaks were referenced to residual
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1H signals from the deuterated solvent and are reported in parts
per million (ppm). 31P{1H} NMR spectra were recorded by using
85% H3PO4 (δ = 0.0 ppm) as an external reference. Tris(chloro-
methyl)methylsilane[28] and {2-[(diphenylphosphanyl)methyl]-2-
methylpropane-1,3-diyl}bis(diphenylphosphane) (Triphos, 5)[32]
were synthesized according to literature procedures. Experimental
data for compounds 8 and 10 are provided within the Supporting
Information. All other compounds were obtained from commercial
vendors and used without further purification. Mass spectra were
obtained with either a Shimadzu QP-2010 or a Bruker Daltonics
Esquire 6000 instrument. UV/Vis spectra were recorded with a
Varian Cary 100 at 25 °C if not specified further. Magnetic mo-
ments were determined by using the Evans method.[33] Prior to use,
all solvents were dried according to standard methods. Thin-layer
chromatography was performed by using Merck TLC aluminum
sheets, silica gel 60 F254. IR spectra were recorded with a Bruker
Tensor IR spectrometer and are reported in cm–1.
Method B: Compound 5 (150 mg, 0.24 mmol) and Ni(PPh3)2Cl2
(156 mg, 0.24 mmol) reacted according to method B (described for
7) to yield an orange solid (158 mg, 88%). ESI-MS: calcd. for
[C41H39Cl2NiP3 – Cl–]+ 717.13; found 717. IR (KBr): ν = 3053,
˜
2961, 1668, 1480, 1432, 1186, 1095, 997, 838, 744, 692 cm–1.
C41H39Cl2NiP3 (754.30): calcd. C 65.29, H 5.21; found C 65.21, H
5.48.
[(TriphosSi)NiBr2] (8): Compound 6 (50 mg, 0.078 mmol) was dis-
solved in dry THF (2 mL), and nickel(II) bromide (22 mg,
0.078 mmol) was subsequently added. The resulting dark red solu-
tion was stirred overnight. The mixture was reduced to half of its
original volume by evaporation of the solvent and subsequently
filtered, and precipitation was initiated by adding Et2O to give a
brownish solid. The solid was recrystallized from DMF by Et2O
diffusion. The red crystals were separated, washed with Et2O, and
dried to afford complex 8 (37 mg, 55%). ESI-MS: calcd. for
C40H39Br2NiP3Si 855.98; found 777.09 [M – Br]+. C40H39Br2NiP3Si
(859.27): calcd. C 55.91, H 4.58; found C 55.62, H 4.73.
[(Methylsilanetriyl)tris(methylene)]tris(diphenylphosphane)
(Tri-
phosSi) (6): Diphenylphosphine (2.1 g, 11.3 mmol) was dissolved in
THF (40 mL) and cooled to –78 °C, and nBuLi (7.1 mL,
11.4 mmol) was added dropwise, which resulted in a deep red reac-
[(Triphos)NiBr2] (10): Compound 5 (50 mg, 0.08 mmol) was dis-
solved in dry THF (2 mL), and nickel(II) bromide (22 mg,
0.08 mmol) was subsequently added. The resulting dark red solu-
tion was stirred overnight. The mixture was reduced to half of its
original volume by evaporation of the solvent and subsequently
filtered. Precipitation was induced by adding Et2O, which gave an
orange solid. The solid was separated, washed with Et2O, and dried
to afford complex 10 (37 mg, 71%). ESI-MS: calcd. for
tion
mixture.
Subsequently,
tris(chloromethyl)methylsilane
(719 mg, 3.76 mmol) was added, and the mixture was stirred for
30 min at –78 °C. The solution was then warmed up overnight,
whereupon the solution decolorized. Quenching of the reaction
with water (1 mL) was followed by extraction with diethyl ether
(3ϫ 100 mL). The organic fractions were combined and dried with
Na2SO4, and the solvents were evaporated to dryness. The residue
was then recrystallized from methanol to afford compound 6 as a
C41H39Br2NiP3 843.19; found 762.85 [M
–
Br]+; calcd. for
C41H39Br2NiP3: 840.00; found 761.09 [M – Br]+. C40H39Br2NiP3Si
(859.27): calcd. C 64.52, H 5.15; found C 64.77, H 5.23.
1
white solid (1.34 g, 56%). H NMR (250 MHz, CDCl3): δ = 7.33–
Sonogashira Coupling. 1-Methyl-4-(phenylethynyl)benzene: In an
exemplary reaction, a degassed solution of 4-iodotoluene (130 mg,
0.6 mmol), phenyl acetylene (73 mg, 0.72 mmol), K2CO3 (165 mg,
1.2 mmol), Ni complex (5mol-%), and [Cu(CH3CN)4]BF4 (10mol-
%) in a mixture of 1,4-dioxane and Distilled water (3:1) was stirred
for 12 h and heated at reflux. The solvent was removed under re-
duced pressure, and the coupling product was isolated by column
7.18 (m, 30 H, C6H5), 1.20 (s, 6 H, CH2), 0.37 (s, 3 H, CH3) ppm.
13C NMR (50 MHz, CDCl3): δ = 142.4, 133.8, 129.7 (C6H5), 15.3
(CH2), 0.0 (CH3) ppm. 31P NMR (250 MHz, CDCl3): δ = –23.1
(PPh2) ppm. ESI-MS: calcd for [C40H39P3Si + Na]+ 663.2; found
663.0. C40H39P3Si (640.76): calcd. C 74.98, H 6.14; found C 74.69,
H 6.49.
[(TriphosSi)NiCl2] (7), Method A: Compound
6
(150 mg,
1
chromatography (PE/CHCl3, 15:1). H NMR (200 MHz, CDCl3):
0.23 mmol) was dissolved in dry THF/DMF (1:1, 5 mL), and NiCl2
(30 mg, 0.23 mmol) was subsequently added. The resulting solution
was stirred for 24 h at room temperature. From the obtained dark
red solution, the solvent was evaporated to dryness, and the residue
was recrystallized from DMF/hexane/diethyl ether. The resulting
dark red crystals were filtered off, washed with hexane and diethyl
ether, and dried in vacuum to afford crystalline material (135 mg,
76%).
δ = 7.48–7.06 (m, 10 H, Haromatic), 2.30 (s, 3 H, CH3) ppm. 13C
NMR (50 MHz, CDCl3): δ = 138.4, 131.1, 131.5, 129.3, 128.2,
123.2, 120.4 (Caromatic), 89.0, 88.3 (Calkyne), 21.5 (CH3) ppm.
X-ray Data Collection and Structure Solution Refinement: Crystals
of 7–9 were obtained by slow diffusion of diethyl ether into solu-
tions of the compounds dissolved in DMF/hexane. Single crystals
suitable for X-ray diffraction analysis were coated with Paratone-
N oil, mounted on a fiber loop, and placed in a cold, gaseous N2
stream on a Rigaku XtlabMini diffractometer performing φ and ω
scans at 170(2) K. Diffraction intensities were measured by using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Data
collection, indexing, initial cell refinements, frame integration, final
cell refinements, and absorption corrections were performed with
the program CrystalClear.[5] Space groups were assigned by analy-
sis of the metric symmetry and systematic absences (determined by
XPREP) and were further checked for additional symmetry by
using PLATON.[34,35] Structures were solved by using direct meth-
ods and refined against all data in the reported 2θ ranges by using
full-matrix least-squares methods on F2 with the SHELXL pro-
gram suite[36] and the OLEX2 interface or the OLEX2 refinement
program.[37] Crystallographic data as well as refinement parameters
are presented in Table S1 in the Supporting Information.
Method B: Compound 6 (150 mg, 0.23 mmol) was dissolved in dry
THF/DMF (1:1), and Ni(PPh3)2Cl2 (153 mg, 0.23 mmol) was sub-
sequently added. The resulting dark red solution was stirred for
24 h. Subsequently, half of the solvent was removed under reduced
pressure. Crystallization was initiated by layering the resulting solu-
tion with hexane/diethyl ether. Red crystals were separated, washed
with hexane and diethyl ether, and dried to yield compound 7
(133 mg, 75%). ESI-MS: calcd for [C40H39Cl2NiP3Si – Cl–]+
733.11; found 733. IR (KBr): ν = 3049, 2923, 2852, 1643, 1481,
˜
1433, 1241, 1122, 982, 813, 697 cm–1. C40H39Cl2NiP3Si·H2O: calcd.
C 60.94, H 5.24; found C 60.98, H 4.86.
[(Triphos)NiCl2] (9): Nickel complex 9 was synthesized according
to a modified literature procedure[30] and as reported for compound
7.
Method A: Compound 5 (200 mg, 0.32 mmol) and NiCl2 (41 mg, Electrochemistry. Instrumentation and Procedures: Cyclic voltam-
0.32 mmol) reacted according to method A (described for 7) to yield
mograms were recorded by using a non-aqueous Ag/Ag+ reference
an orange solid (93 mg, 38%).
electrode [0.1 m (nBu4N)(PF6) and 0.01 m AgNO3 in acetonitrile as
Eur. J. Inorg. Chem. 2015, 2139–2144
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FULL PAPER
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This work was supported by the German Fonds of the Chemical
Industry (Liebig grant to C. M. and U.-P. A.) and the German
Deutsche Forschungsgemeinschaft (DFG) (Emmy Noether grant
to U.-P. A., AP242/2-1). Support by the DFG through the Cluster
of Excellence “Ruhr Explores Solvation” (RESOLV, EXC 1069) is
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Received: February 11, 2015
Published Online: March 18, 2015
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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim