Application of nickel complexes modified by tridentate O,N,O'-ligands as precatalysts in nickel-catalyzed C(sp2)-C(sp3) bond formations

Article abstract of DOI:10.1002/ejic.201101253

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 treated with Ni(OAc) ₂·4H₂O in the presence of an excess of base [NH₃ or 4-(dimethylamino)pyridine (DMAP)] to form the nickel complexes 4a-c. These complexes have been characterized by various techniques, which indicate a tridentate coordination mode of the ligands. X-ray crystallography determined an O,N,O'-coordination of the ligands, in which the ligand is planar, the oxygen donors are trans to each other, and the nitrogen donor is in a cis position. The other coordination sites on the nickel centre are occupied by the added base molecules (NH₃ or DMAP). The number of NH₃ or DMAP ligands depends on the nature of the base; in the case of ammonia, one molecule is coordinated to the nickel centre to form a diamagnetic square-planar complex, whereas with DMAP, an octahedral paramagnetic complex with three additional DMAP ligands was observed. Initial catalytic experiments have been performed by applying the complexes in the nickel-catalyzed C(sp²)-C(sp³) cross-coupling of aryl halides with benzylzinc bromides or dialkylzinc reagents; excellent yields and selectivities have been achieved.

Full text of DOI:10.1002/ejic.201101253

FULL PAPER
DOI: 10.1002/ejic.201101253
Application of Nickel Complexes Modified by Tridentate O,N,OЈ-Ligands as
Precatalysts in Nickel-Catalyzed C(sp2)–C(sp3) Bond Formations
Chika I. Someya,[a] Shigeyoshi Inoue,[b] Sebastian Krackl,[a] Elisabeth Irran,[b] and
Stephan Enthaler*[a]
Keywords: Nickel / Tridentate ligands / Homogeneous catalysis / Cross-coupling
1-Acetyl- [1a; 3,5-CF3, 1-C(=O)CH3] and 1-benzoyl-5-hy-
droxypyrazolines [1b; 3,5-CF3, 1-C(=O)C6H5] have been syn-
thesized and treated with Ni(OAc)2·4H2O in the presence of
an excess of base [NH3 or 4-(dimethylamino)pyridine
pied by the added base molecules (NH3 or DMAP). The
number of NH3 or DMAP ligands depends on the nature of
the base; in the case of ammonia, one molecule is coordi-
nated to the nickel centre to form a diamagnetic square-
(DMAP)] to form the nickel complexes 4a–c. These com- planar complex, whereas with DMAP, an octahedral para-
plexes have been characterized by various techniques, which
indicate a tridentate coordination mode of the ligands. X-ray
crystallography determined an O,N,OЈ-coordination of the li-
gands, in which the ligand is planar, the oxygen donors are
trans to each other, and the nitrogen donor is in a cis position.
The other coordination sites on the nickel centre are occu-
magnetic complex with three additional DMAP ligands was
observed. Initial catalytic experiments have been performed
by applying the complexes in the nickel-catalyzed C(sp2)–
C(sp3) cross-coupling of aryl halides with benzylzinc brom-
ides or dialkylzinc reagents; excellent yields and selectivities
have been achieved.
easy synthesis, high tunability, flexibility and stability of the
ligands have to be considered.[7] We have recently investi-
gated the potential of 1-acetyl- and 1-benzoyl-5-hydroxy-
pyrazoline ligands in coordination chemistry (Figure 1).[8]
Depending on the metal precursor and the reaction condi-
tions, different coordination modes were feasible. In the
case of nickel, 5-hydroxypyrazoline ligands were coordi-
nated in a tridentate O,N,OЈ-coordination mode as reported
by Joshi and co-workers (Figure 1; 4).[9,11] Due to their high
stability and the possibility for the ligand to stabilize
charges by conjugation, the complexes could be suitable for
catalysis.
Introduction
A huge number of methodologies for the formation of
carbon–carbon bonds have been established.[1] Based on the
landmark coupling reactions of the groups of Heck, Suzuki
and Negishi in the 1960s and 1970s, transition-metal-cata-
lyzed reactions have been proven to be a powerful technique
in organic synthesis, which is outlined by countless applica-
tions.[2] Aside from palladium, nickel-based catalysts play
an important role in these transformations.[3] In the last few
decades, various protocols have been established mainly for
the coupling of sp2-carbon electrophiles with sp2-carbon
nucleophiles. In contrast, the coupling of alkyl electrophiles
and nucleophiles is underdeveloped because of various diffi-
culties, e.g. β-hydride elimination, slow reductive elimi-
nation steps and high catalyst loadings, which significantly
reduce the impact of the method.[4] Therefore, the design of
more active and selective catalysts and the search for suit-
able reaction conditions is highly desired. Recently, different
protocols have been developed for the coupling of sp3-car-
bon atoms to sp-, sp2- and sp3-carbon atoms.[5] The ad-
dition of suitable ligands is an important factor to influence
the reaction outcome.[6] Furthermore, low cost, availability,
[a] Technische Universität Berlin, Department of Chemistry,
Cluster of Excellence “Unifying Concepts in Catalysis”,
Straße des 17. Juni 135/C2, 10623 Berlin, Germany
Fax: +49-30-314-29732
Figure 1. Coordination modes of 5-hydroxypyrazolines 1.
E-mail: stephan.enthaler@tu-berlin.de
On this basis, we report the synthesis and characteriza-
tion of easily accessible nickel complexes modified by tri-
dentate O,N,OЈ-ligands and their application in coupling
[b] Technische Universität Berlin, Department of Chemistry:
Metalorganics and Inorganic Materials,
Straße des 17. Juni 135/C2, 10623 Berlin, Germany
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C. Someya, S. Inoue, S. Krackl, E. Irran, S. Enthaler
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chemistry, i.e. the selective nickel-catalyzed C(sp2)–C(sp3) pyrazoline ligand connected to the nickel centre is trans to
cross-coupling of aryl halides with benzylzinc bromides or the ammonia ligand. Conversely, 4b and 4c have octahedral
dialkylzinc reagents.
structures with three DMAP molecules connected to the
nickel atom. The tridentate ligand is coordinated in the
same fashion as seen in 4a. However, the bond lengths to
the nickel atom are slightly elongated. A similar effect was
observed for the DMAP ligands in comparison to the am-
monia ligand.
Results and Discussion
Complex Synthesis and Characterization
The ligands were synthesized according to literature
methods (Scheme 1).[10] Acetohydrazide (7a) and benzo-
hydrazide (7b) were obtained from the reaction of equi-
molar amounts of hydrazine monohydrate with ethyl acet-
ate (6a) or methyl benzoate (6b), respectively, in ethanol
under reflux conditions. The hydrazides 7a and 7b were ob-
tained in good yields as colourless solids and subsequently
heated to reflux with hexafluoroacetylacetone (8) in eth-
anol. 1-Acetyl- and 1-benzoyl-5-hydroxypyrazoline 1 were
obtained as crystalline compounds in good yields (69–87%)
after recrystallization from n-hexane.
Scheme 1. Synthesis of 1.
Having synthesized 1-acetyl- and 1-benzoyl-5-hydroxy-
pyrazoline, the coordination of these ligands to nickel salts
was investigated (Scheme 2).[9,11] A methanol solution of
the ligand and an excess of base [NH3 (17% aqueous solu-
tion) or 4-(dimethylamino)pyridine (DMAP)] was added to
a solution of Ni(OAc)2·4H2O in methanol at room tempera-
ture. After stirring overnight, the volatiles were removed in
vacuo 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.
Scheme 2. Synthesis of 4a–c.
The solid-state structures of 4a–c were characterized by
single-crystal X-ray diffraction. Thermal-ellipsoid plots are
displayed in Figure 2, and selected bond lengths and angles
are listed in Table 1. In agreement with the work of Joshi
and co-workers, a square-planar arrangement was observed
for 4a with similar bond lengths and angles.[9] The triden-
tate ligand is coordinated in an O,N,OЈ-mode to create a
five- and six-membered ring system, which blocks one side
of the metal atom. The ammonia ligand is positioned cis to
Figure 2. Molecular structures of 4a (a), 4b (b) and 4c (c). Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms
the oxygen donors, and the nitrogen atom of the 5-hydroxy- are omitted for clarity.
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Ni Complexes Modified by Tridentate O,N,OЈ-Ligands as Precatalysts
Table 1. Selected bond lengths [Å] and angles [°] for 4a, 4b and 4c. ammonia or DMAP (B), which has been proven by label-
ling studies of the ligand in [D4]methanol.[8] The deproton-
ation process initiates the formation of 4a–c.
4a
4b
4c
Ni–N1
Ni–O1
Ni–O2
Ni–N3
Ni–N4
Ni–N5
O(1)–Ni–O(2)
N(1)–Ni–N(3)
1.816(3)
1.819(3)
1.829(4)
1.919(3)
–
2.055(3)
2.021(3)
2.041(3)
2.118(3)
2.132(3)
2.164(3)
170.16(1)
172.16(1)
2.0440(1)
2.0260(1)
2.0451(2)
2.0950(2)
2.1648(2)
2.1383(2)
169.23(6)
175.43(8)
–
177.36(15)
171.87(17)
Complexes 4a–c were also investigated by 1H NMR spec-
troscopy (Table 2). Paramagnetic 4b and 4c gave broad sig-
nals, which could not be assigned. In contrast, for diamag-
netic 4a, a signal was found at δ = 5.93 ppm, which was
assigned to the C–H proton in the six-membered ring. This
was further proven by the absence of any signal from the
former CH2 group in the ligand. In addition, a broad signal
at δ = 1.50 ppm was attributed to the N–H protons of the
ammonia ligand. The coordination of the acetyl and ben-
zoyl functionalities was investigated by IR spectroscopy
(Table 2). The absence of signals in the 1610–1725 cm–1 re-
gion excludes the possibility of a free or coordinated C=O
functionality in the chelated ligand. In addition, strong
bands were observed in the 1515–1615 cm–1 region, which
are attributed to the C=N functionality. Electron paramag-
netic resonance (EPR) spectroscopy was applied to charac-
terize paramagnetic 4b and 4c. Difficulties are known to
arise with paramagnetic NiII complexes with an integer spin
(S = 1).[3b,12] Indeed, the EPR spectra of 4b and 4c are
silent. Theoretically, in a pure octahedral complex, two un-
paired electrons are expected. However, due to the sizeable
zero-field splitting for the 3d8 configuration usually found
for paramagnetic NiII complexes with an integer spin (S =
1), octahedral 4b and 4c are silent. Nevertheless, SQUID
measurements confirmed the S = 1 state for 4b and 4c.
These compounds both have a constant magnetic moment
(μeff) of ca. 3 μB in the temperature range of 20–300 K.
Although the reaction mechanism for the formation of
4a–c is unknown, a reasonable mechanism is described in
Scheme 3. Firstly, the nickel acetate coordinates to the basic
nitrogen atom of the pyrazoline and to the acetyl function-
ality to form the chelating intermediate A. Deprotonation
of the hydroxy group then occurs in the presence of the
base. The ring opening of the cyclic pyrazoline is initiated
Scheme 3. Potential reaction pathway for the synthesis of 4.
The electrochemistry of 4b and 4c was investigated by
cyclic voltammetry, and the results are summarized in Fig-
ure 3 and Table 3. The first oxidation process is an irrevers-
Figure 3. Cyclic voltammograms of 4b and 4c.
Table 3. Cyclic voltammetric data for 4b and 4c.[a]
EIa [V] (q.r.)[b] EIc [V] (ir.)[b] EIIc [V] (r.)[b] EIIIc [V] (ir.)[b]
4b –1.05
4c –1.11
+1.06
+1.11
+1.49
+1.47
+1.88
+1.87
[a] Electrochemical data for 4b and 4c at room temp., [Bu4N][PF6]
as the supporting electrolyte, scan rate = 10 mV/s, platinum disc
and glassy carbon working electrode; the potentials are referenced
vs. Fc+/Fc. Reference electrode: Ag/Ag+ in CH3CN. [b] r. = revers-
by the deprotonation of the acidic CH2 functionality by ible, ir. = irreversible, q.r. = quasireversible.
Table 2. Analytical properties of 1a, 1b, 4a, 4b, and 4c.
1a
1b
4a
4b
4c
1H NMR [ppm][a]: 4-CH(H)
3.30–3.80
–67.7
–81.4
3.00–3.66
–67.4
–80.5
5.93
–65.3
–71.8
–
–
–
–
19F NMR [ppm][a]
IR:
ν(C=O) [cm–1][b]
ν(C=N) [cm–1][b]
1699 (s)
1645 (s)
1678 (s)
1639 (s)
–
–
–
1615 (s)
1533 (s)
1615 (s)
1533 (s)
1617 (s)
1532 (s)
[a] Chemical shifts were measured in [D1]chloroform at 25 °C. [b] Measured in KBr at 25 °C.
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ible wave with a peak potential at +1.06 V for 4b and EPR measurements was confirmed by DFT calculations
+1.11 V for 4c. The second oxidation process is observed [UB3LYP level 6-31G(d) basis set for N, C, O, F, and H
as a reversible wave with half-wave potentials at +1.49 V atoms and LANL2DZ for the Ni atom]. The calculations
(δE = 0.13 V) for 4b and +1.47 V (δE = 0.15 V) for 4c. The revealed that the highest occupied molecular orbital of 4bЈ
third oxidation process is a low-intensity irreversible wave is localized on the Ni atom, whereas the lowest unoccupied
with a peak potential at +1.88 V for 4b and +1.87 V for molecular orbital comprises the π*-orbital of the ligand.
4c. Only one reduction process was observed with a peak The spin density at the nickel centre (0.9264) may be re-
potential at –1.05 V for 4b and –1.11 V for 4c, which is best garded as the result of a large contribution from the NiI
described as quasireversible.
species (Figure 4b).
DFT calculations have been proven to give reliable pre-
dictions of the electronic structure of 4b [B3LYP/6-31G(d)
for N, C, O, F, and H; LANL2DZ for Ni].[13] The energy-
minimized structure calculated by DFT is in good agree-
ment with that obtained by X-ray diffraction. The molecu-
lar orbitals of 4b are presented in Figure 4. The singly occu-
pied molecular orbitals (SOMOs, –0.157 and –0.186 eV) are
nearly degenerate. SOMO1 mainly originates from the tri-
dentate O,N,OЈ-ligand, whereas SOMO2 comprises d-orbit-
als of the nickel atom with some contribution from the li-
gand (Figure 4a).
Figure 5. EPR spectrum of the mixture of 4b (3.0ϫ10–2 mmol) and
cobaltocene (3.0ϫ10–2 mmol) in THF at 77 K.
Nickel-Catalyzed C(sp2)–C(sp3) Bond Formations
The catalytic abilities of 4a–c were investigated in the
coupling of aryl halides and benzylzinc bromides or dialk-
ylzinc reagents (Table 4). Initial experiments were per-
formed to find suitable reaction conditions. As a model re-
action, the coupling of 1-iodo-4-methoxybenzene (9) with
benzylzinc bromide (10) was performed in the presence of
4b (Table 4, Entries 1–4). The reaction outcome was exam-
ined at different temperatures. Only at an elevated tempera-
ture (70 °C) was the desired coupling product observed in
good yields (Table 4, Entry 4). The nickel complexes oper-
ated as precatalysts, and no reaction took place in their ab-
sence (Table 4, Entry 3). After initial optimization, we es-
tablished that 2.0 mol-% of 4b in THF performed the
C(sp2)–C(sp3) bond formation at 70 °C in excellent yield
(Ͼ 99%, Table 4, Entry 4). A similar result (yield Ͼ 99%)
was achieved with 4c (Table 4, Entry 5). The coupling reac-
tion was also examined by using diethylzinc to produce p-
ethylanisole in a yield of 87% (Table 4, Entry 6). To study
the effect of β-hydride elimination, the coupling reaction
Figure 4. (a) Molecular orbitals of 4b. B3LYP level 6-31G(d): basis
set for N, C, O, F, and H atoms and LANL2DZ for the Ni atom.
(b) Optimized model structure of 4bЈ with an overlay of the com-
puted spin density: UB3LYP level 6-31G(d) basis set for N, C, O, was investigated with α-methylbenzylzinc bromide (Table 4,
F, and H atoms and LANL2DZ for the Ni atom.
Entry 7). The desired coupling product was obtained in a
yield of 27%. However, nonnegligible amounts of the
The one-electron reduction of 4b was confirmed by homocoupled products were detected by GC–MS [18% (2-
chemical reduction using 1 equiv. of cobaltocene. Complex methylbutane-1,3-diyl)dibenzene and 29% 4,4Ј-dimeth-
4b and cobaltocene (1:1) were dissolved in tetrahydrofuran oxybiphenyl]. Furthermore, styrene (4%) was observed as
(THF) at room temperature, transferred to an EPR tube, the β-hydride elimination product.
and the spectrum was immediately recorded at 77 K (Fig-
With these conditions in hand, different aryl iodides were
ure 5). The spectrum shows characteristic signals for a one- treated with benzylzinc bromide in the presence of 4b. Ex-
electron reduction at the nickel atom, which resulted in the cellent yields (Ͼ 99%) were observed for 4-iodotoluene (12)
formation of an NiI species 4bЈ (gx = 2.041, gy = 2.115, gz and iodobenzene (13, Table 4, Entries 9 and 10). In the case
= 2.271). The electronic nature of 4bЈ suggested by CV and of the challenging electron-withdrawing nitro group, no
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Ni Complexes Modified by Tridentate O,N,OЈ-Ligands as Precatalysts
Table 4. Nickel-catalyzed C–C bond formation of benzylzinc brom- product formation was observed (Table 4, Entry 8). More-
ide with aryl halides.[a]
over, some examples of aryl bromides were tested. It was
found that the aryl bromides gave lower yields of the cou-
pling products than aryl iodides (Table 4, Entries 11–13).
However, 1-bromo-4-(trifluoromethyl)benzene (17), which
has a strong electron-withdrawing group, was converted in
excellent yield (Table 4, Entry 14). The reaction of the het-
eroarene 18 with benzylzinc bromide resulted in the quanti-
tative formation of the desired coupling product (Table 4,
Entry 15). On the other hand, the coupling of 1-bromo-4-
fluorobenzene (19) afforded 1-benzyl-4-fluorobenzene (19a)
as the product in 39% yield. No activation of the C–F bond
was observed. To verify the inactivity of the C–F bonds we
tested the coupling reaction of fluorobenzene (20), and no
coupling product was formed (Table 4, Entry 17).
In addition, we investigated the influence of the halide
on the reaction outcome (Figure 6). The coupling reaction
of benzylzinc bromide with substituted anisoles 9, 14 and
21 in the presence of 2 mol-% of 4c was evaluated by using
GC–MS (Scheme 4). Complex 4c was highly active for the
conversion of the 4-iodoanisol (9), and full conversion was
reached within 15 min (turnover frequency = 375 h–1). As
expected for bromo- (14) and chloroanisole (21), the cata-
lyst activity decreased compared to that with 9, e.g. t(50%
yield) = 5.5 min for 9 and t(50% yield) = 160 min for 14.
In particular, the reaction with chloroanisole yielded only
12% of the coupling product after 24 h.
Figure 6. Comparison of different aromatic halides. Reaction con-
ditions: substrate (0.67 mmol), benzylzinc bromide (1.00 mmol,
0.5 m in THF), catalyst 4c (2.0 mol-%), THF (2.0 mL), 70 °C.
Scheme 4. Coupling reaction with different haloanisoles.
[a] Reaction conditions: substrate (0.67 mmol), benzylzinc bromide
(1.00 mmol, 0.5 m in THF), catalyst (2.0 mol-%), THF (2.0 mL),
70 °C, 24 h. [b] Yield of desired product determined by GC–MS
Conclusions
1
and H NMR spectroscopy. [c] Et2Zn (1.00 mmol) in THF/n-hex-
ane solution. [d] 4-Ethylanisole was observed as the product. [e] (α-
Methylbenzyl)zinc bromide (1.00 mmol) in THF. [f] 1-Methoxy-4-
(1-phenylethyl)benzene was observed as the product. (2-Methylbut-
ane-1,3-diyl)dibenzene (18%) and 4,4Ј-dimethoxybiphenyl (29%)
We have synthesized and characterized new nickel com-
plexes with the highly flexible ligands 1-acetyl- [1a; 3,5-CF3,
1-C(=O)CH3] and 1-benzoyl-5-hydroxypyrazoline [1b; 3,5-
were observed as homocoupled products.
CF3, 1-C(=O)C6H5]. The added base is coordinated to the
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3.30–3.80 (m, 2 H, CH2), 2.29 [s, 3 H, C(=O)CH3] ppm. 13C NMR
(50 MHz, CDCl3, 25 °C): δ = 173.3, 143.6 (m), 120.6 (m), 92.5 (m),
41.4 (CH2), 22.4 (H3CC=O) ppm. 19F NMR (188 MHz, CDCl3,
nickel centre as a coligand. Depending on the nature of the
ligand, different numbers of coligands occupy the vacancies
on the nickel atom. Finally, the properties of the complexes
were investigated in cross-coupling reactions. An excellent
performance of the precatalysts was found for C(sp2)–
C(sp3) bond formations.
25 °C): δ = –67.7, –81.4 ppm. IR (KBr): ν = 3393 (br.), 1699 (s),
˜
1645 (s), 1456 (s), 1427 (m), 1378 (m), 1338 (s), 1276 (s), 1198 (s),
1156 (s), 1116 (m), 1099 (m), 1068 (w), 1041 (w), 1026 (w), 996 (w),
924 (m), 875 (m), 760 (m), 734 (s), 658 (w), 621 (w), 581 (m), 521
(w), 501 (w), 474 (w) cm–1. HRMS: calcd. for C7H7F6N2O2 [M +
H]+ 265.04117; found 265.04031.
Experimental Section
1-Benzoyl-5-hydroxy-3,5-bis(trifluoromethyl)pyrazoline (1b): To a
solution of 8 (55.8 mmol) in ethanol (60 mL) was added a solution
of benzohydrazide (55.8 mmol) in ethanol (60 mL). After heating
the mixture to reflux for 5 h, the solvent was removed in vacuo. The
colourless residue was purified by recrystallization from ethanol/
n-hexane (9:1). Yield: 15.9 g (87%, colourless crystals). 1H NMR
(200 MHz, CDCl3, 25 °C): δ = 7.84–7.92 (m, 2 H, Ar), 7.40–7.65
(m, 3 H, Ar), 6.40 (br. s, 1 H, OH), 3.00–3.66 (m 2 H, CH2) ppm.
13C NMR (50 MHz, CDCl3, 25 °C): δ = 171.6, 144.1, 143.7, 133.3,
131.6, 130.5, 128.3, 94.2, 93.8, 41.4 (CH2) ppm. 19F NMR
General: All manipulations with oxygen- and moisture-sensitive
compounds were performed under dinitrogen by using standard
Schlenk techniques. Methanol was used without further purifica-
1
tion. H, 19F and 13C NMR spectra were recorded with a Bruker
AFM 200 spectrometer (1H: 200.13 MHz; 13C: 50.32 MHz; 19F:
188.31 MHz) and
a
Bruker AFM 400 spectrometer (1H:
400.13 MHz; 13C: 100.61 MHz; 19F: 376.07 MHz) by using the pro-
ton signals of the deuterated solvents for reference. Single-crystal
X-ray diffraction measurements were recorded with an Oxford Dif-
fraction Xcalibur S Saphire spectrometer. IR spectra were recorded
either with an Nicolet Series II Magna-IR-System 750 FTIR or a
Perkin–Elmer Spectrum 100 FTIR instrument. UV spectra were
recorded with a SPECORD® S600 instrument (Analytik Jena, Ger-
many). Cyclic voltammetry was measured with a reference 600 sys-
tem (GAMRY instruments, USA). EPR spectra were recorded at
X-band with an ERS 300 spectrometer (ZWG/Magnettech Berlin/
Adlershof, Germany) equipped with a fused quartz Dewar for mea-
surements at 77 K. The g factors were calculated with respect to a
Cr3+/MgO reference (g = 1.9796). Magnetic-susceptibility data
were measured from powder samples of solid material in the tem-
perature range 2–300 K with a SQUID susceptometer with a field
of 1.0 T (MPMS-7, Quantum Design, calibrated with a standard
palladium reference sample, error Ͻ 2%). Multiple-field variable-
temperature magnetization measurements were performed at 1, 4
and 7 T in the range 2–300 K with the magnetization equidistantly
sampled on a 1/T temperature scale. The experimental data were
corrected for underlying diamagnetism by use of tabulated Pascal’s
constants[14] as well as for temperature-independent paramagne-
tism. The susceptibility and magnetization data were simulated
with julX for exchange-coupled systems.[15] The simulations are
based on the usual spin-Hamilton operator for mononuclear com-
plexes with spin S = 5/2:
(188 MHz, CDCl , 25 °C): δ = –67.4, –80.5 ppm. IR (KBr): ν =
˜
3
3391 (br.), 1678 (s), 1639 (m), 1456 (m), 1439 (m), 1336 (s), 1308
(s), 1279 (s), 1175 (s), 1135 (s), 1116 (s), 1077 (s), 1027 (m), 1009
(m), 907 (w), 872 (w), 834 (w), 792 (w), 758 (w), 717 (s), 694 (m),
674 (w), 618 (w), 499 (w) cm–1. HRMS: calcd. for C12H9F6N2O2
[M + H]+ 327.05627; found 327.05569.
Synthesis of 4a: To a methanol (5.0 mL) solution of Ni(OAc)2
(1.20 mmol) was added 1b (0.92 mmol) in methanol (5.0 mL) and
NH3 (17% in water, 5.0 mL). The solution was heated to 60 °C and
stirred overnight. After removal of the volatiles in vacuo, a brown
solid was obtained. The residue was crystallized from ethanol.
Crystals suitable for X-ray diffraction were obtained from an eth-
anol solution at room temperature. Yield: 0.25 g (51%, brown crys-
tals). M.p. 192–193 °C. 1H NMR (200 MHz, CDCl3, 25 °C): δ =
7.83 (m, 2 H, Ar), 7.31 (m, 3 H, Ar), 5.93 (s, 1 H, CH), 1.50 (s, 3
H, NH3) ppm. 13C NMR (100.61 MHz, CDCl3, 25 °C): δ = 171.9,
157.2, 156.9, 143.4, 131.1, 128.8, 128.4, 91.5 ppm. 19F NMR
(188 MHz, C D , 25 °C): δ = –65.3, –71.8 ppm. IR (KBr): ν = 3430
˜
6
6
(br.), 2914 (br.), 1615 (s), 1533 (s), 1444 (m), 1363 (m), 1328 (w),
1266 (s), 1228 (s), 1195 (s), 1159 (s), 1068 (m), 1007 (m), 950 (w),
880 (w), 806 (m), 758 (w), 715 (m), 701 (w), 686 (w), 614 (w), 531
(w) cm–1. HRMS: calcd. for C12H10F6N3NiO2 [M + H]+ 400.00307;
found 400.00305. C12H9F6N3NiO2·H2O (417.9): calcd. C 34.49, H
2.65, N 10.05; found C 34.70, H 2.55, N 9.71. UV/Vis: λ (ε) = 367.5
(11800), 405.0 (11600 m–1 cm–1) nm.
Synthesis of 4b: To a mixture of 1b (0.92 mmol) and DMAP
where g is the average electronic g value and D and E/D are the
axial zero-field splitting and rhombicity parameters, respectively.
Diagonalization of the Hamiltonian was performed with the rou-
tine ZHEEV from the LAPACK Library,[16] and the magnetic mo-
ments were obtained from a first-order numerical derivative dE/dB
of the eigen values. The powder summations were performed by
using a 16-point Lebedev grid.[17] Intermolecular interactions were
considered by using a Weiss temperature, ΘW, as perturbation of
the temperature scale, kTЈ = k(T – ΘW) for the calculation.
(9.2 mmol) in methanol (10 mL) was added
a solution of
Ni(OAc)2 (1.20 mmol) at room temperature. The solution was
stirred overnight. After removal of the volatiles in vacuo, a brown
solid was obtained. The residue was crystallized from ethanol.
Crystals suitable for X-ray diffraction were obtained from ethanol
solution at room temperature. Yield: 0.39 g (57%, first harvest,
brown crystals). M.p. 191–192 °C. 1H, 13C and 19F NMR were not
measured because of paramagnetism. IR (KBr): ν = 3433 (br.),
˜
2921 (br.), 1615 (s), 1533 (s), 1444 (m), 1363 (m), 1328 (w), 1266
(s), 1228 (s), 1196 (s), 1159 (s), 1068 (m), 1007 (m), 950 (w), 880
(w), 806 (m), 759 (w), 715 (m), 701 (w), 685 (w), 646 (w), 613 (w),
585 (w), 531 (w) cm–1. HRMS: calcd. for C33H36F6N8NiO2 [M +
H – 2 dmap]+ 505.06092; found 505.05975. C33H36F6N8NiO2
(749.40): calcd. C 52.89, H 4.84, N 14.95; found C 53.66, H 4.94,
N 15.11. UV/Vis: λ (ε) = 360.5 (2200), 405.5 (1800 m–1 cm–1) nm.
Synthesis of 1-Acetyl-5-hydroxy-3,5-bis(trifluoromethyl)pyrazoline
(1a): To a solution of 1,1,1,5,5,5-hexafluoropentane-2,4-dione (8,
55.8 mmol) in ethanol (60 mL) was added a solution of acetohydra-
zide (55.8 mmol) in ethanol (60 mL). After heating the mixture to
reflux for 5 h, the solvent was removed in vacuo. The colourless
residue was purified by recrystallization from n-hexane or by subli-
mation under high vacuum. Yield: 12.2 g (83%, colourless crystals).
1H NMR (200 MHz, CDCl3, 25 °C): δ = 5.98 (br. s, 1 H, OH),
Synthesis of 4c: To a mixture of 1a (0.37 mmol) and DMAP
(3.70 mmol) in methanol (5 mL) was added a solution of Ni-
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 1269–1277

Ni Complexes Modified by Tridentate O,N,OЈ-Ligands as Precatalysts
(OAc)2 (0.48 mmol) at room temperature. The solution was stirred
overnight. After removing the volatiles in vacuo, a brown solid was
obtained. Crystals suitable for X-ray diffraction were obtained
from ethanol solution at room temperature. Yield: 0.14 g (55%,
brown crystals from ethanol). M.p. 205–206 °C. 1H, 13C and 19F
13C NMR (50 MHz, CDCl3): δ = 159.7, 143.1, 141.5, 129.9, 129.6,
129.0, 126.4, 121.9, 115.4, 112.0, 55.3, 42.0 ppm. MS (ESI): m/z
(%) = 198 (100) [M]+, 183 (16), 167 (36), 153 (13), 121 (28), 91 (12).
1-Benzyl-4-methylbenzene (12a):[18] 1H NMR (200 MHz, CDCl3): δ
= 7.06–7.31 (m, 9 H), 3.98 (s, 2 H, CH2), 2.36 (s, 3 H, CH3)
ppm.13C NMR (50 MHz, CDCl3): δ = 141.5, 138.2, 135.6, 129.3,
129.0, 128.9, 128.8, 128.5, 128.4, 126.1, 41.6, 21.1 ppm. MS (ESI):
m/z (%) = 182 (75) [M]+, 167 (100), 152 (14), 89 (14).
NMR were not measured because of paramagnetism. IR (KBr): ν
˜
= 3438 (br.), 2921 (br.), 1617 (s), 1532 (s), 1446 (m), 1428 (w), 1388
(s), 1324 (m), 1265 (s), 1227 (s), 1182 (s), 1169 (s), 1159 (s), 1138
(s), 1108 (s), 1069 (w), 1054 (m), 1014 (m), 1006 (s), 982 (w), 951
(w), 906 (w), 828 (w), 807 (s), 768 (m), 759 (m), 742 (w), 683 (m),
641 (w), 607 (w), 581 (w), 531 (m), 483 (w) cm–1. HRMS: calcd. for
C28H35F6N8NiO2 [M + H – 2 dmap]+ 443.04527; found 443.04435.
C28H34F6N8NiO2 (687.33): calcd. C 48.93, H 4.99, N 16.30; found
C 49.47, H 5.01, N 16.36. UV/Vis: λ (ε) = 334.5 (5900), 368.0
(5900 m–1 cm–1) nm.
Diphenylmethane (13a):[19] 1H NMR (200 MHz, CDCl3): δ = 7.10–
7.37 (m, 10 H), 3.95 (s, 2 H, CH2) ppm. 13C NMR (50 MHz,
CDCl3): δ = 141.1, 128.9, 128.5, 126.1, 41.6 ppm. MS (ESI): m/z
(%) = 168 (100) [M]+, 153 (25), 91 (22), 83 (13), 65 (14).
1-Benzyl-3-Methoxybenzene (15a):[20] 1H NMR (200 MHz, CDCl3):
δ = 6.85–7.49 (m, 9 H), 4.06 (s, 2 H, CH2), 3.86 (s, 3 H, CH3)
ppm.13C NMR (50 MHz, CDCl3): δ = 159.7, 143.1, 141.5, 129.9,
129.6, 129.0, 126.4, 121.9, 115.4, 112.0, 55.3, 42.0 ppm. MS (ESI):
m/z (%) = 198 (100) [M]+, 183 (20), 167 (40), 152 (15), 121 (12), 91
(19).
General Procedure for the Catalytic C–C Coupling Reaction: A
Schlenk flask was charged with an appropriate amount of 4
(0.014 mmol, 2.0 mol-%) and the corresponding bromo- or iodo-
arene (0.67 mmol). The flask was repeatedly flushed with nitrogen
and evacuated. THF (2.0 mL) was added followed by benzylzinc
bromide (1.00 mmol, 0.5 m in THF). The flask was sealed and
heated at 70 °C for 24 h. The mixture was cooled in an ice bath,
and dichloromethane and water were added. The aqueous layer
was extracted with dichloromethane, and the collected organic lay-
ers were washed with water, brine and dried with Na2SO4. After
filtration and removal of the solvent, the residue was dissolved in
diethyl ether and purified by filtration through a short plug of silica
gel. The analytical properties of the products are in agreement with
literature data.
1-Benzyl-4-(trifluoromethyl)benzene (17a):[21] 1H NMR (400 MHz,
CDCl3): δ = 7.58 (d, J = 8.1 Hz, 2 H), 7.04–7.29 (m, 7 H), 4.07 (s,
2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 145.3, 140.0, 129.2,
128.9, 128.7, 127.0, 126.5, 125.4, 41.7 ppm. 19F NMR (377 MHz,
CDCl3): δ = –62.7 ppm. MS (ESI): m/z (%) = 236 (55) [M]+, 217
(7), 215 (10), 168 (11), 167 (100), 166 (18), 165 (40), 152 (12).
1
2-Benzyl-5-methylpyridine (18a): H NMR (200 MHz, CDCl3): δ =
8.88 (s, 1 H), 7.04–7.29 (m, 7 H), 4.49 (s, 2 H), 2.33 (s, 3 H) ppm.
13C NMR (50 MHz, CDCl3): δ = 157.3, 147.7, 141.5, 137.2, 132.9,
129.4, 128.8, 128.4, 128.3, 42.2, 18.1 ppm. MS (ESI): m/z (%) =
182 (100) [M]+, 167 (15), 40 (42).
1-Benzyl-4-methoxybenzene (9a): 1H NMR (200 MHz, CDCl3): δ =
6.85–7.49 (m, 9 H), 4.06 (s, 2 H, CH2), 3.86 (s, 3 H, CH3) ppm.
Table 5. Data collection and refinement parameters for 4a, 4b and 4c.
4a
4b
4c
Empirical formula
Formula mass [g/mol]
Temperature [K]
Space group
Unit-cell dimensions:
a [Å]
C12H9F6N3NiO2
399.93
150(2)
C34H37Cl3F6N8NiO2
C28H34F6N8NiO2
687.34
150(2)
868.78
150(2)
P21/c
C2/c
P3221
20.259(3)
8.6098(10)
17.6376(11)
12.1712(8)
18.3746(4)
18.3746(4)
b [Å]
c [Å]
α [°]
16.506(2)
90
19.2313(11)
90
15.8986(4)
90
β [°]
γ [°]
97.916(13)
90
2851.6(6)
8
1.863
1600
110.364(7)
90
3870.4(4)
4
1.491
1784
90
120
4648.63(18)
6
1.473
Volume [Å3]
Z
Dcalcd. [g/cm3]
F(000)
2136
Crystal size [mm]
θ range [°]
Index rages
0.34ϫ0.17ϫ0.04
3.42–25.00
–24 Յ h Յ 23
–9 Յ k Յ 10
–19 Յ l Յ 18
5643
2509 [R(int) = 0.0808]
99.8%
0.9445, 0.6395
218
0.41ϫ0.18ϫ0.17
3.37–25.00
–20 Յ h Յ 20
–14 Յ k Յ 13
–22 Յ l Յ 22
16500
6805 [R(int) = 0.0601]
99.8%
0.8788, 0.7403
530
0.68ϫ0.26ϫ0.20
3.39–25.00
–20 Յ h Յ 21
–21 Յ k Յ 19
–18 Յ l Յ 16
32336
5436 [R(int) = 0.0391]
99.7%
0.8722, 0.6465
468
Reflections collected
Independent reflections
Completeness to θ = 25.00°
Relative transmission factors
Parameters
GOF
0.708
0.920
0.987
Final R indices [IϾ2σ(I)][a,b]
R1 = 0.0468, wR2 = 0.0492
R1 = 0.1201, wR2 = 0.0597
R1 = 0.0520, wR2 = 0.0915
R1 = 0.0999, wR2 = 0.0992
R1 = 0.0280, wR2 = 0.0529
R1 = 0.0343, wR2 = 0.0540
R indices (all data)[a,b]
[a] R1 = Σ||Fo| – |Fc||/ΣFo. [b] wR2 = {Σ[(Fo – Fc ) ]/Σ[w(Fo2)2]}1/2
.
2
2 2
Eur. J. Inorg. Chem. 2012, 1269–1277
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1275

C. Someya, S. Inoue, S. Krackl, E. Irran, S. Enthaler
FULL PAPER
1-Benzyl-4-fluorobenzene (19a):[22] 1H NMR (400 MHz, CDCl3): δ
= 7.12–7.30 (m, 9 H), 4.69 (s, 2 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 161.4, 140.9, 136.7, 130.3, 128.8, 128.5, 126.2,
115.2 ppm. 19F NMR (377 MHz, CDCl3): δ = –117.3 ppm. MS
(ESI): m/z (%) = 186 (100) [M]+, 171 (9), 165 (28), 109 (18), 91
(14).
[6]
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Single-Crystal X-ray Structure Determination:[23] Crystals were
mounted on a glass capillary in perfluorinated oil and measured in
a cold N2 flow. The data were collected by using an Oxford Diffrac-
tion Xcalibur S Sapphire at 150(2) K (Mo-Kα radiation, λ =
0.71073 Å). The structures were solved by direct methods and re-
fined on F2 with the SHELX-97 software package. The positions
of the hydrogen atoms were calculated and considered isotropically
according to a riding model (Table 5). CCDC-843194 (for 4a),
-843195 (for 4b) and -843196 (for 4c) 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.
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Acknowledgments
Financial support from the Cluster of Excellence “Unifying Con-
cepts 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. The au-
thors thank Dr. Eckhard Bill (Max Planck Institute for Bioinor-
ganic Chemistry, Mülheim an der Ruhr, Germany) for SQUID
measurements and discussions and Dr. Kallol Ray and Subrata
Kundu (Humbolt University, Berlin, Germany) for the EPR mea-
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