Exploring the coordination chemistry of 1-benzoyl-4,5-dihydro-3,5- bis(trifluoromethyl)-1H-pyrazol-5-ol to copper

Article abstract of DOI:10.1016/j.inoche.2013.10.016

The coordination chemistry of the ligand 1-benzoyl-4,5-dihydro-3,5- bis(trifluoromethyl)-1H-pyrazol-5-ol (1a) has been recently investigated. In dependency of the metal (e.g., nickel, zinc, molybdenum) and the added co-ligand (phosphanes, pyridines, amine

Full text of DOI:10.1016/j.inoche.2013.10.016

Inorganic Chemistry Communications 38 (2013) 131–134
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Exploring the coordination chemistry of
1-benzoyl-4,5-dihydro-3,5-bis(trifluoromethyl)-1H-pyrazol-5-ol to copper
Chika I. Someya a, Shigeyoshi Inoue b, Stephan Enthaler a,
⁎
a
Technische Universität Berlin, Department of Chemistry, Cluster of Excellence “Unifying Concepts in Catalysis”, Straße des 17. Juni 115/C2, 10623 Berlin, Germany
Institute of Chemistry, Metalorganics and Inorganic Materials, Technische Universität Berlin, Straße des 17. Juni 135/C2, D-10623 Berlin, Germany
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The coordination chemistry of the ligand 1-benzoyl-4,5-dihydro-3,5-bis(trifluoromethyl)-1H-pyrazol-5-ol (1a) has
been recently investigated. In dependency of the metal (e.g., nickel, zinc, molybdenum) and the added co-ligand
(phosphanes, pyridines, amines) different coordination modes were feasible (e.g., O,N,O′, O,N, O,O′). Herein we
present the reaction of 1a with Cu(OAc)2 and triphenylphosphane as co-ligand to form the copper complex 2
[Cu(1a-H)(PPh3)2]. The complex was characterized and investigated by various techniques, pointing out a new
bidentate coordination mode of the ligand. In more detail, X-ray crystallography determined a N,O-coordination
in which the ligand is planar and the other coordination sites on the copper centre are occupied by two PPh3 creating
a tetrahedral coordination geometry. Moreover, the complex has been applied as precatalyst in the copper-catalyzed
amination of aliphatic C–H bonds.
Received 26 August 2013
Accepted 11 October 2013
Available online 24 October 2013
Keywords:
Copper
N,O-ligand
Phosphane ligands
DFT calculations
Catalysis
© 2013 Elsevier B.V. All rights reserved.
Catalysis is one key technology for the development of sustainable,
efficient, and selective synthesis. Furthermore, high atom efficiency,
reduced amounts of waste as well as energy, and in consequence
advantageous economics can be realized [1]. In this regard, metal
catalysis turned out to be an excellent methodology illustrated by
countless applications in organic chemistry [2]. In more detail, the
properties of the catalyst is strongly influenced and finetuned by ligands
[3]. Hence, the choice of ligands coordinated to the transition metal
should be considered from chemical and economical points of view,
taking into account inexpensiveness, great availability, easy synthesis,
high tunability, high flexibility and stability. Based on that, the design
of new ligands and the study of their coordination chemistry is an
important research aim [4]. With respect to these requirements an
interesting motif can be the ligand class 1, which is easily accessible
starting from commercially available chemicals (Fig. 1). Recently, the
coordination chemistry of ligand class 1 was investigated. Interestingly,
different coordination modes were observed, depending on the metal
and the added co-ligands (e.g., O,N,O′, O,N, O,O′). For instance, the reaction
of ligand class 1 with Ni(OAc)2•4H2O and 4-dimethylaminopyridine
resulted after double deprotonation in the formation of an octa-
hedral complex with a O,N,O′-coordination (Fig. 1, A), while in
the presence of phosphane co-ligands a square planar geometry
was observed [5]. Interestingly, various applications as precatalysts have
been reported, e.g., C–C cross-coupling reactions, hydrodehalogenations,
hydrodecyanations [6]. On the other hand reacting 1 with ZnMe2 revealed
a O,O′-coordination after deprotonation (Fig. 1, B) [7]. Interestingly,
addition of a base (TMEDA) to the complex allowed the second
deprotonation and created a seven-membered ring system (Fig. 1, C).
Furthermore, the reaction of Mo2(OtBu)6 with ligand class 1 showed O,
O′-coordination and was a useful precatalyst for the reduction of organic
amides to amines (Fig. 1, D) [8]. Based on our ongoing interest in the
coordination chemistry of 5-hydroxypyrazoline ligands we report herein
on the synthesis and characterization of a 5-hydroxypyrazoline copper
complex revealing a new coordination mode and the application as
precatalyst in the copper-catalyzed amination of C-H bonds.
The ligand 1a was synthesized in accordance with methods reported
in the literature [9]. In more detail, benzohydrazide was refluxed with
hexafluoroacetyl acetone in ethanol obtaining ligand 1a after work-up
as crystalline compound. Next, to investigate the coordination abilities
of 1a with copper a mixture of 1a, Cu(OAc)2 and an excess of
triphenylphosphane (3.0 equiv.) was stirred at room temperature for
16 h (Scheme 1). Afterwards all volatiles were removed in vacuum
and the residue was extracted with methanol. The filtrate was
concentrated to result in the formation of a yellow powder in 43%
yield. Crystals suitable for X-ray diffraction analysis were attained
from a toluene solution by slow evaporation of the solvent. The solid-
state structure of complex 2 has been characterized by single-crystal
X-ray diffraction analysis [10]. The thermal ellipsoid plots, selected
bond lengths and angles are shown in Fig. 2. In contrast to other row
4 metals (e.g., Ni, Fe or Zn) a N,O′-coordination was observed. In more
detail, a planar five-membered ring is created by the coordination of
the benzoyl oxygen O1 and the N1 to the copper. In addition, two
triphenylphosphanes are coordinated to the copper center creating a
tetrahedral geometry. Interestingly, the oxygen O2 is not coordinated
to the metal. However, with the obtained data it was not possible to
accurately describe the bond situation in complex 2, since different
isomers (2–1, 2–2, 2–3, 3–4) and a disorder of the non-coordinated
⁎
Corresponding author. Tel.: +49 30314 22039; fax: +49 30314 29732.
E-mail address: stephan.enthaler@tu-berlin.de (S. Enthaler).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.10.016

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C.I. Someya et al. / Inorganic Chemistry Communications 38 (2013) 131–134
Fig. 1. Coordination modes of ligand class 1.
Scheme 1. Synthesis of the copper complex 2 and its possible isomers.
C(O)CF3 are feasible (Scheme 1). Moreover, to clarify the oxidation state
of the copper Electron Paramagnetic Resonance (EPR) measurements
have been carried out. However, no signals for unpaired electrons
were detected excluding the presence of a Cu(II)-d9, and a Cu(I)-d10
centre with no unpaired electrons is assumed. Based on that, during
the complex formation a reduction process of Cu(II) to Cu(I) takes
place, probably with portions of the ligand 1a or the solvent methanol
as sacrificial reagents [11]. The analysis of the filtrate revealed the
formation of triphenylphosphane oxide and numerous unknown
fluorine containing compounds, which originated from the ligand 1a.
The properties of the diamagnetic complex 2 were investigated by
NMR techniques in solution. A signal was found at δ = 5.79 ppm
assigned to the C3–H proton. This was further proven by the absence
of any signal of the former CH2 group in the ligand. In addition, a
broad signal at δ = 17.17 ppm could be attributed to N–H or O–H
protons. Interestingly, the addition of methanol-d4 showed a fast
exchange of the N–H or O–H proton. Noteworthy, a ratio of 1:1 of the
C3–H proton and the N–H or O–H proton was found. Based on that,
the isomer 2–3 with a CH2 group can be excluded (Scheme 1). Moreover,
the tetrahedral structure was confirmed by the occurrence of a single
signal at δ =−0.66ppm in the 31P{1H} NMR for the triphenylphosphane
ligands, which furthermore proves the coordination to the copper (free
PPh3 δ = −4.75 ppm). In order to distinguish between the isomers 2–
1, 2–2 and 2–4 DFT calculations have been performed [12]. The optimized
structures and energies are shown in Fig. 3. As the most stable isomer 2–1
was observed, while for 2–2 and 2–4 an energy difference of 85–95kJ/mol
Fig. 2. Molecular structure of complex 2. Thermal ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are omitted for clarify. Selected bond lengths
(Å) and angle (°) Cu-O1: 2.0937(19); Cu-N1: 2.121(2); Cu-P1: 2.2566(8); Cu-P2:
2.2482(8); O1-C1: 1.239(3); C1-N2: 1.354(3); N1-N2: 1.393(3); N1-C2: 1.311(3); C2-C3:
1.435 (5); O1-Cu1-N1: 77.29(8); P1-Cu1-P2: 124.30(3).

C.I. Someya et al. / Inorganic Chemistry Communications 38 (2013) 131–134
133
Fig. 4. Molecular orbitals of complex 2. B3LYP level 6-31G(d): basis set for N, C, O, F, P and
H atoms and LANL2DZ for the Cu atom.
the metal (−4.99 eV). On the other hand, the lowest unoccupied
molecular orbital (LUMO) is mainly delocalized over the ligand as well
as the aromatic substituent of the ligand (−1.45 eV). The energy gap of
the HOMO and LUMO was 3.54 eV, which is comparable to the work of
Dharmaraj et al. applying a similar ligand motif [11a]. In recent years
the application of copper complexes as precatalysts in the amination of
C-H bonds to form C-N bonds have been reported [13]. Hence, the
catalytic abilities of complex 2 were investigated in the copper-
catalyzed amination of aliphatic C-H bonds. In more detail, an excess
of cyclohexane (3) was reacted with benzamide (4) in the presence of
10 mol% 2 applying di-tert-butyl peroxide as the oxidant (Scheme 2).
After 24 hours at 100 °C a yield of 87% of product 5 and a turnover
number of 8.7 were realized, demonstrating the potential of complex
2 in the amination of aliphatic C-H bonds. In summary we have
investigated the coordination chemistry of the ligand 1-benzoyl-4,5-
dihydro-3,5-bis(trifluoromethyl)-1H-pyrazol-5-ol (1a) with Cu(OAc)2
and triphenylphosphane as co-ligand to form the copper complex 2
[Cu(1a-H)(PPh3)2]. The complex was characterized and investigated
by various techniques, showing a new bidentate coordination mode
and the copper center in the oxidation state +1. In more detail, X-ray
crystallography revealed a N,O′-coordination in which the ligand is
Fig. 3. Optimized structures and energies of the isomers 2–1 (a), 2–2 (b) and 2–4 (c):
B3LYP level 6-31G(d) basis set for N, C, O, F, P and H atoms and LANL2DZ for the Cu atom.
was calculated. To understand the electronic nature of complex 2 a
molecular orbital analysis was performed. Fig. 4 represents the frontier
molecular orbitals. The highest occupied molecular orbital (HOMO) is
mainly delocalized over the ligand and also slightly on the d-orbital of

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C.I. Someya et al. / Inorganic Chemistry Communications 38 (2013) 131–134
Scheme 2. Catalytic abilities of complex 2 in the amidation of aliphatic C-H bonds.
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planar and the other coordination sites on the copper centre are
occupied by two PPh3 creating a tetrahedral coordination geometry.
Moreover, the complex has been successfully applied as precatalyst in
the copper-catalyzed amination of aliphatic C-H bonds. In ongoing
studies the scope and limitations and the reaction mechanism of the
amination reaction will be investigated in more detail.
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Acknowledgement
b) L. Sacconi, P. Paoletti, F. Maggio, J. Am. Chem. Soc. 79 (1957) 4067–4069;
c) K.N. Zelenin, A.R. Tugusheva, S.I. Yakimovich, V.V. Alekssev, E.V. Zerova, Chem.
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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, Dr. Eckhard Bill (Max Planck
Institute for Chemical Energy Conversion) is thanked for EPR
measurements and Dr. Ba Tran (University of California, Berkeley) is
thanked for supporting this work.
[10] CCDC-939571 (for 2) contains 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.
Space group: P-1; unit cell dimensions: a = 11.6984(6) Å, b = 12.9220(7) Å,
c = 14.7020(6) Å, α = 82.872(4)°, β = 77.676(4)°, γ = 77.661(4)°; Volume:
2113.90(18) Å3; GOF: 0.915; R1 = 0.0621; wR2 = 0.133
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Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2013.10.016.
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