Exploring the coordination chemistry of O,N,O′-ligands modified by 2-thienyl-substituents to nickel

Article abstract of DOI:10.1016/j.inoche.2014.03.014

The coordination chemistry of 5-hydroxypyrazoline ligand class 1 modified by 2-thienyl substituents with Ni(OAc)₂ and DMAP as co-ligand to form the nickel complexes 4 [Ni(1-2H)(dmap)_x] was investigated. The complexes were characteriz

Full text of DOI:10.1016/j.inoche.2014.03.014

Inorganic Chemistry Communications 44 (2014) 114–118
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Exploring the coordination chemistry of O,N,O′-ligands modified by
2-thienyl-substituents to nickel
Chika I. Someya a, Shigeyoshi Inoue b, Elisabeth Irran b, Stephan Enthaler a,
⁎
a
Technische Universität Berlin, Institute of Chemistry: Cluster of Excellence “Unifying Concepts in Catalysis”, Straße des 17. Juni 115/C2, D-10623 Berlin, Germany
Technische Universität Berlin, Institute of Chemistry: Metalorganics and Inorganic Materials, 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 5-hydroxypyrazoline ligand class 1 modified by 2-thienyl substituents with Ni(OAc)2
and DMAP as co-ligand to form the nickel complexes 4 [Ni(1-2H)(dmap)x] was investigated. The complexes
were characterized and investigated by various techniques, pointing out different complex geometries,
octahedral vs. square planar, depending on the connectivity of the 2-thienyl substituent. In more detail, X-ray
crystallography revealed an O,N,O′-coordination in which the ligand is planar and the other coordination sites
on the nickel center are occupied by one or three DMAP co-ligands, respectively. Moreover, the complexes
have been applied as precatalyst in nickel-catalyzed hydrodecyanation reactions.
Received 19 January 2014
Accepted 15 March 2014
Available online 22 March 2014
Keywords:
Nickel complexes
Tridentate ligands
Nitrogen ligands
Catalysis
© 2014 Elsevier B.V. All rights reserved.
Hydrodecyanation
Catalysis is one of the key methods for the development of sustainable,
efficient, and selective processes [1]. In this regard, metal catalysis turned
out to be an excellent toolbox for such purposes [2]. In more detail, the
properties or performance of the catalyst is strongly influenced by ligands
[3]. Based on that, the design of new ligands and the study of their coor-
dination chemistry are important research aims [4]. Moreover, the
inexpensiveness, great availability, easy synthesis, high tunability, high
flexibility and stability of the ligands should be considered. 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 studied
and different coordination modes were observed, depending on the metal
and the added co-ligands (e.g., O,N,O′; O,N; O,O′; and N,O′). For instance,
reacting 1 with ZnMe2 revealed an O,O′-coordination after deprotonation
[5]. Interestingly, the addition of a base (TMEDA) to the complex
allowed the second deprotonation and created a seven-membered
ring system. 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 [6]. In contrast to that, an
N,O′-coordination was observed, when ligand class 1 was reacted with
Cu(OAc)2 and triphenylphosphane as co-ligand. Interestingly, the
copper complex was a useful precatalyst in the copper-catalyzed
amination of C\H bonds [7]. Moreover, the reaction of ligand class 1
with Ni(OAc)2·4H2O and 4-dimethylaminopyridine (DMAP) resulted
after double deprotonation in the formation of an octahedral com-
plex with an O,N,O′-coordination (Fig. 1, A), while in the presence of am-
monia or phosphane co-ligands a square planar geometry was observed
(Fig. 1, B) [8]. Interestingly, various applications as precatalysts have
been reported, e.g., C-C cross-coupling reactions, hydrodehalogenations,
hydrodecyanations [9]. It is noteworthy that, the structural appearance
of the complexes (octahedral vs. square planar geometry) strongly de-
pends on the added co-ligands and the control of the geometry by the
tridentate ligand was not studied so far (Fig. 1). Based on that, we report
herein on the synthesis and characterization of 5-hydroxypyrazoline li-
gands modified by 2-thienyl substituents, which allow the control of the
geometry at the nickel center (Fig. 1, D–F). Moreover, the potential of
the complexes was studied in the hydrodecyanation reactions.
The ligands 1a–c were synthesized in accordance with methods re-
ported in the literature [10]. In more detail, the corresponding hydrazides
were refluxed with substituted acetyl acetones in methanol obtaining
ligand class 1 after work-up as crystalline compound (Scheme 1). In
agreement to earlier works, for ligand 1a the cyclic 5-hydroxypyrazoline
form was obtained as proven by NMR investigations [1H NMR δ =
3.34–3.63 ppm (m, CH2)] and single-crystal X-ray diffraction analysis
(Fig. 2a) [11]. The analytical and structural data are comparable to the
recently reported ligand 1 (R1 = Ph, R2 = CF3), showing a minor influ-
ence of the 2-thienyl group on the core of the ligand [5a]. In contrast to
that for ligand 1b two different isomers in a ratio of 1:4 were observed in
solution by NMR methods [1H NMR 1b: δ = 3.29–3.61 ppm (m, CH2);
1b′: δ = 4.13 (s, CH2)], while in the solid state the hydrazone form 1b′
⁎
Corresponding author.
E-mail address: stephan.enthaler@tu-berlin.de (S. Enthaler).
http://dx.doi.org/10.1016/j.inoche.2014.03.014
1387-7003/© 2014 Elsevier B.V. All rights reserved.

C.I. Someya et al. / Inorganic Chemistry Communications 44 (2014) 114–118
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Fig. 1. Coordination modes of ligand class 1 to nickel (dmap = 4-dimethylaminopyridine).
was attained (Fig. 2b). DFT calculations on the B3LYP level of theory and
the 6-31G(d) basis set were carried out and revealed a higher stability
for the hydrazone form 1b′ of 20 kJ/mol compared to 1b [12]. Probably
the carbonyl function attached to the thienyl group (1b/1b′) is less
electrophilic than the carbonyl function attached to the electron-
withdrawing trifluoromethyl group (1a), hence the cyclization is less
favored. Moreover, in case of 1c only the hydrazone form 1c′ was
observed by NMR [1H NMR δ = 4.62 ppm (s, CH2)].
obtained. However, a preliminary structure revealed an octahedral
geometry as observed for complex A with three DMAP ligands along
with the tridentate O,N,O′-ligand (see supporting information). Based
on these observations the C1-substitution in the complexes is crucial
for the geometry and the 2-thienyl substituent can influence the geom-
etry by electronic and steric effects. The complexes were also investigated
by 1H NMR spectroscopy, showing broad signals for the paramagnetic
compound 4a, which could be not assigned. In contrast to that, for the dia-
magnetic complexes 4b and 4c a new signal was found at 6.44 ppm for 4b
(in C6D6) and 6.13 ppm for 4c (in CDCl3), which is assigned as the C–H
proton in the six-membered ring. This was further proven by the absence
of any signal for the former CH2 in the ligand 1. In addition new signals at
2.05 ppm for 4b (in C6D6) and 3.03 ppm for 4c (in CDCl3) were observed
for the methyl protons of the DMAP ligand. Moreover, the chemical shifts
in the 19F NMR for the CF3-groups are slightly high-field shifted compared
to ligand class 1. The coordination of the acyl functionality was investi-
gated by IR measurements. The absence of signals in the region
1600–1725 cm−1 excludes the possibility for a free or coordinated C_O
functionality in the coordinated ligand. Also strong signals were moni-
tored in the region 1532–1618 cm−1, which probably can be attributed
to the C_N functionality. The mechanism for the formation of the com-
plexes is assumed to be similar to that described in an earlier work [8].
Recently a methodology was set up for the nickel-catalyzed
hydrodecyanation of organic cyanides with LiBH4 as hydride source
under mild reaction conditions applying nickel complexes with the O,
N,O′-ligand 1 as precatalyst [9]. In this regard the catalytic abilities of
the complexes 4 were studied in the hydrodecyanation of anisonitrile
with LiBH4 as hydride source (Scheme 2). Interestingly for all complexes
excellent yields were realized within 24 h, while within 1 h a higher
yield was observed for complex 4b.
Afterwards, in agreement to our previously established protocol a
methanol solution of the ligand and an excess of DMAP were added to
a solution of Ni(OAc)2·4H2O in methanol at room temperature [8].
After stirring overnight, all volatiles were removed to obtain brown
powders, which were extracted with toluene and purified by crystalli-
zation to obtain brown crystals. Crystals suitable for X-ray measure-
ments were grown from toluene by slow evaporation of the solvent at
room temperature. The solid-state structures of complexes 4b and 4c
have been characterized by single-crystal X-ray diffraction analysis.
Thermal ellipsoid plots, selected bond lengths and angles are shown in
Fig. 3. 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. In contrast to earlier works only one
DMAP ligand is coordinated to the square planar nickel center. The
DMAP ligand is cis-positioned to the oxygen donors, while the nitrogen
of the 5-hydroxypyrazoline ligand is connected to the nickel center in
the trans-position. Comparing the bond distances of the square planar
coordinated complexes 4b and 4c with the distances of octahedral coor-
dinated complex A (R1 = Ph or Me, R2 = CF3) significant shorter bond
lengths were observed. For instance the Ni1\N1, Ni\O1, Ni\O2 and
Ni1\N3 bond distances are ca. 0.2 Å shorter than those for the
complexes 4b and 4c (complex A: Ni1\N1: 2.055(3) Å, Ni1\O1:
2.021(3) Å, Ni1\O2: 2.041(3) Å, and Ni1\N3: 2.118(3) Å). In the
case of complex 4a no suitable crystals for X-ray measurements were
In summary we have investigated the coordination chemistry of
5-hydroxypyrazoline ligand class 1. modified by 2-thienyl substituents

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C.I. Someya et al. / Inorganic Chemistry Communications 44 (2014) 114–118
a) ligand synthesis
b) complex synthesis
Scheme 1. Synthesis of the ligands and the corresponding nickel complexes.
Fig. 2. Molecular structures of 1a (a) and 1b′ (b). Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) 1a: N1\N2: 1.372(4), N1\C3: 1.504(4), N2\C1: 1.272(4),
C2\C3: 1.533(5), C1\C2: 1.496(5), N1\C6: 1.382(4), C6\O1: 1.217(4), and C3\O2: 1.384(4); 1b′ (the bond lengths are only given for one molecule, because of similarity): N1\N2: 1.358(4)/
1.374(4), N2\C1: 1.275(4)/1.277(4), C2\C3: 1.525(5)/1.523(5), C1\C2: 1.504(5)/1.496(5), N1\C6: 1.370(4)/1.363(4), C6\O1: 1.223(4)/1.231(4), and C3\O2: 1.215(4)/1.217(4).

C.I. Someya et al. / Inorganic Chemistry Communications 44 (2014) 114–118
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Fig. 3. Molecular structures of 4b (a) and 4c (b). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°)
4b: Ni\N1: 1.849(4), Ni\O1: 1.831(3), Ni\O2: 1.842(3), Ni\N3: 1.942(4), O1\Ni\O2: 174.42(15), and N1\Ni\N3: 172.22(16); 4c: Ni\N1: 1.850(3), Ni\O1: 1.825(2), Ni\O2:
1.850(3), Ni\N3: 1.938(3), O1\Ni\O2: 172.75(11), and N1\Ni\N3: 170.72(13).
with Ni(OAc)2 and DMAP as co-ligand to form the nickel complexes 4
[Ni(1-2H)(dmap)x]. The complexes were characterized and investigated
by various techniques, showing different geometries of the nickel center,
octahedral vs. square planar, depending on the connectivity of the
2-thienyl substituent. Moreover, the catalytic abilities of the complexes
were tested in the hydrodecyanation of anisonitrile. In ongoing studies
the post-functionalization of the complexes and the catalytic abilities
will be studied (Fig. 1, G–I) [13].
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Scheme 2. Investigation of the influence of the precatalyst structure on the hydrodecyanation of 5.

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