Inorganic Chemistry Communications
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.
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