Nickel complexes with a O,N,O′-ligand and a phosphane co-ligand - Monometallic versus bimetallic complexes

Article abstract of DOI:10.1016/j.ica.2015.05.008

The coordination chemistry of 5-hydroxypyrazoline ligand precursor 1 with Ni(OAc)₂·4H₂O and 1,1-bis(diphenylphosphino)methane (DPPM) as co-ligand to form the monometallic nickel complex 2 [Ni(1-2H)(DPPM)] was studied. The complex was

Full text of DOI:10.1016/j.ica.2015.05.008

Inorganica Chimica Acta 434 (2015) 37–40
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Nickel complexes with a O,N,O0-ligand and a phosphane
co-ligand – Monometallic versus bimetallic complexes
⇑
Chika I. Someya, Maik Weidauer, Stephan Enthaler
Technische Universität Berlin, Institute of Chemistry: Cluster of Excellence ‘‘Unifying Concepts in Catalysis’’, Straße des 17, Juni 115/C2, D-10623 Berlin, Germany
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 precursor 1 with Ni(OAc)2ꢀ4H2O and 1,1-
bis(diphenylphosphino)methane (DPPM) as co-ligand to form the monometallic nickel complex 2
[Ni(1–2H)(DPPM)] was studied. The complex was characterized and investigated by various techniques,
Received 26 March 2015
Received in revised form 4 May 2015
Accepted 8 May 2015
pointing out a square planar geometry at the nickel center with
Moreover, addition of Fe2(CO)9 revealed the formation of the bimetallic complex
2H)(DPPM)Ni(1–2H)] with DPPM as bridging ligand between two nickel centers and the abstraction of
one DPPM ligand by the iron to form Fe(CO)4(
1-dppm).
g
1-coordination of the DPPM ligand.
[Ni(1–
Available online 21 May 2015
3
Keywords:
g
Nickel complex
Tridentate ligand
Bimetallic complex
Iron complex
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
bidentate co-ligands can be a straightforward target (Fig. 1). For
instance such bimetallic complexes can be useful precatalysts for
catalytic applications (e.g., cooperative catalysis, consecutive catal-
ysis) [18,19]. Based on that, we report herein on the synthesis and
characterization of monometallic and bimetallic nickel(II) com-
plexes with a O,N,O0-ligand and a (bridging) DPPM co-ligand.
Recently, the coordination chemistry of ligand precursor 1 was
examined and different coordination modes (e.g., O,N,O0; O,N; O,O0;
and N,O0) were realized, depending on the metal and the added co-
ligands [1–4]. Moreover, the obtained complexes demonstrated
interesting catalytic potential in various organic transformations.
For instance, the flexibility of the ligand precursor 1 was shown
in the reaction with ZnMe2. On the one hand, an O,O0-coordination
after deprotonation was observed, while the addition of base
revealed a second deprotonation and a O,N-coordination [5,6]. A
similar O,O0-motif was observed for coordination to molybdenum,
resulting in attractive precatalysts for the reduction of organic
amides to amines [7]. In case of copper a N,O0-coordination was
detected with triphenylphosphane as co-ligand and application
in copper-catalyzed amination of CH bond was realized [8]. In con-
trast to that, with iron(II) or iron(III) as complex metal center a
O,N,O0-coordination was observed with 4-dimethylaminopyridine
(DMAP) or triphenylphosphane oxide as co-ligands [9]. Moreover,
the reaction of ligand precursor 1 with Ni(OAc)2ꢀ4H2O and DMAP
resulted after double deprotonation in the formation of an octahe-
dral complex with a O,N,O0-coordination, while in the presence of
ammonia or phosphane co-ligands a square planar geometry was
observed [10–17]. Importantly, at this stage only monometallic
nickel complexes were reported. Based on the flexibility of the
co-ligands the formation of bimetallic complexes with bridging
2. Results and discussion
The 1-benzoyl-5-hydroxypyrazoline
1 was synthesized in
accordance to the procedure reported in the literature [20].
Benzohydrazide was reacted with equimolar amounts of
1,1,1,5,5,5-hexafluoroacetylacetone and heated to reflux for 5 h in
ethanol. After work-up the desired ligand precursor 1 was obtained
as colourless crystalline compounds. With 1 in hand we examined
the coordination to nickel (Scheme 1). In agreement to our previ-
ously established protocol a methanol solution of ligand precursor
1 and an excess of 1,1-bis(diphenylphosphino)methane (DPPM,
3.0 equiv.) was added to a solution of Ni(OAc)2ꢀ4H2O in methanol
at room temperature [10–17]. After stirring overnight, all volatiles
were removed to obtain a brown powder, which was extracted
with toluene and purified by crystallization to obtain red–brown
crystals.
Crystals suitable for X-ray measurements were grown from
toluene by slow evaporation of the solvent at room temperature.
The solid–state structure of complex 2 has been characterized by
single–crystal X-ray diffraction analysis. Thermal ellipsoid plots
and selected bond lengths and angles are shown in Fig. 2.
Interestingly, only one phosphorous unit of the bidentate DPPM
⇑
Corresponding author.
E-mail address: stephan.enthaler@tu-berlin.de (S. Enthaler).
http://dx.doi.org/10.1016/j.ica.2015.05.008
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

38
C.I. Someya et al. / Inorganica Chimica Acta 434 (2015) 37–40
M
PR2
Ph
Ph
N
O
Ni
O
Ni
PR3
CF3
PR2
CF3
N
N
O
N
O
F3C
F3C
Precatalysts in
C-C cross coupling reactions
Hydrodehalogenations
Hydrodecyanations
Bimetallic complexes
Cooperativity in catalysis?
Consecutive catalysis?
Fig. 1. Monometallic vs. bimetallic nickel complexes.
ligand is coordinated to the nickel center, while the other one is
uncoordinated. The tridentate ligand is coordinated in a O,N,O0-
mode creating a five-membered as well as a six-membered ring
system and therefore shielding one side of the metal. The DPPM
ligand is cis-positioned to the oxygen donors, while N1 of the
ligand is connected to the nickel centre in the trans-position.
Overall a square planar nickel center was observed, which is in
accordance to (1–2H)-nickel complexes with monodentate phos-
phane co-ligands. In more detail, the Ni–P1 bond distance of
2.2097(7) Å is to some extend shorter in comparison to nickel(II)
complexes (2.2252–2.2422 Å) having the same coordinating atoms
[12,13,16,17]. For instance, in case of triphenylphosphane as co-li-
gand a Ni–P1 bond distance of 2.2252(10) Å was observed [17]. On
the other hand, the Ni–O and Ni–N1 bond lengths are in the range
of comparable nickel(II) complexes [12,13,16,17]. Moreover, the
N1–Ni–P1 bond angle of 171.70(6)° deviates from the ideal angle
of 180° (comparable complexes: 172.23–177.32°) [12,13,16,17].
The complex 2 was characterized by 1H NMR spectroscopy
applying C6D6 as solvent, showing the C–H proton in the six-mem-
bered ring at 6.16 ppm and the CH2 bridge between the two phos-
phorous with a chemical shift of d = 2.95 ppm. In the 19F NMR
spectrum two sharp signals for the two CF3 groups at ꢁ65.0 ppm
and ꢁ71.2 ppm were observed. The coordination of the DPPM
ligand was confirmed by 31P{1H} NMR; here a signal at 5.85 ppm
for the phosphorous coordinated to the nickel was detected, while
the uncoordinated phosporous has a chemical shift of ꢁ26.1 ppm.
Afterwards attempts were undertaken to coordinate the free phos-
phane unit in complex 2 to other metals (e.g., iron, platinum, cop-
per) to allow access to heterobimetallic complexes. In case of the
reaction of complex 2 with one equivalent of Fe2(CO)9 in refluxing
diethyl ether a new complex was obtained after work-up. Crystals
suitable for X-ray measurements were grown from ethanol by slow
evaporation of the solvent at room temperature. The solid–state
structure of the complex 3 has been characterized by single–crystal
X-ray diffraction analysis. Thermal ellipsoid plots and selected
bond lengths and angles are shown in Fig. 3.
Fig. 2. Molecular structure of 2. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles
(°): Ni1–N1: 1.8672(19), Ni1–O1: 1.8298(16), Ni1–O2: 1.8317(15), Ni1–P1:
2.2097(7), O1–Ni1–O2: 179.61(8), N1–Ni1–P1: 171.70(6).
ring systems and the DPPM ligand is cis-positioned to the oxygen
donors, while the nitrogen of the tridentate ligand is connected
to the nickel centers in the trans-position. The Ni–P bonds (Ni–
P1: 2.2024(9), Ni–P2: 2.2018(8)) are in the same range as observed
for complex 2. In comparison to complex 2 smaller N–Ni–P angles
were found (N1–Ni1–P1: 169.18(8), N3–Ni2–P2: 169.73(10)).
Moreover, the bimetallic complex was characterized by NMR
analysis. The crystals of complex 3 were investigated by 1H NMR
spectroscopy applying C6D6 as solvent, showing the C–H proton
in the six-membered ring at 5.99 ppm. In the 19F NMR spectrum
two sharp signals for the two CF3 groups at ꢁ65.1 ppm and
ꢁ70.9 ppm were observed. The coordination of the DPPM ligand
to both nickel center was confirmed by 31P{1H} NMR; here a signal
at 0.4 ppm was detected. On the other hand, the investigation of
the reaction mixture by 31P{1H} NMR revealed the formation of
other phosphorous containing species. For instance the signals
with a chemical shift of ꢁ26.0 ppm and 66.6 ppm with a coupling
2
constant of JP–P = 84.62 Hz can be assigned to the complex
Fe(CO)4(g
1-dppm) 4 [21,22], which was furthermore proven by
single–crystal X-ray diffraction analysis and is in accordance with
the structure reported by Dias Rodrigues, Lechat and Francisco
[23]. Based on this results the Fe2(CO)9 acts as a DPPM abstraction
reagent from complex 2. The DPPM-free nickel intermediate prob-
ably reacts with an additional complex 2 to form the homobimetal-
Surprisingly, the structure revealed the formation of a homo-
bimetallic complex instead of the desired heterobimetallic com-
plex in which the DPPM ligand acts as bridging ligand between
two (1–2H)-nickel center, which pointing in opposite directions.
In accordance to complex 2 the tridentate ligands are coordinated
in a O,N,O0-mode creating five-membered as well as six-membered
lic complex 3. Moreover,
a singlet signal at 67.0 ppm was
monitored, which can be assigned to the complex
Fe2(CO)5(dppm)2. Interestingly, reacting complex 2 with other
metal sources for instance CpFe(CO)2I or PtCl2(cod) revealed the
formation of complex 3 and the additional complex acts as abstrac-
tion reagent for DPPM.
Ph
Ph
Ph
N
Ph2
P
Ph
N
N
CF3
Ni(OAc)2 4H2O
3.0 equiv. DPPM
O
Ni
PPh2
N
O
F3C
O
O
OH
CF3
N
N
Ni
Fe2(CO)9
diethyl ether, r.t.
-Fe(CO)4(η1-dppm)
Ni
O
N
N
O
P
O
P
MeOH, r.t.
F3C
Ph2
Ph2
F3C
CF3
CF3
F3C
1
2 (yield: 12%)
Scheme 1. Synthesis of nickel complexes 2 and 3.
3 (yield: 56%)

C.I. Someya et al. / Inorganica Chimica Acta 434 (2015) 37–40
39
1275 (s), 1176 (s), 1152 (s), 1078 (m), 1028 (w), 1009 (m), 1009
(m), 905 (w), 792 (w), 757 (w), 715 (w), 672 (w), 631 (w) cmꢁ1
HRMS calc. for C12H8F6N2O2+H: 327.05627, found: 327.05569.
.
4.3. Synthesis of 2
To
a mixture of 1 (3.06 mmol) and bis(diphenylphos-
phino)methane (3 equiv., 9.17 mmol) in methanol (10 mL) was
added a solution of Ni(OAc)2ꢀ4H2O (3.06 mmol) at room tempera-
ture. The solution was stirred overnight. After stirring overnight,
the volatiles were removed in vacuum to obtain a brown powder,
which was extracted from toluene and purified by crystallization.
Crystals suitable for X-ray diffraction analysis were obtained from
toluene solution at room temperature by slow evaporation.
Yield = 290 mg (12%, first crop, red crystals). Mp: 151 °C. 1H NMR
(200 MHz, C6D6, 25 °C) d = 8.03–8.08 (m, 2H), 7.55 (br, 7H), 6.98–
7.08 (m, 16H), 6.16 (s, 1H, CH), 2.95 (s, 2H, CH2) ppm. 13C{1H}
2
NMR (100 MHz, C6D6, 25 °C) d = 173.0, 156.4 (q, JC–F = 33.5 Hz),
2
141.6 (q, JC–F = 30.3 Hz), 133.7, 131.4, 130.7, 129.5, 129.2, 128.8,
Fig. 3. Molecular structure of 3. Thermal ellipsoids are drawn at the 50% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles
(°): Ni1–O1: 1.823(2), Ni1–O2: 1.8356(19), Ni1–N1: 1.854(3), Ni1–P1: 2.2024(9),
Ni2–O3: 1.821(2), Ni2–O4: 1.837(2), Ni2–N3: 1.868(3), Ni2–P2: 2.2018(8), O1–Ni1–
O2: 178.24(10), O3–Ni2–O4: 179.46(11), N1–Ni1–P1: 169.18(8), N3–Ni2–P2:
169.73(10).
1
1
119.5 (q, JC–F = 282.2 Hz), 118.9 (q, JC–F = 282.2 Hz), 91.3,
30.2 ppm (some peaks were overlapped with solvent’s peaks). 19F
NMR (188 MHz, C6D6, 25 °C) d = ꢁ65.0, ꢁ71.2 ppm. 31P{1H} NMR
(81 MHz, C6D6, 25 °C) d = 5.85, ꢁ26.1 ppm. IR (KBr):
m = 3433 (br),
1608 (m), 1592 (m), 1531 (m), 1516 (m), 1483 (w), 1455 (w),
1434 (m), 1366 (m), 1347 (m), 1267 (s), 1203 (s), 1175 (s), 1166
(s), 1156 (s), 1135 (s), 1108 (m), 1067 (m), 1025 (w), 999 (w),
890 (w), 848 (w), 798 (w), 775 (w), 742 (m), 736 (m), 716 (m),
3. Conclusion
695 (s), 585 (w), 581 (w), 484 (w) cmꢁ1
.
HRMS calc. for
In summary we have investigated the coordination chemistry of
C37H29F6N2NiO2P2+H: 767.09619, found: 767.09356. elemental
5-hydroxypyrazoline ligand
1
with Ni(OAc)2ꢀ4H2O and 1,1-
analysis: C37H29F6N2NiO2P2: calc. C 57.92, H 3.68, N 3.65; found
C 57.65, H 3.91, N 3.44.
bis(diphenylphosphino)methane (DPPM) as co-ligand to form the
monometallic nickel complex 2 [Ni(1–2H)(DPPM)]. The complex
was characterized and investigated by various techniques, pointing
4.4. Synthesis of 3
out a square planar geometry at the nickel center with
g
1-coordi-
nation of the DPPM ligand. Moreover, addition of Fe2(CO)9 revealed
the formation of the bimetallic complex 3 [Ni(1–2H)(DPPM)Ni(1–
2H)] with DPPM as bridge between two nickel centers. In ongoing
studies the application of complex 2 and 3 as precatalysts will be
investigated [24].
A mixture of 2 (0.65 mmol) and Fe2(CO)9 (0.65 mmol) in diethy-
lether (50 mL) was stirred for 24 h at room temperature. The vola-
tiles were removed in vacuum to obtain a brown powder, which
was dissolved in a small amount of acetone (10 mL) and purified
by filtration over a plug of silica. After elution of the plug of silica
with acetone (200 mL) and removal of the solvent in vacuum a
red–brown residue was obtained (Note: This procedure removed
4. Experimental section
undesired side products and yields
a mixture of 3, 4 and
4.1. General
Fe2(CO)5(dppm)2). The residue was dissolved in ethanol (50 mL)
and purified by filtration over a plug of silica. After elution of the
plug of silica with ethanol (200 mL) and removal of the solvent
in vacuum a brown residue was obtained, which contains 4
(Note: suitable crystals for X-ray diffraction analysis were obtained
from slow evaporation of the solvent) and Fe2(CO)5(dppm)2. After
elution of the plug of silica with acetone and removal of the solvent
in vacuum a red–brown residue was obtained. The red–brown resi-
due was purified by crystallization with ethanol. Crystals suitable
for X-ray diffraction analysis were obtained from an ethanol solu-
tion at room temperature by slow evaporation. Yield = 210 mg
(56%, red crystals). Mp: 254 °C. 1H NMR (200 MHz, C6D6, 25 °C)
d = 8.02–8.06 (m, 4H), 7.78–7.92 (m, 8H), 6.89–7.05 (m, 18H),
5.99 (s, 2H, CH), 3.10–3.30 (m, 2H, CH2) ppm. 13C{1H} NMR was
not measured, due to poor solubility of complex 3. 19F NMR
(188 MHz, C6D6, 25 °C) d = ꢁ65.1, ꢁ71.2 ppm. 31P{1H} NMR
1H, 19F, 13C and 31P NMR spectra were recorded on a Bruker
Avance 200 spectrometer (1H: 200.13 MHz; 13C: 50.32 MHz; 19F:
188.31 MHz, 31P: 81.01 MHz) and 400 spectrometer
(
13C:
100.61 MHz) using the signals of the deuterated solvents
(1H NMR, 13C NMR), CFCl3 19F NMR) or 85% H3PO4 31P NMR) as
(
(
reference. IR spectra were recorded on a Perkin Elmer Spectrum
100 FT-IR.
4.2. 3,5-di(trifluoromethyl)-1-(benzoyl)-5-hydroxy-pyrazoline (1)
To
a
solution
of
1,1,1,5,5,5-hexafluoropeta-2,4-dione
(55.8 mmol) in ethanol (60 mL) was added a solution of benzohy-
drazide (55.8 mmol) in ethanol (60 mL). After refluxing the mix-
ture for 5 h the solvent was removed in vacuum. 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): d = 7.84–7.92 (m, 2H; Ar), 7.40–7.65 (m, 3H, Ar),
6.40 (s, br, 1H, OH), 3.00–3.66 (m 2H, CH2) ppm. 13C{1H} NMR
(50 MHz, CDCl3, 25 °C) d = 171.6, 144.1, 143.7, 133.3, 131.6,
130.5, 128.3, 94.2, 93.8, 41.4 (CH2) ppm. 19F NMR (50 MHz,
(81 MHz, C6D6, 25 °C) d = 0.37 ppm. IR (KBr):
m = 1945 (w), 1610
(w), 1596 (w), 1535 (m), 1518 (m), 1455 (w), 1437 (m), 1362
(m), 1347 (m), 1267 (s), 1197 (s), 1185 (s), 1154 (s), 1129 (m),
1100 (w), 1065 (w), 801 (w), 784 (w), 744 (w), 735 (w), 716 (w),
691 (m) cmꢁ1
1149.06434, found:
49H34F12N4Ni2O4P2: calc. C 51.17, H 2.98, N 4.87; found C 51.43,
H 3.11, N 4.64.
.
HRMS calc. for C49H34F12N4Ni2O4P2+H:
1149.06678. elemental analysis:
CDCl3, 25 °C) d = ꢁ67.4, ꢁ80.5 ppm. IR (KBr):
m
= 3390 (m), 3324
C
(m), 1680 (s), 1637 (m), 1451 (m), 1434 (m), 1333 (m), 1305 (m),

40
C.I. Someya et al. / Inorganica Chimica Acta 434 (2015) 37–40
[3] P. Sathyadevi, P. Krishnamoorthy, R.R. Butorac, A.H. Cowley, N.S.P. Bhuvanesh,
N. Dharmaraj, Dalton Trans. 40 (2011) 9690.
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Crystals were each mounted on a glass capillary in perfluori-
nated oil and measured in a cold N2 flow. The data were collected
using an Oxford Diffraction Xcalibur S Sapphire at 150(2) K (MoK
a
radiation, k = 0.71073 Å) or an Agilent Technologies SuperNova
(single source) at 150 K CuK radiation, k = 1.5418 Å). The struc-
a
tures were solved by direct methods and refined on F2 with the
SHELX-97 software package. The positions of the hydrogen atoms
were calculated and considered isotropically according to a riding
model. The Figures were created with ORTEP and POV-Ray.
Acknowledgement
[15] C.I. Someya, M. Weidauer, S. Enthaler, Catal. Lett. 143 (2013) 424.
[16] C.I. Someya, E. Irran, S. Enthaler, Inorg. Chim. Acta 421 (2014) 136.
[17] S. Enthaler, M. Weidauer, E. Irran, J.D. Epping, R. Kretschmer, C.I. Someya, J.
Organomet. Chem. 745–746 (2013) 262.
[18] J. Park, S. Hong, Chem. Soc. Rev. 41 (2012) 6931.
[19] I. Bratko, M. Gómez, Dalton Trans. 42 (2013) 10664.
[20] (a) L. Sacconi, Anorg. Allg. Chem. 275 (1954) 249;
(b) K.N. Zelenin, M.Y. Malov, I.V. Zerova, P.B. Terent’ev, A.G. Kalandarishvili,
Khim. Geterotsikl. Soedin. (1987) 1210;
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 adminis-
tered by the Technische Universität Berlin) is gratefully
acknowledged.
(c) K.N. Zelenin, M.Y. Malov, I.P. Bezhan, V.A. Khrustalev, S.I. Yakimovich, Khim.
Geterotsikl. Soedin. (1985) 1000;
Appendix A. Supplementary material
(d) M.Y. Malov, K.N. Zelenin, S.I. Yakimovich, Khim. Geterotsikl. Soedin. (1988)
1358;
(e) A.Y. Ershov, Zhu. Org. Khim. 31 (1995) 1057;
(f) S.I. Yakimovich, I.V. Zerova, Zhu. Org. Khim. 23 (1987) 1433;
(g) V.G. Yusupov, S.I. Yakimovich, S.D. Nasirdinov, N.A. Parpiev, Zhu. Org. Khim.
16 (1980) 415.
CCDC 949640 (for 2), 1050190 (for 3) and 1050191 (for 4) con-
tains 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_re-
quest/cif. Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.ica.
2015.05.008.
[21] S.D. Ittel, C.A. Tolman, P.J. Krusic, A.D. English, J.P. Jesson, Inorg. Chem. 17
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[24] Complexes 2 and 3 were applied as precatalyst in the hydrodecyanation of
benzonitriles with LiBH4 as hydride source, but no activity was observed.
[25] G.M. Sheldrick, SHELXL93, Program for the Refinement of Crystal Structures,
University of Göttingen, Göttingen, Germany, 1997.
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