Ni(II)-POCOP pincer compound [NiCl{C10H5-2,10- (OPPh2)2}] an efficient and robust nickel catalyst for the Suzuki-Miyaura coupling reactions

Article abstract of DOI:10.1016/j.ica.2011.12.052

The Ni(II)-POCOP pincer complex [NiCl{C₁₀H₅-2,10- (OPPh₂)₂}] (2) based on a naphthoresorcinol frame has been synthesized in a very simple and facile manner and efficiently used in the Suzuki-Miyaura cross-coupli

Full text of DOI:10.1016/j.ica.2011.12.052

Inorganica Chimica Acta 387 (2012) 58–63
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Ni(II)–POCOP pincer compound [NiCl{C10H5-2,10-(OPPh2)2}] an efficient
and robust nickel catalyst for the Suzuki–Miyaura coupling reactions
⇑
Fabiola Estudiante-Negrete, Simón Hernández-Ortega, David Morales-Morales
Instituto de Química, Universidad Nacional Autónoma de México, Cd. Universitaria, Circuito Exterior, Coyoacán 04510, México D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 October 2011
Received in revised form 23 December 2011
Accepted 26 December 2011
Available online 2 January 2012
The Ni(II)–POCOP pincer complex [NiCl{C10H5-2,10-(OPPh2)2}] (2) based on a naphthoresorcinol frame
has been synthesized in a very simple and facile manner and efficiently used in the Suzuki–Miyaura
cross-coupling of different para-substituted bromobenzenes producing high yields of the corresponding
biphenyl derivatives.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
POCOP pincer compounds
Nickel compounds
Suzuki–Miyaura couplings
Catalysis
Crystal structures
Nickelacycles
1. Introduction
employing catalysts based on cheaper and safer metals. In this
point nickel could be the answer, as nickel catalysts have been al-
Pincer compounds have had a preponderant importance in
chemistry in the last decade, this being specially important in areas
such as homogeneous catalysis, organometallic chemistry, the acti-
vation of un-reactive or difficult to activate bonds and the activation
of small molecules [1]. The success of these species lying in part on
their high thermal stability, as these complexes are in most of the
cases able to withstand continuous heating at temperatures averag-
ing 200 °C or higher without any apparent decomposition. However,
the attractiveness of these species on occasions has been hampered
by the sometimes, difficulty on their attaining, being the synthesis
of some of these compounds difficult or requiring multi-step reac-
tion paths with the undesirable combination of low yields. Thus,
the discovery of POCOP phosphinite palladium pincer complexes,
offered an answer to this problem, providing a synthetic procedure
based on resorcinols and chlorophosphines to produce the corre-
sponding phosphinito pincer derivatives in a simple manner [2].
On the other hand, the Suzuki–Miyaura cross coupling reaction
[3] has been recognized as a true power tool in organic synthesis
and although palladium species are widely accepted as suitable
catalysts, cost still continues to be one of the main issues for their
employment [4], leading the research involving Pd-based catalysts
to the potential recycling or recovering of the catalysts [5]. Thus,
it would be desirable to have the same catalytic activity by
ready used successfully in other cross coupling reactions such as
the formation of C–S bonds, the thioetherification reaction [6]. In
spite of these, the reports employing Ni compounds for the
Suzuki–Miyaura reaction are limited or the activities reported poor
[7]. Thus, following our continuous interest in the synthesis of
novel pincer complexes and their potential application on indus-
trial relevant transformations [8] we would like to report on this
opportunity the synthesis and successful application of the
phosphinito POCOP pincer complex [NiCl{C10H5-2,10-(OPPh2)2}]
(2) on Suzuki-Miyaura couplings.
2. Experimental
2.1. Material and methods
Unless stated otherwise, all reactions were carried out under an
atmosphere of dinitrogen using conventional Schlenk glassware,
solvents were dried using established procedures and distilled un-
der dinitrogen immediately prior to use. The 1H NMR spectra were
recorded on a JEOL GX300 spectrometer. Chemical shifts are re-
ported in ppm down field of TMS using the residual signals in the
solvent (CDCl3, d 7.27) as internal standard. 31P{1H} NMR spectra
were recorded with complete proton decoupling and are reported
in ppm using 85% H3PO4 as external standards. Elemental analyses
were determined on a Perkin Elmer 240. Positive-ion FAB mass
spectra were recorded on a JEOL JMS-SX102A mass spectrometer
⇑
Corresponding author. Tel.: +52 55 56224514; fax: +52 55 56162217.
E-mail address: damor@servidor.unam.mx (D. Morales-Morales).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.052

F. Estudiante-Negrete et al. / Inorganica Chimica Acta 387 (2012) 58–63
59
operated at an accelerating voltage of 10 kV. Samples were des-
orbed from a nitrobenzyl alcohol (NOBA) matrix using 3 keV xenon
atoms. Mass measurements in FAB+ are performed at a resolution of
3000 using magnetic field scans and the matrix ions as the refer-
ence material or, alternatively, by electric field scans with the
sample peak bracketed by two (polyethylene glycol or cesium io-
dide) reference ions. GC–MS analyses were performed on a Agilent
6890N GC with a 30.0 m DB-1MS capillary column coupled to an
Agilent 5973 Inert Mass Selective detector. The NiCl2ꢀ6H2O, naph-
thoresorcinol (1,3-dihydroxynaphthalene), ClPPh2 and NEt3 were
commercially obtained from Aldrich Chemical Co. All compounds
were used as received without further purification.
indicating that heterogeneous Ni(0) is not involved. This experi-
ment was performed under the optimized conditions for entry 6
(e.g. with bromobenzene).
2.6. Data collection and refinement for [NiCl{C10H5-2,10-(OPPh2)2}]
(2)
Crystalline green-yellow prisms of [NiCl{C10H5-2,10-(OPPh2)2}]
(2) were grown by slow evaporation from a CH2Cl2/n-hexane sol-
vent system, and mounted in random orientation on glass fibers.
In all cases, the X-ray intensity data were measured at 298 K on
a Bruker SMART APEX CCD-based three-circle X-ray diffractometer
system using graphite mono-chromated Mo K
radiation. The detector was placed at a distance of 4.837 cm from
a (k = 0.71073 Å)
2.2. Synthesis of [C10H5-1,9-(OPPh2)2] (1)
the crystals in all cases. A total of 1800 frames were collected with
The title compound was synthesized by a slight modification of
the procedure described for compound [C6H4{1,3-(OPPri2)2}] [9]. A
Schlenk flask was charged with naphthoresorcinol (109 mg,
0.68 mmol), 30 mL of freshly distilled toluene and 0.2 mL of NEt3
(1.43 mmol). The resulting mixture is stirred for 15 min and after
this time chlorodiphenylphosphine (0.18 mL, 1.36 mmol) is added
dropwise under stirring. The mixture is set to reflux overnight and
then allowed to reach room temperature and filtered via canula.
The filtered solution is evaporated under vacuum to afford ligand
(1) as a colorless viscous oil, (88% yield). This compound was used
in the next step without any further purification. 1H NMR
(300 MHz, CDCl3): d = 6.46–7.72 (m, 26H, Ar). 31P{1H} NMR
a scan width of 0.3° in
x and an exposure time of 10 s/frame. The
frames were integrated with the Bruker SAINT software package [11]
using a narrow-frame integration algorithm. The integration of the
data was done using a monoclinic unit cell to yield a total of 24154
reflections to a maximum 2h angle of 50.00° (0.93 Å resolution), of
which 5278 were independent. Analysis of the data showed in all
cases negligible decays during data collections. The structures
were solved by Patterson method using SHELXS-97 [12] program.
The remaining atoms were located via a few cycles of least squares
refinements and difference Fourier maps, using a P21/n space
group, with Z = 4. Hydrogen atoms were input at calculated posi-
tions, and allowed to ride on the atoms to which they are attached.
Thermal parameters were refined for hydrogen atoms on the phe-
nyl groups with Uiso(H) = 1.2 Ueq of the parent atom in all cases.
The final cycle of refinement was carried out on all non-zero data
using SHELXL-97 [13] and anisotropic thermal parameters for all
non-hydrogen atoms. The details of the structure determinations
are given in Table 1. The numbering of the atoms is shown in
Fig. 1 (ORTEP) [14].
(121.379 MHz, CDCl3): d = 111.47 (d, Pab,
2JPaPb = 75.52 Hz). EI-MS
[M]+ = 528 (100%) m/z. Anal. Calc. for C34H26O2P2 (Mr = 528.52): C,
77.27; H, 4.96. Found: C, 77.34; H, 4.84%.
2.3. Synthesis of [NiCl{C10H5-2,10-(OPPh2)2}] (2)
A solution of ligand (1) (465 mg, 0.72 mmol) in toluene (20 mL)
was added dropwise to a suspension of NiCl2 (171 mg, 0.72 mmol)
in toluene (30 mL) and set up to reflux overnight. The solution was
then filtered over a short pad of silica gel, washed with diethyl
ether and pumped off to dryness. The resulting oily product was
extracted with CH2Cl2 (2 ꢁ 5 mL) and then the solvent removed
under vacuum to afford complex (2) as a viscous amber oil which
is recrystallized from a CH2Cl2/hexane solvent system producing
emerald green crystals, (83%). 1H NMR (300 MHz, CDCl3):
d = 6.66–7.77 (m, 22H, Ar). 31P{1H} NMR (121.379 MHz, CDCl3):
3. Results and discussion
The phosphinito POCOP pincer ligand [C10H5-1,9-(OPPh2)2] (1)
was synthesized in a similar manner as that described by our group
for [C6H4-1,3-(OPPri2)2] [9]. Thus, the reaction of 1,3-dihydroxy-
naphthalene with ClPPh2 in a 1:2 M ratio in the presence of a slight
excess of NEt3 under reflux in toluene (Scheme 1) affords ligand (1)
in good yields and pure enough, as shown by 31P{1H} NMR to be
used for the synthesis of the corresponding nickel metal complexes.
Analysis of the ligand by 1H NMR spectroscopy only displays
signals due to the presence of the phenyl rings, between d 6.46
and 7.72 ppm. However, more informative results were obtained
from the analysis by 31P{1H} NMR. In this instance the spectra ob-
tained shows a doublet centered at d 111.47 ppm [111.78 (Pa) and
111.15 (Pb) ppm]. The later results are in agreement with the two
phosphorus being in a slightly different chemical and magnetic
environments, as expected for a non-symmetric pincer ligand
(the two P donor atoms are different by symmetry). Analysis of
compound (1) by mass spectrometry affords the molecular ion.
Elemental analysis was also in agreement with the proposed
formulations.
2
d = 141.69 (d, JPaPb = 8.5 Hz, P). FAB+-MS [M]+ = 621 (12%) m/z,
[MꢂCl]+ = 586 (88%) m/z. Anal. Calc. for C34H25Cl1Ni1O2P2
(Mr = 621.66): C, 65.69; H, 4.05. Found: C, 65.65; H, 4.03%.
2.4. General method for the Suzuki–Miyaura couplings
A DMF solution (4 ml) of 1.0 mmol of halobenzene, 1.3 mmol of
phenyl boronic acid, and the prescribed amount of catalyst (3.0 mg,
4.8 mmol, 1.0% mol) was introduced into a Schlenk tube in the
open air. The tube was charged with a magnetic stir bar and an
equimolar amount of base (Na2CO3, 2.6 mmol), and sealed, and
fully immersed in a 110 °C silicon oil bath. After the prescribed
reaction time (15 h), the mixture was cooled to room temperature
and the organic phase analyzed by gas chromatography (GC–MS)
by duplicate.
The synthesis of the Ni(II) complex was carried out in a similar
manner as we have reported before by direct C–H activation
[6c,15] according to Scheme 2.
2.5. Mercury drop experiments [10]
Thus, the reaction of ligand (1) with NiCl2ꢀ6H2O under reflux
conditions in toluene affords complex [NiCl{C10H5-2,10-(OPPh2)2}]
(2) as an emerald green crystalline product in good yields. Analysis
by proton NMR spectroscopy of this product yields a similar spec-
trum as that determined for the free ligand, with the signals just
been shifted slightly to higher field (d 7.24–8.05 ppm). Further
Following the above described procedures; additionally adding
two drops of elemental Hg to the reaction mixture. After the pre-
scribed reaction times, the solution was filtered and analyzed by
GC–MS: no significant difference in conversion between these
experiments and those in the absence of mercury was observed,

60
F. Estudiante-Negrete et al. / Inorganica Chimica Acta 387 (2012) 58–63
Table 1
OH
O
PPh2
PPh2
Crystal data and structure parameters for [NiCl{C10H5-2,10-(OPPh2)2}] (2).
2 ClPPh2 / NEt3
Toluene, reflux
Identification code
Empirical formula
Formula weight
(2)
C
34H25Cl1O2P2Ni1
621.64
298(2)
0.71073
T (K)
OH
O
(1)
0
k (AÅ)
Crystal system
Space group
monoclinic
P2l/n
Scheme 1. Synthesis of the POCOP phosphinite ligand [C10H5-1,9-(OPPh2)2] (1).
Unit cell dimensions
0
8.8952(7)
a (AÅ)
a
(°)
90
27.561(2)
O
PPh2
PPh2
O
O
PPh2
0
b (AÅ)
b (°)
104.723(2)
12.1216(10)
NiCl2
Toluene, reflux
24 h
0
Ni Cl
c (AÅ)
c
(°)
90
2874.2(4)
4
1.437
0.910
1280
0.26 ꢁ 0.06 ꢁ 0.05
1.48–25.38
ꢂ10 ꢃ h ꢃ 10, ꢂ33 ꢃ k ꢃ 33,
ꢂ14 ꢃ l ꢃ 14
24154
5278 [R(int) = 0.0969]
empirical
0.9559 and 0.7978
V (Å3)
O
PPh2
(2)
Z
Dcalc (Mg/m3)
(1)
Absorption coefficient (mmꢂ1
F(000)
)
Scheme 2. Synthesis of POCOP pincer complex [NiCl{C10H5-2,10-(OPPh2)2}] (2).
Crystal size (mm)
h Range for data collection (°)
Index ranges
Reflections collected
Independent reflections
Absorption correction
Maximum and minimum
transmission
Refinement method
full-matrix least-squares on F2
5278/0/361
1.010
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0633, wR2 = 0.1255
R1 = 0.1091, wR2 = 0.1416
0.467 and ꢂ0.284
Largest difference in peak and hole
(e Åꢂ3
)
Fig. 2.
p–p interactions observed in complex [NiCl{C10H5-2,10-(OPPh2)2}] (2).
of the Cl in the fragmentation process to afford peaks at
[MꢂCl]+ = 586 (88%) m/z. Results obtained from elemental analysis
are also in agreement with the proposed formulation.
Attempts to crystallize [NiCl{C10H5-2,10-(OPPh2)2}] (2) from a
CH2Cl2/hexane solvent system furnished crystals suitable for single
crystal X-ray diffraction analysis. This analysis confirmed unequiv-
ocally the proposed structure and the non-symmetric nature of the
pincer compound (2) (Fig. 1).
The structure shows the nickel center into a slightly distorted
square planar environment, two of the coordination sites being
occupied by the phosphorus donor ligands in a mutually trans con-
formation, and completing the coordination sphere the organome-
tallic Ni–C bond and the chloride ligand trans to the organometallic
carbon. From Fig. 1, it is clear that by symmetry both phosphorus
are different, this observation being in agreement with the results
observed in the 31P{1H} NMR spectrum, the fact that the value of
Fig. 1. An ORTEP representation of the structure of [NiCl{C10H5-2,10-(OPPh2)2}] (2)
at 50% of probability showing the atom labeling scheme. Selected bond lengths (Å):
Ni(1)–C(2) 1.885(4), Ni(1)–P(2) 2.1487(14), Ni(1)–P(1) 2.1544(13), Ni(1)–Cl(1)
2.2000(13). Selected bond angles (°): C(2)–Ni(1)–P(2) 82.12(15), C(2)–Ni(1)–P(1)
81.39(15), P(2)–Ni(1)–P(1) 162.50(5), C(2)–Ni(1)–Cl(1) 177.47(14), P(2)–Ni(1)–
Cl(1) 97.88(5), P(1)–Ni(1)–Cl(1) 98.34(5).
2
the coupling constant is just JPaPb = 8.5 Hz, reflects the close simi-
larity (closely isochronous nuclei) both chemically and magneti-
cally of the two P donor nuclei. A similar phenomenon has been
recently observed by our group in other similar pincer systems
analysis of this sample by 31P{1H} NMR spectroscopy reveals once
more the non-symmetric nature of the ligand, by displaying a spec-
trum for a second order AB system, with a doublet (corresponding
to two different P nuclei) centered at d 141.69 ppm [141.72 (Pa)
and 141.65 (Pb) ppm], having a 2JPaPb coupling constant of 8.5 Hz.
Analysis by FAB+-MS showed the presence of the molecular ion
at [M]+ = 621 (12%) m/z. Other important peaks arose from the loss
[15,16]. Additionally, analysis of the structure reveals weak
p–p
interactions (4.436 Å) among the naphthalene aromatic system
(Fig. 2), this interactions defining the array in the unit cell.
The longstanding search for efficient catalytic systems in transi-
tion metal-catalyzed coupling reactions remains an active subject

F. Estudiante-Negrete et al. / Inorganica Chimica Acta 387 (2012) 58–63
61
O
PPh2
Ni Cl
PPh2
[Ni] =
O
OH
B
(2)
Br
OH
1.0 mol % [Ni], Na2CO3, DMF
110oC, 15 h
+
R
R
R= NH2, OMe, Me, H, Cl, CHO, COMe, CN, NO2
Scheme 3. Evaluation of the catalytic Activity of Complex [NiCl{C10H5-2,10-(OPPh2)2}] (2).
in organic synthesis. Metallacycles possessing the pincer ligand
framework could be an efficient system because of their consider-
able thermal and oxidative stabilities. Various types (e.g., PCP,
NCN, CNC) of pincer complexes of transition metals have been re-
ported to date; among them, palladium–pincer complexes have
been widely investigated and employed as catalysts for cross-cou-
pling reactions [1,2,8,17]. Much less attention has been paid to the
nickel–pincer complexes, although the use of nickel catalysts in-
stead of palladium catalysts may offer interesting advantages, such
as cost effectiveness and distinct catalytic abilities. Thus in an at-
tempt to probe the theory that a robust Ni(II)–POCOP pincer com-
plex would perform well as catalyst we decided to test the air and
moisture stable [NiCl{C10H5-2,10-(OPPh2)2}] (2) pincer complex
and explore its reactivity on the Suzuki–Miyaura C–C cross cou-
pling reactions (Scheme 3), of different p-substituted bromobenz-
enes and phenylboronic acid. The results of these experiments
are presented in Table 2.
The results obtained are notable, particularly for the activated
bromobenzenes attaining quantitative conversions.
Besides, from this table a clear trend can be observed, which is
dependant upon the kind of substituent in the bromobenzene,
thus the more electro-withdrawing is the group the higher the
Table 2
Suzuki–Miyaura couplings using [NiCl{C10H5-2,10-(OPPh2)2}] (2) as catalyst.
Entry
1
Hammet parameter (
0.78
r)
p-Bromobenzene
% Conversiona
2
0.66
3
0.50
4
0.42
(continued on next page)

62
F. Estudiante-Negrete et al. / Inorganica Chimica Acta 387 (2012) 58–63
Table 2 (continued)
Entry
5
Hammet parameter (
r
)
p-Bromobenzene
% Conversiona
0.23
6
0.00
7
8
ꢂ0.17
ꢂ0.27
9
ꢂ0.66
a
Yields obtained by GC are based on bromobenzene and are the average of two runs.
Suzuki-Miyaura Cross-Couplings
100
90
80
70
60
50
40
30
20
10
0
NH2
OCH3
CH3
H
Cl
CHO COCH3
CN
NO2
Hammett Parameter
Graphic 1. Conversion (%) vs. Hammett parameter (r).
conversion to the biphenyl product. This trend can be better ob-
served when the percentage of conversion is plotted against the
Hammett parameter [18] (Graphic 1) where a near linear behavior
can be clearly noted.

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63
As with so many other analogous metallocycle examples, it is
References
possible that the reaction may proceed through the formation of
soluble nickel nanoparticles. Hence, in order to rule out this possi-
bility a mercury drop experiment [10] (see Section 2) was per-
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catalyst with or without the presence of Hg(0). It is noteworthy
the fact that the presence the Cl substituent at the Ni center does
not have to be substituted by a labile group in order to make this
catalysts reactive as has been the case for some palladium pincer
complexes in Heck couplings where silver additives are necessary.
It is possible that the enhanced reactivity observed may be related
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been the case when similar systems are employed for the thioethe-
rification reaction (C–S cross coupling). Efforts aiming to shed
further light on the mechanism through which this catalyst pro-
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In summary, we have successfully synthesized in high yields a
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1619.
We would like to thank Chem. Eng. Luis Velasco Ibarra,
Dr. Francisco Javier Pérez Flores and M.Sc Ma. de las Nieves Zavala
for their invaluable help in the running of the FAB+-Mass and some
NMR spectra, respectively. The financial support of this research by
CONACYT (154732) and DGAPA-UNAM (IN201711) is gratefully
acknowledged.
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CCDC 862571 contains the supplementary crystallographic data
for complex 2, respectively. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.12.052.