Synthesis, characterization and application of pincer-type nickel iminophosphinite complexes

Article abstract of DOI:10.1016/j.poly.2011.01.007

Two pincer-type nickel iminophosphinite complexes, [(2-(CHNR)-6- (OPR′₂)C₆H₃)NiCl] (R = 2,6- ⁱPr₂C₆H₃, R′ = Ph (2a) or ⁱPr (2b)), were synthesized from the reactions of bis(1,5-cyclooctadiene)nickel(0) and corresponding iminophosphinite ligands. The solid state structures of the nickel pincer complexes were determined by X-ray single crystal diffraction studies. They were successfully employed in the Kumada reaction of non-activated aryl chlorides and phenylmagnesium bromide at room temperature.

Full text of DOI:10.1016/j.poly.2011.01.007

Polyhedron 30 (2011) 1091–1094
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization and application of pincer-type nickel
iminophosphinite complexes
Jessica Sanford a, Catlin Dent a, Jason D. Masuda b, Aibing Xia a,
⇑
a Department of Chemistry, Mount Saint Vincent University, Halifax, Nova Scotia, Canada B3M 2J6
b The Maritimes Centre for Green Chemistry and The Department of Chemistry, Saint Mary’s University, Halifax, NS, Canada B3H 3C3
a r t i c l e i n f o
a b s t r a c t
Two pincer-type nickel iminophosphinite complexes, [(2-(CH@NR)-6-(OPR02)C6H3)NiCl] (R = 2,6-
iPr2C6H3, R0 = Ph (2a) or iPr (2b)), were synthesized from the reactions of bis(1,5-cyclooctadiene)nickel(0)
and corresponding iminophosphinite ligands. The solid state structures of the nickel pincer complexes
were determined by X-ray single crystal diffraction studies. They were successfully employed in the
Kumada reaction of non-activated aryl chlorides and phenylmagnesium bromide at room temperature.
Ó 2011 Elsevier Ltd. All rights reserved.
Article history:
Received 1 November 2010
Accepted 11 January 2011
Available online 25 January 2011
Keywords:
Nickel complexes
Pincer types
Unsymmetrical ligands
Iminophosphinites
Cross-coupling reactions
The Kumada reaction
1. Introduction
2. Results and discussion
The chemistry of pincer-type transition metal complexes has
seen tremendous development over the past decade [1–5]. The
pincer ligand systems are relatively easy to access and modify
with desirable electronic and steric features. Pincer-type nickel
complexes have attracted considerable attention in recent years
due to their catalytic and electrochemical properties. They have
been used as (pre)catalysts in a variety of organic transformations
such as cross-coupling reactions [6–15] and olefin oligomerization
reactions [16,17]. The strong donating groups of pincer ligands
have been shown to lower the Ni(III)/Ni(II) reduction potential,
leading to more effective catalysts [18–23]. While early reported
nickel pincer complexes have been predominantly based on sym-
metrical pincer types such as bisphosphine (PCP) and bisamine
(NCN), nickel complexes based on unsymmetrical ligand types
have attracted increasing interest recently [12,15,19,24]. Here
we wish to report two novel nickel complexes bearing unsymmet-
rical PCN-type iminophosphinite pincer ligands. The nickel com-
plexes were easy to prepare and air-stable and have been
employed successfully in Kumada reactions of non-activated aryl
chlorides.
2.1. Synthesis
We have recently reported the synthesis and catalytic
application of a series of palladium complexes based on novel imi-
nophosphinite ligands that were derived from 2-bromo-hydroxy-
benzaldehyde [25]. The pincer-type iminophosphinite ligands
were easy to prepare and modify and could readily form complexes
with palladium(0) or palladium(II) precursors [26]. In a fashion
similar to the preparation of palladium iminophosphinite com-
plexes, the analogous nickel complexes were prepared in simple
steps. The synthetic route of the nickel complexes is outlined in
Scheme 1. Condensation of commercially available 2-chloro-3-
hydroxybenzaldehyde and 2,6-diisopropylaniline in refluxing eth-
anol afforded the corresponding imine (1), which reacted readily
with diphenylchlorophosphine or diisopropylchlorophosphine in
the presence of 4-N,N-dimethylaminopyridine (DMAP). The resul-
tant iminophosphinites were used without further purification to
react with bis(1,5-cyclooctadiene)nickel(0), Ni(COD)2, at room
temperature to give the desired nickel iminophosphinite pincer
complexes (2a and 2b) in good yields. Both nickel complexes are
air- and moisture-stable and can be recrystallized from CH2Cl2/
hexane in the air. They are thermally robust and melt without
decomposition at well over 200 °C. They have been fully character-
ized using NMR and FT-IT spectroscopic methods and X-ray single
crystal analysis. Their elemental analysis results matched well
with expected values.
⇑
Corresponding author. Tel.: +1 902 457 6543; fax: +1 902 457 6656.
E-mail address: aibing.xia@msvu.ca (A. Xia).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.01.007

1092
J. Sanford et al. / Polyhedron 30 (2011) 1091–1094
Scheme 1. Synthesis of pincer-type iminophosphinite nickel complexes.
2.2. Characterization
sive aryl bromides and iodides [7–10,12,13,15]. We chose 4-chlo-
rotoluene, neutral aryl chloride, and 4-chloroanisole,
a
a
The 1H NMR spectra of imine 1 showed single resonance at
8.60 ppm for the proton of the imine (CH@N) group and
5.79 ppm for the proton of OAH group. Its IR spectrum showed
strong absorption at 1631 cmꢀ1, which is typical for the C@N func-
tion group. Both nickel iminophosphinite pincer complexes (2a
and 2b) are diamagnetic. Their 1H NMR spectra showed upfield
chemical shifts of the imine group to 7.98 and 7.95 ppm. The
methyl groups in the isopropylphenyl moieties in the complexes
split to two signals, compared to a single signal of those groups
in the free imine 1. The methyl groups in the isopropylphosphenyl
group showed couplings with phosphorus (JPH = 18.2 Hz) and adja-
cent hydrogen atoms (JHH = 6.9 Hz). The 31P NMR signals of the
complexes are 200 and 148 ppm, respectively. The IR spectra of
the iminophosphinite complexes showed red-shifts of the imine
C@N stretch to 1577 and 1574 cmꢀ1, for 2a and 2b, respectively.
Suitable crystals of 2a and 2b for X-ray analysis were grown
from saturated solutions of dichloromethane and hexane at
ꢀ10 °C. The molecular structures of the complexes were almost
identical. A representative structure of 2b is shown in Fig. 1. The
structure of 2a is included in the Supplementary data. Selected
bond distances and angles for 2b are listed in Table 1. Crystal data
and structure refinement data for 2a and 2b are listed in Table 2. In
the solid state of 2b, the complex exists as a monomer, with Ni(II)
ion adopting a distorted square-planar geometry. The bond angle
of PANiAN is about 163°, suggesting significant strain in the core.
The bond lengths for NiAC (1.847 Å), NiAN (1.986 Å), NiAP
(2.110 Å), and NiACl (2.186 Å) are comparable to the reported val-
ues in related nickel complexes [19,24,27].
deactivated aryl chloride as the organic substrates to react with
phenylmagnesium bromide in THF at room temperature. Under
2 mol% nickel complex loading, 2a gave modest conversion
(ꢁ70%) for both aryl chlorides (Table 3, entries 1 and 3). However,
2b gave near complete conversion (ꢁ99%) for the aryl chlorides
(Table 3, entries 2 and 4). In all cases, the unsymmetrical biphenyls
are the major products while the symmetrical biphenyls are minor
biproducts. The improved efficiency of 2b over 2a may be attrib-
uted to better donating ability of the diisopropylphosphino group
than that of the diphenylphosphino group.
2.4. Conclusion
In summary, we have synthesized and characterized two novel
iminophosphinite
pincer
nickel
complexes.
The
dii-
sopropylphosphinite complex was found to be very efficient in
the Kumada coupling reaction of 4-chlorotoluene and 4-chloroani-
sole at room temperature. The nickel complexes were prepared in
good yields under mild conditions and are air-stable and thermally
robust, making them attractive candidates in catalytic studies. We
are working to expand the scope of the novel complexes in other
cross-coupling reactions. The results will be reported in due
course.
3. Experimental
3.1. General procedures
All manipulations were performed under an argon atmosphere
unless otherwise specified. All anhydrous solvents were purchased
from Sigma–Aldrich Canada Ltd. and stored over 4 Å molecular
sieves prior to use. Diphenylchlorophosphine, diisopropylchloro-
phosphine, 2-chloro-3-hydroxybenzaldehyde, 4-N,N-dimethylami-
nopyridine, and authentic biphenyl samples were purchased from
Sigma–Aldrich Canada Ltd. and used as received. 1H, 13C, and 31P
NMR spectra were recorded on a Bruker AV-500 spectrometer.
The chemical shifts of 1H and 13C NMR spectra were referenced
internally to the solvent signals. The chemical shifts of 31P NMR
spectra were referenced to an external standard using 85%
H3PO4. Infrared spectra were collected on a Nicolet Avatar 330
FT-IR spectrometer. Elemental analyses were carried out by Guelph
2.3. Catalytic studies
The nickel pincer complexes were evaluated in the Kumada
reaction, which is of significant importance among the cross-
coupling reactions because of its relative early discovery and
important applications. The Kumada reaction was initially effected
by nickel complexes [28], but their efficiencies have since been
surpassed by more active palladium catalysts. However, there is
continuing interest in the nickel catalysts due to their low cost
and good activity. Recently important progress has been made in
the nickel-catalyzed coupling of non-activated aryl chlorides,
which are more desirable than the more reactive yet more expen-

J. Sanford et al. / Polyhedron 30 (2011) 1091–1094
1093
Fig. 1. Molecular structure of 2b with 30% probability ellipsoids. Hydrogen atoms have been omitted for clarity.
Table 1
Selected bond lengths (Å) and angles (°) for 2b.
Table 3
Kumada coupling reactions of aryl chloridesa.
Ni1–C7
Ni1–N1
Ni1–P1
Ni1–Cl1
C7–Ni1–N1
C7–Ni1–P1
N1–Ni1–P1
P1–Ni1–Cl1
C7–Ni1–Cl1
N1–Ni1–Cl1
1.847(3)
Ni complex
1.986(2)
2.1100(8)
2.1865(9)
82.55(10)
80.68(8)
163.15(7)
97.19(3)
174.29(8)
99.67(7)
+
R
Cl
PhMgBr
R
Ph
THF
(A)
+
R
R
(B)
Entry
R
Nickel complex
Conversion (%)b
Ratio of A and Bc
1
2
3
4
CH3
CH3
OCH3
OCH3
2a
2b
2a
2b
71
99
69
98
89:11
90:10
88:12
88:12
Table 2
Crystal data and structure refinement for 2a and 2b.
a
1.0 mmol Aryl chloride, 1.5 mmol PhMgBr, 2 mol% nickel complex, 2 mL THF,
room temperature, 22 h.
2a
2b
b
GC yields based on unreacted aryl chlorides, average of two runs.
Calibrated with authentic biphenyl samples.
c
CCDC number
Formula
Formula weight
T (K)
Space group
a (Å)
796787
31H31ClNNiOP
796788
C
C25H35ClNNiOP
490.67
296(2)
P2(1)/c
15.7468(7)
9.8897(5)
16.5849(8)
90
94.100(1)
90
2576.2(2)
4
1.265
0.935
1040
1.30–25.00
16 658
4526
0.0356
0.0800
0.0575
0.0924
558.70
296(2)
P2(1)/n
9.197(1)
21.302(3)
14.281(2)
90
93.174(2)
90
2793.6(7)
4
1.328
0.871
1168
2.39–25.00
18 245
4862
0.0265
0.0677
0.0324
0.0723
Chemical Laboratories in Guelph, Ontario. Melting points were re-
corded on a Mel-Temp (Electrothermal) Apparatus and were
uncorrected.
b (Å)
c (Å)
a
(°)
3.2. Synthesis
b (°)
c
(°)
3.2.1. Synthesis of the imine (1)
V (Å3)
2-Chloro-3-[(2,6-diisopropyl-phenylimino)methyl]phenol (1):
2-chloro-3-hydroxybenzaldehyde (3.13 g, 20.0 mmol) and 2,6-
diisopropylaniline (3.54 g, 20.0 mmol) were dissolved in 30 mL
ethanol. Formic acid (10 drops) was added to the solution. The
resultant yellow solution was refluxed with stirring for 48 h. After
being cooled to room temperature, the reaction mixture was dried
over anhydrous MgSO4 and filtered. The filtrate was concentrated
to dryness under reduced pressure, affording a yellow oil, which
was recrystallized from hexanes to give the pale yellow solid 1.
Yield: 4.61 g (73%). M.p. 132–133 °C. Anal. Calc. for C19H22ClNO: C,
Z
Dcalc (g/cm3)
l
(mmꢀ1
)
F(0 0 0)
h for Data collection (°)
No. of total reflections
No. of unique reflections
R1 [I > 2
wR2 [I > 2
r
(I)]
(I)]
r
R1 [all data]
wR2 [all data]

1094
J. Sanford et al. / Polyhedron 30 (2011) 1091–1094
72.25; H, 7.02; N, 4.43. Found: C, 71.92; H, 7.17; N, 4.38%. 1H NMR
(CDCl3, 500.13 MHz, ppm): d 8.60 (s, 1H, CH@N), 7.87–7.82 (dd,
J = 8.0, 1.6 Hz, 1H, Ar-H), 7.34 (t, J = 8.0 Hz, Ar-H), 7.20–7.13 (m,
4H, Ar-H), 5.79 (broad, 1H, OH), 2.97 (sept, J = 6.9 Hz, CH(CH3)2),
1.21 (d, J = 6.9 Hz, CH(CH3)2). 13C NMR (CDCl3, 125.77 MHz, ppm):
d 158.95, 151.72, 148.87, 137.57, 133.41, 128.04, 121.63, 124.47,
calibrated relative to standards containing authentic samples of
aryl halides and their biphenyl products.
3.4. X-ray structures
Crystals of 2a and 2b were grown by recrystallization methods
using dichloromethane and hexanes as solvents at room tempera-
ture. Single crystals were mounted in thin-walled capillaries.
The data were collected using the Bruker APEX2 software [29]
package on a Siemens diffractometer equipped with an APEXII
123.09, 120.25, 118.57, 27.94, 23.51. IR (KBr, cmꢀ1):
m 3356(s),
2959(s), 2866(m), 1631(s), 1572(s), 1464(s), 1364(m), 1289(s),
1182(m), 1047(m), 780(s), 753(s), 708(m).
3.2.2. Synthesis of nickel pincer complexes, [(2-(CH@NR)-6-(OPR02)-
C6H3)NiCl] (R = 2,6-iPr2C6H3)
CCD detector, a graphite monochromator and Mo Ka radiation
(k = 0.71073 Å). A hemisphere of data was collected in 1664 frames
with 10 s exposure times. Data processing and absorption correc-
tions were applied using APEX2 software package. The structure
was solved (direct methods) and all non-H atoms were refined
anisotropically. Hydrogen atoms were placed in calculated
positions using an appropriate riding model and coupled isotropic
temperature factors. Thermal ellipsoid diagrams (30% probability
level) were produced using Ortep-3 for Windows [30].
General procedure: 1 (0.316 g, 1.00 mmol) and 4-N,N-dimethyl-
aminopyridine (0.122 g, 1.00 mmol) were dissolved in THF
(10 mL). To the solution was added diphenylchlorophosphine or
diisopropylchlorophosphine (1.00 mmol) in THF (5 mL). The resul-
tant mixture was stirred for 17 h at room temperature and filtered.
To the filtrate was added bis(cyclooctadiene)nickel(0) (0.275 g,
1.00 mmol). The mixture was stirred under argon for 24 h. The vol-
atiles were removed under reduced pressure to afford orange sol-
ids, which were washed with hexane (5 mL). The crude products
were recrystallized from CH2Cl2/hexanes to afford the pure nickel
complexes.
Acknowledgements
We thank Natural Science and Engineering Research Council of
Canada (NSERC) and Canada Foundation for Innovation (CFI) for
financial supports.
3.2.3. 2a
R0 = Ph: yield: 0.350 g (63%). M.p.: 269–270 °C. Anal. Calc. for
C
31H31ClNNiOP: C, 66.64; H, 5.59; N, 2.51. Found: C, 66.87; H,
Appendix A. Supplementary data
5.85; N, 2.52%. 1H NMR (CDCl3, 500.13 MHz, ppm): d 8.07 (m, 4H,
Ar-H), 7.98 (d, J = 4.4 Hz, 1H, CH@N), 7.54–7.45 (m, 6H, Ar-H),
7.27–7.11 (m, 6H, Ar-H), 6.88 (m, 1H, Ar-H), 3.47 (sept, J = 6.9 Hz,
2H, CH(CH3)2), 1.39 (d, J = 6.9 Hz, 6H, CH(CH3)0(CH3)00), 1.18 (d,
J = 6.9 Hz, 6H, CH(CH3)0(CH3)00). 13C NMR (CDCl3, 125.77 MHz,
ppm): d 141.06, 131.93, 128.62, 122.95, 28.77, 24.34, 22.96 (some
peaks not shown due to low solubility). 31P NMR (CDCl3,
CCDC 796787 and 796788 contain the supplementary crystallo-
graphic data for 2a and 2b. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
202.47 MHz, ppm):
d m 2957(m),
199.94. IR (KBr, cmꢀ1):
1577(m), 1459(m), 1416(m), 1349(w), 1229(s), 1107(s), 1041(m),
875(m), 739(s), 687(s), 556(m), 478(m).
References
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3.2.4. 2b
R0 = iPr: yield: 0.410 g (84%). M.p. 237–239 °C. Anal. Calc. for
C25H35ClNNiOP: C, 61.20; H, 7.19; N, 2.85. Found: C, 61.41; H,
7.32; N, 2.87%. 1H NMR (CDCl3, 500.13 MHz, ppm): d 7.95 (d,
J = 4.2 Hz, 1H, CH@N), 7.25–7.22 (m, 1H, Ar-H), 7.17–7.15 (m, 2H,
Ar-H), 7.08 (m, 2H, Ar-H), 6.73 (m, 1H, Ar-H), 3.42 (sept, J = 6.9 Hz,
2H, Ar-CH(CH3)2), 2.39 (sept, J = 6.9 Hz, 2H, PCH(CH3)2), 1.51–1.46
(dd, 6H, J = 18.2, 6.9 Hz, PCH(CH3)0(CH3)00), 1.41–1.37 (m, 12H, over-
lapping PCH(CH3)0(CH3)00 and Ar-CH(CH3)0(CH3)00), 1.20 (d, J = 6.9 Hz,
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3.3. General procedures for the Kumada reaction
A 25 mL Schlenk flask was charged with 4-chlorotoluene or 4-
chloroanisole (1.0 mmol), the nickel pincer complex 2a or 2b
(0.020 mmol), and 2 mL THF. To the solution was added phenyl-
magnesium bromide (1.5 mmol) in THF. The mixture was stirred
for 18 h at room temperature under argon. The reaction was
quenched using 10 mL H2O and the reaction mixture was washed
with diethyl ether. The combined organic layer was dried with
MgSO4 and filtered. The volatiles were removed under reduced
pressure. The organic products were analyzed on an Agilent 6890
GC-FID instrument. The conversions and product ratios were