Synthesis and catalytic activity of nickel(II) complexes containing NCN pincer ligands

Article abstract of DOI:10.1016/j.jorganchem.2017.02.025

In this study, nickel(II) complexes [L^meNi(II)Cl]⁺(BPh₄)^?, [L^meNi(II)Cl]⁺(PF₆)^? and [L^et₂Ni(II)]⁺(BPh₄)^?₂ were synthesised and characterized in the solution and solid states. Ligands L^me (C₁₁H₁₂N₆) and L^et (C₁₃H₁₇N₆) featured a central carbene donor linked to pendant pyrazoles via either methylene or ethylene linkers. The catalytic activity of all three complexes was tested in the Kumada cross coupling reaction between phenylmagnesium bromide and aromatic halogen substituted substrates. [L^meNi(II)Cl]⁺(BPh₄)^?was found to be the most active catalyst.

Full text of DOI:10.1016/j.jorganchem.2017.02.025

Accepted Manuscript
Synthesis and catalytic activity of nickel(II) complexes containing NCN pincer ligands
Ashwin G. Nair, Roy T. McBurney, Mark R.D. Gatus, D.Barney Walker, Mohan
Bhadbhade, Barbara A. Messerle
PII:
S0022-328X(17)30095-5
10.1016/j.jorganchem.2017.02.025
JOM 19814
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 16 December 2016
Revised Date: 9 February 2017
Accepted Date: 13 February 2017
Please cite this article as: A.G. Nair, R.T. McBurney, M.R.D. Gatus, D.B. Walker, M. Bhadbhade, B.A.
Messerle, Synthesis and catalytic activity of nickel(II) complexes containing NCN pincer ligands, Journal
of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.02.025.
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ACCEPTED MANUSCRIPT

Nair et al. Synthesis and Catalytic Activity of Nickel(II) Complexes of NCN Pincer Ligands
ACCEPTED MANUSCRIPT
Synthesis and Catalytic Activity of Nickel(II) Complexes Containing
NCN Pincer Ligands
Ashwin G. Nair,a Roy T. McBurney,a Mark R. D. Gatus,a D. Barney Walker,a Mohan Bhadbhadeb
and Barbara A. Messerlea, *
a
Department of Chemistry and Biomolecular Sciences, Macquarie University, Sydney 2109,
Australia
b Mark Wainwright Analytical Centre, University of New South Wales, Sydney 2052, Australia
* Corresponding author
E-mail address: barbara.messerle@mq.edu.au (B. Messerle)
Abstract
In this study, nickel(II) complexes [LmeNi(II)Cl]+(BPh4)-, [LmeNi(II)Cl]+(PF6)- and
[Let Ni(II)]+(BPh4)-2 were synthesised and characterised in the solution and solid states. Ligands Lme
2
(C11H12N6) and Let (C13H17N6) featured a central carbene donor linked to pendant pyrazoles via
either methylene or ethylene linkers. The catalytic activity of all three complexes was tested in the
Kumada cross coupling reaction between phenylmagnesium bromide and aromatic halogen
substituted substrates. [LmeNi(II)Cl]+(BPh4)-was found to be the most active catalyst.
Keywords
Nickel(II) pincer complexes
Organometallic catalysis
Kumada cross coupling
Graphical Abstract Synopsis
1

Nair et al. Synthesis and Catalytic Activity of Nickel(II) Complexes of NCN Pincer Ligands
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A square-planar Ni(II) pincer complex based on a carbene-bis-pyrazole ligand was found to be an
effective catalyst for the Kumada cross coupling of phenylmagnsesium bromide with aryl halides at
room temperature. Synthesis, structural characterization and catalysis behavior of this Ni(II) complex
and related complexes was studied.
1.
Introduction
Among the first reported catalytic C-C bond formation reactions was the Kumada cross
coupling reaction.[1-3] The reaction involves the catalysed cross coupling between a Grignard
reagent and an organic halide, which results in the formation of a new C-C bond. Despite the advent
of alternative reactions, such as Suzuki,[4] Negishi,[5, 6] Stille[7-9] or Sonogashira[10] coupling
reactions, Kumada cross coupling reactions continue to be used in industry due to the ability of the
reaction to directly couple Grignard reagents to a wide variety of halide containing compounds.[11-
13] The Kumada cross coupling is not limited to sp2-hybridised carbons and hence has a greater
versatility compared to other C-C coupling reactions.[14] Also, this reaction has a other advantages
that make it an attractive tool for synthesis: it avoids the use of toxic organotin compounds utilised in
Stille couplings; nickel(II) complexes are typically utilised as catalysts providing a reduced cost
alternative for the same organic transformation compared to palladium; the thermal stability of
Grignard reagents allows a greater range of reaction temperatures to be used for the successful
coupling of both activated and deactivated substrates.[15, 16]
Significant attention has focused on transition metal complexes containing pincer ligands[17]
as catalysts for C-C cross coupling reactions as these complexes can exhibit higher reactivity and
increased stability over complexes containing bidentate or monodentate ligands.[18-20] Ni(II)
complexes containing pincer ligands are widely used in C-C cross coupling reactions, particularly
the coupling of aryl Grignards with aryl or alkyl halides.[21] The ability of the complexes bearing
pincer ligands to efficiently catalyse Kumada cross coupling reactions depends on factors such as the
degree of electron donation to the metal centre, and the lability of the coordinating groups to the
2

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metal centre.[11] Only a few complexes containing a central N-heterocyclic carbene (NHC) donor
and labile donor arms have been reported in the literature, see Fig. 1.[22-25]
Fig. 1 Catalysts featuring ligands containing a central carbene donor and hemilabile pendant donors
and the Ni(II) catalysts presented in this work.
In this work, two pro-ligands bearing a central carbene unit flanked by pyrazole pendant
donor arms were coordinated to nickel(II), generating three novel Ni(II) complexes and their
coordination chemistry was explored in the solution and solid states, see Fig. 1. The ligands differed
in only the length of their linker between imidazole arm and pyrazole arms, pro-ligand LmeH
possessed a methylene bridge, whereas LetH possessed an ethylene bridge. We have successfully
used Rh(I), Ir(I) and Ru(II) complexes containing these ligands for catalysis.[24, 25] The catalytic
activity of all three Ni(II) complexes was investigated in the Kumada cross coupling of phenyl
magnesium bromide with aryl halogens.
2.
Experimental
2.1
Materials
All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques
or in a nitrogen or argon filled glovebox unless otherwise stated. Imidazolium pro-ligands LmeH and
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LetH,[24] [Lme2Ag(I)]+(BPh4)- [25] were prepared according to the literature methods. Commercially
available reagents were purchased from Sigma-Aldrich or Alfa Aesar Inc. and used as received.
Dichloromethane and acetonitrile were dried using a solvent purification system. Tetrahydrofuran
used for catalysis reactions was distilled and dried using a sodium mirror and stored under an inert
atmosphere. All catalysis substrates were purified by distillation and glassware was oven-dried (120-
150 °C) prior to use.
2.2 Equipment
NMR spectra were recorded on a Bruker Avance III 500 MHz and 600 MHz spectrometers.
Chemical shifts (δ) are quoted in ppm and referenced to residual solvent resonances. Mass spectra
were acquired using a Thermo Scientific LCQ Fleet (ESI-MS) mass spectrometer, or using a Thermo
Scientific LTQ Orbitrap XL instrument at the Bioanalytical Mass Spectrometry Facility at University
of New South Wales. ‘M’ is defined as the molecular weight of the compound or cationic fragment
of interest. IR spectra were recorded as KBr discs using an Avatar 370 FTIR spectrometer at the
University of New South Wales. Elemental analyses were carried out at the Campbell
Microanalytical Labaoratory, University of Otago, New Zealand, Elemental Analysis Unit, The
Research School of Chemistry, Australian National University and the Elemental Microanalysis
Service at the Department of Chemistry and Biomolecular Sciences, Macquarie University. Single
crystal X-ray analyses were carried out at the Mark Wainwright Analytical Centre, University of
New South Wales, Sydney. X-ray diffraction measurements were carried out on a Bruker Kappa
APEXII CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.710723 Å). All
structures were solved by direct methods and the full-matrix least-square refinements were carried
out using SHELXL. Absorption correction was performed using Multi-scan SADABS and H-atom
parameters were treated as constrained. CCDC 1513714-1513716 contains supplementary X-ray
crystallographic data for complexes [LmeNi(II)Cl]+(BPh4)-, [LmeNi(II)Cl]+(PF6)- and [Let Ni(II)]
2
+(BPh4)-2.
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2.3
General Procedure for Kumada Cross Couplings
Nickel(II) complexes were pre-dried under vacuum prior to catalysis reactions. The Kumada cross
coupling experiments were carried out under standard Schlenk conditions and performed in duplicate
with the results averaged. GC-MS analyses were performed on a Shimadzu QP2010 Plus gas
chromatograph-mass spectrometer. A BP20 column was used, and the oven temperature was ramped
from 50 to 220 °C at a rate of 10 °C min-1. UHP grade helium was used as the carrier gas. The screw-
cap autosampler vials used were obtained from Agilent Technologies and were fitted with
PTFE/silicone septa and 0.2 mL micro inserts. The identification of products was confirmed using
1
GC-MS spectroscopy and H NMR spectroscopy and the conversion of substrate to product(s) was
monitored by GC-MS by comparing the peak areas of the product(s) to starting materials. A typical
catalysed Kumada coupling experiment was performed as follows: a Schlenk flask was charged with
the catalyst (2 mol%) to which 10 mL of THF was cannulated in an inert atmosphere. The
organohalide substrate (0.5 mmol) and phenylmagnesium bromide (1.5 mmol) were added
subsequently using an air tight syringe. The mixture was stirred at 25 °C for 4 hours. Aliquots were
taken at regular intervals, which were quenched with cold deionised H2O (1 mL) followed by the
addition of Et2O (1 mL). The organic phase was extracted, dried using anhydrous MgSO4 and
filtered using a plug of silica. The crude products (2-3 drops of the Et2O phase) were diluted in 1 mL
of dichloromethane and collected for GC-MS analysis. The organic layers of selected reactions were
also reduced to dryness under vacuum, dissolved in CDCl3 and analysed using 1H NMR spectra.
2.4
Synthesis
2.4.1 Synthesis of [LmeNi(II)Cl]+(BPh4)-
[Lme2Ag(I)]+(BPh4)- (0.050 g, 0.056 mmol) and Ni(PPh3)2Cl2 (0.077 g, 0.118 mmol) were dissolved
in 20 mL of dry THF. The mixture was left stirring overnight at room temperature under an N2
atmosphere. The resulting yellow mixture was filtered using glass fibre (GF/C) filter paper and the
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filtrate reduced to 10 mL. 40 mL of diethyl ether was slowly added to the solution to precipitate
[LmeNi(II)Cl]+(BPh4)- as a yellow powder. Crystals suitable for X-ray crystallography were grown
by vapour diffusion of diethyl ether into a saturated acetone (2 mL) solution of
1
[LmeNi(II)Cl]+(BPh4)-. Yield: 40%. H NMR (600 MHz, (CD3)2CO): δ 8.29 (br d, 2H, H3), 8.18 (br
d, 2H, H5), 7.70 (br s, 2H, H1), 7.37-7.30 (m, 8H, o-BPh4), 6.92 (t, 3J= 7.3 Hz, 8H, m-BPh4), 6.95 (t,
3J= 7.3 Hz, 4H, p-BPh4) 6.81 (br d, 4H, H2), 6.58 (m, 2H, H4). 13C{1H} NMR (150 MHz,
(CD3)2CO): δ 164.3-163.3 (C ipso of BPh4), 147.6 (C3), 143.9 (C6), 136.9 (C1), 136.0 (o-C of
BPh4), 125.6 (m-C of BPh4), 121.8 (p-C of BPh4), 122.0 (C5), 107.6 (C2), 61.9 (C4) ppm. Elemental
analysis found: C, 65.54; H, 5.07; N, 13.04. Calc. for NiC35H32B N6Cl: C, 65.52; H, 5.03; N, 13.10.
ESI-MS (m/z = 303.05) [M-Cl+OH]+.
2.4.2 Synthesis of [LmeNi(II)Cl]+(PF6)-
[LmeH. ]+(PF6)- (0.090 g, 0.242 mmol) and Ag2O (0.150 g, 0.640 mmol) were mixed in 30 mL of dry
CH2Cl2 and the mixture was left stirring overnight at room temperature under an N2 atmosphere.
Filtration of the dark mixture using glass fibre (GF/C) filter paper gave a colourless filtrate which
was reduced to dryness. The resulting white powder was redissolved in 30 mL of dry THF to which
Ni(PPh3)2Cl2 (0.158 g, 0.242 mmol) was added. The orange suspension was left stirring overnight at
room temperature under an N2 atmosphere. The resulting yellow solution was filtered using glass
fibre (GF/C) filter paper and the filtrate reduced to 10 mL. 40 mL of diethyl ether was slowly added
to the solution to precipitate [LmeNi(II)Cl]+(PF6)- as a yellow powder. Crystals suitable for X-ray
crystallography were grown by vapour diffusion of diethyl ether into a saturated acetone (2 mL)
solution of [LmeNi(II)Cl]+(PF6)-. Yield: 62%. 1H NMR (600 MHz, (CD3)2CO): δ 8.36 (br d, 2H, H3),
13
8.16 (br d, 2H, H5), 7.79 (br s, 2H, H1), 6.92 (br d, 4H, H2), 6.60 (m, 2H, H4). C{1H} NMR (150
MHz, (CD3)2CO): δ 147.6 (C3), 143.9 (C6), 136.9 (C1), 122.0 (C5), 107.6 (C2), 61.9 (C4) ppm.
Elemental analysis found: C, 28.24; H, 2.52; N, 17.90. Calc. for Ni1C11H12PN6F6Cl: C, 28.27; H,
2.59; N, 17.98. ESI-MS (m/z = 303.05) [M-Cl+OH]+.
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2.4.3 Synthesis of [Let Ni(II)]+(BPh4)-2
2
[LetH]+(BPh4)- (0.100 g, 0.242 mmol) and potassium bis(trimethylsilyl)amide (0.150 g, 0.640 mmol)
and Ni(PPh3)2Cl2 (0.158 g, 0.242 mmol) were mixed in 30 mL of dry THF and the resulting yellow
mixture was left stirring overnight at room temperature under an N2 atmosphere. The resulting
yellow solution was filtered using glass fibre (GF/C) filter paper and the filtrate reduced to 10 mL.
40 mL of diethyl ether was slowly added to the solution to precipitate [Let Ni(II)]+(BPh4)-2 as a
2
yellow powder. Crystals suitable for X-ray crystallography were grown by vapour diffusion of
diethyl ether into a saturated acetone (2 mL) solution of [Let Ni(II)]+(BPh4)-2. Yield: 18%. 1H NMR
2
2
(600 MHz, (CD3)2CO): δ 8.11 ( d, J = 2.3 Hz, 2H, H13), 7.98 (br t, 2H, H9a), 7.84 (br d, 2H, H1),
7.60 (d, 2J = 2.3 Hz, 2H, H11), 7.53(d, 2J = 2.2 Hz, 2H, H8), 7.49 (br d, 2H, H6), 7.47 (d, 2J = 2.2 Hz,
3
3
2H, H3), 7.35-7.29 (m, 8H, o-BPh4), 6.94 (t, J= 7.3 Hz, 8H, m-BPh4), 6.80 (t, J= 7.3 Hz, 4H, p-
BPh4), 6.42 (dd, 3J = 2.2 Hz 4H, H12), 6.16 (br m, 2H, H2), 5.61(br d, 2H, H10a), 5.58 (br d, 2H, H10b),
5.04 (br m, 2H, H4a), 4.88 (br m, 2H, H9b), 4.63 (br m, 2H, H4b), 4.37 (br m, 2H, H5a), 4.29 (br m,
2H, H5b). 13C{1H} NMR (150 MHz, (CD3)2CO): δ 151.7 (C7), 147.6 (C3), δ142.5 (C11), 141.9 (C1),
140.8 (C10), 136.0 (o-C of BPh4), 135.0 (C13), 131.1 (C3), 127(C8), 125.6 (m-C of BPh4), 123.5
(C6), 121.8 (p-C of BPh4), 108.9 (C12), 105.5 (C2), 50.6 (C5), 50.0 (C4), 49.4 (C9), 48.7 (C10)
ppm. Elemental analysis found: C, 72.07; H, 5.97; N, 13.04. Calc. for Ni1C74H72B2 N12: C, 73.47; H,
6.00; N, 13.89. ESI-MS: (m/z = 889.4) [M+BPh4]+.
3.
Results and Discussion
3.1
Synthesis and characterisation of [LmeNi(II)Cl]+(BPh4)-
Pro-ligands LmeH and LetH were previously synthesised in our group.[24] The Ni(II)
complex of the Lme pincer ligand was synthesised by in situ silver complexation followed by
transmetallation with nickel(II).[25] Pro-ligand [LmeH]+(BPh4)- was first treated with an excess of
silver(I) oxide in dichloromethane to generate [Lme2Ag(I)]+(BPh4)- in situ prior to treatment with two
7

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molar equivalents of Ni(PPh3)2Cl2 to yield complex [LmeNi(II)Cl]+(BPh4)- as a yellow micro-
crystalline solid in 40% yield (Scheme 1a). Column chromatography only yielded one fraction that
was clearly identifiable as the Ni(II) complex, the rest proved to be an intractable mixture. Multiple
attempts were made to increase the yield of complex [LmeNi(II)Cl]+(BPh4)-, such as using other
Ni(II) salts or an external base (Et3N). Whilst complex [LmeNi(II)Cl]+(BPh4)- could be synthesised
using a base, the yield did not improve.
Scheme 1 Synthesis of complexes (a) [LmeNi(II)Cl]+X- (where X = BPh4 or PF6) and (b)
[Let Ni(II)Cl]+(BPh4)-2.
2
The 1H NMR spectrum of complex [LmeNi(II)Cl]+(BPh4)- exhibited broad resonances for all
hydrogens attributed to the pincer ligand at room temperature. A single set of aromatic resonances
between 7.5 and 8.4 ppm were attributed to the pyrazolyl and carbene donors. The low number of
resonances indicated that the complex had a plane of symmetry through the carbene-Ni-Cl axis, with
the ligand likely coordinated in a tridentate fashion to Ni(II) through all three donors (κ3-NCN).
There were significant differences between the chemical shifts of the resonances due to pyrazolyl
hydrogens of the Ni(II) complex in comparison to those observed for the analogous silver(I)
intermediate. The pyrazolyl resonances of intermediate [Lme2Ag(I)]+(BPh4)- occurred at 7.60 ppm
and 8.13 ppm, whereas the pyrazole resonances of the [LmeNi(II)Cl]+(BPh4)- occurred at 8.26 ppm
and 8.29 ppm respectively. The downfield shift of the pyrazolyl resonances of [LmeNi(II)Cl]+(BPh4)-
indicated that the transmetallation reaction resulted in coordination of the two pyrazolyl ligand arms
8

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to the Ni(II) centre, unlike in [Lme2Ag(I)]+(BPh4)- where the N-donors remain unbound. The
integration ratios of ligand proton resonances to those of the BPh4 counterion demonstrated that the
1
ratio of ligand to counterion was 1:1. As the H NMR spectrum indicates that all ligand donors are
bound to the metal centre, it is highly likely that a Cl co-ligand is also bound to the Ni(II) centre.
13C{1H} NMR and 2D NMR spectroscopy did not reveal any valuable information that explained the
1
broad signals in the H NMR spectrum. There are different possibilities for the broad signals in the
NMR spectrum including purity, paramagnetism and conformational flexibility of the complex
[LmeNi(II)Cl]+(BPh4)- in solution state. Further characterisation was therefore carried out to identify
the cause of the broad signals in the NMR spectrum.
Crystals suitable for X-ray analysis were grown by vapour diffusion of diethyl ether into a
saturated solution of [LmeNi(II)Cl]+(BPh4)- in acetone. X-ray crystal structure analysis of
[LmeNi(II)Cl]+(BPh4)- showed that Ni(II) had a square-planar geometry, with the ligand Lme
coordinated in a tridentate fashion via its NCN donors as expected (Fig. 2). The central NHC is
twisted out of the square plane, such that the two methylene linkers are positioned on opposite faces
of the plane at a torsion angle of 30.1o. Mass spectrometry and elemental analysis of crystals of
[LmeNi(II)Cl]+(BPh4)- showed it to be pure. However, 1H NMR analysis of the isolated crystals used
for solid state structure determination of [LmeNi(II)Cl]+(BPh4)- and elemental analysis produced a
spectrum identical to that obtained for the previously synthesised yellow powder with broad signals.
This evidence indicated that the broad resonances in the NMR spectrum of [LmeNi(II)Cl]+(BPh4)-
was not due to the presence of impurities.
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Fig. 2. X-ray crystal structure of (LmeNi(II)Cl]+(BPh4)- as viewed (a) front on and (b) side on.
Ellipsoids are shown at 50% probability level, hydrogens are omitted for clarity, BPh4 anion is
omitted in (b).
As we observed in previous work, facial coordination of ligand Lme to Ru(II) is possible.[25]
Complex [LmeNi(II)Cl]+(BPh4)- could therefore exist in an equilibrium between square planar and
1
tetrahedral conformations (Fig. 3) which could lead to broadened signals in the H NMR spectrum
due to the contribution of a tetrahedral paramagnetic nickel(II) species.
The
Ni(PPh3)2Cl2
precursor used for the synthesis of [LmeNi(II)Cl]+(BPh4)- itself exists in such an equilibrium.[26, 27]
Analysis of complex [LmeNi(II)Cl]+(BPh4)- in acetone solution using UV/Vis spectroscopy revealed
the presence of only one strong signal at 460 nm which is characteristic of a square planar Ni(II)
1
compound.[28] Evan’s method using H NMR at 25 °C demonstrated that there were no
paramagnetic nickel(II) species present.[29, 30] These results confirmed that the broadness observed
in the 1H NMR spectrum was not due to paramagnetism.
Fig. 3. Proposed equilibrium reaction between the square planar and tetrahedral conformations of
complex [LmeNi(II)Cl]+(BPh4)-.
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1
To test our last remaining hypothesis that broadness of the H NMR spectrum is due to the
flexible ligand, [LmeNi(II)Cl]+(BPh4)- was analysed using variable temperature 1H NMR
spectroscopy (Fig. 4). Despite the resonances not completely sharpening up at the lower temperature
1
they did shift position; the H NMR spectrum of complex [LmeNi(II)Cl]+(BPh4)- still showed the
same number of resonances as observed at room temperature. It seems likely that the tridentate
1
coordination of the ligand is preserved at the lower temperatures and that the broadness in the H
NMR spectrum of complex [LmeNi(II)Cl]+(BPh4)- is due to conformational flexibility of the
complex arising from changes in the relative orientations of the pendant pyrazole arms of the ligand,
see Fig. 5.
Fig. 4 Variable temperature NMR (600 MHz, (CD3)2CO) of complex [LmeNi(II)Cl]+(BPh4)-.
Assignments correspond to the labelling shown in the figure.
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Fig. 5 Conformational flexibility arising from changes in the relative orientations of the pyrazole
arms of complex [LmeNi(II)Cl]+(BPh4)-.
3.2
Synthesis and characterisation of [LmeNi(II)Cl]+(PF6)-
Complex [LmeNi(II)Cl]+(BPh4)- proved to be difficult to synthesise in a reasonable yield, requiring a
number of purification steps, and NMR characterisation of the product proved difficult. As the
choice of counter ion used in a metal complex can often affect ease of synthesis and purification, and
-
modify catalytic activity,[31] the analogous PF6 derivative of complex [LmeNi(II)Cl]+(BPh4)- was
synthesised. Pro-ligand [LmeH]+(PF6)- was synthesised following a similar method reported
previously.[24] However, the counterion exchange reaction of the [LmeH]+Cl- salt was performed
with 1.2 equivalents of NH4PF6 instead of NaBPh4. Pro-ligand [LmeH]+(PF6)- was isolated as a
crystalline white solid in 56% yield. The 1H NMR spectrum of [LmeH]+(PF6)- was analogous to that
of [LmeH]+(BPh4)-, exhibiting six 1H resonances with the characteristic imidazolium hydrogen signal
appearing in the expected region at 9.56 ppm. [LmeH]+(PF6)- was reacted with excess Ag2O to
produce [Lme2Ag(I)]+(PF6)- as a pale brown solid (Scheme 1a), to which Ni(PPh3)2Cl2 was added in
1
situ to afford [LmeNi(II)Cl]+(PF6)- as an orange powder in 62% yield. The H NMR spectrum of
-
[LmeNi(II)Cl]+(PF6)- was similar to the analogous BPh4 complex; the 1H NMR spectrum of the PF6
complex exhibited five broad ligand resonances. Unfortunately, just as [LmeNi(II)Cl]+(BPh4)- could
not be characterized using 13C NMR or 2D NMR techniques at 25 °C, neither could
[LmeNi(II)Cl]+(PF6)- as the resonances were too broad. Analysis of [LmeNi(II)Cl]+(PF6)- using
UV/Vis spectroscopy produced a single peak at 460 nm, similar to the UV/Vis spectrum of
[LmeNi(II)Cl]+(BPh4)- which suggested that the broadness was not a result of any paramagnetic
species. Low temperature NMR spectroscopy at -50 °C allowed characterisation of the complex to be
1
achieved using 13C NMR and 2D NMR techniques. The H NMR spectrum of [LmeNi(II)Cl]+(PF6)-
at room temperature indicated a tridentate coordination mode to a square planar Ni(II) centre, similar
1
to of [LmeNi(II)Cl]+(BPh4)-. The only major difference between their H NMR spectra was that the
12

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resonances due to H2 and H4 are well separated in [LmeNi(II)Cl]+(PF6)- compared to those of
[LmeNi(II)Cl]+(BPh4)-, suggesting the change in counterion altered the chemical environment
experienced by the ligand.
Crystals of [LmeNi(II)Cl]+(PF6)- suitable for X-ray analysis were grown by vapour diffusion
of diethyl ether into a saturated solution of [LmeNi(II)Cl]+(PF6)- in acetone. Analysis using X-ray
crystallography confirmed the expected κ3-NCN symmetric structure with the Cl co-ligand
terminally bound to the Ni(II) metal centre, in a trans position relative to the carbene (Fig. 6). Mass
spectrometry shows a dominant signal at 303.05 m/z which was attributed to the cationic fragment
[LmeNi(II)OH]+. The structure with the exemption of the counter ion proved to be isostructural to
[LmeNi(II)Cl]+(BPh4)-.
Fig. 6. X-ray crystal structure of complex [LmeNi(II)Cl]+(PF6)- with ellipsoids at the 50%
probability level, hydrogens have been omitted for clarity.
3.3
Synthesis and characterisation of [Let Ni(II)]+(BPh4)-2
2
The use of a pincer ligand containing a longer alkyl linker between the central imidazolyl and
pendant pyrazolyl moieties can affect the coordination of the ligand to a metal centre. Therefore, the
pro-ligand [LetH]+(BPh4)- with the ethylene linker between the carbene and pyrazole units was
used.[24] As before, we opted for a silver transmetallation route to attempt to generate the final
complex. Complex [Let Ag(I) ]+(BPh4)- was obtained as a white solid by reaction of ligand
2
1
[LetH]+(BPh4)- with excess Ag2O. The H NMR spectrum of the white solid contained nine proton
resonances, six of which were attributed to the ligand protons and the remaining three resonances are
13

Nair et al. Synthesis and Catalytic Activity of Nickel(II) Complexes of NCN Pincer Ligands
ACCEPTED MANUSCRIPT
assigned to the BPh4 protons. In a similar fashion to [Lme2Ag(I)]+(BPh4)-, the absence of the
1
imidazolium proton resonance in the H NMR spectrum of [Let Ag(I)]+(BPh4)- indicated successful
2
complexation of silver(I) to the ligand. The ratio of the integrals of the proton resonances indicate
that the ligand to BPh4 proton ratio is 2 : 1 which was indicative of a homoleptic structure for the
silver complex [Lme2Ag(I)]+(BPh4)-. With the aim of synthesising a Ni(II) complex of ligand Let two
equivalents of Ni(PPh3)2Cl2 were added to [Let Ag(I)]+(BPh4)-. However, the reaction of
2
[Let Ag(I)]+(BPh4)- and Ni(PPh3)2Cl2 resulted in a mixture of unreacted starting material and
2
insoluble silver salts. We attempted numerous repeats of this reaction, however all proved
unsuccessful, hence this route was abandoned. An alternate approach to the synthesis of a Ni(II)
complex with pro-ligand [LetH]+(BPh4)- was to use an external base to deprotonate the imidazolium
salt in situ prior to addition of the nickel precursor (Ni(PPh3)2Cl2). Triethylamine, potassium
carbonate and potassium hexamethyl-disilazide (KHMDS) were all tested as bases for this reaction,
however only KHMDS proved successful at deprotonating [LetH]+(BPh4)-. Thus, the precursor
Ni(PPh3)2Cl2 was reacted with ligand [LetH]+(BPh4)- in the presence of excess KHMDS producing
bis-ligated complex [Let Ni(II)]+(BPh4)-2 as a yellow solid in 18% yield (Scheme 1b). We had hoped
2
to generate a mono-ligated Ni(II) complex rather than the bis-ligated species due to our anticipation
that a bis-ligated complex would be an inferior catalyst. Many attempts were made to improve the
yield of complex [Let Ni(II)]+(BPh4)-2 and to produce a mono-ligated complex, such as altering the
2
reaction temperatures or ligand to metal ratios. However, any deviations from the synthetic method
shown in Scheme 1b resulted in unsuccessful reactions which did not show evidence of any isolable
species.
1
The H NMR spectrum of the nickel(II) complex [Let Ni(II)]+(BPh4)-2 revealed that the
2
1
complex existed as an asymmetric species in solution. The H NMR spectrum contained 19 distinct
resonances, 16 of which were assigned to the protons of two ligands which suggested the complex
had two NHC donors. The analysis of the NMR spectrum indicated that each ligand of complex
14

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[Let Ni(II)]+(BPh4)-2 coordinated through only two donors, and that there was one non-coordinated
2
pyrazolyl arm giving rise to the unsymmetrical structure. The ratio of the integrals of resonances due
to the ligand and counterions indicates that the ratio of ligand to BPh4 is 2:2, again indicating a likely
1
bis-ligated complex. The absence of PPh3 resonances in the H NMR spectra of complex
[Let Ni(II)]+(BPh4)-2 indicated that the two phosphine co-ligands have been displaced from the
2
nickel centre and gave further credence to the proposal of a bis-ligated complex. Further analysis of
1
[Let Ni(II)]+(BPh4)-2 by variable temperature H NMR spectroscopy revealed that the resonance of
2
one of the ethylene hydrogens was shifted to higher ppm at lower temperatures; perhaps indicating a
strong C-H π-interaction with a pyrazolyl unit on the second ligand, see Fig. 7.
Fig 7. Variable temperature 1H NMR spectra (600 MHz, (CD3)2CO) of complex [Let Ni(II)]+(BPh4)-
2
2. ’ in the 1H NMR spectra denotes that an individual 1H resonance is shifted due to interaction with
the opposite ligand. Assignments correspond to the labelling shown in the figure.
Crystals of [Let Ni(II)]+(BPh4)-2 suitable for X-ray analysis were grown by vapour diffusion
2
of diethyl ether into a saturated solution of [Let Ni(II)]+(BPh4)-2 in acetone. X-ray crystal structure
2
analysis revealed that Ni(II) had a square planar geometry, with two ligands coordinated in a
bidentate fashion through their carbene carbon and pyrazolyl nitrogen atoms (κ2-CN) forming a
homoleptic complex, see Fig. 8. The solid-state structure backs up our conclusions drawn from the
15

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1H NMR analysis. The significant shift of the resonances due to H8’ to higher ppm ranges as
1
observed in H NMR was confirmed by X-ray analysis to be due to C-H π-interaction of the proton
to a pyrazolyl ring with a distance of 2.845 Å. This distance is typical of C-H π-interactions of
hydrogen atoms to an aryl ring.[32] A similar bis-ligated Ir(I) complex containing ligand Let was
reported previously by us, where the two ligands were located trans to each other.[24] However,
[Let Ni(II)]+(BPh4)-2 shows a bidentate coordination of the ligand Let to the metal centre whereas the
2
Ir(I) counterpart had a monodentate coordination of the ligand to the metal centre. The solid state
structure of [Let Ni(II)]+(BPh4)-2 confirmed our suspicion that access to the metal centre is severely
2
hindered due the two organic ligands, this may have detrimental effects on catalysis activity.
Fig. 8 X-ray crystal structure of [Let Ni(II)]+(BPh4)-2 as viewed (a) straight on and (b) viewed down
2
C1-Ni-C2 axis. Ellipsoids are shown at the 50% probability level, hydrogen atoms and the two BPh4
anions have been omitted for clarity.
3.4
Comparison of X-ray Crystal Structures
All three Ni(II) complexes, [LmeNi(II)Cl]+(BPh4)-, [LmeNi(II)Cl]+(PF6)- and
[Let Ni(II)]+(BPh4)-2, adopt a square planar geometry. The main difference between the structures of
2
the complexes is that there are two ligands bound to the Ni(II) centre in a bidentate coordination
mode in complex [Let Ni(II)]+(BPh4)-2 (Fig. 8) whereas there is only one ligand coordinated to Ni(II)
2
in [LmeNi(II)Cl]+(BPh4)- (Fig. 2) and [LmeNi(II)Cl]+(PF6)- (Fig. 6). This is clearly as a result of the
extra CH2 in the linker between carbene and pyrazole (methylene vs ethylene). The bond lengths of
16

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the inner coordination sphere of complexes [LmeNi(II)Cl]+(BPh4)- and [LmeNi(II)Cl]+(PF6)- are
nearly identical as expected due to their isostructural solid state structures. Analogous Ni(II) square
planar carbene complexes in the literature where a NHC ligand contains pendant pyridyl arms have
similar Ni(II)-carbene (1.837 Å) and Ni(II)-Cl (2.236 Å) bond lengths to those of complexes
[LmeNi(II)Cl]+(BPh4)- and [LmeNi(II)Cl]+(PF6)-.30 The Ni(II)-carbene bond length in complex
[Let Ni(II)]+(BPh4)-2 is slightly longer than that observed for [LmeNi(II)Cl]+(BPh4)- and
2
[LmeNi(II)Cl]+(PF6)-. As all three complexes consist of two pyrazolyl groups trans to each other, the
Ni(1)-N(1) and Ni(1)-N(2) bond lengths are similar in all three complexes; the maximum variation in
Ni(1)-N bond lengths is 0.012 Å. The two carbene carbons (C1 and C2) and two pyrazolyl nitrogens
(N1 and N2) in complex [Let Ni(II) ]+(BPh4)-2 show no distortion from linearity as the Ni(II) ion lies
2
at an inversion centre (C-Ni-C/N-Ni-N: 180.00(2)°). However, similar alignments in complexes
[LmeNi(II)Cl]+(BPh4)- and [LmeNi(II)Cl]+(PF6)- exhibit slight distortion from linearity
[LmeNi(II)Cl]+(BPh4)-: N-Ni-N: 176.02(2)°, [LmeNi(II)Cl]+(PF6)-: N-Ni-N: 178.88(2)°) which is
likely due to strain arising from the tridentate coordination mode of Lme. There are also significant
rotations of the NHC ligands out of the square plane in all three complexes, giving a torsion angle
(N2-Ni-C1-Imidazolium N4) of 33° in complex [LmeNi(II)Cl]+(BPh4)- and 30° [LmeNi(II)Cl]+(PF6)-
and 59° in complex [Let Ni(II)]+(BPh4)-2.
2
Table 1 Selected bond lengths (Å) and angles (°) for [LmeNi(II)Cl]+(BPh4, [LmeNi(II)Cl]+(PF6)- and
[Let Ni(II)]+(BPh4)2.
2
Atoms
Ni-C1
Ni-C2
Ni-N1
Ni-N2
Ni-Cl
[LmeNi(II)Cl]+(BPh4)- [LmeNi(II)Cl]+(PF6)- [Let Ni(II)]+(BPh4)-2
2
Bond lengths
1.850(4)
----
1.847(2)
-----
1.804(1)
1.804(1)
1.995(8)
1.995(8)
1.908(3)
1.887(3)
1.902(2)
1.900(2)
2.200(1)
89.8(2)
2.220(1)
88.4(1)
-----
Bond angles C1-Ni-N1
91.6(2)
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Nair et al. Synthesis and Catalytic Activity of Nickel(II) Complexes of NCN Pincer Ligands
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C1-Ni-N2
C1-Ni-Cl
C1-Ni-C2
N1-Ni-N2
88.2(2)
176.03(2)
----
88.6(1)
178.88(2)
----
88.4(2)
----
180.00(2)
180.00(2)
177.10(2)
175.68(2)
4. Catalytic Activity of Ni(II) Complexes for Kumada Cross Coupling of Aryl Halides
Ni(II) complexes are known to be efficient catalysts for the Kumada cross coupling reaction.
Among the best nickel catalyst systems for the Kumada cross coupling reaction include nickel ions
complexed to calixarene ligands. These catalytic systems achieve complete conversions of substrate
at room temperature within 1 h, and can catalyse the reaction at low catalyst loadings (0.1-0.02
mol%).[33, 34] However, only a few Ni complexes of weakly coordinating ligands maintain high
activity at elevated temperatures without suffering catalyst decomposition.[15] Thus, our complexes
[LmeNi(II)Cl]+(BPh4), [LmeNi(II)Cl]+(PF6)- and [Let Ni(II)]+(BPh4)-2 based on hemilabile ligands
2
were evaluated as catalysts for the Kumada cross coupling reaction of aryl halides.
4.1
Catalytic Screen for Kumada Cross Coupling Reactions of Chlorobenzene and
Bromobenzene.
Complexes [LmeNi(II)Cl]+(BPh4)-, [LmeNi(II)Cl]+(PF6)- and [Let Ni(II)]2+(BPh4)-2 were initially
2
tested as catalysts for the Kumada cross coupling reactions of chlorobenzene and bromobenzene with
phenylmagnesium bromide (Table 2).
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Table 2 Kumada cross coupling reactions of PhBr and PhCl with PhMgBr catalysed by
[LmeNi(II)Cl]+(BPh4)- , [LmeNi(II)Cl]+(PF6)- and [Let Ni(II)]2+(BPh4)-2.
2
entry
substrate
catalyst
%conv. (3.5 h)
[LmeNi(II)Cl]+(BPh4)-
87 (80)*
1
2
3
[LmeNi(II)Cl]+(PF6)-
90
41
[Let Ni(II)]2+(BPh4)-2
2
[LmeNi(II)Cl]+(BPh4)-
[LmeNi(II)Cl]+(PF6)-
>99 (43)*
40
4
5
6
[Let Ni(II)]2+(BPh4)-2
39
2
Reagents and conditions: Aryl halide (0.5 mmol), phenylmagnesiumbromide (1.5 mmol), catalyst (2
mol%), THF, 25 °C. * 1 mol% catalyst. %Conversions analysed at 3.5 h using GC-MS.
Complex [Let Ni(II)]2+(BPh4)-2 proved to be the least active catalyst of the three catalysts
2
tested, achieving only 40% conversion after 3.5 h of both chlorobenzene and bromobenzene (Table
2, entries 3 and 6). In comparison both [LmeNi(II)Cl]+(BPh4)- and [LmeNi(II)Cl]+(PF6) gave more
than double the conversion after the same reaction time. The low activity of complex
[Let Ni(II)]2+(BPh4)-2 is likely due to its coordinative saturation, as the Ni(II) centre is ligated by two
2
of the ethylene containing ligands, LEt. Any conversion of substrate promoted by
[Let Ni(II)]+(BPh4)-2 likely occurred due to the lability of the weakly coordinating pyrazolyl groups,
2
allowing substrate access to the active metal centre. The pyrazolyl groups are positioned trans to
each other along with the central NHC groups. Previous work reported in the literature has
demonstrated that nickel bis-carbenic systems have reduced activity for the Kumada cross coupling
reaction in relation to mono-carbenic systems.[35]
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Complex [LmeNi(II)Cl]+(BPh4)- proved to be the most effective catalyst overall for the
Kumada cross coupling reactions of chlorobenzene and bromobenzene, achieving high conversions
for both substrates after 3.5 h (Table 2, entries 1 and 4). The BPh4 anion gives similar performance to
PF6 when using bromobenzene but when chlorobenzene was used the conversion was more than
-
doubled. This was attributed to the lower coordinating ability of BPh4 as a counterion than that of
-
PF6 , which typically leads to higher catalyst efficiency.[31, 36] Due to the high conversions
achieved using [LmeNi(II)Cl]+(BPh4)-, the reactions were repeated at a lower catalyst loading of 1
mol% to determine whether the catalyst maintains the activity at a lower loading. Whilst the
conversion of bromobenzene only suffered a slight decrease in conversion, the conversion of
chlorobenzene halved (Table 2, * entries).
4.2
Substrate scope of Kumada cross coupling reaction using [LmeNi(II)Cl]+(BPh4)-
As complex [LmeNi(II)Cl]+(BPh4)- proved to be the most effective catalyst under
investigation here for the cross coupling reactions of bromobenzene and chlorobenzene, it was tested
as a catalyst for the transformation of a selection of substrates with phenylmagnesiumbromide (Table
3). We elected to use a catalyst loading of 2 mol%, as this gave the best conversion. Unsurprisingly,
iodobenzene gave quantitative conversion to biphenyl product. Of all the substrates featuring a
substituent that were tested for conversion to biphenyl products using [LmeNi(II)Cl]+(BPh4)-, the
highest conversion was achieved for the substrate containing an electron withdrawing para-CF3
substituent on the aryl ring (90%). However, the substrate with two meta-CF3 groups gave only 68%
conversion. Disappointingly, para-cyanochlorobenzene gave no conversion to product. In
comparison, substrates with electron donating substituents (i.e. methoxy and methyl) gave lower
conversion to products. It was interesting to observe that the choice of halide altered the selectivity
of cross coupling reactions to form biphenyl or triphenyl products. The catalysed reaction of
phenylmagnesium bromide with the 1,3-dibromobenzene resulted in selective formation of the
monosubstituted 1-bromo-3-phenylbenzene. However, the catalysed reaction of the analogous 1,3-
20

Nair et al. Synthesis and Catalytic Activity of Nickel(II) Complexes of NCN Pincer Ligands
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dichlorobenzene with phenylmagnesium bromide results in the selective production of the
disubstituted 1,3-diphenylbenzene, although the yield was low. This result was unexpected
considering 1,3-dibromobenzene is more likely to produce the disubstituted product due to the
propensity of bromide groups to be more readily substituted in comparison to chloride groups.
Table 3 Range of substrates catalysed by [LmeNi(II)Cl]+(BPh4)- in Kumada cross coupling reactions.
entry
1
R-X
% conv.
entry
5
R-X
% conv.
>99
90
2
78
50
67 (mono)
16 (di)
6
7
8
3
4
68
0
Reagents and conditions: Aryl halide (0.5 mmol), phenylmagnesiumbromide (1.5 mmol), catalyst (2
mol%), THF, 25 °C. %Conversions analysed at 4 h using GC-MS. (m = mono-substituted product,
d= di-substituted product).
Overall, our nickel(II) catalyst [LmeNi(II)Cl]+(BPh4)- has proved to be an effective catalyst
for the room temperature Kumada cross coupling. Complete conversion of iodobenzene and
chlorobenzene was observed after just 4 hours at room temperature. This places this catalyst above
the NHC-Ni(II) catalysis reports from Wang[11, 15] and Louie (though mesitylmagnesium bromide
was used).[35] The levels of conversion of substituted aryl-halides obtained using catalyst
[LmeNi(II)Cl]+(BPh4)- were slightly less than those reported by Wang.[15] The activity of our
21

Nair et al. Synthesis and Catalytic Activity of Nickel(II) Complexes of NCN Pincer Ligands
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catalysts is similar to that reported by Semeril and Matt observed using their calixarene-based Ni(II)
catalysts.[33, 34]
5. Conclusions
Here, we synthesized three Ni(II) complexes using ligands containing both carbene and
pyrazolyl donor groups with either a methylene or ethylene linker joining the pendant pyrazolyls to
the central carbene group. The Ni(II) complexes featuring the shorter methylene linker,
[LmeNi(II)Cl]+(BPh4)- and [LmeNi(II)Cl]+(PF6)-, had similar solid-state structures. The complex with
-
-
the BPh4 counterion outperformed the PF6 analogue when used for the Kumada catalysis reactions,
-
which has been attributed to the weaker coordination of the BPh4 counterion. The Ni(II) complex of
the longer ethylene linker, [Let Ni(II)]+(BPh4)-2, was found to have limited catalytic activity due to
2
the coordinative saturation of the complex, as shown in its solid-state crystal structure. Complex
[LmeNi(II)Cl]+(BPh4)- was found to be the most active catalyst, it proved compatible with different
functional groups and performed more difficult di-substitution reactions. Excellent to moderate
yields were achieved in the Kumada cross coupling reactions at room temperature with substrates
containing electrodonating substituents. Thus, complex [LmeNi(II)Cl]+(BPh4)- was found to be a
good general catalyst for the Kumada cross coupling reaction.
6. Acknowledgements
Some of this work was supported under Australian Research Council’s Discovery Projects
funding scheme (project number DP110101611). AGN is grateful for an Australian Postgraduate
Award (APA) awarded by the Australian Government, administered by the University of New South
Wales and Macquarie University. We thank Macquarie University, the University of New South
Wales and the Australian Government for financial support. We acknowledge Dr Samantha Binding
for useful discussions on the preparation of this manuscript. The authors would like to acknowledge
the NMR Facilities at Macquarie University and within the Mark Wainwright Analytical Centre at
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the University of New South Wales for NMR support. We acknowledge the Campbell
Microanalytical Laboratory, University of Otago, New Zealand, and the Research School of
Chemistry, Australian National University, for elemental analyses.
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24

ACCEPTED MANUSCRIPT
Highlights
•
•
•
Three Ni(II) complexes have been synthesised from a carbene-bis-pyrazole ligand.
The complexes all exhibited a square-planar coordination geometry.
The complexes acted as catalysts in Kumada cross coupling reactions.