Facile Suzuki-Miyaura coupling of activated aryl halides using new CpNiBr(NHC) complexes

Article abstract of DOI:10.1016/j.jorganchem.2016.03.017

Nine new Ni(II)-NHC complexes, [CpNiBr(NHC)], were synthesised from nickelocene and the corresponding symmetric or asymmetric alkyl/-benzyl/phenylethyl imidazolium bromide ligands in relatively high yield. Access to each of the synthesised symmetric or asymmetric alkyl/benzyl/phenylethyl imidazolium bromide salts was obtained through deprotonation of imidazole, followed by treatment with an alkyl- or aryl halide, which is subsequently followed with reaction of a secondary alkyl-, benzyl-, or phenylethyl halide. The series of [CpNiBr(NHC)] exhibited catalytic activity in the Suzuki-Miyaura coupling of activated aryl halides with phenylboronic acid to give the respective biphenyl and biphenyl-containing products. In general, the more electron-donating NHC-bearing Ni complexes showed higher activity with aryl halides bearing electron-withdrawing functionalities including carboxaldehyde moieties. All complexes were characterised by ¹H and ¹³C NMR spectroscopy, FT-IR spectroscopy, CHN and MS analyses, along with six selected single crystal X-ray structures that are reported here.

Full text of DOI:10.1016/j.jorganchem.2016.03.017

Accepted Manuscript
Facile Suzuki-Miyaura coupling of activated aryl halides using new CpNiBr(NHC)
complexes
Frederick P. Malan, Eric Singleton, Petrus. H. van Rooyen, Marilé Landman
PII:
S0022-328X(16)30092-4
10.1016/j.jorganchem.2016.03.017
JOM 19442
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 17 January 2016
Revised Date: 12 March 2016
Accepted Date: 16 March 2016
Please cite this article as: F.P. Malan, E. Singleton, P.H. van Rooyen, M. Landman, Facile
Suzuki-Miyaura coupling of activated aryl halides using new CpNiBr(NHC) complexes, Journal of
Organometallic Chemistry (2016), doi: 10.1016/j.jorganchem.2016.03.017.
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Facile Suzuki-Miyaura coupling of activated aryl halides using new CpNiBr(NHC)
complexes
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Frederick P. Malan, Eric Singleton, Petrus. H. van Rooyen and Marilé Landman*
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Department of Chemistry, University of Pretoria, 02 Lynnwood Road, Hatfield, Pretoria, 0002, South
Africa. *E-mail: marile.landman@up.ac.za
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Abstract
Nine new Ni(II)-NHC complexes, [CpNiBr(NHC)], were synthesised from nickelocene and
the corresponding symmetric or asymmetric alkyl/-benzyl/phenylethyl imidazolium bromide
ligands in relatively high yield. Access to each of the synthesised symmetric or asymmetric
alkyl/benzyl/phenylethyl imidazolium bromide salts was obtained through deprotonation of
imidazole, followed by treatment with an alkyl- or aryl halide, which is subsequently
followed with reaction of a secondary alkyl-, benzyl-, or phenylethyl halide. The series of
[CpNiBr(NHC)] exhibited catalytic activity in the Suzuki-Miyaura coupling of activated aryl
halides with phenylboronic acid to give the respective biphenyl and biphenyl-containing
products. In general, the more electron-donating NHC-bearing Ni complexes showed higher
activity with aryl halides bearing electron-withdrawing functionalities including
carboxaldehyde moieties. All complexes were characterised by 1H- and 13C-NMR
spectroscopy, FT-IR spectroscopy, CHN and MS analyses, along with six selected single
crystal X-ray structures that are reported here.
Keywords: Nickel; N-heterocyclic carbene; Suzuki-Miyaura coupling

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1.
Introduction
N-Heterocyclic carbenes (NHCs) as a class of metal-stabilising “wonder ligands” remain the
ligands of choice in many organometallic complexes, with specific application in
homogeneous catalysis, organocatalysis, and medicinal chemistry [1-4]. This is mainly due to
the support that NHCs provide through the possibility of reliable, predictable, and extensive
steric- and electronic tuning [2,3b,5-7] in the design of a model complex with the specific
application in mind. With the ever-increasing number of transition metal-NHCs reported,
NHC-complexes of Rh [3a,8], Ru [9], Ni [7,10-14], Pd [4a,6b,15,16], Ag [17], and Au
[17,18] remain to be the most abundant in literature, noting that Ni-NHC complexes received
considerable attention only during the last decade [11,13,14]. The reaction of nickelocene
with bis(alkyl/aryl)imidazolium halides to yield the complexes [CpNiX(NHC)] (X = Cl, Br,
I), represents one of the most frequently employed and facile routes into cyclopentadienyl
nickel(II) NHC systems [7,11-13,19,21-22]. Since the discovery of this atom-economical
reaction by Cowley et al. [23] in 2000, the series of substituted [(η5-C5R5)NiX(NHC)] (R =
H, Me; X = Cl, Br, I, SPh) complexes has expanded to constitute a relatively well studied
class of Ni(II) NHC complexes, predominantly employed in synthetic and catalytic
applications [7,11-13,19,21-22].
In homogeneous catalysis, Ni-NHCs occupy an important position in carbon-carbon and
carbon-heteroatom organic transformations offering access to an impressive array of valuable
molecules which could previously be obtained only via expensive Ru and Pd catalysts
[10,11b,12,15,19,20,24,25]. In the plethora of C-C coupling reactions, the Suzuki-Miyaura
reaction has become one of the most studied for catalytic applications due to its tolerance of
functional groups and low toxicity of its by-products [4d,15,19,20a,25,26]. Furthermore,
[CpNiX(NHC)] (X = halide) and related systems have provided equal or improved catalytic
activity in some C-C coupling reactions when compared to the traditional Pd catalysts
[10,12,20,25,27]. In spite of this, the bromo complexes in the series of [CpNiBr(NHC)]
compounds reported are few when compared to their chloro- and iodo-analogues [10,19]. The
known [CpNiBr(NHC)] complexes (NHC = imidazolium backbone) reported to date have
been illustrated in Figure 1.

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Figure 1: Known [CpNiBr(NHC)] complexes reported by various groups [5a,11e,12,21,22b].
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In this study, we expand the existing bromide series by reporting nine novel [CpNiBr(NHC)]
complexes, where both symmetric- and asymmetric NHC ligands are employed. All new
complexes synthesised were characterised, and their catalytic activity evaluated in the
Suzuki-Miyaura (SM) coupling reaction using activated aryl chlorides and bromides. One of
these substrates, 4-chlorobenzaldehyde, bearing an electron-withdrawing functional group,
has to date not been fully investigated previously as a substrate in [CpNiX(NHC)] (X =
halide) catalysed SM reactions.
2.
Material and Methods
2.1
General
All experiments were carried out under an argon atmosphere using standard Schlenk
techniques. Solvents were dried prior to use using standard techniques [28]. Column
chromatography was carried out under inert argon atmospheres using silica gel (particle size
0.063-0.200 mm) as the stationary phase. The imidazolium bromide ligands L1 – L9 (Figure
2) with general formula [HImRR']Br with R, R' = Me, Bn, (CH2)2Ph, 4-NO2Bn were
synthesised and purified according to literature procedures [2,13,14,19,21]. NMR
characterisation confirmed the molecular structures of the synthesised ligands (see
Supplementary Information for data), which correlated with literature data. All other

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chemicals were purchased from Sigma-Aldrich and used without further purification. 1H (300
MHz) and 13C{H} (76 MHz) NMR spectra were recorded on a Bruker ARX-300
spectrometer using either CDCl3, or (CD3)2CO solutions. All measurements were performed
at ambient temperature (~296 K), unless otherwise noted. Chemical shifts were referenced to
the internal residual protio solvent impurity at δH 7.24 (CDCl3) or 2.04 ((CD3)2CO); or
carbon signals at δC 77.0 (CDCl3), or 29.8 and 206.3 ppm ((CD3)2CO). Solid state FT-IR
experiments were carried out on a Perkin Elmer Spectrum RXI FT-IR spectrometer as
pressed KBr pellets in air. Microanalytical analyses (%CHNS) were obtained using a Thermo
Scientific Flash 2000 elemental analyzer fitted with a TCD detector. All GC/MS analyses
were carried out on a Hewlett Packard (HP) GC 1530A coupled to an Agilent 5975C mass
selective detector (MSD). MS (ESI) and MS/MS measurements were performed on a Waters
SYNAPT G2 LC-MS spectrometer.
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2.2
General synthesis of [CpNiBr(NHC)] complexes (1 – 9)
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A suspension of the alkyl/benzyl/phenylethyl imidazolium bromide (3 mmol) in THF (10
mL) with [Ni(C5H5)2] (0.57 g, 3 mmol) was heated under reflux between 3 – 16 hours
(depending on the NHC ligand). The reaction mixture was concentrated in vacuo, and
purified by silica gel column chromatography, using gradient elution with hexane and
dichloromethane. Red to red-brown powders were obtained in relatively high yields.
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[CpNiBr{Im(Bn)2}] (1): Yield: 79%. IR (cm-1): 3168 (ν(=CH), w), 3136 (ν(=CH), w), 3110
(ν(=CH), w), 3028 (ν(-CH), w), 2929 (ν(-CH), w), 1569 (ν(=CH), w), 1495 (δ(-CH), sym, m),
1453 (δ(-CH), sym, s), 1430 (δ(-CH), asym, m), 1404 (δ(-CH), asym, s), 1358 (δ(-CH), s),
1230 (ν(-CN), s), 1028 (ν(CN), w), 794 (δ(=CH), s), 755 (s), 725 (δ(-CH), s), 715 (s), 685 (s).
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1H-NMR (CDCl3, δH) 5.12 (s, C5H5, 5H), 6.09 (dd, JHH = 15 and 65 Hz, CH2, 4H), 6.79 (s,
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NCH, 2H), 7.26 – 7.40 (m, C6H5, 10H). C{1H}-NMR (CDCl3, δC) 55.6 (s, CH2), 91.7 (s,
C5H5), 122.8 (s, NCH), 127.3 (s, C6H5), 127.6 (s, C6H5), 128.0 (s, C6H5), 128.2 (s, C6H5),
128.8 (s, C6H5) 129.2 (s, C6H5), 136.3 (s, ipso-C6H5), 166.9 (s, NCN). CHN (%):
[C22H21BrN2Ni]: C, 58.16 (58.46), H, 4.68 (4.68), N, 5.91 (6.20). MS (ESI): m/z 452.02
(M+).
[CpNiBr{Im(Me)(Bn)}] (2): Yield: 77%. IR (cm-1): 3168 (ν(=CH), w), 3132 (ν(=CH), w),
3104 (ν(=CH), w), 3053 (ν(-CH), w), 3029 (ν(-CH), w), 2924 (ν(-CH), w), 1564 (ν(=CH), w),
1518 (δ(-CH), sym, m), 1495 (δ(-CH), sym, m), 1454 (δ(-CH), sym, s), 1430 (δ(-CH), asym,

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m), 1404 (δ(-CH), asym, s), 1340 (δ(-CH), s), 1230 (ν(-CN), s), 1076 (ν(CN), w), 788
(δ(=CH), s), 725 (δ(-CH), s), 714 (s), 685 (s). 1H-NMR (CDCl3, δH) 4.25 (s, CH3, 3H), 5.16
(s, C5H5, 5H), 5.99 (d, 2JHH = 49 Hz, CH2, 2H), 6.82 (d, 3JHH = 48 Hz, NCH, 2H), 7.20 – 7.31
(m, C6H5, 5H). 13C{1H}-NMR (CDCl3, δC) 38.8 (s, CH3), 55.1 (s, CH2), 91.2 (s, C5H5), 121.9
(s, {Me}NCH ), 123.6 (s, {Bn}NCH), 127.3 (s, C6H5), 127.6 (s, C6H5), 128.4 (s, C6H5), 136.0
(s, ipso-C6H5), 161.8 (s, NCN). CHN (%): [C16H17BrN2Ni]: C, 50.96 (51.12), H, 4.31 (4.56),
N, 7.75 (7.45). MS (ESI): m/z 375.99 (M+).
[CpNiBr{Im(Me)((CH2)2Ph)}] (3): Yield: 74%. IR (cm-1): 3156 (ν(=CH), w), 3124 (ν(=CH),
w), 3100 (ν(=CH), w), 3054 (ν(-CH), w), 2922 (ν(-CH), w), 1604 (ν(=CH), s), 1519 (δ(-CH),
sym, s), 1492 (δ(-CH), sym, m), 1460 (δ(-CH), sym, s), 1408 (δ(-CH), asym, s), 1339 (δ(-
CH), s), 1233 (ν(-CN), s), 1081 (ν(CN), w), 786 (δ(=CH), s), 727 (δ(-CH), s), 714 (s), 697
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(s). H-NMR ({CD3}2CO, δH) 3.41 (br s, NCH2, 2H), 4.28 (s, CH3, 3H), 4.95 (d, JHH = 29
Hz, CH2C6H5, 2H), 5.20 (s, C5H5, 5H), 7.13 (s, NCH, 1H), 7.20 (s, NCH, 1H), 7.23 – 7.38
(m, C6H5, 5H). 13C{1H}-NMR ({CD3}2CO, δC) 37.7 (s, CH3), 39.1 (s, NCH2), 53.9 (s,
CH2C6H5), 92.0 (s, C5H5), 123.4 (s, {Me}NCH ), 124.5 (s, {PhEt}NCH), 127.3 (s, C6H5),
127.6 (s, C6H5), 129.3 (s, C6H5), 129.9 (s, C6H5), 139.7 (s, ipso-C6H5), 161.3 (s, NCN). CHN
(%): [C17H19BrN2Ni]: C, 52.49 (52.36), H, 4.58 (4.91), N, 6.90 (7.18). MS (ESI): m/z 388.19
(M+).
[CpNiBr{Im(Me)(4-NO2Bn)}] (4): Yield: 69%. IR (cm-1): 3168 (ν(=CH), w), 3132 (ν(=CH),
w), 3109 (ν(=CH), w), 3053 (ν(-CH), w), 2929 (ν(-CH), w), 1674 (ν(NO), asym, m), 1516
(δ(-CH), sym, s), 1454 (δ(-CH), sym, s), 1430 (δ(-CH), asym, m), 1404 (δ(-CH), asym, s),
1344 (δ(-CH), s), 1230 (ν(-CN), s), 1166 (ν(NO), sym, m), 1108 (m), 1013 (ν(CN), w), 858
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(m), 795 (δ(=CH), s), 726 (δ(-CH), s), 715 (s), 685 (s). H-NMR (CDCl3, δH) 4.30 (s, CH3,
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3H), 5.19 (s, C5H5, 5H), 6.16 (d, JHH = 39 Hz, CH2, 2H), 6.89 (d, 3JHH = 55 Hz, NCH, 2H),
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7.46 (s, C6H4, 2H), 8.22 (s, C6H4 adjacent to NO2, 2H). C{1H}-NMR (CDCl3, δC) 25.6 (s,
CH3), 39.3 (s, CH2), 91.8 (s, C5H5), 122.2 (s, {Me}NCH ), 124.2 (s, C6H4), 124.6 (s, {4-
NO2Bn}NCH), 128.6 (s, C6H4 adjacent to NO2), 143.8 (s, ipso-C6H4), 147.8 (s, ipso-C6H4
containing NO2), 164.8 (s, NCN). CHN (%): [C16H16BrN3NiO2]: C, 45.64 (45.66), H, 3.52
(3.83), N, 9.61 (9.98). MS (ESI): m/z 418.98 (M+).
[CpNiBr{Im((CH2)2Ph)2}] (5): Yield: 71%. IR (cm-1): 3120 (ν(=CH), w), 3101 (ν(=CH), w),
3048 (ν(-CH), w), 2922 (ν(-CH), w), 2443 (w), 1675 (m), 1603 (ν(=CH), s), 1519 (δ(-CH),

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sym, s), 1459 (δ(-CH), sym, m), 1441 (δ(-CH), sym, s), 1408 (δ(-CH), asym, s), 1341 (δ(-
CH), s), 1234 (ν(-CN), s), 1108 (ν(CN), m), 1013 (w), 859 (s), 786 (δ(=CH), s), 734 (δ(-CH),
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s), 679 (m). H-NMR (CDCl3, δH) 3.32 (m, NCH2, 4H), 4.98 (m, CH2C6H5, 2H), 5.17 (s,
C5H5, 5H), 6.61 (s, NCH, 2H), 7.24 – 7.34 (m, C6H5, 10H). 13C{1H}-NMR (CDCl3, δC) 37.2
(s, NCH2), 53.5 (s, CH2C6H5), 91.8 (s, C5H5), 122.2 (s, NCH ), 126.8 (s, C6H5), 128.7 (s,
C6H5), 129.0 (s, C6H5), 138.1 (s, ipso-C6H5), 161.3 (s, NCN). CHN (%): [C24H25BrN2Ni]: C,
60.21 (60.05), H, 5.38 (5.25), N, 6.13 (5.84). MS (ESI): m/z 480.43 (M+).
[CpNiBr{Im(Bn)(4-NO2Bn)}] (6): Yield: 74%. IR (cm-1): 3163 (ν(=CH), w), 3090 (ν(=CH),
m), 3020 (ν(-CH), w), 2921 (ν(-CH), w), 2846 (w), 1674 (ν(NO), asym, m), 1597 (s), 1517
(δ(-CH), sym, s), 1496 (s), 1455 (δ(-CH), sym, s), 1435 (δ(-CH), asym, m), 1407 (δ(-CH),
asym, s), 1343 (δ(-CH), s), 1230 (ν(-CN), s), 1208 (ν(NO), sym, m), 1108 (m), 1044 (ν(CN),
w), 1011 (m), 859 (m), 838 (m), 788 (δ(=CH), s), 734 (δ(-CH), s), 714 (s), 684 (s). 1H-NMR
(CDCl3, δH) 5.13 (s, C5H5, 5H), 6.08 (m, {Bn}CH2, 2H), 6.23 (m, {4-NO2Bn}CH2, 2H), 6.84
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(d, JHH = 9 Hz, NCH, 2H), 7.28 – 7.50 (m, C6H4 and C6H5, 2H), 8.23 (s, C6H4 adjacent to
NO2, 2H). 13C{1H}-NMR (CDCl3, δC) 55.1 (s, CH2C6H5), 55.9 (s, CH2C6H4NO2), 91.9 (s,
C5H5), 122.5 (s, {Bn}NCH ), 123.4 (s, {4-NO2Bn}NCH), 123.7 (s, C6H5), 124.2 (s, C6H5),
127.8 (s, C6H4), 128.4 (s, C6H5), 128.6 (s, C6H5), 129.1 (s, C6H4 adjacent to NO2), 136.0 (s,
ipso-C6H5), 143.7 (s, ipso-C6H4), 147.8 (s, ipso-C6H5 containing NO2), 165.8 (s, NCN). CHN
(%): [C17H15BrN3NiO2]: C, 53.41 (53.16), H, 4.19 (4.06), N, 8.10 (8.45). MS (ESI): m/z
465.08 (M+).
[CpNiBr{Im(Bn)((CH2)2Ph)}] (7): Yield: 73%. IR (cm-1): 3154 (ν(=CH), w), 3100 (ν(=CH),
w), 3054 (ν(-CH), w), 2922 (ν(-CH), w), 2437 (w), 1674 (m), 1604 (ν(=CH), s), 1518 (δ(-
CH), sym, s), 1459 (δ(-CH), sym, m), 1408 (δ(-CH), asym, s), 1340 (δ(-CH), s), 1233 (ν(-
CN), s), 1108 (ν(CN), m), 1014 (w), 859 (m), 832 (m), 786 (δ(=CH), s), 734 (δ(-CH), s), 700
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(m), 679 (m). H-NMR (CDCl3, δH) 3.36 (q, JHH = 9 Hz, NCH2CH2, 2H), 5.00 (t, JHH = 8
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Hz, CH2C6H5, 2H), 5.12 (s, C5H5, 5H), 6.10 (dd, JHH = 15 and 116 Hz, NCH2C6H5, 2H),
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6.74 (q, JHH = 2 Hz, NCH, 2H), 7.18 – 7.39 (m, C6H5, 10H). C{1H}-NMR (CDCl3, δC)
37.2 (s, CH2CH2C6H5), 53.5 (s, CH2CH2C6H5), 55.7 (s, CH2C6H5), 91.8 (s, C5H5), 122.3 (s,
{PhEt}NCH ), 122.7 (s, {Bn}NCH), 126.9 (s, C6H5), 127.6 (s, C6H5), 128.1 (s, C6H5), 128.7
(s, C6H5), 128.9 (s, C6H5), 129.0 (s, C6H5), 136.6 (s, ipso-C6H5 of PhEt), 138.0 (s, ipso-C6H5
of Bn), 162.9 (s, NCN). CHN (%): [C23H23BrN2Ni]: C, 59.01 (59.28), H, 4.86 (4.97), N, 6.29
(6.01). MS (ESI): m/z 495.45 (M+).

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[CpNiBr{Im((CH2)2Ph)(4-NO2Bn)}] (8): Yield: 70%. IR (cm-1): 3148 (ν(=CH), w), 3116
(ν(=CH), w), 3109 (ν(=CH), w), 2937 (ν(-CH), m), 2840 (ν(-CH), w), 1674 (ν(NO), asym,
m), 1598 (s), 1516 (δ(-CH), sym, s), 1453 (δ(-CH), sym, s), 1412 (δ(-CH), asym, m), 1341
(δ(-CH), s), 1263 (m), 1233 (ν(-CN), s), 1183 (ν(NO), sym, m), 1108 (m), 1015 (ν(CN), w),
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858 (m), 833 (m), 786 (δ(=CH), s), 739 (δ(-CH), s), 721 (s), 700 (s). H-NMR (CDCl3, δH)
3.40 (br s, NCH2CH2, 2H), 5.02 (br s, CH2C6H5, 2H), 5.15 (s, C5H5, 5H), 6.24 (s,
CH2C6H4NO2, 2H), 6.76 (d, 3JHH = 6 Hz, NCH, 2H), 7.24 – 7.33 (m, C6H5, 5H), 7.43 (d, 3JHH
= 7 Hz, C6H4, 2H), 8.25 (d, 3JHH = 8 Hz, C6H4 adjacent to NO2, 2H). 13C{1H}-NMR (CDCl3,
δC) 37.2 (s, CH2CH2C6H5), 53.7 (s, CH2CH2C6H5), 55.3 (s, CH2C6H4NO2), 92.0 (s, C5H5),
123.7 (s, {PhEt}NCH ), 124.3 (s, {4-NO2Bn}NCH), 127.1 (s, C6H5), 128.5 (s, C6H4), 128.9
(s, C6H5), 129.1 (s, C6H4 adjacent to NO2), 129.6 (s, C6H5), 132.3 (s, C6H5), 133.2 (s, C6H5),
137.8 (s, ipso-C6H5 of PhEt), 143.9 (s, ipso-C6H4 of 4-NO2Bn), 147.9 (s, ipso-C6H4
containing NO2), 164.8 (s, NCN). CHN (%): [C18H17BrN3NiO2]: C, 53.67 (54.06), H,
4.09(4.34), N, 8.39 (8.22). MS (ESI): m/z 509.26 (M+).
[CpNiBr{Im(4-NO2Bn)2}] (9): Yield: 59%. IR (cm-1): 3098 (ν(=CH), m), 3071 (ν(=CH), w),
2927 (ν(-CH), m), 2857 (ν(-CH), w), 1679 (ν(NO), asym, m), 1598 (s), 1515 (δ(-CH), sym,
s), 1444 (δ(-CH), sym, s), 1409 (δ(-CH), asym, m), 1342 (δ(-CH), s), 1237 (ν(-CN), s), 1179
(ν(NO), sym, m), 1109 (m), 1014 (ν(CN), w), 858 (m), 798 (δ(=CH), s), 733 (δ(-CH), s). 1H-
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NMR (CDCl3, δH) 5.28 (s, C5H5, 5H), 6.18 (dd, JHH = 12 and 52 Hz, CH2C6H4NO2, 4H),
6.42 (br s, NCH, 2H), 7.30 – 7.53 (m, C6H4, 4H), 8.13 (d, 3JHH = 6 Hz, C6H4 adjacent to NO2,
4H). 13C{1H}-NMR (CDCl3, δC) 41.5 (s, CH2), 43.1 (s, CH2), 92.1 (s, C5H5), 123.6 (s, NCH),
123.7 (s, NCH), 128.6 (s, C6H5), 129.2 (s, C6H5), 144.0 (s, ipso-C6H4 of 4-NO2Bn), 148.1 (s,
ipso-C6H4 adjacent to NO2), 164.8 (s, NCN). CHN (%): [C17H14BrN4NiO4]: C, 48.73 (48.75),
H, 3.38 (3.53), N, 10.52 (10.34). MS (ESI): m/z 539.97 (M+).
2.4
General procedure for the Suzuki-Miyaura coupling reactions
To a round-bottom flask containing the substrate (2.8 × 10-4 mol), K3PO4 (2.6 eq., 7.2 × 10-4
mol) and PhB(OH)2 (1.3 eq, 3.6 × 10-4 mol), was added [CpNiBr(NHC)] (3 mol%, 8.3 × 10-6
mol) and toluene (4 mL) and the subsequent reaction mixture was stirred at 90°C for 60
minutes. The mixture was left to cool, and passed through a pad of silica. 1 mL of the
resulting light yellow solution was then analysed directly by GC/MS using hexadecane as

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internal standard upon which the coupling yields were based. All yields are based on the
average of three runs.
2.5
X-ray crystallography of compounds L3, L4, 1, 2, 3, and 4
Single crystal diffraction studies of compounds L3, L4, 1, 2, 3, and 4 were done using
Quazar multi-layer optics monochromated Mo Kα radiation (k = 0.71069 Å) on a Bruker D8
Venture kappa geometry diffractometer with duo Iµs sources, a Photon 100 CMOS detector
and APEX II control software [29]. All X-ray diffraction measurements were performed at
150(2) K. Data reduction was performed using SAINT+ [29], and the intensities were
corrected for absorption using SADABS [30]. All structures were solved by direct methods
with SHELXS-97 [31] using the OLEX2 [32] interface. All H atoms were placed in
geometrically idealized positions and constrained to ride on their parent atoms. For a table
containing the data collection and refinement parameters, see Supplementary Information.
3.
Results and Discussion
3.1
Synthesis of cyclopentadienyl nickel(II) bromide NHC complexes
Because of the continued interest in the synthesis and interesting application of the large
range of [CpNiX(NHC)] (X = Cl, I) complexes, the inherent need to expand on the few
existing bromo-analogues existed. In order to address this, we have synthesised a range of
flexible symmetric- and asymmetric NHC bromide ligands according to the reported methods
[2,13,21] (see supplementary information), in high yields (> 85%). This synthetic route
involves the initial N-functionalisation of imidazole with RBr (alkyl-, benzyl-, or phenylethyl
halide) in the presence of KOH/K2CO3, which after purification is further reacted with R'Br
to yield the respective N,N'-functionalised imidazolium bromide salts. Each of the resulting
NHC ligands bears N-alkyl, N-benzyl or –N-phenylethylmoieties that provide bulk through
phenyl groups, as well as flexibility through sp3 methylene carbon linkers (Figure 2).

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Figure 2: R and R' groups employed in ligands L1 – L9.
Each of these NHC ligands was reacted with NiCp2 [12,14,19] to form the complexes
[CpNiBr(NHC)] (1 – 9) with the loss of a cyclopentadiene ligand (Table 1). During these
complexation reactions, a typical colour change from emerald green to blood red/red-brown
was observed, with reaction times that varied between 3 and 6 hours. The more electron
withdrawing NHC ligands of 4, 6, 8, and 9 proved to required longer reaction times, ranging
from 6 – 18 hours to give the Ni(II) complexes in notably lower yields (59 – 74%). Nolan et
al. [12] have previously pointed out that longer reaction times were required whenever
bulkier, as opposed to electron-withdrawing, NHC ligands were employed. The
[CpNiBr(NHC)] complexes synthesised showed sufficient stability in deoxygenated
commercial grade solvents of acetone, chloroform, and DCM.
In general the 1H-NMR spectra of complexes 1 – 9 correspond to those of similar complexes
reported in literature [11b,13,14,19,33]. The cyclopentadienyl protons were observed as
singlets between 5.12 and 5.28 ppm, along with the NCH protons appearing either as singlets,
doublets, or quartets (3JHH = 2 – 55 Hz) at 6.42 – 7.20 ppm, depending on the symmetrical
nature of the NHC ligand employed. The 13C-NMR spectra of 1 – 9 also conformed to
literature, with the carbene carbon signal resonating between 161.3 (electron-donating NHCs)
and 166.9 (electron-withdrawing NHCs) ppm.
Table 1: Synthesis data of the [CpNiBr(NHC)] complexes (1 – 9)
Reaction
Complex
[NHC]Br
Yield (%)
time (h)
[HIm(Bn)2]Br
4
3
79
77
1
2
[HIm(Me)(Bn)]Br

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[HIm(Me)((CH2)2Ph)]Br
[HIm(Me)(4-NO2Bn)]Br
[HIm((CH2)2Ph)2]Br
[HIm(Bn)(4-NO2Bn)]Br
[HIm(Bn)((CH2)2Ph)]Br
[HIm((CH2)2Ph)(4-
NO2Bn)]Br
4
6
6
6
6
74
69
71
74
73
3
4
5
6
7
7
70
59
8
9
[HIm(4-NO2Bn)2]Br
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3.2
X-ray crystallographic studies
The molecular structures of complexes [CpNiBr{Im(Bn)2}] (1), [CpNiBr{Im(Me)(Bn)}] (2),
[CpNiBr{Im(Me)((CH2)2Ph)}] (3), and [CpNiBr{Im(Me)(4-NO2Bn)}] (4) have been
confirmed by single crystal X-ray diffraction, and are included in Figures 3 and 4 along with
the structures of L3 and L4. All crystallographic data and data collection parameters are
included in the supplementary information. With the cyclopentadienyl ligand considered as a
single ligand, complexes 1 – 4 may be considered to exhibit a distorted trigonal planar
geometry around the central metal atom, with Cpcent-Ni1-Br1 ≈ Cpcent-Ni-C1 ≈ 133°. In
addition, the angle between the mean imidazolyl (NHC) plane and Ni1-Br1 bond ranges
between 92.865° – 94.519° for 1, 3 and 4. This angle, however, increases to 96.687° for 2,
which contains the least bulky NHC ligand of the four complexes. This is the result of the Cp
and Br ligands being more conveniently arranged around Ni1 with the smaller NHC ligand.
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Figure 3: Perspective views of [CpNiBr{Im(Bn)2}] (1) and [CpNiBr{Im(Me)(Bn)}] (2) with
thermal ellipsoids drawn at 50% probability level.

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The key descriptors of both structures compare well with similar [CpNiBr(NHC)] complexes
[12,14,11e,19,21,33], with Ni1-C1 (1.8688(1) Å (1); 1.8757(1) Å (2)), and Br1-Ni1-C1
(94.107(1)° (1); 96.201(3)° (2)) (Table 2). The molecular structures obtained for ligands
[Im(Me)((CH2)2Ph)]Br (L3) and [Im(Me)(4-NO2Bn)]Br (L4), and their corresponding Ni(II)-
complexes (3 and 4 respectively) are included in Figure 4. The Br1-Ni1-C1 angles of
93.965(3)° (3) and 94.227(6)° (avg.) (4) correspond well with those of 1 and 2, and fall
within the range 92.9(2)° – 98.41(9)° reported for most of the [CpNiX(NHC)] (X = Cl, Br, I)
analogues [12,14,11e,19,21,33]. The flexibility in the range of NHC ligands employed,
before and after coordination is structurally demonstrated through the N1-C1-X and N2-C1-X
bond angles, as well as the C-N1-C1-X and C-N2-C1-X torsion angles, where X = H, Ni. In
sterically demanding environments the N-substituents tend to “fold-out”, such that the bulky
moieties are directed away from the metal centre, which is observed through the C-N-C and
C-N-C-Ni angles. The N1-C1-X angles (123.969(1)° (L3); 125.752(4)° (L4)), as well as the
N2-C1-X angles (127.823(2)° (L3); 125.753(2)° (L4)) in general increased after
complexation to 127.623(4)° (3) and 127.469(9)° (avg.) (4), and 128.052(4)° (3) and
127.950(9)° (avg.) (4), respectively. This shows the compromise the benzyl (1, 2),
phenylethyl (3) and 4-NO2-benzyl (4) groups make to fit around an already crowded nickel
centre bearing cyclopentadienyl and bromide ligands. The C-N1-C1-X and C-N2-C1-X (X =
H, Ni) torsion angles indicated planarity that can be associated with delocalization of electron
density from the ring system to the X atom (4.226(3)° and -5.210(3)° (L3); 2.132(7)° and -
1.030(7)° (L4); 1.020(9)° and 1.577(9)° (3); 2.313(2)° (avg.) and 1.961(2)° (avg.) (4)). No
mentionable changes in the C-N1-C1 and C-N2-C1 bond angles were observed either before
or after complexation, however. Collectively, the ability of the ligand salts (L3, L4) to
compensate for a decrease of spatial freedom when coordinated (3, 4), is shown through the
“folding-out” and flexibility of the N-substituted groups. This functionality of the ligands and
corresponding complexes provide a unique way of introducing the required steric bulk and
flexibility [12] in the ligand systems of Suzuki-Miyaura active catalyst complexes.

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Figure 4: Perspective views of [HIm(Me)((CH2)2Ph)]Br (L3), [HIm(Me)(4-NO2Bn)]Br (L4),
along with their corresponding [CpNiBr(NHC)] complexes (3; 4) with thermal ellipsoids
drawn at 50% probability level. Half a molecule of C6H6 (solvent of crystallization) in 3 has
been omitted for clarity.
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328
Table 2: Selected bond distances (Å) and angles (°) for compounds L3, L4, 1, 2, 3 and 4
3b
4c
Description
L3
L4
1
2
1.8557(1)
1.8806(2)
2.3166(2)
2.3154(2)
94.528(6)
93.925(6)
127.279(8)
127.658(9)
129.040(9)
126.860(8)
123.199(9)
124.046(1)
122.975(1)
124.358(1)
2.343(2)
--
--
1.8688(1) 1.8757(1) 1.8753(1)
2.3427(1) 2.3253(1) 2.3272(1)
94.107(1) 96.201(3) 93.965(3)
Ni-C1
--
--
--
--
Ni1-Br1
Br1-Ni1-C1
123.969(1) 125.752(4) 127.706(2) 127.189(4) 127.623(4)
127.823(2) 125.753(2) 127.706(2) 128.375(4) 128.052(4)
125.375(1) 125.600(3) 123.826(2) 124.104(3) 124.671(6)
125.622(1) 125.994(2) 123.826(2) 124.661(3) 124.123(6)
N1-C1-X (X = Ni, H)
N2-C1- X (X = Ni, H)
Ca-N1-C1
Ca-N2-C1
Ca-N1-C1-X (X = Ni, H)
4.226(3) 2.132(7)
6.267(3)
3.587(7) 1.020(9)

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2.282(2)
0.053(2)
Ca-N2-C1-X (X = Ni, H)
-5.210(3) -1.030(7)
-6.267(3) -4.400(7) 1.577(9)
-3.868(2)
a
b
c
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330
Carbon atom of the substituent on the adjacent N-atom. Contains half of a molecule of C6H6. Two
molecules per asymmetric unit.
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3.4
Suzuki-Miyaura coupling activity
The Suzuki-Miyaura reaction is usually carried out with 4-halo acetophenone and
phenylboronic acid as coupling partners to yield 4-acetylbiphenyl (major, 70 – 100%
conversion) and very low yields of the homo-coupled by-product, 4,4'-diacetylbiphenyl
(minor, 0 – 16%) in less than 60 minutes. Complexes 1 – 9 were employed as catalysts in the
Suzuki-Miyaura (SM) coupling reaction of the aryl chloride, 4-chlorobenzaldehyde and
PhB(OH)2. This is the first instance where the more challenging [14,20b,34] chloro-aryl
carboxaldehydes are employed as substrates using CpNiBr(NHC) complexes, as opposed to
the usual chloro- and bromo aryl ketones with [CpNiX(NHC)] (X = Cl, I) complexes as
catalysts [14,19,20c,33]. All complexes 1 – 9 exhibited catalytic activity under optimized
reaction conditions, i.e. stirred suspensions in toluene with 2.6 eq. K3PO4, 1.3 eq. PhB(OH)2,
and 3 mol% catalyst at 90°C, to give 4-carboxaldehydebiphenyl (CBP) (53 – 79%
conversion) as the major product within one hour (Table 3). Surprisingly, formation of
minute amounts of the homo-coupled product, 4,4'-dicarboxaldehydebiphenyl, and a
significant amount of biphenyl as by-products (15 – 30%) were observed. This result was
found to be directly related to the N-substituted groups of the different Ni(II)-NHC
complexes involved in each isolated reaction, where the more electron-withdrawing benzyl
groups tend to yield slightly higher yields of biphenyl, with consequential lower yields of
CBP.
Table 3: Suzuki-Miyaura reaction of 4-chlorobenzaldehyde with phenylboronic acid
catalysed by [CpNiBr(NHC) complexes (1 – 9)
Entry
Cat.
Solvent
Base
Time Temp
S:PBA:Ba
Conv. (%)b
A:B:C b

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(mol%)
1 (3)
1 (3)
1 (3)
1 (3)
1 (3)
1 (3)
1 (3)
1 (3)
1 (3)
1 (3)
1 (0)
1 (3)
2 (3)
3 (3)
4 (3)
5 (3)
6 (3)
7 (3)
8 (3)
9 (3)
(min)
60
60
60
60
30
120
60
60
60
60
60
60
60
60
60
60
60
60
60
60
(°C)
90
90
90
90
90
90
60
25
90
90
90
90
90
90
90
90
90
90
90
90
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DMP
K3PO4
K2HPO4
K2CO3
NEt3
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.1:2.2
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
1:1.3:2.6
79
33
62
4
79:0:21
76:0:24
75:0:25
24:3:73
73:0:27
69:0:31
65:0:35
58:0:42
65:0:35
43:0:57
9:0:91
1
2
3
4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
43
87
46
24
42
23
5
5
6
7
8
9
CH3CN
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
10
11
12
13
14
15
16
17
18
19
20
59
73
69
71
76
62
73
57
53
81:0:19
79:0:21
70:0:30
83:1:16
78:0:22
78:0:22
78:0:22
66:0:34
77:0:23
354
355
356
General reaction conditions: 4-chlorobenzaldehyde (0.28 mmol), phenylboronic acid (0.36 mmol), base (0.72
b
mmol), catalyst (3 mol%), toluene (4 mL), 90°C, 60 min.a Molar ratio of substrate:phenylboronic acid:base.
Conversion based on internal standard, average value of three runs.
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365
366
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368
In the optimization process of the SM reaction we found, as with other groups [14,19,33],
that K3PO4 as base is by far more effective compared to K2HPO4, K2CO3 or NEt3 (entries 1-
4). An increase in reaction time, or with increased temperature, led to increased yields
(entries 5-6). We focused on a direct comparison of catalytic efficiencies with those reported
and therefore employed similar reaction conditions (90°C, 60 min), for which we obtained
moderate to high conversions. An immediate lowering in conversion was observed upon
lowering of the reaction temperature (entries 7-8). A reaction time of 60 minutes is still
considered fast for Ni-catalysed SM reactions [19], especially without added additives such
as PPh3 [19], whereas product formation occurred within the first 5 minutes. Polar aprotic
CH3CN and dimethoxypropane proved inefficient (entries 9-10), whereas toluene proved
superior as the solvent of choice for aryl coupling reactions that can withstand the

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temperatures involved. Using toluene as solvent, a stabilisation effect might also be
significant, especially if air- and moisture sensitive Ni-NHC catalytic species are involved,
through soft π-interactions of the N-phenyl-containing groups of the NHC ligands. This effect
is further illustrated when lower catalyst concentrations are used providing lower CBP
conversions (entry 11), and might indicate fast catalyst deactivation [19]. In an attempt to
decrease the biphenyl product formation, separate decreases in K3PO4 (2.2 eq.) and
PhB(OH)2 amounts led to an immediate decrease of CBP yield, with no significant
proportional decrease of biphenyl formation (entry 12).
A significant variation of CBP yields was observed among each of the complexes 1 – 9.
Under the optimized conditions, [CpNiBr{Im(Bn)2}] (1) performed best, with 79%
conversion in 60 minutes. An initial turn-over frequency (TOF) of 320 h-1 was observed for
1. This is much higher than the reported 190 h-1 ([Cp*Ni(NCMe){Im(iPr2Ph)2}]PF6) [33],
and comparable to 352 h-1 ([CpNiI{Im(Mes)(nBu)}]), observed to have the best TOF for a
Ni(II)-catalysed SM coupling in the absence of reductant and/or co-catalysts [19]. Amongst
complexes 1 – 9, a general trend was observed where the more electron-withdrawing NHC-
bearing complexes led to the lowest CBP yields, i.e. [CpNiBr{Im(4-NO2Bn)2}] (9) produced
only 53% conversion (entry 20). This could be explained by the relative nickel complex
stabilities once dehalogenation and/or reduction by K3PO4 have taken place. The more
efficient electron-donating ancillary ligands stabilise the sensitive intermediates, and
ultimately depress catalyst decomposition [20b]. This effect was also observed by the groups
of Ritleng [19] and Buchowicz [14], where they found that electron-donating, bulky N-
substituents lead to higher 4-acetylbiphenyl conversions, and the minimization of the
homocoupled product formation. Moreover, Nolan et al. [12] found that the use of bulky
NHC ligands stabilises the intermediate nickel species, while flexible NHC ligands proved
even more useful to allow for unhindered substrate coordination. The latter observation
agrees with our findings, where accessibility to incoming substrates and the reductive
elimination step in the catalytic cycle is better facilitated with bulky, flexible benzyl- and
phenethyl-bearing NHCs. The sp3-methylene carbon-linked NHCs allow for free rotation of
the bulky phenyl groups, which could at any stage of the catalytic process find itself in close
proximity to the metal centre through ligand/substrate interactions. This creates intra-ligand
strain, which could increase the rate of reductive elimination in order to relieve steric strain
before the following substrate coordinates.

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In addition, we also investigated the effect of employing the non-activated bromobenzene, as
well as activated electron-donating substrates in these coupling reactions, and found that the
results conform to those reported [6b,19,33] in that all coupling yields decreased dramatically
(Table 4). Little conversion was observed for bromobenzene, and only 24% conversion for 2-
bromopyridine. The electron-withdrawing 4-bromophenacylbromide showed most promising
with a conversion of 37% and little or no signs of side-reactions, even with an acetyl bromide
functionality present.
Table 4: Suzuki-Miyaura reaction of other substrates with phenylboronic acid catalysed by
complex 1a,b
414
415
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419
420
421
422
423
424
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427
428
Reaction conditions: substrate (0.28 mmol), phenylboronic acid (0.36 mmol), base (0.72
a
mmol), 1 (3 mol%), toluene (4 mL), 90°C, 60 min. GC yields based on internal standard,
average value of three runs. b Molar ratio of products A:B:C, shown in parentheses.
According to the five important points required, as set out by Matsubara et al. [24] when
designing a multi-functional nickel catalyst, the series [CpNiBr(NHC)] (1 – 9) conforms to
the requirements. This is because complexes 1 – 9 (i) are prepared in a facile fashion, (ii) are
thermally and relatively air and moisture stable, (iii) easily form a coordinatively unsaturated
site with mild bases, (iv) possess electron-donating ligands to stabilise the unsaturated site
which accelerates the oxidative addition process, and (v) exhibit a range of steric substituents
on the NHC ligand to enhance the reductive elimination reaction [12, 25].
4.
Conclusion
A new range of flexible, electron-withdrawing and -donating NHC ligands, as well as their
relatively air and moisture stable [CpNiBr(NHC)] complexes were synthesised directly from

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nickelocene, and characterised. Molecular structures of two of these [NHC]Br ligand salts,
and four of the corresponding [CpNiBr(NHC)] complexes were obtained and elucidated. This
series of Ni(II)-NHC complexes serves as an interesting expansion on the current limited
series of bromo-analogues when compared with the vast series of [CpNiX(NHC)] (X = Cl, I)
complexes reported to date. The complexes 1 – 9 efficiently catalysed the SM reaction of,
amongst others, 4-chlorobenzaldehyde, which under optimized conditions gave 4-
carboxaldehydebiphenyl (major) and biphenyl (minor) in an average ratio of 80:20
respectively. The flexibility and bulkiness of the N-substituted groups on the NHC ligands
provided accessibility to incoming substrates, while also facilitating the reductive elimination
of the organic products. Among complexes 1 – 9, the more electron-donating NHC-bearing
complexes were more efficient at the coupling of activated aryl halides and phenylboronic
acid in the SM reactions investigated, as opposed to the electron-withdrawing NHC-bearing
complexes.
Supplementary material
CCDC 1445421-1445426 contain the supplementary crystallographic data for compounds
L3, L4, 1, 2, 3, and 4 respectively. Crystallographic files can be obtained from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
The National Research Foundation of South Africa (NRF) and the University of Pretoria
(UP) are gratefully acknowledged for financial support (ML, FPM). Single crystal X-ray
diffraction collections by Mr D. Liles are gratefully acknowledged as well as assistance with
GC-MS studies by Dr Y. Naudé.
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References
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[2]
[3]
S. Gaillard, J-L. Renaud, Dalton Trans. 42 (2013) 7255-7270.
O. Kühl, Chem. Soc. Rev. 36 (2007) 592-607.
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(a) B.K. Shaw, B.O. Patrick, M.D. Fryzuk, Organometallics. 31 (2012) 783-786. (b)
L. Mercs, G. Labat, A. Neels, A. Ehlers, M. Albrecht, Organometallics. 25 (2006)
5648-5656.
[4]
(a) T. Steinke, B.K. Shaw, H. Jong, B.O. Patrick, M.D. Fryzuk, Organometallics. 28
(2009) 2830-2836. (b) W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290-1309.

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Highlights
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•
Synthesis of nine new Ni(II)-NHC complexes, [CpNiBr(NHC)]
Symmetric and asymmetric alkyl/-benzyl/phenylethyl substituted NHC complexes of
Ni
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Suzuki-Miyaura coupling of activated aryl halides with phenylboronic acid