Earth abundant metal complexes of donor functionalised N-heterocyclic carbene ligands: synthesis, characterisation and application as amination catalysts

Article abstract of DOI:10.1039/c6nj01118g

Six new imidazolium salts of the form 3-R-1-picolylimidazolium bromide (1a: R = 4-nitrophenyl, 1b: R = 4-acetylphenyl, 1c: R = 4-cyanophenyl, 1d: R = allyl, 1e: R = butenyl, 1f: R = pentenyl) were synthesised and isolated in high yields. The corresponding Ag-NHC intermediate complexes 2a, 2d, and 2e were transmetalated to yield [M(NHC)₂Cl₂ M = Co, Ni] complexes in good to excellent yields. The Co-NHC (3a, 3d, 3e) and Ni-NHC (3a′, 3d′, 3e′) complexes were relatively stable in air, insoluble in chlorinated solvents but very soluble in methanol and DMSO. Poorly resolved NMR spectra and magnetic susceptibility values of 2.53 and 2.73 μ_B for 3d and 3e respectively suggest both to be paramagnetic cobalt complexes. All the imidazolium salts, isolated Ag-NHC complexes and the corresponding Co and Ni-NHC complexes were characterised by spectroscopic and analytical techniques. The complexes were found to be active at low catalyst loading (1 mol%) for the C-N coupling of aniline with phenyl bromide under mild reaction conditions. In addition, the in situ generated catalyst obtained from a mixture of NiCl₂/1a (1:2 molar ratio) initiated the C-N coupling of several aryl amines with substituted aryl bromides bearing a wide variety of functional groups. Good to excellent yields of the desired diaryl amine products were obtained. The yields are comparable to data obtained with palladium catalysts or to data obtained under harsher temperature conditions of Cu mediated Ullman reactions.

Full text of DOI:10.1039/c6nj01118g

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DOI: 10.1039/C6NJ01118G
Earth abundant metal complexes of donor functionalised N-heterocyclic
carbene ligands: synthesis, characterisation and application as
amination catalysts
Halliru Ibrahim, Muhammad Dabai Bala
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban
4000, South Africa
Abstract
Six new imidazolium salts of the form 3-R-1-picolylimidazolium bromide (1a: R = 4-
nitrophenyl, 1b: R = 4-acetylphenyl, 1c: R = 4-cyanophenyl, 1d: R= allyl, 1e: R= butenyl, 1f:
R = pentenyl) were synthesised and isolated in high yields. Corresponding Ag-NHC
intermediate complexes 2a, 2d, and 2e were transmetalated to yield [M(NHC)2Cl2 M = Co, Ni]
complexes in good to excellent yields. The Co-NHC (3a, 3d, 3e) and Ni-NHC (3a′, 3d′, 3e′)
complexes were relatively stable in air, insoluble in chlorinated solvents but very soluble in
methanol and DMSO. Poorly resolved NMR spectra and magnetic susceptibility values of 2.53
and 2.73 μB for 3d and 3e respectively suggest both to be paramagnetic cobalt complexes. All
the imidazolium salts, isolated Ag-NHC complexes and the corresponding Co and Ni-NHC
complexes were characterised by spectroscopic and analytical techniques. Low catalyst
loading (1 mol%) of the complexes were active for the C-N coupling of aniline with phenyl
bromide under mild reaction conditions. In addition, the in situ generated catalyst obtained
from a mixture of NiCl2/1a (1:2 mole ratio) initiated the C-N coupling of several aryl amines
with substituted aryl bromides bearing a wide variety of functional groups. Good to excellent
yields of the desired diaryl amine products were obtained. The yields are comparable to data
obtained with palladium catalysts or harsher temperature conditions of Cu mediated Ullman
reactions.
Keywords
NHC, Pyridyl, Imidazolium salts, In situ catalysis, C-N coupling, Co, Ni, Monoarylation
1.
Introduction
Since the pioneering works of Öfele, Wanzlick and Arduengo [1], N-heterocyclic
carbenes (NHCs) have attracted increasing attention as ancillary ligands in organic & inorganic
synthetic transformations and organometallic homogeneous catalysis [2]. Their ability to
tolerate many functional groups, work at ambient temperatures and strongly coordinate to metal
centres in catalytic systems without ligand dissociation due to excellent sigma donating ability
meant NHCs are now the preferred options in organometallics, coordination chemistry and
homogeneous catalysis over other two-electron donor ligands like ethers, amines, and even the
* Correspondence to: e-mail- bala@ukzn.ac.za; fax- +27 31 260 3091

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ubiquitous phosphines [3]. In spite of all these, it is interesting to note that metal-NHC
DOI: 10.1039/C6NJ01118G
complexation especially when applied to homogeneous catalysis, is still dominated by the
platinum group metals (PGM) mainly rhodium, iridium, ruthenium & palladium. While to date
comparatively much fewer complexes are known of earth abundant, non-PGM; first row
transition metals like nickel or cobalt [1a, 3-5].
Although bidentate hemilabile ligands containing a mixture of strong and weak donor
groups have found widespread applications in homogeneous catalysis [6], their complexes with
nickel and especially cobalt are scarce [7]. Hence, except for a few examples that are stabilised
by classical donor ligands like cyclopentadiene, phosphines, heterocyclic systems and CO [8],
few examples of multidentate functionalised Ni- or Co-NHC complexes are available [9]. The
study of multidentate carbene complexes has been an interesting field of study [10a-b], because
during catalysis, the ligand exhibits coordinative and electronic unsaturation due to variability
in binding strengths of the donor atoms to the metal centre. The presence of weak donors help
provide the hemilabile character of such ligands, which accounts for possible reversible
dissociation from the metal centre to generate empty site(s) for the coordination of substrate(s)
without catalyst decomposition [10].
Due to their cost effectiveness and distinct catalytic abilities, interest in the
development of catalyst systems based on earth abundant, non-PGM; first row transition metals
is gaining ground amongst chemists worldwide and several systems based on NHC and Co(II)
[11], Ni(0) [2j,12] and Ni(II) [13] are gradually complementing PGM-based catalysts. Such
complexes have proven to be active in C-S coupling [14], cyclo addition [15], addition
polymerization of norbornane [16], aryl amination [2j, 11b, 13, 17], styrene polymerization
[18], Suzuki and Kumada coupling reactions [19]. More specifically, C-N coupling is very
important in organic synthesis as a route to important groups of chemicals for synthetic,
pharmaceutical and fine chemical industrial applications. Although, most aryl C-N coupling
reactions are catalysed by Pd(0) or copper mediated Ullman reactions; the use of the former is
neither economically nor ecologically advantageous [20] while the latter proceeded via harsh
conditions, high temperatures or requires stoichiometric amounts of a copper catalyst [21,22].
Except for the use of Ni(0)/bis-arylimidazolium salts by Kuhl et al. [2j] and more recent work
by Matsubara and co-workers where they used a nickel(II) halide complex bearing a mixed
PPh3/NHC ligand as a catalyst precursor [13], efficient catalyst systems containing nickel as
the central metal for arylation of aromatic amines are still rare. To the best of our knowledge
the use of functionalised, bidentate and sterically demanding metal-NHC complexes

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incorporating cobalt or nickel as catalysts for the arylation of amines is still novel. Herein, we
DOI: 10.1039/C6NJ01118G
report the synthesis of new functionalised imidazolium salts and their Ag, Co and Ni
complexes. Findings on the application of the Co and Ni complexes as catalysts for the
coupling of aryl amines with aryl halides to selectively yield diaryl amines products are also
reported.
2.
Experimental
2.1. General procedures
All experiments except when otherwise mentioned were performed under an inert
nitrogen atmosphere using standard Schlenk techniques. Solvents were analytical grade and
dried using standard techniques: THF and hexane in benzophenone were distilled over sodium
wire; methanol in iodine was distilled twice over magnesium ribbon; 1,2-dichloroethane was
distilled over CaCl2 and stored with molecular sieve and dichloromethane was distilled over
calcium hydride or P2O5. Methanol washed 4Å molecular sieves were activated by drying in
an oven at 150 °C for 24 h followed by heating in a furnace at 500 °C for 2 d and cooled in a
desiccator. Deuterated solvents were obtained from Merck and are used without further
purification. Both CoCl2(PPh3)2 and NiCl2(PPh3)2 were prepared using a standard procedure
[23]. N-substituted imidazoles were prepared by the modification of established procedures
[24], while the corresponding salts were purified and isolated using a recently reported
technique [25]. The aryl amines used were analytically pure; obtained from Sigma-Aldrich
and distilled prior to use then stored with molecular sieves. Potassium tertiary butoxide, 2-
bromomethyl pyridine hydrobromide, allyl bromide, butenyl bromide, pentenyl bromide and
all aryl halide substrates used were analytically pure; obtained from Sigma-Aldrich and used
without further purification. NMR spectra were recorded on a Bruker AVANCETM 400
Ultrashield spectrometer operating at room temperature and samples were dissolved in CDCl3,
CD3OD or DMSO-d6. The spectra were internally referenced relative to solvent peak(s) and
chemical shift values recorded (δ ppm) with respect to δ = 0 ppm for tetramethylsilane in both
13
1H and C NMR spectra [26]. Hysteresis loop measurement for determination of saturated
magnetization was conducted on vibrating sample magnetometer (VSM) Lakeshore 735.
Infrared spectra were recorded on a PerkinElmer universal ATR Spectrum 100 FT-IR
spectrometer. Mass spectrometry and elemental analysis (where applicable) were recorded on
Waters Micromass LCT Premier TOF MS-ES+ and ThermoScientific Flash2000 elemental
analyser respectively.

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DOI: 10.1039/C6NJ01118G
2.2. General synthetic procedure for the ligand precursors 1a-f
The imidazolium salts which are NHC ligand precursors were synthesised by the
modification of literature procedures [25, 27]. Spectroscopic and analytical data obtained are
presented. A typical and generic procedure for the synthesis of the imidazolium salts 3-R-1-
(2-pyridylmethyl)imidazolium bromide (1a-f) is described:
A saturated solution of Na2CO3 was first used to neutralize 2-(bromomethyl)pyridine
hydrobromide at 0 °C. The neutral pink solution obtained was then extracted with diethyl ether
(3 x 10 ml) at 0 °C and the clear solution of liberated 2-(bromomethyl)pyridine dried over
anhydrous MgSO4 and filtered. The colourless filtrate was then reduced to 1/3 of the initial
volume under reduced pressure and added to a solution of N-substituted imidazole (1 mole
equivalent) in dry acetonitrile (40 ml) at 0 °C. The remaining ether was removed at reduced
pressure and the contents of the flask subjected to gentle reflux for 18 h. Progress of the
reaction was monitored by TLC. The solvent was then removed under reduced pressure and
the TLC of the crude obtained in methanol: ethyl acetate (1: 4) solvent system. This showed
unreacted starting materials with Rf values 0.7-1.0 while the salt remained unmoved at the spot.
Purification was thus obtained with column chromatography. The starting materials were
eluted out with ethyl acetate and the salt obtained as methanol eluent. All volatiles were then
removed at reduced pressure to give the salts in excellent yields.
2.2.1. 3-(4-nitrophenyl)-1-(2-pyridylmethyl)imidazolium bromide (1a)
Prepared from 2-(bromomethyl)pyridine (1.6 mmol) and equimolar para-nitrophenyl
imidazole in acetonitrile. Product obtained as reddish-orange solid, yield (0.52 g, 90%; mp
220-222 °C). IR (ATR cm-1): vmoisture 3404, vC-H aromatic 3017, vC=C 1625, vNO 1538 , vC=N
1349, vAr 744; δH (400 MHz, DMSO-d6): 10.3 (1H, s, NCHN), 8.58-8.50 (4x1H, dd, phenyl),
8.15-8.13 (3x1H, m, pyridyl), 9.91-7.92 (1H, m, pyridyl), 7.60 (1H, d, J 6.9 Hz, CH=CH
imidazoyl), 7.43 (1H, d, J 6.9 Hz, CH=CH imidazoyl) and 5.70 ppm (2H, s, NCH2). δC (400 MHz,
DMSO-d6): 152.9 (NCHN), 149.6 (C pyridyl), 147.6 (CH pyridyl), 139.2 (C Ph), 137.5 (C Ph), 136.9
(CH pyridyl), 125.5 (CH pyridyl), 124.4 (CH Ph), 123.8 (CH imidazoyl), 122.9 (CH Ph), 122.7 (CH
Pyridyl), 121.2 (CH imidazoyl) and 53.6 ppm (NCH2). HRMS (m/z) for C15H13N4O2: calcd 281.1039
[M - Br]+, found 281.1030 [M – Br]+.
2.2.2. 3-(4-acetylphenyl)-1-(2-pyridylmethyl)imidazolium bromide (1b)
Prepared from 2-(bromomethyl)pyridine (0.31 g, 1.21 mmol) and 1-(4-(1H-imidazol-
1-yl)phenyl)ethanone (0.23 g, 1.21 mmol) in acetonitrile. Product obtained as air sensitive

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shiny orange-brown microcrystal, yield (0.39 g, 91%; mp 199-202 °C). IR (ATR cm-1): vmoisture
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DOI: 10.1039/C6NJ01118G
3381, vC-H 3061, 2955, vC=O 1675, vC-N 1237, vAr 836; δH (400 MHz, CDCl3): 11.1 (1H, s,
NCHN), 8.51 (1H, d, J 4.4 Hz, CH=CH imidazoyl), 8.19 (1H, d, J 4.4 Hz, CH=CH imidazoyl),
8.07 (2x2H, dd, J 16.5 Hz, phenyl), 7.97 (1H, s, pyridyl), 7.83 (1H, d, J 7.7 Hz, pyridyl), 7.74-
7.71 (1H, m, pyridyl), 7.35-7.26 (1H, m, pryridyl), 5.96 (2H, s, NCH2 pyridine) and 2.60 ppm
(3H, s, CH3 acetyl). δC (400 MHz, CDCl3): 196.4 (C=O), 152.1 (NCHN), 149.8 (C pyrdyl), 137.8
(CH pyridyl), 137.7 (C Ph), 137.5 (C Ph), 136.0 (CH pyridyl), 130.5 (CH Ph), 124.2 (CH pyridyl), 124.1
(CH imidazoyl), 124.0 (CH Ph), 121.8 (CH pyridyl), 120.6 (CH imidazoyl), 54.2 (NCH2) and 26.8 ppm
(CH3). HRMS (m/z) for C17H16N3O: calcd 278.1289 [M - Br]+, found 278.1293 [M - Br] +.
2.2.3. 3-(4-cyanophenyl)-1-(2-pyridylmethyl)imidazolium bromide (1c)
Dry acetonitrile solution of liberated 2-(bromomethyl)pyridine (0.30 g, 1.18 mmol) was
refluxed with acetonitrile solution of 4-imidazoylbenzonitrile (0.20 g, 1.18 mmol). Crude
product was obtained as distinct off-white precipitate within reddish-pink solution. Precipitate
of the salt was filtered out of the mixture under atmosphere of nitrogen using vacuum filtration,
rinsed with dried acetonitrile and dried in a desiccator. Yield (0.37 g, 93%; mp 265-267 °C).
IR (ATR cm-1): vmoisture 3345, vCH 3050, vCN(nitrile) 1597, vAr 748; δH (400 MHz, DMSO-d6): 10.2
(1H, s, NCHN), 8.57 (1H, d, J 4.4 Hz, pyridyl), 8.49 (1H, d, J 1.4 Hz, pyridyl), 8.23 (2x1H,
dd, CH=CH imidazoyl), 8.12―7.61 (4H, dd, phenyl), 7.59 (1H, d, J 7.8 Hz, pyridyl), 7.42 (1H, d,
J 5.1 Hz, pyridyl) and 5.69 ppm (2H, s, NCH2). δC (400 MHz, DMSO-d6): 153.0 (NCHN),
149.5 (C pyridyl), 137.9 (CH pyridyl), 137.5 (C ), 136.8 (CH pyridyl), 134.4 (CH ), 124.4 (CH
Ph
Ph
pyridyl), 123.8 (CH ), 122.7 (CH imidazoyl), 122.6 (CH pyridyl), 121.0 (CH imidazoyl), 117.8 (CN),
Ph
112.3 (C ) and 53.6 ppm (NCH2). HRMS (m/z) for C16H13N4: calcd 261.1140 [M - Br] +,
Ph
found 261.1131 [M - Br] +.
2.2.4. 3-allyl-1-(2-pyridylmethyl)imidazolium bromide (1d)
Acetonitrile solution of 2-(bromomethyl)pyridine (1.34 g, 5.3 mmol) was added drop
wise to allyl imidazole (0.57 g, 5.3 mmol) in acetonitrile at 0 °C and the mixture subjected to
reflux for 24 h. The salt was obtained as methanol eluent from column chromatography of the
crude. All volatiles were then removed under reduced pressure to yield the pure salt as a
brownish-grey waxy solid. Yield (1.29 g, 87.0%). IR (ATR cm-1): vmoisture 3401, vC-H 3050,
vC=C 1551, vC=N 1497, vAr 759; δH (400 MHz, DMSO-d6): 9.78 (1H, s, NCHN), 8.51 (1H, m,
pyridyl), 8.23 (1H, m, pyridyl), 7.98 (2H, dd, CH=CH imidazoyl), 7.36 (1H, m, pyridyl), 7.22 (1H,
d, J 12.9 Hz, pyridyl), 6.51 (1H, m, CH=CH2 allyl), 5.69 (2H, s, NCH2), 3.44 (2H, s, CH2=CH

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allyl) and 1.78 ppm (2H, s, CH2-N allyl). δC (400 MHz, DMSO-d6): 153.2 (NCHN), 149.5 (C
DOI: 10.1039/C6NJ01118G
pyridyl), 137.5 (CH pyridyl), 135.3 (CH pyridyl), 123.7 (CH allyl), 123.6 (CH pyridyl), 122.9(CH imidazoyl),
122.6 (CH imidazoyl), 121.7 (CH pyridyl), 119.5 (CH2 allyl), 53.0 (NCH2) and 14.4 ppm (CH2-N allyl).
HRMS (m/s) for C12H14N3: calcd 200.1188 [M - Br] +, found 200.1183 [M - Br] +.
2.2.5. 3-butenyl-1-(2-pyridylmethyl)imidazolium bromide (1e)
Liberated 2-(bromomethyl)pyridine (1.34 g, 5.3 mmol,) in acetonitrile was added at 0
°C to acetonitrile solution of butenyl imidazole (0.65 g, 5.3 mmol) and refluxed. Isolated yield
from column (1.23 g, 79%) brown waxy solid. IR (ATR cm-1): vmoisture 3358, vC-H 3050, 2973,
vC=C 1628, vC=N 1450, vAr 785; δH (400 MHz, CD3OD): 9.20 (1H, s, NCHN), 8.57 (1H, d, J 4.6
Hz, pyridyl), 7.90 (1H, m, pyridyl), 7.69 (2x1H, d, J 2.0 Hz, CH=CH imidazoyl), 7.55 (1H, m,
pyridyl), 7.43-7.41 (1H, m, pyridyl), 5.84-5.79 (1H, m, CH=CH2 alkenyl), 5.58 (2H, s, NCH2),
5.10-5.06 (2H, m, CH2=CH alkenyl), 4.37-4.33 (2H, m, CH2 alkenyl) and 2.68-2.66 ppm (2H, m,
CH2 alkenyl). δC (400 MHz, CD3OD): 154.4 (NCHN), 151.0 (C pyridyl), 139.2 (CH pyridyl), 134.2
(CH pyridyl), 125.3 (CH alkenyl), 124.2 (CH pyridyl), 123.6 (CH imidazoyl), 122.9 (CH imidazoyl), 119.6
(CH pyridyl), 54.8 (NCH2), 50.2 (CH2 alkenyl), 47.1 (CH2-N alkenyl) and 35.3 ppm (CH2 alkenyl).
HRMS (m/s) for C13H16N3: calcd 214.1344 [M - Br] +, found 214.1337 [M - Br] +.
2.2.6. 3-pentenyl-1-(2-pyridylmethyl)imidazolium bromide (1f)
Liberated 2-(bromomethyl)pyridine (1.34 g, 5.3 mmol) in acetonitrile was added to
acetonitrile solution of pentenyl imidazole (0.72 g, 5.3 mmol) and refluxed. Isolated yield from
column (1.31 g, 81%) brown waxy solid. IR (ATR cm-1): v(moisture) 3402, vC-H 3072, vC=N 1437,
1158, and vAr 756 δH (400 MHz, DMSO-d6): 9.42 (1H, s, NCHN), 8.54 (1H, d, J 4.8 Hz,
CH=CH imidazoyl), 7.90-7.87 (2x1H, m, pyridyl), 7.84 (1H, d, J 4.8 Hz, CH=CH imidazoyl), 7.52
(1H, m, pyridyl), 7.40-7.38 (1H, m, pyridyl), 5.85-5.81 (1H, m, CH=CH2 alkenyl), 5.60 (2H, s,
NCH2), 5.05-4.95 (2H, m, CH2-N alkenyl), 4.29 (2H, t, J 7.1 Hz, CH2=CH alkenyl), 2.03-1.99 (2H,
m, CH2 alkenyl) and 1.94-1.89 ppm (2H, m, CH2 alkenyl). δC (400 MHz, DMSO-d6): 153.5
(NCHN), 149.5 (C pyridyl), 137.5 (CH pyridyl), 136.9 (CH pyridyl), 123.6 (CH alkenyl), 123.3 (CH
pyridyl), 122.6(CH imidazoyl), 122.4 (CH imidazoyl), 115.7 (CH pyridyl), 52.9 (NCH2), 48.4 (CH2 alkenyl),
29.6 (CH2-N alkenyl), 28.3 (CH2 alkenyl) and 12.4 ppm (CH2 alkenyl). HRMS (m/s) for C14H18N3:
calcd 228.1501 [M - Br] +, found 228.1495 [M - Br] +.

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DOI: 10.1039/C6NJ01118G
2.3. Generic synthesis of Ag complexes 2a, 2d, and 2e
Equimolar quantities of the required imidazolium salt (1a, 1d, or 1e) and Ag2O were
loaded into oven dried Schlenk tube containing 4 Å molecular sieves initially cooled under
nitrogen. To this was then added 1,2-dichloroethane (30 ml) and the mixture subjected to
vigorous stirring and refluxed in the absence of light for 3 d. By then, the dark colour of the
silver(I) oxide had faded. The solvent was then removed under reduced pressure and the
resulting residue extracted with DCM and filtered through a bed of celite. We wanted to
confirm that the Ag-NHC complex was indeed formed; hence, complex 2a was isolated from
the DCM soluble filtrate. The filtrate was first concentrated and the resulting residue was stirred
with hexane or toluene. Removal of solvents to dryness afforded 88% of pure 2a. Compounds
2d and 2e were not as stable as 2a, hence, once generated (confirmed by the absence of
imidazolium C2-H at δ >9 in the 1H NMR spectrum of the crude) they were utilised in situ and
immediately transmetalated to the desired Co or Ni. However characterisation data for the
stable 2a are presented and fully discussed as a representative of the Ag-NHC family of
compounds.
2.3.1. [1-(4-nitrophenyl)-3-(2-pyridyl)imidazol-2-ylidene]silver(I)chloride (2a)
Both 1-(4-nitrophenyl)-3-(pyridine-2-ylmethyl)-1H-imidazol-3-ium bromide 1a, (0.50
g; 1.38 mmol) and silver(I)oxide (0.32 g; 1.38 mmol) were added into a Schlenk tube and
refluxed in 1,2-dichloroethane (30 ml) in the dark for 3 d. Yield: brown microcrystalline
powder, stable in air and unstable in chloroform but soluble in DCM and DMSO (0.510 g,
88%; mp. 173-176 °C). IR (ATR cm-1): vC-H 3084, 2946 vC-N 1515, vN-O 1336, vAr 844, 750; δH
(400 MHz, DMSO-d6): 8.50 (1H, d, J 4.4 Hz, pyridyl), 8.29 (2x1H, d, J 9.0 Hz, phenyl), 8.04-
8.02 (1H, m, pyridyl), 8.02 (2x1H, d, J 1.9 Hz, phenyl), 7.85 (1H, d, J 4.9 Hz, CH=CH imidazoyl),
7.81-7.80 (1H, m, pyridyl), 7.39 (1H, d, J 4.9 Hz, CH=CH imidazoyl), 7.34 (1H, t, J 2.2 Hz,
pyridyl) and 5.58 ppm (2H, s, NCH2). δC (400 MHz, DMSO-d6): 180.4 (NCN; C-Ag), 155.3
(C pyridyl), 149.5 (CH pyridyl), 146.8 (C Ph), 144.4 (C Ph), 137.4 (CH pyridyl), 125.7 (CH pyridyl),
124.9 (CH Ph), 124.4(CH imidazoyl), 123.3(CH Ph), 122.4(CH pyridyl), 120.4 (CH imidazoyl) and 56.4
ppm (NCH2). MS-ES+: m/z(%) 667(10) [Ag(ligand)2]+, 281(100) [(ligand + H)]+.

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2.4. Synthesis of Co and Ni complexes 3a and 3a′
2.4.1. Co[3-(4-nitrophenyl)-1-picolyimidazolin-2-ylidene]2Cl2 (3a)
To a solution of CoCl2(PPh3)2 (0.15 g; 0.24 mmol) in DCM (20 ml) was added drop-
wise a DCM (20 ml) solution of silver-NHC complex 2a (0.10 g, 0.24 mmol). The green
precipitate that formed was continuously stirred at room temperature in the dark for 2 d. At
the expiration of the reaction time, the supernatant liquid was filtered via cannula and the solid
green residue washed with more DCM till the washings became clear. Dry methanol (40 ml)
was then used to extract the solid and the orange-brown extract filtered through a bed of celite.
The clear orange-brown filtrate was then concentrated under reduced pressure to afford green,
air stable microcrystals of the cobalt-NHC complex: 3a. Yield (0.114 g, 69%; mp 275-278
°C). IR (ATR cm-1): vmoisture 3364, vC-H 3092, vNO 1522, vCN 1300, vAr 852, 748; δH (400 MHz,
CD3OD): 8.42 (2x1H, s, pyridyl), 8.25 (2x(1H, 1H), m, pyridyl), 8.02 (2x1H, m, pyridyl),
7.80-7.70 (2x4H, dd, phenyl), 7.42 (2x1H, s, J 4.6 Hz, CH=CH imidazoyl), 7.20 (2x1H, s, CH=CH
imidazoyl) and 5.46 ppm (2x2H, s, NCH2). δC (400 MHz, CD3OD): 153.7 (C pyridyl), 150.8 (CH
pyridyl), 149.6 (C Ph), 140.5 (C Ph), 139.1(CH pyridyl), 137.7 (CH pyridyl), 126.6 (CH Ph), 125.5(CH
imidazoyl), 124.6 (CH Ph), 124.4(CH pyridyl), 122.9 (CH imidazoyl) and 55.5 (NCH2). MS-ES+:
m/z(%) 690(5) [(M + H)]+, 619(10) [(M - 2Cl)]2+, 562(10) [(M - CoCl2 + 2H)]2+, 282(18)
[(ligand + 2H)]2+, 281(100) [(ligand + H)]+.
2.4.2. Ni[3-(4-nitrophenyl)-1-picolyimidazolin-2-ylidene]2Cl2 (3a′)
To a dark blue solution of NiCl2(PPh3)2 (0.15 g; 0.23 mmol) in DCM (20 ml) was added
an equimolar DCM solution (20 ml) of 2a . This prompted a yellow precipitate to be formed
and the mixture stirred in the absence of light at room temperature for 2 d. The DCM soluble
extract was filtered via cannula and the residue washed with more DCM (2 x 20 ml). The
resulting yellow residue was then extracted with dry methanol (40 ml) and filtered through
celite. Removal of all volatiles under reduced pressure yielded 3a′ as orange-yellow
microcrystals that became moist when exposed for a while in air. Yield (0.153 g, 96%; mp
283―285 °C). IR (ATR cm-1): vbroad moisture 3289, vC=C 1523, vC-N 1340, vAr 680; δH (400 MHz,
CD3OD): 8.84 (2x1H, bs, pyridyl), 8.42 (2x2H, d, J 8.8 Hz, phenyl), 8.14 (2x1H, s, CH=CH
imidazoyl), 7.95 (2x1H, s, CH=CH imidazoyl), 7.89-7.73 (2x(1H, 1H), m, pyridyl), 7.54 (2x1H, d, J
7.9 Hz, pyridyl), 7.34 (2x2H, d, J 6.2 Hz, phenyl ), 5.59 (2x2H, s, NCH2). No 13C NMR
spectrum was available due to possible decomposition during the accumulation of FID. HRMS
(m/s) for C30H24N8O4ClNi: calcd 653.0963 (M+ - Cl-), found 653.0983 (M+ - Cl-).

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2.5. Generic procedure for synthesis of Co and Ni complexes 3d, 3d′, 3e and 3DeO′I: 10.1039/C6NJ01118G
Equimolar amounts of the imidazolium salt and Ag2O were mixed together in a Schlenk
tube containing molecular sieves and refluxed in 1,2-dichloroethane (30 ml) in the dark for 3
d. All volatiles were removed and the dark grey solid extracted with DCM (40 ml) and filtered
through a bed of celite. The dark brown filtrate was then reduced to about 1/3 of the initial
volume and added drop wise to a Schlenk tube containing a DCM solution of 1 mole equivalent
of the metal precursor [MCl2(PPh3)2; M= Co, Ni]. The resulting suspension was then stirred
in the dark at room temperature for 2 d. The coloured suspension was filtered via cannula and
the precipitate washed with more DCM and filtered till the washings became colourless, and
then extracted with dry methanol (40 ml). Filtration through a bed of celite and subsequent
removal of all volatiles afforded the desired complex in high yield.
2.5.1. Co(3-allyl-1-picolyimidazolin-2-ylidene)2Cl2 (3d)
Imidazolium salt 1d (0.67 g, 2.39 mmol), Ag2O (0.55 g, 2.40 mmol) and CoCl2(PPh3)2
(1.57 g, 2.40 mmol) were reacted accordingly. Yield: dark green powder that decompose on
long exposure to air (1.20 g, 93%; mp 185-188 °C). IR (ATR cm-1): v(broad moisture) 3378, vC-H
3105, vC=C 1596, vC=N 1438, and vAr 718; δH NMR (400 MHz, CD3OD): 8.32 (2x1H, bs,
pyridyl), 7.59 (2x1H, bm, pyridyl), 7.51 (2x1H, d, J 7.9 Hz, CH=CH imidazoyl), 7.47-7.40 (2x1H,
bm, pyridyl), 7.29 (2x1H, d, J 7.9 Hz, CH=CH imidazoyl), 7.13 (2x1H, d, J 5.2 Hz, pyridyl), 6.59-
6.57 (2x1H, bm, CH=CH2), 5.82 (2H, d, J 7.6 Hz, NCH2), 5.31 (2H, s, NCH2), 5.26 (2x2H, s,
CH2=CH) and 1.56 ppm (2H, d, J 6.9 Hz, CH2-N allyl). μeff = 2.53 μB. MS-ES+: m/z(%) 492(60)
[(M - Cl)]+, 457(20) [(M - 2Cl)]2+, 200(100) [(ligand + H)]+.
2.5.2. Ni(3-allyl-1-picolyimidazolin-2-ylidene)2Cl2 (3d′)
Imidazolium salt 1d (0.67 g, 2.39 mmol), Ag2O (0.55 g, 2.40 mmol) and NiCl2(PPh3)2
(1.57 g, 2.40 mmol) were reacted accordingly. Yield: lemon-green microcrystals relatively
stable in air (1.19 g, 93%; mp 243-245 °C). IR (ATR cm-1): v(broad moisture) 3323, vC=C 1610, vC-
N 1437, vAr 743; δH (400 MHz, DMSO-d6): 8.16 (2x1H, s, pyridyl), 8.05 (2x1H, m, pyridyl),
7.83 (2x1H, s, CH=CH imidazoyl), 7.69 (2x1H, s, pyridyl), 7.37 (2x1H, s, CH=CH imidazoyl), 7.16
(2x1H, m, pyridyl), 6.57 (2x1H, d, J 13.5 Hz, CH=CH2 allyl), 5.97 (2H, s, NCH2), 5.81 (2H,
d, J 13.6 Hz, NCH2), 4.09 (2x2H, s, CH2=CH allyl) and 1.48 ppm (2x2H, s, CH2-N allyl). MS-
ES+: m/z (%) 491(100) [(M – Cl)]+, 455(10) [(M– 2Cl)]2+.

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DOI: 10.1039/C6NJ01118G
2.5.3. Co(3-butenyl-1-picolyimidazolin-2-ylidene)2Cl2 (3e)
Imidazolium salt 1e (0.56 g, 2.0 mmol), Ag2O (0.44 g, 2.0 mmol) and CoCl2(PPh3)2
(1.25 g, 2.0 mmol) were reacted accordingly. Yield: dark green air sensitive powder (0.79 g,
71%; mp 120-123 °C). IR (ATR cm-1): v(broad moisture) 3365, vC=C 1596, vC-N 1437, vAr 762; δH
(400 MHz, DMSO-d6): 8.81-7.80 (2x6H, bm: 4H, pyridyl; 2H, CH=CH imidazoyl), 7.57-7.49
(2x1H, bm, CH=CH2), 5.93 (2H, d, J 5.8 Hz, NCH2), 5.75 (2H, s, NCH2), 5.05 (2x2H, t, J 12.2
Hz, CH2=CH), 4.11-4.01 (2x2H, m, CH2-N alkenyl) and 2.23 ppm (2x2H, s, CH2). μeff = 2.73
μB. MS-ES+: m/z (%) 557(7) [(M + 2H)]2+, 520(22) [(M – Cl)]+, 485(20) [(M – 2Cl)]2+,
214(100) [(ligand + H)]+.
2.5.4. Ni(3-butenyl-1-picolyimidazolin-2-ylidene)2Cl2 (3e′)
Imidazolium salt 1e (0.56 g, 2.0 mmol), Ag2O (0.44 g, 2.0 mmol) and NiCl2(PPh3)2
(1.25 g, 2.0 mmol) were reacted accordingly. Yield: yellowish green air sensitive powder (0.92
g, 83%; mp 108-110 °C). IR (ATR cm-1): v(broad moisture) 3329, vC=C 1610, vC-N 1438, vAr 769; δH
(400 MHz, DMSO-d6): 8.25 (2x1H, s, pyridyl), 8.06 (2x1H, m, pyridyl), 7.84 (2x1H, s,
CH=CH imidazoyl), 7.71 (2x1H, s, CH=CH imidazoyl), 7.44 (2x2H, m, pyridyl), 6.93 (2x1H, m,
CH=CH2), 5.90 (2H, d, J 14.9 Hz, NCH2), 5.65 (2H, s, NCH2), 4.81-4.77 (2x2H, m, CH2=CH),
4.11-3.70 (2x2H, m, CH2-N alkenyl) and 2.32 ppm (2x2H, s, CH2 alkenyl). MS-ES+: m/z (%)
519(50) [(M – Cl)]+, 484(10) [(M – 2Cl-)]2+, 214(100) [(ligand + H)]+.
2.6. Analysis of magnetic susceptibility measurement data
The numeric data obtained from the hysteresis loop measurement using the VSM when
analysed as a plot of magnetic moment/mass (emu/g) against field (Oes) gave an initial
magnetization curve which qualitatively confirmed the paramagnetic nature of the complexes.
The experimentally determined value of saturated magnetization Ms is a function of the number
of moles ‘n’ used per atom (0.025-0.031g). This was used to determine the magnetic
susceptibility (μeff) of each paramagnetic complex in μB which was compared with the
calculated magnetic moments consistent with the number of un-paired electrons. The quotient
Ms/n divided by a constant 5584.9 (derived from the equation: mu_B*N_A = 5584.9
emu/mole; where N_A is Avogadro's number) gave the magnetic susceptibility of the
complexes [28].

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DOI: 10.1039/C6NJ01118G
2.7. Typical procedure for the 3a′ activated arylation of aniline with phenyl bromide
To an oven dried Schlenk tube initially cooled under nitrogen was added aniline (1.0 g,
1 ml, 0.010 mol), phenyl bromide (2.0 g, 1.4 ml, 0.013 mol), KOtBu (1.46 g, 0.013 mol) and
3a′ (0.069 g, 0.0001 mol). The mixture was vigorously stirred in refluxing THF (20 ml) under
an atmosphere of nitrogen for 12 h with reaction progress monitored by TLC. The contents of
the Schlenk tube were then allowed to cool to room temperature after which DCM (30 ml) was
added into the flask, stirred for an hour and filtered through a bed of celite. All volatiles in the
filtrate were removed under reduced pressure and the crude residue obtained was mixed with
silica and loaded onto a column. The desired product (diphenyl amine) was obtained as an
eluent of hexane: DCM (4:1). Removal of all volatiles gave isolated yield of a low melting
yellowish-brown solid (1.13 g, 67%). IR (ATR cm-1): νN-H 3380 (bs, 2o amine), νC-H 3041, νC-
N 1307, νAr 741; δH (400 MHz, DMSO-d6): 8.12 (1H, bs, N-H), 7.21 (2x2H, d, J 7.4 Hz), 7.06
(2x2H d, J 7.6 Hz), 6.81 (2x1H t, J 7.3 Hz,). δC (400 MHz, DMSO-d6): 143.4 (C ), 129.1
Ph
(CH Ph), 119.6 (CH Ph), 116.7 (CH Ph).
2.8. Typical procedure for NiCl2/1a in situ activated arylation of aniline with phenyl
bromide
Anhydrous NiCl2 (0.013 g, 0.0001 mol), imidazolium salt 1a (0.072 g, 0.0002 mol) and
4Å molecular sieves were loaded into a Schlenk tube under an atmosphere of nitrogen and
stirred together in THF for 1 h at room temperature. To the resulting suspension was then
added KOtBu (1.46 g, 0.013 mol), aniline (1.0 g, 0.010 mol) and phenyl bromide (2.0 g, 0.013
mol). The Schlenk tube was then equipped with a condenser and subjected to reflux for 12 h.
After cooling to room temperature, DCM (30 ml) was added and the contents filtered through
a bed of celite. The resulting filtrate was then concentrated and the crude obtained was mixed
with silica and loaded onto a column. The diarylamine product was isolated as an eluent of
hexane: DCM (4:1) after removal of all volatiles.

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3.
Results and discussion
3.1. Synthesis:- picolyl functionalised salts
In the synthesis of the N-substituted imidazoles; conventional synthetic strategy
DOI: 10.1039/C6NJ01118G
applicable to the type of N-substituent were applied [24]. The picolyl functionalised ligand
precursors (1a-f) were synthesised using modification of established procedures [Scheme 1 (a)]
[27].
N
Br
i- 0 oC, Na2CO3(aq)
ii - Et2O
1a: R= C6H4NO2
1b: R=C6H4COCH3
1c: R=C6H4CN
1d: R=allyl
1e: R=butenyl
1f: R=pentenyl
N
N
(a)
+
N
N
HBr
N
iii-CH3CN, reflux,
N2, 24 h
R
R
Br
1a-f
2+
i - Ag2O, 4Å MS,
ClCH2CH2Cl, N2, reflux, 3 d
R
N
i - MCl2(PPh3)2
DCM,4Å MS, rt, N2,3 d
N
N
1a,d,e
N
(b)
N
M
ii - extract with DCM,
N2, rt, 1 h
AgCl
ii - extract with
MeOH
N
N
R
2Cl
N
N
R
2a: R=C6H4NO2
2d: R=allyl
2e: R=butenyl
3a: R=C6H4NO2, M=Co
3a': R=C6H4NO2, M=Ni
3d: R=allyl, M=Co
3d': R=allyl, M=Ni
3e: R=butenyl,. M=Co
3e': R=butenyl, M=Ni
Scheme 1. Synthesis of (a) functionalised imidazolium salts with picolyl moiety and (b)
picolyl functionalised imidazol-2-ylidene(NHC)-Ni(II) and Co(II) complexes.
All the imidazolium salts were obtained in excellent yields via a purification technique
we described for similar salts [25]. In the synthesis of the imidazolium salts, it seems the type
of solvent used has an effect on the reaction rate and we observed that under similar conditions,
when compared to DCM, methanol or toluene as the reaction medium acetonitrile tends to give
higher yields with great ease of purification [27, 29].
Both 3-(4-nitrophenyl)-1-
picolylimidazolium bromide 1a, and 3-(4-cyanophenyl)-1-picolylimidazolium bromide 1c,
precipitated out from the reaction solution after about 12 h of reaction; and were eventually

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obtained by a simple filtration of the salts under an atmosphere of nitrogen which were purViifeiweAdrticle Online
DOI: 10.1039/C6NJ01118G
by simple washing with dry acetonitrile. Alternatively, the crude product may be concentrated
and the salt isolated as methanol eluent via column chromatography [25]. Both 1b and the
alkenyl picolyl imidazolium salts 1d-f, were purified in a similar fashion as methanol eluents
which were then isolated as either grey-brown highly hygroscopic solid (1b), brown waxy
substance (1d) or light brown oils (1e & f).
3.2.Characterisation of salts
The 1H NMR spectrum of each of the salts (1a-f) showed the distinct C-2 proton singlet
peaks at 9-11 ppm, a clear indication of imidazolium salt formation [6a, 30]. Coupling of the
imidazole with the picolyl moiety in all the salts 1a-f was confirmed by the appearance of a
distinct singlet characteristic of the methylene (NCH2) protons observed at 5.5-5.7 ppm [7a,
27]; integrating for 2H. The alkenyl C2 protons (CH2=CH) appeared as a multiplet in 1d-f,
while the distinct singlet at 2.60 ppm corresponded to the methyl group on the acetyl functional
group in 1b (analyses spectra available as supporting information). The rest of the protons in
the 1H NMR spectra of all the salts were within the aromatic region and were consistent with
expected integration and coupling multiplicities for such compounds. The corresponding 13C
NMR spectra of the salts showed the methylene carbon peak (NCH2) at the range 53.5-54.9
ppm with the characteristic down field peak of the imidazolium carbons at 152-154 ppm [27,
30a]. All the salts (1a-f) are extremely hygroscopic and quickly imbibed varying quantities of
moisture (FTIR broad peaks at 3349-3355 cm-1), a common reported problem for related salts
[25]. However, the clean NMR data and exact HRMS results established their purity (see
supporting information). The HRMS (TOF-MS ES+) of all the salts exhibited peaks due to the
molecular ion with the loss of one bromide counter ion. The found values are all within the
acceptable limits of the calculated values of the parent-derived ion peak. The three solid
microcrystalline salts had narrow melting point ranges within expected limits of their molecular
compositions of 220-222, 199-202 and 265-267 °C respectively for 1a-c. These are indicative
of pure solids and comparable to related published data.
3.3.Synthesis and characterisation of metal-NHC complexes
All the nickel and cobalt complexes reported herein were synthesised via
transmetalation from corresponding Ag-NHC complexes (Scheme 1b). It is worthy to note that
for this to succeed we chose to use Ag2O in refluxing 1,2-dichloroethane as a convenient means
to alleviate well documented difficulties [7,10b,27,31] with the formation of Ag-NHC

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compounds of bulky ligand precursors following conventional methods. We also added 4 Å
DOI: 10.1039/C6NJ01118G
molecular sieves to the reaction medium during formation of the Ag-NHC complexes, it served
to improve product yield by the elimination of moisture and other impurities. Hence, Ag-NHC
complexes of 1a, 1d and 1e were prepared by refluxing the corresponding mixtures of the salts
and excess Ag2O in 1,2-dichloroethane for 3 d, within which the dark colour of Ag2O
disappeared. Complete formation of the Ag-NHC complexes was monitored by the
disappearance of the C2 imidazolium protons (9-11 ppm) by 1H NMR of crude products (Fig.
1).
Figure 1: Comparative 1H NMR spectra of 1a (top) and 2a (bottom).
Concentration of the crude and extraction with DCM afforded the Ag-NHC complexes
2a, 2d and 2e [19]. Complex 2a was stable to air and light, hence it was isolated and fully
characterised. The relatively lower melting range of 2a (173-176 °C) as compared to the
imidazolium salt 1a (220-222 °C) is a clear indicator to the formation of a thermally weak Ag-
NHC complex [32]. Compounds 2d and 2e derived from 1d and 1e were less stable to air;
moisture and light, hence were directly transmetalated to Co and Ni. We monitored the
disappearance of the downfield C2 imidazolium protons from the ligand precursors as a guide
and sign of successful deprotonation by the basic Ag2O. Also, further analysis of 2a confirmed

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13
its successful isolation; C NMR data showed a downfield shift of the C2 carbon fromVi1ew5A2rticle Online
DOI: 10.1039/C6NJ01118G
ppm in 1a to 180 ppm in 2a (Figure 2); typical of a carbene-metal bond [33].
Figure 2: Comparative 13C NMR spectra of 1a (top) and 2a (bottom).
The methylene protons in the picolyl moiety also shifted up-field (5.70 to 5.58 ppm) in
1
the H NMR data of 2a with a corresponding downfield shift of the methylene carbon peak
(53.5 to 56.4 ppm; Figure 2) as reported for similar Ag-NHC complexes [27]. Elemental
analysis (CHN) data obtained for 2a agree with the formation of Ag(ligand)Cl and the MS-ES+
results gave the molecular ion as [Ag(NHC)2]+ characteristic of Ag-NHC complexes of the
form Ag(NHC)X [27].
Transmetalation of the Ag-NHC complexes 2a, 2d, and 2e to the desired cobalt or
nickel with either CoCl2(PPh3)2 or NiCl2(PPh3)2 as the metal precursors was achieved by a
slight modification of an established transmetalation procedure [7a]. A distinct colour change
in each of the reactions was an early indicator of successful transmetalation. For example, the
blue colour of the DCM solution of NiCl2(PPh3)2 was observed to gradually fade and develop
a yellow/orange precipitate of the product Ni(NHC)2Cl2. Unreacted Ag-NHC, the metal
precursor and displaced PPh3 were recovered as DCM soluble components. Extraction of the
residue with methanol followed by filtration through a bed of celite and final evaporation of
the solvent under reduced pressure afforded the respective fairly air stable orange powders of
Ni(NHC)2Cl2 (3a′,3d′,3e′) or dark green powders of Co(NHC)2Cl2 (3a, 3d, 3e) complexes.
The cobalt complexes turned into dark orange solutions when dissolved in methanol while the

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nickel complexes maintained their colour in methanol solutions. None of theDOcIo: 1m0.1p0l3e9x/Ce6sNJ01118G
1
showed a peak around 9-11 ppm in their respective H NMR spectra, a confirmation of
successful deprotonation of the imidazolium protons and formation of carbene-metal bonds
(see accompanying supporting information). Absence of a peak around -6 ppm characteristic
of PPh3 in the 31P NMR data is indicative of the purity of the complexes to be free of liberated
phosphine ligand. All the complexes were further characterised by melting point, IR (ATR),
MS-ES+ and HRMS.
1
The H NMR data of 3a accounted for all the expected protons, but interestingly the
expected doublet and singlet peaks that usually account for the diastereotropic methylene
protons in the ligand moiety of similar complexes [7a, 9a]; was observed as a singlet in 3a. The
appearance of the singlet slightly shifted up-field from 5.58 ppm in 2a to 5.47 ppm in 3a was
attributed to possible dynamic atropisomerisation in the three aromatic rings that composed the
ligand backbone as observed in related metal-NHC complexes bearing pincer CNC architecture
[11b, 34]. In such systems, only at a fast rate of inter-conversion would the two methylene
(NCH2) protons be observed on the same plane to generate a singlet, hence giving an insight
into the solution phase geometry of the metal complex.
The related yellow microcrystalline Ni-NHC complex 3a′ gave a 1H NMR data similar
to that obtained for the Co-NHC analogue. Although the peaks were not as sharp, all the
protons were accounted for and the expected diastereotropic protons representing NCH2
methylene protons also appeared as a singlet peak within a similar region to the corresponding
Co complex 3a.
Conversely, both 3d and 3e are ferromagnetic/paramagnetic species as observed from
the respective magnetic susceptibility (2.53 and 2.73 μB) measurements [28]. These values are
in agreement with data reported for related cobalt complexes which are consistent with one
unpaired electron [11]. Hence, only broad 1H NMR spectra were recorded for these compounds
1
(supporting information). However, fully resolved H NMR spectra were obtained for the
corresponding nickel complexes 3d′ and 3e′, with all the expected proton peaks accounted for.
The characteristic doublet and singlet peaks accounting for the diastereotropic protons of each
methylene bridge in the ligand moieties appeared in 3d′ at δ 5.81 and 5.97 ppm and for 3e′ at
δ 5.90 and 5.65 ppm a confirmation of dicarbene complexation to nickel (supporting
information).

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DOI: 10.1039/C6NJ01118G
3.4. Catalytic arylation of aniline
The pioneering works of Migita and co-workers [35] paved the way to transition metal
catalysed arylation of amines with aryl halides, although credit for C-N coupling is attributed
to Buchwald and Hartwig after whom the well-known Buchwald–Hartwig amination is named
[20a, 36]. Most C-N coupling reactions are base initiated with the base serving to reduce the
catalyst precursor to an active state. The organo-alkali metal salts KOtBu and NaOtBu are the
most effective in a variety of catalytic coupling reactions that include C-C coupling of α-
ketones with aryl halide [13], metal-NHC catalysed arylation of aromatic amines [2j, 11b, 13,
20b], and CuTC catalysed arylation of amines and amides [22d]. The organic nature of these
bases and the ease of isolation and removal of their precipitates are some of the advantages that
make them prominent in coupling reactions. Although, the use of commoner bases such as
KOH and K3PO4 may seem advantageous for C-N coupling reactions, they are unsuitable
because KOH for instance is known to react with the substrate yielding mainly phenolic
products while the use of K3PO4 usually resulted in poor yields [37]. However, in Cu-catalysed
aryl C-N coupling reactions, simple inorganic bases like K2CO3 and Na2PO4 are specifically
preferred [37-38].
Results of a preliminary and optimisation study presented in Table 1 investigated the
KOtBu promoted C-N coupling reaction of aniline (4) and phenyl bromide (5) to preferentially
yield the desired monoarylated amine (7).

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DOI: 10.1039/C6NJ01118G
Table 1
Optimization of reaction conditions for arylation of aniline (4) with phenyl bromide (5)
catalyst, base, solvent
N2, reflux, 12 h
+
NH2
+
Br
N
H
6
5
7
4
Entry
Catalyst
Substrates’ mole
Solvent
Yield (%)a
equivalents
5
4
6
7
1
None
1.2
1.2
-
1
THF
THF
THF
THF
THF
THF
THF
13
-
-
2b
None
1
-
3
None
1.2
1.2
1
-
-
4c
None
1
-
-
5
1.2
1
28
27
21
32
40
60
3d
6
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
3d
7
1
3d′
8
1
Acetonitrile
Dioxane
THF
Trace
29
Trace
3d′
9
1
41
3d′
10
11
12
13
14d
15e
16f
17g
18h
19f
20f
21f
22
1
28
67
3a′
1
THF
30
15
3a
1
THF
21
40
3e′
1
THF
13
20
3e
1
THF
12
Trace
20
3a′
1
THF
18
3a′
1
THF
31
68
3a′
1
THF
38
28
3a′
1
THF
23
69
3a′
NiCl2
1
THF
Trace
25
Trace
60
NiCl2/1a (1:1)
NiCl2/1a (1:2)
NiCl2/1a (1:2)
1
THF
1
THF
28
65
1
THF
24
63
Except otherwise mentioned, all reactions were done with 1 mol% of catalyst, 1.2 mole equivalent KOtBu, under
an atmosphere of nitrogen and refluxing solvent for 12 h. Progress of reaction monitored by TLC. a Isolated yield
from column chromatography, average of two runs. b Reaction done at room temperature. c Reaction done in the
absence of a base. d 1.2 mole equivalent of K2CO3 used as the base. e 1.2 mole equivalent of Na2PO4 used as the
base. f Reaction done for 18 h. g 0.5 mol% of catalyst used. h 2 mol% of catalyst used.

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19
As the primary control reaction, the two substrates were reacted together inDtOhIe: 1a0.b10s3Ve9ien/wCcA6erNticJl0e1O1n1l8inGe
of any catalyst (entry 1) and as expected no traces of the desired monoarylated product
(diphenyl amine) were observed. However, a 13% yield of biphenyl, 6, (a yellowish-white
solid, confirmed by NMR, IR and mp = 70-72 °C) was isolated as a by-product in what is
believed to be a competing C-C coupling reaction, which has been identified by others as a
variant of the Hiyama coupling reaction [39]. Further studies were conducted on the blank
control reaction by conducting the reaction at room temperature (entry 2), conducting the
reaction in the absence of the aryl amine (entry 3) or a base (entry 4). The results indicate an
interesting relationship such that heat, base and both substrates are all required for any reaction
to take place.
We then introduced 1 mol% of Co(allylNHC)2Cl2, 3d as the catalyst with a slight molar
excess of the aniline (entry 5). This yielded a modest quantity of the desired product (32%)
along with a corresponding increase in the quantity of the biphenyl product. By changing the
relative mole ratios of the substrates (entry 6) such that a slight excess of the aryl halide was
used with the same catalyst 3d, an increase in the isolated yield of the product 7 and a slight
decrease in that of the side product 6 was observed. This confirms the observation in entry 3
(absence of 4) that the aniline may be responsible for promoting the unwanted reduction of the
aryl halide to biphenyl.
Most of the non-precious metal catalyst systems reported to be active in C-N coupling
reactions are based on nickel metal and its complexes [2j, 2m, 13], hence when the Ni analogue
3d′ was tested under the same conditions as entry 6, 60% of the desired 7 was isolated (entry
7). What is obvious at this stage is that the catalyst selectively promotes the C-N coupling of
the two substrates in competition with the reduction of 5 and its C-C coupling leading to the
formation of the biphenyl 6.
The role of the solvent is important in aryl coupling reactions (entries 7-9). It is clear
that THF is the better of the three solvents tested for the current catalyst systems, as both
acetonitrile (entry 8) and dioxane (entry 9) negatively affected catalyst productivity. This
observation implies that despite the fact that both THF and dioxane are ethereal solvents, mono
N-arylation of aromatic amines is more facile in THF which is consistent with observations in
a related work by Kuhl et al. for the selective mono N-arylation of aromatic diamines in THF,
while in refluxing dioxane they reported N,N’-diarylation of the aromatic diamines [2j]. On
this basis, we adopted THF as the reaction solvent to study the influence of other reaction

New Journal of Chemistry
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parameters such as the steric and electronic nature of the NHC ligand and influencDeOoI:f10v.1a0r3i9o/Cu6sNJ01118G
bases.
Entry 13 illustrates the influence of ligand sterics on the catalysis with the bulkier
butenyl functionalised catalysts 3e yielding lower conversions of the desired product as
compared to the allyl variants (entry 6). Further studies were conducted with 3a′ and in situ
generated catalyst systems obtained from the mixture of imidazolium salt 1a and anhydrous
NiCl2 (entries 15-22). Only traces or low yield (20%) of 7 were obtained with the use of K2CO3
and Na2PO4 as bases (entries 14 and 15 respectively). Increasing the reaction time to 18 h
(entry 16), did not bring any change in yield of 7 (68%) as compared to the standard 12 h (entry
10, 67%). Entries 17 and 18 indicate that the optimum catalyst concentration for the current
system is 1 mol% as the productivity significantly diminished at 0.5 mol% and showed
negligible increase when doubled to 2 mol%.
Finally, the study was extended to investigate in situ generated catalyst systems. The
work of Kuhl et al. [2j], has shown that when combined with appropriate metal sources, bulky
imidazolium salts could generate in situ catalysts. We have recently shown that in situ
generated Fe-NHC complexes are efficient systems for the transfer hydrogenation of ketones
[40]. Therefore, entries 20-22 report on the efficiency of an in situ generated Ni-NHC catalysts
whereby a mixture of NiCl2/1a yielded 63% of the desired product after 12 h which is
comparable to the most active catalyst (3a′) tested thus far. This is a very important finding
given the difficulty and laborious synthetic strategies required for the preparation and isolation
of the metal complexes. To illustrate the scope and limitations of the in situ generated Ni
system, a series of substrates bearing a variety of functional groups were studied. For the
purpose of comparison, the most active isolated complex 3a' was run alongside the in situ
system under the same reaction conditions, the results are presented in Table 2.

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DOI: 10.1039/C6NJ01118G
Table 2
Metal-NHC complexes catalysed N-mono arylation of aryl amines with aryl bromides
R"
3a' or NiCl2/1a (1 mol%)
R"
H
N
NH2
+
Br
KOtBu, THF, N2,
reflux,12 h
R'
R'
Entry
Substrate
Product
Catalyst
Yield (%)†
NH2
Br
N
H
1
67
3a′
5
7
4
1a
2
NiCl2/1a
65
69
3a′
H
N
Br
4
8
9
2a
3
NiCl2/1a
66
71
3a′
H
N
Br
Br
4
11
10
3a
4
NiCl2/1a
69
81
H
N
CN
CN
3a′
4
4
13
12
4a
5
NiCl2/1a
79
73
O
O
3a′
H
N
Br
Br
14
15
5a
6
NiCl2/1a
70
70
H
N
O
O
3a′
4
16
17
6a
7
NiCl2/1a
70
75
H
O
NH2
O
N
5
3a′
17
18

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7a
8
NiCDlO2/I:11a0.1039/C76N3J01118G
H
N
76
3a′
O
18
10
19
8a
9
NiCl2/1a
78
0
NO2
NH2
3a′
5
20
9a
10
NiCl2/1a
0
O
X
Aldol
3a′
4
21
X=Cl, I
10a
NiCl2/1a
Aldol
All reactions were conducted with either 3a′ (1.0 mol%) or a mixture of NiCl2/1a (1:2 molar ratio), KOtBu (1.2
mole equivalent), aryl amine (1.0 mole equivalent), aryl bromide (1.2 mole equivalent), under an atmosphere of
nitrogen and refluxing THF (20 ml) for 12 h. Progress of the reaction was monitored by TLC. †Isolated yield
from column chromatography on average of two runs.
A recent microwave assisted Pd-NHC catalysed C-N coupling reaction [20b] was
reported to display good catalytic activities in Buchwald-Hartwig amination of aryl chlorides.
The data presented by Wang and co-workers is comparatively higher than our values, however,
what this study has demonstrated is that appreciably good yields of monoarylated products are
obtainable under milder conditions based on an earth abundant, non-PGM; nickel catalyst. The
results are also comparable to those reported for monodentate [16], bidentate [2m] NHC-metal
catalyst systems involving cobalt or nickel centre and even the tridentate NHC-metal
complexes of the platinum group of metals [34b]. Hence, an examination of the variety of
substrates reveals that the steric position or electronic nature of the substituent on the aryl halide
had very little overall influence on the coupling reaction as most of the substrates yielded
comparable isolated quantities (circa 70%) of the desired product. A noticeable improvement
was observed with aryl bromide bearing the electron withdrawing CN group as a para-
substituent (entry 4-4a, circa 80% yield) which has been established to render the aryl halide
more reactive and easily susceptible to oxidative attack by the nickel centre [38].
Under the present reaction conditions with either catalyst system, no products were
isolated in the attempted arylation of 2-nitro aniline (entry 9-9a), which is similar to reports on
a CuI catalysed reaction for related substrates under harsher conditions [41]. Both catalysts
(3a' and NiCl2/1a) also failed to activate formation of the desired diarylamine on a mixture of

Page 23 of 27
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23
aniline and p-chloro- or iodo-acetophenone. In both cases, an aldol product (from the VsieewlAfr-ticle Online
DOI: 10.1039/C6NJ01118G
condensation of acetophenone) was isolated in excellent yields which has also been reported
by Matsubara et al. [13] and recently by us [11b] to be more facile in such base promoted
competitive reactions. Finally, it is interesting to note that in all the combinations studied,
comparable catalytic efficiencies were recorded for the isolated metal complex 3a′ and the in
situ generated NiCl2/1a catalyst, hence opening up a simple, robust and functional group
tolerant system for C-N coupling reactions.
As no experimental evidence is available on the mechanism for the catalysed reactions,
we have mentioned earlier that recent and past studies on Ni and Pd-NHC complex catalysed
aryl C-N coupling reactions have been achieved with the metal in the zero (0) oxidation state.
In such studies, the choice of the base was mostly associated with the ease to which it was able
to initiate reduction of the metal centre and thereby create the active specie with an oxidation
state capable of activating the coupling reagents. On this basis a mechanistic pathway followed
by this catalytic reaction could be similar to what we recently reported with similar Co-NHC
complexes bearing pincer ligand architecture [11b].
4.
Conclusion
Herein we have reported the successful synthesis in excellent yields of six new
imidazolium salts bearing a picolyl moiety which were conveniently characterised and some
of which were used as precursors of NHC ligands that afforded Ag-NHC complexes in good
yields. Transmetalation of both the isolated and in situ utilised Ag-NHC complexes afforded
the desired Co and Ni complexes in good to excellent yields based on CoCl2(PPh3)2 or
NiCl2(PPh3)2 as metal precursors respectively.
All the complexes were active in catalysing N-mono arylation of aniline with phenyl
bromide, with the highest yield obtained with the Ni based complex 3a′. The catalyst systems
showed excellent selectivity to N-mono arylation as no traces of N,N-diarylated products were
observed. The good yields of the products, mild reaction conditions and the use of green, earth
abundant, non-PGM; first row transition metals made the current catalyst systems competitive
in comparison to classical Cu mediated or CuI catalysed Ullman coupling and Pd catalysed
systems for aryl C-N coupling reactions.

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DOI: 10.1039/C6NJ01118G
Acknowledgements
We thank the University KwaZulu-Natal and the NRF for financial support. Dr T. Moyo
is acknowledged for the use of VSM lakeshore 735. HI thanks FCE (T) Gusau and TET Fund
for a study fellowship.
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Page 27 of 27
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DOI: 10.1039/C6NJ01118G
Earth abundant metal complexes of donor functionalised N-heterocyclic
carbene ligands: synthesis, characterisation and application as amination
catalysts
Halliru Ibrahim, Muhammad Dabai Bala∗
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban
4000, South Africa
Detailed study on Co and Ni complexes of N-functionalized chelating NHC ligands applied for
the mono-arylation of aniline and substituted aryl amines is presented.
* Correspondence to: e-mail- bala@ukzn.ac.za; fax- +27 31 260 3091