Nickel complexes of 1,2,4-triazole derived amido-functionalized N-heterocyclic carbene ligands: Synthesis, theoretical studies and catalytic application

Article abstract of DOI:10.1016/j.jorganchem.2015.03.007

A series of nickel complexes (1-3)b of 1,2,4-triazole derived amido-functionalized N-heterocyclic carbene ligands were synthesized and structurally characterized. In particular, the [1-(R)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-ylidene]₂Ni [R = Et (1b), i-Pr (2b) and Bn (3b)] complexes were obtained by the direct reaction of the corresponding triazolium chloride salts (1-3)a by the treatment with NiCl₂·6H₂O in presence of K₂CO₃ as a base. The density functional theory studies performed on these complexes reveal highly polar character of the NHC-Ni σ-bonding interaction with corresponding molecular orbital having a maximum contribution (59-69%) from the NHC ligand fragments while that of a minimum contribution (4%) from the central nickel atom. The (1-3)b complexes were found to be moderately active for the catalytic borylation reactions of bromoaryl derivatives by bis(pinacolato)diboron reagent (B₂pin₂) in the presence of Cs₂CO₃ as a base at 70 °C.

Full text of DOI:10.1016/j.jorganchem.2015.03.007

Accepted Manuscript
Nickel Complexes of 1,2,4-Triazole Derived Amido-functionalized N-heterocyclic
Carbene Ligands: Synthesis, Theoretical Studies and Catalytic Application
Anuj Kumar, Linus Paulin Bheeter, Manoj Kumar Gangwar, Jean-Baptiste Sortais,
Christophe Darcel, Prasenjit Ghosh
PII:
S0022-328X(15)00146-1
10.1016/j.jorganchem.2015.03.007
JOM 18958
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 8 September 2014
Revised Date: 1 January 2015
Accepted Date: 4 March 2015
Please cite this article as: A. Kumar, L.P. Bheeter, M.K. Gangwar, J.-B. Sortais, C. Darcel, P. Ghosh,
Nickel Complexes of 1,2,4-Triazole Derived Amido-functionalized N-heterocyclic Carbene Ligands:
Synthesis, Theoretical Studies and Catalytic Application, Journal of Organometallic Chemistry (2015),
doi: 10.1016/j.jorganchem.2015.03.007.
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ACCEPTED MANUSCRIPT
Nickel Complexes of 1,2,4-Triazole Derived Amido-functionalized N-
heterocyclic Carbene Ligands: Synthesis, Theoretical Studies and
Catalytic Application
†
‡
†
‡
Anuj Kumar, Linus Paulin Bheeter, Manoj Kumar Gangwar, Jean-Baptiste Sortais,
,
‡
,†
Christophe Darcel* and Prasenjit Ghosh*
†
Department of Chemistry
Indian Institute of Technology Bombay,
Powai, Mumbai 400 076.
‡
Université de Rennes 1,
UMR CNRS-UR1 6226 Institut des Sciences chimiques de Rennes,
Team “Organometallics: Materials and Catalysis” - Centre for Catalysis and Green
Chemistry
Campus de Beaulieu, 35042 – Rennes, France
Email: pghosh@chem.iitb.ac.in, christophe.darcel@univ-rennes1.fr,
Fax: +91 22 2572 3480, +33 2 23 23 69 39
1

ACCEPTED MANUSCRIPT
Keywords: nickel; N-heterocyclic carbene; borylation; aryl bromide; DFT studies
2

ACCEPTED MANUSCRIPT
Abstract: A series of nickel complexes (1−3)b of 1,2,4-triazole derived amido-
functionalized N-heterocyclic carbene ligands were synthesized and structurally
characterized.
In particular, the [1-(R)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-
triazol-5-ylidene] Ni [R = Et (1b), i-Pr (2b) and Bn (3b)] complexes were obtained by
2
the direct reaction of the corresponding triazolium chloride salts (1−3)a by the
treatment with NiCl •6H O in presence of K CO as a base. The density functional
2
2
2
3
theory studies performed on these complexes reveal highly polar character of the
NHC−Ni σ-bonding interaction with corresponding molecular orbital having a
maximum contribution (59−69 %) from the NHC ligand fragments while that of a
minimum contribution (4 %) from the central nickel atom. The (1−3)b complexes
were found to be moderately active for the catalytic borylation reactions of bromoaryl
derivatives by bis(pinacolato)diboron reagent (B pin ) in the presence of Cs CO as a
2
2
2
3
base at 70 °C.
3

ACCEPTED MANUSCRIPT
Introduction
Recent increase in interest in nickel catalysis has led to the discovery of many
interesting catalytic transformations and has thrown open exciting possibilities for this
metal [1]. In comparison to its immediate congener, palladium, [2] that has seen
phenomenal success in catalysis in recent times, nickel has surprisingly maintained a
low profile all throughout. As a matter of fact, nickel, as a metal, is quite attractive
st
for catalysis mainly for its high nucleophilicity that arise out of its small size as a 1
row transition metal, and for its several accessible oxidation states,[3] that make it
flexible towards various catalytic cycles [4]. In addition to these favorable catalytic
attributes, nickel is inexpensive and thus, as a whole, is ideally suited for catalysis.
One of our interests [5] lies on exploring the catalytic potential of nickel, and towards
this goal, we intended to use the now popular N-heterocyclic carbene ligands that are
widely recognized for their success in homogeneous catalysis [6]. In this context, we
have recently observed the superiority of the nickel N-heterocyclic carbene complexes
over their palladium analogs in the hydroamination reactions of the activated olefins
[
7 ]. We have also designed nickel N-heterocyclic carbene complexes for the
bifunctional catalysis of the Michael addition reaction under base-free conditions by
employing suitably tailored chelating amido-functionalized N-heterocyclic carbene
ligands of the imidazole scaffold [8,9]. Meanwhile, some of us were also interested in
the reactivity of half-sandwich N-heterocyclic carbene (NHC) nickel complexes in
efficient and chemoselective catalytic hydrosilylation of aldehydes, ketones and
imines under mild conditions [10].
4

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Extending even further, we sought to explore the utility of the nickel complexes of the
related amido-functionalized N-heterocyclic carbene ligands of the 1,2,4-triazole
scaffold in various other catalytic transformations like in the catalytic borylation
reactions of bromoaryl derivatives by bis(pinacolato)diboron. The borylation reaction
[11] is important for providing convenient access to the high value intermediates of
the day-to-day organic synthesis, mainly for the Suzuki-Miyaura Cross-Coupling
reaction. In this context it is noteworthy that the nickel catalysts, despite being
among the pioneering ones for the C−C cross coupling reaction, as observed from the
early report of the Corriu-Kumada coupling with Grignard reagents [ 12 ], they
remained less popular till date than their palladium counterparts in various C−C cross
coupling reactions including the Suzuki-Miyaura reaction [ 13 ]. From this
perspective, quite importantly, the recent reports of the nickel catalysis for the
borylation reactions of the organohalide derivatives providing convenient access to
the borylated synthons [14], would ignite interest on the metal. In addition to the
nickel, our other interest in the 1,2,4-triazole based N-heterocyclic carbene ligands
arises from the fact that these ligands are less explored as compared to their imidazole
analogues [15,16].
Hence, in this contribution, we report a series of nickel complexes namely [1-(R)-4-
N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-ylidene] Ni [R = Et (1b), i-Pr (2b) and
2
Bn (3b)] of the 1,2,4-triazole derived N-heterocyclic carbene ligands that show low to
moderate activity in the catalytic borylation reactions of bromoaryl derivatives by
bis(pinacolato)diboron.
5

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Results and Discussions
A series of amido-functionalized N-heterocyclic carbene precursors namely, 1-(R)-4-
N-(furan-2-ylmethyl)acetamido-1,2,4-triazolium chloride [R = Et (1a), i-Pr (2a), Bn
(3a)] were synthesized from the respective 1-(R)-1,2,4-triazoles by the direct reaction
of 2-chloro-N-(furan-2-ylmethyl)acetamide in 52−66 % yield (Scheme 1). The
formation of the 1,2,4-triazolium chloride salts (1−3)a was very much evident from
the appearances of the characteristic (NCHN) resonances at a highly downfield region
1
13
1
at 10.8−10.2 ppm in the H NMR spectrum and at 146.1−145.8 ppm in the C{ H}
NMR spectrum and were further confirmed by High Resolution Mass Spectrometry
+
(
HRMS) studies that showed the [NHC+H] peaks at m/z 235.1206 (1a), m/z
2
49.1356 (2a) and m/z 297.1350 (3a) against the corresponding calculated values of
m/z 235.1195 (1a), m/z 249.1352 (2a) and m/z 297.1352 (3a).
The nickel complexes (1−3)b were then synthesized from the respective triazolium
chloride salts (1−3)a by the treatment with NiCl •6H O in CH CN in presence of
2
2
3
K CO (4.4 equiv.) as a base (Scheme 1 and Figure 1). The diamagnetic nature of the
2
3
(1−3)b complexes indicated a square planar geometry at the metal center in these
1
complexes. The H NMR spectrum showed the absence of the NCHN and amido-NH
resonances, consistent with the formation of a carbene center on the triazolium ring
along with the chelation of the anionic amido-functionalized side arm to the metal
center in these complexes. Quite interestingly, the two methylene moieties of the
(1−2)b complexes were diastereotopic in nature and exhibited four sets of resonances
2
for the 1b and the 2b complexes while displaying a JHH coupling constant of 15 Hz.
In contrast, all of the three methylene groups of the 3b complex were also
6

ACCEPTED MANUSCRIPT
2
diastereotopic in nature displaying six sets of resonances while exhibiting a J
HH
1
coupling constant of 15 Hz in the H NMR spectrum. Lastly, the characteristic
1
3
1
Ni−Ccarbene resonance appeared at 171.6−171.0 ppm region in C{ H} NMR
spectrum of the (1−3)b complexes.
The single crystals of (1-3)b were duly obtained from an acetonitrile solution for the
X-ray single crystal diffraction analysis. The molecular structure of the (1−3)b
complexes determined by the X-ray diffraction studies showed the metal center in a
square-planar geometry (Figure 2−4 and Table 1). The nickel in the (1−3)b
complexes is linked to two chelated N-heterocyclic carbene ligands, which were
arranged in a cis-disposition to each other and were bound to the metal center through
the carbene-C and an amido-N atoms. The Ni−Ccarbene bond distances in the 1b
[
1.860(3) Å and 1.859(3) Å], 2b [1.853(2) Å and 1.853(2) Å] and 3b [1.871(4) Å and
.871(4) Å] complexes compare well with the average d(Ni−Ccarbene) distance [1.864
1
Å] observed for the related structurally characterized nickel(II) complexes of chelated
amido-functionalized N-heterocyclic carbene ligands that are present in the
Cambridge Structural Database [17]. Similarly, the Ni−Namido bond distances in 1b
[
1.9230(2) Å and 1.9220(2) Å], 2b [1.9260(18) Å and 1.9211(18) Å] and 3b [1.927(3)
Å and 1.931(3) Å] are also comparable to the average d(Ni−Namido) [1.932 Å]
observed for the related structurally characterized complexes in the Cambridge
Structural Database [17].
Further, insights into the electronic structures of these (1−3)b complexes were
obtained with the aid of the density functional theory (DFT) study. Specifically, the
geometry optimized structures were computed for the (1−3)b complexes at the
7

ACCEPTED MANUSCRIPT
B3LYP/LANL2DZ, 6-31G(d) level of theory by using the atomic coordinates
obtained from the X-ray diffraction studies (Supporting Information Figure S34−S36
and Table S7−S9). The computed structures are in good agreement with the
experimental ones (Supporting Information Table S10−S12). Subsequently, the
single-point calculations along with the post-wave function analysis using the natural
bond order (NBO) method were performed on the geometry optimized structures at
the same level of theory for a detailed understanding of the electronic properties of
these complexes.
Corroborating the view of a strong electron-donation from the anionic amido-
functionalized N-heterocyclic carbene (NHC) ligands to the metal center in the (1−3)b
complexes, the natural and the Mulliken charge analyses indeed showed significant
increase in the electron density at the nickel center in the (1−3)b complexes relative to
2
+
that of the free Ni ion (Supporting Information Table S1−S3). Along the same line,
a scrutiny of the electronic configuration of the metal center in the (1−3)b complexes
further revealed that the electron donation from the carbene lone pair occurred onto
the 3d and 4s orbitals of the nickel center (Supporting Information Table S4).
Of particular interest are the Ni−Ccarbene bonding interaction in the (1−3)b complexes.
The NBO analysis showed that the Ni−Ccarbene bonds in the (1−3)b complexes
2
2
comprised of an interaction between a sp hybridized Ccarbene orbital with a dsp
hybridized Ni orbital, and this is in agreement with the square-planar geometry of the
central metal atom (Supporting Information Table S5). Additional understanding of
the Ni−Ccarbene interaction were obtained by the Charge Decomposition Analysis
(CDA) that estimated the NHC ligand to the nickel σ−donation, as denoted by d, and
8

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the nickel to the NHC ligand π−back-donation, as denoted by b, occurring in these
(1−3)b complexes. As expected of the anionic amido-functionalized N-heterocyclic
carbene ligands in the (1−3)b complexes to be strongly σ-donating, very high d/b
ratio of 10.95 (1b), 11.30 (2b), and 11.25 (3b) were observed in these complexes
(Supporting Information Table S6). It is worth noting that the forward σ-donation (d)
and the backward π- donation (b) values of the (1−3)b complexes represent the
interactions between the metal center and the two amido functionalized NHC-ligand
fragments, and which individually is composed of two interactions namely, a Ccarbene
Ni interaction and a Namido–Ni interaction.
–
The molecular orbital (MO) correlation diagrams were constructed for the (1−3)b
complexes from the interaction of the individual fragment molecular orbitals (FMOs)
2
+
of the free NHC ligand fragments with that of the free Ni ion. The molecular orbital
(MO) representing the NHC−Ni σ-interactions in the (1−3)b complexes i.e. the
HOMO-36 (1b), HOMO-43 (2b) and the HOMO-42 (3b) is formed from the
interaction between the carbene lone pairs of the two NHC ligand fragments with the
2
+
4
s atomic orbital of the Ni ion (Figure 5 and Supporting Information Figure S37 and
S38). The NHC−Ni σ-bonding molecular orbital in the (1−3)b complexes are
primarily ligand based exhibiting significant contribution [69 % (1b), 69 % (2b) and
5
4
9 % (3b)] from the two NHC ligand fragments and marginal contribution [4 % (1b),
% (2b) and 4 % (3b)] from the central nickel atom and this point towards a
substantial ionic character of the NHC−Ni σ-interaction. Additionally, these
NHC−Ni σ-bonding molecular orbitals namely, the HOMO-36 (1b), HOMO-43 (2b)
and the HOMO-42 (3b) were all found to be deeply buried thereby suggesting highly
stable nature of these Ni−NHC σ-interactions and further indicating that these
9

ACCEPTED MANUSCRIPT
Ni−NHC σ-bonds in the (1−3)b complexes are less susceptible to the attacks by
incoming nucleophiles and electrophiles.
The recent interest on nickel as catalysts in borylation of halogeno derivatives leading
to the corresponding borylated compounds useful in Suzuki-Miyaura cross-coupling
reactions prompted us to examine (1−3)b as potential catalysts for the coupling of
bis(pinacolato)diboron with aryl bromides. In particular, in the presence of the 3
equiv. of Cs CO , the nickel complexes (1−3)b catalyzed the reaction between the
2
3
bis(pinacolato)diboron reagent (B pin ) (1.5 equiv.) with the 1 equiv. of arylbromide
2
2
substrates exhibiting moderate conversions (61−90 %) and yielding the corresponding
borylated compounds in 13 % , 10 % and 8 %, respectively along with the
dehalogenated arenes (Table 2, entries 1−3). It must be noticed that except the
formation of the borylated derivatives, and the unreacted starting reagents, only
dehalogenated arenes were obtained as side products. The formation of the reduced
arene derivatives can be explained by the oxidative addition of the haloarene leading
to an Ni(Ar)(X) intermediate which was then hydrolyzed.
Subsequently, the most active catalyst, 1b, was used in the borylation reactions of
various other arylbromide substrates. (Table 2, Entries 4−9) The ortho- and the meta-
tolylbromide displayed 88 % and 91 % conversions respectively along with low
amount of borylated tolyl derivatives of 12 % and 14 % respectively (Table 2, Entries
4
and 5). However, with the p-anisylbromide and p-tert-butylphenylbromide
substrates, the better yields (20 % and 18%, respectively) were observed at 73 % and
1 % conversions (Table 2, Entries 6 and 7). Notably, the electron-donating
substituted aryl bromide substrates seem to be more reactive than the phenylbromide
7
1
0

ACCEPTED MANUSCRIPT
substrate (83% conv., 10% yield, entry 8). For the 2-naphtylbromide substrate, the
formation of the 20% of the corresponding borylated arene was observed (Table 2,
entry 9).
Conclusion
In summary, a series of nickel complexes (1−3)b of the chelating anionic amido-
functionalized N-heterocyclic carbene ligands have been synthesized and structurally
characterized. The density functional theory studies performed on the (1−3)b
complexes point toward highly stable NHC−Ni interactions that are predominantly σ-
bonding in nature. The (1−3)b complexes were found to be moderately active for the
catalytic borylation reactions of bromoaryl derivatives by the bis(pinacolato)diboron
reagent (B pin ) in the presence of Cs CO as a stoichiometric base at 70 °C, mainly
2
2
2
3
due to a competitive dehalogenation reaction.
1
1

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Experimental Section
General Procedures. All manipulations were carried out using a combination of a
glovebox and standard Schlenk techniques. NiCl •6H O was purchased from SD-fine
2
2
Chemicals (India) and used without any further purification. 1-(ethyl)-1,2,4-triazole
[
(
18], 1-(i-propyl)-1,2,4-triazole [19], 1-(benzyl)-1,2,4-triazole [20] and 2-chloro-N-
furan-2-ylmethyl)acetamide[21] were synthesized according to modified literature
1
13
1
procedures. H and C{ H} NMR spectra were recorded on a Bruker 400 MHz
1
NMR spectrometer. H NMR peaks are labeled as singlet (s), doublet (d), triplet (t),
quartet (q), quartet of doublets (qd), doublet of doublets (dd) septet (sept) and broad
(br). Infrared spectra were recorded on a Perkin Elmer Spectrum One FT-IR
spectrometer. Mass spectrometry measurements were done on a Micromass Q-Tof
spectrometer and Bruker maxis impact spectrometer. Elemental Analysis was carried
out on Thermo Finnigan FLASH EA 1112 SERIES (CHNS) Elemental Analyzer. X-
ray diffraction data for compounds 1b, 2b and 3b were collected on an Oxford
Diffraction XCALIBUR-S diffractometer and Rigaku Hg 724+ diffractometer.
Crystal data collection and refinement parameters are summarized in Table 1. The
structures were solved using direct method and standard difference map techniques,
2
and refined by full-matrix least-squares procedures on F . CCDC-855783 (for 1b),
CCDC-844351 (for 2b) and CCDC-1003751 (for 3b) contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge from
the
Cambridge
Crystallographic
Data
center
via
www.ccdc.cam.ac.uk/data_request/cif.
1
2

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Synthesis of 1-(ethyl)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazolium chloride
(1a)
A mixture of ethyl triazole (1.25 g, 12.9 mmol) and 2-chloro-N-(furan-2-
ylmethyl)acetamide (2.21 g, 12.8 mmol) was refluxed in toluene (ca. 40 mL) for 16
hours during which a black viscous precipitate was formed. After decanting off the
solvent, the precipitate was washed with hot petroleum ether (ca. 3 × 10 mL) and then
dissolved in minimum amount of dichloromethane (ca. 4 mL). The product was
precipitated out by addition of diethyl ether (ca. 15 mL). The product was isolated by
decantation and was vacuum dried to give the product (1a) as a hygroscopic black
1
solid (2.29 g, 66 %). H NMR (CDCl , 400 MHz, 25 °C): δ 10.8 (s, 1H, N-C(5)H-N),
3
9
.40 (br, 1H, NHCO), 9.39 (s, 1H, N-C(3)H-N), 7.28 (br, 1H, C H O), 6.26 (br, 1H,
4 3
3
C H O), 6.23 (br, 1H, C H O), 5.54 (s, 2H, CH ), 4.45 (q, 2H, J = 7 Hz, CH CH ),
4
3
4
3
2
HH
2
3
3
3
13
1
4
.37 (d, 2H, J = 5 Hz, CH NH), 1.57 (t, 3H, J = 7 Hz, CH CH ). C{ H}
HH 2 HH 2 3
NMR (DMSO-d , 100 MHz, 25 °C): δ 164.5 (C=O), 151.4 (C H O), 145.8 (N-C(5)-
6
4
3
N), 143.2 (N-C(3)-N), 142.7 (C H O), 110.9 (C H O), 107.9 (C H O), 49.0 (CH ),
4
3
4
3
4
3
2
-
1
4
7.5 (CH CH ), 36.1 (CH ), 13.9 (CH CH ). IR data (KBr pellet) cm : 1686 (s)
2 3 2 2 3
+
(
ν ). HRMS (ES): m/z 235.1206 [NHC+H] , calcd. 235.1195. Anal. Calcd. for
c=o
C H N O Cl H O: C, 45.76; H, 5.93; N, 19.40. Found: C, 46.58; H, 6.13; N, 19.42
●
11
15
4
2
2
%.
Synthesis
of
[1-(ethyl)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-
ylidene] Ni (1b)
2
A mixture of 1-ethyl-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazolium chloride (1a)
0.640 g, 2.37 mmol), K CO (1.64 g, 11.8 mmol) and NiCl •6H O (0.280 g, 1.18
(
2
3
2
2
mmol) was refluxed in acetonitrile (ca. 40 mL) for 24 hours. The reaction mixture
1
3

ACCEPTED MANUSCRIPT
was filtered and filtrate was reduced in vaccum up to 10 mL. The filtrate was kept at
room temperature for overnight, during which yellow crystals were formed. Finally,
the crystals were separated by filtration and vacuum dried to give the product (1b) as
1
a yellow crystalline solid (0.135 g, 22 %). H NMR (CDCl , 400 MHz, 25 °C): δ 8.02
3
3
4
(
s, 2H, N-C(3)H-N), 7.31 (dd, 2H, J = 2 Hz, J = 1 Hz, C H O), 6.34 (dd, 2H,
HH HH 4 3
3
3
3
JHH = 3 Hz, J = 3 Hz, C H O), 6.12 (d, 2H, J = 3 Hz, C H O), 5.13 (d, 2H,
HH
4
3
HH
4
3
2
2
2
JHH = 15 Hz, CH ), 4.84 (d, 2H, J = 15 Hz, CH ), 4.50 (d, 2H, J = 15 Hz,
2
HH
2
HH
2
3
CH ), 3.61 (d, 2H, J = 15 Hz, CH ), 3.54 (qd, 2H, J =7 Hz, CH CH ), 3.16 (qd,
2
HH
2
HH
2
3
3
3
13
1
2
H, J = 7 Hz, CH CH ), 1.23 (t, 6H, J =7 Hz, CH CH ).
C{ H} NMR
HH
2
3
HH
2
3
(
(
(
CDCl , 100 MHz, 25 °C): δ 171.0 (Ni-NCN), 168.3 (C=O), 156.6 (C H O), 142.4
3
4
3
N-C(3)-N), 141.1 (C H O), 110.9 (C H O), 106.3 (C H O), 54.3 (CH ), 46.9
4
3
4
3
4
3
2
-1
CH CH ), 41.8 (CH ), 15.1 (CH CH ). IR data (KBr pellet) cm : 1590 (s) (ν ).
2
3
2
2
3
c=o
+
HRMS (ES): m/z 525.1506 [M+H] , calcd. 525.1503.
Anal. Calcd. for
C H NiN O ●H O: C, 48.64; H, 5.20; N, 20.63. Found: C, 48.42; H, 5.45; N, 20.30
22
26
8
4
2
%.
Synthesis
chloride (2a)
of
1-(i-propyl)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazolium
A mixture of i-propyl triazole (1.96 g, 17.7 mmol) and 2-chloro-N-(furan-2-
ylmethyl)acetamide (3.06 g, 17.6 mmol) was refluxed in toluene (ca. 30 mL) for 14
hours during which a black viscous precipitate was formed. After decanting off the
solvent, the precipitate was washed with hot petroleum ether (3 × 10 mL) and then
dissolved in minimum amount of dichloromethane (ca. 3 mL). The product was
precipitated out by addition of diethyl ether (ca. 15 mL). The product was isolated by
decantation and was vacuum dried to give the product (2a) as a hygroscopic black
1
4

ACCEPTED MANUSCRIPT
1
solid (2.65 g, 53 %). H NMR (DMSO-d , 400 MHz, 25 °C): δ 10.2 (s, 1H, N-C(5)H-
6
N), 9.14 (s, 1H, N-C(3)H-N), 9.09 (br, 1H, NHCO), 7.58 (br, 1H, C H O), 6.40 (dd,
4
3
3
3
3
1
H, J = 3 Hz, J = 3 Hz, C H O), 6.31 (d, 1H, J = 3 Hz, C H O), 5.11 (s, 2H,
HH HH 4 3 HH 4 3
3
3
CH ), 4.86 (sept, 1H, J = 7 Hz, CH(CH ) ), 4.34 (d, 2H, J = 5 Hz, CH NH),
2
HH
3 2
HH
2
3
13
1
1
1
1
.50 (d, 6H, J = 7 Hz, CH(CH3)2). C{ H} NMR (DMSO-d , 100 MHz, 25 °C): δ
HH 6
64.4 (C=O), 151.4 (C H O), 145.8 (N-C(5)-N), 142.7 (N-C(3)-N), 142.4 (C H O),
4
3
4
3
10.8 (C H O), 107.9 (C H O), 55.5 (CH(CH ) ), 49.0 (CH ), 36.1 (CH ), 21.4
4
3
4
3
3 2
2
2
-1
(
CH(CH ) ). IR data (KBr pellet) cm : 1689 (s) (ν ). HRMS (ES): m/z 249.1356
3
2
c=o
+
[
NHC+H] , calcd. 249.1352. Anal. Calcd. for C H N O Cl
●
0.5H O: C, 49.07; H,
1
2
17
4
2
2
6
.18; N, 19.07. Found: C, 48.63; H, 6.74; N, 17.97 %.
Synthesis
of
[1-(i-propyl)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-
ylidene] Ni (2b)
2
A mixture of 1-(i-propyl)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazolium chloride
2a) (2.33 g, 8.20 mmol), K CO (5.65 g, 40.9 mmol) and NiCl •6H O (0.974 g, 4.09
(
2
3
2
2
mmol) was refluxed in acetonitrile (ca. 70 mL) for 24 hours. The reaction mixture
was filtered and filtrate was reduced in vacuum up to 15 mL. The filtrate was kept at
room temperature for overnight, during which brown crystals were formed. Finally
the crystals were separated by filtration and vacuum dried to give the product (2b) as
1
a yellow crystalline solid (0.684 g, 30 %). H NMR (CDCl , 400 MHz, 25 °C): δ 8.07
3
3
3
(
s, 2H, N-C(3)H-N), 7.29 (br, 2H, C H O), 6.31 (dd, 2H, J = 3 Hz, J = 3 Hz,
4 3 HH HH
3
2
C H O), 6.13 (d, 2H, J = 3 Hz, C H O), 5.24 (d, 2H, J = 15 Hz, CH ), 4.83 (d,
4
3
HH
4
3
HH
2
2
2
2
2
H, J = 15 Hz, CH ), 4.57 (d, 2H, J = 15 Hz, CH ), 3.59 (d, 2H, J = 15 Hz,
HH
2
HH
2
HH
3
3
CH ), 3.54 (sept, 2H, J = 7 Hz, CH(CH ) ), 1.24 (d, 6H, J = 7 Hz, CH(CH3)2),
2
HH
3 2
HH
3
13
1
1
.14 (d, 6H, J = 7 Hz, CH(CH3)2). C{ H} NMR (CDCl , 100 MHz, 25 °C): δ
HH
3
1
5

ACCEPTED MANUSCRIPT
1
71.2 (Ni-NCN), 168.5 (C=O), 156.2 (C H O), 142.3 (N-C(3)-N), 141.1 (C H O),
4
3
4
3
1
10.7 (C H O), 106.7 (C H O), 54.3 (CH(CH ) ), 54.2 (CH ), 41.8 (CH ), 24.3
4
3
4
3
3 2
2
2
-1
(
CH(CH ) ), 20.3 (CH(CH ) ) IR data (KBr pellet) cm : 1596 (s) (ν ). HRMS
3
2
3 2 .
c=o
+
(
ES): m/z 553.1817 [M+H] , calcd. 553.1816. Anal. Calcd. for C H NiN O : C,
2
4
30
8
4
5
2.10; H, 5.47; N, 20.25. Found: C, 51.44; H, 6.04; N, 20.28 %.
Synthesis
of
1-(benzyl)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazolium
chloride (3a)
A mixture of benzyl triazole (2.05 g, 12.9 mmol) and 2-chloro-N-(furan-2-
ylmethyl)acetamide (2.21 g, 12.8 mmol) was refluxed in toluene (ca. 40 mL) for 32
hours during which a black precipitate was formed. After decanting off the solvent,
the precipitate was washed with hot petroleum ether (ca. 3 × 10 mL) and then
dissolved in minimum amount of dichloromethane (ca. 4 mL). The product was
precipitated out by addition of diethyl ether (2 × 15 mL). The product was finally
isolated by decantation and was vacuum dried to give the product (3a) as a black solid
1
(
2.20 g, 52 %). H NMR (DMSO-d , 400 MHz, 25 °C): δ 10.3 (s, 1H, N-C(5)H-N),
6
9
.20 (br, 1H, N-C(3)H-N), 9.16 (s, 1H, NHCO), 7.60 (br, 1H, C H O), 7.43 (br, 5H,
4 3
3
3
3
C H ), 6.41 (dd, 1H, J = 3 Hz, J = 3 Hz, C H O), 6.32 (d, 1H, J = 3 Hz,
6
5
HH
HH
4
3
HH
3
C H O), 5.70 (s, 2H, CH ), 5.17 (s, 2H, CH ), 4.34 (d, 2H, J = 6 Hz, CH NH).
4
3
2
2
HH
2
1
3
1
C{ H} NMR (DMSO-d , 100 MHz, 25 °C): δ 164.3 (C=O), 151.3 (C H O), 146.1
6
4
3
(
N-C(5)-N), 143.8 (C H ), 142.4 (N-C(3)-N), 142.4 (C H O), 133.4 (C H ), 128.9
6 5 4 3 6 5
(
C H ) 128.8 (C H ), 110.5 (C H O), 107.5 (C H O), 54.7 (CH ), 49.1 (CH ), 35.9
6 5 6 5 4 3 4 3 2 2
-
1
(
CH ). IR data (KBr pellet) cm : 1669 (s) (ν ). HRMS (ES): m/z 297.1350
2 c=o
+
[
NHC+H] , calcd. 297.1352. Anal. Calcd. for C H N O Cl 0.5H O: C, 56.22; H,
●
1
6
17
4
2
2
5
.31; N, 16.39. Found: C, 55.93; H, 5.83; N, 15.37 %.
1
6

ACCEPTED MANUSCRIPT
Synthesis
of
[1-(benzyl)-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazol-5-
ylidene] Ni (3b)
2
A mixture of 1-benzyl-4-N-(furan-2-ylmethyl)acetamido-1,2,4-triazolium chloride
3a) (1.20g, 3.60 mmol), K CO (2.48 g, 18.0 mmol) and NiCl •6H O (0.428 g, 1.80
(
2
3
2
2
mmol) was refluxed in acetonitrile (ca. 50 mL) for 24 hours. The reaction mixture
was filtered and the filtrate reduced in vacuum up to 10 mL. The filtrate was kept at
0
°C for 3 hours, during which yellow crystals were formed. Finally the crystals were
quickly separated by filtration and vacuum dried to give the product (3b) as a yellow
1
solid (0.163 g, 14 %). H NMR (CDCl , 400 MHz, 25 °C): δ 8.12 (s, 2H, N-C(3)H-
3
3
N), 7.31 (m, 6H, C H ), 7.19 (br, 2H, C H O), 6.91 (d, 4H, J = 7 Hz, C H ), 6.29
6
5
4
3
HH
6
5
3
2
(
br, 2H, C H O), 6.05 (d, 2H, J = 2 Hz, C H O), 4.97 (d, 2H, J = 15 Hz, CH2),
4
3
HH
4
3
HH
2
2
2
4
1
.79 (d, 2H, J = 15 Hz, CH ), 4.50 (d, 2H, J = 15 Hz, CH ), 4.42 (d, 2H, J
=
HH
2
HH
2
HH
2
2
5 Hz, CH ), 4.39 (d, 2H, J = 15 Hz, CH ), 3.62 (d, 2H, J = 15 Hz, CH2).
2
HH
2
HH
1
3
1
C{ H} NMR (CDCl , 100 MHz, 25 °C): δ 171.6 (Ni-NCN), 168.4 (C=O), 156.0
3
(
(
(
[
C H O), 142.7 (N-C(3)-N), 141.2 (C H O), 133.6 (C H ), 129.3 (C H ), 129.0
4
3
4
3
6
5
6
5
C H ), 127.6 (C H ), 110.7 (C H O), 106.3 (C H O), 55.4 (CH ), 54.0 (CH ), 42.3
6
5
6
5
4
3
4
3
2
2
-1
CH ). IR data (KBr pellet) cm : 1595 (s) (ν ). HRMS (ES): m/z 649.1813
2
c=o
+
M+H] , calcd. 649.1816. Anal. Calcd. for C H NiN O ●H O: C, 57.59; H, 4.83;
3
2
30
8
4
2
N, 16.79. Found: C, 57.71; H, 5.05; N, 16.74 %.
Computational Methods
Density functional theory (DFT) calculations were performed on the metal complexes
1−3)b using GAUSSIAN 09 [22] suite of quantum chemical programs. The Becke
three parameter exchange functional in conjunction with Lee-Yang-Parr correlation
(
1
7

ACCEPTED MANUSCRIPT
functional (B3LYP) has been employed in the study [23]. The polarized basis set 6-
3
1G(d) [24] was used to describe oxygen, carbon, nitrogen and hydrogen atoms. The
Stuttgart−Dresden effective core potential (ECP) along with valence basis sets
LANL2DZ) was used for the nickel [25]. Natural bond orbital (NBO) analysis [26]
(
was performed using NBO 3.1 program implemented in the GAUSSIAN 09 package.
Frequency calculations were performed for all the optimized structures to characterize
the stationary points as minima.
The metal-ligand donor−acceptor interaction were inspected by using the Charge
Decomposition Analysis (CDA) [27] which is a valuable tool for analyzing the
interactions between molecular fragments on a quantitative basis with an emphasis on
the electron donation [28]. The orbital interaction between donor NHC and acceptor
2
+
fragment Ni ion in (1−3)b can be divided into three parts;
(
(
(
i)
σ-donation from two mono−anionic NHC ligand fragments to a
−
2+
nickel(II) ion {2[NHC] →Ni } and designated by (d),
ii)
π- back donation from a nickel(II) in to two mono−anionic NHC ligand
−
2+
fragments {2[NHC] ←Ni } and designated by (b) and
iii) A repulsive interaction (r) between the occupied FMOs of these two
fragments.
The CDA calculations were performed using the AOMix [29,30] program with the
B3LYP/ LANL2DZ, 6-31G(d) wave function. Molecular orbital (MO) compositions
and the overlap populations were calculated using the AOMix program. Analysis of
the MO compositions in terms of occupied and unoccupied fragment orbitals (OFOs
1
8

ACCEPTED MANUSCRIPT
and UFOs, respectively), construction of orbital interaction diagrams, the charge
decomposition analysis (CDA) were performed using the AOMix-CDA [31].
Catalysis studies
General procedure: In a Schlenk tube, under argon atmosphere, was introduced the
arylbromide (0.25 mmol, 1 equiv.), Cs CO (261 mg, 0.80 mmol, 3 equiv.),
2
3
bis(pinacolato)diboron (102 mg, 0.40 mmol, 1.5 equiv.), the nickel complex 1b (7
mg, 1.3 mmol, 5 mol%) and 1 mL of dry THF. The reaction mixture was then stirred
at 70 °C for 20 h. The THF was carefully evaporated. The conversion of the
1
arylbromide and the yield of the borylated arene were determinated by H-NMR using
1
,3,5-trimethoxybenzene (22 mg, 0.13 mmol, 0.5 equiv.) as an internal standard.
Acknowledgements
We thank Indo-French Centre for the Promotion of Advanced Research – IFCPAR
(Project No: 4605-1), New Delhi, for the financial support of this research. PG is
grateful to the Sophisticated Analytical Instrument Facility at IIT Bombay for the
characterization data and to the National Single Crystal X-ray Diffraction Facility, IIT
Bombay, and the Single Crystal X-ray Diffraction Facility at the Chemistry
Department, IIT Bombay, for the crystallographic data. The computational facilities
at the Computer Center, IIT Bombay, and at the Chemistry Department, IIT Bombay,
are gratefully acknowledged. AK thanks CSIR, New Delhi, for research fellowship.
CD and JBS thank the French "Ministère de l’Enseignement Supérieur et de la
Recherche" and the CNRS for financial support. LPB thanks IFCPAR for a PhD
grant.
1
9

ACCEPTED MANUSCRIPT
Electronic supplementary information (ESI) available: The characterization data
of the (1−3)b complexes (Supporting Information Figure S1−S33), the B3LYP
coordinates of the optimized geometries for (1−3)b, NBO tables and CDA table along
with orbital interaction diagrams of 2b and 3b. This material is available free of
charge via the internet at http:www.xxxxxxxxx
2
0

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R
R
N
N
N
N
N
N
N
N
Ni
O
O
O
O
R = Et (1b), i-Pr (2b), Bn (3b)
Figure 1.
2
1

ACCEPTED MANUSCRIPT
Figure 2. ORTEP view of 1b with thermal ellipsoids are shown at the 50 %
probability level. Selected bond lengths (Å) and angles (°): Ni(2)−C(1) 1.860(3),
Ni(2)−C(34)
C(1)−Ni(2)−N(4) 87.54(10), C(12)−Ni(2)−N(8) 85.82(10), C(1)−Ni(2)−C(12)
4.64(11), N(4)−Ni(2)−N(8) 92.43(9).
1.859(3),
Ni(2)−N(4)
1.9230(2),
Ni(2)−N(8)
1.9220(2),
9
2
2

ACCEPTED MANUSCRIPT
Figure 3. ORTEP view of 2b with thermal ellipsoids are shown at the 50 %
probability level. Selected bond lengths (Å) and angles (°): Ni(1)−C(1) 1.853(2),
Ni(1)−C(13) 1.853(2), Ni(1)−N(4) 1.9260(18), Ni(1)−N(8) 1.9211(18),
C(1)−Ni(1)−N(4) 86.67(8), C(13)−Ni(1)−N(8) 86.21(9), C(1)−Ni(1)−C(13) 92.83(8),
N(4)−Ni(1)−N(8) 94.61(7).
2
3

ACCEPTED MANUSCRIPT
Figure 4. ORTEP view of 3b with thermal ellipsoids are shown at the 50 %
probability level. Selected bond lengths (Å) and angles (°): Ni(1)−C(1) 1.871(4),
Ni(1)−C(17) 1.871(4), Ni(1)−N(4) 1.927(3), Ni(1)−N(5) 1.931(3), C(1)−Ni(1)−N(4)
8
6.25(14),
C(17)−Ni(1)−N(5)
88.09(14),
C(1)−Ni(1)−C(17)
94.40(15),
N(4)−Ni(1)−N(5) 91.57(13).
2
4

ACCEPTED MANUSCRIPT
Ni2+
HOMO -7
0
1
2 %
HOMO -23
HOMO -34
HOMO-37
HOMO-41
1
6 %
13 %
1 %
-
5
1
1
7 %
1
0
HOMO-36
HOMO-23
HOMO-34
4 %
-
-
15
20
LUMO+1
HOMO-37
HOMO-41
Figure 5. Simplified orbital interaction diagram showing the major contributions of the NHC−Ni bond in 1b.
25

ACCEPTED MANUSCRIPT
R
R
R
N
N
N
N
N
N
N
N
N
N
R
Cl
N
N
NiCl 6H O
N
2
2
Ni
O
K CO
2
3
Cl
N
O
NH
O
O
R = Et (1), i-Pr (2), Bn (3)
NH
O
O
O
O
(
1-3)a
(1-3)b
Scheme 1.
26

ACCEPTED MANUSCRIPT
Table 1. X-ray crystallographic data for 1b, 2b and 3b.
Compound
1b
2b
3b
Lattice
Monoclinic
Orthorhombic
C H N NiO
Monoclinic
C H N NiO
4
Formula
C H Cl N Ni O
45
53
3
16
2
8.5
24 30
8
4
32 30
8
Formula weight
1177.80
553.27
649.35
Space group
P2 /n
P2 2 2
P2 /c
1
1
1 1
1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
18.8306(3)
10.5000(2)
13.9266(4)
17.9794(4)
90
18.590(5)
9.104(2)
18.316(5)
90
9.25340(10)
30.7706(4)
90
94.472(2)
90
90
109.49(4)
90
90
3
V/Å
5345.36(12)
4
2629.11(11)
4
2922.2(13)
4
Z
Temperature (K)
Radiation (λ,Å)
293(2)
0.71073
1.464
293(2)
0.71073
1.398
100(2)
0.71073
1.476
-3
ρ (calcd.), g cm
θ max, deg.
25.00
25.00
25.00
No. of data
9372
4620
7789
No. of parameters
677
338
406
R [I>2sigma(I)]
0.0419
0.1149
1.038
0.0277
0.0689
1.034
0.0816
0.204
1
wR [I>2sigma(I)]
2
GOF
1.109
2
7

ACCEPTED MANUSCRIPT
Table 2. Selected results for catalytic borylation reactions of arylbromide by
bis(pinacolato)diboron catalyzed by (1−3)b.
(b)
(c)
Entry [Ni] (5 mol%)
Ar-Br
Conversion (%)
Yield (%)
1
2
3
1b
2b
3b
90
75
61
13
10
8
4
5
6
1b
1b
1b
88
91
73
12
14
20
7
8
9
1b
1b
1b
71
83
70
18
10
21
(
a) Ar-Br (50 mg, 0.25 mmol, 1 equiv.), Cs CO (261 mg, 0.80 mmol, 3 equiv.),
2 3
bis(pinacolato)diboron (102 mg, 0.40 mmol, 1.5 equiv.), THF (1 mL), Ni complex (7
mg, 1.3 mmol, 5 mol%), 70 °C for 20 h. (b) Conversion of the arylbromide
1
determinated by H-NMR using 1,3,5-trimethoxybenzene (22 mg, 0.13 mmol, 0.5
equiv.) as an internal standard. (c) NMR yield determined with1,3,5-
trimethoxybenzene as an internal standard.
2
8

ACCEPTED MANUSCRIPT
Graphics for Table of Contents
Nickel Complexes of 1,2,4-Triazole Derived Amido-functionalized N-heterocyclic Carbene Ligands: Synthesis, Theoretical Studies and
Catalytic Application
Anuj Kumar, Linus Paulin Bheeter, Manoj Kumar Gangwar, Jean-Baptiste Sortais, Christophe Darcel* and Prasenjit Ghosh*
R
R
N
N
N
N
N
N
N
N
Ni
O
O
O
O
O
B
Br
O
O
O
O
R = Et (1b), i-Pr (2b), Bn (3b)
Cs CO , THF
O
R
+
B B
R
2
3
70 °C , 17 h
A series of nickel complexes of 1,2,4-triazole derived amido-functionalized N-heterocyclic carbene
ligands were prepared, characterized and explored for their activity in the borylation reaction of
arylbromides.
2
9