Pd and Ni complexes of a novel vinylidene β-diketimine ligand: Their application as catalysts in Heck coupling and alkyne trimerization

Article abstract of DOI:10.1002/aoc.3696

A β-diketimine ligand with vinylidene substitution at γ-carbon, CH₂C(CH₃CNAr)₂ (Ar?=?2,6-diisopropylphenyl) (L²), was synthesized by treating β-diketimine H₂C(CH₃CNAr)₂ with n-BuLi followed by paraformaldehyde. L² formed the homobimetallic ether-bridged β-diketiminate complex [O{(CH₂-β-diketiminate)Pd(OAc)}₂] (1) with (PdOAc)₂. It also gave complexes [L²PdCl₂] (2) and [L²NiBr₂] (3) when treated with PdCl₂(CH₃CN)₂ and NiBr₂(dimethoxyethane), respectively. All the compounds were characterized using ¹H/¹³C NMR spectroscopy and single-crystal X-ray diffraction studies. The catalytic activity of Pd and Ni complexes 1, 2 and 3 was explored in Heck coupling and alkyne trimerization reactions and it was found that they are very good catalysts. The results are reported in detail.

Full text of DOI:10.1002/aoc.3696

Received: 19 September 2016
DOI 10.1002/aoc.3696
Revised: 14 October 2016
Accepted: 31 October 2016
F U L L PA P E R
Pd and Ni complexes of a novel vinylidene β‐diketimine ligand:
Their application as catalysts in Heck coupling and alkyne
trimerization
Raghavendra Beesam | Dastagiri Reddy Nareddula
Department of Chemistry, Pondicherry University,
A β‐diketimine ligand with vinylidene substitution at γ‐carbon, CH2C(CH3CNAr)2
Pondicherry 605 014, India
(Ar = 2,6‐diisopropylphenyl) (L2), was synthesized by treating β‐diketimine
Correspondence
Dastagiri Reddy Nareddula, Department of
H2C(CH3CNAr)2 with n‐BuLi followed by paraformaldehyde. L2 formed the
Chemistry, Pondicherry University, Pondicherry
homobimetallic ether‐bridged β‐diketiminate complex [O{(CH2‐β‐diketiminate)
605 014, India.
Pd(OAc)}2] (1) with (PdOAc)2. It also gave complexes [L2PdCl2] (2) and [L2NiBr2]
Email: ndreddy.che@pondiuni.edu.in
(3) when treated with PdCl2(CH3CN)2 and NiBr2(dimethoxyethane), respectively.
1
All the compounds were characterized using H/13C NMR spectroscopy and sin-
gle‐crystal X‐ray diffraction studies. The catalytic activity of Pd and Ni complexes
1, 2 and 3 was explored in Heck coupling and alkyne trimerization reactions and
it was found that they are very good catalysts. The results are reported in detail.
KEYWORDS
alkyne trimerization, Mizoroki–Heck coupling, Ni complex, Pd complex, vinylidene
β‐diketimine
1
|
INTRODUCTION
Cyclotrimerization of alkynes is a useful tool for synthe-
sizing substituted benzene rings, especially those which are
not easily assembled via conventional aromatic substitution
reactions. Several catalysts containing Ti, Co, Ni, Ru and
Rh have been reported to initialize cyclotrimerization reac-
tions.[39–44] Among these, Ni catalysts have advantages of
being cheaper and having readily available reagents for com-
plexation with a wide range of ligands. It has also been
reported that β‐diketimine‐based Ni complexes can be used
in polymerization reactions.[33,45] Hence we became
interested in synthesizing Pd and Ni complexes of modified
β‐diketimine ligands. In this regard we speculated that the
insertion of a ─CH2OH group at γ‐carbon could enhance
the coordination number as well as water solubility, and
hence an attempt was made to synthesize such a ligand.
Herein, we report our attempts to synthesize a hydroxy-
methyl‐substituted β‐diketimine ligand and the reactions of
the resultant ligand with Pd(OAc)2, PdCl2(CH3CN)2 and
NiBr2(DME) (DME = dimethoxyethane). The catalytic activ-
ities of the formed Pd complexes towards Heck coupling
reactions and of the Ni complex towards cyclotrimerization
of alkynes were explored.
The β‐diketimine ligand (NacNacH) is one of the finest
ligands available to synthetic chemists. Robust and stable
nature of the ligand, and simple synthetic methods of
its numerous variants make it attractive for use in diverse
fields of chemistry such as catalysis,[1–8] main‐group chem-
istry[9–17] and bio‐inorganic chemistry.[18–20] Ligating
groups like ─CN, ─CHO, ─PPh2 and ─NO2 have been
integrated into the β‐diketimine ligand, resulting in the
alteration of its coordination mode and/or its donor ability
towards metals.[21–30] Monoanionic β‐diketiminate and
neutral β‐diketimine complexes of transition metals such
as Fe, Co, Ni, Cu, Zn and Pd[31–37] have been widely used
in organic synthesis for the construction of natural prod-
ucts, biologically significant molecules and polymers
through C─C bond formation. Among these methodolo-
gies, the Heck coupling reaction is a very convenient
method to carry out C─C coupling between aryl halides
and olefins. The utility of a Pd complex of β‐diketimine
as a catalyst in the Heck coupling reaction has been
described.[38]
Appl Organometal Chem 2017; e3696;
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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DOI 10.1002/aoc.3696

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BEESAM AND NAREDDULA
2
| RESULTS AND DISCUSSION
2.1 | Synthesis and structural characterization of
vinylidene β‐Diketimine Ligand (L2)
It has been reported that when a tripodal ligand, tris(1‐pyrazoyl)
methane, is treated with potassium tert‐butoxide and paraformal-
dehyde in tetrahydrofuran (THF), hydroxymethylation occurs at
the bridged carbon.[46] Expecting similar hydroxymethylation at
the γ‐carbon, a β‐diketimine, CH2(CH3C═NAr)2 (Ar = 2,6‐
diisopropylphenyl) (L1H),[33] was deprotonated using n‐butyl
lithium and treated with paraformaldehyde in THF at room tem-
perature. Interestingly, an unexpected vinylidene β‐diketimine,
CH2═C(CH3C═NAr)2 (L2), was formed in moderate yield
(Scheme 1). L2 is a novel variation of the β‐diketimine ligand
in which there is no acidic proton. A thorough search in the
literature revealed no report of the synthesis of simple vinylidene
β‐diketimine type of ligands. However, a Zn complex of a
ferrocenyl‐substituted vinylidene β‐diketimine, which was syn-
thesized by the deprotonation of a γ‐methyl‐β‐diketiminate Zn
complex, has been reported.[47] It is also noteworthy that quite
a few reports have described several macrocyclic ligands con-
taining the C═C at γ‐carbon of the diketimine core as an integral
part.[48–53]
FIGURE
1
Single‐crystal X‐ray structure of ligand L2. All hydrogen
atoms except those of CH2 are omitted for clarity. Thermal ellipsoids are
drawn at the 50% probability level. Selected bond lengths (Å): N1–C2
1.274(2), C1–C2 1.504(3), C2–C3 1.475(3), N2–C4 1.267(2), C3–C6
1.321(3), C4–C5 1.503(3). Selected bond Angles (°): C1–C2–N1
124.52(17), N1–C2–C3 116.91(17), C2–C3–C4 117.57(15), C3–C4–C5
116.53(18), C3–C4–N2 117.55(16), C4–C3–C6 119.69(18), C2–C3–C6
122.72(19), C5–C4–N2 125.84(19)
was employed to synthesize metal complexes. It has been
reported that when an equimolar mixture of L1H and
Pd(OAc)2 is stirred,
a
β‐diketiminate Pd complex,
[L1Pd(OAc)], is formed, in which the diketiminate ligand
is monoanionic.[36] Since there is no acidic proton in L2 it
is difficult to predict how it reacts with Pd(OAc)2 but it is
interesting to probe such a reaction. Treating L2 with one
equivalent of Pd(OAc)2 in acetonitrile at room temperature
for 4 h yields an unusual homobimetallic Pd complex,
[O{(CH2‐β‐diketiminate)Pd(OAc)}2] (1; Scheme 2), in
which two monoanionic β‐diketiminate moieties are bridged
through an ether linkage. CH2 protons appear at 4.26 ppm
in the 1H NMR spectrum of the complex. Single‐crystal
X‐ray diffraction studies confirm the proposed structure.
An ORTEP diagram with selected bond parameters is shown
in Figure 2. Molecular structure of 1 reveals a distorted
square planar geometry around each Pd centre. The acetate
and the diketiminate chelate rings are almost planar and also
coplanar. The whole molecule has a ‘W’ shape with the
bridging O atom present in the centre. The bond distances
and the bond angles compare well with those of the reported
β‐diketiminate complex [L1Pd(OAc)].[36]
The solid‐state structure of L2 was determined using the
single‐crystal X‐ray diffraction technique (Figure 1). The
summation of bond angles around γ‐carbon atom (C3 in
Figure 1), which is 359.98°, suggests that C3 is sp2 hybrid-
ized. The bond distance between C3 and C6 (1.321(3) Å)
1
also indicates a double bond character. The H NMR and
13C NMR spectra of bulk samples are in accordance with
1
the single‐crystal X‐ray structure. The H NMR spectrum
of L2 displays a singlet at 5.97 ppm corresponding to CH2
protons of the vinylidene group in addition to resonances
for isopropyl groups.
2.2 | Synthesis and structural characterization of
palladium and nickel complexes
Recently, in our laboratory, a sulfonate‐substituted β‐diketimine
ligand was synthesized and used to make Ni and Pd com-
plexes.[54] In the present work, vinylidene β‐diketimine L2
The L2 complexes of dichloridopalladium [L2PdCl2] (2)
and dibromidonickel [L2NiBr2] (3) were obtained in moderate
yields by stirring the ligand with [PdCl2(CH3CN)2] and
[NiBr2(DME)] in acetonitrile and dichloromethane, respec-
tively (Scheme 3). Both complexes 2 and 3 were characterized
SCHEME 1 Synthesis of L2 from β‐diketimine L1H
SCHEME 2 Synthesis of palladium complex 1

BEESAM AND NAREDDULA
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FIGURE 2 Single‐crystal X‐ray structure of 1. Solvent molecule and
hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at
the 50% probability level. Selected bond lengths (Å): Pd1–N1 1.967(4),
Pd1–N2 1.969(4), Pd1–O2 2.090(3), Pd1–O3 2.096(4), Pd2–N3 1.963(4),
Pd2–N4 1.958(4), Pd2–O4 2.087(4), Pd2–O5 2.093(4). Selected bond
angles (°): N1–Pd1–N2 91.62(17), N1–Pd1–O2 165.48(16), N1–Pd1–O3
103.03(16), N2–Pd1–O2 102.45(15), N2–Pd1–O3 165.20(15),
O2–Pd1–O3 62.80(14), C6–O1–C7 112.1(4)
FIGURE 3 Single‐crystal X‐ray structure of 2. Solvent molecules and
hydrogen atoms except those of CH2 are omitted for clarity. Thermal
ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å):
Pd1–N1 2.059(17), Pd1–N2 2.068(18), Pd1–Cl1 2.2830(6), Pd1–Cl2
2.2859(6), N1–C2 1.286(3), C2–C3 1.480(3), C3–C6 1.325(4). Selected
bond angles (°): N1–Pd1–N2 90.81(7), N1–Pd1–Cl1 90.61(5), N1–Pd1–Cl2
176.94(5), Cl1–Pd1–Cl2 86.95(2), N1–C2 − C3 120.1(2), C2–C3–C6
119.2(2), C2–C3–C4 120.94(19)
SCHEME 3 Synthesis of palladium and nickel complexes 2 and 3
using single‐crystal X‐ray analysis. ORTEP diagrams of 2
and 3 along with selected bond parameters are shown in
Figures 3 and 4, respectively. In both of the complexes,
the metal β‐diketimine ring adopts a boat shape. In 2, Pd
and γ‐carbon atom deviate from the average plane of the rest
of the carbon atoms by 0.542 and 0.461 Å, respectively. In
the Ni complex 3, Ni is 0.692 Å above the plane and the
carbon atom is 0.381 Å above the plane. The bond angles
around Pd reveal that the geometry around Pd is essentially
square planar. The bond parameters around Pd in 2 are
similar to those of Pd β‐diimine complexes reported in the
literature.[38] The solid‐state structure of 3 shows that the
Ni is in distorted tetrahedral geometry, with the bond angles
around Ni ranging from 92.61° (N1–Ni–N2) to 122.90°
(Br1–Ni–Br2). Ni─N and Ni─Br bond lengths were
compared with those of β‐dimine complexes of Ni and are
in the same range.[33] Further, complex 2 shows a 1H
NMR spectrum that is in compliance with the obtained
structure. However, due to the paramagnetic nature of 3,
its NMR spectrum could not be obtained.
FIGURE 4 Single‐crystal X‐ray structure of 3. All hydrogen atoms except
those of CH2 are omitted for clarity. Thermal ellipsoids are drawn at the
50% probability level. Selected bond lengths (Å): Ni1–N1 1.999(2), Ni1–N2
2.000(2), Ni1–Br1 2.372(5), Ni1–Br2 2.343(4), N1–C2 1.288(3), C2–C3
1.492(4), C3–C6 1.329(4). Selected bond angles (°): N1–Ni1–N2 92.73(9),
N1–Ni1–Br1 103.80(7), N1–Ni1–Br2 114.36(7), Br1–Ni1–Br2 122.906(19)
2.3 | Mizoroki–Heck coupling using complexes 1 and 2
It has been reported that β‐diketimine‐based Pd complexes
show good catalytic activity in various C─C coupling
reactions.[38] Hence complexes 1 and 2 were explored
for their catalytic activity in the Mizoroki–Heck
reaction for C─C bond formation. Initially, the reaction
between 4‐bromoacetophenone and tert‐butyl acrylate was
probed with 1 mol% of 1 in dimethylformamide (DMF)
using K2CO3 as base at 80 °C, which affords the cross‐
coupled product in excellent yields (Table 1, entry 1).
Under similar conditions, 2 performs better than 1,
affording the cross‐coupled product in almost quantitative

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BEESAM AND NAREDDULA
TABLE 1 Optimization of Mizoroki–Heck coupling reaction using com-
plexes 1 and 2a
TABLE 2 Mizoroki–Heck coupling of aryl bromides with tert‐butyl
acrylatea
Yield (%)b
Entry
1
Ar
Catalyst 1
6a, 91
Catalyst 2
6a, 99
Yield of 6a (%)b
Entry
Base
K2CO3
KOtBu
NaOtBu
KOH
Solvent
DMF
Catalyst 1
Catalyst 2
1
2
3
4
5
6
91
—
—
—
80
—
99
—
—
—
82
40
2
3
4
5
6
7
6b, 91
6c, 90
6d, 90
6e, 85
6f, 85
6 g, 67
6b, 99
6c, 97
6d, 96
6e, 91
6f, 90
6 g, 75
DMF
DMF
DMF
NaOAc
K2CO3
DMF
DMF–H2O
aReaction conditions: aryl bromide (1.0 mmol), tert‐butyl acrylate (1.2 mmol),
base (2 mmol), catalysts 1 and 2 (1 mol%), solvent (3 ml), at 80 °C for 8 h.
bIsolated yield.
yields. Encouraged by this result, a few more reactions
were performed to optimize the reaction conditions, espe-
cially using various bases, and the results are summarized
in Table 1. When strong bases such as KOtBu, NaOtBu
and KOH are used, coupling reactions are not facilitated
(Table 1, entries 2–4), but yields are good in the case of
NaOAc (Table 1, entry 5). Further, to check the compatibil-
ity of the catalyst in aqueous medium, a mixture of DMF
and water in 1:1 ratio was used as solvent. Under these
conditions, complex 1 is found to be inactive, whereas
complex 2 affords the product in 40% yield (Table 1, entry
6).
8
6 h, 85
6 h, 90
9
6i, 80
6j, 80
6i, 88
6j, 88
10
After optimizing the reaction conditions, in order to test
the scope of 1 and 2 as catalysts, Mizoroki–Heck reactions
of aryl bromides with various substituents were carried out.
The results are summarized in Table 2. All the aryl bro-
mides used in this study react with tert‐butyl acrylate and
afford the corresponding substituted alkene in excellent
yields. While those with electron‐withdrawing groups give
almost quantitative yields, slightly lower yields are
observed when heteroaryl bromides are employed. Both
complexes 1 and 2 are highly efficient in effecting the
Mizoroki–Heck reaction, as it can be seen that just 1 mol%
of catalyst loading is sufficient to obtain excellent yields;
however, complex 2 is found to produce slightly better
yields in all the reactions carried out in this study. The
catalytic activities of complexes 1 and 2 have been com-
pared with those of reported β‐diketimine Pd complexes.[38]
Though the reported complexes are found to have activity
similar to that of 1 and 2 for 4‐bromoacetophenone, it
was reported that when 4‐bromotoulene was employed,
the yield was reduced markedly to 16% even after the reac-
tion was carried out for 18 h, whereas 1 and 2 afford the
cross‐coupled product in 85 and 91% yields, respectively,
11
6 k, 80
6 k, 88
aReaction conditions: aryl bromide (1.0 mmol), tert‐butyl acrylate (1.2 mmol),
K2CO3 (2 mmol), catalysts 1 and 2 (1 mol%), DMF (3 ml), at 80 °C for 8 h.
bIsolated yield.
within 8 h. While the previously reported complexes
required only 3 h compared to 1 and 2, which need 8 h,
they were used in larger quantities (3.5 mol%) compared
to 1 and 2 (1 mol%).
2.4 | Alkyne trimerization catalysed by complex 3
There are a few literature examples involving Ni(COD)2/PPh3
and Ni(II) salts of regioselective alkyne trimerizations,[42,43]
which prompted us to investigate the catalytic activity of Ni
complex 3 towards [2 + 2 + 2] cyclotrimerization. Initially,
trimerization of phenylacetylene with 2 mol% of complex 3
was carried out using Zn as reductant in acetonitrile at
50 °C for 9 h, affording the two terphenylene isomers 1,2,4‐

BEESAM AND NAREDDULA
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TABLE 3 Optimization of reaction conditions for complex 3‐catalysed
any conversion, which indicates the significance of complex
3 in catalysing the trimerization.
[2 + 2 + 2] cycloaddition reactiona
All the acetylenes that were screened under the
optimized conditions afford very good yields (Table 4).
Except for the case of phenylacetylene, in all other cases
1,2,4‐substituted benzene isomer is formed as the major
product. Catalyst loadings as low as 2 mol%, 50 °C
reaction temperature and high yields are the major
advantages of complex 3 as catalyst.
Entry
Solvent
CH3CN
Toluene
CH3CN
CH3CN
CH3CN
Time (h)
Reductant
Zn
Yield (%)
8a:9ab
40:60
40:60
—
1
2
9
12
9
88
25
—
—
—
Zn
3
Sn
3
| CONCLUSIONS
4
5c
12
12
Mg
—
A serendipitous formation of the vinylidene β‐diketimine
ligand L2 was observed, which on further treatment with
Pd(OAc)2 afforded the unexpected homobimetallic complex
1. Further, reactions of L2 with PdCl2(CH3CN)2 and
NiBr2(DME) yielded L2 complexes of corresponding
dihalidometal 2 and 3. All the metal complexes were charac-
terized using the single‐crystal X‐ray technique. Complexes 1
and 2 were explored for their catalytic activity in Mizoroki–
Heck coupling reactions and found that they are highly
efficient. The reactions do not require an organic base and
K2CO3 is good enough as base. Complex 3 is found to be
a very good catalyst for carrying out [2 + 2 + 2] cycloaddi-
tion of alkyne derivatives leading to cyclotrimerization in
excellent yields.
Zn
—
aReaction conditions: alkyne (1 mmol), catalyst (2 mol%), Zn (3 mmol), solvent
(3 ml).
bRatio determined by 1H NMR.
cWithout catalyst.
triphenylbenzene (8a) and 1,3,5‐substituted benzene (9a) in
40:60 ratio with a total conversion of 88% (Table 3, entry
1). Though the cyclotrimirization takes place in a non‐polar
solvent like toluene, the yield is very low. When Sn and Mg
are used in place of Zn, no reaction takes place even in aceto-
nitrile. Furthermore, phenylacetylene without the catalyst
(Table 3, entry 5) under similar conditions does not undergo
TABLE 4 [2 + 2 + 2] cycloaddition of various alkynes using complex 3a
4
| EXPERIMENTAL
4.1 | General methods and instrumentation
All manipulations were carried out using standard Schlenk
line and glove box techniques under an atmosphere of dry
nitrogen unless specified. Palladium acetate, palladium
chloride, nickel bromide, 2,6‐diisopropylaniline and
acetylacetone were procured from Aldrich and used as
received. CDCl3 and CH3CN were distilled from calcium
hydride. C6D6 was dried over sodium metal and dichloro-
methane was dried over P2O5. Aryl bromides and tert‐butyl
acrylate were purchased from Avra Synthesis and used as
received. NMR spectra were recorded with a Bruker
400 MHz (1H NMR, 400 MHz; 13C NMR, 100.62 MHz)
spectrometer. For 1H NMR and 13C NMR spectra, the residual
solvent peak was used as an internal reference.
Entry
1
Alkyne
8:9b
Yield (%)c
88
40:60
2
68:32
80
3
4
5
70:30
—
82
91
83
68:32
6
7
68:32
70:30
83
79
4.2 | Synthesis of L2
To a stirred solution of L1H (1.00 g, 2.39 mmol) in THF
(20 ml) was added n‐BuLi (1.6 M solution in n‐hexane;
1.64 ml, 2.60 mmol) at −70 °C and the mixture was stirred
for 1 h at the same temperature. The resulting lithiated product
was cannulated into a suspension of paraformaldehyde (0.35 g,
11.66 mmol) in THF (10 ml) at −70 °C, and the mixture was
allowed to warm to room temperature and stirred overnight.
The reaction mixture was filtered using Celite and the volatiles
aReaction conditions: alkyne (1 mmol), catalyst (2 mol%), Zn (3 mmol), CH3CN
(3 ml), at 50 °C for 9 h.
bRatio determined by 1H NMR.
cIsolated yield.

6 of 8
BEESAM AND NAREDDULA
were removed from the filtrate under high vacuum to obtain
yellow oil, which was distilled under vacuum and crystallized
from n‐pentane to afford L2 as pale yellow crystals at 0 °C.
Yield 0.710 g (68%); m.p. 50–52 °C. 1H NMR (CDCl3,
400 MHz, δ, ppm): 7.15–7.13 (d, 4H, Ar H), 7.12–7.06 (m,
2H, Ar H), 5.97 (s, 2H, CH2), 2.91–2.86 (m, 4H, CH(CH3)2),
1.96 (s, 6H, CH3), 1.16–1.15 (d, 12H, CH(CH3)2), 1.14–1.13
(d, 12H, CH(CH3)2). 13C NMR (CDCl3, 100 MHz, δ, ppm):
168.48, 152.88, 145.96, 136.44, 123.80, 123.03, 121.54,
28.10, 23.75, 23.13, 19.32. DEPT‐135 (δ, ppm): 123.66,
122.89, 121.41, 27.97, 23.61, 22.99, 19.18.
mixture turned purple after 2 h of stirring and the stirring
was continued overnight, after which it was filtered using
Celite. The filtrate afforded needle‐shaped purple crystals of
3, three days after storing at 0 °C. Yield 0.160 g (66%); m.
p. 150–153 °C (dec.).
4.6 | General procedure for Mizoroki–Heck reaction
In a glove box maintained under an atmosphere of nitrogen,
catalyst 1 or 2 (1 mol%), aryl bromide (1 mmol) and
K2CO3 (2 mmol) were taken in a Schlenk tube. The tube
was taken out from the glove box and connected to a Schlenk
line. Tert‐Butyl acrylate (1.2 mmol) followed by dry DMF
was syringed into the tube and the mixture was stirred at
80 °C for 8 h. Water was added (5 ml) and the mixture was
extracted with ethyl acetate (2 × 5 ml). The combined organic
extracts were washed with water (2 × 10 ml) followed by
saturated NaCl solution (1 × 10 ml), dried over Na2SO4
and filtered. The ethyl acetate was evaporated and the residue
was dried under vacuum to afford pure cross‐coupled product
4.3 | Synthesis of [O{(CH2‐β‐diketiminate)Pd(OAc)}2] (1)
To a flask charged with L2 (0.100 g, 0.232 mmol) and
Pd(OAc)2 (0.057 g, 0.255 mmol) was added CH3CN
(10 ml) via cannula. A brown‐coloured precipitate started to
form after 1 h. The mixture was stirred at room temperature
for 4 h and the brown solid was collected by filtration, and
dried under high vacuum for 3 h to yield 1 as a brown
powder. Pale yellow crystals were grown from a mixture of
toluene and hexane (1:1). Yield 0.115 g (82%); m.p.
123–125 °C. 1H NMR (CDCl3, 400 MHz, δ, ppm):
7.25–7.19 (m, 2H, Ar H), 7.09 (d, 2H, Ar H), 4.26 (s, 2H,
CH2), 3.43 (m, 4H, CH(CH3)2), 1.84 (s, 6H, CH3), 1.43 (s,
3H, CH3), 1.43 (d, 12H, CH(CH3)2), 1.18 (d, 12H,
CH(CH3)2). 13C NMR (CDCl3, 100 MHz, δ, ppm): 188.79,
156.74, 143.84, 142.11, 126.94, 123.45, 100.19, 69.61,
28.39, 24.10, 23.79, 23.70, 22.86, 20.10. DEPT‐135, δ,
ppm): 127.78, 123.28, 69.44, 28.21, 23.62, 22.70, 19.90.
1
(as determined using H NMR).
4.7 | General procedure for cycloaddition reaction of
Alkynes
In a glove box maintained under an atmosphere of nitrogen,
catalyst 3 (2 mol%) and Zn (3 mmol) were taken in a Schlenk
tube. The tube was taken out from the glove box and con-
nected to a Schlenk line. Acetylene derivatives (1 mmol)
followed by dry CH3CN were syringed into the tube and
the mixture was stirred at 50 °C for 9 h. Water was added
(5 ml) and the mixture was extracted with ethyl acetate
(2 × 5 ml). The combined organic extracts were washed with
water (2 × 10 ml) followed by saturated NaCl solution
(1 × 10 ml), dried over Na2SO4 and filtered. The ethyl acetate
was evaporated and the residue was dried under vacuum.
Column chromatography was performed to isolate the pure
product.
4.4 | Synthesis of [L2PdCl2] (2)
To a flask charged with L2 (0.100 g, 0.232 mmol) and
PdCl2(CH3CN)2 (0.059 g, 0.232 mmol) was added CH3CN
(10 ml) via cannula and the mixture was stirred at room tem-
perature for 6 h. The solvent was removed under vacuum.
The residue was washed with diethyl ether and dried under
high vacuum for 3 h to yield 2 as a brown powder, which
was recrystallized from a mixture of THF and hexane (1:3)
at 0 °C to afford pale yellow crystals. Yield 0.095 g (67%);
4.8 | X‐ray crystallographic studies
Single crystals of 1, 2 and 3 were each mounted on a glass
fibre in paraffin oil and then brought into the cold nitrogen
stream of a low‐temperature device so that the oil
solidified. Data collection was performed with an Oxford
1
m.p. 135–137 °C. H NMR (CDCl3, 400 MHz, δ, ppm):
7.26 (m, 2H, Ar H), 7.17 (d, 4H, Ar H), 6.40 (s, 2H, CH2),
3.26 (m, 4H, CH(CH3)2), 2.12 (s, 6H, CH3), 1.46 (d, 12H,
CH(CH3)2), 1.16 (d, 12H, CH(CH3)2). 13C NMR (CDCl3,
100 MHz, δ, ppm): 172.42, 147.21, 143.77, 139.79, 133.03,
127.99, 123.98, 77.47, 77.16, 76.84, 29.18, 25.90, 24.69,
24.27. DEPT‐135, δ, ppm): 132.89, 127.87, 123.86, 29.03,
25.78, 24.57, 24.16.
Xcalibur diffractometer, equipped with
a
CCD area
detector, using graphite‐monochromated
Mo Kα
(λ = 0.71073 Å) radiation and a low‐temperature device.
Data collection for L2 was done at room temperature. All
calculations were performed using SHELXS‐97 and
SHELXL‐97.[55] The structures were solved by direct
methods and successive interpretation of the difference
Fourier maps, followed by full‐matrix least‐squares refine-
ment (against F2). All non‐hydrogen atoms were refined
anisotropically. The contribution of the hydrogen atoms,
in their calculated positions, was included in the refinement
4.5 | Synthesis of [L2NiBr2] (3)
To a flask charged with L2 (0.150 g, 0.348 mmol) and
NiBr2(DME) (0.107 g, 0.348 mmol) was added dichloro-
methane (10 ml) via cannula. The colour of the reaction

BEESAM AND NAREDDULA
7 of 8
[20] C. E. Tinberg, Z. J. Tonzetich, H. Wang, L. H. Do, Y. Yoda, S. P. Cramer,
using a riding model. Upon convergence, the final Fourier
difference map of the X‐ray structures showed no signifi-
cant peaks. In the molecular structure of complex 1, two
molecules of toluene were found but only one molecule
could be refined and the other molecule could not be
modelled because of serious disorder. Therefore, a new
data set corresponding to omission of the missing solvent
was generated with the SQUEEZE algorithm and the
structure was refined to convergence. Relevant data
concerning crystallographic data, data collection and
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(supporting information).
S1
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ACKNOWLEDGMENTS
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We gratefully acknowledge the Department of Science
and Technology (DST), New Delhi for financial support
and Alexander von Humboldt Stiftung, Germany for the
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donation of
a glove box. We thank the Central
Instrumentation Facility, Pondicherry University for
NMR spectra and DST‐FIST for the single‐crystal X‐ray
diffraction facility.
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How to cite this article: Beesam R, Nareddula DR.
Pd and Ni complexes of a novel vinylidene β‐
diketimine ligand: Their application as catalysts in
Heck coupling and alkyne trimerization. Appl
Organometal Chem. 2017;e3696. doi: 10.1002/
aoc.3696
[54] N. M. Rajendran, N. D. Reddy, Polyhedron 2014, 72, 27.
[55] G. M. Sheldrick, Acta Crystallogr. A 2008, 64, 112.
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