Nickel(II)- and Palladium(II)-NHC Complexes from Hydroxypyridine Functionalized C,O Chelate Type Ligands: Synthesis, Structure, and Catalytic Activity toward Kumada-Tamao-Corriu Reaction

Article abstract of DOI:10.1021/acs.organomet.8b00878

Hydroxypyridine functionalized imidazolium salts (2a-c) have been prepared in a one pot neat reaction between alkyl/aryl imidazoles and 2-chloro-3-hydroxypyridine. The imidazolium salts were used as proligands for the synthesis of new Ni(II) (3a, 3c, and 3c′) and Pd(II) NHC (4a-4c) complexes. Complexes, 3a, 3c, and 4a-4c are four coordinated with square planar geometry around the metal center and feature the C,O chelation of the heterobidentate NHC ligands, using carbene atoms and the O atom of the hydroxypyridine arm. Depending upon the steric bulk of the alkyl/aryl substituents on the ligand, either cis (3a and 4a) or trans (3c, 4b, and 4c) complexes are obtained. For the nickel complex with the 2c ligand, two different isomeric forms (3c and 3c′) were observed. In 3c, both the NHC ligands are bound via C2 and O atoms of the hydroxypyridine arm, whereas, in 3c′, one of the two ligands binds via C4 and N atoms of the hydroxypyridine side arm. 3c′ represents a unique square planar complex with the central metal atom bound simultaneously to both normal and abnormal carbenes. All the five complexes, except 3c′, were evaluated as catalysts for the Kumada-Tamao-Corriu cross-coupling reaction between phenylmagnesium bromide and different aryl chlorides at room temperature.

Full text of DOI:10.1021/acs.organomet.8b00878

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Nickel(II)- and Palladium(II)-NHC Complexes from Hydroxypyridine
Functionalized C,O Chelate Type Ligands: Synthesis, Structure, and
Catalytic Activity toward Kumada−Tamao−Corriu Reaction
Irshad Ahmad Bhat,†,‡ Iruthayaraj Avinash,† and Ganapathi Anantharaman*,†
†Department of Chemistry, Indian Institute of Technology, Kanpur-208016, India
‡S. A. M. Degree College, Budgam, Jammu and Kashmir, 191111, India
S
* Supporting Information
ABSTRACT: Hydroxypyridine functionalized imidazolium salts
(2a−c) have been prepared in a one pot neat reaction between
alkyl/aryl imidazoles and 2-chloro-3-hydroxypyridine. The
imidazolium salts were used as proligands for the synthesis of
new Ni(II) (3a, 3c, and 3c′) and Pd(II) NHC (4a−4c)
complexes. Complexes, 3a, 3c, and 4a−4c are four coordinated
with square planar geometry around the metal center and feature
the C,O chelation of the heterobidentate NHC ligands, using
carbene atoms and the O atom of the hydroxypyridine arm.
Depending upon the steric bulk of the alkyl/aryl substituents on
the ligand, either cis (3a and 4a) or trans (3c, 4b, and 4c)
complexes are obtained. For the nickel complex with the 2c
ligand, two different isomeric forms (3c and 3c′) were observed.
In 3c, both the NHC ligands are bound via C2 and O atoms of the hydroxypyridine arm, whereas, in 3c′, one of the two ligands
binds via C4 and N atoms of the hydroxypyridine side arm. 3c′ represents a unique square planar complex with the central
metal atom bound simultaneously to both normal and abnormal carbenes. All the five complexes, except 3c′, were evaluated as
catalysts for the Kumada−Tamao−Corriu cross-coupling reaction between phenylmagnesium bromide and different aryl
chlorides at room temperature.
is more advantageous in terms of atom economy and its
involvement in the enhancement of the rate of the trans-
metalation step which sometimes tends to be the rate
determining for other reactions like Suzuki coupling involving
milder reagents.6 The disadvantages such as limited substrate
scope due to the greater reactivity of Grignard reagents have
been recently circumvented either by carrying out the reaction
at sufficiently lower temperatures7 or by the slow addition of
Grignard reagent into the reaction mixture.8 Buchwald and
others have shown that the use of special phosphine ligands
could enhance the catalytic activity of palladium based catalysts
toward KTC reaction and the overall KTC coupling reactions
could be carried out even at lower temperature.7a In pursuit of
this, much of the time was devoted toward finding the right
combination of ligand which not only stabilizes the nickel but
also activates the nickel center for the coupling reactions.7b,c
In the last three decades, N-heterocyclic carbenes (NHCs)
have emerged as an excellent alternative to the phosphines due
to their enhanced sigma donation capacity to the metal center
that increases the activity of the transition metal based catalysts
used in cross-coupling reactions. In situ generated or well-
INTRODUCTION
■
The design of new sustainable and efficient catalytic systems
employing cheaper and earth abundant metals is an important
area of ongoing research. In particular, an increasing interest
has been drawn toward the development of 3d-transition metal
based catalytic transformations.1 Besides the cost effectiveness,
a set of accessible oxidation states of nickel (0−IV), facile
oxidative addition of organic substrates on the nickel center,
and the low Ni−C bond strength are some of the key
advantages for the studies on nickel based catalytic trans-
formations.2 However, improving and controlling the reactivity
on the nickel center is one of the challenging tasks. Catalysis
using nickel based complexes is known for several decades
now, and the first set of reports on nickel catalyzed cross-
coupling reactions between aryl halides and Grignard reagents
also known as the Kumada−Tamao−Corriu (KTC) coupling
appeared in 1972.3 Some of the most important advancements
with regard to the application of nickel complexes for KTC
reaction in the recent past include the use of readily available
phenolic salts or aryl sulfonates and triflates as electrophiles.4
Compared to the other cross-coupling reactions involving aryl
boronic acids and similar type of organometallic reagents
which are often themselves prepared from Grignard reagents,5
the direct use of Grignard reagents as the transmetalating agent
Received: December 5, 2018
© XXXX American Chemical Society
A
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defined nickel complexes of NHCs have been successfully
employed as catalysts for the KTC coupling reaction since the
first report by Hermann who utilized in situ generated Ni-
NHC complexes to carry out the challenging cross-coupling
reaction between aryl chlorides/fluorides with Grignard
reagents.9 In contrast to the monodentate NHC ligands,
nickel complexes having bidentate or pincer type ligands
perform better in KTC cross-coupling reactions.10,11 Com-
pared to the neutral NHC ligands, the development of anionic
tethered NHCs and utilization of their metal complexes for
different catalysis have been widely explored.12,13 Use of such
anion tethered NHC-metal complexes in coupling reaction has
been reported by Bouwman’s group with the premise that the
presence of an anionic group accelerates the reaction by
binding to the Ni atom of the catalyst and the Mg atom of the
Grignard reagent together.14a Such a design was also reported
with a phosphine ligand by Nakamura’s group (Chart 1).15
Compared to the anionic nitrogen center, the anionic oxygen
atoms are preferable due to better coordinating ability of
oxygen atom toward both nickel and magnesium centers.
nickel and palladium complexes as well as catalytic applications
of metal complexes in KTC reaction.
RESULTS AND DISCUSSION
■
Synthesis of Imidazolium Salts HIMepyOH·Cl (2a),
HIMespyOH·Cl (2b), HIMespyOH·PF6 (2b·PF6), and HI-
DipppyOH·Cl (2c). 2a−c were prepared in good yield by
heating the 1-alkyl/aryl imidazoles (1a−c) and 2-chloro-3-
hydroxypyridine (Scheme 1) under neat conditions. They were
Scheme 1. Synthesis of 2a−c
characterized by spectroscopic, spectrometric, and elemental
analysis. The imidazolium proton (NCHN) of 2a−c in H
Chart 1. Design of Various Ligands with an Anionic Arm for
KTC Reaction
1
NMR resonates around 9.77 ppm, authenticating the
formation of the expected products. Also the characterization
of 2b by single crystal X-ray diffraction confirms the product
formation and further reveals that the OH group in pyridine is
pointed away from the imidazolium proton (Figure 1).
Functionalization of an NHC with hydroxyl, aryloxy, amido,
and enolate tether are some of the main ways for achieving
anionic NHCs.16 The metal complexes of alcohol and
phenolate functionalized NHCs in particular have been of
interest in the past few years, and this area of work was recently
reviewed by Braunstein and co-workers.12e Synthesis of aryloxy
functionalized NHCs has been a challenge due to the reason
that alkyl/aryl imidazoles or imidazolidine derivatives are too
weak a nucleophile to react with simple aryl halides. Therefore,
it often necessitates the use of metal catalyzed coupling
reactions or tedious cyclization procedure.17 To overcome
such complications, the hydroxypyridine moiety could be
embedded onto the nitrogen of the imidazole using
commercially available 2-chloro-3-hydroxypyridine via quarter-
nization under neat reaction condition. Moreover, the presence
of such hydroxypyridine adjacent to C2/C4 of imidazole is
advantageous in terms of variable coordination of the ligand to
the metal center by means of heterobidentate C,O and/or C,N
chelation in which the carbene carbon atom could be of either
normal or abnormal type (Chart 2). Herein, we describe a
straightforward and convenient synthesis of a series of new
hydroxypyridine functionalized imidazolium salts and their
Figure 1. ORTEP diagram of 2b with ellipsoids in 30% probability.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
(Å) and angles (deg): N(1)−C(1) 1.338(4), N(2)−C(1) 1.333(4),
C(3)−C(2) 1.343(5), O(1)−C(5) 1.354(4), C(1)−N(1)−C(3)
108.7(3).
Interestingly, the solid state structure shows that the bulky
mesityl group is orthogonal to the imidazolium ring, whereas
the hydroxypyridine is twisted with respect to the imidazolium
ring (dihedral angle between the planes of imidazolium and
hydroxypyridine: 29.6(1)°). The twisting of these two rings is
important for a short intramolecular contact between the OH
group and C−H (C3) of the imidazolium unit (2.857(4) Å)
and hydrogen bonding between the O-H···Cl (3.015 Å) and C-
H···Cl of the imidazolium C1-H atom, leading to a
supramolecular network. From 2b, anion exchange with
KPF6 in aqueous medium produced the 2b·PF6 in excellent
yield.
Chart 2. Possible Binding Modes of Carbenes Derived from
Preparation of Ni(II)- and Pd(II)-NHC Complexes.
Complexes of nickel (3a, 3c, and 3c′) were prepared by
treating the imidazolium salts 2a and 2c with either
Ni(PPh3)2Cl2 (for 3a) or NiCl2·6H2O (for 3c and 3c′)
under basic condition in acetonitrile (Scheme 2). The reaction
with the methyl substituted salt (2a) yielded the complex 3a as
a bright yellow-orange solid in near quantitative yield.
However, the reaction with Dipp substituted salt (2c) led to
2a−c
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Information, Figure S1b). After deducting the signals
corresponding to 3c, the remaining peaks were assigned to
the product 3c′. The presence of a highly downfield signal at
9.85 ppm for the C2-H proton, four doublets for methyl
(∼1.0−1.50 ppm), and two septets for CH (2.02 and 3.66
ppm) of isopropyl group protons validates the formation of an
unsymmetrically substituted mesoionic nickel carbene (C4-
bound nickel-NHC) complex in 3c′. Such type of complexity,
especially in the alkyl region in 1H NMR, was also observed in
a closely related mesoionic carbene palladium complex in the
literature.18a Besides, ignoring the signals due to 3c, the
remaining peaks could be accounted for all the protons of 3c′
with proper integral values (Supporting Information, Figure
S1c).
Scheme 2. Synthesis of Metal-NHC Complexes
Structures of 3a, 3c, 3c′, and 4a−4c. The molecular
structures of 3a, 3c, 3c′, and 4a−c were determined by the
single crystal X-ray diffraction (Figures 2−4). All the
the formation of two different nickel complexes (3c and 3c′).
The pink-red colored complex (3c) was exclusively obtained
by washing the crude compound with methanol, whereas the
removal of methanolic filtrate and further workup with a
mixture of diethyl ether and pentane (refer to Experimental
Section) afforded the mixture of pink-red crystals (3c) and
yellow crystals (3c′) which could not be separated out
completely from 3c due to their microcrystalline nature.
However, the crystals of 3c′ were analyzed through single
crystal X-ray diffraction method. The reaction of the mesityl
substituted imidazolium salt (2b) with the nickel precursor
under basic medium resulted in an insoluble precipitate (3b).
In contrast to the above, the reaction of palladium dichloride
with 2a−c under basic medium afforded only two types of
palladium complexes, the cis complex 4a and the trans
complexes 4b and 4c depending upon the steric bulk of the
organic group in the ligand (Scheme 2). All the compounds,
except 3c′, were thoroughly characterized by spectroscopic,
spectrometric, and elemental analysis, besides single crystal X-
ray diffraction studies.
Figure 2. Molecular structures of 3a (left) and 3c (right) with 35%
probability of the ellipsoids. Hydrogen atoms and the solvent
molecules of crystallization have been omitted for clarity.
Figure 3. Molecular structure of 3c′ with 35% probability of the
ellipsoids. Hydrogen atoms and the solvent molecules of crystal-
lization have been omitted for clarity. Selected bond lengths (Å) and
bond angles (deg) for 3c′: M1−C1 1.863(4), M1−O1 1.874(2),
M1−C23 1.865(4), M1−N6 1.952(3), C1−M1−O1 90.90(13), C1−
M1−C23 97.28(15), C23−M1−O1166.67(13), C23−M1−O1
166.67(13), C23−M1−N6 83.69(13), O(1)−Ni(1)−N(6)
89.56(11).
The 1H NMR spectra of 3a and 3c as well as 4a−c were void
of the C2-H and OH proton peaks, and an appearance of a
peak around 160 ppm in 13C NMR indicates the formation of
the metal-carbene complexes. The formation of complexes
with 1:2 metal to ligand ratios were confirmed by the presence
of mass (M + H)+ in the respective ESI-MS and also with
elemental analysis. In contrast, it was difficult to obtain 3c′ in
pure form for its separate characterization either by NMR or
1
by elemental analysis. However, a careful analysis of the H
complexes possess square planar structures with either cis or
trans geometry around the central metal atom, depending upon
the steric hindrance of the alkyl or aryl substituents. Among
them, 3a and 4a exhibit a distorted square planar geometry
around the metal center surrounded by ligands forming the
MC2O2 unit. Here, the chelating bidentate ligands bind to the
metal centers in a cis fashion. The other nitrogen donors of the
ligands remain free.
NMR spectrum of the crude product obtained from diethyl
ether/pentane extractions of the residue from methanolic
filtrate (see Experimental Section) allowed us to distinctly
identify the peaks of 3c′ along with 3c (see Supporting
Information, Figure S1a). The signals of 3c were marked with
an asterisk (*) and can be easily identified by comparing it
1
with the H NMR of the pure sample of 3c (Supporting
C
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Figure 4. Molecular structures of 4a (left), 4b (middle), and 4c (right) with 35% probability of the ellipsoids. Hydrogen atoms have been omitted
for clarity.
Table 1. Selected Bond Distances (Å) and Angles (deg) for 3a, 3c, and 4a−c
3a
4a
3c
4b
4c
M = Ni
M = Pd
M = Ni
M = Pd
M = Pd
M1−C1
M1−O1
M1−C10
M1−O2
C1−M1−O1
C1−M1−C10
C10−M1−O2
O1−M1−O2
C1−M1−O2
C10−M1−O1
1.821(7)
1.897(5)
1.859(6)
1.862(5)
91.1(3)
96.0(3)
91.6(2)
85.9(2)
163.0(3)
163.0(3)
1.969(3)
2.032(2)
1.950(3)
2.054(2)
90.26(11)
96.89(13)
87.33(12)
87.12(9)
166.69(11)
170.42(11)
M1−C1
1.929(2)
1.8492(17)
1.929(2)
1.8492(17)
91.29(9)
180.0
91.29(9)
180.0
88.71(9)
88.71(9)
2.006(3)
1.9925(19)
2.006(3)
1.9925(19)
88.99(10)
180.000(14)
88.99(10)
180.000
2.058(7)
1.976(5)
2.058(7)
1.976(5)
89.1(2)
180.0
89.1(2)
180.0
90.9(2)
90.9(2)
M1−O1
M1−C1i
M1−O1i
C1−M1−O1
C1−M1−C1i
C1i−M1−O1i
O1−M1−O1i
C1−M1−O1i
C1i−M1−O1
91.01(10)
91.01(10)
The average M−C (NHC) bond distances in the bidentate
cis complexes (av. Ni−C1, 1.840(7) Å, 3a and av. Pd−C:
1.959(3)Å, 4a) fall within the range of reported values,
wherein one of the values of Ni1−C1 (1.821(7) Å) and Pd1−
C1 (1.969(3) Å) in 3a and 4a become the shortest (Ni−C:
1.838(2)−1.873(2) Å) and longest (1.946(2)−1.953(3) Å)
distances, respectively, known for these type of complexes
(Table 1).12e,18 The M−O bond distances are 1.8625(5) and
1.897(5) Å for nickel (3a) and 2.032(2) and 2.054(2) Å for
palladium (4a) complexes, respectively. It is noteworthy that
the lowest and highest values of Ni−O and Pd−O bond
distances, respectively, define the new shorter Ni−O and
longer Pd−O distances known for similar type of complexes in
the literature (Ni−O: 1.872(8)−1.915(2) Å; Pd−O 2.011(4)/
2.023(4) Å).18
On the other hand, 3c, 4b, and 4c display a perfect square
planar geometry around the metal centers. Compared to the
smaller methyl substituent in 3a and 4a, the sterically more
demanding mesityl (4b) and diisopropylphenyl substituents
(3c and 4c) preferably direct the chelating bidentate ligands to
bind the metal center in a trans fashion. This enhances the
symmetry within the complexes, featuring only half of the
molecule in the asymmetric unit of the crystal structure. The
M−C distances [Ni−C1: 1.929(2) Å (3c); Pd−C1: 2.006(3)
Å (4b); Pd−C1: 2.058(7) Å (4c)] in the trans complexes are
longer as expected than the corresponding cis complexes (3a
and 4a). The Ni−C bond distances in 3c are longer than the
similarly known trans complex (1.908(5) Å), whereas Pd−C
bond distances fall in the range of the known examples
(2.007(2)−2.023(4) Å).19,18a Further, the M−O bond
distances in 3c (Ni−O: 1.8492(17) Å), 4b (1.9925(19) Å),
and 4c (1.976(5) Å) are shorter, like it is known for such types
of complexes in the literature (Ni−O: 1.835(2) Å; Pd−O:
2.058(1) Å)16i,18a and also to either 3a or 4a which can be
attributed to the higher trans influence of NHC in the latter
complexes.
For all the complexes except 3c′, the ligands bind to the
metal centers through the C2 of the imidazolylidene and the O
atom of the hydroxypyridine arm in a C,O chelating fashion
(Chart 2, binding mode-I). Interestingly, in the case of 3c′, one
of the two ligands was coordinated through C4 of the
imidazolylidene (abnormal carbene) and the N atom of the
hydroxypyridine arm by C,N chelating mode, whereas the
other ligand was bound through C,O chelating mode as
observed for other cases (Chart 2, binding mode-III). Out of
the four different binding modes for this ligand system outlined
in Chart 2, the binding mode-I is the more prevalent, but in
3c′, the same ligand exhibits two different binding modes,
suggesting that the ligand can be regarded as the chelating
ambidentate as it can act as a bidentate ligand in four different
ways. Similar to the other complexes, the nickel atom in 3c′
adopts a square planar geometry. It is interesting to note that,
despite different binding modes, the two metal−carbon
bonding distances in 3c′ are almost similar. Besides, the
molecule has intra- and intermolecular C-H···O interaction
(Table S2, Supporting Information) in the crystal structure of
3c′. It should be pointed out that, though the examples of
metal centers having both normal and abnormal binding
modes are already known in the literature from the single
NHC ligand,20 the formation of the bidentate unit containing
two different chelating heteroatoms (C2^O, C4^N) to a single
metal center is not reported so far.
Kumada−Tamao−Corriu Coupling Reaction. All the
complexes obtained in this study, except 3c′, were evaluated as
catalysts for the Kumada−Tamao−Corriu (KTC) cross-
coupling reaction between phenylmagnesium bromide and 4-
chloroanisole. Preliminary control reactions in THF as the
solvent at room temperature using nickel (3a and 3c) and
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palladium (4a−c) complexes were evaluated. For a direct
comparison of the catalytic activity between the above
synthesized catalysts and the well-known catalysts like
Ni(dppe)2Cl2, Ni(acac)2, and Ni(PPh3)2Cl2, the same
reactions were carried out under similar experimental
conditions, and the results obtained are given in Table 2.
the cases which stands out as a distinct advantage over other
commonly used catalysts for KTC reaction, such as Ni-
(dppe)2Cl2, Ni(acac)2, and Ni(PPh3)2Cl2, under the similar
experimental conditions (Table 2, entries 7, 8, and 9).
Using the above optimized condition (Table 2, entry 4),
different unsubstituted, substituted, and the heteroaromatic
chlorosubstrates were screened for the KTC coupling in the
presence of phenylmagnesium bromide, and the isolated yield
of cross-coupled products are provided in Table 3. Substrates
Table 2. KTC Coupling of 4-Chloroanisole with
a
Phenylmagnesium Bromide
a
Table 3. KTC Coupling Results with 3a as the Catalyst
b
conversion (%)
PhMgBr
(equiv)
time
(h)
C−C
entry
1
catalyst (mol %)
coupled homocoupled
3a (1)
1
1
2
4
1
2
4
1
2
4
1
2
4
1
3
6
2
4
6
2
4
8
2
4
8
2
4
11
7
15
51
33
80
95
43
75
93
4
1
1
7
2
3
5
1
2
2
1
4
5
1
2
4
4
4
2
3
4
5
6
7
8
9
3a (1)
2
3a (2)
1.5
1.5
1.5
1.5
1.5
1.5
1.5
3a (1)
61
93
1
3a (0.5)
13
33
56
62
66
22
33
33
15
50
62
0
3c (1)
9
Ni(dppe)Cl2 (1)
Ni(PPh3)2Cl2 (1)
Ni(acac)2 (1)
14
42
43
13
21
22
5
3
5
9
9
a
Reaction conditions: Substrate 1 mmol, phenylmagnesium bromide
b
1.5 mmol, catalyst 0.01 mmol, THF 4 mL, 30 °C. Isolated yield.
a
b
30 °C, 4 mL of THF, 4-chloroanisole 1 mmol. GC-MS yield.
Only nickel complexes 3a and 3c gave the cross-coupling
products, whereas the corresponding palladium complexes 4a−
c were inactive. Generally, for KTC cross-coupling reaction
which involves the use of the reactive Grignard reagents,
palladium based catalysts tend to be less effective as compared
to the corresponding nickel based catalysts.21 Apart from this,
the performances of the catalysts Ni(dppe)2Cl2, Ni(PPh3)2Cl2,
and Ni(acac)2 were found to be sluggish as compared to other
catalysts with less selectivity for the cross-coupled product
(Table 2, entries 7, 8, and 9).
Further screening of the reaction revealed that the best
yields were obtained when 1 mol % of catalyst 3a and 1.5 equiv
of the Grignard reagent are used for the reaction (Table 2,
entry 4). As reported by Newman and co-workers, a slow
addition of Grignard reagent over the period of 20−30 min
gave a better yield than the bulk addition of the reagent at
once.8 Traces of homocoupled product in the form of biphenyl
were also observed in almost every reaction; however, the
cross-coupled product was obtained as the major product in all
containing methoxy or methyl at the para position gave
excellent yield. Similarly, the yields of monocoupled products
using chloronaphthalene and chloropyrimidine as well as bis-
coupled product of 2,6-dichloropyridine are quantitative,
except in few cases like 2-chloropyridine, 2,5-dichloropyridine,
and 4-trifluoromethyl chlorobenzene.
There are very few nickel complexes containing NHC or
non-NHC ligands in the literature that were found to be
efficient catalysts for KTC cross-coupling of similar substrates
used in Table 3.7b,c,9c,10a,e,11e Each of them are advantageous in
terms of either of these factors such as low catalyst loading,
wider substrate scope, higher yield, and room temperature.
Among them, 3a is promising in terms of (a) shorter reaction
time, (b) room temperature, and (c) low catalyst loading.
Moreover, similar to the observation noted by Bouwman and
co-workers, the anionic oxygen atom in the nickel complex not
only serves as an anchor for bringing the Grignard reagent in
close proximity to the metal center but also might be involved
in increasing the rate of oxidative addition as proposed by
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Yoshikai and co-workers.15 Further understanding of mecha-
nism of the coupling reaction and wider substrate scopes are
underway.
8.05 (dd, J = 4, 1 Hz, 1H, py), 8.28 (s, 1H, Im), 9.79 (s, 1H, NCHN),
11.96 (br s, 1H, OH); 13C NMR (125 MHz, DMSO-d6, δ ppm) 36.7,
121.1, 123.9, 126.6, 127.1, 134.9, 137.1, 139.2, 146.3. IR (KBr): v
̃
3422 (br, m), 3159 (m), 3063 (s), 2949 (s), 2903 (s), 2739 (s), 2607
(s), 2565 (s), 2516 (s), 1769 (w), 1650 (w), 1609 (s), 1578 (s), 1535
(s), 1468 (s), 1408 (w), 1360 (s), 1313 (s), 1297 (s), 1272 (s), 1223
(s), 1169 (m), 1113 (s), 1069 (m), 1057 (s), 969 (m), 826 (s), 798
(s), 763 (s), 746 (s), 692 (m), 645 (m), 633 (w), 610 (s), 559 (w)
cm−1. ESI-MS m/z calcd. for [C9H10N3O]+, 176.0818 [M − Cl]+;
found, 176.0827. Anal. Calcd for C9H10ClN3O: C, 51.07; H, 4.76; N,
19.85. Found: C, 51.34; H, 4.73; N, 19.82.
CONCLUSION
■
In conclusion, a convenient synthesis of new hydroxypyridine
functionalized imidazolium salts has been reported which were
then used as proligands to prepare nickel and palladium NHC
complexes, demonstrating that hydroxypyridine functionaliza-
tion can be one of the efficient ways to prepare heterobidentate
C,O chelate type NHC ligands. The solid state structures of
the complexes reveal that the ligand with a smaller methyl
substituent at the nitrogen atom of the imidazole ring tends to
produce cis complexes with distorted square planar geometry,
whereas, with bulkier substituents on the nitrogen atom of the
ligand, trans complexes with perfect square planar geometries
are produced. For one of the nickel complexes, two different
constitutional isomers were observed (3c and 3c′) and
2b. 1-(2,4,6-Trimethylphenyl)-1H-imidazole (1b) (1.86 g, 10
mmol) and 2-chloro-3-hydroxy pyridine (1.55 g, 12 mmol) were
taken in a Schlenk tube and heated at 130 °C for 72 h to yield a dull
brown residue. The residue was scratched out of the Schlenk tube and
washed with ethyl acetate three times (by stirring the residue in 100
mL of ethyl acetate for 10−12 h), and the purity was checked by
recording a 1H NMR after every wash with the ethyl acetate. The final
suspension was filtered through an open frit to yield 2b as an off-white
powder that was dried under vacuum. Yield 2.25 g (71.4%). Mp:
1
282−285 °C. H NMR (500 MHz, CDCl3, δ ppm): 2.10 (s, 6H, o-
1
characterized by H NMR spectroscopy and single crystal X-
CH3), 2.39 (s, 3H, p-CH3), 7.09 (s, 2H, m-CH), 7.31 (s, 1H, Im-4,5-
CH), 7.36 (dd, J = 8.5, 4 Hz, 1H, py-CH), 8.02 (dd, J = 5, 2 Hz, 1H,
py-H), 8.41 (dd, J = 8.5, 1.5 Hz, 1H, py-H), 8.64 (s, 1H, Im-CH),
9.77 (s, 1H, Im-NCHN). The O-H proton was not observed in
CDCl3 which may be due to the exchange of proton with the solvent.
However, the sample recorded in DMSO-d6 shows the presence of O-
H proton (see Supporting Information, Figure S4).13C NMR (125
MHz, CDCl3, δ ppm): 17.5, 21.2, 120.8, 122.4, 126.8, 129.0, 130.1,
ray diffraction method. 3c has both the NHC ligands bound to
the metal in a normal C2 fashion like all the other complexes,
whereas 3c′ has one NHC binding in an abnormal C4 fashion.
All the complexes were evaluated as catalysts for Kumada−
Tamao−Corriu cross-coupling reaction between phenylmag-
nesium bromide and less reactive aryl chlorides under ambient
conditions, and out of all the complexes, 3a gave the best
yields.
130.4, 133.3, 134.1, 135.8, 138.7, 142.1, 146.6. IR (KBr): ṽ 3432 (br,
m), 3195 (m), 3167 (s), 3064 (m), 3008 (s), 2863 (s), 2766 (s),
2628 (s), 2506 (w), 1617 (s), 1579 (s), 1553 (w), 1526 (s), 1466 (s),
1388 (w), 1346 (m), 1331 (s), 1302 (s), 1270 (s), 1242 (s), 1184
(w), 1165 (w), 1123 (s), 1100 (w), 1084 (m), 1053 (m), 964 (w),
934 (w), 888 (w), 863 (m), 839 (w), 822 (s), 768 (m), 757 (s), 670
(s), 589 (w), 565 (w), 535 (w), 434 (w) cm−1. ESI-MS m/z calcd. for
[C17H18N3O]+, 280.1449 [M − Cl]+; found, 280.1455. Anal. Calcd
for C17H18ClN3O: C, 64.66; H, 5.75; N, 13.31. Found: C, 64.53; H,
5.82; N, 12.96.
EXPERIMENTAL SECTION
■
Materials and Methods. Glassware was dried in an oven at 100
°C prior to use. The chemicals PdCl2, 2-chloro-3-hydroxypyridine,
and 1-methylimidazole (1a) (Sigma-Aldrich) were purchased and
used as received. 1-(2,4,6-Trimethylphenyl)-1H-imidazole (1b) and
1-(2,6-diisopropylphenyl)-1H-imidazol (1c) were prepared by using
the literature procedure.22 Melting points were taken in a capillary
(sealed tube for ligands), and the values reported are uncorrected. IR
spectra were recorded in KBr pellets using a PerkinElmer FTIR model
1320 spectrophotometer operating from 4000 to 400 cm−1. 1H NMR
and 13C NMR spectra were recorded on JEOL-DELTA 400 and 500
MHz spectrometers, and the chemical shifts are referenced with
respect to tetramethylsilane (TMS). ESI-MS (positive mode) spectra
were recorded on a Waters-Q-TOF Premier-HAB213 instrument.
Elemental analyses for the compounds were done using a PerkinElmer
series-II CHNS/O analyzer 2400. For single crystal X-ray
crystallography, the crystal data were collected on a Bruker D8
QUEST CCD diffractometer using graphite-monochromated MoKα
radiation (λ = 0.71073 Å) at 293 K. The data integration and
reduction were processed with SAINT software,23 and the data were
corrected for absorption by using the SADABS program.24 All the
structures were solved by direct methods using the SHELXS program
of the SHELXTL package and refined by full-matrix least-squares
methods with SHELXL-2014.25 All of the hydrogen atoms were
included in idealized positions and a riding model was used. Non-
hydrogen atoms were refined with anisotropic displacement
parameters.
2b·PF6. 2b (2.23g, 7.1 mmol) and KPF6 (1.31g, 7.1 mmol) were
dissolved separately in 70 and 50 mL of distilled water, respectively.
The solution of 2b in water was warmed to completely dissolve the
imidazolium salt and filtered through Whatman paper. The aqueous
KPF6 solution was then added to the aqueous solution of 2b in one
go; a white precipitate was immediately thrown out of the solution.
After properly mixing, the contents were filtered through an open frit.
The white precipitate was washed with water and dried in a 50 °C
oven and then further dried under high vacuum. Yield 2.76 g (92%).
Mp: > 270 °C. 1H NMR (400 MHz, CDCl3, δ ppm): 2.11 (s, 6H, o-
CH3), 2.38 (s, 3H, p-CH3), 7.08 (s, 2H, m-CH), 7.37 (s, 2H, Im-H
+py-H), 7.91 (d, J = 8.4 Hz, 1H, py-CH), 8.04 (d, J = 3.6 Hz, 1H, py-
H), 8.62 (s, 1H, Im-H), 9.64 (s, 1H, Im-NCHN). The O-H proton
was not observed in CDCl3 which may be due to the exchange of
proton with the solvent. 13C NMR (100 MHz, CDCl3, δ ppm):17.5,
21.2, 121.2, 122.7, 127.0, 128.3, 130.1, 133.4, 134.1, 135.7, 135.8,
139.2, 139.3, 142.1. IR (KBr): v 3481 (m), 3195 (w), 3148 (m), 2926
̃
(w), 1619 (w), 1582(w), 1552 (w), 1535 (m), 1479 (m), 1463 (w),
1383 (w), 1331 (w), 1308 (m), 1266 (m), 1250 (m), 1228 (m), 1179
(w), 1166 (w), 1121 (w), 1097 (w), 1057 (w), 968 (w), 903 (w), 852
(s), 813 (s), 747 (m), 662 (w), 644 (w), 583 (w), 556 (s), 462 (w).
ESI-MS m/z calcd. for [C17H18N3O]+, 280.1449 [M − Cl]+; found,
280.1455. Anal. Calcd for C17H18F6N3OP: C, 48.01; H, 4.27; N, 9.88.
Found: C, 48.39; H, 4.09; N, 9.91.
Preparation of Imidazolium Salts HIMepyOH·Cl (2a), HI-
MespyOH·Cl (2b), HIMespyOH·PF6 (2b·PF6), and HIDipppyOH·
Cl (2c). 2a. 1-Methyl imidazole (1a) (0.82 g, 10 mmol) and 2-chloro-
3-hydroxy pyridine (1.55 g, 12 mmol) were taken in a Schlenk tube
and heated at 130 °C for 72 h to yield a brown residue. The residue
was dissolved in a minimum quantity of methanol (20 mL) and
precipitated out by adding diethyl ether (100 mL). This process was
repeated three times to get an off-white solid that was filtered out and
2c. The synthetic procedure of 2c was similar to that of 2b except
that 1-(2,6-diisopropylphenyl)-1H-imidazole (1c) (2.28 g, 10 mmol)
and 2-chloro-3-hydroxy pyridine (1.55 g, 12 mmol) were used. Yield
1
2.10 g (59%). Mp: 269−275 °C. H NMR (400 MHz, CDCl3, δ
1
dried under vacuum. Yield 1.2 g (57%). Mp: 218−225 °C. H NMR
ppm): 1.18 (m, 12H, CH(CH3)2), 2.33 (sep., J = 6.4 Hz, 2H,
CH(CH3)2), 7.32−7.39 (m, 4H, 2Ar-CH + Im-H + py-H), 7.59
(m,1H, Ar-H), 8.00 (d, J = 4 Hz, 1H, py-H), 8.41 (d, J = 8 Hz, 1H,
(500 MHz, DMSO-d6, δ ppm) 3.96 (s, 3H, CH3), 7.45 (dd, J = 8.5,
4.5 Hz, 1H, py), 7.73 (dd, J = 8.5, 1.5 Hz, 1H, py), 7.86 (s, 1H, Im),
F
DOI: 10.1021/acs.organomet.8b00878
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
py-H), 8.68 (s, 1H, Im-H), 9.77 (s, 1H, Im-NCHN). The O-H proton
was not observed in CDCl3 which may be due to the exchange of
proton with the solvent. However, the sample recorded in DMSO-d6
shows the presence of O-H proton (see Supporting Information,
Figure S7). 13C NMR (100 MHz, CDCl3, δ ppm): 24.2, 24.4, 28.9,
120.7, 123.6, 125.0, 126.8, 129.0, 129.8, 132.5, 133.1, 135.9, 138.7,
699.2952 [M + H]+; found, 699.2952. Anal. Calcd for C40H44-
N6NiO2: C, 68.68; H, 6.34; N, 12.01. Found: C, 68.37; H, 6.22; N,
11.89.
Synthesis of Palladium NHC Complexes Pd(IMepyO)2 (4a),
Pd(IMespyO)2 (4b), and Pd(IDipppyO)2 (4c). [Pd(IMepyO)2] 4a.
2a (0.212 g 1 mmol), K2CO3 (0.414 g 3 mmol), and PdCl2 (0.89 g
0.5 mmol) were taken in a Schlenk tube and kept under high vacuum
for 30 min before purging with nitrogen gas. Then 3 mL of dimethyl
sulfoxide (DMSO) was added and the contents were heated at 100 °C
for 15 h. A yellow colored suspension was obtained. The reaction was
stopped, and the solvent was removed under vacuum and the residue
left was isolated with DCM (3 × 20 mL) and filtered through
Whatman paper. Toluene (5 drops) was added to the collected
filtrate. Upon slow evaporation, yellow colored crystals of 4a were
145.2, 146.7. IR (KBr): ṽ 3416 (br), 3179 (w), 3054 (w), 2961 (s),
2868 (m), 2761 (w), 2623 (w), 1617 (s), 1578 (m), 1524 (s), 1478
(s), 1385 (w), 1325 (m), 1301 (m), 1270 (s), 1222 (m), 1178 (m),
1106 (w), 1056 (m), 895 (w), 824 (m), 809 (m), 774 (m), 760 (m),
667 (m), 558 (w), 532 (w) cm−1 ESI-MS m/z calcd. for
.
[C20H24N3O]+, 322.1919 [M − Cl]+; found, 322.1913. Anal. Calcd
for C20H24ClN3O: C, 67.12; H, 6.76; N, 11.74. Found: C, 66.87; H,
6.56; N, 11.88.
1
obtained. Yield 0.080 g, 25%. Mp: > 270 °C. H NMR (400 MHz,
Synthesis of Nickel NHC Complexes, Ni(IMepyO)2 (3a) and
Ni(IDipppyO)2 (3c). 3a. 2a (0.386 g, 1.82 mmol), Ni(PPh3)2Cl2
(0.595 g, 0.0.91 mmol), and K2CO3 (0.753 g, 5.46 mmol) were taken
in a dry Schlenk flask and subjected to vacuum for 30 min.
Acetonitrile (50 mL) was added, and the suspension was stirred at
room temperature for 20 h to obtain a yellow suspension. All the
volatiles were removed under vacuum, and the resultant yellow
colored residue was washed with pentane. The residue was then
dissolved in DCM and filtered. Block shaped crystals of the desired
complex were obtained upon slow evaporation. Yield (0.345 g, 93%).
Mp: > 270 °C. 1H NMR (CDCl3, 400 MHz; δ, ppm): 3.20 (s, 6H, N-
CH3), 6.95 (d, J = 2.4 Hz, 2H, Im-H), 7.07 (dd, J = 8.4, 4.8 Hz, 2H,
py-H), 7.55 (dd, J = 8.4, 1.2 Hz, 2H, py-H), 7.67 (dd, J = 4.8, 1.6 Hz,
2H, py-H), 7.96 (d, J = 2 Hz, 2H, Im-H).13C NMR (CDCl3, 100
MHz; δ, ppm): 37.0, 118.8, 123.7, 123.9, 129.1, 132.7, 140.1, 152.6,
DMSO-d6, δ ppm):δ 3.25 (s, 6H, N-CH3), 7.08 (m, 2H, py-H), 7.20
(d, J = 8.4 Hz, 2H, Im-H), 7.58 (m, 4H, 2py-H+2Im-H), 7.98 (s, 2H,
py-H).13C NMR (100 MHz, DMSO-d6, δ ppm):37.5, 119.3, 124.7,
125.3, 127.9, 132.4, 141.8, 154.7, 156.2. IR (KBr): ṽ 3121 (w), 3062
(w), 1566 (m), 1459 (s), 1428 (s), 1397 (m), 1378 (m), 1346 (m),
1300 (s), 1266 (m), 1241 (s), 1200 (m), 1117 (m), 1078 (m), 966
(m), 852 (w), 797 (w), 786 (m), 744 (m), 732 (m), 651 (w), 618
(w), 604 (m), 582 (w), 526 (w) cm−1. ESI-MS m/z calcd. for
[C18H17N6O2Pd]+, 455.0448 [M + H]+; found, 455.0459 Anal. Calcd
for C18H16N6O2Pd: C, 47.54; H, 3.55; N, 18.48. Found: C, 47.28; H,
3.32; N, 18.15.
[Pd(IMespyO)2] 4b. 2b·PF6 (0.425 g, 1 mmol), K2CO3 (0.414 g, 3
mmol), and PdCl2 (0.89 g 0.5 mmol) were taken in a Schlenk flask
and kept under vacuum for 30 min before purging with nitrogen gas.
Acetonitrile (25 mL) was added, and the contents were refluxed for
12 h. A bright yellow colored suspension was obtained. Upon cooling
to room temperature, the solution was filtered and a pale yellow
precipitate was obtained. The precipitate was washed thoroughly with
water and dried first in a low temperature oven and then under
vacuum. The residue was then extracted with DCM (3 × 40 mL) and
filtered. The yellow DCM filtrates were combined and solvent
removed to give a yellow powder. Suitable crystals were obtained by
slow evaporation of CHCl3 solution at room temperature. Yield
(0.050 g, 15%). 1H NMR (500 MHz, CDCl3; δ ppm): 2.33 (m, 18H,
Meso-CH3+p-CH3), 6.25 (d, J = 8 Hz, 2H, Im-H), 6.79 (s, 4H, Mesm-
CH), 7.00 (s, 4H, 2Im-H+2py-H), 7.57 (s, 2H, py-H), 8.18 (s, 2H,
py-H).13C NMR (125 MHz, CDCl3; δ ppm): 18.5, 21.2, 108.5, 116.9,
119.3, 122.9, 123.8, 128.4, 129.0, 131.3, 132.9, 134.9, 138.2, 153.3. IR
157.0. IR (KBr): ṽ 3170 (w), 3135 (w), 2951 (w), 1635 (w), 1573
(m), 1565 (m), 1463 (s), 1447 (s), 1433 (s), 1401 (m), 1375 (w),
1349 (m), 1310 (s), 1270 (m), 1238 (m), 1205 (w), 1120 (w), 1079
(w), 965 (w), 856 (w), 818 (w), 806 (w), 791 (w), 747 (m), 729
(m), 686 (w), 662 (w), 618 (w), 583 (w), 521 (w) cm−1. ESI-MS m/
z calcd. for [C18H17N6NiO2]+, 407.0766 [M + H]+; found, 407.0774.
Anal. Calcd for C18H16N6NiO2·CH2Cl2: C, 46.38; H, 3.69; N, 17.08.
Found: C, 46.12; H, 3.58; N, 16.81.
3c. 2c (0.089 g, 0.25 mmol), NiCl2·6H2O (0.030 g, 0.125 mmol),
and K2CO3 (0.104 g, 0.75 mmol) were taken in a Schlenk flask and
subjected to vacuum for 30 min. Acetonitrile (20 mL) was added, and
then the reaction mixture was refluxed for 12 h to obtain a yellow
colored suspension. All the volatiles were removed under vacuum,
yielding a pale yellow colored residue. The residue was thoroughly
washed with water to remove the leftover base and inorganic
byproducts. The residue was then washed with methanol (15 mL)
and filtered through an open frit ($). The leftover residue was
dissolved in DCM and filtered. Pink-red colored crystals of 3c were
obtained upon slow evaporation at room temperature. Yield of 3c
0.060 g (69%). Preparation of 3c′: The methanol filtrate obtained
from the above procedure ($) was evacuated to dryness to afford a
yellow colored residue which was dissolved in a mixture of diethyl
ether and pentane (1:2 ratio) and subsequently kept for
crystallization. A mixture of pink red (3c) and yellow colored crystals
of 3c′ were obtained. Even attempts to separate the yellow crystals by
repeated crystallization of the mixture were not successful as the pink
colored crystals (3c) always form along. 3c: Mp: > 270 °C. 1H NMR
(CDCl3, 400 MHz; δ, ppm): 1.13 (d, J = 6.8 Hz,12H, CH(CH3)2),
1.28 (d, J = 6.8 Hz, 12H, CH(CH3)2), 3.32 (sep, J = 6.8 Hz, 4H,
CH(CH3)2), 5.19 (dd, J = 8, 1.2 Hz, 2H, Im-H), 6.53 (dd, J = 8.4, 4.8
Hz, 2H, py-H), 6.69 (d, J = 2 Hz, 2H, Im-H), 7.18 (d, J = 8 Hz, 4H,
Dipp-H), 7.32 (m, 2H, py-H), 7.44 (dd, J = 4.8, 2 Hz, 2H, Dipp-H),
8.25 (d, J = 2 Hz, 2H, py-H). 13C NMR (CDCl3, 100 MHz; δ, ppm):
23.7, 24.5, 28.7, 115.5, 122.0, 123.8, 125.7, 128.2, 128.7, 132.0, 138.1,
(KBr): ṽ 3158 (w), 3073 (w), 2961 (m), 2923 (s), 2851 (m), 1740
(w), 1575 (w), 1466 (m), 1433 (m), 1376 (w), 1338 (m), 1316 (m),
1292 (m), 1261 (s), 109 (s), 1021 (s), 960 (w), 931 (w), 854 (w),
801 (s), 691 (w), 644 (w), 592 (w), 517 (w) cm−1. ESI-MS m/z
calcd. for [C34H33N6O2Pd]+, 663.1700 [M + H]+; found, 663.1703.
Anal. Calcd for C34H32N6O2Pd: C, 61.59; H, 4.86; N, 12.67. Found:
C, 61.28; H, 4.71; N, 12.38.
[Pd(IDipppyO)2] 4c. 2c (0.358 g, 1 mmol), K2CO3 (0.414 g, 3
mmol), KPF6 (0.372 g 2 mmol), and PdCl2, (0.89 g, 0.5 mmol) were
taken in a dry Schlenk flask and kept subjected to vacuum for 30 min
before purging with nitrogen gas. Acetonitrile (25 mL) was added and
the contents were refluxed for 12 h. A bright yellow suspension was
obtained. After filtration through a G-4 frit, a dull yellow precipitate
was obtained which was thoroughly washed with water and dried first
in a low temperature oven and then under vacuum. The precipitate
was then isolated with DCM (25 mL) and filtered to give a clear
yellow DCM filtrate which upon slow evaporation yielded bright
1
yellow crystals of 4c. Yield 0.140 g (38%). Mp: > 270 °C. H NMR
(400 MHz, CDCl3; δ, ppm): 1.10 (d, J = 6.8 Hz, 12H, CH(CH3)2),
1.24 (d, J = 6.8 Hz, 12H, CH(CH3)2), 2.93 (sep, J = 6.8 Hz, 4H,
CH(CH3)2), 5.50 (dd, J = 8.4, 1.6 Hz, 2H,Im-H), 6.69 (dd, J = 8.4, 4
Hz, 2H, py-H), 6.75 (d, J = 2 Hz, 2H, Im-H), 7.26 (d, J = 7.6 Hz, 4H,
Dippm-CH), 7.46 (t, J = 8 Hz, 2H, Dippp-CH), 7.52 (dd, J = 4, 1.6
Hz, 2H, py), 8.21 (d, J = 2 Hz, 2H, Py-H).13C NMR (100 MHz,
CDCl3,δ, ppm): 24.0, 24.4, 28.6, 116.5, 122.8, 123.9, 125.5, 128.9,
138.8, 144.3, 149.7, 160.3. IR (KBr): v 3183 (w), 3136 (w), 3105 (w),
̃
3068 (w), 2966 (s), 2928 (m), 2869 (m), 1692 (w), 1593 (w), 1580
(m), 1567 (m), 1468 (s), 1435 (s), 1400 (m), 1360 (m), 1336 (m),
1310 (s), 1281 (m), 1267 (m), 1222 (m), 1182 (m), 1119 (m), 1103
(m), 1072 (m), 1056 (m), 976 (w), 947 (m), 907 (w), 864 (w), 798
(s), 766 (m), 757 (m), 748 (s), 688 (m), 647 (w), 599 (w), 566 (w),
515 (w), 483 (w) cm−1. ESI-MS m/z calcd. for [C40H45N6NiO2]+,
129.3, 132.5, 137.3, 140.0, 145.4, 152.7, 164.9. IR (KBr): ṽ 3182 (w),
3102 (w), 3135 (w), 3064 (w),2928 (m), 2869 (m), 2965 (m),1688
G
DOI: 10.1021/acs.organomet.8b00878
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(w), 1637 (w), 1591 (w), 1575 (m), 1565 (m), 1463 (s), 1427 (s),
1404 (m), 1360 (m), 1336 (m), 1286 (s), 1262 (m), 1222 (m), 1199
(m), 1118 (m), 1103 (m), 1073 (m),1056 (m), 975 (w), 947 (m),
859 (w), 799 (s), 746 (s), 685 (m), 597 (w) cm−1. ESI-MS m/z calcd.
for [C40H45N6O2Pd]+, 747.2639 [M + H]+; found, 747.2644. Anal.
Calcd for C40H44N6O2Pd: C, 64.29; H, 5.94; N, 11.25. Found: C,
64.01; H, 5.73; N, 11.05.
Procedure for the Kumada−Tamao−Corriu (KTC) Coupling
Reaction. Catalyst, the solid substrates, and a magnetic stirring bar
were charged into a Schlenk flask. The contents were kept under high
vacuum for 30 min before purging with dry nitrogen gas. Then 4 mL
of the dry THF was syringed into the flask and the contents were
stirred for 10 min (the liquid substrates were injected at the time of
adding solvent). Then the required amounts of Grignard reagent (1.5
equiv) were added slowly over the period of 30 min with continuous
stirring. In the majority of the cases, addition of the Grignard reagent
changes the color of the reaction mixture from yellow to deep red.
The reaction was monitored through GC-MS by taking out 0.2 mL of
the reaction mixture after regular intervals of time. Before the GC-MS
measurements, the sample was filtered through a pad of silica gel and
eluted by 3 mL of ethyl acetate.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00878.
Crystallographic data of reported complexes, NMR and
ESI-MS spectra of all the new compounds, and hydrogen
bonding interactions in 3c′ (PDF)
Accession Codes
CCDC 1454595 and 1543389−1543394 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: garaman@iitk.ac.in. Phone: (+91)-0512-259-7517.
■
ORCID
Ganapathi Anantharaman: 0000-0001-7740-5581
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank SERB India for funding. I.A.B. and I.A.
thank their respective funding agencies (UGC and IITK) for
their doctoral fellowship. The contribution of our group
member Sabeeha Parveen in the preparation of this manuscript
is gratefully acknowledged. Dedicated to Professor V. K. Singh
on the occasion of his 60th birthday.
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