Unprecedented synthesis of symmetrical azines from alcohols and hydrazine hydrate using nickel based NNN-pincer catalyst: An experimental and computational study

Article abstract of DOI:10.1016/j.catcom.2019.02.024

Azines are having widespread applications in both industry as well as synthetic chemistry. Thus new catalytic synthetic protocols are desirable as they are greener alternatives than traditional methods of synthesis. Thus, herein a novel earth abundant nickel based NNN-pincer catalyst Ni(BPEA)(Cl₂) is synthesized for the first time for the direct transformation of alcohols and hydrazine hydrate into symmetrical azines. This catalytic reaction is accompanied by dehydrogenative coupling of alcohols and hydrazine hydrate and is carried out in presence of a base. Theoretical calculations supported by experimental evidence have been performed for understanding the mechanistic insights of the reaction.

Full text of DOI:10.1016/j.catcom.2019.02.024

Accepted Manuscript
Unprecedented synthesis of symmetrical azines from alcohols and
hydrazine hydrate using nickel based NNN-pincer catalyst: An
experimental and computational study
Bilal Ahmad Shiekh, Damanjit Kaur, Sachin Kumar Godara
PII:
S1566-7367(19)30058-5
DOI:
https://doi.org/10.1016/j.catcom.2019.02.024
CATCOM 5635
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
20 December 2018
31 January 2019
22 February 2019
Please cite this article as: B.A. Shiekh, D. Kaur and S.K. Godara, Unprecedented synthesis
of symmetrical azines from alcohols and hydrazine hydrate using nickel based NNN-
pincer catalyst: An experimental and computational study, Catalysis Communications,
https://doi.org/10.1016/j.catcom.2019.02.024
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ACCEPTED MANUSCRIPT
Unprecedented Synthesis of Symmetrical Azines from Alcohols and
Hydrazine Hydrate using Nickel Based NNN-Pincer Catalyst: An
Experimental and Computational study.
*
Bilal Ahmad Shiekh , Damanjit Kaur, Sachin Kumar Godara
Department of Chemistry, UGC Sponsored Centre of Advanced Studies-I, Guru Nanak Dev
University, Amritsar India-143005.
*
E-mail: bilal.ahmad459@gmail.com.
Abstract
Azines are having widespread applications in both industry as well as synthetic chemistry. Thus
new catalytic synthetic protocols are desirable as they are greener alternatives than traditional
methods of synthesis. Thus, herein a novel earth abundant nickel based NNN-pincer catalyst
Ni(BPEA)(Cl2) is synthesized for the first time for the direct transformation of alcohols and
hydrazine hydrate into symmetrical azines. This catalytic reaction is accompanied by
dehydrogenative coupling of alcohols and hydrazine hydrate and is carried out in presence of a
base. Theoretical calculations supported by experimental evidence have been performed for
understanding the mechanistic insights of the reaction.
Keywords: Symmetrical azines; Ni-pincer catalyst; Alcohol dehydrogenation; Hydrazine
hydrate; Computational study.

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1
. Introduction
Azines constitute a stereochemically important class of compounds with wide range of
applications in nonlinear optical materials [1], liquid crystals as twisted-nematic displays [2] and
in synthetic chemistry [3]. Because of their modulating behavior towards bio-receptors they
serve as important scaffolds for drug designing and pharmocolgy [4]. They are also important
candidates showing anti-bacterial, anti-malarial, anti-viral, anti-tumor and anti-inflammatory
properties [5a]. Furthermore, azine-based receptors are used as fluorescent imaging reagents in
2
+
living cells which are affected with Hg [5b]. The aromatic substituted azines hold a great
promise in dye industries as coloring agents [6]. Because of conjugation stopping ability, these
compounds can be used as conjugation switches in tunable optical and mechano-optical
molecular devices. Recently, azines have been used in synthesis of covalent organic frameworks
(
COF) which possess high sensitivity in chemosensing [7] and are used in light induced
hydrogen gas generation [8].
There are various conventional approaches of symmetrical azine synthesis including
direct treatment of hydrazine with carbonyl compounds [9]. In recent years, transition metal
catalysts have gained attention for symmetrical azine synthesis as they are greener bond
formation alternatives compared to conventional methods. Amongst these, the first transition
metal catalyst was reported by Michelin in 2002 which involves decomposition of 9-
diazofluorene in the presence of Pt(0) complex [Pt(C2H4)(PPh3)2] [10]. Recently in 2016
Milstein reported a Ru-based pincer catalyst which catalyzes the acceptorless dehydrogenative
coupling of alcohols with hydrazine hydrate [11]. More recently in 2018 while our work was

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under process, Goswami reported a Ni-pincer catalyst for the direct transformation of alcohols
into azines [12]. First two metal-catalyzed methods of symmetrical azine synthesis involve rare
and precious transition metals. Therefore, it’s much desirable to synthesize cheap and earth
abundant transition metal complexes for such important chemical transformations.
So in context of prolonged sustainability and cost efficiency we decided to explore the
dehydrogenative coupling of alcohols with hydrazine hydrate using a more earth abundant and
less costly transition metal complexes. Thus, herein we report Ni-based NNN-pincer catalyst
Ni(BPEA)(Cl2) for one-pot transformation of alcohols into azines which as per our knowledge
has not been reported before (Scheme 1). It’s worthy to mention here that the direct usage of
hydrazine hydrate is challenging [11]. The above synthesized pincer complex is stable in both
solid and in dissolved state. The mechanistic details of the reaction were investigated by carrying
DFT studies without searching for alternate pathways.
2
. Experimental
.1 Catalyst Synthesis
2
The synthesis of the catalyst was carried out in three steps, first step involves the synthesis of
N,N-bis(2-{pyrid-2-ylethyl})hydroxylamine (BPEHA) from vinylpyridine and hyrdoxylamine
hydrochloride, the second step involves synthesis
of N,N-bis(2-{pyrid-2-yl}ethyl)amine
(BPEA) by reduction of BPEHA and in the final step catalyst is synthesised from BPEA using
anhydrous NiCl2. See the supplementary data for detailed information.
Synthesis of N,N-bis(2-{pyrid-2-ylethyl})hydroxylamine: A mixture of 2-vinylpyridine (6.571
g, 62.5 mmol) and hydroxylamine hydrochloride (2.1715 g, 31.25 mmol) was kept for 48 hours
in solvent free environment [13,14]. The resulting mixture was basified with NaHCO3 solution.
The water layer was washed several times with CHCl3 (5x25 ml) and the organic extracts

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collected were dried over sodium sulfate. The mixture was filtered, concentrated and then mixed
with 100 ml hexane. The white coloured product separated out and was purified by
1
recrystallization using chloroform and hexane. The product obtained was characterized by H
1
3
1
NMR and C{ H}NMR spectroscopy. Yield 3.67 g, 42%.
Synthesis of N,N-bis(2-{pyrid-2-yl}ethyl)amine: BPEHA (2.0 g, 8.22 mmol) was reduced
using Zn dust and dilute HCl in absolute ethyl alcohol with constant stirring at 313 K
temperature for 10 hours [13]. Then the mixture was neutralised with dilute NaOH solution
followed by washing with chloroform. After separating from water, the organic layer was dried
in sodium sulfate and then filtered. The solvent was removed by rotary evaporator under reduced
1
13
1
pressure and a pure paye yellow oil collected was characterised by H and C{ H}NMR
spectroscopy. Yield 1.83 g, 92%.
Synthesis of bis(2-{pyrid-2-yl}ethyl)amine, Ni(BPEA)(Cl2): In 100 ml schlenk flask, BPEA
(
1.0 g, 4.4 mmol), anhdrous NiCl (0.57 g, 4.4 mmol) and 75 ml dry THF were mixed under
2
nitrogen atmosphere. The reaction mixture was heated at 313 K temperature with contineous
stirring for 24 hrs. All volatiles were removed leaving behind a greenish coloured complex at the
bottom of the flask which was washed with 50 ml of hexane. The solution was then dried under
vacuum to obtain the desired catalyst. Yield 1.41 g, 89.9%.
2
.2 X-ray Diffraction Analysis: The crystal suitable for x-ray diffraction analysis was obtained
in saturated solution of methanol by slow evaporation of the solvent at room temperature. The
solid state representation of the structure thus obtained by x-ray diffraction technique to confirm
the mode of coordination is portrayed in Figure 1. The unit cell contains four molecules with
with monoclinic space group (P 21/c). The geometry of the complex resembles trigonal
bipyramid in shape with two pyridine rings above and below metal-ligand plane. The Cl which

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lies above the Ni-ligand plane have a larger bond distance by 0.044 Å compared to the one which
lies below plane. The angle between Cl19─Ni1─Cl20 is 124.6ᵒ. Furthermore, the N2 forms a
stronger bond compared to N6 and N14 of two pyrimidine rings as depicted by their bond
distances in the Figure. The x-ray diffraction analysis confirms the structure proposed by
spectroscopic data.
3
. Results and Discussions
3
.1 Exploration of Catalytic Reaction: While exploring the reaction, the pincer
precatalyst was found to be inactive for azine synthesis on its own under standard conditions
which is due to the saturated coordination at the metal center thus making substrate binding
difficult. In order to get an active catalyst, we added NaBH4 and t-BuOK to the solution of
precatalyst before adding to the reaction mixture containing excess alcohol and hydrazine
hydrate which resulted in the formation of azine with the generation of H2 and water.
Encouraged with this attempt, we started searching for standard optimized reaction conditions
and it was revealed that the reaction afforded best conversion (almost 75% yield, Table 1) in
THF with 3 mol% catalyst loading, 5 mol% NaBH4 (with respect to catalyst), 5 mol% t-BuOK
(with respect to catalyst) and 80℃ temperature for 24 hour time. However, when the catalyst
loading was minimized to 1 mol% the conversion of the reactants reduced significantly to 25%.
The reaction conditions needed for this catalyst are entirely different than the conditions
proposed by other research groups [11,12] which suggest the use of molecular sieves. The scope
of this catalytic reaction was analyzed by using various substrates ranging from aliphatic to
aromatic ones in order to understand the steric effects of the substrates used (Table 1). The
treatment of 1-propanol and 2-propanol with hydrazine hydrate for 24 hours yielded the
corresponding azines in almost 60% (entry 1, 2). Similarly, aromatic alcohols like benzyl alcohol

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and 2-hydroxybenzyl alcohol afforded the aromatic azines in 68 and 73% yield, respectively
(entry 3, 6) regardless of any increase in catalyst loadings or reaction timings. Likewise, α-
Methylbenzyl alcohol showed similar efficiency and yielded azine in 72% (entry 4). 4-
Methoxybenzyl alcohol showed the good results and gave azine with 74% yield (entry 5).
3
.2 Optimization of Reaction Conditions: To optimize the reaction conditions of azine
synthesis by Ni-pincer catalyst common reaction conditions such as temperature, solvent, effect
t
of changing percentage of NaBH4, K OBu and PNI etc. were monitored (Supplementary data).
The reaction between benzyl alcohol and hydrazine hydrate was used as model reaction. In the
first few attempts the effect of temperature and solvent was checked (entries 1-8) and on the
basis of solvent screening results, THF gave best results (entry 3) as compared to rest of the
solvents at 80º C temperature. The other solvents used were dioxane, DMF, DMSO, acetonitrile
and toluene but all of them showed no significant results. The solubility of the catalyst was very
low in toluene and hence gave poor results and hence we choose THF as best solvent. Changing
the bases under the identical conditions didn’t gave the good results either (entry 7-11). Amongst
the bases used NEt3 and Cs2O3 showed no activity at all. Furthermore, decreasing the
temperature decreased the yield of the product dramatically (entries 9-13). Next the effect of
percentage of base, NaBH4 and catalyst loading were tested (entries 14-23) and it was delightful
to see that a remarkable improvement in yield was observed by varying the percentage of base
and NaBH4 and best results were obtained for entry 22 in which the product yield was obtained
in ~65% yield after 24 hours time.
3
.3 Cyclic Voltammetry Studies: In order to study the mechanistic insights of the catalytic
reaction of nickel complex, cyclic voltammetry of the reaction mixture (2 ml) was carried out in
-
1
0
.1 M tetra-n-butyl ammonium hexafluorophosphate in THF with scan rate 100-300 mV s . The

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sample studied showed a well-defined voltammogram due to the electron transfer either to metal
or free ligand and is shown in the Figure 2. The voltammogram thus obtained shows two
reduction potentials -1.58 V and -2.32 V, both of which are irreversible. Despite being
irreversible in nature these potentials correspond to Ni(II) and Ni(I). The lower reduction
potential peaks designate the ease with which an electron is added to the metal complex. In
addition to these reduction potentials, there is an additional reversible reduction potential peak at
-
3.4 V which is probably due to the free ligand in the reaction mixture.
.4 Computational Mechanistic Study
3
For understanding the mechanistic insights and characteristic features of this catalytic reaction,
the Density Functional Theory (DFT) calculations were performed at wB97xD [15] level of
theory (See the supplementary data for more details). As illustrated in Figure 3, the entire
catalytic transformation occurs in two steps, the one in which dehydrogenation of alcohol takes
place resulting in the formation of carbonyl compound and the second step involving reaction
between the carbonyl compound and the hydrazine hydrate to form azine. The mechanistic
insights of this two-step chemical transformation studied by DFT studies revealed that reaction
proceeds through the formation of an encounter complex, RALCT between the active catalyst,
PNi and alcohol, RACL. The formation of this encounter complex between the alcohol and the
-
1
catalyst is endergonic in nature with free energy equal to 2.05 kcal mol as portrayed in Figure 3.
In the next step, the dehydrogenation of alcohol takes place in a concerted fashion by
simultaneous transfer hydrogenation to the metal center and the N of ligand through metal-ligand
cooperation. In the next step the liberation of carbonyl compound takes place through a turnover
ꢁ
-1
limiting transition state, TS1 with ꢀ ꢂꢃꢄꢅꢆꢇ kcal mol .The formation of TS1 is the rate-
determining step and the thermodynamic driving force for catalytic chemical transformation. The

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liberation of carbonyl also forms a Ni(I) oxidized state complex, HPNi. In the last and the final
step, the carbonyl compound reacts with the hydrazine hydrate to form the desired azine. The
formation of Ni(I)-HPNi has been confirmed by carrying out cyclic voltammetry of the reaction
mixture in THF as discussed in the experimental section . The voltammogram shows two
irreversible reduction potentials at -1.58 and -2.32 V which are ascribed to for Ni(II) and Ni(I),
respectively. The formation of azine through dehydrogenation of alcohols was also previously
suggested by other research groups [11,12].
4
. Conclusions
In summary, an earth abundant, cheap and significantly valuable pincer catalyst for the first time
has been reported for the convenient one-pot azine synthesis. Not only it adds to the limited
stock of transition metal catalysts of azine synthesis, but it also provides an efficient strategy to
synthesize the symmetrical azines. Notably, the products obtained were in good yield (75 %)
with moderate usage of base. From the DFT perspective, the reaction proceeds via
dehydrogenation of alcohols supported by the cyclic voltammetry which shows two irreversible
reduction potentials at -1.58 V for Ni(II) and at -2.32 V for Ni(I). The reaction between the
carbonyl compound formed and the hydrazine hydrate forms azines and the overall activation
-
1
energy of the reaction is 27.59 kcal mol . Further work is under process to find out the further
scope of the catalyst synthesized.
Acknowledgments
This work was financially supported by UGC, Govt. of India under UGC-BSR scheme as SRF
(Senior Research Fellow) vide notification number, NO.F.25-1/2013-14(BSR)/5-27/2007(BSR).
Supplementary data

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Supplementary material
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Graphical abstract
Scheme 1. Recent Examples and our Newly Designed Ni-Pincer Catalyst for Symmetrical Azine
Synthesis.
Figure 1. Cyclic voltammogram of reaction mixture in THF solution confirming the presence of
-
1
Ni(II) and Ni(I) oxidation states of Ni-pincer complex in the solution (scan rate 300 mV S ).
Figure 2. X-ray structure of the complex Ni(BPEA)(Cl2) with all H-atoms omitted for clarity
(thermal ellipsoids are drawn with 50% probability). Ni1─Cl20=2.30 Å, Ni1─Cl19=2.34 Å,
Ni1─N14=2.17 Å, Ni1─N2=2.10 Å, Cl19─Ni1─Cl20=124.6º, N2─Ni─Cl19=121.8º,
N14─Ni1─N6=176.6º, N2─Ni1─N14=90.9º, N2─Ni1─Cl19=113.5º.
Figure 3. Proposed reaction mechanism for symmetrical azine synthesis from alcohols and
-
1
hydrazine hydrate using Ni-pincer complex. All the given values are reported in kcal mol .

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Table 1. One-pot synthesis of symmetrical azines from alcohols and hydrazine hydrate using
a
nickel catalyst.
a
Optimized Reaction Conditions: Alcohols (3 mmol), hydrazine hydrate (1.6 mmol), PNi catalyst (3 mol %), NaBH4
(
5 mol %) with respect to catalyst, t-BuOK (5 mol %) with respect to the catalyst in THF (5 ml). The reaction was
b
heated under reflux in oil bath at 80℃ for 24 hours time. The yields under parenthesis are GC-MS yields while as
the values outside parenthesis are isolated yields.

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Highlights:





Highly effective protocol is unveiled for the synthesis of symmetrical azines.
The protocol uses an Nickel based NNN-pincer catalyst.
t
Catalytic reaction shows best results in THF in presence of base, K OBu.
The catalytic reaction proceeds through dehydrogenation of alcohols.
Mechanism involves formation of both Ni(I) and Ni(II) complexes during the reaction.

Figure 1

Figure 2

Figure 3