Applications of imino-pyridine Ni(II) complexes as catalysts in the transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.molstruc.2021.129987

Five imino-pyridine Ni(II) complexes: [{Ni(L1)Cl₂}₂] Ni1; [{Ni(L2)Cl₂}₂] Ni2; [{Ni(L3)Cl₂}₂] Ni3; [{Ni(L4)Cl₂}₂] Ni4 and [Ni(L5)₂Cl₂] Ni5 derived from ligands 2,6-diisopropyl-N-[(pyridin-2-yl) methylene] aniline (L1); 2,6-diisopropyl-N-[(pyridin-2-yl) ethylidene]aniline (L2); 2,6-dimethyl-N-[(pyridin-2-yl) methylene] aniline (L3); 2,6-dimethyl-N-[(pyridin-2-yl) ethylidene] aniline (L4) and N-[(pyridin-2-yl) methylene] aniline (L5) were evaluated as catalysts in the transfer hydrogenation of ketones. The Ni(II) complexes demonstrated moderate catalytic activities giving a turnover number (TON) of up to 126 at catalyst loading of 0.5 mol%. The structure of the complexes and nature of ketone substrate influenced the catalytic activities of the complexes. Deactivation studies using mercury and sub-stoichiometric poisoning experiments pointed to the presence of both Ni(0) nanoparticles and Ni(II) homogeneous as the active species.

Full text of DOI:10.1016/j.molstruc.2021.129987

Journal of Molecular Structure 1232 (2021) 129987
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Applications of imino-pyridine Ni(II) complexes as catalysts in the
transfer hydrogenation of ketones
Nokwanda Tsaulwayo, Robert.T. Kumah, Stephen.O. Ojwach∗
School of Chemistry and Physics, College of Agriculture, Engineering and Science, University of KwaZulu-Natal, Private Bag X01, Scottsville, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Five imino-pyridine Ni(II) complexes: [{Ni(L1)Cl2}2] Ni1; [{Ni(L2)Cl2}2] Ni2; [{Ni(L3)Cl2}2] Ni3;
[{Ni(L4)Cl2}2] Ni4 and [Ni(L5)2Cl2] Ni5 derived from ligands 2,6-diisopropyl-N-[(pyridin-2-yl) methy-
lene] aniline (L1); 2,6-diisopropyl-N-[(pyridin-2-yl) ethylidene]aniline (L2); 2,6-dimethyl-N-[(pyridin-2-
yl) methylene] aniline (L3); 2,6-dimethyl-N-[(pyridin-2-yl) ethylidene] aniline (L4) and N-[(pyridin-2-yl)
methylene] aniline (L5) were evaluated as catalysts in the transfer hydrogenation of ketones. The Ni(II)
complexes demonstrated moderate catalytic activities giving a turnover number (TON) of up to 126 at
catalyst loading of 0.5 mol%. The structure of the complexes and nature of ketone substrate influenced
the catalytic activities of the complexes. Deactivation studies using mercury and sub-stoichiometric poi-
soning experiments pointed to the presence of both Ni(0) nanoparticles and Ni(II) homogeneous as the
active species.
Received 1 October 2020
Revised 13 January 2021
Accepted 21 January 2021
Available online 25 January 2021
Keywords:
Nickel
Imino-pyridine
Transfer hydrogenation
Ketone
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
To date, few Ni(II)-based catalysts have been developed and
applied in the transfer hydrogenation of ketones. Notable among
Ni(II) catalysed reactions have occupied a pivotal position in
organic synthesis as supported by significant work performed on
the design of efficient Ni catalysts in the last two decades [1–4].
In many instances, Ni(II) catalysts have been applied as catalysts
in cross-coupling [5], hydrogenation [6], hydrosilylation [7] car-
bene transfer [8], dehalogenation [9] and olefin polymerisation
[10–12] reactions among others. Furthermore, Ni(II) catalysts have
demonstrated unique reactivity in facilitating tandem catalytic
transformations [13]. Transfer hydrogenation of ketones is an
important transformation, due to the production of industri-
ally valuable feedstocks, fine chemicals and biologically active
compounds [14,15]. These transfer hydrogenation reactions are
traditionally catalysed using the more expensive Ru(II) complexes
[16]. This has posed serious challenges in the industrial commer-
cialization and applications of these systems due to their high
level of toxicity and cost of establishment [17,18]. Thus significant
amount of research efforts have been directed towards the design
and development of relatively cheaper metals catalysts such as
Fe(II) [19], Mn(II) [20], Co(II) [21] and Ni(II) [22]. It is therefore
not surprising that Ni(II) complexes have emerged as promising
catalysts in these reactions, due to their ease of preparation and
relatively cheaper costs [12,22].
them include the nickel complex [(dcype)Ni(COD)] derived from
(1,2-bis(dicyclohexyl-phosphine)ethane and 1,5-cyclooctadiene)
reported by Castellanos et al. and found to have a broad scope of
substrates activity. In addition, these catalysts display moderate
catalytic activities of up to 99% yield within 98 h at catalyst
loadings of 2 mol% [23]. Recently, N-heterocyclic carbene Ni(II)
complexes displaying catalytic activities of up to 99% within 4 h in
the transfer hydrogenation of ketones have also been reported by
Bala and co-workers [24]. Although appreciable efforts have been
harnessed in the development of more efficient catalysts for trans-
fer hydrogenation of ketones, it is apparent that less attention has
been paid to the application of Ni(II) imino-pyridine complexes in
the recent past years. Inspired by our recent reports using similar
imine Ni(II) complexes in asymmetric transfer hydrogenation of
ketones, [22], we herein report the applications of readily available
and affordable dinuclear imino-pyridine Ni(II) complexes devel-
oped by Laine et al [25] as catalysts in the transfer hydrogenation
of ketones (Fig. 1). The steric and electronic effects of the complex
structure, substrate scope as well as deactivation profiles in the
transfer hydrogenation of ketones are hereby discussed.
2. Results and discussion
Complexes Ni1 – Ni5 (Fig. 1) were prepared following literature
procedures [22]. Detailed synthetic protocols of both the ligands,
the complexes and their respective spectral data are provided
∗
Corresponding author.
E-mail address: ojwach@ukzn.ac.za (Stephen.O. Ojwach).
https://doi.org/10.1016/j.molstruc.2021.129987
0022-2860/© 2021 Elsevier B.V. All rights reserved.

N. Tsaulwayo, Robert.T. Kumah and Stephen.O. Ojwach
Journal of Molecular Structure 1232 (2021) 129987
Table 1
Preliminary data of complexes Ni1-Ni5 in the TH of acetophenonea
Entry
Complex
Time (h)
bConversion [%]
cTOF/h−1
1
2
3
4
5
Ni1
Ni2
Ni3
Ni4
Ni5
32
32
32
32
32
66
72
75
76
82
2.06
2.25
2.34
2.38
2.56
a
Conditions: acetophenone, (0.23 mL, 2.00 mmol); catalyst, (0.02 mmol, 1.0 mol%); KOH, (5.00 mL of 0.40 M,
2.00 mmol) in 2-propanol); time = 32 h; temperature, 82°C.
b
Determined by 1H NMR spectroscopy.
c
TOF =Turn Over Frequency (mmol. of substrate consumed)/(mmol of catalyst
x time in h). Percentage
conversion is comparable to the yield of 1-phenylethanol.
significant conversions within the first 5 h, after which the kinetic
profiles slowed down (Fig. 2). For instance, complex Ni5 furnished
a conversion of 25% corresponding to TOF of 5.00 h−1 within 4
h, after the rate of the reaction declined. These kinetic profiles
may be due to the original homogeneous nature of the complexes
at the onset of the reactions and subsequent transformation of
homogeneous active species to Ni(0) nanoparticles [26]. From
Table 1 and Fig. 2, it was apparent that the ligand backbone of
the complexes conferred a significant impact on the performance
of the resultant catalysts. In general, complexes bearing less bulky
substituents displayed greater catalytic activities as compared to
those with bulkier substituents. For instance, complex Ni3, con-
taining the methyl substituent on the phenyl ring (L3), achieved
75% conversions, while complex Ni1, containing the isopropyl
substituents, gave conversions of 66% respectively (Table 1 entry
1and 4). This could be ascribed to the high steric effects exhibited
by the alkyl substituents around the coordination sphere of the
metal, which ultimately limits coordination of the substrate to
the metal centre [27]. Indeed, complex Ni5, supported on the un-
substituted ligand L5 was the most active displaying conversions
of 82% (Table 1, entry 5). From an electronic perspective, no dis-
cernable contribution from the ligand was observed. For instance,
the aldimine complex Ni3 and the ketamine analogue complex
Ni4 displayed comparable percentage conversions of 75% and 76%
respectively. While complexes Ni1- Ni5 compare poorly with some
of the most active systems as in trinuclear Ni(II) complexes, as in
Fig. 1. Structure of Ni(II) complexes Ni1-Ni5 used as catalysts in this study.
the case of reports of Bala and co-workers (TOF up to 2000 h−1
)
[24], they showed comparable catalytic activities to other systems
reported in literature [6,23,28–30].
Upon establishing that complexes Ni1-Ni5 show promising
catalytic activities in TH of acetophenone, we then investigated
the optimum reaction conditions. To achieve this, the effects of
catalyst loading, and the role of the nature of base were consid-
ered using the most active complex Ni5, was studied (Table 2).
From the data, we noted that increasing catalyst loading from 0.5
mol% to 1.0 mol%, increased the percentage conversions from 63%
to 82% respectively (Fig. S30). Although an increase in percentage
conversion was observed, the TON decreased drastically from 126
and 82 respectively. More significantly, a further increase of the
catalyst loading from 1.0 mol% to 1.5 mol%, was marked by a
drastic reduction in percentage conversions from 82% (TOF = 2.56
Fig. 2. Conversion vs time graph showing reaction profile of Ni-Ni5 in transfer hy-
drogenation of acetophenone.
as supplementary materials (Figs. S1 – S24). All the complexes
(Ni1 - Ni5) were preliminarily evaluated as catalysts using molar
ratio of 100/100/1 for ketone/KOH/catalyst and acetophenone as
the model substrate (Table 1). The percentage conversions of
the ketone substrate to their respective alcohol products were
monitored using 1H NMR spectroscopy (Figures. S29-S33). All the
complexes (Ni1-Ni5) showed moderate catalytic activities in the
transfer hydrogenation of acetophenone to 1-phenylethanol, giving
conversions between 66% - 82% (Table 1 and Fig. 2). In terms of ki-
netics of the reactions, catalysts Ni1-Ni5 complexes demonstrated
h
−1) to 40% (TOF = 0.84 h−1) respectively (Table 2, entries 4 vs
5). Considering the dinuclear nature of complexes Ni1-Ni5, it is
expected that the active intermediate is likely to be a mononuclear
species (dissociation of the dinuclear pre-catalysts) [20,29]. Thus in
this case, the lower catalytic activities reported with the increase
in catalyst loading, could be ascribed to a shift in equilibrium
2

N. Tsaulwayo, Robert.T. Kumah and Stephen.O. Ojwach
Journal of Molecular Structure 1232 (2021) 129987
Table 2
Optimization of reaction conditions for effective TH of acetophenone using catalysts Ni5.a
.
Entry
Base
Catalyst loading/mol%
bConversion [%]
cTON
dTOF/h−1
1
2
3
4
5
6
7
KOH
0.25
0.50
1.00
1.50
1
39
63
82
40
55
63
49
156
126
82
4.88
3.94
2.56
0.84
1.71
1.97
1.53
KOH
KOH
KOH
27
Li2CO3
K2CO3
Cs2CO3
55
1
63
1
49
a
Conditions: acetophenone, (0.23 mL, 2.00 mmol); KOH, (5.00 mL of 0.4 M, 72 mg, 2.00 mmol) in
2-propanol); temperature = 82 °C, time = 32 h.
b
Determined by 1H NMR spectroscopy.
c
TON = Turn over number (mmol. of substrate consumed/mmol. of catalyst) and
d
TOF = Turn over number (mol. of product/mol. of catalyst x time/h). Percentage conversion equals compa-
rable to the yield of 1-phenylethanol obtained.
towards the inactive dinuclear compounds, in good agreements
with the reports of Kalman and co-workers [31]. In addition, the
influence of the bases was investigated using K2CO3, Li2CO3, and
Cs2CO3 as given in Table 2, entries 5-7. From this study, KOH was
found to be the most active, while Cs2CO3 displayed the lowest
catalytic activity (Table 2, entries 3 and 7). This trend is consistent
with the order of stability and strengths of the bases and agrees
with the findings reported by van Putten et al. [32].
We then shifted our focus to probe the substrate scope using
substituted aromatic and aliphatic ketones under the optimised
conditions (Table 3). From the results in Table 3, the intro-
duction of electron-withdrawing substituents at the ortho- or
para-positions of the acetophenone derivatives resulted in higher
percentage conversions. For example, para-chloroacetophenone at-
tained percentage conversions of 97%, while acetophenone showed
percentage conversions of 82% under similar reaction conditions
(Table 3, entries 1 and 2). On the other hand, the introduction of
electron-donating substituents at either the ortho- or para-position
witnessed lower conversions, consistent with our previous reports
[17]. For instance, 4-methyl acetophenone achieved conversion of
55% compared to 82% attained for acetophenone (Table 3, entries
1 and 5). The position of either electron-donating or withdrawing
groups on the phenyl ring of acetophenone derivatives play a
significant role in controlling the conversions of the substrates.
For example, para-chloroacetophenone showed conversions of 97%
while the analogous ortho-chloroacetophenone gave 90%. This
observation could be attributed to the inductive effect which
ultimately decreases electron density on the carbonyl carbon
resulting in greater activity [22]. However, an opposite trend was
observed in the case of electron-donating groups. For example,
higher conversions of 59% was recorded for 2-methyl acetophe-
none, compared to conversions of 50% obtained for 4-methyl
acetophenone substrate. This observation agreed with the earlier
findings of Wang and co-workers, where 4-methylacetophenone
gave conversions of 64%, compared to conversions of 67% recorded
for 2- methyl acetophenone [29].
Fig. 3. The plot of conversion vs time in the showing TH of acetophenone using
complex Ni5 in the presence of Hg(0) and phosphine poisoning agents.
(Table 3, entries 7 - 9) displayed lower conversions, when com-
pared to the acetophenone substrate. This trend is common and
has been reported by for example Azouzi et al. [33] and Huo et al.
using Fe(II) chiral catalysts [34].
In order to gain insight into the nature of the active species,
we attempted to investigate the homogeneity or formation of
active heterogeneous Ni(0) nanoparticles using Hg(0) and sub-
stoichiometric poisoning tests [34–35]. For mercury poisoning
tests, 3 or 5 drops of Hg(0) was added at the onset of the reac-
tion to a solution of the catalyst, Ni5 and KOH in iPrOH and re-
fluxed at 82 °C in the presence of acetophenone substrate. From
Fig. 3, a significant reduction in the percentage conversions of cat-
alyst Ni5 was observed, thus implicating the presence of active
Ni(0) nanoparticles in the reaction mixture [35]. It is also impor-
tant to note that the addition of 5 drops of Hg(0) resulted in a
further drop in percentage conversion (Fig. 3) consistent with pre-
vious findings [26,35].
Significantly, the complexes also catalysed the transfer hy-
drogenation of heteroaryl ketones substrates, for instance, 2-
acetylpyridine, giving conversions of 66% (Table 3, entry 6). The
use of heterocyclic ketones as in 2-acetylpridine, expectedly
[17] resulted in lower percentage conversion of 66% (Table 2,
entry 6). Similarly, fused aromatic and aliphatic ketone substrates
To confirm the results from the mercury poisoning experiments,
we further conducted sub-stoichiometric poisoning tests by adding
20% and 100% mol. equivalent of PPh3 (with respect to the amount
of catalyst) to the catalyst solution. Indeed, From Fig. 3, we ob-
served a decrease in the percentage conversions in both cases. For
example, using 20% PPh3, conversions of 62% compared to 82%
3

N. Tsaulwayo, Robert.T. Kumah and Stephen.O. Ojwach
Journal of Molecular Structure 1232 (2021) 129987
Table 3
the presence of both homogeneous Ni(II) and Ni(0)-nanoparticle as
the active species [35].
Transfer hydrogenation data of ketone substrate scope study using complex Ni5a
3. Conclusions
In conclusion, a series of imino-pyridine Ni(II) dinuclear com-
plexes were successfully evaluated as catalysts in the transfer
hydrogenation of ketones. The complexes demonstrated moderate
catalytic activities, controlled by the nature of the coordinated lig-
and and reaction conditions. The complexes catalyzed a wide range
of ketone substrates including aromatic, aliphatic and hetero-aryl
ketones. Deactivation studies pointed to the presence of both
homogeneous Ni(II) and Ni(0) nanoparticles as the active species.
Thus, in this contribution, we report a relatively facile, easily
accessible and cheaper Ni(II) catalyst in the transfer hydrogenation
reactions which could form a viable platform to the design of
affordable catalysts for the transfer hydrogenation reactions.
.
Entry
1
Substrate
bConversion[%]
82
dTOF/h−1
2.56
2
3
97
90
3.09
4. Experimental section
4.1. Materials and instrumentation
2.81
The synthetic protocols, unless otherwise mentioned, were per-
formed under N2 gas standard Schlenk or vacuum line techniques.
All other chemical reagents including 2-pyridinecarboxaldehyde
(99%), 2-acetylpyridine (≥99%), 2,6-diisopropylaniline (97%),
2,6-dimethylaniline (99%), NiCl2 (98%), 2-propanol (≥99.5%), para-
toluene sulfonic acid monohydrate (p-TsOH) (≥99.9%), acetophe-
none (≥99.99%), potassium hydroxide, potassium tert-butoxide,
Cs2CO3, 1-acetonaphthone (≥ 99.99%), 2-chloroacetophenone
(≥ 99.99%), 2-methylacephenone (≥ 99.99%), 4-methyl-2-
cycloactephenone (≥ 99.99%),2-hydroxy acetophenone(≥ 99.99%),
4-chloroacetophenone (≥ 99.99%), and 4-methylacetophenone (≥
99.99%), triphenylphosphine (PPh3) (≥ 99.99%), mercury (Hg(0)),
were all purchased from Sigma-Aldrich. Solvents including deuter-
ated NMR solvents were stored in a desiccator. 1H NMR and
13C{1H} NMR (100 MHz) spectra were recorded on a 400 MHz
Bruker Ultra shield NMR spectrometer in CDCl3 solvent. The
infrared spectra were recorded on a Perkin Elmer, Spectrometer
100. LC Premier micro-mass Spectrometer model LC-MS-2002 was
used for mass spectral analyses. Micro-analyses were performed
on a Thermal Scientific Flash 2000. The magnetic moments were
determined using Evans balance (Sherwood MK-1).
4
5
59
55
1.84
1.72
6
66
60
2.06
1.88
7
8
4.2. General procedure for transfer hydrogenation of ketones
The catalytic transfer hydrogenation of ketones was performed
in two-necked round bottom flasks connected to a reflux con-
61
52
1.90
1.62
denser. In
a typical experiment, acetophenone (0.23 mL, 2.00
mmol), a solution of 0.4 M of 5.00 mL of KOH in iPrOH and the
respective Ni(II) complex (0.02 mmol, 1.00 mol %) was refluxed at
82 °C. During the reaction, about 0.1 mL of the reaction mixture
was withdrawn at regular time intervals and analyzed for percent-
age conversion and yield using 1H NMR spectroscopy. The relative
signals of the methyl group of acetophenone (2.50-2.60ppm) and
methyl signal of 1-phenylethanol (1.49 ppm) of the products were
used to calculate the respective percentage conversions.
9
a
Conditions: acetophenone, (0.23 mL, 2.00 mmol); catalyst, Ni5 (6.12 mg, 0.02
mmol, 1 mol%); KOH, (5.00 mL of 0.4 M, 2.00 mmol) in 2-propanol); temperature,
82°C, time 32 h.
4.2.1. Mercury poisoning experiment
b
Determined using 1H NMR spectroscopy.
The mercury poisoning experiment according to the general
procedure described in section 3.2. In this experiment, a solution
of acetophenone (0.23 mL, 2.0 mmol), complex Ni5 (12 mg, 0.02
mmol, 1.00% mmol), and a solution of 0.4 M of 5.00 mL of KOH in
iPrOH and 3 and 5drops of Hg(0) drops were introduced and re-
fluxed for 32 h. During the reaction period, about 0.1 mL of the re-
acting mixture was sampled at regular time intervals, cooled, and
percentage conversion determined using 1H NMR spectroscopy.
d
TOF = (mmol. of substrate consumed)/(mmol. of catalyst x time/h).
in the original reaction was reported. At 100% PPh3 equivalent, a
pure heterogeneous catalyst is expected to undergo complete de-
activation [36,37]. However, in our case, about 40% conversion was
recorded within 32 h (Fig. 3). The incomplete deactivations ob-
served for Hg(0) and PPh3 poisoning experiments, thus point to
4

N. Tsaulwayo, Robert.T. Kumah and Stephen.O. Ojwach
Journal of Molecular Structure 1232 (2021) 129987
4.2.2. Sub-stoichiometric poisoning experiments
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Declaration of Competing Interest
[21] P.E. Sues, K.Z. Demmans, R.H. Morris, Rational development of iron catalysts for
asymmetric transfer hydrogenation, Dalton Trans. 43 (2014) 7650–7667, doi:10.
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The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
CRediT authorship contribution statement
[18] S. Abubakar, M.D. Bala, Transfer hydrogenation of ketones catalyzed by sym-
metric imino-N-heterocyclic carbene Co (III) complexes, ACS Omega 5 (2020)
2670–2679, doi:10.1021/acsomega.9b03181.
Nokwanda Tsaulwayo: Data curation, Investigation, Methodol-
ogy. Robert.T. Kumah: Data curation, Investigation, Software, Vali-
dation, Writing - original draft. Stephen.O. Ojwach: Conceptualiza-
tion, Supervision, Resources, Funding acquisition, Writing - review
& editing.
[23] R.T. Kumah, N. Tsaulwayo, B.A. Xulu, S.O. Ojwach, Structural, kinetics and
mechanistic studies of transfer hydrogenation of ketones catalyzed by chiral
(pyridyl) imine nickel (II) complexes, Dalton Trans. 48 (2019) 13630–13640,
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[22] N. Castellanos-Blanco, A. Arévalo, J.J. García, Nickel-catalyzed transfer hydro-
genation of ketones using ethanol as a solvent and a hydrogen donor, Dalton
Trans. 45 (2016) 13604–13614, doi:10.1039/c6dt02725c.
Acknowledgments
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by a trinuclear Ni (II) complex of a Schiff base functionalized N-heterocyclic
carbene ligand, Inorg. Chim. Acta 484 (2019) 276–282, doi:10.1016/j.ica.2018.
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The authors are grateful to the University of KwaZulu-Natal and
National Research Foundation, South Africa, for financial support.
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mononuclear and dinuclear diimine complexes of late transition metals, Eur. J.
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catalysis? Identification of bulk ruthenium metal as the true catalyst in ben-
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Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.129987.
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