Orientation towards asymmetric transfer hydrogenation of ketones catalyzed by (pyrazolyl)ethyl)pyridine Fe(II) and Ni(II) complexes

Article abstract of DOI:10.1016/j.molstruc.2017.01.068

Compounds 2-[1-(3,5-dimethylpyrazol-1-yl)ethyl]pyridine (L1) and 2-[1-(3,5-diphenylpyrazol-1-yl)ethyl]pyridine (L2) were obtained in a three-step procedure which involved the reduction of acetylpyridine using NaBH₄, chlorination of the alcohol intermediate using SOCl₂ and subsequent reaction with appropriate pyrazoles. Reactions of L1 and L2 with Ni(II) and Fe(II) halides produced the respective complexes Ni(L1)Br₂ (1), Ni(L1)Cl₂ (2), Fe(L1)Cl₂ (3) and Ni(L2)Br₂ (4) as racemic mixtures in moderate yields. The molecular structures of complexes 1 and 4 are dinuclear and mononuclear respectively. All the complexes (1–4) formed active catalysts for the transfer hydrogenation of ketones (THK) in 2-propanol at 82?°C affording conversions of 58%–84% within 48?h. The influence of catalyst structure, reaction conditions and identity of ketone substrates in the TH reactions have been successfully established.

Full text of DOI:10.1016/j.molstruc.2017.01.068

Journal of Molecular Structure 1135 (2017) 197e201
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Orientation towards asymmetric transfer hydrogenation of ketones
catalyzed by (pyrazolyl)ethyl)pyridine Fe(II) and Ni(II) complexes
,
*
Makhosazane N. Magubane a, Mohd Gulfam Alam a, Stephen O. Ojwach a
,
Orde Q. Munro b
a School of Chemistry and Physics, University of KwaZulu-Natal, Pietermaritzburg Campus, Private Bag X01 Scottsville, 3209, South Africa
b School of Chemistry, University of the Witwatersrand, PO WITS, 2050, Johannesburg, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Compounds 2-[1-(3,5-dimethylpyrazol-1-yl)ethyl]pyridine (L1) and 2-[1-(3,5-diphenylpyrazol-1-yl)
ethyl]pyridine (L2) were obtained in a three-step procedure which involved the reduction of ace-
tylpyridine using NaBH4, chlorination of the alcohol intermediate using SOCl2 and subsequent reaction
with appropriate pyrazoles. Reactions of L1 and L2 with Ni(II) and Fe(II) halides produced the respective
complexes Ni(L1)Br2 (1), Ni(L1)Cl2 (2), Fe(L1)Cl2 (3) and Ni(L2)Br2 (4) as racemic mixtures in moderate
yields. The molecular structures of complexes 1 and 4 are dinuclear and mononuclear respectively. All
the complexes (1e4) formed active catalysts for the transfer hydrogenation of ketones (THK) in 2-
propanol at 82 ꢀC affording conversions of 58%e84% within 48 h. The influence of catalyst structure,
reaction conditions and identity of ketone substrates in the TH reactions have been successfully
established.
Received 7 December 2016
Received in revised form
21 January 2017
Accepted 22 January 2017
Available online 26 January 2017
Keywords:
Nickel
Iron
Pyrazolyl
© 2017 Elsevier B.V. All rights reserved.
Transfer hydrogenation
Ketones
1. Introduction
Other reports on the challenges of stereoselective reduction of
acetylpyridine [8,9] have also been published. Herein, we thus
Asymmetric transfer hydrogenation of ketones (ATHK) is a well-
established method used to achieve enantiomerically enriched
secondary alcohols [1,2]. Optically active alcohol products are
useful intermediates in the manufacture of fine chemicals espe-
cially pharmaceuticals and pesticides [3,4]. Noyori and co-workers
report the syntheses of racemic mixtures of these pyrazolyl com-
pounds, their respective Ni(II) and Fe(II) complexes and applica-
tions as catalysts in the transfer hydrogenation of various ketones.
2. Experimental
[1] first reported the Ru(II) complex [RuCl(h6-arene)(N-arylsul-
fonyl-DPEN)] as a catalyst in the transfer hydrogenation of ketones
[5] to afford optically active products with high stereoselectivity.
Since this discovery, there has been an extraordinary chiral catalyst
development and applications in the asymmetric transfer hydro-
genation reactions of ketones [6].
Currently, we have been working on the design and develop-
ment of ruthenium, nickel and iron metal complexes as catalysts in
the transfer hydrogenation of ketones [7]. In this current contri-
bution, we originally aimed to design chiral (pyrazolyl)pyridine li-
gands and their Ni(II) and F(II) complexes as possible stereoselective
catalysts for asymmetric transfer hydrogenation of ketones. How-
ever, out attempts to isolate the chiral ligands using Corey-Bakshi-
Shibata (CBS) as the reducing agent have been so far unsuccessful.
2.1. Materials and methods
The chemicals NiCl2, NiBr2, FeCl2$4H2O, 2-acetylpyridine, SOCl2,
NaBH4 were obtained from Sigma-Aldrich and used as received.
The solvents; dichloromethane, isopropanol, absolute ethanol and
deuterated solvents were bought from Merck Chemicals, distilled
and dried using conventional methods. NMR spectra were recorded
on a Bruker 400 MHz (1H) and a 100 MHz (13C) spectrometer.
Elemental analyses were performed on Thermal Scientific Flash
2000 and mass spectra were recorded on LC Premier micro-mass
Spectrometer. Magnetic moment measurements were performed
in an Evans balance.
2.2. Syntheses2-[1-(3,5-dimethylpyrazol-1-yl)ethyl]pyridine (L1)
Compound L1 was prepared by dissolving 2-(1-chloroethyl)
* Corresponding author.
E-mail address: ojwach@ukzn.ac.za (S.O. Ojwach).
http://dx.doi.org/10.1016/j.molstruc.2017.01.068
0022-2860/© 2017 Elsevier B.V. All rights reserved.

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M.N. Magubane et al. / Journal of Molecular Structure 1135 (2017) 197e201
pyridine, initially prepared by reduction of acetyl pyridine using
NaBH4 and subsequent reaction with SOCl2, (2.02 g, 14.20 mmol)
and 3,5-dimethylpyrazole (1.37 g, 14.20 mmol) in toluene (30 mL),
40% aqueous NaOH (10 mL) and 40% aqueous tetrabutylammonium
bromide (5e6 drops). The reaction mixture was refluxed for 120 h.
The organic layer was then separated from the aqueous layer and
washed three times with deionised water, then dried over anhy-
drous Na2SO4 and solvent removed under vacuum. Purification of
the crude product by column chromatography using a solution of
hexane and diethyl ether mixture (3:2) gave a chrome yellow
2.6. Synthesis of complex [Fe2(L1)2Cl2(m-Cl)2] (3)
3: FeCl2 (0.10 g; 0.50 mmol) and L1 (0.10 g; 0.50 mmol). Dark
brown solid. Yield ¼ 0.08 g (51%). (ESI-MS), m/z (% abundance) 464
(1/2 Mþ ꢁ Cl, 8%). meff ¼ 5 0.00 BM. Anal. Cald for C24H30Cl4Fe2N6: C,
43.94; H, 4.61; N, 12.81. Found: C, 43.49; H, 4.90; N, 13.02.
2.7. Synthesis of complex [Ni2(L2)Br2] (4)
NiBr2 (0.10 g; 0.46 mmol) and L1 (0.15 g; 0.46 mmol) were used.
Purple solid. Yield ¼ 0.17 g (67%). (ESI-MS), m/z (% abundance) 464
(Mþ ꢁ Br, 11%), 384 (Mþ ꢁ Br2, 5%), 325 (Mþ ꢁ NiBr2, 10%), 311
liquid. Yield: 1.43 g (49%). 1H NMR (CDC13):
d 2.14 (s, 3H, CH3, pz);
3
2.30 (s, 3H, CH3, pz); 1.96 (d, 3H, CH3, JHH ¼ 8); 5.46 (q, H, CH3);
5.87 (s, 1H, pz); 6.82 (d, 1H, py, JHH ¼ 7.61 Hz); 7.15 (t, 1H, py,
3
(Mþ
ꢁ
NiBr2,-CH3, 18%) meff
¼
3.62 BM. Anal. Cald for
3JHH ¼ 7.8 Hz); 7.58 (t, 1H, py, JHH ¼ 7.8 Hz); 8.54 (d, 1H, py,
3
C22H19Br2N3Ni: C, 48.58; H, 3.52; N, 7.73. Found: C, 48.78; H, 3.21;
3JHH ¼ 7.8 Hz).13C NMR (100 MHz, CDCl3)
d: 13.71,14.05, 20.19, 59.17,
N, 7.48.
105.64, 120.25, 122.13, 137.07, 139.49, 147.50, 148.77, 162.10. (ESI-
MS) m/z (%) 201 (Mþ, 35%).
2.8. Crystal data collection and structure refinement
Single crystal X-ray diffraction data collection and refinement
for complex 1 and 4 were recorded on a Bruker Apex Duo equipped
with an Oxford Instruments Cryo jet operating at 100 (2) K and an
Incoatec micro source operating at 30 W power. The data were
2.3. Synthesis of 2-[1-(3,5-diphenylpyrazol-1-yl)ethyl]pyridine (L2)
Compound L2 was prepared following the same method
described for L1 using 2-(1-chloroethyl)pyridine (2.00 g,
14.30 mmol) and 3,5-diphenylpyrazole (3.14 g, 14.30 mmol). Light
collected with Mo K
a
(
l
¼ 0.71073 Å) radiation at a crystal-to-
detector distance of 50 mm. The following conditions were used
for the data collection: omega and phi scans with exposures taken
at 30 W X-ray power and 0.50ꢀ frame widths using APEX2 [10]. The
data were reduced with the programme SAINT [10] using outlier
rejection, scan speed scaling, as well as standard Lorentz and po-
larization correction factors. A SADABS semi-empirical multi-scan
absorption correction was applied to the data. Direct methods,
SHELXS-2014 [11] and WinGX [12] were used to solve all three
structures. All non-hydrogen atoms were located in the difference
density map and refined anisotropically with SHELXL-2014 [11]. All
hydrogen atoms were included as idealized contributors in the
least squares process. Their positions were calculated using a
standard riding model with C-Haromatic distances of 0.93 Å and
Uiso ¼ 1.2 Ueq.
red semi-solid. Yield: 1.68 (36%). 1H NMR (CDC13):
d 2.02 (d, 3H,
CH3); 5.67 (q, 1H, CH); 6.67 (s, 1H, CH, pz); 7.16 (d, 1H, py,
3
3JHH ¼ 7.61 Hz); 7.93 (d, 1H, py, JHH ¼ 7.8 Hz); 7.61 (t, 2H, py,
3JHH ¼ 7.73 Hz), 7.36e7.45 (m, 10H, Ph). 13C NMR (100 MHz, CDCl3)
d: 20.04, 59.57, 103.63, 120.93, 122.19, 125.61, 125.63, 125.69, 127.61,
128.40, 128.57, 128.65, 128.69, 128.94, 129.05, 130.50, 133.74, 137.18,
145.69, 148.56, 150.80, 161.95. (ESI-MS) m/z (%) 348 (MþþNa, 75%).
2.4. Synthesis of complex [Ni2(L1)2Br2(m-Br)2] (1)
To a mixture of NiBr2 (0.11 g; 0.50 mmol) in CH2Cl2 (10 mL) was
added a solution of L1 (0.10 g; 0.50 mmol) in CH2Cl2 (10 mL). The
mixture was stirred for 24 h at room temperature to give a purple
solution. Slow evaporation of the solution afforded purple crystals
suitable for single crystal X-ray analyses. Yield ¼ 0.15 g (73%). ESI-
MS), m/z (%) 339 (1/2 Mþ ꢁ Br, 15%), 259 (1/2 Mþ ꢁ Br2, 5%).
meff ¼ 2.96 BM. Anal Cald. C24H30Br4N6Ni2. C, 34.34; H, 3.60; N,
10.01. Found: C, 34.11; H, 3.80; N, 10.32.
3. Results and discussion
3.1. Syntheses and spectroscopic characterization of the compounds
The pyrazolyl ligands L1 and L2 were prepared following a
conventional method for the reduction of ketones using NaBH4 as
shown in Scheme 1. Purification of the crude products using col-
umn chromatography and hexane:diethyl ether (3:2) solvent sys-
tem afforded L1 and L2 as analytically pure compounds in low
yields. 1H NMR spectra of both L1 and L2 showed a quartet at
5.46 ppm and 5.67 ppm respectively which were assigned to the
methine protons. All the expected signals were observed in the 1H
NMR spectra of both compounds L1 and L2 (Fig. S1).
Complexes 2e4 were prepared following the protocol described
for 1.
2.5. Synthesis of complex [Ni2(L1)2Cl2(m-Cl)2] (2)
NiCl2 (0.06 g; 0.50 mmol) and L1 (0.16 g; 0.50 mmol). Orange
solid. Yield ¼ 0.12 g (65%). (ESI-MS), m/z (%) 295 (1/2 Mþ ꢁ Cl, 75%).
meff ¼ 2.95 BM. Anal. Cald for C24H30Cl4N6Ni2: C, 43.56; H, 4.57; N,
12.70. Found: C, 43.29; H, 4.78; N, 12.97.
Treatment of compounds L1 and L2 with the relevant Ni(II) or
Fe(II) salts afforded the corresponding complexes 1e4 in moderate
Scheme 1. The synthetic route for ligands L1 and L2 using NaBH4 as the reducing agent.

M.N. Magubane et al. / Journal of Molecular Structure 1135 (2017) 197e201
199
Scheme 2. Synthesis of Ni(II) and Fe(II) complexes 1e4 of pyrazolyl ligands L1 and L2.
to good yields (Scheme 2). While the Ni(II) complexes 1e3 con-
taining less bulky ligand L1 exhibited dimeric solid state structures,
complex 4 bearing the bulky phenyl ligand L2 was monomeric,
consistent with structural dependence on steric factors as previ-
ously reported for (pyrazol-1-ylmethyl)pyridine Ni(II) complexes
[13]. All the complexes (1e4) were paramagnetic hence we
employed magnetic moment measurements, mass spectroscopy
(Fig. S2), micro-analyses and single crystal X-ray analyses were to
determine their identity and purity.
3.2. Crystal structural description of complexes 1 and 4
Single crystals suitable for X-ray analyses of complexes 1 and 4
were obtained by slow evaporation of their dichloromethane so-
lutions at room temperature. Molecular structures and selected
bond parameters for compounds 1 and 4 are shown in Figs. 1 and 2
respectively, while data collection and structural refinement pa-
rameters are given in Table 1. The solid state structure of 1 (Fig. 1) is
a centrosymmetric dimer and is a five coordinate Ni(II) compound
that contains one ligand unit, one terminal and two bridging bro-
mide ligands. Thus the coordination environment about each Ni(II)
atom is a distorted square pyramidal.
On the other hand, complex 4 is mononuclear containing two
terminal Br ligands and one bidentate L2 unit to give a four-
coordinate number around the metal atom. Thus the coordination
geometry about the Ni atom in complex 4 is a distorted tetrahedral.
The difference in nuclearity in compounds 1 and 4 may be largely
attributed to greater steric demands of the diphenyl ligand L2
compared to the dimethyl ligand L1 [14e16].
Fig. 2. Molecular structure of 4 drawn at 50% probability ellipsoids. Bond lengths (Å):
Ni-N(1),2.006(3); Ni-N(3), 1.984(2); Ni(1)-Br(1), 2.3679(5); Ni(1)-Br(2), 2.3601(4);
N(2)-N(3), 1.364(3). Bond angles (ꢀ): N(1)-Ni(1)-N(3), 92.54(10); N(1)-Ni(1)-Br(2),
104.57(7); N(3)-Ni(1)-Br(2), 108.89(7); N(1)-Ni(1)-Br(1),110.98(7); N(3)-Ni(1)-Br(1),
110.77(7).
Table 1
Data collection and structural refinement parameters of complexes 1 and 4.
Parameter
1
4
Empirical formula
Formula weight
Temperature(K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C24H30Br4N6Ni2
839.60
100
0.71073
Triclinic
P1
8.1757 (5)
8.8004 (6)
11.0344 (7)
82.055 (3)
84.393 (3)
64.290 (2)
707.82 (8)
1
C22 H19 Br2 N3 Ni
543.93
100
0.71073
Triclinic
Pca21
16.5457 (8)
10.4476 (5)
12.0610 (6)
90
The average Ni-Npz and Ni-Br distances of 2.0472 Å and
2.4906(3) Å in 1 are slightly longer than those of compound 4 of
2.006(13) Å and 2.364(5) Å respectively. This could be due to
b (Å)
c (Å)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
90
90
Volume (A3)
2084.90 (18)
4
1.733
4.776
1080
1.95 to 26.04
10162
0.991
Z
Dcalcd (mg/m3)
1.969
7.001
412.0
3.13e28.33
25874
Absorption coefficient (mmꢁ1
F (000)
)
Theta range for data collection (ꢀ)
Reflections collected/unique
Completeness to theta
0.983
1.103
Fig. 1. Molecular structure of 1 drawn at 50% probability ellipsoids. Bond lengths (Å):
Ni-N(1), 2.0472 (13); Ni-N(3), 2.0587 (13); Ni(1)-Br(2), 2.4329 (3); Ni(1)-Br(3), 2.5483
(3); N(1)-N(2), 1.3709 (17). Bond angles (ꢀ):N(1)-Ni(1)-N(3), 89.55 (5); N(1)-Ni(1)-
Br(3), 94.32 (4); N(3)-Ni(1)-Br(2), 92.29 (4); N(1)-Ni(1)-Br(2), 100.87 (4); N(3)-Ni(1)-
Br(3), 166.15 (4).
Goodness-of-fit on F2
0.979
R indices (all data)
R1 ¼ 0.0168
wR2 ¼ 0.0436
0.74 and ꢁ0.47
R1 ¼ 0.0201
wR2 ¼ 0.0411
0.56 and ꢁ0.27
Largest diff. peak and hole (e Åꢁ3
)

200
M.N. Magubane et al. / Journal of Molecular Structure 1135 (2017) 197e201
Table 4
ligand-ligand repulsions arising from greater shielding/crowding
around the Ni atom in 1 compared to 4. Nonetheless, the bond
lengths of Ni-Npy of 2.0587(13) Å, Ni-Br(terminal) of 2.4329(3) Å and
Ni-Br(bridging) 2.5483 (3) for 1 Å are all statistically similar to the
bond lengths reported for similar 2-(pyrazol-1-ylmethyl)pyridine
complexes (Figs. 1 and 2). The Npz-Ni(1)-Npy bond angles about the
central atom in 1 of 89.55(5)ꢀ is slightly less obtuse than the cor-
responding Npz-Ni(1)-Npy bond angle of 92.54(10)ꢀ in 4, consistent
with the presence of the more sterically demanding diphenyl
ligand L2 in complex 4.
Transfer hydrogenation reactions of different ketone substrate using catalyst 4.a
Entry
1
Substrate
%Conversionb
84
TOF (hꢁ1
1750
)
3.3. Catalytic evaluation of complexes 1e4 in transfer
hydrogenation of ketones reactions
2
3
4
5
78
88
51
72
1625
1833
1063
1500
The Ni(II) and Fe(II) complexes 1e4 were investigated as cata-
lysts in the transfer hydrogenation of ketones using KOH as the base
and isopropanol as the hydrogen donor (Table 1). All the complexes
formed active catalysts in the transfer hydrogenation of acetophe-
none giving conversions between 58 and 84% in 48 h (Fig. S3). We
thus investigated the effects of catalyst structure (Table 2), reaction
conditions such as type of base, catalyst concentration (Table 3) and
nature of ketone substrate (Table 4) on these transfer hydrogena-
tion reactions.
The nature of the substituent on the pyrazolyl ring and the
6
35
729
Table 2
Catalysis data for transfer hydrogenation of ketones by 1e4.a
a
Conditions: ketone, 2 mmol; catalyst. 0.02 mmol (1 mol%); base, KOH; 0.4 M of
KOH in 2-propanol (5 mL), temperature 82 C. bDetermined by 1H NMR spectroscopy.
TOF ¼ (mmol of substrate)/(mmol of catalyst)/(h).
coordination environment around the metal was noted to have
appreciable influence on the catalytic behaviour of the resultant
catalysts. While the mononuclear nickel complex 4 gave the high-
est conversion of 84% corresponding to a TOF of 1750 hꢁ1 (Table 2
entry 4), the dinuclear complex 2 was the least active affording
conversions of 58%. This trend could be assigned to the steric de-
mands of the two ligand units in the dinuclear complexes 1e3 as
opposed to one ligand motif in complex 4 [17].
Comparison of complexes 1e3 reveals that the Fe complex 3 was
the most active displaying conversions of 67% (1404 hꢁ1). We
attributed this trend to the high reactivity of Fe(II) complexes
compared to the Ni(II) analogues [18], usually associated with high
electropositivity of Fe(II) [19]. The nature of the halides also
controlled the catalytic performance of complexes 1e3; with the
bromide complex 1 recording higher catalytic activity than the
corresponding dichloride complex 2 (Table 2, entries 1 vs 2).
c
Entry
Catalyst
Conversion [%]b
TOF x 10ꢁ3 (hꢁ1
)
1
2
3
4
1
2
3
4
63
58
67
84
1319
1198
1404
1750
a
Conditions: acetophenone, 2.0 mmol; catalyst; 0.02 mmol (1.0 mol%); base,
0.4 M KOH in 2-propanol (5 ml); time, 48 h, temperature, 82 ꢀC.bDetermined by 1H
NMR spectroscopy. cTurn over frequency (TOF) ¼ (mmol of substrate)/(mmol of
catalyst)/(h).
Table 3
Dependence of THK on the type of base and catalyst concentration using complex 4.a
In order to optimise the catalytic reaction conditions of 1e4 in
̊
the transfer hydrogenation of acetophenone, we studied the in-
fluence of catalyst concentration, nature of base using complex 4
(Table 3). We observed that an increase in catalyst concentration
from 0.5 mol % to 1.0 mol % resulted in improved catalytic activities
from 42% (875 hꢁ1) to 84% (1750 hꢁ1) (Table 3, entries 1 vs 5).
However, a further increase in catalyst loading to 1.5 mol % resulted
in a slight drop in catalytic activity from 1750 hꢁ to 1688 hꢁ1
(Table 3, entries 1 vs 6), usually associated with complex aggrega-
tion, thus limiting the number of active species [20]. The impact of
the nature of the base was probed by examining the catalytic ac-
e
Entry
Base
Conversion (%)b
TOF (hꢁ1
)
1
2
3
4
5
6
KOH
84
80
24
1750
1667
500
2042
875
NaOH
Na2CO3
tBuOK
KOH
98
42c
81d
t
tivities of complex 4 in BuOK, KOH, NaOH and Na2CO3 (Table 2,
KOH
1688
entries 1e4). The most active conditions were realized using tBuOK,
while the least activity was noted using Na2CO3. This trend is
consistent with the order of stability and strengths of the bases and
agrees with literature findings where stronger bases give more
a
Conditions: ketone, 2.00 mmol; catalyst. 0.02 mmol (1 mol%); 0.40 M of base in
2-propanol (5 mL); time, 48 h; temperature, 82 C. bDetermined by 1H NMR spec-
troscopy. c0.01 mmol catalyst (0.50 mol%); d0.03 mmol catalyst (1.50 mol%). eTurn
over frequency (TOF) ¼ (mmol of substrate)/(mmol of catalyst)/(h).

M.N. Magubane et al. / Journal of Molecular Structure 1135 (2017) 197e201
201
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The scope of the ketone substrates that can be converted to the
respective secondary alcohols was also studied for 2-
methylacetophenone,
2-chloroacetophenone,
3-pentanone,
benzophenone and 2-methycyclohexanone using complex
4
(Table 4 and Fig. S4). It was noted that changing the ketone sub-
strate resulted in a significant difference in catalytic activity. For
instance, conversions of 78% (1625 hꢁ1) and 88% (1833 hꢁ1) were
obtained for 2-methylacetophenone and 2-chloroacetophenone
compared to 84% (1750 hꢁ1) recorded for acetophenone respec-
tively (Table 4, entries 1e3) and can be assigned to decreased
electron density on the C]O bond. These findings are in good
agreement with those of Yu et al. [22] in which electron-donating
groups give inferior catalytic activities. Significantly, diminished
catalytic activities of 51% (1063 hꢁ1) and 35% (729 hꢁ1) were
observed for 2-methycyclohexanone and the aliphatic pentan-3-
one respectively. Thus it can be deduced that the aromatic ring
plays a role in increasing substrate reactivity [23]. On the other
hand, the relatively lower reactivity of 72% (1500 hꢁ1) observed for
benzophenone compared to acetophenone 84% (1750 hꢁ) could
attributed to increased ring strain occasioned by the two bulky
phenyl rings in benzophenone substrate.
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drogenation and hydrogenation of ketones catalyzed by iridium complexes,
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and applications, Chem. Soc. Rev. 35 (2006) 226e236.
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(b) M.N. Magubane, G.S. Nyamato, S.O. Ojwach, O.Q. Munro, Structural, kinetic,
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azole) pyridine iron (II) and nickel (II) complexes, RSC. Adv.
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4. Conclusions
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In summary, we have demonstrated that the reactions of
racemic mixtures of 2-[1-(3,5-dimethylpyrazol-1-yl)ethyl]pyridine
ligand (L1) and 2-[1-(3,5-diphenylpyrazol-1-yl)ethyl]pyridine li-
gands (L2)with NiCl2 or NiBr2 salts produce dinuclear and mono-
nuclear complexes respectively. The catalytic behaviour of
complexes 1e4 in the transfer hydrogenation of ketones were
largely controlled by the coordination environment around the
metal atoms in the respective complexes. The optimum conditions
and reactivities of various ketones substrates have been success-
fully established. We believe that this work provides a promising
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The authors would like to the University of KwaZulu-Natal, and
National Research Foundation (NRF), South Africa for financial
support.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2017.01.068.
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