Application of Ni(II) complexes of air stable Schiff base functionalized N-heterocyclic carbene ligands as catalysts for the transfer hydrogenation of aliphatic ketones

Article abstract of DOI:10.1080/00958972.2018.1493199

New air stable N-heterocyclic carbene functionalized Schiff base ligands (L) of the type 2-[-2-[3-(R)imidazol-1-yl]ethyliminomethyl]phenol [R = methyl (2), 2-pyridylmethyl (3)] were synthesized and characterized by NMR, IR, MS, and CHN analysis. Single crystal X-ray structural analysis of their Ni(II) complexes revealed square planar arrangement of the chelating ligands coordinated in tridentate (2, C^N^O) and tetradentate (3, N^C^N^O) modes around the metal. The three new isolated and fully characterized complexes were utilized as catalysts for the catalytic transfer hydrogenation of aliphatic ketones in 2-propanol as solvent and source of hydrogen. Based on 0.2 mol% catalyst concentration, the complexes showed activity for aliphatic ketones and 100% conversion (turnover number of 500) for cyclohexanone and all the aromatic ketones tested.

Full text of DOI:10.1080/00958972.2018.1493199

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Application of Ni(II) complexes of air stable Schiff
base functionalized N-heterocyclic carbene ligands
as catalysts for the transfer hydrogenation of
aliphatic ketones
Samaila Abubakar & Muhammad D. Bala
To cite this article: Samaila Abubakar & Muhammad D. Bala (2018): Application of Ni(II)
complexes of air stable Schiff base functionalized N-heterocyclic carbene ligands as catalysts
for the transfer hydrogenation of aliphatic ketones, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2018.1493199
To link to this article: https://doi.org/10.1080/00958972.2018.1493199
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Application of Ni(II) complexes of air stable Schiff base functionalized
N-heterocyclic carbene ligands as catalysts for the transfer
hydrogenation of aliphatic ketones
SAMAILA ABUBAKAR and MUHAMMAD D. BALA*
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa
*Corresponding author. Email: bala@ukzn.ac.za
Abstract
New air stable N-heterocyclic carbene functionalized Schiff base ligands (L) of the type 2-[-2-[3-
(R)imidazol-1-yl]ethyliminomethyl]phenol [R = methyl (2), 2-pyridylmethyl (3)] were
synthesized and characterized by NMR, IR, MS and CHN analysis. Single crystal X-ray
structural analysis of their Ni(II) complexes revealed square planar arrangement of the chelating
ligands coordinated in tridentate (2, C^N^O) and tetradentate (3, N^C^N^O) modes around the
metal. The three new isolated and fully characterized complexes were utilized as catalysts for the
catalytic transfer hydrogenation of aliphatic ketones in 2-propanol as solvent and source of
hydrogen. Based on 0.2 mol% catalyst concentration, the complexes showed activity for
aliphatic ketones and 100% conversion (turnover number of 500) for cyclohexanone and all the
aromatic ketones tested.
Keywords: Catalytic transfer hydrogenation; Aliphatic ketones; N-Heterocyclic carbene; Nickel
complexes; Solvent free; Schiff base
1. Introduction
The pioneering work of Wanzlick and Öfele in the late 1960s, followed by ground breaking
studies by Arduengo and others, have turned N-heterocyclic carbenes (NHCs) into standard
ligands of choice that rival other two electron donor ligands [1-5]. Hence, NHCs and their metal
complexes have been applied in spheres as varied as catalysis, pharmaceuticals, materials science
and as reagents in fine chemicals synthesis [6-8].
Much before the advent of NHCs, the chemistry of Schiff base ligands was well
established because of their ease of preparation and immense ability to stabilize metal centers in
1

both high and low oxidation states. Hence, Schiff base metal complexes of most metals are
known [9, 10]. Consequently, their utilization as catalysts for the activation of many reactions
including the reduction of ketones, allylic alkylation, Michael addition and a number of coupling
reactions has been reported [11-14]. However, despite the variety of compounds based on either
NHCs or Schiff bases reported over the decades, reports on NHCs incorporating imines in their
structure are quite limited [15].
Nickel is a non-toxic, earth-abundant element that is inexpensive, biocompatible and
relatively benign to the environment. Therefore, nickel-based catalysts could serve as
alternatives to more expensive heavy metal (Pd, Ru and Pt) catalysts that dominate current
literature [16-18]. Increasing numbers of catalytic processes that include reduction of ketones,
olefin polymerization and several coupling reactions have been initiated by NHC-nickel
complexes [19, 20].
Catalytic reduction of ketones to alcohols is a simple but fundamental transformation of
immense importance to both academia and industry [21]. However, the selective reduction of
aliphatic ketones in particular remains a challenging goal because unlike the aromatic types, only
a few substrates have been efficiently converted in good product yields. Thus, the need to
develop new catalysts for efficient and selective reduction of a wide range of aliphatic ketones
remains a challenge. We herein report new nickel complexes of chelating bi-functional ligands
bearing Schiff base and NHC components as cost effective and environmentally benign catalysts
for transfer hydrogenation.
2. Results and discussion
2.1. Synthesis
Detailed route to the synthesis of all the compounds is presented in scheme 1. Although the
identity of ligand 2 is known [22], the high yielding solvent-free approach we adopted as part of
the synthesis strategy for 2 and 3 adds green chemistry credentials to this report [15, 23]. For
complexation to nickel, the prepared ligands were utilized in situ yielding complexes 4, 5 and 6,
which are insoluble in alcoholic solvents, partially soluble in chlorinated ones and soluble in
acetone, acetonitrile, and DMSO. They also showed good air stability; 4 and 6 might be freely
handled and stored in air without decomposition. Complex 5, however, needs to be handled with
more caution as it gradually decomposed on exposure to air and moisture.
2

Scheme 1. Synthetic route to 4-6.
2.2. Spectroscopic analysis
NMR data of 4, 5 and 6 indicated complexation of the ligands to Ni(II) evidenced by the
disappearance (binding) of resonance peaks of key ligand functional groups. These include the
peak associated with the Schiff base OH proton that resonates downfield at circa 12.5 ppm and
that of the carbene NCHN proton, at circa 10.5 ppm. Disappearance of these peaks from the
1H-NMR spectra of all the complexes is diagnostic of ligand coordination [24]. In comparison to
the spectra of 2 and 3, the spectra of 4, 5 and 6 showed a general upfield shift in resonance
frequencies as further evidence of ligand coordination. For instance, the imine proton that
generally resonates at circa 8.5 ppm in the ligands shifted to around 7.8 ppm upon coordination,
except for 4 where a slight downfield shift was observed and this could be attributed to the
deshielding influence of the trans bound pyridyl donor [25-27]. Meanwhile, the two sets of
bridging methylene (CH2) protons α and β to the imine that were observed at circa 4.25 and
4.81 ppm, respectively, in the ligands showed upfield shifts to 3.77 and 3.85 ppm, respectively,
upon ligand coordination to nickel. This is an unequivocal confirmation of imine -C^N-
coordination to the nickel center through the N- lone pair of electrons that usually shielded the
3

neighboring methylene protons. All the aromatic protons also shifted upfield, further confirming
ligand coordination to nickel. The 13C-NMR data of all the complexes were also indicative of
ligand complexation. For instance, a downfield shift was observed for the carbene (NCN) signal
which resonated at 137 ppm in 2, but due to the dπ-pπ interaction between the carbene and the
Ni(II) center in 4 was observed at 152 ppm [28, 29].
The IR spectra of all the complexes distinctly showed disappearance of the ligand
hydroxyl (-OH) absorption (circa 3300 cm-1) and slight shifts in wavenumbers toward lower
frequencies for the imine (C=N) resonance that points to bidentate -O^N- coordination of the
Schiff base moiety to nickel. In addition, positive electrospray mass spectrometric data of 2 and
3 revealed the loss of bromide counter ions from the ligand moieties with base peaks at m/z =
230.1284 and 307.1563, respectively. While for the complexes, m/z = 363.0766 (4), 286.0479
(5), and 366.0883 (6) corresponded to base peaks for [M-PF6, 4] [M+-Cl, 5] and [M-PF6, 6]
respectively. Furthermore, the data recorded for combustion analysis (CHN) of the complexes
were all within acceptable (± 0.3%) ranges of calculated values, a confirmation that the
complexes were isolated in high bulk purity.
2.3. Analysis of the single crystal structures of 4 and 6 by X-ray diffraction
Single crystals of 4 and 6 suitable for X-ray diffraction were grown by slow evaporation of
acetone solutions of the complexes. Crystal and experimental refinement data are shown in table
1, while selected bond lengths and angles are summarized in table 2. ORTEP diagrams showing
unit cell contents with hydrogens omitted for clarity for 4 and 6 are presented in figures 1 and 2,
respectively.
As observed in figures 1 and 2, polydentate 2 and 3 are bound to the nickel centers via
three (C^N^O) and four (N^C^N^O) donors, respectively. Hence, 4 crystallized in the
monoclinic crystal system while 6 crystallized in the lower symmetry triclinic P-1 space group.
More interesting is the terdentate binding mode of 3, coordinated via the O(1), N(3), N(4) and
the carbene C(1) donors in a square planar geometry with angles of 84.29 to 95.02° around the
Ni(II) center in 4. The bond lengths (Å) of the Schiff base donors to nickel, i.e. Ni-O = [1.856
(4); 1.851 (6)], Ni-N(3) = [1.889 (4); 1.888 (6)] and Ni-N(4) = [1.894 (4); 1.933 (6)] are similar
to those reported for salicyl-aldiminato phenyl Ni(II) complexes [30-32]. Furthermore, the
4

imidazolium Ni-C(1) bond lengths (Å) are [1.882 (4) and 1.849 (6)], also within distances
observed in most Ni-NHC complexes [33].
2.4. Catalytic transfer hydrogenation
The three complexes were utilized for catalytic transfer hydrogenation (CTH) of cyclohexanone
as a model substrate in 2-propanol as solvent and hydrogen source. From results of preliminary
optimization studies, all the reactions were conducted with a 0.2 mol% catalyst concentration in
refluxing 2-propanol promoted by KOH. No reactions were observed in the absence of KOH,
while higher concentrations of the catalysts yielded no improvement in conversions and poor
reactivities were observed at lower temperatures. A time dependent reaction profile (figure 3)
indicates that a maximum conversion of 100% with a turnover number of 500 was recorded with
4 after a period of 6 h while 5 and 6, respectively, gave 78% and 95% conversion after 8 h.
The higher turnover number recorded with 4 and 6, as presented in table 3, are the highest
results obtained in nickel catalyzed transfer hydrogenation of ketones and are comparable to
some Ru and Pt catalyzed reactions [34-36]. Following the optimization study, 4 and 6 were
utilized to study the scope of the CTH on aliphatic ketones; both catalysts gave comparable
results for all the substrates presented in figure 4 for complex 4. Three major factors were
considered in substrate reactivity to products, i.e. sterics, electro- and chemo-chemistry around
the carbonyl functionality. For instance, lower steric interaction and stronger electronic
contribution to the C=O bond explains the fact that a para substituted saturated ring substrate
(7a) is significantly easier to reduce than 7b bearing a sterically hindering ortho methyl
substituent. Steric hindrance has also been invoked in the past to explain similar results using
iron and nickel based catalyst systems [37]. In general, the aliphatic acyclic ketones are
significantly more difficult to reduce to products, with conversions ranging from 2% (7c) to 32%
(7j) [38]. The results clearly show dependence of conversion to the alcohol products on
increased steric hindrance to the carbonyl due to lengthening of alkyl chains and stability of
associated carbocations which leads to slight improvements in conversions to product when more
stable carbocations were involved (7i -j) [39]. In terms of chemo-selectivity, it is interesting to
note that no products of C=C bond reduction were recorded for 7i and 7j implying that under the
current conditions, the carbonyl bond was selectively activated and functionalized.
5

Finally, in an effort to put these results into context, we included a selection of aromatic
ketones to this study and all were efficiently and fully converted to respective alcohols (7k–n). It
is thus clear that the complexes are very effective for the CTH of a wide range of carbonyl
containing compounds.
3. Conclusion
Three air stable nickel complexes of Schiff base functionalized N-heterocyclic carbene ligands
have been synthesized and characterized including structural elucidation by X-ray diffraction
analysis. All the complexes were successfully utilized as catalysts in the transfer hydrogenation
of a variety of ketones.
4. Experimental
4.1. Synthesis of 2 and 3
Into a clean Schlenck tube charged with a stir bar and nitrogen gas was placed 1.0 mmol of the
Schiff base and the corresponding substituted imidazole. The mixture was heated for 30 min in
the absence of solvent, leading to isolation of the products in excellent yields. Progress of the
reactions was monitored by TLC over time.
4.1.1. 2-[2-(3-Methylimidazol-1-yl)ethyliminomethyl]phenol (2). The product was isolated as
a pale yellow viscous liquid, yield 0.305 g, 98%, 1H-NMR (CDCl3, 400 MHz) 3.97 (s, 3H), 4.15
(t, 2H), 4.73 (t, 2H), 6.82 (m, 2H), 6.97 (m, 1H), 7.23 (m, 2H), 7.34 (dd, 1H), 8.44 (s, 1H), 10.28
13
(s, 1H), 12.56 (s, 1H, -OH). C-NMR (CDCl3, 400 MHz) 37.26, 51.05, 58.59, 116.90, 118.34,
119.23, 122.92, 132.23, 133.15, 133.76, 137.92, 160.68, 169.08. IR (ATR, cm-1): 3394(OH),
3092(CH, sp2), 1628(C=N), 1575(C=C), 1277(N-C-N). HRMS (ESI) [M+ - Br] calculated for
C13H16N3O: 230.1288, found: 230.1285.
4.1.2. 2-[-2-[3-(2-Pyridylmethyl) imidazol-1-yl]ethyliminomethyl]phenol (3). The product
1
was isolated as a reddish brown moisture sensitive viscous liquid, yield 0.38 g, 98%, H-NMR
(DMSO-d6, 400 MHz) 4.09 (t, 2H), 4.71 (t, 2H), 5.65 (s, 2H), 5.80 (dd, 1H), 6.97 (dd, 2H), 7.45
(m, 3H), 7.91 (m, 3H), 8.20 (m, 1H), 8.58 (s, 1H), 9.41 (s, 1H), 12.69 (s, 1H). 13C-NMR
(DMSO-d6, 400 MHz) 48.46, 49.54, 57.72, 116.31, 118.57, 118.80, 122.15, 122.53, 122.75,
6

123.53, 131.82, 132.68, 137.07, 137.54, 149.48, 153.57, 160.03, 167.79. IR (ATR, cm-1):
3395(OH), 3050(CH, sp2), 1628(C=N), 1575(C=C), 1277(N-C-N). HRMS (ESI) [M+ - Br]
calculated for C18H19N4O: 307.1553, found: 307.1563.
4.2. Synthesis of nickel complexes
4.2.1. 2-[-2-[3-(2-Pyridylmethyl)imidazol-1-yl]ethyliminomethyl]phenol nickel(II) (4). Into a
Schlenck tube containing 3, (0.387 g, 1 mmol) was added methanol (10 ml) followed by KPF6
(0.22 g, 1.2 mmol), and the mixture was allowed to stir for 3 h at room temperature before
triethylamine (0.6 ml, 4 mmol) was slowly added to the mixture; upon stirring for 3 minutes,
nickel(II) chloride diglyme (NiDME) was added to the mixture and allowed to stir overnight.
The precipitated nickel complex was separated by filtration on a filter paper as an orange
1
powder, yield; 0.48 g, 95%. MP = 272–273 C; H-NMR (DMSO-d6, 400 MHz) 3.66 (t, 2H),
4.33 (t, 2H), 5.70 (s, 2H), 6.73 (dd, 2H), 7.42 (m, 3H, Ar), 7.62 (m, 3H, Ar), 8.11 (dd, 1H, Ar),
13
8.31 (dd, 1H, Ar), 8.88 (s, 1H, H-C=N). C-NMR (DMSO-d6, 400 MHz) 47.32, 51.65, 60.06,
115.48, 118.90, 119.99, 122.24, 122.63, 124.38, 124.65, 133.70, 135.06, 140.15, 152.77, 153.32,
162.91, 167.07. IR (ATR, cm-1): 3159(CH, sp2) 1607(C=N), 1534(C=C), 835(N-C-N). HRMS
(ESI) [M+ - Cl] calculated for C18H18N4NiO: 363.0739, found: 363.0766; CHN analysis
calculated for [C18H17NiON4].PF6.H2O: C, 41.02; H, 3.63; N, 10.63; found: C, 41.35; H, 3.42; N,
10.56.
4.2.2. 2-[2-(3-Methylimidazol-1-yl)ethyliminomethyl]phenol nickel(II) (5). Into a Schlenck
tube containing 2, (0.311 g, 1 mmol) was added methanol (10 ml) followed by KPF6 (0.22 g,
1.2 mmol) and the mixture was allowed to stir for 3 h at room temperature before triethylamine
(0.6 ml, 4 mmol) was slowly added to the mixture; upon stirring for 3 minutes, nickel(II)
chloride diglyme (NiDME) was added to the mixture and allowed to stir overnight. The
precipitated nickel complex was separated by filtration on filter paper as an orange solid, yield;
1
0.4 g, 95%. MP = 250 C (dec.); H-NMR (DMSO-d6, 400 MHz) 3.70 (t, 2H), 4.57 (m, 5H),
6.58 (d, 1H), 6.82 (d, 1H), 7.31 (m, 4H), 7.90 (s, 1H). IR (ATR, cm-1): 1612(C=N), 1537(C=C),
829(N-C-N). HRMS (ESI) [M+ - Cl] calculated for C13H14N3NiO: 286.0479, found: 286.0479.
CHN Anal. calculated for [C13H14NiClN3O]·KPF6: C, 30.77; H, 2.98; N, 8.28; found: C, 30.32;
H, 3.20; N, 7.97.
7

4.2.3. 2-[2-(3-Methylimidazol-1-yl)ethyliminomethyl]phenol nickel(II) (6). Into a Schlenck
tube containing 2, (0.311 g, 1 mmol) was added methanol (10 ml) followed by KPF6 (0.22 g,
1.2 mmol) and the mixture was allowed to stir for 3 h at room temperature before triethylamine
(0.6 ml, 4 mmol) was slowly added to the mixture; upon stirring for 3 minutes, nickel(II)
chloride diglyme (NiDME) and one equivalent of methylimidazole were slowly added to the
mixture and allowed to stir overnight. The precipitated nickel complex was separated by
filtration using a filter paper as an orange solid, yield; 0.5 g, 90%. MP = 195 C (melt). 1H-NMR
(DMSO-d6, 400 MHz) 3.13 (s, 3H), 3.73 (m, 5H), 4.59 (t, 2H), 6.64 (dd, 2H), 6.83 (dd, 1H), 7.33
(m, 5H), 7.96 (s, 1H). IR(ATR, cm-1): 1613(C=N), 1539(C=C), 841(N-C-N). LRMS (ESI)
[M+ - PF6] calculated for C17H20N5NiO: 368.1005, found: 366.088. CHN Anal. calculated for
[C20H26F6N5NiO2]·PF6: C, 41.99; H, 4.58; N, 12.24; found: C, 41.67; H, 4.51; N, 12.43.
4.3. Catalytic transfer hydrogenation
All the samples were prepared as follows: into a Schlenck tube fitted with a condenser and stirrer
bar was added the substrate ketone (2.1 mmol), nickel catalyst (0.2 mol%), KOH (2 mmol) and
10 ml 2-propanol. The mixture was heated at 82 C. Conversion of the ketones to corresponding
alcohols was monitored using gas chromatography. Aliquots were taken at given time intervals,
filtered through a pad of cotton wool and then injected (0.5 µl) into a GC equipped with a DB1
Wax polyethylene column. The product was identified by comparison with standards purchased
from Sigma Aldrich. The conversion was calculated from the respective peak area of each
product.
4.4. X-ray data collection and analysis
X-ray quality crystals of 4 and 6 were selected, attached onto a Mitagen loop, and centered in the
X-ray beam by the aid of a video camera. Intensity data were collected on a Bruker APEX2
diffractometer with Mo Kα radiation (λ = 0.71073 Å) equipped with an Oxford Cryostream low-
temperature apparatus operating at 100(1) K. The initial cell matrix was determined from three
series of scans consisting of twelve frames collected at intervals of 0.5° in a 6° range with the
exposure time of ten seconds per frame. Each of the three series of scans was collected at
different starting angles and the APEXII [40] program suite used to index the reflections and
8

refined using SAINT [41]. Data reduction was performed using SAINT software, and the scaling
and absorption corrections were applied using SADABS [42] multiscan technique. The structures
were solved by direct methods using SHELXS [43]. Non-hydrogen atoms were first refined
isotropically and then by anisotropic refinement with full-matrix least squares based on F2 using
SHELXL [43]. All hydrogens were positioned geometrically, allowed to ride on their parent
atoms and refined isotropically.
Supplementary data
Characterization data (NMR, IR, TOF MS and elemental analysis) for all new compounds
accompany this paper as electronic supplementary information. Crystallographic data in .cif
format has been deposited with the Cambridge Crystallographic Data Centre, with numbers
CCDC 1572704 and CCDC 1572683 for 4 and 6, respectively. Copies of this information may
be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK.
Fax: +44(1223)336-033, E-mail: deposit@ccdc.cam.ac.uk or visit http://www.ccdc.cam.ac.uk.
Acknowledgements
We thank the University of KwaZulu-Natal, ESKOM (TESP program) and the National
Research Foundation for financial support. Sizwe Zamisa is highly appreciated for the XRD data
collection. SA also thanks Gombe State University for a study fellowship.
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[37] M.N. Magubane, M.G. Alam, S.O. Ojwach, O.Q. Munro. J. Mol. Struct., 1135, 197
(2017).
[38] M. Boukachabia, S. Zeror, J. Collin, J.-C. Fiaud, L.A. Zouioueche. Tetrahedron Lett., 52,
1485 (2011).
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[40] BRUKER, in: APEXII, BRUKERAXS, Madison,WI (2009).
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[43] G. Sheldrick. Acta Crystallogr., Sect. C, 71, 3 (2015).
11

Figure 1. Thermal ellipsoid plot of the asymmetric unit of 4 shown at the 30% probability level
with hydrogens omitted for clarity.
12

Figure 2. Thermal ellipsoid plot of the asymmetric unit of 6 shown at the 50% probability level
with hydrogens omitted for clarity.
13

Figure 3. Time dependent CTH of cyclohexanone over a period of 8 h.
14

OH
OH
OH
7c
2%
7a
66%
7b
44%
OH
OH
OH
7d
7e
7f
8%
18%
11%
OH
OH
OH
7g
14%
7h
14%
7i
24%
OH
OH
OH
7j
32%
7k
100%
7l
100%
OH
OH
7n
100%
7m
100%
Figure 4. Scope of products in the CTH by 4. Conversions determined by analysis of GC data.
Reaction conditions: (82 C; 6 h; 0.2 mol% concentration of 4).
15

Table 1. Crystal and structure refinement data for 4 and 6.
Parameters
4
6
Empirical formula
Formula weight
Crystal system
Space group
a, Å
C18H17F6N4NiOP
509.03
C20H26F6N5NiO2P
572.14
Monoclinic
P 21/c
Triclinic
P -1
9.4412(2)
16.0341(3)
12.5906(2)
90
9.4647(3)
11.8703(4)
12.0481(4)
108.613(2)
90.1570(10)
112.821(3)
1169.90(7)
2
b, Å
c, Å
α, deg
β, deg
95.0740(10)
90
γ, deg
Cell volume, Å3
1898.51(6)
4
Z
Dcalcd, Mg/m3
µ, mm-1
1.781
1.624
1.184
0.974
F(000)
1032
588
Crystal size, mm3
Ɵmin,Ɵmax, deg
Reflections collected
Independent reflections
Completeness to theta
Goodness-of-fit on F2
Final R indices
R indices (all data)
Largest diff. peak and hole e.Å-3
0.260 × 0.140 × 0.070
2.062 to 27.460
22556
0.250 × 0.240 × 0.130
1.803 to 27.076
22394
4310 [R(int) = 0.0335]
100.0%
5035 [R(int) = 0.0240]
99.9%
1.028
1.054
0.0267, 0.0590
0.0350, 0.0624
0.327 and -0.275
0.0267, 0.0642
0.0317, 0.0664
0.316 and -0.281
16

Table 2. Selected bond lengths [Å] and angles [°] for 4 and 6.
Bond length
4
6
Ni–O (1)
Ni–N (3)
Ni–N (4)
Ni–C (1)
1.8511(12)
1.8880(14)
1.9327(14)
1.8491(17)
1.8586(11)
1.8893(13)
1.8938(15)
1.8818(15)
Bond angle
4
6
C–Ni–O
170.22(7)
88.01(7)
92.09(7)
95.02(6)
85.70(6)
173.35(6)
173.59(6)
90.27(6)
91.93(6)
93.72(5)
84.29(5)
175.68(6)
C–Ni–N4
C–Ni–N3
O–Ni–N3
O–Ni–N4
N4–Ni–N3
17

Table 3. CTH of cyclohexanone catalyzed by 4, 5 and 6.
Catalyst
Conversion (%) a
Time (h)
TON b
100
78
6
8
8
500
390
475
4
5
6
95
a Determined by analysis of GC data. Time = 6 h (4); 8 h (5, 6)
b Turnover number = mol product/mol catalyst.
18

Graphical abstract
19