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ARTICLE IN PRESS
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S.G. Mncube, M.D. Bala / Molecular Catalysis xxx (2017) xxx–xxx
Scheme 1. Synthesis of nickel complexes 3a–d.
Results and discussion
All the neutral triazoles and their corresponding salts were
synthesized by reported methods via the well-established CuAAC
‘click’ protocols [28,29]. For the synthesis of the metal complexes,
we adapted the method of Cowley and co-workers [13] developed
for imidazolium ligand precursors to suit the triazolium com-
pounds reported here. Hence, a series of the triazolium salts were
first signs of ligand coordination and reaction between the carbene
and the nickel centre. In this manner, the various synthesised ligand
precursor salts (2a–d) were all successfully coordinated to nickel
(3a–d, Scheme 1). The aim of this is to study the effects of both steric
and electronic variations on the catalytic activity of the prepared
metal complexes (vide infra).
The complexes were isolated from THF in good to excellent
yields as violet crystalline solids, a clear improvement over ear-
lier methods [30] that reported comparatively lower product yields
when the reactions were conducted in dioxane. All the complexes
were characterised by spectroscopic analysis and the solid-state
structures of 3c and 3d were further analysed by single crystal X-ray
diffraction. The compounds were all stable in air and only soluble
in polar solvents, and 1H NMR spectroscopic analysis of their solu-
tions in CDCl3 showed disappearance of the distinct characteristic
triazolium C-5 proton that normally resonates around 9–10 ppm
for uncoupled triazolium salts, suggesting successful deprotona-
tion and ligand complexation to the Ni(II) centre. It should be noted
that due to residual unreacted nickelocene, we initially encoun-
tered complications in assigning some NMR peaks due to abnormal
chemical shifts, line broadening, and unusual signal integrations.
This is similarly observed and documented in the literature [30,31],
where some 1H-NMR peaks assigned to the triazolium portion of
the complexes were not fully resolved or became more complicated
than expected. Following further purification and recrystallization,
pure Ni-NHC complexes 3a–d were confirmed. From the NMR data,
the presence of an intense singlet peak at circa 5 ppm signifies the
presence of one 5-bonded cyclopentadienyl ligand. The NMR peak
positions of the substituents on the bonded triazolium ligands were
observed to in general have shifted slightly downfields when com-
pared to the chemical shifts of corresponding uncoordinated salts
(e.g. 3b vs 2b, supporting information).
Fig. 1. ORTEP plot of complex 3c with thermal ellipsoids drawn at the 50% proba-
bility level and hydrogen atoms omitted for clarity. Selected bond lengths (Å) and
angles (◦): Ni-I(1) = 2.4951(5); Ni-Ccarbene = 1.881(3); Ni-Cpcentroid = 1.750(4); C(2)-
Ni-I = 97.49(9); C(2)-Ni-Cpcentroid = 131.28(6); I-Ni-Cpcentroid = 131.18(4).
assigned to the bonded Cp ligand clearly indicates formation of
proposed half-sandwich complexes 3a–d from nickelocene and
the respective ligands. Observations from paraffin activation stud-
ies indicates that the incorporation of hydrophobic regions in the
choice of ligand substituents is often favourable to catalytic effi-
ciency of the complex in terms of alkane conversion to oxygenated
products [26]. Hence, we attempted to extend the R substituent
group (Scheme 1) to longer straight alkyl chains beyond propyl,
but under the current reaction conditions, we only isolated traces
of trans-ligated bis-carbene complexes which we believe resulted
from the decomposition (double substitution or second carbene
insertion) of the targeted half-sandwich complexes [30].
Violet single crystals of 3c and 3d suitable for X-ray diffrac-
tion studies were obtained from the slow diffusion of hexane into
concentrated solutions of the complexes in dichloromethane. The
molecular structures of 3c and 3d showing the connectivity of
atoms in the complexes with important bond lengths and angles
highlighted are shown in Figs. 1 and 2 respectively. Complex 3c
crystallised in the monoclinic P21/c space group while 3d crys-
tallised in the orthorhombic Pna21. With the cyclopentadienyl ring
represented by a centroid, both complexes showed trigonal-planar
coordination of the three ligands around each nickel(II) centre. Also,
both structures clearly show the 1,3,4-distribution of substituents
around each 1,2,3-triazolium ligand coordinated to the metal in a
Ccarbene-Ni fashion. The half-sandwich geometry incorporates the
The complexes were also characterised by 13C NMR spec-
troscopy in which all the spectra indicated the presence of bonded
triazolium ligands and a sharp peak resonating at circa 90 ppm
Please cite this article in press as: S.G. Mncube, M.D. Bala, Application of 1,2,3-triazolylidene nickel complexes for the catalytic oxidation