Mixed ligand nickel(II) complexes as catalysts for alkane hydroxylation using m-chloroperbenzoic acid as oxidant

Article abstract of DOI:10.1016/j.ica.2013.07.031

A new family of nickel(II) complexes of the type [Ni(PA)(L)(CH ₃CN)_n]BPh₄ 1-5, where n = 1, 2, H(PA) is 2-picolinic acid and L is N,N′-tetramethylethylenediamine (L1) 1, N,N′,N″-pentamethyldiethylenetriamine (L2) 2, 2,2′-bipyridine (L3) 3, 1,10-phenanthroline (L4) 4 or 2,9-dimethyl-1,10-phenanthroline (L5) 5, has been isolated and characterized using CHN analysis, UV-Vis spectroscopy and ESI-MS. The complex [Ni(PA)(L2)(CH₃CN)](BPh₄) 2 possesses a distorted octahedral coordination geometry in which Ni(II) is chelated to 2-picolinate anion and L2. DFT calculations show that trans isomers of 3-5 are more stable than cis isomers by ca. 4.0 kJ/mol. In contrast, cis-1 is more stable than trans-1 by 15.8 kJ/mol. The complexes catalyze the hydroxylation of cyclohexane efficiently in presence of m-CPBA as oxidant with 244-569 turnover numbers and good alcohol selectivity (A/K, 3.4-7.0). Adamantane is oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone with varying bond selectivity (3 /2, 9.3-14.2) while cumene is selectively oxidized to 2-phenyl-2-propanol. Upon replacing bidentate L1 by tridentate L2 or strongly π-back bonding phen the catalytic activity increases. In contrast, phen is replaced by non-planar bpy or 2,9-dmp with sterically hindering methyl groups the catalytic activity decreases. Thus ligand denticity, Lewis acidity of Ni(II) center and π-back bonding determine the catalytic activity.

Full text of DOI:10.1016/j.ica.2013.07.031

Inorganica Chimica Acta 407 (2013) 98–107
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Mixed ligand nickel(II) complexes as catalysts for alkane hydroxylation
using m-chloroperbenzoic acid as oxidant
Muniyandi Sankaralingam a, Prabha Vadivelu b, Eringathodi Suresh c, Mallayan Palaniandavar a,d,
⇑
a School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
b Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology, CSIR, Trivandrum 695019, India
c Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India
d Department of Chemistry, Central University of Tamil Nadu, Thiruvarur 610004, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new family of nickel(II) complexes of the type [Ni(PA)(L)(CH3CN)n]BPh4 1–5, where n = 1, 2, H(PA) is
2-picolinic acid and L is N,N0-tetramethylethylenediamine (L1) 1, N,N0,N00-pentamethyldiethylenetriamine
(L2) 2, 2,20-bipyridine (L3) 3, 1,10-phenanthroline (L4) 4 or 2,9-dimethyl-1,10-phenanthroline (L5) 5, has
been isolated and characterized using CHN analysis, UV–Vis spectroscopy and ESI-MS. The complex
[Ni(PA)(L2)(CH3CN)](BPh4) 2 possesses a distorted octahedral coordination geometry in which Ni(II) is
chelated to 2-picolinate anion and L2. DFT calculations show that trans isomers of 3–5 are more stable
than cis isomers by ca. 4.0 kJ/mol. In contrast, cis-1 is more stable than trans-1 by 15.8 kJ/mol. The
complexes catalyze the hydroxylation of cyclohexane efficiently in presence of m-CPBA as oxidant with
244–569 turnover numbers and good alcohol selectivity (A/K, 3.4–7.0). Adamantane is oxidized to 1-ada-
mantanol, 2-adamantanol and 2-adamantanone with varying bond selectivity (3°/2°, 9.3–14.2) while
cumene is selectively oxidized to 2-phenyl-2-propanol. Upon replacing bidentate L1 by tridentate L2
Received 10 February 2013
Received in revised form 11 July 2013
Accepted 18 July 2013
Available online 27 July 2013
Keywords:
Nickel complexes
Catalysis
Alkane hydroxylation
Nickel-oxo intermediate
or strongly
nar bpy or 2,9-dmp with sterically hindering methyl groups the catalytic activity decreases. Thus ligand
denticity, Lewis acidity of Ni(II) center and -back bonding determine the catalytic activity.
Ó 2013 Elsevier B.V. All rights reserved.
p-back bonding phen the catalytic activity increases. In contrast, phen is replaced by non-pla-
p
1. Introduction
catalysts. So far, several bio-inspired iron(II)/(III) complexes have
been investigated as catalysts for hydroxylation, epoxidation and
sulfoxidation reactions [17–30]. Though iron complexes are con-
sidered to be one of the most promising catalysts for the industri-
ally important alkane oxidation reactions, a variety of metal-based
homogeneous catalysts for the oxidation reactions has been re-
ported by replacing iron with different transition metals [31–53].
Very recently, nickel has emerged as an attractive transition
metal for the development of catalysts for the industrially impor-
tant hydroxylation of alkanes. Earlier several oxo-bridged dinucle-
ar Ni(II) complexes have been reported to be involved in oxygen
activation chemistry [54–68]. Very recently, Itoh et al. have studied
the effect of ligand and counter anion of many Ni(II) complexes and
found that [Ni(TPA)(OAc)(H2O)](BPh4), where TPA is tris(pyrid-2-
ylmethyl)amine, is a very efficient and robust turnover catalyst
for the catalytic oxidation of cyclohexane with m-chloroperbenzoic
acid (m-CPBA) as oxidant, exhibiting a high alcohol selectivity in
the reaction [69–71]. They suggested the involvement of the highly
reactive nickel-oxo (Ni = O+) intermediate species rather than an
auto-oxidation type free radical species in the catalytic cycle
[70]. Hikichi et al. have crystallized the nickel(II) alkylperoxo com-
plex [NiII(Tpipr)(OOtBu)], where Tpipr is hydrotris(3,5-di-2-propyl-
pyrazolyl)borate, and studied its catalytic oxidation activity
In recent years catalytic oxidation of saturated hydrocarbons
under mild conditions is attracting great attention among academ-
ics and industries as selective hydroxylation of alkanes is an impor-
tant and difficult chemical transformation. Significant efforts have
been made to design efficient catalysts for the hydroxylation of al-
kanes as conventional processes usually require high temperatures
and high pressures [1–6]. In nature, there are many widely inves-
tigated iron-proteins such as methane monooxygenases, which
carry out the unique and selective oxidation of methane to meth-
anol using dioxygen [7–16]. Inspired by the interesting functional
aspects of such metalloenzymes, bioinorganic chemists have paid
much attention to isolation of biomimetic model complexes con-
taining metal ions, and study of reproducing the interesting chem-
ical transformations catalyzed by the enzymes and have made
significant advancements in their efforts. Also, another target is
the understanding of enzyme mechanisms, which may lead to
new insights into biocatalysis and development of new industrial
⇑
Corresponding author. Tel.: +91 9489079402; fax: +91 4366225312.
E-mail
addresses:
palanim51@yahoo.com,
palaniandavarm@gmail.com
(M. Palaniandavar).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.07.031

M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107
99
towards the oxidation of alkanes [72]. Very recently, we have
investigated a series of nickel(II) complexes of tripodal 4N [73]
and 5N [74] ligands as catalysts for alkane hydroxylation reaction
using m-CPBA as the oxidant and found that the total turnover
number (TON) varies linearly with the metal–ligand covalency
parameter (b), supporting the preference of the nickel-oxo
(Ni = O+) intermediate species and the role of metal–ligand cova-
lent bonding [73].
In this study, we explore the use of new mononuclear mixed li-
gand Ni(II) complexes supported by picolinic acid [H(PA)] as the
primary ligand and bidentate N,N0-tetramethylethylenediamine
(L1), 2,20-bipyridine (L3), 1,10-phenanthroline (L4) and 2,9-di-
methyl-1,10-phenanthroline (L5) and the tridentate N,N0,N00-penta-
methyldiethylenetriamine (L2) as co-ligands (Scheme 1) for alkane
hydroxylation using m-CPBA as oxidant. So far Ni(II) complexes of
phenolate and nitrogen donor ligands have been used as catalysts
for alkane hydroxylation reactions. Now we aim at constructing
more efficient and improved alcohol selective nickel(II) catalysts
for alkane hydroxylation using the strongly coordinating picolinic
acid as the primary ligand and different 2N and 3N nitrogen donor
co-ligands using m-CPBA as the oxidant. Also, we wish to investi-
gate the effect of supporting ligands in determining the formation
and stabilization of the still unclear reactive intermediate species
involved in the alkane hydroxylation reaction. The incorporation
Mumbai), acetonitrile and tetrahydrofuran (Merck, India) were dis-
tilled before use.
2.2. Synthesis of Ni(II) complexes
2.2.1. [Ni(PA)(L1)(CH3CN)2](BPh4) 1
A methanol solution (5 mL) of Ni(ClO4)2ꢀ6H2O (0.365 g, 1 mmol)
was added to a mixture of picolinic acid (0.123 g, 1 mmol), L1
(0.116 g, 1 mmol) and triethylamine (0.10 g, 1 mmol) in methanol
solution (5 mL) with stirring at room temperature. After stirring
the resulting indigo colored solution for 30 min, NaBPh4 (0.342 g,
1 mmol) dissolved in acetonitrile (5 mL) was added to this mixture.
The mixture was stirred for additional 30 min. The blue precipitate
obtained was filtered off and washed with cold methanol and then
diethylether. Yield, 0.57 g, 82%. ESI-MS: m/z 296.20 [(M–2CH3CN–
BPh4)+] (Fig. S1). Anal. Calc. C40H46N5O2BNi: C, 68.80; H, 6.64; N,
10.03. Found: C, 68.84; H, 6.61; N, 9.98%.
2.2.2. [Ni(PA)(L2)(CH3CN)](BPh4) 2
The procedure employed for the preparation of 1 was used for 2
also and N,N0,N00-pentamethyldiethylenetriamine (L2, 0.173 g,
1 mmol) L2 was used instead of L1. The blue precipitate obtained
was filtered off and washed with cold methanol and diethylether.
Single crystals suitable for X-ray crystallographic analysis were ob-
tained by slow evaporation of CH3CN/DCM solution of the com-
plex. Yield, 0.62 g, 87%. ESI-MS: m/z 353.20 [(M–CH3CN–BPh4)+]
(Fig. S2). Anal. Calc. C41H50N5O2BNi: C, 68.93; H, 7.05; N, 9.80.
Found: C, 68.89; H, 7.11; N, 9.72%.
of
p-back bonding L3, L4 and L5 ligands are expected to stabilize
the reactive nickel-oxo intermediate species to different extents.
All the present complexes catalyze the hydroxylation of alkanes
like cyclohexane, adamantane and cumene efficiently (244–569
TON) with good alcohol selectivity for cyclohexane (A/K, 5.1–7.0)
within 4 h. The catalytic activity increases when the bidentate li-
gand L1 in 1 is replaced by the tridentate L2 ligand and by phen li-
gand to obtain 2 and 4 respectively. However, when phen is
replaced with bpy (3) and 2,9-dmp (5), the catalytic activity de-
creases because of destabilization of the intermediate [(PA)(L)
(CH3CN)Ni-Oꢀ] radical species. The phen complex 4 selectively
catalyzes the oxidation of adamantane to 1-adamantanol while
the remaining complexes oxidize adamantane to 1-adamantanol,
2-adamantanol and 2-adamantanone (3°/2°, 9.3–14.2), and
cumene is selectively oxidized to 2-phenyl-2-propanol.
2.2.3. [Ni(PA)(L3)(CH3CN)2](BPh4) 3
The procedure employed for the preparation of 1 was used for 3
also and 2,20-bipyridine (L3, 0.156 g, 1 mmol) was used instead of
L1. Yield, 0.59 g, 80%. ESI-MS: m/z 336.07 [(M–2CH3CN–BPh4)+]
(Fig. S3). Anal. Calc. for C44H38N5O2BNi: C, 71.58; H, 5.19; N, 9.49.
Found: C, 71.52; H, 5.13; N, 9.44%.
2.2.4. [Ni(PA)(L4)(CH3CN)2](BPh4) 4
The procedure employed for the preparation of 1 was used for 4
also and 1,10-phenanthroline (L4, 0.198 g, 1 mmol) was used in-
stead of L1. Yield 0.58 g, 76%. ESI-MS: m/z 360.07 [(M–2CH3CN–
BPh4)+] (Fig. S4). Anal. Calcd. for C46H39N5O2BNi: C, 72.38; H,
5.15; N, 9.17. Found: C, 72.32; H, 5.12; N, 9.21%.
2. Experimental
2.1. Materials
2.2.5. [Ni(PA)(L5)(CH3CN)2](BPh4) 5
Nickel(II) perchlorate hexahydrate, picolinic acid, N,N0-tetra-
methylethylenediamine, N,N0,N00-pentamethyldiethylenetriamine,
2,9-dimethyl-1,10-phenanthroline, adamantane, cumene, sodium
tetraphenylborate, m-choloroperbenzoic acid (Aldrich), 2,20-bipyr-
idine, 1,10-phenanthroline (Alfa Aesar), triethylamine, dichloro-
methane, diethylether (Merck, India) and cyclohexane (Ranbaxy)
were used as received. Methanol (Sisco Research Laboratory,
The procedure employed for the preparation of 1 was used for 5
also and 2,9-dimethyl-1,10-phenanthroline (L5, 0.208 g, 1 mmol)
was used instead of L1. Yield 0.56 g, 70%. ESI-MS: m/z 388.07
[(M–2CH3CN–BPh4)+] (Fig. S5). Anal. Calcd. for C48H43N5O2BNi: C,
72.85; H, 5.48; N, 8.85. Found: C, 72.81; H, 5.42; N, 8.90%.
Caution! Perchlorate salts of the compounds are potentially explo-
sive. Only small quantities of these compounds should be prepared and
suitable precautions should be taken when they are handled.
2.3. Reactivity studies
O
N
N
OH
N
N
The oxidation of alkanes was carried out at room temperature
under research grade nitrogen atmosphere. In a typical reaction,
Ni(II) complex (0.35 ꢁ 10–3 mmol dmꢂ3) was added to a mixture
N
N
L1
L2
PA
of alkanes (2.45 mol dmꢂ3) and oxidant m-CPBA (0.35 mol dmꢂ3
)
in CH2Cl2:CH3CN mixture (3:1 v/v). After 4 h the reaction mixture
was quenched with triphenylphosphine, the reaction mixture
was filtered over a silica column and then eluted with diethylether.
An internal standard (bromobenzene) was added at this point and
the solution was subjected to GC analysis. The mixture of organic
products were identified by Agilent GC–MS and quantitatively ana-
lyzed by HP 6890 series GC equipped with HP-5 capillary column
N
N
N
N
N
N
L3
L4
L5
Scheme 1. Structures of ligands employed in the study.

100
M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107
Table 1
(30 m ꢁ 0.32 mm ꢁ 2.5
lm) using a calibration curve obtained
Crystallographic data for complex 2.
with authentic compounds. All of the products were quantified
using GC (FID) with the following temperature program: injector
temperature 130 °C; initial temperature 60 °C, heating rate
10 °C minꢂ1 to 130 °C, increasing the temperature to 160 °C at a
rate of 2 °C minꢂ1, and then increasing the temperature to 260 °C
at a rate of 5 °C minꢂ1; FID temperature 280 °C. GC–MS analysis
was performed under conditions identical to those used for GC
analysis. The averages of three measurements are reported.
2
Empirical formula
C
41H50BN5O2Ni
Formula weight (g molꢂ1
Crystal habit, color
Crystal system
Crystal size
Space group
a (Å)
)
714.38
pink
orthorhombic
0.43 ꢁ 0.38 ꢁ 0.33 mm
Pna21
23.107(2)
13.5882(12)
11.9638(10)
90
b (Å)
c (Å)
a
(°)
2.4. Physical measurements
b (°)
90
c
(°)
90
3756.4(6)
4
1.263
1520
110(2)
21232
8620
0.71073
1.037
V(Å3)
Elemental analyses were performed on a Perkin Elmer Series II
CHNS/O analyzer 2400. The electronic spectra were recorded on an
Agilent 8453 diode array spectrophotometer. ESI-MS analysis was
recorded on a Micro mass Quattro II triple quadrupole mass spec-
trometer. GC–MS analysis was performed on an Agilent GC–MS
equipped with 7890A GC series (HP-5 capillary column) and
5975C inert MSD. The products were quantified by using Hewlett
Packard (HP) 6890 Gas Chromatograph (GC) series equipped with
a FID detector and a HP-5 capillary column (30 m ꢁ 0.32 mm ꢁ
Z
qcalc (g cmꢂ3
F(000)
T (K)
)
No. of Reflections collected
No. of unique reflections
Radiation (Mo K
a
) (Å)
Goodness-of-fit on (GOF) F2
Number of refined parameters
R1/wR2 [I > 2
R1/wR2 (all data)
457
0.0381/0.0989
0.0434/0.1022
r
(I)]a
2.5 lm). All the catalytic reactions were performed under research
grade nitrogen atmosphere using standard Schlenk line technique.
a R1 = [
R
(||Fo| ꢂ |Fc||)/
R
|Fo|]; wR2 = {[
R
(w(Fo2 ꢂ Fc2)2)/
R
(wFo4)]1/2}.
2.5. Crystal data collection and structure refinement
Table 2
Selected bond lengths [Å] and bond angles [°] for 2.
The diffraction experiments were carried out on a Bruker
SMART APEX diffractometer equipped with a CCD area detector.
High quality crystals, suitable for X-ray diffraction was chosen
after careful examination under an optical microscope. Intensity
2
Bond lengths (Å)
Ni(1)–N(1)
Ni(1)–N(2)
2.229(2)
2.105(2)
data for the crystal was collected using Mo Ka (k = 0.71073 Å) radi-
Ni(1)–N(3)
2.210(2)
ation on a Bruker SMART APEX diffractometer equipped with CCD
area detector at 110 K. The data integration and reduction was pro-
cessed with SAINT [75] software. An empirical absorption correction
was applied to the collected reflections with SADABS [76]. The struc-
ture was solved by direct methods using SHELXTL [77] and refined on
F2 by the full-matrix least-squares technique using the SHELXL-97
[78] package. Other non-hydrogen atoms were located in succes-
sive difference Fourier syntheses. The final refinement was per-
formed by full-matrix least-squares analysis. Hydrogen atoms
attached to the ligand moiety were located from the difference
Fourier map. Crystal data and additional details of the data collec-
tion and refinement of the structure are presented in Table 1. The
selected bond lengths and bond angles are listed in Table 2.
Ni(1)–N(4)
Ni(1)–N(5)
Ni(1)–O(1)
2.0706(18)
2.0569(19)
2.0274(16)
Bond angles (°)
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–N(3)
N(1)–Ni(1)–N(4)
N(1)–Ni(1)–N(5)
N(2)–Ni(1)–N(3)
N(2)–Ni(1)–N(4)
N(2)–Ni(1)–N(5)
N(3)–Ni(1)–N(4)
N(3)–Ni(1)–N(5)
N(4)–Ni(1)–N(5)
O(1)–Ni(1)–N(1)
O(1)–Ni(1)–N(2)
O(1)–Ni(1)–N(3)
O(1)–Ni(1)–N(4)
O(1)–Ni(1)–N(5)
83.04(8)
165.37(8)
97.09(7)
91.02(8)
82.59(9)
173.68(8)
93.72(8)
96.90(8)
92.62(8)
92.60(7)
88.35(7)
93.53(8)
89.83(8)
80.17(7)
172.60(7)
3. Results and discussion
3.1. Syntheses and characterization of nickel(II) complexes
3.2. Structures of nickel(II) complexes
The mixed ligand nickel(II) complexes [Ni(PA)(L)(CH3CN)x]
(BPh4) 1–5 were isolated by treating Ni(ClO4)2ꢀ6H2O with a mix-
ture of one equivalent each of H(PA), the corresponding co-ligands
L1–L5 and triethylamine in methanol, adding acetonitrile with stir-
ring and then treating the reaction mixture with stoichiometric
amount of NaBPh4 dissolved in acetonitrile. All the complexes were
characterized by using elemental analysis, electronic spectroscopy
and mass spectrometry. The formulation of the complexes based
on elemental analysis is confirmed by determining the X-ray crys-
tal structure of 2 (Scheme 2, cf. below). Hence, the variations of li-
gand donor functionalities of the complexes are expected to play
an important role in determining the reactivity and stability of
the reactive intermediate species and eventually the catalytic
activity of the complexes.
3.2.1. Description of X-ray structure of [Ni(PA)(L2)(CH3CN)](BPh4) 2
The molecular structure of [Ni(PA)(L2)(CH3CN)](BPh4) 2 is
shown in Fig. 1 together with the atom numbering scheme and
the selected bond lengths and bond angles are collected in Table 2.
The complex molecule contains a NiN5O coordination sphere with
a distorted octahedral coordination geometry constituted by the
three tertiary amine nitrogen atoms of L2 ligand, and the nitrogen
and oxygen atoms of picolinic acid and the remaining coordination
site trans to the carboxylate oxygen atom is occupied by an aceto-
nitrile molecule. The meridional coordination of the tridentate li-
gand L2 is similar that in mixed ligand copper(II) and certain
iron(II) complexes reported already [79,80]. The Ni–Npy (2.0451–
2.1692 Å) and Ni–Namine (1.926–2.196 Å) bond distances are in

M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107
101
N
N
S
N
N
O
N
O
N
O
N
O
Ni
Ni
Ni
Ni
N
S
N
S
N
N
N
S
S
N
S
a
c cis
1, 3, 4, 5
d
5
b trans
1, 3, 4, 5
2
Scheme 2. Possible coordination geometries a–d (S = CH3CN) for 1–5.
complexes (Scheme 2). For the complexes 1, 3 and 4 in both cis and
trans isomeric forms, (see the supporting information for the opti-
mized structures Fig. S6), the equatorial donors are nearly co-planar
and are shown by the torsion angle of Ni–Ni–O1–N3 and Ni–Ni–N4–
N3 which is equal to ca. 180°. The remaining axially disposed donors
are slightly distorted from the axial coordination plane (N1–Ni–N2–
N3 and Ni–Ni–N5–N3 lies around 170–173° respectively). This indi-
cates that the complexes 1, 3 and 4 exhibit a slightly distorted octa-
hedral geometry. In contrast, the complex 5 possesses an octahedral
geometry more distorted than those of 1, 3 and 4. In the case of
N1
O1
N2
Ni
N4
N5
trans-5,
the
equatorial
planes
are
more
distorted
N3
(N1–Ni–O1–N3 and N1–Ni–N4–N3 of 168°) whereas the axial planes
are nearly co-planar. Also, in comparison with the analogous trans-4
structure (the benzene ring in the equatorial plane), the benzene
rings in trans-5 are twisted and are distorted from the equatorial
plane by ca. 165° (Ni–N3–C1–C2, 162°, Ni–N4–C3–C4, 168°) leading
to a twisted boat-like geometry (Scheme 3). This is due to the steric
hindrance of the incorporated methyl group on phen ring at 2,9
positions towards Ni(II) coordination in 5. Similar to 1, 3 and 4,
cis-5 also shows a co-planar equatorial and distorted axial planes
(here the distortion is higher, around 169°). However, as seen with
trans-5, here also the benzene rings in phen are twisted from the
equatorial plane (by Ni–N3–C1–C2 of 163.5° and Ni–N4–C3–C4 of
159.4°).
Overall, a comparison of computational structures of 1–5 indi-
cate that both the cis- and trans-5 are more distorted than those
of 1–4 and even trans-5 looks like a twisted boat in the equatorial
plane. So, the complex 5 has the tendency to release one of the
coordinated solvent (CH3CN) molecules to obtain a stable five-
coordinate square pyramidal geometry (d, Scheme 2). A compari-
son of the energetics reveals that the trans isomer of 3–5 is more
stable than the cis isomer by ca. 4.0 kJ/mol; however, cis-1 isomer
is more stable than trans-1 isomer by 15.8 kJ/mol.
Fig. 1. ORTEP diagram of [Ni(PA)(L2)(CH3CN)](BPh4) 2 showing 50% probability
thermal ellipsoids and the labeling scheme for selected atoms. All hydrogen atoms
are omitted for clarity.
the ranges observed for previously reported nickel(II) complexes
(Ni–Npy, 2.0706; Ni–Namine, 2.105–2.229 Å) [69–71,73,74]. The
Ni–Npy bond (2.0706(2) Å) is shorter than all the Ni–Namine bonds
(2.229(2), 2.210(2), 2.105(2) Å) due to sp2 and sp3 hybridizations
of the pyridyl and tertiary amine nitrogen atoms respectively. Also,
the terminal Ni–Namine bonds (2.229(2), 2.210(2) Å) are longer than
the central Ni–N2amine bond (2.105(2) Å), as expected. The Ni–
N5ACN bond (2.0569(2) Å) is shorter than all other Ni–N bonds
due to sp hybridization of acetonitrile nitrogen (cf. above). The
Ni–O1 bond (2.0274(16) Å) is shorter than all the Ni–N bonds indi-
cating that the carboxylate oxygen is coordinated very strongly to
the nickel(II) centre. For 2 the bond angles 80.17–97.09° and
165.37–173.68° deviate from the ideal octahedral angles of 90°
and 180° respectively revealing the presence of significant distor-
tion in the Ni(II) coordination geometry.
3.3. Electronic spectral properties
3.2.2. Structures of nickel(II) complexes: density functional theory
(DFT) calculations
The electronic spectral data of all the Ni(II) complexes are sum-
marized in Table 3 and the typical electronic absorption spectrum of
2 is shown in Fig. 2. In DCM:ACN (3:1 v/v) solvent mixture, all the
nickel(II) complexes exhibit two broad absorption bands in the
ranges 545–585 and 890–960 nm together with a very weak shoul-
der in the range 772–794 nm. The observed band positions are well
matches with the previously reported octahedral Ni(II) complexes
A DFT study (using g09 program) [81] has been performed to
investigate the geometrical parameters of 1–5 (computed geome-
tries are shown in Scheme 3 and Fig. S6). Depending upon the sol-
vent/acetonitrile coordination, the complexes may exist in both cis
and/or trans forms (Scheme 2). Initially, as a bench mark calcula-
tion,1 the crystal structure of 2 was optimized. The computed geom-
etry (20) is in good agreement with the experimentally determined
crystal structure, except a slight elongation in bond lengths ranging
from 0.03 to 0.1 Å (Fig. S6). Hence, the same computational method-
ology has been followed for optimizing the geometries of other
[70,73]. The lower energy band is assigned to 3A2g ? 3T2g(F) (m1
)
)
transition, and the higher energy band to the 3A2g ? 3T1g(F) (m2
transition in Ni(II) located in an octahedral environment. The
intense shoulder observed around 360 nm in the UV region is as-
signed to 3 2g ? 3T1g(P) (
A m3) transition while the weak shoulder in
the range 772–794 nm to 3A2g ? 1E1g(D) spin-forbidden transition
[82,83]. By fitting the observed m1 and m2 band positions into the
1
All calculations were run with g09 program employing B3LYP basis function. The
quadratic equation connecting the energies of both
m1 and m3 tran-
metal center (Ni) was described by the LANL2DZ basis set along with the associated
RECPs. The rest of the atoms were described by the 6–31G⁄⁄ basis set.
sitions the metal–ligand covalency parameter (b) was calculated

102
M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107
N1-Ni-O1-N3 = 167.6
N1-Ni-N4-N3 = 167.5
N1-Ni-N2-N3 = 177.0
N1-Ni-N5-N3 = 177.0
Ni-N3-C1-C2 = 162.0
Ni-N4-C3-C4 = 167.6
N1-Ni-O1-N3 = 179.0
N1-Ni-N4-N3 = 179.0
N1-Ni-N2-N3 = 169.0
N1-Ni-N5-N3 = 169.0
Ni-N2-C1-C2 = 163.5
Ni-N3-C3-C4 = 159.4
C1
C2
N2
C1
N3
N2
2.17
C3
O1
1.98
2.18
1.98 2.16
2.18
Ni
2.20
N5
N3
C2
C3
O1
2.17
Ni
C4
N4
2.17
N1
N1
2.11
2.19
N4
C4
2.21
N5
trans-5
E =-1521.28529923 a.u
cis-5
E = -1521.28383172 a.u
Scheme 3. Computed geometries of 5 in both cis and trans forms. Bond lengths in Å and torsion angles in deg.
Table 3
UV–Vis spectral data (kmax in nm;
e
in Mꢂ1 cmꢂ1 in parenthesis) of Ni(II) complexes 1–5 in DCM:ACN solvent mixture (3:1 v/v) at 25.0 °C.
3T1g(P) (m3 3A2g 3T1g(F) (m2 3A2g 1E1g (D) 3A2g 3T2g(F) (m1
Complex
3A2g
Found
?
)
?
)
?
?
)
B0
bb (%)
Calcda
1
2
3
4
5
354 (36)
360 (60)
334
335
322
328
359
576 (9)
582 (11)
545 (8)
555 (13)
585 (13)
778 (5)
794 (7)
772 (8)
772 (5)
788 (5)
947 (11)
960 (16)
890 (11)
904 (8)
1041
1055
1045
1018
856
3.5
2.3
3.2
5.7
20.7
–
–
–
936 (7)
a,bCalculated by solving the quadratic equation and using B as 1080 cm–1
.
[83]. The calculated band positions of m3 band agree with the ob-
served ones (Table 3), which confirms the band assignments and
hence the octahedral coordination geometry for Ni(II) complexes
in solution also. The values of metal–ligand covalency parameter
(b) for the present complexes fall in the range b, 2.3-5.7. However,
the 2,9-dmp complex 5 shows a very high b value of 20.7; this is
consistent with the above computational study, which reveals that
the octahedral geometry for 5 is unstable and tends to adopt a stable
five-coordinate square pyramidal geometry in solution [70,84].
0.3
ν1
0.2
ν2
0.1
0
350
500
650
800
950
1100
3.4. Catalytic oxidation of alkanes
Wavelength (nm)
The experimental conditions and results of catalytic oxidation
of different alkanes for all the nickel(II) complexes using different
oxidants are summarized in Tables 4–7. The conversion of alkanes
Fig. 2. Electronic absorption spectra of [Ni(PA)(L2)(CH3CN)](BPh4) (1.0 ꢁ 10ꢂ2 M)
in DCM:ACN solution.
Table 4
Conversion of cyclohexane catalyzeda by 2 with time.
S.No.
Time (min.)
Cyclohexane (TON)
Total TONc
Chlorobenzene
A/Kd
Yielde (%)
-olb
-oneb
e-caprolactone
1
2
3
4
5
6
7
30
60
159
219
342
399
498
510
572
15
13
8
10
17
18
34
20
32
35
40
54
56
67
194
264
385
549
569
584
673
149
192
326
383
491
502
552
4.5
4.9
7.9
8.0
7.0
6.9
5.7
19.4
26.4
38.5
54.9
56.9
58.4
67.3
120
180
240
720
1440
a Reaction conditions: Catalyst (0.35 ꢁ 10ꢂ3 mmol dmꢂ3), Substrate (2.45 mol dmꢂ3), Oxidant (0.35 mol dmꢂ3) in DCM:ACN solvent mixture (3:1 v/v).
b -ol = cyclohexanol and -one = cyclohexanone.
c Total TON = No. of mmol of product/No. of mmol of catalyst.
d A/K = TON of -ol/ (TON of -one + TON of
e Yield based on the oxidant.
e-caprolactone).

M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107
103
Table 5
Products of oxidation of cyclohexane catalyzeda by Ni(II) complexes.
Complex
Cyclohexane (TON)
Total TONc
Chlorobenzene
A/Kd
Yielde (%)
-olb
-oneb
e-caprolactone
Blankf
3
487
498
208
489
250
1
16
17
12
20
13
–
4
557
569
244
585
262
3.0
7.0
7.0
5.8
5.1
6.8
0.4
55.7
56.9
24.4
58.5
26.2
1
2
3
4
5
54
54
24
76
24
461
491
193
446
202
a Reaction conditions: Catalyst (0.35 ꢁ 10ꢂ3 mmol dmꢂ3), Substrate (2.45 mol dm–3), Oxidant (0.35 mol dm–3) in DCM:ACN solvent mixture (3:1 v/v).
b -ol = cyclohexanol and -one = cyclohexanone.
c Total TON = No. of mmol of product/No. of mmol of catalyst.
d A/K = TON of -ol/ (TON of -one + TON of
e Yield based on the oxidant.
e-caprolactone).
f Blank = Substrate (2.45 mol dmꢂ3), Oxidant (0.35 mol dmꢂ3) in DCM:ACN solvent mixture (3:1 v/v).
into hydroxylated products was quantified based on gas chromato-
graphic analysis by using authentic samples and an internal stan-
dard (bromobenzene). The catalytic ability of the complexes
towards oxidation of alkanes like cyclohexane, adamantane and
cumene was studied by using m-CPBA, H2O2 and t-BuOOH as oxi-
dants in dichloromethane/acetonitrile solvent mixture (3:1 v/v)
at room temperature under nitrogen atmosphere. When H2O2/t-
BuOOH was used as the oxidant only trace amounts of the oxidized
products are formed revealing that the complexes are not effective
at all as catalysts in the presence of these oxidants. In the presence
of m-CPBA the oxidation of cyclohexane proceeds to give cyclohex-
anol as the major product (A, 20.8–49.8%) and cyclohexanone (K,
The complex 1 catalyses the oxidation of cyclohexane to 557
TON (487 TON of cyclohexanol, 16 TON of cyclohexanone, 54
TON of e-caprolactone) with an alcohol to ketone selectivity (A/
K) of 7.0. As proposed earlier, we suggest that m-CPBA binds with
Ni(II) center in the catalyst by replacing a labile acetonitrile mole-
cule to form the adduct species [NiII(L)(PA)(OOCOC6H4Cl)]
(Scheme 4), which undergoes either (a) homolysis leading to the
formation of [(L)(PA)NiII-OÅ]/[(L)(PA)NiIII = O] intermediate species
and m-chlorobenzoic acid radical or (b) heterolysis leading to the
formation of [(L)(PA)NiIV = O] intermediate species and benzoic
acid [70]. The species [(L)(PA)NiII-OÅ]/[(L)(PA)NiIII = O] is then
involved in selective hydroxylation of alkanes while m-chloroben-
zoic acid radical undergoes decarboxylation to form chlorobenzene
in more than 79–86% yield. The observation of chlorobenzene
1.2–1.7%) and e-caprolactone (2.4–5.4%) as the minor products. It
has been previously reported that in the absence of any metal cat-
alyst m-CPBA is a very strong oxidizing agent for oxidation of
cyclohexane and adamantane to the corresponding alcohols and
ketones; however, vigorous reaction conditions like very high con-
centration of m-CPBA, high temperature and long reaction time are
employed [85]. In control reactions performed in the absence of
the complexes with m-CPBA as oxidant only very small amounts
of the oxidized products are observed for all the substrates (Cyclo-
hexane, 4 TON; Adamantane, 9 TON; Cumene, 5 TON). This reveals
that all the complexes act as catalysts towards oxidation of alkanes
into alcohols. It is noted that m-CPBA is a quantitative reagent for
converting cyclic ketones into their corresponding lactones (Bae-
Table 7
Oxidation product of cumene catalyseda by Ni(II) complexes.
Complex
2-Phenyl-2-propanol
Total TONb
Yieldc (%)
Blankd
5
257
456
136
428
182
5
257
456
136
428
182
0.5
25.7
45.6
13.6
42.8
18.2
1
2
3
4
5
yer–Villiger oxidation) in the absence of a metal catalyst, e-capro-
a Reaction conditions: catalyst (0.35 ꢁ 10ꢂ3 mmol dmꢂ3), Substrate (2.45 mol dmꢂ3),
Oxidant (0.35 mol dmꢂ3) in DCM:ACN solvent mixture (3:1 v/v).
b Total TON = No. of mmol of product/No. of mmol of catalyst.
c Yield based on the oxidant.
lactone is not the metal-catalyzed product [86]. The latter is the
over oxidized product of oxidation of cyclohexanone by the excess
or unreacted m-CPBA. Also, interestingly, only 50% of the oxidized
products are formed under atmospheric conditions, revealing the
absence of a free radical auto-oxidation reaction.
d Blank = Substrate (2.45 mol dmꢂ3), Oxidant (0.35 mol dmꢂ3) in DCM:ACN sol-
vent mixture (3:1 v/v).
Table 6
Products of oxidation of adamantane catalyzeda by Ni(II) complexes.
Complex
Adamantane (TON)
1-adolb
Total TONc
Selectivityd
Yielde (%)
2-adolb
2-adoneb
3°/2°
Blankf
6.2
2.4
123
113
98
141
37
0.4
–
42
23
–
9
6.6
10.4
9.3
9.4
10.0
14.2
0.9
54.8
63.7
50.2
61.2
33.9
1
2
3
4
5
425
482
381
471
280
548
637
502
612
339
22
a Reaction conditions: Catalyst (0.2 ꢁ 10ꢂ3 mmol dmꢂ3), Substrate (0.4 mol dmꢂ3), Oxidant (0.2 mol dmꢂ3) in DCM:ACN solvent mixture (3:1 v/v).
b 1-adol = 1-adamantanol, 2-adol = 2-adamantanol and 2-adone = 2-adamantanone.
c TON = No. of mmol of product/No. of mmol of catalyst.
d 3°/2° = (TON of 1-adol ꢁ 3)/(TON of 2-adol + TON of 2-adone).
e Yield based on the oxidant.
f Blank = Substrate (0.4 mol dmꢂ3), Oxidant (0.2 mol dmꢂ3) in DCM:ACN solvent mixture (3:1 v/v).

104
M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107
O
Cl
O
HO
O
(PA)(L)Ni II O
II
O
[(PA)(L)Ni(CH3CN)x]
Cl
x = 1 for 1; x = 2 for 2 - 5
O
Cl
Cl
-CO2
O
O
O
Cl
HO
[(PA)(L)NiII O]
(PA)(L)NiIV
or
[(PA)(L)NiIII O]
m-CPBA
O
O
OH
O
O
OH
O
m-CPBA
O
m-CPBA
II
II
[(PA)(L)Ni(CH3CN)x]
[(PA)(L)Ni(CH3CN)x]
Scheme 4. Proposed mechanism of alkane hydroxylation for 1–5.
m-CBA or phenolate [69]. By invoking the formation of such an
intermediate, we have successfully illustrated the trends in cata-
lytic activity of nickel(II) complexes of a new family of 4N and
5N ligands towards alkane hydroxylation [73,74]. The proposed
involvement of the intermediate adduct species [NiII(L)
(PA)OOCOC6H4Cl)] is supported by the successful isolation and
X-ray crystal structure determination of the nickel(II) alkylperoxo
complex [NiII(Tpipr)(OOtBu)], which has been actually employed
for oxidation of alkanes [72]. Also, Kallol Ray et al. have used EPR
and ESI-MS techniques to characterize the intermediate species
[(TMG3tren)Ni-O(H)]n+, where n = 2 for oxo, and n = 3 for hydroxo
and TMG3tren is tris(2-(N-tetramethylguanidyl)ethyl)amine, and
proposed the involvement of [(TMG3tren)NiIII = O] species in al-
kane hydroxylation reaction. Further, the formation of [NiIV
(O)(TMG3tren)](OTf) in the reaction of [Ni(TMG3tren)(OTf)](OTf)
with m-CPBA has been detected by using ESI-MS [86].
The time course of the TON for 2 in cyclohexane oxidation is
shown in Fig. 3 and Table 4, which clearly indicates that the cata-
lytic activity of the Ni(II) complexes gradually proceeds even after
30 min. Also, the catalytic activity increases from the total TON of
194–673 with the alcohol selectivity (A/K) increasing from 4.5 to
8.0. The good A/K value observed indicates the absence of any free
radical auto-oxidation. However, when the number of equivalents
of m-CPBA is increased with an aim to increase the concentration
Cyclohexanol
Cyclohexanone
Caprolactone
700
600
500
400
300
200
100
0
30
60
120
180
240
720
1440
Time (min.)
Fig. 3. Bar chart representation of conversion of cyclohexane catalyzed by 2 with
time in DCM:ACN solvent mixture (3:1 v/v) at room temperature.
supports the involvement of the intermediate species [(L)(PA)NiII-
OÅ]/[(L)(PA)NiIII = O] in catalysis and indicates that m-chlorobenzo-
ate radical is not the reactive intermediate because it readily
undergoes decarboxylation rather than hydrogen abstraction from
alkane. The formation of the remaining alkane hydroxylated prod-
ucts (14–19%) reveals that the gate for formation of another inter-
mediate species, possibly [(L)(PA)NiIV = O], is unlocked. Adducts
with m-CPBA, similar to [NiII(PA)(L)(OOCOC6H4Cl)], and reactive
intermediates, have been proposed previously by Itoh et al. to illus-
trate the catalytic activity of complexes of the type [Ni(L)(X)(X0)],
where L is a tetradentate ligand and X and X0 are OAcꢂ, NO3ꢂ,
of the intermediate and hence the reactivity, the amount of e-cap-
rolactone formed increases as a result of secondary oxidation of
cyclohexanone by the increased oxidant concentration. It is inter-
esting to compare the catalytic activity of 1 towards hydroxylation
of cyclohexane with those of related mixed ligand nickel(II) com-
plexes 2–5 under identical conditions.

M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107
Cyclohexane
105
3.4.1. Effect of ligand denticity
Adamantane
Cumene
700
600
500
400
300
200
100
0
Upon replacing the bidentate ligand L1 in 1 by the tridentate li-
gand L2 to give 2, the oxidation of cyclohexane occurs with a small
increase in the total TON, from 557 to 569 (cyclohexanol, 498 TON;
cyclohexanone, 17 TON; e-caprolactone, 54 TON) and the alcohol
selectivity (A/K, 7.0) remaining the same, which is in accordance
with the small change in the covalency parameter (b: 1, 3.5; 2,
2.3). This is expected of the lower Lewis acidity of the Ni(II) center
in 2 conferred by the increase in ligand denticity from two in 1 to
three in 2. A lower Lewis acidity enhances the ligand exchange pro-
cess with the oxidant m-CPBA, and also facilitates the O–O bond
cleavage in the intermediate species [70]. A similar enhancement
in TON has been observed earlier; the increase in denticity of the
0
1
2
3
4
5
6
(
BzPym2) in
Complex
3N ligand N-benzyl-bis(2-pyridylmethyl)amine
[Ni(BzPym2)(OAc)2(H2O)](BPh4) (524 TON) from three to four of
the 4N ligand TPA in [Ni(TPA)(OAc)(H2O)](BPh4) (656 TON) leads
to enhancement in catalytic activity is increased [70]. So it is clear
that increase in denticity encourages not only binding of the oxi-
dant but also enhances the stability and hence the concentration
of the intermediate species formed by the mixed ligand complex
in solution. It is also possible that the coordinated terminal –
NMe2 group of L2 accepts the proton of m-CPBA and enhances
the reactivity.
Fig. 4. The total TON of Cyclohexane, Adamantane, Cumene catalyzed by the Ni(II)
complexes.
bonding stabilize the reactive intermediate species tune the cata-
lytic efficiency of [Ni(3NO)X2]+ complexes.
3.4.3. Adamantane oxidation
The catalytic activity of the Ni(II) complexes towards oxidation
of adamantane has been also explored and the results are summa-
rized in Table 6. All the complexes catalyze the oxidation of ada-
mantane efficiently to give 1-adamantanol and 2-adamantanol as
the major products along with 2-adamantanone as the minor prod-
uct. Complex 1 catalyzes the adamantane oxidation to give total
TON 548 (1-adamantanol, 425 TON; 2-adamantanol, 123 TON)
with a good bond selectivity (3°/2°, 10.4). Interestingly, 2 catalyzes
the oxidation of adamantane with the total TON (637: 1-adamant-
anol, 482 TON; 2-adamantanol, 113 TON; 2-adamantanone, 42
TON) higher than that for 1 but with a slight decrease in bond
selectivity (3°/2°, 9.3). Upon increasing the denticity from two in
1 to three in 2 the Ni(II) complex increases in stability, leading to
increase in concentration of the reactive intermediate species
and hence the catalytic activity, as observed for the catalytic oxida-
tion of cyclohexane (cf. above). A higher catalytic activity is ob-
served also for 4 with the total TON 612 (1-adamantanol, 471
TON; 2-adamantanol, 141 TON), the selectivity remaining almost
the same (3°/2°, 10.0), as for cyclohexane oxidation. This provides
3.4.2. Effect of ligand donors
Upon replacing the L1 ligand in 1 with phen to give 4, the oxi-
dation of cyclohexane proceeds with increase in catalytic activity
from 557 TON to 585 (cyclohexanol, 489 TON; cyclohexanone, 20
TON;
tivity (A/K, 5.1). The replacement of L1 with two sterically hindered
–NMe2 donors in 1 by phen with two strongly -back bonding het-
e-caprolactone, 76 TON), but with decrease in alcohol selec-
p
erocyclic nitrogen donors to give 4 tends to stabilize the reactive
intermediate species and hence the small increase in catalytic
activity. This is in agreement with the earlier observation of high
catalytic activity for [Ni(TPA)(OAc)(H2O)](BPh4) with three
strongly
p-back bonding pyridyl nitrogen donors [69,73,74]. We
have also shown that the replacement of one of the pyridyl nitro-
gen donors in this complex by the sterically hindered –NMe2 nitro-
gen or weakly
p-back bonding imidazolyl nitrogen decreases the
catalytic activity, illustrating the importance of
p
-back bonding
by heterocyclic nitrogen donor in stabilizing the reactive interme-
diate [(L)(PA)NiII-OÅ] species [73]. Upon replacing phen in 4 with
bpy to give 3 the oxidation of cyclohexane occurs with large de-
crease in catalytic activity (244 TON; cyclohexanol, 208 TON;
support to the importance of ligand p-back bonding in causing en-
hanced stabilization of the reactive intermediate (cf. above). Inter-
estingly, 2-adamantanone product is not observed revealing that 4
catalyzes the oxidation of adamantane very selectively. Thus, when
cyclohexanone, 12 TON;
selectivity remaining almost constant (A/K, 5.8). As the planarity
of bpy (3) is less than that of phen (4), less effective -back bond-
e-caprolactone, 24 TON), the alcohol
the strongly p-back bonding phen (4) is replaced by the non-planar
p
bpy (3) the total TON decreases to 502 (1-adamantanol, 381 TON;
2-adamantanol, 98 TON; 2-adamantanone, 23 TON) with a slight
decrease in bond selectivity (3°/2°, 9.4), as observed for cyclohex-
ane oxidation (Fig. 4), again lending support to the vital role played
ing occurs on moving from 4 (b, 5.7) to 3 (b, 3. 2), leading to desta-
bilization of the reactive intermediate species and hence the
decrease in catalytic activity. Also, it is well-known that phen com-
plexes are more stable than bpy complexes, which will also en-
hance the stability of the complex species in solution. Similarly,
upon replacing phen in 4 with 2,9-dmp to give 5, the oxidation
of cyclohexane takes place with a significant decrease in the cata-
lytic activity from 557 TON to 262 (cyclohexanol, 250 TON; cyclo-
hexanone, 13 TON; e-caprolactone, 24 TON), the alcohol selectivity
remaining almost the same (A/K, 6.8). Upon incorporating two
methyl groups on 2,9 positions of phen in 4 to obtain 5, the pres-
ence of sterically hindering methyl groups renders both
by p-back bonding (as in 4) in stabilizing the reactive intermediate.
Similarly, 5 catalyzes the oxidation of adamantane with a de-
creased total TON 339 (1-adamantanol, 280 TON; 2-adamantanol,
37 TON; 2-adamantanone, 22 TON) but interestingly with a better
bond selectivity (3°/2°, 14.2). Upon introduction of two methyl
groups on 2,9-positions of phen ring the catalytic activity de-
creases up to 45% revealing that the sterically hindering methyl
groups effectively lead to the enhanced destabilization of the reac-
tive intermediate and hence the significant decrease in catalytic
activity. All the complexes catalyze the oxidation with good TON
and high 3°/2° ratio. The high 3°/2° bond selectivity observed indi-
cates that the [(PA)(L)(CH3CN)Ni-OÅ]+ species is the metal-based
r- and
p-back bonding less effective leading to a decrease in stability of
the proposed reactive intermediate (cf. above). Also, the presence
of only one replaceable acetonitrile in 5 (cf. above), in contrast to
two in 3 and 4 would also decrease the ligand exchange process
with the oxidant m-CPBA. Thus both the Lewis acidity of the Ni(II)
oxidant in adamantane oxidation also. Thus, interestingly,
p-back
bonding is one of the predominant factors dictating the stability
center, as modified by the ligand nitrogen donor, and ligand p-back
and selectivity of the reactive intermediate species.

106
M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107
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The catalytic activity of the Ni(II) complexes for the oxidation of
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cumene was also investigated and the results are summarized in
Table 7. All the Ni(II) complexes catalyse the oxidation of cumene
selectively to form 2-phenyl-2-propanol without any side product
formation and the trend in catalytic activity is the same as that ob-
served for cyclohexane and adamantane oxidation. All the above
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observations reveal that ligand
p-back bonding, Lewis acidity of
nickel(II) center and ligand denticity are the factors that influence
the catalytic activity. Systematic studies on well-defined nickel(II)
complexes with varying ligand donor atoms are essential to throw
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[Ni(PA)(L)(CH3CN)n]+, where H(PA) is picolinic acid and L is biden-
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the complexes catalyze the hydroxylation of alkanes like cyclohex-
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p-back bonding phen co-ligand, there
is a small increase in catalytic activity. However, upon replacing
phen by non-planar bpy or 2,9-dmp with sterically hindering
methyl groups on 2,9-positions the catalytic activity decreases,
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We sincerely thank the Council of Scientific and Industrial Re-
search, New Delhi, for a Senior Research Fellowship to M.S. This
work was also supported by Indo-French Centre (IFCPAR)
[Scheme No. IFC/A/4109-1/2009/993]. We also thank the Director,
National Institute for Interdisciplinary Science and Technology (NI-
IST)-CSIR, Trivandrum, for providing the computational facility and
Dr. C. H. Suresh, CSIR-NIIST, Trivandrum, for fruitful discussions.
Professor M. Palaniandavar is a recipient of DST Ramanna Fellow-
ship [Scheme No. SR/S1/RFIC-01/2007 and SR/S1/RFIC-01/2010].
We thank Dr. Balachandran Unni Nair, Central Leather Research
Institute, Chennai for providing the ESI-MS facility.
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CCDC 917086 contains the supplementary crystallographic data
for 2. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-
ta_request/cif. Supplementary data associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
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