Novel nickel(II) complexes of sterically modified linear N4 ligands: Effect of ligand stereoelectronic factors and solvent of coordination on nickel(II) spin-state and catalytic alkane hydroxylation

Article abstract of DOI:10.1039/c7dt00576h

A series of Ni(II) complexes of the types [Ni(L)(CH₃CN)₂](BPh₄)₂1-3, 5 and [Ni(L4)](BPh₄)₂4, where L = N,N′-bis(2-pyrid-2-ylmethyl)-1,4-diazepane (L1), N-(6-methylpyrid-2-ylmethyl)-N′-(pyrid-2-ylmethyl)-1,4-diazepane (L2), N,N′-bis(6-methyl-2-pyridylmethyl)-1,4-diazepane (L3), N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethylenediamine (L5) and L4 = N,N′-bis((1-methyl-1H-imidazole-2-yl)methyl)-1,4-diazepane, have been isolated and characterized. The complex cations of 1 and 4 possess, respectively, distorted octahedral and low-spin square planar coordination geometries in which nickel(II) is meridionally coordinated to all four nitrogen atoms of L1 and L4. DFT studies reveal that L5 with the ethylenediamine backbone coordinates in the cis-β mode in [Ni(L5)(CH₃CN)₂]²⁺5, but in the cis-α mode in [Ni(L5)(H₂O)₂]²⁺. Also, they illustrate the role of ligand donor atom type, diazacyclo backbone and steric hindrance to coordination of pyridyl nitrogen in conferring novel coordination geometries on Ni(II). All these complexes catalyse the oxidation of cyclohexane in the presence of m-CPBA as the oxidant up to 600 turnover numbers (TON) with relatively good alcohol selectivity (A/K, 5.6-7.2). Adamantane is oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone with high bond selectivity (3°/2°, 8.7-11.7). The incorporation of methyl substituent(s) on one (2) or both (3) of the pyridyl rings and the replacement of both the pyridylmethyl arms in 1 by imidazolylmethyl arms to give 4 decrease the catalytic efficiency. Interestingly, 5 with the cis-β mode of coordination provides two labile cis coordination sites for oxidant binding, leading to higher total TON and product/bond selectivity.

Full text of DOI:10.1039/c7dt00576h

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DOI: 10.1039/C7DT00576H
Manuscript Type: DTꢀARTꢀ02ꢀ2017ꢀ000576ꢀR1
Manuscript submitted to: Dalton Trans.
Novel nickel(II) complexes of sterically modified linear N4 ligands: Effect of donor
atom type, steric hindrance, diamine backbone and solvent on nickel(II) spin state
and alkane hydroxylation with mꢀCPBA as oxidant
a
b
a
Muniyandi Sankaralingam, Prabha Vadivelu, and Mallayan Palaniandavar *
a
School of Chemistry, Bharathidasan University, Tiruchirappalli ꢀ 620024, Tamil Nadu, India
b
Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science
and Technology, CSIR, Trivandrumꢀ695019.
*
To whom correspondence should be addressed, eꢀmail: palanim51@yahoo.com and
palaniandavarm@gmail.com, Fax : +91ꢀ431ꢀ2407045
1

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Abstract
A series of Ni(II) complexes of the types [Ni(L)(CH
3
CN)
2
](BPh
4
)
2
1ꢀ3, 5 and [Ni(L4)](BPh
4
2
) 4, where L =
N,N’ꢀbis(2ꢀpyridꢀ2ꢀylmethyl)ꢀ1,4ꢀdiazepane (L1), Nꢀ(6ꢀmethylpyridꢀ2ꢀylmethyl)ꢀN’ꢀ(pyridꢀ2ꢀylmethyl)ꢀ1,4ꢀ
diazepane (L2), and N,N’ꢀbis(6ꢀmethylꢀ2ꢀpyridylmethyl)ꢀ1,4ꢀdiazepane (L3), N,N’ꢀdimethylꢀN,N’ꢀbis(2ꢀ
pyridylmethyl)ethylenediamine (L5) and L4 = N,N’ꢀbis(1ꢀmethylꢀ1Hꢀimidazoleꢀ2ꢀyl)methyl)ꢀ1,4ꢀdiazepane,
has been isolated and characterized. The complex cations of 1 and 4 possess respectively distorted
octahedral and lowꢀspin square planar coordination geometries in which nickel(II) is meridionally
coordinated to all the four nitrogen atoms of L1 and L4. DFT studies reveal that L5 with ethylenediamine
2
+
O)
]2+. Also, it
back bone coordinates in cisꢀβ mode in [Ni(L5)(CH
3
CN)
2
] 5, but in cisꢀα mode in [Ni(L5)(H
2
2
illustrates the role of ligand donor atom type, diazacyclo back bone and steric hindrance to coordination of
pyridyl nitrogen in conferring novel coordination geometries on Ni(II). All the complexes catalyze the
oxidation of cyclohexane in the presence of mꢀCPBA as oxidant up to 600 turnover numbers (TON) with
relatively good alcohol selectivity (A/K, 5.6ꢀ7.2). Adamantane is oxidized to 1ꢀadamantanol, 2ꢀadamantanol
and 2ꢀadamantanone with high bond selectivity (3°/2°, 8.7ꢀ11.7). The incorporation of methyl substituent(s)
on one (2) or both (3) the pyridyl rings as well as and the replacement of both the pyridylmethyl arms in 1
by imidazolylmethyl arms to give 4 decrease the catalytic efficiency. Interestingly, 5 with cisꢀβ mode of
coordination provides two labile cis coordination sites for oxidant binding, leading to higher total TON and
product/bond selectivity.
2

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Introduction
1
2ꢀ4
Nature utilizes a variety of copper and iron containing enzymes to catalyze a variety
of biologically essential transformations. Among the iron enzymes cytochrome P450, bleomycin
and methane monooxygenases, soluble methane monooxygenases (sMMO) are the widely
investigated metalloenzymes that catalyze the oxidation of methane to methanol using molecular
2
ꢀ4
oxygen under ambient conditions. Inspired by the sMMO enzyme activity, several low
molecular weight diiron(II)/(III) complexes have been isolated and investigated as catalysts for
5
ꢀ7
alkane hydroxylation, epoxidation and sulfoxidation reactions. Also, such a study can lead to a
better understanding of the spectral and structural properties of the active sites and shed light on
the catalytic activity of the enzymes. Currently, the study of catalytic oxidation of saturated
hydrocarbons under mild conditions has received greater attention and has become an exciting
and challenging scientific goal, because conventional hydroxylation processes usually require
8
high pressure and high temperature. Though iron complexes are considered as the potential
catalysts for this industrially important reaction, a variety of low molecular weight compounds
of transition metals like Mn, Co, Ni, Cu, Ru and Os has been isolated and investigated as alkane
9
ꢀ13,14ꢀ35
oxidation catalysts.
In very recent times, nickel complexes have attracted much attention as catalysts for
1
4ꢀ15,21,17
alkane hydroxylation. Several dinuclear nickel(II) oxoꢀbridged,
and mononuclear
1
6a,16c,22
16b,18,19,23,24
nickel(III)ꢀperoxo
reactive intermediates in the oxidation. Itoh et. al. have demonstrated
Ni(TPA)(OAc)(H O)](BPh ), where TPA is tris(pyridꢀ2ꢀylꢀmethyl)amine, is a very efficient
and nickel(II)ꢀsuperoxo
complexes have been reported as
2
5
that
[
2
4
and robust turnover catalyst for alkane hydroxylation with mꢀchloroperbenzoic acid (mꢀCPBA)
26
25,26
as oxidant. Later they isolated a series of Ni(II) complexes of linear N3, tripodal N4
and
2
6
27a
tripodal monoꢀ (N3O) , bisꢀ and tris(phenolate) ligands and studied them as catalysts for
oxidation of cyclohexane using mꢀCPBA. Very Recently, they have shown that Ni(II) complexes
of tetradentate N4 ligands catalyze the direct hydroxylation of benzene to phenol using hydrogen
2
7b
peroxide. Recently, Hikichi et. al. have isolated a nickel(II)ꢀalkylperoxo complex to study its
28
catalytic activity towards oxidation of alkanes, and later studied the ability of nickel(II)ꢀ
29
acylperoxo complexes to effect oxygenation of olefins, sulfides and alkane with mꢀCPBA. We
3
0
31
have isolated a few nickel(II) complexes of tripodal N4, and pentadentate N5 ligands and a
3
2
number of mixed ligand nickel(II) complexes as catalysts for alkane hydroxylation using mꢀ
3

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CPBA as oxidant. We observed an interesting linear correlation between the metalꢀligand
3
0
covalency parameter (β) and the total turnover number (TON), and illustrated the importance
of ligand denticity, Lewis acidity of Ni(II) center and πꢀback bonding in determining the
3
2
II
• +
catalytic activity. We established a highꢀspin (S = 3/2) [(N5)Ni ꢀO ] species as the ground
state in cyclohexane hydroxylation reactions, and predicted an overall two spinꢀstate reactivity
3
1
with Hꢀabstraction as the rateꢀdetermining step. Kallol Ray et.al. trapped and characterized the
III
III
33
Ni =O/ Ni ꢀOH intermediate species in the reaction of a nickel(II) salt with mꢀCPBA. Very
III
recently, a Ni ꢀoxyl intermediate has been shown to perform hydrogen atom abstraction (HAA)
3
4
of weak CꢀH bonds and oxygen atom transfer (OAT) to thioanisoles and styrene. Also,
3
5
McDonald et. al. have reported a terminal nickel(III)–oxygen adduct that performs hydrogen
atom abstraction and oxygen atom transfer. A few monoꢀ and dinuclear Ni(II) complexes have
36
been reported now as catalysts for olefin epoxidation.
Thus, nickel(II) complexes have the potential to be developed as efficient alkane
hydroxylation catalysts. So, we continue to isolate nickel(II) complexes, study the effect of
ligand stereoelectronic factors upon the catalytic efficiency and alcohol selectivity in alkane
hydroxylation. Herein we report a few novel nickel(II) complexes of systematically varied
tetradentate N4 ligands containing cyclic diamine backbone and different heterocyclic nitrogen
3
0
atoms as pendants (Scheme 1). Such N4 ligands are expected to make available at least two
labile coordination sites for binding with the oxidant mꢀCPBA, leading to high catalytic activity.
We aim to investigate the effect of incorporating pyridyl, sterically hindered 6ꢀmethylpyridyl,
and strongly σꢀbonding imidazolyl nitrogen in dictating the nickel(II) coordination geometry and
hence the catalytic activity and alcohol/bond selectivity. Also, we have isolated a Ni(II) complex
of the ethylenediamine based ligand L5 to examine the effect of incorporating diazacycloalkane
ligand back bone upon coordination geometry and hence reactivity. Density functional theory
calculations illustrate the ability of bis(imidazolyl) ligand L4 and other related ligands to confer
a lowꢀspin configuration on Ni(II), and also the effect of coordinated solvent on the mode of
coordination of L5. The present ligand systems with diazapane backbone coordinate
meridionally to Ni(II) and tend to form square pyramidal coordination geometry with the axial
site available for binding of oxidant. We have found that all the complexes catalyze the
hydroxylation of cyclohexane and adamantane efficiently within six hours with good alcohol to
ketone selectivity (A/K, 5.6ꢀ7.2) in the presence of mꢀCPBA as oxidant.
4

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N
N
N
N
N
N
N
N
N
N
N
N
L1
L2
H C
L3
CH
3
3
N
N
N
N
N
N
N
N
N
N
L4
L5
Scheme 1. Molecular structures of tetradentate N4 ligands employed in the study
Experimental
Materials
Pyridineꢀ2ꢀcarboxaldehyde,
N,N’ꢀdimethylethylenediamine,
homopiperazine,
nickel(II)
perchlorate hexahydrate, adamantane, mꢀchloroperbenzoic acid, sodium tetraphenylborate, 1ꢀ
methylimidazoleꢀ2ꢀcarboxaldehyde (Aldrich), 6ꢀmethylpyridineꢀ2ꢀcarboxldehyde, sodium
triacetoxyborohydride (Alfa Aesar), acetic acid glacial, ethyl acetate, triethylamine, sodium
sulphate (Merck, India), cyclohexane (Ranbaxy) were used as received. Dichloromethane,
diethylether, tetrahydrofuran, acetonitrile (Merck, India) and methanol (Sisco Research
Laboratory, Mumbai) were distilled before use.
Synthesis of ligands
The ligands N,N’ꢀbis(2ꢀpyridꢀ2ꢀylmethyl)ꢀ1,4ꢀdiazepane (L1), Nꢀ(6ꢀmethylpyridꢀ2ꢀylmethyl)ꢀ
N’ꢀ(pyridꢀ2ꢀylmethyl)ꢀ1,4ꢀdiazepane (L2), and N,N’ꢀbis(6ꢀmethylꢀ2ꢀpyridylmethyl)ꢀ1,4ꢀ
diazepane (L3), N,N’ꢀbis(1ꢀmethylꢀ1Hꢀimidazoleꢀ2ꢀyl)methyl)ꢀ1,4ꢀdiazepane (L4) and N,N’ꢀ
dimethylꢀN,N’ꢀbis(2ꢀpyridylmethyl)ethylenediamine (L5) were prepared using the previously
3
7,38
reported procedures.
Isolation of Ni(II) complexes
Ni(L1)(CH CN) ](BPh ) (1). A methanol solution (5 mL) of Ni(ClO ) ·6H O (0.365 g,
[
3
2
4 2
4 2
2
1
mmol) was added to a methanol solution (5 mL) of L1 (0.282 g, 1 mmol) with stirring at room
temperature. The color of the solution turned to indigo. The solution was stirred for additional 30
min and the color of solution turned to blue. Then followed by the metathesis of the above
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complex by adding NaBPh (0.684 g, 2 mmol) in acetonitrile. The pink colored precipitate was
4
filtrated off, washed with small quantities of iceꢀcold methanol and then dried. Single crystals
suitable for Xꢀray crystallographic analysis were obtained by slow evaporation of
CH CN/CH Cl solution of the complex. Yield, 0.89 g, 84%. Anal. Calcd. for C H N B Ni: C,
3
2
2
69 68
6 2
78.06; H, 6.46; N, 7.92. Found: C, 78.36; H, 6.42; N, 7.83. ESIꢀMS: m/z 170.20
2+
[
(M−2CH CN−2BPh ) ].
3
4
The complexes 2ꢀ4 were prepared by using the procedure employed for isolating 1 and using the
ligands L2–L4.
[
Ni(L2)(CH CN) ](BPh ) (2). Pink color, Yield, 0.849 g, 79%. Anal. Calcd. for C H N B Ni:
3 2 4 2 70 70 6 2
C, 78.16; H, 6.56; N, 7.81. Found: C, 78.36; H, 6.43; N, 7.73. ESIꢀMS: m/z 177.12
2+
[
(M−2CH CN−2BPh ) ].
3
4
[
Ni(L3)(CH CN) ](BPh ) (3). Pink color, Yield 0.81 g, 75%. Anal. Calcd. for C H N B Ni:
3 2 4 2 71 72 6 2
C, 78.26; H, 6.66; N, 7.71. Found: C, 78.14; H, 6.59; N, 7.76. ESIꢀMS: m/z 184.14
2+
[
(M−2CH CN−2BPh ) ].
3
4
[
Ni(L4)](BPh ) (4). Yellow color, Yield 0.89 g, 91%. Anal. Calcd. for C H N B Ni: C, 76.78;
4 2 63 64 6 2
2
+
H, 6.55; N, 8.53. Found: C, 76.81; H, 6.58; N, 8.45. ESIꢀMS: m/z 173.11 [(M−2BPh ) ]. Single
4
crystals suitable for Xꢀray crystallographic analysis were obtained by slow evaporation of
CH CN/CH Cl solution of the complex.
3
2
2
[
Ni(L5)(CH CN) ](BPh ) (5). Pink color, Yield 0.74 g, 71%. Anal. Calcd. for C H N B Ni:
3 2 4 2 68 68 6 2
C, 77.81; H, 6.53; N, 8.01. Found: C, 77.88; H, 6.48; N, 8.10. ESIꢀMS: m/z 164.14
2+
[
(M−2CH CN−2BPh ) ].
3
4
Caution! Perchlorate salts of the compounds are potentially explosive. Only small quantities of
these compounds should be prepared and suitable precautions should be taken when they are
handled.
Physical measurements
1
Elemental analyses were performed on a Perkin Elmer Series II CHNS/O Analyzer 2400. H
NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. All the ligands were
purified on TELEDYNE CombiFlash Rf Flash chromatography. Electronic spectra were
recorded on an Agilent 8453 Diode Array Spectrophotometer. ESIꢀMS analyses were recorded
on a Micromass Quattro II triple quadrupole mass spectrometer. The products were analyzed by
6

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using Hewlett Packard (HP) 6890 GC series Gas Chromatograph equipped with a FID detector
and a HPꢀ5 capillary column (30 m × 0.32 mm × 2.5 ꢀm). GCꢀMS analysis was performed on an
Agilent GCꢀMS equipped with 7890A GC series (HPꢀ5 capillary column) and 5975C inert MSD
under conditions that are identical to that used for GC analysis.
Computational details
The geometry optimization of complexes have been performed with the B3LYP method using
3
9
Gaussian 09 program. The metal center was described with the Stuttgart RECPs and associated
4
0
41
basis sets (SDDALL) and all other atoms with the 6ꢀ31G** basis sets.
Crystallographic data collection, refinement and structure solution
The diffraction experiments were carried out for 1 an OXFORD XCALIBUR
diffractometer and for 4 data was collected using Bruker SMART APEX diffractometer both
equipped with a CCD area. The single crystals of 1 were mounted on a glass fiber in paraffin oil
and then brought into the cold nitrogen stream of a low temperature device so that the oil
solidified. Intensity data for the crystals were collected using MoKα (λ = 0.71073 Å) radiation
on OXFORD XCALIBUR diffractometer equipped with a CCD area detector at low
temperature. Empirical absorption correction using spherical harmonics, implemented in
4
2
SCALE3 ABSPACK scaling algorithm was applied. The structure was solved by direct
2
methods using the SHELXS and refined on F by the fullꢀmatrix leastꢀsquares technique
4
3
SHELXLꢀ2016/6. The single crystals of 4 data were collected at room temperature. An
4
4
empirical absorption correction was applied to the collected reflections with SADABS. The
structure was solved by heavy atom method and other nonꢀhydrogen atoms were located in
successive difference Fourier syntheses. Final cycles of leastꢀsquares refinements with the
modified data set improved both the Rꢀvalues and Goodness of Fit significantly. Hydrogen
atoms attached to the ligand moiety were located from the difference Fourier map. Crystal data
and additional details of the data collection and refinement of the structure are presented in
Table 1. The selected bond lengths and bond angles are listed in Table 2.
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Catalytic oxidations
The oxidation of alkanes was carried out under nitrogen atmosphere at room temperature
−
6
−3
within six hours. In a typical reaction, nickel(II) complex (0.35 × 10 mol dm ) was added to a
−3
−3
mixture of alkanes (2.45 mol dm ) and oxidant mꢀCPBA (0.35 mol dm ) in CH Cl :CH CN
2
2
3
mixture (3:1 v/v). After 6 hr the reaction mixture was quenched with triphenylphosphine and
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 was identified by Agilent GCꢀMS and quantitatively analyzed by
HP 6890 series GC equipped with HPꢀ5 capillary column (30 m × 0.32 mm × 2.5 ꢀm) using a
calibration curve obtained with authentic compounds. All of the products were quantified using
GC (FID) with the following temperature program: injector temperature 130 °C; initial
−1
temperature 60 °C, heating rate 10 °C min to 130 °C, increasing the temperature to 160 °C at a
−
1
−1
rate of 2 °C min , and then increasing the temperature to 260 °C at a rate of 5 °C min ; FID
temperature 280 °C. GCꢀMS analysis was performed under conditions identical to those used for
GC analysis. The averages of three measurements are reported.
Results and discussion
Synthesis and characterization of ligands and complexes
The ligands L1, L3 and L4 were prepared according to a known reductive amination
3
7
procedure by treating one equivalent of homopiperazine with two equivalents of the
corresponding aldehydes and using sodium triacetoxyborohydride as reducing agent. The ligand
L2 (Scheme 1) was synthesized by a two step procedure involving selective substitution of 2ꢀ
(chloromethyl)pyridine with homopiperazine in the presence of triethylamine, followed by
1
0b
reductive amination with 6ꢀmethylpyridineꢀ2ꢀcarboxaldehyde. The ligand L5 was synthesized
by substitution reaction with N,N’ꢀdimethylethylenediamine and picolylchloride hydrochloride
3
8
1
by using the previously reported procedure. All the ligands were characterized by H NMR.
The nickel(II) complexes 1–5 were prepared by adding one equivalent of ligand to a methanol
solution of Ni(ClO ) ·6H O, followed by the addition of sodium tetraphenylborate in acetonitrile
4
2
2
solvent. They are formulated as [Ni(L)(CH CN) ](BPh ) 1ꢀ3, 5 and [Ni(L4)](BPh ) 4 on the
3
2
4 2
4 2
basis of elemental analysis, UVꢀVisible spectroscopy, mass spectrometry and the Xꢀray crystal
structures of 1 and 4 confirm it. The pyridyl and sterically hindering 6ꢀmethylpyridyl and
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imidazolyl moieties are expected to determine the structural and electronic properties and
catalytic activities of the complexes towards alkane hydroxylation. Thus, interestingly, nickel(II)
complexes 1–4 of ligands with diaza cyclic diamine backbone possess trans coordination
geometry (Scheme 2), whereas 5 with ethylenediamine backbone possesses cisꢀα or cisꢀβ
configuration depending on solvent of coordination.
Description of structures of [Ni(L1)(CH CN) ](BPh ) 1 and [Ni(L4)](BPh ) 4
3
2
4 2
4 2
The molecular structures of complex cations of [Ni(L1)(CH CN) ](BPh ) 1 and
3
2
4 2
[
Ni(L4)](BPh ) 4 are shown in Fig. 1 and Fig. 2 respectively, together with atom numbering
4 2
scheme and the principal bond lengths and bond angles are collected in Table 2, respectively.
The complex cation of 1 possesses a NiN coordination sphere with a distorted octahedral
6
coordination geometry constituted by two pyridyl and two tertiary amine nitrogen atoms of the
linear tetradentate ligand and two acetonitrile molecules. The ligand L1 is meridionally
coordinated conferring a trans coordination geometry on Ni(II). The NiꢀN (2.065, 2.076 Å)
py
and NiꢀNamine (2.075, 2.077 Å) bond distances fall in the ranges observed for the previously
2
5ꢀ27,30ꢀ32
reported nickel(II) complexes (NiꢀN , 2.0451 – 2.1692, NiꢀNamine, 1.926 ꢀ 2.196 Å),
and
py
+
are close to those observed (NiꢀN , 2.057 ꢀ 2.060, NiꢀNamine, 2.052 – 2.054 Å) for [Ni(L1)Cl]
py
4
5
already reported. The NiꢀN2 bond length (2.065(2) Å) is shorter than the NiꢀN
bond
amine
py
2
3
lengths (2.075(2), 2.077(2) Å) due to sp and sp hybridization of pyridyl and amine nitrogen
donors respectively. However, the NiꢀN2 bond (2.065(2) Å) is slightly shorter than the NiꢀN6py
py
bond (2.076(3) Å). The nitrogen atoms of both the acetonitrile molecules are coordinated less
strongly than the other four nitrogen atoms of L1, as expected. Further, the NiꢀN8ACN bond
(2.184(3) Å) is longer than the NiꢀN10ACN bond (2.097(3) Å) as it is oriented along the sterically
hindering trimethylene side of diazapane ring. The NꢀNiꢀN (77.63(10)° ꢀ 117.22(10)°) and NꢀNiꢀ
N (159.69(10)° ꢀ 164.92(10)°) bond angles deviate from the ideal octahedral angles of 90° and
1
80° respectively, revealing that the octahedral coordination geometry around Ni(II) is
significantly distorted.
The meridional coordination of L1 resulting in trans coordination geometry in 1 is
1
0c
similar to that found in [Mn(L1)Cl2] with a distorted octahedral coordination geometry, and
2+ 46
[
Mn(L1)(CH CN) ]
with a sevenꢀcoordinate distorted pentagonal bipyramidal geometry.
3
3
6
a
Also, the ligand L1 has been predicted to coordinate meridionally to iron(II) in the in situ
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2+
generated complex species [Fe(L1)(CH CN) ] . The ligand 1,5ꢀbis(2ꢀpyridylmethyl)ꢀ1,5ꢀdiazaꢀ
3
2
cyclooctane, analogous to L1, has been shown to form square planar as well as square pyramidal
4
7
Ni(II) complexes. Interestingly, this is in contrast to the folded geometry of linear N4 ligands
with ethylenediamine backbone, which confer cisꢀα or cisꢀβ octahedral coordination geometries
on Ni(II) (Scheme 2). Thus, the linear ligand L5, which is the ethylenediamine analogue of L1,
48a
2+
48b
forms the highꢀspin complexes [Ni(L5)Cl ]·H O , [Ni(L5)(H O) ]
and
2
2
2
2
+
48b
[
Ni(L5)(OAc)(H O)]
, all of which possess octahedral coordination geometry with cisꢀα
2
Sol
Ni
N
N
Sol
N
Sol
N
N
N
Ni
Ni
N
N
N
Sol
N
N
N
Sol
Sol
cisꢀβ
trans
cisꢀ
α
Scheme 2. Different octahedral configurations adopted by tetradentate N4 ligands
2
+ 5c
configuration. The cisꢀα coordination of L5 is similar to that found in [Fe(L5)(CH CN) ]
and
3
2
4
9
[
[
Mn(L5)Cl ]. On the other hand, interestingly, in the iron(III) tetrachlorocatecholate adduct
2
+
37
Fe(L1)(cat)] , sterically encumbered ligand L1 is bound with a cisꢀβ configuration, revealing
that the mode of coordination of N4 ligands with diazapane backbone depends on the metal ion
and the secondary ligand.
2
+
The molecular structure of [Ni(L4)](BPh ) 4 consists of a discrete [Ni(L4)] cation and
4
2
̅
two BPh counter anions (Fig. 2). The NiN coordination sphere of the complex cation shows a
4
4
distorted square planar coordination geometry constituted by two imidazole and two tertiary
amine nitrogen atoms of meridionally coordinated ligand L4 with its diazacyclo ligand back
bone in normal boat conformation. The observed NiꢀN bond lengths (1.869ꢀ1.915 Å) and bond
angles are in agreement with those observed for the previously reported square planar Ni(II)
45,50
complexes.
1.909(3), 1.915(3) Å) on account of sp and sp hybridization of imidazole and amine nitrogens
respectively. The NꢀNiꢀN (82.83(14)° ꢀ 103.25(16)°) and NꢀNiꢀN (167.46(15)° ꢀ 170.72(14)°)
The NiꢀN bonds (1.869(3), 1.892(3) Å) are shorter than the NiꢀN
bonds
im
amine
2
3
(
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bond angles deviate from the ideal square planar angles of 90° and 180° respectively, revealing
that there is significant distortion in the square planar coordination geometry. Similar lowꢀspin
square planar coordination geometry has been observed for Ni(II) complexes of meridionally
4
5
coordinating tetradentate N4 ligands 1,4ꢀbis(imidazolꢀ4ꢀylmethyl)ꢀ1,4ꢀdiazepane, and 1,4ꢀ
5
0
bis(3ꢀmethylimidazolꢀ4ꢀylmethyl)ꢀ1,4ꢀdiazacycloheptane. Also, the ligand L1, which forms the
octahedral complex [Ni(L1)(CH CN) ](BPh ) 1 (cf. above), also forms the lowꢀspin square
3
2
4 2
2
+
+
planar complex [Ni(L1)] as well as highꢀspin square pyramidal complex[Ni(L1)Cl] (Scheme
4
5
+
3
). As imidazole is a base stronger than pyridine [pK (BH ): pyridine, 5.2; imidazole, 7.01],
a
both the NiꢀN bonds (1.869 ꢀ 1.892 Å) in 4 are shorter than the NiꢀN bonds (1.906 ꢀ 1.927 Å)
im
py
NCCH3
Cl
N
N
N
N
N
N
Clꢀ
+ Clꢀ
+
Ni
Ni
Ni
N
N
N
N
N
N
NCHCH3
2
+
[Ni(L1)Cl]+
[Ni(L1)]2+
[
Ni(L1)(CH CN) ]
3
2
Scheme 3. Different coordination geometries adopted by L1 ligand: [Ni(L1)(CH CN) ](BPh ) 1
3
2
4 2
+
45
2+ 45
(
this work), [Ni(L1)Cl] , and [Ni(L1)]
2
+ 45
in [Ni(L1)] . Further, the L1 and L4 ligands support a square pyramidal coordination
+
45
+ 45
geometry for [Ni(L1)Cl]
and [Ni(L4)Cl]
(τ, 0.028), both with a highꢀspin configuration;
the NiꢀN (2.000, 2.010 Å) and NiꢀN
(2.091, 2.112 Å) bonds are longer than those observed
im
amine
2
+
here for lowꢀspin [Ni(L4)] complex, as expected (cf. above). Thus, the present N4 ligand
systems with diazapane backbone display the ability to confer a range of coordination
geometries (Scheme 3).
DFT studies: Structures of nickel(II) complexes
DFT studies have been carried out to understand the effect of incorporating methyl
substituent(s) on pyridyl ring(s) and replacing pyridyl nitrogen by imidazolyl nitrogen on the
ability of ligands to form complexes with lowꢀspin configuration. The optimized geometries of 1
and 4 agree well with those in their Xꢀray crystal structures (cf. above), suggesting that the
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optimized structures of 1ꢀ5 are reliable in understanding the structural variations (Fig. 3, Fig. 4
and Fig. S1 see in supporting information, ESI †). Upon incorporating a methyl substituent in 1
to obtain 2, the terminal NiꢀN bonds are elongated (1: NiꢀN1 , 2.12; NiꢀN4 , 2.12; 2: Niꢀ
py
py
py
N1 , 2.22; NiꢀN4 , 2.21 Å) with the NiꢀN bonds remaining almost unaffected, and
amine
py
py
concomitantly, the NiꢀNACN bonds are shortened (1: 2.26, 2.17; 2: 2.21, 2.14 Å). Similarly, upon
incorporating a methyl substituent on the other pyridine ring in 2 to get 3, the terminal NiꢀN4py
bond is elongated further though the NiꢀN1 bond is slightly shortened (NiꢀN1 , 2.20; NiꢀN4 ,
py
py
py
2.27 Å) with the NiꢀNamine bonds affected only slightly. Interestingly, the NiꢀN5ACN bond is
shortened further (2.19 Å) and the NiꢀN6ACN bond is elongated slightly (2.17 Å). Thus, the effect
of incorporating sterically hindering methyl substituent(s) on the terminal pyridyl ring(s) in 1 to
obtain 2 and 3 is to shorten the trans axial bonds formed by solvent molecules, which has a very
important consequence on the reactivity of complexes (cf. below). The optimized structure of
2
+
[
[
Ni(L5)(CH CN) ] 5 exhibits a distorted octahedral cisꢀβ coordination geometry; however,
3 2
2
+
Ni(L5)(H O) ] 5a (Fig. S2 in ESI †) exhibits a distorted octahedral cisꢀα coordination
2
2
2
+ 48
geometry, which is found in the Xꢀray crystal structure of [Ni(L5)(H O) ] . This interesting
2
2
observation illustrates that the nature of coordinated solvent determines the mode of
coordination of ethylenediamineꢀbased linear N4 ligands. The calculated bond distances for 5
(
NiꢀN , 2.11, 2.14; NiꢀN
, 2.17,2.20 Å) and 5a (NiꢀN , 2.16, 2.21; NiꢀNamine, 2.18, 2.21 Å)
amine py
py
are in reasonably good agreement with those (NiꢀN , 2.074, 2.104; NiꢀNamine, 2.160 Å)
py
2
+ 48
determined experimentally for [Ni(L5)(H O) ] . Interestingly, the optimized structures of both
2
2
2+
2+
[
Ni(pdao)(H O) ] 6 and [Ni(pdao)(CH CN) ] 6a (pdao = 1,8ꢀbis(2ꢀpyridyl)ꢀ3,6ꢀdimethylꢀ
2 2 3 2
3,6ꢀdiazaoctane) possess an octahedral cisꢀα coordination geometry (Fig. S1, S2 in ESI †), which
4
8a
is in good agreement with the experimentally observed one for 6. This is expected as the
,6,6,5 chelate ring system formed by pdao is more flexible than the 5,5,5,5 chelate ring system
in 5 and 5a.
To understand the ability of L4 ligand with diazapane back bone to confer a lowꢀspin
square planar configuration on Ni(II), the optimized electronic structures of 1ꢀ6 with highꢀ (S =
5
3
9
1
) and lowꢀspin (S = 0) configurations have been computed (Fig. 3, Fig. S1 in ESI†). The
complexes 1ꢀ3, 5 and 6 possess highꢀspin ground states which are more stable than the
corresponding lowꢀspin states by +11.0, +26.1, +19.4, +28.0 and +22.0 kcal/mol respectively. In
the highꢀspin state, all the complexes, as expected, show an octahedral coordination geometry in
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which acetonitrile molecules are coordinated in trans positions and the two unpaired electrons
are found mostly on the Ni(II) center while the lowꢀspin complexes exhibit distorted fourꢀ
coordinate geometries. The optimized lowꢀspin structure (4 ) exhibits a square planar geometry,
ls
as seen in the Xꢀray crystal structure of 4, whereas the optimized highꢀspin structure (4 ) shows
hs
a distorted square planar coordination geometry with all the four NiꢀN bonds being weaker than
those in 4 . It is clear that two solvent molecules are required to coordinate to Ni(II) to form a
ls
stable distorted octahedral geometry as for 1 . And, notably, the 4 structure is computed to be
hs
ls
more stable than 4 by +5.8 kcal/mol only. The computed structures of 1ꢀ5 are useful in
hs
illustrating their potential as catalysts for alkane oxidation (cf. below).
Electronic spectral properties
The electronic spectral data of all the Ni(II) complexes are summarized in Table 3 and
the typical electronic absorption spectrum of 1ꢀ5 is shown in Fig. 5. In CH Cl :CH CN (3:1 v/v)
2
2
3
solvent mixture, all the complexes, except 4, exhibit two wellꢀdefined absorption bands (470–
565, 840–924 nm) together with an absorption spectral feature on the low energy band. The
3
3
lower energy band is assigned to A → T (F) (ν ) transition and the higher energy band to
2
g
2g
1
3
3
A → T (F) (ν ) transition while the feature located on the low energy band is assigned to
2
g
1g
2
3
1
30ꢀ32,51
A → E (D) spinꢀforbidden transition in Ni(II) located in an octahedral environment.
2
g
1g
3
3
The intense shoulder observed in the range 355–385 nm is assigned to A → T (P) (ν )
2
g
1g
3
transition. All these assignments are in good agreement with those observed for the previously
3
0ꢀ32,48a,51,52
reported octahedral Ni(II) complexes.
When one or both the pyridylmethyl arms in 1
are replaced by 6ꢀmethylpyridylmethyl arms to get 2 or 3, the ν band energy decreases, which is
1
expected of the weaker coordination of sterically hindering 6ꢀmethylpyridyl nitrogen. Also, upon
replacing the acyclic diamine in 5 by the cyclic diamine backbone as in 1, the ν band energy
1
increases, which illustrates that the N4 ligands with diazapane backbone possess chelating
ability higher than the analogous acyclic N4 ligand (Fig. 5a). Interestingly, when both the
pyridylmethyl arms in 1 are replaced by imidazolylmethyl arms to get 4, the ν and ν bands
1
2
1
1
disappear and a peak around 446 nm is observed. This feature is assigned to A → A ligand
1
g
2g
4
5,50
field transition in Ni(II) with a square planar coordination geometry,
(Fig. 5b) and this
confirms that N4 ligands with diazapane backbone possess higher ligand field strengths.
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Functionalization of alkanes
The catalytic activity of 1ꢀ5 towards oxidation of cyclohexane and adamantane was
investigated by using mꢀCPBA as oxidant in CH Cl :CH CN solvent mixture (3:1 v/v) at room
2
2
3
temperature and the results are collected in Tables 4ꢀ6. When H O /tꢀBuOOH was used as
2
2
oxidant, only trace amounts of oxidized products were detected (results are not shown here),
revealing that the complexes are not effective as catalysts. In contrast, when mꢀCPBA was used
as oxidant, 1–5 act as good catalysts; cyclohexane is oxidized efficiently with 340ꢀ601 total
TON, with cyclohexanol (A) as the major product (TON, 290ꢀ527) and cyclohexanone (K; TON,
12ꢀ22) and εꢀcaprolactne (L) as the minor product (TON, 34ꢀ70), the alcoholꢀketone selectivity
(A/K, 5.6ꢀ7.2) being appreciable. Thus, only very small amounts of the oxidized products
(cyclohexane, 8 TON; adamantane, 12 TON) were observed in the absence of complexes with
3
1, 53
mꢀCPBA as oxidant.
The product εꢀcaprolactone is the over oxidized product of oxidation of
cyclohexanone, effected by excess or unreacted mꢀCPBA. Also, the total TON for cyclohexane
oxidation under air is lower than that under nitrogen atmosphere, revealing that any radical
intermediate generated during catalytic cycle (cf. below) is quenched by dioxygen/moisture in
air, leading to the decrease in yield. Also, the complexes catalyze the selective oxidation of
adamantane to give up to 676 total TON with good to high regioselectivity between the tertiary
(3°) and secondary (2°) carbons (3°/2° bond selectivity, 11.7).
The complex 1 catalyzes the oxidation of cyclohexane to give a total TON of 542 (TON:
A, 460; K, 12; A/K, 5.6; L,70), which corresponds to 54% conversion of the oxidant to organic
products. In order to illustrate the products formed and the trend in catalytic activity, we propose
that mꢀCPBA binds to nickel(II) by replacing the labile acetonitrile molecules in
2+
II
+
[
Ni(L1)(CH CN) ] to give the intermediate adduct species [Ni (L1)(OOCOC H Cl)] . The
3 2 6 4
II
latter undergoes OꢀO bond homolysis leading to the formation of putative [(L1)Ni ꢀ
•
III
O ]/[(L1)Ni ꢀO] intermediate species and mꢀchlorobenzoic acid radical (Scheme 4). The
3
0ꢀ32
formation of chlorobenzene (69–75% corresponding to total TON) supports this pathway.
We did not observe other products like mꢀchlorophenol or mꢀchlorobenzoic acid, which excludes
the contribution of heterolytic OꢀO bond cleavage pathway. The highꢀspin (S = 3/2) radical
II
•
intermediate [(L1)Ni ꢀO ] (rad ) then abstracts the equatorial hydrogen atom of cyclohexane to
int
II
+
31
form [(L1)Ni (OH)] species and a cyclohexyl radical (Scheme 5). Subsequently, the latter
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3
1
reacts over through the CꢀO rebound step to form the cyclohexanol product. DFT calculations
on Ni(II) complexes of N5 ligands support the homolytic cleavage
O
+
Cl
Cl
HO O
O
O
O
O
O
OH
II
2+
(N4)NiII
[
(N4)Ni (CH CN) ]
O
3 2
m-CPBA
O
Cl
O
-CO2
(N4)Ni (O)].
II
[
Cl
or
(N4)Ni (O)]+
III
[
Scheme 4. Proposed mechanism of alkane hydroxylation
+
+
II
II
+
Ni (L1ꢀL5)
Ni (L1ꢀL5)
I
Ni (L1ꢀL5)
O
H
OH
H
HO
H
H
reactants
rebound
product
radint
Scheme 5. Proposed mechanism for [(L1ꢀL5)Ni ꢀO•]+ catalyzed cyclohexane
II
oxidation/hydroxylation.
II
+
of OꢀO bond in [Ni (N5)(OOCOC H Cl)] to give the highꢀspin (S = 3/2) intermediate species
6
4
II
•
III
[
(N5)Ni ꢀO ]/[(N5)Ni ꢀO]. We have previously invoked such an intermediate species to
3
0
31
successfully illustrate the trends in catalytic activity of nickel(II) complexes of N4 and N5
II
+
ligands towards alkane hydroxylation. The adduct species [Ni (L1)(OOCOC H Cl)] is
6
4
suggested as fiveꢀcoordinate with the bulky mꢀCPBA anion bound to Ni(II) in the axial position
II
+ 45
II
+ 45
on the basis of the Xꢀray structures (cf. above) of [Ni (L1)Cl]
and [Ni (L4)Cl] . Thus, all
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the present N4 ligands with diazapane backbone tend to form fourꢀ, fiveꢀ and sixꢀcoordinate
complexes, but make available only one axial site effectively for binding of oxidant. This is in
2
+
contrast to [Ni(L5)(CH CN) ] 5, which makes available two cis coordination sites, leading to
3
2
higher catalytic activity.
The time course of the TON for 1 in cyclohexane oxidation is shown in Fig. 6 and Table
, which clearly indicates that the catalytic activity of the complex gradually proceeds even after
0 minutes. The catalytic activity increases from the total TON of 75 to 542 with the alcohol
4
3
selectivity (A/K) also increasing from 1.1 to 5.7. When the number of equivalents of mꢀCPBA is
increased with an aim to enhance the concentration of the intermediate and hence the reactivity,
the amount of εꢀcaprolactone formed increases, confirming that εꢀcaprolactone results from
secondary oxidation of cyclohexanone by the oxidant.
Effect of ligand donors
It is interesting that the catalytic activity of 1ꢀ5 towards cyclohexane oxidation varies as
5
> 1 > 2 > 3 > 4. Upon moving from 1 to 2, cyclohexane is catalytically oxidized with decrease
in the total TON from 542 to 494 (TON: A, 422; K, 17; L, 55) and slight increase in alcohol
selectivity (A/K, 5.9). When methyl groups are incorporated on both the pyridyl rings in 1 to get
3
, the total TON decreases further, from 542 to 456 (TON: A, 393; K, 15; L, 48; A/K 6.2) with
slight increase in alcohol selectivity. The methyl group(s) introduced at 6ꢀposition of one (2) or
both (3) the pyridyl rings sterically hinder(s) the coordination of pyridyl nitrogen and stabilizes
the reactive nickelꢀoxo species and hence the lower catalytic activity of 2 and 3. Also, the
incorporation of methyl substituent(s) renders the binding of solvent molecules stronger (cf.
above), making their substitution by the oxidant difficult. Upon replacing both the pyridylmethyl
arms in 1 by two imidazolylmethyl arms to obtain 4, the oxidation of cyclohexane takes place
with substantial decrease in total TON from 542 to 340 (TON: A, 290; K, 16; L, 34) but with
+
enhancement in selectivity (A/K, 7.2). The strongly σꢀbonding [pK (BH ): pyridine, 5.2;
a
imidazole, 7.01], but weakly πꢀbonding imidazolyl nitrogen causes spinꢀpairing in 4, which
II
•
renders the formation of reactive highꢀspin (S = 3/2) intermediate [(L4)Ni ꢀO ] species difficult,
causing a decreased reactivity. It should be noted that only a very small amount of energy (+5.8
kcal/mol) is required to convert the lowꢀspin catalyst to a reactive highꢀspin species.
Interestingly, when the cyclic diamine moiety in 1 is substituted by ethylenediamine moiety to
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obtain 5, cyclohexane is catalytically oxidized with increase in TON from 542 to 601 (TON: A,
27; K, 22: L, 52) as well as alcohol selectivity (A/K, 7.1). The cisꢀβ coordination geometry of 5
5
favors formation and stabilization of the reactive nickelꢀoxo intermediate. It is interesting that
the presence of two cisꢀlabile coordination sites is the key structural aspect for the improved
2+
48,54
catalysis of water oxidation by [Ni(L5)(H O) ] with cisꢀα coordination geometry
We have
2
2
observed that the cisꢀα complex [Ni(isoꢀBPMEN)(CH CN) ](BPh ) , where isoꢀBPMEN is N,Nꢀ
3
2
4 2
3
0
dimethylꢀN’,N’ꢀbis(pyridꢀ2ꢀylmethyl)ethaneꢀ1,2ꢀdiamine, gives enhanced total TON of 622.
2
5ꢀ27,30
Similar enhanced catalytic activity has been observed previously for nickel(II) complexes
with two labile cis positions.
Adamantane oxidation
Adamantane is efficiently oxidized by 1ꢀ 5 to give 1ꢀadamantanol and 2ꢀadamantanol as
the major products along with a small amount of 2ꢀadamantanone as the minor product (Table
6). The trend in the catalytic activity is 5 > 1 > 2 > 3 > 4, which is the same as that for
cyclohexane oxidation; this suggests the involvement of the same highꢀvalent nickelꢀoxo
intermediate species in adamantane oxidation also. Complex 1 catalyzes the oxidation of
adamantane to give a total TON of 589 with high 3°/2° bond selectivity (9.5). Upon moving
from 1 to 2 the total TON decreases to 564 but with a slight increase in 3°/2° bond selectivity
(10.2). Similarly, upon moving from 1 to 3, the total TON decreases further to 525 with slight
increase in 3º/2º bond selectivity (9.9), supporting the steric effect of methyl substituent on
II
•
pyridyl ring (cf. above) in destabilizing the reactive intermediate species [(L1)Ni ꢀO ]. Upon
moving from 1 to 4 the oxidation of adamantane takes place with decrease in both the total TON
(419) and 3°/2° bond selectivity (8.7). The replacement of both the pyridylmethyl arms in 1 by
two imidazolylmethyl arms leads to spinꢀpairing in 4, which renders formation of the reactive
II
•
intermediate [(L4)Ni ꢀO ] species with highꢀspin (S = 3/2) configuration difficult, causing a
decrease in reactivity. Interestingly, as observed for cyclohexane oxidation, the replacement of
the cyclic diazapane backbone in 1 by ethylenediamine backbone to get 5, both the total TON
(676) and 3°/2° bond selectivity (11.7) increase. This supports the above finding that labile cis
sites on nickel(II) facilitate ligand exchange with the oxidant, leading to good catalytic activity
and selectivity.
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Thus, a good πꢀback bonding pyridine donor facilitates the formation and stabilization of
highꢀvalent nickelꢀoxo species; however, incorporation of sterically demanding 6ꢀmethyl
substituent adjacent to pyridyl nitrogen destabilizes it. Also, the ability of the N4 ligands with
diazacyclo back bone have the potential to confer both highꢀ and lowꢀspin configurations on the
Ni(II) catalysts, as dictated by pyridyl and imidazolyl nitrogens, and also steric hindrance to
coordination would determine the catalytic activity towards alkane oxidation.
Conclusions
A few novel nickel(II) complexes of tetradentate N4 ligands with diazacyclo back bone
have been isolated, their coordination geometries assessed and their potential to carry out alkane
hydroxylation using mꢀCPBA as oxidant investigated. The Ni(II) complex of bis(pyridyl)ꢀdiaza
ligand possesses a distorted octahedral coordination geometry whereas its bis(imidazole)ꢀdiaza
analogue possesses a lowꢀspin distorted square planar geometry. DFT studies throw light on the
role of donor atom type, steric hindrance to coordination of pyridyl nitrogen, incorporation of
diaazcyclo back bone and chelate ring sizes in conferring on Ni(II) novel coordination
geometries ꢀ octahedral, highꢀspin square pyramidal and lowꢀspin square planar. The present N4
ligands with diazacyclo back bone meridionally coordinate to Ni(II) but provide only one labile
axial site for binding of the oxidant, and catalyze the oxidation of cyclohexane and adamantane
with appreciably high TON and selectivity. In contrast, the corresponding linear N4 ligand with
ethylenediamine back bone, which favor cisꢀα or cisꢀβ octahedral configuration depending on
solvent of coordination, provides two cis labile sites leading to higher catalytic activity. Thus, a
robust Ni(II) catalyst for alkane oxidation may be designed by incorporating strongly πꢀback
bonding heterocyclic nitrogen donors on a linear ethylenediamineꢀbased tetradentate ligand.
Acknowledgments
This work is supported by the Science and Engineering Research Board (SERB) (Scheme No.
EMR/2015/002222). Professor M. Palaniandavar is Indian National Science Academy (INSA)
Honorary Scientist. We sincerely thank the Council of Scientific and Industrial Research (CSIR),
New Delhi, for the Senior Research Fellowship to M.S and the Department of Science and
Technology, SERB, for the award of Fastꢀtrack Young Scientist Fellowship to P.V. (Ref. No.
CSꢀ232/2012). We thank Department of Science and Technology (FIST program), New Delhi
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for the use of the Xꢀray diffractometer at School of Chemistry. We thank Dr. K. Maheswari
deceased) for collecting the Xꢀray data for complex 1. We thank Dr. Balachandran Unni Nair,
(
Central Leather Research Institute, Chennai for providing the ESIꢀMS facility.
Supporting information
The crystal structures of 1 and 4 have been deposited at the Cambridge Crystallographic Data
Centre and the deposition numbers allotted are CCDC 1400220 and CCDC 1400221. The data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223ꢀ336ꢀ033; or eꢀmail: deposit@ccdc.cam.ac.uk.
References
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2
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Table 1. Crystallographic data for complexes 1 and 4
1
4
Empirical formula
C H B N Ni
C H B N Ni
63 64 2 6
6
9
68
2
6
–
1
Formula weight /g mol
1061.63
Purple
985.53
Crystal habit, color
Yellow
Crystal system
Crystal size
Space group
a, Å
Monoclinic
Monoclinic
0.12 x 0.09 x 0.06 mm
0.23 x 0.10 x 0.09 mm
P2 /c
P2 /n
1
1
11.6625(4)
13.6676(4)
35.5420(11)
90
16.5550(7)
12.0464(4)
27.2008(13)
90
b, Å
c, Å
α, deg
β
, deg
95.268 (3)
90
106.496(5)
90
γ, deg
3
V/Å
5641.4(3)
4
5201.3(4)
21
Z
ꢀ3
ρcalcd/ g cm
F(000)
1.250
1.259
2248
2088
Temperature (K)
150
293(2)
65074
No. of Reflections collected
No. of unique reflections
Radiation(MoKα)/ Å
30859
13214 (R(int) = 0.0757)
14036 (R(int) = 0.1017)
0.7107
0.941
706
0.7107
0.790
650
2
Goodnessꢀofꢀfit on F
Number of refined Parameters
a
R1/ w R2[I > 2σ(I)]
R
1
= 0.0669, wR
2
= 0.0735
R
1
= 0.0698, wR
= 0.1920
2
R1/ w R2 (all data)
R = 0.1458, wR = 0.0921
R = 0.1483, wR = 0.2689
1
2
1
2
a
2
o
2 2
4
o
1/2
R1 = [Σ(||F |ꢀ|F ||)/Σ|F |]; wR2 = {[Σ(w(F ꢀF ) )/Σ(wF )] }
o
c
o
c
24

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Table 2. Selected bond lengths [Å] and bond angles [°] for 1 and 4
1
4
Bond lengths/Å
Ni(1)ꢀN(2)
Ni(1)ꢀN(3)
Ni(1)ꢀN(5)
Ni(1)ꢀN(6)
Ni(1)ꢀN(8)
Ni(1)ꢀN(10)
2.065(2)
2.075(3)
2.077(2)
2.076(3)
2.184(3)
2.097(3)
Ni(1)ꢀN(1)
Ni(1)ꢀN(2)
Ni(1)ꢀN(3)
Ni(1)ꢀN(4)
1.869(3)
1.915(3)
1.909(3)
1.892(3)
Bond angles/ °
N(2)ꢀNi(1)ꢀ N(3)
N(2)ꢀNi(1)ꢀ N(5)
N(2)ꢀNi(1)ꢀ N(6)
N(2)ꢀNi(1)ꢀ N(8)
N(2)ꢀNi(1)ꢀ N(10)
N(3)ꢀNi(1)ꢀ N(5)
N(3)ꢀNi(1)ꢀ N(6)
N(3)ꢀNi(1)ꢀ N(8)
N(3)ꢀNi(1)ꢀ N(10)
N(5)ꢀNi(1)ꢀ N(6)
N(5)ꢀNi(1)ꢀ N(8)
N(5)ꢀNi(1)ꢀ N(10)
N(6)ꢀNi(1)ꢀ N(8)
N(6)ꢀNi(1)ꢀ N(10)
N(8)ꢀNi(1)ꢀ N(10)
82.68(10)
159.94(11)
117.22(10)
86.86(10)
85.55(11)
77.63(10)
159.69(10)
98.70(11)
93.24(11)
82.72(10)
99.79(10)
91.72(10)
79.51(10)
92.43(11)
164.92(10)
N(1)ꢀNi(1)ꢀ N(2)
N(1)ꢀNi(1)ꢀ N(3)
N(1)ꢀNi(1)ꢀ N(4)
N(2)ꢀNi(1)ꢀ N(3)
N(2)ꢀNi(1)ꢀ N(4)
N(3)ꢀNi(1)ꢀ N(4)
87.45(14)
170.27(14)
103.25(16)
82.83(14)
167.46(15)
86.41(16)
25

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a
Table 3. UVꢀvisible spectral data of nickel(II) complexes 1ꢀ5 in CH Cl :CH CN solvent mixture
2
2
3
(3:1 v/v) at 25.0 °C
–
1
–1
Complex
λmax, nm (ε, M cm )
1
2
472 (48),785 (35), 842 (36)
382 (103), 496 (sh), 600 (30),798 (27), 852 (29), 924 (34)
3
4
382 (125), 487 (24), 588 (37),763 (32), 893 (19)
243 (1820), 280 (145), 446 (130)
5
358 (sh), 565 (18), 773 (6), 924 (10)
Concentration of nickel(II) complexes, 1.0 × 10 M
a
−2
26

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a
Table 4. Products of oxidation of cyclohexane catalyzed by 1 with time
d
e
Time
min.)
Cyclohexane (TON)
Total
TON
A/K
Yield
(%)
c
b
b
(
ꢀol
ꢀone
εꢀcaprolactone
3
0
55
176
358
424
460
8
17
18
14
12
12
25
45
63
75(3)
1.1
4.2
5.7
5.5
5.6
7.5
60
218(3)
421(2)
501(2)
542(1)
21.8
42.1
50.1
54.2
1
2
7
20
40
20
70
a
–6
–3
–3
Reaction conditions: Catalyst (0.35 × 10 mol dm ), Substrate (2.45 mol dm ), Oxidant (0.35
–
3
b
mol dm ) in CH Cl : CH CN solvent mixture (3:1 v/v); ꢀol = cyclohexanol and ꢀone =
cyclohexanone; Total TON = no. of mmol of product/no. of mmol of catalyst; A/K = TON of ꢀ
ol/ (TON of ꢀone + TON of εꢀcaprolactone); Yield based on the oxidant.
2
2
3
c
d
e
a
Table 5. Products of oxidation of cyclohexane catalyzed by Ni(II) complexes
d
e
Complex Cyclohexane (TON)
Total
TON
Chloroꢀ A/K
benzene
Yield
(%)
c
b
b
ꢀol
ꢀone
εꢀcaprolactone
f
Blank
6.0
1.0
12
17
15
16
22
1.0
70
55
48
34
8.0
ꢀ
3.0
5.6
5.9
0.8
54.2
49.4
45.6
34.0
1
2
3
4
460
422
393
290
527
542(2) 376
494(3) 352
456(2) 332
340(3) 240
6.2
7.2
7.1
5
52
601(1) 453
60.1
a
–6
–3
–3
Reaction conditions: Catalyst (0.35 × 10 mol dm ), Substrate (2.45 mol dm ), Oxidant (0.35
–
3
b
mol dm ) in CH Cl : CH CN solvent mixture (3:1 v/v); ꢀol = cyclohexanol and ꢀone =
cyclohexanone; Total TON = no. of mmol of product/no. of mmol of catalyst; A/K = TON of ꢀ
ol/ (TON of ꢀone + TON of εꢀcaprolactone); Yield based on the oxidant. Blank = Substrate
(
2
2
3
c
d
e
f
–3
–3
2.45 mol dm ), Oxidant (0.35 mol dm ) in CH Cl :CH CN solvent mixture (3:1 v/v).
2
2
3
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Table 6. Products of oxidation of adamantane catalyzed by Ni(II) complexes
d
e
Complex Adamantane (TON)
Total
TON
Selectivity
3°/2°
6.0
Yield
c
b
b
b
1ꢀadol
2ꢀadol
2ꢀadone
(%)
f
Blank
8.0
2.4
1.6
10
16
12
12
18
12.0
1.2
58.9
56.4
52.5
41.9
67.6
1
2
3
4
5
448
436
403
312
538
131
112
110
95
589(2)
9.5
564(2) 10.2
525(2)
419(3)
9.9
8.7
120
676(2) 11.7
a
–6
–3
–3
Reaction conditions: Catalyst (0.2 × 10 mol dm ), Substrate (0.4 mol dm ), Oxidant (0.2 mol
–3
b
dm ) in CH Cl : CH CN solvent mixture (3:1 v/v); 1ꢀadol = 1ꢀadamantanol, 2ꢀadol = 2ꢀ
adamantanol and 2ꢀadone = 2ꢀadamantanone; TON = no. of mmol of product/no. of mmol of
catalyst; 3°/2° = (TON of 1ꢀadol×3)/(TON of 2ꢀadol+ TON of 2ꢀadone); Yield based on the
oxidant. Blank = Substrate (0.4 mol dm ), Oxidant (0.2 mol dm ) in CH Cl :CH CN solvent
2
2
3
c
d
e
f
–3
–3
2
2
3
mixture (3:1 v/v).
28

Page 29 of 35
Dalton Transactions
View Article Online
DOI: 10.1039/C7DT00576H
2
+
Fig. 1 ORTEP diagram of [Ni(L1)(CH CN) ] complex 1 showing 45% probability thermal
3
2
ellipsoids and the labeling scheme for selected atoms. All hydrogen atoms are omitted for clarity.
29