Nickel(ii) complexes of tripodal 4N ligands as catalysts for alkane oxidation using m-CPBA as oxidant: Ligand stereoelectronic effects on catalysis

Article abstract of DOI:10.1039/c1dt10902b

Several mononuclear Ni(ii) complexes of the type [Ni(L)(CH ₃CN)₂](BPh₄)₂1-7, where L is a tetradentate tripodal 4N ligand such as N,N-dimethyl-N′,N′- bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L1), N,N-diethyl-N′,N′- bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L2), N,N-dimethyl-N′-(1-methyl- 1H-imidazol-2-ylmethyl)-N′-(pyrid-2-ylmethyl)ethane-1,2-diamine (L3), N,N-dimethyl-N′,N′-bis(1-methyl-1H-imidazol-2-ylmethyl)ethane-1, 2-diamine (L4), N,N-dimethyl-N′,N′-bis(quinolin-2-ylmethyl)ethane-1, 2-diamine (L5), tris(benzimidazol-2-ylmethyl)amine (L6) and tris(pyrid-2- ylmethyl)amine (L7), have been isolated and characterized using CHN analysis, UV-Visible spectroscopy and mass spectrometry. The single-crystal X-ray structures of the complexes [Ni(L1)(CH₃CN)(H₂O)](ClO ₄)₂1a, [Ni(L2)(CH₃CN)₂](BPh ₄)₂2, [Ni(L3)(CH₃CN)₂](BPh ₄)₂3 and [Ni(L4)(CH₃CN)₂](BPh ₄)₂4 have been determined. All these complexes possess a distorted octahedral coordination geometry in which Ni(ii) is coordinated to four nitrogen atoms of the tetradentate ligands and two CH₃CN (2, 3, 4) or one H₂O and one CH₃CN (1a) are located in cis positions. The Ni-N_py bond distances (2.054(2)-2.078(3) A) in 1a, 2 and 3 are shorter than the Ni-N_amine bonds (2.127(2)-2.196(3) A) because of sp² and sp³ hybridizations of the pyridyl and tertiary amine nitrogens respectively. In 3 the Ni-N_im bond (2.040(5) A) is shorter than the Ni-N_py bond (2.074(4) A) due to the stronger coordination of imidazole compared with the pyridine donor. In dichloromethane/acetonitrile solvent mixture, all the Ni(ii) complexes possess an octahedral coordination geometry, as revealed by the characteristic ligand field bands in the visible region. They efficiently catalyze the hydroxylation of alkanes when m-CPBA is used as oxidant with turnover number (TON) in the range of 340-620 and good alcohol selectivity for cyclohexane (A/K, 5-9). By replacing one of the pyridyl donors in TPA by a weakly coordinating -NMe₂ or -NEt₂ donor nitrogen atom the catalytic activity decreases slightly with no change in the selectivity. In contrast, upon replacing the pyridyl nitrogen donor by the strongly σ-bonding imidazolyl or sterically demanding quinolyl/benzimidazolyl nitrogen donor, both the catalytic activity and selectivity decrease, possibly due to destabilization of the intermediate [(4N)(CH₃CN)Ni-O] ⁺ radical species. Adamantane is selectively (3°/2°, 12-17) oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone while cumene is selectively oxidized to 2-phenyl-2-propanol. In contrast to cyclohexane oxidation, the incorporation of sterically hindering quinolyl/benzimidazolyl donors around Ni(ii) leads to a high 3°/2° bond selectivity for adamantane oxidation. A linear correlation between the metal-ligand covalency parameter (β) and the turnover number has been observed.

Full text of DOI:10.1039/c1dt10902b

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PAPER
Nickel(II) complexes of tripodal 4N ligands as catalysts for alkane oxidation
using m-CPBA as oxidant: ligand stereoelectronic effects on catalysis†
a
a
b
a
Mani Balamurugan, Ramasamy Mayilmurugan, Eringathodi Suresh and Mallayan Palaniandavar*
Received 12th May 2011, Accepted 11th July 2011
DOI: 10.1039/c1dt10902b
Several mononuclear Ni(II) complexes of the type [Ni(L)(CH
tetradentate tripodal 4N ligand such as N,N-dimethyl-N¢,N¢-bis(pyrid-2-ylmethyl)ethane-1,2-diamine
L1), N,N-diethyl-N¢,N¢-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (L2), N,N-dimethyl-N¢-(1-methyl-
H-imidazol-2-ylmethyl)-N¢-(pyrid-2-ylmethyl)ethane-1,2-diamine (L3), N,N-dimethyl-N¢,N¢-
bis(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (L4), N,N-dimethyl-N¢,N¢-bis(quinolin-
-ylmethyl)ethane-1,2-diamine (L5), tris(benzimidazol-2-ylmethyl)amine (L6) and tris(pyrid-2-
3
CN)
2
](BPh
4
)
2
1–7, where L is a
(
1
2
ylmethyl)amine (L7), have been isolated and characterized using CHN analysis, UV-Visible
spectroscopy and mass spectrometry. The single-crystal X-ray structures of the complexes
[
[
Ni(L1)(CH
Ni(L4)(CH
3
CN)(H
2
O)](ClO
4
)
2
1a, [Ni(L2)(CH
3
CN)
2
](BPh
4
)
2
2, [Ni(L3)(CH
3
CN)
2
](BPh
4
)
2
3 and
3
CN) ](BPh
2
4 2
)
4 have been determined. All these complexes possess a distorted octahedral
coordination geometry in which Ni(II) is coordinated to four nitrogen atoms of the tetradentate ligands
and two CH CN (2, 3, 4) or one H O and one CH CN (1a) are located in cis positions. The Ni–Npy
bond distances (2.054(2)–2.078(3) A) in 1a, 2 and 3 are shorter than the Ni–Namine bonds
3
2
3
˚
2
3
˚
(
2.127(2)–2.196(3) A) because of sp and sp hybridizations of the pyridyl and tertiary amine nitrogens
˚
˚
respectively. In 3 the Ni–Nim bond (2.040(5) A) is shorter than the Ni–Npy bond (2.074(4) A) due to the
stronger coordination of imidazole compared with the pyridine donor. In dichloromethane/acetonitrile
solvent mixture, all the Ni(II) complexes possess an octahedral coordination geometry, as revealed by
the characteristic ligand field bands in the visible region. They efficiently catalyze the hydroxylation of
alkanes when m-CPBA is used as oxidant with turnover number (TON) in the range of 340–620 and
good alcohol selectivity for cyclohexane (A/K, 5–9). By replacing one of the pyridyl donors in TPA by
a weakly coordinating –NMe
2
or –NEt donor nitrogen atom the catalytic activity decreases slightly
2
with no change in the selectivity. In contrast, upon replacing the pyridyl nitrogen donor by the strongly
s-bonding imidazolyl or sterically demanding quinolyl/benzimidazolyl nitrogen donor, both the
catalytic activity and selectivity decrease, possibly due to destabilization of the intermediate
∑
+
◦
◦
[
1
2
(4N)(CH
3
CN)Ni–O ] radical species. Adamantane is selectively (3 /2 , 12–17) oxidized to
-adamantanol, 2-adamantanol and 2-adamantanone while cumene is selectively oxidized to
-phenyl-2-propanol. In contrast to cyclohexane oxidation, the incorporation of sterically hindering
◦
◦
quinolyl/benzimidazolyl donors around Ni(II) leads to a high 3 /2 bond selectivity for adamantane
oxidation. A linear correlation between the metal–ligand covalency parameter (b) and the turnover
number has been observed.
Introduction
monooxygenases, cytochrome P450 and bleomycin. In particular,
the soluble methane monooxygenases (sMMO) are widely inves-
tigated metalloenzymes that catalyze the oxidation of methane to
In Nature a variety of biologically essential transformations
are catalyzed by iron containing enzymes such as methane
1–10
methanol using dioxygen. Inspired by these enzymes, significant
efforts have been made to reproduce the functional aspects of
the non-heme diiron enzymes by designing model complexes,
which are attractive as they catalyze unique selective chemi-
cal transformations such as methane oxidation using dioxygen
or peroxide. Several bio-inspired iron(II)/(III) complexes have
been investigated as catalysts for hydroxylation, epoxidation and
a
Centre for Bioinorganic Chemistry, School of Chemistry, Bharathi-
dasan University, Tiruchirappalli, 620 024, Tamil Nadu, India. E-mail:
palanim51@yahoo.com, palaniandavarm@gmail.com
b
Analytical Science Discipline, Central Salt and Marine Chemicals Research
Institute, Bhavnagar, 364 002, India
†
CCDC reference numbers 805447–805450. For crystallographic data in
11–24
sulfoxidation reactions.
In recent years catalytic oxidation
CIF or other electronic format see DOI: 10.1039/c1dt10902b
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9413–9424 | 9413

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of saturated hydrocarbons under mild conditions has become
an exciting and challenging scientific goal and has received
Experimental
25–30
Materials
greater attention.
Though iron complexes are considered to
be one of the most promising catalysts for this industrially
important reaction, a variety of metal-based homogeneous cat-
alysts for alkane oxidation have been reported by replacing iron
with different transition metals such as Mn, Co, Cu, Ru and
Pyridine-2-carboxaldehyde, 2-aminomethylpyridine, N,N-dime-
thylethylenediamine, N,N-diethylethylenediamine, sodium tri-
acetoxyborohydride, sodium borohydride, 1-methyl-imidazole-2-
carboxaldehyde, nickel(II) perchlorate hexahydrate, adamantane,
cumene, sodium tetraphenylborate, m-choloroperbenzoic acid
31–51
Os.
Among them, ruthenium has attracted much attention
and a few Ru(II/III) complexes of tripodal 4N ligands based
(Aldrich), quinoline-2-carboxaldehyde (Alfa Aesar), acetic acid
52
on tris(2-pyridylmethyl)amine (TPA) and tris(benzimidazol-2-
glacial, dichloromethane, diethylether (Merck, India), ethylben-
zene, nitrilotriacetic acid, o-phenylenediamine (Loba, India) and
cyclohexane (Ranbaxy) were used as received. Methanol (Sisco
Research Laboratory, Mumbai), acetonitrile and tetrahydrofuran
53
ylmethyl)amine (NTB) have been studied as catalysts for alkane
V
hydroxylation, and the involvement of a Ru O intermediate in
the catalytic cycle has been proposed.
Recently, nickel has attracted regarding catalysts for the hy-
droxylation of alkanes in view of its industrial importance.
Earlier several oxo-bridged dinuclear Ni(II) complexes have been
(Merck, India) were distilled before use.
54–66
Synthesis of ligands
reported to be involved in oxygen activation chemistry.
Very
recently, Itoh et al. have demonstrated that the Ni(II) com-
plex [Ni(TPA)(OAc)(H O)](BPh ) is a very efficient and robust
67
N,N-Dimethyl-N¢,N¢-bis(pyrid-2-ylmethyl)ethane-1,2-diamine
71
2
4
(
L1). The ligand was prepared as reported elsewhere. Yield:
turnover catalyst showing a high alcohol selectivity for alkane
hydroxylation with m-chloroperbenzoic acid (m-CPBA) as ox-
idant. Later, they isolated a series of Ni(II) complexes of 3N,
tripodal 4N and 3NO ligands and studied the effect of ligand
and counter anion on the catalytic oxidation of cyclohexane with
m-CPBA, suggesting the involvement of the highly reactive nickel-
1
1
2
2
.19 g (88%). H NMR (400 MHz, CDCl ): d 2.18 (s, 6H), 2.41 (t,
3
H), 2.65 (t, 2H), 3.87 (s, 4H), 7.15 (t, 2H), 7.53 (d, 2H), 7.65 (t,
∑+
H), 8.51 (d, 2H). EI-MS m/z = 270.1 C16
H
22
N
4
.
N ,N -Diethyl-N¢,N¢-bis(pyrid-2-ylmethyl)ethane-1,2-diamine
(L2). The procedure employed for L1 was used for the prepa-
ration of L2. N,N-Diethylethylenediamine was used in the place
of N,N-dimethylethylenediamine. The pale yellow oil formed was
+
oxo (Ni O ) intermediate rather than an auto-oxidation type
68
free radical species in the catalytic cycle. Also, the same group
reported Ni(II) complexes of tripodal phenolate ligands, which are
capable of catalyzing the oxidation of cyclohexane to cyclohexanol
using m-CPBA with up to 100% conversion based on the oxidant
used without further purification for complex preparation. Yield:
1
1.38 g (92%). H NMR (400 MHz, CDCl
3
): d 1.12 (t, 6H), 2.28 (q,
4H), 2.49 (t, 2H), 2.58 (t, 2H), 3.83 (s, 4H), 7.18 (t, 2H), 7.49 (d,
69
∑+
in solvent free conditions. Also, Hikichi et al. crystallized the
2H), 7.68 (t, 2H), 8.61 (d, 2H). EI-MS m/z = 298.21 C18
H
26
N
4
.
II
ipr
nickel(II) alkylperoxo complex [Ni (Tp )(OOtBu)] and studied its
70
N,N-Dimethyl-N¢-(1-methyl-1H-imidazol-2-ylmethyl)-N¢-(py-
rid-2-ylmethyl)ethane-1,2-diamine (L3). N,N-dimethyl-N¢-
pyrid-2-ylmethyl)ethane-1,2-diamine was prepared as reported
and the reductive amination of this compound with 1-
methylimidazole-2-carboxaldehyde using sodium triace-
oxidation activity towards substituted benzaldehydes. However,
the factors determining the selectivity as well as efficiency of
the catalysts and the nature of intermediate species still remain
unclear.
72
(
All the above observations prompted us to isolate Ni(II)
complexes of systematically varied tripodal 4N ligands containing
pyridine, imidazole, sterically demanding quinoline and benzim-
toxyborohydride gave L3 as a pale yellow oil, which was
used without further purification for the isolation of the complex.
1
Yield: 1.12 g (82%). H NMR (400 MHz, CDCl
3
): d 2.13 (s, 6H),
idazole moieties and weakly binding –NMe
2
/–NEt groups, and
2
2
6
8
.44 (t, 2H), 2.63 (t, 2H), 3.67 (s, 3H), 3.58 (s, 2H), 3.78 (s, 2H),
.78 (s, 1H), 6.86 (s, 1H), 7.17 (t, 1H), 7.46 (d, 1H), 7.63 (t, 1H),
study the influence of the ligand stereoelectronic factors upon
the efficiency as well as the alcohol product selectivity of the
complexes as catalysts for alkane hydroxylation reactions. We
aim to construct more efficient and alcohol selective catalysts for
alkane hydroxylation and also collect evidence for the reactive
intermediate involved in the alkane hydroxylation reaction. All the
present nickel(II) complexes catalyse the hydroxylation of alkanes
such as cyclohexane, adamantane, ethylbenzene and cumene
efficiently with good alcohol selectivity using m-CPBA as the
oxidant within two hours. Also, upon increasing the concentration
of substrate the TON reaches 730 with a slight increase in the A/K
ratio (9.0). Furthermore, when the pyridine moiety in the Ni(II)
catalyst is replaced with the strongly s-bonding (benz)imidazolyl
moiety both the efficiency and selectivity of the catalyst decrease,
∑
+
.58 (d, 1H). EI-MS m/z = 273.2 C15
H
23
N
5
.
N,N-Dimethyl-N¢,N¢-bis(1-methyl-1H-imidazol-2-ylmethyl)-
ethane-1,2-diamine (L4). The procedure employed for the prepa-
ration of L1 was also used for L4 and 1-methylimidazole-2-
carboxaldehyde was used instead of pyridine-2-carboxaldehyde.
The colorless oil was used without further purification for complex
1
preparation. Yield: 1.05 g (76%). H NMR (400 MHz, CDCl
3
): d
2
6
.14 (s, 6H), 2.41 (t, 2H), 2.59 (t, 2H), 3.68 (s, 6H), 3.59 (s, 4H),
∑+
.80 (s, 2H), 6.87 (s, 2H). EI-MS m/z = 276.2 C14
H
24
N
6
.
N,N-Dimethyl-N¢,N¢-bis(quinolin-2-ylmethyl)ethane-1,2-dia-
mine (L5). The procedure employed for the preparation of L1
was also used for L5 and quinoline-2-carboxaldehyde was used
instead of pyridine-2-carboxaldehyde. The brown oil was used
and when a –NMe
2
donor group is replaced with a weakly s-
bonding –NEt donor group the selectivity of the catalyst remains
2
approximately the same. In contrast, for adamantane oxidation
the incorporation of sterically hindering quinolyl/benzimidazolyl
donors around Ni(II) leads to a high 3 /2 bond selectivity.
without further purification for the isolation of the complex. Yield:
1
1.28 g (69%). H NMR (400 MHz, CDCl
3
): d 2.13 (s, 6H), 2.42 (t,
◦
◦
2H), 2.62 (t, 2H), 3.80 (s, 4H), 7.15 (d, 2H), 7.42 (t, 2H), 7.57 (t,
9
414 | Dalton Trans., 2011, 40, 9413–9424
This journal is © The Royal Society of Chemistry 2011

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2
C
H), 7.67 (d, 2H), 7.89 (d, 2H), 8.13 (d, 2H). EI-MS m/z = 370.2
6.71; N, 11.49. Found: C, 74.39; H, 6.74; N, 11.45. Single crystals
suitable for X-ray crystallographic analysis were obtained by slow
∑
+
24
H
26
N
4
.
evaporation of a CH CN/DCM solution of the complex.
3
Tris(benzimidazol-2-ylmethyl)amine (L6). This ligand was pre-
pared as reported elsewhere. The H NMR data agreed well with
that reported.
73
1
[Ni(L5)(CH
3
CN)
2
](BPh
4
)
2
5. Yield 0.80 g, 70%. FT-IR (KBr):
3410 (b), 2266 (s), 1595 (s), 1512 (s), 1476 (s), 1425 (s), 1382 (s),
1
6
346 (s), 1143 (s), 1125 (s), 1028 (s), 948 (s), 731 (s), 705 (s),
Tris(pyrid-2ylmethyl)amine (L7). The ligand was prepared as
-
1
2+
08 (s) cm . ESI-MS: m/z 234.48 [(M–CH
Ni: C, 79.39; H, 6.31; N, 7.31. Found: C,
9.28; H, 6.28; N, 7.20.
3
CN–2BPh
4
) ]. Anal.
71
1
reported earlier. Yield: 1.27 g (87%). H NMR (400 MHz,
CDCl ): d 3.85 (s, 6H), 7.16 (t, 3H), 7.49 (d, 3H), 7.72 (t, 3H),
.54 (d, 3H). EI-MS m/z = 290.1 C18
Calcd. for C76
7
H
72
N
6
B
2
3
∑
+
8
H
18
N
4
.
[
Ni(L6)(CH CN) ](BPh
3
2
4
)
2
6. Yield 0.94 g, 79%. FT-IR (KBr)
Synthesis of Ni(II) complexes
Ni(L1)(H O)(CH CN)](ClO
3385 (b), 2804 (b), 2265 (s), 1590 (s), 1450 (s), 1385 (s), 1346
-
1
(
s), 1143 (s), 1111 (s), 1090 (s), 739 (s), 705 (s), 623 (s) cm .
[
2
3
4
)
2
1a.
A
methanol solution
2+
ESI-MS: m/z 252.95 [(M–CH
Ni: C, 76.92; H, 5.69; N, 10.62. Found: C, 76.87; H,
.61; N, 10.54.
3
CN–2BPh
4
) ]. Anal. Calcd. for
(
(
5 mL) of Ni(ClO ·6H O (0.365 g, 1 mmol) was added to L1
4
)
2
2
C
76
H
67
N
9
B
2
0.27 g, 1 mmol) in methanol solution (5 mL) with stirring at
5
room temperature. The colour of the solution turned to indigo.
After stirring the solution for 30 min, 5 mL of acetonitrile was
added. The solution was stirred for an additional 30 min and the
color of the solution turned to blue. Ether diffusion of the blue
coloured solution gave blue crystals, which were suitable for X-ray
crystallographic analysis. Yield, 0.51 g, 87%. FT-IR (KBr) 3411
[Ni(L7)(CH
3
CN)
2
](BPh
4
) 7. Yield 0.89 g, 82%. FT-IR (KBr)
2
-
1
2265 (s), 1595 (s), 735 (s), 707 (s) cm . ESI-MS: m/z 194.49 [(M-
2
+
CH
3
CN–2BPh
4
) ]. Anal. Calcd. for C71
H
68
N
6
2
B Ni: C, 78.55; H,
6.31; N, 7.74. Found: C, 78.48; H, 6.37; N, 7.70.
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.
(
b), 2998 (b), 2265 (s), 1591 (s), 1468 (s), 1439 (s), 1378 (s), 1349 (s),
-
1
1
2
1
093 (s), 764 (s), 625 (s) cm . ESI-MS: m/z 184.52 [(M–CH
3
CN–
Ni: C, 36.83; H, 4.64; N,
1.93. Found: C, 36.75; H, 4.50; N, 11.79.
2
+
BPh
4
) ]. Anal. Calcd. C18
H
27
N
5
O
9
Cl
2
Catalytic oxidations
[
Ni(L1)(CH CN) ](BPh 1. This was obtained from the
3
2
4
)
2
metathesis of 1a by adding NaBPh (0.684 g, 2 mmol) in methanol.
The pink precipitate was filtered off, washed with small quantities
of ice-cold methanol and then dried. Yield, 0.87 g, 83%. FT-IR
4
The oxidation of alkanes was carried out at room temperature
under research grade nitrogen atmosphere. In a typical reaction,
-
3
-3
Ni(II) complex (0.35 ¥ 10 mmol dm ) was added to a mixture
-
3
-3
(
1
CH
KBr) 3435 (b), 2265 (s), 1593 (s), 1476 (s), 1382 (s), 1350 (s),
of alkanes (2.45 mol dm ) and oxidant m-CPBA (0.35 mol dm )
-
1
100 (s), 735 (s), 707 (s), 608 (s) cm . ESI-MS: m/z 184.47 [(M–
in CH Cl : CH CN mixture (3 : 1 v/v). After 1 h the reaction
2
2
3
2
+
3
CN–2BPh
4
) ]. Anal. Calcd. C68
H
68
N
6
B
2
Ni: C, 77.81; H, 6.53;
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 was identified by Agilent GC-MS and
quantitatively analyzed by HP 6890 series GC equipped with HP-5
capillary column (30 m ¥ 0.32 mm ¥ 2.5 mm) using a calibration
curve obtained with authentic compounds. All of the products
were quantified using GC (FID) with the following temperature
N, 8.01. Found: C, 77.89; H, 6.61; N, 7.94.
The complexes 2–7 were prepared by using the procedure
employed for isolating 1 and using the ligands L2–L7.
[
Ni(L2)(CH
3
CN)
2
](BPh
4
)
2
·CH
3
CN 2. Yield, 0.95 g, 85%. FT-
IR (KBr) 3457 (b), 3043 (s), 2268 (s), 1606 (s), 1479 (s), 1443 (s),
425 (s), 1382 (s), 1350 (s), 1097 (s), 735 (s), 705 (s), 610 (s) cm .
ESI-MS: m/z 198.51 [(M–CH
-
1
1
2
+
3
CN–2BPh ) ]. Anal. Calcd. for
4
C
72
H
75
N
7
B
2
Ni: C, 77.30; H, 6.76; N, 8.76. Found: C, 77.20; H,
◦
◦
program: injector temperature 130 C; initial temperature 60 C,
6
.68; N, 8.71. Single crystals suitable for X-ray crystallographic
◦
-1
◦
heating rate 10 C min to 130 C, increasing the temperature to
60 C at a rate of 2 C min , and then increasing the temperature
to 260 C at a rate of 5 C min ; FID temperature 280 C. GC-MS
analysis was performed under conditions identical to those used
for GC analysis. The average of three measurements are reported.
analysis were obtained by slow evaporation of a CH
solution of the complex.
3
CN/DCM
◦
◦
-1
1
◦
◦
-1
◦
[
Ni(L3)(CH
3
CN)
2
](BPh
4
2
) 3. Yield 0.79 g, 76%. FT-IR (KBr)
3
1
7
2
9
470 (b), 2265 (s), 1606 (s), 1511 (s), 1475 (s), 1418 (s), 1382 (s),
350 (s), 1125 (s), 1105 (s), 1030 (s), 978 (s), 842 (s), 818 (s),
-
1
34 (s), 707 (s), 610 (s) cm . ESI-MS: m/z 185.90 [(M–CH
3
CN–
Ni: C, 76.45; H, 6.61; N,
.31. Found: C, 76.37; H, 6.68; N, 9.23. Single crystals suitable for
Physical measurements
2
+
BPh
4
) ]. Anal. Calcd. for C67
H
69
N
7
B
2
Elemental analyses were performed on a Perkin Elmer Series II
CHNS/O analyzer 2400. The electronic spectra were recorded
X-ray crystallographic analysis were obtained by slow evaporation
of a CH CN/DCM solution of the complex.
1
on an Agilent 8453 diode array spectrophotometer. H NMR
3
spectra were recorded on a Bruker 400 MHz NMR spectrometer.
ESI-MS analysis were recorded on a Micromass Quattro II
triple quadrupole mass spectrometer. IR spectra were recorded
on a Perkin Elmer FT-IR spectrometer. GC-MS analysis was
performed on an Agilent GC-MS equipped with 7890A GC
series (HP-5 capillary column) and 5975C inert MSD. The
[
Ni(L4)(CH
IR (KBr) 3497 (b), 3050 (s), 2268 (s), 1607 (s), 1505 (s), 1472 (s),
421 (s), 1382 (s), 1349 (s), 1129 (s), 1105 (s), 1031 (s), 982 (s), 843
3
CN)
2
](BPh
4
)
2
·CH
3
CN 4. Yield 0.78 g, 71%. FT-
1
-
1
(
2
s), 818 (s), 731 (s), 708 (s), 611 (s) cm . ESI-MS: m/z 187.50 [(M–
CH
2
+
3
CN–2BPh
4
) ]. Anal. Calcd. for C68
H
73
N
9
2
B Ni: C, 74.47; H,
This journal is © The Royal Society of Chemistry 2011
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Table 1 Crystallographic data for 1a, 2, 3 and 4
1
a
2
3
4
Empirical formula
Formula weight/g mol-
Crystal habit, colour
Crystal system
C
587.06
Purple
Monoclinic
0.53 ¥ 0.26 ¥ 0.13 mm
P21/c
15.8006(12)
10.4718(8)
15.1299(11)
90.000
106.9420(10)
90
2394.8(3)
4
1.628
18
H
27
N
5
Cl
2
O
9
Ni
C
72
H
72
B
2
N
7
Ni
C
134
H
138
B
4
N
14Ni
2
C
68 73 2 9
H B N Ni
1
1115.70
Pink
Monoclinic
0.39 ¥ 0.37 ¥ 0.21 mm
P 21/c
20.1135(16)
13.4486(11)
23.7510(19)
90.000
110.286(2)
90
6026.1(8)
4
1.230
2105.24
Pink
Monoclinic
0.60 ¥ 0.48 ¥ 0.38 mm
P21/c
19.4952(12)
17.3430(10)
34.986(2)
90.000
92.5370(10)
90
11817.2(12)
4
1.183
1096.68
Violet
Monoclinic
0.20 ¥ 0.15 ¥ 0.13 mm
P21/c
10.9038(2)
14.9757(2)
37.0413(6)
90.000
90.7660(10)
90
6048.01(17)
4
1.204
Crystal size
Space group
a/A˚
b/A˚
c/A˚
◦
a ( )
◦
b ( )
◦
g ( )
V/A˚
3
Z
r
c
/g cm-
F(000)
T/K
3
1216.0
100
14152
2364.0
100
11414
4464.0
293
20794
2328.0
293
14067
No. of Reflections collected
No. of unique reflections
5595
0.71073
1.051
5848
0.71073
0.836
12409
0.71073
1.074
8622
0.71073
1.014
Radiation (Mo-Ka)/A˚
2
Goodness-of-fit on F
Number of refined Parameters
327
732
1397
0.0879/0.1758
0.1507/0.2019
728
a
R
1
/w R2[I > 2s(I)]
/w R (all data)
0.0481/0.1126
0.0558/0.1171
0.0662/0.1157
0.1342/0.1342
0.0489/0.1022
0.1043/0.1227
R
1
2
a
2
2
2
4
1/2
R1 = [R(ꢀF
o
|-|F
c
ꢀ)/R|F
o
|]; wR2 = {[R(w(F
o
-F
c
) )/R(wF
o
)]
}
◦
Table 2 Selected bond lengths [A
˚
] and bond angles [ ] for 1a and 2
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 ¥ 2.5 mm). All the
catalytic reactions were performed under a research grade nitrogen
atmosphere using standard Schlenk line techniques.
1
a
2
Bond lengths/A˚
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(1)–N(5)
Ni(1)–O(9)
2.054(2)
2.090(2)
2.088(2)
2.127(2)
2.044(2)
2.154(2)
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(1)–N(5)
Ni(1)–N(6)
2.069(3)
2.079(3)
2.078(3)
2.196(3)
2.170(3)
2.031(4)
Crystal data collection and structure refinement
◦
Bond angles/
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 were chosen after
careful examination under an optical microscope. Intensity data
N(5)–Ni(1)– N(1)
N(5)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
N(5)–Ni(1)–N(2)
N(1)–Ni(1)–N(2)
N(3)–Ni(1)–N(2)
N(5)–Ni(1)–N(4)
N(1)–Ni(1)–N(4)
N(3)–Ni(1)–N(4)
N(2)–Ni(1)–N(4)
N(5)–Ni(1)–O(9)
N(1)–Ni(1)–O(9)
N(3)–Ni(1)–O(9)
N(2)–Ni(1)–O(9)
N(4)–Ni(1)–O(9)
100.34(9)
97.17(9)
159.57(9)
176.39(9)
82.18(9)
80.85(9)
91.62(9)
92.72(9)
97.25(9)
85.65(9)
84.67(9)
84.92(9)
86.32(9)
98.18(9)
175.16(9)
N(6)–Ni(1)–N(1)
N(6)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
N(6)–Ni(1)–N(2)
N(1)–Ni(1)–N(2)
N(3)–Ni(1)–N(2)
N(6)–Ni(1)–N(5)
N(1)–Ni(1)–N(5)
N(3)–Ni(1)–N(5)
N(2)–Ni(1)–N(5)
N(6)–Ni(1)–N(4)
N(1)–Ni(1)–N(4)
N(3)–Ni(1)–N(4)
N(2)–Ni(1)–N(4)
N(5)–Ni(1)–N(4)
97.12
98.43(14)
160.05(13)
179.11(13)
82.46(13)
81.80(13)
88.49(13)
84.63(12)
83.36(12)
90.69(12)
95.98(13)
92.26(12)
98.45(12)
84.83(12)
174.86(13)
˚
for the crystals were collected using Mo-Ka (l = 0.71073 A)
radiation on a Bruker SMART APEX diffractometer equipped
with CCD area detector at 100 and 293 K. The data integration
74
and reduction was processed with SAINT software. An empirical
absorption correction was applied to the collected reflections with
75
SADABS. The structures were solved by direct methods using
76
2
SHELXTL and refined on F by the full-matrix least-squares
77
technique using the SHELXL-97 package. Even though the data
of 2 was collected at liquid nitrogen temperature (100 K), during
the structure solution it was observed that the carbon atoms of the
coordinated acetonitrile molecule in 2 appeared as diffused peaks
and the methyl carbon is disordered. Both these carbon atoms
were located from the difference Fourier map, and since the peak
heights of the carbon atoms were small and diffused the whole
Results and discussion
Syntheses and characterization of ligands and their nickel(II)
complexes
coordinated CH
3
CN molecule was refined only isotropically. For
The tripodal tetradentate 4N ligands L1–L7 (Scheme 1)
were synthesized according to known procedures, which
involve Schiff base condensation, reductive amination
and fusion reaction. The ligands L1, L2, L4, L5 and
L7 were prepared by reductive amination of N,N-
dimethylethylenediamine/N,N-diethylethylenediamine with two
the disordered methyl carbon the occupancy factor is assigned
using FVAR command. 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.
9
416 | Dalton Trans., 2011, 40, 9413–9424
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Scheme 1 Structures of 4N ligands employed in the study.
moles of pyridine-2-carboxaldehyde (L1, L2)/1-methylimidazole-
-carboxaldehyde (L4)/quinoline-2-carboxaldehyde (L5) using
2
sodium triacetoxyborohydride as reducing agent and were
1
characterized by H NMR spectroscopy and mass spectrometry.
The ligand L3 was prepared by Schiff base condensation of
N,N-dimethylethylenediamine with pyridine-2-carboxaldehyde,
followed by reduction and then reductive amination with 1-
methylimidazole-2-carboxaldehyde. The ligand L6 was prepared
by fusing nitrilotriacetic acid with o-phenylenediamine as
73
previously reported. The nickel(II) complex 1a was isolated by
treating L1 with Ni(ClO O in methanol and then adding
·6H
acetonitrile. The nickel(II) complexes [Ni(L)(CH CN) ](BPh 1–
were isolated by treating Ni(ClO O with the corresponding
4
)
2
2
3
2
)
4 2
7
4
)
2
·6H
2
ligands L1–L7 in methanol, adding acetonitrile with stirring and
then treating the reaction mixture with stoichiometric amounts
of NaBPh . All the complexes were characterized by using
4
elemental analysis and electronic spectroscopy. The formulation
of the complexes based on elemental analysis was confirmed
by determining the X-ray crystal structures of 1a, 2, 3 and 4.
The tripodal ligands with different electron-releasing abilities are
expected to play an important role in determining the stability
of the intermediate involved in the catalytic cycle and hence the
reactivity.
Description of X-ray crystal structures of
2
+
2 3
Fig. 1 Molecular structure of the [Ni(L1)(H O)(CH CN)] 1a (50%
[
[
Ni(L1)(H
Ni(L3)(CH
2
O)(CH
3
CN)](ClO
4
)
2
1a, [Ni(L2)(CH
3
CN)
2
](BPh
4
) 2,
2
probability factor for the thermal ellipsoid). Hydrogen atoms have been
omitted for clarity.
3
CN) ](BPh
2
4
)
2
3 and [Ni(L4)(CH
3
CN)
2
](BPh
4
)
2
4
The molecular structure of [Ni(L1)(H
shown in Fig. 1 together with the atom numbering scheme and the
selected bond lengths and bond angles are collected in Table 2. The
2
O)(CH
3
CN)](ClO
4
)
2
1a is
+
is very similar to that of [Ni(TPA)(OAc)(H
2
O)] in which TPA
occupies four sites of the octahedron and a water molecule and
acetate ion occupy the remaining two sites. Also, the Ni–Npy, Ni–
69
complex contains a NiN
5
O coordination sphere with a distorted
octahedral coordination geometry constituted by two pyridine
and two tertiary amine nitrogen atoms of the tripodal ligand L1,
and the remaining two cis-coordination sites trans to the tertiary
amine nitrogen atoms are occupied by water and acetonitrile
N
amine and Ni–O bond distances in 1a are similar to those observed
+
for the nickel(II) complex cation [Ni(TPA)(OAc)(H
2
O)] .
2
+
The X-ray crystal structures of [Ni(L2)(CH
3
CN)
2
]
2,
2
+
2+
[Ni(L3)(CH
3
CN)
2
]
3 and [Ni(L4)(CH
3
CN) ]
2
4 are shown in
˚
molecules. The Ni–Npy bonds (2.054(2), 2.088(2) A) are shorter
Fig. 2, 3 and 4 respectively together with the atom numbering
scheme, and the selected bond lengths and bond angles are
collected in Table 2 and 3. The distorted octahedral coordination
geometry of 2 is similar to that of 1a. While the two pyridine and
two tertiary amine nitrogen atoms of L2 occupy the four coor-
dination sites of the octahedron in 2, the acetonitrile molecules
2
3
˚
than the Ni–Namine bonds (2.090(2), 2.127(2) A) due to sp and sp
hybridizations of the pyridyl and tertiary amine nitrogen atoms
˚
respectively. Also, the terminal Ni–N4amine bond (2.127(2) A) is
˚
longer than the Ni–N2amine bond (2.090(2) A) formed by the
apical amine nitrogen atom of the tripodal ligand, as expected.
Furthermore, the coordination geometry of the complex cation
occupy the remaining two cis-coordination sites. The Ni–NEt
2
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◦
Table 3 Selected bond lengths [ A˚ ] and bond angles [ ] for 3 and 4
3
4
Ni(1)–N(1)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(1)–N(5)
Ni(1)–N(6)
Ni(1)–N(7)
Ni(2)–N(8)
Ni(2)–N(10)
Ni(2)–N(11)
Ni(2)–N(12)
Ni(2)–N(13)
Ni(2)–N(14)
Bond angles/
N(6)–Ni(1)–N(1)
N(6)–Ni(1)–N(4)
N(1)–Ni(1)–N(4)
N(6)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
N(4)–Ni(1)–N(3)
N(6)–Ni(1)–N(5)
N(1)–Ni(1)–N(5)
N(4)–Ni(1)–N(5)
N(3)–Ni(1)–N(5)
N(6)–Ni(1)–N(7)
N(1)–Ni(1)–N(7)
N(4)–Ni(1)–N(7)
N(3)–Ni(1)–N(7)
N(5)–Ni(1)–N(7)
N(8)–Ni(2)–N(14)
N(8)–Ni(2)–N(11)
N(14)–Ni(2)–N(11)
N(8)–Ni(2)–N(10)
N(14)–Ni(2)–N(10)
N(11)–Ni(2)–N(10)
N(8)–Ni(2)–N(12)
N(14)–Ni(2)–N(12)
N(11)–Ni(2)–N(12)
N(10)–Ni(2)–N(12)
N(8)–Ni(2)–N(13)
N(14)–Ni(2)–N(13)
N(11)–Ni(2)–N(13)
N(10)–Ni(2)–N)13)
N(12)–Ni(2)–N(13)
2.040(5)
2.102(4)
2.074(4)
2.148(4)
2.036(4)
2.167(5)
2.058(4)
2.108(4)
2.073(4)
2.148(4)
2.167(5)
2.065(4)
Ni(1)–N(1)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(1)–N(6)
Ni(1)–N(7)
Ni(1)–N(8)
2.079(2)
2.133(16)
2.0376(19)
2.1481(18)
2.062(2)
2.0735(19)
◦
98.68(19)
98.99(18)
159.8(2)
176.88(18)
82.4(2)
N(4)–Ni(1)–N(8)
N(4)–Ni(1)–N(1)
N(8)–Ni(1)–N(1)
N(4)–Ni(1)–N(3)
N(8)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
N(4)–Ni(1)–N(6)
N(8)–Ni(1)–N(6)
N(1)–Ni(1)–N(6)
N(3)–Ni(1)–N(6)
N(4)–Ni(1)–N(7)
N(8)–Ni(1)–N(7)
N(1)–Ni(1)–N(7)
N(3)–Ni(1)–N(7)
N(6)–Ni(1)–N(7)
99.09(8)
157.51(7)
101.30(8)
81.67(7)
176.12(7)
78.64(7)
91.77(7)
91.23(8)
97.17(8)
84.94(7)
88.43(8)
87.49(8)
83.09(8)
96.34(7)
178.72(7)
80.5(2)
92.11(17)
92.90(18)
96.22(18)
84.89(18)
91.54(16)
86.58(17)
83.17(17)
91.46(18)
176.35(16)
101.52(17)
158.71(17)
97.56(18)
80.21(16)
177.71(17)
80.99(16)
95.45(18)
92.19(17)
93.15(17)
86.14(18)
84.70(17)
91.10(16)
85.59(16)
90.55(17)
176.61(17)
2+
Fig. 2 Molecular structure of [Ni(L2)(CH
3
CN)
2
]
2 (50% probability
factor for the thermal ellipsoid). Hydrogen atoms and lattice acetonitrile
molecule have been omitted for clarity.
˚
bond in 2 (2.196(3) A) is longer than the Ni–NMe
(
2
bond in 1a
˚
2.127(2) A), obviously due to the more sterically hindering –
NEt group forming a coordinate bond to the Ni(II) center that
is weaker than that formed by the –NMe group. In 3 there are
2
2
two crystallographically independent complex molecules in the
same asymmetric unit cell, which exhibit the same coordination
geometry but slightly different bond lengths and bond angles.
Each molecule in the unit cell possesses a distorted octahedral
coordination geometry constituted by one pyridine, one imidazole
and two tertiary amine nitrogen atoms of the tripodal ligand,
and acetonitrile molecules occupy the cis positions as in 2. In
˚
3
the Ni–Nim bond (2.040(5) A) is shorter than the Ni–Npy
˚
2+
bond (2.074(4) A) as the coordination of imidazole is stronger
than that of the pyridine donor. The complex 4 possesses a
distorted octahedral coordination geometry constituted by two
imidazole and two tertiary amine nitrogen atoms of the tripodal
ligand and acetonitrile molecules occupy the cis positions as in
Fig. 3 Molecular structure of [Ni(L3)(CH CN) ] 3 (20% probability
3
2
factor for the thermal ellipsoid). Another molecule on the unit cell and
hydrogen atoms have been omitted for clarity.
imidazole nitrogens. Similarly, the Ni–Namine bond distance in 3
˚ ˚
(2.102(4) A) and 4 (2.133(16) A) is longer than that in 1a (2.090(2)
˚
2
(
and 3. The Ni–NMe
2
bond distances in 3 (2.148(4) A) and 4
2.1481(18) A) are longer than that in 1a (2.127(2) A) due to
˚
˚
˚
˚
˚
A) and 2 (2.079(3) A) due to the stronger Ni–NACN (3: 2.036(4) A;
˚
the replacement of pyridine nitrogen by the strongly s-bonding
4: 2.062(2) A) coordination. The Ni–Npy bond distances
9
418 | Dalton Trans., 2011, 40, 9413–9424
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3 2 4 2
Fig. 5 Electronic absorption spectra of [Ni(L2)(CH CN) ](BPh ) (1.0 ¥
-2
1
0 M) in DCM:ACN (3 : 1 v/v) solvent mixture.
broad absorption bands in the ranges 540–570 and 840–980 nm
together with a very weak shoulder in the range 770–800 nm. The
3
lower energy band in the range 840–980 nm is assigned to A2g
→
3
T
2g(F) (n
1
) transition, and the higher energy band in the range
3
3
5
40–570 nm to A2g → T1g(F) (n
2
) transition in Ni(II) located
in an octahedral environment. The shoulder or band observed
3
around 360 nm in the UV region is assigned to the
A
2g
→
3
T
1g(P) (n
00 nm to the A2g → E1g(D) spin-forbidden transition. The
observed n and n band positions are fitted into the quadratic
equation connecting the energies of both n and n transitions.
The calculated band positions (n ) agree well with the observed
3
) transition while the weak shoulder in the range 770–
3
1
78
8
2+
1
2
Fig. 4 Molecular structure of [Ni(L4)(CH
3
CN)
2
]
4 (20% probability
79
1
3
factor for the thermal ellipsoid). Hydrogen atoms and lattice acetonitrile
molecule have been omitted for clarity.
3
ones (Table 4), which confirms the band assignments and hence
˚
2.069(3)–2.078(3) A) in 2 and 3 are similar to those in 1a and those
the octahedral coordination geometry for Ni(II) in solution also.
An analysis of values of the derived covalency parameter b
reveals interesting trends. A combination of two pyridyl nitrogen
(
80,81
+
69
˚
in [Ni(TPA)(OAc)(H O)] (2.057(6)–2.101(6) A) and are shorter
2
˚
than the Ni–Namine bonds (2.079(3)–2.102(4) A), as expected (cf.
above). The terminal Ni–Namine bonds (2: 2.196(3); 3: 2.148(4)
donors confers metal–ligand covalency higher than that of three
2
+
˚
pyridyl nitrogen donors bound to Ni(II) as in [Ni(tpa)(CH
3
CN) ]
2
A; 4: 2.1481(18)) are longer than the central Ni–Namine bonds (2:
˚
˚
7 indicating that the third pyridyl nitrogen located cis to the
2
.079(3); 3: 2.102(4) A; 4: 2.133(16) A), as expected (cf. above).
◦
67
other two pyridyl nitrogens at a longer distance is not effectively
For 1a, 2, 3 and 4 the N–Ni–N (78.64–101.52 ) and N–Ni–N
157.51–179.11 ) bond angles deviate from the ideal octahedral
angles of 90 and 180 respectively revealing the presence of
significant distortion in the Ni(II) coordination geometry.
◦
involved in p-back bonding. A combination of two pyridyl (1)
or two imidazolyl (4) nitrogen donors confers a higher metal–
ligand covalency than that of two quinolyl nitrogen donors (5)
or that of one pyridyl and one imidazolyl donor (3), revealing
that pyridyl nitrogen is better suited for p-back bonding. Also, a
combination of three pyridyl donors leads to a higher covalency
than that of three benzimidazolyl donors. The incorporation of s-
(
◦
◦
Electronic spectral properties
The electronic spectral data of all the Ni(II) complexes are
summarized in Table 4 and the typical electronic absorption
spectrum of 1 is shown in Fig. 5. In a DCM : ACN (3 : 1 v/v)
solvent mixture, all the present nickel(II) complexes exhibit two
bonding –NMe
2
/–NEt donors also leads to a higher metal–ligand
2
covalency. All these observations reveal that b may be taken as a
measure of the metal–ligand p-back bonding in the complexes.
Table 4 UV-Visible spectral data (lmax in nm; e in M- cm-1 in parenthesis) of Ni(II) complexes 1–7 in DCM : ACN solvent mixture (1 : 3 v/v) at 25 C
1
◦
3
3
A
2g → T1g(P) (n
3
)
a
3
3
3
1
3
3
a
Complex
Found
Calcd
A
2g → T1g(F) (n
2
)
A
2g → E1g(D)
A
2g → T2g(F) (n
1
)
B¢¢
b (%)
1
—
—
—
360 (75)
353 (90)
—
342
342
343
345
360
342
332
331
540 (26)
553 (30)
551 (31)
553 (37)
570 (40)
568 (45)
571 (40)
533 (28)
788 (23)
782 (13)
791 (20)
788 (10)
784 (11)
800 (12)
770 (10)
785 (10)
840 (24)
880 (18)
872 (26)
875 (30)
890 (26)
920 (30)
938 (24)
845 (20)
801
877
855
851
773
947
1038
893
26
19
21
21
28
12
4
1
2
3
4
5
6
7
a
358 (120)
17
a
Calculated by solving the quadratic equation and using B as 1080 cm-1
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a
Table 5 Conversion of cyclohexane catalyzed by 1 with time
Cyclohexane (TON)
b
b
c
d
e
Complex
Time (min)
10
-ol
-one
e-caprolactone
Total TON
A/K
Yield (%)
1
326
475
546
558
19
22
16
12
12
28
45
52
357
525
607
622
10.5
9.5
8.9
8.7
35.7
52.5
60.7
62.2
3
6
1
0
0
20
a
-3
-3
-3
-3
Reaction conditions: Catalyst (0.35 ¥ 10 mmol dm ), Substrate (2.45 mol dm ), Oxidant (0.35 mol dm ) in DCM:ACN solvent mixture (3 : 1 v/v);
b
c
d
-
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 +
e
TON of e-caprolactone); Yield based on the oxidant.
a
Table 6 Variation of substrate concentration for cyclohexane hydroxylation catalyzed by 1
Products (TON)
-3
b
b
c
d
e
f
Complex
Cyclohexane (mol dm )
-ol
-one
e-caprolactone
Total TON
A/K
Yield (%)
Yield (%)
1
0.35
93
18
14
19
17
16
14
12
32
46
55
38
48
42
33
38
51
52
39
27
19
149
222
281
382
498
576
622
656
684
729
1.6
2.5
3.6
6.6
8.2
7.8
8.7
8.2
8.3
8.8
14.9
22.2
28.1
38.2
49.8
57.6
62.2
65.6
68.4
72.9
14.9
11.1
09.3
10.4
09.9
09.6
08.8
08.2
07.6
07.3
0
1
1
1
2
2
2
3
3
.70
.05
.40
.75
.10
.45
.80
.15
.50
160
220
332
444
511
558
585
611
655
a
-3
-3
-3
-3
-3
Reaction conditions: Catalyst (0.35 ¥ 10 mmol dm ), Substrate (0.35 mol dm to 3.5 mol dm ), Oxidant (0.35 mol dm ) in DCM : ACN solvent
b
c
d
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
-
e
f
ol/(TON of -one + TON of e-caprolactone); Yield based on the oxidant; Yield based on the substrate.
a
Table 7 Products of oxidation of cyclohexane catalyzed by Ni(II) complexes
Cyclohexane (TON)
b
b
c
d
e
Complex
-ol
-one
e-caprolactone
Total TON
Chloro-benzene
A/K
Yield (%)
Blank
1.5
0.5
12
42
11
15
13
20
34
20
—
52
62
46
78
50
50
25
35
2
1
558
479
480
485
365
406
280
450
622
583
537
578
428
476
339
505
395
409
324
346
256
265
288
336
8.7
4.6
8.4
5.2
5.7
5.8
4.7
8.1
62.2
58.3
53.7
57.8
42.8
47.6
33.9
50.5
1
2
3
4
5
6
7
a
a
b
-3
-3
-3
-3
Reaction conditions: Catalyst (0.35 ¥ 10 mmol dm ), Substrate (2.45 mol dm ), Oxidant (0.35 mol dm ) in DCM : ACN solvent mixture (3 : 1 v/v);
c
d
-
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 +
e
TON of e-caprolactone); Yield based on the oxidant.
Catalytic oxidation of alkanes
that m-CPBA is a very strong oxidizing agent towards oxidation
of cyclohexane and adamantane to the corresponding alcohols
and ketones in the absence of any metal catalyst, but only under
vigorous reaction conditions including very high concentrations of
m-CPBA, long reaction times and high temperatures. However, in
control reactions performed in the absence of the Ni(II) complexes
with m-CPBA as oxidant we observed only very small amounts
of the oxidized products for all the substrates (Cyclohexane,
2 TON; Adamantane, 5 TON; Ethylbenzene, 10 TON; Cumene,
1 TON). This reveals that all the Ni(II) complexes act as catalysts
towards the oxidation of alkanes to alcohols. In the presence
of the complexes the oxidation of cyclohexane proceeds to give
The experimental conditions and the results of catalytic oxidation
of alkanes into alcohols for all the nickel(II) complexes are summa-
rized in Tables 5–9. The conversion of alkanes into hydroxylated
products was quantified based on gas chromatographic analysis
by using authentic samples and an internal standard. The catalytic
ability of the Ni(II) complexes towards oxidation of alkanes
such as cyclohexane, adamantane, ethylbenzene and cumene was
explored by using m-CPBA, H
dichloromethane/acetonitrile solvent mixture (3 : 1 v/v) at room
temperature under nitrogen atmosphere. It has been reported
2
O
2
and t-BuOOH as oxidants in a
82
9
420 | Dalton Trans., 2011, 40, 9413–9424
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a
Table 8 Products of oxidation of adamantane catalyzed by Ni(II) complexes
d
Adamantane (TON)
Selectivity
b
b
b
c
◦
◦
e
Complex
1-adol
2-adol
2-adone
Total TON
3 /2
Yield (%)
Blank
3.2
1.4
113
68
99
94
67
73
58
0.4
10
18
11
8
11
13
10
5
1
516
390
546
504
426
498
330
639
468
656
606
504
584
398
12.5
13.6
14.8
14.8
16.3
17.3
14.5
63.9
47.6
65.6
60.6
50.4
58.4
39.8
1
2
3
4
5
6
a
a
b
-3
-3
-3
-3
Reaction conditions: Catalyst (0.2 ¥ 10 mmol dm ), Substrate (0.4 mol dm ), Oxidant (0.2 mol dm ) in DCM : ACN solvent mixture (3 : 1 v/v);
c
d
◦
◦
1
-adol = 1-adamantanol, 2-adol = 2-adamantanol and 2-adone = 2-adamantanone; TON = no. of mmol of product/no. of mmol of catalyst; 3 /2 =
TON of 1-adol ¥ 3)/(TON of 2-adol + TON of 2-adone); Yield based on the oxidant.
e
(
a
Table 9 Oxidation products of ethylbenzene and cumene catalysed by Ni(II) complexes
Ethylbenzene (TON)
Cumene
b
c
d
b
d
Complex
1-Phenyl- ethanol
Aceto- phenone
Total TON
A/K
Yield (%) 2-Phenyl-2-propanol
Total TON
Yield (%)
Blank
5.1
4.9
96
88
132
135
129
110
106
10
1
1
1
347
290
303
280
204
212
178
443
378
435
415
333
322
284
3.6
3.3
2.3
2.0
1.6
1.9
1.6
44.3
37.8
43.5
41.5
33.3
32.2
28.4
438
316
361
348
280
298
268
438
316
361
348
280
298
268
43.8
31.6
36.1
34.8
28.0
29.8
26.8
1
2
3
4
5
6
a
a
b
-3
-3
-3
-3
Reaction conditions: Catalyst (0.35 ¥ 10 mmol dm ), Substrate (2.45 mol dm ), Oxidant (0.35 mol dm ) in DCM : ACN solvent mixture (3 : 1 v/v);
c
d
Total TON = no. of mmol of product/no. of mmol of catalyst; A/K = TON of 1-phenylethanol/TON of acetophenone; Yield based on the oxidant.
Scheme 2 Proposed mechanism of alkane hydroxylation.
cyclohexanol as the major product along with cyclohexanone and
e-caprolactone as the minor products, where e-caprolactone is
the over oxidized product of the oxidation of cyclohexanone by
excess or unreacted m-CPBA. It is well known that m-CPBA is
a quantitative reagent for the conversion of cyclic ketones to the
corresponding lactones (Bayer–Villiger oxidation) in the absence
of any metal catalyst and so the e-caprolactone is not the metal
catalyzed product. Interestingly, only 50% of the oxidized products
revealing that the Ni(II) complexes are not effective as catalysts for
these oxidants.
The complex 1 catalyses the oxidation of cyclohexane to
558 TON of cyclohexanol (A) and 12 TON of cyclohexanone
(K) and 52 TON of e-caprolactone (A/K, 8.7). As proposed
68
earlier, we suggest that m-CPBA binds with Ni(II) in the catalyst
by replacing a labile acetonitrile molecule to form the adduct
II
+
[Ni (L)(CH
3
CN)(OOCOC
6
H Cl)] , which undergoes O–O bond
4
are formed under air. Also, when H
2
O
2
/t-BuOOH is used as the
homolysis leading to the formation of the reactive intermediate
oxidant only trace amounts of the oxidized products are formed,
species and m-chlorobenzoic acid radical (Scheme 2). The species
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∑
+
[
(4N)(CH
3
CN)Ni-O ] would then be involved in selective hydrox-
1 by an imidazolylmethyl arm to give 3, cyclohexane is catalytically
oxidized to cyclohexanol (485 TON) and cyclohexanone (15 TON)
and e-caprolactone (78 TON), revealing that the catalytic activity
(Total TON, 578) decreases with a very significant decrease in
alcohol selectivity (A/K, 5.2). Upon lowering the Lewis acidity of
the Ni(II) center by the strongly s-bonding imidazole donor (cf.
above), the catalytic activity is expected to increase by enhancing
the ligand exchange process, but a decrease in catalytic activity is
observed. Also, upon replacing both the pyridylmethyl arms in 1
by imidazolylmethyl arms to give 4, cyclohexane is converted to
365 TON of cyclohexanol and 13 TON of cyclohexanone and 50
TON of e-caprolactone (Total TON, 428; A/K, 5.7). In this case
also, upon decreasing the Lewis acidity of the Ni(II) center, the
catalytic activity is lowered with only a small increase in selectivity.
It is expected that the decrease in Lewis acidity of the Ni(II)
center and the absence of p-back bonding pyridine donors may
ylation of alkanes, while m-chlorobenzoic acid radical undergoes
decarboxylation to form chlorobenzene in greater than 60%
yield. The observation of chlorobenzene supports the involvement
∑
+
of the intermediate species [(4N)(CH CN)Ni-O ] in catalysis
3
and indicates that m-chlorobenzoate radical is not the reactive
intermediate, because it readily undergoes decarboxylation rather
than hydrogen abstraction from the alkane.
The time course of the TON for 1 in cyclohexane oxidation is
shown in Fig. 6, which clearly indicates that the catalytic activity
of the Ni(II) complexes gradually proceeds even after 60 min.
-
3
Also, upon increasing the substrate loading from 2.45 mol dm to
.50 mol dm- (Table 6), the catalytic activity increases from the
total TON of 622 to 730 but with the same alcohol selectivity
A/K, 8.8). The very good A/K value observed indicates the
3
3
(
absence of any free radical auto-oxidation. Interestingly, upon
replacing the tetraphenylborate counter anion in 1 by perchlorate,
as in 1a, the total TON decreases appreciably to 479, with
the selectivity also decreasing enormously to 4.6 and the yield
of e-caprolactone increasing to 62 TON. This shows that the
perchlorate counter anion is coordinated in solution and retards
the ligand exchange process with the oxidant m-CPBA (Scheme 2).
However, when the number of equivalents of m-CPBA is increased,
with an aim to increase the concentration of the intermediate
and hence the reactivity, the amount of e-caprolactone formed
increases as a result of secondary oxidation of cyclohexanone by
the increased oxidant concentration.
destabilize the reactive intermediate species [(4N)(CH CN)Ni–
3
∑
+
O ] leading to a decrease in the reactivity. Upon replacing both
the pyridylmethyl arms in 1 by quinolylmethyl arms to give 5,
cyclohexane is oxidized to 406 TON of cyclohexanol and 20 TON
of cyclohexanone and 50 TON of e-caprolactone (A/K, 5.8) with
a decrease in catalytic activity (Total TON, 476). Obviously, the
steric hindrance of the bulky quinolylmethyl arms to the binding
of m-CPBA becomes significant. So, it is expected that the Ni(II)
complex of a supporting ligand with strongly s-bonding phenolate
donor(s) would act as a poor catalyst. However, it has been
69
reported that the Ni(II)-bis(phenolate) complexes act as better
catalysts than [Ni(TPA)(OAc)(H O)]BPh . It is possible that the
2
4
phenolate donors remain protonated in solution also.
Under conditions identical to those used for 1, the complex
[
Ni(L7)(CH
3
CN)
2
](BPh
4
2
) 7 catalyses the oxidation of cyclohex-
ane to cyclohexanol with a total TON of only 505 with almost
the same selectivity (A/K, 8.1) illustrating that the third pyridyl
nitrogen donor located cis to the other two pyridyl nitrogen
67
donors at a longer distance is not effective in p-back bonding
and hence a decrease in stability of the reactive intermediate
∑
+
species [(4N)(CH
3
CN)Ni–O ] leading to a decrease in reactivity
83
of 7. Also, upon replacing the weakly coordinating –NMe
group in 1 by the pyridyl moiety as in [Ni(L7)(CH
2
2
+
3
CN)
2
]
7,
the Ni(II) center is rendered slightly less Lewis acidic, which
does not facilitate substitution of coordinated CH CN by the
3
Fig. 6 Time dependent oxidation of cyclohexane catalyzed by 1 with
m-CPBA.
oxidant m-CPBA. A similar lower catalytic activity is observed
for 6 with three benzimidazolyl nitrogen donors. Upon replacing
2
+
all the three pyridylmethyl arms in [Ni(L7)(CH
3
CN)
2
]
7 by
benzimidazolylmethyl arms to give 6, cyclohexane is oxidized to
only 280 TON of cyclohexanol, 34 TON of cyclohexanone and
25 TON of e-caprolactone, revealing a large decrease in both
the catalytic activity (Total TON, 339) and selectivity (A/K,
4.7). It is clear that steric hindrance to m-CPBA binding by
the bulky benzimidazolyl moiety and the decrease in both the
Lewis acidity of the Ni(II) center and metal–ligand covalency
become significant in determining the catalysis. However, the
Effect of supporting ligands
It is interesting to compare the catalytic behavior of 1 towards hy-
droxylation of cyclohexane with those of related nickel(II) catalysts
2–7 under identical conditions. Upon replacing the –NMe
2
group
in 1 by an –NEt group to give 2, the product yields observed for
2
cyclohexane oxidation are 480 TON of cyclohexanol, 11 TON of
cyclohexanone and 46 TON of e-caprolactone (Total TON, 537;
A/K, 8.4). This illustrates that the increase in steric bulk of the
nitrogen donor around Ni(II) renders its coordination to Ni(II)
weaker (cf. above), enhancing the Lewis acidity of Ni(II) center
67
complex [Ni(TPA)(OAc)(H
2
O)]BPh oxidizes cyclohexane to
4
cyclohexanol with a higher total TON of 656, but the alcohol
selectivity remains almost the same (A/K, 8.5). It is possible that
∑
+
-
further and decreasing the stability of the [(4N)(CH
3
CN)Ni–O ]
the coordinated acetate ion, unlike ClO
4
in 1a, may a play a role
species and hence the catalytic activity. Also, the steric bulk of
the –NEt group may contribute to the decreased binding of m-
CPBA to Ni(II). Upon replacing one of the pyridylmethyl arms in
in facilitating the binding of m-CPBA.
All the above observations reveal that the replacement of
a pyridyl donor bound to Ni(II) by a weakly coordinating
2
9
422 | Dalton Trans., 2011, 40, 9413–9424
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–
NMe
2
/–NEt
2
group enhances the Lewis acidity of the Ni(II) cen-
oxidation also but with almost the same selectivity. However, 2
catalyses adamantane oxidation to 650 TON, which is higher than
that for 1. All the other complexes also catalyze the oxidation
ter and encourages the binding of m-CPBA, whereas replacement
by a strongly s-bonding (benz)imidazolyl moiety enhances the
electron density on the Ni(II) center leading to the weaker binding
of m-CPBA. Also, a higher Lewis acidity of the Ni(II) center
stabilizes the [(4N)(CH
catalytic activity towards cyclohexane oxidation with m-CPBA as
oxidant. Furthermore, a p-accepting ligand donor such as pyridine
◦
◦
with good TON and high 3 /2 ratio. Thus, upon exchanging a
◦
◦
donor around Ni(II) for a stronger donor, the 3 /2 ratio increases
∑
+
◦
◦
3
CN)Ni-O ] species leading to a higher
gradually from 12.5 up to 17.0. The high 3 /2 selectivity observed
∑
+
indicates involvement of the [(4N)(CH CN)Ni–O ] species as
3
metal-based oxidant in adamantane oxidation also.
stabilizes the reactive radical intermediate [(4N)(CH
3
CN)Ni–
The catalytic activities of the Ni(II) complexes were further
explored for the oxidation of ethylbenzene and cumene and
the results are summarized in Table 9. All the Ni(II) complexes
catalyse the oxidation of ethylbenzene to give 2-phenylethanol and
acetophenone but with moderate selectivity only. Additionally,
they catalyze the oxidation of cumene selectively to form 2-phenyl-
2-propanol without any side product formation.
∑
+
O ] leading to a higher TON in the catalytic oxidation of
hydrocarbons, while a s-donor ligand destabilizes it leading to
a decreased TON. Moreover, a bulky quinolyl/benzimidazolyl
moiety sterically hinders binding of m-CPBA to Ni(II) center
leading to a decrease in the catalytic activity. Very interestingly, a
linear correlation between the metal–ligand covalency parameter
b and TON is obtained (Fig. 7); however, complex 4 shows, in
spite of a higher metal–ligand covalency parameter b due to
two imidazolyl nitrogen donors, a lower TON, and this may be
because of the lower Lewis acidity of the Ni(II) center due to the
strongly s-bonding imidazolyl nitrogen donors (cf. above). Thus, it
is evident that a p-accepting ligand donor such as pyridine and/or
Conclusions
Several nickel(II) complexes derived from tripodal tetradentate
4N ligands have been isolated and their ability to carry out
alkane functionalization was studied using m-CPBA as an oxidant.
All the Ni(II) complexes with distorted octahedral coordination
geometry catalyze the hydroxylation of the alkanes cyclohexane,
adamantane, ethylbenzene and cumene using m-CPBA as oxidant.
The observed variation in alcohol selectivity for cyclohexane
oxidation suggests the involvement of a metal-based oxidant
rather than a freely diffusing radical in the reaction. The ligand
stereoelectronic factors and Lewis acidity of the Ni(II) center
are found to be significant in determining the catalytic activity
as well as the selectivity of alkane oxidation. For adamantane
oxidation, however, the incorporation of a sterically hindering
quinolyl/benzimidazolyl moiety to support Ni(II) leads to a high
a weakly coordinating–NMe
2
/–NEt
2
donor is needed to achieve a
higher catalytic activity by stabilizing the reactive intermediate
II
∑
+
[
(4N)(CH CN)Ni –O ] involved in the catalytic oxidation of
3
hydrocarbons.
◦
◦
3
/2 bond selectivity. A linear correlation between the metal–
ligand covalency parameter (b) and the turnover number has been
observed.
Acknowledgements
We sincerely thank the Department of Science and Technol-
ogy, New Delhi, for a Pre-doctoral Fellowship [Scheme No.
SR/S1/PRD-04/2006] to M.B under new Initiatives in Bioinor-
ganic Chemistry and for supporting this research [Scheme
No. SR/S5/BC-05/2006]. This work was also supported by
Indo-French Centre (IFCPAR) [Scheme No. IFC/A/4109-
Fig. 7 Correlation of TON with metal–ligand covalency parameter b.
Adamantane oxidation
1
/2009/993]. Professor M. Palaniandavar is a recipient of a
The catalytic activity of all the Ni(II) complexes towards oxidation
of adamantane has been also explored and the results are summa-
rized in Table 8. All the Ni(II) complexes catalyse the oxidation of
adamantane efficiently to give 1-adamantanol and 2-adamantanol
as the major products along with 2-adamantanone as the minor
product. Complex 1 catalyzes the oxidation of adamantane to give
DST Ramanna Fellowship [Scheme No. SR/S1/RFIC-01/2007
and SR/S1/RFIC-01/2010]. We thank Department of Science
and Technology (FIST program), New Delhi for the use of the
diffractometer in our School of Chemistry.
5
16 TON of 1-adamantanol, 113 TON of 2-adamantanol and 10
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