Synthesis, characterisation and activity in catalytic epoxidation of olefins of new dinuclear nickel(II) complexes

Article abstract of DOI:10.3184/030823409X12491271131153

Two new dinuclear nickel(II) compounds of formula [NiII₂(μ- L¹₂](ClO₄)₂ MeCN (1 MeCN) and [NiII₂(μ-L²)₂](ClO₄)₂ (2) where HL¹ and HL

Full text of DOI:10.3184/030823409X12491271131153

JOURNAL OF CHEMICAL RESEARCH 2009
AUGUST, 527–532
RESEARCH PAPER 527
Synthesis, characterisation and activity in catalytic epoxidation of olefins
of new dinuclear nickel(II) complexes
Manindranath Bera*
Department of Chemistry, Birla Institute of Technology & Science-Pilani, Hyderabad Campus, Hyderabad – 500078, AP, India
Two new dinuclear nickel(II) compounds of formula [NiII (m-L1)2](ClO4)2·MeCN (1·MeCN) and [NiII (m-L2)2](ClO4)2 (2)
2
2
where HL1 and HL2 stand for 3-(2-(dimethylamino)ethylimino)butan-2-one oxime and 1-(2-(dimethylamino)ethyl-
imino)-1-phenylpropan-2-one oxime respectively, have been synthesised. Single crystal X-ray analyses of the
complexes reveal that the nickel(II) ions are in square-planar N3O environments and form six-membered (NiNO)2
metallacycles. Cyclic voltammetric measurements of 1·MeCN and 2 in MeCN solution show quasirreversible one-
electron oxidations at E1/2 = 0.566 V and 0.603 V (vs Fc+/Fc), respectively, attributed to NiIIINiII/NiII redox couples.
2
Additional reversible NiIII2/NiIIINiII redox responses were observed at relatively higher potential near E1/2 = 0.832 V and
0.850 V (vs Fc+/Fc), respectively, for 1·MeCN and 2.Complexes 1·MeCN and 2 display intense charge-transfer bands
at ~390 and ~345 nm in the visible region. Chemical oxidation of complex 1·MeCN by sodium hexachloroiridate(IV)
III
hexahydrate generates red Ni2 species with characteristic new bands at ~520 and 427 nm in the visible region as
well as the characteristic EPR signals at 77 K with g┴ > gII. Similar phenomena were observed for complex 2 upon
chemical oxidation. The dinickel(II) complexes are catalytically active for epoxidation of olefins using iodosylbenzene
as the terminal oxidant.
Keywords: dinickel(II) complex, oxime ligand, X-ray crystal structure, NMR, olefin epoxidation
There are many nickel(II) complexes with ligands that
incorporate oximate donors that have been shown to
stabilise the nickel(III) and nickel(IV) oxidation states at low
potential.1-5 Such complexes in different oxidation states have
a strong role in bioinorganic chemistry and redox enzyme
systems,6,7 and may provide the basis for models for active
sites of biological systems8,9 or act as catalysts.10-12 Oximes
represent a very important class of ligands in coordination
chemistry.13-16 Recent interest in oxime ligands is mostly
due to the remarkable ability of the deprotonated oximato-
groups to form bridges between metal ions giving rise to
polynuclear complexes of different nuclearity with different
types of oximate bridges.17,18 Recently, Goldcamp et al.
reported19 that the ligand tris(2-hydroxyiminopropyl)amine
binds to Ni(II) ion in multiple protonation states yielding
mononuclear and oximate-bridged dinuclear complexes.
The dinuclear complex of the fully deprotonated ligand
in acetonitrile undergoes oxidation by O2 and this system
oxidises PPh3 with incorporation of oxygen from O2. With a
similar system, they also reported substrate oxidation by the
first Ni(II) + O2 reaction that does not proceed via irreversible
ligand oxidation.20 The first example of olefin epoxidation
by nickel(II) catalysts with iodosylbenzene was shown by
Koola and Kochi.21 Later, Burrows et. al.22 also reported the
syntheses and crystal structures of nickel(II) complexes and
has used them as catalysts in the presence of iodosylbenzene
for epoxidation of a wide variety of olefins. All the above
examples have been studied with mononuclear nickel(II)
systems. The application of dinuclear nickel(II) complexes in
the epoxidation of olefins has received very limited attention.23
So, in this paper, the synthesis, characterisations, electron
transfer behaviour and catalytic activity of oximate-bridged,
dinuclear nickel(II) complexes are reported in detail.
Results
The compounds 1·MeCN and 2 were synthesised in ~90%
yield from a methanolic medium under aerobic conditions at
room temperature by stirring a reaction mixture consisting of
Ni(ClO4)2·6H2O, HL1 (prepared by condensation of 1 mol
of 2,3 butanedione monoxime and 1 mol of N,N-dimethyl
ethylenediamine)andNEt3 andNi(ClO4)2·6H2O,HL2(prepared
by condensation of 1 mol of 1-phenyl-1,2-propanedione-
2-oxime and 1 mol of N,N-dimethyl ethylenediamine) and
NEt3 respectively in a 1:1:1 molar ratio for 1 h. Thus, the
reaction of the preformed tridentate imine-oxime ligands
(HL1 and HL2) with nickel(II) ions in the presence of NEt3
gave the square-planar dinuclear NiII compounds, containing
a Ni2 hexagon formed from bridging N–O group (Scheme 1).
2+
H3C
R
H3C
CH3
CH3
R
N
N
N
O
N
N
Ni(ClO4)2.6H2O
Ni
CH3
Ni
NEt3/MeOH
N
N
O
N
N
H3C
H3C
CH3
OH
R
HL1, when R = CH3
HL2, when R = Ph
CH3
Complex 1, when R = CH3
Complex 2, when R = Ph
Scheme 1 Synthetic procedure for the complexes.
* Correspondent. E-mail: manindra05@yahoo.co.in

528 JOURNAL OF CHEMICAL RESEARCH 2009
The complexes precipitate directly from the reaction mixture
as dark orange-red solids. Copper(II) ions are known to
bind with the similar tridenatate imine-oxime ligand to
give distorted square pyramidal dinuclear complexes.24,25
Three nitrogen atoms of one ligand bind the first nickel(II)
ion in a tridentate N3 mode and the oximate oxygen atom of
the same ligand, binds the second nickel(II) ion. The whole
ligand thereby binds two nickel(II) ions forming a six-
membered metallacycle. During coordination to the nickel(II)
ion, each ligand loses one proton from the oxime OH group
and the resulting oximate ligand exhibits N3O coordination
to each nickel(II) ion. The crystalline cationic complexes
are soluble in more polar solvents such as H2O, MeCN, DMF
and DMSO and are insoluble in less polar solvents such as
CHCl3, THF, Et2O and C6H6. The complexes are highly
stable in air both in solid state and in solution. The elemental
analysis data fit the [NiII (m-L1)2](ClO4)2·MeCN and [NiII (m-
Table 1 Crystallographic data for complexes 1·MeCN and 2
Molecular formula
Molecular weight
Crystal system
Space group
a (Å)
C18H35N7O10Cl2Ni2
697.85
monoclinic
P 21/n
13.1320(3)
12.2045(3)
17.2959(5)
91.6220(14)
2770.89(12)
1.673
4
1448
0.23 ¥ 0.22 ¥ 0.14
1.615
C26H36N6O10Cl2Ni2
780.93
monoclinic
P 21/n
12.5432(5)
8.9714(4)
13.7277(5)
93.745(2)
1541.48(11)
1.682
b (Å)
c (Å)
b (°)
V (Å3)
Dc (g cm-3)
Z
2
808
F(000)
Crystal size/mm
0.45 ¥ 0.44 ¥ 0.32
1.461
m (mm-1)
q Range (°)
R1, wR2 [I > 2s(I)]
1.92-31.50
0.0349, 0.0894
2.13–47.26
0.0229, 0.0599
1.028
Goodness-of-fit on F2 1.056
Final difference map
2
2
max, min (e Å-3)
0.842, -0.479
0.714, -0.807
L2)2](ClO4)2 formulations. The molar conductivity (LM)
values of the complexes in MeCN are in the range of 190–
195 Ω-1cm2mol-1 indicating a behaviour attributable to 1:2
electrolytes.
R1 = S( ||Fo| - |Fc|| )/S|Fo|. wR2 = [ Sw(|Fo| - |Fc|)2/Sw(Fo)2]1/2
.
w = 0.75/(s2(Fo) + 0.0010Fo2).
Table 2 Selected bond lengths (Å) and angles (°) in 1·MeCN (a)
and 2 (b)
Crystals of 1·MeCN and
2
suitable for X-ray
structure determination were obtained from acetonitrile-
dichloromethane and chloroform-hexane solvent mixtures
respectively. Thermal ellipsoid plots of the cationic part of
1·MeCN and 2 are depicted in Fig. 1. Information concerning
the X-ray data acquisition and refinement is given in Table 1.
Selected bond lengths and bond angles are given in Table 2.
Bond lengths (a)
Bond angles (a)
Ni(1)–N(1)
1.8888(13)
1.8519(12)
1.9598(13)
1.8660(10)
1.8866(13)
1.8420(12)
1.9531(14)
1.8471(10)
N(2)–Ni(1)–O(2)
169.20(5)
82.55(6)
101.43(5)
85.17(6)
89.64(5)
166.31(5)
168.98(5)
82.93(5)
101.81(5)
86.59(6)
88.85(5)
169.34(5)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–O(2)
Ni(2)–N(4)
Ni(2)–N(5)
Ni(2)–N(6)
Ni(2)–O(1)
N(2)–Ni(1)–N(1)
O(2)–Ni(1)–N(1)
N(2)–Ni(1)–N(3)
O(2)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
N(5) –Ni(2)–O(1)
N(5)–Ni(2)–N(4)
O(1)–Ni(2)–N(4)
N(5)–Ni(2)–N(6)
O(1)–Ni(2)–N(6)
N(4)–Ni(2)–N(6)
Complex 1·MeCN consists of a dinuclear [NiII (m-L1)2]2+
2
cation, two perchlorate anions and one molecule of acetonitrile
of crystallisation. Complex 2 consists of a dinuclear [NiII
2
(m-L2)2]2+ cation and two perchlorate anions. The coordination
geometries around each nickel(II) ion in complex 1·MeCN
and 2 are best described by distorted square planar N3O
environments. One amine nitrogen, one imine nitrogen, one
oxime nitrogen and one oximate oxygen satisfy the square-
planar coordination geometry of each nickel(II) ion. The
largest deviations from the plane of the four donor atoms are
0.020 (N2) and 0.090 Å (N6) with the metal ion 0.138 (Ni1)
and 0.072 (Ni2) Å out of the mean basal plane for 1·MeCN.
In complex 2, the largest deviation from the plane of the four
donor atoms is 0.014 Å (N2) with the metal ion 0.012 Å out of
the mean basal plane. Three nitrogen atoms of one ligand are
coordinated to one metal centre forming two five-membered
(N-Ni-N) chelate rings. The average chelate bite angles for
the five-membered rings in 1·MeCN and 2 are in the ranges
Bond lengths (b)
Bond angles (b)
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–O(1)
1.8827(4)
1.8439(4)
1.9442(4)
1.8348(4)
O(1)–Ni(1)–N(2)
O(1)–Ni(1)–N(1)
N(2)–Ni(1)–N(1)
O(1)–Ni(1)–N(3)
N(2)–Ni(1)–N(3)
N(1)–Ni(1)–N(3)
174.274(17)
102.525(17)
82.999(17)
88.804(17)
85.674(17)
168.671(17)
82.74(5)–85.88(6)° and 82.99(17)–85.67(17)° respectively.
In complexes 1·MeCN and 2, one central six-membered
dimetallic chelate ring Ni(1)N(1)O(1)Ni(2)N(4)O(2) and Ni
(1)N(1)O(1)Ni(1A)N(1A)O(1A), respectively, is formed
Fig. 1 Molecular structures and numbering schemes for complexes (a) [NiII (m-L1)2](ClO4)2·MeCN (1·MeCN) and (b) [NiII (m-L2)2](ClO4)2 (2).
2
2

JOURNAL OF CHEMICAL RESEARCH 2009 529
throughthecoordinationofoximenitrogenandoximateoxygen
to each nickel(II) ion in a trans arrangement. The average C–N
and N–O distances of the oximate (–(CH3)C=N–O-) moieties
in 1·MeCN are 1.318(17) and 1.338(15) Å, respectively and in
2 are 1.313(6) and 1.326(5) Å, respectively. These distances
are consistent with the deprotonated form of the oxime
functionalities.26-28 The C=Nimine distances are significantly
shorter than the C=Noximate distances in both complexes.
The intradimer Ni–Ni distances are 3.602 Å in 1·MeCN and
3.653 Å in 2 which are consistent with Goldcamp's octahedral
dinickel(II) complex19 but quite larger than distances in other
square planar dinickel(II) systems.23,29,30 The dihedral angle
between the two basal planes is 22.2° in 1·MeCN, whereas in
complex 2 it is 0°.
An interesting feature of the crystal structure lies in the
packing of complex 1·MeCN (Fig. 2), in which the geometry
of the perchlorate groups shows some typical disorders.
The disordered perchlorate anion fills the channels that run
parallel to the b-direction. The other perchlorate anion is
in pockets surrounded by cations. Within these channels
the uncoordinated acetonitrile molecule makes a 2.513 Å
contact with the Ni(1) ion. The Ni–N–C contact angle is
172.75°. There is no evidence of channels in the packing of
complex 2.
The IR spectra of the complexes show strong C=N
stretching of the imine function at ~1630 cm–1 which is
shifted from the free ligand value of ~1639 cm–1. The bands
at ~1580 cm-1 and ~1235 cm-1 in the IR spectrum of the
complexes may be assigned to the oxime C=N groups and NO
–
groups respectively.31,32 The strong unsplit band (nClO4
)
at around 1090 cm–1 suggests absence of coordinated
perchlorate ions.
The complexes are diamagnetic both in solution and in
1
the solid state, as evidenced by sharp, unshifted H NMR
spectra and a magnetic susceptibility measurement consistent
with a square planar geometry. The 1H NMR spectra of
1·MeCN and 2 show two multiplets corresponding to the
two sets of ethylenic protons at d ~3.47–3.65 ppm and 2.65–
2.78 ppm. The sharp singlets corresponding to the amine-
methyl protons and both imine- and oxime-methyl protons
1
in H NMR spectrum appear at d ~1.90 ppm and 2.19 ppm
respectively. The aromatic protons in complex 2 appear as
multiplets in the ranges d = 7.21–7.65 ppm. The absence
of a broad signal due to the oxime proton (in free ligand,
d ~11.55 ppm) in the complex indicates that the ligands
undergo complete deprotonation during complexation and
in solution the dimeric structures exist. The sharpness of
these NMR peaks as well as the small shift of approximately
0.35 ppm for the above proton signals relative to that of free
ligand are indicative of the diamagnetic nickel(II) complex.
That is to say, the NiII ions are in a square-planar coordination
geometry (low-spin d8-electronic configuration for NiII).
These spectroscopic observations were further confirmed by
the single crystal X-ray structure determination.
The electronic absorption spectra of the complexes
1·MeCN and 2 in acetonitrile solution are dominated by two
intense bands in the UV region centred at ~392 and ~340 nm,
corresponding to charge-transfer transitions.33,34 A third, less
intense band appears in the visible region at ~550 nm, which
corresponds to a d–d transition of a low-spin d8–NiII ion.30
Chemical oxidation of 1·MeCN and 2 in MeCN by sodium
hexachloroiridate(IV) hexahydrate in 1:2 molar ratio gives a
III
red-coloured solution developing characteristic Ni2 bands
III
at approximately 520 nm and 427 nm.The Ni2 species is
reasonably stable at ambient temperature only under an argon
atmosphere and the bands are assigned to d–d transition and
charge-transfer transition respectively. In addition to the
427–520 nm bands, some further low intensity bands/
shoulders are observed at longer wavelengths which are
presumably due to axial solvent coordination to the two nickel
centres.34 Representative spectra for complex 1·MeCN are
shown in Fig. 3.
Fig. 2 Molecular packing view of [NiII2(m-L1)2](ClO4)2·MeCN
(1·MeCN) along the b-axis.
Fig. 3 Electronic absorption spectra in acetonitrile solution of (a) [NiII2(m-L1)2](ClO4)2·MeCN (1·MeCN) and (b) chemically oxidised
III
Ni2 species.

530 JOURNAL OF CHEMICAL RESEARCH 2009
The EPR spectrum of the red solution is obtained after
chemical oxidation of 2 by sodium hexachloroiridate(IV)
hexahydrate in 1:2 molar ratio. The spectrum can be
interpreted in terms of mononuclear NiIII chromophores in an
axial geometry, which is in line with the above observation
that, at these temperatures, the ions within the complex are
essentially uncoupled. This is consistent with a tetragonally
distorted octahedral geometry at the metal centres, presumably
with axial coordination of solvent molecules.35,36 The value of
g┴ and gII are 2.11 and 1.99 respectively.
Table
3
Epoxidation of olefin compounds catalysed by
1·MeCN and 2 using iodosylbenzene
Entry Substrates
Catalyst % Conversiona Turnover
numberb
1
2
3
4
5
Styrene
1·MeCN
74
71
81
78
44
49
70
65
53
57
24
21
35
32
14
17
37
35
19
22
2
1-Hexene
Cyclohexene
1-Octene
1·MeCN
2
1·MeCN
2
1·MeCN
With the aim of investigating the extent of stabilisation
of nickel(II) species towards both oxidation and reduction,
and thereby the possibility of accessing variable oxidation
states, cyclic voltammetry experiments were performed
on 1·MeCN and 2. Both the complexes are electroactive
with respect to the metal centres in acetonitrile solution
(tetrabutylammonium perchlorate as supporting electrolyte,
at 298 K). Both the complexes 1·MeCN and 2 display a
quasireversible cyclic voltammetric response with E1/2 near
0.566 V(DEp = 102 mV) and 0.836 V(DEp = 125 mV) (vs Fc+/
Fc) respectively,. Additional quasireversible redox responses
were observed at relatively higher potential near E1/2 = 0.832
V(DEp = 67 mV) and 0.850 V(DEp = 77 mV) (vs Fc+/Fc),
respectively, for 1·MeCN and 2. The lower potential oxidative
2
1·MeCN
2
5-Hexen-1-ol
aDetermined by GC analysis.
bMol of epoxide/mol of catalyst used.
The kinetics of these systems to investigate the reactive
transient intermediate formed during the catalysis has
been studied. This experiment was done using the catalyst
1·MeCN. A transient spectral band at ~325 nm suggests
the formation of a reactive intermediate. These spectral
changes demonstrate that the dicationic NiII catalyst reacts
2
readily with iodosylbenzene to afford a new transient
species, which is most likely a dinickel(III)-oxo species.41-43
Unfortunately, single crystals of this reactive intermediate
could not be obtained despite great efforts, even at –40°C.
The decay of this absorption band in presence of added
styrene follows approximately first-order kinetics at 25°C as
shown in Fig. 4.
response observed is possibly due to the NiIIINiII/NiII /couple.
2
The relatively higher potential oxidative response observed is
due to the NiIII2/NiIIINiII couple. The oxidation processes can
be assigned as follows:
NiIINiII ꢀ
ꢀꢀNiIIINiII ꢀ
ꢀꢀNiIIINiIII
Similar behaviour has been reported for other square-planar
dinuclear nickel(II) complexes.37
The majority of successful catalytic epoxidations have
been developed using mononuclear metal complexes.21,38,39
A major breakthrough in this respect was the discovery by
Jacobsen and co-workers of the enantioselective epoxidation
of unfunctionalised alkenes by chiral manganese salen
catalysts.40 The catalytic efficiency of 1·MeCN and 2 for the
epoxidation of olefins using iodosylbenzene as the terminal
oxidant has been examined. The products were analysed
by GC–MS relative to the internal standard nitrobenzene.
The products were isolated in medium to high yield and
1
characterised by H NMR. In all cases, the products of the
epoxidation using 1·MeCN and 2 were almost identical
and produced in the same ratio. This led us to conclude that
both the complexes catalyse the epoxidation of olefins in
the presence of iodosylbenzene through the formation of a
reactive transient intermediate, most likely, a dinickel(III)-oxo
species, in an intermolecular fashion.21 Thus activated oxygen
and olefin are arranged in such a way that oxygen transfer
to the olefin occurs in the vicinity of the ligand. Therefore,
the overall shuttling of the metal complex between the two
states formally corresponds to an oxidative–addition/
reductive–elimination sequence. But the detailed insight into
the possible cooperative effects of the two nickel centres
in the dinuclear catalysts needs more mechanistic study.
Here, the results of our study on the epoxidation of styrene,
1-hexene, cyclohexene, 1-octene and 5-hexen-1-ol are
reported. The percentage yields and turn-overs from reactions
with these olefins are shown in Table 3. These results indicate
that both 1·MeCN and 2 show almost identical catalytic
efficiency using the same terminal oxidant, iodosylbenzene.
The control experiments have also been performed using
Ni(ClO4)2·6H2O as a catalyst. The experiment did not give rise
to the epoxide formation, which indicates that for oxidation
with iodosylbenzene as the terminal oxidant, nickel activation
by the coordination of a suitable ligand, as in 1·MeCN and 2,
is essential.
(a)
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
0
5
10
15
Time/min
(b)
20
25
30
Fig. 4 (a) decay of the transient absorption band at 325 nm
derived from [NiII2(m-L1)2](ClO4)2·MeCN (1·MeCN) (10-3M) and
excess iodosylbenzene in acetonitrile in presence of 0.03M
styrene; (b) change in ln(At–An) with time at 25°C followed
at 325 nm.

JOURNAL OF CHEMICAL RESEARCH 2009 531
1447(s), 1234(s),1094(s), 924(s), 722(s). Molar conductance, LM:
(MeCN solution) 194.46 Ω-1cm2mol-1. UV-vis spectra [lmax, nm (e,
L mol-1cm-1)]: (MeCN solution) 563 (225), 387(6240), 343 (7550),
231(20435)
Catalysis reactions: To a solution of olefin (5 mmol) and the
catalyst (5 µmol) was added iodosylbenzene (0.5 mmol) in degassed
acetonitrile (5 mL) under an argon atmosphere. The reaction mixture
was stirred vigorously for 4 h. Products were analysed by GC relative
to the internal standard nitrobenzene. A blank experiment was
also performed in a similar way using Ni(ClO4)2·6H2O instead of
catalyst.
Conclusions
Two new square-planar dinuclear nickel(II) complexes
incorporating a six-membered dimetallic chelate ring have
been synthesised and fully characterised. The spectral,
electrochemical and catalytic properties of the complexes
were studied. Both the complexes catalyse the epoxidation of
olefins with almost similar efficiency. An extended work on
manganese, iron and cobalt complexes probing the effects of
structure on reactivities in small ligand binding and catalysis
is on now progress.
CAUTION: Perchlorate salts of metal complexes with
organic ligands are potentially explosive. Only a small amount
of material should be prepared and handled with caution.
Experimental
Materials and physical measurements:
Ni(ClO4)2·6H2O, N,N-dimethyl ethylenediamine, 2,3-butanedione
monoxime and 1-phenyl-1,2-propanedione-2-oxime were purchased
from Aldrich and used as received. HPLC-grade acetonitrile was
used for electrochemical, conductivity and UV/Vis work and all
other chemicals and solvents were of reagent grade and were used
as received. Iodosylbenzene was prepared by the hydrolysis of the
corresponding diacetate with aqueous sodium hydroxide and stored
at –20°C.44 Elemental analyses (C, H, N) were performed with a
Perkin–Elmer model 240 C elemental analyser. 1H and 13C NMR
spectra were recorded on a Bruker AC 300 NMR spectrometer using
TMS as the internal standard. Infrared spectra were recorded on a
Nicolet Nexus 870 FT IR spectrophotometer. The solution electrical
conductivity was obtained with a CDM 80 digital conductivity meter
with a solute concentration of about 10–3 (M). UV-vis spectra and
kinetics were run with a Varian Cary 9 UV-Visible spectrophotometer.
Mass spectra were obtained with a Finnigan MAT 8200 (electron
ionisation, EIMS) instrument. The room temperature magnetic
susceptibilities in the solid state were carried out with a MPMS
XL SQUID susceptometer (Quantum Design Inc). EPR spectra
was recorded at the X-band on a BRUKER Win EPR spectrometer
and calibrated with respect to diphenylpicrylhydrazyl (DPPH,
g = 2.0037). Electrochemical measurements were made with an
AutolabPGSTAT30electrochemicalanalyserwithGPES4.9software.
A platinum working-electrode, a platinum-wire auxiliary electrode,
and an aqueous saturated calomel reference electrode (SCE) were
used in a three-electrode configuration. The supporting electrolyte
was tetrabutylammonium perchlorate (TBAP), and the potentials
are referenced to the ferrocene-ferrocenium redox couple Fc+/Fc as
internal standard. Electrochemical measurements were made under
an argon atmosphere.
X-ray crystallography: Needle-shaped deep violet crystals of
complex 1·MeCN and parallelepiped red crystals of complex
2 suitable for XRD were obtained on recrystallisation from a
acetonitrile/dichloromethane (1:1) and chloroform/hexane (1:1)
mixtures respectively.The intensity data were collected on a Bruker
ApexII CCD X-ray diffractometer with graphite-monochromated
Mo-Ka radiation (l = 0.71073). Precise unit cell dimensions were
determined by least-squares refinement of 25 strong reflections
having 2q values. The structures were solved by the use of direct
methods, and refinement was performed by least-squares methods on
F2 with the SHELXL-97 package.45,46
Supplementary material: CCDC 292398 and 292399 contain the
supplementary crystallographic data for complexes 1.MeCN and 2.
These 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)
1223-336-033; or E-mail: deposit@ccdc.cam.ac.uk.
I thank Prof. Debashis Ray, Department of Chemistry,
IIT-Kharagpur, India for the helpful discussion in writing
throughout the manuscript.
Received 3 June 2009; accepted 24 July 2009
Paper 09/0619 doi: 10.3184/030823409X12491271131153
Published online: 10 August 2009
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Synthesis of the complexes:
[NiII2(m-L1)2](ClO4)2·MeCN (1·MeCN):
A methanolic solution
3
4
5
(20 mL) of N,N-dimethyl ethylenediamine (0.40 mL, 3.8 mmol) and
2,3 butanedione monoxime (0.38 g, 3.8 mmol) was stirred at room
temperature. After stirring for an hour, a methanolic solution (20 mL)
of Ni(ClO4)2·6H2O (1.38 g, 3.8 mmol) was added followed by the
addition of NEt3 (0.52 mL, 3.8 mmol). After complete addition a dark
orange-red compound was seen separating in solution. The whole
reaction mixture was stirred for another hour at room temperature.
The solvent was evaporated in air. The orange-red precipitate was
filtered through a glass frit and washed with ethanol and hexane.
Recrystallisation from acetonitrile/dichloromethane mixture gave
needle-shaped crystals, which were dried in vacuo over fused CaCl2.
Yield: 1.11 g (85%). Anal. Calcd for C18H35N7O10Cl2Ni2: C, 30.98;
H, 5.05; N, 14.05; Ni, 16.75. Found: C, 30.67; H, 4.97; N, 14.27,
Ni, 16.86%. 1H NMR ([D6]DMSO): d = 3.53–3.65 (m, 4H, CH2
attached to imine nitrogen), 2.65–2.69 (m, 4H, CH2 attached to amine
nitrogen), 2.19 (s, 6H, imine CH3), 2.27 (s, 6H, oxime CH3), 1.92
(s, 12H, amine CH3). FT IR (cm-1, KBr disk):n = 3429(b), 2924(m),
1632(s), 1583(s), 1537(s), 1475(s), 1237(s),1091(s), 963(s), 714(s).
Molar conductance, LM: (MeCN solution) 190.34 Ω-1cm2mol-1.
UV-vis spectra [lmax, nm (e, L mol-1cm-1)]: (MeCN solution) 556
(290), 395 (8770), 341 (8565), 225 (18750).
6
7
8
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[NiII2(m-L2)2](ClO4)2 (2): This complex was prepared by similar
method as for complex 1·MeCN using 1-phenyl-1,2-propanedione-
2-oxime instead of 2,3 butanedione monoxime. Yield: 1.38 g (92%).
Anal. Calcd for C26H36N6O10Cl2Ni2: C, 39.98; H, 4.64; N, 10.76;
Ni, 14.96. Found: C, 39.53; H, 4.77; N, 10.57, Ni, 14.75%.1H NMR
([D6]DMSO): d = 7.21–7.65 (m, 10H, C6H5), 3.47–3.65 (m, 4H, CH2
attached to imine nitrogen), 2.68–2.78 (m, 4H, CH2 attached to amine
nitrogen), 2.22 (s, 6H, oxime CH3), 1.90 (s, 12H, amine CH3). FT
IR (cm-1, KBr disk):n = 3425(b), 2936(m), 1627(s), 1587(s), 1520(s),

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