Studies on nickel(II) complexes with amide-based ligands: Syntheses, structures, electrochemistry and oxidation chemistry

Article abstract of DOI:10.1002/ejic.200701122

The present work discusses the nickel chemistry in a set of amide-based open-chain ligands with subtle differences in the backbone or terminal amine substituents. The ligands coordinate to the Ni²⁺ ion through the N_amide and N_amine atoms maintaining a square-planar geometry. Absorption spectra and NMR studies reveal that the solid-state square-planar geometry is retained in solution. The electrochemical results suggest that the Ni^III/Ni^II redox couple primarily depends on the N₄ donors, which is composed of two N_amide and two N_amine atoms and not on the peripheral substituents. All four ligands with variable backbone and substituents are equally competent in stabilizing the Ni^III state. On the basis of electrochemical findings, chemical oxidations were carried out, and they reveal generation of the Ni^III state in two cases, whereas decomposition was observed in others. Preliminary alkene epoxidation reactions suggest that the present nickel complexes transiently stabilize the higher oxidation state of the nickel ion that possibly participates in the oxidation of the substrates. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701122

FULL PAPER
DOI: 10.1002/ejic.200701122
Studies on Nickel(II) Complexes with Amide-Based Ligands:
Syntheses, Structures, Electrochemistry and Oxidation Chemistry
Jyoti Singh,[a] Geeta Hundal,[b] and Rajeev Gupta*[a]
Keywords: Nickel / N ligands / Structure elucidation / Electrochemistry / Oxidation
The present work discusses the nickel chemistry in a set of
amide-based open-chain ligands with subtle differences in
the backbone or terminal amine substituents. The ligands co-
ordinate to the Ni2+ ion through the Namide and Namine atoms
maintaining a square-planar geometry. Absorption spectra
and NMR studies reveal that the solid-state square-planar
geometry is retained in solution. The electrochemical results
suggest that the NiIII/NiII redox couple primarily depends on
the N4 donors, which is composed of two Namide and two
competent in stabilizing the NiIII state. On the basis of elec-
trochemical findings, chemical oxidations were carried out,
and they reveal generation of the NiIII state in two cases,
whereas decomposition was observed in others. Preliminary
alkene epoxidation reactions suggest that the present nickel
complexes transiently stabilize the higher oxidation state of
the nickel ion that possibly participates in the oxidation of
the substrates.
N
amine atoms and not on the peripheral substituents. All four
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
ligands with variable backbone and substituents are equally
The effect of the macrocyclic versus open-chain nature
of the ligands on the metal ions, in particular nickel, was
always considered to be an important feature and has been
frequently addressed.[10] A considerable amount of work
has also been done on the saturated tetraaza systems both
with macrocyclic as well as open-chain ligands.[11] Lampeka
and coworkers[12,13] through their elegant comparative stud-
ies on the dioxotetraaza systems have demonstrated the sig-
nificant changes in the structural, absorption spectral and
the redox properties of the nickel complexes caused by the
macrocyclic versus open-chain nature of the ligands.
Recently, we[14] showed nickel chemistry in a set of
amide-based macrocyclic ligands while keeping the primary
coordination sphere as well as the cavity size of the macro-
cycle fixed (Scheme 1). These macrocyclic ligands have been
shown to stabilize the +2 as well as +3 (transiently) oxi-
dation states of the nickel ion in a square-planar geometry.
The present work stems from our search to prepare and
study nickel complexes in an analogous non-macrocyclic li-
gand environment, and we have commenced a systematic
investigation to shed some light on the differences imposed
by the macrocyclic ligands versus open-chain ligands.
Herein, we report the design and synthesis of four open-
chain amide-based ligands and their coordination chemistry
towards the NiII ion (Scheme 2). The ligands were designed
in a way to keep the primary donor environment around
the metal centre identical, so that the effect of the open-
chain versus macrocyclic nature of the ligands on the physi-
cal, structural, spectroscopic and redox properties of the
nickel centre could be evaluated. Furthermore, we also
show the NiII-catalyzed epoxidation of the alkenes, which
Introduction
The interests in the coordination chemistry of nickel in
variable oxidation state(s) with synthetically designed li-
gands is an area of considerable significance with implica-
tions in chemistry and biology.[1] Our understanding about
nickel biological systems has tremendously been enlight-
ened after the recent spectroscopic and structural investi-
gations of the active-site structure from certain microorga-
nisms.[2] As a result of their biological significance as biomi-
metic models, as well as their potential application in cata-
lytic oxidation reactions,[3] the field of high-valent nickel
chemistry has continued to grow. In addition, the involve-
ment of NiIII species in a number of enzymes, such as [Ni–
Fe] hydrogenases,[4] Ni superoxide dismutase,[5] NiI/NiIII
species in CO dehydrogenases[6] and acetyl coenzyme-A
synthase,[7] demands special attention on the redox chemis-
try of the nickel ion. In this context, designed organic li-
gands hold potential with the unique ability to modulate
the redox chemistry of the nickel ion.[8] The ligand architec-
ture and its mode of coordination[9] is an important factor
responsible for the redox regulation of a metal centre, and
the present work is an attempt to understand the effect of
the macrocyclic versus open-chain nature of the ligands on
the structure and redox chemistry of the nickel ion.
[a] Department of Chemistry, University of Delhi,
Delhi, 110007 India
Fax: +91-11-2766-6605
E-mail: rgupta@chemistry.du.ac.in
[b] Department of Chemistry, Guru Nanak Dev University,
Amritsar, 143005 Punjab, India
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Studies on Nickel(II) Complexes with Amide-based Ligands
thus attests to the ability of the present nickel complexes N2 atmosphere followed by the addition of NiCl2 [or (Et4N)2-
to transiently stabilize higher oxidation states to promote NiCl4 for 4]. Usual workup and recrystallization [vapour
oxidation.
diffusion of diethyl ether into either a CH2Cl2 (for 1 and 3),
DMF (for 2) or MeCN (for 4) solution] afforded a highly
crystalline material in good recrystallized yield (60–80%).
Whereas complexes 1–3 are deep-yellow to orange in col-
our, complex 4 is brown.
In the FTIR spectra, the absence of the ν(N–H) stretch-
ing frequency and the observed bathochromic shift in the
C=Oamide stretch relative to the free ligand confirms the
involvement of the deprotonated amidate group in the
bonding.[14,15] Complexes 1 and 4 also have water as the
solvent of crystallization, which shows the ν(OH) absorp-
tion in the region of 3210–3430 cm–1.[15] Solution conduc-
tivity[16] data confirm the nonelectrolytic nature of all four
complexes, whereas the elemental analysis results authenti-
cate the purity of the bulk samples. In the electrospray mass
spectra, the molecular ion peak corresponding to complexes
1, 2, 3 and 4 was obtained at 287.08 [M + H]+, 357.69 [M
+ Na]+, 392.80 [M + H]+ and 639.76 [M + H]+, respectively,
which thus confirms the integrity of the nickel complexes
in the solution.
Scheme 1. NiII complexes of macrocyclic ligands.
Crystal Structure Studies
Representative complexes 2 and 4 were also charac-
terized by X-ray structure analysis, and their molecular
structures are shown in Figures 1 and 2 (see Figure S1 for
complete numbering scheme of 4), whereas the important
crystallographic and structural details are contained in
Tables 1, 2 and 5. The nickel ion is coordinated by two an-
ionic Namide atoms and two neutral Namine centres in a
square-planar geometry as also noted for macrocyclic ana-
logues 5–8.[14] In both complexes, the NiII centre is sur-
rounded by three five-membered chelate rings.
Scheme 2. NiII complexes of open-chain ligands.
Results and Discussion
Ligand Design and Synthesis
All four open-chain ligands, H2L1–H2L4, are designed to
simultaneously provide coordination through two amide
and two amine atoms while maintaining an identical pri-
mary coordination environment around the nickel centre.
The only difference is the substituents used at the Namine
end with the possibility to alter the electronic and steric
factors. The ligand H2L1 however, differs from the rest due
to the involvement of a flexible ethylene backbone whereas
other three ligands incorporate rigid o-phenylene backbone
as part of the ligand. These ligands were synthesized in two
steps starting from the desired primary diamines. The pri-
mary diamine was first converted into the bis(chloroaceta-
mide),[14] which was further treated with the secondary
amine (dimethyl-, diethyl- or dibenzylamine) to afford the
final ligands. During the synthesis of these ligands, an ex-
cess amount of the secondary amine was used, which also
functioned as a base in the reaction. All new ligands were
thoroughly characterized by using various spectroscopic
measurements and gave satisfactory microanalyses results.
Figure 1. Molecular structure of complex 2. Thermal ellipsoids are
drawn at 30% probability level. Hydrogen atoms are omitted for
clarity.
For complex 2, the average Ni–Namide distance of
1.842(2) Å is 0.144 Å shorter than the average Ni–Namine
Synthesis and Characterization of Nickel Complexes
All four NiII complexes, 1–4, were synthesized by treating distance of 1.986(2) Å. This difference may be attributed
the DMF solution of the ligand with solid NaH under an to the anionic coordination from the deprotonated Namide
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J. Singh, G. Hundal, R. Gupta
FULL PAPER
N2 and N4–C12–C11–N3 dihedral angles, respectively. The
small dihedral angle of ca. 1.6° for the N2–C5–C10–N3
fragment indicates the planarity induced by the rigid o-
phenylene part. This structure is in complete agreement with
that of 7 and in contrast to that of 5 and 6 where a flexible
ethylene fragment was the part of the macrocyclic ligand.[14]
For 4, the average Ni–Namide and Ni–Namine distances at
1.855(4) Å and 2.028(2) Å are 0.013 Å and 0.042 Å longer
than the similar distances in complex 2 (Table 2). The dif-
ference between the amide and amine donors is 0.173 Å.
The bond angles formed by the five-membered chelate rings
are within 83–84° and are at least 27° smaller than the N1–
Ni–N4 angle. In particular, the Namine centres have
stretched out appreciably possibly due to bigger benzyl sub-
stituents attached to it. The nickel ion is almost within the
N4 basal plane with a displacement of only 0.009 Å. All
five-membered chelate planes make angles of 9–13° with
each other as also seen for 2. The C15 and C24 carbon
atoms are significantly out of their respective chelate planes
Figure 2. Molecular structure of complex 4 with partial numbering
scheme. Thermal ellipsoids are drawn at 30% probability level. Sol-
vent molecule and hydrogen atoms are omitted for clarity.
Table 1. Selected bond lengths [Å] and angles [°] for [Ni(L2)] (2) as also noticed in 2. The four N atoms are almost planar
and [Ni(L4)] (4).
with average deviation of 0.1 Å from the plane; however,
[Ni(L2)] (2)
[Ni(L4)] (4)
quite significant deviations for the two Nbenzyl centres lead
them to be displaced opposite to each other. This displace-
ment has resulted in the unique arrangement of the at-
tached phenyl rings. Two phenyl rings A (C2 to C7) and B
(C9 to C14) on N1 and C (C26 to C31) and D (C33 to C38)
on N4 are nearly gauche to each other (dihedral angles of
47 and 58°, respectively). Although A and C are gauche (di-
hedral angle 69°), B and D are syn to each other (dihedral
angle 16°). Thus, A and C face outwards away from each
other and the metal ion, but B and D become parallel to
each other and face the metal ion. The latter give rise to
cation···π interactions with the metal ion, and the phenyl
rings B and D have average centroid-to-metal-ion distance
of ca. 3.8 Å.[17] Thus, these two phenyl rings may be consid-
ered to form two long bonds that occupy the fifth and sixth
coordination positions in a distorted octahedron around
the metal ion (Figure S2). Furthermore, the distance be-
tween the Ni ion and the benzylic carbon centres ranges
from 2.824 to 3.076 Å and the possibility of weak agostic
and/or M···H–bond interactions cannot be ruled out. Sim-
ilar interactions were observed for the coordination com-
plexes in the literature.[18]
Distance/Angle
Ni–N1
Ni–N2
Ni–N3
Ni–N4
N2–Ni–N3
N3–Ni–N4
N2–Ni–N4
N1–Ni–N3
N1–Ni–N2
N1–Ni–N4
1.982(2)
1.844(2)
1.840(2)
1.990(2)
84.24(10)
84.24(9)
167.96(10)
168.52(9)
85.10(10)
106.09(9)
2.034(4)
1.859(4)
1.851(4)
2.022(4)
84.32(18)
83.28(17)
164.69(18)
163.68(16)
82.77(17)
110.90(16)
donors instead of the neutral donation from the Namine
groups. A similar observation was noticed for the macro-
cyclic analogue (complex 7)[14] of this complex (Table 2).
The bond angles formed by the five-membered chelate rings
are smaller than 85° and indicate a tight chelation to the
nickel ion. The Namine–Ni–Namine bite angle at 106.09(9)° is
quite spread out and at least 20° larger than the other N–
Ni–N angles. The nickel ion is 0.072 Å displaced from the
N4 basal plane (defined by N1, N2, N3 and N4). Two lat-
eral five-membered chelate planes make an angle of ca. 15°
with each other, whereas they make an angle of 6–11° with
the central five-membered chelate ring involving two Namide
Weak C–H···X H-bonding interactions (Table S1) create
groups. The C3 and C12 carbon atoms are significantly out an interesting crystal structure. Phenylene C35 and methyl-
of their respective chelate planes as seen by the N1–C3–C4– ene C32 H-bonds with amide oxygen O2 of the centrosym-
Table 2. Comparative structural parameters of open-chain (2 and 4) and macrocyclic Ni2+ complexes 5–8.
Complex Average Ni–Namide Average Ni–Namine
Namide–Ni–Namide Namine–Ni–Namine Displacement of Ni out of N4 basal plane
[Å]
[Å]
[°]
[°]
[Å]
2[a]
4[a]
5[b]
6[b]
7[b]
8[b]
1.842(2)
1.855(4)
1.807(3)
1.818(4)
1.803(2)
1.814(4)
1.986(2)
2.028(4)
1.906(3)
1.910(4)
1.890(2)
1.898(4)
84.24(10)
84.32(18)
87.40(11)
87.90(12)
87.24(10)
87.47(18)
106.09(9)
110.90(16)
92.36(11)
92.08(12)
92.44(11)
91.95(17)
0.072
0.009
0.170
0.136
0.126
0.110
[a] This work. [b] Ref.[14]
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Studies on Nickel(II) Complexes with Amide-based Ligands
metrically related molecules forming a 1D linear polymeric complexes were also titrated with other potential axial li-
–
chain running parallel to the b axis in the ab plane (Fig- gands, such as Cl–, ClO4 , pyridine and 4-dimethylamino-
ure 3). Within this chain two centrosymmetric D phenyl pyridine, and no change in the spectral features was ob-
rings come close to each other and display π···π interactions served. These experiments strongly suggest that the nickel
between them with a centroid-to-centroid distance of ca. centre remains four-coordinated. Noteworthy is that the
3.8 Å. This π···π interaction further stabilizes the formation analogous copper(II) complex of ligand H2L2 [Cu(L2)]
of an H-bonded 1D polymer.
shows the axial coordination either from the Oamide atom
of the neighbouring molecule in the solid state or from the
solvent in solution.[22]
Figure 3. Molecular structure of complex 4 showing the H-bonding
interactions, π···π interactions and solvent molecule (CH3CN,
shown in ball-and-stick fashion), in the ab plane along the b axis.
See text for details.
Amide oxygen O2 is also H-bonded to phenylene C28 of
a symmetry-related molecule along the a axis in the ab
plane, which thus links two such parallel chains along the
plane. Hence, the O2 atom behaves as a triple H-bond ac-
ceptor and is responsible for the 2D extension of the poly-
mer. The acetonitrile solvent molecule is held between two
such chains by various H-bonding interactions. Each sol-
vent molecule acts as a double H-bond donor through its
methyl group and quadruple H-bond acceptor through the
C3, C13, C20 and C37 phenylene carbon atoms. Thus, any
two parallel polymeric chains are linked to each other
through the solvent molecule and also through C4···O1 H-
bonding interactions. Out of these, the C51···O1 and
C37···N50 interactions propagate the crystal structure
along the c axis to extend it in a third dimension along the
bc plane (Figure S3).
Figure 4. UV/Vis spectra of nickel complexes in CH3CN: complex
1 (_____), 2 (-----), 3 (······) and 4 (-··-··-).
The λmax was observed from 435 to 470 nm, and it was
accompanied by a low-energy shoulder in the range 515–
570 nm. The low ε value for these features suggests the d–d
transition nature of the peaks. The high-energy features are
apparent below 310 nm and could tentatively be assigned to
intraligand charge-transfer bands. The observed similarity
between the spectral features of the nickel complexes is also
reflected in their very similar solid-state crystal structures
(for 2 and 4) as well as their proton and carbon NMR spec-
tra (vida infra). The λmax for complexes 1 and 2 are quite
close to their macrocyclic analogues (complexes 5 and 7,
respectively),[14] whereas the absorption maxima differ for 3
and 4 and suggest that the incorporation of the sterically
demanding groups (ethyl or benzyl) tend to alter the chro-
mophore. In fact, the λmax shifts to lower energy for 3 and
4 than for 1 and 2 (and 5 and 7 as well) and point out a
weaker ligand field strength around the nickel centre.
The observed Ni–Namide distances for the present NiII
complexes are within the range of other structurally charac-
terized cases including the Ni2+ complexes of Vagg and co-
workers,[19] Fenton and coworkers[20] and Journaux and co-
workers (with tetraamide ligands).[21] However, these dis-
tances are ca. 0.02–0.05 Å longer than those in the NiII
complexes of the macrocyclic ligands, 5–8.[14] Furthermore,
these distances are marginally longer than the 13-membered
and ca. 0.03 Å smaller than the 14-membered macrocyclic
dioxotetraaza nickel complexes of Lampeka and cowork-
ers.[12a,12b]
NMR Spectral Studies
To investigate the solution state structures of the nickel
complexes, 1H and 13C NMR spectroscopic studies were
performed (Figure S5 and S6). All four complexes, 1–4, are
Absorption Spectral Studies
The absorption spectra of complexes 1–4 were studied in diamagnetic with S = 0 ground state as displayed by their
a number of solvents, such as, MeCN, DMF, DMSO and sharp and ligand-like NMR spectra. Assignments of the
H2O. A comparative spectrum in MeCN is shown in Fig- signals were made through consideration of the relative
ure 4, whereas other spectra are part of the Supporting In- peak area and comparison with the spectrum of the free
formation (Figures S4a and S4b). The λmax as well as other ligand. The interesting aspect of the proton spectra of 3 and
prominent features were found to remain invariant to 4 is the observation of the AB-type or geminal coupling[12]
changes in solvent polarity or the coordinating ability. The for the –CH2R group (where R = CH3 for 3, and Ph for 4)
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J. Singh, G. Hundal, R. Gupta
FULL PAPER
due to the diastereotopicity of the methylene protons. Two peak separation (∆Ep) of 100 mV (Figure 5). The coulome-
methylene protons were observed at two different positions try experiment confirmed the 1e– nature of this response.
for these two complexes. In the case of 3, the pair of protons Complex 2 displays three redox processes at 0.82 (Epa), 0.90
were observed as multiplets at ca. 2.92 and ca. 2.45 ppm. (∆Ep = 50 mV) and 1.1 V (∆Ep = 60 mV) (Figure 6). The
The multiplicity is most likely caused by the presence of number of coulombic electron count was found to be collec-
the methyl group as the immediate neighbour. The adjacent tively one for the first two responses. We believe that the
methyl groups appear as triplets (J = 7.0 Hz). For 4, the first two redox processes are metal-based, whereas the third
pair of protons appear as doublets at δ = 2.59 and 3.66 one is located on the ligand (vide infra). The first redox
ppm with J(HAHB) of ca. 12 Hz.
response, however, disappeared after the coulometric oxi-
The 1H NMR spectra for nickel complexes 1–4 are much dation at 1.0 V, and the remaining two processes now ap-
simpler than those of their macrocyclic analogues (5–8).[14] pear as chemically reversible processes observable at 0.90
This is most likely due to the absence of the structural rigid- and 1.1 V (Figure 6). The coulometric oxidation at 1.35 V
ness, as the chelate rings invert much more rapidly, which also resulted in the identical observation that the two re-
results in a simple time-averaged spectrum. For example, no sponses are now reductive in nature. The exact nature of
AB-type coupling was observed for the protons attached to the species responsible for the feature at 0.82 V is not clear
the –C(O)CH2– group, which was a unique feature for all to us.[23] Complex 3 shows completely irreversible features
four macrocyclic NiII complexes, 5–8.[14] In addition, the (Epa) at 0.8 and ca. 1.0 V with 1e– nature of each process
13C NMR spectra of 1–4 are quite straight forward and a (Figure S7). There is, however, another response at ca. 1.6 V
close similarity in the chemical shifts of the identical carbon that we believe is due to some electrochemically generated
centres was clearly noticeable (Figure S6). Importantly, the species. Three irreversible redox processes (all Epa) were ob-
NMR spectroscopic studies reveal that the solid-state con-
formation of the nickel complexes is retained in the solution
state.
Electrochemical Studies
In order to obtain information about the accessibility of
the NiIII state and/or possible ligand oxidation, cyclic vol-
tammetric (CV) and controlled-potential electrolysis (coul-
ometry) experiments were performed on all NiII complexes.
The electrochemical experiments were done in CH3CN as
well as in the better-coordinating and more-polar solvent
DMF. All potentials (in V) are reported versus saturated
calomel electrode (SCE) throughout the text (Table 3).
Table 3. Electrochemical data for [Ni(L1)] (1), [Ni(L2)] (2), [Ni(L3)]
(3) and [Ni(L4)] (4) and their comparison with macrocyclic com-
plexes 5–8.
[a]
[a]
[a]
[b]
[a]
Figure 5. Cyclic voltammogram of complex 1 in CH3CN.
Complex
E1
E2
E3
E1
ER
Ref.
1
2
3
4
5
6
7
8
0.75
0.82[c], 0.90
0.80[c]
0.78[c]
0.61
–
–
–
–
0.65
0.82[c]
0.80[c]
0.78[c]
0.53
0.67
0.67
0.78
–
this work
1.10
1.00[c]
1.14[c]
–
1.88
1.10
1.13
–1.86 this work
–1.80 this work
–1.67 this work
–
–
1.71[c]
[14]
–
–
–
[14]
0.78
0.71
0.83
[14]
–
–
1.32[c]
[14]
[a] Solvent: CH3CN; Complex ca. 1 m solution, TBAP as sup-
porting electrolyte ca. 100 m solution, potential vs. SCE, glassy-
carbon working electrode, Pt auxiliary electrode, scan rate =
100 mVs–1. [b] Solvent: DMF, other parameters as in [a]. [c] Epa
.
Electrochemistry in CH3CN
Quasireversible to irreversible responses were observed
when the CV studies were performed in CH3CN on the
glassy carbon or platinum working electrode. Complex 1
showed only one oxidative response at 0.75 V with peak-to-
Figure 6. Cyclic voltammograms of complex 2 in CH3CN before
(
____) and after (······) coulometric oxidation at 1.0 V.
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Studies on Nickel(II) Complexes with Amide-based Ligands
served at 0.78, 1.14 and 1.71 V for complex 4 (Figure 7). potential is quite sensitive to the variety of terminal donor
The one-electron nature of each redox process was again groups used along with two Namidate donors. The potentials
confirmed by the coulometric studies.
are typically more negative for the ligands capable of pro-
viding a tetraanionic coordination environment rather than
a dianionic one about the nickel ion. The presence of a
central phenylene bridge (as in 2–4) has made the reduction
potential more positive due to the delocalization of the
negative charge on the deprotonated amide groups into the
benzene ring.[21,24,27] This is in contrast to the cases where
the bridge between two Namidate atoms is an ethylene unit
(as in 1) or a cyclohexane ring.[20]
Interestingly, a reversible reduction process (∆Ep
=
70 mV; ipc/ipa = 0.95–1.0) was also observed in the potential
range from –1.67 to –1.86 V in CH3CN for complexes 2–4
(Figure 8). Similar responses were observed when CV stud-
ies were performed in DMF. We assign these reductive pro-
cess (ER) to be nickel-centred on the basis of the similar
observation for the square-planar NiII complexes of tetra-
dentate amide-based ligands with ER values ranging from
–1.80 to –2.40 V.[20,28] Moreover, the significant shift in the
ER potential of complexes 2–4 towards more positive values
according to 2 Ͼ 3 Ͼ 4 clearly indicates that the reduction
Figure 7. Cyclic voltammogram of complex 4 in CH3CN.
The E1 potentials (the first redox process) for all four process is metal-centred, which leads to a NiI species (Fig-
NiII complexes come in a small potential window of 0.75– ure 8). This observed shift towards positive potential is con-
0.90 V. This process is most likely based on the nickel cen- sistent with the pronounced ligand field effect. If the site of
tre. The reason for a narrow potential window is due to the reduction was ligand then the ER potential would have
identical primary donor groups composed of two anionic come in a narrow range. It is important to mention that
Namide and two neutral Namine atoms. The E1 potential for the macrocyclic NiII complexes do not show any reductive
macrocyclic analogues 5–8 were in the range 0.61–0.83 V response in MeCN or DMF.[14] We speculate that the re-
under identical experimental conditions.[14] The observed duction is accompanied by a conformational change that is
similarity in the E1 potential suggest that the NiII state is not supported by the rigid macrocyclic ligand framework in
evenly stabilized by the open-chain relative to the macro- complexes 5–8.
cyclic systems. Similarly, the E2 potentials (the second redox
process) also fell within the small range 1.0–1.15 V for com-
plexes 2–4. The E2 potentials for macrocyclic analogues 6–
8 were between 1.10–1.13 V that we had assigned as the
oxidation of the o-benzenediamidate fragment generating
the Ni3+-o-benzosemiquinonediimine π-cation species.[14,21,24]
On the basis of the similarity of the two classes of com-
pounds, we suggest that the E2 process is based on the li-
gand. Complex 4, however, shows a third process, E3 at
1.71 V that we tentatively assign as the oxidation of the ben-
zyl substituents.
The +3/+2 reduction potentials (E1) for the NiII square-
planar complexes determined in this work are on the higher
side relative to that of the tetraamidate-NiII complexes of
Journaux and Garcia (0.12 V)[21,24] and other NiII com-
plexes of the ligands with two Namidate atoms in conjunction
with either two pyrrolidine (0.61 V),[20] two acetato
(0.52 V),[21b] two amine groups in macrocyclic ligands and
Figure 8. Cyclic voltammograms of complexes 2 (_____), 3 (----) and
their open-chain analogues (0.495–0.580 V),[12a,12c] two
4 (...) in CH3CN. The feature at –1.75 V for complex 2 is due to
some impurity.
phenolato (0.13 V),[25] two thiophenolato (–0.04 V)[25] and
two thiolato (–0.24 to –0.42 V) groups.[25–26] The E1 poten-
tial is, however, comparable to the ligands where two
Namidate atoms are present in conjunction with two pyr-
idine[20] (0.70 V), two 1-pyrroline[20] (0.71 V) and two amine complexes 2–4 did not result in any observable colour
groups in 13-membered macrocyclic ligand (0.75– change. For complex 1, however, the original yellow colour
0.79 V).[12a,13c] This comparison indicates that the NiIII/II changed to greenish-brown after oxidation at the E1 poten-
Coulometric oxidation at the E1 and E2 potentials for
a
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2057

J. Singh, G. Hundal, R. Gupta
FULL PAPER
tial; the resultant species decomposed immediately. A pos- plexes 3 and 4 did not show any change upon treatment
sible explanation is the unfavourable effect of the excess with Cu(OTf)2 (1 equiv.), whereas decomposition occurred
amount of supporting electrolyte present during the electro- when an excess amount of the oxidant (Ն3 equiv.) was used.
chemical oxidation.[29]
Electrochemistry in DMF
CVs were also recorded in DMF to see the effect of a
better-coordinating and a more-polar solvent. A compara-
tive CV plot is presented in Figure S8. Only complex 1
showed the quasireversible response with an E1 value of
0.65 V (∆Ep = 90 mV), whereas the other three complexes
displayed only the irreversible responses with an Epa value
of ca. 0.8 V (Table 3). On comparison, the NiIII state is bet-
ter stabilized by ca. 0.1 V in DMF than in CH3CN for com-
plexes 1 and 2. This could be due to the enhanced solvation
as the polarity of the medium increased. The other two
complexes, however, did not show any appreciable change
in the E1 potential, possibly due to the hindered accessi-
bility of the solvent towards nickel ions as a result of the
steric crowding. In fact, the crystal structure of 4 shows
long-range cation···π and M···H bond interactions (vide su-
pra) and supports this hypothesis.
Figure 9. Absorption spectra of [1Ox +
] ( ] (– – –) in
_____) and [2Ox +
CH3CN after oxidation of 1 and 2 with Cu(OTf)2.
Chemical Oxidation with Cu(OTf)2
On the basis of the electrochemical findings, we at-
tempted chemical oxidation of the present NiII complexes
by using Cu(OTf)2.[30] Our recent success to generate the
NiIII species of macrocyclic complexes 5–8[14] and the redox
potential of Cu(OTf)2 (0.8 V in CH3CN)[31] depicted its
suitability. The addition of Cu(OTf)2 (1 equiv.) to a CH3CN
solution of 1 immediately resulted in a colour change from
deep-yellow to brownish-green, and new spectral features
were observed at ca. 970 (ε = 200 –1 cm–1) and 635 nm (ε
= 300 –1 cm–1) (Figure 9). These features are quite similar
Figure 10. EPR spectrum of [1Ox +
dation with Cu(OTf)2.
] in CH3CN at 77 K after oxi-
Preliminary Epoxidation Reactions
to those of its macrocyclic analogue [5Ox +
towards a similar electronic chromophore.[14] This species,
]
and point
The chemical oxidation [with Cu(OTf)2] of complexes 1
however, is not very stable and decomposes to product(s) and 2, which resulted in the generation of a metastable NiIII
of unassignable nature. Complex 2 also afforded a greenish- species, suggested that other complexes may also transiently
brown species after its treatment with Cu(OTf)2. The ab- stabilize the higher oxidation state of the nickel ion. It was
sorption spectrum of this new species, [2Ox]+, showed fea- suggested that the diamagnetic square-planar NiII com-
tures at 1020, 890, 790, ca. 700 and 480 nm with the ε value plexes with an accessible NiIII oxidation state display oxi-
between 200–500 –1 cm–1 (Figure 9). The observed absorp- dation activities.[32] This prompted us to probe possible sub-
tion spectral features of [2Ox]+ have some similarity with the strate oxidation reactions. NiII complexes 1–4 were tested
macrocyclic systems [7Ox]+ and [8Ox]+ and again suggests a as catalysts for the epoxidation of alkenes in the presence
similar electronic chromophore.[14] The solution-generated of aldehyde [(CH3)3CHO or (CH3)2CH2CHO] as coreduc-
[1Ox]+ also displayed anisotropic signals in the X-band EPR tor and dioxygen as the oxidizing agent.[33] All catalytic re-
spectrum at 77 K with g values of 2.278, 2.232 and 2.009 actions were carried out in MeCN and the results are sum-
(Figure 10). The corresponding values for [2Ox]+ species are marized in Table 4. This type of transformation is consid-
2.300, 2.230 and 2.015. The EPR spectra of [1Ox + and ered to occur via high-valent metal-oxido intermediates;[34]
[2Ox + are very similar to the macrocyclic systems [5Ox +
] – however, a radical pathway was also suggested in many
]
]
[8Ox]+, which thus suggests a NiIII ion in a square-planar cases.[35] Addition of the aldehyde to an aerated solution of
environment.[14] The large rhombicity and a high average g the nickel complex resulted in distinct colour change. It is to
value of ca. 2.17 suggests that the locus of the oxidation is be noted, however, that no colour change (and apparently
nickel centre as also concluded with macrocyclic systems[14] generation of the active species) was noticed by the reaction
[5Ox]+ – [8Ox]+ and other literature precedents.[21,24,27] Com- between aldehyde and NiII complexes under inert atmo-
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Studies on Nickel(II) Complexes with Amide-based Ligands
sphere. This experiment rules out the possibility of the gen- Table 4. Summary of the NiII complex catalyzed oxidation of al-
kenes.[a]
eration of the acyl radical (RC·O) by the direct reaction
between the NiII complexes and aldehyde as commonly sug-
gested.[34] We believe that the reaction between the NiII
complex with aldehyde and O2 concomitantly produces the
reactive Nix+–O species and the acid. Typically, the colour
changed from yellow to greenish-brown after the addition
of the aldehyde and O2 to the solution of 1, 2 and 4 was
accompanied with distinct features in the absorption spec-
tra. Complex 3, however, immediately decomposed. This
trend also supports the observed reactivity pattern of vari-
ous substrates with the catalysts. Complex 1 was found to
be the most effective catalyst amongst all complexes with
the following trend: 1 Ͼ 2 Ͼ 4. The introduction of the
[e]
Entry Substrate
Complex % Conversion
TON[f]
1
2
3
4
cis-stilbene
cis-stilbene
cis-stilbene
cis-stilbene
trans-stilbene
trans-stilbene
trans-stilbene
trans-stilbene
trans-stilbene
styrene
1
2
3
4
1
1
2
3
4
1
1
2
3
4
1
2
3
4
40
38
56
35
50
100
52
13
97
55
100
93
NR
55
78
54
NR
96
21
20
30
19
400
285
29
5[b]
6[c]
7
8
6
9[c]
10[b]
11[c]
12[d]
13
285
460
285
134
NR
32
210
144
NR
54
styrene
styrene
styrene
electron-withdrawing group in the para position of the 14
styrene
15[c]
p-chlorostyrene
p-chlorostyrene
p-chlorostyrene
p-chlorostyrene
phenyl group in styrene resulted in reduced epoxidation as
observed before[35a] (Table 4, Entries 15–18). For example,
the turn over number (TON) was reduced from 460 to 210
after the introduction of the Cl group in the para position
(Table 4, compare Entries 10 and 15). For the cis-stilbene
epoxidation reaction, trans-stilbene oxide was the only
product,[34b,35a] which thus suggests the free rotation of the
C–C bond in the transition state for an asynchron C–O
bond formation. As anticipated, the percent conversion as
well as the TON for the trans-stilbene epoxidation reaction
were much higher than cis-stilbene (Table 4, Entries 1–4
and 5–9), as the step corresponding to the rotation of the
C–C bond was not required. Catalyst 4 was found to ef-
ficiently epoxidize trans-stilbene with 97% conversion and
a TON of 285, whereas it was moderate with other sub-
strates. We believe that the appended benzyl substituents
help to bring the trans-stilbene closer to the nickel centre
through π–π interactions. In fact, the crystal structure of 4
shows long-range interactions and supports this hypothesis.
Other oxidants, such as, PhIO, tert-butylhydroperoxide,
m-chloroperbenzoic acid, H2O2 and urea·H2O2 adduct were
also used to understand the nature of the oxidizing agent
in the epoxidation reaction. However, only tert-butylhy-
droperoxide was found to cause epoxidation of the trans-
stilbene with ca. 15% conversion. The conversion increased
to ca. 20% when the epoxidation reaction was carried out
under a O2 atmosphere.[35,36] This suggests that a radical
pathway might be involved in the aerobic epoxidation reac-
tion with tert-butylhydroperoxide. However, the high yield
of the epoxide products observed for the aldehyde–O2 com-
bination reactions strongly suggest the involvement of the
Ni complexes in the epoxidation reaction.
16[c]
17
18
[a] Catalyst: 1.66 mol-% (1 mmol), isobutyraldehyde (180 mmol),
substrate (60 mmol), solvent: CH3CN, reaction time: 45 min. [b]
Catalyst: 0.1 mol-% (1 mmol), isobutyraldehyde (3000 mmol), sub-
strate (1000 mmol). [c] Catalyst: 0.33 mol-% (1 mmol), isobutyral-
dehyde (900 mmol), substrate (300 mmol). [d] Catalyst: 0.66 mol-%
(1 mmol), isobutyraldehyde (450 mmol), substrate (150 mmol). [e]
%Conversion = amount of substrate consumed ϫ 100/amount of
initial substrate. [f]. TON = Turn over number; NR = No reaction.
weak interactions (H-bonding, C–H···X and π–π) generate
an interesting structural pattern for complex 4. The electro-
chemical results suggest that the NiIII/NiII redox couple pri-
marily depends upon the N4 donors composed of two
Namide and two Namine atoms and not on the peripheral
substituents. All four ligands with variable backbone and
substituents are equally competent in stabilizing the NiIII
state. This is in contrast with the macrocyclic complexes 5–
8, where a pronounced ligand field effect was observed for
the NiIII/NiII redox couple. Whereas the electrochemical
oxidation of 5–8 generated the respective NiIII species in the
macrocyclic systems, complexes 1–4 were found completely
unresponsive. Chemical oxidation of 1 and 2 with Cu-
(OTf)2, however, transiently generated the NiIII species,
which was characterized by absorption and EPR spectral
studies. These spectral studies indicate that [1Ox +
and
]
[2Ox]+ are quite similar to the macrocyclic systems [5Ox]+ –
[8Ox]+, in which the NiIII ion is in a square-planar geometry.
Epoxidation of the alkenes with O2–aldehyde combination
suggests that the present nickel complexes have the ability
to transiently stabilize the higher oxidation state of the
nickel ion that possibly participates in the substrate oxi-
dation.
Conclusions
We show the synthesis and characterization of four
square-planar nickel(II) complexes with open-chain amide-
based ligands. The crystal structure analysis of 2 and 4 sub-
stantiate the square-planar geometry around the NiII ion.
In addition, complex 4 also shows long-range cation–π in-
teractions from the benzyl substituents, which thus creates
Experimental Section
Materials and Reagents: All reagents were obtained from commer-
cial sources and used as received. N, N-dimethylformamide (DMF)
was dried and distilled from 4 Å molecular sieves and stored over
a pseudooctahedral geometry around the metal ion. Other sieves. Acetonitrile (MeCN) was dried by distillation from anhy-
Eur. J. Inorg. Chem. 2008, 2052–2063
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2059

J. Singh, G. Hundal, R. Gupta
FULL PAPER
drous CaH2. Diethyl ether was dried by refluxing over sodium
metal under an inert atmosphere. Tetrahydrofuran (THF) was dried
by refluxing and distilling from sodium metal and benzophenone.
Ethyl alcohol (C2H5OH) and methyl alcohol (CH3OH) were dis-
tilled from magnesium ethoxide and magnesium methoxide, respec-
tively. Chloroform (CHCl3) and dichloromethane (CH2Cl2) were
purified by washing with 5% sodium carbonate solution followed
by water and finally dried with anhydrous CaCl2 before a final
reflux and distillation. The compounds N, NЈ-bis(chloroacetyl)eth-
ylenediamine and N,NЈ-bis(chloroacetyl)-o-phenylenediamine were
prepared as reported earlier,[14] whereas the ligand H2L2 was syn-
thesized as described recently.[22]
ter of its original volume, and diethyl ether was added to precipitate
the crude product as deep-orange powder. The recrystallization was
achieved by diffusing diethyl ether into a CH2Cl2 solution of the
crude product at room temperature. Yield: 0.20 g (82%). 1H NMR
(300 MHz, [D6]DMSO, 25 °C): δ = 2.52 (s, 12 H, -CH3), 2.69 [s, 4
H, -CH2C(O)-], 2.94 (s, 4 H, -CH2-CH2-) ppm. 13C NMR
(75 MHz, [D6]DMSO, 25 °C): δ = 46 (-CH3), 50 (-CH2CH2-), 70
[-C(O)CH -], 172 (C=O) ppm. FTIR (KBr disk): ν = 3375, 3207,
˜
2
2950, 2923, 2849, 1614 cm–1. Conductivity (CH3CN, ca. 1 m solu-
tion, 298 K): ΛM = 4 Ω–1 cm2 mol–1. UV/Vis (CH3CN): λmax (ε,

–1 cm–1) = 515 sh. (90), 435 (410), 264 sh. (6300), 240 sh. (11700)
nm. UV/Vis (DMF): λmax (ε, –1 cm–1) = 518 sh. (50), 435 (280)
nm. UV/Vis (DMSO): λmax (ε, –1 cm–1) = 518 sh. (50), 435 (280)
nm. UV/Vis (H2O): λmax (ε, –1 cm–1) = 435 (100) nm. MS (EI+,
CH3CN): m/z = 287.08 [M + H]+. C10H20N4O2Ni·1.5H2O (313.69):
calcd. C 38.28, H 7.33, N 17.85; found C 38.27, H 7.83, N 17.56.
Ligand Synthesis
H2L1: To a solution of N, NЈ-bis(chloroacetyl)ethylenediamine
(2.0 g, 9.38 mmol) in ethyl alcohol (25 mL) was added aqueous di-
methylamine (40%, 6.8 g, 56.33 mmol). The resulting mixture was
heated at reflux for 24 h. The solvent was removed under reduced
pressure, followed by the addition of a saturated aqueous solution
(10 mL) of sodium hydrogen carbonate to the oil. The resultant
mixture was extracted with CH2Cl2 (3ϫ10 mL). The CH2Cl2 layers
were combined and dried with anhydrous Na2SO4. The filtrate was
evaporated under reduced pressure to afford a white solid. Yield:
1.5 g (62%). M.p. 98–100 °C. 1H NMR (300 MHz, CDCl3, 25 °C):
δ = 2.2 (s, 12 H, CH3), 2.9 [s, 4 H, -CH2C(O)-], 3.4 (s, 4 H, -CH2-
[Ni(L2)] (2): This compound was synthesized in a similar fashion
to that used for 1. However, the filtrate was directly subjected to
the diethyl ether diffusion to afford an orange-coloured, highly
crystalline product in 70% yield. 1H NMR (300 MHz, CDCl3,
25 °C): δ = 2.63 (s, 12 H, -CH3), 3.26 [s, 4 H, -CH2C(O)-], 6.76 (m,
2 H, Ar-1a), 8.26 (m, 2 H, Ar-1b) ppm. 1H NMR (300 MHz, [D6]-
DMSO, 25 °C): δ = 2.63 (s, 12 H, -CH3), 3.17 [s, 4 H, -CH2C(O)-],
6.57 (m, 2 H, Ar-1a), 8.05 (m, 2 H, Ar-1b) ppm. 13C NMR
(75 MHz, [D6]DMSO, 25 °C): δ = 50 (-CH3), 70 [-C(O)CH2-],120
(-CH, Ar-1a), 122 (-CH, Ar-1b), 142 (Ar-1c), 172 (C=O) ppm.
CH ), 7.4 (br. s, 2 H, NH) ppm. IR (KBr, selected peaks): ν = 3288
˜
2
ν(NH), 2944, 2820, 2772 ν(CH), 1656 ν(CO) cm–1. MS (CH3CN):
m/z = 230.8 [M + H]+. C10H22N4O2·0.5H2O (239.3): calcd. C 50.14,
H 9.6, N 23.14; found C 50.29, H 9.25, N 23.64.
FTIR (KBr disk): ν = 3051, 3003, 2920, 1636, 1572 cm–1. Conduc-
˜
tivity (CH3CN, ca. 1 m solution, 298 K): ΛM = 5 Ω–1 cm2 mol–1.
UV/Vis (CH3CN): λmax (ε, –1 cm–1) = 540 sh. (60), 445 (280), 360
sh. (110), 293 (13500), 268 (17800) nm. UV/Vis (DMF): λmax (ε,
H2L3: This ligand was synthesized in a similar manner to that used
for H2L1 with the following reagents: N,NЈ-bis(chloroacetyl)-o-
phenylenediamine (2.0 g, 7.66 mmol) and diethylamine (3.38 g,
45.97 mmol). The filtrate was evaporated under reduced pressure to
afford a yellow semisolid. Yield: 2.2 g (86%). 1H NMR (300 MHz,
CDCl3, 25 °C): δ = 1.09 (t, J = 7.1 Hz, 12 H, CH3), 2.65 (q, J =
7.1 Hz, 8 H, -CH2-CH3), 3.18 [s, 4 H, -CH2C(O)-], 7.20 (m, 2 H,
aromatic-H), 7.59 (m, 2 H, aromatic-H), 9.4 (br. s, 2 H, NH) ppm.

–1 cm–1) = 540 sh. (50), 445 (250), 355 sh. (120), 296 (16500) nm.
UV/Vis (DMSO): λmax (ε, –1 cm–1) = 530 sh. (60), 446 (330) nm.
UV/Vis (H2O): λmax (ε, –1 cm–1) = 449 (310) nm. MS (EI+,
CH3OH): m/z = 357.69 [M + Na]+. C14H20N4NiO2 (334.3): calcd.
C 50.19, H 5.9, N 16.73; found C 50.15, H 5.85, N 16.48.
[Ni(L3)] (3): This compound was synthesized in a similar fashion
to that used for 1. The recrystallized complex was isolated in 80%
yield as deep-orange blocks after diffusing diethyl ether into a
dichloromethane solution. 1H NMR (300 MHz, [D6]DMSO,
25 °C): δ = 1.87 (t, J = 7 Hz, 12 H, -CH3), 2.45 (m, 4 H, -CH2-3a),
2.92 (m, 4 H, -CH2-3b), 3.16 [s, 4 H, -CH2C(O)-], 6.56 (m, 2 H,
Ar-1a), 8.15 (m, 2 H, Ar-1b) ppm. 1H NMR (300 MHz, CDCl3,
25 °C): δ = 1.99 (s, 12 H, -CH3), 2.45 (q, J = 6 Hz, 4 H, -CH2-3a),
2.84 (q, J = 6 Hz, 4 H, -CH2-3b), 3.18 [s, 4 H, -CH2C(O)-], 6.74
(m, 2 H, Ar-1a), 8.30 (m, 2 H, Ar-1b) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C): δ = 12 (-CH3), 55 (-CH2-CH3), 65 [-C(O)CH2-], 119
(-CH, Ar-1a), 121 (-CH, Ar-1b), 142 (Ar-1c), 173 (C=O) ppm.
IR (KBr, selected peaks): ν = 3244 ν(NH), 2970 ν(CH), 1683
˜
ν(CO), 1596 ν(C=C) cm–1. MS (CH3CN): m/z = 335.5 [M + H]+.
C18H30N4O2 (334.5): calcd. C 64.67, H 8.98, N 16.76; found C
64.78, H 9.20, N 16.73.
H2L4: This ligand was synthesized in a similar manner to that for
H2L1 with the following reagents: N,NЈ-bis(chloroacetyl)-o-pheny-
lenediamine (2.0 g, 7.66 mmol) and dibenzylamine (9.06 g,
45.97 mmol). Yield: 4.24 g (95%). M.p. 150–152 °C. 1H NMR
(300 MHz, CDCl3, 25 °C): δ = 3.20 [s, 4 H, -CH2C(O)-], 3.60 (s, 8
H, -CH2Ph), 7.17 (m, 2 H, aromatic-H), 7.29 (m, 20 H, Ph- H),
7.46 (m, 2 H, aromatic-H), 9.27 (br. s, 2 H, NH) ppm. IR (KBr,
FTIR (KBr disk): ν = 3058, 3001, 2968, 1625, 1566 cm–1. Conduc-
˜
selected peaks): ν = 3278 ν(NH), 3060, 3025, 2811 ν(CH), 1663
˜
tivity (CH3CN, ca. 1 m solution, 298 K): ΛM = 5 Ω–1 cm2 mol–1.
UV/Vis (CH3CN): λmax (ε, –1 cm–1) = 570 sh. (40), 460 (280), 370
sh. (90), 300 sh. (8500), 290 sh. (9800), 270 (17000), 220 (36600)
nm. UV/Vis (DMF): λmax (ε, –1 cm–1) = 565 sh. (60), 460 (360),
374 sh. (120), 308 sh. (12000), 295 sh. (12900). UV/Vis (DMSO):
λmax (ε, –1 cm–1) = 564 sh. (40), 460 (311), 370 sh. (90). UV/Vis
(H2O): λmax (ε, –1 cm–1) = 464 (340). MS (EI+, CH3OH): m/z =
392.8 [M + H]+. C18H28N4NiO2 (391.15): calcd. C 55.22, H 7.16,
N 14.33; found C 55.08, H 7.50, N 14.11.
ν(CO), 1594 ν(C=C) cm–1. MS (CH3CN): m/z = 584.3 [M + H]+.
C38H38N4O2 (582.8): calcd. C 78.35, H 6.52, N 9.22; found C 78.31,
H 7.00, N 9.22.
Synthesis of Nickel Complexes
[Ni(L1)]·1.5H2O (1): The H2L1 ligand (0.20 g, 0.86 mmol) was dis-
solved in DMF (5 mL) and treated with NaH (0.046 g, 1.92 mmol)
under a nitrogen atmosphere. The mixture was stirred until H2 evol-
ution ceased (ca. 15–20 min). To this mixture was added a solution
of NiCl2 (0.112 g, 0.86 mmol) in DMF (8 mL) under inert condi-
tions. The resulting mixture was further stirred for 2 h at room
temperature, which resulted in an orange solution. This solution
was filtered through a pad of Celite atop a medium-porosity frit.
The filtrate was concentrated under reduced pressure to one quar-
[Ni(L4)]·H2O (4): This compound was synthesized in a similar fash-
ion to that used for 1. Instead of NiCl2, (Et4N)2[NiCl4] was used.
The crude complex was recrystallized by diffusing diethyl ether into
a MeCN solution to afford the product in 48% yield. 1H NMR
(300 MHz, [D6]DMSO, 25 °C): δ = 2.50 [s, 4 H, -CH2C(O)-], 2.58
2060
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Studies on Nickel(II) Complexes with Amide-based Ligands
(d, J = 12 Hz, 4 H, -CH2-3a), 3.65 (d, J = 12 Hz, 4 H, -CH2-3b), reflections [I Ͼ 2σ (I)] were used in the structure refinement. Non-
6.65 (m, 2 H, Ar-1a), 7.63 (s, 12 H, Ar-5&6), 8.11 (m, 2 H, Ar-1b), hydrogen atoms were refined anisotropically. All hydrogen atoms
8.14 (s, 8 H, Ar-4) ppm. 13C NMR (75 MHz, [D6]DMSO, 25 °C): were placed in geometrically calculated positions by using a riding
δ = 58 (-CH2-4), 65 [-C(O)CH2-], 119 (-CH, Ar-1a), 121 (-CH, Ar- model. For 4, the intensity data were collected at 295 K with a
1b), 129 (-CH, Ar-7&8), 133 (Ar-5&6), 142 (Ar-1c), 170 (C=O) Siemens P4 X-ray diffractometer by using θ–2θ scanning mode with
ppm. FTIR (KBr disk): ν = 3430, 3033, 2931, 1622, 1569 cm–1. graphite monochromated Mo-Kα radiation. A total of 6835 reflec-
˜
Conductivity (CH3CN, ca. 1 m solution, 298 K): ΛM
=
tions were measured, of which 6446 were unique and 3278 were
5 Ω–1 cm2 mol–1. UV/Vis (CH3CN): λmax (ε, –1 cm–1) = 540 sh. considered observed [I Ͼ 2σ (I)]. The data were corrected for Lo-
(190), 470 (420), 397 sh. (220), 310 sh. (12000), 270 (25000). UV/ rentz and polarization effects and a psi-scan absorption correction
Vis (DMF): λmax (ε, –1 cm–1) = 550 sh. (170), 470 (400), 385 sh. was also applied. The structure was solved by direct methods and
(200), 311 sh. (11700). UV/Vis (DMSO): λmax (ε, –1 cm–1) = 550 refined by full-matrix least-squares refinement techniques on F2.
sh. (120), 470 (300), 387 sh. (140). MS (EI+, CH3OH): m/z = 639.76 All non-hydrogen atoms were refined anisotropically. All hydrogen
[M + H]+. C38H36N4NiO2·H2O (657.41): calcd. C 69.37, H 5.78, N atoms were attached with Uiso values of 1.2 times (for methylene
8.52; found C 69.12, H 6.12, N 8.28.
and phenylene carbon atoms) and 1.5 times (methyl carbon atoms)
the Uiso values of their respective carrier atoms. Details of the
crystallographic data are given in Table 5.
General Procedure for the Epoxidation Reaction: To a solution of
the NiII complex (0.1–1.66 mol-%) in CH3CN (10 mL) was added
isobutyraldehyde (180–3000 mmol) and the substrate (60–
1000 mmol). The reaction mixture was stirred at room temperature
under an O2 atmosphere for 45 min. The progress of the reaction
was monitored by thin-layer chromatography (CHCl3/hexanes, 1:1).
The solvent was removed under reduced pressure, and the oxi-
dation products were separated and purified by column chromatog-
raphy on silica gel (CHCl3/hexanes, 1:1).
Table 5. Crystallographic data for [Ni(L2)] (2) and [Ni(L4)]·CH3CN
(4).
[Ni(L2)] (2)
[Ni(L4)]·CH3CN (4)
Empirical formula
Formula mass
T [K]
Crystal system
Space group
Colour, shape
Size [mm]
a [Å]
C14H20N4O2Ni C40H39N5O2Ni
335.05
298(2)
triclinic
680.48
295(2)
triclinic
Physical Measurements: The conductivity measurements were done
in organic solvents by using the digital conductivity bridge from
the Popular Traders, India (model number: PT – 825). The elemen-
tal analysis data were obtained with an Elementar Analysen Sy-
steme GmbH Vario EL-III instrument. The NMR measurements
were done with either a Hitachi R-600 FT NMR (60 MHz) or an
Avance Bruker (300 MHz) instrument. The infrared spectra (either
¯
¯
P1
P1
orange, block
0.2ϫ0.1ϫ0.2
8.974(2)
9.612(3)
9.688(3)
112.655(4)
91.538(4)
105.001(4)
737.4(4)
2
brown, needle
0.18ϫ0.16ϫ0.16
10.293(3)
11.109(3)
16.194(4)
95.62(2)
107.25(2)
97.82(2)
1733.2(8)
2
b [Å]
c [Å]
as KBr pellet or as a mull in mineral oil) were recorded with a α [°]
β [°]
Perkin–Elmer FTIR 2000 spectrometer. The absorption spectra
were recorded with a Perkin–Elmer Lambda 25 spectrophotometer.
The mass spectra were obtained with a Jeol SX102/DA–6000 in-
strument.
γ [°]
V [Å3]
Z
F [000]
352
1.509
716
1.304
0.602
Dcalcd. [gcm–3]
Electrochemical Measurements: Cyclic voltammetric experiments
were performed by using a PAR model 370 electrochemistry system
consisting of an M-174A polarographic analyzer, M-175 universal
programmer and RE 0074 X-Y recorder or a CH Instruments elec-
trochemical analyzer (600B Series). The cell contained a glassy-
carbon (PAR model G0021) or a Beckman Pt (M-39273) working
electrode, a Pt wire auxiliary electrode and a saturated calomel elec-
trode (SCE) as the reference electrode. A salt bridge (containing
supporting electrolyte, TBAP, dissolved in either MeCN or DMF)
was used to connect the SCE with the experimental solution.[37]
For constant potential electrolysis experiments a Pt mesh was used
as the working electrode. The solutions were ca. 1 m in complex
and ca. 0.1  in supporting electrolyte, TBAP. Under our experi-
mental conditions, the E1/2 values (in Volts) for the couple Fc+/Fc
were 0.40 in MeCN vs. SCE.
Absorption coeff. [mm–1] 1.325
R[a]
[b]
Rw
0.0383
0.0877
1.084
0.0549
0.1247
0.834
GOF on F2
[a] R = Σ||Fo| – |Fc||/Σ|Fo|. [b] Rw = {[Σ(|Fo|2|Fc|2)2]}1/2
.
CCDC-663277 (for 2) and -663278 (for 4) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Crystal structure of complex 4; absorption spectra for com-
Crystal Structure Determination: Single crystals suitable for X-ray
diffraction studies were grown by the vapour diffusion of diethyl
ether into either a DMF solution (for 2) or CH3CN solution (for
4). For complex 2, X-ray diffraction studies of crystal mounted on
a capillary were carried out with a Bruker AXS SMART-APEX
diffractometer with a CCD area detector (Kα = 0.71073 Å, mono-
chromator: graphite).[38] Frames were collected at T = 293 K by ω,
Ͻ?οασ [οο?ϾφϽ?οασ [οξ?Ͼ and 2θ-rotation at 10 s per frame with
SAINT.[39] The measured intensities were reduced to F2 and cor-
1
plexes 1–4 recorded in DMF and DMSO; H and 13C NMR spec-
tra for complexes 1–4 recorded in [D6]DMSO; cyclic voltammo-
grams; H-bonding distances for complex 4.
Acknowledgments
rected for absorption with SADABS.[40] Structure solution, refine- R. G. thanks University Grant Commission (UGC), Department
ment and data output were carried out with the SHELXTL pro- of Science & Technology (DST) both Government of India, New
gram.[40] A total of 2580 reflections were measured, of which 2336 Delhi, for generous financial support, DST-FIST-funded single-
Eur. J. Inorg. Chem. 2008, 2052–2063
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2061

J. Singh, G. Hundal, R. Gupta
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2052–2063

Studies on Nickel(II) Complexes with Amide-based Ligands
[38] SMART: Bruker Molecular Analysis Research Tool, Version
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Received: October 16, 2007
Published Online: March 13, 2008
Eur. J. Inorg. Chem. 2008, 2052–2063
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2063