Nickel(II) complexes incorporating pyridyl, imine and amino chelate ligands: Synthesis, structure, isomer preference, structural transformation and reactivity towards nickel(III) derivatives

Article abstract of DOI:10.1002/ejic.200300865

Facile condensation of 2-acetylpyridine with ethylenediamine in a 1:1 or 2:1 molar ratio yielded two neutral ligands with different denticity: 1-amino-4-(2-pyridyl)-3-azapent-3-ene (tridentate, L¹) and 2,7-bis(2-pyridyl)-3,6-diazaocta-2,6-diene (tetradentate, L³), respectively. Replacing the ketone with 2-pyridinecarboxaldehyde gave a similar set of condensates (L² and L⁴). The tridentate mono-Schiff bases (L¹ and L²) react smoothly with Ni(ClO ₄)₂·6H₂O furnishing the brown bis-chelate complexes (1a and 1b) with an NiN₆ coordination sphere, while the tetradentate bis-Schiff bases (L³ and L⁴) form green pseudo-octahedral complexes (2a and 2b) in which Ni^II is present in an N₄O₂ coordination environment. The isomer specificity for both types of complexes is conspicuous from the representative X-ray structures of 1a and 2a. cis-trans-cis and cis-cis-trans isomers are found exclusively for 1 and 2, respectively. The crystal structure of 2a reveals O-H...O hydrogen bond interactions assembling alternating cations and anions in an infinite chain-like array. Cyclic voltammetric measurements of 1 and 2 in MeCN solution show a quasi-reversible one-electron oxidation near 0.95 and 0.87 V (vs. SCE), respectively, attributed to a Ni^III-Ni ^II redox couple. Another irreversible Ni^IV-Ni ^III redox response was observed at higher potential near 1.70 and 1.80 V (vs. SCE) for 1 and 2, respectively. Complexes 1 and 2 display a weak, broad d-d transition band along with a charge-transfer transition. Magnetic susceptibility measurements (at 298 K) confirmed that the spin states of the Ni^II centres in 1 and 2 are same, S = 1, in agreement with an octahedral configuration. Complexes 1 form stable reddish-brown Ni^III complexes (3) under exhaustive constant-potential electrolysis treatment. The Ni^III complexes display axial EPR spectra both at 298 K and 77 K with g_⊥ > g_∥ indicating the presence of an unpaired electron primarily on the metal centre. Complexes of type 1 and 3 exhibit virtually superimposable cyclic voltammograms in a reverse electrode scan study which confirms no change in the coordination sphere on going from 1 to 3. The effective magnetic moment values (ca. 2.10 μ_B) of 3 suggest a significant orbital contribution to the paramagnetism, consistent with a tetragonally distorted low spin 3d⁷ system. A colour change phenomenon has been observed for type 2 complexes. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200300865

FULL PAPER
Nickel(II) Complexes Incorporating Pyridyl, Imine and Amino Chelate
Ligands: Synthesis, Structure, Isomer Preference, Structural Transformation
and Reactivity Towards Nickel(III) Derivatives
[a]
[b]
[c]
[c]
Suparna Banerjee, Jaydip Gangopadhyay, Can-Zhong Lu, Jiu-Tong Chen, and
Ashutosh Ghosh*[
a]
Keywords: Coordination modes / Isomer specificity / N ligands / Nickel / Structural transformation
Facile condensation of 2-acetylpyridine with ethylenediam- was observed at higher potential near 1.70 and 1.80 V
ine in a 1:1 or 2:1 molar ratio yielded two neutral ligands with
different denticity: 1-amino-4-(2-pyridyl)-3-azapent-3-ene
(vs. SCE) for 1 and 2, respectively. Complexes 1 and 2 display
a weak, broad d-d transition band along with a charge-trans-
fer transition. Magnetic susceptibility measurements (at
1
(
tridentate, L ) and 2,7-bis(2-pyridyl)-3,6-diazaocta-2,6-di-
3
II
ene (tetradentate, L ), respectively. Replacing the ketone
with 2-pyridinecarboxaldehyde gave a similar set of con-
densates (L and L ). The tridentate mono-Schiff bases (L
and L ) react smoothly with Ni(ClO
brown bis-chelate complexes (1a and 1b) with an NiN
dination sphere, while the tetradentate bis-Schiff bases (L
298 K) confirmed that the spin states of the Ni centres in
1 and 2 are same, S = 1, in agreement with an octahedral
2
4
1
III
configuration. Complexes 1 form stable reddish-brown Ni
2
4
)
2
·6H
2
O furnishing the
complexes (3) under exhaustive constant-potential electro-
III
6
coor- lysis treatment. The Ni complexes display axial EPR spectra
3
both at 298 K and 77 K with g
Ќ
Ͼ g|| indicating the presence
4
and L ) form green pseudo-octahedral complexes (2a and 2b) of an unpaired electron primarily on the metal centre. Com-
in which Ni is present in an N
II
4
O
2
coordination environment.
plexes of type 1 and 3 exhibit virtually superimposable cyclic
voltammograms in a reverse electrode scan study which con-
firms no change in the coordination sphere on going from 1
The isomer specificity for both types of complexes is con-
spicuous from the representative X-ray structures of 1a and
2a. cis-trans-cis and cis-cis-trans isomers are found exclus-
B
to 3. The effective magnetic moment values (ca. 2.10 µ ) of 3
ively for 1 and 2, respectively. The crystal structure of 2a re- suggest a significant orbital contribution to the paramagnet-
7
veals O−H···O hydrogen bond interactions assembling al-
ternating cations and anions in an infinite chain-like array.
Cyclic voltammetric measurements of 1 and 2 in MeCN solu-
tion show a quasi-reversible one-electron oxidation near 0.95
ism, consistent with a tetragonally distorted low spin 3d sys-
tem. A colour change phenomenon has been observed for
type 2 complexes.
III
II
and 0.87 V (vs. SCE), respectively, attributed to a Ni -Ni
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
IV
III
redox couple. Another irreversible Ni -Ni redox response
Germany, 2004)
Introduction
neutral mono-Schiff bases and bis-Schiff bases respectively,
as ligands. The electrochemical behaviour of all the mem-
bers of the two families was investigated with the aim of
determining the relative stability of progressively higher oxi-
dation states (ϩ, ϩ and ϩ) and possible transform-
Over the last few decades considerable attention has been
paid to rationalizing the underlying chemical aspects re-
garding the stabilizing and destabilizing factors of Ni
III
[
2Ϫ4]
II
ations thereof.
The NiN complexes smoothly undergo
species derived from Ni precursors in various coordi-
6
nation environments.[
1,2]
electrochemical one-electron oxidation at ambient tempera-
III
ture to give Ni complexes with identical coordination en-
We report here the synthesis and structural features of
new NiN and NiN O coordination spheres derived from
vironments. Hence this type of bis-chelate complex can be
modelled to trigger the reversible electron-transfer reactions
6
4
2
[
5]
[6]
that are essential for biological as well as catalytic pro-
cesses. The molecular packing of a representative of the
NiN O family (2a) has revealed two sterically uncongested
[
[
[
a]
b]
c]
Department of Chemistry, University College of Science and
Technology, University of Calcutta,
9
2, A.P. C. Road, Kolkata 700 009, India
4
2
Department of Inorganic Chemistry, Indian Association for the
Cultivation of Science,
Kolkata 700 032, India
ONN triangular faces forming O···O hydrogen-bond inter-
actions with counter perchlorate anions. Under the appro-
priate conditions these open faces can be utilised to accom-
modate small guest molecules, an area that has been much
less explored than the more common crown ethers,[ imine
The State Key Laboratory of Structural Chemistry, Fujian
Institute of Research on the Structure of Matter, The Chinese
Academy of Sciences,
7]
Fuzhou, Fujian 350002, P. R. China
Eur. J. Inorg. Chem. 2004, 2533Ϫ2541
DOI: 10.1002/ejic.200300865
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2533

S. Banerjee, J. Gangopadhyay, C.-Z. Lu, J.-T. Chen, A. Ghosh
Attempts to perform a template synthesis by conden-
FULL PAPER
Schiff-base macrocycles[8] and cryptands.[9] A thermogravi-
metric experiment followed by spectroscopic characteris- sation of the appropriate aldehyde or ketone with ethylene-
ation showed a change from a six-coordinate octahedral ge- diamine followed by complexation with the metal ion in
ometry to a four-coordinate distorted square planar form situ, maintaining the 2:2:1 and 2:1:1 molar ratio necessary
by releasing two trans water co-ligands. The observed col- for the formation of complexes 1 and 2 gave the exclusive
our change (greenǞ red) is associated with this structural formation of 1 due to the chelate effect.
II
change, as is common for other Ni complexes in different
coordination environments.[
10]
Results and Discussion
Synthesis of the Complexes
The tridentate and tetradentate Schiff bases 1-amino-4-
(2-pyridyl)-3-azapent-3-ene and 2,7-bis(2-pyridyl)-3,6-di-
azaocta-2,6-diene (general abbreviation, L) have been em-
ployed as chelating ligand in the present work. The anal-
ogous 2-pyridylcarboxaldimine ligands have also been used
for a comparative study. The four complexes reported in
this context belong to two different types 1 and 2.
The perchlorate salts act as 1:2 electrolytes in acetonitrile
Ϫ1
2
Ϫ1
solution (Λ ϭ 245Ϫ255 Ω ·cm ·mol ). Complexes of
type 1 give the stable oxidised derivatives 3 under constant
electrolysis treatment, whereas type 2 complexes are in-
herently unstable towards electrolytic treatment.
The cis-trans-cis and cis-cis-trans disposition of the
p
p
i
i
a
a
p
p
a
a
N N -N N -N N (in type 1 complexes) and N N -N N -
w
w
p
O O (in type 2 complexes) donor sets (N is pyridyl nitro-
i
a
w
gen, N is imine nitrogen, N is amino nitrogen and O is
oxygen of water) are the striking features of these com-
plexes.
The brown bis-chelate complexes 1 are formed in excel-
1
2
lent yield upon reacting Ni(ClO ) ·6H O with L or L in Geometrical Preference and Structural Features
4
2
2
a 1:2.2 molar ratio in aqueous methanol under reflux
Scheme 1).
Ni(ClO ) ·6H O reacts smoothly with either L or L in
1
[
Ni(L ) ](ClO )
2 4 2
(
3
4
This complex has a distorted octahedral geometry. The
4
2
2
a 1:1.2 molar ratio in aqueous methanol under reflux to ligand encapsulates the divalent metal ion through six ligat-
give the green complexes 2 in high yield (Scheme 2). Com- ing nitrogen atoms from two pyridyl [N(1) and N(4)], two
plexes 2 undergo an interesting dehydration-rehydration imine [N(2) and N(5)] and two amino [N(3) and N(6)]
process, which is influenced by temperature and pressure. groups (Figure 1, Table 1). Several isomers are theoretically
At ambient temperature and normal pressure the complexes possible considering the relative arrangement of three types
are stable; the effect of heating on the dehydration process of coordinated nitrogen atoms around the metal centre, al-
will be examined later.
though the only one encountered here has the pyridine ni-
Scheme 1
Scheme 2
2534
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Eur. J. Inorg. Chem. 2004, 2533Ϫ2541

Nickel(II) Complexes Incorporating Pyridyl, Imine and Amino Chelate Ligands
FULL PAPER
trogen atoms in the cis position and the imine nitrogen
atoms in the trans positions, thereby leaving two cis posi-
tions available for the amino nitrogen donors. The trans dis-
i
position of two N donor sites in a cis-trans-cis geometry
ensures competition between them for the same metal d-
orbital, thus minimising the back-bonding stabilisation.
i
This cis-trans-cis geometry (no NiN interaction) is steri-
cally more favourable than any of the possible unobserved
i
i
isomers (cis-cis-trans, trans-cis-cis) having a cis NiN N
i
orientation (two NiN interactions). Evidently the steric
relaxation completely offsets the back-bonding disadvan-
tage in the case of the cis-trans-cis isomer and none of the
other isomers are observed.
The N(1), N(3), N(4) and N(6) atoms define a severely dis-
torted basal plane (mean deviation 0.39 A). The five-mem-
bered chelate rings [Ni, N(1), C(5), C(6) and N(2)] and
˚
[Ni, N(4), C(14), C(15), and N(5)] along with the conju-
gated pyridyl moieties constitute satisfactory planes (mean
˚
˚
deviation 0.06 A and 0.07 A respectively). The two remain-
ing saturated five-membered metallo rings are out of plane
˚
(
average mean deviation 0.16 A) and the dimethylene
bridges assume a twisted skew conformation (see inset, Fig-
1
ure 1). As a whole, two Ni(L ) fragments are not planar
˚
(
average mean deviation 0.11 A) essentially due to steric
repulsion between the two dimethylene-bridged chelate
rings. Two pyridylimine metallo rings enjoy extended π-elec-
tron delocalisation, inducing a more rigid nature and a de-
crease in the chelate bite angles (average 77.7°). On the con-
trary, the twisted dimethylene-bridged metallo rings are
flexible with larger (closer to 90°) bite angles (average
8
1.3°). All metal-nitrogen bond lengths are consistent
[
1,11Ϫ14]
i
with literature values.
than the NiϪN and NiϪN bonds by about 0.11 A. This
NiϪN bond-shortening stems from the better π-accepting
ability of imine (CϭN) functions than the N and N coor-
The NiϪN bonds are shorter
p
a
˚
i
p
a
dination sites.
3
[
Ni(L )(H O) ] (ClO )
2
2
4
2
p
i
w
The cis-cis-trans geometry with respect to N , N and O
donor sites is found exclusively rather than another possible
trans-cis-cis isomer for complexes 2. This isomer specificity
i
primarily stems from the better degree of NiϪN back-
Figure 1. A perspective view and atom-labelling scheme of the com-
bonding in the cis-cis-trans isomer due mainly to the pres-
1
plex cation of [Ni(L )
2
](ClO
4
)
2
(1a); all non-hydrogen atoms are
p
ence of a stronger trans σ-donor N atom (in the trans-cis-
represented by their 25% thermal probability ellipsoids; the inset
shows the conformations of the dimethylene-bridged chelate rings
w
i
cis geometry a weaker O atom is trans to the NiϪN bond)
sharing the same metal d-orbital. Moreover the cis-cis-trans
orientation (devoid of NiOw interactions) is better in a
steric sense than the unprecedented trans-cis-cis isomer with
˚
1
Table 1. Selected bond lengths (A) and angles (°) for [Ni(L )
ClO
2
]-
w
w
w
a cis-NiO O disposition (two destabilising NiO interac-
tions). Therefore, we can rationalize the exclusive occur-
rence of the cis-cis-trans isomer in terms of both electronic
(
4 2
)
NiϪN5
NiϪN6
NiϪN1
2.008(5)
2.119(5)
2.123(5)
NiϪN2
NiϪN4
NiϪN3
N5ϪNiϪN6
N5ϪNiϪN4
N6ϪNiϪN4
N2ϪNiϪN1
N4ϪNiϪN1
N2ϪNiϪN3
N4ϪNiϪN3
2.013(5)
2.120(7) and steric factors.
2.143(6)
81.9(2)
77.5(2)
158.6(2)
77.8(2)
87.3(2)
80.7(2)
94.3(2)
N5ϪNiϪN2
N2ϪNiϪN6
N2ϪNiϪN4
N5ϪNiϪN1
N6ϪNiϪN1
N5ϪNiϪN3
N6ϪNiϪN3
N1ϪNiϪN3
179.4(2)
97.5(2)
103.1(2)
102.5(2)
91.7(2)
99.0(2)
94.4(3)
158.2(2)
Eur. J. Inorg. Chem. 2004, 2533Ϫ2541
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2535

S. Banerjee, J. Gangopadhyay, C.-Z. Lu, J.-T. Chen, A. Ghosh
FULL PAPER
3
Figure 2. A perspective view and atom-labelling scheme of the complex cation of [Ni(L )(H
2
O)
2
](ClO
4
)
2
(2a); all non-hydrogen atoms are
represented by their 25% thermal probability ellipsoids; the inset shows the conformation of the dimethylene-bridged metallo ring
A molecular view of 2a is presented in Figure 2, and type, attributable to additional d(metal) Ǟ π*(CϭN) back-
II
selected bond parameters are listed in Table 2. The Ni ion bonding. Indeed this dative back-bonding is facilitated in
p
sits in a reasonably planar equatorial plane (mean deviation presence of the trans N atom (strong σ-donor) sharing the
˚
i
0
.04 A) defined by the N(1), N(2), N(3) and N(4) atoms.
same d orbital of the metal as the N donor. The average
p
˚
NiϪN distance in 2a is appreciably shorter (ca. 0.04 A)
than that observed for 1a. The underlying reason presum-
ably arises from the different relative orientations of the N
˚
3
Table 2. Selected bond lengths (A) and angles (°) for [Ni(L )-
O) ](ClO
(H
2
2
4 2
)
p
i
and N donor atoms around the metal centres (cis for 2a
NiϪN2
NiϪN4
NiϪO2
1.997(6)
2.071(6)
2.100(6)
82.8(2)
78.9(3)
161.2(2)
91.5(3)
87.3(2)
94.4(3)
89.2(2)
NiϪN3
NiϪN1
NiϪO1
N2ϪNiϪN4
N2ϪNiϪN1
N4ϪNiϪN1
N3ϪNiϪO2
N1ϪNiϪO2
N3ϪNiϪO1
N1ϪNiϪO1
2.002(6)
2.088(6) and trans for 1a), inducing the pyridyl moiety to act as a
2.118(5) better σ-donor in 2a. The NiϪO bonds are relatively
w
N2ϪNiϪN3
N3ϪNiϪN4
N3ϪNiϪN1
N2ϪNiϪO2
N4ϪNiϪO2
N2ϪNiϪO1
N4ϪNiϪO1
O2ϪNiϪO1
161.5(3)
78.5(2)
119.9(2)
95.0(2)
88.0(2)
92.4(2)
86.6(2)
[15,16]
II
weak
compared to other aquated Ni systems. The
presence of intermolecular hydrogen-bonding accounts for
the lengthening of the NiϪOw bonds, and is primarily an
electronic effect. The effect of this unsymmetrical hydrogen
bonding through two-coordinated water molecules is re-
flected in a bending of the O ϪNiϪO axis, making an
angle of 171.1(3)°.
w
w
171.1(3)
The uncoordinated perchlorate ions of 2a form a hydro-
gen-bonded network with the two axially coordinated water
molecules of the cationic part of the complex. Two water
Hard donors, like oxygen atoms, coordinated along the
axial direction force the metal ion to sit in the basal plane.
Individual π-conjugated chelate rings are essentially com-
II
molecules of the Ni complex cation are engaged in three
unsymmetrical O···O hydrogen-bond contacts with two
˚
pletely planar (mean deviation 0.01 A in both cases). The
˚
2
open ONN triangular faces (average surface area, ഠ4.1 A )
avoiding unwanted steric interactions. The molecular pack-
ing diagram reveals that the coordinated water molecules of
a cation form hydrogen bonds with perchlorate ions belong-
ing to two different asymmetric units. The relevant O···O
dimethylene-bridged chelate ring [Ni, N(2), C(8), C(9),
˚
N(3)] is only approximately planar (mean deviation 0.08 A)
as compared to the π-conjugated chelate fragments. The
3
overall planarity of the Ni(L ) motif (mean deviation 0.08
˚
A) is disturbed because of the twisted dimethylene bridging
˚
contacts [O(1E)···O(8A): 2.850(6) A, O(2E)···O(3C):
segment (see inset, Figure 2). The chelate bite angles of
N(4)ϪNiϪN(3) and N(1)ϪNiϪN(2) are 78.9(3) and
˚
˚
2
.876(8) A and O(2E)···O(10E): 2.722(8) A] generate a
chain-like cationic assembly in which alternating cations lie
in an almost parallel fashion, as illustrated in Figure 3.
78.5(2)°, respectively, and are less than that observed for
N(2)ϪNiϪN(3) [82.8(2)°]. Like 1a, the difference in bite
angles (Ͼ4°) is fully consistent with the varying degree of
flexibility of the chelate rings. A comparison between the
Electronic Spectra
p
i
NiϪN and NiϪN distances clearly reveals a bond short-
The electronic spectra of the four complexes 1a, 1b, 2a
and 2b were recorded both in the solid state (nujol mull)
˚
i
ening of about 0.08 A (on average) for the latter NiϪN
2536
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 2533Ϫ2541

Nickel(II) Complexes Incorporating Pyridyl, Imine and Amino Chelate Ligands
FULL PAPER
states, cyclic voltammetry experiments were performed on
and 2 (Table 3).
1
Table 3. Electrochemical data for all divalent nickel complexes
Compound
IIE1/2/V (∆E
/mV)[a]
p
NiIII-Ni couple
Ni -Ni couple
IV
III
1
1
2
2
a
b
a
b
0.95 (110)
0.96 (110)
0.85 (100)
0.87 (100)
1.69
1.72
1.78
1.80
[a]
∆E
p
ϭ Peak-to-peak separation.
1
2
[
Ni(L ) ](ClO ) and [Ni(L ) ](ClO ) are electroactive in
2 4 2 2 4 2
acetonitrile solution (TEAP as supporting electrolyte) dis-
playing two one-electron cyclic voltammetric responses with
E1/2 near ϩ0.95 and ϩ1.70 V vs. the saturated calomel elec-
trode (SCE). Type 1 complexes undergo chemically revers-
ible (ipa/ipc ഠ 1) and electrochemically quasi-reversible
(
∆E ϭ 110 mV) one-electron oxidative responses due to
p
III 1/2 3ϩ
II 1/2 2ϩ
the [Ni (L ) ] /[Ni (L ) ] couple, near 0.95 V as ob-
2
2
served for bis-pyridylazonaphthol-chelated complexes of
II [19]
Ni .
Appreciable reversibility is also inferred from the
i ,i vs. ν1 plot, which is nearly constant for different
/2
pa pc
scan rates (0.025, 0.05, 0.075 and 0.1 V/sec). The irreversible
redox signal encountered near ϩ1.70 V is attributed to the
4
ϩ/3ϩ
Ni
with a Ni
response. No voltammetric response consistent
2ϩ/1ϩ
reduction is observed. Further, absence of a
0
2ϩ
Ni (s)ǞNi stripping peak emphasises the reluctance of
II 1/2 2ϩ
[
Ni (L ) ] to undergo a two-electron reduction. The rel-
2
evant electron-transfer processes correlating metal valence
span from ϩ2 to ϩ4 can be represented by Equation (1).
(
1)
Figure 3. Molecular packing view of 2a along the x-direction
Such a relatively high E1/2 value for the Ni3ϩ/2ϩ redox
transformation is typical in complexes where nickel is in
a mixed sulfur-nitrogen coordination environment.[ Even
higher E1/2 values (ca. 1.6 V) have been reported for the
20]
and in acetonitrile solution. No significant differences were
observed. A broad band is observed near 800 nm, well sepa-
rated from the second transition at about 510 nm for both
Ni(bipy)3]3 /[Ni(bipy)3] couple.
ϩ
2ϩ
[21]
The absence of any
[
satellite response is fully consistent with the exclusive occur-
1
and 2. The third d-d band appears near 350 nm and is
rence of one isomer (cis-trans-cis), in solution.
partly obscured by a strong charge-transfer transition. The
3
The redox behaviours of [Ni(L )(H O) ] (ClO ) and
2
2
4 2
three absorption peaks are assigned to transitions from the
4
3
3
3
3
[Ni(L )(H O) ] (ClO ) follow a similar trend as 1a and 1b.
2 2 4 2
A2g ground state to T , T (F) and T (P) excited states.
2g
1g
1g
Complex 2 in acetonitrile exhibits a quasi-reversible (∆E ϭ
p
In addition, a weak peak appears near 750 nm for com-
3
100 mV) one-electron oxidation near 0.87 V consistent with
plexes 2, interpretable on the basis of a spin-forbidden A
2g
III 3/4
3ϩ
II 3/4
2ϩ
1
the [Ni (L )(H
2
O)
2
]
/[Ni (L )(H
2
O)
2
]
couple. The
Ǟ E transition. The calculated values of Dq lie in the
g
Ϫ1
observed shift (in 0.08 V) of the E1/2 value toward negative
range 1250Ϫ1260 cm for all complexes, and are in agree-
1/2 3ϩ/2ϩ
ment with literature values.[
17,18]
III
potential compared to the E1/2 observed for [Ni(L ) ]
2
The reddish-brown Ni
III
accentuates the relative stabilisation of the Ni species de-
complexes 3 are characterised by an LMCT transition at
about 425 nm with a shoulder near 645 nm.
rived from type 2 complexes. The trans disposition of the
1
/2 2ϩ
two imine donor motifs in [Ni(L ) ] which becomes a
2
cis orientation for [Ni(L3 )(H O) ] obviously facilitates
/4
2ϩ
2
2
Electrochemistry
greater delocalisation of the metal electron density, resulting
III
With the aim of investigating the extent of stabilisation in a higher potential barrier for the formation of Ni spec-
of nickel() species towards both oxidation and reduction, ies from the latter complexes. This was observed experimen-
and thereby the possibility of accessing of variable valence tally from the irreversible peak near ϩ1.80 V, ascribed to a
Eur. J. Inorg. Chem. 2004, 2533Ϫ2541
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2537

S. Banerjee, J. Gangopadhyay, C.-Z. Lu, J.-T. Chen, A. Ghosh
FULL PAPER
Ni4ϩ/3ϩ redox process. This oxidative response is shifted to place as this would otherwise result in an averaging of non-
positive potential by 0.10 V in comparison with the equivalent protons.
4
ϩ/3ϩ
II 3/4
2ϩ
Ni
[
couple for 1. Like 1, no [Ni (L )(H O) ]
/
Similarly in 2, the pyridyl and methylene proton signals
2
2
I
3/4
1ϩ
Ni (L )(H O) ]
redox couple is observed in aceto- appear in the region δ ϭ 7Ϫ14 and Ϫ7Ϫ30 ppm respec-
2
2
nitrile solution.
tively. The methyl (2a) and aldimine (2b) hydrogen reson-
In order to exclude any possibility of rapid isomerisation, ances occur in similar regions to those of 1. The aqua hy-
cyclic voltammograms of 1 and 2 were run at various tem- drogen resonances fall in the range δ ϭ 60Ϫ85 ppm. The
3
ϩ/2ϩ
4ϩ/3ϩ
peratures. Strong Ni
and Ni
responses for 1 and large range of the NMR signals of 2 is consistent with its
2
were observed at 288 and 308 K at very nearly the same paramagnetic nature.
formal potential, and with similar current heights, as ob-
served for the cyclic voltammograms recorded at 298 K
Thermal Analysis and Structural Transformation
(
discussed earlier). Therefore, in this context the tempera-
Complexes 2a and 2b are both green and possess an octa-
hedral geometry with axially coordinated water molecules.
The TG curve reveals that upon heating both 2a and 2b
undergo dehydration in the temperature range 50Ϫ95 °C
and 60Ϫ100 °C, respectively, corresponding to the weight
loss of two water molecules (observed: 6.96 and 7.01%; cal-
culated: 6.43 and 6.77% for 2a and 2b, respectively). The
anhydrous compounds begin to decompose at 275 and 270
ture effect upon the redox couples is not significant.
IR Spectra and Magnetism
The two primary NH stretching modes are seen in type
2
1
(
complexes near 3340 and 3290 cmϪ1 as sharp bands
doublet) for the asymmetric and symmetric vibrations,
respectively. The imine (CϭN) stretch appears at about
1
about 20 cm on going from the free ligands (L , L ) to
the complexes indicates the weak π-accepting behaviour of
the chelated ligands. The type 2 complexes exhibit a broad
band in the range 3410Ϫ3440 cm , assigned to the anti-
symmetric and symmetric OϪH stretching vibrations of co-
ordinated water molecules. ν
for 2. The diagnostic CϭN stretch is observed near 1650
°
C, respectively, indicating their high thermal stability.
The colour of the samples changes from green to red dur-
Ϫ1
660 cm , and the red shift of this CϭN absorption of
Ϫ1
1
2
ing dehydration and this colour change is reversed on expo-
sure to air for one day, suggesting a possible rehydration.
To rationalize the observed colour change we performed
magnetic susceptibility and spectral measurements of the
red complexes. The magnetic study (300 K) revealed the
diamagnetic nature of the dehydrated compounds, which is
further supported by the solid-state electronic spectra (nujol
Ϫ1
Ϫ1
appears near 450 cm
NiϪO
[
22]
Ϫ1
Ϫ1
cm , and is shifted to lower frequency by about 30 cm
II
mull), typical of square-planar Ni complexes (single peak
relative to the free ligands. The difference in imine stretch-
ing frequencies of complexes 1 and 2 confirms the greater
electron-withdrawing effect of the chelating bis-Schiff bases
at λmax ഠ 480 nm). Therefore the dehydration process is
definitely accompanied by a structural transformation [oc-
tahedral (green) Ǟ square planar (red)] around the metal
centre. It should be noted that this type of colour change
associated with a structural transformation has been re-
3
4
(L , L ) in the latter. Broad structured ν and sharp ν vi-
3 4
brations of the perchlorate counteranion are observed near
1
100 and 630 cmϪ1 for 1 and 2, respectively.
[
10]
II
ported
for several bis-diamine complexes of Ni , but to
The effective magnetic moment data of complexes 1 and
(polycrystalline state) clearly confirm the presence of two
the best of our knowledge is uncommon in complexes in-
corporating tetradentate bis-Schiff base ligands.
2
6
2
unpaired electrons (t e ) in a pseudo-octahedral environ-
2
ment, in agreement with a high-spin configuration (S ϭ 1).
The magnetic moment values of 3.12, 3.15, 3.06 and 3.05µB
at 298 K for 1a, 1b, 2a and 2b, respectively, suggest an ap-
Electrosynthesis of [Ni(L1/2)2]3؉: EPR and Magnetic
Evidence
The search for trivalent analogues of 1 and 2 prompted
us to carry out electrochemical oxidation (coulometrically)
in dry acetonitrile solvent at 1.25 and 1.20 V respectively.
The ratio of the observed coulomb count and the calculated
[
23]
preciable orbital contribution to the spin-only moments.
NMR Spectra
1
The H NMR spectra of 1 (bulk material, in [D ]DMSO) coulomb count for the one-electron transfer is very close to
6
display paramagnetically shifted resolved signals of the aro- unity (0.97 for 1a and 0.98 for 1b), suggesting an almost
II
matic, aliphatic and amino protons. The pyridyl and meth- quantitative one-electron oxidation of Ni precursors to the
III
1/2 3ϩ
ylene proton resonances occur in the ranges δ ϭ 6Ϫ12 and Ni derivatives. The cyclic voltammograms of [Ni(L ) ]
2
Ϫ12Ϫ45 ppm, respectively. The methyl (1a) and aldimine (initial scan cathodic) are virtually superimposable with
/2
2ϩ
(
1b) hydrogen atom signals appear as singlets in the region those of [Ni(L1 ) ] (initial scan anodic). The observed
2
III
δ ϭ 2.5Ϫ3.5 and 8Ϫ10 ppm, respectively. The amino pro- redox behaviour of the oxidised Ni species confirms that
tons resonate at lower field in the region δ ϭ 75Ϫ95 ppm. the coulometric oxidation of [Ni(L1 ) ] to [Ni(L ) ] is
/2
2ϩ
1/2 3ϩ
2
2
The absence of satellite peaks clearly confirms the exclusive accompanied by no major structural change. The uncon-
occurrence of one isomer. The distinct spin-spin structures trolled decomposition of electrogenerated oxidised species
of the corresponding signals remove any scope for a rapid of 2 in solution ruled out any possibility of isolation and
equilibrium between the possible isomers in solution. These further studies. They exist in solution only on the timescale
results mean that the stereochemistry of 1 in the solid state of the voltammetry measurement, thus demonstrating the
[
24]
is also retained in solution and no interconversion takes role of purely kinetic stabilisation in trivalent nickel.
2538
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org Eur. J. Inorg. Chem. 2004, 2533Ϫ2541

Nickel(II) Complexes Incorporating Pyridyl, Imine and Amino Chelate Ligands
FULL PAPER
The EPR spectroscopic data of the compounds are listed
Further evidence for trivalent nickel complexes is pro-
in Table 4. Polycrystalline samples of 3 at 298 K exhibit an vided by the magnetic moment data. The bulk magnetic
[
25]
axial spectrum
with gЌ and g values of 2.14 and 2.02 moment for 3 at 298 K is consistent with a one-electron
||
respectively (Figure 4). No superhyperfine structure of the paramagnetic behaviour. The effective magnetic moment
1
4
g component attributed to σ-coupling with the two
N
values of 2.12 and 2.09 µ for 3a and 3b, respectively, mean
B
|
|
imine) atoms coordinated in a mutual trans manner is ob- that the Ni complexes are present in a low spin (t26 e )
III
1
(
i
i
served. The bond-angle order N ϪNiϪN (ca. 179°) Ͼ configuration with S ϭ 1/2. A large orbital contribution to
p
a
[24]
N ϪNiϪN (ca. 158°) in the cis-trans-cis isomer of the paramagnetism
is noticed, as expected. Unfortu-
1
2ϩ
[
Ni(L ) ] implies that both steric and electronic require- nately we were unable to grow single crystals of either 3a or
2
i
i
ments will favour the orientation of a linear N ϪNiϪN
3b, although all the EPR and magnetic data unequivocally
moiety along the z-direction. Thus we can reasonably as- suggest the formation of trivalent nickel complexes.
7
sume that this is also the case for 3 (3d ). The observed g
inequality (gЌ Ͼ g||) is fully consistent with a tetragonally
distorted octahedral geometry with elongation of the axial Conclusion
bond along the z-axis.[
2,26,27]
Hence a strong JahnϪTeller
The tridentate and tetradentate ligands presented here
are characterised by their different nitrogen donor sites (py-
effect must be operative to lift the degeneracy of the e or-
2
2
2
bital set following the energy order 3d
Ͼ 3d . The spec-
x Ϫy
z
ridyl, imine and amino), and their different binding abilities
tra of 3 in a frozen (77 K) acetonitrile/toluene (2:1) glass
also follow a similar axial pattern as observed for polycrys-
talline state, strongly indicating that the coordination en-
vironment is retained (in solid and solution). The axial
spectra at 77 K are important as they suggest that the
rhombic field is too weak to split the perpendicular absorp-
tion into individual gxx and gyy tensors. The g values for
these complexes lie well above the spin-only value of a free
electron (2.002). This excludes any possibility of the forma-
II
towards a Ni centre provide us with NiN and NiN O
6
4
2
types of coordination complexes. Bis-chelates were achieved
without deprotonating the amino group. The cis-trans-cis
isomer of 1 is the only isomer obtained due to steric relax-
ation, which completely offsets any electronic disadvantage,
whereas the cis-cis-trans form of 2 is favoured by both steric
and electronic factors. The effect of the low π-acidity of the
ligands is clearly shown by the predominant localised nat-
ure of the bonding in the resulting complexes. Accessible
II
tion of Ni stabilised ligand radicals. Therefore, the ob-
cyclic voltammetric formal potentials of the Ni3 /Ni cou-
ϩ
2ϩ
served EPR signals arise from one metal-centred unpaired
III
[
28]
ple for 1 and 2 allowed us to synthesise the related Ni
electron, in agreement with the trivalent oxidation state.
Table 4. EPR data for trivalent NiIII species.
complexes electrochemically, although the complex formed
from 2 is not particularly stable, unlike that formed from 1.
Proper tuning of the coordination modes of tridentate
ligands containing pyridyl, imine and amino donor atoms
can afford stable trivalent nickel complexes. The design of
such ligands with variable polymethylene spacers and the
study of their efficacy to stabilize the higher oxidation states
of different metals are in progress.
Compound
g
Ќ
g
||
a]
[b]
[b]
[a]
[b]
[b]
3
3
a
b
2.138,[ 2.137
2.018, 2.018
2.139,[ 2.139
a]
2.017, 2.018
[a]
[
a]
Polycrystalline state at 298 K. [a] In acetonitrile/toluene (2:1)
glass at 77 K.
Experimental Section
4 2 2
Starting Materials: Ni(ClO ) ·6H O, 2-acetylpyridine and 2-pyri-
dinecarboxaldehyde were purchased from Aldrich and used as re-
ceived. HPLC-grade acetonitrile was used for electrochemical, con-
ductivity and UV/Vis work and all other chemicals and solvents
were of reagent grade and were used as received.
CAUTION: Perchlorate salts are hazardous and may explode.
Physical Measurements: Spectral measurements were carried out
using the following equipment: UV/Vis (both in acetonitrile solu-
tion and solid), Hitachi U-3501 spectrophotometer fitted with ther-
mostatted cell compartments (sh is shoulder). IR (KBr disc),
PerkinϪElmer RXI FT-IR spectrometer and Nicolet Magna IR
750 Series II. A PerkinϪElmer 2400 Series II elemental analyzer
was used for microanalysis (C, H, N). EPR spectra was recorded
at the X-band on a Varian E-109C spectrometer and calibrated
with respect to diphenylpicrylhydrazyl (DPPH, g ϭ 2.0037). Solu-
tion electrical conductivity was measured in acetonitrile with a
Phillips PR 9500 bridge using a platinized electrode; solute concen-
1
3ϩ
Figure 4. X-band EPR spectrum of [Ni(L )
2
]
(3a) in the polycrys-
talline solid state (298 K); instrument settings: power, 30 dB; fre-
quency, 9.1 GHz; modulation, 100 KHz; scan time, 2 min; sweep
centre, 3200 G
Ϫ3
1
tration was approx. 10 . H NMR spectra were recorded on a
Eur. J. Inorg. Chem. 2004, 2533Ϫ2541
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2539

S. Banerjee, J. Gangopadhyay, C.-Z. Lu, J.-T. Chen, A. Ghosh
FULL PAPER
Bruker FT 300 MHz spectrometer. The thermal analysis (TG-
3288 cmϪ1 (νNH2), 2942Ϫ2862 (νCϪH), 1658 (νCϭN), 1594 (δNϪH),
DTA) was carried out on a Metler Toledo TGA/SDTA 851 thermal 1441Ϫ1307 (νpyridine ring), 1090, 628(νClO4).
analyzer under a dynamic atmosphere of dinitrogen (flow rate: 30
Ϫ1
mL·min ). The sample was heated in an alumina crucible at a rate
3
[
Ni(L )(H
2
O)
2
](ClO
4
)
2
(2a): Ni(ClO
4
)
2
·6H
2
O (0.304 g, 0.83 mmol)
Ϫ1
of 10 °C·min . Magnetic susceptibilities were measured on a PAR
55 vibrating-sample magnetometer. Electrochemical measure-
3
dissolved in 10 mL of water was added to a solution of L (0.266 g,
.99 mmol) in methanol (40 mL), and heated to reflux for 3 h. The
solution turned green. It was then filtered and the filtrate cooled to
room temperature. Solvent removal from the filtrate under reduced
pressure gave a green residue which was washed twice (10 mL each)
with hexane to remove the excess ligand. The compound was then
dried in vacuo over fused CaCl
16 2 4
C H22Cl N
34.26, H 4.01, N 10.04. IR: ν˜ ϭ3435Ϫ3413 cm
982Ϫ2885 (νCϪH), 1647 (νCϭN), 1603 (δOϪH), 1452Ϫ1307
1
0
ments were performed under a dinitrogen atmosphere using a PAR
model Versastat-2 electrochemical analyzer, with a platinum
working electrode. The supporting electrolyte was tetraethyl-
ammonium perchlorate (TEAP), and the potentials are referenced
to the saturated calomel electrode (SCE) without junction correc-
tion.
2
. Yield: 433 mg (93%).
NiO10: calcd. C 34.32, H 3.96, N 10.01; found C
Ϫ1
1
(νOϪH),
L : Ethylenediamine (0.9 g, 15 mmol) and 2-acetylpyridine
1.815 g, 15 mmol) were mixed in 20 mL of methanol and then re-
fluxed for 3 h. The resulting solution was evaporated to 5 mL and
cooled to room temperature. Upon cooling of the solution a yellow
solid precipitated. The solid was collected by filtration and was
recrystallised from hot methanol affording pale yellow cubes, which
2
(
(
ν
r w
pyridine ring), 1096, 626(νClO4), 748 (ρ ), 609 (ρ ), 449 (νNiϪO).
4
[
Ni(L )(H
2
O)
2
](ClO
4
)
2
(2b): This complex was prepared by a simi-
NiO10: calcd. C
1.61, H 3.41, N 10.53; found C 31.55, H 3.48, N 10.50. IR: ν˜ ϭ
2 4
lar procedure. Yield: 407 mg (92%). C14H18Cl N
were dried in vacuo over fused CaCl
2
. Yield: 2.11 g (85%).
3
3
C
9
H
13
N
3
: calcd. C 66.23, H 8.03, N 25.74; found C 66.17, H 8.07,
Ϫ1
437Ϫ3410 cm (νOϪH), 2981Ϫ2879 (νCϪH), 1647 (νCϭN), 1604
(δOϪH), 1449Ϫ1303 (νpyridine ring), 1095, 625(νClO4), 747 (ρ ), 610
Ϫ1
N 25.81. IR: ν˜ ϭ 3345Ϫ3285 cm (νNH2), 2950Ϫ2850 (νCϪH), 1678
CϭN), 1590 (δNϪH), 1447Ϫ1302 (νpyridine ring).
r
(ν
(
ρ
w
), 449 (νNiϪO).
2
1
L : This ligand was prepared by a procedure similar to that for L ,
1
starting with ethylenediamine (0.9 g, 15 mmol) and 2-pyridinecar-
[Ni(L ) ](ClO ) (3a): This complex was prepared by an electro-
2
4 3
1
boxaldehyde (1.605 g, 15 mmol), to obtain a light-yellow semi-solid chemical technique. The complex [Ni(L ) ](ClO ) (0.146 g,
2
4 2
which, on recrystallisation from n-hexane, gave pale-yellow needles.
Yield: 1.84 g (82%). C : calcd. C 64.40, H 7.43, N 28.17;
found C 64.32, H 7.37, N 28.23. IR: ν˜ ϭ 3345Ϫ3281 cm (νNH2),
0.25 mmol) was dissolved in dry acetonitrile (30 mL) and TEAP
was added as supporting electrolyte. The brown solution was then
subjected to coulometric oxidation at 1.25 V vs. SCE under a dini-
8
11 3
H N
Ϫ1
2952Ϫ2847 (νCϪH), 1677 (νCϭN), 1591 (δNϪH), 1450Ϫ1302 trogen atmosphere. The solution gradually turned reddish-brown
(
ν
pyridine ring).
and the oxidation was stopped when the Coulomb count corre-
sponded to a one-electron oxidation. The solvent was then removed
under reduced pressure to obtain a reddish-brown solid mixed with
TEAP. The residue was dissolved in dichloromethane and TEAP
was filtered off. Solvent removal from the filtrate afforded a red-
dish-brown compound which was then dried in vacuo over fused
3
L : Ethylenediamine (0.9 g, 15 mmol) and 2-acetylpyridine
3.630 g, 30 mmol) were mixed in 25 mL of methanol and then re-
(
fluxed for 7 h. The resulting solution was evaporated to obtain a
yellow semi-solid mass which, on recrystallisation from n-hexane,
afforded light-yellow needles. The needles were dried in vacuo over
CaCl
2 3 6
. Yield: 150 mg (88%). C18H26Cl N NiO12: calcd. C 31.63, H
fused CaCl
N 21.04; found C 72.22, H 6.75, N 21.09. IR: ν˜ ϭ 2990Ϫ2880 cm
CϪH), 1680 (νCϭN), 1450Ϫ1307 (νpyridine ring).
2 18 4
. Yield: 2.44 g (61%). C16H N : calcd. C 72.15, H 6.81,
3.83, N 12.30; found C 31.58, H 3.80, N 12.34.
Ϫ1
(
ν
[
Ni(L2)
electrochemical procedure in 89% yield. C16
ployed for L , starting with ethylenediamine (0.9 g, 15 mmol) and C 29.32, H 3.38, N 12.82; found C 29.36, H 3.33, N 12.76.
-pyridinecarboxaldehyde (3.210 g, 30 mmol) to give light yellow
microcrystals. Yield: 2.29 g (64%). C14 : calcd. C 70.57, H
.92, N 23.51; found C 70.51, H 5.98, N 23.45. IR: ν˜ ϭ 2992Ϫ2877
2
](ClO
4
)
3
(3b): This complex was synthesised by a similar
NiO12: calcd.
4
L : This ligand was prepared by a similar procedure to that em-
3 6
H22Cl N
3
2
14 4
H N
X-ray Crystallography: Brown or green single crystals of the com-
plexes 1a and 2a, respectively, were grown by slow evaporation of
solvent from a methanol solution. Data were collected on a Sie-
5
Ϫ1
cm (νCϪH), 1680 (νCϭN), 1451Ϫ1306 (νpyridine ring).
Ni(L1)
2 4 2 4 2 2
](ClO ) (1a): Ni(ClO ) ·6H O (0.165 g, 0.45 mmol) dis-
mens SMART CCD diffractometer with graphite-monochromated
[
˚
Mo-K
α
radiation (λ ϭ 0.71073 A) by the phi and omega scan tech-
solved in the minimum volume of water was added to a solution
1
nique in the range 3° Յ 2θ Յ 50° at 293 K. All the data were
corrected for Lorentz-polarisation and absorption.[ The struc-
tures were generated by direct methods followed by successive
Fourier synthesis and refined by full-matrix least-squares based on
of L (0.163 g, 0.99 mmol) in methanol (30 mL), and heated to re-
29]
flux for 2 h to afford a brown solution. The resulting solution was
cooled to room temperature and filtered. The filtrate was stripped
of solvent under reduced pressure. The residue was washed twice
2
F . All non-hydrogen atoms were made anisotropic. All the hydro-
(
15 mL each) with hexane to remove the excess ligand and then
dried in vacuo over fused CaCl Yield: 242 mg (92%).
NiO : calcd. C 37.02, H 4.49, N 14.39; found C 37.09,
H 4.45, N 14.46. IR: ν˜ ϭ 3340, 3290 cm (νNH2), 2940Ϫ2862
gen atoms (barring water hydrogen atoms) for 2a were added at the
calculated positions. Hydrogen atoms apart from two amino groups
2
.
C
18
H26Cl
2
N
6
8
Ϫ1
(N3, N6) were added at the calculated positions for 1a. Calcu-
TM
lations were performed using the SHELXTAL
v. 5.03 program
(νCϪH), 1658 (νCϭN), 1596 (δNϪH), 1445Ϫ1305 (νpyridine ring), 1089,
package.[ Significant crystal data are listed in Table 5.
30]
627 (νClO4).
CCDC-216308 (for 1a) and -216309 (for 2a) contain the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from
[
Ni(L2)
cedure. Yield: 226 mg (90%). C16
2
](ClO
4
)
2
(1b): This complex was prepared by a similar pro-
NiO : calcd. C 38.57, H
H
22Cl
2
N
6
8
3
.99, N 15.12; found C 38.53, H 4.05, N 15.07. IR: ν˜ ϭ 3341, the Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
2540  2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjic.org Eur. J. Inorg. Chem. 2004, 2533Ϫ2541

Nickel(II) Complexes Incorporating Pyridyl, Imine and Amino Chelate Ligands
R. A. Sheldon and J. K. Kochi), Academic Press: New York,
FULL PAPER
Table 5. Crystal data for complexes 1a and 2a
1
981, 359. [ Catalytic Activation of Dioxygen by Metal Com-
6b]
plexes (Ed.: L. I. Simandi), Kluwer Academic Publishers: Dor-
drecht, 1992, 318.
7]
Complex
1a
2a
[
M. T. Maede, D. H. Busch, in Progress in Inorganic Chemistry
Formula
M
C
18
H
26Cl
2
N
6
NiO
8
16 2 4
C H22Cl N NiO10
(
Ed.: S. J. Lippard), Wiley Interscience: New York, 1985, 33
584.06
monoclinic
P2 /c
15.5216(13)
10.1708(9)
15.7458(14)
90
97.305(2)
90
2465.6(4)
4
559.99
orthorhombic
P2
11.3061(10)
12.9531(12)
15.803(2)
90
90
90
2314.4
4
1.607
293
and references cited therein.
System
Space group
[8]
[9]
V. Alexander, Chem. Rev. 1995, 95, 273 and references cited
1
1 1 1
2 2
therein.
˚
a (A)
J. Lehn, Pure Appl. Chem. 1980, 52, 2441 and references cited
therein.
Y. Ihara, Y. Fukuda, K. Sone, Inorg. Chem. 1987, 26, 3745 and
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S. R. Cooper, Acc. Chem. Res. 1988, 21, 141.
C. R. Lucas, S. Liu, J. Chem. Soc., Dalton Trans. 1994, 185.
D. Shellmann, H.-P. Neuner, F. Knoch, Inorg. Chim. Acta 1991,
˚
b (A)
˚
c (A)
[10]
α (°)
β (°)
γ (°)
[11]
[12]
[13]
˚
3
V (A )
Z
1
90, 61.
D (mg mϪ3)
T (K)
1.573
293
1.059
[14]
I. Castro, M. L. Calatayud, J. Sletten, F. Lloret, M. Julve, J.
Chem. Soc., Dalton Trans. 1997, 811.
µ (mmϪ1)
1.128
3639
0.0347
7779
[15] [15a]
R. Soules, F. Dahan, J. P. Laurent, P. Castan, J. Chem.
Independent reflections 4375
[15b]
Soc., Dalton Trans. 1988, 587.
B. Q. Ma, S. Gao, H. L.
R
int
0.0560
10189
Sun, G. X. Xu, J. Chem. Soc., Dalton Trans. 2001, 130.
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I Ͼ 2σ(I)]
R1, wR2
[16]
J. A. C. van Ooijen, J. Reedijk, A. L. Spek, Inorg. Chem. 1979,
[
1
8, 1184.
0.0685, 0.1452
0.0558, 0.1435
[17] [17a]
E. I. Baucom, R. S. Drago, J. Am. Chem. Soc. 1971, 93,
[17b]
6
469.
P. Krumholtz, Struct. Bonding 1971, 9, 139.
[
18] [18a]
M. A. Robinson, J. D. Curry, D. H. Buch, Inorg. Chem.
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966, 5, 1172.
18b]
H. M. Fischer, R. C. Stoufer, Inorg. Chem.
1
1
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deposit@ccdc.cam.ac.uk].
[
[
19]
20]
S. Mukhopadhyay, D. Ray, J. Chem. Soc., Dalton Trans.
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E. M. Martin, R. D. Bereman, Inorg. Chim. Acta 1991, 188,
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1
2
Acknowledgments
One of the authors (S. Banerjee) is thankful to the University
Grants Commission (UGC), India, for awarding a Junior Research
Fellowship (Sanction no. UGC/548/jr. Fellow Sc.2001/2002).
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Received November 25, 2003
Early View Article
Published Online April 20, 2004
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Metal-Catalyzed Oxidations of Organic Compounds (Eds.:
Eur. J. Inorg. Chem. 2004, 2533Ϫ2541
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2541