Synthesis, crystal structure, electrochemical and kinetic studies on a cyano-bridged CuII-NiII-CuII hetero-trinuclear complex

Article abstract of DOI:10.1016/j.poly.2006.09.095

A cyano-bridged hetero-trinuclear complex, [Cu(HL)]₂[Ni(CN)₄] (2) (H₂L = 3,9-dimethyl-4,8-diazaundec-3,8-diene-2,10-dione-dioxime) has been synthesized and characterized by single crystal X-ray structure determination. The square planar [Cu(HL)]⁺ cation (1), otherwise reluctant to bind axially, is converted to a square pyramidal geometry through axial ligation by the cyano group of [Ni(CN)₄]^2- ion in di-trans fashion. Electrochemical studies in MeCN give (i) E_1/2(Cu^II/I) = -0.52 V and -0.55 V and (ii) E_1/2(Cu^III/II) = 1.15 and 1.23 V versus Ag/AgCl for complexes 1 and 2 respectively, whereas in aqueous solution, the corresponding E_1/2(Cu^II/I) appear at -0.61 V and -0.58 V. The E_1/2(Cu^III/Cu^II) appears at 1.04 V for 2 only; no such process is encountered for the complex 1 at potential ≤1.20 V. In addition to these there is a redox process, occurring in aqueous solution only, for the copper(II) bound ligand oxidation at 0.63 and 0.60 V versus Ag/AgCl for the complex 1 and 2 respectively. Both the complexes undergo acid catalyzed decomposition with a second-order dependence in H⁺. It is found that complex 2 decomposes at a rate ～2 times faster than the complex 1. The kinetic studies on the periodate oxidation reveal that both HIO₄ and IO₄^- are reacting species for the oxidation of 1, while HIO₄ is the sole reacting species for the complex 2. Again, complex 2 reacts faster than 1. An inner-sphere mechanism has been invoked for both the complexes based on large Q, the association constant values.

Full text of DOI:10.1016/j.poly.2006.09.095

Polyhedron 26 (2007) 1012–1022
www.elsevier.com/locate/poly
Synthesis, crystal structure, electrochemical and kinetic studies on
a cyano-bridged CuII–NiII–CuII hetero-trinuclear complex
a
a
b
c
Ambarish Ray , Daipayan Dutta , Parikshit Chandra Mondal , William S. Sheldrick ,
c
a,
*
H. Mayer-Figge , Mahammad Ali
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Saha Institute of Nuclear Physics, Chemical Science Division, 1/AF Bidhan Nagar, Kolkata 700 064, India
b
c
Lehrstuhl fur Analytischie Chemie, Ruhr-Universitat Bochum, D-44780 Bochum, Germany
Received 17 August 2006; accepted 26 September 2006
Available online 10 October 2006
Abstract
A cyano-bridged hetero-trinuclear complex, [Cu(HL)]2[Ni(CN)4] (2) (H2L = 3,9-dimethyl-4,8-diazaundec-3,8-diene-2,10-dione-
dioxime) has been synthesized and characterized by single crystal X-ray structure determination. The square planar [Cu(HL)]+
cation (1), otherwise reluctant to bind axially, is converted to a square pyramidal geometry through axial ligation by the cyano
group of [Ni(CN)4]2ꢀ ion in di-trans fashion. Electrochemical studies in MeCN give (i) E1/2(CuII/I) = ꢀ0.52 V and ꢀ0.55 V and
(ii) E1/2(CuIII/II) = 1.15 and 1.23 V versus Ag/AgCl for complexes 1 and 2 respectively, whereas in aqueous solution, the corre-
sponding E1/2(CuII/I) appear at ꢀ0.61 V and ꢀ0.58 V. The E1/2(CuIII/CuII) appears at 1.04 V for 2 only; no such process is
encountered for the complex 1 at potential 61.20 V. In addition to these there is a redox process, occurring in aqueous solution
only, for the copper(II) bound ligand oxidation at 0.63 and 0.60 V versus Ag/AgCl for the complex 1 and 2 respectively. Both
the complexes undergo acid catalyzed decomposition with a second-order dependence in H+. It is found that complex 2 decom-
poses at a rate ꢁ2 times faster than the complex 1. The kinetic studies on the periodate oxidation reveal that both HIO4 and
ꢀ
IO4 are reacting species for the oxidation of 1, while HIO4 is the sole reacting species for the complex 2. Again, complex 2
reacts faster than 1. An inner-sphere mechanism has been invoked for both the complexes based on large Q, the association
constant values.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Cyano-bridged-hetero-trinucler complex; Crystal structure; Cyclic-voltammetry; Kinetic studies
1. Introduction
In the late seventies, Addison et al. first selected the
pseudo-macrocyclic oxime–imine ligand, H2L (H2L =
3,9-dimethyl-4,8-diazaundec-3,8-diene-2,10-dione-dioxime)
to study the type-2 [4] copper chemistry [5]. However, they
modified the original [Cu(HL)]+ precursor to [Cu(L)BF2]+
to enhance the Lewis acidity of the copper center and
monitored the axial interaction with a series of neutral
or anionic monodentate ligands [5]. Since H2L has the ten-
dency of strong N ! M r-donation; it reduces the Lewis
acidity of the metal center in square planar [Cu(HL)]+
moiety making it reluctant to exhibit coordination
flexibility, which is further assisted due to Jahn Teller
effect.
Oxime–imines represent an important class of ligands
with the capability to stabilize the higher oxidation states
of the central metal ion through strong ligand to metal
(L ! M) r-donation [1]. Though several oxime–imine
complexes of iron and nickel in their higher oxidation
states are well reported [2], but the corresponding copper
complexes are comparatively less [3].
*
Corresponding author.
E-mail address: mali@chemistry.jdvu.ac.in (M. Ali).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.09.095

A. Ray et al. / Polyhedron 26 (2007) 1012–1022
1013
2. Results and discussion
F
O
N
F
B
H
O
OH
N
OH
N
O
O
N
2.1. Description of structure of
[Cu(HL)]2[Ni(CN)4] Æ 4H2O (2)
N
N
Cu
Cu
N
N
N
N
N
N
Complex 2 crystallizes in the monoclinic space group C2/
c and exhibits crystallographic C2 symmetry. The asymmet-
ric unit contains half of the formula unit along with three
water molecules. The molecular structure of the complex
2 consists of a discrete hetero-trinuclear species (Cu–Ni–
+
+
H L
[Cu(HL)]
[Cu(L)(BF2)]
2
˚
Cu) with the copper–copper separation of 9.778 A by
In our earlier report, we have been able to extend the
coordination number of copper(II) under a sterically con-
gested N4 donor macrocyclic environment [6]. In an exten-
severely bent cyano bridging [C(8)–N(5)–Cu, 143.7(4)ꢁ] of
[Ni(CN)4]2ꢀ ion in di-trans fashion; along with four water
molecules. The bonding arrangement of this trinuclear unit
is shown in Fig. 1 and selected bond distances and angles in
Table 1. Such type of cyano bridging of [Ni(CN)4]2ꢀ, leaves
two other trans CN groups as terminal, and is comparable
to [Cu(bappz)]2[Ni(CN)4] Æ (ClO4)2 [11a] [bappz = 1,4-
bis(3-aminopropyl)piperazine]. The coordination geometry
about the copper atom is best described by a square pyra-
mid with the geometrical factor s = 0.01 [14]. The four
nitrogen atoms of the H2L ligand construct the basal plane
and the nitrogen atom of the bridging cyanide occupies the
sion to such efforts, here, we have considered
a
comparatively reluctant [Cu(HL)]+ chromophore to inves-
tigate its axial interaction with the tetracyanonickelate
ion.
Again, the chemistry of cyano-bridged coordination
complexes of transitional metal ions is of contemporary
interest due to remarkable diversity in their structural
topologies and indeed, it is a popular approach for design-
ing the construction of different supramolecular architec-
tures with unusual and interesting properties [7]. Among
various cyanometalates, the tetracyanonickelate anion
[Ni(CN)4]2ꢀ exhibits bridging character involving either
one, two, three or four of the cyano groups and all four
possibilities have been found [7]. Although a few copper(II)
complexes with tetracyanonickelate bridged 1D [8], 2D [9]
and even 3D [9b] structures have been reported recently,
the number of structural studies on discrete molecules is
surprisingly low [10] and such complexes are prepared
mainly by utilizing various N1, N2 or N3 donor ligands.
Moreover, there is only a few reports on structural studies
of such discrete tetracyanonickelate bridged copper(II)
complexes under an N4 donor ligand environment [11]
and to the best of our knowledge this is the first report
on an oxime–imine like N4 donor pseudo-macrocyclic
precursor.
On the other hand, from the chemical redox point of
view, periodate is thought to be a typical inner-sphere oxi-
dant often featuring diverse mechanistic behavior [12].
Though, there are reports on the oxidation of some inor-
ganic species with a few copper(III) oxime–imine com-
plexes [13], corresponding kinetic studies on the oxidation
of copper(II) complexes by inorganic oxidant have not
yet been reported. So a kinetic study on the oxidation of
the cyano-bridged complex, [Cu(HL)]2[Ni(CN)4] Æ 4H2O
(2) along with the parent complex, [Cu(HL)]ClO4 Æ 2H2O
(1) by periodate would be valuable and interesting mecha-
nistically. In this article we report synthesis, crystal struc-
ture, electrochemical and kinetic studies on the periodate
oxidation on a cyano-bridged Cu(II)–Ni(II)–Cu(II) hetero-
trinuclear complex 2. A parallel electrochemical and kinetic
studies on the parent complex 1 reveals that there is a dis-
tinct difference in behavior of 2 over 1 due to the presence
of [Ni(CN)4]ꢀ in 2.
˚
apical position. The copper atom is situated 0.302 A above
the plane towards the apical direction. This makes the basal
angles N(1)–Cu–N(3), 161.81(2)ꢁ and N(2)–Cu–N(4),
162.64ꢁ to deviate significantly from 180ꢁ and the four
N(5)–Cu–N(x) [x = 1, 2, 3 and 4] angles are larger than
90ꢁ (avg. 98.82(2)0) indicating that the coordination geome-
try around each copper(II) atom is a distorted square pyra-
˚
mid. The average Cu–N bond length (1.964(4) A) at the
basal plane is comparable with the complex [Cu(HL)]ClO4 Æ
1/2CH3OH [15]. However, in that complex each copper atom
of monomeric [Cu(HL)]ClO4 Æ 1/2CH3OH unit is further
very weakly coordinated to an oxime oxygen (average Cu–
˚
O distances of 2.492 (4) A) of the adjacent equivalent unit
to form a singly bridged zigzag chain, whereas, a discrete tri-
nuclear structure is prevailing for complex 2 with no possibil-
ity of formation of such oxime oxygen bridged Cu–O bonds.
From the charge balance considerations, it is evident
that out of two cis oxime groups of each asymmetric unit,
one is deprotonated during complex formation and makes
intramolecular H-bonding with the remaining –N–OH
˚
group. The O(1)–O(2) distance (2.520 A) is similar to the
H-bonded oxygen–oxygen distance observed in other cop-
per–oxime complexes and supports this proposition [15,16].
˚
The Cu–N(5) apical bond length (2.155(4) A) is compara-
tively longer than the Cu–N (bridging cyanide)basal
˚
(2.040(7) A) found in [Cu(Me6Cy)][Ni(CN)4] [6]. In con-
trast, Cu–N axial bond lengths for several such cyano
bridged copper(II) complexes fall in the range 2.147–
2.677 [11a,11b] which are comparable to the Cu–N(5) axial
bond length in 2 and indicate the inclination of Cu atom
towards the axial site (vide infra).
The C-end of the cyano ligand produces a strong field
effect, while the N-end behaves as medium ligand with a
ligand field lower than that of NH3. Generally, C–N dis-

1014
A. Ray et al. / Polyhedron 26 (2007) 1012–1022
Fig. 1. ORTEP view of the trinuclear unit of 2. Hydrogen atoms and water molecules are omitted for clarity. (Thermal ellipsoids are represented at the
30% probability level.)
˚
˚
Table 1
groups are 1.134 (6) A and 1.152(7) A respectively and con-
form to the above proposition.
˚
Selected bond lengths (A) and angles (ꢁ) for complex 2
The Ni–C bond distances in [Ni(CN)4]2ꢀ fall in the range
1.856(5)–1.859(5) A for both bridging and terminal cyano
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(4)
Cu(1)–N(5)
N(1)–O(1)
N(4)–O(2)
N(5)–C(8)
N(6)–C(9)
1.961(4)
1.950(4)
1.972(4)
1.972(4)
2.155(4)
1.336(5)
1.339(5)
1.134(6)
1.152(7)
˚
groups and are very close to the ideal value of 1.86 [17].
The [Ni(CN)4]2ꢀ moiety in 2 effectively retains its original
square planar geometry and this is evidenced by the fact
that all the Ni–C–N bond angles fall in the narrow range
of 177.8(5)–178.3(5)ꢁ. Such rigidity of the square planer
structure is generally seen for [Ni(CN)4]2ꢀ in various bridg-
ing structures regardless of different modes of bridging [7].
As it is already known to us that lattice water molecules
present in the crystal play a unique and ubiquitous role, not
only to impart stability of the complex but simultaneously
help to build up the supramolecular as well as the biologi-
cal architectures. The arrangement of lattice water mole-
cules in the present complex is highly ordered and
produces a water column viewed down the b axis (Fig. 2).
N(2)–Cu(1)–N(1)
N(2)–Cu(1)–N(3)
N(1)–Cu(1)–N(3)
N(2)–Cu(1)–N(4)
N(1)–Cu(1)–N(4)
N(3)–Cu(1)–N(4)
N(2)–Cu(1)–N(5)
N(1)–Cu(1)–N(5)
N(3)–Cu(1)–N(5)
N(4)–Cu(1)–N(5)
O(1)–N(1)–Cu(1)
O(2)–N(4)–Cu(1)
C(8)–N(5)–Cu(1)
N(5)–C(8)–Ni(1)
N(6)–C(9)–Ni(1)
80.9(2)
98.7(2)
161.8(2)
162.6(2)
95.1(2)
79.7(2)
97.1(2)
95.2(2)
102.8(2)
100.2(2)
122.2(3)
123.2(3)
143.7(4)
178.3(5)
177.8(5)
2.2. IR study
The positions of the relevant IR bands were listed in the
experimental section. For cyano complexes the absorption
bands due to mCN stretching vibrations are very characteris-
tic. In the free tetracyanonickelate anion this absorption
band appears at 2128 cmꢀ1 [17]. On bridging via N atom
this band shifts to higher wave number. Here two mCN IR
tances fall in the range 1.12–1.16 and no difference is found
whether it is a terminal or bridging ligand [7]. In complex 2,
the C–N distances for the bridging and terminal cyanide

A. Ray et al. / Polyhedron 26 (2007) 1012–1022
1015
g^ = 2.022 0.002. Simulation of this signal using the
Lorentzian function [18] of the individual EPR line gives:
gi = 2.13, g^ = 2.02, Ai = 40 Oe, A^ = 5 Oe with DH =
40 Oe at m = 9.517 GHz. The observed values (gi > g^) con-
firm that the unpaired electron resides in the dx2ꢀy2 orbital.
2.6. Electrochemical studies
The electrochemical properties of the parent complex
[Cu(HL)]ClO4 Æ 2H2O (1) and that of complex [Cu(HL)]2-
[Ni(CN)4] Æ 4H2O (2) were investigated using the cyclic vol-
tammetric technique in aqueous and MeCN solutions.
Tetrabutyl ammonium perchlorate (TBAP, 0.1 mol Lꢀ1
)
was used as the supporting electrolyte in MeCN solution,
while 0.1 M NaClO4 was used in aqueous solution. For
comparison of the data obtained in aqueous solution, cyc-
lic voltammetry of the free ligand (H2L) was also carried
out under similar conditions. All the electrochemical data
are summarized in Table 2.
Fig. 2. Water column of the lattice water molecules (viewed down the b
axis).
bands appeared at 2118(vs) and 2135(s) cmꢀ1 for 2 can be
assigned to the terminal and bridging cyanides respectively
[6]. The broad mO–H band at 3389 cmꢀ1 can assure the pres-
ence of O–Hꢂ ꢂ ꢂO hydrogen bonded oxime proton [16] as
well as the hydrogen bonded lattice water molecules. All
the other bands appear more or less at their usual positions.
Fig. 3a shows the cyclic voltammogram for complex 2
in MeCN (1.0 · 10ꢀ3 mol Lꢀ1) with a well defined cathodic
wave for copper(II) to copper(I) reduction at a potential ca.
ꢀ0.60 V, for which the reverse scan gave an anodic wave at
ca. ꢀ0.50 V and the corresponding E1/2 was estimated as
ꢀ0.55 V versus Ag/AgCl. The peak potentials were invariant
of scan rates and ipc versus m1/2 were linear passing through
origin, where ipc is the cathodic peak current and m is the
scan rate in V sꢀ1. Though the DEp value was slightly higher
than that for the Nernstian behavior it may be assumed as a
reversible process with a ipa/ipc value approximated to unity.
When the scan was continued further to more negative
potential, it gave an irreversible reduction peak at potential
ca. ꢀ1.75 V versus Ag/AgCl (not shown in the figure). How-
ever, this reduction process has not been investigated fur-
ther. In the anodic potential region, a quasi-reversible one
electron oxidation of copper(II) to copper(III) was observed
with DEp = 0.16 V and E1/2 = 1.23 V versus Ag/AgCl. It is
important to note that the both anodic and cathodic peak
positions were invariant with scan rates in the range of
0.05–0.8 V sꢀ1. The diffusion co-efficient [19] value of the
complex 2 was determined and shown in Table 2.
2.3. Thermal analyses
Thermal decomposition of complex 2 starts at ca. 41 ꢁC
and consequently, the complex eliminates its uncoordi-
nated four water molecules and the process continues up
to ca. 58 ꢁC (found/calcd.: 6.45/6.43% with respect to loss
of wt%). This is accompanied by a small endo-effect on
the DTA curve at ca. 54 ꢁC. The anhydrous complex
[Cu(HL)]2[Ni(CN)4] maintains its thermal stability upto
ca. 171 ꢁC, after which it undergoes an immediate exother-
mal decomposition with a DTA curve maxima at ca.
201 ꢁC. The decomposition of the complex is complete at
ca. 200 ꢁC, giving CuO and NiO as the final products
(found/calcd.: 81.65/81.64% with respect to loss of wt%).
2.4. Magnetic property
Electrochemical reduction and oxidation behavior of the
parent complex 1 was found behave similarly in MeCN.
Fig. 3b shows a typical voltammogram for the cyclic vol-
tammetry of the parent complex in MeCN with glassy car-
bon working electrode. The quasi-reversible reduction
process appeared at a slightly more positive potential than
that in complex 2 under the similar condition and also the
voltammetric peak current was consistent with a slightly
greater diffusion co-efficient as shown in Table 2. It is
important to note that the intensity of the anodic peak for
the oxidation of copper(I) to copper(II) in the reverse scan
was greatly reduced with the formation of a desorption
peak at ca. ꢀ0.20 V. Here also the second reduction peak,
which was again irreversible, was not investigated further.
Cyclic voltammetric studies in MeCN indicate that the
complex 2 is reduced at more negative potential (Fig. 3a)
The room temperature (298 K) effective magnetic
moment per monomer of complex 2 is 1.78lB and is com-
parable with the spin only value (1.73lB) for ideally mono-
meric copper(II) complexes. The large Cuꢂ ꢂ ꢂCu separation
˚
(9.778 A) in between two monomeric units via diamagnetic
tetracyanonickelate bridging, weakens the chance of mag-
netic interactions in the complex.
2.5. EPR studies
Room temperature solid state EPR of the polycrystal-
line sample of the complex 2 is typical for mononuclear
copper(II) complexes with axial distortion and the elonga-
tion of axis is characterized by gi = 2.134 0.005 and

1016
A. Ray et al. / Polyhedron 26 (2007) 1012–1022
Table 2
Electrochemical parametersa of the compounds
Compound
Solvent
MeCN
Supporting electrolyte (0.1 M)
TBAP
Epc (V)
Epa (V)
DEp (V)
E1/2 (V)
D (cm2 sꢀ1
)
[CuHL]ClO4 Æ 2H2O (1)]
1.12
ꢀ0.58
0.37
ꢀ0.65
1.15
ꢀ0.60
0.98
1.19
ꢀ0.47
0.89
ꢀ0.58
1.31
ꢀ0.50
1.1
0.07
0.11
0.52
0.07
1.15c
ꢀ0.52d
0.63b
8.9 · 10ꢀ5
2.8 · 10ꢀ5
water
NaClO4
ꢀ0.61d
[Cu(HL)]2[Ni(CN)4] Æ 4H2O (2)
MeCN
water
TBAP
0.16
0.10
0.12
0.48
0.11
1.23c
ꢀ0.55d
1.04c
3.9 · 10ꢀ5
6.8 · 10ꢀ5
2.7 · 10ꢀ5
NaClO4
0.36
ꢀ0.63
0.84
ꢀ0.52
0.60b
ꢀ0.58d
5.3 · 10ꢀ5
H2L
water
NaClO4
0.49
0.73
0.24
0.61b
a
Maximum error 7%; all potentials are measured against Ag/AgCl reference electrode.
E1/2 for ligand oxidation.
b
c
E1/2 for CuIII/CuII couple.
d
E1/2 for CuII/CuI couple. E1/2 is determined as 1/2(Epc + Epa).
Fig. 3. Cyclic voltammograms of complex 1 (b) and 2 (a) in MeCN using
a glassy carbon electrode and TBAP (0.10 M) as supporting electrolyte at
100 mV sꢀ1 scan rate.
Fig. 4. Cyclic voltammograms of complex 1 (c) and 2 (a and b) in water
using a glossy carbon electrode and NaClO4 (0.10 M) as supporting
electrolyte at different scan rates. Inset is the cyclic voltammogram of H2L
under the same conditions.
in comparison to that of the parent complex and this is
attributed due to the axial ligation of [Ni(CN)4]2ꢀ to
[Cu(HL)]+ precursor through nitrogen end (vide supra).
In aqueous solution the cyclic voltammetry of the com-
plex 2 (Fig. 4a and b) showed a reduction behavior very
similar to that observed in MeCN with a quasi-reversible
reduction of CuII/CuI at a E1/2 of ꢀ0.58 V versus Ag/AgCl.
But in contrast, the oxidation behavior in MeCN was com-
pletely different from that in aqueous solution. Oxidation
of copper(II) takes place at a E1/2 of 1.04 V versus Ag/AgCl
which was shifted to a lower potential in comparison to
that observed in MeCN. Oxidation of copper(II) or reduc-
tion of copper(III) involves a fast proton transfer which is
facilitated in aqueous solution, and as a result the redox
potential is shifted to a lower region. This is a kind of pro-
ton-coupled electron transfer reaction and very common
for many nickel(II) and iron(II) complexes of analogous
ligands [2]. Further, in addition to the oxidation of cop-
per(II) to copper(III) another anodic wave was noticed at
Epa = 0.84 V having a reduction wave in the cathodic scan
at ca. 0.36 V with E1/2 = 0.60 V versus Ag/AgCl and was
assigned to a quasi-reversible oxidation of the copper-
bound ligand. It is worth-mentioning here that when the
free ligand H2L was scanned under the similar condition
(Fig. 4 inset), an almost identical oxidation of the ligand
at E1/2 ꢁ 0.61 V was observed. Thus it may be suggested
that this quasi-reversible oxidation process is due to oxida-
tion of the copper-bound ligand.
The electrochemical behavior of 1 (Fig. 4c) was also
studied in aqueous solution under the similar condition.

A. Ray et al. / Polyhedron 26 (2007) 1012–1022
1017
It showed a quasi-reversible reduction of copper(II) to cop-
0.50
0.40
0.30
0.20
0.10
0.00
per(I) with a E1/2 at around ꢀ0.61 V versus Ag/AgCl. It
also showed an oxidation process having E1/2 ꢁ 0.63 V
which is assigned to the quasi-reversible oxidation of the
metal bound ligand. It is however surprising that we could
not get any wave for the oxidation of copper(II) to cop-
per(III) in the potential range up to 1.2 V versus Ag/AgCl.
The chemical and electrochemical oxidation of the par-
ent complex is highly pH sensitive and those were studied
in detail by Sulfab et al. [3]. They claimed that the E1/2 val-
ues for the oxidation of copper(II) to copper(III) of the
parent complex are 0.955 V, 0.933 V and 0.895 V versus
SCE at the pH 3.63, 4.00 and 4.59 respectively [3b]. This
is in agreement with the decrease of electrode potential
with increasing pH. Such consistency holds good over
the pH range 1.2–5.0 [13]. However at higher pH the situ-
ation seems peculiar. Our cyclovoltammetric study of the
parent complex 1 was carried out at pH 6.58 and under
such condition the ligand is very likely to undergo fast
internal oxidation and consequently destabilize the cop-
per(III) complex. Such ligand oxidation may be the prime
reason for the absence of finding any oxidation peak for
copper(II) to copper(III) oxidation at ca 1.20 V. However,
in complex 2 at pH 6.32, the presence of axial r-donation
from [Ni(CN)4]2ꢀ through N atom may be responsible for
stabilization of copper(III) complex under an oxidized
ligand environment. Interestingly, at pH < 5.0, no ligand
oxidation for both 1 and 2 was observed and this resem-
bles the earlier trend [3b]. And finally, it is noteworthy
to mention here that such ligand oxidation in the anodic
part dose not arise in complex 1 as well as in 2 in MeCN,
and hence the previous claim of ligand oxidation at 1.05 V
versus SCE by Addison et al. in the same solvent seems
surprising [5].
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 5. Spectral scan showing the conversion of copper(II) to copper(III)
of the complex 2 under kinetic conditions. First spectrum was for the
complex and second was taken immediately after mixing with
[IVII] = 0.0007 M at [H+] = 0.01 M. Successive spectra were taken in an
arbitrary time scale.
2.7.1. Acid catalyzed decomposition of parent complex (1)
and complex (2)
It was observed that both the complexes 1 and 2
undergo acid catalyzed decomposition. So an attempt
was taken to study the kinetics of acid catalyzed decompo-
sition these complexes. For this purpose, [H+] variation
experiments were conducted in the range 0.01–
0.20 mol Lꢀ1 at l = 0.50 mol Lꢀ1 (Na2SO4) and 25 ꢁC,
where single exponential decay curves manifests a first-
order dependence on the complex. It is interesting to note
that this decomposition rate constant (kobs/sꢀ1) is second-
order in H+ and can best be described as the rapid proton-
ation of oximato-oxygen and then second protonation on
one of the two imine nitrogen followed by decomposition
of the protonated species in a rate determining step and
has been described in Eq. (1). Corresponding data have
been listed in Table 3. Such protonation weaken both the
Cu–N(R)(OH) and Cu–N(R1)(R2) bonds and causes the
decomposition of the complexes.
2.7. Kinetic study
The kinetics of oxidation of the complexes 1 and 2 by
periodate, IVII, was carried out in the range [H+] = (1.0–
50.0) · 10ꢀ3 mol Lꢀ1, [IVII]t = (0.1–10.0) · 10ꢀ3 mol Lꢀ1 (t
represents total) at l = 0.50 mol Lꢀ1 (Na2SO4) and at tem-
perature 25 ꢁC. Kinetics were followed under pseudo-first-
order conditions taking complexes as the minor component
(2 · 10ꢀ4 mol Lꢀ1) and by monitoring the increase in
absorbance due to the formation of copper(III) complexes
at the wavelengths of maximum absorption (515 nm).
Fig. 5 represents the time-resolved spectra generated from
a kinetic trace. The reactions were clean for more than four
half lives, after which kinetic traces become irregular,
maybe due to decomposition or for some other reactions.
As we are interested to note the chemical redox behavior
of these complexes towards periodate as an oxidant, we
looked into the initial oxidation process. Pseudo-first-order
rate constants (kobs/sꢀ1) were obtained by fitting the kinetic
curves with the help of a suitable single exponential com-
puter-fit program and the results obtained in more than
two determinations fall in the error limit 5%.
H
H
O
O
HO
N
O
N
N
N
1
K
a
Cu
Cu
+
+
+ H
+ H
N
N
N
N
+
2+
[
[
Cu(HL)]
Cu(H L)]
2
ð1Þ
H
O
N
HO
N
H
2
k
K
a
Cu
Product
rds
+
N
H
N
3+
Cu(H L)]
[
3
Plots of kobs versus [H+] give non-linear curves (Fig. 6) with
increasing slope which were solved by using suitable com-

1018
A. Ray et al. / Polyhedron 26 (2007) 1012–1022
Table 3
0.020
Summary of pseudo-first-order rate constants for the acid catalyzed
decomposition of complexes 1 and 2 at l = 0.50 mol Lꢀ1 (Na2SO4) and
25 ꢁC
2
1
0.016
0.012
0.008
0.004
0.000
[H+]/M
104 · kobs (1)/sꢀ1
104 · kobs (2)/sꢀ1
0.01
0.05
0.10
0.15
0.20
1.46
3. 12
9.10
25.7
41.6
3.60
9.01
21.6
52.1
90.0
puter-fit program to Eq. (1a) to get kH = 0.098 0.03 L2
molꢀ2 sꢀ1 and n = 1.95 0.17 for 1 and kH = 0.20 0.05
L2 molꢀ2 sꢀ1 and n = 1.93 0.13 for 2
n
0
0.003
0.006
[I(VII)] / M
0.009
kobs ¼ kH½Hþꢃ
ð1aÞ
Fig. 7. Plots of kobs vs. [IVII] for the oxidation of complexes 1 and 2 at
The acid catalyzed decomposition rate constant, kH, for 2
is found to be ꢁ2 times higher than that of complex 1
and can be attributed to the axial coordination of
[Ni(CN)4]2ꢀ to the copper(II) center which probably desta-
bilizes the system and facilitates this process.
[H+] = 0.01 M.
Fig. 8. The presence of measurable intercept on the rate
axis indicates that both protonated (HIO4) and deproto-
nated (IO4ꢀ) species are reactive towards 1. The following
reaction sequence could be framed to explain the above
observation:
2.7.2. Oxidation of 1
The dependence of rate on [IVII]t was carried out at
[H+] = 0.01 mol Lꢀ1 and a plot of kobs versus [IVII]t shows
an initial rapid increase in rate with [IVII]t followed by
slower increase and then becomes almost independent on
[IVII]t (Fig. 7). Such a dependence of rate is best described
by Eq. (2). The factor 2 comes from the stoichiometry of
the reactions
Q1
½CuðHLÞꢃþ þ HIO4 ꢀf½CuðHLÞꢃþ; HIO4g
k1
þ
!½CuðLÞꢃ þ 2Hþ þ IO4
ð3Þ
ð4Þ
ꢁꢀ
Q2
½CuðHLÞꢃþ þ IO4ꢀ ꢀf½CuðHLÞꢃþ; IO4ꢀg
k2
þ
!½CuðLÞꢃ þ Hþ þ IO4
ꢁꢀ
kQ½IVII
ꢃ
kobs ¼ 2
ð2Þ
1 þ Q½IVII
ꢃ
The radical thus generated reacts in a fast stepꢀwith another
copper(II) complex to give [Cu(L)]+ and IO3 as the final
products (Eq. (5)) to account for an overall 2:1 stoichiom-
etry of the reaction with respect to the complex, which is
common to the most of reactions involving periodate as
oxidant in acid medium [12]. The fast decomposition of
the copper(III) complex generated thereby precludes the
A
non-linear least-square computer-fit program
provides k = (1.37 0.06) · 10ꢀ2 sꢀ1 and Q = (1002
166) L molꢀ1. The dependence of kobs on [H+] in the range
0.001–0.01 mol Lꢀ1 at [IVII]t = 0.0007 mol Lꢀ1, where a
strict first-order dependence on [IVII]t holds, is shown in
0.018
0.009
1
0.015
2
2
0.006
0.012
0.009
0.006
0.003
0
1
0.003
0.000
0.00
0.05
0.10
[H+] / M
0.15
0.20
0
0.02
0.04
0.06
[H+] / M
Fig. 6. Plot of pseudo-first-order rate constants (kobs/sꢀ1) vs. [H+] for the
acid catalyzed decomposition of complexes 1 and 2 at l = 0.50 M
(Na2SO4) and 25 ꢁC.
Fig. 8. Plots of kobs vs. [H+] for the oxidation of complexes 1 and 2 at
[IVII] = 0.0007 M.

A. Ray et al. / Polyhedron 26 (2007) 1012–1022
1019
determination of the stoichiometry of the reaction under
the kinetic conditions
the fast decomposition of the copper(III) complex pre-
cludes the determination of the stoichiometry of the reac-
tion under the kinetic conditions
fast
½CuðHLÞꢃþ þ IO4ꢁꢀ þ Hþ !½CuðLÞꢃþ þ IO3ꢀ þ H2O
The rate law derived is shown in Eq. (6)
k1Q1½Hþꢃ þ k2Q2Ka
ð5Þ
kQ½H5IO6ꢃ
ꢀd½Cꢃ=dt ¼ 2
½Cꢃ
ð11Þ
1 þ Q½H5IO6ꢃ
½IVIIꢃt
ð6Þ
Here, C = [Cu(HL)]2[Ni(CN)4]. While expressing H5IO6 in
terms of total periodate concentration Eq. (11) turns to Eq.
(12).
kobs ¼ 2
fKa þ ½Hþꢃg
As, HIO4 is a strong acid, it is imperative that Ka (0.023)
[20] > [H+] and Eq. (6) where, k = kobs/2[IVII]t, turns to
Eq. (7) and easily explains the observed dependence of rate
on [H+],
kQK½Hþꢃ½IVIIꢃt
kobs ¼ 2
ð12Þ
1 þ fK½Hþꢃð1 þ Q½IVIIꢃtÞg
k ¼ ðk1Q1=KaÞ½Hþꢃ þ k2Q2
ð7Þ
Eq. (12) was solved for k and K by fitting kobs as a function
of [IVII]t keeping Q fixed at 100, and the evaluated parame-
ters are: k = (1.97 0.03) · 10ꢀ2 sꢀ1 and Q = 3263
170 L molꢀ1. Again fitting kobs as a function of [H+] with
The linear fit of data to [H+] gives parameters as: k1Q1/
Ka = (6.75 0.54) · 102 sꢀ1, k1Q1 = 15.5 L molꢀ1 sꢀ1 and
k2Q2 = (6.00 · 0.33) L molꢀ1 sꢀ1
.
Q = 3263 as
a
fixed parameter gives parameters:
k = (2.21 0.05) · 10ꢀ2 sꢀ1 and K = (85 7) L molꢀ1. It
is to be mentioned here that very good fitting of two sepa-
rate sets of data for [H+] and [IVII]t indicates the self-consis-
tency of the proposed mechanism. The evaluated proton
2.7.3. Oxidation of 2
The dependence of rate on [IVII]t was carried out at
[H+] = 0.01 M and a plot of kobs versus [IVII]t shows an ini-
tial rapid increase in rate with [IVII]t followed by slower
ꢀ
association equilibrium constant (K = 85) for H4IO6 is in
increase and then becomes almost independent on [IVII
]
t
reasonable agreement with the literature value of K = 61
[12d]. The very large association constant also consistent
with an inner-sphere mechanism which may occur either
through hydrogen bonding between the hydrogen atom
on H5IO6 and Oꢀ of the oxime–imine ligand or the oxygen
atom of H5IO6 and the undissociated H on the oxime–imine
ligand. Another possibility is that H5IO6 may directly coor-
(Fig. 7) and best represented by rate law (2). The effect of
[H+] on the oxidation rate was investigated in the range
[H+] = 0.001–0.05 mol Lꢀ1 at [IVII]t = 0.0007 mol Lꢀ1
,
where a strict first-order dependence on it holds good.
The rate increases initially rapidly with increasing [H+],
then slowly and finally becomes almost independent of
[H+] resulting in a plateau with a negligibly small or no
intercept on the rate axis (Fig. 8). Such a dependence of
rate onꢀ[H+] indicates that anionic forms of periodic acid
(H4IO6 or IO4ꢀ) are un-reactive towards the complex
leaving H5IO6 or HIO4 Æ 2H2O (H2A) as the sole oxidant.
The proton association of periodate anion, which can exist
dinate to the metal center. The main objectionꢀof this prop-
ꢀ
osition is the absence of the reactivity of IO4 or H4IO6
which may be a better coordinating ligand than H5IO6.
,
It is to be mentioned here that the acidity of reaction
media were so chosen for both the reactions that the acid
catalyzed decomposition of the complexes is negligible com-
pare to the electron transfer process. The complex
[Cu(HL)]2[Ni(CN)4] may undergo dissociation to give indi-
vidual ions [Cu(HL)]+ and [Ni(CN)4]2ꢀ; [Cu(HL)]2[-
Ni(CN)4] should then behave identically to that of parent
compound prepared separately. As the dependences of rate
as H4IO and IO4 and ½HAꢀꢃt ¼ ½IO4ꢀꢃ þ ½H4IO ꢀꢃ, can
ꢀ
ꢀ
6
6
be described by Eq. (8) [12d]
K
HAꢀ þ Hþ ꢀ H2A
ð8Þ
thus, K actually represents the apparent proton association
constant reported to have value in the range 43–61 L molꢀ1
at 25 ꢁC [12d,21]. Considering the foregoing discussion, a
reasonable reaction sequence may be proposed to account
for the oxidation of [Cu(HL)]2[Ni(CN)4] by periodic acid
(kobs/sꢀ1) on [IVII
]
and [H+] for the reaction with
t
[Cu(HL)]2[Ni(CN)4]–IVII differ from that of [Cu(HL)]+–IVII
reaction, this may arise due to the presence of [Ni(CN)4]2ꢀ
ions in the former which may form an ion pair with
[Cu(HL)]+ or may be coordinated to the metal center caus-
ing this difference. To establish this proposition we checked
the dependence of kobs on [H+] in presence of [Ni(CN)4]2ꢀ
in the mole ratio 1:2 = [Ni(CN)4]2ꢀ:[Cu(HL)]+ and it was
observed that this dependence was almost identical to that
observed for [Cu(HL)]2[Ni(CN)4]–IVII reaction and support
our proposition. As the acid catalyzed decomposition of
complex 2 is ꢁ2 times higher than complex 1, it manifests
the coordination of [Ni(CN)4]2ꢀ moiety to the copper center
destabilizes the system. It indirectly supports the coordina-
tion of [Ni(CN)4]2ꢀ moiety to the copper center over the
formation of ion pair which rather will stabilize the system.
Q
½CuðHLÞꢃꢄ þ H5IO6 ꢀf½CuðHLÞꢃꢄ; H5IO6g
ð9Þ
k
f½CuðHLÞꢃꢄ; H5IO6g !½CuðLÞꢃþ þ H3IO ꢁꢀ þ H3Oþ ð10Þ
5
Here [Cu(HL)]* represents the modified species of
[Cu(HL)]2[Ni(CN)4] where [Ni(CN)4]2ꢀ may be weakly
coordinated to a copper center in solution. The radical thus
generated reacts with another molecule of the complex in a
2ꢀ
rapid step to yield H3IO5 as the final reduction product
of periodate, which is common to the most of reactions
involving periodate as oxidant in acid medium and corre-
sponds to a 2:1 stoichiometric reactions [12]. Here also

1020
A. Ray et al. / Polyhedron 26 (2007) 1012–1022
It is generally accepted that periodate reacts via inner-
(7) The slightly higher reactivity of 2 over 1 may arise
due to the destabilization of complex 2 through axial
ligation by [Ni(CN)4]2ꢀ. This is in line with the elec-
trochemical behavior in aqueous solution.
sphere mechanism and supported by its failure to oxidize
[Fe(Phen)3]2+ (phen = phenanthroline), which requires an
inner-sphere route [12c]. The oxidation of [Cr(H2O)6]3+
,
an inert complex, was proposed to proceed by inner-sphere
route assuming [Cr(H2O)5(OH)]2+ as the reacting species,
where the ligated OHꢀ increases the lability of the metal
center [12c]. The substantially high formation constant
(Q = 3263 170 L molꢀ1) suggests that the reaction pro-
ceeds through the pre-equilibrium formation of a precursor
complex, an inner-sphere adduct, followed by electron
transfer. Similar proposition was also advocated in the oxi-
dation of dodecatungstocobaltate(II), a robust outer-
sphere reactant, by periodate, where also HIO4 was the sole
reacting species [12d].
4. Experimental
4.1. Materials and methods
All the starting materials like Cu(CH3COO)2 Æ H2O,
CuCO3, KOH, KCl, CH3COONa, NaClO4, HClO4, HCl,
H2SO4, MeOH, diisopropyl ether, N,N-dimethylformam-
ide, standard buffer solutions (ACROS ORGANICS,
USA) of pH 4.0 and 7.0 etc. were of reagent grade and used
as received. 1,3-Propanediamine (E-Merck, India) was dis-
tilled by refluxing over Na metal for about an hour before
use. Analytical grade diacetyl monooxime (Merck Ger-
many), KIO4 (Aldrich) and Na2SO4 (Merck Germany)
were used after recrystallization. All the experimental solu-
tions of the complexes for electrochemical and kinetic stud-
ies were prepared freshly by doubly distilled water.
Elemental analyses were carried out using a Perkin–
Elmer 240 elemental analyzer. Infrared spectra (400–
4000 cmꢀ1) were recorded from KBr pellets on a Nicolet
Magna IR 750 series-II FTIR spectrophotometer. EPR
spectra of polycrystalline samples were recorded using a
Varian Century Series X-band EPR spectrophotometer
(model E 109) with 100 kHz magnetic field modulation.
Room temperature magnetic susceptibilities were recorded
3. Conclusion
(1) A
cyano-bridged
hetero-trinuclear
complex,
[Cu(HL)]2[Ni(CN)4] Æ 4H2O (2) has been synthesized
and characterized by single crystal X-ray structure
determination. The square planar [Cu(HL)]+ cation,
assumes a square pyramidal geometry through axial
ligation by a cyano group of [Ni(CN)4]2ꢀ ion.
(2) The arrangement of lattice water molecules in
[Cu(HL)]2[Ni(CN)4] Æ 4H2O is highly ordered produc-
ing water columns viewed down the b axis.
(3) Both the complexes 1 and 2 electrochemically
behaves very similarly in MeCN as well as in water
showing well defined quasi-reversible redox couples
for CuII/I and CuIII/II conversions, with the exception
that the latter conversion does not arise in 1 in aque-
ous medium.
(4) The copper(II) bound ligand oxidation occurs at
E1/2 = 0.63 and 0.60 V versus Ag/AgCl for the com-
plexes 1 and 2 respectively only in aqueous solution.
The absence of such process in MeCN may be
attributed to the fact that it is a proton-coupled
electron transfer process, which is only feasible in
aqueous medium. Assignment of such a process to
the coordinated ligand oxidation was confirmed by
carrying out parallel study with the free ligand
on
a
PAR vibrating sample magnetometer using
Hg[Co(NCS)4] as the calibrant. Diamagnetic corrections
were estimated from Pascal’s constants. TGA and DTA
curves were recorded simultaneously on a Perkin–Elmer,
Model PyrisDimond TG/DTA in static dry nitrogen at
the heating rate of 5 ꢁC minꢀ1. Electrochemical measure-
ments were carried out using a computer controlled PAR
model 263A VERSASTAT electrochemical instrument
with a glassy carbon disk (4 mm diameter, EG&G) as the
working electrode. All the results were collected at
25 0.5 ꢁC with reference to the Ag/AgCl electrode. All
the solutions were purged with pure argon gas for at least
ten minutes prior to each experiment. Junction potentials
are not corrected. Electronic spectra and kinetic experi-
ments were recorded on an Agilent 8453 UV–Vis diode
array spectrophotometer with Peltier thermostatic temper-
ature control facility at 25 0.10 ꢁC. The pH ( 0.01) mea-
surements were carried out with a Systronics (model 335,
India) digital pH-meter at 25 ꢁC.
which showed similar redox process with E1/2
0.61 V versus Ag/AgCl.
=
(5) Both the complexes 1 and 2 undergo H+ catalyzed
decomposition with a rate second-order in H+ and
can best be described as the rapid protonation of oxi-
mato-oxygen and then second protonation on one of
the two imine nitrogen followed by decomposition of
the protonated species in a rate determining step.
Such protonations weaken both the Cu–N(R)(OH)
and Cu–N(R1)(R2) bonds and causes the decomposi-
tion of the complexes.
4.2. Syntheses
Caution! Since the perchlorate salts are potentially explo-
sive, only small amounts of the materials should be handled
with care.
(6) Periodate oxidation of 1 and 2 reveals that both HIO4
Cu(ClO4)2 Æ 6H2O was prepared from CuCO3 and 50%
HClO4 by the standard method. Na2[Ni(CN)4] Æ 3H2O
was prepared according to standard literature procedure
ꢀ
and IO4 are the reacting species for 1 while HIO4 is
the sole reacting species for 2.

A. Ray et al. / Polyhedron 26 (2007) 1012–1022
1021
[22] taking NaCN instead of KCN, and further character-
ized by CHN analysis. The ligand H2L [23] and the parent
complex [Cu(HL)]ClO4 Æ 2H2O [5] (1) were prepared by
using the literature methods and the purity was also
checked by CHN analysis.
included geometrically and refined isotropically except
the oxime proton and those of water hydrogens of which
the former is randomly disordered over its two possible
ligand sites in the unit cell. Other additional information
concerning the data collection and refinement of the struc-
ture is compiled in Table 4.
4.2.1. [Cu(HL)]2[Ni(CN)4] Æ 4H2O (2)
To a methanolic solution of [Cu(HL)]+ (1.0 mmol, in
40 mL MeOH, generated in situ by mixing equimolar quan-
tities of Cu(ClO4)2 Æ 6H2O and H2L) Na2[Ni(CN)4] Æ 3H2O
(0.158 g, 0.6 mmol) dissolved in a minimum volume of
water, was added drop-wise with constant stirring for
5 min. The precipitate that appeared initially was filtered
off and the dark brown filtrate was allowed to undergo
slow evaporation at ambient temperature whereupon shiny
dark brown crystals suitable for X-ray study were obtained
after two weeks. Yield: 52% based on Cu. Anal. Calc. for
C26H46N12 O8Cu2Ni: C, 37.16; H, 5.52; N, 20.00. Found:
C, 37.26; H, 5.51; N, 19.79%.
Acknowledgements
Financial support from the Council for Scientific and
Industrial Research (CSIR), and University Grant Com-
mission (UGC), New Delhi, India are acknowledged.
Appendix A. Supplementary material
CCDC 278585 contains the supplementary crystallo-
graphic data for the structure reported in the paper. These
data can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.poly.2006.09.095.
Selected IR in cmꢀ1 (KBr): 3389 (br), 2930 (w), 2135 (s),
2118 (vs), 1630 (m), 1541 (br, m), 1500 (w), 1431 (m), 1379
(m), 1358 (m), 1323 (m),1275 (m),1225 (m), 1115 (br), 1089
(w), 1014 (w), 692 (w), 627 (w). leff (per monomer): 1.78lB
at 25 ꢁC.
4.3. Crystal data collection and refinement
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Intensity data for 2 was collected at 293(2) K on a Sie-
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Crystal data and structure refinement for complexes 2
Empirical formula
C26H46Cu2N12NiO8
fw
Crystal system
Space group
840.25
monoclinic
C2/c
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˚
a (A)
13.263(3)
21.255(4)
14.131(3)
˚
b (A)
˚
c (A)
b (ꢁ)
V (A3)
Z
109.44(3)
3756.6(13)
4
T (K)
293(2)
˚
k (A)
0.71073
1.486
1.677
1704
dcalc (g cmꢀ3
l (mmꢀ1
F0 0 0
)
ˇ
´
´ˇ
ˇ ˇ ´
ˇ
´ˇ
´
[7] J. Cernak, M. Orendac, I. Potocnak, J. Chomic, A. Orendacova, J.
)
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´
´
ˇ
´
´
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Reflections collected/unique [Rint
Final R indices [I > 2r(I)]
R indices [all data]
]
4458/4282 [0.0370]
R1 = 0.0583, wR2 = 0.1408
R1 = 0.0937, wR2 = 0.1611
1.014
Goodness-of-fit

1022
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