Indian Jo urn al o f C hemi stry
Vo l. 4 1A, Jul y 2002, pp. 1369- 1373
Electron spin resonance, magnetic and superoxide dismutase activity of
binuclear imidazolate bridged complexes
R N Patel*, K K Shu kla, N Singh & K B Pandeya+
De partment of C he mi stry, A P S University, Rewa (MP) 486 003 , India
Received 18 December 2001; revised 19 February 2002
Elec tro n paramag netic reso nance, magnet ic suscepti bility and superox ide d ismuta se (SO D) activity measure ments o f
imidazo late bri dged [(Sa lva l)C u-lm-C u(Sa lva l)]Na, [(Salva l)C u-lm-Ni (Salva l)]Na and [(Sa lva l)C u-lm-Z n(Sa lva l) 1 a
co mplexes, [w here salva l = salicyledenevalinate, im = imidazo late io n,] are described . Related mo no nuc lea r copper(l l)
compelxes are also sy nt hesized . T he p H-de pe ndent EPR and electroni c spec troscopic stud ies of froze n 50% aq ueous DMSO
so luti on of ho mo- and heterob inucJear compl exes have been fo un d to be stab le over the w ide pH range.
Binuclear imidazo late bridged complexes are of
current interest because of the acti ve site of the
enzy me bo vine erythrocyte. Superoxide dismutase
(SOD)t is known to have a histidine bridged
copper(lI)-zinc(II) centre in each of two indentical
subunits. Consequentl y, a number of model
complexes have been prepared. They in vo lve the pairs
of either identical or dissimilar metals bridged by a
deprotonated imidazole-type molec ule2. t2. Electron
paramag neti c resonance is a powerful too l in study ing
metal-metal interacti ons because it provides direct
infor mati on on the multipli city of the resuiting spin
states and on the value of both the isotropic and
ani sotropic exchange interaction. The va lue of th e
magnetic moment at room te mperature allows one to
su!'pose th at the homobinuclear complex is built up
by the anti fe n'omag neti call y coupl ed binuclear
cl usters (S= I state) and that th e magneti sm of the
heterobinuclear complex is du e to the S = Y2 state.
Mag netic measurements and EPR spectroscopy of the
homobinuclear [(Salval)Cu-lm-Cu(Sa lval)]Na have
shown an antife rromagnet ic interaction . Recentl y, we
have also reported 13· 18 imi dazo late bridged homo- and
heterobinuclear
complexes.
In
the
present
communication, we report the sy nthesis of a number
of homo- and heterobi nu clear comp lexes of
salicy ledenevalinate in wh ich metal centres may be
linked via imidazo le bridging ligands. The va riable
I+Ex-Vi ce Chancell o r. Kanpur Un iversity , Chairman U P P S C
All ahabad, Indi a
p H EPR and visible spectra (50% DMSO) have been
studi ed. EPR spectra of polyc rystalline samples were
also recorded and discussed.
Materials and Methods
Copper perchl orate hexahydrate (A ldrich) and
imidazo le (Sd fin e Chern .) were used as supplied .
Other chemi cals used were of reagent grade.
Caution ! Perchl oride salts are potentiall y explosive
and should be handled in only small quantities.
Synthesis
Complexes 1-5 were prepared by the pu bli shed
meth od20.25 . Their purity was checked by CHN
analys is.
Room
temperature
magnetic
susceptibi lit y
measurements were made on Gouy balance.
Mercury( lI) tetrathiocy nato cobalta te(II) was used as
calibrant (Xg = 16.44 X 10.6 c.g.s. uni t). EPR
measurements were made on a Varian E- line century
series EPR Spectrometer employi ng X-band rad iation
and a cy lindrical cav ity with 100 kHz magnetic
mod ul at ion . TCNE (ge= 2.00277) was used as fi eld
marker. Vi sib le spectra were reco rded in 50% DMSO
on a Systronics UV-v is l 17 spectrophotometer usi ng I
cm path length quartz cell.
Superoxide
dismulase
aClivilies- The SOD
ac tivities were eval uated by the classical nitrob lue
tetrazo li utll essay'9. The concentration of complex
required to yield 50% inhibition of the reduction of
NBT (named IC so was determ ined.
INDI AN J C HEM, SEC A, JU LY 2002
1370
Results and Discussion
The following equ ations desc ribe the sy nth esis of
the mono- and binuclear comp lexes:
acidic/netura l
> [(S alval)Cu OH ]+2H+
H 2Saiva i +C u 2+
u
2
medium
... (1)
ne utral
> [(Salval)Cu-ImH]
[(Salval)Cu-OH 2]+ImH
+H 2 0
... (2)
medium
alkaline
>
[(Salval)Cu-OH 2 ] + [(Salva l)C u-ImH]
[(Salva l)C u "
/
medium
Cu(Salval)]Na + H 20
these complexes. The complex 3 has a substanti all y
reduced magnetic moment, consisten t with proposed
Cu-Im-Cu (dimeric) structure suggesting the
poss ibility of spin-coupled syste m. Thi s behaviour is
ascribed to a paramagnetic species contai ning two
antiferromagnetically coupled Cu(II) ions. The
anti ferromag netic behaviour occurs when Cu -Im-Cu
linkage connects the two copper(II) coordination
spheres at axia l sites of two square-pla nar units, the
proposed structures of [Salval-Cu-lm-Cu-Salva l]N a.
In thi s case, mag netic mome nt data results confirm
that the structures of thi s complex has an imidazo latebridged dicoppe r core, s in ce X-ray crystallographic
verification of these feature s could not be achi eved
ow ing to poor crysta l quality. The magneti c mome nts
of complexes 4 and 5 show magnetic moment values
corresponding to unpaired one spin system (S = I 12).
"1m
EPR spectra
... (3)
We have recorded X- band EPR spectra at room
temperature for polycrysta llin e samples. The EPR
parameters are presented in Tabl e 2. These room
te mperature spectra are unlike that of a tetragonal
copper(Il) and indi cate th e presence of spin-spin
interaction
altho ugh
the
observed
spin-spin
interaction s in these complexes except 3 are sol idsol id interaction s (intermol ec ul ar). In case of complex
3 , the interaction is due to intra mo lecu lar due to th e
20
presence of Cu-lm-Cu bridging uni t. A lso the value
of G is greate r than 4 , which again suggest spin-spin
interaction. In these spectra, ~ s=2 transitions were
also observed at " half-field" . Thi s kind of tran sitions
at " half-field" are again due to mag netic exchange
interactions. A lthough magneti c moment values for
these comp lexes except 3 were found corresponding
alkaline
>
[(Sa lva l)C u-ImH] + [(Salva l)M-OH 2 1
med ium
[(Salval)Cu"" 1m /
M(S alva l)] + H 2 0
.. . (4)
where M= zinc( II ) and nickel(Il ) in heterobinuclear
comp lexes . All
sy nthesi zed
complexes
gave
sati sfactory e le me ntal analysi s (T abl e I ).
Room te mperature magnetic moment va lues are
given in Table I . The room te mperature magnetic
moments of complex 1 a nd 2 just exceed the spinonly va lue for a one unpaired electron system and are
consistent with mononuclear, one-electron nature of
Table I- An alytical and physica l data fo r copper(ll ) compl exes
Complexes
IC so
Colour
mo l/
(~
M.P.
(oq
Yield
(%)
C
H
Found (Calc.), %
N
Cu
~crf
Zn
Ni
(8M)
dm- I)
[(Sa lva l)C u-OH 21
85
Li ght
blue
2 10°C
65
47.92
(47.96)
5.05
(5. 10)
4 .65
(4.72)
2 1.1 4
(2 1.1 0)
1.75
2
[(Sa lval )Cu-lmH]
125
Light
green
160°C
60
51.30
(51.32)
4.80
(4 .78)
11.98
( 11.93)
18. 12
( 18. 17)
1.75
3
[(Sa lva l)C u-lm-Cu(Sa lv al)]Na
26
Dark
blue
205 °C
70
49.40
(49 .35)
4.42
(4.46)
8. 51
(8 .53)
19 .30
( 19.32)
1. 59
4
I(Sa lva l)C u-lm-Ni (Salval)]Na
23
Li ght
blue
209°C
65
49.82
(49.87)
4.35
(4.46)
8.6 1
(8 .69)
9 .77
(9 .73)
5
[(Salva l)Cu-lm-Zn(Sa lva l) )Na
22
Gree n
205 °C
55
49.32
(50.0 1)
4.4 1
(4.35)
8.52
(8.57)
9.90
(9.67)
9.02
(9 .00)
9.85
(9.95)
1.78
1.79
1371
PATEL el 01.: BINUCLEAR IMIDAZOLATE BRIDGED COMPLEXES
to one spin only . Room temperature polycrystall ine
EPR spectra of complexes 4 (C u-lm-Ni) is of an axial
pattern with gl =2.376 and g2 = 1.99 and on the gil
component is superposed as a four-l ine hyperfine
structure due to the copper nucleus, (Fig. I). Axially
symmetric complexes (Cu-lm-Ni) exhibits the
features of S=1I2 , keeping in mind in this regard that
we are observing the doublet ground state (St=ll2)
arising from the anti ferromagnetic exchange coup ling
between Scu=1I2 and SNi=1 in a Cu"Ni" containing
dinuclear unit. The four hyperfine lines in spincoupled Cu"Ni" are easy to understand, as on ly Cu
has isotopes 63CU (69.1 %) and 65CU (30.9%) with lcu
= 3/2, whereas Ni has isotope 61 Ni with INi = 312, but
its natural abundance is very low (1.25%). Thus the
hyperfine structure due to only the copper nucleus is
observed for exchange-coupled CuINi". Similar
. 20n
. d
o bservatlons
.-- were carne
out '111 many C u "N I·"
complexes. The EPR spectral features of the system
compared
to
the
complex
under
study
[Cu(HLOX)Ni"(N 3)h (HLOX = amine oxime ligand)
is given in the inset (Fig. 1).
The X-band EPR spectra for the present complexes
in 100% DMSO solution are nearly axial spectra. The
derived EPR parameters are given in Table 2. T he Xband EPR spectra of the binuclear complexes are also
recorded in 50% aq. DMSO as a function of pH and
derived spectral parameters are given in Table 3.
Solution stability of the [Cul lm) } unit-The
solution
EPR
spectra
of
[(Salval)Cu-lmCu(Salva l)]Na in frozen 50% aq. DMSO so lution s as
a function of pH have been recorded . The EPR
spectra reflects the presence of at least four ax ially or
nearly axially species in solution, poss ibilities for
which are given below :
[(Salval)Cu-Im-Cu(Salval)r, [(Salval)Cu-lmH],
[(Salval)Cu-OH 2] and [Cu(H 20)6f+
~
210
)00
no
lItO
)60
81mT
--,
,
3.00
2600
Magnetic field (Gauss)
Fig. I- X-band EPR spectra (Polycrystalline) at LNT of 4 (inset,
ref. 22, EPR spectra of [Cu"(HLOX) i"(N) h at 77°K
Table 2-EPR parameters for copper(lI ) compl exs in polycrystalline and in 100% DMSO solution
Complexes
1
gt
g2
gil
g.l
AII(G)
[(Salval)C u-OH 2]
2.204
2.043
2.248
2.054
175
170
2
I(Salval)Cu- lmH]
2.203
2.04 1
2.235
2.058
3
[(Salval)Cu-lm-Cu(Salval)]Na
2.207
2.041
2.233
2.060
167
4
[(Sal val)Cu- Im-N i(Sal va l)]Na
2.376
1.990
2.230
2.051
172
5
[(Sal val)Cu- Im-Zn(Salval) ]Na
2.204
2.040
2.235
2.052
170
Table 3- pH-dependent EPR parameters of imidazolatc bridged complexes (i n 50% aqueous DMSO)
3
4
5
[(Salval)Cu-ltll-Cu(Salval)]Na
AII (G)
g.l
Gil
[(Sa lval)Cu-lm-N i(Salval) ]Na
AII (G)
g.l
gil
[(Sal va l)Cu- Im-Zn(Sal va l)]Na
AII (G)
g.l
gil
pH
10.50
2.226
2.053
170
2.213
2.058
172
2.2 11
2.056
8.50
2.232
2.05 1
175
2.211
2.053
173
2.208
2.053
173
7. 15
2.229
2.049
173
2.2 13
2.053
172
2.2 15
2.057
175
6.50
2.2 11
2.052
174
2.2 12
2.055
172
2.209
2.052
173
4.50
2.254
2.056
170
2.248
2.052
170
2.210
2.057
175
2.303
2.059
100
2.304
2.059
157
2.259
2.070
160
2.225
2.068
170
2.307
2.07 1
150
2.231
2.072
150
2.393
2.076
125
2.410
2.078
120
2.385
2.073
125
3.00
170
INDIAN J CHEM, SEC A, JULY 2002
1372
and at lowest pH, it is likely that the salval ligand is
di ssociated according to :
[(Salval)Cu-OH 2]
.. . (5)
The presence of aquo species [Cu(H 20)d+ was
verified by observing gil and g.l signals at 2.283 and
2.076, respecti vely, with All = 125 G for a frozen
so lution of cupric perchlorate dissolved in the sa me
18
so I vent . At pH 4.50, the gil and All para meters reveals
the presence of spec ies [(Salval)Cu-ImH] and
[(Salval)Cu-OH 2], the latter having All = 160G. As the
pH increases, the All value also increae to 174G,
suggesting the formation of imjdazolato bridged
binuclear species [(Salval)Cu-Im-Cu(Salval)]Na. In
the pH- range 6.50-10.50, there is no specific change
in the EPR parameters and in EPR peak features. This
is suggestive that the imidazolate bridge is intact in
thi s pH range.
Solution stability of {Cu -Ni) fmr unit-The EPR
para meters of [(Salval)Cu-Im-Ni(Salval)]Na measured in the ~m s = ±I region as a function of pH in the
frozen aqueous solution, are shown in Fig. 2. The
similarity in spectra in the pH range 6.5-10.5 shows
th at the imidazolate bridge is intact in this pH range.
Similar observations were also' observed in our earlier
work l6 . As pH is lowered to 4.50, the spectra show
more frequent changes, at this pH, the presence of two
species are characterized having gil and All values
corresponding to mononuclear species [(Salval)CuImH], [(Salval)Cu-OH 2]. Evidently, as the pH is
lowered, the imidazole bridge of the binuclear
complex [(Salval)Cu-Im-Ni(Salval)]Na breaks on Ni
side as per dissociation equilibria:
50% aq. DMSO solution. The pH of the so lution was
adjusted to 3.50-10.50 by adding 1M HCl0 4 or 1 M
NaOH. The obtained EPR parameters are given in
Table 3. In the pH range 4.5- 10.5, the complex shows
well-resolved EPR features for this single binuclear
species. This is indicative that the binuclear spec ies
are intact in this pH range . On lowering the pH (i.e.
3.00), the presence of new peak fe atures were
observed. These peak features correspond to EPR
signals for new mononuclear species. One of the peak
correspond
to
mononuclear complex
feature
[(Salval)Cu-ImH]
and
other
features
have
characteristics attributable to species [Cu(OH 2)6]2+.
Thi s aquo species was verified by observing g tensor
at gil = 2.385 and g.l = 2.073 and All = 12SG, which is
quite close to those of [Cu(H 20)6]2+ in the sa me
3o
frozen so lution .
Visible spectra of the imid azo late binuclear
complexes were also recorded in 50% aq . DMSO
solutions at different pH values. Some representa tive
spectra are shown in Fig. 3. The spectra at pH values
6.50-10.5 are nearly identical w ith Amax( nm). This
constancy in Amax corresponds to E PR observations.
As pH lowered to 6.50, the Amax is shifted to the
higher wavelengths. Simultaneous presence of various
species at particular pH values is not reflected in the
spectra. Obviollsly , the equilibria between variou s
species are faster than the spectral ti me scale, so that
only the averaged spectra are obtained.
pH
J·O
L· 5
6·5
H+
[(Salval)Cu-Im-Ni(Salval)]Na
~
[(Salval)Cu-ImH
7·15
OH-
+ [(Salval)Ni-OH 2]
. .. (6)
Further, on lowering the pH upto 3.0, in the EPR
features again two distint species are observed. One of
the peak features corresponding to mononuclear
complex [(Salval)Cu-OH 2] and other features have
characteristics
attributable
to
aquo
species
[Cu(H 20 )6f+.
Solution
stability
of
{Cu-Zn)fmr
unit-The
variable pH EPR spectra of the complex [(Salval)CuIm-Zn(Salval)]Na have also been recorded in frozen
I
8·5
10·5
2500
Magnetic field (Gau5s)
Fog. 2-X-band EPR spectra of the frozen solutio n (0.003 mol
dm·3 in 50% DMSO, LNT) of 4 at different pH va lu es.
PATEL et al. : BINUCLEAR IMID AZOLATE BRIDGED COMPLEXES
1373
In particular EPR,
UV-visible
respectively.
parameters and SOD (Table 1) data are close to those
of proteins. As thi s work herein adds substantial data
to the chemistry of CU2Cu2S0D and CU2Zn2S0D
model and is, therefore, relevent to the bioche mi stry
of thi s element.
Acknowledgement
We are thankful to the UGC New Delhi for
financial assistance.
References
480
560
640
720
800
Wavelength (nm)
Fig. 3- Visible speclra of 50% DMSO of 3 al different pH values.
SOD acti vity
The IC 50 of different co mplexes are given in
Table I . The IC 50 values of these complexes are
hi gher than the value exhibited by native SOD (0.05
11M. ) com pl ex and macrocyclic li gands containing
I 2 18
imidazolate bridged complexes . . The complexes of
present study show high SOD activity th an homo
heterometalli c imidazolate bridged co mpl exes of
diethylenetriamine (dien ) and pentamethyldiethylinetriamine (PMDT)18 as these comp lexes of dien and
PMDT show hi gher ligand field and hence compl exes
of these li gands show less SOD activity (higher 1c5o
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Relationship to fh e biological systems
In the biological systems, the ava ilable li gand
atoms are S, N, and O. The copper(lI ) is a useful
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is important to ascertain the nature of its amino ac id,
peptide and protein comp lex es. The present
synthesized complexes appear to be good models for
active site in [Cu2C u2S0D] and [Cu2Zn2S0D]. These
complexes ex hibits similar features in common with
[Cu2Cu 2S0D]
and
[Cu2Zn2S0D] subunits
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