Spectral studies of dimeric copper(II) complexes of acid amide derivatives as models for type III copper enzymes

Article abstract of DOI:10.1016/S1386-1425(03)00060-X

Dimeric (hydrated and anhydrated) complexes of Cu(II) with N,N'-bis(3-carboxy-1-oxo-2-prop-2-enyl)ethylenediamine (BCOPENH2, A) and N,N'-bis(2-carboxy-1-oxo-phenylenyl)ethylenediamine (BCOPHENH2, B) have been prepared and characerised by elemental analysis, magnetic susceptibility measurements, EPR, thermal and spectral (IR, UV/Vis) studies. EPR parameters and magnetic behaviour indicates that the complexes are antiferromagnetic in nature and most likely adopt the typical carboxylate cage structure. Interesting amide bonding patterns have been observed and various EPR parameters have been evaluated on the basis of these studies, tentative probable structures of the complexes have been proposed.

Full text of DOI:10.1016/S1386-1425(03)00060-X

Spectrochimica Acta Part A 59 (2003) 2775ꢀ2783
/
www.elsevier.com/locate/saa
Spectral studies of dimeric copper(II) complexes of acid amide
derivatives as models for type III copper enzymes
Bhagwan S. Garg *, Deo Nandan Kumar, Meenu Sarbhai, Malladi J. Reddy
Department of Chemistry, University of Delhi, Delhi 110007, India
Received 16 September 2002; received in revised form 21 January 2003; accepted 29 January 2003
Abstract
Dimeric (hydrated and anhydrated) complexes of Cu(II) with N,N?-bis(3-carboxy-1-oxo-2-prop-2-enyl)ethylenedia-
mine(BCOPENH2, A) and N,N?-bis(2-carboxy-1-oxo-phenylenyl)ethylenediamine(BCOPHENH2, B) have been
prepared and characterised by elemental analysis, magnetic susceptibility measurements, EPR, thermal and spectral
(IR, UV/Vis) studies. EPR parameters and magnetic behaviour indicates that the complexes are antiferromagnetic in
nature and most likely adopt the typical carboxylate cage structure. Interesting amide bonding patterns have been
observed and various EPR parameters have been evaluated on the basis of these studies, tentative probable structures of
the complexes have been proposed.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Copper(II); Amide; EPR spectra; IR spectra; Ligand field spectra
1. Introduction
erties [8ꢀ12], such as magnetism and spectroscopy,
/
in type III copper proteins [13,14] a sustained
interest remains. Most of the binuclear copper(II)
complexes which have been reported to be models
of type III copper(II) proteins are antiferromag-
netically coupled or weakly ferromagnetically
coupled [15]. Bridging carboxylate groups of the
peptide chains have been proposed to explain the
antiferromagnetic behaviour of type III copper
enzymes [16]. Very few studies have been con-
ducted with dicarboxylates and virtually no studies
have been made on diamine dicarboxylates [17].
Considering the importance of these complexes as
models for type III copper enzymes, we report the
synthesis and magneto-structural correlations of
Cu(II) complexes of (BCOPENH2, A) and (BCO-
The chemistry of transition metal amides has
attracted much interest over the past decade
because of their important roles in various indus-
trial [1] and biological processes [2ꢀ6]. EPR
/
spectra of Cu(II) cyclic and pendant acid amide
complexes with other anchoring groups are di-
meric, with the two metal ions exchange coupled
(Triplet state) [7]. Although a long history and a
prolific literature exist on the subject of the
relationship between structure and physical prop-
* Corresponding author. Tel./fax: ꢁ
E-mail address: deonandan2002@rediffmail.com (B.S.
Garg).
/
91-11-27666250.
1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1386-1425(03)00060-X

2776
B.S. Garg et al. / Spectrochimica Acta Part A 59 (2003) 2775ꢀ2783
/
PHENH2, B) in their hydrated and anhydrous
forms.
sample magnetometer, Model 155 (Princeton Ap-
plied Research) at 5500 Gauss field strength. UVꢀ
/
Vis spectra were recorded in DMF on a Beckman
DU-64 spectrophotometer. IR spectra were re-
corded on a Perkin-Elmer FT-IR spectrophot-
ometer, Spectrum 2000, using KBr and
polyethylene pellets. X-band EPR spectra were
recorded on a Varian model E-112 EPR spectro-
2. Experimental
All reagents were of analytical grade and were
used without further purification.
meter using DPPH (gꢂ2.0023) as the standard
/
2.1. Synthesis of ligands
with a variable temperature nitrogen cryostat.
TGA and DTA were simultaneously recorded on
Rigaku 8150 thermoanalyser in static air at a
heating rate of 10 deg minꢃ1. A platinum crucible
was used with alumina as the reference material. A
number of decomposition steps were identified
using TGA.
The BCOPENH2 (A) ligand was prepared as
follows: ethylenediamine (3.40 cm3, 0.05 mol) was
dropped to a solution of maleic anhydride (9.8 g,
0.1 mol) in glacial acetic acid (50 cm3) while
cooling in ice. The product was filtered off, washed
thoroughly with acetone, recrystallized twice from
5% sodium carbonate solution and regenerated
with dil. HCl. The BCOPHENH2 (B) ligand was
prepared using the same procedure, but using
phthalic anhydride (14.8 g, 0.1 mol) instead of
maleic anhydride.
3. Results and discussion
All the complexes were coloured, high melting
and microcrystalline or powders, stable in atmo-
spheric conditions, insoluble in most of the organic
solvents and water. They were slightly soluble in
DMSO and DMF. However, in strong acidic and
basic media they are disproportionated reversibly
as shown in the following equation:
2.2. Synthesis of complexes
In a typical preparation of the complexes, 2
mmol of the ligands were suspended in 50 ml of
water. A calculated volume of standard Na2CO3
solution (2 mmol) was added to this suspension
while stirring. A clear Na2L (A or B) solution was
base
base
M
2ꢁ ꢁ2LH2 ꢁ2H2O X(ML)2 ×2H2O X 2L2ꢃ
acid acid
formed. To this solution,
2
mmol of
ꢁ2M(OH)2 ꢁH2O
[Cu(CH3COO)2]2×
/
2H2O in 40 cm3 of water was
The disproportionated products were identified
added. A coloured complex immediately precipi-
tated. The complex was filtered by suction, washed
with warm dilute Na2CO3 solution followed by
warm deionised water, and dried over P4O10 (yield
by atomic absorption spectra. Analytical data,
magnetic moments and other related details for
the complexes are presented in Table 1. The micro-
analysis data obtained are in agreement with the
80ꢀ85%).
/
formula (ML)2×
/
2H2O for hydrated and (ML)2 for
Anhydrous complexes were prepared by heating
the hydrated samples around 100 8C for 1 h.
anhydrous complexes.
2.3. Physical measurements
3.1. Vibrational spectra
The metal content of each complex was deter-
mined by atomic absorption spectroscopy using an
AA-640-13, Shimadzu (Japan) instrument. Ele-
mental analyses were carried out at RSIC, Chan-
digarh. Magnetic measurements were carried out
in the polycrystalline state using a vibrating
The IR spectra of the ligands and their com-
plexes were recorded using KBr disc and their
characteristic bands are summarized in Table 2.
The spectral data showed that the ligands coordi-
nate through its carboxylate oxygen in a bidentate
fashion. The mode of coordination of carboxylate

B.S. Garg et al. / Spectrochimica Acta Part A 59 (2003) 2775ꢀ
/
2783
2777
Table 1
Analytical data and magnetic moments of copper(II) complexes
Compounds
Color
Yield (%)
Analysisꢂ/found/(calculated)%
meff (BM)
ꢃ/2J
C
H
N
Cu
(1) [Cu(A)]2×
/
2H2O
Blue
80
75
78
83
35.74 (35.76)
37.80 (37.80)
49.80 (49.82)
51.28 (51.74)
3.67 (3.58)
3.10 (3.15)
4.08 (4.11)
3.19 (3.35)
8.34 (8.34)
8.62 (8.82)
6.28 (6.39)
6.32 (6.71)
18.42 (18.94)
19.82 (20.00)
14.33 (14.52)
14.98 (15.21)
1.47
1.50
1.50
1.52
287
278
(2) [Cu(A)]2
Pale blue
Blue
Light blue
(3) [Cu(B)]2×
/
2H2O
(4) [Cu(B)]2
groups has often been deduced from the magni-
tude of the observed separation between
nasym(CO2) and nsym(CO2) [18,19]. The separation
between nsym(CO2) and nasym(CO2) was in the
i.e., band I at ꢀ
27 000 cmꢃ1 these are assigned to a dꢀ
and a L0M charge transfer transition respec-
tively. The presence of band II is also considered a
diagnostic criterion for Cuꢀcarboxylate dimer
formation [22,23].
/
14 000 cmꢃ1 and band II at ꢀ
d transition
/
/
/
range of 145ꢀ
tate carboxylate ligands [20]. The IR bands due to
n(NÃH), n(CÄ
O) at 3300 and 1620 cmꢃ1, respec-
/
170 cmꢃ1 as expected from biden-
/
/
/
tively, do not show any shift upon complexation
indicating the non-participation of these groups in
coordination. Hydrated complexes show an addi-
tional band at 3450 cmꢃ1, which suggests the
3.3. Magnetic susceptibility studies
The magnetic moments at room temperature for
all the complexes are found to be subnormal,
presence of the n(OÃ
coordinated water [21]. In the spectra of complexes
n(MÃ
O) bands are observed at 390 cmꢃ1
/H) stretching mode of
following within the range of 1.47ꢀ1.52 BM (Table
/
1). Complex numbers 1 and 3 were selected for
variable temperature magnetic susceptibility stu-
dies, data is presented in Table 3. Diamagnetic
corrections are determined by Pascals method [24]
and subtracted from the measured values of the
total susceptibility. The resulting paramagnetic
susceptibilities per mole are plotted and presented
in Fig. 1. In Fig. 1 the full lines are the best fit to
/
.
3.2. Ligand field spectra
Ligand field spectra of complexes were recorded
in DMF solvent and are presented in Table 2. All
the complexes showed two bands in their spectra,
Table 2
IR (cmꢃ1) and ligand field spectral (cmꢃ1) data of the complexes
Ligand/complexesa
n(NÃ
/
H) or n(OÃ/H)
Amide
I
n(CO2)
Dn
n(MÃO) Ligand field band (cmꢃ1
)
II
Asym. Sym.
Id ꢀ d
pLMCT
(A)
(1)
3300
1620 1480
3300
3450
1620 1480 1580
1410 170 390
1410 165 390
13 896
13 895
27 700
27 700
(2)
(B)
(3)
3300
3280
1620 1480 1575
1620 1470
3280
3450
1618 1475 1565
1415 150 388
1415 145 388
14 285
14 283
27 695
27 698
(4)
3280
1620 1470 1560
a
Table 1.

2778
B.S. Garg et al. / Spectrochimica Acta Part A 59 (2003) 2775ꢀ2783
/
Table 3
Variable temperature magnetic susceptibility data for [Cu(A)]2×
/
2H2O and [Cu(B)]2×2H2O
/
Temperature (K)
[Cu(A)]2×/2H2O
[Cu(B)]2×/2H2O
xg (ꢄ
/
106)
xm (ꢄ
/
106)
xcmorr (ꢄ
/
106)
meff
xg (ꢄ
/
106)
xm (ꢄ
/
106)
xcmorr (ꢄ
/
106)
meff
300
280
260
240
220
200
180
160
140
120
100
2.32
2.36
2.37
2.36
2.35
2.28
2.26
1.95
1.79
1.61
1.32
783
798
800
798
796
769
764
660
604
544
445
903
918
920
918
916
889
884
780
724
664
564
1.47 1.734
1.43 1.819
1.38 1.837
1.32 1.842
1.27 1.837
1.19 1.793
1.13 1.741
1.00 1.603
0.90 1.315
0.70 1.091
0.67 0.689
777
792
800
802
800
784
758
698
573
475
300
939
954
962
964
962
943
920
860
735
637
462
1.50
1.46
1.41
1.36
1.30
1.23
1.15
1.05
0.91
0.78
0.61
the expression of the paramagnetism for a singletꢀ
triplet system [25] given by the following equation:
/
They are presented in Fig. 2(aꢀ
four absorptions of varying intensity were ob-
served as given in Table 4. An absorption at ꢀ
3000 G arises from mononuclear impurities of spin
Sꢂ/12 [29]. The intensity of this absorption in-
/
f). In all the cases,
ꢀꢁ
ꢂꢁ
ꢃ ꢄ
ꢃ
ꢄꢂꢅ
/
Ng2b2
3kT
1
2J
xcmorr
ꢂ
1ꢁ
exp ꢃ
ꢁN2
3
kT
where xcmorr is corrected molar susceptibility, N is
Avogadro’s number, g is the EPR determined
creases as the temperature is lowered whereas the
intensity of the other three absorptions decreases.
value of Lande splitting factor, N2ꢂ
/
60ꢄ
10ꢃ6
/
The other three absorption lines (Hz1, Hz2, HÞ
/
2)
emu molꢃ1 is the assumed temperature indepen-
dent paramagnetism [25], and T is the absolute
temperature. The value of the exchange intensity
2J of both the samples was of the order of 300
cmꢃ1, which indicates a relatively strong magnetic
interaction between the two copper ions.
are due to transitions in the triplet state. It is
noticeable that the line widths of high and low
field absorption are approximately three times that
of the middle one. The intensity of high and low
field absorptions is quite weak whereas intensity of
middle one is very strong. Both hydrated samples
show well defined hyperfine structures in high and
low field absorptions in their low temperature
spectra whereas the anhydrous compounds do not.
At 100 K, the absorptions at the highest and
lowest magnetic fields showed a seven line hyper-
fine splittings from the two equivalent copper(II)
Magnetic susceptibility data support the hy-
pothesis of isolated dimers with a negative intra-
dimer exchange. The 2J values obtained for both
samples are quite comparable to those reported for
similar copper carboxylates [26,27]. Since the
observed 2J values are of the order of ꢀ300
/
cmꢃ1, it follows that alternate Heisenberg chain
model [28] cannot describe properly the magnetic
properties of these compounds of unknown struc-
ture and that the isolated dimer model is strongly
favoured.
(Iꢂ3/2) nuclei with the intensities, 1:2:3:4:3:2:1.
/
In the present complexes only three lines are
observed and D, g , gÞ, Žg have been evaluated
Š
using the Hamiltonian [30,31] set of equations and
presented in Table 4. The crystal field removes the
degeneracy of the triplet state. In the present case,
the crystal field has a quasi axial symmetry and
3.4. EPR spectra
Msꢂ
field splitting parameters D and E (D ꢁ
defined as follows: D arises from the presence of
unique axis Z (the CuꢀCu axis) and E takes into
/
9
/
1 states are almost degenerate. Two zero
EPR spectra of all the complexes were recorded
/
ꢂ/E) are
at X-band frequencies (ꢀ
/
9.5 GHz) at liquid
nitrogen and room temperatures in solid state.
/

B.S. Garg et al. / Spectrochimica Acta Part A 59 (2003) 2775ꢀ
/
2783
2779
Fig. 1. xCmorr versus T curves for (A) [Cu(BCOPEN)]2×
/
2H2O and (B) [Cu(BCOPHEN)]2×2H2O.
/
account of small differences between X and Y
directions. Absence of splitting and HÞ2 absorp-
tion shows little or no rhombic distortion. The
seven line dinuclear hyperfine pattern of g part of
the triplet spectra of hydrated adducts is typical of
copper carboxylates with singletꢀtriplet separa-
tions much larger than the hyperfine component.
/
/
However no such hyperfine interaction was de-
Š

2780
B.S. Garg et al. / Spectrochimica Acta Part A 59 (2003) 2775ꢀ2783
/
Fig. 2. X-band EPR spectra of (a) [Cu(BCOPEN)]2×
RT; (d) [Cu(BCOPHEN)]2×2H2O at LNT; (e) [Cu(BCOPEN)]2 at LNT and (f) [Cu(BCOPHEN)]2 at LNT.
/
2H2O at RT; (b) [Cu(BCOPEN)]2×
/
2H2O at LNT; (c) [Cu(BCOPHEN)]2×2H2O at
/
/
tected for the anhydrous compound, pointing to
inter dimer exchange and consequent broadening
and hence no resultant hyperfine splittings [32].
dimers is comparable to their monomeric impu-
rities indicating that the environment around
Cu(II) may be same.
It is notable that the A value is half the value
Š
that is observed for monomeric complexes. The
3.5. Thermal analysis
absence of hyperfine splitting at HÞ
/
2 may be due
to very a small coupling constant (AÞ) in the
All the complexes have been studied by TGA
and DTA. Hydrated complexes show weight loss
perpendicular region. The Žg values for the
Table 4
Resonance fields (in Gauss) and EPR spectral parameters for complexes
Complex
Triplet state
Hz1 HÞ
Doublet state Hmj
g
Š
gÞ
Žg A ꢄ
/
10ꢃ4 (cmꢃ1
)
D
Žg m.i.a
Š
/
2
Hz2
(1)
(2)
(3)
(4)
460
460
480
480
4600
6060 3000
6060 3000
6100 3050
6100 3050
2.41 2.16 2.24 70
2.41 2.16 2.24
0.419 2.24
2.24
4600
4675
4675
2.40 2.13 2.22 68
2.40 2.13 2.22
0.422 2.21
2.21
a
Monomeric impurity.

Table 5
Thermal decomposition data for complexes
Complex
Decomposition
step
Temperature range
(8C)
Mass loss
(%)
Final residue
(%)
Nature of decomposi- Order of reaction Activation energy (Ea)
kJ molꢃ1
tion
(n)
[CuA]2 ×2H2O
/
I
95ꢀ
/
150
5.30 (5.36)
Endothermic
Exothermic
Exothermic
Endothermic
Exothermic
Exothermic
¡ꢃ2H2O
[CuA]2
/
II
III
I
225ꢀ
/275
43.05 (43.10)
1
27.35
¡ꢃA
[Cu2A]
/
375ꢀ
/475
75.87 (76.45) 23.30 (23.56)
4.10 (4.11)
¡ꢃA
/
2CuO
[CuB]2 ×2H2O
ꢃ2H2O
[CuB]2
/
75ꢀ150
/
/
/
¡
/
/
II
III
225ꢀ
/325
44.70 (44.79)
1
25.75
¡ꢃB
[Cu2B]
¡
/
ꢃB
400ꢀ
/496
81.81 (81.83) 18.16 (18.17)
2CuO

2782
B.S. Garg et al. / Spectrochimica Acta Part A 59 (2003) 2775ꢀ2783
/
thermal decomposition reaction is found to be one
and energy of activation (Ea) is approximately 30
kJ molꢃ1
.
The Ea values follow the order
BCOPENꢁBCOPHEN.
/
4. Conclusions
(i) Both the ligands A and B act as ‘simple’
carboxylic acids in their coordination with Cu(II)
ions.
(ii) All the complexes provide clear examples of
strong antiferromagnetic exchange. This type of
magnetic behaviour and the resulted EPR spectral
features clearly indicate that the complexes are
most likely to adopt the typical carboxylate cage
structure.
(iii) Based on these findings, the present com-
plexes are strongly super-exchange coupled within
an isolated-bridge dimer model.
(iv) This study enables us to propose an
explanation for the properties of a class of
enzymes known to contain the so called type III
copper. We would like to propose exchange
coupling through the appropriate carboxylate
group of the peptide chain as the mechanism
which can account for the observed antiferromag-
netic behaviour of type III copper enzymes. Based
on these studies, the tentative structures for the
complexes shown in Fig. 3 are proposed.
Acknowledgements
One of the authors (Deo Nandan Kumar) is
thankful to CSIR, New Delhi (India) for the
award of Research associateship.
Fig. 3. Proposed structures of the copper(II) complexes (a)
[Cu(BCOPEN)]2×
/
2H2O;
(b)
2H2O and (d) [Cu(BCOPHEN)]2.
[Cu(BCOPEN)]2;
(c)
[Cu(BCOPHEN)]2×
/
at around 150 8C corresponding to loss of two
water molecules, which is further confirmed by
endothermic peaks in the DTA scan. All the
complexes decompose within the 500 8C range.
Thermograms show stepwise decomposition of
ligands from the complexes as shown in Table 5.
The final product of decomposition in all the
complexes is CuO as confirmed by experimental
and calculated weight losses. The order of the
References
[1] H.K. Lee, C.H. Lam, S. Li, Z.Y. Zhang, T.C.W. Mak,
Inorg. Chem. 40 (2001) 4691 (theirin).
[2] T. Bisogno, L.D. Petrocellis, V.D. Marzo, Curr. Pharm.
Des. 8 (2002) 533.
[3] N. Ban, P. Nissen, J. Hansen, P.B. Moore, T.A. Steitz,
Science 289 (2000) 905.
[4] J.K. Kumar, J.S. Oliver, J. Am. Chem. Soc. 124 (2002)
11307.

B.S. Garg et al. / Spectrochimica Acta Part A 59 (2003) 2775ꢀ
/
2783
2783
[5] C.R. Woods, T. Ishii, D.L. Boger, J. Am. Chem. Soc. 124
(2002) 10676.
[18] G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980)
227.
[6] A.T. Vlahos, E.I. Tolis, C.P. Raptopoulou, A. Tsohos,
M.P. Sigalas, A. Terzis, T.A. Kabanos, Inorg. Chem. 39
(2000) 2977.
[19] G.B. Deacon, F. Huber, R.J. Phillips, Inorg. Chim. Acta
104 (1985) 41.
[20] A.B.P. Lever, Inorganic Electronic Spectroscopy, second
ed., Elsevier, Amsterdam, 1984.
[7] P. Arrizabalaga, P. Castan, Polyhedron 2 (1983) 823.
[8] F. Botteman, G.M. Nicolle, L.V. Elst, S. Laurent, A.E.
Merbach, R.N. Muller, Eur. J. Inorg. Chem. (2002) 2686.
[9] S. Zhang, Z. Kovacs, S. Burgess, S. Aime, E. Terreno,
A.D. Sherry, Chem. Eur. J. 7 (2001) 288.
[10] M.M. Bishop, L.F. Lindoy, D.J. Miller, P. Tumer, J.
Chem. Soc, Dalton Trans. (2002) 4128.
[21] B.S. Garg, M.J. Reddy, V. Kumar, J. Coord. Chem. 29
(1993) 33.
[22] L. Dubicki, R.L. Martin, Inorg. Chem. 5 (1966) 2203.
[23] J. Catterick, P. Thronton, Adv. Inorg. Chem. Radio
Chem. 20 (1977) 291.
[24] E. Konig, Magnetic Properties of Transition Metal Com-
pounds, Springer, Berlin, 1966.
[11] R. Rios, C. Jimeno, P.J. Carroll, P.J. Walsh, J. Am. Chem.
Soc. 124 (2002) 10272.
[25] B. Bleany, K.D. Bowers, Proc. R. Soc. 214 (1952) 451.
[26] R.W. Jotham, S.F.A. Kettl, J.A. Marks, J. Chem. Soc.,
Dalton Trans. (1972) 428.
[12] V.K. Aggarwal, G. Hynd, W. Picoul, J.L. Vasse, J. Am.
Chem. Soc. 124 (2002) 9964.
[13] P. Zanello, S. Tamburini, P.A. Vigato, Coord. Chem. Rev.
77 (1987) 165.
[27] A.R. Fortea, P. Alemany, S. Alvarez, E. Ruiz, Chem. Eur.
J. 7 (2001) 627.
[14] T.N. Sorell, Tetrahedron 45 (1989) 3.
[15] P. Cheng, D. Liao, S. Yan, Z. Jiang, G. Wang, Trans. Met.
Chem. 21 (1996) 515.
[28] D.B. Abraham, J. Chem. Phys. 51 (1969) 5195.
[29] P. Sharrock, M. Dartiguenave, Y. Dartiguenave, Bioinorg.
Chem. 9 (1978) 3.
[16] P. Sharrock, C.H. Thibaudeau, A. Caille, Inorg. Chem. 18
(1979) 510.
[30] P. Kottis, R. Lefebvre, J. Chem. Phys. 41 (1964) 379.
[31] M. Shakir, K.S. Islam, A.K. Mohamed, N. Jahan, Trans.
Met. Chem. 22 (1997) 189.
[17] A.K. Patra, R. Mukherjee, Inorg. Chem. 38 (1999)
1388.
[32] R.J. Deodens, Prog. Inorg. Chem. 21 (1976) 209.