Molecular magnetic properties of two-copper(II) containing complexes [Cu(II) (1-phenylamidino-O-n-butylurea) en (H2O)]2(2+) and [Cu(II) sulphato-mono (1-phenylamidino-O-methylurea)]2. An EPR study.

Article abstract of DOI:10.1016/j.saa.2003.08.020

Electron paramagnetic resonance (EPR) investigations were conducted on [Cu(II) (1-phenylamidino-O-n-butylurea) en (H2O)]2(2+) (1) and [Cu(II) sulphato-mono (1-phenylamidino-O-methylurea)]2 (2) respectively, in the temperature range 300-77K. Fine structure characteristics of S = 1 system, was observed in both complexes with zero field splitting of 0.0525 and 0.0225 cm(-1), respectively, suggesting the formation of dimeric complexes. The presence of the half-field signal (DeltaMs= +/-2), in the complex 1, further confirmed the formation of dimer. The temperature dependence of EPR signal intensity has given evidence for the ferromagnetic (FM) coupling between the two Cu2+ ions. The isotropic exchange interaction constants J, were evaluated from this and were found out to be approximately 57 and approximately 27 cm(-1), respectively, for the complexes 1 and 2. The photoacoustic spectra of these complexes had shown a band around 26,400 cm(-1) characteristic of metal-metal bonding giving an independent support for the existence of dimeric Cu2+ species. The high magnetic moment values at room temperature for complex 1 (2.68 microB) and complex 2 (2.00 microB), obtained from the magnetic susceptibility measurements, support the formation of ferromagnetically coupled Cu2+ dimers.

Full text of DOI:10.1016/j.saa.2003.08.020

Spectrochimica Acta Part A 60 (2004) 1593–1600
Molecular magnetic properties of two-copper(II) containing complexes
[Cu(II) (1-phenylamidino-O-n-butylurea) en (H2O)]22+ and
[Cu(II) sulphato-mono (1-phenylamidino-O-methylurea)]2
An EPR study
L. Ajitkumar Sharmaa, O. Ibopishak Singha, AK. Manihar Singha,
R.K. Hemakumar Singha,∗, Ramakant M. Kadamb,
Madhusudan K. Bhideb, Ashok R. Dhobaleb, Medury D. Sastryc
a
Department of Chemistry, Manipur University, Imphal 795003, India
b
Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
c
Department of Physics and Measurement Technology, Lingköping University, S-58183 Lingköping, Sweden
Received 21 July 2003; accepted 20 August 2003
Abstract
2+
Electron paramagnetic resonance (EPR) investigations were conducted on [Cu(II) (1-phenylamidino-O-n-butylurea) en (H2O)]2 (1) and
[Cu(II) sulphato-mono (1-phenylamidino-O-methylurea)]2 (2) respectively, in the temperature range 300–77 K. Fine structure characteristics
of S = 1 system, was observed in both complexes with zero field splitting of 0.0525 and 0.0225 cm−1, respectively, suggesting the formation
of dimeric complexes. The presence of the half-field signal (ꢀMs= 2), in the complex 1, further confirmed the formation of dimer. The tem-
perature dependence of EPR signal intensity has given evidence for the ferromagnetic (FM) coupling between the two Cu2+ ions. The isotropic
exchange interaction constants J, were evaluated from this and were found out to be ∼57 and ∼27 cm−1, respectively, for the complexes 1 and
2. The photoacoustic spectra of these complexes had shown a band around 26,400 cm−1 characteristic of metal–metal bonding giving an inde-
pendent support for the existence of dimeric Cu2+ species. The high magnetic moment values at room temperature for complex 1 (2.68µB) and
complex2(2.00µB), obtainedfromthemagneticsusceptibilitymeasurements, supporttheformationofferromagneticallycoupledCu2+ dimers.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Copper(II) complexes; Phenyldicyandiamide; EPR; PAS; FM coupling; Dimer
1. Introduction
(EPR), with special reference to investigate the nature of
magnetic coupling in dimeric complexes.
Metal–metal interactions in organometallic complexes
particularly those of transition metals have been the sub-
ject of many investigations for a long time [1–5]. This has
been extensively reviewed even two decades ago by Smith
and Pilbrow [1] and Eaton and Zaw [6]. The main interest
of such investigations is to elucidate metal-metal interac-
tion among d electron systems and also to get insight into
the structural aspects by electron paramagnetic resonance
In majority of the cases, where the site symmetry at the
paramagnetic ions is identical and the exchange interaction
is mediated through a shared ligand, there is a greater possi-
bility of formation of antiferromagnetically coupled dimers
resulting in localized S = 0 singlet state. On the other
hand, the ferromagnetic interaction which can arise due to
interaction of unpaired spins at different site symmetries
in bimetallic systems, offers greater variety and richness of
magnetic order at low temperatures. Therefore, the systems
containing two strongly coupled unpaired electrons (with
effective spin S = 1) are attracting considerable attention.
Although there are many reports on EPR of antiferro-
magnetically coupled dimers, relatively few systems with
ferromagnetically coupled dimers are reported [7,8]. Some
∗
Corresponding author. Tel.: +91-385-220787;
fax: +91-385-243-5145.
E-mail addresses: rkhemakumar@rediffmail.com (R.K.H. Singh),
sastry@ifm.liu.se (M.D. Sastry).
1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2003.08.020

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L.A. Sharma et al. / Spectrochimica Acta Part A 60 (2004) 1593–1600
of these aspects are being currently refocused due to the
possibility of manipulating magnetic ordering in molecular
magnets at very low temperatures [9,10] and possibility
of manipulating magnetic order by external means, e.g. by
optical excitation [11]. We have recently observed magnetic
ordering below 40 K in (NBu4)2Co2[Cu(opba)]3. S [opba
is ortho-phenylenebis (oxamato) and S is 3 DMSO·3H2O
or 2H2O], a weakly interacting systems, exibiting well re-
solved 14N hyperfine structure (shfs) on the EPR spectrum
of Cu(II). In these complexes, pseudo-triplet magnetic ex-
cited state formed by antiferromagnetic spin pairing between
Cu2+ and Co2+ linked together in two or three dimensional
network resulted in long range magnetic ordering at lower
temperatures [12].
In our earlier paper on the EPR studies of dichloro-mono
(1-phenylamidino-O-alkylurea) Cu(II) (alkyl: Me, Et, Pr,
Bu or Pe), we have shown that in these complexes, cop-
per(II) ions exist as monomers having square planar geom-
etry. Furthermore, we obtained evidence for the magnetic
field induced “partial molecular alignment” in some of these
field cooled (10 KG) complexes at 77 K [13]. As part of
these investigations we have synthesized two types of copper
complexes, viz. [Cu(II) (1-phenylamidino-O-alkylurea) en
2. Experimental
2.1. Materials and instrumentation
All the chemicals used were of reagent grade. Phenyldicyan-
diamide (PD) (I) was synthesised by the earlier reported
method [14]. The purity of the samples was checked by IR
spectra.
2.2. Preparation of complexes (see Table 1)
(A) Dichloro-mono (1-phenylamidino-O-n-butylurea) cop-
per(II) (DPB) was prepared recently by refluxing
cupric chloride (0.01 mol) and phenyldicyandiamide
(0.01 mol) in n-butanol on a steam bath for 1 h. The
complex was filtered off immediately and washed sev-
eral times with n-butanol and dried in air [13]. Blue
crystals (80%, mp 192 ◦C).
2+
(H2O)]2 (type I complex), and Cu(II) sulphato-mono(1-
(B) The mixed ligand complex 1 was prepared by dissolving
20 mmol of compound (DPB) in hot aqueous ethylene-
diamine solution (20 cm3; 20.1 mmol) and adding 95%
hot ethanol (50 cm3) with constant stirring for 30 min.
The mixture was slowly cooled and kept in a refrig-
erator overnight. Pale pink crystals separated out were
collected and washed repeatedly with ethanol and dried
in air (60%, mp 53 ◦C).
phenylamidino-O-methylurea) (type II complex) where
alkyl: Me, Et, Pr, Bu and Pe. EPR investigations of these
complexes down to 77 K had shown that except for R: Bu
in type I and R: Me in type II other complexes basically
exist as Cu-monomers. Thus more interesting complexes
are the butyl one in type I and methyl one in type II class
which exhibited dimeric EPR spectra. Investigations on
these complexes are presented in this paper. Further, the
results of electronic absorption studies on the complexes 1
and 2 are also presented.
(C) Complex 2 was synthesized by slight modification of
the published method [15]. Bluish green crystals sepa-
rated out by refluxing phenyldicyandiamide (0.01 mol)
Table 1
Analytical data and magnetic moment in different complexes
Code
Colour
Empirical formula
Calculated (%)
µa (B.M.)
Found (%)
C
H
N
Cu
PD
DPB
1
Colourless
Blue
C8N4H8
60.6
59.8
5.0
4.9
35.0
34.9
C
C
12N4OH18CuCl2
14N6O2H28CuCl2
39.1
39.2
4.9
4.9
15.2
15.4
17.2
17.1
1.86
2.68
2.00
Pale pink
Blue
37.62
37.81
6.27
6.24
18.81
18.79
14.22
14.32
2
C9N4O7H16CuCl2
27.87
27.65
4.12
4.11
14.45
14.42
16.38
16.41
PD: phenyldicyandiamide; DPB: dichloro-mono(1-phenylamidino-O-n-butylurea) copper(II).
a
Susceptibility values of the respective complexes were measured at 298 K.

L.A. Sharma et al. / Spectrochimica Acta Part A 60 (2004) 1593–1600
1595
and cupric sulphate (0.01 mol) in methanol on a steam
bath for 3 h, were collected immediately, washed by
methanol and finally by acetone. It was dried in air
(75%, dp 205 ◦C).
2.3. Physical measurements
Photoacoustic spectra (PAS) on solid samples were
recorded using homemade spectrometer in 350–800 nm
range using Tungsten halogen lamp as a source [16] whereas
solution spectra were recorded on a Beckmann DU-640
spectrophotometer. IR spectra were recorded on a Bruker
IFS-66V in KBr disc and far IR spectra using polyethylene
pellets over the range 500–50 cm−1. EPR experiments were
conducted using Bruker ESP-300 spectrometer operated at
X-band frequency (9–10 GHz) with 100 KHz field modu-
lation. DPPH was used as field marker. Temperature was
varied in the range 77–350 K using Bruker variable temper-
ature accessory. The room temperature magnetic moments
(µeff) were measured using PAR vibrating sample mag-
netometer (VSM) Model 155. C, H and N contents were
determined microanalytically on a C, H, N analyzer Perkin-
Elmer 240C model. The amount of copper was determined
by fusing the complex with KHSO4, then extracting with
a mixture of HNO3 and H2SO4 and finally by performing
an iodometric titration. Molar conductances in MeOH were
measured at room temperature on an Elico conductivity
bridge type CM-82T.
Fig. 1. Absorption spectrum of (a) copper acetate (b) complex 1 in solid
state using PAS and (c) complex 1 in methanol solution using Beckman
DU-6 spectrophotometer.
νC band in the range 1590–1600 cm−1 and appearance of
=
N
νC–N band at 1350 and 1251 cm−1 indicates the coordina-
=
tion through C N group of phenylamidine part. The frag-
3. Result and discussion
=
ment N C–O–C of 1-phenylamidino-O-n-butylurea (II) is
=
more delocalized and the bond order of the C–O– group is
raised which gives a new νC band at 1681 cm−1 after co-
3.1. Electronic spectra
=
N
ordination [13,20,21]. These complexes show νa (C–O–C)
stretch at 1188 cm−1 and νs (C–O–C) at 962 cm−1 [21]. The
weak bands in the 920–940, 751–760 and 637–651 cm−1
ranges may be assigned due to co-ordinated water [17]. In
the IR spectrum of complex 1, broad and strong absorption
and strong band at 3231 cm−1 is assigned to νN–H of the
primary amine [22,23]. Two bands at 1060 and 1392 cm−1
are due to νC–C and νC–N of ethylenediamine [24]. The IR
spectrum of complex 2 shows bands at 961, 1002, 1096 and
1184 cm−1, suggesting the presence of a bidentate bridged
sulphate in the complex [15,17]. The bands in the 471–480
and 555–571 cm−1 ranges have been assigned to ν(M–N) and
to ν(M–O) modes [17,24,25].
The photoacoustic (PA) signals of complexes I (Fig. 1b)
and II showed a broad band ca. 15,552 cm−1 due to d–d
transitions. It also showed a weak absorption band around
26,400 cm−1 indicating the possibility of metal–metal
bonding [17]. The PA spectrum of well known antifer-
romagnetically coupled copper acetate (S = 0 ground
state) is also included for comparison. This exhibited
an absorption around 26,500 cm−1 typical of metal-
metal bonding (Fig. 1a) in addition to a broad d–d band
around 16,400 cm−1. The optical absorption spectrum
of complex 1 in methanol (where complex 1 dissoci-
ates into Cu2+ monomers), showing d → d transitions
around 19,000 cm−1. The highly intense absorption around
26,000 cm−1 or 384 nm is due to charge transfer band [18].
3.3. EPR spectra
3.2. IR spectra
The EPR spectra were obtained on polycrystalline sam-
ples in the temperature range 300–77 K using 2 mW power
and saturation of EPR signals was not observed at this mi-
crowave power level. Fig. 2b and d shows the EPR spectra
for complex 1 at 300 K and at 77 K. To resolve the copper hy-
perfine structure on the parallel components the EPR spectra
were also recorded in the second harmonic presentation (not
IR spectrum of phenyldicyandiamide shows a band at
2167 cm−1, νC band at 1656 cm−1 and νC–N at 1396 and
=
N
1301 cm−1 [19]. The absence of a band at 2167 cm−1 in
the IR spectra of the complexes I and II suggests the ab-
sence of a nitrile group in these complexes. Lowering of

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L.A. Sharma et al. / Spectrochimica Acta Part A 60 (2004) 1593–1600
Fig. 2. EPR spectrum of complex 1 at X-band (a) at 300 K higher gain, (b) at 300 K at lower gain, (c) simulated spectrum of copper dimers using the
parameters as given in Table 2 and (d) at 77 K.
shown in figure). The spectrum consists of two intense lines
with features of perpendicular components having a separa-
tion of nearly 400 G. On either side of perpendicular spec-
trum, a signal composed of several narrow lines with nearly
double the separation (790 G) as that of perpendicular sepa-
ration were observed. The seven line structure on the parallel
component with the relative intensities 1:2:3:4:3:2:1 of the
hyperfine splitting (clearly visible on the low field // com-
ponent) is due to hyperfine interaction between two Cu2+
(I = 3/2) nuclei. This signal is assigned to Cu2+ pairs. The
most interesting feature in the EPR spectrum of the complex
1 is the resolved hyperfine structure on parallel components
inspite of the fact that intermolecular dipolar interaction is
expected to cause appreciable line broadening in such con-
centrated paramagnetic complexes. In addition, a weak sig-
nal at g = 3.9000, with unresolved hyperfine coupling was
observed at half the resonance field (H = 1600 G) of that
of the copper monomer. The weak signal corresponds to the
forbidden transition (ꢀMs= 2) among the S = 1 coupled
states, whereas, the signals observed around g = 2.0000
region correspond to the allowed fine structure transitions
(ꢀMs= 1).

L.A. Sharma et al. / Spectrochimica Acta Part A 60 (2004) 1593–1600
1597
Fig. 3. EPR spectrum of complex 1 in methanol solution (a) at room
temperature and (b) at 77 K.
Fig. 4. EPR spectrum of complex 2 (a) at room temperature, (b) at 77 K,
(c) simulated spectrum using parameters given in Table 2 and (d) second
derivative spectrum at 77 K.
The EPR spectrum of complex 1 (Fig. 3) dissolved in
DMF and methanol at 77 K (frozen complex) showed axi-
ally symmetric spectrum. This is shown in Fig. 3. It may be
ial position in solution) [18]. The observed g-values, i.e.
noted that (g )
≈ (g )
whereas (g//)monomer
>
g// > g > 2.000 for monomer and dimer are suggestive
⊥ dimer
⊥ monomer
⊥
(g//)dimer. A substantial differences in g// values of that
of the monomer may be noted (Table 2). On dissolution
in DMF and methanol in addition to breaking of a dimer
into monomers which is supported conductance value in
methanol (ΛM = 158 ꢁ−1 cm2 mol−1 for bi-univalent elec-
trolyte) [26], there can be a change in ligation in axial
position (ligand molecules get attached to complex at ax-
of the dx2−y2 character of the unpaired electrons.
The EPR spectrum of the complex 2 at 300 K was very
different compared to that of complex 1. The EPR spectrum
showed a composite signal, viz. a weak perpendicular dou-
blet (dimer signal) superimposed on perpendicular compo-
nent of copper monomers (that is the coexistence of both
copper monomers and dimer). This is shown in Fig. 4a. On
Table 2
EPR parameters of copper monomers and dimers in various matrices
Matrix
g//
g
giso
A//
A1
Aiso
D (cm−1
)
J (cm−1
)
⊥
1. Cu2+ in complex 1/MeOH
2.1943
2.1200
2.2383
2.2079
2.2079
2.3450
2.0530
2.0525
2.1773
2.0403
2.0403
2.0660
2.1013
2.0750
2.1976
2.0962
2.0962
2.1590
611.74
254.12
–
498.77
249.40
453.30
81.76
–
–
79.22
39.60
Unres.
258.40
–
–
220
109.44
–
–
–
57
27
–
2. (Cu2+
)
in complex 1
0.0525
0.0225
–
0.066
0.048
2
3. (Cu2+)2in complex 2
4. (Cu2+
5. (Cu2+
6. (Cu2+
)
)
)
in CeO2
in CeO2 [30]
in Y zeolite [31]
2
2
2
−52
–
Conversion factor 1 G = 2.8025g/ge MHz. Error in g is 0.0001 and in A is 2.8 MHz.

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L.A. Sharma et al. / Spectrochimica Acta Part A 60 (2004) 1593–1600
lowering the temperature to 77 K, and intense doublet, char-
half-field transition (ꢀMs = 2) to that of total intensity
of the normal transition (ꢀMs = 1) using the follow-
ing equation [27]. Relative intensity = intensity of ꢀMs =
2/total intensity of ꢀMs= 1 = A/r6(9.1/ν)2, where v
is the operating frequency of the EPR spectrometer in GHz
and A is constant and it depends on nature of paramag-
acteristic of fine structure emerged having g// = 2.2383,
g
⊥
= 2.1773, giso = 2.1976, D = 225 G with the simulta-
neous disappearance of the monomer signal (Fig. 4b). These
results indicate that Cu2+ ions are predominantly in dimeric
form at 77 K, whereas at room temperature there are indi-
cations of some monomeric species of Cu2+. This implies
that S = 1 lies lower getting more populated with lowering
of temperature. The EPR spectrum in the second derivative
presentation is shown in Fig. 4b. The EPR spectrum was
simulated using the EPR parameters given in Table 2 and is
shown in Fig. 4c. The most contrasting feature in the EPR
spectrum of complex 2 is the absence of any hyperfine struc-
ture arising from two equivalent coppers and also absence
of half field signal due to (ꢀMs= 2) transition between 77
and 300 K. However, room temperature magnetic moment
netic ion [A = 19.5 + 10.9 ꢀg(ꢀg = |g// − g |)]. This
⊥
method has advantage that the metal–metal distance esti-
mated is independent of the value of exchange coupling J.
The metal–metal distance estimated to be 4 Å is consistent
with that of 3.7 Å obtained from the observed zero field
splitting.
The EPR, magnetic susceptibility, IR and PAS data on
complexes 1 and 2 are presented above clearly established
Cu dimerization in both compounds. The Cu2+–Cu2+ inter-
action appears to be ferromagnetic in nature as the intensity
of dimer signal increases with the lowering of the temper-
ature. And this observation is also supported by the high
values of magnetic moments. The formation of dimeric cop-
per complexes, as mentioned in the introduction, in itself
is not a very new aspect as extensive literature exists. The
important aspect of this investigation, however, is the ex-
istence of ferromagnetic coupling in the copper complexes
and its implications in the possible modeling of the structure
of these complexes. It should be reemphasized that the fer-
romagnetic coupling between two copper ions, in a dimer,
implies that they have different local coordinations around
the paramagnetic ions (that is the nature of unpaired electron
is quite different with symmetry prohibiting direct overlap
of electronic wavefunctions) [28] or relative orientation of
monomer complexes in the crystalline matrix is such that
the metal orbital in the neighboring monomer sites would
be nearly orthogonal to each other. From the g-values given
in Table 2, the structure of monomer species of complex
1 would be elongated octahedron with unpaired electron in
dx2−y2 orbital under the assumption that at least one metal
ion of the dimer, retained its original structure (i.e. as in
complex 1 in MeOH/DMF, the g-values of other complex
of complex 2 is 2.0µ which suggests weak coupling be-
≡
tween two copper ions. Since the complex 2 was insoluble
in organic solvents, and hence was not investigated in the
dissolved form.
The isotropic exchange interaction constant J or the sepa-
ration between the singlet and a triplet, was calculated from
the temperature dependence of the intensity of dimer signal.
The population of the triplet state is governed by Boltzman
distribution and the Curie law following the equation N =
N03/T exp(−J/kβT). The increase in the intensity of EPR
signal with lowering of temperature suggested ferromagnet-
ically coupled copper dimers in complex 1 and complex 2.
J was estimated to be ∼57 and ∼27 cm−1 in the respective
samples.
The average metal–metal distance in the S = 1 dimer
was calculated by using the equation D = 3g2β2/2r3
=
1.39 × 104(g/r3) [26]. The distance between the two un-
paired electrons [Cu(II)–Cu(II)] in the dimers was estimated
to be 3.7 and 4.7 Å, respectively, for complexes 1 and 2 (this
method does not give accurate estimate of r when dipolar
coupling is of the order of nuclear hyperfine coupling).
Alternatively the metal–metal distance r was estimated for
complex 1 from the relative intensity ratio of the forbidden
have been estimated to be g// = 2.0457 and g = 2.0525).
⊥
Fig. 5. Proposed structure of [sulphato mono (1-phenylamidino-O-methylurea) copper(II)]2.

L.A. Sharma et al. / Spectrochimica Acta Part A 60 (2004) 1593–1600
1599
This has very small anisotrophy with g > g// > 2.0000.
one suggestive/tentative structure of complex 2 in the text
and the other as supplementary material (for review purpose
only).
⊥
Thus the unpaired electron has larger 3z2–r2 character in this
complex. If the dimerization is of two identical monomers,
the g-values of monomer and dimer would have been the
same. From the g-values of dimer and monomer of cop-
per complex 1 in MeOH/DMF, it can be inferred as a pos-
sible case of dissimilar ions [29]. The coupling between
two copper monomers one with dx2−y2 and the other with
References:
1. Wasson and Trapp, J. Phys. Chem. 73 (1969) 3763.
2. A. Syamal, Ind. J. Chem. 24A (1985) 946.
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3z2–r2 character would result in ferromagnetic coupling as
observed. It is not clear why the site symmetries at two cop-
per monomers are slightly different. This work only points
to this possibility and the models given by us are purely
suggestive and the structure can possibly be confirmed by
other independent experiment such as X-ray diffraction. For
the complex 2 also, essentially a similar senario appears to
be valid, as the dx2−y2 orbitals at neighboring Cu2+ sites
will be directed in different directions due to tetrahedral na-
Acknowledgements
Authors wish to express their gratitude to Dr. S.B.
Manohar, Head, Radiochemistry Division for his keen
interest in this work. One of the authors R.K. Hemaku-
mar Singh is grateful to Manipur University for a leave
grant. O. Ibopishak Singh is thankful to University Grants
Commission, New Delhi for the Teacher Fellowship
Award.
2−
ture of SO4 ions. The proposed structure of complex 2 is
shown in Fig. 5.
4. Conclusions
References
In summary, this work has given clear evidence for
the formation of ferromagnetically coupled dimers in
copper complexes 1 and 2 having the formula [Cu(II)
(1-phenylamidino-O-n-butylurea) en (H2O)]2 (Cl2)2 and
[Cu(II) sulphato-mono(1-phenylamidino-O-methylurea)]2.
J was estimated to be ∼57 and ∼27 cm−1 for the complexes
1 and 2.
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But very few spectroscopic (EPR) studies [1–3] appeared
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To the best of our knowledge this will be the first spectro-
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presented without a significant effort. So we have included

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