Coordination complexes of bifunctional compounds: I. Synthesis and properties of bis[5-(2-oxyphenyl)-3-phenyl-1,2,4-oxadiazolyl]copper(II). A fluorescent coordination compound of Cu(II)

Article abstract of DOI:10.1016/s0020-1693(99)00108-5

The synthesis of two Cu(II) bis complexes of a substituted 1,2,4-oxadiazole, which acts as part of a bidentate ligand, is reported for the first time. The two coordination compounds, which form an inseparable mixture, are characterized mainly via vibrational and EPR spectroscopy. Room temperature fluorescence is observed from two uncoupled excited states of the complexes, and the respective lifetimes are approximately 600 ps and 5 ns. The fluorescence quantum yields are a function of excitation wavelength, varying between 1.7 and 9.0×10^-3. These values are slightly higher than those of the isolated ligand.

Full text of DOI:10.1016/s0020-1693(99)00108-5

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 292 (1999) 1–6
Coordination complexes of bifunctional compounds
I. Synthesis and properties of
bis[5-(2-oxyphenyl)-3-phenyl-1,2,4-oxadiazolyl]copper(II).
A fluorescent coordination compound of Cu(II)ꢀ
A.S. da Silva a, M.A.A. de Silva a, C.E.M. Carvalho a, O.A.C. Antunes a,
J.O.M. Herrera a, I.M. Brinn a,*, A.S. Mangrich b
a Instituto de Qu´ımica, Uni6ersidade Federal do Rio de Janeiro, Caixa Postal 68.563, Ilha do Funda˜o, 21.941-970 Rio de Janeiro, R.J., Brazil
b Departamento de Qu´ımica, Uni6ersidade Federal do Parana´, Caixa Postal 19.081, 81.531-990 Curitiba, P.R., Brazil
Received 14 July 1998; accepted 17 February 1999
Abstract
The synthesis of two Cu(II) bis complexes of a substituted 1,2,4-oxadiazole, which acts as part of a bidentate ligand, is reported
for the first time. The two coordination compounds, which form an inseparable mixture, are characterized mainly via vibrational
and EPR spectroscopy. Room temperature fluorescence is observed from two uncoupled excited states of the complexes, and the
respective lifetimes are approximately 600 ps and 5 ns. The fluorescence quantum yields are a function of excitation wavelength,
varying between 1.7 and 9.0×10−3. These values are slightly higher than those of the isolated ligand. © 1999 Elsevier Science
S.A. All rights reserved.
Keywords: Fluorescence; Copper complexes; Oxadiazole complexes
sis [9], anti-microbial [10], anti-esquistosome [10],
tranquilizer [11], nervous system depressant [11], anti-
parasitic [12], and muscarinic agonist [13]. Thus, the
behavior of these oxadiazoles in the presence of metal
ions is very important. In spite of this fact, there are
very few reports of either organometallic [14,15] or
coordination compounds [16,17] with the 1,2,4-oxadia-
zolyl moiety. In the latter cases, the 1,2,4-oxadiazolyl
moiety appears as a monodentate ligand.
In addition, room temperature fluorescence is rarely
observed for coordination compounds of transition
metals, which has been attributed [18,19] to highly
efficient internal conversion. What follows is, to the
best of our knowledge, the first report of the 1,2,4-
oxadiazolyl moiety contained in a bidentate ligand, as
well as one of the few reports of room temperature
fluorescence for any transition metal coordination
compound.
1. Introduction
Oxadiazoles have been known for over a century,
but most work has concentrated on the symmetric
1,2,5- and 1,3,4-oxadiazoles. Considerably less work
has been reported on the non-symmetric 1,2,4-oxadia-
zoles, in spite of the fact that they also have been
known [1] for more than a century. The 1,2,4-oxadia-
zolyl moiety is present in several pharmacologically
active compounds, showing a wide range of biological
activities, such as anti-tussigen [2], anastesic [3], an-
thelmintic [4], anti-streptocossus [5], anti-pneumocossus
[6], analgesic [6,7], anti-inflammatory [6,7], anti-spas-
modic [7], anti-cancer [8], anti-lepra [9], anti-tuberculo-
ꢀ Taken, in part, from the M.S. thesis of A.S. da Silva, Universi-
dade Federal do Rio de Janeiro, 1998.
* Corresponding author. Fax: +55-021-290 4746.
E-mail address: irabrinn@iq.ufrj.br (I.M. Brinn)
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00108-5

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A.S. da Sil6a et al. / Inorganica Chimica Acta 292 (1999) 1–6
2. Experimental
2.1. Materials
tion of 720 mg (3.0 mmol) of 5HPPO in 50 ml of
ethanol to a solution of 300 mg (1.5 mmol) of cupric
acetate monohydrate in 50 ml of water, both at 65°C.
The mixture was stirred for 24 h and the light
brown solid formed was filtered, and washed succes-
sively with water, ethanol and dichloromethane, yield-
ing 0.54 g (1.0 mmol, 67%) of Cu5HPPO. Thin layer
chromatography on silica, using dichloromethane as
eluent, produced a single broad spot of Rf=0.42, as
compared to Rf=0.76 for 5HPPO. Thin layer chro-
matography (TLC) was also carried out with chloro-
form, dichloromethane, dichloroethane, acetone,
2-propanol acetonitrile, n-hexane, methanol, ethanol,
tetrahydrofuran or a mixture of these solvents, without
separation of the components. Anal. Calc. for
Cu(C14H9N2O2)2: C, 62.51; H, 3.37; N, 10.08. Found:
C, 62.22; H, 3.64; N, 9.71%.
Chloroform, acetonitrile, dioxane and dimethyl sulf-
oxide (analytical grade, Grupo Qu´ımica) and acetone,
methanol and tetrahydrofuran (analytical grade, E.
Merck) were used in the synthesis of the coordination
compound. Absorption and emission spectroscopy was
carried out in dimethyl sulfoxide (E. Merck,
UVASOL).
Electronic absorption spectra were taken with a
Varian-Cary 1-E spectrophotometer, elemental analyses
were done on a Perkin–Elmer CHN 2400 elemental
analyzer, and atomic absorption was done on a Varian
AA-175 instrument. EPR spectra were taken on a
Bruker ESP 300-E spectrometer equipped with a liquid
N2 Dewar insert and IR spectra were taken on a
Nicolet Magna-IR 760 FTIR instrument. Fluorescence
spectra were taken on a Hitachi F-4500 spectropho-
tofluorimeter, using Rhodamine 6-G as internal stan-
dard. Emission and excitation spectra were decon-
voluted using the self-modeling, factor analysis method
[20]. The single photon counting lifetime determinations
were done on an Edinburgh Analytical Instruments CD
900, using anthracene in n-hexane as the time standard
[21]. The time profiles were deconvoluted using a pro-
gram based on the Marquardt algorithm [22].
3. Results and discussion
3.1. Characterization of the coordination compound
Freezing point depression measurements in camphor,
using naphthalene for calibration, indicated a molecular
weight of 532953 (calculated for Cu(C14H9N2O2)2
537.5). Atomic flame spectroscopy at 327.4 nm indi-
cated a Cu content of 12.890.5% and at 324.7 nm
indicated a Cu content of 11.990.1% (calculated for
Cu(C14H9N2O2)2 12.1%). The flame spectroscopy and
colligative property determinations are interpreted as
indicating that the coordination compound has the
formula Cu(C14H8N2O2)2.
IR spectroscopy on the solid in Nujol (1% by weight)
yielded the following sharp peaks in the far-IR which
were not present in the pure ligand: 268.58 (Cu–O or
Cu–N deformation), 313.03 (Cu–N stretch), 342.93
(Cu–O stretch); as well as the following peaks which
shift in going from pure ligand to complex: 1146.02“
1145.65 (C–O stretch), 1324.12“1351.34 (N–O
stretch) and 1489.02“1468.34 (C–N stretch). The first
four peaks are interpreted as indicating that Cu was
linked to both O and N atoms, presumably two of
each. The slight shifting of the peak at 1146 cm−1 is
interpreted as indicating that the heterocyclic O is not
perturbed by the Cu and therefore the phenolic O, as
expected, is linked to Cu. The increase in frequency of
the peak at 1324 cm−1 is interpreted as indicating that
the N-2 atom of the heterocyclic ring is not also
attached to the Cu. The decrease in frequency of the
peak at 1489 cm−1 is interpreted as indicating that the
N-4 atom of the heterocyclic ring is attached to the Cu.
That it is the N-4 and neither the N-2 nor the O-1 atom
which is linked to the Cu is consistent with previous
[16] findings.
2.2. Synthesis
Benzamidoxime was prepared as described in the
literature [23]. o-Salicylchloride was prepared [24] by
adding 28 g (0.20 mol) of salicylic acid and 1 ml of
pyridine to 150 ml of hexane. This solution was heated
while 15 ml (24.1 g, 0.2 mol) of thionyl chloride were
added. When the solution became clear and gas was no
longer being expelled, the solvent was evaporated in
vacuo, yielding a translucent liquid that was too un-
stable to be characterized. o-Salicylbenzamidoxime was
prepared by adding a solution of 0.34 ml (3.0 mmol) of
salicylchloride in 40 ml of anhydrous tetrahydrofuran
(THF) to a solution of 0.40 g (3.0 mmol) of benzami-
doxime in 80 ml of dry THF. The solution was then
refluxed for 1 h, the solvent evaporated and the solid
recrystallized from acetonitrile, yielding 0.597 g (2.33
mmol, 78%) of white needles; m.p.=172°C (lit. [25]
172°C). 5-(2-Hydroxyphenyl)-3-phenyl-1,2,4-oxadiazole
was prepared [25] by refluxing 604 mg (2.36 mmol) of
o-salicylbenzamidoxime with 100 ml of ethanol, 11.3 ml
of water and 3.5 ml of 1 M HCl for 24 h. Upon
completion, the solvent was evaporated and the solid
was recrystallized from hexane, yielding 534 mg (2.24
mmol, 95%) of a white solid; m.p.=156°C (lit. [25]
155°C). bis[5-(2-Oxyphenyl)-3-phenyl-1,2,4-oxadiazole]-
copper(II) (Cu5HPPO) was prepared by adding a solu-

A.S. da Sil6a et al. / Inorganica Chimica Acta 292 (1999) 1–6
3
EPR spectroscopy at 77 K (Fig. 1) indicated the
presence of two different isomers in the ratio of 2:1.
The parameters corresponding to two Cu2+ ion com-
plexes, obtained by simulation of the experimental spec-
trum using the Bruker Simfonia simulation program,
are given by AÞ=10 G, Aꢀ=118 G, gÞ=2.0780,
gꢀ=2.399 (67%) and AÞ=3 G, Aꢀ=140 G, gÞ=
2.0678, gꢀ=2.3640 (33%). These parameters suggest
that both structures involve weak fields about the cen-
tral Cu, and therefore, the arrangement about the Cu
atom is approximately tetrahedral. However, the sec-
ond has a stronger ligand field as indicated by a lower
gꢀ and a higher Aꢀ, signifying that it has a structure
closer to a square-planar configuration than the first. In
addition, the relative populations of the two configura-
tions allow the calculation of DG=440 J mol−1 for
their interconversion.
TLC, using aluminum oxide as the solid phase, upon
elution with dichloromethane, yielded a sharp spot
for 5 - (2 - hydroxyphenyl) - 3 - phenyl - 1,2,4 - oxadiazole
(5HPPO, Rf=0.76) and a broad spot for the coordina-
tion compound (Rf=0.42). Reversed phase chromatog-
raphy with RP-18 as the phase, upon elution with
dioxane/water (3:1), yielded Rf values of 0.47 for
5HPPO and 0.09 for the complexes.
Molecular mechanics calculations, using the MM+
formalism within the Hyperchem 5.0 program, indicate
that there will be two distinct, quasi tetrahedral struc-
tures belonging to the C2 point group. The one that
originates from the trans planar structure (Fig. 2(a), C2h
symmetry) is calculated to be 649 J mol−1 less stable
than the structure that originates from the folding of
the cis planar structure (Fig. 2(b), C26 symmetry).
The experimental chromatography and EPR results,
as well as the molecular mechanics results, can be
interpreted as indicating that there is more than one
form of the coordination compound present, that they
have a very similar structure and that they are less
polar than 5HPPO. The structures attributed to the
Fig. 2. The transoid (a) and cisoid (b) structures of Cu5HPPO.
complex are two slightly flattened tetrahedra, where the
rotation out of the tetrahedral structure is, in one case,
in the direction of the trans structure and, in the other
case, in the direction of the cis structure. The latter
structure is tentatively assigned as the lower energy
structure, although it must be recognized that the ex-
tremely small calculated energy difference between the
two structures does not allow a definitive assignment to
be made. However, the ordering of the structures is also
consistent with the specific interpretation of the EPR
results that the less stable structure (transoid) is the
more planar of the two, because the cisoid structure
should have more steric hindrance. It is also concluded
that the interconversion between the two structures at
77 K is slower than the time constant of the EPR
instrument (3 ns). However, at ambient temperature it
is faster than the time constant for chromatography
(several minutes) and is probably catalyzed by the silica
surface.
3.2. Electronic spectroscopy and photophysics
UV–Vis spectroscopy (Fig. 3(a,b)) of the complexes
in DMSO showed four broad peaks with maxima at
690 nm (m=5.2×102 cm−1
M
−1), 369 nm (m=3.2×
103 cm−1 M−1), 312 nm (m=8.2×103 cm−1 M−1) and
259 nm (m=1.9×104 cm−1 −1). This is compared to
M
the absorption spectrum (Fig. 3(c)) of the pure ligand in
DMSO which exhibited only two maxima, at 313 nm
(m=6.4×103 cm−1
M
−1) and 263 nm (m=1.1×104
cm−1 M−1). The band at 690 nm is attributed to a d–d
band. There was no evidence for the presence of other
bands in the absorption spectrum. The tentative inter-
pretation given here is that the second band has a
significant amount of mixing of ligand orbitals from
Fig. 1. EPR spectrum of Cu5HPPO in DMSO at 77 K.

4
A.S. da Sil6a et al. / Inorganica Chimica Acta 292 (1999) 1–6
Fig. 3. Absorption spectra of 2×10−4 M Cu5HPPO (a and b) and
2×10−4 M 5HPPO (c). Band a is shown magnified 5×.
both ligands, whereas the third and fourth bands are
much more localized on an individual ligand.
Fluorescence spectroscopy of the complex in DMSO
revealed a band whose maximum is a function of
excitation wavelength, as shown in the upper spectrum
of Fig. 4. For example, at uexc=280 nm, i.e. exciting
into the fourth absorption band, a broad, unstructured
band with maximum at 430 nm was observed (Fig.
4(A)). However, exciting into the third absorption band
at uexc=310 nm produces a broad band whose maxi-
mum is found at 400 nm (Fig. 4(B)) and exciting into
the second band at uexc=350 nm produces a band
whose maximum is at 440 nm (Fig. 4(C)). Taking
emission readings from 380 to 550 nm, at intervals of 5
nm and excitation readings from 250 to 430 nm, at
intervals of 2 nm, it was possible to apply factor
analysis which generated two bands (shown in the
lower portion of Fig. 4), approximately Gaussians, one
with a maximum at 400 nm and the other with a
Fig. 5. (Top) excitation spectra of 2×10−4 M Cu5HPPO in DMSO
at uFl=380 nm (A), 420 nm (B) and 460 nm (C); (bottom) deconvo-
luted excitation spectra (see text).
maximum at 440 nm. (A possible third component,
which was discarded, corresponded to an eigenvalue
that was more than three orders of magnitude smaller
than the smaller one that was retained). The wavelength
maxima of the deconvoluted spectra can be compared
to the fluorescence maximum of the ligand in DMSO of
375 nm. The moderate dislocation of the blue band of
the complex relative to that of the ligand suggests that
the former is largely localized on one of the ligands.
The red fluorescence band of the complex can be tenta-
tively associated with a transition which is predomi-
nantly delocalized over the entire complex.
In an analogous fashion, the excitation spectra gener-
ated were dependent on uFl. Three of these are shown in
the upper part of Fig. 5. At uFl=380 nm the excitation
spectrum shows a shoulder at 275 nm and a maximum
at 314 nm. At uFl=420 nm the excitation spectrum
shows the same shoulder at 275 nm and an additional
one at 380 nm, in addition to maxima at 320 and 356
nm. At uFl=460 nm, the excitation spectrum shows
maxima at 282 and 356 nm and a shoulder at 380 nm.
Again, factor analysis was used, indicating all excita-
tion spectra could be described adequately as some
combination of two components, shown in the lower
portion of Fig. 5. (Once again the eigenvalue of the
discarded third component was approximately three
orders of magnitude less than the smaller of the eigen-
values of the two components that were retained.) In
the case of these two components, one showed a shoul-
der at 268 nm and a single maximum at 316 nm. The
other showed two maxima, a weak one at 280 nm and
a considerably stronger and non-Gaussian band with a
Fig. 4. (Top) fluorescence spectra of 2×10−4 M Cu5HPPO in
DMSO at uExc=280 nm (A), 310 nm (B) and 350 nm (C); (bottom)
deconvoluted fluorescence spectra (see text).

A.S. da Sil6a et al. / Inorganica Chimica Acta 292 (1999) 1–6
5
Table 1
two states can be coupled to one another, or not. The
fluorescence decay time data (Table 2) differentiate
these cases quite nicely. The decay times for 5HPPO in
DMSO of approximately 600 ps are equal to one of the
characteristic times of the complex, the other time being
almost an order of magnitude greater. This is inter-
preted as weighing against two different structures be-
ing responsible for the emission, because the EPR and
chromatography results suggest that the structures
present in the ground state are quite similar, making it
unlikely that they would have such different decay
times.
Fluorescence quantum yields (×103)
Compound
uexc=311 nm
uexc=325 nm
uexc=368
nm
5HPPO
Cu5HPPO
5.190.5
6.390.6
8.290.8
9.090.9
1.790.2
maximum at 356 nm and a shoulder at approximately
385 nm. The deconvoluted band at 316 nm corresponds
to the 312 nm band in the absorption spectrum, the
slight shoulder at 268 nm either indicating a small
contribution from the absorption band at 259 nm, or
else indicating incomplete deconvolution. (It is not
possible to distinguish between these two explanations
at this time.) In the case of the other component, the
band with maxima at 280 and 356 nm corresponds to
the 259 and 369 nm bands in the absorption spectrum.
The large dislocation of the deconvoluted excitation
peak at 280 nm, compared to the absorption maximum
of 259 nm is attributed to the greater intensity of the
xenon lamp of the spectrofluorimeter at the former
wavelength not being properly corrected by the library
instrumental response function. The presence of two
separable components in both the excitation and
fluorescence spectra indicates that equilibrium is not
attained during the lifetime of the two states involved in
the emission.
The fluorescence quantum yields (€fl) at ambient
temperature, using anthracene in cyclohexane [21] as
the standard, are given in Table 1. Even though the €
are all below 0.01, one notices that these values are
slightly higher for the complex than for the ligand alone
and that the value depends on excitation wavelength.
These results lead to the interpretation that two sepa-
rate emissions are being observed, not only because
factor analysis shows the presence of two distinct exci-
tation and fluorescence spectra, but also because the
Whether or not the two states are coupled can be
inferred from a consideration of the kinetic steps of the
overall excited state process. Assuming that there is
coupling:
D1*“Gd
D1*“D2
D2*“Gd
kD1
*kC
kD2
(1)
(2)
(3)
where D1*=excited state 1, D2*=excited state 2,
Gd=ground state and kC=rate constant for coupling.
Assuming a spike light source of infinitesimal width, the
differential equations, which describe the excited state
concentrations after the spike, have been written [26]
for a formally similar system as
d[D1*]/dt= −(kD1+kC) [D1*]
(4)
(5)
d[D2*]/dt=kC[D1*]−kD2 [D2*]
The general solutions have the form:
[D1*]=A exp(−t/~1)+D exp(−t/~2)
[D2*]=B exp(−t/~1)+C exp(−t/~2)
fl
(6)
(7)
Taking the first derivatives, with respect to time, of
Eqs. (6) and (7), and setting them equal to Eqs. (4) and
(5), respectively, it can be shown easily that
dependence of € on uexc suggests that there are two
D=0
(8)
(9)
fl
separate absorption processes which lead to emission.
In principle, the two emissions could be from the two
structures present in the ground state or two different
states of the same structure. If the latter is the case, the
~1=1/(kD1+kC)
~2=1/kD2
(10)
(11)
B=AkC/(~−2 1−~−1 1
)
This solution predicts that state D1* will always
exhibit a monoexponential decay, characterized by ~1,
whereas state D2* will show a biexponential. More
specifically, within the D2* emission band, one sees
from Eq. (11) that when ~2\~1, B will be negative and
~1 will be a rise time, which corresponds to the intu-
itively expected result. However, although ~2\~1 is
observed, both are observed as decay times under the
entire D2* curve. This implies that reactions (1)–(3) do
not describe the kinetics of this system and, therefore,
states D1* and D2* are not coupled. Considering the
Table 2
Characteristic fluorescence decay times a
uem (nm)
5HPPO
~ (ps)
Cu5HPPO
(ps)
~
~ (ps)
2
1
360
375
400
49596
60796
60796
61696
572912 (92)
590910 (77)
40009300 (8)
57009200 (23)
a uexc=325 nm. Numbers in parentheses are the contributions of
each component to the overall decay curve, in %.

6
A.S. da Sil6a et al. / Inorganica Chimica Acta 292 (1999) 1–6
[2] (a) M. de Gregorio, Panminerva Med. 4 (1962) 90. (b) K. Harsanyi,
P. Kiss, D. Korbonita, I.R. Malyata, Arzneim.-Forsch. 16 (1966)
615. [Chem. Abstr.: 70 (1969) 37724k].
[3] J. Sterne, C. Hirsch, Therapie 20 (1965) 89, 95.
[4] (a) B. Silvestrini, C. Pozzatti, Brit. J. Pharmacol. 16 (1961) 209.
[Chem. Abstr.: 56 (1962) 5356b]. (b) B. Silvestrini, C. Pozzatti,
Arch. Inf. Pharmacodyn. 129 (1960) 249. [Chem. Abstr.: 56 (1962)
14874f]. (c) C. Ainsworth, W.E. Buting, J. Davenport, M.E.
Callender, M.C. McCowen, J. Med. Chem. 10 (1967) 208. [Chem.
Abstr.: 66 (1967) 85061f].
[5] G.W. Anderson, H.E. Faith, H.W. Marson, P.S. Winnek, R.O.
Roblin Jr., J. Am. Chem. Soc. 64 (1942) 2902.
[6] G. Palazzo, B. Silvestrini, Bull. Chim. Farm. 101 (1962) 251.
[Chem. Abstr.: 58 (1963) 4548a].
[7] (a) G. Palazzo, M. Tavella, M. Strani, B. Silvestrini, J. Med. Pharm.
Chem. 4 (1961) 351. [Chem. Abstr.: 56 (1961) 1962b]. (b) B.
Silvestrini, G. Corsi, B. Catanese, Boll. Chim. Farm. 103 (1964)
115. [Chem. Abstr.: 61 (1964) 3095e].
similarity of the fluorescence spectra, € and ~ values
fl
between D1* and 5HPPO, one can associate this state
with one where the excitation is heavily localized on the
ligand and the fluorescence decay lifetime is approxi-
mately 600 ps. Thus, D2* can be identified as a state in
which there is greater delocalization of the excitation
and, possibly, more participation of the Cu orbitals,
dislocating the emission band further to the red, with a
fluorescence decay lifetime of approximately 5 ns. The
biexponential decay measured in the red part of the
fluorescence spectrum is then attributed to spectral
overlap, consistent with the Gaussian deconvolution
shown in Fig. 3. Non-coupled excited states have been
reported [18,19] for other coordination complexes.
[8] F. Hoffmann-La-Roche and Co. AG, Fr., 1965, 1,385,856. [Chem.
Abstr.: 62 (1965) 14688b].
4. Conclusions
[9] L.B. Clapp, Adv. Heterocycl. Chem. 20 (1976) 65.
[10] (a) M.E. Dodd, W.B. Stillman, M. Roys, C. Crosby, J. Pharmacol.
Exp. Ther. 82 (1944) 11. (b) A.J. Crovetti, A.M. Von Esch, R.J.
Thill, J. Heterocycl. Chem. 9 (1972) 435. (c) C.H. Robinson, E.
Bueding, J. Fisher, Mol. Pharmacol. 6 (1970) 604. (d) H.H.
Gadebush, H. Brever, G.J. Miraglia, H.I. Basch, R. Semar,
Antimicrob. Agents Chemother. 1969 (1970) 280. [Chem. Abstr.:
75 (1971) 45930t].
[11] B. Cavalleri, G. Lancini, Rom. 63,449, Ital. Appl. 72/26.569.
[Chem. Abstr.: 92 (1980) 58787c].
[12] R.D. Haugwitz, A.J. Martinez, J. Venslavsky, R.G. Angel,
B.V. Maurer, G.A. Jacobs, V.L. Narayanan, L.R. Cruthers, J.
Szanto, J. Med. Chem. 28 (1985) 1234. [Chem. Abstr.: 103 (1985)
87815m].
Of various attempts to form coordination compounds
with 5HPPO, only that with Cu(II) was successful,
suggesting that 5HPPO (and by extension, other 1,2,4-
oxadiazoles which demonstrate biological activity) does
not interact strongly with transition metal ions. The
coordination compound formed was found to be a
mixture of two isomeric forms, almost isoenergetic. At
least one of these forms (probably both) fluoresces at
ambient temperature via two uncoupled states.
[13] J. Saunders, A.M. MacLeod, K. Merchant, G.A. Showell, R.J.
Snow, L.J. Street, R. Baker, J. Chem. Soc., Chem. Commun. (1988)
1618.
[14] L. Prasad, Y. Le Page, F.E. Smith, Acta Crystallogr., Sect. B 38
(1982) 2890.
[15] B.V. Polyakov, V.P. Tverdoklebov, I.V. Tselinskii, Zh. Obshch.
Khim. 55 (1985) 593. [Chem. Abstr.: 103 (1985) 88039v].
[16] M. Massacesi, G. Devoto, G. Gelli, Spectrochim. Acta, Part A 41
(1985) 1433.
[17] C. Moussebois, R. Lenaers, F. Eloy, Helv. Chim. Acta 45 (1962)
446.
[18] M. Zulu, A.J. Lees, Inorg. Chem. 28 (1989) 85.
[19] M. Zulu, A.J. Lees, Inorg. Chem. 32 (1993) 1493.
[20] W.H. Lawton, E.A. Sylvestre, Technometrics 13 (1971) 617.
[21] D.F. Eaton, Pure Appl. Chem. 60 (1988) 1107.
[22] D.W. Marquardt, J. Soc. Ind. Appl. Math. 11 (1963) 431.
[23] F. Eloy, R. Lenaers, Chem. Rev. 62 (1962) 155.
[24] J.F. Boas, J.R. Pilbrow, J. Chem. Soc. (1969) 572.
[25] I.M. Brinn, C.E.M. Carvalho, F. Heisel, J.A. Miehe, J. Phys.
Chem. 95 (1991) 6541.
Acknowledgements
The authors gratefully acknowledge Professors Rajen-
dra Mohan Srivastava, Alistair J. Lees, Harry D. Gafney
and Jose´ A.P. Bonapace for helpful discussions, Mrs
Leonice Coelho for the atomic flame spectroscopy and
elemental analyses, and the Brazilian National Research
Council (CNPq) for partial financial support (to A.S.S.,
O.A.C.A., I.M.B. and A.S.M.), as well as the Foundation
for Studies and Projects (FINEP), the World Bank and
the Jose´ Bonifa´cio Foundation (FUJB) of the Federal
University of Rio de Janeiro for research grants.
References
[1] (a) F. Tiemann, P. Kru¨ger, Berichte 17 (1884) 1685. (b) F. Tiemann,
Berichte 17 (1884) 1689.
[26] R.S. Moog, M. Maroncelli, J. Phys. Chem. 95 (1991) 10359.
.