Proton transfer and coordination properties of aromatic α-hydroxy hydrazones

Article abstract of DOI:10.1016/S0022-2860(01)00668-8

The prototropic properties of two nitro derivatives of aromatic α-hydroxy hydrazones [2,4-dinitro-N-phenyl-N′(2-hydroxy-1-phenylmethylene hydrazine) and 2,4-dinitro-N-phenyl-N′(2-hydroxy-1-naphthylmethylene hydrazine] have been analysed through electronic

Full text of DOI:10.1016/S0022-2860(01)00668-8

Journalof Moel cualr Structure 605 ,2002) 17±26
www.elsevier.com/locate/molstruc
Proton transfer and coordination properties of aromatic a-hydroxy
hydrazones
*
G.A. IbanÄez, G.M. Escandar , A.C. Olivieri
   Â
Departamento de Quõmica Analõtica, Facultad de Ciencias Bioquõmicas y Farmaceuticas, Universidad Nacional de Rosario, Suipacha 531,
Rosario S2002LRK, Argentina
Received 18 April2001; accepted 8 May 2001
Abstract
The prototropic properties of two nitro derivatives of aromatic a-hydroxy hydrazones [2,4-dinitro-N-phenyl-N0,2-hydroxy-
1-phenylmethylene hydrazine) and 2,4-dinitro-N-phenyl-N0,2-hydroxy-1-naphthylmethylene hydrazine] have been analysed
through electronic absorption, 1H and 13C NMR and semiempirical molecular orbital methods. The metal binding properties of
both compounds and of the unsubstituted hydrazones were evaluated from potentiometric and spectrophotometric data in
dioxane±water mixtures. On the basis of the spectral analysis and PM3 semiempirical calculations, the structures of the formed
complexes were proposed. q 2002 Elsevier Science B.V. All rights reserved.
Keywords: Spectroscopy and potentiometry; Proton transfer; Coordination chemistry; Aromatic a-hydroxy hydrazones
1. Introduction
and metalbinding properties of compounds
containing enol±azo or keto±hydrazo functional
groups [4±8], with emphasis on the in¯uence of
different substituents. Speci®cally, the analysis of the
complexes formed between divalent metal ions and N-
phenyl-N0,2-hydroxy-1-phenylmethylene hydrazine)
,PHPH) is carried out. The relationship between the
chelating behaviour and the prototropic properties of
this compound is assessed. In addition, two nitro deri-
vatives, 2,4-dinitro-N-phenyl-N0,2-hydroxy-1-phenyl-
methylene hydrazine) ,2,4,NO2)2PHPH) and 2,4-
dinitro-N-phenyl-N0,2-hydroxy-1-naphthylmethylene
hydrazine) ,2,4,NO2)2PHNH) were synthesised,
studied and compared with the corresponding unsub-
stituted substrates. The studies were performed by a
combination of electronic absorption, NMR, potentio-
metry and semiempiricalcacl ualtions.
Speci®cally for the identi®cation of carbonylic
compounds, hydrazones are extensively used in both
synthetic and analytical chemistry. They are also
employed in the polymer industry ,as plasticising
agents, antioxidants and polymerisation initiators)
and also in the pharmaceutical industry [1,2]. It should
be mentioned that certain hydrazones and their
copper,II) complexes present biological activity as
antitumor agents [3].
In the present work, we continue with the evalua-
tion of aromatic a-hydroxy hydrazones as part of a
generalprogram committed to the study of tautomeric
* Corresponding author. Tel./fax: 154-341-4372-704.
E-mail address: gescanda@fbioyf.unr.edu.ar ,G.M. Escandar).
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860,01)00668-8

18
G.A. IbanÄez et al. / Journal of Molecular Structure 605:2002) 17±26
2. Experimental
were performed on a Brucker AC-200 NMR spectro-
meter operating at nominalfrequencies of 200.1 and
50.3 MHz, respectively, while the corresponding
spectra for 2,4,NO2)2PHNH were recorded on a
Brucker AM-500 spectrometer operating at nominal
frequencies of 500.1 and 125.8 MHz.
2.1. Reagents and solvents
Stock solutions ca. 0.01 M of Cu,II), Co,II) and
Ni,II) were prepared by dissolving the corresponding
nitrates ,AR) in doubly distilled water. The exact
metalconcentrations were determined by titrations
with previously standardised ethylenediaminetetra-
acetic acid solution in the presence of appropriate
indicators [9]. Solutions of carbonate-free NaOH in
30% dioxane±70% water ,v/v) and 70% dioxane±
30% water ,v/v) were standardised with the use of
Gran's plots [10]. Aqueous solutions of carbonate-
free NaOH and HNO3 were standardised against
potassium hydrogenphtalate and sodium carbonate,
respectively. 1,4 Dioxane ,p.a.) was obtained from
Merck. The water employed was puri®ed by ion
exchange and distilled.
2.4. MO calculations
Semiempirical MO calculations in the ground states
,geometry optimisation and calculation of heats of
formation) of the structures of the compounds and
their metalcompelxes were performed using PM3
[12], as implemented in either the HyperChem 5.02
or Spartan packages of programs on a Pentium II 650
microcomputer. Calculations in the excited states for
the nitro derivatives, which are relevant to interpret
luminescent properties, were not be performed
because the ¯uorescence emission of these compound
is negligible.
2.2. Synthesis of the ligands
2.5. Determination of equilibrium constants
The ligands were prepared using standard proce-
dures [11]. PHPH was prepared as previously
described [5]. 2,4,NO2)2PHPH and 2,4,NO2)2PHNH
were prepared according to the following procedure:
to a clear solution obtained by warming 2,4-phenyl-
hydrazine with concentrated HCland ethano,l the
corresponding ethanolic aldehyde solution ,salicyl-
aldehyde or 2-hydroxynaphtalene-1-carbaldehyde)
was added. The mixture was heated to just the boiling
point and shaken. After cooling, the solid obtained
was recrystallised from acetone±ethanol and acetone,
respectively. Elemental analyses were performed by
Atlantic Microlab Inc., and were in agreement with
the calculated values: for PHPH, found: C: 73.54, H:
5.73, N: 13.06%; C13H12N2O requires: C: 73.56, H:
5.70, N: 13.20%; for 2,4,NO2)2PHPH, found: C:
51.79, H: 3.49, N: 18.56%; C13H10N4O5 requires: C:
51.61, H: 3.33, N: 18.53%; for 2,4-,NO2)2±PHNH,
found: C: 57.99, H: 3.45, N: 15.88, C17H12N4O5
requires: C: 59.91, H: 3.41, N: 15.90%. The purity
of the ligands was also checked by thin-layer chroma-
All measurements were carried out at an ionic
strength ,m) of 0.10 M ,NaNO3) and at a temperature
of 20.08C. Due to the low aqueous solubility of both
the compounds under study and their complexes, the
determination of the equilibrium constants was
carried out in dioxane±water mixtures. The effect of
the solvent composition in the pH measurements was
taken into account by correcting each metre reading
with a conversion factor [13]. This factor was
obtained, prior to each experiment, by titration of
both 30% v/v and 70% v/v dioxane±water solutions
of standard HNO3 with successive amounts of stan-
dard NaOH in the corresponding solvent mixture. In
these latter titrations, m was also adjusted to 0.10 M
with NaNO3 and the temperature was maintained at
20.08C. The log of the water hydrolysis constant
values, in our conditions of m and temperature were
found to be 214.2 and 215.5 for 30 and 70% v/v
dioxane±water, respectively. The latter value is in
accordance with that reported for Motekaitis et al. at
258C and m ˆ 0:10 M ,log Kw ˆ 216; Ref. [14]). The
hydrolysis constant values for the metal ions were
obtained from Ref. [15].
1
tography using different solvent systems and by H
NMR.
2.3. NMR measurements
2.5.1. Potentiometric determinations
A Metrohm 713 pH meter ®tted with glass and
1
Both H and 13C NMR spectra for 2,4,NO2)2PHPH

G.A. IbanÄez et al. / Journal of Molecular Structure 605:2002) 17±26
19
Scheme 1.
reference electrodes was used for the potentiometric
measurements. In order to read the hydrogen ion
concentration, the electrode was calibrated with
buffers at the same m to that employed in the equili-
brium constant determinations. Measurements were
made under nitrogen atmosphere in the appropriate
dioxane±water mixture. The initialconcentrations of
proton transfer in both PHPH and PHNH was
described [7]. With the purpose of investigating this
process in the ground state of both 2,4,NO2)2PHPH
and 2,4,NO2)2PHNH ,Scheme 1), we have studied
their prototropic properties and compared them with
those for the unsubstituted a-hydroxy hydrazones.
NMR is widely recognised as a highly diagnostic
toolfor the position of tautomeric equiilbria [18,19].
The 1H NMR spectra of 2,4,NO2)2PHPH and
2,4,NO2)2PHNH show the expected complex patterns
arising from the many aromatic protons. In the case of
2,4,NO2)2PHPH, a signal ascribed to the labile OH
proton was detected at 10.0 ppm. This was not
possible for 2,4,NO2)2PHNH, whose spectrum had
to be recorded in DMSO due to solubility restrictions.
As has been demonstrated [18,19], the 13C NMR
chemicalshift of the C,2)±O carbon is highyl sensi-
tive to the tautomeric structure, with extreme values
of ca. 155 ppm in form a and ca. 180 ppm in form b.
The values obtained for 2,4,NO2)2PHPH and
2,4,NO2)2PHNH, 160.6 and 162.9 ppm, respectively,
clearly indicate that form a is the dominating species
in both cases. These results are in agreement with
those reported for PHPH ,156.9 ppm) and PHNH
,156.2 ppm) [7].
ligand and metal were in the range 8.00 £ 1024
±
1.00 £ 1023 M. Each run was performed at least in
duplicate. The data processing for the computation of
the equilibrium constants and of the species distribution
diagram were performed using the best program [16].
2.5.2. Spectrophotometric determinations
The measurements were made using a Beckman
DU640 specrophotometer in 1.00 cm quartz micro-
cells. Conformity to Beer's law in the range of the
concentrations used was veri®ed prior to the determi-
nations. The initialconcentrations of ligand and metal
ion were in the range 1 £ 1025±1 £ 1024 M. A typical
spectrophotometric titration experiment was previously
described in the literature [6]. The data were converted
into deprotonation constants or equilibrium complex
constants with the use of the program star [17].
The
electronic
absorption
spectra
of
2,4,NO2)2PHPH and 2,4,NO2)2PHNH in both chloro-
form ,with maxima at 381 and 410 nm, respectively)
and in dioxane±water 70±30% v/v at acid pH ,with
maxima at 393 and 415 nm, respectively) indicate that
both compounds exist primarily in the enol a form
3. Results and discussion
3.1. Structural analysis of the protonated forms
In a previous work, the existence of ground state

20
G.A. IbanÄez et al. / Journal of Molecular Structure 605:2002) 17±26
Table 1
Relevant PM3 results for the protonated compounds ,bond lengths are in A)
Ê
Parameter
PHPH
a
PHNH
a
2,4,NO2)2PHPH
2,4,NO2)2PHNH
b
b
a
b
a
b
DHf0 ,kJ mol21
)
177.4
269.5
247.9
289.5
114.5
189.4
184.0
236.9
N±N±C±C±C±O portion
N±N
1.396
1.365
1.377
1.379
1.467
1.249
1.410
1.306
1.457
1.395
1.352
1.458
1.399
1.370
1.474
1.235
1.399
1.306
1.457
1.412
1.356
1.456
1.407
1.367
1.478
1.235
1.401
1.308
1.453
1.397
1.352
1.457
1.412
1.364
1.480
1.233
N±C,7,9)
C,7,9)±C,1)
C,1)±C,2)
C,2)±O
1.304
1.460
1.412
1.356
Aromatic ring bearing the oxygen
C,2)±C,3)
C,3)±C,4)
C,4)±C,5)
C,5)±C,6)
C,6)±C,1)
C,4)±C,4a)
C,4a)±C,8a)
C,8a)±C,1)
1.410
1.382
1.397
1.383
1.403
1.467
1.350
1.439
1.354
1.445
1.430
1.361
1.472
1.341
1.410
1.382
1.399
1.382
1.405
1.469
1.346
1.441
1.348
1.448
1.431
1.361
1.471
1.341
1.425
1.410
1.435
1.449
1.406
1.458
1.426
1.410
1.437
1.450
1.406
1.461
Adjacent aromatic ring
C,4a)±C,5)
1.418
1.370
1.411
1.370
1.421
1.402
1.384
1.395
1.384
1.404
1.418
1.371
1.410
1.371
1.420
1.401
1.385
1.395
1.385
1.403
C,5)±C,6)
C,6)±C,7)
C,7)±C,8)
C,8)±C,8a)
[20]. These results are in agreement with the NMR
data discussed above, and also with those found for
related unsubstituted hydrazones [5].
brium position. Obviously, the aromaticity transfer is
absent in 2,4,NO2)2PHPH, which therefore is forced
to exist in form a.
SemiempiricalMO calculations carried out with the
program PM3 ,Table 1) are also in agreement with the
spectroscopic ®ndings. For comparison, Table 1 lists
heats of formation and relevant bond distances for the
studied dinitro derivatives and also for the parent
compounds PHPH and PHNH. As can be observed,
the bond lengths of the corresponding nitro and
unsubstituted hydrazones are similar. The calculated
heats of formation suggest that the studied compounds
prefer the enolic form a. It can also be seen that C±C
bond lengths in the quinoid ring of the keto form b are
alternating between short and long values, suggesting
that this ring loses aromaticity in going from a to b.
Meanwhile, the adjacent ring in 2,4,NO2)2PHNH
becomes more aromatic as the proton is transferred
from the oxygen to the nitrogen. Although this aroma-
ticity transfer may favour form b, the results suggest
that this effect is not strong enough to shift the equili-
3.2. Deprotonation constants
Due to the low aqueous solubility of both
2,4,NO2)2PHPH and 2,4,NO2)2PHNH, their acidity
constants were spectrophotometrically evaluated in
70% v/v dioxane±water solutions. The pKa values
obtained at 208C and m ˆ 0:10 M were: 11.1 ^ 0.1
and 10.4 ^ 0.1 for 2,4,NO2)2PHPH and 2,4,NO2)2
PHNH, respectively. Fig. 1 shows the absorbance
spectra of 2,4,NO2)2PHPH at different pH values.
As shown in this picture, the maximum at 393 nm
detected at acid pH ,typicalof the enoilc structure
for the protonated compound) shifts towards a
longer wavelength ,470 nm) at high pH. The
presence of an isosbestic point at 425 nm indicates
an equilibrium between two absorbent species ,the
protonated and deprotonated ones). In the case of

G.A. IbanÄez et al. / Journal of Molecular Structure 605:2002) 17±26
21
Fig. 1. Absorbance spectra at different pH values of 2,4,NO2)2PHPH
in 70% v/v dioxane±water; initial C2;4ꢀNO † PHPH ˆ 2:84 £ 1025 M;
2
2
m ˆ 0:10 M ꢀNaNO3†; t ˆ 20:08C:
2,4,NO2)2PHNH, when the pH increases, a shift of
the absorption maximum ,from 415 to 490 nm) is
also observed, while the isosbestic point is
detected at 450 nm. The behaviour of both hydra-
zones with increasing pH indicates conversion of
the enolic hydroxy groups into the enolate ions,
which resemble a keto±hydrazonic anion having a
quinoid structure ,Scheme 2).
Scheme 2.
additionalstabiilty of form b is obtained from this
process ,Table 2).
In agreement with these results, PM3 geometry
optimisation of both anions yields structures with
signi®cant quinoid character ,see the bond lengths
for the N±N±C±C±O moiety quoted in Table 2).
Notice that each anion is in principle able to exist
in two different isomeric forms, identi®ed as A
and B in Table 2: they differ in the con®guration
of the double C,7,9)yN,1) bond. In A, N,2) is
trans with respect to C,7,9), whereas in B the
con®guration is cis. As is apparent in Tabl e 2,
the heats of formation show that B is considerably
more stable than A. This may conceivably due to
the strong hydrogen bond formation N,2)±H´´´O2
,see Scheme 2). As was the case with the
protonated compounds, the anionic form of
2,4,NO2)2PHNH is in principle able to suffer aroma-
ticity transfer, in contrast to the 2,4,NO2)2PHPH
anion. According to the C±C bond lengths of the
adjacent aromatic ring ,intermediate between single
and double bond values) it can be concluded that
3.3. Metal±a-hydroxy hydrazone complexes
Due to signi®cant solubility problems of the metal
systems formed by the naphtholic derivatives, only
the phenolic ones were analysed. Nevertheless, even
these studies required to be performed in dioxane±
water solutions. While the potentiometric technique
is considered as an accurate method for the obtain-
ment of reliable equilibrium constant values, spectro-
photometric measurements are a usefultoolwhen the
former cannot be employed, for instance when preci-
pitate is formed at the concentrations required for
potentiometric analysis. On the other hand, the spec-
trophotometric method is limited by the fact that asso-
ciation reactions are not favoured at low
concentrations of reagents. In the present work, both
methods were applied for the equilibrium constant
determinations, and the limitations found are
discussed in each case.

22
G.A. IbanÄez et al. / Journal of Molecular Structure 605:2002) 17±26
Table 2
Relevant PM3 results for the deprotonated compounds ,bond lengths are in A)
Ê
Parameter
PHPH
A
PHNH
A
2,4,NO2)2PHPH
2,4,NO2)2PHNH
B
B
A
B
A
B
DH0f ,kJ mol21
)
66.7
19.2
116.5
76.1
2 69.3
2 116.7
2 19.7
2 56.6
N±N±C±C±C±O portion
N±N
1.436
1.400
1.429
1.315
1.432
1.452
1.237
1.401
1.404
1.335
1.407
1.460
1.234
1.395
1.403
1.338
1.404
1.460
1.231
1.397
N±C,7,9)
C,7,9)±C,1)
C,1)±C,2)
C,2)±O
1.316
1.431
1.455
1.239
1.303
1.450
1.443
1.261
1.304
1.447
1.436
1.256
1.313
1.432
1.443
1.259
1.314
1.430
1.438
1.255
Aromatic ring bearing the oxygen
C,2)±C,3)
C,3)± C,4)
C,4)±C,5)
C,5)±C,6)
C,6)±C,1)
C,4)±C,4a)
C,4a)±C,8a)
C,8a)±C,1)
1.471
1.356
1.416
1.372
1.410
1.450
1.365
1.406
1.380
1.401
1.480
1.344
1.466
1.349
1.477
1.350
1.425
1.361
1.425
1.457
1.359
1.414
1.369
1.418
1.484
1.341
1.468
1.346
1.437
1.420
1.422
1.431
1.418
1.421
1.443
1.414
1.441
1.435
1.413
1.438
Adjacent aromatic ring
C,4a)±C,5)
1.403
1.379
1.403
1.373
1.424
1.408
1.375
1.407
1.370
1.426
1.402
1.381
1.398
1.378
1.416
1.408
1.376
1.403
1.374
1.421
C,5)±C,6)
C,6)±C,7)
C,7)±C,8)
C,8)±C,8a)
3.3.1. Metal±PHPH complexes
metaland the negative stoichiometric coef®cient
under the H represents the number of protons
displaced upon complexation ,in addition to the
ligand proton), which could arise from coordinated
water molecules. Although both 1:1 and 1:2 metal±
ligand stoichiometric ratios were evaluated, only 1:1
complexes were found. Fig. 2A shows the potentio-
metric pro®le for the Co,II)±PHPH system. A preci-
pitate was observed for about pH 7.3, yet in the
soluble region the [CoLH21] complex was detected
with a maximum concentration of 16% with respect
to the totalanaylticalmetalion concentration. The
obtained equilibrium constant value is shown in
Table 3. The presence of a solid phase in alkaline
medium prevented the evaluation of more deproto-
nated complexes. Since the concentrations of free
cobalt,II) ion and OH2 before solid detection were
insuf®cient to produce Co,OH)2, the solid phase was
We have previously studied the complex formation
between Cu,II) and PHPH [5]. The results showed
that this compound forms cupric chelates with
different degrees of deprotonation and presents a
metalcoordination with the oxygen, nitrogen N,1)
and hydroxylgroups. In the present work, the study
was extended to two additionaldivaelnt ions [Co,II)
and Ni,II)] and to trivalent cations [Fe,III) and
Al,III)]. However, in our working conditions, no
signi®cant complex formation was established with
the latter two metals.
The interaction between the studied ligands and a
divalent metal ion can be explained through the
following global equilibria:
M
21 1 LH O ‰MLH21Š 1 2H1
ꢀ1†
21 1 LH O ‰MLH22Š 1 3H1
ꢀ2†
attributed to the precipitation of the neutral[CoLH 21
complex. The Co,II)±PHPH system was also spectro-
photometrically investigated ,Fig. 3). In this case,
]
M
where LH is the protonated ligand, M21 is the studied

G.A. IbanÄez et al. / Journal of Molecular Structure 605:2002) 17±26
23
Table 3
Log of equilibrium constant values for metal±PHPH complexes,
t ˆ 20:08C; m ˆ 0:10 M ꢀNaNO3†; 30±70% v/v dioxane±water
,charges are omitted)
Quotient
Potentiometry
Spectrophotometry
Co,II)
[CoLH21][H]2/[Co][LH]
[CoLH22][H]3/[Co][LH]
2 11.77 ^ 0.02
sfit ˆ 0:018
2 11.6 ^ 0.1
2 21.7 ^ 0.1
sfit ˆ 0:013
Ni,II)
[NiLH21][H]2/[Ni][LH]
[NiLH22][H]3/[Ni][LH]
2 14.21 ^ 0.02
sfit ˆ 0:022
2 13.9 ^ 0.1
2 25.3 ^ 0.1
sfit ˆ 0:013
Cu,II)a
[CuLH21][H]2/[Cu][LH]
[CuLH22][H]3/[Cu][LH]
2 7.5
2 17.5
a
From Ref. [5].
The potentiometric behaviour of the Ni,II)±PHPH
system is similar to that for the Co,II) one ,Fig. 2),
with the [NiLH21] complex also detected in the
soluble region. Although the yellow±green precipitate
formed is ascribed to the neutral complex, the relevant
product of free concentrations [Ni21] £ [OH2]2 before
precipitation is close to the Ksp of Ni,OH)2, and thus
coprecipitation of this species cannot be discarded. In
the Ni,II)±PHPH spectrophotometric studies, no solid
Fig. 2. ,A) Potentiometric equilibrium curves in 30% dioxane±
water solvent for ,X) Co,II)±PHPH ꢀCCoꢀII† ˆ 9:60 £ 1024 M;
CPHPH ˆ 9:90 £ 1024 M† and ,W) Ni,II)±PHPH ꢀCNiꢀII† ˆ CPHPH
ˆ
1:06 £ 1023 M†: ,B) Potentiometric equilibrium curves in 70%
dioxane±water solvent for ,X) Cu,II)±2,4,NO2)2PHPH ꢀCCuꢀII†
ˆ
9:71 £ 1024 M; C2;4ꢀNO † PHPH ˆ 1:03 £ 1023 M† and ,W) Ni,II)±
2
2
2,4,NO2)2PHPH
ꢀCNiꢀII† ˆ 8:37 £ 1024 M;
C2;4ꢀNO2† PHPH
ˆ
2
9:38 £ 1024 M†; m 2 0:10 M ꢀNaNO3†; t ˆ 20:08C;
F
is the
moles of base per mole of ligand or metal. The solid lines corre-
sponding to calculated pH values and precipitate is indicated by
dashed lines.
since more diluted solutions were used, precipitation
was not detected and the computationalprocessing of
the data indicated the presence of both [CoLH21] and
[CoLH22]2 chelates. The more deprotonated complex
was detected above pH 8 and reached a concentration
of ca. 100% at pH . 10: The values of the formation
constants are displayed in Table 3 and, as can be
appreciated, the one corresponding to the neutral
species is similar to that found using the potentio-
metric method.
Fig. 3. Absorbance spectra at different pH values of the Co,II)±
PHPH system in 30% v/v dioxane±water; initial CCoꢀII† ˆ CPHPH
ˆ
2:04 £ 1025 M; m ˆ 0:10 M ꢀNaNO3†; t ˆ 20:08C: For clarity only
some spectra are illustrated.

24
G.A. IbanÄez et al. / Journal of Molecular Structure 605:2002) 17±26
Table 4
Heats of reaction ,DH0) for metal±PHPH and metal±
2,4,NO2)2PHPH complexes ,I is the arrangement with six-
membered ring, II is the arrangement with seven-membered ring;
values in kJ mol21
)
PHPH
I
2,4,NO2)2PHPH
I
II
2 177.6a
2 376.8b
Co,II)
Ni,II)
2 230.5a
2 400.5b
2 273.4
2 89.4
2 232.8
2 51.3
Cu,II)
a
High spin.
b
Low spin.
involving the proposed arrangements, semiempirical
calculations using PM3 were performed. In all cases,
the heat of reaction corresponding to the following
transformation was computed:
2
‰MLH22
Š
ꢀtetracoordinated† 1 2H2O
Scheme 3.
2
O ‰MLH22
Š
ꢀhexacoordinated†
ꢀ3†
The results are collected in Table 4, including both
A and B isomeric forms of the ligand. As can bee seen,
despite the higher steric compression introduced by
the additional water molecules, the hexacoordinated
species ,arrangement I) are more stable. Also, among
the Co,II) complexes, those of low spin are favoured.
formation was observed, and the results were
explained on the basis of the presence of both
[NiLH21] and [NiLH22]2 complexes. The equilibrium
constant values obtained for these complexes ,Table
3) can be compared with those of Cu,II) and Co,II)
systems. The fact that the equilibrium constants of the
Cu,II) complexes supports larger the conclusion that
this metalion interacts strongyl with the type of
studied donor groups. A similar conclusion was
obtained with related azocompound systems [21].
As was indicated in a previous report [5], both A
and B anionic forms of a-hydroxy hydrazones may
coordinate the metalion. In the ®rst case, a six-
membered ring complex can be formed with a coor-
dination involving the oxygen and the nitrogen N,1)
,arrangement type I). In the second case, the metal
coordination may involve the oxygen atom and the
lone pair electrons of N,2), after proper rotation
about the N,1)±N,2) single bond ,arrangement type
II). Scheme 3 illustrates both type of arrangements. In
both cases, either tetra or hexacoordinated geometries
can be proposed, with the other binding positions
ful®lled with hydroxyl groups and water molecules.
For comparing the relative stabilities of the chelates
3.3.2. Metal±2,4:NO2)2PHPH complexes
Because the metalsystems formed by this ilgand
resulted in low solubilities, both potentiometric and
spectrophotometric evaluations were performed in
mixtures of 70% v/v dioxane±water.
The potentiometric equilibrium pro®le of pH vs F
,F: mol of base added per mol of ligand or metal) for
the 1:1 Cu,II)±2,4,NO2)2PHPH system is shown in
Fig. 2B. Above pH , 6:5; a ®ne precipitate is
observed, which then dissolves at high pH. Neverthe-
less, the data corresponding to pH . 6:5 were not
employed in equilibrium computations. The curve
was satisfactorily ®tted by considering both
[CuLH21] and [CuLH22]2 species ,the latter are
present with concentrations higher than 40% before
precipitations occurs). The appearance of a solid
phase is ascribed to the neutral[CuLH 21] complex

G.A. IbanÄez et al. / Journal of Molecular Structure 605:2002) 17±26
25
Table 5
the corresponding constant values could not be
spectrophotometrically determined for similar
reasons.
Log of equilibrium constant values for metal±2,4,NO2)2PHPH
complexes, t ˆ 20:08C; m ˆ 0:10 M ꢀNaNO3†; 70±30% v/v
dioxane±water ,charges are omitted)
Through the equilibrium constant values shown in
Table 5, it can be seen that the cupric complexes are
more stable than the analogous Co,II) and Ni,II) ones.
In order to optimise the geometries of these
complexes and to calculate the heats of formation of
the structures, semiempiricalcaclualtions using PM3
were performed ,Table 4). The obtained results indi-
cate that while hexacoordinated structures with six-
membered rings are more stable for both Cu,II) and
Ni,II) complexes, the hexacoordinated structure with
a seven-membered ring is favoured for the Co,II)
complex. Also, it can be observed that among the
Co,II) complexes, those of low spin are more
stable.
Quotient
Potentiometry
Spectrophotometry
Co,II)
[CoLH21][H]2/[Co][LH]
[CoLH22][H]3/[Co][LH]
2 12.7 ^ 0.1
2 22.8 ^ 0.1
sfit ˆ 0:011
Ni,II)
[NiLH21][H]2/[Ni][LH]
[NiLH22][H]3/[Ni][LH]
2 13.0 ^ 0.1
2 21.3 ^ 0.1
sfit ˆ 0:037
Cu,II)
[CuLH21][H]2/[Cu][LH]
[CuLH22][H]3/[Cu][LH]
2 8.68 ^ 0.05
2 15.1 ^ 0.1
sfit ˆ 0:032
precipitation, while the dissolution of this solid is
attributed to increasing concentrations of the anionic
[CuLH22]2 complex. The stoichiometries of the
species found ,Table 5) are similar to those for the
PHPH systems. However, since the values were
obtained in different solvent media, they cannot be
strictly compared. The spectrophotometric curves at
different values of pH for the 1:1 Cu,II)±
2,4,NO2)2PHPH system exhibit a similar behaviour
to that of the ligand alone. This can be ascribed either
to low concentrations of the complexes in the experi-
mentalconditions or to the similarity of the extinction
coef®cients for the complexes and the deprotonated
ligand. These facts do not allow the spectrophotome-
tricaldetermination of the equiiblrium constants
without incurring in large errors in the calculated
values.
Due to the insolubility of the Co,II)±
2,4,NO2)2PHPH system in almost all of the potentio-
metric curve, the complexes formed could not be
studied by this method. However, from spectrophoto-
metric titrations, the species [CoLH21] and
[CoLH22]2 were found and their constant values are
shown in Table 5.
Acknowledgements
Financialsupport from the University of Rosario,
Â
CONICET, Fundacion Antorchas and Agencia
Â
Â
Â
Nacionalde Promocio n Cientõ®ca y Tecnologica
,Project PICT99 No 06-06078) is gratefully acknowl-
edged.
References
[1] J.K. Sears, J.R. Darby, The Technology of Plasticizers, Wiley,
New York, 1982.
[2] E. Massarani, D. Nardi, A. Tajana, L. Degen, J. Med. Chem.
14 ,1971) 633.
[3] J.D. Ranford, J.J. Vittal, Y.M. Wang, Inorg. Chem. 37 ,1998)
1226.
[4] J.A. Arancibia, A.C. Olivieri, G.M. Escandar, J. Mol. Struct.
522 ,2000) 233.
[5] G.N. Ledesma, M. Gonzalez Sierra, G.M. Escandar, Polyhe-
dron 17 ,1998) 1517.
[6] G.A. IbanÄez, A.C. Olivieri, G.M. Escandar, J. Chem. Soc.
Faraday Trans. 93 ,1997) 545.
[7] G.N. Ledesma, G.A. IbanÄez, G.M. Escandar, A.C. Olivieri, J.
Mol. Struct. 415 ,1997) 115.
Â
[8] G.A. IbanÄez, A.C. Olivieri, G.M. Escandar, J. Mol. Struct. 435
Finally, in the soluble region of the Ni,II)±
2,4,NO2)2PHPH potentiometric curve ,Fig. 2B), the
[NiLH21] and [NiLH22]2 species were found. The
electronic spectral patterns at different pH values of
the Ni,II)±2,4,NO2)2PHPH system are similar to
those found in the cupric system, and therefore
,1997) 199.
[9] G. Schwanzerbach, Complexometric Titrations, Interscience,
New York, 1960.
[10] F.J. Rossotti, H.J. Rossotti, J. Chem. Ed. 42 ,1965) 375.
[11] A.I. Vogel, Practical Organic Chemistry, Longmans, London,
1961 ,p. 722).
[12] J.J. Steward, J. Comput. Chem. 12 ,1991) 320.

26
G.A. IbanÄez et al. / Journal of Molecular Structure 605:2002) 17±26
Â
[18] S.H. Alarcon, A.C. Olivieri, R.M. Cravero, G. Labadie, M.
Â
Gonzalez-Sierra, Tetrahedron 51 ,1995) 4619.
[13] L.G. Van Uiter, C.G. Hass, J. Am. Chem. Soc. 75 ,1953) 451.
[14] R.J. Motekaitis, A.E. Martell, Inorg. Chem. 27 ,1988) 2718.
[15] C.F. Baes Jr., R.E. Mesmer, The Hydrolysis of Cations, R.E.
Krieger, Malabar, FL, 1986.
Â
[19] S.H. Alarcon, A.C. Olivieri, R.M. Cravero, G. Labadie, J.
Phys. Org. Chem. 8 ,1995) 713.
 Â
[20] S.H. Alarcon, M. Gonzalez-Sierra, A.C. Olivieri, J. Chem.
[16] A.E. Martell, R.J. Motekaitis, The Determination and Use of
Stability Constants, 2nd ed., VHC, New York, 1992 ,p. 143).
Â
[17] J.L. Beltran, R. Codony, M.D. Prat, Anal. Chim. Acta 276
Soc. Perkin Trans. 2 ,1994) 1067.
[21] G.A. IbanÄez, G.M. Escandar, Polyhedron 17 ,1998) 4433.
,1993) 441.