Structural and thermal properties of three cyano-substituted azoderivatives of β-diketones

Article abstract of DOI:10.1016/j.molstruc.2011.02.045

3-(2-Cyanophenylhydrazo)pentane-2,4-dione (HL¹), 3-(4-cyanophenylhydrazo)pentane-2,4-dione (HL²) and 1-ethoxy-2-(4-cyanophenylhydrazo)butane-1,3-dione (HL³), and the Pd(II) complex [Pd(L²)₂] were synt

Full text of DOI:10.1016/j.molstruc.2011.02.045

Journal of Molecular Structure 992 (2011) 72–76
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Structural and thermal properties of three cyano-substituted azoderivatives
of b-diketones
Kamran T. Mahmudov a, Maximilian N. Kopylovich a, Konstantin V. Luzyanin a, Archana Mizar a,
M. Fátima C. Guedes da Silva a,b, Vânia André a, Armando J.L. Pombeiro a,
⇑
a Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, TU Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
b Universidade Lusófona de Humanidades e Tecnologias, ULHT Lisbon, Av. Campo Grande No. 376, 1749-024 Lisbon, Portugal
a r t i c l e i n f o
a b s t r a c t
3-(2-Cyanophenylhydrazo)pentane-2,4-dione (HL1), 3-(4-cyanophenylhydrazo)pentane-2,4-dione (HL2)
and 1-ethoxy-2-(4-cyanophenylhydrazo)butane-1,3-dione (HL3), and the Pd(II) complex [Pd(L2)2] were
synthesized and characterized by IR, 1H and 13C NMR spectroscopies, ESI-MS, elemental and X-ray diffrac-
tion analyses (for HL1). HL3 derived from the unsymmetric 1-ethoxybutane-1,3-dione exists in solution as
a mixture of the enol-azo and hydrazo tautomeric forms and a decrease of the solvent polarity shifts the
tautomeric balance to the hydrazo form, while for HL1 and HL2, derived from the symmetric pentane-2,4-
dione, the hydrazo form is dominating under all the studied conditions. HL1–3 and [Pd(L2)2] show a high
thermal stability with well-defined peaks of phase transition at 438 (HL1), 462 (HL2), 392 (HL3) and 404 K
([Pd(L2)2]).
Article history:
Received 31 January 2011
Accepted 22 February 2011
Available online 25 February 2011
Keywords:
Azoderivatives of b-diketones
Pd(II) complex
Tautomeric balance
Thermal properties
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
On the other hand, the cyano group has a well-recognized
chemical versatility [19]. In particular, in organic chemistry the
It is well known that many azocompounds and b-diketones, un-
der certain conditions, tend to perform tautomeric transforma-
tions. Thus, diazocompounds can exist as a mixture of azo- and
hydrazone tautomeric forms [1], while b-diketones easily switch
to the enol form [2,3]. These tautomeric equilibria are important
from theoretical and practical aspects: about 92% of azo dyes pub-
lished in Colour Index possess this tautomery [4] which affects
their tone, photostability, etc. On the other hand, tautomeric equi-
libria of b-diketones also influence their spectral properties, reac-
tivity, coordination ability, etc. [5,6]. However, there is a lack of
experimental data on the tautomerism of the pigments combining
both of the those functionalities, i.e. azoderivatives of b-diketones
(ADB).
addition of nucleophiles or electrophiles and the asymmetric dipo-
lar cycloaddition to the C„N triple bond offer attractive routes for
the creation of CAC, CAN, CAO and CAS bonds. Therefore, cyano-
substituted ADB are expected to be good candidates towards the
development of a rich organic chemistry.
Thus, taking in mind all the above mentioned considerations,
the main aims of the current work are as follows: (i) to synthesize
new ADB with cyano substituents in the aromatic part of the mol-
ecule (Scheme 1) and prove their coordination ability by synthesiz-
ing a new Pd(II)-ADB complex; (ii) to study the influence of the
introduced substituents on the tautomeric balance and thermal
properties of the synthesized materials.
Moreover, this tautomeric balance can play an important role
for the application of ADB as bistate molecular switches [8–10]
or regulation of the ionophore selectivity in analytical chemistry
[11,12]. Within this view, special attention should be paid to the
nature of the strong intramolecular Oꢀ ꢀ ꢀHAN hydrogen bond and
its influence on the enol-azo ꢀ hydrazo transformation [7]. Addi-
tionally, the synthetic potential of ADB for coordination chemistry
was underestimated, notwithstanding their potentially rich chelat-
ing ability [7,13–18].
2. Experimental
2.1. Materials and instrumentation
All the chemicals were obtained from commercial sources (Al-
drich) and used as received. Infrared spectra (4000–400 cmꢁ1
)
were recorded on a BIO-RAD FTS 3000MX instrument in KBr pel-
lets. 1D (1H, 13C{1H}) and 2D (1H, 1H-COSY, 1H, 13C-HMQC, 1H,
13C-HSQC and 1H, 13C-HMBC) NMR spectra were recorded on
Bruker Avance II + 300 and 400 MHz (UltraShield™ Magnet)
spectrometers at ambient temperature. Chemical shifts (d) are
relative to internal TMS. Carbon, hydrogen, and nitrogen elemental
⇑
Corresponding author. Fax: +351 21 846 4455.
E-mail address: pombeiro@ist.utl.pt (A.J.L. Pombeiro).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.02.045

K.T. Mahmudov et al. / Journal of Molecular Structure 992 (2011) 72–76
73
Z
Z
...
O
C
H..
.
O
C
H
N
X
HL1 = CH3
HL2 = CH3
HL3
Y
H
CN
Z
CN
H
N
Y
N
Y
H3C
O
C
C
H3C
O
C
C
N
= OC2H5 CN
H
enol-azo
hydrazo
X
X
Scheme 1. ADB studied in this work.
analyses were carried out by the Microanalytical Service of the
Instituto Superior Técnico. Electrospray mass spectra were run
with an ion-trap instrument (Varian 500-MS LC Ion Trap Mass
Spectrometer) equipped with an electrospray (ESI) ion source.
For electrospray ionization, the drying gas and flow rate were opti-
mized according to the particular sample with 35 p.s.i. nebulizer
pressure. Scanning was performed from m/z 100 to 1200 in meth-
anol solution. The compounds were observed in the positive mode
(capillary voltage = 80–105 V). Thermal properties were analyzed
with a Perkin–Elmer Instruction system (STA6000) at a heating
rate of 10 K minꢁ1 under a dinitrogen atmosphere.
(ppm): 2.42 (s, 3H, free CH3CO), 2.47 (s, 3H, CH3CO in H-bond),
7.68–7.86 (4H, Ar–H), 13.39 (s, 1H, NAH). 13C–{1H} NMR in
DMSO-d6, d (ppm): 26.24 (CH3), 31.23 (CH3), 106.11 (CN), 116.42
(2Ar–H), 118.95 (Ar–CN), 133.82 (2Ar–H), 135.76 (C@N), 145.83
(Ar–NH–N), 196.44 (C@O), 197.57 (C@O). 1H NMR in CDCl3, d
(ppm): 2.50 (s, 3H, free CH3CO), 2.61 (s, 3H, CH3CO in H-bond),
7.50–7.70 (4H, Ar–H), 14.49 (s, 1H, NAH). 13C–{1H} NMR in CDCl3,
d (ppm): 26.76 (CH3), 31.94 (CH3), 108.45 (CN), 116.41 (2Ar–H),
118.70 (Ar–CN), 134.00 (2Ar–H), 134.73 (C@N), 145.11 (Ar–NH–
N), 196.93 (C@O), 198.76 (C@O).
HL3: yield, 70% (based on 1-ethoxybutane-1,3-dione), yellow
powder, soluble in methanol, ethanol, acetone, dichloromethane,
chloroform, tetrahydrofuran and insoluble in water. Elemental
analysis: C13H13N3O3 (M = 259); C 60.58 (calc. 60.22); H 5.10
(5.05); N 16.25 (16.21)%. IR (KBr): 3448 (NH), 2220 (C„N), 1690
(C@O), 1660 (C@Oꢀ ꢀ ꢀH), 1608 (C@N) cmꢁ1. ESI-MS: m/z: 260
[M+H]+. 1H NMR of a mixture of tautomeric hydrazone and enol-
azo forms in DMSO-d6, d (ppm). Hydrazo, 1.36–1.42 (s, 3H,
CH3CH2O), 2.59 (s, 3H, CH3CO in H-bond), 4.33–4.45 (s, 2H, CH2),
7.61–7.89 (4H, Ar–H), 13.82 (s, 1H, NH). Enol-azo, 1.36–1.42 (s,
3H, CH3CH2O), 2.60 (s, 3H, CH3COH in H-bond), 4.33–4.45 (s, 2H,
CH2), 7.61–7.89 (4H, Ar–H), 11.67 (s, 1H, HO-enol). 13C{1H} NMR
in DMSO-d6, d (ppm): Hydrazo, 13.83 (CH3CH2O), 30.53 (CH3CO),
60.80 (CH2), 104.72 (CN), 114.98 (2Ar–H), 116.32 (Ar–CN), 119.14
(2Ar–H), 133.78 (C@N), 145.75 (Ar–NH–N), 162.18 (C@O), 193.89
(C@O). Enol-azo, 14.10 (CH3CH2O), 30.66 (CH3CO), 61.48 (CH2),
106.25 (CN), 115.32 (2Ar–H), 118.91 (Ar–CN), 128.31 (2Ar–H),
134.18 (C@N), 146.30 (Ar–NH–N), 163.67 (CAO), 196.26 (C@O).
1H NMR of a mixture of tautomeric hydrazone and enol-azo forms
in CDCl3, d (ppm). Hydrazo, 1.34–1.40 (s, 3H, CH3CH2O), 2.47 (s, 3H,
CH3CO in H-bond), 4.29–4.39 (s, 2H, CH2), 7.35–7.65 (4H, Ar–H),
14.46 (s, 1H, NH). Enol-azo, 1.34–1.40 (s, 3H, CH3CH2O), 2.57 (s,
3H, CH3COH in H-bond), 4.29–4.39 (s, 2H, CH2), 7.35–7.65 (4H,
Ar–H), 12.59 (s, 1H, HO-enol). 13C{1H} NMR in CDCl3, d (ppm): Hy-
drazo, 14.07 (CH3CH2O), 26.91 (CH3CO), 61.43 (CH2), 107.33 (CN),
115.61 (2Ar–H), 118.78 (Ar–CN), 129.75 (2Ar–H), 133.89 (C@N),
145.06 (Ar–NH–N), 163.24 (C@O), 194.12 (C@O). Enol-azo, 14.31
(CH3CH2O), 30.89 (CH3CO), 61.96 (CH2), 108.07 (CN), 116.41
(2Ar–H), 128.07 (Ar–CN), 133.76 (2Ar–H), 133.89 (C@N), 145.22
(Ar–NH–N), 164.33 (CAO), 197.57 (C@O).
2.2. Synthesis of HL1–3
HL1–3 were synthesized according to the Japp–Klingemann reac-
tion [20–22] between the diazonium salts of 2- or 4-cyanoaniline
and pentane-2,4-dione or 1-ethoxybutane-1,3-dione.
2.2.1. Diazotization
0.0250 mol of 2- or 4-cyanoaniline were dissolved in 50.00 mL
of water upon addition of 1.000 g of crystalline NaOH. The solution
was cooled in an ice bath to 0 °C, and 0.025 mol of NaNO2 were
added with subsequent addition of 5.00 mL HCl in portions of
0.20 mL for 1 h, under vigorous stirring. During the reaction the
temperature of the mixture must not exceed +5 °C.
2.2.2. Azocoupling
1.000 g of NaOH was added to a mixture of 0.0250 mol of
pentane-2,4-dione or 1-ethoxybutane-1,3-dione with 50.00 mL of
water. The solution was cooled in an ice bath, and a suspension
of 2- or 4-cyanoaniline diazonium (prepared according to the pro-
cedure of Section 2.2.1) was added in two equal portions, under
vigorous stirring for 1 h. On the next day, the formed precipitate
of HL1–3 was filtered off, washed with water, recrystallized from
ethanol and dried in air. The characterization of HL1–3 was under-
taken by elemental analysis, ESI-MS, IR, 1H and 13C NMR spectros-
copies and, in the case of HL1, also by single-crystal X-ray analysis.
HL1: yield 81% (based on pentane-2,4-dione), yellow powder
soluble in DMSO, methanol, ethanol, chloroform and acetone, and
insoluble in water. Elemental analysis: C12H11N3O2 (M = 229); C
62.72 (calc. 62.87); H 4.85 (4.84); N 18.76 (18.33)%. IR (KBr):
3437 (NH), 2220 (C„N), 1677 (C@O), 1639 (C@Oꢀ ꢀ ꢀH), 1601
(C@N) cmꢁ1. ESI-MS: m/z: 230 [M + H]+. 1H NMR in DMSO-d6, d
(ppm): 2.43 (s, 3H, free CH3CO), 2.50 (s, 3H, CH3CO in H-bond,
overlapping with DMSO-d6), 7.28–7.85 (4H, Ar–H), 14.35 (s, 1H,
NAH). 13C–{1H} NMR in DMSO-d6, d (ppm): 26.60 (CH3), 31.31
(CH3), 99.01 (CN), 116.00 (Ar–CN), 116.13 (Ar–H), 125.08 (Ar–H),
133.48 (Ar–H), 134.98 (Ar–H), 135.19 (C@N), 143.94 (Ar–NH–N),
196.48 (C@O), 197.97 (C@O).
2.3. Synthesis of [Pd(L2)2]
229 mg of HL2 were dissolved in 50.00 mL of chloroform and
112 mg of Pd(CH3COO)2 were added, with stirring. The mixture
was refluxed for 5 h and left standing for slow solvent evaporation.
Orange microcrystals started to form in the reaction mixture after
1 d at room temperature, whereafter they were filtered off and
dried in air. Yield, 47% (based on Pd). Elemental analysis:
HL2: yield 92% (based on pentane-2,4-dione), yellow powder
soluble in DMSO, methanol, ethanol, chloroform and acetone, and
insoluble in water. Elemental analysis: C12H11N3O2 (M = 229); C
62.63 (calc. 62.87); H 4.89 (4.84); N 18.65 (18.33)%. IR (KBr):
3447 (NH), 2220 (C„N), 1672 (C@O), 1637 (C@Oꢀ ꢀ ꢀH), 1607
(C@N) cmꢁ1. ESI-MS: m/z: 230 [M + H]+. 1H NMR in DMSO-d6, d
C24H20N6O4Pd (M = 562.87); C 51.10 (calc. 51.21); H 3.45 (3.58);
N 14.86 (14.93)%. IR (KBr): 2229 (C„N), 1670 (C@O), 1638
(C@O), 1608 (C@N) cmꢁ1. ESI-MS: m/z: 596 [M + CH3OH]. 1H
NMR in CDCl3, d (ppm): 2.28 (s, 6H, CH3CO), 2.42 (s, 6H, CH3CO),
7.59–7.73 (8H, Ar–H). 13C–{1H} NMR in CDCl3, d (ppm): 28.66
(2CH3), 29.85 (2CH3), 111.31 (2CN), 118.80 (2Ar–CN), 124.92

74
K.T. Mahmudov et al. / Journal of Molecular Structure 992 (2011) 72–76
(2Ar–H), 132.48 (2Ar–H), 135.45 (2C@N), 152.84 (2Ar–NPd–N),
176.50 (2C@O ? Pd), 197.05 (2C@O).
2.4. X-ray structure determination
Crystals of HL1 suitable for X-ray structural analysis were grown
by slow evaporation at room temperature of its chloroform solution.
The data were collected using a Bruker AXS-KAPPA APEX II diffrac-
tometer with graphite-monochromated Mo Ka radiation. Data were
collected at 150 K using omega scans of 0.5° per frame, and a full
sphere of data was obtained. Cell parameters were retrieved using
Bruker SMART software and refined using Bruker SAINT [23] on all
the observed reflections. Absorption corrections were applied using
SADABS [23]. Structures were solved by direct methods using the
SHELXS-97 package [24] and refined with SHELXL-97 [24] with
the WinGX System-Version 1.80.03. [25] The crystallographic
details are listed in Table 1. Crystallographic data for the structure
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication, CCDC
(805136) HL1. Copies of this information may be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
3. Results and discussion
3.1. Spectroscopic and crystallographic investigation of HL1–3
The new cyano-substituted azoderivatives of b-diketones HL1–3
were synthesized via the Japp–Klingemann reaction [20–22] be-
tween the respective cyano-substituted aromatic diazonium salts
and pentane-2,4-dione or 1-ethoxybutane-1,3-dione in water solu-
tion containing sodium hydroxide. IR spectra of the isolated com-
Fig. 1. 1H,13C-HMQC/1H NMR spectra in DMSO-d6 (20 °C) (a), and determination of
(NH) vibrations at 3437–3448 cmꢁ1
(C@N) and (C„N) are observed at 1672–
, while
tautomeric ratio of HL3 in CDCl3 by 1H NMR (b).
pounds show
m
m(C@O),
m(C@Oꢀ ꢀ ꢀH),
m
m
1690, 1637–1660, 1601–1608 and 2220 cmꢁ1, correspondingly,
what is related to the presence of the H-bonded hydrazone struc-
ture in the solid state (as confirmed by an X-ray diffraction analysis
– see below). This conclusion is supported by 1H and 13C{1H} NMR
data, e.g. the 1H NMR spectra of HL1–3 show a signal at d 13.4–14.5
which can be assigned to the proton of the NH moiety adjacent to
the aryl unit (@NANHA hydrazo, Scheme 1) [7–18].
In addition, the enol-azo form was also detected in solution for
HL3. In fact, its 1H NMR spectra in DMSO-d6, or CDCl3 show the
presence of a mixture of two tautomeric forms. In order to assign
the spectra and determine their ratio, two-dimensional 1H, 1H-
COSY, 1H, 13C-HMQC/1H, 13C-HSQC, 1H, 13C-HMBC and 1H NMR
experiments in both DMSO-d6 and CDCl3 solvents were performed.
The short-range correlations (1H, 1H-COSY and 1H, 13C-HMQC/1H,
13C-HSQC) (Fig. 1a) were useful to assign the signals of the ester
groups of HL3, while the long-range shift correlation experiments
Table 1
Crystallographic data and structure refinement
details for HL1.
Empirical formula
fw
C12H11N3O2
229.24
k (Å)
0.71069
Monoclinic
P21 (No. 4)
8.3358(7)
6.6477(6)
10.2664(9)
92.893(2)
568.18(9)
2
Cryst syst
Space group
a (Å)
b (Å)
c (Å)
b (deg)
V (Å3)
Z
qcalc (Mg/m3)
1.340
0.095
l
(Mo K
a
) (mmꢁ1
)
No. reflns. read
No. reflns. unique
No. reflns. obs.
GOOF
2866
1356
1220
1.080
Rint
0.0181
0.0325
0.0950
R1a (I ꢂ 2
r
)
r
Fig. 2. Thermal ellipsoid plot, drawn at the 50% probability level, with atomic
numbering scheme for HL1. Selected bond lengths (Å) and angles (°): O1AC12
1.212(3); O2AC14 1.231(3); N1AN2 1.311(3); N1AC2 1.394(3); N2AC13 1.309(3);
C1AC10AN10 178.3(3); C2AN1AN2 119.36(16); N1AN2AC13 121.37(17);
O2AC14AC13 119.1(2); C13AC12AO1 121.74(19). Intramolecular hydrogen bond
(dotted line), DAHꢀ ꢀ ꢀA [d(Dꢀ ꢀ ꢀA) Å; \(DHA)°]: N1AH1ꢀ ꢀ ꢀO2 [2.567(2) Å, 132.00°].
wR2b (I ꢂ 2
)
P
P
a
R1 = ||Fo| – |Fc||/ |Fo|.
b
ꢀ
ꢀ
ꢃꢄ
ꢀ
ꢃꢃ
1=2
ꢁ
ꢂ
ꢁ
ꢂ
2
2
P
P
wR2 ¼
w
F2o ꢁ F2c
w
Fo2
.

K.T. Mahmudov et al. / Journal of Molecular Structure 992 (2011) 72–76
75
Fig. 3. Thermal decomposition of HL2 (a) and [Pd(L2)2] (b).
Table 2
Parameters of thermal decomposition of HL1–3 and [Pd(L2)2].
Compound Temperature of phase
transition (K)
Temperature of decomposition
(maximum) (K)
Weight
loss, %
A (sꢁ1
)
Ea
D
àS
D
àH
D
àG
(kJ molꢁ1
)
(J Kꢁ1 molꢁ1
)
(kJ molꢁ1
)
(kJ molꢁ1
)
HL1
438
462
392
404
438–601 (525)
462–612 (513)
418–601 (488)
455–493 (484)
498–540 (506)
39.02
50.18
64.07
9.72
28.4
8.3
49.3
47.3
40.8
48.2
ꢁ229
ꢁ248
ꢁ229
ꢁ146
ꢁ238
42.9
36.6
45.3
75.2
32.4
163
164
157
146
153
HL2
HL3
[Pd(L2)2]
2.4 ꢃ 105 79.2
18.00
3.7
36.6
via 2JH,C and 3JH,C coupling (1H, 13C-HMBC) allowed the discrimina-
tion of the carbon signals of the carbonyl groups from the imine
moieties for each tautomer. These data indicate that a decrease
of the solvent polarity shifts the tautomeric balance towards the
hydrazo form (Fig. 1b and Scheme 1). Thus, the equilibrium be-
tween the enol-azo and hydrazo forms depends on the polarity
of the solvent used (enol-azo 89%, hydrazo 11% in DMSO-d6;
enol-azo 39%, hydrazo 61% in CDCl3). It is worth mentioning that
the obtained data may be useful for some analytical applications
[11,12] which require the knowledge of the distribution of the tau-
tomers in a particular solvent.
methyl protons are shifted to 2.28 and 2.42 ppm (compare with
values for free HL1 – 2.42 and 2.47 ppm, respectively). In the
13C–{1H} NMR spectrum, the resonances of the coordinated C@O
groups (176.5 ppm) and those of the uncoordinated ones
(197.0 ppm) are also shifted in comparison to those of free HL1
(196.9 and 198.8 ppm, correspondingly). Elemental analysis and
ESI-MS in methanol (peak at m/z 596 [M + CH3OH]+) support the
proposed formulation as a diligand monomer, [Pd(L2)2], as previ-
ously reported [27] for other ADB-Pd(II) complexes. Unfortunately,
all the attempts to get crystals suitable for X-ray analysis have
failed.
The single crystal X-ray diffraction study of HL1 (Fig. 2) indi-
cates that the compound crystallizes in the hydrazo form
(Scheme 1). The C13AN2 distance has a typical double bond length
[1.309(3) Å]. The two carbonyl bond distances are slightly different
[1.231(3) and 1.212(3) Å], the longer one concerning the intramo-
lecular hydrogen bonding involving its oxygen atom (see Fig. 2 and
its footnote), where the donor(N)ꢀ ꢀ ꢀacceptor(O) distance of
2.567(2) Å falls within the range (2.50–2.62 Å) observed for other
studied ADBs [7,8,14,22,26].
Thus, based on IR, NMR and X-ray structural analyses, we can
conclude that the studied cyano-substituted ADB derived from
symmetric b-diketones exist in solution and in solid phase mainly
in the hydrazo form, while that derived from the unsymmetric b-
diketone is present in solution as a mixture of hydrazo and enol-
azo forms.
3.3. Thermal behaviour of HL1–3 and [Pd(L2)2]
In order to get more information about their thermal stability,
HL1–3 and [Pd(L2)2] were studied by thermogravimetric (DTA–TG)
techniques in the 303–1073 K temperature range, under dinitrogen
atmosphere. The thermodynamic parameters of decomposition,
namely the activation energy (Ea), enthalpy (
D D
àH), entropy ( àS)
and free energy of decomposition (
D
àG), as well as the pre-expo-
nential factor (A), were evaluated graphically using the Coats–
Redfern relationship [28–30].
The thermograms of HL1–3 and [Pd(L2)2] exhibit a single peak of
phase transition at 438 (HL1), 462 (HL2), 392 (HL3) and 404 K
([Pd(L2)2]) (Fig. 3). TG and DTG curves exhibit a sharp mass loss,
obviously related to decomposition of the compounds. The kinetic
parameters of this process were evaluated (Table 2) showing that
activation energies of decomposition (and hence the rate of
decomposition) follow the sequence: HL3 > HL1 > HL2. The negative
entropy of activation suggests that decomposition is slow [30]. The
values of the pre-exponential factor in the Arrhenius equation (A)
are within the 3.7–2.4 ꢃ 105 sꢁ1 range, thus not allowing to estab-
lish the molecularity of the reaction [26–28]. The sharp thermal
decomposition threshold and the high mass loss rate of HL1–3 make
them good candidates for optical recording media [31,32] or opti-
cal sensors [33], although experiments with polarized light
3.2. Synthesis and characterization of [Pd(L2)2]
The palladium(II) complex [Pd(L2)2] was prepared in 47% yield
upon refluxing, for 5 h, a chloroform solution of HL2 and palla-
dium(II) acetate. The IR spectrum of this complex displays
m
(C„N),
m
(C@O) at 2229, 1670 and 1638 cmꢁ1, values shifted in
relation to those (2220, 1672 and 1637 cmꢁ1) of the free ligand.
m
(NH) was not observed in the complex, confirming the deprotona-
tion of the hydrazone group. Moreover, in the 1H NMR spectrum of
[Pd(L2)2], NAH protons were also not detected, while the peaks of

76
K.T. Mahmudov et al. / Journal of Molecular Structure 992 (2011) 72–76
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4. Conclusions
Three new cyano-substituted ADB, HL1–3, were synthesized and
characterized by IR and multi-nuclear NMR spectroscopies, ESI-MS,
elemental and X-ray diffraction analysis (for HL1). The derivative of
the unsymmetric b-diketone, HL3, exists in solution as a mixture of
the enol-azo and hydrazo tautomeric forms, and a decrease of the
solvent polarity shifts the tautomeric balance to the hydrazo form.
The formation of the heterodienic system, HNAN@CAC@O, and the
fairly weak heteronuclear resonance-assisted hydrogen bond,
NAHꢀ ꢀ ꢀO, is supported by the analytical data. These structural
properties are important for the design of functional materials re-
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structural switching [35]. On the other hand, the thermal stability,
high weight loss rate and sharp thermal decomposition threshold
of the studied ADB make them good potential candidates for
high-density optical recording media [31,32].
The coordination ability of this type of ADB was also demon-
strated by the synthesis of a Pd(II) complex, and the extension of
this study to the preparation of metal complexes with potential
interest on biological and catalytical applications deserves to be
pursued.
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Acknowledgements
This work has been partially supported by the Foundation for
Science and Technology (FCT), Portugal, and its PPCDT (FEDER
funded) and ‘‘Science 2007’’ programs. M.N.K., K.T.M. K.V.L. and
A.M. express gratitude to the FCT for a post-doc fellowship and
working contracts. The authors gratefully acknowledge Dr.
Conceição Oliveira for the ESI-MS analysis, and the Portuguese
NMR Network (IST-UTL Centre) for the NMR facility.
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