Trends in properties of para-substituted 3-(phenylhydrazo)pentane-2,4- diones

Article abstract of DOI:10.1002/poc.1824

Trends between the Hammett's σ_p and related normal, inductive σ_I, resonance σ_R, negative and positive polar conjugation and Taft's σ_p° substituent constants and the distance, δ_N-H NMR chemical shift,

Full text of DOI:10.1002/poc.1824

Research Article
Received: 1 April 2010,
Revised: 27 August 2010,
Accepted: 11 October 2010,
Published online in Wiley Online Library: 6 December 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1824
Trends in properties of para-substituted
3-(phenylhydrazo)pentane-2,4-diones
a
a
´
Maximilian N. Kopylovich , Kamran T. Mahmudov , M. Fatima C. Guedes
da Silvaa,b, Luı´sa M.D.R.S. Martinsa,c, Maxim L. Kuznetsova, Telma
F.S. Silva , Joao J.R. Frau´sto da Silva and Armando J.L. Pombeiro
a
a
a
*
˜
Trends between the Hammett’s sp and related normal snp, inductive sI, resonance sR, negative spS and positive sRp
polar conjugation and Taft’s sop substituent constants and the N—Hꢀ ꢀ ꢀO distance, dN—H NMR chemical shift, oxidation
potential (Eopx=2, measured in this study by cyclic voltammetry (CV)) and thermodynamic parameters (pK, DG0, DH0 and
DS0) of the dissociation process of unsubstituted 3-(phenylhydrazo)pentane-2,4-dione (HL1) and its para-substituted
chloro (HL2), carboxy (HL3), fluoro (HL4) and nitro (HL5) derivatives were recognized. The best fits were found for sp
ox
and/or sSp in the cases of dNꢀꢀꢀO, dN—H and E , showing the importance of resonance and conjugation effects in
p=2
such properties, whereas for the above thermodynamic properties the inductive effects (sI) are dominant. HL2 exists in
the hydrazo form in DMSO solution and in the solid state and contains an intramolecular H-bond with the Nꢀ ꢀ ꢀO
˚
distance of 2.588(3) A. It was also established that the dissociation process of HL1–5 is non-spontaneous, endothermic
and entropically unfavourable, and that the increase in the inductive effect (sI) of para-substitutents
(—H < —Cl < —COOH < —F < —NO2) leads to the corresponding growth of the Nꢀ ꢀ ꢀO distance and decrease of
the pK and of the changes of Gibbs free energy, of enthalpy and of entropy for the HL1–5 acid dissociation process.
The electrochemical behaviour of HL1–5 was interpreted using theoretical calculations at the DFT/HF hybrid level,
namely in terms of HOMO and LUMO compositions, and of reactivities induced by anodic and cathodic electron-
transfers. Copyright ß 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: 3-(4-substituted phenylhydrazo)pentane-2,4-diones; correlation analysis; DFT calculations; redox potential;
tautomers; thermodynamic parameters
between the H-bond strengthening and the p-electron deloca-
lization of conjugated bonds linking the proton-donating
INTRODUCTION
Although azoderivatives of b-diketones (ADB, herein denoted
as HL, Scheme 1) are known since long,[1–3] they still show a
surprisingly modest impact in organic, coordination and organo-
metallic chemistries. In spite of the relatively limited number of
publications on this class of compounds, it has been shown
that ADB can be potentially applied for optical storage,[4]
spectrophotometric determination of some metal ions,[5–7] futher
organic synthesis,[8] as spin-coating films,[9] liquid crystals,[10]
self-assembled layers,[11] antineoplastics,[12,13] semiconducting
materials,[14] antipyretic,[15] analgesic,[15,16] antibacterial[17–19]
drugs, etc. Hence, the study of their physico-chemical properties
and in particular of their tautomery is of interest from both
theoretical and practical points of view.
and proton-accepting groups.[25,26] Since the interacting groups,
—
N—H and C O, can be considered as substituents, their
—
electronic properties can be described by the Hammett’s and
Taft’s s substituent constants,[27–38] which express the degree
of electron-donating or electron-accepting character of the
substituent.[28–43]
In the RAHB systems, the proton-donating N—H group is
an electron-donor when considered as a substituent (thus with a
*
Correspondence to: A. J. L. Pombeiro, Centro de Qu´ımica Estrutural, Complexo
´
I, Instituto Superior Tecnico, TU Lisbon, Av. Rovisco Pais, 1049-001 Lisbon,
Portugal.
E-mail: pombeiro@ist.utl.pt
All the structurally studied ADB have
a characteristic
conjugated heterodienic system forming a strong intramolecular
resonance assisted N—Hꢀ ꢀ ꢀO hydrogen bond (RAHB) linking
one of the carbonyl groups to the NH-moiety of the hydrazo
unit.[20–24] It was shown[22] that this hydrogen bond strongly
influences the properties of ADB and that its strength is
essentially determined by the degree of p-delocalization within
the ketohydrazone hetero-conjugated system and is modulated
by the factors that can affect the degree of conjugation, including
inductive ones and non-bonding intermolecular interactions. On
the other hand, it was assumed that there is a cooperative effect
a
M. N. Kopylovich, K. T. Mahmudov, M. F. C. G. da Silva, L. M.D.R.S. Martins, M. L.
Kuznetsov, T. F.S. Silva, J. J.R. F. da Silva, Armando J.L. Pombeiro
´
Centro de Qu´ımica Estrutural, Complexo I, Instituto Superior Tecnico, TU
Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
b
c
M. F. C. G. da Silva
´
Universidade Lusofona de Humanidades e Tecnologias, ULHT Lisbon, Campo
Grande 376, 1749-024 Lisbon, Portugal
L. M.D.R.S. Martins
Departamento de Engenharia Qu´ımica, ISEL, R. Conselheiro Em´ıdio Navarro,
1959-007 Lisboa, Portugal
J. Phys. Org. Chem. 2011, 24 764–773
Copyright ß 2010 John Wiley & Sons, Ltd.

PROPERTIES OF 3-(PHENYLHYDRAZO)PENTANE-2,4-DIONES
Scheme 1. Possible tautomeric equilibria in azoderivatives of b-diketones HL1–5
—
negative s value), whereas the proton-accepting C O group is
aromatic amines couple successfully with a range of reactive
methylene coupling components in an organic solvent and in
the presence of sodium acetate.[1–3] However, attempts to
prepare our para-substituted HL1,3–5 compounds using this
base either failed completely or gave a low yield of a highly
impure product.[49,53–55] Thus, we modified the reported pro-
cedure and found that good yields of HL could be obtained when
the coupling was undertaken in a sodium hydroxide solution
(described in Experimental section).
The ADB prepared in this study potentially can exist in the
enol-azo, keto-azo and hydrazo tautomeric forms (Scheme 1), but
earlier it was shown experimentally that hydrazones formed from
pentane-2,4-dione exist in the hydrazo form both in solution
and in the solid state.[49,53–55] An intramolecular hydrogen bridge
linking one of the carbonyl groups to the NH-moiety of the
hydrazone unit and forming the six-membered hydrogen
bonding ring was found to be a characteristic feature of the
hydrazo form (Scheme 1) thus stabilizing this particular tautomer.
The infrared spectra of hydrazones based on a pentane-2,4-
dione and, in particular, the position of the carbonyl stretching
vibration, have been of considerable importance in establishing
that the compounds exist in the hydrazo form. The solid state IR
spectrum of the hydrazone HL2 shows two intense carbonyl
bands assignable to the hydrazo form with intramolecular
—
an electron-acceptor substituent (hence with a positive s value).
They may interact via an intramolecular H-bond, forming a very
stable cycle of conjugated bonds. As a result, the N—H bond
length should elongate and the electron-donor properties of this
group as a substituent should become stronger. On the other
—
—
hand, the proton approaches C O with formation of C Oꢀ ꢀ ꢀH
—
—
and, as a consequence, the electron-attracting power, as a
—
—
[44,45]
—
substituent, of C O in C Oꢀ ꢀ ꢀH should increase in comparison
—
—
with C O itself.
—
Another important feature of ADB is their tautomerism.
The keto-enol tautomerism of b-diketones has been extensively
studied during the past five decades[46] and still attracts atten-
tion,[47] showing that the relative amounts of the tautomers are
connected with the intramolecular hydrogen bonding and
depend on the substituents, temperature and media.[48] In the
—
case of ADB one more conjugated N N double bond is present,
—
thus significantly increasing the number of possible tautomers:
(E-, Z-) enol-azo, keto-azo and hydrazo (Scheme 1).[49,50] Further-
more, there are several possible conformers for each category
of tautomers depending on the overall symmetry of the
molecule.[49–52] For example, it has been observed that in
ADB formed by unsymmetrical b-diketones, the six-membered
H-bonded ring will be generated at the more sterically favourable
side of the molecule.[3] Moreover, the substituent in the para
position of the phenyl ring relative to the hydrazone function of
ADB should provide another important factor to influence their
structural properties.
—
—
—
hydrogen bonding (N—H n(C O), n(C Oꢀ ꢀ ꢀH) and n(C N) are
—
—
—
observed at 3449, 1668, 1627 and 1587 cmꢁ1, respectively) in
view of the agreement with published reports on its analogues
[49,53–55]
HL1,3–5
.
Thus, the aim of this paper is to study in a quantitative
way possible correlations among the physico chemical
properties of para-substituted 3-(phenylhydrazo)pentane-2,4-
diones (HL), i.e. (Scheme 1) 3-(phenylhydrazo)pentane-2,4-dione
(HL1), 3-(4-chlorophenylhydrazo)pentane-2,4-dione (HL2), 3-(4-
carboxyphenylhydrazo)pentane-2,4-dione (HL3), 3-(4-fluorophenyl-
hydrazo) pentane-2,4-dione (HL4) and 3-(4-nitrophenylhydrazo)
pentane-2,4-dione (HL5).
In the 1H NMR spectrum of HL2, the hydrazone proton
appears as a broad singlet at d 13.85, which is a clear indication
of a protonated nitrogen atom adjacent to the aromatic unit.
Moreover, the two methyl groups of the pentane-2,4-
dione moiety yield separate singlets at d 2.40 and 2.47. The
stable six-membered H-bonded ring (Scheme 1) turns the
hydrazone tautomeric structure unsymmetrical in accord with
the non- equivalence of the two methyl groups.
Considering all the azoderivatives of pentane-2,4-dione of this
study, i.e. HL1–5 (Scheme 1), one observes that the functional
group (X) in para position of the aromatic part of the molecule
influences dN—H, although no clear linear relationship was found
between the Hammett’s or any related substituent constants
(Tables S1–S3; Tables and figures indicated with the capital letter
S are given as Supplementary Information) and that chemical
shift, the latter taken from the following sources: HL1,[53] HL2
(this work, Experimental section), HL3,[54] HL4 (Supplementary
Information), HL5[55] (all data in CDCl3). The best (although rather
RESULTS AND DISCUSSION
Synthesis of HL2 and spectroscopic study
The para-substituted 3-(phenylhydrazo)pentane-2,4-diones
(HL, Scheme 1) were prepared by aqueous diazotization and
azocoupling. The synthesis and some properties of HL1,3–5 were
reported before.[49,53–55] It has also been found that diazotized
J. Phys. Org. Chem. 2011, 24 764–773
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

M. N. KOPYLOVICH ET AL.
rough) ‘correlation’ was observed for the Hammett’s sp constant
(sp ¼ ꢁ2.55dN—H þ 37.67, r2 ¼ 0.758, n ¼ 5) (Fig. S1). An increase
of the electron-acceptor character of the X substituent corres-
ponds to a proton shift to higher field. The normal snp constant
follows a similar trend, but the inductive (sI) and resonance (sR)
constants do not correlate at all. Hence, both resonance and
inductive substituent effects appear to operate significantly.
Thermodynamic functions of the dissociation process
To determine the dissociation constants of the studied ADB,
pH-metric titration in aqueous-ethanol media was used. The
ionic strength was maintained constant (I ¼ 0.1 M) by adding a
calculated amount of KCl. The dissociation constant of HL5
(Supplementary Information, Table S7) was calculated by the
following equation:[56]
pK ¼ ꢁlogK ¼ pH þ log ½HLꢂꢁlog ½Lꢁꢂ
X-ray diffraction study
and found to be pK ¼ 8.10 ꢃ 0.02. For determination of the
dissociation constants of the diprotic acid HL3, the Schwarzen-
bach algebraic method was used:[57]
Compound HL2 (Fig. 1) exhibits a nearly planar overall con-
formation with structural parameters, bond distances and angles
similar to those found in HL1,3–5[49,53–55], as presented in Tables
S4–S6.
The crystal lattice of HL2 is stabilized by hydrogen bonding
interactions. Indeed, while the chloride atoms act as acceptors of
the H7C hydrogens of vicinal molecules leading to the formation
of a 1D chain, the O2 carbonyl oxygens act as acceptors in
both intramolecular (with N2—H2N) and intermolecular (with the
aromatic C2—H2) hydrogen bondings to the nearest molecules
in the same plane, which thus expand the structure to a
second dimension. Due to the hydrogen bonding involving O2,
the C10—O2 bond length is longer than that of ‘free’ C8—O1
½HþꢂfacH L þ ½Hþꢂꢁ½OHꢁꢂg
2
K1 ¼
;
ð1ꢁaÞcH Lꢁ½Hþꢂ þ ½OHꢁꢂ
2
½Hþꢂfðaꢁ1ÞcH L þ ½Hþꢂꢁ½OHꢁꢂg
2
K2 ¼
;
ð2ꢁaÞcH Lꢁ½Hþꢂ þ ½OHꢁꢂ
2
where cH L is total concentration of the diprotic acid (HL3) and
2
a is neutralization point. The performed calculations (calculation
details and Table S8 at Supplementary Information) gave pK1 ¼
3.98 ꢃ 0.03 and pK2 ¼ 8.24 ꢃ 0.02.
The quantum-chemical calculations at the B3LYP level of
theory indicate that in CH3CN solution the hydrazo-form of HL1
(Scheme 1) is the most stable one followed by the Z-enol-azo,
E-enol-azo and keto-azo forms (less stable by 7.5, 9.2 and
15.4 kcal/mol, respectively). The deprotonated hydrazo-form of
HL3 with the proton removed from the COOH group (HLꢁ3 a) is
more stable than the deprotonated form HLꢁ3 b with the proton
abstracted from the NH group (Table S9 in Supplementary
Information). This indicates that the COOH moiety in HL3 is more
acidic than the NH unit, and the experimentally determined pK1
and pK2 values correspond to the proton elimination from the
COOH and NH groups, respectively.
˚
(1.234(4) and 1.223(3) A, respectively).
The most significant characteristic for this structure is the
—
—
presence of the HN—N C—C O conjugated heterodienic
—
—
system with formation of the heteronuclear N2—H2Nꢀ ꢀ ꢀO2
RAHB. In particular, Table S5 compares the bond lengths and
angles involved in the N—Hꢀ ꢀ ꢀO RAHBs, as well as in other
relevant hydrogen interactions.
In the six-membered H2N—N2—N1—C9—C10—O2 cycle the
angles vary from 103.3(2) to 130.6(2)8, the widest value belonging
to O2—H2N—N2. The para-Cl substituent (with sp ¼ 0.23) causes
a significant shortening of the N—H bond and a slight elongation
The dissociation constants of HL3,5 have been evaluated at 308
and 318 K and the thermodynamic functions for the dissociation
process were calculated using the following well known
relationships:[58,59]
of the Nꢀ ꢀ ꢀO distance, which falls in the Nꢀ ꢀ ꢀO range for HL1,3–5
,
[49,53–55]
˚
2.580–2.603 A.
The plots of the Hammett’s sp, Taft’s sop and related substituent
constants versus the Nꢀ ꢀ ꢀO distance (Tables S1–S3) show that this
distance appears to tend to increase with an increase of the
electron-acceptance of the substituent, though the dNꢀꢀꢀO changes
in some cases are insignificant and a clear correlation cannot be
established. The best ‘relationships’, with r2 ¼ 0.84–78, were found
for sꢁp (sꢁp ¼ 52.25dNꢀꢀꢀO ꢁ 135.01, r2 ¼ 0.842, n ¼ 5, Fig. S2) and
Taft’s sop. Hence, the functional group in para position of the ADB
phenyl ring affects the dNꢀꢀꢀO distance and thus the tautomeric
equilibrium, what can be particularly important for some
applications of this class of compounds (Introduction Section).
DG0 ¼ 2:303RTpK;
ꢂ
ꢀ
ꢁꢃ
DH0 ¼ ꢁR pKðT ÞꢁpK
=½ð1=T3Þꢁð1=T1Þꢂ;
ðT1Þ
3
ꢀ
ꢁ
DS0 ¼ DH0ꢁDG0 =T:
The thermodynamic parameters of the dissociation process of
HL1,2,4 were described earlier[49,60,61] and will be considered
herein for comparative purposes (Table 1). The data thus
obtained testify that with the increase of temperature the acidity
Figure 1. Molecular structure of HL2 with atom numbering scheme and hydrogen bond interactions. Ellipsoids were drawn at 30% probability.
Symmetry operations used to generate equivalent atoms: (i) 1/2 þ x, ꢁ1/5 þ y, þz; (ii) 1/2 ꢁ x, 1/2 ꢁ y, ꢁz; (iii) ꢁ1/2 þ x, 1/2 ꢁ y, ꢁz
wileyonlinelibrary.com/journal/poc
Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 764–773

PROPERTIES OF 3-(PHENYLHYDRAZO)PENTANE-2,4-DIONES
Table 1. Thermodynamic characteristics of dissociation of HL1–5 in water–ethanol solution
HL1–5
T (K)
pK
DG0 (kJ molꢁ1
48.72 ꢃ 0.10
)
DH0 (kJ molꢁ1
46.34 ꢃ 2.43
)
DS0 (J molꢁ1 Kꢁ1
ꢁ7.98 ꢃ 2.53
)
[60]
HL1
HL2
HL3
HL4
HL5
298 ꢃ 0.5
308 ꢃ 0.5
318 ꢃ 0.5
298 ꢃ 0.5
308 ꢃ 0.5
318 ꢃ 0.5
298 ꢃ 0.5
308 ꢃ 0.5
318 ꢃ 0.5
298 ꢃ 0.5
308 ꢃ 0.5
318 ꢃ 0.5
298 ꢃ 0.5
308 ꢃ 0.5
318 ꢃ 0.5
8.54 ꢃ 0.02
8.30 ꢃ 0.03
8.03 ꢃ 0.04
8.36 ꢃ 0.02
8.15 ꢃ 0.04
7.95 ꢃ 0.05
8.24 ꢃ 0.02
8.06 ꢃ 0.06
7.87 ꢃ 0.05
8.12 ꢃ 0.01
7.95 ꢃ 0.03
7.77 ꢃ 0.05
8.10 ꢃ 0.02
7.95 ꢃ 0.03
7.76 ꢃ 0.05
[61]
a
47.70 ꢃ 0.10
47.02 ꢃ 0.10
46.33 ꢃ 0.09
46.22 ꢃ 0.11
ꢁ35.06 ꢃ 2.09
ꢁ44.97 ꢃ 1.88
ꢁ48.72 ꢃ 1.76
ꢁ51.40 ꢃ 1.81
37.25 ꢃ 1.99
33.62 ꢃ 1.78
31.81 ꢃ 1.67
30.90 ꢃ 1.70
[49]
a For HL3, the values of pK2 were used.
0
00
—
compounds of the type ArRC N—NR R can follow different
—
of HL1–5 increases (pK decreases); hence the deprotonation is
endothermic.
electrochemical reduction and also oxidation routes in aprotic
solvents, depending on the substituents[63] which also strongly
influence the redox potentials.[37,38,41,42,64]
To search for correlations of the reduction or oxidation
potentials of HL1–5 with the nature of the substituent X, a cyclic
voltammetric study was performed, at a Pt disk electrode, in 0.2 M
[nBu4N][BF4]/NCMe or THF (or [nBu4N][ClO4]/CH2Cl2 for HL5), at
room temperature. Compounds HL1–5 exhibit one irreversible
oxidation wave (Iox) and one irreversible reduction wave (Ired
(Fig. 2 for HL4) at half-peak potential (Eopx=2 and E ) values (vs. the
ferrocene/ferricinium redox couple) given in Table 2. In the case
of HL5, the reduction wave of the nitro group is observed
at Erped ¼ ꢁ1.53 V. Exhaustive controlled potential electrolysis
(CPE) at a potential slightly anodic (or cathodic, for the reduction)
to that of the peak potential indicates the occurrence of a
single-electron oxidation or of a two-electron reduction at the
corresponding waves, during the extended time scale of CPE.
The introduction of a functional group in the para position of
the aromatic ring of the molecule influences the thermodynamic
characteristics of the dissociation process of HL1–5, i.e. with an
increase of the inductive effect (Table S1) of that substituent
(—H < —Cl < —COOH < —F < —NO2) the O—H and N—H
bonds weaken and as result the values of DH0 decrease and
DS0 becomes more negative (see below), i.e. the acidity and the
number of ions in the system increase. A large positive value of
DG0 indicates that the dissociation process is not spontaneous,
i.e. it should be forced by a base if the deprotonation is needed
(e.g. for coordination).
The Hammett’s inductive sI substituent constant provides
the best relations with the thermodynamic functions of the
dissociation process of HL1–5 (sI ¼ ꢁ1.19pK þ 10.27, r2 ¼ 0.806;
sI ¼ ꢁ0.21DG0 þ 10.29, r2 ¼ 0.805; sI ¼ (ꢁ3.71) ꢄ 10ꢁ2DH0 þ 1.75,
r2 ¼ 0.927; sI ¼ (ꢁ1.33) ꢄ 10ꢁ2DS0 ꢁ 0.08, r2 ¼ 0.941, n ¼ 5; Table
S2, Fig. S3). An increase in sI (i.e. in the substituent electron-
acceptance by inductive effect) leads usually to a decrease of pK,
of DG0 and of DH0, while DS0 becomes more negative. Thus, a
strong electron-withdrawing substituent can significantly
shift the tautomeric equilibrium to the enol-azo form, allowing
its isolation in solid phase what can be useful in some
applications.[8–26] Also an electron-acceptor substituent tends
to promote the acidity of ADB (Fig. S3a), allowing their
complexation with metals in more acidic conditions, what can
favour the analytical selectivity of ADB reagents.[5–7] Hence, one
can regulate some practically important properties of ADB by
inclusion of a suitable functional group with a certain substituent
constant.
)
red
p=2
Electrochemical behaviour of HL1–5
The electrochemical reduction of hydrazone compounds has
been the subject of a number of investigations and the presence
Figure 2. Cyclic voltammogram (n ¼ 0.2 V sꢁ1) of a 5.2 mM solution of
HL4, initiated by the anodic sweep, in 0.2 M [nBu4N][BF4]/NCMe, at a
platinum disk electrode (d ¼ 0.5 mm)
—
of the electro-active group C N—NH— has been shown, e.g.
—
by polarographic methods.[62,63] For instance, the aromatic
J. Phys. Org. Chem. 2011, 24 764–773
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

M. N. KOPYLOVICH ET AL.
the significance of the polar conjugation effects of the
substituent, namely in the HOMO of HL1–5
Table 2. Cyclic voltammetric dataa and theoretical vertical
ionization potential and electron affinity for HL1–5
.
Correlations of the oxidation potential with Hammett’s sp
and related substituent constants have been reported in other
systems, namely for series of coordination compounds bearing
different substituents.[37,38,41,64–68] In contrast, the reduction
potential values do not vary appreciably with a change of the
substituent, being rather close (ꢁ1.75 ꢃ 0.07 V), what suggests an
ox
p=2
red
HL
X
E
IPb
E
p=2
EAc
HL1
HL2
HL3
HL4
HL5
H
Cl
COOHd
F
1.07
1.02
1.18
1.06
1.31
6.44
6.46
6.68
6.46
6.89
ꢁ1.77
ꢁ1.68
ꢁ1.76
ꢁ1.76
ꢁ1.82
2.71
2.78
2.99
2.73
3.34
insignificant involvement of the substituent in the LUMO of HL1–5
.
Aiming to interpret the experimental electrochemical results
and to give an insight into the possible mechanisms of the
oxidation and reduction irreversible processes, theoretical cal-
culations at the B3LYP level of theory were performed.
e
NO2
a Values given in V ꢃ 0.03 relative to the [Fe(h5-C5H5)2]0/þ redox
couple, measured in 0.2 M [nBu4N][BF4]/NCMe (unless stated
The calculations reveal that the HOMOs of HL1–5 are mainly
otherwise), at the scan rate ¼ 0.2 V sꢁ1
b Vertical ionization potentials in eV.
c Vertical electron affinities in eV.
d In THF.
.
—
localized on the C9 and N2 atoms of the C N—N moiety and on
—
the C atoms of the phenyl group (Fig. 4). Furthermore, an orbital
of the aromatic Cl atom also gives a noticeable contribution to
the HOMO of HL2. The delocalized composition of the HOMO is
consistent with the expected (see above and Fig. 4) mesomeric
effect of the substituent. Moreover, the calculated vertical
ionization potentials (IP) of HL1–5 are the highest ones for HL5 and
HL3 (bearing the NO2 and COOH substituents, respectively) and
follow the trend of the experimental oxidation potential values
(Table 2). In the oxidized species HLþ1ꢁ5, the spin density is
delocalized along several atoms and has the highest values on
e In [nBu4N][ClO4]/CH2Cl2; an irreversible reduction wave is also
observed at Erped ¼ ꢁ1.53 V due to the reduction of the nitro
group.
Since the redox processes are irreversible, the redox potentials
are not determined only by thermodynamic factors and thus it
would not be surprising if good correlations with the Hammett’s
and related constants would not be found. Nevertheless, one
—
the aromatic C4 atom and on the N2 and C9 atoms of the C
—
N—N moiety (0.35, 0.30 and 0.26, respectively) and noticeable
values on the aromatic C2 and C6 atoms (0.16). The corres-
ponding resonance structures of HLþ1ꢁ5 (A–D) are shownþin
Scheme 2. Such a distribution provides the possibility of HL1ꢁ5
dimerization (eventually via Hþ loss) through those atoms (in
the case of C4, for the unsubstituted HL1), as reported[69] for some
arylhydrazones. The occurrence of any of such single-electron
anodically induced chemical reactions can account for the
observed irreversibility of the anodic process.[41,70–72]
ox
p=2
observes that the oxidation potential values (E ¼ 1.31 and
1.18 V) for the ADB with the strongest electron-acceptor
substituents, NO2 and COOH (sp ¼ 0.78 and 0.45, or sꢁp ¼ 1.27
and 0.77, respectively), are higher than those with the other
ox
substituents which fall in a narrow range (E at 1.07–1.02 V). The
p=2
2
ox
p=2
best fit (r ¼ 0.903) is found for E versus sꢁp (Fig. 3), suggesting
The LUMOs of HL1–HL4 (Fig. 4) are mostly centred on the N1,
—
N2, C9 and C10 atoms of the H-bonded C—C N—N fragment,
—
with noticeable contributions coming also from orbitals of
the oxygen atoms. Hence, resonance substituent effects are not
expected to play a relevant role. However, for HL5 (with the nitro
substituent), orbitals of the C4 atom and the NO2 group also give
a significant contribution to the LUMO.
The N1C9, N1N2 and CO combinations of the LUMO have
antibonding character (Fig. 4) and hence the one- and, in parti-
cular, two-electron reductions of HL1–5 result in a significant
˚
elongation of the N1C9 and N1N2 bonds (by 0.088 and 0.079 A,
respectively, for the two-electron reduction of HL1). This corre-
lates with experimental results[69,73–76] indicating that the
cleavage of the N1N2 bond is possible as a result of the
Figure 3. Plot of the spꢁ substituent constant versus E for HL1–5
ox
p=2
Figure 4. Plots of the HOMO and LUMO of HL1
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PROPERTIES OF 3-(PHENYLHYDRAZO)PENTANE-2,4-DIONES
Scheme 2. Possible reduction and oxidation processes of HL1–5
reduction, and thus also can account[77–79] for the irreversibility of
the cathodic process. This irreversible character conceivably is the
basis of the lack of correlation of the experimental reduction
potentials with the calculated electron affinities (EA) (Table 2), the
latter predicting a substantial variation with the change of the
substituent (an increase of its electron-withdrawing character
leads to an increase of the EA, thus favouring the reduction), in
red
contrast with the former data (similar E values for all the HL1–5).
p=2
For the mono-reduced species HLꢁ1ꢁ5, the highest spin density
is localized at the N1 atom, and the most stable resonance
structures of the doubly-reduced compounds HL21ꢁꢁ5 are shown in
Scheme 2 (in accord with results of the natural bond orbital (NBO)
analysis, see computational details).
2ꢁ
The dianionic HL
may be protonated,[73] and the O and N1
1ꢁ5
atoms are the most probable sites of proton attack. Indeed, (i) the
effective NBO charges on these atoms are strongly negative
(ꢁ0.79, ꢁ0.75 and ꢁ0.57, respectively in HL21ꢁ), (ii) the regions of
the most negative electrostatic potential are located near these
atoms (Fig. 5) and (iii) the proton attack at such atoms is not
sterically hampered. However, the corresponding diprotonated
structure H3L1a of the enol type (Schemes 2 and 3) is relatively
unstable and should undergo tautomerization. The geometry
optimization of possible diprotonated species showed that the
Figure 5. Distribution of negative electrostatic potential for HL12ꢁ
Scheme 3. Calculated H3L1 structures derived upon reduction (2eꢁ/2Hþ) of HL1. Gibbs free energies in MeCN solution relative to the most stable
structure are indicated in kcal/mol
J. Phys. Org. Chem. 2011, 24 764–773
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M. N. KOPYLOVICH ET AL.
most stable is that of the iminoalcohol type H3L1b followed by
the keto-amino structure H3L1c, H3L1a and two structures with
protonated phenyl group (H3L1d and H3L1e) (Scheme 3). The
calculations also indicate that the experimentally proposed N1N2
and carbon-13, respectively. The chemical shifts are reported in
ppm using tetramethylsilane as an internal reference. The
infrared spectra (4000–400 cmꢁ1) were recorded on a BIO-RAD
FTS 3000MX instrument in KBr pellets. Carbon, hydrogen and
nitrogen elemental analyses were carried out by the Micro-
—
—
2
bond cleavage (Scheme 2) to give the imine (CH CO) C NH and
3
phenylamine C6H5NH2 [73–76,80] is thermodynamically favourable.
analytical Service of the Instituto Superior Tecnico. The acidity of
´
The Gibbs free energy in MeCN solution of the reaction H3L1b !
the solutions was measured using an I-130 potentiometer with an
ESL-43-07 glass electrode and an EVL-1M3.1 silver–silver chloride
electrode adjusted by standard buffer solutions. The pH-metric
titration was carried out in water–ethanol mixtures (see below)
with consideration of the Bates correction[81] at temperatures of
298, 308 and 318 K. The temperature was maintained constant
within 0.5 K by using an ultrathermostat (Neslab 2 RTE 220). The
electrochemical experiments were performed on an EG&G
PAR 273A potentiostat/galvanostat connected to a personal
computer through a GPIB interface. CV studies were undertaken
at room temperature using a two-compartment three-electrode
cell with Pt disc working (d ¼ 0.5 mm) and Pt wire counter
electrodes. Controlled-potential electrolyses (CPE) were carried
out in electrolyte solutions with the abovementioned compo-
sition, in a three-electrode H-type cell. The compartments
were separated by a sintered glass frit and equipped with
platinum gauze working and counter electrodes. For both CV and
CPE experiments, a Luggin capillary connected to a silver wire
pseudo-reference electrode was used to control the working
electrode potential, and a Pt wire was employed as the
counter-electrode for the CV cell. The CPE experiments were
monitored regularly by CV, thus assuring no significant potential
drift occurred along the electrolyses. The solutions were saturated
with dinitrogen by bubbling this gas before each run. The redox
potentials of the HL1–5 were measured in 0.2 M [nBu4N][BF4]/
NCMe or THF (or [nBu4N][ClO4]/CH2Cl2 for HL5), using ferrocene as
an internal potential reference and all reported potentials
(measured at a scan rate of 0.2 V sꢁ1) are given versus the redox
potential for the ferrocene/ferricinium couple (Fc/Fcþ).[82]
—
(CH CO) C NH þ C H NH is ꢁ18.9 kcal/mol.
—
In the case of HL5, with the electroactive nitro substituent that
is shown to contribute to the LUMO (see above), the reduction of
3
2
6
5
2
red
this group occurs at a more favourable potential (E ¼ ꢁ1.53 V)
p=2
red
p=2
than that of the ADB moiety (E ¼ ꢁ1.82 V). This is consistent
with the reported[73–75] redox chemistry of some aromatic nitro
compounds which in aprotic media undergo a one-electron
reduction giving a delocalized radical that is further reduced to a
dianion which finally converts to the nitroso anion radical.
CONCLUSIONS
Trends between Hammett’s sp, normal snp, inductive sI, resonance
sR, negative sꢁp and positive spþ polar conjugation and Taft’s spo
substituent constants and some properties (N—Hꢀ ꢀ ꢀO distance,
dN—H chemical shift, redox potentials and thermodynamic
functions of the dissociation process) of series of para substituted
2
2
ox
p=2
aryl ADB [sp vs. dN—H (r ¼ 0.76); sꢁp vs. dNꢀꢀꢀO (r ¼ 0.84), E
(r2 ¼ 0.90); sI vs. pK (r2 ¼ 0.81), DG0 (r2 ¼ 0.81), DH0 (r2 ¼ 0.93) and
DS0 (r2 ¼ 0.94)] were found. An increase of the electron-
acceptance by inductive effect of the para substituent
(H < Cl < COOH < F < NO2) of HL1–5 enhances the acidity and
the Nꢀ ꢀ ꢀO distance, whereas DG0, DH0 and DS0 of HL1–5
dissociation decrease in the same order, the process being
unspontaneous, endothermic and entropically unfavourable.
IR, NMR and X-ray diffraction analyses showed that HL2 exists
in DMSO solution and in the solid state in the hydrazo form
containing an intramolecular H-bond, similary to the reported
[49,53–55]
Synthesis of the 3-(4-chlorophenylhydrazo)
pentane-2,4-dione
analogs HL1,3–5
.
The electrochemical behaviour of HL1–5 was studied using
CV and the results were interpreted by theoretical calculations at
the DFT/HF hybrid level. The substituent at the aromatic ring
has a stronger influence on the oxidation potential than on the
reduction one, as shown by CV and in accord with the HOMO and
LUMO compositions, the former with an important contribution
of the aromatic component and the latter being essentially
localized at the ADB part. Theoretical studies can account for
the irreversibility of the redox processes and allow proposing
conceivable mechanisms involving single-electron anodically
induced dimerization and two-electron cathodically induced
protonation followed by N—N bond cleavage.
The synthesis and X-ray study of HL1,3–5 were reported
earlier,[49,53–55] and hence will not be discussed in detail here.
The arylhydrazone HL2 was synthesized via the modified Japp–
Klingemann reaction[1–3] between the aromatic diazonium salt of
4-chloroaniline and pentane-2,4-dione in water solution contain-
ing sodium hydroxide. The same method, using NaOH as base,
was also applied in the current study to the synthesis of the other
HL compounds, in good or reasonable yields that are better than
those previously achieved when using sodium acetate.
Diazotization
The obtained correlations or trends should be used to tune
various properties of ADB such as acidity, coordination ability,
biological activity, etc., what can be important for fundamental
studies and for practical applications.
A 3.20 g (0.025 mol) portion of 4-chloroaniline was dissolved in
50 mL water. The solution was cooled in an ice bath to 273 K and
1.725 g (0.025 mol) of NaNO2 was added; then 5.00 mL HCl were
added in 0.5 mL portions for 1 h. The temperature of the mixture
should not exceed 278 K.
EXPERIMENTAL
Materials and methods
Azocoupling
The 1H and 13C NMR spectra were recorded at ambient
temperature on a Bruker Avance IIþ300 (UltraShieldTM Magnet)
spectrometer operating at 300.130 and 75.468 MHz for proton
One gram (0.025 mol) of NaOH was added to a mixture of 2.55 mL
(0.025 mol) of pentane-2,4-dione with 50 mL of water. The
solution was cooled in an ice bath to ca. 273 K, and a suspension
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J. Phys. Org. Chem. 2011, 24 764–773

PROPERTIES OF 3-(PHENYLHYDRAZO)PENTANE-2,4-DIONES
of 4-chloroaniline diazonium (see above) was added in three
portions under rigorous stirring for 1 h.
DFT computational details
The full geometry optimization of all structures has been carried
out at the DFT/HF hybrid level of theory using the B3LYP
functional[87,88] and 6-311þG(d,p) basis set with the help of the
Gaussian-98[89] program package. Restricted approximations for
the structures with closed electron shells and unrestricted
methods for the structures with open electron shells have been
employed. No symmetry operations have been applied. The
Hessian matrix was calculated analytically for the optimized
structures in order to prove the location of correct minima (no
imaginary frequencies), and to estimate the thermodynamic
parameters, the latter being calculated at 25 8C.
Total energies corrected for solvent effects (Es) were estimated
at the single-point calculations on the basis of gas-phase
geometries using the polarizable continuum model[90] in the
CPCM version[91] with CH3CN as solvent. The entropic term in
CH3CN solution (Ss) was calculated according to the procedure
described by Wertz[92] and Cooper and Ziegler[93] using
Eqns. (1)–(4)
Yield 92% (based on pentane-2,4-dione), yellow powder
soluble in DMSO, methanol, ethanol and acetone, and insoluble
in water. Elemental analysis: C11H11Cl1N2O2 (M ¼ 238.67); C 55.53
(calc. 55.36); H 4.58 (4.65); N 11.69 (11.74) %. IR (KBr): 3449 n(NH),
1
1668 n(C O), 1627 n(C Oꢀ ꢀ ꢀH), 1587 n(C N) cmꢁ1. H NMR
—
—
—
—
—
—
(300.130 MHz) in DMSO-d6, internal TMS, d (ppm): 2.40 (s, 3H, CH3),
2.47 (s, 3H, CH3), 7.45, 7.47 (m, 2H, J ¼ 9.0 Hz, Ar—H), 7.58, 7.61 (m,
2H, J ¼ 9.0 Hz, Ar—H), 13.85 (s, 1H, N—H). 13C-NMR (75.468 MHz)
in DMSO-d6, internal TMS, d (ppm): 26.34 (CH3), 31.20 (CH3),
117.91 (Ar—H), 117.91 (Ar—H), 128.97 (Ar—NH—N), 129.42
—
(Ar—H), 129.42 (Ar—H), 133.81 (C N), 140.91 (Ar—Cl), 196.27
—
1
—
—
(C O), 196.81 (C O). H NMR (300.130 MHz) in CDCl , internal
—
—
3
TMS, d (ppm): 2.39 (s, 3H, CH3), 2.51 (s, 3H, CH3), 7.17, 7.23 (m, 2H,
J ¼ 18.0 Hz, Ar—H), 7.27, 7.29 (m, 2H, J ¼ 6.0 Hz, Ar—H), 14.60 (s,
1H, N—H). 13C-NMR (75.468 MHz) in CDCl3, internal TMS, d (ppm):
26.73 (CH3), 31.80 (CH3), 117.46 (Ar—H), 117.46 (Ar—H), 129.90
—
(Ar—NH—N), 131.12 (Ar—H), 131.12 (Ar—H), 133.60 (C N),
—
—
—
140.31 (Ar—Cl), 197.02 (C O), 198.29 (C O).
—
—
.
DS1 ¼ RlnVms ;liq Vm;gas
(1)
(2)
X-ray measurements
.
DS2 ¼ RlnVmꢅ Vms ;liq
The X-ray quality single crystals of HL2 were grown by slow
evaporation at room temperature of its ethanol solution.
They were immersed in cryo-oil, mounted in a Nylon loop and
measured at a temperature of 150 K. Intensity data were collected
using a Bruker AXS-KAPPA APEX II diffractometer with graphite
monochromated Mo-Ka (l 0.71073) radiation. Data were collected
using omega scans of 0.58 per frame and full sphere of data were
obtained. Cell parameters were retrieved using Bruker SMART
software and refined using Bruker SAINT[83] on all the observed
reflections. Absorption corrections were applied using SADABS.
Structures were solved by direct methods by using the
SHELXS-97 package[84] and refined with SHELXL-97.[85] Calcu-
lations were performed using the WinGX System-Version
1.80.03.[86] All hydrogens were inserted in calculated positions.
Least square refinements with anisotropic thermal motion
parameters for all the non-hydrogen atoms and isotropic for
the remaining atoms were employed. Crystallographic data
(excluding structure factors) for the structure reported in this
paper have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication, CCDC (766556) for
compound HL2. Copies of this information may be obtained free
of charge from The Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK. Fax: þ44 1223 336 033. E-mail: data_request@
ccdc.cam.ac.uk. Web page: http://www.ccdc.cam.ac.uk.
ꢄ
.
ꢅ
0;s
liq
0;s
S
ꢁ S þ RlnVs
Vm;gas
gas
m;liq
ꢄ
.
ꢅ
a ¼
(3)
0;s
S
þ RlnVs
Vm;gas
gas
m;liq
Ss ¼ Sg þ DSsol ¼ Sg þ ½DS1 þ aðSg þ DS1Þ þ DS2ꢂ
¼ Sg þ ½ðꢁ12:21 cal=mol KÞꢁ0:23ðSgꢁ12:21 cal=mol KÞ
þ 5:87 cal=mol Kꢂ
(4)
where Sg – gas-phase entropy of solute, DSsol – solvation entropy,
S0,sliq, S0,s and Vsm,liq – standard entropies and molar volume
of the solvent in liquid or gas phases (149.62 and 245.48 J/mol K
gas
and 52.16 mL/mol, respectively, for CH3CN), Vm,gas – molar
volume of the ideal gas at 25 8C (24 450 mL/mol), Vo – molar
volume of the solution corresponding to the standard conditions
(1000 mL/mol). The enthalpies and Gibbs free energies in solution
(Hs and Gs) were estimated using the expressions (5) and (6)
m
Hs ¼ Es þ HgꢁEg
Gs ¼ HsꢁT ꢀ Ss
where Es, Eg and Hg are the total energies in solution and in gas
phase and gas-phase enthalpy.
(5)
(6)
Vertical ionization potentials were calculated as the difference
of the total energies in solution of the oxidized species with
an unrelaxed geometry and the neutral structure HL1–5. Vertical
electron affinities were calculated as the difference of the total
energies in solution of the neutral structure HL1–5 and the
reduced species with an unrelaxed geometry. The nature of the
chemical bonds, electron structures, and effective atomic charges
were calculated and analyzed using the NBO partitioning
scheme.[94]
Potentiometric measurements
The following mixtures, (i)–(iii), were prepared and titrated
potentiometrically against standard 0.01 M NaOH (for HL3 0.04 M
NaOH) in a 40/60% (v/v) water–ethanol mixture at 298 K.
(i) 5 mL of 0.001 M HCl þ 5 mL of 1 M KCl þ 30 mL ethanol;
(ii) 5 mL of 0.001 M HCl þ 5 mL of 1 M KCl þ 25 mL etha-
nol þ 5 mL 0.0l M HL5;
(iii) 5 mL of 0.001 M HCl þ 5 mL of 1 M KCl þ 25 mL of etha-
nol þ 5 mL of 0.02 M HL3.
Acknowledgements
For each mixture (i)–(iii) the volume was made up to 50 mL with
bidistilled water before the titration process. These titrations were
also repeated at temperatures of 308 and 318 K.
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., M.L.K. and K.T.M.
J. Phys. Org. Chem. 2011, 24 764–773
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

M. N. KOPYLOVICH ET AL.
express gratitude to the FCT and IST for working contracts and
a post-doc fellowship. The authors gratefully acknowledge the
Portuguese NMR Network (IST-UTL Centre) for the NMR facility.
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