3,5-Di-tert-butyl-1,4-benzoquinone ferrocenoylhydrazone and its zinc(II), palladium(II), and mercury(II) complexes: Structure and properties

Article abstract of DOI:10.1134/S1070363212010215

A new ligand system, 3,5-di-tert-butyl-1,4-benzoquinone ferrocenoylhydrazone, was synthesized and its complexes with zinc(II), Hg(II), and palladium(II) were prepared. The compounds were studied by IR, electron absorption, and ¹H NMR spectrosco

Full text of DOI:10.1134/S1070363212010215

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 1, pp. 131–137. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © E.A. Raspopova, A.N. Morozov, L.D. Popov, I.N. Shcherbakov, S.I. Levchenkov, V.A. Kogan, 2012, published in Zhurnal Obshchei
Khimii, 2012, Vol. 82, No. 1, pp. 135–141.
3,5-Di-tert-butyl-1,4-benzoquinone Ferrocenoylhydrazone
and Its Zinc(II), Palladium(II), and Mercury(II) Complexes:
Structure and Properties
E. A. Raspopovaa, A. N. Morozova, L. D. Popova,
I. N. Shcherbakova, S. I. Levchenkovb, and V. A. Kogana
a Southern Federal University, ul. Sorge 7, Rostov-on-Don, 344090 Russia
e-mail: mrzv_61@mail.ru
b Southern Scientific Center, Russian Academy of Sciences, Rostov-on-Don, Russia
Received October 28, 2010
Abstract—A new ligand system, 3,5-di-tert-butyl-1,4-benzoquinone ferrocenoylhydrazone, was synthesized
and its complexes with zinc(II), Hg(II), and palladium(II) were prepared. The compounds were studied by IR,
electron absorption, and 1H NMR spectroscopy. Structure of the compounds obtained is discussed.
DOI: 10.1134/S1070363212010215
Hydrazones based on the ferrocene derivatives
attract attention due primarily to their high complexing
ability, as well as in connection with their valuable
practical properties, including biological [1–5], photo-
and electrochemical activities [6]. These compounds
can be divided in two groups: direct (I) and reverse
(II) hydrazones. The compounds of the first group
have been widely studied. This group comprises the
products of condensation of an acylferrocene (mono-
and diacetylferrocene, formylferrocene, benzoylferro-
cene) with carboxylic acid hydrazides. The reverse
hydrazones were obtained by condensation of the
carbonyl compounds with ferocenecarboxylic acid
hydrazide; they there much less studied.
diastereotopic labels, namely 3,5-di-tert-butyl-1,4-
quinone, as a carbonyl component, to investigate pos-
sible dynamic processes caused by the stereochemical
nonrigidity of this segment. Such systems are of
considerable interest in studying the mechanisms and
kinetics of Z–E stereodynamics [24, 25] and contribu-
tion of various isomeric forms to the structure of the
ligands and the complexes based on them. In this
regard, we synthesized 3,5-di-tert-butyl-1,4-benzo-
quinone ferrocenenoylhydrazone III and its complexes
with Pd(II), Zn(II), and Hg(II).
O
C(CH3)3
NH
N
O
R1
R1
Fe
Fc
H
R2
H
C(CH3)3
N N
N N
III
R2
Fc
The IR spectrum of 3,5-di-tert-butyl-1,4-benzo-
quinone ferrocenenoylhydrazone III includes the
bands of stretching vibrations of carbonyl groups of
the hydrazone (1638 cm–1) and quinone fragments, the
hydrazone C=N (1618 cm–1) and NH (3187 cm–1)
bonds. The spectrum also contains the bands of
torsional vibrations of cyclopentadiene rings at 481
and 500 cm–1.
X
X
I
II
X = O, S; R1, R2 = Alk, Ar, Het; Fc = C5H5FeC5H4.
On the basis of certain direct hydrazones a series of
coordination compounds of metals has been syn-
thesized and studied [7–15]. It was of interest to extend
a number of systems based on reverse hydrazones [16–
23] using a spatially hindered fragment containing
In order to clarify the ligand structure and to reveal
the features of its behavior in solution, we have
131

132
RASPOPOVA et al.
recorded its 1H NMR spectra in the solvents of
magnetic nonequivalence and are registered with
different chemical shifts: 6.96 ppm (1H) and 7.64 ppm
(1H). The spectrum includes a signal with chemical
shift 11.80 ppm corresponding to the imine proton,
which disappeares at adding D2O to the sample.
different polarity (CDCl3 and DMSO-d6) at 25°C.
1
In the H NMR spectrum of the hydrazone in
DMSO-d6 a singlet was registered in a strong field
with the chemical shift 1.30 ppm (18H) corresponding
to tert-butyl groups, a singlet with the chemical shift of
4.16 ppm (5H) corresponding to unsubstituted
cyclopentadienyl ring, two signals with the chemical
shifts of 4.45 and 6.5 ppm (2H each) corresponding to
the pairs of equivalent groups of protons of the
substituted cyclopentadienyl ring. In contrast to the
tert-butyl groups, the quinone ortho-protons show
Given the well-known tautomeric equilibria of the
quinone and hydrazone derivatives, the possible
tautomers of compound III in solution (without
accounting for the rotational isomers arising at the
rotation of acylferrocene moiety around the C–N bond)
are azoquinone ketamine IIIa, azoquinone enimine
IIIb, and two zwitterionic forms, IIIc and IIId.
O
N
OH
N
C(CH3)3
O
C(CH3)3
O
Fc
Fc
N
N
H
C(CH3)3
C(CH3)3
IIIа
IIIb
O
O
N
C(CH3)3
O−
C(CH3)3
O−
Fc
Fc
+
+
N
N
N
H
H
C(CH3)3
C(CH3)3
IIIc
IIId
4
It is known that the spin-spin coupling constant J
of the classical aromatic protons is ~1.6 Hz and for the
quinoid structures, ~2.2 Hz [24]. In the spectrum of
quinine III one of the signals of aromatic protons is
split into a doublet with spin-spin coupling constant
2.23 Hz, which allows to exclude from consideration
the tautomeric forms IIIc and IIId, the benzoid forms
of the quinone.
The signal of NH proton also is shifted to the strong
field and broadened. The signals of the ferrocene
moiety underwent minor downfield shifts (~0.15 ppm).
Electron absorption spectrum of the hydrazone III
contains four absorption bands with the maxima at
205, 290, 360, and 421 nm. The bands positions
depend only slightly on the solution pH, in contrast to
the molar extinction coefficient of these bands. The
greatest dependence on pH showed the bands at the
maximum and minimum wavelengths (λ). The most
short-wavelength of these bands (λ 205 nm) apparently
belongs to the π–π* transitions in the aromatic cyclo-
pentadienyl ligands, and the long-wavelength band
corresponds to the d–π* transitions in ferrocene [19].
A twofold opposite change in molar extinction
coefficients of these bands (see the table) can be
attributed to the protonation of the ferrocene fragment.
A feature of the spectrum is the apparent
equivalence of tert-butyl groups, with the obvious non-
equivalence of the aromatic protons. To remove this
contradiction we recorded spectrum of this compound
in a less polar solvent. In the high-field part of this
spectrum were registered two signals of equal intensity
(9H) corresponding to tert-butyl groups, with chemical
shifts 1.33 and 1.38 ppm.
Changing the solvent leads to an additional
modification of the spectrum: the signals are shifted,
particularly those of the quinone protons, which are
shifted relative to each other by 0.15 ppm in CDCl3
and by 0.68 ppm in DMSO-d6.
It is known that, depending on the solution pH, the
proton (H+) is localized either at the iron atom (in
strong acid media) or on the π-system of
cyclopentadiene ring (in weakly acidic solutions) [26].
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3,5-DI-tert-BUTYL-1,4-BENZOQUINONE FERROCENOYLHYDRAZONE
133
The absorption band with the maximum at 360 nm in a
weakly alkaline medium undergoes a slight blue shift
(by 15 nm) and 1.5-fold drop in intensity due to partial
ionization of the ligand.
Dependence of parameters of electronic spectrum of 3,5-di-
tert-butyl-1,4-benzoquinone ferrocenoylhydrazone on рН of
solution (methanol, с 10–5 M)
λ, nm
ε, l mol–1 cm–1
Absorption
band
The presence of several donor centers in the
system, connected with a system of conjugated bonds
and a labile hydrogen atom, suggests to use it as a
ligand for the formation of complexes. At the
interaction of the hydrazone with the ions Pd2+, Zn2+,
and Hg2+ complexes were synthesized of the type IV,
V and VI, respectively, with the composition
corresponding to the general formula ML2. The choice
of the diamagnetic metals is due to the possibility of
studying such complexes by NMR spectroscopy. This
ligand forms also complexes with other metals, in
particular, with cadmium(II), lead(II), and nickel (II),
but chemically pure complexes were obtained only
with zinc(II), palladium(II), and mercury(II).
pH < 7 pH = 7 pH > 7 pH < 7 pH = 7 pH > 7
1
2
3
4
220
295
355
427
205
290
360
421
210
295
345
435
34840 19720 18160
11470 13260 11520
14340 14570
~4000 8500
9870
8300
in the quinoid structures. The form of the signals of
tert-butyl substituents indicates nonequivalence of
these groups. It is known that complexes based on
sterically hindered quinone azomethines are conforma-
tionally labile owing to the Z–E isomerization [24, 25].
The same is possible theoretically in the case of the
discussed complexes. There are two possible
mechanisms of conformational transformation: rotation
(requiring high energy expenditure) and planar in-
version (energetically less costly).
Such processes are temperature-dependent, which
is expressed by the shape of signals in the NMR
spectra of respective compounds. At increase in
temperature coalescence is observed with degenerating
into a singlet of the signals of tert-butyl groups.
In the 1H NMR spectrum of the palladium(II)
complex in CDCl3 compared to that of the original
ligand the strong-field signal of the acidic proton
disappears, which confirms the enolization and
deprotonation of the compound at the complex
formation. Also a significant downfield shift was
detected of the signals of the quinone fragment
protons, while their nonequivalence was retained. The
values of the shift are 0.57 and 1.02 ppm, consistently
with the involvement of the quinoneimine nitrogen
atom in the coordination. The signals of ortho-protons
of the cyclopentadiene ring are shifted upfield by
0.27 ppm. The signals of the quinone protons are split
Since for palladium is typical mainly a planar
structure of the chelate site, one can persume
equilibrium of cis- and trans-forms of the complex in
solution.
4
into doublets with a spin-spin coupling constant J =
2.6 Hz, characteristic of the coupling of meta-protons
Fc
C(CH3)3
Fc
N
C(CH3)3
N
O
O
N
O
O
N
C(CH3)3
C(CH3)3
C(CH3)3
C(CH3)3
Pd
Pd
O
N
O
O
N
O
N
C(CH3)3
N
Fc
C(CH3)3
Fc
IV
cis-Form
IV
trans-Form
These structures are equally probable, since the
intensity ratio of the components in the split signal of
tert-butyl groups in the spectra corresponds to the ratio
of 1:1. However, in the undistorted cis-structure of the
complex IV a significant steric interaction of tert-butyl
groups is possible. In this connection it should be
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134
RASPOPOVA et al.
Fig. 1. Enantiomers of the square-planar complex based on hydrazone III.
noted that the temperature dependence of the signal
shapes, as well as splitting (doubling) of the signals of
each of the nonequivalent protons in the quinone and
ferrocene groups may be associated with the presence
of other conformational equilibrium in the solution,
connected with the existence of two enantiomers
differing by the mode of stabilization of their
structures due to π-conjugation in the cyclopenta-
diene–quinone hydrazone fragment (Fig. 1).
same solvent but under the other conditions. The
spectrum changes with time, and after 120 h the
intensities of the components of the signal of tert-butyl
groups leveled. The broadened signal detected in the
initial spectrum at 7.25 ppm transformed into two
components with the chemical shifts 7.05 and 7.25
ppm. In addition, the signal of ortho-protons of the
substituted cyclopentadiene ligand underwent a down-
field shift by 0.18 ppm. To confirm the assumption of
possible dynamic processes, we registered the spectra
of compounds at elevated temperature (only qualita-
tively). Changing the temperature allowed to note a
tendency to coalesce of the proton signals of tert-butyl
groups
Comparative analysis of the IR spectra of hyd-
razone III and its complexes with zinc(II) and mer-
cury(II) showed that the coordination leads to the
disappearance of the absorption bands of stretching
vibrations of the carbonyl group of hydrazone
fragment and of N–H bond, with the simultaneous
low-frequency displacement by 19 cm–1 of the ν(C=N)
band. This fact indicates the coordination of the
hydrazone as a deprotonated enol form, with involve-
ment in coordination of the azomethine nitrogen atom.
In the IR spectra of zinc(II) and mercury(II) complexes
are registered also the additional absorption bands
ν(C=N) at 1589–1612 cm–1. The presence of these
bands indicates the nonequivalence of azomethine
groups of the hydrazone and quinone fragments.
In the 1H NMR spectrum of the complex of zinc(II)
the following signals were registered: a group of
proton signals of the ferrocene structure, δ 4.26 ppm
(5H), 4.55 ppm (2H), and 4.97 ppm (2H), a broadened
signal at δ 4.25 ppm (2H), and a split high-field signal
with chemical shifts of the components 1.32 and
1.38 ppm, corresponding to tert-butyl groups (Fig. 2).
The components of this signal are not equal in
intensity, and the signals of the spectrum are
broadened, suggesting the existence in the solution of
dynamic equilibrium between different forms of the
complex. This assumption was evidenced indirectly at
the repeated registration of the spectrum using the
Apparently, the equilibrium in the solution of the
complex is associated with a phenomenon of diagonal
twist with rupture of the M–L bond, characteristic of
tetrahedral complexes of zinc(II), palladium(II), and
mercury(II) (Fig. 3) [24].
Similar features were identified for the complexes
of mercury(II), which indirectly indicates the similarity
of structures of the zinc(II) and mercury(II) chelates.
In the electron absorption spectra of these
complexes in acetone solution a charge transfer band
was recorded, which for the zinc(II) complex was
found at a shorter wavelength, λmax = 545 nm, and for
the palladium(II) complex λmax = 605 nm, which
indicates a greater transfer of electron density from the
metal to ligand.
EXPERIMENTAL
IR spectra of the samples prepared as suspensions
in mineral oil and the fluorinated hydrocarbon oil were
obtained on
a
Varian Scimitar 1000 FT-IR
1
spectrometer in the range of 4000–400 cm–1. H NMR
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3,5-DI-tert-BUTYL-1,4-BENZOQUINONE FERROCENOYLHYDRAZONE
135
(a)
δ, ppm
δ, ppm
δ, ppm
δ, ppm
(b)
δ, ppm
δ, ppm
δ, ppm
Fig. 2. Changes with temperature in the 1H NMR spectrum of the zinc(II) complex with hydrazide III. (a) 293 K and (b) 313 K.
spectra were recorded on a Bruker AM-300 instrument
with internal reference TMS. Optical absorption
spectra were recorded on a Unicam Helios Gamma
spectrometer in cells 1 cm thick in the range of 195–
1100 nm, the samples were solutions in methanol or
acetone (c 10–5 M).
ethanol was added a solution of 0.002 mol of 3,5-di-
tert-butyl-1,4-benzoquinone in 3 ml of ethanol. To the
reaction mixture was added 2 drops of acetic acid, and
the resulting solution was refluxed for 3 h. The
solution becomes dark brown. On the next day after
cooling a brown precipitate separated and was filtered
off and washed twice with ethanol. To purify the
hydrazone, it was boiled in a mixture of ethanol and
DMF (4 ml of ethanol + 3 ml of DMF) for 1 h. After
one day the precipitate was filtered off and again
3,5-Di-tert-butyl-1,4-benzoquinone ferrocenoyl-
hydrazone (III). To a solution of 0.002 mol of
ferrocenecarboxylic acid hydrazide [27] in 10 ml of
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136
RASPOPOVA et al.
(a)
(b)
(c)
(d)
Fig. 3. The possible mechanisms of enantiomerization of tetrahedral structures of complex compounds. (a) intramolecular rearran-
gement though diagonal twist, (b) intramolecular enantiomerization through cleavage of the M–L bond, (c) or (b–d) degenerate
ligand exchange.
boiled in 4 ml of methanol. The product was separated,
washed with methanol, and dried in a vacuum at 90°C.
Yield 45%, mp 212–213°C. 1H NMR spectrum
(DMSO-d6), δ, ppm (J, Hz): 1.30 s [18H, C(CH3)3],
4.16 s (5H, C5H5) 4.45 s (2H, ortho-C5H4) , 5.06 s (2H,
meta-C5H4) 6.96 s (1H, quinone), 7.64 s (1H, quinone),
11.80 s (1H, NH). Found, %: C 67.89, H 6.21; N 5.88;
Fe 11.96. C25H30Fe N2O2. Calculated, %: C 67.26, H
6.73; N 6.28; Fe 12.56.
4.55 br.s (2H, ortho-C5H4), 5.14 br.s (2H, meta-C5H4),
7.25 br.s (2H, quinone). Found, %: C 63.51, H 6.34; N
6.12; Fe 12.07. C50H58Fe2N4O4Zn. Calculated, %: C
63.22, H 6.11; N 5.90; Fe 11.80.
Complex of mercury(II) with hydrazone (III)
was prepared similarly. A fine crystalline dark-green
powder. Yield 32%, mp > 200°C. Found, %: C 55.54,
H 5.35; N 4.34; Fe 10.71. C50H58 Fe2HgN4O4. Cal-
culated, %: C 55.30, H 5.35; N 4.61; Fe 10.32.
Complex of zinc (II) with 3,5-di-tert-butyl-1,4-
benzoquinone ferrocenoylhydrazone. To the boiling
suspension of 0.1 g of the hydrazone III in 10 ml of
anhydrous methanol was added a hot solution of zinc
acetate in 3 ml of methanol, the ligand to metal ratio
was 2:1. Complete dissolution of the components
occurred and formation of a dark-brown solution. The
solution was refluxed for 3 h and evaporated to half
volume, then left for 48 h. The separated precipitate
was isolated and washed twice with a small amount of
ethanol. Yield 55%, mp 185–190°C. 1H NMR
spectrum (DMSO-d6), δ, ppm (J, Hz): 1.28 s [9H,
C(CH3)3], 1.32 s [9H, C(CH3)3], 4.26 br.s (5H, C5H5),
Complex of palladium (II) with 3,5-di-tert-butyl-
1,4-benzoquinone ferrocenoylhydrazone was pre-
pared by the similar method, save that palladium
acetate was dissolved in acetone. The complex
precipitated after 5 min boiling the solution. Yield
1
67%, mp > 250°C. H NMR spectrum (DMSO-d6), δ,
ppm (J, Hz): 1.38 s [9H, C(CH3)3], 1.39 s [9H, C(CH3)3],
4.22 s (5H, C5H5), 4.45 t (2H, ortho-C5H4, 3J 1.9), 4.83
t (2H, meta-C5H4, 3J 1.9), 7.68 d (1H, quinone, 4J 2.9),
4
8.28 d (1H, quinone, J 2.9). Found, %: C 60.65, H
5.32; N 5.89; Fe 10.89. C50H58Fe2N4O4Pd. Calculated,
%: C 60.24, H 5.82; N 5.62; Fe 11.24.
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3,5-DI-tert-BUTYL-1,4-BENZOQUINONE FERROCENOYLHYDRAZONE
137
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