Study of electrochemical oxidation of nickel catecholate complexes with bis(diphenylphosphino)ethane by cyclic voltammetry and ESR

Article abstract of DOI:10.1007/s11172-007-0017-0

One-and two-electron electrochemical oxidation of the (dppe)Ni(Cat) complexes (dppe is bis(diphenylphosphino)ethane, Cat is the sterically hindered catechol dianion) was studied. The transfer of the first electron proceeds reversibly to give paramagnetic

Full text of DOI:10.1007/s11172-007-0017-0

Russian Chemical Bulletin, International Edition, Vol. 56, No. 1, pp. 104—107, January, 2007
104
Study of electrochemical oxidation of nickel catecholate complexes
with bis(diphenylphosphino)ethane by cyclic voltammetry and ESR
M. K. Kadirov,aꢀ Yu. H. Budnikova,a T. V. Gryaznova,a O. G. Sinyashin,a
M. P. Bubnov,b A. V. Krashilina,b and V. K. Cherkasovb
aA. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 75 2253. Eꢀmail: kadirov2004@mail.ru
bG. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831 2) 66 1497
Oneꢀ and twoꢀelectron electrochemical oxidation of the (dppe)Ni(Cat) complexes (dppe is
bis(diphenylphosphino)ethane, Cat is the sterically hindered catechol dianion) was studied.
The transfer of the first electron proceeds reversibly to give paramagnetic species; parameters of
their EPR spectra attest to a square planar geometry of oneꢀelectron oxidation products. The
transfer of the second electron is irreversible because of coꢀproportionation of radical cations
involving the initial complexes and the generated dications.
Key words: (diphosphine)nickel(catecholate), radical cation, dication, electron transfer,
electron spin resonance, cyclic voltammetry, coꢀproportionation.
In recent years, metal coordination compounds with
Currently a new method, electron spin resonanceꢀ
detected voltammetry (V—ESR) is being developed. This
method comprises two traditional methods, voltammetry
and ESR, which are combined by an original threeꢀelecꢀ
trode electrolytic ESR cell6 (El—ESR) for investigation
of paramagnetic species and a computer controlling the
ESR spectrometer and the electrochemical setup consistꢀ
ing of a programming device and a potentiostat. The
method allows simultaneous recording of usual voltꢀ
ammetric curves i(E) and also dependences of the ESR
signal intensity (s(E)) and its first derivative (s´(E)) on the
potential. In this work, the V—ESR technique was used to
study nickel catecholate complexes 1 and 2 with bis(diꢀ
phenylphosphino)ethane (dppe).
paramagnetic ligands, including оꢀsemiquinone ligands,
have attracted considerable attention of researchers. The
molecules of these compounds can contain several paraꢀ
magnetic centers of various nature, whose sign and energy
of exchange interactions are determined by the electronic
structure and geometric features.1—3 Paramagnetic ligands
are often electrochemically active; the electrochemical
reduction or oxidation of complexes may involve not only
transformations of the central metal ion but also ligand
transformations, which change the magnetic properties of
the ligands and the system as a whole.
By combining electrochemical methods with ESR
spectroscopy, one can gain additional information about
the electrode processes, electronic structure, geometry,
and reactivity of depolarizers. In some cases, using the
ESR spectrum of a paramagnetic substrate, one can idenꢀ
tify the localization region of the unpaired electron and
the geometry of magnetic nuclei.
Methods for simultaneous recording of ESR signals
and electrochemical characteristics using an electrolytic
cell—ESR in a cavity of the ESR spectrometer in a
potentiostatic mode, by chronopotentiomety, and cyclic
voltammetry (CV) are known.4,5 However, direct comꢀ
parison of CV curves and ESR signal levels is not entirely
valid, because the current is the first derivative of the
charge that has passed through the cell, while the ESR
signal level is proportional to the proper charge related to
the depolarizer under study.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 99—102, January, 2007.
1066ꢀ5285/07/5601ꢀ0104 © 2007 Springer Science+Business Media, Inc.

Oxidation of nickel catecholate complexes
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
105
Experimental
a
Complexes 1 and 2 were prepared at the G. A. Razuvaev
Institute of Organometallic Chemistry of the Russian Academy
of Sciences (Nizhny Novgorod) by the reaction of nickel
tetracarbonyl with dppe and the corresponding оꢀquinone taken
in an equimolar ratio according to a publication.7 The comꢀ
plexes recrystallized from a toluene—dichloromethane mixture
(10 : 1) are darkꢀgreen crystals. Yield 53%. Found (%): C, 73.82;
H, 7.44; Ni, 7.62; P, 8.04. C47H52NiO2P2. Calculated (%):
1
2
C, 73.37; H, 6.76; Ni, 7.64; P, 8.07. IR (mineral oil), ν/cm–1
:
1595, 1485, 1440 s, 1410, 1275, 1255, 1190, 1110 s, 1035, 980,
940, 880, 830, 795, 745, 725 s, 710 s, 690 s, 660, 540, 480, 450.
Acetonitrile was purified by threefold distillation from
KMnO4 and P2O5; NaBF4 was dried for 2 days in vacuo at 100 °C.
Liquid samples were freed from oxygen by liquid nitrogen freezꢀ
ing—evacuation—thawing cycle repeated three times. An inert
atmosphere was created in the El—ESR cell by supplying heꢀ
lium therein under normal pressure. The concentration of the
complexes was 0.005 mol L–1, that of the supporting salt was
0.3 mol L–1. The working an auxiliary electrodes were made of
platinum, and Ag/AgNO3 (0.01 M solution) was used as the
reference electrode.
The measurements were carried out on a hardware/software
unit assembled from an analog electrochemical setup with a
PIꢀ50ꢀ1 potentiostat, a Prꢀ8 programming device, an RE1306
Хꢀrange ESR spectrometer, an ADC module, an Е14ꢀ440 DAC
(LꢀCard), a generation IV computer, and an original threeꢀ
electrode electrolytic spiral cell. The temperature was mainꢀ
tained by a BꢀVTꢀ1000 temperature control unit (Bruker) conꢀ
nected to an RE1306 spectrometer.
0.5 mT
b
1
2
0.5 mT
Fig. 1. Experimental (1) and simulated (2) ESR spectra of radiꢀ
cal cations 1•+ (a) and 2•+ (b) electrochemically generated in
acetonitrile at 293 К in the potentiostatic mode at the firstꢀwave
potential.
ESR spectra were processed by a WinSim (NIEH/NIH)
program, which allows one to determine the key parameters of
the experimental isotropic spectrum by simulating the spectrum
and automatic fitting of the simulated spectrum to the experiꢀ
mental one.
The ESR spectrum of radical cation 1•+ (see Fig. 1, a)
is a quartet with a single line width of 0.105 mT caused by
the HFC of the unpaired electron with two equivalent
P nuclei of the diphosphine ligand with the constant aP =
0.255 mT and with protons in positions 4 and 6 of the
оꢀsemiquinone ring with the constants aH(4) = 0.369 mT
and aH(6) = 0.069 mT.
The spectrum of radical cation 2•+ (see Fig. 1, b) with
a single line width of 0.05 mT was better resolved. The
HFC of the unpaired electron with two equivalent P nuꢀ
clei of the diphosphine ligand, aP = 0.257 mT, and with
two protons, equivalent to within the linewidth, in posiꢀ
Results and Discussion
At the firstꢀpeak potential of the electrochemical oxiꢀ
dation of complexes 1 and 2 (0.6 and 0.7 V, respectively),
ESR signals appear. The ESR spectra of radical cations 1•+
and 2•+ recorded at room temperature in acetonitrile are
shown in Fig. 1, while the magnetic resonance parameters
obtained by automatic fitting with the assumption of
a Lorentzian line shape are in Table 1.
tions 4 and 5 of the оꢀsemiquinone ring, aH(4) = aH(5)
0.366 mT, are observed.
=
Table 1. Parameters of the ESR spectra of radical cations 1•+
and 2•+
The parameters of the isotropic ESR spectra (small
line width, the values of gꢀfactors, HFC constants with
the ligand protons and 31P magnetic isotopes of the
diphosphine ligand) are typical of metal complexes conꢀ
taining a paramagnetic оꢀsemiquinone ligand. The HFC
parameters that we determined for the radical cations
derived from complexes 1 and 2 are consistent with
published data7 on the electrochemical oxidation of
analogous nickel catecholate complexes. The low HFC
Comꢀ
pound
aP
aH
Line width
gꢀFactor
mT
+
1•
0.255 0.369 (H(4)),
0.069 (H(6))
0.257 0.366 (Н(4),
H(5))
0.105
0.050
2.004
2.004
2•+

106
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
Kadirov et al.
The CV (i(E)) and CV—ESR (s´(E)) curves for comꢀ
pound 1 in a broader potential range, i.e., including the
heterogeneous transfer of the second electron, are shown
in Fig. 2, b. According to CV data, the first oxidation
wave is reversible, while reversibility of the second wave is
not manifested in the voltammogram under the experiꢀ
mental conditions. The first oxidation wave i(E) correꢀ
sponds to the s´(E) wave related to the formation of the
radical cation by reaction (1). The second wave (1.3 V)
corresponds to the oxidation of the radical cation to give
the dication
a
1
2
0
0
0.2
0.4
0.6
0.8
E/V
+
1•
12+
,
(2)
1
2
b
as is clearly seen from the shape of the s´(E) curve, which
is lower in intensity. The s´(E) CV—ESR curve explains
why the second oxidation wave is irreversible. After the
second oxidation wave is reached, s´(E) starts to increase.
This implies that some additional process, resulting in
radical cations, occurs after the dication formation poꢀ
tentials are attained. As a result, the rate of formation of
radical cations is higher than the rate of their consumpꢀ
tion according to reaction (2). This third process is the
coꢀproportionation of radical cations with participation
0
of the initial compound 1 and its dication 12+
.
0.2
0.4
0.6
0.8
1.0
1.2
E/V
+
1 + 12+
2 1•
(3)
Fig. 2. Cyclic voltammetry (i(E), 1) and CV—ESR (s´(E), 2)
curves of complex 1 (5•10–3 mol L–1) in acetonitrile in the
presence of a 3•10–1 M solution of NaBF4 at potential sweep
from 0 to 1.0 V (a) and from 0 to 1.6 V (b). The potential sweep
rate was 0.5 V s–1, 293 К.
The increase in the rate of formation of radical cations
continues also after the change in the direction of potenꢀ
tial sweep: the intensity of the s´(E) curve starts to deꢀ
crease at firstꢀwave potentials to reach a minimum at the
cathodic peak.
The oxidation pattern of compound 2 (Fig. 3) does
not differ much from that of complex 1. The first and
second oxidation waves (0.7 and 1.4 V, respectively) are
shifted by ∼0.1 V to more anodic potentials compared to
the waves of complex 1 and the firstꢀwave return peak is
less intense.
Thus, complexes 1 and 2 undergo twoꢀelectron elecꢀ
trochemical oxidation. The transfer of the first electron is
a reversible process affecting the catecholate ligand giving
rise to paramagnetic оꢀsemiquinone complexes 1•+ and
2•+ whose ESR are easily recorded by the hardware/softꢀ
constants with the phosphorus magnetic nuclei
(0.255—0.257 mT) in the diphosphine ligand attest to a
square planar geometry of the resulting cations.7,8
The CV (i(E)) and CV—ESR (s´(E)) curves for comꢀ
pound 1 are shown in Fig. 2, a. The CV curve shows the
final current immediately after application of the potenꢀ
tial and its gradual growth before the oxidation potentials
of the depolarizer are attained, which is mainly due to
charging of the double electric layer and oxidation of
impurities. No substantial increase in s´(E) is noted beꢀ
fore attaining the oxidation potentials of the depolarizer.
This is attributable to the fact that the CV—ESR techꢀ
nique measures the rate of variation of the ESR signal
intensity. This change is due only to depolarizer elecꢀ
trolysis, while the currents of the double electric layer
charging, the adsorption component of the current, and
the currents of impurities do not contribute to the s´(E)
dependence. The CV—ESR curve mainly follows the
CV curve, although does not reproduce its shape in detail.
The s´(E) dependence reflects better the reversibility of
the first electron transfer. The reversible intramolecular
electron transfer related to the catecholate—semiquinone
transition is well known.9—11
1
2
0.2
0.4
0.6
0.8
1.0
1.2 E/V
Fig. 3. Cyclic voltammetry (i(E), 1) and CV—ESR (s´(E), 2)
curves of complex 2 (5•10–3 mol L–1) in acetonitrile in the
presence of a 3•10–1 M solution of NaBF4. The potential sweep
rate was 0.5 V s–1, 293 К.
+
1
1•
(1)

Oxidation of nickel catecholate complexes
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 1, January, 2007
107
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radical cations 1•+ and 2•+ attest to a square planar geꢀ
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voltammetry clarified the nature of the processes involved
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of radical cations involving the initial complexes and their
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Received May 3, 2006;
in revised form October 19, 2006