Study on the spectroelectrochemical properties of S-benzyl-N-(ferrocenyl-1-methyl-metylidene)-dithio-carbazate nickel (II) complex

Article abstract of DOI:10.1016/S1386-1425(02)00036-7

Mechanism of electron transfer of a bridge biferrocene complex, Ni(LSB)₂, derived from the Schiff base ligand, HLSB, S-benzyl-N-(ferrocenyl-1-methyl-metylidene)-dithio-carbazate Ni(LSB)₂, during redox processes is illuminated by in-situ Fourier transform infrared (FTIR) spectroelectrochemistry. The results indicated that two consecutive one-electron steps were involved which gives the corresponding mono- and di-ferricenium cations, respectively. This complex exhibits moderately strong degree of electronic communication between the two-ferrocene moieties, taking place through the skeleton chain of the ligand due to an extensive electron delocalization in the molecule. Possible pathways of electron transfer of Ni(LSB)₂ during the redox processes are postulated.

Full text of DOI:10.1016/S1386-1425(02)00036-7

Spectrochimica Acta Part A 58 (2002) 3113Á3120
/
www.elsevier.com/locate/saa
Study on the spectroelectrochemical properties of
S-benzyl-N-(ferrocenyl-1-methyl-metylidene)-dithio-carbazate
nickel (II) complex
Xiayan Wang a, Zhaoxiang Deng b, Baokang Jin c,*, Yupeng Tian c,d,
Xiangqin Lin a,*
a Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China
b Department of Chemistry, Inorganic Institute, Tsinghua University, Beijing 100084, People’s Republic of China
c Department of Chemistry, Anhui University, Hefei 230039, People’s Republic of China
d State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic of China
Received 28 September 2001; received in revised form 25 January 2002; accepted 25 January 2002
Abstract
Mechanism of electron transfer of a bridge biferrocene complex, Ni(LSB)2, derived from the Schiff base ligand,
HLSB, S-benzyl-N-(ferrocenyl-1-methyl-metylidene)-dithio-carbazate Ni(LSB)2, during redox processes is illuminated
by in-situ Fourier transform infrared (FTIR) spectroelectrochemistry. The results indicated that two consecutive one-
electron steps were involved which gives the corresponding mono- and di-ferricenium cations, respectively. This
complex exhibits moderately strong degree of electronic communication between the two-ferrocene moieties, taking
place through the skeleton chain of the ligand due to an extensive electron delocalization in the molecule. Possible
pathways of electron transfer of Ni(LSB)2 during the redox processes are postulated.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Biferrocene nickel complex; Electronic communication; Delocalization; In situ FTIR spectroscopy; Digital simulation
1. Introduction
Electron donorÁ
[2Á
/
4] and are recently being [5Á7] reported to act
/
as reaction intermediates. The special features of
metal coordinated donors and acceptors have been
/
acceptor (EDA) or chargeÁ
/
transfer (CT) complexes are recently gaining im-
portance as potential high efficiency non-linear
optical materials [1]. Such complexes have been
well documented [8Á12], and ferrocenyl chromo-
phores possess some of the largest microscopic and
macroscopic second order non-linearities reported
/
to date for organometallic compounds [13Á16].
/
Although a good amount of experimental and
theoretical work has been done on such complexes,
a lot more remains to be performed.
* Corresponding authors. Tel.: ꢀ
551-3603388
E-mail address: xqlin@ustc.edu.cn (X. Lin).
/
86-551-3606641; fax: ꢀ86-
/
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 0 3 6 - 7

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X. Wang et al. / Spectrochimica Acta Part A 58 (2002) 3113Á3120
/
In situ Fourier transform infrared (FTIR)
2.3. Electrochemical measurements
spectroelectrochemistry is one of the useful meth-
ods to characterize the intermediates in redox
processes and the mechanism of electron transfer.
Therefore, the effect of other bridging groups on
electron transfer between the two ferrocene moi-
eties of metal bridged biferrocene complexes dur-
ing redox processes and the mechanism for
electron transfer could be determined by the
configurable changes of ligand and/or spacer
group during the redox processes by in-situ
FTIR spectroelectrochemistry.
In this paper, electrochemical measurements,
digital simulation and in situ FTIR were combined
to study the mechanism of electron transfer of a
metal bridged biferrocene trinuclear complex,
Ni(LSB)2, derived from the Schiff base ligand,
HLSB, S-benzyl-N-(ferrocenyl-1-methyl-methyli-
dene)-dithiocarbazate.
Electrochemical measurements were conducted
on an EG&G PAR Model 283 potentiostat/
galvanostat using M270 electrochemical analysis
software in a three-electrode cell configuration
comprising a BAS glassy carbon (GC) (3 mm in
diameter) working electrode, a Ag/AgCl reference
electrode and a coiled platinum wire counter
electrode. Cyclic voltammograms were obtained
in CH2Cl2 solution containing 0.1 M tetrabuty-
lammoniumperchlorate (TBAP) as supporting
electrolyte.
2.4. In situ FTIR spectroscopic measurements
FTIR measurements were performed by an in-
situ method on Nicolet Nexus 870 spectrometer
equipped with a variable angle specular reflectance
accessory (VeeMax II) and a HgCdTe/A (MCT/A)
detector cooled with liquid nitrogen.
The spectroelectrochemical cell was made from
Teflon. A platinum wire and a silver wire were
fixed in the cell body used as counter electrode and
reference electrode, respectively. A disk of sodium
chloride was used as the IR transparent window.
The working electrode was a BAS GC disk (3 mm
in diameter), which was first polished with an
aqueous suspension of 0.1 mm alumina to obtain a
mirrorlike surface and then washed with doubly
distilled water in an ultrasonic bath. In the
measurement of in situ FTIR spectra, the working
electrode was pushed against the window to form a
thin electrolyte layer in the spectroelectrochemical
cell. The incident angle was adjusted to 608. A
2. Experimental
2.1. Preparation of the ligand (HLSB)
S-benzyl dithiocarbazate was synthesized ac-
cording to the literature method [17]. Two drops of
acetic acid were added to a mixture of S-benzyl-
dithiocarbazate and acetylferricene in refluxing
ethanol, after cooling a redÁbrown solid was
/
formed. The solid was collected by filtration and
washed with ethanol and dried under vacuum.
Yield was 80% for the free ligand.
2.2. Preparation of the complex (Ni(LSB)2)
Ethanol solution of the ligand (HLSB), and
nH2O (in a molar ratio of 1:2) were
Ni(OAC)2 ×
/
mixed. A brown crystalline solid was formed after
refluxing for 3 h, which was collected by filtration,
washed with ethanol and dried under vacuum.
Yield was 73% for the nickel complex. The
molecular structure is as shown in Scheme 1.
The details of the design, synthesis, structural
characterization and NLO properties will be
reported in another paper [18].
Scheme 1.

X. Wang et al. / Spectrochimica Acta Part A 58 (2002) 3113Á
/3120
3115
0.449 V). This is due to the strong electron-
withdrawing effect of the dithiocarbazate substi-
tuent when bound directly to the cyclopentadienyl
ring, making oxidation of the ferrocene unit in
HLSB thermodynamically more difficult, than
that of unsubstituted metallocenes.
The voltammetric behavior of Ni(LSB)2 appears
to involve two diffusion-controlled, reversible
redox processes, and the corresponding two half-
wave potentials are close, so that the two voltam-
metric waves are not fully resolved. The determi-
nation of the number of electrons transferred in
the complete oxidation of Ni(LSB)2 was effected
by exhaustive coulometry measurements, carried
out past the anodic wave at the higher potential
Fig. 1. Cyclic voltammograms of 1 mM solution of (a) FcH, (b)
HLSB and (c) Ni(LSB)2 in 0.1 M TBAP/CH2Cl2 (vs. Ag/AgCl).
(Eꢂ1.10 V), resulting in the removal of two
/
Scan rate, 50 mV sꢁ1
.
electrons per molecule. The two consecutive one-
electron oxidation steps give presumably the
corresponding mono- and then di-ferricenium
total of 50 interferometric scans with a resolution
of 4 cmꢁ1 was accumulated for a spectrum with
the electrode under potential control. The spectra
were successively taken while the electrode poten-
tial was scanned at a rate of 2 mV sꢁ1. The
resulting spectra were subtractively normalized to
the base spectrum as:
cations [19] (Eq. (1)) (where Fcꢂ
/
ferrocenyl).
ꢁeꢁ
FcꢀNi unitꢁFc ? (FcꢀNi unitꢁFc)ꢀ
I
Iꢀ
ꢀe
ꢁeꢁ
? (FcꢀNi unitꢁFc)2ꢀ
(1)
ꢀe
I2ꢀ
DR [R(ES) ꢁ R(ER)]
Also, the two steps could be assign to the
oxidation of the Fe(II)ꢀFe(II) complex to the
mixed valence Fe(II)ꢀFe(III) complex, and at a
ꢂ
/
R
R(ER)
/
By subtracting the reflection spectrum at sample
potential ES(R(ES)) from the reflection spectrum
at reference potential ER(R(ER)), the background
due to the absorption of the solution species is
eliminated.
more positive potential, oxidation to the Fe(III)ꢀ
/
Fe(III) complex.
The results also show that it is appreciably more
difficult to remove the second electron than the
first, indicating an electronic interaction between
the ferrocene and ferrienium units in the mono-
cation stage. Two types of interaction may be
envisaged, operating either through the carbon
3. Results and discussion
skeleton of the ligand [20] or via a direct metalÁ
/
metal interaction or field effect [21].
3.1. Electrochemical results
Fig. 2 illustrates the differential pulse voltam-
mogram (DPV) of Ni(LSB)2 in dichloromethane
solution. It shows two sufficiently well separated
peaks of equal height. Half-wave potentials were
calculated from the DPV peak potentials:
The cyclic voltammograms of 1 mM ferrocene
(FcH), HLSB, and Ni(LSB)2 on a GC electrode in
CH2Cl2 solution containing 0.1 M TBAP as
supporting electrolyte are shown in Fig. 1.
E1=2 ꢂEp ꢀ12Ed
For HLSB, a reversible one-electron redox
process has been observed at E1/2
DEꢂ0.118 V. The potential value is considerably
more positive than that of (h5-C5H5)2Fe (E1/2
ꢂ0.608 V with
/
/
where Ep and Ed are peak potential and pulse
height, respectively.
ꢂ
/

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X. Wang et al. / Spectrochimica Acta Part A 58 (2002) 3113Á3120
/
The results obtained were 1E1/2
process corresponding to the first peak and
2E1/2
0.697 V for the second. The corresponding
ꢂ/0.580 V for the
ꢂ
/
DE1/2 value of 117 mV reveals a moderately strong
level of electronic communication between the two
ferrocene residues [22].
3.2. Digital simulation
The experimental voltammogram of Ni(LSB)2
in Fig. 1 is characteristic of an EE electron-transfer
process. Digital simulation was employed to
obtain the corresponding redox half-wave poten-
tials. All the simulations were carried out using a
home-built electrochemical simulator package,
which is capable of simulating virtually any user-
defined electrochemical mechanism. The simulator
is realized on the basis of an exponentially
expanding grid network approach [23,24] employ-
ing a spatial finite-difference discretization of the
governing reaction-diffusion equations and a gear-
trapezoidal temporal integration scheme. The
electrochemical process described in Eq. (1)
coupled with a proportionation reaction was
subjected to the simulations, as shown in Eq. (2).
Fig. 3. Experimental and simulated cyclic voltammograms for
Ni(LSB)2. Solid line, experimental curve; circles, simulated
voltammetric curve considering the distortion by Ru and Cdl;
dashed line, simulated voltammogram in the absence of Ru and
Cdl. The main parameters used in the simulation are nꢂ
sꢁ1
E1/2 0.578 and 0.694 for the first and second
voltammetric waves, respectively, Kcꢂ
sꢁ1, Ruꢂ 10ꢁ6 F.
2000 V, Cdl 20ꢃ
/50 mV
,
ꢂ/
V
/
91.4, k01ꢂ
/
k02ꢂ
/
1 cm
/
ꢂ/
/
k1o
k2o
1ꢁeꢁ?1ꢀ?12ꢀ ꢀeꢁ
(2)
Kc
1ꢀ12ꢀ?21ꢀ
(3)
Fitting of the experimtal voltammogram to
digital simulations (Fig. 3) yields 0.578 and 0.694
V for the two E1/2 values, corresponding to a DE1/2
of 116 mV and the equilibrium constant Kc of 91.4
for the comproportionation solution electron-
transfer (SET) reaction. This value of DE1/2 reveals
a moderately strong electronic interaction between
the two redox-active sites in Ni(LSB)2. It is
noteworthy that the influence of uncompensated
solution resistance (Ruꢂ
/
2000 V) and double layer
capacitance (Cdlꢂ20ꢃ
/
/
10ꢁ6 F) on the resulting
current response was considered during the simu-
lations otherwise no acceptable fitting could be
obtained. As a contrast, the simulated voltammo-
gram without considering the influence of Ru and
Cdl is also provided in Fig. 3, which shows sharper
voltammetric peaks and much less peak separa-
tions.
Fig. 2. Differential pulse voltammogram of 1 mM Ni(LSB)2
solution in 0.1 M TBAP/CH2Cl2 at a scan rate of 10 mV sꢁ1
with pulse height of 10 mV.

X. Wang et al. / Spectrochimica Acta Part A 58 (2002) 3113Á
/3120
3117
3.3. In situ FTIR spectroscopy
larger p-conjugation system favorable for electron
iron dis-
˚
tance is 7.630 A in Ni(LSB)2, which is relatively
transfer through the ligand. The ironꢀ
/
The oxidation of Ni(LSB)2 in the thin layer cell
was conducted. FTIR spectra of Ni(LSB)2 re-
corded during the oxidation process in the spectral
long to minimize the interaction between the iron
˚
nickel distance is 4.830 A in
atoms. The ironꢀ
/
range of 1700Á
the FTIR measurement the reference potential was
set at ꢀ0.2 V, where no oxidation of Ni(LSB)2
occurred.
From ꢀ
/
1300 cmꢁ1 are shown in Fig. 4. In
Ni(LSB)2. Delocalization of Ni electron density
into the spacer chain through a dp0pp interac-
tion is present [26].
/
/
Molecular orbitals of ferrocenyl group consist
of HOMO and LUMO of iron atom and ligand, so
that there is maybe a cooperative bonding action
/
0.5 V, two distinct upward peaks are
observed at 1558 and 1502 cmꢁ1 which can be
ascribed to yCÄN stretching modes [25]. These
within Feꢀ/C5H5. Ligand offers electron density to
iron atom, whereas iron atom feeds back electron
density to ligand simultaneously. The net result of
the two contrast actions is that electron density is
transferred from ligand to iron atom due to weak
p acid of ligand [27]. Iron atoms loss electrons in
the oxidation of Ni(LSB)2, resulting in donor
peaks appear from ꢀ0.5 V, indicating the begin-
/
ning of Ni(LSB)2 oxidation at this potential, and
the peak intensities are increased with the increas-
ing of the electrode potential. These suggest that
the CÄ/N double bond character is decreased
during oxidation process. The decrease is pro-
duced, because, the oxidation of the ferrocene unit
strength of C5H50Fe increased and p-bonding
/
electron density decreased within the ligand. So,
the new strong downward band at 1453 cmꢁ1 is
assigned to yCÄC stretching vibration. The forma-
tion of this band may be due to the change of
cyclopentadienyl resulting from the extensive elec-
tron delocalization of the whole molecule. No new
band appeared when the potential increased to 1.0
V indicating that unlike in other mixed valent
complexes, there is no obvious difference in this
case between mono- and di-ferricenium cations in
in-situ IR spectra.
inductively decrease the electron density on CÄ
group attached to the cyclopentadienyl ring,
resulting in a loss of strength in the CÄN bond.
The structure of Ni(LSB)2 [18] shows that the
Ni(II) atom is coordinated in slight distorted
square-planar configuration with two equivalent
/
N
/
Niꢀ
/
N bonds and Niꢀ/S bonds. The dihedral angle
between the two coordination planes is 17.58. Two
cyclopentadienyl units and Ni(II) moiety almost
are in a planar configuration, and it forms a
As in the case of Ni(LSB)2 oxidation, the wave
number of 1383 cmꢁ1, shifted to a higher value
(1396 cmꢁ1) at 0.6 V, which is indicative of the
chemical environment of ꢀ
/
CH3 group changed.
The spectra for Ni(LSB)2 in the range of 1100Á
/
800 cmꢁ1 during oxidation process, are shown in
Fig. 5.Three positive going bands at around 828,
942, 989 cmꢁ1 and four negative going bands near
859, 952, 1001 cmꢁ1 and 1053Á
clearly observed. The band near 828 cmꢁ1 can be
attributed to vibration modes of a CꢀH bend of
the cyclopentadienyl ring. Oxidation of Ni(LSB)2
H bend from 828 to 859 cmꢁ1 with 31
/
1060 cmꢁ1 are
/
shifts the Cꢀ
/
cmꢁ1 increase. Moreover, it seems that the bands
near 828 and 859 cmꢁ1 have dipolar features.
In ferrocene-based mixed valent systems a band
Fig. 4. In situ FTIR spectra of 5 mM Ni(LSB)2 in 0.1 M
TBAP/CH2Cl2 in the oxidation process. Scan rate, 2 mV sꢁ1
.
ERꢂ
/
ꢀ
/
0.2 V (vs Ag/AgCl).
assigned to a Cꢀ
/
H bend of the cyclopentadienyl

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X. Wang et al. / Spectrochimica Acta Part A 58 (2002) 3113Á3120
/
Fig. 6. In-situ FTIR spectra of 5 mM Ni(LSB)2 in 0.1 M
TBAP/CH2Cl2 in the reduction process. Scan rate, 2 mV sꢁ1
ERꢂ 1.0 V (vs Ag/AgCl).
Fig. 5. In-situ FTIR spectra of 5 mM Ni(LSB)2 in 0.1 M
TBAP/CH2Cl2 in the oxidation process. Scan rate, 2 mV sꢁ1
.
.
/
ꢀ
/
ERꢂ
/
ꢀ/0.2 V (vs Ag/AgCl).
vibration shifts to lower wave number during
oxidation process.
Figs. 6 and 7 illustrate the FTIR spectra of
[Ni(LSB)2]2ꢀ recorded during the reduction pro-
cess. In the measurement of the FTIR spectra, ER
ring has been shown to be diagnostic of the iron
oxidation state. This band shifts from 815 cmꢁ1 in
ferrocene to 851 cmꢁ1 in ferrocenium triiodide
[28]. A mixed-valent diferrocenyle complex having
absorptions at both of these energies has trapped
valence, whereas intrinsically delocalized systems
was set at ꢀ1.0 V, where no reduction of
/
[Ni(LSB)2]2ꢀoccurred.
Fig. 6 shows two upward peaks at around 1453,
display a single Cꢀ/H frequency at close to the
average of the Fe(II) and Fe(III) frequencies, i.e.
ca. 830 cmꢁ1 [28,29]. Since a pair of bands
consistent with an Fe(II)/Fe(III) system is ob-
served in Ni(LSB)2, rather than a band at the
Fe(II)/Fe(III) spectral average, the spectra argue
strongly that [Ni(LSB)2]ꢀ is a trapped-valent
system in solution during oxidation.
1396 cmꢁ1, attributed to yCÄC and yCꢀH of ꢀ
/
CH3
group stretching vibrations of [Ni(LSB)2]ꢀ and/or
[Ni(LSB)2]2ꢀ
respectively. Three downward
,
The bands of 942 and 989 cmꢁ1 are assigned to
Cꢀ
have dipolar characters. Oxidation of Ni(LSB)2
shifts the Cꢀ
S bands from 942 to 952 cmꢁ1 and
989 to 1001 cmꢁ1, respectively. The negative going
band near 1053Á
1060 cmꢁ1 is assigned to a yCÄS
/
S stretching vibration [25], and both seem to
/
/
stretching vibration [25]. The band begins to
appear near 1060 cmꢁ1 at 0.5 V, and shifts to a
lower wave number 1053 cmꢁ1 at 1.0 V during
oxidation of Ni(LSB)2. It is because, Ni(II) moiety
almost is planar configuration, resulting in con-
jugation between Ni(II) moiety and two cyclopen-
tadienyl units. The conjugation effect is increased
along with the increasing of oxidation depth, so
that the wave number of the yCÄS stretching
Fig. 7. In situ FTIR spectra of 5 mM Ni(LSB)2 in 0.1M TBAP/
CH2Cl2 during reduction process. Scan rate, 2 mV sꢁ1. ERꢂ
1.0 V (vs Ag/AgCl).
/
ꢀ
/

X. Wang et al. / Spectrochimica Acta Part A 58 (2002) 3113Á
/3120
3119
peaks are observed near 1558, 1502, 1383 cmꢁ1
corresponding to yCÄN stretching modes and yCꢀH
of CH3 group stretching vibrations of
,
859 and 828 cmꢁ1 are assigned to Cꢀ
/
H bend of
the cyclopentadienyl ring, corresponding to
[Ni(LSB)2]ꢀ and/or [Ni(LSB)2]2ꢀ and Ni(LSB)2.
The spectra of reduction process are consistent
with that of oxidation process which further
confirms that the redox of Ni(LSB)2 is reversible
process, and agrees with the results of electro-
chemistry. Moreover, a band near 1600 cmꢁ1 is
observed both in the spectra during oxidation and
reduction. The contribution of this band is not
clear at present and needs further study.
ꢀ
/
[Ni(LSB)2].
Four positive going bands at around 1052, 1001,
953 and 859 cmꢁ1 and three negative going bands
near 990, 942 and 828 cmꢁ1, are clearly shown in
Fig. 7. The band near 1052 cmꢁ1 can be attributed
to vibration modes of yCÄS of [Ni(LSB)2]ꢀ and/or
[Ni(LSB)2]2ꢀ
The peaks near 1001 and 953 cmꢁ1 are assigned
.
to yCꢀS of [Ni(LSB)2]ꢀ and/or [Ni(LSB)2]2ꢀ
,
Thus, electron transfer through the ligand is
dominant in Ni(LSB)2 complex, and may be
represented schematically as Scheme 2:
shifting to 990 and 942 cmꢁ1 during reduction
process, respectively. A pair of bands at around
Scheme 2.

3120
X. Wang et al. / Spectrochimica Acta Part A 58 (2002) 3113Á3120
/
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2219.
A
novel biferrocene trinuclear complex,
Ni(LSB)2, derived from the Schiff base ligand,
HLSB, S-benzyl-N-(ferrocenyl-1-methyl)-dithio-
carbazate is characterized by cyclic voltammetry,
differential pulse voltammetry, digital simulation,
and in situ FTIR spectroscopy. Results obtained
using different techniques have converged to
suggest that the oxidation of Ni(LSB)2 involves
two consecutive one-electron steps to give corre-
sponding mono- and then di-ferricenium cations in
the potential region between 0.3 and 1.0 V. This
complex exhibits moderately strong degree of
electronic communication between the two-ferro-
cene moieties. The spectra changes in the reduction
process are inversely consistent with that in the
oxidation process, which further confirms that the
redox process of Ni(LSB)2 is chemically reversible,
and agrees with the results of electrochemistry. IR
results indicate that the electron transfer of
Ni(LSB)2 takes place through the skeleton chain
of the ligand due to the extensive electron deloca-
lization of the whole molecule.
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