Electrochemical and in situ SNIFTIR spectroscopic studies of palladium(II) and zinc(II) schiff base complexes: Structural control of the electronic communication between two identical redox sites

Article abstract of DOI:10.1246/bcsj.75.1269

The mechanisms of the electron transfer of S-benzyl-N-(ferrocenyl-1-methyl-methylidene)-dithiocarbazate palladium(II)/zinc(II) complexes [Pd(lsb)₂]/[Zn(lsb)₂] were studied by cyclic voltammetry, differential pulse voltammetry, digital simulation and in-situ subtractively normalized interfacial Fourier transform infrared (SNIFTIR) spectroelectrochemistry. The results indicate that [Pd(lsb)₂], which has a square-planar configuration, involved two consecutive one-electron steps in the redox process, while the tetrahedral configuration of Zn(II) involved a two-electron step. The [Pd(lsb)₂] complex exhibits a moderately strong electronic communication between the two-ferrocene moieties, which occurs through the skeleton chain of the ligand due to extensive electron delocalization of the whole molecule during the redox process, while the [Zn(lsb)₂] complex shows low electron delocalization, and has two almost identical ferrocene moieties.

Full text of DOI:10.1246/bcsj.75.1269

c. Jpn.
© 2002 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 75, 1269–1273 (2002) 1269
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Electrochemical and In Situ SNIFTIR Spectroscopic Studies of
Palladium(Ⅱ) and Zinc(Ⅱ) Schiff Base Complexes: Structural Control of
the Electronic Communication between Two Identical Redox Sites
In Situ SNIFTIR Studies of Schiff Base Complexes
X. Wang et al.
02
69
Xiayan Wang, Zhaoxiang Deng,† Baokang Jin,*,†† Yupeng Tian,††,# Xiangqin Lin*
73
Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
†Department of Chemistry, Inorganic Institute, Tsinghua University, Beijing 100084, China
††Department of Chemistry, Anhui University, Hefei 230039, China
SJA8
09-2673
413
(Received November 13, 2001)
01
02
The mechanisms of the electron transfer of S-benzyl-N-(ferrocenyl-1-methyl-methylidene)-dithiocarbazate palladi-
um(Ⅱ)/zinc(Ⅱ) complexes [Pd(lsb)2]/[Zn(lsb)2] were studied by cyclic voltammetry, differential pulse voltammetry, digi-
tal simulation and in-situ subtractively normalized interfacial Fourier transform infrared (SNIFTIR) spectroelectrochem-
istry. The results indicate that [Pd(lsb)2], which has a square-planar configuration, involved two consecutive one-electron
steps in the redox process, while the tetrahedral configuration of Zn(Ⅱ) involved a two-electron step. The [Pd(lsb)2] com-
plex exhibits a moderately strong electronic communication between the two-ferrocene moieties, which occurs through
the skeleton chain of the ligand due to extensive electron delocalization of the whole molecule during the redox process,
while the [Zn(lsb)2] complex shows low electron delocalization, and has two almost identical ferrocene moieties.
There is a current interest in the synthesis of new materials
Experimental
with large optical nonlinearities, and organometallic com-
Preparation of the Ligand (Hlsb). S-benzyl dithiocarbazate
was synthedized according to a previously reported method.15 Af-
ter two drops of acetic acid were added to a mixture of S-benzyl-
dithiocarbazate and acetylferrocene in refluxing ethanol, upon
cooling a red-brown solid was formed. The solid was collected by
filtration and washed with ethanol and dried in a vacuum.
Preparation of [Pd(lsb)2] and [Zn(lsb)2]. An ethanol solu-
tion of the ligand and [PdCl2(CH3CN)2] or Zn(OAc)2•nH2O were
mixed. A brown or red-brown crystalline solid, which formed af-
ter refluxing for 4h, was collected by filtration, washed with etha-
nol and dried in a vacuum. The molecular structures of the two
complexes are as shown in Scheme 1.
pounds have been shown to be a useful general class of such
materials.1–9 Compared to common organic molecules, they
offer a larger variety of novel structures, possibility of high en-
vironmental stability, and diversity of tunable electronic be-
havior by virtue of coordinated metal centers, which might re-
sult in nonlinear optical (NLO) materials with unique charac-
teristics, such as magnetic and electrochemical properties.10
Ferrocene, owing to its unique and valuable redox and photo-
physical properties, has been widely used in constructing mo-
lecular systems containing multiple redoxative centers.11–12
Metal-bridged biferrocene complexes are one kind of the most
efficient third-order nonlinear optical organometallic com-
pounds.13
The details of the design, synthesis, structural characterization
and NLO properties will be reported elsewhere.15
Because the properties of biferrocenes depend upon the
amount of delocalization or the extent of interaction between
the two or more moieties, the nature and extent of electronic
intramolecular communication between two or more identical
redox centers has received considerable attention.14 In the
present work, electrochemical measurements, digital simula-
tions and in situ SNIFTIR were combined to study the mecha-
nism of electron transfer of metal-bridged biferrocene trinucle-
ar complexes, [Pd(lsb)2]/[Zn(lsb)2], derived from a Schiff-base
ligand, Hlsb, S-benzyl-N-(ferrocenyl-1-methyl-methylidene)-
dithiocarbazate, during a redox process.
Electrochemical Measurements. Electrochemical measure-
ments 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 a CH2Cl2 solution containing
0.1 M tetrabutylammonium perchlorate (TBAP) as a supporting
electrolyte.
In Situ SNIFTIR Spectroscopic Measurements.
Subtrac-
tively normalized interfacial Fourier transform infrared (SNIF-
TIR) spectroscopic measurements were performed by an in-situ
method on a 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.
# State Key Laboratory of Coordination Chemistry, Nanjing Uni-
versity, Nanjing 210093, China

1270 Bull. Chem. Soc. Jpn., 75, No. 6 (2002)
In Situ SNIFTIR Studies of Schiff Base Complexes
Fig. 1. Cyclic voltammograms of a 1 mM solution of (a)
Hlsb, (b) [Zn(lsb)2] and (c) [Pd(lsb)2] in 0.1 M TBAP/
CH2Cl2 at 50 mV s−1
.
small interaction between them. From cyclic voltammogram
of [Pd(lsb)2] and [Zn(lsb)2], it can be seen that the respective
redox responses of [Pd(lsb)2] and [Zn(lsb)2] are shifted to more
positive potentials than those of Hlsb. This indicates that the
coordination of the ligand to palladium or zinc makes the elec-
tron-withdrawing ability of the dithiocarbazate group some-
what stronger. The voltammetric behavior of [Pd(lsb)2] ap-
pears to involve two diffusion-controlled, reversible redox pro-
cesses; also, because the corresponding two half-wave poten-
tials are close, the two voltammetric waves are not fully re-
solved. The determination of the number of electrons trans-
ferred in the complete oxidation of [Pd(lsb)2] was effected by
exhaustive coulometry measurements, carried out past the an-
odic wave at a higher potential (E = 1.10 V), resulting in the
removal of 2 electrons/molecule. The two consecutive one-
electron oxidation steps presumably give the corresponding
mono- and then di-ferrocenium cations16 (Eq. 1) (where Fc =
ferrocenyl):
Scheme 1.
A syringe-type in situ reflection–absorption spectroelectro-
chemical cell was made from Teflon. A platinum wire and a silver
wire were fixed in the cell body used as a counter electrode and a
reference electrode, respectively. A disk of sodium chloride was
used as an IR transparent window. The working electrode was a
BAS GC disk, which was first polished with an aqueous suspen-
sion of 0.1 µm alumina to obtain a mirrorlike surface, and then
washed with doubly distilled water in an ultrasonic bath. While
measuring the in situ SNIFTIR 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
60°. A total of 50 interferometric scans with a resolution of 4 cm−1
were accumulated for an averaged spectrum. The spectra were
successively taken while the electrode potential was scanned at a
rate of 2 mV s−1. The resulting spectra were normalized as
∆R/R = [R(ES)−R(ER)]/R(ER).
By subtracting the reflection spectrum at the sample potential (ES,
R(ES)), from the reflection spectrum at reference potential (ER,
R(ER)), the background due to the absorption of the solvent sys-
tem was eliminated.
(1)
The results also show that it is appreciably more difficult to
remove the second electron than the first one, thus indicating
an electronic interaction between the ferrocene and ferroenium
units in the mono-cation. Two types of interactions may be en-
visaged, operating either through the carbon skeleton of the
ligand17 or via a direct metal-metal interaction or field effect.18
A differential pulse voltammogram (DPV) of [Pd(lsb)2] in a
dichloromethane solution is shown in Fig. 2. It shows two suf-
ficiently well separated peaks of equal height. The half-wave
potentials were calculated from the DPV peak potentials,
Results and Discussion
Electrochemical Results. Cyclic voltammograms for
Hlsb, [Zn(lsb)2] and [Pd(lsb)2] at a glassy carbon electrode in a
CH2Cl2 solution containing 0.1 M TBAP were studied. As il-
lustrated in Fig. 1, the voltammogram of Hlsb shows a revers-
ible one-electron redox process at E1/2 = 0.608 V with ∆Ep =
0.118 V at 50 mV s−1. The voltammogram of [Zn(lsb)2] exhib-
its a reversible two-electron redox step at E1/2 = 0.685V with
∆Ep = 0.174 V. This indicates that the two ferrocenyl moiety
in [Zn(lsb)2] are almost identical and that there exits a very
E1/2 = Ep + Ed/2,

X. Wang et al.
Bull. Chem. Soc. Jpn., 75, No. 6 (2002) 1271
the voltammeric measurements. Fitting the experimental vol-
tammogram to digital simulations (Fig. 3(a)) yielded 0.642 and
0.762 V for the two E1/2 values, corresponding to a ∆E1/2 of 120
mV and an equilibrium constant (KC) of 107 (assuming the dif-
fusion coefficients for species 1, 1+ and 12+ to be equal) for the
comproportionation solution electron-transfer (SET) reaction
(reaction 3). This value of ∆E1/2 coincides with the result ob-
tained from DPV, and reveals a moderately strong electronic
interaction between the two redox-active sites in [Pd(lsb)2],
compared with the statistically derived ∆E1/2 of 36 mV for
identical, non-interacting metal ions in homobinuclear com-
plexes. It is noteworthy that the influence of the uncompensat-
ed solution resistance (Ru = 2000 Ω) and the double-layer ca-
Fig. 2. Differential pulse voltammogram of 1 mM [Pd(lsb)2]
in 0.1 M TBAP/CH2Cl2 at 10 mV s−1 with pulse height of
10 mV.
where Ep and Ed and the peak potential and the pulse height,
respectively.
1
The obtained results were E1/2 = 0.657 V for the process
corresponding to the first peak and 2E1/2 = 0.769 V for the sec-
ond. The corresponding ∆E1/2 value of 112 mV reveals a mod-
erately strong level of electronic communication between the
two ferrocene residues.19
Digital Simulation. The experimental voltammogram of
[Pd(lsb)2] in Fig. 1 is characteristic of an EE electron-transfer
process. A digital simulation was employed to obtain the cor-
responding redox half-wave potentials. All of the simulations
were carried out using a home-built electrochemical simulator
package, which is capable of simulating virtually any user-de-
fined electrochemical mechanism. The simulator is realized
based on an exponentially expanding grid network ap-
proach20,21 employing a spatial finite-difference discretization
of the governing reaction–diffusion equations and a gear-trape-
zoidal temporal integration scheme. The electrochemical pro-
cess subject to the simulations could be denoted as follows:
(1) 1−e → 1+,
(2) 1+−e → 12+
,
(3) 1 + 12+ = 21+.
Fig. 3. (a) Experimental and simulated cyclic voltammo-
grams for [Pd(lsb)2]. Solid line, experimental curve; cir-
cles, simulated voltammetric curve considering the distor-
tion by Ru and Cdl; dotted line, simulated voltammogram
without considering Ru and Cdl. Parameters used in the
simulation are ν = 50 mV s−1, E1/2 = 0.642 and 0.762 V
for the first and second voltammetric waves respectively,
Reaction (3) is usually called a comproportionation reaction,
the equilibrium constant of which is dependent on the differ-
ence of the two formal potentials for reaction (1) and (2) in the
following way:
KC = exp (∆E°F/RT),
Kc = 107, k0 = k0 = 1 cm s−1, Ru = 2000 Ω, Cdl
=
1
2
20×10−6 F. (b) Experimental and simulated cyclic volta-
mmograms for Zn(lsb)2. Solid line, experimental voltam-
mogram; open circles, simulated voltammetric curve con-
sidering the distortion by Ru. Parameters used in the simu-
lation are ν = 50 mV s−1, E1/2 = 0.650 and 0.686 V for the
where F is the Faraday constant, R is the gas constant and T is
the thermodynamic temperature. During the simulation, the
rates of reactions (1) and (2) were assumed to be fast enough to
satisfy Nernstian electron-transfer approximations. Reaction
(3) was assumed to be a fast equilibrium process with extreme-
ly large rate constants for both the forward and backward reac-
tions, so that equilibrium could always be maintained during
two consecutive redox processes respectively. k01 = k0
1 cm s−1, Ru = 2400 Ω.
=
2

1272 Bull. Chem. Soc. Jpn., 75, No. 6 (2002)
In Situ SNIFTIR Studies of Schiff Base Complexes
downward indicates its appearance.
pacitance (Cdl = 20×10−6 F) on the resulting current response
had to be considered during the simulations to account for the
apparent deviation of the voltammetric curve from the Nerns-
tian type; otherwise, no acceptable fitting could be obtained.
As a contrast, the simulated voltammogram without consider-
ing the influence of Ru and Cdl is also provided as shown in Fig.
3(a) (dotted line), which shows sharper voltammetric peaks
and much smaller peak separations. Fig. 3(b) shows the exper-
imental and simulated cyclic voltammograms for Zn(lsb)2.
Fitting of the experimental voltammogram to digital simula-
tions yielded 0.650 and 0.686 V for the two E1/2 values, corre-
sponding to ∆E1/2 ꢀ 36 mV, which indicates that there is a very
small interaction between the two redox sites.
In Situ SNIFTIR Spectroscopy. The SNIFTIR spectra of
[Zn(lsb)2] and [Pd(lsb)2] recorded during the oxidation pro-
cesses in the spectral range of 1700–1300 cm−1 are shown in
Figs. 4 and 5. In measuring of the SNIFTIR spectra, ER was
set at +0.20 V, where no redox reaction of the two complexes
occurred. In the subtractively normalized spectra, the upward
peak indicates the disappearance of a certain group, while
From Fig. 4, it can be seen that two distinct upward peaks
appeared at 1565 cm−1 and 1503 cm−1 which can be ascribed
to the νCwN stretching modes.22 These peaks appear from +0.6
V, indicating the beginning of [Pd(lsb)2] oxidation at this po-
tential; also, the peak intensities increased with the increasing
of the electrode potential. It is clear that the characteristic of
the CwN double bond is reduced during the oxidation process.
This is because oxidation of the ferrocene unit inductively de-
creases the electron density on the CwN group attached to the
cyclopentadienyl ring, resulting in a loss of strength in the
CwN bond.
The structure of [Pd(lsb)2]15 shows that the Pd(Ⅱ) atom is
coordinated in a slightly distorted square-planar configuration
with two equivalent Pd–N bonds and Pd–S bonds. The dihe-
dral angle between the two coordination planes is 12.1°. Two
cyclopentadienyl units and the Pd(Ⅱ) moiety are almost in a
planar configuration, and forms a larger π-conjugation system
favorable for electron transfer through the ligands. The iron-
iron distance is 7.314 Å in [Pd(lsb)2], which is relatively long
so as to minimize the interaction between the iron atoms. The
iron-palladium distance is 4.982 Å in [Pd(lsb)2]. Delocaliza-
tion of the Pd electron density into the spacer chain through a
dπ → pπ interaction is present.23 The Pd(Ⅱ) moiety is avail-
able to form a large π-conjugation system with cyclopentadie-
nyls, which facilitates the transfer an electron through the
ligand.
Because the molecular orbitals of the ferrocenyl group con-
sist of HOMO and LUMO of an iron atom and a ligand, there
is possibly a cooprative bonding action within Fe–C5H5. The
ligand offers electron density to the iron atom, whereas the
iron atom feeds back the electron density to the ligand simulta-
neously. The net result of the two contrast actions is that the
electron density is transferred from the ligand to the iron atom
due to the weak π acid of the ligand.24 The iron atoms lose
electrons in the oxidation of [Pd(lsb)2], resulting in the donor
strength of C5H5 → Fe being increased and the π-bonding
electron density being decreased within the ligand molecule.
Thus, the new strong downward band at 1454 cm−1 is assigned
to νCwC stretching vibration.25 The formation of this band may
be due to a change of cyclopentadienyl resulting from exten-
sive electron delocalization of the whole molecule. Thus, elec-
tron transfer through the ligand is dominant in the [Pd(lsb)2]
complex, and can be represented schematically in Scheme 2.
Figure 5 is the spectra of [Zn(lsb)2], and is obviously differ-
ent from those of [Pd(lsb)2]. From 0.6 V, two obvious positive
peaks appear at 1564 cm−1 and 1485 cm−1, and two negative
peaks at 1621 cm−1 and 1505 cm−1. The peaks at 1564 cm−1
and 1485 cm−1 can be assigned to the νCwN stretching modes of
[Zn(lsb)2]22. Because the two peaks shift to higher wavenum-
bers along with an increase in the electrode potential, the
bands at 1621 cm−1 and 1505 cm−1 can be attributed to the
νCwN stretching modes of [Zn(lsb)2]2+. An upward peak near
1600 cm−1 is observed in both Figs. 4 and 5, which maybe as-
signed to the νCwC stretching mode of phenyl. The certain con-
tribution of this band needs further study.
Fig. 4. In situ SNIFTIR spectra of 5 mM [Pd(lsb)2] in 0.1 M
TBAP/CH2Cl2, Potential scan rate: 2 mV s−1. ER = +0.2
V vs Ag/AgCl.
The structure of [Zn(lsb)2]15 shows that the Zn(Ⅱ) atom is
coordinated in the tetrahedral configuration. The dihedral an-
gle between the two coordination planes is 63.2°. The iron-
Fig. 5. In situ SNIFTIR spectra of 5 mM [Zn(lsb)2] in 0.1 M
TBAP/CH2Cl2. Potential scan rate: 2 mV s−1. ER = +0.2
V vs Ag/AgCl.

X. Wang et al.
Bull. Chem. Soc. Jpn., 75, No. 6 (2002) 1273
Scheme 2.
Soc., 112, 8203 (1990).
iron distance is 7.606 Å in [Zn(lsb)2], which is relatively long
to minimize the interaction between the iron atoms, and the
iron-zinc distance is 5.051 Å. The large space block resulting
from the tetrahedral configuration of [Zn(lsb)2] makes the
Zn(Ⅱ) atom difficult to form a large π-conjugation system,
which brings on a low degree electron delocalization and ob-
structs the electron transfer carryied out through the ligand.
There is therefore a slight electronic communication between
the two redox sites. This is the reason that the band at 1454
cm−1 doesn’t appear in Fig. 5, but appears in Fig. 4.
5
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Conclusion
In this paper, two novel biferrocene trinuclear complexes,
[Pd(lsb)2] and [Zn(lsb)2], derived from the Schiff-base ligand,
Hlsb, S-benzyl-N-(ferrocenyl-1-methyl)-dithiocarbazate, were
characterized by cyclic voltammetry, differential pulse voltam-
metry, a digital simulation, and in-situ SNIFTIR spectrosco-
py. The tetrahedral configuration complex, [Zn(lsb)2], in-
volved one two-electron redox process. The two redox sites
are almost identical in the molecule, and there is a slight elec-
tronic communication between them. The space block hinders
electron transfer between the two ferrocene. The square-pla-
nar configuration complex, [Pd(lsb)2], shows two consecutive
one-electron steps to give corresponding mono- and then di-
ferrocenium cations in the potential region between 0.3 V and
1.0 V vs Ag/AgCl. This complex exhibits a moderately strong
degree of electronic communication between the two-fer-
rocene moieties. In situ SNIFTIR results indicate that the elec-
tron transfer of [Pd(lsb)2] takes place through the skeleton
chain of the ligand due to extensive electron delocalization of
the whole molecule during the redox processes.
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This work was supported by the National Natural Science
Foundation of China (No. 20071001).
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