Combined studies on the surface coordination chemistry of benzotriazole at the copper electrode by direct electrochemical synthesis and surface-enhanced raman spectroscopy

Article abstract of DOI:10.1002/ejic.200700436

The surface coordination chemistry of benzotriazole (BTAH) on a Cu electrode was investigated by electrochemical synthesis of surface complexes and in situ electrochemical surface-enhanced Raman spectroscopy (SERS) in nonaqueous solution. Two different surface complexes were prepared in solution with or without triphenylphosphane (PPh₃). A new mixed-valence Cu-N cluster compound containing BTAH was synthesized by direct electrochemical oxidation of Cu in nonaqueous solutions with PPh₃. The final product, Cu₅Cl(BTA)₅(PPh₃)₄ was crystallized and characterized by microanalysis and Raman spectroscopy together with X-ray crystallographic determinations. The complex crystallized in a monoclinic space group (P2₁/n) with lattice parameters α = 16.739(2) A, b = 18.919(2) A, c = 31.042(4) A, β = 103.194(3)°, V = 9571.0(19) A³, R₁ = 0.0704, wR₂ = 0.1643. The results showed that the complex was a pentanuclear compound. The central Cu^II atom is coordinated to four equatorial and one axial BTA ^- ligands to form a distorted tetragonal pyramidal coordination polyhedron, whereas each surrounding Cu^I ion is in a distorted tetrahedral environment. The in situ SERS studies revealed that the BTA ^- ion was coordinated to a Cu surface through its two N atoms of the triazole ring to form a surface polymer complex of [Cu(BTA)]_n, which suppressed the dissolution and oxidation of Cu effectively. The introduction of PPh₃ blocked the surface coordination of BTAH with the electrode, and a complex of BTA^- and PPh₃ with Cu was formed in the bulk solution. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700436

FULL PAPER
DOI: 10.1002/ejic.200700436
Combined Studies on the Surface Coordination Chemistry of Benzotriazole at
the Copper Electrode by Direct Electrochemical Synthesis and Surface-
Enhanced Raman Spectroscopy
Ya-Xian Yuan,[a] Ping-Jie Wei,[a] Wei Qin,[a] Yong Zhang,[a] Jian-Lin Yao,*[a] and
Ren-Ao Gu[a]
Keywords: Benzotriazoles / Copper / Surface coordination / Electrochemistry / Raman spectroscopy
The surface coordination chemistry of benzotriazole (BTAH)
on a Cu electrode was investigated by electrochemical syn-
thesis of surface complexes and in situ electrochemical sur-
face-enhanced Raman spectroscopy (SERS) in nonaqueous
solution. Two different surface complexes were prepared in
solution with or without triphenylphosphane (PPh3). A new
mixed-valence Cu–N cluster compound containing BTAH
was synthesized by direct electrochemical oxidation of Cu
in nonaqueous solutions with PPh3. The final product,
Cu5Cl(BTA)5(PPh3)4 was crystallized and characterized by
microanalysis and Raman spectroscopy together with X-ray
crystallographic determinations. The complex crystallized in
a monoclinic space group (P21/n) with lattice parameters a =
16.739(2) Å, b = 18.919(2) Å, c = 31.042(4) Å, β = 103.194(3)°,
V = 9571.0(19) Å3, R1 = 0.0704, wR2 = 0.1643. The results
showed that the complex was a pentanuclear compound. The
central CuII atom is coordinated to four equatorial and one
axial BTA– ligands to form a distorted tetragonal pyramidal
coordination polyhedron, whereas each surrounding CuI ion
is in a distorted tetrahedral environment. The in situ SERS
studies revealed that the BTA– ion was coordinated to a Cu
surface through its two N atoms of the triazole ring to form a
surface polymer complex of [Cu(BTA)]n, which suppressed
the dissolution and oxidation of Cu effectively. The introduc-
tion of PPh3 blocked the surface coordination of BTAH with
the electrode, and a complex of BTA– and PPh3 with Cu was
formed in the bulk solution.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
of films (surface complexes) should be based on the known
habits of the components and the structures of the surface
complexes. One reliable and direct way to obtain such com-
ponents is by the preparation of the corresponding surface
complexes under conditions similar to those of the elec-
trode surface. So far, many methods have been used to syn-
thesize the relevant surface complexes. For example, the sol-
uble metal salt and the ligands can simply be mixed in aque-
ous/nonaqueous solutions to produce the corresponding
complexes, or the metal salt and the ligand solid powders
can be mixed to form the complexes, a so-called solid-phase
synthesis.[13] However, the direct electrochemical synthesis
performed for the first time by Tuck and coworkers has
exhibited several advantages for providing the comparable
conditions of the surface.[14–21] This method consists in oxi-
dizing a sacrificial metal anode in the nonaqueous solution
with one or more appropriate kinds of ligands. Therefore,
the synthetic conditions, including the bulk metal electrode,
applied potential/current, solution (aqueous or nonaque-
ous), and electrolyte resemble the real electrode reaction
system. As a consequence, on the basis of the interactions
and chemical bonding between the ligand and the metal, it
is reasonable to deduce the surface adsorption processes
with the participation of some ligands on the electrode in
the complex prepared by the electrochemical synthetic
Introduction
The coordination chemistry of metal surfaces exposed to
a corrosion medium has attracted considerable attention
from the coordination chemists and surface scientists be-
cause of its economic importance.[1–5] For example, some
organic molecules adsorbed on particular metal surfaces
form a compact complex layer that can inhibit the cor-
rosion of the surface effectively.[6–12] Despite the importance
of special organic inhibitors, little attention has been paid
to the investigation of how they bind or coordinate to the
metal surfaces or as to why seemingly minor modifications
in the structures of the active organic compounds can have
a profound effect on their efficiency. Therefore, investiga-
tion into the interaction or the coordination chemistry of
organic ligands with metal surfaces could be beneficial to
obtain deeper insight into the adsorption behavior of li-
gands and their relevant surface coordination processes.
Generally, the deduction of the surface reaction mecha-
nisms that include strong chemisorption or the formation
[a] Department of Chemistry, Suzhou University (Dushu Lake
campus),
Suzhou 215123, China
Fax: +86-512-65880089
E-mail: jlyao@suda.edu.cn
4980
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Studies on the Surface Coordination Chemistry of Benzotriazoles
method. Moreover, no oxidant or reductant is introduced the film forms and protects the metal are the subject of
into the system of the electrochemical synthesis, and this much investigation and debate.
avoids the possible complicated processes resulting from re-
Therefore, a powerful in situ technique with high sur-
dox reactions. Alternatively, the oxidation of the metal elec- face sensitivity that is able to provide specific molecular
trode and the reduction of the ligands were carried out by information on metal surfaces has long been desired.
the “clean” electron, which was removed or added on the Many researchers reported studies on the inhibition effect
anode and cathode by the applied potential/current. This dra- of BTAH by electrochemical SERS in aqueous solutions.
matically decreased the contamination due to extraneous To the best of our knowledge, there is no report on the
chemicals not only in the reaction processes but also in the studies of the adsorption/coordination chemistry and film
separation of the final products. Most importantly, the ad- formation of inhibitors in nonaqueous solutions. However,
justable applied potential/current made the reaction become nonaqueous solvents are extremely important in both pure
highly selective, and thus enabled us to obtain the expected and applied electrochemistry, particularly as a component
final products. However, the introduction of supporting elec- of the electrolyte that is in contact with the electrodes.
trolytes might be found in the final product in the form of Moreover, the involved organic molecules or inorganic
anions or cations. In low concentrations, the appropriate ions can exhibit different inhibition efficiencies.[40,41] For
amount of supporting electrolyte can, to some extent, mini- example, the introduction of I– ions into solutions contain-
mize its negative effect on the complexes. Therefore, the direct ing BTAH makes the inhibition efficiency for both the an-
electrochemical synthetic technique provided an effective way odic and cathodic processes become even more remark-
to simulate the electrochemical adsorption and reaction on able,[42,43] which thus indicates the existence of a synergetic
the electrode surface, which allowed the structure and compo- effect between the I– ions and benzotriazole on the inhibi-
sition of the surface species to be obtained.
tion. However, some organic ligands might play a negative
However, information on the dynamic and mechanistic role in the inhibition when it coexists with BTAH. For
aspects of the surface processes was not obtained by the example, the introduction of ligands such as β-diketonate,
above-mentioned techniques. Information about the pro- acetylacetone, and triphenylphosphane (PPh3) can result
cesses that occur on the electrode surface can be gained by in remarkable differences in the structures of metal com-
several modern surface techniques, such as scanning tunnel- plexes with BTAH, specifically, the transformation of sin-
ing microscopy (STM), X-ray photoelectron spectroscopy gle metal nuclear to multinuclear complexes.[1,44–49] There-
(XPS), electron energy loss spectroscopy (EELS), sum fre- fore, it is worth investigating the inhibition of BTAH in
quency generation (SFG), and infrared and Raman spec- nonaqueous solutions containing some organic molecules
troscopy.[10,11,22–25] Among them, Raman spectroscopy is a that might play an important role in the formation of sur-
powerful tool for the investigation of the surface chemistry face complex films. In contrast, it is highly desirable to
of adsorbed molecules involving adsorption and coordina- extend SERS studies directly to wider applications in sur-
tion at the liquid–solid interface, provided that the surface face science, not only for the various processes of cor-
Raman signals are sufficiently enhanced.[26,27] In particular, rosion and inhibition, but also for the surface coordina-
surface-enhanced Raman scattering (SERS) has been tion chemistry of BTAH together with other ligands. Tri-
shown to be a good method for determining the structure phenylphosphane is one of the most commonly used li-
of molecules adsorbed on electrode surfaces.[28–34] Further- gands in organometallic and coordination chemistry, and
more, unlike EELS and XPS, SERS can yield detailed sur- it is one of the most common ligands found in homogen-
face information free from the interference of the bulk eous transition-metal catalysts for the binding and trans-
phase even at metal–liquid interfaces, and it has been widely formation of organic substrates.[50–53] Phosphanes in gene-
used to investigate the dynamic and mechanisms of surface ral have been mainly used as protecting groups in organic
processes in situ.
synthesis as well as in catalytic processes, in which they
To date, most coatings for metals have contained cor- were responsible for the decisive reaction step.[54] For this
rosion inhibitors as key constituents, and their activity was reason, PPh3 was chosen as the second metal-binding do-
dependent on the surface coordination chemistry of the in- main in studying the surface coordination chemistry of
hibitor. Benzotriazole (BTAH) is one of the most effective BTAH on a Cu electrode.
inhibitors for Cu and its alloys. Since the pioneering work
In the present work, the adsorption behavior and film
of Cotton and coworkers,[35] the adsorption/coordination formation of BTAH were investigated by in situ electro-
behavior and bonding structure between BTAH and a Cu chemical SERS on the surface of a Cu electrode in non-
substrate have been extensively studied by many authors by aqueous solution and by the coordination chemistry be-
using various techniques, including electrochemical, surface tween the Cu surface and BTAH with and without PPh3.
analytical, and spectroscopic techniques.[36–40] Most of the Meanwhile, for the determination of the compositions and
previous work revealed that BTAH inhibited the corrosion structures of the surface complexes, two kinds of complexes
of Cu in a wide pH range and potential region. For exam- of BTAH and Cu with and without PPh3 were synthesized
ple, in different potential regions, BTAH can form a chemi- by direct electrochemical synthesis and characterized by mi-
sorbed layer or a multilayer polymerized structural film croanalysis, X-ray crystallographic measurements, and Ra-
with a larger thickness that increases the inhibition of cor- man spectroscopy. The modes of inhibitor on the Cu sur-
rosion significantly. The details of the mechanism by which face in nonaqueous solution were proposed.
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FULL PAPER
4.18, N 11.47 (CuBTA: calcd. C 59.74, H 4.15, N 10.55)
and revealed that the molar ratio of Cu to the BTA– anion
was 1:1. Previous investigations of BTAH adsorbed on Cu
electrodes showed that compact polymer films of (CuBTA)n
were attached to the surface and inhibited the corrosion
of Cu effectively in the aqueous solution;[12,29,40] this is in
agreement with the synthesized sample that was prepared
by mixing the Cu+ compound with the BTAH solution.
Therefore, it is reasonable to assume that the formula of
the present sample was (CuBTA)n (complex 1). The overall
reaction at the electrode surface is depicted in Equation (5).
On the basis of previous experimental results, its structure
was proposed as that shown in Figure 1.
Results and Discussion
Electrochemical Synthesis of the Complex
The electrochemical oxidation of Cu provided a conve-
nient route to its coordination compound with the title li-
gands in a “one pot” procedure. One of the advantages of
these techniques is that the metal in the product is fre-
quently in a low oxidation state as the starting point is the
element itself. It should be noted that the deposited layer
on the anode resulted in its low conductivity. Thus, a mild
voltage was applied to the system to maintain electrolysis.
However, the electrochemical reaction potential was less
than the applied voltage and the current density was main-
tained with a small value of about 5–8 mAcm–2 during the
electrolytic procedures. Therefore, the conditions used in
the direct electrochemical synthesis were comparable to
those at the Cu electrode surface, which was studied by in
situ SERS. Complexes 1 and 2, which were synthesized by
the electrochemical method, were expected to have the same
composition as the Cu electrode surface in the electrochem-
ical system for the in situ SERS investigation.
(5)
The stoichiometry of the electrochemical reaction is indi-
cated by the electrochemical efficiency outlined in Equa-
tions (1) and (2), where m is the weight lost at the Cu anode,
M is the molar mass of Cu, z is the valence of the Cu ion,
i is the applied current, t is the electrolysis time, and F is
Faraday constant; the value ͐t0idt is determined by the silver
coulomb meter, which is linked into the circuit, and is de-
fined as the number of moles of metal dissolved from the
anode per Faraday of charge. In the present study, the value
of Ef of 0.99 molF–1 and 0.98 molF–1 implied that the
monovalent CuI complex was formed on the electrode sur-
face at the initial stage in solutions without/with PPh3,
respectively. The electrochemical systems can be represented
as Pt(–)|CH3CN, BTAH, electrolyte|Cu(+) and Pt(–)|CH3CN,
PPh3, BTAH, electrolyte|Cu(+), respectively. The electrode
reactions are outlined in Equations (3) and (4).
Figure 1. Proposed structure of the (CuBTA)n polymer complex.
The normal Raman spectrum of (CuBTA)n was pre-
sented with in situ SERS studies. The large blueshift of the
triazole and benzene ring mode indicated the coordination
of the BTA– ion with Cu. The detailed structure infor-
mation will be discussed in the next section.
After the addition of PPh3, no precipitation was ob-
served on the Cu electrode surface or in the solution at the
initial stage of the electrolytic reaction. With an increase in
the electrolysis time, a brown-yellow precipitate was formed
at the bottom of the flask. The absence of precipitation at
the beginning of the electrochemical synthesis in the
CH3CN solution with BTAH and PPh3 indicated that no
film was attached to the surface or that the prepared com-
plex might be soluble in the CH3CN solution (normally,
with a low solubility). To separate the precipitate, the non-
aqueous solution was transferred into the fridge and yellow
crystals were formed. Both the BTA– and PPh3 signals were
detected together in the normal Raman measurement of the
crystal. The frequency of the triazole and benzene ring
mode of BTA– was shifted to 1031 cm–1, which is an indica-
tion of the coordination of the BTA– ion to the Cu elec-
trode. This result revealed that both the BTA– ion and the
PPh3 ligand coordinate to the Cu ion. The final molecular
formula and structure of this crystal should be deduced
from its X-ray crystal structure.
(1)
(2)
cathode: BTAH + eǞBTA– + 1/2H2
anode: CuǞCu+ + e
(3)
(4)
For the solution without PPh3, the BTA– anion coordi-
nated to the Cu+ ion immediately to form a compact film
that attached itself to the electrode surface. Actually, at the
initial stage of the electrochemical synthesis, to be exact, as
soon as the electrolytic current was applied, a yellow pre-
cipitate was produced that attached itself to the Cu elec-
trode surface. At last, a dark yellow product was obtained
after separation of the precipitate from the solution. Unfor-
tunately, no crystal of the final product was obtained be-
cause of its poor solubility in most solvents. Elemental mi-
Crystal Structure of Cu5(BTA)5(PPh3)4Cl
Selected bond lengths and angles of the Cu5(BTA)5-
croanalysis of the sample gave a composition of C 61.35, H (PPh3)4Cl complex are listed in Table 1. The Cu5(BTA)5-
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Studies on the Surface Coordination Chemistry of Benzotriazoles
(PPh3)4Cl complex crystallizes as a pentanuclear complex, BTA– ligands and a phosphorus atom in PPh3, it is also
and its molecular structure is illustrated in Figure 2. Be- coordinated to a chloride ion that bridges two Cu centers
cause the benzotriazolate ion has a charge of –1, the penta- with a Cu···Cu separation of 4.822 Å. The average value of
copper complex is a mixed-valent material that contains all CuI–P bond lengths (about 2.1766 Å) is a little shorter
four CuI ions and one CuII ion. The central CuII ion is than the value in [Cu4(BTA)3(PPh3)4][Cu(PPh3)3](BF4)2
surrounded by four tetrahedrally coordinated CuI ions held [2.200(5) Å],[49] whereas the CuI–Cl bond lengths [Cu1–Cl1
by bridging BTA– ligands. In the title complex, each triden- 2.4456(13) Å, Cu2–Cl1 2.4619(13) Å] are close to the value
tate BTA– ligand bridges two CuI ions in two sides and of about 2.455(1) Å in the Cu2Cl2(PPh3)3 complex together
coordinates to the central CuII ion through its central nitro- with a bridging chloride ion.[56]
gen atom. The central CuII ion is coordinated to four equa-
torial and one axial BTA– ligands at the N2, N5, N8, N11,
and N14 positions, and it has a distorted tetragonal pyrami-
dal coordination geometry with average distances of CuII–
Neq 2.050(1) Å and CuII–Nax 2.294(3) Å. These values are
comparable to the CuII–N bond lengths (2.08 and 2.29 Å,
respectively) in the related Cu5(BTA)6(tfpbd)4 complex
(tfpdb = 4,4,4-trifluoro-1-phenyl-1,3-butanedione).[47] Each
surrounding CuI ion is four-coordinate with two types of
coordination environments. For example, Cu3 (or Cu4) is
in a distorted tetrahedral (CuN3P) environment and is
bound to three nitrogen atoms from different BTA ligands
and a phosphorus atom in PPh3. The CuI–N lengths Cu3–
N9, Cu3–N3, and Cu3–N13 are about 2.061 Å and are
close to those values in the complex [CuL(PPh3)] {L =
bis(2-pyridyl)[2-(6-methoxy)pyridyl]methoxymethane} with
the similar core of CuN3P. [55] In addition to Cu1 (or Cu2)
being coordinated to two nitrogen atoms in two different
Figure 2. ORTEP drawing of the Cu5Cl(BTA)5(PPh3)4 complex.
Hydrogen atoms are omitted for clarity.
Table 1. Selected bond lengths and bond angles of the Cu5(BTA)5-
(PPh3)4Cl complex.
For the CuI ion bridged by the axial benzotriazolate
anion and the chloride ion, the average distance between
Bond lengths [Å]
the surrounding CuI ion is 5.315 Å and the distance in-
creases to about 5.886 Å for the CuI ion bridged by the
equatorial benzotriazolate anions, whereas the average dis-
tance of about 3.500 Å between the surrounding CuI and
central CuII ions is much shorter than the above-mentioned
value. The structure of the complex bears resemblance to
that of Cu5(BTA)6(t-C4H9NC)4,[44] where the latter con-
tains an extra BTA– group instead of the bridging chloride
ion.
Cu1–N1
Cu1–N4
Cu1–P1
Cu1–Cl1
Cu2–N7
Cu2–N10
Cu2–P2
Cu2–Cl1
Cu5–N2
Cu5–N11
Cu5–N8
2.041(3)
2.067(3)
2.1860(12)
2.4456(13)
1.994(4)
Cu3–N9
Cu3–N3
Cu3–N13
Cu3–P3
Cu4–N15
Cu4–N6
Cu4–N12
Cu4–P4
2.060(4)
2.061(3)
2.062(4)
2.1813(12)
2.031(4)
2.081(3)
2.106(4)
2.1956(12)
2.294(3)
2.064(3)
2.006(4)
2.1435(15)
2.4619(13)
2.033(3)
2.050(3)
2.052(4)
Cu5–N14
Cu5–N5
Bond angles [°]
N1–Cu1–N4
N1–Cu1–P1
N4–Cu1–P1
91.89(14)
130.91(10)
127.92(10)
N1–Cu1–Cl1
N4–Cu1–Cl1
P1–Cu1–Cl1
N9–Cu3–P3
N3–Cu3–P3
N13–Cu3–P3
N15–Cu4–N6 99.40(14)
N15–Cu4–N12 96.11(14)
N6–Cu4 –N12 93.10(13)
N15–Cu4–P4
N6–Cu4–P4
N12–Cu4–P4
N2–Cu5–N5
N11–Cu5–N5 88.39(13)
N8–Cu5–N5 172.31(14)
97.69(10)
93.65(10)
105.78(5)
120.67(11)
122.72(10)
117.46(11)
In Situ Electrochemical SERS Investigation
Figure 3 presents a set of potential-dependent SERS
spectra of BTAH from a roughened Cu electrode in a wide
potential range in acetonitrile solution with or without
PPh3 together with the normal Raman spectra of two com-
plexes prepared by an electrochemical method for compari-
son. The series of spectra on the right side of Figure 3 un-
ambiguously demonstrate the change in spectral features
over a narrow range. Because the surface coordination of
the BTA– ion to the Cu electrode occurs in a positive poten-
tial range irreversibly, which was proven by relevant SERS
studies in aqueous solution,[8,12] the potential-dependent
spectra were recorded stepwise starting from a negative li-
mit potential to more positive potentials so that all the ad-
N7–Cu2–N10 96.33(16)
N7–Cu2–P2
N10–Cu2–P2
N7–Cu2–Cl1
N10–Cu2–Cl1 98.03(11)
P2–Cu2–Cl1
N9–Cu3–N3
N9–Cu3–N13 98.83(15)
N3–Cu3–N13 97.63(14)
N2–Cu5–N11 175.34(14)
128.18(11)
126.78(12)
94.95(12)
104.44(5)
94.06(14)
124.37(10)
119.86(10)
117.44(10)
90.64(13)
N2–Cu5–N8
88.28(14)
N11–Cu5–N8 92.08(14)
N2–Cu5–N14 94.02(13)
N11–Cu5–N14 90.59(13)
N8–Cu5–N14 94.17(13)
N5–Cu5–N14 93.49(12)
Cu1–Cl1–Cu2 158.56(6)
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sorption and surface coordination processes could be ob- onto the Cu surface to form an irreversible compact poly-
served. It can be seen that the SERS spectra changed signif- mer film of [Cu(BTA)]n, which increases the inhibition effi-
icantly with the applied potential not only in the intensity, ciency dramatically in a certain potential region in aqueous
but also in the frequency of the relevant bands. The spectra solutions.[8,9,12,35–37] Therefore, it is reasonable to assume
showed a similar spectral feature to that of the BTAH solid that the observed surface Raman spectrum at 0.2 V com-
in the extremely negative potential region both in solution prised contributions from this irreversible polymer film at-
with or without PPh3. For example, the surface spectra at tached to the Cu surface in a wide potential range, that is,
–0.9 V without PPh3 and at –1.0 V with PPh3 exhibited the the BTAH was adsorbed on the surface by the coordination
same frequency as the free BTAH for the triazole and ben- of N atoms with Cu atoms. In the extremely positive poten-
zene ring mode at 1022 cm–1, respectively. This normally tial region, the signal of the film decreased remarkably. This
can serve as direct evidence for the determination of the occurred mainly because of the following two reasons: (1)
coordination configuration of BTAH on the metal surface, the disappearance of or decrease in the SERS activity of
although in this sample this band broadened significantly the Cu surface owing to the oxidation of the Cu surface
because of the complicated surface structure of the Cu elec- and (2) the partial breakage of the polymer film due to the
trode (as shown in Figure 3, right).[12] Our previous results formation of Cu oxide. At 0.6 V, the intensities of the bands
indicated that the BTA– ion of the surface complex film decreased and a broad band at ca. 560 cm–1 was detected
showed a blueshift to ca. 1040 cm–1 for this band. There- with a sharp band at 560 cm–1. The broad band was as-
fore, the high similarity of the SERS spectra of BTAH on signed to the stretching vibrational mode of Cu–O and the
the surface to that from the bulk solution revealed the exis- sharp band was assigned to the triazole ring torsion mode.
tence of an adsorption adlayer of BTAH molecules in the Therefore, the observation of broad bands revealed the for-
relative negative potential region in both solutions.
mation of Cu oxide in this potential region.
It is reasonable to assume that the surface complex (sur-
face film) has the same composition as the synthesized com-
plex. Therefore, on the basis of the fact that the potential-
dependent adsorption behavior in nonaqueous solutions
was similar to that in aqueous solutions, one can assume
that BTAH adsorbs in the form of neutral BTAH molecules
onto the Cu surface to form an adsorption layer in the
negative potential region and in the form of BTA– ions to
form a compact polymer film of the type [Cu(BTA)]n in the
positive potential region; the polymer film partially breaks
in the most positive potential region. The formation of a
surface complex film results in a more compact barrier
layer, which leads to a higher inhibition effect. Most inter-
estingly, when a small amount of PPh3 (the same concentra-
tion as BTAH) was introduced into the solution as above
and the same experimental processes were performed, sig-
nificant spectral changes were found. For example, the Ra-
man signal of BTA/BTAH was not detected at 0.2 V, and
the observed bands in this spectrum could be assigned to
the vibrational modes of CH3CN and PPh3. Therefore, the
detected Raman signal in fact comes mainly from the bulk
solvent rather than from the surface species. The great dif-
ferences in the spectral features in this potential region
might be caused by the differences in the compositions of
the surface films on the surface, that is, the PPh3 might play
Figure 3. Potential dependent SERS spectra of BTAH adsorbed on
the Cu electrode in acetonitrile solution without (bottom) and with
0.01 moldm–3 PPh3 (upper) together with the normal Raman spec-
tra of two surface complexes (left). The figure presented on the
right is for comparison in the range from 950 to 1250 cm–1: (a)
BTAH solid; (b) without PPh3 at –0.9 V; (c) 0.2 V; (d) complex 1;
(e) with PPh3 at –1.0 V; and (f) complex 2.
By moving the potential positively, the signal intensity an important role in the formation of the surface film and
also increased greatly and the frequency of the triazole and it may modify the properties of the surface complex.
benzene ring mode shifted to 1040 cm–1 at 0.2 V in the solu-
tion without PPh3. Moreover, the surface spectrum at 0.2 V
exhibited the same spectral features as that of the electro-
chemically synthesized [Cu(BTA)]n (complex 1; Figure 3,
right). Most interestingly, the color of the electrode surface
The Role of PPh3 in the Surface Coordination Chemistry of
BTAH
changed from brown to gray after the positive potential ex-
It is well known that PPh3 serves as a common ligand in
cursion, and the SERS measurement on the electrode in air metal coordination chemistry. On the basis of the hard–soft
without potential control exhibited similar spectral features acid–base theory, low-valent metal ions (e.g. Cu+) are a
to those in solution in the positive potential region. It was class of soft Lewis acids, whereas PPh3 is a typical soft
reported by many researchers that the BTA– ion can adsorb base.[57] Therefore, stabilization of the low-valent Cu+ metal
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Eur. J. Inorg. Chem. 2007, 4980–4987

Studies on the Surface Coordination Chemistry of Benzotriazoles
ion was increased by the interaction of PPh3 with the metal then the complex was secondly formed in solution. Thus,
ion, which resulted in an increase in the softness by increas- the fact that no BTAH film covered the electrode surface
ing the electron density on the metal ion. Generally, more resulted in the absence of an SERS signal at +0.2 V. There
than one PPh3 molecule coordinated with one Cu+ ion to are two effects that can be attributed to the disappearance
+
form a Cu(PPh3)n cation.[58–60] Therefore, this cation is a of the surface Raman signal of BTAH/BTA– at +0.2 V: (1)
soft acceptor ion that can accommodate a wide range of no compact film was formed on the electrode surface and
coordination anions to form air-stable complexes.[61] In the (2) the decrease in the SERS activity because of the dissol-
+
direct electrochemical synthesis, the electrochemical effi- ution of Cu to form the Cu(PPh3)3 layer in the specific
ciency Ef value of ca. 1.0 molF–1 for both cases indicated potential region. When the potential was increased again
that the Cu metal was oxidized to the Cu+ ion in the elec- to the severe oxidization region, the surface atoms of the
trochemical reaction. As a consequence, the central metal electrode were oxidized to Cu2+ which were located at the
ion was Cu+, which was quickly stabilized by the soft neu- borderline between being a hard acid and a soft acid. Con-
tral PPh3 ligand. Following that, the BTA– anion that was versely, because of the increase in the valence of Cu, an
generated at the cathode participated to form a stable com- increase in the coordination numbers produces steric hin-
plex. In the final product, the separation of crystalline drance that hinders the participation of a large ligand such
+
Cu(PPh3)3Cl indicated the formation of Cu(PPh3)3 (Fig- as PPh3 in the coordination process. Therefore, the observa-
ure 4). However, the structure of the final product in the tion of the SERS signal from the BTA– ion and the absence
latter case of Cu5Cl(BTA)5(PPh3)4 indicated that the central of the SERS signal from the PPh3 ligand in the potential
CuII was coordinated to four equatorial and one axial BTA– region of 0.4 to 0.6 V revealed that the Cu2+ ion could be
ligands to form a distorted tetragonal pyramidal coordina- coordinated by the BTA– anion directly to form a film on
tion polyhedron, whereas each surrounding CuI ion was in the surface again. At +0.6 V, a broad band at ca. 560 cm–1
a distorted tetrahedral environment and coordinated with was detected, which was similar to that obtained in the
one PPh3 ligand. It revealed that one CuI ion was oxidized solution without PPh3. Therefore, it is reasonable to assume
to CuII after the electrochemical reaction, and some of the that the BTA– anion showed a similar adsorption or coordi-
PPh3 ligands that were coordinated to CuI in the initial nation behavior in the most positive potential region, speci-
stage of the electrochemical procedure were exchanged by fically, the formation of surface complex film together with
BTA– ligands in solution. The proposed surface coordina- the Cu oxide.
tion chemistry of BTAH is presented in Equation (6).
The formation of longchains of the insoluble polymer
[Cu(BTA)]n is suppressed because the neutral PPh3 ligand
participates in the formation of the complex and shows a
strong stabilization effect on the complexes. As a conse-
+
quence, the cation of Cu(PPh3)3 dissolves into CH3CN
quickly and hence no strong signal of BTAH from the sur-
face could be detected in a specific potential region. In fact,
it accelerates the dissolution of the Cu electrode rather than
strengthening the inhibition efficiency in that potential re-
gion; explicitly, the existence of PPh3 plays a negative role
in the inhibition of BTAH at the Cu surface.
(6)
Conclusions
The surface coordination of BTAH onto the Cu surface
was deduced to involve chemically adsorbed layers, a BTA–
polymer film, and PPh3-assisted surface dissolution. BTAH
exhibits different adsorbed and filmed behaviors in different
potential regions in solution. In the extremely negative po-
tential region, the neutral BTAH molecules were adsorbed
onto the Cu surface in acetonitrile solutions with or with-
out PPh3. BTAH also formed an attached polymer film on
the surface in the medium without PPh3, whereas a crystal-
line complex formed in solutions containing PPh3. Our pre-
liminary experiment showed that electrochemical synthesis
combined with SERS studies could provide the surface co-
ordination mechanism at the molecular level.
Figure 4. ORTEP drawing of the Cu3Cl(PPh3)3 complex. Hydrogen
atoms are omitted for clarity.
Experimental Section
The in situ SERS measurements were in agreement with
the processes of electrochemical synthesis, namely, the solu-
Physical Measurements: BTAH and PPh3 (Aldrich) were used as
ble Cu(PPh3)3+ cation was firstly formed on the surface and received. Solvents were purified by standard methods. C and H
Eur. J. Inorg. Chem. 2007, 4980–4987
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4985

Y.-X. Yuan, P.-J. Wei, W. Qin, Y. Zhang, J.-L. Yao, R.-A. Gu
FULL PAPER
elemental analysis was performed with an Italy Carlo Erba MOD-
1106 microanalysis instrument. Raman spectra were measured with
a micro-Raman system of LabRam I with a low power He–Ne laser
source.
(0.0739P)2 + 24.2198P], where P = (Fo + 2Fc )/3, S = 1.069. The
final R indices R1 = 0.0704, wR2 = 0.1643[IϾ2σ(I)]. The crystallo-
graphic data is summarized in Table 2, and the crystal structure is
presented in Figure 2. CCDC-283457 contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
2
2
Electrochemical Synthesis of (CuBTA)n, Cu5Cl(BTA)5(PPh3)4: The
electrosynthesis of the metal complex was carried out in acetonitrile
by using Pt as the cathode and Cu as a sacrificial anode according
to the technique described in our previous work.[21] The BTAH
ligand (0.2374 g), with or without PPh3 (0.5230 g), was dissolved
in CH3CN (30 mL) containing a small amount of Et4NClO4 as the
supporting electrolyte. The electrodes were treated with HNO3, and
washed and dried before use. All operations were carried out under
the protection of dry Ar with rigorous exclusion of air by using
standard vacuum line techniques. The voltage was regulated to ob-
tain an initial current of 20 mA. As soon as the current flowed in
the cell, bubbles of hydrogen gas were visible at the cathode. Five
minutes later, pale yellow solids precipitated around the anode
piece. After two hours of electrolysis, the Cu piece was rinsed,
dried, and weighed to determine the electrochemical efficiency (Ef).
A black powder was obtained in the solution without PPh3 and
defined as complex 1. For the system with PPh3, the solid was
isolated by filtration, and then dissolved in a mixture of acetoni-
trile/chloroform (1:1; complex 2). One week later, brown crystals
of Cu5(BTA)5(PPh3)4Cl·2CH3CN were separated out from the
solution in a yield of ca. 63%. A little quantity of Cu(PPh3)3Cl was
separated from the final product and its structure was determined.
The In Situ Electrochemical SERS Measurements: Raman spectra
were recorded with a confocal micro-Raman system (LabRam I
from Dilor). The microscope attachment was based on an Olympus
BX40 system and used a 50ϫ long working length objective
(≈ 8 mm) so that the objective would not be immersed into the
solution. All the experiments were performed with the excitation
line of 632.8 nm from an internal He–Ne laser with a power of
4 mW on the electrode surface. The electrochemical cell for the
SERS measurement was made by Teflon with a quartz window for
introducing the laser and collecting the Raman signal. The working
electrode was a Cu disk with a geometric area of 0.1 cm2 embedded
in a Teflon rod. It was polished successively with 1 and 0.05 µm
alumina powder and sonicated in triply distilled water. Conse-
quently, a double potential step from –0.4 to + 0.4 V (vs. SCE)
was applied to the electrode in 0.1  KCl solution for about 2 min
followed by a potential excursion at –0.4 V for 1 min, and then, it
was rinsed thoroughly and dried with nitrogen to remove traces of
water on the surface. A large Pt ring served as the counter electrode.
The reference electrode was a solid Ag/AgCl electrode, which was
prepared by electrochemical oxidation of an Ag wire with a con-
stant anodic current of 0.4 mAcm–2 in a 0.1  HCl solution for
30 min. A detailed description of the spectroelectrochemical mea-
surements is given elsewhere.[9,40] All experiments were performed
at room temperature. All the chemicals used were analytical reagent
grade. The solutions were prepared by using acetonitrile, which was
pretreated to remove water.
X-ray Structure Determination: C106H86ClCu5N17P4 (2075.04):
calcd. C 61.35, H 4.18, N 11.47; found C 59.74, H 4.15, N 10.55.
A
brown crystal having approximate dimensions of
0.60ϫ0.36ϫ0.35 mm was selected and mounted on a glass fiber.
All measurements were made with a Rigaku Mercury CCD dif-
fractometer equipped with graphite-monochromated Mo-Kα (λ =
0.71070 Å) radiation by using an ω scan mode in the range
3.02ϽθϽ27.48° at 193(2) K. A total of 99572 reflections were col-
lected, of which 17353 with IϾ2σ(I) out of 21901 were unique ones
(Rint = 0.0512) were considered as observed. All of the calculations
were performed by using the SHELXS-97package.[62] The structure
was solved by direct methods and refined by full-matrix least-
squares techniques on F2. All non-hydrogen atoms were refined
anisotropically, whereas the hydrogen atoms included in the final
cycle of refinement were located on the calculated positions boned
to their carrier atoms. The weighting scheme was w = 1/[σ2(Fo2) +
Acknowledgments
The authors gratefully acknowledge the financial support from the
Nature Science Foundation of China (20503019, 20573076) and the
Natural Science Foundation of Jiangsu Province (BK2005032).
Y. X. Y. is partially supported by the Key Academic Discipline of
Organic Chemistry of Jiangsu Province and Suzhou University for
Young Teachers.
Table 2. Crystal data for Cu5(BTA)5(PPh3)4Cl·2CH3CN.
[1] J. Handley, D. Collison, C. D. Garner, M. Helliwell, R. Doch-
erty, J. R. Lawson, P. A. Tasker, Angew. Chem. Int. Ed. Engl.
1993, 32, 1036–1038.
[2] M. Frey, S. G. Haris, J. M. Holmes, D. A. Nation, S. Parsons,
P. A. Tasker, S. J. Teat, R. E. P. Winpenny, Angew. Chem. Int.
Ed. 1998, 37, 2193–2197.
[3] J. M. Thorpe, R. L. Beddoes, D. Collison, C. D. Garner, M.
Helliwell, J. M. Holmes, P. A. Tasker, Angew. Chem. Int. Ed.
1999, 38, 1119–1121.
[4] M. P. Soriaga, Chem. Rev. 1990, 90, 771–793.
[5] C. A. Melendres, N. Camillone, T. Tipton, Electrochim. Acta
1989, 34, 281–286.
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C106H86ClCu5N17P4
2074.95
monoclinic
P21/n
16.739(2)
18.919(2)
31.042(4)
103.194(3)
9571.0(19)
4
β [°]
V [Å3]
Z value
Crystal dimensions [mm]
Dcalcd. [gcm–3]
F(000)
Total reflections measured
Unique
0.60ϫ0.36ϫ0.35
1.440
4252
99572
21901
[6] I. Dugdale, J. B. Cotton, Corros. Sci. 1963, 3, 69–74.
[7] R. Walker, R. C. Benn, Electrochim. Acta 1971, 16, 1081–1088.
[8] G. Xue, J. F. Ding, P. Lu, J. Dong, J. Phys. Chem. 1991, 95,
7380–7384.
[9] J. L. Yao, Y. X. Yuan, R. A. Gu, J. Electroanal. Chem. 2004,
Residuals: R1 [IϾ2σ(I)]
Residuals: wR2 [IϾ2σ(I)]
0.0704
0.1643
573, 255–261.
[10] D. Chadwick, T. Hashemi, Corros. Sci. 1978, 18, 39–51.
[11] J. O’M. Bockris, M. A. Habib, J. L. Carbajal, J. Electrochem.
Soc. 1984, 131, 3032–3033.
Maximum peak in final diff. map [eÅ3] 1.764
Minimum peak in final diff. map [eÅ3]
–1.382
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Studies on the Surface Coordination Chemistry of Benzotriazoles
[12]
trodes (Eds.: J. Lipkowski, P. N. Ross), VCH, New York, USA,
1992, ch. 4, pp. 171–238.
J. Rubim, I. G. R. Gutz, O. Sala, W. J. Orvillethomas, J. Mol.
Struct. 1983, 100, 571–583.
[35] J. B. Cotton, I. R. Scholes, Br. Corros. J. 1967, 2, 1–5.
[36] R. Youda, H. Nishihara, K. Aramaki, Corros. Sci. 1988, 28,
87–96.
[13]
[14]
J. P. Lang, X. Q. Xin, J. Solid State Chem. 1994,108, 118–127.
T. A. Annan, D. G. Tuck, Organometallics 1991, 10, 2159–
2166.
C. Oldham, M. J. Taylor, D. G. Tuck, Inorg. Chim. Acta 1985,
100, L9–L10.
R. Castro, J. Romero, J. A. Garcia-Vazquez, M. L. Duran, A.
Castineiras, A. Sousa, J. Chem. Soc. Dalton Trans. 1990, 3255–
3258.
M. L. Duran, J. Romero, J. A. Garcia-Vazquez, R. Castro, A.
Castineiras, A. Sousa, Polyhedron 1991, 10, 197–202.
R. Castro, M. L. Duran, J. A. Garcia-Vazquez, J. Romero, A.
Sousa, A. Castineiras, W. Hiller, J. Strahle, J. Chem. Soc. Dal-
ton Trans. 1990, 531–534.
T. A. Annan, J. Gu, Z. Tian, D. G. Tuck, J. Chem. Soc. Dalton
Trans. 1992, 3061–3067.
W. Y. Vivian, W. K. Lee, T. F. Lai, Organometallics 1993, 12,
2383–2387.
Y. Yuan, J. Yao, J. Lu, Y. Zhang, R. Gu, Inorg. Chem. Commun.
2005, 8, 1014–1017.
Y. Yamamoto, H. Nishihara, K. Aramaki, Corrosion 1992, 48,
641–648.
[37] J. Kester, T. Furtak, J. Electrochem. Soc. 1981, 128, 112–112.
[38] P. G. Cao, J. L. Yao, J. W. Zheng, R. A. Gu, Z. Q. Tian, Lang-
muir 2002, 18, 100–104.
[15]
[16]
[39] P. G. Cao, R. A. Gu, Z. Q. Tian, Langmuir 2002, 18, 7609–
7615.
[40] R. F. V. Villamil, P. Corio, J. C. Rubim, S. M. L. Agostinho, J.
Electroanal. Chem. 1999, 472, 112–120.
[41] M. Fleischmann, I. R. Hill, G. Mengoli, M. M. Musiani, Elec-
trochim. Acta 1983, 28, 1325–1333.
[42] Y. C. Wu, P. Zhang, H. W. Pickering, D. L. Allara, J. Electro-
chem. Soc. 1993, 140, 2791–2800.
[43] J. L. Yao, B. Ren, Z. F. Huang, P. G. Cao, R. A. Gu, Z. Q.
Tian, Electrochim. Acta 2003, 48, 1263–1271.
[44] V. L. Himes, J. Am. Chem. Soc. 1981, 103, 211–212.
[45] E. G. Bakalbassis, E. Diamantopoulou, S. P. Perlepes, C. P.
Raptopoulou, V. Tangoulis, A. Terzis, T. F. Zafiropoulous, J.
Chem. Soc. Chem. Commun. 1995, 1347–1348.
[46] E. Diamantopoulou, C. P. Raptopoulou, A. Terzis, V. Tang-
oulis, S. P. Perlepes, Polyhedron 2002, 21, 2117–2126.
[47] M. Murrie, D. Collison, C. Garner, M. Helliwell, P. A. Tasker,
S. S. Turner, Polyhedron 1998, 17, 3031–3043.
[48] D. Collison, E. J. L. McInnes, E. K. Brechin, Eur. J. Inorg.
Chem. 2006, 2725–2733.
[49] D. Li, R. Li, Z. Qi, X. Feng, J. Cai, X. Shi, Inorg. Chem. Com-
mun. 2001, 4, 483–485.
[50] G. Kickelbick, H. J. Paik, K. Matyjaszewski, Macromolecules
1999, 32, 2941–2947.
[51] I. Fenger, C. Le Drian, Tetrahedron Lett. 1998, 39, 4287–4291.
[52] K. Dehnicke, F. Weller, Coord. Chem. Rev. 1997, 158, 103–169.
[53] K. Nomiya, R. Noguchi, M. Oda, Inorg. Chim. Acta 2000, 298,
24–32.
[54] R. Pikl, F. Duschek, C. Fickert, R. Finsterer, W. Kiefer, Vib.
Spectrosc. 1997, 14, 189–197.
[55] R. T. Jonas, T. D. P. Stack, Inorg. Chem. 1998, 37, 6615–6629.
[56] T. Kräuter, B. Neumüller, Polyhedron 1996, 15, 2851–2857.
[57] R. G. Pearson, J. Am. Chem. Soc. 1963, 85, 3533–3539.
[58] P. F. Barron, J. C. Dyason, P. C. Healy, L. M. Engelhardt, C. J.
Pakawatchai, J. Chem. Soc. Dalton Trans. 1987, 1099–1106.
[59] D. J. Fife, W. M. Moore, K. W. Morse, Inorg. Chem. 1984, 23,
1684–1691.
[17]
[18]
[19]
[20]
[21]
[22]
[23] L. J. Wan, Acc. Chem. Res. 2006, 39, 334–342.
[24] S. J. Huo, Q. X. Li, Y. G. Yan, Y. Chen, W. B. Cai, Q. J. Xu,
M. Osawa, J. Phys. Chem. B 2005, 109, 15985–15991.
[25] Y. G. Yan, S. J. Huo, Q. J. Xu, W. B. Cai, J. Phys. Chem. B
2006, 110, 14911–14915.
[26] Z. Q. Tian, B. Ren, “Infrared and Raman Spectroscopy in
Analysis of Surfaces” in Encyclopedia of Analytical Chemistry
(Ed.: R. A. Meyers), John Wiley & Sons, Chichester, 2000, pp.
9162–9201.
[27] K. Kneipp, H. Kneipp, I. Itzkan, R. R. Dasari, M. S. Feld,
Chem. Rev. 1999, 99, 2957–2975.
[28] B. Pettinger in Adsorption at Electrode Surface (Eds.: J. Lip-
kowski, P. N. Ross), VCH, New York, 1992, pp. 285–345.
[29] L. W. H. Leung, M. J. Weaver, J. Electroanal. Chem. 1987, 217,
367–384.
[30] Y. Zhang, X. Gao, M. J. Weaver, J. Phys. Chem. 1993, 97, 8656–
8663.
[31] M. Fleischmann, Z. Q. Tian, L. J. Li, J. Electroanal. Chem.
1987, 217, 397–410.
[32] B. Pettinger, H. Wetzel, “Organic and Inorganic Species at Ag,
Cu, and Au Electrodes” in Surface Enhanced Raman Scattering
(Eds.: R. K. Chang, T. E. Furtak), Plenum, New York, USA,
1982, pp. 293–314.
[60] G. Doyle, K. A. Eriksen, D. V. Engen, Organometallics 1985,
4, 830–835.
[33] R. K. Chang, “Raman Spectroscopic Techniques in Interfacial
Electrochemistry” in Spectroscopic and Diffraction Techniques
in Interfacial Electrochemistry (Eds.: C. Gutierrez, C. Me-
lendres), Kluwer Academic, Drodrecht, Netherlands, 1990, pp.
155–180.
[34] J. Lipkowski, L. Stolberg, “Molecular Adsorption at Gold and
Silver Electrodes” in Adsorption of Molecules at Metal Elec-
[61] R. D. Hart, P. C. Healy, G. A. Hope, D. W. Turner, A. H.
White, J. Chem. Soc. Dalton Trans. 1994, 773–779.
[62] G. M. Sheldrick, SHELX-97 and SHELXL-97: Program for
Crystal Structure Refinement, University of Göttingen, Ger-
many, 1997.
Received: April 20, 2007
Published Online: September 10, 2007
Eur. J. Inorg. Chem. 2007, 4980–4987
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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