J. Phys. Chem. 1991, 95, 7380-7384
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SERS, XPS, and Electroanalytlcal Studles of the Chemisorption of Benzotrlazole on a Freshly Etched Surface and an Oxidized Surface of Copper Gi Xue,* Jianfu Ding, Ping Lu, and Jim Dong Department of Chemistry, Nanjing University, Nanjing 21 0008, The People's Republic of China (Received: October 24, 1990) Chemical reaction of benzotriazole on a freshly etched surface of metallic copper has been investigated by surface-enhanced Raman scattering (SERS), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry, and other techniques. We found that the freshly etched copper surface could chemisorb benzotriazole (BTAH) from solution in 15 s forming a surface layer of Cu(0)BTAH. As the system was exposed to air, the surface species easily reacted with the coadsorbed oxygen in 2 min, resulting in a polymeric film of Cu+BTA-. The reaction of benzotriazole with the cleaned surface is faster than with the oxidized surface of copper. SERS study also indicates that the thin complex film formed on a freshly etched copper possesses better anticorrosion properties.
with other techniques, is an excellent spectroscopic tool for the study of thin films since it is very sensitive to the first couple of monolayers and the oxide formation of copper under complex film is clearly visible. We have been able to demonstrate that BTAH on a cleaned surface shows better anticorrosion properties than that on an oxidized surface.
Introduction Azoles such as benzotriazole (BTAH) have found widespread use as corrosion inhibitors for copper and its alloys. BTAH is also of great interest as a ligand in that its presence in many biological systems provides a potential binding site for many metal ions. In a neutral solution, BTAH coordinates with a metal ion via the nitrogen lone pair electrons. In a basic medium, the conjugate base, benzotriazolium anion, BTA-, is formed and may function as a ligand too. The tendency then is the formation of a so-called "inner" complex with stoichiometry M+(BTA-) or M2+(BTA-),, which is usually insoluble in organic solvents and generally considered to be polymeric in nature.' Adsorption of this inhibitor onto copper from aqueous solution results in formation of surface fi1ms.u It appears that this surface film can act as a barrier to cathodic reactions, thus imparting some degree of corrosion resistance to the copper substrate. In recent years there have been considerable efforts devoted to the characterization of the surface films formed during the exposure of copper to corrosion-inhibitive solutions and various possible structures have been proposed.' The mechanisms leading to the film formation, however, still remain uncertain? The ansensus is that BTAH is chemisorbed on the copper oxide surface to form a polymeric complex film. Using ellipsometry, Mansfeld and Smith found that BTAH interacted with copper only when it was covered with oxides.2 More recently, Hashemi and Hogarth investigated surface film formation by BTAH in brine solution using X-ray induced Auger spectroscopy.6 Most of the reports in the literature concerning Cu-BTAH complex formation are related to the reaction of BTAH with copper cations or copper oxides. Rhodin and Tompkins found that exposure of freshly cleaned Cu in air would produce C u 2 0 film. XPS and quartz microbalance measurement indicated the thickness of the Cu20 film reached a constant value, about 10-15 A, after 1 h.'J Tompkins also found that dipping a copper foil into 2-methylbenzimidazole solution for 1 min at room temperature could produce a complex film of hundreds of an stroms thick.* Since the copper foil was only covered with IO of Cu10 before immersion, as reported in his paper? we believe that azoles might react with Cu at zero oxidation state as well as with copper oxides. The past decade witnessed a dramatic growth in the use of the surfaceenhanced Raman scattering (SERS)technique to elucidate the chemical structures and bonding of molecules adsorbed on metal surfaces.e'' We have taken advantage of the high sensitivity of SERS (enhancement factors up to lo6) to study the molecular structure of BTAH species on copper surfaces, resulting in the discovery of a chemisorbed complex film of cuprous benzotriazolate on metallic copper. In this work we will compare the reaction rate of BTAH on a freshly etched surface with that on an oxidized surface of copper. We find that SERS, if combined
Experimental Section 1. Materials and General Procedures. All reagents were purchased from Shanghai Chemical Co. and were reagent grade. BTAH was purified by recrystallization from ethanol before use and dissolved in distilled water/ethanol (1:l) to give a 0.1 M solution at 25 O C . As a sampling substrate for SERS studies, a copper foil (available from Aldrich, 99.99% 0.1 mm thick) was immersed in 2 M H N 0 3 solution. After 10 s, a number of bubbles formed on the surface. Vigorous agitation had been applied for 3-4 min before the roughened copper foil was rinsed with distilled water and dried by blowing with nitrogen gas. The detailed procedure has been reported elsewhere.I2 The etched copper foils were immersed in BTAH solution for 15 s and 2 min, respectively. The surface-adsorbed moieties were characterized by SERS and XPS. In a bulky reaction study, 0.5 g of HN03-etched copper powder was mixed with 100 mL of BTAH solution and stirred for 3 days. After reaction, all of the copper powder was tumed into blue solids which were supposed to be the powdered reaction product and was characterized by IR analysis. 2. A Model Study on the Structure of the Reaction hoduct. In order to determine the chain length of the polymeric complex product, bis(imidazolato)copper(2+) was chosen as a model for structural analysis. CuCI2, imidazole, and pyridine (in 2:101 ratio) were dissolved in a 50% alcoholic solution. After several grams of NaHC03 had been added to adjust the pH value, the reaction solution was heated and stirred for 1 h. The precipitated product was filtered and washed with water and ethanol repeatedly to remove the residual reactants and the pyridine-coordinated cupric chloride. The fully acidified aqueous solution of the product was used to measure its UV absorption spectrum and the relative amounts of imidazole and pyridine ligands in the product may
w
(1) Brown, G. P.; Aftergut, S. J . Polym. Sci. Parr A 1964, 2, 1839. (2) Mansfeld, F.; Smith, T. Corrosion 1973, 29, 105. (3) Notoya, T.; Poling, 0. W. Corrosion 1973, 35, 193. (4) Poling, G. W. Corros. Sci. 1970, 10, 359. (5) Thierry, D.; Leygraf, C. J . Electrochem. Soc. 1986, 133, 2236. (6) Hashemi, T.; Hogarth, C. Electrochim. Acta 1988, 33, 1123. (7) Rhodin, T. N. J . Am. Chem. Sa.1950, 72, 5102. (8) Tompkins, H.; Allara, D.; Pasteur, 0. Surf Inferface Anal. 1983,5, 101. (9) Jeanmaire, D. L.; Van Duyne, R. P. J . Elecfrwnal. Chem. 1977,84, I. (10) Albrecht, M.; Creighton, J. J . Am. Chem. Soc. 1977, 99, 5215. ( I 1) Chang, R. K.; Furtak, T. E., Ms.Surface Enhanced Raman Scatfering. Plenum: New York, 1982. (12) Xue, G.; Dong, J.; Chang, M. Appl. Spectrosc. 1991, 45, 76.
'Author for correspondence.
0022-365419 112095-7380$02.50/0
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1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, NO. 19, 1991 7381
Chemisorption of Benzotriazole on Cu
I
I
-0.4
200
cm-?
900
P o t e n t i a l E (V
-0.2 VS.
SCE)
Figure 1. (Left) SERS spectra of a chemically etched copper foil. (Right) Cyclic voltammograms of a chemically cleaned copper electrode. The potential E was scanned from 0.0 to -0.9 V vs SCE. Scan rate 0.1000 V/s. (A) E x p o d to air for 3 min after etching; (B) exposed to air for 30 min after etching; (C) exposed to air for 60 min after etching.
be obtained by Beer's law on the basis that the maximum absorption bands of imidazole at 256 nm and pyridine at 205 nm scarcely overlap each other. 3. Instrumentation. Raman spectra were collected with a backscattering geometry in air on a SPEX-1403 Raman spectrometer. The incident excitation wavelength was 647.1 nm from a Kr+ laser source with output power of 40 mW. Infrared spectra were recorded on a Nicolet 170SX FT-IR spectrometer. XPS spectra were obtained by an ESCALAB MK-2 spectrometer with as the radiation source. Ultraviolet absorption spectra Mg KO],* were recorded on a Shimadzu UV-240 spectrophotometer. Cyclic voltammetry was performed using a Model 79-1 voltammometer. The working electrode was a polycrystalline copper wire mounted in a conventional three-electrode cell. All potentials were measured and reported with respect to a saturated calomel electrode (SCE) reference. A platinum wire sewed as a counter electrode. The supporting electrolyte was 0.5 M sodium acetate/acetic acid buffer solution (pH = 6.0). The scan rate was 100 mV/s.
Results and Mscussion 1. The Nature of Chemically Cleaned Copper Surfaces. The air oxidation of etched copper can be monitored with SERS. This is illustrated by the SERS spectra shown in Figure 1. The spectra on the left in Figure 1 are from the copper foil which was washed with distilled water after etching. On exposure to air, broad bands centered around 590 and 520 cm-'gradually appeared, which were due to the symmetric and asymmetric stretching vibrations of cuprous oxide. The limiting oxide thickness was reached in about 2 h. This is in agreement with the isotherms found by Rhodin using a vacuum microbalance at 323 K? According to the intensity changes of the Raman bands with prolonged exposure, the oxide bands at the 30-min exposure correspond to 1-2 monolayers of Cu20. The SERS spectrum for the freshly etched copper shows no oxide bands. There are conflicting reports about the thickness of oxide film on copper. Rhodin found a thickness of 10-15 A for CuzO overlayer using microbalance' which was verified by Tompkins in 1983 using XPS techniques! Rhodin claimed that the weight of copper would increase as soon as it exposed to oxygen. The conclusion was that copper was always covered with cuprous oxide. However, at that time there were no advanced techniques that could be used to distinguish the reacted oxygen from the adsorbed oxygen. On the other hand, the surface morphology could not be observed even if oxides exist. Using X-ray induced Auger spectroscopy and low-energy electron diffraction (LEED), Rao reported in 1986 that the weight increment at the initial stage could be attributed to the adsorption of molecular oxygen." The LEED patterns showed that, when Cu( 1 IO) surface was warmed (13)
Prabhakaran, K.;Sen, P.; Rao, C. N. Surf.
Sci. 1986. 177, L971.
up from 80 K, the molecular oxygen dissociated to atomic oxygen. At 426 K, all of the dissociated oxygen desorbed from the Cu( 110) plane without leaving oxides. The desorbed surface could adsorb molecular oxygen again if it were cooled down. Douglass and Alonm reported that as the cleaned surface of copper was exposed to oxygen with a low partial pressure ( 5 X lo-) Torr) at high temperatures, the first visible signs of oxidation observed by a high-resolution electron microscopy appeared after about 2 min: submicrometer size nuclei of CuzO formed randomly on the metal surface. The number density of the nuclei was found to be 2 X 108/cm2at 425 OC, which is approximately 10 monolayers." The surface was completely covered with CuzO in about 30 min of oxidation time.lS So it seems reasonable to propose that the freshly etched polycrystalline copper foil adsorbed the monolayer oxygen as soon as it was exposed to air. After dissociation of the adsorbed molecular oxygen to atomic oxygen, oxidation started at the surface defect sites such as adatoms, kinks, and surface vacancies. The oxide gradually covered the surface at room temperature. The SERS spectrum for the freshly etched copper does not show oxide bands, since the oxide did not cover the surface entirely in the initial exposure. A cyclic voltammetric study on a freshly etched copper wire which was employed as a working electrode corroborates the above SERS results. As shown in the right of Figure 1, the reduction current-potential (i-E) responses for chemically polished copper electrodes illustrate the gradual oxidation process on copper surfaces during prolonged exposure at room temperature. The solid line shows the i-E response lying close to the base line, implying that the oxidation on the copper electrode surface has hardly taken place in the first three minutes. The dashed line in the figure shows the reduction voltammogram for the etched copper exposed to air for 30 min. The small cathodic current indicates a reduction plateau for the limited amount of oxide overlayers on the bare electrode surface. There is a dramatic difference between the i-E responses for the surface oxide at the electrode exposed to air for 60 min (dashed-dotted line) and for 30 min. At the reduction peak potential, E = 4 . 3 0 V, the faradaic current, the current due to the surPfice oxide species presumably regarded as Cu20, became 10 times larger after the 60-min exposure. The voltammogram for the 30-min exposure electrode is markedly different; the current is much lower and most of the current is capacitive (compare solid and dashed lines). The data thus indicate that the surface of a copper electrode exposed to air for about 3 min is not entirely covered with the oxide species, whereas the surface exposed to air for about 60 min becomes a completely oxidized surface. The result from the cyclic voltammograms corelates well with the SERS study about the oxidation process on a chemically cleaned copper surface. 2. Comparison of Chemical Reaction of BTAH on a Freshly Etched Copper Surface with That on an Oxidized Surface. Figure 2 shows the normal Raman spectrum of BTAH in solid state and its SERS spectrum on a roughened copper surface. Before the SERS spectrum was recorded, the sample was washed with ethanol to remove physisorbed material. Characteristic Raman bands of BTAH in the solid state occur at 1388 cm-I for the ring stretching mode, 1025 cm-' for the ring breathing mode, and 784 cm-' for the ring deformation mode.I6 The corresponding bands appear at 1395, 1050, and 790 cm-I, respectively, in the SERS spectrum. Comparing these two Raman spectra, one finds that a few new bands have appeared in the SERS spectrum. The band at 245 cm-I could be assigned to the stretching mode of N-Cu bond which is connected through the nitrogen lone pair of electrons. The band at 410 cm-'is probably due to the overlap of vibrations of benzene ring and N-Cu bond." Figure 3 illustrates SERS spectra of BTAH on a preoxidized surface and on a freshly etched surface of copper. As the copper foil was exposed to air for 2 h after etching, a Cu20 film of 12 (14) Heinemann, K.;Rao, D.; Douglass, D. Oxid. Mer. 1975, 9, 579. (15) Ramanarayanan, T.; Alonzo, J. Oxid. Mer. 1985, 21, 17. (16) Dollish, F. R.; Fateley, W. G.; Bentlcy, F. F., Eds. Characrerlsric Raman Frequencies of Organic Compounds; Wiley: New York, 1973; p 234. (17) Cords, M.M.;Water, J. L. Specrrochim. Acra 1968, ZIA, 1421.
Xue et al.
7382 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 245
4
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b 790
410
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50
w/cm2
initial sweep I
200
1800
1000
successive sweeps
,
Wave Number (cm-')
7 4
Figure 2. Raman spectra: (A) SERS spectrum of benzotriazole on Cu
foil; (B) normal Raman spectrum of benzotriazole in solid state.
1
\
C
d
-0.4
-0.8
oxides
0.0
+0.4
+0.8
P o t e n t i a l (V. v s . SCE)
+
Figure 4. Multiple-sweep cyclic voltammetry of copper at pH 6.0: (A) chemically cleaned copper; (B) copper pretreated with benzotriazole for 15 s.
Oxides h
i/
\
790 1
1050
175
1
4 J
5 0 uA/cm2
790
4 1395
t
-
200
1000
I
4
1
1800
wave number (am-')
Figure 3. SERS spectra: (A) Cu foil exposed to air for 2 h after etching with HN03; (B)benzotriazole on Cu foil which had been exposed to air for 2 h; (C) benzotriazole on Cu foil freshly etched with HN03.
A thickness formed on the surface.* Figure 3A shows the bands of oxides in the 520-590-cm-' region explicitly. When the preoxidized foil was immersed in BTAH solution for 2 min in order that BTAH could be spotted onto the surface, we found that the Raman signal intensities of the adsorbed BTAH (Figure 3B) were rather weak, as compared with those in Figure 1A. Therefore the cleaned surface of copper could adsorb BTAH anchored directly to the metal while the oxidized surface did not reduce the amount of oxides significantly after immersion. in a latter section we will show that the amount of oxide under the film could increase upon heating. Electroanalytical studies provide more evidence about the chemisorption of BTAH onto metallic copper. Figure 4A shows the cyclic voltammetric curves of a cleaned copper electrode. Peaks a and b are associated with the formation of cuprous and cupric
-0.8
-0.4
0.0
t0.4
+0.8
Figure 5. Multiple-sweepcyclic voltammetry of copper, prior immersion in benzotriazole for 15 s. The initial three sweeps scanned from 0.0 to -1 .O V vs SCE. The fourth and successive sweeps scanned from -1 .O to + I .O V vs SCE.
ions by the anodic oxidation, and peaks c and d correspond to the cathodic reduction. The steady-state condition was achieved after 3 cycles. Figure 4 8 shows the cyclic voltammograms of a copper foil which had been immersed in BTAH solution for 15 s before measurement. The initial sweep for BTAH-modified copper electrode shows only one anodic oxidation peak a t +1.1 V and two cathodic reduction peaks in the normal region. The successive cycling produced voltammograms similar to those obtained from the bare copper electrode. This suggests that a film formed on the copper surface when immersd in BTAH solution for only 15 s. The surface film inhibited oxidation of copper somewhat on
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 7383
Chemisorption of Benzotriazole on Cu
initial
-0.8
-0.4
0.0
+0.4
+0.8
P o t e n t i a l ( V . v s . SCE)
Figure 6. Multiple-sweep cyclic voltammetry of copper electrode pretreated with benzotriazole for 2 min.
the first three scans and was later destroyed a t a more positive potential. The electrode used in Figure 5 was prepared in the same method as that used in Figure 4B. However, the initial three sweeps were scanned from 0.0 to -1 .O V vs SCE while the fourth and successive sweeps were scanned from -1 .O to +1 .O V. In the first three sweeps, there are no reduction peaks except a very weak band at -0.86 V in the first cycle, which is due to the reduction of a very small amount of oxide residue on copper surface. This phenomenon means that the surface-modifying film cannot be easily destroyed by electrochemical reduction, indicating that the surface layer formed by quick immersion was composed of Cu(0)-BTAH. The cyclic voltammetric study confirms that the first step of the reaction is the formation of Cu(0)-BTAH complex on the surface. The voltammograms of copper metal pretreated with BTAH for 2 min are shown in Figure 6. The peak current was very small in the initial sweep as compared to that in Figure 4, and it slowly increased in intensity until, after seven sweeps, it became constant. We reckoned that 2 min immersion in BTAH solution produced a more stable and compact film on copper surface. It has long been considered that copper could react with BTAH and other azoles only when it was covered with copper oxides.* However, our experiments show evidence for the direct reaction of metallic copper whose surface oxides had been removed by etching in acid. The difference in the reactivity between metallic copper and copper oxides with BTAH has been investigated by XPS. The XPS survey scan spectrum in Figure 7 for the copper sample, which had been immersed in BTAH ethanol solution for 2 min and then rinsed with ethanol before the spectrum was taken, shows clearly a N( Is) band at the binding energy value of 400 eV but no O( Is) band a t 533 eV. In contrast, the N( Is) peak for the cuprous oxide covered sample in Figure 7,which had been treated in the same way, shows little intensity. The comparison of XPS spectra in Figure 7 indicates that metallic copper can react with BTAH a t a faster rate than cuprous oxide. The corrosion-inhibiting behavior of BTAH on a cleaned surface and on an oxidized surface of copper was also compared by SERS spectra. A freshly etched sample of metallic copper and a sample of copper which had been oxidized in air for 2 h were both immersed in a BTAH solution for 2 min. After withdrawing, they were rinsed with ethanol, heated at 130 O C in air for 20 min, and measured by Raman spectroscopy. Their SERS spectra are shown in Figure 8. One can easily see that the oxide bands increased in intensity significantly for the exposed sample and that the chemically cleaned capper sample shows little intensity of the oxide bands. This comparison by SERS spectra indicates that BTAH chemisorbed on a clean surface of copper shows much better anticorrosion effect than that on the oxidized surface. 3. Surface Reaction Product of Metallic Copper with BTAH in tbe Resence of Oxygen. Attempt has been made to isolate the
i2a
360
600
840
B i n d i n g E n e r g y (ev)
Figure 7. XPS spectra: (A) metallic copper treated with benzotriazole for 2 min; (B) Cu20 treated with benzotriazole for 2 min.
L 200
1000
1800
Wave N u m b e r s (=In-’)
Flgure 8. SERS spectra: (A) air-oxidized Cu foil immersion treated with benzotriazole, then heated at 130 O C for 20 min; (B)freshly etched Cu foil immersion treated with benzotriazole,then heated at 130 O C for 20 min.
surface reaction product formed in the reaction of BTAH and copper metal by agitating copper powder with BTAH solution. After 3 days, a certain amount of reaction product was obtained. XPS analysis shows that the binding energy of C U ( ~ P ~of/ the ~) isolated product is 935.1 eV corresponding to Cu2+ and that of the BTAH-treated surface of copper is 934.1 eV related to Cu+. But the IR reflection-absorption spectrum of the chemisorbed surface and IR transmission spectrum of the product are quite similar to each other, as shown in Figure 9. BTAH has strong intermolecular hydrogen bonding in the solid state, which makes the IR spectrum show strong and broad bands in the region 3500-2400 cm-I, whereas the IR reflection-absorption spectrum
7384 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
nf
(A)
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Xue et al. mation of the complex (benzotriazolato)copper( I+) and water. We found that other azole compounds, such as imidazole and benzimidazole, could react with metallic copper, silver, and other transition metals in a similar We found that as the oxygen was removed from the solution by bubbling with prepurified nitrogen the reaction ceased. This phenomenon is in agreement with previously published results by Hogarth.6 Two of the nitrogen atoms in the benzotriazolium anion are indistinguishable and are equivalent for coordination. Each cuprous cation coordinates with two nitrogen ligands. It is reasonable to propose the structure of (benzotriazolato)copper( l +) to be that of an infinite polymer with the benzotriazolium anions acting as bridging ligands. The third nitrogen atom in the anion can also ligand to copper surface, forming a polymeric protecting layer for copper.
750
It was reported that boiling an imidazole (HIm) solution under weak alkaline condition in the presence of copper ions would produce an “inner complex”, which is usually insoluble in organic solvents and generally considered a polymeric product.’ The Figure 9. Infrared spectra: (A) transmission spectrum of benzotriazole polymerization hypothesis of the copper azolate salt has been in KBr pellet; (B)reflection-absorption spectrum of Cu plate prior imtentatively proved in this laboratory by a model study on bis(immersion in benzotriazole solution; (C) transmission spectrum of the isolated product from Cu powder and benzotriazole mixture. idazolato)copper(2+) through end-group analysis. Since both nitrogen atoms in an imidazolium anion are capable of bonding of the chemisorbed material and the IR spectrum of the isolated to a cupric ion, the imidazole moiety can be treated as product show only very weak bands around 3050 cm-I, which are “ b i f ~ n c t i o n a l ” . ~Pyridine, ~ . ~ ~ however, contains one binding site due to the C-H stretching of the aromatic ring. The complete and will limit the coordination polymerization of bidentate imiabsence of N-H stretching indicates the absence of the imino dazolato anions with cupric ions if present in the weak alkaline hydrogen in the surface material as well as in the isolated product. system of imidazole-copper salt. Thus the use of small amount On the basis of the above studies, we propose that the surface of pyridine in the reaction yields a polymeric cupric bis(imidamoiety is Cu+(BTA-) and the isolated product is CU~+(BTA-)~. zolate) chain with pyridine rings as end groups devoid of a coBTAH has a basicity intermediate between that of saturated ordinate binding site and incapable of further reaction. The amines such as NH3 and aromatic amines such as pyridine. In molecular weight of the polymeric complex can be controlled by addition to its basic properties, BTAH is also a weak acid. But adjusting the concentration of Cu2+, imidazole, and pyridine copper metal at zero oxidation state has not been considered to components in the reaction mixture. The scarce overlap of the be active enough to substitute for the ‘pyrrole”-type proton in strong UV absorption peaks a t 256 nm for pyridine and a t 205 BTAH ethanol solution. Metal benzimidazolates are usually nm for imidazole enables us to determine their relative contents prepared by boiling sufficiently basic solutions of BTAH and metal in a polymeric complex (see details in the Experimental Section). ions.’ The SERS, IR, and XPS studies indicate that BTAH can The experimentally measured number-averaged degree of poreact with copper metal at zero oxidation state under mild conlymerization of Cu2+ with Im- was approximately 50, suggestive ditions to form (benzotriazolato)copper( 1 +). This reaction seems of considerably long chains in the product complex. to follow a new mechanism. On the basis of the reaction conditions Conclusion and the structural properties of the product, we propose the scheme as follows. In neutral solutions, BTAH usually functions as a We have shown that, in the presence of air, benzotriazole can ligand by means of the unshared pair of electrons on the react with a chemically cleaned surface of copper other than copper “pyridine”-type nitrogen. So it seems reasonable to propose that cations or copper oxides in extremely mild conditions. The surface the first step of the reaction is the formation of a Cu(0)-BTAH product is (benzotriazolato)copper(+), which covers the surface complex by ligation of pyridine nitrogen of BTAH with metallic in a shape of polymeric material and shows good anticorrosion copper. In Cu(0)-BTAH complex, both the copper atom and the effect for copper. The reaction rate of BTAH with copper metal imino group are activated. On the other hand, as the solution and oxygen is much faster than that with copper oxides. is exposed to air, copper metal can adsorb oxygen from solution, Acknowledgment. We are grateful for the support from the to form adsorbed oxygen species, eg., undissociated dioxygen, 02. Coordination Chemistry Laboratory and the Solid Microstructure M + O2 = M-02(ads) Laboratory of Nanjing University. 3200
2400 1600 Wave Numbers (an-’)
800
It has been reported that the coadsorbed oxygen shows more basic and oxidizing properties than the oxygen in air or in solution.’* So,under mild conditions, copper metal is prone to donate electrons and BTAH can be deprotonated, resulting in the for(18) Van Ham, N . H. A.; Nieuwenhuys, Carol. 1971, 20, 408.
B. E.;Sachtlcr, W. M. H. J .
(19) Kue, G.; Dai, Q.;Jiang, S. J . Am. Chcm. Soc. 1988, 110, 2393. (20) Xue,G.; Zhang, J.; Shi, G.; Wu,P.J . Chem. Soc., Perkin Truns. 2 1989, 33. (21) Xue, G.; Wu,P.; Bao, Z.; Dong, J.; Cheng, R.J. Chem. Soc., Chcm. Commun. 1990,495. (22) Xue,G.;Jiang, S.;Huang, X.;Shi, G.J . Chem. Soc., Dalton Truns. 1988, 1487. (23) Sundberg, R.;Martin, R. Chem. Reo. 1974, 74,471.
