Langmuir 1994,10, 1477-1481
1477
Complex-Induced Activating Effect on Surface Species: Reactions of Imidazole on Zero Oxidation State Metal Surfaces G. Xue* and J. Dong Department of Chemistry and State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210008, China
Y. Sun Department of Chemistry and Chemical Engineering, Southeast University, Nanjing 210018, China Received July 16, 1993. I n Final Form: February 11, 1994” Chemical reactions of imidazole on metallic silver, copper, and mercury surfaces have been studied by infrared spectroscopy and surface enhanced Raman scattering. It was found that the interactionof imidazole with the metals at zero oxidation state yielded metal imidazolates which covered the metal surfaces as a passive thin layer. As shown by tight-binding extended Huckel molecular orbital calculations, the ease of the reaction suggests that the reactivity of imidazole is increased upon adsorption which involves electron transfer from the metal to imidazole antibonding orbitals and the weakening of the N-H bond in the imidazole molecule.
Introduction Imidazole (IMH) is biologically important since the imidazole nitrogen atoms of histidine residues coordinate to metal ions in many metal1oproteins.l I t is known that in a neutral solution, imidazole coordinates to a metal ion via the lone pair of electrons on pyridine-type nitrogen. In a basic medium, the conjugate base, imidazole anion (IM-1, may function as a ligand too. The tendency then is the formation of a so-called “inner complex” with stoichiometry M+IM- or M2+(IM-)2, which is usually insoluble in organic solvents and generally considered to be polymeric in n a t ~ r e . ~ - 3
IMH
On the other hand, azoles such as imidazole, benzimidazole, and benzotriazole have been recognized as effective corrosion inhibitors for copper and its alloys for a long time. In recent years, great efforts have been 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. Most of the reports in the literature concerning corrosion inhibition mechanism of Cu are associated with the formation of a compact protective layer on Cu surfaces.P7 Our recent research work shows that imidazole can react not only with copper oxide but also with more inert metal
* Correspondence author. Abstract published in Advance ACS Abstracts, April 1, 1994. (1) Sundberg, L.; Martin, R. B. Chem.Rev. 1974, 74,471. (2) Brown, G. P.; Aftergut, S. J . Polym.Sci., Part A 1964,2, 1839. (3)Sigwart, C.; Kroneck, P.; Hemmerich, P. Helu. Chim.Acta 1970, 53,117. (4)Gardiner, D.;Gorvin, A.; Gutteridge, C.; Jackson, A.; Raper, E. Corros.Sci. 1986, 25, 1019. (5) Tompkins, H.; Allara, D.; Pasteur, G. Surf. Interface Anal. 1983, 5,101. (6)Tompkins, H.; Sharma, S. P. Surf. Interface Anal. 1982, 4, 261. (7) Mary, M. L.; Ledung, L.; Carron, K. T. Langmuir 1993,9, 186.
0743-7463/94/2410-1477$04.50/0
surface, as will be described below. It is interesting to note that in 1982, Tompkins et al. found that dipping a copper foil into 2-methylbenzimidazole solution for 1 min a t room temperature could produce a complex film hundreds of angstroms t h i ~ k . ~However a they also noticed that the copper foil was only covered with ca. 10A of Cu20 before immersion. We believe that the increase in the thickness of the surface film after dipping was due to the interaction of benzimidazole with Cu under the oxide layer. In this paper, infrared and surface enhanced Raman scattering (SERS) techniques were used to characterize the surface reaction of imidazole with Ag, Cu, and Hg metals. Since the discovery of the SERS effect in the late 1970s, there has been tremendous interest in developing SERS as a new tool for studying surface chemical processes since SERS is very sensitive to the first couple of monolayers. We have taken advantage of the high sensitivity of the SERS effect on HN03 etched Ag foils to study the chemisorption of IMH on silver. In order to understand the adsorption and reaction features on the metals, extended Huckel tight binding band calculations were performed. Nowadays, surface chemical reactions can be discussed in detail with satisfactory reliability, a t least for simple systems, thanks to recent progress in quantum chemistry. Based on our results, we have been able to demonstrate that the surface of silver, as well as those of copper and mercury, can react with imidazole under mild conditions. This kind of reaction seems to be ignored previously for lack of sufficiently sensitive surface techniques like SERS for the study.
Experimental and Theoretical Procedures Instrumentation. All reagents were purchased from Shanghai Chemical Co. and were reagent grade. IMH was purified by recrystallization from ethanol before use and dissolved in ethanol to give aO.l M solution at 20 “C. Raman spectra were excited by a Kr+ ion laser and collected with a backscattering geometry in air on a SPEX-1403 Raman spectrometer. Mid- and far-infrared spectra were recorded on a Nicolet 170SX FT-IRspectrometer. 1. Materials and
~
(8) Xue,
~~~
~
~
~
G.; Ding, J.; Zhang, M. Chinese Sci. Bull. 1991.36, 1339.
0 1994 American Chemical Society
Xue et al.
1478 Langmuir, Vol. 10, No. 5, 1994 2. General Experimental Procedures. As a sampling substrate for SERS studies, a silver foil (available from Aldrich, 99.99%, 0.025 mm thick) was immersed in a 3.5 M HNOs solution for surface roughening. Agitation had been applied for 3-4 min before the roughened silver foil was rinsed with distilled water and dried by blowing with nitrogen gas. The detailed procedure has been reported elsewhere! The etched silver foils were immersed in IMH solution for 1min for sample doping before SERS measurement. A 1% HNOs etched copper powder (0.5 g) was mixed with 100 mL of IMH solution and stirred in air for 3 days. After reaction, all of the metallic copper powder was turned into purple solids which, insoluble in common solvents, were supposed to be the reaction product and characterized by IR analysis. We found that this reaction may take place in many kinds of solvents such as water, ether, or dimethylformamide. When predried benzyl methyl ether was used, the amount of water was detected quantitatively after reaction. As the oxygen was removed from the solution by bubbling with prepurified nitrogen, the reaction ceased. Elemental analysis confirmed the product had exactly the same composition as the model compound, cupric bis(imidazolate),which was prepared by boiling a basic solution of IMH and Cu(NO& according to ref 10. A total of 1g of molecular silver powder prepared by reducing silver salt was immersed in 20 mL of IMH solution (10% concentration in ethanol) with vigorous stirring for 15days. After separation, 0.3 g of gray product was obtained. In the case of mercury, liquid Hg (1 g) had been stirred vigorously with 0.1 M IMH solution for 10 days before the white product was collected. 3. Theoretical Part. Tight-bindingcalculationsof extended Hackel type"-14 were performed using the same orbital parameters as those reported in ref 15. The bond lengths and angles in an imidazole molecule were taken from Martinez-Carrera's X-ray diffraction results.16 The planar imidazole molecule is so large thatacoverageof 1/3or 1 / 4 o n A g ( l l l ) surfaceisimpossible, no matter what the approach geometry, simply because of excessive steric hindrance between imidazole molecules. The larger unit cell thus brought about forced us to a monolayer Ag film. In calculations of potential energy curves the distance between the pyridine-type nitrogen and the nearest metal atom was set to 2.22 A, whichwas obtained by optimizing the adsorption configuration. This is a reasonable value since in imidazolesilver complexes, the N-Ag distances were found to be varied from 2.12 to 2.13 A.l
Results and Discussions 1. SurfaceReactionof Zero Oxidation State Metals with Imidazole. IMH has strong intermolecular hydrogen bonding in its solid state. The IR spectrum of solid IMH shows prominent N-H--N broad bands in the 26003100 cm-l region (Figure lA), because of linear association of molecules. From the consideration that IMH may adsorb strongly and react with transition-metal surfaces, we made an attempt to isolate the surfacereaction products by vigorously agitating the mixture of metallic Cu, Hg, and Ag in IMH solution. Table 1 illustrate the spectral frequency changes of near IMH and its reaction products with the metals. In order to find out the structure of the products, we prepared a model compound cupric bis(imidazolate) according to a known procedure2J0J7 (9) Xue, G.; Dong, J.; Anal. Chem. 1991, 63, 2393. (10) Bauman, J. E.; Wang, J. C. Inorg. Chem. 1964, 3, 368. (11)Hoffmann, R.; Lipscomb, W. N. J. Chem. Phys. 1962, 36, 2179. (12)Whangbo, M.-H.; Hoffmann, R. J. Am. Chem. SOC.1978, 100, 6093. (13)Saillard, J.-Y.; Hoffmann, R. J. Am. Chem. SOC.1984,106,2006. (14) Calculations were performed using NEWBAND3, a program developed by the Hoffmann group at Cornel1 University. Cf. Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1987,26,846. (15) Dong, J.; Xue, G.; Sun, Y.; Liu, J. Huaxue Xuebao 1993,51,625. (16) Martinez-Carrera, S. Acta Crystallogr. 1966, 20, 783. (17) Elbeck, W.; Holmes, F.; Taylor, C. E. J. Chem. SOC.A 1968,128.
I 2400
3200
4000
Wavenumber
1600 (cm-'
800
)
Figure 1. Infrared spectra of imidazole and its isolated surface reaction products: (A) imidazole in solid state; (B) the reaction product with silver powder; (C)the reaction product with copper powder; (D)the reaction product with mercury. Table 1. Main IR Frequencies (cm-l) of IMH, and Its Reaction Products with Cu. Aa. and HIP IMH solid Cu + IMH Ag + IMH Hg + IMH model compound product product product Cu2+(IM-)2 product 3113w 3095w 3014 2615 s, b 1542 m 1496m 1480m 1449 s 1325 s 1264 s 1147m 1055 vs 936 vs 896 w 841 m 756 s 737 m 660 s 621 m
3155w 3110w
3107m
3107m
3159 w 3110 w
1490s 1474s
1490s 1462s
1490s 1468s
1491 s 1473 s
1320m 1240m 1168m 1083 vs 945m
1300m 1240s 1165m 1088 vs 945m
1300w 1240s 1166m 1088 vs 945m
1318 m 1244 s 1168 m 1083 vs 948 s
844m 755 8
830m 777 s
820m 770 s
846 m 748 s
665 s
663 s
660 s
667 s
337 8
325 s
315 8
337 s
140 m
Intensities: CuC1,
a,
strong; m, mean; w,weak; b, broad.
+ 21MH
-
2KOH
Cu(IM),
+ 2H,O + 2KC1
The IR spectrum of the model compound was found to be quite similar to those of the above reaction products with metals. Table 1also lists the frequency data of the model compound. Elemental analysis revealed that the reactions of IMH with metallic Ag, Cu, and Hg all yielded metal imidazolates (see Table 2). This has made it easy to interpret the spectral changes from Figure 1A to Figure 1B-D. One can easily fiid that the IR spectra of the surface reaction product of IMH on Ag, Cu, and Hg are quite similar to one another. The absence of the broad N-H stretching bands in Figure 1B-D indicates that the imino
Langmuir, Vol. 10, No. 5, 1994 1479
Activating Effect on Surfaces
1150
1265
1150
325
31 5 350 Wavenumbers
150 (cm-' )
Figure 2. (A) Far-infrared spectra of solid imidazole. (B)The surface reaction product of imidazole with copper powder. (C) The surface reaction product of imidazole with silver powder. (D)The surface reaction product of imidazole with mercury. Table 2. Elemental Analysis Result of the Reaction Products calculated ~
~~~
Cu(1M)z Ag(1M) Hg(1M)Z
C, 36.3; H, 3.1; N,28.3 C,20.6; H, 1.7; N,16.0 C, 21.5; H, 1.8;N, 16.7
found C, 36.0; H, 3.0; N,27.6 C, 20.3; H, 1.6;N, 16.2 C, 21.3; H, 1.6;N,17.0
hydrogen has been eliminated after the surface reactions as can be inferred by comparison with the IR spectrum of neat imidazole. We therefore conclude that imidazole exists as a reaction product, metal imidazolates, in the chemisorbed layer. Ring deformation bands in neat IMH at 1449, 896, and 621 cm-l disappeared in IR spectra of the metal imiazolatessince these bands are associated with N-H deformation vibration.18 Observations showed that the reaction product of imidazole with metallic copper has a pronounced far-IR band at 337 cm-l (Figure 2B). Again this product displays a far-IR spectrum quite similar to that of the model compound Cu(1M)z (not shown here, cf. Table 1). The far-IR bands in the products from Ag and Hg appear at 325 and 315 cm-l, respectively (parts C and D of Figure 2). Imidazole shows a strong band a t 141 cm-I, due to N-H-N torsion vibration (Figure 2A).18J9 Using metal isotope techniques, Cornilson and Nakamoto assigned the Cu-N stretching vibrations of imidazole complexes of Cu2+ in the region 350-210 cm-laZoCordes et al. calculated and observed the metal-nitrogen stretching and bending vibration modes in the region 320-380 cm-' for Cu-N, Ni-N, and Co-N bonds.19 Hodgson et al. also assigned these vibrations in the same region.21 We thereby assign the 337-cm-1 band in Figure 2B to N-Cu stretching, 325 cm-' in Figure 2C to N-Ag stretching, and 315 cm-I in Figure 2D to N-Hg stretching. The shoulders near these bands may be assigned to the N-M-N bending mode.lg Far-IR results further corroborate the above conclusion (18) Colombo, L.;Bleckmann, P.;Schrader, B.; Schneider, R.; Plesser, T.J . Chem. Phys. 1974,61,3270.
(19) Corde de N. D., M.; Walter, J. L. Spectrochim. Acta 1968,24A, 237, 1421. (20) Cornilsen, R.; Nakamoto, K. J . Inorg.Nucl. Struct. 1974,36,2467. (21) Hodgson,J.B.;Percy, G. C.;Thornton,D.A. J.Mol. Struct. 1980, 66. 81.
200
1000
Raman shift
1800 (cm-I)
Figure 3. (A) Normal Raman spectrum of imidazole in solid state. (B) SERS spectrum of imidazole adsorbed on silver. (C) SERS spectrum of the sample in (B)washed with ethanol. that these noble metals can react with imidazole, yielding MZ+(IM-)z or M+(IM-). Surface enhanced Raman scattering (SERS) studies provide more evidence about the chemisorbed layer of IMH on metals. Figure 3 shows a normal Raman spectrum of IMH in solid state and its SERS spectra on Ag surfaces after 1min of immersion. Characteristic Raman bands of solid IMH occurring a t 1265 and 1150 cm-1 in Figure 3A are both associated with ring in-plane deformation vibrations.18 In Figure 3B, a strong band a t 1275 cm-' can be attributed to a ring deformation mode, corresponding to the 1265-cm-1band in Figure 3A. The adsorption of IMH on metallic silver led to the frequency shift and band broadening of 1275cm-' and the enhancement of 815- and 410-cm-1 bands. The band a t 815 cm-', also found in the IR spectrum of sodium imidazolate,2 may be related to the 830-cm-1 band in the FTIR spectrum of the reaction product of IMH with metallic silver (see Table 1). In neat IMH, the Raman band a t 810 cm-l was assigned an outof-plane deformation vibration.18 The 410-cm-' band in Figure 3B, having no corresponding band in pure IMH, is probably due to the coupling of a ring out-of-plane mode and N-Ag vibration mode. The great intensity of the outof-plane bands at 815 and 410 cm-l suggests the presence of the flat orientation of the imidazolate anion with respect to the metal surface if SERS selection rules are applied, which are based on the electromagnetic enhancement model. When the sample in Figure 3B was thoroughly rinsed with ethanol and its SERS spectrum measured again, we found that the Raman signal intensity of the N-H inplane deformation mode at 1150 cm-l band was very weak (Figure 3C) as compared with that in Figure 3B. Hence, it seems reasonable that there are two species present on the surface, namely, the adsorbed imidazole before reaction and the product. The vigorous washing procedure is prone to induce the surface reaction and eliminate the unreacted imidazole molecules on the surface layer. Consequently, we propose that the rings of the product imidazolate (IM-) anion are lying down on the surface, in a possibly polymeric structure similar to that previously p o ~ t u l a t e d ,though ~,~
Xue et al.
1480 Langmuir, Vol. 10, No. 5, 1994 the imidazole rings before reaction are favorably standing up on the surface.
Chart 1. Schematic Diagrams of Four Calculated Adsorption Geometries (See Illustrations in Text) H H
L !
L,/H
.............................................
Suppression of the 1150-cm-1 band and the remaining 815- and 410-cm-1 bands in Figure 3C further imply that the normal of the imidazolate ring plane is nearly parallel to the surface normal after washing. Interestingly, conducting SERS experiments on an electrochemically roughened electrode surface revealed almost the same results. It has been reported that the SERS spectra of imidazole adsorbed a t a Ag electrode from solutions of a natural pH show the imidazole to be coordinated uia the pyridine-type nitrogen atom and inclined toward the normal of the surface. At potentials more positive than -0.6 V us SCE, the neutral molecule is deprotonated to give the imidazolate anion, which is strongly adsorbed in a flat orientation toward the surface.22 The assumption that the structure of the salt of Ag+Imis an infinite polymer with imidazolate anions as bridging ligands, as depicted, is based on the fact that imidazolate anion (Im-) contains two equivalent sites for coordination, and each Ag+ ion can coordinate two nitrogens. Numerous reports in the literature have proposed unanimously that imidazolato-metal is a polymeric material. X-ray crystallographic evidence can also be found.= Based on electronic spectral results as well as magnetic moment measurements, different groups of researchers have considered metal imidazolato compounds to be polymeric.24 The coordination ability of one nitrogen atom in a simple
n
A~+(N@N)
salt will not be satisfied unless it coordinates to another Ag+ ion, thus producing a polymerized structure. 2. Discussion of Reaction Mechanism. Imidazole has a basicity intermediate between that of saturated amines and aromatic amines. In addition to its basic properties, IMH is also a weak acid. But copper, silver, and mercury metals a t zero oxidation state are not active enough to substitute for the “pyrrole”-type hydrogen in IMH. Metal imidazolates are usually prepared by boiling sufficiently basic solutions of IMH and metal ions.2J0The above SERS and IR studies indicate that IMH can react with Cu, Ag, and Hg a t zero oxidation state under mild conditions to form metal imidazolates. This seems to follow a new mechanism. In neutral solution, IMH usually functions as a ligand by means of the unshared pair of electrons on the “pyridine”-type nitrogen. So it seems reasonable to suggest that initially a surface complex of M-IMH be formed by ligation of pyridine nitrogen of IMH with surface metal atoms. In this surface complex, the electrons transfer from metal to IMH antibonding ?r* orbitals, thus activating the ring skeleton and making it possible to weaken the N-H bond. On the other hand, as the system is exposed to air, the metal surface can adsorb oxygen to form coadsorbed oxygen species, e.g., undisso(22) Bukowska, J.; Kudelski, A.; Jackowska, K. J . Electroanal. Chem. 1991,309, 251. (23) E.g., Yoshida, S.; Ishida, H. Appl. Surf. Sci. 1990,44,301. Jarvis, J. A. J.; Well, F. A. Acta Crystallogr. 1960, 13, 1027. (24) Examples reported include the imidazolato salt of Fez+ (Seel, F.; Sperber, V. Angew. Chem., Int. Ed. Engl. 1968,7,70), Ni*+and Cu2+(cf. refs 2 and IO), and Ni2+ and Co2+(Eilbeck, W. J.; Holmes, F.; Underhill, A.E. J.Chem.Soc.A 1967,757). ThesaltCu+(Im-)hasalsobeenprepared and a polymeric bridge structure has also been proposed cf. ref 3.
gH
H
I
ciated dioxygen, 02. It has been reported that the coadsorbed oxygen on Cu, Ag, and Au shows more nucleophilic and oxidizing properties than the oxygen in air or in solution.25 So under mild conditions, the metals are prone to donate electrons and the imino H in IMH can be eliminated, resulting in the formation of metal imidazolate complexes and water. In order to illustrate the influence of metal surface on the reactivity of imidazole and gain further insight into the reaction features on the metal surfaces, we performed extended Huckel tight-binding band calculations of adsorption of IMH on Ag.14 Though approximate in nature, this type of calculation can give much chemical insight and has been recently applied to various systems which model the metal, metal oxide surface, or interface. Ag has a fcc structure. For reasons of economy, we model the Ag(ll1) surface by a monolayer slab of Ag atoms. The choice of the unit cell was dictated by the necessity of keeping a relatively low coverage on the surface, otherwise the adsorbate molecules may come into close contact with each other, leading to high repulsive energies. We use a P(3X3) unit cell. The coverage will be 1/9 (nine surface Ag atoms per adsorbate molecule). With the chosen unit cell, no short distances between adjacent molecules are observed on Ag(ll1). Four different adsorption geometries are considered (see Chart l ) , namely, the IMH ring lying down (a) with a silver atom equidistant from the ring C and N atoms or (b) with the pyridine-type N atom adsorbed atop a Ag atom, (c) the ring erect with the pyridine-type nitrogen atom atop adsorbed on a Ag atom, or (d) bridging between two adjacent Ag atoms (nearest neighbors). Detailed analysis of the origination of the bonding characteristics reveals that there is a preference for the bridging fashion (d configuration) over the lying down or atop standing-up fashion (a, b, or c configuration).’6 Here we try to examine the potential energy surface for the reaction on Ag to elucidate the influence of the presence of Ag(ll1) surface on the reactivity of IMH. Similar calculations have been successfully applied to decomposi(25) Outka, D. A.; Madix, R. J. Surf. Sci. 1987,179,361, and references cited therein.
Actiuating Effect on Surfaces
0
1
Langmuir, Vol. 10, No. 5, 1994 1481
2
3
N-H d i s t a n c e
4
(1)
Figure 4. Potential energy curves of imidazole when stretching N-H bond of (a) free imidazole monolayer and (b) imidazole on Ag(ll1) surface. The energies are relative to the unstretched values.
tion of cyclic sulfide on Mo(ll0) and formaldehyde on cr(100).26~27 Curve b in Figure 4 gives the changes in total energy along the dehydrogenation of imino hydrogen for the reaction on Ag(ll1). The dehydrogenation of IMH on Ag(ll1) occurs with a small activation energy (1.55 eV). On the other hand, there is a great energy barrier (4.33 eV) for conversion of IMH to imidazolyl moiety for free IMH monolayer (curve a in Figure 4). Initially, stretching the N-H bond requires energy because the N-H interaction is an attractive two-orbitalltwo-electron type. However, the N-H antibonding orbital drops very rapidly as the N-H bond is stretched and may even fall below the Fermi level. When such a crossing takes place, electrons are transferred into the N-H antibonding orbital, resulting in a two-orbital/four-electron repulsion. If the Fermi level is high in energy, then it may cross with the N-H antibonding orbital early in the reaction, and the elimination of H could have a small energy barrier. Since the Fermi level of the chemisorptive systems (-7.26 eV) (almost the same as that of the clean Ag(ll1)) is higher than that of the IMH monolayer (-12.21 eV) before adsorption, the orbital crossing takes place earlier in the reaction, and dehydrogenation may proceed with a small energy barrier (cf. Figure 4; 1.55 eV versus 4.33 eV). On the other hand, elimination of imino hydrogen on Ag(ll1) was found to be exothermic. Though these results are not likely to be quantitatively correct, they do show qualitatively the dehydrogenation to be much facile on metallic Ag. I t appears possible that the transfer of electrons from metal to oxygen will be followed by a simple deprotonation of imidazole to form the imidazolate anion. Such a mechanism is not strongly supported by either experimental or theoretical results. First, the tendency for surface complex formation for metal imidazole is very strong. We have observed that when benzimidazole was adsorbed on metals, a surface metal-benzimidazole complex, which can exist only on surfaces, was first formed before its reaction with oxygen.% On the other hand, metal surfaces, if preadsorbed by oxygen or even if preoxidized (26) Calhorda, M.J.; Hoffmann, R.; Friend, C. M. J.Am. Chem. SOC. 1990, 112, 50. (27) Cain, S.R.;Emmi, F. Surf. Sci. 1990, 232, 209.
deliberately, showed very weak reactivity toward azole compounds.29 Based on these points as well as the results of the present study, we would rather explain the reaction mechanism with emphasis that the imidazole molecules can react with the coadsorbed oxygen quite easily upon adsorption than say that the adsorbed oxygen, or surface oxide, attacks the active proton in imidazole. This is the main reason that a deprotonation process, which may be easily misunderstood as a sort of the reaction between an active proton and oxide, is not adopted. Secondly, we have compared the energy difference between deprotonation and dehydrogenation of imidazole on Ag(ll1). The results showed that the deprotonation required an energy 6.0 eV higher than the dehydrogenation, indicating that the latter mechanism is preferential. The reason is simple. Upon imidazole adsorption, the imidazole ring is partially negative charged as electrons are transferred from metal to imidazole, thus the formation of a more negatively charged imidazolate on the Ag metal surface by deprotonation is energetically unfavorable. But eliminating a H atom on the surface will not increase the negative charge density on the imidazolyl ring. This surface dehydrogenation process certainly differs from those in aqueous solutions with high pH values where the deprotonation is observed frequently. It must be pointed out that Hoffmann previously proposed that metal-adsorbate bonding is accomplished a t the expense of bonding within the adsorbed molecules.30 This point of view agrees with our observation and may be an important effect on inducing the reaction of IMH with Ag surface easier. Experimentally we call this effect “complex-induced activating effect” in surface reactions of a series of polar organic compounds, such as 1,2- and 1,3-dicarbonylcompounds, nitriles, and ester, on transition metal~.~l-3~
Conclusions We have shown that adsorption of imidazole on chemically clean metal surfaces results in the formation of metal imidazolates. The ease of the reaction suggests that the reactivity of imidazole is increased upon adsorption which is accompanied by the weakening of the N-H bond. It seems that the surface-complex induced activating effect, previously observed in surface reactions of other kinds of organic compounds, also exists in chemisorption of many polar organic compounds.
Acknowledgment. The authors gratefully acknowledge the financial support of the National Science Foundation of China. (28) Xue, G.;Zhang, J.; Shi, G.; Wu, P. J.Chem. SOC.,Perkin Tram. 2 1989, 33.
(29) Xue, G.;Ding, J.; Lu, P.; Dong, J. J.Phys. Chem. 1991,95,7380. (30)Hoffmann, R. Reo. Mod. Phys. 1988,60, 601. (31) Xue, G.; Sheng,Q.;Dong,J.J. Chem. SOC., Faraday Trans. 1991, 87, 1021. (32) Xue, G.;Dong, J.; Sheng, Q. J. Chem. SOC.,Dalton Tram. 1991,
.
An7 -”.
(33) Xue, G.;Zhang, 2.; Dong, J.; Dai, Q.; Qiu, Y. Huaxue Xuebao
1991, 49, 53.
(34) Xue, G.; Huang, X.; Ding, J. J . Chem. SOC.,Faraday Tram. 1991, 87, 1229.
