J. Phys. Chem. C 2007, 111, 11267-11274
11267
Adsorption of Ethanediamine on Colloidal Silver: A Surface-Enhanced Raman Spectroscopy Study Combined with Density Functional Theory Calculations Gengshen Hu, Zhaochi Feng, Jun Li, Guoqing Jia, Difei Han, Zhimin Liu, and Can Li* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China ReceiVed: February 7, 2007; In Final Form: April 29, 2007
The adsorption of ethanediamine on the colloidal silver surface has been studied by means of surface-enhanced Raman spectroscopy (SERS) combined with density functional theory (DFT) calculations. It is deduced from the SERS results that ethanediamine adsorbs on colloidal silver via its two N atoms. The calculation results suggest that the two N atoms of ethanediamine are bound to the same silver atom rather than different silver sites and ethanediamine prefers to absorb on the positively charged sites. Charge transfer between ethanediamine and the silver surface was probed by using SCN- as the probe molecule. The CtN stretching mode of SCNadsorbed on a silver surface shifts from 2133 down to 2120 cm-1 in the presence of ethanediamine, indicating that the charge transfer is from ethanediamine to the silver surface, which is further confirmed by DFT calculations. The ethanediamine adsorption mechanism on silver may shed light on the asymmetric catalysis involving 1,2-diamine ligands coordinated on transition metals.
1. Introduction Due to the great importance of chiral compounds in the manufacture of drugs, vitamins, fragrances, and optical materials, asymmetric catalysis has attracted special attention in recent years. Chiral 1,2-diamines, one type of important ligands, such as chiral 1,2-diamine-cyclohexane, chiral 1,2-diphenyl-ethanediamine, and their derivatives, are widely used in asymmetric catalytic reactions.1 For instance, chiral 1,2-diamine coordinated with a transition metal ion can catalyze asymmetric hydrogenation2-5 and asymmetric transfer hydrogenation5-7 of ketones. Ethanediamine, the simplest 1,2-diamine, has also been used in the asymmetric hydrogenation of ketones1,2 in the presence of phosphine ligands. The properties of 1,2-diamine ligands, such as coordination ability with metal, the steric effect, and electron donor-acceptor ability, can dramatically influence the activity and selectivity of catalysts. Moreover, it is believed that the cleavage of the 1,2-diamine N-H bond is necessary, and then the resulting proton transfers to a prochiral ketone to produce a chiral alcohol in the processes of hydrogenation reaction2-4 and hydrogen transfer reaction.5-7 However, the properties of 1,2-diamine ligands and the interaction between 1,2-diamine ligands and metal are rarely studied and are mainly investigated by theoretical calculations8-10 rather than experimental method. More recently, an Ru/γ-Al2O3 catalyst modified by chiral 1,2diphenyl-1,2-ethylenediamine (S,S-DPEN) was used in the asymmetric hydrogenation of acetophenone and its derivatives, acetylpyridine and five-membered heterocyclic ketones, and up to 86.2% ee value was achieved.11 Other than the homogeneous catalyst, the heterogeneous catalyst was stable in air and could be easily separated from the products and recycled several times without remarkable decrease of the enantioselectivity, which is very attractive for the potential industrial application. However, the mechanism for this heterogeneous asymmetric hydrogenation * To whom correspondence should be addressed. Email:
[email protected]. Homepage: http://www.canli.dicp.ac.cn.
involving 1,2-dianmine ligand is not available. So, it is highly desired to study the adsorption orientation of ethanediamine on a metal surface as well as the interaction between 1,2-diamine ligand and the metal surface. On the other hand, ethanediamine and other 1,2-diamines are widely used as chelating agents for some metal ions and the precursors of biologically relevant polyamines (e.g., spermidine and spermine) which play an essential role in cell growth and differentiation in eukaryotic organisms. For example, some metal complexes (e.g., Pt2+) formed with these amines are known to display anticancer properties through DNA binding. However, the exact nature of the biochemical mechanism involving the 1,2-diamine is still unknown and has been the topic of intense research in recent years.12,13 Surface-enhanced Raman spectroscopy (SERS) is a very sensitive technique that is able to detect molecules at trace concentration level and quench the fluorescence background by radiationless energy transfer between the metal surface and adsorbate.14 It can provide useful information about the adsorption orientation of the molecule on the metal surface and the interaction between the adsorbate and the metal surface, including the coordination information. These are of fundamental interest and importance, particularly for the heterogeneous catalysis.15 Up to now, to the best of our knowledge, using SERS to investigate the adsorption of 1,2-diamine on a metal surface has never been reported. Meanwhile, density functional theoretical (DFT) calculations, based on a simple adsorption model constituted by a ligand bound to one or two metal atoms (or ions), were successfully used to investigate the interaction between adsorbate and the metal surface at the molecular level in recent years.16-22 Although the employed models are very simple, the calculation results are in good agreement with the experimental results and can provide useful information on the interaction between the adsorbate and metal substrate and identify the adsorbed species. In this work, we focused on the adsorption orientation of ethanediamine on metal, the most preferred site for the adsorp-
10.1021/jp071066k CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007
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tion of ethanediamine and the interaction between the adsorbate and the metal surface by using SERS and UV-vis spectroscopy together with DFT calculations. Due to the poor enhancement of Ru metal,23,24 colloidal silver was used as an active substrate to investigate the coordination information of 1,2-diamine with metal and the charge-transfer effect between them. We found that ethanediamine adsorbs on the colloidal silver surface through its two N atoms and with the C-C bond parallel to the surface and that the charge transfer is from ethanediamine to the silver surface. This may be helpful to gain an insight into the modifier-metal interaction in heterogeneous catalysis. 2. Experimental and Computational Details 2.1. Chemicals. Silver nitrate, hydroxylamine hydrochloride, and sodium hydroxide were purchased from Sigma-Aldrich. Ethanediamine (EDA) was obtained from Merck. All chemicals were of analytical grade and were used as received. Tridistilled water was used for the preparation of silver colloids and ethanediamine solution. 2.2. Preparation of Colloidal Silver. The colloidal silver was prepared by reducing AgNO3 with hydroxylamine hydrochloride25 at room temperature. A 225 mL mixture solution of hydroxylamine hydrochloride (1.67 × 10-3 M) and sodium hydroxide (3.33 × 10-3 M) was vigorous stirred, and then a 25 mL silver nitrate solution (0.01 M) was added. The transparent solution changed to gray-green immediately after adding the silver nitrate solution. After stirring for several minutes, the colloidal silver was available for use. 2.3. Methods. The UV-vis absorption spectra measurements were carried out on a JASCO V-550 UV-vis spectrophotometer. Surface-enhanced Raman scattering (SERS) spectra were recorded in backscattering geometry on an Acton Raman Spectrometer equipped with a liquid-nitrogen cooled CCD detector at a resolution of 4 cm-1. A 532 nm semiconductor laser was used as the excitation source, and the laser power at the sample was set as 60 mW. All SERS experiments were performed with a quartz tube at room temperature. For SERS measurements, 0.2 mL sample solutions were mixed with 0.2 mL of colloidal silver to give final concentrations of ethanediamine. The pH values of the ethanediamine solutions were adjusted with H2SO4 solution for acidic conditions. 2.4. Theoretical Calculations. The theoretical calculations on ethanediamine and its silver complexes were carried out by using the Gaussian 03 program package.26 Optimization of the molecular structures and vibrational frequency calculations for the optimized structures were performed by means of density functional theory (DFT) with the Becke three-parameter hybrid functional combined with the Lee-Yang-Parr correlation functional (B3LYP). The 6-31+G(d,p) basis set was used for the calculations of ethanediamine, whereas, in the case of the ethanediamine-silver complexes, a mixed basis set (6-31+G(d,p) for all atoms except silver, LanL2DZ was employed for silver) was used. 3. Results and Discussion 3.1. Raman and SERS Spectra of Ethanediamine. The form of the ethanediamine molecule may be in either gauche or trans conformation. The previous experimental27-29 and theoretical27-30 results have revealed that the gauche conformer is predominant at room temperature. This is due to the intramolecular N‚‚‚H hydrogen bond between the N3 atom and the H12 atom in the gauche conformer as shown in Scheme 1. So here for the Raman frequency calculation of ethanediamine, only the gauche conformer was taken into account.
Figure 1. (A) Raman spectrum of liquid ethanediamine and (B) SERS spectrum of ethanediamine (2 × 10-2 M) on silver surface.
SCHEME 1: Optimized Gauche-conformer Structure by Using the B3LYP/6-31+G(d,p) Method; Detailed Structural Parameters Were Shown in Table 2a
a The arrow in the gauche conformer indicates the N‚‚‚H intramolecular hydrogen bond.
TABLE 1: Assignment of Raman and SERS Bands (cm-1) of the Ethanediamine Molecule Raman
calb
188
170
337 471 839 985 1055 1096 1242 1299 1355 1454 1601 2787 2855 2917 3192 3296 3359
339 484 833 963 1072 1129 1270 1286 1347 1376 1441 1595 2808 2857 2935 3366 3371 3455
SERS 222 287 337 471 845 984 1042 1124 1240 1299 1378 1453 1581 2800 2876 2917 3149 3254 3309
vibrational assignmenta C-C torsion N-Ag symmetric stretch. N-Ag asymmetric stretch. NCCN def. NH2 wag. NH2 rock. CH2 rock. C-N stretch. skeletal def. CH2 and NH2 twist. CH2 and NH2 twist. CH2 wag. CH2 wag. CH2 sciss. NH2 sciss. CH2 symmetrical stretch. CH2 symmetrical stretch. CH2 asymmetrical stretch. NH2 symmetrical stretch. NH2 symmetrical stretch. NH2 symmetrical stretch.
a Stretch., stretching; def., deformation; rock., rocking; twist., twisting; wag., wagging; sciss., scissoring. b The frequencies are predicted at the B3LYP/6-31+G(d,p) level and scaled with 0.96.
The Raman and SERS spectra of ethanediamine are shown in Figure 1. Their peak positions are listed in Table 1 together with the peak assignments predicted by DFT calculations at the B3LYP/ 6-31+G(d,p) level. In the SERS spectrum, several peaks concerned with the N atoms are red-shifted compared with their corresponding peaks in the normal Raman spectrum,
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SCHEME 2: Schematic Representations of the following: (a) N-Ag Symmetric Stretching Mode and (b) N-Ag Asymmetric Stretching Mode
implying that the interaction between ethanediamine and the silver surface is through amino groups. The frequency shifts are most likely due to the change in the electronic structure of the molecule as a result of the charge transfer between the adsorbate and the metal. The C-N stretching mode, at 1055 cm-1 in the Raman spectrum, is markedly enhanced and red-shifted to 1042 cm-1 in the SERS spectrum. On the basis of the surface selection rule,31,32 the vibrational mode with the transition moment perpendicular to the surface should have a large enhancement in Raman signal. Furthermore, the SERS effect is sensitive to the distance33,34 between the adsorbed molecule and the metal surface. The electrical field is the strongest on the metal surface and decreases exponentially with distance from the surface. As a consequence, the part of the adsorbate close to the surface gives rise to more enhanced intensity in Raman peaks than those away from the surface. Therefore, the intense enhancement of the C-N stretching mode indicates that the C-N bond is perpendicular or at least slightly tilted to the surface. The red shift of the C-N stretching mode also reveals that the interaction, between ethanediamine with the silver surface, is through N atoms via its lone pairs of electrons. The peak at 3296 cm-1 in the Raman spectrum assigned to the N-H stretching mode corresponds to the peak at 3254 cm-1 in the SERS spectrum. The peak at 1601 cm-1 in the Raman spectrum assigned to the N-H scissoring mode correlates with the peak at 1581 cm-1 in the SERS spectrum. These two peaks have a red shift about 20-40 cm-1, respectively. The red shifts of these two modes further confirm the assumption that the interaction between ethanediamine and silver is through the N atoms of ethanediamine. Comparing the SERS spectrum with the Raman spectrum, the biggest difference is the appearance of a new strong peak at 222 cm-1, which is attributed to the N-Ag symmetric stretching mode (see Scheme 2a). Moreover, another new peak, assigned to the N-Ag asymmetric stretching mode (see Scheme 2b), appears at about 287 cm-1. For the mono-amino group molecule that adsorbs on the metal surface via the N atom, the latter peak could not be observed due to the lack of this mode. Thus, for ethanediamine, the presence of both peaks indicates that the two amino groups of ethanediamine cocoordinate to the silver surface. Commonly, the Raman intensity of the symmetric stretching mode is stronger than that of the asymmetric stretching mode while the Raman frequency of the symmetric stretching mode is lower than that of the asymmetric stretching mode. Clearly, as revealed from Figure 1, the experimental results are consistent with these. On the other hand, the C-C stretching mode at 839 cm-1 in the Raman spectrum is nearly absent in the SERS spectrum, implying that the C-C bond is parallel to the silver surface. Generally, both electromagnetic enhancement and chargetransfer (chemical) enhancement contribute to the total SERS enhancement. The largest enhancement (105-106) is due to the EM mechanism, with a small contribution from the charge-
Figure 2. SERS spectrum of ethanediamine at different pH values: (A) pH ) 6 and (B) pH ) 2.
SCHEME 3: Proposed Models of the EthanediamineSilver Complex: (a) EDA-Ag, (b) EDA-Ag+, (c) EDAAgAg, (d) EDA-AgAg+, (e) EDA-2Ag, and (f) EDA2Ag+
transfer enhancement (103).35,36 The charge-transfer enhancement leads to frequency shifts of the adsorbed group that are linked directly with the metal surface. As observed in Figure 1 and discussed above, the vibrational frequencies associated with N atoms have a red shift. This indicates that charge-transfer enhancement has a contribution to the total enhancement. Consequently, it is concluded that ethanediamine adsorbs on silver through its N atoms coordinating to the silver surface with a C-C bond parallel to the surface and that both electromagnetic enhancement and charge transfer contribute to the total SERS enhancement. To further confirm the conclusion that adsorption of ethanediamine on the silver surface is through its N atoms via the lone pairs of electrons, the SERS spectra of ethanediamine at various pH values were recorded as shown in Figure 2. The SERS signal of ethanediamine is very strong at pH ) 6 whereas it nearly disappears at pH ) 2, indicating that ethanediamine with protonated N atoms is difficult to adsorb on the silver surface. This provides further evidence that the adsorption of ethanediamine on the silver surface is through the N atoms via their lone pairs of electrons. 3.2. DFT Calculations on the Model Surface Complexes. To gain further insight into the interaction between ethanediamine and the silver surface, DFT calculations were performed. The calculations were mainly focused on the Raman frequencies of ethanediamine in different adsorbed models but not the intensity. Three possible model complexes for DFT calculations were proposed as shown in Scheme 3. The first model was proposed that two nitrogen atoms are bound to one silver atom (see the model in Scheme 3a, denoted as EDA-Ag); the second was
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TABLE 2: Selected Structural Parameters (angstroms for bond length, degrees for dihedral angle) Calculated for Ethanediamine and Ethanediamine-Ag Complexesa EDA C1-C2 C1-N3 C1-H5 C1-H6 C2-N4 C2-H7 C2-H8 N3-H9 N3-H10 N4-H11 N4-H12 N3-H12 N3-Ag13 N4-Ag13 N4-Ag14 Ag13-Ag14 H7-C2-N4-H11 H7-C2-N4-H12 H8-C2-N4-H11 H8-C2-N4-H12
1.528 1.471 1.102 1.095 1.463 1.097 1.107 1.015 1.017 1.015 1.017 2.506 2.670 2.670 2.375 2.634 -67.032 173.220 52.020 -67.728
EDA-g0
EDA-Ag+
1.534 1.467 1.097 1.102 1.467 1.097 1.102 1.016 1.017 1.016 1.017
1.534 1.488 1.095 1.096 1.488 1.095 1.096 1.018 1.019 1.018 1.019
1.534 1.469 1.097 1.101 1.469 1.097 1.101 1.015 1.017 1.015 1.017
1.533 1.484 1.095 1.097 1.484 1.095 1.097 1.017 1.019 1.017 1.019
2.356 2.356
2.554 2.554
2.399 2.399
2.375
2.806 -45.566 -166.778 72.319 -48.893
2.740 -44.021 -161.304 73.249 -44.034
-43.972 -164.549 74.019 -46.558
-44.689 -162.112 72.776 -44.647
EDA-AgAg0
EDA-AgAg+
EDA-2Ag+ 1.539 1.491 1.095 1.096 1.491 1.095 1.096 1.020 1.020 1.020 1.020
-61.222 -175.961 54.668 -60.072
a The structural parameters of the ethanediamine and ethanediamine-silver complexes are estimated at the B3LYP/6-31+G(d,p) and B3LYP/ 6-31+G(d,p)/Lanl2DZ levels, respectively.
proposed that both of the nitrogen atoms of ethanediamine are bound to the same silver atom of a two-atom silver cluster (see the model in Scheme 3c, denoted as EDA-AgAg); and the third one was proposed to consist of two nitrogen atoms which are bound to different silver atoms of the two-atom silver cluster, respectively (see the model in Scheme 3e, denoted as EDA2Ag). Because some silver ions, which may result from inadequate reduction of silver nitrate, are present on the surface of silver colloids, the models for ethanediamine adsorbed on positively charged sites are also calculated. The models, which represent the adsorption of ethanediamine on positively charged sites, are shown in parts b, d, and f of Scheme 3 and denoted as EDA-Ag+, EDA-AgAg+, and EDA-2Ag+, respectively. In fact, for the surface complex of EDA-2Ag, a negative frequency presents in the frequency calculation, indicating that it is not a stable structure. So, we did not take the EDA-2Ag complex into consideration. Table 2 shows the optimized structural parameters of the ethanediamine-Ag complexes. When the structural parameters of ethanediamine-Ag complexes are compared with that of ethanediamine, it is found that the structure of ethanediamine changes dramatically in the process of ethanediamine adsorption on the silver surface. Besides the changes in bond lengths, the most obvious changes take place in the dihedral angles of H7C2-N4-H11, H7-C2-N4-H12, H8-C2-N4-H11, and H8-C2N4-H12, suggesting that the intramolecular hydrogen bond of ethanediamine is broken after the adsorption of ethanediamine and the NH2 group rotates around the C2-N4 bond to coordinate with the silver cluster (compare Scheme 1 with Scheme 3). The trend of change in bond length can also tell us some useful information. The lengths of the N-H bond (N3-H9, N3H10, N4-H11, N4-H12) of EDA-Ag+ and EDA-AgAg+ are longer than those of the free ethanediamine, whereas the lengths of the C-H bond (C1-H5, C2-H7, C2-H8) of these two complexes are shorter than those of the free ethanediamine. This indicates, after the adsorption of ethanediamine on the silver surface, that the frequencies of the N-H stretching and scissoring modes should show a red shift, respectively, whereas the frequencies of the C-H stretching, scissoring, and wagging modes should shift to higher frequencies. These are proved by the experimental results. Experimentally, the N-H asymmetric
stretching mode shifts to lower frequencies from 3359 to 3309 cm-1 and the two N-H symmetric stretching modes shift to lower frequencies from 3296 and 3196 cm-1 to 3254 and 3149 cm-1, respectively. In contrast, the C-H asymmetric stretching mode show a blue shift from 2917 to 2926 cm-1 and the two C-H symmetric stretching modes shift from 2855 and 2787 cm-1 to 2876 and 2800 cm-1, respectively. Besides, the lengths of the C-N bond (C1-N3 and C2-N4) of EDA-Ag+ and EDA-AgAg+ are longer than those of the free ethanediamine, indicating that the C-N stretching mode should show a red shift. This is also confirmed by the Raman and SERS results, as revealed by a shift of ν(C-N) from 1055 to 1042 cm-1. On the other hand, the lengths of the N-H bond (N3-H9, N3-H10, N4-H11, N4-H12) of the EDA-Ag and EDA-AgAg complexes are nearly the same as those of the free ethanediamine, indicating that the N-H stretching mode should not have an apparent shift. This is inconsistent with the experimental results as noted above. The selected frequencies of ethanediamine and ethanediamine-Ag complexes are given in Table 3. In general, the frequency calculations for EDA-Ag+ and EDA-AgAg+ complexes achieved more reasonable results than those for the EDA-Ag, EDA-AgAg, and EDA-2Ag complexes. For example, in the case of the EDA-Ag+ and EDA-AgAg+ complexes, the calculated N-H asymmetric stretching mode shifts from 3455 cm-1 to 3435 and 3439 cm-1, respectively, and the calculated N-H symmetric stretching mode shifts to higher frequencies from 3371 cm-1 to 3363 and 3364 cm-1, respectively. On the other hand, the calculated C-H asymmetric stretching mode shifts from 2935 cm-1 to 2979 and 2971 cm-1 and the two C-H symmetric stretching modes shift from 2855 and 2787 cm-1 to 2876 and 2800 cm-1, respectively. The frequency calculations of the EDA-Ag and EDA-AgAg complexes produced opposite trends in the shift of N-H asymmetric and symmetric stretching modes with the experimental results. For the CH2 wagging mode, the Raman peak position shifts from 1355 to 1378 cm-1 and frequency calculations for all complexes lead to the same trend of frequency shift. As proved by the SERS experiment, the C-N stretching mode shifts to a lower frequency for the strong interaction of N atoms with the silver surface. The changing trends in the length of
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TABLE 3: Selected Frequencies (cm-1) from Experiments and Calculated Resultsa νs(N-A) νas(N-A g) ν(C-C) ν(C-N) ω(CH) δ(CH) ν(NH) νs(CH) νs(CH) νas(CH) νs(NH) νs(NH) νas(NH)
Raman
SER S
EDA
EDA-Ag 0
EDA-Ag+
EDA-AgAg0
EDA-AgAg+
EDA-2Ag+
222 287 839 1055 1355 1453 1601 2787 2855 2917 3192 3296 3359
125 128 845 1042 1378 1454 1581 2800 2876 2926 3149 3254 3309
215 292 833 1072 1347 1452 1582 2808 2857 2935 2963 3371 3455
140 168 840 1072 1370 1442 1595 2862 2877 2937 2955 3371 3459
219 264 815 1061 1377 1446 1598 2936 2940 2979 2992 3363 3435
293 295 816 1069 1369 1444 1598 2875 2890 2941 2960 3376 3462
821 1062 1377 1446 1599 2927 2934 2971 2986 3364 3439
788 998 1359 1449 1596 2927 2938 2981 2985 3342 3411
a B3LYP/6-31+G(d,p)/Lanl2DZ was applied to calculate the frequencies of ethanediamine and ethanediamine-silver complexes. The calculated frequencies were scaled with 0.96 (>300 cm-1) and 1.22 (
