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Langmuir 1999, 15, 5814-5819
The Coordination State of Copper(II) Complexes Anchored and Grafted onto the Surface of Organically Modified Silicates† Andrzej M. Kłonkowski,*,‡ Beata Grobelna,‡ Teresa Widernik,‡ Anna Jankowska-Frydel,§ and Włodzimierz Mozgawa| Faculty of Chemistry, University of Gdan´ sk, ul. J. Sobieskiego 18, 80-952 Gdan´ sk, Poland, Institute of Experimental Physics, University of Gdan´ sk, ul. Wita Stwosza 57, 80-952 Gdan´ sk, Poland, and Faculty of Materials Engineering, Academy of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Krako´ w, Poland Received September 15, 1998. In Final Form: March 31, 1999 Amine derivatives of alkoxides such as (3-aminopropyl)trimethoxysilane and (3-(2-aminoethyl)aminopropyl)trimethoxysilane are reagents in the sol-gel process and, at the same time, versatile ligands forming complexes with transition metal ions (e.g., Cu(II)). One of the major advantages of the sol-gel process is the possibility of preparing Cu(II) complexes in the interior of and on the surface of organically modified silicate xerogels. This paper compares Cu(II) complexes prepared in the interior of silica xerogels with those grafted and anchored onto the surface of such xerogels. Grafting and anchoring require prior preparation of supports with functionalized surfaces. In the former process Cu(II) ions are chemisorbed, and in the latter Cu(II) complexes are immobilized by a condensation reaction. The following spectroscopic results are presented: FT-IR, optical absorption in the visible region, and electron paramagnetic resonance (EPR). The EPR studies are summarized in three identified models of the coordination sites on the modified silica xerogel surfaces. The materials studied are put forward as catalyst precursors.
Introduction Molecular catalysts which might otherwise dissolve in a contacting liquid phase can be bound to an insoluble solid, thus achieving what is termed immobilization. The catalytic entities of the greatest importance in this connection are complex metal species. In this case the entity must be attached firmly enough to the macroscopic surface of the supporting material for it to be resistant to leaching under the conditions of its subsequent use as a catalyst. The general process of attachment, particularly with regard to inorganic species, is sometimes described as the “heterogenation of homogeneous catalysts”. Successful immobilization results in a catalyst which has the physical characteristics of a solid, making for easy retention in a reactor and separation of products, but which preserves the specificity of the corresponding homogeneous catalyst in its catalytic action.1 In general, transition metal ions can be introduced into the interior (bulk) or onto the surface of dry gels (xerogels) by the sol-gel method. In the bulk case metal ions are introduced at the sol stage of preparation. Hydrolysis and polycondensation reactions produce an inorganic or a hybrid organic-inorganic xerogel incorporating a metal complex. The metal ions dissolved in the sol rearrange the local structure to suit their bonding requirements and form complexes of various coordination numbers. During the gelation process the primary unconstrained coordina† Presented at the Third International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland, August 9-16, 1998. ‡ Faculty of Chemistry, University of Gdan ´ sk. § Institute of Experimental Physics, University of Gdan ´ sk. | Faculty of Materials Engineering, Academy of Mining and Metallurgy.
(1) Campbell, I. M. Catalysis at Surfaces; Chapman and Hall: London 1988. (2) Kłonkowski, A. M.; Schlaepfer, C. W. J. Non-Cryst. Solids 1991, 129, 101.
tion environment is subject to distortion by the extended gel network. 2-5 There are alternative techniques of preparing xerogels surface-doped with transition metal complexes, namely grafting and anchoring. A grafted complex is produced when an initial structure bound to a surface is altered considerably by subsequent treatments. Here, the initially bound complex is not usually an active catalyst. In each case the surface of the supports should be functionalized with potentially reactive groups, e.g., hydroxyl or amino. A wide variety of highly active and selective catalysts can be prepared by grafting techniques, commencing with the attachment of suitable organometallic species to a solid surface. Ultimately, the procedure can be extended to the production of highly dispersed, supported metal catalysts. However, after subjection to suitable treatments, the initially grafted species may yield a series of catalytically active materials. By comparison, an anchored complex is created by binding a species, without effecting substantial changes in its structure, to a solid surface. It is becoming clear that an enormous research effort in the field of anchored inorganic catalysts is going to take place in the coming years. These catalysts have the potential of achieving high activity under relatively mild conditions, with sometimes interesting and unusual selectivities. Among the transition metal complexes, Cu(II) entities are very often used in catalytic systems based on silicasupported liquid phases.1 At the same time, Cu(II) is a particularly useful probe ion, since its electron paramagnetic resonance (EPR) spectra are readily observable over a broad temperature range. The Cu(II) ion spectrum is (3) Kłonkowski, A. M.; Schlaepfer, C. W. J. Non-Cryst. Solids 1992, 149, 189. (4) Kłonkowski, A. M.; Koehler, K.; Schlaepfer, C. W. J. Mater. Chem. 1993, 3, 105. (5) Kłonkowski, A. M.; Koehler, K.; Widernik, T.; Grobelna, B. J. Mater. Chem. 1996, 6, 579.
10.1021/la981256+ CCC: $18.00 © 1999 American Chemical Society Published on Web 06/30/1999
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detectable at such low amount as ca. 1015 spins6 and in low-symmetry environments. For this reason, the environment of the metal ion may be usefully studied in the amorphous network.7-9 The aim of the present study is to prepare materials which could possibly be put forward as catalysts following thermal treatment4,5 and to determine the structure of Cu(II) complexes obtained on xerogel surfaces by grafting and anchoring. Experimental Section Materials. All the chemicals used, including methanol, were of guaranteed-reagent grade. Copper(II) nitrate Cu(NO3)2‚3H2O (from Fluka A. G.) and alkoxysilanes, such as tetramethoxysilane Si(OCH3)4 (abbreviated here to T), (3-aminopropyl)trimethoxysilane H2N(CH2)3Si(OCH3)3 (N), and (3-(2-aminoethyl)aminopropyl)trimethoxysilane H2N(CH2)2HN(CH2)3Si(OCH3)3 (NN) (from Aldrich Co.), were used to prepare two series (N and N-N) of xerogel surfaces coordinately modified with Cu(II). The water for the sol-gel procedure and solution of Cu(NO3)2 was doubly distilled. Sample Preparation. In the Bulk. By this technique Cu(II) complexes were prepared in the interior of silica xerogels, as described previously.2-5 On the Surface. In this case Cu(II) complexes can be immobilized on the functionalized surface of the previously prepared xerogel support as follows: (a) Grafting. The aminated xerogel was first prepared by the sol-gel method:10
N series N + xT + (3 + 4x)H2O f H2N(CH2)3SiO3/2‚xSiO4/2
(1)
N-N series N-N + xT + (3 + 4x)H2O f H2N(CH2)2‚HN(CH2)3SiO3/2‚xSiO4/2 (2) where x ) 10. The starting mixture in methanol was stirred vigorously at room temperature. The sol was allowed to gel for 3 days and was then dried at room temperature. The xerogel obtained was heated for 3 h at 120 °C to remove methanol and some of the water from the pores. In the next stage Cu(II) ions from the aqueous solution were chemisorbed by the aminated xerogel sorbent. In this way Cu(II) complexes with amino groups on the surface were created:
N series [Cu(H2O)6](NO3)2 + nN + xSiO4/2 f [Cu(H2O)6-nNn](NO3)2‚xSiO4/2 (3) N-N series [Cu(H2O)6](NO3)2 + m(N-N) + xSiO4/2 f [Cu(H2O)4-m(N-N)m](NO3)2‚xSiO4/2 (4) where N ) H2N(CH2)3SiO3/2 and N-N ) H2N(CH2)2‚HN(CH2)3SiO3/2. Finally, the xerogels with coordinately modified surfaces were dried at 120 °C. (b) Anchoring. Silica xerogel as the support was prepared by the typical sol-gel procedure11 from a starting mixture of T, methanol as diluent, distilled water (T:H2O ) 1:4) and NH3(aq) (6) Alger, R. S. Electron Paramagnetic Resonance. Technique and Applications; Wiley-Interscience: New York, 1968; p 6. (7) Bassetti, V.; Burlamacchi, L.; Martini, G. J. Chem. Soc. 1979, 101, 5471. (8) Kłonkowski, A. M.; Frischat, G. H.; Richter, T. Phys. Chem. Glasses 1983, 24, 47. (9) Kłonkowski, A. M. J. Non-Cryst. Solids 1985, 72, 117. (10) Schmidt, H.; Seiferling, B. In Better Ceramics Through Chemistry II (Mater. Res. Soc. Symp. Proc.); Materials Research Society: Pittsburgh, PA, 1986; Vol. 73, p 739. (11) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, 1990.
catalyst. The next stages of the procedure were as above; however, the mixture was heated for 6 h at 120 °C to remove coordinately active ammonia. Methanol solutions of Cu(II) complexes with N or N-N were prepared separately:
N series Cu(NO3)2 + 4N f [CuN4(NO3)2]
(5)
Cu(NO3)2 + 2N-N f [Cu(N-N)2(NO3)2]
(6)
N-N series
Indeed, Cu(NO3)2 in eqs 5 and 6 is an abbreviation of Cu(NO3)2‚ 3H2O. If so, the reactions are complemented with (CH3O)3Si-R + 3H2O f (OH)3Si-R + 3CH3OH, where R ) N or N-N. The crushed silica xerogel was immersed in one of the solutions and the mixture refluxed at 60 °C. After 5 h, the silica was filtered off, dried at room temperature, and heated at 120 °C, as described above. Apparatus. Optical absorption measurements in the visible region were recorded on a Beckman DU 650 spectrophotometer. Spectra of the crushed xerogel samples were obtained in a silicon oil mull and were collected between 400 and 800 nm. IR absorption spectra were obtained on a Digilab FTS-60V Fourier transform infrared spectrometer. Spectra were collected after 256 scans at 4 cm-1 resolution in the middle-infrared region (MIR, spectral range 4000-400 cm-1). Samples were prepared by the standard KBr pellet method. EPR spectra were recorded on a Bruker ESP-300 spectrometer with a microwave bridge operating in the X-band (9.5 GHz), using sealed quartz capillaries (1 mm diameter) as sample holders. The temperature variation was performed using a Bruker B-ST 100/700 variable-temperature accessory. A magnetic field modulation of 100 kHz was applied. The field was calibrated using diphenylpicrylhydrazyl (DPPH) free radicals. Total band shift analyses were based on the Bruker automation program. Standard deviations of the EPR spectral parameters were estimated as follows: g| (0.003, g⊥ (0.005, and A| (3 × 10-4 cm-1.
Results and Discussion Infrared (IR) spectroscopy is a convenient technique, as it provides information on the backbone structure of both the siloxane network and the organic side groups present within and on the surface of the material. In previous studies on vitreous silica,12 the 1060 cm-1 broad band (with a 1120 cm-1 shoulder), 790, and 450 cm-1 (see Figure 1) were assigned to Si-O-Si vibrations. Specifically, the high-wavenumber band peaking at around 1060 cm-1 was attributed to the Si-O-Si symmetric stretching mode in cyclic structures13 (simultaneously showing that the Si-O-Si bond is present at the surface),14 the second band is assigned to the bending motion of the oxygen atom along the bisector of the Si-O-Si group, and the 450 cm-1 feature to the bending of the bridging oxygen atom perpendicular to the Si-O-Si plane.15 It was also proposed16 that adjacent oxygen atoms in Si-O-Si bridges perform an asymmetric motion 180° out of phase, which is responsible for the shoulder at 1120 cm-1. In the IR spectra of alkoxy-derived SiO2 gel, two or three further absorption bands have been reported in addition to absorption bands observed in fused silica. The band at 947 cm-1 (Figure 1a) is assigned to the stretching vibrations of external Si-O- groups, while the other bands (around 790 and from the 550-580 cm-1 range) (12) Schraml-Marth, M.; Walther K. L.; Wokau, A.; Handy, B. E.; Baiker, A. J. Non-Cryst. Solids 1992, 143, 93. (13) Handke, M.; Mozgawa, W. Vibr. Spectrosc. 1993, 5, 75. (14) Shimizu, I.; Okabayashi, H.; Taga, K.; Nishio, E.; O’Connor, C. J. Vibr. Spectrosc. 1997, 14, 113. (15) Kamitsos, E. I.; Patsis, A. P.; Kordas, G. Phys. Rev. 1993, B48, 12499. (16) Wood, D. L.; Rabinovich, E. M. Appl. Spectrosc. 1989, 43, 263.
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Figure 2. Visible absorption spectra of the aminated oxide materials complexed with Cu(II) prepared by the following methods: (a) in bulk of the N xerogel and (b) grafted to the N xerogel; (c) in bulk of the N-N xerogel and (d) grafted to the N-N xerogel. Figure 1. FT-IR absorption spectra of the oxide supports: (a) silica xerogel compared with (b) silica modified with monoamino groups (N xerogel) and (c) silica modified with diamino groups (N-N xerogel).
were speculatively attributed to the “defect” structure in ref 16. However, it was previously suggested17 that the peak from the latter range is attributable rather to the siloxane backbone, since the 947 cm-1 band is situated near the C-H rocking band at around 960 cm-1 (see Figure 1).18 There is another peak related to OH,19 such as this at 1639 cm-1. This band is presumably caused by H2O deformation. A set of bands corresponding to O-H stretching vibrations, including those of water, appears in the 3000-3500 cm-1 spectral range as one broad band, whereas the shoulder at near 3750 cm-1 (Figure 1a) is attributed to SiO-H stretching in silica xerogel.20 The bands in the 2800-3000 cm-1 region (Figure 1b,c) are assigned to the ν(CH) modes of the methylene groups bound onto the surface of silica and also reflect the conformationally ordered or disordered states of aminopropyl segments participating in hydrogen bonding with the silanol group in the xerogels with N and N-N.14,21,22 (17) Yoshino, H.; Kamiya, K.; Nasu, H. J. Non-Cryst. Solids 1990, 126, 68. (18) Niznansky, D.; Rehspringer, J. L. J. Non-Cryst. Solids 1995, 180, 191. (19) Orcel, G.; Phalippou, J.; Hench, L. L. J. Non-Cryst. Solids 1986, 88, 114. (20) Tripp, C. P.; Veregin, R. P. N.; Hair, M. L. Langmuir 1993, 9, 3518. (21) Vracken, K. C.; Van der Voort, P.; Gillis-D’Hamers, J.; Vansant, E. F. J. Chem. Soc., Faraday Trans. 1992, 88, 3197. (22) Piers, A.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1995, 91, 359.
This structure, also suggested by Vracken et al.,21 indicates that hydrolysis with the methoxy group could have easily been prevented in the case of these gels. The amino group can undergo hydrogen bonding with methoxy, silanol, or surface hydroxy groups. This bonding increases the frequency of the N-H deformation mode. The lowintensity bands in the 1300-1600 cm-1 range are assigned to both associated (bound aminopropyl segment14) and free amino groups;22 this range could be overlapped by the OH band. It has already been reported that the IR band at 1574 cm-1 in the spectrum (Figure 1b) may be assigned to the band mode of the amine acceptor hydrogen bonded to the SiOH group and that there exists an internal hydrogen bonded six-membered ring structure within the SiOH‚‚‚NH2 network. The 1487 cm-1 band has been attributed to the symmetric deformation mode of NH3+ in the SiO-‚‚‚H‚‚‚ NH2+ group. This structure exists also on the surface as previously found14 in the diffuse reflectance infrared Fourier transform (DRIFT) spectra recorded for a material with N. The band at 1665 cm-1 (Figure 1c) is assigned to the NH3+ asymmetric deformation band. Finally, the 1630 cm-1 shoulder (Figure 1b) may be related to the asymmetric NH3+ deformational modes superimposed upon the 1639 cm-1 band arising from the H2O bend modes of water absorbed onto the silica (Figure 1a) and aminated xerogel surfaces. The visible absorption spectra of Cu(II) complexes prepared in bulk and grafted by surface chemisorption were scanned in the 400-800 nm range (Figure 2). They show a broad asymmetric band without a distinct shoulder. (23) Kurth, D. G.; Bein, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 336. (24) Tomlinson, A. A. G.; Hathaway, B. J. J. Chem. Soc. A 1968, 1905.
Copper(II) Complexes on Modified Silicates
Figure 3. X band EPR spectra of the N xerogel complexed with Cu(II) ions: (a) in bulk; (b) by grafting. Recorded at 130 K.
This band seems to be much more characteristic of the tetragonally distorted octahedral environment of Cu(II) ions than of a pentacoordinated species.24 For the most probable tetragonally distorted octahedron (D4h symmetry), the observed absorption can be described as the superimposition of the following three absorption transitions: 2Eg, 2B2g, 2A1g r 2B1g.25 But within experimental error, the shape of the band does not depend on the composition of the xerogel and the method of preparation. In the case of Cu(II) complexes in the N xerogel, the band maximum generally occurs at a higher wavelength than that of the corresponding complexes in the N-N xerogel. However, in the same type of xerogel, the position of the band is shifted toward lower wavelengths for the grafted Cu(II) complex in comparison to the bulk complex (cf. Figure 2a and Figure 2b or Figure 2c and Figure 2d). The shift to higher wavelengths is consistent with smaller crystal field. EPR spectroscopy is a powerful tool for identifying changes in the coordination environment of Cu(II) ions in the xerogels studied. The spectra of the Cu(II) complexes in vitreous samples have been interpreted according to the Kneubu¨hl method.26 The Cu(II) species give rise to typical axially symmetric spectra. However, in the case of the complexes with an N-N ligand, the spectra exhibit two components. The hyperfine structure of the intense perpendicular signal on the high-field side is not resolved (Figures3-6). The EPR spectra of the N samples with Cu(II) species complexed in bulk (Figure 3a) are quite similar to those of the complexes with N grafted on the surface (Figure 3b). In contrast, the N-N ligand in both the materials complexed in bulk and by grafting there exist indeed two coordination forms (aa1 and aa2), but the aa1 predominate distinctly over the aa2 in the bulk prepared xerogel (Figure 4a). Meanwhile, for the N-N grafted sample (Figure 4b) both the forms represent nearly the same content. Previously,5 we identified the aa2 form as a structure present on the N-N xerogel surface. The EPR spectra of the Cu(II) complexes formed on the surface by anchoring (Figure 5a) and grafting (Figure 5b) are similar for the N ligand. In the case of the N-N anchored sample, intensity of lines related to the aa2 form on the surface is higher than for the aa1 existing in the xerogel interior (25) Kivelson, D.; Neiman, R. J. Chem. Phys. 1961, 29, 149. (26) Kneubu¨hl, F. H. J. Chem. Phys. 1960, 33, 1074.
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Figure 4. X band EPR spectra of the N-N xerogel complexed with Cu(II) ions: (a) in bulk; (b) by grafting. Recorded at 130 K.
Figure 5. Comparison of EPR spectra recorded at 130 K. Cu(II) complexes: (a) grafted and (b) anchored on the N xerogel surface.
(Figure 6). Owing to the rather good resolution of the spectra, consisting of two components in the case of the xerogels modified with an N-N ligand, it is possible to determine the spectral parameters for both the coordination species. The set of g|, g⊥, and A| values for the Cu(II) complexes formed in the interior and on the surface of both xerogel series is presented in Table 1. The values of the spectral parameters for the coordination forms termed a2, aa1,
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Figure 7. Proposed structure of the coordination form a2 of Cu(II) in interior and on surface of the N xerogel. (Details described in text.)
Figure 6. Comparison of EPR spectra recorded at 130 K. Cu(II) complexes: (a) grafted and (b) anchored on the N-N xerogel surface. Table 1. EPR Spectral Parameters of Copper(II) Coordination Forms ligand N N-N
preparation method
coordination form
g| (0.003
|A|| 10-4 cm-1 (3
g⊥ (0.003
in bulka grafting anchoring in bulkb
a2 a2 a2 aa1 aa2 aa1 aa2 aa1 aa2
2.334 2.349 2.347 2.124 2.203 2.138 2.216 2.138 2.212
164 150 148 199 177 286 158 188 160
2.038 2.037 2.039 2.036 2.017 2.038 2.018 2.036 2.015
grafting anchoring a
From ref 2. b See ref 5.
and aa2 are the same within experimental error as given in refs 2, 3, and 5. (a1 is an amino-rich form, [CuN4(H2O)2](NO3)2, see ref 3). The fact that all the Cu(II) complexes exhibit a g| > g⊥ > ge ) 2.0023 parameter sequence could suggest that in the xerogels Cu(II) might also exist in a pentacoordinated27 and a tetracoordinated form. It is wellknown that the former (a square pyramidal (C4v)) and the latter (a square planar (D4h) symmetry) are thought to be improbable in the amorphous gels in bulk owing to effective packing.28 Thus, complexes of the types [CuX6-nNn](NO3)2 and [CuX6-2n(N-N)n](NO3)2, where X ) H2O and OH, grafted or anchored onto the surface of the xerogels, can adopt elongated tetragonal-octahedral coordination sites. Thus the results of our study of the xerogel surfaces suggest that the coordination sphere of the form a2 consists of one ligand N, the formula of the complex being [CuX5N](NO3)2 (Figure 7). On the other hand, the bidentate N-N ligand creates more stable complexes with Cu(II) than (27) Hathaway, B. J.; Billing, D. E. Coord. Chem. Rev. 1970, 5, 143. (28) Hathaway, B. J. In Comprehensive Coordination Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford, 1987; Vol. 5, pp 667, 730.
Figure 8. Proposed structures of the coordination spheres of Cu(II) in interior and on surface of the N-N xerogel: (a) coordination form aa1; (b) coordination form aa2 (after ref 5).
does the unidentate ligand N. Therefore, there exist an amino-rich form (aa1) with n ) 2, [CuX2(N-N)2](NO3)2, and an amino-poor form (aa2) with n ) 1, [CuX4(N-N)](NO3)2 in the N-N xerogels (Figure 8). An increase in g| and a simultaneous decrease in |A|| are clearly indicative of the decreasing tetragonality of the Cu(II) environment.7,28 In view of this fact it could be assumed that the coordination forms in the N xerogels are tetragonally less distorted than the forms in the N-N xerogels (Table 1). On the other hand, the complexes grafted and anchored onto the same support display, within experimental error, a similar tetragonal distortion of the coordination sphere. However, these complexes are tetragonally less distorted than the species prepared in the bulk of the xerogel with the same ligand. Finally, the aa2 form is tetragonally less distorted than the aa1 form in each case (cf. ref 3). Conclusions On the surface of the aminated xerogels are identified by the FTIR spectroscopy such groups as CH2, Si-OH, NH3+ in the SiO-‚‚‚H‚‚‚NH2+ structure, as well as absorbed H2O molecules; among them OH and H2O are also coordinatively active.
Copper(II) Complexes on Modified Silicates
The results obtained in this study undertaken to determine the coordination forms in the interior and on the surface of aminated silica xerogels can be summarized as follows: In Cu(II) complexed xerogels of type N there is a coordination site termed a2 with one nitrogen atom in the equatorial plane. This form is present in the interior and on xerogel surface. On the other hand, the N-N xerogel exhibits two coordination forms, aa1 and aa2, the former with two chelate groups (four nitrogen atoms) exists in the interior, while the latter with one chelate group (two nitrogen atoms) in the equatorial plane is formed on the
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surface. All these coordination forms are elongated tetragonal-octahedral sites with D4h symmetry. Complexes with the N ligand are tetragonally less distorted than the species with the N-N ligand. In the N-N xerogel, the amino-poor aa2 site displays less tetragonality than the amino-rich aa1 site and its content predominate over the aa1 form in the anchored sample. Acknowledgment. The financial support of this work by the Polish Scientific Research Council (Grant No. 3 T09A 070 13) is gratefully acknowledged. LA981256+
