Structure-Catalytic Function Relationship of SiO2-Immobilized

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Langmuir 2007, 23, 10407-10418

10407

Structure-Catalytic Function Relationship of SiO2-Immobilized Mononuclear Cu Complexes: An EPR Study Georgia Grigoropoulou,† Konstantinos C. Christoforidis,† Maria Louloudi,‡ and Yiannis Deligiannakis*,† Department of EnVironmental and Natural Resources Management, UniVersity of Ioannina, Seferi 2, 30100 Agrinio, Greece, and Department of Chemistry, UniVersity of Ioannina, Panepisthmiouli Douroutis, Ioannina, Greece ReceiVed March 20, 2007. In Final Form: July 15, 2007 Mononuclear CuL and Cu(2L) complexes, where L is propyl-thiazol-2-ylmethylene-amine, covalently immobilized onto SiO2, can catalyze efficiently the oxidation of 3,5-di-t-butylcatechol (DTBC) to 3,5-di-t-butylquinone (DTBQ) by utilizing ambient O2 as oxidant. By increasing the loading of L on SiO2, the DTBQ formation can be improved up to 400% vs the homogeneous catalyst. Equally important is however that grafting per se at low loading is not adequate for an improved catalytic activity. Appropriate loadings have to be achieved, which then may result in significant catalytic performance. Based on EPR spectroscopy a theoretical method is developed, eq A12, for spinspin distance estimation in heterogeneously dispersed surface complexes. Practical rules including error estimates are provided. By applying this method to the [SiO2-CuL] catalysts it is shown that mononuclear copper complexes fixed on SiO2 with Cu‚‚‚Cu distances as short as 4.9 ( 0.3 Å are responsible for the improved catalytic activity. The present results demonstrate that mononuclear Cu complexes can have considerable catecholase activity, if the proper geometrical proximity can be fixed. Grafting on SiO2 may be an efficient method for engineering catalysts with improved performance.

1. Introduction The heterogenisation of homogeneous catalysts is an area of considerable industrial importance and academic interest, due to the advantages of simplified recovery and reusability of the catalyst. The majority of the novel heterogenised catalysts are based on SiO2 supports,1 primarily because silica displays some advantageous properties, such as excellent chemical and thermal stability and good accessibility due to its high surface area and porosity. Homogeneous metal complexes can be immobilized onto silica2 by chemically modifying the surface of the solid support with organic functionalities.3,4 Bio-inspired metal complexes have been successfully immobilized on solid supports demonstrating their potential use as heterogeneous catalysts.5 Copper complexes represent the active site of many copper proteins involved in transport and activation of molecular oxygen. For example, the type-3 copper protein hemocyanin is an oxygentransport protein, whereas tyrosinase and catechol oxidase use dioxygen to catalyze the oxidation of monophenols to quinones and ortho-diphenols to quinines, respectivelly.6 The active site of these enzymes contains a coupled dinuclear Cu-Cu center with three histidine donors at each Cu center. The determination of the crystal structure of oxidized and reduced form of catechol oxidase7 has shown that, in the oxidized met form of the enzyme, the two Cu (II) ions are bridged by an OH anion which completes * Corresponding author. E-mail: [email protected]. † Department of Environmental and Natural Resources Management, University of Ioannina. ‡ Department of Chemistry, University of Ioannina. (1) Wight, A. P.; Davis, M. E. Chem. ReV. 2002, 102, 3589. (2) Brunel, D.; Bellocq, N.; Sutra, P.; Cauvel, A.; Laspe´ras, M.; Moreau, P.; Di Renzo, F.; Galarneau, A.; Fajula, F. Coord. Chem. ReV. 1998, 1085, 178. (3) Price, P. M.; Clark, J. H.; Macquarrie. D. J. J. Chem. Soc., Dalton Trans. 2000, 101. (4) Corma, A.; Garcia, H. Chem. ReV. 2002, 102, 3879. (5) De Vos, D. E.; Jacobs, P. E. Catal. Today 2000, 57, 105. (6) Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E. Chem. ReV. 1996, 96, 2563. (7) Klabunde, T.; Eicken, C.; Sacchettini, J. C.; Krebs, B. Nat. Struct. Biol. 1998, 5, 1084.

the four-coordinated trigonal pyramidal coordination sphere and the Cu‚‚‚Cu distance is 2.9 Å. In the reduced state of the enzyme, the Cu‚‚‚Cu separation increases to 4.4 Å.7 Several studies on mono- and dinuclear copper complexes as model compounds for catechol oxidase have been reported,5 in order to investigate the catalytic mechanism and to develop new efficient catalysts for oxidation reactions.8-15 A general trend for the catecholase activity of copper complexes is that dinuclear complexes are generally more reactive than mononuclear compounds.16-19 An important conclusion is that, in dinuclear complexes, the Cu‚‚‚Cu distance is a key-factor for the catalytic function. Nishida et al. have reported that several alkoxo or phenoxo bridged dinuclear copper(II) complexes exhibit catalytic activity if the Cu‚‚‚Cu distance is less than 5 Å.20 Meyer et al. have reported the synthesis of dinuclear copper complexes of a series of different pyrazolate-based dinucleating ligands, with Cu-Cu distances as large as 4.53 Å, depending on the side arms (8) Monzani, E.; Battaini, G.; Perotti, A.; Casella, L.; Gullotti, M.; Santagostini, L.; Nardin, G.; Randaccio, L.; Geremia, S.; Zanello, P.; Opromolla, G. Inorg. Chem. 1999, 38, 5359. (9) Selmeczi, K.; Reglier, M.; Giorgi, M.; Speier, G. Coord. Chem. Rev. 2003, 245, 191. (10) Zippel, F.; Ahlers, F.; Werner, R.; Haase, W.; Nolting, H.-F.; Krebs, B. Inorg. Chem. 1996, 35, 3409. (11) Torelli, S.; Belle, C.; Hamman, S.; Pierre, J. L. Inorg. Chem. 2002, 41, 3983. (12) Koval, I. A.; Belle, C.; Selmeczi, K.; Philouze, C.; Saint-Aman, E.; Schuitema, A. M.; Gamez, P.; Pierre, J. L.; Reedijk, J. J. Biol. Inorg. Chem. 2005, 10, 739. (13) Granata, A.; Monzani, E.; Casella, L. J. Biol. Inorg. Chem. 2004, 9, 903. (14) Gupta, M.; Mathur, P.; Butcher, R. J. Inorg. Chem. 2001, 40, 878. (15) Kao, C. H.; Wie, H. H.; Liu, Y. H.; Lee, G.-H.; Wang, Y.; Lee, C. J. J. Inorg. Biochem. 2001, 84, 171. (16) Latif Abuhijleh, A.; Woods, C.; Bogas, E.; and Le Guennioutt, G. Inorg. Chim. Acta 1992, 195, 67. (17) Malachowski, M. P.; Davidson, M. G. Inorg. Chim. Acta 1989, 162, 199. (18) Kida, H.; Okawa, Y.; Nishida, Y. In Copper Coordination Chemistry: Biochemical Inorganic PerspectiVes; Karlin, K. D., Zubieta, J., Eds.; Adenine Guilderland: New York, 1983; p 425. (19) Koval, I. A.; Gamez, P.; Belle, C.; Selmeczi, K.; Reedijk, J. Chem. Soc. ReV. 2006, 35, 814. (20) Oishi, N.; Nishida, Y.; Ida, K.; Kida, S. Bull. Chem. Soc. Jpn. 1980, 53, 2847.

10.1021/la700815d CCC: $37.00 © 2007 American Chemical Society Published on Web 09/01/2007

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of pyrazolate.21 Torelli et al. have described the pH-controlled changes of the metal coordination in a dicopper(II) complex and have shown that the catecholase activity of this complex was dependent on the copper-copper distance which was controlled by pH.22 For short enough Cu...Cu distances a bridging group might also exist and this in turn would play an important role.23-27 Mononuclear copper(II) complexes, have also been examined for catecholase activity.16-18 It has been suggested that mononuclear copper complexes can be efficient catalysts if the copper atoms can be located at a distance of 5 Å or less18 for the bonding of the catechol hydroxyl groups and the subsequent two-electron transfer to dioxygen.18 The requisite of a proper Cu...Cu distance was invoked in other studies of mononuclear Cu complexes.16-17 However, the aforementioned cases16-18 concerned homogeneous complexes in solution where the required Cu‚‚‚Cu distance could not be fixed or measured. Speier et al. reported catecholase activity of an exceptional complex in which the Cu‚‚‚Cu distance is as large as 7.8 Å.9 However, this complex had a large macroligand connecting the two Cu atoms therefore could be taken as dinuclear. In this context, a modern interesting approach is to engineer highly active catalysts, where the Cu centers are kept in close proximity by immobilization of the metal catalyst on a solid support. Indeed, heterogenisation of dinuclear copper complexes coordinated to macroacyclic ligands had led to an increase of the activity of the catalyst in catechol oxidation.29 Furthermore, encapsulation of dinuclear phenanthroline Cu complex in mesoporous silica has also been reported to result in a highly effective catalyst30 where the dinuclear Cu centers were immobilized in the nanochannels of the inorganic matrix leading to stable bimetallic centers. The same strategy was also applied to the preparation of immobilized Cu complexes coordinated to histidine molecules.31 These studies have demonstrated the advantages of immobilizing Cu complexes on inorganic matrices and the potential of these hybrid materials for the development of efficient oxidation Cu catalysts. In the cases where single crystals can be obtained for the homogeneous complexes under study, then X-ray crystallography can provide the Cu coordination as well as the Cu‚‚‚Cu distance. However, in the case of heterogeneous catalysts, single crystals cannot be obtained, and this prohibits the use of X-ray crystallography. In this case, structural information can be obtained by spectroscopic methods. In this context electron paramagnetic resonance (EPR) spectroscopy has been a valuable tool, providing information on the Cu coordination as well as on the Cu‚‚‚Cu distance30,32 for Cu complexes grafted on SiO2. More particularly in a recent work,32 we have presented a mononuclear Cu2+ complex which can achieve considerable catalytic oxidation of 3,5-di-t-butylcatechol (DTBC) in cases (21) Achermann, J.; Meyer, F.; Kaifer, E.; Pritzkow, H. Chem. Eur. J. 2002, 8, 247. (22) Torelli, S.; Belle, C.; Gautier Luneau, I.; Pierre, J. L.; Saint Aman, E.; Latour, J. M.; LePape, L.; Luneau, D. Inorg. Chem. 2000, 39, 3526. (23) Mukherjee, J.; Mukherjee, R. Inorg. Chim. Acta 2002, 337, 429. (24) Gonzalez-Alvarez, M.; Alzuet, G.; Borras, J.; Garc, Z. J. Inorg. Biochem. 2003, 96, 443. (25) Reim, J.; Krebs, B. J. Chem. Soc., Dalton Trans. 1997, 3793. (26) Than, R.; Feldmann, A. A.; Krebs, B. Coord. Chem. Rev. 1999, 182, 211. (27) Belle, C.; Beguin, C.; Gautier-Luneau, I.; Hamman, S.; Philouze, C.; Pierre, J. L.; Thomas, F.; Torelli, S. Inorg. Chem. 2002, 41, 479. (28) He, H.; Linder, D. P.; Rodgers, K. R.; Chakraborty, I.; Arif, A. M. Inorg. Chem. 2004, 43, 2392. (29) Louloudi, M.; Mitopoulou, K.; Evaggelou, E.; Deligiannakis, Y.; Hadjiliadis, N. J. Mol. Catal. A Chem. 2003, 198, 231. (30) Lee, C. H.; Wong, S. T.; Lin, T. S.; Mou, C. Y. J. Phys. Chem. B 2005, 109, 775. (31) Louloudi, M.; Deligiannakis, Y.; Hadjiliadis, N. Inorg. Chem. 1998, 37, 6847. (32) Zois, D.; Vartzouma, C.; Deligiannakis, Y.; Hadjiliadis, N.; Casella, L.; Monzani, E.; Louloudi, M. J. Mol. Catal. A: Chem. 2007, 261, 306.

Grigoropoulou et al.

where geometrical proximity of the Cu2+ complexes, e.g., measured by EPR, can be fixed by grafting on SiO2.32 Accordingly, it was demonstrated that the mononuclear Cu centers can have a catalytic activity comparable to that of a dinuclear Cu‚‚‚Cu catalyst tested under similar conditions.32 Thus, given that mononuclear Cu complexes can be more easily synthesized that dinuclear complexes, this calls for a more systematic survey on the correlation between the catalytic efficiency and the Cu‚‚‚Cu distance of spatially fixed immobilized mononuclear Cu complexes. In the present work, we present a systematic study of mononuclear Cu catalysts immobilized onto SiO2, where the spatial proximity of the Cu centers was controlled by the loading of the SiO2 particle. A novel ligand, herein called L, bearing a thiazole and an imino nitrogen was synthesized and grafted on SiO2. Thiazole was chosen as a well-known robust ligand for copper. A series of organically modified SiO2 samples with different amounts of L were prepared. The aims of the present work are as follows: (a) to study the catalytic activity of mononuclear SiO2-CuL catalysts for the DTBC oxidation by ambient O2, (b) to establish a correlation between the loading and the catalytic activity, and (c) to develop a protocol for the estimation of the Cu‚‚‚Cu distances in heterogeneously dispersed surface complexes, by using EPR spectroscopy. Kinetic aspects or reaction intermediates identification were preliminarily screened; however, their detailed analysis was out of the scope of the present paper. 2. Experimental Section 2.1. Chemical Reagents and Equipment. All reagents were purchased from Aldrich Chemical Co. and were used as received. Silica gel K100 was purchased from Merck and was activated at 200 °C for 12 h before use. Solvents were purchased from Merck, and dry toluene was obtained by standing over preactivated 4 Å molecular sieves. Dry methanol was purchased from Riedel-deHae¨n. UV-vis spectra were recorded using a Perkin-Elmer Lamda 35 spectrometer operating in the range 200-900 nm with a diffuse reflectance setup. Fourier transformed infrared (FT-IR) spectra were recorded using a Spectrum GX Perkin-Elmer FT-IR System with a DRIFT setup. Analytical Measurements. Elemental (C, H, N, and S) analyses were obtained using a Perkin-Elmer Series II 2400 elemental analyzer. The copper amount was determined by flame atomic absorption spectroscopy on a Perkin- Elmer AAS-700 spectrometer. Gas chromatography (GC) analyses were performed on Schimadzu 17A gas chromatograph equipped with a FID detector and an Equity-5 capillary column (30 m, 0.25 mm i.d.) with the following temperature program: initial temperature, 100 °C; heating rate, 10 °C/min; final temperature, 280 °C (final time 3 min); injector temperature, 250 °C; detector temperature, 300 °C. EPR Spectroscopy. Electron paramagnetic resonance (EPR) spectra were recorded with a Brucker ER200D spectrometer at liquid helium or liquid-nitrogen temperatures, equipped with an Agilent 5310A frequency counter. The spectra recorded at 77 or 4 K were comparable. The spectrometer was running under a homemade software based on LabView. EPR spectra for the forbidden, g ∼ 4, transitions were recorded at high microwave power33 typically 64.5 mW. The allowed transitions at g ) 2.3-2.0 were recorded at lower nonsaturating microwave power, typically 12.5 mW. Detailed saturation measurements showed that under these conditions the EPR signals were not saturated, i.e., a behavior typical for Cu(II) complexes.33 The modulation amplitude of 5 G was chosen for the resolution of 14N(I ) 1) supehyperfine couplings, which are resolved in the secondderivative of the spectra. Adequate signal-to-noise was obtained after 20-25 scans for the g ) 4 signals or 3-5 scans for the allowed (33) Bencini, A.; Bertini, I.; Gatteschi, D.; Scozzafava, A. Inorg. Chem. 1978, 17, 3194.

Structure-Catalytic Function Relationship

Langmuir, Vol. 23, No. 20, 2007 10409 Scheme 1. Schematic Representation of the Synthetic Procedure of Supported Materialsa

a Herein the final grafted thiazole-bearing ligand, propyl-thiazol2-ylmethylene-amine (C7H10N2S), is denoted L.

Figure 1. DRIFT spectra of supported materials: (a) aminopropylSiO2, (b) L-SiO2, and (c) CuL-SiO2 catalyst 4.

Figure 2. Effect of SiO2-[L] loading on the DTBC conversion (O, left axis), and the DTBQ formation (9, right axis) indicating the selectivity of the reaction. The solid black bar at the left refers to the DTBC conversion by homogeneous CuL, whereas the open bar at the right refers to the DTBQ conversion by homogeneous CuL. The solid lines through the points are guide to the eye. The same initial [Cu:DTBC] ratio of [1:125] was used in all catalytic experiments. transitions. In the calculations of the relative integrated intensities Iallowed/Iforbidded, these factors, e.g., number of scans and microwave power, were taken into account. The EPR measurements were done in triplicate. Interparticle Cu interactions, e.g., between Cu atoms on neighboring SiO2 particles, were minimized by dispersing the SiO2-CuL powder material in [3:1] acetonitrile-glycerol. Glycerol was used for its known glass-forming properties. Thus, eventual interparticle Cu‚‚‚Cu interactions are less significant than the intraparticle dipolar Cu‚‚‚Cu coupling which, as we show here in detail, are correlated with the Cu loading. 2.2. Grafting of Ligand Structure on the Surface of Silica Gel. Activated silica gel was modified with 3-aminopropyl (triethoxy) silane (APM) following procedures described previously.34 The produced AMP-SiO2 (0.5 g) was then further treated with 2-thiazole carboxaldehyde (0.025mmol-1mmol) in 20 mL of ethanol. The mixture was heated at 60 °C for 12 h, cooled, filtered, washed with ethanol, and dried at 80 °C. Ligand loadings were calculated based on S content of materials estimated from the elemental analysis. (34) Macquarrie, D. J.; Clark, J. H.; Lambert, A.; Mdoe, J. E. G.; Priest, A. React. Funct. Polym. 1997, 35, 153.

2.3. Formation of Copper Catalysts. The catalysts were prepared by stirring a mixture of ligand-grafted silica (0.2 g) and Cu(acetatek)2‚ H2O (Cu-to-ligand molar ratio was kept at 2:1) in ethanol (10 mL) at room temperature for 5 h. After stirring, the catalyst was filtered and washed thoroughly with ethanol and dried at 80 °C for 12 h. The homogeneous complexes were formed by mixing [L:Cu] ) [2:1] in solution. The synthesis of homogeneous L was performed based on the method described in ref 28. In all cases, the amount of Cu2+ was determined by atomic absorption spectroscopy of the remaining Cu2+ in the filtrate or the collected washings. 2.4. Catalyst Testing. The activity of catalysts was tested on the oxidation of 3,5-di-t-butylcatechol. In a typical reaction, the supported catalyst (0.0016 mmol of Cu) was added in a mixture of 2 mL solution (10-1 M acetonitrile solution) of 3,5-di-t-butylcatechol and triethylamine (10 µL). The reaction mixture was stirred at room temperature under open air, and aliquots were taken during the reaction and analyzed by GC. The yields of products were estimated from the peak areas based on the internal standard technique. Blank experiments showed that, in the absence of light, without catalyst the transformation of DTBC to DTBQ takes place at low yield and conversion (5% DTBQ, 8% DTBC). As a control, the catalytic performance of the homogeneous catalyst was also tested. The leaching of the active supported component was carefully evaluated by the “filtration method”.31,32 Accordingly, in typical catalytic experiments, after 1 h, the solid catalysts were filtered. Into the filtrate, the progress of the oxidation reaction was monitoring by GC. In the same sample, the amount of Cu released was determined by atomic absorption spectroscopy. The measurements showed that in the filtrate the catalytic activity was continued achieving ∼30% of conversion that was obtained by the homogeneous CuL catalyst. This shows that some leaching occurs, probably due to the triethylamine added;31,32 however, the main catalytic activity observed is due to the SiO2 immobilized catalysts.

3. Results 3.1. Synthesis and Chemical Analysis of Supported Catalysts. The preparation of supported materials is outlined in Scheme 1. Activated silica was first reacted with 3-aminopropyltriethoxysilane in order to functionalize the surface of support with amino groups required for the further reaction with the desired molecule, 2-thiazole carboxaldehyde. Thiazole was directly anchored to the amine functionalized silica through its aldehyde functionality by an imine bond formation. Herein the thiazole-bearing ligand, propyl-thiazol-2-ylmethylene-amine (C7H10N2S), will be denoted as L. Materials with different amounts of ligand L were prepared, and loadings were determined by considering the elemental analysis data (Table 1). Six different materials with a range of loadings from 0.02 to 1.44 mmol ligand/g material were successfully obtained.

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Table 1. Elemental Composition and Calculated Loadings of L and Cu of Materials catalyst

C (%)

N (%)

S (%)

H (%)

ligand, L (mmol/g)

Cu (mmol/g)

[L:Cu]

a b c d e f

4.0 6.2 7.5 8.0 14.6 17.4

1.5 1.8 2.6 2.5 4.4 4.7

0.05 0.4 1.3 2.2 4.0 4.6

0.8 1.0 1.1 0.9 1.8 2.2

0.02 ( 0.01 0.13 ( 0.01 0.41 ( 0.01 0.59 ( 0.01 1.25 ( 0.01 1.44 ( 0.01

0.02 ( 0.01 0.15 ( 0.01 0.40 ( 0.01 0.54 ( 0.01 0.74 ( 0.01 0.76 ( 0.01

1.00 ( 0.05 0.87 ( 0.05 1.03 ( 0.05 1.09 ( 0.05 1.69 ( 0.05 1.68 ( 0.05

Next, the heterogenised materials were reacted with copper acetate at low loading, i.e., in a [Cu:L] ratio of 2, to form the corresponding anchored SiO2-CuL complexes. After incubation for 24 h, the solid material was washed thoroughly in order to remove physically sorbed copper acetate onto the SiO2 surface. The prepared catalysts present metal loading in the range 0.020.76 mmol Cu/g of SiO2 material, Table 1. The amount of heterogenised Cu increases with the L loading. From Table 1, we observe that the ratio [L:Cu] is ranging from [L:Cu] ∼ 1 to [L:Cu] ∼ 1.7. The structural implication of these ratios will be discussed together with EPR data. Herein the six catalysts are named (a) to (f) as shown in Table 1. 3.2. Spectral Characterizations. 3.2.1. FT-IR Spectra. Figure 1 shows representative DRIFT spectra of the organically modified silica and a supported Cu catalyst. The DRIFTS of 3-aminopropylsilica displays the characteristic -CH2- stretching bands at 2944 and 2870 cm-1 and aliphatic deformation bands at 1474 and 1443 cm-1. The DRIFTS of chemically modified silica (imine) shows a sharp peak at 1640 cm-1 due to the CdN bond. The DRIFT spectrum of supported Cu catalyst displays two broad bands at 1595 and 1415 cm-1 which are assigned to the characteristic symmetric and asymmetric stretching vibrations of COO- group. The CdN stretching band shifts to lower frequency compared to the parent material and is overlapped by the COO- stretching band. The lowering in frequency of the CdN peak is indicative of the formation of a metal-ligand bond. 3.2.2. UV-Visible Spectra. The electronic spectra of the supported catalysts, Figure S1 Supporting Information, were observed three main transitions, see marks in Figure S1. A strong band observed around 270 nm is due to the intraligand π-π* transition. A less intense band observed around 360 nm is assigned to the metal-to-ligand charge-transfer transition. Finally, a weak band around 670 nm is characteristic of the d-d transitions of the Cu(II) metal ion. 3.3. Catalysis Experiments. The catalytic performance of six catalysts was accessed for the oxidation of 3,5-di-t-butylcatechol (DTBC) to 3,5-di-t-butylquinone (DTBQ) under aerobic conditions. The catalytic experiments were performed in a solution of DTBC in CH3CN, oxidized with air in the presence of the Cu supported catalysts. The molar ratio of catalyst to substrate was equal to 1:125 and was kept constant in all experiments. The obtained results for DTBQ formation and DTBC consumption catalyzed by the supported copper catalysts are listed in Table 2. Figure 2 helps to visualize the trends. For comparison, the left-side axis refers to the DTBC conversion, data described by open circles, and the right-side axis refers to the DTBQ formation, data described by solid squares. In addition, the data for the catalytic activity of the homogeneous CuL are also displayed in Figure 2. The solid black bar at the left refers to the DTBC conversion by homogeneous CuL, whereas the hollow bar at the right refers to the DTBQ conversion by homogeneous CuL. DTBC ConVersion. In Figure 2, we observe that the catalytic efficiency of the grafted SiO2-CuL complexes is enhanced as a function of the loading. The DTBC conversion starts from ∼70% at the lowest loading (0.02 mmol Ligand/g material, sample a) and attains >98% at the highest loading (1.44 mmol Ligand /g material, sample f). This is an improvement by ∼50%.

Table 2. Catalytic Results catalyst

DTBQ formation (%) (TON)a

a b c d e f

Heterogeneous 21 (26) 37 (42) 47 (59) 73 (91) 75 (93) 80 (100)

CuL blank

33 5

DTBC conversion (%) (TON)b 69 (86) 98 (122) 98 (122) 93 (116) 100 (125) 96 (120)

Homogeneous 92 8

a TON:moles of DTBQ formed per mole of catalyst b TON:moles of DTBC converted per mole of catalyst. The reactions were completed within 5 h.

When we compare the DTBC conversion between the SiO2CuL catalyst, Figure 2 open circels, vs the homogeneous CuL, Figure 2 black bar, we see that a rather small improvement is achieved by the SiO2-CuL catalyst at increased loading. Noticeably, the DTBC conversion of 69% for the lower loading SiO2-CuL catalyst (sample a) is lower than the DTBC conversion by the homogeneous CuL (92%). This observation bears relevance to the catalytic mechanism that we discuss in the following. DTBQ Formation. In contrast to the DTBC conversion, a significant improvement of the DTBQ formation is observed at increased loading of the SiO2-CuL catalysts. In Figure 2, we see that the DTBQ formation starts from ∼20% at the lowest loading and attains an efficiency of ∼80% for the highest loading, which is a ∼400% improvement. An important observation is that the improvement is nonlinear. Thus, the effect of the enhanced DTBQ conversion upon increasing loading is not simply additive. This observation is one of the main findings of the present work and will be analyzed in more detail in the following. When comparing the DTBQ conversion of the homogeneous CuL vs the heterogenized SiO2-CuL we observe that the low-loading SiO2-CuL catalysts’ efficiency is inferior to that of the CuL, e.g., 33% (CuL) vs 20% by SiO2-CuL (sample a). Superiority of the hetorogenized SiO2-CuL is established after certain loading, i.e., by samples b-f. Thus, both the DTBC conversion and the DTBQ formation data show that grafting at low-loading may eventually result in inferior catalytic activity compared to the homogeneous CuL in solution. Grafting per se at low loading is not adequate for an improved catalytic activity. Appropriate increased loadings have to be achieved, which then may result in significant catalytic performance. Overall the present data demonstrate that the enhancement of the catalytic activity which is achieved by controlling the loading of the SiO2-CuL catalysts is not a simple additive effect. At higher loadings a fundamentally beneficial mechanism takes place which results in a fourfold increase of the DTBQ formation. In this context, in an effort to better understand the determinative parameters for the observed catalytic efficiency enhancement,

Structure-Catalytic Function Relationship

Figure 3. EPR spectra for the SiO2-CuL catalytsts dispersed in acetonitrile:glycerol 3:1. (A) Allowed ∆mS ) 1 transitions. Experimental conditions, T ) 77 K, microwave power 12.5 mW, mod. amp. 5 Gpp. (B). Semiforbidden ∆mS ) 2 transitions at g ) 4. Experimental conditions, T ) 77 K, microwave power 64.5 mW, mod. amp. 5 Gpp. The indices (a) to (f) correspond to L loadings per g of SiO2 material: (a) 0.02 mmol/gr, (b) 0.13 mmol/gr, (c) 0.41 mmol/gr, (d) 0.59 mmol/gr, (e) 1.25 mmol/gr , (f) 1.44 mmol/gr.

Langmuir, Vol. 23, No. 20, 2007 10411

Figure 5. Second derivative of the EPR spectra for sample (a) dotted line and sample (f) solid line. The spectra have been obtained by numerical derivative of the spectra in Figure 4. The “superhyperfine” splittings, due to 14N nuclear couplings from ligands, at the gx,y region are marked by vertical lines as a visual aid. The inset shows a 4-fold expansion of spectra (a) and (f). Table 3. Spin Hamiltonian g and A Tensor Components for the Cu2+ Centers in the SiO2-CuL Catalysts gx gy gz Axa Ay Az Aiso (14N)a a

Figure 4. Analysis of the EPR spectra for the two SiO2-CuL catalysts (a) and (f), taken from Figure 3. The low loading catalyst (a) is described by one rhombic Cu2+ tensor, A, with g and A tensors listed in Table 3. The high loading catalyst (f) is described by two Cu2+ tensors, A and B, with g and A tensors listed in Table 3.

we have used EPR spectroscopy as a tool for an in situ study of the surface Cu complexes. 3.4. EPR Spectroscopy. The X-band EPR spectra for the SiO2-CuL materials are presented in Figure 3. In the left panel, Figure 3A, we present the g ) 2 region where the allowed ∆mS ) 1 transitions of Cu2+ are contributing.33, 35 In the right panel, Figure 3B, we present the g ) 4 region where the semiforbidden ∆mS ) 2 transitions of Cu2+ are contributing.32,33,36,37 Allowed ∆ms ) 1 Transitions: EPR Spectra Show that Cu(II) Complexes Are Mononuclear. The spectra displayed in Figure 3A are typical for mononuclear Cu2+(S ) 1/2, I ) 3/2) complexes.35 Small spectral changes are observed from the lowloading (a) toward the high-loading sample (f). The two extreme cases, spectra (a) and (f), are analyzed in detail in Figure 4. The (35) Hathaway, B. J. Billing, Coord. Chem. ReV. 1970, 5, 143. (36) Eaton, S.; More, K. M.; Sawant, B. M.; Eaton, G. R. J. Am. Chem. Soc. 1983, 105, 6560. (37) Eaton, S.; Eaton, G. R; Chang, C. K. J. Am. Chem. Soc. 1983, 107, 3177.

tensor A

tensor B

2.068 ( 0.002 2.031 ( 0.001 2.245 ( 0.002 13.0 ( 0.5 13.0 ( 0.5 166.0 ( 0.5 14.3 ( 0.5 2 nuclei

2.065 ( 0.002 2.065 ( 0.002 2.277 ( 0.002 20.0 ( 0.5 20.0 ( 0.5 170.0 ( 0.5 14.5 ( 0.5 4 nuclei

Gauss.

spectra can be simulated assuming a spin system with S ) 1/2, I ) 3/2, i.e., for Cu2+, by using the g and A tensors listed in Table 3. The low-loading sample, Figure 5 (a), can be simulated by assuming only one type of Cu2+ g and A tensor, herein called tensor A with (gx, gy, gz) ) (2.068, 2.031, 2.245) and (Ax, Ay, Az) ) (13.0, 13.0, 166.0 G); see Figure 4 and Table 3. In contrast, the EPR spectrum for the high-loading sample (f) can be simulated by using two Cu2+ tensors. One is tensor A, and the second one, herein termed tensor B, is axial with (gx, gy, gz) ) (2.065, 2.065, 2.277) and (Ax, Ay, Az) ) (20.0, 20.0, 170.0 G); see Table 3 and Figure 4. The two tensors can be more easily resolved in the second derivative spectra shown in Figure 5. In a fourfold expansion, see inset in Figure 5, we see that the spectrum of the high-loading sample (f) is composed of a fraction spectrum (a) plus a second spectrum which can be simulated by tensor B. Coordination Sphere of Cu Ions. Interestingly, in the second derivative spectra, presented in Figure 5, we can resolve “superhyperfine” splittings, i.e., due to nuclear couplings from ligands, at the gx,y region. In spectrum (f), solid line in Figure 5, these are well resolved and marked by vertical lines as a visual aid. In Figure 5 (f), we resolve splittings of ∼14 G. In the case of spectrum (a), the gx,y region in the second derivative mode is different, i.e., due to the rhombic gx,y values, see Table 3; however, superhyperfine lines can also be resolved. In first derivative mode, the superhyperfine splittings are masked under the breadth of the gx,y region, see Figure 5; thus, the second derivative has to be used for their proper resolution. Analogous couplings have been observed and analyzed similarly

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Figure 6. Suggested structural model for the formed CuL (left) and Cu(2L) (right) surface complexes depending on the loading of the SiO2-CuL catalysts. According to Table 4, at high-loading type CuL complexes coexist with Cu(2L); however, they have been omitted for clarity. Table 4. Structural Characteristics of the SiO2-CuL Catalysts percentage of CuL and Cu(2L) complexesa catalyst

CuL tensor A

Cu(2L) tensor B

Iforb/Iallwed (× 103)b

Cu‚‚‚Cu distance by eq A1d (Å)c

Cu‚‚‚Cu distance by eq A12 (Å)e

a b c d e f

100 80 65 50 45 45

0 20 35 50 55 55

0.14 0.21 0.29 0.36 0.52 0.68

7.2 ( 1.0 6.8 ( 0.7 6.4 ( 0.5 6.2 ( 0.3 5.8 ( 0.3 5.5 ( 0.3

8.4 ( 1.0 8.3 ( 0.7 7.1 ( 0.5 5.6 ( 0.3 5.2 ( 0.3 4.9 ( 0.3

a Error: (10%. b Error: (0.03. c The errors have been estimated from Figure 9 for an error of 0.03 in Iforb/Iallowed. d Calculated from eq A12, see also graph in Figure 9. e Correction estimated based on the theory described in Appendix I.

by the second derivative for EPR spectra of Cu2+ centers adsorbed in SBA-15 materials.38 Detailed computer simulations, see Figure S2 in the Supporting Information, show that in spectrum (f) four 14N(I ) 1) nuclei are coordinated to the Cu2+ electron spin. The derived A(14N) values are 14 G for four 14N with A(14N) couplings of 14.5 G, listed in Table 3. In spectrum (a), only two 14N(I ) 1) nuclei are coordinated to the Cu2+ electron spin with A(14N) of 14.3 G for both 14N, listed in Table 3. The isotropic 14N(I ) 1)-Cu2+ couplings of 14-15 G i.e. 45-50 MHz, observed for the present Cu2+ complexes are typical for nitrogens directly coordinated to Cu2+ resolved in first-16,17 or second-derivative EPR spectra38 or measured by ENDOR spectroscopy.42 Thus, the EPR spectra provided detailed structural information, indicating that in the SiO2-materials two types of mononuclear Cu complexes are formed. One type, prevailing at low loading, is Cu2+ coordinated by two nitrogens. A second type, formed at high loading, is Cu2+ coordinated by four nitrogens. Additional information about the ligand donor atoms can be obtained by utilizing the g and A values, listed in Table 3. More specifically, based on the Peisach and Blumberg correlation39 of g| vs A| values and pertinent literature data16,30,38,41,42 and taking into account the information on the 14N coupling, we conclude that tensor A is consistent with a 2N2O donor atom coordination in a distorted square planar geometry,35 while tensor B corresponds to a 4N coordination in a distorted square planar geometry.35 Finally, we notice that sulfur coordination, i.e., from the thiazole ring, is not evidenced from the EPR data. In structural terms, the EPR data show the following: In the low-loading sample, spectrum (a) in Figure 4, all of the Cu2+ (38) Murali, A.; Chang, Z.; Ranjit, K. T.; Krishna, R. M.; Kurshev, V.; Kevan, L. J. Phys. Chem. B 2005, 109, 775. (39) Peisach, J.; Blumberg, W. E. Arch. Biochem. Biophys. 1974, 165, 691. (40) Eaton, G. S.; Eaton, S.S. In Biological Magnetic Resonance; Berliner, L. J., Reuben, J., Eds.; Plenum Press: New York, 1987; Vol. 8, p 339. Particularly Chapters 4.2-4.3.1 (41) Barbucci, R.; Campbell, M. J. M Inorg. Chim. Acta 1976, 16, 113. (42) Scholl, H. J.; Hu¨ttermann, J. J. Phys. Chem. 1992, 96, 9684. (43) Coffman, R. E.; Pezeshk, A. J. Magn. Res. 1986, 70, 21.

complexes are coordinated by 2 nitrogens and 2 oxygen atoms. In the high-loading sample spectrum (f) in Figure 5, 45% of the Cu2+ complexes have a 2N2O coordination, which are of the same type as in the low-loading sample. The other 55% of the Cu2+ complexes have 4N coordination. Based on the structure of the ligand L, see Scheme 1, we suggest that at the low loading the two nitrogens originate from one L molecule which coordinates one copper atom in a bidendate mode donating one N from the thiazole ring plus the imino N, see Figure 6. The oxygen atoms can be from acetate and/or solvent. In the 4N coordination, two L atoms per Cu are coordinated donating two N atoms each, see Figure 6. This structural picture is consistent with the measured [L:Cu] ratio, see Table 1. At low loading the [L:Cu] ratio is near 1:1 in accordance with Figure 6. At high-loading, the [L:Cu] ratio 1.68 is consistent with some of the centers having 1 L per 1 Cu, herein termed CuL, and the remaining fraction 2 L per Cu, herein termed Cu(2L). Thus, the EPR spectra show that at low-loading only CuL-type complexes are formed, which are characterized by tensor A. At high-loading, additional Cu(2L)-type complexes are formed, characterized by tensor A. Quantitation of the CuL and Cu(2L) Complexes. From Figure 5, we see that spectrum (f) has comparable contributions from the two subspectra described by tensors A and B. A detailed quantitation by using tensors A and B shows that spectrum (f) contains 55% of tensor B and 45% of tensor A. Thus, the high loading sample has 45% of CuL complexes and 55% Cu(2L) complexes. In an analogous manner, the experimental EPR spectra of all samples, can be deconvoluted based on the tensors A and B, providing quantitative percentages of the CuL and Cu(2L) complexes, see Table 4. The percentages of CuL and Cu2L complexes, from Table 4, are plotted in Figure 7. For completeness, we report that the homogeneous complexes formed by mixing Cu:L at a ratio 1:2 in acetonitrile had an EPR spectrum which is a mixture of tensors

Structure-Catalytic Function Relationship

Figure 7. Species distribution for the CuL and Cu(2L) complexes formed on the SiO2 particles at various loadings of ligand L. Data taken from Table 4a.

A (∼45%) and B (∼45%) plus a small fraction, N1, might be more appropriate, especially at the very low loading sample (a). Thus, if we take as N1 the C(2L) fractions and N2 the CuL fractions, by A12 we obtain the distance estimates quoted in the last column in Table 4. The distances for these samples appear to be consistently >6 Å which is at the limit of the applicability of the EPR method.36 Therefore, the distance estimates for samples (44) Lochmu¨ler, C. H.; Coborn, A. S.; Hunnicutt, M. L.; Harris, J. M. J. Am. Chem. Soc. 1984, 106, 4077. (45) de Monderon, S.; Pottier, A.; Maquet, J.; Babonneau, F.; Sanchez, C. New J. Chem. 2006, 30, 797. (46) Brunel. D.; Cauvel, A.; Di Renzo, F.; Fajula, F.; Fubini, B.; Onida, B.; Garrone, E. New J. Chem. 2000, 24, 807.

10414 Langmuir, Vol. 23, No. 20, 2007

Figure 8. Correlation plot for the catalysis of the DTBQ formation vs the estimated Cu‚‚‚Cu distance. (9) Data for SiO2-CuL taken from Table 3 and Table 4 (calculated by eq A12). (O) literature data from ref 32. For the sake of comparison the data for the dimeric Cu2LA showing high DTBQ formation for both the homogeneous and SiO2-grafted Cu complexes are superimposed. ()) literature data from ref 31. The data at the lower right corner correspond to DTBQ formation by homogeneous Cu-complexes in solution.

(a), (b), and (c) might be error-prone, i.e., indicated by the error margins in Table 4. Despite this, as we show in the following when we put the distances in correlation with the catalytic activity, a consistent correlation emerges which is of particular importance. In the following, the significance of the distance estimates with regard to the catalytic efficiency is discussed.

4. Discussion The present data reveal that improved catalytic activity can be achieved by grafted mononuclear CuL and Cu(2L) copper complexes, upon increased loading. The enhancement of the catalytic activity which is achieved by controlling the loading of the SiO2-CuL catalysts is not a simple additive effect. At higher loadings, a fundamentally beneficial mechanism takes place which results in a fourfold increase of the DTBQ formation. Catalysis Improvement: Coordination vs Proximity. Two structural parameters are found to concur with increased loading: (a) At low loadings, only CuL complexes are formed on the SiO2 particles with an average distance between 7 and 8 Å. (b) At high loadings, a second type of complex is formed, i.e., Cu(2L), with short Cu‚‚‚Cu distances near 5 Å or less. The observed catalytic improvement is potentially correlated with both phenomena, i.e., distance shortening and/or coordination from CuL to Cu(2L). Both CuL and Cu(2L) complexes have a distorted square planer symmetry, with relatively open geometry, i.e., axial positions available for exogenous molecules such as DTBC, to approach the copper atoms, see Figure 6. It is generally accepted that flexibiltiy is a beneficial factor for the catalytic activity, i.e., for geometrical and kinetic reasons.16,17,19 The facile accessibility of the fifth coordination position has been demonstrated to be important for the catalytic activity in mononuclear Cu complexes.16,17 However, on the basis of the low catalytic efficiency obtained for homogeneous CuL/Cu(2L) complexes, we conclude that the formation of the Cu(2L) surface complexes cannot account for the observed improvement. Instead we consider that the proximity of the Cu‚‚‚Cu centers imposed by the shortening of the mononuclear Cu‚‚‚Cu distances is the key factor. A correlation plot of the estimated Cu‚‚‚Cu distances, from Table 4, vs the DTBQ formation, from Table 3, is displayed in Figure 8. In addition, in the same plot, we have included pertinent

Grigoropoulou et al.

reference data published previously, measured under comparable conditions. The picture emerging from this plot is a correlation between the DTBQ formation and the Cu‚‚‚Cu distance of mononuclear complexes. From Figure 8, we see that under the conditions of our experiments, i.e., [Cu:DTBC] ) 1:125, a threshold for DTBQ formation >70% can be achieved for mononuclear complexes with Cu‚‚‚Cu distance near 5 Å or less. As we show here, such a Cu‚‚‚Cu distance can be engineered by increasing the loading via the grafting procedure onto SiO2.The distance limit that we deduce from Figure 8 is in accordance with the distance limit of 5 Å or less, suggested,16-19 though never calculated, previously for optimal performance of mononuclear copper complexes.18 Moreover the present data provide information with regard to the performance of catalysts in nonoptimal distances exceeding considerably 5 Å. Equally important, DTBQ formation data in Figure 8 show that grafting at low-loading may eventually result in inferior catalytic activity compared to the homogeneous CuL in solution. Appropriate loadings have to be achieved, which then may result in significant catalytic performance. In this respect EPR spectroscopy can provide critical guidance with regard to the structure and geometry of the grafted complexes. Mechanistic Aspects. In the present work, we focus mainly on structural aspects in relation to the catalytic efficiency. Although a detailed investigation of mechanistic details is out of the scope of the present work, we attempt here a connection of structural key findings with pertinent mechanistic implications. The catalytic oxidation of DTBC to DTBQ occurs through a four-electron reduction of dioxygen to water.19 In dinuclear Cu complexes, the catalytic cycle starts by the coordination of the anionic DTBC to the dinuclear Cu(II) active core.19,47 The twoelectron redox reaction leads to the formation of the first DTBQ molecule and to the reduced dicopper(I) core. Then O2 binds to the dicopper(I) species forming an oxy state of the dinuclear Cu(II) active center. The oxy [CuII-O22-CuII] intermediate transfers two electrons from a second coordinated substrate to the peroxide resulting in the cleavage of O-O bond and the formation of a second DTBQ molecule and H2O.19,47 In this perspective, the binding mode of the substrate to the copper centers is a question dealt with by different research groups.6,7,48 Within this, the copper-copper distance is considered a determining factor for the catalytic activity. This fact was supported by the general result that two metal centers located in close proximity facilitate the binding of catechol and the twoelectron-transfer step.49 Additionally, the O-O bond cleavage catalyzed by dinuclear cupric systems is much faster than that achieved by mononuclear cupric complexes because of the twoelectron-transfer process which has a larger thermodynamic driving force.30,19,47,50 In the case of homogeneous mononuclear Cu2+ complexes, it has been suggested that a spatial Cu‚‚‚Cu proximity of around 5 Å18 or less is a prerequisite for bonding of the catechol and subsequent transfer of 2-electron to dioxygen.15-18 Our data provide support for this suggestion.18 Particularly, the high loading catalysts with distances near 5 Å (sample e) or 1, 0 e e 1, 0 e e 1 (A12) Ntotal Ntotal

Restimated ) R1

According to eq A12, the values R1 and k, i.e., the distances of the closest approach and the fractions of the Cu‚‚‚Cu subgroups in closest proximity, will determine the estimated distance. In the case of only two fractions, we have N1 + N2 ) 100%; thus, eq A12 gives the results listed by Eaton et al. in ref 40. The difference here is that eq A12 is valid for more general cases, i.e., when the two major fractions are not the total of the centers, i.e., N1 + N2 < 100%. After a systematic scrutiny of various combinations of R1, k, N1, and N2 based on equation A12, we conclude that the limiting cases are the following. (a) N1 g N2. This would correspond to an arrangement where the majority of the Cu centers, N1, are at the shorter distance R1, whereas fewer Cu centers will be at longer distances R2 ) kR1. This is depicted schematically in Figure 10A. In the right panel, we present the distance Restimated calculated by using eq A12. The calculations presented are for k ) 1.5, but similar results are obtained for other k values up to k ) 3. For N1/Ntotal ) 100%, e.g., homogeneous distances, the estimated distance using eq A1, coincides with the Cu‚‚‚Cu distance R1, see squares in Figure 10A, right panel. For increased fraction N2/Ntotal, the Restimated deviates from R1. If we define the deviation as

∆R t Restimated - R1 then

9.1 2 A N1 N2 ν + (A9) 6 Ntotal N R total

( )

∆R g 0

2

that is, if N1 g N2 then Restimated is always an overestimate of R1. The deviation ∆R attains a maximum value

Thus, within the present approximations, the ratio of the integrated intensities is determined (a) by the Cu‚‚‚Cu distances R1 and R2 and (b) the relative populations N1/Ntotal and N2/Ntotal. If we define the ratio

R2 t k, with k >1 R1

∆Rmax ) 10%R1 for N1 ) N2 ) 50% For example Restimated ) 5.5 + 0.6 Å ) 6.1 Å for R1 ) 5.5 Å or Restimated ) 9.0 + 0.9 Å ) 9.9 Å for R1 ) 9.0 Å. Thus, we conclude that within the present approximations

for N1 g N2

then

Restimated ) R1 +∆R

0 e ∆R e 10% R1

[

]

9.1 2 A N1 ν total intergal forbidden 1 N2 = + 6 6 total intergal allowed N R1 k Ntotal total

( )

(A10)

In real experiments, the total intensity ratio would result in an estimate of an apparent Cu‚‚‚Cu distance, according to the relation

9.1 2 total intergal forbidden )A total intergal allowed V R

( )

]

N1 1 1 N2 ‚ + 6 w 6 N R1 k Ntotal total

6

(A8)

Now, by combining eqs A1a and A8, we have

total intergal forbidden = total intergal allowed

By combining eqs A10 and A11, we derive the following expression for the estimated distance Restimated:

1 6 estimated

(A11)

(A13)

(b) N1 < N2. This would correspond to an arrangement where the majority of the Cu centers, N2, are at the longer distance R2, whereas fewer Cu centers, N1, will be at short distance kR1. This is depicted schematically in Figure 10B. In the right panel, we present the distance Restimated calculated by using eq A12. The calculations presented are for k ) 1.5. For N2/Ntotal ) 100%, i.e., when all of the Cu‚‚‚Cu distances are at R2, then the Restimated is R2. The introduction of nonzero N1 population would create a deviation

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∆R t Restimated - R2 which would be

∆R e 0 that is, if N1 < N2, then Restimated is always an underestimate of R2. The deviation ∆R increases rapidly with the N1/Ntotal fraction. For example for N1/Ntotal ) 15%, N2/Ntotal ) 80%, Restimated ) 7.1 Å for R2 ) 8.2 Å that is ∆R ) 1.2 Å which is ∼15% of R2. As a practical rule

for N1 < N2

Restimated ) R2 - |∆R|

(A14)

In summary, on the basis of this theoretical analysis, we may stress the following practical observations: (a) In heterogeneous distance distributions, the fraction of interacting spins with the shortest distance, N1, plays determinant role in the distance which will be estimated from the ratio of the integrated intensity of the forbidden vs allowed EPR signals. (b) In the case where N1 is the dominant fraction, i.e., N1 between 100 and 50%, a maximum of 10% overestimate can be introduced in the estimated distance.

(c) In the case N2 > N1, the minor fraction (N1) with shorter distances (R1) even if the majority of the centers, N2, having distances R2 > R1 would cause an underestimate of the distances. This underestimate my be severe on increasing N1. Thus, an estimate of N1 would be necessary for a reasonable estimate of the underestimate factor. As a practical rule, according to our calculations, for distances between 4 and 9 Å, N1 values of 10, 20, or 30% would introduce an underestimate from R2 by 1.2, 1.5, and 1.8 Å, respectively. Thus, we conclude that case b would lead in small errors less than 10%. Case c can lead to larger errors which will depend on the population N1. An order of magnitude estimate of N1 would suffice for a reasonable estimate of the deviation. Acknowledgment. This work has been supported by Operational Program for Educational and Vocational Training II (EPEAEK II) and particularly the Program PYTHAGORAS I. Supporting Information Available: Figure S1 diffuse reflectance UV-vis spectra for the SiO2-supported catalysts. Figure S2, simulated EPR spectra in second derivative mode. This material is available free of charge via the Internet at http://pubs.acs.org. LA700815D