- Email: [email protected]
Immobilization of triazacyclononane-type metal complexes on inorganic supports via covalent linking: spectroscopy and catalytic activity in olefin oxidation
3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 1997 Elsevier Science B.V.
973
Immobilization of t r i a z a c y c l o n o n a n e - t y p e metal complexes on inorganic supports via covalent linking: spectroscopy and catalytic a c t i v i t y in olefin o x i d a t i o n Y.V. Subba Rao, D.E. De Vos,* B. Wouters, P.J. Grobet and P.A. Jacobs Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 3001 Heverlee (Belgium) Different approaches are tested in the covalent linking of the triazacyclononane (tacn) macrocycle to amorphous or mesoporous siliceous supports. The best catalytic results for the epoxidation of olefins are obtained with a tacn, attached via a 3-oxypropyl-2-hydroxypropyl spacer to the support. The organic structures on the surface are studied with TGA, TPD-MS, 13C-NMR and sorption measurements. ESR is used to probe the details of the metal binding on these surfaces. 1. I N T R O D U C T I O N Catalytic mono-oxygen transfer from first row transition metals to nucleophilic substrates has been the subject of intensive studies since the late seventies [1-2]. The classic procedures of porphyrin-catalyzed oxidations have however obvious disadvantages [3-6]. Chlorinated solvents are often used, either in a two phase system or as co-solvents to dissolve the porphyrin. The reaction mixtures are heavily colored. Catalyst recuperation is not obvious, and often the porphyrin doesn't even survive a single catalytic run. Several groups have attempted with varying success to inlprove the usability of porphyrins by diverse heterogenization techniques [7-10]. As an alternative to porphyrin and phthalocyanine catalysts, complexes of Mn and the cyclic triamine 1,4,7-trimethyl-l,4,7-triazacyclononane (tmtacn) clearly deserve more attention [11]. In acetone and at subambient temperature, the activity of Mn-tmtacn matches that of the more active porphyrins, with 1,000 turnovers within a few hours in the styrene epoxidation [12]. Moreover, Mntmtacn is colorless after reaction, and because of its relatively moderate price, it has even been commercialized for a short while in laundry powders [13]. A heterogeneous version of Mn-tmtacn would obviously offer even more advantages. We have proposed an immobilization of Mn-tmtacn based on zeolite We acknowledge support from K.[LL~dven (YVSR) and F.W.O. (DEDV and PJG). This work was performed in the frame of an interuniversitary attraction pole (I.U.A.P.) program Supramolecular Catalysis. E-mail: [email protected]
974 Y [14]. A major problem is however that this hydrophilic matrix attracts H202. This is a drawback from the peroxide efficiency viewpoint. Therefore a purely siliceous matrix seems more attractive. As pure SiO2 lacks ion exchange capacity, one has to revert to other immobilization strategies, such as the covalent route. The present paper investigates various routes for covalent a t t a c h m e n t of the tacn macrocycle to a pre-formed support matrix. Two different spacers are used to link the surface and the tacn: propyl (P), and glycidoxypropyl (GP). The affinity of the modified surface for metals is probed with the test ions Cu 2§ and Mn 2§ and styrene is the test substrate for the selective hydrocarbon oxidation. A preliminary note on this work has appeared [15]. 2. E X P E R I M E N T A L
MCM-41 was prepared following an existing procedure [16]. The quality of the synthesis was evaluated based on the diffractogram and the N2 sorption isotherm. The material was calcined at 823 K and stored in a desiccator to avoid rehydration. Silica was purchased from Fluka (70-230 mesh) and pretreated under vacuum. 3-Chloropropylsilica was from Aldrich. For the anchoring of the organosilane on the Si matrix, 4.5 mmol of (3glycidyloxypropyl)trimethoxysilane was reacted during 10 h with 3 g of dry support material in 25 ml pre-dried and refluxing toluene. Excess silane was removed by toluene soxhlet extraction. For the reaction of tacn with GP- or Pbearing materials, 80 mg tacn was reacted overnight with 1 g of the vacuumdried support at 323 K in 50 ml toluene, followed by another toluene extraction. Eventually the remaining secondary amine groups on the bound tacn were alkylated with an estimated 5-fold excess of propylene oxide (ethanol, 293 K, 24 h). An overview is given in Scheme 1. H I
S c h e m e 1.
~o
,",,,~o
H i
H H i
H,,~'-~~H /'~,/NCl
~OH
H I
..
.
HOL
975 For TGA, a S e t a r a m TGA-DTA 92 a p p a r a t u s was used. Alternatively, a homebuilt a p p a r a t u s was employed for TPD-MS. GC analysis was on a Chrompack CP-Sil-5 column, eventually coupled to a Fisons mass spectrometer. ESR spectra were recorded with a Bruker ESP-300 and a TEl04 cavity at t e m p e r a t u r e s between 130 and 300 K. N~ sorption experiments were performed with an Omnisorp-100 i n s t r u m e n t . The t-plot method was applied for the analysis of the pore volume. Solid state 13C NMR spectra were recorded using a B r u k e r MSL 400 spectrometer at a resonance frequency of 100.61 MHz. Cross polarization was optimized with glycine as a reference. For the m e a s u r e m e n t of liquid samples, a B r u k e r AMX 300 system was used, operating at 300.13 and 75.47 MHz for 1H and ~3C, respectively. 3. R E S U L T S
Characterization of the functionalized surfaces T h e r m o g r a v i m e t r i c analysis is a basic technique for quantifying surface loading with organic groups. Samples were heated at 5 K per minute up to 1073 K in a He/O~ atmosphere. The weight loss (in %) above 453 K is given for all samples in Table 1. The organic weight increase after the reaction with tacn shows t h a t the linking is successful both for 3-chloropropyl (P) and for glycidyloxypropyl (GP) residues. Tacn surface concentrations are highest with MCM-GP and lowest for the commercial Sil-P. The TGA profiles are highly similar for tacn-containing samples (Sil-P-tacn, Sil-GP-tacn, MCM-GP-tacn) on one h a n d and the tacn-free precursors (Sil-P, Sil-GP, MCM-GP) on the other h a n d (Figure 1). With the latter materials, one main and sharp exotherm is observed around 473 K. With all tacn-containing samples, the combustion occurs over a much broader t e m p e r a t u r e interval (473-823 K).
~
,
Exo
_
-5
-5
Exo I
a
a
-15
2o0
40o
600 (c~
-35 ~,,
200 I
400 I
600 I
(el
F i g u r e 1. Weight loss (%, a) and heat flow (b) for MCM-GP (left) and MCM-GPtacn (right).
976 T a b l e 1. Surface loading (wt. % or tacn concentration) as d e t e r m i n e d by TGA.
Sample
% wt. loss
Sil-P Sil-P-tacn Sil-GP Sil-GP-tacn MCM-GP MCM-GP-tacn
[tacn] (retool / g) 0.20 0.28 0.40
5.9 8.5 12.3 15.9 12.2 17.3
W h e n this t h e r m a l decomposition is assessed by mass spectroscopy, typical f r a g m e n t s of decomposition of e.g. GP are detected. For instance, m/z = 57 is probably due to a 2,3-epoxypropyl group. 13C-NMR can be applied to check the intact nature of the surface groups after the anchoring and the extractions. As an example, we discuss the d a t a for the glycidylated MCM-41 (MCM-GP). *
a
~t
i
,!!
'i
I,,~~i,
!
!
,
i
t I :<,,,',~/4,,j ,,,~~\A,4,f,.,.,v/~,Wr (7, '
i
~i,
ii
W v'W~ r ,
' );i:~
<<~0
l
T
r
--=
....
l
(}
4c}
F i g u r e 2. 13C-NMR spectra of MCM-GP (a) 1H-decoupled, (b) CP with 1H. Table
2. Comparison of 13C-NMR shifts for immobilized GP and its silane
precursor. 6 (ppm)
assignment
6 (ppm)
(liquid)
(liquid)
(solid)
5.26
C2
9 (C2)
0 ......
6
[/~'... 5
7
~-
3
22.85
C3
23
(C3)
4"y~
44.28
C7
44
(C7)
~ i
50.52, 50.88 71.43, 73.55
C6 and C1 C4 and C5
50 (C6) 73.4 (C4,C5)
(CH30)3/
1
:2
977 In the proton-decoupled spectra, the sharp bands of the residual, liquid-like toluene are d o m i n a n t (marked with asterisks in Figure 2a). The signals of the immobilized species only become well-observable when cross-polarized spectra are recorded (Figure 2b). For comparison, the chemical shift values and the a s s i g n m e n t s for the GP silane precursor are also t a b u l a t e d in Table 2, and compared to the shifts of the anchored GP group. The only significant change (within the spectral resolution) is observed for C~, close to the anchoring site. Based on the signals of C6 and C7, the epoxide group is clearly intact after the anchoring. Finally coating of the surface with large fragments such as GP s u b s t a n t i a l l y influences the sorption properties of the material. Figure 3 compares the sorption isotherms of a single batch of MCM-41 before and after GP anchoring. From these plots, the total pore volume with a radius below 2 nm was shown to decrease from 1.8 ml.g -1 to 0.62 ml.g -] Moreover the m a x i m u m in the pore size distribution plot decreases from a pore radius r - 1.7 nm to r - 1.1 nm. V (ml/g)
V (ml/g) 1000
1000 500
500
z
0
0
0.5
P/Po
1.0
0
0.5
P/Po
1.0
F i g u r e 3. N~ sorption isotherms for calcined MCM-41 (left), MCM-GP (right).
Metal b i n d i n g on l i g a n d - f u n c t i o n a l i z e d
surfaces
To probe the metal-binding characteristics of the functional surface, Cu 2+ or Mn ~§ were introduced into the tacn-containing m a t e r i a l by stirring for 2 h in a solution of the corresponding sulfates in m e t h a n o l (90 %) - w a t e r (10%). As this process is f u n d a m e n t a l l y different from ion exchange, ESR was used to confirm the m e t a l u p t a k e and the selective binding of the m e t a l by the N ligand. For Cu ~§ N-coordination (as opposed to T a b l e 3. ESR p a r a m e t e r s for Cu ~§ systems an all oxygen coordination) is (X-band, 130 K). evidenced by a decrease of gll (from -2.40 to <2.30) and an increase of gll All (cm-1) All. The p a r a m e t e r s of Table 3 Cu-MCM-4i 2.409 0.0131 leave no doubt t h a t Cu ~§ binds Cu-Sil-P-tacn 2.289 0.0159 solely on the ligands and not on 2.223 0.0181 the silica or MCM-41 surface. Cu-MCM-GP-tacn 2.229 0.0176 Moreover comparison with [Cu(tacn)] ~§ 2.278 0.0158 literature values for [Cu(tacn)x] ~§ [Cu(tacn)~] ~§ 2.229 0.0177 complexes proves t h a t at low Cu ~§
978 concentrations, MCM-GP-tacn contains exclusively bis complexes, while also m o n o complexes are formed with the P spacer on silica [17]. Thus the length of the spacer seems to determine whether g=2 interaction between neighboring anchored ligands can occur. For Mn 2§ the ESR analysis requires a sample manipulation under N2 to avoid oxidation of the N-coordinated Mn. Nitrogen binding causes a decrease of the originally pseudo-octahedral symmetry of [Mn(H20)6] 2+. For a high-spin d 5 ion such as Mn 2§ this results in zero-field splitting. The non-zero values of the D and E parameters lead to new absorptions in the F i g u r e 4. X-band ESR for spectrum, at apparent g values different from 2 Mn-Sil-GP-tacn (293 K). ( ' n o n - c e n t r a l lines') [18]. We have recently applied such an analysis in the study of the Mn siting in zeolite A [19]. In ligand-free Mn-MCM-41 non-central lines are absent [20]. As an example, Figure 4 shows the X-band spectrum of Mn-Sil-GP-tacn, with non-central absorptions at 260 G, 1400 G and 2450 G (indicated by arrows). This proves that Mn binds selectively on the covalently attached ligand and not on the siliceous surface. Catalytic
activity
of the new
materials
Catalytic oxidations were performed with 1 mmol of substrate, 2 mmol of 35 % aqueous H202, 1 ml of solvent and 20 mg of metallated, functionalized support (containing 4-8 pmol Mn) at 273 K for 1 h. To improve the catalyst performance, 60 24 !ii!ii!~!iiiilililili!!iiiilit
T.O. 30 ~i~:~T~T~I 72 i:~:i:~i~!:?;:i?i~iii~ii~i~l
iiiiiiiiiii!iiiiiiii!iii!)i!iii
I ~1
.................
1
2
3
4
-
PO
-
PO
Sil-P-tacn
Sil-GP-tacn
87 87
5
62
liiiiiil
.................................
6
7
8
PO
PO
PO
MCM-GP-tacn
F i g u r e 5. Turnover numbers in the styrene oxidation with covalently anchored Mn-tacn. The solvent is acetone, except in 7 (CH3OH) and 8 (CH3CN). Numbers above bars are epoxide selectivities.
979 the effect of reacting the anchored tacn with two molecules of propylene oxide (PO) was tested. Turnover numbers in the oxidation of styrene are given in Figure 5 for the various systems. The post-modification with propylene oxide gives the best catalytic results, both with Sil-GP-tacn and MCM-GP-tacn. For the latter system, methanol seems a superior solvent in comparison with acetone and acetonitrile. The epoxide selectivity in reaction 7 is limited, but this is due to secondary reactions of initially formed epoxide to phenylacetaldehyde and the diol. When these secondary products are taken into account, the selectivity for the epoxide and its derived products increases to 76 %. 4. D I S C U S S I O N
The physicochemical characterization of the different intermediates and the final functionalized surfaces demonstrates that the glycidylation approach (Scheme 1) is a valuable alternative to the more familiar 3-chloropropyl method [21]. NMR proves that the oxirane group in the glycidyl residue is intact after the silane-surface c o u p l i n g . The reactivity of this oxirane group allows highly selective subsequent surface reactions under mild conditions. TGA and sorption experiments show the presence of considerable concentrations of GP or GP-tacn groups on the surface. ESR proves the specificity of the metal binding on these materials. Finally the catalytic experiments show that a tacn-type reactivity, with selective mono-oxygen transfer, subsists in the final material. While there is a far-reaching analogy between the catalytic activities of Mnporphyrins and Mn-tacn type complexes, the strategies for their covalent attachment are necessarily very different. The aromaticity of the porphyrin dictates that substitutions can only be made at the periphery of the ligand, which makes the influence of the substituent on the metal activity low. By contrast, the N-substituents in the tacn macrocycle are in the immediate proximity of the metal ion, and might even offer coordinating atoms to the metal center. We have previously studied the catalytic effects of changing the substituents on the tacn macrocycle [22]. With R=H (tacn), the system is almost completely inactive. With R=CH3 (tmtacn), the catalyst has a high activity, but only when acetone is used as a solvent. With hexadentate ligands (R=2-hydroxyalkyl or acetato) activities are somewhat lower, but still considerable when methanol is used as the solvent. For the present heterogeneous system (Mn-support-GP-tacn + PO), the structure and the solvent effects in the catalytic experiments resemble most those of the latter, hexadentate complexes. For such hexadentate complexes, a temporary removal of one of the pendant arms is necessary to create a coordinative vacancy on the metal. The particular role of methanol might be to assist in the temporary dehgation via hydrogen bond formation with 2-OH-alkyl groups. The system is unique in that the covalent link to the surface can participate in the metal coordination via the 2-hydroxy group, as indicated by the arrow in Scheme 1.
980 To our knowledge, this work is the first example of a covalently anchored, catalytically active non-heme Mn compound. Attempts are under way in our laboratory to develop similar catalysts with an even higher activity by immobilization of partially methylated tacn rings.
R E F E R E N C E S AND NOTES 1. B. Meunier, Chem. Rev. 92 (1992) 1411. 2. R. Holm, Chem. Rev. 87 (1987) 1401. 3. P. Battioni, J.P. Renaud, J.F. Bartoli, M. Reina-Artiles, M. Fort and D. Mansuy, J. Am. Chem. Soc. 110 (1988) 8462. 4. B. Meunier, E. Guilmet, M.E. de Carvalho and R. Poilblanc, J. Am. Chem. Soc. 106 (1984) 6668. 5. J.T. Groves and T. Nemo, J. Am. Chem. Soc. 105 (1983) 6243. 6. A.J. Castellino and T.C. Bruice, J. Am. Chem. Soc. 110 (1988) 158. 7. P. Battioni, E. Cardin, M. Louloudi, B. Sch611horn, G.A. Spyroulias, D. Mansuy and T.G. Traylor, Chem. Commun. (1996) 2037. 8. K. Miki and Y. Sato, Bull. Chem. Soc. Jpn. 66 (1993) 2385. 9. A. Sorokin and B. Meunier, J. Chem. Soc. Chem. Commun. (1994) 1799. 10. J.R. Lindsay Smith and R.J. Lower, J. Chem. Soc. Perkin Trans. 2 (1992) 2187. 11. D.E. De Vos and T. Bein, Chem. Commun. (1996) 917. 12. While acetone is less harmful than chlorinated solvents, one should beware of the organic peroxides that result from prolonged contact between acetone and H~O~. These peroxides are potentially explosive (e.g. in distillations). 13. R. Hage, J. Iburg, J. Kerschner, J. Koek, E. Lempers, R. Martens, U.S. Racherla, S.W. Russell, T~ Swarthoff, M. Van Vliet, J. Warnaar, L. van der Wolf and B. Krijnen, Nature 369 (1994) 637. 14. D.E. De Vos, J.L. Meinershagen and T. Bein, Angewandte Chemie, Int. Ed. Engl. 35 (1996) 2211. 15. Y.V.S. Rao, D.E. De Vos, T. Bein and P.A. Jacobs, Chem. Commun. (1997) in press. 16. J.S. Beck, J.C. Vartuli, W. Roth, M. Leonowicz, C.T. Kresge, K.D. Schmitt, C. Chu, D.H. Olson, E.W. Sheppard, S. McCullen, J.B. Higgins and J.L. Schenker, J. Am. Chem. Soc. 114 (1992) 10834. 17. R.D. Bereman, M.R. Churchill, P.M. Schraber and M.E. Winkler, Inorg. Chem. 18 (1979) 3122; P. Chaudhuri, K. Oder, K. Wieghardt, J. Weiss, J. Reedijk, W. Hinrichs, J. Wood, A. Ozarowski, H. Stratemeier and D. Reinen, Inorg. Chem. 25 (1986) 2951. 18. B. Bleaney and D.J.E. Ingram, Proc. Royal Soc. (London) A205 (1951) 336. 19. D.E. De Vos and T. Bein, J. Am. Chem. Soc. 118 (1996) 9615. 20. D. Zhao and D. Goldfarb, Stud. Surf. Sci. Catal. 97 (1995) 181. 21. D. Brunel, A. Cauvel, F. Fajula and F. DiRenzo, Stud. Surf. Sci. Catal. 97 (1995) 173. 22. D.E. De Vos and T. Bein, J. Organometallic Chem. 520 (1996) 195.
973
Immobilization of t r i a z a c y c l o n o n a n e - t y p e metal complexes on inorganic supports via covalent linking: spectroscopy and catalytic a c t i v i t y in olefin o x i d a t i o n Y.V. Subba Rao, D.E. De Vos,* B. Wouters, P.J. Grobet and P.A. Jacobs Center for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, 3001 Heverlee (Belgium) Different approaches are tested in the covalent linking of the triazacyclononane (tacn) macrocycle to amorphous or mesoporous siliceous supports. The best catalytic results for the epoxidation of olefins are obtained with a tacn, attached via a 3-oxypropyl-2-hydroxypropyl spacer to the support. The organic structures on the surface are studied with TGA, TPD-MS, 13C-NMR and sorption measurements. ESR is used to probe the details of the metal binding on these surfaces. 1. I N T R O D U C T I O N Catalytic mono-oxygen transfer from first row transition metals to nucleophilic substrates has been the subject of intensive studies since the late seventies [1-2]. The classic procedures of porphyrin-catalyzed oxidations have however obvious disadvantages [3-6]. Chlorinated solvents are often used, either in a two phase system or as co-solvents to dissolve the porphyrin. The reaction mixtures are heavily colored. Catalyst recuperation is not obvious, and often the porphyrin doesn't even survive a single catalytic run. Several groups have attempted with varying success to inlprove the usability of porphyrins by diverse heterogenization techniques [7-10]. As an alternative to porphyrin and phthalocyanine catalysts, complexes of Mn and the cyclic triamine 1,4,7-trimethyl-l,4,7-triazacyclononane (tmtacn) clearly deserve more attention [11]. In acetone and at subambient temperature, the activity of Mn-tmtacn matches that of the more active porphyrins, with 1,000 turnovers within a few hours in the styrene epoxidation [12]. Moreover, Mntmtacn is colorless after reaction, and because of its relatively moderate price, it has even been commercialized for a short while in laundry powders [13]. A heterogeneous version of Mn-tmtacn would obviously offer even more advantages. We have proposed an immobilization of Mn-tmtacn based on zeolite We acknowledge support from K.[LL~dven (YVSR) and F.W.O. (DEDV and PJG). This work was performed in the frame of an interuniversitary attraction pole (I.U.A.P.) program Supramolecular Catalysis. E-mail: [email protected]
974 Y [14]. A major problem is however that this hydrophilic matrix attracts H202. This is a drawback from the peroxide efficiency viewpoint. Therefore a purely siliceous matrix seems more attractive. As pure SiO2 lacks ion exchange capacity, one has to revert to other immobilization strategies, such as the covalent route. The present paper investigates various routes for covalent a t t a c h m e n t of the tacn macrocycle to a pre-formed support matrix. Two different spacers are used to link the surface and the tacn: propyl (P), and glycidoxypropyl (GP). The affinity of the modified surface for metals is probed with the test ions Cu 2§ and Mn 2§ and styrene is the test substrate for the selective hydrocarbon oxidation. A preliminary note on this work has appeared [15]. 2. E X P E R I M E N T A L
MCM-41 was prepared following an existing procedure [16]. The quality of the synthesis was evaluated based on the diffractogram and the N2 sorption isotherm. The material was calcined at 823 K and stored in a desiccator to avoid rehydration. Silica was purchased from Fluka (70-230 mesh) and pretreated under vacuum. 3-Chloropropylsilica was from Aldrich. For the anchoring of the organosilane on the Si matrix, 4.5 mmol of (3glycidyloxypropyl)trimethoxysilane was reacted during 10 h with 3 g of dry support material in 25 ml pre-dried and refluxing toluene. Excess silane was removed by toluene soxhlet extraction. For the reaction of tacn with GP- or Pbearing materials, 80 mg tacn was reacted overnight with 1 g of the vacuumdried support at 323 K in 50 ml toluene, followed by another toluene extraction. Eventually the remaining secondary amine groups on the bound tacn were alkylated with an estimated 5-fold excess of propylene oxide (ethanol, 293 K, 24 h). An overview is given in Scheme 1. H I
S c h e m e 1.
~o
,",,,~o
H i
H H i
H,,~'-~~H /'~,/NCl
~OH
H I
..
.
HOL
975 For TGA, a S e t a r a m TGA-DTA 92 a p p a r a t u s was used. Alternatively, a homebuilt a p p a r a t u s was employed for TPD-MS. GC analysis was on a Chrompack CP-Sil-5 column, eventually coupled to a Fisons mass spectrometer. ESR spectra were recorded with a Bruker ESP-300 and a TEl04 cavity at t e m p e r a t u r e s between 130 and 300 K. N~ sorption experiments were performed with an Omnisorp-100 i n s t r u m e n t . The t-plot method was applied for the analysis of the pore volume. Solid state 13C NMR spectra were recorded using a B r u k e r MSL 400 spectrometer at a resonance frequency of 100.61 MHz. Cross polarization was optimized with glycine as a reference. For the m e a s u r e m e n t of liquid samples, a B r u k e r AMX 300 system was used, operating at 300.13 and 75.47 MHz for 1H and ~3C, respectively. 3. R E S U L T S
Characterization of the functionalized surfaces T h e r m o g r a v i m e t r i c analysis is a basic technique for quantifying surface loading with organic groups. Samples were heated at 5 K per minute up to 1073 K in a He/O~ atmosphere. The weight loss (in %) above 453 K is given for all samples in Table 1. The organic weight increase after the reaction with tacn shows t h a t the linking is successful both for 3-chloropropyl (P) and for glycidyloxypropyl (GP) residues. Tacn surface concentrations are highest with MCM-GP and lowest for the commercial Sil-P. The TGA profiles are highly similar for tacn-containing samples (Sil-P-tacn, Sil-GP-tacn, MCM-GP-tacn) on one h a n d and the tacn-free precursors (Sil-P, Sil-GP, MCM-GP) on the other h a n d (Figure 1). With the latter materials, one main and sharp exotherm is observed around 473 K. With all tacn-containing samples, the combustion occurs over a much broader t e m p e r a t u r e interval (473-823 K).
~
,
Exo
_
-5
-5
Exo I
a
a
-15
2o0
40o
600 (c~
-35 ~,,
200 I
400 I
600 I
(el
F i g u r e 1. Weight loss (%, a) and heat flow (b) for MCM-GP (left) and MCM-GPtacn (right).
976 T a b l e 1. Surface loading (wt. % or tacn concentration) as d e t e r m i n e d by TGA.
Sample
% wt. loss
Sil-P Sil-P-tacn Sil-GP Sil-GP-tacn MCM-GP MCM-GP-tacn
[tacn] (retool / g) 0.20 0.28 0.40
5.9 8.5 12.3 15.9 12.2 17.3
W h e n this t h e r m a l decomposition is assessed by mass spectroscopy, typical f r a g m e n t s of decomposition of e.g. GP are detected. For instance, m/z = 57 is probably due to a 2,3-epoxypropyl group. 13C-NMR can be applied to check the intact nature of the surface groups after the anchoring and the extractions. As an example, we discuss the d a t a for the glycidylated MCM-41 (MCM-GP). *
a
~t
i
,!!
'i
I,,~~i,
!
!
,
i
t I :<,,,',~/4,,j ,,,~~\A,4,f,.,.,v/~,Wr (7, '
i
~i,
ii
W v'W~ r ,
' );i:~
<<~0
l
T
r
--=
....
l
(}
4c}
F i g u r e 2. 13C-NMR spectra of MCM-GP (a) 1H-decoupled, (b) CP with 1H. Table
2. Comparison of 13C-NMR shifts for immobilized GP and its silane
precursor. 6 (ppm)
assignment
6 (ppm)
(liquid)
(liquid)
(solid)
5.26
C2
9 (C2)
0 ......
6
[/~'... 5
7
~-
3
22.85
C3
23
(C3)
4"y~
44.28
C7
44
(C7)
~ i
50.52, 50.88 71.43, 73.55
C6 and C1 C4 and C5
50 (C6) 73.4 (C4,C5)
(CH30)3/
1
:2
977 In the proton-decoupled spectra, the sharp bands of the residual, liquid-like toluene are d o m i n a n t (marked with asterisks in Figure 2a). The signals of the immobilized species only become well-observable when cross-polarized spectra are recorded (Figure 2b). For comparison, the chemical shift values and the a s s i g n m e n t s for the GP silane precursor are also t a b u l a t e d in Table 2, and compared to the shifts of the anchored GP group. The only significant change (within the spectral resolution) is observed for C~, close to the anchoring site. Based on the signals of C6 and C7, the epoxide group is clearly intact after the anchoring. Finally coating of the surface with large fragments such as GP s u b s t a n t i a l l y influences the sorption properties of the material. Figure 3 compares the sorption isotherms of a single batch of MCM-41 before and after GP anchoring. From these plots, the total pore volume with a radius below 2 nm was shown to decrease from 1.8 ml.g -1 to 0.62 ml.g -] Moreover the m a x i m u m in the pore size distribution plot decreases from a pore radius r - 1.7 nm to r - 1.1 nm. V (ml/g)
V (ml/g) 1000
1000 500
500
z
0
0
0.5
P/Po
1.0
0
0.5
P/Po
1.0
F i g u r e 3. N~ sorption isotherms for calcined MCM-41 (left), MCM-GP (right).
Metal b i n d i n g on l i g a n d - f u n c t i o n a l i z e d
surfaces
To probe the metal-binding characteristics of the functional surface, Cu 2+ or Mn ~§ were introduced into the tacn-containing m a t e r i a l by stirring for 2 h in a solution of the corresponding sulfates in m e t h a n o l (90 %) - w a t e r (10%). As this process is f u n d a m e n t a l l y different from ion exchange, ESR was used to confirm the m e t a l u p t a k e and the selective binding of the m e t a l by the N ligand. For Cu ~§ N-coordination (as opposed to T a b l e 3. ESR p a r a m e t e r s for Cu ~§ systems an all oxygen coordination) is (X-band, 130 K). evidenced by a decrease of gll (from -2.40 to <2.30) and an increase of gll All (cm-1) All. The p a r a m e t e r s of Table 3 Cu-MCM-4i 2.409 0.0131 leave no doubt t h a t Cu ~§ binds Cu-Sil-P-tacn 2.289 0.0159 solely on the ligands and not on 2.223 0.0181 the silica or MCM-41 surface. Cu-MCM-GP-tacn 2.229 0.0176 Moreover comparison with [Cu(tacn)] ~§ 2.278 0.0158 literature values for [Cu(tacn)x] ~§ [Cu(tacn)~] ~§ 2.229 0.0177 complexes proves t h a t at low Cu ~§
978 concentrations, MCM-GP-tacn contains exclusively bis complexes, while also m o n o complexes are formed with the P spacer on silica [17]. Thus the length of the spacer seems to determine whether g=2 interaction between neighboring anchored ligands can occur. For Mn 2§ the ESR analysis requires a sample manipulation under N2 to avoid oxidation of the N-coordinated Mn. Nitrogen binding causes a decrease of the originally pseudo-octahedral symmetry of [Mn(H20)6] 2+. For a high-spin d 5 ion such as Mn 2§ this results in zero-field splitting. The non-zero values of the D and E parameters lead to new absorptions in the F i g u r e 4. X-band ESR for spectrum, at apparent g values different from 2 Mn-Sil-GP-tacn (293 K). ( ' n o n - c e n t r a l lines') [18]. We have recently applied such an analysis in the study of the Mn siting in zeolite A [19]. In ligand-free Mn-MCM-41 non-central lines are absent [20]. As an example, Figure 4 shows the X-band spectrum of Mn-Sil-GP-tacn, with non-central absorptions at 260 G, 1400 G and 2450 G (indicated by arrows). This proves that Mn binds selectively on the covalently attached ligand and not on the siliceous surface. Catalytic
activity
of the new
materials
Catalytic oxidations were performed with 1 mmol of substrate, 2 mmol of 35 % aqueous H202, 1 ml of solvent and 20 mg of metallated, functionalized support (containing 4-8 pmol Mn) at 273 K for 1 h. To improve the catalyst performance, 60 24 !ii!ii!~!iiiilililili!!iiiilit
T.O. 30 ~i~:~T~T~I 72 i:~:i:~i~!:?;:i?i~iii~ii~i~l
iiiiiiiiiii!iiiiiiii!iii!)i!iii
I ~1
.................
1
2
3
4
-
PO
-
PO
Sil-P-tacn
Sil-GP-tacn
87 87
5
62
liiiiiil
.................................
6
7
8
PO
PO
PO
MCM-GP-tacn
F i g u r e 5. Turnover numbers in the styrene oxidation with covalently anchored Mn-tacn. The solvent is acetone, except in 7 (CH3OH) and 8 (CH3CN). Numbers above bars are epoxide selectivities.
979 the effect of reacting the anchored tacn with two molecules of propylene oxide (PO) was tested. Turnover numbers in the oxidation of styrene are given in Figure 5 for the various systems. The post-modification with propylene oxide gives the best catalytic results, both with Sil-GP-tacn and MCM-GP-tacn. For the latter system, methanol seems a superior solvent in comparison with acetone and acetonitrile. The epoxide selectivity in reaction 7 is limited, but this is due to secondary reactions of initially formed epoxide to phenylacetaldehyde and the diol. When these secondary products are taken into account, the selectivity for the epoxide and its derived products increases to 76 %. 4. D I S C U S S I O N
The physicochemical characterization of the different intermediates and the final functionalized surfaces demonstrates that the glycidylation approach (Scheme 1) is a valuable alternative to the more familiar 3-chloropropyl method [21]. NMR proves that the oxirane group in the glycidyl residue is intact after the silane-surface c o u p l i n g . The reactivity of this oxirane group allows highly selective subsequent surface reactions under mild conditions. TGA and sorption experiments show the presence of considerable concentrations of GP or GP-tacn groups on the surface. ESR proves the specificity of the metal binding on these materials. Finally the catalytic experiments show that a tacn-type reactivity, with selective mono-oxygen transfer, subsists in the final material. While there is a far-reaching analogy between the catalytic activities of Mnporphyrins and Mn-tacn type complexes, the strategies for their covalent attachment are necessarily very different. The aromaticity of the porphyrin dictates that substitutions can only be made at the periphery of the ligand, which makes the influence of the substituent on the metal activity low. By contrast, the N-substituents in the tacn macrocycle are in the immediate proximity of the metal ion, and might even offer coordinating atoms to the metal center. We have previously studied the catalytic effects of changing the substituents on the tacn macrocycle [22]. With R=H (tacn), the system is almost completely inactive. With R=CH3 (tmtacn), the catalyst has a high activity, but only when acetone is used as a solvent. With hexadentate ligands (R=2-hydroxyalkyl or acetato) activities are somewhat lower, but still considerable when methanol is used as the solvent. For the present heterogeneous system (Mn-support-GP-tacn + PO), the structure and the solvent effects in the catalytic experiments resemble most those of the latter, hexadentate complexes. For such hexadentate complexes, a temporary removal of one of the pendant arms is necessary to create a coordinative vacancy on the metal. The particular role of methanol might be to assist in the temporary dehgation via hydrogen bond formation with 2-OH-alkyl groups. The system is unique in that the covalent link to the surface can participate in the metal coordination via the 2-hydroxy group, as indicated by the arrow in Scheme 1.
980 To our knowledge, this work is the first example of a covalently anchored, catalytically active non-heme Mn compound. Attempts are under way in our laboratory to develop similar catalysts with an even higher activity by immobilization of partially methylated tacn rings.
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