Different Routes for Preparing Mesoporous Organosilicas Containing

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Different Routes for Preparing Mesoporous Organosilicas Containing € ger’s Base and Their Textural and Catalytic Implications the Tro Evelyne Poli, Estíbaliz Merino, Urbano Díaz, Daniel Brunel, and Avelino Corma* Instituto de Tecnología Química UPV-CSIC, Universidad Politecnica de Valencia, Av. de los Naranjos s/n, E-46022 Valencia, Spain

bS Supporting Information ABSTRACT: Organosilica materials with different contents of Tr€oger’s base (TB) as builder moieties have been synthesized from previously synthesized bis-trialkoxysilylated Tr€oger’s base. Three well-nanostructured hybrid mesoporous materials were prepared through different approachs: (a) anchoring the TB on a preformed silica SBA-15 material by postsynthesis grafting, (b) incorporating the TB fragments into the rigid and ordered periodic mesoporous organosilica (PMO) with hexagonal structure (HMS) by self-assembling process, and (c) using anion fluoride as catalyst for a sol
gel synthesis process in the absence of structural directing agents (SDAs), at neutral pH and low synthesis temperature, to introduce the TB units into the walls of high surface orderless mesoporous materials with flexible structure. The degree of long-range ordering of the materials was determined from X-ray diffraction and transmission electron microscopy and the texture of the various samples were analyzed by nitrogen sorption volumetry. The composition of the materials was determined by elemental analyses and thermogravimetry, and the integrity, topology and structuration level of the Tr€oges base units were characterized by 13C NMR and 29Si MAS NMR spectroscopy. The materials were used as basic organocatalysts in Knoevenagel reaction. The disordered mesoporous hybrid materials show the highest catalytic activity due to the appropriate combination of high accessibility and structural flexibility. These mesoporous organosilica materials are stable upon recycling.

1. INTRODUCTION Organocatalysis has attracted much interest for organic synthesis. Amines (primary, secondary, or tertiary) were probably the first basic organocatalysts reported by Knoevenagel in a series of papers,1 which have been widely used for reactions between aromatic or aliphatic aldehydes with molecules possessing activated methylenic groups.2
7 Other more basic organic molecules such as guanidines,8
10 biguanidines,11 or phosphazenes12,13 have also been used as catalysts for other more-demanding reactions such as Micha€el, Robinson annulation, Beckmann rearrangement, Baylis
Hillman, and transesterification reactions. In the particular case of secondary amines such as piperidine, pyrolidine, morpholine, and proline, the transient formation of electron-rich enamine intermediates14 allows them to react as good nucleophiles with suitable alkyl or acyl halides to give the β-alkylated or acylated iminium halides, which can provide alkylated or acylated recovered carbonyl derivatives through iminium C
N hydrolysis. Furthermore, this intermediate with an electron-poor center at the R-position of N-iminium can also undergo subsequent nucleophilic attack at the R-carbon, under cascade reaction conditions, to produce more complex molecules according to this emerging approach developed in the past decade.15 Tertiary amines such as trimethylamine, as well as DABCO, have attracted special attention as organocatalyst for Baylis
r 2011 American Chemical Society

Hillman reaction.16
20 Though the Tr€oger’s base (TB) has been known for over more than a century,21 it was necessary more than fifty years to elucidate the structure and synthesis mechanism.22 The enantiomers of the C2-symmetrical 2,8-dimethyl-6H,12H5,11-methanodibenzo[b,f][1,5]diazocine molecule (TB) possessing two tertiary bridgehead nitrogen atoms as stereogenic centers (Scheme 1), were resolved by Prelog and Wieland,23 and finally their molecular structures were definitively confirmed by X-ray analysis by Wilcox24 and by complete 1H and 13C NMR analysis by Pardo et al,25 100 years after the pioneering report of Tr€oger. In the past 20 years, there has been a renewal of interest in TB derivatives featuring a chiral, rigid, and V-shaped molecular scaffold for molecular recognition because of the hosting cavity.26
35 However, the applications of TB in catalysis has been scarcely investigated. It was showed that chiral TB was moderately efficient as modifier of Pt/Al2O3 catalyst for heterogeneous enantioselective hydrogenation (ee 65%) of ethyl pyruvate, and in any case, it was less effective than cinchona (ee > 80%).36 Similarly, racemic TB was also used as additive for palladium-catalyzed aerobic Received: January 11, 2011 Revised: March 7, 2011 Published: March 24, 2011 7573

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The Journal of Physical Chemistry C

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Scheme 1. Structure of 2,8-Dimethyl-6H,12H-5,11-methanodibenzo [b,f][1,5]diazocine Molecule (Left) and Chem 3D Representation of the (þ) (S,S)-Enantiomer (Right)

oxidation of alcohols to the corresponding aldehydes and ketones at room temperature and was slightly less efficient than triethyl amine but somewhat more active than other nitrogenbase additives.37,38 The first use of TB as ligand to prepare a transition metal complex with rhodium trichloride provided an air stable, nonhygroscopic system featuring a structural type TB-RhCl3 that revealed interesting catalytic activity for hydrosilylation of terminal alkynes, giving rise in some cases to antiaddition products, ca. cis-alkenylsilanes with selectivities up to 95%.39 Another investigation on the catalytic use of TB derivatives as chiral ligands concerned the asymmetric induction during the addition of diethylzinc to aromatic aldehydes.40 Albeit commercial chiral TB gave a very poor enantioselectivity, (ee 7
22%), TB functionalized at the benzylic position with carbinol chains afforded higher enantiomeric excesses (ee 34
56%) in the produced secondary benzylic alcohol, though much lower than those obtained with chiral amino alcohols.41
43 More recently, (þ) TB was used as precursor of in situ generated aminimide in presence of O-mesitylsulfonylhydroxylamine and cesium hydroxide monohydrate, which acted as effective NH-transfert reagent for the aziridination of chalcones with notable (ee 55
66%) enantioselectivities.44 In the case of stereoselective Mannich reaction of benzaldehyde, aniline, and cyclohexanone, TB and TB derivative (8H,16H-7,15-methanodinaphathol[2,1-b][20 ,10 f][1,5] diazocine) were totally inactive. Interestingly, TB derivative with two substitued pyrazole or isoxyzole gave significant yield with high anti/syn stereoselectivity of products which may come from the characteristic rigid-shape structure of TB.45 Finally, inspired by previous work on Michael reaction, addition of 1
3 addition of carbonyl compounds to nitroolefins and to R,β-unsaturated imides, successfully catalyzed by chiral tertiary amine bearing thiourea, Sergeyev et al. have investigated the use of enantiopure TB bearing thiourea chains as substituent on the aromatic nucleus, as catalyst for the same reactions. As the parent chiral TB, the new thiourea TBs was active, but unfortunately, no enantioselectivity was found.46 The failure of this unique attempt, until now, to use the Tr€oger’s base as a homogeneous enantioselective base catalyst featuring chiral nitrogen as purely basic site, suggests that this class of rigid V-shaped and chiral molecular scaffold would not be appropriated to provide sizable enantioselectivity. However, in contrast to oldest estimation of the basic strength,47 the relatively high basicity of the TB’s nitrogen site (pKHB(N) = 1.15) can be relevant for future investigations as basic organocatalyst. Then, we have explored here the possibilities of TB when into different types of mesoporous silica frameworks providing stable and new heterogeneous basic organic catalysts. We have only found one example in which the TB was heterogeneized on an organic

nanoporous polymer48 that was used as heterogeneous catalyst for the addition of diethylzinc to 4-chlorobenzaldehyde. The supported TB showed comparable activity to the homogeneous TB. Among the different strategies available to covalently incorporate organic groups in the silica network of the nanoporous materials, the first approach is probably to anchor the organocatalyst onto previously synthesized mesoporous materials by postsynthesis grafting methods. With the same scope, co-condensation processes in one-pot synthesis from monosilylated precursors were also performed to prepare mesoporous materials with organic groups pended from the silica surface. Both methods allow preparing interesting catalysts when the active organic function is separated enough from the silica walls, avoiding the undesirable interactions with the surface silanol groups. In this case, the behavior of the organocatalyst can be expected to be more similar to the homogeneous counterpart. However, the hybrid grafted samples may present important drawbacks for the partial blockage of pores and the undesired site
site interactions due to the supramolecular association of the silylated organic-precursor, which are favored in nonpolar solvents.49
51 To overcome these drawbacks, the organocatalyts can be integrated into the framework of periodic mesoporous organosilicas (PMOs) by self-assembling procedures using bis-silylated precursors,52,53 (R0 O)3Si
R
Si(OR0 )3, in the presence of surfactants as structural directing agents (SDAs). The homogeneous distribution of the organic moieties along axial direction of the ordered channels, and the high contents of active sites, enable targeting a novel family of ordered hybrid mesoporous materials with different stabilized functions integrated as builders inside the framework.54
70 Nevertheless, the use of complex SDA molecules during the synthesis of these types of materials and the structural rigidity imposed by the intrinsic topology of some building-blocks, constitute important obstacles in the design of novel functional hybrid solids. There is a third route to obtain organic
inorganic hybrid materials that does not involve the use of surfactants or other SDAs, but still allows preparing less ordered mesoporous hybrid materials with the framework being conformed by organocatalyst and siloxane units alternated and bonded covalently between them. The synthesis route involves a sol
gel procedure catalyzed by NH4F and carried out at neutral pH and room temperature. These soft synthesis conditions together with the higher flexibility exhibited by the network associated to the nonordered organization may encompass some of the mean problems entailed by the grafted materials or by the periodic mesoporous systems.71
75 Here we have prepared mesoporous organosilica hybrid materials based on TB units, using first a postsynthesis grafting 7574

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The Journal of Physical Chemistry C method and second through two one-pot sol
gel route in presence or not of SDA molecules. These as-synthesized materials correspond to three different types of nanostructured solids: (a) Troger’s base-tethered SBA-15 silica resulting from mesoporous silica surface grafted with bis-silylated TB, (b) periodic mesoporous organosilicas featuring HMS type structure with rigid walls through self-assembling process,76 and (c) high surface orderedless mesoporous materials with flexible structure obtained by NH4F sol
gel route in soft conditions, i.e., neutral pH and low temperature. These hybrid mesoporous materials were extensively characterized, and the catalytic performance for a base catalyzed Knoevenagel reaction has been correlated with their physicochemical characteristics.

2. EXPERIMENTAL SECTION 2.1. Reagents. High grade purity (>99%) solvents were used without further treatment except in experiments requiring dry DMF as solvent, which was distilled from CaH2 and stored under argon. In other experiments, all reagents purchased from commercial sources were used without purification. 4-Iodo-aniline (98%), paraformaldehyde (95%), trifluoroacetic acid (99%), bis(acetonitrile) (1,5-cyclooctadiene) rhodium(I) tetrafluorate, triethylamine (g99%), triethylsiloxane (95%), tetramethoxysilane (98%), ammonium fluoride (g98%), hexadecylamine (98%), malononitrile (g99%), redistilled benzaldehyde (g99.5%), and 2,8-dimethyl-6H,12H-5,11-methanodibenzo[b, f][1,5]diazocine (Tr€oger’s base, 98%) were purchased from Aldrich. Tetraethoxysilane (g99%) was from Merck. The SBA-15 sample was prepared according to the method previously reported77 and featured the 100, 110, and 200 reflexions typical of ordered hexagonal (p6mm) symmetry as shown in the Supporting Information, S1. 2.2. Characterization Methods. 2.2.1. Chemical Composition. C%, N%, and H% contents were determined with a Carlo Erba 1106 elemental analyzer. Thermogravimetric and differential thermal analyses (TGA-DTA) where conducted in an air stream with a Metler Toledo TGA/SDTA 851E analyzer. The samples were heated under an air stream from 50 to 800 °C with a heating rate of 10 °C/min NMR spectra were recorded with a Bruker AV300 spectrometer (Lamor frequencies of 75.4, 300.0, and 60 MHz for 13C, 1 H, and 29Si, respectively) for liquids and a Bruker AV400 spectrometer (Lamor frequencies of 79.5 or 100.0 MHz, using 7 or 4 mm MAS probes spinning at 5 or 10 kHz rate) for 29Si and 13 C solid-state MAS
NMR measurements, respectively. The 13 C CP MAS spectra were obtained using 3 ms contact time and 3 s recycling delay. The O-pulse 13C MAS spectra were 40 5 μs corresponding to flip the magnetization angle to 60° with 20 s recycling delay. The 29Si CP MAS NMR pulse duration was 3 ms and 5 s recycling delay. The O-pulse 29Si MAS NMR spectra was 4 μs to flip the magnetization angle to 65° and with 180 s recycling delay. The number of scans was in the range 1000
3000 for 29Si OP-MAS spectra and of 2000
4000 of 13 C CP-MAS spectra. Chemical shifts are referenced with TMS for 29Si, 13C, and 1H. 2.2.2. Textural Characterization. Nitrogen adsorption isotherms where measured at 77 K with a Micrometrics ASAP 2010 volumetric adsorption analyzer. Prior to measurement, the samples were degassed for 12 h at 250 °C for pure silica materials and at 150 °C for the organosilicas. The surface area (SBET) was determined from BET treatment in the range 0.04
0.3 p/p0, and

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assuming a surface coverage of nitrogen molecule 16.2 Å2.78 The mesopore volume Vp was obtained from the nitrogen volume adsorbed when the capillary condensation is ended. Micropore volume was estimated using the t-plot method considering the extrapolated sorbed nitrogen volume before the mesopore filling versus the statistical film thickness for t f 0. When the sample presented micropores, the corrected surface area corresponding to the mesopore and external surfaces was determined from the t-plot slope. The pore size average was determined either by BdB method from assuming a cylindrical model79 or from the pore size distribution (PSD) dV/d log(D) versus log(D) where D was evaluated through the improved KJS method adapted to large mesopores (to see comment in the Supporting Information).80 XRD analysis was carried out with a Philips X’PERT diffractometer equipped with a proportional detector and a secondary graphite monochromator. Data were collected stepwise over the 2° e 2θ e 40° angular region, with steps of 0.02° 2θ, 20s/step accumulation time and Cu KR (λ = 1.54178 Å) radiation. Transmission electron microscopy (TEM) micrographs were obtained with a JEOL 1200X electron microscope operating at 120 keV. The samples were prepared directly by dispersing the powders onto carbon copper grids. 2.3. Synthesis of the Bis-Silylated Precursors. Di-iodo6H,12H-5,11-methanodibenzo[b,f][1,5] Diazocine (1)81. All of the syntheses were carried out in the absence of light. First, 4-iodo-aniline (4.68 g, 21.37 mmol) and then paraformaldehyde (1.45 g, 43.34 mmol) were added with vigorous stirring to trifluoroacetic acid (100 mL) at
15 °C. The resulting mixture was stirred for 20 min at this temperature and then stirred at room temperature for 48 h. The mixture was slowly added on a mixture of NH3 (25% solution in water, 450 mL) in ice at 0 °C. The resulting mixture was extracted with CH2Cl2 (3  200 mL), dried with MgSO4, and concentrated under vacuum. The residue was purified by column chromatography (CH2Cl2/AcOEt, 95:5 to 70:30) to obtain the product as a coloress crystals mp 181 °C (2.96 g, 6.24 mmol; 58.4% yield). 1 H NMR (300 MHz, CDCl3): δ (ppm) = 4.10 (d, J = 16.85 Hz, 2H: H6,12 endo), 4.27 (s, 2H: H13), 4.64 (d, J = 16.85 Hz, 2H: H6,12 exo), 6.90 (d, J = 8.47 Hz, 2H: H4,10), 7.26 (d, J = 1.96 Hz, 2H: H1,7), 7.48 (dd, J = 8.47 and 1.96 Hz, 2H: H3,9). 13 C NMR (100 MHz, CDCl3): δ (ppm) = 58.07 (C6,12), 66.51 (C13), 87.79 (C2,8), 127.01 (C4,10), 130.05 (C6a,12a), 135.75 (C1,7)*, 136.51 (C3,9)*, 147.36 (C4a,10a). * These assignments may be reversed. ESI-MS: Calculated for C15H12I2N2: 474.0781, found 475.00 (100 [MH]þ) (()-2,8-Bis(triethoxysilyl)-6H,12H-5,11-methanodibenzo[b, f][1,5] Diazocine (2)82. All of the syntheses were carried out in the absence of light. A solution of triethylamine (1.75 mL, 23.7 mmol) in anhydrous DMF (20 mL) was added to a mixture of [Rh(CH3CN)2(cod)] BF4 (40 mg, 0.105 mmol) and (()-2,8diiodo-6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine (1 g, 2.11 mmol) under argon. The mixture was stirring 30 min at room temperature and then triethoxysilane (1.54 mL, 15.81 mmol) was added dropwise, and the mixture was heated at 80 °C and stirred for 7 h. The reaction mixture was concentrated under vacuum and then diluted with Et2O (2  100 mL). The solution was filtered through Celite and then through Charcoal. After solvent evaporation under vacuum, the residue correspond to pure (()-2,8-bis(triethoxysilyl)-6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine. (1.012 g ; 1.85 mmol; 87.7% yield. Crystal mp 181 °C). 7575

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The Journal of Physical Chemistry C H NMR (300 MHz, CDCl3): δ (ppm) = 1.25 (t, J = 7 Hz, 18H: CH3
CH2
O), 3.88 (q, J = 7 Hz, 18H: CH3
CH2
O), 4.25 (d, J = 16.6 Hz, 2H: H6,12 endo), 4.39 (s, 2H: H13), 4.76 (d, J = 16.6 Hz, 2H: H6,12 exo), 7.20 (d, J = 7.95 Hz, 2H: H4,10), 7.26 (d, J ∼1.9 Hz, ∼2H: H1,7), 7.50 (dd, J = 7.95 and 1.94 Hz, 2H: H3,9). 13 C NMR (100 MHz, CDCl3): δ (ppm) = 18.21 (CH3
CH2
O), 59.09 (CH3
CH2
O), 58.35 (C6,12), 66.61 (C13), 124.49 (C4,10), 126.70 (C2,8), 127.06 (C6a,12a), 133.82 (C1,7)*, 133.90 (C3,9)*, 149.40 (C4a,10a). * These assignments may be reversed. 29 Si NMR (79 MHz,CDCl3): δ (ppm) =
58.2 Anal. Calcd for C27 H42 O6 N2 Si2: C%, 59.34; N%, 5.13. Found: C%, 59.92 ; N%, 4.14. Theorical N/C molar ratio: 0.741 ; found: 0.736. 2.4. Synthesis of the Mesoporous Organosilicas. 2.4.1. General Synthesis of the Material SBA-15 Grafted with 2 (SBA15TB-G)83. A silica SBA-15 sample (0.3 g) was pretreated during 14 h under vacuum at 150 °C and then cooled at room temperature. Anhydrous toluene (30 mL) and grafting agent as TB precursor (2.5 molecule/nm2) were added in argon atmosphere and stirred during 30 min at room temperature. Thereafter 33.5 mg of water, 0.5 mg of NH4F and 2.6 mg of p-toluene sulfonic acid monohydrate (2: 0.015: 0.015 eq/silane mole) were added under argon. The suspension was stirred 1 h at room temperature, 6 h at 60 °C and 1 h at 120 °C (Dean stark). The powder was collected by filtration and washed successively with toluene, methanol, an aqueous solution of sodium hydrogenocarbonate (5%), methanol/water (1/1), methanol, and diethyl ether and dried for 2 h at 60 °C. 2.4.2. General Synthesis of the Material from 2 with Surfactant Amines as Structural Directing Agents (Si-TB-X
A)76. The molar composition of the reaction mixture was 1SiO2 [(1
x)TEOS þ xSi
TB]:0.25 hexadecylamine:2.05 iPrOH:6.85 EtOH:22.2 H2O. After dissolution of bis-silylated precursor 2 in isopropanol followed by addition of TEOS in ethanol solution, an aqueous solution of hexadecylamine was added slowly to this mixture under vigorous stirring and the reaction mixture was stirred at RT during 24 h. Then, the gel was aged for 24 h at room temperature, filtrated and dried overnight at 60 °C. A fine powder was obtained. The extraction of the surfactant from the powdered solid was achieved with ethanol in Soxhlet apparatus at 90 °C during 24 h. A fine powder was obtained. The recovered sample weights after extraction are listed in the Supporting Information (Table S2). In the next discussion part, the samples as-synthesized are cited as Si-TB
X
A(as) and after the surfactant removing as Si-TB-X
A. 2.4.3. General Synthesis of the Hybrid Materials from 2, in Absence of Structural Directing Agents, Catalyzed by NH4F (SiTB-X
F):75. The molar composition of the reaction mixture was 1SiO2 [(1
x)TMOS þ xSi-TB]:4MeOH:4H2O:2.35  10
4NH4F where xSi
TB is the molar fraction of Si provided by the bis-silylated Troger’s base, (()-2,8-bis(triethoxysilyl)6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine as bridged silsesquioxane. Gelification of the silica precursors by hydrolysis
condensation was carried out at room temperature under vigorous stirring in a glass beaker by slowly addition of a water solution of NH4F. After the addition of the first drops of the aqueous solution of NH4F, a gel formation appears and the stirring is maintained until the total gel formation. Then, the gel was aged for 24 h at 36 °C and finally dried one night at 60 °C and one night at 150 °C. A fine powder was obtained which was subsequently washed with ethanol and with ether. The recovered 1

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Table 1. Molar Compositions of the Synthetic Mixture silica precursor sample

additive content

(1
x)a T.B (x/2) NH4F (10
4 mol) porogen amine

SBA15-TB-G

SBA-15

0.136b

Si-TB-15A(as)

TEOS

0.075

Si-TB-50A(as)

TEOS

0.25

Si-TB-10F

TMOS

0.05

2.35

Si-TB-30F

TMOS

0.15

2.35

Si-TB-50F

TMOS

0.25

2.35

40.5 0.25 0.25

The content of silica precursor was done by the relation (1
x) with x/ 2 the molar fraction of bis-silylated TB. b Values calculated for one mole of final SiO2, and corresponding to 5/2 molecules of bis-silylated TB per nm2 of SBA-15 silica surface. a

sample weights are listed in the Supporting Information (Table S2). 13 C CP MAS NMR (100 MHz): δ (ppm) = 150.0, 133.4, 127.1, 100.2, 83.8, 77.4, 66.2, 58.6, 34.5, 28.1 29 Si NMR (79 MHz): δ (ppm) =
61.4,
70.6,
78.2,
101.8,
109.9,
133.9 In Table 1 the chemical compositions of the synthesis slurries used to obtain the hybrid porous materials studied are shown. 2.5. Catalytic Test. A mixture of benzaldehyde (400 μL, 3.8 mmol), malononitrile (85.8 mg, 1.3 mmol), and the catalyst (1.7% or 0.5 molar) was stirred in a 2 mL glass batch reactor at room temperature under nitrogen. The products were analyzed by GC and GC-MS equipped with an HT-5 column (30 m  0.025 mm  0.025um) and a FID detector. Injector T = 250 °C, detector T = 300 °C. External standard: dodecane in solution. GC response coefficient determined for malonitrile and benzylidene malonitrile in ethyl acetate solution (concentration range: 1.076
3.118 mol 3 L
1) R = 99.44. For catalyst recycling studies, the catalyst was directly reused, without separation and washing, with an additional injection of identical amounts of the malononitrile and benzaldehyde consumed during the previous experiment.

3. RESULTS AND DISCUSSION 3.1. Synthesis of the Bis-Silylated Precursor. An original bis-silylated Tr€oger-base precursor was first synthesized, for preparing mesoporous organosilicas with the TB units uniformly distributed in the silica framework by a co-condensation approach, in the presence or not of structure directing agents. Since the high degree of molecular rigidity of the TB substructure may favor the formation of porous solids with best defined architecture, it was useful to link two trialkoxysilane functions directly to each two aromatic rings. In this regard, the direct route chosen to synthezise the (()-2,8-bis(triethoxysilyl)-6H,12H-5,11methanodibenzo[b,f][1,5] diazocine (2) was the rhodium(I)catalyzed silylation of 2,8-diodo-6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine (1) by trialkoxysilane in the presence of triethylamine (Scheme 2). The preferred TB 2,8-dihalogeno derivative used as starting material was the di-iodo compound84 compared to the more accessible dibromo one.85 Indeed, previous work on rhodium(I)catalyzed cross-coupling reaction of arylhalide and trialkoxysilane has demonstrated the better adequacy of iodide to afford the catalytic cycle, i.e., the silyl addition to give a transient 7576

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Scheme 2. Synthesis of the Di-Iodo Tr€ oger Base Derivative (1) and Di-Silylated Precursor (2) from Iodoanilinea

a

(i) (CH2O)n, THF,
15 °C to room temperature 48 h, NH3 aq.; (ii) [Rh(CH3CN)2(cod)] BF4, Et3N, (EtO)3SiH, DMF anhydrous 7 h, 80 °C.

silyl(hydrico)rhodium(III) intermediate, then the silyl-rhodium(I) by reductive elimination with the aid of triethylamine, followed by the oxidative addition of aryl halide leading to an silylarylrhodium(III)halide that provides the desired product by subsequent reductive elimination. The role of iodide would be to prevent the undesirable reductive elimination of halosilane from the last rhodium(III) halide intermediate in the reaction pathway.86 The synthesis of the 2,8-diodo-6H,12H-5,11methanodibenzo[b,f][1,5] diazocine (1) has already been reported in the literature for the preparation of a variety of functional derivatives87 except for the preparation of the 2,8bis(trialkoxy) compound (2). Actually, the 2,8-di-iodo TB derivative preparation was previously achieved either by condensation of 4-iodoaniline with formaldehyde.88 or by di-iodation of the methanodibenzodiazocine using iodosyl chloride in presence of mercury ditriflate.89 In this work, we have optimized the synthesis of the di-iodo-TB derivative according to the former preparation with slight modifications to the method reported by W€arnmark et al., using 20 mmol scale of iodoaniline reagent and 2 equivalents of formaldehyde leading to 58.4% yield after CC purification. The previously reported yield at 1 mmol scale was of 41
64% and only 37% at 40 mmol scale. Our present result confirms the previous remarks on the high sensitivity of the Tr€oger’s base condensation formation to the reactions conditions, mainly to its scaling.88 The chemical shifts of the 1H and 13 C NMR signals of the (()-2,8-diodo6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine (1) spectra are similar to those reported by W€arnmark et al.81 The complete 13C NMR signal identification, particularly the discrimination between the (C4,10 ) and the quaternary (C6a,12a) nucleus was made using dept program and taking into account the previous accurate spectral assignments of other Tr€ oger’s bases derivatives90 (see the Supporting Information, Figure S3). The conversion of the di-iodo-TB into the bis-(triethoxysilyl)TB compound was based on previous work reported in the literature aiming to subsequently prepare PMOs materials from disilylated functional aromatic moieties. For instance, Shimada et al. successfully prepared 3,6-bis(triethoxysily) carbazole by rhodium-catalyzed disilylation of the 3,6-diodo derivative with triethoxysilane, since the reaction failed when using the 3,6dibromo compound.82 The same strategy was adopted by Hesemann et al. to successfully prepare bis-silylated diarylimidazolium species as precursor of PMOs material containing bisaryl-imidazolium.67 This method was adopted here and 2,8bis-(triethoxysilyl)6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine (2) was obtained in high yields (∼88%; see the Supporting Information, Figure S4). 3.2. Synthesis of the TB-Organomesoporous Materials and Characterization. The incorporation of Tr€ oger-base fragments in the structure of different mesoporous materials from the respective bridged silsesquioxane starting precursor (2), was

achieved through three synthesis routes which are shown in Scheme 3. More specifically, grafted SBA-15 materials with Tr€oger-base units pending from the walls to free channels were first synthesized by postsynthesis modifications (SBA15-TB-G). Then a sol
gel route catalyzed by NH4F as mineralizing agent in absence of structural directing agents, and a micellar self-assembling process in presence of surfactants were employed to obtain disordered (Si-TB-F) and ordered HMS-type (Si-TB-A) mesoporous materials, respectively. Both types of hybrid materials should contain Tr€oger-base fragments as organic builders inserted into the silica walls. These two last methodologies aim to prepare organic
inorganic materials under mild conditions, practically at room temperature and under nonaggressive pH. The resultant hybrids possibly feature different hierarchical structuration which may exhibit either more flexible and disordered porous networks, or organized porous channels exhibiting the typical distribution of conventional mesoporous materials despite the rigidity from the organic builders. The soft synthesis conditions employed were selected to preserve the basic nature of the Tr€oger-base fragments. Because of the particular instability of the Tr€oger’s base in acidic conditions, neutral long chains amines as structure-directing agents were used for the micellar self-assembly route, instead of neutral triblock copolymers, such as P123, that requires strong acidic conditions during the synthesis. The X-ray diffractogram of the grafted material (not shown here), i.e., SBA-15-TB-G, presents a strong reduction in the intensity of the low angle bands due to the contrast loss occurring after the immobilization of the Tr€oger-base fragments onto the surface. On the contrary, the diffraction patterns of the materials containing Tr€oger-base fragments prepared in presence of surfactant (Si-TB-A(as)) present a low angle band (33.2 Å) which is indicative of the long order achieved for these type of materials obtained by self-assembling routes. The XRD pattern obtained after removing the occluded surfactant (Si-TB-A) is similar to the as-synthesized sample, indicating that the surfactant extraction does not modify the organization and structuration of the hybrids. Finally, the XRD of the Si-TB-F sample obtained in fluoride medium does not show the presence of longrange order in the material as should occur for an orderedless hybrid material (Supporting Information, Figure S5). The elemental composition of the hybrid materials has been determined by elemental and thermogravimetrical analyses, and 29 Si MAS NMR spectroscopy. The data are reported in Table 2. The full elemental analyses of the materials are given in the Supporting Information (Table S6.) The set of data shows a good agreement between them, but a discrepancy should be pointed out for the higher values of TB loadings given by means of thermogravimetric analyses than by other analytical techniques, particularly by 29Si MAS NMR. In all cases, the experimental N/C molar ratio obtained by elemental analysis is close to the theoretical N/C molar ratio in the Tr€oger-base units (N/C = 0.13), confirming that the organic fragments are preserved 7577

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Scheme 3. Synthesis of the Hybrid Materials Containing Tr€ oger-Base Units

Table 2. Chemical Composition of the Hybrid Materials elemental analysis sample

molar N/C exp

a

organic contentb TB

Thermogravimetric analysis

solid NMR T/TþQ

mmol/ghybrid (TB mmol/gSiO2)

TB mmol/ghybrid (TB mmol/gSiO2)

(TB mmol/gSiO2)

c

SBA15-TB-G

0.10

0.62 (0.8)

1.08 (1.35)

Si-TB-15A

0.10

0.74 (0.95)

1.54 (2.0)

0.1 (0.83)

Si-TB-50A

0.14

1.75 (3.1)

2.33 (4.15)

0.30 (2.5)

Si-TB-10F Si-TB-30F

0.12 0.13

0.53 (0.65) 0.94 (1.25)

0.80 (1.05) 1.31 (1.75)

0.06 (0.50) 0.13 (1.1)

Si-TB-50F

0.12

1.74 (3.0)

2.17 (3.7)

0.32 (2.7)

0.08 (0.6)

a

Theoretical molar N/C ratio 0.13. b Determined by elemental analysis (N% and C%) considering dry sample (or pure silica content). c Corresponding to 0.93 molecules per nm2.

during the synthesis processes, regardless of the preparation method (Table 2). In the ordered samples (Si-TB-A), after removing the surfactant molecules, the N/C molar ratio is also near to the theoretical value, indicating the effectiveness of the alcohol extraction step. Moreover, the presence of carbon and nitrogen atoms in the composition of the solids synthesized in absence of structural directing agents (Si-TB-F), is an evident probe of the effective incorporation of Tr€oger-base fragments. Regarding the grafting postsynthesis method, the organic moiety loading on the SBA-15 silica is lower than for the other methods (Table 2). Indeed, the grafted sample (SBA15-TB-G) prepared using similar amounts of silyl-Tr€oger-base precursor than during the sol
gel and micellar routes (see experimental conditions), contains a markedly lower amount of the organic moiety (0.62 mmol/g hybrid for SBA15-TB-G versus 1.7 mmol/ g hybrid for Si-TB-50-F and Si-TB-50-A), which confirms the limited effectiveness of the anchoring route to incorporate TB moieties onto the porous surface. In general, it is noteworthy that the organic content in the final hybrid materials is higher for higher contents of the silyl-Tr€oger-base precursor in the synthesis mixture, up to ∼2.33 mmol of organic moieties per gram of solid prepared in presence of surfactants. With the same amount of disilane precursor, both the micellar route and the sol
gel

synthesis catalyzed by NH4F, show a similar effectiveness for incorporating the organic TB linker in the final hybrid materials. The thermogravimetric and differential thermal curves (TGA and DTA) for samples Si-TB-50F, Si-TB-50A(as), Si-TB-50A, and SBA15-TB-G are shown in the Supporting Information (Figure S7). From the TGA curves, the weight loss that corresponds to the decomposition of the organic (org. wt %) has been used to calculate the organic molar content, nTB, per gram of pure silica or nTB versus gram of hybrid according to the method indicated in the Supporting Information S8. This evidence the presence of Tr€oger-base fragments in the resulting hybrid materials, being up to 4 mmol of organic linkers per gram of pure silica for the highest loaded samples. From DTA curves, one can see that the decomposition of organic components in the grafted sample starts at 300 °C, probably due to the presence of residual alkoxysilane groups as will be shown later by 13C MAS NMR analysis. For the ordered and nonordered samples containing the Tr€oger-base builders obtained by one-pot synthesis method, one can see that the thermal decomposition occurs at temperatures in the range 400
600 °C. More specifically, the DTA curve of the ordered HMS-type sample obtained with surfactants, before the acid extraction step (see for Si-TB-50A(as)), exhibits a higher organic 7578

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Table 3. Textural Characteristics of the Tr€ oger’s Base (TB) Hybrid Materials surface area (m2 g
1)

pore diameter (Å)

SBET

[Smesop þ Sext]

total Vp

Vμpa

DBdBb

DKJSc

SBA-15-TB-G

488

397

0.59

0.043

66

81

Pristine SBA-15

870

747

1.28

0.056

86

92

Si-TB-15-A

981

804

1.16

0.053

120

127

Si-TB-50-A

703

466

0.55

0.107

50

83

Si-TB-10-F

872

819

0.55

0.020

36

39

Si-TB-30-F

871

736

0.54

0.064

40

38

Si-TB-50-F

816

633

0.85

0.092

104

122

sample

a

porous volume Vp (cm3 g
1)

Microporous volume. b Determined by the BdB method.79 c Determined by the KJS method.80

Figure 1. N2 adsorption
desorption isotherms of pristine SBA-15 and grafted SBA-15 with bis-silylated Tr€oger-base precursor (SBA15-TB-G and SBA15-TB-G*). This latter derived from the former by standardization of adsorbed nitrogen volume versus pure silica weight.

content loss at 400 °C due to presence of structural directing agents inside the porous channels. After removing the surfactant, the DTA (see for Si-TB-50A) can only be associated to the presence of the Tr€oger-base fragments. 3.3. Textural Properties. The organic content of the hybrid materials and the synthesis methodology employed have an important effect on the textural properties of the final materials. Table 3 reports the surface area, the pore volume, and the mean pore diameter of the hybrid samples. The isotherms of SBA-15 and SBA15-TB-G materials obtained by grafting precursor 2 on the surface of SBA-15 are shown in Figure 1. Even though the BET surface area, pore volume, and pore diameter are lower than for the pristine silica (Table 3), the patterns of the two isotherms are similar. However, the decrease in mesopore volume (standardized versus pure silica) and pore diameter in the grafted SBA-15 with respect to the pristine SBA-15 clearly indicate that the TB precursor has been grafted in the intrapore surface, leading to a strong space reduction within the pore of the original SBA-15 material. The results obtained also show that ordered HMS-type and nonordered hybrid samples prepared with higher content of organic builders show a reduction in the BET surface area, being this reduction more notorious for the materials prepared by selfassembling process whose pore volume is also drastically

Figure 2. N2 adsorption
desorption isotherms of hybrid materials containing Tr€oger-base builders prepared through fluoride sol
gel and micellar routes.

reduced. In contrast, the surface area is very high (800
1000 m2 g
1) for lower contents of the Tr€oger-base, as in samples Si-TB10-F and Si-TB-15-A (Table 3). Interestingly for samples prepared through the NH4F catalyzed sol
gel route, the surface area is always maintained around 800 m2 g
1, being samples Si-TB-50-F and Si-TB-50-A considered as hybrid materials which feature the best compromise between high content of organic builders present in the framework, high surface area (∼700
800 m2 g
1) and pore volume (0.55
0.85 mL g
1; see Table 3 and Figure 2). It is important to point out that, in all cases, the organic
inorganic hybrid materials prepared from silyl-Tr€oger-base precursors exhibit a notable mesoporous volume, as can be deduced from N2 adsorption isotherms (Figure 2) with some microporous contribution determined by t-plot analysis (Table 3). Moreover, this microporous volume is higher for higher TB loading. More specifically, the hybrid samples obtained by the sol
gel or micellar routes show characteristic isotherms of mesoporous materials with hysteresis loops. Nevertheless, the materials obtained in absence of surfactants (Si-TB-50-F) show a marked shift toward higher relative pressures in the inflection point than the hybrids prepared by the self-assembling process with the surfactants (Si-TB-50-A), indicating that the pore diameter is larger (122 Å) for the more flexible and disordered than for the templated hybrid porous materials (83 Å). It can be observed that the mean pore diameter of the flexible samples obtained in 7579

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Figure 3. Solid state 13C CP-MAS NMR spectrum of porous hybrid organosilicas: (a) SBA-15-TB-G, (b) Si-TB-50A, and (c) Si-TB-50F. Liquid 13C NMR spectrum of 2,8-bis(triethoxysilyl)-6H,12H-5,11methanodibenzo[b,f][1,5] diazocine, 2, is shown at the bottom. (* side bands, 1: CDCl3 solvent, 2: DMF solvent and 3: CH3 of residual ethoxysilane).

one-pot sol
gel method catalyzed by NH4F is larger when the amount of Tr€oger-base builders is higher. This is in contrast to the micellar process that leads to a strong porosity reduction when the organic loading is higher (Table 3 and Figures 2, S9, and S10). Regarding the pattern of the desorption branch of isotherm of Si-TB-50-A sample featuring the steep capillary evaporation step at the lower limit of adsorption
desorption hysteresis loop, the evaluation of the pore diameter using the KJS method using the adsorption branch appeared more appropriate than the BdB one (Supporting Information, S10). Transmission electron microscopy (TEM) was used to visualize the morphology of the different hybrid materials. In the case of solids prepared by SDA free sol
gel route (Si-TB-50-F), the micrographs (Figure S11) indicated the presence of disordered free cavities and channels which exhibit a mean pore diameter higher than 80 Å, similar to value calculated from N2 adsorption
desorption isotherms (Table 3). Meanwhile, and as could be expected, the organization observed in the hybrids prepared by self-assembling methods (Si-TB-15-A) is higher. However, when the content of organic linkers incorporated is higher an important long-order organization loss is observed with the ordered hybrid materials (Si-TB-50A). 3.4. NMR Characterization. The 13C CPMAS NMR spectra of samples Si-TB-50A and Si-TB-50F obtained via hydrolysispoly condensation, either in presence or absence of long alkyl chain amine, respectively, and via grafting procedure are shown in Figure 3. The spectra of all porous hybrid organosilicas clearly exhibit peaks at 127 and 133 ppm characteristic of the carbon in the aromatic substructure and signals of both benzylic C6,12 nucleus at 58 ppm and methane C13 nucleus at 66 ppm ascribed to the bicyclo unit of TB. Indeed, these signal positions are in good agreement with those of the bis-silylated precursor 2 obtained in liquid 13C NMR

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Figure 4. 29Si NMR spectra of hybrid porous materials and assignment of T- and Q-type silicon nucleus: (a) SBA15-TB-G, (b) Si-TB-50A, and (c) Si-TB-50F. Liquid 29Si NMR spectrum of 2,8-bis(triethoxysilyl)6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine, 2, is shown at the bottom.

(Figure 3, bottom). This observation evidence the preservation of the structure integrity of Troger’s base molecule during the various experimental conditions used for the preparation of the solid samples. In the case of the grafted SBA-15 silica, the spectrum exhibits signal at 15.5 and an additional signal contribution at 57.3 ppm relevant to ethoxysilyl groups. These groups would correspond to incomplete hydrolysis of the silylating agent 2 during the grafting procedure. Actually, the presence of such residual ethoxy groups should provide an explanation for the highest value of TB loading found by TGA compare to other analysis methods (Table 2). The 29Si one pulse MAS spectra of the various organic
inorganic samples (Figure 4) show signals located at approximately
110,
101, and
93 ppm, characteristic of Q4, Q3, and Q2 silicon sites of the SiO4 substructures in the silica framework. Additional peaks observed around
67 and
59 ppm indicate the presence of the T3 and T2 assigned to the TB-Si bridged subunits.91 These results corroborate that the organic bridges not only remain intact but are also incorporated covalently onto the porous surface or into the network of the solids. In the case of the grafted sample (SBA15-TB-G), it is appreciable the reduced intensity of the T-bands observed in the spectrum (Figure 4a), this being consistent with the low content of Tr€oger-base units versus silica framework (Q-bands; Table 2). Additionally, the spectra of all samples synthesized in this study were modeled using the Dmfit program92 (see the Supporting Information S12). Hence, integration of the modeled ΣTn peaks versus ΣQn þ ΣTn gives a valuable estimation of the tethering Si loadings, and each Qn/ΣQn with n = 2
4 and each Tn/ ΣTn with n = 1
3, provides information on the silane-silanol condensation degree. Taking this account, it was observed that samples containing a low content of Troger’s base, Si-TB-10F, SiTB-30F and Si-TB 15A, present higher condensation degree for both the T-silicon and Q-silicon nucleus, with similar T3:T2:T1 7580

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Scheme 4. Knoevenagel Condensation of Benzaldehyde and Malononitrile

Figure 5. Kinetic curves of malononitrile conversion in presence of benzaldehyde. (a) Catalyzed by homogeneous Troger’s base: (a-1) 1.7% molar TB (blue full line) and without TB (green full line); (a-2) in inset 0.5% (blue dotted line) and 1.7% (full bue line) molar TB. (b) Catalyzed by heterogeneous catalysts containing Tr€oger’s base units: (b-1) using Si-TB-50F (black full line), Si-TB-50A (red full line), Si-TB-G (green full line) with 1.7% molar TB site and without TB (blue full line) in solvent-free conditions at RT temperature; the inset (b-2) shows the kinetic curves of the corresponding solid with 0.5% molar TB site (dotted lines) during the first reaction times.

and Q3:Q2:Q1 ratios whatever the type of one-pot polycondensation used, i.e., 1:0.5
0.7:0.05. In the case of the samples Si-TB50F and Si-TB-50A, the condensation degree Q-silicon nucleus is comparable to that of the sample containing lower TB contents (with a ratio 1:0.65:0.1 for the Si-TB-50F and 1:0.76:0.17 for the Si-TB-50A). In contrast, the condensation degree of the T silicon nucleus differs significantly as a function of the synthesis method employed, the ratio being 1:1.3:0.2 for the nonordered materials and 1:0.76:0.2 for the ordered self-assembled solids. These results confirm the conclusions withdrawn from the nitrogen sorption analyses that indicate that the higher the loading, the more difficult is to achieve framework organization. On the other hand, from 29Si NMR analysis, it is possible to calculate the organic content integrated into the walls of the hybrid materials, giving directly the loading of the truly anchoring C
Si bond versus silica framework. In this way, the values of TB loading obtained considering NMR data (Table 2) and the equation TB loading (mol/gSiO2) = (T/T þ Q)/2 MSiO2, are always lower than those obtained with the other analysis methods (elemental and thermogravimetrical analyses). This would result from partial Si
C bond cleavage that was only observed for phenylene-bridged PMOs prepared under strongly basic conditions.93 (see comment added in the Supporting Information, S13). Another additional confirmation of the effective incorporation of organic TB linkers into the hybrid materials comes from 29Si NMR of the starting bis-silylated precursor, 2, shown in the bottom of Figure 4. The pure disilane precursor exhibits only one peak characteristic of silicon atoms centered at
59 ppm. After the covalent insertion of TB groups into the structure of

mesoporous materials, the signal assigned to silicon atoms bonded to carbon units is shifted from
60 to
80 ppm (Figure 4), something which corroborates the integration of organic linkers within the framework. In this work, we have pointed out the possible Troger’s base units tethered by two anchoring points onto nanostructured SBA-15-type framework as demonstrated by 29Si NMR spectroscopy. Thanks to the postgrafting process, the organic moieties are compelled to be located at the silica surface, the linking silicon atoms owing to the silylating agent being bound to the surface siloxy groups only. Nevertheless, the organic loading versus silica weight is limited at the maximum surface monolayered lining, provided a selective horizontal surface sol
gel polymerization process. Moreover, this methology is unable to avoid the possible supramolecular association of the Troger’s base units during the grafting processing operating in apolar media. Hence, we have successfully prepared PMOS-like materials containing tailor-distributed Troger’s base unit with variable loading into the framework in the range 5
25% versus molar silicon, using sol
gel autoassembly polycondensation in presence of long-chain amines as structuring agents. Nevertheless, the resulting catalytic materials could suffer from relative high framework rigidity due to the notable wall thickness related to its ordered structure. Then we thought that an organocatalyst
supported material with a flexible framework possessing high surface area and large pore volume together with high accessible catalytic sites, may be an even better candidate than grafted or ordered PMOs for catalytic applications. At the end, structural flexibility is a part of the advantages of enzymes during 7581

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Table 4. Catalytic Activity of the Synthesized Materials for the Heterogeneous Knoevenagel Reactiona conversion %b entry

catalyst

1h

2h

8 h yield %c TON 2 hd TOFe (min
1)

1

TB

52.5 73.7 100

75.8

42.5

2.62

2

Si-TB-50-F

40.5 71.6

99.9

99.9

41.4

0.79

3 4

Si-TB-50-A 38.4 60.5 SBA-15-TB-G 27.6 43.3

99.8 91.0

99.0 99.1

35.1 24.9

0.67 0.40

a

Reaction conditions: benzaldehyde (2.4 mmol), malononitrile (1.3 mmol), catalyst (1.7% molar) at room temperature under nitrogen. b Determined by GC. c Total yield after all the catalyst reusing. d The turnover number (TON) = number of malononitrile mole consumed versus mole of catalyst. e Turnover frequency (TOF) in min
1.

catalytic processes.74,75 In the next part of the work, the catalytic performance of the selected samples SBA15-TB-G, Si-TB-50A, and Si-TB-50F for a Knoevenagel condensation model reaction will be used to test the impact of their intrinsic characteristics of the hybrid materials on their final catalytic behavior. 3.5. Catalytic Applications of the Hybrid Materials Based € ger’s Base for Knoevenagel Condensation Reacon the Tro tion. In order to control the accessibility and activity of the Tr€oger’s base molecules that are located at the porous surface of the grafted material, or which are part of the walls in the one-potprepared materials, the catalytic performance of the organomesoporous silica-containing TB moieties here prepared were investigated as base heterogeneous catalysts. First, 2,8-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5] diazocine (Tr€oger’s base) was tested as homogeneous catalyst, showing for the first time up to now the catalytic activity of the Tr€oger’s base for the reaction of malononitrile with benzaldehyde (Scheme 4 and Figure 5a). As there has been a growing interest in developing supported organocatalysts for cleaner organic transformation processing, we first investigated the solvent-free Knoevenagel reaction catalyzed by homogeneous TB in such conditions that will later allow the comparison with the heterogenïzed TB system in terms of catalytic site concentration, malonotrile:benzaldehyde ratio, and recycling conditions. Figure 5a and Tables 4, entry 1, show the results obtained using benzaldehyde (2.4 mmol), malononitrile (1.3 mmol), 1.7% molar homogeneous TB at room temperature under nitrogen. Figure 5a shows that the catalytic activity of TB is significant when the malononitrile conversion is compared to that obtained without TB (blank run). Moreover, the pattern of the kinetic curves obtained with 1.7 and 0.5% molar TB clearly show that the reaction runs with a first order versus TB amount. Three organomesoporous hybrids containing TB have been used as heterogeneous catalyst, the samples Si-TB-50-F and SiTB-50-A which have been considered as featuring the best compromise between high content of catalytic organic sites into the network and best textural properties and the TB-grafted SBA15 for comparison (Figure 5b). First, the experimental kinetic constants obtained with two catalysts samples containing different amount of active sites (amount of base) using the two different catalytic site, i.e., 1.7% and 0.5%, indicates also a first reaction order with respect to the catalyst (Figure 6).

Figure 6. Apparent rate constant as a function of the catalytic site amount for linked Troger’s base using Si-TB-50F (black), Si-TB-50A (red), and Si-TBG (green) with 0.5 and 1.7% molar TB site in solventfree conditions at RT temperature.

Second, the two organomesoporous hybrid materials (ordered and nonordered) containing TB fragments integrated into the framework demonstrates slightly lower catalytic activity than the homogeneous TB (Table 4, entries 2 and 3, and Figure 5). On the other hand, among the solid catalysts, Si-TB-50F gives some higher activity than the ordered Si-TB-50A, while the grafted sample gives a sensibly lower activity (Table 4 and Figure 5). The some higher activity of Si-TB-50F compared to Si-TB50A may be related to the higher surface area of the former sample, favoring a more efficient TB configuration by a lower molecular density limiting some possible undesirable supramolecular association. This catalytic behavior may also be influenced by the slightly lower T-nucleus condensation and higher flexibility at structural level featured by the former sample. Definitively, in the sample Si-TB-50-F, the appropriate combination of high accessibility and structural flexibility due to lower degree of condensation explain the higher catalytic activity exhibited for the nonordered hybrid materials synthesized by one-pot sol
gel method. Moreover, in this last sample the closer proximity of silanol groups and the amino TB fragments should increase the rate of reaction due to the cooperative electrophilic activation.94 It is notheworthy that the catalytic performance of the one-pot samples (Table 4, entries 2 and 3, and Figure 5) are better than that obtained with the TB grafted-material SBA-15-TB (Table 4, entry 4, and Figure 5). The fact that this latter features the best structured and rigid framework would be in line with the beneficial effect of the structural flexibilility of the solid frameworks featured mainly by the material prepared through the fluoride anion-catalytic method (Figure 5). Moreover, the sol
gel one-pot methods performed in polar mixture would turn down the possible undesirable supramolecular association of the silylated TB-precursor which is in contrast favored during the lining process carried out in nonpolar solvent. Another important aspect of green chemical process concerns the stability of the catalyst during recycling runs. This is certainly an important advantage with respect to homogeneous catalysts. Figure 7 presents the catalytic performance of each investigated samples obtained during six cycles. 7582

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’ ASSOCIATED CONTENT

bS

Figure 7. Recyclability of the catalysts for the Knoevenagel reaction between benzaldehyde and malononitrile using 1.7% molar TB site.

Importantly, these results indicate that the catalytic activity of the heterogeneized TB remained unchanged throughout the reuses, confirming the stability of the functional hybrid porous materials described here.

4. CONCLUSION Tr€oger-base units have been covalently incorporated into mesoporous organosilicas framework for the first time. The preparation of these nanostructured hybrid materials were successfully achieved thanks to the prior synthesis of the (()-2,8-bis(triethoxysilyl)-6H,12H-5,11-methanodibenzo-[b,f][1,5]-diazocine bis-silylated precursor from the 2,8-diodo derivative. The incorporation of this organic builder into the structure was done through different synthetic processes from the bridged silylated precursor through (a) postsynthesis grafting method, (b) soft-templating route using long alkyl chain amine as surfactant, and (c) by sol
gel transformation catalyzed by fluoride anion in absence of structural directing agents at neutral pH and low synthesis temperatures. The materials prepared by the different approaches featured different textural properties, topologies, characteristics, and organization levels of the hybrid materials in function of the organic building block content and the synthesis method employed. The accessibility and the activity to the organic TB builders was verified by investigation of their performance as basic catalytic site in Knoevenagel reaction compared to that obtained in homogeneous conditions. The disordered hybrid materials obtained through the fluoride-assisted sol
gel method show the best performance which may be due to the flexibility exhibited by the organic
inorganic framework, which minimizes and make up for the conventional rigidity of TB builders. This fact is also favored by the lower degree condensation showed by the disordered mesoporous hybrid materials and the closer proximity of the silanol and amino groups which increase the rate of reaction due the cooperative assistance. All the hybrid mesoporous organosilica materials containing Troger’s base show a remarkable stability as catalyst during their recycles. This work highlights the advantage of the hybrids prepared from poly silylated building-blocks in terms of stability and versatility, particularly when the active organic builders are integrated into the flexible porous networks.

Supporting Information. S1 reports the synthesis of SBA-15 pure silica and its XRD pattern. Tables S2 and Table S6 depict the composition of the reaction mixture for sample preparation and their crude elemental analysis data, respectively. Figures S3 and S4 show the NMR spectra of the diazocine (1) and (2) derivatives. Figure S5 shows the XRD of the samples SiTB-50A(as), Si-TB-50A and Si-TB-50F. Figure S7 presents the TGA analyses of hybrid materials and S8 presents the formula to determine the TB loading from TGA analyses. Figure S9 shows the nitrogen adsorption
desorption isotherms of the hybrid materials and Figure S10 present the KJS pore size distributions. Figure S11 shows TEM micrographs of the materials Si-TB-50-F, -50-A, and -15-A. Figure S12 shows 29Si one-pulse MAS NMR spectra and results from signals decompositions of all the samples. S13 is a speculative explanation of the under-evaluation of the TB loading content by 29SiMAS NMR. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: þ34 96 3877809. Tel: þ34 96 3877800.

’ ACKNOWLEDGMENT We are thankful financial support by Consolider- Ingenio 2010 (MULTICAT project). E.M. thanks Ministerio de Ciencia y Innovacion by the support through Juan de la Cierva contract. The authors are indebted to Dr. Alejandro Vidal-Moya for technical assistance and fruitful discussion on 13C and 29Si MAS NMR spectroscopies. ’ REFERENCES (1) Knoevenagel, E. Ber. Deuts. Chem. Ges. 1896, 29, 172–174; ibid. 1898, 31, 730–737; ibid. 1898, 31, 738–748; ibid. 1898, 31, 2585–2595; ibid. 1898, 31, 2596–2619. (2) Cope, A. C. J. Am. Chem. Soc. 1937, 59, 2327–2330. (3) Charles, G. Bull. Soc. Chim. Fr. 1963, 8
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