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Predesigned Metal Anchored Building Block for In Situ Generation of Pd Nanoparticles in Porous Covalent Organic Framework: Application in Heterogeneous Tandem Catalysis Mohitosh Bhadra, Himadri Sekhar Sasmal, Arghya Basu, Siba Prasad Midya, Sharath Kandambeth, Pradip Pachfule, Ekambaram Balaraman, and Rahul Banerjee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02355 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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ACS Applied Materials & Interfaces

Predesigned Metal Anchored Building Block for In Situ Generation of Pd Nanoparticles in Porous Covalent Organic Framework: Application in Heterogeneous Tandem Catalysis Mohitosh Bhadraa,b ,Himadri Sekhar Sasmala,b, Arghya Basua, Siba P. Midyab,c, Sharath Kandambetha,b, Pradip Pachfulea, Ekambaram Balaramanb,c* and Rahul Banerjeea,b* a

Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha

Road, Pune-411008, India b

c

Academy of Scientific and Innovative Research (AcSIR), New Delhi, India

CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India

KEYWORDS: covalent organic frameworks, in situ synthesis, Pd nanoparticles, tandem catalysis, benzofuran.

ABSTRACT

The development of nanoparticle-polymer hybrid based heterogeneous catalysts with high reactivity and good recyclability is highly desired for their applications in chemical and

ACS Paragon Plus Environment

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pharmaceutical industries. Herein, we have developed a novel synthetic strategy by choosing predesigned metal anchored building block for in situ generation of metal (Pd) nanoparticles in the stable, porous and crystalline covalent organic framework (COF), without using conventional reducing agents. In situ generation of Pd nanoparticles in COF skeleton is explicitly confirmed from PXRD, XPS, TEM images, and

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N NMR spectral analysis. This hybrid material is found

to be an excellent reusable heterogeneous catalyst for the synthesis of biologically and pharmaceutically important 2-substituted benzofurans from 2-bromophenols and terminal alkynes via tandem process with TON up to 1101. The heterogeneity of the catalytic process is unambiguously verified by mercury poisoning experiment and leaching test. This hybrid material shows superior catalytic performance compared to commercially available homogeneous as well as heterogeneous Pd catalysts.

Introduction The key research objective of the chemical and pharmaceutical industries is to convert homogeneous catalytic reactions into heterogeneous versions through the attachment of catalytic sites on stable supports. Heterogeneous catalysts offer many advantages, over the homogeneous ones which include high recyclability, easy recovery from the reaction mixture and their use in continuous flow processes.1-6 There are different approaches reported in the literature for converting homogeneous catalyst into reusable heterogeneous version (Figure 1a-c).7-9 Furthermore, metal nanoparticles are the excellent candidates for heterogeneous catalysis not only due to their unique properties such as high surface area and narrow size distribution but also due to their vast application in catalytic transformations.10-12 Although metal nanoparticles have already been used to catalyze conventional cross-coupling, oxidation and reduction reactions in


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solution, the synthesis of such metal nanoparticles based heterogeneous catalyst is highly demanding and challenging for keeping the nanoparticles non-aggregated and mono-dispersed during the decoration onto the matrix support.13-18 Among the various catalytic processes, tandem catalysis is a greener process with high atom economy, in which sequential transformation of the substrate occurs via two (or more) mechanistically distinct processes. This process not only minimizes the by-products but also reduces the workup and purification steps.1929

Therefore, it is highly desirable to develop new heterogeneous catalytic systems capable of

performing versatile synthetic transformations via tandem processes. Covalent organic frameworks (COFs) are crystalline porous polymers constructed by reversible linkage of organic building blocks in a periodic manner.30-39 COFs have already been widely used for different applications like gas storage40-42, sensing43-44, opto-electronics45-46 and catalysis.47-55 The availability of the coordination sites for uniform nucleation of nanoparticles within the internal micro environment, along with chemical tunability, high porosity and ordered structural integrities, make COFs as ideal supports for anchoring various catalytically active metal nanoparticles.51-52 However, the poor stability of the COFs is a usual barrier to their applications, especially in the field of catalysis. We could overcome these stability issues by simply introducing an irreversible keto-enol tautomerization within the COF skeleton.37-39 Although, there have been few reports of COF-metal nanoparticle hybrids, in most of the cases metal nanoparticles loading have been done via ex situ processes where metal nanoparticles have been decorated on pre-synthesized COF-materials.52-55 These synthetic procedures have several disadvantages such as additional synthetic steps and usage of the strong reducing agent. The


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Figure 1. Various strategies are used to convert homogeneous catalyst into reusable heterogeneous version (a, b, c are reported approaches, and d is our approach). (e) Synthesis details of the in situ generation of highly dispersed Pd nanoparticles in the TpBpy skeleton. The size of Pd nanoparticles and the COF pore organization is not exactly based on scale to scale. (f) Comparison of PXRD patterns of simulated TpBpy (black) with experimental TpBpy (blue), Pd nanoparticles (cyan) and experimental Pd@TpBpy (red). (g) Schematic representation of tandem catalysis by Pd@TpBpy. usage of the external reducing agent changes the internal structure of COFs, which subsequently affect crystallinity and porosity of these materials. The loss of crystallinity and rigidity of these COF supports decrease the interaction between support and nanoparticles which ultimately results in the loss of catalytic activity due to leaching and sintering of nanoparticles during the repeated cycles. The simple alternative, to overcome this problem, is to load metal nanoparticles in the COF skeleton by in situ synthetic methods, which does not require any external reducing agents.


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Keeping this idea in perspective, herein, we have strategically developed an in situ process to synthesize Pd nanoparticles within a chemically stable COF (Pd@TpBpy) via a judicious choice of Pd metal anchored tailored building units. To the best of our knowledge, this is the first report of in situ generated COF–Pd nanoparticle hybrids and their utilization as a heterogeneous catalyst for the synthesis of biologically important 2-substituted benzofurans via tandem process.56-57 Such type of solution-phase carbon–carbon and carbon heteroatom bond forming reactivity by using single heterogeneous catalyst is rare in literature.8,58,59 Experimental Section Synthesis of 2,2′-bipyridine-5,5′-diamine palladium chloride (Bpy-PdCl2) In a round bottom flask, palladium chloride (177.3 mg, 1 mmol) was added in 30 mL acetonitrile under N2 atmosphere and allowed to stir the reaction mixture at 65 oC for 1h to get clear solution. Then the palladium chloride solution was added drop wise to the solution of 2,2'bipyridine-5,5'-diamine (Bpy) (186.2 mg, 1 mmol) in 20 mL acetonitrile solution under N2 atmosphere. During the addition, the reddish coloured precipitate of 2,2'-bipyridine-5,5'-diaminepalladium chloride (Bpy-PdCl2) complex was found, and for complete conversion the reaction mixture was stirred 5h at 65 oC. The solid precipitate was isolated from reaction mixture and washed with excess acetonitrile solution to get pure 2,2'-bipyridine-5,5'-diamine palladium chloride (Bpy-PdCl2) product, with 86% yield. The pure material (Bpy-PdCl2) was crystallized in DMSO solvent and the crystals were solvent exchanged with acetonitrile for 2 days. The PXRD pattern of 2,2'-bipyridine-5,5'-diamine-palladium chloride (Bpy-PdCl2) solvent exchanged crystals are perfectly matching with simulated one.


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Synthesis of Pd@TpBpy COF The Pd@TpBpy was synthesized by reacting 1,3,5-triformylphloroglucinol (Tp) (21 mg, 0.1 mmol) with 2,2′-bipyridine-5,5′-diamine palladium chloride (Bpy-PdCl2) (54.5 mg, 0.15 mmol) in presence of N,N-dimethylacetamide (DMAc) and 1,4-dioxane solvent combination (2.3 : 0.7 mL) with 0.3 mL of 6M aqueous acetic acid using solvothermal process. The reactants were ultrasonicated for 15 min to disperse homogeneously as well as degassed through three successive freeze-pump-thaw cycles. Then the tube was sealed in vacuum condition and heated at 90 oC in isothermal oven for 3 days. Finally, the phase pure material was filtered out and washed with DMAc and DMSO solvents to remove the residues of the starting materials. The collected reddish black material was solvent exchanged with DMSO and washed with excess acetone. Then the powder material was dried at 110 oC for 12 h under vacuum in order to get as synthesized Pd@TpBpy COF (88% yield). General Procedure for the Tandem Catalysis Phenyl acetylene (0.6 mmol, 66 µL), 2-bromophenol (0.5 mmol, 58 µL), potassium carbonate (34.5 mg, 0.25 mmol) and Pd@TpBpy (2 mg, 0.6 mol%) were added to 1 mL dry DMF under nitrogen atmosphere in a screw cap reaction tube. Then the reaction mixture was allowed to stir at 150 oC for 20h. After the completion of the reaction (monitored by TLC), the reaction mixture was centrifuged to separate the solid and the solid was washed with DCM. The combined organic layer was washed with water to remove potassium carbonate and the organic layer was evaporated under reduced pressure. The crude product was purified by column chromatography to obtain the desired product. Results and Discussion


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Pd@TpBpy was synthesized by reacting 1,3,5- triformylphloroglucinol (Tp) and 2,2′bipyridine-5,5′-diamine palladium chloride (Bpy-PdCl2) in a solvothermal process (Figure 1e, Section S2, S3). The Pd nanoparticles are formed during the COF formation reaction via breakage of Pd–N bond. It is important to mention that the formation of Pd nanoparticles during the COF synthesis does not hamper the overall crystallinity and structural integrity of isolated COF. The intense peak in the PXRD patterns at 3.6 (2θ) corresponding to the reflection from (100) plane show high crystallinity of Pd@TpBpy, whereas the minor peaks arise at 7.2, and 26.9 (2θ) are attributed to (200) and (002) reflections, respectively. The peak at 2θ = 26.9 corresponding to (002) reflection indicates the presence of π-π stacking between the 2D COF layers. Since, the experimental PXRD pattern of Pd@TpBpy matches well with eclipsed model of the simulated COF structure (Figure 1f), we have proposed a structure close to the hexagonal P63 space group (a ꞊ b ꞊ 27.5 Å, c ꞊ 6.8 Å, α ꞊ β ꞊ 90°, γ ꞊ 120°) with the d-spacing of 3.4 Å between the COF layers (Figure S5, Table S2). The additional peaks at higher (2θ) values of 40.2, 46.7, 68.1, 82.1 and 86.6 correspond to the reflections from (111), (200), (220), (311) and (222) planes of Pd (JCPDS#46-1043). In the FTIR spectra, the presence of intense peaks at 1606 (–C=O), 1566 (–C=C) and 1243 cm-1 (–C–N) indicate the keto-enol tautomerised structure of Pd@TpBpy, which is similar to the TpBpy (Figure 2a, S6). The 13C CP-MAS solid state NMR of Pd@TpBpy shows the carbonyl (–C=O) carbon signal at 184.4 ppm. The rest of the sp2 carbons appear together in the range of 108 to 161 ppm (Figure 2b). In order to assess the chemical environments of the nitrogen atoms present in the as-synthesized materials, we have performed the

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N CP-MAS solid state NMR of Pd@TpBPy, TpBpy39 and Bpy-PdCl2

complex, using NH4Cl as an internal standard (Figure 2c). In the case of Bpy-PdCl2, the signal at 67.6 ppm corresponds to the Pd metal coordinated pyridinic nitrogen. In contrast, the signal


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Figure 2. Characterization details of Pd@TpBpy and TpBpy. (a) FTIR spectra of TpBpy (blue) and Pd@TpBpy (red). (b) Comparison of solid state

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C NMR spectra of Bpy-PdCl2 (black),

TpBpy (blue) and Pd@TpBpy (red). (c) Comparison of solid state

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N NMR spectra of Bpy-

PdCl2 (black), TpBpy (blue) and Pd@TpBpy (red). (d) N2 adsorption isotherms of Pd@TpBpy (red) and TpBpy (blue). (e) XPS spectra of Pd in Pd@TpBpy (black) and Bpy-PdCl2 (red). (f) TGA spectra of Pd@TpBpy (red) and TpBpy (blue). corresponding to pyridinic nitrogen of Pd@TpBpy appears at 127.1 ppm, which is well comparable to the signals of pyridinic nitrogens of TpBpy (128.5 ppm), suggesting the pyridinic nitrogens of Pd@TpBpy are not strongly coordinated to the Pd metal centers as like in the BpyPdCl2 complex.60-61 However, the significant up field shift (1.4 ppm) of the pyridinic nitrogens in case of Pd@TpBpy compared to TpBpy suggests that the Pd nanoparticles in Pd@TpBpy are stabilized by the weak nonbonding interactions with the pyridinic nitrogens of the bipyridinic


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Figure 3. Characterization details of Pd@TpBpy. (a,b) SEM images of Pd@TpBpy showing flake-like morphology. (c,d) TEM images of Pd@TpBpy showing a uniform distribution of Pd nanoparticles. (e) HAADF-STEM image of Pd@TpBpy. (f,g) TEM images of Pd@TpBpy showing the lattice fringes of Pd nanoparticles in Pd@TpBpy. (h) Size distribution histogram of Pd nanoparticles in Pd@TpBpy. unit. This result also confirms our initial speculation that during the course of Pd@TpBpy formation, the breakage of Pd–N bond of Bpy-PdCl2 complex takes place, which leads to the in situ synthesis of Pd nanoparticles in Pd@TpBpy hybrid. Pd@TpBpy and TpBpy show type IV reversible N2 adsorption isotherms suggesting the dominant presence of mesopores. The Brunauer–Emmett–Teller (BET) surface areas of Pd@TpBpy and TpBpy are 653 and 1462 m2g-1, respectively (Figure 2d). The pore size distribution of both these COFs are measured on the basis of NLDFT (non-local density functional theory) show the narrow pore size distribution in the range of 1.4 nm and 2.3 nm for Pd@TpBpy and 2.3 nm for TpBpy (Figure S7). TGA of Pd@TpBpy and TpBpy show high


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thermal stability up to 300 and 350 C respectively (Figure 2f). SEM images of Pd@TpBpy indicate a flake-like morphology (Figure 3(a,b), S8a). TEM images clearly show that the Pd nanoparticles are uniformly distributed over the COF matrix (Figure 3(c,d), S8b). The average size of the Pd nanoparticles is found to be 12±4 nm (Figure 3h). Since the Pd nanoparticles are bigger than the pore size of the COF, these nanoparticles distribute around the surface and the interlayer spacing of the COF, which results in the decrease in surface area of Pd@TpBpy with respect to TpBpy. The presence of both high and low-contrast Pd nanoparticles in TEM images clearly suggest good distribution of the Pd nanoparticles in both the surface and interlayer spacing of the COF framework (Figure 3e, S8b). The lattice fringes of the Pd nanoparticles are measured and found to be 0.221 nm from TEM image (Figure 3(f,g)). In order to confirm the oxidation state of Pd, we have carried out detail XPS analysis of both Bpy-PdCl2 complex and Pd@TpBpy COF. In Bpy-PdCl2 complex, the presence of characteristic binding energy peaks at 337 and 342.5 eV of Pd 3d5/2 and Pd 3d3/2 are corresponding to Pd(II) species. Whereas, Pd 3d5/2 and Pd 3d3/2 peaks arising at 335 and 340.2 eV unambiguously confirm the formation of Pd(0) species in Pd@TpBpy (Figure 2e). The elements present in the material are confirmed from the EDX analysis (Figure S9). Whereas, the exact amount of Pd metal present in Pd@TpBpy is quantitatively analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-OES) and the quantity of the Pd metal in the matrix is found up to 15.2 wt%. Benzo[b]furan derivatives are ubiquitous in natural products and exhibit various biological activities such as antimicrobial, antiparasitic, antihyperglycemic, antitumor and so on. 62-66 As a result, there is considerable interest in the construction of 2-arylbenzofuran and its derivatives. Given the stability, high porosity, and crystalline nature of the novel nanostructured catalyst (Pd@TpBpy) with Pd-nanoparticles in the COF skeleton, we set out to evaluate its performance


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in the tandem process (Figure 1g) for the effective synthesis of 2-arylbenzofurans under additive/co-catalyst-free conditions. Table 1. Optimization table for tandem catalysis.a

Entry

Pd Catalyst

Base

Solvent

Yield (%)b

1

Pd@TpBpy

K2CO3

DMF

70

2

Pd@TpBpy

-

DMF

-

3

-

K2CO3

DMF

-

4

Pd/C(10 wt%)

K2CO3

DMF

23

5

PdCl2

K2CO3

DMF

3

6

Bpy-PdCl2

K2CO3

DMF

6

7

TpBpy

K2CO3

DMF

4

a

All the reactions were conducted with 0.5 mmol of 2-bromophenol (1 equiv), 0.6 mmol of phenyl acetylene(1.2 equiv), 0.6 mol% of Pd@TpBpy catalyst, K2CO3 (0.5 equiv) in N,Ndimethylformamide (heated at 150 oC). bBased on GC analysis using m-Xylene as an internal standard. In order to optimize the reaction conditions, 2-bromophenol and phenyl acetylene have been chosen as benchmark substrates. By using catalytic amount of Pd@TpBpy under our optimized conditions 70% yield of the desired product, 2-arylbenzofuran was obtained. The necessity of each of the key reaction components (Pd-catalyst, base, temperature and solvent) was demonstrated through a series of control experiments (Table 1, Table S3-S7). The superiority of


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the catalyst was confirmed by performing control reactions using PdCl2, Bpy-PdCl2 and TpBpy materials as catalysts. It was found that PdCl2, Bpy-PdCl2 and TpBpy are ineffective and yielded only trace amounts of the desired product (Table 1, entries 5-7). Notably, under optimized condition commercially available palladium on carbon, a heterogeneous catalyst, have Table 2. Evaluation of the substrate scope.a,b

a

All the reaction were conducted with 0.5 mmol of 2-bromophenols (1 equiv), 0.6 mmol of alkynes (1.2 equiv), 0.6 mol% of Pd@TpBPy catalyst, K2CO3 (0.5 equiv) in DMF. bIsolated yields. cUsing 5 mmol of 3-bromo-4-hydroxybenzonitril. dBased on GC analysis.


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not shown good activity and gave the expected product in low yield (23% yield) (Table 1, entries 4). With an optimized catalytic system in hand (Table 1), we set out to probe its versatility in the tandem catalysis of various substituted 2-arylbenzofurans. As shown in Table 2, the present catalytic system is compatible with various 2-bromo phenols and alkynes containing an electronrich and electron-deﬁcient substituent and indeed, has a broad substrate scope as well as functional group tolerance. Initially, the scope of 2-bromophenol was demonstrated. Thus, the electron-withdrawing groups such as. –CN (3a), –CHO (3b), –CO2Me (3e) and –NO2 (3d) gave 37% to 77% yields of the desired products. In the case of methyl (3h), a decrease in the yield (26%) of the desired product was observed. Furthermore, different halogen substituents such as – F (3c, 3g), –Cl (3f) and extended conjugated systems like naphthalene (3i) also gave the desired products in 32% to 60% yields (Table 2). Next, we have evaluated the scope of alkyne coupling partner. Thus, electron-donating groups i.e. –CH3 (3l), –OMe (3o), –N(Me)2 (3m), –tBu (3j) and electron-withdrawing groups such as, –F (3k and 3n), –CN (3p) and –COCH3 (3q) gave the corresponding 2-arylbenzofuran with 32% to 79% yields (Table 2). Also, aliphatic substituent like cyclopropyl (3r) and silicon substituent –Si(iPr)3 (3s) yielded 80% and 43% of the corresponding products, respectively. The heterogeneity of the Pd@TpBpy catalyst has been confirmed by typical mercury poisoning experiment and leaching test (Section S13). Further, the Pd@TpBpy catalyst has been recycled (using 3-bromo-4-hydroxybenzonitrile and phenyl acetylene as a coupling partners) for five cycles (Section S14). This can be attributed to the stable and less sintering nature of Pd@TpBpy throughout the catalytic cycles (Section S16, Figure S13). The small decrease in the catalytic activity of Pd@TpBpy in the recyclability test is due to the slight agglomeration of few


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catalytically active Pd nanoparticles during the catalytic process. The superior performance of Pd@TpBpy catalyst with a broad scope of reactants, high catalytic activity with no leaching and extraordinary recyclability confirmed the utility of the Pd@TpBpy catalyst for the tandem reaction. Conclusion In summary, we have established a unique strategy for in situ loading of metal (Pd) nanoparticles in the chemically stable, porous, crystalline COF (Pd@TpBpy), without using a conventional reducing agent. To the best of our knowledge, this is the first report of in situ synthesis of Pd nanoparticles in COF by using Pd anchored building unit for COF synthesis. In situ loading of Pd nanoparticles in COF skeleton is explicitly confirmed from several experimental techniques such as PXRD, XPS, TEM images, and 15N NMR spectral analysis. The stable and crystalline nature of the Pd@TpBpy makes it an ideal hybrid material for the heterogeneous catalyst, for the synthesis of 2-substituted benzofuran derivatives via tandem process with the turnover number up to 1101 (Section S15). The non-sintering behavior and good recyclability of the present catalytic process confirm the robustness and the practical applicability of Pd@TpBpy as a sustainable catalyst. We believe that our developed method for in situ synthesis of metal nanoparticles in the COF will definitely open up a new domain of exciting research area in the field of heterogeneous catalysis.

ASSOCIATED CONTENT Supporting Information. Full experimental procedures, characterization data, NMR spectra and crystallographic data (CIF) are available as a separate file (PDF).


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AUTHOR INFORMATION Corresponding Authors *[email protected] * [email protected] ORCID Rahul Banerjee: 0000-0002-3547-4746 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT M.B., H.S.S., S.P.M., and S.K. acknowledge CSIR India. A.B. acknowledges SERB Young Scientist Scheme (YSS/201S/000606). R.B. acknowledges CSIR (CSC0122 and CSC0102) and DST

(SB/S1/IC-32/2013,

INT/SIN/P-05,

SR/NM/NS-1179/2012G)

for

funding.

E.B.

acknowledges SERB (SB/FT/CS-065/2013). We acknowledge Dr. T. G. Ajithkumar for the NMR facility, Dr. C. S. Gopinath for XPS and Mr. Sandipan Jana for GC-MS data analysis. REFERENCES 1. Benaglia, M.; Puglisi, A.; Cozzi, F. Polymer-Supported Organic Catalysts. Chem. Rev. 2003, 103, 3401–3430.


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2. Ikegami, S.; Hamamoto, H. Novel Recycling System for Organic Synthesis via Designer Polymer-Gel Catalysts. Chem. Rev. 2009, 109, 583–593. 3. Cole-Hamilton, D. J. Homogeneous Catalysis—New Approaches to Catalyst Separation, Recovery, and Recycling. Science 2003, 299, 1702-1706. 4. Mennecke, K.; Cecilia, R.; Glasnov, T. N.; Gruhl, S.; Vogt, C.; Feldhoff, A.; Vargas, M. A. L.; Kappe, C. O.; Kunz, U.; Kirschning, A. Palladium(0) Nanoparticles on Glass-Polymer Composite Materials as Recyclable Catalysts: A Comparison Study on their Use in Batch and Continuous Flow Processes. Adv. Synth. Catal. 2008, 350, 717–730. 5. Huang, W.; Liu, J. H.-C.; Alayoglu, P.; Li, Y.; Witham, C. A.; Tsung, C.-K.; Toste, F. D.; Somorjai, G. A. Highly Active Heterogeneous Palladium Nanoparticle Catalysts for Homogeneous Electrophilic Reactions in Solution and the Utilization of a Continuous Flow Reactor. J. Am. Chem. Soc. 2010, 132, 16771–16773. 6. Moschetta, E. G.; Negretti, S.; Chepiga, K. M.; Brunelli, N. A.; Labreche, Y.; Feng, Y.; Rezaei, F.; Lively, R. P.; Koros, W. J.; Davies, H.M. L.; Jones, C. W. Composite Polymer/Oxide Hollow Fiber Contactors: Versatile and Scalable Flow Reactors for Heterogeneous Catalytic Reactions in Organic Synthesis. Angew. Chem., Int. Ed. 2015, 54, 6470–6474. 7. Jiang, Y.; Gao, Q. Heterogeneous Hydrogenation Catalyses over Recyclable Pd(0) Nanoparticle Catalysts Stabilized by PAMAM-SBA-15 Organic−Inorganic Hybrid Composites. J. Am. Chem. Soc. 2006, 128, 716–717.


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