Active Carbon Functionalized with Chelating Phosphine Groups for

Mar 7, 2008 - Active Carbon Functionalized with Chelating Phosphine Groups for the Grafting of Model Ru and Pd Coordination Compounds. Christopher ...
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J. Phys. Chem. C 2008, 112, 5533-5541

5533

Active Carbon Functionalized with Chelating Phosphine Groups for the Grafting of Model Ru and Pd Coordination Compounds Christopher Willocq, Sophie Hermans, and Michel Devillers* Unite´ de Chimie des Mate´ riaux Inorganiques et Organiques, UniVersite´ catholique de LouVain, Place Louis Pasteur 1/3, B-1348 LouVain-la-NeuVe, Belgium ReceiVed: August 24, 2007; In Final Form: January 23, 2008

An active carbon support has been functionalized in order to introduce at its surface chelating phosphine groups. This has been done in several steps, which have each been studied in detail and optimized in turn. The first step consisted of increasing the number of oxygenated surface groups by oxidative treatment with HNO3. The second step involved the coupling of an amine with the surface carboxylic groups by formation of an amide bond. Various coupling agents were studied, of which SOCl2 was found to be the most efficient. Fluorinated and brominated amines were used as model amines (easier surface quantification by XPS thanks to the presence of heteroatoms such as F or Br), before reacting ethylenediamine. The pending arm of the diamine could then be further transformed into the desired bidentate phosphine in the last step. The success of the procedure was confirmed, and proof for actual surface chemical reactions was obtained. Two coordination compounds, Pd(dba)2 and Ru3(CO)12, were then incorporated on the starting carbon support and on the functionalized one. It was found that the presence of chelating phosphine groups at the surface of the functionalized support allowed to increase the yield of incorporation and the metallic dispersion at the surface, probably via a ligand exchange mechanism. Nanometer-sized Pd and Ru particles were evidenced by TEM.

1. Introduction The surface of a solid is its interface with the medium in which it is placed, hence it defines its properties. In order to tune the properties of a given material, one commonly used strategy is to modify the surface in a second step, the first step being the material’s preparation. In the field of heterogeneous catalysis, when the solid is used as support for an active phase, surface modification usually precedes the incorporation of the active phase, playing thus a double role: (i) modifying the support’s physicochemical characteristics and (ii) allowing a better interaction with the active phase precursors. Quite a few studies dealing with the functionalization of a support, grafting of precursors, activation and then utilization as heterogeneous catalyst have appeared. However, detailed characterization at each step of the preparation and gradual building up of the surface structures at the molecular level is not common. Contributions to methodological aspects are highly welcomed. The present research has been carried out in this context, taking active carbon as support material and noble metals as potential catalytically active phase. Active carbon can be prepared by physical or chemical activation of raw materials such as coal, wood, peat, fruit stones, or coconut shells.1-3 A physical activation implies carbonization of the source material followed by an oxidative treatment, which increases the specific surface area by developing the pore system. The chemical activation produces a high surface area material in one step by the action of chemical agents. Because of the variety of raw materials, of their origins, and of preparation methods, the characteristics of the active carbon obtained vary a lot from batch to batch.1-4 Nevertheless, their * Corresponding author. E-mail: [email protected]. Fax: +3210-47 23 30.

surface is known to comprise heteroatoms such as O, N, or S in the form of functional groups, which remain from the raw material or are introduced during the activation step and which are responsible for their acido-basic character. These functions can be removed by thermal treatment4 or augmented by appropriate reagents. For example, oxygenated functions can be introduced by oxidation and quantified by the so-called Boehm’s titration method.4,5 However, the fast development of carbon surface chemistry is hindered by several major drawbacks: lack of reactivity, hydrophobicity, difficulty to characterize spectroscopically, and so forth. Hence, the examples of carbon functionalization are not very numerous, especially in the area of heterogeneous catalysis. Carbon-supported catalysts are usually prepared by impregnation, deposition-precipitation, ion-exchange, or incipient wetness, without really taking into account the chemistry involved at the surface during the incorporation of the metallic precursor(s). Nevertheless, carbonaceous materials are ideal supports in catalysis because they are widely available, robust, resistant to pressure, temperature, and moisture, and usually present very high specific surface areas.6 Apart from the introduction of oxygenated functions as mentioned above, the chemical functionalization of active carbon is not common, and it is only recently, with the emergence of nanoscopic forms of carbon, that a variety of functional groups have been investigated.7 Reactions of carbon nanotubes with a range of chemical agents (F, O3, Li, ...) gives different surface groups that can be used for a lot of applications. For example, the anchoring of metal species can lead to nanoparticles or nanowires, but the anchoring of chromophores, proteins, or other bio-molecules has also been tested.8-12 As far as we know, only

10.1021/jp076799j CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008

5534 J. Phys. Chem. C, Vol. 112, No. 14, 2008 one short report dealing with the modification of an active carbon in order to introduce phosphines at its surface has appeared.13 Very few cases of true covalent grafting of metallic species onto carbonaceous materials have been reported.14 Some recent studies by Silva et al. present successful carbon-grafted complexes, which can be chiral, such as manganese(III) salen species, for applications in (enantioselective) epoxidation, for example.15-18 Other researchers have attached covalently acetylacetonate complexes by using a carbon sample functionalized by HNO3.19 The obtained material has been used in liquid-phase oxidations and is very stable toward leaching. Here, we want to show that it is possible to functionalize an active carbon, in order to introduce at its surface chelating phosphine groups. These can then be used as anchors for the covalent grafting of metallic complexes, which are potential catalysts precursors. The chosen complexes are a Pd(0) species, Pd(dba)2, and a ruthenium cluster, Ru3(CO)12. The former was selected given the wide applicability of Pd in catalysis,20 and the latter was selected as a “model” polynuclear precursor, knowing that transition metal clusters are ideal precursors for the preparation of catalytically active supported nanoparticles.21-24 Moreover, the use of phosphines in order to immobilize metal clusters had already been investigated with some success on other supports.25,26 The phosphine-based anchoring sites were constructed in several steps, which have each been optimized in turn. The starting point of the functionalization implies surface oxidation in order to increase the number of carboxylic acid groups on the active carbon used. The second step involves a reaction with an amine and the formation of an amide bond. This has been studied in detail and various coupling agents were tested. The P coordination site is then created by derivatization of the pending arm. The final step is the actual grafting of metal complexes. The samples at each stage of the synthesis have been characterized by XPS, N2 physisorption, and/or TEM in order to study the influence of the experimental procedure used. 2. Experimental Section All manipulations were carried out without particular precautions unless otherwise stated. Nevertheless, when the reactions were carried out under nitrogen, Schlenk techniques were implemented, the solvents were distilled before use and stored under nitrogen on molecular sieves, and the obtained products were stored under Ar. All mentioned reactants were commercially available and used as received. Ethylenediamine (H2N(CH2)2NH2), paraformaldehyde ((CH2O)n), triruthenium dodecacarbonyl (Ru3(CO)12), bis(dibenzylideneacetone)palladium (Pd(dba)2), and 2-bromoethylamine hydrobromide (noted H2N-Br) were supplied by Sigma-Aldrich. N-hydroxysuccinimide (NHS), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDAC), 4-dimethylaminopyridine (DMAP), 3,5bis(trifluoromethyl)benzylamine (noted H2N-F6), thionyl chloride (SOCl2), and 4-morpholinoethanesulfonic acid hydrate (MES) were supplied by Acros Organics. Diphenylphosphine (HPPh2) was supplied by Fluka and active carbon (type SX+) was supplied by Norit. Finally, water was purified on a Milli-Q Ultrapure Water Purification System (Milli-Q water). 2.1. Oxidation of CSX+ to Give CCOOH. The starting support was an active carbon of the type SX PLUS (noted CSX+) from Norit. It was seaved in order to keep only the particles comprised between 50 and 100 µm, and it has a specific surface area of ∼900 m2/g. The number of acidic functions of this carbon support (acidity ) 32.0 mmol/100 g) was increased by an acid treatment with 2.5 mol/L HNO3 as described previously.27 In a

Willocq et al. typical experiment, 6 g of CSX+ were placed in a 250 mL trineck round-bottom flask with 150 mL HNO3 2.5 mol/L. The mixture was stirred for 24 h at 110 °C (reflux). Then, it was filtrated and extensively washed with distilled water. The resulting powder was washed on a Soxhlet apparatus for 24 h with water and dried at 70 °C under vacuum for 4 h. The acidity of the resulting CCOOH(a) support was generally around 325 mmol/100 g (determined by Boehm’s titration27). The starting CSX+ was also functionalized with HNO3 0.25 mol/L using the same procedure and, in this case, the obtained support CCOOH(b) presented an acidity of 155.2 mmol/100 g. In the following experiments, the calculated equivalents of reactants involved in the reactions are related to the acidity of the used support. 2.2. Addition of H2N-R on CCOOH to Give CNH2. 2.2.1. Modelization of the Grafting of H2N(CH2)2NH2 with a Fluorinated Amine (H2N-F6) and a Brominated Amine (H2N-Br). Method A. This method was inspired from a literature procedure8 where proteins were immobilized on carbon nanotubes. In a 100 mL round-bottom flask, 100 mg of CCOOH(b) (0.1552 mmol of acidic functions) were introduced together with 25 mL MES buffer solution at pH 6 (5.3 g of MES were dissolved in 50 mL Milli-Q water; 2 g of NaOH were dissolved in 50 mL Milli-Q water, and the NaOH solution was added to the MES one until pH ) 6). Then, 1 equiv NHS (17.9 mg; 0.1555 mmol) was added to the suspension and finally, under quick stirring, 1 equiv EDAC (29.7 mg; 0.1549 mmol) was also added. This mixture was stirred for 30 min and filtrated, and the obtained powder was extensively washed with MES and dried under vacuum at 80 °C for 14 h. The resulting powder was placed in a 100 mL round-bottom flask with 15 mL dichloromethane and 1 equiv H2N-F6 (37.7 mg; 0.1550 mmol). The mixture was stirred for 2 h and filtrated, and the obtained powder was extensively washed with dichloromethane and dried at 60 °C under vacuum for 4 h. Method B. This method was inspired from a literature procedure10 where carbon nanotubes have been functionalized in order to graft nanoparticles. In a 100 mL round-bottom flask, 100 mg of CCOOH(b) (0.1552 mmol of acidic functions) were introduced together with 20 mL of Milli-Q water. Then, 1 equiv EDAC (29.7 mg; 0.1549 mmol), 1 equiv DMAP (18.9 mg; 0.1547mmol), and 1 equiv H2N-Br (29.3 mg; 0.143mmol) were added successively. The mixture was stirred for 6 h and filtrated, and the obtained powder was extensively washed with Milli-Q water and dried at 70 °C under vacuum for 14 h. Method C. This method was also inspired from a literature procedure16 where an active carbon support was functionalized in order to graft copper(II) complexes. In a 100 mL roundbottom flask, 200 mg of CCOOH(b) (0.3104 mmol of acidic functions) was introduced with 9 mL of toluene. Then, 1 mL of SOCl2 was added. The mixture was refluxed (120 °C) for 5 h and filtrated, and the obtained powder was extensively washed with toluene and dried under vacuum at 120 °C for 14 h. Then, 100 mg of the resulting powder was placed in a 100 mL roundbottom flask with 10 mL of dichloromethane, and 1 equiv H2NF6 (37.7 mg; 0.1550 mmol) was added. The mixture was stirred overnight and filtrated, and the obtained powder was extensively washed with dichloromethane and dried at 60 °C under vacuum for 4 h. 2.2.2. Grafting of H2N(CH2)2NH2 on CCOOH to GiVe CNH2Nx. Method A. The procedure was similar to method A described in section 2.2.1, except for the nature of the oxidized carbon used, the amount of reactants, and the use of ethylenediamine. Here, 100 mg of CCOOH(a) (0.3259 mmol of acidic functions) were mixed in a round-bottom flask together with 1 equiv NHS

Phosphine-Functionalized Carbon Support

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5535

TABLE 1: Experimental Conditions Used to Prepare CClx Samples obtained samples

used support

use of SOCl2

CCl1

600 mg CSX+

no

CCl2

600 mg CSX+

yes (3 mL)

CCl3

600 mg CCOOH(a)

no

CCl4

600 mg CCOOH(a)

yes (3 mL)

CCl5

600 mg CCOOH(a)

yes (3 mL)

experimental conditions 30 mL toluene and 5h reflux 27 mL toluene and 5h reflux 30 mL toluene and 5h reflux 27 mL toluene and 5h reflux 27 mL toluene and 5h reflux under nitrogen

(37.5 mg; 0.3258 mmol) and 1 equiv EDAC (62.5 mg; 0.3260 mmol) in 25 mL MES buffer solution (pH 6) for 30 min. Then, the solution was filtrated and the obtained powder was washed with MES and dried. The resulting powder was mixed with 1 equiv ethylenediamine (0.03 mL; 0.449 mmol) in 15 mL MES solution (pH ) 6) for 1 h. The mixture was filtrated, and the obtained powder was extensively washed with MES and dried at 70 °C under vacuum for 4 h. Method B. The procedure was the same as described in section 2.2.1., starting from 100 mg CCOOH(a) (0.3259 mmol of acidic functions), 1 equiv EDAC (62.5 mg; 0.3260 mmol), 1 equiv DMAP (39.8 mg; 0.3258 mmol), and 1 equiv ethylenediamine (0.03 mL; 0.449 mmol). Method C. The procedure was similar to the method C described in section 2.2.1., starting from 250 mg of CCOOH(a) (0.8148 mmol of acidic functions) and 1 mL SOCl2 in 9 mL of toluene in the first step. In the second step, 100 mg of the obtained powder was refluxed (120 °C) with 1 equiv ethylenediamine (0.03 mL; 0.449 mmol) in 10 mL toluene for 4 h. Then, the mixture was filtrated, and the obtained powder was extensively washed with toluene and dried under vacuum at 120 °C for 14 h. 2.2.3. Proof for Chemical Grafting of H2N-F6 or H2N(CH2)2NH2 on CCOOH by Method C. In the first step, 600 mg of support (see Table 1) were refluxed (120 °C) in a 100 mL roundbottom flask with 0 or 3 mL SOCl2 in 30 or 27 mL of toluene, respectively, for 5 h. Then, the mixture was filtrated, and the resulting powder was extensively washed with toluene and dried under vacuum at 120 °C for 14 h to give CClx. The experimental conditions used to obtain the various samples noted CCl1 - CCl5 are given in Table 1. The CCl5 sample was prepared under nitrogen and dried at room temperature under vacuum. In the second step, 100 mg of CClx were introduced into a 100 mL round-bottom flask together with 66.0 mg H2N-F6 (0.2714 mmol) and 10 mL dichloromethane. The mixture was refluxed (50 °C) for 4 h. Then, it was filtrated, and the obtained powder was extensively washed with dichloromethane and dried under vacuum at 60 °C for 14 h to give the series of samples noted CNH2Fx. This experiment was also carried out with ethylenediamine (0.07 mL; 1.0471 mmol) to give the series of CNH2Nx samples. In this case, 400 mg of CClx and 20 mL toluene were used, and the samples were dried at 120 °C under vacuum. 2.3. Addition of HPPh2 onto CNH2 to Give CPPh2 Samples. This procedure was inspired from the literature28 where dendrimers were modified in order to coordinate metal compounds. Under nitrogen, in a 100 mL Schlenk flask, 15.7 mg CH2O (0.5230 mmol) and 0.09 mL HPPh2 (0.5201 mmol) were introduced together with 5 mL of methanol. The mixture was stirred at 70 °C for 10 min and was then cooled to room temperature. Concurrently, 100 mg of CNH2Nx were placed in a

TABLE 2: Constraints Used for XPS Data Treatment peaks considered

area

A

B

A/B

∆(B-A) (eV)

fwhm ratio

Ru 3d5/2 Pd 3d5/2

Ru 3d3/2 Pd 3d3/2

1.5 1.5

4.17 5.26

1 1

100 mL Schlenk flask with 5 mL of methanol. When the first mixture had reached room temperature, it was added to the CNH2 suspension and stirred for 30 min at room temperature. Then, 15 mL of toluene were added and the solution was stirred at 70 °C for 30 min before being stirred at room temperature overnight, to give the CPPh2-1 - CPPh2-5 samples starting from the CNH2N1 - CNH2N5 samples. Alternatively, the solution was stirred at 70 °C for 24 h, to give the CPPh2-6 - CPPh2-10 samples, starting from a new series of CNH2N1 - CNH2N5 samples. Finally, in all cases, the mixture was filtrated under nitrogen and the resulting powder was extensively washed with methanol and dried under vacuum at room temperature. 2.4. Grafting of Metal Complexes. Two model complexes were reacted with the unmodified support (CSX+) and the functionalized one (CPPh2 where P/CXPS ) 0.0181). These were Pd(dba)2 and Ru3(CO)12. The amount of Pd(dba)2 and Ru3(CO)12 incorporated corresponded to a theoretical metal loading of 10 wt % after ligands removal. Practically, 54 mg Pd(dba)2 (0.0939 mmol) or 21 mg Ru3(CO)12 (0.0328 mmol) were brought into contact with 90 mg of support (CSX+ or CPPh2) in a mixture of solvents (10 mL dichloromethane/10 mL toluene) and the suspensions were stirred for a period of 5 days under nitrogen in the dark at room temperature for Pd(dba)2 and at 50 °C for Ru3(CO)12. Then, the mixtures were filtrated under nitrogen and the obtained powders were extensively washed with dichloromethane and dried under vacuum at room temperature for several hours. 2.5. Physicochemical Methods of Characterization. Atomic absorption measurements were carried out on a Perkin Elmer atomic absorption spectrometer 3110. XPS (X-ray photoelectron spectroscopy) analyses were carried out at room temperature on a SSI-X-probe (SSX-100/206) photoelectron spectrometer from Surface Science Instruments (U.S.A.) equipped with a monochromatized microfocus Al X-ray source. Samples were stuck on small troughs with double-face adhesive tape and then placed on an insulating homemade ceramic carousel (Macor Switzerland). Charge effects were avoided by placing a nickel grid above the samples and using a flood gun set at 8 eV. The quantification was based on the C1s, O1s, N1s, Cl2p, P2p, Ru3d, and Pd3d photopeaks. The energy scale was calibrated with reference to the peak Au4f7/2 at 84 eV, and the binding energies were calculated with respect to the C-(C,H) component of the C1s peak fixed at 284.8 eV. Data treatment was performed with the CasaXPS program (Casa Software Ltd, UK). The peaks were decomposed into a sum of Gaussian/Lorentzian (85/15) after substraction of a Shirley type baseline. The constraints used are presented in Table 2. Given the superposition of the C1s and Ru3d3/2 peaks, these constraints were particularly important in order to quantify ruthenium. The following method was used: a Gaussian/Lorentzian (85/15) was placed at the position of the Ru3d5/2 peak, which is visible on the right-hand side (lower binding energy) of the C1s peak. The contribution of the Ru3d3/2 peak to subtract from the carbon component was calculated by reference to the Ru3d5/2 peak by placing another Gaussian/Lorentzian (85/15) at 4.17 eV toward higher binding energy, and imposing an area ratio Ru3d5/2/ Ru3d3/2 ) 1.5 and a fwhm ratio of unity. Because of this problem of overlap, the experimental error on the Ru surface

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SCHEME 1: Functionalization of CCOOH into CPPh2

TABLE 3: Results of the Increase in the Number of Acidic Functions for the CSX+ Support support

acid treatment

Boehm’s acidity (mmol/100 g)

XPS O/C atomic ratio

CSX+ CCOOH(a1) CCOOH(a2) CCOOH(b)

HNO3 2.5 mol/L HNO3 2.5 mol/L HNO3 0.25 mol/L

32.0 325.9 335.4 155.2

0.041 0.188 0.206 0.108

atomic concentration percentage is high, and therefore the Ru/C ratios have to be taken with caution. SEM (scanning electron microscopy) images were acquired on a FEG digital scanning microscope (DSM 982 Gemini from LEO) equipped with a EDXS detector (Phoenix CDU LEAP). The samples were fixed by double-sided conducting adhesive tape onto 5 mm diameter aluminum specimen stubs from Agar Scientific. TEM images were obtained with a LEO 922 OMEGA energy filter transmission electron microscope. The samples were suspended in hexane under ultrasonic treatment, then allowed to settle to discard the biggest particles. A drop of the supernatant was then deposited on a holey carbon film supported on a copper grid, which was dried overnight under vacuum at room temperature before analysis. The nitrogen physisorption experiments were carried out on a ASAP 2000 instrument from MICROMERITICS at 77 K. The sample (0.1 g) was degassed for several hours at 200 °C under a pressure of 5 µmHg before the analysis. 3. Results and Discussion The envisaged functionalization is illustrated in Scheme 1. The first step consists of increasing the number of acidic functions of the SX+ active carbon support as described in the literature.27 The second step deals with the incorporation of ethylenediamine on the support by the formation of an amide bond. To reach that goal, various coupling agents can be used, and, in our case, three different ones were tested. In the last step, phosphines can be grafted on the present terminal amines to lead to the formation of chelating phosphine groups at the surface of the support. These chelating phosphine groups are ideal anchor sites for noble metal based compounds. 3.1. Oxidation of CSX+ to Give CCOOH. The number of acidic functions at the surface of the starting SX+ carbon support was increased by an acid treatment with 0.25 or 2.5 mol/L HNO3 as described in the literature.27 The resulting CCOOH supports (CCOOH(a) obtained by treatment with HNO3 2.5 mol/L and CCOOH(b) obtained by treatment with HNO3 0.25 mol/L) were analyzed by XPS, and their acidity was determined by Boehm’s titration,27 as shown in Table 3 (note: two different batches of carbon treated with 2.5 mol/L HNO3, noted respectively as

CCOOH(a1) and CCOOH(a2), were used, giving in total three different oxidized carbon samples). It can be observed that when HNO3 2.5 mol/L was used, the acidity increased by a factor of 10, which is confirmed by the rise from 0.041 to 0.188-0.206 of the O/C surface atomic ratio obtained by XPS. When HNO3 0.25 mol/L was used, the acidity increased only by a factor of 5, and this is confirmed by an intermediate O/C surface ratio. The results of Boehm’s titration were used to calculate the amount of reactants to be involved in the next steps. 3.2. Addition of H2N-R onto CCOOH to Give CNH2. 3.2.1. Modelization of the Grafting of H2N(CH2)2NH2 with a Fluorinated Amine (H2N-F6) and a Brominated Amine (H2N-Br). In method A (Scheme 2), the first step consisted of mixing CCOOH(b) in a MES solution at pH 6 with NHS and EDAC. The goal of using EDAC was to assist the nucleophilic attack of NHS on the carboxylic functions of the support in order to form an ester (succinimidyl intermediate). This active ester is stable and undergoes nucleophilic substitution reactions with amine groups as explained in the literature.8 In a second step, the support containing this stable active ester was mixed with H2N-F6 in order to link the amine on the support by formation of an amide bond. The resulting powder was analyzed by XPS. In method B, the formation of the amide bond between the carboxylic surface functions of the support and an amine is realized in one step. The support CCOOH(b) was mixed with EDAC, DMAP, and H2N-Br in the same flask, and the resulting powder was again analyzed by XPS. The last method, method C, occurred in two steps. In the first step, CCOOH(b) was refluxed with SOCl2 in toluene. This should lead to the formation of an acyl chloride at the surface of the support (noted CCl) which undergoes more easily nucleophilic attack from amines than carboxylic groups. In the second step, the CCl support was mixed with H2N-F6 in dichloromethane, and the resulting powder was also analyzed by XPS. The two steps are illustrated in Scheme 3. The results obtained by these three methods are shown in the upper part of Table 4. The model amines were grafted with a yield of 8.7, 2.2, and 28.6% for methods A, B, and C, respectively. These values of yield show that the best method to obtain an amide bond between a primary amine and carboxylic functions at the surface of carbonaceous supports is method C (going through an acyl chloride). All of these manipulations were also carried out without coupling agents, to identify their true role (see lower part of Table 4). It means that (i) for method A, the first step was realized without NHS or EDAC; (ii) for method B, the reaction was carried out without DMAP or EDAC, and (iii) for method C, the first step was realized without SOCl2. For method A, it

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J. Phys. Chem. C, Vol. 112, No. 14, 2008 5537

SCHEME 2: Mechanism of Method A

SCHEME 3: Mechanism of Method C

TABLE 5: XPS Results for the Grafting of H2N(CH2)2NH2 on CCOOH(a1) by Methods A, B, and C method

N/Cexp

N/Ccalc

yielda (%)

A B C

0.0409 0.0650 0.0670

0.0782 0.0782 0.0782

36.2 67.0 69.6

a As the N contribution, in the experimental N/C ratio, contains a contamination due to the HNO3 treatment, the yield of the reaction was calculated by subtracting the measured N/C ratio for the starting CCOOH (N/C ) 0.0126) from the obtained N/C ratio after grafting of ethylenediamine and dividing this result by the calculated N/C value.

TABLE 6: XPS Results for the Grafting of H2N-F6 and H2N(CH2)2NH2 on Supports Presenting Different Acidities TABLE 4: XPS Results for the Grafting of H2N-Br and H2N-F6 on CCOOH(b) by Methods A, B, and C method

X/Cexp

X/Ccalc a

yieldb (%)

A (X ) F) B (X ) Br) C (X ) F)

with coupling agent 0.0097 0.1117 0.0004 0.0186 0.0320 0.1117

8.7 2.2 28.6

A (X ) F) B (X ) Br) C (X ) F)

without coupling agent 0.0110 0.1117 0 0.0186 0.0180 0.1117

9.8 0 16.1

a Calculated values are bulk molar ratios. The amount of heteroatom taken into consideration for the calculations corresponds to the amount of amine that would be grafted on the acid functions of the support with a yield of 100% multiplied by the number of heteroatoms present in the considered amine. The amount of C taken into consideration corresponds to the amount of used support by considering that it is only constituted of carbon. b Yields were calculated by dividing the experimental XPS ratios by the calculated ones.

can be observed that, with or without coupling agent, the amine was grafted with the same yield. Indeed, the F/C ratio is approximately the same in both cases. For method B, when no coupling agent was used, no bromine was detected by XPS on the surface. However, even when EDAC and DMAP were used in method B, the amount of detected bromine was very low (near the limit of detection). Finally, for method C, when the carbonaceous support was not treated with SOCl2 prior to the incorporation of the amine, the F/C ratio dropped from 0.0320 to 0.0180, and the yield decreased 13%. This means that a higher amount of fluorinated amine was grafted when the CCOOH(b) support was preliminarily treated with SOCl2. In other

samples

starting support

use of SOCl2

X/Cexp

CNH2F1 CNH2F2 CNH2F3 CNH2F4 CNH2F5 CNH2N1 CNH2N2 CNH2N3 CNH2N4 CNH2N5

CSX+ CSX+ CCOOH(a2) CCOOH(a2) CCOOH(a2) CSX+ CSX+ CCOOH(a2) CCOOH(a2) CCOOH(a2)

no yes no yes yes no yes no yes yes

0.0061 (X ) F) 0.0113 (X ) F) 0.0422 (X ) F) 0.0711 (X ) F) 0.1090 (X ) F) 0.0140 (X ) N) 0.0238 (X ) N) 0.0650 (X ) N) 0.0660 (X ) N) 0.0778 (X ) N)

words, method C seems to be the ideal way to incorporate an amine on the acidified carbon support as it gives the best yield and as it seems that SOCl2 is the only agent that has a chemical influence on the reactivity of supported carboxylic functions. 3.2.2. Grafting of H2N(CH2)2NH2 onto CCOOH to GiVe CNH2. The results of the incorporation of ethylenediamine to give the CNH2 samples are shown in Table 5. The starting CCOOH(a1) support had an acidity of 325.9 mmol/100 g and its O/C and N/C ratios obtained by XPS were respectively 0.1883 and 0.0126. Table 5 shows that ethylenediamine was grafted with a yield of 36% by method A, 67% by method B, and 70% by method C. These results confirm that the best method to create an amide bond between a primary amine and the supported carboxylic acid functions is method C, although method B seems also very efficient. 3.2.3. EVidence for Chemical Grafting of H2N-F6 and H2N(CH2)2NH2 on CCOOH(a) by Method C. To prove that the incorporation of the amines on the support occurred via formation of an amide bond and that it is not the result of a

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TABLE 7: Effect of the Use of SOCl2 and of the Acidity of the Support on the Amount of Grafted Amine

TABLE 8: XPS Results for Grafting of PPh2 on CNH2 samples CPPh2-1 CPPh2-2 CPPh2-3 CPPh2-4 CPPh2-5 CPPh2-6 CPPh2-7 CPPh2-8 CPPh2-9 CPPh2-10

experimental conditions overnight at rt

24h at 70 °C

starting support CNH2N1 CNH2N2 CNH2N3 CNH2N4 CNH2N5 CNH2N1 CNH2N2 CNH2N3 CNH2N4 CNH2N5

yieldb P/Cexp 0.0014 0.0024 0.005 0.0054 0.0096 0.0015 0.0027 0.0062 0.0060 0.0125

P/Ccalc a 0.0077 0.0077 0.0805 0.0805 0.0805 0.0077 0.0077 0.0805 0.0805 0.0805

TABLE 9: Loading and XPS Results for the Incorporation of Pd(dba)2 and Ru3(CO)12 on the CSX+ and CPPh2 Supports atomic absorption

(%) 18.2 31.2 6.2 6.7 11.9 19.5 35.0 7.7 7.4 15.5

a Calculated values are bulk molar ratios. The amount of phosphorus atoms taken into consideration for the calculations corresponds to the amount of phosphorus that would be grafted on the acidic functions of the support with a yield of 100% (two phosphorus atoms by acidic function). The amount of C taken into consideration corresponds to the amount of used support by considering that it is only constituted of carbon. b Yields were calculated by dividing the experimental XPS ratios by the calculated ones.

Figure 1. Nitrogen adsorption isotherms of representative samples at each step of the functionalization: (A) starting carbon (Csx+ - SBET 932 m2/g), (B) carbon oxidized with HNO3 2.5 mol/L (CCOOH - SBET 701 m2/g), (C) CCOOH treated with SOCl2 under nitrogen (CCl - SBET 752 m2/g), (D) CCl reacted with ethylenediamine under nitrogen (CNH2 - SBET 482 m2/g) and (E) CNH2 treated with HPPh2/CH2O under nitrogen 24 h at 70 °C (CPPh2 - SBET 121 m2/g).

simple physisorption, the grafting of ethylenediamine was simulated with the fluorinated amine by incorporating it on supports of different acidities and under variable experimental conditions. This was realized by method C as it gave the best yield. In the first step, the different starting supports were refluxed in toluene with or without SOCl2. The resulting powders were then refluxed in dichloromethane with H2N-F6 or in toluene with H2N(CH2)2NH2. The obtained powders were analyzed by XPS. Table 6 shows the results obtained. The acidified support used CCOOH(a2) had a total acidity of 335.4 mmol/100 g. Its O/C and N/C XPS atomic surface ratios were respectively 0.2065 and 0.0161.

samples

yield of graftinga (%)

metal loading (wt. %)

CSX+Pd (M ) Pd) CPPh2Pd (M ) Pd) CSX+Ru3 (M ) Ru) CPPh2Ru3 (M ) Ru)

97.9 84.7 ∼0 43.4

9.8 8.6 ∼0 4.6

XPS

M/Cexp

M/Ccalcb

0.0584 0.1348 0.0050 0.0425

0.0087 0.0078 0 0.0053

a Yields of grafting were determined by atomic absorption analysis of the reactions filtrates. b Calculated values are bulk molar ratios based on the metal loadings. The amount of metal taken into consideration for the calculations corresponds to the amount grafted on the support (determined by atomic absorption analyses of the filtrates). The amount of C taken into consideration corresponds to the amount of support used by considering that it is only constituted of carbon.

When using the fluorinated amine, it can be observed that the F/C ratio obtained by XPS increases by going from CNH2F1 to CNH2F5, indicating that the amount of grafted amine increases from CNH2F1 to CNH2F5. As mentioned in section 2.2.3., the amount of amine introduced was exactly the same in all cases. The only difference lies in the use of SOCl2 or not and in the use of supports presenting variable acidities. Table 7 summarizes the effect of SOCl2 and of the acidity of the support. Indeed, for supports presenting the same acidity, it can be observed that, when SOCl2 is used, a higher amount of amine is grafted (CNH2F1 vs CNH2F2 and CNH2F3 vs CNH2F4). This confirms once more the facilitated nucleophilic attack of the amine on acyl chlorides compared to the unmodified carboxylic acid functions and the necessity of using of SOCl2 to incorporate a higher amount of amine by formation of an amide bond. Moreover, it can also be observed that, when the number of acidic functions in the starting support increases, the amount of grafted amine increases in parallel (CNH2F1 vs CNH2F3 or CNH2F2 vs CNH2F4). These results show that a real chemical reaction occurred at the surface of the support and that it was not a simple adsorption of the reactants, as the introduced amount of amine in the reaction medium was the same in all cases (see section 2.2.3.). In addition, the reactions to prepare sample CNH2F5 have been carried out under nitrogen (treatment with SOCl2 and addition of the amine), and it can be seen in Table 6 that, in this latter case, a maximal amount of amine was grafted. The same methodology has been applied to the grafting of ethylenediamine, and the trends observed were the same as with the fluorinated amine (Table 6), which takes off all ambiguity about the possibility of physisorption of the reactants at the surface of the support. Ethylenediamine was grafted on the support under nitrogen with a maximal yield of 76%. 3.3. Addition of HPPh2 onto CNH2 to Give CPPh2. Under nitrogen, paraformaldehyde was mixed together with HPPh2 at 70 °C for a short period of time to form the HOCH2PPh2 intermediate in situ. Then, the CNH2Nx supports were introduced in the resulting solution and the obtained powders were analyzed

Phosphine-Functionalized Carbon Support

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5539

Figure 2. (a) TEM image of Pd(dba)2 deposited on CSX+, (b) particle size distribution for Pd(dba)2 deposited on CSX+, (c) TEM image of Pd(dba)2 grafted on CPPh2, and (d) particle size distribution for Pd(dba)2 grafted on CPPh2.

Figure 3. (a,b) TEM images of Ru3(CO)12 deposited on CSX+, (c) TEM image of Ru3(CO)12 grafted on CPPh2, and (d) particle size distribution for Ru3(CO)12 grafted on CPPh2.

by XPS, as shown in Table 8. First, Table 8 shows the same evolution for the P/C ratios than that for the F/C and N/C ratios in Table 6 (for CPPh2-1 to CPPh2-5 and for CPPh2-6 to CPPh2-10). This means that the amount of grafted phosphines is dependent

on the amount of terminal amines present initially at the surface of the support. The amount of grafted phosphines increases when the amount of terminal amines increases. Once more, these results demonstrate that a real chemical reaction occurred at

5540 J. Phys. Chem. C, Vol. 112, No. 14, 2008 the surface of the supports as the introduced amount of phosphines was exactly the same in all cases (see section 2.3.). Second, Table 8 shows that when the incorporation of phosphines occurred at 70 °C for 24 h rather than at room temperature overnight, a higher amount of phosphines is grafted, to reach the best yield of 15.5% related to the acidity of the used support. Because the method used to obtain CPPh2-10 appears to be the most efficient one to functionalize the support, it was used to synthesize all further CPPh2 samples mentioned in the text. 3.4. N2 Adsorption Isotherms. Nitrogen adsorption isotherms were recorded for representative samples at each step of the functionalization (Figure 1). The specific surface area was found to decrease markedly by going from the pristine CSX+ carbon to the phosphine-functionalized one. The surface area calculated using the BET equation is given in the caption of Figure 1 for indicative purpose, even if this method is not optimal for this type of samples, considering that not only the surface area is affected but also the type of pores present. By going from the CSX+ carbon to CCOOH, the specific surface area decreases significantly (from 932 m2/g to 701 m2/g). Preliminary studies on the acidification of CSX+ showed that this loss of surface is due to the chemical treatment (HNO3). Indeed, the specific surface area of the support is sensitive to the concentration in HNO3 and less to the physical variables (temperature, duration, ...). The CCl sample displays no significant textural change (SBET ) 752 m2/g) when compared to the oxidized carbon. The next step of the procedure induces a further decrease of microporous volume (SBET ) 482 m2/g), while the mesopores are unaffected. At the end, a nearly nonporous material is obtained (SBET ) 121 m2/g) because of the loss of both micro- and mesopores. The functionalization is thus deleterious to the textural characteristics of the support, or at least to the accessibility to the internal pore volume. In order to gain some understanding of the mechanisms involved, additional blank experiments were carried out; that is, a sample of CCOOH was submitted to heating for the same duration and in the same mixture of solvents as during functionalization but without the reactants. When compared with the CCOOH sample, the specific surface area was not drastically modified by this treatment (SBET ) 699 m2/g after 9 h at 120 °C in toluene and SBET ) 583 m2/g after a further 24 h at 70 °C in methanol/toluene). This points toward the fact that the chemical modification of the surface is by far the most crucial factor responsible for the loss of surface area, probably because the accessibility of the micro- and mesopores is restricted. Knowing the specific surface area of the CPPh2 (SBET ) 121 m2/g), a number of bidentate phosphine ligands present on the accessible surface per unit area can be calculated: the best yield reported above (15.5%) corresponds to ∼2.5 anchoring sites (chelating phosphines) per nm2. This is a very high density of functional groups for a carbonaceous surface, and shows that, even if the specific surface area is decreased, the chemical functionalization is effective. 3.5. Grafting. Pd(dba)2 and Ru3(CO)12 were incorporated on the unmodified support (CSX+) and on the functionalized one (CPPh2 where P/C (XPS) ) 0.0181). This functionalized support was prepared by the most appropriate way as mentioned above: CSX+ was treated with HNO3 2.5 mol/L; then the acidified support was successively treated under nitrogen with SOCl2, H2N(CH2)2NH2, and CH2O/HPPh2. The complexes were brought into contact with the supports in a mixture of solvents (dichloromethane/toluene 1/1 v/v). Toluene was chosen to “wet” the support, and dichloromethane was chosen to ensure solu-

Willocq et al. bilization of the metal precursors. Table 9 shows the results of metal loading and of XPS characterization, respectively. First, Table 9 shows that Pd(dba)2 was easily adsorbed on CSX+ with a yield of 98% (CSX+Pd) and as easily loaded on the functionalized support (85% for sample CPPh2Pd). Nevertheless, when comparing the Pd/C atomic surface ratios determined by XPS, one can observe that this ratio is higher when Pd(dba)2 was grafted on the functionalized support (CPPh2Pd) than on CSX+. This result indicates that the dispersion of the complex is better on CPPh2. The surface functions act as anchors at the surface of the support for the noble metal-based molecules and favor a good dispersion. Both samples were characterized by transmission electron microscopy (TEM) to visualize particle sizes. Histograms of particle size distributions were constructed by measuring the size of more than 100 particles in each case, arising from 2 to 3 different areas of the samples. In both cases, TEM images (Figure 2a,c) show the presence of well-dispersed nanoparticles of 4-6 nm in size. However, SEM images (not shown) revealed that some agglomerates were also present and mainly on CSX+. Second, Table 9 shows that Ru3(CO)12 was not absorbed on CSX+ (CSX+Ru3) but was grafted with a yield of 43% on CPPh2 (CPPh2Ru3). As Ru3(CO)12 has no affinity for CSX+ (0% grafting), the loading on CPPh2 can only be due to the presence of the phosphine functions on its surface. It has already been observed that Ru3(CO)12 reacts easily with phosphines.29 TEM images in the case of Ru3(CO)12 show the presence of homogeneously dispersed nanoparticles of 2-3 nm on CPPh2 (Figure 3c). This particle size is extremely small, especially knowing that it is more difficult to stabilize small particles on carbon than on inorganic oxides. Moreover, the size distribution is very narrow (Figure 3d). On CSX+, very few particles were found because of the poor loading (Figure 3a and 3b); hence, no histogram was built for this sample. This demonstrates the great advantage of using such a phosphinefunctionalized carbon. The results described here for the grafting of these complexes on the functionalized support indicate that the introduction of phosphine groups on the surface is really efficient to anchor covalently noble metal-based molecules with a good dispersion, thus avoiding the formation of large agglomerates. Nitrogen adsorption isotherms were also obtained for the samples after grafting of the metal (not shown). No major changes were observed when compared to the CPPh2 support. 4. Summary and Conclusion The goal of this study was to graft covalently potential coordination sites at the surface of a high specific surface area active carbon support named SX PLUS. Within the general context of noble metal-based heterogeneous catalysis, it seemed appropriate to build chelating phosphine groups. Indeed, it is known that noble metals have a high affinity for phosphines, and the chelate effect would provide the driving force for surface coordination. To test this hypothesis, two different organometallic compounds were used, that is, Pd(dba)2 and Ru3(CO)12, as precursors for supported palladium or ruthenium nanoparticles. The first step was to identify anchoring sites for the phosphine functions at the surface of the support. Given that active carbons are known to contain at their surface oxygenated groups,1-4 and in particular carboxylic acids, and that these can be quantified by Boehm’s titration,4,5 it was chosen to increase the number of such surface groups by oxidation. This was done by simple treatment with nitric acid. The experimental conditions used for this treatment (concentration of the acid, temperature,

Phosphine-Functionalized Carbon Support duration) have an influence on the number of carboxylic groups introduced. This had already been shown27 but was confirmed here. The acidity determined by Boehm’s titration allows one to calculate a number of moles of anchoring sites per gram of support (knowing the specific surface area). The acidity of the resulting CCOOH(a) support was 325 or 335 mmol/100 g, when using HNO3 2.5 mol/L, while it was 155.2 mmol/100 g when using HNO3 0.25 mol/L (support CCOOH(b)). These samples were also characterized by XPS, and a direct correlation between the O/C surface ratios and the acidity was found, as before.27 Having created reactive sites at the surface of the support, we find that the second step consisted of grafting an organic moiety to build the desired phosphine ligands. It was chosen to use the surface carboxylic groups and to form an amide bond with a selected amine. This step has been studied in detail. In particular, the influence of coupling agents was explored. Three situations were compared: (i) EDAC/NHS in 2 steps, (ii) DMAP + EDAC in one step, and (iii) SOCl2. In order to quantify exactly the yields of reaction at the surface, model amines were used, which comprised heteroatoms easily detected by XPS, here, Br or F. This strategy allowed us to unambiguously identify the situation (iii) as the best compromise. The next step was to graft a fragment of interest, and ethylenediamine was selected. It could successfully be grafted onto the oxidized support, and again, fluorinated amines allowed us to confirm this result. The last step in the building of the surface coordination site was the transformation of a pending amine into a bidentate phosphine, by the action of (CH2O)n/ HPPh2, according to a standard literature procedure.26,28 XPS once more allowed to quantify the yield of reaction via the P/C surface ratio. Textural characterization by nitrogen physisorption however showed that the accessible specific surface area was decreased at each step of the functionalization. These supported phosphines were then used to graft the Pd and Ru complexes. Atomic absorption allowed us to confirm the incorporation of metal at the surface of the functionalized support. In the case of the Pd species, nonspecific adsorption occurred as well, giving a similar loading on the nontreated CSX+ support. With the Ru species, however, no metal was adsorbed on SX+ while 43% yield of grafting could be obtained on the functionalized support. In both cases, the surface M/C ratios where much higher in the case of the functionalized support, and small metallic particles were observed by TEM. In the case of Pd(dba)2, the particles were approximately 4-6 nm in size, while in the case of Ru3(CO)12 they were about 2-3 nm on average. This shows that the phosphine coordination sites have indeed been built at the surface and that they are accessible and reactive toward ligand exchange, hence allowing grafting of a variety of metal complexes. The materials obtained, containing immobilized metal complexes, could find application in a variety of fields: immobilization of homogeneous catalysts, chemical sensors, and composite materials or as precursors for heterogeneous catalysts. In the latter case, a step of prudent thermal treatment should ensure the transformation of these precursor molecules into active nanoparticles, by abstracting their ligand shell, without losing the benefit in terms of size, composition, and dispersion.

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5541 Acknowledgment. The authors gratefully acknowledge the “Fonds National de la Recherche Scientifique” (F.N.R.S.), the “Fonds pour la formation a` la Recherche dans l’Industrie et dans l’Agriculture” (F.R.I.A.) for the research fellowship allotted to C.W. They also acknowledge Prof. E. Gaigneaux for access to XPS, Prof. R. Legras for access to SEM, the NORIT firm for supplying the carbon support and J.-F. Statsijns for technical support. This work has been performed in the frame of the Interuniversity Attraction Poles Programme of the Belgian State, Belgian Science Policy, Project No. INANOMAT, P6/17. References and Notes (1) Ahmadpour, A.; Do, D. D. Carbon 1996, 34, 471. (2) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T. Appl. Catal. A 1998, 173, 259. (3) Moreno-Castilla, C.; Rivera-Utrilla, J. MRS Bulletin 2001, 26, 890. (4) Polania, A.; Papirer, E.; Donnet, J. B.; Dagois, G. Carbon 1993, 31, 473. (5) Boehm, H. P. Carbon 1994, 32, 759. (6) Bansal, R. C.; Donnet, J.-B.; Stoeckli, F. ActiVe Carbon, Marcel Dekker: New York, 1998. (7) Banerjee, S.; Hemraj-Benny, T.; Wong, S. S. AdV. Mater. 2005, 17, 17. (8) Jiang, K. Y.; Schadler, L. S.; Siegel, R. W.; Zhang, X. J.; Zhang, H. F.; Terrones, M. J. Mater. Chem. 2004, 14, 37. (9) Ramanathan, T.; Fisher, F. T.; Ruoff, R. S.; Brinson, L. C. Chem. Mater. 2005, 17, 1290. (10) Sainsbury, T.; Fitzmaurice, D. Chem. Mater. 2004, 16, 3780. (11) Unger, E.; Duesberg, G. S.; Liebau, M.; Graham, A. P.; Seidel, R.; Kreupl, F.; Hoenlein, W. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 735. (12) Zhu, W. H.; Minami, N.; Kazaoui, S.; Kim, Y. J. Mater. Chem. 2004, 14, 1924. (13) Diaz-Aunon, J. A.; Roman-Martnez, M. C.; de Lecea, C. S. M.; Alper, H. Stud. Surf. Sci. Catal. 2002, 143, 295. (14) Krishnankutty, N.; Vannice, M. A. J. Catal. 1995, 155, 312. (15) Jarrais, B.; Silva, A. R.; Freire, C. Eur. J. Inorg. Chem. 2005, 4582. (16) Silva, A. R.; Martins, M.; Freitas, M. M. A.; Figueiredo, J. L.; Freire, C.; de Castro, B. Eur. J. Inorg. Chem. 2004, 2027. (17) Silva, A. R.; Budarin, V.; Clark, J. H.; de Castro, B.; Freire, C. Carbon 2005, 43, 2096. (18) Silva, A. R.; Figueiredo, J. L.; Freire, C.; de Castro, B. Catal. Today 2005, 102, 154. (19) Valente, A.; do Rego, A. M. B.; Reis, M. J.; Silva, I. F.; Ramos, A. M.; Vital, J. Appl. Catal. A 2001, 207, 221. (20) Blaser, H. U.; Indolese, A.; Schnyder, A.; Steiner, H.; Studer, M. J. Mol. Catal. A: Chem. 2001, 173, 3. (21) Femoni, C.; Iapalucci, M. C.; Kaswalder, F.; Longoni, G.; Zacchini, S. Coord. Chem. ReV. 2006, 250, 1580. (22) Hermans, S.; Raja, R.; Thomas, J. M.; Johnson, B. F. G.; Sankar, G.; Gleeson, D. Angew. Chem., Int. Ed. 2001, 40, 1211. (23) Raja, R.; Khimyak, T.; Thomas, J. M.; Hermans, S.; Johnson, B. F. G. Angew. Chem., Int. Ed. 2001, 40, 4638. (24) Raja, R.; Sankar, G.; Hermans, S.; Shephard, D. S.; Bromley, S.; Thomas, J. M.; Johnson, B. F. G. Chem. Commun. 1999, 1571. (25) Feeder, N.; Geng, J. F.; Goh, P. G.; Johnson, B. F. G.; Martin, C. M.; Shephard, D. S.; Zhou, W. Z. Angew. Chem., Int. Ed. 2000, 39, 1661. (26) Judkins, C. M. G.; Knights, K. A.; Johnson, B. F. G.; de Miguel, Y. R.; Raja, R.; Thomas, J. M. Chem. Commun. 2001, 2624. (27) Hermans, S.; Diverchy, C.; Demoulin, O.; Dubois, V.; Gaigneaux, E. M.; Devillers, M. J. Catal. 2006, 243, 239. (28) Reetz, M. T.; Lohmer, G.; Schwickardi, R. Angew. Chem., Int. Ed. 1997, 36, 1526. (29) Phillips, G.; Hermans, S.; Adams, J. R.; Johnson, B. F. G. Inorg. Chim. Acta 2003, 352, 110.