ARTICLE pubs.acs.org/Organometallics
Grafting Reaction of Platinum Organometallic Complexes on Silica-Supported or Unsupported Heteropolyacids Nicolas Legagneux,† Erwann Jeanneau,‡ Amelie Thomas,† Mostafa Taoufik,† Anne Baudouin,† Aimery de Mallmann,† Jean-Marie Basset,† and Frederic Lefebvre*,† †
Universite de Lyon ICL, C2P2 UMR 5265 (CNRS-CPE-Universite Lyon 1), LCOMS-CPE Lyon 43 Boulevard du 11 Novembre 1918 F-69616, Villeurbanne, France ‡ Centre de Diffractometrie Henri Longchambon (UCBL), B^atiment Raulin, 43 Boulevard du 11 Novembre 1918, 69616 Villeurbanne Cedex, France
bS Supporting Information ABSTRACT: Silica-supported Keggin-type heteropolyacids react with platinum(II) complexes displaying a Pt-CH3 group, leading to the evolution of methane and the formation of an organometallic fragment grafted on the polyacid. This species has been characterized by various physicochemical methods such as microanalyses, infrared spectroscopy, solid-state 1D and 2D MAS NMR, and EXAFS. Molecular models were also prepared by reaction of the nonsupported anhydrous polyacid with the platinum(II) methyl complexes. These compounds were characterized in solution by multinuclear NMR (1H, 13C, 183W, and 195Pt) and in the solid state by X-ray diffraction on monocrystals. All data indicate that the interaction between the platinum complex and the polyoxometalate is weak and that this system is better expressed as an organometallic platinum salt of the supported heteropolyacid. This complex can be seen as one of the intermediates in the partial oxidation of methane to methanol, its synthesis being the reverse reaction of the methane activation.
’ INTRODUCTION Polyoxometalates have received great attention in catalysis due to their variety of applications, both in homogeneous and in heterogeneous systems.1-6 Among them, the most studied are those displaying the well-known Keggin structure [XM12O40]n(X = P, Si, Al, ..., and M = W, Mo), probably because some of them are commercially available or easy to prepare and, from a more scientific point of view, they are the most stable thermally. Their applications can be divided in two groups depending on the nature of the M metal in the polyoxometalate. When M = W, these compounds are mainly used in acid catalysis, and due to their acidity (stronger than that of sulfuric acid), they can catalyze many reactions, even requiring strong sites such as the isomerization or the cracking of alkanes.7-21 When M is molybdenum, they are mainly used in oxidation reactions, and here also they can be used for the functionalization of alkanes by peroxides or molecular oxygen.22-24 This classification is very simplistic, and for example the replacement of tungsten by other elements such as noble metals or transition metals should give redox properties to the corresponding product.25-27 However, among all alkanes studied, there is no really interesting result about methane, which is the most important in terms of potential industrial applications. Indeed, direct partial oxidation of methane into methanol, formaldehyde, or C2 hydrocarbons is a challenge, and none of the r 2011 American Chemical Society
systems used in the laboratory scale can actually be applied industrially. As an example, one of these catalysts, first developed by Shilov28 and later improved by Periana,29 is based on a platinum(II) complex stabilized by a bipyrimidine ligand. However the most important problem of this system is that the solvent is sulfuric acid:sulfur trioxide, which is not inactive as the oxidation product, is methyl bisulfate, while SO3 is reduced in SO2 (Scheme 1). Recently Neuman has proposed to increase the efficiency of this system and to avoid the use of sulfuric acid by combining the platinum complex used by Periana, Pt(Bipym)Cl2, and a polyoxometalate, H5PMo10V2O40, in order to facilitate the redox transformation between platinum(IV) and platinum(II) in the catalytic cycle.30 For this purpose he performed a methylation on one nitrogen of the bipyrimidine ligand, leading then to a cationic [(MeBipym)PtCl2]þ platinum complex, which was then used as a countercation for the polyacid, leading to [(MeBipym)PtCl2] 3 H4PMo10V2O40. This system was found to be active in the partial oxidation of methane but without a real improvement compared to the system developed by Periana. One reason could be that in this species the platinum atom is not in close proximity Received: April 27, 2010 Published: March 09, 2011 1783
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heteropolyacids were prepared by impregnation in water. It should be noted that the choice of the solvent is important, as different results could be obtained by using other solvents (see for example ref 31). Typically 1.2 g of the desired tungstic heteropolyacid was dissolved in 10 mL of water, and 2 g of Aerosil silica (S = 200 m2 g -1) was added. After stirring for 10 min water was slowly removed by evaporation at 40 °C. After two days at 40 °C the resulting solid was finely ground. The polyacid content (37.5 wt %) corresponds to a monolayer on the silica surface.
Grafting Reaction of the Platinum Complexes on the Supported Polyoxometalates. Typically, a mixture of platinum complex (0.102 mmol) and H4SiW12O40/SiO2 (500 mg, 0.065 mmol), previously dried under vacuum (10-5 Torr) at 200 °C for 2 h, in 5 mL of the desired solvent (diethyl ether or dichloromethane) was stirred at 25 °C for 3 h in a double Schlenk. After filtration, the solid was washed three times with the same solvent and all volatile compounds were condensed into another reactor (of known volume) in order to quantify the methane evolved during the grafting reaction. After reaction, the resulting powder was dried under vacuum (10-5 Torr). Methane was quantified by gas chromatography.
Reaction of the Platinum Complexes with the Polyoxometalate in Solution. The same procedure was used for all com-
to the polyoxometalate, the positive charge being on a nitrogen atom, which is not in close vicinity to platinum. We describe here the preparation and characterization of a different system, where platinum is directly interacting with the polyacid. For this purpose we used a strategy quite similar to what we described previously in the case of tetraalkyl tin complexes and unsupported or silica-supported heteropolyacids,31,32 the reaction of the anhydrous polyacid with the organometallic complex. In order to have the highest amount of grafted platinum, we used a silica-supported heteropolyacid with a monolayer coverage. Four different platinum complexes were studied, PtMe2(COD), PtMe2(Bipym), PtMe2(DMSO)2, and Pt(Me)(Cl)(DMSO)2. All of them display a platinum-methyl bond that we expect to be be broken by reaction with the polyacid.
’ EXPERIMENTAL SECTION Materials. Pt(CH3)2(COD) was from Strem, while the other organometallic compounds were synthesized according to the following published procedures: cis-Pt(CH3)2(DMSO)2 by the method of Eaborn et al.,33 Pt(CH3)2(Bipym) by that of Sutcliffe et al.,34 and Pt(CH3)(Cl)(COD) according to Clark et al.35 Pt(13CH3)2(COD) was prepared by use of Li(13CH3) according to the above procedure. All products were obtained after recrystallization and were characterized by 1H and 13C NMR (see Supporting Information for the characterization). Preparation of Silica-Supported Heteropolyacids. H3PW12O40 and H4SiW12O40 were from Aldrich. Their purity was checked by 31P and 29Si liquid-state NMR, respectively, and was sufficient (þ99%) for the studies described in this paper. The anhydrous heteropolyacid was obtained by treatment under vacuum at 200 °C of the heteropolyacid and was characterized by 1H MAS NMR (presence of a peak near 10 ppm). H4PMo11VO40 was prepared according to literature procedures.36 The supported
plexes. A typical example is given below for the reaction of PtMe2(COD) with anhydrous H4SiW12O40. The reaction was performed in a 100 mL Schlenk tube. Anhydrous dodecatungstosilicic acid (1 g, 0.347 mmol) was dissolved in anhydrous DMSO (5 mL). PtMe2(COD) (0.463 g, 1.390 mmol) was added after freezing the solvent at 0 °C, and the solution was stirred at room temperature for 2 days. The gas phase was then sampled by gaseous chromatography: 4.09 CH4 evolved for each polyacid introduced. Half of the solvent was then removed by treatment under vacuum at 20 °C. The precipitate was dissolved by heating at 35 °C, and the saturated solution was kept at room temperature overnight. Crystals suitable for X-ray crystallography were then obtained. 1H NMR (DMSO-d6): δ 0.75 (Pt(CH3); JPt-H = 69 Hz), 2.25 and 2.45 (COD CH2-CH2), 5.45, 5.52, 5.65, 5.72 (COD CHdCH). 13C NMR (DMSO-d6): δ 4.4 (Pt(CH3), JPt-C = 604 Hz), 28.5 (COD CH2-CH2, JPt-C = 18 Hz), 31.0 (COD CH2CH2, JPt-C = 15 Hz), 106.7 (COD CHdCH, JPt-C = 130 Hz), 113.8 (COD CH=CH, JPt-C = 44 Hz). Anal. Found (expected values for [SiW12O40][PtMe2(COD)(DMSO)]4(DMSO)3): Pt 16.3 (16.41), W 45.6 (46.45), Si 0.77 (0.59), C 12.04 (13.63), H 2.14 (2.40), S 4.62 (4.71). Microanalysis and Infrared Spectroscopy. Elemental analyses were performed at the CNRS Central Analysis Department of Solaize or at the ICMUB Laboratory of the University of Bourgogne (Dijon). The infrared spectra were recorded on a Nicolet 5700-FT spectrometer by using an infrared cell equipped with CaF2 windows, allowing in situ studies. Typically 32 scans were accumulated for each spectrum, with a resolution of 1 cm-1. The spectra were recorded by transmission, the sample being pressed as a disk (m = 20-50 mg) and kept in a sample holder. NMR Spectrosocopy. 1D MAS 1H and 13C CP-MAS solid-state NMR spectra were recorded on a Bruker DSX-300 spectrometer operating at 300.18 and 75.47 MHz for 1H and 13C, respectively. Samples were introduced under argon in a zirconia rotor, which was then tightly closed. For all experiments, the rotation frequency was set to 10 kHz. Chemical shifts are given with respect to TMS as an external standard, with a precision of 0.2-0.3 ppm and 1 ppm for 1H and 13C NMR, respectively. For carbon both CP-MAS and HPDEC MAS NMR spectra were recorded. For both cases, an apodization function (exponential) corresponding to a line-broadening of 50 Hz was applied. The 2D solid-state NMR spectroscopy experiments (on 13C-enriched samples) were also conducted on a Bruker DSX-300 spectrometer. For the cross-polarization step, a ramped rf field centered at 77 kHz was applied on proton, while the carbon rf field was matched to obtain the optimal signal. During acquisition, the proton decoupling field strength 1784
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Table 1. Evolved Methane during the Reaction of Platinum Complexes with H4SiW12O40/SiO2 Dehydroxylated at 200 °C entry 1
complex PtMe2(COD)
support
reaction time (h)
evolved CH4 per POM
evolved CH4 per grafted Pt
SiO2
15
0
2
H3PW12O40/SiO2
15
0
3
H4SiW12O40/SiO2
3
0.9
1.0
4
H4SiW12O40/MCM-41
15
1.0
1.0 0.9
5
PtMe2(DMSO)2
H4SiW12O40/SiO2
3
1.0
6
PtMe2(Bipym)
H4SiW12O40/SiO2
3
0.3
H4SiW12O40/SiO2
15
0.9
1.1
H4SiW12O40/SiO2 H4PMo11VO40/SiO2
15 24
0.5 0.95
1.0 1.0
7 8 9
PtMeCl(COD) PtMe2(COD)
was also set to 77 kHz. A total of 64 t1 increments with 256 scans each were collected. The spinning frequency was 10 kHz, and the contact time for the cross-polarization step varied from 100 μs to 2 ms. Quadrature detection in ω1 was achieved using the TPPI method.37 The solution NMR spectra were recorded on a Bruker ACX-300 spectrometer. 1H NMR spectra were referenced to C6D5H at 7.15 ppm and 13C NMR to C6D6 at 128.0 ppm. EXAFS Experiments. EXAFS data were acquired on beamline BM29 at the ESRF (Grenoble, France) at the Pt LIII-edge, between 11.25 and 12.4 keV, in the transmission mode, using a double crystal monochromator, Si(111), detuned to minimize higher harmonics and ionization chambers as detectors. The calibration in energy was performed with a platinum foil (E0 = 11 564 eV). The samples were introduced, within a dry and air-preserved glovebox, into a double airtight cell equipped with Kapton windows. The EXAFS spectra were analyzed by standard procedures using the program Athena38 and the suite of programs developed by Alain Michalowicz, in particular the EXAFS fitting program RoundMidnight using calculations with spherical waves.39,40 The post-edge background subtraction was carefully conducted using polynomial and cubic-spline fittings, and the removal of the low-frequency contributions was checked by further Fourier transformation. A cubic-spline fitting gave the best result. The fitting of the spectrum was done with the k3- and k1-weighted data using the following EXAFS equation, where S02 is a scale factor; Ni is the coordination number of shell i; rc is the total central atom loss factor; Fi is the EXAFS scattering function for atom i; Ri is the distance to atom i from the absorbing atom; λ is the photoelectron mean free path; σi is the Debye-Waller factor; Φi is the EXAFS phase function for atom i; and Φc is the EXAFS phase function for the absorbing atom: n N i Fi ðk, R i Þ -2R i expð-2σi 2 k2 Þ sin½2kR i exp χðkÞ = S0 2 r c ðkÞ λðkÞ kR i 2 i¼1
∑
þ Φi ðk, R i Þ þ Φc ðkÞ The program FEFF841 was used to calculate theoretical values for rc, Fi, λ, and Φi þ Φc based on model clusters of atoms. The refinements were performed by fitting the structural parameters Ni, Ri, σi, and the energy shift, ΔE0 (the same for all shells). The fit residue, F (%), was calculated by the following formula:
∑k ½k3 χexp ðkÞ - k3 χcal ðkÞ2 100 F¼ ∑k ½k3 χexp ðkÞ2 As recommended by the Standards and Criteria Committee of the International XAFS Society,42 an improvement of the fit took into account the number of fitted parameters. The number of statistically independent data points or maximum number of degrees of freedom in the signal is defined as Nidp = (2ΔkΔR/π). The inclusion of extra parameters was statistically validated by a decrease of the quality factor,
(Δχ)2/ν, and the values of the statistical errors generated in RoundMidnight were multiplied by [(Δχ)2/ν]1/2 in order to take the systematic errors into account, since the quality factors exceeded 1. The error bars thus calculated are given in parentheses after each refined parameter. As a reference sample, besides the Pt foil, we have studied the Pt(NH3)4(OH)2 complex either diluted in water (Δμ/μ = 0.6) or well dispersed as a solid in boronitride powder and pressed into a selfsupporting wafer (Δμ/μ = 0.8). Both spectra (solution and wafer) are very similar (four Pt-N at 2.04(1) Å). The scale factor, S02, was determined by a method already described by B. Ravel,43 performing several series of fits on the first shell of a Pt(NH3)42þ reference sample and plotting σPt-N2 vs S02 for k1, k2, and k3 weightings. The value thus found for the scale factor, S02 = 0.94, was kept constant in all the fits. X-ray Structure Determination. A suitable crystal was selected under optical microscope and mounted on a four-circle Nonius Kappa CCD diffractometer, using Mo KR radiation (λ ≈ 0.71073 Å) and equipped with a CCD area detector. Intensities were collected by means of the program COLLECT.44 Reflection indexing, Lorentz-polarization correction, peak integration, and background determination were carried out with the program DENZO.45 Frame scaling and unit-cell parameter refinement were performed with the program SCALEPACK.45 Numerical correction absorption was performed by modeling the crystal faces using NUMABS.46 The resulting set of hkl reflections was used for structure refinement. Crystallographic data and details on data collection are given in the Supporting Information as a CIF file.
’ RESULTS AND DISCUSSION Evolved Gases during the Reaction of Platinum Complexes with Supported Heteropolyacids. When the platinum
complexes are contacted with 37.5 wt % (corresponding to a monolayer) H4SiW12O40/SiO2 dehydroxylated at 200 °C, a reaction occurs, as evidenced by a gaseous evolution, even at room temperature and whatever the organometallic complex. GC analysis shows that only methane has been formed, its amount depending on the starting platinum complex (Table 1). No reaction occurs with silica dehydroxylated at 200 or 500 °C or with H3PW12O40/SiO2 dehydroxylated at 200 °C (entries 1 and 2). The methane evolution is greatly dependent on the metal complex: With PtMe2(COD) and PtMe2(DMSO)2 the reaction is complete in 3 h at room temperature (entries 3 and 5), while for PtMe2(Bipym) 15 h is necessary to achieve it (entries 6 and 7). PtMeCl(COD) gives only a 50% reaction rate (based on the expected amount for the same reaction than with the other complexes) after 15 h (entry 8). It should also be pointed out that the nature of silica (Aerosil or MCM-41) has an influence on the reaction time, probably due to diffusion phenomena in the mesoporous silica (entries 3 and 4). 1785
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As no reaction occurs with silica alone, the methane evolution can only be due to the reaction of the platinum complex with the heteropolyacid and more precisely with its strong acidic protons. Indeed we had previously shown that the interaction of H4SiW12O40 or H3PW12O40 with the silica surface could be seen mainly as the protonation of three silanol groups.47 As a consequence, no acidic proton remained on H3PW12O40/SiO2, while one strongly acidic proton per Keggin unit remained in the case of H4SiW12O40/SiO2. These results explained the observations made some years ago in the cracking reaction of n-hexane, H4SiW12O40/SiO2 being a better catalyst than H3PW12O40/SiO2.19 If one excludes PtMeCl(COD), the methane evolution corresponds, when the reaction is complete, to the consumption of ca. one proton per Keggin unit, whatever the amount of platinum complex (between 1.2 and 2 per Keggin unit) that had been introduced, in agreement with a reaction occurring only with the remaining acidic proton on the Keggin heteropolyacid and without migration on the surface as previously observed in the case of the reaction of tetramethyltin.31 A further confirmation of this explanation was obtained in the case of the reaction of PtMe2(COD) with H4SiW12O40/SiO2 at various polyoxometalate loadings. Figure 1 shows the amount of evolved methane as a function of the polyacid amount on the silica support. Clearly a linear relationship is observed for polyacid loadings lower than 0.3 g per gram of silica, while it remains quite constant for higher coverages, for which the formation of bulk polyacid particles on silica can be expected (and shown for example by X-ray powder diffraction studies). Such a behavior is not surprising, as the anhydrous heteropolyacid is not soluble at all in the solvents used for the grafting reaction (diethyl ether or dichloromethane).32 In these conditions, the acidic protons inside the crystallites are not able to react with the platinum complexes. Such a reaction of alkyl platinum complexes with strong acid sites is well documented in classical organometallic chemistry, because the metal atoms are electron-rich centers and the metal-alkyl bonds are susceptible to electrophilic attack.48 It
Figure 1. Evolved methane as a function of H4SiW12O40 amount on silica during the grafting reaction of PtMe2(COD).
leads to the evolution of alkane and to the substitution of one alkyl ligand around platinum. Let us now discuss the case of PtMeCl(COD). The evolved methane is lower than the expected value for a consumption of all strong acid sites (one per POM), but its ratio to the grafted platinum is always equal to 1.0, showing that as in the other complexes the Pt-CH3 bond has been selectively cleaved. There is not, as it could be proposed, a reaction of both the Pt-Cl and Pt-CH3 bonds. The fact that the evolved methane is lower than the expected value is probably related to a slower kinetics, and so the reaction is not complete after 15 h. We can then conclude from these studies that the Pt-Me bond of the four complexes is selectively cleaved by the strong acidic proton remaining on the silica-supported silicotungstic acid. If one excludes the case of PtMeCl(COD), the reaction is also selective in the sense that all accessible protons react, in a reasonable time, with the platinum complex. The reactivity order of the platinum complexes deduced from these studies is PtMe2 ðCODÞ PtMe2 ðDMSOÞ2 > PtMe2 ðBipymÞ >> PtMeClðCODÞ As three of the above complexes have two methyl groups, different reaction products could be expected a priori. The resulting supported complexes were then characterized by various physicochemical methods including microanalyses, infrared spectroscopy, solid-state 1D and 2D MAS NMR, and EXAFS. Characterization of the Supported Platinum Organometallic Complexes by Microanalysis and Infrared Spectroscopy. Elemental analyses (Table 2) show that there is always one platinum per polyoxometalate, with the exception of PtMeCl(COD). The amount of carbon is slightly higher than the expected value, but the difference falls in the experimental errors. More interestingly, when the starting organometallic complex contained one heteroelement (S, N, or Cl), microanalyses show that the amount of this element is quite the same on the grafted complex, in agreement with the absence of reaction of ligands that are not methyl groups (chlorine, bipyrimidine, or DMSO). We will now focus more extensively on the reaction of PtMe2(COD) and PtMe2(Bipym) with 37.5 wt % H4SiW12O40/SiO2. Figure 2 shows the infrared spectra of PtMe2(Bipym), of 37.5 wt % H4SiW12O40/SiO2 dehydroxylated at 200 °C, and of the solid obtained after reaction of these two compounds and removal of the physisorbed complex and of the solvent by filtration, washing, and treatment under vacuum. The infrared spectrum of PtMe2(Bipym) shows, in the 4000-1300 cm-1 range, three groups of bands at ca. 2800 -3100, 1550, and 1400 cm-1 attributed respectively to ν(C-H), ν(CtN), and ν(CdC) vibrations (Table 3). The infrared spectrum of H4SiW12O40/SiO2 shows the classical features of silica dehydroxylated at 200 °C (free and H-bonded ν(O-H) bands of silanols at 3747 and 3700-3500 cm-1) and the three harmonics
Table 2. Elemental Analyses (wt %) of the Reaction Products of Platinum Complexes with 37.5 wt % H4SiW12O40/SiO2 Dehydroxylated at 200 °C entry of Table1
platinum complex
Pt
Mo or W
C
X
Pt/POM
C/Pt
X/Pt
3
PtMe2(COD)
2.66
26.12
1.73
1
4 5
PtMe2(COD) PtMe2(DMSO)2
5.19 1.9
56.33 25.74
3.41 1.09
0.84 (X = S)
1.04 0.9
7
PtMe2(Bipym)
2.54
25.18
1.86
0.80 (X = N)
1.1
12
4.4
8
PtMeCl(COD)
1.13
26.28
1.12
0.23 (X = Cl)
0.5
16
1.1
1786
10.7 9.7 9.3
2.7
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of the framework vibrations of silica at 2000-1600 cm-1. Two additional bands, attributed to ν(O-H) and δ(O-H) vibrations of the heteropolyacid,49 can be seen at 3400-2800 and ca. 2200 cm-1. After reaction, the infrared spectrum shows many modifications: the band at 3747 cm-1, due to free silanols, has considerably decreased and a relatively broad signal near 3700 cm-1 has appeared. This phenomenom is due to the interaction of the silanols with alkyl groups and has been reported previously.50 A new band has also appeared at 3300-3400 cm-1. This band is intense for short reaction times but decreases when the reaction time increases. In order to understand if this band was related to the silica support, an additional experiment was done: The anhydrous heteropolyacid in solution in DMSO was contacted with PtMe2(Bipym) also in solution in DMSO. The system was then rapidly quenched (after less than 3 min) by addition of dichloromethane. Note that no methane had evolved for these short contact times. The infrared spectrum of the resulting solid showed also the broad band at 3300-3400 cm-1, which was then attributed to a ν(N-H) vibration. This observation is in agreement with a rapid protonation of the bipyrimidine ligand of the platinum complex, which further reacts with methane evolution. More informations can be obtained by comparison of the infrared bands of the bipyrimidyl ligands in these various species. Table 3 gives the positions of the ν(C-H) of the CH3 group, the ν(CdN), the ν(CdC), and the ν(C-H) of the cycle infrared frequencies of bipyrimidine, PtMe2(Bipym), PtCl2(Bipym), PtMe2(PPh3)2, the quenched species PtMe2(Bipym)þ H4SiW12O40, and the reaction product of PtMe2(Bipym) with H4SiW12O40/SiO2.
Figure 2. Infrared spectra of (a) PtMe2(Bipym); (b) H4SiW12O40/ SiO2 dehydroxylated at 200 °C; and (c) the solid resulting from the reaction of PtMe2(Bipym) with H4SiW12O40/SiO2 dehydroxylated at 200 °C.
Two types of conclusions can be drawn for these data, one about the bipyrimidine ligand and one about the methyl group. First of all, the ν(C-H) band of the methyl group is not strongly modified upon protonation by the anhydrous heteropolyacid (from 2914 to 2918 cm-1). The three other bands observed in the case of the pure PtMe2(Bipym) complex are probably related to the structure in the solid state where interactions between complexes arise. Such interactions are broken in the presence of the bulky polyoxometalate. Upon reaction with the silica-supported heteropolyacid and evolution of methane, the band is shifted to 2935 cm-1, a value quite similar to that found for PtMe2(PPh3)2. It can then be concluded that (i) methyl groups have been kept on the platinum center and (ii) that these methyl groups are in an environment different from that achieved in the starting complex or in its protonated form. The presence of bands characteristic of the bipyrimidine ligand are also a proof of its presence on the solid after the reaction. The ν(CdN) bands are not modified upon coordination of bipyrimidine to platinum, but the high-frequency band is shifted from 1565 to 1582 cm-1 upon protonation of one nitrogen. After reaction the signal is strongly modified (see Figure 2) as the doublet has been replaced by a strong band and two small shoulders, which could be due to some unreacted protonated complex. No clear information can be deduced from the ν(CdC) bands, which are only slightly shifted upon reaction (from 1403 cm-1 in the free ligand to 1411 cm-1 after reaction with anhydrous H4SiW12O40 or with H4SiW12O40/SiO2). In contrast, the ν(C-C) bands are strongly shifted (by more than 20 cm-1) when the organometallic complex is reacted with the heteropolyacid. In conclusion, infrared spectroscopy shows the presence of methyl groups and of the bipyrimide ligand in the reaction product of PtMe2(Bipym) with H4SiW12O40/SiO2, while microanalysis gives one grafted platinum per supported heteropolyacid. In the case of the reaction of PtMe2(COD) with the supported heteropolyacid, quite the same observations are made for the OH vibrations and for the bands characteristic of the cyclooctadiene ligand (ν(C-H) bands between 2870 and 2930 cm-1 and δ(C-H) bands between 1430 and 1500 cm-1 appear (see Supporting Information)). If chemical analysis shows also the presence of one grafted platinum per polyoxometalate, infrared spectroscopy gives less information about the presence or not of cyclooctadiene and methyl groups coordinated to platinum, as the bands are only slightly modified upon reaction. Characterization of the Supported Platinum Complexes by Solid-State 1H and 13C MAS NMR. The complexes formed by reaction of PtMe2 (COD) and PtMe2(Bipym) with H4SiW12O40/SiO2 were also characterized by 1H and 13C MAS NMR. The 1H MAS NMR spectrum of the reaction product
Table 3. Infrared Frequencies of the Reaction Product of PtMe2(Bipym) with H4SiW12O40/SiO2 and of Some Related Compounds CH3 bipyrimidine PtMe2(Bipym)
2914, 2893, 2851,2785
PtCl2(Bipym) PtMe2(PPh3)2 PtMe2(Bipym) þ H4SiW12O40
2934, 2878, 2806 2918
CdN
CdC
C-H of the cycle
1565, 1557
1403
3049, 2979
1568, 1544
1400
3061, 3036, 3015,2975, 2961
1584, 1555
1406
3066
1582, 1558
1412
3084, 3005, 2963
1411
3089, 2964
1596, 1580 PtMe2(Bipym)/H4SiW12O40/SiO2
2935, 2906, 2822
1559 1787
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Table 4. 13C NMR Chemical Shifts of Some Platinum Complexes Containing the Methyl, Bipyrimidine, or Cyclooctadiene Ligands and the Difference (Δ1H) between the CH2 and dCH 1H Chemical Shifts of Those Containing the Cyclooctadiene Group Complexes with Bipyrimidine (Bipym) -CH3 C1 C2 C3 C4
compound bipyrimidine
163.3
158.2
121.8
156.4 -5.3
PtMe2(Bipym) þ
-5
162.7
156.4
153.2
123.8
153.2
157
124
157
this work
SiW12O40/SiO2
Figure 3. 2D double quanta 1H MAS NMR spectrum of the solid resulting from the reaction of PtMe2(COD) with 37.5 wt % H4SiW12O40/SiO2 dehydroxylated at 200 °C.
with PtMe2(COD) shows mainly a decrease of the signal of the acidic protons at ca. 9 ppm, but it is difficult to observe new peaks due to the broad signal of protonated or H-bonded silanol groups between 1 and 5 ppm. However, it is possible to obtain more information by recording the 2D double quanta 1H MAS NMR spectrum, which will give correlations between protons in a close environment. The spectrum is shown in Figure 3 and gives only two correlation peaks on the diagonal for protons located at ca. 1.3 and 4.3 ppm. These two peaks can be attributed to protons of the (-CH2) and (dCH) groups of cyclooctadiene coordinated to platinum. As these solid-state spectra are referenced by using a solid external reference, a small shift (below 1 ppm) could be observed compared to solution-state spectra, but the chemical shift difference between the two peaks should be independent of the reference used. This difference is listed in Table 4 (column Δ1H) for some platinum complexes bearing a COD ligand. It can be seen that in the surface complex this difference is higher than that achieved in the starting PtMe2(COD) and is similar to that found in free cyclooctadiene or in PtMeCl(COD). This result proves then that a reaction occurred resulting in a modification of the environment of the cyclooctadiene ligand. No correlation is observed for methyl groups and for the protonated silanol groups, probably due to their high mobility during the course of the NMR experiment, and so no indication of the presence of a Pt-CH3 species could be deduced from solidstate 1H NMR. The 13 C CP-MAS NMR spectrum of the same solid is shown in Figure 4a. It displays several peaks at 116, 29.5, and 4 ppm. The two peaks at 116 and 29.5 ppm can be attributed to the cyclooctadiene ligand coordinated to platinum (for PtMe2 (COD) the chemical shifts are at 99 and 30 ppm, while they are at 128.7 and 28.2 ppm for free cyclooctadiene; see Table 4). Note also the presence of a relatively broad signal around 80 ppm, which can be attributed to another dCH group, as observed in PtMeCl(COD), where the two double bonds are not equivalent (Table 4). This should then be an indication of a nonsymmetric environment around platinum. As the signal at 4 ppm (which could be attributed to a Pt-CH3 moiety by analogy with other platinum complexes containing methyl groups; see Table 4) was very small, complementary studies were performed with the 13C-enriched complex Pt(13CH3)2(COD), which was reacted with H 4SiW12O40 /SiO 2 dehydroxylated at 200 °C in order to check the presence of a Pt-CH3 group in the surface organometallic complex.
PtMe2(Bipym)
ref
Complexes with Cyclooctadiene (COD) -CH3 CH2 dCH Δ1H
compound COD PtMe2(COD)
4.8
PtMeCl(COD)
5.5
PtMe2(COD) þ SiW12O40/SiO2
compound
ref
28.2
128.7
3.22
29.9
99.0
2.50
65
28.3
113.7
32.0
83.8
3.15
this work
2.95
Complexes with the -CH3 Ligand -CH3 CH2 dCH Δ1H
cis-[PtMe2{P(O Pr)3}2] i
0.8
ref 66
cis-[PtMe2(DMSO)2]
-4.1
this work
[(μ-SMe2)PtMe2] trans-[Pt(Me)Cl(PPh3)2]
-6.4 -9.5
67 68
5.7
69
(Ph2SiP2)PtMe2
Figure 4. 13C NMR spectra of the solids resulting from the reactions of (a) Pt(CH3)2(COD) (CP-MAS) and (b) Pt(13CH3)2(COD) (HPDEC-MAS) with 37.5 wt % H4SiW12O40/SiO2 dehydroxylated at 200 °C. The asterisks correspond to some remaining diethyl ether (15 and 66 ppm) and to 13CH4 (ca. -10 ppm).
Figure 4b shows the 13C HPDEC-MAS NMR spectrum obtained after reaction with the supported polyoxometalate. The spectrum shows mainly a relatively broad signal at ca. 4 ppm, which can then be attributed unequivocally to a PtCH3 group. A 2D HETCOR 1H-13C NMR study (Figure 5) made with a short contact time (in order to observe only correlations at short distances, i.e., C-H bonds) shows that this signal is correlated to protons giving a peak at 0.6 ppm (which are not seen in the 1788
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Figure 5. 2D 1H-13C HETCOR MAS NMR spectrum of the solid resulting from the reaction of PtMe2(COD) with 37.5 wt % H4SiW12O40/SiO2 dehydroxylated at 200 °C.
Figure 6. 13C CP-MAS NMR spectrum of the reaction product of PtMe2(Bipym) with 37.5 wt % H4SiW12O40/SiO2 dehydroxylated at 200 °C. Spinning sidebands are indicated by asterisks.
classical 1H MAS NMR spectrum, as they are below the signal of silanol groups). This 1H NMR signal can then be attributed to the protons of the Pt-CH3 moiety. When looking more precisely at this spectrum, one can observe the presence of another correlation (of lower intensity) between a carbon signal at ca. 6 ppm and a proton at ca. 1.5 ppm. As only 13C-enriched carbons are seen, this implies that there is not only one platinum species on the surface. This is probably related to the presence of some remaining diethyl ether, which can coordinate the platinum atom (see below). By increasing the contact time, correlations at longer distances can be observed. A new correlation peak appears with the -CH2 groups of cyclooctadiene, probing indirectly the existence of the COD ligand in the coordination sphere of platinum. Solid-state NMR has then proved the existence of methyl and COD ligands with a nonsymmetric environment of the COD ligand and with the methyl groups being near some of the -CH2 groups. It is then reasonable to propose that these two groups are linked on the same platinum atom, leading to a PtMex(COD) species with x = 1 due to the evolution of one methane molecule per grafted platinum. The compound obtained by reaction of PtMe2(Bipym) was also characterized by solid-state 13C CP-MAS NMR. The spectrum is shown in Figure 6. It shows clearly three signals at 157, 124, and -5 ppm, which can be attributed to the bipyrimidine ligand and to a methyl group linked to platinum (Table 4). So, in this case also the platinum has kept at least one methyl group. As there is evolution of one methane molecule
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per grafted platinum, it is reasonable to suppose that there is formation of a PtMe(Bipym) group on the surface. Characterization of the Supported Platinum Complex by EXAFS. In conclusion of all above data, we can reasonably propose that the reaction of PtMe2X (X = Bipym, COD) with H4SiW12O40/SiO2 leads to a platinum species with one methyl group and one X ligand. However these data do not allow to determine the bond between the polyoxometalate and the resulting platinum complex. Is it ionic with a formula like [POM]-[PtMe2X]þ (as observed when reacting SnMe4 with the heteropolyoxometalate in solution32), or is it covalent with a direct bond between the polyoxometalate and platinum? A good method for proposing an answer to that question is EXAFS. As a consequence, the grafted organometallic complex was characterized by EXAFS. This study was carried out with the product resulting from the reaction of PtMe2(COD) with H4PMo11VO40 supported on silica (Table 1, entry 9). A molybdic polyacid was chosen rather than a tungstic one due to the proximity of the L levels of tungsten and the LIII level of platinum, which did not allow the obtention of sufficiently resolved spectra. Instead of H4SiMo12O40, we choose H4PMo11VO40. In this case a preferred location of the platinum complex, near molybdenum or near vanadium, may occur, as previously reported for organometallic complexes of substituted polyoxometalates.51 The experimental and simulated data are shown in Figure 7. The best fit is shown in Table 5. A first shell is composed of 5.4 ( 0.9 carbon atoms at 2.12(1) Å coordinating the platinum center, and the fit was improved considering a second shell composed of 3.8 ( 1.8 carbon atoms at 3.06(3) Å. A fit with k1-weighted data gave similar results with larger statistical errors for the second shell: 5.5(9) C at 2.11(1) Å and 4.0(35) C at 2.95(12) Å. Though EXAFS cannot formally distinguish between C, N, and O light backscatterers surrounding a Pt metal center, it seemed more reasonable, according to the results obtained by 13C MAS NMR (methyl and COD ligands), to first consider σ and π-bonded carbon atoms for the closest contribution, as in the starting molecular complex PtMe2(COD), rather than oxygen or nitrogen contributions. A mixed close shell with C and O atoms was then studied, even if it could not be statistically validated, by substituing Pt-C by a Pt-O contribution at a short distance (1.85 to 2.1 Å) with the same Debye-Waller factor for both PtC and Pt-O contributions and no constraints on the NC and NO coordination numbers, but the program always rejected that possibility, giving the same result as above with NO = 0. The consideration of longer Pt--O distances gave a third shell of ca. three oxygen atoms at 2.73(8) Å, which decreased the value of the residue without much change in the two first shells, but since it increased the quality factor, it could then not be considered as statistically validated. The carbon shell at 2.12(1) Å agrees with the presence of the four ethylenic (dCH-) carbon atoms of cyclooctadiene and the methyl group directly bonded to platinum. This distance is in line with the Pt-CH3 bond length of 2.126(7) Å observed by X-ray diffraction for Pt(CH3)(COD)(OH), but is slightly shorter than those determined for the four Pt-Csp2 bond distances of COD, from 2.216 to 2.247 Å.52 However, shorter distances can be found for the four Pt-Csp2 bonds in platinum(II)-cyclooctadiene complexes, for example, between 2.11 and 2.137(6) Å for Pt(COD)(OSi(OtBu)3)2 and from 2.137 to 2.155 Å for Pt(COD)(OTf)2.54 Even shorter distances can be found in cationic platinum(II)-cyclooctadiene complexes, for instance, 2.08(1) and 2.09(1) Å for the shortest Pt-C sp 2 bonds of the 1789
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Figure 7. Experimental and simulated EXAFS curves at the platinum LIII level (left) and corresponding modulus and imaginary part of the Fourier transform (right) for the reaction product of PtMe2(COD) with H4PVMo11O40/SiO2.
Table 5. EXAFS Parameters for the Reaction Product of Pt(CH3)2(COD) with H4PMo11VO40/SiO2a coordination Pt-neighbor
number
R (Å)
D.W. (σ2, Å2)
Pt-C
5.4 (9)
2.12 (1)
0.0081 (10)
Pt-C
3.8 (18)
3.06 (3)
0.014 (7)
Δk: 2.0-14.2 Å-1; ΔR: 0.8-3.0 Å; S02 = 0.94; ΔE0 = 6.3 ( 0.9 eV (the same for all shells); residue: F = 7.6%; quality factor: (Δχ)2/ν = 4.4 (ν = 11/18). The values in parentheses are the statistical errors generated by the EXAFS fitting program RoundMidNight. a
[Pt(Me)(COD)]þ cation, associated with a Pt-CH3 bond length of 2.08(1) Å,55 and 2.07(1) and 2.09(1) Å for the two shortest Pt-Csp2 bonds of the [Pt(Ph)(bpy)(COD)]þ cation.56 For both cations, an asymmetric coordination of the COD ligand on platinum is observed, with two longer Pt-Csp2 bonds. The carbon atoms bonded to platinum and resulting from a methyl (one σ-bonded C) and a COD ligand (four π-bonded C) can be located at quite similar distances from the metal center, as shown in the examples cited above. These carbon atoms would then not be distinguished since the resolution of the EXAFS spectrum is too low, δRmin = π/2kmax = 0.11 Å. It is then not possible here to resolve the signal of the five carbon atoms, and only one signal can be observed at an average distance of 2.12(1) Å. The distance obtained for the second carbon shell of ca. four carbon atoms at 3.06(3) is compatible with the average distance observed between the platinum atom and the four other nonbonding carbon atoms of a COD ligand in the platinum(II)cyclooctadiene complexes cited above. The results of this EXAFS analysis have to be considered with caution because no potential grafting site (siloxy, different faces of the supported HPA, ...) can be completely excluded for platinum and because of the possibility of several different coexisting structures on the surface. At least it is shown that the grafted platinum seems well-dispersed on the surface and is not agglomerated as metal particles. The contribution of 5.4 ( 0.9 light atoms at a short distance from platinum strongly suggests that the COD ligands are still bonded to the grafted platinum centers, but it is not possible to conclude directly that
Figure 8. Proposed structure for the grafted platinum fragment on silica-supported H4PVMo11O40.
five or six carbon atoms are σ- and π-bonded. However since ca. one methane has been emitted per anchored platinum center during the grafting reaction, it then seems reasonable to consider that the grafted platinum is surrounded by one σ- and four πbonded carbon atoms. An asymmetric bonding of the COD ligand on the platinum center cannot be proved or unproved from this analysis. A structure of the complex that corresponds to what is expected from simple molecular modeling studies is proposed in Figure 8. As often observed in vanado-molybdic compounds, the organometallic complex would be located on the vanadium octahedron, whose oxygen atoms are more nucleophilic. However, since short Pt-O distances have not been clearly identified, this complex may reasonably be seen as a partially cationic [Pt(CH3)(COD)]δþ species in interaction with the polyoxometalate, rather than a covalent complex. Synthesis of Molecular Models by Reaction of Anhydrous Heteropolyacids with Platinum Complexes. In order to better characterize the reaction products of the platinum complexes with the heteropolyacid, the reaction was also studied in homogeneous conditions. For this purpose the four complexes were 1790
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Table 6. Evolved Methane during the Grafting Reaction of Platinum Complexes on Anhydrous H4SiW12O40 in DMSO at Room Temperature complex
reaction time (h)
evolved methane per POM
PtMe2(Bipym)
18
2.88
PtMe2(Bipym)
170
2.71
PtMe2(COD)
15
4.09
PtMe(Cl)(COD)
170
2.03
PtMe2(DMSO)2
15
3.60
contacted in DMSO with anhydrous H4SiW12O40, and the solution was kept at room temperature. The amount of evolved methane was determined and is given in Table 6. It can be seen that for PtMe2(DMSO)2 and PtMe2(COD) the reaction is complete in ca. 15 h, while even for very long reaction times it remains incomplete for the PtMe2(Bipym) and PtMe(Cl)(COD) complexes. For the later complex the behavior is quite similar to that observed in heterogeneous conditions. For the complex with bipyrimidine, the reaction does not seem to proceed to more than 75%, as there is no significant difference between 15 and 170 h. This is probably related to a protonation of the ligand by reaction with the acidic proton. A more detailed characterization of the reaction product (isolated as a microcrystalline solid) of PtMe2(COD) with H4SiW12O40 was made by various techniques such as microanalyses, 1H, 13C, 195Pt, and 183W NMR, and X-ray diffraction on a monocrystal. Unfortunately it was not possible to do a full determination of the structure due to the bad quality of the crystal and the high number of unequivalent atoms. However the coordination sphere around platinum could be determined (see below). Chemical analyses (W 45.6 wt %, Pt 16.3 wt %, Si 0.77 wt %, C 12.04 wt %, and S 4.62 wt %) led to various conclusions. First of all, the W/Pt ratio was 2.97, in agreement with a compound containing four platinums per polyoxometalate. As there was evolution of four methane per polyoxometalate, one can reasonably conclude that only one methyl group of PtMe2(COD) had reacted with the acidic protons. Another key point is the presence of sulfur in a non-negligible amount. As this element is present only in DMSO, it is necessary to conclude that this molecule was present in the solid. The S/Pt ratio (1.72) indicates that there are 1.75 DMSO molecules per platinum. Finally the C/ Pt ratio (12) indicates that the COD ligand has been kept, and the formula of the reaction product could then be written as [SiW12O40][Pt(CH3)(COD)]4 3 7DMSO. The 1H NMR spectrum in DMSO (see Supporting Information) shows signals characteristic of the Pt-CH3 group at 0.75 ppm (with a JPt-H constant of 69 Hz). Compared to PtMe2(COD) (δ = 0.58 ppm and JPt-H = 84 Hz) the signal has been slightly shifted and the J constant has decreased. The spectrum shows also peaks characteristic of the -CH2 and dCH groups of cyclooctadiene at 2.25 and 5.45-5.72 ppm, respectively. The most interesting feature is the presence of at least three peaks for the dCH groups, showing that the double bonds are not equivalent in the platinum complex. The last information is the absence of the signal of the acidic proton of H4SiW12O40 (which should be found around 8 ppm), in agreement with its reaction with PtMe2(COD) and the evolution of methane. The 13C NMR spectrum (see Supporting Information) shows five peaks displaying coupling constants with platinum. First of all, a peak at 4.4 ppm (JPt-C = 604 Hz) can be attributed to a PtMe species. Compared to the starting complex, the main feature
is a decrease of the J constant (from 778 to 604 Hz). As for the proton NMR the 13C signals of the COD ligand show that it is in a dissymmetric environment, as it can be found, for example, in PtMe(Cl)(COD). Two peaks at 28.5 (JPt-C =17.8 Hz) and 31.0 ppm (JPt-C = 14.5 Hz) can be attributed to the -CH2 groups, while two other peaks at 106.7 (JPt-C = 130 Hz) and 113.8 ppm (JPt-C = 44 Hz) are due to the dCH groups. The large difference between the two coupling constants of the two dCH is in agreement with the preferential coordination of one double bond of the COD moiety. In conclusion, 1H and 13C NMR studies show the presence of a PtMe(COD) fragment with a nonsymmetric bonding of the cyclooctadiene ligand. The 195Pt NMR spectrum shows only one signal at -3962 ppm, shifted by ca. 400 ppm compared to the starting PtMe2(COD) complex. The presence of only one peak can be interpreted as (i) the existence of only one platinum species or (ii) several platinum species in rapid exchange. As the solvent was DMSO (for solubility problems), it has not been possible to perform studies at low temperature. However a study at higher temperatures has shown (see Supporting Information) that the platinum chemical shift obeys the Curie law,57 in agreement with a paramagnetic species and thus a platinum(II) species. This study has also shown that the complex decomposes at temperatures higher than 80 °C. In conclusion, the 195Pt NMR study has shown the presence of one peak due to a platinum(II) species. 183 W NMR gives additional information about the polyoxometalate structure in the complex. The spectrum of hydrated H4SiW12O40 in DMSO displays only one signal at -95.5 ppm, in agreement with the R Keggin structure, where all tungsten atoms are equivalent.58 The spectrum of the anhydrous heteropolyacid is quite similar and is only slightly shifted to -93.1 ppm. After reaction with PtMe2(COD) the spectrum shows also only one signal at -92.4 ppm. This result leads to the following conclusions: (i) the polyoxometalate has kept the Keggin structure and all tungsten atoms are equivalent (R isomer). If we suppose that there is a bond between the polyoxometalate and the platinum group, it is then necessary that it is through three oxygen atoms of a face of a W3O13 triad, each platinum coordinating one triad; (ii) the chemical shift of the tungsten atom is quite the same as that of the pure heteropolyacid, so the interaction between the platinum group and the polyoxometalate is very weak, as observed in cation/anion interactions. As a result of these studies, we can reasonably conclude that in DMSO solution the platinum complex acts as a countercation for the polyoxometalate, which has maintained its Keggin structure. Monocrystals were obtained by a diffusion method: pentane (not miscible in DMSO) was first added on the DMSO solution of the complex, and acetone (miscible in DMSO) was then added on pentane. Diffusion of acetone through pentane resulted in a DMSO/acetone mixture where the complex was less soluble. After one week crystals suitable for an X-ray structural determination were obtained. Unfortunately the quality of the crystal was not sufficient to resolve completely the structure, and the best R factor was 8.24%. However these results give sufficient information about the complex, which can be seen as an organometallic platinum salt of [SiW12O40]4- with no direct bond betwen the two species, as the four platinum atoms are located at a relatively long distance from the polyoxometalate. The structure determination shows that the polyoxometalate has retained the R-Keggin structure with W-O and Si-O distances quite comparable to those usually found for this compound.59-61 The coordination 1791
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Organometallics sphere of one of the four unequivalent platinum atoms could be determined. In that species platinum is coordinated to one methyl group (d(Pt-C) = 2.082 Å), one cyclooctadiene via its double bonds (d(Pt-Csp2) = 2.168, 2.171, 2.249, and 2.427 Å), and one DMSO molecule via its sulfur atom (d(Pt-S) = 2.157 Å) in a square-planar arrangement. The Pt-CH3 distance is quite similar to those found for platinum methyl complexes.52,55,62 This distance is also in relatively good agreement with that found by EXAFS for the supported complex even if in that case no solvent molecule was coordinated to platinum. As expected from 13C NMR data, cyclooctadiene is not bonded symmetrically but with one double bond near platinum and the second one at a longer distance. Here also the distances are quite comparable to those found in the literature for such Pt-Csp2 bonds.53-55,63 A drawing of this complex shows clearly also that the methyl group is near one double bond. Finally DMSO is coordinated via its sulfur atom, contrary to what had been observed in the case of the reaction of SnMe4 with anhydrous H4SiW12O40.32 Such a behavior is also in agreement with the literature, as DMSO is generally coordinated to Pt via sulfur in platinum(II) complexes and via oxygen in platinum(IV) complexes.64
’ CONCLUSION We have shown that platinum(II) methyl complexes react with silica-supported heteropolyacids leading to the evolution of methane and the formation of a grafted platinum species, which has been characterized by various physicochemical methods including solid-state NMR and EXAFS. All data indicate that the interaction between the polyoxometalate and the platinum complex is ionic, with the charge being on the platinum atom. Molecular analogues were synthesized by reaction of the corresponding platinum complexes with the anhydrous heteropolyacid in DMSO. The solid obtained in this way displays also an ionic interaction between the inorganic and organometallic entities, with a DMSO molecule coordinated to platinum. When looking at Scheme 1, which displays the partial oxidation of methane to methanol derivatives, the complex described in this paper leads exactly to the intermediate in the first step of the cycle (before the activation of the methane molecule), while the grafting reaction is the reverse of the methane activation. As a consequence, the complexes for which the grafting reaction is the most difficult (typically with a chlorine ligand) should lead to the most active catalysts. The next step in the catalytic cycle displayed in Scheme 1 involves the reduction of the platinum(II) species to a platinum(IV) species. This step could be favored by the presence, in close vicinity of the platinum complex, of a polyoxometalate that could be easily reduced. This could be the driving force allowing the equilibrium to be displaced to the oxidation of methane. ’ ASSOCIATED CONTENT
bS
Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.
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Organometallics
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dx.doi.org/10.1021/om1003539 |Organometallics 2011, 30, 1783–1793
