Article pubs.acs.org/Organometallics
Catalytic Activity of Molybdenum(II) Complexes in Homogeneous and Heterogeneous Conditions Maria Vasconcellos Dias,† Marta S. Saraiva,† Paula Ferreira,*,‡ and Maria José Calhorda*,† †
Departamento de Química e Bioquímica, CQB, Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal ‡ CICECO - Aveiro Institute of Materials, Department of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal S Supporting Information *
ABSTRACT: The new complexes [MoBr(η 3 -C 3 H 5 )(CO)2(L)2] (C1) and [MX2(CO)3(L)2] (M = Mo(II), X = I (C2); M = Mo(II), X = Br (C3); M = W(II), X = I (C4); M = W(II), X = Br (C5)) were synthesized by reaction of 2amino-1,3,4-thiadiazole (L) with [MoBr(η 3 -C 3 H 5 )(CO)2(NCCH3)2] (1), [MoI2(CO)3(CH3CN)2](M = Mo (2); M = W (4)), or [MoBr2(CO)3(CH3CN)2](M = Mo (3); M = W (5)) in 2:1 ratio. The five complexes were immobilized in MCM-41, yielding the materials MCM-Cn (n = 1−5), and C1 was also immobilized in silica gel (Silica-C1) and in a polyhedral oligomeric silsesquioxane (Cube-C1). Complexes and materials were fully characterized by spectroscopic techniques and elemental analysis. DFT calculations showed that several C1 isomers should coexist. The as synthesized and supported complexes were tested as catalysts on the oxidation of geraniol, cis-hex3-en-1-ol, trans-hex-3-en-1-ol, (S)-limonene, and 1-octene. The conversions and TOF significantly depend on the complex and the nature of the substrate. The general conclusions are (i) complex C1 has the highest activity; (ii) tungsten complexes C4 and C5 are more active than the molybdenum analogues; (iii) the immobilization of the catalysts improves the performance; and (iv) silica gel and the polyhedral oligomeric silsesquioxane supports modify the selectivity, leading to products different from the one obtained with MCM for specific substrates.
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INTRODUCTION The need to improve catalysts taking into account not only the chemical process but also the environmental issues remains a challenge. While homogeneous catalysts are usually more controllable in terms of selectivity, heterogeneous systems allow an easier separation of the products and recovery of the catalyst.1,2 The mesoporous silica-based materials MCM-41, developed by Mobil Corporation in 1992,3,4 are among the most efficient supports, not only because they have a high surface area inside the ordered structure of the one-dimensional channels but also because the surface silanol groups make functionalization with organometallic complexes, for instance, easy. The strong covalent bonds thus formed prevent lixiviation of the catalyst, increasing its lifetime,5−9 and lead to active catalysts.10 While the activity is sometimes higher than in the homogeneous parent compound,9 in others secondary reactions take place, damaging the catalyst.7,11−14 The nature of the surface makes these MCM-41-derived materials better catalysts than silica,15,16 but they are more expensive. Other forms of silica, namely, polyhedral oligomeric silsesquioxane (POSS),17−19 have also been shown to coordinate metallic fragments and generate multiple site catalysts.20 These species are based on the (RSiO1.5)8 oligomer, which organizes as a cube held by Si−O−Si bonds and carrying R groups at the vertices.21−33 A suitable choice of these groups will allow © 2015 American Chemical Society
functionalization, as reported, so that the POSS can behave as a ligand.34−38,20 Many examples of applications of these species are known, ranging from catalysis39−51 to analytical chemistry52−55 and special materials.56−60 Epoxides are well known as very important intermediates in the industry of fine chemicals and pharmaceuticals that can be formed by oxidation of the corresponding olefin in the presence of various oxygen sources. Transition metal complexes proved to be effective catalysts for the epoxidation of alkenes, but molybdenum and tungsten are probably the best known transition-metal catalysts for alkene epoxidation using alkyl hydroperoxides, because they are the metals in the industrial ARCO-Halcon process for homogeneous olefin epoxidation with tert-butylhydroperoxide (TBHP) as oxidant.61,62 We reported the functionalization of a POSS with Mo(II) and its application as epoxidation catalyst.20 Since the [MoBr(η3C3H5)(CO)2(L)] complex, with L = 2,2′-pyridylamine, was not a good catalyst, neither in homogeneous conditions nor immobilized in POSS or in MCM-41, we tried to improve the performance of the catalyst and understand the role of the support (POSS, silica, MCM-41). Indeed, many nitrogen donor ligands bound to Mo(II) after substitution of NCMe in Received: October 23, 2014 Published: April 13, 2015 1465
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Organometallics [MoBr(η3-C3H5)(CO)2(NCCH3)2]63 have proved to be very good precursors for olefin epoxidation, being first oxidized to Mo(VI) active species.64 In some supports, heterogeneous catalysts were even more effective than homogeneous catalysts.9 The ligand chosen was 2-amino-1,3,4-thiadiazole, which offers several possibilities for coordination through the ring atoms and bears a NH2 group, which is important for immobilization purposes.65−67 Complexes of Co(II) and Ni(II) with the related 2-aminothiazole were reported to inhibit cancer cell growth, mainly owing to their complex−DNA binding ability, suggesting other applications for the new complexes.68 We report herein the synthesis of the [MoBr(η3-C3H5)(CO)2(L)2] complex with L = 2-amino-1,3,4-thiadiazole, its characterization, and immobilization in MCM-41, POSS, and silica, as well as the catalytic activity of the complex and the materials in the oxidation of several alkenes. Other Mo(II) complexes and their W(II) analogues, with the general formula [MX2(CO)3(L)2] in which X = Br or I,69 were also synthesized and immobilized in MCM-41, in order to extend the scope of the study to other metal fragments.
Figure 1. FTIR spectra of [MoBr(η3-C3H5)(CO)2(NCCH3)2] (1), the ligand 2-amino-1,3,4-thiadiazole (L), and complex [MoBr(η3C3H5)(CO)2(L)2] (C1).
The FTIR spectra of complexes C2−C5 are also characterized by the strong νCO carbonyl vibrations observed, for instance, in complex C2 at 1906, 1938, and 2008 cm−1 and in the precursor 2 at 1921 and 2016 cm−1, while the νCN vibration of L appears at 1597 cm−1 in C2. The different number of carbonyl bands observed is associated with the number of different isomers coexisting in the solid state, owing to the possible geometries for heptacoordinate complexes.70,69 The 1H NMR spectrum of complex C1 is characterized by the signals of the two protons of the ligand L (that of the ring at 8.58 ppm and the amine proton at 7.24 ppm, both shifted relative to their position in the spectrum of the free ligand, at 6.9 and 8.3 ppm, respectively) and those of the allyl group (Hanti at 1.09, Hsyn at 3.37, and Hmeso at 5.77 ppm). These are observed in the spectrum of 1 at 1.24 (Hanti), 3.45 (Hsyn), and 3.68 (Hmeso) for the most abundant isomer. The peaks associated with the two rings and the three allyl carbon atoms are observed in the 13C NMR spectra. In complexes C2− C5, only the ligand L hydrogen and carbon atoms exhibit peaks in the 1H and the 13C NMR spectra. The previous spectroscopic results indicate that L coordinates to the molybdenum and tungsten centers in all the complexes, but only in C1 does the integration of proton signals in the NMR spectrum allow the conclusion that two L ligands are present. This result is confirmed in all complexes C1−C5 by elemental analysis. None of the spectroscopic data, however, indicate which nitrogen atom is bound to the metal. In order to answer this question, DFT calculations (see Computational Details) were performed. We optimized the geometries with L bound through the two ring nitrogen atoms (Figure 2) and considered the possibility of forming the equatorial or the axial isomer (Scheme 2).
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RESULTS AND DISCUSSION Chemical Studies: Complexes. The allyl complex [MoBr(η3-C3H5)(CO)2(NCCH3)2]63 (1) reacts with the ligand 2amino-1,3,4-thiadiazole (L) in a 1:2 ratio to form the new complex [MoBr(η3-C3H5)(CO)2(L)2] (C1). A similar reaction occurs starting from the other Mo(II) and W(II) complexes, [MoI2(CO)3(CH3CN)2] (2), [MoBr2(CO)3(CH3CN)2] (3), [WI2(CO)3(CH3CN)2] (4), and [WBr2(CO)3(CH3C)N)2]69,7 (5), leading to the formation of the new complexes [MX2(CO)3(L)2] (M = Mo, X = I (C2); M = Mo, X = Br (C3); M = W, X = I (C4); M = W, X = Br (C5)), as shown in Scheme 1. Scheme 1. Synthesis of Complexes [MoBr(η3C3H5)(CO)2(L)2] and [MX2(CO)3(L)2]
The new organometallic complexes were characterized by elemental analysis, FTIR, and 1H and 13C NMR spectroscopy. The formation of complex C1 is supported by the appearance in the FTIR spectrum (Figure 1) of the two νCO stretching modes, which appear at 1923 and 1948 cm−1, shifted from their position in the precursor complex 1 (1850 and 1947 cm−1), and the deviation of the νCN stretching mode from 1622 cm−1 in the free L to 1610 cm−1 upon coordination. The characteristic νCN bands at 2320 and 2287 cm−1 in 1 have disappeared.
Figure 2. DFT-optimized structures of the isomers of [MoBr(η3C3H5)(CO)2(L)2] (C1), with relative energies (kcal mol−1). 1466
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Organometallics Scheme 2. More Stable Isomers of Complexes [MoBr(η3C3H5)(CO)2(L)2]
complexes 1−5, affording the functionalized materials MCMC1 to MCM-C5, in the presence of a base. This procedure of immobilizing molybdenum and tungsten complexes of L is illustrated in Scheme 3 and has been used before.11 MCM is characterized by a hexagonal unit cell, leading to four peaks in the 2θ = 2−10° range of the X-ray powder diffractogram, the strongest reflection being assigned to d100, and the three weaker reflections at higher angles to d110, d200, and d210, as shown in Figure 3. These peaks reflect the highly
The energy differences are small, and hydrogen bonds can be formed. In the two equatorial isomers, E1 and E2, there is a N−H···N hydrogen bond, and the only difference is the nitrogen binding the metal. In the lowest energy species (E2), the Mo−N bond is formed with the nitrogen atom adjacent to the C−NH2 bond. The same information results from the comparison between A1 and A2, since the same N atom makes the Mo−N bond in the lowest energy isomer, A2. Weak N− H···Br hydrogen bonds are formed in both A1 and A2, but in A2 there is also a N−H···O one. It seems therefore safe to conclude that this is the preferred coordination mode of the L ligand to molybdenum. The preference for the isomer is not so well defined, as E2 and A2 differ by only 1.3 kcal mol−1, and fluxionality should be observed. Immobilization of Complexes. The five complexes were immobilized in MCM-41 (MCM), and the allyl derivative [MoBr(η3-C3H5)(CO)2(L)2] (C1) was also immobilized in an octasilsesquioxane (Cube) and in silica gel (Silica), in order to prepare heterogeneous catalysts with different properties and compare the role of the support on the catalytic activity of the Mo(II) complexes in homogeneous and heterogeneous conditions. In all cases, 3-chloropropyltriethoxysilane was used as a spacer to react directly with the material walls using the (RO)3Si− functionality, while the Cl side reacts with the NH bond of the 2-amino-1,3,4-thiadiazole ligand. We used a three-step tethering process to functionalize the MCM and the silica gel frameworks. In this procedure, first a silylated spacer reacts with a free silanol group of the silicabased material. This silylation reaction can modify both the physical and chemical properties of a given surface. After, the chloride is replaced by the 2-amino-1,3,4-thiadiazole. Pristine MCM obtained by a template approach8 reacted with an excess of 3-chloropropyltriethoxysilane in dry toluene for 24 h under reflux and with stirring. A white powder (MCMPr) was filtered off and dried under vacuum at room temperature for 4 h. Reaction of this material with the 2amino-1,3,4-thiadiazole led to the formation of the material MCM-L, which was then used as a ligand toward the M(II)
Figure 3. XRD powder patterns of mesoporous materials MCM, MCM-Pr, MCM-L, and MCM-C1.
ordered pore system.71 The lattice parameter a, calculated assuming the hexagonal unit cell (a = 2d100/√3) is equal to 41.0 Å. Upon functionalizing MCM, all X-ray diffraction peaks are still observed in the materials, indicating the retention of the hexagonal structure. However, the intensity of the reflections decreases, resulting from the immobilization of more and more groups inside the wall after reaction with Si-OH groups with consequent reduction of the X-ray scattering contrast between the MCM walls and pore-filled channels.72,73 The same trend is observed after grafting the other metal complexes to form MCM-C2, MCM-C3, MCM-C4, and MCM-C5 (not shown). In all cases there was a clear reduction of the peak intensities when functionalizing the pores. The 77 K N2 adsorption/desorption isotherms of the MCM and MCM-Pr materials (Figure 4) have the type IV shape according to the IUPAC classification74 and are characteristic of
Scheme 3. Synthesis of Materials MCM-Pr, MCM-L, and MCM-C1 to MCM-C5
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Information, SI). The well-defined inflection on the N2 isotherm observed for the precursor material MCM (Figure 4) leads to a very narrow pore size distribution (PSD) curve, with a maximum value of 3.60 nm. Immobilization of bulky groups into the MCM channels is reflected in a shift of the PSD maximum curve (d p) toward smaller pore sizes with concomitant reduction of the intensity, indicating decreased uniformity of the pores. The pore width decreases from 3.60 nm in the pristine MCM to 2.87 nm in MCM-C1. The same observations can be made for all the new materials MCM-C2 to MCM-C5. Elemental analysis shows that an amount of metal between 3.7 and ∼5.1 wt % was introduced into the mesoporous materials. The molybdenum(II) contents ranged from 4.0 wt % (0.40 mmol g−1) in MCM-C1 to 3.7 wt % (0.37 mmol g−1) in MCM-C2, to 5.1 wt % (0.51 mmol g−1) in MCM-C3. The tungsten-containing materials revelead an amount of 4.7 wt % (0.47 mmol g−1) in MCM-C4 and 4.0 wt % (0.40 mmol g−1) in MCM-C5. The FTIR spectra of all the materials show the bands characteristic of the asymmetric νSi−O−Si modes. They appear at 1256, 1002, and 798 cm−1 in MCM-Pr. Stretching νC−H modes at 2800−3000 cm−1 indicate the presence of the chloropropyltriethoxysilane spacer, both in MCM-L and in MCM-C1, and the allyl group in the complex. The characteristic νCO bands reflect the coordination of the Mo(CO)2 fragment, appearing at 1944 and 1860 cm−1, slightly shifted from their position in the spectrum of C1 as the ligand was modified. In the FTIR spectrum of MCM-C2, vibration modes can be assigned, not only to the silicious structure of the material (asymmetric Si− O−Si modes at 1063 and 688 cm−1) but also to the immobilized species: νC−H stretching modes of the propyl chain at 2979 and 2915 cm−1; νCO stretchings at 1886, 1935, and 2015 cm−1; νCN of the ligand at 1627 cm−1. The spectra of materials MCM-C3 to MCM-C5 are comparable. By comparison with the proposed structure of C1−C5, it is considered that two immobilized L ligands in MCM-L coordinate to one metal center of each precursor, substituting two nitrile ligands. Indeed, there is no evidence for coordinated acetonitrile (νCN stretching bands). Further information about the nature of the immobilized species may be obtained from 13C and 29Si NMR spectroscopy (Figures 5 and 6). The 13C CP MAS solid-state resonances in the MCM-Pr material are assigned to the three carbons of the propyl group and to the carbons of unreacted ethoxyl groups of
Figure 4. Nitrogen adsorption isotherms of materials MCM, MCMPr, MCM-L, and MCM-C1 at 77 K.
mesoporous solids. A sharp inflection is observed between the relative pressures (p/p0) of 0.20−0.40 owing to capillary condensation of N2 inside the primary mesoporous channels. The precursor material MCM thus exhibits a narrow pore size distribution, with an average pore diameter (dp) of 3.60 nm, a specific surface area of 1037 m2 g−1, and a pore volume of 0.97 cm3 g−1 (Table 1). All the functionalized materials, MCM-C1 Table 1. Textural Parameters of MCM, MCM-Pr, MCM-L, MCM-C1, MCM-C2, MCM-C3, MCM-C4, and MCM-C5 Materials, from N2 Isotherms at 77 K sample
d100 (Å)
SBET (m2 g−1)
ΔSBETa (%)
Vp (cm3 g−1)
ΔVpb (%)
dBJHc (nm)
MCM MCM-Pr MCM-L MCM-C1 MCM-C2 MCM-C3 MCM-C4 MCM-C5
35.5 33.1 32.9 32.9 32.2 33.0 30.3 30.6
1037 948 929 462 603 502 570 571
9 10 55 42 52 45 45
0.97 0.61 0.57 0.26 0.37 0.31 0.39 0.39
37 41 73 62 68 60 60
3.60 2.87 2.87 2.87 3.03 3.03 2.87 2.87
a
Variation of surface area relative to parent MCM-41. bVariation of total pore volume in relation to parent MCM-41. cMedian pore width determined by the BJH method.
to MCM-C5, also display reversible type IV N2 adsorption isotherms, indicating that the initial material structure is maintained and some of the pores remain accessible after binding of the metal centers. Each successive functionalization of MCM in the three steps shown in Scheme 3 leads to a decrease in N2 uptake, at high p/p0, and both the surface area and pore volume decrease (Table 1), in agreement with the reported values for this type of materials.75,76 These results suggest that the organometallic complexes are grafted in the internal surface of the material. The textural parameters, calculated by the BJH method,77 are summarized in Table 1 and are comparable with others from modified mesoporous materials previously reported.13,9 The calculated pore size distribution curves for MCM-C2 and MCM-C3 and all the materials used in its synthesis (MCM, MCM-Pr, and MCM-L) are shown in Figure S1 (Supporting
Figure 5. 13C CP MAS NMR spectra of materials MCM-Pr, MCM-L, and MCM-C1. 1468
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Qn-type signals, where Qn = Si(OSi)n(OH)4−n, assigned to the characteristic 29Si chemical environments in the MCM-41 material. Unmodified MCM displays two broad convoluted resonances in the 29Si CP MAS NMR spectrum at δ −109.9 and −100.4 ppm, assigned to Q4 and Q3 species of the silica framework. A weak shoulder is also observed at δ −91.2 ppm for the Q2 species. Both Q3 and Q2 sites have free silanol groups that are available for functionalization through esterification reaction. After reaction with 3-chloropropyltriethoxysilane, new organosilica environments arise, identified as Tm signals, with the general formula Tm = R′Si(OSi)m(OR)3−m. The concomitant decrease of the intensity of the peaks assigned to Q2 and Q3 is observed while, comparatively, Q4 intensity is enhanced. T1 is mainly observed for MCM-Pr, indicating that further condensation occurs upon functionalization. To investigate the effect of the support on catalytic activity (see below), complex C1 was immobilized in a POSS cage (Cube) and in silica. Reaction between a Cube previously functionalized with L ligands (Cube-L), containing a Si8O8 core and eight −(CH2)3NHC2HN2S arms, and complex [Mo(η3C3H5)Br(CO)2(NCCH3)2] (1) in a 1:8 ratio led to a new yellow solid (Cube-C1). Elemental analysis indicated that two Mo(η3-C3H5)Br(CO)2(NCCH3) units were attached to each cube and that only one coordinated nitrile was substituted (Scheme 4). The FTIR spectrum of Cube-C1 exhibits the characteristic νSi−O−Si vibrational modes of the Cube cage at 1166 and 681 cm−1, the νC−H modes of the propyl chain at 2934 and 2887 cm−1, and the characteristic νCN, νC−N, and νC−H modes of the L ligand at 1600, 1353, and 3100 and 3170 cm−1. All these bands were observed with small shifts in the FTIR spectrum of the Cube-C1 precursor. Two new bands of Cube-C1 observed at 1856 and 1948 cm−1 identify the νCO stretching modes of the organometallic fragment (1850 and 1947 cm−1 in 1), but the νCN bands are too weak to be observed.
Figure 6. 29Si MAS NMR spectra of materials MCM, MCM-Pr, MCM-L, and MCM-C1.
the 3-chloropropyltriethoxysilane (Figure 5). The L ligand has two carbon atoms, but their signals are not seen in the spectrum of MCM-L or MCM-C1. This observation may be related with a relatively low degree of functionalization or may be due to the presence of tertiary and quaternary carbons, which require a long acquisition and relaxation time in the NMR spectrometer. It should be emphasized that the signal assigned to CH2-Cl of the MCM-Pr seems to broaden. After reaction with the metal fragments the peaks are slightly split and broadened. The resonances of the carbon atoms of the allyl ligand or carbonyl groups of the metal fragment are not evident. 29 Si MAS NMR spectra of MCM, MCM-Pr, MCM-L, and MCM-C1 are shown in Figure 6. It is possible to observe the
Scheme 4. Functionalization of Cube Containing Eight 3-Chloropropyl Arms with L, Followed by Reaction with Complex [MoBr(η3-C3H5)(CO)2(NCCH3)2] to Form Cube-C1
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Organometallics The 13C CP MAS spectrum of Cube-C1 (Figure 7, left) shows the three signals of the propyl chain carbons at 9.8
coordination. This result is in agreement with literature, as, in a similar functionalization, the effect of the metal was also negligible.20 The presence of T2 species was not expected and may be related to the presence of a small quantity of the precursor that does not completely condensate. The possibility that during the reaction with 2-amino-1,3,4-thiadiazole some of silsesquioxane cages open, leading to the formation of T2 species, cannot be completely disregarded. The reaction between complex [Mo(η 3 -C 3 H 5 )Br(CO)2(NCCH3)2] (1) and silica functionalized with the same 3-(2-amino-1,3,4-thiadiazole)propyl ligand (Silica-L,65 Scheme 5) afforded a new material (Silica-C1). Elemental analysis shows approximately 7.63 wt % Mo (0.76 mmol g−1) of molybdenum in Silica-C1, while the values for C, H, N, and S suggest that one nitrile remains coordinated to the metal. The amount of molybdenum immobilized in silica is much larger than in MCM-41 (3.7 wt %), probably owing to the easier access to the surface of silica than to the inside walls of the mesoporous material. The FTIR spectrum of Silica-C1 displays the characteristic νCO bands at 1851 and 1947 cm−1, slightly shifted from their position in the precursor complex 1 (1863 and 1935 cm−1), and the νCN modes from the ligand L as a broad band at 1632 cm−1. Peaks at 2291 and 2317 cm−1 can be assigned to the νCN bands of NCMe, indicating the presence of coordinated ligand and suggesting the proposed coordination geometry of the metal fragment at the silica surface as represented in Scheme 5. The solid-state 13C CP MAS NMR spectra of both Silica-L and Silica-C1 show (Figure 8, left) the signals from the aliphatic carbon chain at 9.4 ppm (SiCH2CH2CH2N), 25.9 ppm (SiCH 2 CH 2 CH 2 N), and 35.4 and 45.4 ppm (SiCH2CH2CH2N), as well as a signal from the SiOCH2CH3 group at 46.2 ppm, resulting from incomplete hydrolysis of the precursor. The signals at 143.4 and 155.3 ppm can be assigned to the thiadiazole ring carbons. All values are consistent with previously reported data for this type of silicon bond.24 No signal from the coordinated nitrile carbon atoms or the allyl carbons can be assigned, owing to the presence of several peaks
Figure 7. 13C CP MAS and 29Si MAS NMR spectra of Cube-L and Cube-C1.
(SiCH2CH2CH2N), 22.9 (SiCH2CH2CH2N), and 41.2 ppm (SiCH2CH2CH2N), shifted upfield compared to those in the precursor Cube-L (signals at 10.3, 23.5, and 52.0 ppm, respectively). Two other resonances at δ 144.3 and 158.3 ppm are observable and are assigned to the two carbons of the thiadiazole ring. After coordination with the metal fragment 1, another peak appears at 215.2 ppm, which may be related with the carbon of the carbonyl groups. Signals at 34.7 and 160.45 ppm, in the Cube-C1 spectrum, are solvent peaks (dimethylformamide, DMF). The assignment of the methyl protons of coordinated acetonitrile and allyl carbons is not conclusive, as the strong peaks from the propyl chain carbons are observed in the same region. 29 Si MAS NMR spectra (Figure 7, right) present only Tm 29Si chemical environments, from which T3 and T2 species can be identified. They are observed at −68.5 and −60.5 ppm, respectively,19,78 and do not change significantly upon metal Scheme 5. Synthesis of Materials Silica-Pr, Silica-L, and Silica-C1
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Figure 8. 13C CP MAS NMR (left) and (right) of Silica-C1 and Silica-L.
29
Si CPMAS NMR spectra
in the respective region. The carbonyl carbon cannot be easily identified on the 13C CP MAS NMR spectrum of Silica-C1. The 29Si CP MAS NMR spectrum (Figure 8, right) of the precursor silica gel material displays signals of Q4 species at −109.2 ppm, Q3 at −99.3 ppm, and a shoulder at about −89.7 ppm from Q2. After reaction with the spacer and the ligand, the functionalized Silica-L shows an enhancement of the Q4 intensity as a result of the quantitative reduction of Q3 and Q2. Reaction with the Mo(II) fragment has a negligible effect of the chemical shifts of Qn species. T3 and T2 species are present in the spectra of Silica-L and Silica-C1 and result from the reaction of the silylated spacer. Upon metal coordination, the intensity of the T2 species becomes less important relative to T3, probably due to further condensation of the germinal silicon species under the reaction conditions. The immobilization of complex 1 in the three supports and 2015 cm−1 occurs in different ways. Experimental evidence indicates that in MCM two acetonitriles of [MoBr(η3C3H5)(CO)2(CH3CN)2] (1) are replaced by two thiadiazole rings hanging from the inner walls, while only one seems to be substituted both in the Cube cage, where the arms are directed away from one another (Cube-C1), and in silica (Silica-C1), where the surface can be considered locally flat. The amount of molybdenum introduced in the support is about twice as large in silica as in MCM-41, probably owing to the absence of constraints. Catalytic Studies. All the new complexes C1−C5 and materials MCM-C1, MCM-C2, MCM-C3, MCM-C4, MCMC5, Cube-C1, and Silica-C1 were tested as precursors in the catalytic oxidation of alkenes: geraniol (ger), cis-hex-3-en-1-ol (cis-3), trans-hex-3-en-1ol (trans-3), (S)-limonene (S-lim), and 1-octene (1-oct). The reaction was carried out in dichloromethane, with tert-butyl hydroperoxide (TBHP, in decane) as oxygen donor, at 328 K (see details in the Experimental Section). No reaction took place in the absence of a catalyst. The total conversions, calculated after 24 h reaction, the turnover frequencies (TOF), and the selectivity of each catalyst are presented in Figures 9 and 10 and in Tables 2−4. The conversions of all substrates with all the catalysts are shown in Figure 9. The first observation is that complex [MoBr(η3-C3H5)(CO)2(L)2] (C1) converts all the substrates, except 1-oct, with significant yields, reaching almost 100% for
Figure 9. Conversions calculated after 24 h reaction for complexes C1, C2, C3, C4, and C5 (top) and for materials MCM-C1, MCM-C2, MCM-C3, MCM-C4, MCM-C5, Silica-C1, and Cube-C1 (bottom).
ger, cis-3, and trans-3. When it is immobilized, conversions become closer to 100% for all substrates, except trans-3 and 1oct in MCM-C1 (86.1%, 4%), S-lim and 1-oct in Cube-C1 (53.7%, 66%), and 1-oct in Silica-C1 (2.5%). For instance, 1oct has been found very difficult to oxidize by several similar catalysts.11 The immobilization in all the tested supports increases the activity of complex C1 in general. The two molybdenum complexes [MoI2(CO)3(L)2] (C2) and [MoBr2(CO)3(L)2] (C3) achieve very low (or no) conversions of all the substrates. The immobilization of C2 in MCM barely improves the performance, while MCM-C3 reaches conversions of 50−60% for ger, trans-3, and S-lim. Substitution of Mo by W has dramatic consequences in the capability of the complexes C4 and C5 to oxidize all the substrates, except 1-oct, with yields close to 100% for trans-3, for instance, in both cases. Enhanced activity of W complexes relative to the Mo ones has been described, for instance, in the epoxidation of cis-cyclooctene catalyzed by N-heterocyclic carbene analogues of C1.79 The material MCM-C4 displays much higher conversions than C4 for all the substrates, but the immobilization of C5 leads to almost complete loss of activity. In general, all heterogeneous catalysts achieve higher conversions of all the substrates than their homogeneous counterparts, although there is a relationship: the best homogeneous catalysts give rise to the best heterogeneous ones. The highest TOF (474 mol molcatalyst−1 h−1), calculated after 10 min reaction, is observed in the oxidation of S-lim in the presence of Silica-C1 (Table 4). Homogeneous catalysts C4 and C5 reach high TOFs of 450 and 331 mol molW−1 h−1 for trans-3 and ger, respectively (Tables 2, 3). The lowest TOFs are obtained for the oxidation of 1-oct (Table 3), associated with conversions under 30% after 24 h reaction. Materials containing the allyl complex C1 as well as the two tungsten 1471
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Organometallics
Figure 10. Kinetic profiles of the oxidation of each of the five substrates in the presence of complex [MoBr(η3-C3H5)(CO)2(L)2] (C1) and materials MCM/Silica/Cube-C1.
Table 2. Conversions (%) and Turnover Frequencies (TOF/mol molMo−1 h−1) in the Oxidation of cis-3 and trans-3 Catalyzed by Complexes C1 to C5, Materials MCM-C1 to MCM-C5, Silica-C1, and Cube-C1 catalyst C1 C2 C3 C4 C5 Cube-C1 a
substrate
TOF*
conv
catalyst
substrate
TOFa
conv
cis-3 trans-3 cis-3 trans-3 cis-3 trans-3 cis-3 trans-3 cis-3 trans-3 cis-3 trans-3
52 26 0 0 0 0 8.5 450 6 330 2 402
98.9 96.2 0 0 0 0 97.9 98.9 (4 h) 89.1 99.6 99.4 99.2 (4 h)
MCM-C1
cis-3 trans-3 cis-3 trans-3 cis-3 trans-3 cis-3 trans-3 cis-3 trans-3 cis-3 trans-3
17 38 19b 0 2 5 191 266 6 17 60 137
99.3 86.1 10.5 0 18.8 (8 h) 56.3 100 95.1 (8 h) 6.1 10.2 97.5 96.3
MCM-C2 MCM-C3 MCM-C4 MCM-C5 Silica-C1
Calculated after 10 min. bAfter 1 h.
complexes C4 and C5 exhibit the highest TOFs and conversions for the majority of the substrates (Tables 2−4). The kinetic profile of the oxidation of the five substrates in the presence of complex C1 or the material obtained by its immobilization (MCM-C1, Silica-C1, Cube-C1) is shown in Figure 10. The kinetic profile curves of oxidation in Figure 10 for MCM-C1, Silica-C1, and Cube-C1, showing the conversions as a function of time, reveal that the maximum rate is achieved very fast, except for the oxidation of 1-oct over Silica-C1, where an induction period of more than 8 h is needed. A similar induction period takes place when the catalyst is C4 or C5. Although the required period to form the active species
containing Mo(VI) is different in each case, similar catalytic species are probably formed.64,81 The immobilization of C1 in MCM-C1 has only a slight effect on the TOF and conversions of all substrates. On the other hand, Cube-C1 can oxidize 1-oct with almost 60% conversion (only 38.6% with C1). The immobilization of the inactive complexes C2 and C3 does not make C2 more active (conversion and TOF), but confers some activity to C3. Complexes C4 and C5 exhibit high TOFs for oxidation of trans-3, but C5 loses its activity upon immobilization in MCM. Faster kinetics also occurs in the oxidation of S-lim with the heterogeneous catalyst MCM-C4 (but not MCM-C5). 1472
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Table 3. Conversions (%), Turnover Frequencies (TOF/mol molMo−1 h−1), and Selectivity in the Oxidation of Geraniol and 1Octene Catalyzed by the Complexes C1 to C5, Materials MCM-C1 to MCM-C5, Silica-C1, and Cube-C1 catalyst
substrate
TOF
ger 1-oct ger 1-oct ger 1-oct ger 1-oct ger 1-oct ger 1-oct ger 1-oct ger 1-oct ger 1-oct ger 1-oct ger 1-oct ger 1-oct
137 4 32 2 32 7 59.4 3 331 2 89 4 46 3 16 3 306 25 55 2 117 66 308 3
C1 C2 C3 C4 C5 MCM-C1 MCM-C2 MCM-C3 MCM-C4 MCM-C5 Cube-C1 Silica-C1
conv 94.4 38.6 25.7 4.5 12.8 2.9 68.0 13.2 96.2 4.9 97.9 61.6 27.9 2.5 51.9 17.8 97.3 19.8 14.2 10.4 97.6 57.4 95.7 2.5
(4 h)
(1.5 h)
(6 h)
C1 C2 C3 C4 C5 MCMC1 MCMC2 MCMC3 MCMC4 MCMC5 CubeC1 SilicaC1
ZLimox
ELimox
Z-LimOH
E-LimOH
diepox
40.2 0 0 39.1 51.0 56.8
48.6 0 0 50.7 42.9 43.0
5.6 0 0 4.5 2.7 0.1
5.6 0 0 5.7 3.4 0.1
0 0 0 0 0 0
2
54.5 0 0 54.1 100 100 (4 h) 17.3
31.4
48.4
8.8
11.4
0
8
58.5
19.1
62.2
9.4
9.4
0
164
79.4
16.2
76.0
3.4
4.4
0
3.1
17.3
46.8
16.1
19.8
0
53.7 (8 h) 100 (0.5 h)
33.3
44.2
10.5
12.0
0
9.2
8.9
0.1
0.2
81.6
TOF
conv
37.9 0 0 43 27 10
3 14 474
Z-2,3
Z-6,7
8.1
87.0
4.9
56.4
43.6
0
43.8
56.2
0
0
93.4
6.6
0
50
50
0
51.0
49.0
29.3
61.5
9.2
26.3
25.5
48.2
0
49.6
50.4
0
85.2
14.8
(4 h)
0
100
0
0
100
0
1-octanal
epox
4.8
95.2
67.7
32.3
56.3
43.7
53.9
46.1
22.0
78.0
2.1
97.9
72.4
27.6
56.6
43.4
21.3
78.7
83.2
16.8
0.8
99.8
22.8
77.2
The remaining factor that might prove determining in the choice of a support is the selectivity. The five substrates and the oxidation products, which were observed, are sketched in Scheme 6. The two substrates cis-hex-3-en-1-ol (cis-3) and trans-hex-3en-1-ol (trans-3) are selectively oxidized to their epoxide by all the catalysts, except the inactive C2 and C3, in the presence of TBHP (Table 4). Two products were detected in the catalytic oxidation of 1octene, namely, 1-octanal and 1,2-epoxyoctane (epox, Table 3). Complex C1 exhibits a high selectivity for the epoxide, 95.2%. Upon immobilization in MCM-C1, the selectivity increases to 97.9% in epoxide. The other two materials, Silica-C1 and Cube-C1, also produce more epoxide than octanal. In particular, Cube-C1 has 99.8% selectivity toward epox. Associated with the 57.4% conversion of the substrate, it is the best catalyst to epoxidize 1-oct. The most selective catalyst for converting 1-oct into octanal (83.2%) is MCM-C5, but the conversion is only 10.4%. Both geraniol and (S)-limonene can be oxidized to a larger number of products (Scheme 6, Tables 3 and 4), as there are two CC bonds in both substrates (yielding several epoxides) and one OH functionality in geraniol (aldehyde). Stereoisomers of some of the possible products can be formed, and some of them can be detected in standard GC-MS conditions (see the Experimental Section). Three products were observed in the oxidation of geraniol (Table 3, Scheme 6): the aldehyde (geranial) and the two epoxides Z-2,3-oxyrane (Z-2,3) and Z-6,7-oxyrane (Z-6,7). Cube-C1 and Silica-C1 afford only one epoxide, Z-2,3 (100%), which is also obtained in yields between 85% and 93% with C4, C1, and MCM-5. Silica-C1 also displays the highest conversion. This system might be promising as a catalyst, since experimental conditions may be optimized with
Table 4. Conversions (%), Turnover Frequencies (*TOF/ mol molMo−1 h−1), and Selectivity in the Oxidation of (S)Limonene Catalyzed by Complexes C1 to C5, Materials MCM-C1 to MCM-C5, Silica-C1, and Cube-C1 catalyst
geranial
The silica gel support leads to materials that perform better than MCM-41 or Cube derivatives, in terms of conversions and TOFs. This may be due not only to a higher percentage of molybdenum loaded in silica gel but also to the absence of structural constraints to access the active species. In MCM-41, the species are supported inside the pores, and in silica gel the active species are located at the surface. Except for oxidizing 1oct, silica gel seems thus a very convenient (also cheap) support material. 1473
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promising catalysts, with ∼100% conversion for the latter, despite the low TOF (slow initiation). In the S-lim oxidation, complex C1 is the most active and selective, as a homogeneous catalyst, supported in MCM-C1 (99.8% limox) and supported in silica. The material Silica-C1 becomes highly selective toward diepoxidation. The oxidation reaction outcome can be tuned by the support. Complexes C2 and C3 containing molybdenum are inactive, reaching a conversion of ∼60% when immobilized (MCM-C3). The tungsten complexes are active, as well as their heterogeneous counterparts, with C5 reaching 100% conversion.
significant results.11 Several catalysts, namely, C4, C5, MCM-1, MCM-C4, and MCM-5, reveal a chemoselectivity for epoxidation but without discrimination between the two types of CC bonds, so that a 50:50 ratio of Z-2,3 and Z6,7, close to statistic, is formed (except for C4 and MCM-5). C1 leads to the three products, with a larger amount of Z-2,3. In MCM-1 the two epoxides are formed in 51:49 ratios, and only Z-2,3 is obtained with Cube-C1 and Silica-C1. The allylic system (C1) shows well the effect of the support. The activity of the other complexes is not influenced in a clear way by the metal (Mo or W) or the halide (Br or I), and grafting them in MCM does not improve much the selectivity of the homogeneous catalysts. Geranial is the one major product when using the molybdenum catalysts C2 or C3 (56.4% and 43.6%). The oxidation of S-lim is in general slow (small TOFs), with the exception of the silica-supported catalyst Silica-C1 with a TOF of 474 mol molMo−1 h−1. This reaction can lead to a variety of products, and five were observed (Scheme 6, Table 4): stereoisomers of 1,2-epoxy-p-meth-8-ene (Z-limox and Elimox), stereoisomers of 2-methyl-5-(1-methylvinyl)cyclohexan-1-ol (Z-lim-OH and E-lim-OH), and 1,2:8,9diepoxy-p-menthane (diepoxide). The diepox formed only with the active Silica-C1 catalyst, with 81.6% selectivity, no trace of this product being detected in any other system. The stereoisomers Z-lim-OH and E-limOH were formed in small amounts or were trace products of the reactions catalyzed by all systems, except C2 and C3 (none). These two complexes were completely inactive and unable to oxidize the S-lim substrate. There is a strong chemoselectivity toward the epoxidation of the inner ring CC bond to give the stereomeric pair Z-limox and E-limox. Similar amounts of the Z and the E isomers are formed in the presence of C1 (40.2%, 48.6%), C4 (39.1%, 50.7%), C5 (51.0%, 42.9%), MCM-C1 (56.8%, 43.0%), MCMC2 (31.4%, 48.4%), and Cube-C1 (33.3%, 44.2%). A larger amount of E-limox is obtained with MCM-C3 (62.2%) or MCM-C4 (76.0%). If the issue is not the discrimination between Z- and E-limox, C1 and MCM-C1 can be considered
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SUMMARY AND CONCLUSIONS The new complexes [MoBr(η3-C3H5)(CO)2(L)2] (C1) and the Mo(II) and W(II) [MX2(CO)3(L)2] (M = Mo, X = I (C2); M = Mo, X = Br (C3); M = W, X = I (C4); M = W, X = Br (C5)) were synthesized and immobilized in MCM-41. Analysis and spectroscopic data suggest that the metal binds two L ligands supported in this material. Complex C1 was also immobilized in two other supports, namely, silica gel and a polyhedral oligomeric silsesquioxane (Cube). In both, the metal is coordinated to only one L ligand of the surface, and one acetonitrile remains. All the complexes and the functionalized materials were tested in catalytic oxidation reactions of a series of substrates with TBHP. In general, the allyl complex C1 is more active than the C2−C5 family. The molybdenum derivatives C2 and C3 are inactive in most conditions. Although their performance improves after immobilization in MCM, they remain poor catalyst precursors. The grafting of C1, C4, and C5 on MCM modifies the results of the catalytic reactions. 1-Octene can be oxidized with almost 60% conversion by C1 supported on the polyhedral oligomeric silsesquioxane (Cube-C1), significantly better than 38.6% with C1. cis-Hex-3-en-1-ol (cis-3) and transhex-3-en-1-ol (trans-3) are selectively oxidized to their epoxide by all the active catalysts. C4 and Cube-C1 have the highest TOFs (>400 mol molMo−1 h−1). The oxidation of geraniol leads to three products. MCM-1 leads to a racemic mixture of the 1474
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University of Aveiro. High-resolution mass spectra were performed at Laboratório Central de Análises, University of Aveiro. 1 H and 13C solution NMR spectra were obtained with a Bruker Avance 400 spectrometer. 29Si and 13C solid-state NMR spectra were performed at University of Aveiro, operating at 79.49 and 100.62 MHz, respectively, on a Bruker Avance 400P spectrometer (9.4 T). 29 Si MAS NMR spectra were recorded with 40° pulses, spinning rates of 5.0−5.5 kHz, and 60 s recycle delays. 29Si CP MAS NMR spectra were recorded with 5.5 μs 1H 90° pulse, 2 ms contact time, a spinning rate of 8 kHz, and 4 s recycle delays. Chemical shifts are quoted in ppm from TMS. 13C spectra (solid state) were recorded at 125.76 MHz on a Bruker Avance 500 spectrometer. Powder XRD measurements were taken on a Philips Analytical PW 3050/60 X’Pert PRO (θ/2θ) equipped with an X’Celerator detector and with automatic data acquisition (X’Pert Data Collector (v2.0b) software), using monochromatic Cu Kα radiation as the incident beam, operating at 40 kV−30 mA. XRD diffraction patterns were obtained by continuous scanning in a 2θ range of 2° to 10° with a 2θ step size of 0.017° and a scan step time of 99.695 s. The N2 sorption measurements were obtained in an automatic apparatus (ASAP 2010; Micrometrics). BET specific surface areas (SBET, p/p0 from 0.03 to 0.13) and specific total pore volume, Vp, were estimated from N2 adsorption isotherms measured at 77 K. The pore size distributions were calculated by the BJH method using the modified Kelvin equation, with correction for the statistical film thickness on the pore walls.81,82 The statistical film thickness was calculated using the Harkins−Jura equation in the p/p0 range from 0.1 to 0.95.83,84 Prior to the measurements, samples were degassed, and physisorbed water was removed by heating at 723 K for MCM and 413 K for the derivatized materials (to minimize the destruction of the functionalities) under vacuum for 2 h. [MoBr(η3-C3H5)(CO)2(C2H3N3S)2] (C1). A solution of [MoBr(η3C3H5)(CO)2(CH3CN)2] (0.36 g, 1 mmol) in CH2Cl2 (15 mL) was added to a heated solution of the L ligand C2H3N3S (0.22 g, 2.2 mmol) in benzene (30 mL). The resulting mixture was refluxed for 10 h under a nitrogen atmosphere. The solvent was concentrated, and the mixture was left in the cold overnight to precipitate. Hexane was added to the remaining solution to obtain more precipitate. The solid was washed with hexane and dried under vacuum. Yield: 0.42 g (88.2%). Anal. Calcd for MoBrC9H11N6O2S2·0.5C2H3N3S (%): C, 22.84; N, 19.98; H, 2.40; S, 16.25. Found: C, 23.05; N, 21.69; H, 2.91; S, 14.61. 1 H NMR (400.13 MHz, (CD3)2SO): δ 1.09 (d, 2H, Hanti), 3.37 (m, 2H, Hsyn), 5.77 (m, 1H, Hmeso), 7.24 (s, 2H, H1), 8.58 (s, 1H, H2). 13C NMR (100.62 MHz, (CD3)2SO): δ 61.6 (Callyl), 75.2 (Callyl), 168.7 (C3), 143.3 (C2). IR (KBr, νcm−1): 3322 (s), 3092 (s), 2786 (s), 2749 (m), 1948 (vs), 1923 (vs), 1610 (s), 1520 (s), 1379 (w), 1339 (m), 1219 (m), 1106 (w), 1022 (w), 930 (w), 892 (w), 807 (m), 784 (w), 638 (w), 574 (w), 485 (m). HR-MS (EI+): observed m/z 397.00 ([M − Br]+), 369.00 ([M − Br − CO]+), 296.00 ([M − Br − (C2H3N3S)]+); calcd m/z for MoC9H11N6O2S2 ([M − Br]+) 396.94, calcd m/z for MoC8H11N6OS2 ([M − Br − CO]+) 368.95, calcd m/z for MoC7H8N3O2S ([M − Br − (C2H3N3S)]+) 295.94. [MX2(CO)3(C2H3N3S)2] (M = Mo or W and X = I or Br) (C2−C5). The ligand L (2-amino-1,3,4-thiadiazole, C2H3N3S (0.22 g, 2.2 mmol)) was completely dissolved in benzene with heating and stirring, and a solution of [MX2(CO)3(CH3CN)2] (1 mmol) in CH2Cl2 (15 mL) was then added. After 12 h of reflux under N2, hexane was added, and a precipitate formed. The product was filtered, washed three times with hexane, and dried under vacuum. [MoI2(CO)3(C2H3N3S)2] (C2). Yield: 0.51 g (80.5%). Anal. Calcd for MoI2C7H6N6O3S2 (%): C, 13.22; N, 13.21; H, 0.95. Found: C, 13.50; N, 12.94; H, 1.04. 1H NMR (400.13 MHz, (CD3)2SO): δ 8.72 (s, H1). 13C NMR (100.62 MHz, (CD3)2SO): δ 169.1 (C3), 144.6 (C2). IR (KBr ν cm−1): 3340 (m) 3263 (s), 3135 (m), 3070 (w), 2008 (s), 1932 (vs), 1906 (s), 1756 (s), 1602 (s), 1597 (vs), 1346 (s), 1226 (m), 1012 (s), 923 (m), 892 (m), 732 (m), 583 (w). [MoBr2(CO)3(C2H3N3S)2] (C3). Yield: 0.43 g (80.0%). Anal. Calcd for MoBr2C7H6N6O3S2 (%): C, 15.51; N, 15.50; H, 1.12; S, 11.83. Found: C, 15.67; N, 14.28; H, 1.46; S, 8.13. 1H NMR (400.13 MHz, (CD3)2SO): δ 8.73 (s, H1). 13C NMR(100.62 MHz, (CD3)2SO): δ
2,3-epoxides with 97% conversion, while in the presence of both Cube-C1 and Silica-C1 only Z-2,3 is detected (100%). C1 and MCM-C1 can be considered promising catalysts in the oxidation of S-lim if the racemic limox is desired, the latter achieving almost 100% conversion. In conclusion, this evaluation of several homogeneous and heterogeneous catalysts containing 2-amino-1,3,4-thiadiazole coordinated to molybdenum or tungsten indicates that the allylic species display higher conversions and TOFs than the others. The polyhedral oligomeric silsesquioxane requires a lengthy synthesis but offers the best way to oxidize 1-octene. The support can significantly modify the selectivity of geraniol oxidation, silica gel (Silica-C1) being the best support to obtain Z-2,3 (much cheaper than the oligomeric silsesquioxane), while MCM-C1 yields the racemic 2,3-epoxide with high conversions. The different performance between the heterogeneous catalysts may be related, at least partly, to the metal coordination sphere and also with the different environment of the metal fragment in the support. It must be considered that in MCM-41 the metal fragment is probably heterogeneously distributed along the material channel with a significant steric confinement and diffusion constraints of reactants and catalytic products. In the cases of Cube-C1 and Silica-C1, the metal centers should be more easily available since no significant steric restraints are expected. We must emphasize that the metal content between heterogeneous catalysts is very different, being much higher in Cube and Silica supports than in MCM-41. This may have an influence on the final catalytic activity. The different reactivity pattern may be associated, at least partly, with the metal coordination sphere.
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EXPERIMENTAL SECTION
General Considerations. All reagents were obtained from Aldrich and used as received. The work involving sensitive compounds was carried out using standard Schlenk techniques. Commercial-grade solvents were dried and deoxygenated by standard procedures (THF, toluene, and benzene over Na/benzophenone ketyl; CH2Cl2 over CaH2), distilled under nitrogen, and kept over 4 Å molecular sieves. The organometallic complexes [MoBr(η3-C3H5)(CH3CN)2(CO)2] (1)63 and [MX2(CH3CN)2(CO)3] (M = Mo, X = I 2; M = Mo, X = Br 3; M = W, X = I 4; M = W, X = Br 5)7,69 were prepared as described before. Silica gel and octasilsesquioxane functionalized with L (2-amino1,3,4-thiadiazole) were obtained from N. L. Dias Filho. The cage polymer octakis(3-chloropropyl)octasilsesquioxane (Cube) was prepared by stirring 3-chloropropyltriethoxysilane in methanol for 6 weeks under acidic conditions. Reaction with the ligand L, in a molar ratio of 1:12, under reflux in anhydrous dimethylformamide, for 12 h, led to octakis[3-(2-amino-1,3,4-thiadiazole)propyl]octasilsesquioxane (Cube-L).65,20 Silica gel functionalized with 3-chloropropyl was obtained by reaction of silica gel, previously activated (heated for 4 h at 150 °C under 10−3 Torr), with 3-chloropropyltriethoxysilane in dry xylene. After drying the resulting white powder, Silica-Cl, under vacuum and degassing (10−3 Torr) for 6 h at room temperature, it was reacted with 2-amino-1,3,4-thiadiazole in dimethylformamide for 40 h at 110 °C.80 MCM-41 and functionalized materials were prepared according to a methodology previously described, using [(C14H29)N(CH3)3]Br as template agent.8 Prior to the grafting experiment, physisorbed water was removed from MCM by heating at 540 °C under vacuum (10−2 Pa) for 6 h. FTIR spectra were obtained as KBr pellets, and diffuse reflectance (DRIFT) measurements (materials) using cm−1 resolution were obtained on a Nicolet 6700 in the 400−4000 cm−1 range. Microanalyses (C, N, S, H) and ICP (Mo and W) were performed at CACTI, University of Vigo, and at Laboratório Central de Análises, 1475
DOI: 10.1021/om501068q Organometallics 2015, 34, 1465−1478
Article
Organometallics 169.4 (C3), 144.4 (C2). IR (KBr ν cm−1): 3407 (s), 3257 (m), 3141 (m), 3084 (w), 1955 (s), 1911 (vs), 1853 (s), 1791 (vs), 1601 (vs), 1507 (vs), 1350 (m), 1228 (m), 1020 (m), 927 (m), 801 (m), 685 (w). [WI2(CO)3(C2H3N3S)2] (C4). Yield: 0.71 g (98.0%). Anal. Calcd for WI2C7H6N6O3S2 (%): C, 11.61; N, 11.61; H, 0.84; S, 8.86. Found: C, 11.36; N, 11.44; H, 1.05; S 9.12. 1H NMR (400.13 MHz, (CD3)2SO): δ 8.64 (s, H1). 13C NMR (100.62 MHz, (CD3)2SO): δ 169.0 (C3),143.8 (C2). IR (KBr ν cm−1): 3480 (m), 3382 (vs), 3264 (vs), 3173 (s), 3092 (vs), 2005 (s), 2000 (vs), 1913 (vs), 1600 (vs), 1516 (s), 1332 (w), 1244 (w), 1008 (m), 924 (m), 886 (m), 822 (m), 593 (m). [WBr2(CO)3(C2H3N3S)2] (C5). Yield: 0.55 g (87.4%). Anal. Calcd for WBr2C7H6N6O3S2 (%): C, 13.35; N, 13.34; H, 0.96. Found: C, 13.55; N, 13.57; H, 1.18. 1H NMR (400.13 MHz, (CD3)2SO): δ 8.66 (s, H1). 13C NMR (100.62 MHz, (CD3)2SO): δ 168.8 (C3),143.9 (C2). IR (KBr ν cm−1): 3242 (s), 3195 (s), 3095 (s), 2006 (m), 1952 (vs), 1855 (vs), 1599 (vs), 1533 (vs), 1335 (m), 1205 (m), 1079 (m), 887 (s), 841 (m), 705 (m), 591 (m). MCM-Pr. A suspension of 1 g of MCM-41 in 30 mL of toluene was treated with an excess of 3-chloropropyltriethoxysilane (2.0 mL) and allowed to reflux for 24 h. The product was filtered, washed with 4 × 20 mL of dichloromethane, and dried under vacuum. 13C CPMAS NMR (δ ppm): 7.7, 9.1 (SiCH2), 16.0 (SiOCH2CH3), 25.2 (CH2CH2CH2), 46.2 (CH2Cl), and 59.3 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −48.3 (T1), −57.5 (T2), −68.1 (T3), −102.0 (Q2), −109.0 (Q4). IR (KBr, ν cm−1): 2979, 2927, 2895 (vs, νN−H), 1873 (s), 1704 (m), 1627 (s). MCM-L. A solution of 2-amino-1,3,4-thiadiazole (L) (0.121 g, 1.2 mmol) in dry toluene (15 mL) was added to a suspension of 1 g of the material MCM-Pr in toluene (20 mL). The mixture was refluxed under N2 for 12 h. The resulting solid was filtered, washed with MeOH, and dried under vacuum for 3 h. 13C CPMAS NMR (δ ppm): 7.6, 9.1 (SiCH2), 16.1 (SiOCH2CH3), 24.7 (CH2CH2CH2), ∼46.0 (CH2Cl and CH2NH), and 59.3 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −59.1 (T2), −65.7 (T3), −101.5 (Q3), −109.9 (Q4). IR (KBr, ν cm−1): 2960, 2870 (s, νN−H), 1880 (m), 1640 (s,νCN), 1490 (m). MCM-C1−MCM-C5. A solution of each complex C1−C5 (1 mmol) in dry toluene (10 mL) was added to a suspension of 1 g of the material MCM-L in dry toluene (20 mL). The mixture was refluxed under N2 overnight. The resulting solid was then filtered, washed with CH2Cl2, and dried under vacuum for 3 h. MCM-[MoBr(η3-C3H5)(CO)2(C2H3N3S)2] (MCM-C1). Anal. Found (%): C, 7.12; N, 1.16; H, 1.68; Mo, 4.02. 13C CPMAS NMR (δ ppm): 7.6, 8.8 (SiCH2), 15.4 (SiOCH2CH3), 25.4 (CH2CH2CH2), 46.8 (CH2Cl and CH2N), 57.9 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −58.9 (T2), −67.6 (T3), −102.9 (Q3), −110.3 (Q4). IR (KBr, ν cm−1): 3254, 3162, 3102 (s, νN−H), 1944, 1860 (m, νCO), 1608 (s,νCN). MCM-[MoI2(CO)3(C2H3N3S)2] (MCM-C2). Anal. Found (%): C, 6.75; N, 1.38; H, 1.78; Mo, 3.7. IR (KBr ν cm−1): 3360, 2979 (s, νN−H), 2015, 1935, 1886 (m, νCO), 1627 (s,νCN). 13C CPMAS NMR (δ ppm): 8.8 (SiCH 2 ), 11.9 (SiOCH 2 CH 3 ), 25.8 (CH2CH2CH2), 45.6 (CH2Cl and CH2N), 58.5 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −58.8 (T2), −67.9 (T3), −103.9 (Q3), −109.9 (Q4). MCM-[MoBr2(CO)3(C2H3N3S)2] (MCM-C3). Anal. Found (%): C, 6.97; N, 1.51; H, 1.52; Mo, 5.05. 13C CPMAS NMR (δ ppm): 9.0 (SiCH2), 16.2 (SiOCH2CH3), 25.9 (CH2CH2CH2), 45.5 (CH2Cl and CH2N), 58.0 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −58.5 (T2), −66.6 (T3), −102.8 (Q3), −109.9 (Q4). IR (KBr ν cm−1): 3311, 2963 (s, νN−H), 2003, 1981, 1893 (s, νCO), 1617 (s, νCN). MCM-[WI2(CO)3(C2H3N3S)2] (MCM-C4). Anal. Found (%): C, 7.36; N, 2.03; H, 1.61; W, 4.7. 13C CPMAS NMR (δ ppm): 8.9 (SiCH2), 14.9 (SiOCH2CH3), 25.7 (CH2CH2CH2), 44.8 (CH2Cl and CH2N), 58.5 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −59.7 (T2), −68.4 (T3), −103.2 (Q3), −110.4 (Q4). IR (KBr ν cm−1): 3366, 2957, 2848 (s, νN−H), 1994, 1903, 1870 (s, νCO), 1626 (s,νCN). MCM-[WBr2(CO)3(C2H3N3S)2] (MCM-C5). Anal. Found (%): C, 6.99; N, 1.43; H, 1.59; W, 3.67. 13C CPMAS NMR (δ ppm): 8.3 (SiCH2), 15.0 (SiOCH2CH3), 25.7 (CH2CH2CH2), 44.8 (CH2Cl and
CH2N), 59.2 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −58.9 (T2), −67.4 (T3), −101.4 (Q3), −109.8 (Q4). IR (KBr ν cm−1): 3386, 2955, 2850 (s, νN−H), 2000, 1960, 1872 (s, νCO), 1627 (s,νCN). Octakis[3-(2-amino-1,3,4-thiadiazole)propyl[MoBr(CO)2(CH3CN)]octasilsesquioxane (Cube-C1). A solution of [MoBr(η3-C3H5)(CO)2(CH3CN)2] (2.27 g, 6.4 mmol) in dry toluene (10 mL) was added to a suspension of Cube-L (1 g, 0.64 mmol) (10:1) in dry toluene (20 mL). The reaction mixture was refluxed under N2 overnight. The resulting material was then filtered, washed with CH2Cl2, and dried under vacuum for 3 h. Yield: 0.83 g (59.2%). Anal. Calcd for Mo2Br2C54H80N26O16Si8S8 (%): C, 29.72; N, 16.69; H, 3.69, S, 11.75; Mo, 8.79. Found: C, 28.68; N, 14.81; H, 3.81; S, 8.31; Mo, 8.21. 13C CPMAS NMR: δ 9.8 (SiCH2), 22.9 (CH2CH2CH2), 41.2 (CH2N), 144.4 and 158.3 (C from the thiadiazole ring). 29Si MAS NMR (δ ppm): −60.5 (T2), −68.5 (T3). IR (KBr ν cm−1): 3170 (w), 3100 (w), 2934 (m), 2887 (m; νN−H), 1942, 1856, (s; νCO), 1794, 1663 (s; νCN), 1600 (m), 1520 (m), 1459 (m), 1421 (m), 1353 (m), 1251 (m), 1166 (s), 1034 (s), 918 (m), 681 (m). Silica-C1. A solution of [MoBr(η3-C3H5)(CO)2(CH3CN)2] (0.5 g, 1.4 mmol) in dry toluene (10 mL) was added to a suspension of 1 g of the material Silica-L in dry benzene (20 mL), and the mixture was refluxed under N2 overnight. The resulting solid was filtered, washed with CH2Cl2, and dried under vacuum for 3 h. Anal. Found (%): C, 6.22; N, 1.97; H, 1.01; S, 0.94; Mo, 7.63. 13C CPMAS NMR: δ 9.4 (SiCH 2 ), 25.9 (CH 2 CH 2 CH 2 ), 35.4, 45.4 (CH 2 N), 46.2 (SiOCH2CH3). 29Si MAS NMR (δ ppm): −60.4 (T2), −69.7 (T3), −89.7 (Q2), −99.3 (Q3), −109.2 ppm (Q3). IR (KBr ν cm−1): 3011 (w), 2940 (w), 2887 (w; νN−H), 2291, 2317 (s; νCN), 1955, 1853 (s; νCO), 1632, 1539 (m; νCN), 1457 (m), 1412 (m), 1280 (m), 1043 (m), 926 (m), 808 (m), 683 (w). Catalytic Studies. The new complexes C1−C5 and materials MCM-C1, MCM-C2, MCM-C3, MCM-C4, MCM-C5, Cube-C1, and Silica-C1 were tested as precursors in the catalytic oxidation of alkenes: geraniol (ger), cis-hex-3-en-1-ol (cis-3), trans-hex-3-en-1ol (trans-3), (S)-limonene (S-lim), and 1-octene (1-oct), using TBHP as oxidant. The catalytic oxidation tests were carried out at 328 K under air in a reaction vessel (25 mL) equipped with a magnetic stirrer and a condenser. In a typical experiment the vessel was loaded with the substrate (100%), internal standard (dibutyl ether), catalyst (1%), oxidant (200%), and 2 mL of solvent. The reaction mixture was refluxed for 24 h, the addition of the oxygen donor determining the initial time. Samples of 100 μL were obtained 10 and 30 min after the reaction started and after 1 h, 1 h 30 min, 2 h, 4 h, 6 h, 8 h, and 24 h. Each sample was diluted in 1 mL of dichloromethane. In order to destroy the hydrogen peroxide and stop the reaction, each sample was treated with a catalytic amount of manganese oxide (Mn2O7). The filtrate was then injected in a GCMS column, and the course of the reactions was monitored by quantitative GC analysis. Blank experiments were made using only substrate and TBHP. A conversion of
