J. CHEM. SOC. FARADAY TRANS., 1995, 91(19), 3511-3517 351 1

Mossbauer and Solid-state NMR Spectroscopic Studies of Tin-modified Rhenium-based Metathesis Catalysts

Regina Buffon* and Ulf Schuchardt lnstituto de Quimica, Universidade Estadual de Campinas, C.P. 6154, 13083-970 Campinas-SP, Brazil Anuar Abras Departamento de Fisica, ICEx, Universidade Federal de Minas Gerais, C.P. 702, 30161-970 Belo H o r i z o n t e M G , Brazil

~ ~~ ~

Mossbauer and solid-state N M R spectroscopic studies have been carried out on Re,O,/SiO,-Al,O,/SnR, meta- thes i s catalysts (R = M e , Et or n-Bu). The effects of both the rhenium loading and t h e SnR, alkyl group are presented. The amoun t s of gas released upon t h e interaction between Re,0,/Si0,-A1,03 and SnR, do not correspond to a simple stoichiometry while chemical reactivity suggests the presence of minor amounts of a rhenium-alkylidene species. Mossbauer and N M R data suggest two kinds of t in sites, with the tin atom linked to t h e surface by one or two S n - 0 bonds. The effects of exposure to air and catalyst regeneration on the tin environment have also been investigated. The nature of the tin-containing surface species is discussed.

Rhenium oxide supported on alumina or silica-alumina is a well known metathesis catalyst for alkenes. In particular the catalyst has a high activity after promotion with a tetra- alkyltin compound. However, the role of the alkyltin pro- moter is not well understood. Reduction of the rhenium atom, modification of the active site (by addition of a tin ligand) and formation of the initiating metal-alkylidene species (via a double alkylation followed by an a-H abstraction) have been postulated as promotion mecha- nisms.' However, only a few studies have been done concern- ing the nature of the interaction between the tin compound and the surface rhenium

In this work, Mossbauer and solid-state NMR spectro- scopies have been used to characterize the surface species resulting from the addition of SnR, (R = Me, Et or n-Bu) to Re,07/Si02-Al,03 catalysts. The effects of air exposure and catalyst regeneration on the tin environment were also inves- tigated. The chemical reactivity of the resulting species towards bulky alkenes and acetone was determined with the aim of identifying a possible metal-alkylidene site.

Experimental Catalysts

All catalysts (with 6, 3 or 1% Re,07 loadings) were prepared by pore-volume impregnation of the support (Si0,-A1203 Akzo, 24.3% A1203, 374 m2 g- ') with calculated amounts of an aqueous solution of ammonium perrhenate (Aldrich, >99%), followed by overnight drying in air at 373 K. The catalysts were calcined at 810 K in a dry air stream (60 mL min-') for 2 h, followed by a nitrogen purge for 1 h, at the same temperature. After cooling to room temperature, the catalysts were transferred to the reaction vessel using Schlenk-tube techniques. Calculated volumes of SnMe, (Fluka, > 99%), SnEt, (Aldrich, 97%) or Sn(n-Bu), (Fluka, 98%) were then added via a syringe, keeping the molar ratio [Sn] : [Re] = 0.55. For chemical reactivity studies, an excess of 2,4,4-trimethylpent-2-ene (Aldrich, 99%), 3,3-dimethylbut- 1-ene (Fluka, >99%) or acetone (Merck, p.a.) was then added to the reaction vessel, via a syringe. Analysis of the gas phase was performed using either a Siemens Sichromat 1 gas chro- matograph with a Porapak Q packed column (3 m x 3.2 mm) for the determination of methane, ethane or butane, or using an H P 5890 gas chromatograph, with an A1203/KCl-

on-fused-silica capillary column (50 m x 0.32 mm), both equipped with a flame ionization detector.

For regeneration experiments, the deactivated catalyst was calcined as previously described, followed by the addition of another portion of SnR, .

MAS NMR Measurements

All MAS NMR spectra were recorded on a Bruker AC 300 spectrometer operating at 75.47 and 11 1.9 MHz for 13C and '' 9Sn, respectively. The probehead was a commercial double- tuned 7 mm double-bearing system from Bruker, allowing spinning frequencies up to 4 kHz. The samples were intro- duced in the zirconia rotors under a dry argon atmosphere in a glove box and tightly closed. For 13C NMR, a typical cross-polarization scheme was used: 90" rotation of the 'H magnetization (pulse length 9.7 p), contact time of 5 ms and recording of the spectrum at high power decoupling. The delay between scans was fixed to 5 s to allow for the complete relaxation of the 'H nuclei. Typically, 11 OOO scans were accu- mulated. Chemical shifts are given with respect to TMS using adamantane as an external reference (6 = 37.7 for the highest chemical shift).

l19Sn NMR spectra were recorded by using a single pulse and high power decoupling, with a delay between each scan of 2.5 s. Typically, 30000-60000 scans were accumulated. The chemical shifts are given relative to SnMe, used as exter- nal reference and with the IUPAC convention for chemical shifts (higher values for higher frequencies).

Miissbauer Measurements

The samples were transferred to the sample holder under an argon atmosphere. "'Sn Mossbauer spectra were run in standard equipment at liquid-nitrogen temperature, using a CaSnO, source maintained at room temperature. The isomer shifts are given with respect to this source.

Results Chemical Reactivity

SnR, reacts promptly with Re207/Si02-A1203, as is evi- denced by a colour change (from white to brownish) and

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3512 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

Table 1 Amount of gas released upon reaction of Re,O,/SiO,-Al,O, with SnR, (mol per mol of tin) for catalysts con- taining 3 and 6% Re,O,

~ ~~

("/.I CH, (R = Me) C,H, (R = Et) C,H,, (R = Bu) ~~

3 0.9 0.6 0.3 6 0.3 0.5 0.3

evolution of RH.? When R = Et or Bu, the amounts of gases evolved per tin atom seem to be dependent on R but not on the concentration of Re,O, (Table 1). However, SnMe, appears to behave in a different way, as the amount of methane released is cu. three times higher for a 3% Re,O, sample than for a 6% Re,O, sample. From the amounts of gas released it is clear that a simple stoichiometric reaction does not account for the interaction between Re,O,/SiO,-Al,O, and SnR, . Moreover, there appears not to be a correlation between the amounts of gases released and the catalytic activity, since it is well established that a 3% Re207 catalyst is more active than a 6% ca ta ly~ t ,~ and that the order of activity of SnR, is: SnEt, > SnBu, > SnMe, .6

In order to verify whether the gas evolution was related to the formation of alkylidene species, the resulting systems were allowed to react with bulky alkenes, viz. 2,4,4-trimethylpent-2- ene or 3,3-dimethylbut-l-ene, which were not expected to undergo metathesis., Nevertheless, most results were ambigu- ous, since both alkenes are able to react with the highly acidic support leading to the formation, among other pro- ducts, of some of the expected first-formed alkenes. For instance, isobutene, which should be produced by the exchange reaction between a methylidene ligand and 2,4,4- trimethylpent-2-ene, is formed from both alkenes, inde- pendent of the presence of Re or SnMe,. Only for the com- bination of Re,07/Si0,-A1,0,/SnMe, and 2,4,4- trimethylpent-2-ene was it possible to reach an unambiguous result, with the formation of 3,3-dimethylbut-l-ene [reaction (l)]. Formation of this alkene, to be expected in the presence of a methylidene ligand, was not detected either in the pres- ence or absence of other SnR, compounds.

A Wittig-type reaction with acetone might help to remove the ambiguity, as blank experiments revealed that the reac- tion between acetone and the support (or Re,O,/support) would only cause the formation of trace amounts of iso- butene (methylpropene). Thus, the reaction of Re207/ Si0,-Al,O,/SnEt, with acetone leads to the formation of 2- methylbut-2-ene, which would be expected in the presence of a Re-ethylidene species, according to reaction (2) :

t Using the same experimental conditions SnR, compounds react with the surface of Si0,-A1,0, with the release of RH (decreasing in the order: methane > butane > ethane), but without any colour change. However, a direct interaction between SnR, and SiO2-A1,O, seems not to take place in the presence of rhenium (cf. NMR results).

Even with an excess of acetone ([acetone] : [Re] = 30), the resulting 2-methylbut-2-ene subsequently reacts with the catalyst to produce ethene, propene and linear butenes, besides an unidentified C7 alkene. Among the reactions sus- ceptible to take place over a metathesis catalyst (Scheme l),

2

J + = r- L7 + x

Scheme 1

the most likely seem to be those depicted by the pathway a,a',c: an isomerization of 2-methylbut-2-ene to 3-methylbut- 1-ene (a), followed by cross-metathesis between these two isomers (a'), would lead to the formation of propene, whose self-metathesis (c) would produce but-2-enes and ethene. However, the total amount of alkenes does not exceed 5% of the ethane released in the gas phase upon the interaction between Re,O,/SiO,-Al,O, and SnEt,.

NMR Experiments

Fig. 1 shows the 13C NMR spectra of Re2O,/SiO,-A1,O,/SnBu, catalysts at different rhenium loadings. For the 6% Re207 sample (Fig. lA), the spectrum

! ' / I

I I I I I I I I

35 3 0 25 20 15 10 5 0 6

Fig. 1 I3C CP MAS NMR spectra of Re,O,/SiO,-Al,O,/SnBu, catalysts: (A) 6% Re,O, (3.6 wt.% Sn); (B) 3% Re,O, (1.8 wt.% Sn); (C) 1% Re,O, (0.6 wt.% Sn); (D) SO,-Al,O, (0.7 wt.Oh Sn)

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J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 3513

Table 2 and l19Sn chemical shifts for Re207/Si02-A120,/SnR4 catalysts and some related compounds

6 ('3C)

compound

6% Re,O,/SiO,-Al,O,/SnBu, 6 % R e 2 0 , /SO ,-A1 ,O ,/SnEt, 6% Re ,O,/SiO,-A1 ,O,/SnMe, s AlOSnBu, =SiOSnBu, SSiOSnEt =SiOSnMe, (Si03-A1,0,)OSnBu, (= SiO) , SnBu (=SiO),SnMe, Bu , SnO Me Bu,SnOSiPh, Me,SnOMe Bu,Sn(OMe), EtReO,

16.4 7.1

- 5.9 15-19 15.1 5.3

- 7.8 15.0 20.2 - 5.3 14.1 16.5 - 5.7 19.5 36.6

27.5 7.1

26.9 26.8

6.6

26.6 26.8

28.4 27.8

27.8 18.2

27.5

26.9 26.8

26.6 26.8

27.4 27.1

27.2

12.3

13.0 12.8

13.1 12.9

13.7 13.7

13.8

103 100 137 80

106 98

136 102

83 94

I29

this work this work this work

8 9 9 9

10 1 1 9

12 9

13 14 15

shows 3 peaks, at 12.3, 16.4 and 27.5 ppm. By comparison with related compounds (Table 2), these peaks can be reason- ably assigned to a C, , C, and C,, ,, respectively, of a species like [Re]-OSnBu,. It is worthwhile to mention here that SnBu, does not react with the surface of SiO, at room tem- perature.' Although SnBu, does react with the surface of SO,-Al,O, under our experimental conditions, for a tin loading of ca. 0.7 wt.% (cf 0.6% found for 1% Re20,/Si0,-A1,0,/SnBu,) the resulting ' ,C NMR spectrum has a different profile (Fig. 1D); for higher tin loadings, for- mation of polymeric species was detected. These results suggest that, in the presence of rhenium, SnBu, (as well as the other SnR, compounds) would react with silica-alumina uia the same sites assumed to be involved in the rhenium-support interaction, i.e. the highly acidic bridging-hydroxyl group^.^.'^ An additional peak at ca. 18 ppm can be attrib- uted to C, of an [ReO],SnBu, species, as a shift to lower fields is observed in related complexes when one butyl ligand is replaced by an alkoxide ligand.',' ',14 For lower Re,O , loadings (Fig. 1B and C), the signal at 16.4 ppm is no longer observed. In related systems, a broadening in this region has been assigned to a variety of surface sites to which the tin atom could be bonded.8." The remaining signals are also broadened and, for the 1 % Re,O, spectrum, it is clear that each peak corresponds to at least two resonances; at 27.5 and 26.9 ppm and at 12.3 and 11.7 ppm, respectively. Although it is not possible to decide whether the peaks at lower fields are due either to C, and C, atoms of the same species or C,+, of different species, the signal at 11.7 ppm can be attributed to a C, interacting with the remaining surface groups uia a hydrogen-type bonding8-' '

The I3C NMR spectra of 3% or 6% Re,O,/SiO2-A1,O,/SnEt, present only one symmetric signal at 7.1 ppm. The fact that C, and C, peaks cannot be separ- ated (as opposed to what was observed' for ~ S i o S n E t , ) may suggest the formation of two or more tin-containing surface organometallic species. In the case of SnMe,, the ' NMR spectrum of a 6% Re207 sample shows an asymmetric peak at -5.9 ppm (Fig. 2A). For the 3% Re,O, sample, this signal is broadened (Fig. 2B) and can be decomposed into at least two peaks; one at -5.9 ppm, assigned to an [RelOSnMe, species, the other at -2.5 ppm.

Note, whatever the SnR, employed, no signal assignable to alkyl groups bonded to a rhenium atom could be detected.

"'Sn spectra of 6% Re,O, samples (tin content: ca. 3.6 wt.%) are poor for all SnR, in spite of high accumulation times, showing only one signal at low field (Fig. 3, Table 2). By comparison with related compounds, these signals can be

assigned to tin atoms attached to the surface by a single Sn-0-Re bond. Since a tin atom linked to the surface by two Sn-0 bonds cannot be detected due to its rigidity and, thus, high relaxation time,16 the poor quality of these spectra may arise from a low concentration of [RelOSnR, species. Although the broadening of these peaks may also be due to quadrupolar splitting, such an effect was not observed for GAlOSnBu, .'

Mossbauer Analyses

Mossbauer spectra for all SnR,-activated systems present two asymmetric absorption peaks. For R = Et, these peaks appear at ca. -0.3 and 3.0 mm sC1, as shown in Fig. 4A. According to computational analyses, these spectra are best fitted through two doublets, corresponding to the presence of two Sn sites, characterized by isomer shifts (IS) in the range 1.34-1.44 mm s- ' (site I) and 1.50-1.83 mm s - ' (site 11) (Table 3). Although the relative concentrations of these sites appear to be independent of the rhenium loading (at least for 3% and 6% Re,O,), there is some dependence on R: site I seems to be favoured when R = Et.

30 25 2 0 15 10 5 0 -5 -10 -15 s

Fig. 2 catalysts: (A) 6% Re,07; (B) 3% R e 2 0 7

13C CP MAS NMR spectra of Re,07/Si0,-A1,03/SnMe,

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3514 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

B

180 140 100 60 20 s

Fig. 3 catalysts with R : (A) Me; (B) Et ; (C) Bu

l19Sn MAS NMR spectra of 6% Re,O,/SiO2-A1,O3/SnR,

By comparison with molecular analogues Bu,SnX and Bu,SnX,," these sites might reasonably be assigned to tin atoms linked to the surface by one and two Sn-0 bonds, respectively, as both IS and quadrupole splitting (QS) values

1 1 1 1 1 1 1 1 1 1 1 1 1

%, < 'f":.: . ..f

s 0 Y ..

v

1 1 1 1 1 1 ~ 1 ] 1 1 1 ~

-6 -4 -2 0 +2 +4 +6 velocity/rnrn s-'

Fig. 4 lI9Sn Mossbauer spectra of 3% Re,O,/SiO,-A1,O3/SnEt4 catalysts: (A) fresh sample; (B) after air exposure; (C) after five regen- erations

are increased when a butyl ligand is replaced by an X ligand. This result is the opposite of that found in the case of =SiOSnBu,, for which the replacement of another butyl group leads to a decrease in the isomer shift." High values for quadrupole splitting reflect a low symmetry of the elec- tronic distribution around the tin atoms, which is expected for atoms near a surface.

After deactivation by exposure to air, the resulting spectra are also best fitted using two doublets, as shown in Fig. 4B. A decrease in the IS and QS values of both sites was observed in all cases (Table 3).

When the catalysts are regenerated by calcination, the resulting spectra show a broad absorption peak at about zero velocity, as shown in Fig. 4C. These spectra are best fitted with two doublets, which also correspond to two Sn sites. The doublet characterized by IS = 0.03-0.05 mm s- l and QS = 0.51-0.60 mm s- ' (site 111) was assigned to SnO, (major species) and the other doublet, having IS = 0.10-0.12 mm s- ' (site IV), corresponds to a polymeric ionic SnIV species.

Discussion Many spectroscopic studies have been carried out in order to characterize the structure of the rhenium oxide phase on Re , 0 ,/y-Al ,O catalysts. 3-2 This sys tem is generally described as a monolayer catalyst and, after calcination, the rhenium oxide is bonded to the alumina surface as a mono- meric species, presenting a C,, symmetry. The structure of the rhenium oxide phase on silica-alumina has been studied less. However, at least up to a 3% Re207 loading, the rhenium oxide supported on silica-alumina has a predomi- nantly monomeric structure. At high loadings, it might also be present as three-dimensional clusters.26 In spite of such differences, Re207/Si02-A1,03 seemed more adequate for spectroscopic studies: in contrast to Re,O,/y-Al,O, , the catalytic activity per rhenium atom for Re,0,/Si0,-A1,03 is higher at low rhenium loadings.' Moreover, at low loadings the catalytic activity is higher for Re,O,/SiO,-Al,O,/SnR, catalysts than for Re,O,/y-A1,O3/SnR4, which may imply a larger number of active sites when silica-alumina is used as a support.

Assuming a monomeric Re species and taking into account all results obtained by our spectroscopic techniques, the surface reactions resulting from the interaction between Re,O,/SiO,-Al,O, and SnR, might reasonably be those depicted in Scheme 2. A single alkylation of an isolated rhenium site would lead to species A, whose presence is accounted for by both Il9Sn NMR and Mossbauer data. The same species has been proposed for alkyltin-promoted Re,07/A1,0, catalyst^.,^ A double alkylation involving only one rhenium site (species B), might also take place. If B, a coordinatively saturated species, is indeed formed, it would not be expected to be stable. Thus, an a-H abstraction, spon- taneous or surface-induced, with elimination of an alkane and formation of a rhenium-alkylidene species (C), might well be the following step. The presence of species like C would be in good agreement with Mossbauer data for site 11: the electron-donor properties of the rhenium ligand would increase the electron density on the tin atom and, thus, its s character, with a resulting increase in the value of the isomer shift (Table 3) when compared with A, as was observed for Bu,SnOPh and Bu,Sn(O)(O)Ph (Table 3, last two rows). The sequence of steps B+C seems to be the most logical pathway leading to the formation of a rhenium-alkylidene species, whose presence (even to a minor extent) is indeed suggested by our studies of the chemical reactivity of the surface species (Wittig-type reaction with acetone and

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J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 3515

Table 3 19Sn Mossbauer data for Re,O,/SiO,-Al,O,/SnR, catalysts and some related compounds

compound IS/mm s - ' QS/mm s - ' * reference

3 Re,O ,/SO ,-A1 ,O ,/SnMe,

deactivated

after five regenerations

6?4 Re,O ,/SiO ,-A1 , 0 ,/SnMe,

3% Re,0,/Si0,-A1,03/SnEt,

deactivated

after five regenerations

6% Re,O,/SiO,-Al,O,/SnEt,

3 YO Re ,O ,/SiO ,-A1 ,O ,/SnBu,

6%) Re2O,/SiO,-A1,O,/SnBu,

deactivated

%3OSnBu, (GSiO),SnBu, Me,SnOH Et3SnOH Bu , SnCl Bu,SnCI, Me,SnCI Me,SnCI , Bu,SnO Bu,SnOSnBu, SnO, Bu,SnOPh

0

(site I) (site 11) (site I) (site 11) (site 111) (site I V ) (site I) (site 11) (site I) (site 11) (site I) (site 11) (site 111) (site I V ) (site I) (site 11) (site I) (site 11) (site I) (site 11) (site I) (site 11)

1.35 1.50 1.26 1.36 0.05 0.12 1.34 1.53 1.36 1.83 1.23 1.75 0.03 0.10 1.34 1.80 1.44 1.58 1.41 1.58 1.35 1.43 1.31 1.23 1.19 1.35 1.36-1.65 1.50-1.75 1.12-1.5 1 1.52-1.6 1 1.15 1.29 0 1.42

1.52

3.05 (54%) 3.96 (46%) 2.98 (62%) 3.88 (38%) 0.60 (71%) 1.33 (29%) 3.41 (56%) 4.12 (44%) 3.55 (65%) 3.90 (35%) 3.20 (67%)

0.51 (54%) 1.16 (46%)

4.03 (43%) 3.13 (50%) 3.85 (50%) 2.90 (59%)

3.65 (33%)

3.76 (57%)

4.04 (41%) 2.65 (65%) 3.39 (35%) 2.50 2.77 2.91 3.00 2.78-3.40 3.25-3.50 3.01 -3.50 3.33-3.85 2.08 1.56 0.53 2.85

3.62

this work this work this work this work this work this work this work this work this work this work this work this work this work this work this work this work this work this work this work this work this work this work

16 16 17 18 18 18 18 18 19 20 21 22

22

Accuracy _+ 0.02 mm s - '. Accuracy & 0.04 mm s - and k 5% for relative concentration in parentheses.

Scheme 2

exchange reaction between an [Re]=CH, species and 2,4,4- trimethylpent-2-ene). Thus, chemical reactivity is in agree- ment with the presence of C, while both NMR and Moss- bauer data would account for the chemical environment of the tin atoms in A and C. Although a double alkylation involving two rhenium sites (species D) might seem highly improbable due to the low density of such sites (ca. 0.2 atoms nmP2 for a 6% Re207 loading), such a possibility cannot be ruled out: the degree of dispersion of rhenium supported on

silica-alumina decreases strongly above a 3% Re,07 loading.26 Moreover, an increase in the rhenium loading cor- responds to an increase in the QS values of site I1 for all SnR, studied, i.e. to a decrease in the symmetry around the tin atom. Therefore, Mossbauer data suggest that higher rhenium loadings favour D, less symmetric than C (the cata- lytically active site). Species C is supposed to be more sym- metric than D because whatever distortion imposed by the surface in the rhenium moiety is evenly distributed to the tin

r -I I

CHR' R

R e SnR, I - - R H \ /O\ R e SnR,

I I / / I \ o / l o 0

0 0 O / S n R 3 1 1 /

I

I I SnR, O = R e - R - I

0 0 Illlllllllr

I I - - L J

"

8 C

\ / R R

Sn

711 I f I f f f I f I f f f f f f I / f / /

D

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3516 J. CHEM. SOC. F A R A D A Y TRANS. , 1995, VOL. 91

atom. This might not be exactly true for D: the tin-attached rhenium atoms can be distorted differently causing uneven influences in the tin atom. A favoured formation of D with higher rhenium loading would be in good agreement with literature data concerning catalytic activity, which is known to decrease when the rhenium loading is in~reased .~ However, in no case was it possible to find a signal in the I3C NMR spectra assignable to an alkyl group bonded to the rhenium atom. That a methyl or methylidene ligand were not observed in the case of Re,07/Si0,-A1,0,/SnMe, was not a surprise, as C, of alkyl groups bonded to metal atoms are hardly detected by solid-state NMR.28,29 However, the non- observation of a C, of an ethyl (or ethylidene) ligand bonded to the rhenium atom, expected to appear at ca. 18 ppm,15 suggests the presence of several rhenium-surface species bearing such ligands. In this case, as the resonance relative to each species would not appear exactly at the same place, and also because of their low concentrations, when compared to those of Sn-Et (one peak for two carbon atoms), the resulting signal would be too small and too broadened to be detected. The same explanation applies for R = Bu.

If the step B -+ C were to account for the total amount of gas released during the interaction between Re,O,/SiO,-Al,O, and SnR, , the relative molar concentra- tion of [Re-alkylidene] : [Re] should vary in the range 0.16- 0.50 (values obtained by multiplying data from Table 1 and 0.55 [Sn] : [Re]). With experimental error, in no case was it possible to find a value higher than 0.05. Moreover, in the case of 3% Re2O7/Si0,-Al2O3/SnMe4, species C would cor- respond to 90Y0 of the tin-containing surface species (see Table l), in total disagreement with Mossbauer data. There- fore, other surface reactions should be considered. A plausible possibility would be an electrophilic attack of a surface OH group on an alkyl ligand of A or D, with liberation of an alkane [reaction (3)]: consumption of some surface OH groups (detected by FTIR spectroscopy) during chemisorp- tion of SnMe,, has already been reported.,'

0 /nR3 OH 0 SnR3 I I / 7Tm

O = R e - R

A A '

Either species A or A' could account for both Mossbauer and NMR data for site I. The extension of such a reaction would be dependent on the bulkiness of the alkyl ligand, in good agreement with experimental data for gas evolution (Table 1) for 3% Re,07/Si0,-A1,0,/SnR, catalysts: the amounts of gases released decrease in the order Me > Et > Bu, in accordance with the increasing bulkiness of R. For 6% Re207 catalysts, the ratio [RH] : [Sn] is more or less con- stant, which might be due to a lower concentration of surface OH groups. The higher value obtained for [EtH] : [Sn] could be associated with the fact that A (site I) is favoured when R = Et.

After exposure to air, a hydrolysis of the Sn-0-Re bonds might be expected, as happens in solution with molec- ular analogues [reactions (4) and (5)] :,

2 R ' O S n R 3 + H,O - R,SnOSnR, + 2 R ' O H

(4)

( R ' O ) , SnR, + H,O - O = S n R , 4- 2 R ' O H

However, at least for R = Bu, the Mossbauer results for both sites I and I1 do not agree with available literature data: values for isomer shifts are too different (Table 3). A possible interaction with the surface (via hydrogen-type bonds or for- mation fif Lewis acid-base adducts) could not account for such a difference. Moreover, O,ReOSnMe, is known to be stable in air.,, In addition, on a low-loading Re,O, catalyst supported on a high surface area SiO,-Al,O,, it is not likely that OSnR, fragments would be close enough to interact with each other. Since Mossbauer data do not support for- mation of R,SnOH (Table 3), hydrolysis of 0-Sn or Re-OSn bonds may be ruled out. Thus, the deactivation process seems not to be directly related to the tin-containing ligand. However, atmospheric water is expected to coordinate to the rhenium atom causing not only its s ~ l v a t i o n ~ ~ but also promoting hydrolysis of the remaining alkyl or alkylidene ligands. Therefore, a change in the environment of the rhenium atom, now solvated and containing an electron- withdrawing alkoxide ligand, might weaken the Re-0-Sn bonds, causing a decrease in the s character of the tin atom and, thus, in the IS values. A weakening of these bonds may also account for the disappearance of a band assigned to a Re-0-Sn stretching in the Raman spectrum of Re20,/A1,0,/SnMe, after exposure to air., However, a pen- tacoordination of the tin atom cannot be ruled out.

Regeneration of the catalyst by calcination under air leads to the formation of SnO, as a major product, in good agree- ment with literature data.27~33 The small differences found for IS and QS values when compared with literature data suggest that SnO, is present as a separate phase and is not bonded to the rhenium atoms. This is in agreement with the fact that, even after several regenerations, most rhenium sublimes when the catalyst is calcined at temperatures >820 K. A minor fraction of the tin atoms which remain bonded to the rhenium (probably as a polymeric ionic Sn" species) might account for the loss in the catalytic activity of the generated systems.,,

Finally, the reactivity of Re,07/Si0,-Al,0, towards 2,4,4- trimethylpent-2-ene deserves a comment: the presence of 3,3- dimethylbut- 1-ene, expected when the initiating carbene species is formed via a n-ally1 mechanism,34 was not detected. Although this result does not support a n-ally1 mechanism, as has been proposed for related system^,^^.^^ it is in good agreement with a recent work by Moloy, who reports metathesis of norbornene with a non-promoted Re,O,/Al,O, catalyst:j7 norbornene is not able to react with rhenium via such a mechanism.

Conclusions Re 20 ,/SiO ,-A1 ,O ,/SnR me tat hesis catalysts have been studied by Mossbauer and solid-state NMR spectroscopies. There appears to be at least three types of tin-containing surface organometallic species, corresponding to two Moss- bauer sites: (-O)SnR, and (-Ox-O)SnR,. Although an alkylidene ligand on the rhenium coordination sphere could not be identified by these spectroscopic techniques, chemical reactivity suggests its presence in minor amounts. Air expo- sure, i.e. catalyst deactivation, does not seem to affect the tin environment directly, as hydrolysis of Sn- 0- Re bonds was not observed. Such bonds, however, may be weakened owing to a change in the coordination sphere of the rhenium.

Catalyst regeneration by calcination leads to the formation of an SnO, phase; an additional polymeric ionic SnIV species, probably bonded to the rhenium, may account for the loss in the catalytic activity observed for such systems.

R.B. thanks Dr. J. C. Mol and Dr. J. H. Zimnoch dos Santos for many fruitful discussions. The authors are deeply

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J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 3517

indebted to Dr. R. M. Matos for his help in preparing samples for Moss bauer analyses. Financial support from CNPq, FINEP, FAPESP and FAPEMIG is gratefully acknowledged.

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Paper 5/01910I; Received 24th March, 1995

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