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Surface organometallic chemistry of zirconium: Application to the stoichiometric activation of the CH bonds of alkanes and to the low-temperature catalytic hydrogenolysis of alkanes
Journal of Molecular Catalysis, 74 (1992) 353-363
353
M2926
Surface organometallic chemistry of zirconium: application to the stoichiometric activation of the C-H bonds of alkanes and to the low-temperature catalytic hydrogenolysis of alkanes Francoise Quignard, Christine Lecuyer, Agnes Choplin, Daniele Olivier and Jean-Marie Basset Institut de Recherches sur la Catalyse, conueruionne a L'Uniuersite Glaude Bernard, Lyon I, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex (France)
Abstract Tetrakis(neopentyl)zirconium reacts with the surface of silica, dehydroxylated at 773 K, to give the surface tris(neopentyl)zirconium complex, ?Si-O-ZrNp3 (1). Under molecular hydrogen (423 K, 400 torr), 1 is transformed into surface zirconium hydrides (2) and silicon hydrides. Complex 2 activates the C-H bonds of cyclooctane and methane via a o-bond metathesis reaction, and selectively at low temperatures hydrogenolyzes alkanes such as neopentane, isobutane and propane, but not ethane. The mechanism proposed involves, inter alia, the activation of C-H bonds of the alkanes and the activation of the C-C bond of the alkyl groups via J3-methyl migration steps.
Introduction
The activation and functionalization of the C-H bonds of alkanes is one of the challenges of modern catalysis. Four different types of C-H bond activation by molecular complexes are now well documented [11. Among them, electrophilic activation via a four-center mechanism [21 (eqn. (1)) by electron-poor d" complexes of the early transition metals (group IV to V), lanthanides and actinides seems particularly promising: Cp*zLuCH3+ 13CH 4 ~ CP*zLu I 3CH 3+CH4
(1)
To our knowledge, no example of a catalytic reaction on these complexes has been reported so far. This may be related to the presence of the electronrich ligands, such as Cp*, necessary to stabilize the dO centers. The substitution of these ligands by surface siloxy ligands may solve this problem: surface functional groups of inorganic oxides may allow the stabilization of electronpoor d? centers by, inter alia, steric effects. We report here that the grafting of a neopentylzirconium complex on the surface of silica followed by hydrogenolysis of the resulting Zr-neopentyl surface fragments leads to highly electron deficient Zr(IV) surface hydrides able to activate the C-H bonds of alkanes.
0304-5102/92/$5.00
© 1992 - Elsevier Sequoia. All rights reserved
354
Results
Synthesis and characterization of supported Zr(IV) dO complexes Synthesis of ~Si-OZrNp3 When ZrNp4 is sublimed on a disk of Si02(500) (Fig. 1(a)), its IR spectrum shows the vCC-H) and S(CH",) vibrational bands typical ofneopentyl ligands at 2953 (vas CH 3 ) , 2864 (vs CH2 ) , 1464 (8 as CH3 ) and 1359 cm- 1 (8s CH3 ) (Fig. 1 (b)). Analysis of the gases evolved under the same experimental conditions on a larger sample (ca. 0.5 g) of deuterated silica reveals the liberation of ca. 1 mol NpD/mol Zr (Table 1), confirming the occurrence of a true surface reaction. The evolution of ca. 10% of nondeuterated NpH can be explained by the fact that the silica surface was ca. 90% deuterated, •as determined by quantitative IR analysis. ~SiOH(D) + ZrNp4 ----+ (~Si-o)",ZrNp4-'"
+ xNpH(D)
(2)
the
Elemental analysis of solid (Table 1) suggests the presence of three neopentylligands per surface Zr atom. This number is confirmed by hydrolysis
4400
3600
2800
2000
1200
WAVENUM8ERS Fig. 1. IR spectra: (a) silica dehydroxylated at 500°C under vacuum; (b) (a), after sublimation of ZrNp4 at 60°C, followed by vacuum treatment at 60 °C, 2h; (c) (b) after reaction with H2 (400 torr, 423 K), followed by evacuation at room temperature.
355
TABLE 1 Chemical characterization of 1 Zr (wt.%)
mole NpH per molZr"
NpD b (%)
C (wt.%)
0.82 1.49 3.12±0.05
0.97 ± 0.05 0.93±0.05
93 91 n.d.
1.73±0.05 n.d. 5.40±0.05
n.d.
"Gas evolved during the reaction of ZrNp4 with Si0 2 (500) . Neopentane is the only detectable hydrocarbon. bAnalyzed by MS.
at room temperature of the alkyl groups of the zirconium surface species with an excess of water (22 torr): ca. 3 mol NpH/mol Zr (exp.: 3.1 ±O.3) are liberated. All these results taken together suggest that the main reaction that occurs between ZrNp4 and the surface of Si0 2(500) is the formation of the surface complex (~Si-O)ZrNp3' (1):. ~Si-OH + ZrNp4 ~ ~SiOZrNp3
+ NpH
(3)
1
It is worth mentioning here that the observed value (x = 3) cannot be taken as the average number of neopentyl ligands per surface Zr atom for the following reasons: (i) all unreacted ZrNp4 has been removed, as shown by IR spectroscopy; (ii) the experimental x value is independent of the Zr content, i.e., of the number of available surface silanol groups per Zr atom (Table 1). The surface complex 1 is very sensitive to trace amounts of O 2 (Scheme
1). ZrNp4
r NpH
H 20
:;; Si-0-ZrlOH)3
:;;SiOH
--1j O2
298K
~ 3 NpH
:;; Si-0-ZrNp3 (1)
\
\(
373K, ,
~ ,.
/ ::::/ Si-O-ZrNp !(CH I Nd
X[
Scheme 1.
2'n
J3-X
~
CH 4
:;; Si-0-ZrlONp)3
356
Ethylene inserts at room temperature into the Zr-C bond of 1, as evidenced by the appearance of strong v(CH 2 ) bands in the IR spectrum. Methane does not react under mild conditions (T < 100°C), as indicated by the lack of neopentane in the gas phase. Synthesis of supported Zr-hydrides 1 reacts with hydrogen (P(H 2 ) = 400 torr, 423 K), as evidenced by IR spectroscopy (Fig. 1(c)). When the reaction is performed in situ in an infrared cell, the following features are observed after 16 h at 150°C: (i) disappearance of the v(C-H) and S(C-H) bands in the 3000-2800 and 1500-1300 cm- I ranges, respectively; (ii) appearance of two moderately intense bands at 2253 and 2191 cm""; (iii) appearance of a strong, broad band centered at 1635 em -1. Deuterium exchange experiments (P(D 2 ) = 90 torr) (Fig. 2(b)) show the total disappearance of the band at 1635 em - 1 and only a minor modification
(c)
4400
1200
4400
3600
2800
2000
1600
WAVENUMBEAS
Fig. 2. m spectra: (a) 2; (b) (a) after treatment with D 2 (90 torr, 298 K) for 15 h followed by evacuation at 298 K; (c) (b), after treatment with H 2 (100 torr, 298 K) for 15 h, followed by evacuation at 298 K. Fig. 3. m spectra: (a) 2; (b) (a) after introduction of CHaI (400 torr, 298 K), followed by evacuation at 298 K; (c) (b) after treatment with KOHIH 20. The 4400-2700 and 1900-1400 cm- I regions are obscured by the presence of water.
357
of the band at 2191 cm - 1. Finally, reintroduction of hydrogen to this system (P(H 2) = 100 torr, RT) restores the initial spectrum (Fig. 2(c)). (It is important to mention here the strict absence of water during the entire experiment (no band around 3600 em -1).) These data suggest that the bands at 1635 cm- 1 are related to the formation of Zr-H bonds (the corresponding Zr-D bond expected at ca. 1169-1151 cm- I cannot be seen owing to the low transmittance of silica in this region). Further chemical characterization with CHaI [3] is in agreement with this hypothesis. The reaction with CHaT (400 torr, RT) liberates methane; simultaneously the band at 1635 cm- 1 completely disappears (Fig. 3(b)). No major modification of the two bands near 2200 em -1 is noticed. Only further alkaline hydrolysis (KOHIH 20 ) provokes the total disappearance of these bands (Fig. 3(c)) and the simultaneous evolution of molecular hydrogen. This chemical reactivity and the position of the vibrational bands* [4] suggest the simultaneous formation on the silica surface of Zr-H (1635 cm- 1) and Si-H (2259, 2191 cm - 1) bonds during hydrogenolysis of ;7Si-OZrNPa. This surface zirconium hydride species is not yet fully characterized and will thus be denoted here [Zr]s-H, (2). The only gaseous products detected during the formation of 2 are methane and ethane (eqn. (4)): ;7Si-O-ZrNpa+H2
---4
[Zr]s-H+ [Si]s-H+9CH 4 +3C 2 H£
(4)
This means that under our conditions, the neopentylligands or the neopentane, initially produced by cleavage of the neopentyl-zirconium bonds, have undergone reactions of hydrogenolysis leading in fine to methane and ethane
(vide infra). C-H activation of alkanes
The supported zirconium hydride, 2, reacts immediately with cyclooctane at 298 K to form a Zr-cyclooctyl surface complex (eqn. (5)): (5)
Evidence for reaction (5) is given by the following experimental results: - after gas-phase evacuation, the infrared spectrum of the solid is modified: two bands characteristic of v(CH 2) vibrational modes appear at 2927 and 2857 cm- I , and two bands typical of o(CH 2 ) vibrational modes appear at 1462 and 1447 em - 1. Simultaneously, the intensity of the v(Zr-H) band at 1635 cm- I decreases strongly (Fig. 4(b)). - the reaction between 2 and cyclooctane leads to the formation of molecular hydrogen in the gas phase. - reaction of the supposedly formed [Zr]s-cyclooctyl species with hydrogen (P(H 2 ) = 400 torr, 423 K) regenerates the zirconium-hydride surface *For example, v(Si-H) variation according to ligation in a series of silicon hydrides: MeaSiH, 2118 cm- 1 , (MeO)aSiH, 2203 cm- 1 , ClaSiH, 2258 cm- 1 •
358
I~ 4400
600
2800
2000
1600
1200
WAVENUMBEAS Fig. 4. IR spectra: (a) 2; (b) (a) after reaction with cyclooctane (45 torr, 298 K), followed by evacuation; (c) (a) after reaction with H 2 (400 torr, 243 K). Asterisks denote bands corresponding to neopentoxy ligands, formed by trace amounts of 02'
complex 2 (Fig. 4(c)); cyclooctane as well as its hydrogenolysis products (methane, ethane, etc.) are detected in the gas phase. Surprisingly, this supported [Zrls-H (2) is also reactive toward methane under moderate conditions (P(CH 4 ) = 400 torr, 423 K): [Zrl s-H+CH 4 - - [Zrls-CH3 + H2
(6)
2
Evidence for reaction (6) comes from the following data: - upon introduction of CH4 on 2 at 423 K, the intensity of the Zr-H band decreases. - molecular hydrogen is formed during this interaction. - analysis by mass spectrometry shows that subsequent hydrolysis with D2 0 (22 torr) liberates, exclusively, monodeuterated methane, CH 3D: (7)
359
Catalytic hydrogerwlysis of alkanes Above 323 K, 2 catalyzes the hydrogenolysis of alkanes such as neopentane, isobutane and propane. At 323 K, neopentane (P(NpH) = 40 torr, p(H z) = 230 torr) is totally converted in 100 h. Up to 50% conversion, the selectivity for isobutane and methane is close to 85% (Fig. 5). At higher conversions, isobutane is transformed into methane and propane. When the reaction is performed at higher temperatures (T> 373 K), the rate of hydrogenolysis is higher, but the selectivity drops, owing to secondary reactions of the previously formed alkanes: isobutane gives propane and methane and subsequently propane gives ethane and methane (Fig. 6). Interestingly, ethane does not undergo any significant further hydrogenolysis; this must have a mechanistic implication. Traces of methyl-2-butane are also observed during the hydrogenolysis of neopentane.
~
50
>-
o
~
c
25-
U1
10 _
methane
85
50 Conversion (%)
ethane
~
propane
~
is obut a r-e
Fig. 5. Hydrogenolysis of neopentane: selectivity/conversion histograms (T = 323 K, p(NpH) = 40 torr, p(H z) = 230 torr, %Zr= 1.5 wt.%).
~
N
'0 o E o
~
t:
100
120
t (h)
Fig. 6. Hydrogenolysis of neopentane at 373 K: evolution of the concentration of various alkanes as a function of time (P(NpH) = 40 torr; p(H z) = 200 torr).
360
Hydrogenolysis of isobutane and of propane have been carried out independently at 423 K. The results confirm the relative simplicity of the reaction path observed with neopentane. In particular, hydrogenolysis of propane is fully selective for methane and ethane at any conversion. The following successive reactions are thus occurring: 2
neopentane + H 2
~
isobutane + H2
2
~
propane + H 2
2
~
isobutane + methane
(8)
propane + methane
(9)
ethane + methane
(10)
The mechanism proposed in Scheme 2 accounts for the products obtained during the hydrogenolysis of neopentane and lower alkanes. The first step of this reaction is a C-H bond activation via sigma-bond metathesis [2] on the surface Zr-hydride. It was deduced from a detailed investigation of the reactivity of 2 with various alkanes (vide supra). The next step is likely a {3-methyl transfer leading to isobutene and a surface zirconium-methyl moiety [5, 6]. Hydrogenolysis of this latter fragment
[ZrlsH CH I 3 CH 3 CH 3
-
[
CH 3
CH. -,
/ ~-CH,-CH.
[zrl s CH.
migration step
CH. cH.-d-CH. - - -~
[Zr( s
Scheme 2.
dH.
Electrophilic C-H activation step
361
would then lead to methane. Isobutane would result from the hydrogenation of isobutene on a surface zirconium hydride (7). Formation of 2-methylbutane is easily explained by a Markovnikov type addition of isobutene into the Zr-Me moiety followed by the hydrogenolysis of the Zr-alkyl bond (Scheme 2). The reason why hydrogenolysis of ethane does not occur may be related to the absence of a f3-methyl group and/or to the possible existence of an agostic interaction with the beta hydrogen, as already observed on a scandium-ethyl fragment (8).
Experimental
Silica (Aerosil from Degussa, 200 m 2 g-l) was treated at 500°C for 16 h, under vacuum (10- 5 torr). After this treatment, the number of surface OH groups was ca. 2 OH nm- 2 , as determined by titration with LiAlH4t . Deuterated silica was prepared by exposing Si0 2(500) to D 20 (22 torr) at 500°C for 2 h and subsequent vacuum evacuation at the same temperature for 2 h; this procedure was repeated three times before evacuation at 500 °C for 16 h. The resulting silica was more than 90% deuterated. Zr(Np)4, prepared according to published procedures (9), was sublimed in situ on Si0 2(500) at 333 K. Elemental analyses were performed by Mikroanalytisches Labor Pascher, Remagen, Germany. IR spectra were recorded on a Nicolet 10MX-FT, equipped with a cell designed for in situ preparations under controlled atmosphere (10). Gas-phase analysis was performed using a combination of volumetric and mass spectrometric measurements. Pressures were accurately determined using a precision gauge (Texas Instruments). Mass spectra were recorded on a quadrupole analyzer (Supravac V.G.), connected to a vacuum system able to maintain a residual pressure of 10- 10 torr.
Conclusion
The reaction oftetrakis(neopentyl)zirconium with the surface of Si0 2(500) leads to the formation of a tris(neopentyl)zirconium surface complex (eqn. (3)). This is the major surface species: it has been fully characterized by IR, NMR and chemical analysis. The observed reactivity is in agreement with previous work [11]: the nature of the surface complex (30Si-O)xZrNp4 -x is dependent upon the degree of hydroxylation of the silica surface. This complex, 1, is the precursor of the Zr hydride surface complex 2. Simultaneously, surface Si-H bonds are formed, which probably result from tAll characteristics of silica are from Degussa (technical note).
362
the opening of a neighbour siloxane bridge by the very oxophilic surface zirconium hydride (eqn. (11)). H \
+ -, Zr---.x ;I
o 010/ SI/ -SI -0
/
\
0
I
/HIR) Zr
/ 0
I
"0 \
H
6\
~I/O / ----0/ \ o I
I
I
0-
SI/ "0
(11)
\
2 Molecular zirconium(1V) hydrides are generally stabilized by two 7J5-cyclopentadienylligands; they are thus 16-electron species [3]. The zirconium(1V) hydride grafted on the surface of Si02(500), 2, can be considered in a first approximation as an 8-electron species. The zirconium center acquires through grafting such high electrophilic properties, that the reaction of o-bond metathesis between the Zr hydride and the C-H bonds of cyclooctane and methane becomes feasible. The hydrogenolysis of various alkanes clearly shows that such a reaction can become catalytic, so that certain types of functionalization of alkanes may be envisaged.
Aclmowledgements The consortium ACTANE (CNR8-French Industries) is gratefully acknowledged for financial support.
References 1 (a) A. E. Shilov, Activation ofSaturated Hydrocarbons by Tramsition Metal Complexes,
2
3 4 5 6 7 8
Reidel, Boston, 1984; (b) C. H. Hill, Activation and Functionalization of Alkanes, Wiley-lnterscience, New York, 1989. (a) P. L. Watson and G. W. Parshall, Ace. Chem. Res., 18 (1985) 51; (b) I. P. Rothwell, Ace. Chern. Res., 21 (1988) 153; (c) C. M. Fendrick and T. J. Marks, J. Am. Chem. Soc., 106 (1984) 2214. J. M. Manriquez, D. R. McAlister, R. D. Sanner and J. E. Bercaw, J. Am. Chern. Soc., 98 (1976) 6733. J. M. Manriquez, D. R. McAlister, R. D. Sanner and J. E. Bercaw, J. Am. Chern. Soc., 100 (1978) 2716. W. Pfohl, Ann. Chem., 207 (1960) 629. P. L. Watson and D. C. Roe, J. Am. Chern. Soc., 104 (1982) 6471. J. Schwartz and M. D. Ward, J. Mol. Catal., 8 (1980) 465. B. J. Burger, M. E. Thompson, W. D. Cotter and J. E. Bercaw, J. Am. Chem. Soc., 112 (1990) 1566.
363 9 P. Davidson, M. F. Lappert and R. Pearce, J. Organometall. Chem., 57 (1973) 269. 10 R. Psaro, R. Ugo, G. M. Zanderighi, B. Besson, A. K. Smith and J. M. Basset, J. Organometall. Chern., 213 (1981) 215. 11 (a) D. G. H. Ballard, Adv. Catal., 23 (1973) 263; (b) J. P. Candlin and H. Thomas, Adv. Chern. Ser., 132 (1974) 212; (c) Yu. I. Yermakov, B. N. Kuznetsov and V. A. Zakharov, Catalysis by Supported Complexes, Elsevier, Amsterdam, 1981; (d) V. A. Zakharov, V. K. Dudchenko, A. M. Min'kov, O. Efimov, L. G. Khomykova, V. P. Babenko and Yu. I. Yermakov. Kinet. Catal., 17 (1976) 38; (e) V. A. Zakharov, E. A. Dudchenko, E. A. Paukshtis, 1. G. Karachiev and Yu. I. Yermakov, J. Mol. Catal., 2 (1977) 421; Cf} S. A. Vasnetsov, A. V. Nosov, V. M. Mastikhin and V. A. Zakharov, J. Mol. Catal., 53 (1989) 37.
353
M2926
Surface organometallic chemistry of zirconium: application to the stoichiometric activation of the C-H bonds of alkanes and to the low-temperature catalytic hydrogenolysis of alkanes Francoise Quignard, Christine Lecuyer, Agnes Choplin, Daniele Olivier and Jean-Marie Basset Institut de Recherches sur la Catalyse, conueruionne a L'Uniuersite Glaude Bernard, Lyon I, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex (France)
Abstract Tetrakis(neopentyl)zirconium reacts with the surface of silica, dehydroxylated at 773 K, to give the surface tris(neopentyl)zirconium complex, ?Si-O-ZrNp3 (1). Under molecular hydrogen (423 K, 400 torr), 1 is transformed into surface zirconium hydrides (2) and silicon hydrides. Complex 2 activates the C-H bonds of cyclooctane and methane via a o-bond metathesis reaction, and selectively at low temperatures hydrogenolyzes alkanes such as neopentane, isobutane and propane, but not ethane. The mechanism proposed involves, inter alia, the activation of C-H bonds of the alkanes and the activation of the C-C bond of the alkyl groups via J3-methyl migration steps.
Introduction
The activation and functionalization of the C-H bonds of alkanes is one of the challenges of modern catalysis. Four different types of C-H bond activation by molecular complexes are now well documented [11. Among them, electrophilic activation via a four-center mechanism [21 (eqn. (1)) by electron-poor d" complexes of the early transition metals (group IV to V), lanthanides and actinides seems particularly promising: Cp*zLuCH3+ 13CH 4 ~ CP*zLu I 3CH 3+CH4
(1)
To our knowledge, no example of a catalytic reaction on these complexes has been reported so far. This may be related to the presence of the electronrich ligands, such as Cp*, necessary to stabilize the dO centers. The substitution of these ligands by surface siloxy ligands may solve this problem: surface functional groups of inorganic oxides may allow the stabilization of electronpoor d? centers by, inter alia, steric effects. We report here that the grafting of a neopentylzirconium complex on the surface of silica followed by hydrogenolysis of the resulting Zr-neopentyl surface fragments leads to highly electron deficient Zr(IV) surface hydrides able to activate the C-H bonds of alkanes.
0304-5102/92/$5.00
© 1992 - Elsevier Sequoia. All rights reserved
354
Results
Synthesis and characterization of supported Zr(IV) dO complexes Synthesis of ~Si-OZrNp3 When ZrNp4 is sublimed on a disk of Si02(500) (Fig. 1(a)), its IR spectrum shows the vCC-H) and S(CH",) vibrational bands typical ofneopentyl ligands at 2953 (vas CH 3 ) , 2864 (vs CH2 ) , 1464 (8 as CH3 ) and 1359 cm- 1 (8s CH3 ) (Fig. 1 (b)). Analysis of the gases evolved under the same experimental conditions on a larger sample (ca. 0.5 g) of deuterated silica reveals the liberation of ca. 1 mol NpD/mol Zr (Table 1), confirming the occurrence of a true surface reaction. The evolution of ca. 10% of nondeuterated NpH can be explained by the fact that the silica surface was ca. 90% deuterated, •as determined by quantitative IR analysis. ~SiOH(D) + ZrNp4 ----+ (~Si-o)",ZrNp4-'"
+ xNpH(D)
(2)
the
Elemental analysis of solid (Table 1) suggests the presence of three neopentylligands per surface Zr atom. This number is confirmed by hydrolysis
4400
3600
2800
2000
1200
WAVENUM8ERS Fig. 1. IR spectra: (a) silica dehydroxylated at 500°C under vacuum; (b) (a), after sublimation of ZrNp4 at 60°C, followed by vacuum treatment at 60 °C, 2h; (c) (b) after reaction with H2 (400 torr, 423 K), followed by evacuation at room temperature.
355
TABLE 1 Chemical characterization of 1 Zr (wt.%)
mole NpH per molZr"
NpD b (%)
C (wt.%)
0.82 1.49 3.12±0.05
0.97 ± 0.05 0.93±0.05
93 91 n.d.
1.73±0.05 n.d. 5.40±0.05
n.d.
"Gas evolved during the reaction of ZrNp4 with Si0 2 (500) . Neopentane is the only detectable hydrocarbon. bAnalyzed by MS.
at room temperature of the alkyl groups of the zirconium surface species with an excess of water (22 torr): ca. 3 mol NpH/mol Zr (exp.: 3.1 ±O.3) are liberated. All these results taken together suggest that the main reaction that occurs between ZrNp4 and the surface of Si0 2(500) is the formation of the surface complex (~Si-O)ZrNp3' (1):. ~Si-OH + ZrNp4 ~ ~SiOZrNp3
+ NpH
(3)
1
It is worth mentioning here that the observed value (x = 3) cannot be taken as the average number of neopentyl ligands per surface Zr atom for the following reasons: (i) all unreacted ZrNp4 has been removed, as shown by IR spectroscopy; (ii) the experimental x value is independent of the Zr content, i.e., of the number of available surface silanol groups per Zr atom (Table 1). The surface complex 1 is very sensitive to trace amounts of O 2 (Scheme
1). ZrNp4
r NpH
H 20
:;; Si-0-ZrlOH)3
:;;SiOH
--1j O2
298K
~ 3 NpH
:;; Si-0-ZrNp3 (1)
\
\(
373K, ,
~ ,.
/ ::::/ Si-O-ZrNp !(CH I Nd
X[
Scheme 1.
2'n
J3-X
~
CH 4
:;; Si-0-ZrlONp)3
356
Ethylene inserts at room temperature into the Zr-C bond of 1, as evidenced by the appearance of strong v(CH 2 ) bands in the IR spectrum. Methane does not react under mild conditions (T < 100°C), as indicated by the lack of neopentane in the gas phase. Synthesis of supported Zr-hydrides 1 reacts with hydrogen (P(H 2 ) = 400 torr, 423 K), as evidenced by IR spectroscopy (Fig. 1(c)). When the reaction is performed in situ in an infrared cell, the following features are observed after 16 h at 150°C: (i) disappearance of the v(C-H) and S(C-H) bands in the 3000-2800 and 1500-1300 cm- I ranges, respectively; (ii) appearance of two moderately intense bands at 2253 and 2191 cm""; (iii) appearance of a strong, broad band centered at 1635 em -1. Deuterium exchange experiments (P(D 2 ) = 90 torr) (Fig. 2(b)) show the total disappearance of the band at 1635 em - 1 and only a minor modification
(c)
4400
1200
4400
3600
2800
2000
1600
WAVENUMBEAS
Fig. 2. m spectra: (a) 2; (b) (a) after treatment with D 2 (90 torr, 298 K) for 15 h followed by evacuation at 298 K; (c) (b), after treatment with H 2 (100 torr, 298 K) for 15 h, followed by evacuation at 298 K. Fig. 3. m spectra: (a) 2; (b) (a) after introduction of CHaI (400 torr, 298 K), followed by evacuation at 298 K; (c) (b) after treatment with KOHIH 20. The 4400-2700 and 1900-1400 cm- I regions are obscured by the presence of water.
357
of the band at 2191 cm - 1. Finally, reintroduction of hydrogen to this system (P(H 2) = 100 torr, RT) restores the initial spectrum (Fig. 2(c)). (It is important to mention here the strict absence of water during the entire experiment (no band around 3600 em -1).) These data suggest that the bands at 1635 cm- 1 are related to the formation of Zr-H bonds (the corresponding Zr-D bond expected at ca. 1169-1151 cm- I cannot be seen owing to the low transmittance of silica in this region). Further chemical characterization with CHaI [3] is in agreement with this hypothesis. The reaction with CHaT (400 torr, RT) liberates methane; simultaneously the band at 1635 cm- 1 completely disappears (Fig. 3(b)). No major modification of the two bands near 2200 em -1 is noticed. Only further alkaline hydrolysis (KOHIH 20 ) provokes the total disappearance of these bands (Fig. 3(c)) and the simultaneous evolution of molecular hydrogen. This chemical reactivity and the position of the vibrational bands* [4] suggest the simultaneous formation on the silica surface of Zr-H (1635 cm- 1) and Si-H (2259, 2191 cm - 1) bonds during hydrogenolysis of ;7Si-OZrNPa. This surface zirconium hydride species is not yet fully characterized and will thus be denoted here [Zr]s-H, (2). The only gaseous products detected during the formation of 2 are methane and ethane (eqn. (4)): ;7Si-O-ZrNpa+H2
---4
[Zr]s-H+ [Si]s-H+9CH 4 +3C 2 H£
(4)
This means that under our conditions, the neopentylligands or the neopentane, initially produced by cleavage of the neopentyl-zirconium bonds, have undergone reactions of hydrogenolysis leading in fine to methane and ethane
(vide infra). C-H activation of alkanes
The supported zirconium hydride, 2, reacts immediately with cyclooctane at 298 K to form a Zr-cyclooctyl surface complex (eqn. (5)): (5)
Evidence for reaction (5) is given by the following experimental results: - after gas-phase evacuation, the infrared spectrum of the solid is modified: two bands characteristic of v(CH 2) vibrational modes appear at 2927 and 2857 cm- I , and two bands typical of o(CH 2 ) vibrational modes appear at 1462 and 1447 em - 1. Simultaneously, the intensity of the v(Zr-H) band at 1635 cm- I decreases strongly (Fig. 4(b)). - the reaction between 2 and cyclooctane leads to the formation of molecular hydrogen in the gas phase. - reaction of the supposedly formed [Zr]s-cyclooctyl species with hydrogen (P(H 2 ) = 400 torr, 423 K) regenerates the zirconium-hydride surface *For example, v(Si-H) variation according to ligation in a series of silicon hydrides: MeaSiH, 2118 cm- 1 , (MeO)aSiH, 2203 cm- 1 , ClaSiH, 2258 cm- 1 •
358
I~ 4400
600
2800
2000
1600
1200
WAVENUMBEAS Fig. 4. IR spectra: (a) 2; (b) (a) after reaction with cyclooctane (45 torr, 298 K), followed by evacuation; (c) (a) after reaction with H 2 (400 torr, 243 K). Asterisks denote bands corresponding to neopentoxy ligands, formed by trace amounts of 02'
complex 2 (Fig. 4(c)); cyclooctane as well as its hydrogenolysis products (methane, ethane, etc.) are detected in the gas phase. Surprisingly, this supported [Zrls-H (2) is also reactive toward methane under moderate conditions (P(CH 4 ) = 400 torr, 423 K): [Zrl s-H+CH 4 - - [Zrls-CH3 + H2
(6)
2
Evidence for reaction (6) comes from the following data: - upon introduction of CH4 on 2 at 423 K, the intensity of the Zr-H band decreases. - molecular hydrogen is formed during this interaction. - analysis by mass spectrometry shows that subsequent hydrolysis with D2 0 (22 torr) liberates, exclusively, monodeuterated methane, CH 3D: (7)
359
Catalytic hydrogerwlysis of alkanes Above 323 K, 2 catalyzes the hydrogenolysis of alkanes such as neopentane, isobutane and propane. At 323 K, neopentane (P(NpH) = 40 torr, p(H z) = 230 torr) is totally converted in 100 h. Up to 50% conversion, the selectivity for isobutane and methane is close to 85% (Fig. 5). At higher conversions, isobutane is transformed into methane and propane. When the reaction is performed at higher temperatures (T> 373 K), the rate of hydrogenolysis is higher, but the selectivity drops, owing to secondary reactions of the previously formed alkanes: isobutane gives propane and methane and subsequently propane gives ethane and methane (Fig. 6). Interestingly, ethane does not undergo any significant further hydrogenolysis; this must have a mechanistic implication. Traces of methyl-2-butane are also observed during the hydrogenolysis of neopentane.
~
50
>-
o
~
c
25-
U1
10 _
methane
85
50 Conversion (%)
ethane
~
propane
~
is obut a r-e
Fig. 5. Hydrogenolysis of neopentane: selectivity/conversion histograms (T = 323 K, p(NpH) = 40 torr, p(H z) = 230 torr, %Zr= 1.5 wt.%).
~
N
'0 o E o
~
t:
100
120
t (h)
Fig. 6. Hydrogenolysis of neopentane at 373 K: evolution of the concentration of various alkanes as a function of time (P(NpH) = 40 torr; p(H z) = 200 torr).
360
Hydrogenolysis of isobutane and of propane have been carried out independently at 423 K. The results confirm the relative simplicity of the reaction path observed with neopentane. In particular, hydrogenolysis of propane is fully selective for methane and ethane at any conversion. The following successive reactions are thus occurring: 2
neopentane + H 2
~
isobutane + H2
2
~
propane + H 2
2
~
isobutane + methane
(8)
propane + methane
(9)
ethane + methane
(10)
The mechanism proposed in Scheme 2 accounts for the products obtained during the hydrogenolysis of neopentane and lower alkanes. The first step of this reaction is a C-H bond activation via sigma-bond metathesis [2] on the surface Zr-hydride. It was deduced from a detailed investigation of the reactivity of 2 with various alkanes (vide supra). The next step is likely a {3-methyl transfer leading to isobutene and a surface zirconium-methyl moiety [5, 6]. Hydrogenolysis of this latter fragment
[ZrlsH CH I 3 CH 3 CH 3
-
[
CH 3
CH. -,
/ ~-CH,-CH.
[zrl s CH.
migration step
CH. cH.-d-CH. - - -~
[Zr( s
Scheme 2.
dH.
Electrophilic C-H activation step
361
would then lead to methane. Isobutane would result from the hydrogenation of isobutene on a surface zirconium hydride (7). Formation of 2-methylbutane is easily explained by a Markovnikov type addition of isobutene into the Zr-Me moiety followed by the hydrogenolysis of the Zr-alkyl bond (Scheme 2). The reason why hydrogenolysis of ethane does not occur may be related to the absence of a f3-methyl group and/or to the possible existence of an agostic interaction with the beta hydrogen, as already observed on a scandium-ethyl fragment (8).
Experimental
Silica (Aerosil from Degussa, 200 m 2 g-l) was treated at 500°C for 16 h, under vacuum (10- 5 torr). After this treatment, the number of surface OH groups was ca. 2 OH nm- 2 , as determined by titration with LiAlH4t . Deuterated silica was prepared by exposing Si0 2(500) to D 20 (22 torr) at 500°C for 2 h and subsequent vacuum evacuation at the same temperature for 2 h; this procedure was repeated three times before evacuation at 500 °C for 16 h. The resulting silica was more than 90% deuterated. Zr(Np)4, prepared according to published procedures (9), was sublimed in situ on Si0 2(500) at 333 K. Elemental analyses were performed by Mikroanalytisches Labor Pascher, Remagen, Germany. IR spectra were recorded on a Nicolet 10MX-FT, equipped with a cell designed for in situ preparations under controlled atmosphere (10). Gas-phase analysis was performed using a combination of volumetric and mass spectrometric measurements. Pressures were accurately determined using a precision gauge (Texas Instruments). Mass spectra were recorded on a quadrupole analyzer (Supravac V.G.), connected to a vacuum system able to maintain a residual pressure of 10- 10 torr.
Conclusion
The reaction oftetrakis(neopentyl)zirconium with the surface of Si0 2(500) leads to the formation of a tris(neopentyl)zirconium surface complex (eqn. (3)). This is the major surface species: it has been fully characterized by IR, NMR and chemical analysis. The observed reactivity is in agreement with previous work [11]: the nature of the surface complex (30Si-O)xZrNp4 -x is dependent upon the degree of hydroxylation of the silica surface. This complex, 1, is the precursor of the Zr hydride surface complex 2. Simultaneously, surface Si-H bonds are formed, which probably result from tAll characteristics of silica are from Degussa (technical note).
362
the opening of a neighbour siloxane bridge by the very oxophilic surface zirconium hydride (eqn. (11)). H \
+ -, Zr---.x ;I
o 010/ SI/ -SI -0
/
\
0
I
/HIR) Zr
/ 0
I
"0 \
H
6\
~I/O / ----0/ \ o I
I
I
0-
SI/ "0
(11)
\
2 Molecular zirconium(1V) hydrides are generally stabilized by two 7J5-cyclopentadienylligands; they are thus 16-electron species [3]. The zirconium(1V) hydride grafted on the surface of Si02(500), 2, can be considered in a first approximation as an 8-electron species. The zirconium center acquires through grafting such high electrophilic properties, that the reaction of o-bond metathesis between the Zr hydride and the C-H bonds of cyclooctane and methane becomes feasible. The hydrogenolysis of various alkanes clearly shows that such a reaction can become catalytic, so that certain types of functionalization of alkanes may be envisaged.
Aclmowledgements The consortium ACTANE (CNR8-French Industries) is gratefully acknowledged for financial support.
References 1 (a) A. E. Shilov, Activation ofSaturated Hydrocarbons by Tramsition Metal Complexes,
2
3 4 5 6 7 8
Reidel, Boston, 1984; (b) C. H. Hill, Activation and Functionalization of Alkanes, Wiley-lnterscience, New York, 1989. (a) P. L. Watson and G. W. Parshall, Ace. Chem. Res., 18 (1985) 51; (b) I. P. Rothwell, Ace. Chern. Res., 21 (1988) 153; (c) C. M. Fendrick and T. J. Marks, J. Am. Chem. Soc., 106 (1984) 2214. J. M. Manriquez, D. R. McAlister, R. D. Sanner and J. E. Bercaw, J. Am. Chern. Soc., 98 (1976) 6733. J. M. Manriquez, D. R. McAlister, R. D. Sanner and J. E. Bercaw, J. Am. Chern. Soc., 100 (1978) 2716. W. Pfohl, Ann. Chem., 207 (1960) 629. P. L. Watson and D. C. Roe, J. Am. Chern. Soc., 104 (1982) 6471. J. Schwartz and M. D. Ward, J. Mol. Catal., 8 (1980) 465. B. J. Burger, M. E. Thompson, W. D. Cotter and J. E. Bercaw, J. Am. Chem. Soc., 112 (1990) 1566.
363 9 P. Davidson, M. F. Lappert and R. Pearce, J. Organometall. Chem., 57 (1973) 269. 10 R. Psaro, R. Ugo, G. M. Zanderighi, B. Besson, A. K. Smith and J. M. Basset, J. Organometall. Chern., 213 (1981) 215. 11 (a) D. G. H. Ballard, Adv. Catal., 23 (1973) 263; (b) J. P. Candlin and H. Thomas, Adv. Chern. Ser., 132 (1974) 212; (c) Yu. I. Yermakov, B. N. Kuznetsov and V. A. Zakharov, Catalysis by Supported Complexes, Elsevier, Amsterdam, 1981; (d) V. A. Zakharov, V. K. Dudchenko, A. M. Min'kov, O. Efimov, L. G. Khomykova, V. P. Babenko and Yu. I. Yermakov. Kinet. Catal., 17 (1976) 38; (e) V. A. Zakharov, E. A. Dudchenko, E. A. Paukshtis, 1. G. Karachiev and Yu. I. Yermakov, J. Mol. Catal., 2 (1977) 421; Cf} S. A. Vasnetsov, A. V. Nosov, V. M. Mastikhin and V. A. Zakharov, J. Mol. Catal., 53 (1989) 37.