820
Langmuir 1986,2, 820-823
bands and therefore the amount of reduced Ruthenium is always very small. Finally, Figure 10 shows that by impregnating MgO with a ruthenium nitrosonitrate solution and then reducing at 623 K, a sample which has the imprinting of “oxidized” Ru/MgO is obtained; the band of the linearly adsorbed CO at Y > 2000 cm-’ (curve a) is very weak and disappears almost completely after outgassing at 473 K (curve b). CO is adsorbed mainly as bridged species (vco between 1960 and 1875 cm-I), with peaks a t 1908 and 1875 cm-l prevailing after outgassing a t 473 K. The faces of Ru microcrystals obtained in this case are therefore the same as those obtained from RU,(CO),~/M~O oxidized samples and the hypothesis of an effect of the MgO support on the morphology of dispersed Ru crystals can be advanced.
Conclusions Some final considerations of the structural effect on the reactivity of the Ru/MgO system compared with the reactivity of Ru/SiO, and A1203systems can be made. The bridged CO species, which are typical of the present Ru/MgO system, are desorbed a t higher temperatures (573-623 K) than the linear ones; as a consequence, the lower activity and the formation of a considerable percent of propene and higher hydrocarbons, instead of methane,
found on Ru/MgO samples6 can be explained simply by this surface structural effect. In fact in the bridged CO species the Ru-C bond is stronger than in linearly adsorbed CO and hydrogen adsorption can be partially inhibited: a lower CO hydrogenation activity accompanied by an increased selectivity toward higher hydrocarbons can therefore be expected. Besides, the higher selectivity toward oxygenated compounds such as alcohols, found on Ru/MgO ~ a m p l e s , ~ J ~ can be explained by the easy formation of an oxygenated, nonstoichiometric, RuO, surface phase which can be reduced, in the temperature range of the CO-H, reaction (473-573 K), to metallic ruthenium. In conclusion this paper gives experimental evidence of the importance of the support and of the type of catalyst preparation on the structural surface morphology of Ru microcrystals. The catalytic activity and the selectivity toward same important catalytic reactions might be influenced by these structural effects.
Acknowledgment. The financial support of the Italian Minister0 Pubblica Istruzione, “Progetti Nazionali di Rilevante Interesse per lo Sviluppo della Scienza”, is acknowledged. Registry No. CO,630-08-0; Ru, 7440-18-8; MgO, 1309-48-4.
[ (CH3)5C5]2Th(CH3)2/Silica Surface Chemistry. High-Resolution 13C CPMAS NMR Evidence for Alkylation
of Surface Silicon Sites Paul J. Toscano and Tobin J. Marks* Department of Chemistry, Northwestern University, Euanston, Illinois 60201 Receiued June 18, 1986. I n Final Form: August 15, 1986
The reaction of Cp’,Th(13CH& (Cr’ = q5-Me6Cs)with dehydroxylated silica (ca. 0.4 surface OH/nm2) has been studied by high-resolution C CPMAS NMR spectroscopy. On the basis of I3C chemical shifts, relative signal intensities, and NMR data from model compounds and model reactions, it is proposed that the predominant adsorption pathway involves methyl transfer from thorium to surface silicon sites. The products of this Si-0 cleavage process are surface Si-CH3 and Cp’,Th(CH&siloxide functionalities. The resulting thorium environment appears to be somewhat more electron rich than in the analogous surface complex on dehydroxylated y-alumina. Elucidating the structure and reactivity of the complexes formed when organometallic molecules are adsorbed on high surface area metal oxides and related inorganic materials is of great current interest in catalytic research.’ We have recently studied2 the surface and catalytic chemistry of organoactinides adsorbed on y-A1203both because of properties (e.g., well-defined and restricted actinide oxidation states) which render such systems useful models for technologically significant early transition(1) (a) Lamb, H. H.; Gates, B. C. J . Am. Chem. SOC.1986,108,81-89 and references therein. (b) Basset, J. M.; Chaplin, A. J. Mol. Catal. 1983, 21, 95-107 and references therein. (c) Yermakov, Yu. I. J. Mol. Catal. 1983, 21, 35-55 and references therein. (d) Iwamoto, M.; Kusano, H.; Kagawa, S. Znorg. Chem. 1983, 22, 3365 and references therein. ( e ) Yermakov, Yu. I.; Kuznetsov, B. N.; Zakharov, V. A. ”Catalysis by Supported Complexes”; Elsevier: Amsterdam, 1981. (0 Bailey, D. C.; Langer, S. H. Chem. Reu. 1981,81, 109. (g) Ballard, D. G. H. J. Polym. Sci. 1975, 13, 2191-2212. (2) (a) He, M.-Y.;Xiong, G.; Toscano, P. J.; Burwell, R. L., Jr.; Marks, T. J. J. Am. Chem. SOC.1985, 107, 641-652. (b) Toscano, P. J.; Marks, T. J. J. Am. Chem. SOC.1985, 107, 653-659.
metal/inorganic support interactions3 and because we have found that adsorbed organoactinides can exhibit high and ligation-sensitive catalytic activity for olefin hydrogenation and polymerization. More conventional techniques employed to characterize the organoactinide adsorbate molecules have included evolved product identification/ quantification, isotopic labeling, and reaction kinetics. In addition, we have shown that significant in situ structural and dynamic information on organometallic adsorbates can be obtained with high-resolution solid-state 13C NMR spectroscopy utilizing cross-polarization (CP), high-power (3) (a) Choi, K.-Y.; Ray, W. H. J . Macromol. Sei., Reu. Macromol. Chem. Phys. 1985, (225, 1-56, 57-97. (b) McDaniel, M. P. Adu. Catal. 1985, 33, 47-98. (c) Pino, P.; Rotzinger, B. Macromol. Chem. Phys. Suppl. 1984, 7, 41-61. (d) Karol, F. J. Catal. Reu.-Sei. Eng. 1984,26, 557-595. ( e ) Firment, L. E. J. Catal. 1983, 82, 196-212 and references therein. (f) Gavens, P. D.; Bottrill, M.; Kelland, J. W.; McMeeking, J. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A,, Abel, E. W., Eds.; Pergamon Press: Oxford, 1982; Chapter 22.5. (g) Galli, P.; Luciani, L.; Checchini, G. Angew. Makromol. Chem. 1981, 94, 63.
0743-746318612402-0820$01.50/0 0 1986 American Chemical Society
Langmuir, Vol. 2, No. 6, 1986 821
[(CH,),CJ,Th(CH,),/Silica Surface Chemistry lH decoupling, variable magnetic field strength, and magic-angle spinning (MAS).4 In t h e case of Cp'zAn(13CH3)z (Cp' = q5-Me5C5,An = Th or U) adsorbed on dehydroxylated y-alumina (DA, ca. 160 mz/g surface area, ca. 0.12 surface OH/nm2,ca. 0.5 An/nm2 coverage; impregnation conditions: flowing H e for 1h / 2 5 "C t h e n 1 h/100 "C, t o liberate 0.3 CH4/An) C P M A S NMR spectrazb revealed t h e transfer of a methyl ligand from thorium t o coordinatively unsaturated surface A1 sites5 (e.g., A, B). T h e chemical shift of t h e residual + C,H3 Cp'ZTh CH3
'
0'AI- 1 A
+
C , H3
Cp'ZTh-CH,
I cp'zlh CH3
'
0 1
'AIL B
0
-AI-
C
Th-13CH3 moiety was in t h e region of Cp',Th(CH,)X compounds where X is relatively electron-withdrawing, suggesting description A or B. If t h e same impregnation conditions as for Cp'2An(CH3)z/DA2aabove are used, ca. 1.6 CH4/An is liberated (mostly by protonolysis) from partially dehydroxylated alumina (PDA, ca. 4 surface OH/nm*). For Cp'2Th(13CH3)z/PDA,surface A1-13CH3 groups are not detectable in t h e CPMAS NMR spectrum a n d t h e weak residual Th-I3CH3 resonance is displaced toward t h e region of Cp',Th(CH3)0R compounds, suggesting structure C. I n contrast t o Cp'zAn(CH3)z/DA, C P ' , A ~ ( C H ~ ) ~ / P DisA n o t an effective hydrogenation catalyst, reasonably reflecting the diminution of An-CH, catalyst precursor sitesza and t h e tendency of a-donating6 alkoxy coligands t o depress An-R hydrogenolysis rates.' As a catalyst support, silica offers an intriguing contrast t o alumina i n that t h e surface OH groups are stronger Brernsted acids, yet surface Lewis acid sites are not present.5a,b,dIn addition, silicas with a wide range of surface OH coverages are now readily accessible? In preliminary chemical studies, we found t h a t supporting organoactinides on partially dehydroxylated silica (PDS, ca. 2 surface O H / n m 2 P resulted in substantial CH4 liberation (>1.2 CH4/An) when activated as on DA above a n d , not surprisingly in view of t h e likely An-CH, protonolysis, negligible hydrogenation activity.& More intriguing, however, was the observation that materials such as Cp',An(CH,),/DS (DS = dehydroxylated silica, ca. 0.4 surface OH/nmz)8which liberated only ca. 0.5 CH4/An upon activation as above, were also catalytically i n a ~ t i v e . T ~his curious result and t h e striking contrast t o Cp',An(4) (a) Thomas, J. M.; Klinowski,J. Adu. Catal. 1985,33,199-374. (b) Maciel, G. E. Science (Washington,D.C.)1984,226, 282-288. (c) Fyfe, C. A. Solid State N M R for Chemists; CRC Press: Guelph, 1983. (d) Mehring, M. Principles of High Resolution N M R in Solids; SpringerVerlag: New York, 1983; Chapters 2 and 4. (e) Yannoni, C. S. Acc. Chem. Res. 1982,15, 201-208. (5) (a) Berhek, L.; Kraus, M. In Comprehensiue Chemical Kinetics; Bamford, C. H., Tipper, C. F. H., Eds.; Elsevier: Amsterdam, 1978 Vol. 20, pp 263-398. (b) Benesi, H. A.; Winquist, B. H. C. Adu. Catal. 1978, 27,97-182. (c) Knozinger, H.; Ratnasamy, P. Catal. Rev.-Sci. Eng. 1978, 17,31-70. (d) Lippens, B. C.; Steggerda, J. J. In Physical and Chemical Aspects of Adsorbents and Catalysts; Linsen, G. B., Ed.; Academic Press: London, 1970; Chapter 4. (6) There is now considerable structural and theoretical evidence for the n-donor role of alkoxide ligands in f-element chemistry. (a) Bursten, B. E.; Casarin, M.; Ellis, D. E.; Fragali, I.; Marks, T. J. Inorg. Chem. 1986, 25,1257-1261. (b) Duttera, M. R.; Day, V. W.; Marks, T . J. J. Am. Chem. Soc. 1984, 106, 2907-2912. (c) Cotton, F. A.; Marler, D. 0.;Schwotzer, W. Inorg. Chim. Acta 1984, 95, 207-209. (d) Cotton, F. A.; Marler, D. 0.; Schwotzer, W. Inorg. Chim.Acta 1984, 85, L31-32. (7) (a) Lin, Z.; Marks, T. J., submitted for publication. (b) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks, T . J. J. Am. Chem. Soc. 1981,103,665C-6667. (8) McDaniel, M. P.; Welch, M. B. J. Catal. 1983, 82, 98-109. (9) Xiong, G.; Burwell, R. L., Jr.; Marks, T. J., unpublished results.
(CHJ,/DA catalytic chemistry prompted the present comparative CPMAS NMR s t u d y of t h e Cp',Th(13CH3)z/DSinteraction. Experimental Section Materials and Methods. All procedures were performed in Schlenk-type glassware interfaced to a high-vacuum (10-4-10-5 torr) line or in a nitrogen-filled Vacuum Atmospheres glovebox equipped with an efficient, recirculating atmosphere purification system. Argon (Matheson, prepurified), helium (Matheson, prepurified), and nitrogen (Matheson, prepurified) were purified further by passage through a supported MnO oxygen removal Pentane column and a Davidson 4-Amolecular sieve (H2S04washed) and diethyl ether (both previously distilled from Na/K/benzophenone) were condensed and stored in vacuo in bulbs on the vacuum line. l3CH,Li.LiI (99% 13C) and C P ' , T ~ ( ' ~ C H , )(99% ~ 13C) were prepared from 13CH31(99% 13C,Cambridge Isotopes Laboratories) according to our published p r o c e d u r e ~ . ~Hexamethyldisiloxane ~J~ (Petrarch) and 1,1,3,3-tetramethyl-2-oxa-1,3-disilacyclopentane (Silar Laboratories) were distilled from molecular sieves in vacuo. The purity of these reagents was checked by 'H NMR (C6D6)using a JEOL FX-9OQspectrometer. Elemental analyses were performed by Dornis und Kolbe Mikroanalytisches Laboratorium (Mulheim a.d. Ruhr, West Germany). Highly dehydroxylated silica (DS) was prepared from Davidson grade 62 silica gel (60-80 mesh, previously washed with 0.1 M HN03, dried in flowing He at 400 "C for 2 h) by heating it in a stream of CO at 900-950 "C for 1 h according to the method of McDaniel and Welch.8 The DS was cooled in a stream of He and stored under Nz. This DS is reported to contain less than 0.4 surface OH/nm' and retains a surface area of ca. 250 m2/g.8 Preparation of Model Organoactinides. Cp',Th(Cl)[OSiMe,(t-Bu)]. A solution of (t-Bu)Me,SiOH'O (0.345g, 2.61 mmol) in toluene (5mL) was added dropwise to a cold (-78 "C) stirred suspension of Cp'2Th(C2H,)C17b(1.47g, 2.59 mmol) in toluene (10mL) over a period of 20 min. The suspension was stirred at -78 "C for an additional 10 min after addition was complete, then allowed to warm to room temperature, and stirred for 2 h. The clear, colorless solution was next filtered and the toluene removed in vacuo. Heptane (10mL) was condensed into the flask, and the solids were broken up by manipulating the stir bar with an external magnet. The suspension was then cooled to -78 "C and the white product collected by filtration. Yield: 1.20g (69%). Anal. Calcd for C26H45C10SiThC, 46.66;H, 6.78; Si, 4.20. Found: C, 46.64;H, 6.56;Si, 4.44. 'H NMR (C,D,): 6 2.02 (s, 30 H, Cp'-CHB), 1.05 (s, 9 H, t-Bu-CH,), 0.20 (s, 6 H, Si-CH,). Cp'zTh(CH3)[OSiMez(t-Bu)]. A solution of CH,Li.LiBr (1.74 mmol; 1.45mL of a 1.2 M solution in diethyl ether, Aldrich) was syringed under Ar flush into a cold (-78 "C), stirred suspension of Cp',Th(C1)[OSiMe2(t-Bu)] (1.15g, 1.72mmol) in diethyl ether (20mL). After addition, the mixture was stirred at -78 "C for 15 min and then at -20 "C for 3 h. At the end of this time, the solvent was removed in vacuo while keeping the contents of the flask below 0 "C to prevent redistribution reactions. Heptane (15 mL) was then condensed into the flask and the suspension quickly filtered. The residual solids collected on the frit were backwashed with cold heptane several times. The heptane solution of the product was next concentrated to 5 mL and cooled to -78 "C. The white product was collected by filtration and dried. Yield 0.45 g (43%). Anal. Calcd for C27H480SiTh:C, 49.98;H, 7.46; Si, 4.43. Found: C, 49.98;H, 7.56;Si, 4.44. 'H NMR (CsD6): 6 1.93 (s, 30 H, Cp'-CH3), 1.00 (s, 9 H, t-Bu-CH,), 0.27 (s, 3 H, Th-CH,), 0.13 (s, 6 H, Si-CH,). Reaction of DS with Organometallic Complexes. On a high-vacuum line, a solution of 20 pmol of Cp',Th(13CH3), in pentane (10 mL) was poured onto 0.20 g of DS. The slurry was stirred under Ar for 1 h at ambient temperature while covered with A1 foil to exclude light. The suspension was then filtered (10) Ciliberto, E.; Doris, K. A.; Pietro, W. J.; Reisner, G. M.; Ellis, D. E.; Fragall, I.; Herbstein, F.H.; Ratner, M. A,; Marks, T. J. J. Am. Chem. SOC.1984, 106, 7748-7761.
822 Langmuir, Vol. 2, No. 6, 1986
Toscano and Marks Table I. Solid-state 13C NMR Chemical Shift Datau Cp’ ring Th-CH, Cp’-CH,
complexb Cp’,Th(13CH3),‘ Cp’zTh(13CH3)z/DSd
123.1 124.2 Cp’2Th(CH3)[OSiMez(t-Bu)]d 123.4, 122.7 Cp’ZTh(CHJ[OCH(t-Bu)z]‘ 123.2 Cp’ZTh(CH3)Cl‘ 126.3 Cp’zTh(CH3)03SCFse 125.8, 122.7 Cp’zTh(’3CH,)z/DA‘ 124.2 Cp’zTh(13CH3)2/PDAc 125.0
68.5 59.0 59.2 58.4 67.6 67.7 71.0 66.3
’3CH,Li.LiI/DSd
others
-5.4 (Si-CH,) 27.1 (t-Bu-CH,), 20.3 (t-Bu-C), 1.1, -0.2 (Si-CH,) 94.4 (0-CH), 38.5 (t-Bu-C), 30.6 (t-Bu-CH,) -20 (AI-CH,) -1.0 (Si-CH,)
ppm downfield from Me&. results.
bFor abbreviations, see text. ‘From ref 2b. dThis work. eToscano, P. J.; Marks, T. J., unpublished
and backwashed several times by condensing small portions of pentane from the filtrate into the upper portion of the filtration apparatus. The pentane was removed in vacuo, and the sample was dried under high vacuum for several hours and then brought into the glovebox for NMR experiments. DS was treated similarly with ‘3CH3Li-LiI except that diethyl ether was employed as solvent. These organometallic loadings correspond to ca. 0.5 metal atom/nm2 of DS. High-Resolution Solid-state 13C NMR Experiments. 13C CPMAS NMR spectra were measured on a JEOL FX-6OQS spectrometer (15.0 MHz, 13C)with Chemagnetics solid-state accessories utilizing high-power ‘H decoupling and magic-angle spinning (2.4-2.7 kHz). The air-sensitive materials were sealed in bullet rotors with silicone high-vacuum grease as previously No I3C features attributable to the grease were observed in blank spectra of pure DS. The magic angle was set by using KBr; chemical shifts are reported in parts per million downfield from Me,% as determined from external hexamethylbenzene.2b The optimum cross-polarization contact time was found to be 3 ms and repetition time, 4 s.
Results Synthesis of Cp’,Th(CH3)[0SiMez(t-Bu)] (1). As a model for possible surface complexes, compound 1 was synthesized via the sequence of eq 1 and 2. It was Cp’,Th(Et)Cl
12.0 9.2 12.6 13.5 12.5 11.5 10.2 10.7
+ (t-Bu)Me,SiOH
toluene
Cp’,Th(Cl)[OSiMe,(t-Bu)] + EtH (1) ether
Cp’,Th(Cl)[OSiMe,(t-Bu)]+ CH3Li Cp’,Th(CH,)[OSiMe,(t-Bu)] + LiCl (2) 1
characterized by standard analytical methodology (see Experimental Section for details). Attempts to prepare 1 from C P ’ , T ~ ( C H ~and ) ~ (t-Bu)Me,SiOH invariably led to mixtures of Cp’,Th(CH,),, 1, and what appeared by NMR to be Cp’,Th[OSiMez(t-Bu)I NMR Spectroscopy. Solid-state I3C CPMAS NMR spectra of neat Cp’,Th(CH3), and C P ’ , T ~ ( ’ ~ C Hare ~)~ shown in ref 2b; data are compiled in Table I. The spectrum of Cp’2Th(13CH3)2/DS(Figure 1A) exhibits resonances straightforwardly assignable to the Cp’ ligands at 6 124.2 and 9.2 (see Table I for assignments) and to the labeled Th-13CH3 functionality a t 6 59.0. An additional resonance of approximately the same intensity as Th-13CH3 is observed a t 6 -5.4 and is assigned (vide infra) to a surface Si-CH3 moiety. The CPMAS NMR spectrum of pure DS is featureless under these experimental conditions. The solid-state 13C spectrum of neat Cp’,Th(CH3)[OSiMe,(t-Bu)] is shown in Figure 1B. Assignments follow readily from spectra of similar compounds.2bJ2 The Th-
,.
(11) Toscano, P. J.; Marks, T. J. Organometallics 1986, 5, 400-402. (12) Bruno, J. W.; Smith, G. M.; Marks, T. J.;Fair, C. K.; Schultz, A. J.; Williams, J. M. J . A m . Chem. SOC.1986, 108, 40-56.
C M I ”
d “ “ “ I ’
’ ’“v, ’
’
100 0 PPm Figure 1. 13C CPMAS NMR spectra (15.0 MHz) of (A) CP’,T~(’~CH~)~/DS at a coverage of ca. 0.5 Th/nm2 (18000 scans), (1000 scans), and (C) (B) Cp’2Th(CH3)[OSiMe2(t-Bu)] 13CH3Li-LiI/DSat a coverage of ca. 0.5 Li/nm2 (16000 scans).
I3CH3resonance is found at 6 59.2, and both the Cp’-C (6 123.4, 122.7) and Si-CH3 (6 1.1, -0.2) signals are split, presumably due to restricted 0-SiMe,(t-Bu) rotation in the solid state. The spectrum of 13CH3Li.LiIadsorbed on DS is shown in Figure 1C. A single, somewhat broadened resonance is observed at 6 -1.0, in a distinctly different resonance position from that of neat I3CH3Li-LiI(6 -16.0).13
Discussion The goal of this investigation was to probe, in situ, CP’,T~(CH~)~/DS coordination chemistry and to compare it to our previous results for Cp’,Th(CH3)z/DA. Perhaps the most distinctive feature of the Cp’,Th( 13CH3)2/DS CPMAS NMR spectrum is the new signal at 6 -5.4 which is roughly comparable in intensity to the Th-l3CH3 signal. We propose that this new feature (clearly Th-WH, derived from the intensity) arises from methylation of surface silicon sites (e.g., eq 3), to yield species such as D and E. The 6 -5.4 signal is assigned to D on the basis of chemical shifts of analogous ‘3CH3Si(OR)3and related compounds.14 Furthermore, the reaction of 13CH3Li.LiIwith DS yields a similar signal and is interpretable in terms of eq 4. The (13) Gurak, J. A,; Chinn, J. W.; Lagow, R. J.;Steinfink, H.; Yannoni, C. S. Inorg. Chem. 1984, 23, 3717-3720. (14) (a) Rakita, P. E.; Worsham, L. S. Inorg. Nucl. Chem. Lett. 1977, 13, 547-550. (b) Harris, R. K.; Kimber, B. J. Org. Magn. Reson. 1975, 7, 460-464. (c) Engelhardt, G.;Jancke, H.; Magi, M.; Pehk, T.; Lippmaa, E. J . Organomet. Chem. 1971, 28, 293-300.
Langmuir, Vol. 2, No. 6, 1986 823
[(CH3),Cd,Th(CH3),/Silica Surface Chemistry
Scheme I
, 13
CH3
Cpt2Th 13 CH3
I
/Si\ 13CH3Li+ +Si-0-Sit
-
‘0
+
Q
E
+ 3Si-O-Li’
+Si-13CH3
(4) breadth of the Si-13CH3 signals likely reflects the heterogeneity of local surface environments (commonly observed in such s p e c t r o ~ c o p y ~ and/or ~ J ~ ) interaction with quadrupolar 7Li+in the case of eq 4. In regard to structure E, the surface Cp’ and Th-13CH3 spectral parameters are in close agreement with those of model complex 1. As noted previously,2b the Th-13CH3 chemical shifts in Cp’,Th(CH,)X compounds are sensitive to the electronic properties of X;16the observed high-field displacement is characteristic of oxygen *-donor ligands such as alkoxides (Table I). The comparable intensities of the 13CH3signals assigned to D and E are also in accord with eq 3,while the relationship of the combined intensities to those of the Cp’ signals indicates that methyl group protonolysis has not been extensive, in accord with the aforementioned chemical i n f ~ r m a t i o n . ~ The present results provide some of the most convincing structural evidence to date that surface alkylation processes as in eq 3 can be a dominant reactivity mode for silica. Previous structural formulations involving systems such as A1Me3/DS were largely based on more tenuous infrared spectral assignments in the C-H stretching region,17while non-MAS NMR studies (e.g., CGH5Li+ DS)” have focused only on adsorbate motional processes. As to solution precedent for eq 4, lithium reagents are known to readily cleave the Si-0 bonds of siloxanes, in analogy to eq 4.19 In contrast, NMR-scale solution experiments indicate negligible reaction between Cp’,Th(CH3), and either hexamethyldisiloxane (F) or 1,1,3,3-tetramethyl-2oxa-1,3-disilacyclopentane( G ) over the course of several /o, Me3Si -0-SiMe3 F
i Me2
G
days at room temperature. While siloxane steric bulk may explain some of the observed diminution in reactivity, it is more likely, as corroborated by other lines of chemical (15) (a) McKenna, W. P.; Eyring, E. M. J. Mol. Catal. 1985, 29, 363-369. (b) Hanson, B. E.; Wagner, G. W.; Davis, R. J.; Motell, E. Inorg. Chem. 1984, 23, 1635-1636 and references therein. (c) Liu, D. K.; Wrighton, M. S.; McKay, D. R.; Maciel, G. E. Inorg. Chem. 1984, 23, 212-220. (16) Similar trends are observed for hydride ‘H chemical shifts in the Cp’,Hf(X)H series: Roddick, D. M.; Fryzuk, M. D.; Seidler, P. F.; Hillhouse, G. L.; Bercaw, J. E. Organometallic 1985,4,97-104. (17) (a) Low, M. J. D.; Severdia, A. G.; Chan, J. J. Catal. 1981, 69, 384-391. (b) Morrow, B. A,; Hardin, A. H. J. Phys. Chem. 1979, 83, 3135-3141. (c) Peglar, R. J.; Hambleton, F. H.; Hockey, J. A. J. Catal. 1971, 20, 309-320. (d) Kunawicz, J.; Jones, P.; Hockey, J. A. Trans. Faraday SOC.1971, 67, 848-853. (18) Slotfeldt-Ellingsen, D.; Resing, H. A. J. Phys. Chem. 1980, 84, 2204-2209. (19) Armitage, D. A. In ref 3f, Vol. 2, pp 154-163.
and spectroscopic evidence, that surface metal-oxygen linkages exhibit an enhanced reactivity which is electronic and strain induced in ~ r i g i n . ~ J ~ , ~ ~ The present results indicate that the course of Cp’,Th(CH3)2 reaction chemistry on DS and the resulting thorium environment is different from that on DA. The NMR data argue that the adsorbed thorium center is qualitatively more electron deficient on DA. The coordinative unsaturation and Lewis acidity of the DA surface is more likely t o promote “methide abstraction” to generate a surface model complex with unsaturated, cationic character21(e.g., A, B), while the coordinatively more saturated DS surface, with silicon less readily assuming a higher coordination number2, and with arguably weaker metal-oxygen bonding,17*20,23 is more likely to undergo Th-CH3 addition to the 0-Si bond (eq 3). In terms of hydrogenation catalysis at f-element centers (e.g., Scheme I), we have previously shown that, among other steps, the four-center “heterolytic” hydrogenolysis of metal-alkyl functionalities (step iii, which is frequently rate-limiting) is accelerated by coordinative unsaturation and electrophilic character a t the metal enter.^^^^ The catalytic properties of Cp’,Th(CH3),/DA vis-&vis Cp’,Th(CH3),/DS appear to reflect just such characteristics. It is apparent that catalytically relevant and rather subtle coordination chemistry can take place between various metal oxide surfaces and organoactinide molecules.
Acknowledgment. We are grateful to the Department of Energy for support of this research under Contract DEAC 02-81ER10980. We thank Prof. R. L. Burwell, Jr., for helpful discussions, Drs. D. Hedden and G. Xiong for samples of DS, and Dr. K. Doris for a sample of dry (tBu)Me,SiOH. Registry No. Cp’2Th(CH3)2, 67506-90-5; Cp’,Th(Cl)[OSiivfe2(t-Bu)], 104835-34-9; Cp’,Th(C2HS)C1, 86727-37-9; Cp’2Th(CH3)[OSiMe2(t-Bu)], 104835-35-0;SiO,, 7631-86-9. (20) (a) Chen, J. G.; Crowell, J. E.; Yates, J. F., Jr. J. Chem. Phys. 1986,84,5906-5909 and references therein. (b) Lavalley, J.-C.; Benaissa, M. J . Chem. SOC.,Chem. Commun. 1984, 908-909. (c) Morrow, B. A.; Cody, I. A. J. Phys. Chem. 1975, 79, 761-762. (d) Bocuzzi, F.; Coluccia, S.; Ghiotti, G.; Morterra, C.; Zecchina, A. J. Phys. Chem. 1978, 82, 1298-1301. (21) Cationic group 4 organometallics are also highly reactive: (a) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J . Am. Chem. SOC.1986,108, 1718-1719. (b) Eisch, J. J.; Piotrowski, A. M.; Brownstein, S. K.; Gabe, 1985, 107, 7219-7221. E. J.; Lee, F. L. J . Am. Chem. SOC. (22) Holmes, R. R.; Day, R. 0.;Sau, A. C.; Holmes, J. M. Inorg. Chem. 1986, 25, 600-606 and references therein. (23) For example, in the bulk materials, AHf0(y-Al2O3)= -395 kcal/ mol vs. AHfo(amorphousSiOJ = -215.9 kcal/mol. Wagman, D.; Evans, W. H.; Halow, I.; Parker, V. B.; Bailey, S. M.; Schumm, R. H. NBS Tech. Note (U.S.) 1966, 270-272. (24) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J . Am. Chem. SOC.1985, 107, 8111-8118.
