Nonradical Mechanisms for the Uncatalyzed Thermal

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Langmuir 2006, 22, 9949-9956

9949

Nonradical Mechanisms for the Uncatalyzed Thermal Functionalization of Silicon Surfaces by Alkenes and Alkynes: A Density Functional Study Cecilia Coletti,† Alessandro Marrone,† Giacomo Giorgi,‡ Antonio Sgamellotti,‡ Gianfranco Cerofolini,§ and Nazzareno Re*,† Dipartimento di Scienze del Farmaco, UniVersita` G. D’Annunzio, Via dei Vestini, I-66100 Chieti, Dipartimento di Chimica, Istituto CNR di Scienze e Tecnologie Molecolari, UniVersita` di Perugia, Via Elce di sotto 8, I-06123 Perugia, and STMicroelectronics, 20041 Agrate Brianza, Milano, Italy ReceiVed January 2, 2006. In Final Form: August 16, 2006 We propose a new concerted mechanism for the uncatalyzed hydrosilylation of terminal alkenes and alkynes, alternative to the conventional radical-based mechanism. Density functional calculations have been carried out on these and on previously proposed alternative mechanisms for the hydrosilylation of ethylene and acetylene by suitable finite size clusters as models of the thermal functionalization of -SiH3, dSiH2, and tSiH groups in flat Si(100) and Si(111) and porous silicon surfaces by alkenes and alkynes. For each step involved in the considered hydrosilylation pathways, we optimized the geometries of reactants and products and located the corresponding transition states. The calculated activation energies for the concerted pathways of ethylene and acetylene are, respectively, 57.6 and 60.9 kcal mol-1 on -SiH3 and in the ranges 62-63 and 58-61 kcal mol-1 on dSiH2 and 64-66 and 56-61 kcal mol-1 on tSiH. These values are much lower than the activation energies calculated for the corresponding homolytic dissociation of the Si-H bond, which is the preliminary step in the radical path, 85.6, 82-83, and 79-81 kcal mol-1, respectively, for -SiH3, dSiH2, and tSiH groups. Our results thus suggest that the thermal hydrosilylation of alkenes and alkynes on silicon surfaces, for which a radical-based mechanism is currently accepted, may occur through a concerted mechanism.

Introduction There is a rapidly growing interest in the functionalization of silicon surfaces in view of the development of new nanoelectronic devices.1,2 Of particular interest has been the alkyne and alkene hydrosilylation of Si-H-terminated surfaces, which leads to chemically stable silicon-carbon bonds.3 Hydrosilylation involves the insertion of the unsatured carbon-carbon bond of an alkene or an alkyne into a silicon-hydride bond and, when occurring on Si-H-terminated surfaces, leads to alkyl and alkenyl terminations; see Scheme 1. Both flat Si(111) and Si(100) and porous silicon surfaces react with terminal alkenes and alkynes under a wide variety of conditions such as in the presence of catalysts (Pt complexes,4 Lewis acids9,10) or radical initiators,5,13 via UV irradiation6a,7,15 * To whom correspondence should be addressed. E-mail: [email protected]. † Universita ` G. D’Annunzio. ‡ Universita ` di Perugia. § STMicroelectronics. (1) Buriak, J. M. Chem. ReV. 2002, 102, 1271-1308. (2) Bent, S. F. Surf. Sci. 2002, 500, 351-360. (3) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48-51. (4) Zazzera, L. A.; Evans, J. F.; Deruelle, M.; Tirrell, M.; Kessel, C. R.; McKeown, P. J. Electrochem. Soc. 1997, 144, 2184-3155. Saghatelian, A.; Buriak, J.; Lin, V. S. Y.; Ghadiri, M. R. Tetrahedron 2001, 57, 5131. (5) Lindford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (6) (a) Effenberger, F.; Go¨tz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. 1998, 37, 2462-2464. (b) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. 1998, 37, 3257-3260. (7) Cicero, R. L.; Lindford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688-5695. (8) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Lindford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759-1758. (9) Buriak, J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 1339-1340. (10) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 1149111502.

Scheme 1. Schematic of 1-Alkene and 1-Alkyne Hydrosilylation of a Hydrogen-Terminated Silicon Surface

or white light irradiation,6b under electrochemically assisted conditions,16 or even by thermally induced reaction.5,8,11,12,14 Among these functionalization approaches, thermal hydrosilylation is one of those avoiding contamination of the silicon surfaces by catalyst or radical-initiator residues and, although requiring more drastic conditions with temperatures around 150-200 °C, is compatible with the high-purity standards of the microelectronic industry. (11) Sieval, A. B.; Opitz, R.; Maas, H. P. A.; Schoeman, M. G.; Meijer, G.; Vergeldt, F. J.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 2000, 16, 10359-10368. (12) (a) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L. Mater. Sci. Eng. C 2002, 1028, 1-5. (b) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L.; Condorelli, G. G.; Fragala`, I. L.; Giorgi, G.; Sgamellotti, A.; Re, N. Appl. Surf. Sci. 2005, 246, 52-67. (13) Lindford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 1263112632. (14) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 13, 6164-6168. (15) Terry, J.; Lindford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. J. Appl. Phys. 1999, 85, 213-221. (16) Henry de Villeneuve, C.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415-2420. Robins, E. G.; Stewart, E. G.; Buriak, J. M. J. Chem. Soc., Chem. Commun. 1999, 2479-2480.

10.1021/la060013b CCC: $33.50 © 2006 American Chemical Society Published on Web 10/18/2006

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Thermal hydrosilylation has been successfully performed on both H-terminated Si(111)5,14 and H-terminated Si(100)8,11,12,14 surfaces and also on porous silicon.17 It has only been observed for long chains, with at least eight carbon atoms, probably due to the higher residence time of the organic molecule at the silanic site on the surface. Although most of the works on the hydrosilylation of Si surfaces have been carried out with 1-alkenes and yield alkyl terminations,5-10 the reaction works equally well with 1-alkynes, leading to alkenyl terminations.11,12 However, in the latter case the reaction may proceed with further hydrosilylation of the initially formed surface-bound alkenyl group, to give an alkyl termination with two Si-C bonds per molecule.11,12 For hydrosilylation promoted by radical initiators, such as diacyl peroxide or UV irradiation, a radical-based mechanism has been clearly evidenced.18 The reaction is initiated by a silicon radical (dangling bond) originated either by a hydrogen abstraction from a surface Si-H group by the initiator-generated radical or by a photochemical Si-H homolytic cleavage. Once formed, the silicon radical may react very rapidly with an unsatured carbon-carbon bond, forming a surface-bound radical which, upon abstraction of hydrogen from a neighboring Si-H bond, leads to the final alkyl or alkenyl termination and regenerates a surface dangling bond which can then propagate the reaction; see Scheme 2. Such a mechanism has been supported by a recent scanning tunneling microscopy (STM) study of styrene addition to H-terminated Si(111) surfaces in ultra-high-vacuum (UHV) conditions which clearly showed that dangling bonds formed by cleavage of Si-H bonds by electrons from the STM tip act as initiating points of chain reactions, leading to the growth of islands of adsorbed styrenes.18 The same radical mechanism has also been proposed for the thermal hydrosilylation reaction since it was first observed for Si(111) surfaces5 and is currently accepted.1 However, it is not clear how homolytic cleavage of a strong Si-H bond with bond energies on the order of 80-90 kcal mol-1 could occur at an appreciable rate at a temperature as low as 150 °C or, in case the radicals were formed during surface preparation, how they can survive in air for the long time required before the reaction starts. Other arguments against the radical mechanism arise from the observation that (i) thermal hydrosilylation by aldehydes

occurs at temperatures much lower than those of thermal hydrosilylation by alkenes, while the rate constants for the addition of silicon-centered radicals to carbonyls are about the same as those for alkenes,17a and (ii) electron-deficient alkynes require longer reaction times, while it is known that they react more rapidly with silicon-centered radicals.6b Two alternative mechanisms to the radical-based path have been recently proposed for the alkene thermal hydrosilylation.1,17a The first one involves catalysis by fluoride ions remaining on the silicon surface after the etching process by aqueous HF; see Scheme 3a. Nucleophilic attack of a surface silicon atom by a F- leads to a pentavalent Si intermediate ion which could then transfer a hydride to the double bond to give a carbanion. This carbanion may then attack the polarized Si center, releasing Fand forming the product. Recent experimental results12b have however shown that thermal hydrosilylation occurs on highly ordered hydrogen-terminated surfaces obtained via exposure of Si(100) surfaces prepared by HF etching to a H2 atmosphere at high temperature (850-1100 °C), which ensures the elimination of any residual fluoride ion, thus ruling out this mechanism, at least for such surfaces. The second alternative mechanism is based on the attack of a surface silicon atom by the electron-rich double bond to form a pentavalent silicon intermediate followed by a [1,3]-hydride shift to the carbocation; see Scheme 3b. A further possibility for the hydrogenated Si(100) surface is that the functionalization may occur at sites where H2 has desorbed. Indeed, it is well-known that the fully hydrogenated Si(100) surface, i.e., the “classic” 1 × 1 dihydride Si(100), may undergo, upon heating, a two-step hydrogen desorption process, leading first to the monohydride 2 × 1 Si(100) surface and then finally to the clean Si(100) surface19 (see Scheme 4a), which easily reacts with alkene through a [2 + 2] addition1,2 (see Scheme 4b). Although the energy barriers of these H2 desorption steps are in the range 50-60 kcal mol-1,25c suggesting that below 200 °C a small fraction of the surface could be activated, the whole desorption process occurs at temperatures above 400 °C mainly because of a high activation entropy and, consequentely, a very low preexponential factor.25c This “surface activation mechanism” can therefore be ruled out as supported by the fact that H2 desorption is not possible through this mechanism for the hydrogenated Si(111) surface, which undergoes hydrosilylation in essentially the same conditions as hydrogenated Si(100). Here, we propose a direct concerted pathway for the hydrosilylation reaction in which the Si-H bond and the carbon-carbon multiple bond approach in a parallel fashion and pass through a four-center transition state which then evolves to the final product; see Scheme 5. Although the analogous insertion reaction in a transition-metal M-H bond is a facile process, and actually constitutes one of the elementary steps of the transition-metalcatalyzed olefin hydrogenation,20 such a concerted process is expected to present a relatively high activation barrier and to the best of our knowledge has never been theoretically considered. In this work we intend to shed light on the mechanism of the thermal hydrosilylation by performing density functional theory (DFT) calculations on the alternative mechanisms to the radicalbased pathways and comparing the relative reaction energies and energy barriers for all the involved steps. The radical-based, the zwitterionic, and the concerted mechanisms will be considered and compared. Since hydrosilylation implies the reaction of only

(17) (a) Boukherroub, R.; Morin, S.; Wayner, D. D. M.; Bensebaa, F.; Sproule, G. I.; Baribeau, J.-M.; Lockwood, D. J. Chem. Mater. 2001, 13, 2002-2011. (b) Bateman, J. E.; Eagling, R. D.; Worall, D. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. 1998, 37, 2683-2685. (18) Cicero, R. L.; Chidsey, C. E. D.; Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Langmuir 2002, 18, 305-307.

(19) Sinnian, K.; Sherman, M. G.; Lewis, L. B.; Weinberg, W. H.; Yates, J. T.; Janda, K. C. J. Chem. Phys. 1990, 92, 5700. Kolasinski, K. W. Int. J. Mod. Phys. B 1995, 9, 2753. Doren, D. J. AdV. Chem. Phys. 1996, 95, 1. Neergaard Waltenburg, H.; Yates, J. Chem. ReV. 1995, 95, 1589-1673. (20) Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988.

Scheme 2 . Mechanism for Radical-Based Hydrosilylation of a Silicon Surface

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Scheme 3. (a) Fluoride-Assisted and (b) Zwitterion-Based Mechanisms for Thermal Hydrosilylation

Scheme 4. Surface Activation Mechanism: (a) Two-Step Hydrogen Desorption from the Hydrogenated Si(100) Surface, Leading to the Clean Si(100) Surface, (b) [2 + 2] Addition of a Terminal Alkene to the Surface Dimer

models of the porous silicon surface and mainly to compare the results with those for the concerted pathway at the same level of theory. Computational Details Methods. All the calculations in this work have been performed using the Gaussian03 program package.23 DFT employing the B3LYP and BPW91 exchange-correlation functionals has been used for all the calculations. The B3LYP functional is based on Becke’s threeparameter exchange functional24a to which the Lee-Yang-Parr (LYP) correlation functional24b was added. B3LYP has been used extensively in the past years to calculate binding and activation energies of organic reactions on Si(100) surfaces using the cluster approximation25a and recently extended to the study of Si(100) and

Scheme 5. Direct Concerted Mechanism for Thermal Hydrosilylation

one surface Si-H unit, involving at most an adjacent unit, the detailed structure of the whole surface is expected not to affect appreciably the overall reaction. Because of this, we performed our DFT calculations on relatively small cluster models of the silicon surface. In particular, SiH3SiH3, (SiH3)2SiH2, and (SiH3)3SiH molecules were employed as models of, respectively, -SiH3, dSiH2, and tSiH groups in porous silicon, while larger Si10H16 and Si13H22 clusters were employed to simulate flat Si(100) and Si(111) surfaces. As the radical pathway for alkene and alkyne hydrosilylation on the flat Si(111) surface has been recently addressed through DFT calculations on a slab or a Si13H22 cluster model,21,22 we will consider this pathway only on the smaller

(21) Kang, J. K.; Musgrave, C. B. J. Chem. Phys. 2002, 116, 9907. Pei, Y.; Ma, J.; Jiang, Y. Langmuir 2003, 19, 7652-7661. Kruse, P.; Johnson, E. R.; DiLabio, G. A.; Wolkow, R. A. Nano Lett. 2002, 2, 807-810. (22) Cho, J.-H.; Oh, D.-H.; Kleinman, L. Phys ReV. B 2002, 65, 0819907(R). Takeuchi, N.; Kanai, Y.; Selloni, A. J. Am. Chem. Soc. 2004, 126, 15890-15896. (23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (24) Becke, A. D. J. Chem. Phys., 1993, 98, 5648. Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1989, B37, 785. (25) (a) Mui, C.; Bent, S. F.; Musgrave, C. B. J. Chem. Phys. 2001, 114, 10170 and references therein. Santos, H. R. R.; Ramos, M. J.; Ferreira Gomes, J. A. N. C. R. Chim. 2005, 8, 1461-1468. Among the huge literature on this field, see also from the authors of this paper: Nunzi, F.; Sgamellotti, A.; Re, N. J. Phys. Chem. B 2004 108, 10881-10886. Nunzi, F.; Sgamellotti, A.; Re, N. Chem. Phys. Lett. 2005, 413, 473-478. Nunzi, F.; Sgamellotti, A.; Re, N. J. Phys. Chem. B 2006, 110, 7682-7687. (b) Reboredo, F. A.; Schwegler, E.; Galli, G. J. Am. Chem. Soc. 2003, 125, 18243-18249. (c) Mui, C.; Bent, S. F.; Musgrave, C. B. J. Phys Chem. B 2004, 108, 18243-18253.

9952 Langmuir, Vol. 22, No. 24, 2006 Si(111) hydrogenated surfaces.11,21,25b,c Binding energies calculated using this method are generally in good agreement with the available experimental data.25 The BPW91 functional is based on the Becke exchange26a and Perdew and Wang correlation26b corrections to the local density approximation (LDA). BPW91 has been proved to be one of the best performing standard generalized gradient approximation (GGA) functionals for the calculation of reaction energies27a and has been recently shown to give reliable results for pericyclic27b and diradical27c reactions. Although it has been found to underestimate the barrier of hydrogen-transfer reactions,27a,28a BPW91 has been employed with success in several studies of chemical processes on the Si(100) surface employing a slab model.29 The use of a standard GGA functional allows a better comparison of our results with those of recent DFT studies of the radical pathway for alkene, alkyne, and aldheyde hydrosilylation on the Si(111) surface performed with a related GGA functional, the Perdew-Burke-Ernzerhof (PBE) functional, and a slab model.21,30 In this context, the performances of several GGA, meta, and hybrid exchange-correlation functionals, including B3LYP and BPW91, have been recently reviewed.28 All the reactants, products, and transition states (TSs) involved in the reactions considered in this study have been optimized without any symmetry constraint with both B3LYP and BPW91 functionals using a 6-31G(d,p) basis set.31 The full Hessian matrix was calculated for each stationary point structure to confirm that a true minimum or transition state had been reached. The frequencies from these calculations have been employed for zero-point energy (ZPE) corrections. An intrinsic reaction coordinate (IRC) analysis was performed for each transition state to check that it is connected with the considered minima.32,33 Single-point B3LYP or BPW91 calculations were then performed on the previously optimized geometries, employing a larger basis set, 6-311++G(3df,3pd).34-36 All the reaction and activation energies reported in the next sections have been obtained from the larger basis set and corrected with the ZPE from the 6-31G(d,p) set. Only the B3LYP values are considered, with the BPW91 values added in parentheses. Indeed, the energy barriers calculated at BPW91 are significantly lower, by 5-8 kcal mol-1, than the B3LYP values and are probably underestimated as recently shown for H-transfer reactions.27a,28 Cluster Models. Two different sets of cluster models for the silicon surface were employed in this work. On one side, the small and unconstrained disilane, SiH3SiH3, disilylsilane, (SiH3)2SiH2, and trisilylsilane, (SiH3)3SiH, molecules were employed as plausible models of, respectively, the surface -SiH3, dSiH2, and tSiH groups on the most floppy edges in porous silicon. On the other side, larger and more rigid Si10H16 and Si13H22 clusters were adopted to mimic the Si(100) and Si(111) flat surfaces. In particular, a Si10H16 cluster is cut from the hydrogenated bulk Si(100) surface and contains five silicon layers: the top layer consists of the surface silicon atom (26) (a) Becke, A. D. Phys. ReV. 1988, A38, 3098. (b) Wang, Y.; Perdew, J. P. Phys. ReV. 1991, B44, 13298. Perdew, J. P.; Wang, Y. Phys. ReV. 1992, B45, 13244. (27) (a) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional Theory; Wiley-VCH: Weinheim, Germany, 2000. (b) Guner, V.; Khuong, K. S.; Leach, A. G.; Lee, P. S.; Bartberger, M. D.; Houk, K. N. J. Phys. Chem. A 2003, 107, 11445-11459. (c) Staroverov, V. N.; Davidson, E. R. J. Mol. Struct.: THEOCHEM 2001, 573, 81-89. (28) (a) Gru¨ning, M.; Gritsenko, O. V.; Baerends, E. J. J. Phys. Chem. A 2004, 108, 4459-4469. (b) Kang, J. K.; Musgrave, C. B. J. Chem. Phys. 2001, 115, 1040-11051. Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Theory Comput. 2006, 2, 364-382. (29) Stich, I. Surf. Sci. 1996, 368, 152. Shen, T.-C.; Steckel, J. A.; Jordan, K. D. Surf. Sci. 2000, 446, 211. Sorescu, D. C.; Jordan, K. D. J. Phys. Chem. B 2000, 104, 8259-8267. (30) Kanai, Y.; Takeuchi, N.; Car, R.; Selloni, A. J. Phys. Chem. B 2005, 109, 18889-18894. (31) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (32) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154. (33) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523. (34) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (35) Blaudeau, J.-P.; McGrath, M. P.; Curtiss, L. A.; Radom, L. J. Chem. Phys. 1997, 107, 5016. (36) Curtiss, L. A.; McGrath, M. P.; Blandeau, J.-P.; Davis, N. E.; Binning, R. C., Jr.; Radom, L. J. Chem. Phys. 1995, 103, 6104.

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Figure 1. Finite size clusters employed to mimic flat silicon surfaces: (a) Si10H16(100), (b) Si10H16(111), and (c) Si16H22. See the text. terminated with two H atoms, while the lower layers consist of, respectively, two, four, two, and one silicon atom whose dangling bonds resulting from the truncation of the bulk are saturated with hydrogen atoms to maintain the sp3 hydridization; see Figure 1a. Two clusters, Si10H16 and Si13H22, are cut from the hydrogenated bulk Si(111) surface. The former contains four silicon layers: the top layer consists of the surface silicon atom terminated with one H atom, while each of the three lower layers consists of three silicon atoms whose dangling bonds are saturated with hydrogen atoms, leading to the same cluster employed for the Si(100) surface, though differently oriented; see Figure 1b. They will be hereafter distinguished as Si10H16(100) and Si10H16(111), respectively. The Si13H22 cluster contains two silicon layers: the top layer consists of seven surface silicon atoms, each one vertically terminated with one H atom, while the remaining lower layer consists of six silicon atoms, and all the dangling bonds are saturated with hydrogen atoms; see Figure 1c. Both sets of cluster models were allowed to fully relax during all the performed geometry optimizations.

Results and Discussion Radical Pathway. In the radical pathway the key step is the formation of the surface silicon radical through the homolytic cleavage of a Si-H bond. This process has been theoretically simulated by lengthening the Si-H distance of one of the Si-H bonds starting from the SiH3SiH3, (SiH3)2SiH2, and (SiH3)3SiH molecules, modeling the porous silicon surface, to give SiH3-

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Table 1. Reaction and Activation Energies (kcal mol-1) for the Ethylene and Acetylene Addition to a (SiH3)nSiH3-n• Radical and for the Abstraction of a Hydrogen Atom from the Terminal SiH3 Group by the β-Carbon Radical in the (H3SiSiH2)(SiH3)n-1SiH3-nCH2CH2• and (H3SiSiH2)(SiH3)n-1SiH3-nCHdCH• Molecules B3LYP ∆E

reaction SiH3SiH2• + CH2dCH2 f SiH3SiH2CH2CH2• (SiH3)2SiH• + CH2dCH2 f (SiH3)2SiHCH2CH2• (SiH3)3Si• + CH2dCH2 f (SiH3)3SiCH2CH2• SiH3SiH2• + CHtCH f SiH3SiH2CHdCH• (SiH3)2SiH• + CHtCH f (SiH3)2SiHCHdCH• (SiH3)3Si• + CHtCH f (SiH3)3SiCHdCH•

BPW91 ∆E

q

Ethylene or Acetylene Addition -14.2 -11.1 -8.2 -22.7 -14.5 -12.0

Hydrogen Abstraction (H3SiSiH2)SiH2CH2CH2• f (•H2SiSiH2)SiH2CH2CH3 -8.2 (H3SiSiH2)(SiH3)SiHCH2CH2• f (•H2SiSiH2)(SiH3)SiHCH2CH3 -8.5 (H3SiSiH2)(SiH3)2SiCH2CH2• f (•H2SiSiH2)(SiH3)2SiCH2CH3 -9.3 (H3SiSiH2)SiH2CHdCH• f (•H2SiSiH2)SiH2CHdCH2 -19.8 (H3SiSiH2)(SiH3)SiHCHdCH• f (•H2SiSiH2)(SiH3)SiHCHdCH2 -19.8 (H3SiSiH2)(SiH3)2SiCHdCH• f (•H2SiSiH2)(SiH3)2SiCHdCH2 -19.3

SiH2•, (SiH3)2SiH•, and (SiH3)3Si• radicals. This distance has been taken as the reaction coordinate and elongated from the equilibrium bond length of ca. 1.49 Å to 4.00 Å. The results show bond dissociation energies of, respectively, 85.6, 83.0, and 80.8 kcal mol-1 (81.5, 79.1, and 76.8 kcal mol-1 at the BPW91 level) without any energy barrier, in agreement with the expected trend of a decrease of the bond dissociation energy on increasing the number of silicon atoms bound to the Si-H unit. We then considered the lengthening of the surface Si-H bond in the Si10H16(100), Si10H16(111), and Si13H22 clusters, obtaining bond dissociation energies of, respectively, 81.9, 79.0, and 80.9 kcal mol-1 (77.9, 74.7, and 76.5 at the BPW91 level) without any energy barrier. The comparison between the two series of data shows that the rigidity of the underlying bulk structure leads to a slight decrease of the Si-H bond dissociation energy, by 1-2 kcal mol-1. For the small cluster models of the porous silicon surface, we addressed the whole radical pathway. We then first considered the reaction of the resulting SiH3SiH2•, (SiH3)2SiH•, and (SiH3)3Si• radicals with ethylene or acetylene to give, respectively, the SiH3SiH2CH2CH2• or SiH3SiH2CHdCH•, (SiH3)2SiHCH2CH2• or (SiH3)2SiHCHdCH•, and (SiH3)3SiCH2CH2• or (SiH3)3SiCHdCH• species with the unpaired electron on the β carbon atom. A geometry optimization has been performed for all reactants and products, and a transition-state search has also been carried out. All the calculated transition states lie very close to the reagents (less than 1 kcal mol-1 for ethylene and 2-3 kcal mol-1 for acetylene), so these reactions can be considered to occur with a negligible energy barrier. The calculated reaction energies for each of the considered reactions are reported in Table 1, at both the B3LYP and BPW91 levels, and show in all cases energetically favored products, by 8-14 kcal mol-1 for ethylene and 13-18 kcal mol-1 for acetylene. We finally considered the hydrogen abstraction by these carbonbased radicals from a neigboring Si-H bond, leading to a terminal ethyl or vinyl group and generating a new silicon radical which may then propagate the chain reaction. To simulate this process, we modified the β-carbon-based radicals by adding a further SiH3 group to one of the terminal SiH3 groups, thus considering the H3SiSiH2SiH2CH2CH2•, H3SiSiH2SiH2CHdCH•, (H3SiSiH2)(SiH3)SiHCH2CH2•, (H3SiSiH2)(SiH3)SiHCHdCH•, (H3SiSiH2)(SiH3)2SiCH2CH2•, or (H3SiSiH2)(SiH3)2SiCHdCH• species; see Scheme 6, left. This allowed us to consider an intramolecular hydrogen abstraction by the β-carbon radical from the added SiH3 group passing through a sterically favored six-membered transition state7 and leading to the final silicon-based radicals

∆E

∆Eq

-14.3 -11.3 -8.6 -17.7 -15.1 -12.7 12.8 12.2 11.8 6.7 6.2 5.9

-9.6 -10.1 -10.9 -21.1 -21.1 -20.5

9.7 9.0 8.2 4.3 3.8 3.6

Scheme 6. Cluster Model of the H Abstraction from a Si-H Bond by a Surface Ethyl or Vinyl Radical

•H2SiSiH2SiH2CH2CH3, •H2SiSiH2SiH2CHdCH2, (•H2SiSiH2)(SiH3)SiHCH2CH3, (•H2SiSiH2)(SiH3)SiHCHdCH2, (•H2SiSiH2)(SiH3)2SiCH2CH3, and (•H2SiSiH2)(SiH3)2SiCHdCH2; see Scheme 6. The calculated reaction and activation energies are reported in Table 1 and show that all the abstraction reactions are slightly exothermic with small energy barriers; in particular, the abstraction reactions by the ethyl radical are exothermic by 8-9 kcal mol-1 with barriers of 12-13 kcal mol-1, while the abstractions by the vinyl radical are more exothermic, by 19-20 kcal mol-1, with smaller barriers, 6-7 kcal mol-1. The results obtained here are in reasonable agreement with those reported in a few recent theoretical investigations of the radical mechanism for the functionalization of Si(111) surfaces with alkenes and alkynes employing larger silicon clusters21 or a slab of several silicon atoms,22 although they calculate slightly higher barriers for hydrogen abstraction (15-16 kcal mol-1 for ethylene and 12 kcal mol-1 for acethylene), indicating that the radical pathway is easier on porous than on flat silicon surfaces, in agreement with experimental evidence.17 In light of the comparison with alternative mechanisms, our calculations confirmsas already known from simple qualitative considerations and previous theoretical investigations21,22sthat the highest barrier in the radical mechanism is by far that associated with the initial homolytic cleavage of a Si-H bond with an energy of 79-86 kcal mol-1, all the following steps showing barriers lower than 13 kcal mol-1. Concerted Pathway. We then considered the concerted pathway for the hydrosilylation reaction in which the Si-H bond and the carbon-carbon multiple bond approach in a parallel fashion and pass through a four-center transition state which then evolves to the final alkyl or vinyl products; see Scheme 5.

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Table 2. Reaction and Activation Energies (kcal mol-1) for the Reaction of (SiH3)nSiH3-n with Ethylene and Acetylene B3LYP fragment

∆E

SiH3SiH3 (SiH3)2SiH2 (SiH3)3SiH Si10H16(100) Si10H16(111) Si13H22

-22.7 -22.0 -21.3 -21.8 -22.0 -20.5

SiH3SiH3 (SiH3)2SiH2 (SiH3)3SiH Si10H16(100) Si10H16(111) Si13H22

-36.1 -36.0 -35.3 -35.5 -35.8 -34.9

BPW91 ∆E

∆Eq

Ethylene 57.6 62.7 66.2 62.8 64.6 66.7

-24.1 -23.5 -22.9 -23.3 -23.4 -22.4

48.7 57.6 58.6 57.7 57.7 58.3

Acetylene 60.9 58.2 60.8 60.5 56.3 57.1

-37.8 -37.6 -37.0 -37.1 -37.5 -36.8

51.5 49.0 50.8 49.2 53.1 54.0

∆Eq

Ethylene. We first considered the reaction between ethylene and the surface Si-H bond of both the SiH3SiH3, (SiH3)2SiH2, and (SiH3)3SiH models of porous silicon and the larger Si10H16(100), Si10H16(111), and Si13H22 cluster models of flat Si(100) and Si(111) surfaces, and the calculated reaction and activation energies are reported in Table 2. In all cases the reaction products, respectively, SiH3SiH2CH2CH3, (SiH3)2SiHCH2CH3, (SiH3)3SiCH2CH3, Si10H15(100)CH2CH3, Si10H15(111)CH2CH3, and Si13H21CH2CH3, are energetically favored over reactants with reaction energies of, respectively, -22.7, -22.0, -21.3, -21.8, -22.0, and -20.5 kcal mol-1 (-24.1, -23.5, -22.9, -23.3, -23.4, and -22.4 kcal mol-1 at the BPW91 level). For all clusters, a four-center transition-state structure has been found, with the Si-H reacting bond coplanar with the C-C bond. All the geometrical parameters are quite similar for the three small clusters, so we will discuss only those for (SiH3)2SiH2 reported in Figure 2a. The distance between Si and the reactive H atom is 1.64 Å, that between the terminal C atom and the reactive H atom is 1.41 Å, the Si-C distance is 2.27 Å, and the C-C bond is 1.43 Å, intermediate between a single and a double bond but closer to the latter. The relatively long distances for the forming Si-C and C-H bonds and short lengths for the vanishing Si-H and CdC bonds indicate an early transition state with a geometry relatively closer to those of the reagents. Energy barriers of 56.3, 62.7, and 66.2 kcal mol-1 (48.7, 57.6, and 58.6 kcal mol-1 at the BPW91 level) have been calculated for the reaction of SiH3SiH3, (SiH3)2SiH2, and (SiH3)3SiH, simulating the ethylene hydrosilylation by, respectively, the -SiH3, dSiH2, and tSiH groups in porous silicon. The latter two values are 6-10 kcal mol-1 higher, probably due to the higher steric hindrance of (SiH3)2SiH2 and (SiH3)3SiH fragments. The transition states for the larger Si10H16(100), Si10H16(111), and Si13H22 clusters are reported in Figure 2b-d. They are all early transition states with geometries quite similar to those of the small clusters discussed above, with the main bond distances within 0.1 Å. Energy barriers of 62.8, 64.6, and 66.7 kcal mol-1 (57.7, 57.7, and 58.3 kcal mol-1 at the BPW91 level) have been calculated, respectively, for the reaction of Si10H16(100), Si10H16(111), and Si13H22 clusters, simulating the ethylene hydrosilylation by the surface dSiH2 groups in Si(100) (the first one) and by the surface tSiH groups in Si(111) (the latter two clusters) Acetylene. We then analyzed the concerted reaction between acetylene and the surface Si-H bond of SiH3SiH3, (SiH3)2SiH2, and (SiH3)3SiH models of porous silicon and the larger Si10H16(100), Si10H16(111), and Si13H22 clusters modeling the flat Si(100) and Si(111) surfaces, and the calculated reaction and activation energies are reported in Table 2.

Figure 2. Optimized structure of the transition states for the ethylene hydrosilylation by (a) (SiH3)2SiH2, (b) Si10H16(100), (c) Si10H16(111), and (d) Si16H22.

Also for the hydrosilylation of acetylene the reaction products, respectively, SiH3SiH2CHdCH2, (SiH3)2SiHCHdCH2, (SiH3)3SiCHdCH2, Si10H15(100)CHdCH2, Si10H15(111)CHdCH2, and Si13H21CHdCH2, are energetically favored over reactants with reaction energies of, respectively, -36.1, 36.0, 35.3, 35.5, 35.8, and 34.9 kcal mol-1 (-37.8, -37.6, -37.0, -37.1, 37.5, and 36.8 kcal mol-1 at the BPW91 level). For the reaction of all the considered clusters a four-center transition-state structure has been found, similar to that for ethylene, with the Si-H reacting bond coplanar with the C-C bond. Since all the geometrical parameters are quite similar for the three small clusters, we will discuss only those for (SiH3)2SiH2 reported in Figure 3a. The

Thermal Functionalization of Silicon Surfaces

Langmuir, Vol. 22, No. 24, 2006 9955 Scheme 7. Structure of the Incipient Vinylcarbene-like Structure

Figure 3. Optimized structure of the transition state for the acetylene hydrosilylation by (a) (SiH3)2SiH2, (b) Si10H16(100), (c) Si10H16(111), and (d) Si16H22.

distance between Si and the reactive H atom is 1.65 Å, that between the terminal C atom and the reactive H atom is 1.63 Å, the Si-C distance is 2.27 Å, and the C-C bond is 1.25 Å, intermediate between a double and a triple bond, indicating again an early transition state. Compared to the transition state for the ethylene hydrosilylation, the main difference, except the shorter C-C distance due to the higher bond order, is the longer distance of the forming C-H bond. Energy barriers of 60.9, 58.2, and 60.8 kcal mol-1 (51.5, 49.0, and 50.8 kcal mol-1 at the BPW91 level) have been calculated, respectively, for the reaction of SiH3SiH3, (SiH3)2SiH2, and (SiH3)3SiH as estimates of the acetylene hydrosilylation by, respectively, the -SiH3, dSiH2, and tSiH groups in porous silicon. At variance with the reaction of ethylene, the barriers for the three fragments are very close, suggesting a lower role of the steric hindrance between the silicon fragment and the smaller acetylene molecule.

The transition states for the larger Si10H16(100), Si10H16(111), and Si13H22 clusters are reported in Figure 3b-d. The first one, simulating the acetylene hydrosilylation by the surface tSiH2 groups on Si(100), is an early transition state with a geometry very close to that of the (SiH3)2SiH2 cluster discussed above, simulating the acetylene hydrosilylation by the surface dSiH2 groups in porous silicon clusters (see Figure 3b), and an energy barrier of 60.5 kcal mol-1 (49.2 kcal mol-1 at the BPW91 level), only slightly higher than that for the same (SiH3)2SiH2 cluster. On the other hand, the transition states for the Si10H16(111) and Si13H22 clusters, simulating the acetylene hydrosilylation by the surface tSiH groups in Si(111), show a geometry significantly different from those for the corresponding (SiH3)3SiH cluster, simulating the acetylene hydrosilylation by the surface tSiH groups in porous silicon. Indeed, the Si-H reacting bond is no longer coplanar with the weakening C-C bond, while the forming C-H bond is shorter (1.34-1.39 Å) and the forming Si-C bond longer (2.48-2.51 Å); see Figure 3c-d. Moreover, the energy barriers of these two transition states are 56.3 and 57.1 kcal mol-1 (52.1 and 53.0 kcal mol-1 at the BPW91 level), i.e., 3-4 kcal mol-1 lower than those for the corresponding (SiH3)3SiH cluster. An IRC calculation on the transition state for Si10H16(111) has shown the reason for this anomalous behavior. In fact, the energy values along the product side of the IRC path exhibit a nearly flat potential region corresponding to an incipient vinylcarbene-like species (see Scheme 7) which then relaxes toward the final Si-vinyl product without any energy barrier, suggesting that the rigidity of this cluster opens an alternative, lower energy, channel to the same product. OVerall Energetics. The energetics of ethylene and acetylene hydrosilylation with the six considered fragments are compared in Table 2, which shows that the acetylene hydrosilylation is energetically and, to a lesser extent and with the exception of SiH3SiH3, also kinetically more favored than that of ethylene, as expected on the basis of the relative strengths of the double and triple C-C bonds. Table 2 also indicates that ethylene hydrosilylation, which has been more throughly investigated, is thermodinamically and kinetically favored passing from tSiH to dSiH2 and -SiH3 groups, consistent with a transmission FTIR study showing that thermal hydrosilylation of porous silicon preferentially consumes SiH3 and SiH2 species.17 Moreover, the comparison between the results for the (SiH3)2SiH2 and (SiH3)3SiH clusters on one side and those for the Si10H16(100) and Si10H16(111) or Si13H22 clusters on the other side allows evaluation of the differencies in the hydrosilylation reactions on the dSiH2 and -SiH3 groups on porous or on flat Si(100) and Si(111) surfaces. The data in Table 2 show, for both ethylene and acetylene, no clear difference between the hydrosilylation on these two kinds of surfaces. What is more important is our results indicate that the energy barriers for the concerted pathway are in the range of 56-66 kcal mol-1 (48-58 kcal mol-1 at the BPW91 level) and therefore much lower than the energy of 79-86 kcal mol-1 (77-82 kcal mol-1 at the BPW91 level) required for the initial homolytic

9956 Langmuir, Vol. 22, No. 24, 2006

cleavage of a Si-H bond in the radical mechanism. This would support the viability of the concerted pathway, though the calculated energy barrier is still higher than the maximum value of ca. 45-50 kcal mol-1, compatible with an appreciable rate at temperatures of 150-200 °C at which thermal hydrosilylation of 1-alkenes and 1-alkynes occurs. Indeed, using the transitionstate theory,37 the kinetic constant for the hydrosilylation reaction can be estimated as

k ) (kBT/h) exp(∆SqR) exp(-∆Hq/RT) where kB is the Boltzmann constant, h the Planck constant, T the temperature, ∆Sq the activation entropy, and ∆Hq the activation enthalpy at absolute zero. Assuming that the reaction occurs between the surface and a physisorbed alkene or alkyne molecule, in the evaluation of the activation entropy we considered only the vibrational degrees of freedom of the reacting molecules and transition state. Using the B3LYP results for the Si10H16-ethylene reaction, we calculated ∆Sq ) +9.4 cal mol-1 K-1, leading to a preexponential factor of 1.1 × 1015 at 200 °C, which for activation enthalpies of 45-50 kcal mol-1 gives kinetic constants compatible with reaction times ranging from a few hours to a few days. We should, however, consider that thermal hydrosilylation of terminal alkenes and alkynes has only been observed for long chains, with at least eight carbon atoms,3 and this has been attributed to the stabilization effects of the physisorption of these molecules to the surface. A recent experimental and theoretical study38 has shown that, for typical 1-alkenes undergoing hydrosilylation to silicon surfaces, such as 1-undecene, the molecule-surface dispersion interaction accounts for as much as 5-6 kcal mol-1 and that a further stabilizing contribution arises from the dispersion interactions with neighboring 1-undecene molecules of the physisorbed layer. Assuming that this stabilization energy lowers the portion of the potential energy surface for the molecule-surface interaction,38 our calculations foresee for the concerted hydrosilylation pathway of a 1-undecene molecule an energy barrier on the order of 50 kcal mol-1 and thus suggest that this could be a viable mechanism for the thermal hydrosilylation of terminal long-chain alkenes and alkynes on Si(100) surfaces. Zwitterionic Pathway. We finally considered the mechanism based on the attack of a surface silicon atom by the electron-rich double bond to form a zwitterionic pentavalent silicon intermediate followed by a [1,3]-hydride shift to the carbocation; see Scheme 3b. Actually, as the zwitterionic silicon pentavalent intermediate involved in the latter mechanism is expected to be quite unstable in the usual reaction conditions (conducted with the neat alkene or in an apolar solvent), it could be a transition state in a concerted pathway. We first tried to optimize the zwitterionic pentavalent intermediates arising from the attack of ethylene or acetylene to the central silicon atom in the SiH3SiH3, (SiH3)2SiH2, and (SiH3)3SiH model molecules. However, despite several attempts with different starting geometries, our calculations do not find any (37) McQuarrie, D. A.; Simon, J. D. Physical Chemistry: A Chemical Approach; University Science Books: Sausalito, CA, 1997. (38) DiLabio, G. A.; Piva, P. G.; Kruse, P.; Wolkow, R. A. J. Am. Chem. Soc. 2004, 126, 16048-16050.

Coletti et al.

evidence for a minimum structure characterized by a pentacoordinated silicon atom. To further check our calculations, we optimized these structures by fixing the Si-C bond at several distances from 4 to 1.5 Å. In all cases, the total energy of the system monotonically increases by decreasing the Si-C distance, without any sign of the presence of a local minimum in the energy, thus indicating that these pentavalent intermediates do not exist. We then tried to optimize these zwitterionic pentavalent structures as transition states in the same three model molecules, and we observed that an optimized structure could only be found if the ethylene or acetylene moiety in the starting geometry was properly oriented toward the reactive hydrogen on the silicon atom. A visual inspection of the resulting optimized geometries shows that the positively charged β-carbon atom has formed a covalent bond with the reactive hydrogen; i.e., the hypothetical zwitterionic pentavalent silicon transition states have collapsed to the same transition states found for the corresponding concerted pathway. These results (assumed to hold true also for the more rigid Si10H16 and Si13H22 clusters for which pentavalent species are expected to be even less stable) show that the zwitterionic and the concerted mechanisms are actually indistinguishable, at least as long as the reaction is conducted in an apolar solvent.

Conclusion For hydrosilylation promoted by radical initiators, such as diacyl peroxide, or UV irradiation, a radical-based mechanism has been clearly evidenced, and the same mechanism is currently accepted also for thermal hydrosilylation. However, it is not clear how homolytic cleavage of a strong Si-H bond with bond energies on the order of 80-90 kcal mol-1 could occur at an appreciable rate at temperatures as low as 150-200 °C required for thermal hydrosilylation. We have proposed a direct concerted pathway in which the Si-H bond and the carbon-carbon multiple bond approach in a parallel fashion and pass through a fourcenter transition state which then evolves to the final product. We have performed density functional calculations on these and on previously proposed alternative mechanisms for the hydrosilylation of ethylene and acetylene by (SiH3)SiH3, (SiH3)2SiH2, (SiH3)3SiH, Si10H16, and Si13H22 as models of the thermal functionalization of -SiH3, dSiH2, and tSiH groups in flat and porous silicon surfaces by alkenes and alkynes. The main result of these calculations is that the activation energy for the concerted pathway, ranging around 60 kcal mol-1 for both ethylene (at least for the -SiH3 fragment) and acetylene, is low enough to suggest that this could be a viable mechanism for the thermal hydrosilylation of terminal alkenes and alkynes on silicon surfaces. The calculations on a previously proposed zwitterionic mechanism have shown that it actually proceeds through the same transition state as for the concerted pathway. In summary, we predict, by means of density functional model calculations, that the thermally induced hydrosilylation of Si(100) and Si(111) surfaces may proceed through a concerted four-atom transition state rather than through a high-energy radical mechanism initiated by a homolytic Si-H f Si• + H• cleavage. Acknowledgment. This work was supported by the Italian Ministry of University and Research (MURST, PRIN 2004, Contract No. 2004033959_003). LA060013B