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Organometallics 2004, 23, 2350-2361
Toward a More Detailed Understanding of Oxidative-Addition Mechanisms: Combined Experimental and Quantum-Chemical Study of the Insertion of Titanium Atoms into C-H, Si-H, and Sn-H Bonds Angela Bihlmeier,† Tim M. Greene,‡ and Hans-Jo¨rg Himmel*,† Institut fu¨ r Anorganische Chemie, Universita¨ t Karlsruhe, Engesserstrasse, Geb. 30.45, 76128 Karlsruhe, Germany, and Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, U.K. Received February 11, 2004
The reactions between Ti atoms and CH4, SiH4, and SnH4 at a temperature of 12 K in Ar matrixes were studied experimentally by IR spectroscopy, taking in the effect of isotopic substitution, and by quantum-chemical calculations. The experiments show that the reactivity changes considerably from CH4 to SiH4 or SnH4. The reaction between Ti and CH4 proceeds inefficiently, and only after photolytic activation of the Ti atoms does insertion occur to give HTiCH3, which features only terminal Ti-H and C-H bonds. On the other hand, reactions with SiH4 and SnH4 occur upon deposition, leading to the products cis- and trans-HTi(µ-H)2SiH and HTi(µ-H)3E (E ) Si, Sn). Selective photolysis can be used to increase the yields of these products. In agreement with the experimental results, quantum-chemical calculations predict the lowest energy form of a molecule with the overall formula TiEH4 to be HTiCH3 with only terminal H atoms for E ) C but HTi(µ-H)3E with three bridging H atoms in the case of E ) Si, Sn. Introduction Oxidative addition is an important reaction, representing a key step in many catalytic cycles. However, the mechanisms of these processes are not fully understood. The activation of C-H bonds is of particular importance. It may be expected that oxidative addition proceeds via a complex, which has in the case of derivatives of silane or methane (HSiR3 and HCR3) the formulas LnM‚SiHR3 and LnM‚CHR3, respectively, where L represents a ligand and M the metal center. Several complexes of SiH4 and even a few of CH4 have been successfully characterized. Examples include the complex [Mo(η2SiH4)(CO)(R2PC2H4PR2)2] (R ) Ph, iBu, Et), which is apparently in thermal equilibrium with the product of oxidative addition, viz. [Mo(H)SiH3(CO)(R2PC2H4PR2)2].1 Complexes featuring BH4- groups have also been the subject of much study. The BH4- unit is isoelectronic with CH4, and therefore, it has been argued that the characterization of the structure, bonding, and conformational variations displayed by derivatives of the BH4anion may hold vital clues to C-H activation processes. The compound Ti(BH4)3 has been characterized in the gas phase via electron diffraction and UV photoelectron spectroscopy and by IR spectroscopy of the matrixisolated molecule.2 Several other species with Ti centers show that the coordination mode of the BH4- unit may vary. Thus, the
compound Ti(BH4)3(PMe3)2, for example, features a sideon coordination of the Ti center to one of the B-H bonds in each of two BH4- groups, while the third BH4group adopts a η2 coordination mode.3 Another example is the compound Ti(BH4)2(dmpe)2 (dmpe ) (CH3)2PCH2CH2P(CH3)2), in which two η2-coordinated BH4units are present.4 There have been numerous previous studies of the interaction of methane with transition-metal atoms, e.g. Fe,5 Co,6,7 and Ni,8,9 to name only a few examples (see also ref 10 and references therein). The reactions of
* To whom correspondence should be addressed. Fax: +49-721-6084854. E-mail:
[email protected]. † Universita ¨ t Karlsruhe. ‡ University of Exeter. (1) Luo, X.-L.; Kubas, G. J.; Burns, C. J.; Bryan, J. C.; Unkefer, C. J. J. Am. Chem. Soc. 1995, 117, 1159.
(2) (a) Dain, C. J.; Downs, A. J.; Rankin, D. W. H. Angew. Chem. 1982, 94, 557. (b) Dain, C. J.; Downs, A. J.; Goode, M. J.; Evans, D. G.; Nicholls, K. T.; Rankin, D. W. H.; Robertson, H. E. J. Chem. Soc., Dalton Trans. 1991, 967. (3) Jensen, J. A.; Girolami, G. S. J. Chem. Soc., Chem. Commun. 1986, 1160.
10.1021/om040016o CCC: $27.50 © 2004 American Chemical Society Publication on Web 04/15/2004
Insertion of Ti into C-H, Si-H, and Sn-H Bonds
Zn,11,12 Cd,12 and Hg12,13 atoms with CH4 in Ar and CH4 matrixes have also been studied, as have those between the main-group atoms Mg,14 B,15 Al,16,17 Ga,18 and In18c and CH4. In all cases, a species of the form HMCH3 with a terminal M-H bond and a methyl group featuring terminal C-H bonds is formed, but the reaction proceeds only after photoactivation of the metal atom. In previous studies some of us have explored the reactions of the group 13 element atoms Al and Ga with SiH4;19,20 in both cases, a spontaneously formed complex featuring a η2-coordinated metal atom was identified and characterized.
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Finally, mechanisms leading to oxidative addition have also been studied in the gas phase by a variety of different techniques. The insertion of metal ions was analyzed, for example, by mass spectrometry, and in some celebrated cases it has proved possible to determine dissociation energies for certain of the species that are generated.22 One important result of these studies is that most transition metals (charged or uncharged) cannot easily be persuaded to react with methane. In the case of some metals, e.g. rhodium,23 not only the possible reaction of the metal atom cation but also the reactions of the positively charged dimer and of small clusters were analyzed. An interesting outcome is that Rh2+ reacts spontaneously with CH4 to give the dehydrogenation product [Rh2CH2]+, but Rh+ and Rhn+ clusters (n g 2) do not show this behavior. Herein we report on the reactions occurring between Ti atoms and CH4, SiH4, and SnH4. In a future publication we will report our investigations on the reactions of Ni atoms with SiH4 and SnH4.24 Hence, it will be shown that the structures of the products of the spontaneous and photolytically activated reactions between Ti atoms and SiH4 or SnH4 differ significantly from those of other MEH4 species that have been studied to date. Experimental Section
The data allowed for an estimate of the differences in the bond distances d(Si-H) of the slightly distorted SiH4 unit. On photolytic activation, the metal atoms inserted into the Si-H bond to give HAlSiH3 and HGaSiH3. In these products, again only terminal H atoms are present. The reaction of the photoexcited group 12 metals with SiH4 yields also the insertion products HMSiH3.21 (4) Jensen, J. A.; Wilson, S. R.; Schultz, A. J.; Girolami, G. S. J. Am. Chem. Soc. 1987, 109, 8094. (5) Kauffman, J. W.; Hauge, R. H.; Margrave, J. L. High Temp. Sci. 1984, 17, 237. (6) Billups, W. E.; Chang, S.-C.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1993, 115, 2039. (7) Billups, W. E.; Chang, S.-C.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1995, 117, 1387. (8) Chang, S.-C.; Hauge, R. H.; Billups, W. E.; Margrave, J. L.; Kafafi, Z. H. Inorg. Chem. 1988, 27, 205. (9) Himmel, H.-J. Chem. Eur. J., in press. (10) Ozin, G. A.; McCaffrey, J. G.; Parnis, J. M. Angew. Chem., Int. Ed. 1986, 25, 1072. (11) Billups, W. E.; Konarski, M. M.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1980, 102, 7393. (12) Greene, T. M.; Andrews, L.; Downs, A. J. J. Am. Chem. Soc. 1995, 117, 8180. (13) (a) Legay-Sommaire, N.; Legay, F. Chem. Phys. Lett. 1994, 217, 97. (b) Legay-Sommaire, N.; Legay, F. Chem. Phys. 1996, 211, 367. (14) McCaffrey, J. G.; Parnis, J. M.; Ozin, G. A.; Breckenridge, W. H. J. Phys. Chem. 1985, 89, 4945. (15) (a) Hassanzadeh, P.; Hannachi, Y.; Andrews, L. J. Phys. Chem. 1993, 97, 6418. (b) Hannachi, Y.; Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1994, 98, 6950. (16) Jeong, G.; Klabunde, K. J. J. Am. Chem. Soc. 1986, 108, 7103. (17) Parnis, J. M.; Ozin, G. A. J. Phys. Chem. 1989, 93, 1204, 1220. (18) (a) Lafleur, R. D.; Parnis, J. M. J. Phys. Chem. 1992, 96, 2429. (b) Xiao, Z. L.; Hauge, R. H.; Margrave, J. L. Inorg. Chem. 1993, 32, 642. (c) Himmel, H.-J.; Downs, A. J.; Greene, T. M.; Andrews, L. Organometallics 2000, 19, 1060.
Evaporation of Ti metal was achieved from a resistively heated Ti filament. The amount of deposited metal was monitored by using a microbalance and by UV/vis spectroscopy. UV/vis and Raman spectroscopy were also used to obtain information about the amount of metal dimers present in the matrix.25 The matrix was deposited on a freshly polished Cu block cooled to 12 K by means of a closed-cycle refrigerator (Leybold LB 510). Other details of the matrix-isolation technique can be found elsewhere.26 SiH4 was used as purchased from Linde (purity >99.99%). SiD4, SnH4, and SnD4 were prepared by the reaction of SiCl4 or SnCl4 with LiAlH4 or LiAlD4 in diglyme and purified by fractional condensation in vacuo. Argon was used as delivered from Messer (purity 99.998%). IR spectra were recorded with a Bruker 113v spectrometer. An MCT detector was used for the spectral region 4000-650 cm-1. Measurements in the region 700-200 cm-1 were carried out with a DTGS detector. Additional spectra between 700 and 50 cm-1 were recorded using a liquid He cooled bolometer. UV/vis spectra were recorded with a Xe arc lamp (Oriel), an Oriel Multispec spectrograph, and a photodiode-array detector. The resolution varied between 0.2 and 0.5 nm. A Hg medium-pressure lamp (Philips LP125) in combination with interference filters was used for photolysis of the matrixes. Quantum-chemical calculations were carried out with the TURBOMOLE program.27 The BP method in combination with an SVP basis set was used for Ti, Si, and H. In the case of Sn, (19) Gaertner, B.; Himmel, H.-J. Angew. Chem. 2002, 114, 1602; Angew. Chem., Int. Ed. 2002, 41, 1538. (20) Gaertner, B.; Himmel, H.-J.; Macrae, V. A.; Downs, A. J.; Greene, T. M. Chem. Eur. J., in press. (21) (a) Legay-Sommaire, N.; Legay, F. J. Phys. Chem. A 1998, 102, 8759. (b) Macrae, V. A.; Greene, T. M.; Downs, A. J. J. Phys. Chem. A 2004, 108, 1393. (22) See, for example: Schwarz, H. Angew. Chem. 2003, 42, 4442 and references given therein. (23) Albert, G.; Berg, C.; Beyer, M.; Achatz, U.; Joos, S.; NiednerSchatteburg, G.; Bondybey, V. E. Chem. Phys. Lett. 1997, 268, 235. (24) Himmel, H.-J. Unpublished results. (25) Himmel, H.-J.; Bihlmeier, A. Chem. Eur. J. 2004, 10, 627. (26) See, for example: Himmel, H.-J.; Downs, A. J.; Greene, T. M. Chem. Rev. 2002, 102, 4191.
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Figure 1. IR spectra in the region around 1500 cm-1 recorded for the reaction of Ti atoms with CH4: (a) spectrum upon deposition; (b) difference between the spectra before and after photolysis at 500 nm; (c) difference between the spectra before and after UV-vis photolysis (200 < λ < 800 nm). an SVP basis set was used only for the two 5s and the two 5p electrons; for the remaining 46 electrons a relativistic ECP (MEFIT, WB)28 was applied.
Results In the following account, the results of the experiments involving codeposition of Ti atoms with CH4, SiH4, and SnH4 will be reported in turn. Ti + CH4. The spectrum taken following deposition of Ti atoms together with 2% CH4 in Ar gave no evidence for an absorption due to a product of any reaction between the two reagents (see Figure 1). Subsequently, the matrix was photolyzed with λmax ) 500 nm light for a period of 30 min. This wavelength can initiate a z(3D1) r a(3F2) or a z(3F2) r a(3F2) transition of the Ti atoms.25,29 The treatment brought about the appearance of a weak but sharp band at 1513.5 cm-1, which can be assigned to the product 1, from the reaction between Ti and CH4. Subsequently, the matrix was subjected to a period of 10 min of photolysis with λmax ) 410 nm, resulting in a decrease of the band due to product 1. No product of the decomposition of 1 was detected. The spectra gave also evidence for weak absorptions due to TiO2 and H2TiO.30 The experiment was repeated, but with CD4 in place of CH4. Again the spectrum taken following deposition showed no sign of any reaction product. Upon photolysis at 500 nm, a weak and sharp band was seen to grow in at 1092.1 cm-1. The absorber responsible for this band is in all probability the perdeuterated version of 1. The behavior of the band toward further photolysis at a different wavelength was similar to that reported for CH4. Thus, photolysis at λmax ) 410 nm or broad-band photolysis led to the rapid decay of 1. Unfortunately, (27) Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel, C. Chem. Phys. Lett. 1989, 162, 165. Eichkorn, K.; Treutler, O.; O ¨ hm, H.; Ha¨ser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 240, 283. Eichkorn, K.; Treutler, O.; O ¨ hm, H.; Ha¨ser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 242, 652. Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc. 1997, 97, 119. Weigend, F.; Ha¨ser, M. Theor. Chem. Acc. 1997, 97, 331. Weigend, F.; Ha¨ser, M.; Patzelt, H.; Ahlrichs, R. Chem. Phys. Lett. 1998, 294, 143. (28) Bergner, A.; Dolg, M.; Kuechle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 80, 1431. (29) Gruen, D. M.; Carstens, D. H. W. J. Chem. Phys. 1971, 54, 5206. (30) Zhang, L.; Dong, J.; Zhou. M. J. Phys. Chem. A 2000, 104, 8882.
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Figure 2. IR spectra in the regions 2170-2145 and 900880 cm-1 of an Ar matrix containing Ti atoms and 0.2% SiH4: (a) upon deposition; (b) after photolysis at λmax ) 500 nm; (c) after photolysis at λmax ) 410 nm.
all attempts to increase the yield of 1 (e.g. by increasing the CH4 or Ti concentrations) failed. We conclude that the reaction between Ti and CH4 needs activation and proceeds inefficiently under the conditions of our experiments to give a low product yield. Ti + SiH4. Unlike the experiments with CH4, the IR spectrum taken upon deposition of Ti atoms together with SiH4 gave evidence for spontaneously formed reaction products. Several experiments showed that not one but four species are formed immediately upon deposition. Selective photolysis proved to be extremely useful to discriminate between these different species. First, a family of absorptions appeared at 2164.3, 2160.4, 893.8, and 888.7 cm-1. These wavenumbers are close to those of free SiH4, and thus, low concentrations of SiH4 (0.2%) in the matrix were necessary in order to observe all of them (see Figure 2). The bands decrease sharply upon photolysis at λmax ) 500 nm and were completely extinguished by photolysis employing broadband visible light (400 < λ < 800 nm). The source of these absorptions is in all probability a complex of the form Ti‚SiH4. Some of the bands observed for the remaining three species have very similar wavenumbers (see Figure 3). Thus, there should be some similarities in the structures of these species. Especially close were the bands of two products, which will be referred to as 2aa and 2ab in the following discussion. The bands of 2aa and 2ab also show almost the same response to the various photolysis cycles. Thus, the bands increase on brief photolysis at λmax ) 500 (1 min) and decrease on photolysis at λmax ) 410. 2aa has bands centered at 2055.0/2047.4, 1541.5, 1507.2, 1144.5, and 854.0 cm-1 and 2ab at 2042.5/ 2040.5, 1514.1, 1120.7, and 937.8/934.2 cm-1. The bands belonging to the fourth species, 3a, appear at 1524.8, 1509.2, 1498.1, and 1129.7 cm-1. They can easily be distinguished from the bands of 2aa and 2ab on the basis of their response to photolysis. Thus, the bands of species 3a were seen to grow upon photolysis at λmax ) 410 at the expense of those due to 2aa and 2ab. The obvious inference is that 2aa and 2ab are precursors to 3a. The bands due to 3a decreased slowly upon photolysis of the matrix with broad-band visible light (400 < λ
