Organometallics 2008, 27, 2123–2127
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Ab Initio and DFT Studies of the Thermal Rearrangement of Trimethylsilylsilylene Bong Hyun Boo,*,† Suk Im,‡ Sungwoo Park,‡ and Sungyul Lee*,‡ Department of Chemistry, Chungnam National UniVersity, Daejeon 305-764, Republic of Korea, and College of EnVironmental Science and Applied Chemistry (BK21), Kyunghee UniVersity, Kyungki-do 446-701, Republic of Korea ReceiVed NoVember 19, 2007
A thermal reaction scheme for the rearrangement of trimethylsilylsilylene (Me3Si-S¨i-H) proposed in a previous study [Organometallics 1986, 5, 698] was studied by the MP2 and DFT methods. The reaction pathways were searched by intrinsic reaction coordinate analysis. We report structures and energies of various silicon species. Activation energies for the C-H insertions by the divalent silicon centers in Me2HSiCH2-S¨i-H and MeHSiCH2-S¨i-Me are predicted to be high, 121 and 110 kJ/mol, respectively, in excellent agreement with an experimental value of 121 kJ/mol. However, the energy barrier for the C-H insertion by the divalent silicon center in Me3S-S¨i-H, predicted to be relatively small, 53 kJ/mol, is lower than that for the 1,2-Me shift (78 kJ/mol) in Me3Si-S¨i-H. 1. Introduction Organosilylenes are known to be primary reactive silicon intermediates in organosilicon chemistry.1–3 In 1986, Boo and Gaspar reported a new rearrangement of an R-silylsilylene, trimethylsilylsilylene (Me3Si-S¨i-H) involving a disilene intermediate (Scheme 1), which is known to compete with wellknown rearrangements (Scheme 2), leading to the formation of the two isomeric β-silylsilylenes.4 The reaction scheme for the rearrangement of the R-silylsilylene to the two β-silylsilylenes is quite reasonable because evidence has already been presented for the similar rearrangement of another R-silylsilylene, trimethylsilylmethylsilylene (Me3Si-S¨i-Me), to two isomeric β-silylsilylenes by Wulff et al., as shown in Scheme 35 It is interesting to note that the stable silylene insertion product 14 is formed in lower yield than that of 15 even if 14 could be formed more efficiently via bfa H shift or afb Me shift than 15. Note that 13 is formed by a single pathway process involving the bfa Me shift. In Scheme 1, the rearrangement of Me3Si-S¨i-H to Me2HSi-S¨i-Me is believed to occur via a Me shift to form Me2SidSiHMe, which in turn reverts to R-silylsilylene, Me2HSi-S¨i-Me. The isomerization of silylsilylene to disilene has been suggested previously by Chen, Cohen, and Gaspar6 (reaction * To whom correspondence should be addressed. (B.H.B.) Tel: 82-42821-6551. Fax: 82-42-821-8896. E-mail:
[email protected]. (S.L.) Tel: 8231-201-2423. Fax: 82-31-202-7337. E-mail:
[email protected]. † Chungnam National University. ‡ Kyunghee University. (1) Gaspar, P. P. React. Intermed. (Wiley) 1978, 1, 229. (2) Gaspar, P. P. React. Intermed. (Wiley) 1981, 2, 335. (3) Gaspar, P. P. React. Intermed. (Wiley) 1985, 3, 333. (4) Boo, B. H.; Gaspar, P. P. Organometallics 1986, 5, 698. (5) Wulff, W. D.; Goure, W. F.; Barton, T. J. J. Am. Chem. Soc. 1978, 100, 6236. (6) Chen, Y. S.; Cohen, B. H.; Gaspar, P. P. J. Organomet. Chem. 1980, 195, C1. (7) Sakurai, H.; Sakaba, H.; Nakadaira, Y. J. Am. Chem. Soc. 1982, 104, 6156. (8) Gaspar, P. P.; Boo, B. H.; Svoboda, D. L. J. Phys. Chem. 1987, 91, 5011.
Scheme 1
Scheme 2
1), Sakurai, Sakaba, and Nakadaira7 (reaction 2), and Gaspar, Boo, and Svoboda8 (reaction 3).
(Me3Si)3SisS¨isSiMe3 f (Me3Si)2SidSi(SiMe3)2 (1) Me3SiMe2SisS¨ isMe f Me2SidSiMeSiMe3
10.1021/om7011612 CCC: $40.75 2008 American Chemical Society Publication on Web 04/02/2008
(2)
2124 Organometallics, Vol. 27, No. 9, 2008
Boo et al.
H3SisS¨ isH f H2SidSiH2
Scheme 3
(3)
Our main purposes in the present study are (1) to explore theoretically the thermal stabilities of the various silicon intermediates because the intermediates are observed only indirectly in the trapping experiments employing several agents,4 (2) to compare the efficiencies of the competing intramolecular C-H insertion and the Me migration concomitant with the formation of disilene 2, (3) to predict the energy barriers for the C-H insertions by the divalent silicon center in Me2HSiCH2-S¨i-H and MeHSiCH2-S¨i-Me because we evaluated the energy barriers by assuming that the energy barrier for the Si-H insertions in HSiMe3 of Me2HSiCH2-S¨i-H and MeHSiCH2-S¨i-Me is 0 (thus significant error could exist), (4) to elucidate the possibilities that the ring opening of the disilirane 4 followed by the H and Me shifts can occur, and (5) finally, to explore the transition states involved in the various silylene rearrangements. In the present study, we evaluated the efficiencies of the reaction routes of the possible rearrangements via ab initio and density functional theory (DFT) studies and elucidated the structures and energies of the silicon intermediates and transition states. The achievements of this study could shed new light on the chemistry of reactive silicon intermediates.
2. Computational Methodologies All calculations were carried out with the Gaussian 03 package suite.9 The ground-state equilibrium geometries and energies were probed by Kohn–Sham DFT.10 Becke’s three-parameter exchange functional11,12 and the gradient-corrected Lee–Yang–Parr correlational functional (B3LYP)13 were used with the 6-311++G(d,p)
basis set. For the promising structures, we further reoptimized the structures with the B3LYP by using the aug-cc-pVDZ basis set of Dunning.14,15 The employment of the cc-pVDZ basis sets16–19 augmented with diffuse functions, aug-cc-pVDZ for the first row, has been demonstrated to be appropriate for describing weak intermolecular interactions such as intermolecular hydrogen bonding.14 For each structure, the complete sets of harmonic vibrational
Table 1. Electronic and Zero-Point Energies, Thermal Corrections, and the Relative Energies and Gibbs Free Energies of the Silicon Intermediates Calculated with the Various Levels of Theorya
ZPE in aub
electronic energy in au
structure MeHSi(CH2)2SiH2 A(L) Me2Si(CH2)SiH2 B(L) MeHSi(CH2)SiHMe C(L) cis-MeHSi(CH2)SiHMe D(L) Me-S¨i-CH2-SiH2Me E(L) Me2HSiCH2-S¨i-H F(L) Me2SidSiHMe G(L) Me3Si-S¨i-H H(L) Me2HSi-S¨i-Me I(L) Me2Si(H)SiMe J(L)
B3LYP/ B3LYP/ 6-311++G(d,p)// aug-cc-pVDZ//B3LYP/ MP2/aug-cc-pVTZ// B3LYP/ B3LYP/6-311++G(d,p) aug-cc-pVDZ B3LYP/aug-cc-pVDZ aug-cc-pVDZ -699.431745 (0)e (0)f -699.401331 (85.8) (70.9) -699.400870 (87.2) (71.1) -699.400523 (88.1) (71.4) -699.387228 (121.9) (85.5) -699.392555 (107.7) (98.8) -699.389929 (122.6) (90.0) -699.381131 (144.7) (107.1) -699.381196 (144.0) (105.1) -699.377667 (151.7) (121.8)
-699.379621 (0) (0) -699.352105 (78.1) (63.2) -699.350977 (81.3) (65.3) -699.350511 (82.5) (65.8) -699.337905 (114.6) (78.2) -699.344829 (96.2) (87.2) -699.342897 (109.2) (76.7) -699.335171 (128.5) (91.0) -699.334555 (129.6) (90.7) -699.331153 (137.0) (107.1)
-698.161989 (0) (0) -698.127294 (97.0) (82.1) -698.125931 (100.8) (84.7) -698.125679 (101.5) (84.7) -698.107677 (147.6) (111.3) -698.117943 (120.5) (111.5) -698.109507 (150.6) (118.0) -698.098847 (177.6) (140.0) -698.097722 (180.0) (141.1) -698.097037 (180.3) (150.3)
therm corr to therm corr to energy at 800 K Gibbs free energy at in kJ/molc 800 K in kJ/mold B3LYP/ aug-cc-pVDZ
B3LYP/ aug-cc-pVDZ
0.110416
118.3
-312.1
0.111417
121.5
-323.7
0.111648
121.1
-325.3
0.111623
121.2
-325.9
0.110796
122.3
-344.4
0.111939
119.1
-320.2
0.113697
122.4
-340.5
0.112346
125.0
-342.9
0.112782
123.3
-345.9
0.112537
122.4
-337.9
a Zero-point and thermal energies (excluding ZPE) were corrected to the energies (Etotal ) Eel + ZPE + Eth) and to the Gibbs free energy. b ZPE was scaled by 0.963.20 c The thermal energy is the sum of translational, rotational, and vibrational energies at 800 K excluding ZPE. d Contribution from translational, rotational, and vibrational energies at 298.15 K except the ZPE, and (pV)()RT) and -TS where T ) 800 K and S800K. e The values denoted in the first parentheses are the relative total energies. f The values denoted in the second parentheses are the relative Gibbs free energies.
Thermal Rearrangement of Trimethylsilylsilylene
Organometallics, Vol. 27, No. 9, 2008 2125
Table 2. Electronic and Zero-Point Energies, Thermal Corrections, and the Relative Energies and Gibbs Free Energies of the Silicon Transition States Calculated with the Various Levels of Theorya
electronic energy in au
structure K(TS) L(TS) M(TS) N(TS) O(TS) P(TS) Q(TS) R(TS) S(TS) T(TS)
B3LYP/ 6-311++G(d,p)// B3LYP/ 6-311++G(d,p)
B3LYP/ aug-cc-pVDZ// B3LYP/aug-cc-pVDZ
MP2/aug-cc-pVTZ// B3LYP/ aug-cc-pVDZ
-699.372960 (153.7)e (144.4)f -699.376781 (147.9) (130.8) -699.367445 (171.7) (142.0) -699.362896 (180.3) (168.7) -699.349190 (214.9) (208.3) -699.348435 (221.4) (192.0) -699.341588 (235.1) (223.9) -699.338047 (238.2) (242.5) -699.331558 (255.3) (256.9) -699.333768 (256.8) (255.3)
-699.325294 (142.0) (132.7) -699.330631 (132.2) (115.2) -699.321658 (155.0) (125.3) -699.314874 (169.5) (157.9) -699.303353 (198.4) (191.8) -699.302534 (205.0) (175.7) -699.294861 (221.0) (209.8) -699.289222 (229.5) (233.8) -699.282571 (247.1) (248.7) -699.286953 (242.8) (241.3)
-698.100670 (160.3) (151.0) -698.094802 (180.0) (162.9) -698.085633 (203.3) (173.6) -698.090500 (187.2) (175.6) -698.073507 (230.4) (223.8) -698.065533 (255.9) (226.5) -698.066222 (249.9) (238.7) -698.067043 (241.4) (245.8) -698.060970 (257.5) (259.1) -698.060099 (267.1) (265.6)
ZPE in aub
therm corr to energy at 800 K in kJ/molc
therm corr to Gibbs free energy at 800 K in kJ/mold
B3LYP/ aug-cc-pVDZ
B3LYP/ aug-cc-pVDZ
B3LYP/ aug-cc-pVDZ
0.110337
117.8
-321.8
0.112101
117.4
-330.0
0.111054
119.5
-340.6
0.110531
117.5
-324.5
0.110001
117.5
-319.5
0.110916
119.6
-340.1
0.109967
117.9
-323.7
0.108363
115.8
-310.2
0.108356
116.0
-312.8
0.110942
116.4
-315.4
a Zero-point and thermal energies (excluding ZPE) were corrected to the energies (Etotal ) Eel + ZPE + Eth) and to the Gibbs free energy. b ZPE was scaled by 0.963.20 c The thermal energy is the sum of translational, rotational, and vibrational energies at 800 K excluding ZPE. d Contribution from translational, rotational, and vibrational energies at 298.15 K except the ZPE, and (pV)()RT) and -TS where T ) 800 K and S800K. e The values denoted in the first parentheses are the relative total energies in kJ/mol. f The values denoted in the second parentheses are the relative Gibbs free energies.
frequencies were then calculated with the analytical second derivative technique by using the B3LYP/6-311++G(d,p) and B3LYP/ aug-cc-pVDZ methods at the geometries optimized with the B3LYP/6-311++G(d,p) and B3LYP/aug-cc-pVDZ methods, respectively. The harmonic vibrational frequencies were also used to determine whether a given structure is a local minimum on the (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; 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.; and Pople, J. A. Gaussian 03, ReVision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (10) Kohn, W.; Sham, L. J. Phys. ReV. A 1965, 140, 1133. (11) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (12) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (13) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (14) Woon, D. E.; Dunning, T. H., Jr. J. Chem. Phys. 1993, 98, 1358. (15) Peterson, K. A.; Woon, D. E.; Dunning, T. H., Jr. J. Chem. Phys. 1994, 100, 7410. (16) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007. (17) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796.
potential energy surface or not. In order to obtain more reliable energies, we carried out additional single-point energy calculations by using the MP2/aug-cc-pVTZ method on the geometries optimized with the B3LYP/aug-cc-pVDZ method. Note that inner-shell electrons were excluded in the electron-correlation calculation to reduce the computational time. The zero-point and thermal energy corrections were made to the electronic and Gibbs free energies by using a scale factor of 0.963.20 All the reaction pathways were confirmed by carrying out the intrinsic reaction coordinate (IRC) analysis,21–23 by which we could check whether the reactants and products used for the prediction of the transition states are correctly reproduced or not. If not, we did not adopt the transition states.
3. Results and Discussion In Tables 1 and 2, we list the relative energies and Gibbs free energies of the various silicon intermediates and transition states, respectively. Note that in Tables 1 and 2, L and TS refer to the local minima and the transition states, respectively, with the letters indicating the order of the Gibbs free energy. All the calculations indicate that A(L) is lowest in energy. Note that (18) Wilson, A. K.; van Mourik, T.; Dunning, T. H., Jr. J. Mol. Struct. (THEOCHEM) 1996, 388, 339. (19) Davidson, E. R. Chem. Phys. Lett. 1996, 260, 514. (20) Rauhut, G.; Pulay, P. J. Phys. Chem. 1995, 99, 3093. (21) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (22) Gonzales, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154. (23) Gonzales, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523. (24) Nagase, S.; Kudo, T. Organometallics 1984, 3, 1320.
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Figure 1. Reaction scheme for the various silylene rearrangements initiated by the 1,2-Me shift. The changes of the total energies and Gibbs free energies (in parentheses) depicted therein were derived from the values listed in Tables 1 and 2 calculated at 800 K using the MP2/aug-cc-pVTZ//B3LYP/aug-cc-pVDZ method.
Figure 2. Reaction scheme for the various silylene rearrangements initiated by C-H insertion. The changes of the total energies and Gibbs free energies (in parentheses) depicted therein were derived from the values listed in Tables 1 and 2 calculated at 800 K using the MP2/ aug-cc-pVTZ//B3LYP/aug-cc-pVDZ method.
the MP2 energies and Gibbs free energies obtained using the same basis set of aug-cc-pVDZ are relatively higher than the DFT values as given in Tables 1 and 2. Figure 1 depicts the feasible reaction pathways for the rearrangement of Me3SiS¨i-H, the initiation of which is a 1,2-Me shift. The energy barrier for the 1,2-Me shift is predicted to be 78 kJ/mol. Nagase and Kudo predicted that there is a 116 kJ/mol energy barrier to the 1,2-Me shift converting methylsilylsilylene to methyldisilene by process 4.24
MeH2SisS¨ isH f H2SidSiHMe
(4)
The reverse activation energy was also predicted to be as high as 145 kJ/mol.24 A local minimum energy structure, a singly bridged form (J(L)), is predicted to exist along the transposition of the hydrogen atom and the methyl group in H(L) by an IRC analysis, although it was experimentally suggested that the process can be completed through a onestep process via the H shift.4 Since the potential well of J(L) is shallow, the further H shift could occur without an energy barrier. However, the entropy effect may place a slight barrier for the H shift converting J(L) to I(L). In Figure 2 are depicted the various isomerizations of Me3Si-S¨i-H that are initiated
via the C-H insertion. The energy barrier for the C-H insertion by the silicon center is predicted to be 53 kJ/mol, somewhat lower than that for the 1,2-Me shift. The energy barrier difference can reflect the trapping product distributions of the silicon intermediates as shown in Schemes 1 and 2. Our previous experiments indicate that the H shift in 4 is a facile process.4 The present calculations also indicate that the energy barrier for the H shift involving the ring opening in B(L) is indeed small (only 63 kJ/mol), implying that the energy barrier could be overcome enough for the further rearrangements to occur by the energy release from the transition state O(TS) being formed. In our previous experimental study, we suggested that Me-S¨i-CH2SiH2Me (6) is formed from disilirane 4 via the Me shift followed by Si-Si bond cleavage.4 The present calculations, however, found that the double shifts of the hydrogen atom and the methyl group concomitant with the formation of β-silylsilylene E(L) are a plausible pathway. However, in the present study we cannot confirm that B(L) undergoes a rearrangement to E(L) only through an intermediate F(L). It is very interesting to note that the yield of the insertion product of 5 is higher than that of 6 even if the Gibbs free energy
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Organometallics, Vol. 27, No. 9, 2008 2127
Figure 3. Reaction scheme for the isomerizations via the C-H insertions by the divalent silicon centers in Me2HSiCH2-S¨i-H and MeHSiCH2-S¨i-Me leading to the stable product 1-methyl-1,3-disilacyclobutane. The changes of the total energies and Gibbs free energies (in parentheses) depicted therein were derived from the values listed in Tables 1 and 2 calculated at 800 K using the MP2/aug-cc-pVTZ// B3LYP/aug-cc-pVDZ method.
evaluated the experimental energy barriers for the C-H insertions by assuming that the energy barrier for the Si-H insertions into HSiMe3 of Me2HSiCH2-S¨i-H and MeHSiCH2-S¨i-Me is 0 and that the silylene insertion products of 8 and 9 are not decomposed during the flash vacuum pyrolysis of 1,1,1,3,3,3-hexamemthyltrisilane in the presence of trimethylsilane,4 and that the C-H insertion barrier in 5 is the same as that in 6. By measuring the temperature-dependent product ratio given in eq 5, we estimated the C-H insertion barrier.
Product ratio )
Figure 4. Molecular parameters of several silicon species optimized with the B3LYP/aug-cc-pVDZ method. Those of the other silicon intermediates and transition states can be reproduced by using the optimized Cartesian coordinates given in the Supporting Information.
of 5 is higher than that of 6. This is evidence that silylene 5 is a kinetically favorable intermediate. As listed in Table 1, it is shown that the thermal correction to the Gibbs free energy is a dominant effect. It is found that the entropy of E(L) (S800K ) 592 J/K) evaluated with the B3LYP/aug-cc-pVDZ//B3LYP/augcc-pVDZ method is significantly larger than that of F(L) (S800K ) 557 J/K) presumably due to the strain arising from the formation of the nonclassical bridged bonding (see Figure 4). In Figure 3, we elucidated the isomerization pathways of silylenes 5 and 6 leading to the stable product 1-methyl-1,3disilacyclobutane A(L) as shown in Scheme 2. Energy barriers for the C-H insertions by the divalent silicon centers in Me2HSiCH2-S¨i-H and MeHSiCH2-S¨i-Me are predicted to be 121 and 110 kJ/mol, respectively, in excellent agreement with an experimental value of 121 kJ/mol.4 Note that we
Product yield of 7 Product yields of 8 + 9
(5)
In Figure 4, we present the molecular parameters of several silicon species. It is interesting to observe in E(L) that one of the terminal hydrogen atoms bridges the two silicon atoms presumably involving a three-center/two-electron Si · · · H · · · Si bond, which makes the H-Si-C-Si subunit greatly strained, thereby giving rise to the decrease of the entropy. In O(TS), one of the hydrogen atoms of the methyl group is significantly shifted to the terminal silicon atom. The other C-H bond lengths are not significantly affected during the elongation process of the C-H bond. In P(TS), the methyl group is also significantly shifted to one of the silicon atoms (bond length ) 2.024 Å). It can be seen in T(TS) that one of the methyl groups and the central carbon atom are significantly shifted from one of the silicon atoms to another. The other molecular structures can be reproduced by using Cartesian coordinates given in the Supporting Information. Supporting Information for the output data is available.
Acknowledgment. This work is partially supported by Chungnam National University in 2007, which is gratefully acknowledged. S.L. thanks the Korea Research Foundation (KRF-2006-311-C00078) for financial support. Supporting Information Available: Cartesian coordinates and output data. This material is available free of charge via the Internet at http://pubs.acs.org. OM7011612
