Chemical Shift Tensors and NICS Calculations for Stable Silylenes Robert West,* ,² Jarrod J. Buffy, ² Michael Haaf, ² Thomas Mu ¨ller, ‡ Barbara Gehrhus, § Michael F. Lappert, § and Yitzhak Apeloig* ,| Department of Chemistry, UniVersity of Wisconsin Madison, Wisconsin 53706 Fachinstitut fu ¨ r Anorganische und Allgemeine Chemie Humboldt UniVersita ¨ t zu Berlin 10115 Berlin, Germany School of Chemistry and Molecular Sciences UniVersity of Sussex, Brighton, U.K. BN1 9QJ Department of Chemistry and Lise Meitner MinerVa Center of Computational Quantum Chemistry Technion-Israel Institute of Technology Haifa 32000, Israel ReceiVed July 14, 1997 Significant efforts have been expended to characterize and understand the electronic nature of silylenes. The diaminosub- stituted silylenes 1, 1 2, 2 and 3 3 enjoy a special interest, both theoretically 1,4,5 and experimentally, 2,6 because of their unusual high thermodynamic and kinetic stability. Silylene 1 can be regarded as a 6π-electron aromatic molecule, and the nature and degree of electron delocalization in this silyene has been controversial. 4-6 We report here the chemical shift tensors for 1-3 (1 δ 11 ) 284.9, δ 22 )-16.1, δ 33 )-43.3; 2 δ 11 ) 350.7, δ 22 )-2.1, δ 33 )-4.5; 3 δ 11 ) 316.4, δ 22 ) 21.1, δ 33 )-60.0 ppm), along with theoretical calculations for model molecules, including nucleus-independent chemical shift (NICS) calcula- tions. 7 The latter support the model that 1 and 3 are cyclically delocalized and have some “aromatic” character. The slow-spinning 29 Si CPMAS NMR spectra of 1-3 were determined at 59.6 MHz, and the results were analyzed using the Herzfeld-Berger method 8 to determine the chemical shielding tensors. The isotropic shifts δ 29 Si of the silylenes in the solid (see Table 1) are very similar to those observed in solution for these compounds (78, 117, and 92 ppm for 1, 2, and 3, respectively). In all three silylenes, the silicon atoms have chemical shielding tensors of nearly axial symmetry. One tensor component (δ 11 ) is significantly deshielded, while δ 22 and δ 33 have nearly the same magnitude and are in the expected shift range for sp 3 -type silicons, +30 to -60 ppm (see Table 1). Therefore, the measured values for the spread of the tensor, Δδ ()δ 11 - δ 33 ) for the silylene silicons are very large. The results for the silylenes parallel those for stable disilenes 9 and analogous diaminocarbenes, 10 reflecting a highly anisotropic electron dis- tribution around the central silicon. The chemical shielding tensors of silylenes 1 and 2 and of model compounds 4-6 were calculated using the DFT-hybrid GIAO method 11 and are summarized in Table 1. 12 Since this theoretical approach is known to overestimate the deshielding contributions to the chemical shielding tensor in cases when electron correlation is important, 12c we also performed MP2/GIAO calculations 13 for the smaller molecules 4 and 5. The chemical shielding tensors of 4 and 5 were also calculated using the IGLO ² University of Wisconsin. ‡ Humboldt University. § University of Sussex. | Technion-Israel Institute of Technology. (1) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Belyakov, A. V.; Verne, H. P.; Haaland, A.; Wagner, M.; Metzler N. J. Am. Chem. Soc. 1994, 116, 2691. (2) West, R.; Denk, M. Pure Appl. Chem. 1996, 68, 785. (3) (a) Gehrhus, B.; Lappert, M. F.; Heinicke, J.; Boese, R.; Blaeser, D. J. Chem. Soc., Chem. Commun. 1995, 1931. (b) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Heinicke, J.; Boese, R.; Blaeser, D. J. Organomet. Chem. 1996, 521, 211. (c) Blakeman, P.; Gehrhus, B.; Green, J. C.; Heinicke, J.; Lappert, M. F.; Kindermann, M.; Veszpremi, T. J. Chem. Soc., Dalton Trans. 1996, 1475. (4) Heinemann, C.; Mueller, T.; Apeloig, Y.; Schwarz, H. J. Am. Chem. Soc. 1996, 118, 2023. (5) Boehme, C.; Frenking, G.; J. Am. Chem. Soc. 1996, 118, 2039. (6) Arduengo, A. J., III.; Bock, H.; Chen, H.; Denk, M.; Dixon, D. A.; Green, J. C.; Hermann, W. A.; Jones, N. L.; Wagner, M.; West, R. J. Am. Chem. Soc. 1994, 116, 6641. (7) Schleyer, P. v. R.; Maercker, C.; Dransfield, A.; Jiao, H.; Hommes, N. J.; Hommes, R. v. E. J. Am. Chem. Soc. 1996, 118, 6317. (8) Herzfeld, J.; Berger, A. E. J. Chem. Phys. 1980, 73, 6021 (9) West, R.; Cavalieri, J.; Buffy, J. J.; Fry, C.; Zilm, K. W.; Duchamp, J.; Kira, M.; Iwamoto, T.; Mueller, T.; Apeloig, Y. J. Am. Chem. Soc. 1997, 119, 4972. (10) Arduengo, A. J., III.; Dixon, D. A.; Kumahiro, K. K.; Lee, C.; Power, W. P.; Zilm, K. W. J. Am. Chem. Soc. 1994, 116, 6361 (11) The calculated geometries of 1-6 are very similar and match closely the experimental structures of 1, 1a 2, 2 and 3. 3a All geometry optimizations and DFT/GIAO calculations, except for the MP2/GIAO calculations, were performed with Gaussian 94: Gaussian, Inc. Pittsburgh, PA, 1995. Because the HF/6-31G* theory proved superior to the DFT-hybrid B3LYP/6-31G* method in predicting the structures of 1 and 2, we used the HF/6-31G* optimized geometries for calculation of the chemical shielding tensors. MP2/ 6-31G* geometries were used for 4 and 5, and the B3LYP/6-31G* geometry was used for 6. (12) (a) Ditchfield, R. Mol. Phys. 1974, 27, 789. (b) Wolinski, K.; Hinton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112, 8251. (c) Cheeseman, J. R.; Trucks, G. W.; Keith, T. A.; Frisch, M. J. J. Chem. Phys. 1996, 104, 5497. (13) Gauss, J. Chem. Phys. 1993, 99, 3629. The MP2/GIAO calculations were done using ACES II (Stanton, J. F.; Gauss, J.; Watts, J. D.; Lauderdale, W. J.; Bartlett, R. J.; University of Florida, 1993). Table 1. Calculated and Observed Chemical Shift Tensors for Silylenes B3LYP/GIAO a δiso δxx δyy δzz Δδ CSA c Θ (deg) 1 d 93.7 334.1 -14.8 -38.3 372.4 360.7 2 d 140.4 432.8 10.0 -20.8 452.6 417.4 4.7 4 e 84.8 326.9 -42.2 -30.2 369.1 363.1 5 e 131.9 430.3 -16.6 -18.1 448.4 447.7 13.5 6 e 90.8 327.4 -15.1 -39.4 366.8 354.6 MP2/GIAO b δiso δxx δyy δzz Δδ CSA Θ (deg) 4 64.0 277.9 -46.2 -39.6 324.1 320.8 5 115.1 390.1 -18.7 -26.0 416.1 412.5 0.5 experimental values δiso δ11 δ22 δ33 Δδ CSA 1 75.2 284.9 -16.1 -43.3 328.2 314.6 2 114.7 350.7 -2.1 -4.5 355.2 354.0 3 92.5 316.4 21.1 -60.0 376.4 335.8 a The 6-311+G(2df,p) basis set was used; for 1, 2, and 6 a 6-311+G(2df,p)(Si), 6-31G*(C,N,H) basis set was used. b A tz2p(Si), tzp(C,N), dz(H) basis set was used. c Chemical shift anisotropy. d Relative to TMS. σ( 29 Si(TMS): B3LYP/GIAO/(6-311+G(2df,p)(Si). 6-31G*(C,N,H))//HF/6-31G*: 332.5. e Relative to TMS. σ( 29 Si(TMS): B3LYP/GIAO/6-311+G(2df,p//MP2/6-31G*: 327.9. σ( 29 Si(TMS): MP2/GIAO/(tz2p(Si)tzp(C,N),dz(H))//MP2/6-31G* 371.1 1639 J. Am. Chem. Soc. 1998, 120, 1639-1640 S0002-7863(97)02328-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/05/1998