Study on Isolable π-Electron Species Containing an Inverted Si=Si Bond and an Unsupported Si-Si π-Bond

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(5) (a) Hehre, W. J.; Pople, J. A. J. Am. Chem. Soc. 1975, 97, 6941–6955. (b) Wagner, H. U.; Szeimies, G.; Chandrasekhar, J.; Schleyer, P. v. R.; Pople, J. A.; Binkley, J. S. J. Am. Chem. Soc. 1978, 100, 1210–1213. (c) Kollmar, H.; Carrion, F.; Dewar, M. J. S.; Bingham, R. C. J. Am. Chem. Soc. 1981, 103, 5292–5303. (d) Hess, B. A. Jr.; Allen, W. D.; Michalska, D.; Schaad, L. J.; Schaefer H. F. III J. Am. Chem. Soc. 1987, 109, 1615–1621. (e) Hess, B. A. Jr.; Michalska, D.; Schaad, L. J. J. Am. Chem. Soc. 1987, 109, 7546–7547. (f) Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 1988, 110, 4710–4718. (g) Wiberg, K. B.; Artis, D. R.; Bonneville, G. J. Am. Chem. Soc. 1991, 113, 7969–7979. (h) Prakash, G. K. S.; Rasul, G.; Reddy, V. P.; Casanova, J. J. Am. Chem. Soc. 1992, 114, 6484–6486. (i) Nguyen, T. L.; Le, T. N.; Mebel, A. M.; Lin, S. H. Chem. Phys. Lett. 2000, 326, 468–476. (j) Dinadayalane, T. C.; Priyakumar, U. D.; Sastry, G. N. J. Phys. Chem. A 2004, 108, 11433–11448. (k) Rasul, G.; Olah, G. A.; Prakash, G. K. S. J. Phys. Chem. A 2006, 110, 7197–7201. (l) Cremer, D.; Kraka, E.; Joo, H.; Stearns, J. A.; Zwier, T. S. Phys. Chem. Chem. Phys. 2006, 8, 5304–5316. (m) Mebel, A. M.; Kislov, V. V.; Kaiser, R. I. J. Chem. Phys. 2006, 125, 133113. (n) Fujita, Y.; Abe, M.; Shiota, Y.; Suzuki, T.; Yoshizawa, K. Bull. Chem. Soc. Jpn. 2016, 89, 770–778.

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(7) Iwamoto, T.; Abe, T.; Sugimoto, K.; Hashizume, D.; Matsui, H.; Kishi, R.; Nakano, M.; Ishida, S. Angew. Chem., Int. Ed. 2019, 58, 4371−4375.

(8) Nied, D.; Köppe, R.; Klopper, W.; Schnöckel, H.; Breher, F. J. Am. Chem. Soc. 2010, 132, 10264−10265.

2-5. Reference

(1) (a) R. Hoffmann, H. Hopf, Angew. Chem., Int. Ed. 2008, 47, 4474–4481. See also: (b) W. T. Borden, Chem. Rev. 1989, 89, 1095–1109; (c) V. S. Mastryukov, J. E. Boggs, Struct. Chem. 2000, 11, 97–103; (d) S. Vázquez, P. Camps, Tetrahedron 2005, 61, 5147–5208.

(2) (a) K. B. Wiberg, Acc. Chem. Res. 1984, 17, 379–386; (b) T. Iwamoto, S. Ishida, Chem. Lett. 2014, 43, 164–170; (c) Y. Heider, D. Scheschkewitz, Dalton Trans. 2018, 47, 7104–7112.

(3) (a) G. Szeimies, J. Harnisch, O. Baumgärtel, J. Am. Chem. Soc. 1977, 99, 5183–5184; (b) U. Szeimies-Seebach, J. Harnisch, G. Szeimies, M. Van Meerssche, G. Germain, J.-P. Declerq, Angew. Chem., Int. Ed. Engl. 1978, 17, 848–850; (c) H.-G. Zoch, G. Szeimies, R. Romer, R. Schmitt, Angew. Chem., Int. Ed. Engl. 1981, 20, 877–878; (d) A.-D. Schlüter, H. Harnisch, J. Harnisch, U. Szeimies-Seebach, G. Szeimies, Chem. Ber. 1985, 118, 3513–3528.

(4) (a) W. J. Hehre, J. A. Pople, J. Am. Chem. Soc. 1975, 97, 6941–6955; (b) H. U. Wagner, G. Szeimies, J. Chandrasekhar, P. v. R. Schleyer, J. A. Pople, J. S. Binkley, J. Am. Chem. Soc. 1978, 100, 1210–1213; (c) H. Kollmar, F. Carrion, M. J. S. Dewar, R. C. Bingham, J. Am. Chem. Soc. 1981, 103, 5292–5303; (d) B. A. Hess, Jr., W. D. Allen, D. Michalska, L. J. Schaad, H. F. Schaefer III, J. Am. Chem. Soc. 1987, 109, 1615–1621; (e) B. A. Hess, Jr., D. Michalska, L. J. Schaad, J. Am. Chem. Soc. 1987, 109, 7546–7547; (f) D. A. Hrovat, W. T. Borden, J. Am. Chem. Soc. 1988, 110, 4710–4718; (g) K. B. Wiberg, D. R. Artis, G. Bonneville, J. Am. Chem. Soc. 1991, 113, 7969–7979; (h) G. K. S. Prakash, G. Rasul, V. P. Reddy, J. Casanova, J. Am. Chem. Soc. 1992, 114, 6484–6486; (i) T. L. Nguyen, T. N. Le, A. M. Mebel, S. H. Lin, Chem. Phys. Lett. 2000, 326, 468–476; (j) T. C. Dinadayalane, U. D. Priyakumar, G. N. Sastry, J. Phys. Chem. A 2004, 108, 11433–11448; (k) G. Rasul, G. A. Olah, G. K. S. Prakash, J. Phys. Chem. A 2006, 110, 7197–7201; (l) D. Cremer, E. Kraka, H. Joo, J. A. Stearns, T. S. Zwier, Phys. Chem. Chem. Phys. 2006, 8, 5304–5316; (m) A. M. Mebel, V. V. Kislov, R. I. Kaiser, J. Chem. Phys. 2006, 125, 133113; (n) Y. Fujita, M. Abe, Y. Shiota, T. Suzuki, K. Yoshizawa, Bull. Chem. Soc. Jpn. 2016, 89, 770–778. For silicon analogues of BBE, see: (o) B. F. Yates, H. F. Schaefer III, Chem. Phys. Lett. 1989, 155, 563–571.

(5) T. Iwamoto, T. Abe, K. Sugimoto, D. Hashizume, H. Matsui, R. Kishi, M. Nakano, S. Ishida, Angew. Chem., Int. Ed. 2019, 58, 4371–4375.

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(8) (a) In this reaction condition, we did not observe the formation of a trisilaallene, which is obtained by the reduction of the corresponding tetrachlorodisilane with potassium graphite in THF. See: S. Ishida, T. Iwamoto, C. Kabuto, M. Kira, Nature 2003, 421, 725–727; (b) For a study on the machine learning concerning the optimization of the reaction conditions, see: M. Fujinami, J. Seino, T. Nukazawa, S. Ishida, T. Iwamoto, H. Nakai, Chem. Lett. 2019, 48, 961–964.

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(10) The 1H NMR spectrum of the reaction mixture (Figure 2-34) was very complicated in the SiMe3 region and it was difficult to identify signals of 4 and 5 clearly.

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(14) The isomerisation of bicyclo[1.1.0]tetrasilane 7 (or 10) to tetrasiladiene 8 (or 11) is predicted to be endergonic [49.7 kJ mol−1 (or 45.6 kJ mol−1 )] (298.15 K) at the B3LYP-D3/6-31G(d) level of theory. The activation barriers for the isomerisation as well as the relative reactivity of the bridgehead Si–Si bonds in 7 and 10 and the Si=Si bond in 8 and 11 toward carbon tetrachloride and methanol should be responsible for the formation of 4 and 5 as well as 9.

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3-5. Reference and Notes

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(2) (a) Adam, W.; Borden, W. T.; Burda, C.; Foster, H.; Heidenfelder, T.; Heubes, M.; Hrovat, D. A.; Kita, F.; Lewis, S. B.; Scheutzow, D.; Wirz, J. J. Am. Chem. Soc. 1998, 120, 593−594. (b) Abe, M.; Adam, W.; Nau, W. M. J. Am. Chem. Soc. 1998, 120, 11304−11310. (c) Abe, M.; Adam, W.; Heidenfelder, T.; Nau, W. M.; Zhang, X. J. Am. Chem. Soc. 2000, 122, 2019−2026. (d) Abe, M.; Adam, W.; Hara, M.; Hattori, M.; Majima, T.; Nojima, M.; Tachibana, K.; Tojo, S. J. Am. Chem. Soc. 2002, 124, 6540−6541.

(3) (a) Abe, M.; Furunaga, H.; Ma, D.; Gagliardi, L.; Bodwell, G. J. J. Org. Chem. 2012, 77, 7612−7619. (b) Harada, Y.; Wang, Z.; Kumashiro, S.; Hatano, S.; Abe, M. Chem. Eur. J. 2018, 24, 14808−14815. (c) Ye, J.; Fujiwara, Y.; Abe, M. Beilstein J. Org. Chem. 2013, 9, 925−933. (d) Akisaka, R.; Abe, M. Chem. Asian J. 2019, 14, 4223−4228.

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(7) In a symposium, Kyushin et al. presented the synthesis of hexakis(tertbutyl)tetrasilabicyclo[1.1.0]butane with a planar structure. Kyushin, S.; Kurosaki, Y.; Matsumoto, N.; Matsumoto, H.; Kyomen, T.; Hanaya, M.; Kudo, T. Silicon−Silicon π Single Bond. In Abstract of the 53rd Symposium on Organometallic Chemistry, Osaka, Japan, September 8−9, 2006; Kinka Chemical Society Japan: Osaka, 2006, A102. (See also, Kyushin, S.; Kurosaki, Y.; Otsuka, K.; Imai, H.; Ishida, S.; Kyomen, T.; Hanaya, M.; Matsumoto, H. Nat. Commun. 2020, 11, 4009.)

(8) Fisher, Frenking, and co-workers reported an N-heterocyclic gallylene-bridged Ge2 species whose [Ge2Ga2] framework is topologically similar to that of 1. A theoretical investigation suggested that the central Ge−Ge bond is a π-bond with a weak σ-type interaction. See: Doddi, A.; Gemel, C.; Winter, M.; Fischer, R. A.; Goedecke, C.; Rzepa, H. S.; Frenking, G. Angew. Chem., Int. Ed. 2013, 52, 450−454.

(9) (a) Iwamoto, T.; Abe, T.; Sugimoto, K.; Hashizume, D.; Matsui, H.; Kishi, R.; Nakano, M.; Ishida, S. Angew. Chem., Int. Ed. 2019, 58, 4371−4375. (b) Nukazawa, T.; Kosai, T.; Honda, S.; Ishida, S.; Iwamoto, T. Dalton Trans. 2019, 48, 10874−10880. (c) Fujinami, M.; Seino, J.; Nukazawa, T.; Ishida, S.; Iwamoto, T.; Nakai, H. Chem. Lett. 2019, 48, 961 −964.

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(12) Compound 2 is stable at room temperature but decomposes after 5 days at 80 °C in the dark to unexpectedly form 1 (46%) and other unidentified products.

(13) Wiberg, N.; Schuster, H.; Simon, A.; Peters, K. Angew. Chem., Int. Ed. Engl. 1986, 25, 79−80.

(14) Uchiyama, K.; Nagendran, S.; Ishida, S.; Iwamoto, T.; Kira, M. J. Am. Chem. Soc. 2007, 129, 10638−10639.

(15) A linear relationship was observed between the experimentally observed isotropic chemical shift (δobs) for several mono-iodinated silicon compounds and the corresponding calculated chemical shift (δcalc) obtained at the M06L/6-311+G(2df,p) [H,C,Si], SDD [I] level of theory (δobs = 1.011δcalc−40.8, R 2 = 0.990) (Figure 3-14 and Table 3-19). The 29Si NMR chemical shifts predicted for 2′c and 2′p in the main text were corrected using this equation with raw δcalc values of 139.1 (2′c) and 191.0 (2′p).

(16) According to the IUPAC definition, a π-bond contains a nodal plane that includes the internuclear bond axis. See: IUPAC. σ, π (Sigma, Pi). In Compendium of Chemical Terminology (the “Gold Book”), 2nd ed. [Online]; McNaught, A. D., Wilkinson, A., Eds.; Blackwell, 1997. DOI: 10.1351/goldbook

(17) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Karafiloglou, P.; Landis, C. R.; Weinhold, F. NBO 7.0; Theoretical Chemistry Institute, University of Wisconsin: Madison, WI, 2018.

(18) The deformation of the bicyclic ring relative to the planar structure in the parent tetrasilabicyclo[1.1.0]butane has been qualitatively explained by a second-order Jahn− Teller distortion. For details, see ref 11k.

(19) We did not observe a significant change in the UV-vis spectrum at lower temperatures (Figure 3-12).

(20) A structure similar to 3S was not located for 4 or 5 as a local minimum.11

(21) Although the origin of the difference between the relative stabilities of 2′p and 2′c remains unclear at this point, it may originate from a balance among the bonding interactions between the bridgehead Si atoms [d = 2.501 Å (2′c), 2.609 Å (2′p)], the strain originating from the Si(bridgehead)−Si(bridge)−Si(bridgehead) angle [α = 64.5° (2′c), 68.0° (2′p)], and the arrangement of the bulky silacyclopentane rings at the bridge positions.

(22) In previously reported theoretical studies, the introduction of bulky substituents at the bridgehead position resulted in destabilization of structures similar to 3L and 3TP with acute bond angles (θ ≈ 100°) and the stabilization of structures similar to 3S; 11f,g,i these predictions are consistent with the reported structures of such species with bulky bridgehead groups.10

(23) During checking the galley proof of the final published version, we were informed that Scheschkewitz et al. obtained a tetrasilicon analogue of a cyclobutane-1,3-diyl. Yildiz, C. B.; Leszczyńska, K. I.; Gonzalez- Gallardo, S.; Zimmer, M.; Azizoglu, A.; Biskup, T.; Kay, C. W. M.; Huch, V.; Rzepa, H. S.; Scheschkewitz, D. Angew. Chem., Int. Ed. 2020, 59, 15087−15092.

(24) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176−2179.

(25) Sheldrick, G. M. SADABS, Empirical Absorption Correction Program; Göttingen, Germany, 1996.

(26) Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3−8.

(27) Wakita, K. Yadokari-XG: Software for Crystal Structure Analyses, 2001; Release of Software (Yadokari-XG 2009) for Crystal Structure Analyses, Kabuto, C.; Akine, S.; Nemoto, T.; Kwon, E. J. Crystallogr. Soc. Jpn., 2009, 51, 218−224.

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(29) GRRM14, Maeda, S.; Harabuchi, Y.; Osada, Y.; Taketsugu, T.; Morokuma, K.; Ohno, K. see https://iqce.jp/GRRM/ (accessed date March 21, 2020); Maeda, S.; Ohno, K.; Morokuma, K. Phys. Chem. Chem. Phys. 2013, 15, 3683−3701.

(30) NBO 7.0, Glendening, E. D.; Badenhoop, J, K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Karafiloglou, P.; Landis, C. R.; Weinhold, F. Theoretical Chemistry Institute, University of Wisconsin, Madison (2018).

(31) Olah, G. A.; Field, L. D. Organometallics 1982, 1, 1485−1487.

(32) (Me3Si)3SiI was prepared according to the published procedure and the 29Si NMR spectrum was recorded measured in C6D6. Bürger, H.; Kilian, W.; Burczyk, K. J. Organomet. Chem. 1970, 21, 291−301.

(33) Hartmann, M.; Haji-Abdi, A.; Abersfelder, K.; Haycock, P. R.; White, A. J. P.; Scheschkewitz, D. Dalton Trans. 2010, 39, 9288−9295.

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(10) T. Nukazawa, T. Iwamoto, J. Am. Chem. Soc. 2020, 142, 9920–9924.

(11) Unfortunately, we have failed to obtain the corresponding 1,3-dibromo derivative from the reaction of 1 with bromoalkanes such as CHBr3, CBr4, (CH2Br)2, and (CHBr2)2.

(12) S. Kyushin, Y. Kurosaki, K. Otsuka, H. Imai, S. Ishida, T. Kyomen, M. Hanaya, H. Matsumoto, Nat. Commun. 2020, 11, 4009.

(13) As 4 undergoes facile isomerisation to cyclotrisilene 6 in solution, the 29Si NMR spectral data of 4 and 6 were taken from the 29Si NMR spectrum of the equilibrium mixture of 4 and 6 recorded at 60 °C and assigned according to the 1H– 29Si HMBC spectrum.

(14) (a) K. Leszczyńska, K. Abersfelder, A. Mix, B. Neumann, H.-G. Stammler, M. J. Cowley, P. Jutzi, D. Scheschkewitz, Angew. Chem., Int. Ed. 2012, 51, 6785–6788; (b) M. J. Cowley, V. Huch, H. S. Rzepa, D. Scheschkewitz, Nat. Chem. 2013, 5, 876–879.

(15) For studies on cyclotrisilenes, see: (a) T. Iwamoto, C. Kabuto, M. Kira, J. Am. Chem. Soc. 1999, 121, 886–887; (b) M. Ichinohe, T. Matsuno, A. Sekiguchi, Angew. Chem., Int. Ed. 1999, 38, 2194–2196; (c) T. Iwamoto, M. Tamura, C. Kabuto, M. Kira, Science 2000, 290, 504–506; (d) T. Iwamoto, M. Tamura, C. Kabuto, M. Kira, Organometallics 2003, 22, 2342–2344; (e) T. Kosai, S. Nishimura, N. Hayakawa, T. Matsuo, T. Iwamoto, Chem. Lett. 2019, 48, 1168–1170; (f) T. Koike, S. Honda, S. Ishida, T. Iwamoto, Organometallics 2020, 39, 4149–4152, see also ref. 14a.

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(17) In the 1H NMR spectrum of a mixture of 4 and 6, the signals due to four SiMe3 groups (36 H) of 6 were substantially broadened and overlapped with that due to eight SiMe3 groups (72H) of 4 (Figure 4-34). Assuming that only the signals of 4 and 6 were overlapped in this region, the equilibrium constants Keq (= [6]/[4]) at 302–333 K were roughly estimated using the integral ratios (Table 4-23). From the Keq, the thermodynamic parameters for the isomerization of 4 to 6 are calculated to be ΔH = +14.6 ± 2.9 kJ mol−1 and ΔS = +49 ± 9 J K−1 mol−1 (Figure 4-35) which are consistent with the calculated relative energy and free energy.

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(20) The isomerisation of a 2,3-disilabutadiene to a long bond isomer of 1,3-disila bicyclobutane was reported. See, D. Motomatsu, S. Ishida, K. Ohno, T. Iwamoto, Chem. Eur. J. 2014, 20, 9424–9430.

(21) The HOMO and HOMO−1 of 8opt are mainly localized on the central chlorinated Si atoms, while LUMO and LUMO+1 are localized on the terminal Si atoms. The orbital feature suggests the presence of two polarized R2Siδ+–Siδ−Cl bonds, which is consistent with the natural atomic orbital (NPA) charges of the silicon atoms [+1.4 and 0.0 for terminal and central Si atoms].

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(23) In contrast to 4, 5 does not exhibit similar interconversion. As 5 slowly decomposes to bicyclo[1.1.0]tetrasil-1(3)-ene 1 and unidentified products at room temperature, the activation barrier for the decomposition may be lower in energy than those of the rate-controlling step of the similar interconversion.

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(25) In the reaction of 1 with CCl4, intermediate 8 should be involved in the formation of acyclic tetrasilane 3.

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(8) (a) S. Masamune, Y. Kabe, S. Collins, D. J. Williams, R. Jones, J. Am. Chem. Soc. 1985, 107, 5552–5553; (b) R. Jones, D. J. Williams, Y. Kabe, S. Masamune, Angew. Chem., Int. Ed. Engl. 1986, 25, 173–174; (c) K. Takanashi, V. Ya Lee, M. Ichinohe, A. Sekiguchi, Chem. Lett. 2007, 36, 1158–1159; (d) K. Ueba-Ohshima, T. Iwamoto, M. Kira, Organometallics 2008, 27, 320–323; (e) T. Iwamoto, N. Akasaka, S. Ishida, Nat. Commun. 2014, 5, 5353.

(9) K. Uchiyama, S. Nagendran, S. Ishida, T. Iwamoto, M. Kira, J. Am. Chem. Soc. 2007, 129, 10638–10639.

(10) During the preparation of this manuscript, Scheschkewitz et al. reported the metathesis reaction of 1-chloro germadisilabicyclo[1.1.0]butane with nucleophiles; for details, see: P. K. Majhi, M. Zimmer, B. Morgenstern, D. Scheschkewitz, J. Am. Chem. Soc. 2021, 143, 8981–8986.

(11) In the crystalline state, apparent π–π interactions were not observed for 4 (the shortest intermolecular CPh∙∙∙CPh distance: 3.75 Å).

(12) J. Bruckmann, C. Krüger, Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1997, 53, 1845–1846.

(13) T. Sato, Y. Mizuhata, N. Tokitoh, Chem. Commun. 2010, 46, 4402–4404.

(14) For details of the calculations, see the Experimental Section.

(15) The values of the 29Si NMR resonances were obtained from the 1H29Si HMBC 2D NMR spectra in C7D8.

(16) In addition to 5c, some conformers of 5 were located at local minima but at higher Gibbs energies than 5p; for details, see the Experimental Section.

(17) G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb, A. Nudelman, B. M. Stoltz, J. E. Bercaw, K. I. Goldberg, Organometallics 2010, 29, 2176–2179.

(18) G. M. Sheldrick, SADABS, Empirical Absorption Correction Program; Göttingen, Germany, 1996.

(19) G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3–8.

(20) K. Wakita, Yadokari-XG: Software for Crystal Structure Analyses, 2001; Release of Software (Yadokari-XG 2009) for Crystal Structure Analyses; C. Kabuto, S. Akine, T. Nemoto, E. Kwon, J. Crystallogr. Soc. Jpn., 2009, 51, 218–224.

(21) M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2009.

(22) GRRM14; (a) S. Maeda, Y. Harabuchi, Y. Osada, T. Taketsugu, K. Morokuma, K. Ohno, see https://iqce.jp/GRRM/, accessed date March 21, 2020; (b) S. Maeda, K. Ohno, K. Morokuma, Phys. Chem. Chem. Phys. 2013, 15, 3683–3701.

(23) NBO 7.0, E. D. Glendening, J. K. Badenhoop, A. E. Reed, J. E. Carpenter, J. A. Bohmann, C. M. Morales, P. Karafiloglou, C. R. Landis, F. Weinhold, Theoretical Chemistry Institute, University of Wisconsin, Madison (2018).

2-5. Reference and Notes

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(7) As an example of a functionalized cyclotrisilene derivative, the generation of a cyclotrisilenyl radical has been suggested as an intermediate in the reaction of a cyclotrisilene with disilenides. See: Leszczyńska, K.; Abersfelder, K.; Majumdar, M.; Neumann, B.; Stammler, H.-G.; Rzepa, H. S.; Jutzi, P.; Scheschkewitz, D. Chem. Commun. 2012, 48, 7820−7822.

(8) Very recently, Scheschkewitz et al. have reported cyclotrimetallenes ligated by a nickel. See: (a) Majhi, P. K.; Zimmer, M.; Morgenstern, B.; Scheschkewitz, D. J. Am. Chem. Soc. 2021, 143, 8981−8986. (b) Majhi, P. K.; Zimmer, M.; Morgenstern, B.; Huch, V.; Scheschkewitz, D. J. Am. Chem. Soc. 2021, 143, 13350−13357.

(9) West et al. have reported that the reaction of tetramesityldisilene with LiAlH4 provided the corresponding 1,2-dihydridodisilane. See: West, R. Science 1984, 225, 1109−1114.

(10) When 1a was treated with 3 equiv of LiAlH4 in THF, a complex reaction mixture was obtained, suggesting that 2 reacts with LiAlH4.

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(15) Optimized structure of 2′opt was calculated at B3LYP-D3/6-311G(d) level of theory.

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(17) Higher migratory aptitude of hydride has been reported for some unsaturated silicon compounds. See: (a) Yamaguchi, T.; Sekiguchi, A. J. Am. Chem. Soc. 2011, 133, 7352−7354. (b) Agou, T.; Sugiyama, Y.; Sasamori, T.; Sakai, H.; Furukawa, Y.; Takagi, N.; Guo, J.-D.; Nagase, S.; Hashizume, D.; Tokitoh, N. J. Am. Chem. Soc. 2012, 134, 4120−4123. For theoretical studies on hydride migration in unsaturated silicon compounds, see: (c) Nagase, S.; Kudo, T. J. Chem. Soc., Chem. Commun. 1984, 1392−1394. (d) Krogh-Jespersen, K. Chem. Phys. Lett. 1982, 93, 327−330. (e) Gordon, M. S.; Truong, T. N.; Bonderson, E. K. J. Am. Chem. Soc. 1986, 108, 1421−1427. (f) Ernst, M. C.; Sax, A. F.; Kalcher, J. Chem. Phys. Lett. 1993, 216, 189−193. (g) McCarthy, M. C.; Yu, Z.; Sari, L.; Schaefer, H. F., III; Thaddeus, P. J. Chem. Phys. 2006, 124, 074303. (h) Maier, G.; Reisenauer, H. P.; Glatthaar, J. Chem. Eur. J. 2002, 8, 4383−4391. (i) Dolgonos, G. Chem. Phys. Lett. 2008, 466, 11−15.

(18) Recrystallization from Et2O at −27 °C also provided orange crystals that exhibit a solvent-separated ion pair structure (see Figure 6-45).

(19) (a) Masamune, S.; Kabe, Y.; Collins, S.; Williams, D. J.; Jones, R. J. Am. Chem. Soc. 1985, 107, 5552−5553. (b) Jones, R.; Williams, D. J.; Kabe, Y.; Masamune, S. Angew. Chem., Int. Ed. Engl. 1986, 25, 173−174. (c) Takanashi, K.; Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A. Chem. Lett. 2007, 36, 1158−1159. (d) Ueba-Ohshima, K.; Iwamoto, T.; Kira, M. Organometallics 2008, 27, 320−323. (e) Iwamoto, T.; Akasaka, N.; Ishida, S. Nat. Commun. 2014, 5, 5353.

(20) (a) Iwamoto, T.; Yin, D.; Kobayashi, A.; Tamura, M.; Motomatsu, D.; Akasaka, N.; Yokouchi, Y.; Ishida, S.; Kira, M. Organometallics 2021, 40, 2557−2566. (b) Lee, V. Ya.; Yokoyama, T.; Takanashi, K.; Sekiguchi, Chem. Eur. J. 2009, 15, 8401−8404. For studies on metalsubstituted cyclotrimetallanes, see: (c) Ichinohe, M.; Toyoshima, M.; Kinjo, R.; Sekiguchi, A. J. Am. Chem. Soc. 2003, 125, 13328−13329. (d) Abersfelder,K.; Scheschkewitz, D. J. Am. Chem. Soc. 2008, 130, 4114−4121. (e) Lee, V. Ya.; Horiguchi, S.; Gapurenko, O. A.; Minyaev, R. M.; Minkin, V. I.; Gornitzka, H.; Sekiguchi, A. Eur. J. Inorg. Chem. 2019, 4224−4227. (f) Heider, Y.; Willmes, P.; Huch, V.; Zimmer, M.; Scheschkewitz, D. J. Am. Chem. Soc. 2019, 141, 19498−19504. (g) Guddorf, B. J.; Feldt, M.; Hepp, A.; Daniliuc, C. D.; Lips, F. Organometallics 2020, 39, 4387−4394. (h) Lee, V. Ya.; Sakai, R.; Takanashi, K.; Gapurenko, O. A.; Minyaev, R. M.; Gornitzka, H.; Sekiguchi, A. Angew. Chem., Int. Ed. 2021, 60, 3951−3955.

(21) (a) Hartmann, M.; Haji-Abdi, A.; Abersfelder, K.; Haycock, P. R.; White, A. J. P.; Scheschkewitz, D. Dalton Trans. 2010, 39, 9288−9295. (b) Willmes, P.; Cowley, M. J.; Hartmann, M.; Zimmer, M.; Huch, V.; Scheschkewitz, D. Angew. Chem., Int. Ed. 2014, 53, 2216−2220. (c) Izod, K.; Evans, P.; Waddell, P. G. Angew. Chem., Int. Ed. 2017, 56, 5593−5597.

(22) Model cyclotrisilene 9, which has an SiMe3 group and a PMe2 group on the Si=Si double bond and methyl groups on the saturated ring silicon atom, adopts a cis-bent structure with pyramidalized unsaturated silicon atoms at the B3LYP-D3/6-311G(d) level of theory (see Figure 6-60). However, the energy difference between the optimized bent structure and a planar structure is only 1 kJ/mol, indicating that the geometry around the Si=Si double bond should be sensitive to the steric demand of the substituents. Therefore, it is difficult to discuss the effects of the PR2 group on the geometry around the Si=Si double bond using the structural characteristics of 8 obtained by XRD analysis.

(23) A natural bond orbital (NBO) analysis suggested an sp1.87 hybridization for the Si orbital in the Si−P bond in 8opt (Table 6-11).

(24) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176−2179.

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(26) Sheldrick, G. M. SADABS, Empirical Absorption Correction Program; Göttingen, Germany, 1996.

(27) Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3−8.

(28) Wakita, K. Yadokari-XG: Software for Crystal Structure Analyses, 2001; Release of Software (Yadokari-XG 2009) for Crystal Structure Analyses, Kabuto, C.; Akine, S.; Nemoto, T.; Kwon, E. J. Crystallogr. Soc. Jpn., 2009, 51, 218−224.

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(30) GRRM14, Maeda, S.; Harabuchi, Y.; Osada, Y.; Taketsugu, T.; Morokuma, K.; Ohno, K. see https://iqce.jp/GRRM/ (accessed date March 21, 2020); Maeda, S.; Ohno, K.; Morokuma, K. Phys. Chem. Chem. Phys. 2013, 15, 3683−3701.

(31) NBO 7.0, Glendening, E. D.; Badenhoop, J, K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Karafiloglou, P.; Landis, C. R.; Weinhold, F. Theoretical Chemistry Institute, University of Wisconsin, Madison (2018).