Electronic Structure of Bis(silyl)carbon-, Bis(silyl)silicon-, and Bis(silyl

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Organometallics 2010, 29, 5596–5606 DOI: 10.1021/om100812b

Electronic Structure of Bis(silyl)carbon-, Bis(silyl)silicon-, and Bis(silyl)germanium-Centered Radicals (R3Si)2XE• (E = C, Si, Ge; X = H, Re(CO)5, F): EPR and DFT Studies† Dennis Sheberla,‡ Boris Tumanskii,*,‡ Dmitry Bravo-Zhivotovskii,‡ Gregory Molev,‡ Victoria Molev,‡ Vladimir Ya. Lee,§ Kazunori Takanashi,§ Akira Sekiguchi,§ and Yitzhak Apeloig*,‡ ‡

Schulich Department of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel, and §Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Received August 19, 2010

Group 14 element bis(silyl)-substituted radicals (R3Si)2XE• (E = C, Si, Ge; X = H, Re(CO)5, F) and (R3Si)(1-Ad)HC• have been studied by EPR spectroscopy and DFT calculations. The significant difference in the kinetic stability at 240 K of the hydrogen-substituted persistent C-centered and analogous short-lived Si- and Ge-centered radicals is explained by different decay mechanisms: H abstraction for E = C and dimerization for E = Si, Ge. The 1HR and 29Siβ hyperfine coupling constants (hfcc) in these radicals have dominating negative spin-polarization (SP) contribution; thus, they have a negative sign. In contrast, in the F-substituted radical, where a(F) results from spin delocalization and positive SP contribution, it has a positive sign. For Si-centered radicals it has been shown by calculations that the 1HR and 29Siβ hfcc’s result from a combination of direct and spinpolarization mechanisms, which vary as a function of the degree of pyramidality around E. As the geometry around E changes from planar to pyramidal, the contribution of the direct mechanism increases and Pthe contribution of spin polarization decreases. The hydrogen-substituted C radicals are planar (P θ(C) = 360.0°), in contrast to the analogous Si and Ge radicals, which are slightly P pyramidal ( θ(Si) = 354.1° and θ(Ge) = 355.5°). Both (R3Si)2XE• species (E = Si, Ge; X = Re(CO)5) are planar around E.

Introduction Group 14 (E = C, Si, Ge, Sn) centered radicals are important reactive intermediates in organic and organometallic chemistry.1 Recently, the isolation and structural characterization of such radicals, including radicals lacking π stabilization, became possible by using bulky substituents, in particular bulky silyl groups.2 These bulky silyl substituents sterically protect the radical center from undergoing typical radical reactions: e.g., dimerization, disproportionation, and hydrogen abstraction. Previous studies of persistent group 14 radicals showed that the kinetic stability of such radicals having similar substituents depends on the central atom E.3,4 The heavier E is, the longer the E-E bond

† Part of the Dietmar Seyferth Festschrift. Dedicated to Prof. Dietmar Seyferth in appreciation of his groundbreaking chemistry and service to the community as the first editor of Organometallics. *To whom correspondence should be addressed. E-mail: tboris@ technion.ac.il (B.T.); [email protected]. (Y.A.). (1) (a) Smith, M. B.; March, J. March’s Advanced Organic Chemistry, 6th ed.; Wiley: Hoboken, NJ, 2007; Chapter 5. (b) Chatgilialoglu, C.; Schiesser, C. H. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 2001; Vol. 1, Chapter 4. (c) Power, P. P. Chem. Rev. 2003, 103, 789. (d) Wentrup, C. Reactive Molecules: the Neutral Reactive Intermediates in Organic Chemistry; Wiley-Interscience: New York, 1984.

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is in the dimer formed by the combination of two radicals, and consequently bulkier substituents are required for stabilizing the radical kinetically. For example, the dimers of C-centered radicals (i.e., substituted ethanes) have a typical C-C bond length of 1.54 A˚, and three Me3Si substituents are sufficiently large to make (Me3Si)3C• persistent, having a lifetime of several days at 298 K.3 In sharp contrast, the (2) (a) Molev, G.; Tumanskii, B.; Sheberla, D.; Botoshansky, M.; Bravo-Zhivotovskii, D.; Apeloig, Y. J. Am. Chem. Soc. 2009, 131, 11698. (b) Becker, M.; F€orster, C.; Franzen, C.; Harthrath, J.; Kirsten, E.; Knuth, J.; Klinkhammer, K. W.; Sharma, A.; Hinderberger, D. Inorg. Chem. 2008, 47, 9965. (c) F€orster, C.; Klinkhammer, K. W.; Tumanskii, B.; Kr€uger, H.-J.; Kelm, H. Angew. Chem., Int. Ed. 2007, 46, 1156. (d) Lee, V. Ya.; Sekiguchi, A. Acc. Chem. Res. 2007, 40, 410. (e) Lee, V. Ya.; Sekiguchi, A. Eur. J. Inorg. Chem. 2005, 1209. (f) Sekiguchi, A.; Fukawa, T.; Lee, V. Ya.; Nakamoto, M. J. Am. Chem. Soc. 2003, 125, 9250. (g) Sekiguchi, A.; Fukawa, T.; Nakamoto, M.; Lee, V. Ya.; Ichinohe, M. J. Am. Chem. Soc. 2002, 124, 9865. (h) Apeloig, Y.; Bravo-Zhivotovskii, D.; Bendikov, M.; Danovich, D.; Botoshansky, M.; Vakul'skaya, T.; Voronkov, M.; Samoilova, R.; Zdravkova, M.; Igonin, V.; Shklover, V.; Struchkov, Y. J. Am. Chem. Soc. 1999, 121, 8118. (i) Lee, V. Ya.; Sekiguchi, A. In Reviews of Reactive Intermediates Chemistry; Moss, R. A., Platz, M. S., Jones, M., Jr., Eds.; Wiley: Hoboken, NJ, 2007; Chapter 2. (j) Lee, V. Ya.; Sekiguchi, A. Organometallic Compounds of Low-Coordinate Si, Ge, Sn and Pb: From Phantom Species to Stable Compounds; Wiley: Chichester, U.K., 2010. (3) Carbon-centered radicals: (a) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1976, 9, 13. (b) Mendenhall, G. D.; Griller, D.; Lindsay, D.; Tidwell, T. T.; Ingold, K. U. J. Am. Chem. Soc. 1974, 96, 2441. (c) Mendnhall, G. D.; Ingold, K. U. J. Am. Chem. Soc. 1973, 95, 3422. r 2010 American Chemical Society

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analogous Si-centered radical (Me3Si)3Si• is highly reactive (τ1/2 < 1 s at 298 K).4m,n This is because the Si-Si bond distance in the corresponding dimer is much longer (2.40 A˚)5 and the Me3Si substituents are too small to prevent radical dimerization. Thus, the kinetic stabilization effect of the Me3Si substituents in the carbon radical dominates over the thermodynamic stabilization of formation of the stronger C-C bond (90.2 kcal mol-1 (1 kcal = 4.184 kJ) for H3C-CH3 vs 76.7 kcal mol-1 for H3Si-SiH3).6 The relationship between the dissociation energy of the central Si-Si bond in R3Si-SiR3 and the kinetic stability of the corresponding R3Si• radicals was recently studied in some detail by some of us.7 The use of very voluminous substituents, such as tBu2MeSi, enabled the isolation and characterization of room-temperature-stable tris(silyl)-substituted Si-, Ge-, and Sn-centered radicals.2a,d-f Isolable tris(silyl)-substituted Pb-centered radicals [R(Me3Si)2Si]3Pb• (R = Et, SiMe3) (Pb-Pb bond distances are typically in the range of 2.872.98 A˚)8 were recently reported.2b,c The major spectroscopic technique used to study radicals is electron paramagnetic resonance (EPR) spectroscopy. An important measured parameter is the isotropic hyperfine coupling constant (hfcc) a(N), which results from the interaction between the magnetic moment of an unpaired electron and the nuclear spin and is a very useful EPR spectroscopy parameter for characterizing radicals; a(N) in T is given by9

aðNÞ ¼

2 μ gN μN FN ð0Þ 3 0

where N = nucleus, μ0 is the vacuum permeability, μN is the nuclear magneton, gN is the nucleus g factor, and FN(0) is the net spin density (i.e., the difference between the R-spin and β-spin electron densities) at nucleus N. Since only the s orbital does not have a node at the nucleus, the unpaired electron must reside in an orbital having an s-orbital contribution in (4) Persistent silyl and germyl radicals: (a) Azinovic, D.; BravoZhivotovskii, D.; Bendikov, M.; Apeloig, Y.; Tumanskii, B.; Veprek, S. Chem. Phys. Lett. 2003, 374, 257. (b) Apeloig, Y.; Bravo-Zhivotovskii, D.; Yuzefovich, M.; Bendikov, M.; Shames, A. I. Appl. Magn. Reson. 2000, 18, 425. (c) Kyushin, S.; Sakurai, H.; Matsumoto, H. Chem. Lett. 1998, 27, 107. (d) Kira, M.; Obata, T.; Kon, I.; Hashimoto, H.; Ichinohe, M.; Sakurai, H.; Kyushin, S.; Matsumoto, H. Chem. Lett. 1998, 1097. (e) McKinley, A. J.; Karatsu, T.; Wallraff, G. M.; Thompson, D. P.; Miller, R. D.; Michl, J. J. Am. Chem. Soc. 1991, 113, 2003. (f) Kyushin, S.; Sakurai, H.; Betsuyaku, T.; Matsumoto, H. Organometallics 1997, 16, 5386. (g) Chatgilialoglu, C. Chem. Rev. 1995, 95, 1229. (h) Chatgilialoglu, C.; Guerrini, A.; Lucarini, M. J. Org. Chem. 1992, 57, 3405. (i) Cotton, J. D.; Cundy, C. S.; Harris, D. H.; Hudson, A.; Lappert, M. F.; Lednor, P. W. J. Chem. Soc., Chem. Commun. 1974, 651. (j) Gynane, M. J. S.; Lappert, M. F.; Riley, P. I.; Riviere, P.; Riviere-Baudet, M. J. Organomet. Chem. 1980, 202, 5. (k) Sakurai, H.; Umino, H.; Sugiyama, H. J. Am. Chem. Soc. 1980, 102, 6837. (l) Hudson, A.; Jackson, R. A.; Rhodes, C. J.; Vecchio, A. L. D. J. Organomet. Chem. 1985, 280, 173. (m) Cooper, J.; Hudson, A.; Jackson, R. A. Mol. Phys. 1972, 23, 209. (n) Bennett, S. W.; Eaborn, C.; Hudson, A.; Jackson, R. A.; Root, K. D. J. J. Chem. Soc. A 1970, 348. (o) Geoffroy, M.; Hammons, J. H. J. Chem. Educ. 1981, 58, 389. (5) Fronczek, F. R.; Lickiss, P. D. Acta Crystallogr. 1993, C49, 331. (6) (a) Lide, D. R. CRC Handbook of Chemistry and Physics, 90th ed. (Internet Version 2010); CRC Press/Taylor and Francis: Boca Raton, FL, 2010. (7) Kravchenko, V.; Bravo-Zhivotovskii, D.; Tumanskii, B.; Botoshansky, M.; Segal, N.; Molev, G.; Kosa, M.; Apeloig, Y. In Organosilicon Chemistry VI: From Molecules to Materials; Auner, N., Weis, J., Eds.; Wiley-VCH: Weinheim, Germany, 2005; p 48. (8) Wang, Y.; Quillian, B.; Wei, P.; Yang, X.-J; Robinson, G. H. Chem. Commun. 2004, 2224. (9) (a) Carrington, A.; McLachlan, A. D. Introduction to Magnetic Resonance; Harper and Row: New York, 1967. (b) Weil, J. A.; Bolton, J. Electron Paramagnetic Resonance, 2nd ed.; Wiley: Hoboken, NJ, 2007.

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order to contribute to FN(0). The 2/3μ0μNgN term is constant for a specific nucleus and can be positive or negative. The spin density at the nucleus, FN(0), can be positive (R > β) or negative (R < β). Thus, the sign of hfcc depends on the sign of the magnetic parameters of the specific nucleus and on the sign of the spin density at that nucleus. For main-group-element radicals, two main mechanisms can contribute to FN(0) and therefore to the isotropic value of the hyperfine coupling constant.9-11 (i) The first is the direct contribution (delocalization or conjugation), which is due to spin density that arises from the spin occupation in the singly occupied molecular orbital (SOMO) having a contribution of an s orbital. This contribution is always positive. (ii) The second is the spin-polarization contribution (indirect), which results from a slightly different interaction of the unpaired R electron with the paired R and β electrons of inner shell orbitals or with those of the neighboring bonds. A positive spin density is induced at the nucleus where the direct spin density resides, and a negative spin density is induced at the adjacent atoms (spin polarization). This contribution is usually smaller than the direct contribution. The interplay of these two contributing mechanisms strongly depends on the radical geometry (e.g., degree of pyramidality of the radical center). In planar radicals, such as H3C•, so-called π radicals, the electron spin is located in a p orbital and therefore the main contribution to 1HR hfcc is by spin polarization and the sign of a(1HR) is negative.9,11 In pyramidal radicals, for example Me2HSi•, the SOMO has a significant s character at the Si nucleus, and therefore the main contribution to 1HR hfcc is by the direct mechanism and the sign of a(1HR) is positive.11b,12 However, it is important to note that ordinary first-order EPR experiments yield only the absolute values of hfcc’s; therefore, to obtain the sign of hfcc’s and to obtain information on the mechanism which contributes to the hfcc, quantum mechanical calculations have to be performed. Although the EPR spectroscopy of the tris(silyl)-substituted radicals of group 14 elements have been studied in some detail,2-4 the EPR spectroscopy of only three bis(silyl)substituted species, i.e., (Me3Si)2HC•,3 (Me3Si)2HSi•,4n and (Me3Si)2MeSi•,4h,m were reported. Furthermore, the effect of the nature of the central atom and the influence of the electronic and steric properties of the silyl substituents on the geometry, on the EPR parameters, and on the reactivity of such radicals were not studied. In order to better understand the difference between carbon-, silicon- and germanium-centered radicals and the effect of substituents on their structure and electronic properties, we present here data on the generation and EPR characterization and identification of several novel types of silyl-substituted group 14 element centered radicals: (R3Si)2HE• (E = C (1a), Si (2a-e), Ge (3a)), (R3Si)(1Ad)HC• (1b) (1-Ad = 1-adamantyl), (R3Si)2[(CO)5Re]E• (E = Si (2g), Ge (3b)), and (R3Si)2FSi• (2f) (Scheme 1). The experimental study is supported by detailed DFT calculations, in order to better understand the correlation between a(E) and a(1H) with the nature of the central atom E and the (10) McConnell, H. M. J. Chem. Phys. 1956, 24, 764. (11) (a) Improta, R.; Barone, V. Chem. Rev. 2004, 104, 1231. (b) Guerra, M. Chem. Phys. Lett. 1995, 246, 251. (c) Chipman, D. M. Theor. Chem. Acc. 1992, 82, 93. (d) Chipman, D. M. J. Chem. Phys. 1983, 78, 3112. (e) Marcellus, D. H.; Davidson, E. R.; Kwiram, A. L. Chem. Phys. Lett. 1975, 33, 522. (12) Krusic, P.; Kochi, J. K. J. Am. Chem. Soc. 1969, 91, 3938.

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Scheme 1

substituents and the radical geometry, as well as the electronic mechanisms which contribute to the hfcc.

Results and Discussion Carbon-Centered Radicals. The carbon-centered radical 1a is generated by a substitution-redox reaction of bromoform with an excess of tBu2MeSiLi in hexane at 240 K (eq 1).

1a is persistent at 240 K, in contrast to (Me3Si)2HC•, which exhibits an extremely rapid second-order decay.3 The higher kinetic stability of 1a results from the effective steric protection of the radical center by the four bulky tert-butyl substituents slowing down its dimerization. Above 240 K 1a decays (the lifetime at 270 K is several minutes), probably via a hydrogen abstraction reaction. The EPR spectrum of 1a (Figure 1) reveals the following hyperfine interactions of the unpaired electron with the 1H, 13 C, and 29Si nuclei: a(1HR) = 18.2 G and a(61HMe) = a(361HtBu) = 0.23 G; 13C a(13CR) = 26.8 G and a(213Cγ) = 6.3 G (13C: I = 1/2, natural abundance 1.1%); a(229Siβ) = 14.5 G (29Si: I = 1/2, natural abundance 4.7%). The value of the hfcc with the central carbon atom of 26.8 G is smaller than those of other known planar carbon radicals: for example, a(13CR) = 38.4 G for H3C• and 39.1 G for H3CH2C•.13 The a(1HR) value of 18.2 G is also smaller than those of other alkyl radicals: i.e., a(1HR) = 23.0 G for H3C• and 22.4 G for H3CH2C•.13 Radical 1b, in which one of the silyl groups is substituted by 1-adamantyl (1-Ad), is generated by halogen abstraction by a photochemically generated B-centered m-carboran-9-yl radical (eq 2).14a-d The photochemical method of halogen abstraction allows repeated photogeneration and measurement of the decay kinetics of radicals at different temperatures. (13) Fessenden, R. W. J. Phys. Chem. 1967, 71, 74. (14) (a) Tumanskii, B. L.; Kampel, V. T.; Bregadze, V. I.; Bubnov, N. N.; Solodovnikov, S. P.; Prokof’ev, A. I.; Kozlov, E. S.; Godovikov, N. N.; Kabachnik, M. I. Russ. Chem. Bull. (Engl. Transl.) 1986, 35, 425. (b) Tumanskii, B. L.; Kampel, V. Ts.; Solodovnikov, S. P.; Bregadze, V. I.; Godovikov, N. N. Russ. Chem. Bull. (Engl. Transl.) 1985, 34, 2450. (c) Tumanskii, B. L.; Valetsky, P. M.; Kabachii, Yu. A.; Bubnov, N. N.; Solodovnikov, S. P.; Korshak, V. V.; Prokof'ev, A. I.; Kabachnik, M. I. Russ. Chem. Bull. (Engl. Transl.) 1984, 33, 2210. (d) Bregadze, V. I.; Kampel, V. Ts.; Godovikov, N. N. J. Organomet. Chem. 1977, 136, 281. (e) Davies, A. G.; Griller, Roberts, B. P. J. Am. Chem. Soc. 1972, 94, 1782.

Figure 1. (a) EPR spectrum of 1a at 240 K in hexane and (b) expanded spectrum of 1a at high resolution, which shows hfcc with the hydrogen atoms on the silyl substituents.

Halogen abstraction from the carbon atom also can be achieved with different radicals, for example Si- or P-centered radicals,1b,14e but the most intense spectrum of 1b was obtained by halogen abstraction by the B-centered radical (eq 2).

The following hfcc’s are measured in the EPR spectrum of 1b (270 K): a(13CR)=32.6 G; a(1HR)=19.5 G; a(29Siβ)=14.0 G; a(71HAd) = 0.72 G; a(31HMe) = 0.48 G; a(181HtBu) = 0.24 G. In 1b a(13CR) and a(1HR) have values intermediate between those of a methyl radical and 1a. This indicates that the silyl groups have a cumulative effect in reducing a(13CR) and a(1HR). 1b exhibits a kinetic stability similar to that of 1a. It is persistent at 240 K and decays at higher temperatures, probably by hydrogen abstraction, as supported by kinetic studies. The decay of the EPR signal of 1b shows the typical3b pseudo-first-order kinetics in the high temperature range of 290-340 K in hexane and can be expressed by

k1 ¼ 108:9 ( 0:3 e - 14:5 ( 0:4=RT where k1 is the rate constant in s-1 and RT is in kcal mol-1 (see the Supporting Information for details, Figure S1).

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DFT quantum mechanical calculations were carried out to determine the structure (UB3LYP/6-31þG(d)) and EPR parameters (UPBE0/TZVP) of radicals 1a,b. According to the calculations, both radicals have a planar radical center. The calculated geometry and spin density of 1a are presented in Figure 2 (radical 1b shows a similar spin density distribution, and it is presented in Figure S2 of the Supporting Information). It should be noted that the calculated geometry of (H3Si)2HC•, with smaller H3Si substituents, is also planar, in contrast to the slightly pyramidal geometry of (H3C)3C•15 and (H3C)2HC•.15b Thus, the planarity of the silyl-substituted radicals is caused by the electronic effect of the silyl groups and is not due to their size. It is well-known that electropositive σ-donating groups decrease the s contribution in the SOMO, resulting in planarization of the radical center and a higher degree of π character.16 The calculated g factor and the 1H and 29Si hfcc’s of radicals 1a,b are, in general, in good agreement with the experimental EPR data (Table 1). However, the calculations underestimate a(13CR) by a factor of ∼1.24. This can be explained by the fact that the calculated geometry corresponds to a hypothetical nonvibrating planar structure at the bottom of the potential energy curve. Consequently, the calculated property (e.q., hfcc) does not include corrections due to the ground vibrational state (T = 0 K).11 For example, for the hypothetical planar H3C• radical a(13CR) ≈ 27 G (calculated11a and experimentally projected value11d), which is lower in comparison to the value of ∼38 G actually observed for the vibrating species at low temperatures13 (the calculated a(13CR) for H3C• including corrections due to anharmonic vibrations is given in Table S1 in the Supporting

Figure 2. DFT calculated spin density of 1a at the 0.002 au contour level. The yellow and blue areas correspond to regions of positive and negative spin densities, respectively. Hydrogen atoms are omitted for clarity, except for HR. Selected calculated bond lengths (A˚) and angles (deg): CR-Si = P 1.883, CR-HR = 1.095; Si-CR-HR 111.7, Si-CR-Si = 136.5, θ(CR) = 360.0.

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Information). Therefore, the calculations that do not include vibrational corrections underestimate the value for a(13CR). In addition, a(13CR) increases with temperature as upper vibrational levels are populated, an effect which is also not included in the calculations. However, for H3C• this dependence is small.17 Silicon-Centered Radicals. The a(29SiR) hfcc is strongly affected by the electronic properties of the substituents and by the geometry at the radical center.4a,18,19 Values of a(29SiR) vary from 498.0 G for F3Si•20 to 55.0 G for (iPr3Si)3Si•.4c We have studied the effect of bulky substituents with similar electronic properties on a(29SiR) and a(1HR) hfcc’s in the series of Si-centered radicals 2a-e and examined the correlation of these parameters with the radical’s calculated geometries. Silyl radicals 2a-e are generated at 180-270 K in pentane by H abstraction from the corresponding dihydrosilanes (R3Si)2SiH2 using photochemically generated tert-butoxy radicals (eq 3).12 The EPR spectra of 2a-e are characterized by the interaction of the unpaired electron with the 1HR nucleus (doublet) and the 29SiR nuclei (satellite lines). Hyperfine coupling of the unpaired electron with the 29Siβ nucleus is not clearly observed in 2a-d due to a low signalto-noise ratio and superposition of the radical EPR signal in

Figure 3. EPR spectra of 2e: (a) experimental spectrum at 270 K (asterisks mark lines belonging to side products); (b) simulated spectrum.

Table 1. Experimental (at 270 K) and Calculated Values of hfcc (G) and Calculated Mulliken Spin Density Populations (in Parentheses) in Radicals 1a,b calcd valuesa

exptl values

1a 1b a

g factor

|a(13CR)|

|a(1HR)|

|a(29Siβ)|

g factor

a(13CR)

a(1HR)

a(29Siβ)b

2.0026 2.0023

26.8 32.6

18.2 19.5

14.5 14.0

2.0028 2.0026

21.7 (112%) 26.2 (118%)

-19.2 (-5.5%) -20.1 (-6.0%)

15.2 (-8.7%) 16.2 (-7.7%)

UPBE0/TZVP//UB3LYP/6-31þG(d). b The magnetogyric ratio of 29Si is negative; thus, the sign of a(29Siβ) is positive.

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Table 2. Experimental (at 270 K) and Calculated Values of hfcc (G) and the Degree of Pyramidality of Radicals 2a-fa calcd valuesb

exptl values

2a 2b 2c 2d 2e 2f a

R3Si

|a(29SiR)|

|a(1HR)|

a(29SiR)

a(1HR)

(iPr3Si)2HSi• (tBu2MeSi)2HSi• (tBuMe2Si)2HSi• (Me3SiMe2Si)2HSi• [(Me3Si)3Si]2HSi• (H3Si)2HSi• (H3C)2HSi• (tBu2MeSi)2FSi•

72.3 77.8 79.5 81.4 83.6

12.4 11.7 11.0 9.6 10.1

183.0d 110.0

17.0d 31.9c

-66.0 -64.3 -73.9 -76.9 -81.6 -108.9 -158.8 -95.3

-10.6 -11.8 -10.8 -10.1 -8.9 -5.5 13.3 27.6c

a(29Siβ) 5.0 6.1 4.4 1.7 2.7 2.0 -3.1

h, A˚

P θ(Si), deg

0.329 0.284 0.362 0.389 0.361 0.482 0.546 0.487

352.1 354.1 350.4 348.9 350.2 342.7 332.0 344.3

Experimental g factor = 2.0045-2.0050 for 2a-e; g = 2.0032 for 2f. UPBE0/TZVP//UB3LYP/6-31G(d). c a(19F); d At 150 K.12 b

the central region of the spectrum with nonidentified secondary reaction products (Figure S3). In the more intense EPR spectrum of 2e additional satellite lines resulting from interaction of the unpaired electron with the two 29Siβ nuclei (4.5 G) and the six 29Siγ nuclei (7.4 G) were observed (Figure 3). These hfcc values are smaller than those measured in the stable planar radical (tBu2MeSi)2HSi(tBu2MeSi)2Si•.2a The measured EPR parameters of radicals 2a-e are given in Table 2. Figure 4. Definition of the h coordinate.

In contrast to the carbon-centered radicals 1a,b, silyl radicals 2a-d are short-lived with lifetimes of only several seconds at 240 K. Note that 1a and 2b carry the same substituents. The main path for the decay of the H-substituted bis(silyl)-silyl radicals is their dimerization to produce the corresponding dimers R2HSi-SiHR2.2a,7 Radical 2f, in which silicon is substituted by the electronegative fluorine, is generated similarly to radicals 2a-e by hydrogen abstraction from (tBu2MeSi)2FSiH by (tBuO)2. Its a(29SiR) and a(19F) values are given in Table 2 (the EPR spectrum is presented in the Supporting Information, Figure S4). The hfcc a(19F) = 31.9 G is less than half as small as that in the more pyramidal radical (CH3)2FSi• (a(19F) = 65.2 G).20 DFT calculations of H-substituted radicals 2a-e and F-substituted 2f show that they all have a pyramidal geometry around the radical center (Table 2), in contrast to the analogous carbon-centered radical 1a, which is planar. The degree of their pyramidality can be represented by the h coordinate, which is defined as the height of a pyramid for which the vertex consists of the radical center and the basis includes the three R-substituent atoms (Figure 4). A planar geometry around the radical center is characterized by h = 0 and a pyramidal geometry by h > 0. The pyramidality of a radical center is affected by the size and the electronic properties of the substituents. The steric interaction between the substituents is minimized in the planar form. Therefore, (15) (a) Noller, B.; Maksimenka, R.; Fischer, I.; Armone, M.; Engels, B.; Alcaraz, C.; Poisson, L.; Mestdagh, J.-M. J. Phys. Chem. A 2007, 111, 1771. (b) Pacansky, J.; Koch, W.; Miller, M. D. J. Am. Chem. Soc. 1991, 113, 317. (c) Krusic, P. J.; Meakin, P. J. Am. Chem. Soc. 1976, 98, 228. (16) (a) Pauling, L. J. Chem. Phys. 1969, 51, 2767. (b) Bickelhaupt, F. M.; Ziegler, T.; Schleyer, P. v. R. Organometallics 1996, 15, 1477. (17) (a) Ellinger, Y.; Pauzat, F.; Barone, V.; Douady, J.; Subra, R. J. Chem. Phys. 1980, 72, 6390. (b) see Table S1, Supporting Information. (18) Guerra, M. J. Am. Chem. Soc. 1993, 115, 11926. (19) Chatgilialoglu, C., Organosilanes in Radical Chemistry; Wiley: Chichester, U.K., 2004; Chapter 1. (20) Merritt, M. V.; Fessenden, R. W. J. Chem. Phys. 1972, 56, 2353.

the larger the substituents, the less pyramidal the radical. Electronegative substituents increase the degree of pyramidalization of the radical center, while electropositive substituents (e.g., silyl groups) reduce it.1d,18,16 The calculations show that the degree of pyramidality decreases when the substituent size increases (Table 2). Radicals 2a-e are much less pyramidal than (H3C)2HSi• and (H3Si)2HSi•. Radical 2f shows a higher degree of pyramidality than 2a-e due the electronegative fluorine substituent.18,16 The calculated geometry and the spin density of radicals 2b,f are given in Figure 5. Table 2 shows that the calculated hfcc’s are underestimated relative to the experimental ones by ca. 10%, as was found also for the carbon-centered radicals 1a,b. This can be attributed to conformational and vibrational averaging11 and to errors in the computational method, as discussed above. A detailed comparison between the calculated and the experimental hfcc’s will be published elsewhere. We calculated the contribution of the direct (D) and the spin-polarization (SP) mechanisms to a(29SiR), a(1HR), and a(19F) as a function of the degree of pyramidality of the radical center (h coordinate) in the model (H3Si)2HSi• and (H3Si)2FSi• radicals, and the results are presented in Figure 6. The geometry of the radicals was fully optimized at each fixed step of h value. The total (D þ SP) values of the hfcc at each step were calculated using the unrestricted UB3LYP/ TZVP method, and they are depicted in Figure 6 by black circles and a solid line. The direct contribution to the hfcc values was calculated using the restricted-open ROB3LYP/ TZVP method, and they are depicted by red squares and a dashed line. The spin-polarization contribution to hfcc was calculated by subtracting the values calculated at the restricted-open method from the total (D þ SP) values, and they are depicted by blue triangles and a dotted line. The degree of pyramidality (h coordinate) of (H3Si)2HSi• and (H3Si)2FSi• at the energy minimum is shown by a green dashed vertical line. The experimental hfcc’s for radicals 2a-e are given by green crosses.

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Figure 5. DFT calculated spin densities at the 0.002 au contour level. Hydrogen atoms are omitted for clarity, except for HR. Calculated g factors, selected values of hfcc, and Mulliken atomic spin densities (in parentheses) at the particular nucleus: (a) radical 2b, g = 2.0047, a(29SiR) = -64.3 G (110.3%), a(229Siβ) = 6.1 G (-3.7%), a(1HR) = -11.8 G (-5.0%); (b) radical 2f, g = 2.0038, a(29SiR) = -95.3 G (94.3%), a(229Siβ) = -3.1 G (2.3%), a(19F) = 27.6 G (1.6%). Selected calculated bond lengths (A˚): 2b, SiR-Siβ = 2.367, SiR-HR = 1.500; 2f, SiR-Siβ = 2.383, SiR-F = 1.660. Calculated geometrical parameters of the radicals are given in Table 2.

At a planar geometry at the radical center (h = 0) the unpaired electron occupies a pure 3p orbital and a(29SiR) results only from a positive spin-polarization contribution (the negative sign of the hfcc is attributed to the negative magnetogyric ratio of the 29Si nucleus). In (H3Si)2HSi• at h = 0, a(1HR) results only from a negative spin-polarization contribution. When the radical pyramidalizes, the s character of the SOMO increases and, consequently, the direct contribution (D) increases and the contribution of the spin-polarization decreases (Figure 6a). In general, the absolute value of a(29SiR) monotonically increases with an increase in the degree of pyramidality of the radical center. The value of a(1HR) increases monotonically from negative to positive (Figure 6b).21 As a consequence, for pyramidal radicals a(1HR) can be zero (this is important to realize, as experimentally it means that a(1HR) is not observed) or have a positive value, as in the case of methylsilyl radicals.11b (H3Si)2FSi• (a model for 2f) shows values of a(29SiR) slightly higher than those of (H3Si)2HSi• with a somewhat (21) Calculations at the PBE0/TZVP level show a slightly higher negative SP contribution, and as a result a(1HR) does not rise above 0 (Figure S8, Supporting Information).


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higher direct contribution (D) and a lower SP contribution (Figure 6c). a(19F) shows a positive contribution of SP (Figure 6d), in contrast to a negative contribution of SP to a(1HR) (Figure 6b). This is explained by the fact that fluorine, being a strong electron acceptor, delocalizes the spin density (Figure 5d).22 According to Figure 6d such delocalization has a maximum efficiency when h ≈ 0.5 A˚. The dependence of the D and SP contributions to a(29Siβ) on the h coordinate is similar to that for a(1HR), but it crosses the zero value at a lower degree of pyramidality (Figure 7). Thus, for radicals 2a-f, which are pyramidal, a(29Siβ) is expected to be low, as observed for 2e, where a(29Siβ)exp = 4.5 G. Calculations show positive low values of a(29Siβ)calc (SP contribution dominating) in 2a-e in the range of þ1.7-6.1 G. For comparison, in the planar (tBu2MeSi)3Si• a(29Siβ)exp = 8.0 G (acalc = þ8.6 G). Even in 2f the calculated a(29Siβ) crosses zero (D contribution dominating), and has a negative low value of -3.1 G. The EPR signal line width (∼2.0 G) is of the same order as a(29Siβ) (Figure S4). This low value explains why we have not experimentally observed a(29Siβ) in 2f. Germanium-Centered Radicals. Radical 3a, the germanium analogue of 2b, was generated by a similar reaction (eq 4). The EPR spectrum of 3a revealed only a doublet originating from the R-proton splitting with a(HR) = 10.9 G (Figure 8). Unfortunately, the low signal-to-noise ratio precluded observation of the 73Ge satellite signals (73Ge: I = 9/2, natural abundance 7.8%). The g factor of 3a (2.019) is typical for stable silyl-substituted Ge-centered radicals. For example, g = 2.0229 for (tBu2MeSi)3Ge•.2g Radical 3a is not persistent, and its half-life is only several seconds at 200 K, similar to the half-life of silyl radicals 2a-e.

The DFT calculations for 3a show a spin density distribution and geometry around the radical center (Figure S6, Supporting Information) similar to that of the analogous silyl radical 2b.23 The calculated value of a(1HR) is in good agreement with the experimental value. The dominating mechanism contributing to a(1HR) is the SP mechanism, similarly to 2a-e. Re-Substituted Radicals. In order to produce germaniumcentered radicals in a higher concentration than that of 3a, we used the photochemically generated (CO)5Re• for H abstraction from (tBu2MeSi)2GeH2, instead of the tertbutoxy radical used in eq 4. Surprisingly, irradiation (λ>300 nm) of a benzene solution, containing equimolar amounts of (tBu2MeSi)2GeH2 and Re2(CO)10, within the cavity of the EPR spectrometer produced an EPR spectrum (Figure 9) which is consistent with that of the Re-substituted germyl radical 3b. We suggest that 3b is produced by the sequence of reactions shown in Scheme 2. The main feature of the EPR spectrum of 3b (g = 2.031) is a sextet arising from the hyperfine interaction of the unpaired electron with the 185,187Re (22) Mitov, S.; Panchenko, A.; Roduner, E. J. Phys. Chem. A 2007, 111, 5294. (23) The calculated g factor, selected values of hfcc, and Mulliken atomic spin densities (in parentheses) at specific nuclei of radical 3a: g= 2.0193; a(73Ge) = -20.2 G (107.4%), a(229Siβ) = 6.0 G (-2.8%), a(1HR) = -11.4 G (-5.1%). Selected calculatedPbond lengths (A˚) and angles (deg): Ge-Si=2.374, Ge-HR = 1.591; θ(Ge) = 355.5, h = 0.254 A˚.

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Figure 6. Calculated direct (D), spin polarization (SP), and total (D þ SP) contributions to the hfcc as a function of pyramidality (h coordinate): (a) a(29SiR) and (b) a(1HR) in (H3Si)2HSi•; (c) a(29SiR) and (d) a(19F) in (H3Si)2FSi•. Experimental values for radicals 2a-e are given by green crosses. The sign of a(29SiR) is inverted for a better comparison with other atoms. The green dashed vertical line indicates the geometry at the energy minimum.

Figure 7. Calculated direct (D), spin polarization (SP), and the total (D þ SP) contribution to a(29Siβ) as a function of pyramidality (h coordinate) in (H3Si)2HSi•. The sign of a(29Siβ) is inverted for better comparison with other atoms. For a similar figure for (H3Si)2FSi• see Figure S5 (Supporting Information).

nuclei (185,187Re: I = 5/2) with a(185,187Re) = 32.8 G. An additional a(73Ge) = 21.5 G is also observed; this value is typical for planar Ge-centered radicals.2g A minor sextet (1:10, a(185,187Re) = 39.0 G) (shown by asterisks in Figure 9)

Figure 8. EPR spectrum of 3a at 200 K.

with a large shift in its g factor (g = 2.006) is also observed, and its identity is not known. Radical 3b is the first example of a transition-metalsubstituted Ge-centered radical. It is relatively stable, having a half-life of several hours at 298 K. The LDI-MS analysis of the reaction mixture revealed peaks corresponding to the parent ion of 3b, and it has the expected isotopic mass distribution (Figures 9c,d).

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Figure 10. DFT calculated spin density of 3b at the 0.002 au contour level. Hydrogen atoms are omitted for clarity. The calculated g factor, selected values of hfcc, and Mulliken atomic spin densities (in parentheses) at the particular nucleus are as follows: g = 2.0275; a(73GeR) = -10.9 G (117.7%), a(185,187Re) = -33.9 G (-15.4%), a(229Siβ) = 7.8 G (-6.1%). Selected calculated bond lengths (A˚) and anglesP (deg): Ge-Si = 2.482, GeRe = 2.802; Si-Ge-Re = 122.0, θ(GeR) = 360.0. Scheme 2

Scheme 3

Figure 9. (a) Experimental and (b) simulated EPR spectra representing a superposition of radical 3b signals and sextet signals (*) of an unidentified radical. (c) Simulated and (d) experimental LDIMS spectra of 3b.

Zero-order regular approximation (ZORA) DFT calculations (UPBE0/TZVP-SARC//UB3LYP/SVP-SARC)24 of the structure and the EPR parameters of radical 3b predict a planar geometry around the Ge atom, which is consistent with the electropositive nature of the Re atom and the bulkiness of the Re(CO)5 group. The calculations also support a spin polarization mechanism for spin-density transfer to the Re nucleus (Figure 10). (24) (a) Buhl, M.; Reimann, C.; Pantazis, D. A.; Bredow, T.; Neese, F. J. Chem. Theor. Comput. 2008, 4, 1449. (b) Pantazis, D. A.; Chen, X.-Y.; Landis, C. R.; Neese, F. J. Chem. Theor. Comput. 2008, 4, 908.

To prepare 2g, the silicon analogue of radical 3b, we irradiated a benzene solution, containing an equimolar mixture of (tBu2MeSi)2SiH2 and Re2(CO)10, but the desired silyl radical was not produced. However, 2g can be generated by UV irradiation of a 1:1 mixture in toluene solution of silylenoid 4a25 and Re2(CO)10 (Scheme 3). 2g is persistent, having a half-life of several hours at 298 K. We suggest that, in the first step of the reaction, silylene (tBu2MeSi)2Si: (4b) is photochemically generated from 4a and its reaction with (OC)5Re• produces 2g. Persistent radical adducts of (25) Molev, G.; Bravo-Zhivotovskii, D.; Karni, M.; Tumanskii, B.; Botoshansky, M.; Apeloig, Y. J. Am. Chem. Soc. 2006, 128, 2784.

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Sheberla et al. Table 3. Experimental (at 298 K) and Calculated Values of hfcc (G) and Calculated Mulliken Spin Density Populations (In Parentheses) in Radical 2g exptl values g factor 2g-A 2g-B

2.0130 2.0127

|a(

185,187

Re)|

|a(29SiR)|

|a(229Siβ)|

59.6 59.0

8.0 8.0

40.0 38.8 calcd valuesa

g factor

a(185,187Re)

a(29SiR)

2g-syn 2.0086 -38.9 (-20.5%) -41.5 (129%) 2g-anti 2.0089 -36.2 (-19.5%) -42.3 (128%) a UPBE0/TZVP-SARC//UB3LYP/SVP-SARC.

Figure 11. EPR spectra: (a) experimental spectrum representing a superposition of the signals from two rotamers (A and B) of silyl radical 2g (signals due to a minor addition radical (tBu2MeSi)3Si• are marked by asterisks); (b) simulated spectrum of rotamer B.

Figure 12. Syn and anti rotamers of 2g, viewed along the Si-Re bond.

Re-centered radicals and N-heterocyclic silylene 26b and germylene26a have been recently observed by us by EPR spectroscopy. The EPR spectrum of 2g shows a set of two very closely placed sextets, which we attribute to the presence of two rotamers of the radical (A and B), with B being the major contributor (Figure 11). The ratio between the signals of A and B does not depend on the irradiation time or the temperature (in the range of 230-320 K). We believe that the two rotamers differ by rotation of the silyl substituents around the Si-Si bond. In one of the rotamers two Me groups on the silyl substituents are in a syn arrangement and (26) (a) Tumanskii, B.; Pine, P.; Apeloig, Y.; Hill, N. J.; West, R. J. Am. Chem. Soc. 2005, 127, 8248. (b) Tumanskii, B.; Pine, P.; Apeloig, Y.; Hill, N. J.; West, R. J. Am. Chem. Soc. 2004, 126, 7786.

a(229Siβ) 8.4 (-8.3%) 8.1 (-7.5%)

in the second rotamer they are in an anti arrangement, as shown in Figure 12. The high signal-to-noise ratio in the EPR spectra of 2g allows a precise observation of the 29SiR satellite lines. The experimental EPR parameters for the two rotamers of 2g are given in Table 3. ZORA DFT calculations of the geometry and the EPR parameters (Table 3) of syn and anti rotamers of radical 2g support the experimental data and predict a planar structure of 2g (see Figure S7 in the Supporting Information). These calculated and experimental a(29SiR) values (Table 3) are typical for planar polysilyl radicals.2a,g For example, for (tBu2MeSi)3Si• the experimental a(29SiR) value is 58.0 G (calculated a(29SiR) = -41.6 G). The calculations predict that the anti rotamer is more stable by 0.6 kcal/mol than the syn rotamer. However, we cannot assign with confidence a specific set of sextets to the syn or anti configuration, because their energy and spectroscopic properties (e.g., hfcc, g factor) are very similar.

Conclusions Novel group 14 element centered silyl-substituted radicals of the types (R3Si)2HE• (E = C, Si, Ge), (R3Si)(1-Ad)HC•, and (R3Si)2[(OC)5Re]E• (E = Si, Ge) were generated by various reactions and were comprehensively studied by EPR spectroscopy and DFT calculations. We showed that two bulky silyl tBu2MeSi substituents at a methyl radical in (tBu2MeSi)2HC• (1a) or one silyl (tBu2MeSi) and one bulky alkyl (1-Ad) substituent in (tBu2MeSi)(1-Ad)HC• (1b) prevent these radicals from dimerization, making them persistent at 240 K. The decay of 1b shows a pseudo-first-order kinetics in the temperature range of 290-340 K, and this is consistent with an H-abstraction decay mechanism. In contrast, two bulky tBu2MeSi substituents at Si- and Ge-centered radicals are not sufficient to prevent their dimerization and they dimerize with half-lifes of several seconds at 240 K. Thus, the decay mechanism is different for C-centered vs Siand Ge-centered radicals carrying the same silyl substituents. C-centered radicals decay mostly by H abstraction, while Siand Ge-centered radicals decay via dimerization. This difference in behavior is due to three main factors: (a) Si-Si and Ge-Ge bonds are much longer than C-C bonds and thus steric effects of substituents are much more effective for C-centered radicals; (b) C-C bonds are generally much stronger than Si-Si or Ge-Ge bonds (e.g., 90.2 kcal mol-1 for H3C-CH3 vs 76.7 kcal mol-1 for H3Si-SiH3, this being also a value for Ge-Ge)6 and thus dimerization of carbon radicals is more exothermic than that of silyl or germyl radicals; (c) a C-H bond produced by H-abstraction

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is significantly stronger than Si-H or Ge-H bonds (e.g., 104.8 kcal mol-1 for H3C-H vs 90.3 kcal mol-1 for H3Si-H and 83.4 kcal mol-1 for H3Ge-H).6,27 The calculated geometries and the experimental hfcc’s show that thePhydrogen-substituted bis(silyl) C-centered radical 1a is planar ( θ(C) = 360.0°; a(13CR) = 26.8 G), in contrast to the analogous are slightly P Si- and Ge-centered radicals, which P pyramidal ( θ(Si) = 354.1°, a(29SiR) = 77.4 G and θ(Ge) = 355.5°). The Si- and Ge-centered radicals (2g and 3b, respectively) substituted with the Re(CO)5 group are persistent at room temperature and are planar around E (a(29SiR) = 59.0, 59.6 G and a(73GeR) = 21.5 G). Two mechanisms, direct and spin polarization, contribute to the spin density at a particular nucleus and therefore to the hfcc’s at that nucleus. Their contributions change as a function of the degree of pyramidality around the radical center, and thus also the total measured hfcc depends on the degree of pyramidality. As the radical geometry changes from a planar geometry to a pyramidal geometry, the absolute value of the contribution of the direct mechanism to the hfcc increases and the spin-polarization contribution decreases. The HR hfcc’s in 1a,b, 2a-e, and 3a and the 185,187Re hfcc’s in 2g and 3b have a dominating negative spin-polarization contribution and thus have a negative sign. In contrast, in the fluorine-substituted radical 2f the spin-polarization contribution to the spin density at F is positive and thus the 19F hfcc has a positive sign. Because two mechanisms may operate in opposite directions, a radical may have a “critical” pyramidal geometry in which a(1HR) and a(29Siβ) are nearly zero and therefore cannot be observed experimentally. This is the case for a(29Siβ) in 2f.

Experimental Section Spectroscopy. EPR spectra were recorded on a Bruker EMX10/12 X-band (ν = 9.4 GHz) digital EPR spectrometer equipped with a Bruker liquid nitrogen temperature controller. Samples were irradiated with the focused and filtered (λ > 300 nm) light of a high-pressure mercury lamp (1 kW) (ARC lamp Model 69920) directly in the microwave cavity of the EPR spectrometer. All EPR spectra were recorded at the nonsaturating microwave power of 1.0-0.5 mW, 100 kHz magnetic field, and modulation of 1.0-0.1 G amplitude. The EPR spectra of radicals 2g and 3b were not saturated at the microwave power of 200 mW. The highresolution EPR spectrum of radicals 1a,b were recorded at the microwave power of 0.05 mW, 20 kHz magnetic field, and modulation of 0.02 G amplitude. Digital field resolution was 2048 points per spectrum, allowing all hyperfine splittings to be measured directly, with the accuracy being better than 0.05 G. EPR spectra processing and simulation were performed with the Bruker WIN-EPR and SimFonia software. The LDI-TOF mass spectrometry experiment was performed without matrix using a Waters MALDI micro MX mass spectrometer equipped with a nitrogen laser (337.1 nm). Mass calibration was performed using polyethylene glycol standards. One microliter of the reaction mixture in a benzene solution was applied to the MALDI plate and dried. NMR spectra were recorded at room temperature in J. Young vacuum NMR tubes equipped with a DMSO-d6 capillary as external standard, using Bruker Avance 300 and Bruker Avance 500 instruments. NMR shifts are given in ppm. Materials. (m-B10C2H9)2Hg. This compound was prepared by the procedure reported elsewhere.14d Adamantyl(di-tert-butylmethylsilyl)bromomethane, Ad(tBu2MeSi)BrCH. To a solution of Ad(CO)Cl (0.2 g, 1 mmol) in (27) Walsh, R. Acc. Chem. Res. 1981, 14, 246.

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10 mL of Et2O was added dropwise (0.5 h) 10 mL of a hexane solution of tBu2MeSiLi (1 mmol) at 0 °C; then the reaction mixture was warmed to room temperature (1 h). The reaction mixture was quenched with water and extracted with hexane. The combined organic extracts were dried, and after evaporation of solvent the acylsilane tBu2MeSi(CO)Ad was obtained as a colorless solid (0.26 g, 81%): 1H NMR (C6D6, δ) 0.294 (s, 3 H, Me), 1.03 (s, 18 H, tBu), 1.35-2.07 (Ad); 13C NMR (C6D6, δ) -5.5, 19.2, 27.8, 28.4 (t-Bu), 36.5, 36.6, 51.8 (Ad); 29Si NMR (C6D6, δ) -7.2. A solution of the acylsilane tBu2MeSi(CO)Ad (0.26 g, 0.81 mmol) in 5 mL of Et2O was added to a suspension of LiAlH4 (0.04 g, 1 mmol) in Et2O (10 mL), and the mixture was refluxed for 3 h. The reaction mixture was quenched with dilute hydrochloric acid and extracted with hexane. The combined organic extracts were dried, and after evaporation of solvent Ad(tBu2MeSi)HCOH was obtained as a colorless solid (0.2 g, 77%): 1H NMR (C6D6, δ) 0.14 (s, 3 H, Me), 1.08 (s, 9 H, tBu), 1.25 (s, 9 H, tBu), 1.41-1.93 (Ad), 3.01 (s, 1 H, broad); 13C NMR (C6D6, δ) -5.7, 19.09, 20.29, 28.12, 28.48, 28.67, 29.76, 36.86, 39.57, 75.25; 29Si NMR (C6D6, δ) 5.3. A mixture of Ad(tBu2MeSi)HCOH (0.2 g, 0.81 mmol) and Br2PPh3 (0.8 g, 0.81 mmol) in CH2Cl2 (5 mL) was refluxed for 1 h. Then after evaporation of the reaction mixture the reddish solid residue was extracted by hexane. After evaporation of hexane the target Ad(tBu2MeSi)HCBr was obtained as a colorless solid (0.21 g, 84%): 1H NMR (C6D6, δ) 0.13 (s, 3 H, Me), 1.08 (s, 9 H, tBu), 1.25 (s, 9 H, tBu), 1.53, 1.63, 1.67, 1.74, 1.78, 1.87 (Ad), 3.35 (s, 1 H, broad); 13C NMR (C6D6, δ) -3.45, 19.9, 22.50 (Me3C), 28.52, 28.83 (tBu), 30.03, 36.31, 41.90 (Ad), 56.48, (CHBr); 29Si NMR (C6D6, δ) 6.24. (tBu2MeSi)2FSiH. This compound was prepared by the procedure reported elsewhere.25 (R3Si)2SiH2. This compound was prepared by the procedure reported for R3Si = tBu2MeSi.28 R3Si = Me3SiMe2Si. 1H NMR (C6D6, δ): 0.16 (s, 12 H, SiMe2Si), 0.29 (s, 18 H, Me3Si), 3.12 (s, 2 H, Si-H). 13C NMR (C6D6, δ): -1.6, 3.6. 29Si NMR (C6D6, δ): -16.5, -43.1, -106.6. R3Si = (Me3Si)3Si. 1H NMR (C6D6, δ): 0.38 (s, 54 H, Me), 3.63 (s, 2 H, Si-H). 13C NMR (C6D6, δ): 2.7. 29Si NMR (C6D6, δ): -9.1, -107.0, -131.1. Bis(di-tert-butylmethylsilyl)germane, (tBu2MeSi)2GeH2. A mixture of bis(di-tert-butylmethylsilyl)dichlorogermane, (tBu2MeSi)2GeCl229 (3.04 g, 6.63 mmol), and LiAlH4 (0.290 g, 7.64 mmol) in Et2O (50 mL) was refluxed for 3 h. The reaction mixture was quenched with dilute hydrochloric acid and extracted with Et2O. The combined organic extracts were dried, and after evaporation of solvent the target (tBu2MeSi)2GeH2 was obtained as a colorless solid (1.70 g, 66%): 1H NMR (C6D6, δ) 0.22 (s, 6 H, Me), 1.08 (s, 36 H, tBu), 2.78 (s, 2 H, Ge-H); 13C NMR (C6D6, δ) -5.7, 20.9, 29.1; 29Si NMR (C6D6, δ) 17.6. Computational Methods. Geometry optimizations were performed using the Gaussian0330 set of programs. All structures

(28) (a) Bravo-Zhivotovskii, D.; Ruderfer, I.; Yuzefovich, M.; Kosa, M.; Botoshansky, M.; Tumanskii, B.; Apeloig, Y. Organometallics 2005, 24, 2698. (29) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A. J. Organomet. Chem. 2003, 685, 168. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Menucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Rega, N.; Salvador, P.; Dannenberg, J. J.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; 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. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 03, Revision D.02; Gaussian, Inc., Wallingford, CT, 2004.

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were confirmed as local minima by calculating second-order derivatives of the energy. Isotropic hyperfine coupling constants and g factors of radicals 1a,b, 2a-f, and 3a were calculated using the PBE0 functional31 in conjunction with a TZVP32 basis set using the ORCA 2.6.35 software package developed by Neese.33 All expectation values of the total spin-squared operator were within 1% of the expected value of 0.75. For the Re-substituted radicals 2g and 3b all-electron ZORA calculations ZORA/ UPBE0/TZVP-SARC//ZORA/UB3LYP/SVP-SARC were done as implemented in ORCA.24

Acknowledgment. We are grateful to Dr. Miriam Karni (Technion) for helpful discussions. This research was supported by the Israel Science Foundation (ISF), by the USA-Israel Binational Science Foundation, by the Fund for the Promotion of Research at the Technion, and by the Minerva Foundation in Munich. Y.A. acknowledges the JSPS Fellowship for research in Japan. B.T. and D.B.-Z. are grateful to The Center for Absorption in Science, Israel, Ministry of Immigrant Absorption, State of Israel, for financial support.

(31) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158. (32) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829. (33) (a) Neese, F. ORCA, Version 2.6.35; University of Bonn, Bonn, Germany, 2008. (b) Neese, F. J. Chem. Phys. 2001, 115, 11080.

Supporting Information Available: Tables giving Cartesian coordinates and energies of stationary points in the calculations of the radicals and Figures S1-S8. This material is available free of charge via the Internet at http://pubs.acs.org.

Electronic Structure of Bis(silyl)carbon-, Bis(silyl)silicon-, and Bis(silyl

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