pubs.acs.org/Organometallics Published on Web 10/14/2010 r 2010 American Chemical Society 5596 Organometallics 2010, 29, 5596–5606 DOI: 10.1021/om100812b Electronic Structure of Bis(silyl)carbon-, Bis(silyl)silicon-, and Bis(silyl)germanium-Centered Radicals (R 3 Si) 2 XE • (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 (R 3 Si) 2 XE • (E = C, Si, Ge; X = H, Re(CO) 5 , F) and (R 3 Si)(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 1 H R and 29 Si β 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 1 H R and 29 Si β hfcc’s result from a combination of direct and spin- polarization 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 the 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 pyramidal ( P θ(Si) = 354.1° and P θ(Ge) = 355.5°). Both (R 3 Si) 2 XE • 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 organome- tallic chemistry. 1 Recently, the isolation and structural char- acterization of such radicals, including radicals lacking π stabilization, became possible by using bulky substituents, in particular bulky silyl groups. 2 These bulky silyl substitu- ents sterically protect the radical center from undergoing typical radical reactions: e.g., dimerization, disproportiona- tion, and hydrogen abstraction. Previous studies of persis- tent 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 is in the dimer formed by the combination of two radicals, and consequently bulkier substituents are required for sta- bilizing 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 Me 3 Si substituents are sufficiently large to make (Me 3 Si) 3 C • persistent, having a lifetime of several days at 298 K. 3 In sharp contrast, the † 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.); apeloig@technion.ac.il. (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. 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