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Tuning the Gap: Electronic Properties and Radical-Type Reactivities of Heteronuclear [1.1.1]Propellanes of Heavier Group 14 Elements

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posted on 28.03.2011, 00:00 by Dominik Nied, Pascual Oña-Burgos, Wim Klopper, Frank Breher
Two heteronuclear [1.1.1]propellanes of group 14, Ge2Si3Mes6 (1) and Sn2Si3Mes6 (2) (Mes = 2,4,6-Me3C6H2), were prepared by reductive coupling of Mes2SiCl2 and GeCl2·dioxane or SnCl2. Both compounds were characterized in detail, including X-ray structure analyses on single crystals. In each case it was found that the E2Si3 cluster core consists of three bridging {SiMes2} units and two ligand-free bridgehead atoms (Eb). As a result of the different size of the bridging units, the distances between the bridgehead atoms are considerably shorter (0.10 Å for 1 and 0.27 Å for 2) than in the homonuclear counterparts Ge5Mes6 and Sn5Dep6 (Dep = 2,6-Et2C6H3) known from the literature. The stronger Eb···Eb interactions in 1 and 2 were confirmed by electrochemical studies using cyclic voltammetry. UV/vis studies, together with density functional theory (DFT) calculations, further supported these findings. A correlation of the Eb···Eb distances and the singlet and triplet A2 transitions for a series of homo- and heteronuclear [1.1.1]propellanes revealed that higher 3A2 excitation wavelengths, and thus lower ΔES→T energies, are obtained either by increasing the distances between the bridgehead atoms or by arranging the involved orbitals in close spatial proximity. Reactivity studies on 1 and 2 using selected reagents showed that Me3SnH or the disulfide FcS−SFc (Fc = ferrocenyl), which are prone to radical-type reactivity, can be readily added across the bridge (the tin hydride reacts only with 1). The resulting 1,3-disubstituted bicyclo[1.1.1]pentane derivatives Me3Sn−Ge(SiMes2)3Ge−H (3) and FcS−E(SiMes2)3E−SFc (4 (E = Ge) and 5 (E = Sn)) were characterized in detail, including X-ray structures of 4 and 5. Interestingly, the homolytic S−S bond addition reactions were found to be susceptible to light. Even though the tin-containing propellane 2 turned out to be more reactive than 1, both conversions can be drastically enhanced simply by using daylight in the lab.