Abstract
The specific reactivity of the tetraradicaloid derivative spirobis(pentagerma[1.1.1]propellane) toward the halogenating reagents (CCl4 and I2) is reported.
Among the nonclassical organic compounds, [1.1.1]propellanes, featuring an inverted tetrahedral geometry at the bridgehead carbons, are particularly challenging to prepare (Wiberg, 1989; Levin et al., 2000). The main interest in such derivatives is the nature of the bond between the bridgehead carbons, which was investigated in a number of experimental and theoretical studies (Wiberg, 1989; Levin et al., 2000; Coppens, 2005; Messerschmidt et al., 2005; Shaik et al., 2009; Wu et al., 2009). The analogs of [1.1.1]propellanes, in which all five skeletal carbons are replaced with the heavier group 14 elements, are equally challenging in the chemistry of the main group elements. Known since the late 1980s by the pioneering contributions of Sita (Sita and Bickerstaff, 1989; Sita and Kinoshita, 1991, 1992), ‘heavy’ [1.1.1]propellanes have now emerged as a rather important class of highly reactive, albeit isolable, organometallics (Drost et al., 2002; Richards et al., 2004; Nied and Breher, 2011; Nied et al., 2009, 2010a,b, 2011). We have recently reported a metal-rich Si2Ge9 cluster 1, manifesting two spiro-fused pentagerma[1.1.1]propellane fragments and thus representing a rare example of a stable tetraradicaloid (Ito et al., 2013). In this communication, we report on the particular reactivity of the bridgehead-to-bridgehead bonds of this Ge-cluster toward the halogenating reagents, carbon tetrachloride, and molecular iodine.
Being proposed the singlet biradicaloid nature of the rather subtle Ge-Ge bridgehead-to-bridgehead interactions, we then probed their reactivity in cluster 1. For the examples of both closed-shell and radical-type reactivity of “heavy” [1.1.1]propellanes, incorporating Si, Ge, and Su atoms at the bridgehead positions, see: Sita and Bickerstaff, 1989; Sita and Kinoshita, 1991, 1992; Drost et al., 2002; Richards et al., 2004; Nied and Breher, 2011; Nied et al., 2009, 2010a,b, 2011. Accordingly, we found that the reaction of 1 tolerates a range of typical radical scavengers, such as CCl4 and I2. Thus, bis(propellane) 1 smoothly reacted with an excess of dry CCl4 (reaction rate is controlled by the low solubility of cluster 1 in benzene), forming a compound 1-[CCl4]2, representing the addition product of two molecules of Cl-CCl3 across the Ge1-Ge1′ bridgehead bonds and isolated in the form of bright-orange crystals (Scheme 1). We suggest addition of the two molecules of CCl4 across the Ge1-Ge1′ bridgehead bonds because alternative addition of CCl4 across other cyclic Ge-Ge bonds would result in less symmetrical environment and accordingly in more complicated NMR spectra. In accord with its molecular symmetry, 1-[CCl4]2 showed two distinct resonances for Me-groups and six signals for tBu-groups in both 1H- and 13C-NMR spectra. Like its precursor 1 (Ito et al., 2013), the CCl4-adduct 1-[CCl4]2 featured notably deshielded skeletal silicons resonating at rather unusual for the tetracoordinate Si atoms low-field at +88.43 ppm (cf., +91.64 ppm in 1) (For comparison, the resonance of the tetracoordinate silicon with the two alkyl and two silyl substituents Me2Si(SiR3)2 can be found in the high-field range from -50 to -100 ppm: Silverstein et al., 2005), which may similarly be ascribed to a specific charge distribution of the electrons caused by the particular bonding situation in cluster 1-[CCl4]2.
Reaction of the Si2Ge9-cluster 1 with carbon tetrachloride.
It is interesting that the alternative product, tetrachloride 1-[Cl2]2, as the product of the addition of four chlorine atoms to the biradicaloid germanium centers, is not detected among the products of this reaction, in contrast to the 1,2-dichlorides that are typically formed upon the radical chlorination of the >E=E< bonds (E=heavier group 14 element) with CCl4 (Lee and Sekiguchi, 2010). This distinction can be realized in terms of the less sterically hindered environment around the bridgehead germanium atoms in 1, allowing direct attack of the bulky CCl3 radical on one of them. Since carbon tetrachloride is a well-known chlorinating source through radical abstraction of its chlorine atoms by low-coordinate heavier group 14 element compounds, we can reasonably explain the reactivity of 1 toward CCl4 as a chemical indicator of the singlet biradicaloid character of its bridgehead Ge-Ge interactions, although one can alternatively explain the observed reactivity of 1 based on the charge-shift bonding of its bridgehead Ge-Ge interactions (Shaik et al., 2009; Wu et al., 2009).
Furthermore, tetraradicaloid 1 also reacted with an excess of molecular iodine, forming a complex mixture of several products, from which one, tetraiodosilatrigermacyclobutane 2, was crystallographically characterized by X-ray diffraction (only a couple of single crystals of 2 were serendipitously crystallized from the reaction mixture, which made characterization of the product by other methods, including NMR spectroscopy, unavailable) (Scheme 2).
Reaction of the Si2Ge9-cluster 1 with iodine.
The tetraiodido-substituted derivative 2 results from a complex transformation, involving breaking/iodination of not only the bridgehead but also several other cyclic Ge-Ge bonds and formation of the SiGe3I4 four-membered ring, in which two germaniums (Ge1 and Ge3) bear one iodine atom each and third germanium (Ge2) bears two iodine substituents (Figure 1).
Crystal structure of the tetraiodido-substituted derivative 2.
ORTEP plot with thermal ellipsoids drawn at the 50% probability level; hydrogen atoms are not shown. Selected bond lengths (Å): Ge1-Ge2=2.4829(3); Ge2-Ge3=2.4869(3); Ge1-Si=2.4810(6); Ge3-Si2=2.4748(6); Ge1-I1=2.5711(3); Ge3-I4=2.5738(3); Ge2-I2=2.5426(2); Ge2-I3=2.5787(3). Selected bond angles (°): Ge1-Ge2-Ge3=92.429(10); Ge2-Ge1-Si2=85.316(15); Ge2-Ge3-Si2=85.360(15); Ge1-Si2-Ge3=92.767(19). Folding of the SiGe3-four-membered ring, as determined by the dihedral angles Ge1-Si2-Ge2-Ge3 and Si2-Ge1-Ge3-Ge2: 21.0° and 22.4°, respectively.
In order to minimize steric repulsion between the bulky organosilyl substituents and the iodine atoms, I1 and I4 are arranged in the cis-diaxial fashion, thus leaving voluminous organosilyl substituents at the cis-diequatorial positions. This results in a distortion of the tetrahedral environment at the Ge1 and Ge3 toward their notable flattening leading to trigonal monopyramidal geometry with the sum of the bond angles around them (excluding I atoms) totaling 340.8° and 337.6°, respectively.
Experimental
General procedures
All experimental manipulations were performed using high-vacuum line techniques or in an argon atmosphere of the MBRAUN MB 150B-G glove-box. All solvents were predried by conventional methods and degassed over potassium mirror in vacuum immediately prior to use. NMR spectra were recorded on Bruker AV-400FT NMR spectrometer (1H NMR at 400.1 MHz; 13C NMR at 100.6 MHz; 29Si NMR at 79.5 MHz). The starting material, cluster 1, was prepared according to a published experimental procedure (Ito et al., 2013). CCl4, dried by the treatment with CaH2, was carefully degassed using vacuum line by several freeze-pump-thaw cycles immediately prior to its use.
Quenching reaction of spirobis(pentagerma[1.1.1]propellane) 1 with CCl4
Cluster 1 (22 mg, 0.014 mmol) was placed in a glass tube with a magnetic stirring bar. Dry oxygen-free benzene (0.5 mL) and excess of dry CCl4 (0.1 mL) were vacuum-transferred into this tube, and the reaction mixture was stirred at room temperature for 4 h to form orange solid. This solid was washed with pentane to afford a pure 1-[CCl4]2 (14 mg, 0.007 mmol, 53%). 1H NMR (C6D6, δ, ppm): 0.51 (s, 6 H, 2 Me), 0.86 (s, 6 H, 2 Me), 1.16 (s, 18 H, 2 tBu), 1.22 (s, 18 H, 2 tBu), 1.30 (s, 18 H, 2 tBu), 1.36 (s, 18 H, 2 tBu), 1.53 (s, 18 H, 2 tBu), 1.58 (s, 18 H, 2 tBu); 13C NMR (C6D6, δ, ppm): -2.2 (Me), -1.2 (Me), 22.9 (C(CH3)3), 23.1 (C(CH3)3), 23.2 (2 C(CH3)3), 28.5 (C(CH3)3), 30.0 (C(CH3)3), 30.1 (C(CH3)3), 30.4 (C(CH3)3), 30.8 (C(CH3)3), 31.1 (C(CH3)3), 33.2 (C(CH3)3), 33.3 (C(CH3)3), 92.1 (CCl3); 29Si NMR (C6D6, δ, ppm): 35.6 (Si substituent), 36.6 (Si substituent), 88.4 (skeletal Si); anal. calcd. for C54H120Cl8Ge9Si6: C, 34.58; H, 6.45. Found: C, 34.65; H, 5.94.
Quenching reaction of spirobis(pentagerma[1.1.1]propellane) 1 with I2
Cluster 1 (11 mg, 0.007 mmol) and excess of I2 (33 mg, 0.130 mmol) were placed in a glass tube with a magnetic stirring bar. Dry oxygen-free benzene (0.5 mL) was vacuum-transferred into this tube, and the reaction mixture was stirred at room temperature for 4 h. The single crystals of 2 were crystallized out of the reaction mixture and were analyzed by X-ray crystallography.
Diffraction data were collected at 150 K on a Bruker AXS APEX II CCD X-ray diffractometer (Mo-Kα radiation, λ=0.71073 Å, 50 kV/30 mA). The structure was solved by the direct method with the SHELXS-97 program (Sheldrick, 1990, 2008) and refined by the full-matrix least-squares method with the SHELXL-97 program (Sheldrick, 1997).
Crystal data for 2: MF=C26H60Ge3I4Si3, MW=1182.38, monoclinic, P2(1)/c, a=22.2494(7), b=10.7480(3), c=18.3510(6) Å, β=107.82°, V=4177.8(2) Å3, Z=4, Dcalcd=1.880 g cm-3. The final R factor was 0.0161 for 8087 reflections with Io>2σ(Io) (Rw=0.0433 for all data, 8601 reflections), GOF=1.087.
Acknowledgments
This work was financially supported by Grant-in-Aid for Scientific Research program (nos. 23655027, 24245007, 24550038, and 90143164) from the Ministry of Education, Science, Sports, and Culture of Japan.
Funding: Ministry of Education, Science, Sports, and Culture of Japan (Grant/Award Number: ‘23655027’, ‘24245007’, ‘24550038’, ‘90143164’).
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