Conformational Control of Polysilanes: Use of CH2 Spacers in the Silicon Backbone
By the reaction of a number of oligosilyl potassium compounds with (trimethylsilyl)chloromethane, derivatives containing the (trimethylsilyl)methyl substituent were prepared. Using X-ray single-crystal structure analysis and UV spectroscopy the conformational properties of some of the compounds were studied. It was found that the (trimethylsilyl)methylated examples exhibit UV absorption properties which correspond to lower energy transitions in comparison to those of analogous trimethylsilylated molecules. The influence of this effect decreases, however, with increasing chain lengths.
Sterically overcrowded or charge perturbed molecules
Hexakis(trimethylsilyl)disilane [(H3C)3Si]3SiSi[Si(CH3)3]3 crystallizes in the rhombohedral space group R3c with six molecules of D3 symmetry per unit cell. The steric overcrowding by the 18 peripheral methyl groups shows up especially in the elongation of the central SiSi bond to 240 pm, in the differing dihedral angles ω(SiSiSiSi) of 43° and 77° and in the rather short non-bonded C⋯C distances of only 352 pm, which reveals an inter-penetration of the methyl groups within their van der Waals radii of about 200 pm. The 1H nuclear magnetic resonance (NMR) spectrum in DCCl3 at 190 K, however, does not exhibit any signal splitting of the 42 equivalent methyl hydrogens and so the rotational barriers must be below 20 kJ mol−1. The photoelectron spectrum shows in its low-energy region between 7.2 eV and 9.3 eV a resolved αSiSi ionization pattern, which can be satisfactorily reproduced by a LCBO-MO model parametrized in terms of the different SiSi/SiSi bond interactions, and the higher ionization energies are assigned on the basis of a radical cation state comparison with [(H3C)Si] 3SiCl, (H3C)3SiCl and (H3C)3CCl. Although the first vertical ionization energy is only 7.70 eV, which is the lowest observed so far for a disilane derivative, and in accord with the cyclovoltammetrically determined irreversible first oxidation potential, no persistent radical cation of hexakis(trimethylsilyl)disilane can be generated in solution.
Photoelectron spectra and molecular properties: CXXII. The low first ionization energies of the β-trimethylsilyl substituted sulfides (H3C)3SiH2CSCH2Si(CH3)3 and ((H3C)3Si)3CSCH3 and their rationalization☆☆☆
Abstract The rather low first vertical ionization energies of bis(trimethylsilylmethyl)sulfide, IE v 1 = 8.04 eV, and tris(trimethylsilyl)methylmethyl sulfide, IE v 1 = 7.66 eV, are compared with those of other alkyl, silylalkyl and silylsulfides and shown further to confirm the small influence of α-trimethylsilyl groups and of the very large donor effect of β-trimethylsilyl substituents in the ground state of radical cations. The overall 2.8 eV (!) lowering of the sulfur τ lone pair ionization and of the individual, mostly non-additive, substituent effects, are rationalized in terms of a qualitative approach based on the electron distribution in the ground state of the neutral molecules.