Jochen Meuret

Sterically overcrowded or charge perturbed molecules: XXIII. Hexakis(trimethylsilyl)disilane: Structure and photoelectron spectrum of a sterically overcrowded molecule☆

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: CXXXVIII. 1,4-Di[tris(trimethylsilyl)silyl]benzene: synthesis, structural analogy, photoelectron spectrum and ESR/ENDOR characterization of its radical anion☆

1,4-Di[tris(trimethylsilyl)silyl)]benzene [(H3C)3Si]3SiC6H4Si[Si(CH3)3]3, which contains eight silicon centers, has been synthesized and some of its molecular properties determined. For a single crystal structure determination, its isovalence-electronic and sterically more crowded carbon analogue, [(H3C)3Si]3CC6H4C[Si(CH3)3]3, was selected. It shows van der Waals interactions due to close lattice packing. Despite its shorter SiC bonds, however, no severe skeletal distortions due to overcrowding are observed; the contraction of the benzene ipso angles to 114° is due to the donor effect of the tris(trimethylsilyl)methyl substituents. The first vertical ionization energy of the title molecule, IE1v = 7.37 eV, is the lowest of all of these recorded up to now for 1,4-dialkyl or 1,4-disilyl substituted benzene derivatives; for the monosubstituted tris(trimethylsilyl)silyl derivative [(H3C)3Si]3SiC6H5 the value is 8.04 eV. Cyclovoltammetric experiments in aprotic solution reveal a velocity-dependent irreversible oxidation above + 1.28 eV and no electron insertion within the measurement range to −2.9 V. The reduction to the radical anion, however, can be achieved in THF solution containing [2.2.2]cryptand at a potassium metal mirror. According to the spin populations deduced from 1H and 29Si ENDOR signal patterns, more than half of the spin is located in the two [(H3C)3Si]3Si groups. Results of molecular orbital (MO) calculations and comparison with equivalent molecular states of chemically related compounds allow rationalization of the effects of the [(H3C)3Si]3Si substituent, which acts as a strong electron donor in radical cations and as a strong electron acceptor in radical anions.

Steric overcrowding in N-trimethylsilyl-substituted-p-phenylenediamines: The origin of a Wurster's Blue radical anion

Abstract The crystal structure of N,N′-bis(trimethylsilyl)-p-phenylene diamine proves an almost planar, though heavily distorted SiHN-C 6 H 4 -NHSi skeleton, whereas in the sterically overcrowded N,N,N′,N′-tetrakis(trimethylsilyl) derivative the p-type electron pairs of the flattened (R 3 Si) 2 N groups are twisted into the plane of the benzene ring and, therefore, provide a rationale for its unexpectedly possible one-electron reduction to a Wursters Blue radical anion.

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.

Structures of sterically overcrowded or charge-perturbed molecules: XXVIII. [(H3C)3Si]3CCCC[Si(CH3)3]3 - a π-system concealed by a hydrocarbon veil☆

Abstract The kinetically shielded di(tris(trimethylsilyl)methyl)acetylene crystallizes in the triclinic space group P 1 with 1 molecule per unit cell in rather dense lattice packing. Its staggered skeletal conformation of close to D3d symmetry represents a sterically advantageous cogwheel gearing of the inner methyl groups of the half-shells of both substituents. The shortest nonbonded C ⋯ C distances of 400 pm between them equal the sum of the van der Waals radii of two H3C groups. A space-filling representation of the molecular structure shows the acetylenic π-system to be almost completely covered by the peripheral HC skin, in keeping with its unusual unreactivity, e.g. towards catalytic hydrogenation. The spatial and electronic effects of the bulky substituents are discussed in terms of structural comparisons with chemically related compounds such as [(H3C)3Si]3SiSi[Si(CH3)3]3, [(H3C)3C]3SiSi[C(CH3)3]3 or [(H3C)3Si]3CC6H4C[Si(CH3)3]3, which also adopt staggered conformations of their sterically congested [(H3C)3X]3Y-half shells along substituent axes of C3-symmetry at different distances Y ⋯ Y.

Photoelektronen-Spektren und Moleküleigenschaften: CXLVIII. Die niedrigen ersten Ionisierungsenergien β-trimethylsilyl-substituierter ether ROCH3-n[Si(CH3)3]n und ROSi[Si(CH3)3]3

Abstract The first vertical ionization energies of dialkylethers are lowered up to 3 eV (!) by β-trimethylsilyl substituents and, therefore, further confirm the powerful electron donor effect of CH 3- n [Si(CH 3 ) 3 ] n and Si[Si(CH 3 ) 3 ] 3 groups. The gas-phase photoelectron spectra are assigned based on geometry-optimized MNDO calculations and the substituent effects are discussed in terms of conformationally dependent hyperconjugative second order perturbations