Johann Hlina

Coordination Chemistry of Disilylated Germylenes with Group 4 Metallocenes.

Reaction of the PEt3 adduct of a disilylated five-membered cyclic germylene with group 4 metallocene dichlorides in the presence of magnesium led to the formation of the respective germylene metallocene phosphine complexes of titanium, zirconium, and hafnium. Attempts to react the related NHC adduct of a disilylated four-membered cyclic germylene under the same conditions with Cp2TiCl2 did not give the expected germylene NHC titanocene complex. This complex was, however, obtained in the reaction of Cp2Ti(btmsa) with the NHC germylene adduct. A computational analysis of the structure of the group 4 metallocene germylene complexes revealed the multiple-bond character of the M–Ge(II) linkage, which can be rationalized with the classical σ-donor/π-acceptor interaction. The strength of the M–Ge(II) bond increases descending group 4.

Formation and Properties of a Bicyclic Silylated Digermene

In the presence of PMe3 or N-heterocyclic carbenes, the reaction of oligosilanylene dianions with GeCl2⋅dioxane gives germylene–base adducts. After base abstraction, the free germylenes can dimerize by formation of a digermene. An electrochemical and theoretical study of a bicyclic tetrasilylated digermene revealed formation of a comparably stable radical anion and a more reactive radical cation, which were characterized further by UV/Vis and ESR spectroscopy.

Basic Reactivity Pattern of a Cyclic Disilylated Germylene

In order to estimate the reactivity of disilylated germylene phosphine adducts, a cyclic version of this compound class was reacted with a number of different reagents. Reactions with the chalcogens sulfur, selenium, and tellurium led to dimers of the heavy ketone analogues. Reactions with water and ethyl bromide proceeded to give the respective oxidized germanol and germyl bromide. Two different reactions with alkynes were observed which led either to a germacyclopropene, by addition of tolane to the germylene, or to a silagermacyclobutene, likely formed by addition of the alkyne across a silagermene. Reaction via the silagermene was also observed in the reaction with benzophenone. Reaction of a germylene phosphine adduct with GeCl2·(dioxane) provided insertion of the silylated germylene into a Ge–Cl bond, leading to a germylated chlorogermylene phosphine adduct.

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.

Catenated Compounds – Group 14 (Ge, Sn, Pb)

This chapter discusses compounds containing Ge–Ge, Sn–Sn, or Pb–Pb bonds. In the cases of germanium and tin, arrays ranging from small oligomers to long-chain polymers are known. In contrast, there are only a few known compounds constructed with more than one Pb–Pb bond. Additional discussion is devoted to homo- and heterocyclic compounds, cage-type molecules, and molecular clusters (excluding Zintl-type anions) of the three elements.

Neutral "Cp-Free" Silyl-Lanthanide(II) Complexes: Synthesis, Structure, and Bonding Analysis.

Complexes featuring lanthanide silicon bonds represent a research area still in its infancy. Herein, we report a series of Cp-free lanthanide (+II) complexes bearing σ-bonded silyl ligands. By reactions of LnI2 (Ln = Yb, Eu, Sm) either with a 1,4-oligosilanyl dianion [K-Si(SiMe3)2SiMe2SiMe2Si(SiMe3)2-K)] (1) or with 2 (Me3Si)3SiK (3) the corresponding neutral metallacyclopentasilanes ({Me2Si(Me3Si)2Si}2)Ln·(THF)4 (Ln = Yb (2a), Eu (2b), Sm (2c)), or the disilylated complexes ({Me3Si}3Si)2Ln·(THF)3 (Ln = Yb (4a), Eu (4b), Sm (4c)), were selectively obtained. Complexes 2b, 2c, 4b, and 4c represent the first examples of structurally characterized Cp-free Eu and Sm complexes with silyl ligands. In both series, a linear correlation was observed between the Ln–Si bond lengths and the covalent radii of the corresponding lanthanide metals. Density functional theory calculations were also carried out for complexes 2a–c and 4a–c to elucidate the bonding situation between the Ln(+II) centers and Si.

Open-Shell Lanthanide(II+) or -(III+) Complexes Bearing σ-Silyl and Silylene Ligands: Synthesis, Structure, and Bonding Analysis

Complexes featuring lanthanide (Ln)–Si bonds represent a highly neglected research area. Herein, we report a series of open-shell LnII+ and LnIII+ complexes bearing σ-bonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands. The reactions of the LnIII+ complexes Cp3Ln (Ln = Tm, Ho, Tb, Gd; Cp = cyclopentadienide) with the 18-crown-6 (18-cr-6)-stabilized 1,4-oligosilanyl dianion [(18-cr-6)KSi(SiMe3)2SiMe2SiMe2Si(SiMe3)2K(18-cr-6)] (1) selectively afford the corresponding metallacyclopentasilane salts [Cp2Ln({Si(SiMe3)2SiMe2}2)]−[K2(18-cr-6)2Cp]+ [Ln = Tm (2a), Ho (2b), Tb (2c), Gd (2d)]. Complexes 2a–2d represent the first examples of structurally characterized Tm, Ho, Tb, and Gd complexes featuring Ln–Si bonds. Strikingly, the analogous reaction of 1 with the lighter element analogue Cp3Ce affords the acyclic product [Cp3CeSi(SiMe3)2SiMe2SiMe2Si(SiMe3)2-Cp3Ce]2–2[K(18-cr-6)]+ (3) as the first example of a complex featuring a Ce–Si bond. In an alternative synthetic approach, the aryloxy-functionalized benzamidinato NHSi ligand Si(OC6H4-2-tBu){(NtBu)2CPh} (4a) and the alkoxy analogue Si(OtBu){(NtBu)2CPh} (4b) were reacted with Cp*2Sm(OEt2), affording, by OEt2 elimination, the corresponding silylene complexes, both featuring SmII+ centers: Cp*2Sm ← :Si(O–C6H4-2-tBu){(NtBu)2CPh} (6) and Cp*2Sm ← :Si(OtBu){(NtBu)2CPh} (5). Complexes 5 and 6 are the first four-coordinate silylene complexes of any f-block element to date. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analysis. In the series 2a–2d, a linear correlation was observed between the Ln–Si bond lengths and the covalent radii of the corresponding Ln metals. Moreover, in complexes 5 and 6, notably long Sm–Si bonds are observed, in accordance with a donor–acceptor interaction between Si and Sm [5, 3.4396(15) Å; 6, 3.3142(18) Å]. Density functional theory calculations were carried out for complexes 2a–2d, 5, and 6 to elucidate the bonding situation between the LnII+ or LnIII+ centers and Si. In particular, a decrease in the Mayer bond order (MBO) of the Ln–Si bond is observed in the series 2a–2d in moving from the lighter to the heavier lanthanides (Tm = 0.53, Ho = 0.62, Tb = 0.65, and Gd = 0.75), which might indicate decreasing covalency in the Ln–Si bond. In accordance with the long bond lengths observed experimentally in complexes 5 and 6, comparatively low MBOs were determined for both silylene complexes (5, 0.24; 6, 0.25) .

σ-Bond Electron Delocalization in Oligosilanes as Function of Substitution Pattern, Chain Length, and Spatial Orientation

Polysilanes are known to exhibit the interesting property of σ-bond electron delocalization. By employing optical spectroscopy (UV-vis), it is possible to judge the degree of delocalization and also differentiate parts of the molecules which are conjugated or not. The current study compares oligosilanes of similar chain length but different substitution pattern. The size of the substituents determines the spatial orientation of the main chain and also controls the conformational flexibility. The chemical nature of the substituents affects the orbital energies of the molecules and thus the positions of the absorption bands.

Using Functionalized Silyl Ligands To Suppress Solvent Coordination to Silyl Lanthanide(II) Complexes

The reaction of the potassium 1,3-trisilanediide Me2Si[Si(Me3Si)2K]2 with SmI2 and YbI2 was found to give the respective disilylated complexes Me2Si[Si(Me3Si)2]2Sm·2THF and Me2Si[Si(Me3Si)2]2Yb·2THF. Desolvation of coordinated solvent molecules in these complexes made their handling difficult. However, using a number of functionalized silanide ligands, complexes with a diminished number or even no coordinated solvent molecules were obtained ((R3Si)2Ln(THF)x (x = 0–3)). The structures of all new lanthanide compounds were determined by X-ray single-crystal structure analysis. NMR spectroscopic analysis of some Yb–silyl complexes pointed at highly ionic interactions between the silyl ligands and the lanthanides. This bonding picture was supported by DFT calculations at the B3PW91/Basis1 level of theory. Detailed theoretical analysis of a disilylated Eu(II) complex suggests that its singly occupied molecular orbitals (SOMOs) are very close in energy to the ligand silicon lone pairs (HOMO), and SQUID magnetometry measurements of the complex showed a deviation from the expected behavior for a free Eu(II) ion, which might be due to a ligand–metal interaction.