Larissa A. Leites

One-electron-mediated rearrangements of 2,3-disiladicarbene.

A disiladicarbene, (Cy-cAAC)2Si2 (2), was synthesized by reduction of Cy-cAAC:SiCl4 adduct with KC8. The dark-colored compound 2 is stable at room temperature for a year under an inert atmosphere. Moreover, it is stable up to 190 °C and also can be characterized by electron ionization mass spectrometry. Theoretical and Raman studies reveal the existence of a Si═Si double bond with a partial double bond between each carbene carbon atom and silicon atom. Cyclic voltammetry suggests that 2 can quasi-reversibly accept an electron to produce a very reactive radical anion, 2(•-), as an intermediate species. Thus, reduction of 2 with potassium metal at room temperature led to the isolation of an isomeric neutral rearranged product and an anionic dimer of a potassium salt via the formation of 2(•-).

Two Modifications Formed by "Sulflower" C16S8 Molecules, Their Study by XRD and Optical Spectroscopy (Raman, IR, UV-Vis) Methods

Sublimation of sulflower, octathio[8]circulene C 16S 8 ( 1), on heating under high vacuum ( approximately 10 (-5) Torr) leads to successive formation of two modifications: a white film ( 1W) and a red polycrystalline solid ( 1R). When kept at room temperature for several weeks, 1W spontaneously turns pink, reflecting the monotropic phase transition 1W --> 1R. The accurate molecular and crystal structure of 1R has been studied using low-temperature (100 K) high-resolution single crystal X-ray analysis. The C 16S 8 molecule in crystal is strictly planar with nearly equalized bonds of each type (C-C, C-S, and CC). The point symmetry group of the free molecule is D 8 h , and the crystal space group is P2 1/ n. These data allowed group-theoretical analysis of vibrational normal modes to be accomplished. Investigation of the charge density distribution of 1R including Baders AIM approach has revealed rather strong intermolecular S...S, S...C, and C...C interactions of charge transfer and pi-stacking types with overall lattice energy of 28.5 kcal/mol. The charge transfer due to the S...S interactions is the reason for the red coloration of 1R. The latter is reflected by its UV-vis spectrum exhibiting absorption bands in the visible region which are absent from that of 1W. Both modifications were studied comparatively by vibrational (Raman, IR) and electronic spectroscopies as well as XRD powder diffraction. All the results obtained are fully consistent and show that 1W is much less ordered than 1R with significantly weakened intermolecular interactions. Rationalizing of these results has led to an idea that 1W could be soluble, in contrast to 1R. Indeed, 1W appeared soluble in common solvents; this finding opens the way to the study of the chemistry of 1 and investigation of its electrooptical properties.

Norbornadiene complexes of transition metals: III. Stereospecificity in the generation of cationic complexes [Rh(2,3,8 : 5,6-η-C7H7CHR)(η-C5H5)]+ (R = Me, Ph, [Rh(2,3,8 : 5,6-η-C7H7CH2)(η-C5H5)]+PF6t-

Abstract Treatment of (2-hydroxymethylnorbornadiene)cyclopentadienylrhodium (V) with sulfuric acid offers the stable cationic complex XI, which can be isolated as PF 6 − or BF 4 − . 1 H and 13 C NMR data indicate that in cation XI the bicyclic ligand is bound to the metal via η 2 -ethylene and η 3 -allyl bonds, including an exocyclic methylene carbon. The structure of complex XI is supported by a single-crystal X-ray diffraction study of its PF 6 − salt. Reaction of diastereomeric carbinols ψ- exo -VI and ψ- endo -Rh {η 4 -C 7 H 7 CH(OH)Me} (η 4 -C 5 H 5 ) (VII) proceeds with absolute stereospecificity to form syn -XII and anti -[Rh(η 5 -C 7 H 7 CHMe)(η 5a -C 5 H 5 )] + (XIII) isomers, respectively. Similarly, cation [Rh(η 5 -C 7 H 7 CHPh)(η 5 -C 5 H 5 )] + (XV), derived from ψ- endo -Rh {η 4 -C 7 H 7 CH(OH)Ph}(η 5 -C 5 H 5 ) (IX), has the syn configuration. However, the same reaction of the ferrocenyl-substituted carbinol ψ- endo -Rh {η 4 -C 7 H 7 CH(OH)Fc} (η 5 -C 5 H 5 ) (X) leads to mixture of two isomeric complexes, syn -(XVI) and anti -[Rh(η 5 -C 7 H 7 CHFc)(η 5 -C 5 H 5 )] + (XVII), in a ratio of 4 1 . The absolute stereospecificity and the relative facility with which the formation of cationic η 3 -allylic complexes from carbinols VI, VII and IX takes place are due to the nucleophilic participation of a rhodium atom in stabilizing the electron-deficient center during the reaction. The disturbance of stereospecificity during the conversion of carbinol X to a cationic complex assumes the formation of an α-ferrocenylcarbenium ion (XIX) as an intermediate (a product of kinetic control). Owing to the relative non-hindrance of rotation around the exocyclic carboncarbon bond C(2)—C(8) in the carbenium ion XIX, the formation of a non-equal mixture of complexes XVI and XVII, the products of thermodynamic control of the reaction, takes place.

Vibrational spectra and structure of cesium salts of icosahedral monocarba-closo-dodecarborate anion, [CB11H12]–, and its nido-derivative, [CB100H13]–

The Raman and IR spectra of the cesium salts of monocarbon carboranes, [closo-CB11H12]– and [nido-CB10H13]–, are reported and the assignment of the normal modes is given. Quantum-chemical calculations of the geometry of undistorted closo-anions B12H122– and CB11H12– were carried out and normal coordinate analysis for the latter was performed. Structural parameters and spectral characteristics of isoelectronic closo-polyhedra [B12H12]2–, [CB11H12]–, and p-C2B10H12 and those of the closo- and nido-structures were compared.

Raman evidence of aromaticity of the thermally stable silylene (tBuNCHCHNtBu)Si

Abstract The Raman, IR and UV spectra of the silylene ( t BuNCH CHN t Bu)Si: ( 1 ) as well as the Raman spectra of related compounds with tetravalent silicon ( t BuNCH CHN t Bu) 2 Si ( 2 ) and ( t BuNCH CHN t Bu)SiCl 2 ( 3 ) and of the heterocycle ( t BuNCH CHN t Bu)C O ( 4 ) are reported. Comparison of the spectra obtained for 1 – 3 reveals a lowered frequency and greatly enhanced intensity of the Raman band corresponding to the C C stretching vibration in the spectrum of 1 which is in accord with the aromatic character of the silylene.

Excitation dependence of Raman spectra of various polydialkylsilane conformations and σ-σ conjugation

Abstract Excitation dependence of the Raman spectra in a wide region (from blue to infrared) was investigated for hmw polysilanes [ n Hex 2 Si] n and [ n Pent 2 Si] n at various temperatures. The results obtained along with analogous data for other polydialkylsilanes lead to the conclusion that among all modifications formed by these polymers, there is only one that exhibits pre-resonance Raman scattering, that is, intensity enhancement and its excitation dependence. This is the crystalline modification with the σ–σ conjugated all- A ( anti ) conformation of the silicon backbone. The Raman spectrum of this modification is enhanced by pre-resonance even when ( ν e − ν ), the difference between the frequency of the lowest energy σ–σ* electronic transition ν e and that of irradiating light ν is as much as ∼17 300 cm −1 , whereas Raman spectra of less-ordered modifications do not exhibit such enhancement even at much smaller ( ν e − ν ) values, e.g. ∼9400 cm −1 . These facts are discussed in terms of Shorygins semi-classical treatment of Raman intensity problem. The Raman patterns observed below 800 cm −1 allow clear distinction between polysilane modifications with all- A , helical (all- D ) and TGTG ′ ( AD + AD − ) main chain conformations, whereas helical (all- D ) and disordered conformations are hardly distinguishable.

Molecular Structures of N,N′-Dimethylbenzimidazoline-2-germylene and -stannylene in Solution and in Solid State by Means of Optical (Raman and UV–vis) Spectroscopy and Quantum Chemistry Methods

X-ray data obtained for germylene 1 evidence its monomeric structure, unlike that of stannylene 2, which had been shown previously to form a coordination dimer. Raman spectra of solid and liquid 1 are identical, whereas the Raman spectra of solid 2 and its solution 2a differ significantly. The spectrum of 2 is complicated and contains the lines corresponding to N → Sn coordination bonds forming a dimer. The spectrum of 2a is simpler and close to that of monomeric 1, thus pointing to the rupture of the dimer in solution. The UV-vis spectrum of solid 2 exhibits a band corresponding to a transition involving the N → Sn coordination bonds. Quantum theory of atoms in molecules data estimate the energy of this bond as ∼19 kcal/mol. The aromaticity of 1 and 2 with their 10 π-electron systems including divalent Ge or Sn atoms is confirmed by negative nucleus-independent chemical shift values.

Cationic hydroxyallylolefin rhodium complexes [Rh(η2, 3-C7H6-3-R-2-CR1OH)(η5-Cp)]+PF6− (R, R1 = H, Alk, Cp = C5H5, C5H4Me, C5Me5) with intramolecular O-H ⋯ Rh hydrogen bonds. Formation of {[CpRh(η2, 3-C7H6-3-R-2-CR1OH)]2-F2POO}+PF6− dimers as a result of partial hydrolysis of the PF6− anion

Abstract Protonation of the 2-acyl derivatives of Rh(η4-norbornadiene)(η5-Cp) (Cp = C5H5, C5H4Me, C5Me5) by HPF6 in diethyl ether yields the stable cationic O-protonated complexes [Rh(η2, 3-C7H6-3-R-2-CR1 OH)(η5-Cp)]+PF6− (R, R1 = H, Me), whose structure was established by IR, 1H and 13C NMR spectroscopy. The formation of the intramolecular hydrogen bond O-H ⋯ Rh was found on the basis of IR and 1H NMR spectra of the cations. CH2Cl2 solutions of the cationic complexes that contained traces of moisture caused the partial hydrolysis of the PF6− anions, to give the novel “dimeric” complexes, {[CpRh(η2, 3-C7H6-3-R-2-CR1 OH}2F2P(O)O}+PF6−. The F2P(O)O group of this complex is involved in the strong intermolecular hydrogen bond, O-H ⋯O-P(F2)-O ⋯ H-O. An X-ray diffraction study has been carried out on one of the “dimeric” cations (R = Me, R1 = H).

Conformational polymorphism of solid tetramesityldisilene Mes2Si=SiMes2 (Raman, UV-vis, IR and fluorescence study)

Conformational polymorphism of solid tetramesityldisilene (1) has been studied by the methods of optical spectroscopy. The three known modifications of 1: orange unsolvated 1a and two 1:1 solvates with toluene (1b) and THF (1c) have been found to transform under specific conditions to a new, most thermodynamically stable polymorph, yellow unsolvated powder 1d. The latter has been characterized by the Raman, IR, UV-vis and fluorescence data. All forms of 1 exhibit Raman spectra differing in details, which reflect their different crystal and molecular structures. Unsolvated 1a and 1d differ significantly in electronic absorption and fluorescence emission. The yellow form 1d can be converted to the orange form 1a upon illumination with laser light in the region 514-457 nm. Similarity of the Raman and UV-vis spectra of 1d to those of the solutions of 1 provides some evidence for a quasi-trans conformation of 1d.

VIBRATIONAL SPECTRA OF DISILENES. NORMAL COORDINATE ANALYSIS AND IDENTIFICATION OF THE V(SI=SI) STRETCHING VIBRATION

Abstract Experimental vibrational spectra are reported for three carbon-substituted disilenes along with data on normal coordinate analysis of several simplified disilene models. Calculations show that there is no normal mode in the disilene spectrum which is well-localized in the Si=Si bond. The Si=Si and Si-C stretching coordinates are heavily mixed. Their in-phase combination results in a normal mode with frequency in the region 450–550 cm −1 which manifests itself as a Raman line of variable intensity (the so-called v (Si=Si)). Their out-of-phase combination gives a mode in the region of about 700 cm −1 which shows itself as an intense Raman line (the so-called v S (Si-C)). Vibrations of an aromatic ring, if present in a disilene molecule, can also couple with the v (Si-Si). Particular eigenvector elements of the two modes in question and thus their potential energy distributions strongly depend on the structure and symmetry of a given disilene molecule. However, the contribution of the Si=Si stretching coordinate to the normal coordinate with frequency in the region 450–550cm −1 is always significant, justifying the designation of this mode as the v (Si=Si).