Holger Fleischer

Molecular “Floppyness” and the Lewis Acidity of Silanes: A Density Functional Theory Study

A comprehensive set of Lewis acid-base adducts of silanes was investigated by means of the density functional theory geometry optimization [B3LYP/6−31G(d)], and thermochemical calculations, [B3LYP/6−311+G(2d,p)//B3LYP/6−31G(d)]. Complex formation was found to weaken Si−Cl and Si−Br bonds more than Si−F or Si−H bonds. Comparable distances between Si and a Lewis base L (L = NH3, OH2, F−) are shorter in hexa- than in pentacoordinated complexes. The molecular structures of the pentacoordinated Si complexes allowed for a mapping of an SN2 reaction pathway by correlating the lengths of the Si−X and Si−L bonds. Complex formation was found to be exothermic for most of the coordination compounds, and the analysis of the natural atomic charges revealed a high ionic character of the dative bonds. Formation of the anionic complexes and of SiH2X2(py)2 is most exothermic with X = Cl or Br and otherwise most exothermic with X = F. Only small enthalpy differences were found between the trans and cis configurations of SiF4(py)2. The standard free enthalpy of complex formation is negative only for complexes between halosilanes and F−, i.e. all other silane Lewis base adducts are thermodynamically unstable under standard conditions with respect to dissociation. It is inferred that the existence of some of the silicon complexes in the solid state or in solution is caused by stabilizing intermolecular forces, and silanes are classified as very weak Lewis acids. The thermochemistry of complex formation was analyzed in terms of molecular “floppyness” of the silanes and the energy of interaction between the deformed silane and the Lewis base. The enhanced complex stability of SiCl4(py)2 compared to SiH4(py)2 and of GeF4(NH3), [GeF5]− and [GeF6]2− compared to the analogues silane adducts does not result from a stronger Lewis acid base interaction but from an increased “floppyness” of SiCl4 and GeF4 compared to SiH4 and SiF4, respectively.

Experimental investigations and ab initio studies of selenium(II) dialkanethiolates, Se(SR)2

Selenium(II) dimethanethiolate, Se(SMe)(2), was synthesized by reaction of SeO(2) with HSMe. Basic spectroscopic data for Se(SMe)(2) and selenium(II) bis(2-methyl-2-propanethiolate), Se(S(t)Bu)(2), were recorded and interpreted with the support of ab initio calculations. Both compounds are thermodynamically unstable relatively to selenium and the corresponding disulfide. The UV/vis spectra of both compounds are qualitatively similar, the two bands being attributed to n(Se)-sigma*(Se-S) transitions. The bands at 369 and 397 cm(-1) in the IR spectra of Se(SMe)(2) and Se(S(t)Bu)(2), respectively, are assigned to nu(as)(SeS(2)). The (77)Se NMR shifts of Se(SMe)(2)(784 ppm) and Se(S(t)Bu)(2)(556 ppm) differ substantially from each other and show positive temperature gradients. Calculations at the GIAO-HF/962+(d) level reproduced the difference of the (77)Se NMR chemical shifts between Se(SMe)(2) and Se(S(t)Bu)(2). At the same level, the effect of conformational changes on (77)Se shifts were studied for Se(SMe)(2). In the solid state Se(SMe)(2) forms long intermolecular SeS contacts while Se(S(t)Bu)(2) does not. Both compounds exhibit anti-conformations of the methyl and tert-butyl groups with respect to the SeS(2) plane. MP2/LANL2DZ(d) geometry optimizations, single point energy and frequency calculations performed for Se(SMe)(2) show, that syn- (C(s)) and anti-conformers (C(2)) represent minima on the potential energy surface, the latter being by 8 kJ mol(-1) lower in energy than the former. Both conformers are stabilized by intramolecular pi-type n(S(1))-sigma*(Se-S(2)) orbital interactions. The energy of the transition state for the mutual conversion of the two conformers was calculated to be 31 kJ mol(-1) above that of the syn conformer, allowing a rapid interconversion of the two conformers at room temperature. Intermolecular interactions between Se(SMe)(2) molecules were also studied by means of calculations at the MP2/LANL2DZ(d) level. For Se(S(t)Bu)(2) MP2/LANL2DZ(d) geometry optimizations and single point energy calculations revealed a C(2)-symmetric anti- and a C(1) symmetric syn-conformer, the latter being 21 kJ mol(-1) higher in energy than the former. Se(SMe)(2) and Se(S(t)Bu)(2) exchange thiolate groups with other selenium(II) dithiolates, tellurium(II) dithiolates and with thiols, if catalytic amounts of p-CH(3)C(6)H(4)SO(3)H are added.

A quantum-chemical study of the structure, vibrations and SiH bond properties of disilylamine, NH(SiH3)2

Quantum-chemical calculations at HF, MP2 and B3LYP levels with 6-31G* and 6-311G** basis sets are reported for disilylamine, NH(SiH3)2. The equilibrium structure is found to vary with both level and basis set, all but one of the structures exhibiting a small lack of planarity of the HNSi2 system. The barrier to inversion, however, is found to be very low, at most 38 cm(-1). Vibration frequencies and intensities are calculated. The frequencies are scaled, where possible, either using updated infrared data or with the aid of factors transferred from N(CH3)(SiH3)2. Unobserved frequencies due to the v(s)NSi2, deltaNSi2 and delta(perpendicular)NH modes are predicted near 610, 210 and 360 cm(-1), respectively. The lower silyl torsion lies below 40 cm(-1). The appearance of a single broad vSiH band in gas-phase samples of both NH(SiH3)2 and NH(SiH3)(SiD3) is suggestive of signal averaging due to internal rotation. The frequencies v(is)SiH, infrared intensities and Raman scattering activities of the bands due to an isolated SiH bond in an otherwise deuterated species are calculated and correlated with the torsional angle of this bond and with the Mulliken charge on the hydrogen atom. The strength of the bond is a minimum, and the infrared intensity and Raman scattering activity are maxima, when the bond direction is roughly orthogonal to the skeletal plane. A major part of the frequency and intensity variations is attributed to n(p)(N)-sigma*(Si-H)) hyperconjugation which, NBO calculations show, reaches a maximum for this conformation. However, systematic smaller variations are found for SiH bonds lying in the skeletal plane, which reflect the proximity of the other silyl group and only partly correlate with Mulliken charge. vSiH-vSiH interaction force constants, f, are calculated for pairs of SiH bonds in different silyl groups and compared with the corresponding dipole-dipole potential energy, the latter calculated using a classical treatment of the interaction between point dipoles arising from delta mu/delta r for the SiH bonds involved. The gradient of the correlation is very close to that expected from the theory, but a negative intercept indicates the presence of additional factors.

Halogen exchange and expulsion: ligand stabilized dihalogen silicon dications

The first ligand stabilized SiCl22+ dications were synthesized using N-methylimidazole as co-ordinating ligand. The compounds SiCl4, SiBr2Cl2, and SiH2Cl2 form six-co-ordinated dicationic compounds of almost octahedral symmetry with similar structures which were investigated by single crystal X-ray analysis and density functional calculations. The structures exhibit particularly short dative Si–N bonds of about 1.90 A. Complexes crystallized from the same solvent are isostructural. A different solvent, though, leads to geometrical variations. It was also discovered that the halogen exchange process among mixed silicon tetrahalides occurs under much milder conditions than previously thought and proceeds with considerable speed even without a catalyst.

BIS(DICHLOROSILYL)METHYLAMINE : SYNTHESIS, CRYSTAL STRUCTURE, AND CONFORMATIONAL ANALYSIS IN THE GAS PHASE

A straightforward preparation has been found for bis(dichlorosilyl)methylamine, (SiHCl2)2NMe (1), involving reaction between H2NMe and an excess of SiHCl3, dissolved either in pentane or THF at 253u200aK. 1 and a side-product, 1,3,5-trichloro-2,4,6-trimethylcyclotrisilazane, (–SiHCl–NMe–)3 (2), were identified by elemental analysis, mass spectrometry and 1H-NMR-spectroscopy. Some physical, NMR- and IR spectroscopical properties of 1 were determined. The molecular and crystal structure of 1 was investigated by single crystal X-ray diffraction. Selected structural parameters: r(Si–N) 169.7(5), r(Si–Cl) 203.1(2)–204.4(2), r(C–N) 150.0(8)u200apm; a(SiNSi) 123.6(3), a(SiNC) 118.3(4)/118.0(4)°. Ab initio force field data and infrared intensities were calculated for four conformers of 1. Comparison of the observed and calculated IR spectra favours the two structures found ab initio provided that their actual abundancies are different from those calculated. n n n nBis(dichlorosilyl)methylamin – Synthese, Kristallstruktur und Konformationsanalyse in der Gasphase n n n nBis(dichlorsilyl)methylamin, (SiHCl2)2NMe (1), kann durch Reaktion von H2NMe bei 253u200aK mit einem Uberschus an SiHCl3 in Pentan oder THF als Losungsmittel dargestellt werden. 1 und ein Nebenprodukt, 1,3,5-Trichlor-2,4,6-trimethylcyclotrisilazan, (–SiHCl–NMe–)3, 2, wurden elementaranalytisch, massenspektrometrisch und durch 1H-NMR Spektroskopie identifiziert. Von 1 wurden einige physikalische, kernresonanz- und infrarotspektroskopische Eigenschaften sowie seine Molekul- und Kristallstruktur durch Einkristallrontgenbeugung bestimmt. Ausgewahlte Strukturparameter: r(Si–N) 169.7(5), r(Si–Cl) 203.1(2)–204.4(2), r(CN) 150.0(8)u200apm; a(SiNSi) 123.6(3), a(SiNC) 118.3(4)/118.0(4)°. Fur vier verschiedene Konformationen von 1 wurden die Ab initio-Kraftfelder und infrarotspektroskopische Intensitaten berechnet. Der Vergleich des experimentellen mit den berechneten IR-Spektren begunstigt die beiden durch Ab initio-Berechnungen gefundenen Strukturen, vorausgesetzt deren tatsachliche, relative Haufigkeiten unterscheiden sich von den berechneten.

Infrared and quantum-chemical studies of the structure and vibrations of methyldisilylamine

Abstract Infrared spectra are reported from labelled species of methyldisilylamine (N(CH 3 )(SiH 3 ) 2 ) in the gas and solid phase. Quantum-chemical (QC) calculations of structure and force field have been carried out at HF, MP2 and B3LYP levels using 6-31G ∗ and 6-311G ∗∗ basis sets. The equilibrium structure belongs to the point group C s with the plane of symmetry at right angles to the NSi 2 angle and a slight non-planarity of the CNSi 2 moiety ( C s ,⊥ ). Spectra in the ν SiH region however suggest an effective C 2 v structure, with two distinct types of SiH bond. Two strong SiH bonds lie in the skeletal plane, eclipsing each other. The single ν is CH band in the CHD 2 species suggests the presence of signal averaging due to internal rotation of the methyl group. The QC calculations indicate the presence of a lower barrier to internal rotation for the methyl group than for the silyl one. Scaled QC-based force fields lead to a number of reassignments of previous Raman and infrared spectra. However, difficulty remains in interpreting the region below 200xa0cm −1 where out-of-plane skeletal bending and torsional modes are expected. In the scaling procedure, a maximum number (19) of factors for different types of motion was determined. Significant variations are found in these factors for similar types of coordinate. Symmetric SiN stretching requires a factor ∼7% greater than that for asymmetric SiN stretching. The spread of these 19 factors is similar in each force field calculation. This indicates that DFT-based force fields require as much careful scaling as do HF or MP2 ones. However, the average DFT factor lies closest to unity.

Experimental and theoretical studies of the molecular and crystal structures of trialkoxy- and chlorodialkoxy-stibanes

The molecular structures of triisopropoxystibane, Sb(OiPr)3, and chlorodiisopropoxystibane, SbCl(OiPr)2, were determined in the solid state by single crystal X-ray diffraction. Sb(OiPr)3 forms discrete centrosymmetric dimers in the solid state via Sb⋯O–Sb interactions, leading to pseudo trigonal bipyramidal configurations of the four co-ordinate Sb atoms, while SbCl(OiPr)2 forms chains via Sb⋯O–Sb and Sb⋯Cl–Sb bridges, resulting in five-co-ordinate Sb atoms with pseudo octahedral configurations. Comparison of the solid state structures and the density functional optimized molecular structures of Sb(OMe)3, SbCl(OMe)2 and their dimers revealed a steady increase of the average Sb–O bond lengths with the co-ordination number of Sb, and mutual trans effects of the ligands. Standard enthalpies of dimer formation from density functional calculations are −23.8 and −69.7 kJ mol−1 for [Sb2(μ-OMe)2(OMe)4] and [Sb2Cl2(μ-OMe)2(OMe)2], respectively, and −42.7 kJ mol−1 for [Sb2(μ-Cl)2(OMe)4]. A natural bond orbital analysis reveals that n(O)–σ*(Sb–O) and n(O)–σ*(Sb–Cl) interactions are the main contributions to the inter-monomer bonding in the O-bridged dimers, of Sb(OMe)3 and SbCl(OMe)2, respectively, while n(Cl)–σ*(Sb–O) plays no significant role in the Cl-bridged dimer of SbCl(OMe)2. IR and Raman spectra of Sb(OiPr)3 indicated molecular association in the solid and liquid phase, but dissociation into monomers in non-polar solvents.

Kinetics and mechanism of the nucleophilic substitution of tellurium(II) dialkanethiolates, Te(SR1)2 with thiols, HSR2

The equilibrium reaction between tellurium(II) dithiolates and thiols, Te(SR1)2 + 2 HSR2 ⇌ Te(SR2)2 + 2 HSR1 was studied by means of 1H- and 125Te NMR spectroscopy and ab initio quantum chemical methods. It was found that the reaction is catalyzed by Brønsted acids and bases, the catalytic activity corresponding to the strength of the respective acid or base. Investigation of the initial step of the reaction, Te(SR1)2 + HSR2 ⇌ Te(SR1)(SR2) + HSR1, showed it to proceed according to first order kinetics for Te(SR1)2, HSR2 and for the catalyst. Ab initio geometry optimizations and frequency calculations suggest [Te(SR1)(HSR1)(HSR2)]+ and [Te(SR1)2(SR2)]− to be stable intermediates and not transition states in the acid and base catalyzed reactions, respectively. The reaction hence proceeds via an additional elimination rather than an S N 2 mechanism. The catalytic activity displayed by acids and bases can be applied to reduce the temperature in synthesis of thermally labile tellurium(II) dithiolates.

trans-Bis(1H-benz­imidazole-1-thione-κS)­tetra­chloro­tellurium methanol disolvate

The Te atom in the title complex, trans-[TeCl4(C6H4N2H2CS)2]·2CH4O or C16H20Cl4N4O2S2Te, occupies a special position at a crystallographic inversion centre and has an octahedral coordination formed by four chloro ligands and the S atoms of two benzxadimidazole–thione molxadecules. The hydrogen-bond system involving the disordered solvent methanol molxadecules links the tellurium complexes into the infinite two-dimensional aggregates in the crystal.

Pyridinium d(+)-10-camphor­sulfonate hemihydrate

In the crystal structure of pyridinium d(+)-10-camphorxadsulfonate hemihydrate, C5H6N+·C10H15O4S−·0.5H2O, a water molxadecule lying on a twofold axis serves as a donor of two hydrogen bonds, thus linking two camphorxadsulfonate anions. Each anion in its turn acts as a hydrogen-bond acceptor for the NH group of a pyridinium cation.