ABSTRACT
By sulfurization of phosphaalkenes (a) either (σ(3),λ(5))-phosphoranes (b) or (σ(3),λ(3))-thiaphosphiranes (c) are formed. In this study, Density Functional Theory (DFT) and coupled cluster (CCSD(T)) calculations have been carried out for model and experimental structures of (σ(3),λ(5))-phosphoranes and (σ(3),λ(3))-thiaphosphiranes to elucidate the factors influencing relative stabilities of b and c. According to the results of quantum chemical calculations, sterically bulky substituents make the phosphorane form more favored. Conversely, electronic effects of the most substituents provide higher stability for thiaphosphirane isomers. The only exception has been found in the cases where the substituent at the phosphorus atom possesses π-donor and σ-acceptor properties (e.g., in the case of amino group) and the substituents at carbon atom exhibit σ-donor/π-acceptor effects (e.g., silyl groups). The stability of the cyclic form c decreases further, if the substituents at the carbon atom are amino groups. In this case, a quite unusual structure has been theoretically predicted, which is considerably different from those of the hitherto known phosphoranes. It indicates a pyramidal configuration at the phosphorus atom and can be conventionally presented as a donor-acceptor adduct of diaminocarbene with thioxophosphine.
Subject(s)
Alkenes/chemistry , Phosphoranes/chemistry , Sulfur Compounds/chemistry , Models, Molecular , Phosphines/chemistry , Quantum TheoryABSTRACT
The salt (eta(5)-pentamethylcyclopentadienyl)silicon(II) tetrakis(pentafluorophenyl)borate (5) reacts at -78 degrees C with lithium bis(trimethylsilyl)amide in dimethoxyethane (DME) as solvent to give quantitatively the compound [bis(trimethylsilyl)amino][pentamethylcyclopentadienyl]silicon(II) 6A in the form of a colorless viscous oil. The reaction performed at -40 degrees C leads to the silicon(IV) compound 7, the formal oxidative addition product of 6A with DME. Cycloaddition is observed in the reaction of 6A with 2,3-dimethylbutadiene to give the silicon(IV) compound 8. Upon attempts to crystallize 6A from organic solvents such as hexane, THF, or toluene, the deep yellow compound trans-1,2-bis[bis(trimethylsilyl)amino]-1,2-bis(pentamethylcyclopentadienyl)disilene (6B), the formal dimer of 6A, crystallizes from the colorless solution, but only after several days or even weeks. Upon attempts to dissolve the disilene 6B in the described organic solvents, a colorless solution is obtained after prolonged vigorous shaking or ultrasound treatment. From this solution, pure 6A can be recovered after solvent evaporation. This transformation process can be repeated several times. In a mass spectroscopic investigation of 6B, Si=Si bond cleavage is observed to give the molecular ion with the composition of 6A as the fragment with the highest mass. The X-ray crystal structure analysis of the disilene 6B supports a molecule with a short Si=Si bond (2.168 A) with efficiently packed, rigid sigma-bonded cyclopentadienyl substituents and silylamino groups. The conformation of the latter does not allow electron donation to the central silicon atom. Theoretical calculations at the density functional level (RI-BP86 and B3LYP, TZVP basis set) confirm the structure of 6B and reveal for silylene 6A the presence of an eta(2)-bonded cyclopentadienyl ligand and of a silylamino group in a conformation that prevents electron back-donation. Further theoretical calculations for the silicon(II) compound 6A, the disilene 6B, and the two species 11 and 11* derived from 6A (which derive from Si=Si bond cleavage) support the experimental findings. The reversible phase-dependent transformation between 6A and 6B is caused by (a) different stereoelectronic and steric effects exerted by the pentamethylcyclopentadienyl group in 6A and 6B, (b) some energy storage in the solid state structure of 6B (molecular jack in the box), (c) a small energy difference between 6A and 6B, (d) a low activation barrier for the equilibration process, and (e) the gain in entropy upon monomer formation.
ABSTRACT
The parent (H(2)N-S-F) and N,N-dialkyl-substituted fluorides of amidosulfoxylic acid (R(2)N-S-F, R=Me or R(2)N=Morph) as well as the related compounds X-S-F (X=CH(3), OH, F, SiH(3), PH(2), SH, Cl) have been investigated with quantum chemical calculations at the ab initio (MP2) level of approximation. The geometries, electronic structures, molecular orbital (MO) energies and NMR chemical shift values have been calculated to evaluate the role and extent of the polarization and delocalization effects in forming of the high-field fluorine NMR resonances within the series of interest. The deltaF magnitudes for all investigated fluorides of amidosulfoxylic acid as well as the deltaN value calculated for Me(2)N-S-F are in the good agreement with the (19)F and (14)N NMR chemical shift values measured experimentally. For the parent compounds, H(2)N-S-F and H(2)N-SO(2)-F, the orientation of principal axes of the magnetic shielding tensors and the corresponding principal sigma(ii) values along these axes have been qualitatively interpreted basing on the analysis of the MO interactions in the presence of the rotating magnetic field.
ABSTRACT
Z,E-isomerization has been investigated for the series of the N-arylthio-1,4-benzoquinonimines using a line shape analysis in the (1)H NMR spectra. Thermodynamic parameters and substituent effects have been analyzed for the isomerization process. It has been shown based on the DFT (B3LYP) calculations that the dynamic transformation for N-arylthio-1,4-benzoquinonimines should be considered as a combination of the two different processes, a rotation about the N-S bond and an inversion at nitrogen via the transition state with the linear C=N-S moiety. The free energies of activation for the isomerization (DeltaG(298 K)) measured experimentally depend on the substitution in the quinonimine moiety and phenyl ring and can be referred either to the inversion of the nitrogen atom or to the hindered rotation about the N-S bond.
