Alexander B. Rozhenko

Reversible Transformation of a Stable Monomeric Silicon(II) Compound into a Stable Disilene by Phase Transfer: Experimental and Theoretical Studies of the System {[(Me3Si)2N](Me5C5)Si }n with n = 1,2

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.

Photodissociation of methyl iodide embedded in a host-guest complex: A full dimensional (189D) quantum dynamics study of CH3I@resorc[4]arene

Accurate full dimensional quantum dynamics calculations studying the photodissociation of CH(3)I@resorc[4]arene on an ab initio based potential energy surface (PES) model are reported. The converged 189D quantum dynamics calculations are facilitated by the multilayer multi-configurational time-dependent Hartree (ML-MCTDH) approach combined with the correlation discrete variable representation (CDVR) for the evaluation of potential energy matrix elements. The potential employed combines an established ab initio PES describing the photodissociation of methyl iodide in the A band with a harmonic description of the resorc[4]arene host and a bilinear modeling of the host-guest interaction. All potential parameters required in the description of the vibrations of the host molecule and the host-guest interaction are derived from ab initio calculations on the host-guest complex. Absorption spectra at 0 K and 300 K are calculated and the electronic population dynamics during the bond breaking process occurring in the first 20-30 fs after the photoexcitation is investigated. Weak but significant effects resulting from the host-guest interaction on this time scale are found and interpreted. The present study demonstrates that accurate fully quantum mechanical dynamics calculations can be preformed for systems consisting of more than 50 atoms using the ML-MCTDH/CDVR approach. Utilizing an efficient statistical approach for the construction of the ensemble of initial wavepackets, these calculations are not restricted to zero temperature but can also study the dynamics at 300 K.

Schrock type transition metal complex formation of push–pull substituted phosphanyl-carbenes, a quantum chemical investigation by density-functional methods

Abstract The formation of coordination compounds of phosphanyl carbenes with a transition metal in a high oxidation state (Schrock type) is evaluated by density functional theory. In the Schrock type carbene complexes the transition metalcarbon bond possesses a multiple bond character. If one of the ligands at the carbene center possesses a leaving ability, e.g. as the diaminophosphenium unit, a facile distortion to a new type of an anionic metallaalkylidene donor-acceptor complex with a diaminophosphenium cation is predicted.

Donor-acceptor interactions with electrophilic terminal phosphinidene complexes

Abstract Donor–acceptor interactions of Lewis bases, such as amine, phosphine, cyanic acid and phosphaalkine with various phosphinidene complexes carrying a W(CO)5 fragment are studied by means of density functional calculations. Accordingly, the donor interactions can be divided into two categories. In terms of energy an amine binds stronger than a phosphine to a phosphinidene, the resulting DoP bond is longer (NP) or equal (PP) with respect to a corresponding single bond. This tendency is also revealed in the corresponding transition metal complexes. In all cases the singlet-triplet energy separation values of the resulting donor-adducts are fairly small. A donor with a π-system, such as HCN yields a shorter NP bond, but the resulting donor-adduct is even less stable with respect to decomposition into a singlet PH complex and cyanic acid. Similar considerations with HCP reveal only a weak donor addition of this fragment to the parent PH complex but a facile rearrangement of the adduct to the diphosphirene transition metal complex. The diaminophosphino–PH possesses a singlet ground state, its corresponding transition metal complex reveals a fairly small singlet–triplet energy separation. It can be considered as a nucleophilic PH complex in contrast to the other homologues which reveal an electrophilic behaviour.

Ab initio calculation of NMR shielding in phosphaalkenes X—PCY2

31P and 13C chemical shielding values, σiso, were calculated for a series of phosphaalkenes X—PCY2 with a variety of substituents such as XH, CH3, F, Cl, Br, OH, NH2, PH2, SiH3, CN and YH, CH3, NH2, SiH3, using the GIAO procedure at the RHF/6–311 + G(d,p) and RHF/6–311 + G(2d,2p) levels. The trends governing the variation of the geometric parameters and the natural charges and Wiberg indices are discussed. The contributions of the various molecular orbitals in 31P and 13C chemical shielding in the phosphaalkene H—PCH2 and ethylene, respectively, were compared. The 31P chemical shielding variation was determined by mixing of the ground and excited states. Good correlations were found between the experimentally measured δ C and δ P values in the series X′—PC(SiMe3)2 and those calculated for the series X—PC(SiH3)2, but on average the calculated 31P chemical shifts seem to be overestimated. A comparison with the calculated data (at the same level) for X—PC(SiMe3)2 indicates only a partial improvement. The 13C chemical shifts in phophaalkenes correlate within the subseries X—PCH2, X—PC(CH3)2 and X—PC(SiH3)2 with the charge variation at the corresponding carbon atom, reflecting the X—(PC) π‐mesomeric interactions. The absence of the corresponding correlation with all phophaalkenes investigated manifestates limited validity of this generally used approach. Copyright

A new procedure for the synthesis of alkyl perfluoroalkanedithiocarboxylates

Abstract Alkyl perfluoroalkanedithiocarboxylates have been synthesized from 1,1-dichloroperfluoroalkyl propyl sulfides by reaction with zinc sulfide or cadmium sulfide.

Cyclochiral resorcin[4]arenes as effective enantioselectors in the gas phase.

The effect of cyclochirality of rccc-2,8,14,20-tetra-n-decyl-4,10,16,22-tetra-O-methylresorcin[4]arene (C) on the enantiodiscrimination of a number of chiral bidentate and tridentate aromatic and aliphatic biomolecules (G) has been investigated by nano-electrospray ionization (nano-ESI)-Fourier transform ion cyclotron resonance mass spectrometry. The experimental approach is based on the formation of diastereomeric proton-bound [C·H·G](+) complexes by nano-ESI of solutions containing an equimolar amount of quasi-enantiomers (C) together with the chiral guest (G) and the subsequent measurement of the rate of the G substitution by the attack of several achiral and chiral amines. In general, the heterochiral complexes react faster than the homochiral ones, except when G is an aminoalcoholic neurotransmitter whose complexes, beyond that, exhibit the highest enantioselectivity. The kinetic results were further supported by both collision-induced dissociation experiments on some of the relevant [C(2) ·H·G](+) three-body species and Density functional theory (DFT) calculations performed on the most selective systems.

A kinetic study of guest displacement reactions on a host–guest complex with a photoswitchable calixarene

The displacement processes of several guests, incorporated in a calixarene host system, were investigated in the gas phase by electrospray ionization-Fourier transform-ion cyclotron resonance (ESI-FT-ICR) mass spectrometry. The complexes resulting from a resorcin[4]arene host with ammonia and sec-butylamine guests were isolated in an ICR-cell, separately using both states of the photoswitch as well as two reference systems for the open and closed forms of the photoswitchable host. The isolated complexes were forced to exchange the guest by using methylamine, ethylamine and sec-butylamine, resulting in different reaction rates for all the measured systems. Especially, the reaction rates of both states of the photoswitch are dependent on the provided guest. Potential side effects like proton exchanges were examined by an H/D-exchange experiment. The results were investigated and supported by quantum chemical calculations (DFT).

Pentacoordination at Fluoro-Substituted Silanes by Weak Lewis Donor Addition

Quantum chemical calculations at the ab initio level indicate that Lewis donors such as amine or water form weak donor-acceptor complexes with fluoro-substituted silanes. The strength of the donor-acceptor formation increases with increasing degree of fluorine substitution. Amine and water form strong adducts only for tetrafluorosilane. The higher element homologues of Lewis donors, such as phosphane and sulfur dihydride, do not in essence coordinate. A considerable role in donor-acceptor formation is exerted by chelation effects which impose intramolecular interactions on functionalized ligands. The corresponding structures are explored.

Multinuclear NMR spectroscopy and semi-empirical MNDO-PM3 quantum chemical investigations of the compounds C6H5XY (X=S, SO, SO2; Y=CF3, CH3)

Abstract The compounds C 6 H 5 XCF 3 and C 6 HXCH 3 (X=S, SO, SO 2 ) have been investigated by multinuclear 1 H, 13 C, 17 O, 19 F and 33 S NMR spectroscopy and quantum chemical methods (MNDO in the PM3 parametrization). Compounds with XCF 3 , SOCH 3 and SO 2 CH 3 groups have been characterized with effective non-planar conformations. A nearly planar effective conformation has been found for C 6 H 5 SCH 3 . As the two lone pairs (LP) of sulphur in the sulphides are essentially non-equivalent with their orbital energies differing considerably, only one LP (with a higher energy level) can participate in the p-π conjugation. In compounds with XCF 3 , SOCH 3 and SO 2 CH 3 groups, the electron-withdrawing π-inductive effect is the main factor determining the π-charge distribution in the benzene ring. The p-π conjugation is essentially distorted in C 6 H 5 SCF 3 due to its various conformations. Electron-donating p-π conjugation only contributes considerably to the π-charge distribution in the benzene ring in C 6 H 5 SCH 3 . An interpretation of the NMR spectra of the considered compounds is given on the basis of the quantum chemical calculations.