Wolfgang W. Schoeller

VALENCE ISOMERIZATION IN THE SOLID STATE : FROM 1,3-DIPHOSPHACYCLOBUTANE-2,4-DIYL TO 1,2-DIHYDRO-1,2-DIPHOSPHETE

The butadiene-like phosphanylcarbene 2 is, according to ab initio calculations, the intermediate in the conversion of 1 into 3 in the solid state [Eq. (a)]; it is only 1.3 kJ mol-1 higher in energy than 1. For this conversion, only minor changes in the endocyclic bonds are required throughout the entire reaction. R2 N=2,2,6,6-Me4 C5 H6 N.

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.

1,3-Diphosphacyclobutane-2,4-diyl-2-ylidenide: A Unique Carbene and Its Trimethylalane Complex.

A unique bonding situation is displayed by the lithium 1,3-diphosphacyclobutane-2,4-diyl-2-ylidenide 2 small middle dot[Li(thf)(n)](+) (Ar=2,4,6-tBu(3)C(6)H(2)) obtained by deprotonation of 1. According to ab initio calculations, the anion 2 can viewed as a cyclic bis(phosphanyl)carbene. Reaction with trimethylaluminum gives the complex 3 small middle dot[Li(thf)(4)](+), whose crystal structure is presented.

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.

Valenzisomerisierung im Festkörper: vom 1,3-Diphosphacyclobutan-2,4-diyl zum 1,2-Dihydro-1,2-diphosphet

Das butadienanaloge Phosphanylcarben 2 ist Ab-initio-Rechnungen zufolge das Intermediat der Umlagerung von 1 zu 3 im Feststoff [Gl. (a)]. Seine Energieliegt nur 1.3 kJ mol−1 hoher als die von 1, und es sind bei dieser Umlagerung wahrend der gesamten Reaktion lediglich kleine Anderungen bezuglich der endocyclischen Bindungen erforderlich. R2N = 2,2,6,6-Me4C5H6N.

Synthesis of Allenylidene Lithium and Silver Complexes, and Subsequent Transmetalation Reactions

Alpha, beta, gamma! Amino substituents in alpha and beta positions allow the isolation of free carbenes, but even in the gamma position, their strong pi-electron-donating properties permit the synthesis of allenylidene lithium adducts and silver complexes (see picture), which are ideal precursors for the preparation of various transition-metal-allenylidene complexes.

Isoelectronic Arduengo-type carbene analogues with the Group IIIa elements boron, aluminum, gallium, and indium

While Arduengo-type carbenes are now well established for the Group IVa elements carbon, silicon and germanium, they are experimentally unknown for the isoelectronic anions with the Group IIIa elements boron, aluminum, gallium, and indium. In the present quantum chemical investigations, the bonding features of this class of compounds are explored. As a result, they are predicted to be stable species with sizeable singlet-triplet energy separations and electron affinities. Hence, they may be considered as valid targets for experimental investigations. An analysis of the electron density distribution in the case of boron and aluminum reveals a strongly polar B-N bond for the former, and a half Al-N bond with positive charge at the aluminum, emphasizing the donor-acceptor formulation for the latter.

Tuning the Nucleophilicity in Cyclopropenylidenes

Cyclopropenylidenes are Hückel aromatic pi systems in which one of the ring atoms is a carbene center. Quantum chemical calculations at the density functional level of theory, supplemented by coupled-cluster calculations, indicate that there is a sizeable energy separation between the lowest-energy singlet and triplet states of these species. Amino groups considerably increase the energy difference between these two states, whereas electron-withdrawing substituents decrease it. The 1,1-dimerization products of cyclopropenylidenes, namely, triafulvalenes, have been investigated. The calculations show that, without steric hindrance and considerable electronic stabilization, cyclopropenylidenes are kinetically unstable and dimerize. Different substituents (alkyl, silyl, terphenyl, amino, and phosphoraneiminato) were probed to tune the frontier orbital energies of cyclopropenylidenes. Accordingly, it is predicted that by a suitable choice of substituents at the olefinic positions, cyclopropenylidenes can be more nucleophilic than their five-membered ring congeners, namely, imidazol-2-ylidenes.

On the Electronic Properties of Substituted Phosphanylcarbenes

A phosphanyl group exerts much less pi-conjugation properties than an amino group. On this basis, corresponding carbene structures exhibit much smaller singlet-triplet energy separations. Of the various structures investigated quantum-chemically the largest singlet-triplet energy separations are predicted for cyclic diphosphanylcarbenes, in which the two functional groups are incorporated into a ring system and the phosphorus atoms are substituted by phosphoraniminato groups. Then the singlet-triplet energy separations become essentially larger than for the Bertrand type (push-pull) carbenes.

Stabilization of Organosilicenium Ions by means of Intramolecular Coordination of O, S, or P Ligands

For the intramolecular stabilization of silicenium ions (R3Si+), O, S, and P donors as well as the known nitrogen-containing systems (as in 1 a) are suitable. The silyl cations in 1 a–d show a trigonal-bipyramidal structure; dynamic processes can be proved by NMR spectroscopy for 1 c, d. Calculations on model compounds document substantial differences in the bonding relationships and support the structural findings. Furthermore, preliminary experiments with 1 b–d indicate significant differences in reactivity.