A lithiomethyl trimethylammonium reagent as a methylene donor.

Straightforward deprotonation of soluble tetramethylammonium salts with alkyllithium reagents gives lithiomethyl trimethylammonium reagents. Coordination of the Li cation is crucial to the stability of these 'N-C ylides'. These reagents were used to prepare epoxides, aziridines and allylic alcohols.

New Benchmark Set of Transition-Metal Coordination Reactions for the Assessment of Density Functionals.

We present the WCCR10 data set of 10 ligand dissociation energies of large cationic transition metal complexes for the assessment of approximate exchange-correlation functionals. We analyze nine popular functionals, namely BP86, BP86-D3, B3LYP, B3LYP-D3, B97-D-D2, PBE, TPSS, PBE0, and TPSSh by mutual comparison and by comparison to experimental gas-phase data measured with well-known precision. The comparison of all calculated data reveals a large, system-dependent scattering of results with nonnegligible consequences for computational chemistry studies on transition metal compounds. Considering further the comparison with experimental results, the nonempirical functionals PBE and TPSS turn out to be among the best functionals for our reference data set. The deviation can be lowered further by including Hartree-Fock exchange. Accordingly, PBE0 and TPSSh are the two most accurate functionals for our test set, but also these functionals exhibit deviations from experimental results by up to 50 kJ mol(-1) for individual reactions. As an important result, we found no functional to be reliable for all reactions. Furthermore, for some of the ligand dissociation energies studied in this work, invoking semiempirical dispersion corrections yields results which increase the deviation from experimental results. This deviation increases further if structure optimization including such dispersion corrections is performed, although the contrary should be the case, pointing to the need to develop the currently available dispersion corrections further. Finally, we compare our results to other benchmark studies and highlight that the performance assessed for different density functionals depends significantly on the reference molecule set chosen.

A quantitative study of intrinsic non-covalent interactions within complexes of α-cyclodextrin and benzoate derivatives.

A novel deconvolution method for energy-resolved reaction cross sections is applied to determine intrinsic gas-phase dissociation energies for non-covalent α-cyclodextrin host-guest complexes. M06-2X//M06-L/6-31+G(d,p) calculations reproduce the experimental results and enable us to quantify the contribution of intermolecular hydrogen bonding.

Intrinsic properties of α-cyclodextrin complexes with benzoate derivatives in the gas phase: an experimental and theoretical study.

The noncovalent interactions in host-guest complexes of α-cyclodextrin (α-CD) with a series of benzoic acid derivatives (RBA) were investigated by electrospray ionization tandem mass spectrometry and density functional theory (DFT) calculations. The 1:1 stoichiometry of the anionic host-guest complexes was unequivocally confirmed by their mass-to-charge ratios (m/z) and isotope patterns. Collision-induced dissociation experiments revealed exclusive fragmentation into [α-CD](-) and neutral RBA and afforded the gas-phase kinetic stability trend [α-CD·3,5-diMeBA](-) < [α-CD·3-MeBA](-) < [α-CD·BA](-) < [α-CD·3-OHBA](-) < [α-CD·3,5-diOHBA](-). This trend follows that of the gas-phase basicities of the guest anions used, indicating that host-guest pairs with more comparable basicities form more stable complexes. DFT calculations at the M06-L/6-31+G(d,p) level of theory provided detailed structural assignments and further elucidated the experimental observations, suggesting that the anionic [α-CD·RBA](-) inclusion complexes are favored over the nonspecific complexes in the gas phase and that hydrogen bonding constitutes the primary host-guest interaction. Additionally, the results provide an estimated gas-phase basicity ΔG(0) = 325-327 kcal mol(-1) for [α-CD](-).

Stereomutation of pentavalent compounds: validating the Berry pseudorotation, redressing Ugi's turnstile rotation, and revealing the two- and three-arm turnstiles.

A general reaction mechanism describes the qualitative change in chemical topology along the reaction pathway. On the basis of this principle, we present a method to characterize intramolecular substituent permutation in pentavalent compounds. A full description of the geometry around five-coordinate atoms using internal coordinates enables the analysis of the structural changes along the stereomutational intrinsic reaction coordinate. The fluxional behavior of experimentally known pentavalent phosphoranes, silicates, and transition-metal complexes has been investigated by density functional theory calculations, and three principal mechanisms have been identified: Berry pseudorotation, threefold cyclic permutation, and half-twist axial-equatorial interchange. The frequently cited turnstile rotation is shown to be equivalent to the Berry pseudorotation. In combination with graph theory, this approach provides a means to systematically investigate the stereomutation of pentavalent molecules and potentially identify hitherto-unknown mechanisms.

Structure and bonding of isoleptic coinage metal (Cu, Ag, Au) dimethylaminonitrenes in the gas phase.

Dimethylaminonitrene complexes of IMesM(+) (IMes =1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; M = Cu, Ag, Au) were prepared in the gas phase and structurally characterized by high-resolution infrared spectroscopy of the cold species, ion-molecule reactions, and DFT computations. We measured the binding energies of the nitrene fragment to the IMesM(+) moiety by energy-resolved collision-induced dissociation experiments in the gas phase, affording a trend in bond strength of M = Cu ≈ Au > Ag. This trend is explained in terms of a detailed metal-nitrogen bonding analysis, from which relativistic effects on the bonding were assessed. Various density functionals were evaluated for reproducing the observed thermochemical data and Truhlar's M06 functional was found to give the best agreement.

Gas-phase energetics of reductive elimination from a palladium(II) N-heterocyclic carbene complex.

Energy-resolved collision-induced dissociation experiments using tandem mass spectrometry are reported for an phenylpalladium N-heterocyclic carbene (NHC) complex. Reductive elimination of an NHC ligand as a phenylimidazolium ion involves a barrier of 30.9(14) kcal mol(-1), whereas competitive ligand dissociation requires 47.1(17) kcal mol(-1). The resulting three-coordinate palladium complex readily undergoes reductive C-C coupling to give the phenylimidazolium pi complex, for which the binding energy was determined to be 38.9(10) kcal mol(-1). Density functional calculations at the M06-L//BP86/TZP level of theory are in very good agreement with experiment. In combination with RRKM modeling, these results suggest that the rate-determining step for the direct reductive elimination process switches from the C-C coupling step to the fragmentation of the resulting sigma complex at low activation energy.

Configurationally rigid pentaorganosilicates.

The intramolecular substituent interchange in recently reported pentaorganosilicates is investigated by B3LYP calculations, which show excellent agreement with the experimental thermochemical data. Two types of ligand permutation are discerned (A and B), which both lead to racemization of the helical, spirocyclic anions. IRC calculations show that stereomutation A bifurcates into two enantiomeric reaction paths, which are inhibited by ortho substitution of the bidentate ligands. The other pathway (B) proceeds through a trigonal bipyramidal transition state with one bisequatorial bidentate ligand and is disfavored by increasing the pi-electron density of the ligand. A more electronegative fifth, monodentate substituent increases the barrier of pathway A and lowers that of pathway B, as in bis(biphenyl-2,2'-diyl)fluorosilicate, which is the first tetraorganofluorosilicate to be isolated and fully characterized. These concepts enabled us to design and synthesize methyl- and ethylbis([2]naphthylpyrrol-2,1'-diyl)silicate as Si-chiral pentaorganosilicates that are configurationally rigid at room temperature.

2,2,6,6-Tetra-kis(biphenyl-2-yl)-4,4,8,8-tetra-methyl-cyclo-tetra-siloxane.

The title compound, [-Si(C(12)H(9))(2)OSi(CH(3))(2)O-](2), was obtained unintentionally as the product of an attempted crystallization of caesium bis-(biphenyl-2,2'-di-yl)fluoro-silicate from dimethyl-formamide. In the crystal, the mol-ecule is located on an inversion center and the siloxane ring adopts a twist-chair conformation with the two dimethyl-substituted Si atoms lying 0.7081 (5) Šout of the plane defined by the two bis-(biphenyl-2-yl)-substituted Si atoms and the four O atoms. In each Si(C(12)H(9))(2) unit, the orientation of one terminal phenyl ring relative to the phenyl-ene ring of the other biphenyl moiety suggests a parallel displaced π-π stacking inter-action [centroid distance = 4.2377 (11) Šand dihedral angle = 15.40 (9)°].

Fused tricyclic phosphiranes--analysis of phosphorus chemical shieldings.

1,2-Addition of transient W(CO)(5)-complexed phosphinidenes exo to hexamethyl Dewar benzene affords the novel 3-phosphatricyclo[3.2.0.0(2,4)]hept-6-ene complexes. The fused tricyclic phosphiranes are obtained as both the Z and the thermally less stable E isomers, the (31)P NMR chemical shifts of which differ by about 60 ppm. A computational investigation shows that the phosphorus pyramidalization and the presence of the gamma double bond are responsible for this effect. The semiquantitative results contribute to a more systematic understanding of the structural influences on (31)P chemical shieldings. The congested double bond of the Z isomer can be epoxidized with m-chloroperbenzoic acid (MCPBA) to afford a fused tetracyclic P,O bis-adduct.

Electronic structure and stability of pentaorganosilicates.

The exceptional stability of recently reported pentaorganosilicates is investigated by bond energy analyses. Experimental coupling constants are used to probe their electronic structure, entailing bonds with mixed ionic-covalent character. Our analyses reconfirm that the axial bonds are more prone to heterolytic cleavage than are the equatorial bonds. Aryl substituents provide substantial electronic stabilization by charge delocalization, but cause steric crowding due to ortho-hydrogen repulsion. In contrast, silicates with two ax,eq biaryl groups are not congested. The remaining substituent is confined to an equatorial site, where it is insensitive to elimination. These concepts adequately explain the experimentally observed stability trends and are valuable for designing other stable pentaorganosilicates.