Alexander F. Sax

Dioxomolybdenum(VI) Complexes with Pyrazole Based Aryloxide Ligands: Synthesis, Characterization and Application in Epoxidation of Olefins

Synthesis, characterization, and epoxidation chemistry of four new dioxomolybdenum(VI) complexes [MoO(2)(L)(2)] (1-4) with aryloxide-pyrazole ligands L = L1-L4 is described. Catalysts 1-4 are air and moisture stable and easy to synthesize in only three steps in good yields. All four complexes are coordinated by the two bidentate ligands in an asymmetric fashion with one phenoxide and one pyrazole being trans to oxo atoms, respectively. This is in contrast to the structure found for the related aryloxide-oxazoline coordinated Mo(VI) dioxo complex 5. This was confirmed by the determination of the molecular structures of complexes 1-3 by X-ray diffraction analyses. Compounds 1-4 show high catalytic activities in the epoxidation of various olefins. Cyclooctene (S1) is converted to its epoxide with high activity, whereas the epoxidation of styrene (S2) is unselective. Internal olefins (S3 and S4) are also acceptable substrates, as well as the very challenging olefin 1-octene (S5). Catalyst loading can be reduced to 0.02 mol % and the catalyst can be recycled up to ten times without significant loss of activity. Supportive DFT calculations have been carried out in order to obtain deeper insights into the electronic situation around the Mo atom.

Mechanistic Insight into the Reactivity of Oxotransferases by Novel Asymmetric Dioxomolybdenum(VI) Model Complexes

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4-bis(2-hydroxybenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-methylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-flurobenzyl)-1,4-diazepane, and 1,4-bis(2-hydroxy-4-chlorobenzyl)-1,4-diazepane (H(2)(L1)-H(2)(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO(2)(L)] 1-6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO(2)(L)] 1-4 have been successfully determined by single-crystal X-ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis-β configuration. The Mo-O(oxo) bond lengths differ only by ≈0.01u2005Å. Complexes 1, 2, 5, and 6 exhibit two successive Mo(VI)/Mo(V) (E(1/2), -1.141 to -1.848u2005V) and Mo(V)/Mo(IV) (E(1/2), -1.531 to -2.114u2005V) redox processes. However, only the Mo(VI)/Mo(V) redox couple was observed for 3 and 4, suggesting that the subsequent reduction of the molybdenum(V) species is difficult. Complexes 1, 2, 5, and 6 elicit efficient catalytic oxygen-atom transfer (OAT) from dimethylsulfoxide (DMSO) to PMe(3) at 65u2009°C at a significantly faster rate than the symmetric molybdenum(VI) complexes of the analogous linear bis(phenolate) ligands known so far to exhibit OAT reactions at a higher temperature (130u2009°C). However, complexes 3 and 4 fail to perform the OAT reaction from DMSO to PMe(3) at 65u2009°C. DFT/B3LYP calculations on the OAT mechanism reveal a strong trans effect.

Is Electrostatics Sufficient to Describe Hydrogen‐Bonding Interactions?

The stability and geometry of a hydrogen-bonded dimer is traditionally attributed mainly to the central moiety A-H⋅⋅⋅B, and is often discussed only in terms of electrostatic interactions. The influence of substituents and of interactions other than electrostatic ones on the stability and geometry of hydrogen-bonded complexes has seldom been addressed. An analysis of the interaction energy in the water dimer and several alcohol dimers--performed in the present work by using symmetry-adapted perturbation theory--shows that the size and shape of substituents strongly influence the stabilization of hydrogen-bonded complexes. The larger and bulkier the substituents are, the more important the attractive dispersion interaction is, which eventually becomes of the same magnitude as the total stabilization energy. Electrostatics alone are a poor predictor of the hydrogen-bond stability trends in the sequence of dimers investigated, and in fact, dispersion interactions predict these trends better.

The ground-state structure of cyclotetrasilane

Abstract The ground-state geometry of cyclotetrasilane has been optimized at the single-determinant Hartree-Fock level by using a local Pseudopotential. Puckered cyclotetrasilane is a local minimum on the potential energy surface lying 2.1 kJ mol below the planar conformer which is characterized as a saddle point at this level of theory. Valence electron density plots, force constants and homodesmotic ring strain energies show that cyclotetrasilane is less strained than cyclobutane.

Stabilities and stationary points for singlet and triplet states of the Si2H− anion

Abstract Various stationary points on the 2A′ and 2A″ potential energy surfaces (PES) of the Si2H radical and the 1A′, 3A′, 3A″, and 1A″ PES of the Si2H− anion are discussed. The ground state electron affinity of Si2H is calculated larger than 200 kJ/mol. The vast majority of the stationary points are strongly non-linear or even symmetrically bridged (C2v) structures.

The isomers of cyclotmsilane

Abstract Geometries of four isomers of Si 3 H 6 have been optimized at the single-determinant Hartree-Fock level by using a local Pseudopotential. Cyclotrisilane is 12 kJ mol more stable than silyldisilene and 16 kJ mol more stable than disilylsilylene and disilanylsilylene. Harmonic vibrational frequencies, symmetry force constants, valence electron-density plots and homodesmotic ring-strain energies are reported for cyclotrisilane, which is characterized by these quantities as a strained molecule.

Numerical determination of pseudorotation constants

We describe a general numerical method for the calculation of pseudorotation constants at an accuracy suited for thermodynamic applications. The pseudorotation is treated using classical mechanics along a pseudorotational pathway, which is spanned by molecular structures obtained from conventional ab initio geometry optimization. We present numerical results for systems with vertical pseudorotation arising from ring puckering (cyclopentane, cyclopentasilane) and for systems with in-plane pseudorotation arising from Jahn–Teller distortion of planar rings (benzene cation, cyclopentadienyl radical, cyclopropenyl radical).

Singlet-triplet splittings and electron affinities of some substituted silylenes

Abstract The lowest electronic states of methyl-, silyl-, and lithium-substituted silylene have been investigated. CH3 acts as an electronegative substituent and increases the singlet-triplet splitting, SiH3 behaves as an electropositive substituent and decreases this energy gap, and the lithium-substituted silylenes even have triplet electronic ground states. It is found that the effects of the electropositive substituents prevail against those of the electronegative ones. Silyl substitution increases the electron affinity significantly, whereas methyl substituents strongly decrease it, or even make it vanish. Despite the highly electropositive lithium substituent, HSiLi and SiLi2 both exhibit unexpectedly large electron affinities, because Li facilitates the delocalization of the extra electron.

Identification and thermodynamic treatment of several types of large‐amplitude motions

We present a partially automated method for the thermodynamic treatment of large‐amplitude motions. Starting from the molecular geometry and the Hessian matrix, we evaluate anharmonic partition functions for selected vibrational degrees of freedom. Supported anharmonic vibration types are internal rotation and inversion (oscillation in a double‐well potential). By heuristic algorithms, we identify internal rotations in most cases automatically from the Hessian eigenvectors, and we also estimate the parameters of anharmonic partition functions (e.g., potential barrier, periodicity, and symmetry number) with thermodynamically sufficient precision. We demonstrate the validity of our schemes by comparison to pointwise calculated ab initio potential curves.

Multiply bonded Si2Hn, (n=0–4) systems. An MC-SCF and CI investigation

Abstract Optimized geometries and harmonic vibrational frequencies are reported for multiply bonded Si 2 H n ( n =0–4) systems. For disilene, disilyne, disilavinylidene and disilynyl energy differences between the two lowest electronic states were estimated using highly correlated wavefunctions. The singlet-triplet splitting in disilene is computed to be 25.6 kcal mol −1 . Trans bent disilyne in its 1 A g state is found to be a local minimum on the potential-energy surface which lies 3.5 kcal mol −1 above singlet disilavinylidene, and the 3 A u state of disilyne is 2.5 kcal mol −1 above the 3 A 2 state of disilavinylidene. For both Si 2 H 2 isomers the singlet states are lower in energy than the respective triplet states; for disilyne the energy difference is 9.5 kcal mol −1 , for disilavinylidene it is 11.0 kcal mol −1 .