PubMed Health⌕ Search

Biomedical subjects

Michael Bühl

Publications and source records attributed to Michael Bühl.

30 records · Page 2Linked to original sources
PubMed2002

Mechanism of olefin metathesis with catalysis by ruthenium carbene complexes: density functional studies on model systems.

Gradient-corrected (BP86) density functional calculations were used to study alternative mechanisms of the metathesis reactions between ethene and model catalysts [(PH(3))(L)Cl(2)Ru[double bond]CH(2)] with L=PH3 (I) and L=C(3)N(2)H(4)=imidazol-2-ylidene (II). On the associative pathway, the initial addition of ethene is calculated to be rate-determining for both catalysts (Delta G(22-25)*[double bond] kcal mol(-1)). The dissociative pathway starts with the dissociation of phosphane, which is rather facile (Delta G(298)* is approximately equal to 5-10 kcal mol(-1)). The resulting active species (L)Cl(2)Ru[double bond]CH(2) can coordinate ethene cis or trans to L. The cis addition is unfavorable and mechanistically irrelevant (Delta G(298)* is approximately equal to 21-25 kcal mol(-1)). The trans coordination is barrierless, and the rate-determining step in the subsequent catalytic cycle is either ring closure of the complex to yield the ruthenacyclobutane (catalyst I, Delta G(298)*=12 kcal mol(-1)), or the reverse reaction (catalyst II, ring opening, Delta G(298)*=10 kcal mol(-1)), that is, II is slightly more active than I. For both catalysts, the dissociative mechanism with trans olefin coordination is favored. The relative energies of the species on this pathway can be tuned by ligand variation, as seen in (PMe(3))(2)Cl(2)Ru[double bond]CH(2) (III), in which phosphane dissociation is impeded and olefin insertion is facilitated relative to I. The differences in calculated relative energies for the model catalysts I-III can be rationalized in terms of electronic effects. Comparisons with experiment indicate that steric effects must also be considered for real catalysts containing bulky substituents.

Journal Article↗
PubMed2002

Synthesis, characterization, and reactivity of monomeric, arylpalladium halide complexes with a hindered phosphine as the only dative ligand.

We report the isolation and structural characterization of several monomeric arylpalladium(II) halide complexes containing tri-tert-butyl phosphine, 1-adamantyl-di-tert-butylphosphine, or 2-adamantyl-di-tert-butylphosphine. X-ray diffraction, IR spectroscopy, and theoretical studies indicated that the complexes may be stabilized by agostic interactions. For example, the distance from the closest hydrogen atom to the palladium metal center in the X-ray structure of the 1-adamantyl-phenylpalladium bromide complex 1 was 2.26(3) A. The calculated Pd-H distance of 2.28 A and harmonic vibrational frequencies were in agreement with the measured distance, but Wiberg bond indices indicated only weak M-H-C interactions. Addition of 2-adamantyl-di-tert-butyl phosphine to 1 led to ligand exchange and formation of 2-adamantyl-di-tert-butyl phosphine complex 2. Addition of P(t-Bu)(3) generated free aryl bromide and Pd[P(t-Bu)(3)](2). Reactivity of complex 1 with nucleophiles provided evidence of the intermediacy of these complexes in palladium-catalyzed cross-coupling reactions. Complex 1 reacted with amine and base to form the corresponding arylamine, with tert-butoxide to form the corresponding ether, with boronic acid and fluoride to form the corresponding biaryl, and with styrene to form stilbene. This complex also catalyzed the reaction of bromobenzene with diphenylamine in the presence of base to produce 94% of the amination product in 45 min at room temperature. This rate is comparable to the reaction rate of in situ generated catalysts.

Halogens↗
PubMed2002

Gas-phase structure of (1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)(eta(2)-1,5-cyclooctadiene)copper(I), Cu(1,5-cod)(hfac), an important precursor for vapor deposition of copper.

The molecular structure of Cu(1,5-cod)(hfac) in the gas phase has been determined by electron diffraction, restrained by parameters calculated ab initio (MP2/AE1 level) or using Density Functional Theory (BP86/AE1 level). The most stable structure is one in which one olefinic group of the cyclooctadiene ligand is coordinated to the square-planar copper atom [refined Cu-C distances 194.0(13) and 194.4(9) pm]. The second C=C double bond is weakly associated with the copper atom [Cu...C distances 267.2(23) and 276.9(25) pm], and the cyclooctadiene ligand has a twist-boat conformation, so that the complex has C(1) symmetry. The nature of the bonding between copper and each of the two olefin moieties has been assessed by topological analysis of the BP86/AE1 total electron density. A form with C(2) symmetry, lying between 2 and 7 kJ mol(-1) above the ground state, is a transition state for exchange of the two olefinic groups. There are also two higher energy conformers, both 10 kJ mol(-1) or more above the ground state. In one of these the cyclooctadiene ligand retains the twist-boat conformation, but the Cu(hfac) moiety is coordinated in the exo position with respect to the noncoordinated olefin, instead of endo, as in the most stable conformer. The molecular symmetry is C(1) in this isomer. In the remaining form the ligand has the chair conformation, and the molecular symmetry is C(s).

Journal Article↗
PubMed2002

Hydronium ion complex of 18-crown-6: where are the protons? A density functional study of static and dynamic properties.

A quantum-chemical study employing the BLYP density functional is reported for the complex of H3O+ with 18-crown-6. According to a Car-Parrinello molecular dynamics (CPMD) study at 340 K, the complex is quite flexible, and is characterized by three quasi-linear (two-center) hydrogen-bond interactions for most of the time. On a time scale of 10 ps, frequent inversions of H3O+ are observed, as well as two 120 degrees rotations switching the hydrogen bonds from one set of crown-ether O atoms to the other. These results are consistent with density-functional studies of stationary points on the potential energy surface, which show how the crown "catalyzes" the guest's inversion. Two close-lying minima are characterized, as well as two distinct transition states connecting them, either via H3O+ inversion or rotation, with barriers of 1.0 and 4.6 kcal/mol, respectively, at the BLYP/II'//BLYP/6-31G level. Orbital interactions between lone pairs on ether O atoms and hydronium sigma(OH) antibonding orbitals are important factors for the directionality of the hydrogen bonds.

Journal Article↗
PubMed2002

Fluxional processes in diamagnetic and paramagnetic allyl dicarbonyl and 2-methylallyl dicarbonyl molybdenum histidinato complexes as revealed by spectroscopic data and density functional calculations.

This work describes a detailed study on the structure and dynamics of pseudooctahedral low-valent complexes of the type [Mo(His-N(epsilon)-R)(eta-2-R'-allyl)(CO)(2)] (His=N(delta),N,O-L-histidinate; R=H, R'=H (1); R=C(2)H(4)CO(2)Me, R'=H (2); R=H, R'=Me (3); R=C(2)H(4)CO(2)Me, R'=Me (4)). These diamagnetic 18-electron complexes were comprehensively characterized spectroscopically and by X-ray crystallography. In the solid state, the (substituted) allyl ligand is in an endo position in all compounds, but it is trans to the His-N(delta) atom in 1 and 2, whereas it is trans to the carboxylate O atom for the 2-Me-allyl compounds 3 and 4. In solution, both isomers are present in a solvent-dependent equilibrium. The third isomer (allyl trans to His-NH(2)) is not spectroscopically observed in solution. This is in agreement with the results from density functional (DFT) computations (BPW 91 functional) for 1 and 3, which predict a considerably higher energy (+6.3 and +5.9 kJ mol(-1), respectively) for this isomer. A likely path for isomerization is calculated, which is consistent with the activation energy determined by variable temperature NMR measurements. At least for 3, the preferred path involves several intermediates and a rotation of the 2-Me-allyl ligand. For the paramagnetic 17-electron congeners, DFT predicts the exo isomer of 3(+) with the 2-Me-allyl ligand trans to the carboxylate O atom to be by far the most stable isomer. For 1(+), an endo-exo equilibrium between the isomers with the allyl ligand trans to the carboxylate O atom is suggested. These suggestions are confirmed by EPR spectroscopy on the electrochemically generated species, which show signals for one- (4) and two- (2) metal-containing compounds. The appearance of the EPR spectra may be rationalized by inspection of the SOMOs from DFT calculations of the species in question. The notion of a metal-centered oxidation is also substantiated by IR spectroelectrochemistry and by UV/Vis spectra of the 17-electron complexes. Upon depleting the metal of electron density, the stretching vibrations of the carbonyl ligands shift more than 100 cm(-1) to higher wavenumbers, and the carbonyl vibration of the metal-coordinated carboxylate shifts by about 50 cm(-1). A color change from yellow to green upon oxidation is observed visually and quantified by the appearance of a new band at 622 nm (2(+)) and 546 nm (4(+)), respectively.

Crystallography, X-Ray↗
PubMed1998

Computational Study of B(4)H(10) Addition to Ethene. Baskets from Boranes.

Three pathways leading from B(4)H(10) + C(2)H(4) to (CH(2)CH(2))B(4)H(8) are examined at the [MP4/6-311+G(d,p)]//MP2/6-31G(d) + ZPC level. The preferred, "addition" path involves transient [B(4)H(8)], [B(4)H(8)(C(2)H(4))], and a cage-opened [(CH(2)CH(2))B(4)H(8)] intermediate with a rate-determining barrier of 33.0 kcal/mol (for [B(4)H(8)] formation). "Indirect hydroboration" of ethene via [B(4)H(8)] is disfavored by entropy and can be excluded. "Direct hydroboration" via a higher-energy [B(4)H(10)] isomer can be competitive; the overall energetic barrier for this process is only 26.9 kcal/mol, but the hydroboration step is disfavored by entropy to such an extent that the "addition" path is the main route at the experimental temperatures. Direct hydroboration affords the intermediate Et-B(4)H(9) which is likely to be involved in side reactions leading to byproducts such as Et-(CH(2)CH(2))B(4)H(7).

Journal Article↗
PubMed1998

Gas-Phase Reaction of Tetraborane(10) and Ethyne: Molecular Structure of nido-1,2-C(2)B(3)H(7) in the Gas Phase.

The molecular structure of nido-1,2-C(2)B(3)H(7), 1, the principal volatile carborane generated in the quenched gas-phase reaction of B(4)H(10) and ethyne at 70 degrees C, has been determined by a combined analysis of gas-phase electron-diffraction data and rotation constants restrained by ab initio computations at the CCSD(T)/TZP' level. The structure is consistent with a geometry having C(s)() symmetry, similar to that of pentaborane(9). The apical position is occupied by a carbon atom, displaced toward B(4) from a position directly above the B(5).B(3) vector, and hydrogen atoms asymmetrically bridge the B-B bonds. The basal atoms are almost coplanar, C(2) lying ca. 2 degrees below the B(3)-B(4)-B(5) plane. Important experimental structural parameters (r(alpha) degrees /pm, angle(alpha)/ degrees ) are r[C(1)-C(2)] = 162.6(6); r[C(1)-B(3)] = 161.4(3); r[C(2)-B(3)] = 154.3(2); r[C(1)-B(4)] = 157.4(5); r[B(3)-B(4)] = 185.7(3); <B(3)-B(4)-B(5) = 80.9(1). In addition to this and the other previously reported carboranes, 2,3-C(2)B(4)H(8), 2-Me-2,3,4-C(3)B(3)H(6), and 4-Me-2-CB(5)H(8), several new derivatives have been identified among the volatile products. These include the dicarbahexaboranes 2,4-Me(2)-2,3-C(2)B(4)H(6) and 5-Et-2,3-C(2)B(4)H(7) (the major volatile products obtained when the reaction is allowed to go to completion, previously reported as tricarbahexaboranes) and the derivatives 2,5-Me(2)-2,3-C(2)B(4)H(6), 4-Et-2,3-C(2)B(4)H(7), 1-Me-2,3,4-C(3)B(3)H(6), 2-Me-2-CB(5)H(8), 3-Me-2-CB(5)H(8), and 2,3,4,5-C(4)B(2)H(6). The complex mechanism of the reaction is discussed in light of these new results.

Journal Article↗
PubMed1997

Molecular Structure of Trifluorophosphine Tetraborane(8), B(4)H(8)PF(3), As Determined in the Gas Phase by Electron Diffraction and ab Initio Computations.

The molecular structure of trifluorophosphine tetraborane(8), B(4)H(8)PF(3), has been studied in the gas phase by electron diffraction. The experimental data can be fitted using a model which represents the gas as consisting solely of the endo conformer with C(s)() symmetry, the PF(3) group staggered with respect to the B(1)-H(1) bond. Important experimental structural parameters (r(alpha) degrees ) are r[B(1)-B(2)] (hinge-wing) = 184.7(9) pm, r[B(1)-B(3)] (hinge-hinge) = 172.2(12) pm, r[B(2)-B(3)] = 179.9(10) pm,r[B(1)-P] = 179.8(9) pm, and r(P-F) (mean) = 152.8(1) pm; B(3)B(1)P = 131.6(11) degrees, and the dihedral ("butterfly") angle between the planes B(1)B(2)B(3) and B(1)B(4)B(3) is 133.9(23) degrees. These values agree well with the ab initio (MP2/TZP level) optimized molecular geometry for the endo conformer; at the MP2/TZP//MP2/TZP + ZPE(HF/6-31G) level, the exo conformer is predicted to represent ca. 2% of the compound vapor, consistent with the experimental (11)B NMR solution spectrum. The experimental and theoretical geometries are supported by comparison of the calculated (IGLO) (11)B NMR chemical shifts with the experimental NMR data.

Journal Article↗