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Andreas W Ehlers

Publications and source records attributed to Andreas W Ehlers.

At least 19 recordsLinked to original sources
PubMed2006

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.

Journal Article↗
PubMed2006

Circumambulatory rearrangement with characteristics of a 2:1 covalent molecular bevel gear.

anti-W(CO)(5)-complexed 9-methyl-9-phosphabicyclo[6.1.0]nonatriene represents a covalently interlocked molecular bevel gear. Correlated movement of the phosphorus atom and the eight-membered ring by way of a "walk" rearrangement makes gear slippage impossible. The gearing motion is transferred to the four-toothed W(CO)(5) propeller connected to the rotating phosphorus atom, enabling a gearing ratio of 2:1 according to B3LYP and Car-Parrinello Molecular Dynamics calculations. Methyl substitution of the eight-membered ring tempers the gearing process, with the PMeW(CO)(5) entity passing the substituted carbon atom only at temperatures above 50 degrees C.

Bridged Bicyclo Compounds↗
PubMed2006

Bidentate phosphorus baskets by intramolecular phosphinidene addition.

Intramolecular phosphinidene addition to the C==C bond of Mo-complexed, seven-membered phosphorus heterocycles affords three novel [(diphos)Mo(CO)(4)] complexes (18-20). The three bidentate phosphorus baskets differ in the composition of the seven-membered ring: one of the phosphorus atoms is flanked by CH(2), NCH(3), or O. The unsaturated tetrahydrophosphepine precursors are synthesized by either ring-closing metathesis (C and N derivatives) or by a cyclization sequence (O derivative). The crystal structures of the nitrogen- (19) and oxygen-containing (20) baskets have relatively small P-Mo-P angles of 76.240(13) degrees and 77.626(12) degrees , respectively, and complex 20 has slightly shortened Mo--P bond lengths.

Molecular Conformation↗
PubMed2006

Valence isomerization of 2-phospha-4-silabicyclo[1.1.0]butane: a high-level ab initio study.

The rearrangements for 2-phospha-4-silabicyclo[1.1.0]butane, analogous to the valence isomerization of the hydrocarbons bicyclobutane, 1,3-butadiene, and cyclobutene, were studied at the (U)QCISD(T)/6-311+G**//(U)QCISD/6-31G* level of theory. The monocyclic 1,2-dihydro-1,2-phosphasiletes are shown to be the thermodynamically preferred product, in contrast to the isomerization of the hydrocarbons, which favors the 1,3-butadiene structure. Furthermore, an unprecedented direct isomerization pathway to the 1,2-dihydro-1,2-phosphasiletes was identified. This pathway is competitive with the isomerization via the open-chain butadienes and becomes favorable when electron-donating substituents are present on silicon. Figure 2-Phospha-4-silabicyclo[1.1.0]butane can isomerize directly into the more stable P,Si-cyclobutene via an unprecedented [sigma2s+sigma2a] process, which becomes favorable over the isomerization via the P,Si-butadiene when electron-donating substituents are present on silicon.

Bridged Bicyclo Compounds, Heterocyclic↗
PubMed2006

Consistent theoretical description of 1,3-dipolar cycloaddition reactions.

The cycloaddition reactions of 18 1,3-dipolar molecules to ethylene and acetylene have been reinvestigated by quantum chemical methods that are based on a second-order perturbation treatment of electron correlation. It is found that SCS-MP2 and the new perturbative B2-PLYP density functional provide accurate reaction barriers and outperform MP2 as well as standard density functionals such as B3-LYP. The new second-order based methods have the additional advantage that they perform better with increasing quality of the one-particle space, as is desired for a good quantum chemical method. The errors for the reaction enthalpies are in general larger than for the barriers when compared to CBS-QB3 literature values, which is related to strong changes in the electronic structures, but the deviations are again smaller than with MP2 or B3-LYP and are also more systematic. The results of a detailed basis set study suggest that properly polarized triple-zeta AO basis sets represent a good compromise between accuracy and computational speed. The combination of very inaccurate density functionals with small (double-zeta) basis sets, which yields good results for the initial part of the reactions due to error compensation, is not recommended.

Letter↗
PubMed2005

3H-benzophosphepine complexes: versatile phosphinidene precursors.

The synthesis of a variety of benzophosphepine complexes [R = Ph, t-Bu, Me; ML(n )()= W(CO)(5), Mo(CO)(5), Cr(CO)(5), Mn(CO)(2)Cp] by two successive hydrophosphinations of 1,2-diethynylbenzene is discussed in detail. The first hydrophosphination step proceeds at ambient temperature without additional promoters, and subsequent addition of base allows full conversion to benzophosphepines. Novel benzeno-1,4-diphosphinanes were isolated as side products. The benzophosphepine complexes themselves serve as convenient phosphinidene precursors at elevated, substituent-dependent temperatures (>55 degrees C). Kinetic and computational analyses support the proposal that the phosphepine-phosphanorcaradiene isomerization is the rate-determining step. In the absence of substrate, addition of the transient phosphinidene to another benzophosphepine molecule is observed, and addition to 1,2-diethynylbenzene furnishes a delicate bidentate diphosphirene complex.

Journal Article↗
PubMed2005

G3(MP2) ring strain in bicyclic phosphorus heterocycles and their hydrocarbon analogues.

[Figure: see text] The ring strain present in 2-aza-1-phosphabicyclo[n.1.0]alka(e)nes (n = 1-5) is calculated at the G3(MP2) level using homodesmotic reactions. The influence of cyclopropa(e)nation and heteroatom substitution is analyzed by a comparison with the corresponding bicyclic hydrocarbons and separate ring systems. It is shown that the strain caused by fusion with a cyclopropane is the sum of the separate rings, whereas the strain resulting from fusion with cyclopropene leads to strain energies much larger than the sum of rings, as a result of the inverted nature of the bridgehead carbon. In all ring structures but one, cyclohexane, substitution by nitrogen and phosphorus is favorable and the effect is most pronounced for the most condensed structures. The calculated strain energies correlate very well with the experimental stability and reactivity of the bicyclic iron-amino phosphirane and phosphirene complexes.

Journal Article↗
PubMed2005

Methylene-azaphosphirane as a reactive intermediate.

Reaction of the transient phosphinidene complexes R-P=W(CO)5 with N-substituted-diphenylketenimines leads unexpectedly to the novel 2-aminophosphindoles, as confirmed by an X-ray crystal structure determined for one of the derivatives. Experimental evidence for a methylene-azaphosphirane intermediate was found by using the iron-complexed phosphinidene iPr2N-P=Fe(CO)4, which affords the 2-aminophosphindole together with the novel methylene-2,3-dihydro-1H-benzo[1,3]azaphosphole. Analysis of the reaction pathways with DFT indicates that the initially formed methylene-azaphosphirane yields both phosphorus heterocycles by way of a [1,5]- or [1,3]-sigmatropic shift, respectively, followed by a H-shift. Strain underlies both rearrangements, which causes these remarkably selective conversions that can be tuned by changing the substituents.

Journal Article↗
PubMed2005

Strained, stable 2-aza-1-phosphabicyclo[n.1.0]alkane and -alkene Fe(CO)4 complexes with dynamic phosphinidene behavior.

The synthesis of highly strained bicyclic phosphirane and phosphirene iron-tetracarbonyl complexes, that is, complexes with 2-aza-1-phosphabicyclo[n.1.0]alkanes and -alkenes (n = 3-5), is explored by using intramolecular cycloaddition of an in situ generated electrophilic phosphinidene complex, [R(iPr)NP=Fe(CO)(4)], to its C=C- and C[triple chemical bond]C-containing R substituent. Saturated bicyclic complexes 7 a-c with n = 4-2 are remarkably stable, as illustrated by the X-ray crystal structure for 7 b (n=3), yet all readily undergo retroaddition to react with phenylacetylene. Shuttling of the phosphinidene iron complex between two equivalent C=C groups is demonstrated for a 1-butene-substituted 2-aza-1-phosphabicyclo[3.1.0]hexane by selective (1)H NMR magnetization transfer from the phosphirane protons to the olefinic protons. Even the more strained unsaturated bicycles 17 a,b (n = 4,3) are surprisingly stable as illustrated by the X-ray crystal structure for 17 a (n = 4), but the smaller phosphabicyclo[3.1.0]hex-5-ene (17 c, n = 2) dimerizes to tricyclic 19 with a unique ten-membered heterocyclic ring; an X-ray crystal structure is reported. Like their saturated analogues also the bicyclic phosphirenes readily undergo retroaddition as shown by the reaction of their phosphinidene iron moiety with phenylacetylene.

Alkenes↗
PubMed2005

Phosphepines: convenient access to phosphinidene complexes.

Reaction of o-diethynylbenzene with transition metal-complexed primary phosphines gives in a single base-induced step stable phosphepine complexes as confirmed by X-ray data. At 75-80 degrees C these phosphepines undergo clean cheletropic elimination of naphthalene to give transient carbene-like phosphinidene complexes that can be trapped in high yield by alkenes, alkynes, and alcohols.

Journal Article↗
PubMed2005

Electronic ground states of iron porphyrin and of the first species in the catalytic reaction cycle of cytochrome P450s.

Electronic structures of iron(II) and iron(III) porphyrins are studied with density functional theory (DFT) using the GGA exchange functional OPTX in combination with the correlation functional PBE (OPBE) and with the correlation functional Perdew (OPerdew) together with a triple zeta-type basis set. These functionals, known for accurately predicting the spin ground state of iron complexes, are evaluated against other functionals for their performance in calculating relative energies for the various electronic states of both the iron porphyrins. The calculated energy orderings are triplet < quintet < singlet for the iron(II) porphyrin and quartet < sextet < doublet for the iron(III) porphyrin cation. Complexation by a thiolate ion (SH-) changes the preferred ground state for both species to high spin. This thiolate complex is used as a mimic for the cytochrome P450s active site to model the first step of the catalytic cycle of this enzyme. This first step is believed to concern the removal of an axial oxygen donating ligand from the hexacoordinated aqua-thiolate-porphyrin-iron(III) resting state. The DFT results suggest that this is not a free water molecule, because of its repulsive nature, but that it has instead hydroxy anion character. These calculations are in line with the experimentally observed change in the spin state from low to high spin upon this removal of the axial hydroxo ligand by binding of the substrate in the heme pocket of cytochrome P450.

Binding Sites↗
PubMed2004

Improved reaction and activation energies of [4+2] cycloadditions, [3,3] sigmatropic rearrangements and electrocyclizations with the spin-component-scaled MP2 method.

A new quantum mechanical scheme to calculate electronic correlation energies, spin-component-scaled MP2, was tested as a tool to predict reaction energies and barriers in computational organic chemistry. Three common pericyclic reactions with known unsatisfactory MP2 descriptions were reinvestigated with the modified MP2 approach, in which the parallel and anti-parallel spin components of the correlation energy are scaled separately. The SCS-MP2 calculated reaction and activation energies of nine Diels-Alder reactions, four [3,3] sigmatropic rearrangements, and ten electrocyclization reactions are compared to those of the MP2, B3 LYP, QCISD(T), and G3 methods. For each, the SCS-MP2 results are in excellent agreement with the experimental data and compare far more favorably to G3 than both MP2 and B3 LYP. Careful evaluation of the effect of the size of the atomic orbital (AO) basis set shows that the larger expansions improve the agreement with experiment for the SCS-MP2 method, while they get worse for both MP2 and B3 LYP.

Journal Article↗
PubMed2004

C-C bond insertion of a complexed phosphinidene into 1,6-methano[10]annulene.

Reaction of electrophilic phosphinidene complex [MePW(CO)5] with 1,6-methano-[10]annulene results in the sole formation of the isomeric C-C insertion products 6 c (main) and 6 d (minor). The single-crystal X-ray structure of the complexed 1,7-methano-3-phospha[11]annulene (6 c) shows a syn-W(CO)5 group at the exo bent phosphorus. The structure displays C-C bond alternation without bonding between the bridgehead carbon atoms. Density functional theory calculations indicate 6 c to result from a concerted disrotatory ring opening of an undetected tricyclic exo-syn phosphirane intermediate. The endo-anti phosphirane cannot undergo ring expansion, due to the high barrier that is associated with an intramolecular antara-antara retro Diels-Alder reaction. The stabilizing effect of transition-metal coordination is discussed.

Journal Article↗
PubMed2004

Generating and dimerizing the transient 16-electron phosphinidene complex [Cp*Ir=PAr]: a theoretical and experimental study.

The properties of the 16-electron phosphinidene complex [CpRIr=PR] were investigated experimentally and theoretically. Density functional theory calculations show a preferred bent geometry for the model complex [CpIr=PH], in contrast to the linear structure of [CpIr=NH]. Dimerization to give [[CpIr=PH]2] and ligand addition to afford [Cp(L)Ir=PH] (L=PH3, CO) were calculated to give compounds that were energetically highly favorable, but which differed from the related imido complexes. Transient 16-electron phosphinidene complex [Cp*Ir=PAr] could not be detected experimentally. Dehydrohalogenation of [Cp*IrCl2(PH2Ar)] in CH2Cl2 at low temperatures resulted in the novel fused-ring systems 17 (Ar=Mes*) and 20 (Ar=Mes), with dimeric [[Cp*Ir=PAr]2] being the likely intermediate. Intramolecular C-H bond activation induced by steric factors is considered to be the driving force for the irreversible formation of 17 and 20. ONIOM calculations suggest this arises because of the large steric congestion in [[Cp*Ir=PAr]2], which forces it toward a more reactive planar structure that is apt to rearrange.

Computer Simulation↗