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Manfred Zabel

Publications and source records attributed to Manfred Zabel.

17 recordsLinked to original sources
PubMed2006

Lewis base stabilized phosphanylborane.

The abstraction of the Lewis acid from [W(CO)(5)(PH(2)BH(2)NMe(3))] (1) by an excess of P(OMe(3))(3) leads to the quantitative formation of the first Lewis base stabilized monomeric parent compound of phosphanylborane [H(2)PBH(2)NMe(3)] 2. Density functional theory (DFT) calculations have shown a low energetic difference between the crystallographically determined antiperiplanar arrangement of the lone pair and the trimethylamine group relative to the P-B core and the synperiplanar conformation. Subsequent reactions with the main-group Lewis acid BH(3) as well as with an [Fe(CO)(4)] unit as a transition-metal Lewis acid led to the formation of [(BH(3))PH(2)BH(2)NMe(3)] (3), containing a central H(3)B-PH(2)-BH(2) unit, and [Fe(CO)(4)(PH(2)BH(2)NMe(3))] (4), respectively. In oxidation processes with O(2), Me(3)NO, elemental sulfur, and selenium, the boranylphosphine chalcogenides [H(2)P(Q)BH(2)NMe(3)] (Q = S 5 b; Se 5 c) as well as the novel boranyl phosphonic acid [(HO)(2)P(O)BH(2)NMe(3)] (6 a) are formed. All products have been characterized by spectroscopic as well as by single-crystal X-ray structure analysis.

Journal Article↗
PubMed2005

Chemistry of transition-metal clusters with mixed Sb/S ligands: evidence for a terminal Sb=S double bond in Cp*3Rh3Sb2S5 (Cp* = C5Me5).

The reaction of [Cp2*Rh2Cl4] (Cp* = C5Me5) with a slight excess of K(3)SbS(3) in boiling THF gave the neutral clusters [Cp*4Rh4S5] (1), [Cp*3Rh3Sb2S5] (2), and after salt metathesis [Cp*3Rh3SbSn]PF6 (3; n = 5 and 6). The structures of 1-3 are heterocubane clusters with CpRh, S, and Sb vertices but with sulfur inserted into one (1 and 2) or two (3) edges. X-ray diffraction analysis of 2 additionally reveals a very short Sb-S distance of 2.297(1) A within the novel mu3-Sb2S4 ligand. Density functional theory calculation of the model compounds [SSbS]3-, [HSSbS]2-, and [HSSbH2S]0 provided strong evidence for the existence of a stable terminal Sb=S double bond in 2.

Journal Article↗
PubMed2005

Enantiomerically pure pentagonal-bipyramidal metal complexes with predetermined helicity in the solid and solution states.

New metal complexes with pentagonal-bipyramidal geometry have been synthesized with the chiral, pentadentate bis(oxazoline) ligand (R,R)-1, including the metal ions magnesium(II), iron(II), and cadmium(II). In the solid state, a complete transfer of chirality from the ligand is observed to exclusively yield enantiomerically pure P-helical, isostructural pentagonal bipyramidal complexes, as determined by X-ray analysis of four compounds. This uncommon coordination geometry is likely to be driven by pi-pi-stacking of the terminal phenyl groups of the linear ligands. The complex cations in [Fe((R,R)-1)(H2O)2](ClO4)2 (3), [Cd((R,R)-1)(H2O)2](ClO4)2 (4), and [Mg((R,R)-1)(H2O)2](ClO4)2 (5) are mononuclear with the two apical positions of the pentagonal bipyramide occupied by two water molecules. In contrast, the structure in dinuclear [Cd((R,R)-1)(MeOH)(mu-I)(CdI3)] (2c) can be described as pentagonal-bipyramidal around cadmium with MeOH and distorted-tetrahedral CdI4 (via one bridging iodo ligand) completing the coordination sphere in axial positions. The crystal packing of 3-5 shows a highly ordered orientation of the mononuclear helical cations into one-dimensional chains along the crystallographic axis a, stabilized by intermolecular pi-pi-stacking. In contrast, the dinuclear helices in 2c are tilted relative to one another, and consequently, directed, one-dimensional helicity in the solid state is not observed. Studies using a combination of mass spectrometry and NMR and CD spectroscopy indicate the presence of only one C2-symmetrical, mononuclear species in acetonitrile for each case, suggesting the formation of diastereo- and enantiomerically pure complexes also in the solution state. All compounds exhibit a very characteristic and almost identical CD pattern between 200 nm and 300 nm. This signal can be attributed to the P-helical, pentagonal arrangement of the ligand.

Journal Article↗
PubMed2004

Helical, nonracemic inorganic-organic hybrid polymers of cadmium halides with pentadentate bis(oxazoline) ligands.

A chiral, nonracemic pentadentate pyridine bis(oxazoline) ligand forms unprecedented P-helical inorganic-organic hybrid polymers with cadmium halides. The one-dimensional chains consist of Lambda2-configured pentagonal-bipyramidal cadmium complexes with metal-centered chirality bridged by [CdX4]2- tetrahedra via shared halide atoms. In the solid state, the overall helicity exhibits strongly directed orientation parallel to the crystallographic axis a.

Journal Article↗
PubMed2004

How delocalized is N,N,N',N'-tetraphenylphenylenediamine radical cation? An experimental and theoretical study on the electronic and molecular structure.

The electronic and molecular structure of N,N,N',N'-tetraphenylphenylenediamine radical cation 1(+) is in focus of this study. Resonance Raman experiments showed that at least eight vibrational modes are strongly coupled to the optical charge resonance band which is seen in the NIR. With the help of a DFT-based vibrational analysis, these eight modes were assigned to symmetric vibrations. The contribution of these symmetric modes to the total vibrational reorganization energy is dominant. These findings are in agreement with the conclusions from a simple two-state two-mode Marcus-Hush analysis which yields a tiny electron-transfer barrier. The excellent agreement of the X-ray crystal structure analysis and the DFT computed molecular structure of 1(+) on one hand as well as the solvent and solid-state IR spectra and the DFT-calculated IR active vibrations on the other hand prove 1(+) adopts a symmetrical delocalized Robin-Day class III structure both in the solid state and in solution.

Journal Article↗
PubMed2004

Synthesis and peptide binding properties of methoxypyrrole amino acids (MOPAS).

[structure: see text] Methoxypyrrole amino acids (MOPAS) have been prepared and were introduced into small peptides with hairpin structures. The intra- and intermolecular binding properties of this heterocyclic amino acid mimicking a dipeptido beta-strand was investigated by NMR titration and X-ray crystal structure analysis. The data reveal a hydrogen bonding pattern that is complementary to a peptide beta-sheet.

Amino Acids↗
PubMed2004

Isochroman derivatives and their tendency to crystallize in chiral space groups.

In methyl [5-methoxy-4-(4-methoxyphenyl)isochroman-3-yl]acetate, C(20)H(22)O(5), (I), and methyl [4-(2,5-dimethoxyphenyl)-8-methoxyisochroman-3-yl]acetate, C(21)H(24)O(6), (II), the heterocyclic rings adopt half-chair conformations. The substituents at the 3- and 4-positions are in a trans configuration in both (I) and (II), being in an axial conformation in (I) and in an equatorial conformation in (II). The crystal structure of (I) is stabilized by weak C-H.O hydrogen bonding, leading to the formation of an infinite three-dimensional network. Compound (II) crystallizes in a chiral space group. This feature, which was also found in previously investigated isochroman derivatives, is related to the arrangement of substituents attached to the isochroman moiety.

Journal Article↗
PubMed2003

Tetrahydroxy 10-membered cyclic enediynes.

The preparation of cyclic 10-membered tetrahydroxy enediynes is reported. The synthesis starts from tartaric acid and allows the control of the relative stereochemistry. Acetal protection of the 2,3-hydroxy groups stabilizes the enediyne during synthesis. Removal of the cyclic protecting group with EtSH/TFA transforms the stable compounds into reactive enediynes, and the rate constants of their cyclization were determined in benzene and water. The cytoxicity of the activated compounds was assayed against tumor cells in vitro, but the growth inhibitory effect was weak compared to cisplatin.

Cell Line, Tumor↗
PubMed2003

Highly substituted azulene dyes as multifunctional NLO and electron-transfer compounds.

Two highly substituted azulene derivatives were synthesised by Pd-mediated dimerisation from the corresponding tolan species. One azulene derivative (2) has donor functionalities (dianisylaminophenyl and dianisylamino) in the 1-, 2-, 3- and 6-positions, while the other (1) has donors (dianisylaminophenyl) in the 2- and 6-positions and acceptors (nitrophenyl) in the 1- and 3-positions. Each azulene derivative shows strong bond length alternation in the solid state, determined by X-ray crystal analysis, and an intense CT band around 450-500 nm in its UV/Vis spectrum. The first-order hyperpolarisability of 1 and of 2 was measured by hyper-Rayleigh scattering and is about that of disperse red DR1. Both azulene derivatives show multiple oxidation processes. The intramolecular adiabatic ET behaviour of the mixed valence radical cations of 1 and of 2 was investigated by UV/Vis/NIR spectroelectrochemistry. The intervalence-CT band of 1(+) could be analysed by the Generalised Mulliken-Hush theory, which yields an electronic coupling V=1140 cm(-1) for the optically induced adiabatic hole transfer.

Journal Article↗
PubMed2003

Preparative and electrochemical investigations on the electron sponge behavior of cobalt telluride clusters: CO substitution in [Co11Te7(CO)10]n- ions (n=1, 2) by PMe2Ph and crystal structure of [Co11Te7(CO)5(PMe2Ph)5].

The reaction of the cluster salts [Cp(2*) Nb(CO)(2)](n)[Co(11)Te(7)(CO)(10)] (Cp*=C(5)Me(5); n=1, 2) with excess PMe(2)Ph gave the neutral, dark brown clusters [Co(11)Te(7)(CO)(6)(PMe(2)Ph)(4)] (5) and [Co(11)Te(7)(CO)(5)(PMe(2)Ph)(5)] (6) with 147 metal valence electrons. The new compounds were characterized by IR spectroscopy, elemental analyses, and mass spectrometry. The molecular structure of 6 was determined by X-ray crystallography. Like its precursor anion, it consists of a pentagonal-prismatic [Co(11)Te(7)] core, but with a ligand sphere composed of five CO and five PMe(2)Ph ligands. Detailed electrochemical studies of both reactions reveal that a stepwise substitution of CO ligands in the initial cluster anions takes place leading to intermediate [Co(11)Te(7)(CO)(10-m)(PMe(2)Ph)(m)](n-) ions (m=1-5; n=1, 2). Each of these intermediates is distinguished by at least one oxidation and two reduction waves, giving rise to a total of 21 redox couples and 27 electroactive species. The electron sponge character of the new compounds is particularly pronounced in 5, which exhibits charges n between +1 and -4 corresponding to metal valence electron counts of between 146 and 151.

Journal Article↗
PubMed2003

A versatile strategy for the synthesis of functionalized 2,2'-bi- and 2,2':6',2' '-terpyridines via their 1,2,4-triazine analogues.

A general synthetic route for the synthesis of functionalized bi- and terpyridines is reported. Functionalized 1,2,4-triazene 4-oxides 7 and 8-obtained from the reaction of hydrazones 1 with pyridine aldehydes and followed by oxidation-are functionalized by introduction of a cyano group via nucleophilic aromatic substitution. The thus-obtained 5-cyano-1,2,4-triazines 9 and 10 undergo facile inverse-electron-demand Diels-Alder reactions with enamines and alkenes to yield functionalized bi- and terpyridines, respectively. The substituent at position 6 of the 1,2,4-triazene 4-oxides must be aromatic or heteroaromatic in order to allow their facile synthesis, but other substituents and reagents may vary. Each step of the synthetic route allows diversification, which makes the approach particularly useful for the facile synthesis of a large variety of functionalized bi- and terpyridines.

Journal Article↗
PubMed2002

Synthesis and structure of a stabilized 10-membered cyclic enediyne.

The synthesis and structure of an acetal protected 10-membered cyclic enediyne-1,2-diol rac-10 is reported. The conformational constrain of the unsaturated macrocycle by the acetal protection group prevents the thermal cyclization reaction of the endiyne during synthesis and purification.

Journal Article↗
PubMed2001

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