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Hans Hartl

Publications and source records attributed to Hans Hartl.

9 recordsLinked to original sources
PubMed2005

Synthesis, crystal structure, and solution stability of Keggin-type heteropolytungstates (NH4)6NiII0.5[alpha-FeIIIO4W11O30NiIIO5(OH2)].nH2O, (NH4)7Zn0.5[alpha-ZnO4W11O30ZnO5(OH2)].nH2O, and (NH4)7NiII0.5[alpha-ZnO4W11O30NiIIO5(OH2)].nH2O (n approximately 18).

Reaction of acidified (pH approximately 7) sodium tungstate solutions with transition metal cations (Fe(3+), Ni(2+), Zn(2+), Co(2+)) leads to the formation of transition-metal-disubstituted Keggin-type heteropolytungstates with 3d-metal ions distributed over three different positions. A detailed investigation of the synthesis conditions confirmed that the complexes could equally be obtained using aqueous solutions of either Na(2)WO(4).2H(2)O (sodium monotungstate) at pH approximately 7, Na(6)[W(7)O(24)]. approximately 14H(2)O (sodium paratungstate A), or Na(10)[H(2)W(12)O(42)].27H(2)O (sodium paratungstate B) as starting materials. Three complexes, (NH(4))(6)Ni(II)(0.5)[alpha-Fe(III)O(4)W(11)O(30)Ni(II)O(5)(OH(2))].18H(2)O, (NH(4))(7)Zn(0.5)[alpha-ZnO(4)W(11)O(30) ZnO(5)(OH(2))].18H(2)O, and (NH(4))(7)Ni(II)(0.5)[alpha-ZnO(4)W(11)O(30)Ni(II)O(5)(OH(2))].18H(2)O were isolated in crystalline form. X-ray single-crystal structure analysis revealed that the solid-state structures of the three compounds consist of four main structural fragments, namely [MO(4)W(11)O(30)M'O(5)(OH(2))](n-) (Keggin-type, alpha-isomer) heteropolytungstates, hexaquo metal cations, [M''(OH(2))(6)](2+), ammonium-water cluster ions, [(NH(4)(+))(8)(OH(2))(12)], and additional ammonium cations and water molecules. The 3d metals occupy the central (tetrahedral, M) and the peripheral (octahedral, M') positions of the Keggin anion, as well as cationic sites (M'') outside of the polyoxotungstate framework. UV-vis spectroscopy, solution ((1)H, (183)W) and solid-state ((1)H) NMR, and also chemical analysis data provided evidence that the 3d-metal-disubstituted Keggin anions do not exist in solution but are being formed only during the crystallization process. Investigations in the solid state and in solution were completed by ESR, IR, and Raman measurements.

Journal Article↗
PubMed2005

Neutral and cationic V(IV)/V(V) mixed-valence alkoxo-polyoxovanadium clusters [V6O7(OR)12]n+ (R = -CH3, -C2H5): structural, cyclovoltammetric and IR-spectroscopic investigations on mixed valency in a hexanuclear core.

The alkoxo-polyoxovanadium clusters [V6O7(OR)12]n+ (R = -CH3, -C2H5) are fully alkylated polyoxometalate derivatives comprising a hexavanadate core with the vanadium ions organized in an octahedral fashion, a classic isopolyoxometalate structure (Lindqvist) which as an entity is not known for vanadium. The clusters are highly redox-active compounds, displaying a large number of thermodynamically stable redox isomers of which the chemical syntheses and structural characterization of the neutral and cationic V(IV)/V(V) mixed-valence species [V(IV)(4-n)V(V)(2+n)O7(OR)12]n+ [SbCl6]n (R = -CH3, n = 0, 1; R = -C2H5, n = 0, 1, 2) are presented here. Neutral and positively charged clusters remain exceptional in the field of polyoxometalate chemistry. Results obtained from cyclic voltammetry, infrared spectroscopy, and from valence sum calculations conducted on X-ray structural data classify these clusters as class II mixed-valence compounds. Their highly symmetrical molecular structures make them particularly interesting as model compounds for the investigation of intervalence charge transfer and electron delocalization in the hexanuclear core. Furthermore, the large number of isostructural redox isomers affords a high variability in d-electron content. Accordingly, a dependency could clearly be established between the extent of electron delocalization and the V(IV)/V(V) ratio in a cluster species. A further interesting observation concerns the neutral ethoxo compound [V(IV)4V(V)2O7(OC2H5)12] (3) which exhibits a crystallographic phase transition accompanied by the conversion from a structure at 173 K with fully localized valencies to a room-temperature modification displaying complete d-electron delocalization.

Cations↗
PubMed2005

Phenylene alkylene dendrons with site-specific incorporated fluorescent pyrene probes.

This report deals with the synthesis and the spectroscopic properties of two second generation (G2) dendrons with site-specific incorporated phenyl pyrene derivatives as solvatochromic fluorescent probes. The generations that do not carry the probe are equipped with volume dummies, pyrene moieties that do not show a solvatochromic effect. Two complementary G2 phenylene alkylene dendrons were synthesized using Suzuki-Miyaura cross coupling. Most of the reactions used in the 10-step sequence generating the target compounds proceeded in good yields. The incorporated probes can be selectively photoexcited and show solvatochromic shifts that are of the same magnitude as for the free probes in a homogeneous solvent environment. In addition to the charge-transfer fluorescence, a broad emission band is observed that is assigned to an intramolecular exciplex formation between the aryl pyrene chromophores.

Journal Article↗
PubMed2004

A new color of the synthetic chameleon methoxyallene: synthesis of trifluoromethyl-substituted pyridinol derivatives: an unusual reaction mechanism, a remarkable crystal packing, and first palladium-catalyzed coupling reactions.

Addition of lithiated methoxyallene to pivalonitrile afforded after aqueous workup the expected iminoallene 1 in excellent yield. Treatment of this intermediate with silver nitrate accomplished the desired cyclization to the electron-rich pyrrole derivative 2 in moderate yield. Surprisingly, trifluoroacetic acid converted iminoallene 1 to a mixture of enamide 3 and trifluoromethyl-substituted pyridinol 4 (together with its tautomer 5). A plausible mechanism proposed for this intriguing transformation involves addition of trifluoroacetate to the central allene carbon atom of an allenyl iminium intermediate as crucial step. Enamide 3 is converted to pyridinol 4 by an intramolecular aldol-type process. A practical direct synthesis of trifluoromethyl-substituted pyridinols 4, 10, 11, and 12 starting from typical nitriles and methoxyallene was established. Pyridinol 10 shows an interesting crystal packing with three molecules in the elementary cell and a remarkable helical supramolecular arrangement. Trifluoromethyl-substituted pyridinol 4 was converted to the corresponding pyridyl nonaflate 13, which is an excellent precursor for palladium-catalyzed reactions leading to pyridine derivatives 14-16 in good to excellent yields. The new synthesis of trifluoromethyl-substituted pyridines disclosed here demonstrates a novel reactivity pattern of lithiated methoxyallene which is incorporated into the products as the unusual tripolar synthon B.

Journal Article↗
PubMed2003

A stereoselective and short total synthesis of the polyhydroxylated gamma-amino acid (-)-detoxinine, based on stereoselective preparation of dihydropyrrole derivatives from lithiated alkoxyallenes.

Based on our earlier results employing lithiated methoxyallene 2 as C(3) building block and imines 3 for the synthesis of dihydropyrrole derivatives 5, we have investigated chiral imines 6, 10, and 15 as electrophilic components. Combined with lithiated alkoxyallenes, these imines provide the corresponding primary adducts and finally the dihydropyrrole derivatives 8, 12, 17, 20, and 22 in good yields and with high to excellent syn selectivities. This stereochemical outcome is interpreted as a result of alpha-chelate control. Treatment with hydrochloric acid converted syn-8 and syn-12 into bicyclic compounds 9 and 13, whereas under more mildly acidic conditions adduct syn-17 was transformed into diol syn-18. The total synthesis of the uncommon gamma-amino acid (-)-detoxinine could be achieved by starting from (S)-malic acid, which was converted into imine 15 in four steps. Lithiated benzyloxyallene added to imine 15 and efficiently furnished the crucial dihydropyrrole derivative syn-22. The hydrogenolysis of this compound did not directly provide the protected triol 29 as anticipated, but a stepwise protocol made the triol available in a fairly satisfactory manner. A second crucial step of the synthesis was the selective oxidation of 29, which could be achieved by employing platinum dioxide and oxygen. The resulting bicyclic lactone 30 was smoothly transformed into enantiopure (-)-detoxinine. Thus, a fairly short synthesis of this natural product based on a lithiated alkoxyallene could be performed, demonstrating the potential of these intermediates for syntheses of interesting functionalized heterocyclic compounds.

Alkanes↗
PubMed1999

Halogenated Carboxonium Salts: Preparation and Structural Analysis.

We have studied the preparation and isolation of the halogenated alkylidene oxonium salts CH(3)OCHX(+)MF(6)(-) (X = Cl, F; M = As, Sb). Addition of dichloromethylmethylether at -78 degrees C to the superacidic media HF/MF(5) resulted in the formation of the chlorine compounds CH(3)OCHCl(+)MF(6)(-), whereas at -65 degrees C the fluorinated salts CH(3)OCHF(+)MF(6)(-) are formed by a chlorine/fluorine exchange. The salts are characterized by their low-temperature IR and Raman spectra. Additionally, the crystal structures of the hexafluoroantimonates are presented. The structures of the cations turned out to be different in some points. The salt CH(3)OCHF(+)SbF(6)(-) crystallizes in the monoclinic space group P2(1)/c with 4 molecules in the unit cell and a = 9.942(2), b = 7.454(2), and c = 10.297(3) Å; beta = 111.27(2) degrees. The salt CH(3)OCHCl(+)SbF(6)(-) crystallizes in the monoclinic space group P2(1)/c with 4 formula in the unit cell and a = 5.970(3), b = 12.019(5), and c = 10.994(5) Å; beta = 92.59(3) degrees.

Journal Article↗