Wulfhard Mickler

Integration of the 1,2,3‐Triazole “Click” Motif as a Potent Signalling Element in Metal Ion Responsive Fluorescent Probes

In a systematic approach we synthesized a new series of fluorescent probes incorporating donor-acceptor (D-A) substituted 1,2,3-triazoles as conjugative π-linkers between the alkali metal ion receptor N-phenylaza-[18]crown-6 and different fluorophoric groups with different electron-acceptor properties (4-naphthalimide, meso-phenyl-BODIPY and 9-anthracene) and investigated their performance in organic and aqueous environments (physiological conditions). In the charge-transfer (CT) type probes 1, 2 and 7, the fluorescence is almost completely quenched by intramolecular CT (ICT) processes involving charge-separated states. In the presence of Na(+) and K(+) ICT is interrupted, which resulted in a lighting-up of the fluorescence in acetonitrile. Among the investigated fluoroionophores, compound 7, which contains a 9-anthracenyl moiety as the electron-accepting fluorophore, is the only probe which retains light-up features in water and works as a highly K(+)/Na(+)-selective probe under simulated physiological conditions. Virtually decoupled BODIPY-based 6 and photoinduced electron transfer (PET) type probes 3-5, where the 10-substituted anthracen-9-yl fluorophores are connected to the 1,2,3-triazole through a methylene spacer, show strong ion-induced fluorescence enhancement in acetonitrile, but not under physiological conditions. Electrochemical studies and theoretical calculations were used to assess and support the underlying mechanisms for the new ICT and PET 1,2,3-triazole fluoroionophores.

Systematic Investigation of Photoinduced Electron Transfer Controlled by Internal Charge Transfer and Its Consequences for Selective PdCl2 Coordination

Fluoroionophores of fluorophore-spacer-receptor format were prepared for detection of PdCl(2) by fluorescence enhancement. The fluorescent probes 1-13 consist of a fluorophore group, an alkyl spacer and a dithiomaleonitrile PdCl(2) receptor. First, varying the length of the alkylene spacer (compounds 1-3) revealed a dominant through-space pathway for oxidative photoinduced electron transfer (PET) in CH(2)-bridged dithiomaleonitrile fluoroionophores. Second, fluorescent probes 4-9 containing two anthracene or pyrene fragments connected through CH(2) bridges to the dithiomaleonitrile unit were synthesized. Modulation of the oxidation potential (E(Ox)) through electron-withdrawing or -donating groups on the anthracene moiety regulates the thermodynamic driving force for oxidative PET (DeltaG(PET)) in bis(anthrylmethylthio)maleonitriles and therefore the fluorescence quantum yields (Phi(f)), too. The new concept was confirmed and transferred to pyrenyl ligands, and fluorescence enhancements (FE) greater than 3.2 in the presence of PdCl(2) were achieved by 7 and 8 (FE=5.4 and 5.2). Finally, for comparison, monofluorophore ligands 10-13 were synthesized.

Voltammetric and Potentiometric Studies on the Stability of Vanadium(IV) Complexes. A Comparison of Solution Phase Voltammetry with the Voltammetry of Microcrystalline Solid Compounds

Formal potentials of non-oxo vanadium complexes obtained by the cyclic voltammetry of solutions and of solid microcrystalline compounds are compared with potentiometrically determined stability constants of the corresponding oxovanadium(IV) complexes. The stability constants show a qualitative correlation with the formal potentials. The non-oxo complexes exhibit a reversible one-electron system VIV/VIII as well as VIV/VV in dichloromethane solution whereas the oxo complexes only show the transition between VIV and VV. Generally, the oxidation of non-oxo vanadium(IV) complexes proceeds at higher potentials than that of the corresponding oxovanadium(IV) species. The shift of the formal potentials is clearly correlated with the structure of the compounds.

Voltammetric studies of non-oxo vanadium(IV) chelates

A series of non-oxo vanadium(IV) chelates with tridentate diacidic ligands was studied by cyclovoltammetry using a platinum electrode and by differential pulse polarography using a hanging mercury drop electrode. The reversibility of the electrode processes was observed for the electrochemical redox reaction. A diffusion-controlled one-electron transfer was mostly found. The influence of structural parameters on the stability of the compounds was discussed considering the shift of the reduction potential of the complexes.

Supramolecular Assemblies with Honeycomb Structures by π‐π Stacking of Octahedral Metal Complexes of 1,12‐Diazaperylene (Eur. J. Inorg. Chem. 8/2006)

The cover picture shows the honeycomb structure formed by π-π stacking of homoleptic octahedral NiII complexes of the “large-surface” ligand 1,12-diazaperylene (dap), [Ni(dap)3](BF4)2 (Ni red, N green, C dark yellow, B blue, F light yellow). The [Ni(dap)3]2+ cation and one of the BF4− counterions form parallel nanosized channels (ca. 9.60 A° in diameter), which run along the crystallographic c axis and are filled with the other disordered BF4− counterion (omitted for clarity). The C3 symmetry of [Ni(dap)3]2+ is a prerequisite for forming the highly symmetrical supramolecular assembly. Details are discussed in the article by H.-J. Holdt et al. on p. 1547 ff.

Separation and Concentration of Pd(II) with Immobilized Maleonitrile‐Dithiocrown Ethers

Especially sulphur containing compounds are suitable for the separation of noble metals[ , ]. 1,2-Dithioethenes are weak chelate-forming ligands [ ]and in the case of bis(methylthio)maleonitrile[ ] the donor power of both of the sulphur atoms is further decreased by the electron withdrawing effect of the cyanogroups. Crowned dithiomaleonitrile are macrocyclic chelate ligands which extract Pd(II) at sufficient rate in a very good yields. The reason for that extraction behaviour is the fact that Pd(II) favours the square planar coordination geometry in opposite to the 3d- elements. The synthesis of the immobilized ligands proceeds from the 2-allyloxy-1,2-propanediol forming the dicarbon acid which is reduced to the diole. With the help of thionylchloride the dichloro compound is synthesized forming together with a dithiolate (1,2-disodium-1,2-dicyanethene-1,2-dithiolate, 1,2-disodium-4-methylbenzene-1,2-dithiolate [ ]) at high dilution conditions the macrocycle. Than the allysubstituted crown ether is sillylated and the resulting alkoxysilane is immobilised onto activated silca gel. The substituent forms in the same time a spacer should be modified in the future. By immobilisation at an inactive matrix the selectivity of the ligand should be applied for the accumulation of palladium from diluted solutions. The extraction was performed from nitric acid solution with a yield of 93% into a ligand solution (chloroform, kerosine). The extraction equilibrium is reached after 10 min. By atomic absorption spectroscopy the metal concentration in the aqueous phase was determined to calculate the extraction rate. By modification of the cavity of the macrocyclus the extraction rate increases from the acyclic compound through maleonitrile-dithio-21-crown-7, maleonitrile-dithio-15-crown-5 and maleonitrile-18-crown-6. The best results can be observed at the maleonitrile-dithio-12-crown-4. The rise of the function lg D= f(lg L) gives the composition of the extracted compounds as 1:1. The separation is unsatisfactory in the case of Ag(I), Hg(II), Pt(II), Tl (I) and the most 3d-elements. Also Ni(II) as a representative for the 3d-elements shows only separations coefficients of 1.43?103 . Summarizing, a very good separation of palladium from the examined elements can be specified. Additional to the extraction experiments, as well as the crystal structures and by UV spectroscopy the formation constants of selected chelates were determined. The observed order corresponds to that found by the extraction of palladium in the system water/chloroform. In the case of maleonitrile-dithio-15-crown-5 Ag(I) is endocyclic coordinated with all donor atoms of the macrocyclus. Already maleonitrile-dithio-18-crown-6 can include the silver cation into its greater cavity. In these cases a 1:1 complex is formed. A 1:2 sandwich structure was noticed only in the case of the smaller ligand maleonitrile- dithio-12-crown-4. Obviously, the formation of that structure is not favoured in the system water/chloroform from which can be explained the unfavourable extraction results.

Komplexbildung und Metallextraktion mit heterocyclischen β-Dicarbonylverbindungen im Vergleich. Struktur von 3-Phenyl-4-benzoyl-isoxazol-5-on / Complex Formation and Metal Extraction with Heterocyclic β-Dicarbonyl Compounds as Comparison. Structure of 3-Phenyl-4-benzoyl-5-isoxazolone

Acid constants and stability constants of nickel and zinc chelates with 2-thenoyltrifluoroacetone, dibenzoylmethane, 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone and 3-phenyl-4-benzoyl-5-isoxazolone were compared. The extraction of copper was studied. Thermoanalytic measurements were made using the ligands and their copper compounds in the solid state. The molecular structure of 3-phenyl-4-benzoyl-5-isoxazolone was determined by X-ray analysis. Crystal data: a = 874,7(2), b = 1959,2(7), c = 897,38(7) pm; β = 101,030(7)°; space group P 121/c 1; Z = 4; R = 0,043; 1920 observed, unique reflexions.