Kaspar Hegetschweiler

4‐[3,5‐Bis(2‐hydroxyphenyl)‐1,2,4‐triazol‐1‐yl]‐ benzoic Acid: A Novel Efficient and Selective Iron(III) Complexing Agent

An exceptionally stable 1:2 complex [FeL2]3− is formed from the ligand H3L and FeIII. In contrast, the affinity of this ligand for other biometals is relatively small. These properties make H3L a highly promising candidate for medical applications (e.g. for the treatment of iron overload).

Recognition of Ionic Guests by Ionic β‐Cyclodextrin Derivatives

Inclusion compounds of cationic, anionic, and neutral p-substituted derivatives of tert-butylbenzene complexed in beta-cyclodextrin and its ionic 6-mono and 6-hepta derivatives were systematically investigated by isothermal titration calorimetry (ITC). All inclusion compounds showed 1:1 stoichiometry with binding constants ranging from 10 to 3 x 10(6) M(-1). The binding free energies could be subdivided into apolar and electrostatic contributions. The electrostatic interactions could be quantitatively described by Coulombs law by taking into account the degree of protonation of hosts and guests, the orientations of the guests within the hosts, and ion shielding as described by the Debye-Hückel-Onsager theory. The orientations of the guests within the cyclodextrin cavities were determined by ROESY NMR spectroscopy.

Complex Formation of NiII, CuII, PdII, and CoIII with 1,2,3,4‐Tetraaminobutane

Complex formation of the two tetraamine ligands (2S,3S)-1,2,3,4-tetraaminobutane (threo-tetraaminobutane, ttab) and (2R,3S)-1,2,3,4-tetraaminobutane (erythro-tetraaminobutane, etab) with Co(III), Ni(II), Cu(II), and Pd(II) was investigated in aqueous solution and in the solid state. For Ni(II) and Cu(II), the pH-dependent formation of a variety of species [Mn(II)xLyHz](2x+z)+ was established by potentiometric titrations and UV/Vis spectroscopy. In sufficiently acidic solutions the divalent cations formed a mononuclear complex with the doubly protonated ligand of composition [M(H2L)]4+. An example of such a complex was characterized in the crystal structure of [Pd(H2ttab)Cl2]Cl2.H2O. If the metal cation was present in excess, increase of pH resulted in the formation of dinuclear complexes [M2L]4+. Such a species was found in the crystal structure of [Cu2(ttab)Br4].H2O. Excess ligand, on the other hand, lead to the formation of a series of mononuclear bis-complexes [Mq(HxL)(HyL)](q+x+y)+. The crystal structure of [Co(Hetab)2][ZnCl4]2Cl. H2O with the inert, trivalent Co(III) center served as a model to illustrate the structural features of this class of complexes. By using an approximately equimolar ratio of the ligand and the metal cation, a variety of polymeric aggregates both in dilute aqueous solution and in the solid state were observed. The crystal structure of Cu2(ttab)3Br4, which exhibits a two-dimensional, infinite network, and that of [Ni8(ttab)12]Br16.17.5H2O, which contains discrete chiral [Ni8(ttab)12]16+ cubes with approximate T symmetry, are representative examples of such polymers. The energy of different diastereomeric forms of such complexes with the two tetraamine ligands were analyzed by means of molecular mechanics calculations, and the implications of these calculations for the different structures are discussed.

VIVO versus VIV complex formation by tridentate (O, Narom, O) ligands: Prediction of geometry, EPR 51V hyperfine coupling constants, and UV-Vis spectra

Systems formed using the V(IV)O(2+) ion with tridentate ligands containing a (O, N(arom), O) donor set were described. Examined ligands were 3,5-bis(2-hydroxyphenyl)-1-phenyl-1H-1,2,4-triazole (H2hyph(Ph)), 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid (H3hyph(C)), 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzenesulfonic acid (H3hyph(S)), and 2,6-bis(2-hydroxyphenyl)pyridine (H2bhpp), with H3hyph(C) being an orally active iron chelator that is commercially available under the name Exjade (Novartis) for treatment of chronic iron overload arising from blood transfusions. The systems were studied using EPR, UV-Vis, and IR spectroscopies, pH potentiometry, and DFT methods. The ligands bind vanadium with the two terminal deprotonated phenol groups and the central aromatic nitrogen to give six-membered chelate rings. In aqueous solution the main species were the mono- and bis-chelated V(IV)O complexes, whereas in the solid state neutral non-oxido V(IV) compounds were formed. [V(hyph(Ph))2] and [V(bhpp)2] are hexacoordinated, with a geometry close to the octahedral and a meridional arrangement of the ligands. DFT calculations allow distinguishing V(IV)O and V(IV) species and predicting their structure, the (51)V hyperfine coupling constant tensor A, and the electronic absorption spectra. Finally, EPR spectra of several non-oxido V(IV) species were compared using relevant geometrical parameters to demonstrate that in the case of tridentate ligands the (51)V hyperfine coupling constant is related to the geometric isomerism (meridional or facial) rather than the twist angle Φ, which measures the distortion of the hexacoordinated structure toward a trigonal prism.

Surface complexation of [Nb6O19]8− with NiII: solvothermal synthesis and X-ray structural characterization of two novel heterometallic NiNb-polyoxometalates ☆

Abstract Solvothermal reaction of NiCl2, Nb(OEt)5 and 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci) in a 1:1:1 ratio under alkaline conditions (NaOH) resulted in the formation of two new NbNi heteropolyoxometalate complexes. [Ni(taci)2]2{trans-[Nb6O19][Ni(taci)]2}·26H2O (1) and [Na(H2O)6]2{cis-[H2Nb6O19][Ni(taci)]2}·18H2O (2), were characterized by single crystal X-ray analysis. In both compounds, two [Ni(taci)]2+ entities are each bonded to three NbONb bridges of a central Nb6O19 unit, resulting in the formation of an approximately C3v symmetric NiN3O3 coordination geometry. The mononuclear analogue [Ni(taci)(H2O)3]2+ was crystallized as the sulfate and structurally characterized for comparison. The two isolated anions {trans-[Nb6O19][Ni(taci)]2}4− and {cis-[H2Nb6O19][Ni(taci)]2}2− have approximate D3d and C2v symmetry, respectively. Compound 2 crystallized, however, in the cubic space group Pn 3 m (No. 224) with the two Ni centers equally distributed over four symmetry equivalent sites. In this structure, the [Na(H2O)6]+ cations and the {cis-[H2Nb6O19][Ni(taci)]2}2− anions represent linearly and tetrahedrally connecting building blocks in a hydrogen bonded supramolecular architecture which can be described in terms of two independent but interpenetrating diamond-type nets.

The reactivity of complexes containing the [Mo3(μ3S)(μS2)3]4+ core. Ligand substitution, sulfur elimination and sulfide binding

The reactivity of complexes, containing the [Mo 3 (μ 3 S)(μS 2 ) 3 ] 4+ core, was investigated in solution. Three types of electrophilic centers, (i) the three Mo atoms, (ii) the three equatorial (in plane) and (iii) the three axial (out of plane) sulfur atoms of the disulfido bridges, interact distinctly different with nucleophilic agents. The selective reactivity enabled the specific synthesis of new complexes containing either the [Mo 3 (μ 3 S)(μS 2 ) 3 ] 4+ or the [Mo 3 (μ 3 S)(μS) 3 ] 4+ cores by using (NEt 4 ) 2 [Mo 3 S(S 2 ) 3 Br 6 ] as starting material. A combination of sulfur abstraction and ligand substitution resulted in the formation of complexes of the composition [Mo 3 S 4 L 3 ] 4+ , where L represents the tridentate N,O donors 1,3,5-triamino-1,3,5-trideoxy- cis -inositol and 1,3,5-trideoxy-1,3,5-tris(dimethylamino)- cis -inositol. The complexes were characterized by one- and two-dimensional NMR spectroscopy and FAB mass spectrometry. In the presence of a weak base and 6-mercaptopurine (Hmp) or 8-hydroxyquinoline (Hoxq), the six Br atoms were replaced by these bidentate ligands forming [Mo 3 S(S 2 ) 3 (oxq) 3 ] + and [Mo 3 S(S 2 ) 3 (mp) 3 ] + . In addition, the three oxq ligands could be quantitatively replaced by diethyldithiocarbamate (dtc − ) in the presence of a slight excess of Na(dtc). The [Mo 3 S(S 2 ) 3 ] 4+ core was not affected under the harsh reaction conditions, but S 2− was liberated by hydrolysis of dtc − . The formed S 2− anion was bound selectively to the three S ax atoms of the [Mo 3 S(S eq S ax ) 3 ] 4+ core, resulting in the formation of the dimer [Mo 3 S(S 2 ) 3 (dtc) 3 ] 2 S. The dimeric structure was established by X-ray analysis, exhibiting a hexacoordinated S 2− atom with a rather short average S ax S 2− distance of 2.71 A and an elongated S eq S ax distance of 2.11 A. Space group Aba 2, a =17.48(1), b =26.13(2), c =16.489(8) A, Z =4, R =0.051.

A new synthetic pathway for tris-μ-disulfido-μ3-thio-triangulo-trimolybdenum(IV) complexes: preparation, characterization and structure of [Mo3S(S2)3(OOCCHSCH2COOH)3]2−

Abstract A new synthetic pathway for complexes containing the polynuclear core [Mo3S7]4+ is described: [Mo3S7(S2)3]2− reacts with HBr to [Mo3S7Br6]2− which seems to be a useful intermediate for further ligand substitution. A new complex with mercapto- succinic acid C4H6O4S (H3msa) as ligand could be prepared. 1H NMR and 13C NMR spectra and X-ray crystal structure analysis are in agreement with the bidentate coordination of mercapto-succinic acid, where S− and COO− are bound to the same molybdenum atom forming a five-membered ring. A monoclinic crystal, space group C2, of the composition [C18H30N3]2[Mo3S7(Hmsa)3][Mo3S7(Hmsa)2(msa)] Br·6H2O was used for X-ray diffraction. The cell parameters are a=21.968(7), b=13.423(13), c= 18.828(3) A, β=94.23°, Z=2, Dc=1.606 g/cm3, μ(Mo Kα)=13.51 cm−1. [Mo3S7(Hmsa)3]2− is stable in water and air, and it can be precipitated from aqueous solution by the addition of large three-fold charged cations. It can be deprotonated reversibly to [Mo3S7(msa)3]5− in water. The pK values are 3.95, 4.41, 5.02 (1 M KCl, 25 °C). To confirm the above described procedure as a useful approach for [Mo3S7]4+ complexes, a similar compound with 2-mercapto-benzoic acid was also prepared.

Characteristic spin-orbit induced 1H(CH2) chemical shifts upon deprotonation of group 9 polyamine aqua and alcohol complexes.

The recently observed nonintuitive pH dependence of methylene (1)H chemical shifts in cobalt(III) polyamine complexes upon deprotonation of coordinated aqua or (poly)alcohol coligands (J. Am. Chem. Soc. 2004, 126, 6728) was attributed to differential spin-orbit effects on the (1)H shifts transmitted over three bonds from the cobalt low-spin d(6) center. These remarkably large spin-orbit effects due to the comparably light Co center have now been examined closely by comparative computations for homologous Rh and Ir complexes, as well as by NMR titrations for a Rh complex. While larger spin-orbit effects (proportional to Z(2)) would have been expected for the heavier metal centers, the characteristic (1)H deshieldings upon deprotonation of [Rh(tren)(OH(2))(2)](3+) [tren = tris(2-aminoethyl)-amine] turn out to be smaller than for the Co homologous Co complex. Systematic computational studies ranging from smaller models to the full complexes confirm these results and extend them to the Ir homologues. Closer analysis indicates that the spin-orbit shift contributions do not follow the expected Z(2) behavior but are modulated dramatically by increasing energy denominators in the perturbation expressions. This is related to the increasing ligand-field splitting from 3d to 4d to 5d system, leading to almost identical differential spin-orbit shifts for the Co and Rh complexes and to only moderately larger effects for the Ir complex (by a factor of about two). Moreover, the differential nonspin-orbit deprotonation shifts cancel the spin-orbit induced contributions largely in the Rh complex, leading to the experimentally observed inverted behavior. The full multidentate polyamine complexes studied experimentally exhibit different three- and four-bond Fermi-contact pathways for transmission of the spin-orbit (1)H shifts. The novel four-bond pathways have different conformational dependencies than the Karplus-like three-bond pathways established previously. Both types of contributions are of similar magnitude. The (1)H NMR deprotonation shift patterns of [Ir(tren)(OH(2))(2)](3+) have been predicted computationally.

The Coordination Chemistry of cis-3, 5-Diaminopiperidine and Substituted Derivatives

An efficient and convenient method for the preparation of cis-3,5-diaminopiperidine (dapi) has been established and the coordination chemistry of this ligand with CoII, CoIII, NiII, CuII, ZnII, and CdII has been investigated in the solid state and in aqueous solution. Potentiometric measurements revealed a generally high stability for the bis complexes of the divalent cations with maximum stability for NiII (log beta2 = 21.2, beta2 = [M(dapi)2][M](-1)[dapi](-2), 25 degrees C, mu = 0.1 mol dm(-3)). Cyclic voltammetry established quasi-reversible formation of [Ni(dapi)2]3+ with a redox potential of 0.91 V (versus NHE) for the Ni(II/III) couple. [Co(dapi)2]3+ was prepared by aerial oxidation of the corresponding CoII precursor. The two isomers trans-[Co(dapi)2]3+ (1(3+), 26%) and cis-[Co(dapi)2]3+ (2(3+), 74%), have been separated and isolated as solid Cl- and CF3SO3- salts. In a non-aqueous medium 1(3+) and 2(3+) reacted with paraformaldehyde and NEt3 to give the methylidene-imino derivatives 3(3+) and 4(3+), in which the two piperidine rings are bridged by two or one N-CH2-O-CH2-N bridges, respectively. Crystal structure analyses were performed for H3dapi[ZnCl4]Cl, 1Cl3 x 2H2O, 2Cl3 x H2O, 3[ZnCl4]Cl, 4[ZnCl4]Cl, [Ni(dapi)2]Cl2 x H2O, [Cu(dapi)2](NO3)2, [Cu(dapi)Cl2], [(dapi)ClCd-(mu2-Cl)2-CdCl(dapi)], and [Co(dapi)(NO2)(CO3)]. The stability of [M(II)(dapi)]2+ and [M(II)(dapi)2]2+ complexes in aqueous solution, particularly the remarkably high tendency of [M(dapi)]2+ to undergo coordinative disproportionation is discussed in terms of the specific steric requirements of this ligand. Molecular mechanics calculations have been performed to analyze the different types of strain in these complexes. A variety of alkylated derivatives of dapi have been prepared by reductive alkylation with formaldehyde, benzaldehyde, salicylaldehyde, and pyridine-2-carbaldehyde. The NiII complexes of the pentadentate N3,N5-bis(2-pyridinylmethyl)-cis-3,5-diaminopiperidine (py2dapi) and the hexadentate N3,N5,1-tris(2-pyridinylmethyl)-cis-3,5-diaminopiperidine (py3dapi) have been isolated as crystalline ClO4- salts [Ni(py2dapi)Cl]ClO4 and [Ni(py3dapi)](ClO4)2 x H2O and characterized by crystal structure analyses.

SCO trimers as high-energy materials ? A density functional study

Abstract Density functional calculations using the hybrid B3LYP functional have been carried out on the fragmentation reaction of cyclic SCO trimers. Analogous calculations for CO2 and HNCO have also been performed for comparison. The energies of the different trimers relative to those of the isolated monomers as well as the energy barriers to fragmentation have been calculated. For all the calculations, a reaction path in which three bonds are simultaneously broken has been considered. It has been found that the fragmentation of cyclic SCO trimers is a rather facile process. The same statement applies to the cyclic CO2 trimer but not to the HNCO one. In addition, the latter is much more stable than three free HNCO monomers, which is not the case for SCO and CO2.