Markus Ströbele

Coordination chemistry of gold catalysts in solution: a detailed NMR study.

Coordination chemistry of gold catalysts bearing eight different ligands [L=PPh(3), JohnPhos (L2), Xphos (L3), DTBP, IMes, IPr, dppf, S-tolBINAP (L8)] has been studied by NMR spectroscopy in solution at room temperature. Cationic or neutral mononuclear complexes LAuX (L=L2, L3, IMes, IPr; X=charged or neutral ligand) underwent simple ligand exchange without giving any higher coordinate complexes. For L2AuX the following ligand strength series was determined: MeOH≪hex-3-yne <MeCN≈OTf(-) ≪Me(2)S<2,6-lutidine<4-picoline<CF(3)CO(2)(-) ≈DMAP<TMTU<PPh(3) <OH(-) ≈Cl(-). Some heteroligand complexes DTBPAuX exist in solution in equilibrium with the corresponding symmetrical species. Binuclear complexes dppf(AuOTf)(2) and S-tolBINAP(AuOTf)(2) showed different behavior in exchange reactions with ligands depending on the ligand strength. Thus, PPh(3) causes abstraction of one gold atom to give mononuclear complexes LLAuPPh(3)(+) and (Ph(3)P)(n)Au(+), but other N and S ligands give ordinary dicationic species LL(AuNu)(2)(2+). In reactions with different bases, LAu(+) provided new oxonium ions whose chemistry was also studied: (DTBPAu)(3)O(+), (L2Au)(2)OH(+), (L2Au)(3)O(+), (L3Au)(2)OH(+), and (IMesAu)(2)OH(+). Ultimately, formation of gold hydroxide LAuOH (L=L2, L3, IMes) was studied. Ligand- or base-assisted interconversions between (L2Au)(2)OH(+), (L2Au)(3)O(+), and L2AuOH are described. Reactions of dppf(AuOTf)(2) and S-tolBINAP(AuOTf)(2) with bases provided more interesting oxonium ions, whose molecular composition was found to be [dppf(Au)(2)](3)O(2)(2+), L8(Au)(2)OH(+), and [L8(Au)(2)](3)O(2)(2+), but their exact structure was not established. Several reactions between different oxonium species were conducted to observe mixed heteroligand oxonium species. Reaction of L2AuNCMe(+) with S(2-) was studied; several new complexes with sulfide are described. For many reversible reactions the corresponding equilibrium constants were determined.

A blue luminescent di-2-pyridylamine cadmium complex with an unexpected arrangement of thiocyanate ligands: a supramolecular layered structure based on hydrogen bonds and π–π stacking interactions

Abstract X-ray crystal structure of [Cd(dpa)2(SCN)2] (dpa=di-2-pyridylamine) is refined in triclinic space group P-1 (no. 2) with a=7.1464(5) A , b=9.4368(7) A , c=18.5843(7) A ,α=77.318(8)° , β=81.487(8)°, γ=71.427(8)° and shows a novel unequivalent coordination of two SCN− and a layered structure built by two types of hydrogen bonding and π–π stacking interactions. The luminescence studies show strong blue photoluminescence in range 360–450 nm as a result of the π → π ∗ transition on the dpa ligand.

Construction of an internally B3N3-doped nanographene molecule

The synthesis of a hexa-peri-hexabenzocoronene (HBC) with a central borazine core is described. The solid-state structure of this BN-doped HBC (BN-HBC) is isotypic with that of the parent HBC. Scanning tunneling microscopy shows that BN-HBC lies flat on Au(111) in a two-dimensional pattern.

A new family of two-dimensional lanthanide(III) coordination polymers: synthesis, structures and properties of [Ln(SIP)(H 2 O) 4 ] n (Ln=Eu, Gd, Ce, and NaH 2 SIP=5-sulfoisophthalic acid monosodium salt)

Abstract A new family of lanthanide coordination polymers with the general formula [Ln(SIP)(H2O)4]n (Ln=Eu, Gd, Ce, 1–3; and NaH2SIP=5-sulfoisophthalic acid monosodium salt) was hydrothermally synthesized and characterized by IR, elemental analysis, single-crystal X-ray crystallography and powder X-ray diffractions. X-ray studies indicated that these compounds are isostructural and adopt an electrically neutral and undulating two-dimensional layer structure constructed from fused {[Eu(H2O)4]3(SIP)3} pseudo-hexagonal grids. Thermogravimetric measurements in N2 revealed that all exhibit weight losses corresponding to four water molecules per formula unit around 120 °C and the dehydrated compounds are stable up to ca 510 °C. The fluorescence spectrum showed that [Eu(SIP)(H2O)4]n displays intense red fluorescence in the solid-state. The paramagnetism for [Gd(SIP)(H2O)4]n in the range 5–300 K reveals no magnetic coupling via SIP ligand.

Total synthesis and biological evaluation of (-)-9-deoxy-englerin A.

An effective total synthesis of (-)-9-deoxy-englerin (4), an analogue of the natural guaiane sesquiterpene englerin A (1), has been achieved. The synthesis features a transannular epoxide opening to construct the 5,7-fused ring system followed by transannular ether formation with mercury(II) trifluoroacetate.

Synthesis of the Guaianolide Ring System via Cycloaddition of a Bicyclic Carbonyl Ylide with Allyl Propiolate

A cyclic carbonyl ylide with a trans-annulated cyclopentane ring was generated by a Rh(2)(OAc)(4)-catalyzed reaction from a diazoketone precursor and trapped with allyl propiolate. The 1,3-dipolar cycloaddition led to the stereoselective formation of an oxygen-bridged polycycle. Via Curtius degradation, the cycloadduct was transformed to the ring skeleton typical of the sesquiterpene family of guaianolides.

Elloxazinones A and B, New Aminophenoxazinones from Streptomyces griseus Acta 2871 †

Two new aminophenoxazinone compounds with antitumor activity, elloxazinone A and B, were isolated from the culture filtrate of Streptomyces griseus Acta 2871. Their chemical structures were determined by mass spectrometry, NMR spectroscopy and X-ray analysis. Elloxazinones A and B showed a moderate inhibition of the proliferation of human cells from gastric adenocarcinoma in vitro but a strong inhibition of hepatocellular carcinoma cells whereas elloxazinone B strongly inhibited the proliferation of human breast carcinoma cells.

Compounds with the electron-rich [W6Cl18]2- cluster anion.

Cluster compounds of the general formula A(2)[W(6)Cl(18)] containing singly charged A cations (A = K, Rb, Ag, Tl, NH(4), N(C(2)H(5))(4), N(n-C(3)N(7))(4), N(n-C(4)H(9))(4)) and [W(6)Cl(18)](2-) anions have been synthesized. Compounds were obtained by W(6)Cl(18) reduction using methanol and the corresponding metal or (alkyl-)ammonium salts. The use of CoCp(2) as reducing agent in inert solvents such as tetrahydrofurane also leads to the ionic compound (Co(C(5)H(5))(2))(2)[W(6)Cl(18)]. All compounds described here evidence the existence of octahedral clusters of the M(6)X(12) type with 20 cluster electrons occupying metal centered states, thus exceeding the conventional number of 16 electrons for this cluster type. Electrospray ionization (ESI) mass spectra were recorded for the neutral compound W(6)Cl(18) and for the ionic salts K(2)[W(6)Cl(18)], Cs(2)[W(6)Cl(18)], and (Co(C(5)H(5))(2))(2)[W(6)Cl(18)], showing that the cluster W(6)Cl(18) unit in these compounds is preserved in solution. The base peak in the ESI spectra for all compounds corresponds to the [W(6)Cl(18)](2-) anion, so that neutral W(6)Cl(18) is prone to undergo a two electron reduction process. This result is confirmed by cyclic voltammetry experiments, which makes of W(6)Cl(18) a very clean mild oxidizing agent. The preparation of the complete series of ionic A(2)[W(6)Cl(18)] (A = K, Cs, CoCp(2)) clusters allows to systematically investigate their structural trends as function of the distinctive cations, which is the main focus of the present work.

The Structures of Fluostatins C, D and E, Novel Members of the Fluostatin Family

Three new fluostatin antibiotics have been isolated from the culture filtrate of Streptomyces strain Acta 1383. The chemical structures of these compounds were determined by mass spectrometry and NMR spectroscopy. The relative configuration has been determined by X-ray crystal structure analysis and the absolute configuration was deduced from NMR data with the help of Helmchen esters. These compounds represent the first reported epoxide fluostatins (1, 2) and underline previously found analogies to the group of kinamycin antibiotics.

Neubestimmung der Kristallstruktur von NbOCl3

NbOCl3 was obtained from a reaction of NbCl5 and Nb2O5 at 260?C. Contrary to the literature data, NbOCl3 crystallizes in the non-centrosymmetric space group P&4macr;21m as determined by single-crystal and powder X-ray diffraction data (crystal: a = b = 1089.59(6) pm, c = 394.79(2) pm, Z = 4, R1 = 0.0229, wR2 = 0.0459, powder: a = b = 1086.36(6) pm, c = 393.65(2) pm). The niobium atoms are surrounded by distorted octahedra built of four chlorine atoms and two oxygen atoms in trans positions. Two such octahedra are edge-bridged through shared chlorine atoms, forming dimers. These units are linked to each other by apical oxygen atoms forming one-dimensional Nb2Cl6O2 chains parallel [001]. Contrary to the literature data two different Nb-O distances are obtained.