Benzene-o-dithiolate ligands as versatile building blocks in supramolecular chemistry

Polydentate ligands with benzene-o-dithiolato donor groups are useful building blocks in supramolecular coordination chemistry. The coordination chemistry of bis- and tris(benzene-o-dithiolato) ligands and mixed benzene-o-dithiolato/catecholato ligands is reviewed. These ligands exhibit a versatile coordination chemistry both in solution and in the solid state.     

Characterization and quantification of Sm(III)/ and Cm(III)/clay mineral outer-sphere species by TRLFS in D2O and EXAFS studies

In order to assess the long-term safety of deep radioactive waste repositories, a precise characterization of the different sorption processes on a molecular basis and the exact definition of geochemical boundary conditions for their relevance are of immense importance. Through sorption on various minerals the migration of radionuclides will be hindered and their retention will be ensured. Using time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure (EXAFS) spectroscopy, it was possible to identify outer-sphere sorbed trivalent lanthanides and actinides onto different montmorillonites and illite. Furthermore, the quantification of Cm(III)/clay outer-sphere sorption in D(2)O at different ionic strengths was shown. The results were confirmed by ion exchange model calculations. Finally, the structural parameters of a Sm(III)/clay outer-sphere complex were obtained by EXAFS measurements.     

A combined theoretical and experimental study of efficient and fast titanocene-catalyzed 3-exo cyclizations

The mechanism of titanocene mediated 3-exo cyclizations was investigated by a combined theoretical and experimental study. A gradient corrected density functional theory (DFT) method has been scaled against titanocene dichloride, the parent butenyl radical, and in bond dissociation energy (BDE) calculations. The BP86 method using density fitting, and a basis set of triple-zeta quality emerged as a highly reliable tool for studying titanocene mediated radical reactions. The computational results revealed important kinetic and thermodynamic features of cyclopropane formation. Surprisingly, the beta-titanoxy radicals, the first intermediates of our investigations, were demonstrated to possess essentially the same thermodynamic stabilization as the corresponding alkyl radicals by comparison of the calculated BDEs. In contrast to suggestions for samarium mediated reactions, the cyclization was shown to be thermodynamically favorable in agreement with earlier kinetic studies. It was established that stereoselectivity of the cyclization is governed by the stability of the intermediates and thus the trans disubstituted products are formed preferentially. The observed ratios of products are in good to excellent agreement with the DFT results. By a combination of computational and experimental results, it was also shown that for the completion of the overall cyclopropane formation the efficiency of the trapping of the cyclopropylcarbinyl radicals is decisive.     

Phosphaniminate von Alkalimetallen: Die Kristallstrukturen von [KNPCy3]4, [KNPCy3]4·2OPCy3, [CsNPCy3]4·4OPCy3 und [Li4(NPPh3)(OSiMe2NPPh3)3(DME)]

The alkalimetal phosphoraneiminates [KNPCy₃]₄, ( 1 ) [KNPCy₃]₄·2OPCy₃ ( 2 ) and [CsNPCy₃]₄·4OPCy₃ ( 3 ) (Cy = cyclohexyl) which are obtainable by the reaction of pottassium amide or cesium amide with Cy₃PI₂ or Cy₃PBr₂ in liquid ammonia, as well as the lithium derivative [Li₄(NPPh₃)(OSiMe₂NPPh₃)₃(DME)] ( 4 ) have been characterized by crystal structure determinations. 4 has been formed by the insertion reaction of silicon greaze (‐OSiMe₂)n into the LiN bonds of [LiNPPh₃]₆ in DME solution (DME = 1, 2‐dimethoxyethane). 1 : Space group P&1macr;, Z = 2, lattice dimensions at 193 K: a = 1389.8(1); b = 1408.1(1); c = 2205.2(2) pm; α = 78.952(10)?; β = 81.215(10)?; γ = 66.232(8)?; R1 = 0.0418. 2 : Space group Pbcn, Z = 4, lattice constants at 193 K: a = 2943.6(2); b = 2048.2(1); c = 1893.8(1) pm; R1 = 0.0428. 3 : Space group Cmc2₁, Z = 4, lattice dimensions at 193 K: a = 2881.6(2); b = 2990.2(2); c = 1883.7(2) pm; R1 = 0.0586. 4 ·1/2DME: Space group R&3macr;c, Z = 12, lattice dimensions at 193 K: a = b = 1583.5(1); c = 11755.3(5) pm; R1 = 0.0495. All complexes have heterocubane structures. In 1‐3 they are formed by four alkali metal atoms and by the nitrogen atoms of the (μ₃‐NPCy₃^‐) groups, whereas 4 forms a "heteroleptic" Li₄NO₃ heterocubane.     

Total Synthesis of Pulvomycin D

A synthetic route to the pulvomycin class of natural products is presented, which culminated in the first synthesis of a pulvomycin, pulvomycin D. Key elements of the strategy include a pivotal aldol reaction which led to bond formation between the C24-C40 and the C8-C23 fragment. The remaining C1-C7 fragment was attached by a Yamaguchi esterification completing the assembly of the 40 carbon atoms within the main skeleton. Ring closure to the 22-membered lactone ring was achieved in the final stages of the synthesis by a Heck reaction. The completion of the synthesis required the removal of six silyl protecting groups in combination with olefin formation at C26-C27 by a Peterson elimination.     

Measurements and Utilization of Consistent Gibbs Energies of Transfer of Single Ions: Towards a Unified Redox Potential Scale for All Solvents

Utilizing the “ideal” ionic liquid salt bridge to measure Gibbs energies of transfer of silver ions between the solvents water, acetonitrile, propylene carbonate and dimethylformamide results in a consistent data set with a precision of 0.6 kJ mol⁻¹ over 87 measurements in 10 half-cells. This forms the basis for a coherent experimental thermodynamic framework of ion solvation chemistry. In addition, we define the solvent independent - and the values that account for the electronating potential of any redox system similar to the value of a medium that accounts for its protonating potential. This scale is thermodynamically well-defined enabling a straightforward comparison of the redox potentials (reducities) of all media with respect to the aqueous redox potential scale, hence unifying all conventional solvents′ redox potential scales. Thus, using the Gibbs energy of transfer of the silver ion published herein, one can convert and unify all hitherto published redox potentials measured, for example, against ferrocene, to the scale.     

Hierarchical dynamics in allostery following ATP hydrolysis monitored by single molecule FRET measurements and MD simulations

We report on a study that combines advanced fluorescence methods with molecular dynamics (MD) simulations to cover timescales from nanoseconds to milliseconds for a large protein. This allows us to delineate how ATP hydrolysis in a protein causes allosteric changes at a distant protein binding site, using the chaperone Hsp90 as test system. The allosteric process occurs via hierarchical dynamics involving timescales from nano- to milliseconds and length scales from Ångstroms to several nanometers. We find that hydrolysis of one ATP is coupled to a conformational change of Arg380, which in turn passes structural information via the large M-domain α-helix to the whole protein. The resulting structural asymmetry in Hsp90 leads to the collapse of a central folding substrate binding site, causing the formation of a novel collapsed state (closed state B) that we characterise structurally. We presume that similar hierarchical mechanisms are fundamental for information transfer induced by ATP hydrolysis through many other proteins.

We report on a study that combines advanced fluorescence methods with molecular dynamics simulations to cover timescales from nanoseconds to milliseconds for a large protein, the chaperone Hsp90.     

Tuning Ruthenium Carbene Complexes for Selective P−H Activation through Metal-Ligand Cooperation

The use of iminophosphoryl-tethered ruthenium carbene complexes to activate secondary phosphine P−H bonds is reported. Complexes of type [(p-cymene)-RuC(SO₂Ph)(PPh₂NR)] (with R = SiMe₃ or 4-C₆H₄−NO₂) were found to exhibit different reactivities depending on the electronics of the applied phosphine and the substituent at the iminophosphoryl moiety. Hence, the electron-rich silyl-substituted complex undergoes cyclometallation or shift of the imine moiety after cooperative activation of the P−H bond across the M=C linkage, depending on the electronics of the applied phosphine. Deuteration experiments and computational studies proved that cyclometallation is initiated by the activation process at the M=C bond and triggered by the high electron density at the metal in the phosphido intermediates. Consistently, replacement of the trimethylsilyl (TMS) group by the electron-withdrawing 4-nitrophenyl substituent allowed the selective cooperative P−H activation to form stable activation products.