Aluminum Alkyl Induced Isomerization of Group IV meso Metallocene Complexes.

The synthesis of group IV metallocene precatalysts for the polymerization of propylene generally yields two different isomers: The racemic isomer that produces isotactic polypropylene (iPP) and the meso isomer that produces atactic polypropylene (aPP). Due to its poor physical properties, aPP has very limited applications. To avoid obtaining blends of both polymers and thus diminish the mechanical and thermal properties of iPP, the meso metallocene complexes need to be separated from the racemic ones tediously -rendering the metallocene-based polymerization of propylene industrially far less attractive than the Ziegler/Natta process. To overcome this issue, we established an isomerization protocol to convert meso metallocene complexes into their racemic counterparts. This protocol increased the yield of iPP by 400% while maintaining the polymer's excellent physical properties and was applicable to both hafnocene and zirconocene complexes, as well as different precatalyst activation methods. Through targeted variation of the ligand frameworks, methoxy groups at the indenyl moieties were found to be the structural motifs responsible for an isomerization to take place -this experimental evidence was confirmed by density functional theory calculations. Liquid injection field desorption ionization mass spectrometry, as well as 1H and 29Si nuclear magnetic resonance studies, allowed the proposal of an isomerization mechanism.

Single-Hydroxide Bridged Dimers of U and Np Actinyls: A Density Functional Study on Their Existence and Structure in Aqueous Solution.

With quantum chemical calculations at the density functional theory level, we examined the structure and the stability of diactinyl monohydroxo complexes [(AnO2)2(OH)]3+/+ in aqueous solution for An = U(VI), Np(VI), and Np(V). In particular, this study contributes to understanding the hydrolysis of Np(VI) and Np(V), which is less well characterized than for U(VI). [(UO2)2(OH)]3+ is a known hydrolysis complex of U(VI) at low pH. Although not yet found in experiments, [(NpO2)2(OH)]3+ is suggested to exist due to the similarity between Np(VI) and U(VI) complexes, while [(NpO2)2(OH)]+ is a hypothetical species thus far. Our calculations suggest that the An(VI) complexes favor the parallel orientation of actinyls, whereas for the Np(V) complex a perpendicular arrangement is stabilized by hydrogen bonds between aqua ligands and the actinyl oxygen atoms. The Np(VI) complex [(NpO2)2(OH)]3+ features a structure and stability similar to its U(VI) analogue. From calculated formation constants for An(VI) diactinyl monohydroxo complexes, we find qualitative agreement with the experiment for U(VI). Both An(VI) complexes are only slightly less stable than the separate mononuclear constituents, the actinyl aqua and the monohydroxo complex. For the Np(V) species [(NpO2)2(OH)]+, we calculated a considerably lower complexation constant than for its An(VI) analogues, but it is more stable against decay into its constituents. Thus, this complex may exist at about the pH where Np(V) hydrolysis starts at not too low Np(V) concentrations.

Hydration Structure and Hydrolysis of U(IV) and Np(IV) Ions: A Comparative Density Functional Study Using a Modified Continuum Solvation Approach.

We studied the hydration and the first hydrolysis reaction of U(IV) and Np(IV) ions in an aqueous environment, applying a relativistic density functional method together with a recently proposed variant of a continuum solvation model where the solute cavities are constructed with effective atomic radii, based on charge-dependent scaling factors. In this way, one obtains improved solvation energies of charged species. We demonstrate that solute cavities, constructed with scaled atomic radii as described, permit one to calculate hydrolysis constants of acceptable accuracy. As a consequence, one is also able to estimate free hydration energies of U(IV) and Np(IV) in adequate agreement with empirical data. According to the model calculations, U(IV) is coordinated by eight to nine water molecules, while the preferred coordination number of Np(IV) is 8. For the highly charged ions under study, the modified solvation model simultaneously yields improved geometries, hydration energies, and hydrolysis constants.

Interdisciplinary Round-Robin Test on Molecular Spectroscopy of the U(VI) Acetate System.

A comprehensive molecular analysis of a simple aqueous complexing system-U(VI) acetate-selected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods was achieved by an international round-robin test (RRT). Twenty laboratories from six different countries with a focus on actinide or geochemical research participated and contributed to this scientific endeavor. The outcomes of this RRT were considered on two levels of complexity: first, within each technical discipline, conformities as well as discrepancies of the results and their sources were evaluated. The raw data from the different experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was high. By contrast, luminescence spectroscopic data turned out to be strongly related to the chosen acquisition parameters. Second, the potentials and limitations of coupling various spectroscopic and theoretical approaches for the comprehensive study of actinide molecular complexes were assessed. Previous spectroscopic data from the literature were revised and the benchmark data on the U(VI) acetate system provided an unambiguous molecular interpretation based on the correlation of spectroscopic and theoretical results. The multimethodologic approach and the conclusions drawn address not only important aspects of actinide spectroscopy but particularly general aspects of modern molecular analytical chemistry.

Metal-free polymerization of phenylsilane: tris(pentafluorophenyl)borane-catalyzed synthesis of branched polysilanes at elevated temperatures.

The strong organoborane Lewis acid B(C6F5)3 catalyzes the polymerization of phenylsilane at elevated temperatures forming benzene and SiH4 as side-products. The resulting polymer is a branched polysilane with an irregular substitution pattern, as revealed by 2D NMR spectroscopy. Having explored the mechanism of this novel metal-free polymerization by computational chemistry methods at the DFT level, we have suggested that unusual cationic active species, namely monomer-stabilized silyl cations, propagate the polymerization. Hydride abstraction of SiH3 moiety by the catalyst in the initiation step was found to be kinetically preferred by around 9 kcal mol(-1) over activation by coordination of the monomer at the aromatic ring. The formation of linear Si-Si bonds during propagation was calculated to be less favorable than branching and ligand scrambling, which accounts for the branched and highly substituted form of the polymer that was obtained. This novel type of polymerization bears the potential for further optimization with respect to degree of polymerization and structure control for both primary as well as secondary silanes, which can be polymerized by sterically less hindered boranes.

An electrochemical study of antineoplastic gallium, iron and ruthenium complexes with redox noninnocent alpha-N-heterocyclic chalcogensemicarbazones.

The electrochemical properties of a series of alpha-N-heterocyclic chalcogensemicarbazones (HL), namely, thiosemicarbazones, selenosemicarbazones, and semicarbazones, and their gallium(III), iron(III), and ruthenium(III) complexes with the general formula [ML(2)][Y] (M = Ga, Fe or Ru; Y = PF(6)(-), NO(3)(-), or FeCl(4)(-)) were studied by cyclic voltammetry. The novel compounds were characterized by elemental analysis, a number of spectroscopic methods (NMR, UV-vis, IR), mass spectrometry and by X-ray crystallography. All complexes show several, mostly reversible, redox waves attributable to the reduction of the noninnocent chalcogensemicarbazone ligands at lower potentials (<-0.4 V vs NHE) than the metal-centered iron or ruthenium redox waves (>0 V vs NHE) in organic electrolyte solutions. The cyclic voltammograms of the gallium complexes display at least two consecutive reversible one-electron reduction waves. These reductions are shifted by approximately 0.6 V to lower potentials in the corresponding iron and ruthenium complexes. The electrochemical, chemical, and spectroscopic data indicate that the ligand-centered reduction takes place at the CH(3)CN double bond. Quantum chemical calculations on the geometric and electronic structures of 2-acetylpyridine (4)N,(4)N-dimethylthiosemicarbazone (HL(B)), the corresponding metal complexes [Ga(L(B))(2)](+) and [Fe(II)(L(B))(2)], and the one-electron reduction product for each of these species support the assignment of the reduction site and elucidate the observed order of the ligand-centered redox potentials, E(1/2)([Fe(II)(L)(2)]) < E(1/2)(HL) < E(1/2)([Ga(L)(2)](+)). The influence of water on the redox potentials of the complexes is reported and the physiological relevance of the electrochemical data for cytotoxicity as well as for ribonucleotide reductase inhibitory capacity are discussed.

Computational Electrochemistry of Ruthenium Anticancer Agents. Unprecedented Benchmarking of Implicit Solvation Methods.

Two ruthenium(III) complexes {(HIm)[trans-RuCl4(DMSO)(Im)] (NAMI-A) and (HInd)[trans-RuCl4(Ind)2] (KP1019), DMSO = dimethyl sulfoxide, Im = imidazole, Ind = indazole} have been tested in phase I clinical trials as potential anticancer drugs. Ru(III) anticancer agents are likely activated in vivo upon reduction to their Ru(II) analogs. Aiming at benchmarking implicit solvation methods in DFT studies of ruthenium pharmaceuticals at the B3LYP level, we have calculated the standard redox potentials (SRPs) of Ru(III/II) pairs that were electrochemically characterized in the literature. 80 SRP values in four solvents were calculated using three implicit solvation methods and five solute cavities of molecular shape. Comparison with experimental data revealed substantial errors in some of the combinations of solvation method and solute cavity. For example, the overall mean unsigned error (MUE) with the PCM/UA0 combination, which is the popular default in Gaussian 03, amounts to 0.23 V (5.4 kcal/mol). The MUE with the CPCM/UAKS combination, which was employed by others for recent computational studies on the hydrolysis of NAMI-A and trans-[RuCl4(Im)2](-), amounts to 0.30 V (7.0 kcal/mol) for all compounds and to 0.60 V (13.9 kcal/mol) for a subset of compounds of the medicinally relevant type, trans-[RuCl4(L)(L')](-). The SRPs calculated with the PCM or CPCM methods in Gaussian 03 can be significantly improved by a more compact solute cavity constructed with Bondi's set of atomic radii. Earlier findings that CPCM performs better than PCM cannot be confirmed, as the overall MUE amounts to 0.19 V (4.3-4.4 kcal/mol) for both methods in combination with Bondi's set of radii. The Poisson-Boltzmann finite element method (PBF) implemented in Jaguar 7 together with the default cavity performs slightly better, with the overall MUE being 0.16 V (3.7 kcal/mol). Because the redox pairs considered in this study bear molecular charges from +3/+2 to -1/-2 and the prediction of solvation free energies is most challenging for highly charged species, the present work can serve as a general benchmarking of the implicit solvation methods.

The first ruthenium-based paullones: syntheses, X-ray diffraction structures, and spectroscopic and antiproliferative properties in vitro.

Two novel paullone derivatives, namely, 6-(alpha-picolylamino)-7,12-dihydroindolo[3,2-d][1]benzazepine (L1) and 9-bromo-6-(alpha-picolylamino)-7,12-dihydroindolo[3,2-d][1]benzazepine (L2), have been prepared. The reaction of cis-[RuCl2(DMSO)4] (DMSO=dimethyl sulfoxide) with L1 and L2 in a 1:1 molar ratio in dry ethanol at 50 degrees C afforded the complexes trans-[RuIICl2(DMSO)2L1] (1a) and trans-[RuIICl2(DMSO)2L2] (1b) in 26 and 30% yield, respectively. The reaction carried out from the same starting compounds in a 1:2 molar ratio at 75 degrees C led to the formation of [RuIICl(DMSO)(L1)2]Cl (2a) and [RuIICl(DMSO)(L2)2]Cl (2b) in 16 and 23% yield, correspondingly. The products were characterized by elemental analysis, one- and two-dimensional NMR spectroscopy, electrospray ionization mass spectrometry, IR spectroscopy, electronic spectra, cyclic voltammetry, and X-ray crystallography (L1, L2, 1a, and 2b). Complexes 2a and 2b exhibit remarkable antiproliferative activity in three human carcinoma cell lines, A549 (non-small cell lung carcinoma), CH1 (ovarian carcinoma), and SW480 (colon carcinoma). The novel complexes show an intercalative mode of interaction with DNA, which may render them attractive alternatives to metal compounds with a coordinative mode of interaction.

Isomeric [RuCl2(dmso)2(indazole)2] complexes: ruthenium(II)-mediated coupling reaction of acetonitrile with 1H-indazole.

Reaction of the antitumor complex trans-[Ru(III)Cl4(Hind)2]- (Hind = indazole) with an excess of dimethyl sulfoxide (dmso) in acetone afforded the complex trans,trans,trans-[Ru(II)Cl2(dmso)2(Hind)2] (1). Two other isomeric compounds trans,cis,cis-[Ru(II)Cl2(dmso)2(Hind)2] (2) and cis,cis,cis-[Ru(II)Cl2(dmso)2(Hind)2] (3) have been obtained on refluxing cis-[Ru(II)Cl(2)(dmso)(4)] with 2 equiv. of indazole in ethanol and methanol, respectively. Isomers 1 and 2 react with acetonitrile yielding the complexes trans-[Ru(II)Cl2(dmso)(Hind){HN=C(Me)ind}].CH3CN (4.CH3CN) and trans,cis-[Ru(II)Cl2(dmso)2{HN=C(Me)ind}].H2O (5.H2O), respectively, containing a cyclic amidine ligand resulting from insertion of the acetonitrile C triple bond N group in the N1-H bond of the N2-coordinated indazole ligand in the nomenclature used for 1H-indazole. These are the first examples of the metal-assisted iminoacylation of indazole. The products isolated have been characterized by elemental analysis, IR spectroscopy, UV-vis spectroscopy, electrospray mass-spectrometry, thermogravimetry, differential scanning calorimetry, 1H NMR spectroscopy, and solid-state 13C CP MAS NMR spectroscopy. The isomeric structures of 1-3 and the presence of a chelating amidine ligand in 4 and 5 have been confirmed by X-ray crystallography. The electrochemical behavior of 1-5 and the formation of 5 have been studied by cyclic voltammetry.