Hierarchical assembly of helicate-type dinuclear titanium(IV) complexes

The ligands 4-7-H(2) were used in coordination studies with titanium(IV) and gallium(III) ions to obtain dimeric complexes Li(4)[(4-7)(6)Ti(2)] and Li(6)[(4/5a)(6)Ga(2)]. The X-ray crystal structures of Li(4)[(4)(6)Ti(2)], Li(4)[(5b)(6)Ti(2)], and Li(4)[(7a)(6)Ti(2)] could be obtained. While these complexes are triply lithium-bridged dimers in the solid state, a monomer/dimer equilibrium is observed in solution by NMR spectroscopy and ESI FT-ICR MS. The stability of the dimer is enhanced by high negative charges (Ti(IV) versus Ga(III)) of the monomers, when the carbonyl units are good donors (aldehydes versus ketones and esters), when the solvent does not efficiently solvate the bridging lithium ions (DMSO versus acetone), and when sterical hindrance is minimized (methyl versus primary and secondary carbon substituents). The dimer is thermodynamically favored by enthalpy as well as entropy. ESI FT-ICR mass spectrometry provides detailed insight into the mechanisms with which monomeric triscatecholate complexes as well as single catechol ligands exchange in the dimers. Tandem mass spectrometric experiments in the gas phase show the dimers to decompose either in a symmetric (Ti) or in an unsymmetric (Ga) fashion when collisionally activated. The differences between the Ti and Ga complexes can be attributed to different electronic properties and a charge-controlled reactivity of the ions in the gas phase. The complexes represent an excellent example for hierarchical self-assembly, in which two different noncovalent interactions of well balanced strengths bring together eleven individual components into one well-defined aggregate.     

Identification of an activated catalyst in the iridium-catalyzed allylic amination and etherification. Increased rates, scope, and selectivity

Studies were conducted to determine possible intermediates in the highly enantioselective, iridium-catalyzed amination and etherification of allylic carbonates, and these studies revealed that cyclometalation of the phosphoramidite ligand is likely to generate the active catalyst. The square-planar [Ir(COD)(L1)Cl] (L1 = P(BINOL)(bisphenethylamine)) did not react with cinnamyl carbonate, but did react with amine to generate an Ir(I) trigonal bipyramidal complex coordinated by COD, a cyclometalated kappa2-phosphoramidite, and a kappa1-phosphoramidite. This complex reacted with phosphines to generate products from replacement of the kappa1-phosphoramidite. These cyclometalated complexes were highly active catalysts for allylic amination and etherification and retained the high selectivity of the original catalyst system. In addition, these complexes combined with [Ir(cod)Cl]2 catalyzed reactions of amines with lower loadings, catalyzed reactions of alkylamines and aromatic amines that did not react with the original catalyst system, and catalyzed reactions of phenoxides under milder conditions.     

Correlation between Solution and the Solid State: The pH Dependent Composition in the Ternary System [Co(H2O)6]2+ or [Ni(H2O)6]2+/Piperazine/Phosphate

The protolytic equilibria of piperazine (C₄H₁₀N₂) and phosphate have been investigated in the presence of cobalt or nickel chloride or nitrate by potentiometric titrations between pH 2 and 8. Potentiometric titrations suggest the presence of [M²⁺(H₂O)₅(C₄H₁₁N₂⁺)]³⁺ and [M²⁺(H₂O)₅(C₄H₁₀N₂)]²⁺ in solution with stability constants logK of 3.1 and 3.8 for M = Co and 3.1 and 3.6 for M = Ni, respectively. Crystallization experiments were then conducted at selected pH values to isolate desired species from the known solution composition. Crystallization afforded [M(H₂O)₆]²⁺(C₄H₁₂N₂²⁺)(HPO₄^2—)₂ at pH 3.5 and 6.2 (M = Co, Ni), and Co₃(PO₄)₂·8H₂O at pH 10.5. No crystals with the dihydrogenphosphate anion or a metal‐bound piperazine ligand could be isolated under the reaction conditions. The solid‐state assembly in the isomorphous structures of [M(H₂O)₆](C₄H₁₂N₂)(HPO₄)₂ with M = Co and Ni is based on an extended hydrogen bonded network between the three ionic building blocks.     

Stepwise retro 1,3-dipolar cycloaddition induced by electron. Impact on 3,5-diphenyl-1,2,4-oxadiazole

The retro 1,3-dipolar cycloaddition induced by electron impact on 3,5-diphenyl-1,2,4-oxadiazole is interpreted as a two-step process on the basis of the energetics and kinetics of the fragments [C₇H₅NO]⁺., [C₆H₅CN]⁺. and [C₆H₅CO]⁺.     

Growth and characterization of Mn- and Co-doped ZnO nanowires

A flow injection chemiluminescence method is proposed for the determination of cobalt, based on the strong catalytic effect of Cobalt(II) (1,10-phenanthroline)₃ complex on the lucigenin-periodate reaction in alkaline medium. Under the optimum experimental conditions, the chemiluminescence signal responded linearly to the concentration of cobalt(II) in the 1.0 × 10⁻⁹–3.0 × 10⁻⁷ g mL⁻¹ range with a detection limit of 4.4 × 10⁻¹⁰ g mL⁻¹ cobalt(II). The relative standard deviation for the determination of 5.0 × 10⁻⁸ g mL⁻¹ of cobalt was 2.3% in eleven replicated measurements. The method was successfully applied to the determination of cobalt(II) in pharmaceutical preparations.     

The conformation of rifamycin S in solution by 1H NMR spectroscopy

The ¹H NMR spectra of rifamycin S in different solvents and at different temperatures strongly suggest the existence of a dominant conformer. The nine dihedral angles of the ansa chain from C-28 to C-19, considered conformationally flexible, were obtained from the vicinal interproton coupling constants by the Karplus equation and the proper alternative for each of them compares well with the corresponding values given by X-ray analysis in solid state.All the possible conformations derivable from the intrinsic alternatives of the NMR method were calculated for the ansa chain between C-28 and C-19 and, by applying geometrical considerations, such as closure of the polygonal path of the ansa and the steric incompatibility between the various atoms of the ansa and of the chromophore, only two of them appeared real. Thus, NMR spectroscopy can be used for studying the conformation of the ansa chain of rifamycins in solution.     

Why are olefins oxidized by RuO4 under cleavage of the carbon-carbon bond whereas oxidation by OsO4 yields cis-diols?

Quantum chemical calculations using gradient-corrected (B3LYP) density functional theory have been carried out to investigate the mechanism of the oxidative cleavage of alkenes by ruthenium tetraoxide. The initial reaction of the tetraoxide with the olefin occurs via a [3+2] cycloaddition as in the case of osmium tetraoxide. The results clearly show that the bond cleavage does not take place at the primary adduct, but much later in the reaction path. After the formation of the ruthenium(VI)dioxo-2,5-dioxolane, the reaction proceeds with the addition of a second olefin to yield ruthenium(IV)-bis(2,5-dioxolane), which in turn becomes oxidized first to rutheniumoxo(VI)-bis(2,5-dioxolane) 6(Ru) and then to ruthenium(VIII)-dioxo-bis(2,5-dioxolane) 7(Ru). Only in complexes containing the metal center in the formal oxidation state +VIII are low activation barriers for C-C bond cleavage and exothermic formation of carbonyl compounds as products calculated. The lowest activation barrier, DeltaH(++) = 2.5 kcal/mol, is calculated for the C-C bond breaking reaction of 7(Ru) which is predicted as the pivotal intermediate of the oxidation reaction. The calculations of the oxidation reaction with OsO(4) show that those reactions where the oxidation state of the metal increases have larger activation barriers for M = Ru than for M = Os, while reactions which reduce the oxidation state have a lower activation barrier for ruthenium compounds. Also, reactions which increase the oxidation state of the metal are in the case of M = Os more exothermic than for M = Ru. In this work, all important points of the potential energy surface (PES) are reported, and the complete catalytic cycle for the oxidative cleavage of olefins by ruthenium tetraoxide is presented.     

Quantum chemical investigations and bonding analysis of iron complexes with mixed cyano and carbonyl ligands

The equilibrium structures and vibrational frequencies of the iron complexes [Fe(CN)(x)(CO)(y)](q) (x = 0-6 and y = 0-5) have been calculated at the BP86 level of theory. The nature of the Fe-CN and Fe-CO has been analyzed with an energy partitioning method. The calculated Fe-CO bond lengths are in good agreement with the results of X-ray structure analysis whereas the Fe-CN bonds are calculated somewhat longer than the experimental values. The theoretically predicted vibrational frequencies of the C-O stretching mode are always lower and the calculated CN(-) frequencies are higher than the observed fundamental modes. The results of the bonding analysis suggest that the Fe-CO binding interactions have approximately 55% electrostatic character and approximately 45% covalent character. There is a significant contribution of the pi orbital interaction to the Fe-CO covalent bonding which increases when the complexes become negatively charged. The strength of deltaE(pi) may even be larger than deltaE(sigma). The Fe-CN(-) bonds have much less pi character. The calculated binding energy of the Fe-CO pi-interactions correlates very well with the C-O stretching frequencies.     

Synthesis and use of alpha-silyl-substituted alpha-hydroxyacetic acids

[reaction: see text] Rhodium-catalyzed oxygen transfer was used to generate benzyl 2-silyl-2-oxoacetates in good yields. The hydrogenation of these compounds led to chiral alpha-silyl-substituted alpha-hydroxyacetic acids. Resolution by means of HPLC using a chiral stationary phase afforded an enantiomerically pure representative of this class of compounds, which was successfully applied as a chiral ligand in an asymmetric aldol-type reaction.     

Theory-enforced Re-investigation of the Origin of the Large Metal–Carbon Bond Strength in PdCH2I+ and its reactions with unsaturated hydrocarbons

State-of-the-art ab initio studies demonstrate that the reaction Pd⁺ + CH₃I → PdCH₂I⁺ + H^. is endothermic by ca. 20 kcal/mol, which translates into a bond dissociation energy (BDE) of ca. 83 kcal/mol for the Pd⁺? CH₂I bond. This figure is in agreement with an experimental bracket of 68 kcal/mol < BDE(Pd⁺? CH₂I) < 92 kcal/mol. Based on these findings, the previously studied Pd⁺/CH₃I system was re-investigated, and double-resonance experiments demonstrate that the formation of PdCH₂I⁺ occurs stepwise via PdCH as a reactive intermediate. Further, ion/molecule reactions of PdCH₂I⁺ with unsaturated hydrocarbons are studied, which reveal the formation of carbon–carbon bonds in the gas phase.