Synthetic and mechanistic aspects of the immortal ring-opening polymerization of lactide and trimethylene carbonate with new homo- and heteroleptic tin(II)-phenolate catalysts

Several new heteroleptic Sn(II) complexes supported by amino-ether phenolate ligands [Sn{LO(n)}(Nu)] (LO(1)=2-[(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)methyl]-4,6-di-tert-butylphenolate, Nu=NMe(2) (1), N(SiMe(3))(2) (3), OSiPh(3) (6); LO(2)=2,4-di-tert-butyl-6-(morpholinomethyl)phenolate, Nu=N(SiMe(3))(2) (7), OSiPh(3) (8)) and the homoleptic Sn{LO(1)}(2) (2) have been synthesized. The alkoxy derivatives [Sn{LO(1)}(OR)] (OR=OiPr (4), (S)-OCH(CH(3))CO(2)iPr (5)), which were generated by alcoholysis of the parent amido precursor, were stable in solution but could not be isolated. [Sn{LO(1)}](+)[H(2)N{B(C(6)F(5))(3)}(2)](-) (9), a rare well-defined, solvent-free tin cation, was prepared in high yield. The X-ray crystal structures of compounds 3, 6, and 8 were elucidated, and compounds 3, 6, 8, and 9 were further characterized by (119)Sn M?ssbauer spectroscopy. In the presence of iPrOH, compounds 1-5, 7, and 9 catalyzed the well-controlled, immortal ring-opening polymerization (iROP) of L-lactide (L-LA) with high activities (ca. 150-550 mol(L-LA) mol(Sn)(-1) h(-1)) for tin(II) complexes. The cationic compound 9 required a higher temperature (100 °C) than the neutral species (60 °C); monodisperse poly(L-LA)s were obtained in all cases. The activities of the heteroleptic pre-catalysts 1, 3, and 7 were virtually independent of the nature of the ancillary ligand, and, most strikingly, the homoleptic complex 2 was equally competent as a pre-catalyst. Polymerization of trimethylene carbonate (TMC) occurs much more slowly, and not at all in the presence of LA; therefore, the generation of PLA-PTMC copolymers is only possible if TMC is polymerized first. Mechanistic studies based on (1)H and (119)Sn{(1)H} NMR spectroscopy showed that the addition of an excess of iPrOH to compound 3 yielded a mixture of compound 4, compound [Sn(OiPr)(2)](n) 10, and free {LO(1)}H in a dynamic temperature-dependent and concentration-dependent equilibrium. Upon further addition of L-LA, two active species were detected, [Sn{LO(1)}(OPLLA)] (12) and [Sn(OPLLA)(2)] (14), which were also in fast equilibrium. Based on assignment of the (119)Sn{(1)H} NMR spectrum, all of the species present in the ROP reaction were identified; starting from either the heteroleptic (1, 3, 7) or homoleptic (2) pre-catalysts, both types of pre-catalysts yielded the same active species. The catalytic inactivity of the siloxy derivative 6 confirmed that ROP catalysts of the type 1-5 could not operate according to an activated-monomer mechanism. These mechanistic studies removed a number of ambiguities regarding the mechanism of the (i)ROPs of L-LA and TMC promoted by industrially relevant homoleptic or heteroleptic Sn(II) species.     

On the spontaneous carboxylation of 1-butyl-3-methylimidazolium acetate by carbon dioxide

The formation of 1-butyl-3-methylimidazolium-2-carboxylate in the mixture of CO(2) with 1-butyl-3-methylimidazolium acetate under mild conditions (298 K, 0.1 MPa) has been put in evidence in the liquid phase using Raman and infrared spectroscopy complemented by DFT calculations and NMR ((1)H, (13)C, (15)N) spectroscopy.     

Long-range effects on the capture and release of a chiral guest by a helical molecular capsule

Helically folded molecular capsules based on oligoamide sequences of aromatic amino acids which are capable of binding tartaric acid in organic solvents with high affinity and diastereoselectivity have been synthesized, and their structures and binding properties investigated by (1)H NMR, X-ray crystallography, circular dichroism, and molecular modeling. We found that elongating the helices at their extremities by adding monomers remote from the tartaric binding site results in a strong increase of the overall helix stability, but it does not influence the host-guest complex stability. The effect of this elongation on the binding and release rates of the guest molecules follows an unexpected non-monotonous trend. Three independent observations (direct monitoring of exchange over time, 2D-EXSY NMR, and molecular modeling) concur and show that guest exchange rates tend to first increase upon increasing helix length and then decrease when helix length is increased further. This investigation thus reveals the complex effects of adding monomers in a helically folded sequence on a binding event that occurs at a remote site and sheds light on possible binding and release mechanisms.     

Expression of GD3 synthase modifies ganglioside profile and increases migration of MCF-7 breast cancer cells

B- and c-series of gangliosides are over-expressed in neuro-ectoderm-related cancers, including breast cancer. It has been shown that G_D3 ganglioside is over-expressed in about 50% of invasive ductal breast carcinoma and the G_D3 synthase (GD3S) gene displays higher expression among estrogen receptor (ER) negative breast tumors. We previously showed that GD3S expression in MDA-MB-231 breast cancer cells induces the expression of G_D2 and increased cell proliferation and migration via a G_D2-dependent activation of c-Met receptor. Here, we show that in ER-positive MCF-7 breast cancer cells, GD3S expression resulted in an increase of G_D1b, which was associated with a decrease of G_M1a and G_M2. Meanwhile, GD3S expressing MCF-7 cells exhibited an increased migration without any modification of proliferation rate. Therefore, GD3S expression can result in different modifications of both ganglioside profiles and cell phenotypes depending on breast cell types.     

In solution sensitization of Er(III) luminescence by the 4-tetrathiafulvalene-2,6-pyridinedicarboxylic acid dimethyl antenna ligand

In the [Er(hfac)(3)(L)](2) complex (1) (L = 4-tetrathiafulvalene-2,6-pyridinecarboxylic acid dimethyl ester), the Er(III) ion is bonded to the tridentate coordination site. Electrochemical and photophysical measurements in solution reveal that the tetrathiafulvalene moiety is a versatile antenna for erbium luminescence sensitization at 6540 cm(-1) upon excitation in the low-energy charge transfer transition (donor to acceptor charge transfer) at 16600 cm(-1) assigned via time-dependent density functional theory calculations.     

Blue TiO_{2 -x}/SiO2 nanoparticles by laser pyrolysis

Composite TiSiOC nanoparticles with Ti/Si ratio varying in a very large range were prepared by laser pyrolysis of a gas–spray mixture of silane and titanium tetra-isopropoxide. The as-formed nanoparticle batches exhibit intense blue colours, varying from dark to light blue while the Ti/Si ratio increases. This blue colour is attributed to the formation of sub-stoichiometric TiOcompounds induced by the presence of reducing agents such as silicon-based radicals and carbon atoms in the reaction medium. The blue colour of the powders is stable for several months at room temperature in normal atmospheric conditions. Elemental analysis, specific surface area and pycnometry measurements, as well as Photon Correlation Spectroscopy allow determining the chemical composition and size of the as-synthesized nanoparticles as a function of the Ti/Si ratio. X-ray diffraction, transmission electron microscopy and IR spectroscopy have been used to analyse their chemical organisation, nanostructure and morphology. Mean grain size is found around 20 nm. The nanoparticles exhibit a core-shell structure TiO/SiO₂, with a core made of titania, surrounded by an amorphous shell, mainly of silica. Crystallites of anatase are present in the core with size increasing with the Ti/Si ratio. Annealing under air at 800°C induces the removal of carbon and the crystallisation of the powders with light beige to white colours.     

Low-frequency fluctuations in two-state quantum dot lasers

We study the feedback-induced instabilities in a quantum dot semiconductor laser emitting in both ground and excited states. Without optical feedback the device exhibits dynamics corresponding to antiphase fluctuations between ground and excited states, while the total output power remains constant. The introduction of feedback leads to power dropouts in the ground state and intensity bursts in the excited state, resulting in a practically constant total output power.     

Potent algicides based on the cyanobacterial alkaloid nostocarboline

[structure: see text] Nostocarboline and seven derivatives were prepared and displayed minimal inhibitory concentration (MIC) values >or=100 nM against the growth of Microcystis aeruginosa PCC 7806, Synechococcus PCC 6911, and Kirchneriella contorta SAG 11.81, probably via the inhibition of photosynthesis. The natural product hybrid nostocarboline/ciprofloxacin displayed additional antibacterial activity against several Gram-negative bacteria (MICs >or=0.7 microM). Nostocarboline can thus be considered a potent, selective, readily available, natural algicide.     

Co(II)-Co(II) paddlewheel complex with a redox-active ligand derived from TTF

A new trimethyltetrathiafulvalene (Me3TTF) derivative Me3TTF-CH=CH-py bearing a pyridyl was synthesized and coordinated to a cobalt(II) benzoate dimer, having paddlewheel core structure, leading to a complex formulated as Co2(PhCOO)4(Me3TTF-CH=CH-py)2. Single-crystal X-ray diffraction studies of the complex performed at 293 and 100 K evidenced the existence of a weak metal-metal interaction. Magnetic studies revealed an antiferromagnetic behavior, which is explained as the result of the direct exchange between metal centers.     

A theoretical investigation into the photophysical properties of ruthenium polypyridine-type complexes

Excited states of ruthenium polypyridine-type complexes have always attracted the interest of chemists. We have recently found evidence of a remarkable long-lived excited state (30 micros) for a Ru(II) complex containing a heteroditopic ligand that can be viewed as a fused phenanthroline and salophen ligand.1 To unravel this intriguing electronic property, we have used density functional theory (DFT) calculations to understand the ground-state properties of [(bpy)(2)Ru(LH(2))](2+), where LH(2) represents N,N'-bis(salicylidene)-(1,10-phenanthroline)diamine. Excited singlet and triplet states have been examined by the time-dependent DFT (TDDFT) formalism and the theoretical findings have been compared with those for the parent complex [Ru(bpy)(3)](2+). The outstanding result is the presence of excited states lower in energy than the metal-to-ligand charge-transfer states, originating from intraligand charge transfer (ILCT) from the phenolic rings to the phenanthroline part of the coordinated LH(2). The spin density distribution for the lowest triplet state provides evidence that it is in fact the lowest triplet state of the free ligand. Correlation between the energy level diagram of orbitals for the ground state and that for the (3)ILCT state clearly establishes that the ruthenium retains its formal Ru(II) oxidation state. The quenching of the luminescence and the evidence of the long-lived excited state observed for [(bpy)(2)Ru(LH(2))](2+) are discussed in the light of the computational results.