Synthesis, characterization, and optical properties of 4H-pyran-4-ylidene donor-based chromophores: the relevance of the location of a thiophene ring in the spacer

A series of new 4H-pyran-4-ylidene donor-based chromophores with a thiophene ring in the spacer has been synthesized. The linear and nonlinear optical (NLO) properties of these compounds have been determined and compared with the results of computational calculations. The position of the thiophene ring proved essential to optimize the figure of merit μβ, with the best results obtained when the heterocyclic system was closer to the donor moiety.     

Alkynyldiphenylphosphine d8 (Pt, Rh, Ir) complexes: contrasting behavior toward cis-[Pt(C6F5)2(THF)2

The synthesis and characterization of a series of mononuclear d(8) complexes with at least two P-coordinated alkynylphosphine ligands and their reactivity toward cis-[Pt(C(6)F(5))(2)(THF)(2)] are reported. The cationic [Pt(C(6)F(5))(PPh(2)C triple-bond CPh)(3)](CF(3)SO(3)), 1, [M(COD)(PPh(2)C triple-bond CPh)(2)](ClO(4)) (M = Rh, 2, and Ir, 3), and neutral [Pt(o-C(6)H(4)E(2))(PPh(2)C triple-bond CPh)(2)] (E = O, 6, and S, 7) complexes have been prepared, and the crystal structures of 1, 2, and 7.CH(3)COCH(3) have been determined by X-ray crystallography. The course of the reactions of the mononuclear complexes 1-3, 6, and 7 with cis-[Pt(C(6)F(5))(2)(THF)(2)] is strongly influenced by the metal and the ligands. Thus, treatment of 1 with 1 equiv of cis-[Pt(C(6)F(5))(2)(THF)(2)] gives the double inserted cationic product [Pt(C(6)F(5))(S)mu-(C(Ph)=C(PPh(2))C(PPh(2))=C(Ph)(C(6)F(5)))Pt(C(6)F(5))(PPh(2)C triple-bond CPh)](CF(3)SO(3)) (S = THF, H(2)O), 8 (S = H(2)O, X-ray), which evolves in solution to the mononuclear complex [(C(6)F(5))(PPh(2)C triple-bond CPh)Pt(C(10)H(4)-1-C(6)F(5)-4-Ph-2,3-kappaPP'(PPh(2))(2))](CF(3) SO(3)), 9 (X-ray), containing a 1-pentafluorophenyl-2,3-bis(diphenylphosphine)-4-phenylnaphthalene ligand, formed by annulation of a phenyl group and loss of the Pt(C(6)F(5)) unit. However, analogous reactions using 2 or 3 as precursors afford mixtures of complexes, from which we have characterized by X-ray crystallography the alkynylphosphine oxide compound [(C(6)F(5))(2)Pt(mu-kappaO:eta(2)-PPh(2)(O)C triple-bond CPh)](2), 10, in the reaction with the iridium complex (3). Complexes 6 and 7, which contain additional potential bridging donor atoms (O, S), react with cis-[Pt(C(6)F(5))(2)(THF)(2)] in the appropriate molar ratio (1:1 or 1:2) to give homo- bi- or trinuclear [Pt(PPh(2)C triple-bond CPh)(mu-kappaE-o-C(6)H(4)E(2))(mu-kappaP:eta(2)-PPh(2)C triple-bond CPh)Pt(C(6)F(5))(2)] (E = O, 11, and S, 12) and [(Pt(mu(3)-kappa(2)EE'-o-C(6)H(4)E(2))(mu-kappaP:eta(2)-PPh(2)C triple-bond CPh)(2))(Pt(C(6)F(5))(2))(2)] (E = O, 13, and S, 14) complexes. The molecular structure of 14 has been confirmed by X-ray diffraction, and the cyclic voltammetric behavior of precursor complexes 6 and 7 and polymetallic derivatives 11-14 has been examined.     

A non-adiabatic quantum-classical dynamics study of the intramolecular excited state hydrogen transfer in ortho-nitrobenzaldehyde

Ab initio surface-hopping dynamics calculations have been performed to simulate the intramolecular excited state hydrogen transfer dynamics of ortho-nitrobenzaldehyde (o-NBA) in the gas phase from the electronic S(1) excited state. Upon UV excitation, the hydrogen is transferred from the aldehyde substituent to the nitro group, generating o-nitrosobenzoic acid through a ketene intermediate. The semiclassical propagations show that the deactivation from the S(1) is ultrafast, in agreement with the experimental measurements, which detect the ketene in less than 400 fs. The trajectories show that the deactivation mechanism involves two different conical intersections. The first one, a planar configuration with the hydrogen partially transferred, is responsible for the branching between the formation of a biradical intermediate and the regeneration of the starting material. The conversion of the biradical to the ketene corresponds to the passage through a second intersection region in which the ketene group is formed.     

[Pd(Fmes)2(tmeda)]: A Case of Intermittent C?H⋅⋅⋅F?C Hydrogen‐Bond Interaction in Solution

The X‐ray structure of the title compound [Pd(Fmes)₂(tmeda)] (Fmes=2,4,6‐tris(trifluoromethyl)phenyl; tmeda=N,N,N′,N′‐tetramethylethylenediamine) shows the existence of uncommon C? H???F? C hydrogen‐bond interactions between methyl groups of the TMEDA ligand and ortho‐CF₃ groups of the Fmes ligand. The ¹⁹F NMR spectra in CD₂Cl₂ at very low temperature (157 K) detect restricted rotation for the two ortho‐CF₃ groups involved in hydrogen bonding, which might suggest that the hydrogen bond is responsible for this hindrance to rotation. However, a theoretical study of the hydrogen‐bond energy shows that it is too weak (about 7 kJ mol^?1) to account for the rotational barrier observed (ΔH^≠=26.8 kJ mol^?1), and it is the steric hindrance associated with the puckering of the TMEDA ligand that should be held responsible for most of the rotational barrier. At higher temperatures the rotation becomes fast, which requires that the hydrogen bond is continuously being split up and restored and exists only intermittently, following the pulse of the conformational changes of TMEDA.     

Conformational flexibility of a synthetic glycosylaminoglycan bound to a fibroblast growth factor. FGF-1 recognizes both the (1)C(4) and (2)S(O) conformations of a bioactive heparin-like hexasaccharide

The first direct NMR determination of the conformation of a conformationally flexible heparin-like hexasaccharide bound to a key receptor, FGF-1, is described. The determination has been based on the use of a 13C-labeled protein and a regular 12C sugar. FGF-1 recognizes several conformations of the iduronic moieties of the hexasaccharide. Therefore, this case is different than that described for the controversial recognition of heparin-like saccharides by AT-III, which seems to recognize just one conformation of the iduronic acid residues.     

Conformational mobility of thianthrene-5-oxide

[reaction: see text] Data on the apparent dipole moment of thianthrene-5-oxide (1) and (1)H NMR spectra in different solvents support the conformational mobility of 1, which flaps between two limit boat conformations with the sulfinyl group in pseudoequatorial and pseudoaxial positions, respectively. The conformational equilibrium of 1 occurs too fast for the (1)H NMR (500 MHz) time-scale even at -130 degrees C, and the equilibrium constant has not been determined. The apparent dipole moments of 1 in n-hexane and 1,4-dioxane and the (1)H NMR spectra of 1 and the model compounds cis- and trans-thianthrene-5,10-dioxides (2) and thianthrene (5) in different solvents and at various temperatures confirm that the relative position of the conformational equilibrium of 1 is solvent-dependent, and more polar solvents favor the conformation with the sulfoxide group in the pseudoaxial position (1(')(ax)). Variable-temperature (1)H NMR spectra have established the interconversion barrier of trans-2 and confirmed that the conformational equilibrium of cis-2 is strongly displaced toward the conformation with both sulfinyl groups in the pseudoequatorial position. The (1)H NMR data support the transannular interaction of the functional groups in 1 and trans-2.     

Technical electrodes catalyzed with PtRu on mesoporous ordered carbons for liquid direct methanol fuel cells

This paper presents the behavior of ordered mesoporous carbon (OMC)-supported catalysts as anodes for direct methanol fuel cells (DMFC), fed with an aqueous methanol solution. OMC samples were prepared by the nanocasting method from a polymerized furan resin using mesoporous silica as a template. Pt and PtRu nanoparticles were supported on OMC with high dispersion, the particle size being 2.4 nm at PtRu loading of 15 wt.%. The resulting catalysts were analyzed using carbon monoxide stripping voltammetry, cyclic voltammetry, and chronoamperometry in three-electrode experiments and recording cell voltage vs. current density curves in practical DMFC. It was found that PtRu-catalyzed technical electrodes exhibited good activity towards methanol electrooxidation in half-cell experiments under fuel-cell-relevant conditions. Specifically, Pt₈₅Ru₁₅/OMC catalyst showed the highest catalytic enhancement compared to Pt/OMC for the steady-state electrooxidation of methanol at 60 °C and 0.5 V, by a factor of 22 in 2-M MeOH solution. DMFC single cells yielded an open-circuit voltage of 0.625 V at 60 °C. Polarization curves indicate that DMFC with OMC-supported Pt₈₅Ru₁₅ catalyst at the anode exhibited the best performance.     

Promoting Difficult Carbon–Carbon Couplings: Which Ligand Does Best?

A Pd complex, cis‐[Pd(C₆F₅)₂(THF)₂] ( 1 ), is proposed as a useful touchstone for direct and simple experimental measurement of the relative ability of ancillary ligands to induce C?C coupling. Interestingly, 1 is also a good alternative to other precatalysts used to produce Pd⁰L. Complex 1 ranks the coupling ability of some popular ligands in the order P^tBu₃>o‐TolPEWO‐F≈tBuXPhos>P(C₆F₅)₃≈PhPEWO‐F>P(o‐Tol)₃≈THF≈tBuBrettPhos?Xantphos≈PhPEWO‐H?PPh₃ according to their initial coupling rates, whereas their efficiency, depending on competitive hydrolysis, is ranked tBuXPhos≈P^tBu₃≈o‐TolPEWO‐F>PhPEWO‐F>P(C₆F₅)₃?tBuBrettPhos>THF≈P(o‐Tol)₃>Xantphos>PhPEWO‐H?PPh₃. This “meter” also detects some other possible virtues or complications of ligands such as tBuXPhos or tBuBrettPhos.     

Visible Light-Gated Organocatalysis Using a RuII-Photocage

The light-gated organocatalysis via the release of 4-N,N-dimethylaminopyridine (DMAP) by irradiation of the [Ru(bpy)₂(DMAP)₂]²⁺ complex with visible light was investigated. As model reaction the acetylation of benzyl alcohols with acetic anhydride was chosen. The pre-catalyst releases one DMAP molecule on irradiation at wavelengths longer than 455 nm. The photochemical process was characterized by steady-state irradiation and ultrafast transient absorption spectroscopy. The latter enabled the observation of the ³MLCT state and the spectral features of the penta-coordinated intermediate [Ru(bpy)₂(DMAP)]²⁺. The released DMAP catalyzes the acetylation of a wide range of benzyl alcohols with chemical yields of up to 99 %. Control experiments revealed unequivocally that it is the released DMAP which takes the role of the catalyst.     

Mono- and Di-Fucosylated Glycans of the Parasitic Worm S. mansoni are Recognized Differently by the Innate Immune Receptor DC-SIGN

The parasitic worm, Schistosoma mansoni, expresses unusual fucosylated glycans in a stage-dependent manner that can be recognized by the human innate immune receptor DC-SIGN, thereby shaping host immune responses. We have developed a synthetic approach for mono- and bis-fucosylated LacdiNAc (LDN-F and LDN-DF, respectively), which are epitopes expressed on glycolipids and glycoproteins of S. mansoni. It is based on the use of monosaccharide building blocks having carefully selected amino-protecting groups, facilitating high yielding and stereoselective glycosylations. The molecular interaction between the synthetic glycans and DC-SIGN was studied by NMR and molecular modeling, which demonstrated that the α1,3-fucoside of LDN-F can coordinate with the Ca²⁺-ion of the canonical binding site of DC-SIGN allowing for additional interactions with the underlying LDN backbone. The 1,2-fucoside of LDN-DF can be complexed in a similar manner, however, in this binding mode GlcNAc and GalNAc of the LDN backbone are placed away from the protein surface resulting in a substantially lower binding affinity. Glycan microarray binding studies showed that the avidity and selectivity of binding is greatly enhanced when the glycans are presented multivalently, and in this format Le^x and LDN-F gave strong responsiveness, whereas no binding was detected for LDN-DF. The data indicates that S. mansoni has developed a strategy to avoid detection by DC-SIGN in a stage-dependent manner by the addition of a fucoside to a number of its ligands.