Regiochemical control of the catalytic asymmetric hydroboration of 1,2-diarylalkenes

The hydroboration of stilbenes and related disubstituted alkenes catalysed by QUINAP complexes may proceed with high enantio- and regioselectivity; rhodium and iridium catalysts give the same product regioisomer but opposite enantiomers.     

Pyridine-based lanthanide complexes combining MRI and NIR luminescence activities

A series of novel triazole derivative pyridine-based polyamino-polycarboxylate ligands has been synthesized for lanthanide complexation. This versatile platform of chelating agents combines advantageous properties for both magnetic resonance (MR) and optical imaging applications of the corresponding Gd(3+) and near-infrared luminescent lanthanide complexes. The thermodynamic stability constants of the Ln(3+) complexes, as assessed by pH potentiometric measurements, are in the range log K(LnL)=17-19, with a high selectivity for lanthanides over Ca(2+), Cu(2+), and Zn(2+). The complexes are bishydrated, an important advantage to obtain high relaxivities for the Gd(3+) chelates. The water exchange of the Gd(3+) complexes (k(ex)(298)=7.7-9.3×10(6) s(-1)) is faster than that of clinically used magnetic resonance imaging (MRI) contrast agents and proceeds through a dissociatively activated mechanism, as evidenced by the positive activation volumes (ΔV(≠)=7.2-8.8 cm(3) mol(-1)). The new triazole ligands allow a considerable shift towards lower excitation energies of the luminescent lanthanide complexes as compared to the parent pyridinic complex, which is a significant advantage in the perspective of biological applications. In addition, they provide increased epsilon values resulting in a larger number of emitted photons and better detection sensitivity. The most conjugated system PheTPy, bearing a phenyl-triazole pendant on the pyridine ring, is particularly promising as it displays the lowest excitation and triplet-state energies associated with good quantum yields for both Nd(3+) and Yb(3+) complexes. Cellular and in vivo toxicity studies in mice evidenced the non-toxicity and the safe use of such bishydrated complexes in animal experiments. Overall, these pyridinic ligands constitute a highly versatile platform for the simultaneous optimization of both MRI and optical properties of the Gd(3+) and the luminescent lanthanide complexes, respectively.     

Thermal yellowing sensitised by pre-photo-oxidation of non-deacidified paper

Characterization of oxidation products responsible for yellowing of paper through absorption and through emission is presented using independently UV-visible absorption spectrophotometry and microspectrofluorimetry to survey the thermal oxidation at 100 °C and the photo-oxidation at λ ≥ 340 nm and at 60 °C of the three types of paper containing low amount of ligneous residues (SA papers) or not (WH and C papers). It was shown that due to secondary oxidation yellowing of product is favoured in the dark. It was shown that pre-photo-oxidation largely increased the yellowing in the dark. The effect of pre-photo-oxidation on yellowing through absorption and on yellowing through emission was shown to be different.     

Combined XRD and infrared studies of pyridinium species in (PyH)(3)[PW(12)O(40)] single crystals

The pyridinium salts of 12-tungstophosphoric acid (PyH)(3)[PW(12)O(40)]·2CH(3)CN (1) and (PyH)(3)[PW(12)O(40)] (2) have been prepared and studied by single crystal and powder X-ray diffraction (XRD) in order to characterize the crystallographic sites occupied by the pyridinium species. The three PyH(+) species are located on two unequivalent sites. Two species are linearly H-bonded to the oxygen atoms of the Keggins unit (α species), whereas the third one (β) forms a bent H-bond. In order to determine the infrared bands characterizing each type of pyridinium species in the 1650-1300 cm(-1) range, infrared spectra have been recorded from room temperature to 100 K. They reveal that only α pyridinium species give rise to the unusual splitting of the PyH(+)ν8b and ν19b modes, whereas β pyridinium species lead to a classical pyridinium spectrum.