Hall N.,University Grenoble alpes |
Hall N.,CNRS Molecular Chemistry Department |
Orio M.,TGE Reseau National de RPE Interdisciplinaire |
Orio M.,University of Lille Nord de France |
And 11 more authors.
The structural and electronic properties as well as the catalytic activity toward sulfoxidation of two new vanadium complexes have been investigated. They both possess in their coordination sphere two alkyl thiolate ligands: a dioxido VV complex [VO2LNS2](HNEt3) (1) (LNS2 = 2,2′-(pyridine-2,6-diyl)bis(1,1′- diphenylethanethiol)) and an oxido VIV complex [VOLN2S2] (2) (LN2S2 = 2,2′-(2,2′-bipyridine-6,6′-diyl)bis(1, 1′-diphenylethanethiol)). The X-ray structure of 1 has revealed that the VV metal ion is at the center of a distorted trigonal bipyramid. The optimized structure of 2 obtained by DFT calculations displays a square-pyramidal geometry, consistent with its EPR spectrum characterized by an axial S = 1/2 signal (gâŠ¥ = 1.988, g â̂¥ = 1.966, Ax(V) = 45 × 10 -4 cm-1, Ay(V) = 42 × 10-4 cm-1, Az(V) = 135 × 10-4 cm -1). DFT calculations have shown that the HOMO (highest occupied molecular orbital) of 1 is notably localized on the two thiolate sulfur atoms (56% and 22%, respectively), consistent with the expected covalent character of the VV-S bond. On the other hand, the SOMO (singly occupied molecular orbital) of 2 is exclusively localized at the VIV ion (92%). Complexes 1 and 2 have shown an ability to catalytically oxidize sulfide into sulfoxide. The oxidation reactions have been carried out with thioanisole as substrate and hydrogen peroxide as oxidant. Yields of 80% and 75% have been obtained in 10 and 15 min for 1 and 2, respectively. However, in terms of conversion, 1 is more efficient than 2 (81% and 44%, respectively). More importantly, the reaction is completely selective with no trace of sulfone produced. While 1 displays a poor stability, catalyst 2 shows the same efficiency after five successive additions of oxidant and substrate. The difference in reactivity and stability between both complexes has been rationalized through a mechanism study performed by means of experimental data (51V NMR and EPR spectroscopy) combined with theoretical calculations. It has been shown that the structure of the cis-oxo peroxo V V intermediate species, which is related to its stability, can partly explain these discrepancies. © 2013 American Chemical Society. Source
Gourier D.,TGE Reseau National de RPE Interdisciplinaire |
Gourier D.,CNRS Laboratory of Condensed Matter Chemistry, Paris |
Delpoux O.,TGE Reseau National de RPE Interdisciplinaire |
Binet L.,TGE Reseau National de RPE Interdisciplinaire |
And 3 more authors.
The search for organic biosignatures is motivated by the hope of understanding the conditions of emergence of life on Earth and the perspective of finding traces of extinct life in martian sediments. Paramagnetic radicals, which exist naturally in amorphous carbonaceous matter fossilized in Precambrian cherts, were used as local structural probes and studied by electron paramagnetic resonance (EPR) spectroscopy. The nuclear magnetic resonance transitions of elements inside and around these radicals were detected by monitoring the nuclear modulations of electron spin echo in pulsed EPR. We found that the carbonaceous matter of fossilized microorganisms with age up to 3.5 billion years gives specific nuclear magnetic signatures of hydrogen ( 1H), carbon (13C), and phosphorus (31P) nuclei. We observed that these potential biosignatures of extinct life are found neither in the carbonaceous matter of carbonaceous meteorites (4.56 billion years), the most ancient objects of the Solar System, nor in any carbonaceous matter resulting from carbonization of organic and bioorganic precursors. These results indicate that these nuclear signatures are sensitive to thermal episodes and can be used for Archean cherts with metamorphism not higher than the greenschist facies. Key Words: Kerogen-Biosignatures-Origin of life-Archean-EPR spectroscopy. © Mary Ann Liebert, Inc. Source