A comparison between models for the substrate−catalyst adduct in hydrodenitrogenation (HDN) catalysis is made with respect to oxygen vs sulfur ancillary ligands. Reacting η2(N,C)-NC5 tBu3H2Ta(OAr)2Cl ...(1, Ar = 2,6-C6H3 iPr2) with KOtBu affords orange crystals of the alkoxide η2(N,C)-NC5 tBu3H2Ta(OAr)2(OtBu) (2), while 1 and LiStBu react to form the red thiolate analogue η2(N,C)-NC5 tBu3H2Ta(OAr)2(StBu) (3). Structural studies of both complexes 2 and 3 are reported and compared with other η2(N,C)-NC5 tBu3H2 derivatives. A trace of the bromide complex η2(N,C)-NC5 tBu3H2Ta(OAr)2Br (4) is isolated from reacting η2(N,C)-NC5 tBu3H2Ta(OAr)2Cl (1) with EtMgBr in THF/Et2O solution and is also structurally characterized for comparison. Complexes 2−4 reveal a severe interruption of aromaticity within the heterocycle, different rotational preferences of the pyridine NC5 plane with respect to the Ta(OAr)2X moiety, and various aryloxide ligand structural differences. From this comparison, arguments will be presented that support the ancillary ligand π-donor ability decreasing as OtBu > OAr > StBu > Cl ≈ Br > Et, although evidence suggests that the StBu ligand is a better σ + π donor overall than OAr or OtBu.
Organochlorine compounds, such as polychlorinated biphenyls (PCBs), pentachlorophenol (PCP), p,p'-DDE and Toxaphene, have been widely used in industry and agriculture for more than fifty years. ...Although they have served their purpose very effectively and at low cost, many of these compounds have been banned in the United States due to their persistence in the environment and their threat to human health. Their natural resistance to degradation has made organochlorine compounds the target of many studies that have been designed to convert them into less toxic compounds. At present there is not a single, simple method than can completely dechlorinate PCBs, PCP, p,p'-DDE and Toxaphene. The work presented here reports the use of a novel bimetallic system, palladized iron (Pd/Fe), to effect the complete dechlorination of these compounds, at ambient temperature and pressure, in a matter of minutes. The dechlorination reaction occurs on the surface of the palladized iron, with removal of all the chlorine atoms from the chlorinated compound and yields the completely dechlorinated molecule and chloride ions as reaction products. The chlorinated compound is reductively dechlorinated while the iron particles are oxidized to Fe²⁺ Water is also reduced in the presence of iron, generating hydrogen, which is collected in the palladium lattice. The palladium is therefore necessary to store hydrogen gas; the "Pd·H₂" acts as a powerful reducing agent and is primarily responsible for the rapid and complete dechlorination of the organochlorine compounds. The Pd/Fe bimetallic system is potentially useful for the large scale remediation of groundwater or soil contaminated with organochlorine compounds. Palladized iron is relatively inexpensive and easy to prepare, and it rapidly and completely dechlorinates organochlorine compounds. For these reasons, the Pd/Fe system should be investigated further for applications in the field.
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