O,N,N-Pincer ligand effects on oxidatively induced carbon–chlorine coupling reactions at palladium

The syntheses of two families of sterically tuneable O,N,N pro-ligands are reported, namely the 2-(phenyl-2′-ol)-6-imine-pyridines, 2-(C6H4-2′-OH),6-(CMe[double bond, length as m-dash]NAr)C5H3N [Ar = 4-i-PrC6H4 (HL1a), 2,6-i-Pr2C6H3 (HL1b)] and the 2-(phenyl-2′-ol)-6-(amino-prop-2-yl)pyridines, 2-(C6H4-2′-OH),6-(CMe2NHAr)C5H3N [Ar = 4-i-PrC6H4 (HL2a), 2,6-i-Pr2C6H3 (HL2b)], using straightforward synthetic approaches and in reasonable overall yields. Interaction of HL1a/c and HL2a/b with palladium(II) acetate affords the O,N,N-pincer complexes, [{2-(C6H4-2′-O)-6-(CMe[double bond, length as m-dash]NAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (1a), 2,6-i-Pr2C6H3 (1b)) and [{2-(C6H4-2′-O)-6-(CMe2NHAr)C5H3N}Pd(OAc)] (Ar = 4-i-PrC6H4 (2a), 2,6-i-Pr2C6H3 (2b)), which can be readily converted to their chloride derivatives, [{2-(C6H4-2′-O)-6-(CMe[double bond, length as m-dash]NAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (3a), 2,6-i-Pr2C6H3 (3b)) and [{2-(C6H4-2′-O)-6-(CMe2NHAr)C5H3N}PdCl] (Ar = 4-i-PrC6H4 (4a), 2,6-i-Pr2C6H3 (4b)), respectively, on reaction with an aqueous sodium chloride solution. Treating each of 3a, 3b, 4a and 4b with two equivalents of di-p-tolyliodonium triflate at 100 °C in a toluene/acetonitrile mixture affords varying amounts of 4-chlorotoluene along with the 4-iodotoluene by-product with the conversions highly dependent on the steric and backbone properties of the pincer complex employed (viz.4a > 3a > 4b > 3b); notably, the least sterically bulky and most flexible amine-containing 4a reaches 90% conversion to 4-chlorotoluene in 15 h as opposed to 17% for imine-containing 3b. In the case of 3a, the inorganic palladium species recovered from the reaction has been identified as the Pd(II) salt [{2-(C6H4-2′-O)-6-(CMe[double bond, length as m-dash]N(4-i-PrC6H4)C5H3N}Pd(NCMe)][O3SCF3] (5a), which was independently prepared by the reaction of 3a with silver triflate in acetonitrile. Single crystal X-ray structures are reported for HL1a, HL2a, 1a, 1b, 2a, 2b, 3a and 5a

An approach for the identification of exemplar sites for scaling up targeted field observations of benthic biogeochemistry in heterogeneous environments

Continental shelf sediments are globally important for biogeochemical activity. Quantification of shelf-scale stocks and fluxes of carbon and nutrients requires the extrapolation of observations made at limited points in space and time. The procedure for selecting exemplar sites to form the basis of this up-scaling is discussed in relation to a UK-funded research programme investigating biogeochemistry in shelf seas. A three-step selection process is proposed in which (1) a target area representative of UK shelf sediment heterogeneity is selected, (2) the target area is assessed for spatial heterogeneity in sediment and habitat type, bed and water column structure and hydrodynamic forcing, and (3) study sites are selected within this target area encompassing the range of spatial heterogeneity required to address key scientific questions regarding shelf scale biogeochemistry, and minimise confounding variables. This led to the selection of four sites within the Celtic Sea that are significantly different in terms of their sediment, bed structure, and macrofaunal, meiofaunal and microbial community structures and diversity, but have minimal variations in water depth, tidal and wave magnitudes and directions, temperature and salinity. They form the basis of a research cruise programme of observation, sampling and experimentation encompassing the spring bloom cycle. Typical variation in key biogeochemical, sediment, biological and hydrodynamic parameters over a pre to post bloom period are presented, with a discussion of anthropogenic influences in the region. This methodology ensures the best likelihood of site-specific work being useful for up-scaling activities, increasing our understanding of benthic biogeochemistry at the UK-shelf scale

Recent advances in homogeneous chromium catalyst design for ethylene tri-, tetra-, oligo- and polymerization

This review focuses on recent progress made using well-defined molecular chromium complexes that, upon suitable activation, can catalyze the tri-, tetra, oligo- and/or polymerization of ethylene. In particular, emphasis will be placed on the tuning of the performance characteristics of these homogeneous catalysts through structural modifications made to the multidentate ligand manifold (e.g., donor atoms, charge, backbone and strain) and the effects these changes have on the resulting ethylene derivatives. While the ability of these catalysts to mediate the formation of high molecular weight linear polyethylene continues to see many developments, their capacity to form polyethylene waxes and oligomers has witnessed some major advances. Moreover, the impressive selectivity of some chromium systems to generate commercially important 1-hexene and more recently 1-octene has seen the implementation of this technology at the industrial level. The types of precatalysts to be discussed will be divided broadly on the basis of their ability to generate either polymers/oligomers or short chain α-olefins; the effects of co-catalyst and reaction conditions (e.g., temperature, pressure, solvent) on catalytic activity and selectivity, will be also developed. In addition, current proposals as to the mechanistic details displayed by these versatile chromium catalysts will be highlighted

Recent advances in homogeneous chromium catalyst design for ethylene tri-, tetra-, oligo- and polymerization

This review focuses on recent progress made using well-defined molecular chromium complexes that, upon suitable activation, can catalyze the tri-, tetra, oligo- and/or polymerization of ethylene. In particular, emphasis will be placed on the tuning of the performance characteristics of these homogeneous catalysts through structural modifications made to the multidentate ligand manifold (e.g., donor atoms, charge, backbone and strain) and the effects these changes have on the resulting ethylene derivatives. While the ability of these catalysts to mediate the formation of high molecular weight linear polyethylene continues to see many developments, their capacity to form polyethylene waxes and oligomers has witnessed some major advances. Moreover, the impressive selectivity of some chromium systems to generate commercially important 1-hexene and more recently 1-octene has seen the implementation of this technology at the industrial level. The types of precatalysts to be discussed will be divided broadly on the basis of their ability to generate either polymers/oligomers or short chain α-olefins; the effects of co-catalyst and reaction conditions (e.g., temperature, pressure, solvent) on catalytic activity and selectivity, will be also developed. In addition, current proposals as to the mechanistic details displayed by these versatile chromium catalysts will be highlighted