Unexpectedly high barriers to M–P rotation in tertiary phobane complexes : PhobPR behavior that is commensurate with tBu2PR

The four isomers of 9-butylphosphabicyclo[3.3.1]nonane, s-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl, have been prepared. Seven isomers of 9-butylphosphabicyclo[4.2.1]nonane (a5-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl; a7-PhobPBu, where Bu = n-butyl, isobutyl, tert-butyl) have been identified in solution; isomerically pure a5-PhobPBu and a7-PhobPBu, where Bu = n-butyl, isobutyl, have been isolated. The σ-donor properties of the PhobPBu ligands have been compared using the JPSe values for the PhobP(═Se)Bu derivatives. The following complexes have been prepared: trans-[PtCl2(s-PhobPR)2] (R = nBu (1a), iBu (1b), sBu (1c), tBu (1d)); trans-[PtCl2(a5-PhobPR)2] (R = nBu (2a), iBu (2b)); trans-[PtCl2(a7-PhobPR)2] (R = nBu (3a), iBu (3b)); trans-[PdCl2(s-PhobPR)2] (R = nBu (4a), iBu (4b)); trans-[PdCl2(a5-PhobPR)2] (R = nBu (5a), iBu (5b)); trans-[PdCl2(a7-PhobPR)2] (R = nBu (6a), iBu (6b)). The crystal structures of 1a–4a and 1b–6b have been determined, and of the ten structures, eight show an anti conformation with respect to the position of the ligand R groups and two show a syn conformation. Solution variable-temperature 31P NMR studies reveal that all of the Pt and Pd complexes are fluxional on the NMR time scale. In each case, two species are present (assigned to be the syn and anti conformers) which interconvert with kinetic barriers in the range 9 to >19 kcal mol–1. The observed trend is that, the greater the bulk, the higher the barrier. The magnitudes of the barriers to M–P bond rotation for the PhobPR complexes are of the same order as those previously reported for tBu2PR complexes. Rotational profiles have been calculated for the model anionic complexes [PhobPR-PdCl3]− using DFT, and these faithfully reproduce the trends seen in the NMR studies of trans-[MCl2(PhobPR)2]. Rotational profiles have also been calculated for [tBu2PR-PdCl3]−, and these show that the greater the bulk of the R group, the lower the rotational barrier: i.e., the opposite of the trend for [PhobPR-PdCl3]−. Calculated structures for the species at the maxima and minima in the M–P rotation energy curves indicate the origin of the restricted rotation. In the case of the PhobPR complexes, it is the rigidity of the bicycle that enforces unfavorable H···Cl clashes involving the Pd–Cl groups with H atoms on the α- or β-carbon in the R substituent and H atoms in 1,3-axial sites within the phosphabicycle

Levallois and non-Levallois blade production at Shuidonggou in Ningxia, North China

The Shuidonggou site is one of the most important for prehistoric research in China, yielding evidence of ancient human colonisations in North China during the Late Upper Pleistocene. Situated in the Ordos desert, it was first discovered and excavated by Teilhard de Chardin during the first half of the 20th century. He noted the originality of the lithic assemblages produced on blades and flakes, comparable to the Upper Palaeolithic in Western Europe. This open-air site in loess context dates from ca. 17,000 to 25,000 BP and may be earlier than 30,000 BP. The principal objective of this paper is to present the technical aspects of the Shuidonggou lithic assemblages using a technological approach based on the dynamic analysis of blanks and cores in a global overview of the chaine operatoire concept. The goal is to understand the knapping strategies used: were both Levallois and non Levallois methods used? The importance of Shuidonggou is thus linked with the question of the use of the Levallois method

Synergic effects between N-heterocyclic carbene and chelating benzylidene-ether ligands toward the initiation step of Hoveyda-Grubbs type Ru complexes

Synergic effects between ancillary N-heterocyclic carbenes [(1,3-bis(2,4,6-trimethylphenyl)-1,3-imidazoline-2-ylidene or 1,3-bis(2,6-diisopropylphenyl)-1,3-imidazoline-2-ylidene] and chelating benzylidene ether ligands were investigated by studying initiation rates and kinetic profiles of Hoveyda-Grubbs (HG) type Ru complexes. A newly designed Ru-benzylidene-oxazinone precatalyst 4 was compared with Grela and Blechert complexes bearing modified isopropyloxy chelating leaving groups and with the standard HG complex to understand how the ancillary and the leaving ligands interact and influence the catalytic activity