Robert Thomas Hembre

A modular family of phosphine-phosphoramidite ligands and their hydroformylation catalysts: steric tuning impacts upon the coordination geometry of trigonal bipyramidal complexes of type [Rh(H)(CO)2(P^P*)]

Four new phosphine-phosphoramidite bidentate ligands have been synthesised and studied in rhodium-catalysed hydroformylation. Variable temperature NMR studies have been used along with HPIR to investigate the coordination mode of the trigonal bipyramidal complexes formed from [Rh(acac)(CO)2], ligand and syngas. It was found that small changes to the ligand structure have a large effect on the geometry of the active catalytic species. The rhodium catalysts of these new ligands were found to give unusually high iso-selectivity in the hydroformylation of propene and 1-octene.

Alternative pathways in the ruthenium catalysed hydrogenation of CO to alcohols

CO hydrogenation in [PBu4]Br in the presence of [Ru3(CO)12] gives predominantly methanol, ethanol and propanol with small amounts of 1,2-ethanediol. Using RuO2 as the catalyst precursor, the same products are formed along with higher alcohols (1-butanol –1-heptanol). Reactions carried out using added 13CH3OH or 13CO show that ethanol and propanol come from homologation reactions of methanol and ethanol respectively, but that the higher alcohols are not formed through the lower alcohols as intermediates.

A Tertiary Phosphonium Salt as a Promoter for the Hydrogenation of CO

Small amounts of [HPBu3]Br that are either present as an impurity in commercial [Bu4P]Br or are added to it promote the hydrogenation of CO catalysed by [Ru3(CO)12] . [HPBu3]Br may be responsible for the irreproducibility that is sometimes observed in similar CO-hydrogenation reactions. The homogeneous conversion of synthesis gas into functional chemicals was first reported by Gresham in the 1950s. In these reactions, metal sources were subjected to high temperatures and very high pressures of syngas to form alcohols and polyols. One goal in the research of syngas has been to form C C bonds from individual molecules of CO whilst retaining some or all of the oxygen functionality. The heterogeneous hydrogenation of CO tends to promote cleavage of the CO bond with the formation of alkanes and alkenes (Fischer–Tropsch chemistry) but the early examples from Gresham showed that homogeneous ruthenium catalysts are able to provide oxygenates. Subsequent research by Dombek (Union Carbide) and Bradley (Exxon) found that the activity could be greatly enhanced by the addition of halide promoters, preferably iodide. Knifton and co-workers independently found good yields when using molten tetraalkylphosphonium halide salts as solvents instead of the usual organic media. Whereas Dombek’s system with N-methylpyrrolidone and iodide salts was particularly useful for spectroscopicand mechanistic analysis, 16] Knifton focused on tuning the selectivity towards a wide scope of products, such as MeOH, 17] EtOH, 1,2ethanediol, and acetic acid 22] and their derivatives. Following Dombek’s work, Ono et al. found that a remarkable increase in selectivity towards EtOH could be achieved by using phosphoric acid or trimethylphosphate as a promoter. 25] Although these systems work well, the problem remains that very high pressures and temperatures are required to achieve reasonable conversions and, therefore, considerable emphasis must continue to be directed towards elucidating the mechanisms of all of these processes and towards finding factors that increase the reaction rates. Herein, we report an interesting promoter that increases the rate of MeOH production. By using the melt chemistry reported by Knifton et al. with tetrabutylphosphonium bromide as the solvent and [Ru3(CO)12] as a catalyst precursor at 200 8C, but under milder pressures (CO/H2, 1:1, 250 bar), MeOH and EtOH are observed as the main products, together with smaller amounts of propanol and 1,2-ethanediol. Qualitative analysis of the gas phase before or after condensing any condensable compounds shows significant amounts of dimethyl-, methylethyl-, and diethyethers, which we have shown elsewhere are formed from the acid-catalysed dehydration of the alcohol products. However, the reproducibility of this system was poor. Marked changes in activity were observed whenever different batches (lot number) of tetrabutylphosphonium bromide were employed. Some batches showed good activity whereas others gave much-poorer activity. Scrutiny of concentrated samples of these different batches of [PBu4]Br by P NMR spectroscopy revealed three peaks (Figure 1). The strongest signal (d= 33.56 ppm) was from tetrabutylphosphonium bromide. In most batches, another peak was present at d= 37.49 ppm, owing to tri-n-butyl(sec-butyl)phosphonium bromide, which was formed by Markovnikoff addition of the P H bond across 1-butene during the synthesis of PBu3 from PH3 and 1-butene. This peak was more intense in the less-active batch of [PBu4]Br. The P{H} NMR spectrum of the active batch revealed an additional peak at d= 11.55 ppm, which split into a doublet (J(H,P) = 487 Hz) in the proton-coupled P NMR spectrum (Figure 1). Likewise, careful scrutiny of the H NMR spectrum (see the Supporting Information, Figure S2) of the same sample revealed a low-intensity doublet (d= 6.83 ppm, J(H,P) = 487 Hz) in addition to the signals from the butyl groups. These signals were assigned to tributylphosphonium bromide, [HPBu3]Br, which is a protonated form of tributyl phosphine.