Doctor of Philosophy (PhD)
Bimetallic cooperativity can potentially increase activity of reactions. This concept is another way to increase reactivity besides simply focusing on the steric and electronic effects of a ligand. A binucleating tetrasphosphine ligand has been developed to showcase bimetallic cooperativity between two rhodium metal centers. Hydroformylation is a widely used industrial process to produce aldehydes from alkenes, H2, and CO. The dirhodium catalyst, [Rh2(μ-CO)(CO)3(rac-et,ph-P4-Ph)](BF4)2,is highly active leading to favorable results when using a DMF/water solvent system, 1-hexene, 90 psi 1:1 H2/CO, and 90° C: initial turnover frequency of 35.4 min-1, linear to branch ratio of 17.6:1, isomerization of 1.9% alkene isomerization, and hydrogenation of < 1%. Unfortunately, this complex was very difficult to make from our usual catalyst starting material, [Rh2(nbd)2(rac-et,ph-P4-P4)](BF4)2 (nbd = norbornadiene).
My research has focused on the synthesis and optimization of the new dirhodium-tetraphosphine catalyst precursors. New catalyst precursors with acetonitrile, pyridine, and cyclooctadiene ligands demonstrate high activity for hydroformylation in water/acetone solvent with results displaying high turnover numbers ranging from 700-800 aldehyde turnovers and low side reactions (alkene isomerization = 5% -7%, hydrogenation = >1%) point to an effective catalyst. The chelator effect of the phenyl linkage ultimately became an issue for stability of the catalyst. The new P4-Ph tetraphosphine ligand, however, has internal phosphines with two P-aryl bonds and only one alkyl group (the central methylene bridge). These are considerably more reactive towards P-aryl group cleavage reactions that leading to rhodium-induced P4-Ph fragmentation. In-situ FT-IR and NMR experiments were performed on the bimetallic catalyst to understand the active catalyst and mechanism for the catalytic cycle. In-situ FT-IR ran in water/acetone illustrates terminal carbonyls at 2120, 2055, 2026, and 2030 cm–1 indicating the presence of the open-mode pentacarbonyl complex at lower temperatures and mostly a monocationic monohydride complex, [Rh2(-H)(CO)x(mixed-P4-Ph)]+, x = 2-4, formed via proton dissociation from the dicationic dihydride complex. The proposed mechanism for the dirhodium-P4-Ph catalyst in water/acetone system is a monocationic monohydride system similar to the previous old catalyst.
Work with the bimetallic cobalt system led to a very active cationic cobalt(II) bisphosphine hydrido-carbonyl catalyst. The cobalt(II) catalyst has a very high alkene isomerization rate that causes a low L:B selectivity for simple alkenes, however, due to the high isomerization rate it shows an exceptional high L:B selectivity to hydroformylate difficult internal branch alkenes. The cobalt(II) catalyst approaches rhodium activity and has a remarkably long lifetime with no signs showing cobalt-induced phosphine ligand degradation.
Johnson, Ryan Alexander, "Investigating Cationic Metal Centers for Hydroformylation" (2019). LSU Doctoral Dissertations. 4820.