Degree

Doctor of Philosophy (PhD)

Department

Chemistry

Document Type

Dissertation

Abstract

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.

Committee Chair

Stanley, George

Available for download on Thursday, March 05, 2020

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