Rare-earth and Zirconium Catalyzed C–H Activation with Applications to Polymer Upcycling

dc.contributor.advisor Sadow, Aaron D
dc.contributor.advisor Huang, Wenyu
dc.contributor.advisor Rossini, Aaron J
dc.contributor.advisor Slowing, Igor I
dc.contributor.advisor Windus, Theresa L
dc.contributor.author Kanbur, Uddhav Ajay
dc.contributor.department Department of Chemistry
dc.date.accessioned 2022-11-09T05:30:36Z
dc.date.available 2022-11-09T05:30:36Z
dc.date.issued 2022-05
dc.date.updated 2022-11-09T05:30:36Z
dc.description.abstract Early transition metal and rare-earth alkyl complexes have a high propensity for activating relatively inert C–H bonds. This could be achieved catalytically or stoichiometrically, using homoleptic or heteroleptic alkyl complexes, as well as by grafting these complexes onto oxide support materials, i.e., the Surface Organometallic Chemistry (SOMC) approach. This thesis describes our efforts to catalyze C–H activation reactions using relatively cheap early transition metal catalysts using earth abundant and inexpensive aluminum reagents. Towards this goal, a set of rare-earth tetramethylaluminate complexes were synthesized, supported by a zwitterionic bisoxazoline cyclopentadienyl borate ligand previously developed by our group. These complexes, {R'2Al(OxMe2)2PhBCp}Ln(AlMe4)2 (Ln = Y, La, Nd; R' = Me, Et, iBu), catalytically mediate the selective and efficient activation and alumination of C–H bonds in terminal alkynes with trialkylaluminums, resulting in the formation of alkynylaluminum species in high yields. Detailed kinetic studies point toward the reversible formation of a unique adduct [Ln](AlR4)2⋅AlR3, followed by the rate limiting metalation of the alkyne. Surprisingly, the addition of AlR3 species across the triple bond of the alkyne, i.e., hydroalumination or carboalumination is strongly disfavored by this process, resulting in a selective metalation step. As a result, this is a unique, general strategy for C–H bond alumination using highly electrophilic, coordinatively saturated metal complexes. C–H activation can also be achieved using heterogeneous catalysts, especially organometallic species immobilized on oxide supports. An example of this technique has been demonstrated with the synthesis of a surface grafted zirconium neopentyl complex on silica-alumina, Zr(CH2CMe3)2@SiAlOx. The combination of this catalyst, polyethylene, and triisobutylaluminum results in the C–C bond cleavage and C-Al bond formation reactions, leading to a mixture of short chain alkanes and functionalized fatty aluminum species. Quenching the reaction mixture with an appropriate electrophile such as O2 or CO2 leads to a biodegradable product distribution of fatty alcohols or acids respectively, which have significant industrial applications such as detergents and surfactants, and offer an end of life alternative to single use plastics. The proposed mechanism involves a C–H activation of the polymer backbone, β-alkyl elimination to give a shorter surface Zr-alkyl species and an olefin, and a chain transfer of the Zr-alkyl group to the AlR3 species, leading to the formation of alkyl-AlR2 species. An extension of this chemistry has also been developed, involving the replacement of the neopentyl ligands on zirconium with bulky tert-butoxide ligands. Surface supported zirconium alkoxides of this type, Zr(OR)3@SiAlOx, in combination with triethylaluminum (AlEt3) mediate the direct C–H alumination of polyolefins without shortening the polymer chains, effectively bypassing the β-alkyl elimination step responsible for C–C cleavage. Thus, this reaction is complementary to the aforementioned C–C alumination chemistry. The catalytic reaction can be extended to shorter alkanes and even methane, effectively transforming methane to methylaluminum species in a single step. Mechanistic studies have shown that the reaction is highly selective towards the activation of methyl groups (i.e., chain ends), and a large kinetic isotope effect is observed, with per-deuterated substrates having little to no reactivity. Aluminated chain ends can be converted to alcohols, acids or halides by quenching with appropriate electrophiles, thereby introducing functional groups on alkane chains directly.
dc.format.mimetype PDF
dc.identifier.doi https://doi.org/10.31274/td-20240329-621
dc.identifier.orcid 0000-0003-1345-6085
dc.identifier.uri https://dr.lib.iastate.edu/handle/20.500.12876/azJ4y9Wv
dc.language.iso en
dc.language.rfc3066 en
dc.subject.disciplines Inorganic chemistry en_US
dc.subject.disciplines Polymer chemistry en_US
dc.subject.disciplines Molecular chemistry en_US
dc.subject.keywords Catalysis en_US
dc.subject.keywords CH Activation en_US
dc.subject.keywords Lanthanide en_US
dc.subject.keywords Organometallic en_US
dc.subject.keywords Polymer Upcycling en_US
dc.subject.keywords Sustainable en_US
dc.title Rare-earth and Zirconium Catalyzed C–H Activation with Applications to Polymer Upcycling
dc.type dissertation en_US
dc.type.genre dissertation en_US
dspace.entity.type Publication
relation.isOrgUnitOfPublication 42864f6e-7a3d-4be3-8b5a-0ae3c3830a11
thesis.degree.discipline Inorganic chemistry en_US
thesis.degree.discipline Polymer chemistry en_US
thesis.degree.discipline Molecular chemistry en_US
thesis.degree.grantor Iowa State University en_US
thesis.degree.level dissertation $
thesis.degree.name Doctor of Philosophy en_US
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