Abstracts Earth and Environmental Sciences

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

by Uddhav Ajay https://orcid.org/0000-0003-1345-6085 Kanbur

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,…