Studies on the Lewis Acidity of Redox-Active Telluranes and the Catalytic Applications of a Low-Valent Iron Complex

Abstract

Recent advances in catalysis have expanded the landscape of both main group and transition metal catalysts, moving beyond traditional boron- and aluminum-based main group systems and 2nd -3rd row transition metal systems to include other p-block and first-row transition metals. Within this context, chalcogen-based Lewis acids, particularly tellurium-centered systems, have recently attracted attention in the realm of chalcogen bonding chemistry. However, this field has been dominated by cationic Te chalcogen bond donors, and neutral Te(IV) systems remain largely underexplored in the literature.1 This thesis presents a comprehensive study of neutral bis(catecholato)telluranes (BCTs), revealing that perchloro- and perbromo-substituted derivatives exhibit pronounced Lewis acidity as determined by Gutmann-Beckett analysis and fluoride ion affinity (FIA) calculations.2 Hard Lewis superacids are compounds whose FIA is greater than that of SbF5 in the gas phase, and experimentally these can defluorinate SbF6–. Notably, despite having FIA lower than that of SbF5, the perchlorinated BCT demonstrates the rare ability to defluorinate SbF6–. So rather than its Lewis acidic potential, there are other mechanistic pathways that enable the defluorination. Based, on cyclic voltammetry and high-resolution mass spectrometry evidence, we hypothesize a fluoride-coupled electron transfer (FCET) mechanism involving redox-active catecholato ligands. Further studies are ongoing in this direction. The BCTs were also found to show covalent binding to hard anions such as fluoride and chloride, along with robust air and moisture stability. Catalytically, the BCTs enable a diverse array of transformations, including aromatic bromination, transfer hydrogenation, hydrodefluorination, Friedel–Crafts alkylation, and Ritter-type solvolysis, achieving high yields under mild conditions and outperforming previously reported chalcogen-based systems in several benchmark reactions.3

Parallel to these developments, the field of sustainable transition metal catalysis has seen significant interest in iron-based systems due to iron’s abundance, low cost, and low toxicity compared to precious metals.4 The second part of my thesis investigates an easily prepared iron(II) amide complex as a catalyst for the hydroboration of aldehydes and ketones, a transformation of growing importance in organic synthesis.5 The iron system operates efficiently under mild, additive-free conditions with low catalyst loadings, displaying broad substrate scope and exceptional chemoselectivity, preferentially reducing aldehydes even in the presence of other reducible groups such as ketones, alkenes, nitriles, esters, amides, acids, and halides. Mechanistic studies suggest the in-situ generation of iron hydride intermediates. The operational simplicity, scalability, and functional group tolerance of this iron(II) amide system position it as a promising sustainable alternative to precious metal catalysts for the synthesis of alcohols from carbonyl compounds.

References

1. P. Pale, V. Mamane, Chalcogen Bonding Catalysis: Tellurium, the Last Frontier? Chem. Eur. J. 2023, 29, e202302755

2. S. Baruah, A. K. Pal, K. Geetharani*, Bis(catecholato)telluranes: probing into the catecholato-effect on “Lewis superacidity” (Manuscript under review).

3. S. Baruah, A. K. Pal, K. Geetharani*, Exploring the catalytic frontiers of bis(catecholato)telluranes. (Manuscript submitted).

4. M. L. Shegavi, S. K. Bose, Recent advances in the catalytic hydroboration of carbonyl compounds. Catal. Sci. Technol., 2019, 9, 3307–3336.

5. A. Baishya, S. Baruah, K. Geetharani*, Efficient hydroboration of carbonyls by an iron(II) amide catalyst. Dalton Trans., 2018, 47, 9231–9237