Activating Mn(III)F3 towards homo- and heterogeneous oxidative catalysts

The work presented in Chapter 2 shows that the complexes [Mn(III)F3(H2O)(L1-3)] (1-3) (where L1 = 1,10-phenanthroline, L2 = 2,2ʹ-bipyridine, L3 = 4,4′-dimethoxy-2,2′bipyridine), [Mn(III)F3(L4)] (4) (L4 = 2,2';6',2"-terpyridine) and[Mn(II)4(μ-F)4(L1)8](NO3)4 (5) are effective homogeneous oxidative catalysts for the epoxidation of trans-stilbene using iodosobenzene as the oxygen source. During these catalytic studies it is shown that acetonitrile is the most suitable solvent and the presence of catalysts 1-5 are vital in successful oxidations, giving extremely low yields and very long reaction times when no catalyst is employed. Attempts at using anhydrous Mn(III)F3 as a catalyst (as opposed to catalysts 1-5) were unsuccessful, giving poor yields presumably due to its poor solubility. These findings indicate that the ligated di- and triimine ligands in 1-5 are vital components within our catalysts. Moreover, the Jahn-Teller elongated Mn-OH2 bonds in complexes 1-3 may also have importance mechanistically. We have also shown that catalysts 1-5 must be added intact when carrying out trans-stilbene epoxidations (as opposed to expecting in-situ self-assembly of the catalyst). After conducting a significant number of epoxidations where various reaction conditions are optimised, we can now convert trans-stilbene quantitatively into trans-stilbene oxide (with no side products) as shown using NMR and GC-MS. Moreover, this can be achieved with reaction times of 7-9 hours. In chapter 3, we demonstrate that this same family of complexes (1-5) can also be employed as oxidative catalysts in the sulphoxidations of both 4-nitrothioanisole and nitrophenyl phenyl sulphide. Blank studies show that without catalysts 1-5, these sulphoxidations are very slow (5-7 days). When 3 equivalents of tetra-n-butylammonium peroxymonosulphate (TBAOX) (oxidant) are employed, conversions are close to 100%, however the major product (as expected) was the sulphone in both cases (giving 1-(methanesulphonyl)-4-nitrobenzene (A2) and 1-(benzenesulphonyl)-4-nitrobenzene (B2), respectively). When 1 equivalent of TBAOX is employed, a mixture of sulphoxide and sulphone products were produced in both cases. However, we were able to limit sulphone production (over oxidation) by lowering reaction temperatures (0 °C) and shortening reaction times (30 mins). The work in Chapter 4 describes our many attempts at introducing Mn(III)F3 (and Mn(II)F2) metal centres at the empty bipyridyl sites found within the Metal-Organic Framework UiO-67-bipy. Thus, introducing {Mn(III)F3(bipy)} active sites within a porous MOF to form a (potential) oxidative catalyst that would represent a heterogeneous analogue material to catalysts 1-4. The UiO-67-bipy precursor was made using two different synthetic methodologies. The first entailed solvothermal synthesis and produced highly crystalline samples, albeit in low yields (∼100 mg; ∼90%). The second method relied on sonochemical methods and provided much larger quantities of product (multi-gram). BET (Brunauer-Emmett-Teller surface area analysis) adsorption studies on the small-scale batch of UiO-67-bipy provided an average particle size of 22 nm and a surface area of 3,485 m²/g, which were significantly more porous than the latter large-scale multigram batch (100-300 nm particle size and surface area of 13.78 m2/g). Although several promising materials were produced during this work, attempts at incorporating Mn(III)F3 (and Mn(II)F2) incorporation gave mixed and inconsistent results. For example, samples produced in duplicate routinely gave rise to significantly different Mn loading levels when measured using ICP-OES (Inductively Coupled Plasma - Optical Emission Spectroscopy). Moreover, powder X-ray diffraction studies on these novel ‘UiO-67-bipy-Mn’ materials showed poor crystallinity when compared to the highly crystalline nature of the precursor UiO-67-bipy.

Citation

Ratanasakprakan, C. (2025) Activating Mn(III)F3 towards homo- and heterogeneous oxidative catalysts. University of Wolverhampton. https://wlv.openrepository.com/handle/2436/625998

Publisher

University of Wolverhampton

URI

https://wlv.openrepository.com/handle/2436/625998

Type

Thesis or dissertation

Language

en

Description

A thesis submitted in fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of Philosophy.

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