Abstract
Although tetravalent cerium compounds, notably ceric ammonium nitrate, have been widely used as oxidizing agents in organic synthesis, there are few studies on the steric and electronic factors that control the oxidation chemistry of well-defined cerium(IV) complexes. In this thesis, cerium(IV) aryloxide, carbonate and carboxylate complexes have been synthesized, and their O-H and C-H oxidations have been studied. The stereoelectronic factors that affect the Ce-O homolysis and H-atom transfer (HAT) chemistry of cerium(IV) complexes have been elucidated.A series of Ce(IV) aryloxide complexes [Ce(LOEt)2(OAr)2] (LOEt- = Kläui tripodal ligand [Co(η5-C5H5){P(O)(OEt)2}3]-) have been synthesized via protonolysis of an in-situ generated Ce(IV)-oxo complex [Ce(LOEt)2(=O)] (1) with aryl alcohols (ArOH) in hexane. The outcome of the reaction between 1 and ArOH was found to be heavily dependent on the steric effect of the aryloxide ligands. Whereas treatment of 1 with non-bulky ArOH such as phenol afforded the Ce(IV) bis(aryloxide) complexes [Ce(LOEt)2(OAr)2], the reactions of 1 with bulky 2,6-disubstituted ArOH led to formation of Ce(III) aryloxides [Ce(LOEt)2(OAr)] and quinones, presumably via Ce-O(aryl) homolysis of unstable, sterically congested Ce(IV) bis(aryloxide) intermediates. [Ce(LOEt)2(OC6H3F2-2,6)2] (2.1f) is the only Ce(IV) bis(2,6-disubstituted phenoxide) complex that has been isolated, apparently owing to the small size of the fluorine substituents. The Ce-O(aryl) homolysis of 2.1f in non-polar solvents, such as dichloromethane and acetonitrile, and hexane in the presence of a substrate has been investigated. In hexane, 2.1f is capable of oxidizing 2,4,6-tri-tert-butylphenol and 9,10-dihydroanthracene to give quinone and anthracene, respectively. Density functional theory calculations indicated that an accessible triplet excited state of 2.1f easily undergoes Ce-O(aryl) homolysis to generate an aryloxyl radical that is responsible for the H-atom abstraction.
The applications of Ce(IV) carboxylate photocatalysts have enjoyed considerable progress in recent years; however, photo-irradiation is indispensable for such Ce(IV) photocatalysis. In this work, we have explored the HAT reactivity of Ce(IV) carbonate and carboxylate complexes which feature an oxidizing Ce(IV) ion and a basic proton-accepting ligand. Under mild, non-photochemical conditions, [Ce(LOEt)2(CO3)] (3.1) is capable of oxidizing O-H bond of phenols and C-H bonds of DHA and xanthene, presumably via a proton-coupled electron transfer (PCET) pathway. Oxidation of pinacol with 3.1 afforded acetone, presumably via -scission of a transient alkoxy radical intermediate. 3.1 has been shown to be a convenient photocatalyst precursor for the decarboxylative oxygenation of arylacetic acids. For instance, irradiation of phenylacetic acid with blue LED light (λ = 450 nm) in acetonitrile in air in the presence of 5 mol% of 3.1 for 17 h afforded a 1: 9 mixture of benzyl alcohol and benzaldehyde quantitatively. Interestingly, irradiation of 3.1 in toluene in air with blue LED light led to the formation of benzaldehyde, presumably via H-atom abstraction of toluene by a transient carbonate radical.
Protonolysis of 3.1 with compounds containing acidic hydrogen atoms has been studied. Treatment of 3.1 with CF3CONH2, CF3SO2NH2 and sulfamic acid led to the isolation of the first Ce(IV) trifluoroacetamide [Ce(LOEt)2(CF3CONH)2] (4.3), trifluoromethanesulfonamide [Ce(LOEt)2(CF3SO2NH)2] (4.4) and sulfamate [Ce(LOEt)2(SO3NH2)2] (4.5) complexes, respectively. Although X-ray diffraction studies could not unambiguously assign the binding mode of the amido ligands in these complexes owing to the similar electron density of oxygen and nitrogen, it is believed that the amido ligands are O-bonded given the hard nature of Ce(IV).
Metal complexes containing hypervalent main group ligands are of interest, owing to their potential applications in selective organic oxidations. With this in mind, we sought to synthesize molecular Ce(IV)-LOEt complexes bearing hypervalent main group oxyanion ligands. Salt metathesis of [Ce(LOEt)2Cl2] with AgBrO3 afforded the bis(bromate) complex [Ce(LOEt)2(BrO3)2] (4.1) which is capable of oxo transfer to PPh3 and thioethers. Treatment of 3.1 with selenous acid led to the formation of the diselenite complex [Ce(LOEt)2(Se2O5)] (4.2). The Ce(IV) diselenite complex can also undergo oxo transfer to PPh3 but it is less active than the bromate analogue.
Finally, we have synthesized Ce(IV) complex containing a tert-butyl substituted imidodiphosphinate ligand [N(tBu2PO)2]- (tbip). Unlike the phenyl and isopropyl-substituted imidodiphosphinate analogs, reaction of K(tbip) with (Et4N)2[CeCl6] afforded a dinuclear Ce(IV) oxo complex, [Et4N]2[{Cl3Ce(μ-tbip)}2(μ-O)] (5.5), which features a linear Ce-O-Ce unit with short Ce-O distances. Also, Ce(IV) bis(aryloxide) complexes such as [Ce(tbip)2(OC6F4H)2] have been synthesized via salt metathesis of [Ce(tbip)2(NO3)2] (5.4) with sodium or silver salts of aryloxides.
| Date of Award | 2024 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Wa Hung LEUNG (Supervisor) |
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