There is an increasing interest in lanthanide compounds due to their diverse applications, ranging from smartphones, fluorescent probes for bioimaging. The past few years have witnessed considerable progress in the synthesis and characterization of lanthanide complexes containing metal-ligand multiple bonds, especially in regards to the isolation of terminal oxo, imido and carbine complexes. Such compounds are expected to be highly reactive due to the high polarity of lanthanide-ligand multiple bonds resulting from the poor orbital energy mismatching between the metal and ligand valence orbitals. Recently, the first lanthanide terminal oxo complex, [Ce^IV(L_OEt)₂(=O)(H₂O)].MeC(O)NH₂ (2.2), has been synthesized by metathesis of [Ce^IV(L_OEt)₂Cl₂] (L_OEt⁻ = [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]⁻)...[ Read more ]

There is an increasing interest in lanthanide compounds due to their diverse applications, ranging from smartphones, fluorescent probes for bioimaging. The past few years have witnessed considerable progress in the synthesis and characterization of lanthanide complexes containing metal-ligand multiple bonds, especially in regards to the isolation of terminal oxo, imido and carbine complexes. Such compounds are expected to be highly reactive due to the high polarity of lanthanide-ligand multiple bonds resulting from the poor orbital energy mismatching between the metal and ligand valence orbitals. Recently, the first lanthanide terminal oxo complex, [Ce^IV(L_OEt)₂(=O)(H₂O)].MeC(O)NH₂ (2.2), has been synthesized by metathesis of [Ce^IV(L_OEt)₂Cl₂] (L_OEt⁻ = [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]⁻) (2.1) with Ag₂O. The isolation of this Ce^IV oxo complex offers us an opportunity to investigate the nucleophilic and redox reactivity of the lanthanide-ligand multiple bond systematically.

In Chapter 2, the reactivity of the Ce^IV oxo complex 2.2 with electrophiles such as acetic anhydride, methyl triflate and tris(pentafluorophenyl)boron has been investigated. The protonation of 2.2 with Brønsted acids, including HCl and triflic acid (HOTf), has been studied. The redox reactions of 2.2 with substituted ferrocenes have been investigated and the redox potential of 2.2 in MeCN has been estimated to lie between -0.11 and -0.48 V vs. Fc^+/0 (Fc = ferrocene). The reaction of 2.2 with electrophilic [Ru^IVL_OEt(N)Cl₂] leading to the formation of a Ce^III/Ru^III complex, [Ce^III(L_OEt)₂(H₂O)(μ−MeC(O)NH)Ru^III(L_OEt)Cl₂] (2.9), presumably via an N−O coupling pathway, has been studied. The oxo-bridged bimetallic complex, [Ce^IV(L_OEt)₂(MeC(O)NH₂)(=O)NaL_OEt] (2.8) with the Ce^IV=O_oxo(Na) distance of 1.953(4) Å, has been obtained from the reaction of 2.2 with NaL_OEt. It was found that the {NaL_OEt} moiety has very little influence on the nucleophilic reactivity of the Ce^IV=O group, but can enhance the stability of the Ce^IV oxo complex in hexane solution with respect to cluster formation.

Chapter 3 reports the synthesis of the first Ce^IV iodosylbenzene and iodylbenzene complexes by reaction of 2.1 with PhIO and PhIO₂, respectively. The crystal structures of [Ce^IV(L_OEt)₂{OI(Cl)Ph}₂] (3.1) and [Ce^IV(L_OEt)₂{OI(O)(Cl)Ph}₂] (3.6) have been determined. Complex 2.1 can catalyse the oxidation of thioanisoles with PhIO. NMR studies indicated that both 3.1 and 3.6 are involved as intermediates in the Ce-catalyzed sulfoxidation.

Chapter 4 reports the synthesis of Ce^IV pseudohalide complexes [Ce^IV(L_OEt)₂(X)₂] by salt metathesis of 2.1 with AgX (X = NCS, N₃, 1/2CO₃). The reaction of 2.1 with [AgCN] afforded an unstable Ce^IV cyanide species that could be trapped with BPh₃ to yield a cyano-borate adduct, [Ce^IV(L_OEt)₂(NCBPh₃)₂] (4.8). The reaction of [Ce^IV(L_OEt)₂(NO₃)Cl] with AgCN led to formation of a bimetallic Ce^IV cyanide complex, [{Ce^IV(L_OEt)₂(NO₃)}₂{u-Ag(CN)₂}] [AgCl₂] (4.13). The redox properties of the Ce^IV pseudohalide complexes have been investigated by cyclic voltammetry.

In Chapter 5, the reduction of the Ce^IV complexes [Ce^IV(L_OEt)₂X₂] (X = 1/2CO₃, OAc, Cl, NO₃, etc.) in tetrahydrofuran (THF) has been investigated. It was found that the Ce^IV carbonate complex [(L_OEt)₂Ce(CO₃)] (5.1) is reduced to a Ce^III complex rapidly in THF containing a trace amount of water. Mechanistic studies suggested that in wet THF [Ce^IV(L_OEt)₂X₂] is readily converted to a Ce^IV aqua complex, which is deprotonated by a base, e.g. CO₃²⁻ in the case of the carbonate complex, to a reactive Ce^IV hydroxide species. Homolytic cleavage of the Ce^IV−OH bond affords H₂O₂ and Ce^III. The H₂O₂ formed can oxidize PPh₃ to O=PPh₃ and has been detected spectrophotometrically.

Chapter 6 describes the synthesis and structures of Rh^III, Ru^III, Cu^II and V^IV complexes supported by both L_OEt⁻ and a bidentate pyridine-alkoxide ligand, 2-pyridyl-2-isopropanoate (pyalk⁻). Cyclic voltammetry indicated that the M^IV state in [M^III(L_OEt)(pyalk)Cl] (M = Rh (6.1), Ru (6.2)) are stabilized by the electron-donating L_OEt⁻ and pyalk⁻ ligands. Oxidation of [M^III(L_OEt)(pyalk)(H₂O)](OTs) with NaIO₄ in water afforded reactive blue and brown species, respectively, which exhibited signals assignable to [{Rh(L_OEt)(pyalk)O}₂]⁺ and [{RuL_OEt(pyalk)O}₂(O)]⁺, respectively, in the mass spectra.