Heterometallic iron and iridium nitrido complexes

Abstract

The nitrido group (N^3-) is a strong π donor ligand that can stabilize metal ions in high oxidation states. Recent studies have shown that diiron μ-nitrido phthalocyanine complexes can catalyze the oxidation of hydrocarbons, including methane, with hydrogen peroxide, presumably via high-valent diiron-oxo intermediates. This prompted us to synthesize heterometallic Ru-N-Fe complexes and explore their oxidation chemistry. We are particularly interested in water-soluble Ru-N-Fe aqua complexes that can be potentially oxidized to hydroxo/oxo species in aqueous media. In this thesis, heterometallic Ru-N-Fe complexes supported by various substituted porphyrins and Schiff base ligands have been synthesized and their redox chemistry and catalytic activity have been studied. In addition, high-valent Ir complexes containing a tripodal bis-cyclometalated C^N^C ligand have been synthesized and their oxidation chemistry has been studied. Chapter 2 describes the synthesis, crystal structure and electrochemistry of the first heterometallic Fe(IV) μ-nitrido complex bearing a water soluble porphyrin ligand, Na₄[(H₂O)(TPPS)Fe(μ-N)RuCl₂(L_OEt)] (TPPS^2-: 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin dianion; L_OEt^- = [(η⁵-C₅H₅)Co{P(O)(OEt)₂}₃]⁻). This water-soluble Ru(IV)-N-Fe(IV) complex can catalyze the oxidation of thioethers by H₂O₂ in aqueous media with over 90% yield, presumably via the intermediacy of dimetallic peroxo [Ru=N=FeO₂] and oxo [Ru=N=Fe=O] species, which have been characterized by mass spectrometry. Chapter 3 describes the synthesis and structures of heterometallic μ-nitrido complexes containing 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (H₂TCPP), [(CH₃CN)(TCPP)Ru(μ-N)RuCl₂(L_OEt)], and protoporphyrin IX dimethyl ester [(PPIXDME)Fe(μ-N)RuCl₂(L_OEt)]. Also, the synthesis of a Fe(IV) μ-nitrido complex containing a polyvalent porphyrin, [(TBHPP)Fe(μ-N)RuCl₂(L_OEt)], where H₂TBHPP = 5,10,15,20-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin, and its oxidation to yield a porphodimethene diquinomethide compound will be presented. Chapter 4 describes the chloride substitution of a heterometallic Fe(IV) μ-nitrido chloride complex bearing a Schiff base ligand. Salt metathesis of [(L_OEt)Cl(py)Ru(μ-N)Fe(Br₄salphen)Cl] (Br₄salphen^2- = N,N’-bis(3,5-dibromosalicylidene)-1,2-phenylenediamine dianion; TPP^2- = tetraphenylporphyrin dianion) with AgX led to formation of Ru-N-FeX complexes (for X^- = CF₃CO₂^- and ReO₄^-) or a tetranuclear mixed-valence [Ru=N=Fe-O-Fe=N=Ru] complex featuring μ-oxo and μ-nitrido bridges (for X^- = BF₄^-). Chapter 5 describes the synthesis of an Ir(III)=N=Ru(IV) carbonyl μ-nitrido complex, [PPh₄][(CO)Cl₂Ir(μ-N)RuCl₂(L_OEt)], and its chloride abstraction with Tl(I) thiolates. Heterometallic Ru-N-M(acac) (M = Rh(III), Ir(III) or Ru(IV); acac^- = acetylacetonate) complexes have been synthesized and structurally characterized. Reaction between [Ru(acac)₂(MeCN)₂] and 1-adamantyl azide (AdN₃) afforded a Ru(II) acetylacetonate complex with a neutral tetraazabutadiene ligand, [Ru(acac)₂(N₄Ad₂)]. Chapter 6 describes the synthesis and characterization of Ir complexes bearing a tripodal bis-cyclometalated C^N^C ligand, 2-(bis(4-(tert-butyl)phenyl)methyl)pyridine (H₂dtbnpy). The Ir(dtbnpy) starting material (6-1) obtained from the reaction of IrCl₃ with H₂dtbnpy has been tentatively formulated as [H₃(dtbnpy)][Ir₂(dtbnpy)₂(μ- Cl)₂Cl(H₂O)]. Ir(dtbnpy) isocyanide, carbonyl and phosphine complexes, as well as a bis(tripod) complex, [Ir(dtbnpy)(L_OEt)], have been synthesized from 6-1. [Ir(dtbnpy)(L_OEt)], which exhibited a low Ir(IV/III) potential (~0 V vs. ferrocene-ferrocenium), can be oxidized to a stable Ir(IV) complex, [Ir(dtbnpy)(L_OEt)]⁺ that has been characterized by electron paramagnetic resonance spectroscopy. Reaction of 6-1 with [Ru(N)Cl₂(L_OEt)] afforded the heterometallic μ-nitrido complex [(H₂O)Cl(dtbnpy)Ir(μ-N)RuCl₂(L_OEt)] that was oxidized by PhICl₂ to form [Cl₂(Cl₂dtbnpy)Ir(μ-N)RuCl₂(L_OEt)], which can be described by the resonance forms Ir(IV)-N-Ru(VI) and Ir(VI)-N-Ru(IV). Ir(dtbnpy) complexes can catalyze the oxidation of cyclooctene with PhIO, presumably via a high-valent Ir=O or Ir-PhIO intermediate.

Date of Award	2022
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Wa Hung LEUNG (Supervisor)