Unique Class of Enantiopure N-Heterocyclic Carbene Half-Sandwich Iridium(III) Complexes with Stable Configurations: Probing Five-Membered versus Six-Membered Iridacycles

Luminescent Complexes of Platinum, Iridium, and Coinage Metals Containing N-Heterocyclic Carbene Ligands: Design, Structural Diversity, and Photophysical Properties

The employment of N-heterocyclic carbenes (NHCs) to design luminescent metal compounds has been the focus of recent intense investigations because of the strong σ-donor properties, which bring stability to the whole system and tend to push the d-d dark states so high in energy that they are rendered thermally inaccessible, thereby generating highly emissive complexes for useful applications such as organic light-emitting diodes (OLEDs), or featuring chiroptical properties, a field that is still in its infancy. Among the NHC complexes, those containing organic chromophores such as naphthalimide, pyrene, and carbazole exhibit rich emission behavior and thus have attracted extensive interest in the past five years, especially carbene coinage metal complexes with carbazolate ligands. In this review, the design strategies of NHC-based luminescent platinum and iridium complexes with large spin-orbit-coupling (SOC) are described first. Subsequent paragraphs illustrate the recent advances of luminescent coinage metal complexes with nucleophilic- and electrophilic-based carbenes based on silver, gold, and copper metal complexes that have the ability to display rich excited state emissions in particular via thermally activated delayed fluorescence (TADF). The luminescence mechanism and excited state dynamics are also described. We then summarize the advance of NHC-metal complexes in the aforementioned fields in recent years. Finally, we propose the development trend of this fast-growing field of luminescent NHC-metal complexes.

Enantiopure Cyclometalated Rh(III) and Ir(III) Complexes Displaying Rigid Configuration at Metal Center: Design, Structures, Chiroptical Properties and Role of the Iodide Ligand

Enantiopure N-heterocyclic carbene half-sandwich metal complexes of the general formula [Cp*M(C^C:)I] (M = Rh, Ir; C^C: = NI-NHC; NI-H = Naphthalimide; NHC = N-heterocyclic carbene) are reported. The rhodium compound was obtained as a single isomer displaying six membered metallacycle and was resolved on chiral column chromatography to the corresponding enantiomers (S)-[Cp*Rh(C^C:)I] (S)-2 and (R)-[Cp*Rh(C^C:)I] (R)-2. The iridium congener, however, furnishes a pair of regioisomers, which were resolved into (S)-[Cp*Ir(C^C:)I] (S)-3 and (R)-[Cp*Ir(C^C:)I] (R)-3 and (S)-[Cp*Ir(C^C:)I] (S)-4 and (R)-[Cp*Ir(C^C:)I] (R)-4. These regioisomers differ from each other, only by the size of the metallacycle; five-membered for 3 and six-membered for 4. The molecular structures of (S)-2 and (S)-4 are reported. Moreover, the chiroptical properties of these compounds are presented and discussed. These compounds display exceptional stable configurations at the metal center in solution with enantiomerization barrier ΔG≠ up to 124 kJ/mol. This is because the nature of the naphthalimide-NHC clamp ligand and the iodide ligand contribute to their configuration’s robustness. In contrast to related complexes reported in the literature, which are often labile in solution.

Visible-Light-Induced Amination of Quinoline at the C8 Position via a Postcoordinated Interligand-Coupling Strategy under Mild Conditions

The postcoordinated interligand-coupling strategy provides a useful and complementary protocol for synthesizing polydentate ligands. Herein, diastereoselective photoreactions of Λ-[Ir(pq)₂(d-AA)] (Λ-d) and Λ-[Ir(pq)₂(l-AA)] (Λ-l, where pq is 2-phenylquinoline and AA is an amino acid) are reported in the presence of O₂ under mild conditions. Diastereomer Λ-d is dehydrogenatively oxidized into an imino acid complex, while diastereomer Λ-l mainly occurs via interligand C-N cross-dehydrogenative coupling between quinoline at the C8 position and AA ligands at room temperature, affording Λ-[Ir(pq)(l-pq-AA)]. Furthermore, the photoreaction of diastereomer Λ-l is temperature-dependent. Mechanistic experiments reveal the ligand-radical intermediates may be involved in the reaction. Density functional theory calculations were used to eluciate the origin of diastereoselectivity and temperature dependence. This will provide a new protocol for the amination of quinoline at the C8 position via the postcoordinated interligand C-N cross-coupling strategy under mild conditions.

Ligand-centred redox activation of inert organoiridium anticancer catalysts

Organometallic complexes with novel activation mechanisms are attractive anticancer drug candidates. Here, we show that half-sandwich iodido cyclopentadienyl iridium(iii) azopyridine complexes exhibit potent antiproliferative activity towards cancer cells, in most cases more potent than cisplatin. Despite their inertness towards aquation, these iodido complexes can undergo redox activation by attack of the abundant intracellular tripeptide glutathione (GSH) on the chelated azopyridine ligand to generate paramagnetic intermediates, and hydroxyl radicals, together with thiolate-bridged dinuclear iridium complexes, and liberate reduced hydrazopyridine ligand. DFT calculations provided insight into the mechanism of this activation. GS- attack on the azo bond facilitates the substitution of iodide by GS-, and leads to formation of GSSG and superoxide if O2 is present as an electron-acceptor, in a largely exergonic pathway. Reactions of these iodido complexes with GSH generate Ir-SG complexes, which are catalysts for GSH oxidation. The complexes promoted elevated levels of reactive oxygen species (ROS) in human lung cancer cells. This remarkable ligand-centred activation mechanism coupled to redox reactions adds a new dimension to the design of organoiridium anticancer prodrugs.

Design and synthesis of chiral spirobifluorenes

Chiroptical spectroscopic methods serve as a practical tool for the structural characterization of chiral systems based on the interaction with polarized light. The higher sensitivity of these methods, compared with their achiral counterparts, not only enables the determination of absolute configuration and conformational preferences, but also supramolecular interactions may be monitored. In order to expand the applicability of chiroptical systems, the development of functional materials exhibiting intense chiroptical responses is essential. As a proof of principle, we previously constructed chiroptical interfaces via thioacetate-derivatized allenes. Because of the photoisomerization issues associated with allenes, we have recently proposed their replacement by spirobifluorenes to achieve robust chiroptical systems. Thus, we hereby present the design and synthesis of chiral spirobifluorenes bearing thioacetates suitable for suface functionalization.

Thermodynamic Resolution and Enantioselective Synthesis of C2-Symmetric Bis-sulfoxides Based on Chiral Iridium(III) Complexes

Enantiopure Λ-[Ir(dfppy)₂(MeCN)₂](PF₆) and Δ-[Ir(dfppy)₂(MeCN)₂](PF₆) (where dfppy is (4,6-difluoropheny)pyridine) were demonstrated to preferentially react with (S,S)-1,2-bis(arylsulfinyl)ethane and (R,R)-1,2-bis(arylsulfinyl)ethane, respectively, under thermodynamic equilibrium. Sequential treatment of Λ-[Ir(dfppy)₂(MeCN)₂](PF₆) and Δ-[Ir(dfppy)₂(MeCN)₂](PF₆) with C₂-symmetric bis-sulfoxides led to diastereoselective formation of the corresponding diastereomers Λ-[Ir(dfppy)₂(R,R)-bis-sulfoxide)](PF₆) in 90-92% and Δ-[Ir(dfppy)₂(S,S)-bis-sulfoxide)](PF₆) in 88-90%, respectively. The uncoordinated (R,S)-bis-sulfoxides were afforded in 45% with >97% de values. Enantiopure (S,S)-bis-sulfoxides and (R,R)-bis-sulfoxides were respectively obtained by the release of sulfoxide ligands from the corresponding complexes in the presence of glycine in yields of 20-21% with 97-99% ee values. The enantioreceptors Λ-[Ir(dfppy)₂(MeCN)₂](PF₆) and Δ-[Ir(dfppy)₂(MeCN)₂](PF₆) can be recycled and reused in the next reaction cycle. Moreover, a protocol for asymmetric oxidation of prochiral bis-sulfide into enantiopure C₂ symmetric bis-sulfoxide was also developed in a high enantioselectivity. The absolute configurations at the metal centers and sulfur atoms were determined by X-ray crystallography.