Strong metal-metal Pauli repulsion leads to repulsive metallophilicity in closed-shell d8 and d10 organometallic complexes.

Metallophilicity is defined as the interaction among closed-shell metal centers, the origin of which remains controversial, particularly for the roles of spd orbital hybridization (mixing of the spd atomic orbitals of the metal atom in the molecular orbitals of metal complex) and the relativistic effect. Our studies reveal that at close M-M' distances in the X-ray crystal structures of d8 and d10 organometallic complexes, M-M' closed-shell interactions are repulsive in nature due to strong M-M' Pauli repulsion. The relativistic effect facilitates (n + 1)s-nd and (n + 1)p-nd orbital hybridization of the metal atom, where (n + 1)s-nd hybridization induces strong M-M' Pauli repulsion and repulsive M-M' orbital interaction, and (n + 1)p-nd hybridization suppresses M-M' Pauli repulsion. This model is validated by both DFT (density functional theory) and high-level coupled-cluster singles and doubles with perturbative triples computations and is used to account for the fact that the intermolecular or intramolecular Ag-Ag' distance is shorter than the Au-Au' distance, where a weaker Ag-Ag' Pauli repulsion plays an important role. The experimental studies verify the importance of ligands in intermolecular interactions. Although the M-M' interaction is repulsive in nature, the linear coordination geometry of the d10 metal complex suppresses the L-L' (ligand-ligand) Pauli repulsion while retaining the strength of the attractive L-L' dispersion, leading to a close unsupported M-M' distance that is shorter than the sum of the van der Waals radius (rvdw) of the metal atoms.

Chiral Iron Porphyrins Catalyze Enantioselective Intramolecular C(sp3 )-H Bond Amination Upon Visible-Light Irradiation.

Iron-catalyzed asymmetric amination of C(sp3 )-H bonds is appealing for synthetic applications due to the biocompatibility and high earth abundance of iron, but examples of such reactions are sparse. Herein we describe chiral iron complexes of meso- and ß-substituted-porphyrins that can catalyze asymmetric intramolecular C(sp3 )-H amination of aryl and arylsulfonyl azides to afford chiral indolines (29 examples) and benzofused cyclic sulfonamides (17 examples), respectively, with up to 93 % ee (yield: up to 99 %) using 410 nm light under mild conditions. Mechanistic studies, including DFT calculations, for the reactions of arylsulfonyl azides reveal that the Fe(NSO2 Ar) intermediate generated in situ under photochemical conditions reacts with the C(sp3 )-H bond through a stepwise hydrogen atom transfer/radical rebound mechanism, with enantioselectivity arising from cooperative noncovalent interactions between the Fe(NSO2 Ar) unit and the peripheral substituents of the chiral porphyrin scaffold.

Self-Assembly of Molecular Trefoil Knots Featuring Pentadecanuclear Homoleptic AuI -, AuI /AgI -, or AuI /CuI -Alkynyl Coordination.

Metal-free and metal-containing molecular trefoil knots are fascinating ensembles that are usually covalently assembled, the latter requiring the rational design of di- or multidentate/multipodal ligands as connectors. In this work, we describe the self-assembly of pentadecanuclear AuI trefoil knots [Au15 (C≡CR)15 ] from monoalkynes HC≡CR (R=9,9-X2 -fluorenyl with X=nBu, n-hexyl) and [AuI (THT)Cl]. Hetero-bimetallic counterparts [Au9 M6 (C≡CR)15 ] (M=Cu/Ag) were self-assembled by reactions of [Au15 (C≡CR)15 ] with [Cu(MeCN)4 ]+ /AgNO3 and HC≡CR. The type of pentadecanuclear trefoil knots described herein is characterized by X-ray crystallography, 2D NMR and HR-ESI-MS. [Au9 Cu6 (C≡CR)15 ] is relatively stable in hexane; its excited state properties were investigated. DFT calculations revealed that non-covalent metal-metal and metal-ligand interactions, together with longer alkyl chain-strengthened inter-ligand dispersion interactions, govern the stability of the trefoil knot structures.

Highly Efficient Thermally Activated Delayed Fluorescence from Pyrazine-Fused Carbene Au(I) Emitters.

Metal-based thermally activated delayed fluorescence (TADF) is conceived to inherit the advantages of both phosphorescent metal complexes and purely organic TADF compounds for high-performance electroluminescence. Herein a panel of new TADF Au(I) emitters has been designed and synthesized by using carbazole and pyrazine-fused nitrogen-heterocyclic carbene (NHC) as the donor and acceptor ligands, respectively. Single-crystal X-ray structures show linear molecular shape and coplanar arrangement of the donor and acceptor with small dihedral angles of <6.5°. The coplanar orientation and appropriate separation of the HOMO and LUMO in this type of molecules favour the formation of charge-transfer excited state with appreciable oscillator strength. Together with a minor but essential heavy atom effect of Au ion, the complexes in doped films exhibit highly efficient (Φ∼0.9) and short-lived (<1 µs) green emissions via TADF. Computational studies on this class of emitters have been performed to decipher the key reverse intersystem crossing (RISC) pathway. In addition to a small energy splitting between the lowest singlet and triplet excited states (ΔEST ), the spin-orbit coupling (SOC) effect is found to be larger at a specific torsion angle between the donor and acceptor planes which favours the RISC process the most. This work provides an alternative molecular design to TADF Au(I) carbene emitters for OLED application.

C-H Activation by an Iron-Nitrido Bis-Pocket Porphyrin Species.

High-valent iron-nitrido species are nitrogen analogues of iron-oxo species which are versatile reagents for C-H oxidation. Nonetheless, C-H activation by iron-nitrido species has been scarcely explored, as this is often hampered by their instability and short lifetime in solutions. Herein, the hydrogen atom transfer (HAT) reactivity of an Fe porphyrin nitrido species (2 c) toward C-H substrates was studied in solutions at room temperature, which was achieved by nanosecond laser flash photolysis (LFP) of its FeIII -azido precursor (1 c) supported by a bulky bis-pocket porphyrin ligand. C-H bonds with bond dissociation enthalpies (BDEs) of up to ≈84 kcal mol-1 could be activated, and the second-order rate constants (k2 ) are on the order of 102 -104  s-1 m-1 . The Fe-amido product formed after HAT could further release ammonia upon protonation.

Tetradentate Gold(III) Complexes as Thermally Activated Delayed Fluorescence (TADF) Emitters: Microwave-Assisted Synthesis and High-Performance OLEDs with Long Operational Lifetime.

Structurally robust tetradentate gold(III)-emitters have potent material applications but are rare and unprecedented for those displaying thermally activated delayed fluorescence (TADF). Herein, a novel synthetic route leading to the preparation of highly emissive, charge-neutral tetradentate [C^C^N^C] gold(III) complexes with 5-5-6-membered chelate rings has been developed through microwave-assisted C-H bond activation. These complexes show high thermal stability and with emission origin (3 IL, 3 ILCT, and TADF) tuned by varying the substituents of the C^C^N^C ligand. With phenoxazine/diphenylamine substituent, we prepared the first tetradentate gold(III) complexes that are TADF emitters with emission quantum yields of up to 94 % and emission lifetimes of down to 0.62 µs in deoxygenated toluene. These tetradentate AuIII TADF emitters showed good performance in vacuum-deposited OLEDs with maximum EQEs of up to 25 % and LT95 of up to 5280 h at 100 cd m-2 .

Controlling Metallophilic Interactions in Chiral Gold(I) Double Salts towards Excitation Wavelength-Tunable Circularly Polarized Luminescence.

Materials exhibiting excitation wavelength-dependent photoluminescence (Ex-De PL) in the visible region have potential applications in bioimaging, optoelectronics and anti-counterfeiting. Two multifunctional, chiral [Au(NHC)2 ][Au(CN)2 ] (NHC=(4R,5R)/(4S,5S)-1,3-dimethyl-4,5-diphenyl-4,5-dihydro-imidazolin-2-ylidene) complex double salts display Ex-De circularly polarized luminescence (CPL) in doped polymer films and in ground powder. Emission maxima can be dynamically tuned from 440 to 530 nm by changing the excitation wavelength. The continuously tunable photoluminescence is proposed to originate from multiple emissive excited states as a result of the existence of varied AuI ⋅⋅⋅AuI distances in ground state. The steric properties of the NHC ligand are crucial to the tuning of AuI ⋅⋅⋅AuI distances. An anti-counterfeiting application using these two salts is demonstrated.

The Metal-Metal-to-Ligand Charge Transfer Excited State and Supramolecular Polymerization of Luminescent Pincer PdII -Isocyanide Complexes.

Pincer PdII -isocyanide complexes are described that display intermolecular interactions and emissive 3 MMLCT excited states in aggregation state(s) at room temperature. The intermolecular PdII -PdII and ligand-ligand interactions drive these complexes to undergo supramolecular polymerization in a living manner. Comprehensive spectroscopic studies reveal a pathway with a kinetic trap that can be modulated by changing the counteranion and metal atom. The PdII supramolecular assemblies comprise two different aggregation forms with only one to be emissive. DFT/TDDFT calculations lend support to the MMLCT absorption and emission of these pincer PdII -isocyanide aggregates.

Counteranion- and Solvent-Mediated Chirality Transfer in the Supramolecular Polymerization of Luminescent Platinum(II) Complexes.

The chirality of supramolecular polymeric materials is subtly affected by a delicate balance among various non-covalent interactions, the details of which are inadequately understood. Now, a fine balance of intermolecular interactions, including closed-shell metal-metal, dispersive and electrostatic interactions, is used to direct the delicate orientation of monomeric building blocks, thereby achieving the successful preparation of different nanostructures with distinct supramolecular chirality. Moreover, kinetically trapped and thermodynamically stable aggregates were monitored over time in the supramolecular polymerization of cationic PtII complexes. The dynamic self-assembly process can be successfully controlled by modifying the counteranion and solvent composition. A chiral doping approach can also be used to induce the chirality of an achiral PtII complex at the supramolecular level.

Luminescent Cyclometalated Gold(III) Alkyl Complexes: Photophysical and Photochemical Properties.

A series of luminescent cyclometalated gold(III) complexes having alkyls as auxiliary ligands has been prepared. The alkyl ligand was found to effectively increase the emission quantum yields and lifetimes of luminescent cyclometalated gold(III) complexes by circumventing the population of LLCT excited states that are found in complexes supported by arylacetylide ligands. These gold(III) alkyl complexes exhibit emission quantum yields and lifetimes of up to 0.40 and 180 µs, respectively, in solution at room temperature. The triplet emission color of these complexes is tunable from yellow to sky blue by modifying the cyclometalating ligand.

Highly Luminescent Pincer Gold(III) Aryl Emitters: Thermally Activated Delayed Fluorescence and Solution-Processed OLEDs.

Herein are described the synthesis, photophysical properties and applications of a series of luminescent cyclometalated AuIII complexes having an auxiliary aryl ligand. These complexes show photoluminescence with emission quantum yields of up to 0.79 in solution and 0.84 in thin films (4 wt % in PMMA) at room temperature, both of which are the highest reported values among AuIII complexes. Thermally activated delayed fluorescence (TADF) is the emission origin for some of these complexes. Solution-processed OLEDs made with these complexes showed sky-blue to green electroluminescence with external quantum efficiencies (EQEs) of up to 23.8 %, current efficiencies of up to 70.4 cd A-1 , and roll-off of down to 1 %, highlighting the bright prospect of AuIII -TADF emitters in OLEDs.

Luminescent Tungsten(VI) Complexes: Photophysics and Applicability to Organic Light-Emitting Diodes and Photocatalysis.

The synthesis, excited-state dynamics, and applications of two series of air-stable luminescent tungsten(VI) complexes are described. These tungsten(VI) complexes show phosphorescence in the solid state and in solutions with emission quantum yields up to 22 % in thin film (5 % in mCP) at room temperature. Complex 2 c, containing a 5,7-diphenyl-8-hydroxyquinolinate ligand, displays prompt fluorescence (blue-green) and phosphorescence (red) of comparable intensity, which could be used for ratiometric luminescent sensing. Solution-processed organic light-emitting diodes (OLEDs) based on 1 d showed a stable yellow emission with an external quantum efficiency (EQE) and luminance up to 4.79 % and 1400 cd m-2 respectively. These tungsten(VI) complexes were also applied in light-induced aerobic oxidation reactions.

Deciphering Photoluminescence Dynamics and Reactivity of the Luminescent Metal-Metal-Bonded Excited State of a Binuclear Gold(I) Phosphine Complex Containing Open Coordination Sites.

Luminescent metal complexes having open coordination sites hold promise in the design of sensory materials and photocatalysts. As a prototype example, [Au2 (dcpm)2)](2+) (dcpm = bis(dicyclohexylphosphanyl) is known for its intriguing environmental sensitive photoluminescence. By integrating a range of complementary ultrafast time-resolved spectroscopy to interrogate the excited state dynamics, this study uncovers that the events occurring in extremely rapid timescales and which are modulated strongly by environmental conditions play a pivotal role in the luminescence behavior and photochemical outcomes. Formed independent of the phase and solvent property within ∼0.15 ps, the metal-metal bonded (3)5dσ*6pσ state is highly reactive possessing strong propensity toward increasing coordination number at Au(I) center, and with ∼510 ps lifetime in dichloromethane is able to mediate light induced C-X bond cleavage.

Luminescent pincer platinum(II) complexes with emission quantum yields up to almost unity: photophysics, photoreductive C-C bond formation, and materials applications.

Luminescent pincer-type Pt(II)  complexes supported by C-deprotonated π-extended tridentate RC^N^NR' ligands and pentafluorophenylacetylide ligands show emission quantum yields up to almost unity. Femtosecond time-resolved fluorescence measurements and time-dependent DFT calculations together reveal the dependence of excited-state structural distortions of [Pt(RC^N^NR')(CC-C6 F5 )] on the positional isomers of the tridentate ligand. Pt complexes [Pt(R-C^N^NR')(CC-Ar)] are efficient photocatalysts for visible-light-induced reductive CC bond formation. The [Pt(R-C^N^NR')(CC-C6 F5 )] complexes perform strongly as phosphorescent dopants for green- and red-emitting organic light-emitting diodes (OLEDs) with external quantum efficiency values over 22.1 %. These complexes are also applied in two-photon cellular imaging when incorporated into mesoporous silica nanoparticles (MSNs).

Selective Ag(I) binding, H2S sensing, and white-light emission from an easy-to-make porous conjugated polymer.

Separating silver (Ag(+)) from lead (Pb(2+)) is one of the many merits of the porous polymer framework reported here. The selective metal binding stems from the well-defined chelating unit of N-heterocycles, which consists of a triazine (C3N3) ring bonded to three 3,5-dimethylpyrazole moieties. Such a rigid and open triad also serves as the distinct building unit in the fully conjugated 3D polymer scaffold. Because of its strong fluorescence and porosity (e.g., BET surface area: 355 m(2)/g), and because of the various types of metal species that can be readily taken up, this versatile framework is especially fit for functionalization. For example, with AgNO3 loaded, the framework solid exhibits a brown color in response to water solutions of H2S, even at the dilution of 5.0 µM (0.17 ppm); whereas cysteine and other biologically relevant thiols do not cause notable change in color. In another example, tunable white-light emission was produced when an Ir(III) complex was doped (e.g., about 0.02% of the polymer weight) onto the framework. Mechanistically, the bound Ir(III) centers become highly emissive in the orange-red region, complementing the broad, bluish emission from the polymer host to result in the overall white-light quality: the color attributes of the emission are therefore easily tunable by the Ir(III) dopant concentration. With this exemplary study, we intend to highlight metal uptake as an effective approach to modify and enrich the properties of porous polymer frameworks and to stimulate interest in further examining metal-polymer interactions in the context of sensing, separation, catalyzes, and other applications.

Water-soluble luminescent cyclometalated gold(III) complexes with cis-chelating bis(N-heterocyclic carbene) ligands: synthesis and photophysical properties.

A new class of cyclometalated Au(III) complexes containing various bidentate C-deprotonated C^N and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized and characterized. These are the first examples of Au(III) complexes supported by cis-chelating bis-NHC ligands. [Au(C^N)(bis-NHC)] complexes display emission in solutions under degassed condition at room temperature with emission maxima (λmax ) at 498-633 nm and emission quantum yields of up to 10.1 %. The emissions are assigned to triplet intraligand (IL) π→π* transitions of C^N ligands. The Au(III) complex containing a C^N (C-deprotonated naphthalene-substituted quinoline) ligand with extended π-conjugation exhibits prompt fluorescence and phosphorescence of comparable intensity with λmax at 454 and 611 nm respectively. With sulfonate-functionalized bis-NHC ligand, four water-soluble luminescent Au(III) complexes, including those displaying both fluorescence and phosphorescence, were prepared. They show similar photophysical properties in water when compared with their counterparts in acetonitrile. The long phosphorescence lifetime of the water-soluble AuIII complex with C-deprotonated naphthalene-substituted quinoline ligand renders it to function as ratiometric sensor for oxygen. Inhibitory activity of one of these water-soluble Au(III) complexes towards deubiquitinase (DUB) UCHL3 has been investigated; this complex also displayed a significant inhibitory activity with IC50 value of 0.15 µM.

A theoretical investigation into the luminescent properties of d8-transition-metal complexes with tetradentate Schiff base ligands.

A theoretical investigation on the luminescence efficiency of a series of d(8) transition-metal Schiff base complexes was undertaken. The aim was to understand the different photophysics of [M-salen](n) complexes (salen = N,N'-bis(salicylidene)ethylenediamine; M = Pt, Pd (n = 0); Au (n = +1)) in acetonitrile solutions at room temperature: [Pt-salen] is phosphorescent and [Au-salen](+) is fluorescent, but [Pd-salen] is nonemissive. Based on the calculation results, it was proposed that incorporation of electron-withdrawing groups at the 4-position of the Schiff base ligand should widen the (3)MLCT-(3)MC gap (MLCT = metal-to-ligand charge transfer and MC = metal centered, that is, the dd excited state); thus permitting phosphorescence of the corresponding Pd(II) Schiff base complex. Although it is experimentally proven that [Pd-salph-4E] (salph = N,N'-bis(salicylidene)-1,2-phenylenediamine; 4E means an electron-withdrawing substituent at the 4-position of the salicylidene) displays triplet emission, its quantum yield is low at room temperature. The corresponding Pt(II) Schiff base complex, [Pt-salph-4E], is also much less emissive than the unsubstituted analogue, [Pt-salph]. Thus, a detailed theoretical analysis of how the substituent and central metal affected the photophysics of [M-salph-X] (X is a substituent on the salph ligand, M = Pt or Pd) was performed. Temperature effects were also investigated. The simple energy gap law underestimated the nonradiative decay rates and was insufficient to account for the temperature dependence of the nonradiative decay rates of the complexes studied herein. On the other hand, the present analysis demonstrates that inclusions of low-frequency modes and the associated frequency shifts are decisive in providing better quantitative estimates of the nonradiative decay rates and the experimentally observed temperature effects. Moreover, spin-orbit coupling, which is often considered only in the context of radiative decay rate, has a significant role in determining the nonradiative rate as well.

A binuclear gold(I) complex with mixed bridging diphosphine and bis(N-heterocyclic carbene) ligands shows favorable thiol reactivity and inhibits tumor growth and angiogenesis in vivo.

In the design of anticancer gold(I) complexes with high in vivo efficacy, tuning the thiol reactivity to achieve stability towards blood thiols yet maintaining the thiol reactivity to target cellular thioredoxin reductase (TrxR) is of pivotal importance. Herein we describe a dinuclear gold(I) complex (1-PF6) utilizing a bridging bis(N-heterocyclic carbene) ligand to attain thiol stability and a diphosphine ligand to keep appropriate thiol reactivity. Complex 1-PF6 displays a favorable stability that allows it to inhibit TrxR activity without being attacked by blood thiols. In vivo studies reveal that 1-PF6 significantly inhibits tumor growth in mice bearing HeLa xenograft and mice bearing highly aggressive mouse B16-F10 melanoma. It inhibits angiogenesis in tumor models and inhibits sphere formation of cancer stem cells in vitro. Toxicology studies indicate that 1-PF6 does not show systemic anaphylaxis on guinea pigs and localized irritation on rabbits.

A robust palladium(II)-porphyrin complex as catalyst for visible light induced oxidative C-H functionalization.

A series of palladium(II)-porphyrin complexes that display dual emissions with lifetimes up to 437 µs have been synthesized. Among the four complexes, PdF20TPP is an efficient and robust catalyst for photoinduced oxidative C-H functionalization by using oxygen as terminal oxidant. α-Functionalized tertiary amines were obtained in good to excellent yields by light irradiation (λ>400 nm) of a mixture of PdF20TPP, tertiary amine, and nucleophile (cyanide, nitromethane, dimethyl malonate, diethyl phosphite, and acetone) under aerobic conditions. Four examples of intramolecular cyclized amine compounds could be similarly prepared. Comparison of the UV-visible absorption spectra before and after the photochemical reaction revealed that PdF20TPP was highly robust (>95 % recovery). The practical application of PdF20TPP has been revealed by the photochemical reactions performed by using a low catalyst loading (0.01 mol %) and on a 10 mmol scale. The PdF20TPP catalyst could sensitize photoinduced oxidation of sulfides to sulfoxides in excellent yields. Mechanistic studies revealed that the photocatalysis proceeded by singlet-oxygen oxidation.

Luminescent organogold(III) complexes with long-lived triplet excited states for light-induced oxidative C-H bond functionalization and hydrogen production.

All that glitters is gold: highly phosphorescent gold(III) complexes with extended π-conjugated cyclometalating ligands exhibit rich photophysical and photochemical properties. They act as efficient photocatalysts/photosensitizers for oxidative functionalizations of secondary and tertiary benzylic amines and homogeneous hydrogen production from a water/acetonitrile mixture.