Strongly Luminescent Cyclometalated Gold(III) Complexes Supported by Bidentate Ligands Displaying Intermolecular Interactions and Tunable Emission Energy

Post-metallation functionalization of the [(C^C)Au(P^P)]+ scaffold through a hydrothiolation reaction

Complex 1 [(C^C)Au(dppv)]PF6 (dppv = cis-1,2-diphenylphosphinoethylene) is reported to react efficiently and selectively with aliphatic and aromatic thiols in the presence of a base. This methodology enables the smooth introduction of various functional groups on the [(C^C)Au(P^P)]+ scaffold.

Bond-Dissociation Energies to Probe Pyridine Electronic Effects on Organogold(III) Complexes: From Methodological Developments to Application in π-Backdonation Investigation and Catalysis

In this work, we report on the synthesis of several organogold(III) complexes based on 4,4'-diterbutylbiphenyl (C^C) and 2,6-bis(4-terbutylphenyl)pyridine (C^N^C) ligands and bond with variously substituted pyridine ligands (pyrR). Altogether, 33 complexes have been prepared and studied with mass spectrometry using higher-energy collision dissociation (HCD) in an Orbitrap mass spectrometer. A complete methodology including the kinetic modeling of the dissociation process based on the Rice-Ramsperger-Kassel-Marcus (RRKM) statistical method is proposed to obtain critical energies E0 of the pyrR loss for all complexes. The capacity of these E0 values to describe the pyridine ligand effect is further explored, at the same time as more classical descriptors such as 1H pyridinic NMR shift variation upon coordination and Au-NpyrR bond length measured by X-ray diffraction. An extensive theoretical work, including density functional theory (DFT) and domain-based local pair natural orbital coupled-cluster theory (DLPNO-CCSD(T)) methods, is also carried out to provide bond-dissociation energies, which are compared to experimental results. Results show that dissociation energy outperforms other descriptors, in particular to describe ligand effects over a large electronic effect range as seen by confronting the results to the pyrR pKa values. Further insights into the Au-NpyrR bond are obtained through an energy decomposition analysis (EDA) study, which confirms the isolobal character of Au+ with H+. Finally, the correlation between the lability of the pyridine ligands toward the catalytic efficiency of the complexes could be demonstrated in an intramolecular hydroarylation reaction of alkyne. The results were rationalized considering both pre-catalyst activation and catalyst reactivity. This study establishes the possibility of correlating dissociation energy, which is a gas-phase descriptor, with condensed-phase parameters such as catalysis efficiency. It therefore holds great potential for inorganic and organometallic chemistry by opening a convenient and easy way to evaluate the electronic influence of a ligand toward a metallic center.

Luminescent Au(III)-M(I) (M = Cu, Ag) Aggregates Based on Dicyclometalated Bis(alkynyl) Gold Anions†

The syntheses and structures of a series of complexes based on the C∧C-chelated Au(III) unit (C∧C = 4,4'-bis(t-butyl) 2,2'-biphenyl-1,1'-diyl) are reported, namely, [{(C∧C)Au(C≡CtBu)2}2M2], (C∧C)Au(C≡CR)(C≡NXyl), and [{(C∧C)Au(C≡CR)2}{M(C≡NXyl)}] (M = Ag, Cu; R = tBu, C6H4tBu-4, C6H4OMe-4; Xyl = 3,5-Me2C6H3). The X-ray structures reveal a broad range of dispositions determined by the different coordination modes of Ag(I) or Cu(I). The complexes are bright photoemitters in the solid state and in poly(methyl methacrylate) (PMMA) films. The photoluminescence is dominated by 3IL(C∧C) transitions, with indirect effects from the rest of the molecules, as supported by theoretical calculations. This work opens up the possibility of accessing Au(III) carbon-rich anions to construct photoluminescent aggregates.

Serum-Stable Gold(III) Bisphosphine Complex Induces Mild Mitochondrial Uncoupling and In Vivo Antitumor Potency in Triple Negative Breast Cancer

The preparation of cyclometalated complexes offers a path to stable materials, catalysts, and therapeutic agents. Here, we explore the anticancer potential of novel biphenyl organogold(III) cationic complexes supported by diverse bisphosphine ligands, Au-1-Au-5, toward aggressive glioblastoma and triple negative breast cancer cells (TNBCs). The [C^C] gold(III) complex, Au-3, exhibits significant tumor growth inhibition in a metastatic TNBC mouse model. Remarkably, Au-3 displays promising blood serum stability over a relevant therapeutic window of 24 h and alteration in the presence of excess L-GSH. The mechanism-of-action studies show that Au-3 induces mitochondrial uncoupling, membrane depolarization, and G1 cell cycle arrest and prompts apoptosis. To the best of our knowledge, Au-3 is the first biphenyl gold-phosphine complex to uncouple mitochondria and inhibit TNBC growth in vivo.

Ligand-enabled oxidation of gold(i) complexes with o-quinones

Chelating P^P and hemilabile P^N ligands were found to trigger the oxidation of Au(i) complexes by o-benzoquinones. The ensuing Au(iii) catecholate complexes have been characterized by NMR spectroscopy, single crystal X-ray diffraction and X-ray absorption spectroscopy. They adopt tetracoordinate square-planar structures. Reactivity studies substantiate the reversibility of the transformation. In particular, the addition of competing ligands such as chloride and alkenes gives back Au(i) complexes with concomitant release of the o-quinone. DFT calculations provide insight about the structure and relative stability of the Au(i) o-quinone and Au(iii) catecholate forms, and shed light on the 2-electron transfer from gold to the o-quinone.

Phosphorescent biaryl platinum(IV) complexes obtained through double metalation of dibenzoiodolium ions

The first series of neutral, tris-chelate, phosphorecent Pt(IV) complexes is reported, which combine two cyclometalated 2-arylpyridine ligands and a dimetalated biaryl. The introduction of biaryl ligands is achieved under mild conditions through the oxidative addition of dibenzoiodolium ions to Pt(II) precursors to give Pt(IV) intermediates with a singly metalated 2-(2-iodoaryl)aryl ligand, followed by the reductive metalation of the C-I bond. The modulation of emission characteristics via derivatization of both types of ligands is demonstrated.

[(C C)Au(N N)]+ Complexes as a New Family of Anticancer Candidates: Synthesis, Characterization and Exploration of the Antiproliferative Properties

A library of eleven cationic gold(III) complexes of the general formula [(C C)Au(N N)]⁺ when C C is either biphenyl or 4,4'-ditertbutyldiphenyl and N N is a bipyridine, phenanthroline or dipyridylamine derivative have been synthesized and characterized. Contrasting effects on the viability of the triple negative breast cancer cells MDA-MB-231 was observed from a preliminary screening. The antiproliferative activity of the seven most active complexes were further assayed on a larger panel of human cancer cells as well as on non-cancerous cells for comparison. Two complexes stood out for being either highly active or highly selective. Eventually, reactivity studies with biologically meaningful amino acids, glutathione, higher order DNA structures and thioredoxin reductase (TrxR) revealed a markedly different behavior from that of the well-known coordinatively isomeric [(C N C)Au(NHC)]⁺ structure. This makes the [(C C)Au(N N)]⁺ complexes a new class of organogold compounds with an original mode of action.

Non-palindromic (C^C^D) gold(III) pincer complexes are not accessible by intramolecular oxidative addition of biphenylenes - an experimental and quantum chemical study

We herein report on the synthesis of biphenylenes substituted with a pyridine (N), a phosphine (P) and a carbene (C') donor as well as a carbene donor with additional pyridine in the lateral position. We describe the synthesis and structures of derived gold(i) complexes, which we tried to use for the synthesis of non-palindromic [(C^C^D)AuIII] pincer complexes by means of an intramolecular oxidative addition of the strained biphenylene ring. However, the anticipated formation of gold(iii) complexes failed due to kinetic and thermodynamic reasons, which we extensively investigated by quantum chemical calculations. Furthermore, we shed light on the oxidative addition of biphenylene to two different gold(i) systems reported in the literature. Our comprehensive quantum-chemical analysis is complemented by NMR experiments.

Chiral Gold(III) Complexes: Synthesis, Structure, and Potential Applications

Since the beginning of the 2000's, homogeneous gold catalysis has emerged as a powerful tool to promote the cyclization of unsaturated substrates with excellent regioselectivity allowing the synthesis of elaborated organic scaffolds. An important goal to achieve in gold catalysis is the possibility to induce enantioselective transformations by the assistance of chiral complexes. Unfortunately, the linear geometry of coordination for gold usually encountered in complexes at the +1 oxidation states renders this goal very challenging. In consequence, the interest toward the synthesis of chiral gold(III) complexes is steadily growing. Indeed, the square planar geometry of the gold(III) cation appears more suitable to promote chiral induction. Beside catalysis, gold(III) complexes have also shown promising potential in the field of pharmacology. Herein, syntheses and applications of well-defined gold(III) complexes reported over the last fifteen years are summarized.

Synthesis of New Donor-Substituted Biphenyls: Pre-ligands for Highly Luminescent (C^C^D) Gold(III) Pincer Complexes

We herein report on new synthetic strategies for the preparation of pyridine and imidazole substituted 2,2'-dihalo biphenyls. These structures are pre-ligands suitable for the preparation of respective stannoles. The latter can successfully be transmetalated to K[AuCl4 ] forming non-palindromic [(C^C^D)AuIII ] pincer complexes featuring a lateral pyridine (D=N) or N-heterocyclic carbene (NHC, D=C') donor. The latter is the first report on a pincer complex with two formally anionic sp2 and one carbenic carbon donor. The [(C^C^D)AuIII ] complexes show intense phosphorescence in solution at room temperature. We discuss the developed multistep strategy and touch upon synthetic challenges. The prepared complexes have been fully characterized including X-ray diffraction analysis. The gold(III) complexes' photophysical properties have been investigated by absorption and emission spectroscopy as well as quantum chemical calculations on the quasi-relativistic two-component TD-DFT and GW/Bethe-Salpeter level including spin-orbit coupling. Thus, we shed light on the electronic influence of the non-palindromic pincer ligand and reveal non-radiative relaxation pathways of the different ligands employed.

High Efficiency Sky-Blue Gold(III)-TADF Emitters*

Highly efficient sky-blue luminescent gold(III) complexes with emission quantum yields up to 82 %, lifetimes down to 0.67 μs and emission peak maxima at 470-484 nm were prepared through a consideration of pincer gold(III) donor-acceptor complexes. Photophysical studies and time-dependent density functional theory (TDDFT) calculations revealed that the emission nature of these gold(III) complexes is most consistent with TADF. Solution-processed OLEDs with these gold(III) complexes as dopants afforded electroluminescence maxima at 465-473 nm with FWHM of 64-67 nm and maximum external quantum efficiencies (EQEs) of up to 15.25 %. This research demonstrates the first example of gold(III)-OLEDs showing electroluminescence maxima at smaller than 470 nm, and highlights the potential of using gold(III)-TADF emitters in the development of high efficiency blue OLEDs and blue emissive dopant in WOLEDs.

The Self-Assembly of a Cyclometalated Palladium Photosensitizer into Protein-Stabilized Nanorods Triggers Drug Uptake In Vitro and In Vivo

Enhanced passive diffusion is usually considered to be the primary cause of the enhanced cellular uptake of cyclometalated drugs because cyclometalation lowers the charge of a metal complex and increases its lipophilicity. However, in this work, monocationic cyclometalated palladium complexes [1]OAc (N^N^C^N) and [2]OAc (N^N^N^C) were found to self-assemble, in aqueous solutions, into soluble supramolecular nanorods, while their tetrapyridyl bicationic analogue [3](OAc)₂ (N^N^N^N) dissolved as isolated molecules. These nanorods formed via metallophilic Pd···Pd interaction and π–π stacking and were stabilized in the cell medium by serum proteins, in the absence of which the nanorods precipitated. In cell cultures, these protein-stabilized self-assembled nanorods were responsible for the improved cellular uptake of the cyclometalated compounds, which took place via endocytosis (i.e., an active uptake pathway). In addition to triggering self-assembly, cyclometalation in [1]OAc also led to dramatically enhanced photodynamic properties under blue light irradiation. These combined penetration and photodynamic properties were observed in multicellular tumor spheroids and in a mice tumor xenograft, demonstrating that protein-stabilized nanoaggregation of cyclometalated drugs such as [1]OAc also allows efficient cellular uptake in 3D tumor models. Overall, serum proteins appear to be a major element in drug design because they strongly influence the size and bioavailability of supramolecular drug aggregates and hence their efficacy in vitro and in vivo.

Bright Luminescent Platinum(II)-Biaryl Emitters Synthesized Without Air-Sensitive Reagents

Transition-metal complexes bearing biaryl-2,2'-diyl ligands tend to show intense luminescence. However, difficulties in synthesis have prevented their further functionalization and practical applications. Herein, a series of platinum(II) complexes bearing biaryl-2,2'-diyl ligands, which have never been prepared in air, were synthesized through transmetalation and successive cyclometalation of biarylboronic acids. This approach does not require any air- or moisture-sensitive reagents and features a simple synthesis even in air. The resulting (Et₄ N)₂ [Pt(m,n-F₂ bph)(CN)₂ ] (m,n-F₂ bph=m,n-difluorobiphenyl-2,2'-diyl) complexes exhibit intense green emissions with high quantum efficiencies of up to 0.80 at 298 K. The emission spectral fitting and variable-temperature emission lifetime measurements indicate that the high quantum efficiency was achieved because of the tight packing structure and strong σ-donating ability of bph.

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 Au^III 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 .

Thermally Stable Donor-Acceptor Type (Alkynyl)Gold(III) TADF Emitters Achieved EQEs and Luminance of up to 23.4% and 70 300 cd m-2 in Vacuum-Deposited OLEDs

Thermally stable, strongly luminescent gold-TADF emitters are the clue to realize practical applications of gold metal in next generation display and lighting technology, a scarce example of which is herein described. A series of donor-acceptor type cyclometalated gold(III) alkynyl complexes with some of them displaying highly efficient thermally activated delayed fluorescence (TADF) with Φ up to 88% in thin films and emission lifetimes of ≈1-2 µs at room temperature are developed. The emission color of these complexes is readily tunable from green to red by varying the donor unit and cyclometalating ligand. Vacuum-deposited organic light-emitting diodes (OLEDs) with these complexes as emissive dopants achieve external quantum efficiencies (EQEs) and luminance of up to 23.4% and 70 300 cd m-2, respectively.

Towards blue emitting monocyclometalated gold(iii) complexes – synthesis, characterization and photophysical investigations

The electronic properties of cyclometalating ligands and ancillary ligands were successfully tailored to achieve blue emission in monocyclometalated gold(iii) complexes.

Rational molecular design for realizing high performance sky-blue-emitting gold(iii) complexes with monoaryl auxiliary ligands and their applications for both solution-processable and vacuum-deposited organic light-emitting devices

Sky-blue-emitting arylgold(iii) complexes have been synthesized for OLEDs with EQEs of up to 11.3%.

A new class of sky-blue-emitting arylgold(iii) complexes containing tridentate bis-cyclometalating ligands derived from 2,6-diphenylpyridine (C^N^C) has been successfully designed and synthesized. By systematically varying the electron-withdrawing groups from cyano, fluoro, and trifluoromethyl to trifluoromethoxy groups on the phenyl ring of the tridentate C^N^C ligands, the emission maxima of these complexes have been significantly blue-shifted from 492 nm to 466 nm in dichloromethane solution. In addition, the higher excited state distortion with respect to the ground state associated with the multiple fluoro substitutions at the tridentate ligand has been successfully reduced by the employment of trifluoromethyl and trifluoromethoxy groups, as revealed by the Huang-Rhys factor. Taking advantage of their high photoluminescence quantum yields of up to 43% in the solid-state MCP thin-films, high performance solution-processable and vacuum-deposited organic light-emitting devices with external quantum efficiencies of up to 5.3% and 11.3%, respectively, have been realized. This work represents the first demonstration of sky-blue-emitting gold(iii) complexes with an x chromaticity coordinate of <0.2.