Dinuclear Au Research Articles

Gold Coordination-Accelerated Multi-Resonance TADF Emission for Efficient Solution-Processible Ultrapure Deep-Blue OLEDs.

Multi-resonance (MR) type emitters have emerged as highly promising candidates for high-resolution organic light-emitting diodes (OLEDs). However, thermally activated delayed fluorescence (TADF) emissions with simultaneous short excited state lifetimes and ultrapure blue color (a CIEy close to 0.046 and an emission peak >440 nm) have rarely been obtained for MR emitters. Herein, we report a design of dual gold-coordinated MR molecules to achieve efficient and short-lived ultrapure blue TADF emission. The dinuclear Au(I) complex, namely iPrAuBN, shows a narrowband deep-blue emission with a peak maximum of 448 nm and a full width at half maximum (FWHM) of 29 nm in doped film. The coordination with two Au atoms significantly shortens the delayed fluorescence lifetime to 7.8 μs in comparison to 60.6 μs for the parental organic analogue. Solution-processed OLED doped with iPrAuBN demonstrates an ultrapure blue electroluminescence with a peak maximum of 442 nm, a FWHM of 19 nm, CIE coordinates of (0.154, 0.036), and a maximum external quantum efficiency of 14.8 %.

Two-Coordinate Dinuclear Donor-Gold(I)-Acceptor Complexes Exhibiting Multiple Excitation Wavelength Dependent Phosphorescence.

Two-coordinate Au(I) complexes with a donor-metal-acceptor (D-M-A) structure have shown rich luminescent properties. However, charge-neutral dinuclear donor-metal-acceptor type Au(I) complexes featuring aurophilic interactions have been seldom explored. Herein, we describe the structures and photoluminescence properties of two dinuclear Au(I) complexes, namely DiAu-Ph and DiAu-Me. Single crystal X-ray structural analysis of DiAu-Ph reveals a short intramolecular Au-Au distance of 3.224 Å. In dilute solution and doped films, excitation wavelength dependent multiple phosphorescence phenomena were observed for these dinuclear complexes. Theoretical calculations reveal that the aurophilic interaction causes increased contribution of the Au d orbital to the highest occupied molecular orbitals. Thus, the gap between singlet and triplet excited states (ΔEST) is enlarged, which disables the thermally activated delayed fluorescence (TADF). Moreover, the large energy separation (0.45-0.52 eV) and the different orbital configurations between the various excited states result in an inefficient internal conversion, accounting for their multiple phosphorescence properties.

Computational Insights into the Photoinduced Dimeric Gold-Catalyzed Divergent Dechloroalkylation of gem-Dichloroalkanes with Alkenes.

The employment of dinuclear Au(I) catalysts in photomediated modern organic transformations has attracted significant attention over the past decade, which commonly demonstrates unique catalytic performance compared with the corresponding mononuclear gold complexes. Nevertheless, detailed mechanisms of dinuclear gold catalysis remain ambiguous, and further mechanistic understanding is highly desirable. Herein, computational studies were carried out to gain mechanistic insights into the photoinduced dinuclear gold-catalyzed divergent dechloroalkylation of gem-dichloroalkanes. Computational results suggest that a proton transfer from the additive, Hantzsch ester (HE), to the base, guanidine, could lead to an ionic pair complex, which is ready to undergo excitation under blue light irradiation to result in the corresponding triplet excited state. Then, the excited complex might undergo oxidative quenching with the dinuclear gold photocatalyst [AuI-AuI]2+, via a single-electron-transfer (SET) step to afford an unusual [Au1/2-Au1/2]+ dinuclear species. The corresponding mononuclear gold catalyst, [AuI]+, however, is not ready to enable the analogous step to give a [Au0] species, which might account for the unique characteristics of dinuclear gold catalysis. Subsequently, the formed [Au1/2-Au1/2]+ intermediate could trigger a Cl-atom transfer from dichloromethane in an inner-sphere manner to furnish a critical chloromethyl radical. Next, the resulting chloromethyl radical could attack the alkenyl moiety of substrates to generate the corresponding alkyl radicals. Then, three possible mechanistic pathways were explored to rationalize the substrate-dependent divergent transformations in this protocol. The main factors responsible for the diversified transformations were discussed.

Valence Tautomerism Induced Proton Coupled Electron Transfer:X-H Bond Oxidation with a Dinuclear Au(II) Hydroxide Complex.

We report the preparation and characterization of the dinuclear AuII hydroxide complex AuII 2(L)2(OH)2 (L=N,N'-bis (2,6-dimethyl) phenylformamidinate) and study its reactivity towards weak X-H bonds. Through the interplay of kinetic analysis and computational studies, we demonstrate that the oxidation of cyclohexadiene follows a concerted proton-coupled electron transfer (cPCET) mechanism, a rare type of reactivity for Au complexes. We find that the Au-Au σ-bond undergoes polarization in the PCET event leading to an adjustment of oxidation levels for both Au centers prior to C(sp3)-H bond cleavage. We thus describe the oxidation event as a valence tautomerism-induced PCET where the basicity of one reduced Au-OH unit provides a proton acceptor and the second more oxidized Au center serves as an electron acceptor. The coordination of these events allows for unprecedented radical-type reactivity by a closed shell AuII complex.

Valence Tautomerism Induced Proton Coupled Electron Transfer:X−H Bond Oxidation with a Dinuclear Au(II) Hydroxide Complex

AbstractWe report the preparation and characterization of the dinuclear AuII hydroxide complex AuII2(L)2(OH)2 (L=N,N′‐bis (2,6‐dimethyl) phenylformamidinate) and study its reactivity towards weak X−H bonds. Through the interplay of kinetic analysis and computational studies, we demonstrate that the oxidation of cyclohexadiene follows a concerted proton‐coupled electron transfer (cPCET) mechanism, a rare type of reactivity for Au complexes. We find that the Au−Au σ‐bond undergoes polarization in the PCET event leading to an adjustment of oxidation levels for both Au centers prior to C(sp3)−H bond cleavage. We thus describe the oxidation event as a valence tautomerism‐induced PCET where the basicity of one reduced Au−OH unit provides a proton acceptor and the second more oxidized Au center serves as an electron acceptor. The coordination of these events allows for unprecedented radical‐type reactivity by a closed shell AuII complex.

Diverse structural reactivity patterns of a POCOP ligand with coinage metals.

We have utilised the 4,6-di-tert-butyl resorcinol bis(diphenylphosphinite) (POCOP) ligand for exploring its coordination ability towards group 11 metal centres. The treatment of the bidentate ligand 1 with various coinage metal precursors afforded a wide range of structurally diverse complexes 2-12, depending upon the metal precursors used. This furnishes several multinuclear Cu(I) complexes with dimeric (2) and tetrameric cores (3, 4, and 5). The tetrameric stairstep complex 4 shows thermochromic behaviour, whereas the dimeric complex 2 and tetrameric complex 3 show luminescence properties at cryogenic temperatures. Interestingly, the halide substitution reaction of the dimeric complex 2 with KPPh2 produces a unique mixed phosphine-based tetrameric Cu(I) complex, 5. Treatment of the POCOP ligand with [CuBF4(CH3CN)4] in the presence of 2,2'-bipyridine afforded heteroleptic complex 6, consisting of tri- and tetra-coordinated cationic Cu(I) centres. Furthermore, we could also isolate cubane (8) and stairstep (9) complexes of Ag(I). The cationic Au(I) complex (12) was obtained from the dinuclear Au(I) complex of POCOP, 11. Complex 12 revealed the presence of a strong intramolecular aurophilic interaction with an Au⋯Au bond distance of 3.1143(9) Å. Subsequently, the photophysical properties of these complexes have been studied. All the complexes were characterised by single-crystal X-ray diffraction studies, routine NMR techniques, and mass spectroscopy.

Dinuclear gold(i) complexes with pyrimidine- and m-carborane-based bisphosphine ligands: synthesis, structure, photoluminescence and cytotoxicity against malignant pleural mesothelioma

Novel luminescent and anticancer dinuclear Au(i) complexes with bisphosphine ligands have been synthesized and studied.

"Dynamical Docking" of Cyclic Dinuclear Au(I) Bis-N-heterocyclic Complexes Facilitates Their Binding to G-Quadruplexes.

With the aim to improve the design of metal complexes as stabilizers of noncanonical DNA secondary structures, namely, G-quadruplexes (G4s), a series of cyclic dinuclear Au(I) N-heterocyclic carbene complexes based on xanthine and benzimidazole ligands has been synthesized and characterized by various methods, including X-ray diffraction. Fluorescence resonance energy transfer (FRET) and CD DNA melting assays unraveled the compounds' stabilization properties toward G4s of different topologies of physiological relevance. Initial structure-activity relationships have been identified and recognize the family of xanthine derivatives as those more selective toward G4s versus duplex DNA. The binding modes and free-energy landscape of the most active xanthine derivative (featuring a propyl linker) with the promoter sequence cKIT1 have been studied by metadynamics. The atomistic simulations evidenced that the Au(I) compound interacts noncovalently with the top G4 tetrad. The theoretical results on the Au(I) complex/DNA Gibbs free energy of binding were experimentally validated by FRET DNA melting assays. The compounds have also been tested for their antiproliferative properties in human cancer cells in vitro, showing generally moderate activity. This study provides further insights into the biological activity of Au(I) organometallics acting via noncovalent interactions and underlines their promise for tunable targeted applications by appropriate chemical modifications.

Dinuclear Au(I), Au(II) and Au(III) Complexes with (CF2 )n Chains: Insights into The Role of Aurophilic Interactions in the Au(I) Oxidation.

New dinuclear Au(I), Au(II) and Au(III) complexes containing (CF2)n bridging chains were obtained. Metallomacrocycles [Au2{μ‐(CF2)4}{μ‐diphosphine}] show an uncommon figure‐eight structure, the helicity inversion barrier of which is influenced by aurophilic interactions and steric constraints imposed by the diphosphine. Halogenation of LAu(CF2)4AuL (L=PPh3, PMe3, (dppf)1/2, (binap)1/2) gave [Au(II)]2 species, some of which display unprecedented folded structures with Au−Au bonds. Aurophilic interactions facilitate this oxidation process by preorganizing the starting [Au(I)]2 complexes and lowering its redox potential. The obtained [Au(II)]2 complexes undergo thermal or photochemical elimination of R3PAuX to give Au(III) perfluorinated auracycles. Evidence of a radical mechanism for these decomposition reactions was obtained.

DNA interaction, anticancer, cytotoxicity and genotoxicity studies with potential pyrazine-bipyrazole dinuclear µ-oxo bridged Au(III) complexes.

Pyrazine-bipyrazole-based µ-oxo bridged dinuclear Au(III) complexes were synthesized and characterized by various spectrometric (1H-NMR, 13C (APT) NMR, FT-IR, Mass spectrometry) and analytical techniques (elemental analysis and conductance measurement). The evaluation of DNA binding activity by UV-Vis absorption spectra and viscosity measurement demonstrated that all the compounds intercalate in between the stacks of DNA base pair and the binding constant values were observed in the range of 5.4 × 104-2.17 × 105M-1. The molecular docking study also supports the intercalation mode of binding. The anti-proliferation activity of complexes on A549 (Lung adenocarcinoma) cells by MTT assay demonstrated IC50 values in the range of 47.46 -298.12μg/mL. The genotoxicity of compounds was checked by smearing observed in the DNA of S. pombe cell under the influence of complexes. The in vivo cytotoxicity of compounds against brine shrimp demonstrated the LC50 values in the range of 4.59-27.22μg/mL. The promising results of the Au(III) complexes received significant attention and make them suitable for the new metallodrugs after the detailed mechanistic biological study.

Dimerization of a mononuclear gold(I) complex to its corresponding dinuclear complex containing a cyclophane-NHC ligand

A mononuclear AuI-NHC complex was synthesized in good yield via the reaction of KAuCl4 with an imidazolium-linked cyclophane salt in the presence of a mild base. The new complex was fully characterized by 1H and 13C NMR spectroscopy and mass spectrometry. The 1H NMR studies, supported by DFT analysis, show how the mononuclear AuI-NHC complex dimerizes to its corresponding dinuclear AuI-NHC analogue slowly in solution (DMSO or CH3CN). The thermodynamic properties show a distinctive preference for dinuclear Au formation over that of the monomer. We have also discussed the unconventional structural contrast between the experimental and computational geometrical analysis of the dinuclear AuI-NHC complex.

Small Variations, Big Impact: Structural Diversity of the Complexes of a Phosphane‐Decorated Benzenedithiol with Group‐11 Metals

AbstractReaction of a phosphane‐decorated benzenedithiol (pbdtH2) with coinage metal salts furnished polynuclear complexes [M2(pbdtH)2] (M=AuI) or [cat][M5(pbdt)3] (cat=unipositive cation, M=AgI, CuI), which were characterized by analytical and spectroscopic techniques and single‐crystal X‐ray diffraction studies. Furthermore, a double salt with an anion [Ag5(pbdt)3(PPh3)]− that proved unstable in solution was characterized crystallographically. The spectroscopic and crystallographic data revealed that the Cu(I) and Ag(I) complexes exhibit, despite their like stoichiometric composition, isomeric molecular structures. The observed disparities were reproduced by DFT studies. The dinuclear Au(I) complex was found to undergo air‐oxidation to furnish a mixed‐valent complex [(AuIII)2(AuI)2(pbdt)4]. The copper(I) – but not the isomeric silver(I) complexes – showed luminescence in the solid state.

Synthesis and enantioseparation of chiral Au13 nanoclusters protected by bis-N-heterocyclic carbene ligands.

A series of chiral Au13 nanoclusters were synthesized via the direct reduction of achiral dinuclear Au(i) halide complexes ligated by ortho-xylyl-linked bis-N-heterocyclic carbene (NHC) ligands. A broad range of functional groups are tolerated as wingtip substituents, allowing for the synthesis of a variety of functionalized chiral Au13 nanoclusters. Single crystal X-ray crystallography confirmed the molecular formula to be [Au13(bisNHC)5Cl2]Cl3, with a chiral helical arrangement of the five bidentate NHC ligands around the icosahedral Au13 core. This Au13 nanocluster is highly luminescent, with a quantum yield of 23%. The two enantiomers of the Au13 clusters can be separated by chiral HPLC, and the isolated enantiomers were characterized by circular dichroism spectroscopy. The clusters show remarkable stability, including configurational stability, opening the door to further investigation of the effect of chirality on these clusters.

Improved Antiproliferative Activity and Fluorescence of a Dinuclear Gold(I) Bisimidazolylidene Complex via Anthracene-Modification.

A straightforward modification route to obtain mono‐ and di‐substituted anthroyl ester bridge functionalized dinuclear Au(I) bis‐N‐heterocyclic carbene complexes is presented. The functionalization can be achieved starting from a hydroxyl‐functionalized ligand precursor followed by transmetallation of the corresponding Ag complex or via esterification of the hydroxyl‐functionalized gold complex. The compounds are characterized by NMR‐spectroscopy, ESI‐MS, elemental analysis and SC‐XRD. The mono‐ester Au complex shows quantum yields around 18%. In contrast, the corresponding syn‐di‐ester Au complex, exhibits significantly lower quantum yields of around 8%. Due to insufficient water solubility of the di‐ester, only the mono‐ester complex has been tested regarding its antiproliferative activity against HeLa‐ (cervix) and MCF‐7‐ (breast) cancer cell lines and a healthy fibroblast cell line (V79). IC50 values of 7.26 μM in the HeLa cell line and 7.92 μM in the MCF‐7 cell line along with selectivity indices of 8.8 (HeLa) and 8.0 (MCF‐7) are obtained. These selectivity indices are significantly higher than those obtained for the reference drugs cisplatin or auranofin.

Mechanisms underlying the cytotoxic activity of syn/anti-isomers of dinuclear Au(I) NHC complexes

The syn- and anti-isomers of dinuclear Au(I) complexes of the type Au2(RLOH)(PF6)2 (R = isopropyl or mesityl) bearing 2-hydroxyethane-1,1-diyl-bridged bisimidazolylidene ligands were separated by reversed phase high performance liquid chromatography (HPLC) and characterized by NMR spectroscopy, elemental analysis, ESI mass spectrometry as well as single crystal X-ray diffraction analysis. Evaluation of the antiproliferative activity of the isolated isomers has shown very small difference in their cytotoxic behavior in various cancer cell lines. Additional counter-anion exchange (hexafluorophosphate to chloride) allows to increase the water solubility of synAu2(MesLOH)(PF6)2 and leads to higher antiproliferative activity when compared to the hexafluorophosphate-complex. Both isomers were treated with l-cysteine as nucleophilic thiol source and only the anti-isomer shows dissociation of one bisimidazolylidene ligand after 24 h. In the case of the syn-isomer, density functional theory calculations indicate a lower reactivity due to the higher steric hindrance of the N-substituents and additional hydrogen bond interaction, which prevents a nucleophilic attack. When the N-substituent is replaced by the bulkier mesityl group, both conformations remain unreactive and result to be the most cytotoxic complexes in the above-mentioned cancer cell lines. Interestingly, synAu2(MesLOH)(PF6)2 exhibits a high selectivity in the MCF-7 cell line with a selectivity index (SI) of 19, which is superior to auranofin (SI < 1), making this compound an ideal candidate for further studies. Preliminary mechanistic studies reveal that the cytotoxic complexes possess mitochondrial-TrxR inhibition properties in the nanomolar range. Additionally, the cellular distribution studies by ICP-MS and nuclear microscopy have shown that the compound accumulates in the membranes. These results suggest that the mitochondrial membrane is the main target for this type of dinuclear complexes, causing oxidative stress by inhibiting mitochondrial thioredoxin reductase.

Dinuclear Gold Complexes Supported by Wide Bite Angle Diphosphines for Preorganization-Induced Selective Dual-Gold Catalysis

The synthesis, reactivity, and potential of well-defined dinuclear gold complexes as precursors for dual-gold catalysis is explored. Using the preorganizing abilities of well-known wide bite angle diphosphine ligands, DBFPhos and DPEPhos, dinuclear Au(I)–Au(I) complexes 1 and 2 are used as precursors to form well-defined monocationic species with either a chlorido- or acetylido-ligand bridging the two gold centers. These compounds are active catalysts for the dual-gold heterocycloaddition of a urea-functionalized alkyne, and the preorganization of both Au-centers affords efficient σ,π-activation of the substrate, even at high dilution, significantly outperforming benchmark mononuclear catalysts.

Gold chloride cluster ions generated by vacuum laser ablation

In this work, a simple way for study the possibility of formation a vapor cluster species of tetrachloroauric acid (HAuCl4), using the laser ablation in the absence of a buffer or reactive atmosphere, and without a postablation supersonic expansion on a commercial matrix assisted laser desorption/ionization time-of-flight mass spectrometer, is reported. Tetrachloroauric acid is known as precursor for the synthesis of gold nanostructures and the complex salts; therefore it is an important task to discover and quantify the species arising from HAuCl4, in order to understand their role in the gold assisted reactions. Mass spectrum of HAuCl4 in a reflector negative-ion mode contains the hydrated mono- and dinuclear gold clusters in the m/z range 286–436, and gold chloride clusters in the m/z range 447–795. In the first part of spectrum, m/z range 286–436, the hydrated gold cluster species of type Au n − (H2O)m (n = 1–2; m = 1, 2, 5, 7, 8) and [Aun(OH)k]−(H2O)m (n = 1–2; k = 1–2; m = 1, 4–8) were found. Besides that, there are gold chloride clusters with general formula [AuHr(HCl)2]−(H2O)m (m = 1–5; 8–9; r = 0–2) in this part of spectrum. In the second part of spectrum, the m/z range 447–795, only gold chloride clusters were obtained. Their general formulae can be written as [AuClt(HCl)v]−(H2O)m (t = 1–4; v = 5–8; m = 2–4, 6–8) and [Aun(HCl)v]−(H2O)m (n = 1–2, v = 4–5, m = 1–2, 5, 7). The analysis of concentration effects on the LDI mass spectra of gold clusters reveals that the relative intensities of signals for the mono- and dinuclear Au clusters increase with decreasing the concentration of water HAuCl4 solutions.

Formation of Dinuclear AuII and AuI/AuIII Mixed‐Valence Complexes is Directed by Structural Constraints Imposed by Cyclophane‐NHC Ligands

The oxidation of dinuclear AuI complexes containing cyclophane‐bis(NHC) ligands by SOCl2, Br2 and I2 has been investigated. The outcome of the oxidation was dependent on the nature of the cyclophane‐NHC framework. The new complexes were fully characterised by X‐ray diffraction and 1H and 13C NMR spectroscopy. When the AuI complex contained an ortho‐cyclophane framework (dictating short intra‐cation Au···Au distances), oxidation resulted in dinuclear AuII complexes of the form X–AuII(NHC)2–AuII(NHC)2–X (X = Cl, Br, I), the Au···Au distance being in the range 2.6055(3)–2.6501(1) Å. These AuII complexes are strongly luminescent. When the AuI complex contained an meta‐cyclophane framework (dictating longer intra‐cation Au···Au distances), oxidation with SOCl2 or Br2 resulted in dinuclear complexes containing AuI and AuIII centres of the form X–AuIII(NHC)2–X–AuI(NHC)2. NMR studies showed that interconversion of the AuI and AuIII sites within the complexes occurred at rates comparable to the NMR timescale.

Controlled Interconversion of a Dinuclear Au Species Supported by a Redox-Active Bridging PNP Ligand Facilitates Ligand-to-Gold Electron Transfer.

Redox non‐innocent ligands have recently emerged as interesting tools to obtain new reactivity with a wide variety of metals. However, gold has almost been neglected in this respect. Here, we report mechanistic investigations related to a rare example of ligand‐based redox chemistry in the coordination sphere of gold. The dinuclear metal‐centered mixed‐valent AuI–AuIII complex 1, supported by monoanionic diarylamido‐diphosphine ligand PNPPr and with three chlorido ligands overall, undergoes a complex series of reactions upon halide abstraction by silver salt or Lewis acids such as gallium trichloride. Formation of the ultimate AuI–AuI complex 2 requires the intermediacy of AuI–AuI dimers 5 and 7 as well as the unique AuIII–AuIII complex 6, both of which are interconverted in a feedback loop. Finally, unprecedented ortho‐selective C−H activation of the redox‐active PNP ligand results in the carbazolyldiphosphine derivative PN*PPr via ligand‐to‐metal two‐electron transfer. This work demonstrates that the redox‐chemistry of gold may be significantly expanded and modified when using a reactive ligand scaffold.

Well‐Defined Dinuclear Gold Complexes for Preorganization‐Induced Selective Dual Gold Catalysis

The synthesis, reactivity, and potential of well-defined dinuclear gold complexes as precursors for dual gold catalysis are explored. Using the preorganizing abilities of the ditopic PN(H) P(iPr) (L(H) ) ligand, dinuclear Au(I) -Au(I) complex 1 and mixed-valent Au(I) -Au(III) complex 2 provide access to structurally characterized chlorido-bridged cationic species 3 and 4 upon halide abstraction. For 2, this transformation involves unprecedented two-electron oxidation of the redox-active ligand, generating a highly rigidified environment for the Au2 core. Facile reaction with phenylacetylene affords the σ,π-activated phenylacetylide complex 5. When applied in the dual gold heterocycloaddition of a urea-functionalized alkyne, well-defined precatalyst 3 provides high regioselectivities for the anti-Markovnikov product, even at low catalyst loadings, and outperforms common mononuclear Au(I) systems. This proof-of-concept demonstrates the benefit of preorganization of two gold centers to enforce selective non-classical σ,π-activation with bifunctional substrates.