Dinuclear Alkynyl Gold(I) Complexes Containing Bridging N-Heterocyclic Dicarbene Ligands: New Synthetic Routes and Luminescence

Dinuclear gold(I) Complexes with Bidentate NHC Ligands as Precursors for Alkynyl Complexes via Mechanochemistry

The use of alkynyl gold(I) complexes covers different research fields, such as bioinorganic chemistry, catalysis, and material science, considering the luminescent properties of the complexes. Regarding this last application, we report here the synthesis of three novel dinuclear gold(I) complexes of the general formula [(diNHC)(Au-C≡CPh)₂]: two Au-C≡CPh units are connected by a bridging di(N-heterocyclic carbene) ligand, which should favor the establishment of semi-supported aurophilic interactions. The complexes can be easily synthesized through mechanochemistry upon reacting the pristine dibromido complexes [(diNHC)(AuBr)₂] with phenylacetylene and KOH. Interestingly, we were also able to isolate the monosubstituted complex [(diNHC)(Au-C≡CPh)(AuBr)]. The gold(I) species were fully characterized by multinuclear NMR spectroscopy and mass spectrometry. The emission properties were also evaluated, and the salient data are comparable to those of analogous compounds reported in the literature.

Significant differences between solid state and solution photochemistry and photophysics of mesogenic organometallic gold complexes

Five new gold complexes 4-ROC6H4C≡CAuN≡CC6H4-4-OR′ (R/R′ = CH3/C9H19 (C1N9 ), C15H31/C9H19 (C15N9 ), C6H13/C15H31 (C6N15 ), C9H19/C15H31 (C9N15 ), C12H25/C15H31 (C12N15 )) were synthesized and characterized (1H and 13C NMR, IR, Raman spectroscopy, and high resolution mass spectrometry). Their organized smectic phases were investigated by TGA, DSC, powder XRD, and polarized light optical microscopy, and the solids are found to have crystalline and amorphous domains. No evidence for Au•••Au interactions was observed. The steady state and time-resolved absorption and emission properties at 298 and 77 K were examined, and surprisingly, the excited lifetime of the triplet excited state in the solid state is extremely short-lived (<100 ps) in comparison with the microsecond time scale recorded for the solution and at 77 K. The photosensitization of 1O2 was observed in solution but not in the solid state. The nature of the singlet (ligand-to-ligand charge transfer) and triplet (ethynyl/intraligand ππ*) excited states were assessed using DFT and TD-DFT computations. The thermal and UV-photochemical formation of gold nanoparticles were performed in solution (slow) and in the solid state (faster). The thermally generated nanoparticles are found to be larger (2–20 nm; TEM) and exhibit well-defined shapes, whereas the photochemically generated ones are smaller (1–10 nm) and show ill-defined shapes.

Synthesis and photophysical properties of multimetallic gold/zinc complexes of (P,N,N,N,P) and (P,N,N) ligands

Orthogonal coordination of gold and zinc yields mono- and bimetallic complexes, which are investigated concerning their photophysics.

Comprehensive Investigation of the Photophysical Properties of Alkynylcoumarin Gold(I) Complexes

Six gold(I) complexes (R₃P-Au-Coum) containing three different alkynylcoumarin chromophores (Coum) with different electron-donating and electron-withdrawing characteristics and two different water-soluble phosphanes (PR₃ = PTA (a) and DAPTA (b)) have been synthesized (1a,b, unsubstituted coumarin; 2a,b, 4-methyl substituted coumarin; 3a,b, 3-chloro and 4-methyl substituted coumarin). A comprehensive study of the photophysical properties of the R₃P-Au-Coum, together with their propynyloxycoumarin precursors 1-3, was performed in solution at room and low temperatures. Spectral and photophysical characteristics of the R₃P-Au-Coum essentially depend on the electronic characteristics of the propynyloxycoumarin ligand. The presence of the Au(I) atom was found to be responsible for an increase of the intersystem crossing, with triplet state quantum yield values, ϕ_T, ranging from ∼0.05 to 0.35 and high coumarin phosphorescence quantum yield values for derivatives 1 and 2; fluorescence dominates the deactivation in derivatives 3. Efficient singlet oxygen photosensitization was observed for the new compounds 3a,b. From TDDFT calculations, the relevant HOMO and LUMO of the compounds, i.e., those involved in the transitions, are dominated by the frontier orbitals associated with the coumarin core. The Au(I)-phosphane structure introduces a new transition assigned to an intraligand transition involving the phosphane ligand, and π(C≡C) system, to the p orbitals of phosphorus and gold atoms.

A Luminescent Two-Coordinate AuI Bimetallic Complex with a Tandem-Carbene Structure: A Molecular Design for the Enhancement of TADF Radiative Decay Rate

A luminescent bimetallic Au^I complex comprised of N-heterocyclic carbene (NHC) and carbazole (Cz) ligands, that is, (NHC')Au(NHC)AuCz has been synthesized and studied. Both carbene ligands in the bimetallic complex act as electron acceptors in tandem to increase the energy separation between the ground and excited state, which is higher than those found in either monometallic analogue, (NHC)AuCz and (NHC')AuCz. A coplanar geometry designed into the tandem complex ensures sufficient electronic coupling between the π-orbitals of the ligands to impart a strong oscillator strength to the singlet intra-ligand charge-transfer (¹ ICT) transition. Theoretical modelling indicates that the emissive ICT excited state involves both NHC ligands. The tandem complex gives blue luminescence (λ_max =480 nm) with a high photoluminescent quantum yield (Φ_PL =0.80) with a short decay lifetime (τ=0.52 μs). Temperature-dependent photophysical studies indicate that emission is via thermally assisted delayed fluorescence (TADF) and give a small singlet-triplet energy difference (ΔE_ST =50 meV, 400 cm^-1 ) consistent with the short TADF lifetime.

Synthesis and Biological Studies on Dinuclear Gold(I) Complexes with Di-(N-Heterocyclic Carbene) Ligands Functionalized with Carbohydrates

The design of novel metal complexes with N-heterocyclic carbene (NHC) ligands that display biological activity is an active research field in organometallic chemistry. One of the possible approaches consists of the use of NHC ligands functionalized with a carbohydrate moiety. Two novel Au(I)–Au(I) dinuclear complexes were synthesized; they present a neutral structure with one bridging diNHC ligand, having one or both heterocyclic rings decorated with a carbohydrate functionality. With the symmetric diNHC ligand, the dicationic dinuclear complex bearing two bridging diNHC ligands was also synthesized. The study was completed by analyzing the antiproliferative properties of these complexes, which were compared to the activity displayed by similar mononuclear Au(I) complexes and by the analogous bimetallic Au(I)–Au(I) complex not functionalized with carbohydrates.

Solution versus solid-state dual emission of the Au(i)-alkynyl diphosphine complexes via modification of polyaromatic spacers

Single molecule luminophores capable of multiple emissions are essential for the development of new materials with unconventional photophysical behavior.

Gold(I) Alkynyls Supported by Mono- and Bidentate NHC Ligands: Luminescence and Isolation of Unprecedented Ionic Complexes

Reactions of NHC·HX (NHC = 1-benzyl-3-methylbenzimidazol-2-ylidene, X = Br^-, PF₆^-) and (AuC≡CR)_n (R = Ph, C₃H₆OH) in the presence of Cs₂CO₃ initially afford compounds of the general formula [(NHC)₂Au]₂[(RC₂)₂Au]X, which can be isolated by crystallization. With increased reaction time, only the expected mononuclear complexes of the type [NHCAuC≡CR] are produced. The crystal structure of [(NHC)₂Au]₂[(PhC₂)₂Au]PF₆ reveals an unprecedented triple-decker array upheld by a remarkably short (2.9375(7) Å) unsupported Au···Au···Au contact. The mononuclear complex [NHCAuC≡CPh] was found to crystallize as three distinct polymorphs and a pseudopolymorph, which depending on the intermolecular Au···Au distances emit blue, green, or yellow light. Two synthetic approaches were employed for the preparation of a series of dinuclear NHC-ligated Au(I) alkynyl complexes of the general formula [NHC-(CH₂)_n-NHC(AuC≡CR)₂], where NHC = N-benzylbenzimidazol-2-ylidene, R = Ph, C₃H₆OH, C₆H₁₀OH, and n = 1-3. In solution, the complexes with aliphatic substituents on the alkynyl fragment are nonemissive, whereas their phenyl-bearing congeners demonstrate characteristic metal-perturbed ³[IL(C≡CPh)] emission. In the solid state, a clear correlation between intermolecular aurophilic interactions and luminescence was established, including their role in the luminescent thermochromism of the phenylalkynyl complexes. The relationship between the Au···Au distance and emission energy was found to be inverse: i.e., the shorter the aurophilic contact, the higher the emission energy. We tentatively attribute this behavior to a smaller extent of excited-state distortion for a structure with a shorter Au···Au separation.

The interplay between fluorescence and phosphorescence with luminescent gold(i) and gold(iii) complexes bearing heterocyclic arylacetylide ligands

The photophysical properties of a series of gold(i) [LAu(C 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CR)] (L = PCy₃ (1a-4a), RNC (5a), NHC (6a)) and gold(iii) complexes [Au(C^N^C)(CCR)] (1b-4b) bearing heterocyclic arylacetylide ligands with narrow band-gap are compared. The luminescence of both series are derived from an intraligand transition localized on the arylacetylide ligand (ππ*(CCR)) but 1a-3a displayed prompt fluorescence (τ_PF = 2.7-12.0 ns) while 1b-3b showed mainly phosphorescence (τ_Ph = 104-205 μs). The experimentally determined intersystem crossing (ISC) rate constants (k_ISC) are on the order of 10⁶ to 10⁸ s^-1 for the gold(i) series (1a-3a) but 10¹⁰ to 10¹¹ s^-1 for the gold(iii) analogues (1b-3b). DFT/TDDFT calculations have been performed to help understand the difference in the k_ISC between the two series of complexes. Owing to the different oxidation states of the gold ion, the Au(i) complexes have linear coordination geometry while the Au(iii) complexes are square planar. It was found from DFT/TDDFT calculations that due to this difference in coordination geometries, the energy gap between the singlet and triplet excited states (ΔE_ST) with effective spin-orbit coupling (SOC) for Au(i) systems is much larger than that for the Au(iii) counterparts, thus resulting in the poor ISC efficiency for the former. Time-resolved spectroscopies revealed a minor contribution (<2.9%) of a long-lived delayed fluorescence (DF) (τ_DF = 4.6-12.5 μs) to the total fluorescence in 1a-3a. Attempts have been made to elucidate the mechanism for the origins of the DF: the dependence of the DF intensity with the power of excitation light reveals that triplet-triplet annihilation (TTA) is the most probable mechanism for the DF of 1a while germinate electron-hole pair (GP) recombination accounts for the DF of 2a in 77 K glassy solution (MeOH/EtOH = 4 : 1). Both 4a and 4b contain a BODIPY moiety at the acetylide ligand and display only ¹IL(ππ*) fluorescence with negligible phosphorescence being observed. Computational analyses attributed this observation to the lack of low-lying triplet excited states that could have effective SOC with the S₁ excited state.

Polynuclear platinum phosphanido/phosphinito complexes: formation of P-O and P-O-P bonds through reductive coupling processes

A mixture of the asymmetric complexes of formula [(RF)2Pt(μ-Ph2PO)(μ-PPh2)Pt(μ-PPh2)2Pt(solv)(solv')] [(1-(solv)(solv')] (solv, solv' = acetone, H2O, CH3CN) has been prepared by reaction of [(RF)2Pt(II)(μ-PPh2)2Pt(II)(μ-PPh2)2Pt(II)(NCCH3)2] with AgClO4 in CH3CN/acetone. The lability of the Pt-solvent bonds allows the displacement of the coordinated solvent molecules by dppm or Cl(-) and the isolation of the tri- or hexanuclear phosphanido/phosphinito Pt(ii) complexes [(C6F5)2Pt(μ-PPh2)(μ-PPh2O)Pt(μ-PPh2)2Pt(dppm)] (2) or [NBu4]2[(C6F5)2Pt(μ-PPh2)(μ-PPh2O)Pt(μ-PPh2)2Pt(μ-Cl)2Pt(μ-PPh2)2Pt(μ-PPh2)(μ-PPh2O)Pt(C6F5)2] (as a mixture of the two possible isomers 4a and 4b). Complex 2 reacts with AgClO4 to form the tetranuclear derivative [(C6F5)2Pt(μ-PPh2)(μ-PPh2O)Pt(μ-PPh2)2Pt(dppm)Ag(OClO3)] (3), which displays two Pt-Ag donor-acceptor bonds. The mixture of the hexanuclear isomers 4a-4b reacts with Tl(acac) producing the acetylacetonato complex [NBu4][(C6F5)2Pt(μ-PPh2)(μ-PPh2O)Pt(μ-PPh2)2Pt(acac)] (5) which, upon reaction with HCl, yields back the mixture of 4a-4b. The reaction of 4a-4b with PPh3 produces [NBu4][(C6F5)2Pt(μ-PPh2)(μ-PPh2O)Pt(μ-PPh2)2Pt(Cl)(PPh3)] (6) as a mixture of isomers with the chloro ligand located syn (6a) or anti (6b) to the PPh2O(-) group. Either the reaction of 6 with AgClO4 or the treatment of 5 with HPPh3ClO4 results in the formation of the species [(C6F5)2Pt(II)(μ-PPh2)2Pt(I)(μ-PPh2OPPh2)Pt(I)(PPh3)] (7) (44 VEC), which can be explained as the consequence of a PPh2O/PPh2 reductive coupling and a rearrangement of ligands in the molecule generating a Pt(ii),Pt(i),Pt(i) compound. All complexes were characterised in the solid state by XRD (only one of the isomers, in the cases of 4 and 6) and in solution by NMR spectroscopy.

Fluorene-based mononuclear gold(i) complexes: the effect of alkyl chain, aggregation-induced emission (AIE) and mechanochromism characteristics

Three gold(i) complexes were synthesized, and these gold(i) complexes exhibit AIE or alkyl chain-dependent mechanochromism characteristics.

A novel carbazole-based gold(i) complex with interesting solid-state, multistimuli-responsive characteristics

A novel carbazole-based gold(ι) complex is reported. The luminogen exhibits significative solid-state reversible mechanochromism, dual-responsive thermochromism and sensitive thin-film vapochromism properties.

Carbazole-based gold(i) complexes with alkyl chains of different lengths: tunable solid-state fluorescence, aggregation-induced emission (AIE), and reversible mechanochromism characteristics

Seven gold(i) complexes with alkyl chains of different lengths were synthesized, these luminogens exhibited AIE and mechanochromism characteristics.

A novel fluorene-based aggregation-induced emission (AIE)-active gold(i) complex with crystallization-induced emission enhancement (CIEE) and reversible mechanochromism characteristics

A new fluorene-based AIE-active gold(i) complex was designed and synthesized. The novel luminogen exhibits a crystallization-induced emission enhancement (CIEE) effect and reversible mechanochromic behavior with fluorescence changes between green and yellow emissions.

Three-coordinate gold(I) N-heterocyclic carbene complexes: a new class of strongly luminescent derivatives

A selected group of cationic three-coordinate Au(I)-NHC complexes of the form [Au(NHC)(dppbz)]OTf have been prepared from a commercially available bidentate phosphine. All complexes have been fully characterised by NMR and mass spectroscopy. The [Au(NHC)](+) fragment shows a pronounced tendency to form linear complexes which is confirmed by the molecular structure of [Au(IPr)(dppbz)]OTf in the solid state. The complexes are brightly luminescent and present very high quantum yield values in the solid state. The assignments of the electronic transitions involved in the emissions are of a phosphorescent nature and it is proposed that the origin of the emissions is derived from the ligand (dppbz) to metal-ligand (Au-NHC) charge-transfer (LML'CT) transition.

N-heterocyclic carbene metal complexes: photoluminescence and applications

This review covers the advances made in the synthesis and applications of luminescent transition metal complexes containing N-heterocyclic carbene (NHC) ligands.

Probing the Viability of Oxo-Coupling Pathways in Iridium-Catalyzed Oxygen Evolution

A series of Cp*IrIII dimers have been synthesized to elucidate the mechanistic viability of radical oxo-coupling pathways in iridium-catalyzed O2 evolution. The oxidative stability of the precursors toward nanoparticle formation and their oxygen evolution activity have been investigated and compared to suitable monomeric analogues. We found that precursors bearing monodentate NHC ligands degraded to form nanoparticles (NPs), and accordingly their O2 evolution rates were not significantly influenced by their nuclearity or distance between the two metals in the dimeric precursors. A doubly chelating bis-pyridine-pyrazolide ligand provided an oxidation-resistant ligand framework that allowed a more meaningful comparison of catalytic performance of dimers with their corresponding monomers. With sodium periodate (NaIO4) as the oxidant, the dimers provided significantly lower O2 evolution rates per [Ir] than the monomer, suggesting a negative interaction instead of cooperativity in the catalytic cycle. Electrochemical analysis of the dimers further substantiates the notion that no radical oxyl-coupling pathways are accessible. We thus conclude that the alternative path, nucleophilic attack of water on high-valent Ir-oxo species, may be the preferred mechanistic pathway of water oxidation with these catalysts, and bimolecular oxo-coupling is not a valid mechanistic alternative as in the related ruthenium chemistry, at least in the present system.

Dinuclear gold(I) complexes with propylene bridged N-heterocyclic dicarbene ligands: synthesis, structures, and trends in reactivities and properties

Four novel dinuclear N-heterocyclic dicarbene gold(I) complexes with a propylene linker between the carbene moieties have been synthesized and their luminescence and electrochemical properties, together with their reactivity towards bromine oxidative addition, have been screened. All the complexes emit in the solid state in the blue-green spectral range (400-500 nm) with appreciable intensities (Φ(em) up to ≈10%). In cyclic voltammetry, the Au(I)/Au(0) peak splits at low temperature into two separate peaks relative to the couples Au(I)-Au(I)/Au(I)-Au(0) and Au(I)-Au(0)/Au(0)-Au(0), thus indicating the presence of an Au···Au interaction in the dinuclear complex. Oxidative addition of bromine affords as a major or unique product Au(II)-Au(II) complexes most likely as a consequence of the interaction between the two gold centres favoured by the propylene linker.