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

Mechanochemical preparation of strongly emissive monosubstituted triarylphosphane gold(i) compounds activated by hydrogen bonding driven aggregations

Gold(i) triarylphosphane compounds are a well-known class of coordination compounds displaying from mild to strong emissive properties. Mechanochemical approaches to the preparation, spectroscopic characterization, X-ray diffraction structural determination, and photophysical studies of green emissive neutral linear monophosphane or neutral pseudo-T-shaped or cationic bis-phosphane gold(i) compounds, are herein discussed. The mechanochemical approach to the preparation of gold(i) derivatives was particularly successful for ligands bearing the carboxylic group, while the preparation with esterified ligands yields better results with solvent-mediated methods. The introduction of carboxyl or ester substituents in one aryl group favors the ligand-centered emissions. The analysis of the origin of the emissions was elucidated on the basis of DFT calculations, addressing the emissive behavior to ligand-centered excited states, strongly affected by supramolecular reversible hydrogen bonding aggregation. The study indicates that the ligand with the carboxylic group is particularly suitable for the mechanochemical preparation of emissive gold(i) complexes for material science applications.

Fundamentals and applications of N-heterocyclic carbene functionalized gold surfaces and nanoparticles

Improved stability and higher degree of synthetic tunability has allowed N-heterocyclic carbenes to supplant thiols as ligands for gold surface functionalization.  This review article summarizes the basic science and applications of NHCs on gold.

Selective formation of a supramolecular coordination complex in the nanometre scale with a ferrocene-based phospholane ligand

A straightforward synthesis of the tetradentate phospholane ligand 1 is reported. The 2 : 1 [M : L] reaction of 1 with [AuCl(tht)] (tht = tetrahydrothiophene) resulted in the 4 : 2 [M : L] supramolecular coordination complex 2 where two ligands 1 are bridging four gold(I) cations. The formation of 2 can be rationalised via a geometrical analysis of the ligand. The coordination mode of the gold atoms was evaluated based on a CSD search, revealing the geometrical changes for a transition from linear to trigonal planar coordination environment.

Main Avenues in Gold Coordination Chemistry

In this contribution, we provide an overview of the main avenues that have emerged in gold coordination chemistry during the last years. The unique properties of gold have motivated research in gold chemistry, and especially regarding the properties and applications of gold compounds in catalysis, medicine, and materials chemistry. The advances in the synthesis and knowledge of gold coordination compounds have been possible with the design of novel ligands becoming relevant motifs that have allowed the preparation of elusive complexes in this area of research. Strong donor ligands with easily modulable electronic and steric properties, such as stable singlet carbenes or cyclometalated ligands, have been decisive in the stabilization of gold(0) species, gold fluoride complexes, gold hydrides, unprecedented π complexes, or cluster derivatives. These new ligands have been important not only from the fundamental structure and bonding studies but also for the synthesis of sophisticated catalysts to improve activity and selectivity of organic transformations. Moreover, they have enabled the facile oxidative addition from gold(I) to gold(III) and the design of a plethora of complexes with specific properties.

Acridine N-Heterocyclic Carbene Gold(I) Compounds: Tuning from Yellow to Blue Luminescence

The synthesis and the luminescence features of three gold(I)-N-heterocyclic carbene (NHC) complexes are presented to study how the n-alkyl group can influence the luminescence properties in the crystalline state. The mononuclear gold(I)-NHC complexes, [(L¹ )Au(Cl)] (1), [(L² )Au(Cl)] (2), and [(L³ )Au(Cl)] (3) were isolated from the reactions between [(tht)AuCl] and corresponding NHC ligand precursors, [N-(9-acridinyl)-N'-(n-butyl)-imidazolium chloride, (L¹ .HCl)], [N-(9-acridinyl)-N'-(n-pentyl)-imidazolium chloride, (L² .HCl)] and [N-(9-acridinyl)-N'-(n-hexyl)-imidazolium chloride, (L³ .HCl)]. Their single-crystal X-ray analysis reveals the influence of the n-alkyl groups on solid-state packing. A comparison of the luminescence features of 1-3 with n-alkyl substituents is explored. The molecules 1-3 depicted blue emission in the solution state, while the yellow emission (for 1), greenish-yellow emission (for 2), and blue emission (for 3) in the crystalline phase. This paradigm emission shift arises from n-butyl to n-pentyl and n-hexyl in the crystalline state due to the carbon-carbon rotation of the n-alkyl group, which tends to promote unusual solid packing. Hence n-alkyl group adds a novel emission property in the crystalline state. Density Functional Theory and Time-Dependent Density Functional Theory calculations were carried out for monomeric complex, N-(9-acridinyl)-N'-(n-heptyl)imidazole-2-ylidene gold(I) chloride and dimeric complex, N-(9-acridinyl)-N'-(n-heptyl)imidazole-2-ylidene gold(I) chloride to understand the structural and electronic properties.

Perhalophenyl Three-Coordinate Gold(I) Complexes as TADF Emitters: A Photophysical Study from Experimental and Computational Viewpoints

We report the synthesis of novel perhalophenyl three-coordinated gold(I) complexes using 1,2-bis(diphenylphosphino)benzene (dppBz) as the chelating ligand and [AuR(tht)] (R = C₆F₅, C₆Cl₂F₃, C₆Cl₅) as the perhalophenyl-gold(I) source, leading to [AuR(dppBz)] (R = C₆F₅ (1), C₆Cl₂F₃ (2), C₆Cl₅ (3)) complexes. The solid-state structures of compounds 2 and 3 consist of discrete three-coordinated Au(I) complexes, which show a distorted trigonal planar geometry for the gold center with dissimilar Au-P distances. The distorted structural arrangement is closely related to its photophysical properties. The studied complexes display very intense emissions at room temperature (RT) and at 77 K in the solid state. Studies of the emissive properties of the complexes at different temperatures suggest that the emissions are phosphorescent at 77 K and exhibit thermally activated delayed fluorescence (TADF) at RT. First-principle calculations of the photophysical processes yielded rate constants for intersystem crossing and reverse intersystem crossing that are in excellent agreement with experimental data.

Unsymmetrical Coordination of Bipyridine in Three-Coordinate Gold(I) Complexes

The unsymmetrical coordination of gold(I) by 2,2'-bipyridine (bipy) in some planar, three-coordinate cations has been examined by crystallographic and computational studies. The salts [(Ph₃P)Au(bipy)]XF₆ (X = P, As, Sb) form an isomorphic series in which the differences in Au-N distances range from 0.241(2) to 0.146(2) Å. A second polymorph of [(Ph₃P)Au(bipy)]AsF₆ has also been found. Both polymorphs exhibit similar structures. The salts [(Et₃P)Au(bipy)]XF₆ (X = P, As, Sb) form a second isostructural series. In this series the unsymmetrical coordination of the bipy ligand is maintained, but the gold ions are disordered over two unequally populated positions that produce very similar overall structures for the cations. Although many planar, three-coordinate gold(I) complexes are strongly luminescent, the salts [(R₃P)Au(bipy)]XF₆ (R = Ph or Et; X = P, As, Sb) are not luminescent as solids or in solution. Computational studies revealed that a fully symmetrical structure for [(Et₃P)Au(bipy)]⁺ is 7 kJ/mol higher in energy than the observed unsymmetrical structure and is best described as a transition state between the two limiting unsymmetrical geometries. The Au-N bonding has been examined by natural resonance theory (NRT) calculations using the "12 electron rule". The dominant Lewis structure is one with five lone pairs on Au and one bond to the P atom, which results in a saturated (12 electron) gold center and thereby inhibits the formation of any classical, 2 e^- bonds between the gold and either of the bipy nitrogen atoms. The nitrogen atoms may instead donate a lone pair into an empty Au-P antibonding orbital, resulting in a three-center, four-electron (3c/4e) P-Au-N bond. The binuclear complex, [μ₂-bipy(AuPPh₃)₂](PF₆)₂, has also been prepared and shown to have an aurophillic interaction between the two gold ions, which are separated by 3.0747(3) Å. Despite the aurophillic interaction, this binuclear complex is not luminescent.

Near-unity thermally activated delayed fluorescence efficiency in three- and four-coordinate Au(i) complexes with diphosphine ligands

The synthesis and photoluminescence properties of three-coordinate Au(i) complexes with rigid diphosphine ligands LMe {1,2-bis[bis(2-methylphenyl)phosphino]benzene}, LEt {1,2-bis[bis(2-ethylphenyl)phosphino]benzene}, and LiPr {1,2-bis[bis(2-isopropylphenyl)phosphino]benzene} are investigated. The LMe and LEt ligands afford two types of complexes: dinuclear complexes [μ-LMe(AuCl)2] (1d) and [μ-LEt(AuCl)2] (2d) with an Au(i)-Au(i) bond and mononuclear three-coordinate Au(i) complexes LMeAuCl (1) and LEtAuCl (2). On the other hand, the bulkiest ligand, LiPr, affords three-coordinate Au(i) complexes, LiPrAuCl (3) and LiPrAuI (4), but no dinuclear complexes. X-ray analysis suggests that both 3 and 4 possess a highly distorted trigonal planar geometry. Moreover, luminescence data reveal that at room temperature, 3 and 4 exhibit yellow-green thermally activated delayed fluorescence in the crystalline state with maximum emission wavelengths at 558 and 549 nm, respectively. The emission yields are close to unity. Quantum chemical calculations suggest that the emission of 4 originates from the (σ + X) → π* excited state that possesses strong intraligand charge-transfer character. The luminescent properties of four-coordinate Au(i) complex (5) possessing a tetrahedral geometry are discussed on the basis of the emission spectra and decay times measured in a temperature range of 309-77 K.

Photoluminescence properties of TADF-emitting three-coordinate silver(i) halide complexes with diphosphine ligands: a comparison study with copper(i) complexes

A series of three- and four-coordinate silver(i) halide complexes exhibiting efficient blue thermally activated delayed fluorescence have been prepared.

Half-sandwich iridium N-heterocyclic carbene anticancer complexes

Half-sandwich iridium N-heterocyclic carbene complexes display potent anticancer activities and are attractive for development as new anticancer agents.

Blue Phosphorescent Zwitterionic Iridium(III) Complexes Featuring Weakly Coordinating nido-Carborane-Based Ligands

We report the development of a new class of phosphorescent zwitterionic bis(heteroleptic) Ir(III) compounds containing pyridyl ligands with weakly coordinating nido-carboranyl substituents. Treatment of phenylpyridine-based Ir(III) precursors with C-substituted ortho-carboranylpyridines in 2-ethoxyethanol results in a facile carborane deboronation and the formation of robust and highly luminescent metal complexes. The resulting nido-carboranyl fragments associate with the cationic Ir(III) center through primarily electrostatic interactions. These compounds phosphoresce at blue wavelengths (450-470 nm) both in a poly(methyl methacrylate) (PMMA) matrix and in solution at 77 K. These complexes display structural stability at temperatures beyond 300 °C and quantum yields greater than 40%. Importantly, the observed quantum yields correspond to a dramatic 10-fold enhancement over the previously reported Ir(III) congeners featuring carboranyl-containing ligands in which the boron cluster is covalently attached to the metal. Ultimately, this work suggests that the use of a ligand framework containing a weakly coordinating anionic component can provide a new avenue for designing efficient Ir(III)-based phosphorescent emitters.

Highly Efficient Catalysis of Retro-Claisen Reactions: From a Quinone Derivative to Functionalized Imidazolium Salts

A new and efficient method for the preparation of several imidazolium salts containing an ester group in the C4 position of the aromatic ring through a retro-Claisen reaction pathway between a quinone derivative and several alcohols is described. This new organic transformation proceeds in the absence of a catalyst, but it is greatly catalyzed by different Lewis acids, especially with AgOAc at a very low catalyst loading and in very short reaction times. The process takes place by the nucleophilic attack of the carbonyl groups by the alcohol functionality, thus promoting a double C-C bond cleavage and C-H and C-O bond formation. This reaction represents the first example of this type between a quinone derivative and alcohols.

A Bis(Diphosphanyl N-Heterocyclic Carbene) Gold Complex: A Synthon for Luminescent Rigid AuAg2 Arrays and Au5 and Cu6 Double Arrays

A mononuclear bis(NHC)/Au(I) (NHC=N-heterocyclic carbene) cationic complex with a rigid bis(phosphane)-functionalized NHC ligand (PC(NHC)P) was used to construct linear Au3 and Ag2 Au arrays, a Au5 cluster with two intersecting crosslike Au3 arrays, and an unprecedented Cu6 complex with two parallel Cu3 arrays. The impact of metallophilic interactions on photoluminescence was studied experimentally.

Stable luminescent iridium(iii) complexes with bis(N-heterocyclic carbene) ligands: photo-stability, excited state properties, visible-light-driven radical cyclization and CO2 reduction, and cellular imaging

Excited state properties, photo-catalysis and cellular imaging of photo-stable bis-NHC Ir(iii) complexes are described.

A new class of cyclometalated Ir(iii) complexes supported by various bidentate C-deprotonated (C^N) and cis-chelating bis(N-heterocyclic carbene) (bis-NHC) ligands has been synthesized. These complexes display strong emission in deaerated solutions at room temperature with photoluminescence quantum yields up to 89% and emission lifetimes up to 96 μs. A photo-stable complex containing C-deprotonated fluorenyl-substituted C^N shows no significant decomposition even upon irradiation for over 120 h by blue LEDs (12 W). These, together with the strong absorption in the visible region and rich photo-redox properties, allow the bis-NHC Ir(iii) complexes to act as good photo-catalysts for reductive C–C bond formation from C(sp³/sp²)–Br bonds cleavage using visible-light irradiation (λ > 440 nm). A water-soluble complex with a glucose-functionalized bis-NHC ligand catalysed a visible-light-driven radical cyclization for the synthesis of pyrrolidine in aqueous media. Also, the bis-NHC Ir(iii) complex in combination with a cobalt catalyst can catalyse the visible-light-driven CO₂ reduction with excellent turnover numbers (>2400) and selectivity (CO over H₂ in gas phase: >95%). Additionally, this series of bis-NHC Ir(iii) complexes are found to localize in and stain endoplasmic reticulum (ER) of various cell lines with high selectivity, and exhibit high cytotoxicity towards cancer cells, revealing their potential uses as bioimaging and/or anti-cancer agents.

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.