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

The Potential of Self-Activating Au(I) Complexes in Gold Catalysis

Gold catalysis has emerged as a groundbreaking field in synthetic chemistry, revolutionizing numerous organic transformations. Despite the significant achieved advancements, the mechanistic understanding behind many gold-catalyzed reactions remains elusive. This Concept article covers the so-called "self-activating" Au(I) complexes, sorting out their pivotal role in gold catalysis. We comment on how Au(I) complexes can undergo self-activation, triggering diverse catalytic transformations without the need for external additives. The most important examples reported so far that underlie the catalytic activity of these species are discussed. This intrinsic reactivity represents a paradigm shift in gold catalysis, offering new avenues for the design of efficient and sustainable catalytic systems. Furthermore, we explore the factors influencing the stability, reactivity, and selectivity of these Au(I) complexes, providing insights into their synthetic utility and potential applications. This area of research not only advances our fundamental understanding of gold catalysis but also paves the way for the development of novel catalytic strategies with broad implications in organic synthesis and the chemical industry.

Redox-Transmetalation Reactions: Easy Access to Homo- and Heterodimetallic d8,d10 Complexes

The ability of the imine PyCH═N-CH2Py (Py = 2-pyridyl, bpi) to behave as a heteroditopic ligand, which is suitable for creating two separate compartments to host metals in different oxidation states, has been developed by studying the reactions of the mixed-valence complexes [(cod)M-Ι(μ-bpi)MΙ(cod)] (M = Rh, Ir) with [M'(Cl)2(PPh3)2] (M' = Pd, Ni). The results depend on the molar ratio of the reagents used (1:1 or 1:2) to give the heterometallic complexes {d10-M',d8-M}-[(PPh3)(Cl)M'0(μ-bpi)MΙ(cod)] (Pd,Rh, 4; Pd,Ir, 5; Ni,Rh, 8; Ni,Ir, 9) and the two-electron mixed-valent compounds [(PPh3)(Cl)M'0(μ-bpi)M'ΙΙ(Cl)] (M' = Ni, 10; Pd, 11), respectively. A redox process occurs in the replacement of the low-valent [(cod)M-I] fragment, whereas the exchange of the [(cod)MI] fragment is redox-neutral. The metal with a d8 configuration in the products exhibits a square-planar geometry coordinated to two (Rh/Ir) or three (Ni/Pd) nitrogen atoms of the bridging bpi ligand. Conversely, the metal with a d10 configuration adopts trigonal-planar geometries, π-bonded to the imine C═N bond. The isolated complexes 4/5 and 10/11, along with the hypothetical heterometallic Pd,Ni compound (12), were studied by DFT methods. Additionally, the T-shaped moiety 'M'ΙΙ(PPh3)(Cl)(η1-CH-N(bpi))', stabilized by a secondary γ-agostic interaction, and the 'M'II(Cl)(κ3N-bpi)' fragment was found to be accessible redoxomers of complexes 10 and 11 by DFT calculations.

Anti-Markovnikov Intermolecular Hydroamination of Alkenes and Alkynes: A Mechanistic View

Hydroamination, the addition of an N-H bond across a C-C multiple bond, is a reaction with a great synthetic potential. Important advances have been made in the last decades concerning catalysis of these reactions. However, controlling the regioselectivity in the amine addition toward the formation of anti-Markovnikov products (addition to the less substituted carbon) still remains a challenge, particularly in intermolecular hydroaminations of alkenes and alkynes. The goal of this review is to collect the systems in which intermolecular hydroamination of terminal alkynes and alkenes with anti-Markovnikov regioselectivity has been achieved. The focus will be placed on the mechanistic aspects of such reactions, to discern the step at which regioselectivity is decided and to unravel the factors that favor the anti-Markovnikov regioselectivity. In addition to the processes entailing direct addition of the amine to the C-C multiple bond, alternative pathways, involving several reactions to accomplish anti-Markovnikov regioselectivity (formal hydroamination processes), will also be discussed in this review. The catalysts gathered embrace most of the metal groups of the Periodic Table. Finally, a section discussing radical-mediated and metal-free approaches, as well as heterogeneous catalyzed processes, is also included.

Gold(i) complexes bearing a PNP-type pincer ligand: photophysical properties and catalytic investigations

The synthesis and characterization of two dinuclear and five tetranuclear gold(i) complexes bearing the 2,6-bis(diphenylphosphinomethyl)pyridine diphosphane ligand (DPPMPY) are herein reported.

From Thioureas to Thioquinolines through Isolated Benzothiazines by Gold Catalysis

New benzothiazine heterocycles have been formed from thiourea derivatives by using different gold catalysts. The catalyst and the conditions were optimised towards the selective synthesis of six‐membered benzothiazine heterocycles, characterised by X‐ray diffraction. Interestingly, these organic compounds evolved under gold catalysis in basic medium to achieve the formation of amino thioquinolines through an unprecedented aromatisation process of the heterocycle. The reaction was also carried out stoichiometrically by reaction with gold complexes to afford thioquinolines coordinated to the gold fragment. Benzothiazine, amino thioquinoline heterocycles and gold‐derived species have a great potential for biological applications.

Dinuclear gold catalysis

This review summarizes the recent achievements of dinuclear gold-catalyzed redox coupling, asymmetric catalysis and photocatalysis. The dinuclear gold catalysts show a better catalytic performance than the mononuclear gold catalysts in certain cases.

Gold-Catalyzed Intermolecular Alkyne Hydrofunctionalizations—Mechanistic Insights

An overview of the current state of mechanistic understanding of gold-catalyzed intermolecular alkyne hydrofunctionalization reactions is presented. Moving from the analysis of the main features of the by-now-generally accepted reaction mechanism, studies and evidences pointing out the mechanistic peculiarities of these reactions using different nucleophiles HNu that add to the alkyne triple bond are presented and discussed. The effects of the nature of the employed alkyne substrate and of the gold catalyst (employed ligands, counteranions, gold oxidation state), of additional additives and of the reaction conditions are also considered. Aim of this work is to provide the reader with a detailed mechanistic knowledge of this important reaction class, which will be invaluable for rapidly developing and optimizing synthetic protocols involving a gold-catalyzed alkyne hydrofunctionalization as a reaction step.

Gold(I) Complexes Nuclearity in Constrained Ferrocenyl Diphosphines: Dramatic Effect in Gold-Catalyzed Enyne Cycloisomerization

Di-tert-butylated-bis(phosphino)ferrocene ligands bearing phosphino substituents R (R=phenyl, cyclohexyl, iso-propyl, mesityl, or furyl) allow tuning the selective formation of Au(I) halide complexes. Thus, dinuclear linear two-coordinate, but also rare mononuclear trigonal three-coordinate and tetrahedral four-coordinate complexes were formed upon tuning of the conditions. Both Au(I) chloride and rarer Au(I) iodide complexes were synthesized, and their X-ray diffraction analysis are reported. The significance of the control of structure and nuclearity in Au(I) complexes is further illustrated herein by its strong effect on the efficiency and selectivity of gold-catalysed cycloisomerization. Cationic linear digold(I) bis(dicyclohexylphosphino) ferrocenes outperform other catalysts in the demanding regioselective cycloisomerization of enyne sulphonamides into cyclohexadienes. Conversely, tetrahedral and trigonal cationic monogold(I) complexes were found incompetent for enyne cycloaddition. We used the two-coordinate linear electron-rich Au(I) complex 2 b (R=Cy) to extend the scope of selective intramolecular cycloaddition of different 1,6-enyne sulfonylamines with high activity and excellent selectivity to the endo cyclohexadiene products.

Structural Characterization and Lifetimes of Triple-Stranded Helical Coinage Metal Complexes: Synthesis, Spectroscopy and Quantum Chemical Calculations

This work reports on a series of polynuclear complexes containing a trinuclear Cu, Ag, or Au core in combination with the fac-isomer of the metalloligand [Ru(pypzH)3 ](PF6 )2 (pypzH=3-(pyridin-2-yl)pyrazole). These (in case of the Ag and Au containing species) newly synthesized compounds of the general formula [{Ru(pypz)3 }2 M3 ](PF6 ) (2: M=Cu; 3: M=Ag; 4: M=Au) contain triple-stranded helical structures in which two ruthenium moieties are connected by three N-M-N (M=Cu, Ag, Au) bridges. In order to obtain a detailed description of the structure both in the electronic ground and excited states, extensive spectroscopic and quantum chemical calculations are applied. The equilateral coinage metal core triangle in the electronic ground state of 2-4 is distorted in the triplet state. Furthermore, the analyses offer a detailed description of electronic excitations. By using time-resolved IR spectroscopy from the microsecond down to the nanosecond regime, both the vibrational spectra and the lifetime of the lowest lying electronically excited triplet state can be determined. The lifetimes of these almost only non-radiative triplet states of 2-4 show an unusual effect in a way that the Au-containing complex 4 has a lifetime which is by more than a factor of five longer than in case of the Cu complex 2. Thus, the coinage metals have a significant effect on the electronically excited state, which is localized on a pypz ligand coordinated to the Ru atom indicating an unusual cooperative effect between two moieties of the complex.

Mild and Efficient Synthesis of Diverse Organo-AuI -L Complexes in Green Solvents

An exceptionally mild and efficient method was developed for the preparation of (hetero)aryl-Au^I -L complexes using ethanol or water as the reaction medium at room temperature and Ar-B(triol)K boronates as the transmetalation partner. The reaction does not need an exogeneous base or other additives, and quantitative yields can be achieved through a simple filtration as the only required purification method, which obviates considerable waste associated with alternative workup methods. A broad reaction scope was demonstrated with respect to both the L and (hetero)aryl ligands on product Au complexes. Despite the polar reaction medium, large polycyclic aromatic hydrocarbon units can be incorporated on the Au complexes in very good to excellent yields. The approach was demonstrated for the chemoselective manipulation of orthogonally protected aryl boronates to afford a new class of N-heterocyclic carbene-Au-aryl complexes. A mechanistic rationale was proposed.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

Monoaurated vs. diaurated intermediates: causality or independence?

Diaurated intermediates of gold-catalysed reactions have been a long-standing subject of debate. Although diaurated complexes were regarded as a drain of active monoaurated intermediates in catalytic cycles, they were also identified as the products of gold-gold cooperation in dual-activation reactions. This study shows investigation of intermediates in water addition to alkynes catalysed by [(IPr)Au(CH3CN)(BF4)]. Electrospray ionisation mass spectrometry (ESI-MS) allowed us to detect both monoaurated and diaurated complexes in this reaction. Infrared photodissociation spectra of the trapped complexes show that the structure of the intermediates corresponds to α-gold ketone intermediates protonated or aurated at the oxygen atom. Delayed reactant labelling experiments provided the half life of the intermediates in reaction of 1-phenylpropyne (∼7 min) and the kinetic isotope effects for hydrogen introduction to the carbon atom (KIE ∼ 4-6) and for the protodeauration (KIE ∼ 2). The results suggest that the ESI-MS detected monoaurated and diaurated complexes report on species with a very similar or the same kinetics in solution. Kinetic analysis of the overall reaction showed that the reaction rate is first-order dependent on the concentration of the gold catalyst. Finally, all results are consistent with the reaction mechanism proceeding via monoaurated neutral α-gold ketone intermediates only.

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.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

Location matters: cooperativity of catalytic partners in porous organic polymers for enhanced CO2 transformation

Functionalities with corrected inter-site distance in porous materials enable them to work in a concerted manner.

Nickel-Alkyl Complexes with a Reactive PNC-Pincer Ligand

Based on previous work related to the design and application of rigid tridentate phosphine-pyridine-phenyl coordination offered by a PNC-pincer ligand upon cyclometalation to nickel, the synthesis, spectroscopic and solid state characterization and redox-reactivity of two NiII(PNC) complexes featuring either a methyl (2CH3 ) or CF3 co-ligand (2CF3 ) are described. One-electron oxidation is proposed to furnish C-C reductive elimination, as deduced from a combined chemical, electrochemical, spectroscopic and computational study. One-electron reduction results in a ligand-centered radical anion, as supported by electrochemistry, UV spectroelectrochemistry, EPR spectroscopy, and DFT calculations. This further attenuates the breadth of chemical reactivity offered by such PNC-pincer ligands.

N-Atom transfer via thermal or photolytic activation of a Co-azido complex with a PNP pincer ligand

Thermal or photolytic activation of well-defined mononuclear [Co(N₃)(PNP)] (PNP = 2,2'-bis(diisopropylphosphino)-4,4'-ditolylamido) results in the structurally characterized dinuclear species [Co(μ-N;κ³-P,N,N-PN^PN)]₂ (3), with two N-bridging phosphiniminato bridgeheads. Density Functional Theory (DFT) calculations indicate the intermediacy of a mononuclear cobalt-nitrido complex, followed by N-migratory insertion into a Co-P bond. Reaction of 3 with two equiv. HCl leads to rupture of the dimer with formation of mononuclear [CoCl(PN^PN^H)] (4) by protonation of the N-bridges.

Bimetallic gold(i) complexes of photoswitchable phosphines: synthesis and uses in cooperative catalysis

The first photoswitchable bimetallic gold catalysts based on an azobenzene backbone have been synthesized and their catalytic properties have been investigated.

Preorganization and Cooperation for Highly Efficient and Reversible Capture of Low-Concentration CO2 by Ionic Liquids

A novel strategy based on the concept of preorganization and cooperation has been designed for a superior capacity to capture low-concentration CO₂ by imide-based ionic liquids. By using this strategy, for the first time, an extremely high gravimetric CO₂ capacity of up to 22 wt % (1.65 mol mol^-1 ) and excellent reversibility (16 cycles) have been achieved from 10 vol. % of CO₂ in N₂ when using an ionic liquid having a preorganized anion. Through a combination of quantum-chemical calculations and spectroscopic investigations, it is suggested that cooperative interactions between CO₂ and multiple active sites in the preorganized anion are the driving force for the superior CO₂ capacity and excellent reversibility.

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.

Fluorescence resonance energy transfer (FRET) for the verification of dual gold catalysis

Two gold complexes with different bodipy-tagged N-heterocyclic carbene ligands, which are a potential FRET pair, were synthesized. It was shown, that the formation of dinuclear intermediates in alkyne transformations (“dual gold catalysis”) are characterized by a FRET signal.

Why can a gold salt react as a base?

A new mass-spectrometry method allows monitoring the key role of the gold–gold interaction in the transformation of gold salts to bases.