Gold(I)-N-Heterocyclic Carbene Synthons in Organometallic Synthesis

The prominent role of gold-N-heterocyclic carbene (NHC) complexes in numerous research areas such as homogeneous (photo)catalysis, medicinal chemistry and materials science has prompted organometallic chemists to design gold-based synthons that permit access to target complexes through simple synthetic steps under mild conditions. In this review, the main gold-NHC synthons employed in organometallic synthesis are discussed. Mechanistic aspects involved in their synthesis and reactivity as well as applications of gold-NHC synthons as efficient pre-catalysts, antitumor agents and/or photo-emissive materials are presented.

Terminal Au-N and Au-O Units in Organometallic Frames

Since gold is located well beyond the oxo wall, chemical species with terminal Au-N and Au-O units are extremely rare and limited to low coordination numbers. We report here that these unusual units can be trapped within a suitable organometallic frame. Thus, the terminal auronitrene and auroxyl derivatives [(CF3 )3 AuN]- and [(CF3 )3 AuO]- were identified as local minima by calculation. These open-shell, high-energy ions were experimentally detected by tandem mass spectrometry (MS2 ): They respectively arise by N2 or NO2 dissociation from the corresponding precursor species [(CF3 )3 Au(N3 )]- and [(CF3 )3 Au(ONO2 )]- in the gas phase. Together with the known fluoride derivative [(CF3 )3 AuF]- , they form an interesting series of isoleptic and alloelectronic complexes of the highly acidic organogold(iii) moiety (CF3 )3 Au with singly charged anions X- of the most electronegative elements (X=F, O, N). Ligand-field inversion in all these [(CF3 )3 AuX]- species results in the localization of unpaired electrons at the N and O atoms.

Spectroscopic Manifestations and Implications for Catalysis of Quasi-d10 Configurations in Formal Gold(III) Complexes

Several gold +I and +III complexes are investigated computationally and spectroscopically, focusing on the d-configuration and physical oxidation state of the metal center. Density functional theory calculations reveal the non-negligible electron-sharing covalent character of the metal-to-ligand σ-bonding framework. The bonding of gold(III) is shown to be isoelectronic to the formal CuIII complex [Cu(CF3 )4 ]1- , in which the metal center tries to populate its formally unoccupied 3dx2-y2 orbital via σ-bonding, leading to a reduced d10 CuI description. However, Au L3 -edge X-ray absorption spectroscopy reveals excitation into the d-orbital of the AuIII species is still possible, showing that a genuine d10 configuration is not achieved. We also find an increased electron-sharing nature of the σ-bonds in the AuI species, relative to their AgI and CuI analogues, due to the low-lying 6s orbital. We propose that gold +I and +III complexes form similar bonds with substrates, owing primarily to participation of the 5dx2-y2 or 6s orbital, respectively, in bonding, indicating why AuI and AuIII complexes often have similar reactivity.

Transition-metal difluorocarbene complexes

Although transition metal carbenes have found widespread applications and difluorocarbene has served as a versatile intermediate, it is still quite challenging to make use of transition-metal difluorocarbenes in synthetic chemistry due to their unpredictable reactivities. In this Highlight, we review recent developments in the transition-metal-catalyzed or -mediated transfer of difluorocarbene and the reactivies and conversions of transition-metal difluorocarbene complexes. We start with the MCF₂ bonding, then provide the progress in the transfer of difluorocarbene, and finally briefly discuss the conversions of MCF₂ into other metal complexes. The understanding of the interesting reactivities of MCF₂ may help design the catalytic transfer of difluorocarbene for various reactions.

Stability of AgIII towards Halides in Organosilver(III) Complexes

The involvement of silver in two-electron AgI /AgIII processes is currently emerging. However, the range of stability of the required and uncommon AgIII species is virtually unknown. Here, the stability of AgIII towards the whole set of halide ligands in the organosilver(III) complex frame [(CF3 )3 AgX]- (X=F, Cl, Br, I, At) is theoretically analyzed. The results obtained depend on a single factor: the nature of X. Even the softest and least electronegative halides (I and At) are found to form reasonably stable AgIII -X bonds. Our estimates were confirmed by experiment. The whole series of nonradiative halide complexes [PPh4 ][(CF3 )3 AgX] (X=F, Cl, Br, I) has been experimentally prepared and all its constituents have been isolated in pure form. The pseudohalides [PPh4 ][(CF3 )3 AgCN] and [PPh4 ][(CF3 )3 Ag(N3 )] have also been isolated, the latter being the first silver(III) azido complex. Except for the iodo compound, all the crystal and molecular structures have been established by single-crystal X-ray diffraction methods. The decomposition paths of the [(CF3 )3 AgX]- entities at the unimolecular level have been examined in the gas phase by multistage mass spectrometry (MSn ). The experimental detection of the two series of mixed complexes [CF3 AgX]- and [FAgX]- arising from the corresponding parent species [(CF3 )3 AgX]- demonstrate that the Ag-X bond is particularly robust. Our experimental observations are rationalized with the aid of theoretical methods. Smooth variation with the electronegativity of X is also observed in the thermolyses of bulk samples. The thermal stability in the solid state gradually decreases from X=F (145 °C, dec.) to X=I (78 °C, dec.) The experimentally established compatibility of AgIII with the heaviest halides is of particular relevance to silver-mediated or silver-catalyzed processes.

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.

Synthesis of Gold(I)-Trifluoromethyl Complexes and their Role in Generating Spectroscopic Evidence for a Gold(I)-Difluorocarbene Species

Readily prepared and bench-stable [Au(CF₃ )(NHC)] compounds were synthesized by using new methods, starting from [Au(OH)(NHC)], [Au(Cl)(NHC)] or [Au(L)(NHC)]HF₂ precursors (NHC=N-heterocyclic carbene). The mechanism of formation of these species was investigated. Consequently, a new and straightforward strategy for the mild and selective cleavage of a single carbon/fluorine bond from [Au(CF₃ )(NHC)] complexes was attempted and found to be reversible in the presence of an additional nucleophilic fluoride source. This straightforward technique has led to the unprecedented spectroscopic observation of a gold(I)-NHC difluorocarbene species.

Trifluoromethylation of [AuF3 (SIMes)]: Preparation and Characterization of [Au(CF3 )x F3-x (SIMes)] (x=1-3) Complexes

Trifluoromethylation of [AuF3 (SIMes)] with the Ruppert-Prakash reagent TMSCF3 in the presence of CsF yields the product series [Au(CF3 )x F3-x (SIMes)] (x=1-3). The degree of trifluoromethylation is solvent dependent and the ratio of the species can be controlled by varying the stoichiometry of the reaction, as evidenced from the 19 F NMR spectra of the corresponding reaction mixtures. The molecular structures in the solid state of trans-[Au(CF3 )F2 (SIMes)] and [Au(CF3 )3 (SIMes)] are presented, together with a selective route for the synthesis of the latter complex. Correlation of the calculated SIMes affinity with the carbene carbon chemical shift in the 13 C NMR spectrum reveals that trans-[Au(CF3 )F2 (SIMes)] and [Au(CF3 )3 (SIMes)] nicely follow the trend in Lewis acidities of related organo gold(III) complexes. Furthermore, a new correlation between the Au-Ccarbene bond length of the molecular structure in the solid state and the chemical shift of the carbene carbon in the 13 C NMR spectrum is presented.

An Organogold(III) Difluoride with a trans Arrangement

The trans isomer of the organogold(III) difluoride complex [PPh₄ ][(CF₃ )₂ AuF₂ ] has been obtained in a stereoselective way and in excellent yield by reaction of [PPh₄ ][CF₃ AuCF₃ ] with XeF₂ under mild conditions. The compound is both thermally stable and reactive. Thus, the fluoride ligands are stereospecifically replaced by any heavier halide or by cyanide, the cyanide affording [PPh₄ ][trans-(CF₃ )₂ Au(CN)₂ ]. The organogold fluoride complexes [CF₃ AuF_x ]^- (x=1, 2, 3) have been experimentally detected to arise upon collision-induced dissociation of the [trans-(CF₃ )₂ AuF₂ ]^- anion in the gas phase. Their structures have been calculated by DFT methods. In the isomeric forms identified for the open-shell species [CF₃ AuF₂ ]^- , the spin density residing on the metal center is found to strongly depend on the precise stereochemistry. Based on crystallographic evidence, it is concluded that Auⁱⁱⁱ and Agⁱⁱⁱ have similar covalent radii, at least in their most common square-planar geometry.

Tuning the Lewis acidity of difluorido gold(iii) complexes: the synthesis of [AuClF2(SIMes)] and [AuF2(OTeF5)(SIMes)]

A route to [Au^IIIClF₂(SIMes)] and [Au^IIIF₂(OTeF₅)(SIMes)] including structural characterization to tune the ligand arrangement and Lewis acidity of gold(iii) centres was developed.

Taming the High Reactivity of Gold(III) Fluoride: Fluorido Gold(III) Complexes with N-Based Ligands

The synthesis of [NMe₄ ][AuF₄ ] and [NEt₄ ][AuF₄ ], as well as an improved one-pot synthesis of Cs[AuF₄ ], is reported. The new [AuF₄ ]^- salts were structurally characterized by single crystal X-ray diffraction, NMR spectroscopy, vibrational spectroscopy, and mass spectrometry. These salts are the first gold(III) fluoride salts that can be isolated in pure form and are convenient to be used in usual organic solvents for subsequent synthesis. The formation of molecular AuF₃ complexes in solution is further reported, characterized as [F₃ Au(NCCH₃ )] at low temperature, as [F₃ Au(py)] and a binuclear derivative which are stable at room temperature. The stability of these species in common organic solvents was investigated and they showed a satisfying stability.