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

An extremely electron poor Au(III) trication bearing acetonitrile ligands

The synthesis and structural characterization of an electron poor Au(III) trication bearing 2 imidazole and 2 acetonitrile ligands is described. The new complex is capable of aryl C-H metalation with the formation of a monomesitylene complex and also demonstrated to be highly oxidizing in the rapid room temperature conversion of cyclohexene to benzene.

Ligand-Enabled Oxidative Fluorination of Gold(I) and Light-Induced Aryl-F Coupling at Gold(III)

MeDalphos Au(I) complexes featuring aryl, alkynyl, and alkyl groups readily react with electrophilic fluorinating reagents such as N-fluorobenzenesulfonimide and Selectfluor. The ensuing [(MeDalphos)Au(R)F]+ complexes have been isolated and characterized by multinuclear NMR spectroscopy as well as X-ray diffraction. They adopt a square-planar contra-thermodynamic structure, with F trans to N. DFT/IBO calculations show that the N lone pair of MeDalphos assists and directs the transfer of F+ to gold. The [(MeDalphos)Au(Ar)F]+ (Ar = Mes, 2,6-F2Ph) complexes smoothly engage in C-C cross-coupling with PhCCSiMe3 and Me3SiCN, providing direct evidence for the oxidative fluorination/transmetalation/reductive elimination sequence proposed for F+-promoted gold-catalyzed transformations. Moreover, direct reductive elimination to forge a C-F bond at Au(III) was explored and substantiated. Thermal means proved unsuccessful, leading mostly to decomposition, but irradiation with UV-visible light enabled efficient promotion of aryl-F coupling (up to 90% yield). The light-induced reductive elimination proceeds under mild conditions; it works even with the electron-deprived 2,6-difluorophenyl group, and it is not limited to the contra-thermodynamic form of the aryl Au(III) fluoride complexes.

Cross-Coupling Reactions Enabled by Well-Defined Ag(III) Compounds: Main Focus on Aromatic Fluorination and Trifluoromethylation

AgIII compounds are considered strong oxidizers of difficult handling. Accordingly, the involvement of Ag catalysts in cross-coupling via 2e- redox sequences is frequently discarded. Nevertheless, organosilver(III) compounds have been authenticated using tetradentate macrocycles or perfluorinated groups as supporting ligands, and since 2014, first examples of cross-coupling enabled by AgI /AgIII redox cycles saw light. This review collects the most relevant contributions to this field, with main focus on aromatic fluorination/perfluoroalkylation and the identification of AgIII key intermediates. Pertinent comparison between the activity of AgIII RF compounds in aryl-F and aryl-CF3 couplings vs. the one shown by its CuIII RF and AuIII RF congeners is herein disclosed, thus providing a more profound picture on the scope of these transformations and the pathways commonly associated to C-RF bond formations enabled by coinage metals.

Reductive Elimination Reactions in Gold(III) Complexes Leading to C(sp3)-X (X = C, N, P, O, Halogen) Bond Formation: Inner-Sphere vs SN2 Pathways

The reactions leading to the formation of C-heteroatom bonds in the coordination sphere of Au(III) complexes are uncommon, and their mechanisms are not well known. This work reports on the synthesis and reductive elimination reactions of a series of Au(III) methyl complexes containing different Au-heteroatom bonds. Complexes [Au(CF₃)(Me)(X)(PR₃)] (R = Ph, X = OTf, OClO₃, ONO₂, OC(O)CF₃, F, Cl, Br; R = Cy, X = Me, OTf, Br) were obtained by the reaction of trans-[Au(CF₃)(Me)₂(PR₃)] (R = Ph, Cy) with HX. The cationic complex cis-[Au(CF₃)(Me)(PPh₃)₂]OTf was obtained by the reaction of [Au(CF₃)(Me)(OTf)(PPh₃)] with PPh₃. Heating these complexes led to the reductive elimination of MeX (X = Me, Ph₃P⁺, OTf, OClO₃, ONO₂, OC(O)CF₃, F, Cl, Br). Mechanistic studies indicate that these reductive elimination reactions occur either through (a) the formation of tricoordinate intermediates by phosphine dissociation, followed by reductive elimination of MeX, or (b) the attack of weakly coordinating anionic (TfO^- or ClO₄^-) or neutral nucleophiles (PPh₃ or NEt₃) to the Au-bound methyl carbon. The obtained results show for the first time that the nucleophilic substitution should be considered as a likely reductive elimination pathway in Au(III) alkyl complexes.

Gold Teflates Revisited: From the Lewis Superacid [Au(OTeF5 )3 ] to the Anion [Au(OTeF5 )4 ]. Chemistry 2023;29:e202203634. [PMID: 36598847 DOI: 10.1002/chem.202203634] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

A new synthetic access to the Lewis acid [Au(OTeF₅ )₃ ] and the preparation of the related, unprecedented anion [Au(OTeF₅ )₄ ]^- with inorganic or organic cations starting from commercially available and easy-to-handle gold chlorides are presented. In this first extensive study of the Lewis acidity of a transition-metal teflate complex by using different experimental and quantum chemical methods, [Au(OTeF₅ )₃ ] was classified as a Lewis superacid. The solid-state structure of the triphenylphosphine oxide adduct [Au(OPPh₃ )(OTeF₅ )₃ ] was determined, representing the first structural characterization of an adduct of this highly reactive [Au(OTeF₅ )₃ ]. Therein, the coordination environment around the gold center slightly deviates from the typical square planar geometry. The [Au(OTeF₅ )₄ ]^- anion shows a similar coordination motif.

Conversion of a AuI Fluorido Complex into an N-Fluoroamido Derivative: N-F versus Au-N Reactivity

The Au^I complex [Au{N(F)SO₂ Ph}(SPhos)] (SPhos=dicyclohexyl(2',6'-dimethoxy[1,1'-biphenyl]-2-yl)phosphane) (2) bearing a fluoroamido ligand has been synthesized by reaction of the fluorido complex [Au(F)(SPhos)] (1) with NFSI (NFSI=N-fluorobenzenesulfonimide). A reaction with CO resulted in an unprecedented insertion into the N-F bond at 2. With the carbene precursor N₂ CH(CO₂ Et) N-F bond cleavage gave the Au-F bond insertion product [Au{CHF(CO₂ C₂ H₅ )}(SPhos)] (7). The presence of CNtBu led to Au-N cleavage at 2 and concomitant amide formation to give the cationic complex [Au(CNtBu)(SPhos)][N(F)SO₂ Ph)] (5), which reacted further to give FtBu as well as the cyanido complex [Au(CN)(SPhos)] (6). These results led to the development of a process for the amination of electrophilic organic substrates by transfer of the fluoroamido group NF(SO₂ Ph)^- .

Crystal Growth from Anhydrous HF Solutions of M2+ (M = Ca, Sr, Ba) and [AuF6]-, Not Only Simple M(AuF6)2 Salts

Crystal growth from anhydrous HF solutions of M²⁺ (M = Ca, Sr, Ba) and [AuF₆]⁻ (molar ratio 1:2) gave [Ca(HF)₂](AuF₆)₂, [Sr(HF)](AuF₆)₂, and Ba[Ba(HF)]₆(AuF₆)₁₄. [Ca(HF)₂](AuF₆)₂ exhibits a layered structure in which [Ca(HF)₂]²⁺ cations are connected by AuF₆ units, while the crystal structure of Ba[Ba(HF)]₆(AuF₆)₁₄ exhibits a complex three-dimensional (3-D) network consisting of Ba²⁺ and [Ba(HF)₂]²⁺ cations bridged by AuF₆ groups. These results indicate that the previously reported M(AuF₆)₂ (M = Ca, Sr, Ba) compounds, prepared in the anhydrous HF, do not in fact correspond to this chemical formula. When the initial M²⁺/[AuF₆]⁻ ratio was 1:1, single crystals of [M(HF)](H₃F₄)(AuF₆) were grown for M = Sr. The crystal structure consists of a 3-D framework formed by [Sr(HF)]²⁺ cations associated with [AuF₆]⁻ and [H₃F₄]⁻ anions. The latter exhibits a Z-shaped conformation, which has not been observed before. Single crystals of M(BF₄)(AuF₆) (M = Sr, Ba) were grown when a small amount of BF₃ was present during crystallization. Sr(BF₄)(AuF₆) crystallizes in two modifications. A high-temperature α-phase (293 K) crystallized in an orthorhombic unit cell, and a low-temperature β-phase (150 K) crystallized in a monoclinic unit cell. For Ba(BF₄)(AuF₆), only an orthorhombic modification was observed in the range 80–230 K. An attempt to grow crystals of Ca(BF₄)(AuF₆) failed. Instead, crystals of [Ca(HF)](BF₄)₂ were grown and the crystal structure was determined. During prolonged crystallization of [AuF]₆^– salts, moisture can penetrate through the walls of the crystallization vessel. This can lead to partial reduction of Au(V) to A(III) and the formation of [AuF₄]⁻ byproducts, as shown by the single-crystal growth of [Ba(HF)]₄(AuF₄)(AuF₆)₇. Its crystal structure consists of [Ba(HF)]²⁺ cations connected by AuF₆ octahedra and square-planar AuF₄ units. The crystal structure of the minor product [O₂]₂[Sr(HF)]₅[AuF₆]₁₂·HF was also determined.

Crystal growth from anhydrous HF solutions of M²⁺ (M = Ca, Sr, Ba) and [AuF₆]⁻ (molar ratio 1:2) gave [Ca(HF)₂](AuF₆)₂ and Ba[Ba(HF)]₆(AuF₆)₁₄. These results indicate that the previously reported M(AuF₆)₂ (M = Ca, Sr, Ba) compounds do not correspond to this chemical formula. When the initial Sr²⁺/[AuF₆]⁻ ratio was 1:1, crystals of [Sr(HF)](H₃F₄)(AuF₆) were grown. Single crystals of M(BF₄)(AuF₆) (M = Sr, Ba) were obtained when a small amount of BF₃ was present during crystallization. The crystal structures of [Ba(HF)]₄(AuF₄)(AuF₆)₇, [O₂]₂[Sr(HF)]₅[AuF₆]₁₂·HF, and Ca(BF₄)(AuF₆) byproducts were also determined.

Synthesis, Reactivity, and Bonding of Gold(I) Fluorido-Phosphine Complexes

Gold(I) fluorido complexes with phosphine ligands have been synthesized from their respective iodido precursors. The bonding situation in comparison between complexes bearing phosphines and N-heterocyclic carbenes (NHCs) was explored quantum-chemically, obtaining similar results for both. Calculations of the ¹⁹F NMR chemical shifts match the experimental values well, including the approximately 40 ppm low-field shifts for the phosphine complexes compared to the NHC complexes, in spite of similar negative charges on fluorine. The reactivity of the highly water-sensitive gold(I) fluorido complexes was studied, resulting in substitution at the metal using trimethylsilyl reagents. The compounds studied were characterized using NMR as well as X-ray diffraction methods.

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 and Characterization of Bidentate (P^N)Gold(III) Fluoride Complexes: Reactivity Platforms for Reductive Elimination Studies

A new family of cationic, bidentate (P^N)gold(III) fluoride complexes has been prepared and a detailed characterization of the gold-fluoride bond has been carried out. Our results correlate with the observed reactivity of the fluoro ligand, which undergoes facile exchange with both cyano and acetylene nucleophiles. The resulting (P^N)arylgold(III)C(sp) complexes have enabled the first study of reductive elimination on (P^N)gold(III) systems, which demonstrated that C(sp² )-C(sp) bond formation occurs at higher rates than those reported for analogous phosphine-based monodentate systems.

Synthesis and Photophysical Properties of T-Shaped Coinage-Metal Complexes

The photophysical properties of a series of T‐shaped coinage d¹⁰ metal complexes, supported by a bis(mesoionic carbene)carbazolide (CNC) pincer ligand, are explored. The series includes a rare new example of a tridentate T‐shaped Ag^I complex. Post‐complexation modification of the Au^I complex provides access to a linear cationic Au^I complex following ligand alkylation, or the first example of a cationic square planar Au^III−F complex from electrophilic attack on the metal centre. Emissions ranging from blue (Cu^I) to orange (Ag^I) are obtained, with variable contributions of thermally‐dependent fluorescence and phosphorescence to the observed photoluminescence. Green emissions are observed for all three gold complexes (neutral T‐shaped Au^I, cationic linear Au^I and square planar cationic Au^III). The higher quantum yield and longer decay lifetime of the linear gold(I) complex are indicative of increased phosphorescence contribution.

Unexpected persistence of cis-bridged chains in compressed AuF3

Raman scattering measurements indicate that cis-bridged chains are retained in AuF3 even at a compression of 45 GPa - in contrast to meta-GGA calculations suggesting that structures with such motifs are thermodynamically unstable above 4 GPa. This metastability implies that novel gold fluorides (e.g. AuF2) might be attainable at lower pressures than previously proposed.

Investigation of Molecular Alkali Tetrafluorido Aurates by Matrix‐Isolation Spectroscopy

Molecular alkali tetrafluorido aurate ion pairs M[AuF₄] (M=K, Rb, Cs) are produced by co‐deposition of IR laser‐ablated AuF₃ and MF in solid neon under cryogenic conditions. This method also yields molecular AuF₃ and its dimer Au₂F₆. The products are characterized by their Au–F stretching bands and high‐level quantum‐chemical calculations at the CCSD(T)/triple‐ζ level of theory. Structural changes in AuF₄⁻ associated with the coordination of the anion to different alkali cations are proven spectroscopically and discussed.

Theoretical Search for the Highest Valence States of the Coinage Metals: Roentgenium Heptafluoride May Exist

We present here a relativistic density functional theory investigation of the penta- and heptavalent states of gold and roentgenium, employing the ZORA (zeroth order regular approximation to the Dirac equation) Hamiltonian, including spin-orbit coupling at the two-component level, and large all-electron relativistic Slater-type quadruple-ζ quadruple polarization (ZORA-STO-QZ4P) basis sets. Unsurprisingly, our calculations confirm the stability of the experimentally known complexes AuF₆^- and Au₂F₁₀ with respect to decomposition to trivalent Au products and F₂. The calculations also predict that RgF₆^- and Rg₂F₁₀ should be even more stable with respect to an analogous decomposition pathway. Like an earlier DFT study ( Inorg. Chem. 2007, 46 (13), 5338-5342), our calculations rule out the true heptavalent Au complex AuF₇ as a stable species, preferring instead a C_s AuF₅···F₂ formulation. Remarkably, our calculations confirm a D_{5 h} pentagonal-bipyramidal structure of RgF₇ as the global minimum, at an energy of approximately half an electron volt below the RgF₅···F₂ form.

Stabilization of Lewis Acidic AuF3 as an N-Heterocyclic Carbene Complex: Preparation and Characterization of [AuF3 (SIMes)]

Two different reaction routes are described to access the unprecedented trifluoridoorganogold(III) complex [AuF₃ (SIMes)]. The compound bears the N-heterocyclic carbene SIMes (1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) as a ligand for a molecular Lewis acidic AuF₃ unit and was characterized by NMR spectroscopy as well as X-ray crystallography. Apart from the use of a [AuF₄ ]^- salt as precursor, the strong oxidizing compound AuF₃ can be employed neat as starting material. The reaction proceeded even in organic solvents in the presence of SIMes as the ligand precursor. Decomposition reactions with the solvent can, therefore, be prevented by using this strategy.

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.

Well defined difluorogold(iii) complexes supported by N-ligands

Rare difluorogold(iii) complexes supported by N-ligands can be synthesized simply either from Au(i) precursors using XeF₂ or Au(iii) using KF.

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.