Isolation of a Reactive Tricoordinate α‐Oxo Gold Carbene Complex

The [(P,P)Au=C(Ph)CO₂Et]⁺ complex 3 [where (P,P) is an o‐carboranyl diphosphine ligand] was prepared by diazo decomposition at ?40 °C. It is the first α‐oxo gold carbene complex to be characterized. Its crystallographic structure was determined and DFT calculations have been performed, unraveling the key influence of the chelating (P,P) ligand. The gold center is tricoordinate and the electrophilicity of the carbene center is decreased. Complex 3 mimics transient α‐oxo gold carbenes in a series of catalytic transformations, and provides support for the critical role of electrophilicity in the chemoselectivity of phenol functionalization (O?H vs. C?H insertion).     

Synthesis and Characterization of a Gold Vinylidene Complex Lacking π‐Conjugated Heteroatoms

Hydride abstraction from the gold (disilyl)ethylacetylide complex [( P )Au{η¹‐C?CSi(Me)₂CH₂CH₂SiMe₂H}] ( P =P(tBu)₂o‐biphenyl) with triphenylcarbenium tetrakis(pentafluorophenyl)borate at ?20 °C formed the cationic gold (β,β‐disilyl)vinylidene complex [( P )Au?C?CSi(Me)₂CH₂CH₂Si (Me)₂]⁺B(C₆F₅)₄^? with ≥90 % selectivity. ²⁹Si NMR analysis of this complex pointed to delocalization of positive charge onto both the β‐silyl groups and the ( P )Au fragment. The C1 and C2 carbon atoms of the vinylidene complex underwent facile interconversion (ΔG^≠=9.7 kcal mol^?1), presumably via the gold π‐disilacyclohexyne intermediate [( P )Au{η²‐C?CSi(Me)₂CH₂CH₂Si (Me)₂}]⁺B(C₆F₅)₄^?.     

π‐Ruthenium Complexes of Hexaphyrins(1.1.1.1.1.1): A Triple‐Decker Complex Bearing Two Ruthenoarene Units

Ruthenium(II) π‐coordination onto [28]hexaphyrins(1.1.1.1.1.1) has been accomplished. Reactions of bis‐Au^III and mono‐Au^III complexes of hexakis(pentafluorophenyl) [28]hexaphyrin with [RuCl₂(p‐cymene)]₂ in the presence of NaOAc gave the corresponding π‐ruthenium complexes, in which the [(p‐cymene)Ru]^II fragment sat on the deprotonated side pyrrole. A similar reaction of the bis‐Pd^II [26]hexaphyrin complex afforded a triple‐decker complex, in which the two [(p‐cymene)Ru]^II fragments sat on both sides of the center of the [26]hexaphyrin framework.     

Hydride Transfer to Gold: Yes or No? Exploring the Unexpected Versatility of Au⋅⋅⋅H−M Bonding in Heterobimetallic Dihydrides

The potential for coordination and H-transfer from Cp₂MH₂ (M=Zr, W) to gold(I) and gold(III) complexes was explored in a combined experimental and computational study. [(L)Au]⁺ cations react with Cp₂WH₂ giving [(L)Au(κ²-H₂WCp₂)]⁺ (L=IPr ( 1 ), cyclic (alkyl)(amino)carbene ( 2 ), PPh₃ ( 3 ) and Dalphos-Me ( 4 ) [IPr=1,3-bis(diisopropylphenyl)imidazolylidene; Dalphos-Me=di(1-adamantyl)-2-(dimethylamino)phenyl-phosphine], while [Au(DMAP)₂]⁺ (DMAP=p-dimethylaminopyridine) affords the C₂-symmetric [Au(κ-H₂WCp₂)₂]⁺ ( 5 ). The Dalphos complex 4 can be protonated to give the bicationic adduct 4 H, showing Au^I⋅⋅⋅H⁺−N hydrogen bonding. The gold(III) Lewis acid [(C^N−CH)Au(C₆F₅)(OEt₂)]⁺ binds Cp₂WH₂ to give an Au-H-W σ-complex. By contrast, the pincer species [(C^N^C)Au]⁺ adds Cp₂WH₂ by a purely dative W→Au bond, without Au⋅⋅⋅H interaction. The biphenylyl-based chelate [(C^C)Au]⁺ forms [(C^C)Au(μ-H)₂WCp₂]⁺, with two 2-electron-3-centre W−H⋅⋅⋅Au interactions and practically no Au−W donor acceptor contribution. In all these complexes, strong but polarized W−H bonds are maintained, without H-transfer to gold. On the other hand, the reactions of Cp₂ZrH₂ with gold complexes led in all cases to rapid H-transfer and formation of gold hydrides. Relativistic DFT calculations were used to rationalize the striking reactivity and bonding differences in these heterobimetallic hydride complexes along with an analysis of their characteristic NMR parameters and UV/Vis absorption properties.     

Expanded‐Ring N‐Heterocyclic Carbenes Efficiently Stabilize Gold(I) Cations,Leading to High Activity in π‐Acid‐Catalyzed Cyclizations

A series of six‐ and seven‐membered expanded‐ring N‐heterocyclic carbene (er‐NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er‐NHC)AuX] (X^?=BF₄^?, NTf₂^?, OTf^?) were generated in solution. According to their ¹³C NMR spectra, the ionic character of the complexes increases in the order X^?=Cl^?<NTf₂^?<OTf^?<BF₄^?. Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er‐NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au⁺] cations. A comparative study has been carried out of the catalytic activities of five‐, six‐, and seven‐membered carbene complexes [(NHC)AuX], [(Ph₃P)AuX], [(Me₂S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD‐Dipp)Au]BF₄ (1,3‐bis(2,6‐diisopropylphenyl)‐3,4,5,6‐tetrahydrodiazepin‐2‐ylidene gold(I) tetrafluoroborate) catalyst bearing seven‐membered‐ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1 mol % catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations.     

Isolation of a Non‐Heteroatom‐Stabilized Gold–Carbene Complex

Gold–carbene complexes are essential intermediates in many gold‐catalyzed organic‐synthetic transformations. While gold–carbene complexes with direct, vinylogous, or phenylogous heteroatom substitution have been synthesized and characterized, the observation in the condensed phase of electronically non‐stabilized gold–carbenes has so far remained elusive. The sterically extremely shielded, emerald‐green complex [IPr**Au=CMes₂]⁺[NTf₂]^? has now been synthesized, isolated, and fully characterized. Its absorption maximum at 642 nm, in contrast to 528 nm of the red‐purple carbocation [Mes₂CH]⁺, clearly demonstrates that gold is more than just a “soft proton”.     

Kinetics and Mechanism of the Gold-Catalyzed Hydroamination of 1,1-Dimethylallene with N-Methylaniline

The mechanism of the intermolecular hydroamination of 3-methylbuta-1,2-diene ( 1 ) with N-methylaniline ( 2 ) catalyzed by (IPr)AuOTf has been studied by employing a combination of kinetic analysis, deuterium labelling studies, and in situ spectral analysis of catalytically active mixtures. The results of these and additional experiments are consistent with a mechanism for hydroamination involving reversible, endergonic displacement of N-methylaniline from [(IPr)Au(NHMePh)]⁺ ( 4 ) by allene to form the cationic gold π-C1,C2-allene complex [(IPr)Au(η²-H₂C=C=CMe₂)]⁺ ( I ), which is in rapid, endergonic equilibrium with the regioisomeric π-C2,C3-allene complex [(IPr)Au(η²-Me₂C=C=CH₂)]⁺ ( I′ ). Rapid and reversible outer-sphere addition of 2 to the terminal allene carbon atom of I′ to form gold vinyl complex (IPr)Au[C(=CH₂)CMe₂NMePh] ( II ) is superimposed on the slower addition of 2 to the terminal allene carbon atom of I to form gold vinyl complex (IPr)Au[C(=CMe₂)CH₂NMePh] ( III ). Selective protodeauration of III releases N-methyl-N-(3-methylbut-2-en-1-yl)aniline ( 3 a ) with regeneration of 4 . At high conversion, gold vinyl complex II is competitively trapped by an (IPr)Au⁺ fragment to form the cationic bis(gold) vinyl complex {[(IPr)Au]₂[C(=CH₂)CMe₂NMePh]}⁺ ( 6 ).     

Probing the Limits of Ligand Steric Bulk: Backbone CH Activation in a Saturated N‐Heterocyclic Carbene

The consequences of extremely high steric loading have been probed for late transition metal complexes featuring the expanded ring N‐heterocyclic carbene 6‐Dipp. The reluctance of this ligand to form 2:1 complexes with d‐block metals (rationalised on the basis of its percentage buried volume, % V_bur, of 50.8 %) leads to C?H and C?N bond activation processes driven by attack at the backbone β‐CH₂ unit. In the presence of Ir^I (or indeed H⁺) the net result is the formation of an allyl formamidine fragment, while Au^I brings about an additional ring (re‐)closure step via nucleophilic attack at the coordinated alkene. The net transformation of 6‐Dipp in the presence of [(6‐Dipp)Au]⁺ represents to our knowledge the first example of backbone C?H activation of a saturated N‐heterocyclic carbene, proceeding in this case via a mechanism which involves free carbene in addition to the Au^I centre.     

N‐Heterocyclic Carbenes and Imidazole‐2‐thiones as Ligands for the Gold(I)‐Catalysed Hydroamination of Phenylacetylene

Gold(I) complexes of 1‐[1‐(2,6‐dimethylphenylimino)alkyl]‐3‐(mesityl)imidazol‐2‐ylidene (C^Imine_R), 1,3‐dimesitylimidazol‐2‐ylidene (IMes) and of the corresponding thione derivatives (S^Imine_R and IMesS) were prepared and structurally characterised. The solid‐state structure of the C^Imine_R and S^Imine_R gold(I) complexes showed monodentate coordination of the ligand and a dangling imine group that could bind reversibly to the metal centre to stabilise otherwise unstable catalytic intermediates. Interestingly, reaction of C^Imine_tBu with [AuCl(SMe₂)] led to the formation of [(C^Imine_tBu)AuCl], which rearranges upon crystallisation into the unusual complex cation [(C^Imine_tBu)₂Au]⁺, with AuCl₂^? as the counterion. The activity of the gold complexes in the hydroamination of phenylacetylene with substituted anilines was tested and compared to control catalyst systems. The best catalytic performance was obtained with [(C^Imine_tBu)AuCl], with the exclusive formation of the Markovnikov addition product in excellent yield (>95 %) regardless of the substituents on aniline.     

Synthesis and Study of Cationic,Two‐Coordinate Triphenylphosphine– Gold–π Complexes

Cationic, two‐coordinate triphenylphosphine–gold(I)–π complexes of the form [(PPh₃)Au(π ligand)]⁺ SbF₆^? (π ligand=4‐methylstyrene, 1? SbF₆), 2‐methyl‐2‐butene ( 3? SbF₆), 3‐hexyne ( 6? SbF₆), 1,3‐cyclohexadiene ( 7? SbF₆), 3‐methyl‐1,2‐butadiene ( 8? SbF₆), and 1,7‐diphenyl‐3,4‐heptadiene ( 10? SbF₆) were generated in situ from reaction of [(PPh₃)AuCl], AgSbF₆, and π ligand at ?78 °C and were characterized by low‐temperature, multinuclear NMR spectroscopy without isolation. The π ligands of these complexes were both weakly bound and kinetically labile and underwent facile intermolecular exchange with free ligand (ΔG^≠≈9 kcal mol^?1 in the case of 6? SbF₆) and competitive displacement by weak σ donors, such as trifluoromethane sulfonate. Triphenylphosphine–gold(I)–π complexes were thermally unstable and decomposed above ?20 °C to form the bis(triphenylphosphine) gold cation [(PPh₃)₂Au]⁺SbF₆^? ( 2? SbF₆).     

Heterobi‐ and ‐trinuclear Complexes with an Amino(ethynyl)carbene as Bridging Ligand

The sequential reaction of the amino(trimethylsilyl)carbene complex [(CO)₅W=C(NH₂)C≡CSiMe₃] ( 1 ) with nBuLi and [I‐Fe(CO)₂Cp] affords the C(carbene)‐N bridged heterobinuclear complex [(CO)₅W=C{NHFe(CO)₂Cp}C≡CSiMe₃] ( 2 ). Desilylation of 1 is achieved by treatment with KF in THF/MeOH. From the reaction of the resulting complex [(CO)₅W=C(NH₂)C≡CH] ( 3 ) with nBuLi and [I‐Fe(CO)₂Cp] two binuclear WFe compounds in a ratio of approximately 1:1 are obtained: the C(carbene)‐C≡C bridged complex 4 and the C(carbene)‐N bridged complex 5 . Repetition of the deprotonation/metallation sequence yields the trinuclear WFe₂ complex 6 . One Fe(CO)₂Cp fragment in 6 is bonded to the amino group and the other one to the terminal carbon atom of the ethynyl substituent. The analogous reaction of 3 with nBuLi and [Br‐Ni(PMe₂Ph)₂Mes] gives a ca. 1:1 mixture of two heterobinuclear complexes ( 7 and 8 ). Complex 7 is bridged by the C(carbene)‐C≡C and complex 8 by the C(carbene)‐N fragment. Subsequent reaction of 7 with BuLi and [Br‐Ni(PMe₂Ph)₂Mes] finally affords the trinuclear WNi₂ complex 9 related to 6 . The solid‐state structure of 2 is established by an X‐ray diffraction analysis. The spectroscopic data of the bi‐ and trinuclear complexes indicate electronic communication between the metal centers through the bridging group.     

Concurrent Stabilization of π‐Donor and π‐Acceptor Ligands in Aromatized and Dearomatized Pincer [(PNN)Re(CO)(O)2] Complexes

Aromatized cationic [(PNN)Re(π acid)(O)₂]⁺ ( 1 ) and dearomatized neutral [(PNN*)Re(π acid)(O)₂] ( 2 ) complexes (where π acid=CO ( a ), tBuNC ( b ), or (2,6‐Me₂)PhNC ( c )), possessing both π‐donor and π‐acceptor ligands, have been synthesized and fully characterized. Reaction of [(PNN)Re(O)₂]⁺ ( 4 ) with lithiumhexamethyldisilazide (LiHMDS) yield the dearomatized [(PNN*)Re(O)₂] ( 3 ). Complexes 1 and 2 are prepared from the reaction of 4 and 3 , respectively, with CO or isocyanides. Single‐crystal X‐ray structures of 1 a and 1 b show the expected trans‐dioxo structure, in which the oxo ligands occupy the axial positions and the π‐acidic ligand occupies the equatorial plane in an overall octahedral geometry about the rhenium(V) center. DFT studies revealed the stability of complexes 1 and 2 arises from a π‐backbonding interaction between the d_xy orbital of rhenium, the π orbital of the oxo ligands, and the π* orbital of CO/isocyanide.     

Structures and Dynamic Solution Behavior of Cationic,Two‐Coordinate Gold(I)–π‐Allene Complexes

A family of seven cationic gold complexes that contain both an alkyl substituted π‐allene ligand and an electron‐rich, sterically hindered supporting ligand was isolated in >90 % yield and characterized by spectroscopy and, in three cases, by X‐ray crystallography. Solution‐phase and solid‐state analysis of these complexes established preferential binding of gold to the less substituted C?C bond of the allene and to the allene π face trans to the substituent on the uncomplexed allenyl C?C bond. Kinetic analysis of intermolecular allene exchange established two‐term rate laws of the form rate=k₁[complex]+k₂[complex][allene] consistent with allene‐independent and allene‐dependent exchange pathways with energy barriers of ΔG^≠₁=17.4–18.8 and ΔG^≠₂=15.2–17.6 kcal mol^?1, respectively. Variable temperature (VT) NMR analysis revealed fluxional behavior consistent with facile (ΔG^≠=8.9–11.4 kcal mol^?1) intramolecular exchange of the allene π faces through η¹‐allene transition states and/or intermediates that retain a staggered arrangement of the allene substituents. VT NMR/spin saturation transfer analysis of [{P(tBu)₂o‐binaphthyl}Au(η²‐4,5‐nonadiene) ]⁺SbF₆^? ( 5 ), which contains elements of chirality in both the phosphine and allene ligands, revealed no epimerization of the allene ligand below the threshold for intermolecular allene exchange (ΔG^≠_298K=17.4 kcal mol^?1), which ruled out the participation of a η¹‐allylic cation species in the low‐energy π‐face exchange process for this complex.     

Stabilized Carbenium Ions as Latent,Z‐type Ligands

Controlling the reactivity of transition metals using secondary, σ‐accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o‐Ph₂P(C₆H₄)Acr)AuCl]⁺ ([ 3 ]⁺; Acr=9‐N‐methylacridinium) and [(o‐Ph₂P(C₆H₄)Xan)AuCl]⁺ ([ 4 ]⁺; Xan=9‐xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [ 4 ]⁺ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative ( 7 ) in which the metal atom is covalently bound to the former carbocationic center. This anion‐induced Au^I/Au^III oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [ 4 ]⁺ into an X‐type ligand in 7 . We conclude that the carbenium moiety of this complex acts as a latent Z‐type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.     

Stabilized Carbenium Ions as Latent,Z‐type Ligands

Controlling the reactivity of transition metals using secondary, σ‐accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o‐Ph₂P(C₆H₄)Acr)AuCl]⁺ ([ 3 ]⁺; Acr=9‐N‐methylacridinium) and [(o‐Ph₂P(C₆H₄)Xan)AuCl]⁺ ([ 4 ]⁺; Xan=9‐xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [ 4 ]⁺ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative ( 7 ) in which the metal atom is covalently bound to the former carbocationic center. This anion‐induced Au^I/Au^III oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [ 4 ]⁺ into an X‐type ligand in 7 . We conclude that the carbenium moiety of this complex acts as a latent Z‐type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.     

Synthesis and molecular structure of biologically significant bis(1,3‐dimesityl‐4,5‐naphthoquinoimidazol‐2‐ylidene)gold(I) complexes with chloride and dichloridoaurate counter‐ions

Diffraction‐quality single crystals of two gold(I) complexes, namely bis(1,3‐dimesityl‐4,5‐naphthoquinoimidazol‐2‐ylidene)gold(I) chloride benzene monosolvate, [Au(C₂₉H₂₆N₂O₂)₂]Cl·C₆H₆ or [(NQMes)₂Au]Cl·C₆H₆, 2 , and bis(1,3‐dimesityl‐4,5‐naphthoquinoimidazol‐2‐ylidene)gold(I) dichloridoaurate(I) dichloromethane disolvate, [Au(C₂₉H₂₆N₂O₂)₂][AuCl₂]·2CH₂Cl₂ or [(NQMes)₂Au][AuCl₂]·2CH₂Cl₂, 4 , were isolated and studied with the aid of single‐crystal X‐ray diffraction analysis. Compound 2 crystallizes in a monoclinic space group C2/c with eight molecules in the unit cell, while compound 4 crystallizes in the triclinic space group P with two molecules in the unit cell. The crystal lattice of compound 2 reveals C—H…Cl^? interactions that are present throughout the entire structure representing head‐to‐tail contacts between the aromatic (C—H) hydrogens of naphthoquinone and Cl^? counter‐ions. Compound 4 stacks with the aid of short interactions between a naphthoquinone O atom of one molecule and the mesityl methyl group of another molecule along the a axis, leading to a one‐dimensional strand that is held together by strong π–η² interactions between the imidazolium backbone and the [AuCl₂]^? counter‐ion. The bond angles defined by the Au^I atom and two carbene C atoms [C(carbene)—Au—C(carbene)] in compounds 2 and 4 are nearly rectilinear, with an average value of ～174.1 [2]°. Though 2 and 4 share the same cation, they differ in their counter‐anion, which alters the crystal lattice of the two compounds. The knowledge gleaned from these studies is expected to be useful in understanding the molecular interactions of 2 and 4 under physiological conditions.     

Gold‐Catalyzed Intermolecular Anti‐Markovnikov Hydroamination of Alkylidenecyclopropanes

The cationic gold phosphine complex [{PCy₂(o‐biphenyl)}Au(NCMe)]⁺SbF₆^? (Cy=cyclohexyl) catalyzes the intermolecular, anti‐Markovnikov hydroamination reaction of monosubstituted and cis‐ and trans‐disubstituted alkylidenecyclopropanes (ACPs) with imidazolidin‐2‐ones and other nucleophiles. This reaction forms 1‐cyclopropyl alkylamine derivatives in high yield and with high regio‐ and diastereoselectivity. NMR spectroscopic analysis of gold π‐ACP complexes and control experiments point to the sp hybridization of the ACP internal alkene carbon atom as controlling the regiochemistry of the ACP hydroamination reaction.     

Efficient Room‐Temperature,Au+‐Mediated Coupling of a Carbene Ligand with Methane To Generate C2Hx (x=4, 6)

The thermal reactions of the closed‐shell, “naked” gold–carbene complex [Au(CH₂)]⁺ with methane have been explored by using FTICR mass spectrometry complemented by quantum chemical (QC) calculations at the CCSD(T)//BMK level of theory. Mechanistic aspects for this unprecedentedly efficient carbene insertion in the C? H bond of methane have been addressed and the origin of the counterintuitive high reactivity of [Au(CH₂)]⁺ towards this most inert hydrocarbon is discussed.     

Synthesis of a Metallo‐Iminosilane via a Silanone–Metal π‐Complex

Facile oxygenation of the acyclic amido‐chlorosilylene bis(N‐heterocyclic carbene) Ni⁰ complex [{N(Dipp)(SiMe₃)ClSi:→Ni(NHC)₂] ( 1 ; Dipp=2,6‐ⁱPr₂C₆H₄; N‐heterocyclic carbene=C[(ⁱPr)NC(Me)]₂) with N₂O furnishes the first Si‐metalated iminosilane, [DippN=Si(OSiMe₃)Ni(Cl)(NHC)₂] ( 3 ), in a rearrangement cascade. Markedly, the formation of 3 proceeds via the silanone (Si=O)–Ni π‐complex 2 as the initial product, which was predicted by DFT calculations and observed spectroscopically. The Si=O and Si=N moieties in 2 and 3 , respectively, show remarkable hydroboration reactivity towards H−B bonds of boranes, in the former case corroborating the proposed formation of a (Si=O)–Ni π‐complex at low temperature.     

Expanded‐Ring N‐Heterocyclic Carbenes for the Stabilization of Highly Electrophilic Gold(I) Cations

Strategies for the synthesis of highly electrophilic Au^I complexes from either hydride‐ or chloride‐containing precursors have been investigated by employing sterically encumbered Dipp‐substituted expanded‐ring NHCs (Dipp=2,6‐iPr₂C₆H₃). Thus, complexes of the type (NHC)AuH have been synthesised (for NHC=6‐Dipp or 7‐Dipp) and shown to feature significantly more electron‐rich hydrides than those based on ancillary imidazolylidene donors. This finding is consistent with the stronger σ‐donor character of these NHCs, and allows for protonation of the hydride ligand. Such chemistry leads to the loss of dihydrogen and to the trapping of the [(NHC)Au]⁺ fragment within a dinuclear gold cation containing a bridging hydride. Activation of the hydride ligand in (NHC)AuH by B(C₆F₅)₃, by contrast, generates a species (at low temperatures) featuring a [HB(C₆F₅)₃]^? fragment with spectroscopic signatures similar to the “free” borate anion. Subsequent rearrangement involves B?C bond cleavage and aryl transfer to the carbophilic metal centre. Under halide abstraction conditions utilizing Na[BAr^f₄] (Ar^f=C₆H₃(CF₃)₂‐3,5), systems of the type [(NHC)AuCl] (NHC=6‐Dipp or 7‐Dipp) generate dinuclear complexes [{(NHC)Au}₂(μ‐Cl)]⁺ that are still electrophilic enough at gold to induce aryl abstraction from the [BAr^f₄]^? counterion.