Gold trifluoromethyl complexes as efficient regioselective catalysts in alkyne hydration.

Gold (III) complexes containing trifluromethyl ligands are efficient catalyst in the hydration of alkynes, operating at low catalyst loadings, without additives, using environmentally friendly solvents and at mild conditions (60 ºC). Hydration of terminal and internal alkynes provide the corresponding ketones in quantitative yields without special precautions as dry solvents or inert atmospheres. Remarkably, hydration of asymmetric internal alkynes proceeds with moderate to notable regioselectivities, providing mixtures of the two possible isomers with ratios up to 90:10.

A Highly Sterically Encumbered Boron Lewis Acid Enabled by an Organotellurium-Based Ligand.

Lewis acidic boron compounds are ubiquitous in chemistry due to their numerous applications, yet tuning and optimizing their properties towards different purposes is still a challenging field of research. In this work, the boron-based Lewis acid B[OTeF3(C6F5)2]3 was synthesized by reaction of the teflate derivative HOTeF3(C6F5)2 with BCl3 or BCl3 ⋅ SMe2. This new compound presents a remarkably high thermal stability up to 300 °C, as well as one of the most sterically encumbered boron centres known in the literature. Theoretical and experimental methods revealed that B[OTeF3(C6F5)2]3 exhibits a comparable Lewis acidity to that of the well-known B(C6F5)3. The affinity of B[OTeF3(C6F5)2]3 towards pyridine was accessed by Isothermal Titration Calorimetry (ITC) and compared to that of B(OTeF5)3 and B(C6F5)3. The ligand-transfer reactivity of this new boron compound towards different fluorides was demonstrated by the formation of an anionic Au(III) complex and a hypervalent iodine(III) species.

Questing for homoleptic mononuclear manganese complexes with monodentate O-donor ligands.

Compounds containing Mn-O bonds are of utmost importance in biological systems and catalytic processes. Nevertheless, mononuclear manganese complexes containing all O-donor ligands are still rare. Taking advantage of the low tendency of the pentafluoroorthotellurate ligand (teflate, OTeF5) to bridge metal centers, we have synthesized two homoleptic manganese complexes with monomeric structures and an all O-donor coordination sphere. The tetrahedrally distorted MnII anion, [Mn(OTeF5)4]2-, can be described as a high spin d5 complex (S = 5/2), as found experimentally (magnetic susceptibility measurements and EPR spectroscopy) and using theoretical calculations (DFT and CASSCF/NEVPT2). The high spin d4 electronic configuration (S = 2) of the MnIII anion, [Mn(OTeF5)5]2-, was also determined experimentally and theoretically, and a square pyramidal geometry was found to be the most stable one for this complex. Finally, the bonding situation in both complexes was investigated by means of the Interacting Quantum Atoms (IQA) methodology and compared to that of hypothetical mononuclear fluoromanganates. Within each pair of [MnXn]2- (n = 4, 5) species (X = OTeF5, F), the Mn-X interaction is found to be comparable, therefore proving that the similar electronic properties of the teflate and the fluoride are also responsible for the stabilization of these unique species.

Silver(i) Perfluoroalcoholates: Synthesis, Structure, and their Use as Transfer Reagents.

Herein we report a general access to silver(i) perfluoroalcoholates, their structure in the solid state and in solution, and their use as transfer reagents. The silver(i) perfluoroalcoholates are prepared by the reaction of AgF with the corresponding perfluorinated carbonyl compounds in acetonitrile and are stable for a prolonged time at -18 °C. X-ray analysis of single crystals of perfluoroalcoholate species showed that two Ag(i) centers are bridged by the alcoholate ligands. In acetonitrile solution, Ag[OCF3] forms different structures as indicated by IR spectroscopy. Furthermore, the silver(i) perfluoroalcoholates can be used as easy-to-handle transfer reagents for the synthesis of Cu[OCF3], Cu[OC2F5], [PPh4][Au(CF3)3(OCF3)], and fluorinated alkyl ethers.

[Xe(OTeF5)(pyF)]+: a strong oxidizing xenonium(II) teflate cation with N-donor bases.

Herein we report on the formation of the adduct salts [Xe(OTeF5)(pyF)][Al(OTeF5)4] (pyF = C5F5N, C5H3F2N) by abstraction of an -OTeF5 group from Xe(OTeF5)2 with the Lewis superacid Al(OTeF5)3 and subsequent adduct formation of the generated [XeOTeF5]+ cation with fluorinated pyridines. These salts represent the first xenonium cations with the weakly coordinating [Al(OTeF5)4]- anion. The strong oxidizing property of these compounds is further assessed.

Further Perspectives on the Teflate versus Fluoride Analogy: The Case of a Co(II) Pentafluoroorthotellurate Complex.

The pentafluoroorthotellurate group (teflate, OTeF5) is considered as a bulky analogue of fluoride, yet its coordination behavior in transition metal complexes is not fully understood. By reaction of [CoCl4]2- and neat ClOTeF5, we synthesized the first cobalt teflate complex, [Co(OTeF5)4]2-, which exhibits moisture-resistant Co-OTeF5 bonds. Through a combined experimental and theoretical (DFT and NEVPT2) study, the properties and electronic structure of this species have been investigated. It exhibits a distorted tetrahedral structure around the cobalt center and can be described as a d7 system with a quartet (S = 3/2) ground state. A comparative bonding analysis of the (pseudo)tetrahedral [CoX4]2- anions (X = OTeF5, F, Cl) revealed that the strength of the Co-X interaction is similar in the three cases, being the strongest in [Co(OTeF5)4]2-. In addition, an analysis of the charge of the Co center reinforced the similar electron-withdrawing properties of the teflate and fluoride ligands. Therefore, the [Co(OTeF5)4]2- anion constitutes an analogue of the polymeric [CoF4]2- in terms of electronic properties, but with a monomeric structure.

Reactivity of [AuF3 (SIMes)]: Pathway to Unprecedented Structural Motifs.

We report on a comprehensive reactivity study starting from [AuF3 (SIMes)] to synthesize different motifs of monomeric gold(III) fluorides. A plethora of different ligands has been introduced in a mono-substitution yielding trans-[AuF2 X(SIMes)] including alkynido, cyanido, azido, and a set of perfluoroalkoxido complexes. The latter were better accomplished via use of perfluorinated carbonyl-bearing molecules, which is unprecedented in gold chemistry. In case of the cyanide and azide, triple substitution gave rise to the corresponding [AuX3 (SIMes)] complexes. Comparison of the chemical shift of the carbene carbon atom in the 13 C{1 H} NMR spectrum, the calculated SIMes affinity and the Au-C bond length in the solid state with related literature-known complexes yields a classification of trans-influences for a variety of ligands attached to the gold center. Therein, the mixed fluorido perfluoroalkoxido complexes have a similar SIMes affinity to AuF3 with a very low Gibbs energy of formation when using the perfluoro carbonyl route.

Gold Teflates Revisited: From the Lewis Superacid [Au(OTeF5 )3 ] to the Anion [Au(OTeF5 )4 ].

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.

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.

Air-stable aryl derivatives of pentafluoroorthotellurate.

We report on two different sets of air-stable derivatives of pentafluoroorthotellurate containing fluorinated and non-fluorinated aryl groups. The acid cis-PhTeF4OH was obtained in gram scale and further transformed to Ag[cis-PhTeF4O], which was used as a cis-PhTeF4O transfer reagent to obtain [PPh4][cis-PhTeF4O]. Furthermore, the synthesis of trans-(C6F5)2TeF3OH was achieved by a selective hydrolysis of trans-(C6F5)2TeF4 in the presence of KF and subsequent protonation by aHF. Quantum-chemical calculations show a higher acidity and robustness against fluoride abstraction for trans-(C6F5)2TeF3OH compared to cis-PhTeF4OH.

Unravelling the Role of the Pentafluoroorthotellurate Group as a Ligand in Nickel Chemistry.

The pentafluoroorthotellurate group (teflate, OTeF5 ) is able to form species, for which only the fluoride analogues are known. Despite nickel fluorides being widely investigated, nickel teflates have remained elusive for decades. By reaction of [NiCl4 ]2- and neat ClOTeF5 , we have synthesized the homoleptic [Ni(OTeF5 )4 ]2- anion, which presents a distorted tetrahedral structure, unlike the polymeric [NiF4 ]2- . This high-spin complex has allowed the study of the electronic properties of the teflate group, which can be classified as a weak/medium-field ligand, and therefore behaves as the fluoride analogue also in ligand-field terms. The teflate ligands in [NEt4 ]2 [Ni(OTeF5 )4 ] are easily substituted, as shown by the formation of [Ni(NCMe)6 ][OTeF5 ]2 by dissolving it in acetonitrile. Nevertheless, careful reactions with other conventional ligands have enabled the crystallization of nickel teflate complexes with different coordination geometries, i.e. [NEt4 ]2 [trans-Ni(OEt2 )2 (OTeF5 )4 ] or [NEt4 ][Ni(bpyMe2 )(OTeF5 )3 ].

Hydrogen bonding to metals as a probe for an inverted ligand field.

Electron-rich, late transition metals are known to act as hydrogen-bonding (HBd) acceptors. In this regard, Pt(ii) centres in square-planar environments are particularly efficient. It is however puzzling that no convincing experimental evidence is currently available for the isoelectronic neighbour Au(iii) being involved in HBd interactions. We report now on the synthesis and characterisation of two series of isoleptic and isoelectronic (d8) compounds [(CF3)3Pt(L)]- and (CF3)3Au(L), where the L ligands are based on the quinoline frame and have been selected to favour HBd with the metal centre. Strong HBd interactions were actually found in the Pt(ii) compounds, based on structural and spectroscopic evidence, and they were further confirmed by theoretical calculations. In contrast, no evidence was obtained in the Au(iii) case. In order to find the reason underlying this general disparity, we undertook a detailed theoretical analysis of the model systems [(CF3)3Pt(py)]- and (CF3)3Au(py). This study revealed that the filled dz2 orbital is the HOMO in the case of Pt(ii), but is buried in the lower energy levels in the case of Au(iii). The sharply different electronic configurations involve ligand-field inversion on going from Pt to the next element Au. This is not a gradual but an abrupt change, which invalidates Au(iii) as a HBd-acceptor wherever ligand-field inversion occurs.

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.

C-N Cross-Couplings for Site-Selective Late-Stage Diversification via Aryl Sulfonium Salts.

We report diverse C-N cross-coupling reactions of aryl thianthrenium salts that are formed site-selectively by direct C-H functionalization. The scope of N-nucleophiles ranges from primary and secondary alkyl and aryl amines to various N-containing heterocycles, and the overall transformation is applicable to late-stage functionalization of complex, drug-like small molecules.

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 AuFx ]- (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.

Gold(II) Trihalide Complexes from Organogold(III) Precursors.

The mononuclear gold(II) halide complexes [AuCl3 ]- and [AuBr3 ]- are formed in the gas phase by collision-induced homolytic splitting of the only Au-C bond in the monoalkylgold(III) precursors [CF3 AuX3 ]- . The geometries of the whole series of [AuX3 ]- complexes (X=F, Cl, Br, I) have been calculated by DFT methods. It has also been found that the neutral AuX2 molecules behave as unsaturated species, showing significant affinity for an additional X- ligand. Moreover, in the open-shell [AuX3 ]- anions, homolytic splitting of one of the Au-X bonds and formation of the lower-valent [AuX2 ]- anions is favored over non-reducing halide dissociation. They should therefore be prone to disproportionation.

(CF3 )3 Au as a Highly Acidic Organogold(III) Fragment.

The Lewis acidity of perfluorinated trimethylgold (CF3 )3 Au was assessed by theoretical and experimental methods. It was found that the (CF3 )3 Au unit is much more acidic than its nonfluorinated analogue (CH3 )3 Au, and probably sets the upper limit of the acidity scale for neutral organogold(III) species R3 Au. The significant acidity increase on fluorination is in line with the CF3 group being more electron-withdrawing than CH3 . The solvate (CF3 )3 Au⋅OEt2 (1) is presented as a convenient synthon of the unsaturated, 14-electron species (CF3 )3 Au. Thus, the weakly coordinated ether molecule in 1 is readily replaced by a variety of neutral ligands (L) to afford a wide range of (CF3 )3 Au⋅L compounds, which were isolated and characterized. Most of these mononuclear compounds exhibit marked thermal stability. This enhanced stabilization can be rationalized in terms of substantially stronger [Au]-L interactions with the (CF3 )3 Au unit. An affinity scale of this single-site, highly acidic organogold(III) fragment was calculated by DFT methods and experimentally mapped for various neutral monodentate ligands. The high-energy profile calculated for the fluorotropic [Au]-CF3 ⇌F-[Au]←CF2 process makes this potential decomposition path unfavorable and adds to the general stabilization of the fragment.

Anionic Derivatives of Perfluorinated Trimethylgold.

The homoleptic compound [PPh4 ][CF3 AuCF3 ] cleanly undergoes photoinduced oxidative addition of CF3 I to afford the organogold(III) derivative [PPh4 ][(CF3 )3 AuI] in good yield and under mild conditions. This compound provides a convenient entry to the chemistry of the perfluorinated (CF3 )3 Au fragment, the properties of which were analyzed with the aid of DFT methods and compared with those of the homologous non-fluorinated (CH3 )3 Au moiety. It was found that reductive elimination of CX3 -CX3 in the former (X=F) requires a much higher energy barrier than in the latter (X=H) and is therefore considerably less favored. This can be considered as one of the main features underlying the significantly higher stability associated to the (CF3 )3 Au fragment and its derivatives. This unsaturated, 14-electron species can be stabilized by coordination of any of the halide ligands, including fluoride. In fact, the whole series of anionic [PPh4 ][(CF3 )3 AuX] complexes (X=F, Cl, Br, I, CN) has now been isolated and conveniently characterized. Evidence for intermolecular decomposition pathways upon thermolysis in the condensed phase is presented.

Gold(I) Fluorohalides: Theory and Experiment.

The anionic trifluoromethylgold(I) derivatives [CF3 AuX]- , which have been prepared and isolated as their [PPh4 ]+ salts in good yield, undergo thermally induced difluorocarbene extrusion in the gas phase, giving rise to the mixed gold(I) fluorohalide complexes [F-Au-X]- (X=Cl, Br, I). These triatomic species have been detected by tandem mass spectrometry (MS2) experiments and their properties have been analyzed by DFT methods. The CF2 extrusion mechanism from the Au-CF3 moiety serves as a model for the CF2 insertion into the Au-F bond, since both reactivity channels are connected by the microreversibility principle.