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.

The CF3 Group as a Synthon of the CF+ Unit in Palladium Chemistry.

The homoleptic trifluoromethyl-palladium(II) complex [Pd(CF3)4]2- (1) is shown to be highly active towards amines. Thus, when treated with primary amines RNH2, it readily undergoes aminolysis of one of the CF3 ligands affording the isocyanide complexes [(CF3)3Pd(CNR)]- (R=aryl). In this process the original CF3 group undergoes total defluorination. Interestingly, the reaction of 1 with secondary amines R2NH proceeds with loss of just two F-substituents, whereby the Fischer-type fluoroaminocarbene complexes [(CF3)3Pd(CFNR2)]- are formed (R=Et, Ph). The reaction of 1 with diamines affords different [(CF3)3Pd(NHC)]- complexes containing sterically non-demanding NHC ligands. Representative examples of various topologies are reported based on the common imidazolidin-2-ylidene or benzimidazolin-2-ylidene rings as well as the expanded-ring perimidin-2-ylidene. This metal-tailored synthetic route, where a CF3 group acts as a pre-carbenic unit, is unprecedented in the vast NHC-chemistry. It takes place under very mild conditions and is envisaged to be extensible to other non-isolable NHC ligands. The key difluorocarbene intermediate [(CF3)3Pd(CF2)]- is experimentally detected.

Synthesis and Lewis Acid Properties of Neutral Silver(III) Adducts Containing the AgIII(CF3)3 Moiety.

The acetonitrile AgIII complex [AgIII(CF3)3(NCCH3)] (2) has been reported independently by Eujen and Naumann in the last century, albeit with intriguing NMR discrepancy. In their reports, 2 was claimed to be obtained starting from either [AgIII(CF3)3Cl]- (3⋅Cl) or [AgIII(CF3)4]- (1) via halide abstraction using AgNO3 or acidic treatment, resp. These two synthetic routes are herein reinvestigated. The feasibility of Naumann's method is demonstrated, thus providing 2 yet accompanied by its s-triazinyl derivative [AgIII(CF3)3(C6H9N3)] (2'). The formation of 2' is unprecedented and was thereby investigated. Both 2 and 2' were isolated in pure fashion and fully characterized. In turn, halide extraction from 3⋅Cl leads to the AgIII-ONO2 anion 5 instead of 2, as evidenced by NMR spectroscopy, EA and Sc-XRD.

The Fluoride Method: Access to Silver(III) NHC Complexes.

We have synthesized the first silver(III) carbene complexes, (CF3 )3 Ag(NHC), by direct reaction of the silver(III) fluoride precursor complex [PPh4 ][(CF3 )3 AgF] with different imidazolium salts. This novel methodology circumvents the use of free NHC molecules. The silver(III) carbene complexes thus prepared are unprecedented and show remarkable thermal stabilities. They display square-planar or square-pyramidal geometry. Following our calculations, the electronic structure of a model representative complex exhibits Inverse Ligand Field (ILF). The compounds reported herein are synthetic analogues of the elusive difluorocarbene and carbonyl species proposed as intermediates in the acidic decomposition of [Ag(CF3 )4 ]- . The synthetic procedure reported is envisaged to enable access to carbene complexes of other late transition-metals in high oxidation states.

Electrophilicity of neutral square-planar organosilver(III) compounds.

Neutral Ag(III) complexes stabilised with just monodentate ligands are here unambiguously established. In a series of square-planar (CF3)3Ag(L) compounds with hard and soft Group 15 donor ligands, L, the metal center has been found to exhibit substantial acidity favouring apical coordination of an additional ligand under no coordination constraints.

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.

Heteropolymetallic Architectures as Snapshots of Transmetallation Processes at Different Degrees of Transfer.

Novel heteropolymetallic architectures have been built by integrating Pd, Au and Ag systems. The dinuclear [(CNC)(PPh3 )Pd-G11 M(PPh3 )](ClO4 ) (G11 M=Au (3), Ag (4); CNC=2,6-diphenylpyridinate) and trinuclear [{(CNC)(PPh3 )Pd}2 G11 M](ClO4 ) (G11 M=Au (6), Ag (5)) complexes have been accessed or isolated. Structural and DFT characterization unveil striking interactions of one of the aryl groups of the CNC ligand(s) with the G11 M center, suggesting these complexes constitute models of transmetallation processes. Further analyses allow to qualitatively order the degree of transfer, proving that Au promotes the highest one and also that Pd systems favor higher degrees than Pt. Consistently, Energy Decomposition Analysis calculations show that the interaction energies follow the order Pd-Au > Pt-Au > Pd-Ag > Pt-Ag. All these results offer potentially useful ideas for the design of bimetallic catalytic systems.

A Five-Coordinate Compound with Inverted Ligand Field: An Unprecedented Geometry for Silver(III).

By using suitable synthetic procedures, we have first isolated the square-planar organosilver(III) compounds [PPh4 ][trans-(CF3 )2 AgX2 ] [X=Cl (1 a), Br (2 a)]. The geometry and stereochemistry of the chloro-derivative 1 a have been unambiguously established by single-crystal X-ray diffraction (SC-XRD) methods. Following our calculations on the relative stability of the cis-/trans-[(CF3 )2 AgX2 ]- couples (X=F, Cl, Br, I), the experimentally obtained compounds 1 a and 2 a appear to be kinetically favored stereoisomers. They display some tendency to associate an additional X- ligand affording rare five-coordinate AgIII species [(CF3 )2 AgX3 ]2- . Interestingly, compound [PPh4 ]2 [(CF3 )2 AgBr3 ] (3) has been identified by SC-XRD methods as the first AgIII derivative with trigonal symmetry in general and trigonal bipyramidal geometry in particular. This unusual five-coordinate species also exhibits inverted ligand field.

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.

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.

Metal-Metal Cooperation in the Oxidation of a Flapping Platinum Butterfly by Haloforms: Experimental and Theoretical Evidence.

The model 1-DFT for the butterfly complex [{Pt(C∧C*)(µ-pz)}2] (1; HC∧C* = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene) shows two minima in the potential energy surface of the ground state in acetone solution: the butterfly-wing-spreading molecules 1-s, (dPt-Pt ≈ 3.20 Å) and the wing-folding molecules 1-f (dPt-Pt ≤ 3.00 Å). Both minima are very close in energy (ΔG° = 1.7 kcal/mol) and are connected through a transition state, which lies only 1.9 kcal/mol above 1-s and 0.2 kcal/mol above 1-f. These very low barriers support a fast interconversion process, resembling a butterfly flapping, and the presence of both conformers in acetone solution. However, the 1-f ratio is so low that it is undetectable in the excitation and emission spectra of 1 in 2-MeTHF of diluted solutions (10-5 M) at 77 K, while it is seen in more concentrated solutions (10-3 M). In acetone solution, 1 undergoes a [2c, 2e] oxidation by CHX3 (X = Cl, Br) in the sunlight to render the Pt2(III,III) compounds [{Pt(C∧C*)(µ-pz)X)}2] (X = Cl (2-Cl), Br (2-Br)). In concentrated solutions, 1 can react with CHCl3 under blue light to give 2-Cl and with CHBr3 in the dark, the latter rendering the compound [BrPt(C∧C*)(µ-pz)2Pt(C∧C*)CHBr2] (3-Br) or mixtures of 2-Br and 3-Br if the reaction is performed under an argon atmosphere or in the air, respectively. Mechanistic studies showed that in concentrated solutions the oxidation processes follow a radical mechanism being the MMLCT-based species 1-f, those which trigger the reaction of 1 with CHBr3 and CHCl3. In the ground state (S0f), it promotes the thermal oxidation of 1 by CHBr3 and in the first singlet excited state (S1f) the blue-light-driven photooxidation of 1 by CHCl3. Complexes, 2-Cl, 2-Br, and 3-Br were selectively obtained and fully characterized, showing Pt-Pt distances (ca. 2.6 Å) shorter than that of the starting complex, 1. They are, together with the analogous [{Pt(C∧C*)(µ-pz)I)}2] and [IPt(C∧C*)(µ-pz)2Pt(C∧C*)CHI2], the only dinuclear metal-metal-bonded PtIII(µ-pz)2PtIII compounds reported to date.

The First Organosilver(III) Fluoride, [PPh4 ][(CF3 )3 AgF].

Organosilver(III) fluoride complexes have been assigned a key role in different fluorination processes. To the best of our knowledge, however, none of them seem to have been isolated or even detected thus far. Here we report on the successful synthesis of the trifluoromethyl derivative [PPh4 ][(CF3 )3 AgF], which has been isolated in high yield. The thermodynamic stability of the Ag-F bond is shown by calculation and demonstrated by multistage mass spectrometry (MSn ) under collision-induced dissociation (CID) conditions. Nevertheless, the substantial elongation found in the Ag-F bond (X-ray) is correlated with a marked nucleophilic character of the terminal F ligand. This Ag-F bond is, in fact, quite reactive: it suffers hydrolysis and is also solvolyzed by thiols.

M-C Bond Homolysis in Coinage-Metal [M(CF3 )4 ]- Derivatives.

A comparative study of the homoleptic [M(CF3 )4 ]- complexes of all three coinage metals (M=Cu, Ag, Au) reveals that homolytic M-C bond cleavage is favoured in every case upon excitation in the gas phase (CID-MS2 ). Homolysis also occurs in solution by photochemical excitation. Transfer of the photogenerated CF3 . radicals to both aryl and alkyl carbon atoms was also confirmed. The observed behaviour was rationalized by considering the electronic structure of the involved species, which all show ligand-field inversion. Moreover, the homolytic pathway constitutes experimental evidence for the marked covalent character of the M-C bond. The relative stability of these M-C bonds was evaluated by energy-resolved mass spectrometry (ERMS) and follows the order Cu

Platinum-mediated monohydration of SO2.

The monohydration of SO2 has been achieved in solution mediated by a platinum-aquo complex. Benzamidate ligands play a key role along the process, acting as versatile proton shuttles. Finally, the platinum center allows the stabilization of benzimidic acid and a hydrogensulphite anion, the disfavored tautomers of benzamide and sulphonate, respectively.

A Cyclometalated N-Heterocyclic Carbene: The Wings of the First Pt2 (II,II) Butterfly Oxidized by CHI3.

The X-ray study on a single crystal of the butterfly-like complex [{Pt(C^C*)(µ-pz)}2 ] (1), containing a cyclometalated N-heterocyclic carbene ligand as wings (HC^C*=1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene), showed three molecules in the asymmetric unit with intermetallic separations (Å) of 3.2294(4) (1A), 3.2834(4) (1B), and 3.1208(6) (1C). From the reaction of 1 with excess of CHI3 in the air and the sunlight, complex [{Pt(C^C*)(µ-pz)I)}2 ] (2) (dPt-Pt =2.6079(2) Å) was obtained as the major product, while [IPt(C^C*)(µ-pz)2 Pt(C^C*)CHI2 ] (3) (dPt-Pt =2.6324(3) Å) was obtained as the major product under argon atmosphere in the dark. Experimental and theoretical investigations showed that an easily accessible radical-like mechanism operates under thermal conditions, with dioxygen acting as an efficient radical (R. ) scavenger. The oxidation of a Pt2 (II,II) "butterfly" by CHI3 to give metal-metal bonded Pt2 (III,III) compounds is described now for the first time.

Pt-M Complexes (M=Ag, Au) as Models for Intermediates in Transmetalation Processes.

Heteropolynuclear complexes [(CNC)(PPh3 )PtM(PPh3 )]ClO4 [M=Au (1), Ag (2)] and [{Pt(CNC)(PPh3 )}2 M]ClO4 [M=Ag (3), Au (4); CNC=2,6-diphenylpyridinate] were prepared and studied by X-ray crystallography, spectroscopic techniques, and DFT calculations. The X-ray crystal structures of 1, 3, and 4 confirmed the existence of Pt-M bonds and M⋅⋅⋅Cipso interactions involving one of the phenyl fragments of CNC. Their NMR spectra showed the persistence of the Pt-M interactions in solution and also revealed an intramolecular metronome-like dynamic process consisting of back-and-forth motion of the acidic M fragments along the C-Pt-C axis. DFT calculations on these complexes identified two main orbital interactions between the [PtCNC] and [M]+ fragments, namely, donation from the former to a vacant orbital of the latter and much weaker backdonation from the acidic M to the Pt fragment. Overall, the strength of the [Pt]⋅⋅⋅M interactions is higher for the gold compounds than for their silver counterparts. The interaction between the acidic center (silver or gold) and the carbon atom of one of the phenylene rings in these heteropolymetallic complexes can be envisaged as the first step in a process of interchange of aryl ligands. However, the ligand exchange cannot progress further due to the polydentate nature of the CNC ligand, and therefore these structures can be considered as frozen snapshots of a transmetalation reaction that has been arrested at different stages of the process.

Homoleptic Trifluoromethyl Derivatives of AgI and AgIII.

The homoleptic silver(I) compound [PPh4 ][CF3 AgCF3 ] (1) provides a convenient entry to the homoleptic silver(III) derivative [PPh4 ][Ag(CF3 )4 ] (2). Once isolated as pure substances, these compounds exhibit marked thermal stabilities. Their structural and spectroscopic properties have been experimentally established. Moreover, their electronic structures have been calculated by theoretical methods. The electronic structure of the oxidized species [Ag(CF3 )4 ]- provides a new case of ligand-field inversion caused by the CF3 ligands.

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.