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.

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.

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.

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.

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.

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

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.

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.

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(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.

(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.

Stereochemistry of complexes with double and triple metal-ligand bonds: a continuous shape measures analysis.

To each coordination polyhedron we can associate a normalized coordination polyhedron that retains the angular orientation of the central atom-ligand bonds but has all the vertices at the same distance from the center. The use of shape measures of these normalized coordination polyhedra provides a simple and efficient way of discriminating angular and bond distance distortions from an ideal polyhedron. In this paper we explore the applications of such an approach to analyses of several stereochemical problems. Among others, we discuss how to discern the off-center displacement of the metal from metal-ligand bond shortening distortions in families of square planar biscarbene and octahedral dioxo complexes. The normalized polyhedron approach is also shown to be very useful to understand stereochemical trends with the help of shape maps, minimal distortion pathways, and ligand association/dissociation pathways, illustrated by the Berry and anti Berry distortions of triple-bonded [X≡ML4] complexes, the square pyramidal geometries of Mo coordination polyhedra in oxido-reductases, the coordination geometries of actinyl complexes, and the tetrahedricity of heavy atom-substituted carbon centers.

Homoleptic organocobalt(III) compounds with intermediate spin.

Homoleptic organocobalt(III) compounds with formula [NBu4][Co(III)(C6X5)4] [X = F (3), Cl (4)] were obtained in reasonable yields by chemical oxidation of the corresponding divalent species [NBu4]2[Co(II)(C6X5)4] [X = F (1), Cl (2)]. The [Co(III)(C6X5)4](-)/[Co(II)(C6X5)4](2-) couples are electrochemically related by quasi-reversible, one-electron exchange processes at moderate potential: E1/2 = -0.29 (X = F) and -0.36 V (X = Cl) versus saturated calomel electrode. The [Co(III)(C6X5)4](-) anions in salts 3 and 4 show an unusual square-planar geometry as established by single-crystal X-ray diffraction methods. According to their stereochemistry, these Co(III) derivatives (d(6)) are paramagnetic non-Kramers systems with a large zero-field splitting contribution and no observable electron paramagnetic resonance (EPR) spectrum. The thermal dependence of their magnetic susceptibilities can be explained in terms of a spin-Hamiltonian formalism with S = 1 ground state (intermediate spin) and substantial spin-orbit contribution. The magnetic properties of the square-planar d(7) parent species [NBu4]2[Co(II)(C6X5)4] were also thoroughly studied both at microscopic (EPR) and macroscopic levels (alternating current and direct current magnetization measurements). They behave as S = 1/2 (low spin) systems with mainly (dz(2))(1) electron configuration and a certain degree of s-orbital admixture that has been quantified. The electronic structures of all four open-shell [Co(C6X5)4](q-) compounds (q = 1, 2) accounting for their respective magnetic properties are based on a common orbital energy-level diagram.

Gold(I) and gold(III) trifluoromethyl derivatives.

Trifluoromethylation of AuCl3 by using the Me3 SiCF3 /CsF system in THF and in the presence of [PPh4 ]Br proceeds with partial reduction, yielding a mixture of [PPh4 ][Au(I) (CF3 )2 ] (1') and [PPh4 ][Au(III) (CF3 )4 ] (2') that can be adequately separated. An efficient method for the high-yield synthesis of 1' is also described. The molecular geometries of the homoleptic anions [Au(I) (CF3 )2 ](-) and [Au(III) (CF3 )4 ](-) in their salts 1' and [NBu4 ][Au(III) (CF3 )4 ] (2) have been established by X-ray diffraction methods. Compound 1' oxidatively adds halogens, X2 , furnishing [PPh4 ][Au(III) (CF3 )2 X2 ] (X=Cl (3), Br (4), I (5)), which are assigned a trans stereochemistry. Attempts to activate CF bonds in the gold(III) derivative 2' by reaction with Lewis acids under different conditions either failed or only gave complex mixtures. On the other hand, treatment of the gold(I) derivative 1' with BF3 ⋅OEt2 under mild conditions cleanly afforded the carbonyl derivative [Au(I) (CF3 )(CO)] (6), which can be isolated as an extremely moisture-sensitive light yellow crystalline solid. In the solid state, each linear F3 C-Au-CO molecule weakly interacts with three symmetry-related neighbors yielding an extended 3D network of aurophilic interactions (Au⋅⋅⋅Au=345.9(1) pm). The high $\tilde \nu $CO value (2194 cm(-1) in the solid state and 2180 cm(-1) in CH2 Cl2 solution) denotes that CO is acting as a mainly σ-donor ligand and confirms the role of the CF3 group as an electron-withdrawing ligand in organometallic chemistry. Compound 6 can be considered as a convenient synthon of the "Au(I) (CF3 )" fragment, as it reacts with a number of neutral ligands L, giving rise to the corresponding [Au(I) (CF3 )(L)] compounds (L=CNtBu (7), NCMe (8), py (9), tht (10)).

Stepwise degradation of trifluoromethyl platinum(II) compounds.

The action of moisture on the homoleptic organoplatinum(II) compound [NBu(4)](2)[Pt(CF(3))(4)] (1) gives rise to the carbonyl derivative [NBu(4)][Pt(CF(3))(3)(CO)] (2), which is itself moisture stable. However, treatment of compound 2 with HCl(aq) results in the formation of [NBu(4)][cis-Pt(CF(3))(2)Cl(CO)] (3), which undergoes degradation of an additional CF(3) group by further treatment with HCl(aq) in large excess, affording [NBu(4)][cis-Pt(CF(3))Cl(2)(CO)] (4). The carbonyl derivatives 2-4 are fairly stable species, in which the CO ligand, however, can be readily extruded by reaction with trimethylamine N-oxide (ONMe(3)). Thus, compound 2 reacts with ONMe(3) in the presence of a number of neutral or anionic ligands affording a series of singly or doubly charged derivatives with the general formulae [NBu(4)][Pt(CF(3))(3)(L)] [L = CNtBu (5), PPh(3) (6), P(o-tolyl)(3) (7), tht (8; tht = tetrahydrothiophene)] and [NBu(4)](2)[Pt(CF(3))(3)X] [X = Cl (9), Br (10), I (11)], respectively. Compound 2 also reacts with ONMe(3) and pyridin-2-thiol (C(5)H(5)NS) giving rise to the five-membered metallacyclic derivative [NBu(4)][Pt(CF(3))(2)(CF(2)NC(5)H(4)S-κC,κS)] (12), which can be viewed as a difluorocarbene species stabilized by intramolecular base coordination. On the other hand, treatment of compound 3 with ONMe(3) in the presence of C(5)H(5)NS yields the four-membered metallacyclic compound [NBu(4)][Pt(CF(3))(2)(NC(5)H(4)S-κN,κS)] (13). The geometries of the metallacycles in compounds 12 and 13 are compared. In the absence of any additional ligand, compound 3 undergoes dimerization producing the dinuclear species [NBu(4)](2)[{Pt(CF(3))(2)}(2)(µ-Cl)(2)] (14). Halide abstraction in the latter compound with AgClO(4) in THF yields the solvento compound cis-[Pt(CF(3))(2)(thf)(2)] (15). The highly labile character of the THF ligands in compound 15 makes this species a convenient synthon of the "cis-Pt(CF(3))(2)" unit.

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.

Solid-state and solution structure of a hypervalent AX5 compound: Sb(C6F5)5.

Eliminating restraints: a trigonal-bipyramidal structure has been found to be the energetically favored geometry of the hypervalent AX(5) molecule Sb(C(6)F(5))(5) in the solid state and also in fluid solution, where molecules move freely and no crystal packing effects operate.