An Aliphatic Solvent-Soluble Lithium Salt of the Perhalogenated Weakly Coordinating Anion [Al(OC(CCl3)(CF3)2)4]−

Structural diversity of mixed polypnictogen complexes: dicationic E2E'2 (E ≠ E' = P, As, Sb, Bi) chains, cycles and cages stabilized by transition metals

The reactivity of the tetrahedral dipnictogen complexes [{CpMo(CO)₂}₂(μ,η²:η²-EE′)] (E, E′ = P, As, Sb, Bi; “Mo₂EE′”) towards different one-electron oxidation agents is reported. Oxidation with [Thia][TEF] (Thia⁺ = C₁₂H₈S₂⁺; TEF⁻ = Al{OC(CF₃)₃}₄⁻) leads to the selective formation of the radical monocations [Mo₂EE′]˙⁺, which immediately dimerize to the unprecedented dicationic E₂E′₂ ligand complexes [{CpMo(CO)₂}₄(μ₄,η²:η²:η²:η²-E′EEE′)]²⁺via E–E bond formation. Single crystal X-ray diffraction revealed that, in the case of Mo₂PAs and Mo₂PSb, P–P bond formation occurs yielding zigzag E₂P₂ (E = As (1), Sb (2)) chains, whereas Mo₂SbBi forms a Sb₂Bi₂ (5) cage, Mo₂AsSb an unprecedented As₂Sb₂ unit representing an intermediate stage between a chain- and a cage-type structure, and Mo₂AsBi a novel planar As₂Bi₂ (4a) cycle. Therefore, 1–5 bear the first substituent-free, dicationic hetero-E₄ ligands, stabilized by transition metal fragments. Furthermore, in the case of Mo₂AsSb, the exchange of the counterion causes changes in the molecular structure yielding an unusual, cyclic As₂Sb₂ ligand. The experimental results are corroborated by DFT calculations.

Unique dicationic hetero-tetrapnictogen E₂E′₂ (E ≠ E′ = P, As, Sb, Bi) chains and cages are obtained via oxidation of the tetrahedranes [{CpMo(CO)₂}₂(μ,η²:η²-EE′)]. Exchange of the counterion causes an unusual cyclization of the As₂Sb₂ ligand.

Hydrostibination of Alkynes: A Radical Mechanism*

The addition of Sb-H bonds to alkynes was reported recently as a new hydroelementation reaction that exclusively yields anti-Markovnikov Z-olefins from terminal acetylenes. We examine four possible mechanisms that are consistent with the observed stereochemical and regiochemical outcomes. A comprehensive analysis of solvent, substituent, isotope, additive, and temperature effects on hydrostibination reaction rates definitively refutes three ionic mechanisms involving closed-shell charged intermediates. Instead the data support a fourth pathway featuring open-shell neutral intermediates. Density-functional theory (DFT) calculations are consistent with this model, predicting an activation barrier that is in agreement with the experimental value (Eyring analysis) and a rate limiting step that is congruent with the experimental kinetic isotope effect. We therefore conclude that hydrostibination of arylacetylenes is initiated by the generation of stibinyl radicals, which then participate in a cycle featuring Sb^II and Sb^III intermediates to yield the observed Z-olefins as products. This mechanistic understanding will enable rational evolution of hydrostibination as a synthetic methodology.

Building blocks for the chemistry of perfluorinated alkoxyaluminates [Al{OC(CF3)3}4]-: simplified preparation and characterization of Li+-Cs+, Ag+, NH4+, N2H5+ and N2H7+ salts

Advanced weakly coordinating anions (WCAs) significantly facilitate synthesis of various exotic chemical compounds and novel, potentially useful materials. One of such anions - [Al{OC(CF₃)₃}₄]^-, denoted [Al(OR^F)₄]^-, appears particularly convenient, as it can be easily prepared from the commercially available alanates and HOC(CF₃)₃. Here we present a thorough characterization of a series of solvent-free M[Al(OR^F)₄] salts, M = Li-Cs, Ag, NH₄, N₂H₅ and N₂H₇, and related compounds of monovalent cations, which are crucial starting materials for further work with these species. Notably, the corresponding synthetic protocols are updated by an improved method for fast, facile and easily scalable synthesis of Li[Al(OR^F)₄], which remains the most useful primary source of the anion. The physico-chemical properties of these salts including crystal structures, thermal stability by TG/DSC, vibrational spectra as well as solubility are discussed in a systematic fashion.

Stable Radical Cation and Dication of an N-Heterocyclic Carbene Stabilized Digallene: Synthesis, Characterization and Reactivity

One- and two-electron oxidation of a digallene stabilized by an N-heterocyclic carbene afforded the first stable gallium-based radical cation and dication salts, respectively. Structural analysis and theoretical calculations reveal that the oxidation occurs at the Ga=Ga double bond, leading to removal of π electrons of the double bond and a decrease of the bond order. The spin density of the radical cation mainly locates at the two gallium centers as demonstrated by EPR spectroscopy and theoretical calculations. Moreover, the reactivity of the radical cation salt toward nBu₃ SnH and cyclo-S₈ was studied; a digallium-hydride cation salt containing a Ga-Ga single bond and a gallium sulfide cluster bearing an unprecedented ladder-like Ga₄ S₄ core structure were obtained, respectively.

Bimolecular vinylation of arenes by vinyl cations

Styrene derivatives can be easily synthesized from vinyl triflates and arenes under mild reaction conditions, using [Li][Al(OC(CF₃)₃)₄] as a catalyst and LiHMDS as a base.

Facile and systematic access to the least-coordinating WCA [(RFO)3Al-F-Al(ORF)3]- and its more Lewis-basic brother [F-Al(ORF)3]- (RF = C(CF3)3)

By reaction of the Lewis acid Me3Si-F-Al(ORF)3 with a series of [PF6]- salts, gaseous PF5 and Me3Si-F are liberated and salts of the anion [F-Al(ORF)3]- ([f-al]-; RF = C(CF3)3) can be obtained. By addition of another equivalent of Me3Si-F-Al(ORF)3 to [f-al]-, gaseous Me3Si-F is released and salts of the least coordinating anion [(RFO)3Al-F-Al(ORF)3]- ([al-f-al]-) are formed. Both procedures work for a series of synthetically useful cations including Ag+, [NO]+, [Ph3C]+ and in very clean reactions with 5 g batch sizes giving excellent yields typically exceeding 90%. In addition, the synthesis of Me3Si-F-Al(ORF)3 has been optimized and scaled up to 85 g batches in an one-pot procedure. These anions could previously only be obtained by difficult to control decomposition reactions of [Al(ORF)4]- or by halide abstraction reactions with Me3Si-F-Al(ORF)3, generating relatively large countercations that are unsuited for further use as universal starting materials. Especially [al-f-al]- is of interest for the stabilization of reactive cations, since it is even weaker coordinating than [Al(ORF)4]- and more stable against strong electrophiles. This bridged anion can be seen as an adduct of [f-al]- and Al(ORF)3. Thus, it is similarly Lewis acidic as BF3 and eventually reacts with nucleophiles (Nu) from the reaction environment to yield Nu-Al(ORF)3 and [f-al]-. This prevents working with [al-f-al]- salts in ethereal or other donor solvents. By contrast, the [f-al]- anion is no longer Lewis acidic and may therefore be used for reactions involving stronger nucleophiles than the [al-f-al]- anion can withstand. Subsequently it may be transformed into the [al-f-al]- salt by simple addition of one equivalent of Me3Si-F-Al(ORF)3.