The Weakly Coordinating Tris(trichlorosilyl)silyl Anion

Closely following the procedure for the preparation of the base‐stabilized dichlorosilylene complex NHC^Dipp⋅SiCl₂ reported by Roesky, Stalke, and co‐workers (Angew. Chem. Int. Ed . 2009 , 48 , 5683–5686), a few crystals of the salt [NHC^Dipp−H⋅⋅⋅Cl⋅⋅⋅H−NHC^Dipp]Si(SiCl₃)₃ were isolated, aside from the reported byproduct [NHC^Dipp−H⁺⋅⋅⋅Cl⁻], and characterized by X‐ray crystallography (NHC^Dipp=N,N‐di(2,6‐diisopropylphenyl)imidazo‐2‐ylidene). They contain the weakly coordinating anion Si(SiCl₃)₃⁻, which was also obtained in high yields upon deprotonation of the conjugate Brønsted acid HSi(SiCl₃)₃ with NHC^Dipp or PMP (PMP=1,2,2,6,6‐pentamethylpiperidine). The acidity of HSi(SiCl₃)₃ was estimated by DFT calculations to be substantially higher than those of other H‐silanes. Further DFT studies on the electronic structure of Si(SiCl₃)₃⁻, including the electrostatic potential and the electron localizability, confirmed its low basicity and nucleophilicity compared with other silyl anions.     

Synthesis and Properties of the Weakly Coordinating Anion [Me3NB12Cl11]−

The weakly coordinating anion [Me₃NB₁₂Cl₁₁]^? has been prepared by a simple two‐step procedure. The anion [Me₃NB₁₂Cl₁₁]^? is easily obtained in batches of up to 20 g by chlorination of the known [H₃NB₁₂H₁₁]^? anion with SbCl₅ at about 190 °C and subsequent N‐methylation with methyl iodide. Starting from Na[Me₃NB₁₂Cl₁₁], several synthetically useful salts with reactive cations ([NO]⁺, [Ph₃C]⁺, and [(Et₃Si)₂H]⁺) were prepared. Full spectroscopic (NMR, IR, Raman, TGA, MS) characterization and single‐crystal X‐ray diffraction studies confirmed the identity and purity of the products. The thermal, chemical, and electrochemical stability as well as the basicity of the [Me₃NB₁₂Cl₁₁]^? anion is similar to that of the structurally related weakly coordinating 1‐carba‐closo‐dodecaborate and closo‐dodecaborate anions. The facile preparation of the [Me₃NB₁₂Cl₁₁]^? anion and its ideal chemical and physical properties make it a cheap alternative to other classes of weakly coordinating anions.     

Silver‐Free Activation of Ligated Gold(I) Chlorides: The Use of [Me3NB12Cl11]− as a Weakly Coordinating Anion in Homogeneous Gold Catalysis

Phosphane and N‐heterocyclic carbene ligated gold(I) chlorides can be effectively activated by Na[Me₃NB₁₂Cl₁₁] ( 1 ) under silver‐free conditions. This activation method with a weakly coordinating closo‐dodecaborate anion was shown to be suitable for a large variety of reactions known to be catalyzed by homogeneous gold species, ranging from carbocyclizations to heterocyclizations. Additionally, the capability of 1 in a previously unknown conversion of 5‐silyloxy‐1,6‐allenynes was demonstrated.     

Isobutene Polymerization Using [CuII(NCMe)6]2+ with Non‐Coordinating Anions as Catalysts

Cu^II compounds coordinated octahedrally with nitriles and associated with bulky, non‐coordinating counter ions can be applied in the polymerization of isobutene at 30 °C. High yields and a high content of terminal double bonds are reached in the resulting highly reactive polyisobutylenes, while the molecular masses are moderate. Two of the coordinating nitriles are more weakly coordinated than the other four, as can be concluded from an exemplary X‐ray structure and from vibrational spectra, thus providing easily accessible sites for substrate coordination.

     

A Highly Hexane Soluble Lithium Salt and other Starting Materials of the Fluorinated Weakly Coordinating Anion [Al{OC(CF3)2(CH2SiMe3)}4]−

The lithium salt of the weakly coordinating alkoxyaluminate anion Li[Al(OC(CF₃)₂(CH₂SiMe₃))₄] ( 2 ) is soluble in polar and even in non‐polar solvents. Especially the solubility in n‐hexane confirms 2 to be an excellent candidate for Li ion catalysis. Its polymeric structure consists of a seven coordinated Li⁺ cation, coordinating a [Al(OC(CF₃)₂(CH₂SiMe₃)]^? anion that serves as hexadentate O₂F₄ ligand and a further bridging F atom of a second anion. Compound 2 reacts with ClCPh₃ giving the [CPh₃]⁺ salt which is at least stable in CD₂Cl₂ over days at 298 K, but decomposes after storage at 333 K for several days.     

Generation and Applications of the Hydroxide Trihydrate Anion, [OH(OH2)3]−, Stabilized by a Weakly Coordinating Cation

The reaction of a strongly basic phosphazene (Schwesinger base) with water afforded the corresponding metastable hydroxide trihydrate [OH(OH₂)₃]^? salt. This is the first hydroxide solvate that is not in contact with a cation and furthermore one of rare known water‐stabilized hydroxide anions. Thermolysis in vacuum results in the decomposition of the hydroxide salt and quantitative liberation of the free phosphazene base. This approach was used to synthesize the Schwesinger base from its hydrochloride salt after anion exchange in excellent yields of over 97 %. This deprotonation method can also be used for the phosphazene‐base‐catalyzed preparation of the Ruppert–Prakash reagent Me₃SiCF₃ using fluoroform (HCF₃) as the trifluoromethyl building block and sodium hydroxide as the formal deprotonation agent.     

Ag[Fe(CO)5]2+: A Bare Silver Complex with Fe(CO)5 as a Ligand

Attempts to prepare Fe(CO)₅⁺ from Ag[Al(OR^F)₄] (R^F=C(CF₃)₃) and Fe(CO)₅ in CH₂Cl₂ yielded the first complex of a neutral metal carbonyl bound to a simple metal cation. The Ag[Fe(CO)₅]₂⁺ cation consists of two Fe(CO)₅ molecules coordinating Ag⁺ in an almost linear fashion. The ν(CO) modes are blue‐shifted compared to Fe(CO)₅, with one band above 2143 cm^?1 indicating that back‐bonding is heavily decreased in the Ag[Fe(CO)₅]₂⁺ cation.     

Silver Salts of the Weakly Coordinating Anion [Me3NB12Cl11]–

Silver(I) salts of weakly coordinating anions (WCA) are commonly applied as oxidizing agents or halide abstracting reagents. The feasibility of a particular silver salt for such applications strongly depends on the “nakedness“ of the silver cation. In this study the reactivity of Ag[Me₃NB₁₂Cl₁₁] in different solvents was investigated. Crystal structures of a variety of complexes were obtained. In several crystal structures two boron clusters are bridged by Ag–Cl contacts. This leads to polymeric structures (e.g. for Ag[Me₃NB₁₂Cl₁₁]·0.5CH₂Cl₂ and Ag[Me₃NB₁₂Cl₁₁]·SO₂). Sterically demanding aromatics like mesitylene, pyrene, and acenaphthene are η¹‐ or η²‐bonded to the silver atom and also form coordination polymers, whereas benzene as a ligand leads to a molecular structure, in which two benzene molecules are η²‐coordinated to the silver cation. In contrast, strong σ donor ligands like pyridine and triphenylphosphine give homoleptic silver complexes and thus cation and anion are separated. Furthermore, the ability of Ag[Me₃NB₁₂Cl₁₁] for performing metathesis reactions was investigated. The reaction with Au^ICl gave the [Au(NCMe)₂]⁺ cation.     

Silver(I) Complexes of the Weakly Coordinating Solvents SO2 and CH2Cl2: Crystal Structures,Bonding, and Energetics of [Ag(OSO)][Al{OC(CF3)3}4], [Ag(OSO)2/2][SbF6], and [Ag(CH2Cl2)2][SbF6]

Pushing the limits of coordination chemistry : The most weakly coordinated silver complexes of the very weakly coordinating solvents dichloromethane and liquid sulfur dioxide were prepared. Special techniques at low temperatures and the use of weakly coordinating anions allowed structural characterization of [Ag(OSO)][Al{OC(CF₃)₃}₄], [Ag(OSO)_2/2][SbF₆], and [Ag(Cl₂CH₂)₂][SbF₆] (see figure). An investigation of the bonding shows that these complexes are mainly stabilized by electrostatic monopole–dipole interactions.

     

A Stable Organic π‐Radical of a Zinc(II)–Copper(I)–Zinc(II) Complex of Decaphyrin

A Zn^II‐Cu^I‐Zn^II heterotrimetal complex of decaphyrin was synthesized by stepwise metalations: metalation of a [46]decaphyrin with Zn^II ions to produce a 46π decaphyrin bis(Zn^II) complex and its subsequent metalation with Cu^II ion. In the second metalation step, it has been shown that Cu^II ion is reduced to a Cu^I ion in the complex and a dianionic bis(Zn^II) containing [46]decaphyrin ligand is oxidized to the corresponding monoanionic [45]decaphyrin ligand, indicating a non‐innocent nature of the decaphyrin ligand. Despite the radical nature, the heterotrimetal complex is fairly stable under ambient conditions and exhibits almost no intermolecular magnetic interaction, owing to extensive delocalization of an unpaired electron in the large π‐conjugated circuit of decaphyrin moiety.     

Fluorinated Weakly Coordinating Anions [M(hfip)6]– (M = Nb,Ta): Syntheses,Structural Characterizations and ­Computations

The synthesis, spectroscopic and structural characterisation of a series of [M(hfip)₆]^– (M = Nb, Ta; hfip = O–C(H)(CF₃)₂) salts that are the typical starting materials to introduce these weakly coordinating anions by metathesis reactions into a given system is described. The salts Li[Nb(hfip)₆] and Li[Ta(hfip)₆] formed in 65 to 77 % yield from freshly sublimed MCl₅ and Li[hfip]. By contrast, several attempts to synthesize Li[Sb(hfip)₆] on the similar route (replace NbCl₅ by SbCl₅) failed to yield a pure product. Upon metathesis of the Li‐niobate with AgF in CH₂Cl₂, the pure Ag[Nb(hfip)₆] formed. Mixing Li[Nb(hfip)₆] with an equimolar amount of Cl–CPh₃ in CH₂Cl₂ gave the yellow [CPh₃][Nb(hfip)₆]. Several of the compounds were characterized by X‐ray analysis. Thus, the crystal structures of the Li⁺‐ and Ag⁺‐solvates 1, 2‐C₆H₄F₂{LiNb(hfip)₆}₂, [Li(H₂O)][Ta(hfip)₆], and [Ag(C₆H₅F)][Nb(hfip)₆] as well as that of [CPh₃][Nb(hfip)₆] were solved and are described in this work.     

H(OEt2)2[P(1,2‐O2C6Cl4)3]: Synthesis,Characterization, and Application as a Single‐Component Initiator for the Carbocationic Polymerization of Olefins

The development of novel Brønsted acids featuring the hexacoordinate phosphorus(V) anion [TRISPHAT]^? {[ 1 ]^?=[P(1,2‐O₂C₆Cl₄)₃]^?} are reported. The title compound, H(OEt₂)₂[ 1 ], was synthesized from 1,2‐(HO)₂C₆Cl₄ (3 equiv) and PCl₅ in the presence of diethyl ether. This compound was fully characterized by ¹H, ³¹P and ¹³C NMR spectroscopy, X‐ray crystallography and elemental microanalysis. Dissolution of H(OEt₂)₂[ 1 ] in acetonitrile results in the slow precipitation of crystalline H(OEt₂)(NCMe)[ 1 ], which was characterized by X‐ray diffraction; however, in CD₂Cl₂ solution the [TRISPHAT]^? anion protonated and ring‐opened. The weighable, solid H(OEt₂)₂ [ 1 ] was found to be a competent initiator for the polymerization of n‐butyl vinyl ether, α‐methylstyrene, styrene and isoprene at a variety of temperatures and monomer‐to‐initiator ratios. At low temperatures, polymers with M_n>10⁵ were obtained for n‐butyl vinyl ether and α‐methylstyrene whereas slightly lower molecular weights were obtained with styrene and isoprene (10⁴<M_n<10⁵). The poly(α‐methylstyrene) synthesized at ?78 °C is syndiotactic‐rich (ca. 87 % rr) whereas the polystyrene obtained at ?50 °C is atactic. The polyisoprene obtained possessed all possible modes of enchainment as well as branched and/or cyclic structures that are often observed in polyisoprene.     

A Janus‐Headed Lewis Superacid: Simple Access to,and First Application of Me3Si‐F‐Al(ORF)3

Upon reaction of gaseous Me₃SiF with the in situ prepared Lewis acid Al(OR^F)₃, the stable ion‐like silylium compound Me₃Si‐F‐Al(OR^F)₃ 1 forms. The Janus‐headed 1 is a readily available smart Lewis acid that differentiates between hard and soft nucleophiles, but also polymerizes isobutene effectively. Thus, in reactions of 1 with soft nucleophiles (Nu), such as phosphanes, the silylium side interacts in an orbital‐controlled manner, with formation of [Me₃Si?Nu]⁺ and the weakly coordinating [F?Al(OR^F)₃]^– or [(^FRO)₃Al‐F‐Al(OR^F)₃]^– anions. If exchanged for hard nucleophiles, such as primary alcohols, the aluminum side reacts in a charge‐controlled manner, with release of FSiMe₃ gas and formation of the adduct R(H)O?Al(OR^F)₃. Compound 1 very effectively initiates polymerization of 8 to 21 mL of liquid C₄H₈ in 50 mL of CH₂Cl₂ already at temperatures between ?57 and ?30 °C with initiator loads as low as 10 mg in a few seconds with 100 % yield but broad polydispersities.     

Silver–Ethene Complexes [Ag(η2‐C2H4)n][Al(ORF)4] with n=1, 2, 3 (RF=Fluorine‐Substituted Group)

Compounds including the free or coordinated gas‐phase cations [Ag(η²‐C₂H₄)_n]⁺ (n=1–3) were stabilized with very weakly coordinating anions [A]^? (A=Al{OC(CH₃)(CF₃)₂}₄, n=1 ( 1 ); Al{OC(H)(CF₃)₂}₄, n=2 ( 3 ); Al{OC(CF₃)₃}₄, n=3 ( 5 ); {(F₃C)₃CO}₃Al‐F‐Al{OC(CF₃)₃}₃, n=3 ( 6 )). They were prepared by reaction of the respective silver(I) salts with stoichiometric amounts of ethene in CH₂Cl₂ solution. As a reference we also prepared the isobutene complex [(Me₂C?CH₂)Ag(Al{OC(CH₃)(CF₃)₂}₄)] ( 2 ). The compounds were characterized by multinuclear solution‐NMR, solid‐state MAS‐NMR, IR and Raman spectroscopy as well as by their single crystal X‐ray structures. MAS‐NMR spectroscopy shows that the [Ag(η²‐C₂H₄)₃]⁺ cation in its [Al{OC(CF₃)₃}₄]^? salt exhibits time‐averaged D_3h‐symmetry and freely rotates around its principal z‐axis in the solid state. All routine X‐ray structures (2θ_max.<55°) converged within the 3σ limit at C?C double bond lengths that were shorter or similar to that of free ethene. In contrast, the respective Raman active C?C stretching modes indicated red‐shifts of 38 to 45 cm^?1, suggesting a slight C?C bond elongation. This mismatch is owed to residual librational motion at 100 K, the temperature of the data collection, as well as the lack of high angular data owing to the anisotropic electron distribution in the ethene molecule. Therefore, a method for the extraction of the C?C distance in [M(C₂H₄)] complexes from experimental Raman data was developed and meaningful C?C distances were obtained. These spectroscopic C?C distances compare well to newly collected X‐ray data obtained at high resolution (2θ_max.=100°) and low temperature (100 K). To complement the experimental data as well as to obtain further insight into bond formation, the complexes with up to three ligands were studied theoretically. The calculations were performed with DFT (BP86/TZVPP, PBE0/TZVPP), MP2/TZVPP and partly CCSD(T)/AUG‐cc‐pVTZ methods. In most cases several isomers were considered. Additionally, [M(C₂H₄)₃] (M=Cu⁺, Ag⁺, Au⁺, Ni⁰, Pd⁰, Pt⁰, Na⁺) were investigated with AIM theory to substantiate the preference for a planar conformation and to estimate the importance of σ donation and π back donation. Comparing the group 10 and 11 analogues, we find that the lack of π back bonding in the group 11 cations is almost compensated by increased σ donation.     

Synthesis,Properties, and Application of Tetrakis(pentafluoroethyl)gallate, [Ga(C2F5)4]−

Weakly coordinating anions (WCAs) are important for academic reasons as well as for technical applications. Tetrakis(pentafluoroethyl)gallate, [Ga(C₂F₅)₄]^?, a new WCA, is accessible by treatment of [GaCl₃(dmap)] (dmap=4‐dimethylaminopyridine) with LiC₂F₅. The anion [Ga(C₂F₅)₄]^? proved to be reluctant towards deterioration by aqueous hydrochloric acid or lithium hydroxide. Various salts of [Ga(C₂F₅)₄]^? were synthesized with cations such as [PPh₄]⁺, [CPh₃]⁺, [(O₂H₅)₂(OH₂)₂]²⁺, and [Li(dec)₂]⁺ (dec=diethyl carbonate). Thermolysis of [(O₂H₅)₂(OH₂)₂][Ga(C₂F₅)₄]₂ gives rise to a dihydrate of tris(pentafluoroethyl)gallane, [Ga(C₂F₅)₃(OH₂)₂]. All products were characterized by NMR and IR spectroscopy, mass spectrometry, X‐ray diffraction, and elemental analysis. Furthermore, an outlook for the application of [Li(dec)₂][Ga(C₂F₅)₄] as a conducting salt in lithium‐ion batteries is presented.