Synthesis of Five‐ and Six‐Coordinate Tris(pentafluoroethyl)fluorosilicates

The research area of perfluoroalkylsilanes is still in its infancy. Although there are already many examples of difluorotriorganylsilicates, the first example of a completely characterized trifluorotriorganylsilicate is presented, the dianion [Si(C₂F₅)₃F₃]^2?. The strongly electron‐withdrawing influence of the pentafluoroethyl groups appears to be a fundamental cause of the stability of this compound. This dianion is also the first structurally characterized example of a tris(pentafluoroethyl)silicon compound. The synthesis and complete characterization of [PPh₄]₂[Si(C₂F₅)₃F₃] and [PPh₄][Si(C₂F₅)₃F₂] along with the precursor [H(OEt₂)₂][Si(C₂F₅)₃F₂] was achieved from SiCl₄ and LiC₂F₅.     

Synthesis and Characterization of Tetrakis(pentafluoroethyl)aluminate

While perfluorinated aryl, aryloxy and alkoxy aluminum species are well-established as weakly coordinating anions (WCAs), corresponding perfluoroalkyl aluminum derivatives are virtually unknown. Reaction of Si(C₂F₅)₃CH₃ with Li[AlH₄] afforded the tetrakis(pentafluoroethyl)aluminate, [Al(C₂F₅)₄]⁻. Several salts of the [Al(C₂F₅)₄]⁻ ion were synthesized and characterized by NMR spectroscopic methods, mass spectrometry, X-ray diffraction studies and elemental analysis.     

Versatile Coordination of AgI and CuI Ions towards cyclo-As5 Ligands

The reactions of the cyclo-As₅ complex [Cp*Fe(η⁵-As₅)] ( B ) with the Ag^I and Cu^I salts of the weakly coordinating anion (WCA) [FAl{OC₆F₁₀(C₆F₅)}₃]⁻ ([FAl]⁻) are studied. These reactions allow the synthesis of the mononuclear complexes [M(η⁵ : η²- B )₂][FAl] (M=Ag ( 1 ), Cu ( 2 )) when a ratio of B /M(FAl) 2 : 1 is used. Compound 1 shows an unusual disorder of the central Ag^I cation between two π-coordinating cyclo-As₅ ligands, which is absent in 2 pointing to a weak interaction of the Ag center towards the cyclo-As₅ ligands in B . When the ratio of B /Ag(FAl) is changed to 3 : 1 or 1 : 1, the respective coordination compounds [Ag(η²- B )₃][FAl] ( 3 ) and [Ag₂(η² : η²- B )₂][FAl]₂ ( 4 ) are accessible. The coordination modes of the cyclo-As₅ units in 1 , 3 and 4 are all different, reflecting the adaptive coordination behavior of B towards Ag^I ions. The optimized geometries in the gas phase of 1 – 4 are determined by DFT calculations to support the bonding situation observed in their solid-state structures.     

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.     

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.     

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.     

Cu[Al(ORF)4] Starting Materials and their Application in the Preparation of [Cu(Sn)]+ (n=12, 8) Complexes

Naked copper…?? A newly developed simple two‐step route to weakly coordinated Cu^I starting materials that were used to prepare novel copper–cyclosulfur adducts, including the first M⁺–S₁₂ complex (see figure, R^F= C(CF₃)₃, C(CH₃)(CF₃)₂, or CH(CF₃)₂). Reactions with the [Al{OC(CF₃)₃}₄]^? counterion mimic gas‐phase chemistry inside a mass spectrometer (to form [Cu(S₁₂)]⁺).

     

A Stable Crystalline Copper(I)–N2O Complex Stabilized as the Salt of a Weakly Coordinating Anion

Nitrous oxide is considered a poor ligand, and therefore only a handful of well‐defined metal–N₂O complexes are known. Oxidation of copper powder with an extreme oxidant, [Ag₂I₂][ An ]₂ ([ An ]^?=[Al(OC(CF₃)₃)₄]^?) in perfluorinated hexane leads to Cu^I[ An ], the first auxiliary ligand‐free Cu^I salt of the perfluorinated alkoxyaluminate anion. The compound is capable of forming a stable and crystalline complex with nitrous oxide, Cu(N₂O)[ An ], where the Cu?N₂O bond is by far the strongest among all other molecular metal–N₂O complexes known. Thorough characterization of the compounds together with the crystal structure of Cu(N₂O)[ An ] complex supported with DFT calculations are presented. These give insight into the bonding in the Cu⁺–N₂O system and confirm N‐end coordination of the ligand.     

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.

     

Hydrogen Storage Materials: Room‐Temperature Wet‐Chemistry Approach toward Mixed‐Metal Borohydrides

The poor kinetics of hydrogen evolution and the irreversibility of the hydrogen discharge hamper the use of transition metal borohydrides as hydrogen storage materials, and the drawbacks of current synthetic methods obstruct the exploration of these systems. A wet‐chemistry approach, which is based on solvent‐mediated metathesis reactions of precursors containing bulky organic cations and weakly coordinating anions, leads to mixed‐metal borohydrides that contain only a small amount of “dead mass”. The applicability of this method is exemplified by Li[Zn₂(BH₄)₅] and M[Zn(BH₄)₃] salts (M=Na, K), and its extension to other systems is discussed.     

Pentafluoroethylaluminates: A Combined Synthetic,Spectroscopic, and Structural Study

Salts of the tetrakis(pentafluoroethyl)aluminate anion [Al(C₂F₅)₄]⁻ were obtained from AlCl₃ and LiC₂F₅. They were isolated with different counter-cations and characterized by NMR and vibrational spectroscopy and mass spectrometry. Degradation of the [Al(C₂F₅)₄]⁻ ion was found to proceed via 1,2-fluorine shifts and stepwise loss of CF(CF₃) under formation of [(C₂F₅)_4−nAlF_n]⁻ (n=1–4) as assessed by NMR spectroscopy and mass spectrometry and supported by results of DFT calculations. In addition, the [(C₂F₅)AlF₃]⁻ ion was structurally characterized.     

Dicationic E4 Chains (E=P,As, Sb,Bi) Embedded in the Coordination Sphere of Transition Metals

The oxidation chemistry of the complexes [{CpMo(CO)₂}₂(μ,η²:η²‐E₂)] (E=P ( A ), As ( B ), Sb ( C ), Bi ( D )) is compared. The oxidation of A – D with [Thia]⁺ (=[C₁₂H₈S₂]⁺) results in the selective formation of the dicationic E₄ complexes [{CpMo(CO)₂}₄(μ₄,η²:η²:η²:η²‐E₄)]²⁺ (E=P ( 1 ), As ( 2 ), Sb ( 3 ), Bi ( 4 )), stabilized by four [CpMo(CO)₂] fragments. The formation of the corresponding monocations [ A ]⁺, [ C ]⁺, and [ D ]⁺ could not be detected by cyclic voltammetry, EPR, or NMR spectroscopy. This finding suggests that dimerization is fast and that there is no dissociation in solution, which was also predicted by DFT calculations. However, EPR measurements of 2 confirmed the presence of small amounts of the radical cation [ B ]⁺ in solution. Single‐crystal X‐ray diffraction revealed that the products 1 and 2 feature a zigzag E₄ chain in the solid state while 3 and 4 bear a central E₄ cage with a distorted “butterfly‐like” geometry. Additionally, 1 can be easily and reversibly converted into a symmetric and an unsymmetric form.     

Univalent Gallium and Indium Phosphane Complexes: From Pyramidal M(PPh3)3+ to Carbene‐Analogous Bent M(PtBu3)2+ (M=Ga,In) Complexes

In a new oxidative route, Ag⁺[Al(OR^F)₄]^? (R^F=C(CF₃)₃) and metallic indium were sonicated in aromatic solvents, such as fluorobenzene (PhF), to give a precipitate of silver metal and highly soluble [In(PhF)_n]⁺ salts (n=2, 3) with the weakly coordinating [Al(OR^F)₄]^? anion in quantitative yield. The In⁺ salt and the known analogous Ga⁺[Al(OR^F)₄]^? were used to synthesize a series of homoleptic PR₃ phosphane complexes [M(PR₃)_n]⁺, that is, the weakly PPh₃‐bridged [(Ph₃P)₃In–(PPh₃)–In(PPh₃)₃]²⁺ that essentially contains two independent [In(PPh₃)₃]⁺ cations or, with increasing bulk of the phosphane, the carbene‐analogous [M(PtBu₃)₂]⁺ (M=Ga, In) cations. The M^I? P distances are 27 to 29 pm longer for indium, and thus considerably longer than the difference between their tabulated radii (18 pm). The structure, formation, and frontier orbitals of these complexes were investigated by calculations at the BP86/SV(P), B3LYP/def2‐TZVPP, MP2/def2‐TZVPP, and SCS‐MP2/def2‐TZVPP levels.     

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.

     

Homoleptic Phosphaalkyne Complexes of Silver(I)

By employing silver salts with a weakly coordinating anion Ag[A] ([A]=[FAl{OC₁₂F₁₅}₃], [Al{OC(CF₃)₃}₄]), two phosphaalkynes could be coordinated side‐on to a bare silver(I) center to form the unprecedented homoleptic complexes [Ag(η²‐P≡CtBu)₂][FAl{OC₁₂F₁₅}₃] ( 1 ) and [Ag(η²‐P≡CtBu)₂][Al{OC(CF₃)₃}₄] ( 2 ). DFT calculations show that the perpendicular arrangement in 1 is the minimum energy structure of the coordination of the two phosphaalkynes to a silver atom, whereas for 2 a unique square‐planar coordination mode of the phosphaalkynes at Ag⁺ was found. Reactions with donor molecules yield the trigonally planar coordinated silver salts [((CH₃)₂CO)Ag(η²‐P≡CtBu)₂][FAl{OC₁₂F₁₅}₃] ( 3 ) and [(C₇H₈)₂Ag(η²‐P≡CtBu)][FAl{OC₁₂F₁₅}₃] ( 4 ). All of the compounds were comprehensively characterized in solution and in the solid state.