Superacids Based on Pentafluoroorthotellurate Derivatives of Aluminum

The new Lewis acid Al(OTeF₅)₃ and its acetonitrile adduct CH₃CN→Al(OTeF₅)₃ were obtained by a simple one-step synthesis in batches of up to 15 g. Al(OTeF₅)₃ and the adduct were characterized by vibrational spectroscopy (IR, Raman) and quantum-chemical calculations. Furthermore, five different salts of the new weakly coordinating anion [Al(OTeF₅)₄]⁻ were prepared in a two-step procedure. [Ph₄P][Al(OTeF₅)₄], Cs[Al(OTeF₅)₄], [Ph₃C][Al(OTeF₅)₄], as well as the protonated benzene derivatives [C₉H₁₃][Al(OTeF₅)₄] and [C₆H₇][Al(OTeF₅)₄] were characterized by low-temperature single-crystal X-ray diffraction and NMR spectroscopy. Arenium salts have rarely been characterized in the solid state and were synthesized in this work in a simplified fashion.     

Energetics of hydride and electron pair attachment to EX3 (E = B, C, Al, Si and X = F, Cl, Br, I) and the study of bonding trends among EX3, EX3, and EX3H by use of ELF and NBO analyses

A theoretical gas-phase “ligand-free” or “electron pair affinity” (EPA) approach, based on CCSD(T)/(SDB-)cc-pVTZ//MP2/(SDB-)cc-pVTZ electronic structure calculations, is introduced as a possible means for determining Lewis acidity trends among planar EX₃^0/+ (E = B, C, Al, Si; X = F, Cl, Br, I) species. In this treatment, the free electron pair is considered to be an extreme Lewis base. The calculated EPA values are compared with experimental Lewis acidities, previously calculated fluoride ion affinity (FIA) and hydride ion affinity (HA) trends, and are found to exhibit reasonable correlations in all cases. The bonding in the planar and trigonal pyramidal conformations of EX₃^0/+ and of the trigonal pyramidal Lewis base EX₃^2−/− anions are assessed by use of natural bond orbital (NBO) and natural resonance theory (NRT) analyses. The NBO charges of the CX₃⁺ (X = Cl, Br, I, OTeF₅) cations are shown to correlate with the cation-anion and cation-solvent contacts in the recently determined crystal structures of [CCl₃][Sb(OTeF₅)₆], [CBr₃][Sb(OTeF₅)₆]·SO₂ClF, [CI₃][Al(OC(CF₃)₃)₄], and [C(OTeF₅)₃][Sb(OTeF₅)₆]·3SO₂ClF and known fluoro-carbocation structures. Topological electron localization function (ELF) basin lobe isosurfaces and volumes are used to rationalize the Lewis acidity trends and bond ionicities of the EX₃^0/+ species, and Lewis basicities of the EX₃^2−/− species.     

Unravelling the Role of the Pentafluoroorthotellurate Group as a Ligand in Nickel Chemistry

The pentafluoroorthotellurate group (teflate, OTeF₅) is able to form species, for which only the fluoride analogues are known. Despite nickel fluorides being widely investigated, nickel teflates have remained elusive for decades. By reaction of [NiCl₄]²⁻ and neat ClOTeF₅, we have synthesized the homoleptic [Ni(OTeF₅)₄]²⁻ anion, which presents a distorted tetrahedral structure, unlike the polymeric [NiF₄]²⁻. This high-spin complex has allowed the study of the electronic properties of the teflate group, which can be classified as a weak/medium-field ligand, and therefore behaves as the fluoride analogue also in ligand-field terms. The teflate ligands in [NEt₄]₂[Ni(OTeF₅)₄] are easily substituted, as shown by the formation of [Ni(NCMe)₆][OTeF₅]₂ by dissolving it in acetonitrile. Nevertheless, careful reactions with other conventional ligands have enabled the crystallization of nickel teflate complexes with different coordination geometries, i.e. [NEt₄]₂[trans-Ni(OEt₂)₂(OTeF₅)₄] or [NEt₄][Ni(bpyMe₂)(OTeF₅)₃].     

Darstellung und elektrochemisches Verhalten von [Nb(OTeF5)6]− und [Ta(OTeF5)6]−-Komplexen

Preparation and Electrochemistry of [Nb(OTeF₅)₆]^? and [Ta(OTeF₅)₆]^? Complexes Nb(OTeF₅)₅ and Ta(OTeF₅)₅ react with Cs[OTeF₅], [Et₄N][OTeF₅], and [(n-Bu)₄N][OTeF₅] to the corresponding Cs[M(OTeF₅)₆], [Et₄N][M(OTeF₅)₆], and [(n-Bu)₄N][M(OTeF₅)₆] complexes, (M = Nb, Ta). The electrochemical reduction of the niobium complex occurs in CH₂Cl₂ at ?0,69 V and in acetonitrile at ?0,60 V (vs. SCE). The tantalum complex is reduced in CH₂Cl₂ at ?1,52 V and in acetonitrile at ?1,42 V (vs. SCE).     

Oxygen-Bridged Ga2(Et)3(OTeF5)3 and the Weakly Coordinating Anions [Ga(Et)(OTeF5)3]− and [Ga(OTeF5)4]−

Salts of the weakly coordinating anions [Ga(OTeF₅)₄]⁻ as well as [Ga(Et)(OTeF₅)₃]⁻ and the neutral Ga₂(Et)₃(OTeF₅)₃ were synthesized and characterized by spectroscopic methods and single-crystal X-ray diffraction. Ga₂(Et)₃(OTeF₅)₃ was formed by treating GaEt₃ with pentafluoroorthotelluric acid (HOTeF₅) and reacted with PPh₄Cl and CPh₃Cl to [PPh₄][Ga(Et)(OTeF₅)₃] and [CPh₃][Ga(Et)(OTeF₅)₃]. In contrast, Ag[Ga(OTeF₅)₄] was prepared from AgOTeF₅ and GaCl₃ and was used as a versatile starting material for further reactions. Starting with Ag[Ga(OTeF₅)₄] the substrates [PPh₄][Ga(OTeF₅)₄] and [CPh₃][Ga(OTeF₅)₄] were formed from PPh₄Cl and CPh₃Cl.     

Formation and Crystal Structure of [(F5TeOXe)+·SO2ClF][Sb(OTeF5)6-]

Xe(OTeF₅)₂ reacts with Sb(OTeF₅)₃ under the formation of [Xe₂(OTeF₅)₃]⁺[Sb(OTeF₅)₆]^-. From SO₂ClF solution a yellow solvate [F₅TeOXe]⁺·SO₂ClF· [Sb(OTeF₅)₆]^- is formed with the crystal data: a = 1028.1(1), b = 1040.9(1), c = 1780.2(3) pm, α = 98.07(1), β = 97.68(1), γ = 105.82(1)°, space group . The O-Xe···O fragment is essentially linear (176.1(2)°), and the two Xe-O distances are quite different 197.1(4) and 242.6(4) pm.     

Vergleichende Untersuchungen zur Struktur von 4‐Dimethylaminopyridin‐Addukten

Comparative Structural Studies on 4‐Dimethylaminopyridine‐Adducts Lewis acid‐base adducts of the type dmap—MMe₃ (M = Al 1 , Ga 2 , In 3 , Tl 4 ) as well as dmap—AlCl₃ ( 6 ) and dmap—Al(t‐Bu)₃ ( 7 ) were synthesized by reaction of MR₃ with 4‐dimethylamino‐pyridine (dmap) whereas dmap—AlH₃ ( 5 ) was obtained from AlH₃·Et₂O. 1 — 7 were characterized by means of NMR (¹H, ¹³C{¹H}) and mass spectrometry and elemental analysis. In addition, their solid state structures were determined by single crystal X‐ray diffraction studies. A comparison of the structural parameters reveales the influence of both electronic (Lewis acidity of the group 13 atom) and steric interactions on the structure and stability of as prepared Lewis acid‐base adducts.     

Reactivity of Lewis Basic Platinum Complexes Towards Fluoroboranes

We herein report detailed investigations into the interaction of Lewis acidic fluoroboranes, for example BF₂Pf (Pf=perfluorophenyl) and BF₂Ar^F (Ar^F=3,5‐bis(trifluoromethyl)phenyl), with Lewis basic platinum complexes such as [Pt(PEt₃)₃] and [Pt(PCy₃)₂] (Cy=cyclohexyl). Two presumed Lewis adducts could be identified in solution and corresponding secondary products of these Lewis adducts were characterized in the solid state. Furthermore, the concept of frustrated Lewis pairs (FLP) was applied to the activation of ethene in the system [Pt(BPf₃)(CH₂CH₂)(dcpp)] (dcpp=1,3‐bis(dicyclohexylphosphino)propane; Pf=perfluorophenyl). Finally, DFT calculations were performed to determine the interaction between the platinum‐centered Lewis bases and the boron‐centered Lewis acids. Additionally, several possible mechanisms for the oxidative addition of the boranes BF₃, BCl_3, and BF₂Ar^F to the model complex [Pt(PMe₃)₂] are presented.     

Cationic-lanthanide-complex-catalyzed reaction of 2-hydroxychalcones with naphthols: Facile synthesis of 2,8-dioxabicyclo[3.3.1]nonanes

Cationic lanthanide complexes [Ln(CH₃CN)₉]³⁺[(AlCl₄)₃]^3–·CH₃CN served as efficient catalysts for the tandem Friedel–Crafts alkylation/ketalization reaction of 2-hydroxychalcones with naphthols/substituted phenols to produce diaryl-fused 2,8-dioxabicyclo[3.3.1]nonanes in moderate to high yields. The high catalytic efficiency of the cationic lanthanide complex [Yb(CH₃CN)₉]³⁺[(AlCl₄)₃]^3–·CH₃CN compared with that of YbCl₃ can be attributed to the increased Lewis acidity of the Yb species as a result of cation formation.     

Synthesis,Characterization, and Application of Two Al(ORF)3 Lewis Superacids

We report herein the synthesis and full characterization of the donor‐free Lewis superacids Al(OR^F)₃ with OR^F=OC(CF₃)₃ ( 1 ) and OC(C₅F₁₀)C₆F₅ ( 2 ), the stabilization of 1 as adducts with the very weak Lewis bases PhF, 1,2‐F₂C₆H₄, and SO₂, as well as the internal C? F activation pathway of 1 leading to Al₂(F)(OR^F)₅ ( 4 ) and trimeric [FAl(OR^F)₂]₃ ( 5 , OR^F=OC(CF₃)₃). Insights have been gained from NMR studies, single‐crystal structure determinations, and DFT calculations. The usefulness of these Lewis acids for halide abstractions has been demonstrated by reactions with trityl chloride (NMR; crystal structures). The trityl salts allow the introduction of new, heteroleptic weakly coordinating [Cl‐Al(OR^F)₃]^? anions, for example, by hydride or alkyl abstraction reactions.     

Friedel–Crafts Type Methylation with Dimethylhalonium Salts

The dimethylchloronium salt [Me₂Cl][Al(OTeF₅)₄] is used to methylate electron-deficient aromatic systems in Friedel–Crafts type reactions as shown by the synthesis of N-methylated cations, such as [MeNC₅F₅]⁺, [MeNC₅F₄I]⁺, and [MeN₃C₃F₃]⁺. To gain a better understanding of such fundamental Friedel–Crafts reactions, the role of the dimethylchloronium cation has been evaluated by quantum-chemical calculations.     

Silver tetrakis(hexafluoroisopropoxy)aluminate as hexafluoroisopropyl transfer reagent for the chlorine/hexafluoroisopropyl exchange in imino phosphanes

Treating 1,3-dichloro-2,4-bis[tris(trimethylsilyl)silyl]-cyclo-diphosphadiazane, [HypNPCl]₂ ((Me₃Si)₃Si = Hyp), or N-(2,4,6-tri-tert-butylphenyl)imino(chloro)phosphane, Mes^∗-NP-Cl (Mes^∗ = 2,4,6-tri-tert-butylphenyl), with Ag[Al(OCH(CF₃)₂)₄] leads to the abstraction of [OCH(CF₃)₂]⁻ from the counter ion [Al(OCH(CF₃)₂)₄]⁻ in a formal Lewis acid/Lewis base reaction. The final products Hyp₂N₂P₂(Cl)(OCH(CF₃)₂), Mes^∗-NP-OCH(CF₃)₂ and the dimeric Lewis acid [Al(OCH(CF₃)₂)₃]₂ have been characterized by means of X-ray analysis.     

From Weakly Coordinating to Non‐Coordinating Anions? A Simple Preparation of the Silver Salt of the Least Coordinating Anion and Its Application To Determine the Ground State Structure of the Ag(η2‐P4)2+ Cation

The unexpected but facile preparation of the silver salt of the least coordinating [(RO)₃Al‐F‐Al(OR)₃]^? anion (R=C(CF₃)₃) by reaction of Ag[Al(OR)₄] with one equivalent of PCl₃ is described. The mechanism of the formation of Ag[(RO)₃Al‐F‐Al(OR)₃] is explained based on the available experimental data as well as on quantum chemical calculations with the inclusion of entropy and COSMO solvation enthalpies. The crystal structures of (RO)₃Al←OC₄H₈, Cs⁺[(RO)₂(Me)Al‐F‐Al(Me)(OR)₂]^?, Ag(CH₂Cl₂)₃⁺[(RO)₃Al‐F‐Al(OR)₃]^? and Ag(η²‐P₄)₂⁺[(RO)₃Al‐F‐Al(OR)₃]^? are described. From the collected data it will be shown that the [(RO)₃Al‐F‐Al(OR)₃]^? anion is the least coordinating anion currently known. With respect to the fluoride ion affinity of two parent Lewis acids Al(OR)₃ of 685 kJ mol^?1, the ligand affinity (441 kJ mol^?1), the proton and copper decomposition reactions (?983 and ?297 kJ mol^?1) as well as HOMO level and HOMO–LUMO gap and in comparison with [Sb₄F₂₁]^?, [Sb(OTeF₅)₆]^?, [Al(OR)₄]^? as well as [B(R^F)₄]^? (R^F=CF₃ or C₆F₅) the [(RO)₃Al‐F‐Al(OR)₃]^? anion is among the best weakly coordinating anions (WCAs) according to each value. In contrast to most of the other cited anions, the [(RO)₃Al‐F‐Al(OR)₃] anion is available by a simple preparation in conventional inorganic laboratories. The least coordinating character of this anion was employed to clarify the question of the ground state geometry of the Ag(η²‐P₄)₂⁺ cation (D_2h, D₂ or D_2d?). In agreement with computational data and NMR spectra it could be shown that the rotation along the Ag‐(P‐P‐centroid) vector has no barrier and that the structure adopted in the solid state depends on packing effects which lead to an almost D_2h symmetric Ag(η²‐P₄)₂⁺ cation (0 to 10.6° torsion) for the more symmetrical [Al(OR)₄]^? anion, but to a D₂ symmetric Ag(η²‐P₄)₂⁺ cation with a 44° twist angle of the two AgP₂ planes for the less symmetrical [(RO)₃Al‐F‐Al(OR)₃]^? anion. This implies that silver back bonding, suggested by quantum chemical population analyses to be of importance, is only weak.     

Pyridine Adducts of Tricyano- and Dicyanoboranes

Cyanoborane adducts of the Lewis acids B(CN)₃, BF(CN)₂, and BH(CN)₂ with pyridine and 4-cyanopyridine have been obtained in high yields. The syntheses were accomplished by oxidation of the readily available potassium salts of the cyano(hydrido)borate anions [BH(CN)₃]⁻ ( MHB ), [BFH(CN)₂]⁻ ( FHB ), and [BH₂(CN)₂]⁻ ( DHB ) with bromine in the presence of the respective pyridine derivative C₅H₅N or 4-CN-C₅H₄N as starting material. All six cyanoborane adducts have been characterized by NMR and vibrational spectroscopy, elemental analysis, and single-crystal X-ray diffraction. The reduction of the cyanoborane adducts has been investigated by cyclic voltammetry and the Lewis acidity of the different cyanoboranes has been assessed using the Gutmann-Beckett method. Selected experimental data and trends are compared to theoretical ones, for example fluoride ion affinities (FIAs).     

Cationic Allyl Complexes of the Rare‐Earth Metals: Synthesis,Structural Characterization,and 1,3‐Butadiene Polymerization Catalysis

Monocationic bis‐allyl complexes [Ln(η³‐C₃H₅)₂(thf)₃]⁺[B(C₆X₅)₄]^? (Ln=Y, La, Nd; X=H, F) and dicationic mono‐allyl complexes of yttrium and the early lanthanides [Ln(η³‐C₃H₅)(thf)₆]²⁺[BPh₄]₂^? (Ln=La, Nd) were prepared by protonolysis of the tris‐allyl complexes [Ln(η³‐C₃H₅)₃(diox)] (Ln=Y, La, Ce, Pr, Nd, Sm; diox=1,4‐dioxane) isolated as a 1,4‐dioxane‐bridged dimer (Ln=Ce) or THF adducts [Ln(η³‐C₃H₅)₃(thf)₂] (Ln=Ce, Pr). Allyl abstraction from the neutral tris‐allyl complex by a Lewis acid, ER₃ (Al(CH₂SiMe₃)₃, BPh₃) gave the ion pair [Ln(η³‐C₃H₅)₂(thf)₃]⁺[ER₃(η¹‐CH₂CH?CH₂)]^? (Ln=Y, La; ER₃=Al(CH₂SiMe₃)₃, BPh₃). Benzophenone inserts into the La? C_allyl bond of [La(η³‐C₃H₅)₂(thf)₃]⁺[BPh₄]^? to form the alkoxy complex [La{OCPh₂(CH₂CH?CH₂)}₂(thf)₃]⁺[BPh₄]^?. The monocationic half‐sandwich complexes [Ln(η⁵‐C₅Me₄SiMe₃)(η³‐C₃H₅)(thf)₂]⁺[B(C₆X₅)₄]^? (Ln=Y, La; X=H, F) were synthesized from the neutral precursors [Ln(η⁵‐C₅Me₄SiMe₃)(η³‐C₃H₅)₂(thf)] by protonolysis. For 1,3‐butadiene polymerization catalysis, the yttrium‐based systems were more active than the corresponding lanthanum or neodymium homologues, giving polybutadiene with approximately 90 % 1,4‐cis stereoselectivity.     

Protonation of CH3N3 and CF3N3 in Superacids: Isolation and Structural Characterization of Long‐Lived Methyl‐ and Trifluoromethylamino Diazonium Ions

The methylamino diazonium cations [CH₃N(H)N₂]⁺ and [CF₃N(H)N₂]⁺ were prepared as their low‐temperature stable [AsF₆]^? salts by protonation of azidomethane and azidotrifluoromethane in superacidic systems. They were characterized by NMR and Raman spectroscopy. Unequivocal proof of the protonation site was obtained by the crystal structures of both salts, confirming the formation of alkylamino diazonium ions. The Lewis adducts CH₃N₃?AsF₅ and CF₃N₃?AsF₅ were also prepared and characterized by low‐temperature NMR and Raman spectroscopy, and also by X‐ray structure determination for CH₃N₃?AsF₅. Electronic structure calculations were performed to provide additional insights. Attempted electrophilic amination of aromatics such as benzene and toluene with methyl‐ and trifluoromethylamino diazonium ions were unsuccessful.     

Influence of Cyclopentadienyl Ring‐Tilt on Electron‐Transfer Reactions: Redox‐Induced Reactivity of Strained [2] and [3]Ruthenocenophanes

In contrast to ruthenocene [Ru(η⁵‐C₅H₅)₂] and dimethylruthenocene [Ru(η⁵‐C₅H₄Me)₂] ( 7 ), chemical oxidation of highly strained, ring‐tilted [2]ruthenocenophane [Ru(η⁵‐C₅H₄)₂(CH₂)₂] ( 5 ) and slightly strained [3]ruthenocenophane [Ru(η⁵‐C₅H₄)₂(CH₂)₃] ( 6 ) with cationic oxidants containing the non‐coordinating [B(C₆F₅)₄]^? anion was found to afford stable and isolable metal?metal bonded dicationic dimer salts [Ru(η⁵‐C₅H₄)₂(CH₂)₂]₂[B(C₆F₅)₄]₂ ( 8 ) and [Ru(η⁵‐C₅H₄)₂(CH₂)₃]₂[B(C₆F₅)₄]₂ ( 17 ), respectively. Cyclic voltammetry and DFT studies indicated that the oxidation potential, propensity for dimerization, and strength of the resulting Ru?Ru bond is strongly dependent on the degree of tilt present in 5 and 6 and thereby degree of exposure of the Ru center. Cleavage of the Ru?Ru bond in 8 was achieved through reaction with the radical source [(CH₃)₂NC(S)S?SC(S)N(CH₃)₂] (thiram), affording unusual dimer [(CH₃)₂NCS₂Ru(η⁵‐C₅H₄)(η³‐C₅H₄)C₂H₄]₂[B(C₆F₅)₄]₂ ( 9 ) through a haptotropic η⁵–η³ ring‐slippage followed by an apparent [2+2] cyclodimerization of the cyclopentadienyl ligand. Analogs of possible intermediates in the reaction pathway [C₆H₅ERu(η⁵‐C₅H₄)₂C₂H₄][B(C₆F₅)₄] [E=S ( 15 ) or Se ( 16 )] were synthesized through reaction of 8 with C₆H₅E?EC₆H₅ (E=S or Se).     

A Highly Lewis Acidic Strontium ansa-Arene Complex for Lewis Acid Catalysis and Isobutylene Polymerization

The potential of a dicationic strontium ansa-arene complex for Lewis acid catalysis has been explored. The key to its synthesis was a simple salt metathesis from SrI₂ and 2 Ag[Al(OR^F)₄], giving the base-free strontium-perfluoroalkoxyaluminate Sr[Al(OR^F)₄]₂ (OR^F=OC(CF₃)₃). Addition of an ansa-arene yielded the highly Lewis acidic, dicationic strontium ansa-arene complex. In preliminary experiments, the complex was successfully applied as a catalyst in CO₂-reduction to CH₄ and a surprisingly controlled isobutylene polymerization reaction.     

Configuration of MFI-type zeolite interacting with the probe molecule P(CH<Subscript>3</Subscript>)<Subscript>3</Subscript> – a theoretical study

With P(CH₃)₃ as the probe molecule adsorbed on titanium silicalite (TS-1) zeolite, the special and important role of T12 site in MFI-type zeolite was clearly elucidated. There are altogether three active sites present in TS-1 zeolite with Ti at the T12 site. Owing to the preferential adsorption of probe molecules on the first Brönsted acidic site, the Ti12 center will probably fail to show Lewis acidity. The ionic [HP(CH₃)₃]⁺ species can be stabilized by the first or second Brönsted acidic site, with the former energetically favored. The latter was formed through the transfer of the ionic [HP(CH₃)₃]⁺ species from the first to the second Brönsted acidic site.     

Cyanohydridoborate Anions: Synthesis,Salts, and Low-Viscosity Ionic Liquids

High-yield syntheses up to molar scales for salts of [BH(CN)₃]⁻ ( 2 ) and [BH₂(CN)₂]⁻ ( 3 ) starting from commercially available Na[BH₄] (Na 5 ), Na[BH₃(CN)] (Na 4 ), BCl₃, (CH₃)₃SiCN, and KCN were developed. Direct conversion of Na 5 into K 2 was accomplished with (CH₃)₃SiCN and (CH₃)₃SiCl as a catalyst in an autoclave. Alternatively, Na 5 is converted into Na[BH{OC(O)R}₃] (R=alkyl) that is more reactive towards (CH₃)₃SiCN and thus provides an easy access to salts of 2 . Some reaction intermediates were identified, for example, Na[BH(CN){OC(O)Et}₂] (Na 7 b ) and Na[BH(CN)₂{OC(O)Et}] (Na 8 b ). A third entry to 2 and 3 uses ether adducts of BHCl₂ or BH₂Cl such as the commercial 1,4-dioxane adducts that react with KCN and (CH₃)₃SiCN. Alkali metal salts of 2 and 3 are convenient starting materials for organic salts, especially for low viscosity ionic liquids (ILs). [EMIm] 3 has the lowest viscosity and highest conductivity with 10.2 mPa s and 32.6 mS cm⁻¹ at 20 °C known for non-protic ILs. The ILs are thermally, chemically, and electrochemically robust. These properties are crucial for applications in electrochemical devices, for example, dye-sensitized solar cells (Grätzel cells).