A model of the mechanism of the chemical interaction of the etchant ion (HF<Subscript>2</Subscript>)<Superscript>–</Superscript> with silicon during its electrochemical etching in hydrofluoric acid solutions

A model was proposed for the mechanism of the chemical interaction of the etchant ion (HF₂)^– with silicon during its electrochemical etching, which explains the possibility of porous silicon etching in the dark and the formation of hydride and hydroxyl groups on the silicon surface.     

The Determining Role of the (HF<Subscript>2</Subscript>)<Superscript>–</Superscript> Ion in the Formation of Pores in Silicon in Its Electrochemical Etching with Hydrofluoric Acid Solutions

A model of the chemical interaction of Si with the (HF₂)^– ion was propsoed to explain some experimental data on the formation of porous silicon.     

Effect of the content of hydrogen fluoride in an etchant on the formation of nanopores in silicon during electrolytic etching

The effect of the HF content on the formation of nanopores in silicon during electrochemical etching was studied. Nanoporous silicon layers were established to be formed only when hydrogen fluoride content in etchants (initial HF content: 49 wt %) was higher than 10–12 vol %. The mass and charge balance of the electrolytic etching of silicon was calculated, and the change in charge number of reaction (effective silicon valence) was determined depending on the HF content. The obtained data were used to propose a silicon etching model with the formation of SiF₄ and nanoporous silicon (where nanopores were formed due to the action of predominantly (HF₂)^? ions).     

Enthalpie de dissolution du fluorure et de l′hydrogenofluorure d'ammonium dans les solutions d'acide fluorhydrique

The standard enthalpies of solution of NH₄F and NH₄HF₂ in aqueous solutions of hydrogen fluoride (in the range of concentration 0–30 mol.1^?1) have been measured and from these results the standard enthalpy of formation of NH₄HF_2(c) has been derived as: δH^o_fNH₄HF_2(c) = ?809.9 ± 0.9 kJ.mol^?1     

Hydrogendifluorokobaltate(II)

Zusammenfassung Aus Kobalt(II)perchlorat und Piperidiniumhydrogendifluorid entstehen in nichtwäßrigen Lösungsmitteln (L) Komplexe, welche HF₂-Einheiten als Liganden enthalten, nämlich [Co(HF₂)L ₅]⁺, [Co(HF₂)₂ L ₄], [Co(HF₂)₃ L ₃]^–, [Co(HF₂)₄ L ₂]^2– und [Co(HF₂)₆]^4–.

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Hydrogendifluorocobaltates(II)

Cobalt(II)perchlorate and piperidinium hydrogendifluoride in non-aqueous solvents (L) yield complex compounds containing an HF₂-group as ligand, e.g. [Co(HF₂)L ₅]⁺, [Co(HF₂)₂ L ₄], [Co(HF₂)₃ L ₃]^–, [Co(HF₂)₄ L ₂]^2– and [Co(HF₂)₆]^4–.

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Mit 3 Abbildungen     

Nitrosylation of Hexafluoridotechnetate(IV)

Potassium hexafluoridotechnetate(IV), K₂[TcF₆], slowly reacts in aqueous solution with acetohydroxamic acid with formation of the ammine nitrosyltechnetium(I) complex [Tc(NO)(NH₃)₄F]⁺. The product crystallizes as mixed TcF₆^2–/HF₂^– salt of the composition [Tc(NO)(NH₃)₄F]₄[TcF₆][HF₂]₂. [Tc(NO)(NH₃)₄F]⁺ represents the first nitrosyltechnetium complex with a fluorido ligand in its coordination sphere. The Tc–F bonds in two crystallographically independent species are 1.987(2) and 2.034(2) Å, respectively. This is slightly longer than in the [TcF₆]^2– counterion.     

Ladder-shaped mono-organooxotin assemblies from n -butylstannanoic acid and 2,4,5-trifluoro-3-methoxybenzonic acid

A new ladder structure of [(n-BuSn(O)O₂CC₆HF₃OCH₃)₂-n-BuSn(O₂CC₆HF₃OCH₃)₃]₂ (1) has been synthesized and characterized by elemental analysis, IR, ¹H, ¹³C, ¹¹⁹Sn NMR spectra and X-ray crystallography. All the tin atoms are six-coordinate and display distorted octahedral geometry. A series of C–H ;··· ;F and π–π stacking interactions play an important function in the supramolecular aggregation.     

Échelle d'acidité dans l'hydrogénodifluorure de potassium (KHF2) fondu à 250°C: Détermination électrochimique

In molten potassium hydrogenodifluoride (KHF₂) fully ionized into K⁺ and HF₂^?, at 250°C, the HF₂^? ion is slightly dissociated according to the equilibrium: HF₂^?HF+F^?. This is a solvent acid base equilibrium, HF being the strongest acid and F^? the strongest base in this system. By means of a voltammetric study we showed that the hydrogen electrode may be used as an acidity indicator electrode in the whole acidity range of the melt. By analysis of the equilibrium potential variation in acidic and in basic media, the HF₂^? dissociation constant (melt autoprotolysis constant): K_D = {HF} {F^?} was determined The experimental value: K_D=10^?2.05 mol² kg^?2 is compared with a calculated one, issued from thermodynamic data. Results obtained with other electrodes (LaF₃ monocrystal electrode and copper electrode) were discussed and compared with those obtained with the hydrogen electrode.     

State of Rh(III) in Hydrofluoric Acid Solutions

A simple method is developed for synthesizing [Rh(H₂O)₆]F₃. 3H₂O with a yield of 80–90%. ¹⁹F, ¹⁰³Rh, and ¹⁷О NMR spectroscopic studies show that the following three processes simultaneously run in the Rh(III)–HF/K–H₂O system via parallel routes: the formation of mononuclear aquafluoro complexes [Rh(H₂O)₆]³⁺ + F–→ [Rh(H₂O)₅F]²⁺ + H₂O; the formation of aquahydrofluoro complexes [Rh(H₂O)₆]³⁺ + HF₂^-→ [Rh(H₂O)₅HF₂]²⁺ + H₂O; and hydrolysis of the aqua ion followed by coordination of fluoride ion and condensation of the hydroxo species [Rh(H₂O)₆]³⁺ + 2F^– → [Rh(H₂O)₄(OH)F]⁺ + HF → condensation. [Rh(H₂O)₆]³⁺ and [Rh(H2O)5F]²⁺ are the two species making a major contribution to the material balance at high acidity under equilibrium conditions. Parameters of the ¹⁹F NMR spectra of individual complex species are presented.     

Penning ionization and photoionization electron spectrometry of hydrogen fluoride

An electron spectrometric study has been performed on HF using metastable helium and neon atoms as well as helium and neon resonance photons. High-resolution electron spectra were obtained for a pure He(2³ S) beam, a mixed He(2¹ S, 2³ S) beam, a mixed Ne(3s,³ P ₂,³ P ₀) beam, and for HeI and NeI VUV light. From the comparison of vibrational populations of HF⁺ (X ²£ _i ,v′) and HF⁺ (A ²Σ⁺,v′) produced by He(2³ S) metastables and HeI resonance photons, we conclude that there is only a slight perturbation of the HF (X ¹Σ⁺) potential in He(2³ S) Penning ionization; no perturbation is found for HF⁺ (X ²Π _i ,v′) formation from Ne(³ P _2,0) metastable ionization of HF. For He(2¹ S)+HF theX- andA-ionic state vibrational peak shapes are substantially broader than in the He(2³ S)+HF case pointing to an additional, charge exchanged interaction (He⁺+HF^?) in the entrance channel of the former system. The vibrational population found for NeI _α photoionization of HF for formation of HF⁺ (X ²Π _i ,v′) is found to differ considerably from that for NeI _β photoionization and from the Franck-Condon factors for unperturbed HF(X ¹Σ⁺) and HF⁺ (X ²Π _i ) potentials suggesting the presence of an autoionizing superexcited state of HF in the energy vicinity of the NeI _α resonance photons. The HF⁺ (X)²Π_3/2:²Π_1/2 fine-structure branching ratios vary significantly with the ionizing agent in a similar way as previously found in HCl and HBr.     

New salts of graphite,C12+HF2− & C24+SiF5− and the treshold for the oxidative intercalation of graphite

Third row transition metal hexafluorides (MF₆) for which the electron affinity exceeds 130 kcal/mole (M = Os, Ir, Pt) have been found to intercalate graphite with electron oxidation of the host lattice, whereas those with inferior electron affinities (M = W, Re) do not intercalate¹. This behavior can be rationalized on kinetic or thermodynamic grounds; arguing for the latter, a simple Born-Haber cycle may be used which suggests an electron affinity threshold of 120–130 kcal/mole for the MF₆ intercalation reaction. For the general case of intercalation reactions by metal fluorides (with or without added fluorine), wherein the graphite lattice is oxidized, the threshold is determined by the free energy of the half-reaction which produces the intercalating fluoro-anion. The lattice energy of the graphite salt must also be taken into account when comparing free energy thresholds for large (e.g., MF₆) and small (e.g. HF₂^?) intercalating species.We have evaluated the free energy of formation of a number of fluoro-anions from the heats of formation and lattice energies of salts which contain them. These studies indicate a threshold free energy of ca. 110 kcal/mole for graphite intercalation. Two ‘borderline’ second stage compounds, C₂₄⁺SiF₅^? and C₁₂⁺HF₂^?, have been synthesized.     

Photoassisted etching of silicon dioxide films

Theoretical approaches to resolving the problem of photoassisted etching of silicon dioxide as the most widely used protective layer in microelectronics are presented. A model of donor-acceptor interaction providing for the desolvation of the F^? ion involved in SiO₂ photoetching is proposed. The etchant compositions were optimized, and the effect of the most important factors on the etching process was examined. It has been shown that the maximal photoetching rate is 0.42 μm/min and the chemical contribution to etching is small, being about 0.02 μm/min.     

Synthesis and structural investigation of the compounds containing HF2 anions: Ca(HF2)2, Ba4F4(HF2)(PF6)3 and Pb2F2(HF2)(PF6)

Three new compounds Ca(HF₂)₂, Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) were obtained in the system metal(II) fluoride and anhydrous HF (aHF) acidified with excessive PF₅. The obtained polymeric solids are slightly soluble in aHF and they crystallize out of their aHF solutions. Ca(HF₂)₂ was prepared by simply dissolving CaF₂ in a neutral aHF. It represents the second known compound with homoleptic HF environment of the central atom besides Ba(H₃F₄)₂. The compounds Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) represent two additional examples of the formation of a polymeric zigzag ladder or ribbon composed of metal cation and fluoride anion (MF⁺)_n besides PbF(AsF₆), the first isolated compound with such zigzag ladder. The obtained new compounds were characterized by X-ray single crystal diffraction method and partly by Raman spectroscopy. Ba₄F₄(HF₂)(PF₆)₃ crystallizes in a triclinic space group P1¯ with a=4.5870(2) Å, b=8.8327(3) Å, c=11.2489(3) Å, α=67.758(9)°, β=84.722(12), γ=78.283(12)°, V=413.00(3) Å³ at 200 K, Z=1 and R=0.0588. Pb₂F₂(HF₂)(PF₆) at 200 K: space group P1¯, a=4.5722(19) Å, b=4.763(2) Å, c=8.818(4) Å, α=86.967(10)°, β=76.774(10)°, γ=83.230(12)°, V=185.55(14) ų, Z=1 and R=0.0937. Pb₂F₂(HF₂)(PF₆) at 293 K: space group P1¯, a=4.586(2) Å, b=4.781(3) Å, c=8.831(5) Å, α=87.106(13)°, β=76.830(13)°, γ=83.531(11)°, V=187.27(18) Å³, Z=1 and R=0.072. Ca(HF₂)₂ crystallizes in an orthorhombic Fddd space group with a=5.5709(6) Å, b=10.1111(9) Å, c=10.5945(10) Å, V=596.77(10) Å³ at 200 K, Z=8 and R=0.028.     

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent-Silyliumylidene Cation

The trapping of a silicon(I) radical with N-heterocyclic carbenes is described. The reaction of the cyclic (alkyl)(amino) carbene [cAAC_Me] (cAAC_Me=:C(CMe₂)₂(CH₂)NAr, Ar=2,6-iPr₂C₆H₃) with H₂SiI₂ in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAAC_Me)₂Si:^.]⁺I⁻ ( 1 ), which features a cationic cAAC–silicon(I) radical, and [cAAC_Me−H]⁺I⁻. In addition, the reaction of the NHC–iodosilicon(I) dimer [I_Ar(I)Si:]₂ (I_Ar=:C{N(Ar)CH}₂) with 4 equiv of I_Me (:C{N(Me)CMe}₂), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(I_Me)₂SiH]⁺I⁻ ( 2 ) comprising the first NHC–parent-silyliumylidene cation. Its further reaction with fluorobenzene afforded the C_Ar−H bond activation product [1-F-2-I_Me-C₆H₄]⁺I⁻ ( 3 ). The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1 . Compounds 1 – 3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X-ray crystallography.     

Crystal Structure of Cesium Dihydrotrifluoride,CsH2F3. Refinement of the Crystal Structures of NMe4HF2 and NMe4H2F3

Single crystals of (NMe₄)(HF₂) were obtained during attempted recrystallization of NMe₄F from fluoroolefin. X‐ray diffraction data show that (NMe₄)(HF₂) crystallizes in the orthorhombic space group Pmmn with unit cell dimensions a = 6.535(2), b = 8.688(3), and c = 5.333(2) Å. The symmetric and virtually linear HF₂ anions exhibit a short F···F distance of 2.256(2) Å. The both crystal structures of (NMe₄)(H₂F₃) (orthorhombic, Pbca, a = 8.509(1), b = 11.273(2), and c = 14.880(2) Å) and CsH₂F₃ (orthorhombic, P2₁2₁2₁, a = 7.345(3), b = 9.126(4), and c = 11.444(4) Å) contain dihydrogentrifluoride anions, H₂F₃^?, which have a bent shape and F···F distances of 2.30‐2.34Å.     

From the Lithium‐2‐anilide‐2‐fluoro‐1,3‐diaza‐2‐sila‐cyclopentene‐GaCl3 Adduct to 1,4,6‐Triaza‐5‐gallium‐7‐sila‐cyclo‐3‐heptene ‐ Experimental and Quantum‐chemical Results

The lithium salt (HC–NCMe₃)₂SiFNLiR ( 1 ) R = C₆H₃(2,6‐CHMe₂)₂ reacts with trichlorogallium under displacement of the lithium ion by GaCl₃ to give the adduct [(HC–NCMe₃)₂SiFN]^– [(GaCl₃)R·Li(thf)₄]⁺ ( 1 ). Compound 1 thermally loses LiCl and forms the bicyclic ring intermediates V and VI . Compound  VI adds the aniline H₂NC₆H₃(2,6‐CHMe₂)₂ and the unsaturated, seven‐membered ring compound –NCMe₃–CH₂–CH=NCMe₃GaCl₂–NR–SiFNHR– ( 2 ) is obtained. The addition is accompanied by an enamine‐imine‐tautomerism and proves the Lewis acid character of the silicon atom in an unknown 3‐center‐2‐electron interaction of one nitrogen atom with the silicon and gallium atoms. Quantum chemical calculations of the thermal isomerisation process and crystal structures of 1 and 2 are reported.     

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent‐Silyliumylidene Cation

The trapping of a silicon(I) radical with N‐heterocyclic carbenes is described. The reaction of the cyclic (alkyl)(amino) carbene [cAAC_Me] (cAAC_Me=:C(CMe₂)₂(CH₂)NAr, Ar=2,6‐i Pr₂C₆H₃) with H₂SiI₂ in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAAC_Me)₂Si:^.]⁺I⁻ ( 1 ), which features a cationic cAAC–silicon(I) radical, and [cAAC_Me−H]⁺I⁻. In addition, the reaction of the NHC–iodosilicon(I) dimer [I_Ar(I)Si:]₂ (I_Ar=:C{N(Ar)CH}₂) with 4 equiv of I_Me (:C{N(Me)CMe}₂), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(I_Me)₂SiH]⁺I⁻ ( 2 ) comprising the first NHC–parent‐silyliumylidene cation. Its further reaction with fluorobenzene afforded the C_Ar−H bond activation product [1‐F‐2‐I_Me‐C₆H₄]⁺I⁻ ( 3 ). The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1 . Compounds 1 – 3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X‐ray crystallography.     

Equilibrium constants of HF,F-, HF-2 and H2F2 species in formic acid.

Equilibrium constants for the fluorinated species HF, F^-, HF^-₂ and H₂F₂ in formic acid and in a 1M potassium formate solution in formic acid have been studied by ¹⁹F NMR. The chemical shifts of these species have been determined from measurements of the shifts for various initial mixtures of differing concentrations of dissolved HF, F^- and HF^-₂. From these values, relative concentrations of HF, F^-, and HF^-₂ and H₂F₂ in each solution have been calculated through a numerical method. The following constants were obtained: K₁ = [H⁺][F^-]/[HF] = 1.1 x 10^-5M; K_D = [HF][F^-]/[HF^-₂] = 0.5 M; K′₁ = [H⁺][HF^-₂]/[H₂F₂]= 1.1 x 10^-5 M; K′_D = [HF]²/[H₂F₂]=0.5 M.     

A benzene‐rich pseudopolymorph of bis[μ‐1,3‐bis(pentafluorophenyl)propane‐1,3‐dionato]‐κ3O,O′:O′;κ3O:O,O′‐bis{aqua[1,3‐bis(pentafluorophenyl)propane‐1,3‐dionato‐κ2O,O′]nickel(II)} benzene tetrasolvate

The title complex comprises two Ni²⁺ ions, four fluorinated ligands and two water molecules in a centrosymmetric dinuclear complex. This compound was crystallized from benzene–CH₂Cl₂, and two types of crystals, viz. the title benzene tetrasolvate, [Ni₂(C₁₅HF₁₀O₂)₄(H₂O)₂]·4C₆H₆, (I), and the previously reported benzene disolvate, [Ni₂(C₁₅HF₁₀O₂)₄(H₂O)₂]·2C₆H₆, (II) [Hori et al. (2009). Bull. Chem. Soc. Jpn, 82 , 96–98], were obtained as pseudopolymorphs. In the crystal structure of (I), the four benzene solvent molecules interact closely with all the pentafluorophenyl groups of the complex through arene–perfluoroarene interactions. The molecular structures of the two compounds show essentially the same conformation, although the benzene molecules are accommodated in a columnar packing in (I), while they are isolated from each other in (II).