Further Insights into the Chemical Synthesis of F2 and on Drying moist HF.

It is well established that solid K2MnF6 reacts with excess SbF5 forming elemental F2. However, if the reaction is carried out in anhydrous HF (aHF) as a solvent, this is surprisingly not the case. Instead, the green Mn(IV) compound K3[(MnIVF)(SbF6)5]F is obtained. The reductive elimination of F2 was not observed under the applied conditions. The compound was characterized by its crystal structure, by Raman spectroscopy, and by quantum-chemical solid-state calculations. It crystallizes in the monoclinic space group P21/c, mP164, with the lattice parameters a = 12.2393(13), b = 12.167(2), c = 20.115(5) Å, ß = 110.805(8)°, V = 2800.1(9) Å3, Z = 4 at T = 200 K. As the use of strictly anhydrous HF is crucial in this and other similar reactions, methods for drying moist HF are discussed.

The Crucial Role of Sb2 F10 in the Chemical Synthesis of F2.

The purely chemical synthesis of fluorine is a spectacular reaction which for more than a century had been believed to be impossible. In 1986, it was finally experimentally achieved, but since then this important reaction has not been further studied and its detailed mechanism had been a mystery. The known thermal stability of MnF4 casts serious doubts on the originally proposed hypothesis that MnF4 is thermodynamically unstable and decomposes spontaneously to a lower manganese fluoride and F2 . This apparent discrepancy has now been resolved experimentally and by electronic structure calculations. It is shown that the reductive elimination of F2 requires a large excess of SbF5 and occurs in the last reaction step when in the intermediate [SbF6 ][MnF2 ][Sb2 F11 ] the addition of one more SbF5 molecule to the [SbF6 ]- anion generates a second tridentate [Sb2 F11 ]- anion. The two tridentate [Sb2 F11 ]- anions then provide six fluorine bridges to the Mn atom thereby facilitating the reductive elimination of the two fluorine ligands as F2 .

Fluoro-nitrogen Cations.

The crystal structures of [NH3 F]+ [CF3 SO3 ]- , [NH2 F2 ]+ [SbF6 ]- , and [N2 F3 ]+ [Sb3 F16 ]- have been determined, representing the first structural characterizations of these simple fluoro-nitrogen cations. The influences of the hybridization of the central nitrogen atom and of the number of fluorine substituents on the N-F bond lengths are evaluated for the series N2 F+ , N2 F3 + , NF2 O+ , NH3 F+ , NH2 F2 + , and NF4 + . It is shown that the N-F bond length decreases from 1.40 Što 1.26 Šwith increasing fluorine substitution and increasing s-character of the nitrogen atom, and that unusual N-F bond lengths reported in the previous literature are caused by disorder problems.

Energetic Properties, Spectroscopy, and Reactivity of NF3O.

The heats of formation of NF3O and similar C, S, and Si systems are predicted using the accurate composite computational chemistry Feller-Peterson-Dixon (FPD) method. The harmonic vibrational frequencies at the CCSD(T)/aug-cc-pVTZ level are reported and compared to the experimental values for NF3O, its isoelectronic species CF3O- and NF4+, and NF3. The infrared intensities were calculated at the MP2/aug-cc-pVTZ level and show that the infrared absorption is predicted to be like those of CF2Cl2 and SF6 within a factor of ∼2. The calculated heats of formation are in good agreement with the available experimental values. These heats of formation are used to calculate a range of bond dissociation energies (BDEs). It is predicted that NF3O is unlikely to decompose either thermally or photolytically in the troposphere. The potential energy curves for the decomposition of NF3O to NF2O + F are all repulsive, as are the channels to form NF3 and either O3P or O1D. The predicted persistence of NF3O in the troposphere is attributed to the high barrier of its reaction with the OH radical and that light with the wavelength needed for its photodissociation will not reach the troposphere. Reliable experimental measurements of the global warming potential of NF3O are needed to confirm our predictions that NF3O is like NF3 in this respect.

Protonation of CH3 N3 and CF3 N3 in Superacids: Isolation and Structural Characterization of Long-Lived Methyl- and Trifluoromethylamino Diazonium Ions.

The methylamino diazonium cations [CH3 N(H)N2 ]+ and [CF3 N(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 CH3 N3 ⋅AsF5 and CF3 N3 ⋅AsF5 were also prepared and characterized by low-temperature NMR and Raman spectroscopy, and also by X-ray structure determination for CH3 N3 ⋅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.

Electronic Structure Predictions of the Energetic Properties of Tellurium Fluorides.

The heats of formation, bond dissociation energies (BDEs), fluoride affinities (FA), fluorocation affinities (FCA), electron affinities (EA), and ionization energies (IP) of TeF n ( n = 1-6) have been predicted using the Feller-Peterson-Dixon (FPD) approach. To benchmark the approach, the bond dissociation energies of Te2 and TeO, the heats of formation of Te2, TeH2, TeO, and TeO2, and the electron affinity for TeO and TeO2 were calculated as there are experimental thermodynamic data available for these tellurium compounds, which allow confirmation of the heat of formation of Te gas as Δ Hf,0K(Te) = 50.7 ± 0.6 kcal/mol. Spin-orbit corrections are required for good results and cannot be ignored. A comparison among fluoride affinities, fluorocation affinities, electron affinities, and ionization energies of TeF n and SeF n is reported.

Formamidinium Nitroformate: An Insensitive RDX Alternative.

Five nitroformate (trinitromethanide) salts featuring nitrogen-containing cations were prepared. The salts were characterized by multinuclear NMR, IR, and Raman spectroscopy, single-crystal X-ray analysis, differential thermal analysis, and friction and impact sensitivity testing. These experimental data are supplemented with thermochemical calculations using the Gaussian-4 composite method, and the performance of these energetic materials was calculated based on the Chapman-Jouguet thermodynamic detonation theory. Out of the five compounds studied by us, the formamidinium salt, [CH(NH2)2]+[C(NO2)3]-, is most interesting. Its performance matches that of RDX (research department explosive, cyclotrimethylenetrinitramine), while it is much less sensitive to impact and friction and, therefore, might be an excellent, less sensitive replacement for RDX.

Synthesis and Characterization of cyclo-Pentazolate Salts of NH4+, NH3OH+, N2H5+, C(NH2)3+, and N(CH3)4.

A breakthrough in polynitrogen chemistry was recently achieved by our bulk synthesis of (N5)6(H3O)3(NH4)4Cl in which the cyclo-pentazolate anions were stabilized extensively by hydrogen bridges with the NH4+ and OH3+ cations. Significant efforts have been carried out to replace these nonenergetic cations and the Cl- anion by more energetic cations. In this paper, the metathetical syntheses of cyclo-pentazolate salts containing the simple nitrogen-rich cations NH4+, NH3OH+, N2H5+, C(NH2)3+, and N(CH3)4+ are reported. These salts were characterized by their crystal structures; vibrational, mass, and multinuclear NMR spectra; thermal stability measurements; sensitivity data; and performance calculations. It is shown that the cyclo-pentazolates are more energetic than the corresponding azides but are thermally less stable decomposing in the range of 80 °C to 105 °C. As explosives, the hydrazinium and hydroxyl ammonium salts are predicted to match the detonation pressure of RDX but exhibit significantly higher detonation velocities than RDX and HMX with comparable impact and friction sensitivities. Although the ammonium salt has a lower detonation pressure than RDX, its detonation velocity also exceeds those of RDX and HMX. As a rocket propellant, the hydrazinium and hydroxyl ammonium salts are predicted to exceed the performances of RDX and HMX. The crystal structures show that the cyclo-pentazolate anions are generally stabilized by hydrogen bonds to the cations, except for the N(CH3)4+ salt which also exhibits strong cation-π interactions. This difference in the anion stabilization is also detectable in the vibrational spectra which show for the N(CH3)4+ salt a decrease in the cyclo-N5- stretching vibrations of about 20 cm-1.

α-Fluoroalcohols: Synthesis and Characterization of Perfluorinated Methanol, Ethanol and n-Propanol, and their Oxonium Salts.

The thermally unstable, primary perfluoroalcohols, CF3 OH, C2 F5 OH, and nC3 F7 OH, were conveniently prepared from the corresponding carbonyl compounds in anhydrous HF solution. Experimental values for the reaction enthalpies and entropies were derived from the temperature dependence of the Rf COF+HF⇄Rf CF2 OH (Rf =F, CF3 , CF3 CF2 ) equilibria by NMR spectroscopy. Electronic structure calculations of the gas-phase and solution reaction energies, gas-phase acidities and heats of formation were carried out at the G3MP2 level, showing that these compounds are strong acids. Protonation of these alcohols in HF/SbF5 produced the perfluoroalkyl oxonium salts Rf CF2 OH2 + SbF6 - .

Perfluoroalcohols: The Preparation and Crystal Structures of Heptafluorocyclobutanol and Hexafluorocyclobutane-1,1-diol.

The first X-ray crystal structure of an α-fluoroalcohol is reported. Heptafluorocyclobutanol was obtained in quantitative yield from hexafluorocyclobutanone by HF addition in anhydrous hydrogen fluoride. The compound was characterized by its X-ray single crystal structure. Heptafluorocyclobutanol readily undergoes hydrolysis to hexafluorocyclobutane-1,1-diol, which was also structurally characterized by X-ray diffraction.

The Uranium(VI) Oxoazides [UO2 (N3 )2 ⋠CH3 CN], [(bipy)2 (UO2 )2 (N3 )4 ], [(bipy)UO2 (N3 )3 ]- , [UO2 (N3 )4 ]2- , and [(UO2 )2 (N3 )8 ]4.

The reaction between [UO2 F2 ] and an excess of Me3 SiN3 in acetonitrile solution results in fluoride-azide exchange and the uranium(VI) dioxodiazide adduct [UO2 (N3 )2 ⋅CH3 CN] was isolated in quantitative yield. The subsequent reaction of [UO2 (N3 )2 ⋅CH3 CN] with 2,2'-bipyridine (bipy) resulted in the formation of the azido-bridged binuclear complex [(bipy)2 (UO2 )2 (N3 )4 ]. The triazido anion [(bipy)UO2 (N3 )3 ]- was obtained by the reaction of [UO2 (N3 )2 ⋅CH3 CN] with stoichiometric amounts of bipy and the ionic azide [PPh4 ][N3 ]. The reaction of [UO2 (N3 )2 ] with two equivalents of the [PPh4 ][N3 ] resulted in the formation of the mononuclear tetraazido anion [UO2 (N3 )4 ]2- as well as the azido-bridged binuclear anion [(UO2 )2 (N3 )8 ]4- . The novel uranium oxoazides were characterized by their vibrational spectra and in the case of [(bipy)2 (UO2 )2 (N3 )4 ]⋅CH3 CN, [PPh4 ][(bipy)UO2 (N3 )3 ], [PPh4 ]2 [UO2 (N3 )4 ], [PPh4 ]2 [UO2 (N3 )4 ]⋅2CH3 CN, and [PPh4 ]4 [(UO2 )2 (N3 )8 ]⋅4CH3 CN by their X-ray crystal structures.

Synthesis and Characterization of Nitro-, Trinitromethyl-, and Fluorodinitromethyl-Substituted Triazolyl- and Tetrazolyl-trihydridoborate Anions.

The problem of preparing energetic, exclusively mono-azolyl substituted hydridoborate anions in high yield and purity from [BH4 ]- and nitroazoles by hydrogen elimination was overcome by reacting the corresponding nitroazolate anions with the BH3 adducts BH3 ⋅S(CH3 )2 or BH3 ⋅THF. The highly-energetic, nitro-, trinitromethyl-, and fluorodinitromethyl- substituted triazolyl- and tetrazolyl-trihydridoborate anions were synthesized in this manner and characterized by vibrational and multinuclear NMR spectroscopy and their crystal structures. The use of excess BH3 resulted in some cases in the addition of a second BH3 molecule bound more-weakly to one of the nitrogen atoms of the azole ring. All monoazolyl-trihydridoborates were thermally less stable than the parent azolate anions. A decomposition product of tetraphenylphosphonium (5-(trinitromethyl)-5H-2λ4 -tetrazol-2-yl)trihydridoborate, the tetraphenyl-phosphonium (dinitro-1H-tetrazol-5-yl)methanide monohydrate, was also structurally characterized, providing some insight into the decomposition pathways of the nitromethyl-substituted azolyltrihydridoborate anions.

Preparation and Characterization of Group†13 Cyanides.

Binary Group 13 cyanides [PPh4 ][Ga(CN)4 ], [PPh4 ]2 [In(CN)5 ], and [PPh4 ]2 [Tl(CN)5 ] were obtained in quantitative yields from the corresponding metal trifluorides MF3 (M=Ga, In, Tl) by fluoride-cyanide exchange reactions with Me3 SiCN in the presence of stoichiometric amounts of [PPh4 ]CN in acetonitrile solution. The 2,2'-bipyridine (bipy) adducts [(bipy)2 Ga(CN)2 ][Ga(CN)4 ], [(bipy)In(CN)3 ], and [(bipy)Tl(CN)3 ] were obtained from the reaction of MF3 with Me3 SiCN and bipy in acetonitrile. While the reaction of the metal trifluorides with Me3 SiCN in acetonitrile resulted in recovery of the starting materials, the reaction of MF3 with Me3 SiCN in pyridine (py) solution resulted in the formation of the pyridine adducts [(py)2 Ga(CN)3 ], [(py)3 In(CN)3 ], and [(py)2 Tl(CN)3 ]. The cyano compounds were characterized by their vibrational spectra and, in most cases, by their X-ray crystal structures.

Formation Mechanism of NF4+ Salts and Extraordinary Enhancement of the Oxidizing Power of Fluorine by Strong Lewis Acids.

Although the existence of the NF4+ cation has been known for 51 years, and its formation mechanism from NF3 , F2 , and a strong Lewis acid in the presence of an activation energy source had been studied extensively, the mechanism had not been established. Experimental evidence had shown that the first step involves the generation of F atoms from F2 , and also that the NF3+ cation is a key intermediate. However, it was not possible to establish whether the second step involved the reaction of a F atom with either NF3 or the Lewis acid (LA). To distinguish between these two alternatives, a computational study of the NF4 , SbF6 , AsF6 , and BF4 radicals was carried out. Whereas the heats of reaction are small and similar for the NF4 and LAF radicals, at the reaction temperatures, only the LAF radicals possess sufficient thermal stability to be viable species. Most importantly, the ability of the LAF radicals to oxidize NF3 to NF3+ demonstrates that they are extraordinary oxidizers. This extraordinary enhancement of the oxidizing power of fluorine with strong Lewis acids had previously not been fully recognized.

Dinitramidoborates: A Fascinating Case of Competing Oxygen and Nitrogen Donors and Tautomerism.

Reactions of the BH4- anion with equimolar amounts of HN(NO2 )2 or of BH3 ⋅THF with K[N(NO2 )2 ]- produced a mono-substituted [BH3 N(NO2 )2 ]- anion, which contains a B-N connected dinitramido ligand. The reaction of BH4- with two equivalents of HN(NO2 )2 afforded the di-substituted borate anion consisting of two isomers, one with both nitramido ligands attached to B through N and the other one with one ligand attached through N and the other one through O. The disubstituted dinitramidoborates are marginally stable under ambient conditions, and the isomer with two N-connected ligands was characterized by its crystal structure. A tri-substituted borate was tentatively identified by NMR in the reaction of BH4- with a large excess of HN(NO2 )2 . All of the anions are highly energetic. Theoretical calculations show that the energy differences between the B-N and B-O tautomers are small, explaining the formation of both.

Protonation of Nitramines: Where Does the Proton Go?

The reactions of nitramine, N-methyl nitramine, and N,N-dimethyl nitramine with anhydrous HF and the superacids HF/MF5 (M=As, Sb) were investigated at temperatures below -40 °C. In solution, exclusive O-protonation was observed by multinuclear NMR spectroscopy. Whereas no solid product could be isolated from the neat HF solutions even at -78 °C, in the HF/MF5 systems, protonated nitramine MF6- salts were isolated for the first time as moisture-sensitive solids that decompose at temperatures above -40 °C. In the solid state, depending on the counterion, O-protonated or N-protonated cations can be formed, in accord with theoretical calculations which show that the energy differences between O-protonation and N-protonation are very small. The salts [H2 N-NO2 H][AsF6 ], [H3 N-NO2 ][SbF6 ], [MeHNNO2 H][SbF6 ], and [Me2 NNO2 H][SbF6 ] were characterized by their X-ray crystal structures.

Preparation and Characterization of Antimony and Arsenic Tricyanide and Their 2,2'-Bipyridine Adducts.

The arsenic(III) and antimony(III) cyanides M(CN)3 (M=As, Sb) have been prepared in quantitative yields from the corresponding trifluorides through fluoride-cyanide exchange with Me3 SiCN in acetonitrile. When the reaction was carried out in the presence of one equivalent of 2,2'-bipyridine, the adducts [M(CN)3 ⋅(2,2'-bipy)] were obtained. The crystal structures of As(CN)3 , [As(CN)3 ⋅(2,2'-bipy)] and [Sb(CN)3 ⋅(2,2'-bipy)] were determined and are surprisingly different. As(CN)3 possesses a polymeric three-dimensional structure, [As(CN)3 ⋅(2,2'-bipy)] exhibits a two-dimensional sheet structure, and [Sb(CN)3 ⋅(2,2'-bipy)] has a chain structure, and none of the structures resembles those found for the corresponding arsenic and antimony triazides.

The Binary Group†4 Azides [PPh4 ]2 [Zr(N3 )6 ] and [PPh4 ]2 [Hf(N3 )6 ].

The binary zirconium and hafnium polyazides [PPh4 ]2 [M(N3 )6 ] (M=Zr, Hf) were obtained in near quantitative yields from the corresponding metal fluorides MF4 by fluoride-azide exchange reactions with Me3 SiN3 in the presence of two equivalents of [PPh4 ][N3 ]. The novel polyazido compounds were characterized by their vibrational spectra and their X-ray crystal structures. Both anion structures provide experimental evidence for near-linear M-N-N coordination of metal azides. The species [M(N3 )4 ], [M(N3 )5 ]- and [M(N3 )6 ]2- (M=Ti, Zr, Hf) were studied by quantum chemical calculations at the electronic structure density functional theory and MP2 levels.

Syntheses of Diphenylaminodiazidophosphane and Diphenylaminofluoroazidophosphane.

Diphenylaminodiazidophosphane (C6H5)2NP(N3)2 was synthesized from the corresponding dihalides (C6H5)2NPX2 (X = F, Cl) and (CH3)3SiN3, and was characterized by vibrational and multinuclear NMR spectroscopy. The intermediate compound (C6H5)2NPF(N3) was also observed by NMR spectroscopy in solution. Some physical properties and reactions of all these compounds are discussed.

Convenient Access to α-Fluorinated Alkylammonium Salts.

A series of novel α-fluoroalkyl ammonium salts was obtained from the corresponding cyano compounds or nitriles by reaction with anhydrous HF. Room-temperature stable trifluoromethyl ammonium salts were obtained in quantitative yield in a one-step reaction at ambient temperature from the commercially available starting materials BrCN or ClCN. The novel cations [CF3CF2NH3](+), [HCF2CF2NH3](+), and [(NH3CF2)2](2+) were obtained from CF3CN, HCF2CN, and (CN)2, respectively, and anhydrous HF. The aforementioned fluorinated ammonium cations were isolated as room temperature stable [AsF6](-) and/or [SbF6](-) salts, and characterized by multi-nuclear NMR and vibrational spectroscopy. The salts [HCF2NH3][AsF6] and [CF3NH3][Sb2F11] were characterized by their X-ray crystal structure.