Erhard Kemnitz

Novel Sol-Gel Synthesis of Acidic MgF2-x(OH)(x) Materials

Novel magnesium fluorides have been prepared by a new fluorolytic sol-gel synthesis for fluoride materials based on aqueous HF. By changing the amount of water at constant stoichiometric amount of HF, it is possible to tune the surface acidity of the resulting partly hydroxylated magnesium fluorides. These materials possess medium-strength Lewis acid sites and, by increasing the amount of water, Brønsted acid sites as well. Magnesium hydroxyl groups normally have a basic nature and only with this new synthetic route is it possible to create Brønsted acidic magnesium hydroxyl groups. XRD, MAS NMR, TEM, thermal analysis, and elemental analysis have been applied to study the structure, composition, and thermal behaviour of the bulk materials. XPS measurements, FTIR with probe molecules, and the determination of N(2)/Ar adsorption-desorption isotherms have been carried out to investigate the surface properties. Furthermore, activity data have indicated that the tuning of the acidic properties makes these materials versatile catalysts for different classes of reactions, such as the synthesis of (all-rac)-[alpha]-tocopherol through the condensation of 2,3,6-trimethylhydroquinone (TMHQ) with isophytol (IP).

Non-aqueous synthesis of high surface area aluminium fluoride-a mechanistic investigation

Fluoride can do it too! Sol–gels of metal fluorides play an important role in the formation of high surface area metal fluorides. The synthesis of amorphous high surface area metal fluorides via a recently discovered two-step synthetic route was investigated in detail, exemplified for aluminium fluoride. The first step is fluorination of aluminium alkoxide with anhydrous HF in organic solvents, which proceeds as a sol–gel process known until now only for metal oxide formation. The reaction pathway is illustrated including crystal structure determination of the intermediate aluminium alkoxide fluoride. The resulting amorphous aluminium alkoxide fluoride has to be freed in a second step of solvating alcohol and of residual alkoxidic groups. This is done by heating in a stream of a mild fluorinating agent like a HCFC or CFC or in HF to obtain high surface area and very high Lewis acidity; an inert gas such as N2 is not sufficient. Using a variety of analytical techniques, including liquid and solid state NMR, X-ray structure analysis and XPS, the reaction pathways have been elucidated.

Two Isomers of C60F48: An Indented Fullerene

Deflated buckyballs: The single-crystal structure of C60 F48 ⋅2 (mesitylene) revealed the presence of both D3 and S6 isomers in the same crystal. C(sp2 )-C(sp2 ) bonds (1.30 Å) are much shorter than C(sp3 )-C(sp3 ) bonds (1.54-1.63 Å). The C60 cage is characterized by concave areas in the regions of six double bonds. Each double bond is effectively shielded by four F atoms, which accounts for the low reactivity of C60 F48 .

New magnesium oxide fluorides with hydroxy groups as catalysts for Michael additions

Amorphous materials—magnesium oxide fluoride with hydroxy groups—of high surface area were prepared for the first time by a soft sol–gel method involving initial fluorination and subsequent hydrolysis of the fluorinated gel. Their structural properties were studied with XRD, FTIR, XPS, and 19F MAS NMR. The magnesium oxide fluorides with hydroxy groups were also tested as heterogeneous catalysts in Michael additions. The incorporation of F into the MgO network led to diverse fluorine coordinations and high surface areas. The introduction of F into the framework enables a tuning of the base properties of the materials and leads to varied catalytic activities and selectivities. A weaker basicity can be obtained by the bridging of isolated OH groups in the MgF2 network with increasing fluorine content. The sample prepared with an F : Mg molar ratio of 1.6 exhibited a catalytic activity similar to that of calcined Mg–Al hydrotalcites and selectively produced Michael addition products in high yields.

Fusing Pentagons in a Fullerene Cage by Chlorination: IPR D2‐C76 Rearranges into non‐IPR C76Cl24

As is well known, fullerenes obtained by conventional arcdischarge synthesis obey the isolated pentagon rule (IPR). Unless fullerene molecules are directly subjected to “fullerene surgery”, exohedral functionalization does not affect the connectivity of their carbon networks. Non-IPR fullerene isomers have been available through appropriate modifications of the arc-discharge methodology to synthesize an already chemically derivatized molecule in which the derivatization stabilizes the pentagon–pentagon junctions. In particular, non-IPR cages are quite common in endohedral metallofullerenes, as encapsulated metal atoms are likely to stabilize the fused pentagon fragments by charge-transfer binding to them. More recently, a number of unconventional exohedral fullerene derivatives, including C50Cl10, [5, 6] C56Cl10, [7] C66H4, [8] C68Cl4, [6] and non-IPR C60Cl8 and C60Cl12, [9] have been obtained by means of an arc-discharge process in presence of additives such as CCl4, Cl2, and CH4. Rare examples of more classical chemical approaches to nonIPR fullerenes are indirectly confirmed transformation of dodecahedrane into C20 [10] and synthesis of a C62 derivative with four-membered cycle in its carbon cage from C60. [11]

Isolation and Structural X‐ray Investigation of Perfluoroalkyl Derivatives of Six Cage Isomers of C84

Perfluoroalkylation of a higher fullerene mixture with CF(3)I or C(2)F(5)I, followed by HPLC separation of CF(3) and C(2)F(5) derivatives, resulted in the isolation of several C(84)(R(F))(n) (n=12, 16) compounds. Single-crystal X-ray crystallography with the use of synchrotron radiation allowed structure elucidation of eight C(84)(R(F))(n) compounds containing six different C(84) cages (the number of the C(84) isomer is given in parentheses): C(84) (23)(C(2)F(5))(12) (I), C(84) (22)(CF(3))(16) (II), C(84) (22)(C(2)F(5))(12) (III), C(84) (11)(C(2)F(5))(12) (IV), C(84) (16)(C(2)F(5))(12) (V), C(84) (4)(CF(3))(12) (VI with toluene and VII with hexane as solvate molecules), and C(84) (18)(C(2)F(5))(12) (VIII). Whereas some connectivity patterns of C(84) isomers (22, 23, 11) had previously been unambiguously confirmed by different methods, derivatives of C(84) isomers numbers 4, 16, and 18 have been investigated crystallographically for the first time, thus providing direct proof of the connectivity patterns of rare C(84) isomers. General aspects of the addition of R(F) groups to C(84) cages are discussed in terms of the preferred positions in the pentagons under the formation of chains, pairs, and isolated R(F) groups.

Variation of sol–gel synthesis parameters and their consequence for the surface area and structure of magnesium fluoride

High surface area MgF2 was prepared by the sol–gel technique from magnesium alkoxides. The influence of different ratios of Mg(OCH3)2 to HF, of different magnesium alkoxides, catalysts, solvents as well as different aging times was investigated. XRD, MAS-NMR, FT-IR, thermal analysis and elemental analysis were applied to study the structure, composition and thermal behaviour of the bulk materials. The determination of the surface properties was accomplished with N2 and Ar adsorption–desorption isotherms.

Surface acidity and catalytic behavior of modified zirconium and titanium dioxides

Abstract Various modified zirconium and titanium dioxides were examined as catalysts for the following test reactions: the gas phase alkylation of 1-butene with isobutane, the double bond isomerization of 1-butene and the disproportionation of dichlorodifluoromethane. The catalysts were prepared by treatment of the precipitated oxide hydrates with either fluorinating, chlorinating, sulfatizing or phosphating agents. The nature of the acid sites was determined by IR spectra of pyridine adsorption complexes and correlated with the catalytic behavior. An activation of zirconium dioxide with sulfur tetrafluoride led to the formation of a larger number of Bronsted acid sites than those obtained for sulfated zirconia. The isooctane yields during the alkylation offered a similar graduation. An enhancement of the number and strength of Lewis acid sites presupposed for the disproportionation test reaction was not observed. Samples based on titanium dioxide exhibited an overall lower catalytic activity. The reaction mechanism of the alkylation of 1-butene with isobutane was proposed.

Halogenometalates with N-containing organic cations structural chemistry and thermal behavior

Abstract The crystal structures, spectroscopic characterization, and thermal behavior of fluoro, chloro, and bromo complexes preferably of aluminium, iron and manganese formed with organic cations are reviewed. Compared with the structural chemistry of complexes with alkali metal cations, the use of quarternary ammonium or protonated N-containing base cations considerably extends the structural variety of halogenocomplexes and, in certain cases, allows the access to new modifications of metal fluorides if a suitable precursor compound is thermally decomposed. For most of the described new compounds, the formation of the final structure is governed by the anionic part. Isolated anions [MF 6 − n (H 2 O) n ] (3 − n)− connected via hydrogen bridges are the predominant structure motifs. However, multi-nuclear units as well as chain structures can be prepared from aqueous HX solutions, too. Due to the hydrogen atoms provided by the organic cations, additional hydrogen bonds stabilize various novel structures. Some special structural features will be presented, e.g. the discrete tetrafluoroaluminate complex as well as pentacoordinated aluminium and iron(II).

Preparation and crystal structure of solvent free C60F18

Abstract C60F18 single crystals were grown by vacuum sublimation from the product of reaction of C60 with K2PtF6 at 460 K in vacuo. Solvent free C60F18 containing only a few percent of C60F18O crystallizes in monoclinic lattice. The molecular structure of C60F18 is very close to that found in the C60F18 solvates with aromatic hydrocarbons. Two C…C distances are slightly elongated in the statistically averaged structure due to the presence of C60F18O.