Dominique Leclercq

The Canted Antiferromagnetic Approach to Single Chain Magnets.

The reaction of manganese(III) acetate meso-tetraphenylporphyrin with phenylphosphinic acid provides the one-dimensional compound of formula [Mn(TPP)O2PHPh] x H2O, which crystallizes in the monoclinic C2/c space group. The chain structure is generated by a glide plane resulting in Jahn-Teller elongation axes of the MnIII octahedra that alternate along the chain. The phenylphosphinate anion transmits a sizable antiferromagnetic exchange interaction that, combined with the easy axis magnetic anisotropy of the MnIII sites, gives rise to a canted antiferromagnetic arrangement of the spins. The static single-crystal magnetic properties have been analyzed with a classical Monte Carlo approach, and the best fit parameters for the exchange and single ion anisotropy are J = -0.68(4) K and D = -4.7(2) K, respectively (using the -2JS(i)S(j) formalism for the exchange). Below 5 K the single-crystal dynamics susceptibility reveals a frequency-dependent out-of-phase signal typical of single-chain magnets. The extracted relaxation time follows the Arrhenius law with delta = 36.8 K. The dynamic behavior has been rationalized and correlated to geometrical parameters of the structure. The contribution of the correlation length to the energy barrier has been investigated, and it has been found that the characteristic length that dominates the dynamics strongly exceeds the correlation length estimated from magnetic susceptibility.

Mixed oxides SiO2–ZrO2 and SiO2–TiO2 by a non-hydrolytic sol–gel route

SiO2–ZrO2 and SiO2–TiO2 mixed oxides with various metal contents have been prepared by a non-hydrolytic sol–gel route involving the condensation between chloride and isopropoxide functions at 110 °C. Well condensed, monolithic gels were obtained in one step, without the use of additives. The Si/M ratio of the oxide may be controlled easily by the composition of the starting mixture. The Si/Zr oxides remained amorphous after calcination for 5 h at 600 °C; IR and 29Si NMR spectroscopy showed a large amount of Si–O–Zr bonds, indicating a homogeneous distribution of the components on the atomic scale. The crystallization of tetragonal zirconia took place at higher temperature; the transformation of tetragonal to monoclinic zirconia was strongly retarded and did not take place after 2 h at 1300 °C. The crystallization of zircon (for a sample containing 50 mol% Zr) started at 1500°C and was completed after 20 h at 1500°C. IR spectroscopy indicated the presence of a limited number of Si–O–Ti bonds in all the Si/Ti oxides after calcination for 5 h at 500 °C. The sample within the stable glass region (5 mol% Ti) appeared perfectly homogeneous: it crystallized at 900 °C as single-phase cristobalite oxide, with Ti4+ ions substituting Si4+ ions at random. On the other hand, the precipitation of anatase was observed for the Si/Ti oxides with a high Ti content (20–50 mol% Ti), which are outside the stable glass region. The transformation of anatase to rutile was not observed even after 2 h at 1300 °C.

Preparation of anatase, brookite and rutile at low temperature by non-hydrolytic sol–gel methods

Titania samples prepared by different non-hydrolytic sol–gel methods, mainly based on the etherolysis and alcoholysis of titanium tetrachloride, have been found to differ in both structure and texture. Thus, the reaction of diethyl ether with TiCl4 at 110 °C affords anatase, which begins to convert into rutile only around 1000 °C. The reaction of TiCl4 with ethanol leads to rutile as early as 110 °C, whereas the reaction of tert-butyl alcohol at 110 °C leads to the singular formation of brookite.

Non-hydrolytic sol–gel routes to silica

Abstract Silica was prepared by solvent-free non-hydrolytic condensation reactions from silicon tetrachloride with different oxygen donors: benzyl alcohol and tetrabenzyloxysilane (no catalysis), tetraethoxysilane, tetraisopropoxysilane, diethylether and diisopropylether (catalyzed by FeCl3, TiCl4, AlCl3 or ZrCl4). Highly condensed gels with a low hydroxyl content were obtained in high yields. A solution 29Si NMR study showed that in the presence of FeCl3 the condensation of SiCl4 with Si(OiPr)4 was accompanied by extensive redistribution of Si–Cl and Si–OiPr bonds, leading to increasingly complex mixtures. Non-hydrolytic silica xerogels were characterized by chemical analysis, IR and 29Si NMR spectroscopy and thermogravimetric analysis. Porosity and specific surface area were determined using nitrogen adsorption–desorption. Xerogels displayed a wide variety of textures depending on the nature of the oxygen donor and of the catalyst.

Preparation of alumina gels by a non-hydrolytic sol-gel processing method

Abstract Monolithic alumina gels were prepared by a novel non-hydrolytic sol-gel route based on the condensation reaction between aluminum alkoxides Al(OR)3 and aluminum halides AlX3, through the formation of alkyl halide RX(X = Br, Cl; R = iso-propyl, sec- or ter-butyl), around 100°C. Samples were then calcined and were found to be amorphous up to about 750°C; they kept a high specific surface area to about 900°C. This delayed crystallization is correlated with the large number of five-coordinate aluminum sites measured in dried gels by 27Al NMR spectroscopy.

Preparation of monolithic gels from silicon halides by a non-hydrolytic sol-gel process

Reactions of organic oxygen compounds, such as ter-butanol, benzylalcohol, dibenzylether and benzaldehyde, with silicon halides provide a novel non-hydrolytic sol-gel route to silica.

Materials chemistry communications. Preparation of monolithic metal oxide gels by a non-hydrolytic sol–gel process

Non-hydrolytic condensation between halide and alkoxide metal precursors leads to oxide gels; thus, Al2O3 and TiO2 were successfully synthesized as monolithic gels.

Organosilicon gels containing silicon-silicon bonds, precursors to novel silicon oxycarbide compositions

Organosilicon gel compounds were obtained from chlorodisilanes with the general formula (CH3)xCl6−xSi2 which may be available as industrial byproducts. Their thermal behavior was investigated by TG-MS analysis and by IR and NMR spectroscopy. Pyrolysis leads to high yields of silicon oxycarbides with novel oxygen-poor compositions. From NMR data, the entire range of tetrahedral units from SiC4 to SiO4 is observed as early as 800°C. Conversion into silicon carbide by carboreduction reaction occurs above 1400°C; it is associated with an extensive loss of CO and SiO needed to meet the SiC stoichiometry.

Silica xerogels containing a functional group at silicon

Modified silica xerogels can be obtained from several trialkoxysilanes containing a specific group attached to silicon ZSi(OR′)3 (where R′ means an alkyl group). We report here the use of Z = H. In basic catalyzed conditions, the hydrolysis induces a release of H2 gas coming from SiH bonds. However, in neutral or acidic conditions it is possible to prevent the cleavage of SiH bonds. Thus, the dried gel contains SiH bonds as shown both by spectroscopy and chemical analysis. This new family of gels reacts with oxygen gas from the atmosphere. During heating, gels show a 7% weight increase in the temperature range 250°C to 600°C.

29Si nuclear magnetic resonance study of the structure of silicon oxycarbide glasses derived from organosilicon precursors

Several silicon oxycarbide glasses with O/Si ratios 1.2 and 1.8 were prepared by pyrolysis below 1000 °C of organosilicon precursors. Their structure was investigated using29Si magic-angle spinning nuclear magnetic resonance. The spectra obtained are consistent with a purely random distribution of Si-O and Si-C bonds, which depends on the O/Si ratio of the glass only, regardless of the structure of the precursors. This result is interpreted by the occurrence, during the pyrolysis of entropically controlled redistribution reactions involving the exchange of Si-O and Si-C bonds.