RESUMEN
Zirconium-containing metal-organic framework (MOF) with UiO-66 topology is an extremely versatile material, which finds applications beyond gas separation and catalysis. However, after more than 10 years after the first reports introducing this MOF, understanding of the molecular-level mechanism of its nucleation and growth is still lacking. By means of in situ time-resolved high-resolution mass spectrometry, Zr K-edge X-ray absorption spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, and X-ray diffraction it is showed that the nucleation of UiO-66 occurs via a solution-mediated hydrolysis of zirconium chloroterephthalates, whose formation appears to be autocatalytic. Zirconium-oxo nodes form directly and rapidly during the synthesis, the formation of pre-formed clusters and stable non-stoichiometric intermediates are not observed. The nuclei of UiO-66 possess identical to the crystals local environment, however, they lack long-range order, which is gained during the crystallization. Crystal growth is the rate-determining step, while fast nucleation controls the formation of the small crystals of UiO-66 with a narrow size distribution of about 200 nanometers.
RESUMEN
The vast structural and chemical diversity of metal-organic frameworks (MOFs) provides the exciting possibility of material's design with tailored properties for gas separation, storage and catalysis. However, after more than twenty years after first reports introducing MOFs, the discovery and control of their synthesis remains extremely challenging due to the lack of understanding of mechanisms of their nucleation and growth. Progress in deciphering crystallization pathways depends on the possibility to follow conversion of initial reagents to products at the molecular level, which is a particular challenge under solvothermal conditions. The present work introduces a detailed molecular-level mechanism of the formation of MIL-53(Al), unraveled by combining in situ time-resolved high-resolution mass-spectrometry, magic angle spinning nuclear magnetic resonance spectroscopy and X-ray diffraction. In contrast to the general belief, the crystallization of MIL-53 occurs via a solid-solid transformation mechanism, associated with the spontaneous release of monomeric aluminum. The role of DMF hydrolysis products, formate and dimethylamine, is established. Our study emphasizes the complexity of MOF crystallization chemistry, which requires case-by-case investigation using a combination of advanced in situ methods for following the induction period, the nucleation and growth across the time domain.
RESUMEN
We report the first synthesis and computational study of Sc3N@C78(CF2) - an analog of the previously reported Sc3N@C80(CF2) with a less common carbon cage whose chemical properties presently remain far less studied. Sc3N@C78 appears to be considerably more reactive toward CF2 addition than Sc3N@C80 and somewhat more reactive than C60. Even though the less symmetric D3h(5)-C78 carbon cage offers far broader opportunities for isomerism than Ih-C80, CF2 addition to Sc3N@C78 proceeds regioselectively, similarly to other common fullerene reactions. A DFT survey of the thermodynamic and kinetic aspects of CF2 addition demonstrates that the regioselectivity is controlled kinetically.
RESUMEN
We report the first successful synthesis of a CF2 derivative of the stable endohedral fullerene Sc3N@Ih-C80. Reaction with CF2ClCOONa yields a single Cs-symmetric Sc3N@C80(CF2) adduct where the CF2 group is inserted into a [6,6]-bond and opens it to 2.3 Å between the bridgehead carbon atoms. As evidenced by absorption and fluorescence spectroscopy as well as cyclic voltammetry, both the HOMO and the LUMO level of Sc3N@C80(CF2) are slightly (ca. 0.1 eV) downshifted with respect to the parent Sc3N@Ih-C80, so the HOMO-LUMO gap remains essentially unchanged. The DFT calculations suggest that the reaction mechanism is not the previously assumed [2 + 1]-cycloaddition of :CF2 carbene but rather nucleophilic addition of CF2Cl- anion followed by elimination of Cl- and closing of the CF2 bridge via intramolecular nucleophilic substitution. Selective formation of the [6,6]-Sc3N@C80(CF2) turns out to be kinetically controlled and promoted by a particular orientation of the endohedral Sc3N cluster with respect to the CF2Cl- addition site. In its turn, the CF2 addend partly hampers the rotation of Sc3N the endohedral cluster compared to its quasi-free reorientations in the parent Sc3N@Ih-C80.
RESUMEN
Stereoselective electrosynthesis of the first individual (f,t A)- and (f,t C)-1,4-fullerene derivatives with a non-inherently chiral functionalization pattern is described, as well as the first example of an optically pure protected primary amino acid directly linked to the fullerene through only the chiral α-amino-acid carbon atom. An application of an auxiliary chiral nickel-Schiff base moiety as derivatizing agent allowed separation of (f,t A)- and (f,t C)-1,4-fullerene derivatives using an achiral stationary phase, a separation which has never been done before.
RESUMEN
The homofullerene compound cis-2-C60 (CF2 )2 , which has an unusual kind of open/closed valence tautomerism undergoes consecutive regioselective hydrogenation at bridgehead carbon atoms upon reduction with Zn/Cu couple in H2 O-toluene mixture. The tautomerism barrier in cis-2-C60 (CF2 )2 is negligible in the neutral state, whereas negative charging both impedes tautomeric transformation and promotes regioselective addition of electrophilic species at the bridgehead carbon atoms. In light of this observation, two novel homofullerene derivatives, mixed [6,6]-open/closed C60 (CF2 )2 H2 and [6,6]-open cis-2-C60 (CF2 )2 H4 , were synthesized and their structures were unambiguously determined by means of single crystal X-ray crystallography and NMR spectroscopy.
RESUMEN
C2 -C70 (CF3 )8 was found to be a very promising substrate in the Bingel and the Bingel-Hirsch reactions combining perfect regioselectivity with much higher reactivity compared to its analogs. The reactions with diethyl malonate yield a single isomer of the monoadduct C70 (CF3 )8 [C(CO2 Et)2 ] and a single C2 -symmetrical bisadduct C70 (CF3 )8 [C(CO2 Et)2 ]2 . The Bingel-Hirsch variation is particularly interesting in that it additionally affords, in a similar regioselective manner, the unexpected alkylated derivatives C70 (CF3 )8 [CH(CO2 Et)2 ]H and C70 (CF3 )8 [C(CO2 Et)2 ][CH(CO2 Et)2 ]H. The novel compounds have been isolated and structurally characterized by means of (1) H and (19) Fâ NMR spectroscopy as well as single-crystal X-ray diffraction. The mechanistic and regiochemical aspects of the reaction are explained with the aid of DFT calculations.
RESUMEN
Novel difluoromethylenated [70]fullerene derivatives, C70(CF2 )n (n=1-3), were obtained by the reaction of C70 with sodium difluorochloroacetate. Two major products, isomeric C70(CF2 ) mono-adducts with [6,6]-open and [6,6]-closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X-ray analysis and high-level spectroscopic techniques. The [6,6]-open isomer of C70(CF2 ) constitutes the first homofullerene example of a non-hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C70(CF2 ) isomers showed that it is substantially higher for the [6,6]-open isomer (the 70-electron π-conjugated system is retained) than the [6,6]-closed form, the latter being similar to the electron affinity of pristine C70. In situ ESR spectroelectrochemical investigation of the C70(CF2 ) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter-conversion between the [6,6]-closed and [6,6]-open forms of a cage-modified fullerene driven by an electrochemical one-electron transfer. Thus, [6,6]-closed C70(CF2 ) constitutes an interesting example of a redox-switchable fullerene derivative.