No abstract

. ‥und doch so verschieden: Nicht‐Häm‐Eisenkomplexe können die stereoselektive Oxidation von C‐H‐ und CC‐Bindungen katalysieren. Die isomeren FeIII‐η1‐Hydroperoxo‐ und FeIII‐η2‐Peroxokomplexe sind dabei wichtige Zwischenprodukte (siehe Bild). Über die spektroskopischen Eigenschaften der FeIII‐Hydroperoxo‐ und Peroxokomplexe wird berichtet.

In an NMR spectroscopic study the cleavage of epoxides [ethylene, propylene, and cis/trans-butylene oxide] by chromyl chloride giving access to oxochromium(VI) alkoxides was shown to proceed via a bimolecular rate-determining step where two molecules of a complex CrO2Cl2···epoxide collide. Subsequently one Cl ligand at the first Cr center attacks the backside of an epoxide molecule complexed at the Cr center of a second CrO2Cl2···epoxide molecule and vice versa. The trans-opening of the epoxides was proved by determining the configuration of the chlorohydrins resulting from hydrolysis of the corresponding alkoxide products in the cases of cis- and trans-butylene oxide. The NMR data provide evidence that each oxochromium(VI) alkoxide adopts one preferred conformation in solution although DFT calculations did not indicate any special stabilizing effects. The product formation was rationalized by DFT calculations concerning the thermodynamics of the reactions.

On photolysis of [CpMo(CO)3(η1‐C3H5)] in solid inert matrices at low temperature, one CO molecule is eliminated yielding the exo and endo isomers of [CpMo(CO)2(η3‐C3H5)] in almost 1:1 proportions, although the fraction of the exo conformer is increased by prolonged broad‐band UV irradiation. If gaseous [CpMo(CO)2(η3‐C3H5)] is isolated in an Ar matrix the proportions of the two conformers are again about 1:1, indicating that they possess almost equal energies, as confirmed by DFT calculations (B3LYP/SDD). This situation changes in a solvent environment, where increasing the polarity of the solvent favours the exo isomer, as indicated by the ratios exo/endo = 1.7:1 (pentane or cyclohexane), 3.0:1 (benzene) and 3.3:1 (CH2Cl2) determined by IR and NMR measurements. Such a finding is consistent with the results of DFT calculations, which reveal that the exo isomer is more polar.

Not only capable of generating organic radicals by H abstraction, chromyl chloride can also trap them again (see scheme). This is confirmed by the crystal structure of a Cr(V) oxo alkoxide, which was formed in three successive steps when bisadamantylidene oxide was allowed to react with CrO(2)Cl(2).