No abstract

Contrary to the (pentamethylcyclopentadienyl)trichloromolybdenum(IV) system, for which the molecular [Cp*MoCl2-(µ-Cl)]2 complex is the only form observed (Abugideiri, F.; Brewer, G. A.; Desai, J. U.; Gordon, J. C.; Poli, R. Inorg. Chem., preceding paper in this issue), the corresponding chemistry of the heavier halide systems is much more complex. Three compounds or mixtures of compounds that correspond to the overall stoichiometry "Cp*MoX3" (X = Br, I) can exist in the solid state and/or in solution [A = Cp*2Mo2X4(i¿-X)2;and various combinations of them can be obtained depending

The mixed-valence quadruply-iodo-bridged [ ( C p * M ~) ~( p -l ) ~] l ~ has been crystallographically characterized; its cyclic voltammogram shows reversible oxidation and reduction processes and suggests that the [ ( C ~* M o ) ~( p -l ) ~] *( n = 0, 1, 2) series of complexes is isostructural.

it must be assumed that heterodiene 1 and Lewis acid 4 form at least two diastereomeric transition structures in which the ratio of 1 to 4 is unknown. The pressure dependence of the enantioselectivity implies that the transition structure that leads to 2 has a smaller (more negative) activation volume and thus is more compact. Therefore the preferential formation of 2 at normal pressure is enhanced at increased pressure. However, this advantageous result for the enantioselective transformation of 1 cannot be generalized. Preliminary investigations[' on the intermolecular cycloaddition of 5 and 6 in the presence of chiral Lewis acid 4 showed a decrease in enantioselectivity when pressure was increased. 6This is what we expected, because it can be assumed that the diene attacks the complex of the dienophile and the chiral Lewis acid from the less hindered side; the corresponding transition structure should therefore have a larger volume than that leading to the other enantiomer, and will thus be disfavored at high pressure.The observed increase in the enantioselectivity of the intramolecular reaction of 1 at higher pressure may be due to the formation of complexes with different stoichiometry. This point will be clarified by further investigations with other substrates, and by varying the Lewis acid concentration. In particular, chiral and achiral Lewis acid/substrate complexes will be studied at high pressure by N M R spectroscopy.

Ein MoIV‐Anion, einen neutralen Mov‐Komplex und ein gemischtvalentes MoIII/MoIV‐Kation im selben Kristall enthält die Titel‐verbindung 1, die durch Umsetzung von [(Cp*MoBr2)2] mit Brom in Benzol hergestellt werden kann. Die Koexistenz von Mo‐Zentren in drei Oxidationszuständen und die Bildung von 1 kann mit den besonderen Redoxeigenschaften der einzelnen Komponenten des Systems Cp*MoBr/Br2 erklärt werden. Das Bild rechts zeigt das Anion [Cp*MoBr4] in 1. Cp* = η5‐C5Me5.