Trimerization of acetylferrocene and of (acetylcyclopentadienyl)tricarbonyl-manganese proceeds efficiently in the presence of ethanol and tetrachlorosilane. The resulting 1,3,5-trisubstituted benzenes have been characterized by X-ray crystallography and compared with the structure of (1,3,5-triphenylbenzene)tris(tricarbonylchromium). The efficacy of EtOH–SiCl4 as a combined reagent for the trimerization of polycyclic ketones is also discussed. Finally, the synthesis and structure of [CpMn(CO)2]2(µ-C=CHPr), derived from the reaction of cymantrene with butyllithium and phenylacetyl chloride at room temperature, is described.Key words: ketone, condensation, cyclization, organometallics, arenes.

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Diels-Alder addition of diphenylacetylene or of 1,2,3-triphenylcyclopropene to 3-ferrocenyl-2,4,5-triphenylcyclopentadienone yields, upon thermolysis, C 6 Ph 5 Fc (5) or C 7 Ph 6 FcH (8), respectively. Subsequent treatment of 8 with triethyloxonium hexachloroantimonate results in the formation of the ferricinium complex [C 7 Ph 6 FcH] + [SbCl 6 ] -(13), rather than the anticipated tropylium cation [C 7 Ph 6 Fc] + . The substituted ferrocene derivatives 5, 8, and 13 have been characterized by X-ray crystallography. From the solid-state structure of 5 it is evident that the peripheral aryl substituents do not adopt a regular propeller type conformation, but instead exhibit an incremental progression of twist angles relative to the central ring. The dynamics of peripheral ring rotations in 5 and 8 have been studied by variable-temperature NMR.

Crystallographically independent structures possessing persubstituted cyclopentadiene and -dienyl moieties were retrieved from the Cambridge Structural Database, and the torsional angles of selected diaryl frames presented in the form of conformational plots. Semiempirical calculations of the corresponding potential energy surfaces reproduced the conformational trends observed in the solid state. By the structure correlation principle, the internal oscillations of nearest (1,2-) and next-nearest (1,3-) aryl rings in all pseudopropeller subunits were found to be only partially correlated. These solid-state data sets, in combination with energetic predictions of the molecules C(5)Ph(5)(-) (1), C(5)Ph(4)H(-) (2), C(4)Ph(4)C=O (3), and C(4)Ph(4)CH(2) (4) reveal that a delayed n-ring-flip (where n = 4 and 5 in C(n)()Ar(n)()(-)(m)()X(m)()), which is otherwise unobservable via NMR spectroscopic methods, is the threshold rotational mechanism.