HETEROBIMETALLIC INDENYL COMPLEXES - MECHANISM OF CYCLOTRIMERIZATION OF DIMETHYL ACETYLENEDICARBOXYLATE (DMAD) CATALYZED BY TRANS-[CR(CO)(3)(HEPTAMETHYLINDENYL)RH(CO)(2)]

The complex trans-[Cr(CO)(3)(heptamethylindenyl)Rh(CO)(2)] (II) is a v ery efficient catalyst precursor in the cyclotrimerization reaction of dimethyl acetylenedicarboxylate (DMAD) to hexacarbomethoxybenzene. Th e formation of the ''true'' catalyst, likely to be the complex trans-[ Cr(CO)(3)-Ind-Rb(DMAD)(2)], is the slow step of the reaction and take s place during the induction period, the length of which is temperatur e dependent. After total consumption of the monomer two organometallic complexes were isolated from the inorganic residue, viz., the catalys t precursor II and the complex trans-[Cr(CO)(3)-Ind-Rh(CO)(FADE)] (II I; FADE = fumaric acid dimethyl ester), which turns out to be active i n the trimerization reaction as II. The hydrogenation of DMAD to FADE is probably occurring via C-H bond activation of the solvent cyclohexa ne.