Heterobimetallic Indenyl Complexes. Kinetics and Mechanism of Substitution and Exchange Reactions of <i>trans</i>-[Cr(CO)₃-indenyl-Rh(CO)₂] with Olefins

The trans coordination of the benzene ring of the indenyl-Rh(CO)2 complex with tricarbonylchromium strongly enhances the rate of substitution of CO's with bidentate olefins, 1,5-cyclooctadiene (COD) and norbornadiene (NBD) (“extra-indenyl effect”). The activation parameters suggest an associative reaction pathway assumed to proceed via the intermediacy of a nonisolable low-hapticity species, η1-indenyl-Rh(CO)2(L2). In addition, the rate of exchange of the Cr(CO)3 group of the complexes trans-[Cr(CO)3-indenyl-Rh(CO)2], 3, and trans-[Cr(CO)3-indenyl-Rh(COD)], 3a, and suitable acceptors (hexamethylbenzene and cycloheptatriene) is markedly increased with respect to that measured for the same reaction in the monometallic complex η-naphthalene-Cr(CO)3 (“extra-naphthalene effect”). These mutual effects of the Cr(CO)3 and RhL2 units are transmitted through the 10 π electron indenyl framework, and the results obtained are in agreement with the existence of an haptomeric ground-state equilibrium between the two isomers trans-[Cr(CO)3-μ,η6:η3-indenyl-RhL2], I, and trans-[Cr(CO)3-μ,η4:η5-indenyl-RhL2], II

Heterobimetallic Indenyl Complexes. Mechanism of Cyclotrimerization of Dimethyl Acetylenedicarboxylate (DMAD) Catalyzed by <i>trans</i>-[Cr(CO)₃(Heptamethylindenyl)Rh(CO)₂ ]^†

The complex trans-[Cr(CO)3(heptamethylindenyl)Rh(CO)2] (II) is a very efficient catalyst precursor in the cyclotrimerization reaction of dimethyl acetylenedicarboxylate (DMAD) to hexacarbomethoxybenzene. The formation of the “true” catalyst, likely to be the complex trans-[Cr(CO)3−Ind*−Rh(DMAD)2], is the slow step of the reaction and takes place during the induction period, the length of which is temperature dependent. After total consumption of the monomer two organometallic complexes were isolated from the inorganic residue, viz., the catalyst precursor II and the complex trans-[Cr(CO)3−Ind*−Rh(CO)(FADE)] (III; FADE = fumaric acid dimethyl ester), which turns out to be active in the trimerization reaction as II. The hydrogenation of DMAD to FADE is probably occurring via C−H bond activation of the solvent cyclohexane

Heterobimetallic Indenyl Complexes. Synthesis and Carbonylation Reaction of <i>anti</i>-[Cr(CO)₃-μ,η:η-indenyl-Ir(COD)]

The reaction of the anti-[Cr(CO)3-μ,η:η-indenyl-Ir(COD)] (I) complex with an excess of CO in CH2Cl2 at 203 K produces quantitatively the η1-[η6-Cr(CO)3-indenyl]-Ir(COD)(CO)2 intermediate which above 273 K converts into the fully carbonylated complex η1-[η6-Cr(CO)3-indenyl]Ir(CO)4; this in turn is stable up to 313 K. Carbonylation of the anti-[Cr(CO)3-μ,η:η-indenyl-Ir(COE)2] analogue (II) gives the η1-[η6-Cr(CO)3-indenyl]-Ir(CO)4 (VII) species in a single fast step. In contrast to the behavior of the corresponding rhodium complexes, for which η1 intermediates have never been observed and the aromatized substitution product is the stable product, the rearomatization of the cyclopentadienyl ring in iridium complexes to give the “normal” substitution product, viz., anti-[Cr(CO)3-μ,η:η-indenyl-Ir(CO)2] (III) is a difficult process which takes place only on bubbling argon through the solution. The final product III is barely stable in solution. If the carbonylation is carried out using a blanket of CO over the solution of complexes I and II, viz., failing CO, the scarcely soluble iridium dimer [η6-Cr(CO)3-indenyl-η3-Ir(CO)3]2 (IX) stable in the solid state is obtained, probably by dimerization of the unstable intermediate anti-[η6-Cr(CO)3-indenyl-η3-Ir(CO)3] (X)

Heterobimetallic (Ferrocenyl)indenyl Rhodium Complexes. Synthesis, Crystal Structure, and Oxidative Activation of [η⁵-(1-Ferrocenyl)indenyl]RhL₂ [L₂ = COD, NBD, (CO)₂]^‖

The binuclear [η5-(1-ferrocenyl)indenyl]Rh(NBD) (1), [η5-(1-ferrocenyl)indenyl]Rh(COD) (1a), and [η5-(1-ferrocenyl)indenyl]Rh(CO)2 (2) complexes have been synthesized (NBD = norbornadiene; COD = cycloocta-1,5-diene). The crystal structure determination showed that the iron and rhodium nuclei are disposed in a transoid configuration in 1 probably to avoid steric repulsions. On the contrary, in 2 the metals are in a cisoid configuration due to the presence of stabilizing π-hydrogen bonds between the CO's and the hydrogens of the unsubstituted cyclopentadienyl ring. The results of the chemical and electrochemical oxidation of 2 are in favor of the existence of an effective interaction between the two metals