Stereoselective Oxidative Additions of Iodoalkanes and Activated Alkynes to a Sulfido-Bridged Heterotrinuclear Early−Late (TiIr₂) Complex

The reactions of the early−late trinuclear complex [Cp(acac)Ti(μ₃-S)₂Ir₂(CO)₄] (1) with electrophiles have been found to occur on the iridium atoms with no other involvement of the early metal than in electronic effects. The reaction with iodine gave two isomers of the diiridium(II) complex [Cp(acac)Ti(μ₃-S)₂Ir₂I₂(CO)₄] differentiated by the relative positions of the iodo ligands on the iridium atoms. The reactions with iodoalkanes are highly stereoselective to give one sole isomer of formula [Cp(acac)Ti(μ₃-S)₂Ir₂(R)(I)(CO)₄] (R = CH₃, CH₂I, CHI₂) with a carbonyl and the iodo ligand trans to the metal−metal bond. The structures of the symmetrical isomer with the iodo ligands trans to the metal−metal bond and that of the compound with R = CHI₂ have been solved by X-ray diffraction methods. The stereoselectivity of the oxidative-addition reactions can be rationalized assuming the influence of steric effects of the groups on the titanium center and a radical-like mechanism. Reactions of 1 with the activated acetylenes, dimethylacetylenedicarboxylate and methylacetylenecarboxylate, gave the complexes [Cp(acac)Ti(μ₃-S)₂Ir₂(μ-η¹-RCCCO₂Me)(CO)₄] (R = CO₂Me, H), with the alkyne bridging the two iridium centers as a cis-dimetalated olefin and the CC bond parallel to the Ir−Ir axis. Two isomers resulting from the disposition of the alkyne along the Ir−Ir vector were observed in solution for the compound with the nonsymmetrical alkyne (R = H), while only one was observed for the compound with R = CO₂Me. An exchange, fast in the NMR time scale, of the apical with the equatorial carbonyls occured in the complexes [Cp(acac)Ti(μ₃-S)₂Ir₂(μ-η¹-RCCCO₂Me)(CO)₄], producing their equivalence in the ¹³C{¹H} NMR spectra.