Versatility of cyclooctadiene ligands in iridium chemistry and catalysis

In the presence of reactants such as acetonitrile, trimethylphosphine, and diphenylacetylene, the 1,5-cyclooctadiene iridium(I) complex [Ir(1,2,5,6-eta-C8H12)(NCCH3)(PMe3)]BF4 (1) has been found to transform into compounds containing cyclooctadiene or cyclooctadienyl ligands in eta(3), eta(2)-; kappa, eta(3)-; kappa(2), eta(2)-; and eta(3)-coordination modes. All these reactions are initiated by an intramolecular C-H activation of the COD ligand and followed by either inter- or intramolecular insertion, or reductive elimination and further C-H activation elementary steps. Compound 1 has also been observed to undergo facile intermolecular oxidative additions of dihydrogen, hydrosilanes, and phenylacetylene to afford iridium(III) hydride complexes. Evidence for the insertion of COD into the Ir-H bonds of these new complexes has been obtained from the isolation of a monohydride complex containing a kappa, eta(2)-cyclooctenyl ligand, from the isomerization of a silyl derivative into analogues containing 1,4- and 1,3-cycloctadiene ligands, and from the occurrence of H/D scrambling among Ir-H and COD C-H sites in the product of DCdropCPh oxidative addition. Si-Si coupling reactions to give disilanes and C-C coupling reactions to give an iridacyclopentadiene complex and 1,2,4-triphenylbenzene have also been observed in silane and phenylacetylene excess, respectively. Competition of all these intra- and intermolecular reactions under the conditions of phenylacetylene hydrosilylation has been found to result in catalytic reactions, the selectivity of which depends on the presence of introduced acetonitrile and its concentration.