Transition metal–carbon bonds. Part IX. Oxidative addition reactions of cyclo-octeneiridium(I) complexes

Abstract

The preparation of [IrCl(CO)(C₈H₁₄)₃] from chloroiridic acid and cyclo-octene reacting in an alcohol, e.g., ethanol, is described. The carbonyl ligand is formed by breakdown of the alcohol and methane is evolved when ethanol is used as reactant (solvent). On washing with ether this triscyclo-octene complex loses cyclo-octene to give [IrCl(CO)(C₈H₁₄)₂]₂. The cyclo-octene ligands can be readily displaced, e.g., by triphenylphosphine, diethylphenylphosphine, or hexa-1,5-diene. Allylic chlorides add on to the complex [IrCl(CO)(C₈H₁₄)₂]₂ with displacement of some cyclo-octene; e.g., with 2-methylallyl chloride the complex [IrCl₂(C₄H₇)(CO)(C₈H₁₄)] is formed; this reacts with more strongly bonding neutral ligands such as pyridine, PPh₃, AsPh₃, etc., and the products [IrCl₂(C₄H₇)(CO)L] contain asymmetrically bonded allylic groups. Acyl chlorides (RCOCl) react with [IrCl(CO)(C₈H₁₄)₂]₂ to give bridged chloro-complexes of the type [IrCl₂R(CO)₂]₂(R = Me, Et, Prⁱ, or Ph). The methyl compound has had its structure determined by X-ray diffraction. This methyl compound reacts with two mol. of dimethylphenylphosphine to give a binuclear acetyl complex [IrCl₂(COMe)CO(PMe₂Ph)]₂ and with a further two mol. to give the mononuclear complex [IrCl₂(COMe)CO(PMe₂Ph)₂] of known stereochemistry. In contrast, the methyl complex [IrCl₂Me(CO)₂]₂ reacts with pyridine to give mixtures of isomers of type [IrCl₂Me(CO)₂Py], which are not acetyl complexes; similarly for the ethyl complex. The ethyl complex [IrCl₂Et(CO)₂py] when heated above its melting point evolves ethylene and pyridine to give bronze needles of what appears to be a hydrido-dicarbonyliridium complex, [IrHCl₂(CO)₂]_x of unknown molecularity. This compound is more easily prepared by treating chloridic acid with carbon monoxide in boiling ethanol. The complex [IrCl(CO)(C₈H₁₄)₂]₂ takes up two mol. of ethylene per iridium atom reversibly; with more ethylene n.m.r. studies show that ethylene exchange between free and complexed ethylene, and also cyclo-octene exchange, is very rapid.