Iridium complexes of a chelating bis(iminoxolene): augmentation of metal–metal π bonding by metal–ligand π bonding

Metalation of the <i>trans</i>-spanning 1,2-ethanediyldianthranilate-bridged bis(iminoquinone) C₂H₄[O₂CC₆H₄-2-(NC₆H₂-3,5-^<i>t</i>Bu₂-2-O)]₂ (Egan) with bis(cyclooctene)iridium(I) chloride dimer results in the formation of a mixture of iridium compounds containing two, one or no chlorides per iridium. The monochloride product is a six-coordinate monomer, with one ester carbonyl of the bridge coordinated to give a <i>mer</i>, κ³ iminoxolene linkage. This compound is formed exclusively as a <i>cis</i>-α isomer with the two iminoxolene nitrogens mutually trans, but it isomerizes upon heating to form an equilibrium mixture with the <i>cis</i>-β isomer. The iridium complex with no chlorides is iridium-iridium bonded dimeric (<i>A</i>,<i>C</i>)-(Egan)₂Ir₂, which is formed as a single <i>S</i>₄-symmetric stereoisomer. The iridium-iridium distance, 2.5584(4) Å, is extremely short for an unsupported iridium-iridium bond. This is attributed to donor-acceptor interactions between filled metal dπ orbitals on one iridium and empty metal-iminoxolene π* orbitals on the other iridium fostering a significant degree of metal-metal π bonding. The dimer can be reduced to a monomeric four-coordinate anion which is alkylated by iodomethane to form a five-coordinate methyliridium complex. In both of these complexes, the ester groups in the bridge are not bound to iridium.