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

Abstract

Metalation of the trans-spanning 1,2-ethanediyldianthranilate-bridged bis(iminoquinone) C₂H₄[O₂CC₆H₄-2-(NC₆H₂-3,5-^tBu₂-2-O)]₂ (Egan) with bis(cyclooctene)iridium(I) chloride dimer results in the formation of a mixture of iridium compounds containing 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 mer, κ³ iminoxolene linkage. This compound is formed exclusively as a cis-α isomer with the two iminoxolene nitrogens mutually trans, but it isomerizes upon heating to form an equilibrium mixture with the cis-β isomer. The iridium complex with no chlorides is iridium-iridium bonded dimeric (A,C)-(Egan)₂Ir₂, which is formed as a single S₄-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.