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 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 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.