Metalation-resistant ligands: Some properties of dibenzocyclooctatetraene complexes of molybdenum, rhodium, and iridium

Article abstract of DOI:10.1021/om00077a009

The chelating diolefinic ligand dibenzo[a,e]cyclooctatetraene (dct) displaces 1,5-cyclooctadiene (cod) from [Ir(cod)Cl]₂ to give [Ir(dct)Cl]₂. This reacts with AgBF₄ and PPh₃ to give [Ir(dct)L₂]BF₄. The addition of H₂ at -80 °C gives cis-[IrH₂(dct)L₂]BF₄, which is stable at 20 °C in CH₂Cl₂ but rearranges with methanol catalysis at -30 °C to cis,trans-[IrH₂(dct)L₂]BF₄. This appears to be the first case of such a catalyzed rearrangement and takes place by a deprotonation/reprotonation sequence. The intermediate [IrH(dct)L₂] can be obtained from the cis,trans dihydride and t-BuOK. Where L₂ is 1,3-bis(diphenylphosphino)propane (dpp), a cis dihydride is obtained at -80 °C, which rearranges with methanol catalysis to a new trans isomer. The analogous rhodium complex [Rh(dct)L₂]PF₆ (L = PPh₃) does not react with H₂, but [RhH₂(dct)L₂]PF₆, the first rhodium dihydride olefin complex, can be obtained from dct and [RhH₂(Me₂CO)₂L₂]PF₆. The strongly electrophilic character imparted to its complexes by the dct ligand is discussed with reference to the IR of (dct)Mo(CO)₄ which suggests that dct is substantially more electron-withdrawing than cod. A Tolman-type electronic parameter for both monodentate and chelating ligands is proposed. The substitution of dct for cod makes the complex cis,trans-[IrH₂(diene)(PPh₃)₂]BF₄ more acidic by at least 8 pK units.