84580-15-4Relevant academic research and scientific papers
Metalation-resistant ligands: Some properties of dibenzocyclooctatetraene complexes of molybdenum, rhodium, and iridium
Anton, Douglas R.,Crabtree, Robert H.
, p. 621 - 627 (2008/10/08)
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.
