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Separation of Isomers and Mechanisms of Inversion of Stereochemistry of Group 9 d6 Tris-Chelate Complexes of Hinokitiol.

Tris-chelate complexes of Co(III), Rh(III), and Ir(III) with 4-isopropyltropolone (hinokitiol or β-thujaplicin) form by the substitution of carbonate and chloride ligands from group 9 trivalent metal salts. The new complexes are neutral, are readily soluble in most organic solvents, and are brightly colored with strong charge transfer bands. The fac isomers of Co(hino)3 and Rh(hino)3 were isolated from the mixture by fractional recrystallization from ethanol. The remaining mixtures were respectively enriched by 5:3 and 4.4:3 for the mer isomer. The 1H NMR data show that the complexes exhibit remarkable stereochemical lability, which is unusual for diamagnetic d6 group 9 metals, with rotational barriers of 14.2 and 18.2 kcal/mol found for the inversion of stereochemistry of Co(hino)3 and Rh(hino)3. The low activation barriers, as well as the analysis of some key structural parameters, suggest that the inversion of stereochemistry occurs via a trigonal-twist (Bailar) mechanism. Facile substitution of a single hinokitiol ligand in the cobalt complex with ethylenediamine to form [Co(en)(hino)2]Cl also indicates that the tris-chelates are substitutionally and configurationally labile.