DFT studies of structures and enatiomerization mechanisms of bis-chelate complexes of Group 12 elements

The enantiomerization processes of bis-chelate complexes of Group 12 elements have been studied, using the model systems involving acetylacetonate and various O2N2-type Schiff bases, by applying both ab initio Hartree-Fock and density functional methods. Both the digonal twist mechanism and bond rupture–formation mechanism have been studied. It was found that the digonal twist pathway including the planar C2h and C2v transition state (TS) structures is predominant over the bond rupture–formation pathway involving the planar, achiral open-ring TS for the Zn(II) complex with malonaldehyde (i.e. 1,3-propanedionato) anion. The replacement of methyl group by hydrogen did not change the energy barriers of enantiomerization via the digonal twist mechanism for Zn(II), Cd(II) and Hg(II). As expected, the barriers decrease proceeding down the group from Zn to Cd and Hg. Surprisingly, in contrast to the tetrahedral ground-states of Zn(II) and Cd(II) counterparts, the bis(imino-acetylacetonato) Hg(II) and related species possess trans-planar C2h ground-state structures, resembling the open-shell Cu(II) and Ni(II) bis-chelate complexes. Solvation effects were also studied using polarizable continuum model for the digonal twist mechanisms and results similar to the gas phase studies were yielded.