Single-crystal, vacuum-ultraviolet, and lon-association circular-dichroism spectra of the bis[(2R)-2-methyl-1,4,7-triazacyclononane]cobalt(III) lon

Abstract

The circular dichroism (c.d.) spectrum of a single crystal of [Co{(2R-Me)-tacn}₂]I₃.5H₂O [(2R-Me)-tacn =(2R)-2-methyl-1,4,7-triazacyclononane] with radiation propagated along the crystal c axis is compared with that of the corresponding perchlorate salt in aqueous solution, the latter spectrum being extended into the vacuum u.v. region. The comparison indicates that the rotational strengths of the D₃ components of the d-electron transition to the octahedral ¹T₁ state of the Co^IIIN₆ chromophore at 476 nm make the relative contribution |R(A₂)| > |R(E)| to the optical activity, in contrast to the converse inequality found in the tris(diamine)cobalt(III) series. The contrasting inequalities are correlated with the predominant location of ligand groups in the polar and in the equatorial D₃, region for the title complex and for the tris(diamine)cobalt(III) series, respectively. The addition of multiply charged oxo-anions to [Co{(2R-Me)-tacn}₂]³⁺ in solution enhances R(E) at the expense of R(A₂), indicating a preferred equatorial location of the anion in the ion-association structure. Ligand-polarisation calculations of the rotational strengths of chiral Co^IIIN₆ complexes with ligand groups in D₃ polar locations, while correct in sign, are not quantitatively satisfactory.