CD Spectra of Polynuclear Complexes of Diimine Ligands: Theoretical and Experimental Evidence for the Importance of Internuclear Exciton Coupling

We have recently reported on dinuclear complexes Λ,Λ-[Co2L2Cl2]CoCl4 of two novel chiral ligands
(1a and 1b) which contain pyridyl-imine chelate groups (Telfer, S. G.; Sato, T.; Kuroda, R. Chem. Commun.
2003, 1064−1065). The absolute configuration of the cobalt(II) centers was unambiguously assigned by
X-ray crystallography. However, the sign of the exciton couplets in their CD spectra was opposite to that
expected on the basis of the stereochemistry of the metal centers. We present a rationalization of these
anomalous spectra in terms of an “internuclear” exciton coupling model which takes into account the coupling
of chromophores located on different metal centers. We have performed a series of semiempirical (ZINDO)
calculations which provide quantitative support to this model. These findings show that the absolute
configuration of the metal centers in a polynuclear complex may be incorrectly assigned on the basis of
CD data if internuclear coupling effects are not taken into consideration. We summarize the CD spectral
data of number of other chiral polynuclear complexes from the literature, including dinuclear complexes
bridged by the 2,2‘-bipyrimidine ligand, complexes of the HAT ligand, and dinuclear triple-stranded helicates.
The amplitude of the CD spectra of many of these complexes is not additive with the number of
chromophores. These anomalous spectra can be accounted for by taking internuclear coupling effects into
consideration.