Solvent-dependent circular dichroism of trans-disubstituted Bis(R-propane-1,2-diamine)cobalt(III) complexes

Abstract

The circular dichroism spectra of trans-disubstituted bis(R-propane- 1,2-diamine)cobalt(III) complexes are reported for an extended series of solvents. The observed variations in the spectra of the dichloro and dibromo complexes are analysed in terms of stereoselective solvation at the diamine N-H protons, and also the ion association between the complexes and their counter-ions. The solvation of these dihalo complexes, which was studied by p.m.r., showed a preference for hydrogen-bond formation with the equatorial N-H protons, rendering the donor nitrogens asymmetric and thus introducing a new source of dissymmetry close to the metal ion chromophore. This affected a decrease in the positive rotational strength of the ¹A_1g → ¹E_g (D_4h) transition, and a positive contribution to the rotational strength of the ¹A_1g → ¹A_2g (D_4h) component. Ion association of the dihalo complexes was evaluated by a comparison of the circular dichroism of the tetraphenylborate and halide salts.     The tendency for ion association was much greater for the tripositive diammine complex. However, stereospecific solvation was of little importance and the circular dichroism spectra did not show any marked solvent dependence for the tetraphenylborate salt.