Effects of electron configuration and coordination number on the vibrational circular dichroism spectra of metal complexes of trans-1,2-diaminocyclohexane

Abstract

Transition metal complexes of ethylenediamine have attracted significant interest as prototype systems for a range of studies related to their chiroptical properties. In order to better elucidate the effects of different central metal ions and also different coordination numbers on the vibrational circular dichroism (VCD) spectra, trans-1,2-diamino cyclohexane (chxn) was chosen as the chiral ligands in the current report. In this case the conformation of the diamino ligand is predetermined by its absolute configuration and the transition from the λ- to the δ-form that can occur in the case of ethylenediamine is no longer possible. The fingerprint region of the vibrational absorption and VCD spectra of three transition metal complexes of chxn have been analysed in detail. For the tris chelate complexes Ni(chxn)₃²⁺ and Cu(chxn)₃²⁺, selective enhancement of some VCD bands in the otherwise almost identical spectra has been observed and explained in terms of a ring current mechanism and of a different number of unpaired electrons of the metal centers. The comparison of the VCD spectra of Cu(chxn)₃²⁺ and Cu(chxn)₂²⁺ reveals the effects of coordination number that manifest as an inversion of the strong bisignate VCD pattern of the NH₂ scissor vibrational modes. This leads to the conclusion that this region can be used to extract information about the ligand environment and the chirality of the metal center.