Dinuclear transition metal complexes as auxiliary chromophores in chiroptical studies on bioactive compounds

Chiroptical methods, the circular dichroism spectroscopy (CD) in particular, appear to be sensitive, fast and convenient methods for determination of absolute configuration of chiral compounds in Solution. For the CID to be applicable, however, the investigated compounds have to be not only optically active but also should absorb circularly polarized light in an accessible spectral range. To carry out the circular dichroic studies on compounds transparent in the UV-vis region, a suitable chromophoric system needs to be introduced into the molecule in the immediate vicinity of stereogenic center. In such manner the inherently transparent compounds are transformed into so-called cottonogenic derivatives. The method proposed in this work consists of the generation of chiral complexes in situ by mixing the chiral but non-absorbing substance with a solution of an achiral transition metal complex acting as auxiliary chromophore. Such complexes of general formula [M-2(COOR)(4)](k+) X-k where M = Mo, Rh, and Ru can exchange in situ one or more of its carboxylate units with other ligands to form chiral complexes of a bridging or a chelating structure. In all cases, i.e., irrespective of the complexation mode, the CD arising within the d-d absorption bands of the metal cluster depends solely upon the chirality of the compound acting as ligand(s). A variety of biologically important compounds like e.g., amino and hydroxy acids, alcohols, amines, diols, aminols etc., are investigated as potential ligands for these studies. The examination of the CID spectra of the in situ formed chiral complexes allows assignment of the absolute configuration and conformational features of the ligands as well.