Spectrum-structure correlation for copper(II) complexes in aqueous solution

In order to study the spectrum-structure correlation of copper(II) equatorially coordinated complexes in aqueous solution, in previous works we investigated two aspects of the visible absorption spectra: (a) the empirical *max estimation, starting by the ligand-field strength value estimated for a series of donor atoms, and (b) the structure – absorptivity relationship. The current study inserts itself in the ambit of our project related to the spectrum-structure correlation for copper(II) ion. Combined measurements of pH and visible absorption spectrophotometry were performed. Visible absorption spectra were recorded during alkalimetric titrations as a function of pH. Further computer-assisted treatment (based of the Lambert-Beer law) allowed us to obtain spectra of single complexes. We tried to draw some general considerations regarding the ability of the visible absorption spectra to provide information on axial co-ordination around copper(II) ion in aqueous solution. With this aim, a series of visible spectra in aqueous solution was recorded on the copper(II) - 2,2’-bipyridine system as a function of pH. 2,2’-bipyridine is a bidentate ligand known for its ability to co-ordinate copper(II) in axial position. The formation constant (T = 25°C, I = 0.1 M) of the dimer Cu2L2H-2 with 2,2’-bipyridine has been re-calculated using pH-metric data. Copper(II) complexes of L-aspartic acid with M:L ratio of 1:2 were also tested. A series of axially coordinated complexes found in the literature has also been considered in order to formulate a simple spectrum-structure correlation test. Structural assignment of planar complexes is based on the empirical equation previously published by our research group. This equation can now help to fortify the discussion and to assess a criterion for the axial co-ordination. A reliable estimation of the red shift per donor group has been tried on a wide set of copper(II) complexes. Particularly low values of red shift are showed by imidazole containing ligands, approximately, 23-30 nm of red shift per axial donor group. As for other donor groups, we can estimate an average red shift ranging between 40 and 60 nm per axial donor group. In spite of this attempt to rationalize the results, it appears rather difficult to reach unambiguous identification of contribution to red shift of single donor atoms. In fact, effects attributable to the overall ligand structure cannot be excluded and, moreover, it seems that also the stability might influence the amplitude of the red shift, being found with the highest red shift those apical-coordinated complexes showing high values of logb. The effect of apical carboxylate, as an example, which exhibits a wide range of values of red shift, suggests such a hypothesis (examples of ligands considered: EDTA, NTA, citric acid).