Theoretical Analysis Of The Chelate-Ring Strain And Vicinal Effects On The ECD Spectra Of Cobalt(Ⅲ) EDTA-type Complexes

The ethylenediamine-N,N,N’,N’-tetraacetate Acid (H4EDTA) and its derivatives can be hexadentated to transition metal ions forming a coordination octahedron due to its strong coordination ability. Since these chiral complexes have been widely applied in biomedicine, chemistry, environment and related fields, they have recevied the much attention.There are many researches on the circular dichroism, UV spectra and NMR of Co(Ⅲ)EDTA series experimentally, but few people correlate circular dichroism to ring strains and vicinal effects from theoretical aspects, and quantitatively analyzes the relationship between them. This paper presents the study of chiral optical properties of Co(Ⅲ)EDTA complexes from the theoretical view. We explain the influence of ring effect on rotational strengths by introducing a new concept of "effective ring strain energy" and analyze the unusual N-vicinal effects of Co(Ⅲ)-ED3A and Co(Ⅲ)-EDDS complexes. The main works are as follows:1、The stable conformation and metastable configuration by structure search have been found and calculated at the B3LYP/6-311++G(2d,p) level using the DFT including solvent effects.2、The ECD spectra have been calculated using the TDDFT at same levels. We found that δ comformational is more stable than λ comformational for A isomers. Analysis shows that the first 3 transitions of Co(Ⅲ)-EDTA series are d-d transition.3、" Effective ring strain energy" has been proposed in this paper to quantitatively explain the affects of rings strain on ECD spectra. Generally, the intensity of the ECD increased when the strain of the effective ring strain energy was increased.4、For Co(Ⅲ)ED3A-type complexes, the ground states and excited states have been calculated based on B3P86/6-311++G(2d,p) level. The calculated ECD curves are in excellent agreement with the observed ones except for some small red or blue shift in the experiment band wavelengths. We focus on the effect of unusual N-vicinal effects.5、The results showed that both the substitutent effects and conformational relaxation effects were sizable and opposite sign, so the vicinal effects due to the chiral nitrogens were significant for the [Co(ED3A-type)(X)]-series but no obvious for the N-substituted [Co(EDDS)]- series.These findings provide a deep insight into the unusual N-vicinal effects as well as the chiroptical properties of the chelates, and the impact of it on circular dichorism has important significance in theory, also have important reference value and a certain significance for the design and development of new chiral material.