Theoretical Analysis Of The Absolute Configurations And The Chiroptical Properties Of Cobalt(?) Complexes With SDDA-type Ligands

In 1972,Maricondi and Douglas studied the optical activity of[Co(EDDA)(L)]+-type complexes with the tetradentate EDDA-type(EDDA =ethylenediamine-N,N?-diacetate)ligands,as well as a bidentate ancillary ligand L,and found that the ECD peak intensities of all the N-alkyl-substituted complexes are less than half those of the corresponding unsubstituted EDDA complexes.This reduction in intensities was thought to be reasonable because the asymmetry of alkyl-substituted nitrogens is less than that of unsubstituted ones.Therefore,this effect is called vicinal effect.In such a sense,it seems impossible to reverse the sign of rotational strengths or ECD spectra only by the vicinal effects.However,just one year later,Maricondi et al found that the isomers of [Co(ED3A)(NO2)]— and[Co(BED3A)(NO2)]—(ED3A is a pentadentate ethylene-diamine-N,N,N?-triacetate,BED3 A is the corresponding N-benzyl-substituted ligand)with comparable structures give ECD curves with opposite signs over most of the d-d absorption region.We call this phenomenon as unusual vicinal effect,and have afforded a theoretical analysis on it.In addition,for cobalt complexes [Co(SDDA)(L)]+ with SDDA-type ligands(i.e.,the N-benzyl-substituted EDDA),Hawn et al found that their ECD intensities are significantly stronger than those in the EDDA counterparts.However,the metal-centered charality of [Co(SDDA)(L)]+ wasassigned to ?,which is different from that of the EDDA chelate.This indicates either a strongly unusual vicinal effect,or an error in the assignment of the metal-centered charality.To account for this experimental phenomenon,the absolute configurations and chiroptical properties of the cobalt complexes with SDDA-type ligands have been systematically investigated at the first principle level.The main work in this thesis is as follows.1.For the 20 isomers of the complexes [Co(SDDA)(en)]+,[Co(EDDA)(stien)]+,[Co(DBEDDA)(en)]+,their geometries have been optimized at the B3LYP/6-311++G(2d,p)level,and verified without imaginary frequencies.Based on the optimized geometries,their13C-NMR spectra have been also computed using the GIAO method with the same functional and basis set.The twist form of E-ring in SDDA has been determined as ? by comparing the calculated and observed 13C-NMR spectra.2.Then,the transition wavelengths,oscillator and rotational strengths of the three series of chelates have been calculated employing the TDDFT method with the same functional and basis set.The assignments of the first 30 low-lying transitions have been made carefully.A comparison of the calculated and observed ECD spectra showed that the metal-centered chirality of [Co(SDDA)(en)]+ is ?,not ? as reported in the reference.Therefore,the N-vicinal effect of [Co(SDDA)(en)]+ is neither unusual,nor same as that in the [Co(EDDA)(L)]+-type complexes.3.To clarify the nature of the vicinal effect,the individual contributions of the chiral arrays to the rotational strengths of the transitions have been also calculated.The results showed that the concept of usual vicinal effects could be further widen to incorporate the two cases: one is for the groups with electron donor groups which lead decrease in the ECD intensities,while another is for those with electron acceptor groups which cause increase in the intensities of ECD bands.Clearly,these findings provide some insight into the N-vicinal effects as well as the chiroptical properties of the chelates,which is important to elucidate the relationship between chiroptical properties and stereochemical features of the chelates.