Complexes of cobalt(III) with phenolate-containing polydentate ligands and bovine serum apo-transferrin: Towards creating spectroscopic models for cobalt(III)-tyrosinate interactions

Reaction of each member of a series of variously substituted N-(2-hydroxybenzyl)glycinate (hbg^2-) ligands [5-bromo- (5-Br-hbg^2-); 5-chloro- (5-Cl-hbg^2-); 3,5-dichloro- (3,5-Cl₂-hbg_2-); 5-chloro-3-methyl- (5,3-Cl,Me-hbg_2-); and 3,5-dimethyl- (3,5-Me₂-hbg_2-)] with cobalt(II) in stoichiometric amounts under ambient conditions afforded the Co^III complex anions of the general formulatrans,trans,trans-[Co(hbg)₂]^-. Single-crystal X-ray analyses of [C₅H₁₀NH₂]-trans,trans,trans-[Co(3,5-Cl ₂-hbg)₂]3 and [C₅H₁₀NH₂]-trans,trans,trans-[Co(3,5-Me ₂-hbg)₂]5 {[C₅H_10- NH₂] piperidinium counter ion} have revealed similar structural motifs of these compounds. Each of the crystal structures shows two non-equivalent complex anions in the unit cell connected through hydrogen bondingvia the piperidinium cation. The two hbg_2- ligands in each complex ion are arranged facially around the cobalt(III) ion and each ligand employs a phenolate oxygen, an amine nitrogen and a carboxylate oxygen to form a distorted octahedral geometry with C_i symmetry. These compounds (1-5) serve as spectroscopic models for the interaction of cobalt(III) with tyrosinate ligands. The electronic spectrum of the compound [C₅H₁₀NH₂]-trans,trans,trans-[Co(5,3-Cl, Me-hbg)₂]4 bears a striking resemblance to that of the complex of cobalt(III) with bovine serum transferrin, [Co₂BTf]. In contrast to previous studies on the spectroscopy of the transferrins, one of the two ligand-field transitions expected to occur in [Co₂BTf] is evident in the electronic spectrum of this derivative of bovine serum transferrin in vitro. One of the fascinating characteristics of thetrans,trans,trans-[Co(hbg)₂]^- model system is the solvent dependence of the optical spectra.