Structure and conformation analyses of two isomeric glycinato[tris-(2-aminoethyl)amine]cobalt(III) complex ions

Abstract

Crystal and molecular structures of the two diastereomeric title complex ions have been determined by X-ray diffraction. Crystals of the orange isomer, [Co(taa)(GlyO)]Cl[ClO₄], are monoclinic, space group P2₁/a, with a = 14.135(4), b= 9.725(4), c = 11.544(4)Å, β= 98.13(13)°, Z= 4, while those of the red isomer, [Co(taa)(GlyO)][ClO₄]₂, are orthorhombic, space group P2₁2₁2₁, with a= 16.990(4), b= 8.382(4), c= 12.034(4)Å, Z= 4. The structure of the orange isomer has been determined by the heavy-atom method, and of the redisomer by direct methods. The structures have been refined to R 0.039 (orange. 2 425 reflections) and 0.035 (red, 1 708 reflections). The glycine is chelated in the octahedral complex through its NH₂, group trans to the tertiary amine of taa in the orange isomer, and cis in the red isomer. These geometries are adequately reproduced by strain-energy minimization calculations and the latter are correlated with the stability relation. Comparison of the two isomeric complexes shows how the tripod ligand is accommodated in the octahedral structure, and reveals a special steric situation which labilizes a substituent cis to N_tert in taa complexes of Co^III. The specific reaction path with which [Co(taa)(OH)(OH,)]²⁺ hydrolyzes the N-terminal residue of peptides to afford the orange isomeric [Co(taa)L]ⁿ⁺(L = amino-acid anion) is rationalized by the present studies.