Dinuclear Titanium(IV) Complexes from Amino Acid Bridged Dicatechol Ligands:Formation, Structure, and Conformational Analysis

摘要

Amino acid bridged dicatechol ligands 3a-e-H_4 form dinuclear double-stranded coordination compounds [(3a-e)_2Ti_2(OCH_3_2)~(2-) with titanium(IV) ions. Due to the directionality of the ligands, the chirality of the strand, and the chiral complex units, up to seven isomers, I-VII, can be obtained for the double-stranded complexes of ligands 3a-e-H_4. The composition of the mixture of isomeric compounds in solution is strongly dependent on the conditions of complex formation. Under thermodynamic control, only a few isomers are obtained, one of which is the major component of the mixture. X-ray structure analyses were performed for K_2[(3b)_2Ti_2(OH)_2] and K_[3d]_2ti_2(OH)]_2](type I), and for the meso complex Na_2[3e0(3e')Ti_2(OCH_3)_2]. A conformational analysis that uses Ramachandrans method revealed that the conformation of the amino acids in the ligand strands can be compared with those found for amino acids in helical peptide structures. The most favored isomer of [(3)_2Ti_2(OCH_3)_2]~(2-) appears to be of type I, with the catecholamide unit located at the N terminus of the ligand strand that binds to a #LAMBDA#-configurated titanium(IV) complex unit and the dihydroxybenzyl group at the C terminus that coordinates to a #DELTA#-configurated titanium(IV) complex unit. The #LAMBDA# configuration at the N terminus induces the conformation of a right-handed helix in the amino acid residue, while the #DELTA# configuration induces the less favored left-handed helix.