Iron and cobalt complexes of 5,5′-di(methylene-N-aminoacidyl)-2,2′-bipyridyl ligands: ligand design for diastereoselectivity and anion binding

Abstract

The syntheses and coordination chemistry of 5,5′-di(methylene-N-aminoacidyl)-2,2′-bipyridyl ligands, where the amino acid is valine (1) or alanine (2), are presented. Complexes [M(1)₃]ⁿ⁺, where M = Co(II), Co(III) and Fe(II), form diastereoselectively when the amine group of the amino acid arm is protonated. At higher pH the diastereoselectivity drops significantly. The solid state structure of [Co^III(1H₂)₃]Cl₂(ClO₄)₇ was determined by X-ray crystallography. Two chloride ions were found to be encapsulated by the amino acid arms of the complex via electrostatic attractions and hydrogen bonding to the protonated amine groups, as seen previously for the Fe(II) complex. No anion binding was detected in aqueous solution, but complexes [Fe^II(1H₂)₂(1H)]⁷⁺ and [Co^III(1H₂)₃]⁹⁺ bind chloride ions in CD₃OD with binding constants of 60(4) and 24(2) M⁻¹ respectively, as determined by ¹H NMR spectroscopy. ¹H NMR spectroscopy suggests considerable conformational change of the ligand sidearms upon chloride binding. Complexes [Fe^II(2)₃]²⁺ and [Co^II(2)₃]²⁺ are formed with d.e.'s of 33 and 56% respectively.