Amino-Alcohol Ligands: Synthesis and Structure of N,N′-bis(2-hydroxycyclopentyl)ethane-1,2-diamine and Its Salts, and an Assessment of Its Fitness and That of Related Ligands for Complexing Metal Ions

Abstract

The synthesis and solid state structure of the amino-alcohol ligand, (1R,2R,1'S,2'S)-N,N'-bis(2-hydroxycyclopentyl)ethane-1,2-diamine (Cyp(2)en) obtained by the reaction of cyclopentene oxide with ethylenediamine, as well as its bromide, chloride, perchlorate, and nitrate salts, is reported. In all five structures, the Cyp(2)en ligand sits on a center of inversion with the two amino groups and two hydroxyl groups of the cyclopentyl moieties trans to each other across that inversion center. Comparing this conformation to that of a density functional theory (DFT)-calculated structure of cis-[Ni(Cyp(2)en)(H(2)O)(2)](2+) suggests that the ligand is not well pre-organized for coordinating a metal ion. The likely solution structure of this ligand, and that of the related ligands bis(2-hydroxy)ethane-1,2-diamine (Bheen) and bis(2-hydroxycyclohexyl)ethane-1,2-diamine (Cy(2)en), was explored using molecular mechanics modeling with the generalized AMBER force field. A Boltzmann distribution of the lowest energy conformations found shows that 36 conformations of Bheen each contribute more than 1% to the population at 300 K; this decreases to 27 conformations for Cyp(2)en and to 12 for Cy(2)en, and Cy(2)en is the most rigid of the three ligands. The population-weighted root-mean-square deviation (rmsd) between these lowest energy conformations and the conformation required for coordinating a metal ion increased in the order Cy(2)en < Cyp(2)en < Bheen, while stability constants for coordinating Ni(II) decrease in the order Cy(2)en > Bheen > Cyp(2)en. The high pre-organization of Cy(2)en is due to an intramolecular N-H....O hydrogen bond, apparently missing in the other two ligands. Pre-organization is likely to be an important, but not the only factor, that controls the value of the stability constants of these ligands with metal ions.