Solution and solid state structures of binuclear zinc(II) complexes of bis(pentadentate) ligands derived from bis(1,4,7-triazacyclononane) macrocycles

Abstract

Binuclear zinc(II) complexes have been prepared for bis(pentadentate) ligands, generated by addition of 2-pyridylmethyl arms to the secondary nitrogens in bis(1,4,7-triazacyclononane) macrocycles, and supported by CH₂CH₂ {tmpdtne, [Zn₂(tmpdtne)Br₂][ClO₄]₂·H₂O 1], CH₂CH₂CH₂ {tmpdtnp, [Zn₂(tmpdtnp)Br₂][ClO₄]₂·3H₂O 2}, CH₂CH₂CH₂CH₂ {tmpdtnb, [Zn₂(tmpdtnb)][ClO₄]₄·3DMSO 3}, m-CH₂C₆H₄CH₂ {tmpdtnm-X, [Zn₂(tmpdtnm-X)][ClO₄]₄ 4} and CH₂CH(OH)CH₂ {tmpdtnp-OH, [Zn₂(tmpdtnp-O)][ClO₄]₃ 5} backbones. The crystal structure of 5 confirmed that the two zinc(II) centres are linked by an endogenous alkoxo bridge generated by deprotonation of the CH₂CH(OH)CH₂ backbone. A short intramolecular Zn· · ·Zn separation of 3.904(2) Å is observed, which matches that found at the active site of alkaline phosphatase. For 1 a binuclear complex with Zn· · ·Zn separation of 6.545(9) Å is formed. The distorted octahedral geometry about each zinc(II) centre is defined by five N-donors from the ligand and a bromide in the sixth position. The transoid angles involving one of the tacn nitrogens and either the bromide or a pyridyl nitrogen (148–169°) highlight the degree of distortion. The arrangement of donor atoms within the distorted octahedral co-ordination sphere of Zn^II is asymmetric with the bromide ligand cis to the bridgehead nitrogen. The series of zinc complexes display interesting variations in their solid state and solution stereochemistries. The ¹H NMR spectrum recorded at 300 K is broad indicating rearrangement between different structures on the NMR timescale. The signals become sharper at higher temperatures but only half the number expected from the solid state structure are observed. A rearrangement to a geometry (square pyramidal through loss of the halide ligand) that preserves the symmetry of the ligand occurs at higher temperatures. Compound 2 exhibits similar solution behaviour while for 3 and 4 the ¹H NMR spectra indicate that the symmetric disposition of the ligand is adopted even at room temperature. A correlation exists between the type of bridge linking the pentadentate compartments of the ligands and the preferred solution structure.