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 (148–1698) highlight the degree of distortion. The arrangement of donor atoms within the distorted octahedral co-ordination sphere of ZnII 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.