Copper(II) complexes of three isomeric bis(tacn) ligands: Syntheses, structures and properties

Two new isomeric bis(tacn) ligands, 3,5-bis(1,4,7-triazacyclonon-1-ylmethyl)-1-(hydroxymethyl)-benzene (HLhyx) and 2,6-bis(1,4,7-triazacyclonon-1-ylmethyl)anisole (Lanx) have been synthesised. In addition, 2,6-bis(1,4,7-triazacyclonon-1-ylmethyl)-4-methylphenol (HLohx) was also synthesised in a considerably higher yield than the previously reported method. The ligands were reacted with two equivalents of Cu(NO3)2·3H2O at pH 5 resulting in the formation of three binuclear complexes. The ligands Lanx and HLhyx form complexes [Cu2Cl3(L)(H2O)](PF6) containing two unique Cu2+ centres coordinated to the ligand. In contrast, the phenolic HLohx ligand forms a bridged complex [Cu2(μ-Cl)(Lohx)](PF6)2, with the two Cu centres further linked by (μ-Cl) and (μ-O) bridges.

Three isomeric m-xylene bridged bis(tacn) ligands have been synthesised in high yield by taking advantage of Boc-based methods using Boc2tacn as a synthetic precursor. Crystallographic analysis of the corresponding copper(II) complexes, with PF6 as the counter-ion, demonstrated that the substituent on the m-xylene ring significantly influenced the structure of the complexes, with two isomeric ligands forming metal complexes of the type [Cu2Cl3(L)(H2O)](PF6), containing two unique Cu2+ centres coordinated to the ligand, whilst the third isomeric ligand formed a bridged complex of the type [Cu2(μ-Cl)(L)](PF6)2, with the two Cu centres further linked by (μ-Cl) and (μ-O) bridges. These results provide a rationale for the differences in binding behaviour observed with recombinant proteins that contain histidine-rich peptide tags when these ligands are immobilised onto support materials and complexed with borderline metal ions, such as Cu(II) ions.Figure optionsDownload as PowerPoint slide