A Metallosupramolecular Approach to Responsive Polymer Gels using Pyridine-Hydrazones

Abstract

The use of metal-induced conformational changes in pyridine-hydrazone crosslinkers to control the volumes of gels was complicated by the adaptive metallosupramolecular behaviour of the ligands. The combination of pyridine-hydrazone ligands with closed-shell Pb(II), Zn(II), Ag(I), Cu(I) and d9 Cu(II) metal ions produced complexes that depended not only on the characteristics of the ligands and metal ions but also on the external conditions.

A mixture of techniques including NMR spectroscopy, ESI mass spectrometry, and X-ray crystallography was used to characterise the metallosupramolecular structures in the solution and solid states and to determine any adaptive behaviour. Chapters 1 and 2 provided an introduction to the metallosupramolecular chemistry and synthesis of pyridine-hydrazone crosslinkers.

Chapter 3 described how the Pb(II) complexes of pyridine-hydrazones were directed solely by the characteristics of the ligand and metal ion. A low coordination number in the short-stranded pyridine-hydrazone ligands activated the stereochemical lone pair in Pb(II) to direct meso-helicates while the higher coordination number in the long-stranded pyridine-hydrazone ligands directed highly-saturated single helicates.

Chapter 4 showed how the higher kinetic lability of Zn(II) led to a more adaptive metallosupramolecular chemistry towards solvent, metal to ligand ratio, and concentration. A steric control due to the small size of Zn(II) favoured meso-helicates in less-coordinating CD3NO2 but double helicates in coordinating CD3CN. Mixtures of meso-helical, double helical, and oligomeric Zn(II) complexes were formed when neither the pyridine-hydrazone ligands nor the Zn(II) ions could select for a single stable structure.

Chapter 5 described how the flexible coordination sphere of Ag(I) benefitted from stabilisation by Ag···Ag and hydrogen bonding interactions in complexes with pyridine-hydrazones. The large difference in thermodynamic stability between the Ag(I) metallosupramolecular structures favoured the formation of a single structure as the solvent, metal to ligand ratio, and reaction time were varied. Meso-helical Ag(I) complexes were directed at high metal to ligand ratios while double helical Ag(I) complexes were directed at low metal to ligand ratios.

Chapter 6 showed how the pyridine-hydrazone ligands could stabilise both Cu(I) and Cu(II) in homometallic and mixed valence double helicates. The stability of the homometallic Cu(I) double helicates towards oxidation increased with the number of coordinated Cu(I) centres. However, no additional stability was found for the Cu(I) double helicate with five Cu(II) centres.

Chapter 7 described the copolymerisation of acryloyl-modified pyridine-hydrazone ligands and methyl acrylate to produce metal-responsive copolymer gels. The swelling of the copolymer gels upon the addition of Pb(II) ions in 2:1 CH3CN/CHCl3 was dominated by charge effects. However, the use of THF as the swelling solvent allowed the metal-induced conformational change of pyridine-hydrazone crosslinkers from linear to helical to reduce the volume of the gels.

The difference in charges due to the adaptive metallosupramolecular chemistry of pyridine-hydrazone ligands does not allow the gel swelling to be directly related to the conformational change of the pyridine-hydrazone crosslinkers. However, if the crosslinkers could be re-designed to form only one kind of complex, then perhaps the metal-induced conformational change of the crosslinkers could solely control the volumes of the polymer gels.