Pyridyldiimine macrocyclic ligands: Influences of template ion, linker length and imine substitution on ligand synthesis, structure and redox properties.

Syntheses are reported for a series of 2,6-diiminopyridine-derived macrocyclic ligands with flexible alkyl linkers, formed via templated Schiff base condensations around alkaline earth metal triflate salts. Systematic comparisons of the physical and electronic properties of the complexes were conducted using NMR spectroscopy, mass spectrometry, solution-phase UV–Vis spectroscopy, cyclic voltammetry and single-crystal X-ray crystallography. [Display omitted] • Templated syntheses of flexible macrocycles with pyridyldiimine fragments. • Crystal structures demonstrate ligand flexibility in adapting to template ion sizes. • UV–vis and electrochemical data gauge differences in the ligand electronic structure. • TD-DFT analyses elucidate the nature of the UV–vis transitions. A series of 2,6-diiminopyridine-derived macrocyclic ligands have been synthesized via [2 + 2] condensation around alkaline earth metal triflate salts. The inclusion of a tert -butyl group at the 4-position of the pyridine ring of the macrocyclic synthons results in macrocyclic complexes that are soluble in common organic solvents, thereby enabling a systematic comparison of the physical properties of the complexes by NMR spectroscopy, mass spectrometry, solution-phase UV–Vis spectroscopy, cyclic voltammetry and single-crystal X-ray crystallography. Solid-state structures determined crystallographically demonstrate increased twisting in the ligand, concurrent with either a decrease in ion size or an increase in macrocycle ring size (18, 20, or 22 membered rings). The degree of folding and twisting within the macrocycle can be quantified using parameters derived from the N pyr -M-N pyr bond angle and the relative orientation of the pyridinediimine (PDI) and pyridinedialdimine (PDAI) fragments to each other within the solid state structures. Cyclic voltammetry and UV–Vis spectroscopy were used to compare the relative energies of the imine π* orbital of the redox active PDI and PDAI components in the macrocycle when coordinated to redox inactive metals. Both methods indicate the change from a methyl to hydrogen substitution on the imine carbon lowers the energy of the ligand π* system. [ABSTRACT FROM AUTHOR]