Complexes of click-derived bistriazolylpyridines: Remarkable electronic influence of remote substituents on thermodynamic stability as well as electronic and magnetic properties

Article abstract of DOI:10.1002/chem.201000721

2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) ligands with substitution patterns ranging from strongly electrondonating to strongly electron-accepting groups, readily prepared by means of Cu-catalyzed 1,3-dipolar cycloaddition (the "click" reaction), were investigated with regard to their complexation behavior, and the properties of the resulting transition-metal compounds were compared. Metal-btp complexes of 1:1 stoichiometry, that is, [Ru-(btp)Cl₂(dmso)] and [Zn(btp)Br₂], could be isolated and were crystallographically characterized: they display octahedral and trigonal-bipyramidal coordination geometries, respectively, and exhibit high aggregation tendencies due to efficient π-π stacking leading to low solubilities. Metal-btp complexes of 1:2 stoichiometry, that is, [Fe-(btp) ₂]²⁺ and [Ru(btp)₂]²⁺, could also be synthesized and their metal centers show the expected octahedral coordination spheres. The iron compounds exhibit quite a complex magnetic behavior in the solid state including spin crossover near room temperature, and hysteresis and locking into high-spin states on tempering at 400 K, depending on the substituents on the btp ligands. Cyclic voltammetry studies of [Ru(btp) ₂]²⁺ reveal strong modulation of the oxidation potentials by more than 0.6 V and a clear linear correlation to the Hammett constant (σ_para) of the substituent at the pyridine core. Isothermal titration calorimetry was used to measure the thermodynamics of the Fe ^II-btp complexation process and enabled accurate determination of the complexation enthalpies, which display a linear relationship with the σ_para values for the terminal phenyl substituents. Detailed NMR spectroscopic studies finally revealed that in the case of Fe^II complexation, dynamics are rapid for all investigated btp derivatives in acetonitrile, while replacing Fe^II by Ru^II or changing the solvent to dichloromethane effectively slows down ligand exchange. The results nicely demonstrate the utility of substituent parameters, originally developed for linear free-energy relationships to explain reactivity in organic reactions, in coordination chemistry, and to illustrate the potential to customdesign btp ligands and complexes thereof with predictable properties. The fast equilibration of the [Fe-(btp)₂]²⁺ complexes together with their tunable stability and interesting magnetic properties should enable the design of dynamic metallosupramolecular materials with advantageous properties.