Complexes of Click‐Derived Bistriazolylpyridines: Remarkable Electronic Influence of Remote Substituents on Thermodynamic Stability as well as Electronic and Magnetic Properties

2,6‐Bis(1,2,3‐triazol‐4‐yl)pyridine (btp) ligands with substitution patterns ranging from strongly electron‐donating 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 custom‐design 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.     

Conditions for occurrence of long-chain oxidation reaction of organic compounds by persulfate in an aqueous medium

Using a kinetic model of the routes of unbranched radical-chain processes, an expression was found for the rate of the aqueous solution. The possible conditions for the occurrence of the process over a wide range of concentrations of persulfate and the organic compound were analyzed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 360–365, May–June, 1990.     

Effects of various halogen anions and cations of alkali metals on energetics of excess charge recombination in stilbene donor-acceptor capped DNA hairpins

DNA hairpin conjugates with a stilbenedicarboxamide (Sa) hole donor and a stilbenediether (Sd) hole acceptor are considered as model systems for studying charge recombination (CR) of excess charges in DNA. Using the method of thermodynamic integration, we estimated the relative free energies of this process in hairpins with three adenine:thymine pairs between Sa and Sd surrounded by 1 M aqueous solutions of ionic compounds M(+)Cl(-) (M = Li, Na, K) and Na(+)X(-) (X = F, Cl, Br, I). The values of this quantity were calculated with respect to the free energy for the same hairpin in the 1 M NaCl aqueous solution. Based on the results obtained, we conclude that halogen anions have no significant influence on the rate of the CR reaction. By contrast, cations of other alkali metals can considerably change the potential barrier of the process, thus affecting the reaction rate. Different results obtained for cations and anions were attributed to the fundamental distinction in the electrostatic interactions of M(+) and X(-) species with negatively charged phosphate groups of the hairpin. In addition, our results show that the relative free energy of CR is larger for cations that are able to be closer to Sd and Sa structural units. The latter correlation suggests that the replacement of Na(+) by cations of other alkali metals enables one to change the CR rate modifying it in either direction.