Though 2,2′:6′,2′′-terpyridine is commonly used as a ligand for forming metallo-supramolecular assemblies in a variety of solvent environments, little is known about how the properties of the medium influence its behavior. We have studied the kinetic stabilities of bis terpyridine complexes with iron(II) and cobalt(II), and found variations in decay rate between commonly employed solvents of more than five orders of magnitude for iron, and thirty-fold for cobalt. While numerous factors likely contribute to these effects, a clear correlation between the decay rates of the iron complexes and the solubility of terpyridine suggests that favorable solvent–ligand interactions may lower the activation barrier for dissociation by stabilizing the transition state. These results illustrate the importance of solvent selection when designing metallo-supramolecular assemblies based on terpyridine, and also provide insights for further tuning the dynamic properties of these complexes.

Kinetic stabilities of bis-terpyridine complexes with iron(II) and cobalt(II) in organic solvent environments

The aquation kinetics of [Co(NH3)5Cl]2+ and [Co(NH3)5Br]2+ ions in acetone-water have been investigated over a wide range of solvent compositions and temperatures. The variation of thermodynamic properties of the activated complex with the mole fraction of acetone reveals the existence of specific solvation. The correlation of the rate constant with dielectric constant of the medium is examined and discussed. A reaction mechanism which describes the solvent effect on reaction rate is proposed.

Aquation kinetics of chloro- and bromo-pentamminecobalt(III) ions in water-acetone mixtures

We report rate constants for reactions of tris(2,2′-bi(4H-5,6-dihydrothiazine)iron(II), Fe(btz)
 3
 2+
 , in acid solution, with hydroxide, with peroxodisulphate, with silver(I), with mercury(II), and with cadmium(II), in aqueous solution, and for the reaction of Fe(btz)3

 3
 2+
 with 1,10-phenanthroline and with 2,2′-bipyridyl in aqueous methanol Mechanisms and relative reactivities are discussed

Kinetics of reactions of the tris(2,2′-bi(4H-5,6-dihydrothiazine))iron(II) cation

The aquation of chloropentamminecobalt(III) ion has been studied in ethylene glycol-water mixtures up to 80% by weight at five temperatures from 35 to 55°C. Specific solvent effects on the reaction rate and mechanism have been investigated and discussed. The validity of our previously proposed equations, correlating the specific reaction rate with the dielectric constant of the medium has been examined. The thermodynamic parameters of activation have been calculated and presented as evidence for the solvation effects and in support of the proposed reaction mechanism.

Kinetics of aquation of chloropentamminecobalt(III) perchlorate in ethylene glycol-water media

The aquation kinetics of the chloropenta-aminecobalt(III) ion in H2O−EtOH mixtures have been determined. A new correlation is described for calculating the chemical potential from kinetic data and molar thermodynamic excess functions for binary mixtures. The rate constants correlate well with Grunwald-Winstein solvating power Y parameter and with the dielectric constant of the medium. The data supports the D mechanism.

Solvent effects on the aquation of chloropentammine-cobalt(III) perchlorate in ethanol-water

Rates of aquation of tris-(1,10-phenanthroline)iron(II) complexes have been determined in series of aqueous ethanol and aqueous formic acid solvent mixtures. Variation of aquation rate with solvent composition for the 5-nitro- and 4,7-dimethyl complexes in the two series of solvents suggests that, in the favourable case of the electron-withdrawing nitro-substituent and of aqueous ethanol as solvent, bond-making between iron and incoming water is of some significance in transition-state formation. An attempt is made to extend the use of the Grunwald–Winstein concept of the relation between reactivity and solvating power of solvents to these inorganic systems.

J. Burgess

Papers

Aquation of tris-(1,10-phenanthroline)iron(II) complexes in aqueous ethanol and in aqueous formic acid