Kinetico-Mechanistic Studies of Nucleoside and Nucleotide Substitution Reactions of Co(III) Complexes of Fully Alkylated Cyclen

Molecular Approach to Alkali-Metal Encapsulation by a Prussian Blue Analogue FeII/CoIII Cube in Aqueous Solution: A Kineticomechanistic Exchange Study

The preparation of a series of alkali-metal inclusion complexes of the molecular cube [{Co^III(Me₃-tacn)}₄{Fe^II(CN)₆}₄]^4- (Me₃-tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), a mixed-valent Prussian Blue analogue bearing bridging cyanido ligands, has been achieved by following a redox-triggered self-assembly process. The molecular cubes are extremely robust and soluble in aqueous media ranging from 5 M [H⁺] to 2 M [OH^-]. All the complexes have been characterized by the standard mass spectometry, UV-vis, inductively coupled plasma, multinuclear NMR spectroscopy, and electrochemistry. Furthermore, X-ray diffraction analysis of the sodium and lithium salts has also been achieved, and the inclusion of moieties of the form {M-OH₂}⁺ (M = Li, Na) is confirmed. These inclusion complexes in aqueous solution are rather inert to cation exchange and are characterized by a significant decrease in acidity of the confined water molecule due to hydrogen bonding inside the cubic cage. Exchange of the encapsulated cationic {M-OH₂}⁺ or M⁺ units by other alkali metals has also been studied from a kineticomechanistic perspective at different concentrations, temperatures, ionic strengths, and pressures. In all cases, the thermal and pressure activation parameters obtained agree with a process that is dominated by differences in hydration of the cations entering and exiting the cage, although the size of the portal enabling the exchange also plays a determinant role, thus not allowing the large Cs⁺ cation to enter. All the exchange substitutions studied follow a thermodynamic sequence that relates with the size and polarizing capability of the different alkali cations; even so, the process can be reversed, allowing the entry of {Li-OH₂}⁺ units upon adsorption of the cube on an anion exchange resin and subsequent washing with a Li⁺ solution.

Activation volumes for cis-to-trans isomerisation reactions of azophenols: a clear mechanistic indicator?

The thermal cis-to-trans isomerisation reaction of a series of hydroxy-substituted azo derivatives was studied kinetico-mechanistically as a function of temperature and pressure in order to investigate the possible role of the solvent in controlling the isomerisation mechanism, viz. inversion versus rotation.

Kinetico-mechanistic Studies on the Substitution Reactivity on the {Ru(II)(bpy)2} Core with Nucleosides and Nucleotides at Physiological pH

The kinetico-mechanistic study of the substitution reactions of the aquo ligands in cis-[Ru(bpy)2(H2O)2](2+) by different nucleotides and nucleosides has been conducted at pH close to the physiological value. The concentration dependence and thermal and pressure activation parameters have been measured to ascertain the activation via which reactions take place. Substitution processes are found associatively activated for nitrogen-bonded nucleosides or nucleotides, with outer-sphere hydrogen-bonded aggregates being determinant. For reactions leading to oxygen-bonded nucleotides, the process is clearly dissociatively activated. A selectively induced lability of the inert {Ru(II)(bpy)2} core is observed on the formation of nitrogen(amide)-bonded complexes at relatively low pH values, which might be relevant for the effective intercalation of designed, ruthenium(II)-bonded, aromatic rings.

Kinetico-mechanistic studies of substitution reactions on cross-bridged cyclen Co(III) complexes with nucleosides and nucleotides

Kinetico-mechanistic studies on the substitution reactivity of the [Co{(μ-ET)cyclen}(H2O)2](3+) complex cation at pH values within the 6.0-7.0 range with biologically significant ligands have been carried out. The substitution processes have been found to occur exclusively on the mono-hydroxobridged [(Co{(μ-ET)cyclen}(H2O))2(μ-OH)](5+) species formed after equilibration of the cobalt complex in the relevant medium. The studies conducted on the substitution of the aqua/hydroxo ligands of this dinuclear species are indicative of a dominant role of outer-sphere complexation, involving hydrogen-bonding interactions. The values of the outer-sphere complex formation equilibrium constant are in line with the intervention of both the exiting aqua ligands and the NH groups at the encapsulating {(μ-ET)cyclen} ligand. These complexes result in the preferential formation of O- or N-bonded nucleotides depending on the structure of the base moiety of the ligand. Even the entry of the different donor bonded nucleotides is hampered by the hydrogen-bonding interaction with the dangling moiety of an already coordinated ligand. In general the overall substitution processes occur at a faster rate than those published for the fully alkylated encapsulating {(Me)2(μ-ET)cyclen} ligand derivative, as expected for the still available base-catalysing NH groups in the {(μ-ET)cyclen} ligand.