Mechanistic information from volume profiles for water exchange and complex-formation reactions of aquated Ni(II). pH, buffer and medium effects

Abstract

Rate and activation parameters for the complex-formation reaction of Ni(2+) with 4-(2-pyridylazo)-N,N-dimethyl aniline (PADA) were studied as a function of pH in different buffers in both aqueous and sodium dodecyl sulfate (SDS) micelle solutions. In aqueous Tris buffer solution, the forward and backward rate constants increased with increasing pH, while the complex-formation constant decreased due to a larger increase in the backward rate constant. The activation entropy, DeltaS(#), and activation volume, DeltaV(#), changed with increasing pH from positive to negative values, suggesting an apparent changeover from a dissociative to a more associative mechanism. Complex-formation reactions with 2,2'-bipyridine in Tris buffer showed almost no increase in the forward and backward rate constants on increasing the pH, but the DeltaS(#) and DeltaV(#) values became more negative. N-ethylmorpholine buffer showed no pH effect on the rate constants and activation parameters. Water exchange reactions of aquated Ni(2+) were also studied as a function of pH under the same conditions. The reported rate and activation parameters for water exchange in Tris and N-ethylmorpholine buffers are consistent with those found for the complex-formation reactions of Ni(2+) with PADA. The observed pH and buffer effects for both the complex-formation and water exchange reactions of aquated Ni(2+) can be accounted for in terms of the formation of a Ni(2+)-Tris complex in Tris buffer and general base catalysis by the buffer components. In SDS micelle solution, the complex-formation reaction with PADA was much faster than in aqueous solution, but the increase in rate constant with increasing pH was less significant, while DeltaS(#) and DeltaV(#) became more positive, pointing to a more dissociative mechanism. For SDS micelle solutions there was no effect on the water exchange rate constant or activation volume. Mechanistic interpretations are offered for all observed pH, buffer and medium effects.

Nickel(II) Complexes as Paramagnetic Shift and paraCEST Agents

“…A fit of the data to the Swift–Connick equations gives a rate constant of 4 × 10 4 s –1 for water ligand exchange for a single bound water. This data is in the range of previously reported water exchange rate constants for Ni II aqua ion but about ten‐fold slower than that of Ni II bound to hexadentate polyaminocarboxylate ligands . These rate constants are about 50‐fold slower than that of an analogous complex, [Co( L5 )] 2+ .…”

Quantitative Analysis of the Interactions of Metal Complexes and Amphiphilic Systems: Calorimetric, Spectroscopic and Theoretical Aspects

“…Fast-reaction kinetic experiments (using the stopped-flow technique) [80,81] The particular case of pressure dependence needs peculiar apparatuses, which enable us to make changes over a quite wide range (until 1500 atm) [82]. The value of the activation volume (∆V # ) can be obtained from the slope of a plot of the natural logarithm of the rate constant over pressure; these values are crucial to unveil the fine details of the metal/ligand interaction in the different media and decide between an associative or dissociative metal ion-binding mechanism (for Ni 2+ /PADA system, see, for instance, reference [83]). Note that the same study highlights that some of the most common buffers (e.g., TRIS, tris(hydroxymethyl)aminomethane) may not be inert toward the reaction (TRIS coordinates Ni 2+ ).…”

Reactivity in organised assemblies

“…The rate constants and the activation parameters for the complex-formation reaction of Ni 2+ with 4-(2-pyridylazo)-N,N,-dimethyl aniline (PADA) bidentate ligand were determined in aqueous solutions in the absence and the presence of SDS micelles under a variety of conditions. 8 The reaction was studied as a function of pH, temperature and pressure. The authors worked in the pH range of 6.0 to 9.0 in water and 8.0 to 10.0 in solutions of SDS micelles, in order to avoid protonation of the ligand at low pH (the pK a of PADA is 4.5 in aqueous solution and 6.7 in SDS micelle solution) and the formation of the insoluble nickel(II) hydroxide in strongly alkaline solutions (a pK a of 9.86 was attributed to Ni 2+ ).…”