Capillary electrophoresis facilitates determination of metal complex stoichiometry by Job’s method of continuous variation

Abstract

Environmental context Knowledge of metal-chelating agent speciation is integral to our ability to predict and interpret the behaviour of synthetic chelating agents in the environment. Capillary electrophoresis can be used to separate metal–ligand complexes with similar spectroscopic characteristics but different stoichiometries, thereby providing insight into metal–ligand speciation that is not possible by any other technique. Here, we demonstrate the utility of capillary electrophoresis for the determination of metal–ligand stoichiometries and evaluate its limitations. Abstract Job’s method of continuous variation is a traditional method used to determine the stoichiometry of metal–ligand complexes. The method is often applied to whole-sample absorbance measurements but its utility is limited when two or more complexes are present at significant concentrations and have similar absorption spectra. Here we employ capillary electrophoresis (CE), which separates complexes on the basis of charge and hydrodynamic radii, to extend the capabilities of Job’s method. Solutions containing nickel(II) and diethylenetriaminepentaacetic acid (DTPA) yield three CE peaks. Job’s method plot maxima, based on areas for each of the three CE peaks, coincide with nickel(II)-to-DTPA ratios of 1 : 1 and 1 : 2, which correspond to two complexes previously identified using whole-sample measurements, plus a ratio of 3 : 2, which corresponds to a previously unreported complex. We demonstrate how CE peak areas and electromigration times can be used to determine complex stoichiometries and formation constants. We discuss the strengths and weaknesses of Job’s Method coupled with CE and implications for speciation determination in environmentally relevant systems.

Insights into microwave‐promoted synthesis of 3‐methyl‐4‐phenyl‐4,9‐dihydro‐1H‐pyrazolo[3,4‐d][1,2,4]triazolo[1,5‐a]pyrimidine derivatives catalyzed using new Pd (II),Cu (II),VO (II), and Ag(I) complexes as a heterogeneous catalyst and computational studies

“…Applying Jobs' approach of (molar ratio and continuous variation), it was used to determine the stoichiometric of the prepared complexes. 1,34 Curves of the continuousvariation method (Figure S7) showed the maximal absorbance at a mole fraction of X-ligand = 0.52, in agreement to 1 M:1 L molar ratio. The formation constants (K f ) were estimated in range of 10 4 :10 5 from the curves of continuous variations and the order of values as DOBPAPd > DOBPAAg > DOBPACu > DOBPAVO complex as shown Table 3.…”

“…This indicates the great stability of studied complexes over a broad range for various applications. 34,35 3.8 | Computational study…”

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“…According to the selected maximum wavelength and the preferred molar ratio in this research, measurements were made for the prepared complex after making some necessary adjustments in the absorbance values according to the preferred and standardized concentration of 0.001 M in 1:2:1 ratio meaning that 1 metal: 2 imidazole: and 1 of 1,10-phenanthroline. This was confirmed by Job's method [37], which showed the best absorbance given by the complex at wavelength 504 nm when the number of moles of imidazole is twice the number of moles of each of 1,10-phenanthroline and the metal, as displayed in (Figure 3). To find out the extent of the stability of the complex over time, the complex was measured at different times and proved its stability for more than three days (Figure 4).…”

Ligand Steric Interactions Modulate Multidentate Ligand Exchange Pathways: Kinetics of Nickel(II) Ion Capture from N-Substituted IDA Complexes by CDTA

“…Therefore, we employ a capillary electrophoresis method to separate and detect free ligands and their nickel complexes. 12,52 We quantified initial rates of Ni(cdta) 2− formation as a function of reactant concentrations and reaction pH and fit observed initial rates with a kinetic model. The proposed kinetic model illuminates the effects of steric interactions between L and Y on reaction pathways and kinetic behavior.…”

“…The structural similarity between these ligands and the presence of excess free ligand and multiple nickel-ligand complexes in reaction solutions impedes quantification by bulk UV–vis spectroscopy. Therefore, we employ a capillary electrophoresis method to separate and detect free ligands and their nickel complexes. , We quantified initial rates of Ni­(cdta) 2– formation as a function of reactant concentrations and reaction pH and fit observed initial rates with a kinetic model. The proposed kinetic model illuminates the effects of steric interactions between L and Y on reaction pathways and kinetic behavior.…”