Ligand values potentiometric titrations

The ligand 6,13-dimethyl-l,4,8,ll-tetra-azacyclotetradecane-6,13-diamine coordinates as a hexadentate ligand to zinc in neutral aqueous solution. Potentiometric titrations were used to determine the stability constant for formation. The pXa values were determined for five of the six possible protonation steps of the hexamine (2.9, 5.5, 6.3, 9.9 and 11.0).697 Studies of the syn and anti isomers of 6,13-dimethyl-1,4,8, ll-tetraazacyclotetradecane-6,13-diamine reveal that they offer different shapes for metal binding, which is reflected in the stability constants for 1 1 zinc ligand ratio complexes. The selectivity of binding to the zinc ion compared to the cadmium(II) ion by both isomers is significant.698... 

Complex stability constants are most often determined by pH-potentiometric titration of the ligand in the presence and absence of the metal ion.100 This method works well when equilibrium is reached rapidly (within a few minutes), which is usually the case for linear ligands. For macrocyclic compounds, such as DOTA and its derivatives, complex formation is very slow, especially for low pH values where the formation is not complete, therefore a batch method is... 

NMR titrations (of anion into ligand at fixed pH) and pH-potentiometric titrations (of pH at fixed anion ligand ratios) provide comparable values of the stability constants for binding of mononegative oxoanions by protonated R3Bm, R3F, and R3P hosts [15,20,21] Table 2. The weak complexation at hexaprotonated levels for tetrahedral monoanionic oxoanions makes it difficult to obtain reliable data for protonation levels below 5. This has however been achieved for nitrate with the cleft binding host R3P as well as for Re O4 with the most basic cryptand R3Bm. 

The kinetics of the hydrolysis of di(2,4-dinitrophenyl) phosphate (DDNPP) were studied in basic solutions buffered with Bis-Tris propane (BTP) in the presence of La3+, Sm3+, Tb3+, and Er3+. Two equivalents of the 2,4-dinitrophenolate ion were liberated for each equivalent of DDNPP and the reaction showed first-order kinetics. Potentiometric titrations showed the formation of dinuclear complexes such as [Ln2(BTP)2(OH) ](6 " i, with values of n varying as a function of pH for all studied metals. Hence the catalytic effect depends on the formation of dinuclear lanthanide ion complexes with several hydroxo ligands.97... 

As for the hydrolysis of the axial aqua ligand, pKa values were determined for the 1-methyluracilate-bridged /TT-02N(9-[Ptra]2-(9H2 by means of potentiometric titration (Xhi,OH2 = 3.2 x 10-4 m, h2,OH2 = 2.0 x 10-7 m) [60]. In addition, it was also suggested that the first ligation in Eqn. 5 selectively occurs at one of the two chemically nonequivalent Pt atoms of HH-[Ptni]2, as a clear isosbestic point was observed when the first ligation was dominating [54][56][59]. 

Tables 3.10 and 3.11, respectively (see Appendix). These tables of data are by no means encyclopedic in nature. In general, it is difficult to ascertain the absolute accuracy of a given set of data and hence several sets of data are given for some ligands. In general, the values obtained by the ion exchange method tend to be less precise and accurate than those obtained by potentiometric titration.

Pre-lab 7.4.b Potentiometric Titrations for Determining Ligand pKa Values and Metal Ligand Stability Constants19... 

A binuclear zinc complex of the 2,6-bis [(2-pyridylmethyl)(2-hydroxyethyl) amino]methyl -4-methylphenol ligand (L33, Fig. 63) catalyzes the hydrolysis of bis(4-nitrophenyl) phosphate in aqueous buffer solution to yield 4-nitrophenol and 4-nitrophenyl phosphate.246 Analysis of the pH-rate profile revealed a sigmoidalshaped curve with a pKa = 7.13. This value is similar to a deprotonation event characterized by potentiometric titration and has been assigned to deprotonation of... 

Most ligand pAa values can be determined via a potentiometric titration, where the pH is monitored after sequential additions of either acid or base. The requirement of millimolar ligand concentration is a major drawback of this method, especially for ligands with poor water solubility. Another common method for determination of protonation constants when a potentiometric titration is not possible is a spectrophotometric titration, where both pH and spectral changes are monitored upon the addition of acid or base. 

Determined by potentiometric titration at 25°, 7 = 0.05 M with Me4NCl. Values are calculated from the cumulative formation constants for the complexes and the ligand pA s. The symbol indicates that the complex was not required to achieve a fit of the experimental data. Uncertainty in log K + 0.2. 

Figure 2.1 Potentiometric titration curves obtained at different stabiiity of ML" complexes (log/f values are given at the respective curves). 0.1 M soiutions of M (20ml) and ligand (volume V) are used. Dilution effects are accounted for.

Ansyin and coworkers have explored the stoichiometric hydrolysis reactivity of zinc complexes of terpyridyl-type ligand having guanidinium or ammonium appendages (Fig. 8.24) using the RNA dimer adenylyl (3 -> 5 )phosphoadenine (ApA) [119]. The hydrolysis of this substrate yields adenosine 3 -monophos-phate, adenosine 2 -monophosphate, adenosine 2, 3 -cAMP, and adenosine. The optimal rate of reaction of Zn(II)-4 with ApA is found at pH 7.5, which is near the pK value of a zinc-coordinated water molecule in this system (pJCa=7.3 determined independently by potentiometric titration). This suggests the involvement of a zinc hydroxide species as the active complex for ApA hydrolysis. 

There are two methods that are considered fundamental in the use of potentiometric titrations to determine the properties of a solution. The first was developed by Bodlander and Fittig (1902). The main feature of this method was to obtain a description of the stoichiometric constant in a reaction between a metal (M) and a ligand (L). However, it is usually desirable to also obtain a value for the stability constant itself since only how the expression looks for the reaction is not sufficient. A method for doing this using potentiometric titrations was introduced by Bjerrum (1941). In this method, Bjerrum usedthe average ligand number defined by... 

Many methods including potentiometry, spectrophotometry, NMR spectroscopy, and reaction kinetics can be used to obtain Kn values in solution. Because ligands are often Arrhenius bases and metal-ligand complexes tend to be soluble in aqueous solution, potentiometric (pH) titration is one of the most widely used procedures. However, for complexes like [Ni(salpd)], where a non-aqueous medium is required, an alternative, spec-trophotometric method is preferred. As you will see when reading through the derivation, this method requires several criteria to be met. One of the most important is that one component, either [Ni(salpd)] or pyridine, must be in excess in our case this is pyridine. Another factor that will simplify the math is that pyridine does not absorb in the region of analysis. 

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