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## Journal of the Chilean Chemical Society

##
*versión On-line* ISSN 0717-9707

### J. Chil. Chem. Soc. v.50 n.3 Concepción sep. 2005

#### http://dx.doi.org/10.4067/S0717-97072005000300008

J. Chil. Chem. Soc., 50, N° 3 (2005), págs: 569-574
Departamento de Química Inorgánica y Analítica, Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Casilla 233, Santiago , Chile
The second stepwise formation constants of binary and ternary copper(II) complexes with a-amino acidate ligands are modeled by using four-descriptor sets consisting of three indexes defined as linear combinations of the E-states of some skeletal groups of the different species involved in the formation equilibria, and the logarithm of the statistical factor. Hydrophobicity and basicity properties of the ligands are also described. Results indicate that both hydrophobic interactions and ligand basicities, as differential factors over the sequence of copper(II) complexes, would operate mainly through changes in the stability of the copper(II)-carboxylate bonds.
Thermodynamic stability of coordination compounds in aqueous solution has been an important topic to almost all areas of chemistry for a long time. The stability of a metal complex is appropriately characterized by the equilibrium constant for its formation process, i.e. by the stability constant (also referred to as formation constant). The knowledge of the stability constants of complexes with ligands of biochemical interest has been relevant for the experimental modeling of both reactions of metal ions with biomolecules In the present study an improvement of the modeling procedure for the logarithms of the K
For the sake of depicting more realistic chemical graphs, the stepwise formation equilibria of binary and ternary copper(II) complexes with a-amino acidates were expressed as: [CuA(H where the water molecules axially coordinated to the metal center have been omitted. Logarithms of the stepwise formation constants K S where I I where N is the principal quantum number, d d where Z DI where r I(>M<) = 0.5874(1/r r = 0.999 ; s = 0.010 ; F = 1309 ; n = 5. where the original slope has been multiplied by the metal ion charge (2+). The intrinsic state value for copper(II) skeletal group was then calculated through this regression equation by using the spherical potential ion radius of copper(II): The E-states of the species involved in the formation equilibrium were combined to define new descriptors, D(i) , by means of the expression D(i) = SS where the E-state indexes are handled as thermodynamic state functions. In this expression the species [CuA] corresponds indeed to [CuA(H Different sets of four descriptors, consisting of the D(i) indexes of some selected skeletal groups and the logarithm of the statistical factor, were used to describe the logarithms of the stepwise formation constants K sf = (r+s)!/r!s! where r and s are the stoichiometric subscripts in the general formula [CuA The modeling of some physicochemical properties of the amino acidate ligands by means of D(i) and SS Multiple regression analyses were carried out by using the software Statgraphics Plus for Window 4.0 on a Pentium III computer.
The calculated E-state values of some skeletal groups, which take part in the formation of chelate rings, are listed in Tables 1-3. The D(i) descriptors calculated from such E-state values are shown in Table 4. Moreover, the four-descriptor combinations tested in multivariable regression analyses against logK
The analysis of variance indicates that there is a statistically significant relationship between the variables at the 99% confidence level (p-value < 0.0001). The r-squared statistic shows that this model accounts for 94.45% of the variability in logK In Table 4 the logK
According to the statistical data listed in Table 5, the role of log(sf) as a descriptor would consist in allowing the feasibility of modeling logK r = 0.9888, s = 0.0258, F = 29.2, Durbin-Watson statistic = 2.206, n = 6, for binary complexes,
Even though the D(i) indexes only import differences in E-state values on passing from the monochelated to the dichelated metal complex, they also encode, to a certain extent, some physicochemical properties of the ligands which are relevant as determining factors for the stability of the metal complexes. Accordingly, the set {D(-O-), D(>C=), D(-NH
where SDG
In these expressions the superscript Seeing that SDG
In Figure 1 the SDG
Obviously, in the above relationship the index S(>Cu<) encodes electronic and topological information upon the amino acidate ligands through its perturbation term, DI(>Cu<).
On the other hand, the set {SS(-O-), SS(>C=)} gives also a rather good description of SDG
Equations 5-7 appear to be in agreement with equation 1. According to the latter equation, the greater is SS(-O-), the lessnegative should be D(-O-) and the greater logK
Here, SDG On the other hand, the E-states of the [CuAB] complexes, SS
According to equations 9 and 10, the ligand basicities should increase as SS(-O-) decreases and SS(>C=) becomes less negative. Seeing that these indexes correspond indeed to the [CuAB] complexes, a decrease in SS(-O-) together with an increase in SS(>C=) would be related with a decrease in polarity of the coordinated oxygen-carbon bonds. In turn, this would be consistent with an increase in the covalent character of the coordinating oxygen-copper(II) bonds and, hence, with an increase in the ligand basicities. Anyways, equations 9 and 10 suggest that the contributions of the ligand basicities, as determining factors for the differences in logK From the above discussion it can be concluded that the D(i) indices appear to be more appropriate than the E-states SS
As can be noticed, the sets of statistical parameters for the above quoted regression equations are closely similar to each other. Thus, the E-state indices appear to be more appropriate for the modeling of physicochemical processes of the type a ® c, or a + (b) ®c, where (b) is a system or chemical species common to all of the considered processes. The above remarks are supported by the successful modeling provided by the E-states for some physical properties and physicochemical processes, namely, boiling points,
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