Research Article | Open Access
Agatino Casale, Concetta De Stefano, Giuseppe Manfredi, Demetrio Milea, Silvio Sammartano, "Sequestration of Alkyltin(IV) Compounds in Aqueous Solution: Formation, Stability, and Empirical Relationships for the Binding of Dimethyltin(IV) Cation by N- and O-Donor Ligands", Bioinorganic Chemistry and Applications, vol. 2009, Article ID 219818, 17 pages, 2009. https://doi.org/10.1155/2009/219818
Sequestration of Alkyltin(IV) Compounds in Aqueous Solution: Formation, Stability, and Empirical Relationships for the Binding of Dimethyltin(IV) Cation by N- and O-Donor Ligands
The sequestering ability of polyamines and aminoacids of biological and environmental relevance (namely, ethylenediamine, putrescine, spermine, a polyallylamine, a branched polyethyleneimine, aspartate, glycinate, lysinate) toward dimethyltin(IV) cation was evaluated. The stability of various dimethyltin(IV) / ligand species was determined in at and at different ionic strengths , and the dependence of stability constants on this parameter was modeled by an Extended Debye-Hückel equation and by Specific ion Interaction Theory (SIT) approach. At mol , for the ML species we have log , 14.2, 12.0, 14.7, 11.9, 7.7, 13.7, and 8.0 for ethylenediamine, putrescine, polyallylamine, spermine, polyethyleneimine, glycinate, lysinate, and aspartate, respectively. The sequestering ability toward dimethyltin(IV) cation was defined by calculating the parameter (the total ligand concentration, as−log , able to bind 50% of metal cation), able to give an objective representation of this ability. Equations were formulated to model the dependence of on different variables, such as ionic strength and pH, and other empirical predictive relationships were also found.
The knowledge of the behavior of organotin(IV) cations in the environment is of great concern for many scientists in several different research fields. The importance of these compounds, from different points of view, was already extensively discussed (e.g., [1–13]). Their environmental and biological activity is mainly related to their chemicophysical behavior in aqueous solution. In fact, their aqueous chemistry is dominated by the formation of various hydrolytic species, even if they also tend to interact with several organic and inorganic ligands, forming a wide number of complex species of different stability. This is particularly relevant in the study of organotin(IV) speciation in natural and waste waters and biological fluids, where other metals and various organic (carboxylic and aminic in particular) and inorganic ligands could be simultaneously present in different concentrations (see, e.g., in [8, 14–18]). In fact, it is well known that organotin(IV) compounds show different biological and environmental activity depending on their speciation: the formation of different species plays an important role in organotin(IV) toxicity and exposure to living organisms and influences their availability, their accumulation, biomodification, and their transport inside the organisms and within and between various environmental compartments [8, 9, 11, 15, 18, 19].
Owing to the objective impossibility of defining the speciation and the sequestration of organotin(IV) compounds in all the different systems where they could be present, since some years we undertook a study on their interactions with various ligand classes, in order to derive general information and empirical relationships to be used for the prediction of both the chemicophysical behavior and the sequestering ability of these ligands toward organotin(IV) cations (e.g., [16, 18–21]. For example, in some of our previous papers we derived some empirical relationships for the modeling of the stability of diethyltin(IV) complexes with O- and N-donor ligands , whilst in others we modeled that of mono-, di-, and trialkyltin(IV) complexes with various carboxylic ligands as a function of simple ligand and metal structural parameters (e.g., the charge of the alkyltin(IV) cation, the number and nature of binding sites, etc.) .
In the present paper, we extended this study to the evaluation of the sequestering ability of polyamines and aminoacids of biological and environmental relevance toward dimethyltin(IV) cation. We opted for the dimethyltin(IV) cation since it is one of the main representatives of diorganotin(IV) compounds. The actual, renewed interest in the chemistry of diorganotin(IV) derivatives is due to the fact that, despite they are less toxic than triorganotin(IV) cations, more recent researches (e.g., [3, 39]) suggest them to possess anticarcinogenic activity, in contrast with the suspected carcinogenicity of other organotin(IV) compounds (triderivatives first) [7, 11].
At the same time, the choice of N-donor ligands (aminoacids and polyamines) was supported by the fact that, despite their importance and their massive presence in natural waters and biological fluids, reported thermodynamic data (stability constants, formation enthalpies, and entropies …) on their interactions with alkyltin(IV) cations are limited (e.g., [7, 16, 22–32]) with respect to contributions on alkyltin interactions with other ligands such as, carboxylates (carefully analyzed, e.g., in ). Furthermore, an accurate analysis of some of those papers evidences that alkyltin(IV) cations preferably bind to ligands via nitrogen groups rather than via oxygen. For example, in the case of lysine and ornithine, which may coordinate as bidentate ligands either by (N, N) or (N, O) donor sets, there is evidence that they bind to dimethyltin(IV) by the former (N, N) donor set .
Since natural waters and biological fluids cover a very wide range of ionic strengths (from for spring waters to for hyper-saline waters), stability constants of various dimethyltin(IV) species were determined in at 25°C and at different ionic strengths, and their dependence on this parameter was modeled by an Extended Debye-Hückel equation and by Specific ion Interaction Theory (SIT) approach [33–35]. Finally, several values of (the total ligand concentration, as , able to bind 50% of metal cation), an empirical parameter used to give an objective representation of the sequestering ability of a ligand [36–38], were calculated for the sequestration of various ligands toward dimethyltin(IV) cation. Equations were formulated to model the dependence of on different variables (e.g., ionic strength and pH), and other empirical predictive relationships were also found between the stability of complexes and the kind and number of functional groups of the ligand(s) involved in the formation equilibria.
2. Experimental Section
Dimethyltin(IV)  dichloride (Alfa-Aesar) was used without further purification, and its purity was checked potentiometrically by alkalimetric titrations, resulting always -diaminoethane (ethylenediamine, en), 1,4-diaminobutane (putrescine, ptr), -bis(3-aminopropyl)-1,4-butanediamine (spermine, sper), polyallylamine ( kDa, paam), and branched polyethyleneimine ( kDa, pei) were used in their hydrochloride forms (di-, di-, tetra-, poly-, and poly- for en, ptr, sper, paam, and pei, resp.). Aspartate () and glycinate () were used as L-aspartic acid and glycine, respectively; lysinate () was used as L-lysine hydrochloride. All ligands were of analytical grade and were purchased from Sigma-Aldrich (and its various brands). They were used without further purification, and their purity was checked potentiometrically by alkalimetric titrations, resulting always . Hydrochloric acid and sodium hydroxide solutions were prepared by diluting concentrated ampoules (Riedel-deHaën) and were standardized against sodium carbonate and potassium hydrogen phthalate, respectively. solutions were preserved from atmospheric by means of soda lime traps. aqueous solutions were prepared by weighing pure salt (Fluka) dried in an oven at 110°C. All solutions were prepared with analytical grade water (Ω) using grade A glassware.
2.2. Apparatus and Procedure
Potentiometric measurements were carried out (at °C) using an apparatus consisting of a Model 713 Metrohm potentiometer, equipped with a combination glass electrode (Ross type 8102, from Thermo/Orion), or a half cell glass electrode (Ross type 8101, from Thermo/Orion) and a double junction reference electrode (type 900200, from Thermo/Orion), and a Model 765 Metrohm motorized burette. Estimated precision was and for e.m.f. and titrant volume readings, respectively. The apparatus was connected to a PC, and automatic titrations were performed using a suitable computer program to control titrant delivery and data acquisition and to check for e.m.f. stability. Some measurements were also carried out using a Metrohm model 809 Titrando apparatus controlled by Metrohm TiAMO 1.0 software for the automatic data acquisition. Potentiometric titrations were carried out in thermostatted cells under magnetic stirring and bubbling purified presaturated N2 through the solution in order to exclude and inside. The titrand solution consisted of different amounts of dimethyltin(IV) dichloride (), ligand (), a slight excess of hydrochloric acid (), and the background salt in order to obtain pre–established ionic strength values ( and 0.5 for gly and lys). The most of measurements were performed considering an metal to ligand ratio, except for some where . Potentiometric measurements were carried out by titrating 25 mL of the titrand solution with standard solutions up to . However, since the formation of sparingly soluble species was never observed in the experimental conditions adopted, some titrations were performed up to . For each experiment, independent titrations of strong acid solution with standard base were carried out under the same medium and ionic strength conditions as the systems to be investigated, with the aim of determining electrode potential () and the acidic junction potential (). In this way, the pH scale used was the total scale, , where  is the free proton concentration (not activity). The reliability of the calibration in the alkaline range was checked by calculating values. For each titration, 80–100 data points were collected, and the equilibrium state during titrations was checked by adopting some usual precautions. These included checking the time required to reach equilibrium and performing back titrations.
The nonlinear least squares computer program ESAB2M  was used for the refinement of all the parameters of the acid-base titration (, , liquid junction potential coefficient, , analytical concentration of reagents). The BSTAC  and STACO  computer programs were used in the calculation of complex formation constants. Both programs can deal with measurements at different ionic strengths. The ES4ECI  program was used to draw speciation and sequestration diagrams and to calculate species formation percentages. The LIANA  program was used to fit different equations.
Protonation, hydrolysis, and complex formation constants are given according to the equilibria (M = dmt and L = fully deprotonated ligand):
Dependence on ionic strength of stability constants of various species, expressed in the molar () concentration scale, was taken into account by a Debye-Hückel type equation:
where is an empirical parameter, and DH is the Debye-Hückel term that, at 25°C, with A = 0.51 and åB = 1.5, is given by
The dependence on medium and on ionic strength of equilibrium thermodynamic parameters has been also taken into account by the Specificion Interaction Theory (SIT) model [33–35]. By using appropriate density values , molar to molal scale conversions of and were performed. When these are expressed in the molal concentration scale, (5) becomes the classical SIT equation [33–35], where is replaced by :
The parameter is the SIT interaction coefficient of the th species (involved in the equilibrium represented by the formation constant ) with the th component (of opposite charge). parameters as well as single interaction coefficients were determined too.
3. Results and Discussion
3.1. Dimethyltin(IV) Hydrolysis and Ligand Protonation
Prior to any study of the binding ability of different ligands toward dimethyltin(IV) cation, an accurate knowledge of the acid-base behavior of both the ligands and dmt is necessary. Protonation constants of polyamines and aminoacids, as well as dimethyltin(IV) hydrolysis constants, were already determined in several experimental conditions, together with the parameters for the modeling of their dependence on medium, ionic strength, and temperature [45–57]. As an example, in Table 1 some of these values are reported, in at and 25°C. In the analysis of this table, it is important to make a brief comment on the protonation constants of paam and pei. Previous studies  demonstrated that, in addition to the classical models used to describe the acid-base behavior of polyelectrolytes (e.g., Högfeldt ), these two polyamines can be considered as a low molecular weight diamine (paam) and a tetraamine (pei). In this way, all calculations and experiments are designed and performed by taking into account the simple dimeric and tetrameric units, respectively. This new model not only maintains the same degree of accuracy of the “classical” ones but also has the evident advantage of facilitating calculations (allowing, e.g., the use of the same computer programs). Furthermore, comparisons between these two polyamines and the used low molecular weight ligands are more immediate, from the point of view of both their acid-base behavior and their binding ability toward dimethyltin(IV) or any other compound.
3.2. Formation and Stability of Dimethyltin(IV)/Amine Species
Calculations performed on potentiometric data of dmt/amine systems gave evidence of the formation of the ML and MLH species for all considered amines. In all investigated systems, further species were formed, with different values of and , depending on the ligand. Values of stability constants determined are reported in Table 2 for all species in each system, at different ionic strengths. This table shows that the species is only formed by the two low molecular weight diamines (i.e., en and ptr), whilst the polyallylamine (another diamine according to the model) forms the . On the contrary, as expected, spermine and polyethyleneimine (the two tetraamines) form two further protonated species, namely, and . Among the investigated diamines, putrescine complexes are much stronger than the corresponding ones of ethylenediamine, whilst paam shows an intermediate behavior. Analogously, species formed by spermine are more stable than those by polyethyleneimine. Globally, the stability of the simple ML species formed by dmt with all investigated amines follows the trend
whilst a slight different order is observed for the other common species, that is, MLH:
From the analysis of Table 2, another interesting aspect is worthy of mention. Among the two investigated low molecular weight diamines (i.e., en and ptr), the stability of ML species is evidently higher for ptr than for en. At first sight, this behavior appears puzzling, considering that ethylenediamine may form with dimethyltin(IV) cation a “five membered” chelate ring, which should be more stable than the analogue “seven membered” ring formed by putrescine. This fact may be interpreted considering that with quite “large” cations, such as organotin cations, ligands with longer alkyl chains (e.g., ptr instead of en) usually form stronger ML species than shorter ligands. With these very large cations, chelation by small ligands is disadvantaged for steric factors, so that these ligands tend to act as monodentate, with a very small contribution of the second N donor group. We also had the same evidence for the interactions of en and ptr with dioxouranium(VI) cation (unpublished work from this laboratory). For similar reasons, the analogies in the stability of the (dmt)(ptr) and (dmt)(sper) species should be an indication that not all the four spermine amino groups are involved in the coordination to dimethyltin(IV). However, further spectroscopic studies were planned to verify these hypothesis and will be the subject of another contribution.
3.3. Formation and Stability of Dimethyltin(IV)/Aminoacid Species
In order to give a more detailed picture of the binding ability of O- and N-donor ligands toward dimethyltin(IV), the speciation of this cation in the presence of three different aminoacids (i.e., glycine, lysine, and aspartic acid) was also investigated. As can be easily noted, in addition to the simplest aminoacid (i.e., glycine), one containing an extra amino group (i.e., lysine) and one with another carboxylic group (i.e., aspartic acid) were selected. Experimental data analysis revealed that all the three ligands form with dimethyltin(IV) cation three main species, namely, , , and the hydroxo-species . In addition to these species, only lysine forms another protonated species, the . Corresponding stability constants are reported in Table 3, at the investigated ionic strengths. As can be observed from the analysis of this table, for all the common species, the order of their stability is
3.4. Influence of Ligand Complexes on Dimethyltin(IV) Speciation
The importance of dimethyltin(IV) complexes with the investigated O- and N-donor ligands on its speciation can be appreciated looking at Figures 1 and 2 where, for example, the percentages of species formed by this cation with two amines (ptr and pei) and two aminoacids (gly and lys) are reported in at and 25°C. As can be noted from these Figures, dmt / ligand species are formed in the whole investigated pH range, with percentages ranging from to . In particular, the highest values are observed for polyethyleneimine species, whilst the lowest value regard complexes formed by glycinate and aspartate. This is a first indication that dimethyltin(IV) cation forms stronger species with N-donor groups than with O-donor. In fact, among the three investigated aminoacids, formation percentages of lysinate species (contain an extra amino-group) are three-four times those reached by glycinate (and aspartate). Worth mentioning is also that, increasing pH, the percentage of dimethyltin complexed by polyamines first increases (more or less sharply, depending on the ligand) and, after a maximum, it decreases. This is due to the fact that, at low pH, investigated polyamines are partially or totally protonated, and their binding ability is significantly reduced. Nevertheless, in the basic pH range, the formation of hydrolytic species (mainly the neutral dmt) is so strong that it inhibits complexation. This trend is less marked for aminoacids, where the carboxylic group is already deprotonated at low pHs.
3.5. Dependence on Ionic Strength of Dimethyltin(IV) Species
Stability constants of dimethyltin(IV) complexes reported in Tables 2 and 3 proved fairly dependent on ionic strength, as shown in Figure 3 where, for example, values for en and asp are plotted as a function of , in (stability constants referred to reactions with , such as of (dmt)(asp), are usually plotted as ). The lines in the same figure represent the dependence on ionic strength expressed by (5), where is taken as reference ionic strength. Refined parameters of this equation are reported in Tables 4 and 5, for species formed by amines and aminoacids, respectively. Of course, parameters related to the dependence on ionic strength of glycinate and lysinate species, based on two ionic strengths only, have no mathematical meaning. Nevertheless, if simultaneously analyzed with those of other systems, these parameters can evenly give a general picture of the dependence on ionic strength of these complexes. In the same tables, corresponding refined parameter is reported for the fitting of stability constants converted in the molal scale. Since differences in the refined at and resulted lower than the error associated to this parameter, only a common value was reported in Tables 4 and 5, valid for both molar and molal datasets. Formation constants and ionic strength values reported in Tables 2 and 3 were converted into the molal concentration scale (data shown in Tables 6 and 7, for dmt/amine and dmt/aminoacid species, resp.) with the aim of modeling the dependence of stability constants of dimethyltin(IV)-ligand species on ionic strength also by the SIT equations, in order to determine SIT interaction coefficients for these species. From the simultaneous analysis of all datasets by LIANA program, classical SIT interaction coefficients of species involved in protonation, hydrolysis, and complex formation equilibria were equally derived and are shown in Table 8 (except for those regarding gly and lys). Water activity and interaction coefficients among proton and chloride ions were taken from literature . Calculations of interaction coefficients reported in Table 8 were only possible fixing some values (otherwise the system is mathematically undetermined): preliminary analysis evidenced that coefficients related to the fully deprotonated, neutral, polyamines were close to “0”, and, for this reason, in successive calculations these values were considered as fixed, and this choice is coherent with the original SIT theory, where only interactions between ions of opposite sign are taken into account. On the other hand, it is possible to use “nonzero” coefficients for the interactions of neutral species with the ionic medium, as suggested by several authors (see, e.g.,  and references therein). Hence, the SIT theory has the potential to describe the activity coefficient and related properties of neutral species . This is the case, for example, of and ML species of aspartate, reported in Table 8.
|log values at I = 0.1 (in both molar or molal concentration scales), taken as reference ionic strength; standard deviation.|
|log values at I = 0.1 (in both molar or molal concentration scales), taken as reference ionic strength; standard deviation; parameters for and species without errors, due to fits based on two experimental points.|