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

Hindawi Publishing Corporation Bioinorganic Chemistry and Applications Volume 2009, Article ID 219818, 17 pages doi:10.1155/2009/219818 Research Article 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 Agatino Casale, Concetta De Stefano, Giuseppe Manfredi, Demetrio Milea, and Silvio Sammartano Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Università di Messina, Salita Sperone, 31, 98166 Messina, Italy Correspondence should be addressed to Silvio Sammartano, Received 20 January 2009; Revised 9 March 2009; Accepted 6 April 2009 Recommended by Claudio Pettinari 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 NaClaq at t = 25◦ C and at different ionic strengths (0.1 ≤ I/mol L−1 ≤ 1.0), 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 I = 0.1 mol L−1 , for the ML species we have log K = 10.8, 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 pL50 (the total ligand concentration, as−log CL , able to bind 50% of metal cation), able to give an objective representation of this ability. Equations were formulated to model the dependence of pL50 on different variables, such as ionic strength and pH, and other empirical predictive relationships were also found. Copyright © 2009 Agatino Casale et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. Introduction 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 2 stability of diethyltin(IV) complexes with O- and N-donor ligands [16], 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.) [19]. 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 [15]). 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 [24]. Since natural waters and biological fluids cover a very wide range of ionic strengths (from I ∼ 0.01 mol L−1 for spring waters to I > 6 mol L−1 for hyper-saline waters), stability constants of various dimethyltin(IV) species were determined in NaClaq at t = 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 pL50 (the total ligand concentration, as − log CL , 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 pL50 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. 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 res (...truncated)