Equilibrium studies of ternary complexes formed by bromide, sulfate, selenite and selenate ions with Zn2+, Mg2+ and 1, 4, 7, 13, 16, 19-hexaaza- 10, 22- dioxacyclotetracosane (obisdien)

Journal of the Brazilian Chemical Society

Lamotte, Michel¹  Nunes, Ricardo J.¹  Silva, Marcos Rivail da¹  Szpoganicz, Bruno¹  Universidade Regional de Blumenau, Blumenau, Brazil¹  Université de Bordeaux I, France¹  Universidade Federal de Santa Catarina, Florianópolis, Brazil¹  Donard, Olivier F.X.¹  Santos, Henrides dos¹ 

关键词: selenite;    selenate;    sulfate;    Obisdien complexes     Introduction The complexation of organic and inorganic anions which results from the association of two or more species through noncovalent bonds yields what has been termed supramolecular chemistry. Although great amount of researches involve the development of ligands for metal ions;    the inorganic and organic anions that play important roles in the environment and biological systems are scarcely studied1. Several reasons may be invoked to explain the unequal treatment between anions and cations complexes chemistry. While the cations have usually a spherical shape;    the anions exist under various geometries. The anions are more strongly solvated than cations of comparable size and the anions are bulkier than cations2. Great interest has been devoted to macrocyclic receptors containing in their framework both oxygen and nitrogen donor atoms3;    4.Firstly synthesized by Lehn5;    6 and co-workers;    the macrocyclic Obisdien has been studied as a ligand that form stables complexes with metal ions and anions7;    8. Their capacity to form stable binuclear complexes with metal ions is remarkable. Several equilibrium constants of metal complexes were determined and due to the fact that this ligand can be hexaprotonated;    the potentiometric titration has been the technique of choice for equilibrium constants determinations9.Studies involving the equilibrium determination with anions as Br-;    SO42-;    AsO2-;    PO43-;    malonic acid;    glycine and others8;    10;    and more recently involving SeO32-;    SeO42-;    11 indicates that this ligand is very important for the selective molecular recognition of these anions. Organic anions also bridges binuclear metal complexes12. In these systems;    the mono- and binuclear complexes of metal ions can set themselves as host;    and the bridging anions are the guests.The imidazolate-;    oxalate- and acetate-bridged binuclear metal ion-Obisdien complexes with Cu2+;    Co2+ and Zn2+ ions have been characterized also by X-ray crystallography13-16. Although the equilibrium constants for some anions are known for Cu(II) and Co(II)-Obisdien complexes4;    these metal ions are not so abundant as Mg(II) and Zn(II) in the environment. The purpose of the present work is to determine the association constants of Mg(II) and Zn(II)-Obisdien with bromide;    sulfate;    selenite and selenate ions. The bromide ion is present in great amount17 in the environment and a previous work10 has shown that this anion forms complexes with Obisdien:Cu2+. The sulfate ion is also present in the environment in great amount and its property are similar to that of selenate ions18. Selenite and selenate ions are present in the environment generally in trace amount and can be essential or toxic when in excess19-21. On the other hand;    metal ions as Zn2+ and Mg2+ are present in the environment in appreciable amount18;    22;    and only a few complexes involving selenite and selenate ions and metal ions with some ligands have been synthesized and characterized23-27.  ExperimentalMaterialsKCl (MERCK) used as a supporting electrolyte in all potentiometric measurements was used without further purification. The macrocyclic Obisdien used was synthesized by a modification of the method previously described5;    6. Zinc (II) nitrate hexahydrate (Zn(NO3)2.6H2O);    magnesium(II) nitrate hexahydrate (Mg(NO3)2.6H2O);    HCl;    sodium sulfate;    sodium selenite and sodium selenate;    were reagents grade materials. Carbonate free solutions of about 0.100M KOH were prepared from Dilut-it ampoules (J.T. BAKER) and were standardized by titration with potassium acid phthalate. The stocks solutions of zinc(II) and magnesium(II) were standardized by titration with EDTA using murexide as the indicator28.Potentiometric equilibrium measurementsIt has been shown29;    30 that in calculating unknown equilibrium constants by the least-squares technique from pH (or potentiometric) titration data;    errors in the chemical model (initial volume;    reactant concentrations;    carbonate or other impurities;    pKw and other known equilibrium constants) and measurement errors (electrode calibration;    drifts in ionic strength;    non-ideal titrant mixing or temperature control) can strongly influence the results. To minimize possible errors;    at the least three titrations were performed with the ligand alone;    and at least one for each anion added. All the potentiometric studies of Obisdien.6HBr in the presence of zinc(II);    magnesium(II) with bromide;    sulfate;    selenite;    and selenate ions were carried out with a Digimed model DMPH3 research pH-meter fitted with blue-glass and Ag/AgCl reference electrodes calibrated with standard HCl 10-2 M (m = 0.100M (KCl)) and CO2 - free KOH solutions to read -Log H+ directly. All systems were studied under anaerobic conditions;    by using a stream of purified nitrogen. The temperature was maintained at 25.00 ± 0.03 °C and the ionic strength (m) was adjusted to 0.100 M by the addition of KCl. Samples of about 0.10 mmol of Obisdien.6HBr (ca. 5% excess) in presence and absence of 0.10 mmol of the anions;    (sulfate;    selenite or selenate) accurately weighted and about 0.20 mmoles of Zn(II) or Mg(II) were diluted with 50 mL of doubly distilled water (KMnO4) in a sealed thermostated weasel in the conditions described above. All the solutions were titrated with 0.100 M standard CO2 - free KOH. The pH range was from 2.8 to 11.2. The pH reproducibility are < 0.002 pH in buffer regions and absolute pH accuracy are < 0.002 pH at low pH and < 0.015 pH at high pH. The brackets in pH is used to emphasize the determination of -Log10H+. Under these conditions;    it appeared that equilibrium was attained within about few minutes (stable pH readings were obtained).Computations The equilibrium constants for Obisdien:Br-;    Obisdien:SO42-;    Obisdien:SeO32-;    Obisdien:SeO42- and Obisdien:Zn(II) systems have been reported earlier9;    11. The proton association constants of sulfate;    selenite and selenate ions were taken from the literature31 and the hydrolysis constants of Zn2+ and Mg2+ ions were reported by Baes and Mesmer32. The dissociation constant of water (pKw) at 25;    00 °C and m = 0.100 M used was 13.78. All these constants were kept fixed during refinement.In the present work some of the constants taken from the literature were determined in background electrolytes different from the one we used. But this is of minor importance as the literature shows that equilibrium constants obtained in different background electrolytes with Obisdien as ligand;    are in good agreement with each one8. The species considered are those which are the most likely to be formed according to the present knowledge in coordination chemistry in solutions and care has been taken to avoid highly improbable unrealistic species. All the equilibrium constants determined in this work were calculated with the aid of the interactive computer program BEST and the species distribution were calculated with the aid of the SPECIES program that used as input the output of BEST program33;    34. The input of BEST program consists of millimoles of each component;    the initial estimates of the equilibrium constants of each species thought to be formed from the solution components and the experimentally determined profiles of pH vs. base added33. At each increment of base added the program sets mass balance equations for all species present and solves for the concentration of hydrogen ions which is compared to the experimental value. The sequence of the calculation begins with the set of known and unknown (estimated) overall stability constants and compute H+ at the equilibrium for each quantity of added base. For each equilibrium points the fitting process consists in the minimization of the differences between the observed and the calculated pH values by using a weighted least squared method. The iterative process is repeated until no further minimization can be obtained for these differences. Further details about the method of calculation are described elsewhere33. Molecular mechanic calculations were performed using the program PC Model running on a IBM compatible PC computer. The molecular mechanics force field presents in the program was used in all molecular mechanics calculations.  Results and DiscussionMolecular mechanics calculationsMolecular mechanic calculations showed that dizinc(II) - and dimagnesium (II) - Obisdien complexes are able to exist under several low energy conformers;    two of them being the most important: the extended and the bowl-shaped conformers (Fig. 1 and 2). The distance between Zn atoms In the extended conformer is 7.32 Å and the Mg - Mg distance is 7.35 Å. The Zn - Zn separation in the bowl shaped conformer is 4.85 Å while the Mg - Mg distance is 4.95 Å for the same conformer. Previous molecular mechanic calculations on dicopper (II)-Obisdien complex have revealed the existence of similar types of conformers35. In all cases such conformers allow the coordination of bridging ligands of differing size as selenate;    selenite;    sulfate;    bromide and hydroxide ions. ;   

DOI  :  10.1590/S0103-50531997000500006

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

The complexation of organic and inorganic anions which results from the association of two or more species through noncovalent bonds yields what has been termed supramolecular chemistry. Although great amount of researches involve the development of ligands for metal ions, the inorganic and organic anions that play important roles in the environment and biological systems are scarcely studied1. Several reasons may be invoked to explain the unequal treatment between anions and cations complexes chemistry. While the cations have usually a spherical shape, the anions exist under various geometries. The anions are more strongly solvated than cations of comparable size and the anions are bulkier than cations2. Great interest has been devoted to macrocyclic receptors containing in their framework both oxygen and nitrogen donor atoms3,4.Firstly synthesized by Lehn5,6 and co-workers, the macrocyclic Obisdien has been studied as a ligand that form stables complexes with metal ions and anions7,8. Their capacity to form stable binuclear complexes with metal ions is remarkable. Several equilibrium constants of metal complexes were determined and due to the fact that this ligand can be hexaprotonated, the potentiometric titration has been the technique of choice for equilibrium constants determinations9.Studies involving the equilibrium determination with anions as Br-, SO42-, AsO2-, PO43-, malonic acid, glycine and others8,10,and more recently involving SeO32-, SeO42-,11 indicates that this ligand is very important for the selective molecular recognition of these anions. Organic anions also bridges binuclear metal complexes12. In these systems, the mono- and binuclear complexes of metal ions can set themselves as host, and the bridging anions are the guests.The imidazolate-, oxalate- and acetate-bridged binuclear metal ion-Obisdien complexes with Cu2+, Co2+ and Zn2+ ions have been characterized also by X-ray crystallography13-16. Although the equilibrium constants for some anions are known for Cu(II) and Co(II)-Obisdien complexes4, these metal ions are not so abundant as Mg(II) and Zn(II) in the environment. The purpose of the present work is to determine the association constants of Mg(II) and Zn(II)-Obisdien with bromide, sulfate, selenite and selenate ions. The bromide ion is present in great amount17 in the environment and a previous work10 has shown that this anion forms complexes with Obisdien:Cu2+. The sulfate ion is also present in the environment in great amount and its property are similar to that of selenate ions18. Selenite and selenate ions are present in the environment generally in trace amount and can be essential or toxic when in excess19-21. On the other hand, metal ions as Zn2+ and Mg2+ are present in the environment in appreciable amount18,22, and only a few complexes involving selenite and selenate ions and metal ions with some ligands have been synthesized and characterized23-27.  ExperimentalMaterialsKCl (MERCK) used as a supporting electrolyte in all potentiometric measurements was used without further purification. The macrocyclic Obisdien used was synthesized by a modification of the method previously described5,6. Zinc (II) nitrate hexahydrate (Zn(NO3)2.6H2O), magnesium(II) nitrate hexahydrate (Mg(NO3)2.6H2O), HCl, sodium sulfate, sodium selenite and sodium selenate, were reagents grade materials. Carbonate free solutions of about 0.100M KOH were prepared from Dilut-it ampoules (J.T. BAKER) and were standardized by titration with potassium acid phthalate. The stocks solutions of zinc(II) and magnesium(II) were standardized by titration with EDTA using murexide as the indicator28.Potentiometric equilibrium measurementsIt has been shown29,30 that in calculating unknown equilibrium constants by the least-squares technique from pH (or potentiometric) titration data, errors in the chemical model (initial volume, reactant concentrations, carbonate or other impurities, pKw and other known equilibrium constants) and measurement errors (electrode calibration, drifts in ionic strength, non-ideal titrant mixing or temperature control) can strongly influence the results. To minimize possible errors, at the least three titrations were performed with the ligand alone, and at least one for each anion added. All the potentiometric studies of Obisdien.6HBr in the presence of zinc(II), magnesium(II) with bromide, sulfate, selenite, and selenate ions were carried out with a Digimed model DMPH3 research pH-meter fitted with blue-glass and Ag/AgCl reference electrodes calibrated with standard HCl 10-2 M (m = 0.100M (KCl)) and CO2 - free KOH solutions to read -Log H+ directly. All systems were studied under anaerobic conditions, by using a stream of purified nitrogen. The temperature was maintained at 25.00 ± 0.03 °C and the ionic strength (m) was adjusted to 0.100 M by the addition of KCl. Samples of about 0.10 mmol of Obisdien.6HBr (ca. 5% excess) in presence and absence of 0.10 mmol of the anions, (sulfate, selenite or selenate) accurately weighted and about 0.20 mmoles of Zn(II) or Mg(II) were diluted with 50 mL of doubly distilled water (KMnO4) in a sealed thermostated weasel in the conditions described above. All the solutions were titrated with 0.100 M standard CO2 - free KOH. The pH range was from 2.8 to 11.2. The pH reproducibility are < 0.002 pH in buffer regions and absolute pH accuracy are < 0.002 pH at low pH and < 0.015 pH at high pH. The brackets in pH is used to emphasize the determination of -Log10H+. Under these conditions, it appeared that equilibrium was attained within about few minutes (stable pH readings were obtained).Computations The equilibrium constants for Obisdien:Br-, Obisdien:SO42-, Obisdien:SeO32-, Obisdien:SeO42- and Obisdien:Zn(II) systems have been reported earlier9,11. The proton association constants of sulfate, selenite and selenate ions were taken from the literature31 and the hydrolysis constants of Zn2+ and Mg2+ ions were reported by Baes and Mesmer32. The dissociation constant of water (pKw) at 25,00 °C and m = 0.100 M used was 13.78. All these constants were kept fixed during refinement.In the present work some of the constants taken from the literature were determined in background electrolytes different from the one we used. But this is of minor importance as the literature shows that equilibrium constants obtained in different background electrolytes with Obisdien as ligand, are in good agreement with each one8. The species considered are those which are the most likely to be formed according to the present knowledge in coordination chemistry in solutions and care has been taken to avoid highly improbable unrealistic species. All the equilibrium constants determined in this work were calculated with the aid of the interactive computer program BEST and the species distribution were calculated with the aid of the SPECIES program that used as input the output of BEST program33,34. The input of BEST program consists of millimoles of each component, the initial estimates of the equilibrium constants of each species thought to be formed from the solution components and the experimentally determined profiles of pH vs. base added33. At each increment of base added the program sets mass balance equations for all species present and solves for the concentration of hydrogen ions which is compared to the experimental value. The sequence of the calculation begins with the set of known and unknown (estimated) overall stability constants and compute H+ at the equilibrium for each quantity of added base. For each equilibrium points the fitting process consists in the minimization of the differences between the observed and the calculated pH values by using a weighted least squared method. The iterative process is repeated until no further minimization can be obtained for these differences. Further details about the method of calculation are described elsewhere33. Molecular mechanic calculations were performed using the program PC Model running on a IBM compatible PC computer. The molecular mechanics force field presents in the program was used in all molecular mechanics calculations.  Results and DiscussionMolecular mechanics calculationsMolecular mechanic calculations showed that dizinc(II) - and dimagnesium (II) - Obisdien complexes are able to exist under several low energy conformers, two of them being the most important: the extended and the bowl-shaped conformers (Fig. 1 and 2). The distance between Zn atoms In the extended conformer is 7.32 Å and the Mg - Mg distance is 7.35 Å. The Zn - Zn separation in the bowl shaped conformer is 4.85 Å while the Mg - Mg distance is 4.95 Å for the same conformer. Previous molecular mechanic calculations on dicopper (II)-Obisdien complex have revealed the existence of similar types of conformers35. In all cases such conformers allow the coordination of bridging ligands of differing size as selenate, selenite, sulfate, bromide and hydroxide ions.    The Obisdien:Metal:Anions complex systemsThe stability constants of normal (deprotonated) complex having 1:2 molar ratios of Obisdien:Zn2+ and Obisdien:Mg2+ to ligands in the 1:2:6 of Br-, and 1:2:1 of SO42-, SeO32-, and SeO42- ions are defined by general Eq. 1. The constants represent the association of Xn- with the binuclear receptor complex LM24+.where Xn- is the anion, LM2X4-n represents the unprotonated, unhydrolysed binuclear chelates. The equilibrium constants for the mononuclear chelates are defined by Eq. 2, where HmLMX2+m-n are the unhydrolized mononuclear chelates and HmLM2+m are the mononuclear receptor complexes. The association constants of theanions with the hydroxo binuclear receptor complexes are defined by Eq. 3.where LM2(OH)xX4-(x+n) represents the binuclear hydroxo chelates and LM2(OH)x 4-x represents the binuclear hydroxo receptor complexes. The hydrolysis of metal ions were considered in the calculations, and are expressed by Eq. 4. The appropriate hydrolysis constants were obtained from the literature32 and we considered the following hydrolytic species: Zn(OH)+, Zn(OH)2, Zn(OH)3-, and Zn(OH)42- for the Obisdien:Zn(II):X systems and MgOH+, and Mg4(OH)44+ for the Obisdien:Mg(II):X systems. Obisdien-Zinc(II)-Bromide complexesPotentiometric equilibrium curves of Obisdien.6HBr in the presence and in the absence of Zn2+ ions under nitrogen in 1:2:6 molar ratio of Obisdien:Zn(II):Br- system were determined and they are shown in theFig. 3. The bromide species is present because the Obisdien sample was crystallized with six molecules of HBr. Due to the great amount of chloride in solution (supporting electrolyte), we checked for the possible competition of this ion with bromide and no complexes were detected. Similarly, minor species involving bromo or chloroselenites or selenates complexes were not considered in this work36. In this system, we observed the formation of a precipitate at about pH 8.0. This precipitate is probably the zinc dihydroxo species Zn(OH)2 which can be to formed in this pH range. The precipitate remains until pH 11.5. The pH data mentioned in this range were not considered in the calculations.   The equilibrium for formation of normal and protonated complexes are indicated by Eqs. 1, 2, and 3. The stability constants determined are presented inTable 1. The curve starts at about pH 3.0, and until pH 4.0 where a = 2 it has the same profile as that for Obisdien.6HBr without Zn(II) ions. This indicates no formation of complex in this pH range until the first two protons of the ligand have been neutralized. Above pH 4.0 the curve of Obisdien:Zn(II):Br- is lower than the Obisdien:Br- curve indicating a displacement of protons. The data values were analyzed considering the formation of both mono- and binuclear species. The formation constants for Obisdien:Zn(II):Br-. species were estimated and refined by the BEST program while leaving fixed the equilibrium constants for the others species34.   The species distribution curves as shown in theFig. 4, were calculated with the aid of the computer program SPECIES and plotted with SPEPLOT program34.The mononuclear diprotonated species (H2LZnBr3+) reaches a maximum at pH 6.4 where it is 38.6% formed, and the mononuclear triprotonated species (H3LZnBr4+) reaches a maximum at pH 6.0 where it is 20.0% formed. The binuclear species (LZn2Br3+) reaches a maximum at pH 7.3 where it is 18.0% formed The species LZn24+ competes with this species. The major species is the binuclear monohydroxo complex (LZn2OHBr2+) that reaches a maximum at pH 9.0 where it is 74.3% formed. This is the major species for Obisdien:Zn(II):Br-system. It predominates in the pH range 7.8 to 10.0. Above pH 10.0 the monohydroxo mononuclear species LZnOH+ and hydrolyzed products takes place (not shown onFig. 4).   The crystal structure of bridging hydroxide ion in the cavity of binuclear macrocyclic and macrobicyclic complexes has been already characterized37, 38 and a suggested structure for LZn2OHBr2+ is schematized in 1, where two Zn(II) ions are coordinated in the two pockets of Obisdien while the bromide and hydroxide ions are bridging the two metal centers. Obisdien-Zinc(II)-Sulfate complexesPotentiometric equilibrium curves of Obisdien.6HBr in the presence and in the absence of Zn2+ and SO42- under nitrogen in 1:2:1 molar ratio of Obisdien.6HBr:Zn(II): SO42- system were carried out and the results are shown in theFig. 3. It was also observed a precipitate at about pH 8.0. The curve shows that until about pH 5.0 the profile is about the same as that of the Obisdien.6HBr curve. This is a indication that non complexes of Zn(II) is formed in this pH range, but above pH 4.0 the curve is below that of the Obisdien.6HBr system, indicating that complexation reaction takes place. The equilibrium constants for all Obisdien:Zn(II):SO42- probable complexes are defined by general Eqs. 1, 2 and 3, and the stability constants are presented inTable 1. The difference in the profile of the Obisdien:Zn(II):SO42- and Obisdien.6HBr:Zn(II) systems in the pH range 5.0 to 6.0 (a equal 2 to 4) is due to the fact that the mononuclear diprotonated species (H2LZnSO42+) compete with the H4LSO42+ species (not shown onFig. 5) and the formation of the last one is more probable, i.e., the equilibrium favor the non metallic species which retains two more protons than the bromide species. The species distribution curves are presented in theFig. 5.   Zn2+ ion does not complex at pH values below 5, and the Obisdien:SO4 species predominate in this region. The major species formed in this system is the (dihydroxo) (m-sulfate) binuclear zinc(II)-Obisdien species, 2, which predominates at pH values above 7.3. The sulfate group is believed to bridge and coordinate the two metal ions in the way suggested by formula 2. The mononuclear monoprotonated species, HLZnSO4+, reaches a maximum at pH 6.9 where it is 24.3% formed, while the mononuclear diprotonated H2LZnSO42+ species reaches a maximum at pH 6.3 where it is only 12.7% formed. The binuclear completely deprotonated species LZn2SO42+ was found in the neutral pH values. It reaches a maximum at pH 7.0 where it is 31.1% formed.Obisdien-Zinc(II)-Selenite complexes Potentiometric equilibrium curves of Obisdien.6HBr in the presence of Zn2+ and selenite ions under nitrogen are illustrated inFig. 3. The curve with selenite shows that the SeO32- ion retain one proton for its protonation by forming HSeO3- ion. While 2 mmoles of KOH are necessary for the observed sharp inflection at a = 2 in the Obisdien.6HBr curve, the presence of SeO32- in the Zn2+, Br-, SO42-, SeO32- curve only about 1 mmol of base is sufficient.The equilibrium constants for all Obisdien:Zn(II): SeO32- probable complexes are defined by Eqs. 1, 2, and 3 and the stability constants are shown inTable 1. At pH values below 8, selenite ion is monoprotonated (Log K = 8), thus the formation constants of zinc(II)-Obisdien complexes with this anion defined by Eqs. 1, and 2 represents the association of zinc(II)-Obisdien complexes with HSeO3- ion. Most of the equilibrium constants necessary for the calculations were taken from literature. For some species, because of the lack of data of the equilibrium constants in the absence of anions, only the overall stability constants of their complexes with anions were determined. These overall stability constants are given in the caption ofTable 1. Minors species (less than 2% ) are not computed in these system.The species distribution curves of the Obisdien:Zn(II): SeO32- system are showed inFig. 6. Zinc(II) ion begins to complex above pH 4.0 forming protonated species of mononuclear zinc(II)-Obisdien complexes. The protonated mononuclear species, H2LZnHSeO33+, HLZnHSeO32+ and LZnHSeO3+ are at their maximum at pH values 6.1, 6.5 and 7.2 where they are 7.2, 44.3 and 27.6% formed respectively. The binuclear species LZn2SeO32+ reaches a maximum at pH 7.5 where it is 43.2% formed, and the dihydroxo species LZn(OH)2SeO3 is the major species above pH 8.0, reaching a maximum of 97.5% at pH 9.7. The selenite group is believed to bridge and coordinate the two zinc(II) ions as suggested by formula 3, in the same way as it was suggested for the sulfate ion in formula 2.    Obisdien-Zinc(II)-Selenate complexes Potentiometric equilibrium curves of Obisdien.6HBr in the presence and in the absence of Zn2+, selenite and selenate ions under nitrogen were determined by the method described in the experimental section and the result is shown in theFig. 3. The curve were interrupted at pH 8.0 due to formation of a precipitate. The curve shows that in the presence of SeO42- ion, the buffer regions extend to almost a = 6 indicating possible formation of hydroxo complexes and binuclear deprotonated complexes.The equilibrium constants for all probable species are defined by Eqs. 1, 2, and 3 and the stability constants are shown in theTable 1. The species distribution curves of Obisdien.6HBr:Zn(II): SeO32-: SeO42- system are showed in theFig. 7. Formation of metal complexes occurs at pH values above 4.0. The zinc(II)-Obisdien complexes are coordinated with SeO32- and SeO42- ions. The mononuclear triprotonated complex (H3LZnSeO43+) reaches a maximum at pH 5.8 where it is 27.6% formed and the diprotonated complex (H2LZnSeO42+) reaches a maximum at pH 6.3 where it is 42.0% formed. The major species formed at neutral pH is the binuclear species LZn2SeO42+ and it is 42% formed. The binuclear dihydroxo species LZn2(OH)2SeO4 predominates at pH values above 7.3. The suggested structure of (dihydroxo)(-selenate) binuclear zinc(II)-Obisdien complex (4) is showed below.  Obisdien-Magnesium(II)-Bromide complexes Potentiometric equilibrium curves of Obisdien.6HBr in the presence and in the absence of Mg2+ ions under nitrogen in 1:2:6 molar ratio of Obisdien:Mg(II):Br- system were determined. They are illustrated inFig. 8.    This figure shows that until about pH 6.5 the profile of the potentiometric titration curve is identical to the profile recorded for the Obisdien.6HBr system. This indicates that no complex formation with Mg(II) ion in this pH range takes place until the first two protons of the ligand have been neutralized. Above this pH, the difference of the profiles for the two curves is small indicating the formation of weak complexes.The equilibrium constants for the Obisdien:Mg(II):Br- complexes defined by Eqs. 1, 2, and 3 were determined and they are reported onTable 2. The stability constants for the receptor Obisdien:Mg2+ complexes were also determined and they are shown in the caption ofTable 2.    The species distribution curves are shown in .Fig 9. Most of the metal species are formed in small amount. The Obisdien:Mg(II) species formed are: the binuclear LMg24+ which is 9.4% formed at pH 10.3, the hydroxo species LMg2OH3+ which is 12.4% formed at pH 11.8 and the mononucleares LMg2+, HLMg3+, H2 LMg4+ and H3 LMg5+ which are 12.3%, 14.5%, 6.2% and 7% formed at pH values 10.5, 9.3, 8.6 and 7.9 respectively. Very careful work were done to evaluate the constants of species which are less than 5% formed and their constants has been determined with the largest errors. Species which are less than 3% formed were not mentioned inFig. 9.   The Obisdien:Mg(II):Br- species are formed even in less amount. The hydroxo binuclear LMg2OHBr2+ is 9.4% formed at pH 11.8, the binuclear LMg2Br3+ is only 3% formed at pH 10.2 and the triprotonated mononuclear H3LMgBr4+ is 3.1% formed at pH 7.9. Obisdien-Magnesium(II)-Sulfate complexes Potentiometric equilibrium curves of Obisdien.6HBr in the presence and in the absence of Mg2+, and SO42- ions under nitrogen in 1:2:1 molar ratio of Obisdien.6HBr:Mg(II):SO42- system were determined and they are shown in theFigure 8. The equilibrium constants for all Obisdien:Mg(II):SO42- complexes detected are defined by Eqs. 1, 2, and 3 and the equilibrium constants are presented inTable 2.The species distribution curves are presented in theFig. 10. The binuclear species are formed at pH values above 8.5 and the mononuclear species are formed in the pH range 7.3-10.0. There are no metal complexes at pH values lower than 7.3. The Mg2+ complexes are formed only in basic region, as it was also observed in the Obisdien:Mg(II):Br- system. The monoprotonated mononuclear species HLMgSO4+ reaches a maximum at pH 9.3 where it is 8.4% formed, and the diprotonated mononuclear H2LMgSO42+ is 9.0% formed at pH 8.6. The binuclear species are formed at pH values above 8.5. They are formed in larger than the mononuclear ones. The deprotonated binuclear species LMg2SO42+ reaches a maximum at pH 10.3 where it is 10.6% formed and the monohydroxo binuclear species LMg2OHSO4+ is 23.1% formed at pH 11.8. The structure suggested for the LMg2OHSO4+ complex (5) is shown below.    Obisdien-Magnesium(II)-Selenite complexes Potentiometric equilibrium curves of Obisdien.6HBr in the presence of Mg2+, sulfate and selenite ions under nitrogen are illustrated inFig. 8. The profile of this system, shown that one proton is retained for formation of HSeO3- ion. The equilibrium constants for all Obisdien:Mg(II):SeO32- complexes are defined by Eqs. 1, 2, and 3, and the stability constants of the species detected are shown inTable 2.The species distribution curves of the Obisdien.6HBr:Mg(II):SO42-:SeO32- system are shown in Fig. 11. The major species in this system are the binuclear species: LMg2SeO32+ and LMg2OHSeO3+ The binuclear species LMg2SeO32+ reaches a maximum at pH 9.9 where it is 32.6% formed and the hydroxo species LMg2OHSeO3+ is 67.8% formed at pH 12.0. As in formula 3 it is suggested that the selenite group bridges and coordinate the two metal ions as in 6. The mononuclear species appears at the pH range 7-10 and they are less than 20% formed (Fig. 9).    Obisdien-Magnesium(II) -Selenate complexes Potentiometric equilibrium curve of Obisdien.6HBr in the presence of Mg(II), sulfate, selenite, and selenate ions under nitrogen is shown inFig. 8. The shape of de curve is about the same as the one in presence of selenite without selenate (previous system), but its buffer region above pH 7.0 is lower, indicating complex formation with selenate in this pH range is stronger. The equilibrium constants for all detected species are defined by Eqs. 1, 2, and 3 and the stability constants determined are shown inTable 2. Minor species (less than 3%) are not considered in this system.The species distribution curves of metal complexes of SeO42- in presence of Br-, SO42- and SeO32- ions are shown in theFig. 12. All metal complex species are SeO42- adducts. One binuclear and three mononucleares species were detected. The hydroxo binuclear species LMg2OHSeO4+ is 91% formed at pH 12.0 and the species LMg2SeO42+ reaches a maximum at pH 9.5 where it is 50.5% formed. The mononuclear species predominate at lower pH values and the triprotonated species is the major species at neutral and moderately basic region. It is 55% at pH 7.6 and the di- and the monoprotonated species are maximum formed at pH values 8.4 and 9.0 where they are 44% and 37% formed respectively. The selenato adducts of these Obisdien complexes are bound by coordinate bonds, and in the

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