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)

Abstracts

Brometo, sulfato, selenito e selenato foram encontrados formando pontes na cavidade dos complexos mononucleares e binucleares de Obisdien:Zn(II) e Obisdien:Mg(II) em solução aquosa a 25 °C e m = 0,100 M (KCl). Medidas de equilíbrio potenciométrico foram realizadas para determinar as constantes de formação das espécies formadas nestes sistemas na faixa de pH de 2 a 12. Para os complexos do Obisdien:Zn(II): ânions, foram detectadas quatro espécies com o íon brometo, cinco espécies com o íon sulfato, seis espécies com o íon selenito e quatro espécies com o íon selenato. Os complexos binucleares hidróxidos predominam sobre todas as outras espécies e a constante de associação diminui na seqüência: SeO4(2-) > SeO3²> SO4(2-) > Br-. O sistema envolvendo os complexos de Obisdien:Mg(II) apresenta três espécies com o íon brometo, quatro espécies com o íon sulfato, cinco espécies com o íon selenito e cinco com o íon selenato. A variação da constante de associação para este sistema é o mesmo observado no sistema anterior: SeO4(2-) > SeO3² > SO4(2-) > Br-. Cálculos de mecânica molecular mostram que os complexos de Obisdien dizinco(II) e dimagnésio(II) adotam várias conformações de baixa energia com diferentes separações Zn-Zn e Mg-Mg permitindo a coordenação dos ânions SeO4(2-), SeO3(2-), SO4(2-), Br- e OH-.

Article

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)⁺

Marcos Rivail da Silvaª, Michel Lamotte^b,

Olivier F.X. Donard^b,Henrides dos Santos^c, Ricardo J. Nunes^c, and Bruno Szpoganicz^c*

^aDepartamento de Química, Universidade Regional de Blumenau, rua Antônio da

Veiga 140, 89035 Blumenau - SC, Brazil

^bLaboratoire de Photophysique et Photochimie Moléculaire, URA CNRS 348,

Université de Bordeaux I, 351, cours de la Libération, 33405 Talence, France

^cDepartamento de Química, Universidade Federal de Santa Catarina,

88040-900 Florianópolis - SC, Brazil

Received: March 26, 1996

Brometo, sulfato, selenito e selenato foram encontrados formando pontes na cavidade dos complexos mononucleares e binucleares de Obisdien:Zn(II) e Obisdien:Mg(II) em solução aquosa a 25 °C e m = 0,100 M (KCl). Medidas de equilíbrio potenciométrico foram realizadas para determinar as constantes de formação das espécies formadas nestes sistemas na faixa de pH de 2 a 12. Para os complexos do Obisdien:Zn(II): ânions, foram detectadas quatro espécies com o íon brometo, cinco espécies com o íon sulfato, seis espécies com o íon selenito e quatro espécies com o íon selenato. Os complexos binucleares hidróxidos predominam sobre todas as outras espécies e a constante de associação diminui na seqüência: SeO₄^2- > SeO₃²> SO₄^2- > Br^-. O sistema envolvendo os complexos de Obisdien:Mg(II) apresenta três espécies com o íon brometo, quatro espécies com o íon sulfato, cinco espécies com o íon selenito e cinco com o íon selenato. A variação da constante de associação para este sistema é o mesmo observado no sistema anterior: SeO₄^2- > SeO₃² > SO₄^2- > Br^-.

Cálculos de mecânica molecular mostram que os complexos de Obisdien dizinco(II) e dimagnésio(II) adotam várias conformações de baixa energia com diferentes separações Zn-Zn e Mg-Mg permitindo a coordenação dos ânions SeO₄^2-, SeO₃^2-, SO₄^2-, Br^- e OH^-.

Bromide, sulfate, selenite and selenate were found to form bridges in the cavity of mononuclear and binuclear Obisdien:Zn(II) and Obisdien:Mg(II) complexes in aqueous solution at 25.0 °C and m = 0.100 M (KCl). Potentiometric equilibrium measurements were used to determine the formation constants of the species formed in these systems in the pH range 2-12. For the Obisdien: Zn²⁺:anions complexes, we have established the existence of four species for bromide ion, five species for sulfate ion, six species for selenite ion and four species for selenate ion. We found that the hydroxo binuclear complexes predominate over the others species, and the binding strength decreases in the sequence: SeO₄^2- > SeO₃^2- > SO₄^2- > Br^-. The Obisdien:Mg(II) system presents three species in case of binding with bromide ion, four with sulfate ion, five with selenite ion and five with selenate ion. Variation of the binding strength in the Obisdien:Mg(II) complexes is the same as in the Obisdien:Zn(II) complexes: SeO₄^2- > SeO₃^2- > SO₄^2- > Br^-.

Molecular mechanics calculations show that the dizinc(II)- and dimagnesium(II)-Obisdien complexes adopt several low energy conformations differing in their Zn to Zn and Mg to Mg separations that allow the coordination of SeO₄^2-, SeO₃^2-, SO₄^2-, Br^- and OH^- anions.

Keywords: 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 atoms^3,4.Firstly synthesized by Lehn^5,6 and co-workers, the macrocyclic Obisdien has been studied as a ligand that form stables complexes with metal ions and anions^7,8. 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^-, SO₄^2-, AsO₂^-, PO₄^3-, malonic acid, glycine and others^8,10,and more recently involving SeO₃^2-, SeO₄^2-,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 crystallography^13-16. 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 excess^19-21. On the other hand, metal ions as Zn²⁺ and Mg²⁺ are present in the environment in appreciable amount^18,22, and only a few complexes involving selenite and selenate ions and metal ions with some ligands have been synthesized and characterized^23-27.

Experimental

Materials

KCl (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 described^5,6. 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 measurements

It has been shown^29,30 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:SO₄^2-, Obisdien:SeO₃^2-, Obisdien:SeO₄^2- and Obisdien:Zn(II) systems have been reported earlier^9,11. 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 program^33,34. 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 Discussion

Molecular mechanics calculations

Molecular 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 systems

The 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 SO₄^2-, SeO₃^2-, and SeO₄^2- ions are defined by general Eq. 1. The constants represent the association of X^n- with the binuclear receptor complex LM₂⁴⁺.

where X^n- is the anion, LM₂X^4-n represents the unprotonated, unhydrolysed binuclear chelates.

The equilibrium constants for the mononuclear chelates are defined by Eq. 2, where H_mLMX^2+m-n are the unhydrolized mononuclear chelates and H_mLM^2+m are the mononuclear receptor complexes. The association constants of the

anions with the hydroxo binuclear receptor complexes are defined by Eq. 3.

where LM₂(OH)_xX^4-(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)₄^2- for the Obisdien:Zn(II):X systems and MgOH⁺, and Mg₄(OH)₄⁴⁺ for the Obisdien:Mg(II):X systems.

Obisdien-Zinc(II)-Bromide complexes

Potentiometric 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 the Fig. 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 in Table 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 the Fig. 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 on Fig. 4).

The crystal structure of bridging hydroxide ion in the cavity of binuclear macrocyclic and macrobicyclic complexes has been already characterized^{37, 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 complexes

Potentiometric equilibrium curves of Obisdien.6HBr in the presence and in the absence of Zn²⁺ and SO₄^2- under nitrogen in 1:2:1 molar ratio of Obisdien.6HBr:Zn(II): SO₄^2- system were carried out and the results are shown in the Fig. 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):SO₄^2- probable complexes are defined by general Eqs. 1, 2 and 3, and the stability constants are presented in Table 1. The difference in the profile of the Obisdien:Zn(II):SO₄^2- 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 on Fig. 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 the Fig. 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 in Fig. 3. The curve with selenite shows that the SeO₃^2- 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 SeO₃^2- in the Zn²⁺, Br^-, SO₄^2-, SeO₃^2- curve only about 1 mmol of base is sufficient.

The equilibrium constants for all Obisdien:Zn(II): SeO₃^2- probable complexes are defined by Eqs. 1, 2, and 3 and the stability constants are shown in Table 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 of Table 1. Minors species (less than 2% ) are not computed in these system.

The species distribution curves of the Obisdien:Zn(II): SeO₃^2- system are showed in Fig. 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 the Fig. 3. The curve were interrupted at pH 8.0 due to formation of a precipitate. The curve shows that in the presence of SeO₄^2- 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 the Table 1. The species distribution curves of Obisdien.6HBr:Zn(II): SeO₃^2-: SeO₄^2- system are showed in the Fig. 7. Formation of metal complexes occurs at pH values above 4.0. The zinc(II)-Obisdien complexes are coordinated with SeO₃^2- and SeO₄^2- 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 in Fig. 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 on Table 2. The stability constants for the receptor Obisdien:Mg²⁺ complexes were also determined and they are shown in the caption of Table 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 in Fig. 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 SO₄^2- ions under nitrogen in 1:2:1 molar ratio of Obisdien.6HBr:Mg(II):SO₄^2- system were determined and they are shown in the Figure 8. The equilibrium constants for all Obisdien:Mg(II):SO₄^2- complexes detected are defined by Eqs. 1, 2, and 3 and the equilibrium constants are presented in Table 2.

The species distribution curves are presented in the Fig. 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 in Fig. 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):SeO₃^2- complexes are defined by Eqs. 1, 2, and 3, and the stability constants of the species detected are shown in Table 2.

The species distribution curves of the Obisdien.6HBr:Mg(II):SO₄^2-:SeO₃^2- 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 in Fig. 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 in Table 2. Minor species (less than 3%) are not considered in this system.

The species distribution curves of metal complexes of SeO₄^2- in presence of Br^-, SO₄^2- and SeO₃^2- ions are shown in the Fig. 12. All metal complex species are SeO₄^2- 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 case of mononuclear complexes H₃LMgSeO₄³⁺, H₂LMgSeO₄²⁺ and HLMgSeO₄⁺, both the coordination bonds and hydrogen bonds are involved (formula 7 for H₃LMgSeO₄³⁺)

In the binuclear complexes, the selenate group is believed to bridge and coordinate the two Mg(II) ions in the manner suggested by formula 8.

Conclusion

Equilibrium constant determination and species distribution curves in the pH 2 - 12 range for the Obisdien:Zn²⁺ and Obisdien:Mg²⁺ systems in the presence of Br^-, SO₄^2-, SeO₃^2-, and SeO₄^2- ions has been carried on by potentiometric studies. For both systems, the strength of binding increases in the order Br^- < SO₄^2- < SeO₃^2- < SeO₄^2-. Studies of a model macrocycle which has the capacity of forming binuclear metal complexes as Obisdien, provide useful results for understanding the behavior of anions and metals involved in complex formation in solution. In all cases metal complex species appears above H 6.0. While the mononuclear species predominates at neutral and moderately basic pH for Mg(II) systems, for the zinc(II) system, the binuclear species are the major species. At more basic pH values, the binuclear species predominate in both metal systems, however the dihydroxo binuclear species are not formed in the magnesium(II) system while it is the major species in the zinc(II) system. Zinc(II) ion has a strong tendency to form hydroxo complexes and the results is consistent with the behavior of zinc(II) in aqueous solution.

The monohydroxo species is the major species in the magnesium(II) systems. The species formed in the magnesium(II) systems are less stable than the ones formed in the zinc(II) systems. Nevertheless, Mg(II) ion are much more abundant in the oceans than Zn(II) ion. Thus, it is expected to play important roles in the complexation of naturally occurring anions as selenite and selenate anions in the presence of ligands.

Acknowledgments

This work was supported by the Laboratoire de Photophysique et Photochimie Moléculaire of Université de Bordeaux I (France) and CNPq (Brazil). Marcos Rivail da Silva thanks CAPES-COFECUB for financial support.

• 1. Bencini, A.; Bianchi, A.; Burguete, I.; Dapporto, P.; Domenéch, A.; Garcia-Espana, E.; Luis Paoli, S.V.P.; Ramirez, J. A. J. Chem. Soc. Perkin Trans. 1994, 2, 569.
• 2. Sell, C.; Galan, A.; de Mendoza, J. Topics in Current Chemistry 1995, 175, 101.
• 3. Andrés, A.; Bazzicalupi, C.; et al.Inorg. Chem. 1994, 33, 617.
• 4. Martell, A. A.; Motekaitis, R. J. J. Am. Chem. Soc. 1988, 110, 8059.
• 5. Lehn, J. M.; Pine, S.H.; Watanabe, E.; Willard, A.K.J. J. Am. Chem. Soc 1977, 99, 6766.
• 6. Comarmond, J.; Plumere, P.; Lehn, J.M.; Agnus, Y.; Louis, R.; Weiss, Kahn, R.; Morgenstern, O.B.I. J. Am. Chem. Soc 1982, 104, 6330.
• 7. Bencini, A.; Bianchi, A.; Paoletti, P.; Paoli, P. Coord. Chem. Rev 1992, 120, 51.
• 8. Motekaitis, R.J.; Martell, A.E. Inorg. Chem 1992, 31, 5534.
• 9. Motekaitis, R.J.; Martell, A.E.; Lecomte, J.P.; Lehn, J.M. Inorg. Chem 1983, 22, 609.
• 10. Rosso, N.D.; Szpoganicz, B.; Debacher, N.; Martell, A.E. Quimica Nova 1995, 18, 421.
• 11. da Silva, M.R.; Szpoganicz, B.; Lamotte, M.; Donard, O.F.X.; Fages, F. Inorg. Chim. Acta 1995, 236, 189.
• 12. Szpoganicz, B.; Motekaitis, R. J., and Martell, A. E. Inorg. Chem 1990, 29, 1467.
• 13. Coughlin,. P.K.; Devan, J.C.; Lippard; S.J.; Watanabe, E.; Lehn, J.M. J. Am. Chem. Soc. 1979, 101, 265.
• 14. Coughlin, P.K.; Martin, A.E.; Dewan, J.C.; Watanabe, E.; Bulkowski, J.R.; Lehn, J.M.; Lippard, S.J. Inorg. Chem 1984, 23, 1004.
• 15. Warzeska, S.; und Krämer, R. Chem. Ber 1995, 128, 115.
• 16. Pierre, J.L.; Chautemps, P.; Refait, S.; Beguin, C.; El Marzouki, A.; Serratrice, G.; Saint-Aman, E.; Rey, P. J. Ame. Chem. Soc 1995, 117, 1965.
• 17. Ahrland, S. In Inorganic Chemistry of the Ocean, in Environment. Inorganic Chemistry; Irgolic, K.J; Martell, A.E., Eds., VCH publisher: Deerfield Beach Florida, 1985.
• 18. O'Neill, P. In Environmental Chemistry, 2° Edition, Chapman & Hall, 1994, pp. 114-130, 221.
• 19. Sugimura, Y.; Suzuki, Y.; Miyake, Y.J. Oceanog. Soc. of Japan 1976, 32, 235.
• 20. Cutter, G.A.; Bruland, K.W. Limnol. Oceanogr 1984, 29(6), 1179.
• 21. Cutter, G.A. Anal. Chim. Acta 1978, 9856.
• 22. Moore, J.W.; Ramamoorthy, S. In Heavy Metals in natural waters. Applied Monitoring and Impact Assessement Spring-Verlag New York 1983, pp. 187.
• 23. Benelli, C.; Vaira, M.D.; Noccioli, G.; Sacconi, L. Inorg. Chem 1977, 16(1), 182.
• 24. Foong, S.W.; Sykes, G. J. Chem. Soc; Dalton Transaction 1973, 3, 504.
• 25. Fowlees, A.D.; Stranks, D.R. Inorg. Chem 1977, 16(6), 1271.
• 26. Fowlees, A.D.; Stranks, D.R. Inorg. Chem 1977, 16(6), 1282
• 27. Haygarth, P.M.; Rowland, P.M.; Sturup, S.; Jones, K.C. .Analyst 1993, 118, 1303.
• 28. Schwarznbach, G.; Flaschka, H. In Complexometric titration Methuen Co.Ltd.; London, 1969, pp.173, 256, and 264.
• 29. Potvin, P.G. Anal. Chim. Acta; 1994, 29, 943.
• 30. Beck, M.T.; Nagypàl, I. In Chemistry of Complex Equilibria Ellis Horwood Limited, 1990, pp 274-280.
• 31. Martell, A.E.; Smith, R.M; Motekaitis, R.J. In NIST Critical Stability Constants; of Metal Complexes, NIST Database 46, Gaithersburg, MD, USA, 1994.
• 32. Baes, C. Jr.; Charles, F.; Mesmer, R.E. In The hidrolysis of cations John Wiley & sons, New York, 1976, pp.99 and 293.
• 33. Martell, A.E.; Motekaitis, R.J. In Determination and use of stability constants; VCH publishers New York, 1992.
• 34. Motekaitis, R.J.; Martell, A.E. Can. J. Chem 1982, 60, 2403.
• 35. Jurek, P.E.; Martell, A.E., Motekaitis, R.J., Hancock, R. Inorg. Chem 1995, 34, 1823.
• 36. Bourdon, S.; Decian, A.; Fischer, J.; Hosseine, M.W.; Lehn, J.M.; Wipff, G.J. J. Coord. Chem 1991, 23, 113.
• 37. Martell, A.E. Mat. Chem. Phys 1993, 35, 273.
• 38. Motekaitis, R.J.; Rudolf, P.R.; Martell, A.E.; Hancock, R. Inorg. Chem. 1989, 28, 112.

*

"

">+Abstracted in part from a dissertation submitted by Marcos Rivail da Silva to the faculty of the Universidade Federal de Santa Catarina in partial fulfillment of the requirements for the degree of Doctor of Chemistry.

Publication Dates

• Publication in this collection
  10 Sept 2010
• Date of issue
  1997

History

• Received
  26 Mar 1996