Equilibrium Studies of Ternary Complexes Formed by Bromide , Sulfate , Selenite and Selenate Ions with Zn 2 + , Mg 2 + and 1 , 4 , 7 , 13 , 16 , 19-Hexaaza-10 , 22-Dioxacyclotetracosane ( Obisdien ) +

ed 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. J. Braz. Chem. Soc. , Vol. 8, No. 5, 459-469, 1997. © 1997 Soc. Bras. Química Printed in Brazil. 0103 – 5053 $6.00 + 0.00


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 studied 1 .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 cations 2 .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 determinations 9 .
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 complexes 12 .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 Cu 2+ , Co 2+ and Zn 2+ ions have been characterized also by X-ray crystallography [13][14][15][16] .Although the equilibrium constants for some anions are known for Cu(II) and Co(II)-Obisdien complexes 4 , 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 amount 17 in the environment and a previous work 10 has shown that this anion forms complexes with Obisdien:Cu 2+ .The sulfate ion is also present in the environment in great amount and its property are similar to that of selenate ions 18 .Selenite and selenate ions are present in the environment generally in trace amount and can be essential or toxic when in excess [19][20][21] .On the other hand, metal ions as Zn 2+ and Mg 2+ 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][24][25][26][27] .

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 indicator 28 .

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 (µ = 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 (µ) 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.
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 one 8 .
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 p[H] vs. base added 33 .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 p[H] 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 elsewhere 33 .
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.

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 bowlshaped 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 conformers 35 .In all cases such conformers allow the coordination of bridging ligands of differing size as selenate, selenite, sulfate, bromide and hydroxide ions.
The equilibrium constants for the mononuclear chelates are defined by Eq. 2, where HmLMX 2+m-n are the unhydrolized mononuclear chelates and HmLM 2+m are the mononuclear receptor complexes.The association constants of the HmLM 2+m + X n- HmLMX 2+m , anions with the hydroxo binuclear receptor complexes are defined by Eq. 3.

Obisdien-Zinc(II)-Bromide complexes
Potentiometric equilibrium curves of Obisdien.6HBr in the presence and in the absence of Zn 2+ 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 work 36 .In this system, we observed the formation of a precipitate at about p[H] 8.0.This precipitate is probably the zinc dihydroxo species Zn(OH)2 which can be to formed in this p[H] range.The precipitate remains until p[H] 11.5.The p[H] 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 p[H] 3.0, and until p[H] 4.0 where a = 2 it has the same profile as that for Obisdien.6HBrwithout Zn(II) ions.This indicates no formation of complex in this p[H] range until the first two protons of the ligand have been neutralized.Above p[H] 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 species 34 .
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 LZn2OHBr 2+ 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 Zn 2+ 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 p[H] 8.0.The curve shows that until about p[H] 5.0 the profile is about the same as that of the Obisdien.6HBrcurve.This is a indication that non complexes of Zn(II) is formed in this p[H] range, but above p[H] 4.0 the curve is below that of the Obisdien.6HBrsystem, indicating that complexation reaction takes place.
[LZnX]  taining Obisdien 1.00 x 10 -3 M and Zn 2+ ion 2.0 x 10 -3 M, in presence of 6.00 x 10 -3 M of bromide.H2LZnBr 3+ , H3LZnBr 4+ , and LZn2Br 3+ represent the di-and triprotonated mononuclear, and binuclear deprotonated Obisdien:Zn 2+ :Br -species respectively.LZn2OHBr 2+ is the binuclear hydroxo Obisdien:Zn 2+ :Br -species.HLZn 3+ and LZn 2+ are the monoprotonated and complete deprotonated Obisdien: Zn(II) species and LZnOH + is the hydroxo one.LZn2 4+ is the binuclear receptor complex.All no metallic species and the hydrolysis product of Zn(II) ion are not represented.Zn 2+ is the free (aquo) zinc(II) ion.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. Zn 2+ ion does not complex at p[H] values below 5, and the Obisdien:SO4 species predominate in this region.The major species formed in this system is the (dihydroxo) (µ-sulfate) binuclear zinc(II)-Obisdien species, 2, which predominates at p[H] 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 p[H] 6.9 where it is 24.3% formed, while the mononuclear diprotonated H2LZnSO4 2+ species reaches a maximum at p[H] 6.3 where it is only 12.7% formed.The binuclear completely deprotonated species LZn2SO4 2+ was found in the neutral p[H] values.It reaches a maximum at p[H] 7.0 where it is 31.1% formed.

Obisdien-Zinc(II)-Selenite complexes
Potentiometric equilibrium curves of Obisdien.6HBr in the presence of Zn 2+ 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.6HBrcurve, the presence of SeO3 2-in the Zn 2+ , 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 p[H] 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.

Obisdien-Zinc(II)-Selenate complexes
Potentiometric equilibrium curves of Obisdien.6HBr in the presence and in the absence of Zn 2+ , 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 p[H] 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 p[H] values above 4.0.The zinc(II)-Obisdien complexes are coordinated with SeO3 2-and SeO4 2-ions.The mononuclear triprotonated complex (H3LZnSeO4 3+ ) reaches a maximum at p[H] 5.8 where it is 27.6% formed and the diprotonated complex (H2LZnSeO4 2+ ) reaches a maximum at p[H] 6.3 where it is 42.0%formed.The major species formed at neutral p[H] is the binuclear species LZn2SeO4 2+ and it is 42% formed.The binuclear dihydroxo species LZn2(OH)2SeO4 predominates at p[H] 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 Mg 2+ 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 p[H] 6.5 the profile of the potentiometric titration curve is identical to the profile recorded for the Obisdien.6HBrsystem.This indicates that no complex formation with Mg(II) ion in this p[H] range takes place until the first two protons of the taining Obisdien 1.00 x 10 -3 M, Zn 2+ ion 2.00 x 10 -3 M, SeO3 2-ion 1.00 x 10 -3 M and SeO4 2-ion 1.00 x 10 -3 M in presence of bromide 6.00 x 10 -3 M. LZn2(OH)2SeO4 and LZn2SeO4 2+ are the dihydroxo and complete deprotonated binuclear Obisdien:Zn 2+ :SeO4 2-species and H2LZnSeO4 2+ and H3LZnSeO4 3+ are the mononuclear ones.LZn2(OH)2SeO3 is the dihydroxo binuclear Obisdien:Zn 2+ :SeO3 2-species and the LZn2SeO3 2+ is the complete deprotonated binuclear species.LZnHSeO3 + and HLZnHSeO3 2+ , are the mononuclear species.Zn 2+ is the free aquo zinc(II) ion and non metallic species are not represented as well as species formed less than 2% for clarity.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:Mg 2+ 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 LMg2 4+ which is 9.4% formed at p[H] 10.3, the hydroxo species LMg2OH 3+ which is 12.4% formed at p[H] 11.8 and the mononucleares LMg 2+ , HLMg 3+ , H2 LMg 4+ and H3 LMg 5+ which are 12.3%, 14.5%, 6.2% and 7% formed at p[H] 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.

Obisdien-Magnesium(II)-Sulfate complexes
Potentiometric equilibrium curves of Obisdien.6HBr in the presence and in the absence of Mg 2+ , 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 p[H] values above 8.5 and the mononuclear species are formed in the p[H] range 7.3-10.0.There are no metal complexes at p[H] values lower than 7.3.The Mg 2+ 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 p[H] 9.3 where it is 8.4% formed, and the diprotonated mononuclear H2LMgSO4 2+ is 9.0% formed at p[H] 8.6.The binuclear species are formed at p[H] values above 8.5.They are formed in larger than the mononuclear ones.The deprotonated binuclear species LMg2SO4 2+ reaches a maximum at p[H] 10.3 where it is 10.6% formed and the monohydroxo binuclear species LMg2OHSO4 + is 23.1% formed at p[H] 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 Mg 2+ , 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: LMg2SeO3 2+ and LMg2OHSeO3 + The binuclear species LMg2SeO3 2+ reaches a maximum at p[H] 9.9 where it is 32.6% formed and the hydroxo species LMg2OHSeO3 + is 67.8% formed at p[H] 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 p[H] 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 p[H] 7.0 is lower, indicating complex formation with selenate in this p[H] 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 p[H] 12.0 and the species LMg2SeO4 2+ reaches a maximum at p[H] 9.5 where it is 50.5% formed.
The mononuclear species predominate at lower p[H] values and the triprotonated species is the major species at neutral and moderately basic region.It is 55% at p[H] 7.6 and the di-and the monoprotonated species are maximum formed at p[H] 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 H3LMgSeO4 3+ , H2LMgSeO4 2+ and HLMgSeO4 + , both the coordination bonds and hydrogen bonds are involved (formula 7 for H3LMgSeO4 3+ ) 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 p[H] 2 -12 range for the Obisdien:Zn 2+ and Obisdien:Mg 2+ systems in the presence of Br -, SO4 2-, SeO3 2-, and SeO4 2-ions has been carried on by potentiomet-ric 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 p[H] for Mg(II) systems, for the zinc(II) system, the binuclear species are the major species.At more basic p[H] 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.

Figure 4 .
Figure 4. Species distribution curves for a supersaturated solution con-

Figure 7 .
Figure 7. Species distribution curves for a supersaturated solution con-

Se 4 ,
Selenate adduct of dihydroxo binuclear Zn(II)-Obisdien complex neutralized.Above this p[H], the difference of the profiles for the two curves is small indicating the formation of weak complexes.
The p[H] range was from 2.8 to 11.2.The p[H] reproducibility are < 0.002 p[H] in buffer regions and absolute p[H] accuracy are < 0.002 p[H] at low p[H] and < 0.015 p[H] at high p[H].The brackets in p[H] is used to emphasize the deter-mination of -