Potentiometric titration of metal ions in methanol - NRC Research Press

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Potentiometric titration of metal ions in methanol Graham Gibson, Alexei A. Neverov, and R.S. Brown

Abstract: The potentiometric titrations of nine lanthanideIII triflates, ZnII and CuII triflate, and the NiII, CoII, MgII, and TiIV perchlorates were obtained in methanol to determine the titration constants (defined as the ss pH at which the [OCH−3 ] /[Mx+]t ratios are 0.5 and 1.5) as well as the apparent ss pK a values for deprotonation of the metal-bound solvent molecules. The titrations were performed under various conditions with and without added salts as electrolytes, and the variations in the titration constants are discussed. In selected cases (La3+, Zn2+) the titration profiles were analyzed using a complex fitting program to obtain information about the species present in solution. Key words: potentiometric titration, methanol, ss pH, metal ion, lanthanides, apparent ss pK a . Résumé : Opérant en solution méthanolique, on a effectué des titrages potentiométriques de neufs triflates de lanthanidesIII, des triflates de ZnII et de CuII et des perchlorates de NiII, de CoII, de MgII et de TiIV afin de déterminer les constantes de titrage (définies comme le ss pH auquel les rapports [OCH−3 ]/[Mx+]t sont égaux à 0,5 et 1,5) ainsi que les valeurs apparentes de ss pK a pour la déprotonation des molécules de solvant fixées au métal. On a effectué les titrages sous diverses conditions, avec et sans addition de sels comme électrolytes, et on discute des variations dans les constantes de titrage. Dans des cas choisis (La3+, Zn2+) on a analysé les profils de titrage à l’aide d’un programme complexe d’ajustement des données afin d’en tirer de l’information sur les espèces présentes en solution. Mots clés : titrage potentiométrique, méthanol, ss pH, ion métallique, lanthanides, ss pK a apparent. [Traduit par la Rédaction]

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Introduction Recently we reported mechanistic aspects of di- and trivalent metal ion catalysis of the methanolysis of activated amides like acetylimidazole (1a) and β-lactams (1b), phosphate diesters (1c, 1g), and carbon-based esters (1d, 1f). Certain metal ions, notably La3+ and Eu3+, give impressive rate enhancements for methanolysis of the substrates, in some cases several million- (1d, 1f) and billion-fold (1c, 1g). These studies revealed a number of interesting aspects about the Mx+/methanol system that proves of practical and fundamental interest in developing other useful catalytic systems for alcoholysis reactions. Among organic solvents, methanol is closest to water in terms of structure and solvation, but its lower dielectric constant of 31.5 vs. 78.5 at 25°C (2) allows a much greater organic substrate solubility than does water and promotes ion-pairing (ion association) of the Mx+ and negatively charged substrates. Furthermore, unlike the situation in water where precipitation of Mx+(OH–)n is a problem above the pKa of the metal-coordinated waters (3), we have observed a greater solubility of most metal ions in MeOH throughout the entire pH region where ionization of Mx+(HOMe)y occurs. While there are several advantages to the Mx+/methanol system, it became evident to us that detailed kinetic and

mechanistic analyses required reliable methods for determining and controlling “pH” in methanol as well as determining equilibrium constants and “pKa” values for the various ionizations of Mx+(HOMe)y. Potentiometric titration methods are relatively simple and effective ways to obtain metal-ion complex stability constants and acid dissociation constants and, under favourable conditions, the speciation of complex mixtures in solution can be estimated with fair confidence (4). Potentiometric titrations have traditionally been done in aqueous solvents (5) and extensive tabulated data are available (6) but titrations in nonaqueous solvents have seen less use, particularly in the cases with metal ions (7a, 7b). A major reason for using nonaqueous solvents has been to determine acidity constants for compounds that could not be reliably measured in the relatively narrow range offered by amphiprotic water, or to better resolve ionizations (8). Since there was no simple way to reliably convert potentiometric readings to absolute pH in nonaqueous solvents referenced to that solvent (denoted ss pH),2 nonaqueous solvents were generally inconvenient to use (8c) and apply, particularly in studies of reaction mechanism. Some years ago, deLigny and Rehbach (9a) empirically determined a method for measuring the ss pH in methanol by adding a correction constant of 2.34 (on the molality scale) to a measured electrode reading. More recently Bosch and co-workers (10) have reported

Received 13 January 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 22 May 2003. Dedicated to Professor Don Arnold for his contributions to chemistry. G. Gibson, A.A. Neverov, and R.S. Brown.1 Department of Chemistry, Queen’s University, Kingston, ON K7L 3N6, Canada. 1

Corresponding author (e-mail: [email protected]). notation is based on the recommendations of the IUPAC, Compendium of analytical nomenclature, definitive rules 1997. 3rd ed. Blackwell, Oxford, U.K. 1998. 2x x pH

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doi: 10.1139/V03-035

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a method for determining ss pH on the molarity scale which for our purposes is relatively simple: if a glass electrode is calibrated using standard aqueous buffers but the potentiometric measurements are taken in a nonaqueous solvent, the values are termed ws pH. Subsequently, one computes ss pH = s w pH – δ, where δ is a correction factor of –2.24 on the molarity scale for measurements made as above in methanol (10). Although dependent on the junction potential of the electrode, there was found to be little difference between readings taken with different electrodes. Rived et al. (10c) have presented an extensive tabulation of ss pKa values in methanol for many organic acids and bases that can be used as buffers to control ss pH, and where previously unknown s s pKa values for materials are needed, these can be determined by titration according to the methods presented. The titration of metal ions, however, is not as straightforward and forms the subject of the present report concerning the potentiometric titration of five common divalent metal ions (ZnII, CoII, MgII, CuII, NiII) and nine LnIII ions, as well as TiIV in MeOH. We have analyzed the potentiometric data in two ways depending upon the level of information required concerning the ss pH-dependent species formed in solution. In addition we have determined qualitatively the effect on titrations of some added counterions often used as anionic components of buffers used for pH control in kinetic experiments and as supporting electrolytes.

Experimental Materials Methanol (anhydrous) was obtained in a SureSeal™ bottle (Aldrich) and used as received. Sodium methoxide was received in a SureSeal™ bottle as a 0.5 M solution in methanol which was diluted to make stock solutions (4 × 10–3 – 2 × 10–2 M). Stock Na+OMe– solutions were stored under argon atmosphere and used within 5 days. All LnIII metal ions were purchased as trifluoromethanesulfonate (OTf –) salts from Aldrich. Stock solutions were prepared in methanol (1 × 10–2 or 5 × 10–2 M) and used within 2 days. Ti(OMe)4 was purchased from Aldrich and a 2 × 10–3 M stock solution was prepared in methanol and converted to the perchlorate salt in situ by the addition of 4 equiv of perchloric acid. Zn(OTf)2, Cu(OTf)2, Co(ClO4)2·6H2O, and Ni(ClO4)2·6H2O were obtained from Aldrich while Mg(ClO4)2 was obtained from Alfa Aesar; all were used as received. Tetrabutylammonium salts were used as purchased from Sigma– Aldrich (except the chloride, from Fluka) to prepare stock solutions in methanol (5 × 10–3 – 2 × 10–2 M). Potentiometric titrations Titrations were performed using a Radiometer Vit90 Autotitrator equipped with an ABU91 Autoburette. The electrodes were Radiometer combination glass electrodes with a double junction design (model pHC2201 had a sleeve junction, and pHC2501 a porous ceramic pin, the latter type being recommended in reference (8a) and found to be most reliable in our studies). The lower reservoir of the pH electrode in contact with the test solution was filled with a 1 M solution of LiClO4 in methanol. The temperature was kept at 25.0°C using a water bath and all titrations were conducted under Ar in a jacketed titration cell.

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The metal ion concentration ranged from 1 × 10–4 to 4 × 10–3 M, and ionic strength was controlled with 0.01 M Bu4 NClO4 unless otherwise noted. Sodium methoxide titrant was standardized by titrating an aliquot of Fisher certified aq HCl, with the endpoint taken to be pH 7.0. Electrode calibration was accomplished by the so-called “practical method” (11), immersing it in standardized aqueous buffers at pH 4.0 and 10.0 followed by rinsing the electrode in anhydrous methanol. Subsequent acidity measurements in methanol gave experimental ws pH readings very close to the actual hydrogen ion concentration, usually ~ 0.03 units higher (11) which was sufficient for our purposes. To the ws pH readings was added the constant 2.24 to give the ss pH as described by Bosch and co-workers (10). Titration data were analyzed by standard computer treatments provided within the program PKAS (4) and, in two selected cases, Hyperquad 2000 (version 2.1 NT) (12) as well, with the autoprotolysis constant of pure methanol taken to be 1 × 10–16.77 at 25°C (10). Since the stated water content of the Aldrich anhydrous methanol is Br– > I–. This trend is not surprising as the notoriously hard cation La3+, which binds via predominantly electrostatic interactions, is expected to bind more weakly to the softer halides (I– and Br–) that have significant covalent character in their bonds (5i, 7a). While we have tried to minimize the salt effects by using weakly nucleophilic anions such as ClO−4 and CF3SO−3 (OTf –), these too raise the ss pG1′ to a certain extent. To investigate further the extent of perturbation of the ss pG1′ we titrated 1 × 10–3 M La(OTf)3 solutions containing 1, 2, and 4 × 10–2 M Bu4NOTf (titration curves not shown). Both the ss pG1′ ( ss pG2′ ) values increase, from 8.0 (10.3) in the absence of added salt to 8.3 (10.3), 8.4 (10.4), and 8.6 (10.5) at the three concentrations employed. This upward shift likely results from an ion association of the triflate with the La3+ which reduces its affinity for OCH−3 , possibly by decreasing the Lewis acidity of the metal ion or by acting as a competitive inhibitor of methoxide binding in the various metal containing species. Perchlorate ion behaves similarly as judged by comparison of the titration ss pG1′ and ss pG2′ values given in Table 2, rows 2 and 3. The divalent transition metal ions, already found to be less acidic than any of the trivalent metal ions studied, are also much less affected by association with added counterions. The titration profiles of 2 × 10–3 M Zn(OTf)2 without and with 0.01 M added Bu4NClO4 (not shown) were essentially superimposable indicating a lack of ion association in this case.

Conclusions The objective of this study was to find a convenient method to obtain titration constants for various metal ions in methanol solution, including the lanthanides and certain transition metal ions. The titration constants are crucial elements for determin-

Can. J. Chem. Vol. 81, 2003 Fig. 7. Potentiometric titration curves of 1 × 10–3 M La(OTf)3 in the presence of 0.04 M Bu4NX salts. (a) (ⵧ) X = NO−3 , (䉭) X = OTf,, (䉮) no added electrolyte; (b) (䊏) X = Cl–, (䉱) X = Br–, (䊊) X = I–, (䉮) no added electrolyte.

ing the ss pH values at which species with a known [OCH−3 ]/Mx+ ratio exist. As such they are essential for interpreting our findings that La3+, Eu3+, and Zn2+ are remarkably active in catalyzing certain methanolysis reactions (1, 21). We have undertaken three different approaches for analyzing the titration data. We have applied the simplified method of Simms for determining certain titration constants, defined as half neutralization constants or ss pG1′ , conveniently interpreted as the solution ss pH where the average OCH−3 /Mx+ ratio is 0.5 and 1.5. This allows one to define approximately the ss pH regions where one can form the maximum concentrations of © 2003 NRC Canada



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certain species that may have kinetic activity. However, this approach does not reveal the intricate details of the acid– base equilibria involved and the constants are complicated composites of various equilibria including ion-pairing between the Mx+ and solution counterions and metal dimerizations and (or) oligomerizations which can affect the ionizations of (Mx+)n(HOCH3)m forms. Through subsequent application of a computer program (PKAS), the shape of the titration profile can be modeled, and apparent ss pKa values can be derived. While these are also complicated functions of the various equilibria in solution, one can identify quickly situations where metal ion association exists because the titration profiles are much steeper than expected for a simple ionization. When necessary, additional detailed information concerning the speciation of the Mx+/OCH−3 forms can be obtained through fits of the potentiometric titration curves to multiequilibrium models using the computer program Hyperquad which gives the microsopic stability constants for the various species in solution. This type of fit is best undertaken for metal ions shown to be catalytically active, and where additional information concerning the likely speciation in solution is available. Through an extensive fitting of the La3+ titration data and comparison with our previous kinetic results for the catalyzed methanolysis of p-nitrophenyl acetate, we have confirmed that the active form of the catalyst is a previously proposed La2(OCH3)2 dimer, giving further credence to the applicability of the titration and (or) kinetic treatment. Moreover, this method reveals a much more complex situation where another dimeric form (La2(OCH3)4) is present and reactive. As such, the advantage of this approach when applied to metal ion catalysis of methanolyses, is the ability to analyze complex kinetic behaviour attributable to a multiequilibrium system which may be difficult, if not impossible, to analyze by standard kinetic approaches. Further work from these laboratories will be aimed at extending the titration and kinetic methodology to other reactions, metal ions, and alcohol solvents.

Acknowledgements The authors acknowledge the financial assistance of Queen’s University and the Natural Sciences and Engineering Research Council of Canada (NSERC). Also acknowledged are Mr. Todd McDonald and Dr. Pedro Montoya-Pelaez for their assistance in determining reliable methods for performing the titrations in methanol.

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