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95Mo NMR study of the effect of structure on complexation of molybdate with alpha and beta hydroxy carboxylic acid ligands
Polyhedron xxx (2015) xxx–xxx
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95
Mo NMR study of the effect of structure on complexation of molybdate with alpha and beta hydroxy carboxylic acid ligands Claude H. Yoder ⇑, Emily L. Christie, Jennifer L. Morford Department of Chemistry, Franklin & Marshall College, Lancaster, PA 17604, United States
a r t i c l e
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Article history: Received 18 April 2015 Accepted 4 September 2015 Available online xxxx Keywords: Molybdate Hydroxy carboxylic acids 95 Mo NMR Glycolic acid Lactic acid
a b s t r a c t The utility of the quadrupolar, spin 5/2 95Mo nucleus in NMR studies of complexation of molybdate have been demonstrated using model alpha hydroxy carboxylic acids. The 95Mo NMR spectra show a down field shift of the resonance upon complexation, presumably due to the smaller 1/DE paramagnetic shift term for the octahedral molybdate complexes relative to the tetrahedral molybdate ion. The 95Mo resonances are sufficiently distinct to establish three solution complexes for the reaction of lactic acid with molybdate. The effect of the structure of hydroxy carboxylic acids on complexation to molybdate (MoO24 ) was studied with 13C and 95Mo NMR spectroscopy. The use of the NMR spectra of both nuclei allows monitoring of the Lewis acidic molybdate site and the Lewis basic ligands. The beta hydroxy carboxylic acids, 3-hydroxypropanoic acid and salicylic acid, did not form complexes with molybdate, suggesting a preference for 5-membered chelated rings. The Lewis acidity of the negatively charged molybdate ion is rationalized with two schemes for the reaction with the chelating bidentate alpha hydroxy carboxylic acids. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The tetrahedral molybdate ion, MoO24 , the main form of molybdenum in neutral and basic aqueous environments [1], is directly related to molybdic acid, the structure of which has been predicted by molecular mechanics to be octahedral with two oxygens, two OH groups, and two water molecules attached to the central molybdenum [2]. Although molybdenum is in the +6 oxidation state in the molybdate ion, the 2 charge of the ion makes it somewhat surprising that it functions as a Lewis acid toward humic and tannic acids [3], the principle organic substances in soil and marine sediments, as well as simple alpha hydroxy carboxylic acids [4]. Humic acids contain many carboxylic acid and (mostly phenolic) hydroxy groups that can complex to metals in solution [5,6], and bind to mineral surfaces to provide new adsorption sites for metal adsorption [7–14]. The 95Mo nucleus, with its natural abundance of 16% and its relative receptivity almost three times greater than that of 13C, should provide a sensitive probe for changes in the electronic environment at the core of the complex. The 95Mo nucleus has I = 5/2 and therefore a quadrupole moment, which makes ⇑ Corresponding author. E-mail address: [email protected] (C.H. Yoder).
observation of the resonance dependent on the electric field surrounding the nucleus. However, the 95Mo resonance can be observed in a variety of environments [17]. The breadth of the resonance can be used to monitor the asymmetry of the electric field produced by the complexing ligands. 13C NMR spectroscopy provides a complementary probe of the changing environment of the ligands upon complexation. The structures of several simple alpha-hydroxy carboxylic acid complexes with molybdate have been reported and characterized using crystallography [4]. These complexes have a 1 to 2 ratio of molybdate to ligand and a distorted octahedral structure that has both oxygens of the ligand bonded to the central molybdenum, which is also attached to two lone oxygens [15]. The 13C NMR resonances of the CH2 group as well as the carboxyl carbon in glycolic acid (lactic acid behaves similarly) shift to higher frequency upon complexation [4,16]. The shift of the IR carboxyl band to lower wavenumbers suggests monodentate bonding of the carboxyl group to the molybdenum center [4]. Our objectives in this study are to (a) determine the utility of 95 Mo NMR for studying the electronic environment of the Mo center in known complexes of molybdate ion with alpha-hydroxy carboxylic acids, (b) utilize 95Mo NMR to determine the structure of molybdate complexes, and (c) provide a coherent rationalization for the apparent Lewis acidity of the molybdate ion.
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2. Materials and methods All ligands, except 3-hydroxypropanoic acid (30% aqueous solution, Sigma–Aldrich), were ACS certified reagents with purities exceeding 95%. The 3-hydroxypropanoic acid was found to be ca. 90% pure by proton and carbon NMR spectroscopy. Molybdate was used as Na2MoO4 (Sigma–Aldrich). Stock solutions (0.50 M) of the ligands—glycolic acid (Sigma– Aldrich), lactic acid (Sigma–Aldrich), salicylic acid (Fisher), 3-hydroxypropanoic acid (Sigma–Aldrich), methoxy acetic acid (Sigma–Aldrich), and methyl glycolate (Sigma–Aldrich)—were prepared using distilled water and volumetric glassware. Various concentrations of the ligand solution were added to the molybdate solution using Eppendorf pipettes and volumetric glassware. The pH of each solution was adjusted to 7.0 ± 0.5 using 1 M NaOH. A 0.50 M solution of dimethylsulfoxide (DMSO) in D2O was added to each solution to provide an internal integration reference of 0.15 M DMSO. NMR spectra were obtained on a Varian INOVA 500 MHz NMR spectrometer at 23 °C. 13C spectra were obtained at 125.597 MHz using a 7 s delay time, sufficient for complete relaxation of all carbon nuclei. 95Mo was observed in 10 mm tubes at 32.548 MHz using a sweep width of 100 kHz, a delay time of 0.01 s, and an acquisition time of 0.8 s. The 95Mo spectra were referenced (0.00 ppm) to Na2MoO4 (0.1 M) in D2O.
3. Results The 95Mo NMR spectrum (Fig. 1) for a solution with a 1 to 2 mole ratio of molybdate to glycolic acid contains a resonance for free molybdate at 0 ppm relative to the Na2MoO4 external reference and a resonance at 114 ppm for the complexed molybdate. Both resonances are relatively sharp, but the 95Mo resonance for the complex is unexpectedly downfield from the molybdate ion. The 13C spectrum (Fig. 2) of the same solution shows the downfield shift of the resonances of both the methylene carbon (74 ppm) and the carboxyl carbon (184 ppm) from the corresponding free glycolic acid peaks (61 and 180 ppm, respectively). For mole ratios of molybdate:glycolic acid from 2:1 to 1:5, the free molybdate ion and the complex are visible in the 95Mo spectrum, and the free ligand and complex are visible in the proton and carbon NMR spectra (Table 1). Chemical shifts of all peaks at different mole ratios are very similar and there is therefore little or no exchange of free ligand with complexed ligand. These shifts
are consistent with those reported [4,16] and are indicative of the formation of a bidentate complex formed by coordination of both the alpha hydroxy group and the carboxyl group [4]. The 1 to 2 (molybdate to ligand) stoichiometry of the complex was confirmed by a Job plot of the concentration of complex as obtained from the 13C NMR spectrum relative to the mole fraction of complex (Fig. 3) as well as a plot of pH versus mole ratio for mixtures of glycolic acid and sodium molybdate (Fig. 4). Fig. 4, like Fig. 3, gives a clear indication of the stoichiometry of the complexes but provides no evidence for the mode of ligand binding—monodentate, bidentate, chelated—nor the structure of the binding sites. The 95Mo NMR spectrum of a solution containing a 1 to 2 mole ratio mixture of molybdate and lactic acid (Fig. 5) contains three peaks of unequal intensities between 80 and 100 ppm downfield of molybdate. The 13C NMR spectrum of the same solution also contains three peaks downfield from the CH(OH), and the COOH carbon resonances in free lactic acid. Consequently, there are three complexes when lactic acid is present in aqueous solution at pH 7. The importance of the carboxyl OH and the alpha OH groups to the complexation of the alpha hydroxy carboxylic acids was explored with mixtures of molybdate and methoxyacetic acid (CH3OCH2COOH), which does not contain an OH group in the alpha position, and methyl glycolate (HOCH2COOCH3), which has no carboxyl OH. No complex was observed for mole ratios of 1 to 2 and 1 to 4 with methoxyacetic acid nor with acetic acid. In contrast, with methyl glycolate the 95Mo and 13C NMR spectra confirmed that the same complex found with glycolic acid was formed after several days. The formation of methanol in the slow reaction with methyl glycolate indicated a de-esterification reaction than proceeded much more slowly (over ca. 12 h) than the complexation with glycolic acid, which was complete within seconds. Thus, it appears that the presence of OH at both positions is necessary for the formation of a complex with molybdate. The minus two charge of the complex [4] in the formula for the complex is consistent with this observation. In order to determine the effect of the structure of the ligand, in particular the relative position of the carboxyl and hydroxyl groups, on the complexation with molybdate, the 13C and 95Mo NMR spectra of solutions of two beta hydroxy carboxylic acids— salicylic acid (o-hydroxybenzoic acid) and 3-hydroxypropanoic acid—were obtained. The 13C and 95Mo NMR spectra of solutions of molybdate and 3-hydroxypropanoic acid at mole ratios varying from 2 to 1 up to 1 to 5 contained only peaks due to starting materials, and, consequently, no complexation was observed in water at pH 7. The 13C and 95Mo NMR spectra of solutions of molybdate and salicylic acid also contained only peaks for starting materials indicating no complex formation at mole ratios from 2 to 1 to 1 to 5.
4. Discussion 4.1.
Fig. 1. 95Mo NMR spectrum of a solution of 1 to 2 mole ratio of molybdate (0.05 M) to glycolic acid.
95
Mo spectra
Generally, the chemical shifts of main group element Lewis acidic centers move upfield upon complexation, a change usually rationalized by the increase in electron density at the Lewis center [18]. However, the chemical shifts for most elements are governed by the paramagnetic shift term, which involves 1/DE, where DE is the average excitation energy for magnetically allowed HOMO–LUMO transitions. This term is difficult to evaluate but for most transition metal centers involves d-orbital contributions [19]. One could imagine that sp3d2 hybridization (simplistically) utilized by Mo(VI) complexes has a smaller HOMO–LUMO gap (and therefore a larger 1/DE term and a greater downfield shift)
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Fig. 2. The 13C NMR spectrum of an aqueous solution of molybdate and glycolic acid at a mole ratio of 1 to 2 (concentration of molybdate = 0.05 M) also containing 0.15 M DMSO at 39.5 ppm.
Table 1 Mo and
95
13
C NMR shifts of the free ligands and complexes with MoO24 . 95
Mo
13
Ligand
d
Glycolic acid Free Complexed
– 114.4
61.3, 179.8 74.0, 184.0
Lactic acid Free Complexed
– 82.3, 87.1, 92.5
19.0, 66.2, 178.0 16.19, 17.58, 17.89, 81.0, 186.7
d
C
than the gap for the sp3 hybridized molybdate ion [17,20]. Although Table 1 only provides shifts at one particular mole ratio, the chemical shifts of each species are dependent only slightly on mole ratio. It is important to notice that the breadth (FWHM) of the Mo peaks are generally greater for the complex than for the molybdate ion, presumably indicating lower symmetry in the electric field surrounding the molybdenum in the complex.
4.2. Rationalization of Lewis acid complexation of molybdate Complexation of molybdate with glycolic acid has several interesting features: (a) alpha hydroxy carboxylic acids have two
Fig. 3. Job plot of the concentration of the molybdate–glycolic acid complex relative to the mole fraction of the complex.
Brønsted–Lowry acidic sites of vastly different acidities—the carboxyl group has a Ka about 1010 greater than that of the hydroxy group, and (b) Lewis acidic sites such as molybdate are infrequently doubly negatively charged. As indicated above, deprotonation of both the carboxyl group and the alpha OH produces the chelation-ready OCH2COO ion. Deprotonation of the alpha hydroxy carboxyl group (Ka ca. 10 4) is expected at neutral pH, and the extent of the NMR shift of the CH2AO carbon indicates that deprotonation of the OH group (Ka ca. 10 15) is also likely during complexation. The formation of the minus two complexes [4] in solutions of the free acid is accompanied by an increase in pH of the aqueous solution (Fig. 4), which suggests that deprotonation of the COOH group occurs followed by partial neutralization of the acidic solution. This deprotonation of the carboxyl group is likely a result of proton transfer from two glycolic acid molecules to the molybdate oxygens, with probable formation of the tetrahedral species MoO2(OH)2. Because of the lower Ka of the alpha hydroxy group, it is helpful to imagine attack of the lone pairs of the CAOH group on the Mo forming an MoAO bond and a protonated hydroxy group. Elimination of water and bidentate complex formation produces an octahedral geometry at Mo (Fig. 6 top). The Lewis acidity of the molybdenum center can be emphasized by representation of molybdate as MoO2+ 2 , a Lewis-acidic entity in
Fig. 4. pH changes in titration of glycolic acid (GA) (0.50 M) with sodium molybdate (0.1 M).
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4.3. Use of
Fig. 5.
95
Mo NMR spectrum of 1 to 2 mixture of molybdate (0.1 M) and lactic acid.
many molybdate complexes [21], accompanied by two ‘‘oxide 2 ions”; i.e., MoO24 = MoO2+ . Complexation of the alpha2 + 2O hydroxy carboxylic acids can be represented using these model ‘‘components” of molybdate ion. Deprotonation of both glycolic acid sites is accomplished by conversion of ‘‘oxide” to water, followed by chelation of the anion OCH2COO to MoO2+ 2 producing the octahedral 1 to 2 complex (Fig. 6 bottom). Neither scheme shown in Fig. 6 is meant to portray a mechanism, but merely to indicate how nucleophilic attack at the molybdenum core and the change in structure at molybdenum can be rationalized.
95
Mo NMR to determine structure of complexes
The geometries possible for the 1:2 complexes are shown in Fig. 7 for glycolic acid. The geometries for lactic acid are the same except that a chiral center exists at each alpha carbon. The three lactic acid complexes in solution clearly have slightly different environments at Mo, as do each of the four possible complexes shown in Fig. 7. The Mo environments in each of the trans-MoO2 complexes are likely similar as are those of the cis-MoO2 complexes [22]. The 13C NMR spectra also contain three peaks downfield from the single peaks for the CHOH and the COOH groups of the free lactic acid. The spectra also contain closely clustered peaks around the CH3 peak of the free ligand. Because the 13C NMR spectrum of the cis complex shown on the bottom left of Fig. 7 would contain two peaks for the CH and carboxyl carbons, whereas the spectra of all of the other geometric isomers would contain only one peak for the CH and one peak for the carboxyl carbons, it is possible that the mixture of isomers present in a 1 to 2 mol ratio of molybdate to ligand contains the two trans isomers as well as the cis isomer with the greater symmetry. However, the presence of the chiral center in lactic acid produces a number of diastereomeric complexes such as:
Fig. 6. Representation of complexation using MoO24 (top) and the MoO2+ 2 entity (bottom).
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Fig. 7. The four possible geometries for the 1:2 glycolic acid complex. The cis complex on the bottom right has a C2 symmetry axis bisecting the cis MoO2 group that makes the CH2 carbons equivalent. For the cis complex on the bottom left, the CH carbons in each of the two chelating ligands (as well as the CH2 carbons in each of the ligands) are not equivalent.
that could also have different 13C CH, C@O, and CH3 NMR shifts and also different 95Mo shifts [22]. The 95Mo spectra of the complexes of both glycolic and lactic acid contain a small, broad peak in the 20–35 ppm region, due possibly to a 1 to 1 complex or the presence of Mo7O624 , which at pH 5.7 produces two peaks, one at 34 ppm and a smaller peak at 210 ppm [21]. 95Mo NMR spectra of mixtures of molybdate and 3-hydroxypropanoic acid, which does not complex with MoO24 , also contain a small broad peak in this area. We tentatively assign the peak to a polymolybdate species, possibly Mo7O624 , which would be expected to form to some limited extent at this pH and molybdenum concentration [1]. Clearly, 95Mo NMR spectroscopy is capable of providing a great deal of information on the electronic surroundings of the Mo center of complexes, including fairly subtle structural and stereochemical nuances. However, for the complexes studied here there does not seem to be a simple relationship between the 95Mo chemical shift and the electron density at the Mo center. 4.4. Structural effects in alpha hydroxy molybdate complexes The explanation of the greater stability of the alpha hydroxy molybdate complexes requires an examination of the difference between the alpha and beta hydroxy ligands and their complexes. Simple molecular models reveal that there would likely be more bond angle distortion in the beta hydroxy complexes, giving rise to a more positive DH° for the formation of these complexes. The DS° of formation of the beta hydroxypropanoic acid complex relative to the alpha analog would also be unfavorable because of the greater internal rotational entropy of the free ligand and its diminution in the complex. The situation with salicylic acid is somewhat different in that the benzene ring makes the OH group more acidic by a factor of about 105 in aqueous solution, and the ligand has less internal rotational entropy than that of 3-hydroxypropanoic acid. Based on these observations, it appears that the lower stability of the beta hydroxy complexes is due primarily to the greater OAMoAO bond angles required in the octahedral geometry of the complexes with beta-hydroxy acids. The frequently studied molybdate chelates with 1,2-dihydroxybenezene (catechol) contain an aromatic ligand with two OH groups four to five orders of magnitude more acidic than the alpha OH group in glycolic acid, for example. Perhaps as importantly, the
catechol ligand is ideally suited for molybdate complexation both because of the planar orientation of its rigid OH groups and because there is little rotational entropy in the ligand. That the loss of rotation conformational entropy can be significant can be illustrated with the absolute entropie of gaseous hexane (S° = 388 J/K-mol) with that of gaseous cyclohexane (S° = 298 J/K-mol). The difference in absolute entropies of 90 J/K-mol is due primarily to the conformational (internal rotational) entropy of hexane that is lost in cyclohexane. At 298 K this difference would be equivalent to a difference in DG° of more than 25 kJ/mol (of course the difference is smaller for alpha hydroxy carboxylic acids). (A difference of about 12 kJ/mol produces more than a 100-fold difference in equilibrium constant.) Other Lewis acidic centers that form complexes with alpha hydroxy carboxylic acids include: Mo(V), which forms both monodentate complexes with HOCH2COO and bidentate chelated complexes containing the OCH2COO entity [23], VO2+ 2 with the bidentate OCH2COO ligand, and VO4+ with both chelating bidentate OCH2COO ligand and monodentate HOCH2COO [24], peroxovanadium(V) with bidentate glycolic acid [25], oxovanadium(IV) with bidentate alpha hydroxy carboxylic acid [26], and the diethyltin(II) cation with both alpha hydroxy- and thiohydroxy carboxylic acids [27]. A variety of metal ions have also functioned as the Lewis acid for complexation with alpha-hydroxy carboxylic acids: Cu(II) with monodentate, bidentate chelating, and the neutral carboxylic acid [28], Mn(II) with glycolic and lactic acids, as well as other acid bidentate anions [29], Ag(I) with monodentate glycolate [30], La(III), Ce(III), and Nd(III) with both bidentate chelating and monodentate chelating glycolate [31], Cm(III) and Cu(III) with mostly monodentate glycolate [32], and Th(IV) and a variety of lanthanides(III) with mostly monodentate glycolate [33]. 5. Conclusion We have shown that 95Mo NMR spectroscopy can be used to detect even subtle structural and stereochemical changes in molybdate complexes. 95Mo NMR resonances of the complex are downfield from the resonance for the molybdate ion. We attribute this downfield shift to the greater HOMO–LUMO gap in the octahedral complexes relative to the sp3-hybridized molybdate ion. These resonances are sensitive to small changes in the electronic environment of the Mo center and are therefore useful in determining the
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number of complexes in solution. Both 95Mo and 13C NMR spectra provide evidence for the existence of only one glycolic acid complex and three lactic complexes with molybdate. In our model complexes, the Lewis acidic molybdate ion prefers to form chelated complexes with ligands that contain two basic oxygen (or likely sulfur) sites located on adjacent carbon atoms. The 5-member rings that form as a result of this chelation are stable and their formation reduces the loss in rotational entropy, relative to larger rings, that accompanies the formation of complexes of most ligands. The monodentate (non-chelated) complexes bound to the molybdenum center only through either the COOH (at least with acetic acid and methoxyacetic acid) or the alpha-OH groups (at least with methyl glycolate) are not present in aqueous solution at pH 7. It is likely that the driving force for chelation is sufficient to cause deprotonation of the hydroxyl group, for which the Ka is presumably quite low (between 10 15 and 10 10). Acknowledgements The authors are indebted to the Camille and Henry Dreyfus Foundation for a Senior Scientist Mentorship award to C.H.Y. and to the Franklin & Marshall College Lucille and William Hackman Research stipends to E.L.C. This material is based upon work supported by the National Science Foundation under Grant No. 0923224. J.L.M. thanks Kelly Murphy for early research on this topic and acknowledges the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. References [1] O.F. Oyerinde, C.L. Weeks, A.D. Anbar, T.G. Spiro, Inorg. Chim. Acta 361 (2007) 1000. [2] X. Liu, J. Cheng, M. Sprik, X. Lu, J. Phys. Chem. Lett. 4 (2013) 2926. [3] T. Wichard, B. Mishra, S.C.B. Myneni, J.-P. Bellenger, A.M.L. Kraepiel, Nat. Geosci. 2 (2009) 625.
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95
Mo NMR study of the effect of structure on complexation of molybdate with alpha and beta hydroxy carboxylic acid ligands Claude H. Yoder ⇑, Emily L. Christie, Jennifer L. Morford Department of Chemistry, Franklin & Marshall College, Lancaster, PA 17604, United States
a r t i c l e
i n f o
Article history: Received 18 April 2015 Accepted 4 September 2015 Available online xxxx Keywords: Molybdate Hydroxy carboxylic acids 95 Mo NMR Glycolic acid Lactic acid
a b s t r a c t The utility of the quadrupolar, spin 5/2 95Mo nucleus in NMR studies of complexation of molybdate have been demonstrated using model alpha hydroxy carboxylic acids. The 95Mo NMR spectra show a down field shift of the resonance upon complexation, presumably due to the smaller 1/DE paramagnetic shift term for the octahedral molybdate complexes relative to the tetrahedral molybdate ion. The 95Mo resonances are sufficiently distinct to establish three solution complexes for the reaction of lactic acid with molybdate. The effect of the structure of hydroxy carboxylic acids on complexation to molybdate (MoO24 ) was studied with 13C and 95Mo NMR spectroscopy. The use of the NMR spectra of both nuclei allows monitoring of the Lewis acidic molybdate site and the Lewis basic ligands. The beta hydroxy carboxylic acids, 3-hydroxypropanoic acid and salicylic acid, did not form complexes with molybdate, suggesting a preference for 5-membered chelated rings. The Lewis acidity of the negatively charged molybdate ion is rationalized with two schemes for the reaction with the chelating bidentate alpha hydroxy carboxylic acids. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The tetrahedral molybdate ion, MoO24 , the main form of molybdenum in neutral and basic aqueous environments [1], is directly related to molybdic acid, the structure of which has been predicted by molecular mechanics to be octahedral with two oxygens, two OH groups, and two water molecules attached to the central molybdenum [2]. Although molybdenum is in the +6 oxidation state in the molybdate ion, the 2 charge of the ion makes it somewhat surprising that it functions as a Lewis acid toward humic and tannic acids [3], the principle organic substances in soil and marine sediments, as well as simple alpha hydroxy carboxylic acids [4]. Humic acids contain many carboxylic acid and (mostly phenolic) hydroxy groups that can complex to metals in solution [5,6], and bind to mineral surfaces to provide new adsorption sites for metal adsorption [7–14]. The 95Mo nucleus, with its natural abundance of 16% and its relative receptivity almost three times greater than that of 13C, should provide a sensitive probe for changes in the electronic environment at the core of the complex. The 95Mo nucleus has I = 5/2 and therefore a quadrupole moment, which makes ⇑ Corresponding author. E-mail address: [email protected] (C.H. Yoder).
observation of the resonance dependent on the electric field surrounding the nucleus. However, the 95Mo resonance can be observed in a variety of environments [17]. The breadth of the resonance can be used to monitor the asymmetry of the electric field produced by the complexing ligands. 13C NMR spectroscopy provides a complementary probe of the changing environment of the ligands upon complexation. The structures of several simple alpha-hydroxy carboxylic acid complexes with molybdate have been reported and characterized using crystallography [4]. These complexes have a 1 to 2 ratio of molybdate to ligand and a distorted octahedral structure that has both oxygens of the ligand bonded to the central molybdenum, which is also attached to two lone oxygens [15]. The 13C NMR resonances of the CH2 group as well as the carboxyl carbon in glycolic acid (lactic acid behaves similarly) shift to higher frequency upon complexation [4,16]. The shift of the IR carboxyl band to lower wavenumbers suggests monodentate bonding of the carboxyl group to the molybdenum center [4]. Our objectives in this study are to (a) determine the utility of 95 Mo NMR for studying the electronic environment of the Mo center in known complexes of molybdate ion with alpha-hydroxy carboxylic acids, (b) utilize 95Mo NMR to determine the structure of molybdate complexes, and (c) provide a coherent rationalization for the apparent Lewis acidity of the molybdate ion.
http://dx.doi.org/10.1016/j.poly.2015.09.009 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
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2. Materials and methods All ligands, except 3-hydroxypropanoic acid (30% aqueous solution, Sigma–Aldrich), were ACS certified reagents with purities exceeding 95%. The 3-hydroxypropanoic acid was found to be ca. 90% pure by proton and carbon NMR spectroscopy. Molybdate was used as Na2MoO4 (Sigma–Aldrich). Stock solutions (0.50 M) of the ligands—glycolic acid (Sigma– Aldrich), lactic acid (Sigma–Aldrich), salicylic acid (Fisher), 3-hydroxypropanoic acid (Sigma–Aldrich), methoxy acetic acid (Sigma–Aldrich), and methyl glycolate (Sigma–Aldrich)—were prepared using distilled water and volumetric glassware. Various concentrations of the ligand solution were added to the molybdate solution using Eppendorf pipettes and volumetric glassware. The pH of each solution was adjusted to 7.0 ± 0.5 using 1 M NaOH. A 0.50 M solution of dimethylsulfoxide (DMSO) in D2O was added to each solution to provide an internal integration reference of 0.15 M DMSO. NMR spectra were obtained on a Varian INOVA 500 MHz NMR spectrometer at 23 °C. 13C spectra were obtained at 125.597 MHz using a 7 s delay time, sufficient for complete relaxation of all carbon nuclei. 95Mo was observed in 10 mm tubes at 32.548 MHz using a sweep width of 100 kHz, a delay time of 0.01 s, and an acquisition time of 0.8 s. The 95Mo spectra were referenced (0.00 ppm) to Na2MoO4 (0.1 M) in D2O.
3. Results The 95Mo NMR spectrum (Fig. 1) for a solution with a 1 to 2 mole ratio of molybdate to glycolic acid contains a resonance for free molybdate at 0 ppm relative to the Na2MoO4 external reference and a resonance at 114 ppm for the complexed molybdate. Both resonances are relatively sharp, but the 95Mo resonance for the complex is unexpectedly downfield from the molybdate ion. The 13C spectrum (Fig. 2) of the same solution shows the downfield shift of the resonances of both the methylene carbon (74 ppm) and the carboxyl carbon (184 ppm) from the corresponding free glycolic acid peaks (61 and 180 ppm, respectively). For mole ratios of molybdate:glycolic acid from 2:1 to 1:5, the free molybdate ion and the complex are visible in the 95Mo spectrum, and the free ligand and complex are visible in the proton and carbon NMR spectra (Table 1). Chemical shifts of all peaks at different mole ratios are very similar and there is therefore little or no exchange of free ligand with complexed ligand. These shifts
are consistent with those reported [4,16] and are indicative of the formation of a bidentate complex formed by coordination of both the alpha hydroxy group and the carboxyl group [4]. The 1 to 2 (molybdate to ligand) stoichiometry of the complex was confirmed by a Job plot of the concentration of complex as obtained from the 13C NMR spectrum relative to the mole fraction of complex (Fig. 3) as well as a plot of pH versus mole ratio for mixtures of glycolic acid and sodium molybdate (Fig. 4). Fig. 4, like Fig. 3, gives a clear indication of the stoichiometry of the complexes but provides no evidence for the mode of ligand binding—monodentate, bidentate, chelated—nor the structure of the binding sites. The 95Mo NMR spectrum of a solution containing a 1 to 2 mole ratio mixture of molybdate and lactic acid (Fig. 5) contains three peaks of unequal intensities between 80 and 100 ppm downfield of molybdate. The 13C NMR spectrum of the same solution also contains three peaks downfield from the CH(OH), and the COOH carbon resonances in free lactic acid. Consequently, there are three complexes when lactic acid is present in aqueous solution at pH 7. The importance of the carboxyl OH and the alpha OH groups to the complexation of the alpha hydroxy carboxylic acids was explored with mixtures of molybdate and methoxyacetic acid (CH3OCH2COOH), which does not contain an OH group in the alpha position, and methyl glycolate (HOCH2COOCH3), which has no carboxyl OH. No complex was observed for mole ratios of 1 to 2 and 1 to 4 with methoxyacetic acid nor with acetic acid. In contrast, with methyl glycolate the 95Mo and 13C NMR spectra confirmed that the same complex found with glycolic acid was formed after several days. The formation of methanol in the slow reaction with methyl glycolate indicated a de-esterification reaction than proceeded much more slowly (over ca. 12 h) than the complexation with glycolic acid, which was complete within seconds. Thus, it appears that the presence of OH at both positions is necessary for the formation of a complex with molybdate. The minus two charge of the complex [4] in the formula for the complex is consistent with this observation. In order to determine the effect of the structure of the ligand, in particular the relative position of the carboxyl and hydroxyl groups, on the complexation with molybdate, the 13C and 95Mo NMR spectra of solutions of two beta hydroxy carboxylic acids— salicylic acid (o-hydroxybenzoic acid) and 3-hydroxypropanoic acid—were obtained. The 13C and 95Mo NMR spectra of solutions of molybdate and 3-hydroxypropanoic acid at mole ratios varying from 2 to 1 up to 1 to 5 contained only peaks due to starting materials, and, consequently, no complexation was observed in water at pH 7. The 13C and 95Mo NMR spectra of solutions of molybdate and salicylic acid also contained only peaks for starting materials indicating no complex formation at mole ratios from 2 to 1 to 1 to 5.
4. Discussion 4.1.
Fig. 1. 95Mo NMR spectrum of a solution of 1 to 2 mole ratio of molybdate (0.05 M) to glycolic acid.
95
Mo spectra
Generally, the chemical shifts of main group element Lewis acidic centers move upfield upon complexation, a change usually rationalized by the increase in electron density at the Lewis center [18]. However, the chemical shifts for most elements are governed by the paramagnetic shift term, which involves 1/DE, where DE is the average excitation energy for magnetically allowed HOMO–LUMO transitions. This term is difficult to evaluate but for most transition metal centers involves d-orbital contributions [19]. One could imagine that sp3d2 hybridization (simplistically) utilized by Mo(VI) complexes has a smaller HOMO–LUMO gap (and therefore a larger 1/DE term and a greater downfield shift)
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Fig. 2. The 13C NMR spectrum of an aqueous solution of molybdate and glycolic acid at a mole ratio of 1 to 2 (concentration of molybdate = 0.05 M) also containing 0.15 M DMSO at 39.5 ppm.
Table 1 Mo and
95
13
C NMR shifts of the free ligands and complexes with MoO24 . 95
Mo
13
Ligand
d
Glycolic acid Free Complexed
– 114.4
61.3, 179.8 74.0, 184.0
Lactic acid Free Complexed
– 82.3, 87.1, 92.5
19.0, 66.2, 178.0 16.19, 17.58, 17.89, 81.0, 186.7
d
C
than the gap for the sp3 hybridized molybdate ion [17,20]. Although Table 1 only provides shifts at one particular mole ratio, the chemical shifts of each species are dependent only slightly on mole ratio. It is important to notice that the breadth (FWHM) of the Mo peaks are generally greater for the complex than for the molybdate ion, presumably indicating lower symmetry in the electric field surrounding the molybdenum in the complex.
4.2. Rationalization of Lewis acid complexation of molybdate Complexation of molybdate with glycolic acid has several interesting features: (a) alpha hydroxy carboxylic acids have two
Fig. 3. Job plot of the concentration of the molybdate–glycolic acid complex relative to the mole fraction of the complex.
Brønsted–Lowry acidic sites of vastly different acidities—the carboxyl group has a Ka about 1010 greater than that of the hydroxy group, and (b) Lewis acidic sites such as molybdate are infrequently doubly negatively charged. As indicated above, deprotonation of both the carboxyl group and the alpha OH produces the chelation-ready OCH2COO ion. Deprotonation of the alpha hydroxy carboxyl group (Ka ca. 10 4) is expected at neutral pH, and the extent of the NMR shift of the CH2AO carbon indicates that deprotonation of the OH group (Ka ca. 10 15) is also likely during complexation. The formation of the minus two complexes [4] in solutions of the free acid is accompanied by an increase in pH of the aqueous solution (Fig. 4), which suggests that deprotonation of the COOH group occurs followed by partial neutralization of the acidic solution. This deprotonation of the carboxyl group is likely a result of proton transfer from two glycolic acid molecules to the molybdate oxygens, with probable formation of the tetrahedral species MoO2(OH)2. Because of the lower Ka of the alpha hydroxy group, it is helpful to imagine attack of the lone pairs of the CAOH group on the Mo forming an MoAO bond and a protonated hydroxy group. Elimination of water and bidentate complex formation produces an octahedral geometry at Mo (Fig. 6 top). The Lewis acidity of the molybdenum center can be emphasized by representation of molybdate as MoO2+ 2 , a Lewis-acidic entity in
Fig. 4. pH changes in titration of glycolic acid (GA) (0.50 M) with sodium molybdate (0.1 M).
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4.3. Use of
Fig. 5.
95
Mo NMR spectrum of 1 to 2 mixture of molybdate (0.1 M) and lactic acid.
many molybdate complexes [21], accompanied by two ‘‘oxide 2 ions”; i.e., MoO24 = MoO2+ . Complexation of the alpha2 + 2O hydroxy carboxylic acids can be represented using these model ‘‘components” of molybdate ion. Deprotonation of both glycolic acid sites is accomplished by conversion of ‘‘oxide” to water, followed by chelation of the anion OCH2COO to MoO2+ 2 producing the octahedral 1 to 2 complex (Fig. 6 bottom). Neither scheme shown in Fig. 6 is meant to portray a mechanism, but merely to indicate how nucleophilic attack at the molybdenum core and the change in structure at molybdenum can be rationalized.
95
Mo NMR to determine structure of complexes
The geometries possible for the 1:2 complexes are shown in Fig. 7 for glycolic acid. The geometries for lactic acid are the same except that a chiral center exists at each alpha carbon. The three lactic acid complexes in solution clearly have slightly different environments at Mo, as do each of the four possible complexes shown in Fig. 7. The Mo environments in each of the trans-MoO2 complexes are likely similar as are those of the cis-MoO2 complexes [22]. The 13C NMR spectra also contain three peaks downfield from the single peaks for the CHOH and the COOH groups of the free lactic acid. The spectra also contain closely clustered peaks around the CH3 peak of the free ligand. Because the 13C NMR spectrum of the cis complex shown on the bottom left of Fig. 7 would contain two peaks for the CH and carboxyl carbons, whereas the spectra of all of the other geometric isomers would contain only one peak for the CH and one peak for the carboxyl carbons, it is possible that the mixture of isomers present in a 1 to 2 mol ratio of molybdate to ligand contains the two trans isomers as well as the cis isomer with the greater symmetry. However, the presence of the chiral center in lactic acid produces a number of diastereomeric complexes such as:
Fig. 6. Representation of complexation using MoO24 (top) and the MoO2+ 2 entity (bottom).
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Fig. 7. The four possible geometries for the 1:2 glycolic acid complex. The cis complex on the bottom right has a C2 symmetry axis bisecting the cis MoO2 group that makes the CH2 carbons equivalent. For the cis complex on the bottom left, the CH carbons in each of the two chelating ligands (as well as the CH2 carbons in each of the ligands) are not equivalent.
that could also have different 13C CH, C@O, and CH3 NMR shifts and also different 95Mo shifts [22]. The 95Mo spectra of the complexes of both glycolic and lactic acid contain a small, broad peak in the 20–35 ppm region, due possibly to a 1 to 1 complex or the presence of Mo7O624 , which at pH 5.7 produces two peaks, one at 34 ppm and a smaller peak at 210 ppm [21]. 95Mo NMR spectra of mixtures of molybdate and 3-hydroxypropanoic acid, which does not complex with MoO24 , also contain a small broad peak in this area. We tentatively assign the peak to a polymolybdate species, possibly Mo7O624 , which would be expected to form to some limited extent at this pH and molybdenum concentration [1]. Clearly, 95Mo NMR spectroscopy is capable of providing a great deal of information on the electronic surroundings of the Mo center of complexes, including fairly subtle structural and stereochemical nuances. However, for the complexes studied here there does not seem to be a simple relationship between the 95Mo chemical shift and the electron density at the Mo center. 4.4. Structural effects in alpha hydroxy molybdate complexes The explanation of the greater stability of the alpha hydroxy molybdate complexes requires an examination of the difference between the alpha and beta hydroxy ligands and their complexes. Simple molecular models reveal that there would likely be more bond angle distortion in the beta hydroxy complexes, giving rise to a more positive DH° for the formation of these complexes. The DS° of formation of the beta hydroxypropanoic acid complex relative to the alpha analog would also be unfavorable because of the greater internal rotational entropy of the free ligand and its diminution in the complex. The situation with salicylic acid is somewhat different in that the benzene ring makes the OH group more acidic by a factor of about 105 in aqueous solution, and the ligand has less internal rotational entropy than that of 3-hydroxypropanoic acid. Based on these observations, it appears that the lower stability of the beta hydroxy complexes is due primarily to the greater OAMoAO bond angles required in the octahedral geometry of the complexes with beta-hydroxy acids. The frequently studied molybdate chelates with 1,2-dihydroxybenezene (catechol) contain an aromatic ligand with two OH groups four to five orders of magnitude more acidic than the alpha OH group in glycolic acid, for example. Perhaps as importantly, the
catechol ligand is ideally suited for molybdate complexation both because of the planar orientation of its rigid OH groups and because there is little rotational entropy in the ligand. That the loss of rotation conformational entropy can be significant can be illustrated with the absolute entropie of gaseous hexane (S° = 388 J/K-mol) with that of gaseous cyclohexane (S° = 298 J/K-mol). The difference in absolute entropies of 90 J/K-mol is due primarily to the conformational (internal rotational) entropy of hexane that is lost in cyclohexane. At 298 K this difference would be equivalent to a difference in DG° of more than 25 kJ/mol (of course the difference is smaller for alpha hydroxy carboxylic acids). (A difference of about 12 kJ/mol produces more than a 100-fold difference in equilibrium constant.) Other Lewis acidic centers that form complexes with alpha hydroxy carboxylic acids include: Mo(V), which forms both monodentate complexes with HOCH2COO and bidentate chelated complexes containing the OCH2COO entity [23], VO2+ 2 with the bidentate OCH2COO ligand, and VO4+ with both chelating bidentate OCH2COO ligand and monodentate HOCH2COO [24], peroxovanadium(V) with bidentate glycolic acid [25], oxovanadium(IV) with bidentate alpha hydroxy carboxylic acid [26], and the diethyltin(II) cation with both alpha hydroxy- and thiohydroxy carboxylic acids [27]. A variety of metal ions have also functioned as the Lewis acid for complexation with alpha-hydroxy carboxylic acids: Cu(II) with monodentate, bidentate chelating, and the neutral carboxylic acid [28], Mn(II) with glycolic and lactic acids, as well as other acid bidentate anions [29], Ag(I) with monodentate glycolate [30], La(III), Ce(III), and Nd(III) with both bidentate chelating and monodentate chelating glycolate [31], Cm(III) and Cu(III) with mostly monodentate glycolate [32], and Th(IV) and a variety of lanthanides(III) with mostly monodentate glycolate [33]. 5. Conclusion We have shown that 95Mo NMR spectroscopy can be used to detect even subtle structural and stereochemical changes in molybdate complexes. 95Mo NMR resonances of the complex are downfield from the resonance for the molybdate ion. We attribute this downfield shift to the greater HOMO–LUMO gap in the octahedral complexes relative to the sp3-hybridized molybdate ion. These resonances are sensitive to small changes in the electronic environment of the Mo center and are therefore useful in determining the
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number of complexes in solution. Both 95Mo and 13C NMR spectra provide evidence for the existence of only one glycolic acid complex and three lactic complexes with molybdate. In our model complexes, the Lewis acidic molybdate ion prefers to form chelated complexes with ligands that contain two basic oxygen (or likely sulfur) sites located on adjacent carbon atoms. The 5-member rings that form as a result of this chelation are stable and their formation reduces the loss in rotational entropy, relative to larger rings, that accompanies the formation of complexes of most ligands. The monodentate (non-chelated) complexes bound to the molybdenum center only through either the COOH (at least with acetic acid and methoxyacetic acid) or the alpha-OH groups (at least with methyl glycolate) are not present in aqueous solution at pH 7. It is likely that the driving force for chelation is sufficient to cause deprotonation of the hydroxyl group, for which the Ka is presumably quite low (between 10 15 and 10 10). Acknowledgements The authors are indebted to the Camille and Henry Dreyfus Foundation for a Senior Scientist Mentorship award to C.H.Y. and to the Franklin & Marshall College Lucille and William Hackman Research stipends to E.L.C. This material is based upon work supported by the National Science Foundation under Grant No. 0923224. J.L.M. thanks Kelly Murphy for early research on this topic and acknowledges the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. References [1] O.F. Oyerinde, C.L. Weeks, A.D. Anbar, T.G. Spiro, Inorg. Chim. Acta 361 (2007) 1000. [2] X. Liu, J. Cheng, M. Sprik, X. Lu, J. Phys. Chem. Lett. 4 (2013) 2926. [3] T. Wichard, B. Mishra, S.C.B. Myneni, J.-P. Bellenger, A.M.L. Kraepiel, Nat. Geosci. 2 (2009) 625.
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