Article Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Theoretical Studies of the Formation Mechanisms, Thermodynamic Stabilities, and Water-Exchange Reactivities of Aluminum-Salicylate Complexes in Aqueous Solution Shaonan Dong, Wenjing Shi, Jing Zhang, and Shuping Bi* School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry of China and Key Laboratory of MOE for Life Science, Nanjing University, Nanjing 210023, China S Supporting Information *
ABSTRACT: The formation mechanisms, thermodynamic stabilities, and water-exchange reactivities of 1:1 monomer aluminum−salicylate (Al−salicylate) complexes in acidic aqueous solution are investigated using the density functional theory-quantum chemical cluster model (DFT-CM) method. (1) The formation pathways for possible monodentate and bidentate Al−salicylate configurations are modeled with the gas phase-supermolecule-polarizable continuum model (GP-SMPCM). It shows that the formation pathways for the Al− salicylate complexes follow the Eigen-Wilkins mechanism, where the dissociation of an inner-shell coordinated water of Al3+ is the rate-determining step. (2) The formation constants Kaq for different Al−salicylate configurations are estimated based on the total Gibbs free energy changes ΔG° for their overall formation pathways. It is indicated that in the acidic aqueous solution at pH ∼ 3, the main existence form of the 1:1 monomer Al−salicylate complex is the phenol-deprotonated bidentate Al(Sal)(H2O)4+ with six-membered ring. Its log Kaq is calculated as 13.8, in good agreement with the literature values of 12.9−14.5. (3) The water-exchange reactions are modeled for different Al−salicylate configurations. The water-exchange rate constant for Al(Sal)(H2O)4+ is estimated as log kH2O = 3.9 s−1, close to the experimental value of 3.7 s−1. It proves again that this configuration is the dominant form under experimental conditions. KEYWORDS: aluminum, salicylate, complex formation, Eigen-Wilkins mechanism, density functional theory similar functional groups.7 The Al−salicylate complexes formed by Al3+ and the salicylate anions (HSal− or Sal2−) are typical species in studying the Al−organic acid complexes. They have been extensively studied by experiments, that is, the macroscopic formation rate constant, thermodynamic stability constant, and water-exchange reaction constant for the 1:1 Al−salicylate complex in weakly acidic (pH ∼ 3) aqueous solution have been measured with stopped-flow method, laser fluorescence spectroscopy, potentiometric titration, and 17O NMR (see S1 in Supporting Information).8−12 However, the dominant existence form of the Al−salicylate complex still remains controversial, as both the monodentate and bidentate configurations have been argued to be the main species.13−18 The key point lies in the difficulty in resolving whether the phenol group is deprotonated or not in the Al−salicylate complex by using available experimental approaches. The microscopic molecular formation kinetics of different Al− salicylate configurations including the elementary reaction steps, reaction sites, and reactant geometries, as well as their
1. INTRODUCTION The acidification of soil solutions caused by acid rain leads to elevated concentrations of aluminum (Al) has serious toxic effects on forests, crops, animals, and humans. The binding of Al by dissolved natural organic matter (NOM) in natural waters and soils not only can alleviate Al toxicity, affect the solubility, mobility, and bioavailability of this metal, but also can influence the fate and bioavailability of organic contaminants in soils and sediments that preferentially associate with NOM.1−3 Environmental chemists are broadly concerned about the formation mechanisms, thermodynamic stabilities, and dynamic reactivities of the Al−organic complexes due to the fundamental importance of these three issues in controlling the fate and behaviors of the Al species in natural environments. A series of experimental techniques have been used to explore the Al−organic chemistry, such as NMR, fluorescence, IR, and UV-Raman spectra and EXAFS.4−6 However, it is impossible to comprehend the microscopic mechanism of thermodynamics and dynamics by one or several experimental techniques due to the low environmental concentrations, rapid reactions, and complicated systems. For instance, the salicylic acid (H2Sal) is a common low-molecular-weight organic acid in nature and is usually used as a simple model for the complex high-molecular-weight fulvic and humic acids due to their © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
December 9, 2017 February 12, 2018 March 8, 2018 March 8, 2018 DOI: 10.1021/acsearthspacechem.7b00141 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry Scheme 1. Formation Pathways of the Monodentate and Bidentate Configurations of the 1:1 Monomer Al−Salicylate Complexes in Aqueous Solution
Figure 1. Optimized geometries of the outer-sphere ion pair and TS in the monodentate and bidentates formation pathways (Nm′ = 4 + 1 for monodentate formation, the stick model for the explicit solvent water molecule of HSal− is labeled gray; Nm′ = 4 for bidentates formation; the dissociating O atoms are labeled white; the attacking O atoms are labeled black).
employed to provide useful information on the molecular structures and microscopic water-exchange reaction kinetics of Al−salicylate complexes except for their microscopic formation mechanisms.16,17,21 In this work, the molecular formation mechanisms, thermodynamic stabilities, and water-exchange reactivities of the Al−salicylate system are studied with the density functional theory−quantum chemical cluster model (DFT-CM) method. The purpose is to show how the three fundamental issues including the formation mechanisms, thermodynamic stabilities, and dynamic reactivities in studying
water-exchange reactivities have not been clearly distinguished either. Nowadays, the lack of a uniform and normative research method that is feasible for comprehensively investigating the chemistry of certain Al−organic systems has become the bottleneck of the deeper understanding of the environmental influences of these Al−organic species.19 One promising route for solving the problems encountered in experiments is the use of quantum chemical methods, such as the quantum chemical cluster model (CM) method.20 This method has been B
DOI: 10.1021/acsearthspacechem.7b00141 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Article
ACS Earth and Space Chemistry
Table 1. Gibbs Free Energy Changes in the Monodentate and Bidentates Formation Pathways and the Estimated Reaction Rate Constants reaction steps step-I, outer-sphere ion pair formation step-II, Al3+ inner-shell dehydration step-III, Φ-CO attacking step-IV, monodentate dehydration step-V, Φ-CO/Φ-OH attacking
reaction sites
ΔG298,a‡a (kJ/mol)
ΔG298,r or ΔG298,r‡b (kJ/mol)
log kTSTc (s−1)
log kH2Od (s−1)
log kH2O(expt)e (s−1)
log k(expt)f (s−1)
−0.1
0.1
−0.8
−47.8
cis to HSal− trans to HSal− Φ-CO, cis Φ-CO, trans Φ-OH, cis Φ-OH, trans
67.6
66.6
0.9
9.9 64.1 68.8 13.4 4.9
−92.2 54.3 56.1 −42.6 −38.2
11.1 1.6 0.7 10.4 11.9
16.8 5.3
−48.5 −53.0 −11.3
9.8 11.9
step-VI, Φ-OH deprotonation
0.6 −0.3
Gibbs free energy differences between TS and R in the ligand-exchange reactions. bΔG298,r: free energy difference between the ion pair [Al3+·HSal−] and the sum of the free ions Al3+ and HSal−. ΔG298,r‡: free energy differences between P and R in the ligand-exchange reactions. cTransition state rate constants for ligand-exchange reactions. dCalculated dehydration reaction rate constants, kH2O = κH2OkTST; κH2O: transmission coefficient, κH2O = 0.1, ref 27. eExperimental water-exchange reaction rate constant for Al3+, ref 28. fExperimental rate constant for inner-sphere Al−salicylate complex formation, ref 8. a
salicylate complex formation mechanism suggested by Secco and Venturini,8 the overall calculated formation pathways for the monodentate and bidentate Al−salicylate configurations are shown in Scheme 1. The free Al3+ and HSal− first form an outer-sphere ion pair, followed by dissociative ligand-exchange reaction pathways to form monodentate and bidentate configurations, consistent with the Eigen-Wilkins mechanism.26 The dissociation ligand-exchange reaction mechanism will be called “D mechanism”. The whole pathway includes six elementary reaction steps: (I) formation of the outer-sphere ion pair; (II) dissociation of an inner-shell coordinated water of Al(H2O)63+; (III) attacking of the carboxyl (Φ-C−O) O atom of HSal− onto the pentacoordinated Al3+; (IV) dehydration of the monodentate; (V) attacking of the carbonyl (Φ-CO) or phenol (Φ-OH) O atoms of HSal− onto the pentacoordinated Al3+; and (VI) deprotonation of the phenol group. In monodentate Al(HSal)(H2O)52+, there are four inner-shell waters adjacent to HSal− (denoted as cis waters) and one innershell water in the para-position of HSal− (denoted as trans water). Thus, in the monodentate dehydration reaction (StepIV), there are two water dissociation sites, cis and trans sites. Correspondingly, after the monodentate dehydration two pentacoordinated trigonal−bipyramidal intermediates can be obtained in which HSal− locates in the axial or the equatorial positions, respectively. Then, bidentates Al(HSal)(H2O)42+ with four- or six-membered rings (denoted as Bidentate-I, and -II) can be formed as the HSal− ligand (Φ-CO or ΦOH) attacks the pentacoordinated Al3+ from the axial or equatorial directions. In Bidentate-II, the phenol proton can further dissociate to form the phenol-deprotonated bidentate configuration (denoted as Bidentate-III). Figure 1 lists the optimized geometries of the outer-sphere ion pair and the TS in the monodentate and bidentates formation pathways. Table 1 lists the Gibbs free energy changes and the estimated reaction rate constants for the reaction steps. The geometries and structural parameters of other reaction species are shown in S3 in Supporting Information. 3.1.2. Formation Mechanism of the Monodentate (Step-I, -II, and III). Formation of the Outer-Sphere Ion Pair (Step-I). The free ion clusters of Al(H2O)63+ and HSal− approach each
the Al−organic complexes can be systematically and effectively elucidated using the quantum chemical calculation method and to provide useful references for studying the chemistry of other environmentally important metal−organic systems in both homogeneous phase and heterogeneous interfaces and in biological systems.
2. COMPUTATIONAL METHOD The formation pathways for different configurations of the 1:1 monomer Al−salicylate complexes from Al3+ and HSal− in weakly acidic (pH ∼ 3) aqueous solution as well as their waterexchange reactions are modeled using the DFT-CM method. All DFT-CM calculations are performed using Gaussian 03 suite of programs.22 Referring to our previous studies,19,21 the gas phase-supermolecule-polarizable continuum model (GPSM-PCM) is used to describe the short-range explicit solvent effects and the long-range bulk solvent effects. Four explicit solvent waters are mainly considered for the Al species (Nm′ = 4) and one additional explicit solvent water is added for the free HSal− in constructing the GP-SM clusters. All the geometry optimizations, frequency analysis and thermodynamic parameter calculations of the free ions, ion pairs, and the reactants (R), transition states (TS), and products (P) in the ligandexchange reactions are conducted in PCM.23 The geometry optimizations and frequency analysis are carried out at the B3LYP/6-311+G(d,p) level,24,25 whereas the single-point energies are calculated using the MP2 method to obtain accurate energy. The Gibbs free energies G298 of the reaction species are calculated based on the single-point energies by adding zero-point energies, thermal corrections, and entropy corrections. The activation Gibbs free energies are denoted as ΔG298,a‡, and the reaction free energies are denoted as ΔG298,r‡ (with TS) or ΔG298,r (without TS). The detailed calculation procedures are the same as in our previous studies19,21 and can also be found in S2 in Supporting Information. 3. RESULTS AND DISCUSSION 3.1. Formation Mechanisms of the 1:1 Monomer Al− Salicylate Complexes. 3.1.1. The Monodentate and Bidentates Formation Pathways. Referring to the Al− C
DOI: 10.1021/acsearthspacechem.7b00141 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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The finally formed monodentate Al(HSal)(H2O)52+ has a Gibbs free energy of 73.4 kJ/mol lower than the sum free energy of the free ions Al(H2O)63+ and HSal−, indicating that the formation of the monodentate configuration is thermodynamically favorable. 3.1.3. Formation Mechanism of the Bidentates (Step-IV, -V and -VI). Monodentate Dehydration (Step-IV). The dissociation processes of the inner-shell coordinated waters cis or trans to the HSal− ligand in the monodentate Al(HSal)(H2O)52+ are similar to our previously modeled water-exchange reactions of the monodentate Al−salicylate complexes (see S3.2 in Supporting Information).21 After the dehydration reactions begin, the leaving waters move far away the central Al3+ and form an H-bond with a neighboring inner-shell coordinated water. From RIV to TSIV, the distances R(Al−OH2)L between Al3+ and the leaving waters lengthen from 1.95−1.96 Å to 2.84−2.99 Å and further lengthen to 3.69−3.88 Å in PIV where the leaving waters move into the second hydration shells. The pentacoordinated PIV are approximate trigonal bipyramids. When the cis water is dissociated, the HSal− ligand locates in the axial position in the trigonal bipyramidal. When the trans water is dissociated, the HSal− ligand locates in the equatorial plane. The ΔG298,a‡ for the cis and trans water dissociations are 64.1 and 68.8 kJ/mol, and the log kH2O are 0.6 and −0.3 s−1, respectively, indicating that both are close each other. The ΔG298,r‡ for the water dissociation reactions at the two sites are 54.3 and 56.1 kJ/mol, also close to each other. The calculation results are similar to our previous modeling results.21 Formation of the Bidentate with Four-Membered Ring (Bidentate-I) (Step-V). After the monodentate dehydrates, the carbonyl (Φ-CO) O atom of the HSal− attacking the pentacoordinated Al3+ from the axial or equatorial directions can form the four-membered chelate ring structure. In RV, the distances R(AlL) between Al3+ and the incoming carbonyl O atoms are about 3.24 Å. In TSV, the distances R(AlL) shrink to 2.64−2.94 Å. In PV, the carbonyl O atoms bind with Al3+ and the four-membered chelate ring forms. In the chelate ring, the two AlO bonds are 1.97 and 1.92 Å, and the bond angles ∠OAlO and ∠OCO are 67° and 114°, respectively. The ΔG298,a‡ for the axial and equatorial direction reactions are 13.4 and 4.9 kJ/mol, and the ΔG298,r‡ are −42.6 and −38.2 kJ/ mol, respectively. Formation of the Phenol-Protonated Bidentate with SixMembered Ring (Bidentate-II) (Step-V). In the trigonal bipyramid formed by monodentate dehydration, the binding between Al3+ and the phenol (Φ-OH) O atom leads to the formation of the six-membered chelate ring structure. Similar to the Bidentate-II formation pathway, Φ-OH can also attack the pentacoordinated Al3+ from the axial or equatorial directions. In RV, the distances R(AlL) between the incoming phenol O atoms and Al3+ are 3.78−3.96 Å. In TSV, these distances shorten to 3.06−3.10 Å. In PV, the phenol O atoms are bonded to Al3+ and the six-membered chelate rings are formed. In the chelate ring, the bond lengths between Al3+ and the carboxyl and phenol O atoms are about 1.81 and 1.97 Å, respectively, and the bond angle ∠OAlO is about 87°. The ΔG298,a‡ for Φ-OH attacking the central Al3+ from the axial and equatorial directions are 16.8 and 5.3 kJ/mol, respectively, close to the ΔG298,a‡ for the Φ-CO attacking cases. The ΔG298,r‡ are −48.5 and −53.0 kJ/mol, respectively. Formation of the Phenol-Deprotonated Bidentate with Six-Membered Ring (Bidentate-III) (Step-VI). It is conventionally viewed that the formation of the six-membered chelate ring
other by fast diffusion and form the outer-sphere ion pair Al(H2O)63+·HSal− by electrostatic effect. In Al(H2O)63+, the asymmetric arrangement of the four explicit solvent waters in the second hydration shell makes the Al3+ inner-shell irregularly octahedral in which six AlOH2 bond lengths are 1.89−1.94 Å. The two carboxyl O atoms in HSal− are equivalent and are equally negatively charged. Thus, one O atom is chosen as the incoming group that will replace an inner-shell coordinated water of Al3+, and the other O atom is H-bonded with the surrounding explicit solvent water. In the ion pair cluster of [Al(H2O)63+·4H2O, HSal−·H2O], the distance R(AlL) between the incoming carboxyl O atom of HSal− and the Al3+ is 3.54 Å. The incoming carboxyl O atom forms two Hbonds with two inner-shell coordinated waters. The explicit solvent water added for HSal− locates between Al3+ and HSal− and forms two H-bonds with an inner-shell coordinated water of Al3+ and the carbonyl O atom of HSal−. The formation of the outer-sphere ion pair Al3+·HSal− by electrostatic interaction between the free ions Al3+ and HSal− reduces the Gibbs free energy of the reaction system by 47.8 kJ/mol. Outer-Sphere Ion Pair Dehydration (Step-II). One innershell coordinated water of Al3+ in the outer-sphere ion pair dissociates. From RII to TSII, the distance R(Al−OH2)L between Al3+ and the leaving water lengths is from 1.94 to 2.99 Å. During the dehydration, the leaving water forms a Hbond with an neighboring inner-shell coordinated water. In PII, the distance R(Al−OH2)L further lengths to 4.00 Å, and the leaving water moves into the second hydration shell, while the inner-shell of Al3+ transforms into an approximate trigonal bipyramid. The dehydration process is similar to previously modeled Al(H2O)63+ dehydration pathways29 and the HSal− remains in the second hydration shell of Al3+ at a distance of about 3.5 Å during the whole dehydration. The ΔG298,a‡ for Al3+ dehydration is 67.6 kJ/mol, close to the ΔG298,a‡ (68.9 kJ/ mol) for the dehydration reaction of Al(H2O)63+ that modeled using the same basis set and solvation model (see S3.1 in Supporting Information). This suggests that the existence of HSal− in the second hydration shell of Al3+ has little influence on the dehydration reaction of Al3+. The log kH2O for step-II is estimated as −0.1 s−1, close to the experimental log kH2O(expt) (0.1 s−1) for the water-exchange reaction of Al3+ measured by 17 O NMR28 and also close to the rate constant log k(expt) (−0.8 s−1) for HSal− penetrating into the Al3+ inner-shell, measured by Secco et al. using the stopped-flow method.8 Formation of the Monodentate (Step-III). The incoming carboxyl O atom of HSal− coordinates to Al3+ from the axial direction in the trigonal bipyramid. From RIII to TSIII, the inner-shell of the trigonal bipyramid undergoes rearrangement. In TSIII, the incoming carboxyl O atom of HSal− locates in the equatorial plane of the trigonal bipyramid, and the distance R(AlL) shortens to 2.40 Å. In PIII, the hexacoordinated monodentate Al−salicylate complex is formed, and the incoming carboxyl O atom is bonded to Al3+ with a AlO bond length of 1.83 Å. The ΔG298,a‡ for carboxyl O atom attacking the penta-coordinated Al3+ is 9.9 kJ/mol and the log kTST is about 11.1 s−1. This ΔG298,a‡ is lower than the ΔG298,a‡ (19.3 kJ/mol) for H2O attacking Al(H2O)53+ (see S3.1 in Supporting Information), showing that the negatively charged HSal− is more nucleophilic than the neutral water molecule. Among the three reaction steps for monodentate formation, the dehydration of the Al3+ (step-II) and Φ-C−O attacking (stepIII) constitute the rate-determining step of the whole D mechanism,30 consistent with Secco et al. and Wang et al.8,9 D
DOI: 10.1021/acsearthspacechem.7b00141 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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ACS Earth and Space Chemistry is accompanied by the phenol proton dissociation.13 The proton dissociation pathway is not found in modeling the ring closure process, but the phenol proton is observed to move into adjacent solvent water molecules during the TS optimizations for the phenol deprotonation in Bidentate-II (see S3.3 in Supporting Information). Therefore, it is proposed that Bidentate-III can be formed from Bidentate-II by phenol proton dissociation. 3.2. Thermodynamic Stabilities and Water-Exchange Reactivities of the Al−Salicylate Complexes. 3.2.1. Phenol Proton Morphology in the Six-Membered Ring. Because the phenol deprotonation pathway for Bidentate-II (step-VI) is not obtained, the stability of Bidentate-III can not be inferred based on the deprotonation energy change. In fact, this is exactly the key issue in determining the main existence form of the Al− salicylate complex in weakly acidic aqueous solution. In conventional viewpoints, the bidentate Al−salicylate complex in aqueous solution mainly exists in the form of the phenoldeprotonated Bidentate-III. The experimental evidence come from two aspects. First, it is supported by the results of electrochemical experiments, NMR, and fluorescence spectra;9,11,14,15 Second, it is found that the solution pH decreases when mixing the Al3+ and HSal− solutions, which is believed to be caused by release of protons during the Al−salicylate complex formation processes.13 However, this experimental evidence can not strongly exclude the existence of other configurations. Here, the pKa for the acid dissociation of the phenol proton in Bidentate-II is estimated on the basis of the calculated deprotonation Gibbs free energy change ΔGH°. To ensure the reliabilities of the calculation results, two forms of deprotonation equations, eqs 1 and 2, are employed.31,32 The pKa values are estimated using equation pKa = ΔGH°/ 2.303RT.31,32
configuration (Figure 2). When a reaction step consists more than one possible reaction paths (e.g., the formation pathway of
Figure 2. Relative Gibbs free energy curves of the formation pathways for different Al−salicylate complexes.
Bidentate-I and -II), the dominant path with the lowest activation energy barrier is selected. According to this principle, the ΔG° of the outer-sphere ion pair and the monodentate are −47.8 and −73.4 kJ/mol, respectively. The ΔG° for monodentate transforming into Bidentate-I and -II through cis ligand-exchange reactions are 11.7 and 5.8 kJ/mol, respectively. The ΔGH° for the phenol deprotonation in Bidentate-II to form Bidentate-III is selected as −11.3 kJ/mol. Table 2 lists the ΔG° and the estimated log Kaq values for different configurations. Table 2 shows that the phenol-deprotonated bidentate with six-membered ring (Bidentate-III) has the largest log Kaq (13.8), which is in good agreement with the experimental log Kaq values of 12.9−14.5 for the 1:1 Al−salicylate complexes.10 For other configurations, the log Kaq values from large to small in turn are monodentate (12.9), Bidentate-II (11.8), BidentateI (10.8), and outer-sphere ion pair (8.4). Secco et al. and Wang et al. proposed that under the experimental condition of solution pH ∼ 3, the monodentate forms as an intermediate.8,9 On the basis of our calculated log Kaq values, the monodentate, Bidentate-I and -II configurations also have high thermodynamic stabilities, thus it is expected that they may also exist in aqueous solution, probably in small amounts, which may be the cause of the controversy on the stable form of the Al−salicylate complex in aqueous solution under different experimental methods and conditions. In previous studies of the Al−salicylate complexes, more attention has been paid on the monodentate and the BidentateIII. Less consideration for the bidentate with the fourmembered ring may be due to the large tension in the fourmembered ring. However, from our estimated log Kaq values, the stability differences between the monodentate and the bidentates with four- and six-membered rings are not very large. It has been proved by experiments and theoretical simulations that the four-membered ring structures formed by Al3+ or other metal ions with the two carboxyl O atoms of carboxylic acids can exist stably in aqueous solution.36 On metal oxide mineral or material surfaces, the bidentate or bridging structures formed by the two carboxyl O atoms of carboxylate ligands bonding onto one or two surface metal sites have also been demonstrated as important surface coordination forms.18 Similarly, the outer-sphere ion pair species, which has lower stabilities and less influences on the Al reactivities, may play
Al(HSal)(H 2O)4 2 + + H 2O → Al(Sal)(H 2O)4 + + H3O+ (1)
Al(HSal)(H 2O)4 2 + → Al(Sal)(H 2O)4 + + H+
(2)
The Gibbs free energy difference between Al(HSal)(H2O)42+ (Bidentate-II) and Al(Sal)(H2O)4+ (Bidentate-III) is calculated as 1127.5 kJ/mol, whereas the Gibbs free energy difference between H3O+ and H2O is calculated as −1135.4 kJ/mol (calculation details are shown in S4 in Supporting Information). Consequently, ΔGH° = −7.9 kJ/mol and pKa = −1.4 are obtained for eq 1. For eq 2, the reported Gibbs free energy Ggas(H+) = −6.28 kcal/mol for gas phase proton and the proton solvation free energy Gsol(H+) −265.9 kcal/mol are used,31,32 and ΔGH° = −11.3 kJ/mol and pKa = −2.0 are obtained for eq 2. The two estimated pKa values are close to each other, proving the reliabilities of the calculation results. The negative pKa values imply that the phenol proton in Bidentate-II can dissociate in acidic aqueous solution, which can be viewed as metal-catalyzed proton transfer.33 3.2.2. Thermodynamic Stabilities of Different Al−Salicylate Configurations. In order to clarify the main existence form of the Al−salicylate species under experimental conditions, the thermodynamic formation constants Kaq of different Al− salicylate configurations are estimated using equation log Kaq = −ΔG°/2.303RT, based on the total Gibbs free energy changes ΔG° of their overall formation pathways.17 Similar to those reported in literature,34,35 for each configuration, ΔG° is obtained by adding the Gibbs free energy changes of all reaction steps in the overall formation pathway of the E
DOI: 10.1021/acsearthspacechem.7b00141 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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Table 2. Formation Constants Kaq and Water-Exchange Reaction Rate Constants kex for Different Al−Salicylate Configurations water-exchange rate constants kexa
formation constants Kaq
ΔG298,a° (kJ/mol)
log kTST (s−1)
log kH2Oc (s−1)
8.4
67.6
0.9
−0.1
−73.4
12.9
64.1
1.6
0.6
−61.7
10.8
59.0
2.5
1.5
−67.6
11.8
64.2
1.5
0.5
−78.9
13.8
44.9
4.9
3.9
ΔG° (kJ/mol)
log Kaq
Al(H2O)63+ + HSal− → Al(H2O)63+·HSal−
−47.8
Al(H2O)63+ + HSal− → Al(HSal)(H2O)52+ + H2O Al(H2O)63+ + HSal− → Al(HSal)(H2O)42+ + 2H2O
b
species Al(H2O)63+·HSal− (outer-sphere ion pair) Al(HSal)(H2O)52+ (monodentate) Al(HSal)(H2O)42+ (bidentate-I) Al(HSal)(H2O)42+ (bidentate-II) Al(Sal)(H2O)4+ (bidentate-III)
formation equations
Al(H2O)63+
−
+ HSal → Al(HSal)(H2O)4
2+
+ 2H2O
Al(H2O)63+ + HSal− → Al(Sal)(H2O)4+ + 2H2O + H+
log Kaq(expt)
12.9−14.5d
log kex(expt) (s−1)
3.7e
Parameters for monodentate and bidentates denote the dehydration of coordinated waters cis to salicylate. bΔG° are the sum of the Gibbs free energy changes for all reaction steps in the pathways forming different Al−salicylate configuration from the free ions Al(H2O)63+ and HSal−. ckH2O = κH2OkTST (κH2O = 0.1), ref 27. dReference 10. eReference 12. a
surface metal sites significantly affects the mineral weathering and diagenetic processes.44−47 Although the surface speciation has been intensely studied in the past, the adsorption kinetics studies are limited and less successful due to the inherent difficulties in data collection and analysis.9 In the past 20 years, an alternative approach has developed whereby surface reaction are studied with nanometer-sized (1−2 nm) aqueous inorganic nanoscale clusters (e.g., Keggin-Al137+) that sever as molecular model for the more complicated oxide mineral interfaces.48,49 With these nanoscale molecular models, the surface adsorption kinetics and ligand-promoted mineral dissolution mechanisms can be probed via the DFT-CM simulations.50−53 2. Implications in biological systems. It is known that Al can cause neurotoxicity in humans and animals and is related to neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis.54 A recent study suggests that Al ions can induce the formation of backbone ring structures in a wide range of peptides through simultaneous coordination to amide nitrogen and carbonyl O atoms on the backbone.55 These ring structures destabilize the protein and result in irreversible denaturation. This particular behavior of the Al ion is not observed for other ions (Na+, K+, Mg2+ and Ca2+), indicating that the ability of Al ions to form chemical bonds with electronegative elements is critical in its ecotoxicological effects. The finding that electrostatic contribution is significant in Al-biomolecule binding is similar to the viewpoints by Washel et al. that the electrostatic preorganization plays key role in enzyme catalyzed reactions.56 One parameter that can be used to measure the electrostatic attraction of a metal ion is its hydration free energy. As predicted by the Born model,57 the large positive charge (+3) and small radius (54 pm) of the Al3+ ion lead to very large hydration free energies. For Al3+, our calculated hydration free energy is −4506.6 kJ/mol, in agreement with the calculated values in literature (e.g., −4523.0 kJ/mol) and the experimental data (e.g., −4525.0 kJ/mol) (see S6 in the Supporting Information for details).58,59 This large hydration free energy of Al3+ implies that Al3+ can bind strongly with biomolecules, such as amino acids, peptides, proteins, and nuclei acids, and thus present biotoxicity. The replacement of hydration waters of the metal ions by biomolecules are proposed to follow similar Eigen-Wilkins mechanism as the formation of aqueous Al−salicylate complexes, where the desolvation abilities of metal ions have been proven closely related to their bioaccessibility.37 Like the aqueous Al−salicylate complexes, the binding of Al ions with biomolecules also meets the issues
important roles in the transformations and migrations of Al species when the ligands are weak anions such as the acetate. 3.2.3. Water-Exchange Reactivities of Different Al− Salicylate Configurations. The elementary water-exchange reaction of metal complexes is known to be fundamental in understanding their reactivities in chemical and biological systems, since it is the initial step in the formation and transformation of the metal species.37,38 For the water-exchange reactions of Al-organic acid complexes, the dissociation of an inner-shell coordinated water is the rate-determining step.19,21 Table 2 lists the estimated dehydration rate constants kH2O for the ion pair, monodentate and bidentate configurations of the Al−salicylate species, while the corresponding geometries and energy parameters of the reaction species are shown in S5 in Supporting Information. The log kH2O for the outer-sphere ion pair, monodentate, Bidentate-I, Bidentate-II and Bidentate-III are −0.1, 0.6, 1.5, 0.5, and 3.9 s−1, respectively, indicating that different Al−salicylate configurations have different waterexchange reactivities. Comparing with the calculated log kH2O(expt) = −0.3 s−1 for the dehydration reaction of Al(H2O)63+, it is obvious that the coordination of the salicylate ligand onto Al3+ enhances the reactivities of the Al3+ inner-shell waters. The log kH2O for Bidentate-III (3.9 s−1) is the closest to the experimental value of log kex(expt) = 3.7 s−1 for the waterexchange reaction of 1:1 Al−salicylate complex in aqueous solution measured with 17O NMR.12 This proves again that the Bidentate-III is the main existence form of the Al−salicylate complexes in weakly acidic aqueous solution. 3.3. Implications in Studying Environmentally Significant Metal−Organic Complexes. The results in this study have important environmental significances: 1. Implications in geochemistry. By extending the DFT-CM method to the studies on other NOM (e.g., carboxylates, phenols, and amino acids) or inorganic ligands (e.g., phosphates, sulfates, nitrate and silicates) binding with Al3+ or other environmentally important metal ions (e.g., Fe3+, Cr3+, Ca2+ and Mg2+), it will be helpful in solving the fundamental environmental chemistry problems such as the morphologies, transformation, and migration mechanisms of typical metal complexes in the natural “water−soil−atmosphere−biology” cycles at the molecular level.39−43 The DFT-CM method can also be applied in studying the surface complexation chemistry. It is believed that the adsorption mechanism for the aqueous organic acid anions and inorganic oxyanions on hydrous metal oxides is conceptually analogous to the aqueous phase EigenWilkins mechanism.9 The coordination of organic acids onto F
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quantum mechanical mixing of different states of solute.69 One practical application example is that in aqueous solution, mineral−water interfaces, and biological enzyme systems, the proton and electron transfer reactions (PT/EP) play important roles in the morphology and reactivity changes of the metal− organic complexes.70−72 Quantum mechanical tunneling is always significant in these reactions, and it is often noticed in large values of H/D kinetic isotope effects.73 The vibrational transition state theory (VTST), MD simulations, and QM/MM methods have been used to evaluate the multidimensional tunneling kinetics in enzyme reactions.74 In order to deal with the challenge of quantizing nuclear motion by using path intergration, it is necessary to combine these computational methods in the dynamic PT/ET reaction modeling and the static pKa estimation.
concerning various binding sites, steric configurations, and even chiral isomerism.60 Our modeled formation mechanisms, thermodynamic stabilities, and water-exchange reactivities of different aqueous Al−salicylate complexes provide good reference for studying the binding of Al ions with biomolecules, and this will be useful in further revealing the toxicological of Al at the molecular level. 3. Applications of combined computational methods. The combinations of Al with natural ligands in aqueous solution have different coordination environments from those on mineral−water interfaces or biological systems. The influence factors include the temperatures, cation coordination structures, solvent polarities, ligand concentrations, and so forth. Modeling the metal−ligand complexes under certain coordination environment requires suitable computational methods. Because of the scope of application of various methods, it may be difficult to obtain complete thermodynamic or kinetic information for a certain system by using a single computational method. The combination of different computational methods would allow one to obtain more comprehensive information and deeper understanding of the studied systems. For example, the CM method can be used in modeling any desired pathway in a straightforward manner, and its strength is that exact geometries and electronic energies of the reactants, transition states, and products can be computed independent of the reactivity of the system.20 The CM method is always combined with implicit sovlation models in simulating condensed phases. For reactions in aqueous solution or organic environments, the geometries and energies of the reaction species are significantly influenced by solvent effects in particular by the dipole properties of the solvent molecules. In addition to the PCM applied in this study, there are many other available solvation models, such as the Langevin dipoles (LD) model and the conductor-like shielding model (COSMO).61,62 Comparing with the continuum models, the LD model may allow one to gain clearer insight into the molecular origin of different solvent effects.61 It would be useful to take CM calculations combined with different implicit solvation models to make comparisons and cross-validations. Its disadvantage is that only a finite number of configurations and reaction pahtways can be simulated without accurate treatment of the thermal averaging issues. Additionally, due to the limited computational power, only a few dozens of atoms can be quantum mechanically treated in the CM clusters under specified precision requirements. These drawbacks make the CM method alone difficult to study the metal−organic complexes in real mineral−water interface or biological systems because modeling bulk mineral or large biomolecules needs to include a large number of atoms. For the metal−organic complexation chemistry under the complicated mineral surface or biological conditions, other computational methods such as the periodic simulations, combined quantum mechanical and molecular mechanical (QM/MM) and molecular dynamics (MD) simulations are very useful tools.53,56,63−67 For instance, in modeling the enzyme reactions the multiscale QM/MM simulation method allows the entire solvated enzyme system to be treated at the atomic level, where QM provides appropriate description of the active chemical bond rearragements and key solvent molecules and MM treats a large fraction of the protein environment and the bulk solvent, to reproduce proper configurations and corresponding activation barriers.68 The empirical valent bond model (EVB) and other QM/MM methods allow one to capture the effect of the solvent on the
4. CONCLUSION The complexation chemistry for possible monodentate and bidentate Al−salicylate configurations are studied with the DFT-CM method. The calculation results indicate that the formation pathways for the monodentate and bidentate Al− salicylate complexes follow the Eigen-Wilkins mechanism. The bidentate Al(Sal)(H2O)4+ is proposed to be the main existence form of the 1:1 monomer Al−salicylate complex in acidic aqueous solution at pH ∼ 3. The estimated formation rate constant, thermodynamic stability constant, and waterexchange reaction rate constant for the Al(Sal)(H2O)4+ complex are in good agreement with the experimental data. This work provides new understanding in the environmental chemistry of the aqueous Al−salicylate system. The longstanding problem of the dominant existence form of the Al− salicylate complex in weakly acidic solution is solved. The various formation pathways and water-exchange rate constants for different Al−salicylate configurations provide a good example of the significant influence of organic acid ligand complexation on the Al morphology and reactivity, and lay the foundation for the study on environmental biological effects of the Al−organic acid complexes as well as their practical applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsearthspacechem.7b00141. (S1) Summarization for the stability constants of aqueous Al−salicylate complexes in literature. (S2) Computational procedures and solvation model tests. (S3) Formation pathways for the aqueous Al−salicylate complexes. (S4) Acid dissociation pKa of the phenol proton in Bidentate-II. (S5) Water-exchange reactions of the bidentates. (S6) Hydration free energy of Al3+. (S7) Cartesian coordinates of all reaction species in the formation and water-exchange reaction pathways for the Al−salicylate complexes (Å) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: 011-86-025-86205840. Fax: +011-86-025-83317761. E-mail:
[email protected]. ORCID
Shuping Bi: 0000-0003-4437-7769 G
DOI: 10.1021/acsearthspacechem.7b00141 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project is supported by the National Natural Science Foundation of China (No. 21177054). We are grateful to the High Performance Computing Center of Nanjing University for doing the numerical calculations in this paper on its Blade cluster system.
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