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Feb 23, 2010 - Electrospray Mass Spectrometry Studies of Purified Aluminum Tridecamer in a 50:50 Water/Acetonitrile Mixture. Ya-Fan Lin and Duu-Jong L...
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J. Phys. Chem. A 2010, 114, 3503–3509

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Electrospray Mass Spectrometry Studies of Purified Aluminum Tridecamer in a 50:50 Water/Acetonitrile Mixture Ya-Fan Lin and Duu-Jong Lee* Department of Chemical Engineering, National Taiwan UniVersity, Taipei, Taiwan 10617 ReceiVed: December 22, 2009; ReVised Manuscript ReceiVed: February 3, 2010

This study conducted electrospray ionization mass spectrometry (ESI-MS) tests on 90-95% pure [AlO4Al12(OH)24(H2O)12]Cl7 (Al13) salt dissolved in a 50:50 v/v water/acetonitrile solution. Acetonitrile typically replaces the ligated aqua ligands and adducts to the aluminum center, leading to aluminum tridecamer breakdown. The chloride anions cannot coordinate with the aluminum center in the presence of acetonitrile molecules. The acetonitrile deconstructs the “cagelike” aluminum tridecamer structure into smaller polymeric species, which are further transformed into a brucite-like structure. The present work obtained the mass spectra of incremental difference of 18u for fresh solution and of 9u for aged solution in the m/z ) 200-900 regime, providing support to the occurrence of water ligand exchange reaction during aluminum tridecamer decomposition. Introduction Aluminum speciation has been probed using electrospray ionization mass spectrometry (ESI-MS) at different solution pH values1 and concentrations.2,3 Various monomeric and polymeric aluminum complexes were identified using different mass spectrometers.1-9 All studies were performed with an aqueous solution and demonstrated that ESI-MS has excellent potential for use as a probe to obtain direct data on metal speciation.10,11 However, chemical information of polyoxoaluminum in the presence of organic solvents is not satisfactorily investigated. It is known that organic solvents, such as acetone,12 acetonitrile,13 and alcohol,14,15 can coordinate with metal centers such as uranil and generate solvent adduct ions. The presence of these ions may help to confirm the molecular structures of aluminum complex.16 Furthermore, previous studies reported that adding inorganic or organic ligands to solutions hinders formation or enhances decomposition of Al13.17-24 With water ligand dissociation, the aluminum tridecamer can be converted to other species in aqueous solutions. The Al13-containing polyaluminum chloride (PACl) is widely utilized for coagulation of natural colloids in water treatment industries.25 The high surface charge of the aluminum tridecamer is assumed effective in neutralizing negative charges of natural colloids. NO3-, Cl-, SO42- and ClO4- ions can coordinate with aluminum center and lead to tridecamer decomposition, as evidenced by the ESI-MS data therein collected.1,9 However, since the coordination ability of these ions with aluminum is relatively weak, the yielded MS spectra incorporated numerous peaks to confuse detailed analysis. On the other hand, the presence of strong ligands such as ethylenediamine tetraacetic acid (EDTA), 2-hydroxynicotinic acid and pyridinecarboxylic acid in water can rapidly convert all aluminum species into stable complex, thereby leading to no useful kinetic data.9,26 A ligand at intermediate coordination ability to aluminum center may be used to partially stabilize the dissociated intermediates for ESI-MS measurements. * Corresponding author: tel, +886-2-23625632; fax, +886-2-23623040; e-mail, [email protected].

No detailed study on behavior of purified Al13 salts in water with strong ligands of intermediate coordination ability is available to the authors’ best knowledge. Acetonitrile is a neutral monodentate that has coordination ability which is comparable to that of a water ligand but is weaker than the strong ligands like EDTA or pyridinecarboxylic acid. Hence, acetonitrile was used in this study as the ligand to incrementally replace water ligands to aluminum tridecamer. This study characterized the aluminum species in the fresh and aged (14 days) 50:50 (v/v) water/acetonitrile mixtures with purified Al13 salts. The chemical species in these solutions were assigned, and the possible structures of the assigned Al species were elucidated. Methods General. All chemicals and solvents were reagent grade and used without further purification. Synthesis and Purification of [AlO4Al12(OH)24(H2O)12]Cl7 (Al13). The synthesis and purification procedures for Al13 were modified versions of those previously reported.27 The prescribed quantity of AlCl3 · 6H2O salt (8.259 g, 34.36 mmol) was dissolved in 20 mL of deionized water at 353 K; aqueous NaOH (3.223 g, 80.75 mmol) was then added dropwisely to the solution and mixed vigorously until the [-OH]/[Al3+] ratio reached 2.42. The resulting solution was initially turbid but became transparent after vigorous mixing for another 2 h. The pH of the final solution was 3.9-4.2. The solution was concentrated using a ¨ CHI Rotavapor R-200; BU ¨ CHI Labortechrotary evaporator (BU nik AG, Switzerland) to reduce the volume to 2-5 mL from 40 mL. The aluminum salt was then precipitated from the solution by adding an n-propanol/acetone mixture (v/v ) 1/5, 25 mL).) Filtration of the suspension using a glass filter (Por. 4; nominal maximum pore size, 10-16 µm; diameter, 100 mm; Robu Glasfilter GeraI¨te GmbH, Hattern, Germany) produced a filter cake that was then washed with tetrahydrofuran (THF). The dried product, purified Al13 salt as a white powder, was obtained with 85% yield. Characterization of Purified Al13. The infrared (IR) spectra were recorded using a Varian 640-IR spectrophotometer (Varianm Inc., Palo Alto, CA, USA) using KBr pellets dispersed with sample powders in the range of 4000-400 cm-1. The

10.1021/jp912101g  2010 American Chemical Society Published on Web 02/23/2010

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Figure 1. ESI-MS spectra generated from the purified Al13 in 50:50 (v/v) CH3CN/H2O solution (fresh solution) in the range of (a) m/z 200 to 300 (b) m/z 300 to 500.

TABLE 1: Compositions of Prominent Ions Observed in ESI MS Spectra of the Purified Al13 in the 50:50 (v/v) Water/ Acetonitrile Solutionsa m/z 203 221 239 257 275 293 329 347 365 383 401 419 437 455 473 491 509 527 545 563 581 a

possible compositions

m/z

possible compositions

Dimer [Al2O2-n(OH)1+2n(H2O)1-n(CH3CN)2]+, n ) 0-1 [Al2O2-n(OH)1+2n(CH3CN)2(H2O)2-n]+, n ) 0-2 [Al2O2-n(OH)1+2n(CH3CN)2(H2O)3-n]+, n ) 0-2 [Al2O2-n(OH)1+2n(CH3CN)2(H2O)4-n]+, n ) 0-2 [Al2O2-n(OH)1+2n(CH3CN)2(H2O)5-n]+, n ) 0-2 [Al2O2-n(OH)1+2n(CH3CN)2(H2O)6-n]+, n ) 0-2 Pentamer [Al5O7(CH3CN)2]+ [Al5O7-n(OH)2n(CH3CN)2(H2O)1-n]+, n ) 0-1 [Al5O7-n(OH)2n(CH3CN)2(H2O)2-n]+, n ) 0-2 [Al5O7-n(OH)2n(CH3CN)2(H2O)3-n]+, n ) 0-3 [Al5O7-n(OH)2n(CH3CN)2(H2O)4-n]+, n ) 0-4 [Al5O7-n(OH)2n(CH3CN)2(H2O)5-n]+, n ) 0-5 [Al5O7-n(OH)2n(CH3CN)2(H2O)6-n]+, n ) 0-6 [Al5O7-n(OH)2n(CH3CN)2(H2O)7-n]+, n ) 0-7 [Al5O7-n(OH)2n(CH3CN)2(H2O)8-n]+, n ) 0-7 [Al5O7-n(OH)2n(CH3CN)2(H2O)9-n]+, n ) 0-7 [Al5O7-n(OH)2n(CH3CN)2(H2O)10-n]+, n ) 0-7 [Al5O7-n(OH)2n(CH3CN)2(H2O)11-n]+, n ) 0-7 [Al5O7-n(OH)2n(CH3CN)2(H2O)12-n]+, n ) 0-7 [Al5O7-n(OH)2n(CH3CN)2(H2O)13-n]+, n ) 0-7 [Al5O7-n(OH)2n(CH3CN)2(H2O)14-n]+, n ) 0-7

599 617 635

[Al5O7-n(OH)2n(CH3CN)2(H2O)14-n]+, n ) 0-7 [Al5O7-n(OH)2-n(CH3CN)2(H2O)15-n]+, n ) 0-7 [Al5O7-n(OH)2n(CH3CN)2(H2O)14-n]+, n ) 0-7 Octamer [Al8O11-n(OH)1+2n(CH3CN)2(H2O)1-n]2+, n ) 0-1 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)2-n]2+, n ) 0-2 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)3-n]2+, n ) 0-3 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)4-n]2+, n ) 0-4 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)5-n]2+, n ) 0-5 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)6-n]2+, n ) 0-6 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)7-n]2+, n ) 0-7 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)8-n]2+, n ) 0-8 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)9-n]2+, n ) 0-9 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)10-n]2+, n ) 0-10 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)11-n]2+, n ) 0-11 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)12-n]2+, n ) 0-11 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)13-n]2+, n ) 0-11 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)14-n]2+, n ) 0-11 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)15-n]2+, n ) 0-11 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)16-n]2+, n ) 0-11 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)17-n]2+, n ) 0-11 [Al8O11-n(OH)1+2n(CH3CN)2(H2O)18-n]2+, n ) 0-11

509 527 545 563 581 599 617 635 653 671 689 707 725 743 761 779 797 815

Samples were tested within 30 min after production (Fresh solutions).

representative IR bands were 3416(s), 2351(w), 1639(s), 999(m), 723(m), 636(m), 559(w), and 451(w) nm. The 27Al nuclear magnetic resonance (NMR) spectra were obtained using a Varian Unity Inova-500 spectrometer (Varian NMR, Inc., Palo Alto, CA, USA) at a resonating frequency of 130 MHz. The 27 Al NMR spectra were referenced to AlCl3 · 6H2O (1 M) at pH 2 (0 ppm). The D2O solution of the purified Al13 was poured into a 5 mm NMR tube and noted δ 62.74 (1Al, [AlO4Al12(OH)24(H2O)12](Cl)7) and trace [Al(H2O)6]Cl3 (93% Al13

species. Tracing the inductively coupled plasma (ICP) of sodium ions indicates that the sodium chloride byproduct was removed during purification. ESI-MS Spectroscopy. The Al13 powder was dissolved in a 50:50 (v/v) water/acetonitrile mixture as a fresh solution, which was immediately tested using an ESI-MS Waters micromass ZQ 4000 spectrometer with a single quadrupole mass analyzer (Waters Corp., Milford, MA, USA). Some of the prepared solution was stored in a sealed vial at room temperature for 14 days. The stored solution is herein called the aged solution. The

Aluminum Tridecamer in Water/Acetonitrile Mixture

Figure 2. A Gaussian shape fitting of (a) aluminum dimers and (b) aluminum pentamers.

concentration of the aluminum solution ([Al13+7]) was adjusted to 67 mM, and the final pH was 4.78. All of the ESI-MS measurements were conducted in the positive ion mode. Results and Discussion The ESI-MS characteristic peaks for Al13 did not exist in samples with the 50:50 (v/v) water/acetonitrile mixture. Instead,

J. Phys. Chem. A, Vol. 114, No. 10, 2010 3505 a few series of signals with m/z 18 increments were observed (see below). On the basis of obtained MS data, some acetonitrile molecules were coordinated to the Al metal. No chloride isotope distribution was identified on the spectra, indicating that no chlorine ions attached to Al in the presence of acetonitrile. ESI-MS Spectra: Fresh Solutions. The sets of m/z sequences on mass spectra were in the m/z ) 200-900 regime (Figure 1 and Table 1), namely, sequence A with m/z ) 203 + 18n (n ) 0-40) and sequence B with m/z ) 226 + 18n (n ) 0-19). The constant increment of 18 in both sequences suggests that the predominant species in this system is monocharged. Furthermore, since no 37Cl- isotope patterns existed on these mass spectra, the constant increment of 18 was not yielded by replacing the hydroxyl group with chlorides (the difference between OH- (m/z 17) and 35Cl- is also 18) as ligands. That is, the incremental difference of 18 m/z in sequences A and B in the m/z ) 200-900 regime is attributable to the water ligand exchange reaction. Furthermore, these series of signals had Gaussian distributions, a result of random fragmentation of water ligands (Figure 2). In sequence A, the series of signals at z/m ) 203, 221, 239, 257, 275, and 293 corresponded to the dimeric aluminum, [Al2O(OH)3(CH3CN)2(H2O)n]+, with n ) 0-5. Since an oxide group plus a water ligand (-O(H2O)) has the same mass as two hydroxide groups (-(OH)2), the sequence can also be assigned as [Al2O2(OH)(CH3CN)2(H2O)n]+, with n ) 1-5 or [Al2(OH)5(CH3CN)2(H2O)n]+, with n ) 0-4. The theoretical maximum number of monodentates, including bridging ligands, surrounding two aluminum centers is 11. The m/z ) 329-615 regime was identified as pentamers, formulated as [Al5O7-n(OH)2n(H2O)m(CH3CN)2]+, with n ) 0-7 and m + n e 15. Likewise, the consecutive peaks of 545 + 18n, with n ) 0-15, were assigned as octamers, such as [Al8O11-n(OH)1+2n(H2O)m(CH3CN)2]+, with n ) 0-11 and m + n e 22. The intensities of signals above 815 were relatively weak; however, they still corresponded to the assignment of [Al11O16-n(OH)2n(H2O)m(CH3CN)2]+ with n ) 0-16 and m + n e 29. When a solvent other than water existed, only the Al polymers of 3n + 2 (n is zero or a natural number), such as Al5 and Al8, were assigned to sequence A. The Al5 and Al8 fragments made

Figure 3. ESI-MS spectra generated from the purified Al13 in 50:50 (v/v) CH3CN/H2O solution (aged solution) in the range of (a) m/z 200-300 (b) m/z 300-500.

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TABLE 2: Compositions of Prominent Ions Observed in ESI MS Spectra of the Purified Al13 in the 50:50 (v/v) Water/ Acetonitrile Solutions Aged for 14 Days (the Aged Solution) m/z 201 210 219 215 224 233 242 247 256 265 274 283 279 288 297 306 315 324 333 342 351

possible compositions Pentamer [Al5O2(OH)9(CH3CN)2]2+ [Al5O2-n(OH)9+2n(CH3CN)2(H2O)1-n]2+, n ) 0-1 [Al5O2-n(OH)9+2n(CH3CN)2(H2O)2-n]2+, n ) 0-2 [Al5O5(OH)3(CH3CN)4]2+ [Al5O5-n(OH)3+2n(CH3CN)4(H2O)1-n]2+, n ) 0-1 [Al5O5-n(OH)3+2n(CH3CN)4(H2O)2-n]2+, n ) 0-2 [Al5O5-n(OH)3+2n(CH3CN)4(H2O)3-n]2+, n ) 0-3 [Al5O6(OH)(CH3CN)6]2+ [Al5O6-n(OH)1+2n(CH3CN)6(H2O)1-n]2+, n ) 0-1 [Al5O6-n(OH)1+2n(CH3CN)6(H2O)2-n]2+, n ) 0-2 [Al5O6-n(OH)1+2n(CH3CN)6(H2O)3-n]2+, n ) 0-3 [Al5O6-n(OH)1+2n(CH3CN)6(H2O)4-n]2+, n ) 0-4 Heptamer [Al7O2(OH)15(CH3CN)2]2+ [Al7O2-n(OH)15+2n(CH3CN)2(H2O)1-n]2+, n ) 0-1 [Al7O2-n(OH)15+2n(CH3CN)2(H2O)2-n]2+, n ) 0-2 [Al7O2-n(OH)15+2n(CH3CN)2(H2O)3-n]2+, n ) 0-2 [Al7O2-n(OH)15+2n(CH3CN)2(H2O)4-n]2+, n ) 0-2 [Al7O2-n(OH)15+2n(CH3CN)2(H2O)5-n]2+, n ) 0-2 [Al7O2-n(OH)15+2n(CH3CN)2(H2O)6-n]2+, n ) 0-2 [Al7O2-n(OH)15+2n(CH3CN)2(H2O)7-n]2+, n ) 0-2 [Al7O2-n(OH)15+2n(CH3CN)2(H2O)8-n]2+, n ) 0-2

Al13 analogues. The ion spray voltage should yield all Al species via random bombardment. Since no species other than Al3n+2 existed, the noted 3n + 2 sequences should be the yield in fresh 50:50 v/v water/acetonitrile solution. Restated, the tridecamer molecule was degraded by the coordination reaction of acetonitrile ligands. This observation correlates well with previous studies indicating that Al13 stability decreased when ligands, such as sulfate,19 phenolic compounds,20 fluoride,21 silicate,22 and organic acid anions23 were present. The signals in sequence B were weak and yielded a series of complexes coordinated with three acetonitrile ligands. The peaks in the m/z ) 226-316 regime with an increment of 18 were assigned as dimeric aluminum, [Al2O2-n(OH)2n(CH3CN)3(H2O)m]+, with n ) 0-2 and m + n e 6. Similarly, the species associated with the signals at 370 + 18n, with n ) 0-11 were likely pentamers, namely, [Al5O7-n(OH)2n(H2O)m(CH3CN)3]+, with n ) 0-5 and m + n e 13. No peaks existed in the m/z > 568 regime. Moreover, isotope distributions of chloride were not present on the mass spectra. Hence, the chloro group presents as a rather weak ligand to the aluminum ion. In other words, in the presence of acetonitrile, chloride has less chance to bind directly with Al3+ and merely behaves as a counterion. A series peaks with an m/z increment of 41/n were not detected, indicating strong binding of acetonitrile ligands compared with the weak binding of water ligands to aluminum ions (Figure 1).

m/z 311 320 329 338 347 356 365 374 383 343 352 361 370 379 388 397 406 415

possible compositions Heptamer [Al7O3(OH)13(CH3CN)4]2+ [Al7O3-n(OH)13+2n(CH3CN)4(H2O)1-n]2+, [Al7O3-n(OH)13+2n(CH3CN)4(H2O)2-n]2+, [Al7O3-n(OH)13+2n(CH3CN)4(H2O)3-n]2+, [Al7O3-n(OH)13+2n(CH3CN)4(H2O)4-n]2+, [Al7O3-n(OH)13+2n(CH3CN)4(H2O)5-n]2+, [Al7O3-n(OH)13+2n(CH3CN)4(H2O)6-n]2+, [Al7O3-n(OH)13+2n(CH3CN)4(H2O)7-n]2+, [Al7O3-n(OH)13+2n(CH3CN)4(H2O)8-n]2+, [Al7O4(OH)11(CH3CN)6]2+ [Al7O4-n(OH)11+2n(CH3CN)6(H2O)1-n]2+, [Al7O4-n(OH)11+2n(CH3CN)6(H2O)2-n]2+, [Al7O4-n(OH)11+2n(CH3CN)6(H2O)3-n]2+, [Al7O3-n(OH)11+2n(CH3CN)6(H2O)4-n]2+, [Al7O3-n(OH)11+2n(CH3CN)6(H2O)5-n]2+, [Al7O3-n(OH)11+2n(CH3CN)6(H2O)6-n]2+, [Al7O3-n(OH)11+2n(CH3CN)6(H2O)7-n]2+, [Al7O3-n(OH)13+2n(CH3CN)6(H2O)7-n]2+,

n n n n n n n n

) ) ) ) ) ) ) )

0-1 0-2 0-3 0-3 0-3 0-3 0-3 0-3

n n n n n n n n

) ) ) ) ) ) ) )

0-1 0-2 0-3 0-3 0-3 0-3 0-3 0-3

ESI-MS Spectra: Aged Solutions. Compared with spectra collected from fresh solutions (Figure 1), the intensities of characteristic peaks were weakened and accompanied the emergence of many noises on the mass spectra for aged solutions (Figure 3). The peak sequence with an m/z increment of 9 was observed, suggesting the presence of double-charged cationic species in the aged solutions. Compared with the compositions of the major species in fresh solutions, a few dicationic polyaluminum species emerged without dimers or trimers in the aged solutions (Table 2). The

Figure 5. Schematic representation of dimeric ion [Al2(OH)2(H2O)8]4+, based on a presentation by Johansson and co-workers.25

Figure 6. Dimeric aluminum species bearing two acetonitriles (a) in a cis axial-axial fashion and (b) in a cis equatorial-axial fashion. (Atoms are represented as red balls for oxygen, blue balls for nitrogen, purple balls for aluminum, and gray balls for carbon. All of the hydrogen atoms are omitted.)

Figure 4. Modeled structures of the aluminum dimers bearing two acetonitrile ligands, of which binuclear aluminum centers were (a) bridged with bis-(µ2-oxo) group, (b) bridged with a bis-(µ2-hydroxo) group, and (c) bridged with a (µ2-oxo) group. (Atoms are represented as red balls for oxygen, blue balls for nitrogen, purple balls for aluminum, gray balls for carbon, and white balls for hydrogen.)

Figure 7. Proposed chain structure of the linear Al5 species.

Aluminum Tridecamer in Water/Acetonitrile Mixture

J. Phys. Chem. A, Vol. 114, No. 10, 2010 3507

Figure 8. Proposed structures of the aluminum trimer: (a) the compact structure; (b) the chain structure.

Figure 9. A fused-ring structure of the pentameric aluminum. (Atoms are represented as red balls for oxygen, blue balls for nitrogen, purple balls for aluminum, and gray balls for carbon. All of the hydrogen atoms are omitted.)

principal aluminum complex cations generated herein were pentamers and heptamers (Table 2 and Figure 3), which could be coordinated by two, four, or six acetonitrile ligands. The pentameric signals had relatively strong intensities, but weaker than those of the heptameric signals. Conversely, the amount of heptamers present was relatively low although a long series of water exchanging signals was noted on the spectra. Compared

with the spectra for the fresh solution, more acetonitrile ligands were associated with polyaluminum species in the aged solution, indicating that more acetonitrile ligands had replaced water ligands over time. Additionally, the intensity of octameric signals, which appeared in the fresh solution, decreased in the aged solution. Structural Identification: Fresh Solution. The ESI-MS measurement offers the gas-phase identification; hence the following discussions are based on the assumption that the structures in the solution phase and solid state are maintained in the gas phase. The molecular geometries of certain aluminum species were optimized to structures with minimum energy using the molecular mechanic method (UFF) with the ArgusLab 4.01 program. Figure 4 shows three possible aluminum dimers bearing two acetonitrile ligands and surrounded by a total of 10 (Figure 4, panels a and b) or 11 ligands (Figure 4, panel c). For the freshwater/acetonitrile mixture, aluminum centers were dimerized by bridging for either the bis-(µ2-oxo) or bis-(µ2hydroxo) ligands (Figure 4, panels a and b), m/z ) 275), which were formulated as [Al2O2(OH)(H2O)5(CH3CN)2]+ and [Al2-

Figure 10. Aluminum cluster growth with a Brucite-like structure: (a) pentamer, (b) octamer, (c) unidecamer.

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Figure 11. ESI MS spectra of the acidified Al(OH)3 in 50:50 (v/v) water/acetonitrile solution: (a) m/z 200 to 300; (b) m/z 300 to 500.

SCHEME 1: The Proposed Pathway of the Aluminum Dimerization: Condensation Reactions Leading to the Formation of Dihydroxo-Bridged Dimers

(OH)5(H2O)3(CH3CN)2]+, respectively. The two proposed structures are in accordance with the structure of the known complex [Al2(OH)2(H2O)8]4+, as demonstrated by Johansson et al.28 (Figure 5). The series of m/z ) 203-275 in Figure 1a is considered the consequence of the two substituted acetonitriles from water molecules as ligands. The two acetonitrile ligands in panels a and b of Figure 4 are adopted in the trans-fashion by the dialuminum complexes. Two isomers exist in each compound, and the bridging ligands were dioxo- or dihydroxogroups (Figure 6). In addition to the aforementioned structures, the two aluminum centers were also bridged by a mono-(µ2oxo) group, forming a dimer with m/z of 293 or 316 in the form of [Al2O(OH)3(H2O)5(CH3CN)2]+ (Figure 4c) or [Al2O(OH)3(H2O)4(CH3CN)3]+, respectively. Few studies have discussed pentaluminum cations in literature. Bi et al.29 established a “continuous” model unifying the “core-links”30 model and “cagelike”27,30 model for characterizing the entire aluminum hydrolysis-polymerization process. We speculate that Al2-Al5 species are in an unstable, chain structure, such that they typically conduct panel conformation to the Al6-Al12 species via polymerization (Figure 7). However,

dimeric and pentametic signals were identified (but not trimeric or tetrameric signals) (Figure 1), suggesting that not all Al2-Al5 species are unstable. Pophristic et al.32 demonstrated that the compact structure revealed in Figure 8a, which consists of a dimer with the third octahedral bound to the dimer by three bridges, yields an Al-Al-Al angle of roughly 60° and is more stable than the linear configuration with an Al-Al-Al angle of nearly 180° (Figure 8b). Therefore, we propose that the pentamer composed of a fused-ring structure (Figure 9) corresponds to species [Al5(OH)14(H2O)10(CH3CN)2]+ with an m/z of 635. The structure of the Al polymer can grow from a pentamer by adding fused-ring units as an octamer and a unidecamer (Figure 10), as found in the mass spectrum. These structures correlate with brucite-like25 fragments. Molecular clusters based on brucite-like cores have a periodic array of alternating Al(III) and hydroxyl linkages in cubane-like moieties33 (Figure 10). Our prediction of the octamer is based on the crystal structures of the brucite-like octamer obtained by Casey et al.34 The aluminum clusters belonging to the brucitelike structure are easier to synthesize when the molecules are

Aluminum Tridecamer in Water/Acetonitrile Mixture coordinated to the edges by organic ligands (generally using aminocarboxylate and pyridine ligands).25,35 As a comparison test, a stock solution was prepared by adding HCl to an Al(OH)3 solution until the pH reached 4.6 for partially dissolving the infinite structure. After passage thorough a 0.45 µm membrane, the filtrate was mixed with the same volume of acetonitrile. Figure 11 shows the mass spectrum of this acidified Al(OH)3 50:50 (v/v) water/acetonitrile solution. Sequences C and D (Figure 11) correlate with sequences A and B in (Figure 1). This observation partially supports the hypothesis that although acetonitrile is considered a neutral monodentate, it easily coordinates with the aluminum center and converts the cagelike aluminum tridecamer into brucite-like fragments as in Al(OH)3 precipitates. Structural Identification: Aged Solution. Species in the aged solution can be different from those in fresh solution. The polymerization process can account for the presence of doublecharged Al7 ligated with two-six acetonitrile molecules in the aged solution. The hypothesized pathway of oligomerization for the hydroxide cluster was Scheme 1.36 In the fresh 50:50 (v/v) water/acetonitrile solution, the major species were identified as single-charged Al2 and Al5 ligated with two acetonitrile molecules (Figure 1). During aging, combination of singlecharged Al2 and Al5 species produced the double-charged Al7 species. Thus, single-charged Al2 and Al5 were both ligated with two acetonitriles and produced the double-charged Al7 species that had four acetonitriles. Conversely, the polyaluminum underwent the solvent exchange process37 during aging. The free acetonitrile replaced the bound water to link with the aluminum center and increase the number of coordinated acetonitriles. Summary The species derived from ligand exchange reactions between purified aluminum tridecamer cluster and the acetonitrile was identified in the fresh and aged (14 days) 50:50 (v/v) water/ acetonitrile mixtures using the ESI-MS tests. Acetonitrile was noted to replace the ligated aqua ligands and to adduct to the aluminum center, which may deconstruct the “cagelike” aluminum tridecamer structure into smaller polymeric species. This work identified the complete mass spectra of incremental difference of 18u for fresh solution and of 9u for aged solution in the m/z ) 200-900 regime, which supports the occurrence of a water ligand exchange reaction during aluminum tridecamer decomposition. References and Notes (1) Sarpola, A.; Hietapelto, V.; Jalonen, J.; Jokela, J.; Laitinen, R. S. J. Mass Spectrom. 2004, 39, 423. (2) Sarpola, A. T; Hietapelto, V. K.; Jalonen, J. E.; Jokela, J.; Ra¨mo¨, J. H. Int. J. EnViron. Anal. Chem 2006, 86, 1007. (3) Urabe, T.; Tanaka, M.; Kumakura, S.; Tsugoshi, T. J. Mass Spectrom. 2007, 42, 591. (4) Sarpola, A.; Hietapelto, V.; Jalonen, J.; Jokela, J.; Laitinen, R. S.; Ra¨mo¨, J. J. Mass Spectrom. 2004, 39, 1209. (5) Sarpola, A.; Hellman, H.; Hietapelto, V.; Jalonen, J.; Jokela, J.; Ra¨mo¨, J.; Saukkoriipi, J. Polyhedron 2007, 26, 2851. (6) Sarpola, A. T; Saukkoriipi, J. J.; Hietapelto, V. K.; Jalonen, J. E.; Jokela, J. T.; Joensuu, P. H.; Laasonen, K. E.; Ra¨mo¨, J. H. Phys. Chem. Chem. Phys. 2007, 9, 377. (7) Urabe, T.; Tsugoshi, T.; Tanaka, M. J. Mol. Liq. 2008, 143, 70.

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