Article pubs.acs.org/JPCC
Investigation of Complex Formation of Cobalt(II) with Molecular Langmuir Layers of Octadecane-2,4-dione I. N. Repina,* M. E. Sokolov, and V. T. Panyushkin Kuban State University, Stavropolskya st. 149, Krasnodar 350040, Russia ABSTRACT: In this Article, we report the process of keto−enol tautomerism and deprotonation of amphiphilic β-diketone (octadecane-2,4-dione) in Langmuir layers. The complex formation of the Co2+ ions with the monolayer octadecane-2,4-dione depending on the pH and the concentration of the Co2+ in the subphase was investigated. Compression of monolayers of the octadecane-2,4-dione and its complex with Co2+ using the Langmuir−Blodgett (LB) method gives rise to monolayer and multilayer thin films with different packing densities. The morphology and optical properties of the LB films were investigated using atomic force microscopy and UV−vis absorption spectroscopy.
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controlled via a computer. The trough was set in an enclosure to be protected from dust and drafts, and the subphase temperature was controlled to within ±0.5 °C. Ultrapure water (resistivity greater than 18 MΩ•cm) was used as the subphase. Monolayers were spread from chloroform solutions on an ultrapure water subphase at 22 °C. After waiting 20 min to allow solvent evaporation, we obtained isotherms by reducing the surface area available to the material at a rate of 100 mm/ min. During the depositions, the transfer surface pressure was fixed at 30 or 40 mN/m. The pH was adjusted to 2.5 by the addition of HCl and between 2.5 and 13.8 by the addition of NaOH. Representative π−A isotherms as a function of pH are shown in Figure 1.
any amphiphilic molecules can form well-structured Langmuir−Blodgett (LB) films. Their hydrophobic part is usually constituted by one or two long aliphatic chains of 12−20 carbon atoms, whereas their hydrophilic part may be a coordinating group, which confers important properties to the amphiphile.1−8 β-Diketonates represent one of the oldest classes of chelating ligands, and have become one of the ligands of choice because of the recent industrial applications of several of their metal complex.9−11 It appears that surfactant β-diketone molecules and their complexes with Co2+ have been little used in a systematic way to date as building blocks of LB films. Thin films of complexes β-diketones with Co2+ may be used to construct materials for optical, catalytic, or sensor application. We have recently synthesized an amphiphilic bidentate ligand (octadecane-2,4-dione), capable of binding a wide range of transition metals. We studied its tautomeric properties in solution and determined the stability of its complexes with Co2+ in the ethanol.12 In this work, we investigated the complex formation of the Co2+ ion with a monolayer of octadecane-2,4-dione.
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EXPERIMENTAL SECTION The detailed synthesis of amphiphilic β-diketone (octadecane2,4-dione) and its complex with Co2+ was reported in a separate manuscript.12 The CoCl2•XH2O (X = 5,6) (99.999%) were obtained from Sigma-Aldrich and vacuum dried at room temperature before using. Octadecane-2,4-dione was spread from chloroform (pure 99.9%) solution in a KSV MINITROUGH 2 LB trough to an area of ∼80 angstroms/molecule on an aqueous subphase or CoCl2 solutions (series with different concentrations of CoCl2: 0.0005, 0.001, 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05 M). pH was held 2.5, 4.0, 7.0, 9.0, 10.5, 11.7, 12.5, and 13.8 for aqueous subphase and 2.5, 3.7, 4.5, 5.3, 6.0, and 7.0 for CoCl2 solution. The complete system was © 2012 American Chemical Society
Figure 1. Isotherms obtained of octadecane-2,4-dione on a aqueous subphase and 0.01 M CoCl2 solution as a function of pH. Received: May 17, 2011 Revised: January 23, 2012 Published: February 8, 2012 5554
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The substrate was a quartz plate (10 mm × 30 mm). It was cleaned with sulfuric acid, dipped in 10% hydrogen peroxide solution, and then rinsed with water. Imaging was performed using a JEOL JSPM-5400 atomic force microscope (AFM) under ambient conditions using a 2 μm × 2 μm scanner and a NSC35/AlBS contilever (MikroMasch) in semicontact mode. Images were obtained from at least five macroscopically separated areas on each sample. Representative images are presented below. UV−vis absorption spectra were taken with a Hitashi U-2900 UV−vis spectrophotometer.
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Figure 3. Plot of collapse pressure of octadecane-2,4-dione as a function of pH of the aqueous subphase (a) and the 0,01 M CoCl2 solution (b).
RESULTS AND DISCUSSION The β-diketones exhibit keto−enol tautomerism. In the enol form, the H-atom of the alcohol function is hydrogen-bonded to the carbonyl O atom. It is common to express the composition of a β-dicarbonyl system by the molar percentage of the enol tautomer at equilibrium rather than by the equilibrium constant K ([enol form]/[keto form]).13 The position of the keto−enol equilibrium influences the complex formation of the β-diketones. The position of the keto−enol equilibrium depends on a variety of factors such as the substituents on the β-dicarbonyl system, the solvent (its nature and pH), the temperature, and the presence of other species in solution that are capable of forming hydrogen bonds. Therefore, we study the keto−enol tautomerism and deprotonation in the Langmuir layers depending on the pH of the aqueous subphase. Figure 1 shows representative surface pressure (π)−area (A) isotherms as a function of pH. The isotherms at pH 2.5 and 7.0 on a water subphase are indistinguishable. The isotherm at pH 13.8 differs from that at a high value of π. The form of isotherms greatly depends on pH of the aqueous subphase. This is related to the processes of keto−enol tautomerism and deprotonation of the molecules octadecane2,4-dione in the Langmuir layers. The molecular area (A) and collapse pressure (π) depending on pH of the aqueous subphase as shown on the Figures 2a and 3a, respectively.
Figure 4. Keto, enol, and deprotonated forms of the octadecane-2,4-dione.
bonds and has a more bulky geometry of the hydrophilic part. In the pH range 9−12, where the molecular area is constant, there is a sharp increase in collapse pressure in the Langmuir monolayer of the octadecane-2,4-dione. It could also be explained by the geometry of the enol form, which forms a loose Langmuir layer. This assumption was proved by calculating the compressibility coefficient (β) of the monolayers in the range of pH β=
1 ⎛⎜ ∂A ⎞⎟ A 0 ⎝ ∂π ⎠T = const
At pH 2.5−7.0, β = 0.007, and at pH 9.0−12.0, β = 0.01. At pH > 12, the molecular area is decreased. This can be explained by deprotonation of the molecules octadecane-2,4dione and the formation of structures similar to the geometry of the structure of the keto form. The process of tautomeric transitions and deprotonation in the Langmuir layers is confirmed by the electronic absorption spectra of the LB films (Figure 5). It is known that the absorption band at 270−290 nm band is attributed to the n → π* transitions of the keto form of acetylacetone.12 This absorption band overlaps the highintensity band with maximum at 250−300 nm and unequivocally be associated with the π → π* transition in the enol form of β-diketone. There are two bands in the electronic absorption spectra at ∼230 and ∼280 nm. Maximum absorption at 230 nm characterizes the enol form of octadecane-2,4-dione. Maximum at 280 nm characterizes the deprotonated form of β-diketone. More detail of the electronic absorption spectra of octadecane2,4-dione in solution is described by us in ref 13. For octadecane-2,4-dione, monolayers on an aqueous subphase at pH 2.5 and pH 9.0 were compressed at a surface pressure of 30 mN/m, and those on a aqueous subphase at pH 13.0 were compressed at a surface pressure of 40 mN/m and then subsequently transferred onto quartz glass substrate. First,
Figure 2. Plot of molecular area of octadecane-2,4-dione as a function of pH of the aqueous subphase (a) and the 0,01 M CoCl2 solution (b).
The molecular area and collapse pressure at pH 2.5 and 9.0 are indistinguishable. At these pH values, the keto form of the molecule dominates (Figure 4). Increasing the pH of the aqueous subphase to pH 9 leads to an increase in molecular area. This is probably due to the displacement of the keto−enol equilibrium toward the predominance of the octadecane-2,4-dione cis-enol form. It is stabilized because of the formation of intramolecular hydrogen 5555
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monolayers of the octadecane-2,4-dione was investigated depending on the pH of the subphase (Figures 2b and 3b) and the concentration of the Co2+ ion in it (Figures 7 and 8).
Figure 5. Plot of UV−vis absorption spectra of 30-layer LB films of the octadecane-2,4-dione. LB films transferred from the aqueous subphase at pH 2.5 (a), 9.0 (b), and 13.8 (c) and from 0.01 M CoCl2 solution at pH 7.0 (d).
Figure 7. Plot of molecular area of octadecane-2,4-dione as a function of concentration Co2+ ion in the subphase (pH of the subphase 7.0).
a very good transfer ratio (TR nearly 1.1 ± 0.1) is observed for octadecane-2,4-dione on a aqueous subphase at pH 2.5 and pH 9.0 in the upward direction (Table 1). In the downward Table 1. Type Transfer and Transfer Ratio of the LB Films TR subphase
N
type transfer
H2O, pH 2.5 H2O, pH 9.0 H2O, pH 13.8 0.01 M CoCl2, pH 7.0
30 30 30 30
Z Z Y Y
down
0.98 ± 0.1 0.97 ± 0.1
up 1.12 1.17 1.14 1.21
± ± ± ±
0.1 0.1 0.1 0.1
Figure 8. Plot of collapse pressure of octadecane-2,4-dione as a function of concentration Co2+ ion in the subphase (pH of the subphase 7.0).
direction, the transfer ratio is very low (TR nearly 0). Therefore, the films are likely to be Z-type. For octadecane-2,4-dione on an aqueous subphase at pH 13.0, the transfer ratio of the LB film is nearly 1.0 during the down and up cycles, indicating the formation of Y-type films. The UV−vis absorption spectra the LB films allow the construction of plot of absorbance of LB films (Aλ) as a function of layer number in two different λ (N) (Figure 6).
In the case of the subphase containing Co2+, the area per molecule was significantly increased from 17.9 to 30.8 Å2 with respect to Co2+-free subphase. It indicates the formation of the complex at pH 7.0. Analyzing the dependences, we can hypothesize that increase in molecular area is result of the complex formation M2+:L 1:2 (Figure 9a). The monolayer creates the steric strain due to the
Figure 9. Expected coordination geometry of complexes M2+:L 1:2 (a) and M2+:L 1:1(b). Figure 6. Plot of absorbance of LB films as a function of layer number in two different λ. LB films transferred from the aqueous subphase at pH 25, pH 9.0, and pH 13.0 and from 0.01 M CoCl2 solution at pH 7.0.
formation of tetrahedral complexes. This leads to a broadening of the monolayer. Formation of the complex M2+:L 1: 2 can be confirmed dependences of molecular area and collapse pressure of octadecane2,4-dione as a function of concentration Co2+ ion in the subphase.
It is known that the complex formation of metal ions with the β-diketones is affected by the pH of the solution.10 Therefore, the complex formation of the Co2+ ion with 5556
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Figure 10. AFM images of 30 layers LB films of octadecane-2,4-dione prepared at pH 2.5 (a), 9.0 (b), and 13.8 (c) from the aqueous subphase and at pH 7.0 from 0.01 M CoCl2 solution (d).
leads to a broadening of the monolayer. With increasing concentration of the Co2+ in the subphase, the complex M2+:L 1:1 is formed because of saturation of the coordination groups by metal ions. AFM measurements indicated the difference in the morphology of 30-layer LB films prepared at pH 2.5, 9.0, and 13.8 from the aqueous subphase and at pH 7.0 from 0.01 M CoCl2 solution.
The increase in the molecular area and collapse pressure at a concentration of the Co2+ = 0.01 M is probably due to the formation of the complex M2+:L 1:2. With increasing concentration of the Co2+ in the subphase, the A decreases, which can be attributed to the formation of the complex M2+:L 1:1 (Figure 9b) because of saturation of the coordination groups by metal ions. The coordination of octadecane-2,4-dione with Co2+ at the air−water interface was confirmed by UV−vis spectra of the LB-film of octadecane-2,4-dione deposited in subphases containing these metal ions. In the absorption spectra of an 30-layer LB film on a quartz substrate on subphases containing Co2+ ion (pH 7.0), respectively, the appearance of an absorption band centered at about 280−290 nm. For octadecane-2,4-dione, monolayers on a 0.01 M CoCl2 solution at pH 7.0 were compressed at a surface pressure of 30 mN/m and then subsequently transferred onto quartz glass substrate. For octadecane-2,4-dione on a 0.01 M CoCl2 solution at pH 13.0, the transfer ratio of the LB film is nearly 1.0 during the down and up cycles, indicating the formation of Y-type films (Table 1). The surface morphology of 30-layer LB films of octadecane2,4-dione prepared at pH 2.5, 9.0, and 13.8 from the aqueous subphase and at pH 7.0 from 0.01 M CoCl2 solution is shown in Figure 10a−d, respectively. In Figure 9a is shown the crystals of the octadecane-2,4dione that are ellipse-shaped with an average grain size of 50− 100 nm. The LB films obtained at pH 9.0 from the aqueous subphase have amorphous structure and at pH 13.8 have very ordered structure. Figure 6d shows the crystals of the complex that are ellipse-shaped, with an average grain size of 10−20 nm.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Petty, C. M. Langmuir−Blodgett Films; Cambridge University Press: Cambridge, U.K., 1996. (2) Okomoto, K.; Taniguchi, M. Langmuir 2001, 17, 195. (3) Tamura, K.; Sato, H. J. Phys. Chem. B 2004, 108, 8287. (4) Kabayashi, K.; Sato, H. J. Phys. Chem. B 2004, 108, 18665. (5) Xu, Y.; Li, H. Thin Solid Films 2000, 375, 257. (6) Wang, X.; Shen, Y. Langmuir 2000, 16, 7538. (7) Serrette, A.; Carroll, P. J. J. Am. Chem. Soc. 1992, 114, 1887. (8) Talham, D. R. Chem. Rev. 2004, 104 (No. 11), 5479. (9) Comprehensive Coordination Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford, U.K., 1987; Vol. 1−7. (10) Garnovskii, A. D.; Kharisov, B. I. Direct Synthesis of coordination and Organometallic Compounds; Elsevier: London, 1999. (11) Mehrotra, R. C.; Rohra, R. Metal β-Diketonates and Allied Derivatives: Academic Press: New York, 1978. (12) Neiland, O. J.; Stradyn, J. P.; Silinsh, A. A. Structure and Tautomeric Transformations β-Carbonyl Compounds; Knowledge: Riga, Latvia, 1977. (13) Arkhipova, I. N.; Sokolov, M. E.; Panyushkin, V. T. News of Higher Educational Institutions. Chemistry and Chemical Technology 2010, 53 (No. 9), 9. (14) Binnemans, K. Rare-Earth Beta-Diketonates; Elsevier, 2005.
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CONCLUSIONS In this conclusion, we have demonstrated keto−enol tautomerism and deprotonation in the monolayers of the amphiphilic β-diketone (octadecane-2,4-dione). This process was investigated depending of the pH subphase. It is shown that at pH 2.5−7.0 in the monolayer is dominated by the keto form of the molecule. Increasing the pH of the aqueous subphase to pH 9.0 displaces the keto−enol equilibrium toward the predominance of the cis-enol form octadecane-2,4-dione, which is stabilized because of formation of intramolecular hydrogen bond. At pH > 12, the monolayer was the stabilized deprotonated form of the β-diketone. We have shown that the monolayers of octadecane-2,4-dione deposited on 0.01 M CoCl2 solution at pH 7 are formed of complex M2+:L 1:2. In the monolayer, the steric strain is created because of the aspiration of the coordination geometry to tetrahedral. This 5557
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