Vanadium - American Chemical Society

Chapter 30

A Vanadium-Based Homogeneous Chemical Oscillator Downloaded by UCSF LIB CKM RSCS MGMT on August 29, 2014 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch030

Kan Kanamori and Yuya Shirosaka Department of Chemistry, Faculty of Science, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan

It has been found that a dichloromethane solution of [V(IV)OCl (bpy)] or [V(III)Cl (CH CN)(bpy)] in the presence of air exhibits a new oscillating reaction. The initial pale green color turned to dark orange after an induction period. The color of the solution changed back to pale green, and this pattern repeated. The dark orange species was revealed to be [{V(V)OCl (bpy)} (µ-O)] by X-ray crystallography. Thus, a redox reaction between vanadium(V) and vanadium(IV) species is responsible for the oscillatory reaction. Dissolved dioxygen may work as an oxidizing agent and formaldehyde included in dichloromethane as a contaminant would be a reducing agent. Addition of chloride to a reaction solution considerably increased the induction period. 2

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© 2007 American Chemical Society

In Vanadium: The Versatile Metal; Kustin, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

425 Manos et al. found that a spontaneous reduction of vanadium(IV) to vanadium(III) occurred when they added 2,2'-bipyridine (bpy) to a solution of a bare vanadium(IV) complex, [V(IV)Cl (acac) ], in dry organic solvent (7). Reduction of vanadium(IV) to vanadium(III) is an important subject with regard to the accumulation and reduction of vanadium by ascidians (tunicates). Since a strictly dry environment is not likely to exist in living organisms, we investigated whether a similar spontaneous reduction would occur in non-dry solvents. We used non-dry ethanol as a solvent and a common vanadyl complex, [V(IV)0(acac) ], instead of [V(IV)Cl (acac) ]. We found that reduction of vanadium(IV) to (III) also occurred when a large excess (up to 60 equivalents) of HC1 gas was introduced into a reaction mixture containing [V(IV)0(acac) ] and bpy (unpublished work). In this experiment, a pale green complex was obtained when more than 80 equivalents of HC1 gas was introduced into an ethanolic solution of [V(IV)0(acac) ]. The pale green complex was found to be [V(IV)OCl (bpy)] where the acac ligands in the starting material were substituted by bpy and chloride. The above compositional assignment is supported by elemental analysis and IR spectroscopy. We dissolved the pale green complex in dichloromethane in order to observe the absorption spectrum. After measurement of a UV-vis spectrum, the remaining solution was kept in a volumetric flask with a stopper. A few days later the color of the solution suddenly turned to dark orange. After that, the color of the solution changed back to pale green, and this pattern repeated, indicating the discovery of a new, vanadium-based oscillator (2). 2

Downloaded by UCSF LIB CKM RSCS MGMT on August 29, 2014 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch030

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Experimental Preparation of [V(IV)OCI (bpy)l 2

[V(IV)0(acac) ] was dissolved in ethanol. A n ethanolic solution of HC1 (more than 80 equivalents) was added to the above solution and the resulting solution was stirred for 15 min. Then, an ethanolic solution containing an equivalent amount of bpy was added. The reaction mixture was stirred for one day. The solution was evaporated to dryness. A n appropriate amount of acetonitrile was added to the residue. The mixture was again evaporated to dryness. This procedure was repeated three times. Pale green powder was collected by filtration. 2

Preparation of [V(III)Cl (CH CN)(bpy)l 3

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[V(III)C1 (THF) ] was dissolved in acetonitrile. A n acetonitrile solution of bpy was added to the solution to yield a green solution. The resulting solution 3

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426 was evaporated to some extent, and a green precipitate was deposited. The green precipitate was collected by filtration.

Preparation of l{V(V)OCl (bpy)} (p-0)] 2

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[V(III)Cl3(CH CN)(bpy)] was dissolved in dichloromethane by stirring under aerobic conditions. After the color of the solution changed to dark orange, the solution was evaporated to some extent, and kept in a freezer at -30 °C. Dark orange crystals were deposited after a few days. Although the dark orange color sometimes disappeared, it generally continues for a long period when the solution is allowed to stand at low temperature without stirring.


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Observation of Oscillations A n appropriate amount of [V(IV)OCl (bpy)] or [V(III)Cl (CH CN)(bpy)] (typically 0.34 mM) was added to dichloromethane under aerobic conditions using a 20-ml volumetric flask with a stopper. The resulting mixture was stirred continuously. Initially the mixture was turbid but became clear, usually after several hours. Since the oscillating reaction we found is very slow, and it takes a long time to observe oscillations for solutions under various conditions, we followed the oscillations by capturing frames of several different reaction solutions simultaneously with a digital video camera. For a selected system, we also followed the oscillations with a UV-vis spectrophotometer. 2

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Results and Discussion Observation of Oscillations with a Digital Video Camera We captured video frames of several dichloromethane solutions of [V(IV)OCl (bpy)] under various conditions every hour. A n example is shown in Fig. 1. To make a graph showing the oscillations, we set a y-value to 0 when the color of the solution was pale green (almost colorless) and set it to I when the color was orange regardless of its depth. The graph thus obtained is shown in Fig. 2(A). As can be seen in Fig. 2(A), the solution exhibited oscillations in color, though the periods corresponding to the orange color and their intervals were irregular. The induction period before the first color change occurred was also irregular. Stirring velocity seems to be one of the factors affecting the induction and oscillating periods. We will discuss other factors below. Videos of an aerobic solution of [V(III)Cl (CH CN)(bpy)] also exhibited oscillations as shown in Fig. 3(A). 2

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In Vanadium: The Versatile Metal; Kustin, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007. 2

Figure 1. Videoframesof the oscillating reaction: [V(IV)OCl (bpy)J (0.34 mM) in 20 ml of dichloromethane; stirring rate = 900 rpm. (See page 4 of color inserts.)


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Figure 3. Oscillating pattern observed by video camera for fV(III)Cl3(CH CN)(bpy)J: End" signifies experiment terminated. A: [V(III)] = 0.34 mM; B: [V(I1I)] = 0.34 mM, [Bza] = 12.5 mM; C: [V(III)J = 0.34 mM, [Bza] = 12.5 mM, Volume ofSoln = Half; D: [V(III)J = 0.34 mM, [BzaJ = 12.5 mM, [TEAC] = 5.0 mM. u

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429 Oscillations in the [V(IV)OCl (bpy)] system were also recorded by measuring UV-vis spectra. In this case, we used a larger vessel (about 50 ml, rather than a 20-ml volumetric) mounted in a temperature-controlled spectrophotometer. The induction period was very long. The reason is not clear at present. During the induction period, a spectral change was observed in the U V region (data not shown), indicating that a pre-reaction occurred before the first color change. We added benzaldehyde after 9 days in order to enhance the reaction. The color of the solution turned immediately to light orange, and the intensity of the orange color changed periodically as shown in Fig. 4. The period of the intensity change was about 24 hours. Downloaded by UCSF LIB CKM RSCS MGMT on August 29, 2014 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch030

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Characterization of the Dark Orange Species The absorption spectrum of the dark orange species formed in a dichloromethane solution of [V(IV)OCl (bpy)] is shown in Fig. 5(A). The observed spectral features (the band positions as well as the band shape) resemble those of a V(III)-0-V(III) dimer. The absorption spectrum of [V(III) (ji-0)(L-his) ] (5) is shown in Fig. 5(B) for comparison. The dark orange species was found to be EPR silent, whereas the initial pale green complex exhibited an 8-line E P R spectrum typical of vanadium(IV) species. Therefore, we first assumed that the dark orange complex would be an oxo-bridged dinuclear vanadium(III) complex, and thus the oscillations were presumed to occur between vanadium(IV) and vanadium(III). This assumption was mistaken, since the X-ray crystal structure analysis shown below revealed that the dark orange species is a vanadium(V) complex, and thus oscillations occurred between vanadium(V) and vanadium(IV) species. A n Ortep perspective view of the dark orange complex's molecular structure is shown in Fig. 6. The complex has an oxo-bridged dinuclear structure as expected. However, the V l - O l and V 2 - 0 2 distances indicate that the oxidation state of the vanadium in not +3 but +5, because these distances are 1.586(4) and 1.582(4) A, respectively. Such short V - 0 distances clearly indicate a double bond character for V l - O l and V 2 0 2 bonds. Therefore, O l and 0 2 atoms should not be water oxygen atoms but oxo ligands, and the oxidation state of the vanadium must be +5. It is surprising that the present oxo-bridged dinuclear vanadium(V) complex exhibited an optical absorption spectrum very similar to oxo-bridged dinuclear vanadium(III) complexes. 2

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Reducing and Oxidizing Agents Since it was found that a redox reaction between vanadium(V) and vanadium(IV) species is responsible for the oscillatory reactions, some reducing


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T/hour Figure 4. Time-dependent Absorbance at 450 nm for V(IV)/Bza System.

Wavelength/nm Figure 5. Abospriton spectra of the dark orange species (A) and [V(IU) (vrO)(L-his) ] (B). 2

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agent as well as some oxidizing agent must participate in the oscillation. Dissolved dioxygen probably functions as an oxidizing agent, since deaeration of a reaction solution strongly inhibits the oscillation. What then is the reducing agent? We simply dissolved [V(IV)OCl (bpy)] or [V(III)Cl (CH CN)(bpy)] in dichloromethane and did not add any reducing agent. The purity report, which was obtained from Wako Chemical, Japan, indicates that the dichloromethane we employed is contaminated by a very small amount of formaldehyde and chlorine. Since formaldehyde has reducing ability, we think at present that it is the reducing agent. In order to examine the effect induced by an addition of aldehyde to a reaction solution, we observed oscillations for a solution containing an excess amount of aldehyde. In this experiment, we used benzaldehyde instead of formaldehyde, because gaseous formaldehyde is difficult to handle in organic solvents. The observed oscillations are shown in Fig. 2(B) and Fig. 3(B) for [V(IV)OCl (bpy)] and [V(III)Cl (CH CN)(bpy)], respectively. As can be seen in these figures, the induction period before the first development of the orange color decreased, which is contrary to our expectation. The reason is not clear at present. There may be two orange species in the reaction solution as suggested by the UV-vis spectra shown in Fig. 4. Other dependencies were investigated. Oscillations were observed for the systems in which the volume of the solution was reduced by one- half, and thus the volume of air was increased. The results are shown in Fig. 2(C) and Fig 3(C). The effect induced by air content is not clear. Addition of chloride (as tetraethylammonium chloride (TEAC)) considerably increases the induction period before the first development of the dark orange color (Fig. 2(D) and Fig. 3(D)). This effect may indicate that direct coordination of dioxygen to vanadium 2

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432 would be requisite for production of the oxo-bridged vanadium(V) dimer that is responsible for the dark orange color, and free chloride suppress this reaction. Further studies are required before a mechanism for this fascinating new chemical oscillator can be developed.


Acknowledgement We wish to thank Prof. Kenneth Kustin, Brandeis University Emeritus, for his valuable suggestions and for encouraging us to embark on a study of vanadium oscillations.

References 1.

2.

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Manos, M. J.; Tasiopoulos, A . J.; Raptopoulou, C.; Terzis, A.; Woollins, J. D.; Slawin, A , M, Z.; Keramidas, A . D.; Kabanos, T. A . J. Chem. Soc., Dalton Trans. 2001, 1556. Epstein, I. R.; Pojman, J. A . An Introduction to Nonlinear Chemical Dynamics: Oscillations, Waves, Patterns, and Chaos; Oxford University Press: New York, 1998. Kanamori, K . ; Teraoka, M.; Maeda, H . ; Okamoto, K . Chem. Lett. 1993, 1731.


Vanadium - American Chemical Society

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