OVERALL: 93/100 Presentation: 9/10 Crystallization of Oxobis(2,4-pentanediono)vanadium(IV) via Reduction of Vanadium(V) Oxide Arya Stark Instructor: Dr. Scott Oliver TA: Yashar Abdollahian Chem 151L, Section 01C Abstract. In this experiment, oxobis(2,4-pentanediono)vanadium(iv) is synthesized from a mixture Comment [YA1]: 8/10 of vanadium(v) oxide, sulfuric acid, acetylacetone, and ethanol. The reaction took place over heat, and came to completion when the original mixture turned blue. The solution was then filtered, neutralized, and recrystallized with the aid of chloroform to yield a turquoise powder. The final product was dried and weighed, then analyzed via IR spectroscopy. Yield was Comment [YA2]: V=O and C=O stretch? relatively high at 10.41 grams, or 71.4 % of the theoretical yield. Introduction Vanadyl acetylacetonate, or oxobis(2, 4-pentanediono)vanadium(IV) by the IUPAC naming system, is a deep blue crystal with several applications beyond the scope of this experiment. A recent study used this compound to catalyze stereoselective synthesis of β- enaminoesters and β-enaminones. These molecules are important building blocks in biologically active compounds, such as peptides, and are also useful in research due to their stability at physiological ph. In addition to acting as a good catalyst for this synthesis, vanadyl acetylacetonate is particularly desirable over other potential catalysts due to its low cost, low toxicity, and ability to dissolve in organic solvents. 1 In addition to having a variety of chemical uses, a recent study found that vanadium(iv) acetylacetonate may have medical applications as Comment [YA3]: Be careful, used the same phrase in the last sentence.
well. Vanadium(IV) acetylacetonate is theorized to act as an insulin mimic on healing fractures, and has been shown to dramatically increase healing time. Unlike insulin, however, vanadium(iv) acetylacetonate will not cause hypoglycemia. 2 In addition to having a variety of applications, vanadyl acetylacetonate and other metal chelate complexes are also interesting to study due to their unique chemical properties. Chelation occurs when a positively charged metal is bound to a ligand by multiple bonds, forming a ring; such ligands are termed polydentate. Chelate rings are well known for their high stability compared to complexes containing similar, monodentate ligands; this stability is often referred to as the chelate effect. 3 The chelate effect is caused by an increase in the number of molecules in solution, which results in greater entropy and thus higher stability. Additionally, some chelating ligands, such as acetylacetonate, experience resonance hybridization which results in greater stability of the complex as well. Resonance hybridization of acetyl acetonate can be seen below, compared to the non-hybridized acetylacetone Figure 1. Resonance hybridization increases stability by allowing electrons to spread out, or delocalize, over a wider space. Delocalization decreases electron-electron repulsion, thus making the molecule more stable. Resonance hybridization in the metal-ligand chelate of this experiment can be seen below in Figure 2. Figure 1: Resonance hybridization in the ligand acetylacetonate (left), compared to the non-hybridized acetylacetone (right).
Figure 2: Resonance hybrid of the chelate complex, vanadyl acetylacetonate. Combined with a basic knowledge of chelation chemistry, the valence shell electron pair repulsion (VSEPR) model makes fairly accurate predictions of the 3D structure of a molecule. VSEPR theory was originally created to account for trends seen in the main group elements., so aalthough some of the more detailed aspects of this theory do not apply to transition metals, basic concepts of molecular geometry are widely applicable. The basic underlying premise of VSEPR theory is that electrons will minimize repulsion by occupying spaces that maximize their distance from other electrons. In this simplified model, the focus is on the spherical central atom and its valence electrons. Another important consideration when using this model is to realize that bonding electrons, which are shared between two atoms, occupy a more restricted space than lone pairs. Thus, the presence of lone pairs on a molecule can distort the molecule s shape. Combining the above principals, the VSEPR model can predict a nearly unlimited amount of molecular shapes for various combinations of lone pairs and bonding electrons. In this experiment vanadyl acetylacetonate is synthesized from a mixture of vanadium(v) oxide, sulfuric acid, and ethanol. Ethanol serves as an electron donor, or a reducing agent, in the reduction of vanadium that takes place through the course of the reaction. The reaction occurs over heat, and came to completion when the original orange mixture turned blue. The solution was then filtered and neutralized with sodium bicarbonate to induce precipitation of the crude Comment [YA4]: It deprotonated the acac. product. This crude powder was further purified and recrystallized with the use of chloroform.
The final product was a turquoise powder which was weighed and analyzed with IR spectroscopy. Experimental. Comment [YA5]: 5/5 Procedure follows the lab handout for this experiment with minor changes. 4 The product was analyzed with IR spectroscopy, which is not mentioned on the handout. Additionally, a large 600 ml beaker was used only up to the point of recrystallization; a 100 ml beaker was used to recrystallize the product in order to prevent excess evaporation of the solvent, chloroform. A Perkin Elmer Spectrum 6S FT-IR Spec (University of California inventory number 11700087) was used to analyze the product via IR. Results and Discussion. The balanced set of equations which summarize this experiment can be seen below in Scheme 1. The final product is commonly referred to as vanadyl acetylacetonate in the literature, but by IUPAC standards is known as oxobis(2,4-pentanediono)vanadium(iv). Based on VSEPR theory described in the introduction, the final product is expected to have either trigonal bipyramidal or square pyramidal geometry. Although the crystal structure of the product was not analyzed in this experiment, the product is known to adopt square pyramidal geometry, with the vanadium slightly displaced from the plane of the chelate ligands. 5 Excess strain on the chelate ligands prevents trigonal bipyramidal geometry. Additional research shows that the vanadium oxygen bond has partial triple bond character. 6 The 3D structure of the product can be seen in Figure 2 in the introduction. 1. V 2 O 5 + 2 H + 2 VO 2 + + H 2 O 2. 10 H 2 O + 2 VO 2 + + CH 3 CH 2 OH 2[VO(H 2 O) 5 ] 2+ + CH 3 COH + 2 H 2 O 3. [VO(H 2 O) 5 ] 2+ + 2[C 5 H 7 O 2 ] - VO(C 5 H 7 O 2 ) 2 + 5 H 2 O Scheme 1: The balanced set of reactions for the synthesis of vanadyl acetylacetonate.
Yield for this experiment was moderately high at 71.4% or 10.41 grams of product collected, of the theoretical 14.58 grams. Most loss resulted from irretrievable residual product following container transfer; this was especially true after filtrations. Recrystallization of the crude product with chloroform is another source of loss. Only half of the solvent was boiled off during recrystallization, thus some product was filtered away dissolved in the reaming Comment [YA6]:? chloroform. The final product was of average quality; the crystals were powder- like in form, likely due to rapid, heat induced crystallization. Additionally, the powder synthesized in this experiment was turquoise in color, rather than the ideal deep blue color described in the procedure. This could be due to unreacted vanadium oxide, which is yellow, remaining in the final product, which would impart a green tint on the otherwise blue product. The IR spectrum of the product, as well as one found in the literature, can be seen below in Figure 4. Additionally, a summary of experimental and literature values for the significant stretches can be found below in Table 1. The experimental values match literature values well, and follow the general trend that heavier elements, such as vanadium, typically have stretches with lower wave numbers due to the fact that they are less mobile. In addition, these results show that the carbonyl stretch in the chelate ligand is lower than the stretch for the free ligand, acetyl acetone. This phenomenon is due to the resonance hybridization of the chelated acetylacetonate, as seen in Figure 1 in the introduction, which weakens the carbonyl stretch, resulting in a lower wave number. The stretch of the chelate is also expected to be lower than free acetyl acetonate, despite the fact that both compounds experience resonance hybridization. In the case of the chelate, there is further delocalization of electron density as referred to in the discussion of the chelate effect in the introduction, which further weakens the carbonyl stretch.
Table 1: IR stretching frequencies of the V=O and C=O bonds of the product. Stretch Experimental Peak Value (cm -1 ) Literature 7 Peak Value (cm -1 ) V=O 998.01 995 N/A C=O 1555.77 1562 1740 Peak Value of Free Ligand Figure 4: IR spectrum of the product (top) and IR found in literature 5 (bottom). Conclusions. Comment [YA7]: 9/10 The synthesis of vanadyl acetylacetonate was successful is certain aspects, and poor in others. The yyield was relatively high, however, the crystals were of average quality. Despite nonideal crystal structure, IR analysis of the product yielded peak values that matched well with Comment [YA8]: colloquial Comment [YA9]: Awkward wording. Not sure what you imply by crystal structure. Crystal quality perhaps?
the literature values and followed the general trend that heavier elements typically have stretches with lower wave numbers. The procedure used in this experiment could be improved by allowing the crystals to forms slowly over time, which would likely improve overall quality. Additionally, usage of a green solvent that could be fully evaporated off the product would improve the yield. Vanadyl acetylacetonate has a wide variety of uses across many fields of study, and the synthesis described in this report could likely be commercially viable with the minor modifications described above. References Comment [YA10]: 15/15 1. Laskar, R., Begum, N., Mir, M., Ali, S., Khan, A. Vanadium(IV) acetylacetonate catalyzed stereoselective synthesis of β-enaminoesters and β-enaminones. Tetrahedron Letters. 2013, 54, 436-440. 2. Paglia, D., Wey, A., Park, A., Breitbart, E., Mehta, S., Bogden, J., Kemp, F., Benevenia, J., O Connor, J., Lin, S. The effects of local vanadium treatment on angiogenesis and chondrogenesis during fracture healing. Journal of Orthopaedic Research. 2012, 30, 1971-1978. 3. Huheey, J., Keiter, E., Keiter, R. Inorganic Chemistry: Principles of Structure and Reactivity, 4 th ed. Harper Collins College Publisher, 1993. 4. Oliver, S. Experiment 3: Synthesis of Vanadium(IV) Oxy(acetylacetonate). Chem 151L, Spring, 2013. 5. Nakamoto, K., Morimoto, Y., Martell, A. Infrared Spectra of Metal Chelate Compounds. IV. Infrared Spectra of Addition Compounds of Metallic Acetylacetonates. Journal of the American Chemical Society. 1961, 83, 4533-4536.
6. Bella, S., Lanza, G., Gulino, A., Fragala, I. Electronic Structure of Bis(2,4- pentanedionato-o,o )oxovanadium(iv). A Photoelecton Spectroscopy, Electronic Spectroscopy, and ab Initio Molecular Orbital Study. Inorganic Chemistry. 1996, 35, 3885-3890. 7. Vlckova, B., Strauch, S., Horak, M. Measurement and Interpretation of Infrared and Raman Spectra of Vanadyl Acetylacetonate. Czechoslovak Chemical Communications. 1986, 686-695. Appendix. Calculations Theoretical yield using vanadium(v) oxide as the limiting reagent 5 g V 2 O 5 x (1 mol V 2 O 5 /181.88 g V 2 O 5 ) x (2 mol VO(C 5 H 7 O 2 ) 2 / 1 mol V 2 O 5 ) x (265.16 g VO(C 5 H 7 O 2 ) 2 / 1 mol VO(C 5 H 7 O 2 ) 2 ) = 14.579 g VO(C 5 H 7 O 2 ) 2 % Yield: actual/theoretical x 100% 10.41 g/14.58 g x 100%= 71.40%