Complex will have ligand that will form coordination bond to the metal center by donating lone pairs of electrons to the empty d-orbital of the metal which is similar to the Lewis acid-base reaction.
In this experiment, Acetylacetone and pyridine is the ligand will act as Lewis base and will be coordinated to the metal center of Vanadium, V, which is the Lewis acid.
Acetylacetone is a β-diketone that contains two alpha hydrogen atoms adjacent to the two carbonyl groups. As carbonyl-group is electron withdrawing group, these two carbonyl functional group will create an inductive effect which cause the alpha hydrogen to be more acidic and be easily deprotonated. Resonance stability of delocalising the anion formed from the deprotonation under basic condition also increases the probability of the deprotonation as shown in Figure 1:
Figure 1: Mechanism of the deprotonation and delocalization of the alpha hydrogen
Acetylacetone exist in both keto and enol isomer form as shown in Figure 2:
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Figure 2: Keto-enol tautomerisation of the Acetylacetone
The enolate anion is able to act as a bidentate ligand as it can chelate onto the oxovanadium metal ion with the two oxygen atoms as the binding sites to form the VO(acac)2 complexes. Two acetylacetonate ligands are able to bind to the oxovanadium metal and form a square pyramidal complex. Addition of the pyridine compound will form an octahedral geometry complex. These can be further proven using IR spectroscopy where the different strengths, lengths and nature as according to Hooke's law, IR absorption spectra is able to be unique for each complex.
Preparation of VO(acac)2
The preparation of VO(acac)2 started with 5ml of concentrated sulfuric acid (H2SO4) slowly added to 5 ml of deionized water in a 50 ml round-bottomed flask. Then, 18 ml of ethanol and followed by 2 g of vanadium pentaoxide (V2O5) was added into the round bottom flask. The solution was refluxed in an oil bath for an hour. The mixture was then cooled and filtered using cotton wool. Subsequently, 6 ml of Acetylacetone was added drop wise to the filtrate. The reaction mixture was carefully added portion by portion to the solution that contained 7.4g of Na2CO3 in 120 ml of deionsed water in a 500 ml conical flask. The mixture was then cooled in ice water and filtered to obtain the dark green product. Product was dried under IR lamp. The product was weighted and percentage yield was calculated.
Preparation of VO (acac)2py
The VO(acac)2py was prepared first by dissolving 0.5 g of the VO(acac)2 products obtained in 20 ml of ethanol in a 50ml of round bottom flask. Then, the mixture was refluxed vigorously with 2 ml of pyridine for one hour which was then concentrated using rotary evaporator. Crystal of crude VO(acac)2py formed upon cooling in ice water which was then filtered and wash with 5 ml of ether. Product was dried under IR lamp. The crude dry product was weighted and percentage yield was calculated. The IR spectrum of product was then determined.
Data Treatment and Analysis
Calculation of Percentage Yield of VO(acac)2
The percentage yield was calculated using the following equation:
V2O5 + 4H+ ïƒ 2(VO)2+ + 2H2O + O2 -------------------------Reaction 1
2(VO)2+ + 2 acacH ïƒ 2H+ + VO(acac)2 ------------------------Reaction 2
Mole of H2SO4 used =
Amount of H+ present = 0.0938 mol x 2 = 0.1876 mol
Mole of acac used =
Mole of V2O5 used = (limiting agent)
The ratio of V2O5 and VO2+ is 1:2,
Mole of VO2+ used = 0.0111 mols x 2 = 0.0222 mols
As ratio of VO2+ and VO(acac)2 is 1:1, then theoretical mole of VO(acac)2 = 0.0222 mols
Theoretical mass of VO(acac)2 = mole of VO(acac)2 x molar mass of VO(acac)2
= 0.0222 mols X 265.16 g/mol = 5.890 g
Percentage yield of VO(acac)2 =
Calculation of Percentage Yield of VO(acac)2py
Mole of pyridine used =
Mole of VO(acac)2 used = (limiting agent)
VO(acac)2 is the limiting agent. The ratio of VO(acac)2 and VO(acac)2py is 1:1.
Mole of VO(acac)2py = 0.00194 mols
Hence, theoretical mass of VO(acac)2py = mole of VO(acac)2py X molar mass of VO(acac)2py = 0.00194 mols x 344.26 g/mol = 0.667 g
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Percentage of VO(acac)2py yield =
Table 1: Summarized Data from IR spectrum of VO(acac)2py
Important peaks observed in the spectrum of VO(acac)2 /cm-1
Important peaks observed in the spectrum of VO(acac)2py /cm-1
Geometry of VO(acac)2 and VO(acac)2py
Vanadium [IV] metal exists as vanadyl, VO2+ in aqueous solution. Before the reaction of the VO2+ with the acetylacetonate, VO2+ is coordinated to five water molecules and existed as octahedral geometry. However, acetylacetonate is a stronger ligand that displaces the water molecules. Since Vanadium is bonded to oxygen with a V=O bond, only 2 acetylacetonate will only be able to form coordinate bond with the vanadium central metal ion to form the VO(acac)2 complex with a square pyramidal geometry as shown in Figure 3.
Figure 3: Reaction diagram of the coordination geometry of VO(acac)2 and VO(acac)2py
The geometry of this complex is in the square pyramidal geometry instead of the trigonal pyramidal geometry is due to the steric effect caused by the acetylacetonate ligands. The geometry of the square pyramidal is to reduce the steric repulsion to the minimum with the optimal distance apart of the two ligands.
Also, from Figure 3, it can be seen that there is a vacant coordination site on the principle z-axis available which allow another more coordination site if there is the presence of strong ligand such as pyridine in this experiment. Therefore, complexation occurred between the pyridine molecules and the VO(acac)2 where nitrogen lone pair in the pyridine molecules act as a π- donor-ligand and strong Lewis base that donate the lone pair of electron to the VO(acac)2 complexes at the vacant coordination site. With the new VO(acac)2py complex formed having a distorted octahedral geometry due to the unequal bond length between the V=O bond and the V=N bond at the axial position of the vanadium metal centre which lead to asymmetry. Also, the octahedral geometry of the VO(acac)2py is a 6-coordinate complex that is of high thermodynamic stability.
Shifting of the V=O stretching band
The complex of the VO(acac)2 has an oxidation complex of +4 which the vanadium(IV) ion exists as a d1. Using the Crystal Field Theory, there is electronic repulsion between the electrons of the acac ligands and the electron on the vanadium metal ion which causes the initially degenerate 5 d-orbitals of vanadium metal ion to split into two different levels. The level with the lower energy has a t2g symmetry and consists of the dxy, dxz, and dyz atomic orbitals. The level with the higher energy has eg symmetry and consists of the dx2-y2 and dx2 orbitals. The difference between the energy levels is the ligand field splitting parameter, âˆ†o. In the octahedral geometry, eg orbitals are on the axial direction and are the most affected by the electrostatic repulsion which destabilize the orbitals and excite them to higher energy level. The t2g orbital is not on the axial position and thus, not much affected by repulsion and thus, stabilized and move to lower energy as shown in Figure 4:
Figure 4: Molecular diagram illustration of Crystal Field Theory
As mentioned in the previous paragraph, the pyridine is a π donor ligand that binds to the VO(acac)2 complex at the vacant site at the axial position of the vanadium(IV) ion. The addition of the pyridine to the complex reduces âˆ†0 which this reduction of the splitting parameter was observed using the comparison of the IR spectrum of the VO(acac)2 and VO(acac)2py.
From the spectrum found in Appendix 2, the spectrum of the VO(acac)2py complex can be seen that the V=O stretching frequency of VO(acac)2py complex is 32 cm-1 lower than the VO(acac)2 complex when the V=O stretching frequency decrease from 998.04 cm-1 to 966.04 cm-1. These reduces of the stretching frequency is proportional to the stretching energy of the V=O bond of the complex. The addition of the pyridine will result in the reduction of the stretching energy of the V=O bond. This shows that the V=O bond of the VO(acac)2py is destabilized upon the addition of pyridine to VO(acac)2. This can be shown using Hooke's Law as shown:
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where v is the stretching frequency in cm-1, k is the force constant, μ is the reduced mass.
The binding of the pyridine has decrease the force constant and the lone pair of the pyridine ligand is electron-donating which increases the mass of vanadium ion that result in larger reduced mass, μ, which according to the Hooke's law, result in lower stretching frequency wavenumber. The wave number is proportional to the amount of energy that is needed for transition where it highly depends on the ligand field splitting parameter. The shifting of the stretching frequency indicate that the ligand field splitting parameter has decreased in magnitude which the addition of the pyridine ligand has caused destabilization of VO(acac)2py complex.
Similarly, the lone pair of electrons of the nitrogen in pyridine is added onto the anti-bonding of the molecular orbitals of VO(acac)2 complex. This causes the pyridine to be trans to the V=O bond. Also, this bonding of the electrons donated will increase the electron density of the vanadium's d orbitals which will decrease the p π → d π donation from oxygen atom to the vanadium atom in the V=O bond. This result in the falls of the bond order of the complex and the lowering of the bond strength which also lengthen the V=O bond length. Overall, there is decrease of energy need to stretch the V=O bond and thus lead to the shifting of the V=O stretching vibration to lower frequency in the spectrum. This also further proven that the VO(acac)2py has an octahedral structure.
Also, there is the delocalization of the π electrons from the aromatic ring of the pyridine which able to stabilize the complex more and decrease the p π → d π donation from oxygen to vanadium in the V=O bond. This make the vanadium-pyridine complexation more feasible and stable that result in the stronger V-N stretching bond which gain partial double bond character. As a result of the inter-electron repulsion, the electron density of the V=O bond is then shifted towards the oxygen atom and cause the weakening of the V=O double bond that cause the bond to loses the characteristic of the double bond nature. Thus, lesser energy is needed to stretch the weakened and lengthened V=O bond and lead to the shifting of the frequency of the spectrum.
There is also the reason of the trans influence where the trans influence is the effect of pyridine ligand that weakens the bond that is trans to itself in the complex. The electrons of the oxo-ligand in the axial position of the complex occupies the dz2 orbital that will cause new binding ligand to form coordinate bond with the vanadium ion to be trans position to the oxo ligand. Pyridine ligand have donate electrons to the VO(acac)2 complex metal center and reside in the 3dz2 orbital from the site trans to the oxygen atom which causes repulsion. Therefore, the pyridine ligand and the oxo-ligand are in the trans position to each other which uses the same orbitals on the metal vanadium ion for bonding. However, the oxygen atom cannot donate the electrons to the metal as well as the pyridine ligand which causes the V=O bond to be weaker. Therefore, both axial bonds to the vanadium are weakened and lengthened which decrease the force constant (k) and decrease the stretching frequency. The shift also helps to confirm the presence of VO(acac)2py.
On the other hand, the VO(acac)2py spectrum found in Appendix 2 have showed peak of 998.04 cm-1 which has a small different to the 997.7cm-1 peak found in the VO(acac)2 spectrum. This shows that there is still presence of VO(acac)2 in the product. Also the literature value of the V=O stretching bond of the VO(acac)2 is found out to be 995 cm-1 which is close to the 997.7 cm-1 in the product spectrum. These further prove that the VO(acac)2 is present. This can explain for the relatively low yield of 58.5% of the VO(acac)2py synthesis as the reaction has not gone into completion. However, comparing the intensity of the peak of 997.7 cm-1 and 966.04 cm-1 which indicate V=O bond in VO(acac)2 and VO(acac)2py complex respectively, there is higher intensity of the 966.04cm-1 which show that more pyridine adduct is formed in product. This also shows that the reaction may be an equilibrium reaction where both product and reactant can be present. This result can also be interpreted as the reaction is not complete and more reflux time or higher temperature for reflux is needed to carry out to ensure more products formed.
Lastly, there is a broad low intensity peak at 3449.72 at the VO(acac)2py spectrum found in Appendix 2 which was due to the presence of water that may be caused by insufficient drying of the VO(acac)2py product.
Observation of Color changes
During the synthesis of the complex, there is a series of observation of color changing of the compound in the solution. The color changes observed during the synthesis of the VO(acac)2 is due to the change of the oxidation state of V2O5 (oxidation state of V = +5). The orange powder of V2O2 is being reacted with the H2O and H2SO4 to reduce to a dark green solution of [V(H2O)5]3+ (oxidation state of V= +3) complex during middle of the vigorous reflux. When the reflux is complete, the solution was in dark blue color which is due to the reduction of the remaining V2O5 to dark blue [VO(SO4)(H2O)5] complex (oxidation state of V = +4). During the forming of the pyridine adduct, the dark blue VO(acac)2 is changed to dark green VO(acac)2py complex.
This color changing phenomenon can also be explained using the Crystal Field Theory mentioned. Electrons of the two different energy level orbitals can be excited with âˆ†o energy. The higher the oxidation state, there will be more different between the two splitting energy levels which therefore lead to higher âˆ†0. For V2O5 (oxidation state of V = +5) will have a higher âˆ†0 and hence will absorbs light of the electromagnetic spectrum of higher energy with higher frequency and reflect low energy light. Therefore, V2O5 will absorb the blue-green light of higher energy and reflect orange of lower energy light which is shown off as orange with naked eyes. When there is a reduction of the compound to VO(acac)2 or VO(acac)2py compound, the light absorbed was at lower frequency and light reflected was of higher frequency such as blue-green and thus, there is the change of orange powder to blue-green product. Therefore, VO(acac)2 and VO(acac)2py appeared as blue-green compounds.
Possible Limitations and Sources of Errors in Experiment
There are several limitations of the experiment that result in not having 100% yield of the product of both VO(acac)2 and VO(acac)2py complexes.
Firstly, there may be loss of product due to the cotton wool filtration that causes some of the filtrate to be trapped in the cotton wool and unable to pass through.
Secondly, according to the Le-Chatelier's principle, for the Reaction 2, if there is an increase in the H+ concentration, reaction will shift the concentration of the reaction to the left which result in lesser product produce. When the reaction mixture is added slowly to the alkaline sodium carbonate solution, incomplete neutralization may occur where not all H+ are removed, and hence lesser product is produced. Therefore, longer cooling time will be needed for re-crystallization so as to allow more crystallization of VO(acac)2 and push the reaction to the right.
Another limitation is of using of the sulphuric acid, H2SO4 may result in the reduction of the VO2+ ions to V3+ ions which decrease VO2+ ions available for product of VO(acac)2 formations.
The lower percentage yield of the VO(acac)2py may be due to incomplete refluxing and thus lead to low yield obtained. Also, impurities may be present in the VO(acac)2 product produced that observed at the bottom of the flaks which lead to false weight measured and inaccurate percentage calculated. The round bottom flask should be swirled every fifteen minutes during the reflux which can prevent the settling of the V2O5 solid at the bottom of the flask and identifying impurities.
The yield of the VO(acac)2py can be improved with addition of excess pyridine and increase the reflux time.
Possible Precaution in Experiment
For the synthesis of the VO(acac)2 complexes, there is a step of the adding of ethanol into the mixture of solution for reflux.
Ethanol was used as solvent. The metal vanadium atom is surrounded by non-polar ligands of acetylacetonate or pyridine in second part of the experiment. Ethanol is more non-polar than water and will form solvent-solute interactions with the complexes and dissolve those ligands for better interaction in the solution. The use of the ethanol also helps to separate the VO(acac)2 from the insoluble V2O5 residue that will be filtered and remain in the cotton wool during filtration. Also, ethanol was used to prevent side reaction that will arise so as to increase the yield of the product. From reaction 1, there is the reaction of the V2O5 and the H2SO4 to produce the intermediate VO2+ ion that besides reacting with the acetylacetonate in reaction 2, VO2+ can react with the water and oxygen molecules by-product formed from reaction 1. It is as shown in Reaction 3. The side reaction will decrease the amount of VO2+ available for reaction with the acetylacetonate in reaction 3 to form product of VO(acac)2. This will decrease the amount of yield of the product produced.
4VO2+ + 2H2O + O2 ï‚® 4VO2+ + 4H+ ---------------------------Reaction 3
However, this side reaction can be reduced with the addition of ethanol where ethanol will react with oxygen and produce acetaldehyde as shown:
C2H5OH + ½O2 ï‚® CH3CHO + H2O ---------------------------Reaction 4
Also, using vacuum to remove the oxygen and water formed can be a good way to reduce problem of side reaction with VO2+.
Another precaution taken was adding drop wise of the acetylacetonate to ensure sufficient reaction time and prevent influx of the H+ concentration and shift the equilibrium to the left and decrease the yield of the product formed.
Anhydrous sodium carbonate, Na2CO3, was dissolved in water so as to form alkaline solution to neutralise H+ ions so as to shift the acetylacetonate toward the enol form. This will increase the formation of the VO(acac)2 products. Also, the mixture solution was added slowly to the sodium carbonate solution so as to prevent the rapid foaming caused by the product of CO2 and lose of product due to overflowing. Also, slowly adjusting the pH with more alkaline environment and less mixture solution allows the reaction to react at steady pace and reduce any side reaction to occur.
The percentage yield of the product of VO(acac)2 and VO(acac)2py complex were 77.8% and 58.5% respectively. From the IR spectrum of VO(acac)2py, it is shown that the V=O bond have shifted from 9998.04 cm-1 to 966.04 cm-1. This shifting of the peak indicate the formation of the pyridine adduct, VO(acac)2py, with the decrease in the wavenumber of the V=O stretching band.