Preparation of bis(acetylacetonato)copper(II)

Experiment 10

Title: Preparation of bis(acetylacetonato)copper(II)

Objectives:

To synthesise Bis (acetylacetanato) copper (II) compound by using Copper (II) Nitrate as starting material.
To recognize the nature of the ligand
To learn the technic of crystallization.

Introduction:

The four most common techniques for the bulk purification of crude organic compounds on a laboratory scale are recrystallization, distillation, extraction, and column chromatography. Numerous other methods are used somewhat less frequently (for example, sublimation) while still others are generally useful only for small scale applications (i.e., preparative gas chromatography). Of the methods useful in the purification of organic solids, recrystallization is usually the most convenient when it is feasible.

In chemistry, recrystallization is a procedure for purifying compounds. A typical situation is that a desired compound X is contaminated by a small amount of compound Y. A chemist can prepare a saturated solution of the mixture X+Y in a warm solvent and subsequently lower the temperature. The cost of this purification method is the loss of the part of compound X that stays in solution.

Like the other methods mentioned above, recrystallization is a separation process. If successfully applied, it will result in the concentration of one component of a mixture in one phase, the solid phase – as crystals, while leaving the other components, the impurities, in a readily separable phase – a liquid solution. The net result is thus the removal of contaminants from a solid material which was originally a component of a mixture.

The basic steps in any recrystallization are, at least in principle,

1) the entire mixture is dissolved in an appropriate solvent at an elevated temperature;

2) the resulting solution is allowed to cool, during which time the desired component crystallizes out of solution as a crystalline solid;

3) the desired solid is separated from the solution by filtration. Any impurity-containing solution adhering to the crystals can often be removed at this point as well by use of a suitable wash solvent.

The key to a successful and efficient recrystallization is the careful choice of an appropriate solvent with which to carry out the process. An ideal recrystallization solvent is characterized by the following properties:

1) it should be unreactive to the substrate being purified;

2) it should dissolve a relatively large amount of the desired substance at a high temperature and a relatively small amount of it at a low temperature;

3) it should dissolve any impurities present well at all temperatures;

4) it should be reasonably volatile, so that excess solvent can be removed from the crystals after filtration.

In this experiment, the method precipitation is also used. Precipitation is the formation of a solid in a solution during a chemical reaction. This can occur when an insoluble substance is formed in the solution due to a reaction or when the solution has been supersaturated by a compound. In most situations, the solid forms "falls" out of the solute phase, and sinks to the bottom of the solution though it will float if it is less dense than the solvent, or form a suspension.

This effect is useful in many industrial and scientific applications whereby a chemical reaction may produce a solid that can be collected from the solution by various methods Precipitation from a solid solution is also a useful way to strengthen alloys.

An important stage of the precipitation process is the onset of nucleation. The creation of a hypothetical solid particle includes the formation of an interface, which requires some energy based on the relative surface energy of the solid and the solution. If this energy is not available, and no suitable nucleation surface is available, supersaturation occurs.

This experiment explores the origin of color in organic molecules. Species that are assemblies of a central metal ion bonded to a group of surrounding molecules or ions are called metal complexes. If the complex carries a net charge, it is generally called a complex ion. Compounds that contain complexes are known as coordination compounds.

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Coordination compound is the number of ligands atoms that are bonded directly to the central metal ion in a particular oxidation state and compound.

The molecules or ions that surround the metal ion in a complex are known as ligands. Ligands are normally either anions or polar molecules. Every ligand has at least one unshared pair of valence electrons. In forming a complex, the ligands are said to coordinate to the metal. For example, the copper (II) nitrate; (Cu(NO3)2⋅3H2O) has two ligand atoms of NO3 because the two molecules are bonded to the central of copper. The three water molecules ...

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Coordination compound is the number of ligands atoms that are bonded directly to the central metal ion in a particular oxidation state and compound.

The bond between a ligand and a metal ion is an example of an interaction between a Lewis base and a Lewis acid. Because the ligands have unshared electrons, they can function as Lewis bases (electron-pair donors). Metal ions (transition-metal ions) have empty valence orbitals, so they can act as Lewis acid (electron-pair acceptors). The formation of metal-ligand bonds can markedly alter the properties observed for the metal ion. A metal complex is a distinct chemical species that has physical and chemical properties different from the metal ion and the ligand from which it is formed.

The ligand geometries are named and described as if the central atom was in the middle of a polyhedron, and the corners of that shape were the locations of the ligands. For example, a complex with four regularly distributed ligands would be described as tetrahedral, while one with six would be hexahedron.The polyhedra need not be regular: there are other possible geometries, such as square pyramidal (four ligands equally distributed in a plane, and one ligand normal to this plane).

Ligands which only bond to the central atom through one site are termed monodentate. Some ligand molecules are able to bind to the metal ion through multiple sites, since they have free lone pairs on more than one atom; these are called polydentate. A ligand with two coordination sites is called bidentate. EDTA is a classic example of a polydentate ligand - it is able to bond through six sites, completely surrounding the central atom. A scorpionate ligand is an example of a tridentate ligand. Complexes of polydentate ligands are called chelated complexes; they tend to be more stable than monodentate complexes as it is necessary to break all of the bonds to the central atom for the ligand to be displaced.

Metal acetylacetonates are used as catalysts for polymerization of olefins and transesterification. It is used as a a stabilizer PVC. They are used as curing agents for epoxy resins, acrylic adhesives and silicone rubbers. They are used as solvents, lubricant additives, paint drier, and pesticides. They are used in glass coatings.

Apparatus and Materials:

Copper (II) nitrate
Ammonia
Acetylacetone
Ethanol
Chloroform
Distilled water
Glass rod
Beaker
Suction filtration set

Procedures:

6g of copper (II) nitrate (Cu(NO3)2.3H2O) dissolved in 60cm3 of water. Concentrated ammonia solution added slowly while stirring until the precipitate first formed has just dissolved.
The solution is continued stirred and 6cm3 of acetylacetone.
The crude product is filtered off using suction filtration, then it is washed well with water and drained thoroughly.
The crude product is recrystallise from a mixture of equal volumes of ethanol and chloroform.
The recrystallised material is filtered off and dried at 90ºc

Data and results:

Analysis and calculation:

Moles of Cu(NO3)2.3H2O

= 6

52 + 2(14.01) + 9(16) + 6(1)

= 2.608 x 10-2 mol

Molar mass of Bis(acetylacetonato)copper(II)

= 10(12.01) + 14(1) + 52 + 4(16)

= 250.1 g/mol

Theoretical Yield

= 2.608 x 10-2 x 250.1

= 6.52 g

Experimental Yield

= 5.30 g

Percent Yield

= 5.30 x 100%

6.52

= 81.29%

Discussion:

The main purpose of this experiment is to produce bis(acetylacetonato)copper(II), [Cu(C5H7O2)2], which is a coordination compound by using precipitation and recrystallization method. Coordination compounds are compounds that contain coordination complex. Coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions (called ligands) bond to a central metal atom (or ion) by coordinate covalent bonds. Bis(acetylacetonato)copper(II), [Cu(C5H7O2)2], has a molecular weight of 261.76, a melting point of 279-2830C, it’s also odorless and is neutral.

Firstly, 6g of Copper (II) nitrate, Cu(No3)2. 3H2O, dark blue crystals, is dissolved in water and a clear blue solution is obtained. This blue solution formed indicates that copper(II)nitrate is soluble in water and produce hexaaquacopper(II) ions and nitrate ions.

Cu(NO3)2 + 6H20 → Cu(H2O)6 2+ + 2NO3 -

After that, small amount of concentrated ammonia is added to the solution with stirring until the precipitate formed has just dissolves. Stirring is done to increase the level of solubility. After concentrated ammonia is added, ammonia will attach itself to the central copper ion using the lone pair electrons on the nitrogen atom which is acting as a Lewis base because it is a lone pair donor to form a metal hydroxide. Therefore, a pale blue precipitate of copper (II) hydroxide is formed.

When more concentrated ammonia is added, the ammonia will replace the molecules bonded to the central atom to give another soluble complex. This process is a ligand exchange reaction and it involves equilibrium such as the one shown below:

However, this is a slightly untypical case, because only four of the six water molecules get replaced to produce the tetraamminediaquacopper(II) ion, [Cu(NH3)4(H2O)2]2+, the structure of the ion is shown below:

This new soluble complex that is formed causes the precipitate to dissolve. The soluble complex formed is tetrammine complex. Addition of ammonia is stopped right after the precipitate has just dissolves. This is because, when more of ammonia is added, a deep blue complex will form. The deep blue colour of this complex is very strong and it is so strong until this reaction is used as a sensitve test for copper(II) ions in solution. Even if we try to reverse the change by adding large amounts of water to the equilibrium, the strength of the deep blue (even highly diluted) always masks the pale blue of the aqua ion. Thus, the solution is stirred after adding only one drop of concentrated ammonia to see if the precipitate dissolves in this experiment.

Let me explain the dissolution of precipitate in excess ammonia for Hexaaquacopper(II) ions. When there is an excess of ammonia, the concentration of Hexaaquacopper(II) ions will fall dramatically which means the position of the equilibrium below will lies to the right.

If the concentration of Hexaaquacopper(II) ions has fallen because of the equilibrium above, the following equilibrium will move to replace it.

The position of the above equilibrium will shift to the left causing the precipitate to dissolve, the amount of neutral complex is lessened to the point where there is no longer enough to form a precipitate. All this has been helpful in explaining why some precipitates dissolve in excess ammonia while others don't. It actually depends on the positions of the equilibria. To get the precipitate to dissolve, you need the ligand exchange equilibrium to lie well to the right, but you need the acid-base equilibrium to be easy to pull back to the left.

Next, acetylacetone is added dropwise into the solution and stirring is continued to improve dissolution. Acetylacetone is a clear, colourless, mobile liquid with a pleasant ketone like odour It is also a valuable starting material for the synthesis of other transition metal compounds. The ammonia in the mixture solution will dissolve the ketone to give a homogeneous solution. This comes about because NH3 a base, encourages the rearrangement of the structure so that one of the carbonyl groups is converted to an acidic –OH group which loses its proton to NH3 and leaves the water-soluble acetylacetonate ion.

acetylacetone acetylacetonate ion

Acetylacetonate ion (acac-) is a bidentate ligand. Bidendate classification (“two teeth”) means that acetylacetonate ion has the ability to bond to a metal ion through 2 oxygen atoms. Another descriptor used for this type of chemistry is chelation. We picture the bidentate ligands as “claw-like” molecules that “bite” the metal ion.

Here, the acetylacetonate ion will react with the tetraamminediaquacopper(II) ion, [Cu(NH3)4(H2O)2]2+. The copper (II) ions will then bind with the oxygen of the acetylacetonate ion after losing all the molecules bounded to it. Two acetylacetonate ion (-2 charged) will bind with one copper (II) ion. Thus, bis(acetylacetonato)copper(II), C10H14CuO4, is formed. The reaction is shown in the equation below.

[Cu(NH3)4(H2O)2]2+. + 2 acac- = Cu(acac)2 + 4NH3 + 2H2O

2 + Cu2+ =

The structure of bis(acetylacetonato)copper(II) has been redetermined from single crystal X-ray diffraction data. The unit cell parameters area=11.331(9),b=4.697(3),c=10.290(9) Å, and=91.84(7)dg. A finalR=0.056 for 727 observed reflections was obtained. The O-Cu-O chelate (bite) angle is 93.2(2)°. The two Cu-O bond lengths are 1.914(4) and 1.912(4) Å. The complex has a slight chair structure and a step angle of 7.05°.

“bis” means that a compound has two identical but separated complex chemical groups in one molecule. In this experiment, the acetylacetonate ion is the complex chemical group.

The structure of bis(acetylacetonato)copper(II) is shown below:

After that, the solution with precipitate is filtered using suction filtration. A filter flask, Buchner funnel and filter paper is used for the filtration. A glass rod is used to direct the solution into the Buchner funnel so that it does not spill. When the solution reaches half the height of the filter funnel, it is allowed to drain before the rest of the solution is poured in to avoid spillage. The level of solution should never exceed the boundaries of the filter paper. This is to avoid loss of precipitate that can creep out from the sides. The remains of precipitate on the glass rod and the beaker is rinsed into the Buchner funnel using distilled water to ensure that we obtain maximum yield of precipitate. The filtered product is bis(acetylacetonato)copper(II) which is a blue coloured powder-like crystalline solid.

Next, the crude product is recrystallised in order to purify the compound. The solubility of the compound in simple available solvents is determined for purification purposes. Usually, water is tried first even though it is not successful most of the times.. The more common solvents used are methanol, ethanol, acetic acid, petroleum ether. Ether, chloroform, ethyl acetate, acetonitrile and dimethylformamide are not so commonly used. If none of the solvent mentioned are found suitable, a mixture of two miscible solvents are used to perform recrystallisation. The crude product should be relatively soluble in one solvent and relatively insoluble in the other solvent. In this experiment, ethanol and chloroform are used as the two miscible solvents to purify bis(acetylacetonato)copper(II). The compound is relatively soluble in chloroform and relatively insoluble in ethanol.

Then, using suction filtration again filters the recrystallised product. The filtered product is then left to dry in the oven to remove water molecules present in the compound. It is then cooled in the desiccator. It is weighed after that. This process is repeated until constant weight of the substance is obtained. Constant weight is defined, as that state when a substance will not change its weight on heating. This is achieved by heating the object. Weighing the cooled object and then heating again, cooling again, and weighing a second time. If the two weights agree within 0.5 mg., the object is said to be at constant weight. If not then the heating, cooling and weighing process is continued until two successive weighings do agree within 0.5 mg.

The mass of bis(acetylacetonato)copper(II) obtained in this experiment was 4.30g. The amount we obtained in this experiment is lesser than the amount produced by the reaction, where the percentage of yield is only 65.95%. This is because, while performing the experiment, we have wasted quite an amount of the yield. It is mostly lost during first suction filtration to obtain the crude product. As filter paper was used during the suction filtration process, the suction pump too strong and suction pressure was very strong. There was once where the filter paper has broken and we have lost some of our crude product. Other than that, the crude product was seeping out the sides of the filter paper while doing suction filtration. This is because the crude product was of a powderlike form and is very difficult to control. Even though we had used a glass rod to lead the solution into the center of the Buchner funnel, the solution with the crude product could have moved to the sides of the filter paper quickly. Therefore, the product may have leaked pass the filter paper into the Buchner funner or the filter flask and causing the final mass to be lesser than it should be.

Other than that, we may have not washed the crude product recrystallised crude product with distilled water thoroughly after suction filtration. Therefore, maybe not all products were rinsed from the beaker and the glass rod into the Buchner funnel. This would have caused the final mass to be lesser than it should be.

Precaution steps:

Advisable not to inhale very close when weighing the products because material is irritating to mucous membranes and upper respiratory tract.
Wear gloves and be patient and careful when conducting with concentrated chemical; to reduce the risk of injury.
Switch off the fans when weighing the material as to prevent any error in reading that may include the mass of the foreign matters.
Tong is used when taking the product out from an oven because the watch glass is hot and injure the hands.
Do not inhale tremendously when handling acetylacetone because is sting to the nose.

Conclusion:

The percentage of yield of preparation of bis(acetylacetonato)copper(II) out of 6g of copper (II) nitrate in this experiment was 81.29%, where theoretical yield is 6.52g and the experiment yield is 5.30g.

References:

Brown LeMay Bursten, Chemistry-The Central Science, 9th Edition, Page 950, 952.
Silberberg, Chemistry-The Molecular Nature of Matter and Change, McGraw Hill, 4th Edition, Page 1017, 1018.
http://www.nsc.org/library/chemical/chlorofo.htm
http://en.wikipedia.org/wiki/Ligand
http://journeytoforever.org/ethanol.html
http://en.wikipedia.org/wiki/Copper_(II)_nitrate

Questions:

Draw the structure of the complex.
Describe the structure by mentioning the ligating atoms of the ligand and its geometry.
Discuss the synthesis of the complex.

(All the questions are answered in the discussion)