D. ORGANOMETALLIC COMPOUNDS

Organometallic complexes are compounds containing both the organic and metal species. They are synthesized by the formation of the metal-carbon covalent bond. C containing part is called as organic ligand. The coordination number of the metal can have a range between 3-9 with a number of chemical substances which have different chemical and physical properties.

Most of the organometallic compounds which have been investigated up to now have coordination number 6 or 4. Complexes with coordination number 5 are less common, but have received intensive study in recent years.i In the following experiments you will synthesize both six-coordinate (tris(acetylacetonato)aluminium(III)) and five-coordinate (vanadyl acetylacetonate) complexes.

Experiment 6: Synthesis of tris-(acetylacetanato) aluminium (III):

Introduction:

Acetylacetonate is a common bidentate ligand, and the source is . Acetylacetone undergoes tautomerization in solution and exists in the keto and enol forms. (Figure 1) In basic medium, enol form loses the hyrdoxyl H so negatively charged acetylacetonate is formed which is resonance stabilized and called as a chelate. (Figure 2)

O O

Figure 1. Figure 2.

Two covalent bonds are formed between the metal atom and the corresponding carbon during the reaction of organometallic compounds of acetylacetonate (bidantate ligand). Since the oxidation state is +3 for Al and -1 acetylacetonate three acetylacetonates react with one Al and then forms a complex with octahedral geometry.

Procedure

1) Dissolve 4 grams of Al(NO3)3·9H2O in about 25 mL of distilled water. 2) Check the pH of the solution. 3) Add 1.25 mL of acetylacetone to the solution, mix and stir well. 4) Measure 10 mL concentrated ammonia and add it to the solution. Then continue to add ammonia dropwise until the solution is just alkaline. 5) Cool the resultant solution in ice. 6) Filter by suction filtration and then dry on a hot plate, which is kept around 700C. 7) To remove any Al(OH)3 impurity, take up the product (while it is still hot) in dichloromethane(DCM) and filter it. Your product and DCM is in the solution form while the impurity Al(OH)3 is in powder form. 8) Evaporate off the DCM to retrieve tris(acetylacetonato)aluminium(III). 9) Record the yield and measure the of your product. 10) Check the solubility of the product in , ether and acetone.

Questions: 2+ 1. The colors of most of the metal complexes are very bright, e.g. deep blue [Cu(NH 3 ) 4 ] , 3− blood red [Fe(SCN) 6 ] . Your product (acetylacetonato)aluminium(III) has a very light yellowish white color. The colors of the metal complexes are disscussed by crystal field theory. By using the basic principles of the crystal field theory, disscuss the light color of your product. (HINT: The nature of the metal ion, the metal's oxidation state, the nature of the ligands, the geometry of the coordination complex) Experiment 7: Preparation of vanadyl acetylacetonate - [VO(acac)2]

Introduction:

Vanadyl acetylacetonate is an organometallic compound which is mostly used for epoxidation reactions. It is synthesized mostly from vanadyl oxides which will also be done in today’s experiment.

Formation of vanadyl acetylacetonate is a two step reaction. First, V2O5 is reduced to vanadyl 2+ ion [VO] which is found as vanadyl sulphate ([V(O)(H2O)4]SO4) V2O5 + 7 H2O + SO2 + H2SO4 → 2 [V(O)(H2O)4]SO4 Then vanadyl sulphate reacts with acetyl acetone in basic medium: [V(O)(H2O)4]SO4 + C5H8O2 + Na2CO3 → [V(O)(C5H7O2)2 + Na2SO4 + 4 H2O + CO2 and the overall reaction is: 2 V2O5 + 9 C5H8O2 → 4 VO(C5H7O2)2 + (CH3CO)2CO + 5 H2O

The product vanadyl acetylacetonate has a square pyramidal geometry –with coordination number 5- as shown below: O

O O V O O

V=O is extremely short and always positioned as axial ligand.

Procedure:

1) Mix 6 ml of H2O, 3 ml of H2SO4, and 9 ml of ethanol in a a 50 ml round bottom flask. 2) To the liquid add ~1.80 g of V2O5. 3) Attach a condenser to the flask and heat it at reflux for 1h with stirring. (In this step, 2+ (IV) oxide (V2O5) is reduced to vanadyl ion ([VO] )). 4) Filter the solution. 5) Add ~3 ml of acetylacetone to the filtrate. 6) Dissolve 7.8 g of Na2CO3 in 16 ml of H2O and add it dropwise to the stirred reaction solution. 7) A blue-green precipitate formed. Fiter the product, wash with H2O, and dry. 8) Record the yield and measure the melting point.

Reference:

1) I.S. Butler and J.F. Harrod, Inorganic Chemistry, (Redwood City, Cf.: Benjamin/Cummings, 1989), p. 221ff. (A simple introduction to ESR spectroscopy.) [QD151.2.B88]