Synthesis and Characterization of Organometallic Mo Complex
Organomettalic chemistry is the chemistry of compounds containing metal-carbon bonds. The interaction of benzene and its derivatives with transition metal to form a stable complexes provides a great understanding of the behavior and the structure of organometallic complex. In this experiment mesitylene, a benzene derivative displaces three carbonyl ligands from Mo(CO)6 complex to form a stable [1,3,5-C6H3(CH3)3](Mo(CO)3 . In this complex, the three π bonds in the benzene ring act as electron donor, forming three σ bonds with the metal. The metal is therefore forms a 6-coordinate complex with an octahedral geometry. This experiment is aimed at synthesizing the octahedral complex and using UV/vis and IR to characterize the product.
Mo(CO) (1.0 g) was added to to 5 mL (36 mmol) mesitylene into a 50 mL round-bottom flask having a side-arm and stopcock. The flask was set up in a refluxing apparatus (Figure 14.1 below) and the side-arm was connected to a nitrogen cylinder via a rubber tubing. The apparatus was then flushed with moderate stream of nitrogen for 5 minutes after which the gas was turned off and the stopcock closed. Refluxing was done for 30 minutes. Immediately refluxing was completed, the heating mantle was removed and nitrogen was turned on again to prevent the mineral oil and mercury from being drawn back into the mixture and to sublime out any remaining Mo(CO)6 from the refluxed mixture.
Nitrogen stream was turned off after the mixture was cooled. 7.5 mL of hexane was then added to the mixture to complete precipitation. The mixture was then filtered (using a suction filter) and 5 mL hexane was further used to rinse the precipitate. To purify the product, it was dissolved in 10 mL of CH2Cl2 and the mixture was the filtered to remove insoluble impurities. 12.5 mL of hexane was then used to precipitate the pure product. The product was filtered and washed twice with 4 mL hexane. It was then allowed to dry under the aspiration of the suction filter. The final product was then weighed in order to obtain the percentage yield.
Result and discussion
The coordination number for Mo is 6 in this complex because at zero oxidation state, the metal has 6d electrons and requires 6 ligands donating 2 sigma electrons each in order to form a stable organometallic configuration which also satisfies the 18 electron rule. The rule is maintained in the aromatic product obtained because the benzene ring has three localized double bonds and each bond behaves as a ligand. So 6 electrons from the ring and the 6 from the three remaining CO ligands complete the 18 electron rule and thus form a stable octahedral complex.
N2 is both a weaker σ donor and a weaker π acceptor than CO ligand. So it is not strong enough to displace or substitute CO in the Mo(CO)6 complex. So there will be no reaction. If the reaction of Mo(CO)6 with mesitylene were conducted in the presence of air, the complex is oxidized to a cation [[1,3,5-C6H3(CH3)3](Mo(CO)3]+and the CO is oxidized to CO2.The reaction of [1,3,5-C6H3(CH3)3](Mo(CO)3 with P(OCH3)3 is by Associative mechanism since the rate is a second order reaction rate. The dissociation of the mesitylene ligand is slow due to steric hindrance caused by the two bulky ligands on the metal. The reaction with PF3 will be slower because it is less bulkier than mesitylene ligand. Bulkier ligands around a metal lead to a more rapid ligand dissociation due to crowding around the metal.
1. RefVijaya, R., Murti, Y.V.G.S., Vijayaraj, T.A. and Sundararajan, G., Opt. Quantum Electron. 25, p. 723.
2 . A. Furlani, S. Licoccia and M.V. Russo. J. Polym. Sci., Polym. Chem. Ed. 24 (1986), p. 991.
3 V. Shivasubramaniam and G. Sundararajan. J. Mol. Catal. 65 (1991), p. 205. |
4. Vogel's Text Book of Practical Organic Chemistry (4th Ed. ed.), English Language Book Society and Longman, London (1978), p. 349.