Synthesis and Reactivity of a Cycloheptatriene Molybdenum Complex

Sesime Akoto.

IntroductionFormation of a metal arene complex is

achieved by the donation of π electrons to an empty metal d-orbital to form a σ-bond.1

There is also electron donation from filled metal d orbitals to vacant π-antibonding orbitals of the arene. Arenes are referred to as σ-donor and π-acceptor ligands. 1

Reaction Details. This reaction involves the synthesis, characterization and reactivity of

η6-cyclohepatriene molybdenum tricarbonyl, (η6-C7H8)Mo(CO)3 (1). 1 is synthesized from the direct reaction of molybdenum hexacarbonyl, Mo(CO)6 with cycloheptatriene.

The oxidation number of molybdenum is 0. Kinetic studies have suggested that the mechanism of arene metal

tricarbonyl formation involves SN1 dissociation: Mo(CO)6 -> Mo(CO)5 + CO.2  

Reaction Details continued.

A hydrogen abstraction reaction is then performed using the triphenylcarbenium ion (triphenylcarbenium hexaflurorphosphate) to form [(η7-C7H7)Mo(CO)3][PF6] (2).

•The final reaction involves CO substitution by I- to form (η7-C7H7)Mo(CO)2I (3). 2 is reacted with NaI in acetone-d6.

Hypothesis

Formation of (η6-C7H8)Mo(CO)3 : It is predicted that that the hydrogens of coordinated cycloheptatriene should be unequivalent and display multiple NMR signals.

Hydrogen abstraction not only increases haptacity but the hydrogens of cycloheptatriene should become equivalent. A single proton signal should be observed.

ExperimentalMo(CO)6 (2.0g) was added to cyclopheptatriene

(8.0mL) in 20 mL of nonane. Mixture was refluxed for 2 hours with stirrings. Water inlet hose was wrapped around the top of the flask to prompt sublimation of unreacted Mo(CO)6 at the top of the flask. The solution is cooled to room temperature under nitrogen. Hexane (15ml) was added to the reaction mixture and filtered to isolate the red crystals. The crystals were washed with hexanes. Further purification involves dissolving the red crystals in chloroform, filtering the solution and reprecipitating the crystals with the addition of hexanes. The crystals were filtered and dried.

Product: red crystals, 28% yield.

Results: IR of 1

υCO 1970, 1904, 1852 cm-1

NMR of 1

Signals: δ5.2,4.15, 2.8, 2.1ppm

Literature NMR and IR

NMR Signals: δ 2.45,3.00, 3.60, 4.92, 6.05 ppm.1

IR: υCO 1984, 1915, 1887 cm-1. 1

Complications A deep red solution was obtained in lab as detailed in

literature. Red crystals were also obtained but NMR signals do not match literature NMR signals.

The procedure followed did not specify performing the reflux under nitrogen. All other synthesis procedures for 1 specified refluxing under inert conditions.3

Mo(CO)6 tends to react with oxygen at high temperatures. More importantly, arene metal complexes are weakly air sensitive in the solid state but are unstable to air. It is reported that metal arenes would react with oxygen at high temperatures.4

Oxygen solubility decreases with increasing solvent temperature. This is probably the reasoning behind the procedure details of refluxing without nitrogen but cooling under nitrogen. However, this procedure proved unsuccessful in this experiment.

Reaction Mixture

Possible Explanations of NMR Spectrum

DecompositionUnreacted cycloheptatriene?

Comparison to Cycloheptatriene NMR Spectrum Cycloheptatriene signals: δ6.58, 6.18, 5.36, 2.42 ppm Product signals: δ5.2,4.15, 2.8, 2.1ppm

What was expected.

If 1 was successful synthesized without decomposition: 1H NMR of 1 should show 5 multiplet peaks for the five unique hydrogens at 2.45, 3.00, 3.60, 4.92, and 6.05ppm. The endo and exo hydrogens are not resolvable on lower field instruments. 1

IR: υCO 1984 cm-1, 1915 cm-1, 1887cm-1.

Expected Results.1 is reacted with an equimolar

quantity of [(C6H5)3C][PF6] to produce a orange air-stable salt, 2. 1

NMR: single peak at δ6.6 ppm. Hydrogen abstraction forms the aromatic tropylium ligand C7H7)+ and all hydrogen sites become equivalent.1

IR υCO 2076 cm-1 and 1963 cm-1. 1

Expected ResultsReaction of 2 with NaI in an NMR tube

(acetone-d6 is the solvent) will produces gas evolution and turns the solution dark green . Substitution of CO by I- in [η7-C7H7)Mo(CO)3]+ does not result in formal metal reduction. 1

1H NMR: single peak at δ5.7 ppm. Singlet confirms retention of the aromatic nature of the ligand. The singlet is upfield because the π donor I- replaces the π acceptor CO, resulting in increased electron density at the metal and π-ligand. 1

Improvements and Conclusion.

The main improvement to this synthesis of 1 is would be to perform the reflux and isolation under nitrogen. Oxygen solubility decreases with increasing temperature while nitrogen gas solubility in common organic solvents is reported to increase with increasing temperature.5

The reaction could be refluxed for a longer period of time to maximize the yield of 1 since molybdenum hexacarbonyl has a tendency to sublime in the condenser.

Works Cited1. Timmers, F.J.; Wacholtz, W.F.; An Advanced Inorganic

Laboratory experiment Using Synthesis and Reactivtiy of Cycloheptatriene Molybdenum Complex. J. Chem. Educ., 1994, 71 (11), p 987-990.

2. Graham, J.R.; Angelici, R.J.; Kinetic Studies of Group VI Metal carbonyl complexes. Substitiion Reactions of Group VI Metal Hexacarbonyls. J. Am. Chem. Soc.,1967, 6 , 88, p2082-2085

3. Experiment 4: Transition Metal Organometallic and Metal-Organic Chemistry. http://www.uiowa.edu/~c004153a/MoCOx-f04rev.pdf

4. Girolami, G.S.; Rauchfuss, T.B.; Angelici, R.J.; Synthesis and Technique in Inorganic Chemistry: A Laboratory manual, 3rd ed.; University Science Books: Sausalito, CA.

5. Why does the solubility of gases usually increase as temperature goes down? http://antoine.frostburg.edu/chem/senese/101/solutions/faq/temperature-gas-solubility.shtml