Organometallic Complexes of Molybdenum Carbonyl



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Organometallic Complexes of Molybdenum Carbonyl
In this experiment, you will synthesize a series of organometallic complexes involving the transition metal molybdenum with the ligands being carbon monoxide (CO), acetonitrile (CH3CN), cycloheptatriene (C7H8), cycloheptatrienyl (C7H7+), and iodide (I). The complexes you synthesize will be spectrally characterized using 1H nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR), and ultraviolet-visible absorption spectroscopy (UV-vis). In addition, you will also perform theoretical quantum chemical calculations to obtain predicted molecular structures and vibrational spectra for these complexes.
Background Information
Transition metals in low oxidation states are able to form complexes with many unsaturated organic ligands, such as CO, acetylenes, olefins, and unsaturated ring systems. The ability of a transition metal to coordinate such molecules has led to a rich area of chemistry known as organometallic chemistry. When a CO ligand or an unsaturated organic ligand bonds to a transition metal, the electron density from the lone pair electrons for CO or from the π-electrons for the unsaturated organic ligand are donated to an empty metal d-orbital forming a σ-bond. Concurrent with the formation of the σ-bond, electron density from a filled metal d-orbital is donated to the empty π-antibonding orbital of appropriate symmetry on the CO or organic ligand forming a π-bond. This is known as metal-ligand back bonding and is illustrated below for a metal-olefin complex. For these types of ligands, the degree of σ-donating and π-accepting ability of the ligand can greatly affect the electron density (and therefore reactivity) at both the organic moiety and the metal center.

When the π-electrons of an unsaturated organic ligand are involved in the bonding to a transition metal in a complex, these compounds are often referred to as π-complexes. The notation ηx is used to describe the number of atoms, x, in the unsaturated organic ligand that are coordinated to the metal center in a π-complex. For example, if all six carbon atoms of benzene (C6H6) were coordinated to a metal, the notation would be η6-C6H6 but if only four carbons were coordinated, the notation used would be η4-C6H6.
In this experiment, the first π-complex you will synthesize will be the (η6-cycloheptatriene) molybdenum tricarbonyl complex, (η6-C7H8)Mo(CO)3. This complex will be synthesized from molybdenum hexacarbonyl in a two-step route through a tris(acetonitrile) molybdenum tricarbonyl intermediate as shown by reactions 1 and 2.
Mo(CO)6 + 3CH3CN → (CH3CN)3Mo(CO)3 + 3CO (1)
(CH3CN)3Mo(CO)3 + C7H8 → (η6-C7H8)Mo(CO)3 + 3CH3CN (2)
In the (η6-C7H8)Mo(CO)3 complex, the C7H8 ring acts as a 6-electron donor to the metal. The second π-complex you will synthesize will be the (η7-cycloheptatrienyl) molybdenum tricarbonyl tetrafluoroborate complex, (η7-C7H7)Mo(CO)3BF4, which is formed by a hydride abstraction from the C7H8 ring of (η6-C7H8)Mo(CO)3 using the triphenylcarbenium tetrafluoroborate, (C6H5)3CBF4, as shown by reaction 3.
6-C7H8)Mo(CO)3 + (C6H5)3CBF4 → (η7-C7H7)Mo(CO)3BF4 + (C6H5)3CH (3)
In the (η7-C7H7)Mo(CO)3BF4 complex, the η7-C7H7 is a planar seven-membered aromatic ring but is still only a 6-electron donor to the metal. The third π-complex you will synthesize will be the (η7-cycloheptatrienyl) molybdenum dicarbonyl iodide complex, (η7-C7H7)Mo(CO)2I, which will be produced from the reaction of (η7-C7H7)Mo(CO)3BF4 with the iodide ion, I, as shown by reaction 4.
7-C7H7)Mo(CO)3BF4 + NaI → (η7-C7H7)Mo(CO)2I + CO + NaBF4 (4)
The structural representations of the three molybdenum π-complexes you will synthesize in this experiment are given below.

Experimental Procedures
The (CH3CN)3Mo(CO)3 and (η6-C7H8)Mo(CO)3 compounds are both air-sensitive compounds and therefore reactions involving these compounds need to be performed under oxygen-free conditions. This will be accomplished by performing the reactions under a nitrogen atmosphere and using a specialized reaction apparatus, which is shown below. The basic apparatus will be assembled the lab period before you start the reaction to allow for a quick start on the day you will actually perform the first reaction.


Preparation of (CH3CN)3Mo(CO)3
The synthesis of the (CH3CN)3Mo(CO)3 complex (reaction 1) requires a reflux time of at least four hours, therefore you will need to come into the lab during the morning scheduled lab time period to get the reaction started and then you will need to come back later in the day to perform the workup of the reaction. Just before you weigh out the Mo(CO)6, you need to flame dry the round bottom flask. Weigh out 1 g of Mo(CO)6 and place it in the 50 mL round bottom flask with a magnetic stir bar. Assemble the round bottom flask with the rest of the apparatus and perform three pump-purge cycles using the Schlenk manifold (use a pinch clamp on the bubbler tubing). After this is completed, flow nitrogen through the apparatus for 2-3 minutes. Then 15 mL of nitrogen-purged CH3CN is added to the apparatus through the rubber septum and the heat setting on the hotplate is turned up to “4”. The mixture will soon begin refluxing and the solution should turn a light yellow color. The mixture needs to be refluxed for approximately four hours. After the reflux is complete, raise the apparatus up from the hotplate, then turn off and drain the condenser water. Fill the Schlenk manifold vacuum trap with liquid nitrogen, put your small vacuum pre-trap in a dry ice/acetone bath and begin to remove the CH3CN by evacuation with the laboratory vacuum line. Keep the bubbler line in place and use the pinch clamp to close off the bubbler line. During the solvent removal process, you will need to lower the apparatus back onto the hotplate after the initial evacuation of the apparatus but the heat setting should be put down to “2” or “3”. In addition, you will also most likely need to use the heat gun to warm up the condenser to prevent the CH3CN from liquefying in condenser. Once the CH3CN has been completely removed, the yellow (CH3CN)3Mo(CO)3 solid should remain in the round bottom flask. At this point, the vacuum pre-trap will contain the liquid CH3CN and this needs to be removed from the apparatus. Using one of the nitrogen lines, add nitrogen to pressurize the apparatus and once pressurized open the pinch clamp to the bubbler to allow the nitrogen to flow through the apparatus.

Preparation of (η6-C7H8)Mo(CO)3
To the round bottom flask containing the solid (CH3CN)3Mo(CO)3 is added 30 mL of nitrogen-purged hexanes and this is heated to reflux (turn heat setting to “4”). Then add 1 mL of nitrogen-purged C7H8 to the reaction mixture. The solution should be refluxed under a nitrogen atmosphere for 36 hours. After a few hours of refluxing, the solution should begin turning a reddish color. If after refluxing overnight there is still yellow solid in the round bottom flask, then you need to turn up the heat a little bit and wrap the round bottom flask in aluminum foil. Once the reflux is complete, filter the hot solution through a glass frit filter using a water aspirator to remove the insoluble material. Because the filtrate solution is the where the (η6-C7H8)Mo(CO)3 product is located, you need to protect the filtrate from potential contamination from water back flowing from the aspirator by setting up a trap between the aspirator and the filter flask. Once the reaction mixture has been filtered, transfer the filtrate to a 50 mL Erlenmeyer flask and reduce the volume of the solution to approximately 15 - 20 mL using gentle heating and a stream of nitrogen. The resulting solution is then cooled in a dry ice/acetone bath for 20 minutes to crystallize the (η6-C7H8)Mo(CO)3 product. Carefully remove the supernatant liquid with a pipet and dry the red (η6-C7H8)Mo(CO)3 solid with a stream of nitrogen for 10 minutes (or more if needed) and then transfer the solid to a pre-weighed vial. Determine the percent yield of the reaction. Obtain the melting point of the (η6-C7H8)Mo(CO)3 solid.
NMR. Obtain the 1H spectrum of (η6-C7H8)Mo(CO)3 in CDCl3. You will also obtain a two-dimensional 1H NMR spectrum (COSY) for this sample to help assign the peaks in the spectrum.

IR. Obtain the IR spectrum of the (η6-C7H8)Mo(CO)3 solid using the attenuated total reflection apparatus (ATR) and as a KBr pellet. You will also obtain the IR spectrum in solution with CCl4 as the solvent using the solution cells with the NaCl windows.

UV-vis. Obtain the UV-visible absorption spectrum of (η6-C7H8)Mo(CO)3 in CH2Cl2 in the range of 800 – 250 nm. As part of the results of the UV-visible spectra, you will determine the molar absorptivity (ε) for each of the spectral bands. In order to do this you will need to make a quantitative solution using your 25 mL volumetric flask. Any dilutions needed after the initial solution can be done with the 10 mL graduated cylinder. For each spectral band, you need to obtain a spectrum in which the maximum absorbance for the band is between 0.8 – 1.0 absorbance units. The literature data is given in the paper by Dauben and Honnen and you should consult this paper to determine the amount of solid needed to make up the initial solution.
Preparation of (η7-C7H7)Mo(CO)3BF4
For this procedure, you will need to first synthesize the triphenylcarbenium tetrafluoroborate, (C6H5)3CBF4, that will be reacted with the (η6-C7H8)Mo(CO)3 to form the desired (η7-C7H7)Mo(CO)3BF4 complex. Weigh out 0.6 g of (C6H5)3COH and place in a 25 mL Erlenmeyer flask (pre-weighed). Then add 6 mL of propionic anhydride to the flask and gently warm the mixture to dissolve the (C6H5)3COH. The solution is then cooled back to room temperature using a room temperature water bath. With the flask in the water bath, 0.7 mL of 50% HBF4 solution is added in 0.1 mL increments with swirling, taking care to keep the solution at room temperature. Once all of the HBF4 is added, the mixture is cooled in an ice bath for 25 minutes. The supernatant liquid is then carefully removed with a pipet and the remaining solid is washed six 1 mL portions of cold, dry diethyl ether (or until washings are clear). Dry the solid (C6H5)3CBF4 with a stream of nitrogen for 10 minutes (or more if needed). Determine the percent yield of the reaction.
Weigh out 0.3 g of (η6-C7H8)Mo(CO)3 and place in a 25 mL Erlenmeyer flask with a stir bar. Fit the flask with a septum and flow nitrogen through the flask for 5 minutes to purge the air in the flask. Add to the flask 8 mL of nitrogen-purged CH2Cl2 and stir to dissolve the solid. Quickly add 0.4 g of (C6H5)3CBF4 to the flask and stir the mixture at room temperature for 30 minutes under a nitrogen atmosphere. Filter the mixture through a glass frit filter to collect the orange/tan precipitate and wash the precipitate with five 1 mL portions of CH2Cl2. Dry the solid (η7-C7H7)Mo(CO)3BF4 with a stream of nitrogen for 15 minutes (or more if needed) and then transfer the solid to a pre-weighed vial. Determine the percent yield of the reaction.
NMR. Obtain the 1H spectrum of (η7-C7H7)Mo(CO)3BF4 in d6-acetone. The solution used to obtain the NMR of this compound will also be used to obtain the NMR and UV-vis spectra of the (η7-C7H7)Mo(CO)2I complex described below. Weigh between 8 – 10 mg of the compound (record exact weight) to prepare the sample for the NMR spectrum and do not discard the solution after the NMR is obtained.

IR. Obtain the IR spectrum of the (η7-C7H7)Mo(CO)3BF4 solid using the ATR apparatus and as a KBr pellet. You will also obtain the IR spectrum in solution with acetone as the solvent using the solution cells with the CaF2 windows.

UV-vis. Obtain the UV-visible absorption spectrum of (η7-C7H7)Mo(CO)3BF4 in CH2Cl2 in the range of 800 – 250 nm. As part of the results of the UV-visible spectra, you will determine the molar absorptivity (ε) for each of the spectral bands. In order to do this you will need to make a quantitative solution using your 25 mL volumetric flask. Any dilutions needed after the initial solution can be done with the 10 mL graduated cylinder. For each spectral band, you need to obtain a spectrum in which the maximum absorbance for the band is between 0.8 – 1.0 absorbance units. The literature data is given in the paper by Dauben and Honnen and you should consult this paper to determine the amount of solid needed to make up the initial solution.
Preparation of (η7-C7H7)Mo(CO)2I
You will not isolate the (η7-C7H7)Mo(CO)2I complex but instead will produce this complex in solution (in situ) from the (η7-C7H7)Mo(CO)3BF4 complex according to reaction 4 and then characterize the (η7-C7H7)Mo(CO)2I complex using NMR, IR, and UV-visible spectroscopy.
NMR. For obtaining the 1H NMR spectrum of the (η7-C7H7)Mo(CO)2I complex, you will use the d6-acetone solution that was used to obtain the NMR spectrum of the (η7-C7H7)Mo(CO)3BF4 complex. Based on the amount of the (η7-C7H7)Mo(CO)3BF4 complex used to prepare the previous NMR solution, calculate the amount of solid sodium iodide (NaI) needed for the stoichiometric reaction and then weigh a slight excess of NaI. Add the NaI solid to the NMR solution in d6-acetone. The solution should immediately change color to a deep green color of the (η7-C7H7)Mo(CO)2I complex. Obtain the 1H NMR spectrum of (η7-C7H7)Mo(CO)2I in d6-acetone. After you have obtained the NMR spectrum of this solution, save the solution for obtaining the UV-visible absorption spectrum.

IR. For obtaining the IR spectrum of the (η7-C7H7)Mo(CO)2I complex, take the acetone solution that was used to obtain the IR spectrum of the (η7-C7H7)Mo(CO)3BF4 complex and add a small amount of solid sodium iodide to the solution. The solution should immediately change color to a deep green color of the (η7-C7H7)Mo(CO)2I complex. Obtain the IR spectrum of (η7-C7H7)Mo(CO)2I in acetone using the solution cells with the CaF2 windows.

UV-vis. Obtain the UV-visible absorption spectrum of the (η7-C7H7)Mo(CO)2I complex in acetone in the range of 800 – 330 nm. As part of the results of the UV-visible spectra, you will determine the molar absorptivity (ε) for each of the spectral bands. In order to do this you will need to make a quantitative solution using your 25 mL volumetric flask, i.e., calculate the concentration of the solution. You will use the solution used to obtain the NMR spectrum to obtain the UV-visible spectrum of this complex. Transfer the entire d6-acetone NMR solution into a 25 mL volumetric flask and dilute the sample with non-deuterated acetone (use some of the non-deuterated acetone to rinse the NMR tube). Any dilutions needed after the initial solution can be done with a 10 mL graduated cylinder.
Theoretical Procedures
You will perform theoretical quantum chemical calculations using a computational chemistry software package to obtain predicted molecular structures and predicted vibrational spectra for the (η6-C7H8)Mo(CO)3, (η7-C7H7)Mo(CO)3+, and (η7-C7H7)Mo(CO)2I complexes. The GaussView 3.0 software program will be used to build each of the complexes and to visualize the molecular structure and vibrational frequency results. The actual computational chemistry calculations are performed using the Gaussian 03W software program. For each of these complexes, you will be performing a geometry optimization to determine the most stable molecular structure of the complex (called a minimum energy structure). Once the most stable structure has been obtained for a given complex, you will then have the Gaussian 03W program perform a calculation to predict all of the vibrational frequency modes of the complex. The theoretical calculations will be performed using a theoretical method called density functional theory (DFT) and in particular you will use the specific B3LYP hybrid functional. This method/functional will be used with a basis set (set of functions representing the atomic orbitals) known as LANL2DZ which has been developed for use with heavy atoms such as Mo and I. After you build each of the complexes using GaussView, then you will need to set the appropriate parameters for the calculation. This is done in GaussView by going to the “Calculate” menu and then selecting “Gaussian”. This will open the “Gaussian Calculation Setup” dialog box. Select the appropriate parameters in the various tabs as described below.
Job Type: Opt+Freq

Method: Ground State, DFT, B3LYP, Basis Set: LanL2DZ, Charge: 0 or 1, Spin: Singlet

Title: Type in a descriptive title of molecule

Link 0: Delete any default text in this section

General: Leave everything as the default choices

Guess: Leave as default choices

NBO: Make sure “None” is selected

Solvation: Make sure “None” is selected
After setting the parameters, click on the “Retain” button at the bottom of the dialogue box. Then go to the “File” menu and choose “Save”. In the “Save As” dialogue box, click “Write Cartesians”, choose Save As “Gaussian Input File”, choose a filename, browse to the appropriate directory to save your file and then click “Save”. Here are some details for the optimization and frequency calculations for each of the complexes you will be performing.
6-C7H8)Mo(CO)3: charge = 0, multiplicity = 1, opt = loose, freq. You need to add the “loose” part to the opt keyword in the .gjf text file by editing the file with Notepad.
7-C7H7)Mo(CO)3+: charge = 1, multiplicity = 1, opt, freq
7-C7H7)Mo(CO)2I: charge = 0, multiplicity = 1, opt, freq
References
“Chapter 16: d- and f-Block Organometallics”, Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry; W. H. Freeman: New York, 1990, pp. 498-536. Handout on Angel.
“Chapter 12: Organometallic Chemistry”, Miessler, G. L.; Tarr, D. A. Inorganic Chemistry; Prentice Hall: Engelwood Cliffs, NJ, 1991, pp. 411-463. Handout on Angel.
“Chapter 27: Computational Chemistry”, Hehre, W. J. in Physical Chemistry; Engel, T.; Reid, P.; Benjamin Cummings: San Francisco, CA, 2006, pp. 597-692. Handout on Angel.
“π-Tropylium-Molybdenum-Tricarbonyl Tetrafluoroborate”, Dauben, H. J.; Honnen, L. R. J. Am. Chem. Soc. 1958, 80, 5570.
“Struktur des Cycloheptatrien-Molybdan-Tricarbonyls”, C7H8Mo(CO)3”, Dunitz, J. D.; Pauling, P. Helv. Chim. Acta 1960, 43, 2188.
“The Crystal and Molecular Structure of π-Cycloheptatrienyltricarbonylmolybdenum(0) Tetrafluoroborate”, Clark, G. R.; Palenik, G. J. J. Organometallic Chem. 1973, 50, 185.
Report Guidelines
The report should contain the following main section headings and subsection headings given below. You will be using the Journal of the American Chemical Society article template (on Angel) to write the report for this experiment. This will produce a report that will look just like a Journal of the American Chemical Society article. Figures should be inserted into the document as either TIFF or JPEG images and should be sized to fit the single column width (3.25 inches) or double column width (7.0 inches). The resolution needs to be high enough that the figures are clear and readable at this size. The final printed report should be single sided. Two articles (one experimental and one theoretical article) from a recent issue of the Journal of the American Chemical Society have been uploaded to Angel for you to use as examples of the report style.
Abstract

  • This should be a concise summary (250 words maximum) of the significant results and conclusions.


Introduction

  • Brief outline of background information pertinent to the experiment and calculations (do not copy the information from my handout verbatim).

  • Cite references as needed.


Experimental and Computational Methods

  • For each compound you synthesized, you should give a brief overview of experimental procedure with the final yields given. This should then be followed by the NMR, IR, and UV-vis, and melting point data for the compound (use the ACS style to present this data, i.e., look at the example journal article).

  • For the computational methods, list the software used to perform the calculations and also described what types of calculations were performed and the specific methods and basis sets used for the calculations.


Results and Discussion

  • This is the section where you will present and discuss the main results, both experimental and theoretical, from this experiment. I want you to present and discuss the results according to the various characterization techniques using the subheadings given below.

NMR Spectra:

Present the 1H NMR data (chemical shifts, integrated areas, peak assignments) for the (η6-C7H8)Mo(CO)3, (η7-C7H7)Mo(CO)3BF4, and (η7-C7H7)Mo(CO)2I complexes. Discuss this data in terms of the structures of each of the individual complexes and also discuss the spectral changes between the three complexes. You do not need to insert the full NMR spectra in this section of the report but I do want you to attach the full NMR spectra of your compounds at the end of the report as supplementary material.



IR Spectra:

Present the IR spectral data for the (η6-C7H8)Mo(CO)3, (η7-C7H7)Mo(CO)3BF4, and (η7-C7H7)Mo(CO)2I complexes. Discuss the spectra of the solids using the ATR and as a KBr pellet for the (η6-C7H8)Mo(CO)3 and (η7-C7H7)Mo(CO)3BF4 complexes and also discuss the carbonyl stretching peaks in the ATR and KBr spectra in relation to the carbonyl stretching peaks in the solution phase spectra for each complex. Discuss the changes in the carbonyl stretching frequencies in the solution phase spectra for all three of the complexes. You do not need to insert the full ATR and KBr pellet IR spectra in this section of the report but I do want you to attach the full ATR and KBr pellet IR spectra of your compounds at the end of the report as supplementary material. However, I do want you insert the IR spectra of the carbonyl region for each type of IR spectrum you obtained (ATR, KBr, and solution) for each complex as a figure in this section. Use absorbance as the y-axis for all inserted spectra.



UV-visible Spectra:

Present the UV-visible spectral data for the (η6-C7H8)Mo(CO)3, (η7-C7H7)Mo(CO)3BF4 and (η7-C7H7)Mo(CO)2I complexes. For the (η6-C7H8)Mo(CO)3 and (η7-C7H7)Mo(CO)3BF4 complexes, compare the molar absorptivities (ε) you determined for the spectral bands to the literature values. Also comment on the general changes in the visible spectral bands for the three complexes and how these correlate to the colors of the solutions. You do not need to insert the UV-vis spectra in this section of the report but I do want you to attach the UV-vis spectra of your compounds at the end of the report as supplementary material.



Computational Results:

Present the theoretical molecular structures for the (η6-C7H8)Mo(CO)3, (η7-C7H7)Mo(CO)3+, and (η7-C7H7)Mo(CO)2I complexes. Compile the Mo–C and C–O bond distances for all three complexes and for the (η6-C7H8)Mo(CO)3 and (η7-C7H7)Mo(CO)3+ complexes compare these to the literature values from the X-ray crystal structures. For each of the structures for which you performed theoretical calculations, you need to insert an image of the optimized structure in this section.



Present the theoretical vibrational frequencies for the carbonyl stretching frequencies for the (η6-C7H8)Mo(CO)3, (η7-C7H7)Mo(CO)3+, and (η7-C7H7)Mo(CO)2I complexes. Compare the theoretical carbonyl stretching frequencies for each compound to the experimental values you obtained from the solution phase IR spectra. For the theoretical carbonyl stretching region, I want you to insert the simulated theoretical vibrational spectrum of this region for each complex as a figure in this section.
Conclusion

  • Summarize the significant experimental and theoretical results and the major conclusions reached. This section should not be a verbatim copy of the abstract.


References

  • Use ACS style (see below for examples, the ACS Style Guide, and any ACS journal). Here are a few examples.

  • Journal article: Amicangelo, J. C., Armentrout, P. B. J. Phys. Chem. A 2000, 104, 11420.

  • Books with editors: Principles of Molecular Recognition; Buckingham, A. D., Ed.; Blackie Academic: Glasgow, 1993, pp 345 - 376.

  • Books without editors: Gilbert, R. G. Theory of Unimolecular and Recombination Reactions; Blackwell Scientific: Oxford, 1990, pp 118 - 124.

  • Lab Handouts: Amicangelo, J. C. Handout for NMR Determination of Keto-Enol Equilibrium Constants, Penn State Erie, Fall 2003.




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