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cis-Mo(CO)4(pip)2
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cis-Mo(CO)4(PPh3)2
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trans-Mo(CO)4(PPh3)2
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last updated
Theory
The experiment aims to investigate the cis-trans isomerization of Mo(CO)4(PPh3)2. In order to obtain these compounds, molybdenum hexacarbonyl is first reacted with piperidine (pip), which leads to the formation of cis-Mo(CO)4(pip)2. The ligand exchange with triphenylphosphine at low temperature (40 oC, boiling dichloromethane) affords the cis-Mo(CO)4(PPh3)2. At elevated temperatures (110 oC, boiling toluene), this compound is converted into the trans-isomer. Thus, the cis isomer can be regarded as the kinetic product, while the trans isomer is the thermodynamic product. The trans isomer can also be obtained directly by the reaction of Mo(CO)6 with two equivalents of triphenylphosphine in a microwave reaction, most likely because the reaction temperature is higher in the microwave reaction favoring the thermodynamic product.
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cis-Mo(CO)4(pip)2
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cis-Mo(CO)4(PPh3)2
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trans-Mo(CO)4(PPh3)2
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Kinetic studies have shown that the cis-Mo(CO)4(PR3)2 compounds lose the phosphine ligand at very different rates. While phosphine ligands with small cone angles are lost very slowly, phosphine ligands with large cone angles are lost very fast. This step determines the overall rate for the isomerization because the initial step appears to be the dissociation of the phosphine ligand leads to Mo(CO)4(PPh3). The average Mo-P bond distances seem to be a good measure for the dissociation kinetics as well.
| PR3 | Cone angle | Average Mo-P distance | Rate of PR3 loss from Mo(CO)4(PR3)2 | 95Mo-NMR shift |
| PPhMe2 | 122o | 252.9 pm | very slowly | -1637 ppm (cis, CH2Cl2) -1655 ppm (trans, CH2Cl2) |
| P(OPh)3 | 128o | 244.3 pm | very slowly | -1754 ppm (cis, CH2Cl2) -1792 ppm (trans, CH2Cl2) |
| P(n-Bu)3 | 132o | 255.2 pm | very slowly | -1742 ppm (cis, CH2Cl2) -1741 ppm (trans, toluene) |
| PPh3 | 145o | 257.7 pm | fast | -1556 ppm (cis, CH2Cl2) |
| PCy2Ph | ~162o | very fast | ||
| PCy3 | 170o | 265.5 pm | ??? | -1765 ppm (trans, CH2Cl2) |
| Average Mo-N distance | ||||
| Piperidine | 234.5 pm | -1075 ppm (cis, CH2Cl2) | ||
| Pyrrolidine | -1140 ppm (cis, CH2Cl2) | |||
| Pyridine | ~225 ppm | -1046 ppm (cis, CH2Cl2) | ||
Phosphite ligand (i.e., P(OPh)3) display a stronger bond than phosphine ligands (i.e., PPh3). The Mo-P bond distances are shorter and the Mo-P coupling constants are higher as well: cis-Mo(CO)4(P(OPh)3)2: 250 Hz; cis-Mo(CO)4(PPh3)2: 140 Hz.
Experiment
Safety: Molybdenum hexacarbonyl and its derivative are highly toxic and volatile. Make sure to handle the compounds in the hood. Toluene is flammable and dichloromethane is a suspected carcinogen. All procedures have to be performed in the hood.
a. cis-Mo(CO)4(pip)2
Note that the piperidine has to be probably dried over potassium hydroxide first to remove the water and oxidation products.
While it is not really required, the reaction should be carried out under the exclusion of air to reduce the oxidation products. During the reflux, a bright yellow precipitate will form. It is very important to filter the solution while it is still hot in order to keep the Mo(CO)4(pip) in solution.
b. cis-Mo(CO)4(PPh3)2
While it is not really required, the reaction should be carried out under the exclusion of air to reduce the oxidation products. cis-Mo(CO)4(pip)2 is reacted with 2.2 equivalents of triphenylphosphine in dry, boiling dichloromethane. After the refllux, the volume of the solution has to be reduced to ~20 mL before methanol is added. Upon storage in an ice-bath, light yellow crystals form.
c. trans-Mo(CO)4(PPh3)2
While it is not really required, the reaction should be carried out under the exclusion of air to reduce the oxidation products. cis-Mo(CO)4(PPh3)2 is refluxed in toluene for 30 minutes. After cooling the mixture, chloroform is added to keep the unreacted cis isomer in solution. The mixture is filtered and methanol is added to the filtrate. Upon cooling, the off-white (or very pale yellow) solid is formed.
Characterization
a. Infrared spectroscopy
The infrared spectrum are aquired on the spectrometer in YH 6076 (ATR setup). Make sure to get an expansion of the carbonyl range.
b. 13C- and 31P-NMR spectrum
The solvent here is CDCl3. Both measurements can be set up together on the 400 MHz autosampler because this spectrometer possesses a quad probe that can measure 1H, 13C, 19F and 31P-NMR spectra. Make sure to follow the directions for preparing NMR samples. Make sure to follow the directions given by the staff in the NMR lab (i.e., Dr. Strouse, Dr. Taylor, etc.).
c. 95Mo-NMR spectrum
The samples should be a saturated solution of the compound in dry dichloromethane or dry toluene. Make sure to purge that the NMR tube is dry and purged with nitrogen or argon prior placing the sample in it. The samples have to be prepared and measured immediately in order to have a chance to observe a signal. The solution should be as concentrated as possible.
Hints to pre-lab questions
ad 2: What governs how many signals and appear in the NMR spectrum? What determines how the signal appears?
ad 4: How can one rationalize different bond distance in coordination compounds?
ad 7: Which geometric isomers are possible for MX3L3? What determines with of these isomers is favored?