The experiment aims to inspection the cis-trans isomerization the Mo(CO)4(PPh3)2. In bespeak to acquire these compounds, molybdenum hexacarbonyl is first reacted through piperidine (pip), which leads to the development of cis-Mo(CO)4(pip)2. The ligand exchange through triphenylphosphine at low temperature (40 oC, boiling dichloromethane) affords the cis-Mo(CO)4(PPh3)2. At elevated temperatures (110 oC, cook toluene), this compound is converted into the trans-isomer. Thus, the cis isomer deserve to be concerned as the kinetic product, when the infectious diseases worldwide isomer is the thermodynamic product. The trans isomer can additionally be derived directly through the reaction of Mo(CO)6 through two equivalents of triphenylphosphine in a microwave reaction, many likely since the reaction temperature is greater in the microwave reaction favoring the thermodynamic product.

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cis-Mo(CO)4(pip)2
cis-Mo(CO)4(PPh3)2
trans-Mo(CO)4(PPh3)2

Kinetic studies have shown that the cis-Mo(CO)4(PR3)2 compounds shed the phosphine ligand at really different rates. When phosphine ligands with little cone angles are lost really slowly, phosphine ligands with big cone angles are lost an extremely fast. This action determines the all at once rate because that the isomerization because the initial step appears to it is in the dissociation that the phosphine ligand leader to Mo(CO)4(PPh3). The mean Mo-P bond distances seem to be a good measure because that the dissociation kinetics as well.

PR3Cone angleAverage Mo-P distanceRate that PR3 loss native Mo(CO)4(PR3)295Mo-NMRshift
PPhMe2122o252.9 pmvery slowly-1637 ppm (cis, CH2Cl2)-1655 ppm (trans, CH2Cl2)
P(OPh)3128o244.3 pmvery slowly-1754 ppm (cis, CH2Cl2)-1792 ppm (trans, CH2Cl2)
P(n-Bu)3132o255.2 pmvery slowly-1742 ppm (cis, CH2Cl2)-1741 ppm (trans, toluene)
PPh3145o257.7 pmfast -1556 ppm (cis, CH2Cl2)
PCy2Ph~162overy fast
PCy3170o265.5 pm???-1765 ppm (trans, CH2Cl2)
Average Mo-N distance
Piperidine234.5 pm-1075 ppm (cis, CH2Cl2)
Pyrrolidine-1140 ppm (cis, CH2Cl2)
Pyridine~225 ppm-1046 ppm (cis, CH2Cl2)

Phosphite ligand (i.e., P(OPh)3) screen a stronger bond 보다 phosphine ligands (i.e., PPh3). The Mo-P bond ranges are much shorter and the Mo-P coupling constants are greater 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 manage the compound in the hood. Toluene is flammable and also dichloromethane is a suspected carcinogen. Every procedures have to be carry out 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 also oxidation products.

While the is not really required, the reaction have to be brought out under the exclusion of wait to reduce the oxidation products. During the reflux, a glowing yellow precipitate will form. The is an extremely important to filter the systems while it is still hot in bespeak to store the Mo(CO)4(pip) in solution.

b. Cis-Mo(CO)4(PPh3)2

While it is not really required, the reaction must be lugged out under the exclusion of wait to minimize the oxidation products. Cis-Mo(CO)4(pip)2 is reacted v 2.2 equivalents the triphenylphosphine in dry, boil dichloromethane. After the refllux, the volume the the solution needs to be lessened to ~20 mL before methanol is added. Upon storage in one ice-bath, irradiate yellow crystals form.

c. Trans-Mo(CO)4(PPh3)2

While that is no really required, the reaction should be lugged out under the exemption of waiting to mitigate the oxidation products. Cis-Mo(CO)4(PPh3)2 is refluxed in toluene because that 30 minutes. ~ cooling the mixture, chloroform is included to save the unreacted cis isomer in solution. The mixture is filtered and also methanol is included to the filtrate. Upon cooling, the off-white (or an extremely pale yellow) heavy is formed.

Characterization

a. Infrared spectroscopy

The infrared spectrum are aquired top top the spectrometer in YH 6076 (ATR setup). Make sure to obtain an development of the carbonyl range.

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b. 13C- and 31P-NMR spectrum

The solvent here is CDCl3. Both measurements have the right to be collection up with each other on the 400 MHz autosampler since this spectrometer possesses a quad probe that have the right to measure 1H, 13C, 19Fand 31P-NMR spectra. Make sure to follow the directions because that preparing NMRsamples. Make certain to monitor the directions offered by the staff in the NMRlab (i.e., Dr. Strouse, Dr. Taylor, etc.).

c. 95Mo-NMR spectrum

The samples have to be a saturated equipment of the compound in dry dichloromethane or dry toluene. Make sure to purge the the NMRtube is dry and purged v nitrogen or argon former placing the sample in it. The samples have to be prepared and also measured instantly in order to have a opportunity to observe a signal. The solution must be as concentrated as possible.

Hints come pre-lab questionsad 2: What governs how numerous signals and appear in the NMRspectrum?What determines just how the signal appears?ad 4: How deserve to one rationalize various bond distance in coordination compounds?ad 7: i beg your pardon geometric isomers are feasible for MX3L3? What determines v of this isomers is favored?