UTMUTMUNIVERSITI TEKNOLOGI MALAYSIA

GROUP MEMBERS:GROUP MEMBERS:NG MEI XIANGNG MEI XIANG

NOOR HIDAYAH MOHD JUSOHNOOR HIDAYAH MOHD JUSOHLEE YEE VOONLEE YEE VOON

NORADILA AWANG NORADILA AWANG MOHD NORULAKMAL AB MALIK MOHD NORULAKMAL AB MALIK

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EXPERIMENT 5:EXPERIMENT 5:IDENTIFICATION OF STEREOCHEMICAL IDENTIFICATION OF STEREOCHEMICAL

ISOMERS OF [Mo(CO)ISOMERS OF [Mo(CO)44(L)(L)22] BY INFRARED ] BY INFRARED SPECTROSCOPYSPECTROSCOPY

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Complex [Mo(CO)4(pip)2] can be synthesized from the reflux reaction between molybdenum hexacarbonyl, Mo(CO)6 with piperidine.

The yellow brownish precipitate formed was then react with triphenylphosphine (PPh3) with reflux and the cis-[Mo(CO)4(PPh3)2] formed was pale yellow in colour.

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After the thermal isomerization, the trans-[Mo(CO)4(PPh3)2] formed was brownish-off white in colour.

The infrared spectrum have few sharp absorbance at about 2100-1750 cm-1 as the C=O bond present in the complex.

The structure of [Mo(CO)4(pip)2] and [Mo(CO)4(PPh3)2] were cis and in thermal isomerization, the structure of [Mo(CO)4(PPh3)2] was trans. Trans- structure was more stable than Cis- structure.

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Three molybdenum tetracarbonyl complexes and the molecular geometry of the complexes was determined by using infrared spectroscopy.

Metal carbonyls and their derivatives display sharp, intense CO stretching vibrations in the 1750 - 2100 cm-1 region.

Intensity data for the CO stretching motion in substituted metal carbonyls are presented for compound of the types [Mo(CO)4(L)2] , trans-[Mo(CO)4(L)2]and cis-[Mo(CO)4(L)2].

Four carbonyl absorption bands are to be expected from a cis-[Mo(CO)4(L)2] complex whereas only one band is expected from trans isomer.

Most of the organometallic compounds are air sensitive and tend to decompose if not handled properly.

Those compounds can be oxidized easily, especially when they are in solution, because of their sensitivity to oxygen and water.

Consequently, handling of organometallic compounds should be carried out under dry and deoxygenated atmosphere or under vacuum.

Mo(CO)6 is a popular reagent in organometallic synthesis because one or more CO ligands can be displaced by other donor ligands.

Mo(CO)6(1.0g)Weighted Mo(CO)6(1.0g)Weighted

+ 40ml dry heptane + 5ml piperidine + 40ml dry heptane + 5ml piperidine

+ few pieces boiling stones & magnetic stirring bar

+ few pieces boiling stones & magnetic stirring bar

Filter precipitate while hot

Filter precipitate while hot

Wash twice with 10 ml (60-80C) petroleum ether that has been cooled

Wash twice with 10 ml (60-80C) petroleum ether that has been cooled

1)Weight2)Calculate the % yield 3) Record IR

1)Weight2)Calculate the % yield 3) Record IR

Heated for two hours (reflux temp= 110oC-120oC) with stirring.

Heated for two hours (reflux temp= 110oC-120oC) with stirring.

Suck dry by vacuum pump

Suck dry by vacuum pump

0.5g Mo(CO4)(pip)2 + 20ml dichloromethane in N2 + 0.75g PPh3 → Reflux for 20 mins

0.5g Mo(CO4)(pip)2 + 20ml dichloromethane in N2 + 0.75g PPh3 → Reflux for 20 mins

Cool to room temperature Cool to room temperature

Filter orange solution by gravity filtration

Filter orange solution by gravity filtration

Concentrate filtrate by rotary evaporator

Concentrate filtrate by rotary evaporator

Collect by pump filtration

Collect by pump filtration

Add 8 drops methanol

Add 8 drops methanol

Solution put in ice bath first, then cooled in freezer (<0oC) for 15 minutes

Solution put in ice bath first, then cooled in freezer (<0oC) for 15 minutes

1)Weight2)Calculate the % yield 3) Record IR

1)Weight2)Calculate the % yield 3) Record IR

0.5g molybdenum carbonyl complex+ 10ml dry toluene under N2→Reflux for 30 minutes

0.5g molybdenum carbonyl complex+ 10ml dry toluene under N2→Reflux for 30 minutes

Cool to room temperature Cool to room temperature

Reduce volume on rotary evaporator Reduce volume on rotary evaporator

Purify the product by 10ml dichloromethane Purify the product by 10ml dichloromethane

Collect product by gravity filtration Collect product by gravity filtration

Concentrate the filtrate, collect precipitateConcentrate the filtrate, collect precipitate

Weight product, calculate % yield, and record IRWeight product, calculate % yield, and record IR

(A) Synthesis of [Mo(CO)(A) Synthesis of [Mo(CO)44(HNC(HNC55HH1010))22]]

Functional Group Wavenumber, cm-1

N-H (°2 amine) 3264.59

C-H (sp3 stretching) 2926.20, 2855.26

C=O stretching 1839.16, 1890.44,1995.98, 2011.68

Table 1: Data from Infrared Spectrum [Mo(CO)4(HNC5H10)2]

Weight of product = 1.14gColour of product = Yellow colourMelting point = 1250CPercentage yield = 79.72%

((B) Synthesis of cis-[Mo(CO)B) Synthesis of cis-[Mo(CO)44(PPh(PPh33))22]]Table 2: Data from Infrared Spectrum cis-[Mo(CO)4(PPh3)2]

Functional Group Wavenumber, cm-1

C-H (sp2 stretching) 2925.34

C=O stretching 1827.84,1871.46, 2012.45

C=C stretching 1461.50

Weight of product = 0.84gColour of product = Pale yellow Melting point = 1600C– 1650CPercentage yield = 86.60%

(C) Synthesis of trans-[Mo(CO)(C) Synthesis of trans-[Mo(CO)44(PPh(PPh33))22]]

Functional Group Wavenumber, cm-1

C-H (sp2 stretching) 2924.79

C=O stretching 1889.17, 1947.80,2021.88,2071.59

C=C stretching 1463.91, 1585.12

Table 3: Data from Infrared Spectrum trans-[Mo(CO)4(PPh3)2]

Weight of product = 0.44gColour of product = Brownish off-whiteMelting point = 1630C-1680C Percentage yield = 88%

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All the apparatus used in this experiment must totally dry.

Piperidine is a reagent for the reaction to occur while dry heptane acted as a catalyst.

All the apparatus used in this experiment must totally dry.

Piperidine is a reagent for the reaction to occur while dry heptane acted as a catalyst.

1) Mo(CO)6 is very sensitive to water 1) Mo(CO)6 is very sensitive to water

2) has high tendency to form complex with aqua

ligand if water was present

2) has high tendency to form complex with aqua

ligand if water was present

Reflux

Paraffin oil• its boiling point (>110.6 0C) was higher

than heptanes• suitable for heat transfer

The temperature should not be too high (slightly above 110.6 0C) to avoid the solution turn to brown color

After reflux for 2 hours, a yellow brownish precipitate was formed.

to allow the mixture to undergo

complete reaction

Precipitate was filtered while it was still hot

• to prevent the formation of impurities • interferences on the surface of precipitate

Wash with cool ether petroleum • Avoid the precipitate to dissolve

Ether petroleum • Good eluent which did not dissolve most of the organic compounds

• Acted as washing agent• Has low dissolution in water

Physical change of colour product, because organometallic compound is ease oxidized and sensitivity to oxygen and water.

Handling of these compounds suggest to be carried out under dry and deoxygenated atmosphere or under N2 gas.

The function of N2 gas was to eliminate oxygen gas and to prepare the suitable medium for chemical reaction.

The mechanism of formation [Mo(CO)4(HNC5H10)2] by reaction of Mo(CO)6

and HNC5H10 by using SN2 mechanism.

The reaction is carried out dissolve in dry heptane, so the CO ligand is highly solvated.

One of the CO bond was weakening while the pip was coming to approach the metal. This complex is cis-complex due to the strong trans effect of CO bond.

Due to the Mo(CO)6 complex is an inert complex because of CO has more electrons occupied in t2g orbitals and prevent the pip easily approach to the central metal atom.

Therefore, the solution was refluxed for two hours with stirring to provide extra energy to complete the reaction.

The SThe SNN2 2 mechanism of formation mechanism of formation [Mo(CO)[Mo(CO)44(HNC(HNC55HH1010))22] is shown as ] is shown as followingfollowing::

Mo

CO

CO

CO

CO

OC

OC

+ pip pip------Mo(CO)5----CO Mo

CO

CO

CO

OC

OC

pipslow

+Mo

CO

CO

CO

OC

OC

pip

pip Mo

CO

CO

OC

OC

pip

pip

intermediate 

Cis-[Mo(CO)4(HNC5H10)2]

Mixture(0.5g of [Mo(CO)4(pip)2] + 20 mL

of dry dichloromethane + 0.75 g of PPh3 ) was heated at reflux temperature for 20 minutes to make sure that the reaction was fully completed.

Orange solution was left to cool to room temperature for the precipitation process before filtered through gravity filtration.

Mixture(0.5g of [Mo(CO)4(pip)2] + 20 mL

of dry dichloromethane + 0.75 g of PPh3 ) was heated at reflux temperature for 20 minutes to make sure that the reaction was fully completed.

Orange solution was left to cool to room temperature for the precipitation process before filtered through gravity filtration.

Rotary evaporator • to remove the solvent from the product

formed

8 drops of methanol was added and the solution was cooled in a freezer (<00C) • to induce the formation of the

precipitate

There are 4 absorption bands of C=O show that this complex has Cis- structure.

Pale yellow precipitate formedPale yellow precipitate formed

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For the synthesis of [Mo(CO)4(PPh3)2], dry dichloromethane was used as a solvent. Since piperidine was highly solvated in dichloromethane, the synthesis reaction became faster and undergoes dissociation process.

PPh3 was a ∏ acceptor ligand which was a stronger ligand than pip which was only a σ donor ligand. Therefore, the reaction go more faster than the synthesis of [Mo(CO)4(pip)2] from Mo(CO)6.

Moreover, this complex was is cis-complex which was kinetically stable because PPh3

just substitutes the ligand of pip which had weak field ligand .

Mo

CO

CO

OC

OC pip

Mo

CO

CO

OC

OC

+PPh3 Mo

CO

CO

OC

OC

PPh3PPh3 PPh3

PPh3

Mo

CO

CO

OC

OC

pip

pip

Mo

CO

CO

OC

OC pip

+PPh3 Mo

CO

CO

OC

OC pip

PPh3

The reaction for the formation of cis-[Mo(CO)4(PPh3)2] complex by using SN1 mechanism. 

Cis-isomer

• Solution (0.5 g of [Mo(CO)4(PPh3)2] + 10 mL of dry toluene under N2) was heat at reflux temperature for 30 minutes to obtained complete reaction.

• The solvent volume was reduced through rotary evaporator to yield a product as a brownish off-white residue.

• Solution (0.5 g of [Mo(CO)4(PPh3)2] + 10 mL of dry toluene under N2) was heat at reflux temperature for 30 minutes to obtained complete reaction.

• The solvent volume was reduced through rotary evaporator to yield a product as a brownish off-white residue.

The solution was heat at reflux temperature for 30 minutes to make sure that the reaction was completed.

There are 4 absorption bands of C=O show that this complex has Cis- structure.

Theoretically, this was a complex with trans-isomer with one absorption peak in the spectrum.

This is because during the 30 minutes reflux, the cis isomer would convert to the trans-isomer due to the considerable steric hinderance inherent in using PPh3 as ligands, making the trans isomer thermodynamically more stable.

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This showed that the reaction does not go to completion in this part and there is a mix of cis and trans isomers in our product.

The product was purified by dissolving in 10mL of dichloromethane and was collected by gravitational filtration.

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Brownish off-white precipitate was [Mo(CO)4(PPh3)2]

For the trans synthesis of [Mo(CO)4(PPh3)2], toluene was used as a solvent. The PPh3 was first dissolved in the toluene because PPh3 was a bulky ligand.

Although PPh3 dissociation occurs in this trans-[Mo(CO)4(PPh3)2] species, this dissociative process is less facile than in the cis-[Mo(CO)4(PPh3)2].

After dissociation of PPh3, the PPh3 will form back as in the trans position due to the steric repulsion in the PPh3

compounds.

The reaction of dissociation mechanism for the The reaction of dissociation mechanism for the formation of trans-[Mo(CO)formation of trans-[Mo(CO)44(PPh(PPh33))22] complex by ] complex by using Susing SNN1 mechanism.1 mechanism.

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• Through the infrared spectrum obtained, we found that the tetracarbonyldipiperidine molybdenum,[Mo(CO)4(HNC5H10)2] complex formed has Cis structure, while [Mo(CO)4(PPh3)2] complex for part B has the Cis- structure and [Mo(CO)4(PPh3)2] complex for part C has the Trans- structure.

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1) By interpreting the absorption bands of the CO group in the infrared spectra, determine the stereochemistry of the complexes that you have prepared.

 

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Complexes C=O absorption band(cm-1) stereochemistry

[Mo(CO)4(HNC5H10)2] 2011.68,1995.98,1890.44,

1839.16

Cis isomer

[Mo(CO)4(PPh3)2], b 2012.45,1871.46, 1827.84 Cis isomer

[Mo(CO)4(PPh3)2], c 1947.80 Trans isomer

2. Write a reaction scheme to show clearly the stereochemistry of isomers formed in the complexes.

(i) The reaction for the formation of [Mo(CO)4(HNC5H10)2] complex.

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OC H10C5HN

CO

CO

Mo Mo

OC

CO

H10C5HN

CO

CO

CO

+ 2 HNC5H10

Cis-isomer

(ii) The reaction for the formation of cis- [Mo(CO)4(PPh3)2] complex

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CO

CO

OC

H10C5HN

Mo Mo

OC

CO

H10C5HN

PPh3

PPh3

CO

+ 2 PPh3

CO

OC

cis-isomer

(iii) The reaction for the formation of trans [Mo(CO)4(PPh3)2] complex

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CO

OC

OC

CO

Mo

CO

Mo

OC

COPPh3

OC

PPh3

PPh3

Thermal Isomerization

PPh3

cis-isomer trans-isomer

4. State the type of mechanism involved in the carbonyl substitution reaction.Answer:

Type of mechanism involved in the carbonyl substitution reaction is outer sphere mechanism.

5. Name the isomer of complex [Mo(CO)4(PPh3)2] which is thermodynamically more stable.Answer:Trans isomer of complex [Mo(CO)4(PPh3)2] is more thermodynamically stable.

 

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