migratory insertion reactions in organo transition

8/11/2019 Migratory Insertion Reactions in Organo Transition

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CHEM 352 Binh Nguyen

Migratory Insertion Reactions in Organo Transition-Metal Chemistry

Synthesis and Characterization of Mo(II) Alkyl and Acyl Complexes

Analysis

The three dimensional structure of the starting molybdenum

material, [5CpMo(CO)3]2, is shown in Figure 1

1. Assuming the

cyclopentadienyl rings are rotating very fast, this structure has C2hsymmetry

because it has a C2axis coming out of the paper, a hreflection plane

passing through OC-Mo-Mo-CO plane and an inversion center in the middle of

the Mo-Mo bond. . Note that this assumption is reasonable, because NMR

spectra cannot distinguish the protons (in1H-NMR, Table XXX) and the

carbon atoms (in13

C-NMR, table XXX) of the rings. This proposed

structure is confirmed by Infrared Spectroscopy: IR spectrum shows 3

absorptions bands in the carbonyl region (see attached spectrum). Derivingthe vibrational normal modes for CO stretches in the C2h symmetry yields

2Ag+ Bg+ Au+ A2u. Because Agand Bgare not IR-active, we would expect 3

IR bands for CO stretches. This is exactly what we see in IR spectrum

mentioned above. Another possible structure is the cis-isomer shown in Figure 2, which has C2v

symmetry. This isomer would exhibit 5 IR active stretching bands.

From the structure of [5CpMo(CO)3]2, one likely structure for

[MoMe(CO)3(5C5H5)] is shown in Figure 3, which has Cssymmetry (it has a

reflection plane passing through OC-Mo-Mo-CO plane). By symmetry, we would

expect to see two equivalent carbonyl ligands. This observation is supported by

13C-NMR, which only shows 2 types of Mo-CO resonances (see Table XXX).

With Cssymmetry, group theory predicts that there are three IR-active

stretching CO bands (2A + A). However, IR spectrum only shows 2 bands in

the carbonyl stretching region (see attached spectrum). It seems that we might

have an overlap between two of three expected bands (maybe between A and

A because two A bands cannot have the same energy due to the lack of degeneracy). But this

disagreement cannot be resolved without any further information.

The product of migratory insertion has two possible structures shown in Figure 4: cis and

trans isomers. The trans isomer has Cs symmetry and the cis isomer has C1 symmetry. Group

theory predicts 2 IR active Mo-CO stretching bands for both of the structures, which is exactly

what we see in the obtained spectrum (see the attached spectrum). However, these two structures

have a fundamental difference: two CO ligands are equivalent in the trans isomer but not

equivalent in the cis isomer. Therefore, we would expect to see 2 distinct Mo-CO resonances in

for the cis isomer and only 1 Mo-CO resonance for the trans isomer in13

C-NMR. The13

C-NMR

of the product shows only one Mo-CO resonance, so the correct structure of

[Mo(COMe)(CO)2(5C5H5)(PPh3)] is the trans structure in Figure 4.

Figure 1. Correct structure o

[5CpMo(CO)3]2

Figure 2. Possible Cis-isome

Figure 3. Structure of

[MoMe(CO)3(5C5H5)]


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Figure 4. Two possible structure for [Mo(COMe)(CO)2(5C5H5)(PPh3)]

Experimental Section

All the syntheses in this experiment were done under an inert-atmosphere of Nitrogen gas

inside an MBraun Unilab 2000 glove box. The starting materials, including methyl iodide,

triphenylphosphine, the 1M solution of Super-Hydride (LiEt3BH) and anhydrous solvents, were

purchased from Sigma-Aldrich or Strem and used as received. The starting molybdenum

material [5CpMo(CO)

3]

2was purchased from Strem and used as received. The neutral alumina

was purchased from Sigma-Aldrich and dried at 150oC under vacuum for 12 hours before use.

NMR solvent, Chloroform-d, was purchased from Cambridge Isotope Laboratories and filtered

through neutral alumina before use. Infrared spectra were taken with a solution IR cell in

dichloromethane by a Mattson Model 4020 Galaxy Series FTIR Spectrometer. NMR spectra

were obtained with a Varian Unity-Plus 400 MHz FT Nuclear Magnetic Resonance Spectrometer

at 399.95 MHz for1H-NMR, 100.58 MHz for

13C-NMR and 161.90 MHz for

31P-NMR. Both

13C and

31P spectra were acquired with proton decoupling. The chemical shifts for

1H and

13C

were referenced against CDCl3the residual proton at 7.24ppm and the13

C at 77 ppm. For31

P,

the chemical shifts were referenced to concentrated phosphoric acid, which comes at 0ppm.

The synthesis of [MoMe(CO)3(5

C5H5)] was performed in an inert-atmosphere glovebox. 214.9 mg (0.4386 mmol) of [

5CpMo(CO)3]2was dissolved in 10mL anhydrous THF in a

20mL scintillation vial with a flea-sized magnetic stir bar. 2 equivalent of Super-Hydride (1.1

mL of a 1.0 M solution in THF, 1.1 mmol) was slowly added into the vial by syringe. The

mixture was stirred for 20 minutes and 88.5 L (1.39 mmol) of methyl iodide (CH3I) was then

added by a calibrated pipet. The reaction mixture was stirred for 2 hours. The volatiles in the

mixture were removed in vacuoand the crude product was stored in a freezer at -35oC over the

course of a week. The crude product was extracted into solution by 15 mL pentane and filtered

through 1.5 cm of neutral alumina in a 30mL frit. The solution was pulled through the frit by

vacuum and the frit was washed with extra pentane until the eluent was colorless. The pentane

solvent was then removed in vacuo. The yellow solid products mass was 56.6 mg,

corresponding to a yield of 24.8%.

The migratory insertion to synthesize [Mo(COMe)(CO)2(5C5H5)(PPh3)] was also done

in an inert-atmosphere glove box. 87.3 mg (0.333 mmol) of PPh3was dissolved in 5mL

acetonitrile and the resulting solution was then transferred into a 20mL scintillation vial

containing 55.2 mg (0.217 mmol) of [MoMe(CO)3(5C5H5)] and a flea-sized magnetic stir bar.

The mixture was stirred overnight and stored until next lab period. The yellow solid product was


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collected by a 30 mL frit and washed with 10mL pentane and then dried under vacuum. The

yellow product was scratched off the frit and transfer to a clean scintillation vial. 80.0 mg of

product was collected, corresponding to a yield of 71.2%.

Spectral Assignments

1.

[5CpMo(CO)3]2

2. [MoMe(CO)3(5C5H5)]

Figure XXX:Numbering scheme for

[MoMe(CO)3(5C5H5)]

3. [Mo(COMe)(CO)2(5C5H5)(PPh3)]

IR

Frequency (cm-1

) Type Reference

2018.12 CO Piper

1925.46 CO Piper

Carbona

Chemical

Shift (ppm)b

Pattern Notes on Assignment

1 92.4 s

2 226.5 s

3 239.8 s

This peak was assigned by comparison with 13C-NMR of

[Mo(COMe)(CO)2(5C5H5)(PPh3)]. Chemical shifts of C3

carbons should remain relatively unchanged.

4 -22.3 s

Protona

Chemical Shift

(ppm)b

Pattern

(Rel Int) Reference

a 5.28-5.29 m (5H) Piper

b 0.35 s (3H) Piper


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Figure XXX: Numbering scheme for Figure XXX:Numbering scheme for PPh3.

[Mo(COMe)(CO)2(5C5H5)(PPh3)] Note that H atoms are not specified due to

inability to distinguish them in the obtained1H-NMR. This symmetry is not the same as

the symmetry of PPh3bounded in the

complex.

IR

Frequency (cm-1) Type Reference

1938.13 CO Barnett

1858.1 CO Barnett

1613.43 Acetyl Barnett

Carbon

a

Chemical

Shift (ppm)b Pattern Notes on Assignment1 96.7 s

2 266.1 s

3 238.3 d

This peak was assigned by comparison with 13C-NMR of

[Mo(COMe)(CO)2(5C5H5)(PPh3)]. Chemical shifts of C3

carbons should remain relatively unchanged.

J = 24 Hz

4 51.4 s

5 135.47 d JPC= 44 Hz

6 128.5 d JPC= 10 Hz

7 133 d JPC = 10.7 Hz8 130.35 s

Protona

Chemical

Shift (ppm)b

Pattern

(Rel. Int.)Reference

a 4.99 s (5H) Barnett

b 2.59 s (3H) Barnett

phenyl protons 7.4 m(15H) Barnett


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Reference

1. Wong, R.; Ooi, M.; Tan, G.; Ng, S.Inorganica Chimica Acta, 2007, 360 (9),3113-

3118.

2.

migratory insertion reactions in organo transition

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