Synthesis and Determination of [1,3,5-C6H3(CH3)3]Mo(CO)3
Author: Adam Capriola
CHM 2521 Section 151, Department of Chemistry, Saint Joseph’s University
Date Submitted: March 18, 2010
Abstract
The reaction of mesitylene with Mo(CO)6 under reflux yields [1,3,5C6H3(CH3)3]Mo(CO)3 in low percent yield (around 1%). 1H NMR of [1,3,5C6H3(CH3)3]Mo(CO)3 shows singlets at δ 2.25 and 5.23 with absorption ratios of 9:3,
respectively. 1H NMR of mesitylene shows singlets at δ 2.25 and 6.78, also with absorption
ratios of 9:3, respectively. This suggests addition of the metal complex to mesitylene causes
downfield shifting of the signal for protons attached directly to the ring as they are unshielded
from the backbonding of carbonyl groups. The IR spectrum of [1,3,5-C6H3(CH3)3]Mo(CO)3
shows a strong antisymmetric C-O stretch at 1852 cm-1 and a medium symmetric C-O stretch at
1942 cm-1 with peak areas of 64.462 cm-1 and 9.111 cm-1 respectively. The calculated OC-MoCO bond angle is 108.32°.
Introduction
The reaction of mesitylene with Mo(CO)6 under reflux yields [1,3,5C6H3(CH3)3]Mo(CO)3. The reaction specifically takes place in the following manner:
Scheme 1
CH3
CH3
CO
OC
CO
Mo
OC
+
H3C
H3C
CO
CO
OC
CH3
CH3
Mo
OC
CO
This is compound of interest because it a metal-arene complex and can be considered to be an
octahedral rather than tetrahedral complex. This is because the OC-Mo-CO bond angles are
close to 90° instead of the expect 109.5° for tetrahedrals.1 In order to determine the structure of
said substance from its 1H NMR, the peaks must be compared to the same spectrum of
mesitylene for indication of identical methyl group peaks and downfield shifting a peak
indicative of protons attached directly to the ring. The IR spectrum of [1,3,5C6H3(CH3)3]Mo(CO)3 can be analyzed for peaks indicative of symmetrical and antisymmetrical
carbonyl stretching, whose areas can be used to calculate the bond angle between the carbonyl
groups attached the to metal.
Experimental
All syntheses were carried out in nitrogen and the reagents and solvents were purchased
from commercial sources and used as received unless otherwise noted. The synthesis of [1,3,5C6H3(CH3)3]Mo(CO)3 (1) was based on reports published previously.1
[1,3,5-C6H3(CH3)3]Mo(CO)3 (1). Mo(CO)6 (2.083 g, 7.92 mmol) and mesitylene (10
mL, 72 mmol) were added subsequently to a 100 mL 3 neck round-bottom flask along with a
small magnetic stir bar. A sand bath was constructed and set to 50% power. A greased sidearm,
stopcock, and 30 cm cold water condenser were attached to the round-bottom flask. A greased
gas inlet was then attached to the condenser and connected to a bubbler. The condenser was not
connected to a cold water source; it was used only to allow air to circulate. The sidearm was
connected to a nitrogen source, and the system was allowed to degas for 5 minutes. The system
was then put on the sand bath and the stir bar was spun at a moderate speed via a magnetic
stirring instrument.
After 5 minutes, the solution in the round-bottom flask was not boiling as outlined, so the
sand bath was turned up to 70% power. The sand bath was turned up to 85% another 5 minutes
later. A rigorous boil was achieved when the sand bath was set to 95% 5 minutes after that. It
was then set to 85% power in efforts to obtain a less extreme boil. After a total of 0.33 h of
reflux, the solution was taken off the sand bath and allowed to cool to room temperature. When
the apparatus was removed from the sand bath, it was dropped and roughly more than 60% of the
solution was lost. The remaining solution cooled to a blackish yellow color.
The following and final procedures took place in the presence of air. Once cool, the
solution was washed with 15 mL of hexane via suction filtration in a 15 mL frit. The solution
was then washed with another 5 mL of hexane. About 10 mL CH2Cl2 was added to the blackish
yellow powder precipitate remaining in the frit. The powder was washed with 25 to 30 mL of
hexane and vacuum dried. This powder was discarded and the collected yellowish washings
were rotovapped for about 0.33 h to obtain the desired product. The product was vacuum filtered,
as it would not completely dry under the rotovap. The resulting yellowish powder was
determined to be 1 (0.028 g, 1.18% yield based on the amount of Mo(CO)6 used). 1H NMR
(CH2Cl2): δ 2.25 (s, -CH3), 5.23 (s, C-H). FTIR (ATR) ν(C-O) 1852 cm-1 (s, C-O linkage), ν(C-O)
1942 cm-1 (m, C-O linkage).
1,3,5-C6H3(CH3)3 (2). The 1H NMR spectrum of 2 was extrapolated from the literature.1
1
H NMR (CHCl3): δ 2.25 (s, -CH3), 6.78 (s, C-H).
Results
The reaction of Mo(CO)6 with mesitylene yielded 0.028 g of the product, [1,3,5C6H3(CH3)3]Mo(CO)3. This translated to 0.09328 mmol, and thus a 1.18% yield based on the
amount of Mo(CO)6 used, which was the limiting reagent in the reaction. Mo(CO)6 reacted to
form the product in a 1:1 ratio, and 7.92 mmol of Mo(CO)6 was used to start, so that proportion
was taken into account when calculating the percent yield. Proton NMR spectroscopy yielded a
two peaks of interest. A peak found at δ 2.25 was indicative of methyl hydrogens and a peak
noted at δ 5.23 was suggestive of hydrogens attached directly to the aromatic ring.1 These peaks
were noted with relative intensities of 9 to 3, respectively. Peaks seen at δ 7.25 and 1.54 were
attributed to solvent and hexane, respectively. The 1H NMR spectrum of mesitylene in CHCl3
showed absorptions at δ 2.25 and 6.78 with relative intensities of 9 to 3, respectively.1 The peak
at δ 2.25 hinted of methyl protons and the peak at δ 6.78 was suggestive of protons bonded
directly the ring. The mass spectrum of [1,3,5-C6H3(CH3)3]Mo(CO)3 showed a peak at m/z =
302.0, which is nearly equal to the molar mass of said substance.1 The IR spectrum showed a
strong peak around 1852 cm-1 of area 64.462 cm-1 indicative of antisymmetrical C-O stretching
and a medium peak around 2942 cm-1 of area 9.111 cm-1 indicative of symmetrical C-O
stretching. These areas were used to calculate a OC-Mo-CO bond angle of 108.32°.
Discussion
The percent yield of [1,3,5-C6H3(CH3)3]Mo(CO)3 is very poor. Much of this quantitative
shortcoming can be attributed to clumsiness, as much of the solution containing future product
was lost when the reaction vessel was dropped. This not only resulted in a direct loss of solution,
but also exposed the solution to air. The solution had been kept under nitrogen as to prevent
decomposition of the products. This exposure to air undoubtedly had a contribution to the poor
percent yield. The solution was also not heated as desired because it was difficult to control the
sand bath. The solution was to be brought to a moderate boil, but could not be controlled to do
so. It would not boil, then boiled rigorously a moment later. Attempts were made to subdue the
boiling, but were unsuccessful. This overheating was probably not favorable for the reaction.
More adept control of the sand bath would have resulted in a better yield of product.
Washing the product with excess amounts of hexane and CH2Cl2 also may have added to
the loss of product. Excess washing would make it difficult to extract the product from the
solution, as there would have been a relatively small amount of product compared to the amount
of solution it was dissolved in. The solution could not be completely dried with the rotovap,
which means there was an excess of hexane and/or mesitylene in the solution. Vacuum filtration
then had to be used to collect the product, which was not ideal. Best case scenario, the rotovap
would have completely dried the product and it would have been scraped out of the flask.
Vacuum filtration gives a better chance for loss of product.
The product seemed pure as it produced clear 1H NMR and IR spectra readings. The 1H
NMR spectrum shows a methyl peak at δ 2.25 and a C-H peak at δ 5.23, and the reagent in the
reaction, mesitylene, also gives a peak at δ 2.25. This seems to confirm the structure and
addition of the metal complex, as only the C-H peak was shifted downfield. The methyl protons
are too shielded to be affected by the metal. The downfield shift is caused by backbonding of the
carbonyls. IR spectroscopy revealed 3 C-O stretches, 2 of which were accounted for by a strong
peak at 1852 cm-1 and the 3rd of which was accounted for by a medium peak at 1942 cm-1. Two
peaks were seen because of symmetrical and antisymmetrical stretching of the carbonyls.1 The
two antisymmetical modes have exactly identical absorption frequencies, and the symmetrical
mode has a different absorption frequency than them, which means that a total of two peaks
should be seen.1 The areas of these peaks, 64.462 cm-1 for antisymmetical stretch and 9.111 cm-1
for symmetrical stretch, allowed for discovery of the bond angle between the CO ligands. The
calculated angle was 108.32°. This seems to make sense as [1,3,5-C6H3(CH3)3]Mo(CO)3 is
predicted to be a tetrahedral complex and the expected bond angle for tetrahedral complexes is
109.5°, but in reality [1,3,5-C6H3(CH3)3]Mo(CO)3 acts more like an octahedral complex and the
bond angle should be slightly less than 90°.1 This error could be due to excess solvent or impure
an sample, which resulted in skewed IR spectrum readings and thus incorrect peak areas. The
oxidation state and electron count of Mo in [1,3,5-C6H3(CH3)3]Mo(CO)3 are 0 and 6 electrons, so
it is an 18 electron complex.
Conclusion
The main purpose of the experiment was to interpret the 1H NMR and IR spectra of
[1,3,5-C6H3(CH3)3]Mo(CO)3 to confirm the product and to decipher the bond angle between the
carbonyls. The 1H NMR spectrum of the product showed peaks at δ 2.25 and 5.23, while the 1H
NMR spectrum of mesitylene gave peaks at δ 2.25 and 6.78, both in ratios of 9:3, respectively.
This ratio seems to confirm methyl groups and single protons attached to the ring. The
downfield shifting of the second peak is attributed to the coordination of mesitylene to the metal
complex. The protons attached directly to the ring are affected by backbonding of the carbonyl
groups. The methyl hydrogens are shielded, and thus are not affected by the metal complex.
The IR spectrum yielded two peaks near 2000 cm-1. These two peaks account for 3 C-O
stretches, 2 of which are accounted for by a strong peak at 1852 cm-1 and the 3rd of which are
accounted for by a medium peak at 1942 cm-1. The strong peak accounts for antisymmetrical
stretching of the carbonyls and the medium peak accounts for symmetrical stretching of the
carbonyls. The areas of these peaks, 62.462 cm-1 and 9.111 cm-1 respectively, provide for a
theoretical angle between the carbonyls of 108.32°. In reality, this angle should be only nearly
90°. The percent yield for the reaction was poor and could have improved with a more steady
hand, more precise heating of the reagents, less exposure of the solution to air, and less solvent
used for washing.
References
(1) Angelici, R. J.; Girolami, G. S.; Rachufuss T. B. Synthesis and Technique in Inorganic
Chemistry: A Laboratory Manual; University Science Books: Sausilito, CA, 1999; pp 161-170.