Michael Ludden L3Report2016

Michael Ludden Level 3 Project AH2

1

Synthetic and Kinetic Studies of Migratory Insertion Reactions using

varying Molybdenum Complexes

Michael Ludden

Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK.

Email: [email protected]

Abstract

A variety of Molybdenum complexes were synthesised from a Molybdenum carbonyl dimer

starting material and characterised using 1H NMR, 13C NMR and IR spectroscopy. From these

complexes, kinetic measurements were taken for the reaction between [CpMo(CO)3R]

(R = Me, Et) and a phosphine nucleophile, PR3, using the software TimeBaseTM. The effects of

varying individual aspects of the reaction under pseudo-first-order conditions (with respect to

the nucleophile) were investigated, and it was found that changing the R group, the solvent and

the temperature of reaction all produced a change in the rate constant, Kobs. Activation

parameters for the reaction were calculated and compared to literature values, with a negative

ΔSǂ value obtained indicating an associative mechanism.

Introduction

A migratory insertion reaction involves a

metal complex with unsaturated co-

ordinated ligands and a nucleophilic X-type

ligand effectively undergoing nucleophilic

attack by either an incoming nucleophile, L,

or a co-ordinating solvent such as THF.1 This

prompts migration of the X-type ligand,

commonly an alkyl group, into one of the

unsaturated ligands cis to it, hence the term

‘migratory insertion’ (see Fig. 1). It is

possible for the insertion itself to occur two

ways: either 1,2-insertion or 1,1-insertion.

The process of migratory insertion forms a

new carbon-carbon or carbon-hydrogen

bond. It is this characteristic that makes it

1 I. S. Butler, F. Baloso, R. G. Pearson, Inorganic Chemistry, 1967,

6, 2074. 2 L. S. Hegedus, Transition Metals in the Synthesis of Complex

Organic Molecules, University Science Books, Sausalito, CA, 1999.

very useful when combined with a catalyst

in industrial applications, as it allows alkyl

fragments to have useful functional groups

added to them.2 A well-known example of

this is Alkene Hydrogenation using

Wilkinson’s Catalyst, [RhCl(PPh3)3].3

A process that utilises carbonyl migratory

insertion, and is more relevant to the topics

discussed in this report, is the Monsanto

process. This involves the carbonylation of

methanol to produce acetic acid and is a

process used industrially on a phenomenal

scale, with ca. 7 million tonnes being

produced annually.4

The rate at which migratory insertion will

occur depends on many different

experimental factors, most of which can be

varied reasonably easily. The rate

determining step in the overall mechanism

is the migration of the X-type ligand itself,

3 J. A. Osborn, G. Wilkinson and J. F. Young, Chem. Commun.

(London), 1965, 17. 4 A. Haynes, in Catalytic Carbonylation Reactions, ed. Beller and

Matthias, Springer, Berlin, 2006, ch. 4.

Figure 1 - Solvent assisted migration of alkyl group into neighbouring CO ligand

mailto:[email protected]


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followed by the incoming L-type ligand

binding (which is relatively much quicker).

This is detailed in Fig. 2, seen below.

This report aims to investigate the kinetics

of migratory insertion reactions with

Molybdenum carbonyl complexes by

performing a series of experiments to

determine the rate law for the reaction, the

activation parameters ΔHǂ and ΔSǂ, and the

dependence of rate on factors such as the

solvent and nature of the incoming ligand.

Results and Discussion

1. Synthesis and Characterisation of

[CpMo(CO)3Me]

The first compound produced in the lab was

the methyl-group containing complex

[CpMo(CO)3Me]. This was done by a simple

reduction of a molybdenum carbonyl

dimer, [CpMo(CO)3]2 using Super Hydride

and then reacting the anion produced with

methyl iodide to add the methyl ligand.

After purification, the crystals produced

were small and bright yellow in colour. A

yield of 62% was calculated for this

5 G. Miessler, D. Tarr, Inorganic Chemistry, Pearson Education, St

Quezon City, 2007.

synthesis. The crystals were determined to

be the anticipated product through IR and

NMR characterisation – note the singlet

peak at ≈ 0.4 ppm in the 1H NMR spectrum

indicating the presence of the methyl

ligand. The protons on the methyl group are

very shielded due to them being bound to

the electropositive Molybdenum centre,

hence the lower than usual ppm shift (0.4

compared to 0.9).

A similar situation is seen in the 13C NMR

spectrum. The methyl group is again at a

much lower ppm shift than would usually

be expected – in this case it is at -22 ppm

(lit. value -29.8 – 23.5 ppm).5 The peak for

the Cp ring can clearly be seen at 92 ppm

(lit. value 91 – 129 ppm) and the upwards

pointing triplet at 77 ppm is that of the

solvent, CDCl3.

The IR spectrum recorded for this complex

shows two carbonyl peaks; the one at

around 1942 cm-1 is due to both the A’ and

A” stretching bands overlapping whilst the

peak at 2024 cm-1 is the symmetric

vibration. Note the absence of any peaks

lower down the spectrum, in the ketone

C=O stretch region. This points to all the

carbonyl ligands being terminal and triple-

bonded as in free CO; this is what would be

expected for this compound.

Another point worth expanding on is the

presence of what could be called ‘satellite’

peaks on the IR spectrum for this

compound and the ethyl and iodide

compounds also. The reason for these very

low intensity peaks is the presence of the 13C isotope in the sample. With a natural

abundance of 1%, this matches up with the

relative intensities of the peaks seen on the

spectrum. They are seen at a slightly lower

wavenumber due to the slightly higher

Figure 2 - Reaction pathway for a migratory insertion reaction involving an incoming ligand, L.

Figure 3 - The two-step reaction schematic for the synthesis of [CpMo(CO)3Me].


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mass of 13C – as the CO molecule can be

treated as two atoms on a vibrating ‘spring’,

the frequency of vibration will decrease

when the mass of one of the carbon atoms

is increased.6 This explains why the

satellites are seen at a lower wavenumber

as opposed to a higher one.

Upon studying the characterisation data, it

was decided the product was indeed

[CpMo(CO)3Me].


[CpMo(CO)3Et]

The next complex to be synthesised was

chosen to be [CpMo(CO)3Et], as it could be

synthesised following the route for the

previous compound but by substituting

methyl iodide for iodoethane.

This reaction afforded small, dark green

crystals with a yield of 54%. This was only

for the crude product, however, as the

vacuum sublimation step (detailed further

in the experimental) was very time-

consuming and was therefore not

performed for this compound. The spectra

were still collected as before.

The indications of the ethyl group being

present in the complex echoed those seen

in the methyl complex both in the 1H and 13C NMR spectra.

The IR spectra of this complex is near-

identical to that of the methyl complex.

Again, two distinct peaks are seen, but this

time at 1936 and 2019 cm-1, about 5

wavenumbers lower than those seen in the

6 H. Haas, R. K. Sheline, J. Chem. Phys, 1967, 47, 2996.

methyl complex. This difference can be

explained by bond strengths and the impact

of the bound ligand. An ethyl ligand is a

stronger electron donor than a methyl

ligand due to inductive effects, and so will

increase the electron density on the metal

centre. This leads to the bond between the

metal centre and the bound CO ligands

becoming stronger due to increased

backbonding (and stronger forward

bonding, taking synergic bonding into

account).7 In turn, this means the C≡O bond

is weakened, resulting in a lower vibrational

frequency in the IR spectrum. The

difference in this case is about 5 cm-1, as

mentioned earlier.

The NMR spectra obtained for this complex

also lent weight to the notion the synthesis

had been successful without the additional

purification step.

Whilst the 1H NMR spectrum for the ethyl

complex was quite cluttered with what

appeared to be contaminants such as water

and silicone grease, the triplet at 1.45 ppm

and the neighbouring quartet at 1.7 ppm

are indicative of a CH3 and CH2 group

respectively. This points very strongly to the

presence of an ethyl ligand, and further

agrees with a successful synthesis.

7 F. A. Cotton, C. S. Kraihanzel, J. Am. Chem. Soc., 1962, 84 (23),

pp 4432–4438.

Figure 4 - The complex [CpMo(CO)3Et]

Figure 5 - The forward and backbonding seen between a metal centre and any bound CO ligands. Both of these will be strengthened by the presence of a donor ligand eg. Et


4

The 13C NMR spectrum continues to

support a successful synthesis through the

peaks at -5 ppm and 20 ppm being typical

of a metal-bound ethyl group. The negative

ppm value again is due to the shielding

provided through bonding to the metal

centre.8 The large peak at 92 ppm shows

the Cp ring is still present in the product

also. After investigating the product’s

obtained spectra, it was deemed to be

[CpMo(CO)3Et], as anticipated.


[CpMo(CO)2(COMe)(PPh3)]

As a large section of this experiment would

involve chemical kinetic investigations

producing compounds similar to

[CpMo(CO)2(COMe)(PPh3)], it seemed

sensible to synthesise and analyse this

product, to give some spectra to compare

the kinetic results to.

This product was synthesised by reaction of

the methyl complex synthesised earlier in

the experiment, [CpMo(CO)3Me], with

triphenylphosphine, PPh3. This was done

over a long period of time to ensure the

reaction would go to completion. A bright

yellow precipitate was formed overnight,

and this was filtered off and washed with

light petroleum. This gave dull yellow

crystals, with a calculated yield of 65%.

Initially the plan was to recrystallise this

precipitate, to purify the product and

8 Y. Ruiz-Morales, G. Schreckenbach, T. Ziegler, Organometallics, 1996, 15 (19), pp 3920–3923. 9 A. G. Orpen; N. G. Connelly, “Structural systematics: the role of

P-A σ* orbitals in metal-phosphorus π-bonding in redox-related

obtain a more accurate yield. However,

upon attempted recrystallisation no

crystals were formed. The solvent of the

solution was removed under reduced

pressure and crystals of the original, crude

product were afforded. It is likely the

recrystallisation failed as too much hot

solvent was added to the impure crystals.

Both IR and NMR spectra were run for the

crude product to confirm its identity. The IR

spectra for this complex differs from those

seen before as there is a new carbonyl

environment present. As seen previously,

there are still two carbonyl peaks visible at

the higher end of the spectrum – this time

at 1938 and 1855 cm-1. These are lower

than the peaks in this region for both the

methyl and ethyl complexes; this is likely

due to the replacement of an electron

donating X-type ligand (Me, Et) with a L-

type ligand, PPh3. The PPh3 ligand acts as

both a σ-donor and a π-acceptor, in the

same manner as a CO ligand.9 This results in

stronger M-C bonds and this in turn results

in weaker C≡O bond, and a lower

wavenumber.

The IR spectrum also displays a peak around

1620, lower than any seen previous. The

acyl ligand, COMe, is responsible for this

peak. As the ligand contains only a C=O

double bond, as opposed to a triple bond

seen in the other ligands, it is at a lower

wavenumber in a region more commonly

associated with ketones, aldehydes and

other such carbonyl containing compounds.

The presence of this peak confirms that the

product was likely to be

[CpMo(CO)2(COMe)(PPh3)].

The 1H NMR spectrum for this complex did

not yield much information as the sample

pairs of M-PA3 complexes”, Organometallics, 1990, 9 (4): 1206–1210.

Figure 6 - The reaction scheme for the synthesis detailed above


5

was of an insufficient concentration. The

lower end of the spectrum is cluttered with

unidentifiable peaks, many of which could

be contaminants such as grease or water.

Further up the spectrum, however, are

peaks which can be labelled as the Cp ring,

seen at 5.0 ppm, and the phenyl rings of the

triphenylphosphine ligand, seen at roughly

7.5 ppm.

The 13C NMR spectrum also suffered from a

lack of quality due to another low

concentration sample. The signal-to-noise

ratio for the spectrum is poor and the peak

for the methyl group is not apparent. What

the spectrum does possess, however, is the

peak indicating the Cp ligand and numerous

separate environments for the phenyl

groups on the PPh3 ligand. The Cp ligand is

the peak seen at 97 ppm and if the

expansion is studied closely, three distinct

peaks can be identified, all related to

carbons in each phenyl ring. The only

disadvantage to this expansion is that the

fourth peak, a doublet with a large J value,

is not observable amongst the noise. This

missing doublet would relate to C1, seen in

Fig. 7, as it would have the largest J value.

This is because phosphorus contains spin-

active nuclei with spin = ½, and so 13C can

couple to it, hence the doublets observed

on the spectrum. The nearer the carbon to

the coupled phosphorus, the larger the

splitting will be.

The presence of phosphorus in this

compound meant that for the first time in

this experiment, 31P NMR could be used.

The spectrum obtained contained a single

peak, which was expected for this

compound, as there was only one

phosphorus atom present, in the PPh3

ligand. This contributed evidence to suggest

the synthesis had been completed

successfully.


[CpMo(CO)3I]

The final compound synthesised was one

containing an atom of Iodine as a ligand.

This was done by the relatively

straightforward synthesis route of reacting

the molybdenum dimer with a

stoichiometric quantity of I2. The relevant

equation is shown in Eqn. 1, below.

[CpMo(CO)3]2 + I2 → 2[CpMo(CO)3I]

Equation 1 - The reaction between the molybdenum dimer and Iodine

This reaction produced a small quantity of

dark purple crystals, with a calculated yield

of 36%. Characterisation was carried out on

these crystals without further purification.

The IR spectrum for this compound is

similar to that of both the methyl and ethyl

complexes, but with one notable

difference; the peaks that originally

overlapped on both aforementioned

spectra are visibly separate for this

complex. An explanation for this

observation is the electronegativity of

Iodine – this property means electron

density is pulled away from the metal

centre, producing weaker backbonding to

the CO ligands. This gives a higher CO bond

order and therefore stronger bonds, hence

Figure 7 - A diagram of the product complex with labelled 13C environments on the phenyl ring. All 3 phenyl rings

can be considered the same.


6

the higher wavenumbers linked to the

peaks (a shift of about 20 cm-1).

The 1H NMR spectrum yields little other

than peaks seen before in other complexes.

The peak at 5.6 ppm indicates the presence

of the Cp ring and there is little else shown

on the spectrum, as expected.

The 13C NMR spectrum gives something

more to discuss in the more distinct CO

environments present in this complex.

There are two distinct CO peaks, leading to

the conclusion that two environments are

present. Looking at the predicted structure

of the product, this is in agreement with the

spectra obtained; two of the CO ligands can

be considered ‘cis’ to the electronegative

Iodine ligand whereas the third is opposite

and can be considered ‘trans’.10 Of course,

these terms are only used loosely as the Cp

ring causes distortion meaning the

remaining ligands are not truly ‘trans’ or

‘cis’. The ratio of the peaks also matches up

with the 2:1 cis to trans ratio of the

complex.

All the evidence detailed here pointed to

the complex synthesised indeed being

[CpMo(CO)3I].

5. Kinetics of Migratory Insertion reactions

of [CpMo(CO)3R] (where R = Me, Et)

The kinetic experiments for the complexes

synthesised involved producing samples of

both the incoming phosphine/phosphite

and the metal complex dissolved in the

chosen solvent. Initially, a background

spectrum was run of the phosphine (or

phosphite) solution using TimeBaseTM IR

software, and then the solution of metal

complex was mixed with the

phosphine/phosphite solution. Upon

10 L.J. Todd, J.R. Wilkinson, J.P. Hickey, D.L. Beach and K. W.

Barnett, Journal of OrganometaIlic Chemistry, 1978, 154, p. 151-157.

mixing, the software was set to run in the

region of 20+ spectra over a given time, to

allow observation of the reaction

proceeding. The focus of the spectra was

the carbonyl region, between 2200 and

1500 cm-1, as this allowed both the

decrease in reactant and increase in

product to be seen through the IR spectra;

as the alkyl complex carbonyl peak

decreases the acyl complex carbonyl peak

would increase. Once the time set for the

experiment had elapsed or the reaction was

deemed to have gone to completion, the

data was exported to KaleidagraphTM and

from this the rate constant was calculated.

y = ((m2 − m3) e−(m1mo)) + m3

Equation 2 - The equation used in Kaleidagraph to calculate the line of best fit for the data gathered.

The equation shown above was used to

produce a trendline to fit the data obtained

from the spectra. Absorbance was plotted

against time and this produced a graph

displaying an exponential curve, as seen in

the example graph, Fig. 8.

Figure 8 - An example of the graphs produced by Kaleidagraph. This particular example is for the methyl

complex reacting with PMe3 in THF at 48.8 °C


7

Equation 2 could be related to another

equation, labelled as equation 3, to

determine a value for the rate constant, k.

𝐴 = (𝐴∞ − 𝐴0) 𝑒−𝑘 𝑡 + 𝐴0

Equation 3 - The modified version of Eqn. 2; this links the recordable values for x and y - these being time and

absorbance, respectively.

By comparing the two equations it can be

seen that the rate constant, k, can be

compared to m1 in the first equation (Eqn.

2). KaleidagraphTM produced a value for m1

in the table seen above the graph, and

these were taken and recorded using

Microsoft ExcelTM to compare the values for

different complexes, solvent, incoming

ligand and varying temperature. These will

now be looked at in more detail

individually.

5.a. Effect of complex (variation of R

group)

Two complexes were compared:

[CpMo(CO)3Me] and [CpMo(CO)3Et]. All

factors other than temperature and the

alkyl substituent were kept constant

throughout this sub-experiment.

As Table 1 displays, the rate constants for

the ethyl complex are a factor of 22.5 to

27.5 times larger than those for the methyl

complex. The difference in k with increasing

11 S. P. Nolan, R. L. de la Vega, S. L. Mukerjee, and C. D. Hoff,

Inorg. Chem. 1986, 25, p. 1160-1165.

temperature is roughly double for both

complexes.

It is likely that the rate constant is larger for

the ethyl complex because of the increased

steric strain of the ethyl complex. As ethyl is

a bulkier ligand than methyl, incorporating

it into a CO ligand to form an acyl ligand will

relieve this steric strain, and give a more

stable product. It seems logical, therefore,

that the rate constant for ethyl will be

higher.11

It was also suggested that due to the ethyl

complex’s ability to undergo β-elimination,

it would be more unstable than the methyl

complex (which cannot undergo β-

elimination). This lower stability results in a

more reactive species and therefore a

faster rate of reaction.

Past studies on migratory insertion

involving alkyls report that the σ-bond

strength of the alkyl-Mo bond may also

affect the rate; ethyl forms a weaker bond

to the Mo centre and also reacts quicker.12

5.b. Effect of incoming ligand (variation of

phosphine/phosphite)

The next variable tested was the nature of

the incoming ligand that promoted

migratory insertion on the complexes. Four

different ligands – two phosphines and two

phosphites – were tested and the rate

constants compared.

12 P. J. Craig and M. Green, J. Chem. Soc (A), 1968, p. 1978-1981.

Complex Temp. / K Kobs/s-1

[CpMo(CO)3Me] 311.0 1.73x10-4

[CpMo(CO)3Me] 321.2 3.94x10-4

[CpMo(CO)3Et] 311.0 4.76x10-3

[CpMo(CO)3Et] 321.2 8.77x10-3

Complex Nucleophile Kobs/s-1

[CpMo(CO)3Me] PPh3 3.94x10-4

[CpMo(CO)3Me] PCy3 3.89x10-4

[CpMo(CO)3Me] P(OEt)3 3.40x10-4

[CpMo(CO)3Me] P(OMe)3 3.50x10-4

Table 1 - Data for the variation of alkyl substituent at two different temperatures.

Table 2 - Data for the variation of the incoming nucleophile.


8

Variation of the nucleophile had very little

effect on the rate constant, with a total

range of 0.54x10-4 between the smallest

and largest values. This is a great contrast

from the variation seen by varying the alkyl

ligand on the complex. The values are close

enough that it could be said with

confidence that there was no effect on the

rate constant by varying the incoming

nucleophile. There is a slight difference

seen between the phosphines and

phosphites – about 0.4x10-4 s-1 separates

them. This could be explained by the

presence of electronegative oxygen atoms

in phosphites, drawing electron density

away from the phosphorous atom and

causing the nucleophile to be weaker

overall. Other studies of this experiment

prior to this paper have reached the same

conclusion.13

The lack of variation of Kobs between ligands

points to the nucleophile co-ordination step

of the mechanism not being rate-limiting.

As migratory insertion is a two-step

process,14 it suggests that the methyl

migration to form an acyl ligand is in fact

the rate-limiting step.

5.c. Effect of ligand concentration (varying

the concentration of the nucleophile)

In addition to varying the nature of the

incoming nucleophile, the concentration

was also varied in an attempt to determine

whether the rate depended on this factor.

13 I. S. Butler, F. Baloso, R. G. Pearson, Inorganic Chemistry,

1967, 6, 2074.

Perhaps the most obvious first impression

from the data shown in Table 3 is that the

rate was faster when the nucleophile was at

the same concentration as the complex,

and that a 2:1 ratio produced an almost

identical value for Kobs as having the

nucleophile in excess. This was an

unpredictable outcome for this experiment,

but could potentially be explained by the

manipulation on the rate law caused by the

new concentrations used.

When the nucleophile is in excess, the rate

law is pseudo first order, and takes the form

seen in equation 4:

Rate = k′[Complex]

Equation 4 - Rate law for the migratory insertion process.

And as 𝑘′ = 𝑘[𝑁𝑢], the nucleophile’s

concentration is not involved in the rate, as

it is considered a constant. When a 1:1 ratio

is used, however, the nucleophile is no

longer in excess and the rate law becomes

second order:

Rate = k[Nu][Complex]

Equation 5 - Second order rate law for the 1:1 [Nu]:[Complex] case.

This could explain the anomalously high Kobs

observed for this reaction.

Sadly, compared to the other variables

tested during this experiment, varying the

ligand concentration did not yield much

useful information.

14 M. J. Wax and R. G. Bergman, J. Am. Chem. Soc. 1981, 103, p.

7028-7030.

Complex [Nu]:[Complex] Kobs/s-1

[CpMo(CO)3Me] excess 1.80x10-3

[CpMo(CO)3Me] 1:1 2.95x10-3

[CpMo(CO)3Me] 2:1 1.82x10-3

Table 3 – Data for the variation of the

incoming nucleophile concentration.


9

5.d. Effect of temperature (variation of

reaction temperature)

In this series of tests, the temperature of

reaction was varied between 30 and 60 °C

on the apparatus display. These

corresponded to a slightly lower actual

operating temperature due to heat loss in

the tubes etc. This was accounted for in the

data table:

Table 4 - The variation of the rate constant with changing

temperature.

It was found that an increase in

temperature resulted in an increase in Kobs.

The relationship between the two was

determined as exponential, as seen in the

Arrhenius equation (eqn. 6).

k = Ae−EART

Equation 6 - The Arrhenius equation linking a reaction's rate constant and the temperature of reaction.

The results obtained from this section of

the experiment agree with previous studies

in this area.15 As the temperature is

increased, more molecules in the reaction

mixture will possess an energy greater than

EA, allowing the reaction to proceed.

Figure 9 shows the link between the two

values with an R2 value of 0.9976. This is a

very strong correlation and confirms that

the rate of reaction is dependent upon

temperature.

15 F. Calderazzo, F. A. Cotton, Inorg. Chem., 1962, 1 (1), p. 30–

36.

5.e. Effect of solvent (variation of solvent

reaction carried out in)

The reaction was carried out in co-

ordinating solvents, these being either THF

or MeCN. The co-ordinating effect of the

solvents means that the 16-electron

intermediate of the reaction is stabilised

and will react with the incoming

nucleophile at a higher rate than in non-co-

ordinating solvents.16 It was considered

that the solvent’s strength as a donor could

affect the rate of reaction, as the solvent co-

ordination occurs before the nucleophilic

attack of the new ligand.

As table 5 shows, the rate constant was

about 4.5 times lower in THF than in MeCN.

This observation agrees with theory

regarding donor strength; as MeCN is a

linear donor whereas THF is bulkier, MeCN

is expected to be a better donor. The ability

of MeCN to accept backbonding into its π*

16 A. Haynes, Longitudinal Ligands in Organometallic Chemistry,

CHM3104, University of Sheffield, 2015, p. 34-38.

Complex Display Temp/°C

Operating Temp/°C

Kobs/s-1

[CpMo(CO)3Me] 30.0 28.8 6.56x10-5

[CpMo(CO)3Me] 40.0 38.0 1.73x10-4

[CpMo(CO)3Me] 50.0 48.2 3.94x10-4

[CpMo(CO)3Me] 60.0 58.0 9.94x10-4

Complex Solvent Kobs/s-1

[CpMo(CO)3Me] THF 3.94x10-4

[CpMo(CO)3Me] MeCN 1.80x10-3

y = 0.0489e0.0917x

R² = 0.9976

0123456789

1011

20 30 40 50 60

Ko

bs

/ s-1

Temperature / °C

Comparison of T and Kobs

Figure 9 - The relationship between T and Kobs in graphical form.

Table 5 – Comparison of solvent effects on the rate constant. Both solvents used were co-ordinating.


10

orbital was also suggested to contribute to

its strength as a donor (THF does not have

this ability).17

Having the rate constants display this effect

lends weight to the proposal that the

mechanism is solvent-assisted and will

occur faster in more co-ordinating solvents.

It also points to the rate limiting step

involving solvent co-ordination, otherwise

the solvent would have little to no effect on

Kobs.

5.f. Effect of adding Lewis Acid (catalyst)

The final variation tested during this

experiment was addition of a catalyst in the

form of a Lewis Acid, AlCl3. It was predicted

that the rate would increase by a factor up

to 108 times upon addition of the catalyst.

The effect of the AlCl3 group is to polarise

the CO ligand and promote migration of the

alkyl group. Once migration has occurred

the AlCl3 group remains bound to the CO

ligand and donates a lone pair to the new

vacant site present on the metal centre,

negating the need for a co-ordinating

solvent to stabilise the 16-electron

intermediate.

17 K. Burger, Solvation, Ionic and Complex Formation Reactions

in Non-Aqeuous Solvents, Elsevier Scientific, Amsterdam, 1983.

Table 6 indicates how the results from this

experiment did not display the large

increase in rate predicted before the

readings were taken. There was no notable

difference in Kobs with the catalyst added –

in fact there was a slight decrease observed.

This was attributed to the aluminium

chloride reactant being aged and

potentially unreactive due to moisture

contamination (aluminium trichloride can

hydrolyse upon contact with water).

6. Activation Parameters for the reaction

Having calculated rate constants for the

reaction between the methyl complex

[CpMo(CO)3Me] and PPh3, it was possible to

calculate the activation parameters, ΔHǂ

and ΔSǂ, for this reaction. These parameters

are useful indicators of the reaction

mechanism, whether it be associative or

dissociative.

The rate constant is linked to the Gibbs’ free

energy of the reaction, and using the Eyring

equation produces equation 7, seen below:

𝑙𝑛 k = −∆Hǂ

RT+ 𝑙𝑛 (

kBT

h) +

∆Sǂ

R

Equation 7 - The modified Eyring equation, allowing us to calculate values for the enthalpy and entropy of reaction.

kB is Boltzmann’s constant.

This equation is conveniently in a form comparable to the equation for a linear plot, 𝑦 = 𝑚𝑥 + 𝑐. By plotting ln k on the y-axis and 1/T on the x-axis, values for the enthalpy and entropy of reaction can be

Complex Nucleophile Kobs/s-1

[CpMo(CO)3Me] PPh3 1.73x10-4

[CpMo(CO)3Me] PPh3/AlCl3 1.67x10-4

Figure 10 - A diagram of how the 16 electron intermediate is stabilised by a co-ordinating solvent.

Here, THF is donating into the metal centre.

Figure 11 - AlCl3 acting as a catalyst during the migratory insertion reaction.

Table 6 - Comparison of the rate constant with and without the catalyst added.


11

calculated, allowing determination of the mechanism for the reaction.

The linear trendline for this plot gives values

that can be incorporated into calculations

to produce the values for ΔHǂ and ΔSǂ. A

sample calculation for this will be provided

in the appendix to this report.

The first point to make relative to the data

shown in table 7 is the trend for the entropy

of activation. This experiment produced a

lower ΔSǂ value for the methyl complex

than for the ethyl complex; a trend not

18 I. S. Butler, F. Baloso, R. G. Pearson, Inorganic Chemistry,

1967, 6, 2074. * This value is for reaction with P(n-OC4H9)3 19,19,20 S. P. Nolan, R. L. de la Vega, S. L. Mukerjee, and C. D.

Hoff, Inorg. Chem. 1986, 25, p. 1160-1165.

reflected in the literature. This could be due

to experimental error in the lab and/or the

quality of the ethyl complex sample; as it

was not purified, impurities may have been

present.

The negative value of ΔSǂ for both

complexes is indicative of an associative

mechanism.22 As the transition state is

more ordered, its entropy is lower and so

ΔSǂ would be expected to decrease. This

agrees with the notion mentioned earlier

involving solvent co-ordination in the rate

limiting step, which is itself an associative

process.

The data obtained from this experiment

does not correlate with the data found in

the literature for values of ΔHǂ. It is

reported in previous studies of this area

that the barrier for insertion involving metal

carbonyls is greater for larger R groups.23

This would assume a larger ΔHǂ value for

the ethyl complex, as the ethyl migrating

involves a larger group sterically relative to

methyl and therefore a larger barrier to

insertion.

All reactions carried out were thermally

assisted to encourage the reaction to run to

completion due to a lack of spontaneity

caused by a positive Gibbs’ free energy. The

positive status of both enthalpy values

indicates that the process is endothermic

and agrees with the prior statement.

It is regrettable that the trends between

this experiment and previous studies did

not agree. A potential lack of data points

may have contributed to the activation

parameters calculated being different to

those found in the literature.

* These values are for reaction with PMePh2 22 A. Haynes, Transition Metal Reaction Mechanisms, CHM 2104,

University of Sheffield, 2014. 23 L. Luan, P. S. White, M. Brookhart, J. L. Templeton, J. Am.

Chem. Soc. 1990, 112, p. 8190.

Complex ∆S / J K-1 mol-1 ∆S lit.

∆H / kJ mol-1 ∆H lit.

[CpMo(CO)3Me] -75.9 -104.618* 75.1 53.619*

[CpMo(CO)3Et] -138.0 -94.620 47.2 68.221*

y = -9.0319x + 14.627R² = 0.9968

-16.0

-15.5

-15.0

-14.5

-14.0

-13.5

-13.0

-12.5

-12.0

3.00 3.10 3.20 3.30 3.40

ln(k

/T)

103.(1/T) / K

Eyring Plot for the reaction of [CpMo(CO)3Me] with excess PPh3 to

determine activation parameters

Table 7 - A comparison of the activation parameters

obtained using data from this experiment to values

quoted in the literature.

Figure 12 - Eyring plot produced using data from this experiment. All factors other than temperature were kept

constant.


12

Conclusions

This experiment set out to characterise a

number of Molybdenum carbonyl

complexes synthesised in the lab and to

then perform kinetic investigations upon

the complexes to determine the rate law,

dependence of the rate on any factors of

the reaction and values for the activation

parameters, ΔHǂ and ΔSǂ. The complexes

that were attempted were all synthesised

successfully. These were:

1) [CpMo(CO)3Me]

2) [CpMo(CO)3Et]

3) [CpMo(CO)3I]

4) [CpMo(CO)2(COMe)(PPh3)]

Compounds 1) and 2) were then used for a

series of kinetic experiments involving a

phosphine nucleophile reacting with the

complex in solution through the mechanism

of migratory insertion. As the concentration

of the nucleophile greatly exceeded the

concentration of the complex, pseudo-first-

order conditions were apparent and this

allowed any dependences of the rate on

factors of the reaction to be highlighted. It

was found that the rate increased by a

factor of roughly 22 times (Kobs = 8.77x10-3

compared to 3.94x10-4 at 40°C) when the R

group present on the Molybdenum

complex was ethyl rather than methyl.

Other factors positively influencing the rate

include the solvent – MeCN produced a rate

constant 4.5 times greater than observed in

THF – and the temperature, which caused

an increase in an exponential manner, as

predicted by the Arrhenius equation.

Varying both the identity and the

concentration of the nucleophile had no

effect on the value of Kobs great enough for

them to be considered an influential factor.

Introducing a catalyst was predicted to

increase the rate of reaction but this was

not observed during this experiment due to

reasons discussed earlier.

The rate law was deemed to be first order

with respect to complex and zero order

with respect to the incoming nucleophile

after consideration of the above

information gathered from this experiment.

This was reinforced by the values calculated

for the entropy and enthalpy of activation –

both compounds displayed negative values

of ΔSǂ (-75.9 and -138.0 for R = Me, Et

respectively) meaning the mechanism for

the reaction was associative and solvent

assisted. In this case, migration of the alkyl

group occurs before the nucleophile binds

and is the rate-determining step. Much of

the evidence acquired from this study lent

weight to this being the situation for this

reaction. The calculated activation

parameters are an area that would require

further study, as the data from this

experiment did not correlate with trends

and values found in the literature.

Experimental

1. Synthesis and characterisation of

[CpMo(CO)3Me]

[CpMo(CO)3]2 (0.51 g, 1.04 mmol) was

dissolved in 25 cm3 dry THF under an inert

N2 atmosphere and constant stirring. To

this solution, Li[BEt3H] (3 cm3, 2.5 mmol)

was added by syringe. A colour change from

dark red to dark green after 20 minutes was

observed. Methyl iodide (0.93 cm3, 14.94

mmol) was added and left under constant

stirring for 1 hour. The solvent was then

removed under reduced pressure and the

resulting solid product extracted using pet.

ether (4 x 20 cm3 portions). After extraction,

the now colourless sample was filtered

through an Al2O3 plug under gravity and

more pet. ether (≈ 40 cm3) was run through


13

the column to remove the yellow

colouration attained by the solution. The

solvent was removed from the filtrate again

using reduced pressure, and the solid

obtained then further purified by vacuum

sublimation between 40 - 60 °C for

approximately 8 hours.

Produced bright yellow crystals (0.33 g, 62

%), found ʋmax/cm-1 (solution cell; THF) 2025

(C≡O stretch) 1942 (C≡O stretch); δH

(400MHz; CDCl3) 0.39 (3H, s, CH3) 5.30 (5H,

s, C5H5) 7.30 (s, CDCl3); δC (100MHz; CDCl3) -

22.2 (CH3) 30.9 (acetone impurity) 76.7,

77.0, 77.3 (CDCl3) 92.5 (C5H5).


[CpMo(CO)3Et]

[CpMo(CO)3]2 (0.50 g, 1.02 mmol) was

dissolved in 25 cm3 dry THF under an inert

N2 atmosphere and constant stirring. To

this solution, Li[BEt3H] (3 cm3, 2.5 mmol)

was added by syringe. Again, a colour

change from dark red to dark green was

observed. Iodoethane (0.8 cm3, 10 mmol)

was added and left under constant stirring

for 1 hour. The solvent was then removed

under reduced pressure and the resulting

solid product extracted using pet. ether (4 x

20 cm3 portions). The colourless solution

was filtered through an Al2O3 plug under

gravity, and more pet. ether (≈60 cm3) was

allowed to run through the column to

ensure maximum product extraction. The

solvent was then again removed under

reduced pressure, and the solid obtained

dried under vacuum for 10 minutes.

Dull green crystals were afforded (0.30g,

54%), found ʋmax/cm-1 (solution cell; THF)

2019 (C≡O stretch) 1937 (C≡O stretch); δH

(400MHz; CDCl3) 1.42 (3H, t, CH3) 1.70 (2H,

q, CH2) 5.30 (5H, s, C5H5) 7.29 (s, CDCl3); δC

(100MHz; CDCl3) -3.8 (CH2) 20.2 (CH3) 76.7,

77.0, 77.3 (CDCl3) 92.8 (C5H5) 227.7 (CO).


[CpMo(CO)3I]

[CpMo(CO)3]2 (0.20 g, 0.408 mmol) was

reacted with Iodine (0.104 g, 0.408 mmol)

in 25cm3 dry THF under an inert N2

atmosphere and left under constant stirring

for 1 hour. The solvent was removed under

reduced pressure and the solid obtained

was extracted using 4 x 20 cm3 portions of

pet. ether before being filtered using an

Al2O3 plug. The solvent was again removed

under reduced pressure and the solid

product dried under vacuum for 10

minutes.

Obtained dark red/purple crystals (0.11 g,


2044 (C≡O stretch) 1977, 1962 (C≡O

stretch); δH (400MHz; CDCl3) 5.62 (5H, s,

C5H5) 7.29 (s, CDCl3); δC (100MHz; CDCl3)

76.7, 77.0, 77.4 (CDCl3) 94.2 (C5H5) 220.2,

236.3 (CO).


[CpMo(CO)2(COMe)(PPh3)]

[CpMo(CO)3Me] (0.13 g, 0.5 mmol) was

added to PPh3 (0.2 g, 0.76 mmol) in 10 cm3

MeCN under N2 for approximately 16 hours.

A yellow precipitate was formed which was

collected on a sinter and washed

thoroughly with pet. ether (3 x 15 cm3)

portions. The solid was dried over vacuum

and then a recrystallisation attempted

using boiling pet. ether. No crystals were

formed in the recrystallisation and so the

solvent was removed under reduced

pressure and the solid produced dried

through vacuum desiccation for 30

minutes.

Dull yellow crystals were produced (0.17 g,


1938 (C≡O stretch) 1855 (C≡O stretch) 1620


14

(C=O stretch); δH (400MHz; CDCl3) 1.22 (3H,

s, CH3) 1.52 (water impurity) 2.17 (acetone

impurity) 5.0 (5H, s, C5H5) 7.29 (s, CDCl3)

7.47 (15H, m, C6H5); δC (100MHz; CDCl3)

76.7, 77.0, 77.3 (CDCl3) 96.7 (C5H5) 129.3 (d,

C2 of C6H5) 131.2 (s, C4 of C6H5) 133.8 (d, C3

of C6H5); δP (100MHz; CDCl3) 67.9 (PPh3).

5. Kinetic Measurements for CO migratory

insertion

0.2g PR3 was dissolved in 2cm3 solvent

(MeCN / THF) in a 5cm3 graduated cylinder.

The solution was made up to the mark by

adding more solvent. A sample of this

solution was placed in a solution cell and

allowed to equilibrate with the

temperature of the thermal jacket. A

background spectrum was then run using

TimeBaseTM software. The cell was rinsed

thoroughly with DCM and also the chosen

solvent in two separate processes.

2cm3 of the phosphine/phosphite solution

made up earlier was added to 5mg

[CpMo(CO)3R] and shaken to dissolve all

solid. A sample of this solution was again

placed in the solution cell and multiple

spectra were run over a period of time,

usually 3-4 hours. The reactions were

stopped prior to the full time elapsing if

they could be considered to have reached

completion.

The TimeBaseTM data for each individual

carbonyl peak was saved and exported to

KaleidagraphTM, where a first order

formation/regression graph was plotted

(for each peak). The rate constant for each

graph was calculated by the software and

recorded as a spreadsheet on Microsoft

ExcelTM.

Thanks go to the other members of this research group (J. Hill, H. Pan, J. Skade, L. Thomas and

L. Wattam) for their contribution and effort during the course of this study, and Dr. Haynes for

his time and guidance throughout.

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Michael Ludden L3Report2016