Homolytic Bond Strength and Radical Generation from (1-Carbomethoxyethyl)pentacarbonylmanganese(I)

Article abstract of DOI:10.1002/ejic.201900429

Compound (1-carbomethoxyethyl)pentacarbonylmanganese(I), [MnR(CO)₅] (R = CHMeCOOMe, 1), was synthesized from K⁺[Mn(CO)₅]^– and methyl 2-bromopropionate and isolated in pure form. Upon thermal activation, the Mn–R bond is homolytically cleaved and the resulting 1-carbomethoxyethyl radical is able to initiate the polymerization of methyl acrylate (MA). A kinetic study of the decomposition of 1 in the presence of tris(trimethylsilyl)silane, TTMSS (10 equiv., saturation conditions) at 70, 65 and 60 °C yielded the T-dependent activation rate constant, k_a, which allowed the calculation of the activation enthalpy (ΔH^? = 35.3 ± 2.8 kcal mol^–1) and entropy (ΔS^? = 27.2 ± 8.1 cal mol^–1 K^–1) through the use of the Eyring relationship. The ΔH^? value can be taken as an upper limit for the thermodynamic bond dissociation enthalpy, which was estimated as 36.9 kcal mol^–1 by DFT calculations. The higher thermal stability of 1 relative to that of simpler R derivatives that form stronger Mn–R bonds can be attributed to more difficult CO dissociation, precluding the decomposition by β-H elimination.

Full text of DOI:10.1002/ejic.201900429

A Journal of
Accepted Article
Title: Homolytic bond strength and radical generation from (1-
carbomethoxyethyl)pentacarbonyl-manganese(I)
Authors: Roberto Morales-Cerrada, Christophe Fliedel, Jean-Claude
Daran, Florence Gayet, Vincent Ladmiral, Bruno Ameduri,
and Rinaldo Poli
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900429
Link to VoR: http://dx.doi.org/10.1002/ejic.201900429

10.1002/ejic.201900429
European Journal of Inorganic Chemistry
FULL PAPER
Homolytic bond strength and radical generation from (1-
carbomethoxyethyl)pentacarbonylmanganese(I)
Roberto Morales-Cerrada,[a,b] Christophe Fliedel,[a] Florence Gayet,[a] Vincent Ladmiral,[b] Bruno
Améduri,*[b] and Rinaldo Poli*[a]
Abstract:
Compound
(1-carbomethoxyethyl)pentacarbonyl-
the reactivity of the free radicals of growing chains scales
inversely to that of their associated monomers. Therefore,
radicals produced from less reactive monomers lead to stronger
bonds with the moderating agent and the resulting dormant
species are consequently less easily reactivated.
manganese(I), [MnR(CO)5] (R = CHMeCOOMe, 1), was synthesized
from K+[Mn(CO)5]- and methyl 2-bromopropionate and isolated in pure
form. Upon thermal activation, the Mn-R bond is homolytically cleaved
and the resulting 1-carbomethoxyethyl radical is able to initiate the
polymerization of methyl acrylate (MA). A kinetic study of the
decomposition of 1 in the presence of tris(trimethylsilyl)silane, TTMSS
(10 equiv, saturation conditions) at 70, 65 and 60 °C yielded the T-
dependent activation rate constant, ka, which allowed the calculation
of the activation enthalpy (∆H‡ = 35.3±2.8 kcal·mol-1) and entropy
ka
Pn
T
kp
+ M
T + Pn
kt
kda
(∆S‡
=
27.2±8.1 cal·mol-1·K-1) through the use of the Eyring
bimolecular
terminations
relationship. The ∆H‡ value can be taken as an upper limit for the
thermodynamic bond dissociation enthalpy, which was estimated as
36.9 kcal·mol-1 by DFT calculations. The higher thermal stability of 1
relative to that of simpler R derivatives that form stronger Mn-R bonds
can be attributed to more difficult CO dissociation, precluding the
decomposition by β-H elimination.
Scheme 1. Moderating equilibrium for a controlled radical polymerization.
It was previously shown that the controlled polymerization of VDF
by another RDRP method termed “iodine transfer polymerization”
(ITP),[5] which generally suffers from irreversible deactivation of
the iodine-capped chains,[5-6] can be improved by the presence of
•
[Mn(CO)5 ] radicals generated from [Mn2(CO)10] and visible light.[7]
Introduction
This phenomenon suggested the likelihood that [Mn(CO)5]• may
also directly trap the growing PVDF• chains, generating
organometallic [(CO)5Mn-PVDF] dormant species of type
[(CO)5Mn-CF2CH2PVDF] (for regular growing chains) or
[(CO)5Mn-CH2CF2PVDF] (for chains obtained after an inverted
monomer addition). To learn about this possibility, we recently
investigated model compounds with a variety of F-containing alkyl
groups, [Mn(RF)(CO)5] (RF = CF3, CHF2, CH2CF3) in terms of their
We have recently become interested in using organometallic
compounds of transition metals with homolytically weak metal-
carbon bonds as initiators for controlled radical polymerization,
also known as Reversible Deactivation Radical Polymerization
(RDRP).[1] A dynamic equilibrium between the growing radical
•
chain (Pn ) and a dormant species (Pn-T), formed by reversible
trapping with a moderating agent (T), as shown in Scheme 1,
decreases the likelihood of bimolecular radical terminations
(persistent radical effect),[2] leading to a more predominant
monomer (M) addition and controlled chain growth. The strategies
that use a transition metal complex as moderating agent have
been termed “organometallic-mediated radical polymerizations”
(OMRP).[3] We have been particularly attracted to the controlled
polymerization of challenging monomers such as vinylidene
fluoride (VDF).[4] As is well-known from basic polymer chemistry,
•
homolytic bond strength and aptitude to generate RF radicals
under thermal or photochemical (visible, UV) conditions.[8] The
experimental studies were carried out by a kinetic approach,
trapping the thermally generated radicals with tris(trimethyl-
silyl)silane (TTMSS) to produce RF-H. These investigations
yielded estimates for the bond dissociation enthalpies (BDEs) that
are in agreement with those of previous[9] and more recent[10] DFT
investigations. Furthermore, these investigations have shown that
the Mn-RF bond cannot be thermally cleaved at significant rates
under the conditions previously used for the ITP of VDF in the
presence of [Mn2(CO)10] (50 °C). On the other hand, cleavage
[a]
Mr. R. Morales Cerrada, Dr. C. Fliedel, Dr. F. Gayet, Prof. R. Poli
CNRS, LCC (Laboratoire de Chimie de Coordination)
Université de Toulouse, UPS, INPT
205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4,
France.
•
with production of RF radicals occurs at higher temperature or
under visible or UV light irradiation and, under these conditions,
these compounds successfully initiated the polymerization of
VDF.[8]
E-mail: rinaldo.poli@lcc-toulouse.fr
https://rinaldo-poli.jimdo.com/
The present contribution extends the homolytic BDE determi-
nation to the alkyl compound [Mn(CHMeCOOMe)(CO)5], 1, which
[b]
Mr. R. Morales Cerrada, Dr. V. Ladmiral, Dr. B. Améduri
ICGM
•
generates the ester-stabilized radical CHMe(COOMe) (a model
Univ Montpellier, CNRS, ENSCM
Montpellier, France.
E-mail: bruno.ameduri@enscm.fr,
https://www.icgm.fr/bruno-ameduri
of the growing poly(methyl acrylate) radical chain). We were also
interested in establishing whether this compound could function
as a suitable initiator for the radical polymerization of methyl
acrylate.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201900429
European Journal of Inorganic Chemistry
FULL PAPER
Results
neglected and the experimentally measured pseudo-first order
rate constant for the decay of the starting complex (kobs
)
corresponds to the rate constant of the Mn-C bond homolytic
cleavage (ka in equation 1). It was established that 10 equivalents
of TTMSS per Mn are sufficient, even for the strongest bond with
CF3, to achieve saturation conditions.[8]
(a) Synthesis
The title compound has been previously described,[11] but was
only obtained as an alkyl rearrangement by-product during the
decarbonylation of [Mn(COCH2CH2COOCH3)(CO)5], which yields
the expected [Mn(CH2CH2COOCH3)(CO)5] as the main product
by reverse migratory insertion. We have synthesized compound
ka
[(CO)5Mn-R]
R
+
[(CO)5Mn]
(1)
(2)
1, following
a
well-established general procedure,[12] by
kda
1
(R = CHMeCOOMe)
ktrap
nucleophilic substitution of bromide in methyl 2-bromopropionate
by pentacarbonylmanganate, which was made in situ by reduction
of [Mn2(CO)10] with Na-K alloy, see Scheme 2. We prefer this
reducing agent[8] over the more commonly employed sodium
amalgam.
R-H
+
(R = CHMeCOOMe)
+
(SiMe3)3Si
(SiMe3)3Si-H
R
2
kdim,Mn
2 [(CO)5Mn]
2 (SiMe3)3Si
[Mn2(CO)10]
(3)
(4)
(5)
kdim,Si
Si2(SiMe3)6
ktrap,Mn
Br
+
[Mn2(CO)10]
[(CO)5Mn-Si(SiMe3)3] + H2
(SiMe3)3Si-H
OMe
Mn(CO)5
Scheme 3. Thermal decomposition of alkylpentacarbonylmanganese(I)
compounds in the presence of TTMSS as a radical trap. In the present study, R
= CH(CH3)COOCH3.
Na/K
THF, r.t.
O
OMe
K+[Mn(CO)5]-
[Mn2(CO)10
]
THF, r.t.
O
1
39%
The experimental protocol used for 1 (see Experimental Section)
was the same as in our recent study of the Mn-RF bond strengths
in compounds [(CO)5Mn-RF] (RF = CF3, CHF2 and CH2CF3),[8]
except that the increased thermal lability of the Mn-C bond in 1
allowed using lower temperatures. The kinetic monitoring was
Scheme 2. Synthesis of compound 1.
Compound [Mn(CHMeCOOEt)(CO)5], which is rather similar to 1
(ethyl instead of methyl in the ester group), was previously
synthesized using the same strategy.[13] In that contribution, the
influence of the halogen nature (X) in the R-X substrate on the
product yield was investigated in detail. It was established that the
bromide substrate gives the best yields, whereas Cl led to better
results for analogous primary alkyl derivatives. These syntheses
are affected by side reactions with generation of [Mn2(CO)10] and
[MnX(CO)5], in addition to the desired [MnR(CO)5]. The reactions
of Na+[Mn(CO)5]- or PPN+[Mn(CO)5]- with MeCH(Br)COOEt in
THF at 25 °C gave respectively 63% and 76% spectroscopic
(55Mn NMR) yields of [Mn(CHMeCOOEt)(CO)5], but no isolated
yields were reported.[13] In our hands, K+[Mn(CO)5]- and methyl 2-
bromopriopionate gave, after chromatographic purification, a fair
39% yield of pure product. The spectroscopic properties of the
isolated compound matched those reported in the previous
contribution (see SI, Figures S1 and S2).[13]
1
based on H NMR measurements in C6D6. The expected main
product is methyl propanoate, CH3CH2COOCH3 (2). However,
several by-products were also formed, leading to resonance
overlap and complicating the extraction of rate data. The
formation of the expected 2 was suggested by the observation of
a growing triplet at δ 0.96 with J = 6 Hz (cf. δ 1.17, J = 7.5 Hz in
CDCl3).[16] The relative amounts of compound 1 at different times
were calculated from the relative integral of the signals of the
1,3,5-trioxane internal standard (δ 4.45 to 4.55) and the CHCH3
proton doublet (δ 1.40 to 1.55), which is located in a clean spectral
region with no overlap with the resonances of other species, see
SI, Figure S3. The first-order decay plots at 70, 65 and 60 °C are
shown in Figure 1 and the resulting ka values are reported in Table
1. Analysis of these data by the Eyring equation (Figure 2) yields
‡
the activation parameters ∆Ha‡ = 35.3±2.8 kcal·mol-1 and ∆Sa
27.2±8.1 cal·mol-1·K-1.
=
(b) Experimental investigation of the Mn-C bond cleavage
Table 1. Kinetics parameters and half-lives for the decay of compound 1 in
the presence of TTMSS (10 equiv) in C6D6.
The Mn-C homolytic bond strength in compound
1 was
investigated by monitoring the rate of its disappearance at
different temperatures in the presence of a sufficient amount of
trapping agent for the produced •CHMe(COOMe) radical to insure
saturation conditions. Using the ubiquitous TEMPO as a trapping
agent[14] turned out unsuitable for this type of system, whereas
TTMSS gave good results.[8] The proposed sequence of reactions
for the produced [Mn(CO)5]• and (Me3Si)Si• radicals, which is
based on literature reports,[15] is shown in reactions 1-5 (Scheme
3).
T/°C
60
65
70
kobs/s-1
t1/2/min
(4.1±0.4)·10-5
282±27
(9.6±0.5)·10-5
120±6
(2.00±0.03)·10-4
58±1
Saturation conditions are achieved when the rate of the trapping
process (ktrap in equation 2) is much faster than that of the reverse
radical recombination (kda in equation 1). Hence, the latter may be
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10.1002/ejic.201900429
European Journal of Inorganic Chemistry
FULL PAPER
indicating its low aptitude to add ligands as expected for a low-
valent 17-electron complex.[21] Although this radical has never
been studies in a benzene matrix, benzene is certainly a poorer
ligand than CO for low-valent metal centers.
Table 2. Comparison of experimental activation enthalpies and calculated
BDEs for different [MnR(CO)5] compounds, in kcal mol-1.
‡
R
∆Ha
Ref.
BDEa
55.1
50.5
48.0
36.9
Ref.
[8]
[10]
CF3
53.8±3.5
50.6±0.8
46.3±1.6
35.3±2.8
[8]
[8]
[10]
[10]
CH2CF3
CHF2
CHMeCOOMe
This work
This work
a Difference between the enthalpies of ([Mn(CO)5]• + R•) and [MnR(CO)5]
calculated by the DFT method.
Figure 1. First order decay of [Mn(CHMeCOOMe)(CO)5] (1) in C6D6 at 70, 65
and 60 °C in the presence of TTMSS.
(c) DFT calculation of the Mn-C BDE in compound 1
Calculations of the BDE in compound 1 were based on the DFT
approach with the BPW91* functional, which has given us
excellent results in previous applications the thermochemistry of
22]
first-row paramagnetic complexes.[10,
This functional is a
reparametrized version of B3PW91 with the same parameters
previously optimized for B3LYP.[23] Geometry optimizations were
run in the gas phase and the energies were corrected for
dispersion forces. No solvation corrections were applied, since
the experimental study was carried out in the low-polarity benzene
solvent. This approach is identical to that of the recent study of
the analogous [MnR(CO)5] complexes with R = CF3, CHF2 and
CH2CF3.[10] The BDE is the difference between the enthalpies of
the [Mn(CO)5]• and •CH(CH3)COOCH3 fragments and that of the
starting complex 1. The calculated energies and the Cartesian
coordinates of the three optimized geometries are given in the SI,
Table S1, whereas views of the optimized structures and the bond
breaking thermochemical parameters are given in Figure 3. The
enthalpy value is also reported in Table 2. A comparison of the
experimental activation enthalpy and the calculated BDE shows
an excellent match, as for the previously published fluorinated
alkyl derivatives.[8, 10]
Figure 2. Eyring plot of kobs as function of temperature for the decomposition of
compound 1.
Under the reasonable hypothesis that the radical recombination
process (kda in equation 1, Scheme 3) has an associated
‡
negligible activation enthalpy, the determined ∆Ha parameter
may be taken as
a reasonable upper estimate of the
thermodynamic bond dissociation enthalpy. This assumption is
commonplace, made also in other studies of metal-carbon bond
dissociation energies by a similar kinetic approach in non-
coordinating solvents.[14a-d, 17] The value obtained for compound 1
is much lower than those obtained for the fluorinated alkyl
analogues, see Table 2. This is consistent with the greater
stabilization of the carbomethoxyethyl radical, which is a model of
the poly(methyl acrylate) growing chain, relatively to fluorinated
alkyl radicals. It must be underlined that using a coordinating
solvent would not provide a legitimate approximation of the metal-
carbon bond strength, because the thermodynamics and kinetics
would be skewed by the solvent coordination process.[18] The
[Mn(CO)5] radical was shown to have a 5-coordinate C4v structure
both in an Ar[19] and in a CO[20] low-temperature matrix, thus
Figure 3. Views of the optimized geometries of [Mn(CH(CH3)COOCH3)(CO)5]
(1), [Mn(CO)5]• and •CH(CH3)COOCH3, and DFT-calculated thermochemical
parameters.
(d) Initiation of methyl acrylate radical polymerization
Since compound 1 is soluble in methyl acrylate (MA), bulk
polymerizations using 1 as initiator were initially carried out at
different temperatures (75, 50 and 30 °C) with three different
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10.1002/ejic.201900429
European Journal of Inorganic Chemistry
FULL PAPER
monomer/initiator ratios (70, 30 and 20). All tests led to rapid (0.5
- 1 h) viscosity increase and even to a solid, proving the
occurrence of MA polymerization and the efficiency of 1 as a
radical initiator. The high viscosity suggests the formation of very
high molar mass macromolecules, which made it impossible to
of methyl acrylate (the expected β-H elimination product), was
never observed in the degrading solutions of 1. A faster β-H
elimination from the ethyl and isopropyl derivatives, relative to 1,
may be caused by their greater aptitude to dissociate CO, a
possible consequence of the stronger Mn-C bond and hence
stronger trans effect of the Et and iPr groups relative to the ester-
substituted alkyl in 1. It is also likely that the homolytic bond
cleavage, which is easier for 1 than for the ethyl derivative
according to the DFT predictions, is more favored than CO
dissociation, thus precluding a decomposition pathway by β-H
elimination for this compound.
perform
a detailed kinetic study. Therefore, an additional
polymerization was carried out at 70 °C in a dilute C6D6 solution.
This solvent was selected because it dissolves all components
including the polymer, it does not promote chain transfer to
solvent, and it allows direct monitoring by 1H NMR spectroscopy.
A polymerization also took place under these conditions (results
in the SI, Table S2), although at a much slower rate as expected
from the lower monomer concentration. The 1H NMR spectrum of
the polymer (SI, Figure S4) confirmed its identity as PMA by
comparison with the published spectrum.[24] A conversion vs. time
plot is shown in Figure S5. In agreement with the determined half-
life of compound 1 at 70°C in C6D6 (ca. 1 h, Table 1), the
polymerization essentially stops after 8 h (ca. 8 half-lives), when
the polymer yield is only 14%. The recovered polymer samples
have high average molar masses and high dispersities (results
are also in Table S2), even for the first sample recovered after 1
h. This shows that the manganese complex does not exert any
kind of control (i.e. a persistent radical effect, as in OMRP) for this
polymerization. This result is quite expected, because the
released [Mn(CO)5]• radicals rapidly disappear by dimerization
(equation 3, Scheme 3, kdim,Mn = 1.9·109 M-1·s-1)[25] and, although
they can be thermally generated in small equilibrium amounts at
70°C,[26] they are not able to add to methyl acrylate and initiate
the polymerization. The SEC trace of the first polymer sample (SI,
Figure S6) shows the immediate production of a high molar mass
PMA. This remains essentially unchanged in the subsequent
samples, while a new lower average molar mass component
starts to grow, resulting in the decrease of the overall average
molar mass and in a dispersity increase.
Conclusions
The present contribution has proven that compound 1 is thermally
activated to generate the stabilized radical, •CHMeCOOMe, which
is a model of a polyacrylate radical chain. This generated radical
successfully initiates the radical polymerization of MA. The Mn-R
homolytic bond dissociation enthalpy has been quantitatively
assessed by a kinetic approach. Notably, the experimentally
measured activation enthalpy matches rather closely the DFT
estimate of the BDE. The same was recently found for similar
compounds with fluorinated R groups,[8] for which the BDEs are
10-20 kcal/mol higher than that of 1. These comparisons confirm
that the chosen DFT methodology may be used as a predictive
tool for further thermochemical studies of Mn-C bond breaking in
[MnR(CO)5] complexes.
The recently introduced[8] protocol for the experimental
determination of the metal-carbon homolytic bond cleavage rate
using TTMSS as radical trapping agent, developed for the strong
Mn-R bonds in [MnR(CO)5] (R = CF3, CHF2, CH2CF3), has been
extended here to compound 1, which contains a more weakly
bonded alkyl group leading to the ester-stabilized acrylate radical
•CHMeCOOMe. The experimentally determined activation
enthalpy is in close agreement with the predicted thermodynamic
BDE from DFT calculations. The compound was proved to
successfully initiate the radical polymerization of methyl acrylate
under thermal activation conditions. A comparison of the thermal
stability of 1 with that of other thermally fragile pentacarbonyl-
manganese(I) alkyl derivatives shows that 1 is less susceptible to
decompose by initial CO dissociation followed by β-H elimination,
leaving the homolytic bond breaking as the dominant
decomposition pathway. Further developments on the complex
scaffold are now under investigation to improve the OMRP
equilibrium / polymerization control.
Discussion
While compound 1 has a homolytically weak Mn-R bond (see half-
lives at different temperatures in Table 1), it is relatively stable at
room temperature when protected under an inert atmosphere.
Only slow darkening was witnessed when the compound was left
as a solid at room temperature in air. This is in stark contrast with
the high thermal instability of other alkylpentacarbonylmanga-
nese(I) compounds, such as [Mn(CO)5(C2H5)] or [Mn(CO)5(n-
C3H7)], which were reported to slowly decompose even at -10 °C
in the dark under an inert atmosphere.[27] The increased thermal
sensitivity of those compounds cannot be attributed to an easier
homolytic bond cleavage, because ethyl and isopropyl are non-
stabilized radicals that form stronger bonds than an acrylate
radical. Indeed, the previously published DFT investigation of the
Mn-R BDE in [MnR(CO)5] included the compound with R = C2H5
(BDE = 41.9 kcal/mol, i.e. 5.0 kcal/mol stronger than the Mn-C
bond in 1).[10] Such low thermal stability has been attributed to the
available β-H elimination pathway following the dissociation of a
CO ligand.[27b] In this respect, the stability of 1 is striking because
this compound also contains β-H atoms. However, the formation
Experimental Section
General. All operations were carried out under an atmosphere of argon
except for the column chromatography purification, which was carried out
in air. Compounds [Mn2(CO)10] (98%, Strem Chemicals), methyl 2-
bromopropionate (98%, Sigma-Aldrich), tris(trimethylsilyl)silane (TTMSS,
97%, Fluorochem), 1,3,5-trioxane (99%, Fluorochem), benzene-d6 (99.5%
D, Euriso-top), acetone-d6 (99.5% D, Euriso-top) and silica gel (40-63 µm,
VMR Chemicals) were used as received. Methyl acrylate (99%, Sigma-
Aldrich) was filtered through activated basic alumina Brockmann I (Sigma-
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10.1002/ejic.201900429
European Journal of Inorganic Chemistry
FULL PAPER
Aldrich) prior to use to remove the monomethyl ether hydroquinone
inhibitor. Laboratory Reagent grade (≥ 99.5%) diethyl ether, pentane and
THF were purchased from Sigma-Aldrich. THF was purified by percolation
through a dry activated alumina column. Potassium (98%, Aldrich) and
sodium (≥ 99.8%, Aldrich) were washed in n-pentane to remove the
mineral oil prior to use. The nuclear magnetic resonance (NMR) spectra
were recorded on a Bruker Avance™ III 400 MHz spectrometer. The
instrumental parameters for recording 1H NMR spectra were as follows:
flip angle 30°, acquisition time 5.7 s, pulse delay 2 s, number of scans 64
(4 for the kinetic study), and a pulse width of 3.05 µs. The Fourier transform
infrared (FTIR) spectra on the pentane solutions were recorded in
integrated from 4.65 ppm to 4.85 ppm and the resulting value was set to 1
in all spectra. Then, the complex signal was integrated from 1.39 ppm to
1.53 ppm. Knowing the initial concentration of the manganese complex in
each tube (C0), the concentration at time t (Ct) was determined as Ct =
C0It/I0, where I0 and It stand for the integral value of the NMR signal of the
complex at t = 0 and at t, respectively. The ln[C0/Ct] vs. time plots led to
straight lines, the slopes of which represents k.
Computational Details. The computational work was carried out using
the Gaussian09 suite of programs.[28] Gas-phase geometry optimizations
were performed without any symmetry constraint using the BPW91*
functional, which is a reparametrized version of B3PW91 with the same
parameters previously optimized for B3LYP.[23] The basis sets used were
6-31G(d,p) for all light atoms (H, C, F, O, S) and SDD, augmented by an f
polarization function (α = 2.195),[29] for the Mn atom. The unrestricted
formulation was used for open-shell molecules, yielding only minor spin
contamination (<S2> at convergence was very close to the expected value
of 0.75 for the radical species). All final geometries were characterized as
local minima by verifying that all second derivatives of the energy were
positive. Thermochemical corrections were obtained at 298.15 K on the
basis of frequency calculations, using the standard approximations (ideal
gas, rigid rotor and harmonic oscillator). Corrections for dispersion were
carried out at the fixed BPW91* optimized geometries using Grimme’s D3
empirical method (BPW91*-D3) with SR6 and S8 parameters identical to
those optimized for B3PW91.[30]
transmission mode with
a
PerkinElmer Spectrum One FT-IR
Spectrophotometer using a CaF2 window with a 4 mm thick and 0.05 mm
path length. The apparent number average molar masses and dispersities
(Ð) of the synthesized polymers were determined using a PL-GPC 50 Plus
apparatus from Polymer Laboratories (Varian Inc.) equipped with two 300
mm PL-gel 5 µm, mixed D (200–300000 g mol−1) columns thermostated
at 35 °C and a refractive index detector. Calibration was performed with
PMMA standards. THF was used as the eluent at a flow rate of 1 mL·min−1
.
The GPC system was calibrated using narrow poly(methyl methacrylate)
(PMMA) standards ranging from 550 to 1,568,000 g mol-1 (EasiVial-
Agilent). The typical sample concentration was 10 mg/mL.
Synthesis of (1-carbomethoxyethyl)pentacarbonylmanganese(I), 1. In
a Schlenk tube were introduced 400 mg (10.23 mmol) of metallic
potassium and 250 mg (10.87 mmol) of metallic sodium under argon. They
were crushed together to generate the liquid “NaK” alloy. A solution of
dimanganese decacarbonyl (2.00 g, 5.13 mmol) in 25 mL of dry THF was
added and the resulting mixture was stirred for 3 h at room temperature.
The mixture was then filtered through Celite to yield a greenish-brown
solution, rinsing the Celite with 10 mL of dry THF. Then, methyl 2-
bromopropionate (1.71 g, 10.24 mmol) was added dropwise at room
temperature, generating a brown solution, which subsequently turned to
yellow. After stirring at room temperature for 3 h, the solvent was
evaporated under reduced pressure to yield on oily residue. The product
was purified by column chromatography through a silica gel column. To
facilitate the transfer of the oily product onto the chromatographic column,
a small amount of silica gel was added to the solution from the synthesis
before solvent removal. After evaporation, the silica gel containing the
complex was laid on top of the column chromatography as a solid deposit.
The chromatography was carried out using n-pentane as the initial mobile
phase. After elimination of a first yellow fraction corresponding to
[Mn2(CO)10], the mobile phase polarity was increased using a 9:1 mixture
of n-pentane and diethyl ether. A light-yellow fraction was collected,
followed by evaporation to dryness under reduced pressure to afford the
product as a yellow oil 1.13 g, 4.00 mmol, yield 39%). The spectroscopic
properties of the isolated product (1H NMR, FTIR in pentane) matched
those previously reported by Casey et al. 1H NMR (actone-d6, δ): 3.62 (3H,
Acknowledgments
This work was supported by the Agence Nationale de la
Recherche (ANR, French National Agency) through the project
FLUPOL (grant No. ANR-14-CE07-0012). We also gratefully
acknowledge additional financial support from the Centre National
de la Recherche Scientifique (CNRS). This work was granted
access to the HPC resources of IDRIS under the allocation 2016–
086343 made by GENCI (Grand Equipement National de Calcul
Intensif) and to the resources of the CICT (Centre
Interuniversitaire de Calcul de Toulouse, project CALMIP).
Keywords: bond dissociation enthalpy • pentacarbonyl-
manganese(I) alkyl • methyl acrylate • DFT calculations • radical
polymerization
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European Journal of Inorganic Chemistry
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Entry for the Table of Contents
FULL PAPER
The metal-carbon bond in
Weak Mn-carbon bond
[Mn(CHMeCOOMe)(CO)5] is
sufficiently weak to be homolytic
Roberto Morales-Cerrada, Christophe
Fliedel, Florence Gayet, Vincent
Ladmiral, Bruno Améduri,* and Rinaldo
Poli*
cleaved, releasing the acrylate radical
‡
under mild conditions (∆Ha
=
35.3±2.8 kcal mol-1, ∆Sa‡ = 27.2±8.1
cal mol-1 K-1) to initiate the radical
polymerization of methyl acrylate.
Page No. – Page No.
Homolytic bond strength and radical
generation from (1-carbomethoxy-
ethyl)pentacarbonylmanganese(I)
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