Task-Dependent Coordination Levels of SmI2

Article abstract of DOI:10.1021/acs.joc.8b02989

Ligation plays a multifaceted role in the chemistry of SmI₂. Depending on the ligand, two of its major effects are increasing the reduction potential of SmI₂, and in the case of a ligand, which is also a proton donor, it may also enhance the reaction by protonation of the radical anion generated in the preceding step. It turns out that the number of ligand molecules that are needed to maximize the reduction potential of SmI₂ is significantly smaller than the number of ligand molecules needed for a maximal enhancement of the protonation rate. In addition to the economical use of the ligand, this information can also be utilized as a diagnostic tool for the reaction mechanism in differentiating between single and multistep processes. The possible pitfalls in applying this diagnostic tool to PCET and cyclization reactions are discussed.

Full text of DOI:10.1021/acs.joc.8b02989

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Article
Task-Dependent Coordination Levels of SmI2
Sandeepan Maity, Amey Nimkar, and Shmaryahu Hoz
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02989 • Publication Date (Web): 22 Jan 2019
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Task-Dependent Coordination Levels of SmI2
Sandeepan Maity, Amey Nimkar and Shmaryahu Hoz*
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Department of Chemistry, Bar-Ilan University; Ramat Gan-5290002, Israel.
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Abstract: Ligation plays a multifaceted role in the chemistry of SmI2. Depending on the ligand,
two of its major effects are increasing the reduction potential of SmI2, and in the case of a ligand
which is also a proton donor, it may also enhance the reaction by protonation of the radical anion
generated in the preceding step. It turns out that the number of ligand molecules which are
needed to maximize the reduction potential of SmI2 is significantly smaller than the number of
ligand molecules needed for maximal enhancement of the protonation rate. In addition to the
economical use of the ligand, this information can also be utilized as a diagnostic tool for the
reaction mechanism in differentiating between single and multi-step processes. The possible
pitfalls in applying this diagnostic tool to PCET and cyclization reactions are discussed.
*E-mail: shoz@biu.ac.il
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Introduction
The educated use of additives has made SmI2 a unique and versatile reducing agent.1,2 The role of
the additive in SmI2-mediated chemistry differs according to its nature. For example, HMPA, one
of most common additives in SmI2 chemistry, is well known for its high affinity to SmI2, which
leads to formation of a thermodynamically more powerful reductant.3 On the other hand,
coordinating proton donors, such as methanol, water, ethylene glycol (EG) etc. enhance the
reactivity of SmI2 by unimolecular protonation of the resultant radical anion within the ion-pair.4
This process is much more efficient than the bimolecular protonation by proton donors from the
bulk and successfully competes with the back electron transfer from the radical anion to Sm3+ and
hence ‘locks’ the reaction.5 Some strongly coordinating proton donors such as glycols combine
the two features; they raise the reduction potential of SmI2 and also function as efficient proton
donors.1g,5a,6
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In the present paper, we employ two additives – EG and N-methylethanolamine (NMEA), which
are known to provide both thermodynamic (increase in reduction potential) and kinetic
(protonation) driving forces to the reactions.1g,5a,6a By following reduction potentials and reaction
rates as a function of additive concentration, better insight was obtained into the chemistry of SmI2.
Results and discussion
The effect of an EG ligand on reactions of SmI2 was studied by several groups.7 It coordinates
very well to SmI2 in THF, as can be seen from its effect on the spectrum of SmI2 (Figure 1).
0 M
0.01 M
0.04 M
0.1 M
0.02 M
0.05 M
0.2 M
0.03 M
0.08 M
0.3 M
0.3
0.2
0.1
0
480
580
λ nm
680
Figure 1: The effect of EG on the spectrum of SmI2 in THF.
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In addition, it increases the reduction potential of SmI2 (Table 1)6a and once bound to the SmI2 it
serves as an efficient protonating agent of the radical anion within the ion pair consisting of the
radical anion and Sm3+.8 In Figure 2, the rate constants for the reduction of anthracene are given
as function of EG concentration, along with the corresponding reduction potentials of SmI2.
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Table 1: Cyclic voltammetric data of SmI2-EG complexa
[EG]/
M
E1 (V) E2 (V) E1/2 (V)
0
-1.13
-1.35
-1.43
-1.5
-1.48
-1.85
-1.89
-1.89
-1.9
-1.3
0.01
0.02
0.03
0.04
0.05
0.1
-1.6
-1.66
-1.69
-1.71
-1.73
-1.77
-1.79
-1.81
-1.82
-1.83
-1.52
-1.52
-1.59
-1.6
-1.94
-1.95
-1.99
-2
0.2
0.3
-1.63
-1.64
-1.65
0.4
-2.01
0.5
-2.01
aE1 and E2 are the oxidative (Sm2+ to Sm3+) and reductive peak (Sm3+ to Sm2+) potential respectively. E1/2 = (E1+E2)/2.
E1 values are used for constructing plots with kinetic data.
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0.025
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0.015
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0.005
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0
0.2
0.4
[EG] M
0.6
Figure 2: Reduction potentials (blue) and rate constants (orange) of SmI2 for anthracene as
function of [EG].
It is very noticeable for anthracene that the increase in reaction rate constants with EG
concentration lags much behind the increase in reduction potential of SmI2. While the rate
constants level off at relatively high EG concentrations (>0.5 M), the reduction potential levels off
at much lower concentrations (around 0.1 M). The leveling off of the rate constant could originate
from a change in the rate determining step, from protonation of the radical anion at low EG
concentrations to electron transfer at high EG concentrations. This possibility is ruled out by
kinetic H/D isotope effects at 0.05, 0.2 and 0.5 M EG, which were found to be 2.0, 1.9 and 2.0,
respectively.7a Hence, protonation is rate determining also at the plateau region. Therefore, a
plateau in the rate constants is expected upon completion of the coordination shell of EG molecules
around the samarium ion. We have already shown that there is a huge preference for unimolecular
protonation of the radical anion internally within the ion pair by proton donors ligated to the SmI2
over bimolecular protonation by proton donors from the bulk.5 Thus, once the coordination sphere
is completed, the system attains the maximum number of efficient proton donors, and the rate
constant levels off. Additional proton donors will contribute in a bimolecular manner and hence
will only marginally affect the protonation rate.
This discrepancy between the concentrations at which the rates of reduction and reduction
potentials level off deserves some discussion. As the coordination sphere of the samarium ion is
only completed at high EG concentrations (> 0.5 M), it is clear that the coordination of SmI2 is
only partial when the reduction potential levels off (at [EG] ~ 0.1 M). Yet, this partial coordination
is sufficient to induce the maximum reduction potential. It is highly likely that this dichotomous
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behavior stems from the significantly different ionic radii of Sm2+ (1.22 Å) and Sm3+ (0.958 Å),9
which translates to a much larger surface area of Sm2+ (by more than 50%). The ligand-induced
enhancement of the reduction potential of SmI2 stems from the higher exothermicity of its binding
to Sm3+ than to the Sm2+.6a,10 This shifts the equilibrium toward Sm3+, providing a greater driving
force for electron transfer (equation 1).
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S+m3 I2
+ SmI2
Substrate
Substrate
(1)
Due to the smaller ionic radius, Sm3+ can accommodate fewer ligand molecules than Sm2+,
therefore the reduction potential levels off at a lower coordination level, corresponding to the
maximal effective coordination number of Sm3+. On the other hand, efficient protonation depends
on the availability of protons within the ion pair, which increases with the number of proton donors
coordinated to the SmI2.2i,5,7aThe reaction rate therefore levels off at significantly higher EG
concentrations where full coordination is attained. Even if some EG molecules are shed as the
Sm2+ transfers an electron and shrinks considerably to the size of Sm3+, these proton donors remain
in the reaction cage and are available for a unimolecular protonation of the radical anion.
Based on this understanding, a sterically hindered ligand, which coordinates reasonably well to the
larger Sm2+, is expected to coordinate poorly to Sm3+ at the concentration in which the maximal
reduction potential is attained. Therefore, its effect on the reduction potential will be diminished.
Indeed, we find that pinacol (tetramethylethylene glycol) coordinates well to SmI2 (Figure S4), but
hardly raises the reduction potential (from -1.13 to -1.18 V vs. Ag/AgNO3), compared to EG,
which increases the reduction potential of SmI2 to -1.65 V.
Interestingly, as can be seen in Figure 1, the changes in the spectra of SmI2 converge around an
EG concentration of 0.1 M, similar to the reduction potential.6a This may be explained by
considering the cloud of negative charge, which becomes more diffused in the excited state and
moves away from the nucleus, mimicking to some extent the smaller radius of Sm3+.11
In order to substantiate the assumption that the slow protonation of the radical anion is responsible
for the gap in Figure 2, the same experiment was repeated with benzyl chloride. As can be seen in
Figure 3, the rate constants for the reduction of benzyl chloride and the reduction potentials match
nicely, both leveling off at an EG concentration of about 0.1 M. This is expected, as benzyl chloride
reacts by a mechanism known as a dissociative electron attachment.12 That is, no intermediate
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radical anion is obtained, and the expulsion of the chloride ion takes place concurrently with the
electron transfer. Consequently, further addition of EG, beyond the concentration in which the
reduction potential, and the electron transfer, level off (~0.1 M) are expected to have no significant
effect on the rate of C-Cl bond cleavage.
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1.7
1.5
1.3
1.1
0.012
0.01
0.008
0.006
0.004
0.002
0
0
0.1 0.2 0.3 0.4 0.5 0.6
[EG] M
Figure 3: Reduction potentials (blue) and rate constants (orange) of SmI2 for benzyl chloride as
function of EG concentration.
The two types of behavior exhibited by anthracene and benzyl chloride suggest that plots similar
to those presented in Figures 2 and 3 may serve as a reliable diagnostic tool to identify the rate-
determining step in the reactions of SmI2.
In order to further support the validity of the concept, it is worthwhile to consider the kinetics of a
general substrate for which protonation is rate determining (Eq. 2). Assuming a steady state
concentration of radical anion, the kinetics of the reaction is described by Equation 3.
ket
kp
S+m3 I2
+ SmI2
-H
Substrate
(2)
(3)
Substrate
Substrate
k-et
H
RO
푘푒푡 ∙ 푘푝
푘표푏푠 =
푘 ―푒푡 + 푘푝
If kp is increased so that kp >> k-et, kobs becomes equal to ket, which is expected to respond to changes
in the concentration of EG similarly to the reduction potential, as observed for benzyl chloride
(Fig. 3). Such a change from anthracene to benzyl chloride behavior is expected to occur gradually,
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along with an increase in the protonation rate. However, since we cannot enhance the protonation
rate on anthracene, another substrate, cyclohexanone, was employed, which undergoes protonation
on the oxygen, a process known to be much faster than protonation on carbon.13 For comparison,
we use normalized plots (where the maximum value is set to be 1) as shown in Figure 4, which
demonstrates that the curve for cyclohexanone has indeed moved in the expected direction relative
to anthracene.
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1
0.8
0.6
0.4
0.2
0
0
0.1
0.2
0.3
0.4
0.5
[EG] M
Figure 4: Normalized graphs of SmI2 reduction potentials (blue), rate constants with anthracene
(orange), and rate constants with cyclohexanone (red) as function of EG concentration.
Figure 5 shows the same type of graph for the ligand NMEA, which increases the reduction
potential of SmI2 much more than EG (Table S1),6a reacting with anthracene and 3-methyl-2-
butanone. The figure clearly shows that the gap observed for anthracene shrinks significantly with
3-methyl-2-butanone as a substrate, in line with our suggestion. Needless to say, the rate constants
for benzyl chloride match very well with the reduction potentials (Figure S5). It is concluded from
Figures 4 and 5 that the faster the protonation, the smaller the gap between the two curves.
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0
0.02
0.04
0.06
0.08
[NMEA] M
Figure 5: Normalized graphs (maximum value taken as 1) of reduction potential (blue), rate
constants with anthracene (orange), and rate constants with 3-methyl-2-butanone (red) as function
of NMEA concentration.
Finally, at the suggestion of a referee to examine the effect of the anions associated with Sm2+
which is often over looked, we have studied the effect of EG on the Sm(OTf)214 reactions with
benzyl chloride and anthracene. First, we have compare the rate of reduction of these two
presentative substrates by SmI2 and Sm(OTf)2 in the presence of EG as additive. As evident clearly
from Figure S8 and S9 (numerical values of rate constants are given in Table S4), that effect of
EG on the reaction of Sm(OTf)2 is very similar to SmI2. In addition, rates of benzyl chloride
reduction by SmI2 and Sm(OTf)2 in the absence of EG are also very close, which suggests that
reduction potential of these two Sm(II) salts are comparable.
With this mechanistic understanding at hand, we have applied above discussed diagnostic tool in
Sm(OTf)2 mediated reactions (Figure 6). To construct these plots, we have used reduction potential
values of SmI2. Overall, these results show that the same behavior is obtained regardless of whether
the counter ion is iodide or triflate suggesting that, at least in this case, the anions associated with
the samarium cation are either bystander spectators or that the role they play in the reaction is not
much dependent on their identity. It could be of interest to broaden the scope of this paper to other
solvents such as DME. However, in doing so one must not transfer the data from THF to the other
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solvent since the solvent coordination free energy to the SmI2 and Sm3+ will change and therefore,
will affect all the related parameters including the reduction potential.
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1.7
1.1
0.02
0.006
0.002
0
[EG]/M
0
0.5
0
0.5
[EG]/ M
1
Figure 6: Reduction potentials (blue) and rate constants (orange) of Sm(OTf)2 for benzyl chloride
as function of EG concentration. Insert shows similar plot using anthracene as substrate.
Conclusions.
In conclusion, we have shown that different levels of ligand coordination to SmI2 are needed for
the fulfillment of different tasks. Partial coordination is sufficient for reaching the maximal
reduction potential of SmI2, concomitantly attaining the maximal rate of electron transfer. On the
other hand, when the coordinated ligands play a role in the rate determining step of the reaction,
for example when they protonate the radical anion, protonation is facilitated by completion of the
coordination sphere at significantly higher ligand concentrations. Plots such as those shown in
Figures 2 and 3 may serve as a very good diagnostic tool for assessing the role played by the
ligands in the reaction.
A few words of caution: in the case of benzyl chloride, the rate determining step is the electron
transfer, whereas in the reduction of anthracene, the electron transfer is a pre-equilibrium step,
which is followed by a rate determining proton transfer. It should be emphasized that it is possible
that a reaction may exhibit a benzyl chloride-like behavior, and yet the electron transfer can still
be reversible. This case will be encountered if the rate determining step is independent of the
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ligands surrounding the SmI2, for example if the rate determining step is cyclization. The kinetic
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equation then becomes similar to Eq. 2, with kcyc replacing kp (Eq. 4):
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푘푒푡 ∙ 푘푐푦푐
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(4)
푘표푏푠 =
푘 ―푒푡 + 푘푐푦푐
When kcyc << k-et, it will be reduced to Equation 5:
푘푒푡
(5)
푘표푏푠 =
푘푐푦푐 = 퐾푒푡 ∙ 푘푐푦푐
푘 ―푒푡
Since kcyc is independent of the abundance of ligands around the Sm3+, and as Ket is related to the
reduction potential in a similar manner as ket, this case will also exhibit a benzyl chloride-like
behavior, although electron transfer is not rate determining.
The second point to be considered is the recent suggestion that SmI2 reacts with anthracene in the
presence of water by a hydrogen atom transfer (HAT) mechanism (Eq. 6)2i,15 which is sometimes
referred to as proton-coupled electron transfer (PCET).
H H
SmI2 H2O
(6)
In this case, the rate determining step is also the first step, as the electron and proton transfer steps
are coupled into a single step. The behavior may be similar to that observed for anthracene, rather
than for benzyl chloride, as the HAT depends also on the availability of protons around the SmI2,
which in turn correlates with the number of ligand molecules that are attached to SmI2.
Experimental Section
General: All the reagents were purified prior to use by following standard procedures.16 Liquid
reagents such as substrates and additives were degassed with argon prior to use. THF was dried
and freshly distilled off sodium/benzophenone under argon atmosphere. SmI2 was freshly prepared
prior to use by stirring of samarium metal and 1,2-diiodoethane at room temperature.4d The
concentration of SmI2 was determined by UV-visible spectroscopic measurements ( 619 nm;  =
635).6a Kinetic reactions were carried out in clean and dry glassware under nitrogen atmosphere.
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Kinetics and UV-VIS measurements: Stopped flow kinetic and UV-visible spectral
measurements were carried out in a Hi-Tech Stopped Flow Spectrometer. In kinetic measurements,
all the reactions were performed under pseudo first order conditions (SmI2: 2 mM and Subs: 10
mM). Rate of reactions were monitored by following the disappearance of the SmI2-additive
complex absorbance. Reduction of anthracene, 3-methyl-2-butanone and cyclohexanone resulted
9,10-dihydroanthracene17, 3-methyl-2-butanol17 and cyclohexanol2h respectively. Reduction of
benzyl chloride provided mixture of toluene and 1,2-diphenylethane.17
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Spectral measurements were performed with a Stopped Flow Spectrometer. To record the
spectrum of SmI2 in the presence of additives, SmI2 and the corresponding additive were taken in
two different syringes and mixed within the stopped flow machine. The wavelength was scanned
over a range to obtain the spectrum. The concentration of SmI2 in all the experiments was 2 mM.
Cyclic Voltammetry: Cyclic voltammetry was performed in a single potentiostat from Bio Logic
Scientific Instruments. Glassy carbon, Ag/AgNO3 in acetonitrile and Pt wire were used as working,
reference and counter electrode, respectively. The glassy carbon electrode was polished with
polishing alumina and then washed thoroughly before each set of measurements. The reference
electrode had a potential of 0.542V with respect to SHE. Tetrabutylammonium
hexafluorophosphate (0.1 M) was used as a supporting electrolyte. The SmI2 concentration was 2
mM for all sets of experiments.
Supporting Information
Cyclic voltammetry data, rate constants of reductions, UV-vis spectra, additional CV vs kinetics
plots and kinetic traces used to determine rate constants are provided in Supporting Information.
The Supporting Information is available free of charge on the ACS Publications website at DOI:
Corresponding Author
*E-mail: shoz@biu.ac.il
Acknowledgements
This research was supported by the Israel Science Foundation, grant number 583/15. We are
grateful to Prof. Flowers for allowing Dr. Maity to carry out the triflate experiments in his
laboratory.
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TOC graphics
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