The Journal of Organic Chemistry
Note
equilibrium constant of 0.6 M−1. As most of the alcohols
studied here did not affect the VIS absorption of SmI2, their
equilibrium constants were determined only by NMR.
Another prominent difference between the two methods is
that VIS follows changes in the spectrum of SmI2, whereas
NMR monitors the chemical shift of the substrate. In order to
shift the equilibrium in eq 1 to the right, the concentration of
either SmI2 and/or the ligand (L) should be increased.
Addition of alcohols to SmI2 in THF, with the exclusion of
MeOH, did not affect much the visible electronic spectrum of
SmI2. Even with MeOH, a significant change in the spectrum
commences only around 1 M. At this high concentration, the
properties of the medium are probably significantly changed, as
both homo and hetero hydrogen bonding are formed.
Therefore, it is impossible to determine the equilibrium
constant based on the changes in the visible spectrum even for
MeOH. Thus, for alcohols, only the NMR method, where
much lower concentrations of the ligand are needed was
applied. Addition of SmI2 to a solution of an alcohol in THF
causes a significant change in the chemical shift, enabling to
determine the equilibrium constant. Although the constants
are small, the sheer fact that an alcohol molecule whose
binding site is oxygen can efficiently replace a THF molecule
whose binding atom is also oxygen is surprising, all the more so
in light of the 1000-fold higher concentration of THF (12.3 M
vs ∼10 mM for the alcohol). Apparently, this ability of the
alcohol to replace THF molecules stems from two origins.
First, an alcohol complexed to Sm2+ has an enhanced ability to
hydrogen bond to a THF molecule. Second, the lower steric
size of the alcohol molecules around the binding atoms plays
an important role in the complexation to SmI2. Thus, in THF,
two CH2 units flank the oxygen atom whereas in alcohol one
CH2 unit and one H unit are involved.
SmI2 + L F SmI2 − L
(1)
The NMR complexation shift depends on the molar ratio of
the complex to the ligand ([SmI2−L]/[L]), while VIS depends
on the complex concentration [SmI2−L].Therefore, increasing
the ligand concentration to shift the equilibrium to the right
and produce more of the complex would be counterproductive,
as the lower complex-to-ligand ratio would produce a smaller
complexation shift. This is clear from eq 2, which shows that
the ratio decreases proportionately with the decreasing
concentration of SmI2 that is converted to the complex.
Thus, in the NMR method where the chemical shift of the
ligand is followed, the concentration of SmI2 should be
increased in order to shift the equilibrium to the right while
increasing the complex-to-ligand ratio.
K[SmI2] = [SmI2−L]/[L]
(2)
On the other hand, increasing the concentration of L fits
very nicely into VIS where the absorption of SmI2 is followed,
as it induces the desired change in absorption. On the technical
side, it is clearly much easier to gather VIS than NMR data.
The number of data points collected in each measurement is
very high (number of nm over which the spectrum is
measured) as opposed to the much fewer data points (number
of observed chemical shifts) in NMR. Also, unlike NMR
measurements, VIS spectra are taken inside the glovebox,
accounting for the sensitivity of SmI2 to air. The determination
of two successive equilibria was enabled only with NMR.
Based on the lack of any effect on the VIS spectrum, we
assumed in the past that alcohols such as t-BuOH and
trifluoroethanol (TFE) do not bind to SmI2.10 While the NMR
spectrum of t-BuOH was indeed not affected by SmI2, the data
(Table 1) shows that TFE binds to SmI2 with an equilibrium
constant of 0.6 M−1. Nevertheless, the aforementioned
assumption stays nearly correct, as under typical conditions
(2 mM SmI2 and 0.1 M of TFE used in most of our kinetic
studies), only 6% of the SmI2 molecules are bound to a single
TFE molecule. It should be noted that the binding site of TFE
may be the CF3 group rather than the oxygen due to the higher
accumulated negative charge.
In THF solution, SmI2 is bound to two ligands, THF and
iodide ions. In ligand exchange, THF is the first to be replaced,
as evidenced from the crystal structure of SmI2(DME)2(THF)
where THF rather than iodide molecules are replaced by
DME.11a Increasing the concentration of the ligand or its
affinity to SmI2 will eventually replace the iodidesas
demonstrated by Flowers et al.11b,c
ALCOHOLS
■
Chart 1 shows the alcohols for which equilibrium constants
were determined. The values of K1 are given in Table 1.
Chart 1
An interesting phenomenon is revealed upon examination of
different alcohol chain lengths (Cn in Figure 2). As the length
increases, the equilibrium constant decreases, but the decrease
levels off very rapidly. The reason seems to be of an entropic
origin. The first shell of THF molecules bound to SmI2
hampers the free rotation of the tails of the alcohol molecules
embedded in the complex, resulting in a decrease in entropy.
For short alcohols this affects the whole molecule, while for
alcohols with longer tails, the part which protrudes outside of
this shell is not significantly affected and hence does not lead
to a substantial further reduction in the equilibrium constant.
Table 1. NMR Determined First Equilibrium Constants for
Alcohols with SmI2
alcohols
K1, M−1
TFE
0.63
1.03
1.4
1.8
4.4
n-octanol
n-BuOH
EtOH
MeOH8
t-BuOH
AMINES
■
Chart 2 shows the cyclic and acyclic amines for which
equilibrium constants with SmI2 were determined. The values
of K1 are given in Table 2.
∼0
Unlike alcohols, the addition of amines caused a significant
change in the spectrum of SmI2, as shown for n-BuNH2
10862
J. Org. Chem. 2021, 86, 10861−10865