fastest substrates to be activated. After this, the kinetic selectivity largely follows
steric accessibility to the C–H bond. The range for thermodynamic preference spans
a much larger range, 220 million:1 or 11.5 kcal/mol. It is also noteworthy that the
most preferred product is the one in which the strongest C–H bond has been broken,
the phenyl hydride. This thermodynamic preference for breaking the strongest C–H
bond can only be accounted for by the formation of an even more favorable
rhodium–phenyl bond. It is the strength of the metal–carbon bond that is formed
that drives these equilibria, not the strength of the C–H bond that must be broken.
These are product driven equilibria, so the focus on the C–H bond strength to
predict favorability is not warranted.
While all of the substrates discussed above are not shown in Fig. 2, the same
analysis can be performed with all of them (alkynes, substituted methanes). One
caveat that we encountered was that many of these substituted derivatives proved to
be very stable. Loss of alkane from the n-pentyl hydride complex has a half-life of
about an hour at 25
C. Methane loss from 3 has a half-life of about 5 h. Loss of
benzene from 2, however, is extremely slow (months), and therefore, the rate of
benzene reductive elimination at 25
C was determined by extrapolation from the
rate at higher temperatures. The Eyring plot of ln(k/T) vs. 1/T gave activation
parameters for reductive elimination of benzene ΔH
{
¼ 37.8 (1.1) kcal/mol and
ΔS
{
¼ 23 (3) e.u., which can be used to calculate the rate at other temperatures. As
mentioned above, the substituted derivatives are much more stable. Reductive
elimination of the alkynyl hydrides was examined at 100
C, as was the elimination
of many of the substituted methyl derivatives. In these cases, the rate of benzene
elimination was calculated from the Eyring parameters at the same temperature as
that where the rate of reductive elimination was measured, so that the barriers could
be directly compared as in Fig. 2. The determination of ΔG
0 for all substrates
allows Eq. 7 to be used to determine relative metal–carbon bond strengths for these
compounds. Table 1 summarizes these data, giving ΔΔG
{ , ΔG
0 , and D rel (Rh–C) for
all substrates.
With D rel (Rh–C) now available for all substrates, the data can be compared
visually by plotting D rel (Rh–C) vs. the C–H bond strength of the substrate. Figure 3
shows the resulting plot. The data fall into two classes of substrates. The parent
hydrocarbon data are shown in blue, with the M–C sp bonds being strongest and then
the M–C sp2 , followed by the M–C sp3 . The line has a slope of 1.4, indicating that the
range of metal–carbon bond strengths is about 40% greater than the range of
carbon–hydrogen bond strengths. The data for the substituted methanes is shown
in red. It is parallel with a slope of 1.4 also but is offset vertically by about 7 kcal/
mol. This offset reflects the fact that the metal–carbon bonds are about 7 kcal/mol
stronger than what you would expect based upon the strength of the C–H bond that
is being broken. Also, while chloro and fluoro substituents are seen to strengthen the
metal–methyl bond, all of the other substituents actually weaken the metal–methyl
bond. This is actually to be expected, as bond strengths are based on homolysis, and
these radicals are all stabilized by resonance. The unexpected 7 kcal/mol “increase”
in bond strength is believed to be attributable to a greater ionic contribution to the
metal–carbon bond with these substituents on the α-carbon.
The Effects of Ancillary Ligands on Metal–Carbon Bond Strengths as. . .
75
steric accessibility to the C–H bond. The range for thermodynamic preference spans
a much larger range, 220 million:1 or 11.5 kcal/mol. It is also noteworthy that the
most preferred product is the one in which the strongest C–H bond has been broken,
the phenyl hydride. This thermodynamic preference for breaking the strongest C–H
bond can only be accounted for by the formation of an even more favorable
rhodium–phenyl bond. It is the strength of the metal–carbon bond that is formed
that drives these equilibria, not the strength of the C–H bond that must be broken.
These are product driven equilibria, so the focus on the C–H bond strength to
predict favorability is not warranted.
While all of the substrates discussed above are not shown in Fig. 2, the same
analysis can be performed with all of them (alkynes, substituted methanes). One
caveat that we encountered was that many of these substituted derivatives proved to
be very stable. Loss of alkane from the n-pentyl hydride complex has a half-life of
about an hour at 25
C. Methane loss from 3 has a half-life of about 5 h. Loss of
benzene from 2, however, is extremely slow (months), and therefore, the rate of
benzene reductive elimination at 25
C was determined by extrapolation from the
rate at higher temperatures. The Eyring plot of ln(k/T) vs. 1/T gave activation
parameters for reductive elimination of benzene ΔH
{
¼ 37.8 (1.1) kcal/mol and
ΔS
{
¼ 23 (3) e.u., which can be used to calculate the rate at other temperatures. As
mentioned above, the substituted derivatives are much more stable. Reductive
elimination of the alkynyl hydrides was examined at 100
C, as was the elimination
of many of the substituted methyl derivatives. In these cases, the rate of benzene
elimination was calculated from the Eyring parameters at the same temperature as
that where the rate of reductive elimination was measured, so that the barriers could
be directly compared as in Fig. 2. The determination of ΔG
0 for all substrates
allows Eq. 7 to be used to determine relative metal–carbon bond strengths for these
compounds. Table 1 summarizes these data, giving ΔΔG
{ , ΔG
0 , and D rel (Rh–C) for
all substrates.
With D rel (Rh–C) now available for all substrates, the data can be compared
visually by plotting D rel (Rh–C) vs. the C–H bond strength of the substrate. Figure 3
shows the resulting plot. The data fall into two classes of substrates. The parent
hydrocarbon data are shown in blue, with the M–C sp bonds being strongest and then
the M–C sp2 , followed by the M–C sp3 . The line has a slope of 1.4, indicating that the
range of metal–carbon bond strengths is about 40% greater than the range of
carbon–hydrogen bond strengths. The data for the substituted methanes is shown
in red. It is parallel with a slope of 1.4 also but is offset vertically by about 7 kcal/
mol. This offset reflects the fact that the metal–carbon bonds are about 7 kcal/mol
stronger than what you would expect based upon the strength of the C–H bond that
is being broken. Also, while chloro and fluoro substituents are seen to strengthen the
metal–methyl bond, all of the other substituents actually weaken the metal–methyl
bond. This is actually to be expected, as bond strengths are based on homolysis, and
these radicals are all stabilized by resonance. The unexpected 7 kcal/mol “increase”
in bond strength is believed to be attributable to a greater ionic contribution to the
metal–carbon bond with these substituents on the α-carbon.
The Effects of Ancillary Ligands on Metal–Carbon Bond Strengths as. . .
75