4
0
540 Organometallics, Vol. 20, No. 22, 2001
Morkin and Leigh
-60 °C range and are shown in the form of Arrhenius
Sch em e 1
plots in Figure 1. For comparison, the temperature
dependence of the diffusional rate constant (calculated
using the modified Debye equation (kdiff ) 8RT/3000η)
3
9
and published temperature-viscosity data
(Ea )
-
1
-1
2
.4 ( 0.5; log(A/M
s
) ) 12.1 ( 0.4) is also plotted in
Figure 1. Table 1 lists the Arrhenius parameters
obtained from linear least-squares analysis of the two
sets of data, along with “best fit” values of kdim at
The near-zero activation energies for dimerization of
both 16a and 16b are similar to those reported previ-
26,29
ously for 2a in the gas phase
and, given the high
2
5 °C, calculated from the Arrhenius parameters. We
degree of exothermicity associated with the process, are
note that the errors in the Arrhenius parameters are
reported as (2σ, which represents confidence intervals
close to 95% in both cases.
46-48
9
consistent with either concerted
or stepwise mech-
anisms for the reaction. The preexponential factors are
q
associated with entropies of activation of ∆S ) -18 (
The similar absolute reactivities and Arrhenius pa-
rameters for dimerization of 16a and 16b are quite
2 and -13 ( 2 cal/mol‚K for 16a and 16b, respectively,
at 300 K.
interesting, considering the substantial differences in
While we cannot yet conclusively decide between
concerted and stepwise dimerization mechanisms in
either case, it is possible to define for the stepwise
mechanism what the kinetic behavior of the putative
reaction intermediates must be in order to be consistent
with the experimentally observed activation energies.
Scheme 1 defines the form of a stepwise mechanism
involving a single intermediate, analogous to the mech-
anisms that have been previously established for the
their reactivity toward nucleophiles.1
3,40
For example,
1
6a reacts at close to the diffusion-controlled rate with
methanol in solution, via a stepwise mechanism that is
overall first-order in alcohol. In contrast, germene 16b
reacts by a mechanism that is strictly second-order in
methanol, with the first-order pathway being too slow
to be detected. The similarities in the rates of dimer-
ization and methanol addition to 16a are not surprising,
considering what is known of the thermochemistries of
the two reactions in simpler derivatives. For example,
the heats of dimerization and hydration of 1,1-dimeth-
ylsilene (2a ), calculated from experimental heats of
4
9
addition of alcohols and other nucleophiles to transient
5
0
silenes in solution. Mechanisms of this type, when the
reaction is strongly exothermic, will exhibit a bell-
shaped Arrhenius behavior provided that a large enough
temperature range can be spanned experimentally.9
This behavior is caused by the fact that the rates of the
two reaction pathways available to the intermediates
reversion to reactants (k-I) and collapse to products
(kP)svary in different ways with temperature. The
activation energy is negative over the (high) tempera-
ture range where k-I > kP, positive over the (low)
temperature range where k-I < kP, and zero at the
temperature where k-I ≈ kP. If the dimerizations of both
16a and 16b are nonconcerted, then the near-zero
activation energies that are observed indicate that k-I
and kP must be of similar magnitudes to one another
over the 0-60 °C temperature range; however, the
intermediate from 16a must revert to reactants margin-
ally faster than collapse to products (since its Ea is
slightly negative), while the opposite must be true for
the intermediate from 16b.
4
1
formation for 2a (∆Hf° ) 9 kcal/mol ), 3a (∆Hf° )
4
2
-
51.6 kcal/mol ), trimethylsilanol (∆Hf° ) -119 kcal/
4
3
39
mol ), and water (∆Hf° ) -56 kcal/mol ), are ∆Hdim )
69.6 and ∆Hhyd ) -70.0 kcal/mol, respectively. These
-
compare reasonably well with high-level theoretical
estimates of the corresponding parameters for the
parent silene, H2SidCH2: ∆Hdim ) -77 to -83 kcal/
mol1
4,18-21
and ∆Hhyd ) -77 kcal/mol. A self-consistent
44
set of data for germene dimerization and hydration
are unfortunately not available. However, values of
2
2
45
∆
Hdim ) -75.3 kcal/mol and ∆Hhyd ) -61.7 kcal/mol
have been reported for head-to-tail dimerization and
hydration of the parent molecule (H2GedCH2) on the
basis of RHF/6-311G(d,p) and MP3/DZ ab initio calcula-
tions, respectively. These data indicate that the dimer-
ization of silenes and germenes of analogous structure
have comparable overall exothermicities, consistent
with the similarities in the absolute rate constants for
dimerization of 16a and 16b. Nucleophilic addition, on
the other hand, maintains a strong exothermicity in
silenes, but is significantly less exothermic in the case
of germenes. This too is reflected in the relative reac-
tivities of 16a and 16b toward methanol addition.
In our view, the most reasonable stepwise reaction
mechanism for dimerization of the 1,1-diphenyl-substi-
tuted metallaenes 16a and 16b is one that involves the
intermediacy of the 1,3-dimetalla-1,4-butanediyl biradi-
cals 18, and so further examination of the viability of
this mechanism requires that we have independent
information on how such biradicals behave. In principle,
this information should be available from studies of the
photochemistry of the corresponding 1,3-dimetallacy-
clobutanes, since the photolysis of these compounds is
(
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