A new mechanism for metal-catalyzed stannane dehydrocoupling based on α-H-elimination in a hafnium hydrostannyl complex

Article abstract of DOI:10.1021/ja017495s

The hafnium hydrostannyl complex CpCp*Hf(SnHMes₂)Cl (Cp* = η⁵-C₅Me₅) has been synthesized from the reaction of CpCp*Hf(H)Cl with Mes₂SnH₂. This compound has been identified as an intermediate in the metal-catalyzed dehydrocoupling of Mes₂SnH₂ to the distannane Mes₂HSnSnHMes₂ (Mes = 2,4,6-trimethylphenyl). The dehydrocoupling in this system appears to occur by elimination of :SnMes₂ from CpCp*Hf(SnHMes₂)Cl, with Sn-Sn bond formation proceeding via insertion of the stannylene into a Sn-H bond. Copyright

Full text of DOI:10.1021/ja017495s

Published on Web 03/20/2002
A New Mechanism for Metal-Catalyzed Stannane Dehydrocoupling Based on
r-H-Elimination in a Hafnium Hydrostannyl Complex
Nathan R. Neale and T. Don Tilley*
Department of Chemistry, UniVersity of California, Berkeley, Berkeley, California 94720-1460
Received November 8, 2001
Scheme 1
The transition metal-catalyzed dehydropolymerization of second-
ary stannanes by zirconocene catalysts provided the first examples
of high-molecular weight polystannanes,1 which exhibit interesting
optical and electronic properties.2 Further applications of dehydro-
coupling methodologies in tin chemistry should be facilitated by
knowledge of the mechanism by which the metal center mediates
Sn-Sn bond formation. A possible mechanism for the d0 metal-
catalyzed dehydrocoupling of hydrostannanes involves σ-bond
metathesis steps operating via concerted, four-center transition
states, as described for the analogous dehydrocoupling of hydrosi-
lanes.3 Our efforts to probe this question have previously been
thwarted by unsuccessful attempts to isolate examples of the
presumed σ-bond metathesis intermediate, a d0 metal hydrostannyl
(M-SnHR2) derivative. Furthermore, the low Sn-H bond strength
suggests the possible operation of dehyrocoupling mechanisms
involving tin-centered radicals.4 Here we describe a metal-catalyzed
dehydrocoupling of Mes2SnH2 to Mes2HSnSnHMes2 (Mes ) 2,4,6-
trimethylphenyl), for which an intermediate d0 metal hydrostannyl
complex has been isolated and studied. The dehydrocoupling in
this system appears to occur by elimination of :SnMes2 from Cp-
Cp*Hf(SnHMes2)Cl (Cp* ) η5-C5Me5), with Sn-Sn bond forma-
tion proceeding via insertion of the stannylene into a Sn-H bond.
The reaction of CpCp*Hf(H)Cl3a (1) with Mes2SnH2 (2) resulted
in the elimination of hydrogen and the formation of the hydrostannyl
complex CpCp*Hf(SnHMes2)Cl (3) (benzene-d6, 10 °C, in the dark,
by 1H NMR spectroscopy). After 2 h the concentration of 3 reached
a maximum (64% of Hf species present), and after complete
conversion of the stannane 2 (12 h) the products remaining were 3
(59%) and the distannane Mes2HSnSnHMes2 (4, 31%). The
presence of a significant amount of 1 (26%) at this point suggested
that this complex might be a catalyst for the dehydrocoupling of 2
to 4, and this was confirmed by observation of this conversion (in
98% yield) over 24 h with a catalytic amount of 1 (4 mol %,
benzene-d6, dark, 25 °C). Further investigation (vida infra) sug-
gested that 3 is an intermediate in the catalysis, as indicated by the
mechanism of Scheme 1.
Scheme 2
values for the free stannane (1765, 1864 Hz), as is the νSnH
stretching frequency (1728 cm-1; 1864 cm-1 for 2).
The dehydrogenative condensation of 1 and 2 is reversible, as
indicated by the addition of H2 (1 atm) to a solution of isolated 3,
which resulted in the conversion of 3 to 1 (92%) and 4 (21%) over
2 days. The expected stannane 2 was observed only in trace amounts
during the reaction, since it is converted to 4 under these conditions.
Thus, complex 3 appears to possess an unusually reactive metal-
tin bond, as zirconium and hafnium stannyl derivatives that have
been studied previously do not undergo hydrogenolysis.8
To probe the Hf-Sn bond-forming process of Scheme 1, the
kinetics of the reaction between 1 and 2 were studied at 9.3 °C
(benzene-d6) under pseudo-first-order conditions of excess stannane.
Despite the reversibility of this reaction, well-behaved kinetics were
observed for ca. 75% conversion of 1 in the presence of excess 2.
The dependence of the rates on the concentration of 2 established
a second-order rate law, rate ) k[1][2], with k(9.3 °C) ) 6.4 (2) ×
10-3 M-1 s-1. The activation parameters determined by an Eyring
analysis (over the temperature range 0.7-33.2 °C), ∆Hq ) 7.4 (3)
kcal mol-1 and ∆Sq ) -42 (1) eu, are consistent with a concerted
σ-bond metathesis process in which severe restriction of vibrational
or rotational degrees of freedom accompanies formation of the
transition state.
The hafnium hydrostannyl complex 3 was isolated as thermo-
chromic crystals (yellow j10 °C; orange J10 °C) in 64% yield
from the stoichiometric reaction between 1 and 2 (1.5 h, 10 °C,
dark, benzene). The molecular structure of 3 (Scheme 2) is
characterized by a Hf-Sn bond length of 3.0073(6) Å. For
comparison, the other d0 group 4 M-Sn bond lengths to be reported
are 2.843(1) Å for Cp2Ti(SnPh3)Cl,5 and 3.0231(2) (M ) Zr) and
2.9956(3) Å (M ) Hf) for {MeSi[SiMe2N(4-CH3C6H4)]3}SnMCp2-
Cl.6 There is no apparent interaction between the Hf center and
the Sn-H bond of 3 (the Sn-H hydrogen was located in the
A further probe of the mechanism involved isotopic labeling
studies with deuterium. However, the kinetic isotope effect mea-
sured for the reaction of the deuterated stannane 2-d2 with 1, kHH
1
difference Fourier map). As expected,7 the J117/119SnH coupling
/
constants (918, 961 Hz) are greatly reduced from the corresponding
kHD ) 1.2 (1), is much lower than what might be expected for a
concerted transition state A (Scheme 2).9 Deuteration at hafnium
(1-d1) results in a small, inverse isotope effect, kHH/kDH ) 0.89 (3),
* To whom correspondence may be sent. E-mail : tdtilley@socrates.berkeley.edu.
9
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C O M M U N I C A T I O N S
and deuteration at both Hf and Sn results in a cancellation of the
previous effects: kHH/kDD ) 1.0 (1). These results may be explained
by a mechanism involving rate-determining coordination of the Sn-
H bond of the stannane to the Hf center of 1 to form a σ-complex
(Scheme 2). Cleavage of the Sn-H bond would then occur in a
subsequent, relatively rapid step involving the 4-center transition
state A. In our view, the inverse isotope effect is best explained as
a secondary effect resulting from loss of the normal bending modes
for the Hf-H(D) bond in the sterically crowded Sn-H σ-complex.10
The kinetic behavior described above has not been previously
observed for σ-bond metathesis processes, but it appears to reflect
the highly crowded nature of the reactants 1 and 2.
course of 11 days (benzene-d6, by 1H NMR spectroscopy),
suggesting that this distannane may also exist in equilibrium with
2 and :SnMes2. This elimination and reinsertion of stannylene has
previously been observed for distannanes.13 Thus, the observed 4-d1
was most likely formed from scrambling via the equilibria of
Scheme 2. These data lead us to believe that Sn-Sn bond formation
results from stannylene insertion into the Sn-H bond of the
stannane (Scheme 2). Similar stannylene insertions have been
proposed for related Sn-Sn bond-formations,14 including the
stepwise synthesis of linear chains up to 15 Sn atoms in length.14f
In summary, isolation of a reactive d0 metal hydrostannyl
complex has allowed mechanistic studies on a metal-catalyzed Sn-
Sn bond formation which features an R-H-elimination process. This
elimination produces the free stannylene :SnMes2, which appears
to insert into a Sn-H bond to produce the distannane. Interestingly,
the experimental data may be interpreted as evidence for coordina-
tion of a σ bond prior to a σ-bond metathesis step. Future efforts
will focus on establishing whether this mechanism may be operative
in metal-catalyzed dehydropolymerizations of secondary stannanes
to polystannanes. In addition, we have begun to explore R-elimina-
tion as a general decomposition mode for compounds containing
d0 metal-main group element bonds.
Given the sterically encumbered natures of 2 and 3, it seemed
unlikely that they could react in a concerted manner to produce
the distannane. Attempts to study the kinetics of this process were
complicated by the relatively rapid decomposition of 3 in solution.
Interestingly, the thermal decomposition of 3 (toluene-d8, in the
dark) cleanly produced the hydride 1 (20% after 10 min), and over
the course of 3 days a temperature-dependent equilibrium was
established. At room temperature, the 3/1 ratio was 1:10, while at
-30 °C complex 3 was favored by a ratio of 2:1. Thus, it appears
that 3 decomposes via the elimination of dimesitylstannylene;
1
however, H NMR spectra of the reaction mixture suggested the
Acknowledgment. We are grateful to National Science Founda-
tion for their generous support of this work, and to R. G. Bergman
and D. A. Singleton for valuable discussions.
Supporting Information Available: Experimental details of the
synthesis of compounds 2, 3, 4, and 5, kinetic measurements, tables
and plots of kinetic data, and tables of crystal, data collection, and
refinement parameters, bond distances and angles, and anisotropic
displacement parameters for 3 (PDF and CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
presence of a number of mesityl-containing products. To facilitate
their identification, the concentration of tin species in solution was
increased by adding a significant amount of 2 to 3, and analysis of
this reaction mixture (13 equiv 2, benzene-d6, dark, 25 °C) by 119Sn
NMR spectroscopy revealed the presence of three tin species (δ
-348.7, -316.8, -225.7). By comparison to known values for other
(R2Sn)n compounds,1b,11 we attribute these shifts to the cyclics
(Mes2Sn)n, with n ) 3, 4, and 5. In addition, a FAB-MS analysis
of this solution gave ions corresponding to the n ) 3 and 4 cyclics;
fragmentation of (Mes2Sn)5 likely prevented its detection by mass
spectrometry. It therefore appears that complex 3 exists in equi-
librium with 1 and a mixture of cyclic species derived from the
oligomerization of :SnMes2 (eq 1).
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Further evidence in support of the R-H-elimination of stannylene
from 3 was obtained by a trapping experiment in which 3 was
allowed to decompose in the presence of the trapping agent 2,3-
dimethylbutadiene (23 equiv), which resulted in a 92% yield (by
1H NMR spectroscopy) of the stannylene-trapped product 1,1-
dimesityl-3,4-dimethylstannacyclopent-3-ene (5). An attempt to
measure the rate of this R-H-elimination using the highly efficient
stannylene trap 1,2-bis(methylene)cylcopentane12 indicated that the
first-order rate constant was 2 × 10-3 s-1 (200 equiv, -25 °C,
toluene-d8). However, doubling the amount of the diene from 200
to 400 equiv led to an increase of the apparent rate constant by a
factor of 1.7. The lack of an efficient trap for :SnMes2 has prevented
us from directly probing the kinetics of R-elimination, although it
seems clear that this process is quite rapid.
Insight into the mechanism for Sn-Sn bond formation was
derived from the reaction of 3 with the dideuteriostannane Mes2-
SnD2 (2-d2, 1.0 equiv) in benzene-d6. At room temperature, 2-d2
was completely consumed in less than 100 min and was converted
to the dideuteriodistannane Mes2DSnSnDMes2 (4-d2) and the
monodeuteriodistannane Mes2HSnSnDMes2 (4-d1) in 67 and 6%
yields, respectively (the remaining tin species were 3 (8%) and
(Mes2Sn)n (19%)). However, when 4 was mixed with 2-d2 (1.2
equiv), 4-d1 (21%), 2-d1 (39%), and 2 (13%) were formed over the
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