C. Adams et al. / Journal of Organometallic Chemistry 749 (2014) 266e274
273
Fig. 8. Diagram of the equilibrium between the two possible bonding modes for each metallic fragment.
for proton and silicon nuclei can be evidenced, indicative that these
rhodium catalysts boost their selectivity values when working with
silanes with a dipole moment with a negative charge centered on
entry of the each analyzed substrates, whether the silane in an
oxidative addition plus the coordination of styrene.
The remaining complexes, the anti isomers, all have identical
behavior, if taken into consideration the fact that complex 4 has
two rhodium centers that may equally react within the catalytic
cycle, thus shortening the reaction time. A similar catalytic test
with half the concentration of this catalyst (considering equivalents
of rhodium instead), required 65 min for reaction time. This result
may allow us to consider the behavior of all s-indacendiide catalysts
similar, if not identical.
the hydrogen atom and a positive charge on silicon, as in SidþeHdꢁ
.
The three last entries in Table 5 present a dipolar moment of the
siliconehydrogen bond closer to: SidꢁeHdþ, possibly hardening the
oxidative addition process on the Rh center Nevertheless, Table 5
also lists some cone angles for some trialkylsilanes (correspond-
ing to each respective phosphine analog [38], revealing both fac-
tors, steric and electronic are responsible for the outcome of
product selectivity, though steric influence seems to be the most
significant factor when determining product selectivity.
Therefore, these catalysts seem capable of presenting very high
selectivity values with a fairly limited number of silanes, more
specifically, trialkylsilanes. Aryl and Alcoxy groups seem unsuitable,
at least for these Rh catalysts.
If the 103Rh-NMR chemical shifts for mono- and bimetallic
rhodium complexes (Table 7) are taken into consideration, these
have similar bonding modes between all complexes to the five
membered ring. This can imply that rhodium is not severely
affected when changing the secondary metallic group to another,
which can explain the catalytic results in Table 6, and their
similarities.
Because of this, it is feasible to assume the existence of a
cooperative effect in these heterobinuclear complexes in the form
of a metallic fragment forcing rhodium to have a greater ring
slippage, thus bonding closer to an allylic towards s-indacene,
similar to the effect seen in literature [21,20,43] and also shown in
3.5. Effect of the secondary metallic center
Several rhodium-based complexes, which differ only by the
main ligand, have been tested [16,39e42]. Comparison between
catalytic results of runs with mono- and binuclear complexes show
that in the latter case, the second metallic center created a
remarkable effect, the most pronounced consequence being on the
activity of the catalyst. Indeed, concentration of Rh was kept con-
stant for all catalytic tests and, this allowed direct assessment of
relative activity. Mononuclear complexes 3 presented a reaction
time of 600 min (see Table 6), while complete silane consumption is
observed within 35 and 180 min, for 4 and 40, respectively. The
strong electronic interaction between both metals channeled
through the bridging ligand can be invoked to account for the
higher activities of dinuclear complexes. The differences of effi-
ciency between 4 and 40, can be rationalized in terms of steric
hindrance due to the proximity of the two Rh(COD) entities in the
syn isomer.
Fig. 8. This reaction is surely favoring a (M)h h
5:(Rh) 3 configuration.
4. Conclusion
These studied catalytic systems shown a high selectivity to
dehydrogenative silylation (DS) of olefins, particularly with styrene
and tri(isopropyl)silane, at 80 ꢀC. Only olefins with electron-
acceptor groups seem suitable for these organometallic rhodium
complexes, together with only trialkylsilanes. These results prove
neutral rhodium complexes are equally effective (if not more) than
other cationic species, unlike what has been previously reported.
Also, each previously listed catalyst have turnover values over 2000
in each case, data which have not been reported for simple com-
plexes, also indicative of the presence of a very active catalytic
species.
Binuclear complexes showed the presence of a cooperative ef-
fect between both metallic centers, which is clearly evidenced
when compared with complex 3, possibly explained to the different
bonding mode Rh has towards indacene once a secondary metallic
fragment is bonded to the spacer ligand, regardless of the different
behavior these present in ESR and cyclic voltammetry tools.
The selectivity was also found to depend on the nature (mono-
vs. bimetallic) of the catalyst. Indeed, homobinuclear compounds
lead to similar product distribution, i.e., 85e88% and 12e15% of
dehydrogenative silylation and hydrosilylation products, respec-
tively. In contrast, the selectivity of the reaction in the presence of
mononuclear complex 3 drops to 75%. However, these differences
remain weak and hazardous to rationalize. Thus, the difference of
activity provides direct evidence for the involvement of cooperative
effect between both metal centers during the catalysis. Dehydro-
genative silylation and hydrosilylation products result from two
competitive catalytic cycles and electronic communication may
have stabilized intermediaries, which may have not taken place
with mononuclear complexes. Detailed mechanism remain to be
clarified and in particular, the precise role of the electronic
communication during the reaction. Further exploration in this
direction is currently in progress.
Acknowledgments
The authors thank projects FONDECYT Nos. 11110273, 1060588,
1060589; 1100283, 1110758, 11100027, and project ECOS-CONICYT
C01E06 and C08E01, ECO-NET Program No.18824SD.
References
Both syn isomers (complexes 40 and 20) have higher reaction
times compared with the anti-isomers (complexes 4, 1, and 2). This
difference can be explained due to steric hinderance, concealing
rhodium with the secondary metallic fragment, hardening the
Adams
