ACS Catalysis
Research Article
reoriented with an elongated Ca−C bond (2.58 Å) in order to
have the lone pair pointing toward the calcium atom, and the
interaction between the calcium and the alkyl group is ensured
by a strong α-agostic interaction. The Ca−H bond is not yet
formed (2.49 Å) and the activated Si−H one is elongated
(from 1.49 to 1.59 Å) as the hydrogen interacts with Ca (Ca−
H distance, 2.27 Å). Following the intrinsic reaction
coordinate, it yields the calcium hydride (whose dimerization
is computed to be favorable by 13.2 kcal/mol, but that is quite
unlikely to occur under catalytic conditions due to the low
concentration of the formed hydride and also because the
dimerization barrier is computed to be 15.5 kcal/mol) with the
release of the tertiary silane (Ph)(Me)Si(H)(CH2SiMe3) that
is observed experimentally. This step is exothermic by 4.7 kcal/
mol. The formed hydride can thus react with another molecule
of Ph(Me)SiH2 (see also Figure S169). The reaction begins by
the formation of a stable silane adduct (−6.2 kcal/mol). Rather
than the unproductive Si−H activation (H/H exchange), the
system undergoes a Si−C activation reaction. Such a reaction
was also reported to be kinetically accessible and favored over
Si−H activation with the silicon at the α position by Perrin et
al. in their study of hydrosilylation of alkene catalyzed by
Cp2*SmH.26 The associated transition state is again an σ-bond
metathesis one with the silicon at the β position. The geometry
is also a slightly distorted TBP around the silicon where the
hydride lies in the equatorial plane and the phenyl ring is in the
apical position. The Si−H bond almost formed (1.67 Å), while
the Ca···C distance remains long (3.04 Å). However, the ipso
carbon of the phenyl ring is bent in the direction of the calcium
indicating that although small, an interaction exists between
the two centers. The barrier is slightly lower than the first one
(17.4 kcal/mol) in line with a kinetically accessible step. The
system further evolves with the formation of a calcium phenyl
complex and the liberation of MeSiH3 that is observed
experimentally, and whose formation is exothermic by 5.4
kcal/mol from the entrance channel (almost athermic +0.8
kcal/mol from calcium hydride formation). Finally, the calcium
phenyl complex reacts with a molecule of silane Ph(Me)SiH2.
After weak coordination of silane (0.1 kcal/mol), the system
reaches a Si−H activation transition state (TS) with silicon at
the β position. This TS has similar features as those of the Si−
C activation one which was just described above. Indeed, the
hydrogen lies in the equatorial plane, whereas the phenyl is in
the apical position. The Si−H bond is elongated to 1.63 Å
allowing a Ca−H interaction at 2.26 Å. The Ca−phenyl
distance has also been strongly elongated to 2.93 Å (2.47 Å in
the calcium phenyl complex) but remains bent toward Ca. The
second phenyl ring lies in the equatorial plane, whereas the
methyl is in the apical position. The associated barrier is 19.1
kcal/mol, which is similar to the first barrier, indicating that the
hydride formation, which is the actual catalyst, is the rate-
determining step of the reaction. Following the intrinsic
reaction coordinate, it yields the silane distribution product
SiH(Me)(Ph)2 and regenerates the calcium hydride. The
overall redistribution reaction is exothermic by 9.1 kcal/mol.
The substituent effect has been investigated by computing the
redistribution reaction for (p-CF3-C6H4)(Me)SiH2 (Table 2,
entry 6). The computed reaction profile (Figure S170) is quite
similar to that reported for PhSi(Me)H2 (Figure 3), but the
energies are found to be lower. Following the Curtin−
Hammett principle, since the rate-determining step of the
reaction is the last step (redistribution TS), the difference in
the activity of the two substrates only depends on the TS
energies (13.6 for R′ = p-H vs 9.5 kcal/mol for R′ = p-CF3) so
that the activity for the latter is greater than that of the former.
CONCLUSIONS
■
In summary, organocalcium complexes supported on a β-
diketiminato-based tetradentate ligand (L1 or L2) are
synthesized and structurally characterized, and their applica-
tion as catalysts for the redistribution of hydrosilanes is
explored. The supporting ligands L1 and L2 can both stabilize
the calcium alkyl complex, while for calcium hydride, only the
one with L2 which contains a bulky tert-butyl group is
obtained. The calcium alkyl complex supported by L2
efficiently catalyzes the redistribution of ArSiH3 or Ar(alkyl)-
SiH2 to Ar3SiH and SiH4 or Ar2(alkyl)SiH and alkylSiH3,
respectively, and its catalytic performance is much better than
those of the calcium alkyl complex supported by L1 and a
reported divalent ytterbium alkyl complex. This calcium alkyl
complex also catalyzes the cross-coupling between the
electron-withdrawing, substituted Ar(R)SiH2 and the elec-
tron-donating, substituted Ar′(R)SiH2 to provide ArAr′(alkyl)-
SiH in good yields, and the synthesized ArAr′(alkyl)SiH can be
easily transferred to other organosilicon compounds based on
its Si−H bond transformation. Therefore, this protocol
provides new opportunities to synthesize various arylsilanes.
Both experimental and theoretical studies indicate that calcium
hydride is the real catalytic species in the catalytic cycle. The
reaction sequence is demonstrated to involve three steps
(catalyst formation, Ca−Ph formation, and catalyst regener-
ation) which exhibit similar low activation barriers (17−19
kcal/mol) in line with a facile reaction.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
■
sı
Experimental and computational details and X-ray
crystallography structure data (PDF)
Crystallographic information on complex 3 (CCDC
Crystallographic information on complex 4 (CCDC
Crystallographic information on complex 5 (CCDC
Crystallographic information on complex 6 (CCDC
AUTHOR INFORMATION
Corresponding Authors
■
Laurent Maron − LPCNO, CNRS & INSA, Université Paul
Yaofeng Chen − State Key Laboratory of Organometallic
Chemistry, Shanghai Institute of Organic Chemistry,
University of Chinese Academy of Sciences, Chinese Academy
of Sciences, Shanghai 200032, People’s Republic of China;
Authors
Tao Li − State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, University of
6354
ACS Catal. 2021, 11, 6348−6356