which strongly suggests that the presence of both early and late transition metals is
indispensable for the reaction to occur. The stoichiometric reaction of complexes
43b and 43c with NaB(Ar
F ) 4 and propargylic alcohol gave the heterobimetallic
allenylidene complexes 45. These reactions proceed via the formation of chloridobridged complexes 44. The structures of both Ru/Zr complexes 44b and 45b have
been confirmed by X-ray diffraction study. In the mechanism of the catalytic
etherification of propargylic alcohols proposed by the authors, the reaction takes
place at the ruthenium center and involves 44 and 45 as key intermediates. The
formation of the chlorido-bridged complex 44 during the catalytic reaction would
promote the final dissociation of the propargylic ether.
3.2 Group 9/Group 4–5 Heterobimetallic Complexes
3.2.1 Co/Ti, Co/Zr, and Co/Hf
As mentioned in Sect. 3.1.1, Green has described a variety of homo- and heterobimetallic complexes containing a methylene-bridged (cyclopentadienyl)(indenyl)
ligand [80]. The Co/Zr heterobimetallic complex 46 has been examined as catalyst
precursor for ethylene polymerization with MAO as cocatalyst and shows activities
similar to the Fe/Zr complex 22 or [Cp 2 ZrCl 2 ]–MAO (Scheme 30).
In a pioneering paper [97], Osakada has reported ethylene polymerization trials
with a series of early–late heterobimetallic complexes, including a Co/Zr combination, and showed that some of these complexes enable the enchainment of the
α-olefin or of the oligomer formed at the late metal center to the polymer grown at
the Zr center. The synthesis of the Co/Zr heterobimetallic complex 49 is elegant and
involves as key step the Ru-catalyzed cross metathesis of an ansa-zirconocene
complex 47 with an allyl substituent and a Co complex 48 having a pendant acrylate
(Scheme 31). In the solid state, both Zr and Co atoms were found far away from
Ph
OH
Ph
+ EtOH
M
Cl
Cl
Et 2
P
PEt 2
Ru
Cl
43a: M = Ti
43b: M = Zr
43c: M = Hf
M
Cl
PEt 2
PEt 2
Ru
Cl
B(Ar F )4
44
Ph
OEt
Ph
M
Cl
Cl
Et 2
P
PEt 2
Ru
C
C
CPh 2
45
B(Ar F )4
cat. 43 (10 mol%)
NaB(Ar F ) 4 (10 mol%)
60°C, 72h
cat. 43a: 56%
cat. 43b: 57%
cat. 43c: 71%
M
Cl
Cl
Et 2
P
PEt 2
Ru
B(Ar F )4
Ph
Ph
OEt
Ph
OH
Ph
H 2 O
EtOH
Ph
OEt
Ph
Scheme 29 Etherification of propargylic alcohols catalyzed by heterobimetallic complexes and
proposed mechanism
“Early–Late” Heterobimetallic Catalysis and Beyond
157
indispensable for the reaction to occur. The stoichiometric reaction of complexes
43b and 43c with NaB(Ar
F ) 4 and propargylic alcohol gave the heterobimetallic
allenylidene complexes 45. These reactions proceed via the formation of chloridobridged complexes 44. The structures of both Ru/Zr complexes 44b and 45b have
been confirmed by X-ray diffraction study. In the mechanism of the catalytic
etherification of propargylic alcohols proposed by the authors, the reaction takes
place at the ruthenium center and involves 44 and 45 as key intermediates. The
formation of the chlorido-bridged complex 44 during the catalytic reaction would
promote the final dissociation of the propargylic ether.
3.2 Group 9/Group 4–5 Heterobimetallic Complexes
3.2.1 Co/Ti, Co/Zr, and Co/Hf
As mentioned in Sect. 3.1.1, Green has described a variety of homo- and heterobimetallic complexes containing a methylene-bridged (cyclopentadienyl)(indenyl)
ligand [80]. The Co/Zr heterobimetallic complex 46 has been examined as catalyst
precursor for ethylene polymerization with MAO as cocatalyst and shows activities
similar to the Fe/Zr complex 22 or [Cp 2 ZrCl 2 ]–MAO (Scheme 30).
In a pioneering paper [97], Osakada has reported ethylene polymerization trials
with a series of early–late heterobimetallic complexes, including a Co/Zr combination, and showed that some of these complexes enable the enchainment of the
α-olefin or of the oligomer formed at the late metal center to the polymer grown at
the Zr center. The synthesis of the Co/Zr heterobimetallic complex 49 is elegant and
involves as key step the Ru-catalyzed cross metathesis of an ansa-zirconocene
complex 47 with an allyl substituent and a Co complex 48 having a pendant acrylate
(Scheme 31). In the solid state, both Zr and Co atoms were found far away from
Ph
OH
Ph
+ EtOH
M
Cl
Cl
Et 2
P
PEt 2
Ru
Cl
43a: M = Ti
43b: M = Zr
43c: M = Hf
M
Cl
PEt 2
PEt 2
Ru
Cl
B(Ar F )4
44
Ph
OEt
Ph
M
Cl
Cl
Et 2
P
PEt 2
Ru
C
C
CPh 2
45
B(Ar F )4
cat. 43 (10 mol%)
NaB(Ar F ) 4 (10 mol%)
60°C, 72h
cat. 43a: 56%
cat. 43b: 57%
cat. 43c: 71%
M
Cl
Cl
Et 2
P
PEt 2
Ru
B(Ar F )4
Ph
Ph
OEt
Ph
OH
Ph
H 2 O
EtOH
Ph
OEt
Ph
Scheme 29 Etherification of propargylic alcohols catalyzed by heterobimetallic complexes and
proposed mechanism
“Early–Late” Heterobimetallic Catalysis and Beyond
157