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Scheme 5.20
Synthesis of
2
-
bridged dinuclear Ru-complexes.
B(3,5-(CF
3
)
2
-C
6
H
3
)
4
) was reported by Werner et al. [267]. Unfortunately, again no
data on ROMP activity are available.
Heterobimetallic ruthenium alkylidenes, which may be prepared by reaction of
Cl
2
Ru(PCy
3
)
2
(CHR) with [Ru(
p-
cymene)Cl
2
]
2
, [Os(
p-
cymene)Cl
2
]
2
and [Rh(
t
-butylcy-
clopentadienyl)Cl
2
]
2
(Scheme 5.21), were reported to possess significantly elevated
activities in the ROMP of 1,5-cyclooctadiene (COD) and 2,2-bis(trifluoromethyl)-
norbornene [268]. Other ROMP-active initiators are the bimetallic complexes (
p-
cymene)RuCl(
-Cl)
2
RuCl(CHPh)(NHC)
(NHC=N-heterocyclic carbene, Cp*=pentamethylcyclopentadienyl) [236, 237].
It is worth mentioning that the same compounds have been reported to be
highly active in ring-closing metathesis (RCM) [238]. The reactivity of such sys-
tems increases in the order Ru<Os<Rh. Assuming that the coordination of the
second metal center to the bridging chlorides directly influences their electron
withdrawing capability, this finding is consistent with the observed order of activ-
ity I <Br <Cl in the mononuclear complexes [170].
-Cl)
2
RuCl(CHPh)(NHC) and (Cp*)RhCl(
Scheme 5.21
Synthesis of heterobimetallic ruthenium carbenes R=Ph.
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