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ticular the importance of
-donation related with the exchange of phosphines, the
influence of electronic as well as steric effects of this type of compounds have
been studied in detail [172]. The stability as well as the reactivity order that may
be deduced therefrom is PPh
3
<PBz
3
<PCyPh
2
<PCy
2
Ph<P-i-Bu
3
<P-i-Pr
3
<PCy
3
.
The influence of different phosphines on the reactivity of ruthenium-based sys-
tems of the general formula Cl
2
Ru(CHCHCPh)
2
(PR
3
)
2
was additionally investi-
gated in the ring-closing metathesis (RCM) of dipropargylmalonate [170]. These
investigations again revealed increasing reactivity in the order I <Br <Cl and
PPh
3
<P-i-PPr
2
Ph<PCy
2
Ph<P-i-Pr
3
<PCy
3
. In principle, two different mecha-
nisms for the reaction of the initiator with an olefinic substrate were postulated
on the basis of these experiments. These are an
associative
, with both phosphines
on the metal center, and a
dissociative
, with only one phosphine attached to the
ruthenium core. The finding that the addition of CuCl as a phosphine scavenger
resulted in significantly elevated catalytic activities provided support for a dis-
sociative mechanism.
=456 nm of
Ru(CHPh)Cl
2
(PCy
3
)
2
revealed similar behavior, that is dissociation of one phos-
phine to form a trigonal bipyramidal structure [173]. Mechanistic studies by elec-
trospray ionization (ESI) tandem-MS in the gas-phase support dissociation of one
phosphine and suggested that the metallacyclobutane ring was a transition state
rather than an intermediate [174]. Monophosphine adducts were also found to be
the active species in ROMP carried out in the gas-phase [175]. Monophosphine or,
generally speaking, mono-ligand adducts are unstable. Since such mono adducts
are the actual catalytic species in metathesis-based reactions, the utility of any cat-
alyst strongly depends on the ratio of the rate of catalysis to the rate of decomposi-
tion. Consequently, catalyst design has to focus on accelerating the catalytic pro-
cess instead of accelerating both processes.
Interestingly, quantum molecular dynamics studies only suggested that both
mono- and diphosphine adducts were stable enough to support both mechanisms
[176]. Nevertheless, these calculations underlined the importance of the use of
sterically crowded phosphines for the preparation of highly active ruthenium alky-
lidenes as they lead to longer and consequently less stable Ru-P bonds. The im-
portant effect of phosphine size and basicity on metathesis performance [170, 172]
was further underlined by the finding that even small changes in the PCy
3
ligand
allow the fine-tuning of this catalytic system. Thus, the use of the PCy
2
CH
2
SiMe
3
ligand allows the synthesis of the initiator Ru(CHPh)Cl
2
(PCy
2
CH
2
SiMe
3
)
2
that
turned out to be highly active in the polymerization of norbornene imides [177].
Quantitative measurements of the rate constants of phosphine dissociation and re-
binding as well as on olefin insertion confirmed the dissociative mechanism.
These studies further revealed that, in systems of the general formula
Cl
2
Ru(CHPh)(PR
3
)
2
, phosphine exchange is much faster than the rate of reaction
with olefin [178]. In addition, complexes with labile phosphines (PPh
3
) exhibit the
highest initiation rates [168] yet lowest polymerization activities. In order to be
able to use the comparably reactive bis(tricyclohexyl) derivatives for ROMP in a
way that quantitative initiation was achieved, a possibility to enhance initiation ef-
ficiency was greatly needed. The solution to this problem was found by addition
Investigations on the photoreactivity at
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