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J. Handzlik
products of reaction between monomeric Cr(II) site and ethene, are not effective in
chain propagation [63]. However, easily generated Cr(IV) chromacyclopentane intermediate can lead, after ethylene insertion, to experimentally observed [66] 1-hexene
formation. The latter is a product of intramolecular β-H transfer, which competes
with the ring expansion, and the kinetic preference depends on the Cr site strain. On
the other hand, Cr(III) alkyl site was predicted to be the active propagating species,
although the mechanism of its formation was not explained [63]. Instead, generation
of propagating Cr(IV) alkyl site through hydrogen transfer from a surface silanol to
Cr(II) species was later proposed [64]. Another initiation route postulated involves
dimeric Cr(II) species which might react with ethene to form cyclic dimeric Cr(III)
propagating species without external sources of hydrogen [65].
Various initiation pathways for the Phillips catalyst, starting from Cr(II)
site modelled by chromasiloxane ring (2 Si atoms), were calculated with
the B3LYP functional [67]. The key intermediate was Cr(IV) chromacyclopentane
species. According to their experimental results [43, 68], the authors took into account
the presence of formaldehyde, which can be formed during reduction of the surface
Cr(VI) species by ethene. They found that adsorption of one formaldehyde molecule
on the Cr site favours ethene dimerization to 1-butene through Cr(IV) hydride intermediate and metathesis to form propene/ethene through a Cr(IV) chromacyclobutane intermediate. This proposal might explain experimentally observed production
of propene and butene during the induction period of ethene polymerization over
the Phillips catalyst [43, 68]. On the other hand, after formaldehyde desorption,
a ring expansion pathway to Cr(IV) chromacycloheptane and subsequent one-step
reductive elimination of 1-hexene were predicted to be kinetically preferred.
Conley et al. [69, 70] showed that well-defined Cr(III)/SiO 2 catalysts are active
in ethene polymerization. They proposed that heterolytic C–H bond activation of
ethene forms the first Cr–C bond, which was supported by computational (B3LYPD3) studies [70]. Two F-terminated cluster models were constructed (Fig. 9), representing major tri-coordinate Cr(III) species and minor species with Cr(III) coordinated to an additional siloxane bridge, which was confirmed by comparison
of experimental and computed vibrational shifts after CO adsorption. The calculated reaction pathways included initiation by hydrogen transfer from ethene to
generate (≡SiO) 2 (≡SiOH)Cr(III)-vinyl species, subsequent propagation according to the Cossee–Arlman mechanism and less kinetically favoured termination
via reverse hydrogen transfer to chromium alkyl. Coordination of two ethene
molecules to the Cr(III) site facilitates the C–H bond activation, but it is prevented
if the additional siloxane ligand is present. On this basis, it was suggested that
Cr(III) sites coordinating siloxane bridge are inactive in polymerization. The same
computational approach was used for investigations of propane dehydrogenation
over the Cr(III)/SiO 2 system [70].
The C–H bond activation mechanism was further re-examined by Fong et al. [6],
who used the same cluster model representing Cr(III) species and concluded that the
initiation step is too slow. In addition, they found a termination route which is faster
than propagation, so only oligomers would be formed. Similar results were obtained
for the analogous mechanism on Cr(II) site represented by the chromasiloxane cluster
J. Handzlik
products of reaction between monomeric Cr(II) site and ethene, are not effective in
chain propagation [63]. However, easily generated Cr(IV) chromacyclopentane intermediate can lead, after ethylene insertion, to experimentally observed [66] 1-hexene
formation. The latter is a product of intramolecular β-H transfer, which competes
with the ring expansion, and the kinetic preference depends on the Cr site strain. On
the other hand, Cr(III) alkyl site was predicted to be the active propagating species,
although the mechanism of its formation was not explained [63]. Instead, generation
of propagating Cr(IV) alkyl site through hydrogen transfer from a surface silanol to
Cr(II) species was later proposed [64]. Another initiation route postulated involves
dimeric Cr(II) species which might react with ethene to form cyclic dimeric Cr(III)
propagating species without external sources of hydrogen [65].
Various initiation pathways for the Phillips catalyst, starting from Cr(II)
site modelled by chromasiloxane ring (2 Si atoms), were calculated with
the B3LYP functional [67]. The key intermediate was Cr(IV) chromacyclopentane
species. According to their experimental results [43, 68], the authors took into account
the presence of formaldehyde, which can be formed during reduction of the surface
Cr(VI) species by ethene. They found that adsorption of one formaldehyde molecule
on the Cr site favours ethene dimerization to 1-butene through Cr(IV) hydride intermediate and metathesis to form propene/ethene through a Cr(IV) chromacyclobutane intermediate. This proposal might explain experimentally observed production
of propene and butene during the induction period of ethene polymerization over
the Phillips catalyst [43, 68]. On the other hand, after formaldehyde desorption,
a ring expansion pathway to Cr(IV) chromacycloheptane and subsequent one-step
reductive elimination of 1-hexene were predicted to be kinetically preferred.
Conley et al. [69, 70] showed that well-defined Cr(III)/SiO 2 catalysts are active
in ethene polymerization. They proposed that heterolytic C–H bond activation of
ethene forms the first Cr–C bond, which was supported by computational (B3LYPD3) studies [70]. Two F-terminated cluster models were constructed (Fig. 9), representing major tri-coordinate Cr(III) species and minor species with Cr(III) coordinated to an additional siloxane bridge, which was confirmed by comparison
of experimental and computed vibrational shifts after CO adsorption. The calculated reaction pathways included initiation by hydrogen transfer from ethene to
generate (≡SiO) 2 (≡SiOH)Cr(III)-vinyl species, subsequent propagation according to the Cossee–Arlman mechanism and less kinetically favoured termination
via reverse hydrogen transfer to chromium alkyl. Coordination of two ethene
molecules to the Cr(III) site facilitates the C–H bond activation, but it is prevented
if the additional siloxane ligand is present. On this basis, it was suggested that
Cr(III) sites coordinating siloxane bridge are inactive in polymerization. The same
computational approach was used for investigations of propane dehydrogenation
over the Cr(III)/SiO 2 system [70].
The C–H bond activation mechanism was further re-examined by Fong et al. [6],
who used the same cluster model representing Cr(III) species and concluded that the
initiation step is too slow. In addition, they found a termination route which is faster
than propagation, so only oligomers would be formed. Similar results were obtained
for the analogous mechanism on Cr(II) site represented by the chromasiloxane cluster