5 Stereorigidity of Bridged Metallocenes
and Stereoselectivity of the Catalysts
The lower stereorigidity and structural flexibility in complexes 5, 10, and 11 were
associated with the more frequent occurrence of site epimerization and the lower
stereospecificity of the resulting catalyst systems. In complex 5, the conformational
interconversion due to the fluxional behavior of the ethano-bridge, and in
complexes 10 and 11, the highly flexible structure as a result of the replacement
of a strongly η
5 -bonded ligand moiety (an aromatic ring) by a single atom η
1 -
bonded –N–R amido group (and the resulting amine-type umbrella interconversion
6
were considered to be the underlying reasons for lower skeletal rigidity and a
probable source for higher site epimerization rate. However, one should be aware
that even for bridged metallocene structures that have doubly η
5 -bonded ligand
systems such as in complexes 1 (2), 6 (7), and 9 the idea of high rigidity conveyed
from solid state, X-ray images are somehow deceptive and should be taken as
relative. The images extracted from X-ray crystal structural analysis project
molecules contained in the unit cells in the solid state under the crystal packing
effects. These rigid and static images reflect only a snapshot of the constantly
vibrating, bending, and “breathing” molecules (the inward/outward movement of
the two centroids towards and from the transition metal). In solution, the individual
molecules freely float in the solvent and interact with the solvent molecules
(the medium); they are much more mobile and dynamic than one would imagine.
Thus the question arises as to how rigid the stereorigid metallocene structures
are in solution. In this context, it is important to be aware of at least two dynamic
phenomena regarding the fluxional behavior of metallocene molecules in solution,
namely, the phenomena of haptotropy [149–156] and ring slippage. The haptotropy
and ring slippage [157–160, 161, and references therein] [162, 163], depicted
schematically in Fig. 19, could influence the electronic properties of the active
site and the steric environment surrounding it, particularly in the cationic state and
in the presence of a counter-ion.
In Sect. 2.3 it was shown by the example of complexes 1 (2) and 6 (7), that the
phenomenon of hapticity change or bond order variation between the transition metal
and the aromatic ligands can change the kinetic pathways of the polymerization
process and its outcome dramatically. The haptotropic behavior of metallocenes is
a known phenomenon in transition organometallic chemistry and homogeneous
catalysis [149–156].
The hapto-flexible aromatic ligands bonded to the transition metal can facilitate
the ligand exchange reaction by “temporarily” lowering the hapticity in the transition
state and permitting the increase in the formal coordination number without breaking
the canonic electronic rules [157–160, 161, and references therein; 162, 163]. As a
general rule, the hapticity change should be considered to be involved in almost all
6 See footnote 5.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
81
and Stereoselectivity of the Catalysts
The lower stereorigidity and structural flexibility in complexes 5, 10, and 11 were
associated with the more frequent occurrence of site epimerization and the lower
stereospecificity of the resulting catalyst systems. In complex 5, the conformational
interconversion due to the fluxional behavior of the ethano-bridge, and in
complexes 10 and 11, the highly flexible structure as a result of the replacement
of a strongly η
5 -bonded ligand moiety (an aromatic ring) by a single atom η
1 -
bonded –N–R amido group (and the resulting amine-type umbrella interconversion
6
were considered to be the underlying reasons for lower skeletal rigidity and a
probable source for higher site epimerization rate. However, one should be aware
that even for bridged metallocene structures that have doubly η
5 -bonded ligand
systems such as in complexes 1 (2), 6 (7), and 9 the idea of high rigidity conveyed
from solid state, X-ray images are somehow deceptive and should be taken as
relative. The images extracted from X-ray crystal structural analysis project
molecules contained in the unit cells in the solid state under the crystal packing
effects. These rigid and static images reflect only a snapshot of the constantly
vibrating, bending, and “breathing” molecules (the inward/outward movement of
the two centroids towards and from the transition metal). In solution, the individual
molecules freely float in the solvent and interact with the solvent molecules
(the medium); they are much more mobile and dynamic than one would imagine.
Thus the question arises as to how rigid the stereorigid metallocene structures
are in solution. In this context, it is important to be aware of at least two dynamic
phenomena regarding the fluxional behavior of metallocene molecules in solution,
namely, the phenomena of haptotropy [149–156] and ring slippage. The haptotropy
and ring slippage [157–160, 161, and references therein] [162, 163], depicted
schematically in Fig. 19, could influence the electronic properties of the active
site and the steric environment surrounding it, particularly in the cationic state and
in the presence of a counter-ion.
In Sect. 2.3 it was shown by the example of complexes 1 (2) and 6 (7), that the
phenomenon of hapticity change or bond order variation between the transition metal
and the aromatic ligands can change the kinetic pathways of the polymerization
process and its outcome dramatically. The haptotropic behavior of metallocenes is
a known phenomenon in transition organometallic chemistry and homogeneous
catalysis [149–156].
The hapto-flexible aromatic ligands bonded to the transition metal can facilitate
the ligand exchange reaction by “temporarily” lowering the hapticity in the transition
state and permitting the increase in the formal coordination number without breaking
the canonic electronic rules [157–160, 161, and references therein; 162, 163]. As a
general rule, the hapticity change should be considered to be involved in almost all
6 See footnote 5.
Syndiotactic Polypropylene: Discovery, Development, and Industrialization. . .
81