A comparison of the
1 H NMR spectra [19, 20, 128] of complexes 1 and 6 reveals
further that the structural differences may be more substantial in solution phase.
Comparison of
1 H NMR spectra of the complexes 1 and 6 shows that for both
molecules the proton signals related to the cyclopentadienyl groups of both catalyst
systems have very similar chemical shift values and present the same coupling
pattern (two virtual triplets) [20]. Examination of the fluorenyl’s proton signals
indicates that some of the protons belonging to the fluorenyl part of complex 6 have
been subjected to major different shielding and deshielding forces, giving rise to
completely different chemical shifts and signal patterns. Fluorenyl protons attached
to carbons 3,3
0 and 4,4
0 (see Fig. 13 for numbering) closest to the bridge
substituents, the phenyl groups, in complex 6 experience the most dramatic
up-field shift. Their signals appear more than 1.5 and 0.5 ppm shifted to higher
field, respectively, whereas the chemical shift variations for protons 5,5
0 and 6,6
0
(attached to carbon 5,5
0 and 6,6
0 in Fig. 13), more distant from the bridge, are
negligible (see Sect. 8) [20, 129, 130]. Apparently, the introduction of the phenyl
groups in the C1 bridge (vide supra) causes important changes in the electron density
distribution of the aromatic π-system of the fluorenyl’s six-membered rings. This in
turn provokes a redistribution of the electron densities concentrated on the different
C–H atom groups and their chemical shift repositioning [20, 129, 130]. It is also
possible that the magnetic field anisotropy caused by the ring currents of the two
phenyl substituents in the bridge is responsible for the observed differences in the
chemical shifts and signal patterns for the said protons.
The slight outward repositioning of the metal center and dramatic differences in
chemical shifts and signal pattern of the fluorenyl protons for compounds 1 and 6,
on the other hand, could be indicative of fact that the fluorenyl moieties of the
ligands of these complexes are engaged in different bonding relationship or
hapticities with the transition metal in the solution phase. In other words, the
redistribution of the electron densities and changes in the Zr–C bonds in fluorenyl
carbon atoms may be associated with, or a result of, an eventual hapticity change
from η
5 -bonding to η
3 -bonding. It is in essence speculated that different Zr–C flu
bond hapticities, η
5 or η
3 , (or even a η
1 -type bonding) are at the origin of the
different catalytic performances of (1 and 2)/MAO, and (6 and 7)/MAO catalyst
systems [129, 130]. To prove the veracity of the hypothesis concerning different
η-bonding in complexes 1 and 6 in solution phase, a hydrogenation experiment was
undertaken that proved to be very revealing.
The aim of the hydrogenation experiment was, first, to verify the assumption that
whether in either one of the complexes a η
3 Zr–C flu is operative and, if so, to assign
the correct Zr–C flu centroid hapticities to each of the two metallocene complexes 1
and 6. Theoretically, the η
3 nature of the five-membered ring of Zr–fluorenyl bond in
one of the metallocenes would basically imply that the corresponding two
six-membered rings are fully aromatic and not easily subject to hydrogenation. On
the contrary, if in one of the metallocenes the Zr–C flu centroid bond is η
5 in nature,
then the fused six-membered rings are not “quite” aromatic and more hexadiene like
in nature and therefore subject to facile hydrogenation. Figure 14 shows the
hydrogenation scheme for the two metallocene complexes 1 and 6 and the formation
68
A. Razavi
1 H NMR spectra [19, 20, 128] of complexes 1 and 6 reveals
further that the structural differences may be more substantial in solution phase.
Comparison of
1 H NMR spectra of the complexes 1 and 6 shows that for both
molecules the proton signals related to the cyclopentadienyl groups of both catalyst
systems have very similar chemical shift values and present the same coupling
pattern (two virtual triplets) [20]. Examination of the fluorenyl’s proton signals
indicates that some of the protons belonging to the fluorenyl part of complex 6 have
been subjected to major different shielding and deshielding forces, giving rise to
completely different chemical shifts and signal patterns. Fluorenyl protons attached
to carbons 3,3
0 and 4,4
0 (see Fig. 13 for numbering) closest to the bridge
substituents, the phenyl groups, in complex 6 experience the most dramatic
up-field shift. Their signals appear more than 1.5 and 0.5 ppm shifted to higher
field, respectively, whereas the chemical shift variations for protons 5,5
0 and 6,6
0
(attached to carbon 5,5
0 and 6,6
0 in Fig. 13), more distant from the bridge, are
negligible (see Sect. 8) [20, 129, 130]. Apparently, the introduction of the phenyl
groups in the C1 bridge (vide supra) causes important changes in the electron density
distribution of the aromatic π-system of the fluorenyl’s six-membered rings. This in
turn provokes a redistribution of the electron densities concentrated on the different
C–H atom groups and their chemical shift repositioning [20, 129, 130]. It is also
possible that the magnetic field anisotropy caused by the ring currents of the two
phenyl substituents in the bridge is responsible for the observed differences in the
chemical shifts and signal patterns for the said protons.
The slight outward repositioning of the metal center and dramatic differences in
chemical shifts and signal pattern of the fluorenyl protons for compounds 1 and 6,
on the other hand, could be indicative of fact that the fluorenyl moieties of the
ligands of these complexes are engaged in different bonding relationship or
hapticities with the transition metal in the solution phase. In other words, the
redistribution of the electron densities and changes in the Zr–C bonds in fluorenyl
carbon atoms may be associated with, or a result of, an eventual hapticity change
from η
5 -bonding to η
3 -bonding. It is in essence speculated that different Zr–C flu
bond hapticities, η
5 or η
3 , (or even a η
1 -type bonding) are at the origin of the
different catalytic performances of (1 and 2)/MAO, and (6 and 7)/MAO catalyst
systems [129, 130]. To prove the veracity of the hypothesis concerning different
η-bonding in complexes 1 and 6 in solution phase, a hydrogenation experiment was
undertaken that proved to be very revealing.
The aim of the hydrogenation experiment was, first, to verify the assumption that
whether in either one of the complexes a η
3 Zr–C flu is operative and, if so, to assign
the correct Zr–C flu centroid hapticities to each of the two metallocene complexes 1
and 6. Theoretically, the η
3 nature of the five-membered ring of Zr–fluorenyl bond in
one of the metallocenes would basically imply that the corresponding two
six-membered rings are fully aromatic and not easily subject to hydrogenation. On
the contrary, if in one of the metallocenes the Zr–C flu centroid bond is η
5 in nature,
then the fused six-membered rings are not “quite” aromatic and more hexadiene like
in nature and therefore subject to facile hydrogenation. Figure 14 shows the
hydrogenation scheme for the two metallocene complexes 1 and 6 and the formation
68
A. Razavi