To gain insight on the activity and stereoregulating ability
of these systems we investigated the polymerization of
neat 1-hexene by the two dibenzylzirconium complexes
following activation with tris(pentafluorophenyl)borane.
L1ZrBn2 featuring the bulky tert-butylphenolate substituents
led to a polymerization catalyst of low activity of ca.
1 g mmolꢀ1
h
ꢀ1, and to a polymer of relatively high
Scheme 2
isospecificity ([mmmm] = 87%). The pentad distribution
supports an enantiomorphic site control mechanism for this
polymerization. L2ZrBn2 featuring the electron-withdrawing
chlorophenolate substituents led to a catalyst of considerably
that gave diastereomer mixtures in this reaction. According to
the ligand chirality and the arguments portrayed above, we
propose that mononuclear octahedral complexes had formed in
which the (S,S)-Salan ligands wrap in a fac–fac manner around
the titanium centre to give L-chiral-at-metal diastereomers, as
drawn in Scheme 1. The same high diastereoselectivity
was found for zirconium. Reacting L1H2 and L2H2 with
Zr(OtBu)4 and with ZrBn4 (Bn = benzyl) led to the complexes
L1,2Zr(OtBu)2 and L1,2ZrBn2, respectively, obtained as single
higher activity of ca. 150 g mmolꢀ1
h
ꢀ1, and, as expected
for the non-directing nature of the non-bulky groups,
to a stereo-irregular polymer. The lower activity of the
bipyrrolidine-based Salan complexes relative to those of
the diaminoethane-based Salan complexes may result from
the enhanced rigidity of the former. A related structure–activity
trend was observed for the polymerization catalysis of
1-hexene by the dibenzylzirconium complexes of the Salan
ligands based on the 1,2-trans-diaminocyclohexane bridge.11
In conclusion, we have synthesized the first Salan ligands
assembled around chiral 2,20-bipyrrolidine. These ligands were
shown to lead to single diastereomers around octahedral metal
centres, unlike the diaminocyclohexane based Salan ligands
that gave diastereomer mixtures around titanium. The
relatively high isopecificity in 1-hexene polymerization reflects
on the potential applications of these chiral complexes in
asymmetric catalysis. We are investigating the construction
of related ligands around this strongly directing structural
motif, which are expected to wrap in a predictable and
diastereoselective manner in cases where ligands constructed
around the trans-1,2-diaminocyclohexane lead to mixtures.
We thank Prof. Alexandre Alexakis (Geneve) for a generous
gift of (S,S)-2,20-bipyrrolidine. We thank the Israel Science
Foundation for financial support.
isomers of C2-symmetry according to H NMR.18
1
Single crystals of L1Zr(OtBu)2 suitable for X-ray analysis
were grown from cold pentane, and the structure was solved.
The structure is very similar to that of a zirconium complex of
the non-chiral Salan ligand, particularly apparent in the ligand
fac–fac wrapping and cis geometry between the two OtBu
groups as may be appreciated in Fig. 1. Similar bond lengths
and bond angles around the zirconium, and N–C–C–N
dihedral angles in the two structures of 55.2 and 59.11,
respectively, reveal that the binding of the bipyrrolidine-Salan
to the metal is not strained. The wide angles of ca. 1661 of the
labile alkoxo groups indicate a significant p-donation to the
zirconium. The (S,S)-chirality of the ligand backbone was
established in this chiral space group. Most importantly, the
predicted L-wrapping of this enantiomer of the Salan ligand is
clearly observed in the structure. We presume that the Salan
ligands in the other zirconium and titanium complexes wrap
analogously.
Notes and references
1 E. Y. Tshuva, I. Goldberg and M. Kol, J. Am. Chem. Soc., 2000,
122, 10706; S. Segal, I. Goldberg and M. Kol, Organometallics,
2005, 24, 200; S. Segal, A. Yeori, M. Shuster, Y. Rosenberg and
M. Kol, Macromolecules, 2008, 41, 1612.
2 V. Busico, R. Cipullo, R. Pellecchia, S. Ronca, G. Roviello and
G. Talarico, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 15321;
P. Corradini, G. Guerra and L. Cavallo, Acc. Chem. Res., 2004, 37,
231.
3 (a) For lactide polymerization by group 4 complexes of Salan
ligands, see: S. Gendler, S. Segal, I. Goldberg, Z. Goldschmidt and
M. Kol, Inorg. Chem., 2006, 45, 4783; (b) P. Hormnirun,
E. L. Marshall, V. C. Gibson, A. J. P. White and D. J. Williams,
J. Am. Chem. Soc., 2004, 126, 2688; A. J. Chmura,
M. G. Davidson, M. D. Jones, M. D. Lunn, M. F. Mahon,
A. F. Johnson, P. Khunkamchoo, S. L. Roberts and S. S.
F. Wong, Macromolecules, 2006, 39, 7250. For related transforma-
tions by related complexes, see: M. G. Davidson, C. T. O’Hara,
M. D. Jones, C. G. Keir, M. F. Mahon and G. Kociok-Kohn,
¨
Inorg. Chem., 2007, 46, 7686.
4 J. Balsells, P. J. Carroll and P. J. Walsh, Inorg. Chem., 2001, 40,
5568.
5 A. Yeori, S. Groysman, I. Goldberg and M. Kol, Inorg. Chem.,
2005, 44, 4466.
6 A. Yeori, I. Goldberg and M. Kol, Macromolecules, 2007, 40,
8521.
Fig. 1 ORTEP representation of (S,S)-L1Zr(OtBu)2 (left) and a
related complex prepared from non-chiral Salan ligand3a with 50%
probability ellipsoids, and hydrogen atoms omitted for clarity.
Selected bond lengths (A) and angles (1): Zr–O2 2.050(3), Zr–O3
2.063(3), Zr–O4 1.940(3), Zr–O5 1.944(3), Zr–N6 2.464(4), Zr–N7
2.450(4); O2–Zr–O3 162.1(1), N6–Zr–N7 70.6(1), O4–Zr–O5 106.0(1),
C46–O4–Zr 167.5(3), C50–O5–Zr 166.2(3).
7 Y. Sawada, K. Matsumoto, S. Kondo, H. Watanabe, T. Ozawa,
K. Suzuki, B. Saito and T. Katsuki, Angew. Chem., Int. Ed., 2006,
ꢁc
This journal is The Royal Society of Chemistry 2009
3054 | Chem. Commun., 2009, 3053–3055