Article
Organometallics, Vol. 30, No. 4, 2011 755
been known for a long time, but the true breakthrough was
achieved by stereospecific polymerization of conjugated
dienes with transition metal-based coordination catalysts.21
Homogeneous Ziegler-Natta catalyst systems composed of
complexes of various transition metals, such as Ti,22,24d V,23
Fe,24 Co,24d,25 Ni,24d,26 and Nd,27 and the coactivators alu-
minum alkyls or the aluminum alkyl chloride have been ext-
ensively investigated. Some chromium((III) systems such as
Cr(acac)3/AlEt3, Cr(allyl)3/MAO, Cr(allyl)2Cl//MAO, and
Cr(acac)3/MAO have been reported to polymerize butadiene
to polybutadiene with predominantly 1,2- or cis-1,4-units,
though their catalytic activities are rather low.22f,23a,28 Re-
cently some Cr(II) complexes supported by phosphine ligands,
such as CrCl2(dmpe)2 (dmpe =1,2-bis(dimethylphosphino)
ethane)29 and Cr(CH3)2(dmpe)2,29 were found to show high
catalytic activity for butadiene polymerization in the pre-
sence of MAO, affording predominantly 1,2-polybutadiene.
The isoprene polymerization by chromium complexes is rare so
29
far. Only some Cr-allyl,28b-d derivatives and CrCl2(dmpe)2
were known to show low catalytic activity for isoprene polym-
erization to form polyisoprene with predominantly 3,4-units,
and some chromium complexes supported by N,N-bis-
(diarylphosphino)amine30 were reported to display moder-
ate catalytic activity for isoprene trimerization.
Figure 3. Perspective view of 1c with thermal ellipsoids drawn
at 30% probability level. Hydrogens and uncoordinated solvent
are omitted for clarity. The selected bond lengths (A) and angles
(deg): Cr(1)-C(1) 1.985(6), Cr(1)-N(1) 2.200(4), Cr(1)-N(2)
2.186(5), Cr(1)-Cl(1) 2.4875(16), Cr(1)-Cl(2) 2.4624(16), Cl-
(1)-Li(1) 2.306(12), Cl(2)-Li(1) 2.372(11), O(1)-Li(1) 1.872
(13), O(2)-Li(1) 1.973(13), Cl(1)-Cr(1)-Cl(2) 91.70(6), Cl(1)-
Li(1)-Cl(2) 98.8(4), N(1)-Cr(1)-N(2) 151.50(18), O(1)-Li-
(1)-O(2) 100.3(6).
All new chromium complexes 1a-1c, 2a-2c were evalu-
ated as catalysts for isoprene polymerization. The polymer-
ization data are summarized in Table 2. Upon activation
with trialkylaluminum, all complexes were found to be
inactive toward isoprene polymerization and no polymer
was obtained even the polymerization experiments were
carried out at high temperatures and for long times, which
may be attributed to the low Lewis acidity of the resulted
alkyl chromium complexes. The Cr(II) complexes 1a-1c are
still inactive even they were activated with AlR3/Ph3CþB-
(C6F5)4- or MAO. Such a result is understandable consider-
ing that the cationic species formed from these complexes
during the activation reaction carries no alkyl group and thus
is unable to form a polymer chain though the isoprene molecule
can coordinate to the metal center as shown in Scheme 3.
The Cr(III) complexes 2a-2c show moderate to high
catalytic activity and good trans-1,4 selectivity at room tem-
perature when activated with AlR3/Ph3CþB(C6F5)4-. Com-
plex 2b show higher activities than that of complex 2a and 2c
under similar conditions, and a complete conversion could
in Cr(II) complexes 1a-1c (2.4624(16)-2.5338(8) A). The
two chlorides arrange in trans-positions to form a large
Cl-Cr-Cl angle of 178.83(3)° in 2a, 177.34(3)° in 2b, and
179.17(4)° in 2c.
Isoprene Polymerization Studies. Polybutadiene and poly-
isoprene have been the most widely used synthetic rubbers.
Methods for the polymerization of conjugated dienes have
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