Enhancement of Chain Growth
and Chain Transfer Rates in Ethylene Polymerization by (Phosphine-sulfonate)PdMe
Catalysts by Binding of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> to
the Sulfonate Group
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Abstract
Binding of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> to
a sulfonate oxygen of (<i>ortho</i>-phosphino-arenesulfonate)PdR
catalysts results in a 3–4 fold increase in the rate of chain
growth and a larger increase in the rate of chain transfer. The reaction
of (PO-Et)PdMe(py) (<b>1a</b>, [PO-Et]<sup>−</sup> = <i>ortho</i>-{(2-Et-Ph)<sub>2</sub>P}-<i>para</i>-toluenesulfonate)
with 1 equiv of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> yields the
base-free dimer {(PO-Et)PdMe}<sub>2</sub> (<b>2a</b>), in which
the (PO-Et)PdMe units are linked through an eight-membered [PdSO<sub>2</sub>]<sub>2</sub> ring. The reaction of {(PO-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>)PdMe}<sub>2</sub>(TMEDA) (<b>4b</b>; [PO-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>]<sup>−</sup> = <i>ortho</i>-{(3,5-<sup><i>t</i></sup>Bu<sub>2</sub>-Ph)<sub>2</sub>P}-<i>para</i>-toluenesulfonate,
TMEDA = <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine) with BF<sub>3</sub>·Et<sub>2</sub>O yields the soluble base-free dimer {(PO-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>)PdMe}<sub>2</sub> (<b>2b</b>), in which the (PO-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>)PdMe units are linked through a four-membered Pd<sub>2</sub>O<sub>2</sub> ring. <b>2b</b> reacts with 2 equiv of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> to yield {[PO·B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>]PdMe}<sub>2</sub> (<b>5b</b>, [PO·B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>]<sup>−</sup> = [2-{(3,5-<sup><i>t</i></sup>Bu<sub>2</sub>-Ph)<sub>2</sub>P}-4-Me-C<sub>6</sub>H<sub>3</sub>SO<sub>2</sub>OB(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>]<sup>−</sup>),
which crystallizes from Et<sub>2</sub>O as the monomeric complex [PO·B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>]PdMe(Et<sub>2</sub>O) (<b>6b</b>). In both <b>5b</b> and <b>6b</b>, the B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> binds to a sulfonate oxygen. In toluene solution at 60 °C, <b>2b</b> polymerizes ethylene (80 psi) to linear polyethylene with <i>M</i><sub>n</sub> = 3,000, while the B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> adducts <b>5b</b> and <b>6b</b> yield ethylene
oligomers (<i>M</i><sub>n</sub> = 160–170). <b>5b</b> and <b>6b</b> are 3–4 times more active than <b>2b</b>. Similarly, <b>1a</b> polymerizes ethylene to linear
polyethylene with <i>M</i><sub>n</sub> = 29,300 (toluene,
80 °C, 435 psi), while <b>1a</b>-4 B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> yields polymer with <i>M</i><sub>n</sub> = 2,520 with a 4 fold increase in activity. <b>2b</b> reacts
with ethylene at 7 °C to form the ethylene adduct (PO-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>)PdMe(CH<sub>2</sub>CH<sub>2</sub>) (<b>7b</b>) followed by multiple insertions to generate
(PO-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>)Pd(CH<sub>2</sub>CH<sub>2</sub>)<sub><i>n</i></sub>CH<sub>3</sub> species.
In contrast, <b>5b</b> reacts with ethylene to form [PO·B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>-3,5-<sup><i>t</i></sup>Bu<sub>2</sub>]PdMe(CH<sub>2</sub>CH<sub>2</sub>) (<b>8b</b>) followed by insertion and β-H transfer to yield
propene with subsequent catalytic formation of 1-butene and higher
olefins. The rate of ethylene insertion of <b>8b</b> is 3 times
greater than that of <b>7b</b>, consistent with the batch polymerization
results. The polymer yield and molecular weight data show that binding
of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> to <b>2b</b> and <b>1a</b> increases the chain transfer rates by a factor of 80 and
42, respectively
