Synthesis and Reactivity of Palladium(II) Alkyl Complexes that Contain Phosphine-cyclopentanesulfonate
Ligands
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Abstract
The synthesis of the phosphine-cyclopentanesulfonate
pro-ligands
Li/K[2-PPh<sub>2</sub>-cyclopentanesulfonate] (Li/K[<b>2a</b>]), Li/K[2-P(2-OMe-Ph)<sub>2</sub>-cyclopentanesulfonate] (Li/K[<b>2b</b>]), and H[<b>2b</b>], and the corresponding Pd(II)
alkyl complexes (κ<sup>2</sup>-<i>P</i>,<i>O</i>-<b>2a</b>)PdMe(pyridine) (<b>3a</b>) and (κ<sup>2</sup>-<i>P</i>,<i>O</i>-<b>2b</b>)PdMe(pyridine)
(<b>3b</b>) is described. The sulfonate-bridged base-free dimer
{(<b>2b</b>)PdMe}<sub>2</sub> (<b>4b</b>) was synthesized
by abstraction of pyridine from <b>3b</b> using B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>. The borane-coordinated base-free dimer
[{<b>2b·</b>B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>}PdMe]<sub>2</sub> (<b>5b</b>), in which B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> binds to a sulfonate oxygen, was prepared by addition of
1 equiv of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> per Pd to <b>4b</b> or addition of 2 equiv of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> to <b>3b</b>. Compounds <b>3b</b>, <b>4b</b>, and <b>5b</b> polymerize ethylene with low activity (up to
210 kg mol<sup>–1</sup> h<sup>–1</sup> at 250 psi and
80 °C) to linear polyethylene (<i>M</i><sub>n</sub> = 1950–5250 Da) with predominantly internal olefin placements. <b>3b</b> and <b>4b</b> copolymerize ethylene with methyl acrylate
to linear copolymers that contain up to 11.7 mol % methyl acrylate,
which is incorporated as −CH<sub>2</sub>CH(CO<sub>2</sub>Me)CH<sub>2</sub>– (80%) in-chain units and −CH<sub>2</sub>CH(CO<sub>2</sub>Me)Me (8%) and −CH<sub>2</sub>CHCH(CO<sub>2</sub>Me) (12%) chain-end units. <b>3b</b> and <b>4b</b> also
copolymerize ethylene with vinyl fluoride to linear copolymers that
contain up to 0.41 mol % vinyl fluoride, which is incorporated as
−CH<sub>2</sub>CHFCH<sub>2</sub>– (∼80%) in-chain
units and −CH<sub>2</sub>CF<sub>2</sub>H (7%), −CH<sub>2</sub>CHFCH<sub>3</sub> (5%), and −CH<sub>2</sub>CH<sub>2</sub>F (8%) chain-end units. Complexes <b>3b</b> and <b>4b</b> are more stable and active in ethylene polymerization than analogous
(PAr<sub>2</sub>-<i>CH</i><sub>2</sub><i>CH</i><sub>2</sub>SO<sub>3</sub>)PdR catalysts, but are less active than
analogous (PAr<sub>2</sub>-<i>arene</i>sulfonate)PdR catalysts.
Low-temperature NMR studies show that <b>4b</b> reacts with
ethylene below −10 °C to form the ethylene adduct <i>cis</i>-<i>P</i>,<i>R</i>-(<b>2b</b>)PdMe(ethylene) (<b>7b</b>), which undergoes ethylene insertion
at 5 °C. DFT calculations for a model (PMe<sub>2</sub>-cyclopentanesulfonate)Pd(Pr)(ethylene)
species show that ethylene insertion proceeds by <i>cis</i>-<i>P</i>,<i>R</i>/<i>trans</i>-<i>P</i>,<i>R</i> isomerization followed by migratory
insertion, and that the lower activity of <b>3b</b> and <b>4b</b> vis-à-vis analogous (PAr<sub>2</sub>-arenesulfonate)PdR
catalysts results from a higher barrier for migratory insertion of
the <i>trans</i>-<i>P</i>,<i>R</i> isomer
