Combined Experimental and Theoretical Study of Bis(diphenylphosphino)(<i>N</i>‑thioether)amine-Type Ligands in Nickel(II) Complexes
for Catalytic Ethylene Oligomerization
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Abstract
Starting from the new ligands bis(diphenylphosphino)(<i>N</i>-4-(methylthio)phenyl)amine (<b>4</b>, N(PPh<sub>2</sub>)<sub>2</sub>(<i>p-</i>C<sub>6</sub>H<sub>4</sub>)SMe) and its
monosulfide derivative (Ph<sub>2</sub>P)N{P(S)Ph<sub>2</sub>}(<i>p-</i>C<sub>6</sub>H<sub>4</sub>)SMe (<b>4·S</b>),
we have prepared and characterized, including by X-ray crystallographic
studies, their Ni(II) complexes [NiCl<sub>2</sub>{(Ph<sub>2</sub>P)<sub>2</sub>N(<i>p-</i>C<sub>6</sub>H<sub>4</sub>)SMe-<i>P</i>,<i>P</i>}] (<b>5</b>) and [NiCl<sub>2</sub>{(Ph<sub>2</sub>P)N{P(S)Ph<sub>2</sub>}(<i>p-</i>C<sub>6</sub>H<sub>4</sub>)SMe-<i>P</i>,<i>S</i>}]
(<b>6</b>), respectively. The bis-sulfide compound N{P(S)Ph<sub>2</sub>}<sub>2</sub>(<i>p-</i>C<sub>6</sub>H<sub>4</sub>)SMe (<b>4·S</b><sub><b>2</b></sub>) was also prepared
and structurally characterized. Computational studies showed that
the combined influence of stronger P donors and a four-membered-ring <i>P</i>,<i>P</i> chelate leads to complex <b>5</b> being thermodynamically more stable than <b>6</b>, which contains
one weaker PS donor group but a five-membered <i>P</i>,P<i>S</i> chelate ring. For comparison, the bis-chelate
complex [Ni{(Ph<sub>2</sub>P)N{P(S)Ph<sub>2</sub>}(<i>p-</i>C<sub>6</sub>H<sub>4</sub>)SMe-<i>P</i>,<i>S</i>}<sub>2</sub>](BF<sub>4</sub>)<sub>2</sub> (<b>7</b>), the
monochelate complexes [NiBr<sub>2</sub>{(Ph<sub>2</sub>P)N{P(S)Ph<sub>2</sub>}(<i>p-</i>C<sub>6</sub>H<sub>4</sub>)SMe-<i>P</i>,<i>S</i>}] (<b>8</b>) and the Pd(II) analogue
of <b>6</b>, [PdCl<sub>2</sub>{(Ph<sub>2</sub>P)N{P(S)Ph<sub>2</sub>}(<i>p-</i>C<sub>6</sub>H<sub>4</sub>)SMe-<i>P</i>,<i>S</i>}] (<b>9</b>), were synthesized
and structurally characterized and their solution behavior was investigated.
The catalytic activity and selectivity in ethylene oligomerization
of the Ni(II) complexes <b>5</b> and <b>6</b> and their
known <i>N</i>-(methylthio)propyl analogues [NiCl<sub>2</sub>{(Ph<sub>2</sub>P)<sub>2</sub>N(CH<sub>2</sub>)<sub>3</sub>SMe-<i>P</i>,<i>P</i>}] (<b>2</b>) and [NiCl<sub>2</sub>{(Ph<sub>2</sub>P)N{P(S)Ph<sub>2</sub>}(CH<sub>2</sub>)<sub>3</sub>SMe-<i>P</i>,<i>S</i>}] (<b>3</b>), which
were obtained from the bis(diphenylphosphino)(<i>N</i>-(methylthio)propyl)amine
ligand N(PPh<sub>2</sub>)<sub>2</sub>(CH<sub>2</sub>)<sub>3</sub>SMe
(<b>1</b>) and its monosulfide derivative (Ph<sub>2</sub>P)N{P(S)Ph<sub>2</sub>}(CH<sub>2</sub>)<sub>3</sub>SMe (<b>1·S</b>),
respectively, revealed a significant influence of the nature of the <i>N</i>-substituent (aryl vs alkyl thioether) and of the chelate
ring size (<i>P</i>,P vs <i>P</i>,P<i>S</i>). DFT calculations showed that the trend in Δ<i>E</i><sub>rel</sub>, [NiCl<sub>2</sub>(<i>P</i>,<i>P</i>)] > [NiCl<sub>2</sub>(<i>P</i>,P<i>S</i>)] > [NiCl<sub>2</sub>(P<i>S</i>,P<i>S</i>)], results from the stronger covalent character of the
Ni–P vs Ni–S bond. Using AlEtCl<sub>2</sub> as cocatalyst,
mostly ethylene dimers were produced, with significant amounts of
trimers (selectivity in the range 11–36%). Productivities up
to 40400 and 48200 g of C<sub>2</sub>H<sub>4</sub>/((g of Ni) h),
with corresponding TOF values of 84800 and 101100 mol of C<sub>2</sub>H<sub>4</sub>/ ((mol of Ni) h), were obtained with precatalysts <b>2</b> and <b>3</b>, respectively
