<i>N</i>‑Phosphanyl- and <i>N</i>,<i>N</i>′‑Diphosphanyl-Substituted
N‑Heterocyclic Carbene Chromium Complexes: Synthesis, Structures,
and Catalytic Ethylene Oligomerization
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Abstract
The chromium(II) complexes [CrCl<sub>2</sub>(<sup><i><b>t</b></i><b>‑Bu</b></sup><b>NHC,P</b>-κ<i>C</i>)<sub>2</sub>]
(<b>1</b>), [CrCl<sub>2</sub>(<sup><b>Mes</b></sup><b>NHC,P</b>-κ<i>C</i>)<sub>2</sub>] (<b>2</b>), [CrCl<sub>2</sub>(<sup><b>Dipp</b></sup><b>NHC,P</b>-κ<i>C</i>)<sub>2</sub>]
(<b>3</b>), and [CrCl<sub>2</sub>(<b>P,NHC,P</b>-κ<i>C</i>)<sub>2</sub>] (<b>4</b>) containing the <i>N</i>-phosphanyl- or <i>N,N</i>′-diphosphanyl-substituted
N-heterocyclic carbene (NHC) hybrid ligands <sup><i><b>t</b></i><b>‑Bu</b></sup><b>NHC,P</b> (1-(di-<i>tert</i>-butylphosphino)-3-<i>tert</i>-butylimidazol-2-ylidene), <sup><b>Mes</b></sup><b>NHC,P</b> (1-(di-<i>tert</i>-butylphosphino)-3-mesitylimidazol-2-ylidene), <sup><b>Dipp</b></sup><b>NHC,P</b> (1-(di-<i>tert</i>-butylphosphino)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene),
and <b>P,NHC,P</b> (1,3-bis(di-<i>tert</i>-butylphosphino)imidazol-2-ylidene),
respectively, were prepared from Cr<sup>II</sup> ([CrCl<sub>2</sub>(thf)<sub>2</sub>]) or Cr<sup>III</sup> ([CrCl<sub>3</sub>(thf)<sub>3</sub>] or [Cr(Me)Cl<sub>2</sub>(thf)<sub>3</sub>]) precursors.
The solid-state structures of these four complexes show square-planar
Cr<sup>II</sup> centers, with two trans chloride and two monodentate
C<sub>NHC</sub> donors. Alkylation of <b>3</b> and <b>4</b> with [Mg(benzyl)<sub>2</sub>(thf)<sub>2</sub>] led to the formation
of the σ complexes [Cr(benzyl)<sub>3</sub>(<sup><b>Dipp</b></sup><b>NHC,P</b>-κ<i>C</i>,κ<i>P</i>)] (<b>5</b>) and [Cr(benzyl)<sub>3</sub>(<b>P,NHC,P</b>-κ<i>C</i>,κ<i>P</i>)] (<b>6</b>), respectively, with five-coordinate distorted-square-pyramidal
Cr<sup>III</sup> coordination, comprising a chelating ligand through
the C<sub>NHC</sub> and one P donor and three benzyl groups. These
complexes were used as precatalysts in ethylene oligomerization, and
it was found that the nature of the cocatalyst used and the metal
oxidation state have a remarkable influence on the catalytic properties.
The Cr<sup>III</sup>/MAO systems displayed superior catalytic performance
(TOF values up to 16320 mol of C<sub>2</sub>H<sub>4</sub>/((mol of
Cr) h) for <b>6</b>) and gave mostly oligomers. Interestingly,
the oligomers obtained with complex <b>3</b> were almost exclusively
1-hexene and 1-butene when the reaction was initiated at 30 °C.
The overall activities and selectivities were also affected by the
initial reaction temperature and the nature of the solvent. With AlEtCl<sub>2</sub> (EADC) as cocatalyst, polyethylene was predominately formed
