Chromium(III) scorpionate complexes supported by tris(2-pyridyl)phosphine-derived ligands: synthesis, structure, and electrochemical and catalytic properties.

Abstract

The first examples of chromium(III) scorpionate-type complexes supported by tris(2-pyridyl)phosphine-derived ligands are reported. Reactions of [Cr(THF)₃Cl₃] with tris(2-pyridyl)phosphine (Py₃P) and its chalcogen derivatives, Py₃PS and Py₃PSe, afforded the scorpionates [Cr(Py₃PX)Cl₃] (X = none, S, Se), featuring facial N,N',N″-coordination of the ligand. In contrast, reaction with tris(2-pyridyl)phosphine oxide (Py₃PO) results in an unexpected ligand transformation, yielding the phosphinite complex [Cr(Py₂PO₂)(H₂O)Cl₂] containing the in situ generated [Py₂P(O)O]^- ligand. X-Ray diffraction analysis established the molecular structures of the complexes, while EPR spectroscopy confirmed the high-spin state of the Cr(III) centres and revealed pronounced rhombic distortion of octahedral Cr³⁺ ions for the phosphinite derivative. Cyclic voltammetry demonstrates that the PX functionality significantly influences the redox behaviour of the complexes. Preliminary catalytic studies show that only the phosphine complex [Cr(Py₃P)Cl₃] is active in ethylene oligomerization and polymerization. Notably, replacing the co-catalyst from MAO to Et₃Al dramatically switches the reaction outcome from selective oligomerization to exclusive polymerization. These results establish tris(2-pyridyl)phosphine-derived ligands as a new platform for Cr(III) scorpionate chemistry and provide insight into their structural and functional properties.