Coordination chemistry of sterically encumbered pyrrolyl ligands to chromium(ii): mono(pyrrolyl)chromium and diazachromocene formation

Abstract

A series of diazachromocenes with sterically demanding pyrrolyl ligands, 2,5-(Me₃C)₂C₄H₂N (1), 2,5-(Me₃C)₂-3,4-Me₂C₄N (2) and 2,3,5-(Me₃C)₃C₄HN (3), was prepared and investigated by various spectroscopic techniques, and in some cases by X-ray diffraction and magnetic susceptibility studies. The diazachromocenes exhibit an S = 1 ground state; no indication of a spin-equilibrium was obtained. With the same ligands mono(pyrrolyl)chromium(II) complexes are accessible, [{(κN-2,5-(Me₃C)₂C₄H₂N)Cr(thf)}₂(μ-Cl)₂] (1-CrCl(thf)), [(κN-2,5-(Me₃C)₂-3,4-Me₂-C₄H₂N)Cr(Cl)(tmeda)] (2-CrCl(tmeda) and [(η⁵-2,3,5-(Me₃C)₃C₄HN)Cr(μ-Cl)₂] (3-CrCl), which show either η⁵- or η¹-(κN) coordination depending on the substitution pattern. ¹H NMR spectroscopy serves as a valuable tool to distinguish between these coordination modes. The Cr(II) atoms in the mono(pyrrolyl) complexes adopt a high spin configuration (S = 2) and in dimeric species antiferromagnetic coupling between the spin carriers was observed. However, none of these mono(pyrrolyl)chromium complexes is an effective or selective ethylene oligomerization catalyst on activation with MAO or AlMe₃, supporting the importance of a Cr(I)/Cr(III)-based catalytic cycle.