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-(Me 3C)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-(Me 3C)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 AlMe 3, supporting the importance of a Cr(i)/Cr(iii)-based catalytic cycle. © the Partner Organisations 2014.