New structural motifs in chromium(iii) calix[4 and 6]arene chemistry

Abstract

The reactions of either p-tert-butylcalix[4]areneH₄ [Cax(OH)₄], 1,3-dimethoxy-p-tert-butylcalix[4]areneH₂ [Cax(OH)₂(OMe)₂] or p-tert-butylcalix[6]areneH₆ [Cax[6](OH)₆] with alkali metal cations and the chromium complexes [CrCl₃(THF)₃] or [CrCl₂(THF)] (THF = tetrahydrofuran) are described. In particular, the treatment of [Cax(OH)₄] with in situ generated M[Cr(Ot-Bu)₄] (M = Li or K) affords, after work-up, the complex {[CrCax(O)₄(NCMe)]₂[Li(THF)]₂[Li(NCMe)]₂Cl₂(μ₄-O)}·2.25(MeCN) (1) and two forms of {K(MeCN)_x[CrCax(O)₄(NCMe)][CrCax(OH)(O)₃(NCMe)]}·MeCN (x = 3 or 4) (2), respectively. The potassium salt of [Cax(OH)₄] reacts with [CrCl₂(THF)] to afford a highly sensitive complex, which on exposure to air gives rise to the compound {[CrCax(O)₄K(THF)₂]₂H(μ-O)(H₂O)K(THF)(H₂O)(μ-THF)}₂·6THF (3), that possesses four chromium calixarene units, two sets of K⁺ ions that are coordinated in two very different environments together with terminal and bridging THF ligands. The reaction of n-BuLi with [Cax(OH)₄], followed by the addition of [CrCl₃(THF)₃] affords, after work-up, the complex {Cr[μ-Cax(OH)(O)₃](NCMe)}₂·4Me₂CO·0.5H₂O (4). Complex 4 is also available via the hydrolysis of complex 1. Interaction of [Cax(OH)₂(OMe)₂] with NaH or n-BuLi in THF affords, following the addition of [CrCl₃(THF)₃], the demethylated dimer {Cr[μ-Cax(OH)(O)₃](NCMe)}₂·4MeCN (5), which is a different solvate of 4, or the monomeric salt {Cax(O)₂(OMe)₂CrCl₂[Li(NCMe)₂]}·2MeCN (6), respectively. The reaction of 1,3-dipropoxy-p-tert-butylcalix[4]areneH₂, [Cax(OH)₂(On-Pr)₂], with n-BuLi followed by [CrCl₃(THF)₃] affords the monomeric complex [CrCl₂Cax(O)₂(On-Pr)₂Li(NCMe)₂]·3MeCN (7). Interaction of the mono-methoxy ligand [Cax(OH)₃(OMe)] with n-BuLi in diethyl ether followed by addition of [CrCl₃(THF)₃] in THF affords, upon work-up, the complex [CrCax(OMe)(O)₃(NCMe)]₂·MeCN (8). Interaction of p-tert-butylcalix[6]areneH₆ [Cax[6](OH)₆] with Na[Cr(Ot-Bu)₄] (formed in situ from [CrCl₃(THF)₃] and excess NaOt-Bu) affords the complex {CrCax[6](O)₃(OH)₃Na(NCMe)₂(μ-OH)}₂·4MeCN (9). Complexes 1–9 have been structurally characterised; for 3 and 8, it was necessary to use synchrotron radiation. Screening of 1–9 for ethylene polymerisation using either methylaluminoxane (MAO) or dimethylaluminium chloride (DMAC) as co-catalyst afforded only very poor catalytic activities (≤ 1 g mmol⁻¹ hr⁻¹bar⁻¹).