Reactivity of Mg(AlMe4)2 towards neutral tris(pyrazolyl)alkanes

Abstract

Various new tris(pyrazolyl)alkanes of the class R′CTp^3-R (R′ = Me, Et, nPr, iBu; R = Et, cyPr, Cy, p-tBuPh, Ph; cyPr = cyclopropyl, Cy = cyclohexyl, p-tBuPh = para-tert-butylphenyl) were synthesised and their reactivity towards Mg(AlMe₄)₂ was examined. Along with new examples of recurring structural motifs, such as separated ion pairs and “metal in a box” complexes, e.g., [(MeCTp^3-Et)₂Mg][AlMe₄]₂, several magnesium complexes with new structural features/compositions were obtained. Treatment of the “metal in a box” species [(MeCTp^3-R)Mg][AlMe₄]₂ with THF donor gave the terminal methyl complex [(MeCTp^3-cyPr)MgMe(thf)₂][AlMe₄]. Variation of the backbone alkyl substituent R′ in the tris(pyrazolyl)alkane R′CTp^3-Ph gave ionic liquids (R′ = Et, nPr) and the methyl-bridged dimagnesium complex [({iBuCTp^3-Ph}Mg{AlMe₄})₂(μ-Me)][AlMe₄]. The bulky Cy and p-tBuPh moieties at the pyrazolyl 3 position gave the new structural motif [MeCTp^3-RMg(ηⁿ-AlMe₄)][AlMe₄] (η³, R = Cy; η², R = p-tBuPh), stabilising a “[Mg(AlMe₄)]⁺” entity. The AlMe₃ group can be reversibly displaced under reduced pressure affording the new separated ion pair [(MeCTp^3-p-tBuPh)MgMe][AlMe₄] with a terminal “[MgMe]⁺” moiety. Moderate thermal treatment of both [(MeCTp^3-p-tBuPh)MgMe][AlMe₄] and [(MeCTp^3-p-tBuPh)Mg(η²-AlMe₄)][AlMe₄] resulted in selective C–H-bond activation in the 5 position of one of the pyrazolyl moieties and the formation of an AlMe₃-modified anionic tris(pyrazolyl)alkane and hence the neutral complex [MeC(pz^3-p-tBuPh)₂(pz^{3-p-tBuPh,5-AlMe₃})]MgMe.