Rare-earth metalation of calix[4]pyrrole/calix[4]arene free of alkali-metal companions

The redox transmetalation/protolysis (RTP) reactions of ytterbium or neodymium metal with calix[4]H₄ (5,11,17,23-tetra-tert-butylcalix[4]arene-25,26,27,28-tetrol) in the presence of bis(pentafluorophenyl)mercury under ultrasonication yielded [Ln^III(calix[4]H)(thf)]₂ (1, Ln = Yb; 2, Ln = Nd). The characterization of both 1 and 2, including an X-ray single-crystal structural determination for 2, suggests triple deprotonation of the macrocyclic ligand on metalation. The related RTP reaction of H₄N₄Et₈ (meso-octaethylcalix[4]pyrrole) with ytterbium metal and Hg(C₆F₅)₂ at ambient temperature, however, resulted in quadruple deprotonation and afforded the ytterbium(II) calix[4]pyrrolide complex [Yb₂(N₄Et₈)(thf)4] (3) in good yield. Subsequent oxidation of 3 by dioxygen generated the novel tetranuclear ytterbium(III) complex [Yb₄(μ-O)₂(N₄Et₈)₂(thf)₂] (4). The structures of the ytterbium(II) complex 3 and the ytterbium(III) complex 4 incorporate endo (3) and endo/exo (4) pyrrolide sandwich and half-sandwich units, respectively, with metal centers η¹ bound by nitrogen and η⁵ bonded by pyrrolide rings. The RTP reaction of lanthanum metal using diphenylmercury in place of bis(pentafluorophenyl)mercury gave the triply deprotonated and N-confused pyrrolide (with an alkyl substituent of one pyrrolide ring migrated to a β-position) macrocyclic complex [La₂(HN₃N'Et₈)₂] (5). The triple deprotonation of the macrocyclic ligand H4N4Et8 was also achieved through its reaction with 3 molar equiv of potassium metal, giving colorless crystals of [{K₃(HN₄Et₈)(thf)(PhMe)₂}_n] (6). However, an attempt to isolate the corresponding partially deprotonated calix[4]pyrrolide ytterbium(III) complex through the metathesis reaction of potassium precursor 6 with ytterbium triiodide was unsuccessful.