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 [LnIII(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.