Rare-Earth Metalation
of Calix[4]pyrrole/Calix[4]arene
Free of Alkali-Metal Companions
- Publication date
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Abstract
The redox transmetalation/protolysis (RTP) reactions
of ytterbium
or neodymium metal with calix[4]H<sub>4</sub> (5,11,17,23-tetra-<i>tert</i>-butylcalix[4]arene-25,26,27,28-tetrol) in the presence
of bis(pentafluorophenyl)mercury under ultrasonication yielded [Ln<sup>III</sup>(calix[4]H)(thf)]<sub>2</sub> (<b>1</b>, Ln = Yb; <b>2</b>, Ln = Nd). The characterization of both <b>1</b> and <b>2</b>, including an X-ray single-crystal structural determination
for <b>2</b>, suggests triple deprotonation of the macrocyclic
ligand on metalation. The related RTP reaction of H<sub>4</sub>N<sub>4</sub>Et<sub>8</sub> (<i>meso</i>-octaethylcalix[4]pyrrole)
with ytterbium metal and Hg(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> at ambient temperature, however, resulted in quadruple deprotonation
and afforded the ytterbium(II) calix[4]pyrrolide complex [Yb<sub>2</sub>(N<sub>4</sub>Et<sub>8</sub>)(thf)<sub>4</sub>] (<b>3</b>)
in good yield. Subsequent oxidation of <b>3</b> by dioxygen
generated the novel tetranuclear ytterbium(III) complex [Yb<sub>4</sub>(μ-O)<sub>2</sub>(N<sub>4</sub>Et<sub>8</sub>)<sub>2</sub>(thf)<sub>2</sub>] (<b>4</b>). The structures of the ytterbium(II) complex <b>3</b> and the ytterbium(III) complex <b>4</b> incorporate
endo <b>(3)</b> and endo/exo (<b>4</b>) pyrrolide sandwich and half-sandwich
units, respectively, with metal centers η<sup>1</sup> bound
by nitrogen and η<sup>5</sup> 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 [La<sub>2</sub>(HN<sub>3</sub>N′Et<sub>8</sub>)<sub>2</sub>] (<b>5</b>). The
triple deprotonation of the macrocyclic ligand H<sub>4</sub>N<sub>4</sub>Et<sub>8</sub> was also achieved through its reaction with
3 molar equiv of potassium metal, giving colorless crystals of [{K<sub>3</sub>(HN<sub>4</sub>Et<sub>8</sub>)(thf)(PhMe)<sub>2</sub>}<sub>n</sub>] (<b>6</b>). However, an attempt to isolate the corresponding partially
deprotonated calix[4]pyrrolide ytterbium(III) complex through the
metathesis reaction of potassium precursor <b>6</b> with ytterbium
triiodide was unsuccessful
