Reactivity of Organothorium Complexes with TEMPO

Reactions
of the 2,2,6,6-tetramethylpiperidin-1-oxyl radical (TEMPO)
with thorium metallocenes have been examined to investigate both the
radical reaction patterns for organothorium complexes and the coordination
chemistry of TEMPO with thorium. (η5-C5Me5)2ThMe2 reacts with 2 equiv of
TEMPO to generate 1-methoxy-2,2,6,6-tetramethylpiperidine (Me-TEMPO)
and (η5-C5Me5)2ThMe­(η1-TEMPO), which contains a TEMPO– anion coordinated
to thorium through oxygen only. (η5-C5Me5)2Th­(η1-C3H5)­(η3-C3H5), synthesized
from (η5-C5Me5)2ThBr2 and (C3H5)­MgBr, reacts with
2 equiv of TEMPO to form 1-(2-propen-1-yloxy)-2,2,6,6-tetramethylpiperidine
(allyl-TEMPO) and (η5-C5Me5)2Th­(η1-C3H5)­(η1-TEMPO). Although bis­(TEMPO) metallocenes were not obtained
in these reactions, the methyl group in (η5-C5Me5)2ThMe­(η1-TEMPO)
is reactive with 1 equiv of CuBr to form (η5-C5Me5)2ThBr­(η1-TEMPO).
The bis­(TEMPO) metallocene (η5-C5Me5)2Th­(η1-TEMPO)2 is
accessible in the reaction of [(η5-C5Me5)2ThH2]2 with 4 equiv of
TEMPO. In contrast, (η5-C5Me5)2ThBr2 reacts with 2 equiv of TEMPO by loss
of C5Me5 to form (C5Me5)2 and (η2-TEMPO)2ThBr2, in which the TEMPO– anions bind through
oxygen and nitrogen. The bromide ions in (η2-TEMPO)2ThBr2 can be replaced by an additional 2 equiv
of TEMPO in the presence of 2 equiv of KC8 to form the
per­(TEMPO) complex Th­(η1-TEMPO)2(η2-TEMPO)2. ThBr4(THF)4 reacts
with TEMPO to form ThBr4(THF)2(η1-TEMPO), which contains an oxygen-bound TEMPO radical. The Th3+ complex (η5-C5Me4H)3Th is oxidized in the presence of TEMPO, without ligand
loss, to afford the Th4+ species (η5-C5Me4H)3Th­(η1-TEMPO).
The reactions show that TEMPO can react with organothorium complexes
in several ways including coordination, anion substitution, and cyclopentadienyl
replacement.