Yttrium Complexes Featuring Different Y–C Bonds. Comparative Reactivity Studies: Toward Terminal Imido Complexes

The
reactions of 2,6-diisopropylaniline with equimolar amounts of alkyl–heteroaryl
yttrium complexes containing Y–C­(sp3, alkyl) along
with Y–C­(sp2, heteroaryl) bonds resulting from intramolecular
C–H bond activation of the amido–pyridinate ligands
[NNOBzFur]­YCH2SiMe3(THF)2, [NNSBzTh]­YCH2SiMe3(THF)2, and [NNSEtTh]­YCH2SiMe3(THF)2 have been scrutinized with the aim of synthesizing yttrium
terminal imido species. These reactions occur at ambient temperature
with the protonolysis of the Y–C­(sp3, alkyl) bond,
thus affording anilido–heteroaryl species and maintaining the
residual Y–C­(sp2, heteroaryl) bond untouched. However,
the subsequent transformation of the as-synthesized anilido–heteroaryl
complexes depends on the nature of the substituent on the 6-position
of the pyridyl ring. In the case of the benzofuryl yttrium derivative
[NNOBzFur]­YNH-2,6-iPr2C6H3(THF)2 (4), heating to 50 °C
results in benzofuran ring opening with the formation of an anilido
species supported by a dianionic amido–yne–phenolate
ligand framework, [NNCCO]­YNH-2,6-iPr2C6H3(THF) (6). In contrast, a
complex containing a benzothiophenyl moiety, [NNSBzTh]­YNH-2,6-iPr2C6H3(THF)2 (7), slowly undergoes protonation of the Y–C­(sp2, heteroaryl) bond and a ligand redistribution reaction takes
place, affording an yttrium bis­(anilido) species supported by a monoanionic
amido–pyridinate ligand featuring intramolecular Y–S
heteroaryl coordination, [NNSBzTh]­Y­[NH-2,6-iPr2C6H3]2 (9). It is worth noting that an yttrium complex containing α-thiophenyl
fragment, [NNSEtTh]­YNH-2,6-iPr2C6H3(THF)2 (10), turned out
to be extraordinarily robust and no transformation was ever detected
even upon heating the complex at 100 °C for prolonged times.