CNC-Pincer Rare-Earth Metal Amido Complexes with a
Diarylamido Linked Biscarbene Ligand: Synthesis, Characterization,
and Catalytic Activity
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Abstract
In
preparation of CNC-pincer rare-earth metal amido complexes with
a diarylamido linked biscarbene ligand, it is found that conditions
have a key influence on final products. Reaction of a THF suspension
of bis[2-(3-benzylimidazolium)-4-methylphenyl]amine
dichlorides (H<sub>3</sub><b>L</b>Cl<sub>2</sub>) with [(Me<sub>3</sub>Si)<sub>2</sub>N]<sub>3</sub>RE(μ-Cl)Li(THF)<sub>3</sub> (RE = Yb, Eu, Sm) in THF at room temperature afforded the
only unexpected fused-heterocyclic compound 8,9-dibenzyl-3,14-dimethyl-8<i>a</i>,9-dihydro-8<i>H</i>-benzo[4,5]imidazo[2′,1′:2,3]imidazo[1,2-<i>a</i>]imidazo[2,1-<i>c</i>]quinoxaline
(<b>1</b>) containing an imidazolyl ring and a piperidyl ring,
which formed through carbene C–C and C–N coupling. However,
the reaction of H<sub>3</sub><b>L</b>Cl<sub>2</sub> with [(Me<sub>3</sub>Si)<sub>2</sub>N]<sub>3</sub>Er(μ-Cl)Li(THF)<sub>3</sub> in toluene afforded the CNC-pincer erbium amido complex incorporating
a diarylamido linked biscarbene ligand <b>L</b>Er[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> (<b>2</b>) in low yield and
the above fused-heterocyclic compound <b>1</b>. The stepwise
reaction of H<sub>3</sub><b>L</b>Cl<sub>2</sub> with strong
bases (<i>n</i>-BuLi or LiCH<sub>2</sub>SiMe<sub>3</sub>) in THF for 4 h, followed by treatment with [(Me<sub>3</sub>Si)<sub>2</sub>N]<sub>3</sub>RE(μ-Cl)Li(THF)<sub>3</sub>, generated zwitterion complexes [<b>L</b><sub>2</sub>RE][RECl{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] (<b>L</b> = [4-CH<sub>3</sub>-2-{(C<sub>6</sub>H<sub>4</sub>CH<sub>2</sub>-[N(CH)<sub>2</sub>CN]}C<sub>6</sub>H<sub>3</sub>]<sub>2</sub>N; RE = Y (<b>3</b>), Er (<b>4</b>), Yb (<b>5</b>)) in less than 20% yields together with fused-heterocyclic
compound <b>1</b>. Additionally, the reaction of H<sub>3</sub><b>L</b>Cl<sub>2</sub> with 6 equiv of NaN(SiMe<sub>3</sub>)<sub>2</sub> in THF for 4 h, followed by treatment with YbCl<sub>3</sub>, generated a novel discrete complex [<b>L</b><sub>2</sub>Yb][{Na(μ-N(SiMe<sub>3</sub>)<sub>2</sub>)}<sub>5</sub>(μ<sub>5</sub>-Cl)] (<b>6</b>). The one-pot
reaction of H<sub>3</sub><b>L</b>Cl<sub>2</sub> with <i>n</i>-BuLi, followed by reaction with [(Me<sub>3</sub>Si)<sub>2</sub>N]<sub>3</sub>RE(μ-Cl)Li(THF)<sub>3</sub> in THF at −78 °C, generated the CNC-pincer lanthanide
bisamido complexes <b>L</b>RE[N(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> (RE = Er (<b>2</b>), Y (<b>7</b>), Sm (<b>8</b>), Eu (<b>9</b>)) in moderate yields.
These kinds of biscarbene supported pincer bisamido complexes could
also be prepared by a one-pot reaction of bis(imidazolium) salt (H<sub>3</sub><b>L</b>Cl<sub>2</sub>) with 5 equiv of NaN(SiMe<sub>3</sub>)<sub>2</sub>, followed by treatment with RECl<sub>3</sub>, in good yields at −78 °C. Investigation of the catalytic
activity of complexes <b>2</b> and <b>7</b>–<b>9</b> indicated that all complexes showed a high activity toward
the addition of terminal alkynes to carbodiimides producing propiolimidines,
which represents the first example of rare-earth metal CNC-pincer-type
catalysts applied for catalytic C–H bond addition of terminal
alkynes to carbodiimides at room temperature
