Synthesis and Characterization of Rare-Earth Metal
Complexes Supported by 2‑Imino or Amino Appended Indolyl Ligands
with Diverse Hapticities: Tunable Selective Catalysis for Isoprene
Polymerization
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Abstract
The reaction of 2-(2,6-DippNHCH<sub>2</sub>)C<sub>8</sub>H<sub>5</sub>NH (Dipp = 2,6-<sup><i>i</i></sup>PrC<sub>6</sub>H<sub>3</sub>, C<sub>8</sub>H<sub>5</sub>NH
= indolyl) with 1 equiv
of (Me<sub>3</sub>SiCH<sub>2</sub>)<sub>3</sub>Yb(THF)<sub>2</sub> at room temperature generated mononuclear ytterbium complex <b>1</b> having the indolyl ligands in η<sup>1</sup>:η<sup>1</sup> mode with reduction of Yb<sup>3+</sup> to Yb<sup>2+</sup> and oxidation of the amino to imino group. In the case of Er and
Y, the reactions produced dinuclear complexes <b>2</b> and <b>3</b> having the indolyl ligands in μ-η<sup>2</sup>:η<sup>2</sup>:η<sup>1</sup> modes with the central metals.
When the rare-earth metal is dysprosium, the reaction afforded mixed
ligated dinuclear complex <b>4a</b> having indolyl ligands in
μ-η<sup>5</sup>:η<sup>1</sup>:η<sup>1</sup> and μ-η<sup>6</sup>:η<sup>1</sup>:η<sup>1</sup> modes with Dy, and its isomer <b>4b</b> having the
indolyl ligands only in μ-η<sup>5</sup>:η<sup>1</sup>:η<sup>1</sup> modes with Dy. However, when the rare-earth
metal is Gd, the reaction only produced the mixed ligated dinuclear
gadolinium complex [(μ-η<sup>5</sup>:η<sup>1</sup>:η<sup>1</sup>)-2-(2,6-DippNCH<sub>2</sub>)Ind(μ-η<sup>6</sup>:η<sup>1</sup>:η<sup>1</sup>)-2-(2,6-DippNCH<sub>2</sub>)Ind][Gd(CH<sub>2</sub>SiMe<sub>3</sub>)(thf)]<sub>2</sub> (<b>5</b>), having indolyl ligands in μ-η<sup>5</sup>:η<sup>1</sup>:η<sup>1</sup> and μ-η<sup>6</sup>:η<sup>1</sup>:η<sup>1</sup> modes with Gd. In
addition, treatment of 2-(2,6-DippNHCH<sub>2</sub>)C<sub>8</sub>H<sub>5</sub>NH with 1.25 equiv of (Me<sub>3</sub>SiCH<sub>2</sub>)<sub>3</sub>Gd(THF)<sub>2</sub> produced the alkoxido-bridged trinuclear
gadolinium complex [(μ-η<sup>3</sup>:η<sup>2</sup>:η<sup>1</sup>:η<sup>1</sup>)-2-(2,6-DippNCH<sub>2</sub>)Ind(μ-η<sup>2</sup>:η<sup>1</sup>:η<sup>1</sup>)-2-(2,6-DippNCH<sub>2</sub>)Ind<i>-</i>(η<sup>1</sup>:η<sup>1</sup>)-2-(2,6-DippNCH<sub>2</sub>)Ind]-Gd<sub>3</sub>[(μ<sub>3</sub><i>-</i>O(CH<sub>2</sub>)<sub>5</sub>SiMe<sub>3</sub>)(μ<sub>2</sub>-O(CH<sub>2</sub>)<sub>5</sub>SiMe<sub>3</sub>)(thf)<sub>3</sub>] (<b>6</b>) having
indolyl ligands in η<sup>1</sup>:η<sup>1</sup>, μ-η<sup>2</sup>:η<sup>1</sup>:η<sup>1</sup>, and μ-η<sup>3</sup>:η<sup>2</sup>:η<sup>1</sup>:η <sup>1</sup> modes with metals, respectively. In complex <b>6</b>, sp<sup>2</sup> C–H activation is observed at the 7-indolyl position
producing unique 2-amido substituted indolyl-1,7-dianions having a
μ-η<sup>3</sup>:η<sup>2</sup>:η<sup>1</sup>:η<sup>1</sup> bonding modes with three metals. The O(CH<sub>2</sub>)<sub>5</sub>SiMe<sub>3</sub> arises from the ring-opening
of THF by attack of CH<sub>2</sub>SiMe<sub>3</sub>. Moreover, when
2-(2,6-DippNHCH<sub>2</sub>)C<sub>8</sub>H<sub>5</sub>NH was treated
with 1 equiv of (Me<sub>3</sub>SiCH<sub>2</sub>)<sub>3</sub>Sm(THF)<sub>2</sub>, a dinuclear samarium complex [μ-η<sup>3</sup>:η<sup>1</sup>:η<sup>1</sup>-2-(2,6-DippNCH<sub>2</sub>)Ind]<sub>3</sub>Sm<sub>2</sub>(thf)<sub>3</sub> (<b>7</b>)
having a bridged indolyl ligand in μ-η<sup>3</sup>:η<sup>1</sup>:η<sup>1</sup> hapticities was isolated. All structures
of the complexes have been determined by X-ray crystallographic analyses.
Dinuclear alkyl complexes <b>2</b>–<b>5</b> have
been tested as isoprene polymerization initiators in the presence
of Al<sup><i>i</i></sup>Bu<sub>3</sub> and [Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]. The regioselectivity
for isoprene polymerization is tunable from 1,4-<i>cis</i> (up to 93.5%) to 3,4- (up to 86.2%) selectivity by these catalysts
simply by adjusting the addition order of Al<sup><i>i</i></sup>Bu<sub>3</sub> and [Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]
