Unusual Si–H Bond
Activation and Formation
of Cationic Scandium Amide Complexes from a Mono(amidinate)-Ligated
Scandium Bis(silylamide) Complex and Their Performance in Isoprene
Polymerization
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Abstract
Amine elimination of scandium tris(silylamide) complex
Sc[N(SiHMe<sub>2</sub>)<sub>2</sub>]<sub>3</sub>(THF) with 1 equiv
of the amidine
[PhC(N-2,6-<sup><i>i</i></sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>]H in toluene afforded the neutral mono(amidinate)
scandium bis(silylamide) complex [PhC(N-2,6-<sup><i>i</i></sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>]Sc[N(SiHMe<sub>2</sub>)<sub>2</sub>]<sub>2</sub> (<b>1</b>) in 93% isolated
yield. When <b>1</b> was activated with 1 equiv of [Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] in the presence of
THF, the unexpected cationic amidinate scandium amide complex [{PhC(N-2,6-<sup><i>i</i></sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>}ScN{SiHMe<sub>2</sub>}{SiMe<sub>2</sub>N(SiHMe<sub>2</sub>)<sub>2</sub>}(THF)<sub>2</sub>][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] (<b>2</b>) was generated. Treatment of <b>1</b> with excess AlMe<sub>3</sub> gave the Sc/Al heterometallic methyl
complex [PhC(N-2,6-<sup><i>i</i></sup>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>2</sub>]Sc[(μ-Me)<sub>2</sub>AlMe<sub>2</sub>]<sub>2</sub> (<b>3</b>). All these complexes were well-characterized
by elemental analysis, NMR spectroscopy, and X-ray crystallography.
The combination <b>1</b>/[Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] in toluene showed activity toward isoprene
polymerization. Addition of excess AlMe<sub>3</sub> to the <b>1</b>/[Ph<sub>3</sub>C][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] catalyst
system switched the regioselectivity of isoprene polymerization from
3,4-specific to cis-1,4-selective
