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

Amine elimination of scandium tris­(silylamide) complex
Sc­[N­(SiHMe2)2]3(THF) with 1 equiv
of the amidine
[PhC­(N-2,6-iPr2C6H3)2]H in toluene afforded the neutral mono­(amidinate)
scandium bis­(silylamide) complex [PhC­(N-2,6-iPr2C6H3)2]­Sc­[N­(SiHMe2)2]2 (1) in 93% isolated
yield. When 1 was activated with 1 equiv of [Ph3C]­[B­(C6F5)4] in the presence of
THF, the unexpected cationic amidinate scandium amide complex [{PhC­(N-2,6-iPr2C6H3)2}­ScN­{SiHMe2}­{SiMe2N­(SiHMe2)2}­(THF)2]­[B­(C6F5)4] (2) was generated. Treatment of 1 with excess AlMe3 gave the Sc/Al heterometallic methyl
complex [PhC­(N-2,6-iPr2C6H3)2]­Sc­[(μ-Me)2AlMe2]2 (3). All these complexes were well-characterized
by elemental analysis, NMR spectroscopy, and X-ray crystallography.
The combination 1/[Ph3C]­[B­(C6F5)4] in toluene showed activity toward isoprene
polymerization. Addition of excess AlMe3 to the 1/[Ph3C]­[B­(C6F5)4] catalyst
system switched the regioselectivity of isoprene polymerization from
3,4-specific to cis-1,4-selective.