s-Block metal complexes of superbulky (tBu3Si)2N−: a new weakly coordinating anion?

Abstract

Sterically hindered amide anions have found widespread application as deprotonation agents or as ligands to stabilize metals in unusual coordination geometries or oxidation states. The use of bulky amides has also been advantageous in catalyst design. Herein we present s-block metal chemistry with one of the bulkiest known amide ligands: (tBu₃Si)₂N⁻ (abbreviated: ^tBuN⁻). The parent amine (^tBuNH), introduced earlier by Wiberg, is extremely resistant to deprotonation (even with nBuLi/KOtBu superbases) but can be deprotonated slowly with a blue Cs⁺/e⁻ electride formed by addition of Cs⁰ to THF. (^tBuN)Cs crystallized as a separated ion-pair, even without cocrystallized solvent. As salt-metathesis reactions with (^tBuN)Cs are sluggish and incomplete, it has only limited use as an amide transfer reagent. However, ball-milling with LiI led to quantitative formation of (^tBuN)Li and CsI. Structural characterization shows that (^tBuN)Li is a monomeric contact ion-pair with a relatively short N–Li bond, an unusual T-shaped coordination geometry around N and extremely short Li⋯Me anagostic interactions. Crystal structures are compared with Li and Cs complexes of less bulky amide ligands (iPr₃Si)₂N⁻ (^iPrN⁻) and (Me₃Si)₂N⁻ (^MeN⁻). DFT calculations show trends in the geometries and electron distributions of amide ligands of increasing steric bulk (^MeN⁻ < ^iPrN⁻ < ^tBuN⁻) and confirm that ^tBuN⁻ is a rare example of a halogen-free weakly coordinating anion.