Structural Characterization of Lithium and Sodium Bulky Bis(silyl)amide Complexes

Abstract

Alkali metal amides are vital reagents in synthetic chemistry and the bis(silyl)amide {N(SiMe₃)₂} (N′′) is one of the most widely-utilized examples. Given that N′′ has provided landmark complexes, we have investigated synthetic routes to lithium and sodium bis(silyl)amides with increased steric bulk to analyse the effects of R-group substitution on structural features. To perform this study, the bulky bis(silyl)amines {HN(Si^tBuMe₂)(SiMe₃)}, {HN(SiⁱPr₃)(SiMe₃)}, {HN(Si^tBuMe₂)₂}, {HN(SiⁱPr₃)(Si^tBuMe₂)} and {HN(SiⁱPr₃)₂} (1) were prepared by literature procedures as colourless oils; on one occasion crystals of 1 were obtained. These were treated separately with ⁿBuLi to afford the respective lithium bis(silyl)amides [Li{μ-N(Si^tBuMe₂)(SiMe₃)}]₂ (2), [Li{μ-N(SiⁱPr₃)(SiMe₃)}]₂ (3), [Li{N(Si^tBuMe₂)₂}{μ-N(Si^tBuMe₂)₂}Li(THF)] (4), [Li{N(SiⁱPr₃)(Si^tBuMe₂)}(DME)] (6) and [Li{N(SiⁱPr₃)₂}(THF)] (7) following workup and recrystallization. On one occasion during the synthesis of 4 several crystals of the ‘ate’ complex [Li₂{μ-N(Si^tBuMe₂)₂}(μ-ⁿBu)]₂ (5) formed and a trace amount of [Li{N(SiⁱPr₃)₂}(THF)₂] (8) was identified during the recrystallization of 7. The reaction of {HN(Si^tBuMe₂)₂} with NaH in the presence of 2 mol % of NaO^tBu gave crystals of [Na{μ-N(Si^tBuMe₂)₂}(THF)]₂ (9-THF), whilst [Na{N(SiⁱPr₃)₂}(C₇H₈)] (10) was prepared by deprotonation of 1 with ⁿBuNa. The solid-state structures of 1⁻10 were determined by single crystal X-ray crystallography, whilst 2⁻4, 7, 9 and 10 were additionally characterized by NMR and FTIR spectroscopy and elemental microanalysis.

Keywords: bis(silyl)amide; ligand; lithium; sodium.