Synthesis and structure of lithium amides and solvated derivatives containing bulky bis(silylamide) ligands

Abstract

Dimeric, non-solvated lithium amides containing the sterically crowded silylamide ligands {Li[N(SiMe 3)(SiPh 2 t Bu)]} 2 ( 1) and {Li[N(SiMe 3)(SiPh 3)]} 2 ( 2) have been synthesized by the reaction of the secondary amines with n-butyl lithium in hexane. Addition of excess donor solvents to hydrocarbon solutions of 1 yields Li[N(SiMe 3)(SiPh 2 t Bu)](L) 2, where L = thf ( 3), py ( 4), and hmpa ( 5), while pyridine addition to 2 produces Li[N(SiMe 3)(SiPh 3)](py) 2 ( 6). A related lithium amide, Li{N(SiMe 3)[Si(SiMe 3) 3]}(py) 2 ( 7), was prepared from the treatment of the in situ generated dimeric compound with excessive pyridine. In a less-obvious synthesis, reaction of the lithium amide LiN(ad)(SiMe 3) (ad = adamantyl) with 0.5 equiv CaI 2 in thf did not yield the expected calcium bis(amide), but rather the unexpected product [(ad)(SiMe 3)N] 2Li 3(μ 3-I)(thf) 2 ( 8) was produced. All of the complexes have been fully characterized by 1H and 13C NMR spectra, infrared spectroscopy, elemental analyses, and single crystal X-ray diffraction. The structures of 1 and 2 were comprised of four-membered rings with amido ligands bridging two lithium atoms. The structures of 3– 7 feature monomeric molecules with three-coordinate lithium centers. Compound 8 exhibited an unusual structure with two four-membered rings fused by a Li–I bond edge. The μ 3-I atom bridged three lithium atoms with two significantly different I–Li bond lengths of 2.742(9) (average) and 3.12(1) Å.