New Alkali Metal Primary Amide Ladder Structures Derived fromtBuNH2: Building Cisoid and Transoid Ring Conformations into Ladder Frameworks

Abstract

Two novel alkali metal amide ladder complexes have been synthesised and crystallographically characterised. Derived from the same primary amine precursor (tBuNH 2 ), they represent important additions to the series of ladder arrangements previously established within secondary amide chemistry. Thus the sodium amide´amine complex [{[tBuN(H)Na] 3´H2 NtBu} I ] forms an infinite wavelike ladder structure. Covering three nitrogen ± sodium rungs, its curved sections display a cisoid conformation of amide substituents; but where these curved sections fuse, a transoid conformation is found. Every third sodium cation along the ladder framework is ligated by a tert-butylamine solvent molecule. In contrast, the heterobimetallic derivative, [{[tBuN(H)] 2 LiNat meda} 2 ], adopts a finite oligomeric ladder structure limited to only four nitrogen ± metal rungs in length. The central rungs contain lithium, while the outer rungs contain sodium. As in the all-sodium structure, the ladder is curved; there is a mixture of cisoid and transoid ring conformations within its framework. TMEDA solvent molecules complete the structure by chelating the sodium cations at the ladder ends.

Developing a Hetero‐Alkali‐Metal Chemistry of 2,2,6,6‐Tetramethylpiperidide (TMP): Stoichiometric and Structural Diversity within a Series of Lithium/Sodium, Lithium/Potassium and Sodium/Potassium TMP Compounds

“…Multicomponent TMP bases generally have an alkali-metal–non-alkali-metal (e.g., Mg, Zn, Al, Mn) combination integrated with ligands in molecular assemblies. Surprisingly, there appears to have been no progress made in trying to merge ideas from each era, that is in constructing multicomponent TMP complexes8 based on mixed alkali-metal–alkali-metal pairings, a fact made all the more perplexing because hetero-alkali-metal imide,9 alkoxide,10 primary amide,11 heteroanionic alkoxide/primary amide12 and other secondary amide13 complexes have all received attention. The excellent recent studies of O’Shea et al have implicated TMP in a hetero-alkali-metal base for performing selective vinyl14 and alkyl/aryl15 deprotonation in a series of substituted toluenes, albeit using a heteroanionic alkyl/alkoxide/amide base.…”

Applications of the Laddering Principle — A Two-Stage Approach to Describe Lithium Heterocarboxylates

“…Laddered structures generally exhibit curvature, twisting, or other deviations from planarity in order to accommodate the steric demands of the substituents. The terminology to describe this effect in primary and secondary lithium amides was first introduced by Mulvey,5 and focused on the relative orientation of substituents about the Li 2 N 2 rings. We have adopted a natural extension of these terms, using cisoid and transoid to describe arrays of any three contiguous four‐membered rings as well (Figure 2).…”

“…Where laddering is limited to dimerization by bulky substituents and/or solvation, a transoid geometry is most often observed,7 as would be expected on steric grounds. However, in ladders with three or more rungs there is often a preference for regions of cisoid geometry, either exclusively or intermixed with transoid regions 5,6,8. Presumably this is because a cisoid geometry sometimes provides a greater separation of substituents that are two or more rungs apart than would the corresponding transoid arrangement.…”

Mixed Lithium Complexes: Structure and Applicationin Synthesis

“…THF molecules complete the coordination sphere of each metal: one bonding to the Li and two to the larger Na atom. When the less sterically demanding primary amide t BuNH − is employed, the nuclearity increases, as now a tetranuclear dimer [(TMEDA) 2 ⋅Li 2 Na 2 {μ-N(H) t Bu} 4 ] 4 ensues, which adopts a ladder-shaped motif [12]. Its molecular structure consists of two N-Li-N-Na rings that conjoin via a central N-Li-N-Li ring and the Na atoms are each bound to a TMEDA ligand ( Figure 5 [13].…”

“…During this work, the structures of three lithium-sodium TMP [11][12][13]. The crystallographically characterized structure of a mixed unsolvated Li and Na alkoxide [Li 4 Na 4 (μ-O t Bu) 8 ] 9 has been reported and due to its eye-catching distinctive shape ( Figure 5.5), it has been referred to as a molecular breastplate.…”