Lithium amides ring laddering

Scheme 3.6-2. Homo-aggregation of lithium amide rings showing the general preference for ladder, as opposed to stack, structures.

Considering the importance of alkali metal phosphanides it is not surprising that numerous review articles have dealt with this subject [34-36]. The solid state and solution structures vary from dimers with central M2 P2 cycles to larger rings and from chain to ladder structures as described for the lithium amides (see Sections 3.6.1 and 3.6.2). Cage compounds in the field of lithium phosphanides are unusual... 

Dimeric and higher aggregate lithium amides can generally be classified into the coordination motifs illustrated in Scheme 2.2. The four-membered (LiN)2 ring is ubiquitous in lithium amide chemistry and is observed both in discrete dimeric structures in either planar (Scheme 2.2, A) or non-planar (Scheme 2.2, B) geometries as well as in oligomeric and polymeric (ladder) frameworks (Scheme 2.2, C). Trimeric six-membered (LiN), ring... 

III. Simple Lithium Amides (Amidolithiums) and Their Complexes Ring Laddering... 

Fig. 26. Structural types for uncomplexed lithium amides (RR NLi) .- (a) dimeric ring (n = 2), showing the projection of R,R groups above and below the ring plane (b) rings with n = 2,3, and 4 (c) further association of rings into ladders.

The major structural types found for lithium amide complexes in the solid state are illustrated in Fig. 34. These comprise ladders of limited extent when the L Li ratio is less than 1 1 (Fig. 34a), dimeric (NLi)2 rings, when this ratio is 1 1 and, usually, when the complexants are monodentate (Fig. 34b), and monomers, both contact-ion pairs (CIPs) and solvent-separated ion pairs (SSIPs) (Fig. 34c). Monomers occur always when there are two or more monodentate complexants per Li. This also is usual with bidentate ligands, and is always found when the ligands have higher denticity. 

Fig. 34. Structural types for complexed lithium amides (RR NLixLi, L = a Lewis base (a) ladders with n = 2 and 3 (b) dimeric rings (c) monomers (CIPs and SSIPs). ...

Lateral association is not restricted to lithium amides. Lithium phosphide rings (RR PLi) will have a stereochemistry similar to (RR NLi) rings. The R,R groups perpendicular to the (PLi) ring plane will preclude stacking, but facilitate laddering. The presence of a deficiency of Lewis base (less than one per Li) already precludes the formation of... 

These calculational results concur extremely well with the experimental findings. Lithium amide complexes are dimeric rings (or monomers) if the complexant Li ratio is 1 1 or greater, and if every Li bears (at least) one complexant. However, there is a switch of structural preference from ring to ladder if a base is not coordinated to each Li. In such cases, only the end-Li centers of the ladder are complexed, even though a 1 1 complexant Li ratio may be present under the experimental conditions. 

By using a combination of gas-phase synthesis and millimeter/submillimeter-wave spectroscopy, LiNH2 was found to be a monomeric unsolvated planar molecule. The lithium amide [H2NCH2CH2N(H)Li]co has a polymeric ladder structure with two types of (NLi)2 ring which alternate throughout its infinite length. ... 

While the puckered (LiN)3 ring is the central structural unit in these unsolvated amides, a ring-ladder interconversion may be observed upon addition of coligand to the lithium amides. This structural rearrangement follows the same pattern as observed in the chemistry of monofunctional... 

Supramolecular self-assembly of lithium amides can occur by a process known as ring-laddering [71] by which ladder structures are generated. An illustrative recent example is octameric [LiNHBu ]g, 126, (Figure 6.7) [109]. 

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