Aggregation ladder

Addition of anhydrous LiX (X = OH, Cl, Br, 1) to Li[Bu"C(NBu%] in THF afforded laddered aggregates in which two neutral lithium amidinates chelate one LiX unit. When the added salt is Lil, the monomeric laddered aggregate is isolated as a bis-THF adduct. In the case of LiOH, LiCl, and LiBr, the ladders dimerize about their external LiX edges. This process is highlighted in Scheme 10 for LiOH. The molecular structure of the resulting dimeric ladder complex is depicted in Figure 2. °... 

Anderson has produced a range of improved zinc porphyrin-DABCO ladder complexes where an aromatic substituent in the meso position reduces aggregation (57). The 1H NMR spectra show a characteristic... 

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

The above data indicate that the association energy for dimerization of stacked dimeric species G is —26.3 (—29.2) kcalmoU (energies at the 6-31G and 3-21G level of theory). The energies for the polymerization of the species C and F are —40.4 (—47.0) kcalmoU and —40.6 (—46.1) kcalmoU, respectively. This calculated energy of aggregation suggests the formation of a polymeric ladder as more favorable than the stacked dimer. In the experiment, it is seen that the stacked dimer crystallizes only on rapid cooling... 

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... 

The ladder structures formed by lithium amides and their heavier group 15 analogues stand in contrast to those formed by the related lithium alkyls which generally prefer aggregates with three-dimensional or one-dimensional polymeric structures. 

---