Tetramers ladder

LiN(CH2)3CH2)5] have been investigated structurally, spectroscopically and by computation " The tetramer has a Li. ladder structure in which a lithium at each end of the... 

Fig. 3. Structural options for uncomplexed organolithiums (a) infinite stack of (LiX)2 rings, (b) infinite ladder of (LiX)2 rings, (d) hexamer, a limited stack of two (LiX>3 rings, (e) tetramer, a limited stack of two (LiX)2 rings, and (f) isolated (LiX)4, (LiX)3, and (LiX)2 rings. For contrast, (c) illustrates a segment of an infinite three-dimensional lattice, X being an inorganic atomic anion such as CK.

The lithium derivative of the chiral chelating diamine (3 )-2-(l-pyrrolidinylmethyl)-pyrrolidine (6) has been used extensively in stereoselective synthesis, i.e. in the deprotonation of ketones and rearrangement of epoxides to homoallylic alcohols. The lithium amide has been crystallized from toluene solution, and X-ray analysis revealed that it forms a ladder-type tetramer with the two pyrrolidine nitrogens solvating the two lithiums at the end of the ladder38, (Li-6)4. 

In summary, chelating chiral lithium amides exist in either of four major structural motifs or mixtures of them (Scheme 3). Non-coordinating solvents generally favor cyclic trimers, A. Ladder tetramers are favored for pyrrolidide amides in the absence of coordinating solvents. 

Lithium amide 4 has been crystallized from toluene and X-ray crystallography has shown that the crystals are made up of tetramers of ladder type35. 

Opening the cube leads to ladder-like structures such as Li4Cl4(azetidine)2[N-(3-aminopropyl)-azetidine]2, Li4l4(2,4,6-trimethylpyridine)6, and Li4Br4(2,6-dimethylpyridine)6. The latter, prepared by recrystallizing LiBr in pyridine, has a structure typical for the class i.e., a stepped tetramer (see Pigure 97). A more complex species, LiLLiClLiL(THP) 2, is formed as a side product of the reaction of [LiL(THP)J (L = A,A-dimethyl-V -trimethylsilylethane-1,2-diamide)... 

The four-membered ring unit Hg2Cl2 can occur as a part of a ladder structure. Thus, (2-pyridylphenyl)mercury(II) chloride, 15a, is a tetramer, with the skeleton shown in 15b (only atoms directly bonded to mercury are shown for clarity). The mercury-chlorine interatomic distances are in the range 3.184-3.442 A [75]. 

In most cases of compounds having a C(0)NHP(0)(NH)2 skeleton (containing two H-acceptors-three H-donors), the HBs lead to a 1-D chain. Different 1-D ladder arrangements with tetramer motifs and a linear arrangement with two different kinds of motifs (dimer and tetramer) were also observed. Therefore, two H-donor sites (HNc(o)nhp(O) and one of the HNR) participate with two O atoms in the intermolecular HBs, the other HNR may act as the three following manners (a) in an intramolecular HB with C(0), (b) in a weaker HB with P(0) as the above mentioned tri-centered HB and (c) without cooperation in any HB. 

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