Lithium amides structural types

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.

Very few studies on uncompleted, lithium amides have been carried out in solution, e.g., in hydrocarbon solvents. Only five oligomeric, ligand-free compounds of this type have been characterized structurally f (54)—(58) Table VI Section III,B,1]. Studies carried out on solutions in polar solvents would, in effect, involve complexes. For example, the unsolvated crystalline trimers (55) and (56) both form dimeric etherate complexes (19, 20, 76, 81). Solution studies of com-plexed lithium amides will be described in Section III,C,3. 

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). ...

Unhindered aliphatic ketones selectively yield E-enolates if they are deprotonated by a lithium amide via a transition state structure of type A of Figure 13.15. This occurs, for example, when the B-type transition state is destabilized because of the use of a base that is even more sterically demanding than LDA such as, for example, LTMP (for structure, see Figure 4.18). For example, diethyl ketone and LTMP form the if-enolate with ds = 87 13. 

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. ... 

Recent structural investigations on lithium organo(fluorosilyl)amides have revealed that the lithium cation can form aggregates with internal lithium coordination to fluorine, and mixed aggregates of the amide and LiF. Structural types such as (205), (206), (207) and (208) have been found for these compounds. 

If we neglect the H atom in LiOH then its structure corresponds to the anti-PbO type. Since the axial ratio and the free positional parameter of this tetragonal structure can vary in a certain range, different isopuntal structure [9] types are possible. Thus for c/a = V2 and z(anion) = j, a cubic close-packing of the anions results with the cations in tetrahedral holes. An axial ratio c/a = 1/V2 and an anion parameter z =, on the other hand, correspond to the CsCl type with coordination number 8. As follows from Table 56, LiOH approximates a cubic close-packing with Li in deformed tetrahedral coordination. The position of the lone electron pair of PbO is here taken by H (corrected O—H distance 0.98 A [325] similar to the lone pair-cation distance). The electron density corresponds to Li 0 ° H and one electron smeared between the layers [1012]. In Table 55, LiOH is compared with chemically related compounds. Lithium amide has a closely related structure in which the layers of tetrahedral cation sites are alternately I and i occupied (5T1 + IT2 and ti, respectively) instead of the completely occupied and completely empty layers of LiOH. This is obviously a consequence of the weaker dipole character of NHJ. LiF, with no dipole moment, crystallizes in the rocksalt structure. The structure of LiSH is similar to chalcopyrite whereas that of the hydrosulfides and hydroselenides of Na, K and Rb is a rhombohedrally deformed rocksalt type. 

Highly enantioselectivity assumed to originate from a mixed aggregate 171 of the trans-lithium enolate of t-butyl propionate 169 and the chiral lithium amide 170 was observed in aldol additions to various aldehydes, as exemplified in Scheme 5.55. Thus, the acylated aldols obtained with benzaldehyde formed in a diastereomeric ratio of 92 8 in favor of the anti-product, with an enantiomeric excess of 94% ee [83]. More recent studies on the structures of mixed aggregates between lithium enolates and chiral amide bases (see also Chapter 3) provided an insight in this type of enantioselective conversion. 

The regioselectivity, reactivity, and structure of the aluminum adducts of type 65 and of the aluminum ate base 64 have been carefully studied [22, 23]. Alternatively, LiCl-monomerized TMP bases such as 68 and 69 allow the smooth deprotonation of various aromatic and heterocyclic compounds. These bases are soluble in THF (ca. 0.3 M) and decompose in THF at 25°C within 12 h. They are prepared in almost quantitative yield from corresponding lithium amides (Scheme 4) [24]. 

A very large number of mixed metal aluminium amides has been reported. The majority are lithium-aluminium amide salts that exhibit a variety of different structures. Only a small number of ese compounds are discussed here.39,52,57,72,118-138 simplest is LiAl-(NH2)4 produced from the reaction of lithium and aluminium in liquid ammonia at 80 to 100 °C. The atomic arrangement of LiAl(NH2)4 has been studied by IR-spectroscopy and single crystal X-ray crystallography and was found to be a new variant of the GaPS4-type structure. 

A triple anion complex containing enolate, amide, and halide functionalities can be isolated from the mixture of n-butyl bromide, hexamethyldisilazane, TMEDA, Bu Li and pinacolone (Bu COMe). The resulting solution of LiBr, LiN(SiMc3)2, LiOC(Bu )=CH2, and TMEDA produces crystals of Li4(/.t4-Br)( u-OC(Bu )=CH2)2(M-N(SiMe3)2)(TMEDA)2, which, instead of forming a ladder-type structure, consists of a planar butterfly of four lithium atoms bonded to a //4-Br the stability of this arrangement has been studied with semi-empirical (PM3) and ab initio HE/ LANL2DZ computations. ... 

A theoretical evaluation of ligands that would stabilize a vanadium-vanadium triple bond was undertaken and concluded that O- and N-donors in amidates would yield the greatest stability.890 A lantern-type metal-metal bonded dinuclear V11 complex was synthesized by the reduction of [VCl3(thf)3] with NaHBEt3 followed by addition of the lithium salt of V,V -di-p-tolylformamidate (215). 65 The crystal structure shows the V V distance to be 1.98 A, which is consistent with a fairly strong metal-metal triple bond.865... 

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