Lithium amides solid-state structures

On this basis, information about the aggregation state and the solid state structure of lithium amides becomes available. A similar relationship is known for x( O) and the Si-O-Si angle in silicates . ... 

B. Uncomplexed Lithium Amides 1. Solid-State Structures... 

III,C,1, which discussed solid-state structures of lithium amide complexes. 

Self-organization as a result of the formation of amide-bridged dimers is not limited to rfiorganoamides of the alkali metals. Occasionally alkah metal derivatives of primary amines have been shown to be dimeric in the solid state. Structurally characterized examples are [LiNHPh(thf)2]2, 79a [77a], [LiNHC6F5(thf)2]2, 79b, and [K NHC6H2(CF3)3-2,4,6 (thf)3]2, 105 [77b]. These three examples nicely illustrate how the composition of the solvates depends on the size of the alkali metal ion. In 79a and 79b two additional THF ligands are sufficient to saturate the coordination sphere of lithium (coordination number 4), whereas the potassium ions in 105 are coordinated by three THF ligands and by two weak K- - F interactions. 

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... 

An investigation of lithium diisopropyl amide (LDA) by solid state NMR led to the observation of dramatic differences between the spectra of the solid polymer and the complex crystallized from THF. Li as well as "C and "N MAS spectra showed large sideband patterns in the former case and only a few sidebands in the latter. For both materials X-ray data are available and establish a helix structure for the polymeric material, which is insoluble in hydrocarbon or ethereal solvents, and a dimer structure of the THF complex (25, 26, Scheme 4). The obvious difference between both structures, apart from the solvent coordination in the THF complex, is the magnitude of the structural N-Li-N angle, which is close to 180° in the first case and close to 90° in the second (176° and 107°, respectively). Thus, a large difference for the electric field gradient around the Li cation is expected for the different bonding situations. 

Isolated uncomplexed lithium amide dimers are not known in the solid state. However, electron diffraction shows (56) to be dimeric in the gas phase (80). The major change in structural parameters with ring... 

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. 

Section III(B,2 described how results from ab initio and MNDO calculations can explain many of the structural features found in the solid state for uncomplexed lithium amides (RR NLi) . In particular, they explain (1) why rings formed experimentally, with n = 2,3, and 4, have R,R groups perpendicular to the (NLi) ring plane (2) why a trimer (n = 3) is favored over a dimer (n = 2) and (3) why the only known... 

Although the first bis(iminophosphorano)methanides were reported by Elsevier,15 the first structurally characterised examples were reported by Cavell.16 Solvent-free 1 and 2 were prepared from the reaction between the parent methane and lithium or sodium bis(trimethylsilyl)amides in aromatic solvents. By avoiding Lewis base solvents such as ethers, dimeric complexes were isolated. Treatment with excess quantities of alkali metal amide did not effect a second deprotonation, even under reflux conditions over days, which was attributed to the less basic nature of amides compared to alkyls (see section 3.2 below). In addition to the expected methanide-alkali metal bonds, methine C H — Li interactions were observed in 1 in the solid state but the analogous C-H — Na interactions appeared to be weak in 2. 

The ladder structures found in 5-block amides are common structural motifs in phosphides as well. Solvent free [LiP(SiMe3)2]6 displays such an arrangement in the solid state, with four five-coordinate and two four-coordinate P atoms and four three-coordinate and two two-coordinate Li atoms the Li—P distances range from 2.38 A to 2.63 A (see Figure 52)." Li4(/X2-PR2)20 3 PR2)2(TFIF)2, formed from the reaction of P(SiMe3)3 with Bu Li in TFIF, has a fused tricyclic (LiP)4 ladder skeleton. The Li atoms are three-coordinate, with each of the two terminal lithiums bound to two P atoms and one THF, while the two internal lithiums have three phosphorus atoms as neighbors (see Figure 53)." " In solution, there is no NMR evidence for Xi- P... 

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